The thermal Hall effect has been observed in a wide variety of magnetic insulators1–9, yet its origins remain controversial. Although some studies attribute the effect to intrinsic mechanisms10–14, such as heat carriers with Berry curvature, others propose extrinsic mechanisms15–17, such as heat carriers scattering off crystal defects. Even the nature of the heat carriers is unknown: magnons, phonons and fractionalized spin excitations have all been proposed. Resolving these issues is essential for the study of quantum spin liquids, and particularly for α-RuCl3, in which a quantized thermal Hall effect has been attributed to Majorana edge modes18,19. Here we use ultrasonic measurements of the acoustic Faraday effect to demonstrate that the phonons in α-RuCl3 have Hall viscosity—a non-dissipative viscosity that rotates phonon polarizations and deflects phonon heat currents. We show that phonon Hall viscosity produces an intrinsic thermal Hall effect that quantitatively accounts for a substantial fraction of the measured thermal Hall effect in α-RuCl3: the thermal Hall effect in α-RuCl3 is due to phonons, and it is intrinsic. More broadly, we demonstrate that the acoustic Faraday effect is a powerful tool for detecting phonon Hall viscosity and the associated phonon Berry curvature, offering a new way to uncover and study exotic states of matter that elude conventional experiments.
