PHONON MODE-COUPLING IN SUPERCONDUCTING LUNI2B2C

Recently, Kawano et al. reported that an interesting phononlike excita tion, localized in q and w, appears below T-c, in the superconducting compound YNi2B2C. This interesting observation led us to perform a ser ies of experiments on the previously studied LuNi2B2C superconducting compound covering a large range of q and w. We demonstrate that there is no new excitation and that the lowest lying excitations can be asso ciated with the Delta(4)[xi 00] acoustic and optical phonon branches w hich exhibit pronounced dips at low temperatures for wave vectors clos e to the Fermi surface nesting vector, xi(m), characteristic of these compounds. Above T-c, as the temperature decreases the frequencies of both modes in the vicinity of xi(m) decrease and there is a shift of i ntensity from the upper to the lower mode, an effect characteristic of mode coupling. Below T-c, there is a dramatic decrease in the linewid th of the acoustic mode due to the opening of the superconducting gap.