How massless are Weyl fermions in Weyl semimetals?

TL;DR

This paper investigates the apparent masslessness of Weyl fermions in Weyl semimetals, revealing that nonlinear dispersion effects near the Weyl nodes can induce asymmetries in current generation, challenging the assumption of their massless nature.

Contribution

It uncovers how nonlinear energy dispersion causes asymmetries in Weyl fermion responses, providing a method to control these effects using a tailored electromagnetic field.

Findings

Nonlinear dispersion leads to asymmetry in current generation at Weyl nodes.

A trefoil field scheme can control the induced asymmetry from positive to negative.

Weyl fermions exhibit effective mass-like behavior due to nonlinear effects.

Abstract

Circularly polarized light fails to generate currents in inversion-symmetric Weyl semimetals with degenerate Weyl nodes. While each node generates current with the direction depending on its chirality, the two currents in the two degenerate nodes of opposite chirality cancel each other. By extension, it is also generally expected that the currents generated at the same Weyl node by the fields of opposite helicity should also observe mirror symmetry and cancel. Surprisingly, here we find that this is not the case. The origin of this effect lies in the nonlinear energy dispersion, which manifests strongly already very close to the Weyl nodes, where linear dispersion is expected to hold and the Weyl fermions are thus expected to be massless. A scheme based on using a trefoil field composed of a counterrotating fundamental and its second harmonic is proposed to control the induced asymmetry at a chiral node from positive to negative, including zero.