Vortex pinning by natural defects in thin films of YBa₂Cu₃O_7-δ

Although vortex pinning in laser-ablated YBa₂Cu₃O_7-δ films on (100) SrTiO₃ is dominated by threading dislocations (Dam B et al (1999) Nature 399 439), many other natural pinning sites are present. To identify the contribution from twin planes, surface corrugations and point defects, we manipulate the relative densities of all defects by post-annealing films with various as-grown dislocation densities, n_disl. While a universal magnetic field B dependence of the transport current density j_s (B, T) is observed (independently of n_disl, temperature T and the annealing treatment), the defect structure changes considerably. Correlating the microstructure to j_s (B, T), it becomes clear that surface roughness, twins and point defects are not important at low magnetic fields compared to linear defect pinning. Transmission electron microscopy indicates that threading dislocations are not part of grain boundaries nor are they related to the twin domain structure. We conclude that j_s (B, T) is essentially determined by pinning along threading dislocations, naturally induced during the growth process. Even in high magnetic fields, where the vortex density outnumbers n_disl, it appears that linear defects stabilize the vortex lattice by means of the vortex-vortex interaction.