Axionic Acoustic Phonons from Weyl Semimetals

TL;DR

This paper investigates how dynamical axion fields in Weyl semimetals influence sound propagation, revealing modifications in sound speed and attenuation due to axial electron-phonon coupling, with implications for experimental detection.

Contribution

It introduces the effects of dynamical axion fields on acoustic phonons in Weyl semimetals, highlighting unique modifications in sound properties and the role of axial electron-phonon interactions.

Findings

Axion modifies sound speed for longitudinal waves parallel to ACDW

Attenuation effects are negligible beyond RPA

Sound propagation experiments can probe axionic effects without electric fields

Abstract

The sound propagation properties resulting from a dynamical axion insulator generated from a Weyl semimetal are explored. Due to the axial electron-phonon coupling, the speed of sound in the material and its attenuation is seen to be modified. At the Random Phase Approximation (RPA) level, it is seen that the axion obstructs a stronger modification of the speed of sound for longitudinal waves propagating parallel to the Axionic Charge Density Wave (ACDW) wavevector but not for transverse waves. On the other hand, the attenuation is modified by effects beyond the RPA, but the axionic contribution is negligible. These effects can be probed in sound propagation experiments without invoking the axial anomaly, which would require an additional electric field. We also discuss the importance of axial electron-phonon interactions with respect to the more conventional vector interactions in these systems.