948 research outputs found
Thermal conductivity of graphene in Corbino membrane geometry
Local laser excitation and temperature readout from the intensity ratio of
Stokes to anti-Stokes Raman scattering signals are employed to study the
thermal properties of a large graphene membrane. The concluded value of the
heat conductivity coefficient \kappa ~ 600 W/m \cdot K is smaller than
previously reported but still validates the conclusion that graphene is a very
good thermal conductor.Comment: 4 pages, 3 figure
Camera motion estimation through planar deformation determination
In this paper, we propose a global method for estimating the motion of a
camera which films a static scene. Our approach is direct, fast and robust, and
deals with adjacent frames of a sequence. It is based on a quadratic
approximation of the deformation between two images, in the case of a scene
with constant depth in the camera coordinate system. This condition is very
restrictive but we show that provided translation and depth inverse variations
are small enough, the error on optical flow involved by the approximation of
depths by a constant is small. In this context, we propose a new model of
camera motion, that allows to separate the image deformation in a similarity
and a ``purely'' projective application, due to change of optical axis
direction. This model leads to a quadratic approximation of image deformation
that we estimate with an M-estimator; we can immediatly deduce camera motion
parameters.Comment: 21 pages, version modifi\'ee accept\'e le 20 mars 200
Few layer graphene on SiC, pyrolitic graphite and graphene: a Raman scattering study
The results of micro-Raman scattering measurements performed on three
different ``graphitic'' materials: micro-structured disks of highly oriented
pyrolytic graphite, graphene multi-layers thermally decomposed from carbon
terminated surface of 4H-SiC and an exfoliated graphene monolayer are
presented. Despite its multi-layer character, most parts of the surface of the
graphitized SiC substrates shows a single-component, Lorentzian shape, double
resonance Raman feature in striking similarity to the case of a single graphene
monolayer. Our observation suggests a very weak electronic coupling between
graphitic layers on the SiC surface, which therefore can be considered to be
graphene multi-layers with a simple (Dirac-like) band structure.Comment: 4 pages, 3 Figures Structure of the paper strongly modified, small
changes in Fig 2 and 3. Same interpretation and same result
Dirac fermions at the H point of graphite: Magneto-transmission studies
We report on far infrared magneto-transmission measurements on a thin
graphite sample prepared by exfoliation of highly oriented pyrolytic graphite.
In magnetic field, absorption lines exhibiting a blue-shift proportional to
sqrtB are observed. This is a fingerprint for massless Dirac holes at the H
point in bulk graphite. The Fermi velocity is found to be c*=1.02x10^6 m/s and
the pseudogap at the H point is estimated to be below 10 meV. Although the
holes behave to a first approximation as a strictly 2D gas of Dirac fermions,
the full 3D band structure has to be taken into account to explain all the
observed spectral features.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Let
Fine structure of "zero-mode" Landau levels in HgTe/HgCdTe quantum wells
HgTe/HgCdTe quantum wells with the inverted band structure have been probed
using far infrared magneto-spectroscopy. Realistic calculations of Landau level
diagrams have been performed to identify the observed transitions.
Investigations have been greatly focused on the magnetic field dependence of
the peculiar pair of "zero-mode" Landau levels which characteristically split
from the upper conduction and bottom valence bands, and merge under the applied
magnetic field. The observed avoided crossing of these levels is tentatively
attributed to the bulk inversion asymmetry of zinc blend compounds.Comment: 5 pages, 4 figure
Anisotropy of effective masses in CuInSe2
Anisotropy of the valence band is experimentally demonstrated in CuInSe2, a key component of the absorber layer in one of the leading thin-film solar cell technology. By changing the orientation of applied magnetic fields with respect to the crystal lattice, we measure considerable differences in the diamagnetic shifts and effective g-factors for the A and B free excitons. The resulting free exciton reduced masses are combined with a perturbation model for non-degenerate independent excitons and theoretical dielectric constants to provide the anisotropic effective hole masses, revealing anisotropies of 5.5 (4.2) for the A (B) valence bands
Excitation power and temperature dependence of excitons in CuInSe2
Excitonic recombination processes in high quality CuInSe2 single crystals have been studied by photoluminescence (PL) and reflectance spectroscopy as a function of excitation powers and temperature. Excitation power dependent measurements confirm the identification of well-resolved A and B free excitons in the PL spectra and analysis of the temperature quenching of these lines provides values for activation energies. These are found to vary from sample to sample, with values of 12.5 and 18.4meV for the A and B excitons, respectively, in the one showing the highest quality spectra. Analysis of the temperature and power dependent PL spectra from the bound excitonic lines, labelled M1, M2, and M3 appearing in multiplets points to a likely assignment of the hole involved in each case. The M1 excitons appear to involve a conduction band electron and a hole from the B valence band hole. In contrast, an A valence band hole appears to be involved for the M2 and M3 excitons. In addition, the M1 exciton multiplet seems to be due to the radiative recombination of excitons bound to shallow hydrogenic defects, whereas the excitons involved in M2 and M3 are bound to more complex defects. In contrast to the M1 exciton multiplet, the excitonic lines of M2 and M3 saturate at high excitation powers suggesting that the concentration of the defects involved is low. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4709448
Electron-phonon coupling in the two phonon mode ternary alloy quantum well
We have investigated the infrared transmission of a two-dimensional (2DEG)
electron gas confined in a single
quantum well in order to study the electron optical phonon interaction in a two
phonon mode system. Infrared transmission experiments have been performed in
both the perpendicular Faraday (PF) and tilted Faraday (TF) configurations for
which the growth axis of the sample is tilted with respect to the incident
light propagation direction and to the magnetic field direction. The
experimental results lead to question the validity of the concept of polaron
mass in a real material.Comment: 7 pages, 3 figure
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