Characteristics of Nb/Al/AlOx/Al/AlOx/Nb junctions based on the proximity effect

We present the results of a systematic study of Nb/Al/AlOx/Al/AlOx/Nb junctions having a structure in which an Al/AlOx film is added into a Nb/Al/AlOx/Nb junction, in this letter. A Josephson current flows across the junctions because the second Al layer has superconductivity due to the proximity effect, The critical current density J(c) and quasiparticle characteristics strongly depend on the mean thickness of a second Al film and the transparency of the AlOx barriers. The junctions have relatively large conductance due to the proximity effect, This conductance results in a small hysteresis on its current-voltage curve compared to that of the Nb/Al/AlOx/Nb junction at the same J(c). Furthermore, the spread of the critical current for the 64 junctions connected in series is +/-1.2%. This shows that the junctions have uniform enough characteristics to be used in integrated circuits.