PARITY EFFECTS IN JOSEPHSON TUNNELING THROUGH A METALLIC ISLAND

We study the Josephson current through a superconducting or normal gra in coupled by two tunnel junctions to macroscopic superconducting lead s and capacitively to a gate. For a superconducting grain the critical Josephson current has a threshold behavior - due to the Coulomb block ade - when the superconducting gap Delta is suppressed below the charg ing energy E(C) by a magnetic field. This effect is a direct consequen ce of a discrimination by the superconducting ground state between odd and even number of electrons on the grain. The critical current is pe riodically modulated by the gate voltage; it is 2e-periodic in the gat e-induced grain charge if Delta > E(C), but tends to become e-periodic as Delta --> 0. In the intermediate region the gate-modulated critica l current develops a double-peak structure; the gap between peaks incr eases as Delta decreases. If the gap energy on the grain Delta(g) is s maller than in the leads, the Josephson current is reduced, but reache s a finite, temperature dependent asymptotic value corresponding to a normal grain if Delta(g) --> 0.