arXiv subjects

V. I. Anisimov

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

At least 19 recordsLinked to original sources

arXiv2026

Magnetism and symmetry of superconducting gap in LaFeAsO from dynamical mean-field theory

By employing a combined method of density functional theory and dynamical mean field theory (DFT+DMFT) we investigate the effect of electronic correlations on the magnetic and superconducting properties of the iron-based parent compound LaFeAsO. We find that the static non-local susceptibility $χ({\bf q})$ and the dynamical spin structure factor $S({\mathbf q},ω)$ exhibit a peak at the in-plane wave vector ${\mathbf Q}=(π,π)$, which is strongly enhanced upon inclusion of dynamical vertex corrections in the ladder approximation, leading to magnetic instability. Considering the eigenfunctions of the Bethe-Salpeter equation with the vertex, obtained within the second order perturbation theory, as well as the ladder approach containing dynamic interaction vertices, in agreement with earlier weak-coupling-based studies of LaFeAsO, we obtain a close competition between $d$-wave and $s_{\pm}$ order parameters, dominating in the second-order and ladder approach, respectively. We argue that the dominating $s_{\pm}$ instability in the ladder DFT+DMFT approach is related to the reduced degree of magnetic frustration by itinerant degrees of freedom due to only partially formed local magnetic moments. Our study shows that dynamic correlation effects do not change the type of the leading superconducting instability in LaFeAsO.

cond-mat.str-el↗

arXiv2026

Magnetic fluctuations and superconducting pairing in $\varepsilon$-iron

We study Coulomb correlation effects and their role in superconductivity of $\varepsilon$-iron under pressure from 12 to 33 GPa by using a combination of density functional and dynamical mean-field theory. Our results indicate a persistence of the Fermi-liquid behavior below the temperature $\sim$1000 K. The Coulomb correlations are found to substantially renormalize the density of states, reducing the distance from the peak to the Fermi level to 0.4 eV compared to 0.75 eV obtained in DFT calculations. We find significant antiferromagnetic correlations, which are accompanied by the formation of short-lived local magnetic moments. We use the obtained results as a starting point for construction of the multi-band Bethe-Salpeter equation, which eigenvalues indicate that antiferromagnetic spin fluctuations may result in the superconducting pairing in $\varepsilon$-Fe. Moreover, the tendency to superconducting instability becomes weaker with the increase of pressure, which may explain the disappearance of superconductivity at $\sim$30 GPa.

cond-mat.str-el↗

arXiv2023

Electron correlation effects in paramagnetic cobalt

We study the influence of Coulomb correlations on spectral and magnetic properties of fcc cobalt using a combination of density functional theory and dynamical mean-field theory. The computed uniform and local magnetic susceptibilities obey the Curie-Weiss law, which, as we demonstrate, occurs due to the partial formation of local magnetic moments. We find that the lifetime of these moments in cobalt is significantly less than in bcc iron, suggesting a more itinerant magnetism in cobalt. In contrast to the bcc iron, the obtained electronic self-energies exhibit a quasiparticle shape with the quasiparticle mass enhancement factor ${m^*/m}\sim$1.8, corresponding to moderately correlated metal. Finally, our calculations reveal that the static magnetic susceptibility of cobalt is dominated by ferromagnetic correlations, as evidenced by its momentum dependence.

cond-mat.str-el↗

arXiv2023

Transition from Pauli paramagnetism to Curie-Weiss behaviour in vanadium

We study electron correlations and their impact on magnetic properties of bcc vanadium by a combination of density functional and dynamical mean-field theory. The calculated uniform magnetic susceptibility {in bcc structure} is of Pauli type at low temperatures, while it obeys the Curie-Weiss law at higher temperatures. Thus, we qualitatively reproduce the experimental temperature dependence of magnetic susceptibility without introducing the martensitic phase transition. Our results for local spin-spin correlation function and local susceptibility reveal that the Curie-Weiss behavior appears due to partial formation of local magnetic moments, which originate from $t_{2g}$ states and occur due to local spin correlations caused by Hund's rule coupling. At the same time, the fermionic quasiparticles remain well-defined, while the formation of local moments is accompanied by a deviation from the Fermi-liquid behavior. In particular, the self-energy of the $t_{2g}$ states shows the non-analytic frequency dependence, which is a characteristic of the spin-freezing behavior, while the quasiparticle damping changes approximately linearly with temperature in the intermediate temperature range $200$--$700$~K. By analyzing the momentum dependence of static magnetic susceptibility, we find incommensurate magnetic correlations, which may provide a mechanism for unconventional superconductivity at low temperatures.

cond-mat.str-el↗

arXiv2022

Coulomb correlations and magnetic properties of L1$_0$ FeCo: a DFT+DMFT study

We consider electronic correlation effects and their impact on magnetic properties of tetragonally distorted chemically ordered FeCo alloys (L1$_0$ structure) being a promising candidate for rare-earth-free permanent magnets. We employ a state-of-the-art method combining density functional and dynamical mean-field theory. According to our results, the predicted Curie temperature reduces with increase of lattice parameters ratio $c/a$ and reaches nearly 850 K at ${c/a=1.22}$. For all considered $c/a$ from 1 to $\sqrt{2}$, we find well-localized magnetic moments on Fe sites, which are formed due to strong correlations originating from Hund's coupling. At the same time, magnetism of Co sites is more itinerant with a much less lifetime of local magnetic moments. However, these short-lived local moments are also formed due to Hund's exchange. Electronic states at Fe sites are characterized by a non-quasiparticle form of self-energies, while the ones for Co sites are found to have a Fermi-liquid-like shape with quasiparticle mass enhancement factor ${m^*/m\sim 1.4}$, corresponding to moderately correlated metal. The strong electron correlations on Fe sites leading to Hund's metal behaviour can be explained by peculiarities of the density of states, which has pronounced peaks near the Fermi level, while weaker many-body effects on Co sites can be caused by stronger deviation from half-filling of their $3d$ states. The obtained momentum dependence of magnetic susceptibility suggests that the ferromagnetic ordering is the most favourable one except for the near vicinity of the fcc structure and the magnetic exchange is expected to be of RKKY type.

cond-mat.str-el↗

arXiv2021

Itinerant magnetism of chromium under pressure: a DFT+DMFT study

We consider electronic and magnetic properties of chromium, a well-known itinerant antiferromagnet, by a combination of density functional theory (DFT) and dynamical mean-field theory (DMFT). We find that electronic correlation effects in chromium, in contrast to its neighbours in the periodic table, are weak, leading to the quasiparticle mass enhancement factor ${m^*/m \approx 1.2}$. Our results for local spin-spin correlation functions and distribution of weigths of atomic configurations indicate that the local magnetic moments are not formed. Similarly to previous results of DFT at ambient pressure, the non-uniform magnetic susceptibility as a function of momentum possesses close to the wave vector ${{\mathbf Q}_{\rm H}=(0,0,2π/a)}$ ($a$ is the lattice constant) sharp maxima, corresponding to Kohn anomalies. We find that these maxima are preserved by the interaction and are not destroyed by pressure. Our calculations qualitatively capture a decrease of the Néel temperature with pressure and a breakdown of itinerant antiferomagnetism at pressure of $\sim$9 GPa in agreement with experimental data, although the Néel temperature is significantly overestimated because of the mean-field nature of DMFT.

cond-mat.str-el↗

arXiv2021

Orbital-selective coherence-incoherence crossover and metal-insulator transition in Cu-doped NaFeAs

We study the effects of electron-electron interactions and hole doping on the electronic structure of Cu-doped NaFeAs using the density functional theory plus dynamical mean-field theory (DFT+DMFT) method. In particular, we employ an effective multi-orbital Hubbard model with a realistic bandstructure of NaFeAs in which Cu-doping was modeled within a rigid band approximation and compute the evolution of the spectral properties, orbital-dependent electronic mass renormalizations, and magnetic properties of NaFeAs upon doping with Cu. In addition, we perform fully charge self-consistent DFT+DMFT calculations for the long-range antiferromagnetically ordered Na(Fe,Cu)As with Cu $x=0.5$ with a real-space ordering of Fe and Cu ions. Our results reveal a crucial importance of strong electron-electron correlations and local potential difference between the Cu and Fe ions for understanding the \textbf{k}-resolved spectra of Na(Fe,Cu)As. Upon Cu-doping, we observe a strong orbital-dependent localization of the Fe $3d$ states accompanied by a large renormalization of the Fe $xy$ and $xz$/$yz$ orbitals. Na(Fe,Cu)As exhibits bad metal behavior associated with a coherence-to-incoherence crossover of the Fe $3d$ electronic states and local moments formation near a Mott metal-insulator transition (MIT). For heavily doped NaFeAs with Cu $x \sim 0.5$ we obtain a Mott insulator with a band gap of $\sim$0.3 eV characterized by divergence of the quasiparticle effective mass of the Fe $xy$ states. In contrast to this, the quasiparticle weights of the Fe $xz$/$yz$ and $e_g$ states remain finite at the MIT. The MIT occurs via an orbital-selective Mott phase to appear at Cu $x\simeq0.375$ with the Fe $xy$ states being Mott localized. We propose the possible importance of Fe/Cu disorder to explain the magnetic properties of Cu-doped NaFeAs.

cond-mat.str-el↗

arXiv2020

Electronic correlations, spectral and magnetic properties of ZrZn$_2$

We present results of a theoretical study of a prototypical weak ferromagnet ZrZn$_2$. We use the density-functional theory (DFT)+dynamical mean-field theory (DMFT) method to study the electronic and local magnetic properties. The obtained DFT+DMFT electronic self-energies are Fermi-liquid like, indicating a small effective mass enhancement of the Zr $4d$ states $m^*/m\sim 1.1 - 1.3$ accompanied by partly formed local moments within the electronic states of $t_{2g}$ symmetry. The effect of electronic interaction is shown to be essential for determining the correct topology of some of the Fermi surface sheets. To study in detail the pressure dependence of the Curie temperature $T_{\rm C}$ and corresponding pressure-induced quantum phase transition, we consider an effective single-band model, constructed using the Zr $4d$ contribution to the total density of states. The model is studied within static and dynamic mean-field theory, as well as spin-fermion approach. We show that the spin-fermion approach yields the temperature dependence of susceptibility at ambient pressure and the pressure dependence $T_{\rm C}(p)$, including the first-order quantum phase transition at $p\approx 1.7$~GPa, comparable well with the experimental data.

cond-mat.str-el↗

arXiv2020

Electronic correlation effects and local magnetic moments in L1$_0$ phase of FeNi

We study the electronic and magnetic properties of L1$_0$ phase of FeNi, a perspective rare-earth-free permanent magnet, by using a combination of density functional and dynamical mean-field theory. Although L1$_0$ FeNi has a slightly tetragonally distorted fcc lattice, we find that magnetic properties of its constituent Fe atoms resemble those in pure bcc Fe. In particular, our results indicate the presence of well-localized magnetic moments on Fe sites, which are formed due to Hund's exchange. At the same time, magnetism of Ni sites is much more itinerant. Similarly to pure bcc Fe, the self-energy of Fe $3d$ states is found to show the non-Fermi-liquid behavior. This can be explained by peculiarities of density of Fe $3d$ states, which has pronounced peaks near the Fermi level. Our study of local spin correlation function and momentum dependence of particle-hole bubble suggests that the magnetic exchange in this substance is expected to be of RKKY-type, with iron states providing local-moment contribution, and the states corresponding to nickel sites (including virtual hopping to iron sites) providing itinerant contribution.

cond-mat.str-el↗

arXiv2020

Emergence of quantum critical charge and spin-state fluctuations near the pressure-induced Mott transition in MnO, FeO, CoO, and NiO

We perform a comprehensive theoretical study of the pressure-induced evolution of the electronic structure, magnetic state, and phase stability of the late transition metal monoxides MnO, FeO, CoO, and NiO using a fully charge self-consistent DFT+dynamical mean-field theory method. Our results reveal that the pressure-induced Mott insulator-to-metal phase transition in MnO-NiO is accompanied by a simultaneous collapse of local magnetic moments and lattice volume, implying a complex interplay between chemical bonding and electronic correlations. We compute the pressure-induced evolution of relative weights of the different valence states and spin-state configurations. Employing the concept of fluctuating valence in a correlated solid, we demonstrate that in MnO, FeO, and CoO a Mott insulator-metal transition and collapse of the local moments is accompanied by a sharp crossover of the spin-state and valence configurations. Our microscopic explanation of the magnetic collapse differs from the accepted picture and points out a remarkable dynamical coexistence (frustration) of the high-, intermediate-, and low-spin states. In particular, in MnO, the magnetic collapse is found to be driven by the appearance of the intermediate-spin state (IS), competing with the low-spin (LS) state; in FeO, we observe a conventional high-spin to low-spin (HS-LS) crossover. Most interestingly, in CoO, we obtain a remarkable (dynamical) coexistence of the HS and LS states, i.e., a HS-LS frustration, up to high pressure. Our results demonstrate the importance of quantum fluctuations of the valence and spin states for the understanding of quantum criticality of the Mott transitions.

cond-mat.str-el↗

arXiv2018

Magnetic exchange and susceptibilities in fcc iron: A supercell dynamical mean-field theory study

We study the momentum- and temperature dependencies of magnetic susceptibilities and magnetic exchange in paramagnetic fcc iron by a combination of density functional theory and supercell dynamical mean-field theory (DFT+DMFT). We find that in agreement with experimental results the antiferromagnetic correlations with the wave vector close to $(0,0,2π)$ dominate at low temperatures (as was also obtained previously theoretically), while the antiferromagnetic and ferromagnetic correlations closely compete at the temperatures $T\sim 1000$ K, where $γ$-iron exists in nature. Inverse staggered susceptibility has linear temperature dependence at low temperatures, with negative Weiss temperature $θ_{\rm stagg} \approx -340$ K; the inverse local susceptibility is also linear at not too low temperatures, showing well formed local moments. Analysis of magnetic exchange shows that the dominant contribution comes from first two coordination spheres. In agreement with the analysis of the susceptibility, the nearest-neighbor exchange is found to be antiferromagnetic at low temperatures, while at temperature of the $α$-$γ$ structural phase transition its absolute value becomes small, and the system appears on the boundary between the regimes with strongest antiferro- and ferromagnetic correlations.

cond-mat.str-el↗

arXiv2018

Paraorbital ground state of trivalent Ni ion in LiNiO$_2$ from DFT+DMFT calculations

In LiNiO$_2$ Ni$^{3+}$ ion has $d^7$ configuration in cubic crystal field with one electron on double degenerate $e_g$ orbitals, and such ion is considered to be Jahn-Teller (JT) active. However despite the fact, that this compound is an insulator, and hence $d$-electrons are localized, a cooperative JT lattice distortion was not observed. This problem was usually supposed to be resolved by the presence of local JT-distortions that do not order in cooperative JT distorted crystal structure. In the present work DFT+DMFT approach, combining Density Functional Theory with Dynamical Mean-Field Theory, was applied to study electronic and magnetic properties of LiNiO$_2$. In the result, insulating solution with a small energy gap value was obtained in agreement with experimental data. However, in contrast to previous calculations by other methods, the symmetry was not broken and the calculated ground state is a thermodynamical mixture of $αd^7 + βd^8L $ ($α\approx 60\%, β\approx 40\%$) ionic states. The $d^8L $ state is JT inactive and we have found that for the nickel $d^7$ state two configurations with an electron on the Ni $d_{x^2-y^2}$ or $d_{3z^2-r^2}$ orbital have equal statistical weights. So the orbital degeneracy of Ni$^{3+}$ ion is not lifted and that explains the absence of the cooperative JT lattice distortion in this compound. Also, the temperature dependence of inverse magnetic susceptibility of LiNiO$_2$ has been calculated and a good agreement with experimental data was obtained.

cond-mat.str-el↗

arXiv2018

Effect of density of states peculiarities on Hund's metal behavior

We investigate a possibility of Hund's metal behavior in the Hubbard model with asymmetric density of states having peak(s). Specifically, we consider the degenerate two-band model and compare its results to the five-band model with realistic density of states of iron and nickel, showing that the obtained results are more general, provided that the hybridization between states of different symmetry is sufficiently small. We find that quasiparticle damping and the formation of local magnetic moments due to Hund's exchange interaction are enhanced by both, the density of states asymmetry, which yields stronger correlated electron or hole excitations, and the larger density of states at the Fermi level, increasing the number of virtual electron-hole excitations. For realistic densities of states these two factors are often interrelated because the Fermi level is attracted towards peaks of the density of states. We discuss the implication of the obtained results to various substances and compounds, such as transition metals, iron pnictides, and cuprates.

cond-mat.str-el↗

arXiv2018

Phonon mode softening and elastic properties of hafnium under pressure

The effect of pressure on the zone-center optical phonon modes and the elastic properties of hafnium has been studied by both experimental (Raman scattering) and theoretical (DFT) approaches. It was found an anomaly dependence of the phonon frequency of E$_{2g}$ mode in the pressure range from 0 to 67 GPa. The calculated electronic structure of hafnium shows significant changes under pressure, which have a pronounced spatial anisotropy similar to the anisotropy of the observed phonon properties. The dependencies of the elastic properties on pressure including the components of the elasticity tensor, bulk and shear modulus, Poisson ratio and Universal anisotropy index obtained during the calculations has characteristic features indicating the occurrence of the structural phase transition.

cond-mat.mtrl-sci↗

arXiv2018

Correlation strength, Lifshitz transition and the emergence of a two- to three-dimensional crossover in FeSe under pressure

We report a detailed theoretical study of the electronic structure, spectral properties, and lattice parameters of bulk FeSe under pressure using a fully charge self-consistent implementation of the density functional theory plus dynamical mean-field theory method (DFT+DMFT). In particular, we perform a structural optimization and compute the evolution of the lattice parameters (volume, $c/a$ ratio, and the internal $z$ position of Se) and the electronic structure of the tetragonal (space group $P4/nmm$) paramagnetic FeSe. Our results for the lattice parameters are in good quantitative agreement with experiment. The $c/a$ ratio is slightly overestimated by about $3$~\%, presumably due to the absence of the van der Waals interactions between the FeSe layers in our calculations. The lattice parameters determined within DFT are off the experimental values by a remarkable $\sim$$6$-$15$~\%, implying a crucial importance of electron correlations. Upon compression to $10$~GPa, the $c/a$ ratio and the lattice volume show a decrease by $2$ and $10$~\%, respectively, while the Se $z$ coordinate weakly increases by $\sim$$2$~\%. Most importantly, our results reveal a topological change of the Fermi surface (Lifshitz transition) which is accompanied by a two- to three-dimensional crossover. Our results indicate a small reduction of the quasiparticle mass renormalization $m^*/m$ by about $5$~\% for the $e$ and less than $1$~\% for the $t_2$ states, as compared to ambient pressure. The behavior of the momentum-resolved magnetic susceptibility $χ({\bf q})$ shows no topological changes of magnetic correlations under pressure, but demonstrates a reduction of the degree of the in-plane $(π,π)$ stripe-type nesting. Our results for the electronic structure and lattice parameters of FeSe are in good qualitative agreement with recent experiments on its isoelectronic counterpart FeSe$_{1-x}$S$_x$.

cond-mat.str-el↗

arXiv2017

The role of temperature and Coulomb correlation in stabilization of CsCl-phase in FeS under pressure

The iron-sulfur system is important for planetary interiors and is intensely studied, particularly for better understanding of the cores of Mars and Earth. Yet, there is a paradox about high-pressure stability of FeS: ab initio global optimization (at DFT level) predicts a Pmmn phase (with a distorted rocksalt structure) to be stable at pressures above ~120 GPa, which has not yet been observed in the experiments that instead revealed a CsCl-type phase which, according to density functional calculations, should not be stable. Using quasiharmonic free energy calculations and the dynamical mean field theory, we show that this apparent discrepancy is removed by proper account of electron correlations and entropic effects.

cond-mat.str-el↗

arXiv2017

Momentum-dependent susceptibilities and magnetic exchange in bcc iron from supercell DMFT calculations

We analyze the momentum- and temperature dependences of the magnetic susceptibilities and magnetic exchange interaction in paramagnetic bcc iron by a combination of density functional theory and dynamical mean-field theory (DFT+DMFT). By considering a general derivation of the orbital-resolved effective model for spin degrees of freedom for Hund's metals, we relate momentum-dependent susceptibilities in the paramagnetic phase to the magnetic exchange. We then calculate non-uniform orbital-resolved susceptibilities at high-symmetry wave vectors by constructing appropriate supercells in the DMFT approach. Extracting the irreducible parts of susceptibilities with respect to Hund's exchange interaction, we determine the corresponding orbital-resolved exchange interactions, which are then interpolated to the whole Brillouin zone. Using the spherical model we estimate the temperature dependence of the resulting exchange between local moments.

cond-mat.str-el↗

arXiv2017

Effect of electron correlations on the electronic structure and phase stability of FeSe upon lattice expansion

We present results of a detailed theoretical study of the electronic, magnetic, and structural properties of the chalcogenide parent system FeSe using a fully charge self-consistent implementation of the density functional theory plus dynamical mean-field theory (DFT+DMFT) method. In particular, we predict a remarkable change of the electronic structure of FeSe which is accompanied by a complete reconstruction of the Fermi surface topology (Lifshitz transition) upon a moderate expansion of the lattice volume. The phase transition results in a change of the in-plane magnetic nesting wave vector from $(π,π)$ to $(π,0)$ and is associated with a transition from itinerant to orbital-selective localized magnetic moments. We attribute this behavior to a correlation-induced shift of the van Hove singularity of the Fe $t_{2}$ bands at the M-point across the Fermi level. Our results reveal a strong orbital-selective renormalization of the effective mass $m^*/m$ of the Fe $3d$ electrons upon expansion. The largest effect occurs in the Fe $xy$ orbital, which gives rise to a non-Fermi-liquid-like behavior above the transition. The behavior of the momentum-resolved magnetic susceptibility $χ({\bf q})$ demonstrates that magnetic correlations are also characterized by a pronounced orbital selectivity, suggesting a spin-fluctuation origin of the nematic phase of paramagnetic FeSe. We conjecture that the anomalous behavior of FeSe upon expansion is associated with the proximity of the Fe $t_{2}$ van Hove singularity to the Fermi level and the sensitive dependence of its position on external conditions.

cond-mat.str-el↗