Superconductivity of topological surface states in bulk insulating Bi₂Te₃ films on niobium

Topological insulators (TI) coupled to s‑wave superconductors (SC) can theoretically host topological superconducting states with emergent Majorana zero modes. However, prior works of TI/SC have overlooked TI with a thickness‑mediated transition between bulk conducting and insulating phases, which can impact the quantum‑mechanical coupling between surface and bulk states responsible for long‑range proximity effects. Employing a unique cleavage‑based “flip‑chip” method, we report the preparation of topological Bi₂Te₃ films with predetermined thicknesses (3–7 quintuple layers) on the elemental superconductor Nb. Synchrotron band mappings of Bi₂Te₃/Nb reveal an unexpected dimensionality‑mediated crossover from a bulk conducting to bulk insulating system with increasing Bi₂Te₃ thickness. Proximity‑induced gaps are quantified as a function of Bi₂Te₃ thickness using ultralow‑temperature scanning tunneling spectroscopy, and the results are compared with our prior findings on degenerately n‑doped Bi₂Se₃/Nb, degenerately p‑doped Sb₂Te₃/Nb, and bulk insulating (Bi_1−xSb_x)₂Te₃/Nb. Our study completes the picture for superconducting proximity effects in TI/Nb while offering methods for realizing accessible pairing needed in topological qubits.