Study of Strong Superconducting Coupling between High Tc and Low Tc Superconducting layers by Interface-Control

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An enhancement of superconducting coupling between high temperature superconducting (HTS) layer and low temperature one, which is seriously suppressed in usual interfaces by structural defects and interface-reactions, has been attempted by controlling electronic structure of the HTIS-side region. In the present project, YBa_2Cu_3O_<7-δ> (YBCO) films was selected as the HTS layer. In the first stage, for developing an intrinsic surface nature with the original amplitude of the order parameter, we have adopted a cleaning of the YBCO surface by an annealing under an intensive atomic oxygen beam. This treatment yielded an elimination of carbon-related impurities and a development of metallic feature in in-situ ultraviolet photoemission spectra. The height of the Fermi edge of the treated a-, and c-YBCO films was as high as 10 % of Cu 3d characterized peak in their valence band, which was comparable with that of vacuum-cleaved single crystals at a cryogenic temperature. The optimum treatmen … More t condition in the present study was as follows : an annealing temperature of 450 ℃ and a flux density of the atomic oxygen above 5x10^<15> atoms/cm^2・sec. Because of the high electron density of states, a contact resistibity between the optimally treated c-YBCO surface and in-situ deposited Au reached in the range of 1〜5x10^<-9> Ω・cm^2. A high critical current density up to 7x10^5 a/cm^2 at 4.2 K was also achieved in the Pb-In/Ag ; 1〜2 nm /Au ; 1〜2 nm /c-YBCO (optimally treated) multiplayer structure. A suppression of tunneling conductance around zero bias was observed in scanning tunneling spectroscopy on the (ultra thin Au)/c-YBCO surface at 4.2 K, which could be explained by propagation of superconducting proximity effect through the Au layer. The obtained transport properties meet requirements in design of integrated circuits. And the results also indicate an usefulness of the control of the surface of HTS layers by using both the atomic oxygen treatment and in-situ characterization of the electronic structure. Less