Characterization of Superconducting Localized Structures

We studied electron localization effects in superconducting Nb/Al bilayers. These effects are characterized by a decrease in resistivity with increasing temperature typical of a semiconductor and by an enhancement of the critical temperature Tc. The samples, made by superposition of Nb and Al films, each 50- 200 nm thick, show a sharp superconducting transition between 8 and 9 K. The electron localization effects are detectable only when the resistivity at 10 K is larger than about 100 mu?cm. We discuss the fabrication conditions leading to the observed electron localization. We also present some measurements of the current-voltage characteristics in localized and metallic Nb/Al bilayers and we analyse them in terms of a simple thermal picture.