Max-Planck Inst. Festkoerperforsch., Stuttgart , D-7000/80, Germany

A review with 37 refs. The use of electron spectroscopies for establishing a simple microscopic model and for detg. its parameters is discussed. Such models can be used as a starting point for studying other properties, e.g., thermodn. and transport properties. As an example Ce compds. are discussed. The valence photoemission and the Bremsstrahlung Isochromat spectra suggest that the Anderson model is a good starting point. It is discussed how the parameters can be estd. directly from expt., or by comparing model calcn. and expt. Based on these parameters some thermodn. properties are deduced. The need for methods for calcg. parameters is discussed. Currently used models of strongly correlated systems are often drastic simplifications of reality. Such models can provide a reasonable description of the system only if some of the interactions not included explicitly in the model are nevertheless included implicitly as a renormalization of the parameters. The calcn. of parameters therefore requires an understanding of these renormalizations. The Coulomb interaction U between localized electrons, e.g., 3d or 4f electrons, is strongly screened by the delocalized electrons, which tend to keep an atom neutral when the no. of localized electrons is changed. This charge transfer screening depends crucially on the character of the system, and how it renormalizes U. The hopping matrix elements V between localized and delocalized electrons are discussed for 3d, 4f and 5f compds. V Depends strongly on the configuration of the atom, i.e. the no. of localized electrons and the possible presence of a core hole. A core hole, for instance, leads to a large redn. of V. A prescription for how to choose the hopping matrix elements for different expts. is given. The renormalization of V due to various coulomb interactions not explicitly included in the model is discussed. The results are compared with expt. for CdMnTe and some Ce compds. While the general agreement with expt. is satisfactory, the calcd. hopping matrix elements tend to be too large for the Ce compds.
 

Conf. Proceedings Vol. 25, "2nd European Conf. on Progress in X-Ray Synchrotron Radiation Research", Eds. A. Balerna, E. Bernieri, S. Mobilio, Bologna: SIF (1990), 77-91.