Lifting the spin-momentum locking in ultra-thin topological insulator films

TL;DR

This study investigates how reducing the thickness of topological insulator films affects their electronic properties, revealing a transition that lifts spin-momentum locking and opens a gap, advancing understanding of quantum spin Hall states.

Contribution

It demonstrates the thickness-dependent lifting of spin-momentum locking and gap opening in ultra-thin topological insulator films through transport measurements.

Findings

Exponential conductivity drop below critical thickness

Presence of spin-conserving backscattering

Lifting of spin-momentum locking and gap opening

Abstract

Three-dimensional (3D) topological insulators (TIs) are known to carry 2D Dirac-like topological surface states in which spin-momentum locking prohibits backscattering. When thinned down to a few nanometers, the hybridization between the topological surface states at the top and bottom surfaces results in a topological quantum phase transition, which can lead to the emergence of a quantum spin Hall phase. Here, we study the thickness-dependent transport properties across the quantum phase transition on the example of (Bi$_{0.16}$Sb$_{0.84}$)$_2$Te$_3$ films, with a four-tip scanning tunnelling microscope. Our findings reveal an exponential drop of the conductivity below the critical thickness. The steepness of this drop indicates the presence of spin-conserving backscattering between the top and bottom surface states, effectively lifting the spin-momentum locking and resulting in the opening of a gap at the Dirac point. Our experiments provide crucial steps towards the detection of quantum spin Hall states in transport measurements.