Microstructure and superconductivity in ultrathin layers of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}

The authors have studied microstructures and superconductivity in ultrathin YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} (YBCO) films. They focus on the issues of terminating layer and superconductivity in surface layer of the film. Atomic force microscope (AFM) observation has revealed that the surface of the YBCO thin films in the initial stage consists of two-dimensional nuclei with a height of unit cell of YBCO. Cross-sectional transmission electron microscope (TEM) observation has revealed that the terminating layer of the YBCO (001) film is a CuO chain layer. In order to investigate the superconductivity in the surface of the YBCO (001) thin film, they have grown one-unit-cell thick (1-UCT) YBCO on PrBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} (PrBCO) layer and covered the surface of 1-UCT YBCO layer with various cap layers and examined the conditions required for the cap layers in order to produce the superconductivity of 1-UCT YBCO layer. They have found that MO-type atomic layer is needed to grow on the CuO terminating layer for the formation of BaO-CuO-MO charge reservoir layer and to make ordering in the CuO chain layer for the hole donation into the underlying CuO{sub 2} layers. In this case, MO layers are atomic layers of BaO and SrO inmore » NaCl-type structure or BaO, SrO, CaO, PbO, and CdO in perovskite-type structure, where the valence of M is 2+ and the lattice mismatch between the cap oxide and YBCO is sufficiently small (< 5%). They have also found that two CuO chain layers above and below the CuO{sub 2} bilayer interposing Y layer are needed for the occurrence of the superconductivity of the CuO{sub 2} bilayer.« less