Theory of first-order isostructural valence phase transitions in the mixed valence compounds YbIn1-xAgxCu4 -: art. no. 155109

For describing the first-order isostructural valence phase transition in mixed-valence compounds we develop an approach based on the lattice Anderson model. We take into account the Coulomb interaction between localized f and conduction band electrons and two mechanisms of electron-lattice coupling. One is related to the volume dependence of the hybridization. The other is related to local deformations produced by ionic radius fluctuations of rare-earth ions accompanying valence fluctuations. The large f-state degeneracy allows us to use the 1/N expansion method. Within the model we develop a mean-field theory for the first-order valence phase transition in YbInCu4. It is shown that the Coulomb interaction enhances the exchange interaction between f and conduction-band electron spins and is the driving force of the phase transition. In the framework of our approach the first-order isostructural Valence phase transition is a transition from a paramagnetic state with localized f electrons coupled weakly with conduction electrons to a ground state with strongly hybridized f and conduction-electron states. A comparison between the theoretical calculations and experimental measurements of the valence change, susceptibility, specific heat, entropy, elastic constants, and volume change in YbInCu4 and YbAgCu4 are presented, and a good quantitative agreement is found. On the basis of the model we describe the change of character of the transition into the ground state from a first-order valence phase transition to a continuous transition in the series of compounds YbIn1-xAgxCu4. We want to stress, however, that in all cases the ground state exhibits the properties that are typical of heavy fermions. The effect of pressure on physical properties of YbInCu4 is studied and the H-T phase diagram is found.