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 CeCu_<b>2</b>Ge_<b>2</b>and Related Compounds

Abstract

Properties of an extended periodic Anderson model with the f-c Coulomb\ninteraction $\\Ufc$ is studied as a model for \\Ge and related compounds which is\ncosidered to exhibit a sharp valence change under pressure. The problem is\ntreated by extending the {\\it slave-boson} and large-N expansion method to\ntreat the present system. It is shown that, as the f-level $\\epsilon_{\\rm f}$\nis increased relative to the Fermi level, the sharp valence change is caused by\nthe effect of $\\Ufc$ with moderate strength of the order of the bandwith of\nconduction electrons. The superconducting transition temperature $T_\\rc$ due to\nthe valence-fluctuation exchange is estimated on the slave-boson fluctuation\napproximation. In the model with spherical Fermi surface, $T_\\rc$ exhibits\nsharp peak as a function of $\\epsilon_{\\rm f}$, simulating the effect of\npressure, for the d-wave pairing channel.\n