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Perron, Jonathan. "Diffusion résonante des rayons x mous dans la glace de spins artificielle". Thesis, Paris 6, 2014. http://www.theses.fr/2014PA066562/document.
Pełny tekst źródłaFrustration is a concept in physics which appears when all interactions in a system cannot be satisfied at the same time. Artificial spin ice is an example of a magnetic system that is frustrated by the competition between the dipolar interactions. So far, the properties of this metamaterial has been studied by means of microscopic techniques with the disadvantage of a limited dynamical range. At a time when dimensions of the nanomagnets allow the access to fast dynamics using magnetic fluctuations, the time-resolution of scattering techniques could make them relevant for such studies.Soft x-ray resonant magnetic scattering (SXRMS) is a magnetic-sensitive technique with a resolution compatible with the dimensions of artificial spin ice. The scope of this thesis is to demonstrate that SXRMS can bring relevant information about the magnetic organisation in artificial spin ice. It is organized in two parts. The fabrication of scattering-compatible artificial spin ice samples is described step by step. Then, the studies of both artificial square and kagome spin ice are presented. Signals of magnetic origin have been recorded, bringing insights in the magnetic organization of the system. While most of the studies have been performed on static systems, a small chapter is presenting a serie of studies using non-microscopic techniques which include systems with fluctuating magnetic moments.The different studies performed within this thesis demonstrate that artificial spin ice can not only be studied by microscopy techniques, but that scattering and non-microscopic methods are also relevant. In particular, it opens the way to more advanced experiments such as, in the x-ray domain, the illumination of the system with a coherent beam in order to resolve the nature of the very short magnetic correlations using x-ray photon correlation spectroscopy technique. The presented work is therefore a first step toward the study of dynamics in artificial spin ice
Massouras, Maryam. "Artificial spin ice : from statistical physics to stochastic computing". Electronic Thesis or Diss., Université de Lorraine, 2021. http://www.theses.fr/2021LORR0143.
Pełny tekst źródłaSince their introduction in 2006, artificial spin ice systems have sparked an interest for their similarity with frustrated systems such as water ice or some pyrochlores. These networks of interacting nanomagnets can be observed using simple microscopy techniques and be tailored at will. In this work, we have studied customised geometries in order to observe the evolution of dipolar interactions and develop a device. First, we have studied the tuning of the dipolar interactions induced by rotation of the elements of the square geometry. For an as-grown system that the ground state evolution from antiferromagnetic to ferromagnetic orders observed is consistent with dipolar computations. Taking into account the long-range interactions in our Monte Carlo simulations, we have described the total dipolar energy along with the vertex populations of our system with all its tilted networks. This has led to a study of the thermodynamics of our system and a unique effective could be ascribed. Secondly, we have taken an interest into the possibility of tailoring these artificial spin systems for applications in random number generation. We have seen that these systems are a better fit exploiting domain wall propagation rather than reversals by coupling. We have then established that the main challenge to overcome in large connected structures is the trade-off between nucleation field from the outputs and depinning field of the upper part of the nanostructure. Indeed, it is necessary to ensure than the domain wall propagating in the structure is indeed the injected one and that no parasitic nucleation occurs which is overcome this issue. The domain wall propagation has been characterised in terms of propagation fields and types of reversals. We have shown that our optimised system exhibits reversal induced by the injected domain wall mainly in unidimensional fashion with clear stochastic behaviour. The characterisation of this behaviour has shown that a mean weight value describes the choices observed which do not exhibit any correlation. Our artificial spin system therefore behaves as a Galton board, the prime device for random number generation. This PhD work has shown the versatility of artificial spin systems as spin ice for understanding the influence of tuned interactions in their thermodynamics. This work has also demonstrated their versatility for an application with the characterisation of their stochastic behaviour through an optimisation of the geometry
Chioar, Ioan-Augustin. "Glaces kagomé de spins artificiels : de la dégénérescence à courte-portée vers l'ordre dipolaire". Thesis, Université Grenoble Alpes (ComUE), 2015. http://www.theses.fr/2015GREAY033/document.
Pełny tekst źródłaArtificial spin networks were initially proposed as toy-spin models destined for the investigation of magnetic frustration effects in two-dimensional spin lattices, a complementary approach to the study of the magnetic frustration encountered in spin ice pyrochlores. Generally fabricated via lithography techniques, these arrays of nano-scale magnetic islands can be designed at-will. Given the size and shape of the elements, their magnetization is almost uniform throughout their volume, thus making these islands act like classical Ising spins. Combined with the possibility of individually imaging the magnetic degrees of freedom in real space, these systems offer an almost infinite playground for the investigation of competing interactions in magnetostatic frameworks and potential for the experimental discovery of novel and exotic magnetic phases. However, unlike their condensed matter counterparts, first-generation artificial spin networks are insensitive to thermal fluctuations, requiring other driving mechanisms for accessing their complex low-energy manifolds. A field-protocol has been employed for driving such networks towards their ground-state configurations, although they only partially manage to accommodate pair-island interactions. More recently, thermally-active artificial spin networks have been introduced, surpassing the limits of demagnetized arrays in the quest for exotic low-energy spin textures.This thesis presents experimental and numerical studies performed on artificial kagome spin arrays, one of the most frustrated two-dimensional lattices. The kagome spin ice geometry has received most of the community's attention as it presents highly degenerate manifolds and unconventional spin textures. Within a dipolar long-range framework, it displays a low-temperature regime characterized by the coexistence of a crystalline phase, associated to the magnetic charge, and a disordered spin lattice. While demagnetizing such artificial kagome arrays cannot access this exotic state, thermally-active networks can locally retrieve such a phase, creating crystallites of antiferromagnetically-ordered magnetic charges. The first part of this work presents the experimental protocol employed to this purpose. A kinetic model is also proposed that successfully captures the observed experimental features and explains the efficiency of this approach.The second part of the current thesis presents a study of a novel artificial spin ice system, the artificial kagome Ising network. This network primarily differs from the kagome spin ice array by having its magnetic moments pointing along the vertical axis. A recent study of this system has concluded that, after demagnetization, these two artificial kagome networks display similar pairwise spin correlation development and their final frozen states can be well characterized by short-range interaction models. Through the use of demagnetization protocols, magnetic force microscopy and Monte Carlo simulations, it is demonstrated that long-range dipolar interactions between the magnetic elements cannot be neglected when describing the remanent states of demagnetized artificial kagome Ising networks. These results assess the limits of the reported universal behavior of artificial kagome lattices and enrich the spectrum of magnetic phases that could be achieved with such nanostructured systems. Indeed, Monte Carlo simulations indicate that this kagome Ising network presents a different low-energy behavior than kagome spin ice, the incipient stages of which have been accessed experimentally, but its dipolar ground-state configuration remains unknown. Nevertheless, by inspecting the low-temperature thermodynamic features of this array and through the use of a geometrical construction, a ground-state candidate is provided
Raban, Valentin. "Dynamique hors équilibre des monopôles magnétiques dans la glace de spin". Thesis, Lyon, 2018. http://www.theses.fr/2018LYSEN052/document.
Pełny tekst źródłaSpin ices, such as Dy2Ti2O7 and Ho2Ti2O7, are materials exhibiting exotic magnetic properties. They were the first frustrated ferromagnetic crystalline compounds to be discovered. The frustration leads to the fractionnalisation of the spin degrees of freedom and the emergence of magnetic monopoles, whose physics is formalised in the dumbbell model. In this thesis, we study the full phase diagram of this model in analogy with theS=2 Blume-Capel model. We identify in this diagram the fragmented phase observed experimentally in Ho2Ir2O7, and we localise the critical point of the transition between the spin ice phase and the fragmented phase.In a second part, we show numerically that the dynamics of this system at thecritical point belongs to the 3D Ising university class. We use for this two tools :the Kibble-Zurek scaling law and the fluctuation-dissipation ratio. For the latter, ithas been necessary to introduce a novel method to measure response functions. Wealso emphasize that these tools are specifically interesting for spin ice materials, as the unusually long microscopic time scale (1 μs) should make it possible to experimentallyobserve out-of-equilibrium phenomena related to critical slowing down.In a third part, we use the violation of the fluctuation-dissipation theorem to characterise a strongly out-of-equilibrium regime of spin ice - a thermal quench from high to low temperature, where degrees of freedom are kinetically blocked because ofthe Coulombic attraction between the monopoles
Henry, Louis-Paul. "Glace bidimensionnelle classique et quantique : phases de Coulomb et phases ordonnées". Phd thesis, Ecole normale supérieure de lyon - ENS LYON, 2013. http://tel.archives-ouvertes.fr/tel-00932367.
Pełny tekst źródłaGuitteny, Solène. "Etude de systèmes frustrés par diffusion neutronique : Pr2Zr2o7 et Tb2Ti2o7 sont-ils des glaces de spin quantiques ?" Thesis, Paris 6, 2015. http://www.theses.fr/2015PA066370/document.
Pełny tekst źródłaThis work is the neutron scattering study of the pyrochlores Tb2Ti2O7 and Pr2Zr2O7. These compounds where magnetic ions are Non-Kramers ions are expected to be quantum spin ices. In Pr2Zr2O7, the study of the elastic and inelastic response together with the study of the magnetic structures in applied magnetic field lead to the conclusion that the magnetic ground state is a mixing of the wave functions of the crystal-field ground state doublet because of quadrupolar terms in the Hamiltonian. These terms would originate from a coupling to the lattice. Using the mean-field approximation, a model based on a local structural distortion reproduces quite well our measurements. Despite strong differences with Pr2Zr2O7, our measurements provide evidence for a similar mechanism in Tb2Ti2O7. Again, this would be caused by multipolar terms in the Hamiltonian reflecting a strong coupling of the magnetic moments to the lattice. Then, these pyrochlores would not be quantum spin ices. Instead, the extreme sensibility to the environment characteristic of the Non-Kramers ions would lead to these fluctuations. Our measurements of samples slightly off-stoichiometry emphases the strong reactivity of the magnetic behavior of these compounds
Jaubert, Ludovic D. C. "Topological Constraints and Defects in Spin Ice". Phd thesis, Ecole normale supérieure de lyon - ENS LYON, 2009. http://tel.archives-ouvertes.fr/tel-00462970.
Pełny tekst źródłaHarman-Clarke, Adam. "Contraintes Topologiques et Ordre dans les Systèmes Modèle pour le Magnétisme Frustré". Thesis, Lyon, École normale supérieure, 2011. http://www.theses.fr/2011ENSL0659.
Pełny tekst źródłaIn this thesis a series of model frustrated magnets have been investigated. Their common parent is the spin ice model, which is transformed into the kagome ice and square ice models in two-dimensions, and an Ising spin chain model in one-dimension. These models have been examined with particular interest in the spin ordering transitions induced by constraints on the system: a topological constraint leads, under appropriate conditions, to the Kasteleyn transition in kagome ice and a lattice freezing transition is observed in square ice which is due to a ferromagnetic ordering transition in an Ising chain induced solely by finite size effects. In all cases detailed Monte Carlo computational simulations have been carried out and compared with theoretical expressions to determine the characteristics of these transitions. In order to correctly simulate the kagome ice model a loop update algorithm has been developed which is compatible with the topological constraints in the system and permits the simulation to remain strictly on the groundstate manifold within the appropriate topological sector of the phase space. A thorough survey of the thermodynamic and neutron scattering response of the kagome ice model influenced by an arbitrary in-plane field has led to a deeper understanding of the Kasteleyn transition, and a computational model that can predict neutron scattering patterns for kagome ice materials under any experimental conditions. This model has also been shown to exhibit quantised thermodynamic properties under appropriate conditions and should provide a fertile testing ground for future work on the consequences of topological constraints and topological phase transitions. A combined investigation into the square ice and Ising chain models has revealed ordering behaviour within the lattice that may be decoupled from underlying ferro- magnetic ordering and is particularly relevant to magnetic nanoarrays
Levis, Demian. "Two-dimensional Spin Ice and the Sixteen-Vertex Model". Phd thesis, Université Pierre et Marie Curie - Paris VI, 2012. http://tel.archives-ouvertes.fr/tel-00763350.
Pełny tekst źródłaLevis, Demian. "Two-dimensional spin ice and the sixteen-vertex model". Phd thesis, Paris 6, 2012. http://www.theses.fr/2012PA066414.
Pełny tekst źródłaIn this thesis we present a thorough study of the static and dynamic properties of the 2D sixteen-vertex model or, in other words, a simplified version of the dipolar spin-ice model. After a general discussion on frustrated magnets, and spin-ice in particular, we motivate the introduction of our sixteen-vertex model in order to study the collective behaviour of spin-ice. We use a rejection-free continuous-time Monte Carlo algorithm with local spin-flip updates to analyse the equilibrium phases and the critical properties of the 2D model. We compare our results with the integrable cases. We extend the model to be defined on carefully chosen trees and employ a Bethe-Peierls approximation to study the equilibrium properties of generic vertex models. The range of validity of the approximation is discussed by comparing the results obtained analytically for the model defined on trees with the exact and numerical results obtained for the 2D model. Motivated by advent of artificial spin-ice realisations, we set the parameters of the model in order to reproduce the experimental situation. We show that the sixteen-vertex model gives an accurate description of the thermodynamics of artificial spin-ice samples. Our theoretical results are in quasi-quantitative agreement with experimental data obtained in as-grown samples away from the critical point. The phase diagram of the sixteen-vertex model and the nature of the equilibrium phases is presented in detail. Our vertex model is build as a stochastic extension of the integrable six-vertex model in order to include thermal fluctuations in the form of defects. We study the ordering dynamics of the system following different kind of quenches by means of Monte Carlo simulations. We analyse the evolution of the density of defects and we identify the dynamical mechanisms leading the different ordering processes. We show that the dynamics proceed through coarsening accordingly to the dynamical scaling picture. The interplay between localised and extended topological defects is discussed. We study in detail the existence of a dynamical arrest following a quench as observed in 3D dipolar spin-ice
Rousset, Elodie. "Architectures radiales hétéro-poly-métalliques pour la photosynthèse artificielle et le stockage de l'information". Thèse, Paris 6, 2015. http://hdl.handle.net/1866/13569.
Pełny tekst źródłaBy a supramolecular strategy, radial hetero-poly-metallic architectures were obtained for applications in artificial photosynthesis and molecular magnetism. The first part is devoted to the synthesis, as well as the photophysical characterization (UV-vis absorption, emission, electrochemistry, X-ray diffraction) of ruthenium(II) complexes bearing a wide range of polypyridine ligands. Theoretical calculations were performed to support the interpretation of the photophysical properties. Through their pendant pyridine moieties, these complexes were used as core of architectures, bearing rhenium tris-carbonyl (for antenna effects), and cobaloximes (for catalytic properties) complexes. The nuclearities obtained vary from two to seven according to the core involved. These systems were engaged in photo-production of hydrogen, demonstrating more efficient systems than the reference in the field, the archetypal [Ru(bpy)3]2+. The second part concerns the study of transition metal couples, built on polycyanometallate cores, or oxamide-bridged lanthanide-based assemblies. These “complex as ligand” and “assemblies as ligand” approaches allow us to obtain high nuclearity systems on which we seek single molecule magnet (SMM) properties or good magnetocaloric effect (based on CrNi, GdCu, DyCu). Photo-magnetic properties have been studied on the RuCu and MoCu couples, which can serve as molecular switches in complex systems. Finally, a tri-functional hetero-tetra-metallic architecture was obtained containing a MoCu switch, a CuTb SMM entity, and a ruthenium complex.
Kaiser, Vojtech. "The Wien Effect in Electric and Magnetic Coulomb systems - from Electrolytes to Spin Ice". Thesis, Lyon, École normale supérieure, 2014. http://www.theses.fr/2014ENSL0942/document.
Pełny tekst źródłaA Coulomb gas or fluid comprises charged particles that interact via the Coulomb interaction. Examples of a Coulombic systems include simple and complex electrolytes together with magnetic monopoles in spin ice. The long-range nature of the Coulomb interaction leads to a rich array of phenomena.This thesis is devoted to the study of the non-equilibrium behaviour of lattice based Coulomb gases and of the quasi-particle excitations in the materials known as spin ice which constitute a Coulomb gas of magnetic charges. At the centre of this study lies the second Wien effect which describes the linear increase in conductivity when an electric field is applied to a weak electrolyte. The conductivity increases due to the generation of additional mobile charges via a field-enhanced dissociation from Coulombically bound pairs.The seminal theory of Onsager gave a detailed analysis of the Wien effect. We use numerical simulations not only to confirm its validity in a lattice Coulomb gas for the first time but mainly to study its extensions due to the role of the ionic atmosphere and field-dependent mobility. The simulations also allow us to observe the microscopic correlations underlying the Wien effect.Finally, we look more closely at the emergent gas of monopoles in spin ice—the magnetolyte. The magnetic behaviour of spin ice reflects the properties of the Coulomb gas contained within. We verify the presence of the Wien effect in model spin ice and in the process predict the non-linear response when exposed to a periodic driving field, or to a field quench using Wien effect theory. We use a straightforward extension of the lattice Coulomb gas simulations to refine our predictions. It is a highly unusual result to find an analytic theory for the non-equilibrium behaviour of a highly frustrated system beyond linear response
Chapuis, Yann. "Frustration géométrique, transitions de phase et ordre dynamique". Phd thesis, Grenoble 1, 2009. http://www.theses.fr/2009GRE10130.
Pełny tekst źródłaThis PhD thesis is about geometrically frustrated magnetic materials which crystallise in the pyrochlore structure, made of corner sharing regular tetrahedra. Firstly, we show that the growth of the spin liquid Tb2Ti2O7 presents many difficulties : some physical properties are sample dependent. Anyhow, two characteristic temperatures (2 and 50 K) are deduced by different experimental techniques, irrespective of the sample. Then, about the ordered spin ice Tb2Sn2O7, the study of spin dynamics suggests the existence of a complex relaxation mechanism, characterized by several fluctuation times. One of them is tau ≈ 10^{-10} s. Microscopic and macroscopic measurements carried out on these two compounds show that the lowest two crystal field level are singlets, and they are separated by an energy of about 2 K. About the XY antiferromagnet Er2Ti2O7, some results suggest the existence of exitations, responsible for low temperature spin dynamics. Finally, there inconsistencies between the interpretation of some experimental results about the existence of a low temperature spin dynamics in the Heisenberg antiferromagnet Gd2Sn2O7
Chapuis, Yann. "Frustration géométrique, transitions de phase et ordre dynamique". Phd thesis, Grenoble 1, 2009. http://tel.archives-ouvertes.fr/tel-00463643.
Pełny tekst źródłaTalatchian, Philippe. "Bio-inspired computing leveraging the synchronization of magnetic nano-oscillators". Thesis, Université Paris-Saclay (ComUE), 2019. http://www.theses.fr/2019SACLS008/document.
Pełny tekst źródłaSpin-torque nano-oscillators are non-linear, nano-scale, low power consumption, tunable magnetic microwave oscillators which are promising candidates for building large networks of coupled oscillators. Those can be used as building blocks for neuromorphic hardware which requires high-density networks of neuron-like complex processing units coupled by tunable connections. The neuromorphic approach allows to overcome the limitation of nowadays computers and to reduce their energy consumption. Indeed, in order to perform cognitive tasks as voice recognition or image recognition, the brain is much more efficient in terms of energy consumption. Due to the large number of required neurons (100 billions), a neuromorphic chip requires very small oscillators such as spin-torque nano-oscillators to emulate neurons. Recently a first demonstration of neuromorphic computing with a single spin-torque nano-oscillator was established, allowing spoken digit recognition with state of the art performance. However, to realize more complex cognitive tasks, it is still necessary to demonstrate a very important property of neural networks: learning an iterative process through which a neural network can be trained using an initial fraction of the inputs and then adjusting internal parameters to improve its recognition or classification performance. One difficulty is that training networks of coupled nano-oscillators requires tuning the coupling between them. Here, through the high frequency tunability of spin-torque nano-oscillators, we demonstrate experimentally the learning ability of coupled nano-oscillators to classify spoken vowels with a recognition rate of 88%. To realize this classification task, we took inspiration from the synchronization of rhythmic activity of biological neurons and we leveraged the synchronization of spin-torque nano-oscillators to external microwave stimuli. The high experimental recognition rates stem from the weak-coupling regime and the high tunability of spin-torque nano-oscillators. Finally, in order to realize more difficult cognitive tasks requiring large neural networks, we show numerically that arrays of hundreds of spin-torque nano-oscillators can be designed with the constraints of standard nano-fabrication techniques
Schmidt, Hinnerk Christian. "Statistical Physics of Sparse and Dense Models in Optimization and Inference". Thesis, Université Paris-Saclay (ComUE), 2018. http://www.theses.fr/2018SACLS366.
Pełny tekst źródłaDatasets come in a variety of forms and from a broad range of different applications. Typically, the observed data is noisy or in some other way subject to randomness. The recent developments in machine learning have revived the need for exact theoretical limits of probabilistic methods that recover information from noisy data. In this thesis we are concerned with the following two questions: what is the asymptotically best achievable performance? And how can this performance be achieved, i.e., what is the optimal algorithmic strategy? The answer depends on the properties of the data. The problems in this thesis can all be represented as probabilistic graphical models. The generative process of the data determines the structure of the underlying graphical model. The structures considered here are either sparse random graphs or dense (fully connected) models. The above questions can be studied in a probabilistic framework, which leads to an average (or typical) case answer. Such a probabilistic formulation is natural to statistical physics and leads to a formal analogy with problems in disordered systems. In turn, this permits to harvest the methods developed in the study of disordered systems, to attack constraint satisfaction and statistical inference problems. The formal analogy can be exploited as follows. The optimal performance analysis is directly related to the structure of the extrema of the macroscopic free energy. The algorithmic aspects follow from the minimization of the microscopic free energy (that is, the Bethe free energy in this work) which is closely related to message passing algorithms. This thesis is divided into four contributions. First, a statistical physics investigation of the circular coloring problem is carried out that reveals several distinct features. Second, new rigorous upper bounds on the size of minimal contagious sets in random graphs, with bounded maximum degree, are obtained. Third, the phase diagram of the dense Dawid-Skene model is derived by mapping the problem onto low-rank matrix factorization. The associated approximate message passing algorithm is evaluated on real-world data. Finally, the Bayes optimal denoising mean square error is derived for a restricted class of extensive rank matrix estimation problems
Castonguay, Steve. "Traumatismes et décès en escalade au Québec". Mémoire, Université de Sherbrooke, 2016. http://hdl.handle.net/11143/8818.
Pełny tekst źródłaAbstract : INTRODUCTION: This study aims to produce a portrait of injuries associated with rock climbing in Quebec by: 1) describing the characteristics of deaths associated with climbing in Quebec; 2) describing the characteristics of emergency room visits associated with climbing; 3) estimating the lifetime prevalence and the incidence rate for climbing injuries, and by exploring the associated risk or protective factors. METHODS: 1) A case series of climbing related deaths was analysed by extracting data from coroners’ investigation reports, between 1986 and 2013. 2) A case series of emergency room consultations was analysed from data provided by the Canadian Hospitals Injury Reporting and Prevention Program, between 1992 and 2011. 3) A cross-sectional descriptive study using a self-administered online survey sent to members of the Fédération québécoise de la montagne et de l’escalade (FQME), from April 8th, 2014 to June 3rd, 2014. RESULTS: 1) and 2) Of the 14 deaths and 29 cases of emergency room visits selected and analysed, the majority were men aged between 18 and 29 years. Climber falls were the leading cause of death or acute injury. 3) With a participation rate of 19 % (765/4,109), the lifetime prevalence for trauma was estimated at 11.1% on artificial climbing structures, 11.3% for rock climbing, and 7.6% for ice climbing. An approximation of the incidence in the last 12-months was estimated at 0.37 events causing injuries per 1,000 hours on artificial climbing structures (ACS), 0.17 for rock climbing and 0.10 for ice climbing. Younger age was associated with the occurrence of acute injuries (p < .01) for all types of climbing and the small number of climbing practice years with the occurrence of acute injuries during rock climbing (p = .03). Routes of higher climbing difficulty (p = .01) or a greater intensity of climbing practice (p < .01) and the occurrence of acute injuries on ACS. CONCLUSION: Climbing is a safe physical activity, with very low prevalence and incidence of acute injury. Results will help interventions in order to improve climbing safety.
Mizrahi, Alice. "Jonctions tunnel magnétiques stochastiques pour le calcul bioinspiré". Thesis, Université Paris-Saclay (ComUE), 2017. http://www.theses.fr/2017SACLS006/document.
Pełny tekst źródłaMagnetic tunnel junctions are promising candidates for computing applications. But when they are reduced to nanoscale dimensions, maintaining their stability becomes an issue. Unstable magnetic tunnel junctions undergo random switches of the magnetization between their two stable states and thus behave as stochastic oscillators. However, the stochastic nature of these superparamagnetic tunnel junctions is not a liability but an asset which can be used for the implementation of bio-inspired computing schemes. Indeed, our brain has evolved to function in a noisy environment and with unstable components. In this thesis, we show several possible applications of superparamagnetic tunnel junctions.We demonstrate how a superparamagnetic tunnel junction can be frequency and phase-locked to a weak oscillating voltage. Counterintuitively, our experiment shows that this is achieved by injecting noise in the system. We develop a theoretical model to understand this phenomenon and predict that it allows a hundred-fold energy gain over the synchronization of traditional dc-driven spin torque oscillators. Furthermore, we leverage our model to study the synchronization of several coupled junctions. Many theoretical schemes using the synchronization of oscillators to perform cognitive tasks such as pattern recognition and classification have been proposed. Using the noise-induced synchronization of superparamagnetic tunnel junctions would allow implementing these tasks at low energy.We draw an analogy between superparamagnetic tunnel junctions and sensory neurons which fire voltage pulses with random time intervals. Pushing this analogy, we demonstrate that populations of junctions can represent probability distributions and perform Bayesian inference. Furthermore, we demonstrate that interconnected populations can perform computing tasks such as learning, coordinate transformations and sensory fusion. Such a system is realistically implementable and could allow for intelligent sensory processing at low energy cost
Aizen, Kevin. "Prédiction de la performance au hockey sur glace avec des évaluations de terrain". Thesis, 2020. http://hdl.handle.net/1866/25652.
Pełny tekst źródłaIn ice hockey, the physical assessments used to date are not very specific to the task of ice hockey players and may have several limitations for predicting performance. Acceleration ability and maximum speed are highly related to player performance and only some off-ice physical assessments showed associations with these variables. Few studies have considered involvement of specific hip muscles, which are particularly solicited during the skating cycle. The data science and the use of machine learning tools have grown in recent years in the sports community. It has shown to improve prediction and has the ability to recognize complex and non-linear relationships between variables, and thus, can outperform standard statistical models and human intuition. Our main objective is the prediction of skating performance of ice hockey players with physical assessments and the use of machine learning models. We used data from physical assessment of 72 athletes who attended at the combine of the Quebec Major Junior Hockey League. We used acceleration and speed in forward and backward direction as target variables. The standard physical assessment performed on that day and isometric hip joint strength developed on its six degrees of freedom were input variables for a linear (LassoCV) and a non-linear (xgboost) model. Our machine learning models show good prediction errors by showing a mean absolute percentage error (MAPE) of less than 2.96±2.15% for the forward speed variables, while the prediction errors for acceleration and backward speed variables remain high (MAPE: > 4.46±4.15%). The broad jump and the isometric strength developed in hip flexion showed the most importance for acceleration over 5 meters (5F and 5B) and sprint over 30 meters (30F and 30B) in both skating directions. After validation of the models, coaches could use the results of these physical assessments to develop maximum forward speed performance of ice hockey players.