Wachter, Intermediate valence and heavy fermions

Wachter, P. Intermediate valence and heavy fermions, in Gschneidner Jr., K.A., Eyring, L., editors. Handbook on the Physics and Chemistry of Rare Earths. Amsterdam North-Holland Publishing Company 1994, chap. 132. 

The chapter by Wachter (132), which is one of the most extensive and comprehensive ones in the entire Handbook series, reviews intermediate valence and heavy fermions in a wide variety of lanthanide and actinide compounds, ranging from metallic to insulating materials. The behaviors of these materials are discussed from the basic idea that a gap exists between two narrow f-like subbands, i.e. the hybridization gap model, which can account for the observed physical and electronic properties. As pointed out by Wachter, heavy fermions are intermediate valence materials by virtue of the fact that they have nonintegral f occupation values. The main difference between normal intermediate valence materials is that their Fermi energies are in the hybridization gap, while for the heavy fermion materials the Fermi energy is not in the gap. 

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