148 research outputs found
Intramolecular Imino Diels-Alder Reaction: Progress toward the Synthesis of Uncialamycin
We herein described an intramolecular imino Diels-Alder reaction promoted with BF3.OEt2/DDQ affording substituted quinolines. Using this procedure, we prepared the chiral quitioline moiety of the uncialamycin, a new enediyne natural product
Interplay between interferences and electron-electron interactions in epitaxial graphene
We separate localization and interaction effects in epitaxial graphene
devices grown on the C-face of a 4H-SiC substrate by analyzing the low
temperature conductivities. Weak localization and antilocalization are
extracted at low magnetic fields, after elimination of a geometric
magnetoresistance and subtraction of the magnetic field dependent Drude
conductivity. The electron electron interaction correction is extracted at
higher magnetic fields, where localization effects disappear. Both phenomena
are weak but sizable and of the same order of magnitude. If compared to
graphene on silicon dioxide, electron electron interaction on epitaxial
graphene are not significantly reduced by the larger dielectric constant of the
SiC substrate
Magnetoresistance of disordered graphene: from low to high temperatures
We present the magnetoresistance (MR) of highly doped monolayer graphene
layers grown by chemical vapor deposition on 6H-SiC. The magnetotransport
studies are performed on a large temperature range, from = 1.7 K up to room
temperature. The MR exhibits a maximum in the temperature range K.
The maximum is observed at intermediate magnetic fields ( T), in between
the weak localization and the Shubnikov-de Haas regimes. It results from the
competition of two mechanisms. First, the low field magnetoresistance increases
continuously with and has a purely classical origin. This positive MR is
induced by thermal averaging and finds its physical origin in the energy
dependence of the mobility around the Fermi energy. Second, the high field
negative MR originates from the electron-electron interaction (EEI). The
transition from the diffusive to the ballistic regime is observed. The
amplitude of the EEI correction points towards the coexistence of both long and
short range disorder in these samples
In_(0.75)Ga_(0.25)As on GaAs submicron rings and their application for coherent nanoelectronic devices
Electron-phase modulation in magnetic and electric fields will be presented
in In_(0.75)Ga_(0.25)As Aharonov-Bohm (AB) rings. The zero Schottky barrier of
this material made it possible to nanofabricate devices with radii down to
below 200 nm without carrier depletion. We shall present a fabrication scheme
based on wet and dry etching that yielded excellent reproducibility, very high
contrast of the oscillations and good electrical gating. The operation of these
structures is compatible with closed-cycle refrigeration and suggests that this
process can yield coherent electronic circuits that do not require cryogenic
liquids. The InGaAs/AlInAs heterostructure was grown by MBE on a GaAs substrate
[1], and in light of the large effective g-factor and the absence of the
Schottky barrier is a material system of interest for the investigation of
spin-related effects [2-4]} and the realization of hybrid
superconductor/semiconductor devices [5].Comment: 7 pages, 4 figures, to appear in Physica
The hole Fermi surface in BiSe probed by quantum oscillations
Transport and torque magnetometry measurements are performed at high magnetic
fields and low temperatures in a series of p-type (Ca-doped) BiSe
crystals. The angular dependence of the Shubnikov-de Haas and de Haas-van
Alphen quantum oscillations enables us to determine the Fermi surface of the
bulk valence band states as a function of the carrier density. At low density,
the angular dependence exhibits a downturn in the oscillations frequency
between and , reflecting a bag-shaped hole Fermi surface.
The detection of a single frequency for all tilt angles rules out the existence
of a Fermi surface with different extremal cross-sections down to ~meV.
There is therefore no signature of a camel-back in the valence band of our bulk
samples, in accordance with the direct band gap predicted by calculations.Comment: A supplemental material file giving a more detailed description of
our work is available upon reques
Magneto-transport in high g-factor, low-density two-dimensional electron systems confined in In_0.75Ga_0.25As/In_0.75Al_0.25As quantum wells
We report magneto-transport measurements on high-mobility two-dimensional
electron systems (2DESs) confined in In_0.75Ga_0.25As/In_0.75Al_0.25As single
quantum wells. Several quantum Hall states are observed in a wide range of
temperatures and electron densities, the latter controlled by a gate voltage
down to values of 1.10^11 cm^-2. A tilted-field configuration is used to induce
Landau level crossings and magnetic transitions between quantum Hall states
with different spin polarizations. A large filling factor dependent effective
electronic g-factor is determined by the coincidence method and cyclotron
resonance measurements. From these measurements the change in
exchange-correlation energy at the magnetic transition is deduced. These
results demonstrate the impact of many-body effects in tilted-field
magneto-transport of high-mobility 2DESs confined in
In_0.75Ga_0.25As/In_0.75Al_0.25As quantum wells. The large tunability of
electron density and effective g-factor, in addition, make this material system
a promising candidate for the observation of a large variety of spin-related
phenomena.Comment: 7 pages, 5 figure
Système de supervision d'une centrale photovoltaïque pour les particuliers
International audienceNous présentons un système de supervision dédié aux centrales photovoltaïques des particuliers reliées au réseau d'énergie. Le module est piloté par un microcontrôleur, qui lit les informations relatives à la production d'électricité à partir du compteur et de l'onduleur de production. Ces données sont ensuite enregistrées automatiquement, à intervalles réguliers et de manière pérenne, sur une carte SD intégrée au module. Cet enregistreur a deux fonctions principales : informer et alerter. En effet, les données enregistrées sont consultables à tout moment par internet sous forme de graphes sur des pages web, pour connaître les productions journalières, mensuelles ou annuelles. Aussi, ce système autonome peut alerter, via l'envoi d'un SMS (Short Message Service) ou d'un email, en cas de problème détecté par l’onduleur de production. Ce projet propose une interface simple d'utilisation, pratique et évolutive pour la supervision d'une centrale photovoltaïque
Growth of monolayer graphene on 8deg off-axis 4H-SiC (000-1) substrates with application to quantum transport devices
Using high temperature annealing conditions with a graphite cap covering the
C-face of an 8deg off-axis 4H-SiC sample, large and homogeneous single
epitaxial graphene layers have been grown. Raman spectroscopy shows evidence of
the almost free-standing character of these monolayer graphene sheets, which
was confirmed by magneto-transport measurements. We find a moderate p-type
doping, high carrier mobility and half integer Quantum Hall effect typical of
high quality graphene samples. This opens the way to a fully compatible
integration of graphene with SiC devices on the wafers that constitute the
standard in today's SiC industry.Comment: 11 pages, 4 figures , Submitted in AP
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