Penetration of magnetic flux in YBa2Cu3O7 superconducting thin films in an external magnetic field is visualized using a magneto-optic technique. A variety of flux patterns due to non-linear vortex diffusion is observed: (1) Roughening of the flux front with scaling exponents identical to those observed in burning paper including two distinct regimes where respectively spatial disorder and temporal disorder dominate. In the latter regime Kardar-Parisi-Zhang behavior is found. (2) Fractal penetration of flux with Hausdorff dimension depending on the critical current anisotropy. (3) Penetration as 'flux-rivers'. (4) The occurrence of commensurate and incommensurate channels in films with anti-dots as predicted in numerical simulations by Reichhardt, Olson and Nori. It is shown that most of the observed behavior is related to the non-linear diffusion of vortices by comparison with simulations of the non-linear diffusion equation appropiate for vortices.

In a recent paper [1], it has been shown from transport measurements that artificial reversible pinning can be created by the application of a magnetic tape on the surface of a thin superconducting film. We study the influence of a periodic array of magnetic stripes, generated from a signal pre-recorded on a magnetic tape, on the pinning of vortices. With our high resolution magneto-optical technique [2], we visualize in two dimensions the penetration of the flux in a thin YBa2Cu3O7-δ film partially covered by such a magnetic tape. In particular, we investigate the evolution of flux entry in the superconducting strip under and next to the magnetic tape. We present results obtained for various temperatures and various applied fields.

Kinetic roughening of flux fronts penetrating in superconducting thin films are studied by means of a high resolution magneto-optic technique. The roughening exponent (α = 0.64) and growth exponent (β = 0.65) obtained from a dynamic scaling analysis of the initial stage of flux penetration and, at small length scales, are characteristic for a static disorder dominated nonlinear diffusion such as also observed in the directed percolated depinning model. At large length scale, α = 0.46 indicates a transition towards dynamic stochastic disorder, similar to the behavior of Kardar-Parisi-Zhang systems. There is a striking similarity with the behavior of combustion fronts in burning paper.

In this study the role of anisotropy on flux penetration in c-axis epitaxial Tl2⁢Ba2⁢CuO6+𝑥 and YBa2⁢Cu3⁢O7−𝑥 films is investigated by magneto-optics. We study thin films of Tl2⁢Ba2⁢CuO6+𝑥 on substrates with vicinal angles of 0° (well-oriented), 0.5°, 2.5°, and 4° and YBa2⁢Cu3⁢O7−𝑥 films as a function of chain-conduction-induced anisotropy. A crossover from fractal to nonfractal flux penetration is observed with increasing anisotropy. Numerical simulations of anisotropic flux motion are compared with experiment.

High quality twin-free c-axis oriented YBa₂Cu₃O_7-δ films were grown by pulsed laser deposition on NdGaO₃. We find resistive anisotropies between 1 < ρ_a/ ρ_b < 1.6. The resistive anisotropy above T_c appears to be correlated to the anisotropy in the critical current below T_c. The normal state properties of the chains along b seem to have a significant influence on the superconducting properties of YBa₂Cu₃O_7-δ.

In an attempt to clarify the origin of the large critical current densitiesJ c observed in Laser Ablated and Sputtered YBa2Cu3O7−δ (YBCO) thin films, we make a systematic study of the low temperatureJ c in samples carefully analysed using STM and AFM.J c (B) is determined from torque-magnetometry performed in ring-patterned thin films. Epitaxial YBCO films nucleate in c-axis oriented single-crystalline islands with sizes ranging between 200 and 700 nm. We show thatJ c can be mainly explained by vortex pinning localised in the island boundaries.

Epitaxial YBa2Cu3O7-δ films nucleate in c-axis oriented single-crystalline islands. The surface of the single-crystalline SrTiO3 substrates exhibit steps of one third of the YBa2Cu3O7-δ c-axis. These steps generate a mismatch in the island boundaries between the CuO2 superconducting blocks. We show that these defect regions are strong candidates for being the pinning centers responsible for the large critical currents observed in Laser Ablated and Sputtered thin films.