96 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
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
Conception d'un épandeur de fumures organiques pour les exploitations à 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
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
Quantum Transport in Chemically-modified Two-Dimensional Graphene: From Minimal Conductivity to Anderson Localization
An efficient computational methodology is used to explore charge transport
properties in chemically-modified (and randomly disordered) graphene-based
materials. The Hamiltonians of various complex forms of graphene are
constructed using tight-binding models enriched by first-principles
calculations. These atomistic models are further implemented into a real-space
order-N Kubo-Greenwood approach, giving access to the main transport length
scales (mean free paths, localization lengths) as a function of defect density
and charge carrier energy. An extensive investigation is performed for epoxide
impurities with specific discussions on both the existence of a minimum
semi-classical conductivity and a crossover between weak to strong localization
regime. The 2D generalization of the Thouless relationship linking transport
length scales is here illustrated based on a realistic disorder model.Comment: 14 pages, 18 figures, submitte
Screening and interlayer coupling in multilayer graphene field-effect transistors
With the motivation of improving the performance and reliability of
aggressively scaled nano-patterned graphene field-effect transistors, we
present the first systematic experimental study on charge and current
distribution in multilayer graphene field-effect transistors. We find a very
particular thickness dependence for Ion, Ioff, and the Ion/Ioff ratio, and
propose a resistor network model including screening and interlayer coupling to
explain the experimental findings. In particular, our model does not invoke
modification of the linear energy-band structure of graphene for the multilayer
case. Noise reduction in nano-scale few-layer graphene transistors is
experimentally demonstrated and can be understood within this model as well.Comment: 13 pages, 4 figures, 20 reference
Localization of Dirac electrons by Moire patterns in graphene bilayers
We study the electronic structure of two Dirac electron gazes coupled by a
periodic Hamiltonian such as it appears in rotated graphene bilayers. Ab initio
and tight-binding approaches are combined and show that the spatially periodic
coupling between the two Dirac electron gazes can renormalize strongly their
velocity. We investigate in particular small angles of rotation and show that
the velocity tends to zero in this limit. The localization is confirmed by an
analysis of the eigenstates which are localized essentially in the AA zones of
the Moire patterns.Comment: 4 pages, 5 figure
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