Microscopic superconducting parameters from tunneling inA15Nb-Sn

High-quality tunnel junctions have been fabricated on thin films of $A15\mathrm{}$ Nb-Sn (20-25 at.%) using oxidized $a$-Si tunnel barriers and Pb counterelectrodes. These junctions have been used to measure changes in the gap, the transition temperature $T_c$, and the tunneling density of states with composition in this important high-$T_c$ superconductor. With the use of the proximity-effect-modified data reduction scheme developed by Arnold and Wolf, values for ${\alpha }^{2}F\left(\omega \right)$ and ${\mu }^{*}$ are obtained. As the Sn content approaches stoichiometry, the lowest-energy phonon branch in ${\alpha }^{2}F\left(\omega \right)$ both increases in weight and shifts to lower energy. These two effects combine to produce the increases in $\lambda$ as stoichiometry is approached, and at the same time can account for the observed increase in $\frac{2\Delta }{k_B{T}_c}$. The values of ${\mu }^{*}$ remain essentially constant as a function of composition, and hence show no evidence for an increased Coulomb interaction with increasing disorder as recently proposed by Anderson et al.