Eliashberg theory applied to the study of an Nb-Ge series

We use strong coupling theory of superconductivity to perform a detailed analysis of the Eliashberg functions α2F(ω), for thirteen samples of Nb-Ge with critical temperatures ranging from 7.0 K to 21.1K. As critical temperature increases, we analyze the general trends of the electron-phonon coupling parameter λ, of the integral of α2F(ω)''A'', and of other characteristics of α2F(ω) on the basis of qualitative or empirical criteria. While we find that the samples have in general the behavior expected, a closer analysis points to an overall attenuation in α2F(ω) of unclear origin. Though the gap edge, Δ0, appears to be well described by the α2F(ω) obtained, the thermodynamic critical field agrees poorly with that reproduced by α2F(ω) in the only case where the necessary (tunneling α2F(ω), critical field measurements) data are available. Our analysis suggests there is a gross uncertainty in the measured Hc(0). The overall analysis shows that the samples obtained are of enough quality to already give meaningful results upon inversion of the tunneling data. (author). 23 refs, 14 figs, 6 tabs