Studying Weyl Fermions by Terahertz Experiments

Weyl fermions are massless chiral particles. They do not exist in the Universe, but appear as quasiparticles in solids. The chirality of Weyl fermions is a source of fundamentally unusual properties of materials possessing these particles – Weyl semimetals. The project aims to investigate Weyl semimetals by the means of terahertz optics.

Background

Weyl semimetals are three-dimensional gapless topological materials, in which election bands intersect at arbitrary points in the Brillouin zone - the Weyl nodes. Each Weyl node is characterized by a quantum number, the chirality. Thus, the Weyl fermions (i.e. the electrons near the nodes) have their spins and momenta either in the same (chirality is +1) or in the opposite (chirality is -1) directions. Under the application of parallel static electric and magnetic fields, charge is transferred between points of opposite chirality. This violation of the chiral charge conservation law, called the chiral anomaly, is the source of most unusual properties of Weyl semimetals. Theory predicts that the chiral anomaly can be detected in optical experiments at low frequencies (terahertz) via measuring differences in absorption of the light with different circular polarizations. The goal of the project is to check these predictions experimentally.

For further information on this topic take a look at Phys. Rev. B 91, 081106 or Phys. Rev. B 89, 245121.

The experiments will be performed on the THz spectrometer, available at the Institute. Here, an overview of the method is shown.