Double-barrier Josephson structures as the novel elements for superconducting large-scale integrated circuits

Abstract

An overview of the current status of different types of non-hysteretic Josephson junctions is given with emphasis on double-barrier structures. The results of theoretical work on double-barrier SIS′IS Josephson junctions (I is a tunnel barrier, S′ is a thin film with T C′< T C) are presented. The microscopic model for the supercurrent is developed for two cases: the S′ interlayer in the clean and in the dirty limit. The model describes the cross-over from direct Josephson coupling of the external S electrodes to the regime of two serially connected SIS′ junctions. We calculate the I C R N product as a function of the T C′/ T C ratio, the interlayer thickness and the barrier strengths and compare the theory with experimental data for Nb/AlO x /Al/AlO x /Nb junctions. We argue that these junctions are very promising in rapid single flux quantum (RSFQ) and programmable voltage standard applications, since they are intrinsically shunted and have controllable interfaces. We formulate the requirements for materials and interface barriers in order to increase critical current densities and I C R N products in double-barrier junctions.