Quadrupolar correlations and spin freezing in S = 1 triangular lattice antiferromagnets

TL;DR

This paper investigates the finite temperature properties and disorder effects in S=1 triangular lattice antiferromagnets, revealing robust quadrupolar correlations, specific heat features, and spin freezing phenomena relevant to NiGa2S4.

Contribution

It introduces a novel semiclassical Monte Carlo approach to study quadrupolar correlations and analyzes the impact of weak disorder on magnetic order and spin freezing.

Findings

Quadrupolar correlations lead to a two-peak specific heat structure.

Weak disorder disrupts long-range magnetic order, inducing spin-glass-like freezing.

The study provides insights into experimental observations in NiGa2S4.

Abstract

Motivated by experiments on NiGa2S4, we discuss characteristic (finite temperature) properties of spin S = 1 quantum antiferromagnets on the triangular lattice. Several recent theoretical studies have suggested the possibility of quadrupolar (spin-nematic) ground states in the presence of sufficient biquadratic exchange. We argue that quadrupolar correlations are substantially more robust than the spin-nematic ground state, and give rise to a two peak structure of the specific heat. We characterize this behavior by a novel T > 0 semiclassical approximation, which is amenable to efficient Monte Carlo simulations. Turning to low temperatures, we consider the effects of weak disorder on incommensurate magnetic order, which is present when interactions beyond nearest neighbor exchange are substantial. We show that non-magnetic impurities act as random fields on a component of the order parameter, leading to the disruption of long-range magnetic order even when the defects are arbitrarily weak. Instead, a gradual freezing phenomena is expected on lowering the temperature, with no sharp transition but a rapid slowing of dynamics and the development of substantial spin-glass-like correlations. We discuss these observations in relation to measurements of NiGa2S4.