arXiv subjects

Z. V. Pchelkina

Publications and source records attributed to Z. V. Pchelkina.

36 records · Page 2Linked to original sources

arXiv2009

Magnetic ground state and multiferroicity in BiMnO$_3$

We argue that the centrosymmetric $C2/c$ symmetry in BiMnO$_3$ is spontaneously broken by antiferromagnetic (AFM) interactions existing in the system. The true symmetry is expected to be $Cc$, which is compatible with the noncollinear magnetic ground state, where the ferromagnetic order along one crystallographic axis coexists with the the hidden AFM order and related to it ferroelectric polarization along two other axes. The $C2/c$ symmetry can be restored by the magnetic field $B \sim 35$ Tesla, which switches off the ferroelectric polarization. Our analysis is based on the solution of the low-energy model constructed for the 3d-bands of BiMnO$_3$, where all the parameters have been derived from the first-principles calculations. Test calculations for isostructural BiCrO$_3$ reveal an excellent agreement with experimental data.

cond-mat.mtrl-sci↗

arXiv2008

Electronic Structure of New AFFeAs Prototype of Iron Arsenide Superconductors

This work is provoked by recent discovery of new class prototype systems AFFeAs (A=Sr,Ca) of novel layered ironpnictide High-Tc superconductors (Tc=36K). Here we report ab initio LDA results for electronic structure of the AFFeAs systems. We provide detailed comparison between electronic properties of both new systems and reference LaOFeAs (La111) compound. In the vicinity of the Fermi level all three systems have essentially the same band dispersions. However for iron fluoride systems F(2p) states were found to be separated in energy from As(4p) ones in contrast to La111, where O(2p) states strongly overlaps with As(4p). Thus it should be more plausible to include only Fe(3d) and As(4p) orbitals into a realistic noninteracting model than for La111. Moreover Sr substitution with smaller ionic radius Ca in AFFeAs materials leads to a lattice contruction and stronger Fe(3d)-As(4p) hybridization resulting in smaller value of the density of states at the Fermi level in the case of Ca compound. So to some extend Ca system reminds RE111 with later Rare Earths. However Fermi surface of new fluorides is found to be nearly perfect two-dimensional. Also we do not expect strong dependence of superconducting properties with respect to different types of A substitutes.

cond-mat.supr-con↗

arXiv2008

Electronic Structure of New LiFeAs High-Tc Superconductor

We present results of it ab initio LDA calculations of electronic structure of "next generation" layered ironpnictide High-Tc superconductor LiFeAs (Tc=18K). Obtained electronic structure of LiFeAs is very similar to recently studied ReOFeAs (Re=La,Ce,Pr,Nd,Sm) and AFe2As2 (A=Ba,Sr) compounds. Namely close to the Fermi level its electronic properties are also determined ma inly by Fe 3d-orbitals of FeAs4 two-dimensional layers. Band dispersions of LiFeAs are very similar to the LaOFeAs and BaFe2As2 systems as well as the shape of the Fe-3d density o f states and Fermi surface.

cond-mat.supr-con↗

arXiv2008

Electronic Structure of Prototype AFe_2As_2 and ReOFeAs High-Temperature Superconductors: a Comparison

We have performed ab initio LDA calculations of electronic structure of newly discovered prototype high-temperature superconductors AFe_2As_2 (A=Ba,Sr) and compared it with previously calculated electronic spectra of ReOFeAs (Re=La,Ce,Pr,Nd,Sm). In all cases we obtain almost identical densities of states in rather wide energy interval (up to 1 eV) around the Fermi level. Energy dispersions are also very similar and almost two-dimensional in this energy interval, leading to the same basic (minimal) model of electronic spectra, determined mainly by Fe d-orbitals of FeAs layers. The other constituents, such as A ions or rare earths Re (or oxygen states) are more or less irrelevant for superconductivity. LDA Fermi surfaces for AFe_2As_2 are also very similar to that of ReOFeAs. This makes the more simple AFe_2As_2 a generic system to study high-temperature superconductivity in FeAs - layered compounds.

cond-mat.supr-con↗

arXiv2008

Orbital Ordering and Magnetic Interactions in BiMnO$_3$

This work is devoted to the analysis of orbital patterns and related to them interatomic magnetic interactions in centrosymmetric monoclinic structures of BiMnO$_3$, which have been recently determined experimentally. First, we set up an effective lattice fermion model for the manganese 3d bands and derive parameters of this model entirely from first-principles electronic structure calculations. Then, we solve this model in terms of the mean-field Hartree-Fock method and derive parameters of interatomic magnetic interactions between Mn ions. We argue that although nearest-neighbor interactions favors the ferromagnetism, they compete with longer-range antiferromagnetic (AFM) interactions, the existence of which is directly related with the peculiar geometry of the orbital ordering pattern realized in BiMnO$_3$ below 474 K. These AFM interactions favor an AFM phase, which breaks the inversion symmetry. The formation of the AFM phase is assisted by the orbital degrees of freedom, which tend to adjust the nearest-neighbor magnetic interactions in the direction, which further stabilizes this phase. We propose that the multiferroelectric behavior, observed in BiMnO$_3$, may be related with the emergence of the AFM phase under certain conditions.

cond-mat.mtrl-sci↗

arXiv2008

Origin of "hot-spots" in the pseudogap regime of Nd(1.85)Ce(0.15)CuO(4): LDA+DMFT+Sigma_k study

Material specific electronic band structure of the electron-doped high-Tc cuprate Nd(1.85)Ce(0.15)CuO(4) (NCCO) is calculated within the pseudo gap regime, using the recently developed generalized LDA+DMFT+Sigma_k scheme. LDA/DFT (density functional theory within local density approximation) provides model parameters (hopping integral values, local Coulomb interaction strength) for the one-band Hubbard model, which is solved by DMFT (dynamical mean-field theory). To take into account pseudogap fluctuations LDA+DMFT is supplied with "external" k-dependent self-energy Sigma_k, which describes interaction of correlated conducting electrons with non-local Heisenberg-like antiferromagnetic (AFM) spin fluctuations responsible for pseudo gap formation. Within this LDA+DMFT+Sigma_k approach we demonstrate the formation of pronounced "hot-spots" on the Fermi surface (FS) map in NCCO, opposite to our recent calculations for Bi(2)Sr(2)CaCu(2)O(8-d) (Bi2212), which have produced rather extended region of FS "destruction". There are several physical reasons for this fact: (i) the "hot-spots" in NCCO are located closer to Brillouin zone center; (ii) correlation length of AFM fluctuations ξis larger for NCCO; (iii) pseudogap potential Δis stronger, than in Bi2212. Comparison of our theoretical data with recent bulk sensitive high-energy angle-resolved photoemission (ARPES) data for NCCO provides good semiquantitative agreement. Based on that comparison alternative explanation of the van-Hove singularity at -0.3 eV is proposed. Optical conductivity both for Bi2212 and NCCO is also calculated within LDA+DMFT+Sigma_k and compared with experimental results, demonstrating satisfactory agreement.

cond-mat.str-el↗

arXiv2008

High Temperature Superconductivity in Transition Metal Oxypnictides: a Rare-Earth Puzzle?

We have performed an extensive ab initio LDA and LSDA+U calculations of electronic structure of newly discovered high-temperature superconducting series ReO(1-x)F(x)FeAs (Re=La,Ce, Pr, Nd, Sm and hypothetical case of Re=Y). In all cases we obtain almost identical electronic spectrum (both energy dispersions and the densities of states) in rather wide energy interval (about 2 eV) around the Fermi level. We also debate that this fact is unlikely to be changed by the account of strong correlations. It leads inevitably to the same critical temperature Tc of superconducting transition in any theoretical BCS-like mechanism of Cooper pair formation. We argue that the experimentally observed variations of Tc for different rare-earth substitutions are either due to disorder effects or less probably because of possible changes in spin-fluctuation spectrum of FeAs layers caused by magnetic interactions with rare-earth spins in ReO layers.

cond-mat.supr-con↗

arXiv2007

Dominance of many-body effects over one-electron mechanism for band structure doping dependence in Nd$_{2-x}$Ce$_x$CuO$_4$: LDA+GTB approach

In the present work we report the band structure calculations for the high temperature superconductor Nd$_{2-x}$Ce$_x$CuO$_4$ in the regime of strong electronic correlations within an LDA+GTB method, which combines the local density approximation (LDA) and the generalized tight-binding method (GTB). The two mechanisms of band structure doping dependence were taken into account. Namely, the one-electron mechanism provided by the doping dependence of the crystal structure, and the many-body mechanism provided by the strong renormalization of the fermionic quasiparticles due to the large on-site Coulomb repulsion. We have shown that in the antiferromagnetic and in the strongly correlated paramagnetic phases of the underdoped cuprates the main contribution to the doping evolution of band structure and Fermi surface comes from the many-body mechanism.

cond-mat.str-el↗

arXiv2007

Comparative study of electron and hole doped High-Tc compounds in pseudogap regime: LDA+DMFT+Sk approach

Pseudogap regime for the prototype high-Tc compounds hole doped Bi2Sr2CaCu2O8-x (Bi2212) and electron doped Nd2-xCexCuO4 (NCCO) is described by means of novel generalized LDA+DMFT+Sk approach. Here conventional dynamical mean-field theory (DMFT) equations are supplied with additional (momentum dependent) self-energy Sk. In the present case Sk describes non-local dynamical correlations induced by short-ranged collective Heisenberg-like antiferromagnetic spin fluctuations. Material specific model parameters of two neighboring CuO2 layers of Bi2212 and single CuO2 layer of NCCO were obtained within local density approximation (LDA) and constrained LDA method. We show that Fermi surface in presence of the pseudogap fluctuations have perfectly visible "hot-spots" for NCCO while in Bi2212 there is just rather broad region with strong antiferromagnetic scattering. Results obtained are in good agreement with recent ARPES and optical experiments.

cond-mat.str-el↗

arXiv2006

Pseudogap behaviour in Bi2212: Results of Generalized DMFT Approach

To describe pseudogap regime in Bi2212 we employ novel generalized ab initio LDA+DMFT+Sigma_k hybrid scheme. This scheme is based on the strategy of one of the most powerfull computational tool for real correlated materials LDA+DMFT. Here the LDA+DMFT equations are supplied by an additional (momentum dependent) self-energy Sigma_k in the spirit of our recently proposed DMFT+Sigma_k approach, accounting for pseudogap fluctuations. In the present model Sigma_k describes non-local correlations induced by short-ranged collective Heisenberg-like antiferromagnetic spin fluctuations. The effective single impurity problem of the DMFT is solved by NRG. Material specific model parameters for effective x2-y2 orbital of Cu-3d shell of Bi2212 compound, e.g. hopping integrals, local Coulomb interaction U and pseudogap potential Delta were obtained within LDA and LDA+DMFT. We report theoretical LDA+DMFT+Sigma_k quasiparticle bands dispersion and damping, Fermi surface renormalization, momentum anisotropy of (quasi) static scattering, densities of states, spectral densities and angular resolved photoemission (ARPES) spectra accounting for pseudogap and bilayer splitting effects for normal (slightly) underdoped Bi2212 (delta=0.15). We show that LDA+DMFT+Sigma_k successfully describes strong (pseudogap) scattering close to Brillouin zone boundaries. Our calculated LDA+DMFT+Sigma_k Fermi surfaces and ARPES spectra in presence of the pseudogap fluctuations are almost insensitive to the bilayer splitting strength. The LDA-calculated value of bilayer splitting is found to be rather small to describe experimentally observed peak-dip-hump structure.

cond-mat.str-el↗

arXiv2006

Construction of Wannier functions from localized atomic-like orbitals

The problem of construction of the Wannier functions (WFs) in a restricted Hilbert space of eigenstates of the one-electron Hamiltonian $\hat{H}$ (forming the so-called low-energy part of the spectrum) can be formulated in several different ways. One possibility is to use the projector-operator techniques, which pick up a set of trial atomic orbitals and project them onto the given Hilbert space. Another possibility is to employ the downfolding method, which eliminates the high-energy part of the spectrum and incorporates all related to it properties into the energy-dependence of an effective Hamiltonian. We show that by modifying the high-energy part of the spectrum of the original Hamiltonian $\hat{H}$, which is rather irrelevant to the construction of WFs in the low-energy part of the spectrum, these two methods can be formulated in an absolutely exact and identical form, so that the main difference between them is reduced to the choice of the trial orbitals. Concerning the latter part of the problem, we argue that an optimal choice for trial orbitals can be based on the maximization of the site-diagonal part of the density matrix. The main idea is illustrated for a simple toy model, consisting of only two bands, as well as for a more realistic example of $t_{2g}$ bands in V$_2$O$_3$. An analogy with the search of the ground state of a many-electron system is also discussed.

cond-mat.str-el↗

arXiv2006

Evidence for strong electronic correlations in the spectra of Sr$_2$RuO$_4$

The importance of electronic correlation effects in the layered perovskite Sr$_2$RuO$_4$ is evidenced. To this end we use state-of-the-art LDA+DMFT (Local Density Approximation + Dynamical Mean-Field Theory) in the basis of Wannier functions to compute spectral functions and the quasiparticle dispersion of Sr$_2$RuO$_4$. The spectra are found to be in good agreement with various spectroscopic experiments. We also calculate the $\textbf{k}$-dependence of the quasiparticle bands and compare the results with new angle resolved photoemission (ARPES) data. Two typical manifestations of strong Coulomb correlations are revealed: (i) the calculated quasiparticle mass enhancement of $m^*/m \approx2.5$ agrees with various experimental results, and (ii) the satellite structure at about 3 eV binding energy observed in photoemission experiments is shown to be the lower Hubbard band. For these reasons Sr$_2$RuO$_4$ is identified as a strongly correlated 4$d$ electron material.

cond-mat.str-el↗

arXiv2005

A new hybrid LDA and Generalized Tight-Binding method for the electronic structure calculations of strongly correlated electron systems

A novel hybrid scheme is proposed. The {\it ab initio} LDA calculation is used to construct the Wannier functions and obtain single electron and Coulomb parameters of the multiband Hubbard-type model. In strong correlation regime the electronic structure within multiband Hubbard model is calculated by the Generalized Tight-Binding (GTB) method, that combines the exact diagonalization of the model Hamiltonian for a small cluster (unit cell) with perturbation treatment of the intercluster hopping and interactions. For undoped La$_2$CuO$_4$ and Nd$_2$CuO$_4$ this scheme results in charge transfer insulators with correct values of gaps and dispersions of bands in agreement to the ARPES data.

cond-mat.str-el↗

arXiv2005

Crystal-field splitting for low symmetry systems in ab initio calculations

In the framework of the LDA+U approximation we propose the direct way of calculation of crystal-field excitation energy and apply it to La and Y titanates. The method developed can be useful for comparison with the results of spectroscopic measurements because it takes into account fast relaxations of electronic system. For titanates these relaxation processes reduce the value of crystal-field splitting by $\sim30%$ as compared with the difference of LDA one electron energies. However, the crystal-field excitation energy in these systems is still large enough to make an orbital liquid formation rather unlikely and experimentally observed isotropic magnetism remains unexplained.

cond-mat.str-el↗

arXiv2004

Full orbital calculation scheme for materials with strongly correlated electrons

We propose a computational scheme for the ab initio calculation of Wannier functions (WFs) for correlated electronic materials. The full-orbital Hamiltonian H is projected into the WF subspace defined by the physically most relevant partially filled bands. The Hamiltonian H^{WF} obtained in this way, with interaction parameters calculated by constrained 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 self-energy operator Σ(e) 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(r,r',e). Using G(r,r',e) 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 SrVO3 and V2O3. Comparisons are made with previous results obtained by the LDA+DMFT approach where the LDA DOS was used as input, and with new bulk-sensitive experimental spectra.

cond-mat.str-el↗

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↗

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↗