"DC and AC Magnetic Properties of a Thin Walled Superconducting Niobium Cylinder"

Nucleation of the superconducting phase in a thin surface sheath in a decreasing magnetic field parallel to the sample surface was predicted by Saint-James and de Gennes fifty years ago. This prediction was confirmed a short time after the paper’s publication.  In spite of this, a model for the ac losses in bulk samples in magnetic fields above H_c2 is still absent. Experiments with thin-walled superconducting cylinders can provide an essential simplification of this problem. Our new data on the dc and ac magnetic properties of such thin-walled superconducting Nb cylinders will be presented. A phenomenological model that describes the ac response of the surface superconducting states is proposed on the basis of these data. This model assumes that the observed ac response in dc fields larger than H_c2 is due to the relaxation of surface superconducting states with nonzero current in the walls to states with zero current, and the existence of a critical current below which this relaxation is absent.