Time-resolved terahertz magneto-optical study of the Weyl semimetal TaAs

Weyl semimetals host pairs of singly-degenerate electronic states with linear energy dispersion and opposite chirality that are separated in momentum space. For a typical Weyl semimetal, linear bands and electron pockets with opposite chirality exist only at low energies (<100 meV). This makes the terahertz frequency regime exceptionally attractive for exploring novel optical/magneto-optical effects predicted for Weyl semimetals.

Here we investigate the temporal response of photo-excited Weyl electrons in a magnetic field. We present terahertz pump-probe experiments in reflection geometry at a photon energy of 14 meV (≡3.4 THz) on the Weyl semimetal TaAs in the presence of an in-plane static magnetic field.

Our measurements reveal several transient magneto-optical features, including a significant reduction in the hot electron cooling rate when a magnetic field is applied. We also observe a much slower (>1 ns) relaxation process that only exists when the terahertz pulses are co-polarized in the direction of the static magnetic field, which could be associated with dynamical chiral charge pumping between the Weyl nodes with opposite chirality.