749 research outputs found
A micro-magneto-Raman scattering study of graphene on a bulk graphite substrate
We report on a magneto-Raman scattering study of graphene flakes located on
the surface of a bulk graphite substrate. By spatially mapping the Raman
scattering response of the surface of bulk graphite with an applied magnetic
field, we pinpoint specific locations which show the electronic excitation
spectrum of graphene. We present the characteristic Raman scattering signatures
of these specific locations. We show that such flakes can be superimposed with
another flake and still exhibit a graphene-like excitation spectrum.
Two different excitation laser energies (514.5 and 720 nm) are used to
investigate the excitation wavelength dependence of the electronic Raman
scattering signal.Comment: 6 pages, 5 figure
Multiple magneto-phonon resonances in graphene
Our low-temperature magneto-Raman scattering measurements performed on
graphene-like locations on the surface of bulk graphite reveal a new series of
magneto-phonon resonances involving both K-point and Gamma-point phonons. In
particular, we observe for the first time the resonant splitting of three
crossing excitation branches. We give a detailed theoretical analysis of these
new resonances. Our results highlight the role of combined excitations and the
importance of multi-phonon processes (from both K and Gamma points) for the
relaxation of hot carriers in graphene.Comment: 20 pages, 11 figure
Circular dichroism of magneto-phonon resonance in doped graphene
Polarization resolved, Raman scattering response due to E phonon in
monolayer graphene has been investigated in magnetic fields up to 29 T. The
hybridization of the E phonon with only the fundamental inter Landau
level excitation (involving the n=0 Landau level) is observed and only in one
of the two configurations of the circularly crossed polarized excitation and
scattered light. This polarization anisotropy of the magneto-phonon resonance
is shown to be inherent to relatively strongly doped graphene samples, with
carrier concentration typical for graphene deposited on SiO
Probing the band structure of quadri-layer graphene with magneto-phonon resonance
We show how the magneto-phonon resonance, particularly pronounced in sp2
carbon allotropes, can be used as a tool to probe the band structure of
multilayer graphene specimens. Even when electronic excitations cannot be
directly observed, their coupling to the E2g phonon leads to pronounced
oscillations of the phonon feature observed through Raman scattering
experiments with multiple periods and amplitudes detemined by the electronic
excitation spectrum. Such experiment and analysis have been performed up to 28T
on an exfoliated 4-layer graphene specimen deposited on SiO2, and the observed
oscillations correspond to the specific AB stacked 4-layer graphene electronic
excitation spectrum.Comment: 11 pages, 5 Fi
Autocalibration with the Minimum Number of Cameras with Known Pixel Shape
In 3D reconstruction, the recovery of the calibration parameters of the
cameras is paramount since it provides metric information about the observed
scene, e.g., measures of angles and ratios of distances. Autocalibration
enables the estimation of the camera parameters without using a calibration
device, but by enforcing simple constraints on the camera parameters. In the
absence of information about the internal camera parameters such as the focal
length and the principal point, the knowledge of the camera pixel shape is
usually the only available constraint. Given a projective reconstruction of a
rigid scene, we address the problem of the autocalibration of a minimal set of
cameras with known pixel shape and otherwise arbitrarily varying intrinsic and
extrinsic parameters. We propose an algorithm that only requires 5 cameras (the
theoretical minimum), thus halving the number of cameras required by previous
algorithms based on the same constraint. To this purpose, we introduce as our
basic geometric tool the six-line conic variety (SLCV), consisting in the set
of planes intersecting six given lines of 3D space in points of a conic. We
show that the set of solutions of the Euclidean upgrading problem for three
cameras with known pixel shape can be parameterized in a computationally
efficient way. This parameterization is then used to solve autocalibration from
five or more cameras, reducing the three-dimensional search space to a
two-dimensional one. We provide experiments with real images showing the good
performance of the technique.Comment: 19 pages, 14 figures, 7 tables, J. Math. Imaging Vi
Electronic excitations and electron-phonon coupling in bulk graphite through Raman scattering in high magnetic fields
We use polarized magneto-Raman scattering to study purely electronic
excitations and the electron-phonon coupling in bulk graphite. At a temperature
of 4.2 K and in magnetic fields up to 28 T we observe -point electronic
excitations involving Landau bands with and with that can be selected by controlling the angular momentum of the
excitation laser and of the scattered light. The magneto-phonon effect
involving the optical phonon and -point inter Landau bands
electronic excitations with is revealed and analyzed within a
model taking into account the full dispersion. These polarization
resolved results are explained in the frame of the Slonczewski-Weiss-McClure
(SWM) model which directly allows to quantify the electron-hole asymmetry.Comment: 13 pages, 10 figure
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
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