The invention relates to a superconductive thin layer of YBa.sub.2 Cu.sub.3 O.sub.7-.delta..
This material, in which .delta.=0.1-0.5, is superconductive at temperatures lower than approximately 90 K.
Such a superconductive thin layer is described, for example, in "Thin superconducting oxide films" by R. B. Laibowitz a.o., Physical Review B, 35 (16), pp. 8821-8823 (1987). Sapphire (aluminum oxide) or magnesium oxide is used as a substrate. The thin layer is manufactured by means of electron beam evaporation in a vacuum.
The manufacture of superconductive thin layers on aluminum oxide is also described in "Superconducting thin films of the perovskite superconductors by electronbeam deposition" by R. H. Hammond a.o., Extended Abstracts for MRS Symposium on High Temperature Superconductors, Anaheim, Apr. 23-24 (1987). In order to suppress diffusion between the substrate and the superconducting thin layer, buffer layers are used, for example, consisting of MgO, Ta.sub.2 O.sub.5, LaF.sub.3 or ZrO.sub.2.
Other substrate materials which are used for providing superconductive thin layers of YBa.sub.2 Cu.sub.3 O.sub.7-.delta. are strontium titanate and noble metals, for example gold and silver.
After the deposition of the superconductive thin layer this layer must still be subjected to a prolonged treatment at high temperature in oxidizing conditions. The problem then occurs that the superconductive compound in many cases reacts with the substrate which is at the expense of the superconductive properties. The transition temperature T.sub.c to superconductive behaviour becomes lower and the transition becomes less steep. The incorporation in particular of metals, for example, aluminum and silicon has proved detrimental. Noble metals and strontium titanate are more suitable, but strontium titanate may also react with YBa.sub.2 Cu.sub.3 O.sub.7-.delta..