Tunnelingα2F(ω)as a function of composition inA15NbGe

We have fabricated high-quality $A15\mathrm{}$ NbGe-$\mathrm{Si}{\mathrm{O}}_x$-Pb tunnel junctions on electron-beam-codeposited oxygen-doped ${\mathrm{Nb}}_3$Ge. $\frac{2\Delta }{k_B{T}_c}$ rises from the Bardeen-Cooper-Schrieffer limit in Ge-poor samples to become strong coupled ($\frac{2\Delta }{k_B{T}_c}\sim 4.35$) as stoichiometry is approached. These junctions have satisfactory features for taking derivative measurements. The data were reduced by the modified McMillan-Rowell proximity-gap inversion analysis developed by Arnold and Wolf to generate ${\alpha }^{2}F\left(\omega \right)$ and related microscopic parameters. As the $T_c$ and gap increase, a movement of the lowest phonon branch to lower energies is observed with a resultant decrease in $〈{\omega }^2〉$. Thus $\lambda$ increases with $T_c$ while $〈{\omega }^2〉$ decreases, in agreement with the idea that mode softening is a major factor in the increase of $T_c$ with approach to stoichiometry, as previously found in ${\mathrm{Nb}}_3$Al by Kwo and Geballe.