In a recent Letter, Wang and Brady (WB) [1] analyse the rheology of Brownian hard spheres using constant stress and pressure Brownian dynamics simulations. The main observable is the shear viscosity, η( ̇γ,Π), expressed as a function of the shear rate ̇γ and adimensional pressure ̄Π = Πa3/(kBT), where Π is the pressure, kBT the thermal energy, and a the average particle diameter. The central conclusion is the discovery of a “universal viscosity divergence”

We use molecular dynamics simulations to investigate dynamic heterogeneities and the potential energy landscape of the Gaussian core model (GCM). Despite the nearly Gaussian statistics of particles' displacements, the GCM exhibits giant dynamic heterogeneities close to the dynamic transition temperature. The divergence of the four-point susceptibility is quantitatively well described by the inhomogeneous version of the mode-coupling theory. Furthermore, the potential energy landscape of the GCM is characterized by large energy barriers, as expected from the lack of activated, hopping dynamics, and display features compatible with a geometric transition. These observations demonstrate that all major features of mean-field dynamic criticality can be observed in a physically sound, three-dimensional model.

In this theoretical work, we study the polarized infrared spectra of double-walled carbon nanotubes (DCNTs) as a function of their diameters, chiralities and lengths. For infinite DCNTs, mathematical relations are derived to estimate the diameter of the inner and outer tubes from the knowledge of their experimental mid-infrared spectra. The infrared spectra of DCNTs with a finite length are also calculated using the spectral moments method. We observe additional infrared bands as a function of the relative lengths of the inner and outer tubes. Bands located below 900 cm-1 give reliable information about these lengths.

Deep ultra-violet semiconductor lasers have numerous applications for optical storage and biochemistry. Many strategies based on nitride heterostructures and adapted substrates have been investigated to develop efficient active layers in this spectral range, starting with AlGaN quantum wells on AlN substrates and more recently sapphire and SiC substrates. Here we report an efficient and simple solution relying on binary GaN/AlN quantum wells grown on a thin AlN buffer layer on a silicon substrate. This active region is embedded in microdisk photonic resonators of high quality factors and allows the demonstration of a deep ultra-violet microlaser operating at 275 nm at room temperature under optical pumping, with a spontaneous emission coupling factor $\beta=(4±2) 10^{-4}$. The ability of the active layer to be released from the silicon substrate and to be grown on silicon-on-insulator substrates opens the way to future developments of nitride nanophotonic platforms on silicon.

The dynamics of the free-exciton capture by boron acceptors and phosphorus donors in diamond is observed in the picosecond range by time-resolved photoluminescence experiments at low temperature. The formation of boron-bound excitons is observed with a delay of 410 ps after the formation of free excitons. For phosphorus, this delay is 120 ps. This is the result of the free-exciton capture by B° and P° impurities. The lifetimes of boron- and phosphorus-bound excitons are measured and found to be equal to 270 and 70 ps, respectively. These values are about four orders of magnitude shorter than for the same impurities in silicon. Ei being the ionization energy of dopants, these results scale well with the Ei4 dependence of the Auger recombination rate expected for bound excitons in indirect band-gap semiconductors.

Polymer nanocomposites are used widely, mainly for the industrial application of car tyres. The rheological behavior of such nanocomposites depends in a crucial way on thedispersion of the hard filler particles – typically silica nanoparticles embedded in a soft polymer matrix. It is thus important to assess the filler structure, which may be quitedifficult for aggregates of nanoparticles of high polydispersity, and with strong interactions at high loading. This has been achieved recently using a coupled TEM/SAXSstructural model describing the filler microstructure of simplified industrial nanocomposites with grafted or ungrafted silica of high structural disorder. Here, wepresent an original method capable of reducing inter-aggregate interactions by swelling of nanocomposites, diluting the filler to low-volume fractions. Note that this isimpossible to reach by solid mixing due to the large differences in viscoelasticity between the composite and the pure polymer. By combining matrix crosslinking,swelling in a good monomer solvent, and post-polymerization of these monomers, it isshown that it is possible to separate the filler into small aggregates. The latter have then been characterized by electron microscopy and small-angle X-ray scattering,confirming the conclusions of the above mentioned TEM-SAXS structural model applied directly to the highly loaded cases.

Single crystals of α-quartz type Si1−xGexO2 with x < 0.2 were grown in 0.05 M NaOH solution. Infrared measurements confirmed that crystals grown at high pressure and high temperature have a low –OH group defect content. After a few millimeters of crystal growth under these conditions, α3500 reaches 0.1 cm−1 and no free –OH are present. The d11 value measured on an X-cut from the crystal with x = 0.0375 is 3.08 pC N−1 (±0.15), in good agreement with the value of 2.97 pC N−1 obtained using density functional theory based calculations. Piezoelectric measurements were performed on the Si1−xGexO2 crystal with x = 0.0375, both at room temperature and after annealing at various temperatures. The piezoelectric signal of the crystal with x = 0.0375 and pure SiO2 disappears after annealing at 635 °C and 545 °C, respectively. Nonlinear optical (NLO) properties of Si1−xGexO2 crystals were measured by Maker's fringe technique on Z-cut χ11(2) and their corresponding values for x = 0.023 and 0.028 are 1.3(2) pm V−1 and 1.6(2) pm V−1, respectively. These values are in good agreement with density functional theory based calculations. The light induced damage threshold values measured on Si1−xGexO2 crystals with x = 0.023 and 0.028 are very similar to that of α-quartz.