574 research outputs found

    Consistent low-energy reduction of the three-band model for copper oxides with O-O hopping to the effective t-J model

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    A full three-band model for the CuO2_{2} plane with inclusion of all essential interactions - Cu-O and O-O hopping, repulsion at the copper and oxygen and between them - is considered. A general procedure of the low-energy reduction of the primary Hamiltonian to the Hamiltonian of the generalized tt-t′t'-JJ model is developed. An important role of the direct O-O hopping is discussed. Parameters of the effective low-energy model (the hopping integral, the band position and the superexchange constant JJ are calculated. An analysis of the obtained data shows that the experimental value of JJ fixes the charge transfer energy Δ=(ϵp−ϵd)\Delta =(\epsilon_{p}-\epsilon_{d}) in a narrow region of energies.Comment: 32 pp. (LATEX), two figures (PostScript) appende

    The dimerized phase of ionic Hubbard models

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    We derive an effective Hamiltonian for the ionic Hubbard model at half filling, extended to include nearest-neighbor repulsion. Using a spin-particle transformation, the effective model is mapped onto simple spin-1 models in two particular cases. Using another spin-particle transformation, a slightly modified model is mapped into an SU(3) antiferromagnetic Heisenberg model whose exact ground state is known to be spontaneously dimerized. From the effective models several properties of the dimerized phase are discussed, like ferroelectricity and fractional charge excitations. Using bosonization and recent developments in the theory of macroscopic polarization, we show that the polarization is proportional to the charge of the elementary excitations

    Spectral function of the 1D Hubbard model in the U→+∞U\to +\infty limit

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    We show that the one-particle spectral functions of the one-dimensional Hubbard model diverge at the Fermi energy like ∣ω−εF∣−3/8|\omega-\varepsilon_F|^{-3/8} in the U→+∞U\to +\infty limit. The Luttinger liquid behaviour ∣ω−εF∣α|\omega-\varepsilon_F|^\alpha, where α→1/8\alpha \to 1/8 as U→+∞U\to +\infty , should be limited to ∣ω−εF∣∼t2/U|\omega-\varepsilon_F| \sim t^2/U (for UU large but finite), which shrinks to a single point, ω=εF\omega=\varepsilon_F,in that limit. The consequences for the observation of the Luttinger liquid behaviour in photoemission and inverse photoemission experiments are discussed.Comment: 4 pages, RevTeX, 2 figures on reques

    Anisotropic Spin Hamiltonians due to Spin-Orbit and Coulomb Exchange Interactions

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    This paper contains the details of Phys. Rev. Lett. 73, 2919 (1994) and, to a lesser extent, Phys. Rev. Lett. 72, 3710 (1994). We treat a Hubbard model which includes all the 3d states of the Cu ions and the 2p states of the O ions. We also include spin-orbit interactions, hopping between ground and excited crystal field states of the Cu ions, and rather general Coulomb interactions. Our analytic results for the spin Hamiltonian, H, are corroborated by numerical evaluations of the energy splitting of the ground manifold for two holes on either a pair of Cu ions or a Cu-O-Cu complex. In the tetragonal symmetry case and for the model considered, we prove that H is rotationally invariant in the absence of Coulomb exchange. When Coulomb exchange is present, each bond Hamiltonian has full biaxial anisotropy, as expected for this symmetry. For lower symmetry situations, the single bond spin Hamiltonian is anisotropic at order t**6 for constant U and at order t**2 for nonconstant U. (Constant U means that the Coulomb interaction between orbitals does not depend on which orbitals are involved.)Comment: 50 pages, ILATEX Version 2.09 <13 Jun 1989

    Phonon Assisted Multimagnon Optical Absorption and Long Lived Two-Magnon States in Undoped Lamellar Copper Oxides

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    We calculate the effective charge for multimagnon infrared (IR) absorption assisted by phonons in the parent insulating compounds of cuprate superconductors and the spectra for two-magnon absorption using interacting spin-wave theory. Recent measured bands in the mid IR [Perkins et al. Phys. Rev. Lett. {\bf 71} 1621 (1993)] are interpreted as involving one phonon plus a two-magnon virtual bound state, and one phonon plus higher multimagnon absorption processes. The virtual bound state consists of a narrow resonance occurring when the magnon pair has total momentum close to (Ï€,0)(\pi,0).Comment: 4 page

    Forecasts and assimilation experiments of the Antarctic ozone hole 2008

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    The 2008 Antarctic ozone hole was one of the largest and most long-lived in recent years. Predictions of the ozone hole were made in near-real time (NRT) and hindcast mode with the Integrated Forecast System (IFS) of the European Centre for Medium-Range Weather Forecasts (ECMWF). The forecasts were carried out both with and without assimilation of satellite observations from multiple instruments to provide more realistic initial conditions. Three different chemistry schemes were applied for the description of stratospheric ozone chemistry: (i) a linearization of the ozone chemistry, (ii) the stratospheric chemical mechanism of the Model of Ozone and Related Chemical Tracers, version 3, (MOZART-3) and (iii) the relaxation to climatology as implemented in the Transport Model, version 5, (TM5). The IFS uses the latter two schemes by means of a two-way coupled system. Without assimilation, the forecasts showed model-specific shortcomings in predicting start time, extent and duration of the ozone hole. The assimilation of satellite observations from the Microwave Limb Sounder (MLS), the Ozone Monitoring Instrument (OMI), the Solar Backscattering Ultraviolet radiometer (SBUV-2) and the SCanning Imaging Absorption spectroMeter for Atmospheric CartograpHY (SCIAMACHY) led to a significant improvement of the forecasts when compared with total columns and vertical profiles from ozone sondes. The combined assimilation of observations from multiple instruments helped to overcome limitations of the ultraviolet (UV) sensors at low solar elevation over Antarctica. The assimilation of data from MLS was crucial to obtain a good agreement with the observed ozone profiles both in the polar stratosphere and troposphere. The ozone analyses by the three model configurations were very similar despite the different underlying chemistry schemes. Using ozone analyses as initial conditions had a very beneficial but variable effect on the predictability of the ozone hole over 15 days. The initialized forecasts with the MOZART-3 chemistry produced the best predictions of the increasing ozone hole whereas the linear scheme showed the best results during the ozonehole closure

    Angle resolved photoemission spectroscopy of Sr_2CuO_2Cl_2 - a revisit

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    We have investigated the lowest binding-energy electronic structure of the model cuprate Sr_2CuO_2Cl_2 using angle resolved photoemission spectroscopy (ARPES). Our data from about 80 cleavages of Sr_2CuO_2Cl_2 single crystals give a comprehensive, self-consistent picture of the nature of the first electron-removal state in this model undoped CuO_2-plane cuprate. Firstly, we show a strong dependence on the polarization of the excitation light which is understandable in the context of the matrix element governing the photoemission process, which gives a state with the symmetry of a Zhang-Rice singlet. Secondly, the strong, oscillatory dependence of the intensity of the Zhang-Rice singlet on the exciting photon-energy is shown to be consistent with interference effects connected with the periodicity of the crystal structure in the crystallographic c-direction. Thirdly, we measured the dispersion of the first electron-removal states along G->(pi,pi) and G->(pi,0), the latter being controversial in the literature, and have shown that the data are best fitted using an extended t-J-model, and extract the relevant model parameters. An analysis of the spectral weight of the first ionization states for different excitation energies within the approach used by Leung et al. (Phys. Rev. B56, 6320 (1997)) results in a strongly photon-energy dependent ratio between the coherent and incoherent spectral weight. The possible reasons for this observation and its physical implications are discussed.Comment: 10 pages, 8 figure

    Photoinduced metallic properties of one-dimensional strongly correlated electron systems

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    We study photoinduced optical responses of one-dimensional strongly correlated electron systems. The optical conductivity spectra are calculated for the ground state and a photoexcited state in the one-dimensional Hubbard model at half filling by using the exact diagonalization method. It is found that, in the Mott insulator phase, the photoexcited state has large spectral weights including the Drude weight below the optical gap. As a consequence, the spectral weight above the optical gap is largely reduced. These results imply that a metallic state is induced by photoexcitation. Comparison between the photoexcited and hole-doped states shows that the photoexcitation is similar to chemical doping.Comment: 4 pages, 4 figures, submitted to J. Phys. Soc. Jp

    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

    Magnetic properties of NaV2O5, a one-dimensional spin 1/2 antiferromagnet with finite chains

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    We have performed measurements of the magnetic susceptibility of NaV2_2O5_5 between 2 and 400 K. The high temperature part is typical of spin 1/2 chains with a nearest--neighbour antiferromagnetic exchange integral JJ of 529 K. We develop a model for the susceptibility of a system with finite chains to account for the low temperature part of the data, which cannot be fitted by a standard Curie-Weiss term. These results suggest that the next nearest--neighbour exchange integral J2J_2 in CaV4_4O9_9 should be of the order of 500 K because, like JJ in NaV2_2O5_5, it corresponds to corner sharing VO5_5 square pyramids.Comment: An early version of the manuscript was mistakenly submitted. Although relatively minor, the changes concern the list of authors, the main text, the references and the figure captions. 10 pages of latex, 2 figure
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