Influence of Coulomb and proximity effects on electron tunneling through normal metal-superconductor interfaces

We develop a microscopic description of single and double electron tunneling through normal metal - superconductor interfaces in the presence of proximity and charging effects. In both ballistic and diffusive limits we derive the effective action for the system and show that its behavior can essentially depend on the physical boundary conditions in both normal and superconducting electrodes far from the tunnel barrier. We study quantum fluctuations of the charge at the NS interface due to virtual Andreev reflection which can substantially modify the Coulomb blockade for double electron tunneling. We also investigate the charge transfer through mesoscopic NS interfaces and NSN transistors in the presence of Coulomb effects for various physical situations. We describe a novel charge transfer mechanism in NSN structures - double electron cotunneling - and show that this mechanism can play an important role in the limit of relatively small external voltages. The results of our theory agree well with available experimental data. ((orig.))