Abstract
The interface between topological and normal insulators hosts metallic states that appear due to the change in band topology. While 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 embedded topological states (ETSs) in heterostructures of GeTe (normal insulator) and hbox {Sb}_2hbox {Te}_3 (topological insulator). We analyse their dependence on the interface and their confinement characteristics. First, to characterise the heterostructures, we evaluate the GeTe-Sb_2Te_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. In addition, 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 et al. Sci. Rep. 6, 27716 (2016)] where ETSs were seen to couple over thick layers.
Highlights
The interface between topological and normal insulators hosts metallic states that appear due to the change in band topology
We present a detailed materials characterisation of the heterostructures including X-ray photoemission spectroscopy (XPS) to evaluate the band offset, and Atom Probe Tomography (APT) to visualise the spatial atomic distribution
We have studied the band structure of bulk-like heterostructures of Sb2Te3 and GeTe as a prototypical topological insulator (TI)-normal insulator (NI) system
Summary
We discuss the band offset between the GeTe and Sb2Te3 layers in the GST heterostructure. Following Refs.[23,24,25], this is done by evaluating the difference of the core electron energy levels in bulk samples, and comparing this to the difference in the core energy levels in a heterostructure This would require XPS spectra of three separate samples: a bulk GeTe film, a bulk Sb2Te3 film, and a heterostructure of the two in which the top layer is sufficiently thin (∼ 5 nm) that the X-rays can penetrate it fully and sample both materials. We present results for the band offset obtained from electronic structure calculations of bulk Sb2Te3 and GeTe via density functional theory (DFT) and non-equilibrium Green function (NEGF) theory. Within the Sb (Ge)-rich region there is a relative concentration of < % Ge (Sb) whereas in the interfacial regions (centred at ≈ 8 nm and ≈ nm) the relative concentrations of Ge and Sb are equal
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