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

From a large number of T/T-oxide/Ag tunneling junctions, with T = Ta and 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, ..mu..* = 0.15) which deviate markedly from values accepted hitherto and stronger high-energy (longitudinal phonon) contributions to ..cap alpha../sup 2/F(..omega..). For niobium, high-energy contributions to ..cap alpha../sup 2/F(..omega..) 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 A, and at least 10 A in the tantalum and niobium cases, respectively.