Single-particle (Giaever) tunneling has been performed on $A15\mathrm{}$ ${\mathrm{Nb}}_3$Al thin films with goodquality junctions formed by oxidized amorphous-silicon barriers. The tunneling measurement is used as a diagnostic probe for assessing the material properties of ${\mathrm{Nb}}_3$Al, including changes in the progressive film growth. The electron-phonon spectral function ${\alpha }^{2}F\left(\omega \right)$ and related physical parameters are generated by the proximity-effect inversion analysis of the tunneling density of states. We find that ${\mathrm{Nb}}_3$Al is a strong-coupled superconductor with $\frac{2\Delta }{k_B{T}_c}>~4.4$ only when the composition approaches the $A15\mathrm{}$ phase boundary (24 at.% Al). A difference in the lowfrequency behavior of ${\alpha }^{2}F\left(\omega \right)$, observed between two junctions of different coupling strength, strongly suggests the importance of a mode softening mechanism, and implies that the average $〈\omega 〉\text{'}\mathrm{s}$ are rather sensitive to composition, or perhaps disorder. The lattice of ${\mathrm{Nb}}_3$Al being softer than ${\mathrm{Nb}}_3$Sn and ${\mathrm{V}}_3$Si contributes significantly to the large $\lambda$ and the high $T_c$ of ${\mathrm{Nb}}_3$Al.

• Received 1 October 1980

DOI:https://doi.org/10.1103/PhysRevB.23.3230