Mesoscopic Transport Characteristics of a Normal-Metal-Superconducting-Grain- Superconductor System

We investigate transport properties of a normal-metal-superconducting-grain-superconductor system by the use of the equivalent single-particle multi-channel networks, taking into account the multi-level structure, the Coulomb interaction, and the pair potential on the grain. The dependence of the current on the gate voltage shows oscillating behaviour with a period related to 2e of the charge on the grain, reflecting the charge transfer in units of Cooper pairs. The conductance can be enhanced when the pairing parameter is near the Coulomb energy e²/2C, due to the resonance of the Andreev reflection through the grain. The magnitude of the Andreev reflection as a function of the bias voltage exhibits complicated structures, reflecting the multiple levels, the spin orientations, and the interaction energy on the grain.