470 research outputs found
Carbon cage-like materials as potential low work function metallic compounds: Case of clathrates
We present an ab-initio calculation of the electronic affinity of the
hypothetical C-46 clathrate by studying its bare and hydrogenated (100)
surfaces. We show that such a system shares with the diamond phase a small
electronic affinity. Further, contrary to the diamond phase, the possibility of
doping endohedrally these cage-like systems allows to significantly raise the
position of the Fermi level, resulting in a true metal with a small work
function. This is illustrated in the case of the Li8@C-46 doped compound. Such
a class of materials might be of much interest for the design of
electron-emitting devices.Comment: 4 pages, 3 figures, RevTe
Room temperature Peierls distortion in small radius nanotubes
By means of {\it ab initio} simulations, we investigate the phonon band
structure and electron-phonon coupling in small 4-\AA diameter nanotubes. We
show that both the C(5,0) and C(3,3) tubes undergo above room temperature a
Peierls transition mediated by an acoustical long-wavelength and an optical
phonons respectively. In the armchair geometry, we verify that the
electron-phonon coupling parameter originates mainly from phonons at
and is strongly enhanced when the diameter decreases. These results
question the origin of superconductivity in small diameter nanotubes.Comment: submitted 21oct2004 accepted 6jan2005 (Phys.Rev.Lett.
Electron-phonon coupling in the C60 fullerene within the many-body GW approach
We study the electron-phonon coupling in the C60 fullerene within the
first-principles GW approach, focusing on the lowest unoccupied t1u three-fold
electronic state which is relevant for the superconducting transition in
electron doped fullerides. It is shown that the strength of the coupling is
significantly enhanced as compared to standard density functional theory
calculations with (semi)local functionals, with a 48% increase of the
electron-phonon potential Vep. The calculated GW value for the contribution
from the Hg modes of 93 meV comes within 4% of the most recent experimental
values. The present results call for a reinvestigation of previous density
functional based calculations of electron-phonon coupling in covalent systems
in general.Comment: 4 pages, 0 figur
Exploring approximations to the GW self-energy ionic gradients
The accuracy of the many-body perturbation theory GW formalism to calculate
electron-phonon coupling matrix elements has been recently demonstrated in the
case of a few important systems. However, the related computational costs are
high and thus represent strong limitations to its widespread application. In
the present study, we explore two less demanding alternatives for the
calculation of electron-phonon coupling matrix elements on the many-body
perturbation theory level. Namely, we test the accuracy of the static
Coulomb-hole plus screened-exchange (COHSEX) approximation and further of the
constant screening approach, where variations of the screened Coulomb potential
W upon small changes of the atomic positions along the vibrational eigenmodes
are neglected. We find this latter approximation to be the most reliable,
whereas the static COHSEX ansatz leads to substantial errors. Our conclusions
are validated in a few paradigmatic cases: diamond, graphene and the C60
fullerene. These findings open the way for combining the present many-body
perturbation approach with efficient linear-response theories
Short to long-range charge-transfer excitations in the zincbacteriochlorin-bacteriochlorin complex: a Bethe-Salpeter study
We study using the Bethe-Salpeter formalism the excitation energies of the
zincbacteriochlorinbacteriochlorin dyad, a paradigmatic photosynthetic complex.
In great contrast with standard timedependent density functional theory
calculations with (semi)local kernels, charge transfer excitations are
correctly located above the intramolecular Q-bands transitions found to be in
excellent agreement with experiment. Further, the asymptotic Coulomb behavior
towards the true quasiparticle gap for charge transfer excitations at long
distance is correctly reproduced, showing that the present scheme allows to
study with the same accuracy intramolecular and charge transfer excitations at
various spatial range and screening environment without any adjustable
parameter.Comment: 5 pages, 2 figures, 1 tabl
First-principles GW calculations for DNA and RNA nucleobases
On the basis of first-principles GW calculations, we study the quasiparticle
properties of the guanine, adenine, cytosine, thymine, and uracil DNA and RNA
nucleobases. Beyond standard G0W0 calculations, starting from Kohn-Sham
eigenstates obtained with (semi)local functionals, a simple self-consistency on
the eigenvalues allows to obtain vertical ionization energies and electron
affinities within an average 0.11 eV and 0.18 eV error respectively as compared
to state-of-the-art coupled-cluster and multi-configurational perturbative
quantum chemistry approaches. Further, GW calculations predict the correct \pi
-character of the highest occupied state, thanks to several level crossings
between density functional and GW calculations. Our study is based on a recent
gaussian-basis implementation of GW with explicit treatment of dynamical
screening through contour deformation techniques.Comment: 5 pages, 3 figure
Theoretical Study of One-dimensional Chains of Metal Atoms in Nanotubes
Using first-principles total-energy pseudopotential calculations, we have
studied the properties of chains of potassium and aluminum in nanotubes. For BN
tubes, there is little interaction between the metal chains and the tubes, and
the conductivity of these tubes is through carriers located at the inner part
of the tube. In contrast, for small radius carbon nanotubes, there are two
types of interactions: charge-transfer (dominant for alkali atoms) leading to
strong ionic cohesion, and hybridization (for multivalent metal atoms)
resulting in a smaller cohesion. For Al-atomic chains in carbon tubes, we show
that both effects contribute. New electronic properties related to these
confined atomic chains of metal are analyzed.Comment: 12 pages + 3 figure
Excitons in boron nitride nanotubes: dimensionality effects
We show that the optical absorption spectra of boron nitride (BN) nanotubes
are dominated by strongly bound excitons. Our first-principles calculations
indicate that the binding energy for the first and dominant excitonic peak
depends sensitively on the dimensionality of the system, varying from 0.7 eV in
bulk hexagonal BN via 2.1 eV in the single sheet of BN to more than 3 eV in the
hypothetical (2,2) tube. The strongly localized nature of this exciton dictates
the fast convergence of its binding energy with increasing tube diameter
towards the sheet value. The absolute position of the first excitonic peak is
almost independent of the tube radius and system dimensionality. This provides
an explanation for the observed "optical gap" constancy for different tubes and
bulk hBN [R. Arenal et al., to appear in Phys. Rev. Lett. (2005)].Comment: 5 pages, 2 figure
Infinite-layer fluoro-nickelates as model materials
We study theoretically the fluoro-nickelate series NiF ( Li, Na,
K, Rb, Cs) in the tetragonal infinite-layer structure. We use density
functional theory to determine the structural parameters and the electronic
band structure of these unprecedented compounds. Thus, we predict these
materials as model systems where the Ni oxidation is realized and
the low-energy physics is completely determined by the Ni-3 bands only.
Fluoro-nickelates of this class thus offer an ideal platform for the study of
intriguing physics that emerges out of the special electronic
configuration, notably high-temperature unconventional superconductivity.Comment: 6 pages, 4 tables, and 6 figure
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