Laboratoire Structure et Dynamique par Résonance Magnétique

After a general introduction, two sections are devoted to a survey of the theories of spin temperature and of nuclear relaxation in solids. This is followed by a short section on the so-called well-resolved solid effect, and a much more detailed one on DNP by thermal mixing in the non-linear low spin-temperature domain. One then analyses the various methods of measurement of the nuclear polarisation, as well as an indirect method of detecting the electronic resonance based on the existence of large nuclear polarisations. The final section describes briefly several applications of dynamic nuclear polarisation.

We have fabricated an array of ultrasmall tunnel junctions which acts like a turnstile for single electrons. When alternating voltage of frequency f is applied to a gate, one electron is transferred per cycle through the device. This results in a current plateau in the current-voltage characteristic at I=ef. The overall behavior of the device is well explained by the theory of Coulomb blockade of electron tunneling. We discuss the accuracy limitations of this device.

Résumé. 2014 Nous analysons le comportement de solutions en bon solvant de chaines macro-moléculaires et confinées dans des tubes (ou des lamelles) de diamètre D comparable au rayon de la pelote. Les interactions répulsives entre monomères sont prises en compte grâce à une méthode d’échelle qui va au-delà de la théorie usuelle de Flory-Huggins. Pour le problème des lamelles, nous trouvons cinq régimes différents (dépendant de la concen-tration C et du diamètre D) avec des changements de comportement réguliers à toutes les frontières. Dans le problème des tubes un de ces régimes disparait et deux lignes de cross-over se confondent, donnant lieu à une discontinuité plus marquée. Ceci est lié au fait que lorsque D devient de l’ordre de la taille du monomère, les différentes pelotes ne peuvent plus s’interpénétrer. Les propriétés thermodynamiques, les corrélations locales et la taille de la chaine sont estimées dans chaque cas. Toutefois les arguments d’échelle prédisent seulement les lois de puissance (en C et D) pour toutes ces quantités mais ne donnent pas les coefficients numériques. Abstract. 2014 We analyse the theoretical behaviour of macromolecular chains dissolved in a good solvent, and confined into tubes (or slits) of diameter D comparable to the coil radius. The repulsive interactions between monomers are taken into account by a scaling method which goes beyond the

The current-voltage characteristic of an ultrasmall tunnel junction is calculated for arbitrary frequency dependence of the impedance presented to the junction by its electromagnetic environment. It is shown that the Coulomb blockade of tunneling is washed out by quantum fluctuations of the charge on the junction capacitor except for ultrahigh impedance environments. Two simple cases where the environment can be treated as an inductor or resistor are examined in detail. Effects of finite temperatures are discussed.

The transition to turbulence via spatiotemporal intermittency observed in a partial differential equation displays statistical features typical of critical phenomena. Analogy with directed percolation is drawn.

Calcium silicate hydrate (C-S-H) is the main constituent of hydrated cement paste and determines its cohesive properties. Because of the environmental impact of cement industry, it is more and more common to replace a part of the clinker in cement by secondary cementitious materials (SCMs). These SCMs are generally alumina-rich and as a consequence some aluminum is incorporated into the C-S-H. This may have consequences on the cohesion and durability of the material, and it is thus of importance to know the amount and the location of Al in C-S-H and what the parameters are that control these features. The present paper reports the (29)Si and (27)Al MAS NMR analyses of well-characterized C-A-S-H samples (C-S-H containing Al). These samples were synthesized using an original procedure that successfully leads to pure C-A-S-H of controlled compositions in equilibrium with well-characterized solutions. The (27)Al MAS NMR spectra were quantitatively interpreted assuming a tobermorite-like structure for C-A-S-H to determine the aluminum location in this structure. For this purpose, an in-house written software was used which allows decomposing several spectra simultaneously using the same constrained spectral parameters for each resonance but with variable intensities. The hypothesis on the aluminum location in the C-A-S-H structure determines the proportion of each silicon site. Therefore, from the (27)Al NMR quantitative results and the chemical composition of each sample, the intensity of each resonance line in the (29)Si spectra was set. The agreement between the experimental and calculated (29)Si MAS NMR spectra corroborates the assumed C-A-S-H structure and the proposed Al incorporation mechanism. The consistency between the results obtained for all compositions provides another means to assess the assumptions on the C-A-S-H structure. It is found that Al substitutes Si mainly in bridging positions and moderately in pairing positions in some conditions. Al in pairing site is observed only for Ca/(Si+Al) ratios greater than 0.95 (equivalent to 4 mmol.L(-1) of calcium hydroxide). Finally, the results suggest that penta and hexa-coordinated aluminum are adsorbed on the sides of the C-A-S-H particles.

BACKGROUND: TiO2 particles are commonly used as dietary supplements and may contain up to 36% of nano-sized particles (TiO2-NPs). Still impact and translocation of NPs through the gut epithelium is poorly documented. RESULTS: We show that, in vivo and ex vivo, agglomerates of TiO2-NPs cross both the regular ileum epithelium and the follicle-associated epithelium (FAE) and alter the paracellular permeability of the ileum and colon epithelia. In vitro, they accumulate in M-cells and mucus-secreting cells, much less in enterocytes. They do not cause overt cytotoxicity or apoptosis. They translocate through a model of FAE only, but induce tight junctions remodeling in the regular ileum epithelium, which is a sign of integrity alteration and suggests paracellular passage of NPs. Finally we prove that TiO2-NPs do not dissolve when sequestered up to 24 h in gut cells. CONCLUSIONS: Taken together these data prove that TiO2-NPs would possibly translocate through both the regular epithelium lining the ileum and through Peyer's patches, would induce epithelium impairment, and would persist in gut cells where they would possibly induce chronic damage.

The theoretical framework and the details of calculations of nonasymptotic critical behavior, above ${T}_{c}$, previously published for $n=1$, are presented and extended to the cases $n=2 \mathrm{and} 3$. The complete description of the real preasymptotic critical domain ${\mathcal{D}}_{\mathrm{preas}}$ needs only three adjustable parameters. We show that the ${\ensuremath{\varphi}}^{4}$ model at infinite cutoff is sufficient to obtain this description. The higher-transient and finite-cutoff effects in ${\mathcal{D}}_{\mathrm{preas}}$ affect only the adjustable parameters. A precise nonperturbative treatment of the ${\ensuremath{\varphi}}^{4}$ field theory exactly for $d=3$, in the spirit of the Parisi work, yields nonasymptotic functions of temperature which, including all the quantitative universal characteristics of the critical behavior in ${\mathcal{D}}_{\mathrm{preas}}$, are adapted to a suitable comparison with experiments. We emphasize that this work allows a determination, within the experimental accuracy, of the size of ${\mathcal{D}}_{\mathrm{preas}}$ and an appreciation of the effects of the higher corrections to scaling. We also give the estimates for $n=1, 2, \mathrm{and} 3$ of a new universal combination of amplitudes (${{R}_{B}}_{\mathrm{cr}}$) which concerns the specific heat alone.

We have shown that the collective modes of a bounded two-dimensional plasma in a magnetic field exhibit unexpected features which are dynamical manifestations of the Hall effect. In high fields we observed the appearance of a novel magnetically localized one-dimensional wave which propagates along the perimeter of the plasma.

The 2D quantum system of electrons at a GaAs/GaAlAs heterojunction in high magnetic field at low temperature is shown to exhibit conduction typical of pinned charge-density waves. Crossover from Ohmic conduction occurs on the same boundary at which radio-frequency resonances signal the onset of transverse elasticity. A further small non-Ohmic region is isolated from the main area by a v=1/5 quantum-Hall-effect phase. The relationship found between the threshold conduction field and the resonance frequency is well accounted for by a model of pinned electron crystallites.

Some properties of a linear chain of harmonically coupled double wells are studied by the technique of the transfer operator. Exact values for the chain specific heat, the displacement probability density, and the first six even moments of the dynamical displacement correlation function are obtained. The model is shown to be a prototype for displacive as well as for order–disorder transitions depending on the strength of the harmonic coupling. Criteria for the harmonic behavior of the system are derived. By this method, it was not possible to find the exact form of the dynamical correlation functions for very anharmonic cases; this is so in general for order–disorder systems, but also for a small wave vector and temperature region in displacive systems.

Starting from a tricritical approach to the polymer theta point, we deduce a (T, C) diagram for polymer solutions where T is the temperature, C the monomer concentration. We define four different regions in this diagram. For each of these regions we calculate the T, N, C, dependence of the mean square end to end distance of a chain, the screening length, the osmotic pressure, and the second virial coefficient. We also give a scaling form for the equation of state.

Silica spheres of 1600-\AA{} diameter immersed in the binary fluid water+2-6-lutidine have been studied by light-scattering techniques. The scattered light was seen to be strongly dependent on temperature and concentration. This effect is due to the appearance of a lutidine layer on the spheres. The layer thickness dramatically increases near a transition line were the spheres aggregate as a result of attractive interactions. This transition exhibits all the features predicted for the prewetting transition between high and low adsorption of a fluid on a surface.

The response to a boundary heating of a very compressible, low-diffusivity, supercritical fluid (${\mathrm{CO}}_{2}$) under zero-gravity is studied by solving numerically the full non-linear one-dimensional Navier-Stokes equations. Both short (acoustic) and long (diffusion) time scales are investigated. A new mechanism of heat transport is seen, where the thermal energy is transformed into kinetic energy in a hot expanding boundary layer (the piston), which in turn is transformed in the bulk into internal energy. Steeply profiled waves are observed. In contrast to the ``critical slowing down'' behavior, the enhancement of heat transport is so important that it is nearly completed after 1% of the diffusion time.

We have analyzed an Ising-like model, in the mean-field approach, involving two “antiferromagnetically” coupled sublattices. This model simulates the so-called “two-step” spin-crossover transition, for which a precise definition is given. If both sublattices are equivalent, it implies a spontaneous breaking of symmetry which may occur within a temperature range limited by two “Néel températures”. It, also predicts a simultaneous reversal of the magnetization of the sublattices (if they are unequivalent) at a “characteristic” value of temperature. These features are analyzed simultaneously with some details. The present model fits and explains well the available experimental data concerning [ Fe(2-pic)3] Cℓ2- EtOH and FeII[ 5NO2 – sal – N(1, 4, 7, 10)] .

Up to 2 million tons per year of titanium dioxide (TiO₂) nanoparticles (NP) are produced worldwide. This extensive production is postulated to result in release into the environment with subsequent contamination of soils and plants; however, few studies have examined TiO₂-NP uptake and impact on plants. In this study, wheat and rapeseed plantlets were exposed to 14 nm or 25 nm anatase TiO₂-NP in hydroponics conditions, either through root or leaf exposure. Microparticle-induced x-ray emission (μPIXE) coupled with Rutherford backscattering spectroscopy (RBS) was used to quantify absorbed titanium (Ti). Micro x-ray fluorescence (μXRF) based on synchrotron radiation was used to evaluate Ti distribution in roots and leaves. Our results show that both TiO₂-NP are accumulated in these plantlets upon root exposure and that Ti content is higher in rapeseed than wheat. Ti distribution in root cross sections depended on NP agglomeration state. NP are also accumulated in plantlets upon leaf exposure. Finally, it was found that TiO₂-NP exposure induced increased root elongation but did not affect germination, evapotranspiration, and plant biomass. Taken together, these results confirm that TiO₂-NP may be accumulated in plant crops but may only moderately impact plant development.

Treatment of [Fe2(mu-pdt)(CO)6] [pdt=S(CH2)3S] with dppe (Ph2PCH2CH2PPh2) in refluxing toluene affords the asymmetric complex [Fe2(mu-pdt)(CO)4(dppe)] (1). Protonation of 1 with HBF4-Et2O in CH2Cl2 gives at room temperature the mu-hydrido derivative [Fe2(mu-pdt)(CO)4(dppe)(mu-H)](BF4) (2). Monitoring the reaction by 1H, 31P, and 13C NMR at low temperature reveals unambiguously that the process of the protonation of 1 implies terminal hydride intermediates.

The frequency dependence of viscoelastic properties of a branched polymer sample near its gelation threshold has been studied in oscillatory measurements. The results are in agreement with De Gennes' analogy relating the critical behaviour of the elastic moduli G' and G'' to the electrical conductivity of percolation clusters. In particular the increase of both G' and G'' with frequency (0.1 to 10 Hz) can be described by power laws with a common exponent u = t/s + t, and the loss angle δ assumes near the gelation point the universal value δc = πu/2 (t and s denote, respectively, the exponents of elastic modulus and viscosity at zero frequency). The value of u determined in these experiments (u = 0.70 ± 0.02) is in very good agreement with percolation theory.

Titanium dioxide nanoparticles (TiO(2)-NPs) are produced in large quantities, raising concerns about their impact for human health. The aim of this study was to deeply characterize TiO(2)-NPs genotoxic potential to lung cells, and to link genotoxicity to physicochemical characteristics, e.g., size, specific surface area, crystalline phase. A549 cells were exposed to a panel of TiO(2)-NPs with diameters ranging from 12 to 140 nm, either anatase or rutile. A set of complementary techniques (comet and micronucleus assays, gamma-H2AX immunostaining, 8-oxoGuanine analysis, H2-DCFDA, glutathione content, antioxidant enzymes activities) allowed us to demonstrate that small and spherical TiO(2)-NPs, both anatase and rutile, induce single-strand breaks and oxidative lesions to DNA, together with a general oxidative stress. Additionally we show that these NPs impair cell ability to repair DNA, by inactivation of both NER and BER pathways. This study thus confirms the genotoxic potential of TiO(2)-NPs, which may preclude their mutagenicity and carcinogenicity.

In the displacive limit of a one-dimensional coupled double-well model, three sets of propagating solutions of the corresponding nonlinear field equation are exhibited. It is shown that knowledge of their amplitude and their energy suffices to interpret the variation with the temperature and the wave vector of the numerically calculated three-peaks structure of the dynamical correlation function. In addition, Lorentz invariance of the nonlinear field equation shows that localized solutions (propagating walls) have the properties of a relativistic particle, the limit velocity of which is a sound velocity. These propagating walls exhibit nontrivial interactions and, therefore, do not have the usual soliton properties. The behavior of the central peak and specifically the nonzero frequency of the latter, for a nonzero wave vector, is explained by assuming a perfect relativistic gas of such moving walls.