Abstract
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.