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V. I. Anisimov

Publications and source records attributed to V. I. Anisimov.

At least 145 records · Page 8Linked to original sources
arXiv2001

Effect of local Coulomb interactions on the electronic structure and exchange interactions in Mn12 magnetic molecules

We have studied the effect of local Coulomb interactions on the electronic structure of the molecular magnet Mn12-acetate within the LDA+U approach. The account of the on-site repulsion results in a finite energy gap and an integer value of the molecule's magnetic moment, both quantities being in a good agreement with the experimental results. The resulting magnetic moments and charge states of non-equivalent manganese ions agree very well with experiments. The calculated values of the intramolecular exchange parameters depend on the molecule's spin configuration, differing by 25-30% between the ferrimagnetic ground state and the completely ferromagnetic configurations. The values of the ground-state exchange coupling parameters are in reasonable agreement with the recent data on the magnetization jumps in megagauss magnetic fields. Simple estimates show that the obtained exchange parameters can be applied, at least qualitatively, to the description of the spin excitations in Mn12-acetate.

cond-mat.mes-hall↗
arXiv2001

Band structure approach to the resonant x-ray scattering

We study the resonance behaviour of the forbidden 600 and 222 x-ray Bragg peaks in Ge using LDA band structure methods. These Bragg peaks remain forbidden in the resonant dipole scattering approximation even taking into account the non local nature of the band states. However they become allowed at resonance if the eigenstates of the unoccupied conduction band involve a hybridization of p like and d like atomic states. We show that the energy dependence of the resonant behaviour, including the phase of the scattering, is a direct measure of this p-d hybridization.and obtain quantitative agreement with experiment. A simple physical picture involving a product of dipole and quadrupolar transition matrix elements explains this behaviour and shows that it should be generally true for cases where the resonating atom is not at an inversion center. This has strong implications for the description of the resonance behavior of x-ray scattering in materials where the resonant atom is not at an inversion center such as V2O3 and in ferro and antiferro electric and piezo electric materials in general.

cond-mat.str-el↗
arXiv2001

Nature of insulating state in NaV$_{2}$O$_{5}$ above charge-ordering transition: a cluster DMFT study

The nature of insulating state driven by electronic correlations in the quarter-filled ladder compound $α'$NaV$_{2}$O$_{5}$ is investigated within a cluster dynamical mean-field approach. An extended Hubbard model with first-principle tight-binding parameters have been used. It is shown that the insulating state in the charge-disordered phase of this compound is formed due to the transfer of spectral density and dynamical charge fluctuations where for the latter, the role of inter-site Coulomb interaction is found to be of crucial importance.

cond-mat.str-el↗
arXiv2001

Mott-insulator transition in Ca(2-x)Sr(x)RuO4: A study of the electronic and magnetic properties

The electronic structures of the metallic and insulating phases of the alloy series Ca(2-x)Sr(x)RuO4 (0 <= x <= 2) are calculated using LDA, LDA+U and Dynamical Mean-Field Approximation methods. In the end members the groundstate respectively is an orbitally non-degenerate antiferromagnetic insulator (x=0) and a good metal (x=2). For x>0.5 the observed Curie-Weiss paramagnetic metallic state which possesses a local moment with the unexpected spin S=1/2, is explained by the coexistence of localized and itinerant Ru-4d-orbitals. For 0.2<x<0.5 we propose a state with partial orbital and spin ordering. An effective model for the localized orbital and spin degrees of freedom is discussed. The metal-insulator transition at x=0.2 is attributed to a switch in the orbital occupation associated with a structural change of the crystal.

cond-mat.str-el↗
arXiv2001

Acoustic Plasmons in MgB2

We present strong evidence for the existence of an acoustic plasmon mode, that is, a quadrupolar charge collective mode with linear dispersion, in MgB2. This mode may be responsible for the anomalously small value of the Coulomb pseudopotential required to explain the high superconducting transition temperature of 40K.

cond-mat.supr-con↗
arXiv2001

The spectral and magnetic properties of $α$- and $γ$-Ce from the Dynamical Mean-Field Theory and Local Density Approximation

We have calculated ground state properties and excitation spectra for Ce metal with the {\it ab initio} computational scheme combining local density approximation and dynamical mean-field theory (LDA+DMFT). We considered all electronic states, i.e. correlated f-states and non-correlated s-, p- and d-states. The strong local correlations (Coulomb interaction) among the f-states lead to typical many-body resonances in the partial f-density, such as lower and upper Hubbard band. Additionally the well known Kondo resonance is observed. The s-, p- and d-densities show small to mediate renormalization effects due to hybridization. We observe different Kondo temperatures for $α$- and $γ$-Ce ($T_{K,α}\approx 1000 K$ and $T_{K,γ}\approx 30 K$), due to strong volume dependence of the effective hybridization strength for the localized f-electrons. Finally we compare our results with a variety of experimental data, i.e. from photoemission spectroscopy (PES), inverse photoemission spectroscopy (BIS), resonant inverse photoemission spectroscopy (RIPES) and magnetic susceptibility measurements.

cond-mat.str-el↗
arXiv2000

Mott-Hubbard Metal-Insulator Transition in Paramagnetic V_2O_3: a LDA+DMFT(QMC) Study

The electronic properties of paramagnetic V_2O_3 are investigated by the ab-initio computational scheme LDA+DMFT(QMC). This approach merges the local density approximation (LDA) with dynamical mean-field theory (DMFT) and uses numerically exact quantum Monte Carlo simulations (QMC) to solve the effective Anderson impurity model of DMFT. Starting with the crystal structure of metallic V_2O_3 and insulating (V_{0.962}Cr_{0.038})_2O_3 we find a Mott-Hubbard metal-insulator-like transition at a Coulomb interaction U\approx 5eV. The calculated spectrum is in very good agreement with experiment. Furthermore, the occupation of the (a_{1g},e_{g1}^π,e_{g2}^π) orbitals and the spin state S=1 determined by us agree with recent polarization dependent X-ray-absorption experiments.

cond-mat.str-el↗
arXiv2000

Localization in ruthenates: magnetic and electronic properties of Ca(2-x)Sr(x)RuO(4)

The electronic structures of the metallic and insulating phases of Ca(2-x)Sr(x)RuO(4) (0<= x <= 2) are calculated using LDA, LDA+U and Dynamical Mean-Field Approximation methods. For x=0 the ground state is an orbitally non-degenerate antiferromagnetic insulator. For 0.2 0.5 the observed Curie-Weiss paramagnetic metallic state possessing a local moment with the unexpected spin S=1/2 is explained by the localization of only two of the three Ru-4d-orbitals.

cond-mat.str-el↗
arXiv2000

Realistic modeling of strongly correlated electron systems: An introduction to the LDA+DMFT approach

The LDA+DMFT approach merges conventional band structure theory in the local density approximation (LDA) with a state-of-the-art many-body technique, the dynamical mean-field theory (DMFT). This new computational scheme has recently become a powerful tool for ab initio investigations of real materials with strong electronic correlations. In this paper an introduction to the basic ideas and the set-up of the LDA+DMFT approach is given. Results for the photoemission spectra of the transition metal oxide La_{1-x}Sr_xTiO_3, obtained by solving the DMFT-equations by quantum Monte-Carlo (QMC) simulations, are presented and are found to be in very good agreement with experiment. The numerically exact DMFT(QMC) solution is compared with results obtained by two approximative solutions, i.e., the iterative perturbation theory and the non-crossing approximation.

cond-mat.str-el↗
arXiv2000

Coulomb interaction in oxygen \textit{p}-shell in LDA+U method and its influence on calculated spectral and magnetic properties of transition metal oxides

Coulomb interaction between electrons on p-orbitals of oxygen atom in strongly correlated compounds is not negligible, since its value (U_p) has comparable order of magnitude with the value of Coulomb interaction on d-orbitals of transition metal atom (U_d). We investigate the effect of taking into account Coulomb correlations in oxygen p-shell in addition to the correlations in the transition metal d-shell in frame of the LDA+U method. Our calculations for NiO, MnO and La2CuO4 show that this additional correction in general improves the agreement with experimental data for the spectral (energy gap values, relative position of the main peaks in X-ray photoemission spectroscopy (XPS) and Bremsstrahlung isohromate spectroscopy (BIS)) and magnetic properties (magnetic moment values and intersite exchange interaction parameters values).

cond-mat.str-el↗
arXiv2000

Calculation of photoemission spectra of the doped Mott insulator La_{1-x}Sr_xTiO_3 using LDA+DMFT(QMC)

The spectral properties of La$_{1-x}$Sr$_{x}$TiO$_{3}$, a doped Mott insulator with strong Coulomb correlations, are calculated with the ab initio computational scheme LDA+DMFT(QMC). It starts from the non-interacting electronic band structure as calculated by the local density approximation (LDA), and introduces the missing correlations by the dynamical mean-field theory (DMFT), using numerically exact quantum Monte-Carlo (QMC) techniques to solve the resulting self-consistent multi-band single-impurity problem. The results of the LDA+DMFT(QMC) approach for the photoemission spectra of La$_{1-x}$Sr$_{x}$TiO$_{3}$ are in good agreement with experiment and represent a considerable qualitative and quantitative improvement on standard LDA calculations.

cond-mat.str-el↗
arXiv2000

Comment on "Electronic fine structure in the electron-hole plasma of SrB6"

In this comment we point out several misleading conclusions reached by Rodriguez et al. [PRL 84, 3903 (2000) and cond-mat/0003201] about the details of the band structure in SrB_6 and their role in interpreting the high-temperature weak ferromagnetism, which develops in this material upon doping.

cond-mat.str-el↗
arXiv2000

Magnetic Susceptibilities of Spin-1/2 Antiferromagnetic Heisenberg Ladders and Applications to Ladder Oxide Compounds

A comprehensive theoretical and experimental study is presented of the magnetic susceptibility versus temperature χ(T) of spin S = 1/2 two- and three-leg Heisenberg ladders and ladder oxide compounds. Extensive quantum Monte Carlo simulations of χ(T) were carried out for both isolated and coupled two-leg ladders with spatially anisotropic intraladder exchange. Accurate fits to these and related literature QMC data were obtained. We have also calculated the one- and two-magnon dispersion relations and the dynamical spin structure factor for anisotropic isolated 2 x 12 ladders. The exchange constants in the two-leg ladder compound SrCu2O3 are estimated from LDA+U calculations. We report the detailed crystal structure of SrCu2O3 and of the three-leg ladder compound Sr2Cu3O5. New experimental χ(T) data are reported for the two-leg ladder cuprates SrCu2O3 and LaCuO_{2.5}, and for the (nominally) two-leg ladder vanadates CaV2O5 and MgV2O5. The new and literature χ(T) data for these compounds and for Sr2Cu3O5 are modeled using our QMC χ(T) simulation fits, and the exchange coupling constants between the spins-1/2 are thereby estimated for each material. The surpisingly strong spatial anisotropy of the bilinear intraladder exchange constants in the cuprate compounds is discussed together with the results of other experiments sensitive to this anisotropy. Recent theoretical predictions are discussed including those which indicate that a four-spin cyclic exchange interaction within a Cu4 plaquette is important to determining the magnetic properties and which can significantly influence the exchange interactions estimated from χ(T) data assuming the presence of only bilinear exchange.

cond-mat.str-el↗
arXiv1999

LDA+DMFT study for $La_{1-x}Sr_{x}TiO_{3}$

The dynamical mean-field theory together with the non-crossing approximation is used to set up a novel scheme to study the electronic structure of strongly correlated electron systems. The non-interacting band structure is obtained from a density functional calculation within the local density approximation. With this method the doped Mott insulator $\rm La_{1-x}Sr_x Ti O_3$ is studied. Starting from first-principle calculations for a cubic and an orthorhombic system we determined the one-particle spectrum. Both one-particle spectra show a lower Hubbard band (seen as $d^1 \to d^o$ transitions in photo emission experiments), a Kondo resonance near the Fermi energy and the upper Hubbard band ($d^1\to d^2$ transitions in an inverse photoemission experiment). The upper Hubbard band develops a multipeak structure, a consequence of the consideration of all local two-particle correlations, which leads to the full multiplet structure in the atomic limit. The calculation for the orthorhombic system shows qualitative good agreement when compared with experimental photoemission spectra.

cond-mat.str-el↗
arXiv1999

Excitonic ferromagnetism in the hexaborides

A ferromagnet with a small spontaneous moment but with a high Curie temperature can be obtained by doping an excitonic insulator made from a spin triplet exciton condensate. Such a condensate can occur in a semimetal with a small overlap or a semiconductor with a small bandgap. We propose that it is responsible for the unexpected ferromagnetism in the doped hexaboride material Ca_{1-x}La_xB_6.

cond-mat.str-el↗
arXiv1999

The electronic structure of the heavy fermion metal $LiV_2O_4$

The electronic structure of the first reported heavy fermion compound without f-electrons LiV_2O_4 was studied by an ab-initio calculation method. In the result of the trigonal splitting and d-d Coulomb interaction one electron of the $d^{1.5}$ configuration of V ion is localized and the rest partially fills a relatively broad conduction band. The effective Anderson impurity model was solved by Non-Crossing-Approximation method, leading to an estimation for the single-site Kondo energy scale T_K. Then, we show how the so-called exhaustion phenomenon of Nozières for the Kondo lattice leads to a remarkable decrease of the heavy-fermion (or coherence) energy scale $T_{coh}\equiv {T_K}^2/D$ (D is the typical bandwidth), comparable to the experimental result.

cond-mat.str-el↗
arXiv1999

SiC(0001): a surface Mott-Hubbard insulator

We present ab-initio electronic structure calculations for the Si-terminated SiC(0001)$\sqrt{3}\times\sqrt{3}$ surface. While local density approximation (LDA) calculations predict a metallic ground state with a half-filled narrow band, Coulomb effects, included by the spin-polarized LDA+U method, result in a magnetic (Mott-Hubbard) insulator with a gap of 1.5 eV, comparable with the experimental value of 2.0 eV. The calculated value of the inter-site exchange parameter, J=30K, leads to the prediction of a paramagnetic Mott state, except at very low temperatures. The observed Si 2p surface core level doublet can naturally be explained as an on-site exchange splitting.

cond-mat.str-el↗