Phonon profiles in superconducting YNi2B2C and LuNi2B2C

We performed a systematic inelastic neutron scattering, between 4 and 620 K , of the profiles of the anomalous acoustic and optic phonon observed in bo th compounds in the vicinity of <(xi)over right arrow>(m) congruent to (0.5 0 0). Above T-c we find that, in both compounds, the frequencies of the ac oustic modes decrease with decreasing temperature and there is intensity tr ansfer from-the upper (optic) to the lower (acoustic) mode. The frequencies of the optic modes of the Y compound, on the other hand, decrease slower t han those of the Lu compound as the temperature decreases towards T-c and, as a result, the two branches approach each other in the Lu compound, where as they separate in the Y compound. In both compounds the observed phonon p rofiles above T-c can be described satisfactorily in terms of a simple coup le-mode model. Comparison of the phonon spectra of the two compounds below T-c demonstrates that the sharp peak at approximately 4 meV with a broad sh oulder at the higher-energy side, observed in both compounds, arises from t he change in the acoustic phonon profile upon entering the superconducting state. [S0163-1829(99)07641-9].