Resonant Josephson tunneling in S-I-S′-I-S multilayered devices

It was observed experimentally that the magnitude of the Josephson current through a single junction in the double-barrier Nb/Al-AlO_x-Nb/Al-AlO_x-Nb stack is lower than that for the nwhole device. This effect is explained by a model that takes into account Andreev reflection in the S-I-S′-I-S double-barrier stack, where S and S′ are superconductors and I is an insulator. The middle S′ layer is very thin (with the thickness of order the coherence length) and has reduced superconducting parameters as compared with the external S layers. The model predicts bound state levels in the middle electrode. The bound states provide an additional channel of Josephson tunneling which results in enhanced Josephson critical current density through the stack as com-pared with that for the single S-I-S′ junction.