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

Double tunnel junctions Nb/Al−AlOx−Nb′/Al−AlOx−Nb and NB/Al/Nb′/Al−A10x–Nb″/Al−A10x−Nb′/Al/Nb have been fabricated and investigated. It is found that if the thickness of the Nb′/Al(Nb″/Al) bilayers is of the order of the coherence length in Nb, and the superconductivity of the thin Nb′ (Nb″) layers is originally suppressed, the enhancement of superconductivity of the middle Nb′/Al bilayers in the first type of devices occurs at T = 4.2 K. The effect of enhancement of the superconductivity of the Nb′/Al bilayer in the first type of structures as compared to the superconductivity of the Nb ″/Al bilayer in the second type of structures involves an increase in both the superconducting energy gap and the dc Josephson critical current density measured across the layers at V = 0. These effects are supposed to be due to induction of superconductivity (proximity effect) in the thin Nb′/Al bilayer from the outermost Nb electrodes exhibiting stronger superconductivity through the tunnel barriers.