1,571 research outputs found
New Lessons from an Old Park
Recent restoration projects in New York\u27s Central Park have produced a series of notable designs carried out under the leadership of the Parks Department and Central Park Administrator Elizabeth Barlow
Determination of screened Coulomb repulsion energies in organic molecular crystals: A real space approach
We present a general method for determining screened Coulomb parameters in
molecular assemblies, in particular organic molecular crystals. This allows us
to calculate the interaction parameters used in a generalized Hubbard model
description of correlated organic materials. In such a model only the electrons
in levels close to the Fermi level are included explicitly, while the effect of
all other electrons is included as a renormalization of the model parameters.
For the Coulomb integrals this renormalization is mainly due to screening. For
molecular materials we can split the screening into intra- and inter-molecular
screening. Here we demonstrate how the inter-molecular screening can be
calculated by modeling the molecules by distributed point-polarizabilities and
solving the resulting self-consistent electrostatic screening problem in real
space. For the example of the quasi one-dimensional molecular metal TTF-TCNQ we
demonstrate that the method gives remarkably accurate results.Comment: Submitted to Special Issue - ISCOM 2009 - Physica B Condensed Matte
Spectral functions for strongly correlated 5f-electrons
We calculate the spectral functions of model systems describing 5f-compounds
adopting Cluster Perturbation Theory. The method allows for an accurate
treatment of the short-range correlations. The calculated excitation spectra
exhibit coherent 5f bands coexisting with features associated with local
intra-atomic transitions. The findings provide a microscopic basis for partial
localization. Results are presented for linear chains.Comment: 10 Page
Optical sum rule in metals with a strong interaction
The restricted optical sum rule and its dependence on the temperature, a
superconducting gap and the cutoff energy have been investigated. As known this
sum rule depends on the cutoff energy and the relaxation rate even for a
homogeneous electron gas interacting with impurities or phonons. It is shown
here that additional dependence of the spectral weight on a superconducting gap
is very small in this model and this effect disappears totally when the
relaxation rate is equal zero. The model metal with a single band is considered
in details. It is well known that for this model there is the dependence of the
sum rule on the temperature and the energy gap even in the case when the
relaxation is absent. This dependence exists due to the smearing of the
electron distribution function and it is expressed in the terms of Sommerfeld
expansion. Here it is shown that these effects are considerably smaller than
that of related with the relaxation rate if the band width is larger than the
average phonon frequency. It is shown also that the experimental data about the
temperature dependence of the spectral weight for the high- materials can be
successfully explained in the framework approach based on the temperature
dependence of the relaxation rateComment: 13 pages, 7 figures, the talk given on Internatinal coference on
theoretical physics, april 11-16,2005, Mosco
Coulomb parameters and photoemission for the molecular metal TTF-TCNQ
We employ density-functional theory to calculate realistic parameters for an
extended Hubbard model of the molecular metal TTF-TCNQ. Considering both intra-
and intermolecular screening in the crystal, we find significant longer-range
Coulomb interactions along the molecular stacks, as well as inter-stack
coupling. We show that the long-range Coulomb term of the extended Hubbard
model leads to a broadening of the spectral density, likely resolving the
problems with the interpretation of photoemission experiments using a simple
Hubbard model only.Comment: 4 pages, 2 figure
Weak-coupling expansions for the attractive Holstein and Hubbard models
Weak-coupling expansions (conserving approximations) are carried out for the
attractive Holstein and Hubbard models (on an infinite-dimensional hypercubic
lattice) that include all bandstructure and vertex correction effects. Quantum
fluctuations are found to renormalize transition temperatures by factors of
order unity, but may be incorporated into the superconducting channel of
Migdal-Eliashberg theory by renormalizing the phonon frequency and the
interaction strength.Comment: 10 pages, (five figures available from the author by request) typeset
with ReVTeX, preprint NSF-ITP-93-10
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