Nonlocal Characteristics of the Bulk Upper Critical Field of Niobium

Abstract
Measurements of the anisotropic critical field Hc2 of Nb at temperatures down to 0.06 K show that its temperature dependence contains a logarithmic factor as predicted by Gor'kov on the basis of a nonlocal theory for an isotropic type-II superconductor. The critical field extrapolated to zero temperature, when averaged over all crystallographic directions and normalized to the slope of the critical field for TTc, has an enhancement close to that predicted by the Hohenberg-Werthamer calculation for the nonlocal effects of Fermi-surface anisotropy, as evaluated by Mattheiss for a band model of niobium.