Lattice vibrations and electron-phonon coupling in superconducting quarternary borocarbides: An inelastic neutron-scattering investigation

The phonon densities of states of the borocarbides ${\mathrm{RNi}}_2$ ${\mathrm{B}}_2$C (R=La, Y, and Lu) have been investigated using inelastic neutron-scattering techniques. These vibrational spectra are analyzed with the help of lattice dynamical model calculations. In addition experiments were performed on materials for which superconductivity has been suppressed by substituting 30% Ni by Co. The superconducting compounds display strong anomalous frequency shifts when compared to these Co-doped reference systems. This applies in particular to in-plane boron excitations found between 40 and 60 meV. Furthermore, the low-frequency part of the phonon spectra (up to 38 meV) of the superconducting systems shows a pronounced soft-mode behavior of the Ni-B vibrations and, unexpectedly, also of the heavy R-atoms involved excitations. The latter effect also appears upon cooling ${\mathrm{LuNi}}_2$ ${\mathrm{B}}_2$C from room temperature to below 30 K. In contrast to these findings, the high frequency ${\mathrm{A}}_{1\mathrm{g}}$ mode which Mattheiss et al. predicted to couple particularly strongly to the electronic system does not display any unusual frequency shift.