Probing embedded topological modes in bulk-like GeTe-Sb$_2$Te$_3$ heterostructures

TL;DR

This study combines experimental and theoretical methods to investigate embedded topological states in GeTe-Sb$_2$Te$_3$ heterostructures, revealing their delocalization, robustness to disorder, and potential for manipulation in solid-solid interfaces.

Contribution

It provides a comprehensive analysis of embedded topological states at solid-solid interfaces, including experimental characterization and theoretical modeling, which was less explored before.

Findings

Embedded topological modes are delocalized over many lattice spacings.

ETSs are relatively robust to disorder.

Insights into manipulating topological modes in heterostructures.

Abstract

The interface between topological and normal insulators hosts metallic states that appear due to the change in band topology. While these topological states at a surface, i.e., a topological insulator-air/vacuum interface, have been studied intensely, topological states at a solid-solid interface have been less explored. Here we combine experiment and theory to study such \textit{embedded} topological states (ETSs) in heterostructures of GeTe (normal insulator) and Sb$_2$Te$_3$ (topological insulator). We analyse their dependence on the interface and their confinement characteristics. To characterise the heterostructures, we evaluate the GeTe-Sb$_2$Te$_3$ band offset using X-ray photoemission spectroscopy, and chart the elemental composition using atom probe tomography. We then use first-principles to independently calculate the band offset and also parametrise the band structure within a four-band continuum model. Our analysis reveals, strikingly, that under realistic conditions, the interfacial topological modes are delocalised over many lattice spacings. Interestingly, the first-principles calculations indicate that the ETSs are relatively robust to disorder and this may have practical ramifications. Our study provides insights into how to manipulate topological modes in heterostructures and also provides a basis for recent experimental findings [Nguyen \textit{et al.}, Sci. Rep. \textbf{6}, 27716 (2016)] where ETSs were seen to couple over large distances.