Critical Spin Liquid versus Valence Bond Glass in Triangular Lattice Organic $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$

TL;DR

This paper investigates the low-temperature magnetic properties of the organic triangular-lattice compound $ppa$-(ET)$_2$Cu$_2$(CN)$_3$, showing that disorder-induced valence bond defects, rather than quantum criticality, explain experimental observations.

Contribution

It demonstrates that disorder-induced valence bond defects account for the low-temperature behavior, challenging the quantum critical scenario in this frustrated magnetic system.

Findings

Disorder-induced spin defects explain experimental data.

Valence bond glass phase emerges at low temperature.

Theoretical framework applies broadly to frustrated magnets.

Abstract

In the quest for materials with unconventional quantum phases, the organic triangular-lattice antiferromagnet $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$ has been extensively discussed as a quantum spin liquid (QSL) candidate. Recently, an intriguing quantum critical behaviour was suggested from low-temperature magnetic torque experiments. Through microscopic analysis of all anisotropic contributions, including Dzyaloshinskii-Moriya and multi-spin scalar chiral interactions, we highlight significant deviations of the experimental observations from a quantum critical scenario. Instead, we show that disorder-induced spin defects provide a comprehensive explanation of the low-temperature properties. These spins are attributed to valence bond defects that emerge spontaneously as the QSL enters a valence bond glass phase at low temperature. This theoretical treatment is applicable to a general class of frustrated magnetic systems and has important implications for the interpretation of magnetic torque, nuclear magnetic resonance, thermal transport and thermodynamic experiments.