2,380 research outputs found
Impact of local stacking on the graphene-impurity interaction: theory and experiments
We investigate the graphene-impurity interaction problem by combining
experimental - scanning tunneling microscopy (STM) and spectroscopy (STS) - and
theoretical - Anderson impurity model and density functional theory (DFT)
calculations - techniques. We use graphene on the SiC(000-1)(2x2)_C
reconstruction as a model system. The SiC substrate reconstruction is based on
silicon adatoms. Graphene mainly interacts with the dangling bonds of these
adatoms which act as impurities. Graphene grown on SiC(000-1)(2x2)_C shows
domains with various orientations relative to the substrate so that very
different local graphene/Si adatom stacking configurations can be probed on a
given grain. The position and width of the adatom (impurity) state can be
analyzed by STM/STS and related to its local environment owing to the high bias
electronic transparency of graphene. The experimental results are compared to
Anderson's model predictions and complemented by DFT calculations for some
specific local environments. We conclude that the adatom resonance shows a
smaller width and a larger shift toward the Dirac point for an adatom at the
center of a graphene hexagon than for an adatom just on top of a C graphene
atom.Comment: 13 pages, 6 figures, Accepted for publication in Phys. Rev.
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
Synthesis and studies of new organic semiconductors based on furan moieties coming from biomass
Date du colloque : 08/2010International audienc
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
Semi-conducteurs organiques obtenus à partir de matières premières dérivées de la biomasse
Date du colloque : 05/2009</p
Quasiparticle scattering off phase boundaries in epitaxial graphene
We investigate the electronic structure of terraces of single layer graphene
(SLG) by scanning tunneling microscopy (STM) on samples grown by thermal
decomposition of 6H-SiC(0001) crystals in ultra-high vacuum. We focus on the
perturbations of the local density of states (LDOS) in the vicinity of edges of
SLG terraces. Armchair edges are found to favour intervalley quasiparticle
scattering, leading to the (\surd3\times\surd3)R30{\deg} LDOS superstructure
already reported for graphite edges and more recently for SLG on SiC(0001).
Using Fourier transform of LDOS images, we demonstrate that the intrinsic
doping of SLG is responsible for a LDOS pattern at the Fermi energy which is
more complex than for neutral graphene or graphite, since it combines local
(\surd3\times\surd3)R30{\deg} superstructure and long range beating modulation.
Although these features were already reported by Yang et al. Nanoletters 10,
943 (2010), we propose here an alternative interpretation based on simple
arguments classically used to describe standing wave patterns in standard
two-dimensional systems. Finally, we discuss the absence of intervalley
scattering off other typical boundaries: zig-zag edges and SLG/bilayer graphene
junctions
Role of pseudospin in quasiparticle interferences in epitaxial graphene probed by high-resolution scanning tunneling microscopy
Pseudospin, an additional degree of freedom related to the honeycomb
structure of graphene, is responsible of many of the outstanding electronic
properties found in this material. This article provides a clear understanding
of how such pseudospin impacts the quasiparticle interferences of monolayer
(ML) and bilayer (BL) graphene measured by low temperature scanning tunneling
microscopy and spectroscopy. We have used this technique to map, with very high
energy and space resolution, the spatial modulations of the local density of
states of ML and BL graphene epitaxialy grown on SiC(0001), in presence of
native disorder. We perform a Fourier transform analysis of such modulations
including wavevectors up to unit-vectors of the reciprocal lattice. Our data
demonstrate that the quasiparticle interferences associated to some particular
scattering processes are suppressed in ML graphene, but not in BL graphene.
Most importantly, interferences with 2qF wavevector associated to intravalley
backscattering are not measured in ML graphene, even on the images with highest
resolution. In order to clarify the role of the pseudospin on the quasiparticle
interferences, we use a simple model which nicely captures the main features
observed on our data. The model unambiguously shows that graphene's pseudospin
is responsible for such suppression of quasiparticle interferences features in
ML graphene, in particular for those with 2qF wavevector. It also confirms
scanning tunneling microscopy as a unique technique to probe the pseudospin in
graphene samples in real space with nanometer precision. Finally, we show that
such observations are robust with energy and obtain with great accuracy the
dispersion of the \pi-bands for both ML and BL graphene in the vicinity of the
Fermi level, extracting their main tight binding parameters
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