Superconducting transition temperature in a Nb/NbxSi1-x bilayer system

We investigated the effect of amorphous NbxSi1-x on the superconducting transition temperature (Tc) of two-dimensional single-crystal Nb films, as a function of thickness (dNb-Si) and of x varied across the metal-insulator transition (MIT), xc=11.5%. When dNb-Si100, for insulating Nb-Si compositions, Tc as a function of dNb-Si displays an enhancement. We shall discuss this result in the framework of existing theories. When dNb-Si100, Tc 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 Tc enhancement at any thickness was observed for Nb-Si overlayers very close to the MIT or at the metallic side.