Repulsively bound magnon excitations of a spin-1/2 XXZ chain in a staggered transverse field

TL;DR

This paper investigates the excitation spectrum of a spin-1/2 XXZ chain with a staggered transverse field, revealing bound magnon states and matching experimental terahertz spectroscopy data.

Contribution

It combines analytical and numerical methods to identify high-energy bound magnon states in a staggered field, advancing understanding of quantum phase transitions in such systems.

Findings

Identification of high-energy two-magnon and three-magnon bound states

Dominance of single magnons at high fields and low temperatures

Excellent agreement with terahertz spectroscopy experiments

Abstract

We study the excitation spectrum of the one-dimensional spin-1/2 XXZ chain with antiferromagnetic Ising anisotropy across a magnetic quantum phase transition induced by the application of a site-dependent transverse magnetic field. Motivated by the chain antiferromagnet BaCo$_2$V$_2$O$_8$, we consider a situation where the transverse magnetic field has a strong uniform component and a weaker staggered part. To determine the nature of the excitations giving rise to the spin dynamical structure factor, we use a combination of analytical approaches and the numerically exact time-dependent matrix product state method. We identify below the quantum phase transition high-energy many-body two-magnon and three-magnon repulsively bound states which are clearly visible due to the staggered component of the magnetic field. At high magnetic fields and low temperature, single magnons dominate the dynamics. Our theory results are in very good agreement with terahertz spectroscopy experimental results presented in [Wang et al., Nature 631, 760 (2024)].