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Contrary to previous studies that classify Na(2)IrO(3) as a realization of the Heisenberg-Kitaev model with a dominant spin-orbit coupling, we show that this system represents a highly unusual case in which the electronic structure is dominated by the formation of quasimolecular orbitals (QMOs), with substantial quenching of the orbital moments. The QMOs consist of six atomic orbitals on an Ir hexagon, but each Ir atom belongs to three different QMOs. The concept of such QMOs in solids invokes very different physics compared to the models considered previously. Employing density functional theory calculations and model considerations we find that both the insulating behavior and the experimentally observed zigzag antiferromagnetism in Na(2)IrO(3) naturally follow from the QMO model.

We discuss the role of electronic correlations in the iron-based superconductor LiFeAs by studying the effects on band structure, mass enhancements, and Fermi surface in the framework of density functional theory combined with dynamical mean field theory calculations. We conclude that LiFeAs shows characteristics of a moderately correlated metal and that the strength of correlations is mainly controlled by the value of the Hund's rule coupling J. The hole pockets of the Fermi surface show a distinctive change in form and size with implications for the nesting properties. Our calculations are in good agreement with recent angle-resolved photoemission spectroscopy and de Haas-van Alphen experiments.

We propose a microscopic description of the bond-disproportionated insulating state in the bismuth perovskites XBiO3 (X=Ba, Sr) that recognizes the bismuth-oxygen hybridization as a dominant energy scale. It is demonstrated using electronic structure methods that the breathing distortion is accompanied by spatial condensation of hole pairs into local, molecular-like orbitals of the A1g symmetry composed of O-2pσ and Bi-6s atomic orbitals of collapsed BiO6 octahedra. Primary importance of oxygen p-states is thus revealed, in contrast to a popular picture of a purely ionic Bi 3+ /Bi 5+ charge-disproportionation. Octahedra tilting is shown to enhance the breathing instability by means of a non-uniform band-narrowing. We argue that formation of localized states upon breathing distortion is, to a large extent, a property of the oxygen sublattice and expect similar hybridization effects in other perovskites involving formally high oxidation state cations.PACS numbers: 74.20. Pq,71.30.+h,71.45.Lr The physics of perovskite compounds, featuring BO 6 octahedra (B=cation) as building blocks, is exceptionally rich. The perovskites can be driven through a variety of structural, electronic, and magnetic phase transitions and are hosts to such intriguing states of matter as high-transition-temperature (T c ) superconductivity [cuprates [1], bismuth perovskites XBiO 3 (X=Ba, Sr) [2-4]], a pseudo-gap state with strongly violated Fermiliquid properties (cuprates[5]), and a spin/charge density wave [rare-earth nickelates RNiO 3 (R=rare-earth atom) [6]], to name a few. It is appealing to relate the diverse physical properties observed across the perovskite family of materials with the individual characteristics of the cation B. The latter can be magnetic (Cu in the cuprates) or not (Bi in the bismuth perovskites), orbitally active (Mn in the manganites [7]) or not (Cu, Bi), prone to strong electronic correlations (transition-metal elements Cu and Ni) or not (Bi). With the due appreciation of the cation factor, there are, however, many striking similarities among different perovskite families suggesting an equally important role of their common structural framework. A vivid example is the transition into a bond-disproportionated insulating phase found in both the bismuthates and the rare-earth nickelates, in which oxygen plays an extremely important role [8][9][10][11]. Observations of this kind have given rise to theories aimed at a unified description of the perovskites, where the various competing phases emerge from polaronic and bipolaronic excitations of the polarizable oxygen sublattice.While it is a common practice within this approach to assume that the only effect of electron-phonon coupling is variation of on-site energies, in this Letter we demonstrate that the effects due to hybridization between the oxygen-p orbitals and cation orbitals can be even more important. For this purpose, we focus on the bismuth perovskites XBiO 3 where the analysis is greatly facilitated by the fact that the Bi ion valence states are non-m...

Motivated by the recently renewed interest in the superconducting bismuth perovskites, we investigate the electronic structure of the parent compounds ABiO3 (A = Sr, Ba) using ab initio methods and tight-binding (TB) modeling. We use the density functional theory (DFT) in the local density approximation (LDA) to understand the role of various interactions in shaping the ABiO3 bandstructure near the Fermi level. It is established that interatomic hybridization involving Bi6s and O-2p orbitals plays the most important role. Based on our DFT calculations, we derive a minimal TB model and demonstrate that it can describe the properties of the bandstructure as a function of lattice distortions, such as the opening of a charge gap with the onset of the breathing distortion and the associated condensation of holes onto a1g-symmetric molecular orbitals formed by the O-2pσ orbitals on collapsed octahedra. We also derive a single band model involving the hopping of an extended molecular orbital involving both Bi-6s and a linear combination of six O-2p orbitals which provides a very good description of the dispersion and band gaps of the low energy scale bands straddling the chemical potential.