Intra-atomic Correlation Research Articles

Energy spectrum of the organic quasi-1D conductors with NNN and correlated hopping

A model for organic quasi-one-dimensional conductors (TMTTF)2X and (TMTSF)2X is considered. The anisotropic character of these compounds is modelled by two different hopping parameters: t between nearest neighbors (NN) in a chain of tetramethyl-tetrathiafulvalene (TMTTF) or tetramethyl-tetraselenfulvalene (TMTSF) molecules and t between the chains (NNN between next nearest neighbors). Taking into account the correlated hopping of electrons allows us to describe the effect of site occupancy on hopping processes. In a regime of strong intraatomic correlation, high energy processes are cut off by applying two successive canonical transformations. An effective model is obtained for concentration of electrons n < 1 which contains kinetic exchange terms of antiferromagnetic (AF) nature. Oppositely, NNN hopping and correlated hopping disfavor the AF order. The energy spectrum of the effective model is calculated. Application of the obtained results to quasi-one-dimensional conductors is discussed.

Variational ground state of a two-orbital Hubbard model

An optimization variational Monte Carlo method is formulated for a two-dimensional Hubbard model with doubly degenerate atomic orbitals. We use generalized Gutzwiller-type wave functions, which include all the possible intra-atomic correlations. As a demonstration, we show results of energy and some correlation functions for a special case; at a quarter filling and for large Coulomb interactions, a ferromagnetic state with an orbital order overcomes paramagnetic and superconducting states. However, this ferromagnetism is destabilized by electron doping or by introducing a diagonal transfer in one direction, both of which work as substantial frustrations.

Exchange parameters in Fe-based molecular magnets

The calculation of interatomic magnetic exchange interactions entering the Heisenberg model is outlined from the standpoint of the density functional theory (DFT) for two Fe-based molecular magnets: a trinuclear complex with a Schiff base ligand, which is an antiferromagnetically coupled frustrated system, and a model bipyrimidine-connected planar network of Fe ions. First-principles electronic structure calculations are performed using the real-space method Siesta and the full-potential linearized augmented plane wave FLAPW method FLEUR, correspondingly. We discuss the application of fixed spin moment technique for preparing the system in a given magnetic configuration, and the effect of intraatomic Coulomb correlation, approximated by the LDA + U technique, on the values of interaction parameters.

Electron correlations in systems with orbital degeneracies: The dual nature of 5f electrons, heavy fermions and spectral functions in U-compounds

The present paper focuses on the dual nature of 5f electrons in U compounds and the concept of partial localization. The latter can arise due to intra-atomic correlations described by Hund's rules. To derive a microscopic picture, the electronic correlations in orbitally degenerate systems are studied by exact diagonalizations for finite clusters. Results for excitation spectra are presented. The cluster calculations form the basis for an approximate treatment of extended systems. The excitation spectra of U-based compounds are calculated for models which account for the local atomic correlations and realistic band structures. We discuss how high-energy shoulders observed in photoemission can be understood and how heavy fermions can emerge.

Strongly correlated crystal-field approach to Mott insulator [formula omitted

Our success in description of recent electron spin resonance results on Mott insulator LaCoO 3 (Phys. Rev. B 67 (2003) 172401) lies in taking into account strong electron correlations among d electrons and the relativistic spin–orbit coupling. In the developed quantum atomistic solid state theory (QUASST) we assume that the atomic-like integrity of the 3 d 6 system is preserved in the Co 3 + ion in LaCoO 3 and that intra-atomic correlations are stronger than crystal-field interactions. We conclude that in LaCoO 3 there is no intermediate spin state as came out from band-structure calculations. The excited states originate from the high-spin 5T 2g term, being 12 meV above the ground 1A 1 state. We are convinced that many-electron CEF approach with strong correlations and the atomic-scale orbital magnetism is physically adequate approach to 3d oxides.

Approximative treatment of5f-systems with partial localization due to intra-atomic correlations

Increasing experimental and theoretical evidence points towards a dual nature of the $5f$ electrons in actinide-based strongly correlated metallic compounds, with some $5f$ electrons being localized and others delocalized. In a recent paper [Phys. Rev. B 69, 115114 (2004)], we suggested the interplay of intra-atomic correlations as described by Hund's rules and a weakly anisotropic hopping (hybridization) as a possible mechanism. The purpose of the present work is to provide a step towards a microscopic description of partial localization in solids by analyzing how well various approximation schemes perform when applied to small clusters. It is found that many aspects of partial localization are described appropriately both by a variational wave function of Gutzwiller type and by a treatment which keeps only those interactions which are present in so-called $\mathrm{L}\mathrm{D}\mathrm{A}+U$ calculations. In contrast, the energies and phase diagram calculated within the Hartree-Fock approximation show little resemblance with the exact results. Enhancement of hopping anisotropy by Hund's rule correlations is found in all approximations.

Dual nature of5felectrons: Effect of intra-atomic correlations on hopping anisotropies

We study the effect of intra-atomic correlations on hopping anisotropies of 5f electrons. Results are presented for two- and three-sites clusters. They include phase diagrams in the presence and absence of an external magnetic field as a function of anisotropic hopping. It is shown that intra-atomic correlations may considerably enhance hopping anisotropies to the extent that electrons in some of the 5f orbitals remain localized. This provides a microscopic basis for a previously made assumption that for some Uranium-based heavy-fermion materials the 5f electrons must be treated as partially localized when Fermi surfaces and effective masses are calculated.

Electron correlation effects in scattering of atom on the atom adsorbed on the metal surface

The role of Coulomb correlations on the resonant charge transfer in atom scattering on an atom adsorbed on a metal surface has been studied theoretically within the Hartree–Fock approximation and the time-dependent Anderson–Newns model. Using the evolution operator technique the probabilities of the different charge states of the back-scattered atoms were calculated for broad range of parameters. In particular, the effect of the constant or z-dependent ( z being the distance of the moving atom from the surface) effective intra-atomic Coulomb correlations was analysed.

Molecular adsorption on the surface of strongly correlated transition-metal oxides: A case study for CO/NiO(100)

It is well known that the physical properties of some transition-metal compounds (mostly oxides) are strongly affected by intra-atomic correlations. Very recently, investigations of the adsorption of small molecules such as CO on the surfaces of transition-metal oxides have led to rather surprising results: the weak adsorbate-substrate bonding and the asymmetric (tilted) adsorption geometries contrast sharply the strong bonding and symmetric geometries characteristic for metallic surfaces. Calculations based on either Hartree-Fock or density-functional methods have failed to explain these observations. For bulk transition-metal oxides it has been demonstrated that the addition of a Hubbard-type on-site Coulomb repulsion U to the local-density Hamiltonian leads to an improved description of the electronic structure of these materials, but a consistent description of all physical properties proved to be elusive. In the present work, we present a comprehensive investigation of bulk NiO and of clean and CO-covered NiO(100) surfaces. We demonstrate that adding the on-site Coulomb repulsion to the spin-polarized gradient-corrected density-functional Hamiltonian leads to a consistently improved description of a wide range of cohesive, electronic, and magnetic properties of NiO (bulk and surface) and a very accurate description of the adsorption properties of CO. The effects of the strong electronic correlations in the substrate on the adsorbate-substrate bonding are discussed in detail.

Ground state, electronic structure and magnetism of LaMnO3

We have calculated the discrete low-energy electronic structure in LaMnO3 originating from the atomic-like states of the strongly correlated 3d4 electronic system occurring in the Mn3+ ion. We take into account very strong intra-atomic correlations, crystal field interactions and the intra-atomic spin-orbit coupling. We calculated magnetic and paramagnetic state of LaMnO3 within the consistent description given by Quantum Atomistic Solid State Theory (QUASST). Our studies indicate that the intra-atomic spin-orbit coupling and the orbital magnetism are indispensable for the physically adequate description of electronic and magnetic properties of LaMnO3. Keywords: 3d oxides, crystal field, spin-orbit coupling, LaMnO3 PACS: 71.70Ej, 75.10Dg

Generalization of the shell model and correlation effects in strongly correlated oxides

Direct analytical and numerical calculation show that the two-electron atomic configuration can be unstable with respect to a static or dynamic displacement of the electron shells. This enables us to develop a so-called nonrigid shell model for a partial account of the intra-atomic electron correlations. In frames of the model a correlated state of two-electron molecular configuration is described by a set of symmetrized shell displacements similarly to the well-known shell model developed for a description of the lattice dynamics. The former relate to the latter as the optical phonon modes relate to the acoustical one. The effect under consideration is expected to be of particular importance for anionic ${2p}^{6}$ background in oxides. The nonrigid anionic background forms an unconventional magnetoelectric system characterized by a set of independent orbital order parameters and competition of paramagnetic and diamagnetic responses. The model allows an introduction of the pseudospin formalism and effective ``spin Hamiltonian'' for a description of the short- and long-range ordering of nonrigid atomic backgrounds in crystals. We predict a number of unconventional correlation effects including: (i) a formation of polaronlike holes and charge-transfer excitons; (ii) the mechanism of the self-trapping for holes and/or charge-transfer excitons; (iii) a purely electronic (pseudo)Jahn-Teller effect due to a coupling between valent hole and a nonrigid anionic background; (iv) a combined (pseudo)Jahn-Teller effect provided by a joint account both of the electron shell displacements and conventional nuclear ones; (v) an appearance of the soliton chiral correlation states; (vi) unconventional magnetic response; (vii) unconventional hyperfine coupling.

Coulomb correlations and magnetic anisotropy in orderedL10CoPt and FePt alloys

We present results of the magneto-crystalline anisotropy energy (MAE) calculations for chemically ordered $L1_0$ CoPt and FePt alloys taking into account the effects of strong electronic correlations and spin-orbit coupling. The local spin density + Hubbard U approximation (LSDA+U) is shown to provide a consistent picture of the magnetic ground state properties when intra-atomic Coulomb correlations are included for both 3$d$ and 5$d$ elements. Our results demonstrate significant and complex contribution of correlation effects to large MAE of these material.

Large orbital moments and internal magnetic fields in lithium nitridoferrate(I).

The iron nitridometalates Li2[(Li(1-x)Fe(I)(x))N] display ferromagnetic ordering and spin freezing. Large magnetic moments up to 5.0mu(B)/Fe are found in the magnetization. In Mössbauer effect studies huge hyperfine magnetic fields up to 696 kOe are observed at specific Fe sites. These extraordinary fields and moments originate in an unusual ligand field splitting for those Fe species leading [within local spin density approximation (LSDA)] to a localized orbitally degenerate doublet. Including spin-orbit interaction and strong intra-atomic electron correlation (LDA+SO+U) gives rise to a large orbital momentum.

Simple master equation for the description of charge transfer in atom–surface scattering

We have developed a new simple master equation for the description of charge transfer in atom–surface scattering. The equation includes some effects of strong intra-atomic correlation such as the Coulomb blockade. Comparisons with an accurate numerical solution obtained using a non-equilibrium Green's function technique show that the proposed simple master equations (SMEs) represent a convenient, computationally fast and semi-accurate means of describing charge transfer in atom–surface scattering.

Theoretical calculation of the optical properties of Y3Fe5O12

The electronic structure and the optical properties of Y3Fe5O12 crystal are calculated using the local spin density approximation+U approach. The intra-atomic correlation effect of the Fe 3d electrons is shown to be important in describing the insulating nature of the crystal. With a choice of the parameters U=3.5 eV and J=0.8 eV in the model Hamiltonian, a gap of 2.66 eV is obtained and a significant lowering of the occupied potion of the Fe 3d states is observed. The calculated optical absorption in the range 3–6 eV is from the bulk O 2p to the Fe 3d band states. The calculated spin magnetic moments are in good agreement with experiment.

Ground state theory of delta-Pu

Correlation effects are important for making predictions in the delta phase of Pu. Using a realistic treatment of the intra-atomic Coulomb correlations we address the long-standing problem of computing ground state properties. The equilibrium volume is obtained in good agreement with experiment when taking into account Hubbard U of the order 4 eV. For this U, the calculation predicts a 5f(5) atomiclike configuration with L = 5, S = 5/2, and J = 5/2 and shows a nearly complete compensation between spin and orbital magnetic moments.

Triplet superconductivity in Sr2RuO4 in terms of the t-J-I-model

A generalized t-J-I-model is proposed for Sr2RuO4 that takes the strong intra-atomic correlations of the d electrons and the features of the electronic structure of Sr2RuO4 into account. It is shown that, in the limit of strong correlations, there are no singlet s-type solutions for the superconducting state, but triplet solutions exist because of ferromagnetic spin correlations. For typical values of the model parameters, T c ∼1 K, consistent with the value of T c for Sr2RuO4.

Electronic friction in the presence of strong intra-atomic correlations for atoms moving near metal surfaces

Electronic friction in the presence of strong intra-atomic correlations for atoms moving near metal surfaces

Analytic first-order properties from explicitly correlated many-body perturbation theory and Gaussian geminal basis

Theory of analytic first-order properties is formulated in a basis set independent way using the first-quantized many-body perturbation theory. This formulation allows the correlation effects to be described with explicitly correlated basis sets. The basis of Gaussian geminals is employed to calculate the second- and third-order correlation corrections to the lowest multipole moments of the H2 and LiH molecules. The same formalism is also utilized to compute the intra-atomic correlation contribution to the first-order interaction energy for the helium dimer. The results compare favorably with the literature data obtained using the conventional, orbital basis approach.

Analysis of the shift in the superconducting transition under pressure in the Anderson–Hubbard two-orbital model

The two-orbital Hubbard model is used to obtain formulas for the fermion excitation spectrum in the energy bands hybridized by Anderson’s interaction. A transition to the Hubbard operators, which diagonalizes the one-site part of the Hamiltonian, allows us to use the Green’s temperature function technique to take into account the interstitial jump term while studying the superconducting properties of the model. An analysis of the lower part of the energy spectrum leads to a formula for the superconducting transition temperature Tc associated with the pairing of quasiparticles in one of the correlated bands. The dependence of Tc on electron concentration and energy parameters determining the intraatomic correlation is studied. Proposing a simple relation between the value of pressure (P) and width of the correlated band, the dependence of Tc on the pressure was defined. Good agreement between the theoretical calculation of the dependence of Tc on the pressure and the experimental results for Y1−xPrxBa2Cu3O7−δ is found. Comparison of the theoretical and experimental results for the dependence of Tc and its derivative d(ln Tc)/dP on Sr and Bi-content (x) in La2−xSrxCuO4 has been made. It is concluded that the model under consideration can be used for the description of the shift in Tc under pressure for a number of superconductors.