Phonon mode coupling in superconducting ${\mathrm{LuNi}}_2$${\mathrm{B}}_2$C

Recently, Kawano et al. reported that an interesting phononlike excitation, localized in q and ω, appears below ${\mathrm{T}}_{\mathrm{c}}$ in the superconducting compound ${\mathrm{YNi}}_2$${\mathrm{B}}_2$C. This interesting observation led us to perform a series of experiments on the previously studied ${\mathrm{LuNi}}_2$ ${\mathrm{B}}_2$ C superconducting compound covering a large range of q and ω. We demonstrate that there is no new excitation and that the lowest lying excitations can be associated with the ${\mathrm{\Delta }}_4$ [ξ00] acoustic and optical phonon branches which exhibit pronounced dips at low temperatures for wave vectors close to the Fermi surface nesting vector, ${\xi }_{\mathrm{m}}$ , characteristic of these compounds. Above ${\mathrm{T}}_{\mathrm{c}}$ , as the temperature decreases the frequencies of both modes in the vicinity of ${\xi }_{\mathrm{m}}$ decrease and there is a shift of intensity from the upper to the lower mode, an effect characteristic of mode coupling. Below ${\mathrm{T}}_{\mathrm{c}}$ , there is a dramatic decrease in the linewidth of the acoustic mode due to the opening of the superconducting gap.