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

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.

6 pages, 4 figures, accepted for publication in Phys. Rev. Res. rapid communication

References in corpus (6)

• Mott Transition from a Spin Liquid to a Fermi Liquid in the Spin-Frustrated Organic Conductor kappa-(ET)2Cu2(CN)3
• Amperean Pairing Instability in the U(1) Spin Liquid State with Fermi Surface and Application to $\kappa -(BEDT-TTF{)}_{2}C{u}_2(CN{)}_3$
• Emergence of inhomogeneous moments from spin liquid in the triangular-lattice Mott insulator $\kappa$-(ET)${}_2$Cu${}_2$(CN)${}_3$
• Absence of charge order in the dimerized κ-phase BEDT-TTF salts
• Field-induced staggered magnetic moment in the quasi-two-dimensional organic Mott insulator $\kappa$-(BEDT-TTF)${}_2$Cu[N(CN)${}_2$]Cl
• Insulator-metal transition on the triangular lattice

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• Charge-Transfer Complex $\kappa$-(BEST)${}_2$Cu${}_2$(CN)${}_3$ Analogous to Organic Spin Liquid Candidate
• Absence of superconductivity in the Hubbard dimer model for kappa-(BEDT-TTF)_2X
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