arXiv subjects

A. F. Barabanov

Publications and source records attributed to A. F. Barabanov.

At least 19 recordsLinked to original sources

arXiv2026

Spherically symmetric approaches in the theoretical study of low-dimensional magnets

The main ideas and some of the most important results of the spherically symmetric self-consistent approach and a number of related theoretical algorithms are presented. These methods make it possible to study low-dimensional Heisenberg-type spin models, including frustrated ones, with careful consideration of the theoretic (Mermin-Wagner and Marshall) theorems, as well as the site spin constraint. Thus, the difficulties that may arise in the traditional analysis of low-dimensional magnetic systems are avoided. The approach can also be applied to the spin-pseudospin model, and is also embedded in more complex constructions when considering spin models with free carriers, such as the basic and three-band Hubbard models, t-J and s-d models, and the Kondo lattice.

cond-mat.str-el↗

arXiv2020

Spectral characteristics of the antiferromagnetic spin-1/2 Heisenberg model on the square lattice in a magnetic field

We predict that spin-waves in an ordered square quantum antiferromagnet in a transverse magnetic field (h) may demonstrate three modes of spin excitations. Starting from the self-consistent rotation-invariant Green's function method, a new mean-field theory is constructed for h not equal to 0. The method preserves the translational and the axial symmetries, and provides exact fulfillment of the single-site constraint for each of the three modes. We examine the dynamical structure factors S(aa)(k,w), a= x, y, z. It is shown, that the introduction of h leads to the hybridization of two degenerate spin modes due to the appearance of a nondiagonal on a, b spin-spin Green's functions. The comparison of the theory with the exact diagonalization study and with results on inelastic neutron scattering experiments is discussed at T = 0. We discuss also the correspondence of the theory to the existing theories, which allow only two spin excitations modes for the total S(k,w).

cond-mat.str-el↗

arXiv2019

Magnetic spiral order in the square-lattice spin system (CuBr)Sr$_{2}$Nb$_{3}$O$_{10}$

We address quantum spin helical states in the strongly frustrated Heisenberg model. Contrary to conventional Dzyaloshinskii-Moriya approach we show that such states appear without central symmetry breaking. As an example, we demonstrate that the magnetic and thermodynamic properties of the quasi-two-dimensional square-lattice compound $\mathrm{(CuBr)Sr_{2}Nb_{3}O_{10}}$ can be interpreted within 2D $S = 1/2$ $J_1-J_2-J_3$ Heisenberg model. In this compound neutron experiment indicates helical spin order while central symmetry does hold.

cond-mat.mtrl-sci↗

arXiv2019

Continuous transformation between ferro and antiferro circular structures in $J_1-J_2-J_3$ frustrated Heisenberg model

Frustrated magnetic compounds, in particular low-dimensional, are topical research due to persistent uncover of novel nontrivial quantum states and potential applications. The problem of this field is that many important results are scattered over the localized parameter ranges, while areas in between still contain hidden interesting effects. We consider $J_1-J_2-J_3$ Heisenberg model on the square lattice and use the spherically symmetric self-consistent approach for spin-spin Green's functions in "quasielastic" approximation. We have found a new local order in spin liquids: antiferromagnetic isotropical helices. On the structure factor we see circular concentric dispersionless structures, while on any radial direction the excitation spectrum has "roton" minima. That implies nontrivial magnetic excitations and consequences in magnetic susceptibility and thermodynamics. On the $J_1-J_2-J_3$ exchange parameters globe we discover a crossover between antiferromagnetic-like local order and ferromagnetic-like; we find stripe-like order in the middle. In fact, our "quasielastic" approach allows investigation of the whole $J_1-J_2-J_3$ globe.

cond-mat.dis-nn↗

arXiv2019

Thermodynamics of symmetric spin--orbital model: One- and two-dimensional cases

The specific heat and susceptibilities for the two- and one-dimensional spin--orbital models are calculated in the framework of a spherically symmetric self-consistent approach at different temperatures and relations between the parameters of the system. It is shown that even in the absence of the long-range spin and orbital order, the system exhibits the features in the behavior of thermodynamic characteristics, which are typical of those manifesting themselves at phase transitions. Such features are attributed to the quantum entanglement of the coupled spin and orbital degrees of freedom.

cond-mat.str-el↗

arXiv2017

Coulomb repulsion of holes and competition between d_{x^2-y^2}-wave and s-wave parings in cuprate superconductors

The effect of the Coulomb repulsion of holes on the Cooper instability in an ensemble of spin-polaron quasiparticles has been analyzed, taking into account the peculiarities of the crystallographic structure of the CuO$_2$ plane, which are associated with the presence of two oxygen ions and one copper ion in the unit cell, as well as the strong spin-fermion coupling. The investigation of the possibility of implementation superconducting phases with d-wave and s-wave pairing of the order parameter symmetry has shown that in the entire doping region only the d-wave pairing satisfies the self-consistency equations, while there is no solution for the s-wave pairing. This result completely corresponds to the experimental data on cuprate HTSC. It has been demonstrated analytically that the intersite Coulomb interaction does not affect the superconducting d-wave pairing, because its Fourier transform $V_q$ does not appear in the kernel of the corresponding integral equation.

cond-mat.supr-con↗

arXiv2016

Stability of the d-wave pairing with respect to the intersite Coulomb repulsion in cuprate superconductors

Within the spin-fermion model for cuprate superconductors, the influence of the intersite Coulomb interactions $V_2$ and $V_2'$ between holes located at the next-nearest-neighbor oxygen ions of CuO$_2$ plane on the implementation of the d_{x^2-y^2}-wave pairing is studied. It is shown that d-wave pairing can be suppressed only for unphysically large values of $V_2$ and $V_2'$.

cond-mat.supr-con↗

arXiv2016

The generalized Kondo lattice model and its spin-polaron implementation for cuprates by projection method

It is shown that the spin-fermion model found to be an effective low-energy implementation of the three-band Emery model after Wannier transformation of $p_x$- and $p_y$-orbitals in the oxygen ions subsystem is reduced to the generalized Kondo lattice model. Its essential feature is the presence of spin-correlated hoppings of the current carriers between distant cells. Numerical calculations of the spin-polaron spectrum demonstrate the important role of the distant hoppings.

cond-mat.supr-con↗

arXiv2016

Stability of the superconducting $d_{x^2-y^2}$-wave pairing towards the intersite Coulomb repulsion between oxygen holes in high-T$_c$ superconductors

It is shown that an account for the space separatedness of the two-orbital subsystem of the oxygen holes and the subsystem of the localized spins of copper ions in high-T$_c$ cuprate superconductors leads to the stability of the superconducting $d_{x^2-y^2}$-wave pairing towards the strong Coulomb repulsion between holes located at the nearest oxygen ions. This effect is due to the fact that the Coulomb potential slips out of the equation for the Cooper pairing in the $d_{x^2-y^2}$-wave channel owing to the properties of symmetry.

cond-mat.supr-con↗

arXiv2015

Thermodynamic properties of the 2D frustrated Heisenberg model for the entire $J_{1}-J_{2}$ circle

Using the spherically symmetric self-consistent Green's function method, we consider thermodynamic properties of the $S=1/2$ $J_1$-$J_2$ Heisenberg model on the 2D square lattice. We calculate the temperature dependence of the spin-spin correlation functions $c_{\mathbf{r}}=\langle S_{\mathbf{0}}^{z}S_{\mathbf{r}}^{z}\rangle $, the gaps in the spin excitation spectrum, the energy $E$ and the heat capacity $C_{V}$ for the whole $J_{1}$--$J_{2}$-circle, i.e. for arbitrary $φ$, $J_1=cos(φ)$, $J_2=sin(φ)$. Due to low dimension there is no long-range order at $T\neq 0$, but the short-range holds the memory of the parent zero-temperature ordered phase (antiferromagnetic, stripe or ferromagnetic). $E(φ)$ and $C_{V}(φ)$ demonstrate extrema "above" the long-range ordered phases and in the regions of rapid short-range rearranging. Tracts of $c_{\mathbf{r}}(φ)$ lines have several nodes leading to nonmonotonic $c_{\mathbf{r}}(T)$ dependence. For any fixed $φ$ the heat capacity $C_{V}(T)$ always has maximum, tending to zero at $T\rightarrow 0$, in the narrow vicinity of $φ= 155^{\circ}$ it exhibits an additional frustration-induced low-temperature maximum. We have also found the nonmonotonic behaviour of the spin gaps at $φ=270^{\circ}\pm 0$ and exponentially small antiferromagnetic gap up to ($T\lesssim 0.5$) for $φ\gtrsim 270^{\circ}$.

cond-mat.str-el↗

arXiv2014

Elementary Excitations in the Symmetric Spin--Orbital Model

Possible types of elementary excitations in the symmetric spin-orbital model on the a square lattice are analyzed using a spherically symmetric self-consistent approach. The excitation spectra are calculated. The behavior of the corresponding correlation functions depending on the temperature and parameters of the model is studied. A schematic phase diagram is plotted. It is shown that the thermodynamics of the system is mainly determined by elementary excitations with the entangled spin and orbital degrees of freedom.

cond-mat.str-el↗

arXiv2014

d-Wave Pairing in an Ensemble of Spin Polaron Quasiparticles in the Spin-Fermion Model of the Electronic Structure of the CuO2 Plane

It is demonstrated for the first time that the strong coupling between spin moments of copper ions and oxygen holes, which arises upon hybridazation mixing of two hole subsystems in the Emery model, not only affects the formation of spin polaron quasiparticles but also ensures effective attraction between them via the exchange interaction. This results in the Cooper instability with d-wave pairing in a 2D ensemble of spin polaron quasiparticles. The T-x-phase diagram obtained using this approach agrees well with the available experimental data.

cond-mat.supr-con↗

arXiv2013

Phase transitions in 2D $J_{1}-J_{2}$ model with arbitrary signs of exchange interactions

The ground state of the $S=1/2$ $J_{1}-J_{1}$ Heisenberg model on the 2D square lattice with arbitrary signs of exchange constants is considered. States with different spin long-range order types (antiferromagnetic checkerboard, stripe, collinear ferromagnetic) as well as disordered spin-liquid states are described in the frames of one and the same analytical approach. It is shown inter alia, that the phase transition between ferromagnetic spin liquid and long-range order ferromagnet is a second-order one. On the ordered side of the transition the ferromagnetic state with rapidly varying condensate function is detected.

cond-mat.str-el↗

arXiv2009

Unconventional state with two coexisting long-range orders for frustrated Heisenberg model at quantum phase transition

For the frustrated two-dimensional $S=1/2$ antiferromagnetic Heisenberg model close to quantum phase transition we consider the singlet ground states retaining both translational and SU(2) symmetry. Besides usually discussed checkerboard, spin-liquid and stripe states an unconventional state with two coexisting long-range orders appears to be possible at sufficiently large damping of spin excitations. The problem is treated in the frames of self-consistent spherically symmetric approach.

cond-mat.str-el↗

arXiv2009

Singlet-triplet Hamiltonian for spin excitation in the Kondo-insulator

Within the framework of periodic asymmetric Anderson model for Kondo isoulators an effective singlet-triplet Hamiltonian with indirect antiferromagnetic f-f exchange interaction is introduced which allows to study analytically the dynamic magnetic susceptibilities of f-electrons. The approach allows to describe the three-level spin excitation spectrum with a specific dispersion in $YbB_{12}$. Distinctive feature of the consideration is the introduction of small radius singlet and triplet collective f-d excitations which at movement on a lattice form low - and high-energy spin bands.

cond-mat.str-el↗

arXiv2007

On the Kinetic Equation and Electrical Resistivity in Systems with Strong Spin- Hole Interaction

The problem of constructing the kinetic equation with the description of motion of a hole in systems with strong spin- hole interaction (such as high- temperature superconductors) in terms of the spin polaron has been considered in the framework of the regular antiferromagnetic $s-d$ model. It has been shown by the example of the electrical resistivity that kinetics is determined by the properties of the bands of the spin polaron (rather than "bar hole") and their quasiparticle residues $Z_{k}$. The cases of low and optimal doping of the $CuO_{2}$ plane have been considered. It has been shown that the rearrangement of the spectrum of the lower polaron band, as well as the strong doping dependence of the quasiparticle residues $Z_{k}$ is decisive in the unified consideration of these cases.

cond-mat.str-el↗

arXiv2006

Theory of Spin Susceptibility in Frustrated Layered Antiferromagnets

The self-consistent treatment of real and imaginary renormalizations in the dynamic spin susceptibility for the frustrated Heisenberg model reproduces for cuprates at low doping: a spin spectrum, a saddle point for q near (pi/2,pi/2), nearly constant q-integrated susceptibility for energy less than 150 meV and a scaling law. Frustration increase (optimally doped case) leads to a stripe scenario with a saddle point at q near (pi,pi/2) and $χ_{2D}(ω)$ peak near 30meV. The obtained $χ(\mathbf{q},ω)$ describes neutron scattering results and leads to well-known temperature transport anomalies in doped cuprates.

cond-mat.str-el↗

arXiv2003

Dispersion of the dielectric function of a charge-transfer insulator

We study the problem of dielectric response in the strong coupling regime of a charge transfer insulator. The frequency and wave number dependence of the dielectric function $ε({\bf q},ω)$ and its inverse $ε^{-1}({\bf q},ω)$ is the main object of consideration. We show that the problem, in general, cannot be reduced to a calculation within the Hubbard model, which takes into account only a restricted number of electronic states near the Fermi energy. The contribution of the rest of the system to the longitudinal response (i.e. to $ε^{-1}({\bf q},ω)$) is essential for the whole frequency range. With the use of the spectral representation of the two-particle Green's function we show that the problem may be divided into two parts: into the contributions of the weakly correlated and the Hubbard subsystems. For the latter we propose an approach that starts from the correlated paramagnetic ground state with strong antiferromagnetic fluctuations. We obtain a set of coupled equations of motion for the two-particle Green's function that may be solved by means of the projection technique. The solution is expressed by a two particle basis that includes the excitonic states with electron and hole separated at various distances. We apply our method to the multiband Hubbard (Emery) model that describes layered cuprates. We show that strongly dispersive branches exist in the excitonic spectrum of the 'minimal' Emery model ($1/U_d=U_p=t_{pp}=0$) and consider the dependence of the spectrum on finite oxygen hopping $t_{pp}$ and on-site repulsion $U_p$. The relationship of our calculations to electron energy loss spectroscopy is discussed.

cond-mat.str-el↗