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    Wyświetlanie 1-8 z 8
    Tytuł:
    Checkerboard-Like Charge Ordering in Underdoped Cuprates
    Autorzy:
    Wróbel, P.
    Powiązania:
    https://bibliotekanauki.pl/articles/2046738.pdf
    Data publikacji:
    2006-04
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    74.20.Mn
    71.10.Fd
    Opis:
    Recent scanning tunneling microscopy measurements which indicate the formation of two-dimensional density modulations at some doping levels in cuprates were reviewed. A model of hard-core bosons which represent bound hole pairs in the two-dimensional doped antiferromagnet was discussed in the context of these experimental results. By means of an exact numerical diagonalization it was shown that the Coulomb repulsion between bosons brings about the formation of charge modulations.
    Źródło:
    Acta Physica Polonica A; 2006, 109, 4-5; 533-539
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    In the Search of Electron Correlation Effects in DNA
    Autorzy:
    Wróbel, A.
    Wróbel, P.
    Powiązania:
    https://bibliotekanauki.pl/articles/1537235.pdf
    Data publikacji:
    2010-08
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    87.14.gk
    75.20.-g
    71.10.Fd
    Opis:
    We discuss the energy scale separation induced in DNA by gaps between molecular orbital states of individual bases and by electron correlations. We also demonstrate how this separation gives rise to effective low energy models of electron transport in DNA.
    Źródło:
    Acta Physica Polonica A; 2010, 118, 2; 409-412
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Nonfrustrated Antiferromagnet in a Frustrated Lattice Due to Charge Ordering: A Scenario for Pairing in Layered Cobaltates
    Autorzy:
    Wróbel, P.
    Suleja, W.
    Powiązania:
    https://bibliotekanauki.pl/articles/2047314.pdf
    Data publikacji:
    2007-04
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.10.Fd
    74.20.Mn
    75.50.Ee
    Opis:
    The mechanism of pairing-symmetry selection in the weakly electron doped t-J model on the honeycomb lattice has been analyzed. The discussion of that problem has been motivated by some recent suggestions that due to charge ordering which may take place in the unconventional superconductor Na$\text{}_{x}$CoO$\text{}_{2}$· yH$\text{}_{2}$O at doping levels near x=1/3 the physics of CoO$\text{}_{2}$ planes may be effectively described in terms of a model for a weakly electron doped antiferromagnet on the honeycomb lattice. In the current publication the main emphasis has been put on reviewing experimental and theoretical work, the results of which support the scenario of charge order. In the calculation, the so-called string picture has been used. It has been demonstrated that spin fluctuations may induce in the honeycomb lattice the formation of an unconventional two-particle bound state. Upon the condensation of bound particles this mechanism may give rise to unconventional pairing. The critical value of the ratio J/t which is sufficient to induce binding has not been evaluated. It has been assumed instead that in the case of cobaltates some additional isotropic attractive interaction, for example phonon mediated, is active. Despite that the exchange of spin fluctuations is not a dominating interaction, it selects the symmetry of the paired state because it is anisotropic. C$\text{}_{3v}$ is the relevant point group for the t-J model on the honeycomb lattice. It has been shown that the bound state of two additional electrons doped to the half-filled antiferromagnetically ordered system has zero total momentum and p-wave symmetry of the irreducible representation E. The expected paired state is a mixture of a singlet and a triplet because the honeycomb lattice does not possess the inversion symmetry.
    Źródło:
    Acta Physica Polonica A; 2007, 111, 4; 563-572
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Asymmetric Tunneling Conductance in Doped Antiferromagnets
    Autorzy:
    Maciąg, A.
    Wróbel, P.
    Powiązania:
    https://bibliotekanauki.pl/articles/2046776.pdf
    Data publikacji:
    2006-04
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.10.Fd
    74.20.Mn
    74.25.Fy
    Opis:
    We calculate the tunneling conductance in the framework of the spin polaron model, which is an effective model for the t-J model, in the limit where at least short-range antiferromagnetic correlations exist. We show that both in the normal state and in the superconducting state the asymmetry of tunneling conductance is present. This fact is the implication of the particle-hole asymmetry of the spin polaron Hamiltonian in the limit of low density of spin polarons which are quasiparticles emerging in a hole doped antiferromagnet. Experimental evidence of analogous asymmetric tunneling conductance was found in the tunneling spectroscopy measurements of high T$\text{}_{c}$ superconductors.
    Źródło:
    Acta Physica Polonica A; 2006, 109, 4-5; 607-610
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Pairing-Symmetry Selection in a Weakly Doped Canted Antiferromagnet on the Triangular Lattice
    Autorzy:
    Wróbel, P.
    Suleja, W.
    Powiązania:
    https://bibliotekanauki.pl/articles/1812329.pdf
    Data publikacji:
    2008-07
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.10.Fd
    74.20.Mn
    75.50.Ee
    Opis:
    The mechanism of superconductivity generation by spin fluctuations in the electron doped canted antiferromagnet on the triangular lattice was analyzed. The underlying assumption is that the formation of the bound state is the prerequisite of pairing. The outcome of this analysis is also valid if an additional isotropic attraction is active but the anisotropic spin-fluctuation mediated force decides on the symmetry of the two-particle bound state. When the canted antiferromagnetic state is generated, the symmetry of the point group $C_{6v}$ for the triangular lattice is lowered to the symmetry of $C_{3v}$. It is demonstrated that spin fluctuations definitely favor the p-wave bound state, which transforms according to the E representation of $C_{3v}$. Since the inversion is not an element of $C_{3v}$, the parity is not a good quantum number and thus the predicted paired state will be a mixture of singlet and triplet. Such a scenario may be relevant to physics of superconducting triangular cobaltates or organics.
    Źródło:
    Acta Physica Polonica A; 2008, 114, 1; 225-228
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Influence of Electronic Structure on the Phase Diagram of Weakly Doped Superconducting Cuprates
    Autorzy:
    Maciąg, A. M.
    Wróbel, P.
    Powiązania:
    https://bibliotekanauki.pl/articles/2047334.pdf
    Data publikacji:
    2007-05
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.10.Fd
    74.20.Mn
    74.25.Dw
    Opis:
    Motivated by the difference between the experimental phase diagram of cuprates and expectations of the t-J model, we analyze the influence of the electronic structure on superconducting state generation for small levels of doping. Following some theoretical studies of the Fermi surface for hole doped superconducting cuprates we base our calculations on the t-t'-t''-J model. We construct the spin polaron model, which is an effective model for the t-J model, and we expand it by adding the new terms related to the next neighbor hopping integrals t' and t''. In the framework of this model we study evolutions of superconducting correlation functions with doping. As a result of numerical calculations we find out that superconducting state vanishes for small levels of doping and finite values of t' and t''. This is in qualitative agreement with experimental results.
    Źródło:
    Acta Physica Polonica A; 2007, 111, 5; 721-725
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Spin Polarons in Weakly Doped Antiferromagnets: Experimental Evidence Obtained for Cuprates
    Autorzy:
    Wróbel, P.
    Suleja, W.
    Eder, R.
    Powiązania:
    https://bibliotekanauki.pl/articles/1812270.pdf
    Data publikacji:
    2008-07
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.27.+a
    71.10.Fd
    74.72.-h
    Opis:
    The mechanism of spin polaron formation in moderately doped cuprates is discussed. These objects represent holes embedded into heavy clouds formed by spin fluctuations. Wave functions of spin polarons are spatially confined due to the increase in the exchange energy which is induced by hole motion giving rise to the creation of spin fluctuations. These wave functions are eigenstates of an "unperturbed" Hamiltonian which is defined by processes responsible for the tendency toward confinement. The eigenstates transform according to different irreducible representations of the point group reflecting the symmetry of the problem. Thus, the spin polarons being local wave functions resemble orbital states. The spectrum of optical conductivity in the mid-infrared range is determined by transitions between s-wave and p-wave spin polarons. The hybridization between different spin polarons which is induced by some high order processes gives rise to the formation of energy bands. The pronounced transfer of the spectral weight between different bands is induced by the coupling between spin fluctuations created by the hopping hole and local quantum fluctuations in the empty antiferromagnetic background from which an electron has been rejected, for example during the photoemission process. The form of the energy dispersion for spin polarons gives rise to the formation of a small Fermi surface at the low hole-doping range. These three above mentioned phenomena were observed in cuprates which seems to confirm the spin polaron scenario. The discussion of related experiments is the additional objective of this paper.
    Źródło:
    Acta Physica Polonica A; 2008, 114, 1; 51-58
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Unconventional Superconductivity, Spontaneous Time-Reversal Symmetry Breaking and Lowering of Kinetic Energy in Doped, Short-Range Antiferromagnets
    Autorzy:
    Wróbel, P.
    Maciąg, A.
    Eder, R.
    Fulde, P.
    Micnas, R.
    Powiązania:
    https://bibliotekanauki.pl/articles/2038385.pdf
    Data publikacji:
    2004-11
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    74.20.Mn
    71.10.Fd
    Opis:
    In this paper we focus on the anomalous temperature dependence of the in-plane conductivity and symmetry mixing of the superconducting order parameter observed in various experiments on cuprates. We show that the one-band Hubbard model is not capable of describing the physics of cuprates because the kinetic energy is lowered in this model in the superconducting state, which contradicts experimental observations. The proper model to investigate doped, short-range antiferromagnets is the t-J model, for which our results agree with experiments. We analyze a spin polaron model, that is an effective model for a doped antiferromagnet. In the framework of this model we also study the superconducting order-parameter symmetry-mixing phenomenon. We show that the expected mixing of d-wave symmetry with p-wave symmetry takes place in the superconducting order-parameter at a finite value of the doping parameter. This symmetry mixing brakes the time-reversal symmetry.
    Źródło:
    Acta Physica Polonica A; 2004, 106, 5; 575-581
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
      Wyświetlanie 1-8 z 8

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