Infrared Electrodynamics of Dirac Materials

This dissertation reports on infrared optical spectroscopic studies of a novel class of materials named Dirac materials, which cover a broad range of materials including Topological Insulators (TI) and Dirac/Nodal-line semimetals. These materials share a similar low-energy Hamiltonian that can be described by massless/massive Dirac fermions. Adding out-of-plane magnetic field generates additional features in the optical spectra that allow us to distinguish Dirac fermions with usual fermions with parabolic bands. I will first demonstrate identifications of surface states (SS) of TI using Faraday rotation spectroscopy, where both the top and bottom SS can be identified and found to host carriers of opposite sign. Secondly, I will generalize the power-law behavior for two-dimensional (2D) and three-dimensional (3D) Dirac semimetals to dispersive nodal-line semimetals. This leads to the discoveries of Dirac nodal-lines in topological semimetal NbAs2. Finally, the optical signatures of electronic correlations are discussed and the unexplored overlapping area between strongly correlated systems and Dirac semimetals are studied. The prominent correlation effects in nodal-line semimetal ZrSiSe uncovered by a combination of optical and magneto-optical spectroscopies will be discussed.