Electronic correlations and crystal structure distortions in BaBiO3.

BaBiO3 is a material where Bi4+ ions with half-filled 6s-states form an alternating set of Bi3+ and Bi5+ ions resulting in a charge ordered insulator. The charge ordering is accompanied by breathing distortion of the BiO6 octahedra (extension and contraction of the Bi-O bond lengths). Standard density functional theory (DFT) calculations fail to obtain the crystal structure instability caused by the pure breathing distortions. Combining effects of the breathing distortions and tilting of the BiO6 octahedra allows DFT to reproduce qualitatively an experimentally observed insulator with monoclinic crystal structure but strongly underestimates the breathing distortion parameter and energy gap values. In the present work we reexamine the BaBiO3 problem within the GGA + U method using a Wannier function basis set for the Bi 6s-band. Due to the high oxidation state of bismuth in this material, the Bi 6s-symmetry Wannier function is predominantly extended spatially on surrounding oxygen ions and hence differs strongly from a pure atomic 6s-orbital. That is in sharp contrast to transition metal oxides (with exclusion of high oxidation state compounds) where the major part of the d-band Wannier function is concentrated on the metal ion and a pure atomic d-orbital can serve as a good approximation. The GGA + U calculation results agree well with experimental data, in particular with experimental crystal structure parameters and energy gap values. Moreover, the GGA + U method allows one to reproduce the crystal structure instability due to the pure breathing distortions without octahedra tilting.

Spin state of negative charge-transfer material SrCoO(3).

We employ the combination of the density functional theory and the dynamical mean-field theory to investigate the electronic structure and magnetic properties of SrCoO(3), monocrystals of which were prepared recently. Our calculations lead to a ferromagnetic metal in agreement with experiment. We find that, contrary to some suggestions, the local moment in SrCoO(3) does not arise from intermediate spin state, but is a result of coherent superposition of many different atomic states. We discuss how the attribution of magnetic response to different atomic states in solids with local moments can be quantified.

Ab initio studies on the interplay between spin-orbit interaction and Coulomb correlation in Sr2IrO4 and Ba2IrO4.

Ab initio analyses of A(2)IrO(4) (A=Sr,Ba) are presented. Effective Hubbard-type models for Ir 5d t(2g) manifolds downfolded from the global band structure are solved based on the dynamical mean-field theory. The results for A=Sr and Ba correctly reproduce paramagnetic metals undergoing continuous transitions to insulators below the Néel temperature T(N). These compounds are classified not into Mott insulators but into Slater insulators. However, the insulating gap opens by a synergy of the Néel order and significant band renormalization, which is also manifested by a 2D bad metallic behavior in the paramagnetic phase near the quantum criticality.

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 Hund's 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] eV and J = 0.8 eV 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] eV, J = 0.5 eV. 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.

Mutual experimental and theoretical validation of bulk photoemission spectra of Sr1-xCaxVO3.

We report high-resolution high-energy photoemission spectra together with parameter-free LDA + DMFT (local density approximation + dynamical mean-field theory) results for Sr1-xCaxVO3, a prototype 3d(1) system. In contrast to earlier investigations the bulk spectra are found to be insensitive to x. The good agreement between experiment and theory confirms the bulk sensitivity of the high-energy photoemission spectra.