Abstract
Abstract3D topological insulators 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, the thickness‐dependent transport properties across the quantum phase transition are studied on the example of (Bi0.16Sb0.84.)2Te3 films, with a four‐tip scanning tunneling microscope. The 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. The experiments provide a crucial step toward the detection of quantum spin Hall states in transport measurements.
Highlights
This allows spin-conserving scattering from k to −k if top and bottom surface states interact (Fig. 1b, right), which effectively corresponds to a lifting of spin-momentum locking in the topological surface states (TSS)
While the gap opening at the Dirac point has been detected in angleresolved photoemission spectroscopy and scanning tunnelling spectroscopy experiments on ultra-thin topological insulators (TIs) films[15,16,17], the second effect has yet not been observed, since it requires the systematic measurement of the transport properties of pristine samples, which is difficult in lithographically patterned samples, because the processing tends to degrade the ultrathin films
Samples are prepared by molecular beam epitaxy (MBE) with a shadow mask, which allows the deposition of a (Bi1−xSbx)2Te3 wedge in which the film thickness increases in steps of single quintuple layers from 1 QL ≈ 1 nm at the edge of the film to 12 QL in its centre (Fig. 1c, see Methods for details)
Summary
The (Bi0.16Sb0.84)2Te3 thin films were grown on a silicon-on-insulator (SOI) substrate, the latter consisting of a degenerately doped Si(100) handle wafer, a 300 nm oxide layer, and an undoped 70 nm Si(111) template layer. The STM tips are individually and precisely positioned on a chosen terrace of specific thickness L (which can be determined by counting step edges) in a linear four-point configuration with a distance s ≈ 250 nm between the tips. Once the four tips have been positioned laterally, they are driven from the tunnelling regime into contact with the sample surface In this electrical point contact regime, the sheet conductivity σ of the TI film is measured by repeatedly recording the voltage drop V between the two inner tips as a function of the current I injected between the two outer tips. After completion of the electrical measurements, the tips are retracted and the sample area is imaged again with STM In these images, the contact points of the tips are usually discernible as small spots of typically only ∼ 1 nm height (see Supplementary Fig. 1)
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