Magnetic topological insulators exhibit a surface phonon Hall viscosity – an acoustic analogue of the Hall effect where phonons acquire a transverse response to strain. Chatterjee and Liu show that this viscosity creates a phonon polarization-filtering mechanism: interface phonon modes develop at frequencies below the bulk modes, and these interface modes carry specific circular polarization, enabling the material to selectively transmit one handedness of circularly polarized phonons while reflecting the other.

The scattering framework they develop examines acoustic waves injected from a trivial insulator into a magnetic topological insulator under both normal and oblique incidence. At normal incidence, the phonon polarization filter acts cleanly, separating left- and right-circular modes. At oblique angles, the situation enriches: surface acoustic Faraday rotation appears, where the polarization plane of a linearly polarized phonon beam rotates upon transmission, and longitudinal-transverse mode conversion occurs, coupling compressional and shear waves through the surface Hall viscosity.

The connection to axion electrodynamics is structural rather than metaphorical. The same topological field theory that describes the electromagnetic response of these materials – the axion term in the action – governs their acoustic response when reformulated in terms of strain fields. The phonon Hall viscosity is the strain-sector manifestation of the topological magnetoelectric effect. This means that phononic devices built from magnetic topological insulators inherit the robustness of topological protection: the filtering and rotation effects are determined by bulk topology rather than surface details, making them resistant to disorder and fabrication imperfections.

(arXiv:2603.17274)