Raman and Transient Reflectance Spectroscopy in the Type II Weyl Semimetal NbIrTe₄

There has been intense interest in understanding how topological electronic structure in Weyl semimetals interact with trivial (non-topological) electron and phonon excitations.  There have been a number of experimental and theoretical publications which suggest that phonons may couple strongly with the low energy topological electronic states. The van der Waals layered material NbIrTe₄ is a Type II Weyl semimetal which has 8 pairs of Weyl nodes which are very close to the Fermi energy.  It has broken inversion symmetry and 24 atoms per unit cell which result in 69 possible phonon modes.  We use micro-Raman scattering on a single nanoflake to observe the angular polarization dependence for 19 modes with frequencies which extend from 40 to 300 cm^-1.  We find for the A₁ symmetric phonon modes it is possible to extract directly a measure of the electron-phonon coupling and many frequencies display a strong sensitivity to only slightly different excitation energies (e.g. 2.0 eV vs 2.4 eV) .  This raises the question of how one might show that a particular trivial electronic state well above the Fermi energy is strongly coupled to a low energy topological electronic excitation near the Fermi energy.  I discuss very recent transient polarized reflectivity measurements at energies ranging from 0.3 to 1 eV which might show a way to answer this question.