Macroscopic degeneracy of zero-mode rotating surface states in 3D Dirac and Weyl semimetals under radiation

TL;DR

This paper explores how circularly polarized radiation induces highly degenerate, rotating surface states with angular currents in 3D Dirac and Weyl semimetals, revealing novel topological phenomena.

Contribution

It uncovers the emergence of macroscopic degenerate surface states with angular currents under radiation, a new topological effect in 3D semimetals.

Findings

Midgap surface states appear as evanescent waves.

Degeneracy of states is proportional to radiation flux.

Surface states exhibit angular current and rotation.

Abstract

We investigate the development of novel surface states when 3D Dirac or Weyl semimetals are placed under circularly polarized electromagnetic radiation. We find that the hybridization between inverted Floquet bands opens in general a gap, which closes at so-called exceptional points found for complex values of the momentum. This corresponds to the appearance of midgap surface states in the form of evanescent waves decaying from the surface exposed to the radiation. We observe a phenomenon reminiscent of Landau quantization by which the midgap surface states get a large degeneracy proportional to the radiation flux traversing the surface of the semimetal. We show that all these surface states carry angular current, leading to an angular modulation of their charge that rotates with the same frequency of the radiation, which should manifest in the observation of a macroscopic chiral current in the irradiated surface.