ac Response and losses of differently pinned type-II superconductors

Measurements of the penetration of flux and losses in ac fields (20–2000 cps) at 4.2 K together with quasistatic magnetization curves on three samples of Ta ₉₂ Nb ₈ having different degrees of pinning are presented. The results are explained in terms of surface shielding currents taking into account both the pinning strength and the flux-flow resistivity exhibited by the bulk. A quantitative approach is used in which the concept of flux-flow resistivity proves applicable when the sample is subjected to an ac magnetic field.