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 (Bi0.16Sb0.84)2Te3 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.
@article{arxiv.2106.06217,
title = {Lifting the spin-momentum locking in ultra-thin topological insulator films},
author = {Arthur Leis and Michael Schleenvoigt and Vasily Cherepanov and Felix Lüpke and Peter Schüffelgen and Gregor Mussler and Detlev Grützmacher and Bert Voigtländer and F. Stefan Tautz},
journal= {arXiv preprint arXiv:2106.06217},
year = {2024}
}