Crossover from Ising- to Rashba-type superconductivity in epitaxial Bi 2 Se 3 /monolayer NbSe 2 heterostructures.

A topological insulator (TI) interfaced with an s-wave superconductor has been predicted to host topological superconductivity. Although the growth of epitaxial TI films on s-wave superconductors has been achieved by molecular-beam epitaxy, it remains an outstanding challenge for synthesizing atomically thin TI/superconductor heterostructures, which are critical for engineering the topological superconducting phase. Here we used molecular-beam epitaxy to grow Bi 2 Se 3 films with a controlled thickness on monolayer NbSe 2 and performed in situ angle-resolved photoemission spectroscopy and ex situ magnetotransport measurements on these heterostructures. We found that the emergence of Rashba-type bulk quantum-well bands and spin-non-degenerate surface states coincides with a marked suppression of the in-plane upper critical magnetic field of the superconductivity in Bi 2 Se 3 /monolayer NbSe 2 heterostructures. This is a signature of a crossover from Ising- to Rashba-type superconducting pairings, induced by altering the Bi 2 Se 3 film thickness. Our work opens a route for exploring a robust topological superconducting phase in TI/Ising superconductor heterostructures.