北海道大学エンレイソウの会、2010年10月5日

しょっぱい砂糖とスピンの開放--- Charge-Lattice-Coupled Quantum Phase Transition--K. Kanoda, Applied Physics, University of Tokyo

Collaborators

Univ. TokyoAISTOsaka City Univ.

R. Takehara, Hosoda, F. Iwase ( present;IMS), K. Miyagawa
Y. Tokura, H. Okamoto
S. Horiuchi
K. Murata

Outline0.

Introduction to NIT

1.

Observation of quantum critical fluctuations in NIT

2.

Separation of charge transfer and lattice symmetry breaking

TTF-QCl4 の結晶構造• 擬一次元交互積層型電荷移動錯体
TTF (tetrathiafulvalene)
ドナー

QCl4 (p-chloranil)
アクセプターe-7.40 Å数値は Le Cointe et al., Phys. Rev. B, 51, 3374 (1995) による

7.62 Å

Neutral-Ionic transition (NIT)TTF-QCl4
Torrance et al., PRL. (1981)

TTF

QCl4

TTF QCl4

NeutralparaelectricBand insulator

Temperature
Pressure

Charge transfer
Lattice dimerization+ - + - + -IonicCosts electronic energy
Gains Mardelung energy+
TTF QCl4

ferroelectric

Spin-Peierls
polarized

Dimerized
Mott insulator

NIT phase diagram
TTF-QCl4

Lemee-Cailleau et al.
PRL 79, 1690
(1997)

Neutral phase

Ionic phase

Okamoto et al.

1st-order transitionof simultaneous CT and dimerizationNIT in DMTTF-QCl4 –continuous and quantum by Horiuchi, Tokura
Chemical pressure
DMTTF-2,6-QBr2Cl2
QCl4

DMTTF

QCl4
QBr4

Physical pressure
NeutralIonic

Continuous transition
Horiuchi et al., J. Am. Chem. Soc., 123, 665 (2001).

Nuclear quadrupole resonance (NQR)
probes the charge state and symmetry of the lattice
I ≥1

I=3/2m = ±3 / 2m = ±1 / 2
nucleusResonance frequency

Charge transfere-e-

Cl or Br nuclei

rf pulsehν Q
Electric field gradient (EFG)e 2 qQη21+νQ =2=3line shifteq = Vzz
η = (Vxx − V yy ) Vzz

Dimerization

νQ

Cl or Br nuclei

line splitting

νQNQR

Charge transfer

Spectral shift
Spectral splitting

Symmetry breaking (dimerization)

Iwase et al., JPSJ (2007)
NeutralIonic

Ionic
TTF-QCl4 Cl-NQR

QCl4

NeutralJ. Gallier et al., Phys. Rev. B, 47,11688 (1993).

NQR
spectraLinewidthLine width (FWHM)

Spin-lattice relaxation rate 1/T1

DMTTF-QCl4 35Cl NQR

DMTTF-QBr4 Br NQR

Barrett formula:ε = A+B(T1 2) coth(T1 2T ) − T0

T0：Curie-Weiss temperature
T1：characteristic crossover temperature
dividing the quantum and classical region

NQR spectra at 5K

Pressure-induced NIT at low T
dimerization Ps ( P,0) ∝ ν split ∝ (P − Pc )βPc(20K)=1.41 kbar

20 K

β=0.55Pc(5K)=1.32 kbar

5K

β=0.55mean-field behavior
R. Morf et al., PRB 16462 (1977).

Pressure-induced NIT; continuous and quantum

79Br NQR

Spins are hidden
all the way across the transition
QBr4 molecule

Quantum criticalfluctuations

NMR緩和
電場揺らぎか磁気揺らぎか？
I ≥1

I=3/2m = ±3 / 2m = ±1 / 2hν Qnucleus
同位体核位置における環境は同じ、

γ n, Q79Br(1/T )/81Br(1/T )11

違いは核の定数 γnとQ

H M ' = −γ n =I ⋅ δH

1 T1 ∝ Aγ n2(79γ/81γ)2=(10.67/11.50)2=0.861

H Q ' = −eQ ⋅ δq

1 T1 ∝ BQ 2(79Q/81Q)2= (1.197)2=1.43

R. E. Alonso et al., Phys. Rev. B 69, 125101 (2004).
J. Bieron et al., Phys. Rev. A 64, 052507 (2001).

1/T1は磁気相互作用によるものではない
つまり、転移近傍でもスピンは顔を出さない
20 K 79Br &81Br NQR

臨界緩和は電荷/格子ゆらぎによるもの（四重極緩和）であることが明らかになった．

スピンは、ずっと閉じこめられたままか？

NIT phase diagram
TTF-QCl4

Lemee-Cailleau et al.
PRL 79, 1690(1997)

Neutral phase

Ionic phase

Okamoto et al.

1st-order transitionof simultaneous CT and dimerization

Separation of charge transfer and dimerization transition
Shift without splitting

TTF-QCl4

Charge transfer

Splitting
Dimerization

NeutralIonic

M. H. Lemee-Cailleau et al., PRL, 79, 1690 (1997)

Extended phase diagram of TTF-QCl4
N phaseN-I domain wallcharge soliton

Short-range order
Spin soliton

Dimer gas

Dimer liquidor Heisenberg modelwith short-ranged spinPeierls order

Dimer crystal
I phasepolarization

Long-range order

Conclusion
Charge transfer

Temperature

Spins deconfined

Uniform
Neutral phase

Uniform
Ionic phase
Dimerization

Dimerized
Ionic phase(Ferroelectric)

Quantum criticalfluctuation

0Charge transfer
Dimerization

Pressure