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.

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.