Superconducting proximity effects and pairing in (Bi_1-xSb_x)₂Te₃ films on niobium

Interfacing a topological insulator (TI) with an s-wave superconductor (SC) can favor the formation of helical-Cooper pairing in the topological surface states. However, realizing this system is often challenging, due to the impracticality of growing TIs on most SCs and possibly defects and Fermi surface/lattice mismatch at the TI/SC interface. By developing a novel cleavage-based “flip chip” approach, we have successfully fabricated single-crystalline (Bi_1-xSb_x)₂Te₃ thin films of a predetermined thickness (3 – 10 quintuple layers) on top of bulk Nb film. Moreover, by carefully controlling the growth conditions of the prototypical TI Bi₂Te₃ films, we find from our angle-resolved photoemission spectroscopy (ARPES) measurements of Bi₂Te₃/Nb that the Fermi level can lie in the band gap, giving rise to a lightly n-doped TI on a SC substrate. Ongoing laser ARPES and STM measurements would clarify the mechanism of coupling between the SC and the topological surface states in these doped TI/SC thin-film heterostructures. Our study underlines methods for realizing superconductivity in TIs and for clarifying the pairing in these heterostructures for applications in topological quantum computing.