Post on 01-Feb-2016
description
Structure and dynamics of spin polarons induced by doping a Haldane
spin-1 chain
Collin Broholm*
Johns Hopkins University and NIST Center for Neutron Research
* Supported by the National Science Foundation
Ying Chen LANLGuangyong Xu BNLG. Aeppli UCLJ. F. DiTusa LSU I. A. Zaliznyak BNLC. D. Frost ISIST. Ito Tsukuba
M. Kenzelmann JHUK. Oka TsukubaT. G. Perring ISISH. Takagi ISSP M. E. Bisher NECM. M. J. Treacy NECR. Paul NIST
Xu et al., Science 289, 419 (2000)Kenzelmann et al., PRL 90, 087202 (2003)
Outline
1. Introduction to pure Y2BaNiO5
2. Site impurities (Mg2+↔ Ni2+)
3. Bond impurities (Ca2+↔ Y3+)
4. Conclusions and outlook
Sendai 11/8/03SPINS cold neutron spectrometer at NCNR
kikf Q
''R
)'( )0(S)(S1
2
1),(
RRR
RRQiti teN
edtQ
S
''R
)'( )0(S)(S1
2
1),(
RRR
RRQiti teN
edtQ
S
Sendai 11/8/03
MAPS Spectrometer at ISIS in UK
Sendai 11/8/03
Haldane gap in Y2BaNiO5
40
20
60
00 0.5 1 1.5 2
q ()
h
(m
eV
)S(q)~ (-(q))1-cosq(q)
Ba
Y
O2-
Ni
T. Ito, K. Oka and H. Takagi
MAPS (ISIS)
Sendai 11/8/03
in
ten
sity
(co
utn
s p
er
15
min
.)
I(q
,w)
(1/m
eV
)
a b c
Inter-chain coupling and anisotropy
I(q
,w)
(1/m
eV
)
in
t. (
cts
per
15
min
.)
Chain
Q
Sendai 11/8/03
Facts on Y2BaNiO5
, 1,
2 2 2
, , ,
' , , '''
n m n mnm
z x yn m n m n mnm nm
mm n m n mnmm
J
D S E S S
J
S S
S S
HIntra-chain exchangeIntra-chain exchange
AnisotropyAnisotropy
Inter-chain exchangeInter-chain exchange
Chemical Formula Common name J meV
D/J 10-3
E/J 10-3
gBH/J J’/J 10-3
TN K
AgVP2S6 58(4) 5.8 0.020 0.01 < 2 Y2BaNiO5 21 -39 12 0.055 < 0.5 < 0.05 Ni(C3H10N2)2N3(ClO4) NINAZ 10.7 170 0.11 <0.7 <0.06 Ni(C3H10N2)2NO2(ClO4) NINO 4.5 250 0.21 <1.2 Ni(C2H8N2)2NO2(ClO4) NENP 4.1(3) 180 0.28 0.8 <0.0003 Ni(C5D14N2)2N3(PF6) NDMAP 2.85 250 0.41 0.6 <0.25 CsNiCl3 2.275 -1.9 0.45 17 4.9
J meV
D/J 10-3
E/J 10-3
J’/J 10-3
TN K
21 -39 12 < 0.5 < 0.05
Sendai 11/8/03
Outline
1. Introduction to pure Y2BaNiO5
2. Site impurities (Mg2+↔ Ni2+)
3. Bond impurities (Ca2+↔ Y3+)
4. Conclusions and outlook
Sendai 11/8/03
Cutting chains in Y2BaNi1-xMgxO5
• Mg2+on Ni2+ sites finite length chains
Kojima et al. (1995)
Pure
Ni2+Ni2+
Mg2+Mg2+
3
Mg2+ Y3+
O2-
Sendai 11/8/03
Haldane gap with impurities
Kenzelmann et al. PRL (2003)
Sendai 11/8/03
An ensemble of finite length chains
Chain length L
P(L
)
2%
4%
8%
P(
)/m
ax(
P(
))
2%
4%
8%
2 1L
P L Lc c Probability spin in isolated segment of length L
Gap from chain of length L 2
2L vL
Ensemble averaged scattering LLS q P L S q
Sendai 11/8/03
Haldane gap with impurities
Kenzelmann et al. PRL (2003)
Sendai 11/8/03
ESR with neutrons
Kenzelmann et al. PRL (2003)
Sendai 11/8/03
Minimal model for chain end spins
Bg H
Bg H
1 expL LJ L
L is odd
Bg H
Bg HL is even
0 2 2 20 0L L L L L z x y z B zE J D DS E S S g HS HSinglet-triplet Single ion anisotropy Zeeman
C. D. Batista et al., PRB (1999)
Affleck, Kennedy, Lieb, and Tasaki PRL (1987)
Sendai 11/8/03
Chain-end composite spin
Kenzelmann et al. PRL (2003)
Sendai 11/8/03
Form factor for chain-end spin
Kenzelmann et al. PRL (2003)
Sendai 11/8/03
Outline
1. Introduction to pure Y2BaNiO5
2. Site impurities (Mg2+↔ Ni2+)
3. Bond impurities (Ca2+↔ Y3+)
4. Conclusions and outlook
Sendai 11/8/03
Hole doping Y2-xCaxBaNiO5
•Ca2+ on Y3+ sites mobile bond defects
Kojima et al. (1995)
Pure
Ca2+Ca2+
Y3+Y3+
FMCa2+
Ni
O2-
Sendai 11/8/03
Transport in Ca doped Y2BaNiO5
T. Ito et al. PRL (2002)
1D conductivity, no Charge ordering1D conductivity, no Charge ordering
Sendai 11/8/03
Gap modes in 4% Ca-doped Y2BaNiO5
q
Sendai 11/8/03
Does q vary with calcium concentration?
q not strongly dependent on x
q not strongly dependent on x
single impurity effectsingle impurity effect
Xu et al., Science 289, 419 (2000)
Sendai 11/8/03
Bond Impurities in a spin-1 chain: Y2-
xCaxBaNiO5
Y3+
Ni
O
FM
Ca2+
Form-factor for FM-coupled chain-end spins
2/ 2( ) 2Re ( ) iqS q M q e
A symmetric AFM droplet
22/*2/ )()()(
ll
iql
iqlll eqMeqMPqS
Ensemble of independent randomly truncated AFM droplets
Sendai 11/8/03
Gap modes in 4% Ca-doped Y2BaNiO5
Excited state ?Excited state ?
Quasi-elasticFrom deg. GSQuasi-elasticFrom deg. GS
Sendai 11/8/03
Minimal model of spin polaron
'J'J
2 2 212
2 2 212
12
12
'
'
' 1
' 1LR LOR
L R O L R
LR L R
LOR O LR
S S LR LR
LOR LOR
J J
J
J
E J J S S
J S S cst
S S S S S
S S S
S S S
H
32 'J
'J J
32 1LOR LRS S
12 1LOR LRS S
12 0LOR LRS S
1 12 20,1LR LR LOR LRS S S S
J
LS RS
OS
Sendai 11/8/03
Intra polaron dynamics in Ca-doped Y2BaNiO5
0 5 10 15 20 250
meV
Norm
ali
zed
In
ten
sity
(A
rb)
Pure
4% Ca
Clean gapClean gap
32 'J
?
'J JIntra quartetIntra quartet
Sendai 11/8/03
Conclusions:Experimental observation of increase in
Haldane gap with decreasing chain length Dilute impurities in the Haldane spin chain
create sub-gap composite spin degrees of freedom.
Edge states have an AFM wave function that extends into the bulk over distances of order the Haldane length.
Holes in Y2-xCaxBaNiO5 are surrounded by AFM spin polaron with central phase shift of
Low energy spin polaron excitations could be―oxygen spin flip between aligned chain-end spins―Or anisotropy-split quartet
Viewgraphs and publications at http://www.pha.jhu.edu/~broholm/homepage
Sendai 11/8/03
Outlook
1. Determine spin polaron level assignment in field
2. What is localization length for spin polaron?
3. Spin polaron structure in frustrated magnets
4. Spin polarons in high TC superconductors?
5. Magnetism of interacting spin polarons
Design by T. D. Pikehttp://www.pha.jhu.edu/~broholm/MACS