We investigate freely expanding viscoelastic sheets. The sheets are produced by the impact of drops on a quartz plate covered with a thin layer of liquid nitrogen that suppresses shear viscous dissipation as a result of the cold Leidenfrost effect. The time evolution of the sheet is simultaneously recorded from top and side views using high-speed cameras. The investigated viscoelastic fluids are Maxwell fluids, which are characterized by low elastic moduli, and relaxation times that vary over almost two orders of magnitude, thus giving access to a large spectrum of viscoelastic and elastocapillary effects. For the purposes of comparison, Newtonian fluids, with viscosity varying over three orders of magnitude, are also investigated. In this study, d(max), the maximal expansion of the sheets, and t(max) the time to reach this maximal expansion from the time at impact, are measured as a function of the impact velocity. By using a generalized damped harmonic oscillator model, we rationalize the role of capillarity, bulk elasticity and viscous dissipation in the expansion dynamics of all investigated samples. In the model, the spring constant is a combination of the surface tension and the bulk dynamic elastic modulus. The time-varying damping coefficient is associated to biaxial extensional viscous dissipation and is proportional to the dynamic loss modulus. For all samples, we find that the model reproduces accurately the experimental data for d(max) and t(max).

We exhibit a set of six explicit higher nonlocal integral invariants of free-surface deep-water gravity waves conservedin lowest nontrivial orders of perturbation in the amplitude of the surface displacement.

We have elaborated a new preparation method of ternary AgInS2 and alloyed quaternary AgInS2-ZnS nanocrystals which consisted of two consecutive injections of sulfur (S dissolved in oleylamine, OLA) and then silver (AgNO3 or Ag2CO3 dissolved in dichlorobenzene, DCB) precursors to a mixture of indium(III) chloride, zinc stearate and 1-dodecanethiol (DDT) dissolved in 1-octadecene (ODE). In these conditions nucleation of cubic In2S3 seeds took place followed by the growth of orthorhombic AgInS2 or alloyed AgInS2-ZnS cubic phases to yield a heterodimer type of nanocrystals. In both types of nanocrystals clearly separated photoluminescence peaks could be observed, confirming their heterogenic nature. The first one at 430 nm originated from the luminescence of the In2S3 phase. The second one was ascribed to the presence of ternary Ag-In-S or quaternary Ag-In-Zn-S phases and its position, within the spectroscopic range from 515 nm to 710 nm, strongly depended on the nanocrystals composition. The registered two-dimensional excitation-emission topographical maps clearly indicated that the observed emissions in two different spectral regions were related to the excitations in the same spectral range (300-400 nm), however their photoluminescence mechanisms were distinctly different. The photoluminescence lifetime of 3 ns measured for the emission at shorter wavelengths was typical of the simple mechanism of excitons radiative recombination in the In2S3 phase. Significantly longer lifetime of the longer wavelengths emission (26 s) seemed to clearly indicate that in this case the photoluminescence mechanism was more complex, involving exciton trap states whose positions depended on the composition of the ternary (or quaternary) phase.

Nuclear magnetism inn-doped semiconductors with a positive hyperfine constant is revisited. Two kindsof nuclear magnetic ordering can be induced by resident electrons in a deeply cooled nuclear spin system.At positive nuclear spin temperature below a critical value, randomly oriented nuclear spin polarons similarto that predicted by Merkulov [Phys. Solid State40, 930 (1998)] should emerge. These polarons are orientedrandomly, and within each polaron, nuclear and electron spins are aligned antiferromagnetically. At negativenuclear spin temperature below a critical value, we predict another type of magnetic ordering—a dynamicallyinduced nuclear ferromagnet. This is a long-range ferromagnetically ordered state involving both electrons andnuclei. It can form if electron spin relaxation is dominated by the hyperfine coupling, rather than by the spin-orbitinteraction. Application of the theory to then-doped GaAs suggests that ferromagnetic order may be reached atexperimentally achievable nuclear spin temperatureN≈−0.5μK and lattice temperatureTL≈5K.

Spatiotemporal spin noise spectroscopy is combined with dynamic light scattering in order to reach spatial resolutions down to ∼λ/10. Applied to a system of localized electron spins, an insulating n-doped CdTe layer, this allows us to reveal long spin jump distances ∼ 2.7 μm. Spin noise spectra at large wave vectors q (q 1) provide a snapshot of the spin dynamics before jump (therefore not affected by spin motion), while at smaller q, spin motion sets in. This allows us to unravel the contributions of spin-orbit and hyperfine fields in the electron spin relaxation and to determine self-consistently all parameters relevant to the spin dynamics. We propose a phenomenological equation inspired by studies of atomic jump diffusion by neutron scattering, which includes the relevant spin relaxation mechanisms and the effect of time of flight of the spin fluctuation across the laser spot. This modeling reproduces all experimental results.

Downsizing electronic and electric equipment requires the optimization of electric field distributions in order to avoid localized dielectric breakdown (also called partial discharges). This paper presents a novel dielectric composite material aimed at grading electrical local surface stress. This functional material has a conductivity which increases by several orders with the applied electric field giving the ability to distribute the field by itself. It is prepared for the first time by dispersing graphite nanoplatelets in a polymer and may be used as a resistive or capacitive field grading material in electronic and electrical applications. Mechanisms at the origin of the nonlinear behavior are discussed.

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