161 research outputs found

    A Non-Crossing Approximation for the Study of Intersite Correlations

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    We develop a Non-Crossing Approximation (NCA) for the effective cluster problem of the recently developed Dynamical Cluster Approximation (DCA). The DCA technique includes short-ranged correlations by mapping the lattice problem onto a self-consistently embedded periodic cluster of size NcN_c. It is a fully causal and systematic approximation to the full lattice problem, with corrections O(1/Nc){\cal{O}}(1/N_c) in two dimensions. The NCA we develop is a systematic approximation with corrections O(1/Nc3){\cal{O}}(1/N_c^3). The method will be discussed in detail and results for the one-particle properties of the Hubbard model are shown. Near half filling, the spectra display pronounced features including a pseudogap and non-Fermi-liquid behavior due to short-ranged antiferromagnetic correlations.Comment: 12 pages, 11 figures, EPJB styl

    Magnetism and Phase Separation in the Ground State of the Hubbard Model

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    We discuss the ground state magnetic phase diagram of the Hubbard model off half filling within the dynamical mean-field theory. The effective single-impurity Anderson model is solved by Wilson's numerical renormalization group calculations, adapted to symmetry broken phases. We find a phase separated, antiferromagnetic state up to a critical doping for small and intermediate values of U, but could not stabilise a Neel state for large U and finite doping. At very large U, the phase diagram exhibits an island with a ferromagnetic ground state. Spectral properties in the ordered phases are discussed.Comment: 9 pages, 11 figure

    On the Analyticity of Solutions in the Dynamical Mean-Field Theory

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    The unphysical solutions of the periodic Anderson model obtained by H. Keiter and T. Leuders [Europhys. Lett. 49, 801(2000)] in dynamical mean-field theory (DMFT) are shown to result from the author's restricted choice of the functional form of the solution, leading to a violation of the analytic properties of the exact solution. By contrast, iterative solutions of the self-consistency condition within the DMFT obtained by techniques which preserve the correct analytic properties of the exact solution (e.g., quantum Monte-Carlo simulations or the numerical renormalization group) always lead to physical solutions.Comment: 4 pages, 1 figur

    Dynamical Magnetic Susceptibility for the tt-JJ Model

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    We present results for the {\em dynamical}\/ magnetic susceptibility of the tt-JJ model, calculated with the dynamical mean field theory. For J=0J=0 we find enhanced ferromagnetic correlations but an otherwise relatively q⃗\vec{q}-independent dynamical magnetic susceptibility. For J>0J>0 the explicit antiferromagnetic exchange leads to a dynamic spin structure factor with the expected peak at the antiferromagnetic Bragg point.Comment: 3 pages LaTeX, postscript figures included, submitted as contribution to SCES' 96, to appear in Physica

    Transport Properties of the Infinite Dimensional Hubbard Model

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    Results for the optical conductivity and resistivity of the Hubbard model in infinite spatial dimensions are presented. At half filling we observe a gradual crossover from a normal Fermi-liquid with a Drude peak at ω=0\omega=0 in the optical conductivity to an insulator as a function of UU for temperatures above the antiferromagnetic phase transition. When doped, the ``insulator'' becomes a Fermi-liquid with a corresponding temperature dependence of the optical conductivity and resistivity. We find a T2T^2-coefficient in the low temperature resistivity which suggests that the carriers in the system acquire a considerable mass-enhancement due to the strong local correlations. At high temperatures, a crossover into a semi-metallic regime takes place.Comment: 14 page

    Phase diagram of the frustrated Hubbard model

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    The Mott-Hubbard metal-insulator transition in the paramagnetic phase of the one-band Hubbard model has long been used to describe similar features in real materials like V2_2O3_3. Here we show that this transition is hidden inside a rather robust antiferromagnetic insulator even in the presence of comparatively strong magnetic frustration. This result raises the question of the relevance of the Mott-Hubbard metal-insulator transition for the generic phase diagram of the one-band Hubbard model.Comment: 4 pages, 6 figure

    The Dynamical Cluster Approximation (DCA) versus the Cellular Dynamical Mean Field Theory (CDMFT) in strongly correlated electrons systems

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    We are commenting on the article Phys. Rev. {\bf B 65}, 155112 (2002) by G. Biroli and G. Kotliar in which they make a comparison between two cluster techniques, the {\it Cellular Dynamical Mean Field Theory} (CDMFT) and the {\it Dynamical Cluster Approximation} (DCA). Based upon an incorrect implementation of the DCA technique in their work, they conclude that the CDMFT is a faster converging technique than the DCA. We present the correct DCA prescription for the particular model Hamiltonian studied in their article and conclude that the DCA, once implemented correctly, is a faster converging technique for the quantities averaged over the cluster. We also refer to their latest response to our comment where they argue that instead of averaging over the cluster, local observables should be calculated in the bulk of the cluster which indeed makes them converge much faster in the CDMFT than in the DCA. We however show that in their original work, the authors themselves use the cluster averaged quantities to draw their conclusions in favor of using the CDMFT over the DCA.Comment: Comment on Phys. Rev. B 65, 155112 (2002). 3 pages, 2 figure

    Localization of strongly correlated electrons as Jahn-Teller polarons in manganites

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    A realistic modeling of manganites should include the Coulomb repulsion between ege_g electrons, the Hund's rule coupling to t2gt_{2g} spins, and Jahn-Teller phonons. Solving such a model by dynamical mean field theory, we report large magnetoresistances and spectra in good agreement with experiments. The physics of the unusual, insulating-like paramagnetic phase is determined by correlated electrons which are-due to strong correlations-easily trapped as Jahn-Teller polarons.Comment: 4 pages, 3 figure
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