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Observation of Bethe strings in the quantum spin chain antiferromagnet BaCo2V2O8

Konrad Puzniak1,2,3,4,*, A. T. M. Nazmul Islam1, Xiaotong Chen5, Jiahao Yang6, Paul Steffens7, Martin Boehm7, Jianda Wu8,5,9, and Bella Lake1,2,†

  • *Contact author: konrad.puzniak@psi.ch
  • †Contact author: bella.lake@helmholtz-berlin.de
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Phys. Rev. B 113, 174419 – Published 18 May, 2026

DOI: https://doi.org/10.1103/5n46-c5k4

Abstract

Despite being predicted almost a century ago as the excitations of the spin-1/2 antiferromagnetic chain, Bethe strings were overshadowed by the more dominant multi-spinon-pair excitations and were only observed recently. Here we confirm their existence in the quasi-one-dimensional quantum antiferromagnet BaCo2V2O8 using a magnetic field to separate them from the spinons. We find excellent quantitative agreement over an extended field range between these experimental results and our theoretical predictions based on the algebraic Bethe ansatz formalism. This work proves the existence of Bethe strings (multimagnon bound states) up to length-3 in BaCo2V2O8.

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Phys. Rev. B 113, 174419– Published 18 May, 2026
  • Received 28 November 2025
  • Revised 24 March 2026
  • Accepted 16 April 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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