Structure of velocity distributions in shock waves in granular gases with extension to molecular gases

International audienceVelocity distributions in normal shock waves obtained in dilute granular flows are studied. These distributions cannot be described by a simple functional shape and are believed to be bimodal. Our results show that these distributions are not strictly bimodal but a trimodal distribution is shown to be sufficient. The usual Mott-Smith bimodal description of these distributions, developed for molecular gases, and based on the coexistence of two subpopulations (a supersonic and a subsonic population) in the shock front, can be modified by adding a third subpopulation. Our experiments show that this additional population results from collisions between the supersonic and subsonic subpopulations. We propose a simple approach incorporating the role of this third intermediate population to model the measured probability distributions and apply it to granular shocks as well as shocks in molecular gases

Azimuthal instability of the radial thermocapillary flow around a hot bead trapped at the water-air interface

We investigate the radial thermocapillary flow driven by a laser-heated microbead in partial wetting at the water-air interface. Particular attention is paid to the evolution of the convective flow patterns surrounding the hot sphere as the latter is increasingly heated. The flow morphology is nearly axisymmetric at low laser power P. Increasing P leads to symmetry breaking with the onset of counter-rotating vortex pairs. The boundary condition at the interface, close to no-slip in the low-P regime, turns about stress-free between the vortex pairs in the high-P regime. These observations strongly support the view that surface-active impurities are inevitably adsorbed on the water surface where they form an elastic layer. The onset of vortex pairs is the signature of a hydrodynamic instability in the layer response to the centrifugal forced flow. Interestingly, our study paves the way for the design of active colloids able to achieve high-speed self-propulsion via vortex pair generation at a liquid interface

Time-resolved PhotoEmission Spectroscopy on a Metal/Ferroelectric Heterostructure

In thin film ferroelectric capacitor the chemical and electronic structure of the electrode/FE interface can play a crucial role in determining the kinetics of polarization switching. We investigate the electronic structure of a Pt/BaTiO3/SrTiO3:Nb capacitor using time-resolved photoemission spectroscopy. The chemical, electronic and depth sensitivity of core level photoemission is used to probe the transient response of different parts of the upper electrode/ferroelectric interface to voltage pulse induced polarization reversal. The linear response of the electronic structure agrees quantitatively with a simple RC circuit model. The non-linear response due to the polarization switch is demonstrated by the time-resolved response of the characteristic core levels of the electrode and the ferroelectric. Adjustment of the RC circuit model allows a first estimation of the Pt/BTO interface capacitance. The experiment shows the interface capacitance is at least 100 times higher than the bulk capacitance of the BTO film, in qualitative agreement with theoretical predictions from the literature.Comment: 7 pages, 10 figures. Submitted to Phys. Rev.

Interface Electronic Structure in a Metal/Ferroelectric Heterostructure under Applied Bias

The effective barrier height between an electrode and a ferroelectric (FE) depends on both macroscopic electrical properties and microscopic chemical and electronic structure. The behavior of a prototypical electrode/FE/electrode structure, Pt/BaTiO3/Nb-doped SrTiO3, under in-situ bias voltage is investigated using X-Ray Photoelectron Spectroscopy. The full band alignment is measured and is supported by transport measurements. Barrier heights depend on interface chemistry and on the FE polarization. A differential response of the core levels to applied bias as a function of the polarization state is observed, consistent with Callen charge variations near the interface.Comment: 9 pages, 8 figures. Submitted to Phys. Rev.

Evidence for the formation of two phases during the growth of SrTiO3 on silicon

International audienceEpitaxial SrTiO3 (STO)/Si templates open a unique opportunity for the integration of ferroelectric oxides, such as BaTiO3 on silicon and for the realization of new devices exploiting ferroelectricity. STO itself has been shown as ferroelectric at room temperature when deposited in thin layers on Si, while bulk STO is tetragonal and, thus, ferroelectric below 105 K. Here, we demonstrate the coexistence, at room temperature, of strained cubic and tetragonal phases in thin STO/Si layers. The tetragonal STO phase presents a pronounced tetragonality for thicknesses up to 24 ML. Above this thickness, the strained cubic STO phase starts relaxing while the tetragonal STO phase progressively transits to cubic STO. The origin of the simultaneous formation of these two phases is analyzed and is attributed to oxygen segregation at the early stages of the growth

Chemistry and structure of BaTiO3 ultra-thin films grown by different O2 plasma power

International audienceWe present a study of the chemical and atomic properties of 5 nm TiO2-terminated BaTiO3 (001) epitaxialfilms on Nb-doped SrTiO3, as a function of the atomic oxygen plasma power for film growth. Lowerplasma power produces non-stoichiometric films with oxygen vacancies and Ti3+ ions. The larger Ti3+ion radius and the in-plane clamping gives rise to an increase in the out-of-plane lattice parameter.XPS measures the Ti3+ concentration and the concomitant increase in dissociative water uptake in thefilm, giving rise to on-top OH adsorption on surface Ti, proton adsorption on surface oxygen, and a nearsurface Ba-OH environment

In vivo stem cell tracking using scintigraphy in a canine model of DMD

One of the main challenges in cell therapy for muscle diseases is to efficiently target the muscle. To address this issue and achieve better understanding of in vivo cell fate, we evaluated the relevance of a non-invasive cell tracking method in the Golden Retriever Muscular Dystrophy (GRMD) model, a well-recognised model of Duchenne Muscular Dystrophy (DMD). Mesoangioblasts were directly labelled with 111In-oxine, and injected through one of the femoral arteries. The scintigraphy images obtained provided the first quantitative mapping of the immediate biodistribution of mesoangioblasts in a large animal model of DMD. The results revealed that cells were trapped by the first capillary filters: the injected limb and the lung. During the days following injection, radioactivity was redistributed to the liver. In vitro studies, performed with the same cells prepared for injecting the animal, revealed prominent cell death and 111In release. In vivo, cell death resulted in 111In release into the vasculature that was taken up by the liver, resulting in a non-specific and non-cell-bound radioactive signal. Indirect labelling methods would be an attractive alternative to track cells on the mid- and long-term

A Review on the Efficiency of Graphene-Based BHJ Organic Solar Cells

Graphene, a material composed of one-atom-thick planar sheets of sp2-bonded carbon atoms with a two-dimensional honeycomb structure, has been proposed for many applications due to its remarkable electronic, optical, thermal, and mechanical properties. Its high transparency, conductivity, flexibility, and abundance make it an excellent material to be applied in the field of organic photovoltaic cells, especially as a replacement for transparent conducting oxide electrodes. However, graphene has been demonstrated to be useful not only as substitute for indium tin oxide electrodes, but also as cathode, electron acceptor, hole transport, and electron extraction material. Thus, in this work, we summarize and discuss the efficiency of bulk heterojunction devices using graphene as a main constituent.El grafeno, un material compuesto de hojas planas de un átomo de espesor de átomos de carbono unidos a sp2 con una estructura bidimensional en forma de panal, ha sido propuesto para muchas aplicaciones debido a sus notables propiedades electrónicas, ópticas, térmicas y mecánicas. Su alta transparencia, conductividad, flexibilidad y abundancia lo convierten en un excelente material para ser aplicado en el campo de las células fotovoltaicas orgánicas, especialmente como reemplazo de los electrodos conductores transparentes de óxido. Sin embargo, se ha demostrado que el grafeno es útil no sólo como sustituto de los electrodos de óxido de indio y estaño, sino también como catodo, aceptor de electrones, transporte de agujeros y material de extracción de electrones. Por lo tanto, en este trabajo, resumimos y discutimos la eficiencia de los dispositivos de heterojunción a granel utilizando grafeno como constituyente principal