LATTICE-VIBRATIONS AND ELECTRON-PHONON COUPLING IN SUPERCONDUCTING QUARTERNARY BOROCARBIDES - AN INELASTIC NEUTRON-SCATTERING INVESTIGATION

The phonon densities of states of the borocarbides RNi2B2C (R = La, Y, and Lu) have been investigated using inelastic neutron-scattering tec hniques. These vibrational spectra are analyzed with the help of latti ce dynamical model calculations. In addition experiments were performe d on materials for which superconductivity has been suppressed by subs tituting 30% Ni by Co. The superconducting compounds display strong an omalous frequency shifts when compared to these Co-doped reference sys tems. This applies in particular to in-plane boron excitations found b etween 40 and 60 meV. Furthermore, the low-frequency part of the phono n spectra (up to 38 meV) of the superconducting systems shows a pronou nced soft-mode behavior of the Ni-B vibrations and. unexpectedly, also of the heavy R-atoms involved excitations, The latter effect also app ears upon cooling LuNi2B2C from room temperature to below 30 K. In con trast to these findings, the high frequency A(1g) mode which Mattheiss et al. predicted to couple particularly strongly to the electronic sy stem does not display any unusual frequency shift.