COMPETITION BETWEEN CRYSTALLINE ELECTRIC-FIELD SINGLET AND ITINERANT STATES OF F ELECTRONS

A new kind of phase transition is proposed for lattice fermion systems with simplified f(2) configurations at each site. The free energy of the model is computed in the mean-field approximation for both the iti nerant state with the Kondo screening, and a localized state with the crystalline electric field (CEF) singlet at each site. The presence of a first-order phase transition is demonstrated in which the itinerant state changes into the localized state toward lower temperatures. In the half-filled case, the insulating state at high temperatures change s into a metallic state, in marked contrast with the Mott transition i n the Hubbard model. For comparison, corresponding states are discusse d for the two-impurity Kondo system with f(1) configuration at each si te.