Y. Boughaleb

Nonlinear refraction and absorption activity of dimethylaminostyryl substituted BODIPY dyes

Optical and nonlinear optical properties of difluoroboradiazaindacene (BODIPY) models with attached dimethylaminostyryl substituents were studied. The optical absorption and fluorescence emission of BODIPY compounds were found to be dependent on the number of dimethylaminostyryl substituents. In order to investigate the nonlinear refraction and absorption activity, the Z-scan technique was used employing a laser generating a wavelength at 532 nm with 30 ps pulse duration. The studied dimethylaminostyryl containing BODIPY compounds showed considerable nonlinear refraction and reverse saturable absorption. The BODIPY containing a pair of dimethylaminostyryl substituents demonstrated an increased third order nonlinear optical response mostly due to the extension of its conjugated length.

SURFACE DIFFUSION IN SYSTEMS OF INTERACTING BROWNIAN PARTICLES

The paper reviews recent results on diffusive phenomena in two-dimensional periodic potential. Specifically, static and dynamic properties are investigated by calculating different correlation functions. Diffusion process is first studied for one-dimensional system by using the Fokker–Planck equation which is solved numerically by the matrix continued fraction method in the case of bistable potential. The transition from hopping to liquid-like diffusion induced by variation of some parameters is discussed. This study will therefore serve to demonstrate the influence of this form of potential. Further, an analytical approximation for the dc-conductivity is derived for a wide damping range in the framework of the Linear Response Theory. On the basis of this expression, calculations of the ac conductivity of two-dimensional system with Frenkel–Kontorova pair interaction in the intermediate friction regime is performed by using the continued fraction expansion method. The dc-conductivity expression is used to determine the rest of the development. By varying the density of mobile ions we discuss commensurability effects. To get information about the diffusion mechanism, the full width at half maximum λω(q), of the quasi-elastic line of the dynamical structure factor S(q,ω) is computed. The calculations are extended up to large values of q covering several Brillouin zones. The analysis of λω(q) with different parameters shows that the most probable diffusion process in good two-dimensional superionic conductors consists of a competition between a back correlated hopping in one direction and forward correlated hopping in addition to liquid-like motions in the other direction.

Mechanical characterization of an electrostrictive polymer for actuation and energy harvesting

Electroactive polymers have been widely used as smart material for actuators in recent years. Electromechanical applications are currently focused on energy harvesting and actuation, including the development of wireless portable electronic equipment autonomous and specific actuators such as artificial muscles. The problem to be solved is to make its devices the most efficient, as possible in terms of harvested energy and action. These two criteria are controlled by the permittivity of the electrostrictive polymer used, the Young’s modulus, and their dependence on frequency and level of stress. In the present paper, we presented a model describing the mechanical behaviour of electrostrictive polymers with taking into account the mechanical losses. Young’s modulus follows a linear function of strain and stress. However, when the elongation becomes higher, the data obtained from this strain linear trend and significant hysteresis loops appear the reflections on the existence of mechanical losses. In this wo...

COLLECTIVE BEHAVIOR OF INTERACTING PARTICLES: RADIUS-DEPENDENT PHASE TRANSITION

The aim of this paper is to study and discuss the effect of three zones (repulsion zone, orientation zone and attraction zone) on the phase transition in 2D-collective moving particles. Our main motivation is to better understand the complex behavior of non-equilibrium multi-agent system by extending the earlier and original model proposed by Viscek et al. [T. Viscek et al., Phys. Rev. Lett.75 (1995) 1226] for one zone. The analysis is performed over different situations by using a numerical simulation method. It is found that the radius R2 of orientation zone plays an important role in the system. In effect, by varying the parameter R2 a phase transition can be achieved from disordered moving of individuals to a group to highly aligned collective motion. The results also show that, the critical value of R2 at which the transition emerges depends strongly on the size of the repulsion zone but not on the size of attraction one.

Nonlinearity and scaling behavior in a soft lead zirconate titanate piezoceramic

Lead oxide-based ferroelectrics, represented by lead zirconate titanate [Pb(Zr,Ti)O3] or PZT), are the most widely used materials for piezoelectric actuators, sensors, and transducers due to their excellent piezoelectric properties. Most of these piezoelectric materials are employed under a variety of strains (stress, electrical field, and temperature). It would thus be interesting to predict their behaviors under different excitations without having to perform too much experimental work, i.e., just carry out a single experiment and still be able to provide the other experimental values. The purpose of this paper has thus been to propose several behavioral laws linking the electrical field, temperature and mechanical stress. The first law rendered it possible to express the mechanical stress by an equivalent electric field [ΔE≡αΔT×P(E,T0)]. Subsequently, a law linking the electrical field and temperature {ΔE≡[2β×P(E,θ0)]×Δθ} was proposed. From these two laws, a third law was identified reflecting the mech...

Evaluation by fast Fourier transforms analysis of energy harvesting in electrostrictive polymers driven by an electric field and a mechanical excitation

Electrostrictive polymers offer the promise of energy harvesting with few moving parts where power can be produced simply by stretching and contracting a relatively low-cost rubbery material. The use of such polymers for energy harvesting is a growing field, which has great potential from an energy density viewpoint. Basically, the relative energy gain depends on the current induced by the mechanical strain and frequency. A previous study in the Laboratoire de Génie Electrique et Ferroélectricité laboratory has indicated that one can measure the dielectric constant, the Young’s modulus, and the electrostrictive coefficient of a polymer film by determining the current flowing through the sample when the polymer film was simultaneously driven by an electrical field and mechanical excitation. The goal of this study has thus been to develop a solution for artificially increasing the coupling factor of electrostrictive materials, based on the optimization of the frequency of the electric field and the amplitude strain of the mechanical excitation leading to an increase in the generated current. When relating this parameter with a transverse strain of 5% and a bias field of 10 V/µm, it was found that such a process rendered it able to increase the converted power to 14 µW at a mechanical frequency of 6 Hz. The converted power was much higher than for the frequency of 3 Hz for which a low power was consumed by the polarization of the polymer. The theoretical analysis was supported by the experimental investigations. The contribution of this study provides a framework for developing energy harvesting techniques that should improve the overall performance of the system.

Non-universality in the 2D flocking model

In this paper, we discuss the universality of the critical exponents β and δ found in the Viscek model for one zone of interaction in the 2D flocking model. Within the framework of this model, the particles move with the same absolute velocity v0 and interact locally by trying to align their direction with that of neighbors. In this paper, we include a second zone of interaction named zone of repulsion R1, where each agent attempts to maintain a minimum distance from the others. Our model results show that in order to maintain an order in a flock with higher density, it is necessary to decrease the region of repulsion around each individual. Depending on the noise and on the density, the order parameter va is found to scale as and , respectively. Our findings show that the exponents β and δ depend strongly on the size of the repulsion zone and on the density ρ, indicating the non-universality of these critical exponents. The analysis is performed over different situations by using a numerical simulation technique.

COMMENSURABILITY EFFECTS ON DIFFUSION PROCESS IN STEPPED STRUCTURES

This study deals with the investigation of diffusion process of one-dimensional system with steps for adsorbates interacting via the nearest-neighbor harmonic forces. The results are obtained from numerical studies, utilizing the method of stochastic Langevin dynamics. To study commensurability effects and the role of steps in the behavior of the diffusing particles, we have computed the diffusion coefficient for large concentrations and several interaction strengths. Our numerical results show that the diffusive behavior is reduced for commensurate structure case when the ground state has only one particle per one period of the substrate potential and enhanced for incommensurate density. Furthermore, the dynamic is qualitatively similar to that obtained in the case of no steps but with a clear reduction of the diffusion rate. Implications of these findings are discussed.

Numerical simulation of the one-dimensional incommensurate charge density wave dynamics

Abstract The effect of impurities on the dynamic properties of the charge density wave (CDW) system is explored through numerical simulations based on the one-dimensional Fukuyama, Lee and Rice (FLR) elastic model of weakly pinned incommensurate CDW. In agreement with previous experimental work, the current density J CDW transported by the sliding CDW and the associated excess conductivity σ CDW are found to obey respectively the laws J CDW ∝ (E/E T −1) α and σ CDW ∝ (E T /E) (E/E T −1 α in a wide range of fields and impurity concentrations; α is an impurities dependent exponent and E T is the threshold field. The spatial mean velocity fluctuations present a pic which broadens with increasing field for E ⪢ E T .

Relaxation and diffusion barriers at step edges of Cu, Ag and Au homo- and heterogeneous systems: Case of (100) facet

Abstract Using molecular dynamics simulations based on the embedded atom method, we present the calculations of adsorption and activation energies for the diffusion of adatoms (Cu, Ag or Au) on Cu (100), Ag (100) or Au (100) surfaces with steps. We have also examined the relaxation trends and bond lengths of the adatoms for both fourfold and bridge sites. We note that the vertical distance of the adatom relaxation to the first nearest neighbors is the largest (1.56 %) for Ag on Cu (100) and the shortest (–14.58 %) for Cu on Au (100) as compared to other systems. On the other hand, for Cu on the Au (100) system, we find the adatom barrier for hopping along the step edges to be 0.44 eV, which is the highest for this process among the systems studied here, but the lowest barrier is found for Ag on Cu (100) compared to other systems and costs only 0.20 eV. Attention has also been focused on the evaluations of the adsorption and activation energies for the nine systems in the presence of step edges. The diffusion barriers over and along step edges are interpreted in terms of the cohesive energies of the adatoms and substrates. Moreover, these results can offer some basic rules for forecasting precise atomic surface morphologies in homo- and hetero-epitaxial growth.