Variational method study of the spin-1 Ising model with biaxial crystal-field anisotropy

TL;DR

This paper investigates the phase diagram of the spin-1 Ising model with biaxial crystal-field anisotropy using a variational approach, revealing the nature of phase transitions and tricritical points across different lattice structures.

Contribution

It introduces a variational method based on the Bogolyubov inequality to analyze the phase transitions in the spin-1 Ising model with biaxial anisotropy, highlighting the presence of first-order and tricritical points.

Findings

Only continuous phase transitions for certain parameters on small lattices.

First-order and continuous transitions identified by the variational approach.

Tricritical points appear for lattices with coordination number z ≥ 7.

Abstract

The phase diagram of the spin-1 Ising model in the presence of a biaxial crystal-field anisotropy is studied within the framework of a variational approach, based on the Bogolyubov inequality for the free energy. We have investigated the effects of a transverse crystal-field $D_{y}$ on the phase diagram in the $T-D_{x}$ plane. Results obtained by using effective-field theory (EFT) on the honeycomb (% $z=3$), square ($z=4$) and simple cubic ($z=6$) lattices ($z$ is the coordination number) show only continuous phase transitions, while the variational approach presents first-order and continuous phase transitions for $D_{y}=0$% . We have also used the EFT for larger values of $z$ and we observe the presence of tricritical points in the phase diagrams, for $z\geq 7$, in accordance with the variational approach results.