Anisotropic superconducting properties of YBa2Cu3O7-δ (δ=0.1 and 0.4) untwinned single crystals

We have performed a detailed anisotropy study of the superconducting properties of untwinned single crystals of YBa2Cu3O7-δ for δ∼0.1 and δ∼0.4 in magnetic fields up to 22 T using a rf method giving critical fields H* which are determined by the onset of flux-line motion. For δ∼0.1 we find the highest H* along the b axis; the a-b anisotropy amounts to 20% at 85 K. The 60-K superconductivity for δ∼0.4 is characterized by the largest H* along the a axis and pronounced 2D behavior as manifested by a cusp in the angular dependence of H* and by the extremely steep slope of H*(T) perpendicular to c. We discuss our experimental results with respect to the significance of the Cu-O chains for the high-Tc superconductivity of YBa2Cu3O7-δ