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Wyświetlanie 1-15 z 89
Tytuł:
Superconductivity of the Weak Hopping Anderson Lattice with Generalized Electron Interactions
Autorzy:
Trieu, Duong Hai
Powiązania:
https://bibliotekanauki.pl/articles/1891689.pdf
Data publikacji:
1991-10
Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Tematy:
74.20.-z
Opis:
We study the Anderson lattice with a weak hopping amplitude t and generalized electron interactions. The interactions between f electrons, between f and conduction electrons and between conduction electrons are taken into account. An effective Hamiltonian in a second order perturbation in t is derived. Using this Hamiltonian, we study superconductivity. Singlet d-like superconductivity may occur in presence of the above interactions. In some cases the interactions lead to suppressing superconductivity.
Źródło:
Acta Physica Polonica A; 1991, 80, 4; 601-610
0587-4246
1898-794X
Pojawia się w:
Acta Physica Polonica A
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
On the Anharmonic Polaronic Model
Autorzy:
Konior, J.
Powiązania:
https://bibliotekanauki.pl/articles/1923721.pdf
Data publikacji:
1992-11
Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Tematy:
71.38.+i
74.20.-z
33.10.Cs
Opis:
Anharmonic polaronic model is considered. The model consists of the electronic subsystem, described within the extended Hubbard model, coupled to anharmonic local phonons. By the canonical Holstein-Lang-Firsov transformation the electron-phonon interaction term is ruled out and, then, the effective electronic Hamiltonian is obtained by averaging over the ground state of the phononic subsystem. It is shown that anharmonicity introduces two main additional factors as compared with the harmonic case. First, the band narrowing factor is less rapidly decaying function of the electron-phonon interaction strength. Second, anharmonicity introduces further renormalization of the on- and intersite interaction between fermions.
Źródło:
Acta Physica Polonica A; 1992, 82, 5; 773-776
0587-4246
1898-794X
Pojawia się w:
Acta Physica Polonica A
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
From Weak to Strong Coupling Superconductivity: Collective Modes in a Model with Local Attraction
Autorzy:
Kostyrko, T.
Micnas, R.
Powiązania:
https://bibliotekanauki.pl/articles/1929358.pdf
Data publikacji:
1993-06
Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Tematy:
74.20.-z
71.28.+d
73.20.Mf
Opis:
We discuss the density-density response functions and the collective excitation spectrum for the extended Hubbard model with on-site attraction and arbitrary electron density, in the superconducting ground state. For the short-range intersite interaction we find the sound wave-like modes with the velocity interpolating smoothly between the weak (v ∝ 2Zt, |U| ≪ 2Zt) and strong coupling (v ∝ Zt$\text{}^{2}$/|U|,|U| ≫ 2Zt) limits. The latter agrees with the results obtained from an effective pseudospin Hamiltonian valid in the strong coupling limit. In the weak coupling regime, we obtain that apart from a commensurate charge density wave instability, an increase in the intersite Coulomb repulsion can give rise to a charge density wave incommensurate with the lattice period, away from half-filling. The ground state phase diagram for 2D square lattice, including singlet superconducting ground state, electronic droplet formation, and charge density waves is determined. It is also shown that the energy of collective excitations evolves smoothly from weak to strong coupling limit for a 2D lattice, in the case of a long-range Coulomb interaction. Finally, the mode coupling effects in the frequency behavior of the density-density response functions are analyzed.
Źródło:
Acta Physica Polonica A; 1993, 83, 6; 837-848
0587-4246
1898-794X
Pojawia się w:
Acta Physica Polonica A
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
A Pairwise Approximation in the Hubbard Model
Autorzy:
Szeftel, J.
Powiązania:
https://bibliotekanauki.pl/articles/1930569.pdf
Data publikacji:
1994-02
Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Tematy:
71.10.+x
71.45.-d
74.20.-z
Opis:
The Hubbard Hamiltonian is projected onto a representation consisting of electron pairs characterised by the momentum of their centre of mass. Within this approximation the electron gas can be viewed as a collection of subsets, each of which contains a constant number of electron pairs, all having the same centre of mass momentum. As these subsets are decoupled, the Hubbard Hamiltonian is diagonalised to give two types of many-body eigenstates: correlated and uncorrelated. The uncorrelated pairs build up an ideal Fermi gas. Excellent agreement is found for the uncorrelated energy calculated at zero temperature in one dimension between this model and the Bethe ansatz, for arbitrary electron concentration and magnitude of the electron interaction. The correlated states turn out to be of the BCS type.
Źródło:
Acta Physica Polonica A; 1994, 85, 2; 329-335
0587-4246
1898-794X
Pojawia się w:
Acta Physica Polonica A
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Antiferromagnetism and Superconductivity in Three-Band Hubbard Model
Autorzy:
Dutka, J.
Oleś, A.
Powiązania:
https://bibliotekanauki.pl/articles/1930567.pdf
Data publikacji:
1994-02
Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Tematy:
75.50.Ee
74.20.-z
Opis:
We determined the phase diagram of the three-band model for CuO$\text{}_{2}$ planes of high temperature superconductors with an attractive interoxygen interaction U$\text{}_{pp}$. The found magnetic moments of 0.47µ$\text{}_{B}$ in the undoped system, the disappearance of antiferromagnetic order at the doping of δ ≈ 0.06 hole per CuO$\text{}_{2}$ unit, as well as the enhanced tendency to superconductivity with the decreasing value of charge-transfer gap, are in good agreement with the experimental findings.
Źródło:
Acta Physica Polonica A; 1994, 85, 2; 323-327
0587-4246
1898-794X
Pojawia się w:
Acta Physica Polonica A
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
On Aharonov-Bohm Effect in Multiconnected Superconductor
Autorzy:
Sazonov, S. N.
Powiązania:
https://bibliotekanauki.pl/articles/1931826.pdf
Data publikacji:
1994-12
Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Tematy:
74.55.+h
74.20.-z
Opis:
The system of co-axial cylinders: superconductor-ferromagnet- superconductor is considered. The temperature of the outer superconductor transition to the normal state, derived from the Ginzburg-Landau energy functional, depends on the state of the inner one. The quantity of heat is evaluated, which is liberated at the inner cylinder pushing out of core.
Źródło:
Acta Physica Polonica A; 1994, 86, 6; 987-993
0587-4246
1898-794X
Pojawia się w:
Acta Physica Polonica A
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Parametric Equations for the Energy Gap. Extended BCS Case
Autorzy:
Gonczarek, R.
Mulak, M.
Powiązania:
https://bibliotekanauki.pl/articles/1931831.pdf
Data publikacji:
1994-12
Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Tematy:
74.20.-z
Opis:
The generalised form of the gap equation which comprises also some non-BCS approaches is defined and solved. The parametric gap equations have been derived in a few equivalent cases and discussed in$\text{}_{}$dissimilar limits. It is shown that the relations between the critical temperature and zero-temperature energy gap can be extended for all systems under consideration. The main benefits and incontestable advantage of the presented method are demonstrated with regard to the general BCS case.
Źródło:
Acta Physica Polonica A; 1994, 86, 6; 1007-1014
0587-4246
1898-794X
Pojawia się w:
Acta Physica Polonica A
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Pseudospin Models of Superconductors with Very Short Coherence Length
Autorzy:
Robaszkiewicz, S.
Powiązania:
https://bibliotekanauki.pl/articles/1860991.pdf
Data publikacji:
1994-01
Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Tematy:
74.20.-z
05.30.Jp
71.28.+d
Opis:
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.
Źródło:
Acta Physica Polonica A; 1994, 85, 1; 117-131
0587-4246
1898-794X
Pojawia się w:
Acta Physica Polonica A
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Temperature Dependence of the Electron Raman Light Scattering in Normal Metals and Superconductors
Autorzy:
Falkovsky, L.
Klama, S.
Powiązania:
https://bibliotekanauki.pl/articles/1930519.pdf
Data publikacji:
1994-02
Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Tematy:
74.20.Fg
74.20.-z
78.30.Er
Opis:
Within the anisotropic BCS model the temperature dependence of the electron Raman light scattering in normal metals and superconductors is investigated. In either case analytical expressions describing the intensity of scattered light as a function of the Raman frequency and temperature are derived.
Źródło:
Acta Physica Polonica A; 1994, 85, 2; 311-315
0587-4246
1898-794X
Pojawia się w:
Acta Physica Polonica A
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
The Polaron Scenario for High-T$\text{}_{c}$ Superconductivity
Autorzy:
Ranninger, J.
Powiązania:
https://bibliotekanauki.pl/articles/1860957.pdf
Data publikacji:
1994-01
Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Tematy:
74.20.+z
71.38.+i
67.20.+k
Opis:
On the basis of experimental evidence we conclude that high-T$\text{}_{c}$ superconducting materials are composed of two distinct subsystems: one in which electrons are bound together in form of localized pairs of small polarons (bipolarons) and one in which electrons exist as strongly correlated Fermions. Upon doping one passes at a critical doping rate from the insulating parent compounds to metallic compounds. There, a charge transfer mechanism between the two subsystems sets in abruptly by which bipolarons can decay into pairs of electrons and vice versa. Such a picture can be modelled by a mixture of interacting Bosons and Fermions. A superconducting ground state for such a system develops at low temperature which is characterized by the opening of a gap in the single electron spectrum and the appearance of collective Boson excitations with a linear spectrum which is confined to the energy regime of the gap. Above T$\text{}_{c}$, a pseudo-gap remains upon closing the superconducting gap. There are strong indications that the normal state has non Fermi liquid behaviour.
Źródło:
Acta Physica Polonica A; 1994, 85, 1; 89-102
0587-4246
1898-794X
Pojawia się w:
Acta Physica Polonica A
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Structures of Thermodynamic Functions for S-Paired Fermi Systems in Parametric Equations Approach
Autorzy:
Mulak, M.
Gonczarek, R.
Powiązania:
https://bibliotekanauki.pl/articles/1945431.pdf
Data publikacji:
1996-05
Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Tematy:
74.20.-z
Opis:
The general method of investigation of S-paired Fermi systems is given and discussed as generalization of parametric equation for the energy gap. The additional parametric equations for thermodynamic potential and heat capacity are obtained as functions of introduced parameters. The detailed calculations are given for a few exemplary chosen magnetic fields and superflows. Moreover, we consider the case of so-called statistical spin liquid, which constitutes non-BCS model.
Źródło:
Acta Physica Polonica A; 1996, 89, 5-6; 689-698
0587-4246
1898-794X
Pojawia się w:
Acta Physica Polonica A
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Superconductivity, Charge Orderings and Phase Separations in Systems with Local Electron Pairing
Autorzy:
Robaszkiewicz, S.
Pawłowski, G.
Powiązania:
https://bibliotekanauki.pl/articles/1946347.pdf
Data publikacji:
1996-09
Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Tematy:
74.20.-z
05.30.Jp
71.28.+d
Opis:
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.
Źródło:
Acta Physica Polonica A; 1996, 90, 3; 569-586
0587-4246
1898-794X
Pojawia się w:
Acta Physica Polonica A
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Crossover from BCS to Bose-Einstein in Hubbard Model
Autorzy:
Pedersen, M. H.
Schneider, T.
Beck, H.
Powiązania:
https://bibliotekanauki.pl/articles/1955835.pdf
Data publikacji:
1997-02
Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Tematy:
71.27.+a
71.10.Fd
74.20.-z
71.10.-w
Opis:
We generalize the standard attractive Hubbard, having an on-site pair creation operator Q$\text{}_{i}$ = a$\text{}_{i↑}$a$\text{}_{i↓}$, to one with n components Q$\text{}_{iα}$ = a$\text{}_{iα↑}$a$\text{}_{iα↓}$, α=1,2,...,n. In the limit n → ∞ we obtain the Ginzburg-Landau functional. On this basis we explore the crossover from weak (BCS) to strong coupling (Bose-Einstein condensation) superconductivity. The associated self-consistent equations for the Ginzburg-Landau parameters are similar to those of the T-matrix approach. The evolution of the band structure with increasing interaction strength is studied and correlated with the behavior of the pair propagator and the transition temperature. We find that the pairing interaction creates a new band which moves downwards in energy as the interaction strength increases and separates into a lower Hubbard band when the interaction strength becomes comparable to the band width. In the strong coupling regime, a third band with small spectral weight is also found in between the lower and upper Hubbard bands.
Źródło:
Acta Physica Polonica A; 1997, 91, 2; 419-422
0587-4246
1898-794X
Pojawia się w:
Acta Physica Polonica A
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Discontinuous Phase Transitions in S-Paired Fermi Systems
Autorzy:
Mulak, M.
Gonczarek, R.
Powiązania:
https://bibliotekanauki.pl/articles/1968488.pdf
Data publikacji:
1997-12
Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Tematy:
74.20.-z
Opis:
The most general S-paired systems of fermions for which a gap equation can be separated according to the specific redefined variables, are considered. Moreover, a solution of this gap equation (gap energy) is obtained in a parametric form. It allows us to investigate thermodynamic properties of such systems i.e. the thermodynamic potential, entropy, heat capacity and critical magnetic induction. We show also that the heat capacity difference between the superconducting and the normal phase is entirely determined by the order parameter. The obtained results reveal possibilities of discontinuous phase transition to the normal state and moreover, the theorem on existence of such transitions is formulated and proved. The condition for the BCS-type energy gap is given, which additionally classifies properties of the heat capacity of S-paired phase by virtue of the shape of the energy gap. In order to illustrate the developed formalism, the numerical results for the so-called statistical spin liquids and application of the established methods to the experimental data conversion, are presented and discussed.
Źródło:
Acta Physica Polonica A; 1997, 92, 6; 1177-1189
0587-4246
1898-794X
Pojawia się w:
Acta Physica Polonica A
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Effects of Coulomb Interactions Between Fermions on Superconductivity in Boson-Fermion Model
Autorzy:
Kostyrko, T.
Powiązania:
https://bibliotekanauki.pl/articles/1955831.pdf
Data publikacji:
1997-02
Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Tematy:
67.40.Db
74.20.-z
05.30.-d
Opis:
Influence of Coulomb repulsion between fermions on stability of the superconducting state in the model of boson-like local electron pairs hybridized with wide-band fermions is studied. It is shown rigorously that the Bose condensation of the local pairs implies a divergence of the pairing susceptibility in the fermion subsystem. Charge carrier concentration dependence of the critical temperature T$\text{}_{c}$ is evaluated with the help of random phase approximation and spectral properties of the bosons at T$\text{}_{c}$ are analyzed.
Źródło:
Acta Physica Polonica A; 1997, 91, 2; 399-403
0587-4246
1898-794X
Pojawia się w:
Acta Physica Polonica A
Dostawca treści:
Biblioteka Nauki
Artykuł
Wyświetlanie 1-15 z 89

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