Clusters and Ising droplets in the antiferromagnetic lattice gas

A definition of clusters of particles and holes with antiferromagnetic order is given for a lattice gas with coupling constant K < 0. In two dimensions it is shown that the Ising antiferromagnetic critical line is also a percolation line if P_b = 1 - exp(-|K|/2). Along this line these clusters called “droplets” diverge with Ising exponents.     

A mathematical model for pattern formation in biological systems

A system of reaction-diffusion equations with applications in biological problems is presented and studied theoretically and numerically. Solutions demonstrating the following types of behaviour have been obtained: (i) time constant, spatially inhomogeneous, (ii) time periodic, spatially homogeneous, and (iii) time periodic, travelling wave-like. Some applications in embryological problems are discussed.     

Aging of polyamide 12 in oil at different temperatures and pressures

Polyamide 12 (PA 12) is an alternative to steel in the manufacture of pipes for transporting oil and gas. However, polyamide pipes suffer degradation, triggered by factors such as temperature and exposure to chemicals. In this research, the authors studied the variation of mechanical and thermal properties and of structural characteristics of a PA 12 grade after aging in oil. The aging process lasted 3360 h. It was made at ambient temperature and at 70 °C, under pressures of 1 and 17 atm. At ambient temperature and atmospheric pressure, PA 12 increased in volume, weight and rigidity, and decreased in damping. At 70 °C and atmospheric pressure, there was a smaller increase in volume and weight. PA 12 samples also darkened, showed an increase in crystallinity and damping, and a decrease in rigidity. At 70 °C and 17 atm, a decrease in volume and weight was observed; the samples also darkened, increased in crystallinity and rigidity, and decreased in damping. In all aging conditions, the results of the thermal analysis and the analysis of functional groups showed that the chemical structure of the material was not significantly changed. According to the results, PA 12 has a good resistance to aging under these conditions. Copyright © 2017 John Wiley & Sons, Ltd.     

Polymer monolayers with a small viscoelastic linear regime: equilibrium and rheology of poly(octadecyl acrylate) and poly(vinyl stearate)

The equilibrium properties of monolayers of two polymers: poly(octadecyl acrylate) and poly(vinyl stearate) on water have been measured. The surface pressure (Pi) versus surface concentration (Gamma) curves indicate that the water-air interface is a poor solvent for both polymers. The thermal expansivity shows a sharp change near room temperature. This behavior is typical of a glass transition; this is the first time that such a plot is observed for Langmuir films. The Pi vs Gamma curves measured by the continuous compression method show strong anisotropy effects. They also show that the monolayer is brought into nonequilibrium states depending on the compression rate. Within the linear regime, the relaxation experiments were bimodal. The longest relaxation time strongly increases as T is decreased, which might be compatible with the high increase of viscosity in the glass transition. The oscillatory barrier experiments showed that the maximum strain of the linear regime is smaller than 3% for both monolayers. The Fourier-transform analysis of the oscillatory experiments beyond the linear regime points out the contribution of different harmonics in the response function. Oscillations in the nonlinear regime show hysteresis cycles. The results obtained indicate that some of the previously published data for these polymer monolayers correspond to nonequilibrium states.     

Dissipative curvature fluctuations in bilayer vesicles: Coexistence of pure-bending and hybrid curvature-compression modes

We have studied the relaxation dynamics of shape fluctuations in unilamellar lipid vesicles by neutron spin echo (NSE). The presence of a hybrid curvature-compression mode coexisting with the usual bending one has been revealed in the experimental relaxation functions at high q . Differently to the conventional relaxation ∼ q ³ typical for bending modes, the hybrid mode was found to relax as ∼ q ² , which is compatible with a dissipation mechanism arising from intermonolayer friction. Complementary data obtained from flickering spectroscopy (FS) in giant unilamellar vesicles confirm the existence of both modes coexisting together. By combining NSE and FS data we have depicted the experimental bimodal dispersion diagram, which is found compatible with theoretical predictions for reliable values of the material parameters. From the present data two conventional dynamical methods (NSE and FS) have been shown to be suitable for measuring intermonolayer friction coefficients in bilayer vesicles.     

VCSEL-based gigabit IR-UWB link for converged communication and sensing applications in optical metro-access networks

We report on experimental demonstration of an impulse radio ultrawideband (IR-UWB) based converged communication and sensing system. A 1550-nm VCSEL-generated IR-UWB signal is used for 2-Gbps wireless data distribution over 800-m and 50-km single mode fiber links which present short-range in-building and long-reach access network applications. The IR-UWB signal is also used to simultaneously measure the rotational speed of a blade spinning between 18 and 30 Hz. To the best of our knowledge, this is the very first demonstration of a simultaneous gigabit UWB telecommunication and wireless UWB sensing application, paving the way forward for the development and deployment of converged UWB VCSEL-based technologies in access and in-building networks of the future.     

Increment of lateral resolution in digital holography by speckle noise removal

Experimental features such as wavelength, camera specifications and reconstruction distance determine the theoretical limit for lateral resolution in digital holography. However, the actual experimental resolution limit is about 50% below such theoretical limit due to the high-contrast speckle noise presented in the reconstructed holograms. Recently, a technique has been introduced to reduce the contrast of speckle noise that is based on the superposition of uncorrelated hologram reconstructions of the same static object [5] (Garcia-Sucerquia et al., 2006). By this approach of reducing the contrast of the speckle noise, it is experimentally shown that an improvement of the order of 50% can be reached when 100 reconstructed images are superimposed.     

Influence of deposition conditions on nanocrystalline InN layers synthesized on Si(1 1 1) and GaN templates by RF sputtering

We present a detailed investigation on the influence of deposition conditions on morphological, structural and optical properties of InN films deposited on Si(1 1 1) and GaN-on-sapphire templates by reactive radio-frequency (RF) sputtering. The deposition parameters under study are nitrogen content in the sputtering gas, substrate–target distance, substrate temperature and RF power. X-ray diffraction measurements confirm the (0 0 0 1) preferred growth orientation and the wurtzite crystallographic structure of the material. For optimized deposition conditions, InN on Si(1 1 1) substrates presents smooth surface with root-mean-square roughness ∼1 nm. Surface quality of the InN films can be further improved by deposition on GaN-on-sapphire templates, achieving root-mean-square roughness as low as ∼0.4 nm, comparable to that of the underlying substrate. The room-temperature absorption edge is located at 1.70 eV. Intense low-temperature photoluminescence peaking at 1.60 eV is observed.     

Vertical electron transport study in GaN/AlN/GaN heterostructures

In this work, we investigate the electronic structure and vertical electron transport through GaN/AlN/GaN single-barrier structures with different AlN thickness, grown by plasma-assisted molecular beam epitaxy. Conductive and capacitive characterization has been performed, and the experimental results are interpreted by comparison with 1D self-consistent simulations. Capacitive measurements reveal a complete depletion of the top GaN layer, and the formation of a two-dimensional electron gas at the bottom interface of the AlN barrier, even for barrier thicknesses of 0.5 nm (2 monolayers of AlN). Conductive atomic force microscopy reveals discrete leakage current locations with a density of 10⁷ cm⁻², more than one order of magnitude lower than the dislocation density in these samples. These results are promising for the fabrication of resonant tunnelling diodes using the GaN/AlN material system.     

The variational approximation for 2‐dimension solitons at cubic‐quintic nonlinearity in quasiperiodic potentials

We apply the variational approximation to study the dynamics of solitary waves of the nonlinear Schrödinger equation with compensative cubic‐quintic nonlinearity for asymmetric 2‐dimension setup. Such an approach allows to study the behavior of the solitons trapped in quasisymmetric potentials without an axial symmetry. Our analytical consideration allows finding the soliton profiles that are stable in a quasisymmetric geometry. We show that small perturbations of such states lead to generation of the oscillatory‐bounded solutions having 2 independent eigenfrequencies relating to the quintic nonlinear parameter. The behavior of solutions with large amplitudes is studied numerically. The resonant case when the frequency of the time variations (time managed) potential is near of the eigenfrequencies is studied too. In a resonant situation, the solitons acquire a weak time decay.