Quantum Spin-Liquid Behavior in the Spin-1/2 Random Heisenberg Antiferromagnet on the Triangular Lattice

Abstract

Experimental quest for the hypothetical "quantum spin liquid" state has\nrecently met a few promising candidate materials including organic salts\n\\kappa-(ET)2Cu2(CN)3 and EtMe3Sb[Pd(dmit)2]2, S=1/2 triangular-lattice\nHeisenberg antiferromagnets consisting of molecular dimers. These compounds\nexhibit no magnetic ordering nor the spin freezing down to very low\ntemperature, while various physical quantities exhibit gapless behaviors.\nRecent dielectric measurements revealed the glassy dielectric response\nsuggesting the random freezing of the electric polarization degrees of freedom.\nInspired by this observation, we propose as a minimal model of the observed\nquantum spin-liquid behavior the S=1/2 antiferromagnetic Heisenberg on the\ntriangular lattice with a quenched randomness in the exchange interaction. We\nstudy both zero- and finite-temperature properties of the model by an exact\ndiagonalization method, to find that when the randomness exceeds a critical\nvalue the model exhibits a quantum spin-liquid ground state. This\nrandomness-induced quantum spin-liquid state exhibits gapless behaviors\nincluding the temperature-linear specific heat. The results provide a\nconsistent explanation of the recent experimental observations on organic\nsalts.\n