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Charge Ordering and Phase Separations in the Spinless Fermion Model with Repulsive Intersite Interaction

100%
Czart W., Robaszkiewicz S., Tobijaszewska B.
Acta Physica Polonica A
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2008
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vol. 114
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issue 1
129-134
EN
We study a simple effective model for description of charge orderings in narrow band materials, i.e. the spinless fermion model with repulsive intersite interaction W. The analysis is concentrated on the problem of phase separations and the effects of next-nearest neighbor hopping t_2 on the charge ordered states in this system. The cases of d-dimensional (d≥2) hypercubic lattices are considered for arbitrary particle concentration (0 < n < 1). Within the broken symmetry Hartree-Fock approximation the phase diagrams as a function of W and n are evaluated for representative cases. The results for t_2 ≠ 0 are compared with those found for the case with nearest neighbor hopping only.
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Effects of Frustrating Hopping on Charge Ordered States in Itinerant Fermion Systems for Arbitrary Concentration in 2D Lattice

100%
Czart W., Grzybowski P., Nogala M., Robaszkiewicz S.
Acta Physica Polonica A
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2012
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vol. 121
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issue 4
828-831
EN
There is ongoing, intense, research in the field of electron charge orderings (CO) and charge density waves phenomena, due to experimental discovery of such phases in numerous important compounds. The aim of this work is to extend recent advances in the field by studying two simple effective paradigmatic models used to describe CO in narrow band materials i.e. (i) a model of correlated electrons: the so-called t-W model of spinless fermions with repulsive interaction W and (ii) the molecular crystal model with the coupling of electrons to intramolecular (crystal field) vibrations in the static limit. The finite temperature phase diagrams are evaluated at arbitrary carriers concentration for several representative cases. Our calculations are performed within the (broken symmetry) HFA for d=2 square lattice and arbitrary carriers concentration. In this contribution we focus on the effects of next-nearest-neighbor hopping on the CO states in these systems and the problem of phase separations involving checkerboard CO with the nesting vector Q=(π,π). The results we show here are an extension of our previous work on the subject.
3
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On the Phase Diagram of the Zero-Bandwidth Extended Hubbard Model with Intersite Magnetic Interactions for Strong On-Site Repulsion Limit

100%
Murawski S., Kapcia K., Pawłowski G., Robaszkiewicz S.
Acta Physica Polonica A
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2012
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vol. 121
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issue 5-6
1035-1037
EN
In this report we have analyzed a simple effective model for a description of magnetically ordered insulators. The Hamiltonian considered consists of the effective on-site interaction (U) and the intersite Ising-like magnetic exchange interaction (J) between nearest neighbors. For the first 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-field 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: F-NO).
4
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Charge Orderings and Phase Separations in Itinerant Fermion Systems at Half Filling

100%
Czart W., Grzybowski P., Nogala M., Robaszkiewicz S.
Acta Physica Polonica A
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2012
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vol. 121
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issue 5-6
1042-1044
EN
We analyse the ground state phase diagrams of the charge orderings in narrow band materials using two effective models: (1) the spinless fermion model (t - W) with repulsive intersite interaction (W_{ij} > 0) and (2) the molecular crystal model with the coupling of electrons to intramolecular (crystal field) vibrations. We present results for the case of half filled bands for d = 2 square lattice. The calculations are performed within the (broken symmetry) Hartree-Fock approximation. The study takes into consideration the effects of frustrating next-nearest-neighbour hopping (t_2) on the charge ordered states in these systems. We focus on the two cases: (i) homogeneous phases and phase separations involving checkerboard charge ordering with the nesting vector Q= (π,π) only and (ii) homogeneous phases and phase separations involving two types of charge ordering: (a) checkerboard charge ordering with the nesting vector Q = (π,π), and (b) collinear (CL) charge ordering with Q = (0,π) or Q = (π,0).
5
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Mechanical Characteristics of As₂S₃ Glasses Induced by Doping with Bismuth

100%
Šiljegović M., Lukić-Petrović S., Petrović D., Videnović I., Turyanytsa I.
Acta Physica Polonica A
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2016
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vol. 129
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issue 4
488-491
EN
This paper presents and discusses the results of the instrumented indentation test of the samples of the system Bi_{x}(As₂S₃)_{100-x}, x= 1.5, 3, 5, and 7 at.%. Measurements of mechanical parameters were performed using a Fischerscope HM2000 S nanoindentation device. The experimental data obtained by measuring the microhardness parameters were used to determine some other mechanical quantities that are important for the characterization of the examined materials in terms of their potential applications. For the first three compositions, the results indicated an increase in the microhardness with the increase in the content of doping atoms, which can be interpreted as an enhancement of the strength and stiffness of the structural network. The lower value of microhardness of the sample with the maximum content of Bi can BE associated with the specific structure of this composition. The pronounced indentation size effect was also detected on the indentation curve in the range of smaller loads. According to the model of elastic-plastic deformation, applied for the description of indentation size effect measured for the investigated chalcogenides, the largest value of the elastic recovery was observed for the sample Bi₇(As₂S₃)₉₃. The calculated values of the elasticity modulus show that the glass with x= 5 at.% Bi is characterized with the highest atomic packing density.
6
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Charge Orderings and Phase Separations in the Atomic Limit of the Extended Hubbard Model with Intersite Density-Density Interactions

100%
Kapcia K., Kłobus W., Robaszkiewicz S.
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EN
A simple effective model of charge ordered insulators is studied. The tight binding Hamiltonian consists of the effective on-site interaction U and the intersite density-density interactions W_{ij} (both: nearest-neighbour and next-nearest neighbour). In the analysis of the phase diagrams we have adopted the variational approach, which treats the on-site interaction term exactly and the intersite interactions within the mean-field approximation. The phase separated states have not been taken into account in previous analyses. Our investigations of two cases of the on-site interaction: attraction (U/(- W_{Q}) = - 10) and repulsion (U/(- W_{Q}) = 1.1) show that, depending on the values of the next-nearest neighbour attraction, the system can exhibit not only homogeneous phases: charge ordered and nonordered, but also various phase separated states (charge ordered-nonordered, charge ordered-charge ordered).
7
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Charge Ordering and Phase Separations in the Molecular Crystal Model

100%
Czart W., Grzybowski P., Robaszkiewicz S.
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issue 2
369-372
EN
We analyse the ground state phase diagrams and thermodynamic properties of charge orderings in narrow band materials using the molecular crystal model in the static limit. We present results for the hypercubic lattices in dimensions d = 2 and d = ∞. We focus our study on the problem of phase separations involving charge orderings and the effects of next-nearest-neighbor hopping (t_2) on the charge ordered states in these systems. The ground state phase diagrams are evaluated for a few representative cases. Results for the molecular crystal model are compared with those obtained previously for the spinless fermion model with repulsive intersite interaction W.
8
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Some Properties of Two-Dimensional Extended Repulsive Hubbard Model with Intersite Magnetic Interactions - A Monte Carlo Study

88%
Murawski S., Kapcia K., Pawłowski G., Robaszkiewicz S.
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vol. 126
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issue 4a
A-110-A-114
EN
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.
9
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Electron Phase Separations Involving Superconductivity in the Extended Hubbard Models with Pair Hopping Interaction

88%
Kapcia K.
Acta Physica Polonica A
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2015
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vol. 127
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issue 2
204-206
EN
In this work the extended Hubbard models with pair hopping interaction (at the atomic limit) are investigated within the variational approach, which treats the on-site interaction term exactly and the intersite interactions within the mean-field approximation (exact in d → +∞). We analyze mutual stability of the superconducting phase and charge or (ferro/antiferro-)magnetic orderings as well as homogeneous mixed phases. Our preliminary results for U=0 show that the superconducting phase can coexist with the charge or (ferro/antiferro-)magnetic phases only in states with electron phase separation.
10
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Phase Separations in the Narrow-Bandwidth Limit of the Penson-Kolb-Hubbard Model at Zero Temperature

88%
Kapcia K., Czart W.
Acta Physica Polonica A
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2018
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vol. 133
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issue 3
401-404
EN
In this work we study the ground state of the Penson-Kolb-Hubard model in the limit of narrow-bandwidth. We present phase diagrams of the model for fixed chemical potential and concentration (involving various phase separations). The results are derived within the Hartree-Fock approximation (HFA) in the narrow-bandwidth regime and compared with the exact ones in the atomic limit and the high-dimension regime. Our investigation reveals that the HFA can reconstruct the exact diagram at the ground state when the bandwidth approaches to zero.
11
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Interplay between Charge and Magnetic Orderings in the Zero-Bandwidth Limit of the Extended Hubbard Model for Strong On-Site Repulsion

88%
Kapcia K., Kłobus W., Robaszkiewicz S.
Acta Physica Polonica A
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2012
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vol. 121
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issue 5-6
1032-1034
EN
A simple effective model of charge ordered and (or) magnetically ordered insulators is studied. The tight binding Hamiltonian analyzed consists of (i) the effective on-site interaction U, (ii) the intersite density-density interaction W and (iii) intersite magnetic exchange interaction J^{z} (or J^{xy}) between nearest-neighbors. The intersite interaction are treated within the mean-field approximation. One shows that the systems considered can exhibit very interesting multicritical behaviors, including among others bicritical, tricritical, tetracritical and critical end points. The analysis of the model has been performed for an arbitrary electron concentration as well as an arbitrary chemical potential in the limit of strong on-site repulsion (U → +∞). The phase diagrams obtained in such a case are shown to consist of at least 9 different states, including four homogeneous phases: nonordered (NO), ferromagnetic (F), charge ordered (CO), ferrimagnetic (intermediate, I) and five types of phase separation: NO-NO, F-NO, F-F, CO-F, CO-I.
12
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Stable and Metastable Phases in the Atomic Limit of the Extended Hubbard Model with Intersite Density-Density Interactions

88%
Kapcia K., Robaszkiewicz S.
Acta Physica Polonica A
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2012
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vol. 121
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issue 5-6
1029-1031
EN
We have studied a simple effective model of charge ordered insulators. The tight binding Hamiltonian consists of the effective on-site interaction U and the intersite density-density interaction W_{ij} (both: nearest-neighbor and next-nearest-neighbor). In the analysis of the phase diagrams and thermodynamic properties of this model we have adopted the variational approach, which treats the on-site interaction term exactly and the intersite interactions within the mean-field approximation. Our investigations of the general case (as a function of the electron concentration n) have shown that the system exhibits various critical behaviors including among others bicritical, tricritical, critical-end, and isolated critical points. In this report we concentrate on the metastable phases and transitions between them. One finds that the first- and second order transitions between metastable phases can exist in the system. These transitions occur in the neighborhood of first as well as second order transitions between stable phases. For the case of on-site attraction the regions of metastable homogeneous phases occurrence inside the ranges of phase separated states stability have been also determined.
13
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Monte Carlo Study of Phase Separation in Magnetic Insulators

88%
Murawski S., Kapcia K., Pawłowski G., Robaszkiewicz S.
Acta Physica Polonica A
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2015
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vol. 127
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issue 2
281-283
EN
In this work we focus on the study of phase separation in the zero-bandwidth extended Hubbard with nearest-neighbors 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. antiferromagnetic-non-ordered transitions) can be of first 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.
14
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Magnetic Orderings and Phase Separations in the Zero-Bandwidth Limit of the Extended Hubbard Model with Intersite Magnetic Interactions

88%
Kłobus W., Kapcia K., Robaszkiewicz S.
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issue 2
353-355
EN
A simple effective model for a description of magnetically ordered insulators is analysed. The tight binding Hamiltonian consists of the effective on-site interaction (U) and intersite magnetic exchange interactions (J^{z}, J^{xy}) between nearest neighbours. The phase diagrams of this model have been determined within the variational approach, which treats the on-site interaction term exactly and the intersite interactions within the mean-field approximation. We show that, depending on the values of interaction parameters and the electron concentration, the system can exhibit not only homogeneous phases: (anti-)ferromagnetic (F_α) and nonordered (NO), but also phase separated states (PS_α: F_α-NO).
15
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Superconductivity, Metastability and Magnetic Field Induced Phase Separation in the Atomic Limit of the Penson-Kolb-Hubbard Model

88%
Kapcia K.
|
EN
We present the analysis of paramagnetic effects of magnetic field (B) (Zeeman term) in the zero-bandwidth limit of the extended Hubbard model for arbitrary chemical potential μ and electron density n. The effective Hamiltonian considered consists of the on-site interaction U and the intersite charge exchange term I, determining the hopping of electron pairs between nearest-neighbour sites. The model has been analyzed within the variational approach, which treats the on-site interaction term exactly and the intersite interactions within the mean-field approximation (rigorous in the limit of infinite dimensions d → +∞). In this report we focus on metastable phases as well as phase separated states involving superconducting and nonordered phases and determine their ranges of occurrence for U/I_0=1.05 (I_0=zI) in the presence of magnetic field B≠0. Our investigations of the general case for arbitrary U/I_0 show that, depending on the values of interaction parameters (for fixed n), the phase separating state can occur in higher fields than the homogeneous superconducting phase (field-induced phase separated). Moreover, a first-order superconducting-nonordered transition occurs between metastable phases and these metastable phases can exist inside the regions of the phase separated state stability. Such behaviour is associated with the presence of tricritical line on the phase diagrams of the system.
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