28 research outputs found
Hidden non-Fermi liquid behavior due to crystal field quartet
We study a realistic Kondo model for crystal field quartet ground states
having magnetic and non-magnetic (quadrupolar) exchange couplings with
conduction electrons, using the numerical renormalization group method. We
focus on a local effect dependent on singlet excited states coupled to the
quartet, which reduces the non-magnetic coupling significantly and drives
non-Fermi liquid behavior observed in the calculated quadrupolar
susceptibility. A crossover from the non-Fermi liquid state to the Fermi liquid
state is characterized by a small energy scale very sensitive to the
non-magnetic coupling. On the other hand, the Kondo temperature observed in the
magnetic susceptibility is less sensitive. The different crystal-field
dependence of the two exchange couplings may be related to the different
dependence of quadrupolar and magnetic ordering temperatures in
CeLaB.Comment: 7 pages, 5 EPS figures, REVTe
Theory of the first-order isostructural valence phase transitions in mixed valence compounds YbIn_{x}Ag_{1-x}Cu_{4}
For describing the first-order isostructural valence phase transition in
mixed valence compounds we develop a new 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 f- shell size fluctuations 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 YbInCu_{4}. 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. A
comparison between the theoretical calculations and experimental measurements
of the valence change, susceptibility, specific heat, entropy, elastic
constants and volume change in YbInCu_{4} and YbAgCu_{4} are presented, and a
good quantitative agreement is found. On the basis of the model we describe the
evolution from the first-order valence phase transition to the continuous
transition into the heavy-fermion ground state in the series of compounds
YbIn_{1-x}Ag_{x}Cu_{4}. The effect of pressure on physical properties of
YbInCu_{4} is studied and the H-T phase diagram is found.Comment: 17 pages RevTeX, 9 Postscript figures, to be submitted to Phys.Rev.