Massive Dirac fermions and spin physics in an ultrathin film of topological insulator

TL;DR

This paper investigates the unique spin-dependent transport and optical phenomena in ultrathin topological insulator films, revealing a thickness-dependent topological phase transition and novel surface state properties.

Contribution

It demonstrates how tunneling in ultrathin films induces a mass gap in surface Dirac states and uncovers spin-dependent effects and a topological phase transition.

Findings

Surface states form massive Dirac hyperbolas due to tunneling.

Spin Hall effects and spin-dependent optical transitions are observed.

A topological phase transition occurs with varying film thickness.

Abstract

We study transport and optical properties of the surface states which lie in the bulk energy gap of a thin-film topological insulator. When the film thickness is comparable with the surface state decay length into the bulk, the tunneling between the top and bottom surfaces opens an energy gap and form two degenerate massive Dirac hyperbolas. Spin dependent physics emerges in the surface bands which are vastly different from the bulk behavior. These include the surface spin Hall effects, spin dependent orbital magnetic moment, and spin dependent optical transition selection rule which allows optical spin injection. We show a topological quantum phase transition where the Chern number of the surface bands changes when varying the thickness of the thin film.