The electron-coupled phonon spectrum o? F 9 X, and /i* 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 X are BCS-like and M* is always negative. Computer-modeling studies describing a niobiumlike material cannot generate an acceptable set of strong-coupling parameters.
Superconducting tunneling experiments on niobium single crystals with indium and lead-bismuth counterelectrodes show an isotropic energy-gap value of 3.12±0.05 meV, or 3.93kTc, for 26 junction orientations. Self-consistent analysis criteria have been developed for the determination of gap values. These criteria are not met in the existing literature on transition-metal tunneling.
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