Cherenkov sound on a surface of a topological insulator

TL;DR

This paper explores unique Cherenkov sound phenomena on topological insulator surfaces, revealing exotic propagation behaviors influenced by anisotropy, including forward, backward, and localized sound beams, offering new insights into surface state properties.

Contribution

It demonstrates the existence of anomalous Cherenkov sound and anisotropy-dependent propagation features on topological insulator surfaces, providing potential experimental tools for surface state characterization.

Findings

Forward Cherenkov sound can occur without isotropic systems.

Backward Cherenkov sound appears above a critical anisotropy.

Sound localizes into specific beams at high anisotropy.

Abstract

Topological insulators are currently of considerable interest due to peculiar electronic properties originating from helical states on their surfaces. Here we demonstrate that the sound excited by helical particles on surfaces of topological insulators has several exotic properties fundamentally different from sound propagating in non-helical or even isotropic helical systems. Specifically, the sound may have strictly forward propagation absent for isotropic helical states. Its dependence on the anisotropy of the realistic surface-states is of distinguished behavior which may be used as an alternative experimental tool to measure the anisotropy strength. Fascinating from the fundamental point of view backward, or anomalous, Cherenkov sound is excited above the critical angle $\pi/2$ when the anisotropy exceeds a critical value. Strikingly, at strong anisotropy the sound localizes into a few forward and backward beams propagating along specific directions.