In Cerium intermetallic compounds of the “122” family, 4f levels can hybridize with surrounding atoms’ wavefunctions leading to peculiar physical properties, whose full understanding requires the knowledge of the 4f levels’ splitting and symmetry.
The crystal electric field (CEF) effects lift the degeneracy of the spin-orbit multiplets and the resulting energy splittings (tens of meV) as well as the symmetry of the ground and excited states must be determined experimentally. Unfortunately, the techniques usually employed to study the CEF in lanthanides show limitations which can prevent an accurate characterisation of the 4f levels.
This thesis discusses the potential of Resonant Inelastic soft X-ray Scattering (soft-RIXS) to become a new probe of the CEF in cerium compounds. The latest advances in instrumentation have pushed the energy resolution in soft RIXS down to 30 meV at the Ce M5 edge, good enough to resolve the electronic transitions between the lowest lying 4f levels.
The energies of the spin-orbit and CEF excitations provide a direct measurement of the 4f splittings and the comparison of the experimental spectra with full-multiplet single-ion calculations unambiguously identifies the symmetry of the lowest CEF levels.
The orientation of the wave-functions in the crystal ab plane can also be determined by measuring the polarisation of the inelastically scattered photons. Moreover, the interaction with itinerant states can lead to a momentum dependence of the 4f levels' energy and this leaves signatures in the excitation spectra that can be observed in RIXS.
This thesis lays the groundwork for new studies on the physics of 4f electrons. With better energy resolution and more advanced modelling, RIXS studies of the CEF in Cerium and other lanthanide ions will become even more accurate in the future. With the present work, a new field of research opens up and brings a wealth of new possibilities to probe the fascinating properties of rare-earth compounds.