The generalization of the Local Density Approximation (LDA) method for the systems with strong Coulomb correlations is presented which gives a correct description of the Mott insulators. The LDA+U method is based on the model hamiltonian approach and allows to take into account the non-sphericity of the Coulomb and exchange interactions. parameters. Orbital-dependent LDA+U potential gives correct orbital polarization and corresponding Jahn-Teller distortion. To calculate the spectra of the strongly correlated systems the impurity Anderson model should be solved with a many-electron trial wave function. All parameters of the many-electron hamiltonian are taken from LDA+U calculations. The method was applied to NiO and has shown good agreement with experimental photoemission spectra and with the oxygen Kα X-ray emission spectrum.

https://iopscience.iop.org/article/10.1088/0953-8984/9/4/002/pdf

First-principles calculations of the electronic structure and spectra of strongly correlated systems: the LDA+ U method

Although known since the late 19th century, organic–inorganic perovskites have recently received extraordinary research community attention because of their unique physical properties, which make them promising candidates for application in photovoltaic (PV) and related optoelectronic devices. This review will explore beyond the current focus on three-dimensional (3-D) lead(II) halide perovskites, to highlight the great chemical flexibility and outstanding potential of the broader class of 3-D and lower dimensional organic-based perovskite family for electronic, optical, and energy-based applications as well as fundamental research. The concept of a multifunctional organic–inorganic hybrid, in which the organic and inorganic structural components provide intentional, unique, and hopefully synergistic features to the compound, represents an important contemporary target.

Organic–Inorganic Perovskites: Structural Versatility for Functional Materials Design

An electron in a solid, that is, bound to or nearly localized on the specific atomic site, has three attributes: charge, spin, and orbital. The orbital represents the shape of the electron cloud in solid. In transition-metal oxides with anisotropic-shaped d-orbital electrons, the Coulomb interaction between the electrons (strong electron correlation effect) is of importance for understanding their metal-insulator transitions and properties such as high-temperature superconductivity and colossal magnetoresistance. The orbital degree of freedom occasionally plays an important role in these phenomena, and its correlation and/or order-disorder transition causes a variety of phenomena through strong coupling with charge, spin, and lattice dynamics. An overview is given here on this "orbital physics," which will be a key concept for the science and technology of correlated electrons.

http://science.sciencemag.org/content/sci/288/5465/462.full.pdf

Orbital Physics in Transition-Metal Oxides

In this Review, we discuss the state-of-the-art understanding of non-precious transition metal oxides that catalyze the oxygen reduction and evolution reactions. Understanding and mastering the kinetics of oxygen electrocatalysis is instrumental to making use of photosynthesis, advancing solar fuels, fuel cells, electrolyzers, and metal–air batteries. We first present key insights, assumptions and limitations of well-known activity descriptors and reaction mechanisms in the past four decades. The turnover frequency of crystalline oxides as promising catalysts is also put into perspective with amorphous oxides and photosystem II. Particular attention is paid to electronic structure parameters that can potentially govern the adsorbate binding strength and thus provide simple rationales and design principles to predict new catalyst chemistries with enhanced activity. We share new perspective synthesizing mechanism and electronic descriptors developed from both molecular orbital and solid state band structure principles. We conclude with an outlook on the opportunities in future research within this rapidly developing field.

/pdf/toward-the-rational-design-of-non-precious-transition-metal-462cs631ks.pdf

Toward the rational design of non-precious transition metal oxides for oxygen electrocatalysis

Constructing magnetic molecular solids by employing three-atom ligands as bridges

The electronic structure of the perovskite ${\mathrm{LaCoO}}_{3}$ for different spin states of Co ions was calculated in the local-density approximation LDA+U approach The ground state is found to be a nonmagnetic insulator with Co ions in a low-spin state Somewhat higher in energy, we find two intermediate-spin states followed by a high-spin state at significantly higher energy The calculations show that Co 3d states of ${\mathit{t}}_{2\mathit{g}}$ symmetry form narrow bands which could easily localize, while ${\mathit{e}}_{\mathit{g}}$ orbitals, due to their strong hybridization with the oxygen 2p states, form a broad \ensuremath{\sigma}* band With temperature variation which is simulated by a corresponding change of the lattice parameters, a transition from the low- to intermediate-spin state occurs This intermediate-spin (occupation ${\mathit{t}}_{2\mathit{g}}^{5}$${\mathit{e}}_{\mathit{g}}^{1}$) can develop an orbital ordering which can account for the nonmetallic nature of ${\mathrm{LaCoO}}_{3}$ at 90 KT500 K Possible explanations of the magnetic behavior and gradual insulator-metal transition are suggested \textcopyright{} 1996 The American Physical Society

Intermediate-spin state and properties of LaCoO3.

Two new compounds of the Bi/Ca/Co/O and Bi(Pb)/Ca/Co/O systems have been prepared. Their structure is built up from the intergrowth of four rock-salt-type layers and one [CoO2] hexagonal layer. Both cobaltites exhibit large thermopower values (S-300 K similar to 140 muV K-1), low resistivity values (rho(300) (K) 40-60 mOmega cm) and small thermal conductivities (kappa(300) (K) similar to I W K-1 m(-1)). Furthermore, these compounds exhibit a negative magnetoresistance, (MR = rho(H)-rho(H=0)/rho(H=0)), reaching, at 2.5 K, -85% in 7 T for the Bi/Ca/Co/O misfit cobaltite. A large negative magnetothermopower is also found for these cobaltites in the same temperature range. A qualitative explanation of the observed behaviour is proposed.

http://iopscience.iop.org/article/10.1088/0953-8984/15/17/323/pdf

Magnetoresistance and magnetothermopower properties of Bi/Ca/Co/O and Bi(Pb)/Ca/Co/O misfit layer cobaltites

A mechanism is suggested which can modify in certain cases the Goodenough-Kanamori-Anderson rules determining the character of superexchange in magnetic insulators; namely, side groups coupled to ligands, which are often present but are usually ignored, may contribute significantly, and may in certain cases even lead to a change in sign of the superexchange interaction. Thus this factor can make the 90\ifmmode^\circ\else\textdegree\fi{} superexchange of half-filled shells antiferromagnetic, in contrast to the usual case. Qualitative arguments and numerical estimates show that this mechanism may be important in the inorganic spin-Peierls compound ${\mathrm{CuGeO}}_{3}$. \textcopyright{} 1996 The American Physical Society.

/pdf/influence-of-side-groups-on-90-degrees-superexchange-a-1xfeq18cxg.pdf

Influence of side groups on 90 degrees superexchange: A modification of the Goodenough-Kanamori-Anderson rules.

Magnetic phase diagram of Ru-doped Sm 1 − x Ca x MnO 3 manganites: Expansion of ferromagnetism and metallicity

The role of elastic interactions between Jahn-Teller ions in formation of various orbital- and charge-ordered structures in manganites and related compounds is analyzed. It is shown that such interactions alone are often sufficient to reproduce the structures observed in different regions of the phase diagram. Special attention is focused on stripe structures at high doping levels.

http://arxiv.org/pdf/cond-mat/0112340.pdf

DI Khomskii

Papers

Intermediate-spin state and properties of LaCoO3.

Magnetoresistance and magnetothermopower properties of Bi/Ca/Co/O and Bi(Pb)/Ca/Co/O misfit layer cobaltites

Influence of side groups on 90 degrees superexchange: A modification of the Goodenough-Kanamori-Anderson rules.

Magnetic phase diagram of Ru-doped Sm 1 − x Ca x MnO 3 manganites: Expansion of ferromagnetism and metallicity

Elastic interactions and superstructures in manganites and other Jahn-Teller systems