78 research outputs found
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
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
Angle-resolved photoemission spectra of graphene from first-principles calculations
Angle-resolved photoemission spectroscopy (ARPES) is a powerful experimental
technique for directly probing electron dynamics in solids. The energy vs.
momentum dispersion relations and the associated spectral broadenings measured
by ARPES provide a wealth of information on quantum many-body interaction
effects. In particular, ARPES allows studies of the Coulomb interaction among
electrons (electron-electron interactions) and the interaction between
electrons and lattice vibrations (electron-phonon interactions). Here, we
report ab initio simulations of the ARPES spectra of graphene including both
electron-electron and electron-phonon interactions on the same footing. Our
calculations reproduce some of the key experimental observations related to
many-body effects, including the indication of a mismatch between the upper and
lower halves of the Dirac cone
Raman spectra of epitaxial graphene on SiC and of epitaxial graphene transferred to SiO2
Raman spectra were measured for mono-, bi- and trilayer graphene grown on SiC
by solid state graphitization, whereby the number of layers was pre-assigned by
angle-resolved ultraviolet photoemission spectroscopy. It was found that the
only unambiguous fingerprint in Raman spectroscopy to identify the number of
layers for graphene on SiC(0001) is the linewidth of the 2D (or D*) peak. The
Raman spectra of epitaxial graphene show significant differences as compared to
micromechanically cleaved graphene obtained from highly oriented pyrolytic
graphite crystals. The G peak is found to be blue-shifted. The 2D peak does not
exhibit any obvious shoulder structures but it is much broader and almost
resembles a single-peak even for multilayers. Flakes of epitaxial graphene were
transferred from SiC onto SiO2 for further Raman studies. A comparison of the
Raman data obtained for graphene on SiC with data for epitaxial graphene
transferred to SiO2 reveals that the G peak blue-shift is clearly due to the
SiC substrate. The broadened 2D peak however stems from the graphene structure
itself and not from the substrate.Comment: 27 pages, 8 figure
Symmetry Breaking in Few Layer Graphene Films
Recently, it was demonstrated that the quasiparticle dynamics, the
layer-dependent charge and potential, and the c-axis screening coefficient
could be extracted from measurements of the spectral function of few layer
graphene films grown epitaxially on SiC using angle-resolved photoemission
spectroscopy (ARPES). In this article we review these findings, and present
detailed methodology for extracting such parameters from ARPES. We also present
detailed arguments against the possibility of an energy gap at the Dirac
crossing ED.Comment: 23 pages, 13 figures, Conference Proceedings of DPG Meeting Mar 2007
Regensburg Submitted to New Journal of Physic
Laser-induced etching of few-layer graphene synthesized by Rapid-Chemical Vapour Deposition on Cu thin films
The outstanding electrical and mechanical properties of graphene make it very
attractive for several applications, Nanoelectronics above all. However a
reproducible and non destructive way to produce high quality, large-scale area,
single layer graphene sheets is still lacking. Chemical Vapour Deposition of
graphene on Cu catalytic thin films represents a promising method to reach this
goal, because of the low temperatures (T < 900 Celsius degrees) involved during
the process and of the theoretically expected monolayer self-limiting growth.
On the contrary such self-limiting growth is not commonly observed in
experiments, thus making the development of techniques allowing for a better
control of graphene growth highly desirable. Here we report about the local
ablation effect, arising in Raman analysis, due to the heat transfer induced by
the laser incident beam onto the graphene sample.Comment: v1:9 pages, 8 figures, submitted to SpringerPlus; v2: 11 pages,
PDFLaTeX, 9 figures, revised peer-reviewed version resubmitted to
SpringerPlus; 1 figure added, figure 1 and 4 replaced,typos corrected,
"Results and discussion" section significantly extended to better explain
etching mechanism and features of Raman spectra, references adde
Grain Boundaries in Graphene on SiC(000) Substrate
Grain boundaries in epitaxial graphene on the SiC(000) substrate are
studied using scanning tunneling microscopy and spectroscopy. All investigated
small-angle grain boundaries show pronounced out-of-plane buckling induced by
the strain fields of constituent dislocations. The ensemble of observations
allows to determine the critical misorientation angle of buckling transition
. Periodic structures are found among the flat
large-angle grain boundaries. In particular, the observed highly ordered grain boundary is assigned to the previously
proposed lowest formation energy structural motif composed of a continuous
chain of edge-sharing alternating pentagons and heptagons. This periodic grain
boundary defect is predicted to exhibit strong valley filtering of charge
carriers thus promising the practical realization of all-electric valleytronic
devices
Properties of Graphene: A Theoretical Perspective
In this review, we provide an in-depth description of the physics of
monolayer and bilayer graphene from a theorist's perspective. We discuss the
physical properties of graphene in an external magnetic field, reflecting the
chiral nature of the quasiparticles near the Dirac point with a Landau level at
zero energy. We address the unique integer quantum Hall effects, the role of
electron correlations, and the recent observation of the fractional quantum
Hall effect in the monolayer graphene. The quantum Hall effect in bilayer
graphene is fundamentally different from that of a monolayer, reflecting the
unique band structure of this system. The theory of transport in the absence of
an external magnetic field is discussed in detail, along with the role of
disorder studied in various theoretical models. We highlight the differences
and similarities between monolayer and bilayer graphene, and focus on
thermodynamic properties such as the compressibility, the plasmon spectra, the
weak localization correction, quantum Hall effect, and optical properties.
Confinement of electrons in graphene is nontrivial due to Klein tunneling. We
review various theoretical and experimental studies of quantum confined
structures made from graphene. The band structure of graphene nanoribbons and
the role of the sublattice symmetry, edge geometry and the size of the
nanoribbon on the electronic and magnetic properties are very active areas of
research, and a detailed review of these topics is presented. Also, the effects
of substrate interactions, adsorbed atoms, lattice defects and doping on the
band structure of finite-sized graphene systems are discussed. We also include
a brief description of graphane -- gapped material obtained from graphene by
attaching hydrogen atoms to each carbon atom in the lattice.Comment: 189 pages. submitted in Advances in Physic
Correlating Raman Spectral Signatures with Carrier Mobility in Epitaxial Graphene: A Guide to Achieving High Mobility on the Wafer Scale
We report a direct correlation between carrier mobility and Raman topography
of epitaxial graphene (EG) grown on silicon carbide (SiC). We show the Hall
mobility of material on the Si-face of SiC [SiC(0001)] is not only highly
dependent on thickness uniformity but also on monolayer strain uniformity. Only
when both thickness and strain are uniform over a significant fraction (> 40%)
of the device active area does the mobility exceed 1000 cm2/V-s. Additionally,
we achieve high mobility epitaxial graphene (18,100 cm2/V-s at room
temperature) on the C-face of SiC [SiC(000-1)] and show that carrier mobility
depends strongly on the graphene layer stacking. These findings provide a means
to rapidly estimate carrier mobility and provide a guide to achieve very high
mobility in epitaxial graphene. Our results suggest that ultra-high mobilities
(>50,000 cm2/V-s) are achievable via the controlled formation of uniform,
rotationally faulted epitaxial graphene.Comment: 13 pages including supplimental material. Submitted to Nature
Materials 2/23/200
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