Clarifying superconducting proximity effects in topological insulating (Bi_1-xSb_x)₂Te₃ films on niobium

Coupling a topological insulator (TI) with an s-wave superconductor (SC) can initiate p-wave-like pairing in the nontrivial surface states. However, this proximity-induced pairing is still not well understood and challenging to realize, possibly due to a short superconducting coherence length and/or Fermi surface/lattice mismatch at the TI/SC interface. Using a novel cleavage-based “flip-chip” method, we have successfully prepared single-crystalline (Bi_1-xSb_x)₂Te₃ thin films with a predetermined thickness (2-5 quintuple layers) on bulk Nb substrate. By carefully tuning the composition x, we confirm using angle-resolved photoemission spectroscopy (ARPES) that the Fermi level lies in the band gap for all (Bi_1-xSb_x)₂Te₃ films, thus yielding a lightly n-doped TI on a SC even in the thin-film limit. Proximity-induced superconducting gaps in these samples are measured by ultrahigh-resolution laser ARPES. The results will be compared with those from prior studies of heavily n-doped TI films on Nb. Our study helps to reveal the mechanisms of proximity-induced pairing in TIs and also underscores the limitations of using the proximity effect to realize topological superconductivity.