Strong-Coupling Saturation of Superconductivity inNb3Al0.8Ge0.2

Abstract

The recent suggestion, based on the tunneling experiments, that the superconducting transition temperature ${T}_{c}$ of ${\mathrm{Nb}}_{3}$${\mathrm{Al}}_{0.8}$${\mathrm{Ge}}_{0.2}$ is saturated with respect to the variation of the phonon frequencies, is critically examined. This is first done within the McMillan set of ideas concerning the high ${T}_{c}$, which was used in formulating the suggestion. The suggestion is thus shown to be in contradiction with the known pressure dependence of ${T}_{c}$. The observed weak variation of ${T}_{c}$ in the martensitic transition is not explained by the saturation effect but rather by the inefficiency of the low-frequency phonons in building up the superconductivity. It is further pointed out that the models of ${\mathrm{Nb}}_{3}X$ compounds, based on the singularity in the electron density of states, are not expected to yield a saturation of ${T}_{c}$ with respect to the phonon frequencies.