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Search Results: 1 - 10 of 297698 matches for " J. Mallet "
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Quasiparticle scattering off phase boundaries in epitaxial graphene
A. Mahmood,P. Mallet,J. -Y. Veuillen
Physics , 2012, DOI: 10.1088/0957-4484/23/5/055706
Abstract: We investigate the electronic structure of terraces of single layer graphene (SLG) by scanning tunneling microscopy (STM) on samples grown by thermal decomposition of 6H-SiC(0001) crystals in ultra-high vacuum. We focus on the perturbations of the local density of states (LDOS) in the vicinity of edges of SLG terraces. Armchair edges are found to favour intervalley quasiparticle scattering, leading to the (\surd3\times\surd3)R30{\deg} LDOS superstructure already reported for graphite edges and more recently for SLG on SiC(0001). Using Fourier transform of LDOS images, we demonstrate that the intrinsic doping of SLG is responsible for a LDOS pattern at the Fermi energy which is more complex than for neutral graphene or graphite, since it combines local (\surd3\times\surd3)R30{\deg} superstructure and long range beating modulation. Although these features were already reported by Yang et al. Nanoletters 10, 943 (2010), we propose here an alternative interpretation based on simple arguments classically used to describe standing wave patterns in standard two-dimensional systems. Finally, we discuss the absence of intervalley scattering off other typical boundaries: zig-zag edges and SLG/bilayer graphene junctions.
Universality of Crystallographic Pinning
A. Hoffman,J. Mallet-Paret
Mathematics , 2008,
Abstract: We study traveling waves for reaction diffusion equations on the spatially discrete domain $\Z^2$. The phenomenon of crystallographic pinning occurs when traveling waves become pinned in certain directions despite moving with non-zero wave speed in nearby directions. Mallet-Paret has shown that crystallographic pinning occurs for all rational directions, so long as the nonlinearity is close to the sawtooth. In this paper we show that crystallographic pinning holds in the horizontal and vertical directions for bistable nonlinearities which satisfy a specific computable generic condition. The proof is based on dynamical systems. In particular, it relies on an examination of the heteroclinic chains which occur as singular limits of wave profiles on the boundary of the pinning region.
Composición en ácidos grasos de los aceites de semillas de especies mediterráneas
Ucciani, E.,Mallet, G.,Gamisans, J.,Gruber, M.
Grasas y Aceites , 1994,
Abstract: In the context of research into the lipids of higher plant, the fatty acid composition of eleven species is reported. These species belong to nine families, and most of them are common in the Mediterranean flora. Oils from some plants (Arctium tomentosum, Hypericum perforatum, Linaria vulgaris. Pulicaria dysenterica, Smilax áspera) were partially hydrolyzed, and contained free fatty acids. Arctium tomentosum seed oil and, to a lesser extent, Smilax áspera seed oil, showed a trans absorption band in their IR spectra. Small quantities of waxes were present in Pulicaria dysenterica and Silybum marianum seed oil. The fatty acid composition of the oils from these plants approaches levels found in food oils as well as industrial oils. Three species showed high levels of their predominant fatty acid: Alliaria petiolata (erucic acid, 41.5%), Linaria vulgaris and Pulicaria dysenterica (linoleic acid, 70 and 72% respectively). In the seed oil of Arctium tomentosum four fatty acids were not identified: one of them could be 3(t), 9(c), 12(c)- octadecatrienoic acid. But the most complex composition was that of Atriplex hortensis oil, which contained thirty three fatty acids, of which fifteen were unknown. En el marco de investigaciones sobre los lípidos de las plantas superiores, se presenta la composición en ácidos grasos de once especies. Estas especies pertenecen a nueve familias diferentes y la mayoría son frecuentes en la flora mediterránea. Numerosos aceites (Arctium tomentosum, Hypericum perforatum,Linaria vulgaris, Pulicaria dysenterica y Smilax áspera) contienen glicéridos parciales y ácidos grasos libres. El aceite de Arctium tomentosum, y en menor grado, el de Smilax áspera muestran una banda espectral trans en espectrometría IR. En los aceites de Pulicaria dysenterica y de Silybum marianum peque as cantidades de ceras han sido puestas de manifiesto. Las composiciones en ácidos grasos se aproximan a las de los aceites alimenticios e industriales conocidos. Tres especies contienen un ácido graso útil a un nivel elevado: Alliaria petiolata (41,5% de ácido erúcico). Linaria vulgaris y Pulicaria dysenterica (respectivamente 70 y 72% de ácido linoléico). En el aceite de Arctium tomentosum cuatro ácidos grasos menores no han sido identificados: uno de ellos podría ser el octadeca-3(t), 9(c), 12(c) trienóico. Pero se alcanza el máximo de complejidad con el aceite de Atriplex hortensis con treinta y tres ácidos grasos de los cuales quince son desconocidos.
Terrain surfaces and 3-D landcover classification from small footprint full-waveform lidar data: application to badlands
F. Bretar,A. Chauve,J.-S. Bailly,C. Mallet
Hydrology and Earth System Sciences Discussions , 2009,
Abstract: This article presents the use of new remote sensing data acquired from airborne full-waveform lidar systems. They are active sensors which record altimeter profiles. This paper introduces a set of methodologies for processing these data. These techniques are then applied to a particular landscape, the badlands, but the methodologies are designed to be applied to any other landscape. Indeed, the knowledge of an accurate topography and a landcover classification is a prior knowledge for any hydrological and erosion model. Badlands tend to be the most significant areas of erosion in the world with the highest erosion rate values. Monitoring and predicting erosion within badland mountainous catchments is highly strategic due to the arising downstream consequences and the need for natural hazard mitigation engineering. Additionaly, beyond the altimeter information, full-waveform lidar data are processed to extract intensity and width of echoes. They are related to the target reflectance and geometry. Wa will investigate the relevancy of using lidar-derived Digital Terrain Models (DTMs) and to investigate the potentiality of the intensity and width information for 3-D landcover classification. Considering the novelty and the complexity of such data, they are presented in details as well as guidelines to process them. DTMs are then validated with field measurements. The morphological validation of DTMs is then performed via the computation of hydrological indexes and photo-interpretation. Finally, a 3-D landcover classification is performed using a Support Vector Machine classifier. The introduction of an ortho-rectified optical image in the classification process as well as full-waveform lidar data for hydrological purposes is then discussed.
Dust emission size distribution impact on aerosol budget and radiative forcing over the Mediterranean region: a regional climate model approach
P. Nabat,F. Solmon,M. Mallet,J. F. Kok
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2012, DOI: 10.5194/acp-12-10545-2012
Abstract: The present study investigates the dust emission and load over the Mediterranean basin using the coupled chemistry–aerosol–regional climate model RegCM-4. The first step of this work focuses on dust particle emission size distribution modeling. We compare a parameterization in which the emission is based on the individual kinetic energy of the aggregates striking the surface to a recent parameterization based on an analogy with the fragmentation of brittle materials. The main difference between the two dust schemes concerns the mass proportion of fine aerosol that is reduced in the case of the new dust parameterization, with consequences for optical properties. At the episodic scale, comparisons between RegCM-4 simulations, satellite and ground-based data show a clear improvement using the new dust distribution in terms of aerosol optical depth (AOD) values and geographic gradients. These results are confirmed at the seasonal scale for the investigated year 2008. This change of dust distribution has sensitive impacts on the simulated regional dust budget, notably dry dust deposition and the regional direct aerosol radiative forcing over the Mediterranean basin. In particular, we find that the new size distribution produces a higher dust deposition flux, and smaller top of atmosphere (TOA) dust radiative cooling. A multi-annual simulation is finally carried out using the new dust distribution over the period 2000–2009. The average SW radiative forcing over the Mediterranean Sea reaches 13.6 W m 2 at the surface, and 5.5 W m 2 at TOA. The LW radiative forcing is positive over the basin: 1.7 W m 2 on average over the Mediterranean Sea at the surface, and 0.6 W m 2 at TOA.
Atomic and electronic structure of monolayer graphene on 6H-SiC(000-1)(3 x 3) : a scanning tunneling microscopy study
F. Hiebel,P. Mallet,L. Magaud,J. -Y. Veuillen
Physics , 2009, DOI: 10.1103/PhysRevB.80.235429
Abstract: We present an investigation of the atomic and electronic structure of graphene monolayer islands on the 6H-SiC(000-1)(3x3) (SiC(3x3)) surface reconstruction using scanning tunneling microscopy (STM) and spectroscopy (STS). The orientation of the graphene lattice changes from one island to the other. In the STM images, this rotational disorder gives rise to various superlattices with periods in the nm range. We show that those superlattices are moir\'e patterns (MPs) and we correlate their apparent height with the stacking at the graphene/SiC(3x3) interface. The contrast of the MP in STM images corresponds to a small topographic modulation of the graphene layer. From STS measurements we find that the substrate surface presents a 1,5 eV wide bandgap encompassing the Fermi level. This substrate surface bandgap subsists below the graphene plane. The tunneling spectra are spatially homogeneous on the islands within the substrate surface gap, which shows that the MPs do not impact the low energy electronic structure of graphene. We conclude that the SiC(3 x 3) reconstruction efficiently passivates the substrate surface and that the properties of the graphene layer which grows on top of it should be similar to those of the ideal material.
Ripples in epitaxial graphene
F. Varchon,P. Mallet,J. -Y. Veuillen,L. Magaud
Physics , 2007,
Abstract: Graphene outstanding properties directly come from its pecular electronic structure and thus from the honeycomb lattice symmetry. The way interaction with the substrate impact this lattice is of primary importance. This is peculiarly true for epitaxial graphene because of the SiC substrate. The advantage of this system that produces macroscopic samples in registry with a substrate could turn to a major drawback if the graphene lattice reveals to be strongly distorted. Extensive ab initio calculations supported by Scanning Tunneling Microscopy experiments, demonstrate here that the substrate indeed induces a strong nanostructuration of the interface carbon layer. It propagates to the above C layer where it generates incommensurate ripples in the honeycomb lattice.
Graphene on the carbon face of SiC: electronic structure modification by hydrogen intercalation
F. Hiebel,P. Mallet,J. -Y. Veuillen,L. Magaud
Physics , 2011, DOI: 10.1103/PhysRevB.83.075438
Abstract: It has been shown that the first C layer on the SiC(0001)(2{\times}2)C surface already exhibits graphene-like electronic structure, with linear pi bands near the Dirac point. Indeed, the (2{\times}2)C reconstruction, with a Si adatom and C restatom structure, efficiently passivates the SiC(0001) surface thanks to an adatom/restatom charge transfer mechanism. Here, we study the effects of interface modifications on the graphene layer using density functional theory calculations. The modifications we consider are inspired from native interface defects observed by scanning tunneling microscopy. One H atom per 4 {\times} 4 SiC cell (5 {\times} 5 graphene cell) is introduced in order to saturate a restatom dangling bond and hinder the adatom/restatom charge transfer. As a consequence, the graphene layer is doped with electrons from the substrate and the interaction with the adatom states slightly increases. Native interface defects are therefore likely to play an important role in the doping mechanism on the C terminated SiC substrates. We also conclude that an efficient passivation of the C face of SiC by H requires a complete removal of the reconstruction. Otherwise, at variance with the Si terminated SiC substrates, the presence of H at the interface would increase the graphene/substrate interaction.
Note on a 'hint' for an annual modulation signature of a 60 GeV WIMP
G. Gerbier,J. Mallet,L. Mosca,C. Tao
Physics , 1997,
Abstract: In a recent presentation at the TAUP97 conference about the analysis of data from a NaI(Tl) 100 kg underground detector run at the Gran Sasso Laboratory by the DAMA/NaI group, the speaker concluded there was a hint for a WIMP of mass 60 GeV with a cross section on proton of 10-5 pb (Spin Independent coupling) (see also ROM2F/97/33). Even if no claim of any definite signal is made, such a statement is strong enough to deserve a critical look. The present note puts forward a few arguments which point towards a largely overestimated statistical significance of the effect, an inconsistency in the energy distribution and several experimental effects which could easily mimic the observed excess. The conclusion is that, in no way, the presented result can be considered as a hint of a WIMP annual modulation.
A compact design for the Josephson mixer: the lumped element circuit
J. -D. Pillet,E. Flurin,F. Mallet,B. Huard
Physics , 2015, DOI: 10.1063/1.4922188
Abstract: We present a compact and efficient design in terms of gain, bandwidth and dynamical range for the Josephson mixer, the superconducting circuit performing three-wave mixing at microwave frequencies. In an all lumped-element based circuit with galvanically coupled ports, we demonstrate non degenerate amplification for microwave signals over a bandwidth up to 50 MHz for a power gain of 20 dB. The quantum efficiency of the mixer is shown to be about 70$\%$ and its saturation power reaches $-112$ dBm.
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