A semi-classical approach to electron spin resonance in quantum spin systems

TL;DR

This paper introduces a semi-classical method using rotor models and Monte Carlo simulations to analyze electron spin resonance in quantum spin systems, particularly effective in one-dimensional large-spin systems at intermediate temperatures.

Contribution

It develops a semi-classical approximation framework for ESR in quantum spin systems, validated against quantum Monte Carlo results and applied to compute ESR spectra.

Findings

Semi-classical approximation matches quantum Monte Carlo magnetization results.

Calculated ESR spectra show broad paramagnetic and spin wave resonances.

Method is especially effective for 2D systems, demonstrated in 1D large-spin chains.

Abstract

We develop a semi-classical approximation to electron spin resonance in quantum spin systems, based on the rotor or non-linear sigma model. The classical time evolution is studied using molec- ular dynamics while random initial conditions are sampled using classical Monte Carlo methods. Although the approximation may be especially powerful in two dimensions, we apply it here to one- dimensional systems of large spin at intermediate temperatures, in the presence of staggered and uniform magnetic fields. We first test the validity of the semi-classical approximation by comparing the magnetization to quantum Monte Carlo results on S = 2 chains. Then we calculate the ESR spectrum, finding broad coexisting paramagnetic and spin wave resonances.