Characteristics of Nb/Al/AlOx/Al/AlOx/Nb Junctions Based on the Proximity Effect

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Abstract
We present the results of a systematic study of Nb/Al/AlO x /Al/AlO x /Nb junctions having a structure in which an Al/AlO x film is added into a Nb/Al/AlO x /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 AlO x 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/AlO x /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.