J. Kuneš

Coulomb repulsion and correlation strength in LaFeAsO from density functional and dynamical mean-field theories

The LDA+DMFT (local density approximation combined with dynamical mean-field theory) computation scheme has been used to calculate spectral properties of LaFeAsO-the parent compound of the new high-T(c) iron oxypnictides. The average Coulomb repulsion [Formula: see text] and Hunds exchange J parameters for iron 3d electrons were calculated using the first-principles constrained density functional theory scheme in the Wannier functions formalism. Resulting values strongly depend on the number of states taken into account in the calculations: when the full set of O-2p, As-4p and Fe-3d orbitals and the corresponding bands are included, the interaction parameters [Formula: see text]xa0eV and J = 0.8xa0eV are obtained. In contrast, when the basis set is restricted to the Fe-3d orbitals and bands only, the calculation gives much smaller values of [Formula: see text]xa0eV, J = 0.5xa0eV. Nevertheless, DMFT calculations with both parameter sets and the corresponding basis sets result in a weakly correlated electronic structure that is in agreement with the experimental x-ray and photoemission spectra.

NiO: Correlated Band Structure of a Charge-Transfer Insulator

The band structure of the prototypical charge-transfer insulator NiO is computed by using a combination of an ab initio band structure method and the dynamical mean-field theory with a quantum Monte-Carlo impurity solver. Employing a Hamiltonian which includes both Ni d and O p orbitals we find excellent agreement with the energy bands determined from angle-resolved photoemission spectroscopy. This brings an important progress in a long-standing problem of solid-state theory. Most notably we obtain the low-energy Zhang-Rice bands with strongly k-dependent orbital character discussed previously in the context of low-energy model theories.

Spin state transition and covalent bonding in LaCoO3

We use the dynamical mean-field theory to study a p-d Hubbard Hamiltonian for LaCoO3 derived from ab initio calculations in local density approximation (LDA+DMFT scheme). We address the origin of local moments observed above 100 K and discuss their attribution to a particular atomic multiplet in the presence of covalent Co-O bonding. We show that in solids such attribution, based on the single ion picture, is in general not possible. We explain when and how the single ion picture can be generalized to provide a useful approximation in solids. Our results demonstrate that the apparent magnitude of the local moment is not necessarily indicative of the underlying atomic multiplet. We conclude that the local moment behavior in LaCoO3 arises from the high-spin state of Co and explain the precise meaning of this statement.

Metal-insulator transition inNiS2−xSex

The origin of the gap in NiS2 as well as the pressure- and doping-induced metal-insulator transition in the NiS2-xSex solid solutions are investigated both theoretically using the first-principles band structures combined with the dynamical mean-field approximation for the electronic correlations and experimentally by means of infrared and x-ray absorption spectroscopies. The bonding-antibonding splitting in the S-S (Se-Se) dimer is identified as the main parameter controlling the size of the charge gap. The implications for the metal-insulator transition driven by pressure and Se doping are discussed.

Density-functional calculation of the Coulomb repulsion and correlation strength in superconducting LaFeAsO

V. I. Anisimov, Dm. M. Korotin, S. V. Streltsov, A. V. Kozhevnikov, 2 J. Kuneš, A. O. Shorikov, and M. A. Korotin Institute of Metal Physics, Russian Academy of Sciences, 620041 Yekaterinburg GSP-170, Russia Joint Institute for Computational Sciences, Oak Ridge National Laboratory P.O. Box 2008 Oak Ridge, TN 37831-6173, USA Theoretical Physics III, Center for Electronic Correlations and Magnetism, Institute of Physics, University of Augsburg, Augsburg 86135, GermanyConstrained density functional theory scheme in Wannier functions formalism has been used to calculate Coulomb repulsion U and Hund’s exchange J parameters for Fe-3d electrons in LaFeAsO. Results strongly depend on the basis set. When O-2p, As-4p, and Fe-3d orbitals are included, computation results in U = 3–4 eV. With the basis set restricted to Fe-3d orbitals only, computation gives parameters corresponding to F0 = 0.8 eV, J = 0.5 eV. Local Density Approximation combined with Dynamical Mean-Field Theory calculation with these parameters results in weakly correlated electronic structure.

Temperature-dependent correlations in covalent insulators: Dynamical mean-field approximation

Motivated by the peculiar behavior of FeSi and

Correlations in a Band Insulator

{text{FeSb}}_{2}

Correlation effects in the triangular lattice single-band system Li x Nb O 2

, we study the effect of local electronic correlations on magnetic, transport, and optical properties in a specific type of band insulator, namely a covalent insulator. Investigating a minimum model of covalent insulator within a single-site dynamical mean-field approximation, we are able to obtain the crossover from low-temperature nonmagnetic insulator to high-temperature paramagnetic metal with parameters realistic for FeSi and

Material-Specific Investigations of Correlated Electron Systems

{text{FeSb}}_{2}

Metal-insulator transition in NiS2-xSex

systems. Our results show that the behavior of FeSi does not imply microscopic description in terms of Kondo insulator (periodic Anderson model) as can be often found in the literature, but in fact reflects generic properties of a broader class of materials.