Giant two-phonon Raman scattering from nanoscale NbC precipitates in Nb
2015 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 91, no 9, 094302Article in journal (Refereed) Published
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.
Place, publisher, year, edition, pages
American Physical Society , 2015. Vol. 91, no 9, 094302
IdentifiersURN: urn:nbn:se:liu:diva-115403DOI: 10.1103/PhysRevB.91.094302ISI: 000351036900002OAI: oai:DiVA.org:liu-115403DiVA: diva2:795358
The authors thank G. Ciovati of Jefferson Laboratory for supplying Nb samples used in this study. Calculations (H.L.) were performed with financial support by the SSF-project Designed multicomponent coatings, MultiFilms and the Swedish Research Council. Calculations were carried out at the Swedish National Infrastructure for Computing (SNIC), Argonne LCRC and Argonne Center for Nanoscale Materials. The work at Argonne National Laboratory and the use of the Center for Nanoscale Materials and the Electron Microscopy center at Argonne National Laboratory were supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-06CH11357. This work was also supported by the Department of Energy, Office of Science, Office of High Energy Physics, early career award FWP#50335 to T.P.2015-03-162015-03-162015-04-21Bibliographically approved