Energy spectrum and broken spin-surface locking in topological insulator quantum dots

TL;DR

This paper investigates the energy spectrum and spin structure of topological insulator quantum dots, revealing broken spin-surface locking and localized states, supported by both numerical and analytical models.

Contribution

It provides a detailed analysis of the energy spectrum and spin textures in topological insulator quantum dots, highlighting broken spin-surface locking at low energies.

Findings

Spin direction is not locked to the surface in topologically protected modes.

Identification of zero-momentum and subgap localized states.

Analytical surface Dirac fermion model reproduces numerical results.

Abstract

We consider the energy spectrum and the spin-parity structure of the eigenstates for a quantum dot made of a strong topological insulator. Using the effective low-energy theory in a finite-length cylinder geometry, numerical calculations show that even at the lowest energy scales, the spin direction in a topologically protected surface mode is not locked to the surface. We find "zero-momentum" modes, and subgap states localized near the "caps" of the dot. Both the energy spectrum and the spin texture of the eigenstates are basically reproduced from an analytical surface Dirac fermion description. Our results are compared to microscopic calculations using a tight-binding model for a strong topological insulator in a finite-length nanowire geometry.