Nonmagnetic insulator state in Na1CoO2 and phase separation of na vacancies

Crystallographic, magnetic, and NMR properties of a Na1CoO2 single crystal with x approximately = 1 are presented. We identify the stoichiometric Na1CoO2 phase, which is shown to be a nonmagnetic insulator, as expected for homogeneous planes of Co3+ ions with S = 0. In addition, we present evidence that, because of slight average Na deficiency, chemical and electronic phase separation leads to a segregation of Na vacancies into the well-defined, magnetic, Na0.8CoO2 phase. The importance of phase separation is discussed in the context of magnetic order for x approximately = 0.8 and the occurrence of a metal-insulator transition for x --> 1.     

Shear localization in a model glass

Using molecular dynamics simulations, we show that a simple model of a glassy material exhibits the shear localization phenomenon observed in many complex fluids. At low shear rates, the system separates into a fluidized shear band and an unsheared part. The two bands are characterized by a very different dynamics probed by a local intermediate scattering function. Furthermore, a stick-slip motion is observed at very small shear rates. Our results, which open the possibility of exploring complex rheological behavior using simulations, are compared to recent experiments on various soft glasses.     

Characterization of Polypropylene Surface Treated in a CO2 Plasma

The polypropylene modification in CO₂ plasma mainly contributes to degradation, functionalization, and cross-linking. The degradation, whose rate is depending on CO₂ dissociation and oxygen atom formation, is a quite slow reaction and it is associated with surface topography alteration, especially of the amorphous phase of the polypropylene. The surface roughness increases with the treatment duration and the amorphous phase is more degraded than the crystallized part. The functionalization, corresponding to an increase of the surface energy (57.3 mJ m ^{– 2} in 30 s), and to an oxidation (23 oxygen at.%) with the appearance of alcohol, ketone, and acid functions is a much faster phenomenon. Cross-linking takes also place during this type of treatment and will reinforce the stability of the modified surface.     

Multiscaled polarization effects in <Emphasis Type="Italic">Suneve coronata</Emphasis> (Lepidoptera) and other insects: application to anti-counterfeiting of banknotes

The scales of many Lepidoptera and the elytra of quite a number of Coleoptera possess specialized micro- and nano-structures that produce special polarization effects. They are constituted by concave multilayered cavities. This leads to two different effects: (1) interferential non-polarized coloration by reflection near normal incidence in the middle of the cavities and (2) polarized interferential colouration at lower wavelength after double reflection near the Brewster incidence at the periphery of the cavities. The macroscopic appearance resembles the “pointillist effect” with one of the component polarized while the other one is not. The first one can be extinguished with linear polarizer so that the colour is modified. In most insects, the structure is locally symmetric; hence, no macroscopic effects can be seen. In certain species, this symmetry is partly broken, and a slight effect can be observed. In the wing dorsal surface of the fascinating neotropical butterflies genus Suneve, perpendicular structures of two different kinds in size polarize the reflected light. The larger one is constituted by the convex cover scales whose apex falls perpendicularly on the bases of the following scales, creating long polarized valley (50 μm width) transversally running across the wing. The smaller one is constituted by the ridges of the scales (2 μm apart) that polarize light in the perpendicular direction.Adapted multilayered structures can be deposited onto banknotes to create anti-counterfeiting patterns as a further development of protection and security. Different effects can be produced by the use of such structures. (1) Changes of luminosity: A specific pattern will be constituted by two different areas: one with horizontal concave multilayered structures, and the other one with vertical structures. Under unpolarized light, the reflected spectra of these different areas are identical and no pattern appears. Under polarized light, i.e., through a linear polarizer, one or the other area is partly extinguished, according to the orientation of the polarizer, and the pattern appears. (2) Changes in colours: a compound colour can be produced by the juxtaposition of planed and grooved surfaces. Due to the change of incidence of the incoming light, the colours of these two zones are different, but that produced by the first one is unpolarized while the second one is partly polarized and can be modified with a linear polarizer. PACS 42.25.Ja; 42.25.Hz; 42.66.Ne; 78.40.Me     

Convenient access to C-2 or C-5 substituted 4-azaindole derivatives

Functionalization at C-2 and C-5 of N-benzenesulfonyl-4-azaindole 1 was performed by lithiation reactions and original palladium-catalyzed chemistry. It led to very useful new substituted 4-azaindole derivatives in fair to high yields.     

Electronic peculiarities of the excited states of [RuN5C]+ vs. [RuN6]2+ polypyridine complexes: insight from theory

The ground state, oxidized ground state, (3)MLCT and (3)MC excited states have been studied by DFT and TDDFT for two Ru(II) complexes bearing an N(6) or N(5)C coordination sphere. The effect of replacing one Ru-N dative bond by one Ru-C covalent bond have been studied and quantified on their ground state by the means of geometry optimization, NBO analysis and calculation of their IR vibrations. IR fingerprints of the Ru-C bond have been found at 945 and 1113 cm(-1). In addition, this study confirmed and quantified the effects of N→C(-) substitution on the spectroscopic properties of the [RuN(5)C](+) complex: a broader and bathochromically-shifted absorption spectrum, a smaller ground-(3)MLCT energy gap and a highly energetic (3)MC state are the major characteristics of the carbon-containing monocationic complex.     

Highly nonlinear dynamics in a slowly sedimenting colloidal gel

We use a combination of original light scattering techniques and particles with unique optical properties to investigate the behavior of suspensions of attractive colloids under gravitational stress, following over time the concentration profile, the velocity profile, and the microscopic dynamics. During the compression regime, the sedimentation velocity grows nearly linearly with height, implying that the gel settling may be fully described by a (time-dependent) strain rate. We find that the microscopic dynamics exhibit remarkable scaling properties when time is normalized by the strain rate, showing that the gel microscopic restructuring is dominated by its macroscopic deformation.     

Suppressed compressibility at large scale in jammed packings of size-disperse spheres

We analyze the large-scale structure and fluctuations of jammed packings of size-disperse spheres, produced in a granular experiment as well as numerically. While the structure factor of the packings reveals no unusual behavior for small wave vectors, the compressibility displays an anomalous linear dependence at low wave vectors and vanishes when q→0. We show that such behavior occurs because jammed packings of size-disperse spheres have no bulk fluctuations of the volume fraction and are thus hyperuniform, a property not observed experimentally before. Our results apply to arbitrary particle size distributions. For continuous distributions, we derive a perturbative expression for the compressibility that is accurate for polydispersity up to about 30%.