Coupling behavior of Cu-O₂ layers in oxygen-deficient YBCO films and ultrathin YBCO films

We have measured the resistivity as a function of temperature in a magnetic field at various field angles θ for oxygen-deficient YBCO films and ultrathin YBCO films; θ is the angle between the crystal c axis and the direction of the magnetic field. The angular dependence of the critical field, H_c2, shows a cusplike behaviour at θ=90° for the oxygen-deficient 60 K YBCO films indicating that the Cu-O₂ layers are two-dimensional in nature. Both T_c versus θ in a constant field and H_c2 as a function of θ show a plateau at θ=90° for 83 K (zero resistance) ultrathin YBCO films with a thickness of 20 units of the YBCO cell; these results indicate that there is a coupling between the Cu-O₂ layers. The dimensionality of the Cu-O₂ layers in oxygen-deficient YBCO films and ultrathin YBCO films is discussed in terms of the anisotropic Ginzburg-Landau model and the Tinkham thin-film model.