488 research outputs found
A new type of photoionized code required for the new era of X-ray spectroscopy
With the advent of the present and future spatial X-ray missions, it becomes
crucial to model correctly the line spectrum of X-ray emitting media. We have
built a photoionization code, Titan, solving the transfer of a thousand lines
and of the continuum with the "Accelerated Lambda Iteration" method, which is
most reliable for line transfer. In all other photoionization codes the line
intensities are computed with the so-called "escape probability formalism",
used in its simplest approximation. In a previous paper (Dumont et al. 2003),
it was shown that this approximation leads to errors which can exceed one order
of magnitude in the case a Thomson thickness of the order of unity. We show
here that it also happens in the case of a Thomson thickness of 0.001 to 0.1.
The errors on the line fluxes and line ratios are of the order of 30% for a
column density of 10^{20} cm^{-2}, and a factor five for a column density of
10^{23} cm^{-2}, in conditions appropriate for Seyfert 2 and for the Warm
Absorber of Seyfert 1.Comment: 11 pages, 7 figures. accepted by A&
Non-linear Frequency Transduction of Nano-mechanical Brownian Motion
We report on experiments addressing the non-linear interaction between a
nano-mechanical mode and position fluctuations. The Duffing non-linearity
transduces the Brownian motion of the mode, and of other non-linearly coupled
ones, into frequency noise. This mechanism, ubiquitous to all weakly-nonlinear
resonators thermalized to a bath, results in a phase diffusion process altering
the motion: two limit behaviors appear, analogous to motional narrowing and
inhomogeneous broadening in NMR. Their crossover is found to depend
non-trivially on the ratio of the frequency noise correlation time to its
magnitude. Our measurements obtained over an unprecedented range covering the
two limits match the theory of Y. Zhang and M. I. Dykman, Phys. Rev. B 92,
165419 (2015), with no free parameters. We finally discuss the fundamental
bound on frequency resolution set by this mechanism, which is not marginal for
bottom-up nanostructures.Comment: Article plus Supplementary Materia
Classical decoherence in a nanomechanical resonator
SI not providedInternational audienceDecoherence is an essential mechanism that defines the boundary between classical and quantum behaviours, while imposing technological bounds for quantum devices. Little is known about quantum coherence of mechanical systems, as opposed to electromagnetic degrees of freedom. But decoherence can also be thought of in a purely classical context, as the loss of phase coherence in the classical phase space. Indeed the bridge between quantum and classical physics is under intense investigation, using classical nanomechanical analogues of quantum phenomena. In the present work, by separating pure dephasing from dissipation, we quantitatively model the classical decoherence of a mechanical resonator: through the experimental control of frequency fluctuations, we engineer artificial dephasing. We report on the methods available to define pure dephasing in these systems, which are prerequisite in the understanding of decoherence processes in mechanical devices, both classical and quantum
On the link between mechanics and thermal properties: mechanothermics
We report on the theoretical derivation of macroscopic thermal properties
(specific heat, thermal conductivity) of an electrically insulating rod
connected to two reservoirs, from the linear superposition of its mechanical
mode Brownian motions. The calculation is performed for a weak thermal
gradient, in the classical limit (high temperature). The development is kept
basic as far as geometry and experimental conditions are concerned, enabling an
almost fully analytic treatment. In the modeling, each of the modes is subject
to a specific Langevin force, which enables to produce the required temperature
profile along the rod. The theory is predictive: the temperature gradient (and
therefore energy transport) is linked to motion amplitude cross-correlations
between nearby mechanical modes. This arises because energy transport is
actually mediated by mixing between the modal waves, and not by the modes
themselves. This result can be tested on experiments, and shall extend the
concepts underlying equipartition and fluctuation-dissipation theorems. The
theory links intimately the macroscopic size of the clamping region where the
mixing occurs to the microscopic lengthscale of the problem at hand: the phonon
mean-free-path. We believe that our work should impact the domain of thermal
transport in nanostructures, with future developments of the theory toward the
quantum regime
Modal Decomposition in Goalpost Micro/nano Electro-mechanical Devices
We have studied the first three symmetric out-of-plane flexural resonance
modes of a goalpost silicon micro-mechanical device. Measurements have been
performed at 4.2K in vacuum, demonstrating high Qs and good linear properties.
Numerical simulations have been realized to fit the resonance frequencies and
produce the mode shapes. These mode shapes are complex, since they involve
distortions of two coupled orthogonal bars. Nonetheless, analytic expressions
have been developed to reproduce these numerical results, with no free
parameters. Owing to their generality they are extremely helpful, in particular
to identify the parameters which may limit the performances of the device. The
overall agreement is very good, and has been verified on our nano-mechanical
version of the device.Comment: Journal of Low Temperature Physics (2013
Measuring frequency fluctuations in nonlinear nanomechanical resonators
Advances in nanomechanics within recent years have demonstrated an always
expanding range of devices, from top-down structures to appealing bottom-up
MoS and graphene membranes, used for both sensing and component-oriented
applications. One of the main concerns in all of these devices is frequency
noise, which ultimately limits their applicability. This issue has attracted a
lot of attention recently, and the origin of this noise remains elusive up to
date. In this Letter we present a very simple technique to measure frequency
noise in nonlinear mechanical devices, based on the presence of bistability. It
is illustrated on silicon-nitride high-stress doubly-clamped beams, in a
cryogenic environment. We report on the same dependence of the frequency
noise power spectra as reported in the literature. But we also find unexpected
{\it damping fluctuations}, amplified in the vicinity of the bifurcation
points; this effect is clearly distinct from already reported nonlinear
dephasing, and poses a fundamental limit on the measurement of bifurcation
frequencies. The technique is further applied to the measurement of frequency
noise as a function of mode number, within the same device. The relative
frequency noise for the fundamental flexure lies in the range
ppm (consistent with literature for cryogenic MHz devices), and
decreases with mode number in the range studied. The technique can be applied
to {\it any types} of nano-mechanical structures, enabling progresses towards
the understanding of intrinsic sources of noise in these devices.Comment: Published 7 may 201
Spécificité des PME de services supérieurs françaises à l'international : le rôle des technologies de l'information et de la communication
International audienceThis article deals with International SMEs and their strategic management linked with information and communication technologies. We would like to know how International SMEs manage their distance and proximity with clients, when they are themselves in peripherical area. We will take as an example the case of French International SMEs. We will compare the difference in terms of ICT management between local or regional SMEs and European or International SMEs. First of all, we will present the state of Art of International Management Studies, then we will focus on the relations between ICT and Entreprises. Secondly, we will focus on the DISCOTEC study, with the development of the methodology, the sample chosen and the analysis of the ground. Finally we will discuss the results and open new perspectives and we will compare our work with other countries such as the Czech RepublicCet article s'intéresse aux PME Internationales et à la façon dont leur management mobilise les technologies de l'information et de la communication. Il s'agit de voir comment des PME à dimension internationale gèrent la distance et leur éloignement vis-à -vis de leurs clients, alors même qu'elles sont elles-mêmes situées dans des zones d'activités périphériques, dispersées sur un territoire donné en prenant le cas de la France comme exemple. Nous pourrons ainsi voir les différences existantes entre des entreprises à dimension régionale, voire nationale et des entreprises à dimension européenne ou mondiale, et comment elles gèrent leurs technologies de l'information et de la communication sur un territoire donné. Nous présenterons dans un premier temps un état de l'art sur la théorie du management des entreprises internationales et leurs relations à la proximité et/ou distance avec les TIC dans différentes régions françaises, surtout lorsque ces PME sont dans des zones blanches, c'est-à -dire exclues des grands axes et réseaux de communication (privées de fibre optique par exemple). Puis nous nous présenterons la méthodologie de l'enquête, le questionnaire et son traitement par le cabinet ENOV Research, les analyses menées et enfin nous nous concentrons sur les perspectives de recherche et notamment envisager la poursuite de ces travaux avec d'autres pays européens comme la république tchèque
The thermal conductivity of silicon nitride membranes is not sensitive to stress
We have measured the thermal properties of suspended membranes from 10 K to
300 K for two amplitudes of internal stress (about 0.1 GPa and 1 GPa) and for
two different thicknesses (50 nm and 100 nm). The use of the original 3 \omega
-Volklein method has allowed the extraction of both the specific heat and the
thermal conductivity of each SiN membrane over a wide temperature range. The
mechanical properties of the same substrates have been measured at helium
temperatures using nanomechanical techniques. Our measurements show that the
thermal transport in freestanding SiN membranes is not affected by the presence
of internal stress. Consistently, mechanical dissipation is also unaffected
even though Qs increase with increasing tensile stress. We thus demonstrate
that the theory developed by Wu and Yu [Phys. Rev. B 84, 174109 (2011)] does
not apply to this amorphous material in this stress range. On the other hand,
our results can be viewed as a natural consequence of the "dissipation
dilution" argument [Y. L. Huang and P. R. Saulson, Rev. Sci. Instrum. 69, 544
(1998)] which has been introduced in the context of mechanical damping.Comment: 15 pages, 6 figures. Submitted to PR
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