Observation of the extreme enhancement of superconducting properties of the middle Nb/Al layer in the double-barrier tunnel structures

Double tunnel junctions Nb/Al-Al Ox-Nb'/Al-Al Ox-Nb and Nb/Al/Nb'/Al-Al Ox-Nb''/Al-Al Ox/Al/Nb have been fabricated and investigated. It is found that if the thicknesses of the Nb'/Al(Nb''/Al) bilayers is of order of the coherence length in Nb, and the superconductivity of the thin Nb'(Nb'') layers is originally reduced, the enhancement of superconductivity of the middle Nb'/Al bilayer in the first type of devices occurs at T = 4,2 K. The effect involves both a considerable enhancement of the superconducting energy gap and an increase of the dc Josephson critical current density when measured across all the layers. The effect is supposed to be due to induction of superconductivity (proximity effect) in the Nb'/Al bilayer from the outermost Nb electrodes through the tunnel barriers