Unconventional Thermal Magnon Hall Effect In A Ferromagnetic Topological Insulator

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 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 kappa(xy) as a function of the model parameters, associated with the local Berry curvature and occupation probability of the bulk bands. Throughout, kappa(xy) can be easily tuned with external parameters such as the magnetic field and temperature.