Angle-dependent planar thermal Hall effect by quasi-ballistic phonons in black phosphorus

TL;DR

This paper investigates the angle-dependent planar thermal Hall effect in black phosphorus, revealing that quasi-ballistic phonons and anharmonicity contribute to a non-magnetic origin of the effect, with implications for phonon transport understanding.

Contribution

It demonstrates that phonon mean-free-paths near sample thickness and anharmonicity are key to the thermal Hall effect in black phosphorus without involving spin.

Findings

Transverse temperature gradient observed with in-plane magnetic field.

Angular dependence matches sinusoidal components peaking at high symmetry.

Anharmonicity and electric dipolar waves influence phonon behavior.

Abstract

The origin of the phonon thermal Hall effect in insulators is a matter of ongoing debate. The large amplitude of the signal in an elemental non-magnetic solid, such as black phosphorus (BP), calls for a minimal mechanism not invoking the spin degree of freedom. Here, we show that a longitudinal heat flow generates a transverse temperature gradient in BP even when the magnetic field, the heat current and the thermal gradient lie in the same plane. The phonon mean-free-path is close to the sample thickness. Therefore, it is unlikely that scattering by point-like symmetry-breaking defects play a major role. We show that the angular dependence of the signal can be mapped to the sum of two sinusoidal components each peaking when the magnetic field is parallel to a high symmetry. We propose that anharmonicity may play a major role and argue that the magnetic field can exert a torque on electric dipolar waves traveling with phonons.