Virtual levels, mixed valence phase and electronic phase transitions in rare earth compounds

The influence of the Coulomb interaction between localized and conduction electrons on the properties of magnetic impurities in metals and on electronic phase transitions such as the γ-α-α ' transitions in Ce and the insulator-metal transition in SmS is discussed. Due to excitonic pairing between f-holes and s-electrons, similar to that in excitonic insulators, the virtual f-levels in metals may acquire an extra width, in contrast to the width in the Anderson model, depends upon the position of the f-level, the width being the largest when the f-level crosses the Fermi-level. This effect stabilizes the intermediate valence phase. As a result, in the Falicov model one gets either a gradual phase transition (like that found in SmTe), or a first order one, followed by the intermediate valence phase (SmS), or, which is most interesting, two successive jump-like transitions with a mixed valence in between, similar to the γ-α-α ' transitions in Ce. The mixed valence phase is described here as a kind of an 'excitonic insulator'. The theory also predicts the correct slopes of the phase equilibrium lines for both Ce and SmS. (author)