Upper-Critical-Field Limits for Bulk Type-Ii Superconductors

Approximate zero-temperature upper-critical-field limits Hc20 (max), consistent with the recent extreme-type-II theory of Werthamer, Helfand, and Hohenberg (WHH), are deduced by assuming (1) an electron mean free path l0 comparable to the interatomic spacing, (2) a spin-orbit-coupling-induced spin-flip mean free path lso=2l0, with approximate applicability of the WHH theory in this domain, and (3) Tc and l-independent parameters typical of present-day high-field superconductors. Highest Hc20(max) ≈ 850 kG are obtained for high-Tc β-tungsten-structure compounds.