Synchronous switching in vertically stacked Josephson junctions

Vertically stacked Josephson junctions with different thickness of 2, 4, 8, 14, 20, 40 nm for the intermediate electrode have been experimentally investigated. In the double junction, the synchronous switching between the lower and upper junctions is observed. Though simultaneous switching is also observed in the series-connected Josephson junction, some differences only in the double junction with thin intermediate Nb layer such as 4 nm or 8 nm are observed. We have also fabricated three stacked junctions and four stacked junctions and have investigated the magnetic field dependence of the critical currents.<>