M.N. Mikheev Institute of Metal Physics

Evidence is presented that within the density-functional theory orbital polarization has to be treated on an equal footing with spin polarization and charge density for strongly interacting electron systems. Using a basis-set independent generalization of the LDA+U functional, we show that electronic orbital ordering is a necessary condition to obtain the correct crystal structure and parameters of the exchange interaction for the Mott-Hubbard insulator ${\mathrm{KCuF}}_{3}$.

A generalization of the local density approximation (LDA) method for systems with strong Coulomb correlations is described which gives a correct description of the Mott insulators. The LDA+U method takes into account orbital dependence of the Coulomb and exchange interactions which is absent in the LDA. The scheme can be regarded as a `first-principles' method because there are no adjustable parameters. When applied to the transition metal and rare-earth metal compounds, the LDA+U method gives a qualitative improvement compared with the LDA not only for excited-state properties such as energy gaps but also for ground-state properties such as magnetic moments and interatomic exchange parameters. The orbital-dependent rotationally invariant LDA+U potential gives a correct orbital polarization and a corresponding Jahn - Teller distortion as well as polaron formation.

The generalization of the local-density-approximation method for the systems with strong Coulomb correlations is proposed, which restores the discontinuity in the one-electron potential as in the exact density functional. The method is based on the model-Hamiltonian approach and allows us to take into account the nonsphericity of the Coulomb and exchange interactions. The calculation scheme could be regarded as a first-principle method due to the absence of adjustable parameters. The method was applied to the calculation of the photoemission (x-ray photoemission spectroscopy) and bremsstrahlung isochromat spectra of NiO.

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.

We propose an energy functional for localized electron systems that corresponds to the constrained-local-density approximation (LDA) but includes some corrections for spin and orbital polarization to take Hund's first and second rules into account. The discontinuity of the one-electron potential known for an exact density functional can be easily incorporated in LDA in the scope of our formalism. Applications of the method to the electronic structure and configurational stability of d impurities in Rb are presented.

We discuss a general approach to a realistic theory of the electronic structure in materials containing correlated $d$ or $f$ electrons. The main feature of this approach is the taking into account of the energy dependence of the electron self-energy with the momentum dependence being neglected (local approximation). It allows us to consider such correlation effects as the non-Fermi-step form of the distribution function, the enhancement of the effective mass including Kondo resonances,'' the appearance of the satellites in the electron spectra, etc. To specify the form of the self-energy, it is useful to distinguish (according to the ratio of the on-site Coulomb energy $U$ to the bandwidth $W$) three regimes---strong, moderate, and weak correlations. In the case of strong interactions ($U/W>1$---rare-earth system) the Hubbard-I approach is the most suitable. Starting from an exact atomic Green function with the constrained density matrix ${n}_{{\mathrm{mm}}^{\ensuremath{'}}}$ the band-structure problem is formulated as the functional problem on ${n}_{{\mathrm{mm}}^{\ensuremath{'}}}$ for $f$ electrons and the standard local-denisty-approximation functional for delocalized electrons. In the case of moderate correlations ($U/W\ensuremath{\sim}1$---metal-insulator regime, Kondo systems) we start from the $d=\ensuremath{\infty}$ dynamical mean-field iterative perturbation scheme of Kotliar and co-workers and also make use of our multiband atomic Green function for constrained ${n}_{{\mathrm{mm}}^{\ensuremath{'}}}$. Finally for the weak interactions ($U/W<1$---transition metals) the self-consistent diagrammatic fluctuation-exchange approach of Bickers and Scalapino is generalized to the realistic multiband case. We present two-band, two-dimensional model calculations for all three regimes. A realistic calculation in the Hubbard-I scheme with the exact solution of the on-site multielectron problem for $f(d)$ shells was performed for mixed-valence $4f$ compound TmSe, and for the classical Mott insulator NiO.

We present Mn $3s$ x-ray photoelectron spectra of manganese oxides with the Mn formal valency from $2+$ to $4+.$ We found that the ${\mathrm{Sr}}^{2+}$ doping or cation deficiency in manganites do not change the Mn $3s$ splitting in manganites with the Mn formal valency from $3.0+$ to $3.3+.$ We suggest that doping holes are localized in O $2p$ states.

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 La_{1-x}Sr_xTiO_3 was studied by the new method and the results showed qualitative improvement in agreement with experimental photoemission spectra.

Severe plastic deformation (SPD) is effective in producing bulk ultrafine-grained and nanostructured materials with large densities of lattice defects. This field, also known as NanoSPD, experienced a significant progress within the past two decades. Beside classic SPD methods such as high-pressure torsion, equal-channel angular pressing, accumulative roll-bonding, twist extrusion, and multi-directional forging, various continuous techniques were introduced to produce upscaled samples. Moreover, numerous alloys, glasses, semiconductors, ceramics, polymers, and their composites were processed. The SPD methods were used to synthesize new materials or to stabilize metastable phases with advanced mechanical and functional properties. High strength combined with high ductility, low/room-temperature superplasticity, creep resistance, hydrogen storage, photocatalytic hydrogen production, photocatalytic CO2 conversion, superconductivity, thermoelectric performance, radiation resistance, corrosion resistance, and biocompatibility are some highlighted properties of SPD-processed materials. This article reviews recent advances in the NanoSPD field and provides a brief history regarding its progress from the ancient times to modernity. Abbreviations: ARB: Accumulative Roll-Bonding; BCC: Body-Centered Cubic; DAC: Diamond Anvil Cell; EBSD: Electron Backscatter Diffraction; ECAP: Equal-Channel Angular Pressing (Extrusion); FCC: Face-Centered Cubic; FEM: Finite Element Method; FSP: Friction Stir Processing; HCP: Hexagonal Close-Packed; HPT: High-Pressure Torsion; HPTT: High-Pressure Tube Twisting; MDF: Multi-Directional (-Axial) Forging; NanoSPD: Nanomaterials by Severe Plastic Deformation; SDAC: Shear (Rotational) Diamond Anvil Cell; SEM: Scanning Electron Microscopy; SMAT: Surface Mechanical Attrition Treatment; SPD: Severe Plastic Deformation; TE: Twist Extrusion; TEM: Transmission Electron Microscopy; UFG: Ultrafine Grained. © 2022 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.

With the help of band structure calculations $(\mathrm{LSDA}+U)$ a clear picture of the physics behind the metallic ferromagnetic properties of ${\mathrm{CrO}}_{2}$ is revealed. It is concluded that ${\mathrm{CrO}}_{2}$ is a negative charge transfer gap material which leads to self-doping and explains why it is a metal in spite of the large Coulomb interactions. We find that there exist in ${\mathrm{CrO}}_{2}$ both localized and itinerant $d$ electrons, resulting in ferromagnetic ordering due to double exchange similar to colossal magnetoresistance manganates.

The electronic structure of various nickel oxides with nickel valence varying from 1+ to 3+ was investigated with the aim to find similarities and differences to the isoelectronic cuprates. Only if the Ni ions are forced into a planar coordination with the O ions can a $S=1/2$ magnetic insulator be realized with the difficult ${\mathrm{Ni}}^{+}$ oxidation state and possibly doped with low spin $(S=0){\mathrm{Ni}}^{2+}$ holes directly analogous to the superconducting cuprates. The more common ${\mathrm{Ni}}^{3+}$ oxidation state cannot be used to make a parent magnetic insulator as it forms rather as localized $S=1 {\mathrm{Ni}}^{2+}$ embedded in a sea of itinerant O holes. Strong coupling of these holes to the localized spins via $2p\ensuremath{-}3d$ hybridization leads to a heavy-fermion system with a large Kondo temperature, which was confirmed experimentally for ${\mathrm{LaNiO}}_{3}.$

We found direct experimental evidence for an orbital switching in the V 3d states across the metal-insulator transition in VO2. We have used soft-x-ray absorption spectroscopy at the V L2,3 edges as a sensitive local probe and have determined quantitatively the orbital polarizations. These results strongly suggest that, in going from the metallic to the insulating state, the orbital occupation changes in a manner that charge fluctuations and effective bandwidths are reduced, that the system becomes more one dimensional and more susceptible to a Peierls-like transition, and that the required massive orbital switching can only be made if the system is close to a Mott insulating regime.

We develop a diagrammatic approach with local and nonlocal self-energy diagrams, constructed from the local irreducible vertex. This approach includes the local correlations of dynamical mean-field theory and long-range correlations beyond. It allows us, for example, to describe (para)magnons and weak localization effects---in strongly correlated systems. As a first application, we study the interplay between nonlocal antiferromagnetic correlations and the strong local correlations emerging in the vicinity of a Mott-Hubbard transition.

We present an approach to investigate the interplay of antiferromagnetism and d-wave superconductivity in the two-dimensional Hubbard model within a numerically exact cluster dynamical mean-field approximation. Self-consistent solutions with two nonzero order parameters exist in a wide range of doping level and temperatures. A linearized equation for the energy spectrum near the Fermi level has been solved. The resulting d-wave gap has the correct magnitude and k dependence, but some distortion compared to the pure ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ superconducting order parameter due to the presence of underlying antiferromagnetic ordering.

We propose a computational scheme for the ab initio calculation of Wannier functions (WFs) for correlated electronic materials. The full-orbital Hamiltonian $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{H}$ is projected into the WF subspace defined by the physically most relevant partially filled bands. The Hamiltonian ${\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{H}}^{WF}$ obtained in this way, with interaction parameters calculated by constrained local-density approximation (LDA) for the Wannier orbitals, is used as an ab initio setup of the correlation problem, which can then be solved by many-body techniques, e.g., dynamical mean-field theory (DMFT). In such calculations the matrix self-energy $\mathrm{\ensuremath{\sum}}^{\ifmmode \hat{}\else \^{}\fi{}}(\ensuremath{\epsilon})$ is defined in WF basis which then can be converted back into the full-orbital Hilbert space to compute the full-orbital interacting Green function $G(\mathbf{r},{\mathbf{r}}^{\ensuremath{'}},\ensuremath{\epsilon})$. Using $G(\mathbf{r},{\mathbf{r}}^{\ensuremath{'}},\ensuremath{\epsilon})$ one can evaluate the charge density, modified by correlations, together with a new set of WFs, thus defining a fully self-consistent scheme. The Green function can also be used for the calculation of spectral, magnetic, and electronic properties of the system. Here we report the results obtained with this method for $\mathrm{Sr}\mathrm{V}{\mathrm{O}}_{3}$ and ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$. Comparisons are made with previous results obtained by the LDA+DMFT approach where the LDA density of states was used as input, and with new bulk-sensitive experimental spectra.

We report a careful and systematic study of thermal and photochemical degradation of a series of complex haloplumbates APbX3 (X = I, Br) with hybrid organic (A+ = CH3NH3) and inorganic (A+ = Cs+) cations under anoxic conditions (i.e., without exposure to oxygen and moisture by testing in an inert glovebox environment). We show that the most common hybrid materials (e.g., MAPbI3) are intrinsically unstable with respect to the heat- and light-induced stress and, therefore, can hardly sustain the real solar cell operation conditions. On the contrary, the cesium-based all-inorganic complex lead halides revealed far superior stability and, therefore, provide an impetus for creation of highly efficient and stable perovskite solar cells that can potentially achieve pragmatic operational benchmarks.

A full-potential version of a recently proposed generalization of the local-density method (LDA+U) is used to study the influence of impurities, lattice, and magnetic relaxation in ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$. The carriers form mean-field analogs of Zhang-Rice singlets, which are only slightly affected by impurity and in-plane lattice relaxation effects. On the other hand, an ``anti-Jahn-Teller'' polaron, characterized by a 0.26 \AA{} shorter Cu to apical-oxygen bond length and triplet spin, is nearly stable. Although the impurity potentials are similar, the self-localization effects are much stronger in ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{NiO}}_{4}$.

We present a method to calculate the effective exchange interaction parameters based on the realistic electronic structure of correlated magnetic crystals in local approach with the frequency dependent self-energy. The analog of ``local force theorem'' in the density-functional theory is proven for highly correlated systems. The expressions for effective exchange parameters, Dzialoshinskii-Moriya interaction, and magnetic anisotropy are derived. The first-principles calculations of magnetic excitation spectrum for ferromagnetic iron, with the local correlation effects from the numerically exact QMC scheme, are presented.

In recent years, spin-orbit effects have been widely used to produce and detect spin currents in spintronic devices. The peculiar symmetry of the spin Hall effect allows creation of a spin accumulation at the interface between a metal with strong spin-orbit interaction and a magnetic insulator, which can lead to a net pure spin current flowing from the metal into the insulator. This spin current applies a torque on the magnetization, which can eventually be driven into steady motion. Tailoring this experiment on extended films has proven to be elusive, probably due to mode competition. This requires the reduction of both the thickness and lateral size to reach full damping compensation. Here we show clear evidence of coherent spin-orbit torque-induced auto-oscillation in micron-sized yttrium iron garnet discs of thickness 20 nm. Our results emphasize the key role of quasi-degenerate spin-wave modes, which increase the threshold current.

The electronic properties of paramagnetic ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ are investigated by the computational scheme $\mathrm{LDA}+\mathrm{DMFT}(\mathrm{QMC})$. This approach merges the local density approximation (LDA) with dynamical mean-field theory (DMFT) and uses quantum Monte Carlo simulations (QMC) to solve the effective Anderson impurity model of DMFT. Starting with the crystal structure of metallic ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ and insulating $({\mathrm{V}}_{0.962}{\mathrm{Cr}}_{0.038}{)}_{2}{\mathrm{O}}_{3}$ we find a Mott-Hubbard transition at a Coulomb interaction $U\ensuremath{\approx}5\mathrm{eV}$. The calculated spectrum is in very good agreement with experiment. Furthermore, the orbital occupation and the spin state $S\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1$ determined by us agree with recent polarization dependent x-ray-absorption experiments.