Interacting Dirac magnons in honeycomb ferromagnets CrBr$_3$

Abstract: We study the effects of magnon-magnon interactions in the two-dimensional van der Waals ferromagnet CrBr$_3$ focusing on its honeycomb lattice structure. Motivated by earlier theoretical predictions of temperature-induced spectral shifts and van Hove singularities in the magnon dispersion~\cite{PhysRevX.8.011010}, we go beyond the commonly used thermal magnon approximation by applying second-order perturbation theory in a fully numerical framework. Our analysis uncovers significant deviations from previous analysis: in particular, the predicted singularities are absent, consistent with recent inelastic neutron scattering measurements~\cite{PhysRevLett.129.127201}. Moreover, we find that the temperature dependence of the renormalized magnon spectrum exhibits a distinct $T^3$ behavior for the optical magnon branch, while retaining $T^2$ behavior for the acoustic or down magnon band. This feature sheds new light on the collective dynamics of Dirac magnons and their interactions. We further compare the honeycomb case with a triangular Bravais lattice, relevant for ferromagnetic monolayer MnBi$_2$Te$_4$, and show that both systems lack singular features while displaying quite distinct thermal trends.