SOFT PHONONS IN SUPERCONDUCTING LUNI2B2C

Inelastic-neutron-scattering techniques have been used to measure the low-lying phonon-dispersion curves of superconducting LuNi2B2C along t he[xi 00] and [00 xi] symmetry directions. The most important result o f this experiment is that the phonon frequencies of the acoustic and f irst optical Delta(4) [xi 00] branches in the vicinity of the zone-bou ndary point G(1) decrease with decreasing temperature. Actually these two branches exhibit pronounced dips at low temperatures, close to G(1 ). This shows that the electron-phonon interaction is quite strong and causes an incipient lattice instability, a behavior typical of strong ly coupled conventional superconductors. Furthermore, the phonon anoma lies occur at wave vectors close to those of the incommensurate magnet ically ordered structures observed in the magnetic compounds of this f amily, which suggests that both the magnetic ordering and the incipien t lattice instabilities are influenced by common nesting features of t he Fermi surfaces of the rare-earth nickel boride carbides.