Studies on Valence Fluctuation and Orbital Occupancy in an Impurity Anderson Model with f2 Local-Singlet Ground State

Abstract

An interplay of valence fluctuation and orbital occupancy is studied for a two-orbital impurity Anderson model having f2 singlet ground and triplet excited states in the localized limit. Employing the numerical renormalization group method, we identify the existence of a quantum phase transition between the local-singlet and the Kondo-singlet states in a variation of the c–f hybridization, and clarify how it depends on the f2 singlet–triplet energy splitting. It is found that the transition takes place definitely at a finite strength of the hybridization even when the singlet–triplet splitting is infinitely large. It is also found that as the splitting becomes small, the occupancies of the singlet and triplet states display a drastic change in the vicinity of the transition point. The origin of these findings is discussed in view of the features of valence fluctuation from the local many-body singlet state.