In this report we have analyzed a simple eective model for a description of magnetically ordered insulators.The Hamiltonian considered consists of the eective on-site interaction (U ) and the intersite Ising-like magnetic exchange interaction (J ) between nearest neighbors. For the rst time the phase diagrams of this model have been determined within Monte Carlo simulation on 2D-square lattice. They have been compared with results obtained within variational approach, which treats the on-site term exactly and the intersite interactions within mean-eld approximation. We show within both approaches that, depending on the values of interaction parameters and the electron concentration, the system can exhibit not only homogeneous phases: (anti-)ferromagnetic (F) and nonordered (NO), but also phase separated states (PS: FNO).
In this paper the two dimensional extended Hubbard model with intersite magnetic Ising-like interaction in the atomic limit is analyzed by means of the classical Monte Carlo method in the grand canonical ensemble. Such an effective simple model could describe behavior of insulating (anti)ferromagnets. In the model considered the Coulomb interaction (U ) is on-site and the magnetic interactions in z-direction (J > 0, antiferromagnetic) are restricted to nearest-neighbors. Simulations of the model have been performed on a square lattice consisting of N = L × L = 400 sites (L = 20) in order to obtain the full phase diagram for U/(4J) = 1. Results obtained for on-site repulsion (U > 0) show that, apart from homogeneous non-ordered (NO) and ordered magnetic (antiferromagnetic, AF) phases, there is also a region of phase separation (PS: AF/NO) occurrence. We present a phase diagram as well as some thermodynamic properties of the model for the case of U/(4J) = 1 (and arbitrary chemical potential and arbitrary electron concentration). The AF-NO transition can be second-order as well as first-order and the tricritical point occurs on the diagram. PACS numbers: 71.10.Fd -Lattice fermion models (Hubbard model, etc.), 75.10.-b -General theory and models of magnetic ordering, 75.30.Fv -Spin-density waves, 64.75.Gh -Phase separation and segregation in model systems (hard spheres, Lennard-Jones, etc.), 71.10.Hf -Non-Fermi-liquid ground states, electron phase diagrams and phase transitions in model systems
In this work we focus on the study of phase separation in the zero-bandwidth extended Hubbard with nearestneighbors intersite Ising-like magnetic interactions J and on-site Coulomb interactions U . The system has been analyzed by means of the Monte Carlo simulations (in the grand canonical ensemble) on two-dimensional square lattice (with N = L × L = 400 sites) and the results for U/(4J) = 2 as a function of chemical potential and electron concentration have been obtained. Depending on the values of interaction parameters the system exhibits homogeneous (anti-)ferromagnetic or non-ordered phase as well as phase separation state. Transitions between homogeneous phases (i.e. antiferromagneticnon-ordered transitions) can be of rst or second order and the tricritical point is also present on the phase diagrams. The electron compressibility K is an indicator of the phase separation and that quantity is of particular interest of this paper.
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