Abstract
Superconductivity in high-Tc oxides originates from the presence of (CuO2)-planes which lead to highly anisotropic normal and superconducting transport properties. The short coherence length ξc ≈ 1 to 3Å causes a spatial variation of the order parameter along the c-direction with dramatic consequences on the vortex dynamics. As model systems to study the influence of structural changes we prepared epitaxial YBa 2( Cu 1−x TM x)3 O 7 films ( TM = Zn and Ni), Bi 2 Sr 2 CaCu 2 O 8 films and coherent YBa 2 Cu 3 O 7/ PrBa 2 Cu 3 O 7 superlattices. Measurements of the critical current density [Formula: see text] clearly reveal the intrinsic pinning mechanism in YBa 2 Cu 3 O 7 for B ⊥ c at low temperatures which disappears approaching Tc. Small transition metal dopings act as pinning centers reducing dissipation due to thermally activated flux movement. The decoupling of the (CuO2)-layers in the superlattices causes a transition from anisotropic 3d to 2d behavior. Therefore the superconducting properties in external magnetic fields, which resemble closely those of Bi 2 Sr 2 CaCu 2 O 8 films, are dominated by the field component parallel to the c-axis. For B ⊥ c the resistive transitions ρ (B, T) and the critical current density jc (B, T) are nearly field independent.
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