Tunnelingα2F(ω) as a function of composition inA15 V-Si

High-quality A15 V-Si/${\mathrm{SiO}}_{\mathrm{x}}$/Pb tunnel junctions on electron-beam codeposited ${\mathrm{V}}_3$Si have been fabricated. 2Δ/$k_B$ $T_c$ remains near the weak-coupled Bardeen-Cooper-Schrieffer limit as stoichiometry is approached. These junctions have satisfactory characteristics for taking derivative measurements. The data were reduced with use of the modified McMillan-Rowell proximity-gap inversion analysis developed by Arnold and Wolf to generate ${\alpha }^2$F(ω) and related microscopic parameters. As $T_c$ and the gap increase, there is no evidence of any qualitative change in the phonon spectra (such as phonon mode softening seen in other A15 compounds). Thus while in other A15 compounds such as ${\mathrm{Nb}}_3$Ge, ${\mathrm{Nb}}_3$Al, and ${\mathrm{Nb}}_3$Sn there is evidence that mode softening is a major factor in the high transition temperatures, in ${\mathrm{V}}_3$Si the high density of states at the Fermi surface (and not mode softening) is probably primarily responsible for the high $T_c$.