Electronic structure and magnetic anisotropy of CrO_2

The problem of importance of strong correlations for the electronic structure, transport and magnetic properties of half--metallic ferromagnetic CrO_2 is addressed by performing density functional electronic structure calculations in the local spin density approximation (LSDA) as well as using the LSDA+U method. It is shown that the corresponding low--temperature experimental data are best fitted without accounting for the Hubbard U corrections. We conclude that the ordered phase of CrO$_2 is weakly correlated.Comment: 5 pages, 7 EPS figures, in RevTex forma

Spin and orbital excitation spectrum in the Kugel-Khomskii model

We discuss spin and orbital ordering in the twofold orbital degenerate superexchange model in three dimensions relevant to perovskite transition metal oxides. We focus on the particular point on the classical phase diagram where orbital degeneracy is lifted by quantum effects exclusively. Dispersion and damping of the spin and orbital excitations are calculated at this point taking into account their mutual interaction. Interaction corrections to the mean-field order parameters are found to be small. We conclude that quasi-one-dimensional Neel spin order accompanied by the uniform d_{3z^2-r^2}-type orbital ordering is stable against quantum fluctuations.Comment: 4 pages with 3 PS figures, 1 table, RevTeX, accepted to Phys. Rev. B. Rapid Communicatio

Interpolative Approach for Solving the Anderson Impurity Model

A rational representation for the self--energy is explored to interpolate the solution of the Anderson impurity model in general orbitally degenerate case. Several constrains such as the Friedel's sum rule, positions of the Hubbard bands as well as the value of quasiparticle residue are used to establish the equations for the coefficients of the interpolation. We employ two fast techniques, the slave--boson mean--field and the Hubbard I approximations to determine the functional dependence of the coefficients on doping, degeneracy and the strength of the interaction. The obtained spectral functions and self--energies are in good agreement with the results of numerically exact quantum Monte Carlo method.Comment: 15 pages, 9 figure

Dynamical correlations in multiorbital Hubbard models: Fluctuation-exchange approximations

We study the two band degenerate Hubbard model using the Fluctuation Exchange approximation (FLEX) method and compare the results with Quantum Monte-Carlo calculations. Both the self-consistent and the non-self-consistent versions of the FLEX scheme are investigated. We find that, contrary to the one band case, in the multiband case, good agreement with the Quantum Monte-Carlo results is obtained within the electron-electron T-matrix approximation using the full renormalization of the one-particle propagators. The crossover to strong coupling and the formation of satellites is more clearly visible in the non-self-consistent scheme. Finally we discuss the behavior of the FLEX for higher orbital degeneracy.Comment: 18 pages with 12 PS figure

Orbital-selective Mott transitions in the degenerate Hubbard model

We investigate the Mott transitions in two-band Hubbard models with different bandwidths. Applying dynamical mean field theory, we discuss the stability of itinerant quasi-particle states in each band. We demonstrate that separate Mott transitions occur at different Coulomb interaction strengths in general, which merge to a single transition only under special conditions. This kind of behavior may be relevant for the physics of the single-layer ruthenates, Ca$_{2-x}$Sr$_x$RuO$_4$.Comment: 4 pages, 4 figure

Electronic spectrum in high-temperature cuprate superconductors

A microscopic theory for electronic spectrum of the CuO2 plane within an effective p-d Hubbard model is proposed. Dyson equation for the single-electron Green function in terms of the Hubbard operators is derived which is solved self-consistently for the self-energy evaluated in the noncrossing approximation. Electron scattering on spin fluctuations induced by kinematic interaction is described by a dynamical spin susceptibility with a continuous spectrum. Doping and temperature dependence of electron dispersions, spectral functions, the Fermi surface and the coupling constant are studied in the hole doped case. At low doping, an arc-type Fermi surface and a pseudogap in the spectral function are observed.Comment: 13 pages (revtex), 18 figures, to be published in JET

Thermoelectric properties of the degenerate Hubbard model

We investigate the thermoelectric properties of a system near a pressure driven Mott-Hubbard transition. The dependence of the thermopower and the figure of merit on pressure and temperature within a degenerate Hubbard model for integer filling n=1 is calculated using dynamical mean field theory. Quantum Monte Carlo method is used to solve the impurity model. Obtained results can qualitatively explain thermoelectric properties of various strongly correlated materials.Comment: RevTex, 7 pages, 6 figure

Phase diagram of orbital-selective Mott transitions at finite temperatures

Mott transitions in the two-orbital Hubbard model with different bandwidths are investigated at finite temperatures. By means of the self-energy functional approach, we discuss the stability of the intermediate phase with one orbital localized and the other itinerant, which is caused by the orbital-selective Mott transition (OSMT). It is shown that the OSMT realizes two different coexistence regions at finite temperatures in accordance with the recent results of Liebsch. We further find that the particularly interesting behavior emerges around the special condition $U=U'$ and J=0, which includes a new type of the coexistence region with three distinct states. By systematically changing the Hund coupling, we establish the global phase diagram to elucidate the key role played by the Hund coupling on the Mott transitions.Comment: 4 pages, 6 figure