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    Wyświetlanie 1-15 z 15
    Tytuł:
    Pairing Scenarios for the Hubbard Model in the Strong Coupling Limit
    Autorzy:
    Apinyan, V.
    Kopeć, T.
    Powiązania:
    https://bibliotekanauki.pl/articles/1812294.pdf
    Data publikacji:
    2008-07
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.27.+a
    71.10.Fd
    74.20.Rp
    Opis:
    We consider a spin-rotationally invariant Hubbard model. The original electron operator is presented in the charge-spin-fermion U(1)×SU(2) factorized form. The factorization procedure is given in terms of the emergent gauge fields. As a result, the electron appears like a composite object consisting of strongly fluctuating phase field and spatially rotating spin axis. Furthermore, we elaborate microscopically on the form of possible pairing states.
    Źródło:
    Acta Physica Polonica A; 2008, 114, 1; 159-163
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Metals and Semimetals with Strongly Correlated Electrons
    Autorzy:
    Fulde, P.
    Powiązania:
    https://bibliotekanauki.pl/articles/1953157.pdf
    Data publikacji:
    1997-01
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    75.20.Hr
    71.27.+a
    71.10.Fd
    75.30.Mb
    Opis:
    A characteristic feature of strongly correlated electrons is the appearance of low energy scales. They may result in heavy-quasiparticle excitations at low temperatures. Our understanding of these excitations has been considerably modified over the last few years. While it was originally thought that the Kondo effect is the sole source of heavy-fermion behavior, two additional mechanisms have been identified meanwhile. One is based on the Zeeman effect and occurs in Nd$\text{}_{2-x}$Ce$\text{}_{x}$CuO$\text{}_{4}$. The other is operative in the semimetal Yb$\text{}_{4}$As$\text{}_{3}$ and is based on nearly half filled one-dimensional Hubbard chains. In all three cases the low energy excitations result from spin degrees of freedom.
    Źródło:
    Acta Physica Polonica A; 1997, 91, 1; 67-76
    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:
    On the BCS-BEC Crossover in the 2D Asymmetric Attractive Hubbard Model
    Autorzy:
    Kujawa-Cichy, A.
    Powiązania:
    https://bibliotekanauki.pl/articles/1426931.pdf
    Data publikacji:
    2012-05
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.10.Fd
    74.20.Rp
    71.27.+a
    71.10.Hf
    Opis:
    We analyze the evolution from the weak coupling (BCS-like limit) to the strong coupling limit of tightly bound local pairs in the 2D asymmetric attractive Hubbard model, in the presence of the Zeeman magnetic field (h). The broken symmetry Hartree approximation is used. We also apply the Kosterlitz-Thouless scenario to determine the phase coherence temperatures. We obtain that for the spin dependent hopping integrals $(t^{↑} ≠ t^{↓})$ the homogeneous polarized superfluid $(SC_{M})$ phase in the ground state for the strong attraction and lower filling can be stabilized. We find a topological quantum phase transition (the Lifshitz type) from the unpolarized superfluid phase $(SC_0)$ to $SC_{M}$ and tricritical point in the (h - μ) and spin polarization (P) vs. attraction (U < 0) ground state phase diagrams. The finite temperatures phase diagrams for $t^{↑} ≠ t^{↓}$ are constructed.
    Źródło:
    Acta Physica Polonica A; 2012, 121, 5-6; 1066-1069
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    On the Imbalanced d-Wave Superfluids Within the Spin Polarized Extended Hubbard Model: Weak Coupling Limit
    Autorzy:
    Kujawa-Cichy, A.
    Powiązania:
    https://bibliotekanauki.pl/articles/1431324.pdf
    Data publikacji:
    2012-04
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.10.Fd
    74.20.Rp
    71.27.+a
    71.10.Hf
    Opis:
    We investigate the superfluid properties of d-wave pairing symmetry within the Extended Hubbard Model (EHM) in a magnetic field. We analyze the temperature and magnetic field dependencies of the order parameter. We find that in the two-dimensional case, the spatially homogeneous spin polarized superfluidity (SC(P≠0)) is stable in the weak coupling limit, at T=0, as opposed to the s-wave pairing symmetry case in 2D. We construct the ground state phase diagrams both for fixed chemical potential and electron concentration. Furthermore, we obtain the temperature vs. magnetic field and temperature vs. spin polarization phase diagrams.
    Źródło:
    Acta Physica Polonica A; 2012, 121, 4; 824-827
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Properties of the Bound States on a Hexagonal Lattice
    Autorzy:
    Bak, M.
    Powiązania:
    https://bibliotekanauki.pl/articles/2047341.pdf
    Data publikacji:
    2007-05
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.10.Fd
    74.20.Rp
    71.27.+a
    71.10.Hf
    Opis:
    Two electrons on a lattice pose an interesting problem which can be solved exactly. When the effective interactions are strong enough, the bound states are formed. The properties of such bound pairs in an extended Hubbard model are examined for both signs of nearest neighbor hopping integral t. The stabilization of s*-wave pairing is found for t<0 and of f-wave for t>0. Phase diagrams for the existence of none, one or two s*-wave solutions are calculated.
    Źródło:
    Acta Physica Polonica A; 2007, 111, 5; 781-789
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Anderson-Kondo Lattice Hamiltonian from the Anderson-Lattice Model: A Modified Schrieffer-Wolff Transformation and the Effective Exchange Interactions
    Autorzy:
    Kądzielawa-Major, E.
    Spałek, J.
    Powiązania:
    https://bibliotekanauki.pl/articles/1374041.pdf
    Data publikacji:
    2014-10
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.10.Fd
    71.20.Eh
    71.27.+a
    71.70.Gm
    Opis:
    We derive the Anderson-Kondo lattice model by applying canonical perturbation expansion for the Anderson-lattice model in direct space. The transformation is carried out up to the fourth order by a modified Schrieffer-Wolff transformation: we separate the part of hybridization term responsible for the high-energy processes (involving the largest in-the-system intraatomic Coulomb interaction between f electrons) and replace it with the virtual processes in higher orders. The higher-order processes lead to three separate exchange interactions. The obtained Hamiltonian contains both the Kondo (f-c) and the superexchange (f-f) interactions, as well as a residual hybridization responsible for the heavy-quasiparticle formation. This effective Hamiltonian can be used to analyze the magnetic or the paired states, as well their coexistence in heavy-fermion systems. The magnitudes of both the Kondo exchange and the superexchange integrals are estimated as a function of bare hybridization magnitude.
    Źródło:
    Acta Physica Polonica A; 2014, 126, 4a; A-100-A-103
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Mixed State in t-U-J-V Model
    Autorzy:
    Tobijaszewska, B.
    Bak, M.
    Powiązania:
    https://bibliotekanauki.pl/articles/1810406.pdf
    Data publikacji:
    2009-01
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    74.20.-z
    74.20.Rp
    74.25.Dw
    71.10.Fd
    71.27.+a
    Opis:
    The t-U-J-V model in the U → ∞ limit is treated by renormalized mean-field theory for arbitrary electron density. Superconductivity of s+id symmetry is found in the large parts of the phase diagram. Calculations also show a possibility of phase separation.
    Źródło:
    Acta Physica Polonica A; 2009, 115, 1; 135-137
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    On the BCS-BEC Crossover in the Spin-Polarized Attractive Hubbard Model at T=0
    Autorzy:
    Kujawa-Cichy, A.
    Powiązania:
    https://bibliotekanauki.pl/articles/1537284.pdf
    Data publikacji:
    2010-08
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.10.Fd
    74.20.Rp
    71.27.+a
    71.10.Hf
    Opis:
    The influence of the Zeeman magnetic field (h) on the ground state evolution of superfluid properties from the weak coupling (BCS-like) to the strong coupling limit of tightly bound local pairs with increasing attraction has been studied. The analysis was carried out within the spin-polarized attractive Hubbard model, for a fixed number of particles and a d = 3 simple cubic lattice. The broken symmetry Hartree approximation was used. For strong attraction and in the dilute limit, the homogeneous magnetized superconducting phase $(SC_{M})$ was found in the h-|U| and P-|U| diagrams.
    Źródło:
    Acta Physica Polonica A; 2010, 118, 2; 423-425
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    On the Phase Diagram of Spin-Polarized Attractive Hubbard Model: Weak Coupling Limit
    Autorzy:
    Kujawa, A.
    Micnas, R.
    Powiązania:
    https://bibliotekanauki.pl/articles/1812269.pdf
    Data publikacji:
    2008-07
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.10.Fd
    74.20.Rp
    71.27.+a
    71.10.Hf
    Opis:
    The superfluid properties of the attractive Hubbard model in a Zeeman magnetic field, and in the weak coupling regime were investigated. The temperature and magnetic field dependences of the order parameter were analyzed. Furthermore, the temperature vs. magnetic field and temperature vs. spin polarization phase diagrams for the 2D and 3D lattices were obtained. For some parameters a reentrant transition was found.
    Źródło:
    Acta Physica Polonica A; 2008, 114, 1; 43-50
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Some Properties of the Spin-Polarized Attractive Hubbard Model
    Autorzy:
    Kujawa, A.
    Micnas, R.
    Powiązania:
    https://bibliotekanauki.pl/articles/1810407.pdf
    Data publikacji:
    2009-01
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.10.Fd
    74.20.Rp
    71.27.+a
    71.10.Hf
    Opis:
    The influence of a Zeeman magnetic field on the superconducting characteristics of the attractive Hubbard model was investigated. The ground state and temperature phase diagrams were obtained for a fixed number of particles. Two critical magnetic fields were found for the first order phase transition from the superconducting to the normal state for n ≠ 1. For some range of parameters a reentrant transition was found and gapless superconductivity was obtained. The significance of the Hartree term was also analyzed.
    Źródło:
    Acta Physica Polonica A; 2009, 115, 1; 138-140
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    The BCS-BEC Crossover and Superconductivity in a Lattice Fermion Model with Hard Core Repulsion
    Autorzy:
    Kujawa, A.
    Powiązania:
    https://bibliotekanauki.pl/articles/2047337.pdf
    Data publikacji:
    2007-05
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.10.Fd
    74.20.Rp
    71.27.+a
    71.10.Hf
    Opis:
    We considered the BCS-BEC crossover at T=0 in the framework of a lattice fermion model (the extended Hubbard model). We analyzed the case of on-site hard core repulsion (U=∞) and intersite attraction (W<0) in a 3D system with extended s-wave pairing symmetry. The mechanisms of density-driven and attraction-strength-driven crossover were analyzed. We obtained the ground state phase diagram. The analyzed quantities are: chemical potential, order parameter, coherence length, and fraction of condensed particles.
    Źródło:
    Acta Physica Polonica A; 2007, 111, 5; 745-752
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Critical Values for Electron Pairing in t-U-J-V and t-J-V Models
    Autorzy:
    Bak, M.
    Powiązania:
    https://bibliotekanauki.pl/articles/1812342.pdf
    Data publikacji:
    2008-07
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    74.20.-z
    74.20.Fg
    74.20.Rp
    74.25.Dw
    71.10.Fd
    71.27.+a
    Opis:
    The critical values for pairing of s*- and d-wave symmetry are calculated within the t-U-J-V and the t-J-V models in renormalized mean field theory. It is shown that the first model yields the proper critical values in the low density limit while the other does not. Renormalized mean field theory strongly decreases the absolute value of (negative) critical values for larger electron densities.
    Źródło:
    Acta Physica Polonica A; 2008, 114, 1; 275-279
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Universal Relations in the Attractive Hubbard Model with Next-Nearest Neighbor Hopping
    Autorzy:
    Bak, M.
    Powiązania:
    https://bibliotekanauki.pl/articles/2046748.pdf
    Data publikacji:
    2006-04
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    74.25.Fy
    71.10.Fd
    74.20.Fg
    71.27.+a
    72.10.-d
    Opis:
    Recently a new universal relation between superfluid densityρ$\text{}_{s}$ andσ$\text{}_{dc}$T$\text{}_{c}$, where σ$\text{}_{dc}$ is direct current conductivity and T$\text{}_{c}$ - superconducting critical temperature, was found by Homes et al. (Nature 430, 539 (2004)). The theoretical derivation of the relation based on BCS theory is correct only qualitatively. In the present paper this relation is calculated theoretically, using Hartree-Fock approximation and second-order perturbation theory. It is found that although correct qualitatively, quantitatively the results are too small. Inclusion of the second neighbor hopping improves the results.
    Źródło:
    Acta Physica Polonica A; 2006, 109, 4-5; 565-568
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Electronic Correlations within Fermionic Lattice Models
    Autorzy:
    Matlak, M.
    Grabiec, B.
    Krawiec, S.
    Powiązania:
    https://bibliotekanauki.pl/articles/2047738.pdf
    Data publikacji:
    2007-09
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.10.-w
    71.10.Ca
    71.10.Fd
    71.20.Be
    71.27.+a
    71.28.+d
    73.21.La
    78.67.Bf
    Opis:
    We investigate two-site electronic correlations within generalized Hubbard model, which incorporates the conventional Hubbard model (parameters: t (hopping between nearest neighbours), U (Coulomb repulsion (attraction))) supplemented by the intersite Coulomb interactions (parameters: J$\text{}^{(1)}$ (parallel spins), J$\text{}^{(2)}$ (antiparallel spins)) and the hopping of the intrasite Cooper pairs (parameter: V). As a first step we find the eigenvalues E$\text{}_{α}$ and eigenvectors |E$\text{}_{α}$〉 of the dimer and we represent each partial Hamiltonian E$\text{}_{α}$|E $\text{}_{α}$〉〈 E$\text{}_{α}$| (α=1,2,...,16) in the second quantization with the use of the Hubbard and spin operators. Each dimer energy level possesses its own Hamiltonian describing different two-site interactions which can be active only in the case when the level will be occupied by the electrons. A typical feature is the appearance of two generalized t-J interactions ascribed to two different energy levels which do not vanish even for U=J$\text{}^{(1)}$=J{(2)}=V=0 and their coupling constants are equal to ±t in this case. In the large linebreak U-limit for J$\text{}^{(1)}$=J$\text{}^{(2)}$=V=0 there is only one t-J interaction with coupling constant equal to 4t$\text{}^{2}$/|U| as in the case of a real lattice. The competition between ferromagnetism, antiferromagnetism and superconductivity (intrasite and intersite pairings) is also a typical feature of the model because it persists in the case U=J$\text{}^{(1)}$=J$\text{}^{(2)}$=V=0 and t≢0. The same types of the electronic, competitive interactions are scattered between different energy levels and therefore their thermodynamical activities are dependent on the occupation of these levels. It qualitatively explains the origin of the phase diagram of the model. We consider also a real lattice as a set of interacting dimers to show that the competition between magnetism and superconductivity seems to be universal for fermionic lattice models.
    Źródło:
    Acta Physica Polonica A; 2007, 112, 3; 537-547
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
      Wyświetlanie 1-15 z 15

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