Phonon circular birefringence and polarization-filter in Magnetic Topological Insulators

TL;DR

This paper introduces a novel phonon polarization-filter mechanism in magnetic topological insulators driven by surface phonon Hall viscosity, enabling selective transmission of circularly polarized phonons and advancing topological phononic device engineering.

Contribution

It uncovers a new effect where surface PHV creates an interface phonon mode with specific polarization, acting as a phonon polarization filter, and develops a scattering framework for phonon manipulation.

Findings

Surface PHV induces a circularly polarized interface phonon mode.

Magnetic topological insulators can selectively transmit polarized phonons.

The study reveals phenomena like surface acoustic Faraday rotation.

Abstract

The surface phonon Hall viscosity (PHV)-an acoustic analog of axion electrodynamics-emerges from the strain response of magnetic topological insulators and gives rise to novel acoustic phenomena. In this work, we propose a previously unexplored effect: a phonon polarization-filter mechanism induced by the surface PHV, which generates an interface phonon mode with its frequency below the bulk mode frequency. This interface mode possesses a specific circular polarization and therefore acts as a polarization filter, confining only phonons with the matching polarization at the interface. Magnetic topological insulators can thus selectively transmit one type of circularly polarized phonon mode, enabling the manipulation of phonon polarization and angular momentum. In addition, we further develop a generalized scattering framework to study the effect of an injected acoustic wave from a trivial insulator to a magnetic topological insulator with both normal and oblique incidence, and discuss the phenomena of surface acoustic Faraday rotation and longitudinal-transverse mode conversion. Our results establish surface Hall viscosity as a powerful mechanism for engineering axial phonon states and open new avenues for topological phononic devices based on phonon angular momentum.