Probing charge pumping and relaxation of the chiral anomaly in a Dirac semimetal

TL;DR

This study uses magneto-terahertz spectroscopy on Dirac semimetal films to observe a sharp Drude response linked to chiral charge dynamics, revealing insights into charge pumping and relaxation mechanisms related to the chiral anomaly.

Contribution

It provides experimental evidence of a new transport channel in Dirac semimetals associated with the chiral magnetic effect, highlighting the role of chiral charge conservation.

Findings

Observation of a sharp Drude response under magnetic field.

Spectral weight correlates with net chiral charge.

Chiral relaxation rate remains field-independent.

Abstract

The linear band crossings of 3D Dirac and Weyl semimetals are characterized by a charge chirality, the parallel or anti-parallel locking of electron spin to its momentum. Such materials are believed to exhibit a ${\bf E} \cdot {\bf B}$ chiral magnetic effect that is associated with the near conservation of chiral charge. Here, we use magneto-terahertz spectroscopy to study epitaxial Cd$_3$As$_2$ films and extract their conductivities $\sigma(\omega)$ as a function of ${\bf E} \cdot {\bf B}$. As field is applied, we observe a remarkably sharp Drude response that rises out of the broader background. Its appearance is a definitive signature of a new transport channel and consistent with the chiral response, with its spectral weight a measure of the net chiral charge and width a measure of the scattering rate between chiral species. The field independence of the chiral relaxation establishes that it is set by the approximate conservation of the isospin that labels the crystalline point-group representations.