CRITICAL-CURRENT OF A BALLISTIC ASYMMETRIC SNSNS JUNCTION

The critical current through an asymmetrically stacked, double superco nductor-normal-metal-superconduct (SNS) junction is studied in the mes oscopic and ballistic regime. The asymmetry in such SNSNS junctions ex cludes any intuitive choice for the phase phi(2) of the middle superco nductor S-2. We propose to determine phi(2) from the condition that th e currents in the two normal regions are the same. The Bogoliubov-de G ennes equation is solved analytically within the Andreev approximation . In the long S2 limit, we find that the current-phase relation (CPR) can be constructed directly from the CPRs of the two constituent SNS j unctions, of which the longer SNS junction imposes a cutoff to the cri tical current. In short S-2 cases, the cutoff feature remains, but the cutoff value is affected by the Andreev-level tunneling. The dependen cies of the SNSNS critical current on the length of S-2, and the asymm etry in the configuration are studied. (C) 1998 Elsevier Science Ltd. All rights reserved.