Surface Phonon Hall Viscosity Induced Phonon Chirality and Nonreciprocity in Magnetic Topological Insulator Films

TL;DR

This paper explores how surface phonon Hall viscosity in magnetic topological insulators induces phonon chirality and nonreciprocity, revealing novel surface acoustic phenomena linked to topological and magnetic properties.

Contribution

It introduces the concept of surface phonon Hall viscosity arising from the Nieh-Yan action, connecting phonon dynamics with surface magnetization in topological insulators.

Findings

Chiral phonons occur with parallel surface magnetization.

Nonreciprocal phonon propagation occurs with anti-parallel magnetization.

Potential experimental signatures include thermal Hall effect and magnon-polarons.

Abstract

The surface half-quantum Hall effect, a hallmark consequence of axion electrodynamics, can be induced by gapping out the surface states of topological insulators through surface magnetization, and has led to a variety of topological response phenomena observed in experiments. In this work, we investigate phonon dynamics originating from an acoustic analog - the surface phonon Hall viscosity - that can also occur at the surface of magnetic topological insulators. This surface phonon Hall viscosity stems from the Nieh-Yan action in the strain response of topological insulators, where strain acts as the effective vierbein field for the bulk low-energy massive Dirac fermions. Crucially, this viscosity term entangles phonon dynamics with surface magnetization. In magnetic topological insulator films, we find that this interaction causes acoustic phonons to become chiral when the magnetization at the top and bottom surfaces is parallel, and nonreciprocal when it is anti-parallel. We further discuss potential experimental signatures of phonon dynamics induced by surface phonon Hall viscosity, specifically the phonon thermal Hall effect and magnon-polarons.