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1
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Influence of Coulomb Interactions on the Properties of Induced Pairing Model

100%
Czart W., Robaszkiewicz S.
Acta Physica Polonica A
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2001
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vol. 100
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issue 1
75-88
EN
We study the superconducting properties of a model of coexisting itinerant carriers and local pairs with finite binding energy, taking into account the effects of Coulomb (density-density) and direct pair hopping interactions. The evolution of the phase diagrams and superfluid characteristics with electron concentration, interaction parameters and the relative position of the bands is examined. The model is found to exhibit several kinds of superconducting behaviors ranging from the BCS-like to the local-pair-like. The relevance of the obtained results to the interpretation of experimental data for the doped bismuthates (Ba_{1-x}K_xBiO_3 and BaPb_{1-x}BiO_3) is pointed out.
2
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Superconducting Characteristics of the Penson-Kolb Model

100%
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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Phase Separations in the Narrow-Bandwidth Limit of the Penson-Kolb-Hubbard Model at Zero Temperature

100%
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.
4
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Phase Diagrams of the Penson-Kolb-Hubbard Model with Repulsive Pair-Hopping Interaction

100%
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.
5
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Ground State Phase Diagram of the Extended Hubbard Model with Pair-Hopping Interaction in the Limit of Very Narrow Bandwidth

100%
Kapcia K., Czart W.
Acta Physica Polonica A
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2016
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vol. 130
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issue 2
617-620
EN
The extended Hubbard model with the pair-hopping interaction, i.e. the Penson-Kolb-Hubbard model, is one of the conceptually simplest phenomenological models for studying correlations and for description of superconductivity in very narrow-band systems with short-range, almost unretarded pairing. We present ground state phase diagrams of the model derived within the broken-symmetry Hartree-Fock approximation in the narrow-bandwidth regime and compare these results with the exact ones in the atomic limit for the limit of high dimensions. The investigation of the diagrams of the model show that results obtained within both approaches are consistent, although for the case of the finite single-electron hopping phases with magnetic long-range order also occur.
6
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Superconducting Properties of theη-Pairing State in the Penson-Kolb-Hubbard Model

100%
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 Phase Properties of the Penson-Kolb-Hubbard Model with Attractive Pair-Hopping Interaction

100%
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.
8
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Ground State Phase Diagrams of the Extended Hubbard Model with Repulsive Pair-Hopping Interaction for Nonfrustrated 2D Lattice

100%
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.
9
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Electron Orderings of Half-Filled Extended Hubbard Models with Spin- and Charge-Exchange Interaction

100%
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.
10
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Superconductivity of the Two-Dimensional Penson-Kolb Model

100%
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.
11
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Charge Ordering and Phase Separations in the Spinless Fermion Model with Repulsive Intersite Interaction

81%
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.
12
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Effects of Frustrating Hopping on Charge Ordered States in Itinerant Fermion Systems for Arbitrary Concentration in 2D Lattice

81%
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.
13
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Charge Orderings and Phase Separations in Itinerant Fermion Systems at Half Filling

81%
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).
14
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Charge Ordering and Phase Separations in the Molecular Crystal Model

81%
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.
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