Good-quality tunnel junctions have been fabricated on ultrathin films of Nb whose thicknesses ranged from 9 to 900 Å. As the film thickness is reduced below 50 Å, $T_c$ decreases, $\frac{2\Delta }{k{T}_c}$ increases, and mode softening is observed through $\frac{d^{2}V}{d{I}^2}$ measurements. These results imply that as the thickness decreases below 50 Å the electron-phonon constant, $\lambda$, becomes larger than expected from the drop in $T_c$.

• Received 14 April 1986

DOI:https://doi.org/10.1103/PhysRevLett.57.901