487 research outputs found
The dimerized phase of ionic Hubbard models
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
Consistent low-energy reduction of the three-band model for copper oxides with O-O hopping to the effective t-J model
A full three-band model for the CuO 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
-- 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 are calculated. An
analysis of the obtained data shows that the experimental value of fixes
the charge transfer energy in a narrow
region of energies.Comment: 32 pp. (LATEX), two figures (PostScript) appende
Spectral function of the 1D Hubbard model in the limit
We show that the one-particle spectral functions of the one-dimensional
Hubbard model diverge at the Fermi energy like
in the limit. The Luttinger liquid behaviour
, where as ,
should be limited to (for large but
finite), which shrinks to a single point, ,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
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
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 .Comment: 4 page
Magnetic properties of NaV2O5, a one-dimensional spin 1/2 antiferromagnet with finite chains
We have performed measurements of the magnetic susceptibility of NaVO
between 2 and 400 K. The high temperature part is typical of spin 1/2 chains
with a nearest--neighbour antiferromagnetic exchange integral 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 in CaVO should be of the
order of 500 K because, like in NaVO, it corresponds to corner
sharing VO 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
Forecasts and assimilation experiments of the Antarctic ozone hole 2008
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
Projected Wavefunctions and High Temperature Superconductivity
We study the Hubbard model with parameters relevant to the cuprates, using
variational Monte Carlo with projected d-wave states. For doping 0 < x < 0.35
we obtain a superconductor whose order parameter tracks the observed
nonmonotonic Tc(x). The variational parameter Delta_{var}(x) scales with the
(pi,0) ``hump'' and T* seen in photoemission. Projection leads to incoherence
in the spectral function, and from the singular behavior of its moments we
obtain the nodal quasiparticle weight Z which vanishes linearly in x, though
the Fermi velocity remains finite as x approaches zero. The Drude weight
D_{low} and superfluid density are consistent with experiments, and D_{low}
scales with Z.Comment: 4 pages, 5 EPS figures. (1) Many improvements including discussion of
n(k) and superfluid density. (2) Added comparison with slave boson mean field
theory. (3) Added new reference
Polaron Effects on Superexchange Interaction: Isotope Shifts of , , and in Layered Copper Oxides
A compact expression has been obtained for the superexchange coupling of
magnetic ions via intermediate anions with regard to polaron effects at both
magnetic ions and intermediate anions. This expression is used to analyze the
main features of the behavior of isotope shifts for temperatures of three types
in layered cuprates: the Neel temperatures (), critical temperatures of
transitions to a superconducting state (), and characteristic temperatures
of the pseudogap in the normal state ().Comment: 4 pages, 1 figur
Quasiparticle photoemission intensity in doped two-dimensional quantum antiferromagnets
Using the self-consistent Born approximation, and the corresponding wave
function of the magnetic polaron, we calculate the quasiparticle weight
corresponding to destruction of a real electron (in contrast to creation of a
spinless holon), as a funtion of wave vector for one hole in a generalized
model and the strong coupling limit of a generalized Hubbard model. The
results are in excellent agreement with those obtained by exact diagonalization
of a sufficiently large cluster. Only the Hubbard weigth compares very well
with photoemission measurements in Sr_2CuO_2Cl_2.Comment: 11 pages, latex, 3 figure
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