# Lattice-matched heterojunctions between topological and normal insulators: A first-principles study

## TL;DR

This study uses first-principles calculations to analyze lattice-matched heterojunctions between topological and normal insulators, revealing localized topological interface states with preserved spin textures, aiding device design.

## Contribution

It provides the first detailed theoretical analysis of specific TI/NI heterojunctions, demonstrating the localization and preservation of topological states at these interfaces.

## Key findings

- Topological interface states are strictly localized at the heterojunctions.
- Spin textures of interface states are preserved as in surface states.
- Band offsets are accurately evaluated using GW and DFT methods.

## Abstract

Gapless boundary modes at the interface between topologically distinct regions are one of the most salient manifestations of topology in physics. Metallic boundary states of time-reversal-invariant topological insulators (TIs), a realization of topological order in condensed matter, have been of much interest not only due to such a fundamental nature, but also due to their practical significance. These boundary states are immune to backscattering and localization owing to their topological origin, thereby opening up the possibility to tailor them for potential uses in spintronics and quantum computing. The heterojunction between a TI and a normal insulator (NI) is a representative playground for exploring such a topologically protected metallic boundary state and expected to constitute a building block for future electronic and spintronic solid-state devices based on TIs. Here, we report a first-principles study of two experimentally realized lattice-matched heterojunctions between TIs and NIs, Bi$_2$Se$_3$(0001)/InP(111) and Bi$_2$Te$_3$(0001)/BaF$_2$(111). We evaluate the band offsets at these interfaces from many-body perturbation theory within the $GW$ approximation as well as density-functional theory. Furthermore, we investigate the topological interface states, demonstrating that at these lattice-matched heterointerfaces they are strictly localized and their helical spin textures are as well preserved as those at the vacuum-facing surfaces. These results taken together may help in designing devices relying on spin-helical metallic boundary states of TIs.