Unconventional electromagnetic mode in neutral Weyl Semimetals

TL;DR

This paper investigates a novel electromagnetic excitation in neutral Weyl semimetals, revealing a massless mode with linear dispersion at low energies and a crossover to an overdamped regime at higher momenta.

Contribution

It introduces a new gapless electromagnetic mode in Weyl semimetals, showing its dispersion, damping, and crossover behavior, expanding understanding of topological materials' electromagnetic responses.

Findings

Discovery of a massless, slightly damped electromagnetic mode with linear dispersion.

Identification of a crossover momentum where the mode becomes overdamped.

Both the novel mode and the unscreened gauge field exhibit attenuation.

Abstract

We study light propagation in a neutral Weyl semimetal with the Fermi level lying at the Weyl nodes in the weak self-interacting regime. The nontrivial topology induces a screening effect in one of the two transverse gauge fields, for which we find two branches of attenuated collective excitations. In addition to the known topologically gaped photon mode, a novel massless and slightly damped excitation appears. Strikingly, at low energies this new excitation has a linear dispersion and it propagates with the same velocity than the electrons, while at energies well above the electron-hole continuum threshold it behaves as a massive attenuated photon with velocity similar to the speed of light in the material. There is a crossover at certain momentum in the direction perpendicular to the separation of the Weyl nodes above which the novel gapless mode enters into an overdamped regime. Regarding the unscreened gauge field we show that it is also attenuated, which is a non-topological property shared by Dirac semimetals as well.