Strong-Coupling Calculation of the Upper Critical Field of Amorphous Lead

A calculation of the upper critical field ${B}_{c2}(T)$ is presented for the amorphous superconductor ${\mathrm{Pb}}_{0.75}$${\mathrm{Bi}}_{0.25}$. Experimentally measured ${\ensuremath{\alpha}}^{2}F(\ensuremath{\Omega})$ and ${\ensuremath{\mu}}^{*}$ are used in the strong-coupling theory for ${B}_{c2}(T)$. We show that the anomalous behavior of ${B}_{c2}(T)$ at low temperatures, which has been found recently, can be explained quantitatively by the influence of the anomalously enhanced low-frequency part in ${\ensuremath{\alpha}}^{2}F(\ensuremath{\Omega})$. In addition, spin-polarization effects are considered and it is shown that due to strong-coupling effects their contribution is unimportant for ${\mathrm{Pb}}_{0.75}$${\mathrm{Bi}}_{0.25}$.

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