arXiv subjects

I. A. Nekrasov

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

At least 37 records · Page 2Linked to original sources

arXiv2013

Density Functional Theory and Generalized Tight-Binding combined method for Hubbard fermion-phonon coupling study in strongly correlated LSCO-system

We present ab initio results for the electron-phonon interaction of the Γ-point phonons in the tetragonal high-temperature phase of La2CuO4. Eigenfrequencies and eigenvectors for the symmetry-allowed phonon modes are calculated with the full-potential augmented plane wave+local orbitals method using the frozen phonon approach. It is found that the Γ-point phonons with the strongest electron-phonon interaction are the A{2u} modes with 236 cm^{-1}, 131 cm^{-1} and 476 cm^{-1}. To take effect of strong electron on-site interaction into account we use generalized tight-binding method that results in the interaction of phonons with Hubbard fermions forming quasiparticle's band structure. Finally, the matrix elements of Hubbard fermion-phonon interaction and their reduction due to strong electron correlation are obtained.

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arXiv2013

Comparative Study of Electronic structure of New Superconductors (Sr,Ca)Pd2As2 and related compound BaPd2As2

This paper presents the comparative study of LDA calculated electronic structure of new isostructural to iron based systems superconductors (Sr,Ca)Pd2As2 with Tc about 1K and similar but structurally different system BaPd2As2. Despite chemical formula looks similar to iron superconductors and even main structural motif is the same - layers of Fe square lattices, electronic structure of (Sr,Ca)Pd2As2 and BaPd2As2 differs from Fe(As,Se)-HTSC completely. All these systems have essentially three dimensional Fermi surfaces in contrast to Fe(As,Se) materials. The Fermi level is crossed by low intensive tails of Pd-4d and As-4p states. However (Sr,Ca)Pd2As2 and BaPd2As2 materials have rather well developed peaks of Pd-4d(x2-y2) band. Thus by doping of about 2 holes per unit cell one can increase density of states at the Fermi level by a factor about 2.5. Since experimentally these compounds were found to be simple BCS superconductors the hole doping may considerably increase Tc. LDA calculated total densities of states at the Fermi level for stoichiometric systems perfectly agree with experimental estimates signifying rather small role of electronic correlations.

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arXiv2013

Doping Dependence of Correlation Effects in K{1-x}Fe{2-y}Se2 Superconductor: LDA'+DMFT Investigation

We present detailed LDA'+DMFT investigation of doping dependence of correlation effects in novel K{1-x}Fe{2-y}Se2 superconductor. Calculations were performed at four different hole doping levels, starting from hypothetical stoichiometric composition with total number of electrons equal to 29 per unit cell through 28 and 27.2 electrons towards the case of 26.52, which corresponds to chemical composition K{0.76}Fe{1.72}Se2 studied in recent ARPES experiments. In general case the increase of hole doping leads to quasiparticle bands in wide energy window +/-2 eV around the Fermi level becoming more broadened by lifetime effects, while correlation induced compression of Fe-3d LDA' bandwidths stays almost the same and of the order of ~1.3 for all hole concentrations. However close to the Fermi level situation is more complicated. Here in the energy interval from -1.0 eV to 0.4 eV the bare Fe-3d LDA' bands are compressed by significantly larger renormalization factors up to 5 with hole doping increase, while the value of Coulomb interaction remains the same. This fact manifests the increase of correlation effects with hole doping in K{1-x}Fe{2-y}Se2 system. Moreover in contrast to typical pnictides K{1-x}Fe{2-y}Se2 does not have well defined quasiparticle bands on the Fermi leves but pseudogap like "dark" region instead. We also find that with the growth of hole doping Fe-3d orbitals of various symmetries are affected by correlations in a different way in different parts of Brillouin zone. To illustrate this we determine quasiparticle mass renormalization factors and energy shifts, which transform the bare Fe-3d LDA' bands of various symmetries into LDA'+DMFT quasiparticle bands. These renormalization factors effectively mimic more complicated energy dependent self-energy effects and can be used to analyze the available ARPES data.

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arXiv2012

LDA'+DMFT Investigation of Electronic Structure of K{1-x}Fe{2-y}Se2 Superconductor

We investigate electronic structure of the new iron chalcogenide high temperature superconductor K{1-x}Fe{2-y}Se2 (hole doped case with x=0.24, y=0.28) in the normal phase using the novel LDA'+DMFT computational approach. We show that this iron chalcogenide is more correlated in a sense of bandwidth renormalization (energy scale compression by factor about 5 in the interval +/-1.5 eV), than typical iron pnictides (compression factor about 2), though the Coulomb interaction strength is almost the same in both families. Our results for spectral densities are in general agreement with recent ARPES data on this system. It is found that all Fe-3d(t2g) bands crossing the Fermi level have equal renormalization, in contrast to some previous interpretations. Electronic states at the Fermi level are of predominantly xy symmetry. Also we show that LDA'+DMFT results are in better agreement with experimental spectral function maps, than the results of conventional LDA+DMFT. Finally we make predictions for photoemission spectra lineshape for K{0.76}Fe{1.72}Se2.

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arXiv2012

Consistent LDA'+DMFT approach to electronic structure of transition metal oxides: charge transfer insulators and correlated metals

We discuss the recently proposed LDA'+DMFT approach providing consistent parameter free treatment of the so called double counting problem arising within the LDA+DMFT hybrid computational method for realistic strongly correlated materials. In this approach the local exchange-correlation portion of electron-electron interaction is excluded from self consistent LDA calculations for strongly correlated electronic shells, e.g. d-states of transition metal compounds. Then the corresponding double counting term in LDA+DMFT Hamiltonian is consistently set in the local Hartree (fully localized limit - FLL) form of the Hubbard model interaction term. We present the results of extensive LDA'+DMFT calculations of densities of states, spectral densities and optical conductivity for most typical representatives of two wide classes of strongly correlated systems in paramagnetic phase: charge transfer insulators (MnO, CoO and NiO) and strongly correlated metals (SrVO3 and Sr2RuO4). It is shown that for NiO and CoO systems LDA'+DMFT qualitatively improves the conventional LDA+DMFT results with FLL type of double counting, where CoO and NiO were obtained to be metals. We also include in our calculations transition metal 4s-states located near the Fermi level missed in previous LDA+DMFT studies of these monooxides. General agreement with optical and X-ray experiments is obtained. For strongly correlated metals LDA$^\prime$+DMFT results agree well with earlier LDA+DMFT calculations and existing experiments. However, in general LDA'+DMFT results give better quantitative agreement with experimental data for band gap sizes and oxygen states positions, as compared to the conventional LDA+DMFT.

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arXiv2012

Cuprates, Manganites, and Cobaltites: Multielectron Approach to the Band Structure

High-$T_c$ superconductors with CuO$_2$ layers, manganites La$_{1-x}$Sr$_x$MnO$_3$, and cobaltites LaCoO$_3$ present several mysteries in their physical properties. Most of them are believed to come from the strongly-correlated nature of these materials. From the theoretical viewpoint, there are many hidden rocks in making the consistent description of the band structure and low-energy physics starting from the Fermi-liquid approach. Here we discuss the alternative method -- multielectron approach to the electronic structure calculations for the Mott insulators -- called LDA+GTB (local density approximation + generalized tight-binding) method. Its origin is a straightforward generalization of the Hubbard perturbation theory in the atomic limit and the multiband $p-d$ Hamiltonian with the parameters calculated within LDA. We briefly discuss the method and focus on its applications to cuprates, manganites, and cobaltites.

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arXiv2012

Electronic Structure of New Multiple Band Pt-Pnictide Superconductors APt3P

We report LDA calculated band structure, densities of states and Fermi surfaces for recently discovered Pt-pnictide superconductors APt3P (A=Ca,Sr,La), confirming their multiple band nature. Electronic structure is essentially three dimensional, in contrast to Fe pnictides and chalcogenides. LDA calculated Sommerfeld coefficient agrees rather well with experimental data, leaving little space for very strong coupling superconductivity, suggested by experimental data on specific heat of SrPt3P. Elementary estimates show, that the values of critical temperature can be explained by rather weak or moderately strong coupling, while the decrease of superconducting transition temperature Tc from Sr to La compound can be explained by corresponding decrease of total density of states at the Fermi level N(E_F). The shape of the density of states near the Fermi level suggests that in SrPt3P electron doping (such as replacement Sr by La) decreases N(E_F) and Tc, while hole doping (e.g. partial replacement of Sr with K, Rb or Cs, if possible) would increase N(E_F) and possibly Tc.

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arXiv2012

Consistent LDA'+DMFT -- an unambiguous way to avoid double counting problem: NiO test

We present a consistent way of treating a double counting problem unavoidably arising within the LDA+DMFT combined approach to realistic calculations of electronic structure of strongly correlated systems. The main obstacle here is the absence of systematic (e.g. diagrammatic) way to express LDA (local density approximation) contribution to exchange correlation energy appearing in the density functional theory. It is not clear then, which part of interaction entering DMFT (dynamical mean-field theory) is already taken into account through LDA calculations. Because of that, up to now there is no accepted unique expression for the double counting correction in LDA+DMFT. To avoid this problem we propose here the consistent LDA'+DMFT approach, where LDA exchange correlation contribution is explicitly excluded for correlated states (bands) during self-consistent band structure calculations. What is left out of Coulomb interaction for those strongly correlated states (bands) is its non-local part, which is not included in DMFT, and the local Hartree like contribution. Then the double counting correction is uniquely reduced to the local Hartree contribution. Correlations for strongly correlated states are then directly accounted for via the standard DMFT. We further test the consistent LDA'+DMFT scheme and compare it with conventional LDA+DMFT calculating the electronic structure of NiO. Opposite to the conventional LDA+DMFT our consistent LDA'+DMFT approach unambiguously produces the insulating band structure in agreement with experiments.

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arXiv2011

Electronic and Magnetic Structure of Possible Iron Based Superconductor BaFe2Se3

We present results of LDA calculations (band structure, densities of states, Fermi surfaces) for possible iron based superconductor BaFe2Se3 (Ba123) in normal (paramagnetic) phase. Results are briefly compared with similar data on prototype BaFe2As2 and (K,Cs)Fe2Se2 superconductors. Without doping this system is antiferromagnetic with T_N^{exp}~250K and rather complicated magnetic structure. Neutron diffraction experiments indicated the possibility of two possible spin structures (antiferromagnetically ordered "plaquettes" or "zigzags"), indistinguishable by neutron scattering. Using LSDA calculated exchange parameters we estimate Neel temperatures for both spin structures within the molecular field approximation and show τ_1 ("plaquettes") spin configuration to be more favorable than τ_2 ("zigzags").

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arXiv2011

Generalized dynamical mean-field theory in physics of strongly correlated systems

This review is devoted to generalization of dynamical mean-field theory (DMFT) for strongly correlated electronic systems towards the account of different types of additional interactions, necessary for correct physical description of many experimentally observed phenomena in such systems. As additional interactions we consider: (1) interaction of electrons with antiferromagnetic (or charge) fluctuations of order parameter in high-Tc superconductors leading to the formation of pseudogap state, (2) scattering of electrons on static disorder and its role in general picture of Anderson-Hubbard metal-insulator transition, (3) electron-phonon interaction and corresponding anomalies of electronic spectra in strongly correlated systems. Proposed DMFT+Sigma approach is based on taking into account above mentioned interactions by introducing additional self-energy Sigma (in general momentum dependent) into conventional DMFT scheme and calculated in a self-consistent way within the standard set of DMFT equations. Here we formulate general scheme of calculation of both one-particle (spectral functions and densities of states) and two-particle (optical conductivity) properties. We examine the problem of pseudogap formation, including the Fermi arc formation and partial destruction of the Fermi surface, metal-insulator transition in disordered Anderson-Hubbard model, and general picture of kink formation within electronic spectra in strongly correlated systems. DMFT+Sigma approach is generalized to describe realistic materials with strong electron-electron correlations based on LDA+DMFT method. General scheme of LDA+DMFT method is presented together with some of its applications to real systems. The LDA+DMFT+Sigma approach is employed to modelling of pseudogap state of electron and hole doped high-T_c cuprates. Comparison with variety of ARPES experiments is given.

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arXiv2011

Electronic structure of two-dimensional hexagonal diselenides: charge density waves and pseudogap behavior

We present theoretical study of electronic structure (spectral functions and Fermi surfaces) for incommensurate pseudogap and charge density wave (CDW) and commensurate CDW phases of quasi two dimensional diselenides 2H-TaSe2 and 2H-NbSe2. Incommensurate pseudogap regime is described within the scenario based on short-range order CDW fluctuations, considered within the static Gaussian random field model. In contrast e.g. to high-Tc cuprates layered dichalcogenides have several different CDW scattering vectors and electronic spectrum with two bands at the Fermi level. To this end we present theoretical background for the description of multiple scattering processes within multiple bands electronic spectrum. Thus obtained theoretical spectral functions and Fermi surfaces are compared with recent ARPES experimental data, demonstrating rather good qualitative agreement.

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arXiv2011

Iron Based Superconductors: Pnictides versus Chalcogenides

We present a brief review of the present day situation with studies of high-temperature superconductivity in iron pnictides and chalcogenides. Recent discovery of superconductivity with T_c > 30 K in A_xFe_{2-x/2}Se_2 (A=K,Cs,Tl,...) represents the major new step in the development of new concepts in the physics of Fe - based high-temperature superconductors. We compare LDA and ARPES data on the band structure and Fermi surfaces of novel superconductors and those of the previously studied FeAs superconductors, especially isostructural 122 - superconductors like BaFe_2As_2. It appears that electronic structure of new superconductors is rather different from that of FeAs 122 - systems. In particular, no nesting properties of electron and hole - like Fermi surfaces is observed, casting doubts on most popular theoretical schemes of Cooper pairing for these systems. The discovery of Fe vacancies ordering and antiferromagnetic (AFM) ordering at pretty high temperatures (T_N> 500 K), much exceeding superconducting T_c makes these systems unique antiferromagnetic superconductors with highest T_N observed up to now. We discuss the role of both vacancies and AFM ordering in transformations of band structure and Fermi surfaces, as well as their importance for superconductivity. In particular, we show that system remains metallic with unfolded Fermi surfaces quite similar to that in paramagnetic state. Superconducting transition temperature T_c of new superconductors is discussed within the general picture of superconductivity in multiple band systems. It is demonstrated that both in FeAs - superconductors and in new FeSe - systems the value of T_c correlates with the value of the total density of states (DOS) at the Fermi level.

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arXiv2010

Electronic Structure, Topological Phase Transitions and Superconductivity in (K,Cs)xFe2Se2

We present LDA band structure of novel hole doped high temperature superconductors (Tc~30K) KxFe2Se2 and CsxFe2Se2 and compare it with previously studied electronic structure of isostructural FeAs superconductor BaFe2As2 (Ba122). We show that stoichiometric KFe2Se2 and CsFe2Se2 have rather different Fermi surfaces as compared with Ba122. However at about 60% of hole doping Fermi surfaces of novel materials closely resemble those of Ba122. In between these dopings we observe a number of topological Fermi surface transitions near the G point in the Brillouin zone. Superconducting transition temperature Tc of new systems is apparently governed by the value of the total density of states (DOS) at the Fermi level.

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arXiv2010

Novel multiple-band superconductor SrPt2As2

We present LDA calculated electronic structure of recently discovered superconductor SrPt2As2 with Tc=5.2K. Despite its chemical composition and crystal structure are somehow similar to FeAs-based high-temperature superconductors, the electronic structure of SrPt2As2 is very much different. Crystal structure is orthorhombic (or tetragonal if idealized) and has layered nature with alternating PtAs4 and AsPt4 tetrahedra slabs sandwiched with Sr ions. The Fermi level is crossed by Pt-5d states with rather strong admixture of As-4p states. Fermi surface of SrPt2As2 is essentially three dimensional, with complicated sheets corresponding to multiple bands. We compare SrPt2As2 with 1111 and 122 representatives of FeAs-class of superconductors, as well as with isovalent (Ba,Sr)Ni2As2 superconductors. Brief discussion of superconductivity in SrPt2As2 is also presented.

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arXiv2010

Theoretical investigation of TbNi_{5-x}Cu_x optical properties

In this paper we present theoretical investigation of optical conductivity for intermetallic TbNi_{5-x}Cu_x series. In the frame of LSDA+U calculations electronic structure for x=0,1,2 and on top of that optical conductivities were calculated. Disorder effects of Ni for Cu substitution on a level of LSDA+U densities of states (DOS) were taken into account via averaging over all possible Cu ion positions for given doping level x. Gradual suppression and loosing of structure of optical conductivity at 2 eV together with simultaneous intensity growth at 4 eV correspond to increase of Cu and decrease of Ni content. As reported before [Knyazev et al., Optics and Spectroscopy 104, 360 (2008)] plasma frequency has non monotonic doping behaviour with maximum at x=1. This behaviour is explained as competition between lowering of total density of states on the Fermi level N(E_F) and growing of number of carriers. Our theoretical results agree well with variety of recent experiments.

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arXiv2010

Limited influence of diluted ferromagnetic dimers on Curie temperature in complex magnetic systems

In this work we investigate Ising and classical Heisenberg models for two and three dimensional lattices in presence of diluted ferromagnetic dimers. For such models the Curie temperature as a function of ratio of intra-dimer exchange coupling constant I_A and other inter-site coupling constants I_B is calculated. In case dimer is treated exactly and environment within the mean-field approach it was found that even for I_A/I_B\to\infty T_C remains finite. Similar analysis is proposed for rhombohedral phase of intermetallic compound Gd2Fe17 where so-called Fe1-Fe1 "dumbbell" forms the diluted ferromagnetic dimer. It was shown that for such complex magnetic systems T_C is determined by all variety of exchange interactions and for the interval 0 < I_A/I_B < \infty T_C changes are not more than +/-10%.

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arXiv2010

Ab Initio Exchange Interactions and Magnetic Properties of Intermetallic Compound Gd(2)Fe(17-x)Ga(x)

Intermetallic compounds R2Fe17 are perspective for applications as permanent magnets. Technologically these systems must have Curie temperature Tc much higher than room temperature and preferably have easy axis anisotropy. At the moment highest Tc among stoichiometric R2Fe17 materials is 476 K, which is not high enough. There are two possibilities to increase Tc: substitution of Fe ions with non-magnetic elements or introduction of light elements into interstitial positions. In this work we have focused our attention on substitution scenario of Curie temperature rising observed experimentally in Gd(2)Fe(17-x)Ga(x) (x=0,3,6) compounds. In the framework of the LSDA approach electronic structure and magnetic properties of the compounds were calculated. Ab initio exchange interaction parameters within the Fe sublattice for all nearest Fe ions were obtained. Employing the theoretical values of exchange parameters Curie temperatures Tc of Gd(2)Fe(17-x)Ga(x) within mean-field theory were estimated. Obtained values of Tc agree well with experiment. Also LSDA computed values of total magnetic moment coincide with experimental ones.

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arXiv2010

Pseudogap phase of high-Tc compounds described within the LDA+DMFT+Sigma approach

LDA+DMFT+Sigma_k approach was applied to describe pseudogap phase of several prototype high-Tc compounds e.g. hole doped Bi2212 and LSCO systems and electron doped NCCO and PCCO, demonstrating qualitative difference of the Fermi surfaces (FS) for these systems. Namely for Bi2212 and LSCO the so called "hot-spots" (intersection of a bare FS and AFM Brillouin zone (BZ) boundary), where scattering on pseudogap fluctuations is most intensive were not observed. Instead here we have Fermi arcs with smeared FS close to the BZ boundary. However for NCCO and PCCO "hot-spots" are clearly visible. This qualitative difference is shown to have material specific origin. Good agreement with known ARPES data was demon strated not only for FS maps but also for spectral function maps (quasiparticle bands in cluding lifetime and interaction broadening).

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