74 research outputs found
Ab initio calculations of response properties including electron-hole interaction
We discuss the current status of a computational approach which allows to
evaluate the dielectric matrix, and hence electronic excitations like optical
properties, including local field and excitonic effects. We introduce a recent
numerical development which greatly reduces the use of memory in such type of
calculations, and hence eliminates one of the bottlenecks for the application
to complex systems. We present recent applications of the method, focusing our
interest on insulating oxides.Comment: 11 pages, 5 figures, 1999 MRS Proceedin
First-principles GW calculations for fullerenes, porphyrins, phtalocyanine, and other molecules of interest for organic photovoltaic applications
We evaluate the performances of ab initio GW calculations for the ionization
energies and HOMO-LUMO gaps of thirteen gas phase molecules of interest for
organic electronic and photovoltaic applications, including the C60 fullerene,
pentacene, free-base porphyrins and phtalocyanine, PTCDA, and standard monomers
such as thiophene, fluorene, benzothiazole or thiadiazole. Standard G0W0
calculations, that is starting from eigenstates obtained with local or
semilocal functionals, significantly improve the ionization energy and band gap
as compared to density functional theory Kohn-Sham results, but the calculated
quasiparticle values remain too small as a result of overscreening. Starting
from Hartree-Fock-like eigenvalues provides much better results and is
equivalent to performing self-consistency on the eigenvalues, with a resulting
accuracy of 2~4% as compared to experiment. Our calculations are based on an
efficient gaussian-basis implementation of GW with explicit treatment of the
dynamical screening through contour deformation techniques.Comment: 10 pages, 3 figure
Can molecular projected density-of-states (PDOS) be systematically used in electronic conductance analysis?
Using benzene-diamine and benzene-dithiol molecular junctions as benchmarks,
we investigate the widespread analysis of the quantum transport conductance
in terms of the projected density of states (PDOS) onto
molecular orbitals (MOs). We first consider two different methods for
identifying the relevant MOs: 1) diagonalization of the Hamiltonian of the
isolated molecule, and 2) diagonalization of a submatrix of the junction
Hamiltonian constructed by considering only basis elements localized on the
molecule. We find that these two methods can lead to substantially different
MOs and hence PDOS. Furthermore, within Method 1, the PDOS can differ depending
on the isolated molecule chosen to represent the molecular junction (e.g.
benzene-dithiol or -dithiolate); and, within Method 2, the PDOS depends on the
chosen basis set. We show that these differences can be critical when the PDOS
is used to provide a physical interpretation of the conductance (especially,
when it has small values as it happens typically at zero bias). In this work,
we propose a new approach trying to reconcile the two traditional methods.
Though some improvements are achieved, the main problems are still unsolved.
Our results raise more general questions and doubts on a PDOS-based analysis of
the conductance.Comment: 12 pages, 9 figure
Ground-state correlation energy of beryllium dimer by the Bethe-Salpeter equation
Since the '30s the interatomic potential of the beryllium dimer Be has
been both an experimental and a theoretical challenge. Calculating the
ground-state correlation energy of Be along its dissociation path is a
difficult problem for theory. We present ab initio many-body perturbation
theory calculations of the Be interatomic potential using the GW
approximation and the Bethe-Salpeter equation (BSE). The ground-state
correlation energy is calculated by the trace formula with checks against the
adiabatic-connection fluctuation-dissipation theorem formula. We show that
inclusion of GW corrections already improves the energy even at the level of
the random-phase approximation. At the level of the BSE on top of the GW
approximation, our calculation is in surprising agreement with the most
accurate theories and with experiment. It even reproduces an experimentally
observed flattening of the interatomic potential due to a delicate correlations
balance from a competition between covalent and van der Waals bonding.Comment: 6 pages, 2 figures, 1 tabl
Beyond time-dependent exact-exchange: the need for long-range correlation
In the description of the interaction between electrons beyond the classical
Hartree picture, bare exchange often yields a leading contribution. Here we
discuss its effect on optical spectra of solids, comparing three different
frameworks: time-dependent Hartree-Fock, a recently introduced combined
density-functional and Green's functions approach applied to the bare exchange
self-energy, and time-dependent exact-exchange within time-dependent
density-functional theory (TD-EXX). We show that these three approximations
give rise to identical excitonic effects in solids; these effects are
drastically overestimated for semiconductors. They are partially compensated by
the usual overestimation of the quasiparticle band gap within Hartree-Fock. The
physics that lacks in these approaches can be formulated as screening. We show
that the introduction of screening in TD-EXX indeed leads to a formulation that
is equivalent to previously proposed functionals derived from Many-Body
Perturbation Theory. It can be simulated by reducing the long-range part of the
Coulomb interaction: this produces absorption spectra of semiconductors in good
agreement with experiment.Comment: 12 pages, 3 figures, 1 tabl
Many-body correlations and coupling in benzene-dithiol junctions
Most theoretical studies of nanoscale transport in molecular junctions rely
on the combination of the Landauer formalism with Kohn-Sham density functional
theory (DFT) using standard local and semilocal functionals to approximate
exchange and correlation effects. In many cases, the resulting conductance is
overestimated with respect to experiments. Recent works have demonstrated that
this discrepancy may be reduced when including many-body corrections on top of
DFT. Here we study benzene-dithiol (BDT) gold junctions and analyze the effect
of many-body perturbation theory (MBPT) on the calculation of the conductance
with respect to different bonding geometries. We find that the many-body
corrections to the conductance strongly depend on the metal-molecule coupling
strength. In the BDT junction with the lowest coupling, many-body corrections
reduce the overestimation on the conductance to a factor two, improving the
agreement with experiments. In contrast, in the strongest coupling cases,
many-body corrections on the conductance are found to be sensibly smaller and
standard DFT reveals a valid approach.Comment: 9 pages, 4 figure
Coherent electronic transport through graphene constrictions: sub-wavelength regime and optical analogies
Graphene two-dimensional nature combined with today lithography allows to
achieve nanoelectronics devices smaller than the Dirac electrons wavelength.
Here we show that in these graphene subwavelength nanodevices the electronic
quantum transport properties present deep analogies with classical phenomena of
subwavelength optics. By introducing the concept of electronic diffraction
barrier to represent the effect of constrictions, we can easily describe the
rich transport physics in a wealth of nanodevices: from Bethe and Kirchhoff
diffraction in graphene slits, to Fabry-Perot interference oscillations in
nanoribbons. The same concept applies to graphene quantum dots and gives new
insigth into recent experiments on these systems.Comment: 5 pages, 4 figures submitted PR
Ab initio study of the optical absorption and wave-vector-dependent dielectric response of graphite
We performed ab initio calculations of the optical absorption spectrum and the wave-vector-dependent dielectric and energy-loss functions of graphite in the framework of the random-phase approximation. In the absorption spectrum, the most prominent peaks were analyzed in terms of interband transitions from specific regions of the Brillouin zone. The inclusion of the crystal local-field effects (LFE) in the response had an important influence on the absorption spectrum for light polarization parallel to the c axis. The calculated electron energy-loss spectra, even without LFE, were in very good agreement with existing momentum-dependent energy-loss experiments concerning the peak positions of the two valence-electron plasmons. Important aspects of the line shape and anisotropy of the energy-loss function at large momentum transfer q were also well described: the splitting of the total (π+σ) plasmon and the appearance of peaks originating from interband transitions. Finally, the role of the interlayer interaction was studied, in particular with regard to its effect on the absorption spectrum for light polarization parallel to c, and to the position of the higher-frequency π+σ plasmon.This work was supported by the EC-RTN program
NANOPHASE (Contract No. HPRN-CT-2000-00167). A.R. acknowledges support from the Ecole Polytechnique during a sabbatical leave in 2001 where this work was started and partial support from Spanish MCyT(MAT2001-0946), University of the Basque Country (9/UPV 00206.215-13639/2001) and COMELCAN (HPRN-CT-2000-00128). Computer time was granted by IDRIS (Project No. 544).Peer reviewe
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