arXiv subjects

A. K. Arzhnikov

Publications and source records attributed to A. K. Arzhnikov.

At least 19 recordsLinked to original sources

arXiv2025

Effect of thermal fluctuations on topological crossover in the chiral d+id superconducting phase

The effect of thermal fluctuations on the temperature dependence of the topological index C1 of the chiral d+id superconducting phase of a two-dimensional single-band model on a triangular lattice is investigated. Thermal fluctuations are taken into account within the framework of the self-consistent functional-integral theory. It is established that when the nodal points are located far inside (outside) the Fermi contour of the normal phase, thermal fluctuations expand the relative temperature ranges in which the values of the topological index are close to integer values C1=4(-2). This expansion depends both on the value of the topological index and on the magnitude of the effective attraction between the electrons. However, as the nodal points approach the Fermi contour, topological crossovers to new C1 values are observed, which can persist over a wide temperature range. The nature and degree of influence of thermal fluctuations on these crossovers are established. It is assumed that the observed effects may also manifest in the edge state behavior of a similar system with open boundaries.

cond-mat.supr-con↗

arXiv2023

Effect of thermal fuctuations on the nontrivial topology of the d+id superconducting phase

The behavior of the topological index, characterizing the properties of superconducting phases of quasi-two-dimensional systems with nontrivial topology, is investigated depending on the temperature and parameters of the effective non-Hermitian Hamiltonian. For this purpose, a method of calculating the topological index, based on a self-consistent functional-integral theory, is proposed. The method makes it possible to take into account thermal fluctuations and study the behavior of the topological index as a function of temperature and Hamiltonian parameters. The chiral d+id superconducting phase of a quasi-two-dimensional model with effective attraction between the electrons located at the nearest sites of a triangular lattice is considered. It is shown that the characteristic features in the energy dependence of the self-energy part, which arise when thermal fluctuations are taken into account, have a structure that does not lead to a change in the topological properties of the system. It is found that thermal fluctuations, as well as an increase in effective attraction in this system, contribute to the expansion of the temperature region, in which the value of the topological index is close to the integer C1=-2.

cond-mat.supr-con↗

arXiv2022

Interplay of magnetism and superconductivity in 2D extended Hubbard model

The interplay between magnetic and superconducting states on a square lattice is studied using the extended Hubbard model, which takes into account the attraction of electrons located at nearest neighbor sites. Ferro-, antiferro-, and spiral magnetic states with all possible sets of spiral wave vectors, as well as singlet superconducting states with s-wave and d-wave pairing order parameters, are considered. Formation of a state with the intermediate s+id-symmetry and phase separation between different phases are allowed. The results of the Hartree-Fock and slave boson approximations are compared in order to study the role of correlation effects. Both macroscopic phase separation and microscopic coexistence of the superconducting and magnetic phases are found under certain model parameters.

cond-mat.str-el↗

arXiv2021

Generalization properties of restricted Boltzmann machine for short-range order

The restricted Boltzmann machine (RBM) is used to investigate short-range order in binary alloys. The network is trained on the data collected by Monte Carlo simulations for a simple Ising-like binary alloy model and used to calculate the Warren--Cowley short-range order parameter and other thermodynamic properties. We demonstrate that RBM not only reproduces the order parameters for the alloy concentration at which it was trained, but can also predict them for any other concentrations.

cond-mat.dis-nn↗

arXiv2020

Thermal fluctuations in superconducting phases with chiral $ d + id $ and $ s $ symmetry on a triangular lattice

The behavior of thermal fluctuations of a superconducting order parameter with extended $ s $ and chiral $ d + id $ symmetry is investigated. The study is carried out on a triangular lattice within the framework of the quasi-two-dimensional single-band model with attraction between electrons at neighboring sites. The method of consistent consideration of the order parameter fluctuations and the charge carrier scattering by fluctuations of coupled electron pairs, based on the theory of functional integration is used. The distribution functions of the phase fluctuation probabilities depending on temperature and charge carrier concentration are obtained. The temperature dependences of the amplitudes of the averaged superconducting order parameter are calculated. A phase diagram of superconducting states is constructed for the entire range of variation in the charge carrier concentration $ 0 <n <2 $. Near the boundaries of this range, topologically trivial superconducting states with extended $ s $ symmetry are realized, while a superconducting state with topologically nontrivial chiral $ d + id $ symmetry is realized between them. The calculated anomalous self-energies are compared with the experimental ones obtained using machine learning techniques.

cond-mat.str-el↗

arXiv2018

Phase diagrams of singlet superconducting states with mixed symmetry

The competition between the singlet superconducting states with $s$- and d-wave symmetry of the order parameter is studied within a single-band model with nearest-neighbor attractive interaction. The zero- and finite-temperature ground state phase diagrams are constructed for different ratios between the nearest- and next-nearest-neighbor electron transfer integrals. The mixed $s+id$ pairing state is shown to form in the intermediate region between $s$- and d-waves. The temperature phase transitions between the pure and the mixed pairing state are found.

cond-mat.supr-con↗

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↗

arXiv2014

Canted spiral magnetic order in layered systems

Formation of a canted spiral magnetic order is studied in the framework of a mean-field approximation of the Hubbard model. It is revealed that this magnetic state can be stabilized under certain conditions in layered systems with a relatively small interplane electron hopping. Example of an experimentally observed magnetic structure of La$_{2-x}$Sr$_x$CuO$_4$ is considered. It is shown that the canting magnetic order can be described in terms of a simple non-relativistic band magnetism.

cond-mat.str-el↗

arXiv2012

Phase diagram of t-t' Hubbard model taking into account spin-spiral waves and phase separation at finite temperature

The effect of temperature on the magnetic phase separation and the parameters of spin-spiral waves (SSW) is studied using a two-dimensional (2D) single-band $t-t'$ Hubbard model and the Hubbard-Stratonovich transformation. Both commensurate (antiferromagnetic (AF)) and incommensurate (helical) magnetic phases are considered. It is shown that the temperature significantly affects the collinear and helical magnetic phases. With an increase in the temperature, the phase-separation (PS) regions (AF$+[Q,Q])$, $([Q,Q]+[Q,π])$ get substantially reduced but new regions $([Q_{1},π]+[Q_{2},π])$, (AF$+[Q,π])$ arise. The results are used for the interpretation of the magnetic properties of cuprates.

cond-mat.str-el↗

arXiv2011

Incommensurate spin-density wave in two-dimensional Hubbard model

We consider the magnetic phase diagram of the two-dimensional Hubbard model on a square lattice. We take into account both spiral and collinear incommensurate magnetic states. The possibility of phase separation of spiral magnetic phases is taken into consideration as well. Our study shows that all the listed phases appear to be the ground state at certain parameters of the model. Relation of the obtained results to real materials, e.g. Cu-based high-temperature superconductors, is discussed.

cond-mat.str-el↗

arXiv2009

Incommensurate magnetic order and phase separation in the two-dimensional Hubbard model with nearest and next-nearest neighbor hopping

We consider the ground state magnetic phase diagram of the two-dimensional Hubbard model with nearest and next-nearest neighbor hopping in terms of electronic density and interaction. We treat commensurate ferro- and antiferromagnetic, as well as incommensurate (spiral) magnetic phases. The first-order magnetic transitions with changing chemical potential, resulting in a phase separation (PS) in terms of density, are found between ferromagnetic, antiferromagnetic and spiral magnetic phases. We argue that the account of PS has a dramatic influence on the phase diagram in the vicinity of half-filling. The results imply possible interpretation of the unusual behavior of magnetic properties of one-layer cuprates in terms of PS between collinear and spiral magnetic phases. The relation of the results obtained to the magnetic properties of ruthenates is also discussed.

cond-mat.str-el↗

arXiv2008

Conditions for the spin-spiral state in itinerant magnets

The spin-spiral (SS) type of magnetization is studied with the Hubbard model. Consideration of noncollinearity of the magnetic moments results in a phase diagram which consists of regions of the SS and paramagnetic states depending on the number of electrons and the parameter U/t (U is the Hubbard repulsion, and t is an overlap integral). A possibility of stabilization of the SS state with three nonzero components of magnetic moment is considered.

cond-mat.mtrl-sci↗

arXiv2007

The formation and ordering of local magnetic moments in Fe-Al alloys

With density functional theory, studied are the local magnetic moments in Fe-Al alloys depending on concentration and Fe nearest environment. At zero temperature, the system can be in different states: ferromagnetic, antiferromagnetic and spin-spiral waves (SSW) which has a minimum energy. Both SSW and negative moment of Fe atoms with many Al atoms around them agree with experiments. Magnetization curves taken from literature are analysed. Assumption on percolation character of size distribution of magnetic clusters describes well the experimental superparamagnetic behaviour above 150 K.

cond-mat.mtrl-sci↗