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Bilaplacian problems with a sign-changing coefficient
No full textInternational audienceWe investigate the properties of the operator ∆(σ∆·) : H^2_0 (Ω) → H^{−2}(Ω), where σ is a given parameter whose sign can change on the bounded domain Ω. Here, H^2_0 (Ω) denotes the subspace of H^2(Ω) made of the functions v such that v = ν · ∇v = 0 on ∂Ω. The study of this problem arises when one is interested in some configurations of the Interior Transmission Eigenvalue Problem. We prove that ∆(σ∆·) : H^2_0 (Ω) → H^{−2}(Ω) is a Fredholm operator of index zero as soon as σ ∈ L^∞(Ω), with σ^{−1}∈ L^∞(Ω), is such that σ remains uniformly positive (or uniformly negative) in a neighbourhood of ∂Ω. We also study configurations where σ changes sign on ∂Ω and we prove that Fredholm property can be lost for such situations. In the process, we examine in details the features of a simpler problem where the boundary condition ν · ∇v = 0 is replaced by σ∆v = 0 on ∂Ω
The dual beam spectropluviometer
No full textInternational audienceThe Dual Beam spectropluviometer (DBS) measures the equivalent diameter, the fall velocity and the time of arrival of raindrop
Tropical bounds for eigenvalues of matrices using Hungarian dual variables
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Novel Route to Triethylsilyl-Substituted Cyclopropanes
No full textInternational audienceRadical adducts of S-α-ketonyl dithiocarbonates (xanthates) to triethyl vinylsilane are converted into triethylsilyl-substituted cyclopropanes upon ozonolysis followed by exposure to 1,4-dibromobutane and Cs2CO3/18-crown-6 in refluxing acetonitrile. The sequence works best with phenacyl type adducts, and the use of the less bulky trimethysilyl (TMS) derivatives results in extensive desilylation
Using Optimal Matching Analysis in Sociology: Cost Setting and Sociology of Time
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Amélioration de la production photovoltaïque par l'emploi de miroirs plans
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Reductive Alkylation of Thioamides with Grignard Reagents in the Presence of Ti(OiPr)4: Insight and Extension
No full textInternational audienceThe reductive alkylation of thioamides by Grignard reagents in the presence of Ti(OiPr)4 is the subject of a study involving 20 different substrates. The influence of various parameters has been evaluated, showing notably that the yields of this moderately efficient process can be improved in several cases by applying a slow addition of the Grignard reagent. The results presented in this contribution also provide new insight into the reactivity of the proposed key intermediates, namely, a metalated iminium species and, ultimately, an α-metalated amine. Interestingly, by control of the temperature and the amount of Grignard reagent engaged, the reaction can be directed toward the selective formation of the former titanium intermediate complex. This represents an extension of the original method, allowing the synthesis of various previously inaccessible substituted amines by subsequent addition of a nucleophilic reagent. This role can be played not only by organomagnesium compounds but also by alkyllithium reagents, alkyltitanium(IV) complexes, and lithium aluminum hydride. The properties of the α-metalated amine final intermediate have also been explored, demonstrating that this complex is a poor nucleophile but can act as a radical precursor, which is especially evidenced when the resulting radical species are stabilized. Overall, this chemistry thus proves unexpectedly rich and can plausibly lay the basis for the development of new applications in the future
Latent heat and nonlinear vortex liquid in the vicinity of the first-order phase transitionin layered high-Tc superconductors
No full textInternational audienceIn this work we revisit the vortex matter phase diagram in layered superconductors solving still open questions by means of ac and dc local magnetic measurements in the paradigmatic Bi2Sr2CaCu2O8 compound. We show that measuring with ac magnetic techniques is mandatory in order to probe the bulk response of vortex matter, particularly at high temperatures where surface barriers for vortex entrance dominate. From the TFOT evolution of the enthalpy and latent heat at the transition we find that, contrary to previous reports, the nature of the dominant interlayer coupling is electromagnetic in the whole temperature range. By studying the dynamic properties of the phase located at T TFOT, we reveal spanning in a considerable fraction of the phase diagram of a nonlinear vortex phase suggesting that bulk pinning might play a role even in the liquid vortex phase
Evidence for anisotropic dielectric properties of monoclinic hafnia using high-resolution TEM valence electron energy-loss spectroscopy and ab initio time-dependent density-functional theory
No full textInternational audienceThe effect of nanocrystal orientation on the energy loss spectra of monoclinic hafnia (m-HfO2) is measured by high resolution transmission electron microscopy (HRTEM) and valence energy loss spectroscopy (VEELS) on high quality samples. For the same momentum-transfer directions, the dielectric properties are also calculated ab initio by time-dependent density-functional theory (TDDFT). Experiments and simulations evidence anisotropy in the dielectric properties of m-HfO2, most notably with the direction-dependent oscillator strength of the main bulk plasmon. The anisotropic nature of m-HfO2 may contribute to the differences among VEELS spectra reported in literature. The good agreement between the complex dielectric permittivity extracted from VEELS with nanometer spatial resolution, TDDFT modeling, and past literature demonstrates that the present HRTEM-VEELS device-oriented methodology is a possible solution to the difficult nanocharacterization challenges given in the International Technology Roadmap for Semiconductors