Alpha effect in helically forced turbulence

The effect on large-scale dynamics of small-scale helicity injection in three-dimensional resistive magnetohydrodynamic turbulence is investigated for a weak initial magnetic field. The magnetic configuration of α² dynamo prototype flows is such that, generally, the energy concentrates on one large-scale mode. However, we obtain that alpha effect is not limited to the dominant mode and that a non-local equation (in Fourier space) is more appropriate to describe it. It gives some insights into the non-local theory of Pouquet et al. [J. Fluid Mech. 77 (1976) 321] where the inverse cascade results from a competition between the helicity and Alfvén effects.     

Self-interacting diffusions

 This paper is concerned with a general class of self-interacting diffusions {X _t } _{t ≥0} living on a compact Riemannian manifold M. These are solutions to stochastic differential equations of the form : dX _t = Brownian increments + drift term depending on X _t and μ _t , the normalized occupation measure of the process. It is proved that the asymptotic behavior of {μ _t } can be precisely related to the asymptotic behavior of a deterministic dynamical semi-flow Φ = {Φ _t } _{t ≥0} defined on the space of the Borel probability measures on M. In particular, the limit sets of {μ _t } are proved to be almost surely attractor free sets for Φ. These results are applied to several examples of self-attracting/repelling diffusions on the n-sphere. For instance, in the case of self-attracting diffusions, our results apply to prove that {μ _t } can either converge toward the normalized Riemannian measure, or to a gaussian measure, depending on the value of a parameter measuring the strength of the attraction. Received: 21 July 2000 / Revised version: 12 December 2000 / Published online: 15 October 2001     

A mathematical model for the transient evolution of a resonant tunneling diodeModélisation d'une diode à effet tunnel résonant en régime transitoire

A mathematical model of quantum transient transport is derived and analyzed. The model describes the evolution of electrons injected into the “device” by reservoirs having a stationary statistics. The electrostatic potential in the device is modified by electron presence through electrostatic interaction. The wave functions are computed in the device region and satisfy non homogeneous open boundary conditions at the device edges. A priori estimates are deduced from the “dissipative properies” of the boundary conditions and from the repulsive character of the electrostatic interaction. To cite this article: N. Ben Abdallah, O. Pinaud, C. R. Acad. Sci. Paris, Ser. I 334 (2002) 283–288.     

Activation of oxaziridines by lewis acids: application in enantioselective sulfoxidation

The preparation of new and easily accessible chiral N-alkyloxaziridines for asymmetric sulfides oxidation was investigated. Most significantly, we report here that inert N-alkyloxaziridines can be activated by Lewis acids to achieve a fast, selective, and efficient oxygen atom transfer to sulfides. Asymmetric sulfoxidation by three new chiral oxaziridines (two of them were structurally characterized by X-ray analysis) afforded enantioselectivities ranging from 22% to 63% ee with the simplest aryl alkyl sulfides.     

Solid-phase synthesis of "mixed" peptidomimetics using Fmoc-protected aza-beta3-amino acids and alpha-amino acids

[structure: see text] A solid-phase fluorenylmethyloxycarbonyl (Fmoc)-based synthesis strategy is described for "mixed" aza-beta3-peptides as well as a convenient general approach for their required building blocks, the aza-beta3-amino acid residues (aza-beta3-aa). These monomers allow the synthesis of relatively large quantities of pure mixed aza-beta3-peptides. The required Fmoc-substituted aza-beta3-amino acids are accessible by convenient synthesis, and a number of monomers including those containing side chains with functional groups have been synthesized. The method was applied toward the solid-phase synthesis of aza-beta3-peptide mimetics of a biologically active histone H4 sequence.     

Synthesis, characterization of dihydrolipoic acid capped gold nanoparticles, and functionalization by the electroluminescent luminol

The use of gold nanoparticles as biological probes requires the improvement of colloidal stability. Dihydrolipoic acid (DHLA), a dithiol obtained by the reduction of thioctic acid, appears therefore very attractive for the stabilization and the further functionalization of gold nanoparticles because DHLA is characterized by a carboxylic acid group and two thiol functions. The ionizable carboxylic acid groups ensure, for pH > or = 8, the water solubility of DHLA-capped gold (Au@DHLA) nanoparticles, prepared by the Brust protocol, and the stability of the resulting colloid by electrostatic repulsions. Moreover almost all DHLA, adsorbed onto gold, adopts a conformation allowing their immobilization by both sulfur ends. It is proved by sulfur K-edge X-ray absorption near edge structure spectroscopy, which appears as an appropriate tool for determining the chemical form of sulfur atoms present in the organic monolayer. Such a grafting renders the DHLA monolayers more resistant to displacement by dithiothreitol than mercaptoundecanoic acid monolayers. The presence of DHLA on gold particles allows their functionalization by the electroluminescent luminol through amine coupling reactions assisted by 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride and N-hydroxysuccinimide. As a luminol-functionalized particle is nine times as bright as a single luminol molecule, the use of the particles as a biological probe with a lower threshold of detection is envisaged.     

NO2 disproportionation for the IR characterisation of basic zeolites

NO2 disproportionation on alkaline zeolites is used to generate nitrosonium (NO+) and nitrate ions on the surface, and the infrared vibrations observed are very sensitive to the cation chemical hardness and to the basicity of zeolitic oxygen atoms.     

Sub-shot-noise heterodyne polarimetry

We report an experimental demonstration of a heterodyne polarization rotation measurement with a noise floor 4.8 dB below the optical shot noise by use of classically phase-locked quantum twin beams emitted above threshold by an ultrastable type II Na:KTP cw optical parametric oscillator. We believe that this is the largest noise reduction achieved to date in optical phase-difference measurements.     

Open- and closed-loop aberration correction by use of a quadrature interferometric wave-front sensor

Experimental results are presented for an adaptive optics system based on a quadrature Twyman-Green interferometric wave-front sensor. The system uses a circularly polarized reference beam to form two interferograms with a pi/2 phase shift. The experiments conducted used Kolmogorov phase screens to simulate atmospheric phase distortions. Strehl ratio improvements by a factor of 8, to an absolute value of 0.45, are demonstrated.