Electrical contact to superconductors

Electrical contacts to conventional superconductors and high-temperature oxide superconductors are reviewed. The technologically important conventional superconductors are Nb and its alloys and compounds. The oxide superconductors are typified by YBa₂Cu ₃O₇. Three distinct forms in which the superconductor can be fabricated are considered: thin films for electronics applications, and monofilaments or tapes for measurements of the current-carrying capability of a superconducting material. The author discusses the physics of interfaces with superconductors and length scales on each side of the interface over which degradation of the material will affect the performance of the contact. It is noted that materials issues arise, in part, because the characteristic length scales are different for each superconductor and for each type of interface. Contacting the high-temperature oxide superconductors is difficult not only because the characteristic lengths are short, 0.1 to 10 nm, but because they are strongly dependent on crystal orientation and the oxides tend to be chemically unstable within that distance of the surface