Theoretical studies of the phase transition in the anisotropic 2-D square spin lattice

TL;DR

This paper compares two theoretical methods to study the phase transition in a 2-D anisotropic Heisenberg spin lattice, revealing a first-order transition and domain-dependent behaviors.

Contribution

It introduces and compares the Dressed Cluster Method and Real Space Renormalization Group approach for analyzing phase transitions in anisotropic 2-D spin systems.

Findings

Dressed Cluster Method confirms first-order transition via energy discontinuity.

RG method shows phase transition as due to two domains with fixed points.

Spin gap depends on anisotropy in the Heisenberg-Ising domain.

Abstract

The phase transition occurring in a square 2-D spin lattice governed by an anisotropic Heisenberg Hamiltonian has been studied according to two recently proposed methods. The first one, the Dressed Cluster Method, provides excellent evaluations of the cohesive energy, the discontinuity of its derivative around the critical (isotropic) value of the anisotropy parameter confirms the first-order character of the phase transition. Nevertheless the method introduces two distinct reference functions (either N\'eel or XY) which may in principle force the discontinuity. The Real Space Renormalization Group with Effective Interactions does not reach the same numerical accuracy but it does not introduce a reference function and the phase transition appears qualitatively as due to the existence of two domains, with specific fixed points. The method confirms the dependence of the spin gap on the anisotropy parameter occurring in the Heisenberg-Ising domain.