Overdamped Josephson junctions for applications to precision measurement

Overdamped Nb/Al-AlOx/Nb Josephson junctions are an intermediate state between the SIS and SNS Josephson junctions. With a proper choice of the fabrication parameters, stable and reproducible nonhysteretic current-voltage characteristics have been obtained Featuring critical current densities up to 70 kA/cm2 and characteristic voltages up to 450 muV at 4.2 K, overdamped junctions are suitable for many applications ranging from RSFQ electronic circuits to programmable and ac voltage standards. Tests on single junctions and arrays of series-connected junctions have shown a suitable behaviour when driven by a microwave with frequencies around 75 GHz. The quantized steps have proven to be stable and flat at nV level. These junctions have also shown an improved temperature dependence, which can be interesting in view of the development instrumentation with an integrated quantum voltage reference operated in a closed cycle refrigerator.