Strong Correlations, Frustration, and why you should care
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Transcript of Strong Correlations, Frustration, and why you should care
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Strong Correlations, Frustration, and why
you should care
Workshop on Future Directions
For some other perspectives, see http://motterials07.wikispaces.com/Seminar+Schedule#current Discussion Monday “Grand Challenges in oxides”
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Two reasons to study condensed matter Intellectual adventure
Understand nature Uncover basic
mechanisms and organization of matter
Explore neat phenomena
Usefulness Create and use
functional materials Make devices Change the world
These are not independent (or shouldn’t be!) Major progress in useful parts of condensed matter
involves plenty of intellectual adventure With infinite variety of natural phenomena, we need some
guidance
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What makes a material/device useful? Semiconductors:
Sensitivity: can control charge with modest doping, electric fields
Quality: clean materials and great interfaces Understanding: semiconductor modeling is
simple! GMR
Sensitivity: control resistance with modest B field
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What do correlated electrons have to offer? New capabilities:
Superconductivity Diverse magnetism Spin-charge coupling, e.g. multiferroics Large thermopower Controlled many-electron coherence in
nanostructures Sensitivity:
Competing/coexisting ordered states, very close in energy
Balance between these states is easily altered
Archetype: frustrated magnets
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Frustrated Magnets
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Cr: d3
Spinel: ACr2X4
A=Zn,Cd,HgX=O
Antiferromagnet Multiferroic
A=Mn,Fe,CoX=O
A=CdX=S
Colossal magnetocapacitance
Data from S.-H. Lee, Takagi, Loidl groups
Sensitivity of Frustrated Magnets
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Challenge: spin liquid regime Frustration leads to suppressed order
“Frustration parameter” f=CW/TN & 5-10 System fluctuates between competing ordered
states for TN<T<CW
What is the nature of the correlated liquid?
Spin liquid
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Frustration: Degeneracy When kBT ¿ J, system is constrained to ground
state manifold Triangular lattice Ising antiferromagnet
One dissatisfied bond per triangle Entropy 0.34 kB / spin
Pyrochlore Heisenberg antiferromagnet
Pyrochlore “Spin ice”: 2 in/2 out Ising spins Pauling entropy ¼ ½ ln(3/2) kB / spin
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A rare example of understanding Pyrochlore spin liquids are “emergent
diamagnets” Local constraint: Dipolar correlations
Youngblood and Axe, 1980Isakov, Moessner, Sondhi 2003
Y2Ru2O7: J. van Duijn et al, 2007
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Problem: develop “spin liquid theory” Details of dipolar correlations are too
subtle for current experiments (SNS?) Need other probes of the liquid state
Impurities How does a defect affect the correlated
medium? Analog of Friedel oscillations? How do they couple?
Phase transitions What is the nature of ordering phenomena out
of the spin liquid? Constraint can change critical behavior
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Strange spin glasses in HFMs SCGO: SrCr9pGa12-9pO19 s=3/2 kagome
• Tg independent of disorder at small p?• Unusual T2 specific heat?
• nearly H-independent!
Ramirez et al, 89-90.
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A simple model of constrained criticality Classical cubic dimer model
Hamiltonian
Model has unique ground state – no symmetry breaking.
Nevertheless there is a continuous phase transition! - Without constraint there is only a crossover.
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Numerics (courtesy S. Trebst)
Specific heat
C
T/V
“Crossings”
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Other spin liquids? A-site spinels Many materials!
1 900
FeSc2S4
10 205
CoAl2O4
MnSc2S4
MnAl2O4
CoRh2O4 Co3O4
s = 5/2
s = 3/2
f À 1: “Spiral spin liquid” Q-fluctuations constrained to “spiral surface” Very different from dipolar spin liquid
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Quantum Spin Liquids f = CW/TN =1 : quantum paramagnetism RVB and gauge theory descriptions
developed theoretically but Recent flurry of experimental QSLs do not
match theory very well! Herbertsmithite kagome Na3Ir4O8 hyperkagome NiGa2S4 triangular s=1 -(BEDT) organic triangular lattice FeSc2S4 diamond lattice spin-orbital liquid
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Na3Ir4O7 Hyperkagome A quantum paramagnet:
CW¼ -650K2500
2000
1500
1000
500
0
1 (
mol
Ir/
cm3 )
3002001000T (K)
Na4Ir3O8
H = 1 T
5d5 LS
Ir4+
S = 1/2
60
40
20
0Cm
/T (
mJ/
Km
ol I
r+T
i)
200150100500T (K)
8
6
4
2
0
S m (
J/K
mol
Ir+
Ti)
1.8
1.6
1.4
(1
0-3em
u/m
ol I
r)
1086420T (K)
2.0
1.8
1.6
1.4
10-3
x = 0
0.01 T0.1 T1 T5 T
Tg
1
0-3
em
u/m
ol Ir
» Const
C » T2
inconsistent with quasiparticle picture?
Same behavior in other s=1/2 materials!
0 10K
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What is frustration good for? Obtain coexisting orders
Multiferroics: (ferro)magnetism and ferroelectricity Strong spin-lattice coupling effects in frustrated magnets Non-collinear spiral magnetism very generic and couples
(often) to electric polarization
Control magnetism by engineering interactions Only small changes need be made even when dominant
exchange is large Interesting to try by oxide interface engineering
c.f. J. Tchakalian, La(Cr/Fe/Mn)O3 layers already under study Can “generic” spiral states of frustrated magnets be
disrupted in interesting ways by interfaces?
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Orbital Frustration
Spinel FeSc2S4
CW=50K, TN<30mK: f>1600! Integrated entropy indicates
orbitals are involved
Orbital degeneracy is a common feature in oxides (perovskites, spinels, etc.) Often removed by Jahn-Teller effect Can JT be avoided by frustration and
fluctuations? Can orbitals be quantum degrees of freedom?
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The Future Controlling correlations and frustration
Understand the mechanisms behind competing/coexisting orders and correlated liquids
In magnets and other contexts Learn to control them by
Chemistry and materials processing (e.g. oxide heterostructures)
External means (gates, fields, strain, etc.)
Tremendous improvements in our understanding of correlated materials Improved probes (SNS, tunneling, Inelastic x-rays) Improved materials (laser MBE…) Improved theory: synergy of ab initio and
phenomenological methods