# Faraday rotation due to surface states in the topological insulator (Bi$_{1-x}$Sb$_{x}$)$_{2}$Te$_{3}$

## TL;DR

This study uses magneto-infrared spectroscopy to investigate the charge dynamics and surface states in topological insulator thin films, revealing the presence of Dirac fermions on opposite surfaces and their distinct cyclotron resonances.

## Contribution

We provide the first comprehensive Faraday rotation and cyclotron resonance data set that isolates surface state responses in (Bi,Sb)$_2$Te$_3$ thin films, highlighting the coexistence of electron and hole Dirac fermions.

## Key findings

- Identification of three distinct cyclotron resonance energies.
- Surface states host both electron- and hole-type Dirac fermions.
- Surface Dirac fermions are distinguishable from bulk responses.

## Abstract

Using magneto-infrared spectroscopy, we have explored the charge dynamics of (Bi,Sb)$_2$Te$_3$ thin films on InP substrates. From the magneto-transmission data we extracted three distinct cyclotron resonance (CR) energies that are all apparent in the broad band Faraday rotation (FR) spectra. This comprehensive FR-CR data set has allowed us to isolate the response of the bulk states from the intrinsic surface states associated with both the top and bottom surfaces of the film. The FR data uncovered that electron- and hole-type Dirac fermions reside on opposite surfaces of our films, which paves the way for observing many exotic quantum phenomena in topological insulators.