Low temperature value of the upper critical field Hc2(0) of isotropic single band metals

The strong coupling Schossmann-Schachinger theory for the upper critical field is considered methodically from the user point of view. We check the accuracy of approximate formulas for the upper critical field. In particular, we explain in detail as a recently proposed convenient expression (Shulga et al. cond-mat/0103154) can be justified. The connection of Hc2(0) with the critical temperature Tc is considered and the similarity with the Allen-Dynes Tc-formula is shown explicitly.