Free energy variational approach for the classical anisotropic XY model in a crystal field

TL;DR

This paper develops a variational method to analyze the free energy and phase diagrams of the three-dimensional anisotropic XY model with a crystal field, comparing results with Monte Carlo simulations and other approaches.

Contribution

It introduces a variational approach that simplifies the analysis of the anisotropic XY model in a crystal field, providing insights into phase behavior and magnetization.

Findings

Phase diagrams as a function of Hamiltonian parameters

Agreement with Monte Carlo and series expansion results at low temperatures

Simplification of self-consistent equations through assumptions

Abstract

A variational approach for the free energy is used to study the three-dimensional anisotropic XY model in the presence of a crystal field. The magnetization and the phase diagrams as a function of the parameters of the Hamiltonian are obtained. Some limiting results for isotropic XY and planar rotator models in two and three dimensions are analyzed and compared to previous results obtained from analytical approximations as well as from those obtained from more reliable approaches such as series expansion and Monte Carlo simulations. It is also shown that from this general variational approach some simple assumptions can drastically simplify the self-consistent implicit equations. The validity of the low temperature region of this approach is analyzed and compared to Monte Carlo results as well.