Multimagnon Bound States in Li₂CuO₂

Interactions between collective excitations can lead to fascinating emergent phenomena which cannot be reproduced in a non-interacting picture. Understanding the role of such interactions has become a central theme in condensed matter physics. In the specific context of interacting magnons, Bethe [1] showed the existence of multimagnon bound states in the Heisenberg ferromagnetic (FM) chain. Subsequent theoretical work has revealed a rich array of bound state-related physics in various models and exciting experimental demonstrations have been achieved in real quasi-one-dimensional materials. In this vein, we report a study of the S = ½ FM chain material Li₂CuO₂ [2]. We resolve additional interchain interactions and present a refined exchange model from inelastic neutron scattering (INS). We also present the observation and characterization of two- and, potentially, three-magnon bound states stabilized by exchange anisotropy. This represents, to the best of our knowledge, the first observation of such states by INS. Accompanying Raman scattering data is shown providing further confirmation of magnon-magnon interactions. Finally, thermodynamic measurements and additional INS relevant to the recent proposal of an exotic zero-field multimagnon Bose-Einstein condensate ground state in Li₂CuO₂ are shown [3].

1. H. Bethe, Z. Phys. 71, 205 (1931).

2. E. Zoghlin, M. B. Stone, and S.D. Wilson, Phys. Rev. B 108.6 (2023): 064408.

3. C.E. Agrapidis, , S.-L. Drechsler, and S. Nishimoto, Phys. Rev. Res. 7.4 (2025): 043051.