Phonon Thermal Hall Effect: The Roles of Disorder, Annealing, and Metallic Contacts

TL;DR

This study demonstrates that the phonon thermal Hall effect in insulators like SrTiO$_3$ is intrinsic and highly sensitive to disorder, with annealing partially restoring the effect and metallic contacts ruling out parasitic signals.

Contribution

It shows that disorder suppresses the phonon thermal Hall effect and annealing can partially restore it, clarifying the intrinsic nature of the phenomenon.

Findings

High thermal conductivity correlates with a significant thermal Hall angle.

Disordered samples show negligible thermal Hall effect.

Annealing partially restores the thermal Hall effect without affecting thermal conductivity.

Abstract

The phonon thermal Hall effect (THE) is a ubiquitous yet poorly understood phenomenon in insulators. Its microscopic origin remains debated, partly due to significant sample-dependent variations that hint at uncontrolled experimental parameters. Using SrTiO$_3$ as a model system, we identify disorder and uncontrolled strain as suppressors of a thermal Hall signal. Crystals with high thermal conductivity exhibit a substantial thermal Hall angle $\nabla T_y / \nabla T_x$ (up to 0.3\% at 9 T), whereas the effect is virtually absent in disordered samples. Crucially, annealing (in air atmosphere) these disordered samples partially restores the THE (approximately 0.1\% at 9 T) with little effect on the longitudinal thermal conductivity. This decoupling reveals that the amplitude of THE is not simply set by the phonon mean free path. Furthermore, measurements performed with metallic and insulating contacts yield identical results on the same sample. This definitively rules out parasitic signals as the effect's origin. Our work, by establishing the phonon THE as an intrinsic property of the crystal lattice and extremely sensitive to disorder, sharply constrains theoretical scenarios.