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Berlin Brandenburg

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  • 1
    Language: English
    In: Journal of Superconductivity and Novel Magnetism, 2016, Vol.29(12), pp.3093-3097
    Description: Cluster perturbation theory is used to calculate band structure, spectral functions, Fermi surface, and spin and charge susceptibilities for the two-orbital model of iron pnictides with the on-site multiorbital Hubbard interactions. Susceptibilities are calculated within the approximation combining the cluster perturbation theory for the self-energy corrections and the random-phase approximation (RPA) for the vertex renormalizations. Calculations for the small values of Hubbard repulsion U ≤ 2 eV confirm that the rigid band approximation and RPA for the spin and charge susceptibilities are suitable approaches for the case of weak interactions.
    Keywords: Fe-based superconductors ; Mutiorbital models ; Cluster perturbation theory
    ISSN: 1557-1939
    E-ISSN: 1557-1947
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  • 2
    Language: English
    In: Journal of Superconductivity and Novel Magnetism, 2016, Vol.29(4), pp.1089-1095
    Description: The s ± and s ++ models for the superconducting state are subject of intense studies regarding Fe-based superconductors. Depending on the parameters, disorder may leave intact or suppress T c in these models. Here, we study the special case of disorder with equal values of intra- and interband impurity potentials in the two-band s ± and s ++ models. We show that this case can be considered as an isolated point and T c there has maximal damping for a wide range of parameters.
    Keywords: Multiband superconductivity ; Impurity scattering ; Fe-based superconductors
    ISSN: 1557-1939
    E-ISSN: 1557-1947
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  • 3
    Language: English
    In: Journal of Superconductivity and Novel Magnetism, 2013, Vol.26(8), pp.2665-2668
    Description: We formulate and study the three-orbital model for iron-based superconductors. Results for the band structure, Fermi surface, and the spin susceptibility in both normal and superconducting s ± states are presented. We also discuss the pairing interaction and show that the dominant part of it should come from the intraorbital scattering.
    Keywords: Fe-based superconductors ; Spin susceptibility ; Low-energy model
    ISSN: 1557-1939
    E-ISSN: 1557-1947
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  • 4
    Language: English
    In: Journal of Superconductivity and Novel Magnetism, 2016, Vol.29(4), pp.1127-1132
    Description: Slow oscillations (SlO) of magnetoresistance is a convenient tool to measure electronic structure parameters in quasi-two-dimensional metals. We study the possibility to apply this method to multi-band conductors, e.g., to iron-based high-temperature superconducting materials. We show that SlO can be used to measure the interlayer transfer integral in multi-band conductors similar to single-band metals. In addition, the SlO allow to measure and compare the effective masses or the electron scattering rates in various bands.
    Keywords: Fermi surface ; Quantum oscillations ; Fe-based superconductors ; Slow oscillations ; Magnetoresistance
    ISSN: 1557-1939
    E-ISSN: 1557-1947
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  • 5
    Language: English
    In: Journal of Superconductivity and Novel Magnetism, 2013, Vol.26(8), pp.2607-2609
    Description: We consider the doping dependence of the normal and superconducting properties of La 2 Sr 2− x CuO 2 and YBa 2 Cu 3 O 7 in the low energy effective model based on the ab initio LDA+GTB calculations. With doping we have found a concentration region with electronic instability of the uniform state. We have shown that two quantum phase transitions (QPT) of the Lifshitz type correspond well to the experimental phase diagram. For superconducting state we have considered both magnetic and phonon mechanisms of pairing.
    Keywords: Strong electron correlations ; Fermi surface reconstruction
    ISSN: 1557-1939
    E-ISSN: 1557-1947
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  • 6
    Language: English
    In: Journal of Superconductivity and Novel Magnetism, 2013, Vol.26(9), pp.2873-2874
    Description: We study the spin resonance peak in recently discovered iron-based superconductors. The resonance peak observed in inelastic neutron scattering experiments agrees well with predicted results for the extended s -wave ( s ± ) gap symmetry. Recent neutron scattering measurements show that there is a disparity between transverse and longitudinal components of the dynamical spin susceptibility. Such breaking of the spin-rotational invariance in the spin-liquid phase can occur due to spin-orbit coupling. We study the role of the spin-orbit interaction in the multiorbital model for Fe-pnictides and show how it affects the spin resonance feature.
    Keywords: Fe-based superconductors ; Spin-resonance peak ; Spin-orbit coupling
    ISSN: 1557-1939
    E-ISSN: 1557-1947
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  • 7
    Language: English
    In: Journal of Superconductivity and Novel Magnetism, 2013, Vol.26(9), pp.2831-2835
    Description: We consider the doping dependence of the normal and superconducting properties of La 2− x Sr x CuO 4 in the low energy effective model based on the ab initio LDA+GTB calculations. We have found that two quantum phase transitions (QPT) of the Lifshitz type correspond well to the experimental phase diagram. For superconducting state, we have considered both magnetic and phonon mechanisms of pairing. Finally, we compare the true Fermi surface and the spectral intensity map seen in ARPES within a new norm conserving cluster perturbation theory (NC-CPT).
    Keywords: Strong electron correlations ; Fermi surface reconstruction ; Cluster perturbation theory
    ISSN: 1557-1939
    E-ISSN: 1557-1947
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  • 8
    Language: English
    In: Journal of Superconductivity and Novel Magnetism, 2013, Vol.26(8), pp.2637-2640
    Description: We study optical properties of the multiband superconductors with an s ± order parameter symmetry. By comparing results of our theory with experimental data on optical conductivity for Ba 0.68 K 0.32 Fe 2 As 2 single crystals, we show that satisfactory description of the novel superconductors can be obtained only considering a strong electron-boson coupling. We reexamine the effect of disorder and demonstrate that multiband superconductors are more robust with respect to it than naively expected by simple analogy with paramagnetic impurities in single-band superconductors. Moreover, disorder may give rise to new effects, in particular to a phase transition s ± → s ++ . We discuss how the systematic study of disorder impact on the density of states and the optical conductivity may provide information on the underlying order parameter structure.
    Keywords: Multiband superconductivity ; Optical conductivity ; Effects of disorder
    ISSN: 1557-1939
    E-ISSN: 1557-1947
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  • 9
    Language: English
    In: Journal of Superconductivity and Novel Magnetism, Jan, 2009, Vol.22(1), p.37(4)
    Description: Byline: A. Donkov (1), I. Eremin (1,2), J. Knolle (1), M. M. Korshunov (1,3) Keywords: Electron--phonon interaction; Superconductivity; Sodium cobaltate Abstract: We study theoretically the electron--phonon interaction in [Na.sub.x]CoO.sub.2. For the A .sub.1g and E .sub.1g phonon modes found in Raman experiments, we calculate the matrix elements of the electron--phonon interaction. Analyzing the feedback effect of the conduction electrons on the phonon frequency [omega], we investigate the doping dependence of these two phonon modes. Due to the momentum dependence of the electron--phonon interaction, we find the strongest renormalization of the E .sub.1g mode around the Brillouin zone boundary which should be observed in the neutron scattering. At the same time, the A .sub.1g mode shows the strongest coupling to the conducting electrons around the I point and reveals its doping dependence in the Raman experiments. Our results shed light on the possible importance of the electron--phonon interaction in the lamellar sodium cobaltates. Author Affiliation: (1) Max-Planck-Institut fur Physik komplexer Systeme, 01187, Dresden, Germany (2) Institute fur Mathematische und Theoretische Physik, TU Braunschweig, 38106, Braunschweig, Germany (3) Siberian Branch of Russian Academy of Sciences, L.V. Kirensky Institute of Physics, 660036, Krasnoyarsk, Russia Article History: Registration Date: 13/10/2008 Received Date: 10/10/2008 Accepted Date: 13/10/2008 Online Date: 25/10/2008
    Keywords: Phonons ; Doping ; Boundaries ; Sodium ; Neutron Scattering ; Mathematical Analysis ; Conduction ; Joining ; Conduction Electrons ; Brillouin Zone ; Sheds ; Feedback ; Superconductors (General) (So);
    ISSN: 1557-1939
    E-ISSN: 15571947
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  • 10
    Language: English
    In: Journal of Superconductivity and Novel Magnetism, 2009, Vol.22(1), pp.37-40
    Description: We study theoretically the electron–phonon interaction in Na x CoO 2 . For the A 1 g and E 1 g phonon modes found in Raman experiments, we calculate the matrix elements of the electron–phonon interaction. Analyzing the feedback effect of the conduction electrons on the phonon frequency ω , we investigate the doping dependence of these two phonon modes. Due to the momentum dependence of the electron–phonon interaction, we find the strongest renormalization of the E 1 g mode around the Brillouin zone boundary which should be observed in the neutron scattering. At the same time, the A 1 g mode shows the strongest coupling to the conducting electrons around the Γ point and reveals its doping dependence in the Raman experiments. Our results shed light on the possible importance of the electron–phonon interaction in the lamellar sodium cobaltates.
    Keywords: Electron–phonon interaction ; Superconductivity ; Sodium cobaltate
    ISSN: 1557-1939
    E-ISSN: 1557-1947
    Source: Springer Science & Business Media B.V.
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