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Magnetic flux front and induction contours in superconducting YBa2Cu3O7-d films with defect size s ~ l (superconducting coherence length) and s > l are studied by magneto-optical imaging. Robust self-affine spatial correlation was observed using scaling analysis in the small pinning-dominated (s ~ l) disorderd films. The roughness exponent was determined to be ~ 0.66, independent of numbers of defects (or the film thickness). When the disorder landscape also included a distribution of large defects (s >> l), the flux front and induction contours exhibited self-similarity, with a fractal dimension to be ~ 1.33 using the box-counting method. The remarkably different flux penetration patterns were shown to be the manifestation of self-organized criticality at different length scales., Comment: 20 pages, 4 figures, submitted to Physical Review B

We report a detailed study of the microstructure and defects in sintered polycrystalline MgB2. Both TEM and x-ray data reveal that MgO is the major second-phase in our bulk samples. Although MgB2 and MgO have different crystal symmetries, being P6/mmm and Fm-3m, respectively, their stacking sequence of Mg and B (or O) and lattice spacings in certain crystallographic orientations are very similar. The size of MgO varies from 10~500nm, and its mismatch with the MgB2 matrix can be a source for dislocations. Dislocations in MgB2 often have a Burgers vector of <100>. 1/3<1-10> and 1/3<210> partial dislocations and their associated stacking faults were also observed. Since both dislocations and stacking faults are located in the (001) basal plane, flux pinning anisotropy is expected. Diffuse scattering analysis suggests that the correlation length along the c-axis for defect-free basal planes is about 50nm. (001) twist grain-boundaries, formed by rotations along the c-axis, are major grain boundaries in MgB2 as a result of the out-of-plane weak bonding between Mg and B atoms. An excess of Mg was observed in some grain boundaries. High-resolution nano-probe EELS reveals that there is a difference in near edge structure of the boron K-edge acquired from grain boundaries and grain interiors. The change at the edge threshold may be suggestive of variation of the hole concentration that would significantly alter boundary superconductivity., Comment: 20 pages and 12 figures

A large shift of the onset of flux-flow resistivity and the irreversibility line H(irr)(T) to higher temperatures is observed in Tl2Ba2Ca2Cu3O10 films containing linear defects created by Ag(+21) ion irradiation. The H(irr)(T), which has a characteristic L shape in highly anisotropic Tl and Bi based cuprates, becomes more like that of YBa2Cu3O7 in the presence of these defects. The Jc at 77 K also shows a large increase as a result of flux localization at the defects. The transport data indicate that in the H-T plane above H(irr)(T) of the unirradiated material, an ensemble of unoccupied defects is required for effective pinning of each flux line in the system.

The muon relaxation rate (sigma) was measured in the high critical temperature superconductors YBa2Cu3O(x) for x = 6.66, 6.95, 7.0, and La1.85 SrO.15 CuO4 in transverse external magnetic fields 1 is approximately 4 kG. A simple relation is found which connects the transition temperature T(c), the magnetic field penetration depth lambda(L), the carrier concentration n(s) and the effective mass m* as T(c) varies as sigma which varies as 1/lambda(L) squared which varies as n(s)/m*. The linear dependence T(c) varies as n(s)/m* suggests a high energy scale for the coupling between superconducting carriers.