Biconical structures in two-dimensional anisotropic Heisenberg antiferromagnets

TL;DR

This paper investigates the role of biconical structures in phase transitions of two-dimensional anisotropic Heisenberg antiferromagnets, revealing how added anisotropy influences phase behavior through simulations.

Contribution

It provides a detailed analysis of how biconical structures affect phase transitions in anisotropic Heisenberg models, including the impact of single-ion anisotropy.

Findings

Biconical structures are crucial in the transition region between antiferromagnetic and spin-flop phases.

Adding single-ion anisotropy can lead to either an intervening biconical phase or a direct first-order transition.

A narrow disordered phase dominated by bidirectional fluctuations appears at the critical field.

Abstract

Square lattice Heisenberg and XY antiferromagnets with uniaxial anisotropy in a field along the easy axis are studied. Based on ground state considerations and Monte Carlo simulations, the role of biconical structures in the transition region between the antiferromagnetic and spin--flop phases is analyzed. In particular, adding a single--ion anisotropy to the XXZ antiferromagnet, one observes, depending on the sign of that anisotropy, either an intervening biconical phase or a direct transition of first order separating the two phases. In case of the anisotropic XY model, the degeneracy of the ground state, at a critical field, in antiferromagnetic, spin--flop, and bidirectional structures seems to result, as in the case of the XXZ model, in a narrow disordered phase between the antiferromagnetic and spin--flop phases, dominated by bidirectional fluctuations.