Browsing by Author "Albrecht, J."

Magneto-optical investigation of flux penetration into high-temperature superconducting thin films allows the determination of the local critical current density j(c) by an inversion scheme of Biot-Savart's law. This method is used to examine the influence of silver sheeting on j(c) in thin films of YBa2Cu3O7-delta (YBCO) quantitatively. It can be found that a feasible silver covering layer on top of a YBCO thin film can enhance the critical current density by up to 50%. Spatially resolved measurements of the magnetic-flux density distribution in partly silver covered YBCO films show the influence of the cover layer on the current pattern in the superconductor. The measured enhancement of the critical current density, that is induced by the silver layer, has its origin in a spatially varying proximity effect between superconductor and silver layer which leads to a strong variation of the flux-line energies on a small length scale. This variation is directly related to an additional pinning force density on the flux lines. A detailed model is developed that can explain the measured enhancement of the critical current density by considering this additional pinning.

In the past few years magneto-optical flux imaging (MOI) has come to take an increasing role in the investigation and understanding of critical current densities in high-T-c superconductors (HTS). This has been related to the significant progress in quantitative high-resolution magneto-optical imaging of flux distributions together with the model-independent determination of the corresponding current distributions. We review in this article the magneto-optical imaging technique and experiments on thin films, single crystals, polycrystalline bulk ceramics, tapes and melt-textured HTS materials and analyse systematically the properties determining the spatial distribution and the magnitude of the supercurrents. First of all, the current distribution is determined by the sample geometry. Due to the boundary conditions at the sample borders, the current distribution in samples of arbitrary shape splits up into domains of nearly uniform parallel current flow which are separated by current domain boundaries, where the current streamlines are sharply bent. Qualitatively, the current pattern is described by the Bean model; however, changes due to a spatially dependent electric field distribution which is induced by flux creep or flux flow have to be taken into account. For small magnetic fields, the Meissner phase coexists with pinned vortex phases and the geometry-dependent Meissner screening currents contribute to the observed current patterns. The influence of additional factors on the current domain patterns are systematically analysed: local magnetic field dependence of j(c)(B), current anisotropy, inhomogeneities and local transport properties of grain boundaries. We then continue to an overview of the current distribution and current-limiting factors of materials, relevant to technical applications like melt-textured samples, coated conductors and tapes. Finally, a selection of magneto-optical experiments which give direct insight into vortex pinning and depinning mechanisms are reviewed.

Local magnetic relaxation experiments are carried out on YBa2Cu3O7-delta thin films by magneto-optics. The current density is calculated quantitatively by an inversion scheme of Blot Savart's law which gives direct information about the time evolution of the current density with a high spatial resolution of about 5 mum. We determine the local activation energies U-0(x,y) by fitting different relaxation laws at different positions (x, y) in the film plane. This analysis yields a spectral distribution of the activation energies that is independent of the applied model. Possible mechanisms which could lead to such a distribution are discussed.