The thermal Hall response of the phonon gas – (Kamran Behnia / LNCMI / Seminar). – 2/07/2026, 14H

Seminar LNCMI

Kamran Behnia, Laboratoire de Physique et Etude de Matériaux (ESPCI, Université Paris Sciences & Lettres, CNRS)

Seminar LNCMI, 2/07/2026, 14H, LNCMI, Toulouse

Abstract
First observed two decades ago [1], the phonon Hall effect, is nowadays observed in numerous insulators, including non-magnetic ones [2-4].
The observation implies a field-induced misalignment between the heat flux and the entropy flux vectors. Phonon thermal Hall conductivity peaks when most phonon-phonon collisions become normal, indicating a link with phonon hydrodynamics and a magnetic field induced change in the balance of three phonon events.
This conjecture is backed by the case of molecular gases subject to magnetic field. In their case, the Seftleben-Beenakker effect [5] arises due to the field-induced modification of the molecular collision cross sections. The breakdown of the Born-Oppenheimer approximation in a magnetic field can offer a geometric [Berry] phase to ordinary acoustic phonons. A picture, which would explain the experimentally observed bound to the thermal Hall of elemental insulators [6].

[1] C. Strohm, G. L. J. A. Rikken and P. Wyder, Phys. Rev. Lett. 95, 155901 (2005).
[2] X. Li, B. Fauqué, Z. Zhu, and K. Behnia, Phys. Rev. Lett. 124, 105901 (2020).
[3] X. Li, Y. Machida, A. Subedi, Z. Zhu, L. Li & K. Behnia, Nature Comm. 14, 1027 (2023).
[4] X. B. Jin et al., Phys. Rev. Lett. 135, 196302 (2025).
[5] L. Hermans, A. Schutte, H. Knaap, and J. Beenakker, Physica 46, 491 (1970).
[6] K. Behnia, Phonon thermal Hall as a lattice Aharonov-Bohm effect, SciPost Phys. Core 8, 061 (2025).

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