Faraday rotation in birefringent nonsymmorphic 2D materials

It has been recently reported that materials like Bi monolayer (MBi) and α-bismuthene, with crystal lattices described by a nonsymmorphic symmetry group, possess Dirac-like band dispersion around specific high symmetry momenta. At variance with graphene, two-dimensional nonsymmorphic Dirac semimetals (NSDS) have strongly anisotropic Dirac cones that remain robust in the presence of strong spin-orbit coupling and are very sensitive to the application of magnetic fields. In a recent paper, we studied the optical absorption spectra of NSDS based on a minimal model that includes all the relevant aspects of nonsymmorphic symmetry[1], under the assumption that a single Dirac cone dominates the absorption spectrum at low frequency. The absorption spectra were strongly dependent on polarization and frequency near a magnetic-field induced van Hove singularity. In this paper, we present a complete study of the magneto-optical properties of the NSDS model, i.e., we calculate both the real and the imaginary parts of all the components of the conductivity tensor in the presence of a magnetic field. From this, we extract the Faraday rotation angle as a function of frequency below the magnetic field-induced gap. Our results reveal that the longitudinal conductivities depend on the anisotropy, which gives rise to birefringence. However, the transverse component (the Hall conductivity) is only frequency-dependent and proportional to the Faraday rotation angle. At the zero-frequency limit, we get a finite value of Faraday rotation angle, which is 2α, where α is the fine structure constant. This value increases significantly as the frequency approaches the magnetic field-induced absorption edge. These results indicate that material like MBi can be suitable polarization rotators with high transmittance overall. More broadly, nonsymmorphic 2D Dirac materials appear to be good platforms for the realization of magneto-optic devices.