ac magnetic susceptibility, Meissner effect, and bulk superconductivity

Ac magnetic-susceptibility data for several Ba(${\mathrm{Pb}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Bi}}_{\mathrm{x}}$)${\mathrm{O}}_3$ samples indicate behavior quite different from that usually regarded as ‘‘typical’’ for alloys and compounds. These results focused attention on the criteria for bulk superconductivity in general and in particular on two recently proposed criteria for deducing bulk superconductivity from zero-field, ac magnetic-susceptibility data. These are (1) the appearance of an ‘‘excessive loss’’ as measured by a maximum or ‘‘peak’’ in plots of the loss component of the complex ac magnetic susceptibility as a function of temperature and (2) an observed dependence of the magnitude of the ‘‘ac signal’’ upon the ac measuring frequency. A series of measurements were performed on elements and alloys in both singly and multiply connected geometries which clearly reveal that the recently proposed criteria are not applicable in general. In addition, the ability of normal-state electrodynamics to account for the frequency dependence of the magnitude of the ‘‘ac signal’’ and the observation of an ‘‘excessive loss’’ peak in all samples under suitable experimental conditions suggest that the above criteria are not valid ones. It is concluded that zero-field, ac magnetic-susceptibility data are insufficient for discerning bulk from nonbulk superconductivity and that such data should not be used as a basis for Meissner-effect measurements or discussions.