Consequences of anisotropy in electrical charge storage: application to the characterization by the mirror method of TiO2 rutile

This article is devoted first to anisotropic distributions of stored electric charges in isotropic materials, second to charge trapping and induced electrostatic potential in anisotropic dielectrics. On the one hand, we examine the case of anisotropic trapped charge distributions in linear homogeneous isotropic (LHI) insulators, obtained after an electron irradiation in a scanning electron microscope. This injection leads to the formation of a mirror image

Electron Beam Charging of Insulators with Surface Layer and Leakage Currents

International audienceThe electron beam induced selfconsistent charge transport in layered insulators is described by means of an electron-hole fight-drift model FDM and an iterative computer simulation. Ballistic secondary electrons and holes, their attenuation and drift, as well as their recombination, trapping, and detrapping are included. Thermal and field-enhanced detrapping are described by the Poole-Frenkel effect. Furthermore, an additional surface layer with a modified electric surface conductivity is included which describes the surface leakage currents and will lead to particular charge incorporation at the interface between the surface layer and the bulk substrate

Electron beam charging of insulators: A self-consistent flight-drift model

International audienceElectron beam irradiation and the self-consistent charge transport in bulk insulating samples are described by means of a new flight-drift model and an iterative computer simulation. Ballistic secondary electron and hole transport is followed by electron and hole drifts, their possible recombination and/or trapping in shallow and deep traps. The trap capture cross sections are the Poole-Frenkel-type temperature and field dependent. As a main result the spatial distributions of currents j(x,t), charges, the field F(x,t) and the potential slope V(x,t) are obtained in a self-consistent procedure as well as the time-dependent secondary electron emission rate sigma(t) and the surface potential V0(t) For bulk insulating samples the time-dependent distributions approach the final stationary state with j(x,t)=const=0 and sigma=1. Especially for low electron beam energies E0=4 keV the incorporation of mainly positive charges can be controlled by the potential VG of a vacuum grid in front of the target surface. For high beam energies E0=10, 20, and 30 keV high negative surface potentials V0=−4, −14, and −24 kV are obtained, respectively. Besides open nonconductive samples also positive ion-covered samples and targets with a conducting and grounded layer (metal or carbon) on the surface have been considered as used in environmental scanning electron microscopy and common SEM in order to prevent charging. Indeed, the potential distributions V(x) are considerably small in magnitude and do not affect the incident electron beam neither by retarding field effects in front of the surface nor within the bulk insulating sample. Thus the spatial scattering and excitation distributions are almost not affected

Direct Observation of Propagating Gigahertz Coherent Guided Acoustic Phonons in Free Standing Single Copper Nanowires

We report on gigahertz acoustic phonon waveguiding in free-standing single copper nanowires studied by femtosecond transient reflectivity measurements. The results are discussed on the basis of the semianalytical resolution of the Pochhammer and Chree equation. The spreading of the generated Gaussian wave packet of two different modes is derived analytically and compared with the observed oscillations of the sample reflectivity. These experiments provide a unique way to independently obtain geometrical and material characterization. This direct observation of coherent guided acoustic phonons in a single nano-object is also the first step toward nanolateral size acoustic transducer and comprehensive studies of the thermal properties of nanowires

Phonons in Slow Motion: Dispersion Relations in Ultra-Thin Si Membranes

We report the changes in dispersion relations of hypersonic acoustic phonons in free-standing silicon membranes as thin as \sim 8 nm. We observe a reduction of the phase and group velocities of the fundamental flexural mode by more than one order of magnitude compared to bulk values. The modification of the dispersion relation in nanostructures has important consequences for noise control in nano and micro-electromechanical systems (MEMS/NEMS) as well as opto-mechanical devices.Comment: 5 page

Intensity fluctuations of spherical acoustic waves propagating through thermal turbulence*

International audienc

SIMULATION NUMÉRIQUE DE LA PROPAGATION D'UNE ONDE ACOUSTIQUE DANS UNE TURBULENCE BIDIMENSIONNELLE PAR UNE MÉTHODE DE RAYONS

Nous présentons des résultats de simulation numérique de la propagation d'une onde acoustique dans une turbulence cinématique bidimensionnelle. Le milieu turbulent est représenté par une superposition de modes de Fourier aléatoires. Pour chaque réalisation, on détermine les rayons issus d'une source ponctuelle, on calcule la variance du temps de parcours le long de ces trajectoires ainsi que la distance pour laquelle se forme la première caustique.A numerical technique for simulating the behavior of an acoustic wave propagating through a turbulent medium is presented. A bidimensional velocity field is generated using a superposition of random Fourier modes. The time variance along the rays and the probability density for the occurence of the first caustics are computed

Etude numérique de la focalisation aléatoire d'une onde se propageant dans une turbulence thermique 2D

We consider the propagation of acoustic waves, in either the geometric or the parabolic approximation, through an ensemble of realizations of a random field modeling a 2D homogeneous isotropic thermal turbulence. Our results for a plane wave demonstrate that the occurrence of caustics as well as the maximum peak in the scintillation index are governed by the same universal parameter related to the correlation function of the temperature fluctuations