Physics

The Effect of Fe Substitution on Pyroelectric Properties of 0.62Pb(Mg[sub ]Nb[sub ])O-0.38PbTiO Single Crystals

L.H , W.X , Z.X , C.HLW , C.CL , C.K


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Fe-doped 0.62Pb(Mg[sub ⅓]Nb[sub ⅔]O₃-0.38PbTiO₃ (PMN-0.38PT) single crystals with two different dopant concentrations, 0.2 and 1.0 mol %, were grown by a modified Bridgman technique. The effect of Fe substitution on dielectric and pyroelectric properties was examined. The dielectric constant of the 0.2 mol % Fe-doped PMN-0.38PT single crystal was controlled successfully, indicating the possibility of tuning the dielectric properties by doping with a small concentration of iron ions. The mechanism of doping effect was also discussed based on the principles of crystal chemistry. The pyroelectric response of the single crystals was measured using a dynamic method from 20 to 50 °C. Compared with the 1.0 mol % Fe-doped single crystal, the 0.2 mol % Fe-doped sample possesses the better pyroelectric properties. At room temperature, the pyroelectric coefficient and detectivity figure of merit (F[sub D) for the 0.2 mol % Fe-doped sample are 568 µC/m² K and 53 µPa[sup −1/2], respectively, and they increase slightly with temperature increasing. These excellent pyroelectric properties as well as being able to produce large-size and high-quality single crystals make this kind of single crystal a promising candidate for high-performance infrared detectors and other pyroelectric applications.Department of Applied Physic

A dynamic regularized gradient model of the subgrid-scale stress tensor for large-eddy simulation

A.Vollant , G.Balarac , C.Corre


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Large-eddy simulation solves only the large scales part of turbulent flows by using a scales separation based on a filtering operation. The solution of the filtered Navier-Stokes equations requires then to model the subgrid-scale stress tensor to take into account the effect of scales smaller than the filter size. In this work, a new model is proposed for the SGS stress model. The model formulation is based on a regularization procedure of the gradient model to correct its unstable behavior. The model is developed based on a priori tests to improve the accuracy of the modeling for both structural and functional performances, i. e. , the model ability to locally approximate the SGS unknown term and to reproduce enough global SGS dissipation, respectively. LES is then performed for a posteriori validation. This work is an extension to the SGS stress tensor of the regularization procedure proposed by Balarac et al. to model the SGS scalar flux. A set of dynamic regularized gradient models is thus made available for both the momentum and the scalar equations. The second objective of this work is to compare this new set of DRG models with direct numerical simulations , filtered DNS in the case of classic flows simulated with a pseudo-spectral solver and with the standard set of models based on the dynamic Smagorinsky model. Various flow configurations are considered: decaying homogeneous isotropic turbulence,turbulent plane jet, and turbulent channel flows. These tests demonstrate the stable behavior provided by the regularization procedure, along with substantial improvement for velocity and scalar statistics predictions.

Turbulence heating observer - satellite mission proposal

M.Andre , J.De , D.B. , C.Norgren , A.Vaivads , A.Retino , J.Soucek , Y.V. , F.Valentini , C.Philippe , O.Alexandrova , S.D. , M.A. , D.Burgess , E.Camporeale , D.Caprioli , C.H. , E.Clacey , C.M. , J.P. , A.N. , S.Eriksson , M.L. , S.Haaland , M.Hoshino , H.Ji , H.Karimabadi , H.Kucharek , B.Lavraud , F.Marcucci , W.H. , T.E. , R.Nakamura , Y.Narita , Z.Nemecek , H.Opgenoorth , M.Palmroth , D.Perrone , J.Pincon , P.Rathsman , H.Rothkaehl , F.Sahraoui , S.Servidio , L.Sorriso-Valvo , R.Vainio , Z.Voros , R.F.


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The Universe is permeated by hot, turbulent, magnetized plasmas. Turbulent plasma is a major constituent of active galactic nuclei, supernova remnants, the intergalactic and interstellar medium, the solar corona, the solar wind and the Earth's magnetosphere, just to mention a few examples. Energy dissipation of turbulent fluctuations plays a key role in plasma heating and energization, yet we still do not understand the underlying physical mechanisms involved. THOR is a mission designed to answer the questions of how turbulent plasma is heated and particles accelerated, how the dissipated energy is partitioned and how dissipation operates in different regimes of turbulence. THOR is a single-spacecraft mission with an orbit tuned to maximize data return from regions in near-Earth space -magnetosheath, shock, foreshock and pristine solar wind -featuring different kinds of turbulence. Here we summarize the THOR proposal submitted on 15 January 2015 to the `Call for a Medium-size mission opportunity in ESAs Science Programme for a launch in 2025 '. THOR has been selected by European Space Agency for the study phase.

La repartition dans l'espace des directions d'emission des photoelectrons

P.Auger , F.Perrin


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Resume : Lorsqu'on superpose deux ondes lumineuses de memes direction, frequence et intensite, mais de phases independantes, polarisees lineairement dans des plans perpendiculaires, on doit obtenir une onde de lumiere naturelle dont les effets doivent avoir une symetrie de revolution autour de la direction de propagation. L'application de ce principe donne une condition necessaire a laquelle doit satisfaire la repartition dans l'espace des directions d'emission des electrons projetes par l'effet photoelectrique. On peut ainsi montrer d'abord l'impossibilite de certaines formes de repartition, en particulier de celles qui resultent des theories de W. Bothe et F. -W. Bubb. En admettant de plus que, quand la frequence du rayonnement excitateur est faible, l'effet photoelectrique produit par une onde polarisee est de revolution autour de la direction du champ electrique de l'onde incidente, on acheve la determination de la loi de repartition. Quand la frequence du rayonnement excitateur devient grande, la quantite de mouvement absorbe introduit une dissymetrie de plus en plus importante qui se traduit par un deplacement vers l'avant de la repartition. En admettant que la quantite de mouvement est communiquee au photoelectron, on peut determiner completement comment se modifie la loi de repartition, du moins lorsque l'energie d'arrachement des electrons est negligeable. Lorsque cette energie est importante, des hypotheses plus particulieres sont necessaires ; l'ancienne conception des orbites electroniques conduit a une dispersion supplementaire considerable, en opposition avec les resultats experimentaux, qui sont, au contraire, completement expliques par des hypotheses deduites des nouvelles conceptions de la mecanique ondulatoire. Les formules obtenues sont rassemblees dans un resume, et comparees aux donnees experimentales de P. Auger, W. Bothe, F. -W. Bubb. La verification est tout a fait satisfaisante dans tous les cas etudies.

Frequency conversion with superconducting point contacts operating as oscillator-mixers and its application to the measurement of the linewidth of the josephson current at very high frequency

G.Vernet , R.Adde


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The Josephson frequency conversion in the millimeter and submillimeter wave region is studied with superconducting Nb-Nb point contact junctions at 4. 2 K, using the contact both as the local oscillator and the mixer with intermediate frequencies of 0. 1 and 9. 2 GHz. Frequency conversion signals are only detected in the latter case with a microwave receiver at 9. 2 GHz in a range of Josephson frequencies between 145 and 1 600 GHz. The experiments with intermediate frequency fi of 0. 1 GHz indicate for this small value of f i that the observed signals are not related to frequency conversion but noise effects. An analysis of the frequency conversion mechanism in a Josephson junction operated as an oscillator-mixer is proposed which includes the noise effects and also the feedback effects induced in a junction driven by a constant current source. We have then investigated the noise properties up to 450 GHz of superconducting point-contacts using the junction as an oscillator-mixer where the Josephson current is mixed with harmonics of an externally applied signal. The emitted intermediate frequency is detected at 9. 2 GHz and its linewidth is measured. In the very high frequency range the dc quasiparticle current dominates in a Josephson weak-link and we compare the corresponding theoretical linewidth calculated by Scalapino with our experimental results.
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