ac Susceptibility Transition in Type-II Superconductors and Surface Critical Currents

The ac susceptibility transition below $H_{c3\mathrm{}}$ has been investigated as a function of ac field amplitude, in cylindrical samples of Pb-Bi alloys ($l<~\kappa <~5$). Converting the ac fields to critical surface currents $J_c$ permits comparison with the values calculated by Abrikosov and Park. The present results agree with those of Swartz and Hart, and of Rollins and Silcox. Although above $H_{c2\mathrm{}}$ the qualitative behavior is similar to the theoretical calculations, the magnitudes are found to be lower. At $H_{c2\mathrm{}}$ a sharp break is found in $J_c$, as reported previously. $\frac{J_c\kappa }{H_c}$ is found to be independent of $\kappa$ within experimental error. Surface roughness and slight sample misalignment could have lowered the observed $J_c$.