JOSEPHSON EFFECT IN LOW-CAPACITANCE SUPERCONDUCTOR NORMAL-METAL SUPERCONDUCTOR SYSTEMS

The transport properties of a small superconductor-normal-metal-superconducting tunnel junction can be controlled by a gate electrode coupled capacitively to the central island. We evaluate the critical Josephson current I(c) through such a system as a function of the gate voltage V(g) taking into account parity effects in the superconductors. The dependence I(c)(V(g)) shows resonant singularities and has a qualitatively different character for DELTA < E(C) and DELTA > E(C), DELTA being the superconducting gap and E(C) = e2/2C being the charging energy. Due to the sweeping of the gate voltage, the system can be driven to states which are metastable with respect to a change of the number of electrons on the central island. Since the lifetime of the metastable states can be macroscopically large, one can observe a remarkable bistability of the Josephson current. A weak magnetic field supresses the interference of the electrons propagating through the central electrode, and, consequently, the magnitude of the critical current.