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1
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Pseudospin Models of Superconductors with Very Short Coherence Length

100%
Robaszkiewicz S.
Acta Physica Polonica A
|
1994
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vol. 85
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issue 1
117-131
EN
We discuss a few models developed for description of superconductivity with very short coherence length and point out their formal equivalence to specific spin models, such as an anisotropic Heisenberg model (s = 1/2) with two- and four-spin exchange coupling and with fixed magnetization in the z-direction, an XY-Z model with single ion anisotropy and a Kondo-lattice model with anisotropic on-site exchange. New results concerning the phase diagram of the Hubbard model with strong attraction, the electromagnetic properties of systems with the first order superconducting transition and a possibility of local charge moment compensation in systems of coexisting bound pairs and itinerant electrons are presented.
2
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Superconducting Characteristics of the Penson-Kolb Model

64%
Czart W., Robaszkiewicz S.
Acta Physica Polonica A
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2000
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vol. 97
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issue 1
217-220
EN
We study superconducting properties of the Penson-Kolb model, i.e. the tight-binding model with the pair-hopping (intersite charge exchange) interaction J. The evolution of the critical fields, the coherence length, the Ginzburg ratio, and the London penetration depth with particle concentration n and pairing strength are determined. The results are compared with those found earlier for the attractive Hubbard model.
3
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Superconductivity, Charge Density Wave and Charge Kondo States in Systems of Coexisting Itinerant Electrons and Local Pairs

64%
Sidowski M., Robaszkiewicz S.
Acta Physica Polonica A
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2006
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vol. 109
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issue 4-5
647-652
EN
The properties of a system of coexisting local pairs and itinerant electrons described by the (hard-core) boson-fermion model are discussed. For the first time we include into analysis of the model not only the superconducting and non-ordered (normal) states but also the charge density wave phases as well as the so-called charge Kondo state. Within an extended mean-field approximation, a mutual stability of charge density wave, superconducting and charge Kondo states are determined at T = 0 in the case of half-filled fermionic and bosonic bands.
4
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Superconductivity, Charge Orderings and Phase Separations in Systems with Local Electron Pairing

64%
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.
5
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Phase Diagrams of the Penson-Kolb-Hubbard Model with Repulsive Pair-Hopping Interaction

64%
Czart W., Robaszkiewicz S.
Acta Physica Polonica A
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2016
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vol. 130
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issue 2
613-616
EN
We study the extended Hubbard model with on-site density-density U and intersite pair hopping J interactions, i.e. the Penson-Kolb-Hubbard model. This report focuses mainly on the properties of the model at T ≥q 0 in the case of repulsive J (J < 0) which may stabilize superconductivity with η-pairing. The analysis is performed within the (broken symmetry) Hartree-Fock approximation for arbitrary interaction parameters (J < 0 and U) and electron concentration (0 < n < 2) on the d = 2 square lattice. The phase diagrams of the model at T=0 and at finite temperatures are examined taking into account magnetic and charge-ordered phases and superconducting states with η- and s-wave pairing.
6
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Superconducting Properties of theη-Pairing State in the Penson-Kolb-Hubbard Model

64%
Czart W., Robaszkiewicz S.
Acta Physica Polonica A
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2004
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vol. 106
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issue 5
709-713
EN
The Penson-Kolb-Hubbard model, i.e. the Hubbard model with the pair-hopping interaction J is studied. We focus on the properties of the superconducting state with the Cooper-pair center-of-mass momentum q= Q (η-phase). The transition into theη-phase, which is favorized by the repulsive J (J<0) is found to occur only above some critical value |J_c|, dependent on band filling, on-site interaction U and band structure, and the system never exhibits standard BCS-like features. This is in obvious contrast with the properties of the isotropic s-wave state, stabilized by the attractive J and attractive U, which exhibit at T=0 a smooth crossover from the BCS-like limit to that of tightly bound pairs with increasing pairing strength.
7
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Ground State and Thermodynamic Properties of an Induced-Pairing Model

64%
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.
8
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Ground State Phase Properties of the Penson-Kolb-Hubbard Model with Attractive Pair-Hopping Interaction

64%
Czart W., Robaszkiewicz S.
Acta Physica Polonica A
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2015
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vol. 127
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issue 2
278-280
EN
We study the ground state phase diagrams of the extended Hubbard model with the pair-hopping interaction J, i.e. the Penson-Kolb-Hubbard model. We concentrate on the case of attractive J (J > 0) which can stabilise s-wave pairing superconductivity in the system and competes with the on-site repulsive U. The evolution of the phase diagrams and order parameters for arbitrary electron density and representative values of the interaction parameters are analyzed within the (broken symmetry) Hartree-Fock approximation. The calculations are performed for the case of nearest-neighbors electron hopping t on the d=2 square lattice.
9
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Stable and Metastable Phases in the Atomic Limit of the Extended Hubbard Model with Intersite Density-Density Interactions

64%
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.
10
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Ground State Phase Diagrams of the Extended Hubbard Model with Repulsive Pair-Hopping Interaction for Nonfrustrated 2D Lattice

64%
Czart W., Robaszkiewicz S.
Acta Physica Polonica A
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2015
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vol. 127
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issue 2
275-277
EN
We analyze the extended Hubbard model with pair-hopping interaction J, i.e. the Penson-Kolb-Hubbard model, in the case of repulsive J (J < 0) within the (broken symmetry) Hartree-Fock approximation. We focus our study on the effects of on-site U interaction on the mutual stability of magnetic, charge-ordered, and superconducting states including the eta-pairing phase, i.e. the state with the Cooper-pair center-of-mass momentum q=Q. The ground state phase diagrams are obtained for arbitrary particle concentration n on nonfrustrated d=2 square lattice.
11
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Electron Orderings of Half-Filled Extended Hubbard Models with Spin- and Charge-Exchange Interaction

64%
Czart W., Robaszkiewicz S.
Acta Physica Polonica A
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2006
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vol. 109
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issue 4-5
577-582
EN
The electron orderings of the half-filled extended Hubbard models (i) with anisotropic spin-exchange interactions (the t-U-J_ǁ-J_⊥ model) and (ii) with charge-exchange interaction (the t-U-I model) are discussed. In particular, we present new results concerning the phase diagrams of the model (i) in the case of uniaxial spin exchange (J_ǁ≢0) for d-dimensional hypercubic lattices (1≤d≤∞) and conclude about the basic effects of the particular spin- and charge-exchange interaction terms (J_ǁ, J_⊥, and I) in both models.
12
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Superconducting Properties and Phase Separation Effects in Systems with Local Pairing

64%
Pawłowski G., Robaszkiewicz S.
|
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.
13
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Superconductivity of the Two-Dimensional Penson-Kolb Model

64%
Czart W., Robaszkiewicz S.
Acta Physica Polonica A
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2001
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vol. 100
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issue 6
885-895
EN
Two-dimensional (d=2) Penson-Kolb model, i.e. the tight-binding model with the pair-hopping (intersite charge exchange) interaction, is considered and the effects of phase fluctuations on the s-wave superconductivity of this system are discussed within the Kosterlitz-Thouless scenario. The London penetration depth λ at T=0, the Kosterlitz-Thouless critical temperature T_c, and the Hartree-Fock approximation critical temperature T_p are determined as a function of particle concentration and interaction. The Uemura type plots (T_c vs.λ^{-2}(0)) are derived. Beyond weak coupling and for low concentrations they show the existence of universal scaling: T_c ~1/λ^2(0), as it was previously found for the attractive Hubbard model and for the models with intersite electron pairing.
14
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Superconductivity versus Diagonal Disorder in the (Hard-Core) Boson-Fermion Model

52%
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.
15
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Some Properties of Two-Dimensional Extended Repulsive Hubbard Model with Intersite Magnetic Interactions - A Monte Carlo Study

52%
Murawski S., Kapcia K., Pawłowski G., Robaszkiewicz S.
|
|
vol. 126
|
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.
16
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Superfluid Characteristics of Induced-Pairing Model

52%
Czart W. R., Szkudlarek M., Robaszkiewicz S.
Acta Physica Polonica A
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1997
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vol. 91
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issue 2
415-418
EN
We study the electromagnetic and thermodynamic properties of a model of coexisting local electron pairs and itinerant carriers coupled via the intersubsystem charge exchange. The calculations of the London penetration depth, the energy gap, the magnetic critical fields and the coherence length in the superconducting phase are performed. The effects of reduced binding energy of local pairs are discussed.
17
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Charge Ordering and Phase Separations in the Spinless Fermion Model with Repulsive Intersite Interaction

52%
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.
18
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Effects of Frustrating Hopping on Charge Ordered States in Itinerant Fermion Systems for Arbitrary Concentration in 2D Lattice

52%
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.
19
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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

52%
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.
20
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Some Exact Results for the Zero-Bandwidth Extended Hubbard Model with Intersite Charge and Magnetic Interactions

52%
Kapcia K., Kłobus W., Robaszkiewicz S.
Acta Physica Polonica A
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2015
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vol. 127
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issue 2
284-286
EN
The extended Hubbard model in the zero-bandwidth limit is studied. The effective Hamiltonian consists of (i) on-site U interaction, (ii) intersite density-density interaction W, and (iii) Ising-like magnetic exchange interaction J between the nearest-neighbors. We present rigorous (and analytical) results obtained within the transfer-matrix method for 1D chain in two particular cases: (a) W=0 and n=1; (b) U → +∞ and n=1/2 (W ≠ 0, J ≠ 0). We obtain the exact formulae for the partition functions which enables to calculate thermodynamic properties such as entropy, specific heat (c), and double occupancy per site. In both cases the system exhibits an interesting temperature dependence of c involving a characteristic two-peak structure. There are no phase transitions at finite temperatures and the only transitions occur in the ground state.
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