[halshs-01137597] Entrée : Savoir politique

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[hal-01137502] Enjeux contemporains de la recherche et des formations en journalisme, à partir des "révolutions arabes

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[hal-01137502] Enjeux contemporains de la recherche et des formations en journalisme, à partir des "révolutions arabes

[...]



from HAL : Dernières publications http://ift.tt/19AGNNM

[halshs-01009969] Pietro Verri et Jean-Baptiste Say : valeur, monnaie et loi des débouchés

J.-A. Schumpeter dans son Histoire de l'Analyse Economique présente Say comme le continuateur de Turgot et de Cantillon. Il ajoute que ces derniers sont ceux qui ont probablement le plus marqué son analyse de. Si cette appréciation ne nous paraît pas discutable en terme de filiation, l'influence que Schumpeter leurs attribue doit être nuancée au profit du comte Pietro Verri qui est, selon nous, celui qui a le plus influencé la vision de JB.Say après Smith. Il faut cependant préciser que Schumpeter cite Verri dans la chaîne qui conduit de Galiani à Walras en passant par Say. Nous voulons tenter ici de déterminer quelle influence la lecture de Verri a pu exercer sur Say. Faut-il limiter cette influence à ce que Say lui même indique dans une note du Traité, à propos du commerce, ou bien faut-il donner une place toute autre et bien plus fondamentale dans la constitution de sa vision théorique générale ? (...)



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[halshs-01009969] Pietro Verri et Jean-Baptiste Say : valeur, monnaie et loi des débouchés

J.-A. Schumpeter dans son Histoire de l'Analyse Economique présente Say comme le continuateur de Turgot et de Cantillon. Il ajoute que ces derniers sont ceux qui ont probablement le plus marqué son analyse de. Si cette appréciation ne nous paraît pas discutable en terme de filiation, l'influence que Schumpeter leurs attribue doit être nuancée au profit du comte Pietro Verri qui est, selon nous, celui qui a le plus influencé la vision de JB.Say après Smith. Il faut cependant préciser que Schumpeter cite Verri dans la chaîne qui conduit de Galiani à Walras en passant par Say. Nous voulons tenter ici de déterminer quelle influence la lecture de Verri a pu exercer sur Say. Faut-il limiter cette influence à ce que Say lui même indique dans une note du Traité, à propos du commerce, ou bien faut-il donner une place toute autre et bien plus fondamentale dans la constitution de sa vision théorique générale ? (...)



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[hal-01136651] Whittle Index Policy for Crawling Ephemeral Content

We consider a task of scheduling a crawler to retrieve content from several sites with ephemeral content. A user typically loses interest in ephemeral content, like news or posts at social network groups, after several days or hours. Thus, development of timely crawling policy for such ephemeral information sources is very important. We first formulate this problem as an optimal control problem with average reward. The reward can be measured in the number of clicks or relevant search requests. The problem in its initial formulation suffers from the curse of dimensionality and quickly becomes intractable even with moderate number of information sources. Fortunately, this problem admits a Whittle index, which leads to problem decomposition and to a very simple and efficient crawling policy. We derive the Whittle index and provide its theoretical justification.



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[hal-00012910] Mesoscopic Spin Tunneling in Molecular Crystals

The phenomenon of Quantum Tunneling of Mesoscopic Spins is reviewed in the light of the behavior of the archetype of these systems: the molecular complex Mn12-ac. Most observations can be understood in the framework of the reduced Hilbert space dimension 2S+1=21. Due to the large spin S=10, the energy barrier preventing spin rotation is large, and as a consequence, quantum relaxation is very slow. The application of a magnetic field of a few Tesla below 1 K allows to observe tunneling (i) between the states m=-10 and m=10-n with n=8 to 11 if the field is longitudinal, or (ii) between the two ground-states m~-10 and m~10 if the field is transverse. The crossover temperature between ground-state and thermally assisted tunneling in a longitudinal field extrapolates in zero field at ~1.7 K. The observation of square root relaxation at short-times/low-temperatures and of exponential relaxation at long-times/high-temperatures, as observed previously above 1.5 K, confirms the important role of the spin bath dynamics which is out of equilibrium in the first regime and at equilibrium in the second one. In a second part of this paper a new molecule, so-called V15, with resultant spin S=1/2 is investigated. Contrary to high spin molecules, the energy barrier of low spin molecules is small or null, and the splitting between the symmetrical and anti-symmetrical states is sufficiently large to allow spin-phonon transitions during spin rotation. In low spin molecules the coupling to the environment is quite different from the one found in large spin molecules in low fields.



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[hal-00081868] Niobium and niobium nitride SQUIDs based on anodized nanobridges made with an Atomic Force Microscope

We present a fabrication method of superconducting quantum interference devices (SQUIDs) based on direct write lithography with an Atomic Force Microscope (AFM). This technique involves maskless local anodization of Nb or NbN ultrathin films using the voltage biased tip of the AFM. The SQUIDs are of weak-link type, for which two geometries have been tested: Dayem and variable thickness nanobridges. The magnetic field dependence of the maximum supercurrent Ic(flux) in resulting SQUIDs is thoroughly measured for different weak link geometries and for both tested materials. It is found that the modulation shape and depth of Ic(flux) curves are greatly dependent on the weak link size. We analyze the results taking into account the kinetic inductance of nanobridges and using the Likharev-Yakobson model. Finally we show that the present resolution reached by this technique (20nm) enables us to fabricate Nb weak-links which behavior approaches those of ideal Josephson junctions.



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[hal-00130701] Superconductivity in Doped column-IV sp3 Semiconductors: The Case of t he Clat hrates

We present a joint experimental and theoretical study of the superconductivity in doped silicon clathrates. The critical temperature in Ba8@Si-46 is shown to strongly decrease with applied pressure. These results are corroborated by ab initio calculations using MacMillan's formulation of the BCS theory with the electron-phonon coupling constant λ calculated from perturbative density functional theory. Further, the study of I8@Si-46 and of gedanken pure silicon diamond and clathrate phases doped within a rigid-band approach show that the superconductivity is an intrinsic property of the sp3 silicon network. As a consequence, carbon clathrates are predicted to yield large critical temperatures with an effective electron-phonon interaction much larger than in C60.



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[hal-00012831] Determination of the magnetic anisotropy axes of single-molecule magnets

Simple methods are presented allowing the determination of the magnetic anisotropy axes of a crystal of a single-molecule magnet (SMM). These methods are used to determine an upper bound of the easy axis tilts in a standard Mn12-Ac crystal. The values obtained in the present study are significately smaller than those reported in recent high frequency electron paramagnetic resonance (HF-EPR) studies which suggest distributions of hard-axes tilts.



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[hal-01136651] Whittle Index Policy for Crawling Ephemeral Content

We consider a task of scheduling a crawler to retrieve content from several sites with ephemeral content. A user typically loses interest in ephemeral content, like news or posts at social network groups, after several days or hours. Thus, development of timely crawling policy for such ephemeral information sources is very important. We first formulate this problem as an optimal control problem with average reward. The reward can be measured in the number of clicks or relevant search requests. The problem in its initial formulation suffers from the curse of dimensionality and quickly becomes intractable even with moderate number of information sources. Fortunately, this problem admits a Whittle index, which leads to problem decomposition and to a very simple and efficient crawling policy. We derive the Whittle index and provide its theoretical justification.



from HAL : Dernières publications http://ift.tt/1IdGt3s

[hal-00012910] Mesoscopic Spin Tunneling in Molecular Crystals

The phenomenon of Quantum Tunneling of Mesoscopic Spins is reviewed in the light of the behavior of the archetype of these systems: the molecular complex Mn12-ac. Most observations can be understood in the framework of the reduced Hilbert space dimension 2S+1=21. Due to the large spin S=10, the energy barrier preventing spin rotation is large, and as a consequence, quantum relaxation is very slow. The application of a magnetic field of a few Tesla below 1 K allows to observe tunneling (i) between the states m=-10 and m=10-n with n=8 to 11 if the field is longitudinal, or (ii) between the two ground-states m~-10 and m~10 if the field is transverse. The crossover temperature between ground-state and thermally assisted tunneling in a longitudinal field extrapolates in zero field at ~1.7 K. The observation of square root relaxation at short-times/low-temperatures and of exponential relaxation at long-times/high-temperatures, as observed previously above 1.5 K, confirms the important role of the spin bath dynamics which is out of equilibrium in the first regime and at equilibrium in the second one. In a second part of this paper a new molecule, so-called V15, with resultant spin S=1/2 is investigated. Contrary to high spin molecules, the energy barrier of low spin molecules is small or null, and the splitting between the symmetrical and anti-symmetrical states is sufficiently large to allow spin-phonon transitions during spin rotation. In low spin molecules the coupling to the environment is quite different from the one found in large spin molecules in low fields.



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[hal-01136656] Broad band setup for magnetic field-induced domain wall motion in cylindrical nanowires

In order to improve the precision of domain wall dynamics measurements, we develop a coplanar waveguide-based setup where the domain wall motion should be triggered by pulses of magnetic field. The latter are produced by the Oersted field of the waveguide as a current pulse travels toward its termination, where it is dissipated. Our objective is to eliminate a source of bias in domain wall speed estimation while optimizing the field amplitude. Here, we present implementations of this concept for magnetic force microscopy (MFM) and synchrotron-based investigation.



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[tel-01135694] Stratégie de fusion pour des signaux écrits et sonores

L’être humain dans sa quête de mise en œuvre d’un dialogue le plus naturel possible avec sa machine s’inspire continuellement de la machine la plus perfectionnée connue à ce jour : l’être humain lui-même. Une caractéristique forte du dialogue entre humains est le recours à la multi-modalité. Le travail rapporté dans ce manuscrit porte sur l’étude, la conception et la validation d’un système de reconnaissance des expressions mathématiques, classe particulière de structures bidimensionnelles. Ce système est développé dans un cadre bimodal où l’on considère de façon complémentaire l’écriture manuscrite et la parole. La complémentarité qui existe entre ces deux modalités a été vérifiée et exploitée à profit dans notre système, d’abord dans un cadre simplifié qui est celui de la reconnaissance des symboles mathématiques isolés, puis dans un cadre plus général et plus réaliste, celui des expressions mathématiques complètes. La mise en place de ce système bimodal et sa validation requérant la disponibilité de données bimodales, nous avons collecté, complètement annoté et mis à disposition une base, nommée HAMEX, contenant 4350 expressions bimodales couvrant différents domaines. Nous montrons comment utiliser la transcription automatique de la dictée d’une expression pour guider le système de reconnaissance du signal manuscrit pour obtenir des résultats supérieurs au système monomodal. Les performances de ce système s’avèrent être meilleures que celles d’un système mono-modal basé uniquement sur le signal manuscrit seul.



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[hal-01136079] CEDIMS: cloud ethical DICOM image Mojette storage

Dicom images of patients will necessarily been stored in Clouds. However, ethical constraints must apply. In this paper, a method which provides the two following conditions is presented: 1) the medical information is not readable by the cloud owner since it is distributed along several clouds 2) the medical information can be retrieved from any sufficient subset of clouds In order to obtain this result in a real time processing, the Mojette transform is used. This paper reviews the interesting features of the Mojette transform in terms of information theory. Since only portions of the original Dicom files are stored into each cloud, their contents are not reachable. For instance, we use 4 different public clouds to save 4 different projections of each file, with the additional condition that any 3 over 4 projections are enough to reconstruct the original file. Thus, even if a cloud is unavailable when the user wants to load a Dicom file, the other 3 are giving enough information for real time reconstruction. The paper presents an implementation on 3 actual clouds. For ethical reasons, we use a Dicom image spreaded over 3 public clouds to show the obtained confidentiality and possible real time recovery.



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[hal-00012830] Comment on 'Quantum Coherence between High Spin Superposition States of Single Molecule Magnet Ni4

In a recent paper, cond-mat/0405331 (Ref. 1), del Barco et al. reported experimental studies on a Ni4 molecular system. They used an experimental method (combining microwave spectroscopy with high sensitivity magnetic measurements) that we have introduced before, cond-mat/0404410 (see also Ref. 2 and 3). Among other things, our technique allows us to monitor spin-state populations in the presence of microwave magnetic fields. Absorption line-widths give rough 'upper bounds' on the decoherence rate similar to 'standard' high frequency electron paramagnetic resonance (HF-EPR) techniques. In the case of quasi continuous radiation our technique does NOT give directly the spin-lattice relaxation time T1. For measurements like those of del Barco et al., it simply gives access to the phonon-bottleneck time, a parameter that is many orders of magnitude longer than the spin-lattice relaxation time. Any conclusion concerning quantum coherence is preliminary.



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[hal-00012829] Comment on 'Pulsed field studies of the magnetization reversal in molecular nanomagnets

In a recent paper, cond-mat/0404041, J. Vanacken et al. reported experimental studies of crystals of Mn12-ac molecular nanomagnets in pulsed magnetic fields with sweep rates up to 4000 T/s. Steps in the magnetization curve were observed. The data were explained by collective dipolar relaxation. We give here an alternative explanation that is based on thermal avalanches triggered by defect molecules (faster relaxing species). These species are always present in Mn12-ac molecular nanomagnets. We propose a simple method to test this interpretation. Note that we do not question the possibility of collective effects that are bassed on spin--spin interactions.



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[hal-00012825] Quantum dynamics of exchange biased Single Molecule Magnets

We present a new family of exchange biased Single Molecule Magnets in which antiferromagnetic coupling between the two components results in quantum behaviour different from that of the individual SMMs. Our experimental observations and theoretical analysis suggest a means of tuning the quantum tunnelling of magnetization in SMMs. See also: W. Wernsdorfer, N. Aliaga-Alcalde, D. Hendrickson, G. Christou, Nature 416 (2002) 406.



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[hal-00743042] Semi-Lagrangian discontinuous Galerkin schemes for some first and second order partial differential equations

Explicit, unconditionally stable, high order schemes for the approximation of some first and second order linear, time-dependent partial differential equations (PDEs) are proposed. The schemes are based on a weak formulation of a semi-Lagrangian scheme using discontinuous Galerkin (DG) elements. It follows the ideas of the recent works of Crouseilles, Mehrenberger and Vecil (2010), Rossmanith and Seal (2011), for first order equations, based on exact integration, quadrature rules, and splitting techniques for the treatment of two-dimensional PDEs. For second order PDEs the idea of the scheme is a blending between weak Taylor approximations and projection on a DG basis. New and sharp error estimates are obtained for the fully discrete schemes and for variable coefficients. In particular we obtain high order schemes, unconditionally stable and convergent, in the case of linear first order PDEs, or linear second order PDEs with constant coefficients. In the case of non-constant coefficients, we construct, in some particular cases, "almost" unconditionally stable second order schemes and give precise convergence results. The schemes are tested on several academic examples.



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[hal-00012823] Spin-spin cross-relaxation in single-molecule magnets

It is shown that dipolar and weak superexchange interactions between the spin systems of single-molecule magnets (SMM) play an important role in the relaxation of magnetization. These interactions can reduce or increase resonant tunneling. The one-body tunnel picture of SMMs is not always sufficient to explain the measured tunnel transitions. We propose to improve the picture by including also two-body tunnel transitions such as spin-spin cross-relaxation (SSCR). A Mn4 SMM is used as a model system to study the SSCR which plays also an important role for other SMMs like Mn12 or Fe8. At certain external fields, SSCRs can lead to quantum resonances which can show up in hysteresis loop measurements as well defined steps. A simple model allows us to explain quantitatively all observed transitions. Including three-body transitions or dealing with the many-body problem is beyond the slope of this paper.



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