Ambipolar magneto-optical response of ultra-low carrier density topological insulators

TL;DR

This study explores the magneto-optical properties of ultra-low carrier density topological insulator films, revealing quantized responses and parity anomaly effects in the THz range, advancing understanding of Dirac fermion behavior.

Contribution

It demonstrates the ability to produce ultra-low carrier density Sb₂Te₃ films with observable quantized magneto-optical responses, highlighting the parity anomaly in 2D Dirac systems.

Findings

Observation of quantized Hall response at ultra-low carrier densities

Detection of parity anomaly through Faraday angle discontinuity

Confirmation of Dirac fermion behavior in topological insulator films

Abstract

We have investigated the THz range magneto-optical response of ultralow carrier density films of Sb$_2$Te$_3$ using time-domain THz polarimetry. Undoped Sb$_2$Te$_3$ has a chemical potential that lies inside the bulk valence band. Thus its topological response is masked by bulk carriers. However, with appropriate buffer layer engineering and chemical doping, Sb$_2$Te$_3$ thin films can be grown with extremely low electron or hole densities. The ultralow carrier density samples show unusual optical properties and quantized response in the presence of magnetic fields. Consistent with the expectations for Dirac fermions, a quantized Hall response is seen even in samples where the zero field conductivity falls below detectable levels. The discontinuity in the Faraday angle with small changes in the filling fraction across zero is manifestation of the parity anomaly in 2D Dirac systems with broken time reversal symmetry.