Quantum Spin Liquid Behaviors in the Random Spin-1/2 Heisenberg Antiferromagnets on the Triangular and the Kagome Lattices H. KAWAMURA Osaka Univerisity Collaborators: K. WATANABE, H. NAKANO, T. SAKAI, T. SHIMOKAWA NQS 2014, Nov. 5, 2014 KIKEN, Kyoto
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Quantum Spin Liquid Behaviors in the Random Spin-1/2 Heisenberg Antiferromagnets on the Triangular
and the Kagome Lattices
H. KAWAMURA Osaka Univerisity
Collaborators: K. WATANABE, H. NAKANO, T. SAKAI, T. SHIMOKAWA
NQS 2014, Nov. 5, 2014 KIKEN, Kyoto
Possible quantum spin liquid state in frustrated magnets
RVB state [P.W. Anderson (‘73)]
Realizable in frustrated systems?
Novel liquid-like quantum spin state without magnetic long-range order ?
Long quest for the quantum spin liquid
Resonating Valence Bond (RVB) state
Resoting state of quantum-mechanical singlet state generally with a finite gap
long-range)RVB possibly gapless
“gapless” or “gapped”
Ground state of the simplest nearest- neighbor bilinear Heisenberg model
AF magnetic LRO at T=0 even for S=1/2
[ 120 degree structure ]
+ +・・・
Triangular lattice
Kagome lattice
Liquid-like ground state without magnetic long-range order nor the spin freezing
Z2 spin liquid U(1) spin liquid, valence bond crystal, etc
Experimental discovery of the “quantum spin liquid” state
Quantum spin liquid states observed in certain S=1/2 frustrated AFs
κ-(ET)2Cu2(CN)3
EtMe3Sb[Pd(dmit)2]2
S=1/2 organic salts Mott insulator
What is its nature ? → still remains controvertial
Spin is noncoplanar locally even at T =0 ⇒ quantum fluctuations But no chiral LRO
< Chiral freezing parameter qχ >
< Local scalar chirality amplitude χlocal >
[Scalar chirality]
Chiral freezing parameter
Coplanar or noncoplanar ?
Magnetization curve
Near linear magnetization curve for strong randomness.
The 1/3 -plateau disappears
magnetization
< N = 30 >
1/3 plateau
[Exp. dmit] [D. Watanabe et al ‘12]
T=0
Energy gap
Gapless (or very small gap) in the spin-liquid regime at Δ≧Δc
Ground state is a spin singlet for most of samples, with a small fraction of triplets.
First excited state is a spin triplet for most of samples.
Energy gap Rate of triplet ground states
1st excited states
Specific heat
< Overall >
T –linear low-T specific heat in the spin-liquid regime at Δ≧Δc
T - Linear
γ-term calc. ~ 20 mJK-2 exp. κ-ET ~ 12 mJK-2
[Exp: κ-ΕΤ]
T
Specific heat
Susceptibility
[Y.Shimizu, et al . PRL, 2003 ]
Susceptibility exhibits a gapless behavior.
For sufficiently strong randomness, An intrinsic Curie tail is observed.
Susceptibility
NMR relaxation rate T1-1
NMR relaxation rate T1-1
Gapless behavior at low T characterized by an exponent 1.5~2
[Exp. dmit]
∝T1.5
∝T2
A weak finite-T anomaly : Its origin ?
Experimentally, an anomaly in NMR T1
-1
(in specific heat) observed
[Y.Shimizu et al ‘03]
[S.Yamashita et al ‘08]
[T.Itou et al ’08,’ 10] [Y. Shimizu et al ‘03]
NMR relaxation rate T1-1
Broad peak appears for L>18
Some cooperative effect
[Exp. dmit]
A possible candidate of the weak finite-T anomaly
characterized by a parity-like two-valued topological quantum number : no distinction between “R ” & ”L” (no “circulation” in the usual sense)
Vortex formed by chirality vectors
Topological transition (or a crossover) within the spin-liquid state
κ
Z2 vortex
vortex binding-unbinding
[]
[H.K. & S. Miyashita, ‘84]
T < Tv T > Tv
?
Random Singlet Phase ( Valence Bond Glass )
Nature of the quantum spin liquid state
Gapless behavior reflecting the distribution of singlet binding energy due to the distribution of Jij . Nearly free spins can exist as `orphan‘ spins.
Rubust against perturbations. No QCP !
Relation to experiments
1. Stabilization of quantum spin-liquid state
2. Gapless behavior, including the T –linear low-T specific heat, the power-law behavior of the NMR relaxation rate, and gapless (occasionally Curie-like) low-T susceptibility. 3. Near T -linear magnetization curve
4. Robustness against various perturbations e.g., magnetic fields, deuteration, pressure etc. 5. Intrinsic inhomogeniety ! weak inhomogeneous moment induced by fields
NMR [Shimizu et al ’06]
µSR [Nakajima et al ’12] ! `microscopic phase separation’ suggested
Summary (triangular) * S=1/2 random Heisenberg AF on the triangular lattice exhibits a randomness-induced quantum spin-liquid ground state for sufficiently strong randomness.
Random Singlet Phase ( Valence Bond Glass )
* The state is gapless (or nearly gapless), with a T –linear low-T specific heat .
* The spin-liquid state realized here is a “random-singlet” state or a “valence bond glass (VBG) ” state, rather than the RVB state. The state is robust against various perturbations, with no direct relevance to quantum criticality. * The random-singlet phase picture seems to explain various features of experimental results on organic κ-ET and dmit salts.
Quantum spin-liquid-like behavior in the S =1/2 random kagome-lattice Heisenberg AF
herbertsmithite
ZnCu3(OH)6Cl2
Random-bond kagome model as a minimal model of herbertsmithite
Bond-random modulation of the effective exchange coupling J on the kagome plane
[D.E. Freedman et al `10] Zn2+ on the triangular layer replaced by Cu2+
Spin is noncoplanar locally even at T =0 ⇒ quantum fluctuations But no chiral LRO at any Δ
< Chiral freezing parameter qχ >
< Local scalar chirality amplitude χlocal >
[Scalar chirality] Chiral freezing parameter
Specific heat
T - Linear
T
C
Change of behavior around Δc ~ 0.4 within the nonmagnetic state
N=12 & 18
[exp.] ∝T
∝T2/3
The low-T peak (structure) gone for Δ>Δc T-linear low-T specific heat
[J.S. Helton et al. 2007]
Regular kagome (Δ=0)
[S.Sugiura and A.Shimizu, 2013]
Susceptibility
χ
T
gapless susceptibility with a Curie-tail for stronger randomness Change of behavior around Δc ~ 0.4
within the nonmagnetic state
T=0 [J.S. Helton et al, 2010]
[exp.]
“Phase transition” within the non-magnetic state
R = [total number of samples]
[Number of samples with triplet ground states]
R becomes nonzero for Δ > Δc ~0.4, suggesting some sort of transition
AF
Random Singlet
Random Singlet
Z2 (or U(1) ?) spin liquid
Dynamical structure factor S(q,ω) At Γ point T = 0
Δ = 1
(Calc.)
(Exp.) single crystal Very broad
intensity
No gap
Δ = 0
[T.-H. Han et al, ’12]
[A.M. Lauchli et al, 2009 ]
For details, see poster by T.Shimokawa
magnetic
Summary (kagome) * S=1/2 random Heisenberg AF on the kagome lattice exhibits within the non-magnetic state a phase transition from the randomness-irrelevant to the randomness-relevant quantum spin-liquid state with increasing the randomness.
* The randomness-relevant state is gapless (nearly gapless), with a T –linear low-T specific heat .
* The spin-liquid state realized here is a “random-singlet” state or a “valence bond glass (VBG) ” state, with no direct relevance to quantum criticality.
* The random-singlet phase picture seems to explain various features of experimental results on herbertsmithite.
Valence-bond glass picture [R.R.P. Singh, ’10] Site-random model
Related systems Is frustration essential ? S =1/2 random square-lattice Heisenberg AF Corresponding classical system ? Classical random triangular-lattice Heisenberg AF
unfrustrated system --- square lattice
sublattice mag. ms2 spin freezing parameter q
Random-bond S=1/2 AF Heisenberg model on the square lattice
AF LRO persists up to the maximal randomness
Frustration plays a role
Classical system ー random triangular AF Monte Carlo simulations
For sufficiently strong randomness, AF LRO gives way to the spin-glass LRO.
<T = 0 >
planar noncoplanar
3-‐subla)ce AF LRO SG
Δ 0 0.6 0.8 1.0 0.7 0.9 Δc1
( Δ = 0.6 ~ 0.7 ) Δc2 ( Δ = 0.8~0.9 )
Deduced from finite-T Monte Carlo
Summary * S=1/2 random Heisenberg AFs on the triangular and the kagome lattices exhibit a quantum spin-liquid-like behavior, if the randomness exceeds a certain critical value. * The randomness-relevant spin-liquid state is gapless (or nearly gapless), with a T –linear low-T specific heat .
* The spin-liquid state realized here is a “random-singlet” state or a “valence bond glass (VBG) ” state, with no direct relevance to quantum criticality.
* The random-singlet phase picture seems to explain various features of available experimental data on organic salts (triangular) and herbertsmithite (kagome).
* Quantum effect, randomness and frustration are all essential to stabilize the random singlet state.