The Heisenberg antiferromagnet on an anisotropic triangular lattice: linear spin-wave theory

TL;DR

This paper uses linear spin-wave theory to analyze quantum fluctuations in the Heisenberg antiferromagnet on an anisotropic triangular lattice, revealing potential quantum disordered states near phase transitions.

Contribution

It provides a detailed analysis of quantum spin fluctuations and their impact on ground state properties across different regimes of the anisotropic triangular lattice model.

Findings

Magnetization vanishes near J2/J1=0.5, indicating possible quantum disordered state.

Quantum corrections are large but finite near phase transition, contrasting with square lattice models.

Large J2/J1 ratios suggest a quantum disordered state due to significant quantum fluctuations.

Abstract

We consider the effect of quantum spin fluctuations on the ground state properties of the Heisenberg antiferromagnet on an anisotropic triangular lattice using linear spin-wave theory. This model should describe the magnetic properties of the insulating phase of the kappa-(BEDT-TTF)_2 X family of superconducting molecular crystals. The ground state energy, the staggered magnetization, magnon excitation spectra and spin-wave velocities are computed as a function of the ratio between the second and first neighbours, J2/J1. We find that near J2/J1 = 0.5, i.e., in the region where the classical spin configuration changes from a Neel ordered phase to a spiral phase, the staggered magnetization vanishes, suggesting the possibility of a quantum disordered state. In this region, the quantum correction to the magnetization is large but finite. This is in contrast to the frustrated Heisenberg model on a square lattice, for which the quantum correction diverges logarithmically at the transition from the Neel to the collinear phase. For large J2/J1, the model becomes a set of chains with frustrated interchain coupling. For J2 > 4 J1, the quantum correction to the magnetization, within LSW, becomes comparable to the classical magnetization, suggesting the possibility of a quantum disordered state. We show that, in this regime, quantum fluctuations are much larger than for a set of weakly coupled chains with non-frustated interchain coupling.