Evidence for Interband Circularly Polarized Optical Transitions in the Type II Weyl Semimetal NbIrTe₄

NbIrTe₄ is a material with broken inversion symmetry which has 8 Weyl points lying on the kz = 0 plane, all of whjch are all more than ~120 meV above the Fermi surface, and 4 Type II Weyl points lying in each of the kz = ± 0.2 planes which are ~80 meV below the Fermi energy.  We use circularly polarized mid-IR light to excite a two-terminal nanoflake device which shows that the signal associated with the circular photogalvanic effect increases by two orders of magnitude as the excitation energy decreases from 1 eV down to 0.3 eV.  In addition, an extremely strong peak is observed between 0.6-0.8 eV which we associate with a band-to-band transition.  Using circularly polarized transient reflectivity, we show that this same transition is seen in the transient circular dichroism signal which undergoes a sign change with the zero near the same energy.  Because the 1.5 eV pump pulse can only modulate electronic states near the Fermi level, this allows us to show that either the initial or final state in the optical transition must lie within 80 meV of the Fermi level.  A simple model shows that such a transient circular dichroism signal can result from optical transitions from a Type-II Weyl point to a higher lying parabolic-like state.  This suggests that these results may be connected with the 8 Weyl points lying in the kz = ± 0.2 planes which are near the Fermi level and are Type-II.