Surface effects in the spectroscopy of mixed-valent Ce compounds

The electronic properties of correlated mixed-valent Ce compounds have been studied in detail over the last two decades by means of a variety of electron spectroscopies. This work has been carried out in connection with their peculiar ground state properties, deriving from the hybridization of the Ce 4f wavefunctions with the conduction band states. However, the relevance of surface effects, in the interpretation of the spectral line-shapes has been recognized only very recently. These are related to the reduction of 4f hybridization at the surface. In this review I will give a summary of the main achievements in this area, showing the procedures to separate the surface from the bulk spectral contributions for both the Ce 4f spectral functions and the Ce 3d core levels. In addition, a systematic analysis of the surface and bulk electronic properties of a number of different compounds is reported. This new information can in some cases reconcile the results obtained by different spectroscopies in terms of theoretical models based on the Anderson single impurity Hamiltonian. However, when comparing spectroscopies which rise to considerably different final states this may no longer be valid and some possible solutions are outlined. Furthermore, preliminary results on single crystals are presented. They seem to show somewhat unexpected features which need to be studied in deeper detail in the future. (C) 1998 Elsevier Science B.V.