A recently developed dynamical mean-field theory in the iterated perturbation theory approximation was used as a basis for construction of the "first principles" calculation scheme for investigating electronic structure of strongly correlated electron systems. This scheme is based on Local Density Approximation (LDA) in the framework of the Linearized Muffin-Tin-Orbitals (LMTO) method. The classical example of the doped Mott-insulator La1−xSrxTiO3 was studied by the new method and the results showed qualitative improvement in agreement with experimental photoemission spectra.

One-electron Green's functions of a narrow-band paramagnetic insulator are calculated within the framework of the s-d exchange and Hubbard models with strong correlations. The agreement with the l/z-perturbation theory is discussed for the approximations employed. It is demonstrated that the analytical properties of the retarded Green's functions are violated for simple approximations in the Hubbard model and the s-d model with a negative exchange parameter I (and, for self-consistent versions, at any sign of I). Numerical results for the corresponding densities of states are presented. Im Rahmen von s-d-Austausch und Hubbard-Modellen mit starker Korrelation werden Ein-Elektronen-Greenfunktionen cines. schmalbandigen paramagnetischen Isolators berechnet. Fur die benutzten Naherungen wird die Ubereinstimmung mit der l/z-Storungstheorie diskutiert. Es wird gezeigt, dal3 die analytischen Eigenschaften der retardierten Greenfunktionen fur simple Naherungen im Hubbardmodell und s-d-Modell mit einem negativen Austauschparameter I (und, fur selbstkonsistente Versionen, fur jedes Vorzeichen von I) verletzt sind. Numerische Ergebnisse fur die entsprechenden Zustandsdichten werden angegeben.

A critical consideration of some approaches to the metal-insulator transition problem,startingfrom the many-electronrepresentation, iscarriedout within the framework of the Hubbard and classical HI models in the far-paramagnetic region. The analytical propertiesof the corresponding one-electron Green functions are discussed. the importance of terms of sufficiently high orders in l/z being demonstrated. The total energy, electronic specific heat and corrections to the local moment are calculated. The Hubbard-111 approximation in the Hubbard model (but not in the s d model) is shown to lead to difficultieswhen calculating thermodynamic properties

A model of alloy is considered which includes both quenched disorder in the electron subsystem ("alloy" subsystem) and electron-phonon interaction. For given approximate solution for the alloy part of the problem, which is assumed to be conserving in Baym's sense, we construct the generating functional and derive the Eliashberg-type equations which are valid to the lowest order in the adiabatic parameter. The renormalization of bare electron-phonon interaction vertices by disorder is taken into account consistently with the approximation for the alloy self-energy. For the case of exact configurational averaging the same set of equations is established within the usual T -matrix approach. We demonstrate that for any conserving approximation for the alloy part of the selfenergy the Anderson's theorem holds in the case of isotropic singlet pairing provided disorder renormalizations of the electron-phonon interaction vertices are neglected. Taking account of the disorder renormalization of the electron-phonon interaction we analyze general equations qualitatively and present the expressions for Tc for the case of weak and intermediate electron-phonon coupling. Disorder renormalizations of the logarithmic corrections to the effective coupling, which arise when the effective interaction kernel for the Cooper channel has the second energy scale, as well as the renormalization of the dilute paramagnetic impurity suppression are discussed.

Using ab initio band structure and model calculations we studied magnetic properties of one of the Mn4 molecular magnets (Mn4(hmp)6), where two types of the Mn ions exist: Mn 3+ and Mn 2+ . The direct calculation of the exchange constants in the GGA+U approximation shows that in contrast to a common belief the strongest exchange coupling is not between two Mn 3+ ions (J bb ), but along two out of four exchange paths connecting Mn 3+ and Mn 2+ ions (J wb ). Within the perturbation theory we performed the microscopic analysis of different contributions to the exchange constants, which allows us to establish the mechanism for the largest ferromagnetic exchange. In the presence of the charge order the lowest in energy virtual excitations, contributing to the superexchange, will be not those across the Hubbard gap ∼ U , but will be those between the Mn 3+ and Mn 4+ ions, which cost much smaller energy V (≪ U ). Together with strong Hund's rule coupling and specific orbital order this leads to large ferromagnetic exchange interaction for two out of four Mn 2+ -Mn 3+pairs.