Charge disproportionation in the spin-liquid candidate $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$ at 6 K revealed by $^{63}$Cu NQR measurements

TL;DR

This study reveals a charge disproportionation transition at 6 K in the spin-liquid candidate $$-(ET)$_2$Cu$_2$(CN)$_3$, detected via $^{63}$Cu NQR, which clarifies the origin of the mysterious 6 K anomaly.

Contribution

The paper demonstrates that the 6 K anomaly is due to charge disproportionation, identified through $^{63}$Cu NQR measurements, providing new insight into the electronic state of this spin-liquid candidate.

Findings

Sharp peak in $T_1^{-1}$ at 6 K for $^{63}$Cu NQR

No anomaly in $^{13}$C and $^{1}$H NMR $T_1^{-1}$

Charge disproportionation likely causes the 6 K anomaly

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.