Band structure and end states in InAs/GaSb core-shell-shell nanowires

TL;DR

This study explores the electronic band structure and end states in InAs/GaSb core-shell-shell nanowires, revealing that while true topological edge states are absent, localized end states exhibit some robustness under disorder.

Contribution

It provides a detailed analysis of the band structure and end states in InAs/GaSb nanowires, showing the absence of topological edge states but the presence of localized end states with partial protection.

Findings

End states are localized at wire ends in the inverted band-gap regime.

Topological edge states do not appear due to lack of 1D edges.

End states are robust to angular disorder if the bulk gap remains open.

Abstract

Quantum wells in InAs/GaSb heterostructures can be tuned to a topological regime associated with the quantum spin Hall effect, which arises due to an inverted band gap and hybridized electron and hole states. Here, we investigate electron-hole hybridization and the fate of the quantum spin Hall effect in a quasi one-dimensional geometry, realized in a core-shell-shell nanowire with an insulator core and InAs and GaSb shells. We calculate the band structure for an infinitely long nanowire using $\mathbf{k \cdot p}$ theory within the Kane model and the envelope function approximation, then map the result onto a BHZ model which is used to investigate finite-length wires. Clearly, quantum spin Hall edge states cannot appear in the core-shell-shell nanowires which lack one-dimensional edges, but in the inverted band-gap regime we find that the finite-length wires instead host localized states at the wire ends. These end states are not topologically protected, they are four-fold degenerate and split into two Kramers pairs in the presence of potential disorder along the axial direction. However, there is some remnant of the topological protection of the quantum spin Hall edge states in the sense that the end states are fully robust to (time-reversal preserving) angular disorder, as long as the bulk band gap is not closed.