Abstract
Single-particle (Giaever) tunneling has been performed on A15 Nb3Al 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 Nb3Al, including changes in the progressive film growth. The electron-phonon spectral function 2F() and related physical parameters are generated by the proximity-effect inversion analysis of the tunneling density of states. We find that Nb3Al is a strong-coupled superconductor with 2kBTc>~4.4 only when the composition approaches the A15 phase boundary (24 at.% Al). A difference in the lowfrequency behavior of 2F(), observed between two junctions of different coupling strength, strongly suggests the importance of a mode softening mechanism, and implies that the average ?'s are rather sensitive to composition, or perhaps disorder. The lattice of Nb3Al being softer than Nb3Sn and V3Si contributes significantly to the large and the high Tc of Nb3Al. © 1981 The American Physical Society.