Giant two-phonon Raman scattering from nanoscale NbC precipitates in Nb

High-purity niobium (Nb), subjected to the processing methods used in the fabrication of superconducting rf cavities, displays micrometer-sized surface patches containing excess carbon. High-resolution transmission electronmicroscopyandelectronenergy-lossspectroscopymeasurementsarepresentedwhichrevealthepresence 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 twophononscattering.Theunprecedentedstrengthandsharpnessofthetwo-phononsignalhaspromptedatheoretical 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 spectroscopy. The exact origin of the stronger two-phononresponseisnotknown atpresentbutitsuggeststhepossibilityof 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.