131 research outputs found
Ultrathin oxides: bulk-oxide-like model surfaces or unique films?
To better understand the electronic and chemical properties of wide-gap oxide
surfaces at the atomic scale, experimental work has focused on epitaxial films
on metal substrates. Recent findings show that these films are considerably
thinner than previously thought. This raises doubts about the transferability
of the results to surface properties of thicker films and bulk crystals. By
means of density-functional theory and approximate GW corrections for the
electronic spectra we demonstrate for three characteristic wide-gap oxides
(silica, alumina, and hafnia) the influence of the substrate and highlight
critical differences between the ultrathin films and surfaces of bulk
materials. Our results imply that monolayer-thin oxide films have rather unique
properties.Comment: 5 pages, 3 figures, accepted by PR
Quasiparticle band structure based on a generalized Kohn-Sham scheme
We present a comparative full-potential study of generalized Kohn-Sham
schemes (gKS) with explicit focus on their suitability as starting point for
the solution of the quasiparticle equation. We compare quasiparticle
band structures calculated upon LDA, sX, HSE03, PBE0, and HF functionals for
exchange and correlation (XC) for Si, InN and ZnO. Furthermore, the HSE03
functional is studied and compared to the GGA for 15 non-metallic materials for
its use as a starting point in the calculation of quasiparticle excitation
energies. For this case, also the effects of selfconsistency in the
self-energy are analysed. It is shown that the use of a gKS scheme as a
starting point for a perturbative QP correction can improve upon the
deficiencies found for LDA or GGA staring points for compounds with shallow
bands. For these solids, the order of the valence and conduction bands is often
inverted using local or semi-local approximations for XC, which makes
perturbative calculations unreliable. The use of a gKS starting point
allows for the calculation of fairly accurate band gaps even in these difficult
cases, and generally single-shot calculations following calculations
using the HSE03 functional are very close to experiment
Benchmarking GW against exact diagonalization for semi-empirical models
We calculate groundstate total energies and single-particle excitation
energies of seven pi conjugated molecules described with the semi-empirical
Pariser-Parr-Pople (PPP) model using self-consistent many-body perturbation
theory at the GW level and exact diagonalization. For the total energies GW
captures around 65% of the groundstate correlation energy. The lowest lying
excitations are overscreened by GW leading to an underestimation of electron
affinities and ionization potentials by approximately 0.15 eV corresponding to
2.5%. One-shot G_0W_0 calculations starting from Hartree-Fock reduce the
screening and improve the low-lying excitation energies. The effect of the GW
self-energy on the molecular excitation energies is shown to be similar to the
inclusion of final state relaxations in Hartree-Fock theory. We discuss the
break down of the GW approximation in systems with short range interactions
(Hubbard models) where correlation effects dominate over screening/relaxation
effects. Finally we illustrate the important role of the derivative
discontinuity of the true exchange-correlation functional by computing the
exact Kohn-Sham levels of benzene.Comment: 9 pages, 5 figures, accepted for publication in Phys. Rev.
Quasiparticle Self-Consistent GW Theory
In past decades the scientific community has been looking for a reliable
first-principles method to predict the electronic structure of solids with high
accuracy. Here we present an approach which we call the quasiparticle
self-consistent GW approximation (QpscGW). It is based on a kind of
self-consistent perturbation theory, where the self-consistency is constructed
to minimize the perturbation. We apply it to selections from different classes
of materials, including alkali metals, semiconductors, wide band gap
insulators, transition metals, transition metal oxides, magnetic insulators,
and rare earth compounds. Apart some mild exceptions, the properties are very
well described, particularly in weakly correlated cases. Self-consistency
dramatically improves agreement with experiment, and is sometimes essential.
Discrepancies with experiment are systematic, and can be explained in terms of
approximations made.Comment: 12 pages, 3 figure
Many-Body Approximation Scheme Beyond GW
We explore the combination of the extended dynamical mean field theory
(EDMFT) with the GW approximation (GWA); the former sums the local
contributions to the self-energies to infinite order in closed form and the
latter handles the non-local ones to lowest order. We investigate the different
levels of self-consistency that can be implemented within this method by
comparing to the exact QMC solution of a finite-size model Hamiltonian. We find
that using the EDMFT solution for the local self-energies as input to the GWA
for the non-local self-energies gives the best result.Comment: 4 pages, 8 figure
Renormalization of Molecular Electronic Levels at Metal-Molecule Interfaces
The electronic structure of benzene on graphite (0001) is computed using the
GW approximation for the electron self-energy. The benzene quasiparticle energy
gap is predicted to be 7.2 eV on graphite, substantially reduced from its
calculated gas-phase value of 10.5 eV. This decrease is caused by a change in
electronic correlation energy, an effect completely absent from the
corresponding Kohn-Sham gap. For weakly-coupled molecules, this correlation
energy change is seen to be well described by a surface polarization effect. A
classical image potential model illustrates trends for other conjugated
molecules on graphite.Comment: 4 pages, 3 figures, 2 table
Quasiparticle Electronic structure of Copper in the GW approximation
We show that the results of photoemission and inverse photoemission
experiments on bulk copper can be quantitatively described within
band-structure theory, with no evidence of effects beyond the
single-quasiparticle approximation. The well known discrepancies between the
experimental bandstructure and the Kohn-Sham eigenvalues of Density Functional
Theory are almost completely corrected by self-energy effects.
Exchange-correlation contributions to the self-energy arising from 3s and 3p
core levels are shown to be crucial.Comment: 4 pages, 2 figures embedded in the text. 3 footnotes modified and 1
reference added. Small modifications also in the text. Accepted for
publication in PR
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