Sharp valence transition caused by f-c Coulomb interaction in an extended periodic Anderson model

An extended periodic Anderson model with U-ff = infinity and U-fc, f-c Coul omb interaction, is investigated by the variational Monte Carlo (VMC) metho d. In order to parameterize the effect of charge transfer between f- and c- electron states caused by U-fc, a projection operator Pi(i sigma sigma') [1 - (1 - h)n(i sigma)(f)n(i sigma')(c)] is introduced in the variational fun ction together with the conventional Gutzwiller projection operator Pi(i)[1 - n(i up arrow)(f)n(i down arrow)(f)]. It is shown that the Valence of f-e lectrons decreases rapidly as the level of f-electrons epsilon(f) is increa sed, if U-fc is large enough comparable to half of the bandwidth of conduct ion electrons. The result implies a possibility of valence fluctuation mech anism as an interpretation of the coincidence of peaks of the residual resi stivity and superconducting transition temperature observed in CeCu2Ge2 und er pressure. (C) 2000 Elsevier Science B.V. All rights reserved.