# Experimental Observation of Bethe Strings

## TL;DR

This study experimentally observes Bethe string states in a one-dimensional quantum magnet using terahertz spectroscopy, confirming theoretical predictions and revealing their role near quantum criticality.

## Contribution

First experimental identification of Bethe string states in a condensed-matter system, validating Bethe ansatz predictions in a real material.

## Key findings

- Observation of complex bound states (strings) in SrCo2V2O8
- Identification of fractional magnetic excitations (psinons and antipsinons)
- String states dominate quantum spin dynamics near criticality

## Abstract

Almost one century ago, string states - complex bound states (Wellenkomplexe) of magnetic excitations - have been predicted to exist in one-dimensional quantum magnets and since then become a subject of intensive theoretical study. However, experimental realization and identification of string states in condensed-matter systems remains an unsolved challenge up to date. Here we use high-resolution terahertz spectroscopy to identify string states in the antiferromagnetic Heisenberg-Ising chain SrCo2V2O8 in strong longitudinal magnetic fields. We observe complex bound states (strings) and fractional magnetic excitations (psinons and antipsinons) in the field-induced critical regime, which are precisely described by the Bethe ansatz. Our study reveals that two-string and three-string states govern the quantum spin dynamics close to the quantum criticality, while the fractional excitations are dominant at low energies, reflecting the antiferromagnetic quantum fluctuations.

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Source: https://tomesphere.com/paper/1706.04181