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The Global ETD Search service is a free service for researchers
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Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
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Showing 681 to 690 of 691 (0.071 seconds)| 681 |
<b>Probabilistic Computing Through Integrated Spintronic Nanodevices</b>John Arnesh Divakaruni Daniel (20360574) 10 January 2025 (has links)
<p dir="ltr">Probabilistic computing is a novel computing scheme that offers a more efficient approach than conventional complimentary metal-oxide-semiconductor (CMOS)-based logic in a variety of applications ranging from Bayesian inference to combinatorial optimization, and invertible Boolean logic. These applications, which have found use in the rapidly growing fields of machine learning and artificial intelligence, are traditionally computationally-intensive and so make the push for novel computing schemes that are intrinsically low-power and scalable all the more urgent.</p><p dir="ltr">The probabilistic bit (or p-bit, the base unit of probabilistic computing) is a naturally fluctuating entity that requires <i>tunable </i>stochasticity; low-barrier nanomagnets, in which the magnetic moment fluctuates randomly and continuously due to the presence of thermal energy, are a natural vehicle for providing the core functionality required. This dissertation describes the work done in mining the rich field of spintronics to produce devices that can act as natural hardware accelerators for probabilistic computing algorithms.</p><p dir="ltr">First, experiments exploring Fe<sub>3</sub>O<sub>4</sub> nanoparticles as naturally stochastic systems are presented. Using NV center measurements on an array of such nanoparticles, it is shown that they fluctuate intrinsically at GHz frequencies at room temperature; these fluctuations could be harnessed to act as a stochastic noise source, and would, in principle, enable fast computation.</p><p dir="ltr">The focus then shifts to the development of a platform that allows for easier <i>electrical</i> readout: the low-barrier magnetic tunnel junction (MTJ). We show the work done in the development and characterization of these devices, how they respond to non-ideal environments, such as elevated temperatures and exposure to high-energy electromagnetic radiation, how their intrinsic stochasticity might be tuned with electrical currents and external magnetic fields, and then how these might be integrated with a simple transistor circuit to produce a compact low-energy implementation of a p-bit.</p><p dir="ltr">Next, by integrating our stochastic MTJs with 2D-MoS<sub>2</sub><sup> </sup>field-effect transistors (FETs), the first <i>on-chip </i>realization of a key p-bit building block, displaying voltage-controllable stochasticity, is demonstrated. This is followed by another key demonstration through the fabrication of stochastic MTJs directly on top of an integrated circuit platform, where the transistor circuitry is provided by 180nm-node CMOS technology.</p><p dir="ltr">In addition, supported by circuit simulations, this work provides a careful device-level analysis of the three transistor-one magnetic tunnel junction (3T-1MTJ) p-bit design, evaluating how the characteristics of each component can influence the overall p-bit’s output. In particular, we show that – against common wisdom – a large tunnel magnetoresistance (TMR) is not the best choice for p-bits; bimodal telegraphic fluctuations are highly undesirable and are a sign of a slow device; and an ideal inverter with a large gain is unsuitable for p-bit applications due to the higher likelihood of unwanted plateaus in the resulting p-bit’s output.</p><p dir="ltr">This analysis is extended to consider the impact of such non-ideal p-bits when used to construct probabilistic circuits, with the focus on the emulation of the Boolean logic AND gate through a three p-bit correlated system. It is found that a probabilistic circuit made with ideal p-bits can accurately emulate the function of an AND gate, while the non-ideal p-circuits suffer from an increased error rate in emulating the AND gate’s truth table.</p><p dir="ltr">The understanding gained at the individual device level, in what makes a good or bad MTJ, to how the different components of the 3T-1MTJ p-bit can affect its output, and subsequently how non-ideal p-bits can impact circuit performance, can be important for the future realization of scaled on-chip p-bit networks.</p>
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Contribution à la théorie des ondelettes : application à la turbulence des plasmas de bord de Tokamak et à la mesure dimensionnelle de cibles / Contribution to the wavelet theory : Application to edge plasma turbulence in tokamaks and to dimensional measurement of targetsScipioni, Angel 19 November 2010 (has links)
La nécessaire représentation en échelle du monde nous amène à expliquer pourquoi la théorie des ondelettes en constitue le formalisme le mieux adapté. Ses performances sont comparées à d'autres outils : la méthode des étendues normalisées (R/S) et la méthode par décomposition empirique modale (EMD).La grande diversité des bases analysantes de la théorie des ondelettes nous conduit à proposer une approche à caractère morphologique de l'analyse. L'exposé est organisé en trois parties.Le premier chapitre est dédié aux éléments constitutifs de la théorie des ondelettes. Un lien surprenant est établi entre la notion de récurrence et l'analyse en échelle (polynômes de Daubechies) via le triangle de Pascal. Une expression analytique générale des coefficients des filtres de Daubechies à partir des racines des polynômes est ensuite proposée.Le deuxième chapitre constitue le premier domaine d'application. Il concerne les plasmas de bord des réacteurs de fusion de type tokamak. Nous exposons comment, pour la première fois sur des signaux expérimentaux, le coefficient de Hurst a pu être mesuré à partir d'un estimateur des moindres carrés à ondelettes. Nous détaillons ensuite, à partir de processus de type mouvement brownien fractionnaire (fBm), la manière dont nous avons établi un modèle (de synthèse) original reproduisant parfaitement la statistique mixte fBm et fGn qui caractérise un plasma de bord. Enfin, nous explicitons les raisons nous ayant amené à constater l'absence de lien existant entre des valeurs élevées du coefficient d'Hurst et de supposées longues corrélations.Le troisième chapitre est relatif au second domaine d'application. Il a été l'occasion de mettre en évidence comment le bien-fondé d'une approche morphologique couplée à une analyse en échelle nous ont permis d'extraire l'information relative à la taille, dans un écho rétrodiffusé d'une cible immergée et insonifiée par une onde ultrasonore / The necessary scale-based representation of the world leads us to explain why the wavelet theory is the best suited formalism. Its performances are compared to other tools: R/S analysis and empirical modal decomposition method (EMD). The great diversity of analyzing bases of wavelet theory leads us to propose a morphological approach of the analysis. The study is organized into three parts. The first chapter is dedicated to the constituent elements of wavelet theory. Then we will show the surprising link existing between recurrence concept and scale analysis (Daubechies polynomials) by using Pascal's triangle. A general analytical expression of Daubechies' filter coefficients is then proposed from the polynomial roots. The second chapter is the first application domain. It involves edge plasmas of tokamak fusion reactors. We will describe how, for the first time on experimental signals, the Hurst coefficient has been measured by a wavelet-based estimator. We will detail from fbm-like processes (fractional Brownian motion), how we have established an original model perfectly reproducing fBm and fGn joint statistics that characterizes magnetized plasmas. Finally, we will point out the reasons that show the lack of link between high values of the Hurst coefficient and possible long correlations. The third chapter is dedicated to the second application domain which is relative to the backscattered echo analysis of an immersed target insonified by an ultrasonic plane wave. We will explain how a morphological approach associated to a scale analysis can extract the diameter information
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Mouvement et interaction d'un ensemble de particules dans un écoulement supercritiqueBoehm, Tobias 18 February 2005 (has links) (PDF)
Le transport solide par charriage dans les cours d'eau est un problème de longue date dans l'étude des écoulement biphasiques. Les processus physiques impliqués sont en effet mal compris et complexes. Nous présentons une étude expérimentale du mouvement de billes de verre sphériques grossières. Elles étaient entraînées dans un mouvement quasi-bidimensionnel par un écoulement d'eau turbulent peu profond dans un canal à forte pente sur un lit mobile. Nous avons étudié le cas de l'équilibre du transport solide, c'est-à-dire qu'il n'y avait ni érosion ni dépôt de particules sur des intervalles de temps suffisamment longs. Les écoulements ont été filmés par le côté avec une caméra rapide. L'utilisation d'un logiciel de traitement d'images a permis de déterminer les caractéristiques des écoulements comme les trajectoires des particules, leur état de mouvement (arrêt, roulement ou saltation) et la hauteur d'eau. Le premier résultat frappant de nos expériences est que, sur des périodes de temps courtes, le transport par charriage apparaît comme un processus très intermittent. Les grandes fluctuations du débit solide résultent essentiellement de l'arrangement dans le lit mobile de particules. Alors que, pour des pentes modérées, les particules sont principalement transportées en saltation, le régime de roulement est prédominant pour des pentes fortes.
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Der glazifluviale Formenschatz im Gletschervorfeld des Himalaya und der Versuch einer relativ-zeitlichen Einordnung / Glaciofluvial sequence of forms in the glacier foreland of the Himalayas and their chronological classificationTombrink, Gerrit 23 November 2017 (has links)
No description available.
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Théorie des options et fonctions d'utilité : stratégies de couverture en présence des fluctuations non gaussiennes / Options theory and utility functions : hedging strategies in the presence of non-gaussian fluctuationsHamdi, Haykel 04 March 2011 (has links)
L'approche traditionnelle des produits dérivés consiste, sous certaines hypothèses bien définies, à construire des stratégies de couverture à risque strictement nul. Cependant,dans le cas général ces stratégies de couverture "parfaites" n'existent pas,et la théorie doit plutôt s'appuyer sur une idée de minimisation du risque. Dans ce cas, la couverture optimale dépend de la quantité du risque à minimiser. Dans lecadre des options, on considère dans ce travail une nouvelle mesure du risque vial'approche de l'utilité espérée qui tient compte, à la fois, du moment d'ordre quatre,qui est plus sensible aux grandes fluctuations que la variance, et de l'aversion aurisque de l'émetteur d'une option vis-à-vis au risque. Comparée à la couverture endelta, à l'optimisation de la variance et l'optimisation du moment d'ordre quatre,la stratégie de couverture, via l'approche de l'utilité espérée, permet de diminuer lasensibilité de la couverture par rapport au cours du sous-jacent. Ceci est de natureà réduire les coûts des transactions associées / The traditional approach of derivatives involves, under certain clearly defined hypothesis, to construct hedging strategies for strictly zero risk. However, in the general case these perfect hedging strategies do not exist, and the theory must be rather based on the idea of risk minimization. In this case, the optimal hedging strategy depends on the amount of risk to be minimized. Under the options approach, we consider here a new measure of risk via the expected utility approach that takes into account both, the moment of order four, which is more sensitive to fluctuations than large variance, and risk aversion of the investor of an option towards risk. Compared to delta hedging, optimization of the variance and maximizing the moment of order four, the hedging strategy, via the expected utilitiy approach, reduces the sensitivy of the hedging approach reported in the underlying asset price. This is likely to reduce the associated transaction costs.
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| 686 |
Supraleitung in Gallium-implantiertem Silizium / Superconductivity in gallium-implanted siliconSkrotzki, Richard 21 July 2016 (has links) (PDF)
Die vorliegende Arbeit beschäftigt sich mit der elektrischen Charakterisierung 10 nm dünner Schichten bestehend aus amorphen Ga-Nanoclustern eingebettet in Ga-dotiertes polykristallines Si. Die Herstellung der Schichten geschieht via Ionen-Implantation in Si-Wafer samt anschließender thermischer Ausheilung. Elektrische Transportmessungen in Magnetfeldern von bis zu 50 T zeigen, dass die Schichten durch Variation der Ausheilparameter zwei strukturelle Supraleiter-Isolator-Übergänge durchlaufen. TEM-gestützte Strukturanalysen decken auf, dass den Übergängen eine Gefügetransformation zugrunde liegt, die das Wechselspiel zwischen supraleitender Cluster-Kopplung und kapazitiver Ladungsenergie sowie dem Ausmaß von thermischen und Quantenfluktuationen beeinflusst. Im supraleitenden Regime (Tc = 7 K) wird ein doppelt reentrantes Phänomen beobachtet, bei dem Magnetfelder von mehreren Tesla in anisotroper Form die Supraleitung begünstigen. Eine qualitative Erklärung gelingt via selbstentwickeltem theoretischen Modell basierend auf Phaseslip-Ereignissen für Josephson-Kontakt-Netzwerke. Für Anwendungen im Bereich der Sensor-Technologie und Quanten-Logik werden die Schichten erfolgreich via Fotolithographie und FIB (focused ion beam) mikro- und nanostrukturiert. Dadurch gelingt die erstmalige Beobachtung des Little-Parks-Effektes in einer Nanostruktur aus amorphem Ga. / The following thesis is devoted to the electrical characterization of 10 nm thin layers consisting of amorphous Ga nanoclusters embedded in Ga-doped polycrystalline Si. The preparation of the layers is realized via ion implantation in Si wafers plus subsequent thermal annealing. Electrical-transport measurements in magnetic fields of up to 50 T show that the layers undergo two structural superconductor-insulator transitions upon variation of the annealing parameters. Structural analyzes based on TEM investigations reveal an underlying transformation of the size and distance of the clusters. This influences the interplay of the superconducting cluster coupling and capacitive charging energy as well as the extent of thermal and quantum fluctuations. In the superconducting regime (Tc = 7 K) a double-reentrant phenomenon is observed. Here, magnetic fields of several Tesla facilitate superconductivity in an anisotropic way. A qualitative explanation is given via a self-developed theoretical model based on phase-slip events for Josephson-junction arrays. With respect to applications regarding sensor technology and quantum logic circuits the layers are successfully micro- and nanostructured via photolithography and FIB. This allows for the first observation of the Little-Parks effect in a nanostructure of amorphous Ga.
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Scale-free Fluctuations in in Bose-Einstein Condensates, Quantum Dots and Music Rhythms / Skalenfreie Fluktuationen in Bose-Einstein Kondensaten, Quantenpunkten und MusikrhythmenHennig, Holger 27 May 2009 (has links)
No description available.
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Beyond the paired-catchment approach : isotope tracing to illuminate stocks, flows, transit time, and scalingHale, V. Cody 19 December 2011 (has links)
This dissertation integrates a process-based hydrological investigation with an
ongoing paired-catchment study to better understand how forest harvest impacts
catchment function at multiple scales. We do this by addressing fundamental questions
related to the stocks, flows and transit times of water. Isotope tracers are used within a
top-down catchment intercomparison framework to investigate the role of geology in
controlling streamwater mean transit time and their scaling relationships with the
surrounding landscape. We found that streams draining catchments with permeable
bedrock geology at the Drift Creek watershed in the Oregon Coast Range had longer
mean transit times than catchments with poorly permeable bedrock at the HJ Andrews
Experimental Forest in the Oregon Cascades. We also found that differences in
permeability contrasts within the subsurface controlled whether mean transit time
scaled with indices of catchment topography (for the poorly permeable bedrock) or
with catchment area (for the permeable bedrock). We then investigated the process-reasons
for the observed differences in mean transit time ranges and scaling behavior
using a detailed, bottom-up approach to characterize subsurface water stores and
fluxes. We found that the mean transit times in catchments underlain by permeable
bedrock were influenced by multiple subsurface storage pools with different
groundwater ages, whereas storage in the poorly permeable catchments was limited to
the soil profile and that resulted in quick routing of excess water to the stream at the
soil bedrock interface, leading to mean transit times that were closely related to
flowpath lengths and gradients. Finally, we examined how and where forest trees
interacted with subsurface storage during the growing season using a forest
manipulation experiment, where we tested the null hypothesis that near-stream trees
alone influenced daily fluctuations in streamflow. We felled trees within this zone for
two 2.5 ha basins and combined this with isotopic tracing of tree xylem water to test if
water sources utilized by trees actively contributed to summer streamflow. We
rejected our null hypotheses and found that diel fluctuations in streamflow were not
generated exclusively in the near-stream zone. We were unable to link, isotopically,
the water sources trees were utilizing to water that was contributing to streamflow.
Our results provide new process-insights to how water is stored, extracted, and
discharged from our forested catchments in Western Oregon that will help better
explain how forest removal influences streamflow across multiple scales and
geological conditions. / Graduation date: 2012
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Akumulace energie z OZE / Accumulation of Energy from Renewable Energy SourcesHeller, Ondřej January 2010 (has links)
The objective of the first part of master’s thesis is mapping the potential of various types of renewable sources in Europe and Czech Republic, especially solar energy, wind energy, water energy and biomass. There are described principals and ways of energy generation from these sources, brief overview of current technologies, and also their advantages and limitations. An important part is electric supply continuity from renewable sources, there are large differences and the resulting to restrictions on construction and connecting the units to the power system. In this work there are mentioned some impacts on network and rates of change of supply, some sources are also evaluated in terms of maximum power, that can be connected to the power system in our country. The conclusion of the first part is dedicated to energy storage technologies, which are suitable and usable for renewable sources, there are described their principals, properties, status of development and types of aplications, in which these technologies are used. This chapter also focusses on the price level of each technology. The second part of the thesis deals with 1 MWp on-grid photovoltaic power plant design. This design includes also the redox flow batteries accumulation, the first variant calculates on 24-hour steady energy supply, the second optimalized variant calculates on daily energy supply. There are the accumulation system costs estimated and also the payback period for the both variants. Additionally there is also determined minimum penalization for cost-effective operation. The last part is dedicated to changes of impact on the local grid and changes of system impacts, after the accumulation system is installed.
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Supraleitung in Gallium-implantiertem SiliziumSkrotzki, Richard 12 July 2016 (has links)
Die vorliegende Arbeit beschäftigt sich mit der elektrischen Charakterisierung 10 nm dünner Schichten bestehend aus amorphen Ga-Nanoclustern eingebettet in Ga-dotiertes polykristallines Si. Die Herstellung der Schichten geschieht via Ionen-Implantation in Si-Wafer samt anschließender thermischer Ausheilung. Elektrische Transportmessungen in Magnetfeldern von bis zu 50 T zeigen, dass die Schichten durch Variation der Ausheilparameter zwei strukturelle Supraleiter-Isolator-Übergänge durchlaufen. TEM-gestützte Strukturanalysen decken auf, dass den Übergängen eine Gefügetransformation zugrunde liegt, die das Wechselspiel zwischen supraleitender Cluster-Kopplung und kapazitiver Ladungsenergie sowie dem Ausmaß von thermischen und Quantenfluktuationen beeinflusst. Im supraleitenden Regime (Tc = 7 K) wird ein doppelt reentrantes Phänomen beobachtet, bei dem Magnetfelder von mehreren Tesla in anisotroper Form die Supraleitung begünstigen. Eine qualitative Erklärung gelingt via selbstentwickeltem theoretischen Modell basierend auf Phaseslip-Ereignissen für Josephson-Kontakt-Netzwerke. Für Anwendungen im Bereich der Sensor-Technologie und Quanten-Logik werden die Schichten erfolgreich via Fotolithographie und FIB (focused ion beam) mikro- und nanostrukturiert. Dadurch gelingt die erstmalige Beobachtung des Little-Parks-Effektes in einer Nanostruktur aus amorphem Ga. / The following thesis is devoted to the electrical characterization of 10 nm thin layers consisting of amorphous Ga nanoclusters embedded in Ga-doped polycrystalline Si. The preparation of the layers is realized via ion implantation in Si wafers plus subsequent thermal annealing. Electrical-transport measurements in magnetic fields of up to 50 T show that the layers undergo two structural superconductor-insulator transitions upon variation of the annealing parameters. Structural analyzes based on TEM investigations reveal an underlying transformation of the size and distance of the clusters. This influences the interplay of the superconducting cluster coupling and capacitive charging energy as well as the extent of thermal and quantum fluctuations. In the superconducting regime (Tc = 7 K) a double-reentrant phenomenon is observed. Here, magnetic fields of several Tesla facilitate superconductivity in an anisotropic way. A qualitative explanation is given via a self-developed theoretical model based on phase-slip events for Josephson-junction arrays. With respect to applications regarding sensor technology and quantum logic circuits the layers are successfully micro- and nanostructured via photolithography and FIB. This allows for the first observation of the Little-Parks effect in a nanostructure of amorphous Ga.
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