We have investigated the disorder of epitaxial graphene close to the charge neutrality point (CNP) by various methods: (i) at room temperature, by analyzing the dependence of the resistivity on the Hall coefficient; (ii) by fitting the temperature dependence of the Hall coefficient down to liquid helium temperature; (iii) by fitting themagnetoresistances at low temperature. All methods converge to give a disorder amplitude of (20±10) meV. Because of this relatively low disorder, close to the CNP, at low temperature, the sample resistivity does not exhibit the standard value~h/4e^2 but diverges. Moreover, themagnetoresistance curves have a unique ambipolar behavior,which has been systematically observed for all studied samples.This is a signature of both asymmetry in the density of states and in-plane charge transfer. The microscopic origin of this behavior cannot be unambiguously determined. However, we propose a model in which the SiC substrate steps qualitatively explain the ambipolar behavior.

We measure the spin-resolved transport of dipolar excitons in a biased GaAs double quantum well structure. From these measurements we extract both spin lifetime and mobility of the excitons. We find that below a temperature of 4.8K there is a sharp increase in the spin lifetime of the excitons, together with a sharp reduction in their mobility. Below a critical power the spin lifetime increases with increasing mobility and density, while above the critical power the opposite trend is observed. We interpret this transition as evidence of the interplay between two different spin dephasing mechanisms: at low mobility the dephasing is dominated by the hyperfine interaction with the lattice nuclei spins, while at higher mobility the spin-orbit interaction dominates and a Dyakonov-Perel spin relaxation takes over. The excitation power and temperature regime where the hyperfine interaction induced spin dephasing is observed correlates with the regime where a dark dipolar quantum liquid was reported recently on a similar sample.

La prégnance de la dégradation de l’environnement par l’homme doit nous amener à envisager de profonds changements de nos modes de vie, de mobilité, de production et de consommation pour aller vers une humanité plus harmonieuse avec l’environnement. La finalité de la technologie peut servir la résilience écologique.Ainsi, l’avènement d’applications permettant d’envisager un covoiturage dynamique apporterait deux effets bénéfiques à la réduction de la pollution : l’un, lié à une décroissance de la consommation de carburant via l’optimisation du remplissage des véhicules et, l’autre, lié à la diminution du nombre d’automobiles en ville grâce à leur partage.Nous présentons l’embryon d’une application de carsharing reposant sur un algorithme génétique hermaphrodite (actuellement accessible sur github) en espérant pouvoir fédérer des développeurs libres sur le projet

Graphene and AlN are promising materials, interesting to combine together. In this study, we will present first results for direct growth of graphene on bulk AlN and on AlN templates using chemical vapor deposition, including the annealing of these substrates at high temperatures. Atomic force microscopy (AFM) enabled us to study the evolution of the AlN surface morphology after annealing and growth. Few-layer graphene deposition is demonstrated on the basis of X-ray photoemission and Raman spectroscopy

The Hall hole density pH and Hall mobility µH in highly aluminum-doped 4H-SiC have been investi-gated in the wide temperature range between 100 K and 900 K. After an overview of the literature data related to the analysis of electrical transport experiments, a model taking into account the two upper-most valence bands of heavy and lights holes, one single acceptor energy EA and the empirical Hall factor rHexp has been discussed in details. The limits of the applied model as a function of temperature and acceptor concentration have been considered. For each analyzed sample, the acceptor density NA and the compensation ratio ND/NA have been experimentally assessed by capacitance-voltage and secondary ion mass spectroscopy measurements. Finally, the Hall carrier concentration vs doping level NA-ND as well as ionization energy of acceptor EA vs acceptor concentration NA in 4H-SiC have been discussed.

Silicon carbide (SiC) sublimation is the most promising option to achieve transfer-free graphene at the wafer-scale. We investigated the initial growth stages from the buffer layer to monolayer graphene on SiC(0001) as a function of annealing temperature at low argon pressure (10 mbar). A buffer layer, fully covering the SiC substrate, forms when the substrate is annealed at 1600°C. Graphene formation starts from the step edges of the SiC substrate at higher temperature (1700°C). The spatial homogeneity of the monolayer graphene was observed at 1750°C, as characterized by Raman spectroscopy and magneto-transport. Raman spectroscopy mapping indicated an AG-graphene/AG-HOPG ratio of around 3.3%, which is very close to the experimental value reported for a graphene monolayer. Transport measurements fromroom temperature down to 1.7 K indicated slightly p-doped samples (p~10^10cm-2) and confirmed both continuity and thickness of the monolayer graphene film. Successive growth processes have confirmed the reproducibility and homogeneity of these monolayer films.