Zero-bias conductance dip and phonon features in the superconductor density of states observed on tantalum- and niobium-based tunneling junctions

From a large number of $T-T$-oxide—Ag tunneling junctions, with $T=\mathrm{Ta}\mathrm{and}\mathrm{Nb}$, an empirical correlation was observed between the slope $S$ of the normal conductance-versus-voltage characteristics and the strength of the phonon-induced features of the superconductor density of states. This correlation allows for a tentative extrapolation to the bulk tantalum density of states which, when inverted according to the Rowell-McMillan scheme, leads to parameters ($\lambda =0.80\mathrm{}$, ${\mu }^{*}=0.15\mathrm{}$) which deviate markedly from values accepted hitherto and stronger high-energy (longitudinal phonon) contributions to ${\alpha }^{2}F\left(\omega \right)$. For niobium, high-energy contributions to ${\alpha }^{2}F\left(\omega \right)$ stronger than recently assumed are highly probable. The slope $S$ is governed by a zero-bias conductance dip. This anomaly as well as the nonideal features of the tunneling density of states in the superconducting state are attributed to a metal-semiconductor transition layer between the superconductor and its oxide with spatial extent of 3-10 Å, and at least 10 Å in the tantalum and niobium cases, respectively.