Does Strong-Coupling Theory Describe Superconducting Nb?

The electron-coupled phonon spectrum ${\alpha }^{2}F$, $\lambda$, and ${\mu }^{*}$ have been determined for polycrystalline and high-purity, single-crystal niobium from tunnel junctions fabricated with three different counterelectrodes. All spectra compare well with phonon spectra extracted from neutron-scattering experiments. Values of $\lambda$ are BCS-like and ${\mu }^{*}$ is always negative. Computer-modeling studies describing a niobiumlike material cannot generate an acceptable set of strong-coupling parameters.