arXiv subjects

S. G. Ovchinnikov

Publications and source records attributed to S. G. Ovchinnikov.

At least 19 recordsLinked to original sources

arXiv2026

Doping- and temperature-dependent electronic structure and spin dynamics in the Hubbard model on a square lattice within cluster perturbation theory

The doping and temperature dependencies of the electronic structure and the dynamic spin susceptibility of the Hubbard model on a square lattice are studied within cluster perturbation theory (CPT). The most general features of both electronic and spin spectra qualitatively agree with the experimental data on cuprates. Generalized mean-field calculations of the electronic structure using static short-range magnetic correlations from CPT are also implemented and compared with the results of CPT, allowing us to discuss the role of short-range antiferromagnetism in the formation of the pseudogap.

cond-mat.str-el↗

arXiv2025

Diagonal superexchange in a simple square CuO$_2$ lattice

Many microscopic models with the interaction between the next-nearest neighbours as a key parameter for cuprate physics have inspired us to study the diagonal superexchange interaction in a CuO$_2$ layer. Our investigation shows that models with extended hopping provide a correct representation of magnetic interactions only in a hypothetical square CuO$_2$ layer, where the diagonal superexchange interaction with the next-nearest neighbors always has the AFM nature. The conclusions are based on the symmetry prohibition on FM contribution to the diagonal superexchange between the next-nearest neighbors for a simple square CuO$_2$ layer rather than for a real CuO$_2$ layer, where diagonal AFM superexchange may be overestimated. We also discuss the reasons for magnetic frustration effects and high sensitivity of spin nanoinhomogeneity to square symmetry breaking.

cond-mat.str-el↗

arXiv2025

Pseudogap in the lightly hole-doped triangular-lattice moiré Hubbard model

The electronic structure of the lightly hole-doped triangular-lattice moiré Hubbard model is studied within cluster perturbation theory (CPT) using 13-site clusters for a fixed doping concentration $p=1/13$ varying the Coulomb parameter $U$ and the hopping phase parameter $ϕ$ related to the spin-orbital interaction. We have also developed a rather simple generalized mean-field approximation (GMFA) containing the amplitude of the spin correlations as a free parameter to fit the CPT data.The evolution of the Fermi surface and the pseudogap with the parameters $ϕ$ and $U$ is explained from the viewpoint of the short-range magnetic order. The geometric frustration and the additional model parameter related to the spin-orbital interaction result in a more rich physics of the pseudogap state compared to the case of a more conventional square lattice.

cond-mat.str-el↗

arXiv2025

Evolution of the pseudogap band structure in a system of electron-correlated lattice polarons

The evolution of the role of lattice vibrations in the formation of the pseudogap state in strongly correlated electron systems has been investigated concerning changes in the electron-phonon coupling parameters and the concentration of doped charge carriers. We apply the polaronic version of the generalized tight-binding method to analyze the band structure of a realistic multiband two-dimensional model that incorporates the electron-lattice contributions of both Holstein and Peierls types. It has been demonstrated that the emergence of polaronic effects begins with the modulation of spectral function intensity. However, within a specific region of the phase diagram, a significant transformation of the electron band structure and pseudogap state occurs. It results from coherent polaron excitations that create a partially flat band near the Fermi level. This process leads to a change in the topology of the Fermi surface and the emergence of corresponding features in the density of states.

cond-mat.str-el↗

arXiv2025

Non-monotonic dependence of $T_c$ on the c axis compression in the HTSC cuprate La$_{2-x}$Sr$_x$CuO$_4$

The effect of the the $c$ axis compression on the electronic structure and superconducting properties of the HTSC cuprate La$_{2-x}$Sr$_x$CuO$_4$ at different doping is investigated. The electronic structure of quasiparticle excitations is obtained within the effective five-band Hubbard model using the equation of motion method for Green's functions builded on the Hubbard operators. The superconducting gap and ${T_c}$ are calculated taking into account the exchange pairing mechanism involving not only the Zhang-Rice singlet but also excited two-hole triplet and singlet states. The energy of the $a_{1g}$ orbitals increases with increasing compression and the $a_{1g}$ quasiparticle bands begin to strongly interact with the $b_{1g}$ bands at the top of the valence band. The reconstruction of the region of states determining the superconducting properties results in the high density of states near the Fermi level. This mechanism leads to an increase in $T_c$ in the underdoped region with increasing compression. The pairing constants renormalizations under compression results in ${T_c}$ decreasing. The competition of these two effects leads to non-monotonic behavior of $T_c$ under the $c$-axis compression near optimal doping.

cond-mat.str-el↗

arXiv2025

Electronic structure of the CuO monolayer in the paramagnetic phase considering the Coulomb interactions

The electronic structure of the CuO monolayer is investigated taking into account the intra- and interatomic Coulomb interactions on copper and oxygen atoms. Local Coulomb interactions and covalence effects are treated exactly when constructing quasiparticle excitations using the generalized tight-binding method (GTB). The electronic system is described in the hole representation within the eight-band p-d model including long-range hoppings up to four nearest neighbors with parameters obtained from calculations using the density functional theory. Using the multiband Hubbard model, we calculated the dependencies of the band structure of quasiparticle excitations, Fermi surfaces, constant energy maps at the top of the valence band, band gap values and contributions of different orbitals to states at the top of the valence band in the regime of strong intra-atomic Coulomb interactions on copper for different values of the intra-atomic interaction on oxygen $U_p$ and the interatomic copper-oxygen interaction $V_{pd}$. It is shown that the system goes from the insulating state with d-states at the top of the valence band to the metallic state in which the main contribution to low-energy excitations is made by oxygen orbitals depending on the values of the parameters $U_p$ and $V_{pd}$.

cond-mat.str-el↗

arXiv2025

Dependence of the energy and orbital structure of local states in CuO monolayer on Coulomb parameters

The dependence of the energies and orbital structure of local states in the CuO monolayer on intra- and interatomic Coulomb interactions on copper and oxygen orbitals is studied. The electronic system is described within the eight-band p-d model in the hole representation with the on-site energies and hopping integrals obtained using density functional theory. CuO cluster multiparticle eigenstates are calculated using exact diagonalization. The difference between the energy dependencies on the Coulomb parameters for the states with the predominant probability density on the d-orbital and the states in which hole occupies p-orbitals leads to crossover of d- and p-states. The ground single-hole and two-hole states which determine the electronic structure of the low-energy excitations have the character of d- or p-orbitals in the different regions of the Coulomb parameters space. The gap between the energies of the dispersionless quasiparticles forming the top of the valence band and conductivity band also have different values in these two regions. The magnitude of this gap and the orbital character of the local multiparticle states change sharply even with an insignificant change in the Coulomb interactions within the boundary region of parameters between the regions in which the local states are formed by the d- or p-orbitals.

cond-mat.str-el↗

arXiv2024

Electronic structure and minimal models for flat and corrugated CuO monolayers: an ab initio study

$\mathrm{CuO}$ atomic thin monolayer ($\mathrm{mlCuO}$) was synthesized recently. Interest in the $\mathrm{mlCuO}$ is based on its close relation to $\mathrm{CuO_2}$ layers in typical high temperature cuprate superconductors. Here, we present the calculation of the band structure, the density of states and the Fermi surface of the flat $\mathrm{mlCuO}$ as well as the corrugated $\mathrm{mlCuO}$ within the density functional theory (DFT) in the generalized gradient approximation (GGA). In the flat $\mathrm{mlCuO}$, the $\mathrm{Cu}$-$3d_{x^2-y^2}$ band crosses the Fermi level, while the $\mathrm{Cu}$-$3d_{xz,yz}$ hybridized band is located just below it. The corrugation leads to a significant shift of the $\mathrm{Cu}$-$3d_{xz,yz}$ hybridized band down in energy and a degeneracy lifting for the $\mathrm{Cu}$-$3d_{x^2-y^2}$ bands. Corrugated $\mathrm{mlCuO}$ is more energetically favorable than the flat one. In addition, we compared the electronic structure of the considered $\mathrm{CuO}$ monolayers with bulk $\mathrm{CuO}$ systems. We also investigated the influence of a crystal lattice strain (which might occur on some interfaces) on the electronic structure of both $\mathrm{mlCuO}$ and determined the critical strains of topological Lifshitz transitions. Finally, we proposed a number of different minimal models for the flat and the corrugated $\mathrm{mlCuO}$ using projections onto different Wannier functions basis sets and obtained the corresponding Hamiltonian matrix elements in a real space.

cond-mat.supr-con↗

arXiv2023

One- and two-particle correlation functions in the cluster perturbation theory for cuprates

Physics of high-$T_c$ superconducting cuprates is obscured by the effect of strong electronic correlations. One way to overcome the problem is to seek for an exact solution at least within the small cluster and expand it to the whole crystal. Such an approach is in the heart of the cluster perturbation theory (CPT). Here we develop CPT for the dynamic spin and charge susceptibilities (spin-CPT and charge-CPT), within which the correlation effects are explicitly taken into account by the exact diagonalization. We apply spin-CPT and charge-CPT to the effective two-band Hubbard model for the cuprates obtained from the three-band Emery model and calculate one- and two-particle correlation functions, namely, spectral function and spin and charge susceptibilities. Doping dependence of the spin susceptibility was studied within spin-CPT and CPT-RPA that is the CPT generalization of the random phase approximation (RPA). Both methods produce the low energy response at four incommensurate wave vectors in qualitative agreement to the results of the inelastic neutron scattering on overdoped cuprates.

cond-mat.str-el↗

arXiv2023

Orthogonal magnetic structures of Fe4O5:representation analysis and DFT calculations

The magnetic and electronic structures of Fe4O5 have been investigated at ambient and high pressures via a combination of representation analysis, density functional theory (DFT+U) calculations, and Mössbauer spectroscopy. A few spin configurations corresponding to the different irreducible representations have been considered. The total-energy calculations reveal that the magnetic ground state of Fe4O5 corresponds to an orthogonal spin order. Depending on the magnetic propagation vector k two spin ordered phases with minimal energy differences are realized. The lowest energy magnetic phase is related to k = (0, 0, 0) and is characterized by the ferromagnetic ordering of the iron magnetic moments at prismatic sites along the b axis and antiferromagnetic ordering of iron moments at octahedral sites along the c axis. For the k = (1/2, 0, 0) phase, the moments in the prisms are antiferromagnetically ordered along the b axis and the moments in the octahedra are still antiferromagnetically ordered along the c axis. Under high pressure, the Fe4O5 exhibits magnetic transitions with corresponding electronic transitions of the metal-insulator type. At a critical pressure PC ~ 60 GPa the Fe ions at the octahedral sites undergo a high-spin to low-spin state crossover with a decrease in the unit-cell volume of ~ 4%, while the Fe ions at the prismatic sites remain in the high-spin state up to 130 GPa. This site-dependent magnetic collapse is experimentally observed in the transformation of Mössbauer spectra measured at room temperature and high pressures.

cond-mat.mtrl-sci↗

arXiv2022

Light-induced ultrafast dynamics of spin crossovers in LaCoO3

Ultrafast quantum dynamics relaxation of a photoexcited state in a strongly correlated spin crossover system LaCoO3 under a sudden perturbation is considered with the density matrix generalized master equation. The magnetization and cobalt-oxygen bond length oscillations were found. The evolution of the electronic band structure during relaxation is calculated in the framework of the LDA+GTB method.

cond-mat.str-el↗

arXiv2022

Magnetic collapse in Fe$_3$Se$_4$ under high pressure

Electronic structure and magnetic properties of Fe$_3$Se$_4$ are calculated using the density functional approach. Due to the metallic properties, magnetic moments of the iron atoms in two nonequivalent positions in the unit cell are different from ionic values for Fe$^{3+}$ and Fe$^{2+}$ and are equal to $M_1=2.071 μ_B$ and $M_2=-2.042 μ_B$, making the system ferrimagnetic. The total magnetic moment for the unit cell is $2.135 μ_B$. Under isotropic compression, the total magnetic moment decreases non-monotonically and correlates with the non-monotonic dependence of the density of states at the Fermi level $N(E_F)$. For 7% compression, the magnetic order changes from the ferrimagnetic to the ferromagnetic. At 14% compression, the magnetic order disappears and the total magnetic moment becomes zero, leaving the system in a paramagnetic state. This compression corresponds to the pressure of 114 GPa. The magnetic ordering changes faster upon application of an isotropic external pressure due to the sizeable anisotropy of the chemical bondings in Fe$_3$Se$_4$. The ferrimagnetic and paramagnetic states occur under pressures of 5.0 and 8.0 GPa, respectively. The system remains in the metallic state for all values of compression.

cond-mat.mtrl-sci↗

arXiv2022

Excitonic ordering in strongly correlated spin crossover systems: induced magnetism and excitonic excitation spectrum

The effects associated with interatomic hoppings of excitons and the excitonic Bose condensate formation in the strongly correlated spin crossover systems are considered in the framework of the effective Hamiltonian for the two-band Kanamori model. The appearance of antiferromagnetic ordering due to the exciton order is found even in the absence of interatomic exchange interaction. The spectrum of excitonic excitations is calculated at various points of the "temperature vs. crystal field" phase diagram. Outside the region of exciton ordering, the spectrum has a gap, which vanishes at the boundary of the exciton condensate phase. The non-uniform spectral weight distribution over the Brillouin zone is found. The role of electron-phonon interaction is discussed as well.

cond-mat.str-el↗

arXiv2021

The enhanced s*-wave component of the superconducting gap in the overdoped HTSC cuprates with orthorhombic distortion

In this work, the concentration and temperature dependences of the superconducting gap of the HTSC cuprate in the orthorhombic phase are obtained within the framework of the Hubbard model taking into account the exchange mechanism of pairing. The enhanced s*-wave component of the superconducting gap against the background of the prevailing d-wave component in a narrow range of hole concentrations in the overdoped region is found. To elucidate the reasons for such unusual behavior the electronic structure of the low-energy excitations and the structure of the contributions of the pair states to the superconducting gap in momentum space were investigated at different doping. It is shown that the presence of a shallow pocket in the region of the maximum value of the s*-wave component of the gap results in different symmetry of the superconducting gap in different regions of the k-space and an increase in the s*-wave component.

cond-mat.supr-con↗

arXiv2021

Phonon-assisted insulator-metal transitions in correlated systems driven by doping

We consider how electron-phonon interaction influences the insulator-metal transitions driven by doping in the strongly correlated system. Using the polaronic version of the generalized tight-binding method, we investigate a multiband two-dimensional model taking into account both Holstein and Su-Schrieffer-Heeger types of electron-lattice contributions. For adiabatic ratio of the hopping parameter and the phonon field energy, different types of band structure evolution are observed in a wide electron-phonon parameter range. We demonstrate the relationship between transition features and such properties of the system as the polaron and bipolaron crossovers, pseudogap behavior of various origin, orbital selectivity, and the redistribution of the spectral weight due to the electron-phonon interaction.

cond-mat.str-el↗

arXiv2020

Spin state crossover in Co3BO5

The magnetic contribution of the Co3+ ions in Co3BO5 has been investigated using the Co (K-edge) XMCD, dc magnetic susceptibility, and heat capacity measurements. The crystal structure of Co3BO5 single crystal has been solved in detail at the T range 296-703 K. The results have been supplemented by the GGA+U calculations.

cond-mat.mtrl-sci↗

arXiv2020

Polaron transformations in the realistic model of the strongly correlated electron system

Electron-phonon coupling, diagonal in a real space formulation, leads to polaron paradigm of smoothly varying properties. However, fundamental changes, namely the singular behavior of polarons, occur if non-diagonal pairing is involved into consideration. The study of polaron transformations and related properties of matter is of particular interest for realistic models, since competition between diagonal and non-diagonal electron-phonon contributions in the presence of other strong interactions can result in unconventional behavior of the system. Here we consider the multiband pd-model of cuprate superconductors with electron-phonon interaction and analyze the features of the systems that are caused by the competition of diagonal and non-diagonal electron-phonon contributions in the limit of strong electron correlations. Using the polaronic version of the generalized tight-binding method, we describe the evolution of the band structure, Fermi surface, density of states at Fermi level, and phonon spectral function in the space of electron-phonon parameters ranging from weak to strong coupling strength of the adiabatic limit. On the phase diagram of polaron properties we reveal two quantum phase transitions and show how electron-phonon interaction gives rise to Fermi surface transformation (i) from hole pockets to Fermi arcs and (ii) from hole to electron type of conductivity. We also demonstrate the emergence of new states in the phonon spectral function of the polaron and discuss their origin.

cond-mat.str-el↗

arXiv2020

Ultrafast dynamics of light-induced spin crossovers under high pressure

Within the multielectron model of magnetic insulator with spin crossover under high pressure we have studied the mean field phase diagrams in pressure temperature plane and dynamics of a sudden excited non equilibrium spin state. We obtain the different relaxation of the magnetization, high spin/low spin occupation numbers, and the metal oxygen bond length for different values of the external pressure. We found the long living oscillations of magnetization without pressure and at small external pressure. Close to crossover pressure the smooth relaxation is accompanied with a set of sharp strongly non linear oscillations of magnetization and HS/LS occupation numbers that are accompanied by the Franck Condon resonances.

cond-mat.str-el↗