Bulk Upper Critical Field of Clean Type-II Superconductors: V and Nb

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