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

Abstract

In general, elementary excitations in solids such as crystal field (CF)transitions and phonons are considered decoupled and the determination andinterpretation of the measured spectra of the two phenomena, i.e. theCF level schemes and the phonon dispersion relations, are performedindependently of each other. In addition, the spectra of these two excitations aregenerally quite complex and hence any unusual features are difficult todetect. A signature of a strong coupling between the two phenomena is theobservation of an unusual behaviour in both subsystems. To prove the couplingunambiguously it is therefore necessary to investigate e.g. the phonondispersion relations of an isostructural compound where the magneticrare-earth ion (Ce, Yb) is replaced by a non-magnetic, but chemicallyequivalent ion (Y, La, Lu) and to determine the CF schemes of the samecompound with the rare-earth ion replaced by a ‘normal’ magnetic rare earth.This requires, of course, time-consuming, detailed investigations. Withthese considerations in mind, it is not a surprise that there are only a fewexamples known where a coupling between electronic (CF transitions)and lattice (phonons) degrees of freedom have been reported. Here wewill discuss results on three rare-earth compounds where the couplingbetween CF transitions and phonons has been unambiguously shown byinelastic neutron scattering experiments: CeAl2, YbPO4 and CeCu2.