Unconventional thermal magnon Hall effect in a ferromagnetic topological insulator

Abstract

We present theoretically the thermal Hall effect of magnons in a ferromagnetic lattice with a Kekule-O coupling (KOC) modulation and a Dzyaloshinskii-Moriya interaction (DMI). Through a strain-based mechanism for inducing the KOC modulation, we identify four topological phases in terms of the KOC parameter and DMI strength. We calculate the thermal magnon Hall conductivity ${\ensuremath{\kappa}}^{xy}$ at low temperature in each of these phases. We predict an unconventional conductivity due to a nonzero Berry curvature emerging from band proximity effects in the topologically trivial phase. We find sign changes of ${\ensuremath{\kappa}}^{xy}$ as a function of the model parameters, associated with the local Berry curvature and occupation probability of the bulk bands. Throughout, ${\ensuremath{\kappa}}^{xy}$ can be easily tuned with external parameters such as the magnetic field and temperature.