Temperature-dependence of anomalous superconducting nucleation fields in rolled niobium foils

Abstract

We have measured critical fields (Hc2) of rolled niobium foils by ac susceptibility and by critical current measurements in parallel and perpendicular field (Hsparallel and Hsperpendicular). Hc2 agrees with GLAG theory. Hs/Hc2 increases from a constant value near Tc to a higher constant value at lower temperatures. We attribute Hsparallel and Hsperpendicular to high-κ regions at the surface and in the bulk respectively. We assume that the temperature dependence is governed by the ratio d/ξ(T), where d is a characteristic thickness of the high-κ structure. For the internal structure we estimate d similar 1000 Angstrom, in agreement with the wall thickness of the dislocation cell structure. Assuming that the internal high-κ regions are cell walls, we estimate for the dislocation ND in the walls ND < 7 × 1011 cm-2, in agreement with previous published estimates.