Bulk upper critical field of clean type-II superconductors: V and Nb

The bulk upper critical field H-c2 of single crystals of Nb and V has been measured for various crystallographic orientations within the temperature range between the respective critical temperature and T approximate to 0.06 degrees K. The normalized critical field averaged over all crystallographic directions (h(t)) = (H-c2(t))/(-dH(c2)/dt)(t=1), where t = T/T-c, and the relative anisotropy of H-c2 for the two metals are found to be similar. The measurements support predictions of the Hohenberg-Werthamer calculation that attributes the observed anisotropy of H-c2 to the effects of an anisotropic Fermi surface. Some differences in the behavior of H-c2(t) for the present samples can be understood qualitatively to arise from the effects of a shorter electron-collision time tau for the less pure V specimen. At very low temperatures, H-c2 (0) - H-c2(t) for V does not follow a t(2)ln(t) temperature dependence, previously observed for Nb, but can be characterized by an empirical formula t(2) In(t + alpha) in which the constant alpha is conjectured to be inversely proportional to tau.