Coupling between electronic and lattice degrees of freedom in 4f-electron systems investigated by inelastic neutron scattering

In general, elementary excitations in solids such as crystal field (CF) transitions and phonons are considered decoupled and the determination and interpretation of the measured spectra of the two phenomena, i.e. the CF level schemes and the phonon dispersion relations, are performed independently of each other. In addition, the spectra of these two excitations are generally quite complex and hence any unusual features are difficult to detect. A signature of a strong coupling between the two phenomena is the observation of an unusual behaviour in both subsystems. To prove the coupling unambiguously it is therefore necessary to investigate e.g. the phonon dispersion relations of an isostructural compound where the magnetic rare-earth ion (Ce, Yb) is replaced by a non-magnetic, but chemically equivalent ion (Y, La, Lu) and to determine the CF schemes of the same compound with the rare-earth ion replaced by a 'normal' magnetic rare earth. This requires, of course, time-consuming, detailed investigations. With these considerations in mind, it is not a surprise that there are only a few examples known where a coupling between electronic (CF transitions) and lattice (phonons) degrees of freedom have been reported. Here we will discuss results on three rare-earth compounds where the coupling between CF transitions and phonons has been unambiguously shown by inelastic neutron scattering experiments: CeAl2, YbPO4 and CeCu2