# Quantum Critical Dynamics of a Heisenberg-Ising Chain in a Longitudinal Field: Many-Body Strings versus Fractional Excitations

## TL;DR

This study uses terahertz spectroscopy to explore quantum spin dynamics in a Heisenberg-Ising chain, revealing the interplay of many-body string and fractional excitations across different phases.

## Contribution

It provides the first detailed experimental observation of many-body string excitations in a quantum spin chain, validated by Bethe-Ansatz calculations.

## Key findings

- Confined spinon excitations below 5.5 K
- Identification of many-body string and fractional excitations above 3.8 T
- String excitations dominate near the critical field B_c

## Abstract

We report a high-resolution terahertz spectroscopic study of quantum spin dynamics in the antiferromagnetic Heisenberg-Ising spin-chain compound BaCo$_2$V$_2$O$_8$ as a function of temperature and longitudinal magnetic field. Confined spinon excitations are observed in an antiferromagnetic phase below $T_N\simeq 5.5$ K. In a field-induced gapless phase above $B_c=3.8$ T, we identify many-body string excitations as well as low-energy fractional psinon/antipsinon excitations by comparing to Bethe-Ansatz calculations. In the vicinity of $B_c$, the high-energy string excitations are found to be dynamically dominant over the fractional excitations.