Search for High T_c Superconductors Capable Selective Doping

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The purpose of the project is to search for high T_c marerials suitable for the electronic device application. Two approaches have been adopted. One is a search for materials with less anisotropy and longer coherence length which would relrese the difficulty in utilizing the high-T_c materials for conventional superconducting devices such as Josephson and three-terminal devices. It has been demonstrated that a perovskite oxide, Ba_<1-x>K_xBiO_3 with T_c=30K, is most promising and there is a possibility to fabricate all-oxide-SIS junctions using this material.The second is to search for potential materials which might lead to the development of a novel high T_c electronic device using properties unique to high T_c's. PrBa_2Cu_3O_7 - Pr-substituted compound of YBa_2Cu_3O_7(Y123) - is found to be a most interesting candidate. The present study has revealed that the CuO_2 planes in Pr123 are not doped with holes, which is the reason for non-metallic property, due to relatively small Madelung energy difference between in-plane O and Cu site as well as to the valence fluctuation of Pr. The magnitude of the Madelung energy difference is comparable with that in Pr_2CuO_4, a parent material of N-type high T_c superconductors. This fact strongly suggests that Pr123 might be doped with electrons. Attempts for N-type doping have been made by substituting Ce for Pr, La for Ba or F for O, though N-type doping has not been successful yet. If Pr123 were made an N-type superconductor, it would open a door to the development of a novel high T_c device with superconducting PN junctions in a Pr123/Y123 superlattice. Pr123/Y123 superlattice.