Microscopic origin of an exceptionally large phonon thermal Hall effect from charge puddles in a topological insulator

We present the experimental observation of a drastically enhanced thermal Hall effect in the topological insulator material TlBi$_{0.15}$Sb$_{0.85}$Te$_2$. Although heat transport is dominated by phonons, moderate magnetic fields generate a thermal Hall ratio ($\kappa_{xy}/\kappa_{xx}$) above 2\%, an unprecedented value for a nonmagnetic material. The transverse thermal conductivity $\kappa_{xy}$ exhibits a pronounced maximum in fields of a few Tesla. This characteristic field dependence allows us to identify the microscopic origin of the thermal Hall effect in this system. Small densities of charged impurities induce locally conducting regions, so-called charge puddles, within the bulk insulating matrix. Via electron-phonon coupling, these charge puddles imprint a large thermal Hall effect onto the phonons accounting for both the magnitude and the magnetic-field dependence of the observed effect.