Magneto-infrared Spectroscopy of Topological Materials

The topological nature of a material is not only reflected on the surfaces or along the edges but also hidden inside the material in the bulk electronic structure. In this talk, I will describe how the bulk-sensitive magneto-infrared spectroscopy technique can be used to probe the electronic structure topology. I will use transition-metal pentatellurides (ZrTe₅ and HfTe₅, Dirac semimetals) and monopnictides (NbP, Weyl semimetals) as material examples. For the Dirac semimetals, we find that the observed Landau level transitions are similar to that in graphene but with a finite mass and the Zeeman effect opens the inverted band gap due to the large g-factor in the materials. For the Weyl semimetals, we find that the magnetic field opens a sizable gap at the charge neutrality point (Weyl annihilation) due to the finite coupling between the Weyl points and a new optical transition rule appears when the magnetic field breaks the axial symmetry. For both material systems, a semiquantitative agreement between the experiment and the effective Hamiltonian model calculation is achieved.