Relation between micro-structure and transport properties of epitaxial YBa₂Cu₃O_7-δ thin films

In order to understand the origin of the high critical current densities observed in thin superconducting films compared to single crystals we measured in several epitaxial YBa₂Cu₃O_7-δ thin films the magnetic field and temperature dependence of the superconducting current density j_s and the dynamical relaxation rate Q≡dlnj_s/dln(dB/dt). For all samples we found that (i) the superconducting current density j_s is independent of magnetic field up to a certain field B_Φ. The value of this field, which is sample dependent, decreases with increasing temperature; (ii) for fields B>B_Φ, the superconducting current density falls off as B_-1/2 over several decades; (iii) Q exhibits a sharp decrease for B<B_Φ and (iv) Q is essentially constant for B>B_Φ. These results can be understood in terms of strong pinning. Based on structural information obtained by AFM and STM we propose a model to explain both the high value and the magnetic field dependence of j_s and Q.