1,945 research outputs found
Tuning the electron-phonon coupling in multilayer graphene with magnetic fields
Magneto Raman scattering study of the E optical phonons in multi-layer
epitaxial graphene grown on a carbon face of SiC are presented. At 4.2K in
magnetic field up to 33 T, we observe a series of well pronounced avoided
crossings each time the optically active inter Landau level transition is tuned
in resonance with the E phonon excitation (at 196 meV). The width of the
phonon Raman scattering response also shows pronounced variations and is
enhanced in conditions of resonance. The experimental results are well
reproduced by a model that gives directly the strength of the electron-phonon
interaction.Comment: 4 pages, 3 figure
Effect of a magnetic field on the two-phonon Raman scattering in graphene
We have studied, both experimentally and theoretically, the change of the
so-called 2D band of the Raman scattering spectrum of graphene (the two-phonon
peak near 2700 cm-1) in an external magnetic field applied perpendicular to the
graphene crystal plane at liquid helium temperature. A shift to lower frequency
and broadening of this band is observed as the magnetic field is increased from
0 to 33 T. At fields up to 5--10 T the changes are quadratic in the field while
they become linear at higher magnetic fields. This effect is explained by the
curving of the quasiclassical trajectories of the photo-excited electrons and
holes in the magnetic field, which enables us (i) to extract the electron
inelastic scattering rate, and (ii) to conclude that electronic scattering
accounts for about half of the measured width of the 2D peak.Comment: 11 pages, 7 figure
High-Energy Limit of Massless Dirac Fermions in Multilayer Graphene using Magneto-Optical Transmission Spectroscopy
We have investigated the absorption spectrum of multilayer graphene in high
magnetic fields. The low energy part of the spectrum of electrons in graphene
is well described by the relativistic Dirac equation with a linear dispersion
relation. However, at higher energies (>500 meV) a deviation from the ideal
behavior of Dirac particles is observed. At an energy of 1.25 eV, the deviation
from linearity is 40 meV. This result is in good agreement with the theoretical
model, which includes trigonal warping of the Fermi surface and higher-order
band corrections. Polarization-resolved measurements show no observable
electron-hole asymmetry.Comment: 4 pages,3 figure
Semiclassical theory for spatial density oscillations in fermionic systems
We investigate the particle and kinetic-energy densities for a system of
fermions bound in a local (mean-field) potential V(\bfr). We generalize a
recently developed semiclassical theory [J. Roccia and M. Brack, Phys. Rev.\
Lett. {\bf 100}, 200408 (2008)], in which the densities are calculated in terms
of the closed orbits of the corresponding classical system, to
dimensions. We regularize the semiclassical results for the U(1) symmetry
breaking occurring for spherical systems at and near the classical
turning points where the Friedel oscillations are predominant and well
reproduced by the shortest orbit going from to the closest turning point
and back. For systems with spherical symmetry, we show that there exist two
types of oscillations which can be attributed to radial and non-radial orbits,
respectively. The semiclassical theory is tested against exact
quantum-mechanical calculations for a variety of model potentials. We find a
very good overall numerical agreement between semiclassical and exact numerical
densities even for moderate particle numbers . Using a "local virial
theorem", shown to be valid (except for a small region around the classical
turning points) for arbitrary local potentials, we can prove that the
Thomas-Fermi functional reproduces the oscillations in
the quantum-mechanical densities to first order in the oscillating parts.Comment: LaTeX, 22pp, 15 figs, 1 table, to be published in Phys. Rev.
Multi-shell gold nanowires under compression
Deformation properties of multi-wall gold nanowires under compressive loading
are studied. Nanowires are simulated using a realistic many-body potential.
Simulations start from cylindrical fcc(111) structures at T=0 K. After
annealing cycles axial compression is applied on multi-shell nanowires for a
number of radii and lengths at T=300 K. Several types of deformation are found,
such as large buckling distortions and progressive crushing. Compressed
nanowires are found to recover their initial lengths and radii even after
severe structural deformations. However, in contrast to carbon nanotubes
irreversible local atomic rearrangements occur even under small compressions.Comment: 1 gif figure, 5 ps figure
Symmetry breaking in commensurate graphene rotational stacking; a comparison of theory and experiment
Graphene stacked in a Bernal configuration (60 degrees relative rotations
between sheets) differs electronically from isolated graphene due to the broken
symmetry introduced by interlayer bonds forming between only one of the two
graphene unit cell atoms. A variety of experiments have shown that non-Bernal
rotations restore this broken symmetry; consequently, these stacking varieties
have been the subject of intensive theoretical interest. Most theories predict
substantial changes in the band structure ranging from the development of a Van
Hove singularity and an angle dependent electron localization that causes the
Fermi velocity to go to zero as the relative rotation angle between sheets goes
to zero. In this work we show by direct measurement that non-Bernal rotations
preserve the graphene symmetry with only a small perturbation due to weak
effective interlayer coupling. We detect neither a Van Hove singularity nor any
significant change in the Fermi velocity. These results suggest significant
problems in our current theoretical understanding of the origins of the band
structure of this material.Comment: 7 pages, 6 figures, submitted to PR
Detection of Coxiella burnetii in complex matrices by using multiplex quantitative PCR during a major Q fever outbreak in the Netherlands
Q fever, caused by Coxiella burnetii, is a zoonosis with a worldwide distribution. A large rural area in the southeast of the Netherlands was heavily affected by Q fever between 2007 and 2009. This initiated the development of a robust and internally controlled multiplex quantitative PCR (qPCR) assay for the detection of C. burnetii DNA in veterinary and environmental matrices on suspected Q fever-affected farms. The qPCR detects three C. burnetii targets (icd, com1, and IS1111) and one Bacillus thuringiensis internal control target (cry1b). Bacillus thuringiensis spores were added to samples to control both DNA extraction and PCR amplification. The performance of the qPCR assay was investigated and showed a high efficiency; a limit of detection of 13.0, 10.6, and 10.4 copies per reaction for the targets icd, com1, and IS1111, respectively; and no crossreactivity with the nontarget organisms tested. Screening for C. burnetii DNA on 29 suspected Q fever-affected farms during the Q fever epidemic in 2008 showed that swabs from dust-accumulating surfaces contained higher levels of C. burnetii DNA than vaginal swabs from goats or sheep. PCR inhibition by coextracted substances was observed in some environmental samples, and 10- or 100-fold dilutions of samples were sufficient to obtain interpretable signals for both the C. burnetii targets and the internal control. The inclusion of an internal control target and three C. burnetii targets in one multiplex qPCR assay showed that complex veterinary and environmental matrices can be screened reliably for the presence of C. burnetii DNA during an outbreak. © 2011, American Society for Microbiology
A wide band gap metal-semiconductor-metal nanostructure made entirely from graphene
A blueprint for producing scalable digital graphene electronics has remained
elusive. Current methods to produce semiconducting-metallic graphene networks
all suffer from either stringent lithographic demands that prevent
reproducibility, process-induced disorder in the graphene, or scalability
issues. Using angle resolved photoemission, we have discovered a unique one
dimensional metallic-semiconducting-metallic junction made entirely from
graphene, and produced without chemical functionalization or finite size
patterning. The junction is produced by taking advantage of the inherent,
atomically ordered, substrate-graphene interaction when it is grown on SiC, in
this case when graphene is forced to grow over patterned SiC steps. This
scalable bottomup approach allows us to produce a semiconducting graphene strip
whose width is precisely defined within a few graphene lattice constants, a
level of precision entirely outside modern lithographic limits. The
architecture demonstrated in this work is so robust that variations in the
average electronic band structure of thousands of these patterned ribbons have
little variation over length scales tens of microns long. The semiconducting
graphene has a topologically defined few nanometer wide region with an energy
gap greater than 0.5 eV in an otherwise continuous metallic graphene sheet.
This work demonstrates how the graphene-substrate interaction can be used as a
powerful tool to scalably modify graphene's electronic structure and opens a
new direction in graphene electronics research.Comment: 11 pages, 7 figure
- …