ESR investigation on the Breather mode and the Spinon-Breather dynamical crossover in Cu Benzoate

TL;DR

This study uses ESR to observe breather excitations in Cu benzoate, revealing a crossover from spinon to breather regimes and confirming the sine-Gordon model's applicability to high-field spin dynamics.

Contribution

First experimental observation of breather excitations in a S=1/2 Heisenberg chain under high fields, validating the sine-Gordon model for spin dynamics.

Findings

Detection of a new narrow ESR line indicating breather excitation

Anomalies in linewidth and resonance field at the crossover regime

Agreement of field dependence with theoretical breather formula

Abstract

A new elementary-excitation, the so called "breather excitation", is observed directly by millimeter-submillimeter wave electron spin resonance (ESR) in the Heisenberg quantum spin-chain Cu benzoate, in which a field-induced gap is found recently by specific heat and neutron scattering measurements. Distinct anomalies were found in line width and in resonance field around the "dynamical crossover" regime between the gap-less spinon-regime and the gapped breather-regime. When the temperature becomes sufficiently lower than the energy gap, a new ESR-line with very narrow line-width is found, which is the manifestation of the breather excitation. The non-linear field dependence of the resonance field agrees well with the theoretical formula of the first breather-excitation proposed by Oshikawa and Affleck. The present work establishes experimentally for the first time that a sine-Gordon model is applicable to explain spin dynamics in a S=1/2 Heisenberg spin chain subjected to staggered field even in high fields.