Anisotropic flux penetration, pinning and critical fields in high T_c superconductors

We discuss the superconducting anisotropy of high temperature oxide superconductors, particularly YBaCuO and BiSrCaCuO, based on measurements of the zero-field-cooled dc magnetization vs temperature. This technique yields apparent H⊥c1 values for BiSrCaCuO in the range of 80-140 Oe, within a factor of two of the values for YBaCuO crystals. The analysis based on the anisotropic Ginzburg-Landau theory implies an effective mass anisotropy of less than 10. We attribute previously reported large anisotropies to the extrinsic effect of intergrowths. This has important implications for the possibility of two-dimensional statistical phenomena, like Kosterlitz-Thouless transitions, in these materials. © 1990.