The influence of local correlations on the phase states in the model of semi-hard-core bosons on a square lattice

The work considers a model of charged "semi-hard-core" bosons on a square lattice with a possible filling number at each node, ranging from 0 to 2. Temperature phase diagrams of the model are obtained using numerical Monte Carlo quantum simulation methods, and the influence of local charge correlations is examined. Comparison with results from mean-field methods shows that local charge correlations contribute to an increased role of quantum fluctuations in the formation of phase states.Comment: 5 pages, 2 figure

Unconventional phase transitions in strongly anisotropic 2D (pseudo)spin systems

We have applied a generalized mean-field approach and quantum Monte-Carlo technique for the model 2D S = 1 (pseudo)spin system to find the ground state phase with its evolution under application of the (pseudo)magnetic field. The comparison of the two methods allows us to clearly demonstrate the role of quantum effects. Special attention is given to the role played by an effective single-ion anisotropy (»on-site correlation»). © 2018 The Authors, published by EDP Sciences.The research was supported by the Government of the Russian Federation, Program 02.A03.21.0006 and by the Ministry of Education and Science of the Russian Federation, projects Nos. 2277 and 5719

Critical Behavior of a 2D Spin-Pseudospin Model in a Strong Exchange Limit

We study the 2D static spin-pseudospin model equivalent to the dilute frustrated antiferromagnetic Ising model with charge impurities. We present the results of classical Monte Carlo simulation on a square lattice with periodic boundary conditions in a “strong” exchange limit. In the framework of the finite-size scaling theory we obtained the static critical exponents for the specific heat α and the correlation length ν for a wide range of the local density–density interaction parameter ∆ and charge density n. It was shown that the system exhibits non-universal critical behavior depending on these parameters. © 2020 Polish Academy of Sciences. All rights reserved.Russian Foundation for Basic Research, RFBR: 18-32-00837/18Ministry of Education and Science of the Russian Federation, Minobrnauka: 2277, 5719Government Council on Grants, Russian FederationThe research was supported by the Government of the Russian Federation, Program 02.A03.21.0006 and by the Ministry of Education and Science of the Russian Federation, projects No. 2277 and 5719 by RFBR according to the research project No. 18-32-00837/18, scholarship of the president of the Russian Federation No. SP-2278.2019.1

Magnetocaloric Effect in Two-Dimensional Diluted Ising Model: Appearance of Frustrations in the Ground State

Abstract: The magnetocaloric effect in a two-dimensional Ising model is considered for different ratios between parameters of inter-site repulsion of nonmagnetic impurities and exchange coupling. Classical Monte Carlo simulations on a square lattice show that in case of weak coupling and at sufficiently high concentrations of nonmagnetic impurities the long-range ferromagnetic ordering breaks down to give isolated spin clusters in the ground state of the system. This leads to appearance of a paramagnetic response in the system at the zero temperature and nonzero entropy of the ground state. The feasibility to detect frustration of ground state using data on the magnetic entropy variation is discussed. © 2020, Pleiades Publishing, Ltd.The work was supported within the Competitiveness Enhancement Program of the Ural Federal University (Russian Federation Government Act 211, agreement no. 02.A03.21.0006), by the Ministry of Education and Science, Russian Federation (project FEUZ-2020-0054), and by the Russian Foundation for Basic Research (research project no. 18-32-00837/18)

Magnetocaloric effect in the 2D dilute Ising system

We consider the magnetocaloric effect (MCE) for the 2D Ising system with a fixed concentration of non-magnetic mobile charged impurities. Pseudospin formalism and highperformance Monte Carlo simulations are used to describe the system and, in particular, to obtain the temperature dependences of the MCE parameters for our system. The effect of the impurityimpurity interaction strength on the concentration dependence of the MCE parameters is discussed, as well as the possibility of using the MCE effect to reveal a hidden frustration in the system. © Published under licence by IOP Publishing Ltd.Ministry of Education and Science of the Russian Federation, Minobrnauka: 2277, 5719Government Council on Grants, Russian Federation: A03.21.0006, 02.This work was supported by Program 211 of the Government of the Russian Federation (Agreement 02.A03.21.0006), the Ministry of Education and Science of the Russian Federation (projects nos. 2277 and 5719)

Phase diagrams of a 2D Ising spin-pseudospin model

We consider the competition of magnetic and charge ordering in high-Tc cuprates within the framework of the simplified static 2D spin-pseudospin model. This model is equivalent to the 2D dilute antiferromagnetic (AFM) Ising model with charged impurities. We present the mean-field results for the system under study and make a brief comparison with classical Monte Carlo (MC) calculations. Numerical simulations show that the cases of strong exchange and strong charge correlation differ qualitatively. For a strong exchange, the AFM phase is unstable with respect to the phase separation (PS) into the charge and spin subsystems, which behave like immiscible quantum liquids. An analytical expression was obtained for the PS temperature.Comment: 14 pages, 3 figure

Critical Properties of a 2-D Frustrated Magnet With Non-Magnetic Impurities

We report on the classical Monte Carlo (MC) study of phase transitions (PTs) and critical behavior of a 2-D spin-pseudospin model describing a dilute magnet with competing charge and spin interactions. The static critical exponents of the specific heat and correlation length are calculated using the finite-size scaling theory in a wide range of model parameters. The order of PTs is analyzed within the energy histogram method. It is found that approaching the frustration point and increasing the density of non-magnetic impurities lead to non-universal critical behavior and first-order PTs. Features of non-universal critical behavior are shown to depend on the relationship between the parameters of the spin and pseudospin interactions. © 1965-2012 IEEE.This work was supported in part by the Competitiveness Enhancement Program of the Ural Federal University (Act 211 of the Government of the Russian Federation) under Grant 02.A03.21.0006 and Grant CEP 3.1.1.1-20 and in part by the Ministry of Science and Higher Education of the Russian Federation under Project FEUZ-2020-0054. The authors would like to thank Alexander Moskvin for fruitful discussions

Nontrivial Ground State Degeneracy of the Spin–Pseudospin Model of a Two-Dimensional Magnet Near the Frustration Point

Abstract: The classical Monte Carlo method is used to study the properties of the ground state and phase transitions of the spin–pseudospin model, which describes a two-dimensional Ising magnet with competing charge and spin interactions. This competition leads to the ground state degeneracy and the frustration of the system. It is shown that the ground state degeneracy is observed in the frustration area with nonzero probabilities of the formation of two different ordered states. Based on the histogram analysis of the Monte Carlo data, the type of phase transitions is analyzed. It is found that, near the frustration point, first-order phase transitions are observed in the dependence on the ratio between the spin (s = 1/2) and pseudospin (S = 1) interactions. © 2021, Pleiades Publishing, Ltd.Ministry of Education and Science of the Russian Federation, Minobrnauka: FEUZ-2020-0054; Government Council on Grants, Russian Federation: CEP 3.1.1.1.g-20; Ural Federal University, UrFUThis work was supported by the Competitiveness Enhancement Program of the Ural Federal University (Act 211 of the Government of the Russian Federation, Agreement no. 02.A03.21.0006 and CEP 3.1.1.1.g-20) and the Ministry of Science and Higher Education of the Russian Federation (project FEUZ-2020-0054)

Monte Carlo Simulation of a Model Cuprate

We develop a classical Monte Carlo algorithm based on a quasi-classical approximation for a pseudospin S = 1 Hamiltonian in real space to construct a phase diagram of a model cuprate with a high Tc. A model description takes into account both local and nonlocal correlations, Heisenberg spin-exchange interaction, correlated single-particle, and two-particle transport. We formulate a state selection algorithm for a given parameterization of the wave function in order to ensure a uniform distribution of states in the phase space. The simulation results show a qualitative agreement with the experimental phase diagrams. © 2021 Institute of Physics Publishing. All rights reserved.The research was supported by the Ministry of Education and Science of the Russian Federation, project FEUZ-2020-0054, and by scholarship of the president of the Russian Federation No. SP-2278.2019.1