Interplay of Mott Transition and Ferromagnetism in the Orbitally Degenerate Hubbard Model

A slave boson representation for the degenerate Hubbard model is introduced. The location of the metal to insulator transition that occurs at commensurate densities is shown to depend weakly on the band degeneracy M. The relative weights of the Hubbard sub-bands depend strongly on M, as well as the magnetic properties. It is also shown that a sizable Hund's rule coupling is required in order to have a ferromagnetic instability appearing. The metal to insulator transition driven by an increase in temperature is a strong function of it.Comment: 5 pages, revtex, 5 postscript figures, submitted to Phys. Rev.

Slave-Boson Functional-Integral Approach to the Hubbard Model with Orbital Degeneracy

A slave-boson functional-integral method has been developed for the Hubbard model with arbitrary, orbital degeneracy $D$. Its saddle-point mean-field theory is equivalent to the Gutzwiller approximation, as in the case of single-band Hubbard model. Our theory is applied to the doubly degenerate ($D = 2$) model, and numerical calculations have been performed for this model in the paramagnetic states. The effect of the exchange interaction on the metal-insulator (MI) transition is discussed. The critical interaction for the MI transition is analytically calculated as functions of orbital degeneracy and electron occupancy.Comment: Latex 20 pages, 9 figures available on request to [email protected] Note: published in J. Physical Society of Japan with some minor modification

Metal-insulator transition in a doubly orbitally degenerate model with correlated hopping

In the present paper we propose a doubly orbitally degenerate narrow-band model with correlated hopping. The peculiarity of the model is taking into account the matrix element of electron-electron interaction which describes intersite hoppings of electrons. In particular, this leads to the concentration dependence of the effective hopping integral. The cases of the strong and weak Hund's coupling are considered. By means of a generalized mean-field approximation the single-particle Green function and quasiparticle energy spectrum are calculated. Metal-insulator transition is studied in the model at different integer values of the electron concentration. With the help of the obtained energy spectrum we find energy gap width and criteria of metal-insulator transition.Comment: minor revisions, published in Phys. Rev.

Analysis of some physical properties of cerium compounds in the Anderson model

Using the theory of Lacroix-Lyon-Caen et al., who have calculated the magnetic susceptibility of a cerium Kondo system in the Anderson model, including the crystal field effects, we have determined the value of the exchange parameter Γn(EF) and the Kondo temperature TK of some cerium compounds from susceptibility measurements. We observe that | Γ| n(EF) increases when the electronic specific heat coefficient γ decreases and that the product γ.Γn(EF) varies only slightly. The analysis of the resistivity curves of these compounds (in particular, we have measured the electrical resistivity of CePb3) shows the validity of the calculations performed by Cornut and Coqblin with the Anderson Hamiltonian for T ⪢ TK.Utilisant la théorie de Lacroix-Lyon-Caen et al. qui ont calculé la susceptibilité magnétique d'un système Kondo de cérium dans le modèle d'Anderson en tenant compte des effets du champ cristallin, nous avons déterminé la valeur du paramètre d'échange Γn(EF) et la température de Kondo TK de quelques composés de cérium à partir de mesures de susceptibilité. On peut remarquer que | Γ| n(E F) augmente quand le coefficient de chaleur spécifique électronique γ décroît et que le produit γ.Γn(EF) varie peu. L'analyse des courbes de résistivité de ces composés (nous avons notamment mesuré la résistivité électrique de CePb3) montre la validité des calculs effectués par Cornut et Coqblin avec l'Hamiltonien d'Anderson lorsque T⪢ TK

THEORIES OF CORRELATION AND COLLECTIVE DISTORTION IN NARROW BANDS.DÉGÉNÉRESCENCE ORBITALE ET STRUCTURE MAGNÉTIQUE ET CRISTALLOGRAPHIQUE

Nous étudions quelques effets dus à la dégénérescence orbitale dans les bandes étroites : lorsque l'énergie coulombienne intra-atomique est grande il se forme un superréseau orbital. En utilisant le modèle de Hubbard dégénéré, nous obtenons un Hamiltonien effectif pour le spin et l'orbite qui généralise le superéchange d'Anderson. Nous appliquons cet Hamiltonien effectif à quelques composés de transition et nous discutons la relation entre structure magnétique et structure cristallographique.We study some effects of the orbital degeneracy in narrow band solids : when the intraatomic Coulomb energy is large an orbital superlattice is set up. Using the degenerate Hubbard model we derive an effective Hamiltonian for spin and orbit which is a generalization of Anderson superexchange. We apply this Hamiltonian to some transition metal compounds and we discuss the relation between magnetic and cristallographic structure

Magnetic properties of CeAl2 at low temperature

Un simple modèle s-f permet d'obtenir les principales caractéristiques d'un réseau Kondo. En particulier la diminution de la température d'ordre, due à l'effet Kondo, a été évaluée en fonction de la pression et comparée à des mesures de susceptibilités effectuées jusqu'à 6 kbar.A simple s-f model allows one to obtain the main characteristics of a Kondo lattice. In particular the lowering of the ordering temperature, due to the Kondo effect, has been evaluated as a function of pressure and compared with the susceptibility measured up to 6 kbar