566 research outputs found

    Fluctuation Exchange Analysis of Superconductivity in the Standard Three-Band CuO2 Model

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    The fluctuation exchange, or FLEX, approximation for interacting electrons is applied to study instabilities in the standard three-band model for CuO2 layers in the high-temperature superconductors. Both intra-orbital and near-neigbor Coulomb interactions are retained. The filling dependence of the d(x2-y2) transition temperature is studied in both the "hole-doped" and "electron-doped" regimes using parameters derived from constrained-occupancy density-functional theory for La2CuO4. The agreement with experiment on the overdoped hole side of the phase diagram is remarkably good, i.e., transitions emerge in the 40 K range with no free parameters. In addition the importance of the "orbital antiferromagnetic," or flux phase, charge density channel is emphasized for an understanding of the underdoped regime.Comment: REVTex and PostScript, 31 pages, 26 figures; to appear in Phys. Rev. B (1998); only revised EPS figures 3, 4, 6a, 6b, 6c, 7 and 8 to correct disappearance of some labels due to technical problem

    High temperature superconductivity in dimer array systems

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    Superconductivity in the Hubbard model is studied on a series of lattices in which dimers are coupled in various types of arrays. Using fluctuation exchange method and solving the linearized Eliashberg equation, the transition temperature TcT_c of these systems is estimated to be much higher than that of the Hubbard model on a simple square lattice, which is a model for the high TcT_c cuprates. We conclude that these `dimer array' systems can generally exhibit superconductivity with very high TcT_c. Not only dd-electron systems, but also pp-electron systems may provide various stages for realizing the present mechanism.Comment: 4 pages, 9 figure

    Towards analytic description of a transition from weak to strong coupling regime in correlated electron systems. I. Systematic diagrammatic theory with two-particle Green functions

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    We analyze behavior of correlated electrons described by Hubbard-like models at intermediate and strong coupling. We show that with increasing interaction a pole in a generic two-particle Green function is approached. The pole signals metal-insulator transition at half filling and gives rise to a new vanishing ``Kondo'' scale causing breakdown of weak-coupling perturbation theory. To describe the critical behavior at the metal-insulator transition a novel, self-consistent diagrammatic technique with two-particle Green functions is developed. The theory is based on the linked-cluster expansion for the thermodynamic potential with electron-electron interaction as propagator. Parquet diagrams with a generating functional are derived. Numerical instabilities due to the metal-insulator transition are demonstrated on simplifications of the parquet algebra with ring and ladder series only. A stable numerical solution in the critical region is reached by factorization of singular terms via a low-frequency expansion in the vertex function. We stress the necessity for dynamical vertex renormalizations, missing in the simple approximations, in order to describe the critical, strong-coupling behavior correctly. We propose a simplification of the full parquet approximation by keeping only most divergent terms in the asymptotic strong-coupling region. A qualitatively new, feasible approximation suitable for the description of a transition from weak to strong coupling is obtained.Comment: 17 pages, 4 figures, REVTe

    Fourth Order Perturbation Theory for Normal Selfenergy in Repulsive Hubbard Model

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    We investigate the normal selfenergy and the mass enhancement factor in the Hubbard model on the two-dimensional square lattice. Our purpose in this paper is to evaluate the mass enhancement factor more quantitatively than the conventional third order perturbation theory. We calculate it by expanding perturbatively up to the fourth order with respect to the on-site repulsion UU. We consider the cases that the system is near the half-filling, which are similar situations to high-TcT_c cuprates. As results of the calculations, we obtain the large mass enhancement on the Fermi surface by introducing the fourth order terms. This is mainly originated from the fourth order particle-hole and particle-particle diagrams. Although the other fourth order terms have effect of reducing the effective mass, this effect does not cancel out the former mass enhancement completely and there remains still a large mass enhancement effect. In addition, we find that the mass enhancement factor becomes large with increasing the on-site repulsion UU and the density of state (DOS) at the Fermi energy ρ(0)\rho(0). According to many current reseaches, such large UU and ρ(0)\rho(0) enhance the effective interaction between quasiparticles, therefore the superconducting transition temperature TcT_c increases. On the other hand, the large mass enhancement leads the reduction of the energy scale of quasiparticles, as a result, TcT_c is reduced. When we discuss TcT_c, we have to estimate these two competitive effects.Comment: 6pages,8figure

    Knight Shift Anomalies in Heavy Electron Materials

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    We calculate non-linear Knight Shift KK vs. susceptibility χ\chi anomalies for Ce ions possessing local moments in metals. The ions are modeled with the Anderson Hamiltonian and studied within the non-crossing approximation (NCA). The Kvs.χK-vs.- \chi non-linearity diminishes with decreasing Kondo temperature T0T_0 and nuclear spin- local moment separation. Treating the Ce ions as an incoherent array in CeSn3_3, we find excellent agreement with the observed Sn K(T)K(T) data.Comment: 4 pages, Revtex, 3 figures available upon request from [email protected]

    An electron correlation originated negative magnetoresistance in a system having a partly flat band

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    Inspired from an experimentally examined organic conductor, a novel mechanism for negative magnetoresistance is proposed for repulsively interacting electrons on a lattice whose band dispersion contains a flat portion (a flat bottom below a dispersive part here). When the Fermi level lies in the flat part, the electron correlation should cause ferromagnetic spin fluctuations to develop with an enhanced susceptibility. A relatively small magnetic field will then shift the majority-spin Fermi level to the dispersive part, resulting in a negative magnetoresistance. We have actually confirmed the idea by calculating the conductivity in magnetic fields, with the fluctuation exchange approximation, for the repulsive Hubbard model on a square lattice having a large second nearest-neighbor hopping.Comment: RevTex, 5 figures in Postscript, to be published in Phys. Rev.

    Superconductivity in quantum-dot superlattices composed of quantum wire networks

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    Based on calculations using the local density approximation, we propose quantum wire networks with square and plaquette type lattice structures that form quantum dot superlattices. These artificial structures are well described by the Hubbard model. Numerical analysis reveals a superconducting ground state with transition temperatures TcT_c of up to 90 mK for the plaquette, which is more than double the value of 40 mK for the square lattice type and is sufficiently high to allow for the experimental observation of superconductivity.Comment: 10 pages, 4 figure

    Andreev Scattering and the Kondo Effect

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    We examine the properties of an infinite-UU Anderson impurity coupled to both normal and superconducting metals. Both the cases of a quantum dot and a quantum point contact containing an impurity are considered; for the latter, we study both one and two-channel impurities. Using a generalization of the noncrossing approximation which incorporates multiple Andreev reflection, we compute the impurity spectral function and the linear-response conductance of these devices. We find generically that the Kondo resonance develops structure at energies corresponding to the superconducting gap, and that the magnitude of the resonance at the Fermi energy is altered. This leads to observable changes in the zero-bias conductance as compared to the case with no superconductivity.Comment: 8 pages, 7 figures; expanded version to appear in PR

    Theory for the excitation spectrum of High-T$_c superconductors : quasiparticle dispersion and shadows of the Fermi surface

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    Using a new method for the solution of the FLEX-equations, which allows the determination of the self energy Σk(ω)\Sigma_{\bf k}(\omega) of the 2D2D Hubbard model on the real frequency axis, we calculate the doping dependence of the quasi-particle excitations of High-Tc_c superconductors. We obtain new results for the shadows of the Fermi surface, their dependence on the deformation of the quasi particle dispersion, an anomalous ω\omega-dependence of ImΣk(ω){\rm Im}\Sigma_{\bf k}(\omega) and a related violation of the Luttinger theorem. This sheds new light on the influence of short range magnetic order on the low energy excitations and its significance for photoemission experiments.Comment: 4 pages (REVTeX) with 3 figure

    Effects of Spin Fluctuations in Quasi-One-Dimensional Organic Superconductors

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    We study the electronic states of quasi-one-dimensional organic conductors using the single band Hubbard model at half-filling. We treat the effects of the on-site Coulomb interaction by the fluctuation-exchange (FLEX) method, and calculate the phase diagram and physical properties. The calculated pressure dependence of the Neel temperature coincides well with the experimental one. We also show that a pseudogap is formed in the density of states near the chemical potential and that d-wave superconductivity appears next to the antiferromagnetic state. Moreover the NMR relaxation rate increases on cooling in the low-temperature region.Comment: 4 pages, 5 figures, to apprear in J. Phys. Soc. Jp
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