Electrostatic control of the proximity effect in the bulk of semiconductor-superconductor hybrids

The proximity effect in semiconductor-superconductor nanowires is expected to generate an in-
duced gap in the semiconductor. The magnitude of this induced gap, together with the semicon-
ductor properties like the spin-orbit coupling and g - factor, depends on the coupling between the
materials. It is predicted that this coupling can be adjusted through the use of electric fields. We
study this phenomena in InSb/Al/Pt hybrids using nonlocal spectroscopy. We show that these
hybrids can be tuned such that the semiconductor and superconductor are strongly coupled. In this
case, the induced gap is similar to the superconducting gap in the Al/Pt shell and closes only at
high magnetic fields. In contrast, the coupling can be suppressed which leads to a strong reduction
of the induced gap and critical magnetic field. At the crossover between the strong-coupling and
weak-coupling regimes, we observe the closing and reopening of the induced gap in the bulk of a
nanowire. Contrary to expectations, it is not accompanied by the formation of zero-bias peaks in
the local conductance spectra. As a result, this cannot be attributed conclusively to the anticipated
topological phase transition and we discuss possible alternative explanations.