Vanishing Wilson ratio as the hallmark of quantum spin-liquid models

Abstract

We present numerical results for finite-temperature T>0 thermodynamic quantities, entropy s(T), uniform susceptibility χ_{0}(T), and the Wilson ratio R(T), for several isotropic S=1/2 extended Heisenberg models, which are prototype models for planar quantum spin liquids. We consider in this context the frustrated J_{1}-J_{2} model on kagome, triangular, and square lattice, as well as the Heisenberg model on a triangular lattice with the ring exchange. Our analysis reveals that typically in the spin-liquid parameter regimes the low-temperature s(T) remains considerable, while χ_{0}(T) is reduced consistent mostly with a triplet gap. This leads to vanishing R(T→0), being the indication of a macroscopic number of singlets lying below triplet excitations. This is in contrast to the J_{1}-J_{2} Heisenberg chain, where R(T→0) either remains finite in the gapless regime, or the singlet and triplet gap are equal in the dimerized regime.