COMPLEX MAGNETIC-FIELD DEPENDENCE OF CRITICAL CURRENTS IN MULTIPLE-STACKED TUNNEL BARRIERS

We have performed magnetic field experiments on devices containing the double tunnel barrier structure Nb-(Al-AlOx-Al)-Nb-(Al-AlOx-Al)-Nb, where the thickness of the common electrode is reduced to below the London penetration depth of Nb. The dimensions of the devices are smaller than the Josephson penetration length of the barriers and the field is applied in a direction parallel to the junctions. In this limit we find that it is possible for Abrikosov vortices to enter the middle Nb region and that the critical current of the junctions can be used as a sensitive indicator of the presence of one or more vortices in the common electrode.