EFFECTS OF CHARGE PARITY IN TUNNELING THROUGH A SUPERCONDUCTING GRAIN

Abstract

The Coulomb blockade in a small superconducting grain, connected to two macroscopic electrodes by tunnel junctions, is discussed. The ground state energy of the grain with an odd number of electrons is shifted by the value of the superconducting gap due to the presence of one unpaired electron. In the case of superconducting leads, the charge parity effects give rise to a threshold behavior of the critical Josephson current when the superconducting gap is suppressed below the charging energy, e.g. by a magnetic field. The critical current is periodically modulated by the voltage of a gate electrostatically coupled to the grain. We find a characteristic change in this modulation at the threshold. In the case of normal leads, the low-bias conductance of this system is due to electrons passing through the grain in pairs (Andreev reflection). The linear conductance is periodic in the gate voltage. The period and the conductance activation energy are determined by charge 2e, rather than e. At resonance, the current first grows linearly with the applied bias, and then drops when an odd electron is allowed to enter the grain. Due to the high entropy of a state with an unpaired electron, this "poisoning" may also be stimulated by a fairly low temperature.