Emergent quantum confinement at topological insulator surfaces

TL;DR

This paper demonstrates that a simple band bending model can explain complex surface electronic states in topological insulators and reveals a rich 3D spin texture resulting from bulk topology and quantum confinement effects.

Contribution

It introduces a parameter-free tight-binding model that explains the electronic hierarchy and uncovers a complex spin texture on topological insulator surfaces.

Findings

Quantitative explanation of surface electronic states

Discovery of a rich 3D spin texture

Modification of surface-bulk connectivity by quantum confinement

Abstract

Bismuth-chalchogenides are model examples of three-dimensional topological insulators. Their ideal bulk-truncated surface hosts a single spin-helical surface state, which is the simplest possible surface electronic structure allowed by their non-trivial $\mathbb{Z}_2$ topology. They are therefore widely regarded ideal templates to realize the predicted exotic phenomena and applications of this topological surface state. However, real surfaces of such compounds, even if kept in ultra-high vacuum, rapidly develop a much more complex electronic structure whose origin and properties have proved controversial. Here, we demonstrate that a conceptually simple model, implementing a semiconductor-like band bending in a parameter-free tight-binding supercell calculation, can quantitatively explain the entire measured hierarchy of electronic states. In combination with circular dichroism in angle-resolved photoemission (ARPES) experiments, we further uncover a rich three-dimensional spin texture of this surface electronic system, resulting from the non-trivial topology of the bulk band structure. Moreover, our study reveals how the full surface-bulk connectivity in topological insulators is modified by quantum confinement.