Anomaly-induced sound absorption in Weyl semimetals

TL;DR

This paper presents a semiclassical theory explaining how Berry monopoles and the chiral anomaly influence sound absorption in Weyl semimetals under magnetic fields, proposing acoustic magneto-chiral dichroism as a topological probe.

Contribution

It introduces a novel semiclassical framework linking band topology to sound absorption, highlighting the role of Weyl node motion and magneto-chiral effects in these materials.

Findings

Sound absorption primarily arises from Weyl node motion in energy space.

Acoustic magneto-chiral dichroism can serve as a topological band structure probe.

Negative magneto-absorption at low frequencies can change sign with frequency increase.

Abstract

We develop a semiclassical theory of sound absorption in Weyl semimetals in magnetic fields. We focus on the contribution to the absorption that stems from the existence of Berry monopoles in the band structure of such materials, or, equivalently, due to the chiral anomaly and chiral magnetic effect. Sound absorption is shown to come primarily from the motion of Weyl nodes in energy space, associated with the propagation of a sound wave. We argue that acoustic magneto-chiral dichroism, which occurs when absorption of sound is different for opposite propagation directions, and for opposite directions of the magnetic field, can be a definitive probe of band topology. The part of the monopole-related sound magnetoabsorption that is even in the magnetic field is negative in time-reversal Weyl materials. The difference in the sign of the effect as compared to the positive anomaly-related transport magnetoconductance stems from the existence of valley electrochemical imbalances without magnetic field in the sound propagation problem. In centrosymmetric Weyl semimetals with few Weyl nodes, the magneto-absorption is negative at low frequencies, but can change sign with increasing frequency.