Dirac-screening stabilized surface-state transport in a topological insulator

TL;DR

This study demonstrates that Dirac-screening effects stabilize surface-state transport in a topological insulator, maintaining quantized Hall conductance over a wide density range and revealing the influence of steep band bending on surface states.

Contribution

It provides experimental evidence of Dirac-screening stabilizing surface transport in a topological insulator and introduces a model accounting for steep band bending effects.

Findings

Surface state transport remains dominant over a wide density range.

Quantum Hall effect contributions from individual surfaces are identifiable.

Steep band bending influences Dirac point positions via gate voltage.

Abstract

We report magnetotransport studies on a gated strained HgTe device. This material is a threedimensional topological insulator and exclusively shows surface state transport. Remarkably, the Landau level dispersion and the accuracy of the Hall quantization remain unchanged over a wide density range ($3 \times 10^{11} cm^{-2} < n < 1 \times 10^{12} cm^{-2}$). This implies that even at large carrier densities the transport is surface state dominated, where bulk transport would have been expected to coexist already. Moreover, the density dependence of the Dirac-type quantum Hall effect allows to identify the contributions from the individual surfaces. A $k \cdot p$ model can describe the experiments, but only when assuming a steep band bending across the regions where the topological surface states are contained. This steep potential originates from the specific screening properties of Dirac systems and causes the gate voltage to influence the position of the Dirac points rather than that of the Fermi level.