Optical Response from Charge-Density Waves in Weyl Semimetals

TL;DR

This paper investigates how collective excitations in Weyl semimetals with charge-density waves influence their optical responses, revealing unique effects of cone tilting and amplitude modes on nonlinear conductivity and harmonic generation.

Contribution

It provides a systematic analysis of the optical response in Weyl-CDW systems, highlighting the role of tilt and collective modes in nonlinear conductivity, which was not previously understood.

Findings

Gapless collective mode does not contribute unless Weyl cones are tilted.

Third-order conductivity is the lowest nontrivial contribution in untilted cones.

A sharp third harmonic peak appears below the single-particle gap.

Abstract

The study of charge-density wave (CDW) distortions in Weyl semimetals has recently returned to the forefront, inspired by experimental interest in materials such as (TaSe4)2I. However, the interplay between collective phonon excitations and charge transport in Weyl-CDW systems has not been systematically studied. In this paper, we examine the longitudinal electromagnetic response due to collective modes in a Weyl semimetal gapped by a quasi one-dimensional charge-density wave order, using both continuum and lattice regularized models. We systematically compute the contributions of the collective modes to the linear and nonlinear optical conductivity of our models, both with and without tilting of the Weyl cones. We discover that, unlike in a single-band CDW, the gapless CDW collective mode does not contribute to the conductivity unless the Weyl cones are tilted. Going further, we show that the lowest nontrivial collective mode contribution to charge transport with untilted Weyl cones comes in the third-order conductivity, and is mediated by the gapped amplitude mode. We show that this leads to a sharply peaked third harmonic response at frequencies below the single-particle energy gap. We discuss the implications of our findings for transport experiments in Weyl-CDW systems.