Probing the Transition in Bulk Ce under Pressure : A Direct Investigation by Resonant Inelastic X-Ray Scattering
This Transition in Ce is archetypical of localization-delocalization phenomena encountered in f-electron systems. This isostructural phase transition accompanied by a large volume contraction (14%) is a manifestation of subtle interactions between f levels self-consistently embedded in a sea of conduction electrons, this transition is commonly described by either a Mott transition or a Kondo hybridization.
Important information about this transition is contained not only in the occupation number, nf, but also in the probability of double occupancy of the f sites. It is well known that the f states are clearly identifiable in spectroscopies (absorption, XAS or photoemission, XPS). Many proofs of the mixed-valent behaviour has been accumulated unambiguously from such experiments. Well separated features, each assigned to different f states, indicate that the f configurations are mixed in the ground state. But the different screenings of the core hole by the |f 0>, |f 1> and |f 2> states, lead to strong difficulties to estimate the various f-electron weights and therefore the degree of hybridisation of the f electron at the origin of the heavy-fermion-like behaviour of Ce.
Recently, Resonance Inelastic x-ray Scattering (RIXS) has emerged as a means of probing the mixed-valent behaviour in rare earths (RE) systems in considerable details. The experiment consists in measuring the 3d->2p decay following a resonant excitation close to the RE-L2,3 edges (2p->5d transition).
The RIXS process benefits from the selective resonant enhancement of the different states. Previous series of experiments  have revealed a well-defined feature associated with the |f2> configuration, which is normally hidden in XAS spectra. The f 1/f 2 ratio, as a function of temperature, closely resemble the magnetization loop of the transition induced by chemical pressure in Ce-Th alloy. However, electron interactions with the dopant element necessarily intervene in the Ce-4f electronics properties.
We have investigated elemental Ce, by 2p3d-RIXS, directly subjected to high pressure to induce the transition  (ID12, ESRF).