Gapped magnetic ground state in the spin-liquid candidate $\kappa$-(BEDT-TTF)$_2$Ag$_2$(CN)$_3$ suggested by magnetic spectroscopy

TL;DR

This study uses multifrequency ESR measurements to investigate the magnetic ground state of a frustrated quantum spin liquid candidate, revealing a gapped, singlet-rich state with impurity contributions.

Contribution

First detailed ESR analysis indicating a gapped, singlet-dominated ground state in $$-(BEDT-TTF)$_2$Ag$_2$(CN)$_3$, a rare triangular lattice quantum spin liquid.

Findings

Spin susceptibility drops sharply at low temperatures, indicating a spin gap.

ESR linewidth decreases monotonically, suggesting strong singlet correlations.

Impurity spins contribute to the weak Curie-like susceptibility at low temperatures.

Abstract

The nature of the magnetic ground state of highly frustrated systems remained puzzling to this day. Here, we have performed multifrequency electron spin resonance (ESR) measurements on a putative quantum spin liquid compound $\kappa$-(BEDT-TTF)$_2$Ag$_2$(CN)$_3$, which is a rare example of $S = 1/2$ spins on a triangular lattice. At high temperatures, the spin susceptibility exhibits a weak temperature dependence which can be described by the Heisenberg model with an antiferromagnetic exchange interaction of strength $J/k_B \approx 175$ K. At low temperatures, however, the rapid drop of the static spin susceptibility, together with monotonic decrease of the ESR linewidth indicates that strong singlet correlations develop below a pairing energy scale $T^*$ accompanied by a spin gap. On the other hand, a weak Curie-like spin susceptibility and the angular dependence of the linewidth suggest additional contribution from impurity spins. We propose the gradual formation of spin singlets with an inhomogeneous spin gap at low temperatures.