Influence of microscopic defects in type-II superconducting thin films on the magnetic flux penetration - art. no. 104503

The magnetic flux penetration into thin type-H superconducting films with c ircular defects is investigated. The artificial circular defects (diameter =40 mum) in an YBa2Cu3O7-delta thin film (thickness approximate to 300 nm) were prepared by pulse-laser irradiation. The flux penetration into the zer o-field-cooled superconducting film was visualized by means of the magneto- optic method. A stepwise increase of the external magnetic field allowed a detailed investigation of the influence of local defects on the flux penetr ation. For a magnetic field parallel to a long sample (longitudinal geometr y) with a long cylindrical defect a single parabolic discontinuity line app ears. Also in the case of a thin superconducting film exposed to a transver se magnetic field (transverse geometry), a single parabolic discontinuity l ine has been supposed in the vicinity of a local defect. On the contrary, o ur investigations show that the flux and current distribution around a sing le defect in a superconducting thin film can be determined not by a single, but by two discontinuity parabolas. In thin superconducting films in trans verse geometry screening currents in the Meissner region (j<j(c)) are prese nt in contrast to extended infinitely long samples, in the longitudinal geo metry. We explain our experimental results by the influence of these Meissn er screening currents on the temporal formation of the shape of an approach ing flux front.