SUPERCONDUCTIVITY AND ELECTRICAL-PROPERTIES IN SINGLE-CRYSTALLINE ULTRATHIN NB FILMS GROWN BY MOLECULAR-BEAM EPITAXY

High-quality superconducting ultrathin Nb films were grown on sapphire single-crystal substrates by molecular-beam epitaxy, of which thickne sses ranged from 1.2 to 9.0 nm. They were found to be single crystalli ne by reflection high-energy electron diffraction. Size effects appear ed remarkably in the normal conduction range. Superconducting transiti on temperatures (T-c's) of these films were higher than in the past re ports for the same thicknesses. The thickness (d) dependence of T-c wa s In[T-c(d)]proportional to-1/d, which could be explained in terms of the proximity effect. A theory of localization effects in two-dimensio nal superconductors by Maekawa and Fukuyama, on the other hand, could not be applied to the present experiments, since the prepared ultrathi n films had better crystallinity than those discussed in the theory.