Topologically protected states in $\delta$-doped junctions with band inversion

TL;DR

This paper proposes a method to detect topologically protected surface states in topological insulators by depositing a delta-doped layer of donors near the boundary, enhancing optical signals and revealing their robustness.

Contribution

It introduces a novel approach of delta doping to isolate and study gapless surface states in topological insulators through optical measurements.

Findings

Surface states are robust against delta doping perturbations.

Delta doping enhances intraband optical transitions.

The method aids in understanding surface-bulk state interplay.

Abstract

A topological boundary can be formed at the interface between a trivial and a topological insulator. The difference in the topological index across the junction leads to robust gapless surface states. Optical studies of these states are scarce in the literature, the reason being the difficulty to isolate their response from that of the bulk. In this work, we propose to deposit a $\delta$ layer of donor impurities in close proximity to a topological boundary to help detecting gapless surface states. As we will show, gapless surface states are robust against this perturbation and they enhance intraband optical transitions as measured by the oscillator strength. These results allow to understand the interplay of surface and bulk states in topological insulators.