Dissipation in BSCCO Superconductors: Influence of Pinning Centres
The critical current densities Jc and the activation energies U(T, B) for the thermally activated motion of vortices in four well-characterized Ag-clad Bi2223 and Bi2212 tapes have been determined. The observed Jc(77 K, B=0)≤39000 A/cm2 belong to the upper class of the results obtained so far for these materials. Like in other BSCCO samples U shows a (Tc-T) and ~B-α variations with T and B respectively. The comparison of pinning potentials U0(B)=U(T=0, B) for tapes with those for the other types (single crystal, thin film) of Bi2212 and Bi2223 samples shows that at lower field U0 depends more on the type of the sample than on the particular compound. For the same B, U0 systematically increase in order of single crystal, thin film and tape which indicates that in BSCCO compounds, in addition to anisotropy, the specific pinning centres (introduced during processing of the sample) affect the flux motion at lower B. This conclusion is supported with a weaker field dependence of Jc in tapes exhibiting higher U0. At elevated B (≈10 T) all U0 data seem to converge reaching values (U0~300 K) typical for pinning by oxygen vacancies. At lower B, the variations of U0, Jc and the volume pinning force Fp with B for tapes seem consistent with the collective pinning of vortices. The lack of correlation between the magnitudes of Jc, U0 and Fpmax is explained in terms of the percolative current path in these inhomogeneous materials.
KeywordsOxygen Vacancy Critical Current Density Epitaxial Films3 Flux Line Vortex State
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