Current distribution and degradation of superconductivity in large contacts containing a type II superconductor

Phenomena associated with electron transport in normal metal-type II superconductor contacts of diameter 1–70 μm are studied. The superconductivity is destroyed as the current density attains the critical value for which the vortex motion becomes continuous. Contrary to the earlier assumptions, the voltage Vc1 is not directly connected with the energy gap of the superconductor, but is rather determined by the normal electrode and depends on the ratio of the electron mean free path Ii in the normal metal and the contact diameter d. Various current distributions may be realized in point contacts. For relatively clean contacts made of normal metals (Ii = d), as well as clean contacts with a thin (t ≪ d) damaged layer at the boundary between electrodes, the current density is constant over the sample cross-section. In extremely “dirty” contacts (Ii ≪ d), annular current flow is realized, the current density being low at the center and increasing towards the contact periphery. In the absence of an external magnetic field, the current in S–c–S contacts also flows in a ring whose diameter begins to increase as the field generated by the current exceeds Hc1 and the vortices penetrate the contact cross-section.

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