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

Abstract

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