In the paper, we study the thermodynamic and electromagnetic properties of the Penson-Kolb (PK) model, i.e., the tight-binding model for fermionic particles with the pair-hopping interaction J. We focus on the case of repulsive J (i.e., J < 0), which can stabilize the eta-pairing superconductivity with Cooper-pair center-of-mass momentum q = Q, Q = (π/a,π/a,. . . ). Numerical calculations are performed for several d-dimensional hypercubic lattices: d = 2 (the square lattice, SQ), d = 3 (the simple cubic lattice) and d = ∞ hypercubic lattice (for arbitrary particle concentration 0 < n < 2 and temperature T ). The ground state J versus n phase diagrams and the crossover to the Bose-Einstein condensation regime are analyzed and the evolution of the superfluid characteristics are examined within the (broken symmetry) Hartree-Fock approximation (HFA). The critical fields, the coherence length, the London penetration depth, and the Ginzburg ratio are determined at T = 0 and T > 0 as a function of n and pairing strength. The analysis of the effects of the Fock term on the ground state phase boundaries and on selected PK model characteristics is performed as well as the influence of the phase fluctuations on the eta-pairing superconductivity is investigated. Within the Kosterlitz-Thouless scenario, the critical temperatures TKT are estimated for d = 2 SQ lattice and compared with the critical temperature Tc obtained from HFA. We also determine the temperature Tm at which minimal gap between two quasiparticle bands vanishes in the eta-phase. Our results for repulsive J are contrasted with those found earlier for the PK model with attractive J (i.e., with J > 0).

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 t2 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.

We study the basic thermodynamic and electromagnetic properties of the superconductor described by the negative-U Hubbard model ( gap parameter ∆, critical temperature T C , London penetration depth λ, thermodynamic critical field H C and Ginzburg-Landau correlation length ξ G-L ).

PACS 74.20. ÀZ, 71.28.þD, 74.20.Mn Penson-Kolb model, i.e. the tight-binding model with the pair-hopping (intersite charge exchange) interaction J is considered. In the analysis we focus on the properties of the superconducting state with Cooper-pair center-of-mass momentum q ¼ Q (h-phase), which can develop in this model for the case of repulsive J(J < 0). The evolutions of the phase diagrams, the critical temperatures and superfluid characteristics with particle concentration and pairing strength are discussed.General formulation The Penson-Kolb (PK) model is one of the conceptually simplest models for studying superconductivity of the narrow band systems with short coherence length [1][2][3][4][5]. The model Hamiltonian has the form:where n is ¼ c þ is c is ; t is the single electron hopping integral, J is the pair hopping (intersite charge exchange) interaction, m is the chemical potential, the limit hiji restricts the sum to nearest neighbors (nn).The model includes a nonlocal pairing mechanism (the pair hopping term J) that is distinct from the on-site interaction in the attractive Hubbard (AH) model and that is the driving force of pair formation and also of their condensation.In our recent work [2] we have studied thermodynamic and electromagnetic properties of the PK model in the case of attractive J(J > 0), which stabilizes s-wave pairing state (S) analogous to that driven by the on-site attraction in the AH model. Here, we focus on the case of repulsive J(J < 0), which favorizes the h-phase, i.e. superconducting state with Cooper-pair center of mass momentum As in Ref.[2] our analysis is based on the (broken symmetry) HFA and the linear response theory. The free energy of the h-phase F h is calculated to be:wherecos k a , a ¼ x; y; . . . ;J 0 ¼ zJ, z is the number of nn, b ¼ 1=k B T. The h-pairing order parameter x h ¼

Phase diagrams and electromagnetic properties of the system of coexisting itinerant (c)-electrons and localized (d)-electrons which can form real space local pairs are analyzed. The model considered assumes arbitrary value of the on-site density interaction of d-electrons U d , which allows investigation of the effects of reduced d-pair binding energy. The phase diagrams of the system have been evaluated within the approach which treats the on-site interaction term U d exactly and the remaining interactions within the broke symmetry HFA. Within the linear response theory the electromagnetic kernels are calculated and basic superfluid characteristics of the system are determined as a function of electron concentration, interactions and relative position of the bands. Depending on parameters the model is found to exhibit several kinds of superconducting behaviors ranging from the BCS-like to the local pair-like. The relevance of obtained results to the interpretation of experimental data for short-coherence length superconductors is pointed out.

PACS 71. 28.+d, 74.20.Mn We study the two-dimensional Penson -Kolb model, i.e the tight-binding model with the pair-hopping interaction J, and analyze the effects of next-nearest neighbour hopping 2 t on the superconductivity in this system. In the analysis we focus on the properties of the superconducting state with Cooper-pair centerof-mass momentum q Q = (eta-phase), which can develop in this model for the case of repulsive J ( 0 J < ) and 0 q = (s-wave phase), which develops for the case of attractive J ( 0 J > ). The evolutions of the phase diagrams, ground state characteristics and the transition temperatures with particle concentration and pairing strength are determined for square lattice. Within the Kosterlitz -Thouless scenario the Uemura-type plots ( c T vs.) are also derived. The results for 2 0 t π are compared with those found for the case with nn hopping only. General formulationThe Penson-Kolb (PK) model is one of the conceptually simplest phenomenological models for studying superconductivity in systems with short-range, almost unretarded pairing [1][2][3][4][5][6][7]. It includes a nonlocal pairing mechanism (the pair hopping term J ) that is distinct from the on-site interaction in the attractive Hubbard (AH) model and that is the driving force of pair formation and also of their condensation. The model Hamiltonian has the following form:where ij t are the single electron hopping integrals, J is the pair hopping (intersite charge exchange) interaction, the limit ij 〈 〉 restricts the sum to nearest neighbors (nn). The Peierls factors in Eq. (1) account for the coupling of electrons to the magnetic field via its vector potential ( )Ú r A r , and e is the electron charge.In our recent works we have studied the phase diagrams and thermodynamic and electromagnetic characteristics of this model for d -dimensional hypercubic lattices 1 d £ £ • assuming hopping ij t restricted to nn [2,4,6]. Here we extend those investigations and discuss the effects of second neighbour hopping 2 0 t π , which have not been studied yet.

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.PACS numbers: 74.20.-z, 71.28.-1d, 74.25.Ηa General formulationThe Penson-Kolb (PK) model is one of the conceptually simplest phenomenological models for studying superconductivity in systems with short-range, almost unretarded pairing [1,2]. It includes a nonlocal pairing mechanism (the pair hopping term J) that is distinct from the on-site interaction in the attractive Hubbard (ΑΗ) model and that is the driving force of pair formation and also of their condensation. Thus, the superconducting properties can be essentially different in these two models [2]. In the paper we focus on the PK model with arbitrary particle concentration and discuss its superfluid characteristics which have not been considered up to now. In the analysis we have used a linear response theory [3,4] and the electromagnetic kernel has been evaluated within HFA-RPA scheme. The model Hamiltonian has the following form:where t is the single electron hopping integral, J is the pair hopping (intersite charge exchange) interaction, the limit (ii) restricts the sum to nearest neighbors (nn). The Peierls factors in (1) account for the coupling of electrons to the magnetic fleld via its vector potential A(r): Φij = (-e/ħc) fRjRi dvA(r), and e is the electron charge.(217)

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