Magneto-optical conductivity in a topological insulator

TL;DR

This paper investigates how a quadratic momentum term affects the magneto-optical properties of topological insulator surface states, revealing modifications in Landau levels, optical absorption, and Hall conductivity spectra.

Contribution

It introduces the impact of a quadratic term on the magneto-optical response of topological insulators, highlighting differences from the pure Dirac case.

Findings

Splitting of optical absorption lines into two peaks due to Landau level differences.

Distinct contributions of opposite signs in the Hall conductivity's imaginary part.

Shifted peaks in circularly polarized optical conductivity with a modified cyclotron frequency.

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 case 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 $\mu$. Its optical spectral weight is found to decrease with increasing $\mu$ rather than increase as it does in the pure Dirac limit.