Topological insulators and topological superconductors have been the subject of intensive research in recent years due to their exotic behaviors as well as the possibility to host Majorana Fermions, which may have applications in fault tolerant quantum computing. The signatures of topological surface states have been demonstrated by Angle Resolved Photoemmision, however, they are often less clear in electrical transport measurements due to contributions from the bulk conduction. In his talk, I will discuss our studies on Bi2Te3 nanotubes in order to maximally increase the surface-to-volume ratio as well as revealing topological properties in nanoscales. The nanotubes in the study have an outer diameter in the range of 70-120 nm and the wall width 9-12 nm, thicker than the critical thickness for outer-inner surface state hybridization. At low temperatures, surface states dominate the transport behavior in these samples as remaining bulk conduction are suppressed by disorder as demonstrated by resistance quantum oscillations in magnetic field.1 Detailed numerical simulations support that the resistance oscillations are arising from the topological surface states which have substantially longer localization length than that of other non-topological states. This result demonstrates the inherent nature of the topological surface states protected from strong disorder. We have previously studied superconductor/topological insulator heterostructures (NbSe2/Bi2Se3), and observed both proximity-induced bulk and two-dimensional surface superconductivity.2, 3 We have tried to induce superconductivity in the nanotubes with superconducting Nb contacts. However, contrary to the results in the bilayer films, inducing superconductivity in the nanotubes results in an anomalous resistance increase when the Nb contact becomes superconducting. The experimental results, though still lacking explanation, will be discussed in this talk.
[1] R. Z. Du et al. Phys. Rev. B 93, 195402 (2016)
[2] S.-Y. Xu et al. Nature Physics 10, 943-950 (2014)
[3] W. Dai et al. Scientific Report 7, 7631 (2017)