Hyperbolic plasmons in massive tilted two-dimensional Dirac materials

Abstract

We explore topological transitions in the type of propagation of surface electromagnetic modes in massive anisotropic tilted Dirac systems. The presence of tilting and mass gives rise to an indirect band gap that strongly modifies the joint density of states compared to the gapless system. New van Hove singularities appear, and the interplay between intra and interband transitions leads to an anisotropic optical conductivity with imaginary parts acquiring opposite signs in orthogonal directions, opening the possibility of having hyperbolic propagation of plasmons. Isofrequency contours and low plasmon losses, as obtained from the dispersion relation, show that transitions between purely anisotropic quasi-elliptical and well-defined, highly directional, hyperbolic modes are attainable only when tilt and mass coexist. Fine-tuning in the THz regime suggests that this might be useful as a dynamical probe of concurring tilt and mass in Dirac materials.