Temperature behavior of the magnon modes of the square lattice antiferromagnet

TL;DR

This paper develops a spin-wave theory for the square lattice Heisenberg antiferromagnet, revealing temperature-dependent changes in magnon modes and matching Monte Carlo results without fitting parameters.

Contribution

It introduces a new spin-wave theoretical framework that captures short-range order and temperature effects in the antiferromagnet, aligning with previous modified theories and simulations.

Findings

A gapless magnon mode appears with increasing temperature.

Spectral intensity shifts from conventional to gapless modes.

Theory agrees with Monte Carlo results for T < 0.8J.

Abstract

A spin-wave theory of short-range order in the square lattice Heisenberg antiferromagnet is formulated. With growing temperature from T=0 a gapless mode is shown to arise simultaneously with opening a gap in the conventional spin-wave mode. The spectral intensity is redistributed from the latter mode to the former. For low temperatures the theory reproduces results of the modified spin-wave theory by M.Takahashi, J.E.Hirsch et al. and without fitting parameters gives values of observables in good agreement with Monte Carlo results in the temperature range 0 <= T < 0.8J where J is the exchange constant.