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Properties of an extended periodic Anderson model with the f-c Coulombinteraction $\Ufc$ is studied as a model for \Ge and related compounds which iscosidered to exhibit a sharp valence change under pressure. The problem istreated by extending the {\it slave-boson} and large-N expansion method totreat the present system. It is shown that, as the f-level $\epsilon_{\rm f}$is increased relative to the Fermi level, the sharp valence change is caused bythe effect of $\Ufc$ with moderate strength of the order of the bandwith ofconduction electrons. The superconducting transition temperature $T_\rc$ due tothe valence-fluctuation exchange is estimated on the slave-boson fluctuationapproximation. In the model with spherical Fermi surface, $T_\rc$ exhibitssharp peak as a function of $\epsilon_{\rm f}$, simulating the effect ofpressure, for the d-wave pairing channel

journal of the physical society of japan

Enhanced Valence Fluctuations Caused by f-c Coulomb Interaction in Ce-Based Heavy Electrons: Possible Origin of Pressure-Induced Enhancement of Superconducting Transition Temperature in CeCu2Ge2and Related Compounds