Dynamical properties of the sine-Gordon quantum spin magnet Cu-PM at zero and finite temperature

TL;DR

This study combines numerical simulations and ESR experiments to analyze the spin dynamics of Cu-PM, revealing deviations from ideal models, boundary-bound states, and temperature-induced crossover phenomena in a quantum spin magnet.

Contribution

It provides a comprehensive analysis of the dynamical properties of Cu-PM beyond the low-energy sine-Gordon model, including finite temperature effects and boundary-bound states.

Findings

Deviation from Lorentz invariant dispersion for single-soliton resonance

Confirmation of boundary-bound state presence from field theory

Temperature-induced crossover and emergence of interbreather transitions

Abstract

The material copper pyrimidine dinitrate (Cu-PM) is a quasi-one-dimensional spin system described by the spin-1/2 XXZ Heisenberg antiferromagnet with Dzyaloshinskii-Moriya interactions. Based on numerical results obtained by the density-matrix renormalization group, exact diagonalization, and accompanying electron spin resonance (ESR) experiments we revisit the spin dynamics of this compound in an applied magnetic field. Our calculations for momentum and frequency-resolved dynamical quantities give direct access to the intensity of the elementary excitations at both zero and finite temperature. This allows us to study the system beyond the low-energy description by the quantum sine-Gordon model. We find a deviation from the Lorentz invariant dispersion for the single-soliton resonance. Furthermore, our calculations only confirm the presence of the strongest boundary bound state previously derived from a boundary sine-Gordon field theory, while composite boundary-bulk excitations have too low intensities to be observable. Upon increasing the temperature, we find a temperature-induced crossover of the soliton and the emergence of new features, such as interbreather transitions. The latter observation is confirmed by our ESR experiments on Cu-PM over a wide range of the applied field.