Dissipation in BSCCO Superconductors: Influence of Pinning Centres

Abstract

The critical current densities J_c 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 J_c(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 (T_c-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 J_c 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, J_c and the volume pinning force F_p with B for tapes seem consistent with the collective pinning of vortices. The lack of correlation between the magnitudes of J_c, U₀ and F_pmax is explained in terms of the percolative current path in these inhomogeneous materials.