Soft phonons in superconducting ${\mathrm{LuNi}}_2$${\mathrm{B}}_2$C

Inelastic-neutron-scattering techniques have been used to measure the low-lying phonon-dispersion curves of superconducting ${\mathrm{LuNi}}_2$${\mathrm{B}}_2$C along the [ξ00] and [00ξ] symmetry directions. The most important result of this experiment is that the phonon frequencies of the acoustic and first optical ${\mathrm{\Delta }}_4$ [ξ00] branches in the vicinity of the zone-boundary point ${\mathit{G}}_1$ decrease with decreasing temperature. Actually these two branches exhibit pronounced dips at low temperatures, close to ${\mathit{G}}_1$. This shows that the electron-phonon interaction is quite strong and causes an incipient lattice instability, a behavior typical of strongly coupled conventional superconductors. Furthermore, the phonon anomalies occur at wave vectors close to those of the incommensurate magnetically ordered structures observed in the magnetic compounds of this family, which suggests that both the magnetic ordering and the incipient lattice instabilities are influenced by common nesting features of the Fermi surfaces of the rare-earth nickel boride carbides.