Boundary Resonances in S = 1/2 Antiferromagnetic Chains under a Staggered Field

TL;DR

This paper develops a boundary field theory approach to analyze electron spin resonance in open S=1/2 antiferromagnetic chains with a staggered field, revealing boundary bound states and explaining previously unknown experimental modes.

Contribution

It introduces a boundary sine-Gordon field theory framework to identify boundary resonances and interpret experimental ESR modes in antiferromagnetic chains.

Findings

Identification of boundary bound states in the sine-Gordon model.

Explanation of unknown ESR modes as boundary resonances.

Modification of selection rules at the boundary.

Abstract

We develop a boundary field theory approach to electron spin resonance in open $S=1/2$ Heisenberg antiferromagnetic chains with an effective staggered field. In terms of the sine Gordon effective field theory with boundaries,we point out the existence of boundary bound states of elementary excitations, and modification of the selection rules at the boundary. We argue that several "unknown modes" found in electron spin resonance experiments on KCuGaF$_6$ [I. Umegaki et al., Phys. Rev. B 79, 184401 (2009)] and Cu-PM [S. A. Zvyagin et al., Phys. Rev. Lett. 93, 027201 (2004)] can be understood as boundary resonances introduced by these effects.