  The interface between two-dimensional topological Dirac states and an s-wave
superconductor is expected to support Majorana bound states that can be used
for quantum computing applications. Realizing these novel states of matter and
their applications requires control over superconductivity and spin-orbit
coupling to achieve spin-momentum locked topological surface states which are
simultaneously superconducting. While signatures of Majorana bound states have
been observed in the magnetic vortex cores of bulk FeTe0.55Se0.45,
inhomogeneity and disorder from doping makes these signatures unclear and
inconsistent between vortices. Here we report superconductivity in monolayer
FeTe1-ySey (Fe(Te,Se)) grown on Bi2Te3 by molecular beam epitaxy. Spin and
angle resolved photoemission spectroscopy directly resolve the interfacial spin
and electronic structure of Fe(Te,Se)/Bi2Te3 heterostructures. We find that for
y = 0.25 the Fe(Te,Se) electronic structure overlaps with the topological
Bi2Te3 interfacial states which disrupts the desired spin-momentum locking. In
contrast, for y = 0.1 a smaller Fe(Te,Se) Fermi surface allows for clear
spin-momentum locking observed in the topological states. Hence, we demonstrate
the Fe(Te,Se)/Bi2Te3 system is a highly tunable platform for realizing Majorana
bound states where reduced doping can improve characteristics important for
Majorana interrogation and potential applications.
