Terahertz properties of Dirac fermions in HgTe films with optical doping

TL;DR

This study investigates the terahertz optical properties of HgTe films with critical thickness, demonstrating optical doping control of charge carriers and providing evidence of Dirac fermions through magneto-optical measurements.

Contribution

It introduces a method to control charge carrier density in HgTe films using optical doping and confirms the presence of Dirac fermions via terahertz magneto-optical analysis.

Findings

Electron density can be increased by over an order of magnitude with light.

The terahertz response becomes purely electronic after illumination.

Electron cyclotron mass shows a square root dependence on charge concentration.

Abstract

Terahertz properties of mercury telluride (HgTe) films with critical thickness are presented and discussed. The density of the charge carriers is controlled using contact-free optical doping by visible light. In the magneto-optical response of HgTe the contribution of two types of carriers (electrons and holes) can be identified. The density of the electrons can be modified by light illumination by more than one order of magnitude. As the hole density is roughly illuminationindependent, the terahertz response of the illuminated samples becomes purely electronic. In some cases, light illumination may switch the qualitative electrodynamic response from hole-like to the electron-like. The cyclotron mass of the electrons could be extracted from the data and shows a square root dependence upon the charge concentration in the broad range of parameters. This can be interpreted as a clear proof of a linear dispersion relations, i.e. Dirac-type charge carriers.