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École doctorale Physique de la région parisienne (....-2013)

Overview
Works: 95 works in 96 publications in 2 languages and 96 library holdings
Roles: Other
Publication Timeline
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Most widely held works by École doctorale Physique de la région parisienne (....-2013)
Etude à l'échelle atomique de l'ordre local et de la structure électronique aux interfaces d'une jonction tunnel magnétique métal / oxyde : du dispositif fonctionnel à un système modèle by Katia March( Book )

2 editions published in 2008 in French and held by 2 WorldCat member libraries worldwide

The work detailed in this thesis represents an experimental contribution to the development of a new magnetic random access memory (MRAM) in which the main element is a magnetic tunnel junction (MTJ). Interfaces play a crucial role in determining the transport properties and magnetic coupling in these systems, which consist of layers having thicknesses on the order of nanometers. As such, investigation at the nanometer or even atomic scale is rnandatory. This is achieved here using spatially resolved electron energy loss spectroscopy (BELS), the dominant technique in this study. The purpose of the investigation on ALTIS Semiconductor functional industrial systems is to evaluate their stability over time as well as during the steps involved in their production. This work highlights effects restricting their industrial production and suggests solutions to counteract them. In parallel, in order to better understand the importance of the MTJ interfaces themselves, a monocrystalline Co₀,₆Fe₀,₄ / MgO / Co₀,₆Fe₀,₄ (001) system, deposited using molecular beam epitaxy, was also investigated. The Co₀,₆Fe₀,₄ alloy exhibits a half-metallic character with regards to the symmetries of the Bloch states which lead to strong spin polarization. The very high quality of the crystalline structure at the interfaces offers the possibility to consider the sample as an ideal system, allowing a detailed analysis of the fine structure of the transition metal L-edges and the oxygen K-edge. These experimental spectra can thus be simulated using multiple scattering calculations in order to test the influence of the local environment and to characterize the p-d hybridization effects at the interfaces
Dynamique quantique hors-équilibre et systèmes désordonnés pour des atomes ultrafroids bosoniques by Bruno Sciolla( )

1 edition published in 2012 in English and held by 1 WorldCat member library worldwide

The fast progress of cold atoms experiments in the last decade has allowed to explore new aspects of strongly correlated systems. This thesis deals with two such general themes: the out of equilibrium dynamics of closed quantum systems, and the impact of disorder on strongly correlated bosons at zero temperature. Among the different questions about out of equilibrium dynamics, the phenomenon of dynamical transition is still lacking a complete understanding. The transition is typically signalled, in mean-field, by a singular behaviour of observables as a function of the parameters of the quench. In this thesis, a mean field method is developed to give evidence of a strong link between the quantum phase transition at zero temperature and the dynamical transition. We then study using field theory techniques a relativistic O(N) model, and show that the dynamical transition bears similarities with a critical phenomenon. In this context, the dynamical transition also appears to be formally related to the dynamics of symmetry breaking. The second part of this thesis is about the disordered Bose-Hubbard model and the nature of its phase transitions. We use and extend the cavity mean field method, introduced by Ioffe and Mezard to obtain analytical results from the quantum cavity method and the replica trick. We find that the conventional transition, with power law scaling, is changed into an activated scaling in some regions of the phase diagram. Furthermore, the critical exponents are continuously varying along the conventional transition. These intriguing properties call for further investigations using different methods
Decentralized network control, optimization and random walks on networks by Caterina De Bacco( )

1 edition published in 2015 in English and held by 1 WorldCat member library worldwide

In the last years several problems been studied at the interface between statistical physics and computer science. The reason being that often these problems can be reinterpreted in the language of physics of disordered systems, where a big number of variables interacts through local fields dependent on the state of the surrounding neighborhood. Among the numerous applications of combinatorial optimisation the optimal routing on communication networks is the subject of the first part of the thesis. We will exploit the cavity method to formulate efficient algorithms of type message-passing and thus solve several variants of the problem through its numerical implementation. At a second stage, we will describe a model to approximate the dynamic version of the cavity method, which allows to decrease the complexity of the problem from exponential to polynomial in time. This will be obtained by using the Matrix Product State formalism of quantum mechanics. Another topic that has attracted much interest in statistical physics of dynamic processes is the random walk on networks. The theory has been developed since many years in the case the underneath topology is a d-dimensional lattice. On the contrary the case of random networks has been tackled only in the past decade, leaving many questions still open for answers. Unravelling several aspects of this topic will be the subject of the second part of the thesis. In particular we will study the average number of distinct sites visited during a random walk and characterize its behaviour as a function of the graph topology. Finally, we will address the rare events statistics associated to random walks on networks by using the large-deviations formalism. Two types of dynamic phase transitions will arise from numerical simulations, unveiling important aspects of these problems. We will conclude outlining the main results of an independent work developed in the context of out-of-equilibrium physics. A solvable system made of two Brownian particles surrounded by a thermal bath will be studied providing details about a bath-mediated interaction arising for the presence of the bath
Surface Plasmon modes revealed by fast electron based spectroscopies : from simple model to complex by Arthur Losquin( )

1 edition published in 2013 in English and held by 1 WorldCat member library worldwide

Surface Plasmons (SP) are elementary excitations mixing electrons and photons at metal surfaces,which can be seen in a classical electrodynamics framework as electromagnetic surface eigenmodesof a metal-dielectric system. The present work bases on the ability of mapping SP eigenmodes withnanometric spatial resolution over a broad spectral range using spatially resolved fast electron basedspectroscopies in a Scanning Transmission Electron Microscope (STEM). Such an ability has beenseparately demonstrated during the last few years by many spatially resolved experiments of ElectronEnergy Loss Spectroscopy (EELS), which measures the energy lost by fast electrons interactingwith the sample, and CathodoLuminescence (CL), which measures the energy released by subsequentlyemitted photons. In the case of EELS, the experimental results are today well accountedfor by strong theory elements which tend to show that the quantity measured in an experiment canbe safely interpreted in terms of the surface eigenmodes of the sample. In order to broaden thisinterpretation to fast electron based spectroscopies in general, I have performed combined spatiallyresolved EELS and CL experiments on a simple single nanoparticle (a gold nanoprism). I have shownthat EELS and CL results bear strong similarities but also slight differences, which is confirmed bynumerical simulations. I have extended the theoretical analysis of EELS to CL to show that CLmaps equally well than EELS the radiative surface eigenmodes, yet with slightly different spectralfeatures. This work is a proof of principle clarifiying the quantities measured in EELS and CL,which are shown to be respectively some nanometric equivalent of extinction and scattering spectroscopieswhen applied to metal-dielectric systems. Based on this interpretation, I have applied EELSto reveal the SP eigenmodes of random metallic media (in our case, semicontinuous metal films beforethe percolation threshold). These SP eigenmodes constitute a long standing issue in nanooptics.I have directly identified the eigenmodes from measurements and data processing. I havefully characterized these eigenmodes experimentally through an electric field intensity pattern, aneigenenergy and a relaxation rate. Doing so, I have shown that the fractal geometry of the medium,which grows towards the percolation, induces random-like eigenmodes in the system at low energies.Keywords: Surface plasmons, fast electron based spectroscopies, scanning transmission electronmicroscopy, disordered media
Quelques aspects du chaos quantique dans les systèmes de N-corps en interaction : chaînes de spins quantiques et matrices aléatoires by Yasar Yilmaz Atas( )

1 edition published in 2014 in French and held by 1 WorldCat member library worldwide

My thesis is devoted to the study of some aspects of many body quantum interacting systems. In particular we focus on quantum spin chains. I have studied several aspects of quantum spin chains, from both numerical and analytical perspectives. I addressed especially questions related to the structure of eigenfunctions, the level densities and the spectral properties of spin chain Hamiltonians. In this thesis, I first present the basic numerical techniques used for the computation of eigenvalues and eigenvectors of spin chain Hamiltonians. Level densities of quantum models are important and simple quantities that allow to characterize spectral properties of systems with large number of degrees of freedom. It is well known that the level densities of most integrable models tend to the Gaussian in the thermodynamic limit. However, it appears that in certain limits of coupling of the spin chain to the magnetic field and for finite number of spins on the chain, one observes peaks in the level density. I will show that the knowledge of the first two moments of the Hamiltonian in the degenerate subspace associated with each peak give a good approximation to the level density. Next, I study the statistical properties of the eigenvalues of spin chain Hamiltonians. One of the main achievements in the study of the spectral statistics of quantum complex systems concerns the universal behaviour of the fluctuation of measure such as the distribution of spacing between two consecutive eigenvalues. These fluctuations are very well described by the theory of random matrices but the comparison with the theoretical prediction generally requires a transformation of the spectrum of the Hamiltonian called the unfolding procedure. For many-body quantum systems, the size of the Hilbert space generally grows exponentially with the number of particles leading to a lack of data to make a proper statistical study. These constraints have led to the introduction of a new measure free of the unfolding procedure and based on the ratio of consecutive level spacings rather than the spacings themselves. This measure is independant of the local level density. By following the Wigner surmise for the computation of the level spacing distribution, I obtained approximation for the distribution of the ratio of consecutive level spacings by analyzing random 3x3 matrices for the three canonical ensembles. The prediction are compared with numerical results showing excellent agreement. Finally, I investigate eigenfunction statistics of some canonical spin-chain Hamiltonians. Eigenfunctions together with the energy spectrum are the fundamental objects of quantum systems: their structure is quite complicated and not well understood. Due to the exponential growth of the size of the Hilbert space, the study of eigenfunctions is a very difficult task from both analytical and numerical points of view. I demonstrate that the groundstate eigenfunctions of all canonical models of spin chain are multifractal, by computing numerically the Rényi entropy and extrapolating it to obtain the multifractal dimensions
Entre métal et isolant : dynamique ultrarapide dans l'isolant topologique Bi2Te3 et domaines microscopiques à la transition De Mott Dans V203 by Mahdi Hajlaoui( )

1 edition published in 2013 in English and held by 1 WorldCat member library worldwide

This thesis presents the study of metal-insulator coexistence in two very different systems for the scientific community of condensed matter: the 3D topological insulator Bi2Te3 and the prototype compound of the Mott transition V2O3. Both systems were studied by techniques based on photoelectron spectroscopy. The first technique is the TR- ARPES (time and angle resolved photoemission spectroscopy), with a temporal resolution of 80 fs, applied to the 3D topological insulator Bi2Te3 to distinguish the ultrafast dynamics of metallic surface states from that of the insulating bulk states. This allows us to understand the different mechanisms of scattering between the surface and the bulk, as well as the amelioration on the Dirac cone relaxation due to the preexistence of subsurface band bending. The second technique used in this thesis is the SPEM (scanning photoelectron microscopy), with a spatial resolution of 150 nm, which was used to study the coexistence of metallic and insulating domains at the Mott transition on V2O3. This coexistence takes its origin from the first order character of the transition. The measurement shows the metal-insulator coexistence on the Cr-doped: metal domains are due to nucleation centers < 150 nm and the shape of the domains is clearly linked to the shape of the cleavage steps
Thermoelectric conversion in disordered nanowires by Riccardo Bosisio( )

1 edition published in 2014 in English and held by 1 WorldCat member library worldwide

This thesis is focused on thermoelectric conversion in disordered semiconductor nanowires in the field effect transistor configuration. We first consider a low temperature regime, when electronic transport is elastic. For a 1D Anderson model, we derive analytical expressions describing the typical thermopower of a single nanowire as a function of the applied gate voltage, and we show that it is largely enhanced at the nanowire band edges. Our results are confirmed by numerical simulations based on a Recursive Green Function calculation of the thermopower. We then consider the case of inelastic transport, achieved by phonon-assisted hopping among localized states (Variable Range Hopping). By solving numerically the Miller Abrahams random resistor network, we show that the thermopower can attain huge values when the nanowire band edges are probed. A percolation theory by Zvyagin extended to nanowires allows to qualitatively describe our results. Also, the mechanism of heat exchange between electrons and phonons at the band edges lead to the generation of hot and cold spots near the boundaries of a substrate. This effect, of interest for cooling issues in microelectronics, is showed for a set of parallel nanowires, a scalable and hence promising system for practical applications. The power factor and figure of merit of the device are also estimated.Finally, we characterize a general three-terminal system within the linear response (Onsager) formalism: we derive local and non-local transport coefficients, as well as generalized figures of merit. The possibility of improving the performance of a generic quantum machine is discussed with the help of two simple examples
Imagerie par nappe laser de l'activité neuronale dans l'ensemble du cerveau d'un poisson-zèbre by Thomas Panier( )

1 edition published in 2014 in French and held by 1 WorldCat member library worldwide

One third of the manuscript deals with the design of a set-up to deliver tactile stimulation to a human subject. Using microneurography, we recorded the neural response of the mecanoreceptors during a controlled stimulation. The set-up provides precise control over the position, the orientation, the speed and the loading force of a stimulus that we machined on a micro-mill. Thus we can study the transduction of the information, from initially contained in the surface geometry up to the neural coding. The second part shows the construction of Single Plane Illumination Microscope designed to record the neuronal activity of a zebrafish larva. We aim at recording the whole chain of transmission from the sensor to the brain. The fish are genetically modified to express the GCaMP3 calcium indicator in each of their neurons. With the SPIM, where the excitation light comes from the side of the specimen in the shape of a lightsheet and the observation is made from the top at a 90 degrees angle, we can increase the frame rate and the size of the field of view simultaneously, compared to usual techniques (confocal or 2-photons microscopes). These improvements are used to study the correlations between signals from neurons distributed all over the larva?s brain, and discover the underlying networks
Synchronization of spin trasnsfer nano-oscillators by Abbass Hamadeh( )

1 edition published in 2014 in English and held by 1 WorldCat member library worldwide

Spin transfer nano-Oscillators (STNOs) are nanoscale devices capable of generating high frequency microwave signals through spin momentum transfer. Although they offer decisive advantages compared to existing technology (spectral agility, integrability, etc.), their emitted power and spectral purity are quite poor. In view of their applications, a promising strategy to improve the coherence and increase the emitted microwave power of these devices is to mutually synchronize several of them. A first step is to understand the synchronization of a single STNO to an external source. For this, we have studied a circular nanopillar of diameter 200~nm patterned from a Cu60|Py15|Cu10|Py4|Au25 stack, where thicknesses are in nm. In the saturated state (bias magnetic field > 0.8 T), we have identified the auto-Oscillating mode and its coupling to an external source by using a magnetic resonance force microscope (MRFM). Only the uniform microwave field applied perpendicularly to the bias field is efficient to synchronize the STNO because it shares the spatial symmetry of the auto-Oscillation mode, in contrast to the microwave current passing through the device. The same sample was then studied under low perpendicular magnetic field, with the two magnetic layers in the vortex state. In this case, it is possible to excite a highly coherent mode (F/∆F>15000) with a linewidth below 100 kHz. By analyzing the harmonic content of the spectrum, we have determined that the non-Linear amplitude-Phase coupling of the excited mode is almost vanishing, which explains the high spectral purity observed. Moreover, the oscillation frequency can still be widely tuned thanks to the Oersted field created by the dc current. We have also shown that the synchronization of this mode to a microwave field source is very robust, the generation linewidth decreasing by more than five orders of magnitude compared to the autonomous regime. From these findings we conclude that the magneto-Dipolar interaction is promising to achieve mutual coupling of vortex based STNOs, the dipolar field from a neighboring oscillator playing the role of the microwave source. We have thus experimentally measured a system composed of two STNOs laterally separated by 100 nm. By varying the different configurations of vortex polarities, we have observed the mutual synchronization of these two oscillators
Tensorial methods and renormalization in Group Field Theories by Sylvain Carrozza( )

1 edition published in 2013 in English and held by 1 WorldCat member library worldwide

In this thesis, we study the structure of Group Field Theories (GFTs) from the point of view of renormalization theory.Such quantum field theories are found in approaches to quantum gravity related to Loop Quantum Gravity (LQG) on the one hand,and to matrix models and tensor models on the other hand. They model quantum space-time, in the sense that their Feynman amplitudes label triangulations, which can be understood as transition amplitudes between LQG spin network states. The question of renormalizability is crucial if one wants to establish interesting GFTs as well-defined (perturbative) quantum field theories, and in a second step connect them to known infrared gravitational physics. Relying on recently developed tensorial tools, this thesis explores the GFT formalism in two complementary directions. First, new results on the large cut-off expansion of the colored Boulatov-Ooguri models allow to explore further a non-perturbative regime in which infinitely many degrees of freedom contribute. The second set of results provide a new rigorous framework for the renormalization of so-called Tensorial GFTs (TGFTs) with gauge invariance condition. In particular, a non-trivial 3d TGFT with gauge group SU(2) is proven just-renormalizable at the perturbative level, hence opening the way to applications of the formalism to (3d Euclidean) quantum gravity
Edge states in Chern Insulators and Majorana fermions in topological superconductors by Doru Sticlet( )

1 edition published in 2012 in English and held by 1 WorldCat member library worldwide

Mécanismes moléculaires de la friction aux interfaces polymères souples by Celine Cohen( )

1 edition published in 2011 in French and held by 1 WorldCat member library worldwide

En dépit de leur importance pratique considérable, et bien que de nombreuses expériences établissent une corrélation certaine entre les hétérogénéités d'interaction de surface (rugosité ou inhomogénéités chimiques) et les propriétés de friction des surfaces, le rôle de ces interactions sur la friction n'est encore pas bien décrit par les modèles et les expériences existants. Dans ce travail de thèse, nous nous sommes intéressés à l'identification des mécanismes moléculaires de la friction aux interfaces polymères souples. Dans ce contexte, nous avons réalisé deux études complémentaires. La première partie du travail concerne le mouvement d'une ligne triple solide-liquide-vapeur qui se déplace sur une surface solide sous l'effet de différentes forces (gravité, forces capillaires et tensions interfaciales), et en particulier le lien entre le piégeage et le dépiégeage de la ligne triple et l'hystérèse de l'angle de contact. Cette méthode permet de mesurer des angles de contact d'avancée et de reculée avec une précision sans précédent (0,1°)et s'avère être particulièrement sensible aux mécanismes qui tendent à ancrer la ligne triple. Ceci en fait un outil de choix pour étudier la friction liquide/solide. Dans la seconde partie du travail, nous avons cherché à comprendre comment des chaînes de polymère flexibles, fortement ancrées sur une surface solide, dans le régime des fortes densités de greffage affectent la friction entre une telle surface et un élastomère réticulé constitué du même polymère. Nous avons montré que le comportement en friction de cette couche confinée suit exactement le comportement rhéofluidifiant observé pour des couches de fondu de masses molaires équivalentes mais avec un temps de relaxation beaucoup plus long que celui des chaînes en fondu,la reptation n'étant pas permise pour les chaînes ancrées. Enfin, en comparant les résultats obtenus pour des couches greffées chimiquement à une extrémité et des couches fortement adsorbées, ayant par ailleurs les mêmes caractéristiques moléculaires (masse molaire des chaînes et épaisseur de la couche ancrée), nous avons mis en évidence que la friction est remarquablement sensible à l'organisation moléculaire au sein de la couche ancrée
Quantum Shot Noise in Graphene by Andrey Mostovov( )

1 edition published in 2014 in English and held by 1 WorldCat member library worldwide

We have conducted an experimental study of the quantum shot noise in a mono-layer graphene device. Conductance of the device and the quantum Hall effect were also investigated. A theoretical model, describing conductance and quantum shot noise in ideal (ballistic) graphene was proposed by Tworzydlo et al., 2006. In diffusive graphene, that is much easier achievable experimentally, shot noise was investigated numerically by several authors (San-Jose et al., 2007, Lewenkopf et al., 2008, Logoteta et al., 2013). Conclusions of the first experimental works (DiCarlo et al., 2008 and Danneau et al., 2008), addressing this problem, didn't lead to an enough broad understanding of it and a further investigation was required. In our experiment we intended to maximally reduce the contributions of the measurement system to the detected signal by performing four-point voltage noise measurement as well as by using cross-correlation detection. In addition to that, our measurement system include home-made cryogenic low-noise amplifiers combined with band-pass filters, while our experimental device carries a constriction in the center of graphene layer and side-gates are used instead of back-gate. First, using the results of the conductance and of the quantum Hall effect measurements we determined the mean free path in our sample and concluded that it was in diffusive regime. The extracted values of the Fano factor show a good agreement with the above-mentioned simulations for this regime, in particular, the peak at Dirac point, predicted by Lewenkopf et al., was observed. Moreover our results are consistent with those of Danneau et al. and DiCarlo et al
Synthèse et étude des propriétés mésomorphes d'espaceurs pour la fonctionnalisation des nanotubes de carbone by Loïc Louise( )

1 edition published in 2012 in French and held by 1 WorldCat member library worldwide

Synthesis and study of mesomorphic properties of grafts for carbon nanotubes functionalizationIn order to study the toxicity of carbon nanotubes, cholesterol based derivatives were synthesized. Due to their exceptional properties, a wide field of applications is opened for carbon nanotubes. Regarding this field, knowing their toxicity becomes a key issue.The work presented here aims towards the synthesis of chemical derivatives designed to increase interactions between carbon nanotubes and a biological environment. The synthesis of a series of cholesterol derivatives aimed for the functionalization of carbon nanotubes has been developed. Such derivatives are intended to interact with cell membranes.Moreover, many synthetic intermediates have mesomorphic properties. These properties have been studied using different methods such as polarized optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray scattering (MWAXS).HiPco nanotubes have been purified, functionalized and characterized with techniques such as Raman spectroscopy and X-ray diffraction (XRD).Finally, a study of carbon nanotubes toxicity is presented
Fluctuations quantiques et effets non-linéaires dans les condensats de Bose-Einstein : des ondes de choc dispersives au rayonnement de Hawking acoustique by Pierre-Élie Larré( )

1 edition published in 2013 in French and held by 1 WorldCat member library worldwide

This thesis is devoted to the study of the analogue of Hawking radiation in Bose-Einstein condensates. The first chapter presents new configurations of experimental interest making it possible to realize the acoustic equivalent of a gravitational black hole in the flow of a one-dimensional atomic condensate. In each case we give an analytical description of the flow pattern, the associated quantum fluctuations, and the spectrum of Hawking radiation. Analysis of the two-body density correlations in position and momentum space emphasizes the occurrence of signals revealing the Hawking effect in our systems. By demonstrating a sum rule verified by the connected two-body density matrix we show that the long-range density correlations have to be associated to the diagonal modifications of the two-body density matrix when the flow of the condensate presents an acoustic horizon. Motivated by recent experimental studies of wave patterns generated in semiconductor microcavity polariton condensates we analyze in a second chapter superfluid and dissipative characteristics of the flow of a nonresonantly pumped one-dimensional polariton condensate past a localized obstacle. We examine the response of the condensate in the weak-perturbation limit and by means of Whitham theory in the nonlinear regime. We identify a time-dependent regime separating two types of stationary and dissipative flow: a mostly viscous one at low velocity and another one characterized by Cherenkov radiation of density waves at large velocity. Finally we present polarization effects obtained by including the spin of polaritons in the description of the condensate and show in the third chapter that similar effects in the presence of an acoustic horizon could be used to experimentally demonstrate Hawking radiation in polariton condensates
Limites quantiques dans les mesures de distance à l'aide de peignes de fréquences by Pu Jian( )

1 edition published in 2014 in English and held by 1 WorldCat member library worldwide

In many fields of physics, the determination of the space-time position of an object is performed at high levels of accuracy and precision by the exchange of light pulses. In this thesis, we investigate the quantum limits in a parameter estimation scheme using light in a practical point of view, and we study how these limits apply in a range-finding scheme using optical frequency combs. In a first part, we study the quantum limits in a general parameter estimation problem by the means of the quantum Cramér-Rao bound. We focus on schemes involving multimode Gaussian states and derive the limits of sensitivity in the estimation of any parameter encoded in such states. We show that a simple experimental setup allows to optimally measure the parameter carried by the light. In a second part, we study how these limits apply in a range-finding protocol using optical frequency combs. In a well-controlled environment such as vacuum, we show that there exists an optimal scheme, requiring pulse shaping techniques, which sensitivity is better than the usual phase interferometry and time-of-flight measurements. We present experimental results that exhibits the standard quantum limit in space-time positioning. In the last part, we study the limitations introduced to this optimal scheme when the environment is weakly dispersive, like in air. We demonstrate that the loss of accuracy caused by such environmental fluctuations can be compensated at the expense of a reduced sensitivity. We propose an experimental scheme that allows to perform a real-time ranging measurement immune from atmospheric perturbations, without any knowledge of the values of the environmental parameters
Quantum Hall effect in graphene for resistance metrology : Disorder and quantization by Fabien Lafont( )

1 edition published in 2015 in English and held by 1 WorldCat member library worldwide

L'effet Hall quantique (EHQ) apparaissant dans des gaz bidimentionnels d'électrons places à basse température et sous fort champ magnétique a révolutionné la métrologie des résistances depuis sa découverte en 1980 par Klaus von Klitzing. Cet effet apporte une représentation de l'ohm uniquement basé sur la constante de Planck et la charge de l'électron. En 2004, le graphène, un arrangement purement bi-dimensionnel d'atomes de carbone en nid d'abeille, dans lequel les porteurs de charge se comportent comme des fermions de Dirac, a permis de mettre à jour une nouvel effet Hall quantique. Du point de vue de la métrologie des résistances l'EHQ dans le graphène est très prometteur car plus robuste que celui apparaissant dans les hétérostructures semi-conductrices. Ceci pourrait mener à la création d'un étalon de résistance plus pratique, fonctionnant à plus haute température et plus faible champ magnétique ce qui serait un avantage notable pour une dissémination accrue d'un étalon de résistance précis vers les acteurs industriels. Dans ce manuscrit une étude complète de l'impact des défauts linéaires, omniprésent dans le graphène crû par dépôt chimique en phase vapeur sur métal, dans le régime d'effet Hall quantique est menée. Nous avons montré que ces défauts linéaires mènent à des processus de dissipation non-conventionnels qui viennent altérer la quantification de la résistance de Hall. Cette étude pointe vers l'utilisation de monocristaux pour les prochaines investigations du graphène CVD pour une application en métrologie des résistances. La deuxième partie de ce manuscrit est dédiée à l'étude du graphène crû par dépôt chimique en phase vapeur sur carbure de silicium. Nous avons comparé précisément la résistance de Hall d'un échantillon de graphène entre 10 et 19 T à la température de 1.4 K avec celle donnée par un étalon de résistance en GaAs/AlGaAs avec une incertitude relative de ( -2 ± 4 ) × 10⁻¹⁰. Pour la première fois un étalon de résistance en graphène a pu fonctionner dans les mêmes conditions de température et de champs magnétique que celui fabriqué en GaAs/AlGaAs et de plus sur un intervalle de champ magnétique plus de dix fois plus grand. Nous avons également étudié les processus de dissipation apparaissant dans cet échantillon de graphène. Cette étude montre que la longueur de localisation des porteurs de charge sature à une valeur proche de l'extension de la fonction d'onde et ce sur une grande plage de champs magnétique, ce qui soulève des questions intéressantes concernant le désordre présent dans ce type de graphène. Finalement dans un second échantillon provenant de la même technique de fabrication nous avons comparé précisément la résistance de Hall de l'échantillon de graphène avec celle d'un étalon de résistance en GaAs/AlGaAs. Il apparait que la résistance de Hall dans l'échantillon de graphène est quantifié avec une précision métrologique pour des champs magnétiques allant jusqu'à 3.5 T, des températures atteignant 9 K et reste dans un état non dissipatif jusqu'à des courants de 500 µA. Ceci ouvre une voie directe à la réalisation d'étalons quantiques de résistance réalisés en graphène
Dynamique par transfert de spin et synchronisation d'oscillateurs couplés à base de vortex magnétiques by Nicolas Locatelli( )

1 edition published in 2012 in French and held by 1 WorldCat member library worldwide

My PhD work is dedicated to the spin transfer induced self-sustained dynamics of two coupled vortices, in nano-pillars spin-valves structures (Py/Cu/Py). A first objective was to understand the spin-polarized transport processes as well as spin transfer mechanisms associated to highly non-homogeneous magnetic configurations. This study allows me to identify and then precisely tune the vortex based magnetic configurations, and notably to observe the influence of spin transfer on reversal mechanisms of the vortex core. Combining analytical calculations and micro-magnetic simulations, we determine the conditions on relative parameters for the two vortices (chiralities and polarities) necessary to obtain self-sustained gyrotropic oscillations of the coupled vortices in a single pillar. A very interesting case is predicted for the pillars with larger diameters (typically over 200nm) for which the critical current is reduced to zero. The experimental results confirm the predictions that a coupled dynamics exists with linewidths as narrow as 200kHz, that is a record at zero field (corresponding to a quality factor Q ≈ 5000, an order of magnitude over the self-sustained oscillations of a single vortex), and even down to 50kHz under external field.A second objective was to investigate the synchronization of two vortex based spin transfer oscillators. We demonstrate theoretically that the phase locking through dipolar coupling of two identical oscillators can be achieved for any parameters of the two vortex. However, the coupling is three times stronger when vortices have opposite core polarities. From an experimental point of view, the synchronization capability for two oscillators having a frequency mismatch reaching up to 10 % of the auto-oscillation frequency has been demonstrated. This work, being part of the research effort made to improve the rf properties of spin transfer nano-oscillators emphasizes how the excitation of coupled magnetizations modes is important to reach lower and lower linewidths
Dark Matter Indirect Detection with charged cosmic rays by Gaelle Giesen( )

1 edition published in 2015 in English and held by 1 WorldCat member library worldwide

Overwhelming evidence for the existence of Dark Matter (DM), in the form of an unknownparticle filling the galactic halos, originates from many observations in astrophysics and cosmology: its gravitational effects are apparent on galactic rotations, in galaxy clusters and in shaping the large scale structure of the Universe. On the other hand, a non-gravitational manifestation of its presence is yet to be unveiled. One of the most promising techniques is the one of indirect detection, aimed at identifying excesses in cosmic ray fluxes which could possibly be produced by DM annihilations or decays in the Milky Way halo. The current experimental efforts mainly focus in the GeV to TeV energy range, which is also where signals from WIMPs (Weakly Interacting Massive Particles) are expected. Focussing on charged cosmic rays, in particular antiprotons, electrons and positrons, as well as their secondary emissions, an analysis of current and forseen cosmic ray measurements and improvements on astrophysical models are presented. Antiproton data from PAMELA imposes contraints on annihilating and decaying DM which are similar to (or even slightly stronger than) the most stringent bounds from gamma ray experiments, even when kinetic energies below 10 GeV are discarded. However, choosing different sets of astrophysical parameters, in the form of propagation models and halo profiles, allows the contraints to span over one or two orders of magnitude. In order to exploit fully the power of antiprotons to constrain or discover DM, effects which were previously perceived as subleading turn out to be relevant especially for the analysis of the newly released AMS-02 data. In fact, including energy losses, diffusive reaccelleration and solar modulation can somewhat modify the current bounds, even at large DM masses. A wrong interpretation of the data may arise if they are not taken into account. Finally, using the updated proton and helium fluxes just released by the AMS-02 experiment, the astrophysical antiproton to proton ratio and its uncertainties are reevaluated and compared to the preliminarly reported AMS-02 measurements. No unambiguous evidence for a significant excess with respect to expectations is found. Yet, some preference for thicker halos and a flatter energy dependence of the diffusion coefficient starts to emerge. New stringed constraints on DM annihilation and decay are derived. Secondary emissions from electrons and positrons can also be used to constrain DM annihilation or decay in the galactic halo. The radio signal due to synchrotron radiation of electrons and positrons on the galactic magnetic field, gamma rays from bremsstrahlung processes on the galactic gas densities and from Inverse Compton scattering processes on the interstellar radiation field are considered. With several magnetic field configurations, propagation scenarios and improved gas density maps and interstellar radiation field, state-of-art tools allowing the computaion of synchrotron and bremssttrahlung radiation for any WIMP DM model are provided. All numerical results for DM are incorporated in the release of the Poor Particle Physicist Cookbook for DM Indirect Detection (PPPC4DMID). Finally, the possible GeV gamma-ray excess identified in the Fermi-LAT data from the Galactic Center in terms of DM annihilation, either in hadronic or leptonic channels is studied. In order to test this tantalizing interprestation, a multi-messenger approach is used: first, the computation of secondary emisison from DM with respect to previous works confirms it to be relevant for determining the DM spectrum in leptonic channels. Second, limits from antiprotons severely constrain the DM interpretation of the excess in the hadronic channel, for standard assumptions on the Galactic propagation parameters and solar modulation. However, they considerably relax if more conservative choices are adopted
Stochastic modeling of intracellular processes : bidirectional transport and microtubule dynamics by Maximilian Ebbinghaus( )

1 edition published in 2011 in English and held by 1 WorldCat member library worldwide

Dans cette thèse, des méthodes de la physique statistique hors équilibre sont utilisées pour décrire deux processus intracellulaires. Le transport bidirectionnel sur les microtubules est décrit à l'aide d'un gaz sur réseau stochastique quasi-unidimensionnel. Deux espèces de particules sautent dans des directions opposées en interagissant par exclusion. La présence habituelle d'accumulations de particules peut être supprimée en rajoutant la dynamique du réseau, c'est-à-dire de la microtubule. Un modèle simplifié pour la dynamique du réseau produit une transition de phase vers un état homogène avec un transport très efficace dans les deux directions. Dans la limite thermodynamique, une propriété de l'état stationnaire limite la longueur maximale des accumulations. La formation de voies peut être causée par des interactions entre particules. Néanmoins, ces mécanismes s'avèrent peu robustes face à une variation des paramètres du modèle. Dans presque tous les cas, la dynamique du réseau a un effet positif et bien plus important sur le transport que la formation de voies. Par conséquent, la dynamique du réseau semble un point-clé pour comprendre la régulation du transport intracellulaire. La dernière partie introduit un modèle pour la dynamique d'une microtubule sous l'action d'une protéine qui favorise les sauvetages. Des phénomènes intéressants de vieillissement apparaissent alors, et devraient être observables dans des expériences
 
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Alternative Names
École doctorale 107

ED 107

ED107

Languages
English (13)

French (8)