arXiv subjects

I. A. Nekrasov

Publications and source records attributed to I. A. Nekrasov.

86 records · Page 5Linked to original sources

arXiv2004

Parameters of the Effective Singlet-Triplet Model for Band Structure of High-$T_c$ Cuprates by Different Approaches

The present paper covers the problem of parameters determination for High-$T_c$ superconductive copper oxides. Different approaches, {\it ab initio} LDA and LDA+U calculations and Generalized Tight-Binding (GTB) method for strongly correlated electron systems, are used to calculate hopping and exchange parameters of the effective singlet-triplet model for $CuO_2$-layer. The resulting parameters are in remarkably good agreement with each other and with parameters extracted from experiment. This set of parameters is proposed for proper quantitative description of physics of hole doped High-$T_c$ cuprates in the framework of effective models.

cond-mat.supr-con↗

arXiv2003

Explanation of the similarity of the experimental photoemission spectra of SrVO3 and CaVO3

We present parameter-free LDA+DMFT results for the many-particle density of states of cubic SrVO3 and orthorhombic CaVO3. Both systems are found to be strongly correlated metals, but em not on the verge of a metal-insulator transition. In spite of the considerably smaller V-O-V bonding angle in CaVO3 the photoemission spectra of the two systems are very similar, their quasiparticle parts being almost identical. This is in contrast to earlier theoretical and experimental conclusions, but in agreement with recent bulk-sensitive photoemission experiments.

cond-mat.str-el↗

arXiv2003

Influence of Co$^{3+}$ spin-state on optical properties of LaCoO$_3$ and HoCoO$_3$

Optical properties of the isoelectronic compounds LaCoO$_3$ and HoCoO$_3$ has been experimentally and theoretically investigated. We've measured the real $ε_1(ω)$ and imaginary $ε_2(ω)$ parts of the dielectric function, reflectance $R(ω)$ and optical conductivity at room temperature. The shift of the most pronounced spectral features to the high energy region on 0.3 eV associated with larger distortions due to the smaller rare earth ionic radii in HoCoO$_3$ in comparison with LaCoO$_3$ was observed. Also there was found an enhancement of absorption intensity in the range 1.3-2.3 eV in all kinds of spectra in HoCoO$_3$, which can be attributed basing on the results of LDA+U calculations to the different spin-states of Co$^{3+}$ ion in these compounds. The shift of the onset of the absorption from less than 0.1 eV in LaCoO$_3$ to 0.7 eV in HoCoO$_3$ and an absorption intensity enhancement in a narrow spectral range 1.2-2.6 eV in HoCoO$_3$ are clearly seen from the calculated convolution of partial densities of states obtained in the LDA+U approach. Such changes are assumed to be induced by the different Co$^{3+}$ spin-state in these compounds at room temperature.

cond-mat↗

arXiv2003

The influence of the rare earth ions radii on the Low Spin to Intermediate Spin state transition in lanthanide cobaltite perovskites: LaCoO3 vs. HoCoO3

We present first principles LDA+U calculations of electronic structure and magnetic state for LaCoO3 and HoCoO3. Low Spin to Intermediate Spin state transition was found in our calculations using experimental crystallographic data for both materials with a much higher transition temperature for HoCoO3, which agrees well with the experimental estimations. Low Spin state t6e0 (non-magnetic) to Intermediate Spin state t5e1 (magnetic) transition of Co(3+) ions happens due to the competition between crystal field t_2g-e_g splitting and effective exchange interaction between 3$d$ spin-orbitals. We show that the difference in crystal structure parameters for HoCoO3 and LaCoO3 due to the smaller ionic radius of Ho ion comparing with La ion results in stronger crystal field splitting for HoCoO3 (0.09 eV ~ 1000 K larger than for LaCoO3) and hence tip the balance between the Low Spin and Intermediate Spin states to the non-magnetic solution in HoCoO3.

cond-mat.str-el↗

arXiv2002

Orbital state and magnetic properties of LiV_2 O_4

LiV_2 O_4 is one of the most puzzling compounds among transition metal oxides because of its heavy fermion like behavior at low temperatures. In this paper we present results for the orbital state and magnetic properties of LiV_2 O_4 obtained from a combination of density functional theory within the local density approximation and dynamical mean-field theory (DMFT). The DMFT equations are solved by quantum Monte Carlo simulations. The trigonal crystal field splits the V 3d orbitals such that the a_{1g} and e_{g}^{pi} orbitals cross the Fermi level, with the former being slightly lower in energy and narrower in bandwidth. In this situation, the d-d Coulomb interaction leads to an almost localization of one electron per V ion in the a_{1g} orbital, while the e_{g}^{pi} orbitals form relatively broad bands with 1/8 filling. 2The theoretical high-temperature paramagnetic susceptibility chi(T) follows a Curie-Weiss law with an effective paramagnetic moment p_{eff}=1.65 in agreement with the experimental results.

cond-mat.str-el↗

arXiv2002

First Principle Electronic Model for High-Temperature Superconductivity

Using the structural data of the La2CuO4 compound both in the low temperature tetragonal phase and in the isotropic phase we have derived an effective t-J model with hoppings t and superexchange interactions J extended up to fourth and second neareast neighbors respectively. By numerically studying this hamiltonian we have then reproduced the main experimental features of this HTc compound: d-wave superconductivity is stabilized at small but finite doping delta>6% away from the antiferromagnetic region and some evidence of dynamical stripes is found at commensurate filling 1/8.

cond-mat.supr-con↗

arXiv2001

The LDA+DMFT Approach to Materials with Strong Electronic Correlations

LDA+DMFT is a novel computational technique for ab initio investigations of real materials with strongly correlated electrons, such as transition metals and their oxides. It combines the strength of conventional band structure theory in the local density approximation (LDA) with a modern many-body approach, the dynamical mean-field theory (DMFT). In the last few years LDA+DMFT has proved to be a powerful tool for the realistic modeling of strongly correlated electronic systems. In this paper the basic ideas and the set-up of the LDA+DMFT(X) approach, where X is the method used to solve the DMFT equations, are discussed. Results obtained with X=QMC (quantum Monte Carlo) and X=NCA (non-crossing approximation) are presented and compared. By means of the model system La$_{1-x}$Sr$_{x}$TiO$_{3}$ we show that the method X matters qualitatively and quantitatively. Furthermore, we discuss recent results on the Mott-Hubbard metal-insulator transition in the transition metal oxide V$_2$O$_3$ and the $α$-$γ$ transition in the 4f-electron system Ce.

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

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

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↗

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↗