Electron tunneling spectroscopy of superconductingA15 V-Ga alloy films

The results of a tunneling study of the high-$T_c$, A15 superconductor ${\mathrm{V}}_3$Ga are presented. The superconducting electron-phonon interaction spectral function ${\alpha }^2$F(ω) is obtained for a set of samples with compositions spanning stoichiometry. These results are of particular interest since ${\mathrm{V}}_3$Ga is unusual among the high-$T_c$, A15 superconductors in that single-phase material can be formed on either side of stoichiometry. The results demonstrate that increased weight of ${\alpha }^2$F(ω) (or so-called mode softening) occurs at low energies as stoichiometry is approached either from above or below. Concomitantly ${\omega }_{\log }$ is found to show a minimum at stoichiometry. Interestingly, although $T_c$ is a symmetric function of composition around stoichiometry, ${\omega }_{\log }$ is asymmetric. Hence, some other factor beyond mode softening is playing a significant factor in the systematics of the high $T_c$’s exhibited by this superconductor as a function of composition.