Heat capacity of anisotropic Heisenberg antiferromagnet within the spin Hartree-Fock approach in quasi-1D Regime

TL;DR

This paper develops a spin Hartree-Fock theory to accurately model heat capacity in anisotropic quantum Heisenberg antiferromagnets, capturing the crossover from 1D to 2D and identifying a quantum transition in triangular lattices.

Contribution

It introduces a quantitative Hartree-Fock approach that reproduces exact 1D results and describes the 1D-2D crossover and quantum transition in anisotropic antiferromagnets.

Findings

The theory matches Bethe Ansatz results in 1D.

No spurious phase transitions occur at finite temperature.

Identifies a quantum transition point at anisotropy ~0.6 in triangular lattices.

Abstract

We study the anisotropic quantum Heisenberg antiferromagnet for spin-1/2 that interpolates smoothly between the one-dimensional (1D) and the two-dimensional (2D) limits. Using the spin Hartree-Fock approach we construct a quantitative theory of heat capacity in the quasi-1D regime with a finite coupling between spin chains. This theory reproduces closely the exact result of Bethe Ansatz in the 1D limit and does not produces any spurious phase transitions for any anisotropy in the quasi-1D regime at finite temperatures in agreement with the Mermin-Wagner theorem. We study the static spin-spin correlation function in order to analyse the interplay of lattice geometry and anisotropy in these systems. We compare the square and triangular lattice. For the latter we find that there is a quantum transition point at an intermediate anisotropy of $\sim0.6$. This quantum phase transition establishes that the quasi-1D regime extends upto a particular point in this geometry. For the square lattice the change from the 1D to 2D occurs smoothly as a function of anisotropy, i.e. it is of the crossover type. Comparing the newly developed theory to the available experimental data on the heat capacity of $\rm{Cs}_2\rm{CuBr}_4$ and $\rm{Cs}_2\rm{CuCl}_4$ we extract the microscopic constants of the exchange interaction that previously could only be measured using inelastic neutron scattering in high magnetic fields.