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  • Letter
  • Open Access

Nonlinear spectroscopy as a magnon breakdown diagnosis and its efficient simulation

David A. S. Kaib*, Marius Möller, and Roser Valentí

  • Institut für Theoretische Physik, Goethe-Universität Frankfurt, Max-von-Laue-Strasse 1, 60438 Frankfurt am Main, Germany

  • *Contact author: kaib@itp.uni-frankfurt.de
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Phys. Rev. Research 8, L012052 – Published 5 March, 2026

DOI: https://doi.org/10.1103/h5sd-flyf

Abstract

Identifying quantum spin liquids, magnon breakdown, or fractionalized excitations in quantum magnets is an ongoing challenge due to the ambiguity in interpreting excitation continua occurring in linear-response probes. Recently, it was proposed that techniques measuring higher-order response, such as two-dimensional coherent spectroscopy (2DCS), could resolve such ambiguities. Numerically simulating nonlinear response functions can, however, be computationally very demanding. We present an efficient Lanczos-based method to compute second-order susceptibilities χ2(ωt,ωτ) directly in the frequency domain. Applying this to extended Kitaev models describing α−RuCl3, we find qualitatively different nonlinear responses between intermediate magnetic field strengths and the high-field regime. To put these results into context, we derive the general 2DCS response of partially polarized magnets within the linear spin-wave approximation, establishing that χ2(ωt,ωτ) is restricted to a distinct universal form if the excitations are conventional magnons. Deviations from this form, as predicted in our Lanczos-based simulations for α−RuCl3, can hence serve in 2DCS experiments as direct criteria to determine whether an observed excitation continuum is of conventional two-magnon type or of different nature.

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Phys. Rev. Research 8, L012052– Published 5 March, 2026
  • Received 9 April 2025
  • Revised 2 September 2025
  • Accepted 23 December 2025
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