arXiv subjects

V. I. Anisimov

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

At least 37 records · Page 2Linked to original sources

arXiv2016

Nonlocal correlations in the vicinity of the $α$-$γ$ phase transition in iron within a DMFT plus spin-fermion model approach

We consider nonlocal correlations in iron in the vicinity of the $α$-$γ$ phase transition within the spin-rotationally-invariant dynamical mean-field theory (DMFT) approach, combined with the recently proposed spin-fermion model of iron. The obtained nonlocal corrections to DMFT yield a decrease of the Curie temperature of the $α$ phase, leading to an agreement with its experimental value. We show that the corresponding nonlocal corrections to the energy of the $α$ phase are crucially important to obtain the proximity of energies of $α$ and $γ$ phases in the vicinity of the iron $α$-$γ$ transformation.

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arXiv2016

Doping induced spin state transition in Li$_x$CoO$_2$ as studied by the GGA+DMFT calculations

Magnetic properties of Li$_x$CoO$_2$ for $x = 0.94, 0.75, 0.66$ and $0.51$ were investigated in frames of method combining Generalized Gradient Approximation with Dynamical Mean--Field Theory (GGA+DMFT). We found that a delicate interplay between Hund's exchange energy and $t_{2g}-e_g$ crystal field splitting is responsible for the high spin to low spin state transition for Co$^{4+}$ ions. The GGA+DMFT calculations show that at small doping level the Co$^{4+}$ ions adopt high spin state, while delithiation results in increase of the crystal field splitting and low spin state becomes preferable. The Co$^{3+}$ ions were found to stays in the low spin configuration for any $x$.

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arXiv2016

Correlations induced orbital ordering and cooperative Jahn-Teller distortion in the paramagnetic insulator KCrF$_3$

We investigate the origin of the orbital ordering in the paramagnetic phase of KCrF$_3$. All previous studies described structural parameters of the paramagnetic phase using a magnetic ordering in the compound. Our simulations of real paramagnetic KCrF$_3$ were performed within an approach combining density functional theory and dynamical mean field theory (DFT+DMFT). As a result, it was found that the experimentally observed cooperative Jahn-Teller effect is successfully described in a lattice relaxation calculation for structure without any long-range magnetic ordering. It is established that the existence of the orbital ordering even in undistorted perovskite structure clearly confirms the electronic origin of the orbital ordering in KCrF$_3$.

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arXiv2016

Quantum states entanglement in hemoglobin molecule active center

An ab initio study of the electronic and spin configuration for the iron ion in the active center of the human hemoglobin molecule is presented. It is well known that the iron ion, being surrounded by the porphyrin ring and the ligands, plays the key role in the realization of the basic oxygen-transport functions of the molecule. This work is focused on the investigation the features of the 3$d$-shell electronic states of the iron ion located inside the active center of the hemoglobin molecule. Also in this paper we study in detail the changes in these states occurring during the oxidation process. We use a combination of the Density Functional Theory (DFT) method and the Dynamical Mean Field Theory (DMFT) approach. This method allows to consider dynamic correlation effects that are important in the description of systems containing transition metal ions. It was found that the state of the valence electrons of the iron ion of the active center of hemoglobin molecule is the entangled quantum state. This state is a mixture of several electronic states with comparable statistical probability. Furthermore, it was found that the process of the bond formation between the iron-porphyrin complex and the oxygen molecule is more complex than a simple high-spin to low-spin state of the Fe ion transition. The transition metal ion oxidation is accompanied by substantial redistribution of the states probabilities and the increasing of the entanglement degree. This process also leads to the reduction of the total spin moment from $s\approx$2.1 for the FeP(Im) to $s\approx$1.7 for the FeP(Im)(O$_2$).

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arXiv2015

Structural ${γ\textrm{-}\varepsilon}$ phase transition in Fe-Mn alloys from CPA+DMFT approach

We present a computational scheme for total energy calculations of disordered alloys with strong electronic correlations. It employs the coherent potential approximation combined with the dynamical mean-field theory and allows one to study the structural transformations. The material-specific Hamiltonians in the Wannier function basis are obtained by density functional theory. The proposed computational scheme is applied to study the ${γ\textrm{-}\varepsilon}$ structural transition in paramagnetic Fe-Mn alloys for Mn content from 10 to 20 at. %. The electronic correlations are found to play a crucial role in this transition. The calculated transition temperature decreases with increasing Mn content and is in a good agreement with experiment. We demonstrate that in contrast to the ${α\textrm{-}γ}$ transition in pure iron, the ${γ\textrm{-}\varepsilon}$ transition in Fe-Mn alloys is driven by a combination of kinetic and Coulomb energies. The latter is found to be responsible for the decrease of the ${γ\textrm{-}\varepsilon}$ transition temperature with Mn content.

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arXiv2015

Calculation of the exchange constants of the Heisenberg model in the plane-wave based methods using the Green's function approach

An approach to compute exchange parameters of the Heisenberg model in plane-wave-based methods is presented. This calculation scheme is based on the Green's function method and Wannier function projection technique. It was implemented in the framework of the pseudopotential method and tested on such materials as NiO, FeO, Li2MnO3, and KCuF3. The obtained exchange constants are in a good agreement with both the total energy calculations and experimental estimations for NiO and KCuF3. In the case of FeO our calculations explain the pressure dependence of the Néel temperature. Li2MnO3 turns out to be a Slater insulator with antiferromagnetic nearest-neighbor exchange defined by the spin splitting. The proposed approach provides a unique way to analyze magnetic interactions, since it allows one to calculate orbital contributions to the total exchange coupling and study the mechanism of the exchange coupling.

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arXiv2015

Metal-Insulator Transition and Lattice Instability of Paramagnetic V2O3

We determine the electronic structure and phase stability of paramagnetic V$_2$O$_3$ at the Mott-Hubbard metal-insulator phase transition, by employing a combination of an ab initio method for calculating band structures with dynamical mean-field theory. The structural transformation associated with the metal-insulator transition is found to occur upon a slight expansion of the lattice volume by $\sim 1.5$ %, in agreement with experiment. Our results show that the structural change precedes the metal-insulator transition, implying a complex interplay between electronic and lattice degrees of freedom at the transition. Electronic correlations and full charge self-consistency are found to be crucial for a correct description of the properties of V$_2$O$_3$.

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arXiv2015

Pressure-driven metal-insulator transition in BiFeO$_3$ from Dynamical Mean-Field Theory

A metal-insulator transition (MIT) in BiFeO$_3$ under pressure was investigated by a method combining Generalized Gradient Corrected Local Density Approximation with Dynamical Mean-Field Theory (GGA+DMFT). Our paramagnetic calculations are found to be in agreement with experimental phase diagram: Magnetic and spectral properties of BiFeO3 at ambient and high pressures were calculated for three experimental crystal structures $R3c$, $Pbnm$ and $Pm\bar{3}m$. At ambient pressure in the $R3c$ phase, an insulating gap of 1.2 eV was obtained in good agreement with its experimental value. Both $R3c$ and $Pbnm$ phases have a metal-insulator transition that occurs simultaneously with a high-spin (HS) to low-spin (LS) transition. The critical pressure for the $Pbnm$ phase is 25-33 GPa that agrees well with the experimental observations. The high pressure and temperature $Pm\bar{3}m$ phase exhibits a metallic behavior observed experimentally as well as in our calculations in the whole range of considered pressures and undergoes to the LS state at 33 GPa where a $Pbnm$ to $Pm\bar{3}m$ transition is experimentally observed. The antiferromagnetic GGA+DMFT calculations carried out for the $Pbnm$ structure result in simultaneous MIT and HS-LS transitions at a critical pressure of 43 GPa in agreement with the experimental data.

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arXiv2015

Hellmann-Feynman Forces within the DFT+U in Wannier functions basis

The most general way to describe localized atomic-like electronic states in strongly correlated compounds is to utilize Wannier functions. In the present paper we continue the development of widely-spread DFT+U method onto Wannier function basis set and propose the technique to calculate the Hubbard contribution to the forces. The technique was implemented as a part of plane-waves pseudopotential code Quantum-ESPRESSO and successfully tested on a charge transfer insulator NiO.

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arXiv2014

Influence of Hund's exchange and spin-orbit coupling on spectral properties of fcc-Ce: DFT+DMFT study

Spectral properties of fcc-Ce have been calculated in frames of modern DFT+DMFT method with Hybridization expansion CT-QMC solver. The influence of Hund's exchange and spin-orbit coupling (SOC) on spectral properties of Ce were investigated. SOC is responsible for the shape of spectra near the Fermi level and Hund's exchange interaction doesn't change the obtained spectra and can be neglected.

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arXiv2014

Correlation-driven topological Fermi surface transition in FeSe

The electronic structure and phase stability of paramagnetic FeSe is computed by using a combination of ab initio methods for calculating band structure and dynamical mean-field theory. Our results reveal a topological change (Lifshitz transition) of the Fermi surface upon a moderate expansion of the lattice. The Lifshitz transition is accompanied with a sharp increase of the local moments and results in an entire reconstruction of magnetic correlations from the in-plane magnetic wave vector $(π,π)$ to $(π,0)$. We attribute this behavior to a correlation-induced shift of the Van Hove singularity originating from the $d_{xy}$ and $d_{xz}/d_{yz}$ bands at the M-point across the Fermi level. We propose that superconductivity is strongly influenced, or even induced, by a Van Hove singularity.

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arXiv2014

Effect of correlations and doping on the spin susceptibility of iron pnictides: the case of KFe2As2

The temperature dependence of the paramagnetic susceptibility of the iron pnictide superconductor KFe2As2 and its connection with the spectral properties of that material is investigated by a combination of density functional theory (DFT) in the local density approximation and dynamical mean-field theory (DMFT). Unlike other iron pnictide parent compounds where the typical oxidation state of iron is 2, the formal valence of Fe in KFe2As2 is 2.5, corresponding to an effective doping with 0.5 hole per iron atom compared to, for example, BaFe2As2. This shifts the chemical potential and thereby reduces the distance between the peaks in the spectral functions of KFe2As2 and the Fermi energy as compared to BaFe2As2. The shift, which is clearly seen on the level of DFT as well as in DMFT, is further enhanced by the strong electronic correlations in KFe2As2. In BaFe2As2 the presence of these peaks results [Phys. Rev. B 86, 125124 (2012)] in a temperature increase of the susceptibility up to a maximum at ~1000 K. While the temperature increase was observed experimentally the decrease at even higher temperatures is outside the range of experimental observability. We predict that in KFe2As2 the situation is different. Namely, the reduction of the distance between the peaks and the Fermi level due to doping is expected to shift the maximum in the susceptibility to much lower temperatures, such that the decrease of the susceptibility should become visible in experiment.

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arXiv2014

Electronic correlations determine the phase stability of iron up to the melting temperature

We present theoretical results on the high-temperature phase stability and phonon spectra of paramagnetic bcc iron which explicitly take into account many-body effects. Several peculiarities, including a pronounced softening of the [110] transverse (T$_1$) mode and a dynamical instability of the bcc lattice in harmonic approximation are identified. We relate these features to the $α$-to-$γ$ and $γ$-to-$δ$ phase transformations in iron. The high-temperature bcc phase is found to be highly anharmonic and appears to be stabilized by the lattice entropy.

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arXiv2014

CPA for strongly correlated systems: Electronic structure and magnetic properties of NiO-ZnO solid solutions

The method of electronic structure calculations for strongly correlated disordered materials is developed employing the basic idea of coherent potential approximation (CPA). Evolution of electronic structure and spin magnetic moment value with concentration $x$ in strongly correlated Ni$_{1-x}$Zn$_x$O solid solutions is investigated in the frame of this method. The obtained values of energy gap and magnetic moment are in agreement with the available experimental data.

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arXiv2014

Electronic Structure and Spectra of CuO

We report the electronic structure of monoclinic CuO as obtained from first principles calculations utilizing density functional theory plus effective Coulomb interaction (DFT + U) method. In contrast to standard DFT calculations taking into account electronic correlations in DFT + U gave antiferromagnetic insulator with energy gap and magnetic moment values in good agreement with experimental data. The electronic states around the Fermi level are formed by partially filled Cu 3$d_{x^2-y^2}$ orbitals with significant admixture of O 2$p$ states. Theoretical spectra are calculated using DFT + U electronic structure method and their comparison with experimental photoemission and optical spectra show very good agreement.

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arXiv2014

The nature of the ferromagnetic ground state in the Mn4 molecular magnet

Using ab initio band structure and model calculations we studied magnetic properties of one of the Mn$_4$ molecular magnets (Mn4(hmp)6), where two types of the Mn ions exist: Mn3+ and Mn2+. The direct calculation of the exchange constants in the GGA+U approximation shows that in contrast to a common belief the strongest exchange coupling is not between two Mn3+ ions (J_{bb}), but along two out of four exchange paths connecting Mn3+ and Mn2+ ions (J_{wb}). The microscopic analysis performed within the perturbation theory allowed to establish the mechanism for this largest ferromagnetic exchange constant. The charge ordering of the Mn ions results in the situation when the energy of the excited state in the exchange process is defined not by the large on-site Coulomb repulsion U, but by much smaller energy V, which stabilizes the charge ordered state. Together with strong Hund's rule coupling and specific orbital order this leads to a large ferromagnetic exchange interaction for two out of four Mn2+ --Mn3+ pairs.

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arXiv2013

First-principles Calculation of Atomic Forces and Structural Distortions in Strongly Correlated Materials

We introduce a novel computational approach for the investigation of complex correlated electron materials which makes it possible to evaluate interatomic forces and thereby determine atomic displacements and structural transformations induced by electronic correlations. It combines \textit{ab initio} band structure and dynamical mean-field theory and is implemented with the linear-response formalism regarding atomic displacements. We apply this new technique to explore structural transitions of prototypical correlated systems such as elemental hydrogen, SrVO$_3$, and KCuF$_3$.

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arXiv2013

Magnetic fluctuations and effective magnetic moments in γ-iron due to electronic structure peculiarities

Applying the local density and dynamical mean field approximations to paramagnetic γ-iron we revisit the problem of theoretical description of magnetic properties in a wide temperature range. We show that contrary to α-iron, the frequency dependence of the electronic self-energy has a quasiparticle form for both, t_{2g} and e_g states. In the temperature range T=1200-1500 K, where γ-iron exist in nature, this substance can be nevertheless characterized by temperature-dependent effective local moments, which yield relatively narrow peaks in the real part of the local magnetic susceptibility. At the same time, at low temperatures γ-iron (which is realized in precipitates) is better described in terms of itinerant picture. In particular, the nesting features of the Fermi surfaces yield maximum of the static magnetic susceptibility at the incommensurate wave vector q_{max} belonging the direction q_X-q_W (q_X=(2π/a)(1,0,0),q_W=(2π/a)(1,1/2,0), a is a lattice parameter) in agreement with the experimental data. This state is found however to compete closely with the states characterized by magnetic wave vectors along the directions q_X-q_L-q_K, where q_L=(2π/a)(1/2,1/2,1/2), q_K=(2π/a)(3/4,3/4,0). From the analysis of the uniform magnetic susceptibility we find that contrary to α-iron, the Curie-Weiss law is not fulfilled in a broad temperature range, although the inverse susceptibility is nearly linear in the moderate-temperature region (1200-1500 K). The non-linearity of the inverse uniform magnetic susceptibility in a broader temperature range is due to the density of states peak located close to the Fermi level. The effective exchange integrals in the paramagnetic phase are estimated on the base of momentum dependent susceptibility.

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