Anisotropic superconducting properties of YBa_{2}Cu_{3}O_{7-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 ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ for δ≊0.1 and δ≊0.4 in magnetic fields up to 22 T using a rf method giving critical fields ${\mathrm{H}}^{\mathrm{*}}$ which are determined by the onset of flux-line motion. For δ≊0.1 we find the highest ${\mathrm{H}}^{\mathrm{*}}$ 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 ${\mathrm{H}}^{\mathrm{*}}$ along the a axis and pronounced 2D behavior as manifested by a cusp in the angular dependence of ${\mathrm{H}}^{\mathrm{*}}$ and by the extremely steep slope of ${\mathrm{H}}^{\mathrm{*}}$(T) perpendicular to c. We discuss our experimental results with respect to the significance of the Cu-O chains for the high-${\mathrm{T}}_{\mathrm{c}}$ superconductivity of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$.