Quantum interference and supercurrent in multiple-barrier proximity structures

We analyze an interplay between the proximity effect and quantum interference of electrons in the hybrid structures superconductor–normal-metal–superconductor which contain several insulating barriers. We demonstrate that the dc Josephson current in these structures may change qualitatively due to quantum interference of electrons scattered at different interfaces. In junctions with few conducting channels mesoscopic fluctuations of the supercurrent are significant and its amplitude can be strongly enhanced due to resonant effects. In the many-channel limit averaging over the scattering phase effectively suppresses interference effects for systems with two insulating barriers. In that case a standard quasiclassical approach describing scattering at interfaces by means of Zaitsev boundary conditions allows us to reproduce the correct results. However, in systems with three or more barriers the latter approach fails even in the many-channel limit. In such systems interference effects remain important in this limit as well. For short junctions these effects result in additional suppression of the Josephson critical current, indicating the tendency of the system towards localization. For relatively long junctions interference effects may—on the contrary—enhance the supercurrent with respect to the case of independent barriers.