Evidence for Topological Band-to-Band Transitions in a Type-II Weyl Semimetal

The ternary van der Waals material NbIrTe$_4$ is a Type-II Weyl semimetal. We use a tunable circularly polarized mid-infrared laser to investigate the existence of band-to-band excitations using transient reflectivity in an exfoliated nanoflake and photothermoelectric and photogalvanic signals in a device. Unpolarized photothermoelectric spectroscopy shows that the absorption in the Weyl semimetal increases rapidly above 0.3 eV as expected from the increase in the density of states from DFT calculations. However, the reflectivity shows a sign change in the circular dichroism of the reflected light which is dominated by $\sigma^-$ light for energies below 0.5 eV and $\sigma^+$ light for energies above that energy. Using an intense pulse to perturb the electrons near the Fermi level, we show that this 0.5 eV energy is associated with a band-to-band transition from a band slightly below the Fermi energy to a higher lying empty band. We use spectroscopy of the circular photogalvanic effect (CPGE) from 0.3 to 1 eV to show a strong peak near 0.5 eV which lies on top of a two orders-of-magnitude increase at the lowest energies as the Weyl points are approached. We conclude that this band-to-band optical transition enhances the Berry Curvature responsible for the CPGE, and may involve the Weyl points just below the Fermi energy.