Anomalous phonons below T-c in superconducting RNi2B2C (R = Lu, Y)

The phonons that exhibit anomalous temperature dependence in these compound s have wave vectors close to <(xi)over right arrow>(m) = (xi,0,0), with xi congruent to 0.5, and belong to two Delta(4)[xi 00] branches (one acoustic and the other optic) which cannot cross by symmetry. The behavior of these phonon modes was studied systematically between 2 and 620 K. We find that a bove T-c the frequencies of the acoustic modes decrease with decreasing tem perature, and there is intensity transfer from the upper (optic) to the low er (acoustic) mode in both compounds. As the temperature decreases towards T-c, the two branches approach each other in the vicinity of <(xi)over righ t arrow>(m) for the Lu compound, whereas they separate in the Y compound. T his behavior, above T-c, can be satisfactorily explained by a couple-mode m odel. Below T-c the phonon spectrum changes dramatically: in both compounds , it consists of a sharp peak at approximately 4 meV with a broad weak shou lder on the high-energy side. The width of the sharp peak remains practical ly constant but its intensity decreases with increasing temperature and van ishes in the vicinity of T-c. The results below T-c are in qualitative agre ement with recent theoretical calculations. (C) 1999 Elsevier Science B.V. All rights reserved.