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

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