High-Frequency Surface Impedance of Dirty Type-II Superconductors in the Surface-Sheath Regime

Abstract
Surface-resistance measurements in the superconducting surface-sheath regime of Pb-In and Pb-Bi alloys are analyzed in the light of the recent theories of dirty type-II superconductors. From the changes of slope of the surface resistance R(H) at Hc3, the temperature dependence of the κ2 parameter can be deduced. For the alloys investigated, it is found that κ2 varies more strongly with temperature than expected from the dirty-limit theory. It is believed that this reflects strong-coupling effects. From the value of R(H) at Hc2, the absolute value of the sheath thickness can also be derived. This is found to agree well, near Tc, with the theoretical predictions of Fink and Kessinger. The analysis of the experimental data suggests that weak-coupling formalism remains applicable in the strong-coupling limit, and that deviations appear as modified temperature dependences of the critical fields. The Hc2(t) and Hc3(t) found for the two alloys are nevertheless rather close to the weak-coupling predictions.