Thermal Evolution of Dirac Magnons in the Honeycomb Ferromagnet CrBr$_3$

TL;DR

This study uses advanced neutron scattering to analyze the thermal behavior of Dirac magnons in CrBr3, confirming ideal dispersion features and revealing universal temperature dependence, while challenging some theoretical predictions.

Contribution

It provides the first definitive experimental confirmation of Dirac magnons in CrBr3 and details their thermal evolution, updating previous data with modern techniques.

Findings

CrBr3 exhibits ideal Dirac magnon dispersion at the K point.

Magnon lifetime and thermal band renormalization follow a universal T^2 trend.

Dispersion lacks the predicted van Hove singularities.

Abstract

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 (INS) 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, highlighting the need for a deeper theoretical analysis.