Abstract
The dc characteristics of tunnel junctions between weak and strong superconductors characterized by transition temperatures and , respectively (), exhibit an excess pair current. This current is proportional to the imaginary part of the generalized pair-field susceptibility , which characterizes the dynamics of the weak superconductor, where and are proportional, respectively, to the voltage across and the magnetic field in the junction. We show here that this result follows in a simple way from the interaction that gives rise to the usual Josephson current below , and can in fact be viewed as a second-order Josephson effect. Above the order parameter on the strong side acts as an effective conjugate field to induce an order parameter on the weak side, yielding a proportional Josephson current between the two electrodes. We also interpret this result to yield a charge-imbalance relaxation time for the effective gapless super-conductor on the weak side. Below the induced part of the order parameter interacts with the usual Josephson current to produce an excess current in a manner analogous to the way an external oscillator coupled to a junction results in a constant-voltage step in the characteristic. In both temperature regimes the magnitude of this excess current is related to an appropriate time-dependent Ginzburg-Landau equation. Finally, the effect of thermal voltage noise on the excess current is considered. Throughout, a heuristic approach is used to bring out new aspects of the problem and to make the physical content of the theory accessible.
- Received 15 October 1981
DOI:https://doi.org/10.1103/PhysRevB.25.6701
©1982 American Physical Society