544 research outputs found
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
Dynamical density-density correlations in one-dimensional Mott insulators
The dynamical density-density correlation function is calculated for the
one-dimensional, half-filled Hubbard model extended with nearest neighbor
repulsion using the Lanczos algorithm for finite size systems and analytically
for large on site repulsion compared to hopping amplitudes. At the zone
boundary an excitonic feature exists for any finite nearest neighbor repulsion
and exhausts most of the spectral weight, even for parameters where no exciton
is visible at zero momentum.Comment: 5 pages, REVTeX, epsf, 3 postscript figure
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
Hyperbaric Oxygen Therapy: Solution for Difficult to Heal Acute Wounds? Systematic Review
Hyperbaric oxygen therapy (HBOT) is used to treat various wound types. However, the possible beneficial and harmful effects of HBOT for acute wounds are unclear. We undertook a systematic review to evaluate the effectiveness of HBOT compared to other interventions on wound healing and adverse effects in patients with acute wounds. To detect all available randomized controlled trials (RCTs) we searched five relevant databases up to March 2010. Trial selection, quality assessment, data extraction, and data synthesis were conducted by two of the authors independently. We included five trials, totaling 360 patients. These trials, with some methodologic flaws, included different kinds of wound and focused on different outcome parameters, which prohibited meta-analysis. A French trial (n = 36 patients) reported that significantly more crush wounds healed with HBOT than with sham HBOT [relative risk (RR) 1.70, 95% confidence interval (CI) 1.11-2.61]. Moreover, there were significantly fewer additional surgical procedures required with HBOT (RR 1.60, 95% CI 1.03-2.50), and there was significantly less tissue necrosis (RR 1.70, 95% CI 1.11-2.61). In one of two American trials (n = 141) burn wounds healed significantly quicker with HBOT (P <0.005) than with routine burn care. A British trial (n = 48) compared HBOT with usual care. HBOT resulted in a significantly higher percentage of healthy graft area in split skin grafts (RR 3.50, 95% CI 1.35-9.11). In a Chinese trial (n = 145) HBOT did not significantly improve flap survival in patients with limb skin defects. HBOT, if readily available, appears effective for the management of acute, difficult to heal wound
Excitons in Mott insulators
Motivated by recent Raman and resonant inelastic X-ray scattering experiments
performed for Mott insulators, which suggest formation of excitons in these
systems, we present a theory of exciton formation in the upper Hubbard band.
The analysis based on the spin polaron approach is performed in the framework
of an effective t-J model for the subspace of states with one doubly occupied
site. Our results confirm the existence of excitons and bear qualitative
resemblance to experimental data despite some simplifications in our approach.
They prove that the basic underlying mechanismof exciton formation is the same
as that which gives rise to binding of holes in weakly doped antiferromagnets.Comment: 4 pages, 1 figur
Uma abordagem probabilística para quantificar o movimento de solutos no solo em uma escala de bacia hidrográfica.
Apresenta uma metodologia probabilística para quantificar a lixiviação de solutos em uma escala de bacia hidrográfica. O conceito de função transferência é empregado, mas ao invés de utilizar funções transferências analíticas, foi desenvolvido um modelo numérico, permitindo-se obter muito mais flexibilidade no emprego deste conceito, quando se pretende calcualr o transporte de soluto em escala de campo. O modelo numérico estocástico desenvolvido pode ser caracterizado como um modelo Lagrangeano descontínuo.bitstream/CNPDIA/9690/1/PA09_96.pd
Pairing Correlations on t-U-J Ladders
Pairing correlations on generalized t-U-J two-leg ladders are reported. We
find that the pairing correlations on the usual t-U Hubbard ladder are
significantly enhanced by the addition of a nearest-neighbor exchange
interaction J. Likewise, these correlations are also enhanced for the t-J model
when the onsite Coulomb interaction is reduced from infinity. Moreover, the
pairing correlations are larger on a t-U-J ladder than on a t-Jeff ladder in
which Jeff has been adjusted so that the two models have the same spin gap at
half-filling. This enhancement of the pairing correlations is associated with
an increase in the pair-binding energy and the pair mobility in the t-U-J model
and point to the importance of the charge transfer nature of the cuprate
systems
Twelve years of global observations of formaldehyde in the troposphere using GOME and SCIAMACHY sensors
This work presents global tropospheric formaldehyde columns retrieved from near-UV radiance measurements performed by the GOME instrument onboard ERS-2 since 1995, and by SCIAMACHY, in operation on ENVISAT since the end of 2002. A special effort has been made to ensure the coherence and quality of the CH<sub>2</sub>O dataset covering the period 1996–2007. Optimised DOAS settings are proposed in order to reduce the impact of two important sources of error in the derivation of slant columns, namely, the polarisation anomaly affecting the SCIAMACHY spectra around 350 nm, and a major absorption band of the O<sub>4</sub> collision complex centred near 360 nm. The air mass factors are determined from scattering weights generated using radiative transfer calculations taking into account the cloud fraction, the cloud height and the ground albedo. Vertical profile shapes of CH<sub>2</sub>O are provided by the global CTM IMAGES based on an up-to-date representation of emissions, atmospheric transport and photochemistry. A comprehensive error analysis is presented. This includes errors on the slant columns retrieval and errors on the air mass factors which are mainly due to uncertainties in the a priori profile and in the cloud properties. The major features of the retrieved formaldehyde column distribution are discussed and compared with previous CH<sub>2</sub>O datasets over the major emission regions
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 anisotropies and general on--site Coulomb interactions in the cuprates
This paper derives the anisotropic superexchange interactions from a Hubbard
model for excitations within the copper 3d band and the oxygen 2p band of the
undoped insulating cuprates. We extend the recent calculation of Yildirim et
al. [Phys. Rev. B {\bf VV}, pp, 1995] in order to include the most general
on--site Coulomb interactions (including those which involve more than two
orbitals) when two holes occupy the same site. Our general results apply when
the oxygen ions surrounding the copper ions form an octahedron which has
tetragonal symmetry (but may be rotated as in lanthanum cuprate). For the
tetragonal cuprates we obtain an easy--plane anisotropy in good agreement with
experimental values. We predict the magnitude of the small in--plane anisoComment: 25 pages, revte
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