arXiv subjects

A. A. Katanin

Publications and source records attributed to A. A. Katanin.

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.

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arXiv2026

Suppression of local magnetic moment formation and paramagnetic exchange interactions in monolayer Fe$_3$GeTe$_2$

We study the electronic and magnetic properties of monolayer Fe$_3$GeTe$_2$ within the DFT+DMFT approach in the paramagnetic phase. We argue that this compound is sufficiently far from the local magnetic moment limit, demonstrating non-linear temperature dependencies of the partial inverse local and uniform magnetic susceptibilities in a broad temperature range. We find that in the regime of moderate Coulomb interactions ($U=3-4$ eV), the iron atoms located above and below the Ge plane carry a substantial local magnetic moment ($μ\gtrsim 4.5 μ_B$), while the iron atom located within the Ge plane does not exhibit any pronounced magnetic moment. At the same time, the RKKY-type exchange interactions between these two symmetry-nonequivalent types of atoms turn out to be crucial for stabilizing long-range ferromagnetic order in Fe$_3$GeTe$_2$. The estimated spin-wave stiffness and Curie temperature are in good agreement with the experimental data, indicating that a dynamical treatment of electron correlations in Fe$_3$GeTe$_2$ is essential to properly describe its partially itinerant magnetic behavior.

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arXiv2026

Hubb_DMFT and Wan2mb_DMFT: Continuous-Time Quantum Monte Carlo Solvers for Single- and Multi-Orbital Hubbard Model

We present two related customized software packages, Hubb_DMFT and Wan2mb_DMFT, designed to solve the Dynamical Mean-Field Theory (DMFT) equations for strongly correlated electron systems. Hubb_DMFT is adjusted for the single-band Hubbard model, providing a fast way to calculate the local self-energy, as well as the two-particle fermion and triangular fermion-boson charge and spin vertices, while Wan2mb_DMFT extends this capability to realistic multi-orbital systems, directly interfacing Wannier tight-binding Hamiltonians with many-body solvers. Both codes are based on iQIST v.0.7 impurity solver and utilize a modified and internally integrated Continuous-Time Quantum Monte Carlo (CT-QMC) core with the hybridization expansion (CT-HYB) framework and improved self-energy and vertex estimators for density-density interaction, ensuring numerically exact solutions for quantum impurity problems.

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arXiv2026

Magnetic quantum criticality: The role of the Fermi surface geometry

We investigate magnetic quantum phase-transitions in bulk correlated metals. To this end, we focus on the Hubbard model on different cubic lattices as a function of temperature and electronic density, determining the relevant regimes around its quantum magnetic transition, i.e. classical, quantum critical, and quantum disordered, as well as the corresponding (thermal/non-thermal) quantum critical exponents. Our numerical results, based on dynamical mean-field theory, together with supporting analytical derivations, rigorously demonstrate how and why the presence of different kinds of Kohn anomalies on the underlying Fermi surface (i) drives the quantum critical behavior above the quantum critical point and (ii) shapes the whole phase diagram around it. Our findings highlight the importance of an explicit inclusion of such Fermi surface geometrical properties into the universality class definition for magnetic quantum phase-transitions in correlated metals.

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arXiv2026

Short-range magnetic order and multi-stage phase transitions in the easy-plane van der Waals magnet CrCl$_3$

We investigate the evolution of spin correlations and the nature of the multi-stage magnetic transition in the quasi-two-dimensional easy-plane van der Waals magnet $\rm CrCl_3$. By combining broadband ferromagnetic resonance (FMR) spectroscopy and DC SQUID magnetometry on mechanically exfoliated micro-flakes with non-local dynamical mean-field theory (DFT+DMFT) calculations, we analyze both long- and short range magnetic order in CrCl$_3$. Experimentally, SQUID and FMR measurements confirm the presence of the crossover to a spin polarized phase with the subsequent transition into an antiferromagnetic ground state upon cooling, but show robust short-range correlations at temperatures far above the magnetic ordering temperatures. Theoretically, we show the existence of highly stable local magnetic moments at room temperature, with a giant room temperature lifetime $τ$ of 130--300 ps due to a wide Mott bandgap. Below room temperature, a rapid growth of the in-plane correlation length $ξ$ signals the formation of strong short range magnetic order consistent with the experimental observations. We also obtain a temperature-driven crossover of the interlayer exchange interaction, which changes from positive (ferromagnetic) at high temperatures to negative (antiferromagnetic) in the low-temperature ordered phase.

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arXiv2026

The pseudogap in high-$T_c$ superconductors from SU(2) gauge symmetry and dynamic correlation effects

We consider the spectral properties of the two-dimensional Hubbard model, describing the electronic properties of high-$T_c$ compounds, within the SU(2) gauge theory, which assumes the separation of electronic degrees of freedom into those of spinon and chargon subsystems. We use the dynamic mean-field theory (DMFT) approach to describe magnetic long-range order in the chargon subsystem while also treating spinon fluctuations on top of this state. We show that DMFT supplemented by long-wavelength magnetic fluctuations is essential for describing the asymmetry in the damping between the inner and outer regions of the hole pockets and the resulting formation of Fermi arcs in the underdoped regime, especially at low hole doping. The underlying hole pockets in the chargon subsystem can be associated with those observed in quantum oscillation measurements.

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arXiv2026

Splitting of electronic spectrum in paramagnetic phase of itinerant ferromagnets and altermagnets

We study self-energy effects induced by strong magnetic fluctuations in the paramagnetic phase of strongly-correlated itinerant magnets within the density functional theory combined with the dynamical mean field theory (DFT+DMFT approach) and its non-local extension. We show that both local and non-local magnetic correlations yield a splitting of the electronic spectrum in the paramagnetic phase, such that it closely resembles the DFT band structure in the ordered phase. We demonstrate these effects on $α$-iron, half-metal CrO$_2$, van der Waals material CrTe$_2$, and altermagnet CrSb. Although the obtained split bands do not possess a certain spin projection, their splitting suppresses spectral weight at the Fermi level. Even when originating from local magnetic correlations, the splitting is strongly momentum dependent as a consequence of the orbital selectivity of non-quasiparticle states. The relative importance of non-local vs. local correlations depends on the proximity to half filling of $d$ states: closer to half filling, the role of local correlations increases.

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arXiv2026

Magnetic properties of monolayer, multilayer, and bulk CrTe$_2$

We investigate magnetic properties of CrTe$_2$ within the density functional theory (DFT) approach in ferromagnetic phase and combination of DFT and dynamical mean field theory (DFT+DMFT) approach in paramagnetic phase. We show that few layer CrTe$_2$ possesses well formed local magnetic moments. In the single layer CrTe$_2$ we find antiferromagnetic exchange with 120\degree antiferromagnetic structure most preferable. In the bilayer and trilayer systems electronic correlations in DFT+DMFT approach yield ferromagnetic exchange interaction within each layer, but the interaction between the layers is antiferromagnetic, such that alternation of the direction of magnetization of the layers is expected. In bulk CrTe$_2$ we find the tendency to ferromagnetic order at low temperature, but with increase of temperature antiferromagnetic correlations between the layers dominate. Determination of the critical number of layers at which the interlayer antiferromagnetic order changes to the ferromagnetic one, likely requires consideration of the non-local Coulomb interactions. Erratum: In our study [A. A. Katanin, E. M. Agapov, Phys. Rev. B 111, 035118 (2025)] we found for the monolayer CrTe$_2$ the most preferable 120\degree spin spiral structure. While this order is expected for freely suspended CrTe$_2$, for experimentally realized single layer CrTe$_2$ on a substrate, which is characterized by larger lattice constant, the momentum dependence of the susceptibilities drastically changes. In particular, the ferromagnetic ground state is expected for this compound, in agreement with the experimental data.

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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.

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arXiv2025

Exchange interactions in itinerant magnets: the effects of local particle-hole irreducible vertex corrections and SU(2) symmetry of Hund interaction

We study exchange interactions in iron and nickel within the DFT+DMFT approach with density-density and SU(2) symmetric Coulomb interaction. In particular, we analyze the representation of exchange interactions through the electron non-local Green's functions, connecting the local particle-hole (ph) irreducible interaction vertices. While the neglect of the local ph-irreducible vertex corrections, suggested previously within the dual fermion approach [E. A. Stepanov, et.al., Phys. Rev. Lett. {\bf 121}, 037204 (2018)], yields the result corresponding to the generalization of the magnetic force theorem approach, we argue that these vertex corrections are in fact less important for strong localized magnets, such as iron, but become more essential for weak itinerant magnets, such as nickel. At the same time, the overall account of the local vertex and self-energy corrections in the so-called renormalized magnetic force theorem approach is essential for iron, and less important for nickel. The difference of the results of density-density and SU(2) symmetric Coulomb interaction is found to be relatively small, in contrast to the Curie temperature and the value of the local magnetic moment.

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arXiv2025

Frequency dependence of temporal spin stiffness and short-range magnetic order in the doped two-dimensional Hubbard model

We study doping and temperature dependencies of temporal and spatial spin stiffnesses of the Hubbard model within the mean field approach for incommensurate magnetic order. We show that the frequency dependence of temporal spin stiffness within the considered mean-field approach is crucial to obtain small values of correlation length, comparable to those observed in cuprates. Using the obtained spin stiffnesses, we obtain the temperature and doping dependence of correlation length within the large-$N$ limit of the respective nonlinear sigma model. In agreement with the experimental data on La$_{2-x}$Sr$_x$CuO$_4$ we obtain short range magnetic order with relatively small correlation length at $0.1 \lesssim x\lesssim 0.2$, and magnetically ordered ground state in the narrow doping region $0.05\lesssim x \lesssim 0.1$. The latter state may correspond to the spin-frosen state, observed in the experimental data on La$_{2-x}$Sr$_x$CuO$_4$.

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arXiv2025

Two distinct quantum critical behaviors in the doped two-dimensional periodic Anderson model

We study quantum criticality in the doped two-dimensional periodic Anderson model with the hybridization acting as a tuning parameter. Employing the dynamical vertex approximation we find two distinct quantum critical behaviors. One is a quantum critical point between the antiferromagnetically ordered and the Kondo state, both metallic with itinerant $f$ electrons. Here, we obtained the critical exponent $γ\approx 1$ for the temperature dependence of the antiferromagnetic susceptibility. We observe a \emph{second} quantum critical behavior with $γ=2$ above the continuing zero-temperature magnetic order, at a quantum critical point where the $f$ electrons turn from localized to itinerant.

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arXiv2024

Magnetic properties of half metal from the paramagnetic phase: DFT+DMFT study of exchange interactions in CrO$_2$

We study magnetic properties of CrO$_2$ within the density functional theory plus dynamical mean-field theory (DFT+DMFT) approach in the paramagnetic phase. We consider the 3-orbital (per Cr site) model, containing only $t_{2g}$ states, the $5$-orbital model, including all $d$-states, as well as the model including also the oxygen $p$-states. Using the recently proposed approach of calculation of exchange interactions in paramagnetic phase, we extract exchange interaction parameters and magnon dispersions for these models. While the magnon dispersion in the 3-orbital model possesses negative branches in accordance with previous studies in ferromagnetic phase, this drawback is removed in the $5$-orbital model. The model including oxygen states (with purely local interaction at chromium sites) overestimates the exchange interactions and spin wave stiffness. While this overestimate is partly corrected by including non-local interaction between chromium and oxygen states within mean-field approximation, the $5$-orbital model appears as most adequate for describing magnetic properties of CrO$_2$ with local Coulomb interaction. The possibility of describing magnetic properties of this material starting from paramagnetic phase points to the correspondence of magnetic properties in this phase and ferromagnetic phase, as well as important contributions of double exchange in paramagnetic phase of CrO$_2$.

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arXiv2024

Local magnetic moment formation and Kondo screening in the presence of Hund exchange: the two-band Hubbard model analysis

We study formation and screening of local magnetic moments in the two-band Hubbard model in the presence of Hund exchange interaction using dynamic mean field theory approach. The characteristic temperatures of the formation, beginning and full screening of local magnetic moments are obtained from the analysis of temperature dependencies of the orbital, charge, and spin susceptibilities, as well as the respective instantaneous correlation functions. At half filling we find the phase diagram, which is similar to the single-orbital case with wide region of formation of local magnetic moments below the orbital Kondo temperature. Similarly to the single-orbital case, with decreasing temperature the screening of local magnetic moments is preceded by appearance of fermionic quasiparticles. In the two-band case the quasiparticles are however present also in some temperature region above the Mott insulating phase. At finite doping we find broad regime of presence of local magnetic moments, which coexist with incoherently or coherently moving holes. We also find, similarly to the half filled case, finite temperature interval in the doped regime, when the fermionic quasiparticles are formed, but do not yet screen local magnetic moments.

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arXiv2024

Antiferromagnetic and spin spiral correlations in the doped two-dimensional Hubbard model: gauge symmetry, Ward identities, and dynamical mean-field theory analysis

We reconsider the derivation of Ward identities for spin stiffnesses, which determine the non-linear sigma model of magnetic degrees of freedom of interacting electrons in the presence of antiferromagnetic or incommensurate correlations. We emphasize that in the approaches, which do not break explicitly spin symmetry of the action, the spatial components of gauge kernel, which is used to obtain spin stiffnesses, remain gauge invariant even in case of spontaneous spin symmetry breaking. We derive the corrected Ward identities, which account for this gauge invariance. We emphasize that the frequency dependence of temporal spin stiffnesses is not fixed by the obtained identities, and show that the infinitesimally small external staggered field is crucially important to obtain finite static uniform transverse susceptibility. On the other hand, we find that the spatial spin stiffnesses are determined by the gauge kernel of the Legendre transformed theory, which is in general {\it different} from the gauge kernel of the original theory and obtain an explicit expressions for spatial spin stiffnesses through susceptibilities and current correlation functions. We verify numerically the obtained results within dynamic mean field theory, and obtain doping dependencies of the resulting spin stiffnesses for antiferromagnetic and incommensurate phase.

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arXiv2023

Effect of vacancies on magnetic correlations and conductance in graphene nanoflakes with realistic Coulomb interaction

We study the effect of various configurations of vacancies on the magnetic properties of graphene nanoflake (GNF) with screened realistic long-range electron interaction [T. O. Wehling, et. al., Phys. Rev. Lett. 106, 236805 (2011)] within the functional renormalization group approach. In agreement with previous studies, the presence of vacancies in GNF yields to a strong enhancement of spin-density-wave (SDW) correlations. We show however that only some part of the considered configurations of vacancies posses SDW ground state. The probability of a system with a random configuration of vacancies to be in the SDW ground state increases with increase of vacancy concentration. The disorder-averaged sublattice magnetization increases linearly with the concentration of vacancies. The ratio of the sublattice magnetizations at the center and edges of GNF, averaged over various realizations of disorder, depends only weakly on the number of vacancies. The effects of vacancies on the linear conductance and charge properties of GNF are discussed.

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arXiv2023

MXene Fe2C as a promising candidate for the 2D XY ferromagnet

Monolayer Fe$_2$C is expected to possess strong electronic correlations, which can significantly contribute to electronic and magnetic properties. In this work we consider electronic and magnetic properties of MXene Fe$_2$C within the DFT+DMFT approach. We establish the existence of local magnetic moments $μ=3.2μ_B$ in this compound, characterized by sufficiently long lifetime of $τ\sim 350$ fs. We also calculate exchange interaction parameters accounting for electronic correlations using the recently developed approach for paramagnetic phase. We obtain the strongest exchange interaction $11$ meV between next nearest neighboring Fe atoms above (and below) the carbon plane, and the subleading interaction $6$ meV between the next to next nearest neighboring atoms across the carbon plane. The resulting dependence of the Berzinskii-Kosterlitz-Thouless (BKT) and Curie temperatures on magnetic anisotropy is obtained. The BKT temperature for the pristine Fe$_2$C is $T_{\rm BKT}\simeq 290$ K, which makes this compound a good candidate for the two-dimensional ferromagnet with XY anisotropy.

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arXiv2023

DFT+DMFT study of exchange interactions in cobalt and their implications for the competition of hcp and fcc phases

We reconsider magnetic properties of fcc and hcp cobalt within the density functional theory plus dynamical mean-field theory (DFT+DMFT) approach in the paramagnetic phase. Using recently proposed approach of calculation of exchange interactions in paramagnetic phase, we extract exchange interaction parameters of fcc and hcp cobalt and show that the hcp phase possesses larger spin stiffness, in agreement with the experimental data, showing stronger tendency to ferromagnetism. Accordingly, the DMFT Curie temperature of the hcp phase appears to be higher, than that of the fcc phase. The disappearance of magnetic order in the fcc phase well below the cobalt Curie temperature is expected to affect its structural stability and make this phase energetically unfavourable near the experimental Curie temperature. This may explain the "revival" of the hcp phase near the Curie temperature of cobalt, observed in the recent experimental results of perturbed angular correlation study [Sci. Rep. {\bf 12}, 10054 (2022)].

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