Theory of the first-order isostructural valence phase transitions in mixed valence compounds YbIn_{x}Ag_{1-x}Cu_{4}

摘要

For describing the first-order isostructural valence phase transition inmixed valence compounds we develop a new approach based on the lattice Andersonmodel. We take into account the Coulomb interaction between localized f andconduction band electrons and two mechanisms of electron-lattice coupling. Oneis related to the volume dependence of the hybridization. The other is relatedto local deformations produced by f- shell size fluctuations accompanyingvalence fluctuations. The large f -state degeneracy allows us to use the 1/Nexpansion method. Within the model we develop a mean-field theory for thefirst-order valence phase transition in YbInCu_{4}. It is shown that theCoulomb interaction enhances the exchange interaction between f and conductionband electron spins and is the driving force of the phase transition. Acomparison between the theoretical calculations and experimental measurementsof the valence change, susceptibility, specific heat, entropy, elasticconstants and volume change in YbInCu_{4} and YbAgCu_{4} are presented, and agood quantitative agreement is found. On the basis of the model we describe theevolution from the first-order valence phase transition to the continuoustransition into the heavy-fermion ground state in the series of compoundsYbIn_{1-x}Ag_{x}Cu_{4}. The effect of pressure on physical properties ofYbInCu_{4} is studied and the H-T phase diagram is found.