Puzzles on Fermi Surfaces in CeRu2Si2: Bonding Kondo State

Abstract

Mechanism of the unusual continuous transition in CeRu 2 Si 2 from valence fluctuation regime to Kondo regime, in which the Fermi surface volume changes exactly by one electron, is studied in detail. The transition occurs due to the singlet bonding Kondo band, which becomes sharper and more dispersionless as it approaches the Fermi energy with weakening quasiparticle character and crosses the Fermi energy at the transition. Various anomalous properties are related also with the mixed valence character in the valence fluctuation regime, disappearing in the Kondo regime. An unusual behavior of Hall effect with a large and sharp peak at low temperature, which has been misunderstood as the anomalous Hall effect, is explained from the mechanism that there are two kinds of Fermi surfaces, the first group formed from small holes with weak c - f mixing, showing a little change at the transition, and the second group of large Fermi surfaces with strong c - f mixing changing nearly discontinuously at the transition...