Phase Transitions in Bilayer Heisenberg Model with General Couplings

TL;DR

This paper investigates the phase diagram of the bilayer square-lattice Heisenberg model with various couplings, revealing classical and quantum phases, including ordered and canted states, through multiple theoretical approaches.

Contribution

It provides a comprehensive analysis of phase transitions in the bilayer Heisenberg model across broad parameters, incorporating quantum fluctuations with advanced methods.

Findings

Identification of classical phases with specific ordering wave vectors

Discovery of a canted phase stabilized by competing interactions

Quantum fluctuations significantly alter the phase boundaries

Abstract

The ground state properties and phase diagram of the bilayer square-lattice Heisenberg model are studied in a broad parameter space of intralayer exchange couplings, assuming an antiferromagnetic coupling between constituent layers. In the classical limit, the model exhibits three phases: two of these are ordered phases specified by the ordering wave vectors (pi,pi;pi) and (0,0;pi), where the third component of each indecates the antiferromagnetic orientation between layers, while another one is a canted phase, stabilized by competing interactions. The effects of quantum fluctuations in the model with S=1/2 have been explored by means of dimer mean-field theory, small-system exact diagonalization, and high-order perturbation expansions about the interlayer dimer limit.