Superconducting regime in ultra thin Niobium nanofilms

We have investigated the metallic state and superconducting properties of high quality Niobium nano-films as a function of their thickness, in order to study the transport properties and the quantum size oscillations of the superconducting critical temperature, Tc, in the ultranarrow regime. We have also characterized the interaction between the substrate and the Niobium nano-films which is responsible for an overall trend of suppression of Tc due to an inverse proximity effect. Layers with several thicknesses, varying from about 8 nm to 80 nm, have been deposited by sputtering on the oxidized surface of silicon wafers and then characterized by measurements of the temperature dependent resistivity (see figure below) and current-voltage characteristics. We found that Tc, as an overall trend, lowers for decreasing thickness while the width of the Tc resulted at least one order of magnitude lower than data reported in the literature. These very narrow widths of the superconducting phase transition indicate remarkable high structural quality of our Nb nanofilms, which is a key prerequisite for further nanostructuring pointing toward Nb nanostripes. On top of the overall decrease we found a small oscillatory behavior of Tc which we attribute to the quantum size effects and the shape resonances due to the strong confinement of the nanometer thick films along the transversal direction. This demonstrates a sizable interplay between quantum size effects and (anti)proximity effects in the superconducting phase of the nanofilms.