22 research outputs found
Local-Ansatz Approach with Momentum Dependent Variational Parameters to Correlated Electron Systems
A new wavefunction which improves the Gutzwiller-type local ansatz method has
been proposed to describe the correlated electron system. The ground-state
energy, double occupation number, momentum distribution function, and
quasiparticle weight have been calculated for the half-filled band Hubbard
model in infinite dimensions. It is shown that the new wavefunction improves
the local-ansatz approach (LA) proposed by Stollhoff and Fulde. Especially,
calculated momentum distribution functions show a reasonable momentum
dependence. The result qualitatively differs from those obtained by the LA and
the Gutzwiller wavefunction. Furthermore, the present approach combined with
the projection operator method CPA is shown to describe quantitatively the
excitation spectra in the insulator regime as well as the critical Coulomb
interactions for a gap formation in infinite dimensions.Comment: To be published in Phys. Soc. Jpn. 77 No.11 (2008
Numerical Calculations of the B1g Raman Spectrum of the Two-Dimensional Heisenberg Model
The B1g Raman spectrum of the two-dimensional S=1/2 Heisenberg model is
discussed within Loudon-Fleury theory at both zero and finite temperature. The
exact T=0 spectrum for lattices with up to 6*6 sites is computed using Lanczos
exact diagonalization. A quantum Monte Carlo (QMC) method is used to calculate
the corresponding imaginary-time correlation function and its first two
derivatives for lattices with up to 16*16 spins. The imaginary-time data is
continued to real frequency using the maximum-entropy method, as well as a fit
based on spinwave theory. The numerical results are compared with spinwave
calculations for finite lattices. There is a surprisingly large change in the
exact spectrum going from 4*4 to 6*6 sites. In the former case there is a
single dominant two-magnon peak at frequency w/J appr. 3.0, whereas in the
latter case there are two approximately equal-sized peaks at w/J appr. 2.7 and
3.9. This is in good qualitative agreement with the spinwave calculations
including two-magnon processes on the same lattices. Both the Lanczos and the
QMC results indicate that the actual infinite-size two-magnon profile is
broader than the narrow peak obtained in spinwave theory, but the positions of
the maxima agree to within a few percent. The higher-order contributions
present in the numerical results are merged with the two-magnon profile and
extend up to frequencies w/J appr. 7. The first three frequency cumulants of
the spectrum are in excellent agreement with results previously obtained from a
series expansion around the Ising limit. Typical experimental B1g$ spectra for
La2CuO4 are only slightly broader than what we obtain here. The exchange
constant extracted from the peak position is J appr. 1400K, in good agreement
with values obtained from neutron scattering and NMR experiments.Comment: 15 pages, Revtex, 13 PostScript figure
The Korringa-Kohn-Rostoker Non-Local Coherent Potential Approximation (KKR-NLCPA)
We introduce the Korringa-Kohn-Rostocker non-local coherent potential
approximation (KKR-NLCPA) for describing the electronic structure of disordered
systems. The KKR-NLCPA systematically provides a hierarchy of improvements upon
the widely used KKR-CPA approach and includes non-local correlations in the
disorder configurations by means of a self-consistently embedded cluster. The
KKR-NLCPA method satisfies all of the requirements for a successful cluster
generalization of the KKR-CPA; it remains fully causal, becomes exact in the
limit of large cluster sizes, reduces to the KKR-CPA for a single-site cluster,
is straightforward to implement numerically, and enables the effects of
short-range order upon the electronic structure to be investigated. In
particular, it is suitable for combination with electronic density functional
theory to give an ab-initio description of disordered systems. Future
applications to charge correlation and lattice displacement effects in alloys
and spin fluctuations in magnets amongst others are very promising. We
illustrate the method by application to a simple one-dimensional model.Comment: Revised versio
Complete chemoenzymatic synthesis of the Forssman antigen using novel glycosyltransferases identified in Campylobacter jejuni and Pasteurella multocida
Transduction du signal et adaptation lors de changements de régime lumineux chez une cyanobactérie (rôle des nucléotides cycliques et du complexe protéine-chlorophylle CP43' )
PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF