Charge disproportionation in the spin-liquid candidate κ−(ET)2Cu2(CN)3 at 6 K revealed by Cu63 NQR measurements

T Kobayashi,Y Furukawa,H Taniguchi,A Kawamoto,Q.-P Ding,K Satoh

doi:10.1103/physrevresearch.2.042023

Abstract

The spin-liquid candidate $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$ [ET: bis(ethylenedithio)tetrathiafulvalene] does not exhibit magnetic ordering down to a very low temperature, but shows a mysterious anomaly at 6 K. The origin of the so-called 6 K anomaly is still under debate. We carried out nuclear quadrupole resonance (NQR) measurements on the copper sites of the insulating layers, which are sensitive to the charge dynamics unlike the conventional spin-1/2 nuclear magnetic resonance (NMR). The main finding of this study is that the observation of a sharp peak behavior in the nuclear spin-lattice relaxation rate $T_1^{-1}$ of $^{63}$Cu NQR at 6 K while $T_1^{-1}$ of both $^{13}$C and $^{1}$H NMR show no clear anomaly. This behavior can be understood as a second-order phase transition related to charge disproportionation in the ET layers.

Highlights

Quasi-two-dimensional organic charge transfer salts κ(ET)2X [ET and X denote bis(ethylenedithio) tetrathiafulvalene and monovalent anion, respectively] possess half-filled bands owing to the strong dimeric structures of the donor molecules
We carried out nuclear quadrupole resonance (NQR) measurements on the copper sites of the insulating layers, which are sensitive to the charge dynamics unlike conventional spin-1/2 nuclear magnetic resonance (NMR)
This material does not show long-range magnetic ordering down to T = 32 mK despite a large antiferromagnetic exchange interaction of ∼250 K [6], and it has attracted much attention as a quantum spin liquid. κ-(ET)2Cu2(CN)3 showed a relaxorlike dielectric response below 60 K, and intradimer charge disproportionation (CD) was proposed [7] the mechanism is under debate [8,9,10]

Summary

Introduction

Quasi-two-dimensional organic charge transfer salts κ(ET)2X [ET and X denote bis(ethylenedithio) tetrathiafulvalene and monovalent anion, respectively] possess half-filled bands owing to the strong dimeric structures of the donor molecules. Charge disproportionation in the spin-liquid candidate κ-(ET)2Cu2(CN)3 at 6 K revealed by 63Cu NQR measurements We carried out nuclear quadrupole resonance (NQR) measurements on the copper sites of the insulating layers, which are sensitive to the charge dynamics unlike conventional spin-1/2 nuclear magnetic resonance (NMR).

Results

Conclusion