Tunneling α2F(ω) as a function of composition in A15 NbGe

We have fabricated high-quality A15 NbGe-SiO/sub x/-Pb tunnel junctions on electron-beam-codeposited oxygen-doped Nb3Ge. 2Δ/k/sub B/T/sub c/ rises from the Bardeen-Cooper-Schrieffer limit in Ge-poor samples to become strong coupled (2Δ/k/sub B/T/sub c/approx.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 α2F(ω) and related microscopic parameters. As the T/sub c/ and gap increase, a movement of the lowest phonon branch to lower energies is observed with a resultant decrease in <ω2>. The lambda increases with T/sub c/ while <ω2> decreases, in agreement with the idea that mode softening is a major factor in the increase of T/sub c/ with approach to stoichiometry, as previously found in Nb3Al by Kwo and Geballe