Quantifying superconducting proximity pairing in topological (Bi_1-xSb_x)₂Te₃ films on niobium substrates

Topological insulator films proximally coupled to simple isotropic superconductors are theoretically promising candidates for the 2D topological superconductor phase with exotic emergent phenomena, but fabricating these systems is often exceedingly difficult. By employing a novel cleavage-based “flip-chip” technique, we report the fabrication of single-crystalline (Bi_1-xSb_x)₂Te₃ thin films (x = 0 to 1) with predetermined layer thicknesses (2-10 layers) on superconducting Nb. While bulk conducting topological Bi₂Se₃ films interfaced with Nb films exhibit strong superconducting order in the nontrivial surface states, bulk insulating topological (Bi_1-xSb_x)₂Te₃ films on Nb (x = 0.62) instead show a massive suppression of proximity pairing at the surface, even for films only two layers in thickness. This giant attenuation of surface superconductivity is revealed by our recent ultrahigh-resolution band mappings and clarified by our ongoing scanning tunneling spectroscopy measurements, the results of which are compared with those of bulk conducting n-doped topological Bi₂Se₃/Nb and Bi₂Te₃/Nb. The results further highlight the design constraints of employing the superconducting proximity effect to generate topological superconductivity in nearly intrinsic systems.