Thermal Evolution of Dirac Magnons in the Honeycomb Ferromagnet CrBr_{3}.

Nikitin, S E; Fåk, B; Krämer, K W; Fennell, T; Normand, B; Läuchli, A M; Rüegg, Ch (2022). Thermal Evolution of Dirac Magnons in the Honeycomb Ferromagnet CrBr_{3}. Physical review letters, 129(12), p. 127201. American Physical Society 10.1103/PhysRevLett.129.127201

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CrBr_{3} is an excellent realization of the two-dimensional honeycomb ferromagnet, which offers a bosonic equivalent of graphene with Dirac magnons and topological character. We perform inelastic neutron scattering measurements using state-of-the-art instrumentation to update 50-year-old data, thereby enabling a definitive comparison both with recent experimental claims of a significant gap at the Dirac point and with theoretical predictions for thermal magnon renormalization. We demonstrate that CrBr_{3} has next-neighbor J_{2} and J_{3} interactions approximately 5% of J_{1}, an ideal Dirac magnon dispersion at the K point, and the associated signature of isospin winding. The magnon lifetime and the thermal band renormalization show the universal T^{2} evolution expected from an interacting spin-wave treatment, but the measured dispersion lacks the predicted van Hove features, pointing to the need for more sophisticated theoretical analysis.

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