Optical properties of magnetically doped ultra-thin topological insulator slabs

TL;DR

This paper investigates the optical properties of ultra-thin topological insulator slabs with magnetic doping, revealing how surface coupling and exchange fields influence optical absorption, conductivity, and magneto-optical effects like Kerr and Faraday rotations.

Contribution

It provides a theoretical analysis of how magnetic exchange fields affect optical and magneto-optical properties in ultra-thin topological insulator slabs, highlighting quantized Hall conductivity and large Kerr/Faraday angles.

Findings

Exchange field splits hyperbolic bands and modifies optical absorption.

Quantized Hall conductivity occurs for exchange fields larger than surface coupling.

Large Kerr and Faraday rotation angles are maintained over wide frequency ranges.

Abstract

Starting from a three dimensional Hamiltonian, we study the optical properties of ultra-thin topological insulator slabs for which the coupling between Dirac fermions on opposite surfaces results in two degenerated gapped hyperbolic bands. The gap is a threshold for the optical absorption and translates in a peak in the imaginary part of the optical conductivity. An exchange field applied perpendicular to the slab splits the degenerated hyperbolic bands and a double step structure come out in the optical absorption, whereas a double peak structure appears in the imaginary part of the longitudinal optical conductivity. The exchange field breaks time-reversal symmetry and for exchange fields larger than the surfaces coupling gap, the zero frequency Hall conductivity is quantized to $e^2/h$. This result implies large values of the Kerr and Faraday rotation angles. In ultra-thin slabs, the absence of light multiple scattering and bulk conductivity, makes the Kerr and Faradays angles to remain rather large in a wide range of frequencies.