214 research outputs found
Asymptotics of an optimal compliance-location problem
We consider the problem of placing n small balls of given radius in a certain
domain subject to a force f in order to minimize the compliance of the
configuration. Then we let n tend to infinity and look at the asymptotics of
the minimization problem, after properly scaling the functionals involved, and
to the limit distribution of the centres of the balls. This problem is both
linked to optimal location and shape optimization problems.Comment: 20 pages with 2 figures; final accepted version (minor changes, some
extra details on the positivity assumption on
How the SiC substrate impacts graphene atomic and electronic structures
Graphene, the two-dimensional form of carbon presents outstanding electronic
and transport properties. This gives hope for the development of applications
in nanoelectronics. However, for industrial purpose, graphene has to be
supported by a substrate. We focus here on the graphene-on-SiC system to
discuss how the SiC substrate interacts with the graphene layer and to show the
effect of the interface on graphene atomic and electronic structures.Comment: 3 pages, 3 figure
Conception d'un Ă©pandeur de fumures organiques pour les exploitations Ă traction animale d'Afrique
Design of an Animal-drawn Manure Spreader for Small Farms in Africa. Les bilans azotés des farines d'asticots de 2 jours séchés pendant 12, 24 et 48 heures à 70 °C ont été étudiés. Parmi elles, la farine dérivée d'asticots séchés durant 24 heures a le meilleur bilan azoté, du reste inférieur à celui de la caséine. Toutefois, au regard de la valeur de leurs bilans azotés, ces farines d'asticots séchés apparaissent globalement comme une source potentielle de protéine pour l'alimentation animale
Graphene on the C-terminated SiC (000 ) surface: An ab initio study
The atomic and electronic structures of a graphene layer on top of the
reconstruction of the SiC (000) surface are studied from
ab initio calculations. At variance with the (0001) face, no C bufferlayer is
found here. Si adatoms passivate the substrate surface so that the very first C
layer presents a linear dispersion characteristic of graphene. A small
graphene-substrate interaction remains in agreement with scanning tunneling
experiments (F.Hiebel et al. {\it Phys. Rev. B} {\bf 78} 153412 (2008)). The
stacking geometry has little influence on the interaction which explains the
rotational disorder observed on this face.Comment: 4 pages, 3 figures, additional materia
Electron states of mono- and bilayer graphene on SiC probed by STM
We present a scanning tunneling microscopy (STM) study of a
gently-graphitized 6H-SiC(0001) surface in ultra high vacuum. From an analysis
of atomic scale images, we identify two different kinds of terraces, which we
unambiguously attribute to mono- and bilayer graphene capping a C-rich
interface. At low temperature, both terraces show
quantum interferences generated by static impurities. Such interferences are a
fingerprint of -like states close to the Fermi level. We conclude that the
metallic states of the first graphene layer are almost unperturbed by the
underlying interface, in agreement with recent photoemission experiments (A.
Bostwick et al., Nature Physics 3, 36 (2007))Comment: 4 pages, 3 figures submitte
Quasiparticle Chirality in Epitaxial Graphene Probed at the Nanometer Scale
Graphene exhibits unconventional two-dimensional electronic properties
resulting from the symmetry of its quasiparticles, which leads to the concepts
of pseudospin and electronic chirality. Here we report that scanning tunneling
microscopy can be used to probe these unique symmetry properties at the
nanometer scale. They are reflected in the quantum interference pattern
resulting from elastic scattering off impurities, and they can be directly read
from its fast Fourier transform. Our data, complemented by theoretical
calculations, demonstrate that the pseudospin and the electronic chirality in
epitaxial graphene on SiC(0001) correspond to the ones predicted for ideal
graphene.Comment: 4 pages, 3 figures, minor change
Electronic structure of epitaxial graphene layers on SiC: effect of the substrate
Recent transport measurements on thin graphite films grown on SiC show large
coherence lengths and anomalous integer quantum Hall effects expected for
isolated graphene sheets. This is the case eventhough the layer-substrate
epitaxy of these films implies a strong interface bond that should induce
perturbations in the graphene electronic structure. Our DFT calculations
confirm this strong substrate-graphite bond in the first adsorbed carbon layer
that prevents any graphitic electronic properties for this layer. However, the
graphitic nature of the film is recovered by the second and third absorbed
layers. This effect is seen in both the (0001)and 4H SiC
surfaces. We also present evidence of a charge transfer that depends on the
interface geometry. It causes the graphene to be doped and gives rise to a gap
opening at the Dirac point after 3 carbon layers are deposited in agreement
with recent ARPES experiments (T.Ohta et al, Science {\bf 313} (2006) 951)
Few layers graphene on 6H-SiC(000-1): an STM study
We have analyzed by Scanning Tunnelling Microscopy (STM) thin films made of
few (3-5) graphene layers grown on the C terminated face of 6H-SiC in order to
identify the nature of the azimuthal disorder reported in this material. We
observe superstructures which are interpreted as Moir\'e patterns due to a
misorientation angle between consecutive layers. The presence of stacking
faults is expected to lead to electronic properties reminiscent of single layer
graphene even for multilayer samples. Our results indicate that this apparent
electronic decoupling of the layers can show up in STM data.Comment: 20 page
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