arXiv subjects

P. A. Igoshev

Publications and source records attributed to P. A. Igoshev.

At least 19 recordsLinked to original sources

arXiv2026

Quantum magnetic phase transitions in a Kugel-Khomskii model including spin-orbit coupling

Using the formalism of pseudospin and isospin operators the Hamiltonian of an effective Kugel-Khomskii model with spin-orbit coupling is derived with an exact account of the $t_{2g}$ multiplet splitting by the crystal field. An analytical solution is obtained for an arbitrary relation between the Hubbard repulsion and crystal field splitting, i.e., interpolating the cases of Mott-Hubbard and charge-transfer insulators. A description of orbital orders is given in terms of octupole moments. The ground-state phase diagram is constructed in the parameter space spanned by spin-orbit coupling, Hund's exchange, and Hubbard interaction. We investigate a quantum phase transition between a state exhibiting hidden magnetic and orbital long-range order and a ferromagnetic state with a reduced magnetic moment accompanied by antiferroorbital order. It is shown that the cooperative effect of Hund's and spin-orbit interactions gives rise to an easy-plane-type anisotropy.

cond-mat.str-el↗

arXiv2025

Itinerant electron metamagnetism for lattices with van Hove density-of-states singularities near the Fermi level

Itinerant-electron metamagnetism is investigated within the Hubbard model for various lattices having van Hove singularities (vHS) in the electronic spectrum: face-centered cubic and orthorhombic lattices. The remarkable itinerant-electron metamagnetic transition occurs provided that the Fermi level is in the region with a strong positive curvature of the density of electron states typically positioned between two close van Hove singularities. Orthorhombic distortion of a~tetragonal lattice is a promising mechanism for generating two closely split vHS with strong density-of-states curvature between them. A phase diagram in terms of electron filling and Hubbard interaction parameter is presented, which shows the paramagnetic-metamagnetic-ferromagnetic phase transition and regions of saturated and non-saturated magnetism. The standard Landau theory expansion based on~the~electron density of states in the vicinity of the Fermi level is demonstrated to be insufficient to describe the whole magnetic phase diagram including the itinerant-electron metamagnetic transition.

cond-mat.str-el↗

arXiv2023

Ferromagnetic instability in itinerant fcc lattice electron systems with higher order van Hove singularities: Functional renormalization group study

We investigate the possibility of ferromagnetic ordering in the non-degenerate Hubbard model on the face-centered cubic lattice within the functional renormalization group technique using temperature as a scale parameter. We assume the relations between nearest, next-nearest, and next-next-nearest hopping parameters providing higher order (giant) van Hove singularity of the density of states. The ferromagnetic instability formation with lowering temperature is described consistently in the one-loop approximation for a one-particle irreducible vertex of two-particle electron interaction. The chemical potential versus temperature phase diagrams are calculated. We find ferromagnetic order only for sufficiently strong divergence of the density of states and fillings in the vicinity of van Hove singularity. The obtained Curie temperature is more than an order of magnitude smaller than the results of the random-phase approximation. The main origin of the suppression of ferromagnetism is the screening of interaction in the particle-particle channel. We also do not find the pronounced tendency towards incommensurate order when the Fermi level is moved away from a van Hove singularity, such that the first order quantum phase transitions from the ferro- to paramagnetic phase are obtained.

cond-mat.str-el↗

arXiv2022

Giant density-of-states van Hove singularities in the face-centered cubic lattice

All van Hove singularities in the density of states (DOS) of face-centered cubic lattice in the nearest and next-nearest neighbour approximation, focusing on higher-order ones, are found and classified. At special values of the ratio $τ$ of nearest and next-nearest neighbour hopping integrals, $t$ and $t'$, giant DOS singularities, caused by van Hove lines or surfaces, are formed. An exact formula for DOS which provides efficient numerical implementation is proposed. The standard tetrahedron method is demonstrated to be inapplicable due to its poor convergence in the vicinity of kinks caused by van Hove singularities. A comparison with the case of large space dimensionality (infinite coordination number) including next-nearest neighbours is performed.

cond-mat.str-el↗

arXiv2022

Electron Spectrum Topology and Giant Density-of-States Singularities in Cubic Lattices

The topology of isoenergetic surfaces in reciprocal space for simple (sc), body-centered (bcc), and face-centered (fcc) cubic lattices is investigated in detail in the tight-binding approximation, taking into account the transfer integrals between the nearest and next neighbors $t$ and $t'$. It is shown that, for values $τ= t'/t = τ_\ast$ corresponding to a change in the topology of surfaces, lines and surfaces of $\mathbf k$-van Hove points can be formed. With a small deviation of $τ$ from these singular values, the spectrum in the vicinity of the van Hove line (surface) is replaced by a weak dependence on $\mathbf k$ in the vicinity of several van Hove points that have a giant mass proportional to $|τ- τ_ \ast|^{-1}$. Singular contributions to the density of states near peculiar $τ$ values are considered; analytical expressions for the density of states being obtained in terms of elliptic integrals. It is shown that in a number of cases the maximum value of the density of states is achieved at energies corresponding not to $\mathbf{k}$-points on the Brillouin zone edges, but to its internal points in highly symmetrical directions. The corresponding contributions to electron and magnetic properties are treated, in particular, in application to weak itinerant magnets.

cond-mat.str-el↗

arXiv2021

Giant van Hove Density of States Singularities and Anomalies of Electron and Magnetic Properties in Cubic Lattices

Densities of states for simple (sc) and base-centered (bcc) cubic lattices with account of nearest and next-nearest neighbour hopping integrals $t$ and $t'$ are investigated in detail. It is shown that at values of $τ\equiv t'/t = τ_\ast$, corresponding to the change of isoenergetic surface topology, the formation of van Hove $\bf k$ lines takes place. At small deviation from these special values, the weakly dispersive spectrum in the vicinity of van Hove lines is replaced by a weak $\bf k$-dependence in the vicinity of few van Hove points which possess huge masses proportional to $|τ- τ_\ast|^{-1}$. The singular contributions to the density of states originating from van Hove points and lines are considered, as well as the change in the topology of isoenergetic surfaces in the $\bf k$-space with the variation of $τ$. Closed analytical expressions for density of states as a function of energy and $τ$ in terms of elliptic integrals, and power-law asymptotics at $τ= τ_\ast$ are obtained. Besides the case of sc lattice with small $τ$ (maximum of density of states corresponds to energy level of X $\bf k$-point), maximal value of the density of states is always achieved at energies corresponding to \textit{inner} $\bf k$-points of the Brillouin zone positioned in high-symmetry directions, and not at zone faces.

cond-mat.mtrl-sci↗

arXiv2021

Emerging mechanisms of magnetocaloric effect in phase-separated metals

We present a study of the magnetocaloric effect in metallic systems exhibiting first-order magnetic transitions and focus on consequences of magnetic phase separation. We account for ferrimagnetic, ferromagnetic, and Neel antiferromagnetic order. Based on the archetypal Hubbard model being treated within the mean-field approximation, we provide and explore its implications on the field-induced entropy change in metallic system with phase separation. Chosen framework allows us to properly analyze phase volumes' dependence on parameters of phase-separated (PS) system. Moreover, an account for phase separation boundaries as functions of magnetic field provides a natural splitting of the PS region, where each subregion corresponds to a different temperature dependence of entropy change: moving from one subregion to the other produces a kink, followed by a strong linear growth of entropy change. We encounter a second-order magnetic transition from paramagnetic to antiferromagnetic phase in PS region that occurs for particular parameter values. Despite the fact that both phases have zero total magnetization, the transition has a strong impact on entropy change.

cond-mat.str-el↗

arXiv2021

Incommensurate magnetic order in rare earth and transition metal compounds with local moments

Within the framework of the $s$-$d(f)$ exchange model in the mean-field approximation for square, simple cubic, body-centered and face-centered cubic lattices, the formation of a ferromagnetic, spiral, and commensurate antiferromagnetic order is investigated. The possibility of the formation of inhomogeneous states (magnetic phase separation), which necessarily arises during first-order phase transitions in the electron filling parameter, is taken into account. The saturation of the antiferromagnetic and spiral states is studied depending on the parameters of the model. The results obtained include a rich variety of magnetic structures and phase transitions, allowing the interpretation of magnetic properties of semiconducting and metallic systems containing magnetic atoms.

cond-mat.str-el↗

arXiv2021

Metal-insulator transition and antiferromagnetism in the generalized Hubbard model: Treatment of correlation effects

The ground state for the half-filled $t-t'$ Hubbard model is treated within the Hartree-Fock approximation and the slave boson approach including correlations. The criterium for the metal-insulator transition in the Slater scenario is formulated using an analytical free-energy expansion in the next-nearest-neighbor transfer integral $t'$ and in direct antiferromagnetic gap $Δ$. The correlation effects are generally demonstrated to favor the first-order transition. For a square lattice with a strong van Hove singularity, accidental close degeneracy of antiferromagnetic and paramagnetic phases is analytically found in a wide parameter region. As a result, there exists an interval of $t'$ values for which the metal-insulator transition is of the first order due to the existence of the van Hove singularity. This interval is very sensitive to model parameters (direct exchange integral) or external parameters. For the simple and body-centered cubic lattices, the transition from the insulator antiferromagnetic state with increasing $t'$ occurs to the phase of an antiferromagnetic metal and is a second-order transition which is followed by a transition to a paramagnetic metal. These results are quantitatively modified when taking into account the intersite Heisenberg interaction, which can induce first-order transitions. A comparison with the Monte Carlo results is performed.

cond-mat.str-el↗

arXiv2019

Investigation of magnetocaloric effect: Stoner approximation vs DMFT

A comparative study of the magnetocaloric effect (MCE) in metals within the single-band Hubbard model on the face-centered cubic (fcc) lattice using both mean-field (Stoner) approximation (MFA) and dynamical mean-field theory (DMFT) is done. The MCE is investigated in the case of second order magnetic phase transition from ferromagnet to paramagnet. To ensure presence of itinerant ferromagnetism in the Hubbard model the special case of spectrum parameters generating giant van Hove singularity at the bottom of the band is considered, while the Fermi level $E_{\rm f}$ is in the vinicity of the band bottom. To compare MCE within MFA and DMFT temperature dependence of magnetization, total energy and finally entropy for a set of Coulomb interactions $U$ at zero and finite values of magnetic field $h$ for both methods were performed. Also one of the MCE potentials, isothermal entropy change, as a function of temperature $ΔS (T)$ for both MFA and DMFT is calculated. In the MFA, the expected maximum value of $ΔS (T)$ at the Curie temperature $T_C$ ($ΔS_{\rm max}$) quite significantly decreases while $U$ grows. Similar but much weaker decreasing of $ΔS_{\rm max}$ is found for DMFT results. The account of local quantum fluctuations results in larger values of $ΔS_{\rm max}$ within DMFT than within MFA. A peak width of $ΔS (T)$ at half height is approximately the same for both methods. Another effect of DMFT local quantum fluctuations is the destruction of anomalous Curie temperature $T_C$ dependence on $U$ present in MFA, which is invoked by an effect of giant van Hove singularity. However the relative cooling power (RCP) is very close in DMFT and MFA for the same model parameters and goes down upon $U$ increase.

cond-mat.str-el↗

arXiv2019

Electron states and magnetic phase diagrams of strongly correlated systems

Various auxiliary-particle approaches to treat electron correlations in many-electron models are analyzed. Applications to copper-oxide layered systems are discussed. The ground-state magnetic phase diagrams are considered within the Hubbard and $s$-$d$ exchange (Kondo) models for square and simple cubic lattices vs. band filling and interaction parameter. A generalized Hartree-Fock approximation is employed to treat commensurate ferro-, antiferromagnetic, and incommensurate (spiral) magnetic phases, and also magnetic phase separation. The correlations are taken into account within the Hubbard model by using the slave-boson approach. The main advantage of this approach is correct estimating the contribution of doubly occupied states number and therefore the paramagnetic phase energy.

cond-mat.str-el↗

arXiv2017

Kohn anomalies in momentum dependence of magnetic susceptibility of some three-dimensional systems

We study a question of presence of Kohn points, yielding at low temperatures non-analytic momentum dependence of magnetic susceptibility near its maximum, in electronic spectum of some three-dimensional systems. In particular, we consider one-band model on face centered cubic lattice with hopping between nearest and next-nearest neighbors, which models some aspects of the dispersion of ZrZn$_2$, and the two-band model on body centered cubic lattice, modeling the dispersion of chromium. For the former model it is shown that Kohn points yielding maxima of susceptibility exist in a certain (sufficiently wide) region of electronic concentrations; the dependence of the wave vectors, corresponding to the maxima, on the chemical potential is investigated. For the two-band model we show existence of the lines of Kohn points, yielding maximum of the susceptibility, which position agrees with the results of band structure calculations and experimental data on the wave vector of antiferromagnetism of chromium.

cond-mat.str-el↗

arXiv2017

Magnetic phase transitions and unusual antiferromagnetic states in the Hubbard model

Ground state magnetic phase diagrams of the square and simple cubic lattices are investigated for the narrow band Hubbard model within the slave-boson approach by Kotliar and Ruckenstein. The transitions between saturated (half-metallic) and non-saturated ferromagnetic phases as well as similar transition in antiferromagnetic (AFM) state are considered in the three-dimensional case. Two types of saturated antiferromagnetic state with different concentration dependences of sublattice magnetization are found in the two-dimensional case in the vicinity of half-filling: the state with a gap between AFM subbands and AFM state with large electron mass. The latter state is hidden by the phase separation in the finite-U case.

cond-mat.str-el↗

arXiv2016

Spiral magnetic order, non-uniform states and electron correlations in the conducting transition metal systems

The ground-state magnetic phase diagram is calculated within the Hubbard and $s$-$d$ exchange (Kondo) models for square and simple cubic lattices vs. band filling and interaction parameter. The difference of the results owing to the presence of localized moments in the latter model is discussed. We employ a generalized Hartree-Fock approximation (HFA) to treat commensurate ferromagnetic (FM), antiferromagnetic (AFM), and incommensurate (spiral) magnetic phases. The electron correlations are taken into account within the Hubbard model by using the Kotliar-Ruckenstein slave boson approximation (SBA). The main advantage of this approach is a correct qualitative description of the paramagnetic phase: its energy becomes considerably lower as compared with HFA, and the gain in the energy of magnetic phases is substantially reduced.

cond-mat.str-el↗

arXiv2016

Magnetic States, Correlation Effects and Metal-Insulator Transition in FCC Lattice

The ground-state magnetic phase diagram (including collinear and spiral states) of the single-band Hubbard model for the face-centered cubic lattice and related metal-insulator transition (MIT) are investigated within the slave-boson approach by Kotliar and Ruckenstein. The correlation induced electronic spectrum narrowing and a comparison with a generalized Hartree-Fock approximation allow one to estimate the strength of correlation effects. This, as well as the MIT scenario, depends dramatically on the ratio of the next-nearest and nearest electron hopping integrals $t'/t$. In contrast with metallic state, possessing strong band narrowing, insulator one is only weakly correlated. The magnetic (Slater) scenario of MIT is found to be superior over the Mott one. Unlike simple and body-centered cubic lattices, MIT is the first order transition for most $t'/t$. The insulator state is type-II or type-III antiferromagnet, and the metallic state is spin-spiral, collinear antiferromagnet or paramagnet depending on $t'/t$. The picture of magnetic ordering is compared with that in the standard localized-electron (Heisenberg) model.

cond-mat.str-el↗

arXiv2016

Metal-Insulator Transition in the Hubbard Model: Correlations and Spiral Magnetic Structures

The metal--insulator transition (MIT) for the square, simple cubic, and body-centered cubic lattices is investigated within the $t-t'$ Hubbard model at half-filling by using both the Hartree-Fock approximation (HFA) generalized for the case of spiral order and Kotliar-Ruckenstein slave-boson approach. It turns out that magnetic scenario of MIT becomes superior over non-magnetic one. The electron correlations lead to some suppression of the spiral phases in comparison with HFA. We found the presence of metallic antiferromagnetic (spiral) phase in the case of three-dimensional lattices.

cond-mat.str-el↗

arXiv2015

Spiral magnetism in the single-band Hubbard model: the Hartree-Fock and slave-boson approaches

The ground-state magnetic phase diagram is investigated within the single-band Hubbard model for square and different cubic lattices. The results of employing the generalized non-correlated mean-field (Hartree-Fock) approximation and generalized slave-boson approach by Kotliar and Ruckenstein with correlation effects included are compared. We take into account commensurate ferromagnetic, antiferromagnetic, and incommensurate (spiral) magnetic phases, as well as phase separation into magnetic phases of different types, which was often lacking in previous investigations. It is found that the spiral states and especially ferromagnetism are generally strongly suppressed up to non-realistically large Hubbard $U$ by the correlation effects if nesting is absent and van Hove singularities are well away from the paramagnetic phase Fermi level. The magnetic phase separation plays an important role in the formation of magnetic states, the corresponding phase regions being especially wide in the vicinity of half-filling. The details of non-collinear and collinear magnetic ordering for different cubic lattices are discussed.

cond-mat.str-el↗

arXiv2015

Correlation Effects and Non-Collinear Magnetism in the Doped Hubbard Model

The ground--state magnetic phase diagram is investigated for the two-- and three--dimensional $t$--$t'$ Hubbard model. We take into account commensurate ferro--, antiferromagnetic, and incommensurate (spiral) magnetic phases, as well as phase separation into magnetic phases of different types, which was often missed in previous investigations. We trace the influence of correlation effects on the stability of both spiral and collinear magnetic order by comparing the results of employing both the generalized non-correlated mean--field (Hartree--Fock) approximation and generalized slave boson approach by Kotliar and Ruckenstein with correlation effects included. We found that the spiral states and especially ferromagnetism are generally strongly suppressed up to non-realistic large Hubbard $U$, if the correlation effects are taken into account. The electronic phase separation plays an important role in the formation of magnetic states and corresponding regions are wide, especially in the vicinity of half--filling. The details of magnetic ordering for different cubic lattices are discussed.

cond-mat.str-el↗