arXiv subjects

A. S. Moskvin

Publications and source records attributed to A. S. Moskvin.

At least 19 recordsLinked to original sources

arXiv2025

Classical Monte Carlo algorithm for simulation of a pseudospin model for cuprates

A classical Monte Carlo algorithm based on the quasi-classical approximation is applied to the pseudospin Hamiltonian of the model cuprate. The model takes into account both local and non-local correlations, Heisenberg spin-exchange interaction, single-particle and correlated two-particle transfer. We define the state selection rule that gives both the uniform distribution of states in the phase space and the doped charge conservation. The simulation results show a qualitative agreement of a phase diagrams with the experimental ones.

physics.comp-ph↗

arXiv2025

The classical-quantum disproportionation transition and magnetic ordering in RNiO$_3$ nickelates

The insulator-quasi-metal (bad metal) transition observed in Jahn-Teller (JT) magnets orthonickelates RNiO$_3$ (R = rare earth, or yttrium Y) is considered a canonical example of the Mott transition, traditionally described in the framework of Hubbard's $U-t$ model. However, in reality, the insulating phase of nickelates is the result of charge disproportionation (CD) with the formation of a system of spin-triplet ($S = 1$) electron [NiO$_6$]$^{10-}$ and spinless ($S = 0$) hole [NiO$_6$]$^{8-}$ centers, equivalent to a system of effective spin-triplet composite bosons moving in a nonmagnetic lattice. The effective CD-phase Hamiltonian takes into account local ($U$) and nonlocal ($V$) correlations, and the transfer of composite bosons ($t_b$). Within the framework of the effective field approximation, we have shown the existence of two types of CD phases: the high-temperature classical paramagnetic CO-phase of charge ordering of electron and hole centers, and the low-temperature magnetic quantum CDq phase with charge and spin density transfer between electron and hole centers, with ''uncertain valence'' [NiO$_{6}$]$^{(9\pmδ)-}$ ($0 \le δ\le 1$) and spin density $(1 \pm δ)/2$ NiO$_6$-centers. In the classical CO phase, spin-triplet electron centers are surrounded by the nearest nonmagnetic hole centers, which ''turns off'' the strong superexchange interaction of the nearest neighbors. The magnetic ordering in the quantum CDq phase is determined by a strong traditional superexchange and an unusual bosonic double exchange mechanism.

cond-mat.str-el↗

arXiv2025

Monte Carlo simulation of spin-reorientation transition in weak ferrimagnets YFeCrO3

This work presents the modeling of the magnetic 3d sublattice in mixed orthoferrites-orthochromites YFe1-xCrxO3 using classical Monte Carlo methods. It is shown that, when taking into account the competition of the Dzyaloshinskii vectors in the mixed compositions, magnetic moment compensations are observed, as well as angular magnetic configurations corresponding to the spin reorientation.

cond-mat.str-el↗

arXiv2025

Modified Monte Carlo method with thermostat algorithm for model orthonickelates

The results of numerical simulation using a modified Monte Carlo method with a thermostat algorithm for a pseudospin model of orthonickelates are presented. Temperature phase diagrams are constructed for various degrees of filling and for various parameters of the model, and the effect of local correlations on the critical temperatures of the model orthonickelate is investigated. The possibility of detecting phase inhomogeneous states is shown. The numerical simulation results show good qualitative agreement with the analytical results in the mean field approximation.

cond-mat.stat-mech↗

arXiv2025

Modified Monte Carlo method with the heat bath algorithm for a model cuprate

The results of numerical simulation using a modified Monte Carlo method with a heat bath algorithm for the pseudospin model of cuprates are presented. The temperature phase diagrams are constructed for various degrees of doping and for various parameters of the model, and the effect of local correlations on the critical temperatures of the model cuprate is investigated. It is shown that, in qualitative agreement with the results of the mean field, the heat bath algorithm leads to a significant decrease in the estimate of critical temperatures due to more complete accounting of fluctuations, and also makes it possible to detect phase inhomogeneous states. The possibility of using machine learning to accelerate the heat bath algorithm is discussed.

cond-mat.supr-con↗

arXiv2025

Insulator-bad metal transition in RNiO$_3$ nickelates beyond Hubbard model and density functional theory

The insulator-bad metal transition observed in the Jahn-Teller (JT) magnets orthonickelates RNiO$_3$ (R = rare earth or yttrium Y) is considered to be a canonical example of the Mott transition, traditionally described in the framework of the Hubbard $U$-$t$-model and the density functional theory. However, actually the real insulating phase of nickelates is the result of charge disproportionation (CD) with the formation of a system of spin-triplet (S=1) electron [NiO$_6$]$^{10-}$ and spinless (S=0) hole [NiO$_6$]$^{8-}$ centers, equivalent to a system of effective spin-triplet composite bosons moving in a nonmagnetic lattice. Taking account of only charge degree of freedom we develop a novel minimal $U$-$V$-$t_b$-model for nickelates making use of the charge triplet model with the pseudospin formalism and effective field approximation. We show the existence of two types of CD-phases, high-temperature classical CO-phase with the G-type charge ordering of electron and hole centers, and low-temperature quantum CDq-phase with charge and spin density transfer between electron and hole centers, uncertain valence and spin value for NiO$_6$ centers. Model $T$-R phase diagram reproduces main features of the phase diagram found for RNiO$_3$.

cond-mat.str-el↗

arXiv2024

The Monte Carlo Method for the Orthonickelate Model

The peculiarities of phase states of the triplet boson model for orthonickelates are investigated analytically and by means of numerical simulations. The conditions of thermodynamic stability of homogeneous phases are found. It is shown that the description of the phase inhomogeneous state in the mean-field approximation qualitatively agrees with the observed state of the system in numerical simulations by the classical Monte Carlo method

cond-mat.stat-mech↗

arXiv2024

Weak ferrimagnets of the YFe1-xCrxO3 type: negative magnetization and spin reorientation

In this work, we present an analysis of magnetic properties of weak ferrimagnets of the YFe$_{1-x}$Cr$_x$O$_3$ type. Taking into account the main spin interactions - isotropic Heisenberg superexchange, antisymmetric Dzyaloshinskii-Moriya exchange, single-ion spin anisotropy - calculations of the free energy, concentration and temperature dependences of magnetization were carried out within a framework of a molecular field model. In particular, the model demonstrates the phenomena of temperature compensation and negative magnetization, as well as spin reorientation. The compensation temperature reaches room temperature T = 300 K in a composition with x = 0.45. The existence of magnetic structures of the $G_{xyz}$ type with the spatial orientation of the Neel vector is predicted.

cond-mat.str-el↗

arXiv2024

The mean field approximation for a system of triplet bosons in nickelates

Rare-earth orthonickelates RNiO3 are Jahn-Teller magnets, unstable with respect to the anti-Jahn-Teller disproportionation reaction with the formation of a system equivalent to a system of effective spin-triplet composite bosons moving in a non-magnetic lattice. Within the framework of the two-sublattice approximation, we have developed a mean field theory for a model nickelate with competition between phases of charge ordering, an antiferromagnetic insulator and a spin-triplet superconductor, and constructed phase diagrams taking into account phase separation.

cond-mat.stat-mech↗

arXiv2024

Anti-Jahn-Teller disproportionation and prospects for spin-triplet superconductivity in d-element compounds

We argue that the unusual properties of a wide class of materials based on Jahn-Teller 3d and 4d ions with different crystal and electronic structures, from quasi-two-dimensional unconventional superconductors (cuprates, nickelates, ferropnictides/chalcogenides, ruthenate SrRuO4), manganites with local superconductivity to 3D ferrates (CaSr)FeO3, nickelates RNiO3 and silver oxide AgO with unusual charge and magnetic order can be explained within a single scenario. The properties of these materials are related to the instability of their highly symmetric Jahn-Teller "progenitors" with the ground orbital E-state to charge transfer with anti-Jahn-Teller disproportionation and the formation of a system of effective local composite spin-singlet or spin-triplet, electronic or hole bosons moving in a non-magnetic or magnetic lattice. These unusual systems are characterized by an extremely rich variety of phase states from non-magnetic and magnetic insulators to unusual metallic and superconducting states.

cond-mat.supr-con↗

arXiv2023

Jahn-Teller magnets

A wide class of materials with different crystal and electronic structures from quasi-two-dimensional unconventional superconductors (cuprates, nickelates, ferropnictides/chalcogenides, ruthenate SrRuO$_4$), 3D systems as manganites RMnO$_3$, ferrates (CaSr)FeO$_3$, nickelates RNiO$_3$, to silver oxide AgO are based on Jahn-Teller $3d$ and $4d$ ions. These unusual materials called Jahn-Teller (JT) magnets are characterized by an extremely rich variety of phase states from non-magnetic and magnetic insulators to unusual metallic and superconducting states. The unconventional properties of the JT-magnets can be related to the instability of their highly symmetric Jahn-Teller "progenitors" with the ground orbital $E$-state to charge transfer with anti-Jahn-Teller $d$-$d$ disproportionation and the formation of a system of effective local composite spin-singlet or spin-triplet, electronic or hole $S$-type bosons moving in a non-magnetic or magnetic lattice. We consider specific features of the anti-JT-disproportionation reaction, properties of the electron-hole dimers, possible phase states of JT-magnets, effective Hamiltonians for single- and two-band JT-magnets, and present a short overview of physical properties for actual JT-magnets.

cond-mat.str-el↗

arXiv2023

Simple Realistic Model of Spin Reorientation in 4f-3d Compounds

Spin reorientation is an important phenomenon of rare-earth perovskites, orthoferrites and orthochromites. In this study, we consider a simple but realistic microscopic theory of the spontaneous spin-reorientation transitions induced by the 4f-3d interaction, more specifically, the interaction of the main Kramers doublet or non-Kramers quasi-doublet of the 4f ion with an effective magnetic field induced by the 3d sublattice. The obtained results indicate that the cause of both the temperature and the character of the spin-reorientation transition is a competition between the second and fourth order spin anisotropy of the 3d sublattice, the crystal field for 4f ions, and the 4f-3d interaction.

cond-mat.str-el↗

arXiv2023

The Parallel Monte Carlo Algorithm Implementation on GPU for the Systems with an Ising Hamiltonian under the Condition of a Constant Charge Density

This paper is devoted to computational algorithms designed to describe the classical Ising magnet in some specific cases when an additional macroscopic restriction in form of constant charge density exists in the system. We developed and implemented a parallel algorithm for modeling such a systems on GPU with CUDA technology. This work focuses on technical aspects of implementing the algorithm.

physics.comp-ph↗

arXiv2023

Topological structures in unconventional scenario for 2D cuprates

We introduce a minimal model to describe the charge degree of freedom in cuprates with the on-site Hilbert space reduced to only the three valence states CuO$_4^{7-,6-,5-}$ (nominally Cu$^{1+,2+,3+}$) and make use of the S=1 pseudospin formalism. The formalism constitutes a powerful method to study complex phenomena in interacting quantum systems characterized by the coexistence and competition of various ordered states. Overall, such a framework provides a simple and systematic methodology to predict and discover new kinds of orders. In particular, the pseudospin formalism provides the most effective way to describe different topological structures, in particular, due to a possibility of a geometrical two-vector description of the on-site states. We introduce and analyze effective pseudospin Hamiltonian with on-site and inter-site charge correlations, two types of a correlated one-particle transfer and two-particle, or the composite boson transfer. The 2D S=1 pseudospin system is prone to a creation of different topological structures, which form topologically protected inhomogeneous distributions of the eight local S=1 pseudospin order parameters. We present a short overview of localized topological structures, typical for S=1 (pseudo)spin systems, focusing on unexpected antiphase domain walls in parent cuprates and so-called quadrupole skyrmion, which are believed to be candidates for a topological charge excitation in parent or underdoped cuprates. Puzzlingly, these unconventional structures can be characterized by an uniform distribution of the mean on-site charge, that makes these invisible for X-rays. Quasiclassical approximation and computer simulation are applied to analyze localized topological defects and evolution of the domain structures in "negative-$U$" model under charge order-superfluid phase transition.

cond-mat.str-el↗

arXiv2023

DFT, L(S)DA, LDA+U, LDA+DMFT..., whether we do approach to a proper description of optical response for strongly correlated systems?

I present a critical overview of so-called "{\it ab initio}" DFT (density fuctional theory) based calculation schemes for the description of the electronic structure, energy spectrum, and optical response for strongly correlated 3$d$ oxides, in particular, crystal-field and charge transfer transitions as compared with an "old"\, cluster model that does generalize crystal-field and ligand-field theory. As a most instructive illustration of validity of numerous calculation techniques I address the prototypical 3$d$ insulator NiO predicted to be a metal in frames of a standard LDA (local density approximation) band theory.

cond-mat.str-el↗

arXiv2023

Influence of Local Correlations on the "Homogeneous Insulator-Superconductor" Transition in the Domain Boundaries of the Charge-Order Phase of a 2D System of a Mixed Valence

It is demonstrated in the (pseudo)spin S=1 formalism that the structure of antiphase domain boundaries in the phase of charge ordering of a mixed-valence system of the Cu1+, 2+, 3+ "triplet" type in cuprates on a two-dimensional square lattice depends to a considerable extent on on-site correlation parameter U. The results of computer modeling on large square lattices illustrate the change in the boundary structure (from a homogeneous monovalent nonconducting structure of the Cu2+ type to a filamentary superconducting one) induced by a relatively small variation of positive U values.

cond-mat.supr-con↗

arXiv2023

Features of the Domain Boundaries of a Highly Anisotropic (S = 1) Antiferromagnet near the Transition to the Quantum Paramagnet Phase

It is shown that the structure of antiphase domain boundaries in the antiferromagnetic (AFM) phase of a highly anisotropic magnet with S = 1 on a two-dimensional square lattice depends greatly on single-ion anisotropy parameter D. Computer modeling on large square lattices illustrates the changes in the boundary structure from the quantum paramagnet (QP) to the XY phase, including the intermediate QP-XY phase at fairly small variations in positive D.

cond-mat.str-el↗

arXiv2021

Model of charge triplets for high-T$_c$ cuprates

Starting with a minimal model for the CuO$_2$ planes with the on-site Hilbert space reduced to a charge triplet of the three effective valence centers [CuO$_4$]$^{7-,6-,5-}$ (nominally Cu$^{1+,2+,3+}$) with different conventional spin, different orbital symmetry, and different local lattice configuration, we develop a unified non-BCS spin-pseudospin model to describe the main phase states of doped cuprates. We argue that antiferromagnetic insulating, charge ordered, superconducting, and Fermi-liquid phases are possible phase states of a model parent cuprate, while typical phase state of a doped cuprate, in particular mysterious pseudogap phase, is a result of a phase separation. Superconductivity of cuprates is not a consequence of pairing of doped holes, but the result of quantum transport of on-site composite hole bosons, whereas main peculiarities of normal state can be related to an electron-hole interplay for unusual Fermi-liquid phase and features of the phase separation. Puzzlingly, but it is the electron-lattice interaction, which in the BCS model determines $s$-wave pairing, in the model of local composite bosons gives $d_{x^2-y^2}$-symmetry of the superconducting order parameter, thus showing once again a substantial involvement of the lattice in the cuprate's HTSC.

cond-mat.supr-con↗