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Superconductivity, Charge Orderings and Phase Separations in Systems with Local Electron Pairing

100%
Robaszkiewicz S., Pawłowski G.
Acta Physica Polonica A
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1996
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vol. 90
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issue 3
569-586
EN
We study two effective models developed for description of superconductors with short-coherence length: (i) the extended Hubbard model with on-site attraction and intersite repulsion, (ii) the model of hard-core charged bosons on a lattice. The analysis is concentrated on the problem of phase separations and competition between superconductivity (SS) and charge-density-wave (CDW) orderings. The phase diagrams of the systems are shown to consist of at least seven different states, including 3 types of phase separated (PS) states: CDW-SS (PS1), CDW-normal (PS2) and the state of electron droplets (PS3). By taking into account the PS states and the effects of longer-range density-density interactions (beyond nearest neighbors) our paper substantially generalizes and modifies the conclusions of previous papers concerning the models considered.
2
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Ground State and Thermodynamic Properties of an Induced-Pairing Model

100%
Pawłowski G., Robaszkiewicz S.
Acta Physica Polonica A
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1998
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vol. 94
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issue 4
683-699
EN
We study the phase diagrams and thermodynamic properties of a system of coexisting wide-band current carriers (c-particles) and narrow-band electrons (d-electrons) which can form local pairs. There are two distinct mechanisms of superconductivity in the model considered: (i) the intersubsystem charge exchange, which leads to the superconducting state involving both types of electrons and (ii) the pair hopping interaction of d-electrons, leading to the superconducting state of d-particles only. In contrast to previous works on the subject, we assume an arbitrary value of on-site density interaction of d-electrons U, which allows us to study the effects of reduced d-pair binding energy. Within the approach in which the U term is exactly treated we determine the evolution of superconducting properties as a function of interactions and relative position of the bands. In a definite range of parameters the system shows features which are intermediate between those of a local pair superconductor and those of a classical BCS model. The increasing on-site Coulomb repulsion U competing with the intersubsystem charge exchange and the pair hopping interaction reduces the superconducting critical temperature. Moreover, it can induce a change of the superconducting transition into the first order one. Above a critical value of U, dependent on concentration of electrons and other interactions, the superconducting state cannot be stable at any temperature.
3
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Superconducting Properties and Phase Separation Effects in Systems with Local Pairing

100%
Pawłowski G., Robaszkiewicz S.
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EN
We examine the phase separation effects in a class of models developed for description of superconductors with local electron pairing: (i) the extended attractive Hubbard model, (ii) the model of hard-core charged bosons on a lattice. We analyse the behaviour of various superfluid characteristics as well as the evolution of the phase diagrams with increasing external magnetic field.
4
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Superconductivity versus Diagonal Disorder in the (Hard-Core) Boson-Fermion Model

81%
Pawłowski G., Robaszkiewicz S., Micnas R.
Acta Physica Polonica A
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2004
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vol. 106
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issue 5
745-749
EN
The influence of randomness of local pair site energies on superconducting properties of a system of coexisting local pairs and itinerant electrons described by the (hard-core) boson-fermion model is analyzed within variational BCS Hartree-Fock approach. Assuming the rectangular distribution function of the random potential the phase diagrams, critical temperatures, and the chemical potential behaviours are determined.
5
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Some Properties of Two-Dimensional Extended Repulsive Hubbard Model with Intersite Magnetic Interactions - A Monte Carlo Study

81%
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.
6
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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

81%
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).
7
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Monte Carlo Study of Phase Separation in Magnetic Insulators

81%
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.
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