PHONHALL · Resolving the origin of the phonon thermal Hall Effect

Phonons, the quasiparticles of lattice vibrations, are the primary heat carriers in insulators. In many insulators, it has been observed that applying a heat current and a magnetic field perpendicular to it creates a transverse heat gradient via phonons, leading to the phonon thermal Hall effect. Despite the growing number of quantum materials exhibiting this effect, the underlying cause remains unclear, since phonons are neutral quasiparticles. A key unresolved question is whether this effect is due to an extrinsic mechanism, such as skew scattering that could cause asymmetric phonon scattering, or an intrinsic mechanism, like a Berry phase and generate a thermal Hall response. The PHONHALL project aims to identify the responsible mechanism. It will determine if the effect is intrinsic or extrinsic by examining two different aspects: the impact of introducing defects (via electron irradiation) and angle-resolved thermal Hall measurements. Understanding the phonon thermal Hall effect in quantum materials could enable controllable heat flow and low-power thermal devices, including materials that dissipate heat efficiently under high magnetic fields as required in high-field fusion magnets.