Double barrier long Josephson junctions with a contact to the intermediate superconducting layer

We demonstrate a double barrier long Josephson tunnel junction device of overlap geometry with an external electric contact to the middle electrode. The junctions were fabricated with a standard Nb/Al-AlO/sub x//Nb process. To exhibit fluxon coupling effects in stacked junctions, the intermediate Nb layer thickness between two barriers has to be of the order of the London penetration depth, i.e. 90 nm for sputtered Nb films. By using a planarization process, we were able to avoid steep steps on the wafer and so to reduce the intermediate layer thickness to 90 nm. Double-junction stacks with areas from 10/spl times/10 /spl mu/m/sup 2/ to 20/spl times/600 /spl mu/m/sup 2/ were fabricated. In the best stacks, the difference in the critical currents of two junctions was less than 3%. Clear voltage locking effects were observed in current-voltage (I-V) characteristics.