Cavendish Laboratory · HFM2014 3 GENERAL 3.1 CONFERENCEOBJECTIVES...
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Cavendish Laboratory Welcome to the 7th International Confer- ence on Highly Frustrated Magnetism 2014 Cambridge University July 7 th - 11 th , 2014
Transcript of Cavendish Laboratory · HFM2014 3 GENERAL 3.1 CONFERENCEOBJECTIVES...
CavendishLaboratory
Welcome to the 7th International Confer-
ence onHighly FrustratedMagnetism 2014
Cambridge University
July 7th − 11th, 2014
This event is organised with the kind help of Conference Cambridge:
HTTPS://WWW.CONFERENCECAMBRIDGE.COM/
This event is sponsored by:
HTTP://GLOBAL.OUP.COM/?CC=GB
HTTP://WWW.ALMAX-EASYLAB.COM/
HTTP://WWW.CRYOGENIC.CO.UK/
HTTP://WWW.PHOTONIC-SCIENCE.COM/
HTTP://WWW.NATURE.COM/NMAT/INDEX.HTML
HTTP://WWW.LOT-QD.COM/
HFM2014
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Contents
1 Organisers 5
2 Speakers 6
3 General 7
3.1 ConferenceObjectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2 Presentation Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.3 Conference Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.3.1 Tutorial Day . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.3.2 Arrival Information &Conference Registration . . . . . . . . . . . . . . . . . . . . 10
3.3.3 Conference Dinners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.3.4 Optional Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4 Travel to Cambridge Information 13
4.1 Getting to Cambridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.2 Travelling around the Surrounding Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.3 Further Travel Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5 Things to do in Cambridge 17
5.1 Food &Drink . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.2 University of Cambridge - Stride Guides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6 Conference Programme&Abstracts 18
6.1 Full programme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
6.2 Day byDay programme&Abstracts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
6.2.1 Sunday July 6th 2014 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
6.2.2 Monday July 7th 2014 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
6.2.3 Tuesday July 8th 2014 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
6.2.4 Wednesday July 9th 2014 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
6.2.5 Thursday July 10th 2014 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
6.2.6 Friday July 11th 2014 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
6.3 Poster Session Programme&Abstracts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
6.3.1 Poster Session I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
6.3.2 Poster Session II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
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1 ORGANISERS
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ORGANISINGCOMMITTEE
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• Claudio Castelnovo (Chair)
• Steve Blundell
• Steve Bramwell
• John Chalker
• Oleg Petrenko
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SCIENTIFIC PROGRAMMEEDITOR
.• Laura Bovo
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ADMINISTRATIVE SUPPORT
.• Alan Clarke
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WEBSITEDESIGN
.• Daniel Corbett
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CONFERENCE SUPPORT
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• Felix Flicker
• James Hamp
• Marianne Haroche
• Franz Lang
• Bruno Tommasello
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CONTACT
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SCIENTIFIC ADVISORYBOARD:
F. Becca A. Keren
C. Broholm R. Kremer
K. Damle C. Lacroix
J. Gardner B. Lake
B. Gaulin P.A. Lee
M. Gingras C. Lhuillier
J. Greedan P.Mendels
S. Grigera F.Mila
C. Henley I. Mirabeau
L. Heyderman R.Moessner
C. Hooley S. Nakatsuji
K. Kanoda P. Schiffer
H. Kawamura O. Tchernyshyov
C.Wiebe
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2 SPEAKERS
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PLENARY SPEAKERS
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C. Broholm
M. Gingras
R.Moessner
S. Nakatsuji
X.-G.Wen
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INVITED SPEAKERS
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• F. Bert
• G.-W. Chern
• R. Coldea
• J. Fujioka
• B. Gaulin
• L. Heyderman
• P. Holdsworth
• H. C. Jiang
• A. Laeuchli
• M. Lees
• T.McQueen
• I. Mirebeau
• Y.Motome
• A. Rosch
• R. Stewart
• A.Wills
• M. Yamashita
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3 GENERAL
3.1 CONFERENCEOBJECTIVES
The 2014 International Conference onHighly FrustratedMagnetismwill be held at CambridgeUniver-
sity in Cambridge, UK during the week of July 7-11, 2014. This international conference will focus on
recent developments in the study of the phenomenon of frustration in magnets. It will feature presen-
tations reporting on experimental and theoretical studies of magnetic frustration, in all of its manifes-
tations.
This conference follows in the series of HFM 2012 (Hamilton, Canada), HFM 2010 (Baltimore, USA),
HFM 2008 (Braunschweig, Germany), HFM 2006 (Osaka, Japan), HFM 2003 (Grenoble, France) and
HFM2000 (Waterloo, Canada).
CambridgeUniversity is conveniently located45minutesnorth-eastof Londonbyexpress train (leaving
King’s Cross Station). The conferencewill be hosted atQueens’ College (http://http://www.queens.cam.ac.uk/), withinwalking distance of Cambridge city centre. For information on the climate, the his-tory of the City and University, local orientation, and other details please refer to the ever-so-useful
Wikipedia page (http://en.wikipedia.org/wiki/Cambridge).
Claudio Castelnovo
HFM2014 Conference Chair
Cambridge University
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3.2 PRESENTATION INFORMATION
ORAL PRESENTATIONS
(PLENARY, INVITEDANDCONTRIBUTED TALKS)
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The scheduled presentation time is:
Plenary Lectures (45min.: 35min. presentation + 10minutes for discussion).
Invited Talks (30min.: 25min. presentation + 5minutes for discussion).
Contributed Talks (15min.: 12minute presentation + 3minutes for discussion).
All talks will be held in the Fitzpatrick Hall of Queens’ College. Common PC projection devices will be
available for all oral presentations. Speakers are advised to bring their talk on a USB device and with
the assistanceof theAudioVisual Technician load it onto thepresentation computerwhichwill be avail-
able(*). It is possible to connect yourWindowsLaptoporMacat the stagehowever this can lead to short
delays whilst these devices are connected.
(*) presentations should be brought on a USB device to the technicians gallery (”Machine Room”) in the
Fitzpatrick Hall, where they can be previewedwith the help of the technician and loaded on to the con-
ference computer system.
POSTER PRESENTATIONS
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Posters will be displayed on Tuesday, July 8th (Poster Session I) and Thursday, July 10th (Poster
Session II).
For Poster Session I, you can start setting up your posters onMondaymorning and you can leave
them up untilWednesday at lunch time.
For Poster Session II, you can start setting up your posters onWednesday afternoon and you can
leave them up until Friday at noon.
Posters will be mounted on the poster boards provided. The boards are 2m high x 1m wide, they are
Velcro compatible, and they take posters up to A0 portrait. Please prepare your posters so they can
easily be read froma fewmeters distance. Theboardswill be labelledwithposter ID referencenumbers
assigned.
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3.3 CONFERENCE INFORMATION
3.3.1 TUTORIALDAY
The tutorial daywill be held on the Sunday preceding the start of the conference
HFM2014, in the Bowett room at Queens’ College. It is a graduate focus work-
shop supported by the EPSRCNEtworkPlus on Emergence and Physics far from
Equilibrium (http://qr.net/ywsp).
Frustration inmanybodysystemsdenotes the inability tofindstates that simultaneouslyminimiseall in-
teraction energy terms. Contrary to conventional systems, frustrated systems do not order in a unique
ground state at low temperatures. Whilst they remain disordered, they often develop non-trivial cor-
relations that lead to new and unexpected properties.
The concepts of emergence and far fromequilibriumphenomena play amajor role in understanding the
physics of frustrated systems. For instance, frustration introduced by disorder has been known to give
rise to extraordinarily long time scales and glassiness, which have been and are currently the subject
of significant research efforts world-wide. More recently, frustrated spin systems have been found to
exhibit emergent gauge symmetries and excitations, leading to discovery of new phases known as spin
liquids, often displaying topological order and fractionalisation.
This one-day workshop will present some of the key concepts and techniques relating to the study of
emergence and far from equilibrium phenomena in frustrated magnetic systems. The level of the talks
and discussions will be aimed at junior faculty, postdocs and graduate students (not necessarily from
the specific area of research), with the aim to stimulate interdisciplinary discussions.
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DETAILS
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Speakers: C. Broholm (JohnsHopkins), B. Gaulin (McMaster), L. Heyderman (PSI), P. Holdsworth
(ENS Lyon), E. Y. Vedmedenko (Hamburg) and X-G.Wen (Perimeter).
The talkswill be held in theBowett roomatQueens’ College (No.7on theCollegemap in Sec.4.3).
They will start in the morning at 10:15 and coffee will be available beforehand from 9:30. The
event will come to a close around 18:30.
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3.3.2 ARRIVAL INFORMATION&CONFERENCE REGISTRATION
Delegates should arrive at Queens’ College via the Porters Lodge on Silver street (see Sec.4.3 for more
details on how to get there). The Porters Lodge staff will direct them to the registration area and/or
their accommodation, as appropriate. There will also be posted bespoke signs.
All accommodation booked through the conferencewebsite is on site andwithinwalking distance from
the Porters Lodge.
..
The registration area is in the lobby of Cripps Hall (No.2 on the Collegemap provided in Sec.4.3).
The registration desk will be open from 9:00 to 19:00 on the Sunday and from 8:00 until 17:00,
Monday through Thursday.
All meals will be served in the Cripps diningHall, steps away from the lecture theatre (Fitzpatrick
Hall, No.2 on the Collegemap in Sec.4.3).
3.3.3 CONFERENCEDINNERS
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WELCOMEDINNER - SUNDAY JULY 6TH
.
A welcome dinner will be held on Sunday evening, 6th of July, at 19:00 in the Cripps dining Hall
(No.2 on the Collegemap provided in Sec.4.3).
There is no seating plan for the dinner.
If youdid not express your intention to attend the dinner, aswell as dietary requirements, in your
accommodation booking for the conference, please contact the organisers as soon as possible.
Wemay not be able to accommodate people who do not provide this information in advance.
..
CONFERENCEDINNER -WEDNESDAY JULY 9TH
.
The conference dinner will be held on Wednesday evening, 9th of July, at 19:00 in the Cripps
dining Hall (No.2 on the Collegemap provided in Sec.4.3).
There is no seating plan for the dinner.
If youdid not express your intention to attend the dinner, aswell as dietary requirements, in your
accommodation booking for the conference, please contact the organisers as soon as possible.
Wemay not be able to accommodate people who do not provide this information in advance.
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3.3.4 OPTIONAL EVENTS
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GUIDED TOUROF THECITYOFCAMBRIDGE
.
Guided walking Tours of the City of Cambridge and its Colleges have been arranged, for those
of you who are interested, during the free afternoon on Wednesday, 9th of July. They will be
conducted by certified VisitCambridge guides.
The cost per person is of GBP 5.00 for a 1.5 hour tour of the City or GBP 12.00 for a 2 hour tour
of the City + King’s College. Each guide will take a group of approximately 15-20 people and the
choice of tour will be arranged directly with your guide and the other groupmembers.
We have tentatively booked four guides, according to the number of participantswho expressed
an interest in the city tour at registration. If you have not expressed an interest andwould like to
be included in the initiative, please let the organisers know as soon as possible.
The guides will meet you at the conference location, in the Cripps Court of Queens’ College, at
14:30 onWednesday, 9th of July. They will collect payments and assemble groups for each tour
guide. Please be ready to leave at 15:00.
Please look out for the ’HFMCity Tour’ sign. You should bring cash to pay the guide directly on
the day. If at all possible, please bring exact change for your preferred tour option.
The tourswill operate also inmildly adverseweather conditions. Should this be the case, the use
of a rain coat and/or umbrella may be advisable.
..
Our Historic University and College Tours cover the most important aspects of the city, University and
colleges. Your guide will relate some of the fascinating stories regarding the famous people connected
with Cambridge whilst looking at some of the best known and impressive sights Cambridge has to offer.
In your private tour, there will be plenty of opportunity for questions en route and time to explore areas
of particular interest in detail.
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PUNTINGONTHERIVERCAM
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Punting in the riverCam is a typical Cambridge leisure activity and allows to enjoy some spectac-
ular views of the City and Colleges from a unique perspective. It can be easily arranged at one’s
own convenience, although the river does become rather busy depending on time of the day and
weather conditions; pre-bookingapuntmaybeadvisable. Punting caneitherbechauffeured (i.e.,
with a driverwho doubles as a tour guide) at a higher price and usually in a group; or one can rent
a punt by the hour and attempt their luck at propelling and steering such unconventional means
of transportation.
Images, technical details as well as anecdotal information can be found on the Wikipedia page:
http://en.wikipedia.org/wiki/Punt_(boat).
All punting activities are to be arranged with Scudamore’s at one’s own convenience. Scud-
amore’s has several punting stations across town, ofwhich one (Mill Lane Punting Station, http://www.scudamores.com/punting-mill-lane) is conveniently located just across the road (andthe river) with respect to Queens’ College Porter’s Lodge. We have negotiated discount vouch-
ers for conference participants, which will be included in your registration package upon arrival.
Each voucher gives a discount for one person plus an optional guest. Please arrange all further
details such at times, type of punting experience, as well as bookings directly with Scudamore’s
(http://www.scudamores.com/).
The punting vouchers are valid all week long and Scudamore’s is open well into the late evening.
It is alsowell within reach to enjoy a punting trip after the city tour onWednesday and be back in
time for the conference dinner!
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Chauffeured punt tour:
Our private punt tours offer the ideal introduction to Cambridge’s eventful history. Discover how the city
has evolved frommedievalmarket town to its position today as aworld-famous centre of academic excel-
lence whilst enjoying the spectacular views. From the gothic splendour of King’s College, to the classical
elegance of Clare College, your punting guide will make sure that you don’t miss a thing.
A private tour offers you the exclusivity of your own punt, reserved for your preferred time and bypassing
the public tour queue. With the chauffeur all yours, you can decide how much of the commentary you
wish to hear and ask asmany questions as you like. Choose between a 45minute or a 60minute private
punt tour.
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4 TRAVEL TOCAMBRIDGE INFORMATION
4.1 GETTING TOCAMBRIDGE
TOURDE FRANCE - IMPORTANT INFO
Please note that the first day of the conference, Monday the 7th of
July, the Tour de France will be passing through Cambridge. Much of
the city centre, including the streets around the conference venue, will
be closed to car access from the very early morning until late after-
noon.
We strongly encourage participants to arrive on the Sunday to avoid possible disruptions. If you re-
quire vehicle access to Queens’s College, you can find further details about the Tour and related street
closures here http://qr.net/yT6S.
ARRIVINGBYAIR
CambridgeAirport (http://www.cambridgeairport.com/) is convenientlylocated only a few miles from the city centre and the conference venue.
Direct connections are available between Cambridge and a handful of na-
tional and international locations (please consult the airport website for up
to date routes information http://www.cityjet.com/).
The second closest airport is London Stansted (http://www.stanstedairport.com/) which is situated30 miles south of Cambridge and offers a number of routes to and from Europe and other UK destina-
tions. Direct routes to Cambridge by National Express bus services (http://www.nationalexpress.com/home.aspx) and trains are available and taxis are plentiful.
LondonGatwick (http://www.gatwickairport.com/), LondonLuton (http://www.london-luton.co.uk/) andLondonHeathrow(http://www.heathrowairport.com/) areallwithin1-2hoursdriveofCam-bridge and accessible by public transport via connecting bus and train services.
ARRIVINGBY TRAIN
Cambridge has a fast train service to and from London Kings’ Cross, which
is adjacent to St Pancras for Eurostar connections (http://www.eurostar.com/). The journey takes approximately 45 minutes and several fast trainsdeparteachhour, to checkexactarrival anddeparture timespleasevisitNa-
tional Rail (http://www.nationalrail.co.uk/).
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TO/FROMCAMBRIDGEAIRPORT
There isashuttleservice (http://airportlynx.co.uk/shuttle/shuttlebus.html) to/from the Cambridge Airport that will drop off / pick up at any de-
sired location in town. Tickets can be pre-booked online. AirportLynx oper-
ates also a normal taxi service from their desk in theArrivals hall. The same
companyprovides shuttle services to/fromall othermajor LondonAirports.
Alternatively, connecting bus services (http://www.nationalexpress.com/home.aspx) and taxis areplentiful.
TRAVELLINGBYCAR
For delegates that prefer to drive, Cambridge offers a number of Park & Ride sites on the outskirts of
the City and journey times into the centre take just a fewminutes.
Few parking spaces are available near the conference venue and advance booking is strictly required.
Please contact theorganisers at [email protected] toenquire about cost (approximately3.50
GBP/day), availability and to reserve a parking space.
4.2 TRAVELLINGAROUNDTHE SURROUNDINGAREA
You canfinddetailed information about public transport and routes intoCambridge at TransportDirect
(http://qr.net/y0ec) and Traveline East Anglia (http://qr.net/y0eF).
If you have a mobility disability, Cambridge and Cambridgeshire Passenger Transport has useful infor-
mation such as which services have low-floor buses. Contact them by telephone on 01223 717 740.
Many people now travel into the city on the Park & Ride bus service (http://qr.net/y0f5).This operates frequent buses from five car park locations on the city out-
skirts into the city centre.
For information on the guided busway that runs between St Ives and Cam-
bridge, and between Huntingdon and Cambridge, visit the county council’s
Guided Buswaywebpages (http://qr.net/y0if).
There is an excellent Citi Bus Network (http://qr.net/y0jx) around thecity, includinganightbusonFridayandSaturdaynights. Therearealsogood
services into the city from themain local towns.
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4.3 FURTHER TRAVEL INFORMATION
The conferencewill be hosted byQueens’ College. This is a summary of es-
sential informationextracted fromtheQueens’Collegevisitors information
website (http://qr.net/y0mA), which you are kindly referred to, shouldyou need further details.
Post Code: CB3 9ET (in the UK, the post code is sufficient to identify a building, say using Google maps
or other navigation devices).
The site plan of the College is reported below. All lunches, as well as the welcome and conference din-
ners are served in the Cripps Dining Hall (No.10). The auditorium is the Fitzpatrick Hall (No.5). Coffee
breaks are served in the Conservatory andCollege Bar areas (No.8&9), next to the LyonCourt. Accom-
modation is arranged in the Fisher building, as well as in the buildings surrounding Cripps Court.
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t Gra
ntch
este
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until
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reac
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roun
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om 0
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o st
raig
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cros
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rst e
xit)
into
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traf
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ghts
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to S
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et a
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thro
ugh
bolla
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ens'
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ntra
nces
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on
the
left.
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ther
tim
es, t
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right
(sec
ond
exit)
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n C
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t of t
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out b
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ambr
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left
and
keep
in th
e le
ft la
ne;
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o st
raig
ht a
cros
s an
imm
edia
te s
econ
d m
ini-r
ound
abou
t (at
Len
sfie
ld R
oad)
, to
head
nor
th u
p Tr
umpi
ngto
n St
reet
; *
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stra
ight
pas
t the
Mill
Lan
e/Pe
mbr
oke
Stre
et ju
nctio
n;
* At
Silv
er S
treet
, tak
e a
forc
ed le
ft tu
rn (K
ing'
s Pa
rade
is c
lose
d to
mot
or tr
affic
); *
Follo
w S
ilver
Stre
et o
ver t
he R
iver
Cam
: Que
ens'
Col
lege
ent
ranc
es a
re im
med
iate
ly
on th
e rig
ht.
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ES
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T R U M P I N G T O N S T
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S R D
R E G E N T ST
DR
UM
ME
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T
P
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Queen’s College Cambridge 16
HFM2014
Queens’ College is located where Silver Street crosses the River Cam. The
main entrance and Porters’ Lodge is on Silver Street, close to the river (in-
dicated by the red arrow and black bowler hat within the red circle on the
map: http://qr.net/y0tZ).
All participants should go to the Porters’ Lodge on arrival.
The College is right in the centre of town, 0.6 miles from Central Bus station on Drummer street (with
connections to all major airports in the London area), and 1.3 miles from the Railway station (with fast
and frequent connections to LondonKing’s Cross Station; this provides an alternative route to the Lon-
don airports).
5 THINGS TODO INCAMBRIDGE
5.1 FOOD&DRINK
This link http://qr.net/CHxF includes an introductory paragraph and a
search and book function that delegates might find useful. The easiest way
to find places to eat in Cambridge is to check out the SEARCH FOOD &
DRINK on the right hand side of this web page.
5.2 UNIVERSITYOF CAMBRIDGE - STRIDEGUIDES
TheUniversity of Cambridge has commissioned these podcast /MP3 audio
walks. You can listen to themonline, or you can download the audio to your
iPod or mobile player for your own guided audio tour around Cambridge.
The walks vary in length - you can go at your own pace, stopping between
tracks as you go. You can download the audio walks along with maps here:
http://www.strideguides.com/cu/.
Queen’s College Cambridge 17
HFM2014
6 CONFERENCE PROGRAMME&ABSTRACTS
6.1 FULL PROGRAMME
Foro
nda
FuRum
iny
10:1
5 -
10:3
0G
off
Shi
miz
uO
noda
10:3
0 -1
0:45
11:1
5 -
11:3
0Q
&A
Gui
tten
y11
:15
- 11
:30
11:4
5 -
12:0
0Kim
Qui
nter
o-Cas
tro
Dun
sige
rPr
att
12:0
0 -
12:1
5Sta
rykh
Cla
rkLh
otel
Yosh
ida
12:1
5 -
12:3
0M
ilaW
iebe
Bov
oSm
irno
v12
:30
- 12
:45
Q&
AJa
uber
t
14:3
0 -
14:4
5M
orle
yD
epen
broc
k14
:45
- 15
:00
Q&
A14
:45
- 15
:00
Bra
nfor
dM
essi
oKov
rizh
inte
a
16:0
0 -
16:1
5Q
&A
16:0
0 -
16:1
5U
daga
wa
16:1
5 -
16:3
0G
hosh
17:4
5 -
18:0
0Q
&A
18:3
0Clo
sing
Laeu
chli
McQ
ueen
Ber
t
Wen
Jian
g
Nak
atsu
ji
Wed
nesd
ay
Hol
dsw
orth
Moe
ssne
r
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lin
coffee
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gras
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eau
Frid
ay
Lees
Th
urs
day
Mo
nd
ay
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esd
ay
Bro
holm
Col
dea
Will
sRos
ch
09:0
0 -
09:4
5
09:4
5 -
10:1
5
10:4
5 -
11:1
5
Yam
ashi
ta11
:30
- 12
:00
09:3
0 -
10:1
5
10:1
5 -
11:1
5
Su
nd
ay
Tu
tori
al
12:3
0 -
14:0
0
14:0
0 -
14:3
0
09:0
0 -
09:4
5
09:4
5 -
10:1
5
10:1
5 -
10:4
5
11:1
5 -
11:4
5
10:4
5 -
11:1
5
11:3
0 -
12:3
0
12:4
5 -
13:4
5
13:4
5 -
14:4
5
lunc
h
dinn
er
free
aft
erno
on
Ste
war
t
tea
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ka
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derm
anChe
rn
post
er
sess
ion
I
12:0
0 -
13:0
0
13:0
0 -
14:0
0
post
er
sess
ion
II
Mot
ome
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mar
y a
nd
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sing
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dsw
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lunc
h
tea
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lin
15:0
0 -
15:3
0
15:3
0 -
16:0
0
19:0
019
:00
Vedm
eden
ko
16:4
5 -
17:4
5
18:0
0 -
18:3
0
16:1
5 -
16:4
5
Hey
derm
an
Bro
holm
dinn
er
15:0
0 -
16:0
0
Queen’s College Cambridge 18
HFM2014
6.2 DAYBYDAYPROGRAMME&ABSTRACTS
6.2.1 SUNDAY JULY 6TH 2014
11:15 - 11:30
12:30 - 12:45
14:45 - 15:00
16:00 - 16:15
17:45 - 18:00
18:30
11:30 - 12:30 Xiao-Gang Wen, Perimeter Institute
10:15 - 11:15 Peter C.W. Holdsworth, Ecole Normale Superieure de Lyon
09:30 - 10:15
Closing
15:00 - 16:00 Collin L. Broholm, Johns Hopkins University
19:00
Unraveling the complex dynamics of frustrated magnets with neutrons
Artificial Frustrated Spin Systems from Dipolar-Coupled Nanomagnets
Questions and Answers
Questions and Answers
tea
dinner
16:15 - 16:45
16:45 - 17:45 Laura J. Heyderman, ETH Zurich, Paul Scherrer Institut
Sunday, 6 July 2014
Special Topic: Elena Y. Vedmedenko, University of Hamburg, Institute for
Applied Physics18:00 - 18:30
Application of the string theory to the two-dimensional dipolar spin ice:
How to store energy in Dirac strings.
Questions and Answers
Tutorial
coffee
Classical frustrated magnetism
The magic of many-body entanglement: A unification quantum information,
quantum matter, and elementary particles
From new materials to new understanding: phase behaviour,
structure, and ground state selection in frustrated magnets
Questions and Answers
Questions and Answers
lunch12:45 - 13:45
13:45 - 14:45 Bruce D. Gaulin, McMaster University
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Session Chairs of the day:
10:15 -12:45 G.Moeller
13:45 -16:00 P. Holdsworth
16:45 -18:30 S. Blundell
Queen’s College Cambridge 19
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KEYNOTE
.
Classical FrustratedMagnetism.
Peter C.W. Holdsworth (École Normale Supérieure de Lyon)
In this talk Iwill outlinesomeof thekey features thathavemade frustratedmagnetismoneof the leadingareasofmany
body research over the last 25 years. Frustration can be driven, either bymagnetic disorder, or by lattice geometry. In
either case, it can be thought of as the inability to satisfy pairwise interactions around closed loops on themicroscopic
scale. Theresult is thesuppressionofmagneticordering, leadingultimately tospinglassphysics, in thecaseofdisorder
and classical spin liquid behaviour for geometrically frustrated systems. I will review the similarities and differences
between spin glasses and spin liquids, putting emphasis on the remarkable range of dynamic response observed both
experimentally and theoretically.
Classical spin liquids are characterized by extensive ground state degeneracy, so that fluctuations play a key role in
their low temperature behaviour. As a consequence, they have proved to be a laboratory for a huge array of emergent
many body phenomena. I will review some of these, including entropic selection of ordered phases and emergent
gauge physics. Time permitting I will try to give some pointers to the future, where among other things, quantum
fluctuations, couplingofmagnetic and itinerantelectronicdegreesof freedom, confinementand the returnofdisorder
promise to be important issues.
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KEYNOTE
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TheMagic ofMany Body Entanglement: A UnificationQuantum Information, QuantumMatter, and ElementaryParticles.
Xiao-GangWen (Perimeter Institute)
Many-body entangled spins (qubits) can lead to new topological states ofmatter, and I will explain that how fermions,
anyons, gauge fields can emerge from those topologically ordered states. If our vacuumhappen to be proper topolog-
ically ordered state of qubits, then the emergent quasiparticlesmay behave like the elementary particles in ourworld.
This will represent a unificationmatter and (quantum) information.
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KEYNOTE
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FromNewMaterials to NewUnderstanding: Phase Behaviour, Structure, and Ground State Selection inFrustratedMagnets.
Bruce D. Gaulin (McMaster University)
Newmagneticmaterials generatemuch of the interest and excitement in geometrically frustratedmagnets. Typically,
a particular research thread starts withmaterials synthesis in polycrystal form, followed bymeasurements which es-
tablish the broad-brush characteristics of the relevant low temperature phases. Single crystal growth may be subse-
quently possible, which opens the field up to themost sophisticated experimental studies. However, along thewaywe
often learn that the materials so produced are not perfect, but possess defects and non-stoichiometries. These typi-
cally occur at a sufficiently low level that theywould not be expected to affect the ground state properties of conven-
tionalmagnetic ground states - but certain frustrated ground states havebeen shown tobe surprisingly susceptible to
these effects. I’ll describe several programs of materials preparation and characterization of the rare earth titanates
Yb2Ti2O7, Er2Ti2O7 and Tb2Ti2O7, mostly using neutron scattering and low temperature heat capacity techniques,
which illustrate these and related issues. Along the way I’ll introduce neutron scattering techniques and how these
inform on chemical structure andmagnetic phases, both conventional and otherwise.
Queen’s College Cambridge 20
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KEYNOTE
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Unraveling the Complex Dynamics of FrustratedMagnets with Neutrons.
Collin Broholm* (Johns Hopkins University)
Neutron scattering canprovidedetailed information about the atomic scale dynamics of frustratedmagnets. This talk
will provide thebackground information tobetter understand theexperimental results tobepresentedat theensuing
conference. It will also be an invitation to employ this rapidly developing technique in your own scientific work as an
experimentalist or a theorist.
I shall start by describing the theoretical framework of inelastic neutron scattering. The focus shall be on aspects of
the theory that help to understand the nature and capabilities of the technique and the methods of analysis that are
in use.
Then I shalldescribe threetypesofneutronspectrometers thatcoversixordersofmagnitude inenergy: Thedirectand
inverse geometry time of flight spectrometer, the triple axis spectrometer, and the neutron spin echo spectrometer.
The capabilities of each instrument type and the character of the data produced is reviewed.
The talkwill emphasize the latest developments in instrumentation and thepromise of various ongoing developments
in neutron sources and instrumentation.
*Supported by U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under
award No. DE-FG02-08ER46544.
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KEYNOTE
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Artificial Frustrated Spin Systems fromDipolar-Coupled Nanomagnets.
Laura J. Heyderman (ETH Zurich, Paul Scherrer Institute)
Artificial spin systems [1] have received much attention in recent years, largely due to the pioneering work of Peter
Schiffer and his group in 2006 [2], who created a systemof dipolar-coupled nanomagnets arranged in a frustrated ge-
ometry with magnetic behaviour analogous to that of the rare earth titanate pyrochlores, and appropriately named
artificial spin ice [3]. In the square ice, elongatedmagnets are placed on the sites of a square lattice and, at every vertex
where four magnets meet, the lowest energy configuration consists of two-moments pointing into and twomoments
pointing away from the vertex, obeying the so-called ice rule. With modern lithography techniques, one can create
artificial spin systemswith various intricate designs that go beyond the square ice geometry, and particularly appeal-
ing is the possibility to directly observe the magnetic configurations resulting, for example, from the application of a
magnetic field, usingmagnetic microscopymethods.
In this tutorial, I will give a brief historical perspective of the field of dipolar-coupled nanomagnets and discuss the
three key areas that have so far been addressed by research into artificial spin ice. First, I will discuss attempts to
achieve the ground statewith various protocols involving theuseof alternatingmagnetic fields to provide aneffective
thermal anneal. Secondly, I will cover field reversal experiments in extended nanomagnet arrays and, in particular,
discuss the observation of emergent magnetic monopoles and their associated Dirac strings. Finally, I will outline
the recent work on thermally active systems, demonstrating how thermal annealing can provide a successful route
to the ground state and how the magnetic moment reorientations can be directly observed with synchrotron x-ray
photoemission electronmicroscopy.
I will conclude by highlighting possible areas for future research, which involve the study of different geometries, the
pursuit of device applications and observation of fast dynamics. I will also indicate the experimental details that one
should consider when constructing artificial spin systems and compare the various microscopy techniques available
for the observation of themagnetic configurations.
[1] L.J. Heyderman and R.L. Stamps, J. Phys.: Condens. Matter 25, 363201 (2013); [2] R.F. Wang, C. Nisoli, R. S. Freitas, J. Li, W. Mc-
Conville, B J.Cooley,M.S. Lund,N. Samarth, C. Leighton, V.H.Crespi, andP. Schiffer, Nature439, 303 (2006); [3]C.Nisoli, R.Moessner
and P. Schiffer, Reviews ofModern Physics 85 (4), 1473-1490 (2013)
Queen’s College Cambridge 21
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SPECIAL TOPIC
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Application of String Theory to the Two-Dimensional Dipolar Spin Ice: How to Store Energy in Dirac Strings.
Elena Y. Vedmedenko (University of Hamburg)
String theories are elegant mathematical concepts that can be applied in different ways, but are best known as can-
didates to explain the laws of the physical universe in a unified manner. Here, an effective string theory is applied to
two-dimensional dipolar spin ices (2D-DSI), artificially created analogs of bulk spin-ice (3D-DSI). In contrast to 3D-
DSI their 2D counterparts on a square lattice should not retain a Pauling entropy plateau at low temperature, but
spin ice configurations with emerging monopole defects have been reported in real systems. The theory of magnetic
monopoles applies to 3D-DSI because the band width of Pauling states - the states that make up the monopole vac-
uum - is 100mKandmuch smaller than themonopole creation costs of fewK. In 2D-DSI this bandwidth is comparable
with the energy scale for monopole creation , hence calling into question themonopole picture.
In this work analytical and numerical analysis are used to show that an effective string theory provides the natural
description of the finite-width band of Pauling states in 2D-DSI. The fine-structure constant α ≈ 1/137 comes out
naturally from the calculation of the Dirac string tension. Furthermore, it is demonstrated that the string charge con-
servationmaybeused toachieve spontaneous, long-lasting transientmagnetic currents; i.e.,magnetricity. Thedriving
force of spontaneous current is the natural decay of open strings towards their closed counterparts. Based on the dis-
covered effect an application of strings for the energy and charge storage as well as information transfer is proposed.
Specifically, energy can be stored in a saturated square 2D-DSI with low coercivity by pinning the rim dipoles. This
is illustrated by an experimental demonstration of a set of interacting magnetic elements. In general these results
establish a simple physical system for the laboratory test of string theories.
Queen’s College Cambridge 22
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6.2.2 MONDAY JULY 7TH 2014
11:45 - 12:00 A.L. Chernyshev, UC Irvine
Dynamics, lifetime, and disorder in quantum magnets and liquids
12:00 - 12:15 O. A. Starykh, University of Utah
Unusual ordered phases of magnetized frustrated antiferromagnets
12:15 - 12:30 F. Mila, EPFL, Lausanne
Crystals of bound states in the magnetization plateaus of the Shastry-Sutherland model
14:30 - 14:45 S. A. Morley, University of Leeds
Thermal and field-driven dynamics of Artificial Spin Ice in real and reciprocal Space
14:45 - 15:00 W. R. Branford, Imperial College London
Symmetry breaking in Artificial Spin Ice nanostructures
16:00 - 16:15 M. Udagawa, University of Tokyo
Transport theory of itinerant spin ice
16:15 - 16:30 S. Ghosh, LASSP, Cornell University
Complex incommensurate orders in the kagome Kondo lattice model
T.M. McQueen, Johns Hopkins University
10:45 - 11:15
Monday, 7 July 2014
Metastability and incommensurability in frustrated magnets
Neutron and x-ray scattering studies of candidate Kitaev materials
09:45 - 10:15 R. Coldea, Oxford University
09:00 - 09:45C. L. Broholm,
Johns Hopkins University
Doping of a triangular lattice antiferromagnet: Magnetic properties and electrical transport
coffee
10:15 - 10:45
lunch 12:30 - 14:00
J. Fujioka, University of Tokyo
Skyrmions and monopoles in chiral magnets
15:00 - 15:30
15:30 - 16:00
14:00 - 14:30 L. J. Heyderman, ETH Zurich, Paul Scherrer Institut
Magnetic field driven and thermally active behaviour of Artificial Spin Ice
Metal-insulator transition in pyrochlore-type iridates with anomalous magnetic domain-wall conductance
tea
11:15 - 11:45 A. Rosch, University of Cologne
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Session Chairs of the day:
9:00 -10:45 S.T. Bramwell
11:15 -12:30 M. Gingras
14:00 -15:00 E.Y. Vedmedenko
15:30 - 16:30 C. Castelnovo
Queen’s College Cambridge 23
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PLENARY
.
Metastability and Incommensurability in FrustratedMagnets.
C. Broholm (Johns Hopkins University)
A possible ground state of a frustrated magnet is a modulated spin configuration with a period that balances com-
peting interactions. Beyond breaking inversion symmetry and therefore potentially being multiferroic, long-range
modulated phases can form intricate domain walls and skyrmion lattices.
In this talk I discuss slow dynamics and metastability associated with incommensurate phases of spin systems with
near and next nearest interactions that compete along certain crystalline directions. SrHo2O4 turns out to be a
frustrated quasi-one-dimensional Ising spin system that beautifully illustrates the key characteristics. These help to
understand the slow spin dynamics and metastability observed in quasi-two-dimensional NiGa2S4 and multiferroic
Ni3V2O8.
The talk is based on recent neutron scattering experiments including spin echo andwide angle polarized diffraction.
[Supported by U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under
award No. DE-FG02-08ER46544.]
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INVITED
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Neutron and x-ray scattering studies of candidate Kitaevmaterials.
R. Coldea, Sunkyun Choi, A. Biffin, R.D. Johnson, A.N. Kolmogorov, S.J. Blundell, P.G. Radaelli (Oxford Univ.), T. Lancaster (DurhamUniv.), I.I.
Mazin (NRL), Y. Singh, S. Manni, P. Gegenwart (Univ. Goettingen), J.G. Analytis (Univ. Berkeley), K. R. Choi, S.W. Cheong (Rutgers Univ.), C.
Stock, J. Taylor, P. Manuel, P.J. Baker (ISIS), A. Bombardi (Diamond)
This talk will describe neutron and x-ray scattering studies of the crystal structure and cooperativemagnetism of the
frustrated layered honeycomb magnets Na2IrO3 [1] and Li2IrO3, and a polytype of Li2IrO3 with a three-dimensional
honeycomb network [2]. Those materials are candidates to display composite spin-orbital J = 1/2moments at the
Ir4+ ions, coupled by strongly-frustrated bond-directional anisotropic exchanges, the so-called Kitaev spin model,
which has in its phase diagram novel magnetically-ordered phases and a quantum spin liquid with exotic excitations.
In search for such physics we have performed x-ray and neutron scattering studies of the crystal structure and mag-
netic order, and the spin dynamics using a optimized setup tominimize the strong neutron absorption by the Ir nuclei.
We have been successful in observing strongly dispersive excitations of the Ir moments in both layered honeycomb
magnets Na2IrO3 [1] and Li2IrO3 and results are comparedwith predictions for a Kitaev-Heisenbergmodel as well as
a Heisenbergmodel with further neighbour couplings.
[1] Sungkyun Choi, R. Coldea, A.N. Kolmogorov, T. Lancaster, I. I. Mazin, S. J. Blundell, P. G. Radaelli, Yogesh Singh, P. Gegenwart, K. R.
Choi, S.-W. Cheong, P. J. Baker, C. Stock, and J. Taylor, Phys. Rev. Lett. 108, 127204 (2012); [2] K.A. Modic, T.E. Smidt, I. Kimchi, N.P.
Breznay, A. Biffin, Sungkyun Choi, R.D. Johnson, R. Coldea, P. Watkins-Curry, G.T. McCandless, F. Gandara, Z. Islam, A. Vishwanath,
J.Y. Chan, A. Shekhter, R.D.McDonald, J.G. Analytis, arXiv:1402.3254 (2014); [3] J. Chaloupka, G. Jackeli, andG. Khaliullin, Phys. Rev.
Lett. 105, 027204 (2010).
Queen’s College Cambridge 24
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INVITED
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Doping of a Triangular Lattice Antiferromagnet: Magnetic Properties and Electrical Transport.
T.M. McQueen (Johns Hopkins University), J.P. Sheckelton (Johns Hopkins University)
LiZn2Mo3O8 is an insulating material comprised of Mo3O8 layers separated by disordered LiZn2 layers. The Mo3O8
layers consist of edge-sharingMoO6 octahedra that formdiscreteMo3O13 clusters in a triangular arrangement. A for-
mal electron count suggests eachMo3O13 cluster collectively yields a S = 1/2 spin. Prior experiments have suggested
LiZn2Mo3O8 exhibits a gapless spin excitation spectrum with spin fluctuations that slow down as the temperature is
lowered. These data are consistent with short-range spin correlations from the formation of a condensed valence-
bond state. In this talk, I will provide a brief summary of previous results, and then present our findings of the effect
of hole doping on this triangular lattice antiferromagnet system, over the entire accessible electron count (0 to 1 un-
paired electrons per site) in comparisonwith theoretical expectations. If time permits, related new frustrated cluster
systemswill also be discussed.
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INVITED
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Skyrmions andmonopoles in chiral magnets.
A. Rosch (University of Cologne)
In cubicmagnetswithout inversion symmetry so-called skyrmion lattices, stable arrays ofmagneticwhirls, form in the
presence of small magnetic fields. Thesewhirls are characerized by awinding number and are therefore topologically
quantized. The skyrmions couple efficiently to electrons by Berry phaseswhich can be described by sizable emergent
magnetic and electric field which have been detected in a series of experiments. We investigate the dynamics of a
first-order phase transition where the skyrmions are destroyed and the topolgy of the magnetic state changes. This
transition can be described by themotion of hedgehog defects, which can be interpreted asmagnetic monopoles and
antimonopoles. We discuss the forces on suchmonopoles and their dynamics.
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CONTRIBUTED
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Dynamics, lifetime, and disorder in quantummagnets and liquids.
Sasha Chernyshev, Mike Zhitomirsky
High-resolution neutron resonance spin-echo experiments allow for precise measurements of lifetime and energy
shift of elementary excitations in quantum magnets and superfluid 4He. In the latter, the roton energy shift vs tem-
perature differs from that of the roton’s inverse lifetime, in contradiction to the Landau-Khalatnikov theory. We find
that the interactionof rotonswithphonons, allowedby thebrokengauge symmetryof theBosecondensate, is respon-
sible for the effect. For a prototypical 2D XY antiferromagnet, we show that local modulations of magnetic couplings
have a profound effect on the temperature-dependence of the inverse lifetime of excitations. The disorder-induced
relaxation at low temperatures can greatly exceed the effect of conventionalmagnon-magnon scattering. Our results
agree with experimental data and generalizations to the other systems are offered.
Queen’s College Cambridge 25
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CONTRIBUTED
.
Unusual ordered phases of magnetized frustrated antiferromagnets.
Oleg A. Starykh (University of Utah), Leon Balents (KITP), Andrey V. Chubukov (University ofWisconsin)
Frustrated antiferromagnets in magnetic field host a number of unusual magnetically ordered phases. In my talk I
describe an interesting interplay between two such states - spin nematic state (SN) and collinear spin density wave
state (SDW), both of which were recently observed experimentally in frustrated spin-1/2 materials. Both phases are
characterized by fully gapped transverse spin excitations with S = 1 but different broken symmetries. I first show
that, somewhat unexpectedly, weak interchain exchange interaction betweennematic spin chains actually stabilizes a
two-dimensional collinearSDWphase, andbrieflydescribe itsproperties. I then turn to the1/3magnetizationplateau
phaseof the triangularantiferromagnet,whichcanbethoughtofasacommensurateSDWstate, anddescribe its insta-
bility, via a novel two-magnon condensation transition, towards a nematic spin-current state. This gapped state spon-
taneously breaks lattice inversion symmetry and is characterized by a dynamically generated Dzyaloshinskii-Moriya
interaction.
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CONTRIBUTED
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Crystals of bound states in themagnetization plateaus of the Shastry-Sutherlandmodel.
F. Mila (EPFL, Lausanne, Switzerland) and P. Corboz (ETHZ, Zürich, Switzerland)
Using infiniteprojectedentangled-pair states (iPEPS),we showthat theShastry-Sutherlandmodel in anexternalmag-
netic field has low-magnetization plateauswhich, in contrast to previous predictions, correspond to crystals of bound
states of triplets, and not to crystals of triplets. The first sizable plateaus appear at magnetization 1/8, 2/15 and 1/6,
in agreementwith experiments on the orthogonal-dimer antiferromagnet SrCu2(BO3)2, and they can be naturally un-
derstood as regular patterns of bound states, including the intriguing 2/15 one. We also show that, even in a confined
geometry, two triplets bind into a localized bound statewithSz = 2. Finally, we discuss the role of competing domain-
wall and supersolid phases as well as that of additional anisotropic interactions.
Queen’s College Cambridge 26
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INVITED
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Magnetic field driven and thermally active behaviour of Artificial Spin Ice.
L.J. Heyderman (ETH Zurich, Paul Scherrer Institute)
Artificial spin ice consists of two-dimensional arrangements of dipolar-coupled monodomain nanomagnets arranged
on the sites of the square or kagome lattice [1-3]. The nanomagnet moments mimic the spins in pyrochlore spin ice
[4], with the anisotropy arising from the crystal field replaced by the shape anisotropy associatedwith each elongated
nanomagnet. In order to investigate these systems, wemainly employ synchrotron x-ray photoemission electronmi-
croscopy (PEEM), which provides high contrast images of themagnetic configurations, but have also shown that x-ray
resonant magnetic scattering can give important insights into themagnetic correlations [5].
Using PEEM, we have demonstrated the existence of emergent magnetic monopoles in the artificial kagome spin ice
[6]. In an applied magnetic field, monopole-antimonopole pairs nucleate and separate in an avalanche-type man-
ner forming one-dimensional Dirac strings consisting of reversed nanomagnet moments, and the behaviour has been
quantitatively explained by Monte Carlo simulations [6,7,8]. With careful modification of the shape of particular is-
lands it is possible to control string progression, so providing a first step towards novel spintronic devices.
The finite building blocks of the artificial kagome spin ice, consisting of one, two and three hexagonal rings [9], are
also of interest for device applications that take advantage of the multiple states in such finite coupled nanomagnet
structures [10]. In addition, on attempting to apply an “effective thermal anneal” via demagnetisation, we find that
the percentage of low energy states decreases as the number of rings increases, indicating that it will be impossible
to achieve the ground state in an extended array using such a partially deterministic demagnetisation protocol. We
have therefore established a method to create thermally active artificial spin ice with fluctuating magnetic moments
and to observe the evolution of the magnetic configurations with time in the PEEM, allowing us to study relaxation
processes and providing a controlled route to the lowest-energy state.
The thermally active behaviour observed in the building blocks of artificial kagome spin ice can be understood by con-
sidering the low energy sector of the dipolar energy landscape [11]. We have also observed the thermal relaxation
of extended arrays of artificial square ice [12], which progresses first via a string regime, then by a regime containing
ground state domains, and finally ending up in one of the two ground state configurations. The vertex type statistics
as a function of time reflect the two regimes and are in good agreement with kinetic Monte Carlo simulations when
disorder is taken into account.
[1] R.F. Wang, C. Nisoli, R. S. Freitas, J. Li, W. McConville, B J.Cooley, M.S. Lund, N. Samarth, C. Leighton, V.H. Crespi, and P. Schiffer,
Nature 439, 303 (2006); [2] L.J. Heyderman andR.L. Stamps, J. Phys.: Condens. Matter 25, 363201 (2013); [3] C. Nisoli, R.Moessner
and P. Schiffer, Reviews of Modern Physics 85 (4), 1473-1490 (2013); [4] M.J. Harris, S.T. Bramwell, D.F. McMorrow, T. Zeiske, and
K.W. Godfrey, Phys. Rev. Letts. 79, 2554 (1997); [5] J. Perron, L. Anghinolfi, B. Tudu, N. Jaouen, J.-M. Tonnerre, M. Sacchi, F. Nolting,
J. Lüning, and L.J. Heyderman, Phys. Rev. B 88, 214424 (2013) (2013); [6] E. Mengotti, L.J. Heyderman, A. Fraile Rodríguez, F. Nolt-
ing, R.V. Hügli, H.B. Braun Nature Physics 7, 68 (2011); [7] R.V. Hügli, G. Duff, B. O’Conchuir, E. Mengotti, L.J. Heyderman, A. Fraile
Rodríuez, F. Nolting, and H. B. Braun, J. Appl. Phys. 111, 07E103 (2012); [8] R.V. Hügli, G. Duff, B. O’Conchuir, E. Mengotti, A. Fraile
Rodríguez, F. Nolting, L.J. Heyderman, and H.B. Braun, Phil. Trans. Roy. Soc. A 370, 5767(2012); [9] E. Mengotti, L.J. Heyderman, A.
Fraile Rodríguez, A. Bisig, L. Le Guyader, F. Nolting, and H.B. Braun, Phys. Rev. B 78, 144402 (2008); [10] R.V. Chopdekar, G. Duff,
R.V. Hügli, E. Mengotti, D. A. Zanin, L.J. Heyderman and H.B. Braun, New Journal of Physics 15, 125033 (2013); [11] A. Farhan, P.
Derlet, A. Kleibert, A. Balan, R.V. Chopdekar, M. Wyss, L. Anghinolfi, F. Nolting and L.J. Heyderman, Nature Physics 9, 375 (2013);
[12] A. Farhan, P.M. Derlet, A. Kleibert, A. Balan, R.V. Chopdekar, M. Wyss, J. Perron, A. Scholl, F. Nolting, and L.J. Heyderman,Phys.
Rev. Lett. 111, 057204 (2013)
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CONTRIBUTED
.
Thermal and Field-driven dynamics of Artificial Spin Ice (ASI) in Real and Reciprocal Space.
S. A. Morley, A. Stein, D. A. Venero, M. C. Rosamond, A. Hrabec, P. Shepley, S. Riley, P. J. Fischer, M-Y.Im, S. Langridge, C. H. Marrows
Artificial spin icearenanomagnetic islandsconfinedtoa two-dimensionalplane, their shapeanisotropyandsizemeans
they can be considered as single domain Ising-like spins. As they are lithographically defined, they provide a tuneable
toy statisticalmodel for the frustratedmagnetismof real spin icematerials. There is an energy barrier associatedwith
flipping the magnetic moment of an island and in order to overcome this barrier with experimental temperatures we
have reduced their volume, yielding true thermodynamics of the ASI system. The size of the islands was reduced to
much smaller than has previously been studied and varied from the largest having lateral dimensions of 80× 250 nm
down to the smallest, 30×75nm,with thicknesses between10 and15nm. Theywere designed to have a lattice spac-
ing twice their longest side and as such should be strongly interacting. We have made real space observations using
transmission xray microscopy (TXM) and also developed a reciprocal space method to measure the smallest islands
using synchrotron xray photon correlation spectroscopy (XPCS). Some recent papers have shown thermal behaviour
either using macroscopic measurements such as magneto-optic kerr effect (MOKE) or remanent imaging methods
such as photo-emission electron microscopy (PEEM) and magnetic force microscopy (MFM). However, they tend to
use, laterally, much larger but very thin islands, and as such represent less rigid analogues of atomic magnetic mo-
ments. In the TXMstudy, as full fieldmicroscopy is used, a large area and consequently a significant number of islands
have been studied simultaneously. Also, due to the �photon-in-photon-out� method its possible to apply a magnetic
field during imaging. For the 80× 250 nm islands, real-time magnetic reversal of the spin ice was studied as a func-
tion of field step. We also observed changes in the coercive field with temperature, reducing the coercivity to half its
original value with a temperature increase of∆T = 100K. The energy scale is compared with relaxation times calcu-
lated from XPCS studies of 30× 75 nm islands of 1600 and 900 s, at T= 300 and 320 K, respectively. The impact of
island size and thicknesson the reducedbarrier height is discussed in termsof curlingmagnetisationwithin the islands
themselves, having implications for themodelling of real spin icematerials.
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CONTRIBUTED
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Symmetry breaking in Artificial Spin Ice nanostructures.
W. R. Branford (Imperial), K. Zeissler (Imperial), S. K. Walton (Imperial), L. F. Cohen (Imperial).
We study magnetotransport in artificial spin ice array. A symmetry change occurs at low temperatures, indicating a
collective response of the nanomagnets. [1] We explore this symmetry breaking in arrays of different ferromagnetic
material. We have also observed asymmetry at room temperature [2] in magnetic charge transport in artificial spin
ice [3-5] and show that domain wall topology controls the direction of propagation.
[1]W. R. Branford, S. Ladak, D. E. Read, K. Zeissler & L. F. Cohen. Science 335, 1597, (2012); [2] K. Zeissler, S. K.Walton, S. Ladak, D.
E. Read et al. Sci. Rep. 3, 01252, (2013); [3] S. Ladak, D. E. Read, G. K. Perkins, L. F. Cohen &W. R. Branford. Nature Physics 6, 359,
(2010); [4] S. Ladak, D. Read, T. Tyliszczak, W. R. Branford & L. F. Cohen. New Journal of Physics 13, 023023, (2011); [5] S. Ladak, S.
K.Walton, K. Zeissler, T. Tyliszczak et al. New Journal of Physics 14, 045010, (2012).
Queen’s College Cambridge 28
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INVITED
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Metal-insulator transition in Pyrochlore-type iridates with anomalousmagnetic domain-wall conductance.
J. Fujioka, K. Ueda, Y. Takahashi, M. Urai (Univ. of Tokyo), T. Suzuki (RIKEN Center for Emergent Matter Science, Japan), S. Ishiwata (Univ. of
Tokyo), Y. Taguchi (RIKEN Center for Emergent Matter Science, Japan), M. Kawasaki and Y. Tokura (Univ. of Tokyo; RIKEN Center for
Emergent Matter Science, Japan)
Recently, the interplay between relativistic spin-orbit interaction and electron correlation in 5d-transition metal ox-
ides has received of an enormous attention. In these class of materials, the strong spin-orbit interaction entangles
spin andorbital degree of freedomof correlated electron,which opens avenue toward a realization of unconventional
electronic phases such as quantum spin liquid or topological insulator/semimetal. For example, in the pyrochlore iri-
datesR2Ir2O7 (R=rare earth elements), a variety of unconventional phases includingWeyl semimetal and topological
insulator is predicted to emerge on the verge of metal-insulator transition. In Weyl semimetals, the bulk electronic
state is characterized by Dirac-type linear band dispersions and topologically protected edge states emerges at sam-
ple surface. Despite extensive theoretical research, the systematic experimental investigation on the bulk electronic
property and edge state inR2Ir2O7 has been left elusive.
In this study, we have investigated the evolution of the electronic structure upon the metal-insulator transition for
the pyrochlore-type Nd2Ir2O7 by means of transport and magnetic measurements as well as optical spectroscopy.
The metal-insulator transition is induced by the partial substitution of Ir-ion with Rh-ion, i.e. by the effective tuning
of the spin-orbit interaction and electron correlation. We have identified that the unconventional zero-gap semicon-
ducting state emerges on the verge ofmetal-insulator transition, where the boundary of antiferromagnetic (all-in all-
out magnetic structure) domain is highly conductive than bulk domain. These results can be discussed in terms of the
emergence ofWeyl semimetal.
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CONTRIBUTED
.
Transport theory of itinerant spin ice.
M. Udagawa (Univ. of Tokyo)
Spin ice is a prototypical frustrated magnet defined on a pyrochlore lattice. Its ground state is described by a simple
rule called “ice rule”: out of four spins on a tetrahedron, two spins point inward, while the other two outward. This
simple rule is not sufficient to determine the spin configuration uniquely, but it leavesmacroscopic degeneracy in the
ground state. Despite themacroscopic degeneracy, however, the ground state is not completely disordered, but it ex-
hibits algebraic spatial correlation,which characterizes this stateas “Coulombphase”wherevariousexotic properties,
such asmonopole excitations and unusual magnetic responses are observed. Given the peculiar spatial correlation, it
is interesting to ask what happens if itinerant electrons coexist and interact with spin ice. Indeed, this setting is rel-
evant to several metallic Ir pyrochlore oxides, such as Ln2Ir2O7 (Ln=Pr, Nd), where Ir 5d itinerant electrons interact
with Ln 4f localized moments. In these compounds, anomalous transport phenomena have been reported, such as
non-monotonic magnetic field dependence of Hall conductivity [1], low-temperature resistivity upturn [2] and spon-
taneous Hall effect [3].
Toaddress these issues,weadoptaspin-ice-type IsingKondo latticemodelonapyrochlore lattice, andsolve thismodel
by applying the cluster dynamical mean-field theory and the perturbation expansion in terms of the spin-electron
coupling. As a result, we found that (i) the resistivity shows a minimum at a characteristic temperature below which
spin ice correlation develops [4]. This scenario also explains the thermodynamic behavior of the system in a unified
way, which cannot be captured by conventional theory of Kondo effect. Moreover, (ii) the Hall conductivity shows
anisotropic and non-monotonic magnetic field dependence due to the scattering from the spatially extended spin
scalar chirality incorporated in spin ice manifold [5]. We also discuss the thermal transport and optical conductiv-
ity of the system, and compare themwith typical behaviors of double-exchange systems [6]. These results give unified
understanding to the thermodynamic and transport properties of Ln2Ir2O7 (Ln=Pr, Nd), and give new insights into the
role of geometrical frustration in itinerant systems. This work is in collaboration with H. Ishizuka, Y. Motome and R.
Moessner.
[1] Y. Machida et al., Phys. Rev. Lett. 98, 057203 (2007); [2] S. Nakatsuji et al., Phys. Rev. Lett. 96, 087204 (2006); [3] Y. Machida et
al., Nature 463, 210 (2010); [4] M. Udagawa, H. Ishizuka and Y. Motome, Phys. Rev. Lett. 108, 066406 (2012); [5] M. Udagawa and
R.Moessner, Phys. Rev. Lett., 111, 036602 (2013); [6]M. Udagawa et al., in preparation.
Queen’s College Cambridge 30
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CONTRIBUTED
.
Complex incommensurate orders in the KagomeKondo latticemodel
C. L. Henley and S. Ghosh (Cornell)
In the Kondo lattice model (KLM), a single band of itinerant electrons have a nearest-neighbor hopping t and are ex-
change coupledJK to classicalmoments on sites of aKagome lattice. Wehave used two approaches to determine the
phase diagram at all electron fillings n except when the Fermi energy falls in the flat bands. RKKY approach: Our first
approach is valid in theweak coupling limitJK ≪ t . in the “RKKY limit” of themodel,We integrate out electron states
(by numerical integration in the zone) at T = 0 to obtain the “RKKY” effective Heisenberg interactions Jij between
pairs of classical spins, using lattice sizes up toN = 3 × 482. Adapting a method previously described for other sys-
tems [1], we find energy minima of the pair Hamiltonian (with interactions out to many neighbors), using both zero-
and finite-temperature Monte Carlo. The spin configuration found is characterized using the common-origin plot of
spin directions and by Fourier transforming the spins on each sublattice 1. At almost all fillings, we find the spin pat-
tern is mostly a linear combination of the optimal (“Luttinger-Tisza” =L.T.) eigenmodes of the large matrix (Jij) – a
behavior that is known rigorously for Bravais lattices, but is not exact for the Kagome. In the case of special commen-
surate fillings, such that the ordering wavevectorQ is a symmetry point in the zone, commensurate orders are found:
at n = 1/3, Q falls at the Dirac point and the well-known√3 ×
√3 order is found; for n = 5/12 and n = 3/12, the
L.T. eigenmodes haveQ at the Van-Hove points and (we conjecture) the twelve-sublattice cuboc1 and cuboc2 orders
[2] are respectively obtained. At all other fillings, we find incommensurate orders which have been ignored in previous
studies of the KLM [3]. Two common kinds of phases are “1Q” coplanar spirals, in which all three sublattices use the
same ordering vector from the zone, and “3Q” spirals, in which each sublattice comes a different wavevector out of
the star of equivalent ones. The orderingwavevectorQ evolves continuously alongmirror lines in the zone except for
jumpsat a coupleof fillings. Variational approach for anyJK/t: Our secondapproach is “variational” in thatwecalculate
theexact ground state energy (bynumerical diagonalizationof the single-electronenergies) for a long list of candidate
spin configurations, taking the winner of this competition to approxaimte the true ground state. This list includes all
the configurations found for JK/t ≪ 1 using the first approach. We obtain a rich two-parameter phase diagram as a
function of (n, JK), which has three distinct regimes (i) JK ≪ t (RKKY regime), dominated by incommensurate orders;
(ii) JK ∼ t, dominated by commensurate antiferromagnetic orders; and (iii) JK ≫ t, the double-exchange limit, in which
the trivial ferromagnetic phase is found for all n.
[1] Sklan and Henley, Phys. Rev. B 88, 024407 (2013); Lapa and Henley, arXiv:v:1210.6810; [2] Domenge et al., Phys. Rev. B 72,
024433 (2005); [3] Martin and Batista, Phys. Rev. Lett. 101, 156402 (2008); Akagi and Motome, J. Phys. Soc. Jpn. 79, 083711
(2010).
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6.2.3 TUESDAY JULY 8TH 2014
11:45 - 12:00 D. L. Quintero-Castro, Helmholtz-Zentrum Berlin
Investigating the complex magnetic phase diagram of the highly frustrated compound SrYb2O4
12:00 - 12:15 L. Clark, McMasterUniversity
From spin glass to spin liquid ground states in molybdate pyrochlores
12:15 - 12:30 C. R. Wiebe, Univ. of Winnipeg, Univ. of Manitoba
Magnetic ordering in the XY pyrochlores Yb2Ge2O7 and Er2Ge2O7
14:30 - 15:00 Stewart, ISIS, STFC Magnetic structures of Heisenberg pyrochloreantiferromagnets
poster session I (drinks reception sponsored in part by LOP-Quantum Design)
15:30 - 17:45
12:30 - 14:00 lunch
14:00 - 14:30 Gia-Wei Chern, Los Alamos National Laboratory
Dipolar order by disorder in the classical Heisenberg antiferromagnet on the kagome lattice
15:00 - 15:30 tea
Alpha, beta or gamma? The crystallography of the 'vesignieites' - S=1/2 kagome antiferromagnets
Tuesday, 8 July 2014
09:00 - 09:45Xiao-Gang Wen,
Perimeter InstituteSymmetry protected topological order
09:45 - 10:15 Hong-Chen Jiang, UC Berkeley
Quantum spin liquid, topological order and entanglement entropy
10:15 - 10:45 A. M. Laeuchli, University of Innsbruck
A journey from the XX to the XXZ limit of the S=1/2 kagome antiferromagnet
10:45 - 11:15 coffee
11:15 - 11:45 A. S. Wills, University College London
Details about the Tuesday Poster Session can be found in Sec. 6.3.1
..
Session Chairs of the day:
9:00 -10:45 J. Chalker
11:15 -12:30 I. Mirebeau
14:00 -15:00 S Chernyshev
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PLENARY
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Symmetry protected topological order.
Xiao-GangWen (Perimeter Institute)
I will discuss the recent progresses on Symmetry protected topological (SPT) order. In particular, themodels that can
realize SPT orders, and the probes that may allow us to detect the SPT orders.
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INVITED
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Quantum Spin Liquid, Topological Order and Entanglement Entropy.
Hong-Chen Jiang
Quantum spin liquids (QSLs) are elusive magnets without magnetism, resisting symmetry breaking even at zero tem-
perature due to strong quantum fluctuations and geometric frustration. The simplest QSLs known theoretically are
characterized by topological order, i.e., topological QSL, and support fractionalized excitations. However, there is no
practical way to directly determine the topological nature of the states. In my talk, I will introduce a simple and prac-
tical approach, i.e., cylinder construction, to identify topological order by entanglement entropy. As an example, by
extracting accurate topological entanglement entropy (TEE), we identify the quantum spin liquid ground states with
topological order in the antiferromagnetic spin-1/2 Heisenberg model on the Kagome lattice. Finally, I will also try
to talk about the finite-size corrections to TEE, and its relevance to QSLs, as well as future searches for topological
ordered phases.
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INVITED
.
A journey from the XX to the XXZ limit of the S=1/2 KagomeAntiferromagnet.
AndreasM. Läuchli (University of Innsbruck)
n this talkwenumerically explore the properties of the S=1/2KagomeAntiferromagnet from the easy plane (XX) limit
to the easy axis (XXZ) limit. In contrast to many other perturbations of the Heisenberg point (such as DM interac-
tions or further neighbor couplings) the present deformations do basically not change the structure of the low-energy
spectrum, and thereforemost likely describe the same phase of matter.
We then discuss some important consequences of this finding: i) in the easy plane (XX) limit the system reduces to
simple hard core bosons with frustrated hopping. We further explore the energy spectrum in the half-filled Bose
Hubbard model away from the infinite on-site repulsion limit, and find an extended region of stability. These results
demonstrate that quantum simulations of the spin liquid phase of the kagome antiferromagnet with ultracold atoms
are possible with present-day experimental technology.
ii) In the opposite limit (Ising+ small in-plane exchange) we identify a long-sought low-energy effective model, with a
reduced Hilbert space, which is able to quantitatively explain the occurrence of the many low-lying energy levels as
well as their quantum numbers. In this limit the effect model can bemapped at themicroscopic level to aU(1) lattice
gauge theory coupled to bosonic charge-twomatter fields. This effective model has the potential to finally shed light
on themicroscopic nature of the spin liquid phase at the Heisenberg point.
This work has been performed in collaboration with R.Moessner (MPI Dresden).
Queen’s College Cambridge 33
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INVITED
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Alpha, beta or gamma? The crystallography of the ‘vesignieites’ – S = 12 kagome antiferromagnets.
A. S. Wills, D. Boldrin
The ‘vesignieites’ are a relatively new family of frustratedmagnetswith the general formula,ACu3V2O8(OH)2, where
A2+= Ba, Sr, Pb. With very similar crystal structures, they allow access to a selection of closely related model S = 12
kagomemagnets. In recentyears therehasbeenmuchdiscussionover thenatureof thecrystal structureofvesignieite
itself,A2+= Ba, with the original monoclinic structure (C2/m) proposed by Okamoto et al. [1] being thought by later
workers to be a low symmetry phase (α-phase) with single crystals of a higher symmetry rhombhedral vesignieite (β-
phase) realisablebyhydrothermal synthesis [2]. In sodoing, the limitationsof theslightlydistortedmonoclinickagome
lattice could be removed, as the rhombohedral structure would involve a perfect kagome lattice with 3× symmetry.
Our work into the crystal structure of vesignieite originally showed it to possess less of a monoclinic distortion than
originally thought,withonlyaminute< 0.07%bond-lengthdifferencebetween inequivalentCusites [3]. More recent
studies of the relatedA2+= Sr, Pbmaterials have led us to propose a new trigonal structure for the vesignieites and an
unusual trimerised structure [4]. This presentation will summarise the crystal chemistry of the vesignieites and draw
relations to their magnetic respones.
[1] Y.Okamoto, H. Yoshida, andZ.Hiroi, J. Phys. Soc. Jpn. 78, 033701 (2009); [2] H. Yoshida, Y.Michiue, E. Takayama-Muromachi, and
M. Isobe, J. Mater. Chem. 22, 18793 (2012); [3] R. H. Colman, F. Bert, D. Boldrin, A. D. Hillier, P. Manuel, P. Mendels, and A. S. Wills,
Phys. Rev. B 83, 180416(R) (2011); [4] D. Boldrin and A.S.Wills, in preparation (2014)
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CONTRIBUTED
.
Investigating the ComplexMagnetic Phase Diagram of the Highly Frustrated Compound SrYb2O4.
D. L. Quintero-Castro (HZB), A. T. M. N. Islam (HZB), M. Reehuis (HZB) and B. Lake (HZB)
SrYb2O4 is an insulating magnet, consisting of two types of zigzag chains running along the c-axis and forming a hon-
eycomb structure in the ab-plane. The similar first and second-neighbor distances suggest high geometrically frus-
trated magnetic interactions. This frustration sums up to a strong single ion anisotropy to produce a highly degener-
ate ground state manifold reflected by a very complex and anisotropic magnetic phase diagram. Despite of SrYb2O4
having aCurieWeiss temperatureof−110K, the compoundonly orders at0.9Kat zerofield, themagnetic structure is
found tobenoncollinearwitha reductionof theorderedmagneticmoment fromthe full ionicmoment. In thezerofield
phase, two clear gappedmagnetic excitationmodes are present, dispersing from 0.2meV up to 2meV. The excitations
branches are broad and coexist with magnetic diffuse scattering, indicating a sort of short range spin correlations.
The presence of broad modes and diffuse scattering is typical of antiferromagnets where the magnetic moments are
only partially ordered in the ground state. The different magnetic phases have been investigated by neutron diffrac-
tion at HZB at different fields and orientations, new magnetic Bragg peaks appear at 5T, suggesting a formation of a
newmagnetic structure. The difference between the zero field diffraction data and the 2T and 5T fields show a large
change on the diffuse scattering when increasing themagnetic field, suggesting a change in themagnetic excitations.
Here, we will discuss about the general magnetic properties of this compound, the competition between the single
ion anisotropy and the frustrated magnetic interactions and how the external magnetic field induces new magnetic
phases.
[D. L Quintero-Castro, et al., Phys. Rev. B 86, 064203 (2012)]
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CONTRIBUTED
.
From Spin Glass to Spin Liquid Ground States inMolybdate Pyrochlores.
L. Clark (McMaster University), G. J. Nilsen, E. Kermarrec, G. Ehlers, A. Harrison, J. P. Attfield and B. D. Gaulin.
The rare earth (R) molybdate pyrochlores, R2Mo2O7, are a well studied family of magnetic materials. In particular,
the spin glass ground stateof theY2Mo2O7 in the apparent absenceof disorder continues tobeof interest [1], [2]. One
material thatmay give further insight into the nature of this unusual ground state is the pyrochlore Lu2Mo2O7, which
is based on the smallest cation in the rare earth series, non-magnetic Lu3+. Here, we will present the first magnetic
study of this system, which displays the onset of a spin glass state below Tf ≈ 16 K and a surprising T 2 dependence
of low temperature heat capacity [3]. Our neutron scattering studies reveal a build-up of diffuse magnetic scattering
due to frustrated near- and next-near-neighbour antiferromagnetic interactions in the spin glass state (D7, ILL) and a
collapse of inelastic neutron scattering into the elastic line at the spin freezing transition (CNCS, SNS). Materials that
combine strong geometric frustration with low spin magnetic ions are especially fascinating since these properties
tend to induce exotic quantum spin liquid behaviour. In the rare earthmolybdate pyrochlores, this can be achieved by
nitridation of the oxide phases. Synthesis of oxynitridematerials with the compositionR2Mo2O5N2 containMo5+ d1
S = 12cations on the geometrically frustrated pyrochlore network of corner sharing tetrahedra and are, therefore,
excellent candidates for the studyof quantumspin liquid phenomena. Wewill report the synthesis of a newoxynitride
material, Lu2Mo2O5N2, and show howmagnetic susceptibilty and heat capacity measurements reveal a marked dif-
ference in the nature of the ground state in comparison to its precusor oxide, Lu2Mo2O7 [4]. Despite the persistence
of strong antiferromagnetic exchange, there is no evidence for spin freezing down to 2Kandheat capacity data follow
a linear temperature dependence down to 0.5 K, which suggest that the role of quantum fluctuations becomes more
significant in the selection of a ground state in the oxynitride pyrochlore. Neutron scattering (D7, ILL andCNCS, SNS)
reveals an absenceof thebuild-upof short range correlations thatwas found for the spin glass state of Lu2Mo2O7. We
also observe a persistence of inelastic scattering, indicating that theMo5+ spins in Lu2Mo2O5 remain dynamic to the
lowest temperature of our experiment, on an energy scale≈ θ/100. Furthermore, the nature of the inelastic neutron
scattering spectra of Lu2Mo2O5N2 appears to be gapless on an energy scale of≈ 0.1meV.
[1] J. S. Gardner et al., Phys. Rev. Lett. 82, 221 (1999); [2] H. J. Silverstein et al., Phys. Rev. B 89, 054433 (2014); [3] L. Clark et al., J.
Solid State Chem. 203, 199 (2013); [4] L. Clark, PhD Thesis, University of Edinburgh (2013).
..
CONTRIBUTED
.
Magnetic ordering in the XY pyrochlores Yb2Ge2O7 and Er2Ge2O7.
C. R.Wiebe (University ofWinnipeg, University of Manitoba), X. Li (Beijing National Laboratory for CondensedMatter), W. M. Li (Beijing
National Laboratory for CondensedMatter), K. Matsubayashi (University of Tokyo), Y. Sato (Kyushu University), C. Q. Jin (Beijing National
Laboratory for CondensedMatter), Y. Uwatoko (University of Tokyo), T. Kawae (Kyoto University), A. M. Hallas (McMaster University), A. M.
Arevalo-Lopez (University of Edinburgh), J. P. Attfield (University of Edinburgh), J. S. Gardner (National Synchrotron Research Center,
Taiwan), R. S. Freitas (Universidade de Sao Paulo), Z. L. Dun (University of Tennessee), H. D. Zhou (University of Tennessee) and J. G. Cheng
(University of Tokyo)
The XY pyrochlore magnets Yb2Ge2O7 and Er2Ge2O7 have been synthesized using a high pressure technique and
were found to exhibit long ranged magnetic ordering at low temperatures via susceptibility and neutron scattering
experiments. Yb2Ge2O7 has an antiferromagnetic transition at 0.62 K, which is in sharp contrast to the Yb2Ti2O7
quantum spin ice state. Er2Ge2O7 has an antiferromagnetic transition with a slightly higher transition at TN = 1.4
K compared to TN = 1.2 K in Er2Ti2O7, albeit with enhanced magnetic exchange interactions. Both XY systems are
shown to be very sensitive to the effect of chemical pressure, which provides an excellent opportunity to study the
underlying physics for the quantum spin ice state in Yb2Ti2O7 and the order-by-disorder mechanism in Er2Ti2O7.
Queen’s College Cambridge 35
HFM2014
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INVITED
.
Dipolar Order by Disorder in the Classical Heisenberg Antiferromagnet on the Kagome Lattice.
Gia-Wei Chern, RoderichMoessner
Ever since theexperimentswhich founded thefieldof highly frustratedmagnetism, the kagomeHeisenberg antiferro-
magnet hasbeen the archetypical setting for the studyoffluctuation inducedexotic ordering. To this day thenatureof
its classical low-temperature state has remained amystery: the nonlinear nature of the fluctuations around the expo-
nentially numerous harmonically degenerate ground states has not permitted a controlled theory, while its complex
energy landscape has precluded numerical simulations at low temperature, T . Here we present an efficient Monte
Carlo algorithmwhich removes the latter obstacle. Our simulations detect a low-temperature regime inwhich corre-
lations asymptote to a remarkably small value as T → 0. Feeding these results into an effective model and analyzing
the results in the framework of an appropriate field theory implies the presence of long-range dipolar spin order with
a tripled unit cell.
..
INVITED
.
Magnetic structures of Heisenberg pyrochlore antiferromagnets.
J. R. Stewart (STFC), J. A. M. Paddison (Oxford/STFC), D. D. Khalyavin (STFC), PManuel (ISIS), G. Ehlers (SNS, Oak Ridge), O. Petrenko
(Warwick), A. L. Goodwin (Oxford) and J. S. Gardner (Ansto)
Canonical examples of highly frustrated antiferromagnets are the gadolinium pyrochlores Gd2Ti2O7 (GTO) and
Gd2Sn2O7 (GSO) both of which order at around 1 K, but with different structures. GSO forms the so-called Palmer-
Chalker structure with a k = 0 propagation vector [1], while GTO forms a more complicated - and still controversial
- partially ordered ground state with a propagation vector of k = (1/2, 1/2, 1/2), resulting in a split of the crystallo-
graphically equivalent Gd sites into two non-equivalentmagnetic sites one of which (kagome site, 3/4 of all spins) has
an ordered moment close to the maximum allowed of 7 µB while the other (interstitial site, 1/4 of all spins) remains
mostly disordered [2]. While powder diffraction doesn’t enable unambiguous determination of the possible k-states,
(i.e. one or a combination of the 4 possible propagation vectors), analysis of the diffuse scattering from the interstitial
site was used to determine that the ground state was a 4k-structure [3]. We have recently grown high quality sin-
gle crystals of GTO in order to attempt to confirm this finding. However, using in-field diffraction data taken on the
WISH diffractometer at ISIS we find that the 4k-structure is definitively ruled out for GTO. This has lead us to return
to the original powder diffraction and xyz polarisation analysis data in order to understand how the observed diffuse
scattering can be compatible with a 1k (or other alternative) structure.
[1] Wills, A. S. et al. J. Phys.: Condens. Matter 18, L37�L42 (2006); [2] Champion, J. D. M., Wills, A. S., Fennell, T. & Bramwell, S. T.
Phys. Rev. B 64 140407 (2001); [3] Stewart, J. R., Ehlers, G., Wills, A. S., Bramwell, S. T. & Gardner, J. S. J. Phys.: Condens. Matter 16,
L321�L326 (2004).
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6.2.4 WEDNESDAY JULY 9TH 2014
11:45 - 12:00 S. R. Dunsiger, Technische Universitat Munchen
Depth resolved local susceptibility of spin liquid and spin ice
12:00 - 12:15 E. Lhotel, Institut Neel CNRS, Grenoble
Far from equilibrium monopole dynamics inspin ice
12:15 - 12:30 L. Bovo, University College London
Spin ice thin-films of Dy2Ti2O7
12:30 - 12:45 L. D. C. Jaubert, Okinawa Institute (OIST)
Breaking spin ice symmetry
Wednesday, 9 July 2014
09:00 - 09:45 R. Moessner, MPI-PKS Dresden
Disorder and dynamics in spin liquids
09:45 - 10:15 B. D. Gaulin, McMaster University
Quantum ordered and disordered ground states in XY pyrochlores
10:15 - 10:30 F. R. Foronda, Oxford University
Quantum spin ice states in Pr-based pyrochlores
10:45 - 11:15 coffee
10:30 - 10:45 J. Goff, Royal Holloway Defects and monopole dynamics in spin ice
11:15 - 11:45 P. C. W. Holdsworth, Ecole Normale Superieure de Lyon
The Monopole Physics of Spin Ice
free afternoon (city tour, punting)
19:00 dinner
12:45 - 14:00 lunch
..
Session Chairs of the day:
9:00 -10:45 M. Udagawa
11:15 -12:30 B.D. Gaulin
Queen’s College Cambridge 37
HFM2014
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PLENARY
.
Disorder and dynamics in spin liquids.
R. Moessner (MPI-PKS Dresden)
Beyond their deceptively featureless ground states, spin liquids are particularly remarkable in the exotic nature of
their excitations. While dynamical correlations carry information about the full excitation spectrum, disorder can in
turn help in nucleating or localising defects with unusual properties. Both are thus in their own way capable of pro-
viding insights into the unusual properties of spin liquids. This talk reviews recent progress on both fronts.
..
INVITED
.
QuantumOrdered andDisordered Ground States in XY Pyrochlores.
B.D. Gaulin (McMaster University) and K.A. Ross (Johns Hopkins University)
The XY pyrochlores Er2Ti2O7 and Yb2Ti2O7 can be described by effective spin 1/2 degrees of freedom decorating
the cubic pyrochlore lattice. Time-of-flight neutron scattering in high field, polarized states have allowed a very pre-
cise determination of spin Hamilonians, based on anisotropic exchange allowed by spin-orbit coupling, in each of
Er2Ti2O7 and Yb2Ti2O7. In the case of Yb2Ti2O7, these measurements show that this ”ferromagnetic” XY system
possesses a ground state close to that expected of quantum spin ice. Measurements on the ”antiferromagnetic” XY
pyrochlore, Er2Ti2O7, helped resolve a decade-long mystery as to how its ordered ground state is selected by the
order-by-quantum disorder mechanism. Very recent high resolution measurements observe the small energy gap in
its Goldstone-like excitation spectrum required by this mechanism for ground state selection.
..
CONTRIBUTED
.
Quantum spin ice states in Pr-based pyrochlores.
F. R. Foronda, S. J. Blundell, J. S. Möller (Oxford University), T. Lancaster (DurhamUniversity), F. Lang (Oxford University), S. R. Giblin, F. L.
Pratt (ISIS), D. Prabhakaran (Oxford University)
Muon-spin rotation is an ideal tool for probing the ground states of frustrated magnets such as pyrochlore oxides.
However, one issue to consider in these studies is the extent to which the muon perturbs its local environment. In
some cases this could induce a ground state that differs from the intrinsic one. This scenario has been suggested by a
recent µSR investigation on the metallic compound Pr2Ir2O7 [1,2] in which the observed muon relaxation was inter-
preted asmuon-induced effect. To address this issuewe have performed ZF and LF µSR on the insulating compounds
Pr2B2O7 (B = Sn, Zr, Hf) in the range 0.05 – 280 K. Kubo-Toyabe relaxation functions are observed in zero-field for all
temperatures, which indicate strong nuclear moments and weak dynamics. We find that the static muon relaxation
rate∆ and dynamic relaxation rate λ exhibit similar behaviour in all insulating andmetallic compounds and develop a
model in which themuon relaxation can be interpreted as an intrinsic effect.
[1]D.MacLaughlin, Y.Ohta, Y.Machida, S.Nakatsuji, G. Luke,K. Ishida, R.Heffner, L. Shu, andO.Bernal, PhysicaB:CondensedMatter.
404, 667 (2009); [2]D.E.MacLaughlin,Y.Nambu,Y.Ohta,Y.Machida, S.Nakatsuji, andO.O.Bernal, J.Phys.: Conf. Series225, 012031
(2010).
Queen’s College Cambridge 38
HFM2014
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CONTRIBUTED
.
Defects andmonopole dynamics in spin ice.
J. Goff, G. Sala, D. Porter (Royal Holloway) M. Gutmann (ISIS) D. Prabhakaran (Oxford) D. Pomaranski, C. Mitchelitis, J. Kycia (Waterloo) C.
Castelnovo (Cambridge)
The idea of magnetic monopoles in spin ice has enjoyed much success at intermediate temperatures, but at low tem-
peratures a description in terms of monopole dynamics alone is insufficient. Recently, numerical simulations were
used to argue that magnetic impurities account for this discrepancy by introducing a magnetic equivalent of residual
resistance in the system. Here we propose that oxygen deficiency is the leading cause of magnetic impurities in as-
grown samples, and we determine the defect structure and magnetism in Y2Ti2O7 using diffuse neutron scattering
andmagnetizationmeasurements. These defects are eliminated by oxygen annealing. The introduction of oxygen va-
cancies causes Ti4+ to transform tomagnetic Ti3+ with quenched orbital magnetism, but the concentration is anoma-
lously low. In the spin-ice material Dy2Ti2O7 we find that the same oxygen-vacancy defects suppress moments on
neighbouring rare-earth sites, and that these magnetic distortions dramatically slow down the long-time monopole
dynamics at sub-Kelvin temperatures.
..
INVITED
.
TheMonopole Physics of Spin Ice.
Peter C.W. Holdsworth (École Normale Supérieure de Lyon)
II will review a number of recent results concerning the static and dynamic properties of monopoles in a “magne-
tolyte�� - a lattice Coulomb gas constrained by the “Dirac strings�� of magnetic moments that characterize spin ice.
I will show that the spin ice phase diagram can be accurately reproduced by considering the energetics of monopole
crystallization. In this description the “all-in-all-out�� antiferromagnetic phase appears as a crystal of double charged
monopoles. Anew, intermediatephasemayappearnear theboundarybetweenspin iceandorder - amonopole crystal
in which themagnetic moments appear to be fragmented by the local constraints intomonopole and local gauge field
contributions [1]. Newspecificheat data [2]will bepresented forbothDysprosiumandHolmiumTitanate in the range
0.5−12K. Iwill shown that it comparesquantitativelywithboth simulationdata andDebye-Huckle theory specifically
adapted to the magnetolyte. Detailed comparison provides evidence of the emergent entropic charge carried by the
dirac string constraints. Finally I will discuss numerical simulations of theWien effect in model electrolytes [3] and in
spin ice.
[1] M. Brooks-Bartlett, A. Harman-Clarke, S. Banks, L. Jaubert, P.C.W. Holdsworth, Phys. Rev. X, 4, 011007 (2014); [2] L. Bovo, V.
Kaiser, J.Bloxsom, S.T.Bramwell et. al., inpreparation; [3]V.Kaiser, S. T.Bramwell, P.C.W.Holdsworth, R.Moessner,NatureMaterials,
12, 1033-1037 doi :10.1038/nmat3729, (2013)
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CONTRIBUTED
.
Depth Resolved Local Susceptibility of Spin Liquid and Spin Ice.
S. R. Dunsiger (Technische Universität München), S. J. Blundell (Oxford University), P. Böni (Technische Universität München), A. Dabkowski
(McMaster University), M. J. P. Gingras (University ofWaterloo), T. Goko (Paul Scherrer Institut), R. F. Kiefl (University of British Columbia),
T. Lancaster (DurhamUniversity), L. Liu (Columbia University), H. Luetkens (Paul Scherrer Institut), G. Luke (McMaster University),
E. Morenzoni, G. J. Nieuwenhuys (Paul Scherrer Institut), D. Prabakharan (Oxford University), J. Preston (McMaster University), A. Suter
(Paul Scherrer Institut) and Y. J. Uemura (Columbia University)
Spin ices, such as Dy2Ti2O7, are topical geometrically frustrated model magnetic systems. Specifically, the Dy mag-
netic ions reside on the vertices of a pyrochlore lattice of corner-sharing tetrahedra. A large single ion anisotropy
forces themoment to point along local< 111 > crystalline axes, making themoments classical local Ising spins. Mag-
netic frustration in spin ices stems fromthe1/r3 long-rangenatureof themagnetic dipolar interaction. Theirminimum
energy spin configurations are characterised by two spins pointing in and two spins pointing out on each tetrahedron.
The problem of the dynamical response of themoments remainsmuch less studied and understood. However, a clas-
sical thermal fluctuation causing the flip of an Ising spin from an “in” to an “out” direction on the pyrochlore lattice has
beendescribed in termsof thenucleationof amagneticmonopole. Themagneticmonopoles contribute to the internal
magnetic fields in spin ice,whichmaybemeasuredusing local experimental probes likeµSR. Proximal to thematerials,
Sala et al [1] predict themonopole fields should dominate over themuch smaller fields present in their absence–given
that the field of a monopole decays less slowly (1/r2) than that of an isolated spin. Unlike stray fields set up by the
induced magnetisation due to a uniform external field, the monopole density grows with temperature. We report
an investigation of the local susceptibility of the archetypal spin ice compound Dy2Ti2O7 using depth resolved low
energy µSR, both within the compound and proximally, in a silver overlayer. The results are compared with measure-
ments of the bulk magnetisation and wave vector dependent susceptibility in Ho2Ti2O7 by Jaubert et al [2]. A major
unresolved question concerns the fate of the spin ice state, when the strength of the local Ising anisotropy is reduced,
whichmayboost the relative importanceofquantumfluctuations. TheoxidepyrochloreTb2Ti2O7 is perhaps themost
well recognised experimental realisation of a classical spin liquid. Theoretical models predict qualitative changes in
magnetic ground state depending on the dimensionality of the system. Using low energy µSR, we explore the local
susceptibility as a function of implantation on the nanometre depthscale and examine the effect of a crossover from
3 to 2 dimensions within the same material, without the drastic change of altering the crystal structure or magnetic
species involved.
[1] G. Sala et al, PRL 108, 217203 (2012); [2] L. D. C. Jaubert et al, PRX 3, 011014 (2013)
Queen’s College Cambridge 40
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CONTRIBUTED
.
Far From EquilibriumMonopole Dynamics in Spin ice.
E. Lhotel (Institut Néel CNRS), C. Paulsen (Institut Néel CNRS), M. Jackson (Institut Néel CNRS), B. Canals (Institut Néel CNRS), D.
Prabhakaran(Oxford University), K. Matsuhira (KIT, Kitakyushu), S. R. Giblin (Cardiff University), S. T. Bramwell (UCL)
In spin ice materials such as Dy2Ti2O7 and Ho2Ti2O7, the frustrated pyrochlore lattice geometry and Ising-like
anisotropyof themagnetic ions along their local< 111 > axes combinewith a self-screeningdipole-dipole interaction
togive a local ’ice rule’ [1]. Simply stated, theminimumenergy state corresponds to two spins pointing in and twospins
pointing out of each tetrahedron, which stabilises a degenerate and disorderedmagnetic ground state. Thermally ac-
tivated defects (i.e three spins in and one out, or three out and one in) take the form of effectivemagneticmonopoles.
The density of themonopoles in zero applied field goes to zerowith temperature as 2 exp(µ/kBT )whereµ/kB ≈ 4.5
KforDy2Ti2O7. Nevertheless, theoretically it hasbeen shown that a fast thermal quench in the ’dipolar spin ice’model
could createmonopole-rich states at low temperature [2]. Recentlywe demonstrated experimentally the importance
of the quench rate on the dynamic properties at low temperature. In addition, we proposed a new protocol which we
call the ’magnetothermal avalanche quench’ technique that results in a density of monopoles more than an order of
magnitude greater than the fastest conventional zerofield cooling [3]. The trick is to exploit the heat that is createdby
themagnetic work done on the sample when the field is changed, whichwill causes a sudden increase in temperature
solely inside the sample. The sample then finds itself at relatively high temperature but connected to a cold thermal
bath. Theensuingquench is asefficient and rapidaspossible. The relaxationof themagnetisationmeasured fromsuch
amonopole-rich state is found to exhibit spontaneous dynamical effects that typify far-from-equilibrium systems, yet
are captured by simplemodels.
[1] M.J. Harris, S.T. Bramwell, D.F. McMorrow,T. Zeiske and K.W. Godfrey, Phys. Rev. Lett. 79, 2554 (1997); [2] C. Castelnovo, R.
Moessner, and S. L. Sondhi, Phys. Rev. Lett. 104, 107201 (2010); [3] C. Paulsen,M. J. Jackson, E. Lhotel, B. Canals, D. Prabhakaran, K.
Matsuhira, S. R. Giblin and S. T. Bramwell, Nature Phys 10, 135-139 (2014).
..
CONTRIBUTED
.
Spin Ice Thin-Films of Dy2Ti2O7.
L. Bovo (UCL), X. Moya (Cambridge), D. Prabhakaran (Oxford), Yeong-Ah Soh (Imperial College London), A. T. Boothroyd (Oxford), N. D.
Mathur (Cambridge), G. Aeppli (UCL), S. T. Bramwell (UCL).
Spin ice [1] illustrates much novel science, including unusual phases, degeneracies, quasiparticles and topology [1-4].
A characteristic featureof spin ice is its apparent violationof theThirdLawof thermodynamics. This leads toanumber
of interesting properties including the emergence of an effective vacuum for ‘magneticmonopoles’ and their currents
- ‘magnetricity’. We fabricate [5] thin epitaxial films of Dy2Ti2O7 on an inert substrate, adding a new dimension to the
experimental study of spin ice. The films show the distinctive characteristics of spin ice at temperatures greater than
2K, but at lower temperaturewefind evidence of a zero entropy state. This restoration of the third law in spin ice thin
films is consistent with a predicted [6] strain-induced ordering. In more general terms, our results illustrate how the
fabrication and study of thin films open up new possibilities for the control andmanipulation of the unusual magnetic
properties of spin icematerials and related frustratedmagnets.
[1] Harris, M.J.et al. PRL 79, 2554 (1997); [2] Ramirez, A.P. et al. Nature 399, 333 (1999); [3] Ryzhkin, I.A. J. Exp. and Theor. Phys. 101,
481 (2005); [4] Castelnovo, C. et al. Nature 451, 42 (2008); [5] Bovo, L. at al Nature Commun 5, 3439( 2014); [6] Jaubert, L.D.C. PRL
105, 087201 (2010)
This work was supported by EPSRC grant EP/I034599/1
Queen’s College Cambridge 41
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CONTRIBUTED
.
Breaking spin ice symmetry.
L.D.C. Jaubert (OIST, Japan)
Symmetry is beautiful. But once broken, an exotic diversity can emerge, especially with a hint of frustration. In this
talk, we will investigate how lattice distortions can break the finely tuned spin-ice symmetry. Motivated by recent
experimentsonpyrochloreoxides, spin-lattice couplinghasbecomeapromisingnewsetof degreesof freedomto take
into account, either as a signature of magnetic monopoles in Dy2Ti2O7 [1], as a way to stabilize spin liquid regimes in
Tb2Ti2O7 [2], or as a new source of magnetic ordering [3]. We shall introduce a realistic model of magneto-elastic
coupling displayingmany aspects of such exotic phenomena, such as topological defects stabilization, and extend our
work to describe the spin freezing occurring in presence of lattice disorder.
[1] Grams et al. arXiv:1307.8287; [2] Fennell et al. Phys. Rev. Lett. (2014); [3] Tchernyshyov et al. Phys. Rev. Lett. (2002)
Queen’s College Cambridge 42
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6.2.5 THURSDAY JULY 10TH 2014
10:15 - 10:30 M. Fu, McMaster University Revealing the local magnetism of S=1/2 kagome lattice ZnCu3(OH)6Cl2 using single crystal NMR
10:30 - 10:45 Y. Shimizu, Nagoya University
Spin order and disorder in organic diamond and triangular lattices
11:45 - 12:00 F. Pratt, ISIS, STFC muSR of propagating spin excitations in quantum disordered magnets
12:00 - 12:15 M. Yoshida, ISSP,University of Tokyo
High field magnetic phases of volborthite studied by single crystal NMR
12:15 - 12:30 A. I. Smirnov, Kapitza Institute RAS
Two crossovers between spinons and magnonsin S=1/2 frustrated antiferromagnet Cs2CuCl4
14:30 - 14:45 S. Depenbrock, UC Irvine Dynamics of the Z2 spin liquid on the kagome lattice
14:45 - 15:00 L. Messio, UPMC Paris Kapellasite: a gapless chiral spin liquid
15:00 - 15:15 D. Kovrizhin, Cambridge University
Dynamics of a two-dimensional quantum spin liquid: signatures of emergent Majorana fermions and fluxes
15:30 - 17:45 poster session II (drinks reception sponsored in part by LOP-Quantum Design)
10:45 - 11:15 coffee
11:15 - 11:45 M. Yamashita, ISSP, University of Tokyo
Study of elementary excitations of two-dimensional quantum spin liquids
12:30 - 14:00 lunch
14:00 - 14:30 Y. Motome, University of Tokyo
How does the Kitaev spin liquid evaporate?
15:15 - 15:30 tea
Thursday, 10 July 2014
09:00 - 09:45S. Nakatsuji, ISSP, University of Tokyo
Emergent excitations in spin liquids through coupling with electrons and orbitals
09:45 - 10:15 F. Bert, Universite Paris-Sud
Spin liquid state in the vanadium based kagomecompound [NH4]2[C7H14N][V7O6F18]
Details about the Thursday Poster Session can be found in Sec. 6.3.2
..
Session Chairs of the day:
9:00 -10:45 S. Blundell
11:15 -12:30 M. Hagiwara
14:00 -15:00 F.Mila
Queen’s College Cambridge 43
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PLENARY
.
Emergent excitations in spin liquids through coupling with electrons and orbitals.
S. Nakatsuji, J.J. Ishikawa, M. Halim, Y. Matsumoto(ISSP, Univ. of Tokyo)
One of the highlights in the study of geometrically frustrated magnets is the emergence of nontrivial excitations in
spin liquid states, in particular through the interplay between different degrees of freedom, such as spins and mo-
bile electrons, and/or orbitals. Such interesting phenomena have been found in Pr2Ir2O7, a metallic spin ice with two
electronic sectors. One is Pr based spin ice, where strong quantum effects are expected for ferromagnetic exchange
couplingandmanysimilaritiesare foundto thequantumspin icecandidatePr2Zr2O7. Anothersector is the5dconduc-
tion electrons forwhich novel topological phases have been predicted based on the semimetallic half-filled Jeff = 1/2
banddue to strong spin-orbit coupling. Wewill discuss our observations of chiral spin liquid andquantumcriticality in
Pr2Ir2O7 andpossible topological effects due to semimetallic electronic structure. If timepermits, wewill also discuss
a strikingly isotropic spin liquid state found in a spin-orbital quantummagnet Ba3CuSb2O9. This work is based on the
collaboration with K. Kimura, Y. Machida, C. Broholm, J.Wen, T. McQueen, Y. Tokiwa, P. Gegenwart, T. Kondo, S. Shin,
M. Takigawa, R. Chen, E-G.Moon, L. Balents, H. Sawa, E. Nishibori, Y. Han,M.Hagiwara, T. Sakakibara, D.MacLaughlin,
Y.Wakabayashi,W. Higemoto, S. Onoda, S. Ishihara.
..
INVITED
.
Spin liquid state in the vanadium based kagome compound [NH4]2[C7H14N][V7O6F18].
F. Bert (Laboratoire de Physique des Solides, UMR 8502 CNRS, Université Paris-Sud)
Despite remarkable recent progresses in the understanding of spin liquid phases, it remains puzzling that in the few
known experimental candidates themagnetic excitations appear to be gapless at variance with most theoretical pre-
dictions. Clearly more and different experimental cases have to be investigated. While most quantum magnets are
based on Cu2+ (3d9, S=1/2), F.H. Aidoudi et al [1] have recently synthesized a new vanadium based quantummaterial
showingan interesting frustratedmagnetic lattice. Themagnetic structure consists ofwell separatedkagomebilayers
where V4+ (3d1, S=1/2) quantum spins sits on kagome-like layers bridged by V3+ (3d2, S=1) ions forming a triangular
interlayer. Despite sizeable antiferromagnetic interactions (θ = 58(4) K), µSR revealed no magnetic freezing down
to at least 40mK. Interestingly both bulk magnetization and heat capacity, and local probe 19F NMR experiments
show that the interlayer S=1 spins (V3+) do not couplemagnetically to the kagome-like layers so that thematerial can
be considered to a good approximation as a quantum kagome candidate. While macroscopic susceptibility is mainly
dominated by the curie-like contribution of V3+ ions in the low T regime, 19F NMR shift experiment unravel a non-
monotonic behavior quite similar to the one observed in Herbersmithite. After subtraction of the nuclear and nearly
freeV 3+ contribution to themagnetic specificheat, the latter showsaquasi-linear increaseversus temperaturepoint-
ing again to a gapless excitation spectrum in this new spin liquidmaterial[2].
[1] F.H. Aidoudi et al, Nat. Chem. 3, 801 (2011); [2] L. Clark et al, PRL 110, 207208 (2013)
Queen’s College Cambridge 44
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CONTRIBUTED
.
Revealing the LocalMagnetism of S=1/2 Kagome Lattice ZnCu3(OH)6Cl2 using Single Crystal NMR.
M.Fu (McMaster University),T.Imai (McMaster University; Canadian Institute for Advanced Research), T. H. Han (MIT), Y. S. Lee (MIT)
HerbersmithiteZnCu3(OH)6Cl2 is knowntobeapromising candidatematerial hostingaquantumspin liquidground
state. The recent success in single crystal growth of ZnCu3(OH)6Cl2 as well as the discovery of a continuum of
spinon excitations using inelastic neutron scattering [1] have opened a new chapter in the study of highly frustrated
magnetism. However, the mechanism behind the realization of the non-magnetic ground state in ZnCu3(OH)6Cl2
remains controversial, mainly due to the difficulty in understanding the role of defects in its physical properties. To
distinguish the intrinsic magnetism of the kagome lattice from the defect contribution, we used 17O, 63Cu and 2D
NMR to probe the local behavior of spin susceptibility and spin dynamics, which provided invaluable insights into the
nature of defects and their potential influence on the kagome spin lattice [2].
[1] T. H. Han et al., Nature 492, 406 (2012); [2] M. Fu, T. Imai et al., in preparation. Also see T. Imai. et al., Phys. Rev. B 84, 020411(R)
(2011); Phys. Rev. Lett. 100, 077203 (2008).
..
CONTRIBUTED
.
Spin order and disorder in organic diamond and triangular lattices.
Y. Shimizu (Nagoya Univ.), T. Hiramatsu, Y. Yoshida (Meijo Univ.), A. Ootsuka, M. Maesato (Kyoto Univ.), M. Yoshida, M. Takigawa (Tokyo
Univ.), M. Itoh (Nagoya Univ.), G. Saito (Meijo Univ.)
Versatile magnetic ground states have been investigated by NMR in organic Mott insulators with triangular and dia-
mond lattices. We find that a new compound κ-(ET)2Ag2(CN)3 with triangular lattice (t′/t ∼ 1) exhibits no indication
of long-range magnetic order down to 0.2 K despite the antiferromagnetic exchange of 170 K. Unlike the previous
spin liquid candidates κ-(ET)2Cu2(CN)3 and β′-EtMe3Sb[Pd(dmit)2]2, the low-lying excitations are homogeneous for
the Ag complexwith the steep temperature dependence in the nuclear spin-lattice relaxation rate, 1/T1 ∼ T 4, imply-
ing gapped spinons. In contrast, the coproduct (ET)Ag4(CN)5with diamond lattice exhibits long-range ordering at 101
K. The other κ-(ET)2X (X = B(CN)4 and CF3SO3) with t′/t > 1 shows spin-Peierls and antiferromagnetic transitions
below 4 K. These results highlight the frustrated ground states and quantum phase transition in κ-(ET)2X.
Queen’s College Cambridge 45
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INVITED
.
Study of elementary excitations of two-dimensional quantum spin liquids.
M. Yamashita (ISSP, Univ. of Tokyo)
Study of disordered states of quantum spins in two-dimensions, so-called quantum spin liquids (QSLs), has been at-
tracting attentionbecause2DQSLcanbeanewstate ofmatter characterizedbyunknownquasiparticles. The ground
states of QSLs and its exotic phenomena, such as fractionalized excitation with an artificial gauge field, have been
extensively discussed for decades, yet to be identified by lack of any real materials. This is why the recent discov-
eries of materials possessing an ideal 2D triangular or a kagomé lattice have spurred a great deal of interest. Espe-
cially, identifying the elementary excitation characterizing the ground state has been the central focus of attention.
In this presentation, I will introduce our studies of elementary excitations of an organicMott insulator with 2D trian-
gular lattice EtMe3Sb[Pd(dmit)2]2 , in which no magnetic ordering has been observed down to very low temperature
(T ∼ J/10, 000) [1]. From our thermal transport measurements, we have reported that a sizable temperature-linear
termof thermal conductivity is clearly resolved in the zero-temperature limit [2], showing gapless excitationwith long
mean free path (∼ 1, 000 lattice distances). Further, fromourmagnetic torquemeasurements, we have found that the
gapless excitation is a magnetic excitation [3]. Moreover, this gapless QSL state is found to be stable against reducing
the strength of frustration. I will discuss that these results point toward the emergence of a quantum critical phase in
which the spins of electrons remainmobile, despite the frozen charge degree of freedom.
I will also present our recent study of thermal transport measurements of kagomé material Volborthite
Cu3V2O7(OH)2·2H2O. From NMR studies of the slightly distorted kagomé material, multiple ordered phases have
been found in low temperatures and under high fields [4]. Upon entering one of the ordered phases under field, we
have found an increase of the thermal conductivity, showing an additional thermal transport due to spin wave excita-
tions. I will also talk about our attempts to observe a thermal-Hall effect of spinons suggested from the theory [5].
[1]K.KanodaandR.Kato, Annu. Rev. Condens. MatterPhys. 2, 167 (2011); [2]M.Y. et al. Science328, 1246 (2010); [3]D.Watanabe,
M.Y., et al., Nature communications 3, 1090 (2012); [4] M. Yoshida et al.: J. Phys. Soc. Jpn. 81 (2012) 024703; [5] H. Katsura et al.:
Phys. Rev. Lett. 104, 066403 (2010).
..
CONTRIBUTED
.
µSR of Propagating Spin Excitations in QuantumDisorderedMagnets.
F. Pratt (ISIS), P. Baker (ISIS), S. Blundell (Oxford), T. Lancaster (Durham)
Propagating spin excitations in quantum disordered magnets such as spin chains or quantum spin liquids (QSL) can
provide important information about their underlying nature. Local spin probe techniques such as NMR or µSR pro-
vide a means to extract the spectral density of fluctuations associated with the spin motion via field dependence of
the relaxation rate of the local spin probe. This technique is relatively well established for the case of a Heisenberg
antiferromagnetic spin chain such asDEOCC-TCNQF4 where 1D diffusivemotion of spinonswas observed using lon-
gitudinal field (LF) µSR [1]. The method can be extended to 2D QSL systems such as (ET)2Cu2(CN)3. In this case LF
µSRmeasurements give a 2D diffusion rate that increases with temperature following a weak power law. The exper-
imental results can be comparedwith two theoretical predictions that have beenmade for spinon transport in such a
QSL.Onemodel is based on Fermionic spinonswith a finite Fermi surface [2] and the othermodel assumes a quantum
critical regime for the spinon dynamics [3]. Bothmodels predict aweak power law for the diffusion rate, however only
the quantum critical model is consistentwith both the sign andmagnitude of the power law observed in theµSR data.
[1] F.L. Pratt et al, Phys. Rev. Lett. 96, 247203 (2006); [2] C.P. Nave and P.A. Lee, Phys. Rev. B 76, 235124 (2007); [3] Y. Qi, C. Xu and
S. Sachdev, Phys. Rev. Lett. 102, 176401 (2009).
Queen’s College Cambridge 46
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CONTRIBUTED
.
High fieldmagnetic phases of volborthite studied by single crystal NMR.
M. Yoshida, K. Nawa, M. Takigawa, H. Ishikawa, Y. Okamoto, Z. Hiroi (ISSP, Univ. of Tokyo), M. Jeong, S. Krämer, M. Horvatić, C. Berthier
(LNCMI, Grenoble), J. Yamaura (MCES, Tokyo Inst. Tech.), H. Yoshida (Hokkaido Univ., Japan)
The possibility of exotic quantum states in two-dimensional spin systems with frustrated interactions has attracted
strong attention. Volborthite Cu3V2O7(OH)2·2H2O, which has distorted kagome layers, is an example of interesting
frustrated spin systems. The absence of magnetic order down to 2 K, much lower than the Curie-Weiss temperature
115 K, indicates strong effects of frustration [1]. 51VNMR experiments on powder samples of volborthite have been
performed [1-4]. Three distinct magnetic phases were found at low temperatures, phase I below 4.5 T, II up to 26 T,
and III above [3, 4]. Phase I shows anomalies such as a Lorentzian line shape and a large spin-echo decay rate 1/T2
pointing to unusually slow fluctuations [3].
Quite recently, single crystals of volborthite were successfully prepared [5, 6]. We have performed 51V NMR mea-
surements on a single crystal in magnetic fieldsB up to 31 T. In phase I, distribution of the internal fields and unusual
spin fluctuations are observed in the same way as the 51V NMR experiments of the powder sample. Below 22 T in
phase II, the NMR spectra have a double-horn type line shape, which is characteristic of an incommensurate helical
or spin-density-wave order. Above 26 T, the spectra show almost a single peak, indicating a simple spin structure in
phase III. The center of gravity of the spectrum indicates that a 1/3magnetization plateau is realized in phase III. The
magnetization plateau is confirmed by the magnetization measurements of single crystals [7]. We also found a new
magnetic phase in the field region between phases II and III (23 < B < 26 T). This phase shows unusual distribution
of the internal fields similar to that in phase I.Wewill discuss whichmodel can explain the 1/3magnetization plateau
and the various magnetic phases observed in the single crystal of volborthite. Wewill also discuss how to explain the
difference between themagnetization of the single crystal and that of the previous powder sample [4, 8].
[1] Z. Hiroi et al., J. Phys. Soc. Jpn. 70, 3377 (2001); [2] F. Bert et al., Phys. Rev. Lett. 95, 087203 (2005); [3] M. Yoshida et al., Phys.
Rev. Lett. 103, 077207 (2009); [4] M. Yoshida et al., J. Phys. Soc. Jpn. 81, 024703 (2012); [5] H. Yoshida et al., Nat. Commun. 3, 860
(2012); [6] H. Ishikawa et al., Acta Cryst. C68, i41-i44 (2012); [7] H. Ishikawa et al., unpublished; [8] Y.Okamoto et al., Phys. Rev. B83,
180407(R) (2011).
..
CONTRIBUTED
.
Two crossovers between spinons andmagnons in S=1/2 frustrated antiferromagnet Cs2CuCl4.
A.I. Smirnov, K.Yu. Povarov, T.A. Soldatov (Kapitza Institute for Physical Problems RAS)
The S=1/2 dielectric antiferromagnet Cs2CuCl4 with a distorted triangular lattice is a quasi 2Dmagnet with the spin
ordering greatly reduced by zero-point fluctuations. The ordering at temperatureTN=0.62K is strongly delayedwith
respect to Curie-Weiss temperature TCW=4 K. In the temperature range TN<T<TCW there is a correlated spin-
liquid statewith a continuumofmagnetic excitations [1]. Our electron spin resonance (ESR) study is performed in the
frequency range 9<f<350GHz. In the spin-liquid phase, it reveals a fine structure of the spinon spectrum in the Bril-
louin zone center in form of a resonance doublet. This doublet is a signature of the spinon continuum of S=1/2 chains
with the uniformDzyaloshinskii-Moriya interaction [2]. At cooling down to T=0.05 K, for f=60–120GHz this spinon
doublet is found to survive deep in the ordered phase. On the other hand, at f<50 GHz the doublet is transformed
to an antiferromagnetic resonance (AFMR) signal. The frequency of this lower crossover corresponds approximately
to themain exchange integral. The coexistence of a low-frequency AFMR and of a spinonmode at a higher frequency
may be ascribed to the proximity of a quantum critical point, where oscillations of an order parameter and spinons
coexist. However, a consistent theory of such a combined spectrum is still absent. At the increase of the frequency
and, hence, of the magnetic field, the spinon doublet collapses at about a half of the saturation field (i.e. above 150
GHz). Further increase of the magnetic field causes magnetic saturation and reveals a new spectrum of q=0 excita-
tions in form of an intensive Larmor precession, coexisting with a much weaker mode of the exchange origin. This
high-frequency crossover to a doublet of another kind is due to the transition from the fluctuating spin liquid to a fully
polarized saturated phase.
[1] R. Coldea et al., Phys. Rev. B. 68, 134434 (2003); [2] K. Yu. Povarov et al., Phys.Rev. Lett. 107, 037204 (2011).
Queen’s College Cambridge 47
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INVITED
.
Howdoes the Kitaev spin liquid evaporate?
Y. Motome, J. Nasu, M. Udagawa, T. Kaji, and K. Matsuura (Univ. of Tokyo)
Quantum spin liquid (QSL) is a new state of matter in insulating magnets. Experimental candidates for QSL were re-
cently discovered in several quasi-2D and 3D compounds, which have been stimulating the study of QSL. A prevalent
common belief in the exploration of QSL is that the absence of a thermodynamic phase transition down to the lowest
temperature (T )manifests a symptomofQSL.Weraise aquestionon thismyth: could suchan indirect evidencebe the
only way to reach QSL? In general, liquid and gas have the same symmetry and they are not necessarily distinguished
by a phase transition. In classical fluids, however, there is a first-order transition between liquid and gas, which termi-
nates at a critical end point in the pressure-temperature phase diagram; beyond that liquid and gas are adiabatically
connected with each other by a crossover. Hence, our questions are what the case of QSL is, and if the high-T para-
magnet is always adiabatically connected to the low-T QSL. To address these issues, we investigate the Kitaev model
[1] and its generalization to 3D on a hyperhoneycomb lattice [2], both of which possess gapful and gapless QSL as the
exact ground states dependingon theparameters. We startwith the anisotropic limitwhere themodels are described
by emergent Ising-type variables. By an extensiveMonte Carlo simulation, we find that in the 3D case the model ex-
hibits a phase transition from the high-T paramagnetwith aCoulombic nature to the low-T gapfulQSL phase [3]. This
finite-T transition is caused by the constraints on the Ising-type variables, and hence, can be characterized by a topo-
logical property of the excitations. We show that the transition is of second order, which presumably belongs to the
3D Ising universality class. These are in sharp contrast to the 2D case where no constraint exists and QSL is contin-
uously connected to the high-T paramagnet. We also find that the magnetic susceptibility shows a characteristic T
dependence from high-T Curie law to low-T Van Vleck behavior with showing a broad hump in between. We also
discuss an extension of our analysis to a generic case including the gapless QSL region and provide comprehensive
understanding of how the Kitaev spin liquid evaporates.
[1] A. Kitaev, Ann. of Phys. 321, 2 (2006); [2] S.Mandal andN. Surendran, Phys. Rev. B 79, 024424 (2009); [3] J. Nasu et al., to appear
in Phys. Rev. B (arXiv:1309.3068).
..
CONTRIBUTED
.
Dynamics of the Z2 Spin Liquid on the Kagome Lattice.
S. Depenbrock (UC Irvine), S. White (UC Irvine)
We calculate the Green’s function of the nearest-neighbor Heisenberg model on the kagome lattice with the time-
dependent density-matrix renormalization group. Using a novel decomposition we are able to extend the accessible
time frame and to compute the dynamical structure factor of the Z2 spin liquid and compare it with experimental
results for Herbertsmithite.
..
CONTRIBUTED
.
Kapellasite: a gapless chiral spin liquid.
L. Messio, B. Bernu, S. Biéri, C. Lhuillier (LPTMC, UPMC, Paris)
The ground state of the kagomeantiferromagnet remains the subject of intense debates. Herewe take somedistance
with it, considering an other model on the kagome lattice: the one derived from kapellasite. Recent experiences on
kapellasite (a kagome compound synthetised in 2008) have evidenced a gapless spin liquid behavior ([1]). Inelastic
neutron scattering results on a powder sample show a maximal intensity near 0.5A−1, which is a surprising result.
High temperature expansions ([2]) indicate a ferromagnetic first neighbor exchange with frustrating antiferromag-
netic second and third neighbor exchanges. The derivated classical model leads to propose a chiral ground state with
aBraggpeakat0.5A−1. Wewill seehowtodescribeaneventual gaplesschiral spin liquidandobtain thecorresponding
structure factor using fermionic spinons.
[1] B. Fåk et al., Phys. Rev. Lett. 109, 037208 (2012); [2] B. Bernu et al., Phys. Rev. B 87, 155107 (2013)
Queen’s College Cambridge 48
HFM2014
..
CONTRIBUTED
.
Dynamics of a two-dimensional quantum spin liquid: signatures of emergentMajorana fermions and fluxes.
D. Kovrizhin (TCM, Cambridge)
We provide a complete and exact theoretical study of the dynamical structure factor of a two-dimensional quantum
spin liquid in gapless and gapped phases. We show that there are direct signatures – qualitative and quantitative – of
theMajorana fermionsandgaugefluxesemerging inKitaev’s honeycombmodel. These includecounterintuitivemani-
festationsof quantumnumber fractionalisation, suchas aneutron scattering responsewith a gapeven in thepresence
of gapless excitations, and a sharp component despite the fractionalisation of electron spin. Our analysis identifies
new varieties of the venerable X-ray edge problem and explores connections to the physics of quantum quenches.
[1] J. Knolle, D. L. Kovrizhin, J. T. Chalker, R. Moessner, arXiv:1308.4336
Queen’s College Cambridge 49
HFM2014
6.2.6 FRIDAY JULY 11TH 2014
Summary and Closing. Announcment of the Poster Prize by Nature Materials.12:00 - 13:00
13:00 - 14:00 lunch
10:15 - 10:30 M. Ruminy, Paul Scherrer Institut
Quasielastic neutron scattering in Tb2Ti2O7 and Y1.9Tb0.1Ti2O7
10:30 - 10:45 S. Onoda, RIKEN
Phase transitions, crossovers, and excitations in quantum spin ice
10:45 - 11:15 coffee
11:30 - 12:00 M. R. Lees University of Warwick
Studies of Magnetic Order in Yb2Ti2O7
11:15 - 11:30 S. Guitteny, LLB, CEA Saclay
Neutron scattering study of the quantum spin ice candidate Pr2Zr2O7
Friday, 11 July 2014
09:00 - 09:45M. J. P. Gingras,
University of WaterlooRare-earth magnetic pyrochlore oxides: Moving beyond the Ising era
09:45 - 10:15 I. Mirebeau, LLB, CEA Saclay
Magnetic structures and anisotropic excitations in Tb2Ti2O7 spin liquid
..
Session Chairs of the day:
9:00 -10:45 O. Petrenko
11:15 -12:00 C.. Broholm
Queen’s College Cambridge 50
HFM2014
..
PLENARY
.
Rare-EarthMagnetic PyrochloreOxides: Moving Beyond the Ising Era.
Michel Gingras (University ofWaterloo)
Magnetic materials have long offered physicists the opportunity to study some of the general principles that govern
collective phenomena in Nature. For example, different materials can possess distinct effective spin symmetry, al-
lowing experimentalists and theorists to explore the combined role of spin symmetry and spatial dimensionality on
the spontaneous development, or not, of long-range order. Because of the wide variety of interesting and complex
behaviours that they display, highly frustrated magnetic pyrochlore oxides have attracted much interest for nearly
thirty years. Since the late 1990s, significant progress has been made in understanding pyrochlore oxides with mag-
neticmoments having one spin components, or Ising systems. This is the case of the popular ’spin ices’. More recently,
combined experimental and theoretical efforts have been directed at understanding pyrochlores described by two
component spins (XY systems) and three component spins (Heisenberg systems). These two classes may be as rich
as the Ising spin ices, displaying intriguing phenomena such as spin liquid behavior, order-by-disorder and persistent
spin dynamics down to very low-temperature. In this seminar, I will present an overviewof the current understanding
of some of themost interesting non-Ising pyrochlores that has emerged over past couple of years.
..
INVITED
.
Magnetic structures and anisotropic excitations in Tb2Ti2O7 spin liquid.
I. Mirebeau1, S. Petit1, J. Robert1, S. Guitteny1, A. Gukasov1, P. Bonville2, A. Sazonov1,3, C. Decorse4. 1Laboratoire Léon Brillouin; 2Service
de Physique de l’Etat Condensé; 3RWTHAachen University; 4ICMMO, UMR 8182, Univ. Paris Sud.
Among the pyrochlores, Tb2Ti2O7 spin liquid, also called quantumspin ice, remains themostmysterious, in spite of 15
years of intense investigation. Our recent single crystal experiments using neutron diffraction and inelastic neutron
scattering down to 50mK yield new insight on its hotly debated ground state. By applying a highmagnetic field along
a [111] anisotropy axis [1], the Tb moments gradually reorient without showing the magnetization plateau observed
in classical spin ices. Quantitative comparison of the neutron andmagnetization datawithmean field calculation sup-
ports a dynamical symmetry breaking akin to a dynamic Jahn-Teller distortion, preserving the overall cubic symmetry.
In the non-Kramers Tb3+ ion it induces a quantummixing of thewave-functions of the ground state crystal field dou-
blet enabling the formation of a spin liquid, viewed as a non-magnetic two-singlet ground state in this mean-field pic-
ture [2]. The spin lattice coupling also shows up in the spin fluctuations in zero field [3]. Dispersive excitations emerge
from pinch-points in the reciprocal space, with anisotropic spectral weight. This is the first evidence of such excita-
tions in a disordered ground state. They reveal the breaking of some conservation law ruling the relative orientations
of the fluctuatingmagneticmoments in a given tetrahedron, as for themonopole excitations in classical spin ices. The
algebraic character of the correlations shows that Tb2Ti2O7 ground state is akin to a Coulomb phase. Finally, the first
excited crystal field level and an acoustic phonon mode interact, repelling each other. The whole results show that
the magneto-elastic coupling is a key feature to understand the surprising spin liquid ground state. They call for an
interaction between quadrupolar moments, for which the Jahn-Teller distortion is a first (single site) approximation.
[1] A. Sazonov et al. PRB 88, 184428, (2013); P. Bonville et al. PRB 89, 085115, (2014); [2] P. Bonville et al. PRB 84, 184409 (2011);
[3] S. Petit et al. PRB 86, 174403, (2012); S. Guitteny et al. PRL 111, 087201 (2013); [4] T. Fennell et al. PRL 112, 017203 (2013).
Queen’s College Cambridge 51
HFM2014
..
CONTRIBUTED
.
Quasielastic neutron scattering in Tb2Ti2O7 and Y1.9Tb0.1Ti2O7.
M. Ruminy (Paul Scherrer Institut), E. Pomjakushina (Paul Scherrer Insitut), M. Kenzelmann (Paul Scherrer Institut) and T. Fennell (Paul
Scherrer Institut)
The rare earth pyrochlore Tb2Ti2O7 remains a conundrum after more than a decade [1]. Experimentally, a spin liq-
uid phase with strong magnetoelastic effects survives to lowest temperatures and recent neutron scattering experi-
ments have shown a strong coupling between spin and lattice excitations at relatively high energies [2]. The neutron
spectrum of Tb2Ti2O7 also contains significant low energy scattering, which has variously been interpreted as a low-
lying crystal field excitation associated with a departure from cubic symmetry [3] or quasielastic scattering originat-
ing fromquantumfluctuations [4,5,6]. Recent experiments suggest that this scattering has a small gap [7], or contains
both quasielastic scattering and a low energy dispersive mode [8]. Here we report on new high resolution neutron
experiments on polycrystalline Tb2Ti2O7 and Y1.9Tb0.1Ti2O7. In the concentrated sample, a temperature-dependent
lineshape indicates a crossover from single-ion to cooperative fluctuations, while in the diluted sample two sharp ex-
citations appear alongwith the single-ion process. Thewavevector and temperature dependence of the sharpmodes
indicates that they originate from ground and excited state transitions of Tb3+ ions dimers, not crystal field excita-
tions. Taken together, our findings show that the quasielastic scattering in the magnetoelastic spin liquid phase of
Tb2Ti2O7 must be attributed to collective excitations of the Tb3+ ions [9].
[1] Gardner et al., Rev. Mod. Phys. 82, 53 (2010); [2] Fennell et al., Phys. Rev. Lett. 112, 017203 (2014); [3] Bonville et al., Phys. Rev.
B 84, 184409 (2011); [4] Yasui et al., J. Phys. Soc. Japan 71, 599 (2002); [5] Takatsu et al., J. Phys. Condens. Mat. 24, 052201 (2012);
[6] Gaulin et al., Phys. Rev. B 84, 140402(R) (2001); [7] Fritsch et al., arXiv:1312.0847 (2013); [8] Guitteny et al., Phys. Rev. Lett. 111,
087201 (2013); [9] Ruminy et al., in preparation.
..
CONTRIBUTED
.
Phase transitions, crossovers, and excitations in quantum spin ice.
S. Onoda (RIKEN), Y. Kato (RIKEN)
Finite-temperature properties of quantum spin ice and related systems have attracted great interest from both the-
oretical and experimental viewpoints because of the emergent analogous lattice quantum electrodynamics hosting
“electric” and “magnetic” monopole excitations as well as “photons”. From the theoretical side, it may show confine-
ment to deconfinement transitions at zero temperature and crossovers at finite temperatures, which are still contro-
versial beyond a naive level ofWilson’smean-field theory. From the experimental side, it is crucial to identify “photon”
and “Higgsfield” (monopole in thecontextof spin ice) excitations in relevantmaterials suchasYb2Ti2O7 andPr2Zr2O7.
Here, we report a state-of-the-art quantumMonte-Carlo study on a quantum spin icemodel. The specific heat shows
two broad peaks, signaling two crossovers from a local moment regime at high temperatures, through a confinement
(classical spin ice) regime at intermediate temperatures, to a deconfinement (Coulombic spin liquid) regime at low
temperatures. ThishasalsobeenconfirmedbycalculatingWilson loopdistributions. Wealsopresentfield-theoretical
calculations on the lattice AbelianHiggsmodel derived from generic quantum spin icemodel and compare the results
on thecrossovers to thenumericalones. Wealso revealquantumexcitationspectraaswell asdynamical spin structure
factors relevant to inelastic neutron-scattering experiments, which feature Higgs modes and continuum excitations
in Higgs phases expected for Yb2Ti2O7.
Queen’s College Cambridge 52
HFM2014
..
CONTRIBUTED
.
Neutron scattering study of the quantum spin ice candidate Pr2Zr2O7.
Solène Guitteny, Julien Robert, Arsen Goukassov (CEA, Centre de Saclay, DSM/IRAMIS/ Laboratoire Léon Brillouin), Pierre Bonville (CEA,
Centre de Saclay, DSM/IRAMIS/ Service de Physique de l’Etat Condensé), Elsa Lhotel (Institut Néel C.N.R.S – Universite Joseph Fourier),
Claudia Decorse (LPCES, Université Paris-Sud), Monica Ciomaga Hatnean, Geetha Balakrishnan (University ofWarwick), Sylvain Petit,
Isabelle Mirebeau (CEA, Centre de Saclay, DSM/IRAMIS/ Laboratoire Léon Brillouin)
Thepyrochlore compoundPr2Zr2O7 is suggested tobeagoodcandidate fordisplayingquantumspin icephenomenol-
ogy [1]. This is supported by recent measurements by Kimura et al showing that the elastic magnetic pattern displays
a large pinch point broadening and fast fluctuations down to 50mK. They also showed that the first excited doublet of
the crystal electric field of the non-Kramers ion Pr3+ is located at 9.5 meV above the doublet ground state [2]. In this
work, we report local susceptibility measurements carried out with polarised neutrons. Our measurements reveal
the strong Ising-like anisotropy of the magnetic moments of the Pr3+ ions along the <111> axis. Inelastic neutron
scattering also allowed us to investigate the evolution of the quasi-elastic signal versus the temperature, indicating
the existence of fast fluctuations within the ground state doublet down to 1.5K at least. We also show the existence
of an additional CEF mode at 5 meV not reported in [2] and which must be taken into account to refine the crystal
field scheme. These observations recall the case of the Ising-like non-Kramers Tb3+ ions in Tb2Ti2O7. In that case, the
coupling with the lattice written in terms of quadrupoles is likely at play involving the existence of a magneto-elastic
excitation [3,4]. In Pr2Zr2O7 however, preliminary study of the phonon modes around the CEF levels does not sug-
gest the existenceof such features as observed inTb2Ti2O7. Then, a quadrupolar couplingmaybe at playbut via other
process.
[1] M.J.P. Gingras and P.A. McClarty, arXiv1311:1817v1 (2013); [2] K. Kimura et al,Nat. Commun., 5, 1934 (2013); [3] T. Fennell et al,
Phys. Rev. Lett., 112, 017203, (2013); [4] S. Guitteny et al, Phys. Rev. Lett., 111, 087201, (2013).
..
INVITED
.
Studies ofMagnetic Order in Yb2Ti2O7.
M.R. Lees (University ofWarwick), L.-J. Chang (National Cheng Kung University), I. Watanabe (AdvancedMeson Science Laboratory, RIKEN),
A.D. Hillier (ISIS), Y. Yasui (Meiji University), S. Onoda (CondensedMatter Theory Laboratory, RIKEN), G. Balakrishnan (University of
Warwick), E. Lhotel (Institut Néel, Grenoble), S.R. Giblin (Cardiff University).
The nature of the ground state of the pyrochlore magnet Yb2Ti2O7 is much debated. We present muon-spin relax-
ation (µSR) spectra, specific-heat versus temperatureC(T )measurements, aswell as detailedmagnetization studies,
on polycrystalline and single-crystal samples of Yb2Ti2O7. C(T ) exhibits a sharp peak at a TC of 0.21 and 0.25 K for
the single-crystal andpolycrystalline samples, respectively. Forboth samples, themagnetic entropy releasedbetween
50mKand30Kamounts toR ln 2 per Yb. At temperatures belowTCweobserve a steep drop in the asymmetry of the
zero-fieldµSR time spectra at short time scales, aswell as a decoupling of themuon spins from the internal field in lon-
gitudinalmagnetic fieldsof≤ 2500Oe forboth thepolycrystalline and single-crystal samples. Ourmagnetizationdata
also indicate that there is a first-order ferromagnetic transition in both forms of Yb2Ti2O7. The first-order character
of the transition is preserved in applied fields up to 200Oe. The transition stabilizes a ferromagnetic component and
involves slow dynamics of the magnetization. Our results are consistent with the onset of long-range ferromagnetic
magnetic order in single-crystal and polycrystalline Yb2Ti2O7.
Queen’s College Cambridge 53
HFM2014
6.3 POSTER SESSIONPROGRAMME&ABSTRACTS
The two best posters of the conference will be awarded a GBP 200 prize each, cour-
tesy of NatureMaterials (npg). The announcement of the winners will take place at the
closing of the conference on Friday.
6.3.1 POSTER SESSION IRe
f$#Bo
ard$#
S10101
1
S10102
2
S10103
3
S10104
4
S10105
5
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6
S10107
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S10110
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S10112
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S10201
13
POSTER
'SESSION'I''**'Tue
sday'Ju
ly'8th'2014,'from
'15.30'to
'17.45'
02_Spin$Ice$and$Re
lated$
Materials$(experim
ent)
LocalEp
robe
$investigations$of$the
$reEentrant$sp
inEliqu
id$groun
d$state$of$th
e$no
vel$kagom
e$antiferromagne
t$ZnC
u3(OH)6SO4.
M.$G
omilsek
Single$Crystal$Growth$and
$Magne
tic$Prope
rEtie
s$of$N
ovel$Kagom
e$Co
mpo
unds,$
KV3G
e2O9,$KMn3
Ge2O
9,$and
$K2V
2Mo3
O11.$
Magne
tic$Prope
rties$o
f$the
$Kagom
eETriangular$Lattice$An
tiferromagne
t$NaB
a2Mn3
F11.$
Microscop
ic$m
agne
tic$m
odeling$for$the
$spinE1/2$kagom
e$compo
und$
[NH4
]2[C7H
14N][$V7
O6F18].
Grou
nd$States$o
f$the
$$S$=$1/2$Heisenb
erg$kagome$antiferromagne
t$(Rb{1Ex}Cs{x})2
Cu3SnF12$determined
$via$m
agne
tic$m
easuremen
ts.
Gapless$S
pin$Liqu
id$Groun
d$State$in$a$S=1/2$Vanadium$Kagom
e$An
tiferromagne
t.
Spin$wave$excitatio
ns$und
er$strong$geo
metrical$frustration$in$CaB
aCo2
Fe2O
7.$
Incommen
surate$antife
rrom
agne
tic$order$in$th
e$layered$kagome$system
$CaBa
Co2Fe2O7.$
S.$Hara
H.$Ishikawa
O.$Janson
L.$Bovo
$01_Kagome$Ba
sed$
System
s
ReEevaluation$of$th
e$crystal$structure$of$the
$S$=$1/2$$kagom
e$antiferromagne
t$vesig
nieite,$B
aCu3
V2O8(OH)2.$
Magne
tic$excita
tions$in$th
e$frustrated
$kagom
e$qu
antum$fe
rrom
agne
t$haydeeite
$stud
ied$by$inelastic$neu
tron
$scatterin
g.B.$Fàk
K. K
atay
ama
J.C.
Ora
in
J. R
eim
J. R
eim
FieldEIndu
ced$Quantum
$Critical$phe
nomen
a$in$Kagom
eELattice$
Antiferromanget.$
T.$Sakai
Novel$The
rmod
ynam
ics$in$Dy
2Ti2O7$Spin$Ice:$Two$expe
rimen
tal$case$stud
ies.$
Subject
Poster$Title
Presen
ter
Muo
n$spin$re
laxatio
n$stud
ies$o
f$the
$pinwhe
el$kagom
e$compo
unds$
Rb2C
u3SnF12$and$Cs2C
u3SnF12.
P.$Baker
D.$Boldrin
Queen’s College Cambridge 54
HFM2014
S10202
14
S10203
15
S10204
16
S10205
17
S10206
18
S10207
19
S10208
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S10209
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S10210
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S10211
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S10212
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S10214
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S10215
27
S10216
28
S10217
29
B..Klemke
G.M
..Luke
S..Petit
D..Pom
aranski
02_Spin.Ice.and.Re
lated.
Materials.(e
xperim
ent)
Prob
ing.the.spin.liqu
id.state.of.T
b2Ti2O
7.with.chem
ical.pressure.
A.M
..Hallas
The.Nature.of.the
.finiteUtem
perature.transition.in.the
.anisotrop
icpyrochlore.Er2Ti2O
7.P.C.W..H
oldsworth
Origin.of.the
.(1/2,1/2,1/2).o
rder.in..Tb2
Ti2O
7.Y.J..Kao
M..CiomagaUHatne
an
Calorimetric.stud
ies.of.the
.antife
rrom
agne
t.Yb2G
e2O7.
R..Freitas
Coexistence.of.M
agne
tic.Orders.and.Glassy.Ph
ases.in.Pyrochlore.
Antife
rrom
agne
ts.M
3Co(CO
3)$_2C
l.(M.=.Na,.Li)..
Z..Fu
Spin.correlation
s.and.magne
toelastic.excitation
s.in.Tb2
Ti2O
7.T..Fen
nel
Mod
elling.Neu
tron
.Scattering.in.Y2M
o2O7.Using.the
.LargeUN.M
etho
d..
F..Flicker
Gappe
d.and.Gapless.Low
.Field.States.in.the
.Quantum
.Spin.Ice.Cand
idate.
Tb{2+x}Ti{2
Ux}O{7+y}.
E..Kermarrec
Thermal.transpo
rt.in.spin.ice.Dy2Ti2O
7..
Quantum
.Magne
tism
.in.Yb2
Ti2O
7.
Order.by.disorder.or.en
ergetic.selection.of.the
.groun
d.state.in.the
.XY.
pyrochlore.antife
rrom
agne
t.Er2Ti2O7.?.A.neu
tron
.scattering.stud
y.
Low.tem
perature.spe
cific.heat.measuremen
ts.of.the
.spin.ice.material.
Dy2Ti2O
7.do
wn.to.340.m
K.
Stuffin
g.and.substitution
s.in.the
.Pyrochlore.compo
unds.
D..Prabh
akaran.
Vibrating.coil.m
agne
tometry.in.Dy2Ti2O
7..at.milliUK
elvin.tempe
ratures..
C..Duvinage
Static.m
agne
tic.ph
ase.revealed
.by.muo
n.spin.relaxation.and.thermod
ynam
ic.
measuremen
ts.in.quantum
.spin.ice.Yb2Ti2O7..
L.J..Chang
Stud
y.of.the
.Magne
tic.Prop
erties.of.Single.Crystals.of.the
.Geo
metrically.
Frustrated
.ZirUcon
ate.Pyrochlores,.A2Zr2O7..
Queen’s College Cambridge 55
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30
S10219
31
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32
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33
S10222
34
S10301
35
S10302
36
S10303
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S10402
40
S10403
41
S10404
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S10405
43
S10406
44
03_Perovskites,7Spinels,7
and7Related7M
aterials
Investigation7on7the7low7temperature7distorted7phase7of7MgCr2O4.
S.7Gao
Magnetic7Orders7in7Heisenberg7pyrochlore7antiferromagnets.
T.7Higo
Therm
al7spin7liquid7in7Sr2CuWO67with7frustrated7Cu(II)72D7square7lattice.7
O.7Burrows
Long7range7m
agnetic7order7in7spinVorbitVcoupled7double7perovskites7
Ba2YRuO_67and7Ba2CaOsO67probed7with7neutron7scattering7and7m
uon7spin7
relaxation:7Comparison7with7theory7and7disordered7Ba2YReO67and7Ba2YMoO6.
J.P.7Carlo
Evolution7of7the7M
agnetic7Excitations7in7the7Low7Temperature7Phase7of7
Yb2Ti2O77as7a7Function7of7Applied7M
agnetic7Field.
02_Spin7Ice7and7Related7
Materials7(experiment)
Evidence7for7a7longVrange7m
agnetic7order7in7Er2Sn2O7.
I.7Zivkovic
J.7Knolle
04_Quantum7Spin7Liquids7
(theory)
Quantum7Kagome7Ice.7
J.7Carrasquilla
Effective7flux7Hamiltonians7for7Kagome7systems.
S.7Ghosh
Doping7a7topological7quantum7spin7liquid:7slow7holes7in7the7Kitaev7honeycomb7
model.
Confinemed7and7deconfined7phases7of7quantum7square7ice.
Quantum7spin7liquid7with7a7M
ajorana7Ferm
i7surface7on7the7threeVdim
ensional7
hyperoctagon7lattice.
Dynamics7of7a7twoVdim
ensional7quantum7spin7liquid:7signatures7of7emergent7
Majorana7ferm
ions7and7fluxes.
G. H
alas
z
L.P.
Hen
ry
M. H
erm
anns
Low7temperature7Therm
al7conductivity7and7therm
al7expansion7of7Spin7Ice7
materials.
High7M
agnetic7Field7Phase7Diagram7of7Pyrochlore7Tb2Ti2O77Along7[111].
J.D.7Thompson
W.7Toews
L.7Yin
Low7temperature7m
agnetic7properties7of7a7Ce3+7pyrochlore.777
R.7Sibille
Queen’s College Cambridge 56
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S10408
46
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48
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05_G
eneral3(the
ory)
Entrop
y3change3and
3the
3magne
tocaloric3effect3in3antife
rrom
agne
tic3clusters.
N.A.3de3Oliveira
Investigations3of3M
agne
tic3Fractal3Structures3of3Diluted3Antife
rrom
agne
tic3
Materials.
A.N.3B
azhan
Novel3m
agne
tic3mod
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Queen’s College Cambridge 58
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01 Kagome Based Systems
S10101 Muon spin relaxation studies of the pinwheel
kagome compounds Rb2Cu3SnF12 and Cs2Cu3SnF12.
P. Baker (ISIS), F. Pratt (ISIS), J. Möller (Oxford University), S. Blundell
(Oxford University), D. Prabhakaran (Oxford University), W. Hayes (Ox-
ford University), T. Lancaster (Durham University), — The compounds
Rb2Cu3SnF12 and
Cs2Cu3SnF12 have a distorted kagome lattice. Previous neu-
tron scatteringmeasurements have established that
Rb2Cu3SnF12 has magnetic excitations consistent with those
of a valence bond solid and that Cs2Cu3SnF12 ordersmagnet-
ically at 20K.We have carried outmuon spin relaxationmea-
surements on single crystal samples of both materials and a
polycrystalline sample of Rb2Cu3SnF12 prepared separately.
The resultsof themeasurementsonsingle crystal samplesare
consistentwith previously reported results, with nomagnetic
ordering in Rb2Cu3SnF12 evident above 50mK andmagnetic
ordering at 20 K in Cs2Cu3SnF12. However, in the polycrys-
talline sample of Rb2Cu3SnF12 we found well-defined mag-
netic ordering present below230Kwith comparable internal
fields to the ordered state of single crystal Cs2Cu3SnF12. The
oscillations present in the muon data are remarkably well-
defined and occur at up to four frequencies in different tem-
perature ranges. The combination of internal fields changes
at 60, 80, and 210 K, with no oscillations above 230 K, and
the fields are∼ 100mT rather than the∼ 1mT typically ex-
perienced by muons in F-µ-F bound states. It is remarkable
that the temperatures at which the internal fields change are
in good agreement with the predictions of numerical calcula-
tions based on the parameters obtained by inelastic neutron
scattering on the single crystals of Rb2Cu3SnF12.
S10102 Re-evaluation of the crystal structure of the S = 12
kagome antiferromagnet vesignieite, BaCu3V2O8(OH)2 D.
Boldrin (UCL), A.S. Wills (UCL) —S = 12kagome antiferromagnets
(KAFMs) provide excellent experimental opportunities to ex-
plore highly frustratedmagnetic states such as quantum spin
liquidsandresonatingvalencebondstates. Themineralvesig-
nieite
BaCu3V2O8(OH)2, first synthesised in 2009 by Okamoto et
al. [1], is one of only a handful of model S = 12KAFM ma-
terials. Despite a Weiss temperature of θW ≈ −80K and a
slight distortion of the kagome lattice, the material does not
order until TN ≈ 10K [2]. Furthermore, a dominant in-plane
Dzyaloshinskii-Moriya anisotropy suppresses quantum fluc-
tuations and promotes a classical magnetic order at TN [3].
However, depending on sample quality the behaviour at TN
varies frompartial spin-freezing to long-rangeorder [4]. Here
we propose a new structure for vesignieite from synchrotron
dataand relate thechanges in samplequalitywithanordering
of a trigonal structure. The data agree with P3121 symmetry
which results in nearest-neighbour Cu2+ ions superimposing
chevrons on the kagome lattice, leaving open the possibility
of a trimer spin structure.[1] Okamoto et al., J. Phys. Soc. Jpn., 2009, 78, 033701; [2] Quilliam et
al., Phys. Rev. B, 2011, 84, 180401; [3] Zorko et al., Phys. Rev. B, 2013,
88, 144419; [4] Yoshida et al., J. Phys. Soc. Jpn, 2013, 82, 013702
S10103 Magnetic excitations in the frustrated kagome
quantum ferromagnet haydeeite studied by inelastic neu-
tron scattering. B. Fåk (SPSMS, INAC, CEA), D. Boldrin and A. S.
Wills (UCL), M. Enderle, J. Ollivier andM. Zbiri (Institut Laue-Langevin) —
Quantum S=1/2 spins localized on the corner-sharing trian-
gular network of a kagome lattice are highly frustrated mag-
neticmaterials and are among themost promisingmodel sys-
tems to display novel exotic and highly degenerate ground
states, such as (chiral) spin liquids or resonating valence
bond states. Unfortunately, there are only few experimen-
tal realizations of undistorted spin-1/2 kagome lattices, the
most noteworthy being herbertsmithite [1] and kapellasite
[2]. We present experimental results from a third example of
an undistorted S=1/2 kagome lattice, the mineral haydeeite,
α-Cu3Mg(OD)6Cl2 [3], which is the magnesium-analogue of
kapellasite. The Curie-Weiss constant is close to zero as a
consequence of competing ferro- and anti-ferromagnetic in-
teractions. Static susceptibility measurements indicate a fer-
romagnetic transition at TC = 4.2 K but no long-range mag-
netic order has been observed by neutron diffraction. Ab
initio calculations have difficulties in obtaining accurate esti-
mates of the main exchange interactions in this type of sys-
tems; even the sign of the nearest-neighbor exchange is un-
known. Wehaveperformed inelastic neutron scatteringmea-
surements on a fully deuterated powder sample of haydeeite
at low temperatures to determine the magnetic excitations
and extract the dominating exchange integrals.[1] T.-H. Han et al., Nature 492 (2012) 406; [2] B. Fåk et al., Phys. Rev.
Lett. 109 (2012) 037208; [3] R. H. Colman, A. Sinclair, and A. S.Wills,
Chem. Mater. 22 (2010) 5774.
S10104 Local-probe investigations of the re-entrant spin-
liquid ground state of the novel kagome antiferromagnet
ZnCu3(OH)6SO4 M.Gomilšek (Jožef Stefan Institute), A. Zorko (Jožef
Stefan Institute), Q.-M. Zhang (Renmin University of China) — The
recently discovered compound ZnCu3(OH)6SO4 is a novel
quantum kagome antiferromagnet potentially lacking spin
freezing in the ground state [1]. It exhibits an unusual re-
entrant spin-liquid state, characterized by a paradigmatic
crossover between two gapless spin-liquid regimes with low-
ering temperature. Understanding of this unprecedented
phenomenon is currently missing as it is irreconcilable with
established theories. Local-probe magnetic techniques are
uniquely suited for the study of the re-entrance phenomenon
and for assessing the nature of both spin-liquid regimes at a
microscopic level. Wewill present the results fromour recent
Queen’s College Cambridge 59
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ESR and µSR investigations of this compound.[1] Y. Li et al., arXiv:1310.2795.
S10105 Single Crystal Growth and Magnetic Properties of
Novel Kagome Compounds, KV3Ge2O9, KMn3Ge2O9, and
K2V2Mo3O11. S. Hara (Kobe University), H. Sato (Chuo University),
T. Sakurai (Kobe University), and H. Ohta (Kobe University) — Single
crystals of the novel potassium oxide compounds KV3Ge2O9,
KMn3Ge2O9, and
K2V2Mo3O11 have been synthesized by a hydrothermal
method. These compounds have a hexagonal unit cell with
the space group P63/mmc for the formers and P63mc for the
latter. The lattice constants are a = 5.8624(4) Å and c =
13.7094(7) Å, a = 5.8832(4) Å and c = 13.7705(7) Å, and a =
5.7713(3) Å and c = 13.8144(7) Å, respectively. KV3Ge2O9
andKMn3Ge2O9 contains layersof edge-sharedV(Mn)O6 oc-
tahedra, where V(Mn)3+ ions with S = 1(2) spin form kagome
lattice layers separated by double layers of GeO4 tetrahedra
each other. In contrast to these, K2V2Mo3O11 contains lay-
ers of edge-sharedMoO6 octahedra,whereMo ions formdis-
torted kagome lattice layers separated by double layers of
VO6 octahedra each other.
Wemeasured temperature dependence of the magnetic sus-
ceptibilities on these single crystals. TheWeiss temperatures
are estimated over − 100 K for each material. These nega-
tive Weiss temperatures indicate that the nearest-neighbor
exchange interaction among magnetic ions is antiferromag-
netic, and suggest the existence of magnetic frustration. The
magnetic susceptibility of KV3Ge2O9 shows a broad maxi-
mum at approximately 70 K, suggesting that magnetic tran-
sition is suppressed in spite of the presence of strong anti-
ferromagnetic interactions. Below approximately 20 K, the
susceptibility under a magnetic field perpendicular to the
kagome plane steeply drops toward zero, while the in-plane
susceptibility divergeswith reduction in temperature. On an-
other,
KMn3Ge2O9, the one shows a broad maximum at about 80
K as like KV3Ge2O9. However, a small cusp was observed at
around 40 K under 0.1 T for DC susceptibilities without any
anomaly for AC susceptibilities between 2 to 60 K. A mag-
netic anisotropy is observed below 45 K; the susceptibility
under a magnetic field perpendicular to the kagome plane
decreases toward zero with reducing temperature. It indi-
cates antiferromagnetic transition. On the other hand, the in-
plane susceptibility increases with reducing temperature. In
contrast, themagnetic susceptibility ofK2V2Mo3O11 shows a
Curie-like behavior above 100 K. Below approximately 60 K,
however, the susceptibility suggests cluster-like magnetism
in consideration of shift of Curie-constant, and under 10 K,
this material shows ferrimagnetic behavior.
S10106Magnetic Properties of theKagome-Triangular Lat-
tice Antiferromagnet NaBa2Mn3F11 H. Ishikawa, T. Okubo Y.
Okamoto, K. Nawa, M. Yoshida, M. Takigawa, Z. Hiroi (ISSP, University
of Tokyo) —NaBa2Mn3F11 is a layered fluoride in which Mn2+
ions (S = 5/2) comprise a kagome-like lattice.1 The kagome-
like lattice deforms so as to generate the next-nearest-
neighbor interaction J2 between three out of six spins in
the hexagon of a normal kagome lattice, in addition to the
nearest-neighbor interaction J1. As a function of J2/J1, this
lattice can interconnect the kagome (J2 = 0) and the triangu-
lar (J2 = J1) lattices and thus is called the kagome-triangular
(KT) lattice.2 We have synthesised a polycrystalline sample
ofNaBa2Mn3F11 and studied itsmagnetic properties bymag-
netic susceptibility, heat capacity and 23Na NMR measure-
ments. Furthermore, magnetic orders for classical Heisen-
berg spins in the KT lattice have been theoretically exam-
ined. We will discuss the magnetic interactions and order of
NaBa2Mn3F11 from the structural considerations and analy-
sis based on classical Monte-Carlo simulation.[1] J. Darriet et al. J. Solid StateChem. 98, 379 (1992); [2]H. Ishikawa
et al. J. Phys. Soc. Jpn. 83, 043703(2014).
S10107 Microscopic magnetic modeling for the spin- 12
kagome compound [NH4]2[C7H14N][V7O6F18]. O. Janson
(NICPB,MPI CPfS), A. A. Tsirlin (NICPB,MPI CPfS), I. Rousochatzakis (MPI
PKS), H. Rosner (MPI CPfS) — In the recently synthesised com-
pound
[NH4]2[C7H14N][V7O6F18], magneticS= 12V4+ atoms forman
ideal kagome lattice [1]. Very recent µSR studies indicate the
emergence of a gapless spin liquid state as a result of mag-
netic frustration [2]. Using density functional theory calcula-
tions,weaddress themicroscopicmagneticmodel of this low-
dimensional compound. We show that its peculiar symme-
try gives rise to two inequivalent nearest-neighbor couplings.
The behavior of the resulting quantum spin model is studied
usingexactdiagonalizationandcomparedto theexperiments.
OJ and AT were supported by the Mobilitas program of the
ESF, grant numbersMJD447 andMTT77, respectively.[1] F. H. Aidoudi et al., Nature Chem. 3, 810 (2011); [2] L. Clark et al.,
Phys. Rev. Lett. 110, 207208 (2013).
S10108 Ground States of the S=1/2 Heisenberg kagome
antiferromagnet (Rb1−xCsx)2Cu3SnF12 determined via
magnetic measurements. K. Katayama, N. Kurita, and H. Tanaka
(Tokyo Institute of Technology) — Kagome-lattice Heisenberg
antiferromagnets (KLHAFs) have been attracting growing
attention as one of the most intriguing frustrated systems.
It has been proposed that, in S = 1/2 HKLAF, a synergistic
effect of the geometric frustrations and the quantum fluc-
tuations leads to quantum disordered state. However, the
nature of its ground state is highly controversial.
Our recent works have revealed that A2Cu3SnF12 (A=Rb [1]
andCs [2]) is apromising familyof theS = 1/2KLHAFsystem
Queen’s College Cambridge 60
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for comprehensive study. Since the high purity and sizable
single crystals are available, the detailedmagnetic properties
of the two compounds have been probed by inelastic neutron
scattering experiments [3, 4]. Rb2Cu3SnF12 is found to show
aquantumdisordered ground state [1, 2], inwhich a pinwheel
valence bond solid (VBS) is realized [3]. The kagome network
of Rb2Cu3SnF12 is spatially anisotropic with four kinds of ex-
change nearest neighbour interactions. On the other hand,
Cs2Cu3SnF12 has a uniform kagome lattice at room tempera-
ture [2]. As temperature decreases, a structural phase transi-
tionoccurs atTt =184Kand theunit cell is enlarged to2a×2a
[2, 4]. Cs2Cu3SnF12 exhibits an antiferromagnetic ordering at
TN =20 K [4]. Thus, it is of great interest to investigate the
mixed system (Rb1−xCsx)2Cu3SnF12, because the end mem-
bers exhibit different quantum ground states and the quan-
tum phase transition is expected to occur at someCs concen-
tration x. Elucidating the systematic changes of the ground
state could provide an important clue todetailedmechanisms
of the pinwheel VBS phase. Here, we report magnetic mea-
surements of (Rb1−xCsx)2Cu3SnF12 performed on various x.
It was found that, with increasing x, the non-magnetic sin-
glet ground sate changes to an antiferromagnetic ordered
state at x≃ 0.53. The magnitude of spin gap decreases to
zero atx≃ 0.53, and the ordered ground state is observed for
x> 0.53, which is indicative of a quantum phase transition at
x≃ 0.53.[1] K. Morita et al., J. Phys. Soc. Jpn. 80, 043707 (2008); [2] T. Ono et
al., Phys. Rev. B 79, 174407 (2009); [3] K. Matan et al., Nature Phys.
6, 865 (2010); [4] T. Ono et al., J. Phys. Soc. Jpn. 83, 043701 (2014).
S10109 Gapless Spin Liquid Ground State in a S=1/2 Vana-
dium Kagome Antiferromagnet. J.C. Orain (CNRS, Université
Paris-Sud), L. Clark (University of Edimburgh), F. Bert, P. Mendels (CNRS,
Université Paris-Sud), F.H. Aidoudi, P. Lightfoot, R.E Morris (University of
St. Andrews, St. Andrews) —Among the rareexperimental realiza-
tionsof theKAFMmodel the recently synthesizedcompound,
[NH4]2[C7H14N ][V7O6F18] (DQVOF) [1], is thefirstone tobe
built on V 4+ (d1) ions rather than more usual Cu2+ (d9) thus
allowing to investigate the effects of different perturbations
to the ideal Heisenberg Hamiltonian. Our low temperature
magnetization and specific heat results suggest that DQVOF
is a good candidate for the S=1/2 KAFM physics despite a
complexstructure [2]. Thecompound ismadeof twodifferent
Vanadium ions, V3+ S=1 interlayer Vanadium ions and V4+
S=1/2 Kagome layer Vanadium ions. The low temperature
magnetization measurements show that the V3+ are decou-
pled from theKagome layers. Furthermore, the low tempera-
ture specific heat andµSR studies evidence a gapless spin liq-
uid behaviour down to 40mK. Recent NMR studies allowed
us to unreveal the intrinsic susceptibility of the Kagome lay-
ers which is hidden in the macroscopic measurements by the
Curie like interlayer V3+ contribution.
[1] F. H. Aidoudi and al, Nat. Chem. 3, 801 (2011); [2] L. Clark and al,
Phys. Rev. Lett. 110, 207208 (2013).
S10110 Spin wave excitations under strong geometrical
frustration in CaBaCo2Fe2O7. J. Reim (Forschungszentrum
Jülich), L. Fritz (Utrecht), J. Robert (Laboratoire Léon Brillouin), M. Vall-
dor (Max-Planck Institut für Chemische Physik fester Stoffe), W. Schweika
(European Spallation Source and Forschungszentrum Jülich) — Due to
strong geometrical frustration, the layered kagome system
of the hexagonal swedenborgite structure [1] exhibits an un-
usual large range of disordered ground states as observed in
a number of isostructural compounds [2-4], which can be de-
scribed by a classical 3D spin liquid regime (see contribution
Buhrandt, Fritz). Recently, in the compound CaBaCo2Fe2O7,
we observed a√3 ×
√3 antiferromagnetic ordered ground
state and Monte Carlo simulations show that the system is
close to the phase boundary of the spin liquid phase. Our in-
elastic neutron scattering studyon large single crystals shows
strong influences of the peculiar geometric frustration, with
a relatively strong damping in the kagome layers and spin
wave propagations of larger correlation length perpendicu-
lar to the kagome layers. The experimental results have been
discussed in comparison with theoretical calculations of the
spin dynamics and numerical simulations [5] based on a near-
est neighborHeisenbergmodel. It is found that the excitation
spectrum isvery sensitive to small changes inorderingandac-
cordingly exchange interactions.[1] M. Valldor et al. Solid State Sci., 4(7):923–931, July 2002; [2] W.
Schweika et al. Phys. Rev. Lett., 98(6):067201, February 2007; [3] J. R.
Stewart et al. Phys. Rev. B,83(2):024405, January 2011; [4] P.Manuel
et al. Phys. Rev. Lett., 103(3):037202, July 2009; [5] J. Robert et al.
Phys. Rev. Lett., 101(4):117207, September 2008
S10111 Incommensurate antiferromagnetic order in
the layered kagome system CaBaCo2Fe2O7. J. Reim
(Forschungszentrum Jülich), M. Valldor (Max-Planck Institut für Chemis-
che Physik fester Stoffe), W. Schweika (European Spallation Source and
Forschungszentrum Jülich) — The layered kagome system in the
hexagonal swedenborgite structure [1] displays similarly to
the pyrochlores a highly frustrated network of tetrahedral
coordinated magnetic ions. However, its broken inversion
symmetry raises further the complexity of ordering due to
non-vanishing Dzyaloshinski-Moriya interactions. Recently
investigated compounds of this family show various signs for
unusual geometric frustration and disordered ground states
despite of the typically strong antiferromagnetic exchange.
[2-5] We have studied single crystals of the compound
CaBaCo2Fe2O7, by neutron diffraction including polarization
analysis, and observed an antiferromagnetic ordering below
TN ≈ 160K in a√3 ×
√3 larger supercell, which essentially
agrees with expectations from a simple Heisenberg model
with only in-plane (J1) and out-of (kagome) plane (J2) nearest
neighbor interactions.[5] A gradual spin reorientation with
Queen’s College Cambridge 61
HFM2014
temperature is evidenced by polarization analysis. A further
particular intriguing result is the chiral interference observed
as an apparent asymmetry of the magnetic Bragg intensities,
from which a cycloidal character of the antiferromagnetic
order can be concluded and related to small Dzyaloshinski-
Moriya interactions. At low temperatures, high resolution
powder and single crystal diffraction experiments reveal an
incommensurate splitting of the antiferromagnetic peaks,
with a propagation vector τ = 0.016Å−1
corresponding to
a periodicity of about 400Å. The incommensurate ordering
is discussed within the Heisenberg model for J2/J1 < 1.5,
which is the phase boundary to commensurate ordering.[[1] M. Valldor et al. Solid State Sci., 4(7):923–931, July 2002; [2] W.
Schweika et al. Phys. Rev. Lett., 98(6):067201, February 2007; [3] J. R.
Stewart et al. Phys. Rev. B,83(2):024405, January 2011; [4] P.Manuel
et al. Phys. Rev. Lett., 103(3):037202, July 2009; [5] D. D. Khalyavin et
al. Phys. Rev. B, 82(9):094401, September 2010.
S10112 Field-Induced Quantum Critical phenomena in
Kagome-Lattice Antiferromanget. T. Sakai (JAEA SPring-
8 and University of Hyogo), H. Nakano (University of Hyogo) — The
kagome-lattice antiferromagnet is one of interesting frus-
trated systems. It exhibits some exotic field-induced phe-
nomena, like a magnetization plateau, jump etc. Our previ-
ous large-scale numerical diagonalization study on the S=1/2
kagome-lattice antiferromagnet revealed that a new field-
induced phenomenon, ”the magnetization ramp”, occurs at
1/3 of the saturation magnetization[1]. It is characterized by
different critical exponents between the lower-field and the
higher-field sides of the magnetization curve[2]. In order to
clarify unconventional properties around the 1/3 magnetiza-
tion,weconsideredsomeextended latticemodels; adistorted
kagome lattice andadistorted triangular lattice etc. including
the perfect kagome lattice, and invesitigated quantum phase
transitions with respect to several exchange couplings. As a
result, the 1/3 magnetized kagome-lattice antiferromagnet
was revealed to belong to an unconventional phase different
from any well-known ones[3,4]. The ground-state magneti-
zation curve recently obtained by the numerical diagonaliza-
tion up to the 42-spin cluster is also presented to estimate
the shape in the thermodynamic limit. The relation of the
present work with the recent other works using the density
matrix renormalization group[5] and the numerical diagonal-
ization[6]will be discussed. In addition some interestingmag-
netization processes of other related frustrated systems will
be presented[7,8].[1] H. Nakano and T. Sakai, J. Phys. Soc. Jpn. 79 (2010) 053707;
[2] T. Sakai and H. Nakano, Phys. Rev. B 83 (2011) 100405(R); [3]
H. Nakano, T. Shimokawa and T. Sakai, J. Phys. Soc. Jpn. 80 (2011)
033709; [4] T. Sakai and H. Nakano, Physica Status Solidi B 250
(2013) 579; [5] S. Nishimoto, N. Shibata and C. Hotta, Nature Comm.
4 (2013) 2287; [6] S. Capponi et al., Phys. Rev. B 88 (2013) 144416;
[7] H. Nakano and T. Sakai, J. Phys. Soc. Jpn. 82 (2013) 083709; [8] H.
Nakano,M. Isoda and T. Sakai, to appear in J. Phys. Soc. Jpn.
02 Spin Ice and RelatedMaterials (experiments)
S10201 Novel Thermodynamics in Dy2Ti2O7 Spin Ice: Two
experimental case studies. L. Bovo (LCN,UCL), L. D. C. Jaubert
(OIST), P. C. W. Holdsworth (ENS) and S. T. Bramwell (LCN,UCL). — Spin-
ice systems [1,2] can be described by a network of corner-
shared tetrahedra of localised magnetic moments: geometri-
cal spin frustrationarises. Thisproblem is topologicallyequiv-
alent to proton ordering in water ice: to minimise the energy
the spins obey the ‘ice-rule’. Emergent magnetic monopoles
[3,4] have been modelled as deconfined excitations carrying
a magnetic Coulomb charge which are associated with vio-
lations of the ice rule. Spin ices show a variety of proper-
ties some of which are better described by spins, other by
monopoles. Magnetic susceptibility is a spin property and it
shows a peculiar crossover [5]. Here [6] we present a care-
ful experimental observation for spherical crystals. Themag-
netic entropy [2] is another signature that can be described
in terms of magnetic monopoles. Here [7] we show an alter-
nativemethodbasedonMaxwell’s thermodynamic equations
that can yield to themagnetic entropy on an absolute scale.[1] Harris, M.J. et al. PRL 79, 2554(1997); [2] Ramirez, A. P.et al. Na-
ture 399, 333(1999); [3] Ryzhkin, I.A. J. Exp. and Theor. Phys. 101,
481(2005); [4]Castelnovo, C. et al. Nature451, 42(2008); [5] Jaubert,
L.D.C. et al. Phys. Rev. X 3, 011014(2013); [6] Bovo, L. et al. JPCM 25,
386002(2013); [7] Bovo, L.et al. JPCM 25, 356003(2013). This work
was supported by EPSRC grant EP/I034599/1.
S10202 Staticmagnetic phase revealed bymuon spin relax-
ation and thermodynamic measurements in quantum spin
ice Yb2Ti2O7. L.J. Chang (National Cheng Kung University), M. R,
Lees (University of Warwick), I. Watanabe (RIKEN ), A. D. Hillier (ISIS), Y.
Yashi (Meiji University), S. Onoda (RIKEN) —The pyrochloremagnet
Yb2Ti2O7 has attracted great interest as the first candidate
to be a magnetic analogue of the Higgs phase or a U(1) quan-
tum spin liquid, depending on different cryststals used for
the experiments. Via comprehensive polarized diffuse neu-
tron scattering experiments on a high quality single-crystal
Yb2Ti2O7 sample,wehadobservedthefirst-orderphasetran-
sition occurred at TC 0.21 K, separating a high-temperature
magnetic Coulomb phase, that is characterized by pinch-
point features, and a low-temperature magnetically ordered
phase, that exhibits finite planar components of pseudospins
[1]. Since this planar component can be described as the
Bose condensation of spinons, carrying emergent magnetic
monopoles, coupled to the U(1) gauge field within the frame-
work of a quantum spin icemodel, and the transition temper-
ature is lowenough for aquantumCoulomb liquidbehavior to
appear, this first-order phase transition has been regarded as
a magnetic analogue of a Higgs transition. In this talk, we will
present our new muon spin relaxation (muSR) and specific-
Queen’s College Cambridge 62
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heatmeasurementsonpolycrystallineandsingle-crystal sam-
ples of Yb2Ti2O7. The specific heat exhibits a sharp peak at
TC between 0.21 and 0.26 K. The magnetic entropy released
between 50 mK and 30 K amounts to Rln 2 per Yb. Obser-
vations of a steep drop in the asymmetry of zero-field muSR
time spectra at short time scales, as well as a decoupling of
themuonspins fromthe internalfield in longitudinalmagnetic
fields of ≤ 0.25 T, below TC demonstrate the formation of
static magnetic moments. These evidences strongly support
the scenario of themagnetic Higgs phase.[1] L. J. Chang et al., Nat. Commun. 3, 992 (2012).
S10203 Study of the Magnetic Properties of Single Crys-
tals of the Geometrically Frustrated Zirconate Pyrochlores,
A2Zr2O7. M. Ciomaga Hatnean (University of Warwick), C. Decorse
(Université Paris-Sud), M. R. Lees, O. A. Petrenko, D. S. Keeble, G. Bal-
akrishnan (University of Warwick) — Geometrically frustrated py-
rochlore oxides of the type A2B2O7 (where A= Rare Earth,
B= Ti or Zr) have been the subject of extensive investigations
because of their interesting and rather unconventional mag-
netic properties, such as spin liquid, spin glass, or spin ice be-
haviour. Recently, the Zr containing pyrochlore, Pr2Zr2O7,
has been shown to exhibit spin freezing at low temperatures.
We report the structural characterization and investigation
of the magnetic properties of this new class of geometrically
frustrated zirconium pyrochlore oxides. High quality single
crystals of Pr2Zr2O7 have been grown using the floating-
zone technique. Themagnetization of thePr2Zr2O7 crystals
has beenmeasured along the three principal crystallographic
axes. Other crystals of oxides belonging to the zirconate py-
rochlores family have been also been grown using the same
technique and their magnetic properties are also reported.
S10204 Stuffing and substitutions in the Pyrochlore com-
pounds. D. Prabhakaran, S.Wang and A.T. Boothroyd (University of Ox-
ford) —Pyrochlore structure compounds are extensively stud-
ied for their spin-ice properties. However, these compounds
are very sensitive to structural defects and disorder. For ex-
ample, a small degree of stuffing changes the heat capacity
valueofYb2Ti2O7dramatically [1]. Defects in thepyrochlore
structure may be caused by site mixing or oxygen deficiency.
The effect is stronger in crystals compared to the powder
samples. To address this problem, we have stuffed the rare
earth element onto the B site with different percentages and
investigated the structural andmagnetic properties. Wehave
also varied the oxygen concentration in the system by adjust-
ing the valence of the B site and by substitution [2]. We grew
single crystals using different techniques andwill present our
most recent findings.[1]. K. A. Ross, Th. Proffen, H. A. Dabkowska, J. A. Quilliam, L. R.
Yaraskavitch, J. B. Kycia and B. D. Gaulin, Phys. Rev. B 86 (2012)
174424; [2]. G. Sala, M. J. Gutmann, D. Prabhakaran, D. Pomaranski,
C. Mitchelitis, J. B. Kycia, D. G. Porter, C. Castelnovo and J. P. Goff,
NatureMaterials (in press).
S10205 Vibrating coil magnetometry in Dy2Ti2O7 at milli-
Kelvin temperatures. C. Duvinage (TUM, Munich), D. Prab-
hakaran (Oxford University), C. Pfleiderer (TUM,Munich), A. T. Boothroyd
(Oxford University) — Spin ice attracts great interest as a state
in which emergent fractionalized excitations and magnetic-
field induced topological forms of order may occur. An im-
portant characteristic of the spin ice systems Dy2Ti2O7 and
Ho2Ti2O7, as well as the spin liquid system Tb2Ti2O7, is the
observation of spin freezing below a few hundred mK [1,2].
Wereport vibrating coilmagnetometrydowntomKtempera-
tures of Dy2Ti2O7, addressing the evidence for field-induced
phase transitions. Of particular interest is the observation
of putativemagnetisation avalanches in the spin-frozen state
which depend sensitively in number and size on the sweep
rate of the applied magnetic field. These avalanches have
been interpreted in terms of monopole excitations [3].[1] Krey et al., PRL 108, 257204 (2012); [2] Legl et al., PRL 109,
047201 (2012); [3] Slobinsky et al., PRL 105, 267205 (2010).
S10206 Spin correlations andmagnetoelastic excitations in
Tb2Ti2O7. T. Fennell (PSI), M. Ruminy (PSI), M. Kenzelmann (PSI), B.
Roessli (PSI), H. Mutka (ILL), J. Ollivier (ILL), L.-P. Regnault (CEA), U. Stuhr
(PSI), O. Zaharko (PSI), L. Bovo (UCL), A. Cervellino (PSI), M. Haas (Prince-
ton), R. Cava (Princeton) — In the rare earth pyrochlore Tb2Ti2O7,
a three-fold puzzle exists - themechanismbywhich Tb2Ti2O7
escapes both magnetic order and/or a structural distortion,
and furthermore, the nature of the spin liquidwhich exists in-
stead, are long standing questions in the field of frustrated
magnetism. Using polarized neutron scattering we have re-
cently shown that at low temperature Tb2Ti2O7 has power-
law correlations, manifested by pinch point scattering, some-
what similar to a spin ice. We have also discovered that an
acoustic phonon is coupled to an excited crystal field state,
producing a hybrid excitation with both propagating spin and
phonon components, a magnetoelastic mode. Our results im-
ply that the Hamiltonian of Tb2Ti2O7 must incorporate both
spin and lattice degrees of freedom, and that it must produce
a type of Coulomb phase.
S10207 Modelling Neutron Scattering in Y2Mo2O7 Using
the Large-N Method. F. Flicker (U. of Bristol, Perimeter Insti-
tute, U. of Waterloo), M. J. P. Gingras (U. of Waterloo, Perimeter Insti-
tute, Canadian Institute forAdvancedResearch) —ThepyrochloreYt-
triumMolybdenate (Y2Mo2O7) exhibits a spin glass transition
at 22.5K, but themechanism behind the transition has eluded
characterization for over thirty years. The first single-crystal
sample of this material was recently grown. Elastic neutron
scattering experiments revealed the presence of a liquid-like
isotropic ring in the (hhl) plane at radius 0.44Å−1. Such a ring
has never previously been observed arising frommagnetic in-
teractions.
Queen’s College Cambridge 63
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Using the large-N method we model the neutron scattering
for Heisenberg spins on a pyrochlore lattice. Including inter-
actions to fourth nearest neighbour we use a combination of
analytic andnumerical techniques to conclusively ruleout the
possibility that this simple model can reproduce the experi-
mental results. We speculate on more complicated mecha-
nismswhich could potentially reproduce this unexpected and
novel observation.
S10208 Calorimetric studies of the antiferromagnet
Y b2Ge2O7. R. S. Freitas (IF-USP, Brazil), E. Arrighi (IF-USP, Brazil),
Z. L. Dun (UT, Knoxville, USA), E. S. Choi (UT, Knoxville, USA), M. Lee
(NHMFL, Tallahassee, USA), H. D. Zhou (UT, Knoxville, USA; NHMFL,
Tallahassee, USA), A. M. Hallas (UM, Winnipeg, Canada), C. R. Wiebe
(NHMFL, Tallahassee, USA; UM, Winnipeg, Canada), J. S. Gardner
(NSRRC, Taiwan), A. M. Arevalo-Lopez (CSEC-SC, Edinburgh, United
Kingdom), J. P. Attfield (CSEC-SC, Edinburgh, United Kingdom) and J. G.
Cheng (BNLCMP-IP, Beijing, China) —The hunt for strong quantum
effects in geometrically frustrated pyrochlores has found in
Y b2Ti2O7 a promising candidate. The reduced effective spin
of Yb enhances quantum fluctuations and may disturb the
delicate balance of crystal-field, exchange and dipolar effects
resulting in a magnetic Coulomb quantum liquid state. From
the experimental point of view, the nature of the ground state
in this material is under intense debate. Some reports point
to a ferromagnetic order while others are more consistent
with a quantum spin liquid state with no long-range order.
We will report on precise low-temperature specific-heat
measurements on Y b2Ge2O7. This sample has a smaller
lattice parameter than the Ti-based counterpart, resulting
in stronger exchange correlations. Our results indicate
long-range antiferromagnetic order below TN = 0.58K . A
detailed analysis of the critical exponent α around TN , along
with high field magnetization measurements, allows us to
extract important information regarding the nature of the
ground state of this highly correlated compound.
S10209Coexistence ofMagneticOrders andGlassy Phases
in Pyrochlore AntiferromagnetsM3Co(CO3)2Cl (M =Na, Li).
Z. Fu (Forschungszentrum Juelich GmbH), Y. Zheng (Xi’an Jiaotong Uni-
versity), Y. Xiao (Forschungszentrum Juelich GmbH), S. Bedanta (National
Institute of Education and Research), A. Senyshyn (Technische Universi-
taet Muenchen), G. Simeoni (Technische Universitaet Muenchen), Y. Su
(Forschungszentrum Juelich GmbH), P. Koegerler (RWTH Aachen Univer-
sity), Th. Brueckel (Forschungszentrum Juelich GmbH) — Pyrochlore
antiferromagnets Na3Co(CO3)2Cl and Li3Co(CO3)2Cl pos-
sess Co2+ (s = 3/2) pyrochlore magnetic structure. For
Na3Co(CO3)2Cl, the DC magnetization and AC susceptibil-
ity measurements suggest a spin-glass-like phase transition
at ∼4.5 K and indicate a long-range collective magnetic be-
havior at 17 K [1,2] The temperature dependence of the spe-
cific heat shows a sharp peak at 1.5 K, which is attributed to
a long-range magnetic phase transition. The frustration fac-
tor of Na3Co(CO3)2Cl is then determined to be ∼22.5. TheNa3Co(CO3)2Cl sample proves to be chemically ordered by
means of neutron diffraction. The diffuse neutron scatter-
ing with polarization analysis reveals short-range spin corre-
lations characterized by dominating antiferromagnetic cou-
pling between nearest neighbors and weak ferromagnetic
coupling between next nearest neighbors. The magnetic re-
flections observed at 50 mK can be well explained within an
all-in-all-out spin configuration. Inelastic neutron scattering
of Na3Co(CO3)2Cl exhibits spin-wave-like excitations at 3.7
K, indicating that the spin-glass-like transition at Tg = 4.5 K is
not a conventional spin glass transition. The peak observed
in magnetic susceptibility at 17 K is attributed to the onset
of an intermediate partially-ordered phase transition, quali-
tatively consistent with the theoretical predictions. As a ho-
molog of Na3Co(CO3)2Cl, Li3Co(CO3)2Cl shows some new
features, different from Na3Co(CO3)2Cl although both com-
pounds have similar magnetic structures. According to DC
magnetization, Li3Co(CO3)2Cl has two magnetic phase tran-
sitions at 16K and10K. AC susceptibility shows a frequency-
dependent peak at ∼5 K, which probably relates to a re-
entrant spin-glass phase. Unlike Na3Co(CO3)2Cl, there is no
long range order below 2 K in Li3Co(CO3)2Cl.[1] Y. Zheng, A. Ellern and P. Koegerler, Acta Cryst. C 67, i56 (2011);
[2] Z. Fu et al., Phys. Rev. B 87, 214406 (2013).
S10210 Probing the spin liquid state of Tb2Ti2O7 with
chemical pressure. A. M. Hallas (McMaster University), A. M.
Arevalo-Lopez (University of Edinburgh), J. G. Cheng (Chinese Academy of
Sciences), H. J. Silverstein (University of Manitoba), P. M. Sarte (Univer-
sity of Manitoba), H. D. Zhou (University of Tennessee), G. M. Luke (Mc-
Master University), C. R. Wiebe (University of Winnipeg) — The ori-
gin of the spin liquid state in Tb2Ti2O7 has challenged exper-
imentalists and theorists alike for nearly 20 years. Despite
classical predictions of a Néel state below ∼1 K, Tb2Ti2O7
avoids static order to temperatures below 50 mK. Recently,
experimentalevidencehasmounted foramagnetoelastic spin
liquid state [1] or a quantum spin ice state [2] in Tb2Ti2O7.
One popular avenue of investigating this spin liquid state has
been to probe its destruction. For example, externally ap-
plied pressures can induce long-range antiferromagnetic or-
der in Tb2Ti2O7 [3]. To gain a better understand of the ex-
otic magnetism in Tb2Ti2O7, we have perturbed this system
with chemical pressure through the synthesis of Tb2Ge2O7.
In contrast to the stannate pyrochlores, germanium substitu-
tion results in a lattice contraction and enhanced exchange
interactions. We have characterized the magnetic ground
state of Tb2Ge2O7 with specific heat, ac and dcmagnetic sus-
ceptibility, and polarized neutron scattering measurements.
Akin to Tb2Ti2O7, there is no long-range order in Tb2Ge2O7
down to 18 mK. The Curie-Weiss temperature of -19.2(1) K,
Queen’s College Cambridge 64
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which is more negative than Tb2Ti2O7, supports the picture
of stronger antiferromagnetic exchange. Polarized neutron
scattering of Tb2Ge2O7 reveals that at 3.5 K liquid-like cor-
relations dominate in this system. However, the liquid-like
correlations give way to intense short-range ferromagnetic
correlations below 1 K. Despite stronger antiferromagnetic
exchange, the ground state of Tb2Ge2O7 has ferromagnetic
character, in stark contrast to the pressure-induced antifer-
omagnetic order observed in Tb2Ti2O7. The striking differ-
ences between externally applied and chemical pressuremay
hint at the origin of the exotic magnetism in Tb2Ti2O7.[1] T. Fennell et al., Phys. Rev. Lett. 112, 017203 (2014); [2] K. Fritsch
et al., Phys. Rev. B 87, 094410 (2013); [3] I. Mirebeau et al., Nature
420, 54 (2002).
S10211 The Nature of the finite-temperature transition in
the anisotropic pyrochlore Er2Ti2O7. P. C. W. Holdsworth
(ENSLyon),M. E. Zhitomirsky (CEAGrenoble), R.Moessner (MPIPKS,Dres-
den) —We use classical Monte Carlo simulation to study the
finite-temperature transition in a model XY antiferromag-
net on a pyrochlore lattice, relevant for the pyrochloremate-
rial Er2Ti2O7 [1]. The orderedmagnetic structure selected by
thermal fluctuations is six-fold degenerate. Despite this the
critical behavior corresponds to the three-dimensional XY
universality class. We determine an additional critical expo-
nent ν6 = 0.75 > ν , which determines the width of a domain
wall in theordered state and is characteristic of adangerously
irrelevant scalingvariable. Thepresenceof several character-
istic length scales in theorderedphaseyields theperculier co-
existence ofmagnetic Bragg peaks and diffusemagnetic scat-
tering observed experimentally in neutron diffraction mea-
surements [2].[[1] M. E. Zhitomirsky, P. C. W. Holdsworth, R. Moessner,
arXiv:1402.4689; [2] J. P. C. Ruff et. al Phys. Rev. Lett. 101,
147205 (2008)
S10212 Origin of the (1/2,1/2,1/2) order in Tb2Ti2O7 Y.-J.
Kao, Y.-C. Yang (National Taiwan University) — The pyrochlore ma-
terial Tb2Ti2O7 has been the focus of intensive research as
it does not show any conventional long-range order down to
50mK, and remains in a dynamic spin-liquid state. Recent
experiments observed signatures of a (1/2,1/2,1/2) order in
both neutron scattering and specific heat measurements [1-
3]. We derive an effective pseudospin-1/2 Hamiltonian for
Tb2Ti2O7 byprojectingamicroscopicHamiltonianwith thef -
p hybridization to the lowest crystal field doublets [4,5]. We
present a mean-field phase diagram of this Hamiltonian, and
estimate thepossibleparameters in the spinHamiltonian that
give rise to the (1/2,1/2,1/2) in the real material.[1] K. Fritsch, K. A. Ross, Y. Qiu, J. R. D. Copley, T. Guidi, R. I. Bewley,
H. A. Dabkowska, B. D. Gaulin, Phys. Rev. B, 87, 094410 (2013); [2] K.
Fritsch, E. Kermarrec, K.A. Ross, Y.Qiu, J. R.D.Copley,D. Pomaranski,
J. B. Kycia, H. A. Dabkowska, and B. D. Gaulin, arXiv:1312.0847; [3] T.
Taniguchi, H. Kadowaki, H. Takatsu, B. Føak, J. Ollivier, T. Yamazaki,
T. J. Sato, H. Yoshizawa, Y. Shimura, T. Sakakibara, T. Hong, K. Goto,
L. R. Yaraskavitch, J. B. Kycia, Phys. Rev. 87, 060408(R) (2013); [4]
ShigekiOnodaandYoichiTanaka, Phys. Rev. B,83, 094411 (2011); [5]
Hamid R. Molavian, Michel J.P. Gingras, and Benjamin Canals, Phys.
Rev. Lett., 98, 157204 (2007)
S10213 Gapped and Gapless Low Field States in the Quan-
tum Spin Ice Candidate Tb2+xTi2−xO7+y . E. Kermarrec (Mc-
Master University), K. Fritsch (McMaster University), D.Maharaj (McMas-
ter University), M. Couchman (McMaster University), A. Morningstar (Mc-
Master University), D. Pomaranski (Waterloo University), J. B. Kycia (Wa-
terloo University), K. A. Ross (John Hopkins University), Y. Qiu (NIST),
J. R. D. Copley (NIST), H. A. Dabkowska (McMaster University), B. D.
Gaulin (McMaster University) — Among the rare-earth magnetic
frustrated pyrochlore compounds which display exotic quan-
tumphenomena, the antiferromagnet Tb2Ti2O7 has been the
subject of intense research for more than 15 years due to
the mysterious nature of its ground-state. An exciting pos-
sibility would be that Tb2Ti2O7 realizes a Quantum Spin Ice
state at low temperature, where quantum fluctuations – in-
troduced via either relatively strong anisotropic exchange in-
teractions[1] or through virtual crystal field excitations[2] –
prevent the formation of any long-range order. Recently,
Taniguchi et al.[3] discovered that a minute off-stoichiometry
x in polycrystalline samples, leading to the nominal formula
Tb2+xTi2−xO7+y , can tune thesystemfromaspin-liquid state
to a partially ordered state with a small antiferromagnetic
frozen moment of ∼ 0.1 µB . We investigate with high reso-
lution time-of-flight inelastic neutron scattering the ground-
state of two single crystals of Tb2+xTi2−xO7+y . X-ray diffrac-
tion and magnetic susceptibility measurements confirmed
their different stoichiometry levelx. Previousmeasurements
revealed the existence of diffuse magnetic elastic scattering
at ( 12, 12, 12) positions in reciprocal space at T = 70 mK, as-
cribed to a short range, frozen, antiferromagnetic spin ice
state[4]. We perform a parametric study in field and tem-
perature of this low-T state and construct a refined H − T
phase diagram for Tb2Ti2O7 under small applied [110] mag-
netic fields, in good agreement with recent ac-susceptibility
studies[5]. This frozen antiferromagnetic spin ice state ap-
pears to be gapped (∆ ∼ 0.06 − 0.08meV) and is destroyed
by the application of small critical field Hc ≃ 0.075 T for
T = 80 mK, revealing its fragility. Our high resolution neu-
tron scatteringmeasurements reveal the sensitivity of such a
ground state to stoichiometry x at the 0.005 level.[[1] S. HCurnoe, Phys. Rev. B 88, 014429 (2013); [2]H. R.Molavian et
al. Phys. Rev. Lett. 98, 157204 (2007); [3] T. Taniguchi et al., Phys. Rev.
B 87, 060408(R) (2013); [4] K. Fritsch et al., Phys. Rev. B 87, 094410
(2013); [5] L. Yin et al., Phys. Rev. Lett. 110, 137201 (2013)
S10214 Thermal transport in spin ice Dy2T i2O7. B.
Klemke (Helmholtz-Zentrum Berlin, Germany), P. Strehlow (Physikalisch-
Technische Bundesanstalt, Berlin, Germany), A. Sokolowski (Helmholtz-
Queen’s College Cambridge 65
HFM2014
Zentrum Berlin, Germany), M. Meissner (Helmholtz-Zentrum Berlin, Ger-
many; European Spallation Source, Lund, Sweden) — Geometrical
frustration is a common featureof condensedmatter systems
in which the lattice geometry inhibits the formation of a sin-
gle groundstate configuration. In order to analyse low tem-
perature heat transport in the spin ice compoundDy2Ti2O7,
we derive thermodynamic field equations that are based on
the kinetic theory of phonons and their interaction with lo-
calised magnetic excitations [1]. It is shown that the solution
of the derived field equations for given boundary and initial
values of heat-pulse experimentswell describes all measured
temperatureprofiles recorded in the temperature range from
0.3 to 15 K and in magnetic fields up to 1.5 T. Thus, the data
of both the specific heat and the thermal conductivity, which
were obtained in thermal relaxation and steady-state heat
transport measurements, are in agreement with the thermo-
dynamicmodelling of heat-pulse experiments. The evaluated
temperature and field dependencies of both the specific heat
contributions and their corresponding relaxation times indi-
cate that the magnetic excitations above the ground-state
manifold of the spin ice compound Dy2T i2O7 take the form
ofmagnetic monopoles.[1] P. Strehlow, S.Neubert, B. Klemke,M.Meissner, ContinuumMech.
Thermodyn. 24, 347 (2012).
S10215 Quantum Magnetism in Yb2Ti2O7. G.M. Luke,
H. Dabkowska, R. D’Ortenzio, E. Kermarrec, T. Medina, T.J.S. Munsie, T.J.
Williams (McMaster), B. Frandsen, T. Goko, L. Liu, Y.J. Uemura (Columbia),
S.R. Dunsiger (Munchen), — The pyrochlore magnet Yb2Ti2O7 is
believed to possess a spin Hamiltonian which can support
a Quantum Spin Ice (QSI) ground state. Specific heat mea-
surements at very low temperatures exhibit notable sample
variation, with some samples displaying a sharp thermody-
namic phase transition at about 265mK.We report zero field
muon spin relaxation measurements of polycrystalline and
single crystal specimens of Yb2Ti2O7 which demonstrate an
absence of static magnetism in this system. Transverse field
measurements for the polycrystal and single crystals (with
fields along [111], [110] and [100]) reflect the transitions or
cross-overs seen in specific heat measurements. The precise
nature of the ground state of Yb2Ti2O7 is not yet completely
determined, but is characterized by an absence of static mag-
netic moments as would be found for periodic long-range or-
der or a frozen spin glass state. Yb2Ti2O7 remains a candidate
for a quantum spin liquid state.
S10216 Order by disorder or energetic selection of
the ground state in the XY pyrochlore antiferromagnet
Er2Ti2O7? A neutron scattering study.
Sylvain Petit, Julien Robert, Solène Guitteny (CEA, Centre de Saclay,
DSM/ IRAMIS/ Laboratoire Léon Brillouin), Pierre Bonville (CEA, Centre de
Saclay, DSM/ IRAMIS/ Service de Physique de l’Etat Condensé), Claudia
Decorse (LPCES, Université Paris-Sud), JacquesOllivier, HannuMutka (In-
stitut Laue Langevin),Michel J.P. Gingras (University ofWaterloo), Isabelle
Mirebeau (CEA, Centre de Saclay, DSM/ IRAMIS/ Laboratoire Léon Bril-
louin) — Examples of materials where an “order by quantum
disorder” mechanism is at play to select a particular ground
state are scarce [1,2]. It has been recently proposed that the
anti-ferromagnetic XY pyrochlore Er2Ti2O7 reveals a most
convincing case of this mechanism [3,4,5]. Observation of a
spin gap at zone centers was interpreted as a definitive proof
of this physics [6]. We argue, however, that the magnetic
anisotropy provided by the interaction-induced admixing be-
tween the crystal-field ground and excited levels gives an al-
ternative energetic mechanism [7,8]. Random phase approx-
imation RPA calculations based on a mean field model taking
into account explicitly the crystal-electric field anisotropy re-
producequitewell newhigh resolution inelastic neutron scat-
tering data. These experiments point out the existence of a
spin gap of∆ ≈ 43 µeV which can be well understood within
this scenario: in this energetic selection mechanism, the gap
originates from the anisotropy rather than quantum fluctua-
tions effects. The present study revives the issue of the phys-
ical origin of the experimentally broken discrete symmetry
ground state in Er2Ti2O7. It raises the question of the quan-
tumorderbydisorderas the soleorevenprincipalmechanism
for the selection of themagnetic ground state in thismaterial.[1] C. Lacroix, Introduction to Frustrated Magnetism (Springer-
Verlag, Berlin, 2011); [2] J. Villain, R. Bidaux, J.-P. Carton, R. Conte,
J. Phys 41, 1263 (1980); [3] J. D. M. Champion et al, Phys. Rev. B,
68, 020401 (R), (2003); [4] M. E. Zhitomirsky et al, Phys. Rev. Lett.
109, 077204 (2012); [5] L. Savary et al, Phys. Rev. Lett. 109, 167201
(2012); [6] K. A. Ross et al, Phys. Rev. Lett. 112 057201 (2014); [7] S.
Petit et al, in preparation; [8] P. A. McClarty et al, Journal of Physics:
Conference Series 145, 012032 (2009).
S10217 Low temperature specific heat measurements of
the spin icematerial Dy2Ti2O7 down to 340mK. D. Pomaran-
ski (U. Waterloo), L. R. Yaraskavitch (U. Waterloo), S. Meng (U. Waterloo),
K. A. Ross (McMaster U.), H.M.L. Noad (McMaster U.), H. A. Dabkowska
(McMaster U.), B.D. Gaulin (McMaster U.), J. B. Kycia (U. Waterloo) —
Experimental work on the low temperature spin dynamics of
dysprosium titanate suggests that by carefully tracking the
flow of heat into and out of the material over extremely long
periods of time (up to 1 week at a single temperatures), the
material enters a state that is more ordered then previously
observed.[1] This work raises the question: What is the true
ground state of spin ice? While there are theoretical mod-
els that agree qualitatively with our work, there is also com-
pelling evidence that a quantummechanical model of spin ice
might be responsible for the observed phenomena.[1] D. Pomaranski et al., Absence of Pauling’s residual en-
tropy in thermally equilibrated Dy2Ti2O7. Nature Physics
doi:10.1038/nphys2591 (2013)
S10218 Low temperaturemagnetic properties of a Ce3+ py-
rochlore. R. Sibille (PSI, Switzerland), E. Lhotel (Institut Néel, France),
Queen’s College Cambridge 66
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V. Pomjakushin (PSI, Switzerland), M. Frontzek (PSI, Switzerland), and M.
Kenzelmann (PSI, Switzerland) —Over the last two decades or so,
researchers have shown an increasingly strong interest in the
magnetic behavior of pyrochlore oxides having the general
formula A23+B2
4+O7 where A is a rare-earth ion that forms a
network of corner-sharing tetrahedra, the most famous ex-
ample of three-dimensional geometrically frustrated lattice.
However, up to know, there exists - to the best of our knowl-
edge - onlyworks on effective spin 1/2 pyrochlores but no re-
port on a ‘true’ spin 1/2 material to probe quantum effects.
For this reason,wehave focused on the synthesis of newCe3+
pyrochlore compounds, which appear as good candidates to
study frustration with S=1/2 spins. In this poster we will
present our results on Ce2Sn2O7. No sign of magnetic tran-
sition could be observed in magnetic measurements down to
the lowest investigated temperature, 70mK.
S10219 Evolution of the Magnetic Excitations in the Low
Temperature Phase of Yb2Ti2O7 as a Function of Applied
Magnetic Field. J.D. Thompson (University of Oxford), I. Cabrera
(University of Oxford), P.A. McClarty (University of Oxford and ISIS), D.
Prabhakaran (University of Oxford), T. Guidi (ISIS), R. Coldea (University
ofOxford) —Yb2Ti2O7 has recently attracted a great deal of at-
tention due to the presence of local Ising exchange in combi-
nation with a local planar crystal field anisotropy, making this
material a potential quantum mechanical version of the clas-
sical spin ice materials. The nature of the ground state of the
crystal field in thismaterial allowsquantumfluctuations away
from the two-in-two-out spin ice states that are not possible
in classical spin ice,which theoreticalworks indicatemay lead
to the existence of novel low temperature phases supporting
exotic excitations. Yb2Ti2O7 does display a phase transition
at Tc ∼ 200mK in zero field, but the nature of this transition
is still controversial despite it’s existence being known for 55
years. Compounding the difficulty in understanding the low
temperature phase in Yb2Ti2O7 is the fact that experiments
have shown that the conditions under which samples of this
material areprepared canhavea significant impacton the low
temperature physics of the material. In this talk I will discuss
the evolution of the magnetic excitations in an applied mag-
netic field along the [001] direction up to 9 T observed dur-
ing a recent inelastic neutron scattering experiment on a sin-
gle crystal of Yb2Ti2O7 at T< 160 mK. This crystal has been
characterised using low temperature heat capacity measure-
ments and found to show a single sharp phase transition at
Tc ∼ 200 mK, consistent with the best single crystal pre-
sented in the literature to date.
S10220 Low temperature Thermal conductivity and ther-
mal expansion of Spin Ice materials. W. Toews (University
of Waterloo), S. Zhang (University of Waterloo), L. Clark (McMaster Uni-
versity), K. Ross (McMaster University), H. Dabkowska (McMaster Uni-
versity), B. Gaulin (McMaster University), R. Hill (University of Water-
loo) — Thermal conductivity, thermal expansion and magne-
tostrictionmeasurementsweremadeon single crystals of the
frustratedmagneticmaterialsHo2Ti2O7 (HTO)andDy2Ti2O7
(DTO) along the [111] crystallographic direction. Measure-
ments were made quasi-statically at temperatures from 250
mK to 1.4 K and in fields up to 8 T in the [111] direction. Ex-
treme care was taken to ensure the samples were well equili-
brated at each temperature and field step bywaiting for up to
1.5 · 104 s at the lowest temperatures before measuring andby slowly cooling the sample over the course of 48 hours. The
results of the measurements will be discussed, highlighting
the differences and similarities between the two materials.
In particular, the temperature dependence of the thermal ex-
pansion in both DTO and HTO is contrasted with recent heat
capacitymeasurements inDTO [1], which are consistentwith
the onset of magnetic order. The insight gained is unique be-
cause equivalent heat capacity measurements in HTOwould
be very difficult due to the dominating nuclear contribution.
This isnot thecase in thermalexpansionmeasurementswhich
are not as sensitive to nuclear contributions.[1] D. Pomaranski et al., Nature Physics 9, 353-356 (2013).
S10221 High Magnetic Field Phase Diagram of Pyrochlore
Tb2Ti2O7 Along [111]. L. Yin, J.S. Xia, Y. Takano and N.S.Sullivan
(High B/T Facility, NHMFL, The University of Florida), Q.J. Li, X.F. Sun
(USTC, China) —By means of ac magnetic-susceptibility and dc
magnetization measurements in magnetic fields along [111],
we establish the magnetic phase diagram of Tb2Ti2O7 up to
35Tanddownto16mK. In thezerofield limit, anewphaseex-
isting below 50mK is identified as quantum kagome ice, sup-
porting that the ground stateof Tb2Ti2O7 is quantumspin ice.
A very slow spin relaxation behavior is observed in the low-
field limit. Furthermore, a new magnetic transition is discov-
ered in 14-16T, implying that the high-filed phase along [111]
may not be the simple ”3-in, 1-out / 3-out, 1-in” state.
S10222 Evidence for a long-range magnetic order in
Er2Sn2O7. I. Zivkovic (Institute of Physics, Zagreb, Croatia), J. Lago
(University of the Basque Country, UPV-EHU, Spain), J. Piatek (LQM,
ICMP, EPFL, Switzerland), S. T. Bramwell (LCN, UCL, UK), M. Shirai (UCL,
UK), T. Rojo (University of the Basque Country, UPV-EHU, Spain) — The
network of tetrahedrons formed by magnetic moments on
rare-earth ions in pyrochlore systems with a general formula
RE2(Ti,Sn)2O7 provides a rich arena for investigations of
variousmagnetic phenomena related tomagnetic frustration.
The presence of different types of magnetic interactions of
similar magnitudes (exchange interactions, dipole-dipole
interaction, single-ion anisotropy,...) ensures that the ground
state of any specific compound is often very susceptible to
small perturbations.
The change of the composition from Ti to Sn has been
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shown to drastically influence the ground state. For example
Tb2Sn2O7 is shown to exhibit an ordered spin ice state while
Tb2Ti2O7 is considered as a spin-liquid candidate since no or-
dering has been observed down to the lowest temperatures.
Likewise, Er-based compounds are reported to substantially
differ in their ground state: Er2Ti2O7, has been shown to
order below 1.17 K, stabilized by quantum fluctuations
(order-by-disorder), while for Er2Sn2O7 it has been indicated
that a long-range magnetic order is absent, and that a
short-range magnetic correlations of Palmer-Chalker type,
characteristic for dipolar interactions, are present.
Here we present results of ac susceptibility, µSR and neu-
tron diffraction experiments on two different batches of
Er2Sn2O7. In both cases, ac susceptibility shows a kink
around 130 mK with no detectable frequency dependence,
thus signaling long-range magnetic order, in contrast to
previously published results. Long-range order is corrob-
orated by neutron diffraction, which shows that magnetic
Bragg intensity consistent with a Palmer-Chalker structure
develops below Tc on top of the magnetic diffuse scattering.
As in the case of its Ti analogue and many other frustrated
pyrochlores, no characteristic wiggles can be detected in the
ZF µSR signal of the ordered phase of Er2Sn2O7. The transi-
tion at Tc is, however, marked by a peak in the temperature
dependence of themuon relaxation rate.
03 Perovskites, Spinels, and Related Systems
S10301 Thermal spin liquid in Sr2CuWO6 with frustrated
Cu(II) 2D square lattice. O. Burrows, M. de Vries (Edinburgh)
—We studied the magnetic properties of Sr2CuWO6 as part
of a programme of study of W(VI) and W(V) double per-
ovskites. Sr2CuWO6 has rocksalt-ordered Cu(II)(S = 1
2
)and W(VI)
(5d0
)cations on the perovskite B sites. Cu(II)(
3d9)is Jahn-Teller active, giving rise to a cubic to tetrag-
onal structural transition below 920 °C [1]. This leads to a
quasi-2D square lattice with competing near neighbour and
further neighbour antiferromagnetic interactions between
the Cu(II) spins. Heat capacity and SQUID measurements
on Sr2CuWO6 showed a broad maximum below 100K, but
no sharp anomalies that would indicate any magnetic tran-
sitions, and so suggested the possibility of a (quantum) spin
liquid at low temperatures. Hence, inelastic neutron scat-
tering and �SR at ISIS were used to examine the microscopic
magnetic properties over a range of temperatures. Below
20K, an additional reflection is observed in neutron scatter-
ing at q = 0.7Å−1 that can be indexed as(12, 12, 12
)(in the
face-centredsetting), as also seen in theanalogouscompound
Ba2CuWO6 by Todate et al. [2] Our neutron spectroscopy
data obtained atMARI, ISIS, reveals that themagnetic disper-
sion curves corresponding to this antiferromagnetic ordering
persist at temperaturesup to100K, andonly a gradual reduc-
tion of the dynamic correlations is observed above the order-
ing temperature. These dynamic correlations are sufficiently
strong toserveasevidenceofa thermal spin liquidandexplain
the absence of visible anomalies in the bulk SQUID and heat-
capacity data at the freezing transition. �SR further confirms
an ordering transition takes place at 24K. The muon relax-
ation at low temperatures further indicates that even in the
frozenstate themagneticordering is short tomediumranged.
Further neutron spectroscopy studies will be needed to fully
characterise this thermal spin liquid stateat temperaturesbe-
tween 25 K and∼ 100K.[1] Vasala, S., Cheng, J.-G., Yamauchi, H., Goodenough, J. B., andKarp-
pinen, M. July 2012 Chem. Mater. 24(14), 2764–2774; [2] Todate, Y.,
Higemoto, W., Nishiyama, K., and Hirota, K. November 2007 J. Phys.
Chem. Solids 68(11), 2107–2110.
S10302 Long range magnetic order in spin-orbit-coupled
double perovskites Ba2YRuO6 andBa2CaOsO6 probedwith
neutron scattering and muon spin relaxation: Comparison
with theory and disordered Ba2YReO6 and Ba2YMoO6. J.
P. Carlo (Villanova University), J. P. Clancy (University of Toronto), C. M.
Thompson (McMaster University), Y. J. Uemura (Columbia University), J.
E. Greedan (McMaster University), and B. D. Gaulin (McMaster Univer-
sity) —Frustrationmanifests in thedouble perovskite (DP) lat-
tice A2BB’O6, in which the antiferromagnetically correlated
B’ ions comprise a network of edge-sharing tetrahedra. Per-
ovskitesmaybe synthesizedwithmost elements from the pe-
riodic table, enabling systematic studies of frustration as a
function of structural distortion, lattice parameter, ionic size,
moment size, and spin-orbit coupling (SOC), for example. The
latter, in particular, has been explored by Chen et al. [1,2],
who found that sizable SOC in d1 and d2 systems yields rich
phase diagramswith diverse ground states.
Here we report inelastic neutron scattering and muon
spin relaxation (µSR) experiments on the undistorted DPs
Ba2YRuO6 [3] and Ba2CaOsO6 [4], finding evidence for
long-range order in both. Ba2YRuO6 (4d3 Ru5+), orders
antiferromagnetically with TN = 47K (although ΘCW =
−522K, so f = 11) and exhibits a 4 meV gap below 36K; such
a gap is unexpected for an d3 orbital singlet and suggestive of
exotic physics induced by SOC. Ba2CaOsO6 has been found
via susceptibility, heat capacity and µSR measurements to
exhibit long-range order below 50K (ΘCW = −150K, f =
3), consistent with theoretical expectations, although the
precise long-range nature of the ground state has not yet
been determined.
Both compounds are isostructural to Ba2YReO6 [5] and
Ba2YMoO6 [6]. The latter (4d1 Mo5+) system possesses a
singlet low-temperature state with a 28 meV gap. While
Ba2YReO6 is isoelectronic to Ba2CaOsO6 (Os6+ vs. Re5+,
both 5d2 and neighbors in the periodic table with similar
λSOC ∝ Z4), the rhenate exhibits a disordered spin-frozen
Queen’s College Cambridge 68
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ground state below 50K, in contrast to theoretical predic-
tions, despite a lack of evidence for structural disorder.
This research was supported in part by NSERC (Canada), a
Villanova Faculty Development Grant, and the NSF (USA) via
the DMR and PIRE programs. We wish to thank the TRI-
UMFCMMSstaff for invaluable technical assistancewithµSR
experiments. Research at Oak Ridge National Laboratory’s
Spallation Neutron Source was sponsored by the Scientific
User Facilities Division, Office of Basic Energy Services, US
Department of Energy.[1]G.Chen, R. Pareira and L. Balents, Phys. Rev. B82, 174440 (2010);
[2] G. Chen and L. Balents, Phys. Rev. B 84, 094420 (2011); [3] J. P.
Carlo et al. Phys. Rev. B 88, 024418 (2013); [4] C.M. Thompson et al.
arXiV:1312.6553; [5] T. Aharen et al. Phys. Rev. B81, 064436 (2010);
[6] J. P. Carlo et al. Phys. Rev. B 84, 100404(R) (2011).
S10303 Investigation on the low temperature distorted
phase of MgCr2O4. S. Gao (LNS, PSI); O. Zaharko (LNS, PSI); M. Ru-
miny (LNS, PSI); T. Fennell (LNS, PSI); V. Tsurkan (Dept. Phys., Uni. Augs-
burg); Ch. Rüegg (LNS,PSI) —ACr2O4 spinel compoundswithCr3+
ions forming a highly frustrated pyrochlore lattice have been
extensively studied as a possible realization of the pyrochlore
Heisenberg antiferromagnet. It has been found that geomet-
rical frustration leads to a fluctuating cooperative paramag-
netic state characterized by spatially localized spin correla-
tions and at low temperatures this frustration is released by
a spin-driven Jahn-Teller distortion [1].
Despite of these extensive studies some basic questions still
remain to be addressed. What is the low temperature (LT)
nuclear structure? How does this structural distortion influ-
ence theCr3+ spin configuration? And, perhaps themost cru-
cial one for understanding of their spin dynamics, what are
the exchange couplings in the distorted phase? Here, using x-
ray synchrotron and neutron diffraction togetherwith inelas-
tic neutron scattering, we tried to answer these questions for
MgCr2O4 compound. Powder synchrotron diffraction shows
that the LT structure belongs to the Fddd space group, which
contradicts previously determined I41/amd space group, but
is consistent with magnetic resonance measurements on the
related ZnCr2O4 compound [2]. Based on the Fddd space
group, magnetic structure is solved from single crystal neu-
tron diffraction data with both symmetry analysis and simu-
lated annealingmethods. Finally, spinwave dispersion for co-
aligned MgCr2O4 crystals is measured on both hot and cold
neutron triple-axis spectrometers. Fitting thedispersionwith
linear spin wave theory reveals a significant contribution of
the further-neighboring interaction, lending a direct support
for first-principle calculations.[1] S.-H. Lee et al., PRL 84 3718 (2000); [2] V. N. Glazkov et al., PRB 79
024431 (2009).
S10304MagneticOrders inHeisenberg pyrochlore antifer-
romagnets. T. Higo, K. Iritani, M. Halim, and S. Nakatsuji (University
of Tokyo) — Geometrically frustrated magnets have attracted
great interest due to the possible emergence of novel spin-
disordered states by suppressing conventional magnetic or-
der. In three dimensions, the frustrated magnetism on the
pyrochlore lattice which is based on an Ising spin with a fer-
romagnetic coupling has been intensively studied because of
their variety of exotic properties such as the spin ice and
spin liquid [1-3]. On the other hand, an antiferromagnetic
pyrochlore magnet with a Heisenberg spin, which is another
type of the frustrated pyrochlore system, has been predicted
to have an equally exotic magnetic ground state. Indeed, a
various experimental studies have revealed strong frustra-
tion effects in AB2O4 [4] and LiXCr4O8 [5]. However, mag-
netic long range order owing to structural distortion occurs
in thesematerials. Therefore, new systemwhich does not ex-
hibit a structural phase transition is desired. In this presenta-
tion, wewill show result of our investigation on various types
of magnetic orders found in the spinel chalcogenide which
would be a good candidate ofHeisenberg antiferromagnet on
a pyrochlore lattice.[1] M. Saito, et al, Phys. Rev. B 72, 144422 (2005); [2] S. Nakatsuji,
et al., Phys. Rev. Lett. 96, 087204 (2006); [3] K. Kimura, et al., Nat.
Commun. 4, 1934 (2013); [4] S.-H. Lee, et al., J. Phys. Soc. Jpn. 79,
011004 (2010); [5] Y. Okamoto, et al., Phys. Rev. Lett. 110, 097203
(2013).
04Quantum Spin Liquids (theory)
S10401QuantumKagome Ice. J. Carrasquilla (Perimeter), Z. Hao
(U. Waterloo), R. Melko (U. Waterloo and Perimeter). —We study the
ground-state phase diagram of a frustrated spin-1/2 XY Z
model on the kagome lattice in presence of a magnetic field
along the z-axis. Using quantum Monte Carlo simulations
we find that a strong first-order phase transition separates
a magnetically ordered state where spins align along the x-
axis from an exotic paramagnetwith nonzero pair-spin corre-
lationsatfinitemagneticfield,whichwe identify as aquantum
kagome ice. We shed light on the nature of the phases of this
model through analytical arguments and a careful analysis of
several correlation functions. We speculate on the possible
consequences of these results for quantum spin icematerials
constrained by an external magnetic field.
S10402EffectivefluxHamiltonians forKagomesystems. S.
Ghosh, C. L. Henley (LASSP, Cornell University) — Several kinds of in-
teracting problems are reduced, in standard treatments, to
non-interacting electrons or bosonshoppingona lattice, such
that the phases in the hopping amplitudes are parametrized
by “classical” (non fluctuating) degrees of freedom. The
summed energy of the quantum particles is thus, implicitly,
a function of the classical variables (more precisely of the
gauge-invariant fluxes they specify). Whenever there are nu-
merous nearly degenerate classical configurations, one faces
Queen’s College Cambridge 69
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a typical frustrated problem. We propose a general approach
whereby one numerically fits the total energy to an effective
Hamiltonian Heff =∑
ℓ Jℓhℓ, organized as a sum over dif-
ferent loops ℓ on the lattice, where hℓ is a function of classi-
cal variables on loop ℓ proportional to cos(φℓ), where φℓ is a
U(1) flux. Using Heff we then find the optimum among the
competing flux patterns. Such an approach was applied in
the past to SBMFT in the large κ limit and Heisenberg spins
in the large-S limit [1]. In this contribution, we demonstrate
this approach in two models on the Kagome lattice: Dou-
ble Exchange (DE) and Schwinger Boson Mean Field theory
(SBMFT). In either case, there is an important parameter, on
which the fitted couplings Jℓ and the optimum states depend
implicitly – respectively electron filling n, and the κ param-
eter (analogous to spin length). Kagome spin ice DE model-
Here, the classical degrees of freedom are Ising spins, each
alignedwith the local triangle-triangle axis, with the “ice con-
straint” that either one or two point inwards in every triangle.
To these local moments are coupled electrons with nearest-
neighbor hopping; the electron spins, in the usual DE limit,
are constrained to follow the direction of the local moments,
so the hopping phase factors are the Berry phases induced
whenever an electron hops to a site with a different spin di-
rection. Kagome-ice DE models have been of recent interest
[2]. We find that state selection within the Kagome ice man-
ifold occurs at the level of loops of length six: the hexagons
and bowtie loops. However, the degeneracy is lifted only par-
tially, with no unique lowest energy state at any filling. SBMFT
saddle points-SBMFT is a large-N route to approximate sin-
glet ground states of antiferromagnets, here applied in the
small κ limit. We decouple in the “pairing” channel, such that
link amplitudes Qij on each nearest-neighbor bond are de-
fined self-consistently. There are known to be exponentially
many mean-field saddle-point solutions, many of which spon-
taneously break spatial and/or time reversal symmetries. Our
effective Hamiltonian terms are products of the link ampli-
tudes around the loops, hℓ = |Qij | ∗ |Qjk|... ∗ |Qai| cos(φℓ).
We fit the Jℓ’s precisely enough to resolve the small energy
differences between saddle points: it appears there is an in-
terval of κ values in which chiral solutions are stabilized.[1] Hizi, Sharma, and Henley PRL 95, 167203 (2005); Hizi and Hen-
ley, PRB 73, 054403 (2006); [2] Ishizuka and Motome, PRB 87,
081105(R), (2013); Chern et al, arXiv:1212.3617v1.
S10403 Doping a topological quantum spin liquid: slow
holes in the Kitaev honeycombmodel. G. Halász, J. Chalker, R.
Moessner —Wepresent a controlledmicroscopic study of mo-
bile holes in the spatially anisotropic (abelian) gapped phase
of the Kitaev honeycomb model. We address the proper-
ties of (i) a single hole – its internal degrees of freedom as
well as its hopping properties; (ii) a pair of holes – their (rel-
ative) particle statistics and interactions; (iii) the collective
state for a finite density of holes. We find that each hole
in the doped model has an eight-dimensional internal space,
characterized by three internal quantum numbers: the first
two ‘fractional’ quantumnumbers describe the binding to the
hole of the fractional excitations (fluxes and fermions) of the
undoped model, while the third ‘spin’ quantum number de-
termines the local magnetization around the hole. The ‘frac-
tional’ quantum numbers also encode fundamentally distinct
particle properties, topologically robust against small local
perturbations: some holes are free to hop in two dimensions,
while others are confined to hop in one dimension only; dis-
tincthole typeshavedifferentparticle statistics, and inpartic-
ular, someof themexhibit non-trivial (anyonic) relative statis-
tics. These particle properties in turn determine the physi-
cal properties of the multi-hole ground state at finite doping,
andwe identify two distinct ground stateswith different hole
types that are stable for different model parameters. The re-
spective hopping dimensionalities manifest themselves in an
electrical conductivity approximately isotropic in one ground
state and extremely anisotropic in the other one. We also
compareourmicroscopic studywith relatedmean-field treat-
ments, and discuss the main discrepancies between the two
approaches,which inparticular involve thepossibilityofbind-
ing fractional excitations as well as the particle statistics of
the holes. On a technical level, we describe the hopping of
mobile holes via a quasi-stationary approach, where effec-
tive hoppingmatrix elements are calculated between ground
states with stationary holes at different positions. This ap-
proach relies on the fact that themodel remains exactly solv-
able in the presence of stationary holes, and that the motion
of sufficiently slow holes does not generate bulk excitations
in a gapped phase. When the bare hopping amplitude ismuch
smaller than the energy gap, many of our results – in particu-
lar those on the hopping properties and the particle statistics
– are exact.
S10404 Confinemed and deconfined phases of quantum
square ice. L-P. Henry (ENS Lyon), T. Roscilde (ENS Lyon) —
“Quantum spin-ice” materials have attracted a lot of inter-
est as examples of quantum spin liquids. We consider the
particular case of the antiferromagnetic Ising model on the
checkerboard lattice with quantum fluctuations induced by
the means of a transverse field. We show that it exhibits a
thermally induced, deconfined quantum Coulomb phase of a
two-dimensional lattice gauge theory, supporting fractional-
ized spinons. It emerges at finite, yet exceedingly low tem-
peratures from the melting of two distinct confined phases :
a plaquette valence-bond solid for low magnetic field, and a
canted Néel state for larger field. These latter phases appear
via thehighlynon-lineareffectofquantumfluctuationswithin
the degenerate manifold of ice-rule states, and they can be
identified as the two competing ground states of a discrete
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lattice gauge theory (quantum linkmodel) emerging as the ef-
fective Hamiltonian of the system within degenerate pertur-
bation theory.
S10405 Quantum spin liquid with a Majorana Fermi sur-
face on the three-dimensional hyperoctagon lattice. M.
Hermanns, S. Trebst —Motivated by the recent synthesis of β-
Li2IrO3 – a spin-orbit entangled j = 1/2Mott insulator with
a three-dimensional lattice structure of the Ir4+ ions – we
consider generalizations of the Kitaevmodel believed to cap-
ture some of the microscopic interactions between the Irid-
ium moments on various trivalent lattice structures in three
spatial dimensions. Of particular interest is the so-called hy-
peroctagon lattice– thepremedial latticeof thehyperkagome
lattice, for which the ground state is a gapless quantum spin
liquid where the gapless Majorana modes form an extended
two-dimensional Majorana Fermi surface. We demonstrate
that this Majorana Fermi surface is inherently protected by
lattice symmetries and discuss possible instabilities. We thus
provide the first example of an analytically tractable micro-
scopic model of interacting SU(2) spin-1/2 degrees of free-
dom in three spatial dimensions thatharborsa spin liquidwith
a two-dimensional spinon Fermi surface.
S10406 Dynamics of a two-dimensional quantum spin liq-
uid: signatures of emergent Majorana fermions and fluxes.
J. Knolle (MPI-PKS), D. Kovrizhin (U Cambridge), J. Chalker (U Oxford),
R. Moessner (MPI-PKS) — Topological states of matter present a
wide variety of striking new phenomena. Prominent among
these is the fractionalisation of electrons into unusual parti-
cles: Majorana fermions, Laughlin quasiparticles or magnetic
monopoles. Their detection, however, is fundamentally com-
plicated by the lack of any local order, such as, for example,
themagnetisation in a ferromagnet. While there arenowsev-
eral instances of candidate topological spin liquids, their iden-
tification remains challenging. Here, we provide a complete
and exact theoretical study of the dynamical structure fac-
tor of a two-dimensional quantum spin liquid in gapless and
gapped (abelian andnon-abelian) phases. Weshowthat there
aredirect signatures–qualitative andquantitative–of theMa-
jorana fermions and gauge fluxes emerging in Kitaev’s honey-
comb model. These include counterintuitive manifestations
of quantum number fractionalisation, such as a neutron scat-
tering response with a gap even in the presence of gapless
excitations, and a sharp component despite the fractionalisa-
tion of electron spin. Our analysis identifies new varieties of
the venerable X-ray edge problem and explores connections
to the physics of quantum quenches.
S10407 Study of vison-spinon bound states on the kagome
lattice. M. Lawler (Binghamton U.), J. Shao (Binghamton U.), S. Ghosh
(Cornell U.), Gil-Young Cho (UIUC) — We study electric (bosonic
spinon), magnetic (vison) and electric-magnetic bound state
(fermionic spinon) excitations in large-N theories of Z2 spin
liquids. We do so by numerically constructing vison saddle
point solutions in large-N Schwinger boson models of quan-
tumspin liquidphaseson thekagome latticeand their bosonic
spinon excitation spectrum. Remarkably, the lowest energy
bosonic spinon excitations in the presence of these visons are
bound states fromwhich fermionic spinon excitationsmay be
constructed. These results provide strong evidence that a
quantitative description of Z2 spin liquids beyond mean field
theoryandconsistentwith their topological order is possible.
S10408 Magnon condensation with finite degeneracy on
the triangular lattice. G. Marmorini (RIKEN), T. Momoi (RIKEN)
—Wepresent a thorough study of the J1-J2-J3 triangular lat-
tice antiferromagnet close to the saturation field, where the
magnetic structure is determined in termsof adiluteBosegas
of magnons. We focus on the case of ferromagnetic J1, that
is particularly rich because frustration effects can allow for
magnons of different wave-vectors to condense simultane-
ously. Our calculation includes an interlayer coupling J0, that
can be taken as small as 10−4 (in units of |J1|), in which casethe system is effectively two-dimensional. Besides the well-
known spiral and fan phases, we find a new double-q phase
(superposition of twomodes), dubbedQ0 −Q1 phase (or sim-
ply “01”), that features striped chiral order and a new kind
of multiferroicity. Furthermore, in some regions of the pa-
rameter space, we show that a dilute gas of magnon can not
be stable and phase separation (corresponding to a magne-
tization jump) is expected. In the J1-J2 model two-magnon
bound states also appear, and an exact-diagonalization anal-
ysis suggests the stability of the corresponding spin-nematic
state. We additionally discuss the peculiar runaway behav-
ior of certain phase boundaries in the crossover from three to
two dimensions, which can lead to the sudden disappearance
of some phases.
S10409 Transitions between Z2 topologically ordered
phases. S. Morampudi (Max-Planck-Institute for the Physics of
Complex Systems,), C. v. Keyserlingk (Rudolf Peierls Centre for Theoretical
Physics), F. Pollmann (Max-Planck-Institute for the Physics of Complex
Systems) — Distinguishing different topologically ordered
phases and characterizing phase transitions between them
is a difficult task due to the absence of local order param-
eters. We use a combination of analytical and numerical
approaches to distinguish two such phases and characterize a
phase transition between them. The “toric code” and “double
semion” models are simple lattice models exhibiting Z2
topological order. Although both models express the same
topological ground state degeneracies and entanglement
entropies, they are distinct phases of matter because their
emergent quasi-particles obey different statistics. We use
exact diagonalization to study this model and find indications
Queen’s College Cambridge 71
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of a first-order transition. We show that the quasi-particle
statistics provides a robust indicator of the distinct topolog-
ical orders throughout the whole phase diagram. Using the
same technique, we identify both phases in a quantum dimer
model on the kagome lattice.
S10410 Persisting topological order via geometric frustra-
tion. K.P. Schmidt (TU Dortmund, Germany) — Correlated quan-
tumsystems in twodimensionsdisplayavarietyof fascinating
properties. One of the most intriguing issues is the concept
of topological order which goes beyond the paradigm of clas-
sifying the ground states of nature by spontaneous symme-
try breaking. Topologically-ordered quantum systems pos-
sesselementaryexcitationswith fractionalquantumnumbers
and unconventional particle statistics. One of the standard
models displaying topological order is the so-called toric code
which is an exactly solvablemodel of interacting spins. In this
talkwe study the stability of the topological phase in the toric
code model in the presence of a uniform magnetic field. Fur-
thermore, we introduce a toric code on the dice lattice which
is also exactly solvable and topologically ordered at zero tem-
perature. In the presence of a magnetic field, the flux dynam-
ics is mapped to the highly frustrated transverse field Ising
model on the kagome lattice. This correspondence suggests
an intriguing disorder by disorder phenomenon in a topolog-
ically ordered system implying that the topological order is
extremely robust due to the geometric frustration. A gen-
eral connectionbetween fully frustrated transversefield Ising
models and topologically ordered systems is demonstrated.
S10411 Investigation of the phase diagramofS = 1/2 spins
on the triangular lattice with ring exchange couplings, by
means of exact diagonalization. P. Sindzingre (Sorbonne Univer-
sites) — I report an investigationof theground-statephasedia-
gramof twospin−1/2modelson the trianglar latticeobtained
bymeans of exact diagonalization on finite clusters:
1) the J1 − J2 − J3 model with pairwise couplings up to 3rd
neigbors,
2 the J1 − J2 − J3 − K model with additional ring exchange
coupling for small 2nd and 3rd neigbor coupling.
S10412 Exploring the spin-orbital ground state of
Ba3CuSb2O9. A. Smerald and F. Mila (EPFL, Lausanne, Switzer-
land) — Recent experiments have shown that the material
Ba3CuSb2O9 fails to order magnetically at temperatures as
low as 20mK [1,2,3]. Furthermore, no Jahn-Teller distortion
is observed [1], leading to speculation that the ground state
may be a spin-orbital liquid, as has been shown to be the
case for the SU(4) symmetric Khugel-Khomskii model on thehoneycomb lattice [4]. Starting fromaHubbardmodel for de-
generate Cu eg-orbitals, we derive a spin-orbital Hamiltonian
in the limit of strongon-siteCoulombrepulsion. Wesolve this
Hamiltonian for small clusters by decoupling spin and orbital
degrees of freedom [5], and also by exact diagonalisation.
We find that the inclusion of out-of-plane Cu′-sites, which
decorate the honeycomb lattice, are crucial to understand
the low-energy physics. The resulting ground state involves
nearest-neighbour spin singlets coexisting with orbital order,
and can be understood in terms of dimer coverings of an
emergent square lattice. While the experimental picture
is complicated by structural disorder, we find qualitative
agreement between our theory andNMR experiments [2].[1] S. Nakatsuji et al, Science 336, 559 (2012); [2] J. A. Quilliam
et al, Phys. Rev. Lett. 109, 117203 (2012); [3] H. D. Zhou et al,
Phys. Rev. Lett. 106, 147204 (2011); [4] P. Corboz, M. Lajko,
A. Lauchli, K. Penc and F. Mila, Phys. Rev. X 2, 041013 (2012); [5]
F. Vernay, K. Penc, P. Fazekas and F. Mila, Phys. Rev. B 70, 014428
(2004).
S10413 Topological defects in a spin-nematic phase on the
triangular lattice Hiroaki. T. Ueda, Nic Shannon — Topologi-
cal defects play an important role in the theory of nematic
phases in liquid crystals. However, relatively little is known
about their role in quantum spin nematics[1,2,3]. Here we
consider the topological defects which could arise in such a
state. Themodelweconsider is thespin-1bilinearbiquadratic
model on the triangular lattice, tuned to an SU(3) point[4,5,6].
We classify defects by homotopy theory, and explore how
they evolve into the neighboring anti-ferroquadrupolar spin-
nematicphase. The stable configurationand theenergyof the
defect are discussed.[1]B. A. Ivanov, R. S. Khymyn, and A. K. Kolezhuk, Phys. Rev. Lett.
100, 047203 (2008); [2]T. Grover and T. Senthil, Phys. Rev. Lett. 107,
077203 (2011); [3]C. Xu and A. W. W. Ludwig, Phys. Rev. Lett, 108,
047202 (2012); [4]A. Lauchil, F. Mila and K. Penc, Phys. Rev. Lett. 97,
087205 (2006); [5]H. Tsunetsugu and M. Arikawa, J. Phys. Soc. Jpn.
75, 083701 (2006); [6]A. Smerald and N. Shannon, Phys. Rev. B 88,
184430 (2013).
S10414 Quantum phase diagram of triangular-lattice anti-
ferromagnets with XXZ anisotropy and magnetic field. D.
Yamamoto (Waseda Institute for Advanced Study), G.Marmorini (RIKEN),
I. Danshita (Yukawa Institute for Theoretical Physics) —Wediscuss the
quantumeffectson theground-statemagneticphasediagram
of the spin-1/2 antiferromagnetic XXZ model on a triangu-
lar lattice using a large-size cluster mean-field method com-
bined with a scaling scheme. We find that the region of the
high-field coplanar phase is significantly extended toward the
easy-plane side in the quantum phase diagram compared to
the classical counterpart. This makes possible a field-induced
transition from the umbrella to the high-field coplanar state,
which interprets themagnetization process of the triangular-
lattice antiferromagnet Ba3CoSb2O9 for applied magnetic
field parallel to the c axis. We also find that another non-
classical coplanar phase appears for strong fields and large
easy-planeanisotropydue toaparticular liftingmechanismof
classical ground-state degeneracy by quantum fluctuations.
Queen’s College Cambridge 72
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Furthermore,wegiveafirstmicroscopicdemonstration thata
weak inter-layer coupling is needed to explain themagnetiza-
tion anomaly observed in Ba3CoSb2O9 for strong transverse
magnetic field.
S10415 Bond randomness in Kitaev’s honeycomb spin-
liquid model. F. Zschocke (TU Dresden), M. Vojta (TU Dres-
den) — The Kitaev model on the honeycomb lattice realizes
a spin liquid whose emergent excitations are gapless Majo-
rana fermions and static Z2 gauge fluxes. Upon introduction
of bond randomness the model remains exactly solvable, via
an equivalent tight-binding model of canonical fermions. We
use this to study a number of observables as function of dis-
order strength, payingparticular attention toproperly select-
ing physical states within the canonical-fermion description.
Specifically, we calculate the distribution of local suscepti-
bilities, extract the NMR lineshape, and make contact with
known results on the problem of disordered Dirac fermions.
05General (theory)
S10501 Entropy change and the magnetocaloric effect in
antiferromagnetic clusters. N. A. de Oliveira — The mag-
netocaloric effect[1,2], which is the heating or cooling of a
magnetic material upon magnetic field variation, is the basis
of magnetic refrigeration. It can be characterized by the en-
tropychange inan isothermalprocessandbythe temperature
change in an adiabatic process. In the normal effect (or direct
effect) the sample heats up while in the inverse effect it cools
down for∆B > 0. Whether or not themagnetocaloric effect
proofs its technological application it is important to under-
stand it from the fundamental physics point of view. In this
work, we discuss the entropy change and themagnetocaloric
effect in a cluster of magnetic ions. For this purpose, we con-
sider a model Hamiltonian of interacting magnetic moments.
We perform the calculations considering different situations.
In the first one, we consider two non equivalent sites with
an antiferromagnetic arrangement. In this case, the entropy
change and the magnetocaloric effect, which depend on the
direction of the appliedmagnetic field, can exhibit the normal
or the inverse behavior. In the second scenario, we consider
three non equivalent sites where the frustration is naturally
introduced. In this case, themagnetocaloric effect,which also
depends on the direction of the application of the magnetic
field, exhibit a similar behavior as theprevious case. However,
its values are somewhat smaller.[1] K. A. Gschneidner Jr et al, Rep. Prog. Phys. 68 (2005) 1479; [2] N.
A. deOliveira and P. J. von Ranke, Phys. Rep. 489 (2010) 89.
S10502 Investigations ofMagnetic Fractal Structures ofDi-
luted Antiferromagnetic Materials. A.N.Bazhan (P.L.Kapitza
Institute for Physical Problems) — Investigations of diluted anti-
ferromagnetic materials near percolation limit such as (Mn
or Ni)1−x(Zn or Mg)xF2 crystals of tetragonal structure with
twomagnetic ions in magnetic unit cells and competing diag-
onal and in basic plane, but rather small, antiferromagnetic
exchange interactions of magnetic ions were of interest with
aim of indications of magnetic frustration effects in magnetic
fractal structuresofdilutedantiferromagneticmaterials, con-
cerned with possible influence of magnetic frustrations on
disappearance of antiferromagnetic long range orderings in
magnetic fractal structures and frustration effects in mag-
netic nano-cluster systems in destroyed fractal structures,
connected with randomly distributed triangles and chains
of magnetic ions with antiferromagnetic exchange interac-
tions. Binomial distributions of magnetic ions, with probabil-
ity of magnetic ions, having n less then eight nearest neigh-
bors, determined by, p(n) = xn(1-x)8−n8!/(8-n)!n! expres-
sion, were confirmed by concentration dependencies of con-
stant, θx=x·θ0 in temperature dependence of paramagnetic
susceptibility, [1]. Appearance of temperature dependencies
of magnetic susceptibilities in weakmagnetic fields, less then
∼1-2kOe, andweakly temperature andmagnetic field depen-dencies of magnetic susceptibilities in magnetic fields, higher
then ∼40-60kOe, at the concentrations higher then x∼0.6,less then percolation limit xc∼0.75-0.77 and disappearance
of antiferromagnetic orderings indicate that magnetic fractal
structures, with effective, less then three, dimension in such
tetragonal structures are formed, where two dimensional tri-
angles and one dimensional chains of magnetic ions make up
elements of magnetic fractal structures, that continues up to
x∼0.8 in destroyed long rang antiferromagnetic orderings.
Difference of magnetic field dependencies of samples mag-
netic moments with x∼0.8 from paramagnetic magnetic field
dependencies, M∗(H,T)=M0BJ (gµBJH/kT), [1], indicates con-
tributions of spin-spin correlation functions to magnetic sus-
ceptibilities ofmagnetic ions inmagnetic fractal systems, that
inaddition tocontributionsandparticularities, determinedby
short range antiferromagnetic orderings with finite spin-spin
correlation lengths and their distributions, does not exclude,
determined by frustrations in distributions of exchange inter-
actions, contributions and particularities of spin-spin correla-
tions, leading tomagnetic states, suchas spinglass stateswith
own magnetic susceptibility at low magnetic fields and zero
temperatures, if, higher then in discussed materials, antifer-
romagnetic in plane exchange interactions, that determines
degeneracy of magnetic ground states, and corresponding
conditions of such magnetic states appearance take place in
parts of fractal and destroyed fractal structures. Despite the
difficulties, arising due to individual paramagnetic ions of the
systems, particularities of the spin-spin correlations in mag-
netic fractal systems can be indicated in studies of magnetic
field dependencies of samples magnetic moments and sus-
ceptibilities, using vector v.s.magnetometers, where contri-
Queen’s College Cambridge 73
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butions of linear susceptibilities of individual paramagnetic
ions can be eliminated in experiments.[1]. A.N.Bazhan, V.N.Bevz, S.V.Petrov. Journal of Experimental and
Theoretical Physics, JTEP, 95 985 (1989).
S10503 Topological-sector fluctuations at the Berezinskii-
Kosterlitz-Thouless transition. M. Faulkner (UCL and ENS
de Lyon), S. Bramwell (UCL), P. Holdsworth (ENS de Lyon) — The
Berezinskii-Kosterlitz-Thouless phase transition [1,2] is topo-
logical in its nature and has high relevance to a diverse range
of physical systems [3-9]. We perform the classification of
its topological signaturesusing thetwo-dimensionalCoulomb
gas on a torus in the context of the local-field algorithm
for Coulombic systems developed by Maggs and co-workers
[10,11] and later by Raghu et al. [12]. This algorithm cir-
cumvents the technical difficulties of computing long-ranged
forces by representing the system in termsof a lattice field. In
analogy with the Kasteleyn transition in spin ice [13], fluctu-
ations between topological sectors in this lattice-field repre-
sentation are strictly zero in the low-temperature phase but
finite above the transition temperature.[1]Berezinskii V L1971 Sov. Phys.–JETP 32493; [2]Kosterlitz JMand
Thouless D J 1973 J. Phys. C: Solid State Phys. 6 1181; [3] Trombet-
toni A, Smerzi A and Sodano P 2005New J. Phys. 7 57; [4] Hadzibabic
Z, Krüger P, Cheneau M, Battelier B and Dalibard J 2006 Nature 441
1118–1121; [5] Beasley M R, Mooij J E and Orlando T P 1979 Phys.
Rev. Lett. 42 1165; [6] Resnick D J, Garland J C, Boyd J T, Shoemaker
S and Newrock R S 1981 Phys. Rev. Lett. 47 1542; [7] Bramwell S T
and Holdsworth P CW 1993 J. Phys.: Condens. Matter 5 4; [8] Taroni
A, Bramwell S T andHoldsworth PCW2008 J. Phys.: Condens. Matter
20 L53–L59; [9] Salzberg A M and Prager S 1963 J. Chem. Phys. 38
2857; [10] Levrel L, Alet F, Rottler J andMaggs AC1999 Pramana 53
6; [11] Maggs A C and Rossetto V 2002 Phys. Rev. Lett. 88 19; [12]
Raghu S, Podolsky D, Vishwanath A and Huse D A 2008 Phys. Rev. B
78 184520; [13] Jaubert et al. 2013 Phys. Rev. X 3 011014.
S10504 Novel magnetic model arising from the ordering of
side chains of a tetraphilic liquid crystal. C.Bentham (Uni-
versity of Sheffield), S.P.George (University of Sheffield, Sheffield; CERN),
J.Poulton, G.A.Gehring (University of Sheffield) — A novel form of
ordering has been observed and is modelled using an Ising
model.
A tetraphilic liquid crystal molecule has two side chains one
containing fluorine (F) and the other silicon (Si) and these
chains repel each other. The rigid liquid crystal molecules are
arranged in hexagons stacked in planes. Each hexagon con-
tains six chains each attached to amid points of each of its six
sides. In a magnetic representation the chains can be repre-
sented as Ising spins arranged on a Kagome lattice pointing
into, and out of, adjoining hexagons. The energetics are char-
acterised by one parameter, U, given in terms of the interac-
tion energies between different types of chain U = uSi−Si +
uF−F − 2uSi−F within one hexagon [1].
This is a frustrated system as it is not possible to have all
hexagons occupied by either all Si or all F chains. The ground
state isonewith threesublatticesofhexagons, onecontaining
all Si chains, one with all F chains and the third with three of
each. Theorderparameterof the ithhexagon isnSi(i)−nF(i).
The model has some features in common with an antiferro-
magnetically coupled S = 3 Ising model on a triangular sub-
lattice where the spin component on the ith site are given by
2m(i) = nSi(i)− nF(i).
Monte Carlo simulations have been done for the disordering
chains. As the two chains on one liquid crystal molecule in-
terchange this will change the order parameters on the two
neighbouring hexagons by one unit each so that the value of∑i m(i) remains equal to zero since the total number of Si
and F chains are fixed to be equal by the chemistry. This is
equivalent to using Kawasaki dynamics for the Stot = 0 state
of an antiferromagnet.
We find three phases with divergent susceptibilities at both
transitions: the ordered phase with long range order, a novel
fluctuating phase in which there are three sublattices but the
order parameters are interchanging between sublattices [2]
and a disordered phase. It is unusual to have two phase tran-
sitions in a systemwith one energy parameter.
Therelationbetweenthis liquidcrystalphaseandthe instabil-
ity of theS = 3 Isingmodel on a triangular sublatticewhere a
Kosterlitz-Thouless phase exists down to zero [3] is discussed
including the reasons for a second transition to an ordered
phase for the liquid crystal.[1] X. B. Zeng, R. Kieffer, B. Glettner, C. Numberger, F. Liu, K. Pelz,
M. Prehm, U. Baumeister, H. Hahn, H. Lang, G. A. Gehring, C. H. M.
Weber, J. K. Hobbs, C. Tschierske, G. Ungar, Science 331, 1302-1306
(2011); [2] S.P.George,C.Bentham,X.Zeng,G.UngarandG.A.Gehring,
To be published; [3] Adam Lipowski, Tsuyoshi Horiguchi and Dorata
Lipowska, Phys Rev Lett 74, 3888 (1995). Cen Zeng and Christopher
L. Henley, Phys Rev B 55, 14935 (1997).
S10505 Simulations of crystal fields and magnetization in
the multi-site ladder oxides SrRE2O4 (RE=Dy, Ho, Er). B.
Malkin, D. Nabiullin (Kazan Federal University) — The orthorhom-
bic rare earth (RE) strontium oxides with the Pnam space
group, SrRE2O4, exhibit a largevarietyofmagneticproperties
at low temperatures and attract considerable interest due
to complex arrangement of magnetic ions in the crystal lat-
tice (see [1,2] and references therein). A unit cell contains 8
RE ions distributed equally between two crystallographically
nonequivalent positions. Both sites, S1 and S2, with the point
symmetry Cs are surrounded by 6 oxygen ions. The identical
RE ions in the S1 (or S2) sites form chains connected in the
zig-zag manner ladders along the crystallographic c-axis and
the honeycomb network of edge sharing hexagons in the ab-
plane.
We report on the results of simulations of crystal field en-
ergies of RE ions, temperature dependencies of components
of the magnetic dc-susceptibility tensor and field dependen-
Queen’s College Cambridge 74
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cies of the magnetization along the crystallographic axes at
low temperatures in SrDy2O4, SrHo2O4 and SrEr2O4 based
on calculations of crystal field parameters in the framework
of the exchange charge model. The obtained sets of 15 crys-
tal field parameters for RE ions in sites S1 and S2 (30 param-
eters in total) which vary monotonously along the RE series
will bepresented. Foreach site, theeffective single ionHamil-
tonian which operates in the total space of C14n states of a
4fn configuration is considered. The magnetic dipole inter-
actions are accounted for exactly by using the Evald summa-
tion method, and the bilinear anisotropic exchange interac-
tions between the nearest neighbor RE ions with the varied
parameters are introduced to match the experimental data
on the magnetization field dependencies available from lit-
erature [1-3]. Conclusions concerning possible magnetic or-
dering and low-temperaturemagnetic structures in the titled
compoundswill bediscussed. This researchwas supportedby
the RFBRGrant 14-02-00826.[1] PetrenkoO.A., Low Temp. Phys. 40, 106 (2014); [2] Poole A. et al.,
cond-mat arXiv:1401.3265 (2014); [3] Hayes T.J. et al. , J. Phys. Soc.
Jpn. 81, 024708 (2012).
S10506 Quantitative Model-Independent Refinement of
Magnetic Diffuse ScatteringData. J. Paddison (Oxford/STFC), R.
Stewart (STFC), P. Manuel (STFC), P. Courtois (ILL), G. McIntyre (ANSTO),
B. Rainford (Southampton), S. Agrestini (MPI), C. Fleck (Warwick), M. Lees
(Warwick), P. Deen (ILL/ESS), O. Petrenko (Warwick), A. Goodwin (Oxford)
— Traditionally, spin correlations in frustrated systems have
usually been studied by calculating the neutron scattering
pattern anticipated fromamodelHamiltonian and comparing
with experimental single-crystal neutron scattering data. De-
spite the undoubted success of this approach, it also has two
important limitations: (i) the interaction model must be an-
ticipated, and (ii) large single-crystal samples must be avail-
able. Here, we evaluate the alternative approach of reverse
Monte Carlo (RMC) refinement [1], in which the orientations
of spins in a large configuration are fitted directly to experi-
mental data. First, we demonstrate the effectiveness of RMC
refinementof powderdiffuse scatteringdatabyfitting to sim-
ulated “data” for several canonical frustrated models. We
showthat, in each case, the three-dimensional (single-crystal)
diffuse scattering pattern is recovered accurately from the fit
to one-dimensional (powder) scattering data [2]. We present
results from application of RMC refinement to two real sys-
tems. First, we refine experimental powder diffuse scatter-
ing data for the Ising-like spin chain system Ca3Co2O6 (see,
e.g., [3]) above TN = 25K, revealing a decoupling of frus-
trated correlations between chains and one-dimensional cor-
relations within chains. Second, we refine single-crystal dif-
fuse scattering data for Co-doped β-Mn, a metal which does
not show magnetic order to T = 50mK [4]. The spin corre-
lations we identify are shown to describe an emergent spin
structure which mimics the triangular lattice antiferromag-
net, one of the canonical models of frustratedmagnetism. Fi-
nally, we discuss the advantages and disadvantages of RMC
refinement compared to traditional approaches, and discuss
potential future applications.[1] R. L. McGreevy and L. Pusztai,Mol. Simul. 1, 359 (1988); [2] J. A.
M. Paddison and A. L. Goodwin, Phys. Rev. Lett. 108, 017204 (2012);
[3] S. Agrestini et al., Phys. Rev. Lett. 101, 097207 (2008); [4] J. A. M.
Paddison et al., Phys. Rev. Lett. 110, 267207 (2013).
S10507 Spin Glass Field Theorywith Replica Fourier Trans-
forms. I. R. Pimentel (Universityof Lisbon), C.DeDominicis (CEASaclay)
— We develop a field theory for spin glasses using Replica
Fourier Transforms (RFT). We present the formalism for the
case of replica symmetry (RS) and the case of replica symme-
try breaking (RSB) on an ultrametric tree, with the number
of replicas n and the number of replica symmetry breaking
stepsRgeneric integers. WeshowhowtheRFTapplied to the
two-replicafieldsallows toconstructanewbasiswhichblock-
diagonalizes the four-replica mass-matrix, into the replicon,
anomalousand longitudinalmodes. Theeigenvaluesaregiven
in termsof themassRFTandthepropagators in theRFTspace
are obtained by inversion of the block-diagonal matrix. The
formalism allows to express any i-replica vertex in the new
RFTbasis and hence enables to perform a standard perturba-
tion expansion. We apply the formalism to calculate the con-
tribution of the Gaussian fluctuations around the Parisi’s so-
lution for the free-energy of an Ising spin glass.
S10508 Investigation of static, dynamic and temperature-
dependent properties of nanoparticle ensembles for novel
magnetoresistive sensor devices by means of Monte Carlo
and stochastic spin dynamics simulations. L. Teich, C. Schröder,
C. Müller, A. Patel (University of Applied Sciences Bielefeld), J. Meyer, A.
Hütten (Bielefeld University) — A new concept for magnetoresis-
tive sensors based on magnetic nanoparticles in novel silica
gels shows promising features regarding sensor sensitivity,
complexityof theproductionprocessandflexibility inapplica-
tion fields. It has been shown [1] that, by dispersing commer-
cially available cobalt nanoparticles in novel silica gels [2] and
agarose, giant magnetoresistance effect amplitudes of more
than60%canbeobserved in transportmeasurements. More-
over, comparing the sensor concept on the basis of gels to
conventional conceptspreparedby lithographic techniques, a
remarkabledecrease inproduction costs canbeexpecteddue
to the printability of the gels. Besides the promising technical
capabilites of the sensor concept, theoretical investigations
of the magnetic nanoparticle arrangements reveal notable
properties. The structures that have been prepared for the
sensor setup exhibit a strong dipolar coupling between the
particles, resulting in a characteristic behaviour. Stochastic
spin dynamics methods [3] have been used to investigate the
static and dynamic properties of the nanoparticle arrange-
Queen’s College Cambridge 75
HFM2014
ments at T = 0. Simulations reveal the existence of a multi-
tude of low energy configurations separated by energy barri-
ers, reminiscent ofmagnetic dipole glass behaviour. Here, we
present efficient strategies to find the lowest energy configu-
rations by means of stochastic spin dynamics simulations, in-
spired by experimental demagnetisation routines, that have
been applied succesfully to artificial spin ice [4]. Further-
more, we have studied the thermodynamic properties of the
nanoparticle arrangements bymeans ofMonte Carlo simula-
tions [3]. Here we find characteristic properties, resembling
dipole glass behaviour as well.[1] J. Meyer, T. Rempel, M. Schäfers, F.Wittbracht, C. Müller, A. V. Pa-
tel and A. Hütten: Giant magnetoresistance effects in gel-like matrices.
Smart Mater. Struct., 2013, 22, 025032; [2] C. Müller, K. Kraushaar,
A. Doebbe, J. H. Mussgnug, O. Kruse, E. Kroke and A. V. Patel: Syn-
thesis of transparent aminosilane-derived silica based networks for the en-
trapment of sensitive materials. Chem. Commun., 2013, 49, 10163-
10165; [3] L. Engelhardt and C. Schröder: Simulating computation-
ally complex magnetic molecules. In R. E. P. Winpenny (Ed.), Molecular
Cluster Magnets, 2011, World Scientific Publishers, Singapore; [4] Z.
Boudrikis, J. P. Morgan, J. Akerman, A. Stein, P.Politi, S. Langridge, C.
H.Marrows and P. L. Stamps: Disorder strength and field-driven ground-
state domain formation in artificial spin ice: experiment, simulation and
theory. Phys. Rev. Lett, 2012, 109, 037203.
06 ClassicalMagnetism, eco. Spin Ice (theory)
S10601 Spin-liquid phase and order-by-disorder on the
swedenborgite lattice. S. Buhrandt (Utrecht University, Univer-
sity of Cologne), L. Fritz (Utrecht University, University of Cologne) —The
phenomenon of frustration refers to the inability to satisfy
competing interactions simultaneously. Often, strong frus-
tration leads to a large number of degenerate ground states
with fluctuations suppressing the ordering tendencies. A
challenging task is to characterize the spin-liquid phase re-
sulting fromthe inability toorderandtheeventualbreakingof
ground state degeneracy. While this is usually accomplished
by small perturbations, an intrinsic effect is entropic order-
by-disorder. We present evidence that a classical nearest-
neighbor Heisenberg model on the swedenborgite lattice
hosts both an extended spin-liquid phase as well as a version
of entropic order-by-disorder taking place at very low tem-
peratures. We argue that this observation renders magnetic
insulators on the swedenborgite lattice a prime candidate for
displaying spin liquid and order-by-disorder physics.
S10602 Magnon pairing in pyrochlore antiferromagnets.
T. Momoi (RIKEN), E. Takata (ISSP), M. Oshikawa (ISSP) — Chromium
spinel oxides ACr2O4 (A = Zn, Cd, Hg) provide simple
S = 3/2 pyrochlore antiferromagnetic spin systems where
detailed comparison between experimental and theoretical
studies is possible. Some magnetic structures of these com-
pounds, includingahalf-magnetizationplateau, havebeende-
scribedby theclassicalHeisenbergmodelwithbiquadratic in-
teraction, which originates from spin-lattice coupling [1]. Re-
cent high-field measurements [2,3,4] revealed a new phase
near the saturation field, which has not been predicted by
the classical spin model. Motivated by these observations,
we study the quantum S = 3/2 pyrochlore Heisenberg an-
tiferromagnet with biquadratic interaction. Our calculation
concludes that a very small biquadratic interaction can in-
duce magnon pairing in pyrochlore antiferromagnets near
the saturation field, which leads to the appearance of ferro-
quadrupolar phase or equivalently spin nematic phase. We
will present a phase diagram for the quantum spin model in
appliedmagnetic field.[1] K. Penc, N. Shannon, and H. Shiba, Phys. Rev. Lett. 93, 197203
(2004); [2] S. Kimura et al., Phys. Rev. B 83, 214401 (2011); [3] A.
Miyata et al., Phys. Rev. Lett. 107, 207203 (2011); [4] A. Miyata, S.
Takeyama, andH.Ueda, arXiv:1302.3664; [5] E. Takata, T.Momoi, and
M. Oshikawa, in preparation.
S10603 Hidden frustration in multiple-Q ordered metals.
Y. Motome and S. Hayami (Univ. of Tokyo) —Noncoplanar multiple-
Qmagnetic orders, inwhich spins align neither in a line nor on
aplane, often lead tonew low-energyexcitationsand/or topo-
logically nontrivial states. In particular, triple-Q magnetic
orders, which are characterized by three different ordering
wave vectors, have drawn much interest. A skyrmion lattice
in the A phase of MnSi, is a typical example of such triple-Q
orders, stabilized by competition between the ferromagnetic
exchange interaction and the Dzyaloshinskii-Moriya interac-
tion. Another example is found in geometrically frustrated
lattices,whichgives rise to a topological (Chern) insulator and
associated quantum anomalous Hall effect. For instance, a
four-sublattice triple-Q order was found in a Kondo lattice
model on a triangular lattice [1,2], and the importance of an
effective biquadratic interaction mediated by itinerant elec-
tronswaspointedoutbytheperturbation in termsof thespin-
charge coupling [3]. On the other hand, a noncoplanar triple-
Q order was recently found even in an unfrustrated cubic-
lattice model without any competing interactions [4]. This
suggests a general mechanism for stabilizing such multiple-
Q states in itinerant magnets. Stimulated by these obser-
vations, we here investigate the stabilization mechanism of
multiple-Q states from a wider viewpoint by carefully reex-
amining the perturbation calculations. We derive the gen-
eral form of fourth-order perturbations with respect to the
spin-charge coupling, and compare the free energy for single-
, double-, and triple-Q states on various lattice structures. As
a result, we find that the instability toward the multiple-Q
states is summarized in a general phase diagram given by two
parameters appearing in the fourth-order perturbations irre-
spective of lattice structures. The instability on each lattice
can be traced by calculating the parameters as functions of
electron filling and temperature. From this general analysis,
Queen’s College Cambridge 76
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we show that the triple-Q instabilities in the triangular and
cubic lattice systems are understood in a unified way. More-
over, we apply the analysis to other lattices and find similar
instabilities in the square and face-centered cubic lattices.[1] I. Martin and C. D. Batista, Phys. Rev. Lett. 101, 156402 (2008);
[2] Y. Akagi and Y. Motome, J. Phys. Soc. Jpn. 79, 083711 (2010); [3]
Y. Akagi, M. Udagawa, and Y. Motome, Phys. Rev. Lett. 108, 096401
(2012); [4] S. Hayami, T.Misawa, Y. Yamaji, and Y.Motome, Phys. Rev.
B 89, 085124 (2014).
S10604 Vortex domain walls in helical magnets. Fuxi-
ang Li, T. Nattermann and V.L. Pokrovsky — We show that helical
magnets exhibit a specific type of domain wall consisting of
a regular array of vortex lines except of few distinguished
orientations. This result follows from topological considera-
tion and is independent of the microscopic models. We used
simple models to calculate the shape and energetics of vor-
tex walls in centrosymmetric and non-centrosymmetric crys-
tals. Vorticesarestronglyanisotropic, deviating fromthecon-
ventional Kosterlitz-Thouless form. The width of the domain
walls depend only weakly on themagnetic anisotropy, in con-
trast to ferromagnets and antiferromagnets. We show that
vortex walls can be driven by external currents and in multi-
ferroics also by electric fields. Some of textures predicted
by our theory appeared earlier in experimental pictures, but
were not identified.
S10605 Orphan Spins and Disorder on the Coulomb Phase.
J. Rehn, A. Sen, A. Andreanov, A. Scardicchio, R.Moessner —Under cer-
tain conditions on the lattice geometry it is known that highly
frustrated antiferromagnets exhibit the nontrivial “Coulomb
phase”, where the excitations behave as effective charges
with Coulomb interactions. An interesting scenario occurs
when adding nonmagnetic impurities in such systems (as is
the case in thematerial SCGO). In this case the excitations (vi-
olations of the “ice rules”) happens as an effect of the pres-
ence of impurities, giving rise to the so called “Orphan spins”
which also are predicted to interact as Coulomb charges. I
will present somenumerical results for these twodimensional
systems of interacting Orphan spins, where the possibility of
a glassy phase is studied.
S10606 Multi-boson theory for the magnetoelectric heli-
magnet Cu2OSeO3. J. Romhanyi (IFW Dresden), J. van den Brink
(IFW Dresden), I. Rousochatzakis (IFW Dresden) — The Cu2OSeO3 is
the first insulating system exhibiting skyrmion-lattice phase.
In addition it has finite magnetoelectric coupling enabling
technological applications. In Cu2OSeO3 the magnetic Cu2+
ions form a distorted pyrochlore lattice. Recent ab initio den-
sity functional calculations revealed that due to the two well
separated exchange energy scales, this pyrochlore lattice is
divided into strongandweak tetrahedra. Buildingon this fact,
we perform a microscopic multi-boson theory which keeps
the strong tetrahedra quantum mechanically entangled, and
treats theweakcouplingsbetween themonameanfield level.
This theory captures theexperimentally observed spin reduc-
tion and provides the excitation spectrum as well as the dy-
namical structure factors, pertinent to inelastic neutron scat-
tering, Raman, and other spectroscopic probes. We also dis-
cussmagnetoelectriceffectbasedonthis theoryandcompare
our results with recent electron spin resonance spectrum.
S10607 Spatial dimension dependence of first-order phase
transition nature in stacked triangular lattice system. Ryo
Tamura (NIMS) and Shu Tanaka (Univ. of Tokyo) — We studied
the phase transition nature in frustrated classical Heisenberg
model on a stacked triangular lattice by Monte Carlo simu-
lations. In the model, there are three types of interactions:
the first nearest-neighbor ferromagnetic interaction J1 and
the third nearest-neighbor antiferromagnetic interaction J3
in each triangular layer, and the first nearest-neighbor ferro-
magnetic interaction J⊥ in interlayer. Here, we investigated
thephase transitionnature in the case that theorder parame-
ter space is SO(3)×C3, where SO(3) andC3 correspond to the
spin and lattice rotational symmetries, respectively. When
the interlayer interaction is absent (J⊥ = 0), the system ex-
hibits a first-order phase transition withC3 symmetry break-
ing and the dissociation of Z2 vortices at the same temper-
ature [R. Tamura and N. Kawashima, J. Phys. Soc. Jpn. 77,
103002 (2008); Ibid. 80, 074008 (2011)]. On the other hand,
we found that in the systemwith finite J⊥, a first-order phase
transitionwithbreakingofSO(3)andC3 symmetriesoccursat
finite temperature [R. Tamura and S. Tanaka, Phys. Rev. E 88,
052138 (2013)]. Furthermore, we discovered that the tran-
sition temperature increases but the latent heat decreases as
interlayer interaction increases, which is opposite to the be-
havior observed in typical unfrustrated stacked systems. In
addition, this fact is not obtained by a naivemean-field analy-
sis.
S10608 Second-order phase transition in two-dimensional
frustrated systems. S. Tanaka (Univ. of Tokyo), R. Tamura (NIMS),
and N. Kawashima (ISSP) — We investigated nature of phase
transition in two dimensional (2D) Heisenberg model with
competing interactions, where the order parameter space is
SO(3)×Z2. The model we considered is the classical Heisen-
berg model on a uniaxially distorted triangular lattice with
the nearest-neighbor interaction J1 and the third nearest-
neighbor interaction J3. Depending on the interaction ratio
J3/J1 and distortion parameterλ, the order parameter space
can be changed [R. Tamura, S. Tanaka, and N. Kawashima, to
appear in J. Phys. Soc. Jpn. Conf. Proc.]. We focused on the
case that theorder parameter space is describedby thedirect
product between spin and lattice rotation symmetries, espe-
cially, SO(3)×Z2 [R. Tamura, S. Tanaka, and N. Kawashima,
Phys. Rev. B 87, 214401 (2013)]. Using theMonte Carlo sim-
Queen’s College Cambridge 77
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ulations, we found that a second-order phase transition with
Z2 symmetry breaking occurs at a finite temperature. In ad-
dition, Z2 vortex dissociation was observed at the same tem-
perature. To our best knowledge, this is the first example that
the simultaneous emergence of the second-order phase tran-
sition and Z2 vortex dissociation. By the finite-size scaling
analysis, we confirmed that the phase transition belongs to
the 2D Ising universality class. Thus, it is concluded that Z2
vortex dissociation does not affect the critical phenomena in
this model.
S10609 Magnetism in rare-earth quasicrystals: RKKY in-
teractions and low temperature behaviour. S. Thiem, J.
Chalker (Theoretical Physics, University of Oxford) — Magnetism in
rare-earth quasicrystals presents many basic open questions
despite the considerable research efforts since the discovery
of this material class. Here we take a two-fold theoretical ap-
proach, with results that match much of the experimentally
observed phenomenology. First, we compute RKKY interac-
tions between localised moments, using tight binding mod-
els on quasiperiodic tilings. Second, we study the statistical
mechanics of Ising spins coupled via theseRKKY interactions.
We find the emergence of strongly coupled spin clusters with
significantly weaker inter-cluster coupling. Spins freeze in an
apparentlydisorderedconfigurationat lowtemperatures, but
without evidence of the multiple low-lying states and glassi-
ness that are characteristic of conventional spin-glasses.
07Organics
S10701 Paired electron crystal state on the 2D triangular
lattice inκ - (BEDT-TTF)2Hg(SCN)2Cl. N.Drichko (JohnsHopkins
University, USA), M. Mourigal (Johns Hopkins University, USA), R. Beyer
(Universität Stuttgart,Germany), E.Rose (Universität Stuttgart,Germany),
M. Dressel (Universität Stuttgart, Germany), J. A. Schlueter (Argonne Na-
tional Laboratory, USA), S. A. Turunova (IPCP, Chernogolovka, Russia),
E. I. Zhilyaeva (IPCP, Chernogolovka, Russia), R. N. Lyubovskaya (IPCP,
Chernogolovka, Russia). —We present an experimental study of
the quasi-2D organic conductor κ-(BEDT-TTF)2Hg(SCN)2Cl
in the 10-300 K temperature range by optical techniques, re-
sistivity and specific heat. At temperatures above 30 K the
material shows properties of a highly-correlated 1/2-filled
metal on an anisotropic triangular lattice of (BEDT-TTF)+12
close to Mott insulating state. We focus on the charge-order
insulating state and argue that it shows evidence of spin sin-
glet formation. This ground state is a “paired electron crys-
tal”, a state analogous to a valence bond solid. We discuss
our results in a context of the spin liquid candidate κ-(BEDT-
TTF)2Cu2(CN)3.
Queen’s College Cambridge 78
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6.3.2 POSTER SESSION II
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POSTER
'SESSION'II''**'T
hursda
y'July'10th'2014,'from'15.30'to
'17.45'
Parity$violation$of$charge$do
main$siz
es$in$artificial$sp
in$ice.
F.$M
ontaigne
Frustration$stud
y$in$artificial$quasiIcrystal.
D.$Shi
Electrical$transport$m
easuremen
ts$on$ho
neycom
b$artificial$spin$ice.
K.$Zeissler
The$magne
toIoptical$re
spon
se$in$th
e$ultrathin$films$o
f$Fe/Au
(001).$
M.$B
oukelkou
l
Prop
agation$of$m
agne
tic$dom
ain$walls$in$artificial$sp
in$ice.$
D.M.$B
urn
Magne
tic$frustration$in$dipolar$cou
pled
$nanoIdisk$sy
stem
s.$
M.$Chadh
a
Non
Iuniversality
$of$artificial$frustrated
$spin$sy
stem
s.I.A
.$Chioar
Effective$Thermod
ynam
ics$throu
gh$Dem
agne
tization$in$Artificial$Spin$Ice.
I.A.$Chioar
Subject
Poster$Title
Presen
ter
01_Itin
erant$S
ystems$
(the
ory)
Evolution$of$M
agne
tic$Order$in$Fe(1+y)Te$Com
poun
ds$with
$Increasin
g$Interstitial$Iron.
S.$Ducatman
InteractionIindu
ced$anom
alou
s$quantum
$Hall$state$on$the$ho
neycom
b$lattice.
T.$Duric
Charge$and
$magne
tic$correlatio
ns$near$the
$Mott$transition
$of$the
$Hub
bard$
mod
el$on$the$aniso
trop
ic$kagom
e$lattice.$
M.$Enjalran
Magne
tic$Transition
s$of$D
irac$Ferm
ions$with
$SpinIOrbit$Co
upling.
Interplay$of$charge$and$spin$fluctuations$of$stron
gly$interacting$electron
s$on$
the$kagome$lattice$at$one
Ithird$filling.
M.$H
ohen
adler
K.$Pen
c
M.$U
dagawa
Role$of$d
omain$walls$in$allIin/allIou
t$ordered
$states.
02_A
ritifical$Spin$Ice$and$
Other$Nanostructured$
System
s
Resonant$Soft$X
IRay$Scattering$On$Artificial$Spin$Ice.
L.$Aghinolfi
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O. P
etro
va
Unp
aired4Majorana4ferm
ions4on4dislocations4in4Kita
ev's4hon
eycomb4spin4
mod
el.
O.Petrova
Impu
ritie
s4in4Spin4Ice4Crystals.
Theo
retical4study4of4spe
ctral4and
4magne
tic4prope
rties4of4Tb2
Ti2O
7:4im
pact4of4
rand
om4strains.4
B.4M
alkin
Spin4ice4is4not4highly4correlated
.4Is4it?
T.4Yavors'kii
G.4Sala
A4dynam
ic4Ja
hnTTeller4cou
pling4to4describe4the4very4low4te
mpe
rature4fieldT
indu
ced4magne
tic4structures4in4th
e4Tb2Ti2O74spin4liqu
id.
A.P.Sazon
ov
Mon
opole4Hop
ping4th
rough4Quantum
4Spin4Tunn
elling4in4Spin4Ice.
B.4Tom
masello
A4Unifying4Field4Theo
ry4fo
r4The
4Pyrochlore4Lattice.
H.4Yan
03_Spin4Ice4(the
ory)
Coulom
bic4spin4liqu
ids:4re
loaded
.O.4B
enton
Magne
tic4m
omen
t4fractionalisation4in4a4m
onop
ole4crystal.
M.4B
rooksTBa
rlett
Far4from4equ
ilibrium4beh
aviour4of4spin4ice4materials.
C.4Calstelno
vo
Quantum
4spinTliquid4ph
ases4of4q
uantum
4spinTice.
A.G.R.4D
ay
Emergence4of4Loo
ps4in4SpinTIce.
K.4Essafi
Critical4beh
avior4in4cubic4dimer4m
odel4at4finite
4mon
opole4fugacity.
G.4Sreejith
Wien4Effect4in4Spin4Ice.
V.4Kaiser
Saturatio
n4fie
ld4entropies4of4Ising4antife
rrom
agne
ts:4app
lication4to4spinTice4
Dy2Ti2O
7.V.K.4Varma
Quantum
4Effects4in4a4Realistic4M
odel4of4Spin4Ice.
P.4M
cClarty
Excitatio
ns4in4quantum
4spin4ice:4an4analytical4study.
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Lattice2distorcions2in2frustrated2sysmtes:2spin2ice.
D.2Cabra
Spin2fluctuations2in2frustrated2m
etal2LiV2O42with2heavy2ferm
ion2behavior.2
05_Other2Frustrated2Spin2
Systems2(experiments)
Order2in2the2short2ranged2ordered2state2of2Gd3Ga5O12.
P.2Deen
Updated2phase2diagram2of2the2frustrated2m
agnet2Gd3Ga5O12.
O.2Florea
High2m
agnetic2field2studies2on2frustrated2ferric2antiferromagnets.2
M.2Hagiwara
Observation2of2magnon2decay2in2LuMnO3.
M.D.2Le
Frustration2in2the2Cairo2pentagonal2lattice2antiferromagnet2Bi2Fe4O9.
M.D.2Le
Universal2memory2effects2observed2after2temporary2heating/cooling2in2
Heisenberg2spin2glasses2and2spontaneous2restoration2of2the2spin2configuration2
existing2before.2
H.2Mamiya
Phase2transition2of2the2first2order2in2the2field2of2magnetic2frustration2in22
Heusler2alloys2NiVMnVIn.2
A.V.2Mashirov
G.G.2Morgan
M.2Pregelj
R.2Sibille
K.2Tomiyasu
Ordering2Phenomena2in2Spin2Crossover2Solids.2
Persistent2spin2dynamics2in2amplitude2m
odulated2m
agnetic2ground2states.
Magnetic2frustration,2hierarchy2of2exchange2interactions,2and2idle2spin2
behavior2in2a22D2lattice2of2bowVties.2
04_Low2Dim
ensional2
Systems2(theory)
FiniteVtemperature2dynamics2of2highly2frustrated2quantum2spin2chains.
A.2Honecker
Phase2diagram2of2the2alternatingVspin2Heisenberg2chain2with2extra2isotropic2
threeVbody22exchange2interactions.
N.B.2Ivanov
GroundVstate2phase2diagram2of2an2anisotropic2rungValternating
S=1/22ladder.
T.2Tonegawa
SymmetryVprotected2topological2phases2in2frustrated2spinV1/22zigzag2chain.
H.2Ueda
03_Spin2Ice2(theory)
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S.-Toth
D.-Yoshizawa
07_5d-Electron
-Systems-
(experim
ent)
Novel-m
agne
tism
-in-the
-spinJorbit-driven
-Mott-insulator-Ba
2YIrO6.-
T.-Dey
08_M
ultiferroics
Dom
ains-and
-multiferroicity-in-CuC
rO2:-a-single-crystal-neu
tron
-diffraction-
stud
y.M.-Frontzek
06_Low
-Dim
ension
al-
System
s-(experim
ents)
Characterization
-of-Q
uasiJOne
JDim
ension
al-Cup
rates.
K.-Caslin
Neu
tron
-studies-of-P
bCuSO4(OH)2,-a-candidate-fo
r-a-no
vel-quantum
-phase.-
E.-Cem
al
Expe
rimen
tal-evide
nce-for-a-magne
tization
-plateau-and
-for-fie
ldJsup
pressed-
lowJene
rgy-spin-fluctuations-in-the
-triangular-antiferrom
agne
t-Ba
3CoN
b2O9.-
J.-Dai
ZeroJfield-sho
rtJrange-order-in-the
-quasiJone
-dim
ension
al-frustrated
-ferrom
agne
t-LiCu
VO4.
M.-End
erle
The-spin-ladd
er-com
poun
d-Ba
2Cu(2+)Te(6+)O6:-an-intriguing-interplay-
betw
een-crystal-and
-magne
tic-dimen
sion
alities.
A.S.-G
ibbs
UltraJhigh-magne
tic-fie
ld-indu
ced-1/3-magne
tization
-plateau-in-a-frustrated
-triangularJla
ttice-magne
t-Cu
CrO2.-
A.-M
iyata
Preten
ded-qu
antum-spinJliquid-be
havior-in-a-m
ixed
-triangular-
antiferrom
agne
tic-system
.T.-Ono
PressureJdriven-dimen
sion
ality-change-in-a-quantum
-magne
t.M.-Skoulatos
Raman-study-of-m
agne
tic-excitation
s-and-magne
toJelastic-cou
pling-in-SrCr2O4.
N.-D
richko
Microscop
ic-Study-of-the
-Field-Indu
ced-Ph
ases-of-the
-Magne
tic-Frustrated
-Quantum
-Spin-Ch
ain-System
-Linarite.-
Frustration-in-the
-J1-J-J2-chain-of-N
aCuM
oO4(OH).
K.-Naw
a
Magno
n-mod
es-in-αJCaCr2O4-measured-by-neu
tron
-scattering-and-far-infrared
-absorption
.MultiJfrequ
ency-ESR-in-the
-S=5/2-triangular-lattice-antiferrom
agne
t-Cu
FeO2-in-
an-inJplane
-magne
tic-fie
ld.
S.-Sullow
05_O
ther-Frustrated-Spin-
System
s-(experim
ents)
Frustrated
-diamon
d-lattice-antiferrom
agne
t-Co
Al2O4-by-neu
tron
-scattering-
and-classical-M
onteJCarlo-m
odeling.
T.-Yam
azaki
Magne
tic-Re
spon
se-of-G
d(1Jx)La(x)-to-Low-Field.
O.-Zaharko
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09_D
imerise
d3Mod
els3
and3Materials
Grow
th3and
3Characterisa
tion3of,3the
3frustrated
3ShastryCSuthe
rland
3Magne
ts,3
the3Ra
re3Earth3Tetrabo
rides.3
D.3Brunt
Spin3dynam
ics3in3the3mineral3com
poun
d3Malachite,3Cu2
(OH)2C
O3.
E.3Canevet
Triplon3Ha
ll3effect3in3th
e3Shastry3Sutherland
3Material.
R.3Ganesh
Spin3su
persolid3on3the3ShastryCSutherland
3lattice.3
P.3Sen
gupta
08_M
ultiferroics
Theo
ry3of3m
agne
toelectric3effe
cts3in3multiferroics3
CuFeO23and33BiFeO
3.PressureCin
duced3magne
tic3phase3transitions3in3a3m
ultiferroic3delafossite3
CuFeO23as3observed3by33highCpressure3neu
tron
3diffraction.
S.3M
iyahara
N.3Terada
Queen’s College Cambridge 83
HFM2014
01 Itinerant Systems (theory)
S20101 Evolution of Magnetic Order in Fe1+yTe Com-
pounds with Increasing Interstitial Iron. S. Ducatman (UW-
Madison), N. Perkins (UW-Madison), R. Fernandes (UM-Twin Cities) —
We studied the electronic and magnetic properties of iron-
chalcogenide Fe1+yTe compounds based on the multiband
model, in which localized spins and itinerant electrons co-
exist and are coupled by Hund’s rule coupling. Integrating
out the conduction electrons, we computed additional cou-
plings between localized spins similarly to the conventional
Ruderman-Kittel-Kasuya-Yosida (RKKY) theory. We found
that resulting RKKY-like interactions have oscillating charac-
ter andare substantial up to the thirdneighbors, but arenegli-
gible beyond this. We computed themagnetic phase diagram
of themodifiedJ1−J2−J3model and showed it captures the
evolution of themagnetic order in Fe1+yTe as a function of Fe
excess y.
S20102 Interaction-inducedanomalousquantumHall state
on the honeycomb lattice. T. Duric (LCN), N. Chancellor
(LCN), I. F. Herbut (SFU, MPIPKS) —We examine the existence of
the interaction-generated quantum anomalousHall phase on
the honeycomb lattice. For the spinless model at half fill-
ing, the existence of a quantum anomalous Hall phase (Chern
insulator phase) has been predicted using mean-field meth-
ods. However, recent exact diagonalization studies for small
clusters with periodic boundary condition have not found
a clear sign of an interaction-driven Chern insulator phase.
We use exact diagonalization method to study properties of
small clusters with open boundary condition and, contrary
to previous studies, we find clear signatures of the topolog-
ical phase transition for finite size clusters. We also exam-
ine applicability of the entangled-plaquette state (correlator-
product state) ansatz to describe the ground states of the
system. Within this approach the lattice is covered with
plaquettes and the ground state wave-function is written in
terms of the plaquette coefficients. Configurational weights
can then be optimized using a variational Monte Carlo algo-
rithm. Using the entangled-plaquette state ansatz we study
the ground state properties of the system for larger system
sizes and show that the results agree with the exact diago-
nalization results for small clusters. This confirms validity of
the entangled-plaquette state ansatz to describe the ground
states of the system and provides further confirmation of the
existence of the quantum anomalous Hall phase in the ther-
modynamic limit, as predicted by the mean-field theory cal-
culations.
S20103 Charge and magnetic correlations near the Mott
transition of the Hubbardmodel on the anisotropic kagome
lattice. M. Enjalran (Southern CT State University) — The exper-
imental work seven years ago on the herbertsmithite mate-
rial, ZnCu3(OH)6Cl2, has generated enormous excitement in
the field of frustrated magnetism because it presented the
strongest candidate yet for a 2D quantum spin liquid. The
material is an excellent realization of a spin-1/2 kagome lat-
tice antiferromagnet with nearest neighbor exchange as the
dominate energy scale. Numerous theoretical and numeri-
cal studies of the quantumHeisenberg model on the kagome
lattice have been performed since and have produced a gen-
eral consensus that the ground state of the nearest neighbor
model is a spin liquid with a small gap, in contrast to the gap-
less phaseobserved in theherbertsmithite. The experimental
data for ZnCu3(OH)6Cl2 has also motivated theoretical and
numerical work on the kagome lattice Hubbard model. Al-
though there is not currently amaterial systemwith itinerant
electrons on the kagome lattice, a situation that is unlikely to
persist indefinitely, it is important to investigate the proper-
ties of simple models of itinerant electrons on this lattice in
orderdevelopadeeperunderstandingof correlatedphases in
the presence of frustration. We contribute to this pursuit by
studying the single band Hubbard model on the anisotropic
kagome lattice. The frustration in our model is tuned by ad-
justing the hopping along different bonds, t1 and t2. Within
the Hartree-Fock approximation, we study the development
of charge andmagnetic correlations in the half filledmodel as
a function of frustration and interaction strength as the sys-
tempasses through themetal-to-insulator transition. Our re-
sults are compared to findings from similar calculations of the
anisotropic triangular lattice Hubbard model and to theoret-
ical/numerical data for itinerant electrons and quantummag-
nets on the kagome lattice.
S20104 Magnetic Transitions of Dirac Fermions with Spin-
Orbit Coupling. M. Hohenadler (University of Würzburg) —Moti-
vated by experimental progress with graphene and topolog-
ical insulators, we use exact quantum Monte Carlo methods
to study magnetic Mott transitions of fermions on the hon-
eycomb lattice. At weak interactions, the ground state is ei-
ther a semimetal (without spin-orbit coupling) or a quantum
spin Hall insulator (with spin-orbit coupling). A Hubbard re-
pulsion drives a transition to a state with antiferromagnetic
order and broken time-reversal invariance. In the absence of
spin-orbit coupling, it has been shown that the transition is of
the Gross-Neveu type [Assaad and Herbut, Phys. Rev. X 3,
031010 (2013)]. Spin-orbit coupling reduces the spin symme-
try from SU(2) to U(1), and the transition instead falls into
the 3D XY universality class [Hohenadler et al., Phys. Rev. B
85, 115132 (2012) ]. The corresponding effective spin-model
shows frustration in the longitudinal direction but not in the
transverse direction, and the magnetic state is an easy-plane
antiferromagnet. Here, we report exact numerical results for
a more realistic 1/r Coulomb potential, which reveal that the
universality class remains unchanged for both transitions.
Queen’s College Cambridge 84
HFM2014
S20105 Interplayof chargeandspinfluctuationsof strongly
interacting electrons on the kagome lattice at one-third fill-
ing. K. Penc (ISSPO, Wigner RCP, Budapest), F. Pollmann (MPI-
PKS, Dresden), K. Roychowdhury (MPI-PKS, Dresden), C. Hotta (Kyoto
Sangyo University, Kyoto) — We study electrons hopping on a
kagome lattice at third filling described by an extended Hub-
bard Hamiltonian with on-site and nearest-neighbour repul-
sions in the strongly correlated limit. As a consequence of the
commensurate filling and the large interactions, each trian-
gle has precisely two electrons in the effective lowenergy de-
scription, andtheseelectrons formchainsofdifferent lengths.
The effective Hamiltonian includes the ring exchange around
the hexagons as well as the nearest-neighbor Heisenberg in-
teraction. Using large scale exact diagonalization, we find
that the effective model exhibits two different phases: If the
charge fluctuations are small, the magnetic fluctuations con-
fine the charges to short loops around hexagons, yielding a
gapped charge ordered phase. When the charge fluctuations
dominate, the system undergoes a quantum phase transition
to a resonating plaquette phase with ordered spins and gap-
less spin excitations. Wefind that a peculiar conservation law
is fulfilled: the electron in the chains can be divided into two
sublattices, andthisdivision is conservedbytheringexchange
term. [arXiv:1402.4932]
S20106Roleofdomainwalls in all-in/all-outordered states.
M. Udagawa and Y. Motome (Univ. of Tokyo) — Recently, all-in/all-
out-type magnetic ordering has been found in several py-
rochlore conductors, and draws considerable attention, due
to their novel response to external fields[1] and possible re-
alization of topologically non-trivial electronic states[2]. This
novelorderedphasehasbeenfound, for instance, inNd2Ir2O7
and Cd2Os2O7, by neutron[3] and X-ray scattering exper-
iments[4]. While the existence of all-in/all-out-type order
has been established for these compounds, however, their
thermodynamic and transport properties remain to be un-
derstood. In particular, several experiments imply the exis-
tence of a characteristic temperature scale below the transi-
tion temperature, Tc. For
Cd2Os2O7, the Hall signal develops abruptly at T ∼ 200K[5],
which is substantially lower than Tc ∼ 225K. The µSR sig-
nal also appears well below Tc, around T = 150K[6]. An-
other interesting aspect is that the resistivity exhibits dra-
matic magneto-hysteresis at T ∼ 2K, much below Tc ∼ 40K
in Nd2Ir2O7[7]. In this contribution, we focus on the role of
domainwalls in the all-in/all-out ordered state, as a key to un-
derstanding the origin of this temperature scale, and further
elucidating the detailed properties of this phase. We adopt
the Hubbard model with Ising-like exchange coupling, and
solve this model with an unrestricted Hartree-Fock approxi-
mation by taking into account real space modulations due to
the domain formation. As a result, we found a crossover in
thedomainwall structurebelowTc[8]. Weassociate thechar-
acteristic temperature scale with this domain wall crossover,
and address the thermodynamic and transport properties of
the all-in/all-out-type insulator phase.[1] Y. Arima, J. Phys. Soc. Jpn. 82, 013705 (2012); [2] X. Wan et al.,
Phys. Rev. B 83, 205101 (2011), W. Witczak-Krempa and Y. B. Kim,
Phys. Rev. B 85, 045124 (2012); [3] K. Tomiyasu et al., J. Phys. Soc.
Jpn. 81, 034709 (2012); [4] M. Yamaura et al., Phys. Rev. Lett. 108,
247205 (2012); [5]D.Mandrus et al., Phys. Rev. B63, 195104 (2001);
[6]A.Kodaetal., J.Phys. Soc. Jpn. 76, 063703 (2007); [7]K.Matsuhira
et al., J. Phys. Soc. Jpn. 82, 023706 (2013); [8] M. Udagawa and Y.
Motome, in preparation.
02Artificial Spin Ice andOther Nanostructured Systems
S20201 Resonant Soft X-Ray Scattering On Artificial Spin
Ice. L. Anghinolfi (ETH Zürich, Paul Scherrer Institut), J. Perron (LCPMR
(UMR 7614 UPMC/CNRS)), B. Tudu (LCPMR (UMR 7614 UPMC/CNRS)),
N. Jaouen (Synchrotron SOLEIL), J.M. Tonnerre (Institut Néel), F. Nolt-
ing (Paul Scherrer Institut), J. Lüning (LCPMR (UMR 7614 UPMC/CNRS),
Synchrotron SOLEIL), L.J. Heyderman (ETH Zürich, Paul Scherrer Institut)
—Artificial spin ice consists of dipolar-coupled nanomagnets
placed at the sites of a square or kagome planar lattice [1].
These particular geometries prevent the dipolar interactions
to be simultaneously satisfied at the vertices where the is-
lands meet, making the system magnetically frustrated. Mi-
croscopy techniques [2,3] are usually employed to investigate
such systems and to directly resolve the magnetic configura-
tion of the islands. In contrast, scattering is a complementary
methodwhich provides informationonmagnetic correlations
over length and time scales not accessible with microscopy
[4]. In the present work [5], we employ soft x-ray resonant
magnetic scattering with circularly-polarized light to study
artificial square ice. The scattering patterns are recorded
with a CCD camera, providing an extended picture of the re-
ciprocal space in two dimensions. Puremagnetic Bragg peaks
observed inas-grownsamples indicate thepresenceof a long-
range antiferromagnetic ordered phase [3], which is subse-
quently destroyed by orienting the magnetic moments with
an applied field. In order to examine the evolution of themag-
netic configuration with the applied magnetic field, we track
the variations of the dichroic contrast intensity at the Bragg
peakpositions. Our numerical simulations, basedon the kine-
matical approach, correctly reproduce the experimental scat-
tering patterns, and allow us to estimate the number nano-
magnets in each of the two sublattices with reversed mo-
ments. This work is a first step toward the study of magnetic
correlations in thermally active frustrated nanomagnet sys-
tems, with the possibility to observe how such correlations
develop with time.[1] R.F,Wang et al., Nature 439, 303-306 (2006); [2] E.Mengotti et al.,
Nature Physics 7, 68-74 (2011); [3] J.P. Morgan et al., Nature Physics
7, 75-79 (2011); [4] K. Chesnel et al., Physical Review B 66, 172404
(2002); [5] J. Perron et al., Physical Review B 88, 214424 (2013).
Queen’s College Cambridge 85
HFM2014
S20202Themagneto-optical response in the ultrathin films
of Fe/Au(001). M. Boukelkoul (Sétif1 university , Algeria), A.
Haroun (Sétif1 university,Algeria) — By means of the first prin-
ciple calculation, we have investigated the structural, mag-
netic and magneto-optical properties of the ultrathin films
of Fen/Au(001) with (n=1, 2, 3). Calculations are performed
within the framework of relativistic approach using DFT the-
orem with local spin density approximation (LSDA). To cal-
culate the band structure, we have used the Spin-Polarized
Relativistic (SPR) LinearMuffin-Tin Orbitals (LMTO) with the
Atomic Sphere Approximation (ASA) method. The Hamilto-
nian and the overlap matrices corresponding to Dirac equa-
tionareexpressed in termsof thebasis setof the so-called rel-
ativistic muffin-tin orbitals centered on the atomic sites. The
crystalline structure was calculated by a relaxation process.
A body centered tetragonal (bct) pseudomorphic crystalline
structure with a tetragonality ratio c/a larger than unity was
found. The magnetic calculations revealed that the inter-
plane coupling is ferromagnetic with an enhanced magnetic
moment. Using the Kubo-Greenwood linear response; the
dispersion of the optical conductivity tensor as a function
of the frequency of the incoming electromagnetic radiation
is calculated from the energy band structure and the polar
magneto-optical Kerr effect spectra are given over a photon
energy range extended to 15eV. Themicroscopic origin of the
most interesting features ofKerr rotations are interpretedby
interband transitions. Unlike thin layers, the Kerr response
showed that the anisotropy in the ultrathin films is charac-
terized by a magnetization which is perpendicular to the film
plane.
S20203 Propagation of magnetic domain walls in artificial
spin ice. D.M. Burn, S.K.Walton,M. Chadha, L.F. Cohen andW.R. Bran-
ford (Imperial College London, UK) — Nanoscale patterning tech-
niques can be used to fabricate magnetic structures with di-
mensions comparable with those of magnetic domains and
domain walls (DWs). This can give control over the interac-
tions taking place within the material allowing complex frus-
trated geometries to be designed such as artificial spin ice. In
addition toadvances in the fundamental physical understand-
ing of magnetism from studying these nanoscale systems, re-
search in this areamay lead to the development of novel spin-
tronic devices for technological applications. In artificial spin
ice, magnetic nanobars can support magnetic DWs and the
magnetisation reversal in an appliedfield ismediated through
DW propagation. Interactions between the micromagnetic
structure of a DW and the spin structure at the nanobar ver-
ticesdetermine thepathofaDWand it’sde-pinningfield from
the vertex. In this work the trajectory of a DW at a nanobar
vertex is investigated as a function of the dynamic behavior
of the propagating DW. This includes the time dependent pe-
riodic changes in the DW micromagnetic structure that re-
sults from Walker breakdown. These results have implica-
tions for future technological applications as well as suggest-
ing processes that may governmagnetisation reversal in arti-
ficial spin ice structures.
S20204 Magnetic frustration in dipolar coupled nano-disk
systems. Megha Chadha (Imperial College London), Stephanie K.
Walton (Imperial College London), David M. Burn (Imperial College Lon-
don), KatharinaZeissler (Imperial College London), Solveig Felton (Queen’s
University, Belfast), Lesley F. Cohen (Imperial College London), and Will
R. Branford (Imperial College London). — Ferromagnetic nano-dot
arrays are interesting for data storage applications, but as
the density of disks becomes high the dipolar interactions be-
tweendisks become strong. In thisworkwe study lithograph-
ically prepared arrays of densely packed single domain perm
alloy nano-disks where the dipolar correlations are signifi-
cant. We study the collective magnetic array properties for
different array geometries and varying disk separation and
explore the effect of magnetic frustration in these systems.
S20205 Non-universality of artificial frustrated spin sys-
tems. I.A. Chioar, N. Rougemaille, A. Grimm, O. Fruchart, E. Wag-
ner, M. Hehn, D. Lacour, F. Montaigne, B. Canals — Artificial spin ice
systems provide an exciting playground for the study of spin
models. In the case of 2D lithographically-patterned arrays
of nanomagnets, magnetic imaging techniques have success-
fully been used to directly observe, in real space, howeach in-
dividual spin locally accommodates frustration [1]. Further-
more, artificial spin systems offer the opportunity to change
the geometry of the array at will and to explore new phe-
nomena. However, in most cases, these pseudo-spins can
present highmagnetic anisotropy barriers, preventing the re-
versal of their magnetization and thus rendering them in-
sensitive to thermal fluctuation. Therefore, a demagnetiza-
tion procedure has been generally used to make this type of
systems evolve towards lower energy manifolds where ex-
otic magnetic behavior is expected [2–5]. So far, most ef-
forts have been focused on the square and kagome lattices
of in-plane magnetized nanomagnets. However, Zhang and
coworkers [6] have recently investigated the properties of
an artificial frustrated spin system in which the nanomagnets
have out-of-plane magnetization. In this study, a carefully
comparison is performed between the development of pair-
wise spin correlations for two different artificial realizations
of the kagome topology: the ferromagnetic kagome spin ice
model (ksi), with in-plane magnetized spins, and the antifer-
romagnetic kagome Ising model (kI), with out-of-plane mag-
netized spins. One important conclusion has been drawn:
the two systems described by spin models based solely on
nearest-neighbor interactions showstrikingsimilarities in the
development of moment pair correlations, indicating a uni-
versality in artificial spin ice behavior. The physics of demag-
Queen’s College Cambridge 86
HFM2014
netized artificial spin ice systems thus seems to transcend the
particular material realization, and even the geometry of the
magneticmoments. Investigating the properties of similar ar-
tificial kagomearraysof nanomagnetswithout-of-planemag-
netization, we end up with a different conclusion [7]: our ex-
perimental findings can only be described by spinmodels that
include long range dipolar interactions, breaking the appar-
ent universality between the ksi and kI frustrated systems, as
they develop clearly distinctive pairwise spin and charge cor-
relations. Therefore, we can assess the limits of the reported
universality and we also provide arguments to explain why
these two systems show, at first sight, striking similarities in
the development of pairwise spin correlations.[1] R. F. Wang et al, Nature 439, 303 (2006); [2] R. F. Wang et al, J.
Appl. Phys. 101, 09J104 (2007); [3] X. Ke et al, Phys. Rev. Lett.
101, 037205 (2008); [4] G. Muller and R. Moessner, Phys. Rev. B 80,
140409 (2009); [5]N.Rougemaille et al, Phys. Rev. Lett. 106, 057209
(2011); [6] S. Zhang et al, Phys. Rev. Lett. 109, 087201 (2012); [7] I.A.
Chioar et al, submitted to Phys. Rev. B.
S20206 Effective Thermodynamics through Demagnetiza-
tion in Artificial Spin Ice. I.A. Chioar, N. Rougemaille, M. Hehn,
D. Lacour, F. Montaigne, B. Canals —Artificial spin ice systems can
offer new insights into how frustrated spin systems gradu-
ally accommodate magnetic frustration effects [1]. By using
lithography techniques, artificial networks of nanomagnets
can be manufactured with the desired shape, size and net-
work topology, while nano-characterization techniques can
yield an exhaustive knowledge of the system’s magnetic con-
figuration in real space. However, in most cases, such net-
works are insensitive to thermal fluctuations due to the rel-
atively highmagnetic anisotropy barrier of the nano-magnets
that prevents the reversal of their magnetization. On the
bright side, this can ease the measuring process, as the mag-
netic state of a pseudo-spin is stable during probing, but, on
theotherhand, itmakes it difficult to explore thedifferent en-
ergetic manifolds and observe how the system gradually ac-
commodates frustration effects. Therefore, for such arrays,
demagnetization protocols have been mostly used to make
the systems evolve towards their ground states where exotic
phases are expected [2-4]. To which extend this procedure
mimics thebehaviorof an identical, but thermalized systemof
spins, is ahighly intriguingandaddressed issue [5-8]. Wehave
investigated numerically the links between demagnetized ar-
tificial arrays of nanomagnets and their equivalent thermal-
ized versions made of Ising spins. To this end, Monte Carlo
simulations have been performed for the study of the ther-
modynamic behaviour of such systems, while numerical de-
magnetization codes have beendeveloped to investigate how
different systemcharacteristics, particularly thedisorder, de-
termine the final magnetic configuration after a demagneti-
zation session. Results show that the behavior of these ar-
tificial networks miraculously resembles that of thermalized
systems as if our networks would present an effective ther-
modynamics. We report our findings in this poster.[1] R. F. Wang et al - Nature, vol. 439, no. 7074, pp. 303–306, Jan.
2006; [2] R. F. Wang et al - J. Appl. Phys., vol. 101, no. 9, p. 09J104,
May 2007; [3] X. Ke et al - Phys. Rev. Lett., vol. 101, no. 3, p. 037205,
Jul. 2008; [4] N. Rougemaille et al - Phys. Rev. Lett., vol. 106, no. 5,
p. 057209, Feb. 2011; [5] C. Nisoli et al - Phys. Rev. Lett., vol. 98, no.
21, p. 217203,May2007; [6] C. Nisoli et al - Phys. Rev. Lett., vol. 105,
no. 4, p. 047205, Jul. 2010; [7] Z. Budrikis et al - Phys. Rev. Lett., vol.
109, no. 3, p. 037203, Jul. 2012; [8] P. E. Lammert et al - New J. Phys.,
vol. 14, no. 4, p. 045009, Apr. 2012.
S20207 Parity violation of charge domain sizes in artificial
spin ice. F. Montaigne, D. Lacour (Institut Jean Lamour, Universit�
de Lorraine and CNRS), I. A. Chioar, N. Rougemaille (CNRS, Inst. NEEL
and Univ. Grenoble Alpes), D. Louis, S. Mc Murtry, H. Riahi (Institut Jean
Lamour, Universit� de Lorraine andCNRS), B. Santos Burgos, T. O.Mentes,
A. Locatelli (Elettra-Sincrotrone Trieste), B. Canals (CNRS, Inst. NEELand
Univ. Grenoble Alpes), M. Hehn (Institut Jean Lamour, Universit� de Lor-
raine andCNRS) —Acrystal of emergingmagnetic charges is ex-
pected in the phase diagram of the dipolar kagome system. A
partial observation of charge crystallites in thermally demag-
netized artificial spin ice arrays has been recently reported
by S. Zhang [ZHA] and coworkers and explained through the
thermodynamics of the system as it approaches a charge-
ordered state.
Following a similar approach, we have generated a partial or-
der ofmagnetic charges in an artificial kagom� spin ice lattice
made out of the ferrimagnetic alloyGdCo. Themagnetic con-
figurationsaredeterminedbyXMCD-PEEMafter thermalde-
magnetizationof thenanomagnets above theirCurie temper-
ature. It appears that the size distribution of charge domain
size is peculiar. Considering the smallest domains, their distri-
bution isnotmonotonic: domainsof1, 3and5verticesappear
to be underrepresented. This parity violation in the domain
size distribution is unexpected. Monte-Carlo simulations of
the dipolar spin ice model do not indicate any special behav-
ior for the charge domain distribution.
We show that only an out-of-equilibrium kinetic process can
explainourexperimentaldata. Asimplemodelbasedonapro-
gressive (and irreversible) ”remagnetisation” of the network
reproduces qualitatively the observed features. This work
shows that despite thermal ”demagnetisation” is a very effi-
cient process to probe the behavior of artificial spin systems,
kinetic process can have a major importance when the mate-
rials is heated above its Curie temperature.
S20208 Frustration study in artificial quasi-crystal. Dong
Shi, G.Burnell, C.H.Marrows, A.Stein — Frustration, especially ge-
ometry frustration plays important role in understanding the
groundstate, phase transitionanddynamicprocessofphysics
systems. As intermediate system between periodic crystal
Queen’s College Cambridge 87
HFM2014
and disorder system, magnetic quasi-crystal, the mechanism
of spin order still mysterious. Here, with the help of electron-
beam lithography, we have build a nano-fabricated artificial
magnetic quasi-crystal model which possess similar frustra-
tion in ”real” material. The magnetic moment of each island
which mimic the ”spin” can be imaged directly by microscopy
technique. The collective behaviour of array in as fabricated
state and rotational demagnetized state was investigated.
Both of these methods which have been performed on con-
ventional artificial spin ices, and are regard as themost effec-
tiveways could bring the system close to its ground state. On
theotherhand, a groundstate candidate is predicted theoret-
ically, which is constructed using unit decagons and consist of
skeleton part and flippble part. It turned out that the ground
state ismulti-degeneracy causedbygeometry frustration. No
closematch to the ground statewas found in our experiment.
The order parameter calculation shows short range order be-
haviour only. More experiment method seeking the ground
state is undergoing.
S20209 Electrical transport measurements on honeycomb
artificial spin ice. K. Zeissler (Imperial College London), M. Chadha
(Imperial College London), L. Cohen (Imperial College London), W. Bran-
ford (Imperial College London) —Artificial spin ice is amacroscopic
playground for magnetically frustrated systems. We have
previously shown that in a cobalt honeycombartificial spin ice
composed of 1µm long nanowires there are unusual features
in themagnetotransport below50K.Hereweexplore the low
temperature transport of equivalent artificial spin ice struc-
tures fabricated from permalloy. We discuss the extent to
which the phenomenon is generic to the honeycomb artificial
spin ice geometry and the effect of changing the constituent
material on the onset temperature and the magnitude of the
magnetotransport effect.
03 Spin Ice (theory)
S20301 Coulombic spin liquids: reloaded. O. Benton (Ok-
inawa IST), H. Yan (Okinawa IST), L. Jaubert (Okinawa IST), N. Shannon
(Okinawa IST) — The discovery and subsequent understanding
of the Coulombic spin liquid state in the “spin ices” Ho2Ti2O7
and Dy2Ti2O7 has been a landmark achievement in the study
of frustrated magnetism [1-3]. Systems realizing Coulom-
bic ground states avoid conventional order down to T = 0
and possess a set of conserved fluxes, with thermally excited
topological defects interacting according to Coulomb’s law.
These systems are thus able to provide beautiful examples of
emergent electromagnetism. Here, we report the discovery
of a new type of Coulombic spin liquid on the pyrochlore lat-
tice. This spin liquid arises naturally on the phase diagram of
the most general nearest neighbour exchange model for py-
rochlore materials. We show that this spin liquid reproduces
the location, shape and neutron polarisation dependence of
the pinch points observed in Tb2Ti2O7 [4] and the apparent
“dimensional reduction” seen in Yb2Ti2O7 [5]- both features
which are not predicted by other pyrochlore spin liquid mod-
els. Our work shows that Coulombic spin liquid phenomena
are a more general feature of the pyrochlore phase diagram
than previously thought and opens up new avenues for ex-
ploring the rich physics of pyrochlorematerials.[1] M. J. Harris, S. T. Bramwell, D. F. McMorrow, T. Zeiske and K. W.
Godfrey, Phys. Rev. Lett. 79, 2554, (1997); [2] S. T. Bramwell and M.
J. P. Gingras, Science 294, 1495, (2001); [3] C. Castelnovo, R. Moess-
ner and S. L. Sondhi, Nature 451, 42, (2008); [4] T. Fennell, M. Ken-
zelmann, B. Roessli, M. K. Haas and R. J. Cava, Phys. Rev. Lett. 109,
017201, (2012); [5] K. A. Ross, L. R. Yaraskavitch,M. Laver, J. S. Gard-
ner, J. A. Quilliam, S. Meng, J. B. Kycia, D. K. Singh, T. Proffen, H. A.
Dabkowska and B. D. Gaulin, Phys. Rev B 84, 17442 (2011).
S20302 Magnetic moment fractionalisation in a monopole
crystal. M. Brooks-Bartlett, S. Banks, P. Holdsworth, L. Jaubert
and A. Harman-Clarke — The emergent Coulomb phase in spin
ice materials is now well known. Likewise, the monopoles
that emerge as excitations from themagnetic charge vacuum
ground state of spin ice have been extensively studied. In
this work we take a slightly different perspective, tuning the
chemical potential of theemergentmonopoles such that their
formation is favoured over magnetically charge neutral spin
configurations at low temperatures. By arbitrarily excluding
doubly chargedmonopoles ourmodel reveals not only the ex-
pected charge crystallization transition but also a remarkable
juxtaposition of antiferromagnetic Bragg peaks and spin liq-
uid like pinch points in the magnetic structure factor. We ex-
plain these observations in terms of the intrinsic fragmenta-
tion of themagnetic moments.
S20303 Far from equilibrium behaviour of spin ice mate-
rials. C.Castelnovo (Cambridge), R.Moessner (MPIPKS), S.Mostame
(Harvard), S.L.Sondhi (Princeton) — Non-equilibrium physics in
spin ice is a novel setting which combines kinematic con-
straints, emergent topological defects, and magnetic long
range Coulomb interactions. In spin ice, magnetic frustra-
tion leads tohighly degenerate yet locally constrainedground
states. Together, they form a highly unusual magnetic state –
a ”Coulombphase” –whose excitations are pointlike defects –
magnetic monopoles – in the absence of which effectively no
dynamics ispossible. At lowtemperatures, themonopolesare
sparse and dynamics becomes very slow. These systems are
therefore prone to falling out of equilibrium at low temper-
atures, for instance following comparatively rapid changes
in temperature or applied magnetic field. In this regime, a
wealth of dynamical phenomena occur, including reaction
diffusion behaviour, slow dynamics due to kinematic con-
straints, as well as behaviour that mimic the deposition of in-
teracting dimers on a lattice. The situation is further compli-
cates by thepresenceof disorder that, evenat small densities,
Queen’s College Cambridge 88
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appears to have a sizeable effect on the low-temperature dy-
namics of these systems. Here we investigate some of these
phenomena and we propose how to effectively extend exist-
ing theories to to describe spin ice far from equilibrium.
S20304 Quantum spin-liquid phases of quantum spin-ice.
A.G.R. Day (Univ. of Waterloo), Z. Hao (Univ. of Waterloo), M.J.P Gin-
gras (Univ. of Waterloo, CIFAR, Perimeter Institute) —Quantum spin-
ice offers an exciting playground for supporting exotic phases
of matter characterized by fractionalized excitations such as
emergent magnetic monopoles. The quantum spin-ice model
has been recently successfully applied to describe some of
the insulating rare-earth pyrochlore oxides. I will discuss the
possible spin-liquid phases that may be stabilized in quan-
tum spin-ice systems and their possible experimental signa-
tures. Amongst many different implications, quantum spin-
ice physics is believed to be relevant for the description of
rare-earth iridates, that exhibit anomalous Hall effect, and
for thewell experimentally-characterizedmaterialsYb2Ti2O7
and Pr2R2O7 (R = Sn,Zr), for which the low-temperature
state is still a matter of debate.
S20305Emergence of Loops in Spin-Ice. K. Essafi (OIST), L.D.C.
Jaubert (OIST) — In spin-ice systems, both on the pyrochlore
and checkerboard lattices (e.g. artificial spin-ice), the ground
state, called the Coulomb phase, obeys the 2-in - 2-out ice
rule. This constraint leads to the emergence of one dimen-
sional degrees of freedom, loops, which can reach macro-
scopic sizes. These loops can be described by the stochas-
tic (Schramm)-Loewner evolution (SLE)which gives a descrip-
tion of the continuum limit of lattice curves. The SLE is
parametrised by κ which is directly connected to the fractal
dimensionDf of the loops and indicates how dense and wig-
gly the loopsare. Differentvaluesofκcorrespondtodifferent
universality classes. A variant of theCoulombphase conserv-
ing a high degree of degeneracy can be obtained by applying
a magnetic field or a uniaxial pressure. This can lead to the
appearance of a phase transition, e.g. with the uniaxial pres-
sure there is a so-called KDP phase transition between the
Coulomb phase and a ferromagnetic one. We are interested
in the evolution of the loop size distribution and the structure
factor, in particular the pinch points, at the transition.
S20306 Critical behavior in cubic dimer model at finite
monopole fugacity. G. J. Sreejith (NORDITA), S. Powell (Univer-
sity of Nottingham) — The cubic dimer model is a constrained
classical statistical system in which degrees of freedom cor-
respond to the presence or absence of ‘dimers’ on edges be-
tween nearest neighbour lattice points in a cubic lattice. The
dimers fluctuate subject to the constraint that each lattice
point has one and only one edge occupied by a dimer. In the
presence of an aligning interaction between dimers, the sys-
temundergoes a transition fromahigh temperatureCoulomb
phase of fluctuating dimers to a low temperature columnar
ordered phase. This transition is believed to be an uncon-
ventional continuous phase transition, in NCCP1 universality
class. We present a numerical study of the transition when a
small fractionof latticepoints areallowed toviolate thedimer
constraints. Points where dimer constraints are not satisfied
correspond tomonopoles in the critical gauge theory. Wefind
that the finite size scaling behaviour of the systemunder vari-
ation of the monopole fugacity agrees quantitatively with a
spinor deconfinement transition in the fully constrained limit.
In addition, by using Monte Carlo simulations of the statisti-
cal interaction between a pair of monopoles in an otherwise
constrained system, we calculate the scaling dimensions for
monopoles of chargeQ = 1, 2 and 3 at theNCCP1 fixed point.
These monopoles correspond to lattice points with 2, 3 and 4
overlapping dimers respectively.
S20307 Wien Effect in Spin Ice. V. Kaiser (MPI PKS Dres-
den / ENS Lyon), S. Bramwell (UCL / LCN), P. Holdsworth (ENS Lyon), R.
Moessner (MPI PKS Dresden) — Electrolyte theory lends itself to
explaining physics of spin ice, through its mapping to a gas
of magnetic monopoles. This facilitates computing quanti-
tieswhich are otherwise very hard to capture, given the long-
range nature of the interactions. Here, we focus on non-
equilibrium and non-linear response properties, in particular
non-linear susceptibility. The Second Wien effect dominates
magnetolyte dynamics at low temperatures, where the ap-
plication of magnetic field increases the monopole density.
The increase isdrivenbyfield-enhanceddissociationofbound
monopolepairs,whichare subsequently replenished fromthe
spin ice manifold. We extend the seminal theory by Onsager
to describe corrections due to Debye-Hückel screening and
variablemobility in a lattice electrolyte, obtaining a quantita-
tive agreement of theory and simulations. Although theDirac
string network constrains themagnetolyte,we showhowone
can observe and characterize theWien effect in spin ice.
S20308 Saturation field entropies of Ising antiferromag-
nets: application to spin-ice Dy2Ti2O7. Vipin Kerala Varma
— Saturation field entropies Sres. of antiferromagnetic Ising
modelsonquasi-one-dimensional latticesandthekagome lat-
tice are calculated and presented, along with an elucidation
of the zero-temperature field-induced phases in the systems.
We employ Binder’s algorithm on the kagome lattice for effi-
ciently and exactly computing the partition function of over
1300 spins to give Skag.res. /kB = 0.393589(6). Finally we com-
ment on the relation of the kagome lattice to the situation in
the spin-ice compoundDy2Ti2O7, comparingwith earlier the-
oretical and experimental estimates of this value.
S20309QuantumEffects in aRealisticModel of Spin Ice. P.
McClarty (ISIS), R. Moessner (MPIPKS, Dresden), K. Penc (Research Insti-
tute for Solid State Physics and Optics, Budapest), F. Pollmann (MPIPKS,
Queen’s College Cambridge 89
HFM2014
Dresden), N. Shannon (OIST), O. Sikora (National Taiwan University) —
The dipolar spin ice model provides an excellent means of
understanding the leading behaviour of the classical spin liq-
uid in Ho2Ti2O7 and Dy2Ti2O7. However, there is evi-
dence from experiment that there are perturbations in the
form of further neighbour couplings of exchange origin. In
this study, we use exact diagonalization and quantumMonte
Carlo simulation together with large N and classical Monte
Carlo to explore the phase diagram of a realistic model for
the spin ice materials including the effects of quantum tun-
nelling between different ice states. The phase diagram turns
out to be fairly simple despite the large number of compet-
ing states. We find that the competition between long-range
dipolar interactions and second-neighbour exchange inter-
actions helps to stabilize a quantum spin liquid phase and,
for large enough exchange, a ferromagnetic ground state.
We discuss the implications of these results for the spin ice
Dy2Ti2O7.
S20310Excitations in quantumspin ice: an analytical study.
O. Petrova, R. Moessner (Max Planck Institute for the Physics of Complex
Systems) —Typical spin icematerials canbemodeledusingclas-
sical Ising spins. The geometric frustration of the pyrochlore
lattice causes the spins to satisfy ice rules,whereas aviolation
of the ice constraint constitutes an excitation. Flipping adja-
cent spins fractionalizes the excitation into two monopoles.
Long-range dipolar interactions between magnetic moments
give rise to an effective Coulomb interaction between the
emergent magnetic charges. In a classical setting, the spin
flips arise due to thermal effects and applied magnetic fields.
Recent experimental evidence points to quantum fluctua-
tions as another likely source of spin flips. We study the fea-
tures of quantumspin ice expected tobevisible in experiment
which distinguish it from the purely classical setting, focusing
on both features of the spectrum and signatures in neutron
scattering experiments.
S20311 Unpaired Majorana fermions on dislocations in Ki-
taev’s honeycomb spin model. O. Tchernyshyov (Johns Hopkins
University), P. Mellado (Adolfo Ibáñez University), O. Petrova (Max Planck
Institute for the Physics of Complex Systems) —Kitaev’s honeycomb
model is an example of an exactly solvable quantum spin liq-
uidwith aZ2 gauge structure. The response of a spin liquid to
external perturbations is an important topic that has not yet
been explored in sufficient depth. Willans et al. have shown
that even trivial lattice defects such as vacancies induce un-
usual local excitations in the form of free spins possessing
only one Cartesian component (the other two are nonlocal
degrees of freedom). Here we show that topological lattice
defects such as dislocations give rise to fractionalized excita-
tions in the form of unpairedMajorana fermions. Physical ex-
citations associatedwith these defects are (complex) fermion
modes made out of two (real) Majorana fermions connected
by a Z2 gauge string. The quantum state of these modes is
robust against local noise and can be changed by winding a
Z2 vortex around a dislocation. The exact solution respects
gauge invariance and reveals a crucial role of the gauge field
in the physics ofMajoranamodes.
S20312 Impurities in Spin Ice Crystals. G. Sala (TCM group,
Royal Holloway University of London and ISIS Facility), C. Castelnovo
(TCM group, Cavendish Laboratory, University of Cambridge), D. J. Porter
and J. P. Goff (Royal Holloway University of London), M. J. Gutmann
(Rutherford Appleton Laboratory, ISIS Facility), D. Prabhakaran (Claredon
Laboratory, University of Oxford), D. Pomaranski, C. Michelitis and J. Ky-
cia (University of Waterloo). — Spin ice crystals (and pyrochlore
oxides in general) have raised a lot of interest of late thanks
to their exotic properties, including emergent gauge symme-
tries, possible spin liquid behaviour, and magnetic monopole
excitations. Theoretical and experimental efforts in the study
of these materials have benefited from the relative ease of
growth of large clean single crystals. Even in such clean
systems, however, impurities can play a crucial role in de-
termining the properties at very low temperatures (see e.g.,
H. M. Revell, Nature Phys. 9, 34-37 (2013) or C. Henley,
http://arxiv.org/abs/1210.8137,). Here we investigate this is-
sue both experimentally and theoretically. We study how
oxygen deficiency affects spin ice samples, altering the effec-
tivemonopoledescriptionandthethermodynamicproperties
of the system at low temperatures. We also discuss how the
oxygen stoichiometry canbequantifiedand controlled exper-
imentally.
S20313Adynamic Jahn-Teller coupling todescribe thevery
low temperature field-induced magnetic structures in the
Tb2Ti2O7 spin liquid. A.P.Sazonov (RWTHAachen University; JCNS
Outstation at MLZ; CE-Saclay, Laboratoire Léon Brillouin), I.Mirebeau,
A.Gukasov (CE-Saclay, Laboratoire Léon Brillouin), H.B.Cao (CE-Saclay,
Laboratoire Léon Brillouin; ORNL, Quantum Condensed Matter Division),
P.Bonville (CE-Saclay, Service de Physique de l’Etat Condensé), S.Petit,
J.Robert (CE-Saclay, Laboratoire Léon Brillouin), E.Ressouche (SPSMS,
UMR-E CEA/ UJF-Grenoble 1), B.Grenier (SPSMS, UMR-E 9001, CEA-
INAC/UJF-Grenoble 1), C.Decorse, G.Dhalenne (ICMMO UMR 8182,
Université Paris-Sud). — We have studied the field-induced
magnetic structures of Tb2Ti2O7 pyrochlore by single-crystal
neutron diffraction under a field applied along the [111] and
[110] axes, up toH = 12T and down to T = 40mK. We re-
fined the magnetic structures with k = 0 propagation vec-
tor by performing a symmetry analysis in the space groups
R3̄m for H ∥ [111] and I41/amd for H ∥ [110], significantly
reducing the number of free parameters. For a field applied
along [111], the Tb moments gradually reorient towards the
field direction, keeping close to a “3-in, 1-out / 1-in, 3-out”
spin structure in the whole measured field range 0.05–12T.
Queen’s College Cambridge 90
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Our results rule out the “all-in / all-out” structure previously
proposedanddonot support theexistenceof amagnetization
plateau [1]. For a field along [110], the spin-ice-like structures
are observed for a misalignment of a few degrees, whereas
other structures, where the Tb-β moments flip by “melting”
on the field axis, occur when the field is perfectly aligned [2].
We perform a quantitative comparison with mean-field cal-
culations and we propose the presence of a low-temperature
dynamic symmetry breaking of the local trigonal symmetry,
akin to adynamic Jahn-Teller effect, i.e. preserving theoverall
cubic symmetry. The evolution of the magnetic structure de-
duced from our neutron diffraction data below 1K and in the
field range0.05-12T, forbothH ∥ [111]andH ∥ [110], arewell
accounted for by thismodel. Wediscuss the possible origin of
this off-diagonal mixing term in the crystal field hamiltonian
in terms of quadrupole-quadrupole interaction or magneto-
elastic effects.[1] A.P.Sazonov et al. PRB 88, 184428, (2013); [2] A.P.Sazonov et al.
PRB 82, 174406, (2010).
S20314 Monopole Hopping through Quantum Spin Tun-
nelling in Spin Ice. B. Tomasello, G. Sala, J.Quintanilla, C. Castelnovo,
R. Moessner —The low temperature dynamics in spin icemate-
rials is governedby thedensity andmobility of elementaryex-
citations that behave as emergent magnetic monopoles. The
diffusion of such monopoles proceeds via flipping of large
electronic spins with Ising-like anisotropy (due to their crys-
tal field environment). Experimental evidence suggests that,
at temperatures relevant for spin ice physics, this flipping
occurs as a quantum-mechanical tunnelling through a large
anisotropy barrier. Here we investigate this process at the
microscopic, single-ion level by computing the quantum dy-
namics resulting from the interplay between the crystal field
Hamiltonian and the Zeeman coupling with magnetic fields
(either applied or due to other spins). We interpret our re-
sults in termsofmonopolehopping rates, andwecompareour
predictions with existing experiments for bothHo2Ti2O7 and
Dy2Ti2O7.
S20315 A Unifying Field Theory for The Pyrochlore Lat-
tice. H. Yan, O. Benton, L.D.C. Jaubert, N. Shannon —Different py-
rochlore materials have distinct properties, for example, the
apparent dimensional reduction of Yb2Ti2O7 and the order-
by-disorder phase transition of Er2Ti2O7. Here we present a
field theorybasedonsymmetryanalysis of thepyrochlore lat-
tice, and show how it provides a general approach to explain
various properties of different pyrochlorematerials. The the-
ory provides straight forward method to identify the ground
stateofall phases fornearest-neighbouranisotropiccoupling.
It also describes the exotic physics emerging on the phase
boundaries, and its entropic influence in the ordering proce-
dure of neighbouring phases.
S20316 Spin ice is not highly correlated. Is it? T. Yavors’kii
(AMRC, Coventry University) —The beauty, richness and complex-
ity of the frustratedmagnetismbecomes apparent at temper-
atures T , lower than the scale J of the leading spin interac-
tions, where spin systems can form highly correlated, degen-
erate states. In this presentation I use Monte Carlo simula-
tions on graphics processing units [1,2] to study the classical
nearest-neighbor antiferromagnetic Ising spin models on the
pyrochlore, aswell askagomeandgarnet lattices, fromT ≫ J
down to T ≪ J .
I show that, down to the degenerate ground state manifolds
at T ≪ J , the spin correlations in the models show features
consistent with the picture, usually thought of as describing
spin models in their non-correlated regimes at T ≫ J . Statis-
tical physics properties of the models, such as the pair corre-
lation function, are demonstrated to bewell described by the
variational single-particle mean-field theory [3] (MFT) ansatz
at all T ≥ 0, provided the MFT temperature scaleΘ, where
Θc < Θ < ∞, is mapped onto the physical temperature scale
0 ≤ T < ∞ by considering Θ as a suitable function of T .
The models are thus completely “transparent” to the param-
agnetic MFT treatment deep below the MFT critical temper-
ature Θc > 0, which might speak in favor of using MFT as a
simple andpowerful tool for the studyof perturbations at low
T in these, and other [4], systems.[1] Computer Simulations on Graphics Processing Units, eds. M.Weigel,
A. Arnold and P. Virnau, Eur. Phys. J. Special Topics 210, (2012);
[2] The GPU code used here has many common elements with the
code described in: T. Yavors’kii and M. Weigel, p. 159 of Ref. [?]; [3]
P.M.Chaikin andT.C. Lubensky,Principles of CondensedMatter Physics
(Cambridge University Press, Cambridge, UK, 1995); [4] Taras Ya-
vors’kii, Matthew Enjalran, Michel J.P. Gingras, Phys. Rev. Lett. 97,
267203 (2006).
S20317 Theoretical study of spectral andmagnetic proper-
ties of Tb2Ti2O7 : impact of random strains. B. Malkin, V.
Klekovkina (Kazan Federal University) — The crystal field ground
state of the Tb3+ ions in the geometrically frustrated py-
rochlore oxide Tb2Ti2O7 is a non-Kramers doublet with the
g-factor of about 10. A specific feature of this compound
is very strong magnetoelastic coupling that manifests itself
in the low-temperature giant forced magnetostriction and
anisotropic lattice softening. Contrary to expectations, nei-
ther magnetic long range ordering, nor structural phase tran-
sitions have been observed down to 0.05K. The results of dif-
fuse and inelastic neutron scattering measurements and ex-
perimental data onfield dependencies of the isothermalmag-
netization at very low temperatures have been interpreted
recently by P. Bonville et al. (see [1] and references therein) in
the frameworkof amodelbasedon theassumption for tetrag-
onal lattice distortions of fixed value. This assumption lead-
ing to a singlet ground state of the Tb3+ ions agrees with the
spin-liquid behavior of Tb2Ti2O7 but it contradicts to the ob-
Queen’s College Cambridge 91
HFM2014
servedmagnetic diffuse neutron scattering and the hyperfine
contribution to the specific heat. We showhere that the rein-
terpretation of this model allows to remove the contradic-
tions mentioned above. More than half a century ago, elec-
tron paramagnetic resonance measurements gave evidence
for continuous distribution of non-Kramers doublet splittings
in rare earth compounds [2] due to random lattice strains. It
is important that there are at least two independent strain
modes contributing into thedoublet splitting. So, the splitting
value introduced in [1] can be considered as the most proba-
bleone. Weanalyzedifferentphysical propertiesofTb2Ti2O7
accounting for low-symmetry random crystal fields induced
by point defects. The extensive observables are averaged
over the generalized distribution function of local strains pro-
duced by point defects in the elastic continuum [3]. Parame-
ters of the electron-deformation interaction were estimated
in the framework of the exchange charge model and verified
by the analysis of magnetoelastic effects [4]. The derived mi-
croscopic approach operates with a single varied parameter,
that is the sample dependent width of the strain distribution
function proportional to a concentration of lattice defects.
The results of simulations of the inelastic neutron scatter-
ing intensity vs energy transfer, paramagnetic and staggered
field dc susceptibilities (single site or tetrahedron mean field
approximations are employed), temperature dependencies of
the specific heat and elastic constants, field dependencies of
magnetic moments at different Tb3+ sites will be presented.
As follows from calculations, magnetic and structural phase
transitions are suppressed in crystals with plausible concen-
trationsof latticedefects. This researchwas supportedby the
RFBRGrant 14-02-00826.[1] Bonville P. et al., Phys. Rev. B 89, 085115 (2014); [2] Baker J.M.
and Bleaney B., Proc. Roy. Soc. Lond. A 245, 156 (1958); [3] Malkin
B.Z. et al., Phys. Rev. B 86, 134110 (2012); [4] Klekovkina V.V. et al., J.
Phys.: Confer. Ser. 324, 012036 (2011).
S20318 Lattice distortions in frustrated systems: spin ice.
R.A. Borzi, F. Albarracin, H. D: Rosales, A. Stepke, G. Rossini, D. Prab-
hakaran, A.P. Mackenzie, D.C. Cabra, S.A: Grigera —We study the ef-
fects of lattice deformations on spin ice, with Ising spins cou-
pled by nearest neighbor exchange and long range dipolar in-
teractions in the presence of a magnetic field. We describe
the lattice energy according to the Einstein model and inte-
grate out the phonon degrees of freedom. The resulting ef-
fectiveHamiltonian is studiedbyMonteCarlosimulations, un-
der different directions of themagnetic field. We find that, as
theeffectof thedeformation is increased, a richplateaustruc-
ture appears in the magnetization curves for a restricted 2D
version (Kagome ice). In the 3D case this simplemodel repro-
duces several experimentally observed features, which can-
not be accounted for within a pure spinmodel.
04 LowDimensional System (theory)
S20401 Finite-temperature dynamics of highly frustrated
quantum spin chains. A. Honecker (Göttingen University, Germany),
B. Normand (RenminUniversity of China, Beijing) —Highly-frustrated
magnets are characterized by a (nearly) flat one-triplet exci-
tation band at zero temperature, resulting in correspondigly
high degeneracies in the spectrum. Little is known from
theoretical studies about the temperature-dependence of
this single-particle dispersion and less still concerning multi-
particle dynamics at finite temperature. Experimentally, in-
elastic neutron scattering studies of low-dimensional frus-
trated systemssuchasSrCu2(BO3)2 requirean interpretation
of the thermal evolution of scattering intensities. We investi-
gate these issues using the example of a highly frustrated spin
ladder and present numerical results from exact diagonalisa-
tion for the dynamic structure factor as a function of temper-
ature. We find anomalously rapid transfer of spectral weight
out of the single-particle band to a wide range of energies.
Nevertheless, single- andmany-particle excitations persist as
sharp spectral features to surprisingly high and even infinite
temperatures.
S20402 Phase diagram of the alternating-spin Heisenberg
chain with extra isotropic three-body exchange interac-
tions. N. B. Ivanov (Fakultät für Physik, Uni-Bielefeld,; ISSP, Bul-
garian Academy of Sciences), J. Ummethum, and J. Schnack (Fakultät
für Physik, Uni-Bielefeld) — For the time being the impact of
extra three-body exchange interactions on Heisenberg spin
systems is scarcely explored. These interactions have been
mostly used as a tool for constructing various exactly solv-
able one-dimensional models although they can be expected
to support specific quantum effects and phases. In this work,
based on numerical density-matrix renormalization group
and exact diagonalization calculations, we explore the quan-
tumphase diagramof theHeisenberg chain constructed from
alternating S = 1 and σ = 12site spins which de-
fines a realistic prototype model of this kind admitting extra
three-body exchange terms. We demonstrate that the addi-
tional exchange termsstabilize avarietyofpartially-polarized
(plateau) states as well as two specific non-magnetic states,
i.e., a critical spin-liquid phase controlled by two Gaussinal
conformal theories as well as a critical nematic-like phase
characterized by dominant quadrupolar S-spin fluctuations.
Most of the established effects reflect specific features of the
three-body exchange interaction such as the promotion of lo-
cal collinear spin configurations and the enhanced tendency
towards nearest-neighbor clustering of the spins. It may be
expected that most of the predicted effects of the isotropic
three-body interaction persist in higher space dimensions.
S20403 Ground-state phase diagram of an anisotropic
rung-alternating S=1/2 ladder. T. Tonegawa (Kobe
Queen’s College Cambridge 92
HFM2014
Univ. and Osaka Pref. Univ.), K. Okamoto (Tokyo. Inst. Tech.), T. Hik-
ihara (Gunma Univ.), T. Sakai (SPring-8 and Univ. Hyogo), J. Mor-
ishige (Kyushu Univ.), K. Nomura (Kyushu Univ.) — We numer-
ically explore, with the help of some physical considera-
tions, the ground-state phase diagramof an anisotropic rung-
alternating S=1/2 ladder. We express the Hamiltonian de-
scribing the system as H = Jl∑
j
{S⃗j,a · S⃗j+1,a + S⃗j,b ·
S⃗j+1,b
}+ Jr
∑j
{λx,yr (Sx
2j−1,aSx2j−1,b + Sy
2j−1,aSy2j−1,b) +
λzr S
z2j−1,aS
z2j−1,b
}+ J ′
r
∑j
{λx,yr (Sx
2j,aSx2j,b + Sy
2j,aSy2j,b) +
λzr S
z2j,aS
z2j,b
}, where S⃗j,α is the S=1/2 operator at site j
on rung α(= a or b); Jl the magnitude of the isotropic leg
interaction; Jr and J ′r those of the two kinds of anisotropic
rung interactionswhicharealternating;λxyr andλz
r theXXZ-
type anisotropy parameters of the rung interactions. This
system has a frustration when JrJ′r <0 irrespective of the
sign of Jl. Assuming Jl=0.2, Jr=−1 and |J ′r |≤1, we de-
termine the ground-state phase diagrams in the case where
the anisotropy of the rung interactions is of the Ising-type
(λzr =1, 0≤λx,y
r ≤1) and in the case where it is of the XY -
type (λxyr =1 and 0≤λz
r ≤1). The obtained phase diagram in
the former case consists of the ‘ferromagnetic’-‘singlet dimer’,
Haldane and antiferromagnetic stripe Néel phases. In par-
ticular, we find that when the system has the frustration
(0 <J ′r ≤1), the incommensurate state becomes the ground
statewithin the region of theHaldanephase. The appearance
of the Haldane phase in the case of Ising-type interactions is
contrary to the ordinary situation, and this is called the inver-
sion phenomenon concerning the interaction anisotropy. On
the other hand, the phase diagram in the latter case includes
the ‘triplet dimer’-‘singlet dimer’, Néel, Haldane and triplet
dimer phases. We find that the phase transitions between
the Néel phase, the region for which is within the frustrated
region, and one of the other three phases are the first-order
transitions. The appearance of the Néel phase in the case of
XY -type interactions is also the inversion phenomenon con-
cerning the interaction anisotropy. The inversion phenomena
which we have found are attributed to the frustration effect.
S20404 Symmetry-protected topological phases in frus-
trated spin-1/2 zigzag chain. H. Ueda (RIKEN), S. Onoda (RIKEN)
—Our recent density-matrix renormalization group (DMRG)
study on a frustrated spin-1/2 XXZ zigzag chain has revealed
that a bondalternation in ferromagnetic nearest-neighbor in-
teractions drives an otherwise gapless vector-chirality long-
range ordered phase into two gapped phases (VCD+ and
VCD−) with finite vector-chirality and dimer long-range or-
ders [1]. These two phases can be discriminated from each
other by the sign of the z-component dimer order parame-
ter, a string order parameter, and the entanglement spec-
tra but not by the symmetry. Here, using the infinite-size
DMRG method, we show these phases are indeed classified
as distinct symmetry-protected topological phases according
tohowthestate transformsunderZ2×Z2 (aC2 rotationanda
mirror) and time-reversal symmetry operations. We will also
clarify the criticality of a possible topological transition be-
tween the VCD+ and VCD− phases. A relevance to a spin-
gapped material Rb2Cu2Mo3O12 are discussed, including a
field-induced gapped vector-chirality, and thus ferroelectric,
order through a staggered scalar chiral ring-exchange inter-
action.[1] H. Ueda and S. Onoda, Phys. Rev. B 89, 024407 (2014).
05Other Frustrated Spin Systems (experiments)
S20501 Order in the short ranged ordered state of
Gd3Ga5O12. P. Deen (European Spallation Source/Niels Bohr
Institute, University of Copenhagen), H. Jacobsen (Niels Bohr Institute,
University of Copenhagen/European Spallation Source), O.A.Petrenko
(Warwick University), J. Paddison (Oxford University/STFC), M.T.
Fernandez-Diaz (Institut Laue-Langevin) — Despite considerable
theoretical and experimental attention to the magnetic state
of the frustrated compound with the highest frustration
index, Gd3Ga5O12 (GGG), there is little insight into the
spin correlations of the low temperature disordered phase
[1,2,3,4]. We present powder and single crystal neutron
scattering results on the short ranged ordered magnetic
state of this archetypal frustrated hyperkagome compound
using the recently developedReverseMonteCarlo technique
to compare the expected single crystal scattering profile
determined from powder neutron scattering to single crystal
neutron diffraction profiles obtained using the hot neutron
four circle diffractometer of the ILL, D9. These results reveal,
for the first time, the neutron scattering profile of GGG in the
[h k 0] plane. The magnetic order is composed of fluctuating
dimerised magnetic ions and magnetic triangular loops. The
spin directions are strongly perturbed with strong magnetic
components along the crystalline axes and in the [1 11] plane.
Bergholtz [5] indicates that the magnetic ground state of a
hyperkagome crystalline structure are interlinked decagons
loops surrounded by dimers and trimers. It is possible that
the magnetic correlations observed in GGG at 0.175 K are a
precursor. These salient results shed much light on magnetic
frustration in GGG.[1] P. Schiffer, A. Ramirez, D. Huse, P. Gammel, U. Yaron, D. Bishop
and A. Valentino, Phys. Rev. Lett 74 2379 (1995); [2]O.A. Petrenko.
D. Mck Paul, C. Ritter, T. Zeiske and M. Yethiraj, Physica B 266, 41
(1999); [3]W. I. Kinney andW. P.Wolf, J. Appl. Phys. 50, 2115 (1979);
[4] P. P. Deen, O. Petrenko, G. Balakrishnan, B. Rainford, C. Ritter, L.
Capogna, H. Mutka and T. Fennell, Phys. Rev. B 82, 174408 (2010);
[5]E. J. Bergholtz. A.M. Läuchli andR.Moessner, Phys. Rev. Lett. 105,
237202 (2010).
S20502 Updated phase diagram of the frustrated magnet
Gd3Ga5O12. O. Florea (Institut Néel, CNRS, Grenoble, France), E.
Lhotel (Institut Néel, CNRS, Grenoble France), P. Deen (ESS, Lund, Swe-
den), H. Jacobsen (Niels Bohr Institute, University of Copenhagen, Den-
Queen’s College Cambridge 93
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mark), O. Petrenko (Department of Physics, University ofWarwick, United
Kingdom), H. Mutka (ILL, Grenoble, France) — Gd3Ga5O12 (GGG)
is one of the very few realizations of the highly frustrated
hyperkagome lattice with antiferromagnetic interactions. In
this three-dimensional structure, the magnetic Gd ions are
on two interpenetrating corner sharing triangular sublattices.
No long range order has been found down to 25 mK [1], de-
spite a Curie-Weiss temperature of -2 K, but a spin freezing
was reported below 0.14 K [2].
The (H,T) phase diagram has long been understood in terms
of distinct regions with short range order below 800mK sup-
plementedwithdistinct clustered regionsof incommensurate
(IC) correlations below 0.14 K [2,3]. According to this phase
diagram the application at very low temperature (60 mK) of
only 0.7 T results in a stable antiferromagnetic (AF) region up
to 1.2 T. This region extends out to 400mK.
Our polarized neutron diffraction could not be reconciled
with this phase diagram: the scattering profiles show a
smooth and subtle variation with applied magnetic field, in
contrast with the distinct regions described above. To under-
stand these scattering features, we performed a systematic
study of themagnetization at very low temperature on 160Gd
powder and 157Gdsingle crystal samples to redefineprecisely
the phase diagram. It appears that the phase diagram ismuch
more complicated than previously realized [4] with several
new phases in competition.[1] S. Hov et al, J.Mag. andMagn. Mat. 15-18 (1980); [2] P. Schiffer et
al, Phys. Rev. Lett. 73, 2500 (1994); [3] O. Petrenko et al, Phys. Rev.
Lett. 80,4570 (1998); [4] O. Petrenko et al. J.Phys: Conf. Ser. 145,
012026 (2008).
S20503 High magnetic field studies on frustrated ferric an-
tiferromagnets. M. Hagiwara, T. Fujita (CAHMFS, Grad. Sch. Sci.,
Osaka Univ., Japan) —We will report on the results of magne-
tization and multi-frequenciy electron spin resonance (ESR)
experiments on some ferric frustrated antiferromagnets in
high magnetic fields of up to 55 T measured at the Center
for Advanced High Magnetic Field Science, abbreviated as
CAHMFS, (previous affiliation: KYOKUGEN) in Osaka Uni-
versity. We plan to give two topics, namely, the triangu-
lar lattice CuFe1−xGaxO2 that shows interesting successive
field induced transitions along the c-axis for x=0, and the
Kagome lattice K-Fe-jarosite in which the origin of magnetic
anisotropy was clearly found to be the DM interaction from
our experiment and its analysis.
S20504 Observation of magnon decay in LuMnO3. M.D.
Le (Institute for Basic Science, Korea), J. Oh (Seoul National University),
J. Jeong (SNU), J.-H. Lee (SungKyunKwan University), W.-Y. Song (SKKU),
H. Woo (ISIS Facility), T.G. Perring (ISIS), W.J.L. Buyers (Canadian Neutron
Beam Centre), Z. Yamani (CNBC), S.-W. Cheong (Rutgers University), J.-G.
Park (IBS & SNU) —We report the observation of magnon de-
cay in the frustrated triangular lattice Heisenberg antiferro-
magnet LuMnO3. BelowTN=90K, it adopts the non-collinear
120o structure, and this non-collinearity is predicted by the-
ory to allow coupling between one-magnon and two-magnon
states and thus decay of single magnons into two. At certain
points in the Brillouin zone, we observed that the highest en-
ergy spin wave mode becomes exceptionally broad, indica-
tive of this decay. Additional features of the spin wave dis-
persion, including a dispersion minima at the zone boundary
and anomalous polarisation dependenceof themagnons near
thisQ also point to a departure from linear spin wave theory.
Not all the observed differences can be yet explained by the-
ory, however, which may be due to the trimerisation of the
spins arising from the giant off-centering of theMn ions in the
triangular plane. Interestingly, recent inelastic neutron scat-
tering experiments show that YMnO3, in which the Mn ions
are displaced in the opposite direction (away from the central
O rather than towards as is the case for LuMnO3) resulting
in an ”anti-trimerisation”, also shows evidence of magnon de-
cay. Magnon-magnon interactions are thus evidently insensi-
tive to the details of the spin Hamiltonian, and also do not re-
quire low spin systems to manifest, which suggests that simi-
lar behaviourmay be observed in other non-collinear antifer-
romagnets.
S20505 Frustration in the Cairo pentagonal lattice antifer-
romagnet Bi2Fe4O9. M.D. Le (Institute for Basic Science, Korea), J.
Jeong (Seoul National University), K. Ramesh-Kumar (IBS), Y. Jo (Kyung-
pook National University), J.-S. Kim (Pohang University of Science and
Technology), K. Kindo (University of Tokyo), U. Stuhr (Paul Scherrer Insti-
tut), J.-G. Park (IBS & SNU). —We present the spin exchange in-
tegrals of Bi2Fe4O9, as determined by inelastic neutron scat-
tering, and its high field magnetic phase diagram from mag-
netisation measurements in pulsed magnetic fields. Frustra-
tion in the Cairo pentagonal lattice arises from competition
between the two inequivalent nearest neighbour exchange
interactions in the lattice, J33 and J43. When the ratio x =
J43/J33 is low, anorthogonal antiferromagnetic structure re-
sults, as is the case forBi2Fe4O9, even though it deviates from
the ideal Cairo lattice in that there are two differentJ43 cou-
plings. We find that for both, x < 1 in agreementwith theory.
The spin Hamiltonian, in a mean-field treatment, also quali-
tatively reproduces the experimentallymeasuredmagnetisa-
tion and magnetic phase diagram. Remarkably, the frustra-
tion effects (e.g. θCW/TN ≈ 7) persist despite a large inter-
layer coupling, which may be due to an unexpectedly large
easy-plane anisotropy term (and corresponding gap in the ex-
citations at Γ) for the Fe3+ ion, preserving the essential 2D
physics of the frustrated geometry.
S20506 Universal memory effects observed after tempo-
rary heating/cooling in Heisenberg spin glasses and spon-
taneous restoration of the spin configuration existing be-
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fore. H. Mamiya (NIMS), N. Tsujii (NIMS), N. Terada (NIMS), S. Ni-
mori (NIMS), H. Kitazawa (NIMS), A. Hoshikawa (Ibaraki Univ.), T. Ishi-
gaki(Ibaraki Univ.) — Spin-glass has been one of the most-
studied glassy systems; however, little is known regarding
their nature. One of the reasons is that non-equilibrium
relaxations continue throughout experiments over a period
of weeks whereas various intriguing equilibrium states have
been theoretically proposed till now. In this study, we care-
fully reexamine the features of the slow relaxations them-
selves, in order to clarify the nature by a detailed comparison
with the energy landscapes predicted by the theories. Sam-
ples arematerials regarded as a kindofHeisenberg spin glass:
a dilute magnetic alloy Cu97Mn3 [1], a dilute magnetic semi-
conductor Cd55Mn45Te [2], and a geometrically frustrated
magnet ZnFe2O4 [3]. In our typical experiments, each sam-
ple was initially subjected to a magnetic field for a signifi-
cant duration (aging.) Then the field was cut off and decay
of thermoremanent magnetization was recorded on various
thermal histories. Consequently, we found that the decay
was extremely accelerated when the sample was temporar-
ily heated/cooled midway through the isothermal relaxation.
In contrast, in the cases that the sample was sufficiently aged
in themagneticfield, the thermoremanentmagnetizationsur-
prisingly increased despite absence of magnetic field when
the temperature returned to the original after the temporary
heating/cooling. In other words, the memory in the aging pe-
riod returns despite once being rejuvenated. These phenom-
ena were commonly observed in the three samples. Because
the magnetization mirrors evolution of the spin configura-
tion, these acceleration and reversion of the decay indicate
that the configuration is destabilized when the temperature
changes and it is spontaneously restored when the tempera-
ture is returned to the original. Whereas such destabilization
and restoration do not occur if the spin glass is simply frozen,
it is possible in an energy landscape with a temperature- sen-
sitive funnel-like structure in the explored region. This ex-
planation agrees well with the ghost domain scenario of the
droplet picture but not in the other cogent models proposed
over the last fewdecades [1-3]. This findinghas thusprovided
fresh insight into the stereotype of glassy systems: a disor-
dered configuration frozen in numerousmeta-stable states.[1] H. Mamiya and S. Nimori, New J. Phys. 12, 083007 (2010); [2]
H. Mamiya and S. Nimori, J. Appl. Phys. 111, 07E147 (2012); [3] H.
Mamiya et al. unpublished.
S20507Phasetransitionof thefirstorder in thefieldofmag-
netic frustration in Heusler alloys Ni-Mn-In. A.V. Mashirov
(IRE RAS), A.P. Kamantsev (IRE RAS), V.V. Koledov (IRE RAS), V.G. Shavrov
(IRE RAS) — In the last seven years theHeusler alloysNi-Mn-In
are being actively studied for the purpose of magnetocaloric
effect [1] and magnetic shape memory effect. In the follow-
ing alloys with a certain concentration of chemical elements
a metamagnetic structural transition (phase transition of the
first order) as well as magnetic frustration are observed [2].
The following samples of Heusler system alloysNi-Mn-In had
a certain elements’ concentration. Proportion of this con-
centration in samples demonstrated a decrease in the mag-
netic frustration, when Ni concentration increased. In this
experimental work an example of possibility of existence of
such samples are given. These samples with the thermomag-
netic analysis display extra phase transition of the first or-
der as is in the magnetic frustration case, and as in the field
of spontaneous magnitisation below Curie temperature. In
this work thermomagnetic analysis M=f(T) and differential
scanning calometry C=f(T) methods were used. Thus, when
making the samples and in the process of thermal curing it is
possible to make samples of Heusler system alloys Ni-Mn-In
with an additional phase transition of the first order as in the
fieldof spontaneousmagnitisationand in thecaseofmagnetic
frustration.1] Tishin A.M. and Y.I. Spichkin The magnetocaloric effect and its ap-
plication. Institute of Physics Publishing, Bristol and Philadelphia,
2003, 475pp; [2] S. Pramanick, S. Chatterjee, S. Giri, S.Majumdar, V.V.
Koledov, A. Mashirov, A.M. Aliev, A.B. Batdalov, B. Hernando, W.O.
Rosa, L.Gonzalez-Legarreta. Multiplemagneto-functionalproperties
of Ni46Mn41In13 shape memory alloy. Journal of Alloys and Com-
pounds, 578, pp. 157-161 (2013).
S20508 Ordering Phenomena in Spin Crossover Solids. G.
G. Morgan, H. Müller-Bunz, M. Harris, M. Griffin — In the last
decade structural symmetry breaking and spin state order-
ing has emerged as a structural feature in many examples
of two-step spin crossover (SCO) in Fe2+,[1] Fe3+[2] and
Co2+.[3] In correlated magnetic materials the underlying
driving force for such structural phase transitions is often
magnetic frustration, such as the ferrimagnetic ordering ob-
served in CsCoCl3.[4] However despite the increasing num-
ber of symmetrybreaking events in SCOcomplexes theorigin
of the phenomenon in these systems is not well understood.
Here we present some new examples of symmetry breaking
two-step SCO transitions in Fe3+ and in the less well-studied
Mn3+ ion, and examine the nature of the ordering in the in-
termediate phase.[1] a) D. Chernyshov, M. Hostettler, K. W. Törnroos, H.-B. Bürgi,
Angew. Chem. Int., Ed. Eng. 2003, 42, 3825-3830; b) S. Bonnet, M.
A. Siegler, J. S. Costa, G. Molnar, A. Bousseksou, A. L. Spek, P. Gamez,
J. Reedijk, Chem. Commun. 2008, 5619-5621; c) S. Pillet, E.-B. Ben-
deif, S. Bonnet, H. J. Shepherd, P. Guionneau, Phys. Rev. B. 2012,
86, 064106; d) N. Brefuel, H. Watanabe, L. Toupet, J. Come, N. Mat-
sumoto, E. Collet, K. Tanaka, J.-P. Tuchagues, Angew. Chem. Int. Ed.
Eng., 2009, 48, 9304-9307; [2] M. Griffin, S. Shakespeare, H. J. Shep-
herd, C. J. Harding, J.-F. Létard, C. Desplanches, A. E. Goeta, J. A. K.
Howard, A. K. Powell, V. Mereacre, Y. Garcia, A. D. Naik, H. Müller-
Bunz, G. G. Morgan, Angew. Chem. Int. Ed. Eng., 2011, 50, 896-
900; [3] a) M. G. Cowan, J. Olguín, S. Narayanaswamy, J. L. Tallon, S.
Brooker, J. Am. Chem. Soc., 2012, 134, 2892-2894; b) S. Hayami,
Queen’s College Cambridge 95
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Y. Komatsu, T. Shimizu, H. Kamihata, Y. H. Lee, Coord. Chem. Rev.,
2011, 255, 1981-1990; [4] J. P. Goff, D. A. Tennant, S. E. Nagler, Phys.
Rev. B, 1995, 52, 15992-16000.
S20509 Persistent spin dynamics in amplitude modulated
magnetic ground states. M. Pregelj (Jožef Stefan Institute, Slove-
nia), A.Zorko (Jožef Stefan Institute, Slovenia),D.Ar�on (Jožef Stefan Insti-
tute, Slovenia), O. Zaharko (Paul Scherrer Institute, Switzerland), H. Berger
(Ecole polytechnique fédérale de Lausanne, Switzerland) —Exoticmag-
netic ground states are often induced by geometrical frustra-
tion. In particular, frustrated systems may exhibit dynamic
spin liquid phases, frozen spin glass states or in rare cases
even developmagnetic long-range order (LRO) that is accom-
panied by persistent spin dynamics (PSD). There are only a
few reports of such coexistence and they still lack a com-
prehensive description. We focus on two systems, a layered
FeTe2O5Br compound with magnetic Fe3+ (S=5/2) ions [1]
and a quasi-one-dimensional β-TeVO4 compound with mag-
netic V4+ (S=1/2) ions [2] that both undergo several low-
temperature magnetic transitions [2,3] and finally establish
complex incommensurate magnetic ground states [3,4]. De-
spite the layered topology of FeTe2O5Br, recent investigation
revealed surprisingly strong Fe-O-Te-O-Fe exchanges, form-
ing an exchange network of spin chains coupled by weaker
frustrating interactions [5]. Here we present our neutron
diffraction andmuon spin relaxation results, which show that
incommensurate amplitude-modulated magnetic LRO is ac-
companied by spin fluctuations, persisting at lowest accessi-
ble temperatures in FeTe2O5Br [6] as well as in β-TeVO4 [4].
Hence, the observed behavior appears to be a more general
feature of spin chains, which thus offer a well-defined frame-
work to study the coexistence of LRO and PSD.[1] R. Becker et al., JACS 128, 15470, (2006); [2] Y. Savina et al., PRB
84, 104447 (2011); [3] M. Pregelj et al., PRL 103, 147202 (2009); [4]
M. Pregelj et al. in preparation; [5] M. Pregelj et al., PRB 86, 054402
(2012); [6]M. Pregelj et al., PRL 109, 227202 (2012).
S20510 Magnetic frustration, hierarchy of exchange inter-
actions, and idle spin behavior in a 2D lattice of bow-ties.
R. Sibille (PSI, Switzerland), V. Simonet (Institut Néel, France), M. Ken-
zelmann (PSI, Switzerland), E. Lhotel (Institut Néel, France), B. Malaman
(IJL, France), G. Venturini (IJL, France), T. Mazet (IJL, France), M. Frontzek
(PSI, Switzerland), V. Pomjakushin (PSI, Switzerland), M. Bartkowiak (PSI,
Switzerland),M.Zolliker (PSI, Switzerland), andM.François (IJL, France) —
The lattice of Fe2+ cations interconnected by oxygen atoms in
the Fe5(OH)2(C4H4O4)4metal-organic framework is found to
form an original two-dimensional (2D) frustrated lattice. This
network of low connectivity forms a 2D array of bow-ties (a
5-spins unit formed by two corner-sharing triangles) which
are connected to each other by additional Fe2+-O-Fe2+ links.
Within a bow-tie unit the exchange interactions connecting
the central spin to its four neighbors are ferromagnetic-like,
while the exchange interactions coupling pairs of spins at the
two extremities of a bow-tie are antiferromagnetic-like; the
central spinbeing consequently frustrated. This lattice canbe
viewed as derived from a Kagome net with four extra bonds
at the four corners of every bow-tie unit. Besides reveal-
ing the original topology of this 2D array of magnetic cations,
we present various experimental results from macroscopic
measurements (magnetization, AC susceptibility, heat capac-
ity) and neutron diffraction experiments. The compound or-
ders at 8.9 K and its magnetic structure displays a quasi-
orthogonal arrangement of the magnetic moments. We have
shown theexistenceof fluctuating spins in theorderedphase.
The magnetic arrangement and the spin dynamics are ratio-
nalized by a mean field analysis. We could evidence that, in
the ordered phase, the topology of the lattice together with
the magnetic frustration induce a very weak molecular field
on the central spins that remain substantially dynamical. The
hierarchy of the intraplane magnetic exchange interactions
plays a significant role in the peculiar magnetic behavior of
this material. Finally, the properties and magnetic structures
of isostructural materials based on Co2+ and Mn2+ will be
tackled, as well as their similarities and differences with the
ferrous compound.
S20511 Spin fluctuations in frustrated metal LiV2O4 with
heavy fermion behavior. K. Tomiyasu, K. Iwasa (Tohoku Univer-
sity, Japan), H. Ueda (Kyoto University, Japan), S. Niitaka (RIKEN, Japan),
S. Iikubo (Kyusyu Institute of Technology, Japan), S. Kawamura, T. Kikuchi,
K. Nakajima (JAEA, Japan) — Spinel-type oxide LiV2O4 is a rep-
resentative system for itinerant frustration. This material, in
whichmagneticV ions (nominally 3.5+) formapyrochlore lat-
tice, exhibits no magnetic order above 2 K at least, a heavy
fermion behavior below∼ 20 K in specific heat, and metallic
low electrical resistivity [1]. Powder inelastic neutron scat-
tering revealed that spin fluctuations with Qc ∼ 0.6 Å−1
growbelow∼20K,whicharemostprobablyconnected to the
heavy fermion behavior [2,3]. However, the overall picture in
(Q,E) space is still veiled. Here, we report the overall data,
measured for apowder specimenon state-of-the-art inelastic
neutron scattering spectrometer AMATERAS at J-PARC, and
the analysis results.[[1] Y. Matsushita et al., Nature 4, 845 (2005). [2] S.-H. Lee et al., PRL
86, 5554 (2001). [3] A. P. Murani et al., J. Phys.: Condens.Matter 16,
S607 (2004).
S20512 Magnetic Response of Gd1−xLax to Low Field. T.
Yamazaki, M. Kurihara, M. Onishi, and H. Yaguchi — A magnetic al-
loy systemGd1−xYx have been studied extensively for a long
timebecause they shows variousmagnetic phases such as he-
limagnetic phase and tricritical points whichwere candidates
for Lifshitz points. Recently, we observed anomalous slow
spin dynamics in helimagnetic phase and strong nonlinear-
ity of magnetization at paramagnetic-helimagnetic transition
temperatureTN inGd0.62Y0.38. Weconsider that the slowdy-
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namics and nonlinearity observed inGd1−xYx occur owing to
the veryweakmagnetic anisotropyofGd+3-ion. On theother
hand, there are only a few reports on themagnetic properties
of a similar alloy systemGd1−xLax. Larica et al. haveobserved
a very sharp peak of the AC susceptibility at 135K suggest-
ing a magnetic transition in Gd0.73La0.27. The peak of the AC
susceptibility was very sensitive to theDCmagnetic field and
was strongly suppressed by applying lowDC field. They have
also observed a weak peak of the AC susceptibility at 113K.
In order to investigate more detailed magnetic property of
Gd1−xLax and to search intriguing phenomena as observed in
Gd1−xYx, we performed the DC and AC magnetization mea-
surements for Gd1−xLax alloys at low field. We found weak
ferromagnetic behavior in Gd0.73La0.27 at the intermediate
temperature range 115K< T < 145K corresponding to the
range between the two peaks observed in the AC suscepti-
bility mentioned above. Below 115K, the DCmagnetizations
measuredunder the zero-field-cooled andfield-cooled condi-
tions were clearly different from each other. In this confer-
ence, we will report the detail including the experimental re-
sults for other La-concentration samples and discuss on the
magnetic structures and their origins.
S20513 Frustrated diamond lattice antiferromagnet
CoAl2O4 by neutron scattering and classical Monte-Carlo
modeling. O. Zaharko (Paul Scherrer Institute, Villigen, Switzerland),
S. Toth (Paul Scherrer Institute, Villigen, Switzerland), O. Sendetskyi (Paul
Scherrer Institute, Villigen, Switzerland), A. Cervellino (Paul Scherrer Insti-
tute, Villigen, Switzerland), V.Tsurkan (University of Augsburg, Augsburg,
Germany), A. Maljuk (Leibniz Institute, Dresden, Germany) — Due to
magnetic frustration the diamond lattice can host an exotic
highly degenerate state consisting of coplanar spirals, the
so-called spiral spin liquid (SSL). The degeneracy of the SSL
can be lifted by thermal or quantum fluctuations resulting
to an ordering transition via the ’order by disorder’ mecha-
nism [1]. For realization of the SSL the ratio J2/J1 between
the antiferromagnetic frustrating next-nearest-neighbor
interaction J2 and the nearest-neighbor interaction J1 should
exceed 1/8. In the CoAl2O4 antiferromagnet magnetic
Co2+ ions form the diamond lattice, but to which extend
the theoretically predicted SSL physics is realized in real
samples? From one side, the experimental observations by
specific heat [2], muon spin relaxation [3], neutron diffraction
[4] are consistent with a spin liquid state. From the other
side, the J2/J1 ratio is only 0.109(2) [4] and sample properties
are affected by the Co/Al antisite disorder [5]. We performed
neutron scattering experiments on single crystal CoAl2O4
and by classical Monte-Carlo modeling try to distinguish
which mechanism - ’order by disorder’ or ’order by quenched
disorder’ - is relevant for this material.[1]D.Bergman, J.Alicea, E.Gull, et al. NaturePhysics3, 487(2007); [2]
N. Tristan, V. Zestrea, G. Behr, et al. Phys. Rev. B 77, 094412 (2008);
[3] T. Suzuki, N. Nagai, M. Nohara, H. Takagi, J. Phys.: Condens. Mat-
ter 19, 145265 (2007); [4] O. Zaharko, A. Cervellino, V. Tsurkan, et al.
Phys. Rev. B 81, 064416 (2010), Phys. Rev. B 84, 094403 (2011); [5]
K. Hanashima, Y. Kodama, D. Akahoshi, et al. J. Phys. Soc. Jpn. 82,
024702(2013).
06 LowDimensional Systems (experiments)
S20601 Characterization of Quasi-One-Dimensional
Cuprates. K. Caslin, R.K. Kremer, A. Schulz, A. Munoz, F. Pertlik, J.
Liu, M.H. Whangbo, F.S. Razavi, J.M. Law — Many Cu2+ (S=1/2)
linear-spin-chains systems have been shown to exhibit low-
dimensional magnetism. Spin-chains are formed when CuX6
(X=O,Cl,Br,...) Jahn-Teller distorted octahedra link together
via the trans-edges of their basal planes. Most often, these
spin-chains support ferromagnetic (FM) nearest-neighbor
(NN) and antiferromagnetic (AFM) next-nearest-neighbor
(NNN) spin-exchange interactions leading to a frustrated
system. Spin-chain systems with competing interactions
are known to develop AFM incommensurate spin-spiral
structures and sometimesmultiferroic behavior. There exists
a magnetic phase diagram which can predict the intra-chain
behaviour of spin-chain compounds using the ratio of the NN
over the NNN spin exchange constants, α=Jnn/Jnnn. Critical
points exists on the boundaries atα=- 4 andα=2,where small
spin-exchange perturbations on a system in the vicinity of
one of these pointsmay induce a pronounced response of the
system. We report on Cu2+ frustrated spin-chain systems
exhibiting α ratios close to critical points. Measurements of
magnetic susceptibility, heat capacity, Raman spectroscopy,
and electron paramagnetic resonance were performed.
DFT calculations of the spin-exchange constants and TMRG
simulations on themagnetic susceptibilitieswere also carried
out.
S20602 Neutron studies of PbCuSO4(OH)2, a candidate for
a novel quantum phase. E. Cemal (ILL, Grenoble, France,
and Royal Holloway, London, UK), M. Enderle (ILL, Grenoble, France), B.
FÅK (CEA, Grenoble, France), R.K. Kremer (MPI Stuttgart, Germany) —
One dimensional frustrated ferromagnets have been receiv-
ing theoretical and experimental interest due to the predic-
tion of novel multipolar quantum phases under an applied
magnetic field [1,2]. These novel phases are described as ne-
maticmulti-magnonbound stateswhich appear close to satu-
ration. There has already been suggestions of a two-magnon
bound state from magnetisation measurements of LiCuVO4
close to saturation (Hs∼ 45T) [4,5]. While quadrupolar spin-
density short-range order was evidenced by neutron scatter-
ing at low fields [3], the character of the high-field phase can-
not be explored with neutron scattering due to the strength
of the field. PbCuSO4(OH)2 could be a candidate for a direct
neutron study due to its much lower saturation field Hs∼8T[6]. Our inelastic neutron scattering measurements allow
Queen’s College Cambridge 97
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to determine the relevant exchange parameters and locate
PbCuSO4(OH)2 in another multipolar quantum phase than
LiCuVO4.[[1] L. Kecke et al., Phys. Rev. B, 76, 060407(R) (2007); [2] T. Hikihara
et al., Phys. Rev. B, 78, 144404 (2008); [3] M. Mourigal et al., Phys.
Rev. Lett., 109, 027203 (2012); [4] L. E. Svistov et al., JETP Lett., 93,
24 (2011); [5] M. E. Zhitomirsky and H. Tsunetsugu, Europhys. Lett.
92, 37001 (2010); [6] A. U. B. Wolter et al., Phys. Rev. B, 85, 014407
(2012).
S20603Experimental evidence for amagnetization plateau
and for field-suppressed low-energy spin fluctuations in
the triangular antiferromagnet Ba3CoNb2O9 J.Dai, S.S.Sun,
P.S.Wang (Renmin University of China), S. L. Li, Ding. Hu (Beijing Na-
tional Laboratory for Condensed Matter Physics), B. Normand, Weiqiang
Yu (Renmin University of China) — A polycrystalline sample of
Ba3CoNb2O9, a compound forming a triangular lattice of
Co2+ ions in their low-spin (S = 1/2) state, has been synthe-
sized by a solid-state reaction technique and investigated by
heat capacity, magnetization, and NMR measurements. At
zero field, the low-temperature susceptibility shows Curie-
Weiss behavior with θcw=-52K, whereas the specific heat
shows a sharp magnetic transition at T = 1.3 K, suggesting
a highly frustrated antiferromagnetic system. With increas-
ing field, the line shifts in our low-temperature NMR spec-
tra show a continuous increase of the magnetization until a
plateau is reached at µ0H 4.5 T, indicating a change of mag-
netic structure. Our measurements of the spin-lattice re-
laxation rate show that the low-energy spin fluctuations dif-
fer dramatically for the two magnetic phases, a result con-
sistent with the specific-heat data. A strong spin response
at low fields is characteristic of (frustrated) magnetic or-
der, while a large spin gap is observed in the magnetization-
plateau regime. This gap closes and reopens at the transi-
tion to a fully polarized state above 5 T. Our data suggest that
Ba3CoNb2O9 is an excellent system for studying the highly
frustrated ground state and magnetic excitations of the S =
1/2 triangular lattice.
S20604 Zero-field short-range order in the quasi-one di-
mensional frustrated ferromagnet LiCuVO4. M. En-
derle (ILL, Grenoble, France), E. Hirtenlechner (ILL, Grenoble, France,
and PSI, Switzerland), B. Fåk (CEA, Grenoble, France), R.K. Kremer (MPI
Stuttgart, Germany) — Ferromagnetic spin 12Heisenberg chains
with frustrating next-nearest-neighbour antiferromagnetic
exchange develop novel multipolar quantum phases close to
their saturation field [1,2]. In LiCuVO4, the ratio of fer-
romagnetic nearest-neighbour and antiferromagnetic next-
nearest-neighbour interaction should lead to quadrupolar
correlations and consequently spin density dipolar correla-
tions already at very small magnetic field, if not zero-field
[3]. Close to saturation, the multipolar order can be un-
derstood as condensation of bound multi-magnon states. In
the presence of a two-dimensional frustrated interchain in-
teraction as in LiCuVO4 [4], bound-two magnon states have
been shown to lead to quadrupolar long-range order at very
elevated magnetic fields [5], and evidence for this high-
field phase was found in high-field magnetization measure-
ments [6]. However, the picture of a bound two-magnon
state breaks down at low magnetic fields due to the ex-
plosing density of bound quasi-particles. Nevertheless, in
LiCuVO4, quadrupolar correlations reflected in spin density
short-ranged dipolar correlations have been demonstrated
bypolarizedneutron scattering inmagneticfields as lowas20
%of the saturationfield [7]. Clearly, thepictureof bound two-
magnon states cannot hold any more at such low magnetic
fields. We have discovered zero-field short-range magnetic
order and have characterized it by polarized elastic neutron
scattering. The zero-field short- and long-range magnetic or-
der and their polarisation as a function of temperature are
discussed in a novel picture of low-density fractional quasi-
particles.[1] L. Kecke et al., Phys. Rev. B 76, 060407(R) (2007); [2] T. Hikihara et
al., Phys. Rev. B 78, 144404 (2008); [3] J. Sudan, et al., Phys. Rev. B 80,
140402 (2009); [4] M. Enderle et al., Europhys. Lett., 70 237 (2005);
[5] M. E. Zhitomirsky and H. Tsunetsugu, Europhys. Lett. 92, 37001
(2010); [6] L. E. Svistov et al., JETP Lett. 93, 24 (2011); [7]M.Mourigal
et al., Phys. Rev. Lett. 109, 027203 (2012).
S20605 The spin ladder compound Ba2Cu2+Te6+O6: an in-
triguing interplay between crystal andmagnetic dimension-
alities. A. S. Gibbs (The Univ. of Tokyo, RIKEN, Univ. of St Andrews), A.
Yamamoto (RIKEN), K. S. Knight (ISIS), D. Hashizume (RIKEN), A. Yaresko
(MPI-FKF Stuttgart), M. Nakamura (MPI-FKF Stuttgart), H. Takagi (MPI-
FKF Stuttgart, The Univ. of Tokyo, RIKEN) —Motivated by the dis-
covery of spin-liquid-type behaviour in Ba3CuSb2O9, we in-
vestigated a range of materials with related structures. This
led us to the compounds Ba2M2+Te6+O6 (M2+=Ni, Cu, Zn).
These compounds adopt structures composed of triplets of
face-sharing TeO6 andMO6 octahedra (in contrast to pairs of
face-sharing octahedra in Ba3CuSb2O9). These are linked by
corner-sharing TeO6 octahedra. This leads to a crystal struc-
turecomposedof1Dchainsalong the c-axis anda2Dnetwork
in the ab-plane. We have synthesised large-volume polycrys-
talline samples suitable for neutron scattering and also single
crystals allowing investigationof anisotropic physical proper-
ties. We performedmagnetisation, specific heat and neutron
diffractionexperimentsalongwithLSDA+Ucalculations to in-
vestigate thephysicalpropertiesof thesematerials. Ourstud-
ies reveal that Ba2ZnTeO6 is nonmagnetic as expected for a
Zn2+ S=0 system with a structural transition at T ≈ 157K
whilst Ba2NiTeO6, with S=1Ni2+, is an antiferromagnet with
TN = 8.5K and no structural transitions below room tem-
perature. Most interesting is Ba2CuTeO6, with S=1/2 Cu2+,
which shows quasi-one-dimensional behaviour. Surprisingly,
Queen’s College Cambridge 98
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the one-dimensional character is strongest along the b-axis
in contrast to the crystal structure which shows 1D charac-
ter along the c-axis. The susceptibility of Ba2CuTeO6 shows a
broad overturn near T = 100K which can be well fitted by a
Bonner-Fishermodel down to50K. That this behaviour is due
to effective lowdimensionalmagnetic interactions is strongly
supported by LSDA+U calculations which indicate the mag-
netic structure to be based on S = 1/2 Cu2+ ladders. The
rungs and legs of the ladders have almost equal effective ex-
changecouplingswith thediagonaland inter-laddercouplings
an order of magnitude weaker. Surprisingly, this is followed
at lower temperatures by novel non-trivial low-temperature
magnetic behaviour. A small kink in susceptibility at T ∗ =
16K and an increase in anisotropy indicates ordering but
no transition is seen in specific heat. While the structural
phase diagram including a Jahn-Teller transition at high tem-
perature and a further structural transition just below room
temperature have been identified using neutron diffraction,
the magnetic structure of the novel low-temperature phase
has not yet been determined. The interplay between crystal
structure, Jahn-Teller distortions and magnetic dimensional-
ity in these compounds will be discussed.
S20606 Ultra-high magnetic field induced 1/3 magneti-
zation plateau in a frustrated triangular-lattice magnet
CuCrO2 . A. Miyata, K. Ohgushi, S. Takeyama (ISSP, Univ. of Tokyo) —
CuCrO2, which belongs to a delafossite oxide, is a typicalmul-
tiferroic compound in triangular-lattice magnets [1]. In ad-
dition to the multiferroicity, CuCrO2 shows a rich magnetic
phase diagram in high magnetic fields due to the geometrical
frustration [2]. However the full magnetic phase diagram of
CuCrO2 has not yet been obtained due to its strongmagnetic
interactions (Curie-Weiss temperature is about -210 K [1].)
We investigatedmagneticphasesofCuCrO2, byusingmagne-
tization and magneto-optical measurements under ultrahigh
magnetic fields of up to 120 T. As a result, 1/3 magnetization
plateauwas observed around 95 T.[1] K. Kimura et al., Phys. Rev. B 78, 140401(R) (2008); [2] E. Mun et
al., Phys. Rev. B 89, 054411 (2014).
S20607Frustration in theJ1−J2 chain ofNaCuMoO4(OH).
K. Nawa, Y. Okamoto, A. Matsuo, K. Kindo, and Z. Hiroi (ISSP, Unversity of
Tokyo, Kashiwa, Chiba, Japan) — In a one-dimensional frustrated
magnet with the competing nearest neighbor ferromagnetic
interaction (J1) and the next-nearest neighbor antiferromag-
netic interaction (J2), novel multipolar states such as a spin-
nematic state are theoretically expected near the magneti-
zation saturation[1,2]. In order to obtain experimental evi-
dence for a spin-nematic order, a new candidate compound
foraone-dimensional frustratedmagnetwithaJ1−J2 chain is
explored. Here we show that NaCuMoO4(OH)[3] can be the
candidate. Wesucceeded toprepare small yellow-greencrys-
tals of NaCuMoO4(OH) by a hydrothermal synthesis. The
saturation magnetic field was found to be 26 T in high-field
pulsemagnet experiments, which ismuch smaller than 44.4 T
for a previous candidate compound LiVCuO4[4]. We will dis-
cuss the magnitudes of J1 and J2 using magnetic susceptibil-
ity, magnetization, and heat capacity data with the assistance
of calculations by the exact diagonalizationmethod.[1] T. Vekua, et al., Phys. Rev. B 76, 174420 (2007); [2] T. Hikihara, et
al., Phys. Rev. B 78, 144404 (2008); [3] A. Moini, et al., Inorg. Chem.
25, 3782 (1986); [4] L. E. Svistov, et al. JETP Lett. 93, 21 (2011).
S20608Pretendedquantumspin-liquid behavior in amixed
triangular antiferromagnetic system. T. Ono, H. Tomi-
naga, K. Iwase, H. Ishibashi, H. Yamaguchi, Y. Hosokoshi (Osaka Pref.
Univ.), N. Kurita, H. Tanaka (Tokyo Inst. of Tech.) — The ori-
gin of the quantum spin-liquid behavior observed on S =
1/2 triangularantiferromagnets (TAF)κ-(ET)2Cu2(CN)31) and
EtMe3Sb[Pd(dmit)2]22) have been explained by using the
multiple-spin exchange interactions3). Stimulated by the re-
cent dielectric studies on κ-(ET)2Cu2(CN)3 which shows the
glassy dielectric response at low temperature4), Watanabe et
al. demonstrated the physical properties of an S = 1/2 TAF
with randomness in the exchange interaction5). According
to their results, the ground state of the system become dis-
ordered state with introducing the randomness to a certain
magnitude. In the disordered region, the magnetic suscepti-
bility tends to diverge for T → 0 limit, and the specific heat
exhibits a T -liner behavior at low temperature.
In order to inspect the prediction experimentally, we have
investigated magnetic and thermodynamic properties of a
mixed TAF system Cs2CuCl4−xBrx. From the magnetic mea-
surementsof thepresent system, itwas found that thesystem
does not have the long range ordered state for the samples
with intermediate concentration region 1.8 ≤ x ≤ 3.0. The
ground state of the samples with intermediate region should
not be the quantum spin-liquid state but the random singlet
state6) where the static spin singlets are arranged randomly.
Themagnetic susceptibility increasesmonotonouslywith de-
creasing temperature, and the specific heat exhibits T -linear
behavior at low temperature for the sampleswith that region.
We will show the present system acts like as if it has a quan-
tum spin-liquid ground state.[1] Shimizu et al. Phy. Rev. Lett. 91 107001 (2003); [2] Itou et al. Na-
ture Phys. 6 673 (2010); [3] Montrunich Phys. Rev. B 72 045105
(2005); [4] Abdel-Jawad et al. Phys. Rev. B 82 125119 (2010); [5]
Watanabe et al. J. Phys. Soc. Jpn. 83 034714 (2014); [6] Imada, J.
Phys. Soc. Jpn. 56 881 (1987).
S20609 Pressure-driven dimensionality change in a quan-
tum magnet. M. Skoulatos, Ch. Fiolka, M. Mansson, J. White, K.
Krämer and Ch. Rüegg — Low-dimensional quantum magnetism
has long been at the center of attention in condensed matter
physics mainly due to the simplicity and beauty of such phys-
ical systems.
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In CuF2(D2O)2(pyz) (pyz=pyrazine) and at ambient pressure,
the S=1/2 spins of Cu2+ ions form a 2D square lattice. Bulk
properties imply that above some critical pressure the sys-
tem changes dimensionality due to a complete reorientation
of the Jahn-Teller axis [1,2]. Wewere able to directlymeasure
such a transition for the first time ever, employing inelastic
neutron scattering.[1] G.J. Halder et al., Angew. Chem. Int. Ed. 50, 419 (2011); [2] S.
Ghannadzadeh et al., Phys. Rev. B 87, 241102(R) (2013).
S20610 Microscopic Study of the Field Induced Phases of
theMagnetic FrustratedQuantumSpinChain SystemLinar-
ite. S. Süllow(TU Braunschweig), B. Willenberg (TU Braunschweig, HZ
Berlin), M. Reehuis (HZBerlin), M. Schäpers (IFWDresden), A. U. B.Wolter
(IFW Dresden), B. Büchner (IFW Dresden), K. C. Rule (ANSTO), B. Oulad-
diaf (ILLGrenoble) —Aneutrondiffraction andNMRstudyof the
field induced phases of linarite PbCuSO4(OH)2 is presented
for temperatures down to 1.7K and fields applied along the
b axis. A spin flop transition in two steps resulting into a
collinear structurewas observed. Furthermore, an extraordi-
nary sine-wavemodulated structurewithmagneticmoments
parallel to the field direction was found, which encloses the
other long-range ordered phases at higher fields and temper-
atures. This phase exhibits a shift of the propagation vector
upon changing the magnetic field and is discussed in terms of
a 3D spin density wave phase, which tentatively can be de-
scribedwith density waves of bound three-magnons.
S20611Magnonmodes inα-CaCr2O4 measured by neutron
scattering and far infrared absorption. S. Tóth and Ch.
Rüegg (Paul Scherrer Institut, CH), B. Lake (Helmholtz-Zentrum Berlin,
DE), M. Schmidt and J. Deisenhofer (University of Augsburg, DE) — α-
CaCr2O4 is a spin-3/2, distorted triangular latticeHeisenberg
antiferromagnet. It develops long-range magnetic order be-
low TN = 42 K where the angles between nearest neighbor
spins are 120◦ on the triangular planes. This simple magnetic
structure masks the complex pattern of exchange interac-
tions [1]. The magnetic excitation spectrum measured by in-
elastic neutron scattering reveals unusual low energymodes,
which can be explained by linear spin-wave theory assuming
a complex set of 1st and 2nd neighbor exchange interactions
[2,3]. The fitted first neighbor exchange interactions strongly
correlate with the Cr3+–Cr3+ distances, as expected for di-
rect exchange interactions in the triangular planes. UsingTHz
optical spectroscopy, several low energymagnonmodes have
been found that interact with the far-infrared light. We use
the THz data to improve our spin wavemodel.[1] S. Toth, B. Lake et. al., PRB, 84, 054452 (2011); [2] S. Toth, B. Lake
et. al., PRL, 109, 127203 (2012); [3] D. Wulferding, et al., JPCM, 24,
435604 (2012)
S20612Multi-frequency ESR in the S = 5/2 triangular lat-
tice antiferromagnet CuFeO2 in an in-plane magnetic field.
D. Yoshizawa, T. Kida, T. Fujita (Center for Advanced High Magnetic Field
Science, Graduate School of Science, Osaka University, Toyonaka, Osaka,
Japan), S. Kimura (Institute for Materials Research, Tohoku University,
Sendai, Miyagi, Japan), M. Hagiwara (Center for Advanced HighMagnetic
Field Science, Graduate School of Science, Osaka University, Toyonaka,
Osaka, Japan) —The triangular lattice antiferromagnetCuFeO2
has been regarded as a frustrated magnet with a Heisenberg
spin (Fe3+: S = 5/2, L = 0). However, below TN2 = 11 K,
CuFeO2 exhibits an Ising-like four-sublattice collinear order
in contrast to a noncollinear 120◦ spin configuration in an
usual Heisenberg triangular lattice antiferromagnet [1]. On
applying the magnetic field to the [001] direction, successive
phase transitions including a ferroelectric phase are exhib-
ited below TN2 [2, 3]. This unusual transitionswere suggested
to be caused by a strong spin-lattice coupling and distant ex-
change interactions [4]. Owing to these interesting features,
CuFeO2 turns tobeoneof themostattractive frustratedmag-
nets.
In this study, we have performed the magnetization and
the multi-frequency electron spin resonance (ESR) measure-
ments onCuFeO2 single crystals in pulsedmagnetic fields ap-
plied to the [110] direction. Field-induced transitions were
seen at Hc1 = 25.2 T, Hc2 = 30.3 T, Hc3 = 52.7 T in the
magnetization curve. We observed the ESR branchs below
Hc1, and analyzed them with the parameters obtained from
the ESR analysis for the [001] direction [5].[1]M.Mekata et al., J. Phys. Soc. Jpn. 62, 4474 (1993); [2] S. Mitsuda
et al., J. Phys. Soc. Jpn. 69, 3513 (2000); [3] S. Kimura et al., Phys. Rev.
B 84, 104449 (2011); [4] R. S. Fishman et al., Phys. Rev. B 85, 020405
(2012); [5] T. Fujita et al., J. Phys. Soc. Jpn. 82, 064712 (2013).
S20613Raman study ofmagnetic excitations andmagneto-
elastic coupling in SrCr2O4. M.Valentine (Johns Hopkins Uni-
versity, USA), S. Koohpayeh (Johns Hopkins University, USA), M. Mourigal
(Johns Hopkins University, USA), T. M. McQueen (Johns Hopkins Univer-
sity, USA), C. Broholm (Johns Hopkins University, USA), N. Drichko (Johns
Hopkins University, USA), S. Dutton (Prinston University, USA), R. J. Cava
(Prinston University, USA), T. Birol (Cornell University, USA), H. Das (Cor-
nell University, USA), C. J. Fennie (Cornell University, USA). —Using Ra-
man spectroscopy, we investigate magneto-elastic coupling
and magnetic excitations in the antiferromagnetic triangular
lattice SrCr2O4, which develops helical magnetic order be-
low43K. Temperature dependentRamanphonon spectra are
compared to Density Functional Theory (DFT) calculations
which allow us to assign normal modes and identify weak ef-
fects arising from interacting lattice andmagnetic degrees of
freedom. Raman bands associated with two-magnon excita-
tions are observed at 15meVand38meV.We show that their
position is in agreementwith ourDFT calculations of the spin
exchange constant J for SrCr2O4 and theoretical predictions
for two-magnon excitations on a triangular lattice. By follow-
ing the temperature dependence of the two-magnon excita-
tions, we show that two-dimensional spin correlations persist
Queen’s College Cambridge 100
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up to at least 70 K.
07 5d Electron Systems (experiment)
S20701Novelmagnetisminthespin-orbitdrivenMott insu-
lator Ba2YIrO6. T. Dey (IFW Dresden), A. Maljuk (IFW Dresden), O.
Kataeva (IFW Dresden+Kazan Federal University Russia), F. Steckel (IFW
Dresden), D. Gruner(IFWDresden), T. Ritschel (IFWDresden), C. G. F. Blum
(IFW Dresden), A. U. B. Wolter (IFW Dresden), J. Geck (IFW Dresden), C.
Hess (IFW Dresden+TU Dresden), S. Wurmehl (IFW Dresden+TU Dres-
den), and B. Buechner (IFW Dresden+TU Dresden)) —We have syn-
thesized single crystals of Ba2YIrO6 for the first time. Sin-
gle crystal and powder (crushed crystals) x-ray diffraction re-
sults confirm that thematerial crystalizes in cubic double per-
ovskite structure (space group Fm-3m). In this material, the
only magnetic ion Ir is expected to be in +5 oxidation state
with 5d4 electronic configuration. While materials with 5d4
configuration are generally expected to be non-magnetic, it is
proposed that an interplay between Coulomb interaction (U)
and spin-orbit-coupling (SOC) can create novel singlet-triplet
excitonic magnetism. Recently, Cao et. al. reported the dou-
ble perovskite material Sr2YIrO6 with Ir5+ ions to exhibit an
antiferromagnetic long-range order below 1.3K. This mate-
rial crystallizes in a monoclinic structure (space group P21/n)
with highly distorted IrO6 octahedra. Cao et. al. claimed that
thisdistortion isactually responsible for themagnetism in this
compound. To verify this statement it is therefore essential
to study a material without this crystallographic distortion.
This motivates the detailed study of the Ba-analog Ba2YIrO6
which has a cubic structure. From our magnetization mea-
surement, we found Ba2YIrO6 is paramagnetic in the tem-
perature range 2-300K. Curie-Weiss fitting (30-300K) of sus-
ceptibility data gives an effective magnetic moment µeff =
0.47µB/Ir and aWeiss constant θCW = −15K (AF) but nomag-
netic ordering is found down to 1.8K.Our study suggests that
Ba2YIrO6 is a spin-orbit drivenMott insulatorwith frustrated
magnetic interactions. In this presentation, details of crystal
growthalongwithstructural,magnetic, thermalandelectrical
transport measurement results will be discussed.
08Multiferroics
S20801 Domains and multiferroicity in CuCrO2: a single
crystal neutron diffraction study. M. Frontzek, A. Podlesnyak, G.
Ehlers, S. Barilo, S. Shiryaev, V. Zapf, E. Mun —Multiferroic materials
have become of interest for their unusual low-temperature
properties ingeneral, and the tunabilityof themagnetic struc-
ture through an electric field and the electric polarization
through a magnetic field in particular. The most promis-
ing candidates for such controllable multiferroics have been
found among the materials with inherent geometric mag-
netic frustration. Among these, the delafossite CuCrO2,
which crystallizes in the rhombohedral R3m space group, is
a multiferroic compound with an apparent strong coupling
of spin and charge. In contrast to other multiferroic com-
pounds CuCrO2 shows a spontaneous electric polarization
upon antiferromagnetic ordering without an accompanying
structural phase transition, thus the spiral magnetic ordering
alone breaks the inversion symmetry. The peculiar magnetic
structureofCuCrO2 allows thedirect quantitative analysis of
the domain population. In our contribution, we present a de-
tailed study on CuCrO2 single crystals using neutron diffrac-
tion in applied electric and magnetic fields. With the fields
we were able to tune the multiferroic states in CuCrO2 and
could directly relate them to the underlying domain physics.
Surprisingly, the domain population is changed only slightly
by the electric field and the observed multiferroic proper-
ties arise therefore only from a fraction of the whole crys-
tal. Further, the sign reversal of the electric polarization
through a reversed electric field is not accompanied by a do-
main re-distribution. This indicates a coupling of the elec-
tric polarization to the chirality of the magnetic spiral. We
will also present evidence that the three domain state is the
ground state and that the one- or two-domain state is a non-
equilibrium state. The underlying mechanism is speculative,
but its existence is important to understand the multiferroic
properties of CuCrO2.
S20802 Theory of magnetoelectric effects in multiferroics
CuFeO2 and BiFeO3 . S. Miyahara (Fukuoka Univ.), N. Furukawa
(Aoyama Gakuin Univ.) — In multiferroics, there is a strong cou-
pling between magnetization M and electric polarization P.
Due to the magnetoelectric (ME) coupling, ME effects and
electromagnon, electro-active magnon, processes arise. As
a typical ME coupling, spin dependent electric polarization
due to spin current mechanism, PKNB = deij × (Si × Sj),
is well known and a cycloidal spin structure couples to an
electric polarization. We investigate an anti-symmetric spin
pairsdependentelectricpolarizationpαAS ind-pmodelonadis-
torted crystal structure and show that it is given by a generic
form pαAS = dαβ(Si × Sj)β (α, β = x, y, z) with a tensor
d̂. As a result, proper screw and canted antiferromagnetic
spin structures can couple to electric polarizations. As typi-
cal examples,we show that static anddynamicalMEeffects in
proper screw states in frustrated triangularmagnets CuFeO2
and field-induced antiferromagnetic states in a distorted per-
ovskite BiFeO3 are explained well by generic anti-symmetric
spin pairs dependent electric polarization.
S20803 Pressure-induced magnetic phase transitions in
a multiferroic delafossite CuFeO2 as observed by high-
pressure neutron diffraction. N. Terada, D. D. Khalyavin, P.
Manuel, T. Osakabe, P. G. Radaelli, H. Kitazawa —We are develop-
ing the high-pressure neutron diffraction experimental sys-
Queen’s College Cambridge 101
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temby combining a hybrid-anvil-typepressure cell devicewith
high flux cold neutron time-of-flight diffractometer WISH in
ISIS neutron facility. This systempotentially enables us to ob-
serve high quality magnetic diffraction data under high pres-
sure up to 10 GPa and low temperature. Recently, we have
studied the pressure effect on strongly frustrated magnetic
system of delafossitemultiferroic CuFeO2, by using the pres-
sure system, and obtained the pressure-induced magnetic
phase transitions. A magnetic ground state with the com-
mensurate propagation vector k = (0, 12, 12) below 3 GPa in
CM1 phase turns into proper screw magnetic ordering with
k = (0, q, 12; q ≃ 0.4) and polar 21′ group at P =3.2 GPa.
(ICM2 phase) The polar ICM2 phase is almost identical to the
ferroelectric phase induced by eithermagnetic field or chem-
ical doping under ambient pressure. Above 4.3 GPa, another
new phase with incommensurate k = (qa, qb, qc; qa ≃ 0, qb ≃0.34, qc ≃ 0.43) and 11′ is stabilized in ICM3 phase, and fi-
nally the thermally induced incommensurate phase (ICM1)
with collinear spin-density-wave (nonpolar 2/m1′ group) is
realized as a ground state in higher pressure region. Com-
paring the crystal lattice volume and degree of lattice distor-
tion among themagnetic phases in ambient pressure, we con-
clude that the pressure induced phase transitions originate
fromthepressure-suppressionof the spontaneous latticedis-
tortion to release spin-frustration in CuFeO2.
09DimerisedModels andMaterials
S20901 Growth and Characterisation of, the frustrated
Shastry-Sutherland Magnets, the Rare Earth Tetraborides.
D. Brunt, O. A. Petrenko, G. Balakrishnan (University of Warwick) —Up
until the discovery of SrCu2(BO3)2, the Shastry-Sutherland
lattice (SSL) was of purely theoretical interest. The SSL is an
example of a frustrated lattice that has an exact solution for
the ground state [1]. The lattice itself is rarely experimentally
realised, with it appearing in only a hand full of compounds.
One such family is the rare earth tetraborides (REB4). The
REB4 crystallise intoa tetragonal structure,where theRE ions
forma sublattice consisting of squares and triangles in the ab-
plane which can be mapped to the SSL. The typical type of
ordering within this family is antiferromagnetic, with the ex-
ception of PrB4, which orders ferromagnetically. The compe-
tition between the magnetic and quadrupolar interactions is
believed to be key in establishing a ground state and there-
fore novel magnetic properties in these systems have been
expected. Previous investigation have provided some inter-
esting results, such as fractional magnetisation plateaux in
TmB4, ErB4 and HoB4 [2] as well as complex magnetic phase
diagrams found in NdB4 and HoB4 [3,4]. We have growth
large single crystals of NdB4, GdB4 and HoB4 using the float-
ing zone technique. Laue photographs of these crystals have
proven they are of good quality. We have also fully charac-
terised the crystals, inmagnetic fields parallel and perpendic-
ular to the c-axis, using magnetisation and specific measure-
ments.[1] S. Shastry, and B. Sutherland, Physica 108 B (1981) 1069; [2] S.
Mat’aš et al., J. Phys.: Conf. Series 200, (2010) 032041; [3] J.Y. Kim,
B. K. Cho, and S. H. Han, J. Appl. Phys. 105 (2009) 07E116; [4] R.
Watanuki et al. J. Phys. Conf. Series 150 (2009) 042229.
S20902 Spin dynamics in themineral compoundMalachite,
Cu2(OH)2CO3. E. Canévet (Laboratoire de Physique des Solides),
B. Fåk (SPSMS /INAC), M. Enderle (Institut Laue-Langevin), J.H. Chun
and R.K. Kremer (Max Planck Institute for Solid State Research). —
The mineral compound malachite, with the chemical formula
Cu2(OH)2CO3, is a green pigment known since the ancient
Egypt. Surprisingly, its magnetic properties have not at-
tractedmuchattention. Fromastructural point of view,mala-
chite is composed of Cu2O6 dimers forming chains running in
the (a, c) plane of the monoclinic space group P21/a. Along
the b-axis, the chains are well separated by carbonate CO3
groups. The investigations of its magnetic properties began
recently [1,2]. Thermodynamical measurements on natural
polycrystalline samples show no evidence of long rangemag-
netic order down to T = 2 K and a spin gap of 125 K. From
these measurements, the model proposed to describe mala-
chite is the alternating antiferromagnetic Heisenberg chain
with an alternation parameter α = 0.5. This suggests that
malachite could be the first candidate situated in the middle
between weakly coupled dimers (0 ≤ α < 0.5) and strongly
interacting dimers (0.5 < α ≤ 1). Recent LDA calculations
[2] highlight a more complex network of interactions in mala-
chite. Indeed, the magnetic interactions between chains via
thecarbonategroupscannotbeneglectedand induce frustra-
tion. Malachite is therefore as a rare Cu2+ system presenting
a complex network of competing interactions. We have used
inelastic neutron scattering to characterize the ground state
of malachite on fully deuterated polycrystalline samples. The
measured energy and wave vector dependence of the mag-
netic excitations at zero field is used to obtain a description of
the ground state and themagnetic interactions of malachite.[1] E. Janod et al., Solid State Commun., 11, 513 (2000); [2] S.
Lebernegg et al., Phys. Rev. B 88, 224406 (2013).
S20903 Triplon Hall effect in the Shastry SutherlandMate-
rial. R. Ganesh, J. Romhányi, K. Penc —We demonstrate that
SrCu2(BO3)2 (SCBO), the celebrated realization of the Shas-
try Sutherland (SS) model, hosts triplon bands with topolog-
ical character. While the SS model has an exact dimer sin-
glet ground state, thematerial SCBOhas smallDzyaloshinskii
Moriya (DM) interactions which admix a triplet component
into the ground state. Furthermore, the DM interactions act
as a vector potential that couples to the triplon excitations.
An applied magnetic field then opens band gaps, leaving us
with topologicalmagnonbandswithnon-zeroChernnumbers
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(±2). Thus, SCBOsupports protected triplon edgemodes and
is amagnetic analogueof the integer quantumHall effect. We
predict a measurable thermal Hall signal in SCBOwherein an
applied thermal gradient leads to a transverse heat current.
S20904 Spin supersolid on the Shastry-Sutherland lattice.
P. Sengupta, K. Wierschem (NTU) —We have studied an extended
version of the canonical Shastry-Sutherland model – includ-
ing additional interactions and exchange anisotropy in an ap-
plied magnetic field. The model is appropriate for describ-
ing the low energy properties of some members of the rare
earth tetraborides. In the limit of large Ising-like exchange
anisotropy, we demonstrate the stabilization of columnar an-
tiferromagnetic order in the ground state at zero field and an
extendedmagnetizationplateauat1/2 the saturationmagne-
tization in the presence of an applied longitudinal magnetic
field. Our results show that for an optimal range of exchange
parameters, a spin supersolid ground state is realized over a
finite range of an applied field between the columnar antifer-
romagnetic phase and themagnetization plateau. Our results
provide crucial guidance in designing further experiments to
search for the interesting spin supersolid phase in ErB4.
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