Thermal Hall conductivity of semimetallic graphite dominated by ambipolar phonon drag

TL;DR

This study reveals that in highly oriented pyrolytic graphite, the thermal Hall effect is predominantly driven by ambipolar phonon drag, resulting in a record-high Hall Lorenz number and distinct temperature dependence from electronic contributions.

Contribution

It demonstrates that ambipolar phonon drag, not electronic carriers, dominates the thermal Hall response in graphite, challenging previous assumptions about the origin of the effect.

Findings

Thermal Hall conductivity exceeds electronic expectations by two orders of magnitude.

Record Hall Lorenz number of approximately 67 times the Lorenz number.

Ambipolar phonon drag is identified as the primary mechanism behind the thermal Hall effect.

Abstract

It is now known that in addition to electrons, other quasi-particles 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 ($\kappa_{xy}$) is two 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 $164.9\times10^{-8}V^2 K^{-2}$ ($\sim$67$L_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 non-electronic 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.