Disorder effects in topological insulator thin films

TL;DR

This paper investigates how Coulomb impurities and dielectric effects cause energy smearing in topological insulator thin films, impacting their topological properties and the observability of the quantum spin Hall effect.

Contribution

It introduces a modified Coulomb potential model accounting for dielectric effects, showing how impurity-induced smearing depends on film thickness and dielectric environment.

Findings

Energy smearing $$ is large and thickness-independent for $d > 5$ nm.

Smearing increases as film thickness decreases below 5 nm.

Maximum film thickness for observing quantum spin Hall effect is approximately 7 nm.

Abstract

Thin films of topological insulators (TI) attract large attention because of expected topological effects from the inter-surface hybridization of Dirac points. However, these effects may be depleted by unexpectedly large energy smearing $\Gamma$ of surface Dirac points by the random potential of abundant Coulomb impurities. We show that in a typical TI film with large dielectric constant $\sim 50$ sandwiched between two low dielectric constant layers, the Rytova-Chaplik-Entin-Keldysh modification of the Coulomb potential of a charge impurity allows a larger number of the film impurities to contribute to $\Gamma$. As a result, $\Gamma$ is large and independent of the TI film thickness $d$ for $d > 5$ nm. In thinner films $\Gamma$ grows with decreasing $d$ due to reduction of screening by the hybridization gap. We study the surface conductivity away from the neutrality point and at the neutrality point. In the latter case, we find the maximum TI film thickness at which the hybridization gap is still able to make a TI film insulating and allow observation of the quantum spin Hall effect, $d_{\max} \sim 7$ nm.