Abstract
Excess currents have been observed in electron tunnelling between superconductors which cannot be explained on a model assuming the tunnelling of single particles. They take the form of a current step at applied voltages equal to half the energy gap of either superconductor, followed by an exponential-like rise in current. The size of the step varies greatly between samples and is sometimes too small to detect, but it is typically 10−4 of the current in the normal state at the same voltage. The excess currents are attributed to processes in which two particles tunnel simultaneously through the insulating barrier (Schrieffer and Wilkins 1963). The conditions for the occurrence of the various tunnelling processes are examined, and higher-order processes are discussed.

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