Superconducting transition temperature in a Nb/NbxSi1−xbilayer system

We investigated the effect of amorphous ${\mathrm{Nb}}_{\mathit{x}}$ ${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$ on the superconducting transition temperature (${\mathit{T}}_{\mathit{c}}$) of two-dimensional single-crystal Nb films, as a function of thickness (${\mathit{d}}_{\mathrm{Nb}\mathrm{-}\mathrm{Si}}$) and of x varied across the metal-insulator transition (MIT), ${\mathit{x}}_{\mathit{c}}$=11.5%. When ${\mathit{d}}_{\mathrm{Nb}\mathrm{-}\mathrm{Si}}$≤100 Å, for insulating Nb-Si compositions, ${\mathit{T}}_{\mathit{c}}$ as a function of ${\mathit{d}}_{\mathrm{Nb}\mathrm{-}\mathrm{Si}}$ displays an enhancement. We shall discuss this result in the framework of existing theories. When ${\mathit{d}}_{\mathrm{Nb}\mathrm{-}\mathrm{Si}}$≥100 Å, ${\mathit{T}}_{\mathit{c}}$ systematically decreases as the Nb-Si composition approaches the MIT from the insulating side. We can interpret this phenomenon, using the proximity-effect theory. No ${\mathit{T}}_{\mathit{c}}$ enhancement at any thickness was observed for Nb-Si overlayers very close to the MIT or at the metallic side.