Magneto-optical conductivity in a topological insulator

Abstract

Adding a small subdominant quadratic-in-momentum term to a dominant linear Dirac dispersion curve affects conduction and valence band differently and leads to an hourglass-like structure for energy as a function of momentum. This applies to the protected surface states in topological insulators. The energies of the conduction and valence band Landau levels are also different and this leads to the splitting of optical absorption lines produced by the magnetic field, which acquire a two-peak structure. It also changes the peaks in the imaginary part of the Hall conductivity into two distinct contributions of opposite signs. The real part of the circularly polarized optical conductivity, however, retains its single-peak structure but the peaks in right and left handedness cases are shifted in energy with respect to each other in contrast to the pure Dirac case. The magnitude of the semiclassical cyclotron frequency is significantly modified by the presence of a mass term, as is its variation with value of the chemical potential $\ensuremath{\mu}$. Its optical spectral weight is found to decrease with increasing $\ensuremath{\mu}$ rather than increase as it does in the pure Dirac limit.