  A topological insulator (TI) interfaced with an s-wave superconductor has
been predicted to host an unusual form of superconductivity known as
topological superconductivity (TSC). Molecular beam epitaxy (MBE) has been the
primary approach in the scalable synthesis of the TI/superconductor
heterostructures. Although the growth of epitaxial TI films on s-wave
superconductors has been achieved, it remains an outstanding challenge for
synthesizing atomically thin TI/superconductor heterostructures, which are
critical for engineering the TSC phase. Here, we used MBE to grow Bi2Se3 films
with the controlled thickness on monolayer NbSe2 and performed in-situ
angle-resolved photoemission spectroscopy and ex-situ magneto-transport
measurements on these Bi2Se3/monolayer NbSe2 heterostructures. We found that
the emergence of Rashba-type bulk quantum well bands and spin-nondegenerate
surface states coincides with a marked suppression of the in-plane upper
critical magnetic field of the superconductivity in Bi2Se3/monolayer NbSe2
heterostructures. This is the signature of a crossover from Ising- to
Rashba-type superconducting pairings, induced by altering Bi2Se3 film
thickness. Our work opens a new route for exploring a robust TSC phase in
TI/Ising superconductor heterostructures.
