Finite-temperature order-disorder phase transition in a frustrated bilayer quantum Heisenberg antiferromagnet in strong magnetic fields

TL;DR

This paper studies the phase transition in a frustrated bilayer quantum Heisenberg antiferromagnet near saturation, revealing a mapping to a hard-square model and identifying a 2D Ising universality class transition with specific heat singularity.

Contribution

It introduces a mapping of the low-energy spin degrees of freedom to a hard-square gas and characterizes the associated phase transition in the quantum antiferromagnet.

Findings

Identification of a phase transition in the spin system.

Mapping to a hard-square lattice gas model.

Observation of a specific heat singularity near saturation.

Abstract

We investigate the thermodynamic properties of the frustrated bilayer quantum Heisenberg antiferromagnet at low temperatures in the vicinity of the saturation magnetic field. The low-energy degrees of freedom of the spin model are mapped onto a hard-square gas on a square lattice. We use exact diagonalization data for finite spin systems to check the validity of such a description. Using a classical Monte Carlo method we give a quantitative description of the thermodynamics of the spin model at low temperatures around the saturation field. The main peculiarity of the considered two-dimensional Heisenberg antiferromagnet is related to a phase transition of the hard-square model on the square lattice, which belongs to the two-dimensional Ising model universality class. It manifests itself in a logarithmic (low-)temperature singularity of the specific heat of the spin system observed for magnetic fields just below the saturation field.