No abstract

No abstract

The effect of point defects created by γ-irradiation and extended defects created by Pb-ion irradiation on the behavior of the critical current density, J c in several superconductors have been investigated. We have found a limitation of the role of both types of defects on enhancing J c . We report an explanation of the effect of γ-irradiation and Pb-ion irradiation doses on J c in these materials. The explanation is based on combining several competing mechanisms of irradiation which we believe to take place mainly in the regions of the grain boundaries. The influences of these mechanisms were found to vary with the irradiation dose level.

Measurements of the magnetization vector M of a polycrystalline (Ba,K)BiO3 sample turned in a fixed field H at 4.2 K show that the rotational behavior of MP, the penetrating vortex-flux component of M, depends sensitively on the magnetic state of the sample before rotation. For the field-cooled state at 240 Oe, the vortex lines represented by MP are seen to bifurcate into two groups, one of which rotates rigidly with the sample while the other is held by H and turns frictionally relative to the sample. This behavior, seen earlier in polycrystalline YBa2Cu3O7, is directly ascribable to a distribution in the strength of the vortex pinning forces. For the hysteretic state in which H is raised from a large negative value to +240 Oe, a vortex-flux bifurcation is again observed but with an important difference. The strongly pinned vortices (rotating rigidly) reflect the original negative polarity of H, whereas the weakly pinned vortices (turning frictionally) are those produced by the positive H. Hence, in this hysteretic state before rotation, there is a coexistence of vortices of opposing polarity.

No abstract