Thermal Hall Conductivity of Semimetallic Graphite Dominated by Ambipolar Phonon Drag

It is now known that in addition to electrons, other quasiparticles such as phonons and magnons can also generate a thermal Hall signal. Graphite is a semimetal with extremely mobile charge carriers of both signs and a large lattice thermal conductivity. We present a study of the thermal Hall effect in highly oriented pyrolytic graphite (HOPG) samples with electronic, phononic, and phonon drag contributions to the thermal Hall signal. The measured thermal Hall conductivity (κ_{xy}) is 2 orders of magnitude higher than what is expected by electronic carriers according to the electrical Hall conductivity and the Wiedemann-Franz law, yielding a record Hall Lorenz number of 164.9×10^{-8}  V^{2} K^{-2} (∼67L_{0})-the largest ever observed in a metal. The temperature dependence of the thermal Hall conductivity significantly differs from its longitudinal counterpart, ruling out a purely phononic origin of the nonelectronic component. Based on the temperature dependence and the amplitudes of the Seebeck and Nernst responses, we demonstrate that ambipolar phonon drag dominates the thermal Hall response of graphite.