Upper bound on strong-coupling corrections to the second upper critical field

An upper bound for the strong-coupling correction eta/sub H//sub >//sub c//sub =/ to the second upper critical magnetic field of an Eliashberg superconductor is established as a function of Coulomb pseudopotential (μ/sup */) and of impurity content (t+). Results are also given for many actual superconductors for which the electron-phonon spectral densities are known. These are found to always fall below our upper bound although in some cases they can fall fairly close to the maximum, which varies with μ/sup */ and t+