Skin Effect in a Type-II Superconductor

A single-crystal cylinder of Pb-2.6% In is placed in a dc magnetic field parallel to its axis, and a low-amplitude (1-mG to 1-G) low-frequency alternating field is superposed parallel to the dc field. Between Hc1 and Hc2, a volume flux penetration into the sample is observed by means of a one-layer coil of 20-μ wire wound directly on the sample. When the excitation-field level is larger than a certain "threshold" value, the signal contains odd harmonics of the excitation frequency. Below this threshold, the flux penetration is linear with excitation field, as in the case of a normal metal, and can thus be described by an equivalent resistivity ρ. At the lowest excitation levels employed, we do not observe any effect of a superconducting surface sheath, as though the critical surface currents were extremely small. Furthermore, above Hc2, as determined by the magnetization curve, the persistence of nonlinear effects suggests the existence of vortices, even when the total magnetization is found to be negligible.