Mixed axial-torsional anomaly in Weyl semimetals

TL;DR

This paper predicts a novel mixed axial-torsional anomaly in Weyl semimetals, where strain-induced torsion causes nonconservation of chiral currents, leading to measurable electrical signals under magnetic fields.

Contribution

It introduces the concept of a mixed axial-torsional anomaly in Weyl semimetals and proposes an experimental setup to observe it via strain-induced torsion and sound waves.

Findings

Prediction of axial-torsional anomaly in Weyl semimetals.

Proposal of strain and sound waves to activate the anomaly.

Expected measurable alternating current in magnetic field.

Abstract

We show that Weyl semimetals exhibit a mixed axial-torsional anomaly in the presence of axial torsion, a concept exclusive of these materials with no known natural fundamental interpretation in terms of the geometry of spacetime. This anomaly implies a nonconservation of the axial current---the difference in current of left- and right-handed chiral fermions---when the torsion of the spacetime in which the Weyl fermions move couples with opposite sign to different chiralities. The anomaly is activated by driving transverse sound waves through a Weyl semimetal with a spatially varying tilted dispersion, which can be engineered by applying strain. This leads to sizable alternating current in presence of a magnetic field that provides a clear-cut experimental signature of our predictions.