Photon absorption of two-dimensional nonsymmorphic Dirac semimetals

Two-dimensional Dirac semimetals have attracted a great deal of attention because of their linear energy dispersion and nontrivial Berry phase. These materials are rare because the nodal band structure is fragile against perturbations such as the spin-orbit coupling (SOC). Recently, it has been reported that nonsymmorphic crystal lattices possess symmetry-enforced Dirac-like band dispersion around certain high symmetry momenta even in the presence of SOC. Here we calculate the optical absorption spectra of the nonsymmorphic semimetals, which hosts anisotropic Dirac cones, with different Fermi velocities along the x and y directions. Our calculations show that the optical absorption coefficient depends strongly on the anisotropy factor and the photon polarization. By rotating the latter, one can change the absorption coefficient by more than an order of magnitude, giving rise to birefringence. When a magnetic field is applied, the absorption coefficient also depends on an internal parameter, which we term the "mixing angle" of the band structure. This parameter becomes therefore accessible to experimental investigation. We further find that an in-plane magnetic field, while leaving the system gapless, can induce a Van Hove singularity in the joint density of states: this causes a significant enhancement of the optical absorption at the frequency of the singularity for one direction of polarization but not for the orthogonal one, making the optical properties even more strongly dependent on polarization and anisotropy. These results suggest that a very pure nonsymmorphic two-dimensional Dirac semimetal can be an excellent candidate material for tunable magneto-optic devices.