Quantum fluctuations in d9 model

Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe

Effective spinless fermions in the strong coupling Kondo model

Starting from the two-orbital Kondo-lattice model with classical t_2g spins, an effective spinless fermion model is derived for strong Hund coupling J_H with a projection technique. The model is studied by Monte Carlo simulations and analytically using a uniform hopping approximation. The results for the spinless fermion model are in remarkable agreement with those of the original Kondo-lattice model, independent of the carrier concentration, and even for moderate Hund coupling J_H. Phase separation, the phase diagram in uniform hopping approximation, as well as spectral properties including the formation of a pseudo-gap are discussed for both the Kondo-lattice and the effective spinless fermion model in one and three dimensions.Comment: Revtex4, 10 pages, 15 figures, typos correcte

Orbital Ordering in Paramagnetic LaMnO3 and KCuF3

{\it Ab-initio} studies of the stability of orbital ordering, its coupling to magnetic structure and its possible origins (electron-phonon and/or electron-electron interactions) are reported for two perovskite systems, LaMnO$_3$ and KCuF$_3$. We present a new Average Spin State (ASS) calculational scheme that allowed us to treat a paramagnetic state. Using this scheme, we succesfully described the experimental magnetic/orbital phase diagram of both LaMnO$_3$ and KCuF$_3$ in crystal structures when the Jahn-Teller distortions are neglected. Hence, we conclude that the orbital ordering in both compounds is purely electronic in origin.Comment: 10 pages, 5 figure

Effective d8 model: phase diagram

Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe

Spin stiffness and quantum fluctuations in C-type and A-type antiferromagnets

We present a systematic study of quantum fluctuations in the C-type and A-type antiferromagnetic (AF) phases in cubic lattices and in bilayer systems. Using the linear spin-wave theory, we show that the spin stiffness and the quantum corrections to the order parameter and energy obtained for C-AF and A-AF phases decrease with the increasing number of ferromagnetic bonds. Therefore, the quantum spin effects in LaMnO_3 and in LaVO_3 are rather small, suggesting the magnetic moments of 3.91 and 1.89 Bohr's magneton, respectively. They cannot explain the strong reduction of the magnetic order parameter observed in cubic vanadates.Comment: 6 pages, 4 figure

Resonant x-ray diffraction study of the magnetoresistant perovskite Pr0.6Ca0.4MnO3

We report a x-ray resonant diffraction study of the perovskite Pr0.6Ca0.4MnO3. At the Mn K-edge, this technique is sensitive to details of the electronic structure of the Mn atoms. We discuss the resonant x-ray spectra measured above and below the charge and orbital ordering phase transition temperature (TCOO = 232 K), and present a detailed analysis of the energy and polarization dependence of the resonant scattering. The analysis confirms that the structural transition is a transition to an orbitally ordered phase in which inequivalent Mn atoms are ordered in a CE-type pattern. The Mn atoms differ mostly by their 3d orbital occupation. We find that the charge disproportionation is incomplete, 3d^{3.5-\delta} and 3d^{3.5+\delta} with \delta\ll0.5 . A revised CE-type model is considered in which there are two Mn sublattices, each with partial e_{g} occupancy. One sublattice consists of Mn atoms with the 3x^{2}-r^{2} or 3y^{2}-r^{2} orbitals partially occupied, the other sublattice with the x^{2}-y^{2} orbital partially occupied.Comment: 15 pages, 15 figure

Ordered Phases and Magnetic Excitations in Doped Manganites

We present theoretical models concerning magnetic and charge ordering in half-doped manganites. The new approach to microscopic description of this group of compounds, taking into consideration the oxygen degrees of freedom, is discussed. The spin wave dispersion relations are shown both in the model of effective spins on Mn ions and in the new approach when two Mn ions form a large effective spin S=7/2

EFFECTIVE EXCHANGE INTERACTIONS AND MAGNETIC GROUND STATE OF STRONGLY CORRELATED ELECTRONS

Nous appliquons une nouvelle transformation canonique à deux systèmes : i/ electrons dans une bande étroite, et ii/ impureté magnétique. Cette transformation nous donne un Hamiltonien magnétique pour chacun de ces systèmes ; une méthode de fonctions de Green avec découplage conservant le moment, nous permet alors d'obtenir leurs propriétés. Nous obtenons entre autres une loi de Curie-Weiss pour la susceptibilité de l'impureté, et prouvons qu'une bande à moitié pleine - correspondant à un isolant de Mott - a un état fondamental antiferromagnétique de Heisenberg.A new canonical transformation is applied to the systems of electrons in a narrow band and magnetic impurity, which enable us to derive effective Hamiltonians for those systems. Further we study their properties via single-particle Green functions with the moment conserving decoupling. The most interesting results obtained are the Curie-Weiss law for the impurity susceptibility and a proof that for a particular case of a half-filled band. Corresponding to the Mott insulator, the ground state is that of Heinsenberg antiferromagnet (i.e. the bandwidth of quaisparticle states vanishes)