  High purity niobium (Nb), subjected to the processing methods used in the
fabrication of superconducting RF cavities, displays micron-sized surface
patches containing excess carbon. High-resolution transmission electron
microscopy and electron energy-loss spectroscopy measurements are presented
which reveal the presence of nanoscale NbC coherent precipitates in such
regions. Raman backscatter spectroscopy on similar surface regions exhibit
spectra consistent with the literature results on bulk NbC but with
significantly enhanced two-phonon scattering. The unprecedented strength and
sharpness of the two-phonon signal has prompted a theoretical analysis, using
density functional theory (DFT), of phonon modes in NbC for two different
interface models of the coherent precipitate. One model leads to overall
compressive strain and a comparison to ab-initio calculations of phonon
dispersion curves under uniform compression of the NbC shows that the measured
two-phonon peaks are linked directly to phonon anomalies arising from strong
electron-phonon interaction. Another model of the extended interface between Nb
and NbC, studied by DFT, gives insight into the frequency shifts of the
acoustic and optical mode density of states measured by first order Raman. The
exact origin of the stronger two-phonon response is not known at present but it
suggests the possibility of enhanced electron-phonon coupling in transition
metal carbides under strain found either in the bulk NbC inclusions or at their
interfaces with Nb metal. Preliminary tunneling studies using a point contact
method show some energy gaps larger than expected for bulk NbC.
