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, Hc2, 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 Tc versus θ in a constant field and Hc2 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.
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