Site occupation constraints in mean-field approaches of quantum spin systems at finite temperature

TL;DR

This paper investigates how site occupation constraints affect mean-field descriptions of quantum spin systems at finite temperature, comparing fixed-site occupation to average occupation methods and analyzing their impact on order parameters.

Contribution

It introduces and compares two different constraints for site occupation in mean-field models, highlighting their quantitative differences in describing quantum spin systems.

Findings

Significant quantitative differences between fixed and average occupation constraints.

Different mean-field representations lead to varying order parameter behaviors.

The choice of site occupation constraint impacts the accuracy of quantum spin system descriptions.

Abstract

We study the effect of site occupation on the description of quantum spin systems at finite temperature and mean-field level. We impose each lattice site to be occupied by a single electron. This is realized by means of a specific prescription. The outcome of the prescription is compared to the result obtained by means of a projection procedure which fixes the site occupation to one particle per site on an average. The comparison is performed for different representations of the Hamiltonian in Fock space leading to different types of mean-field solutions. The behaviour of order parameters is analyzed for each choice of the mean-field and constraint which fixes the occupation rate at each site. Sizable quantitative differences between the outcomes obtained with the two different constraints are observed.