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Burchell, T. D. "Studies of fracture in nuclear graphite". Thesis, University of Bath, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.374615.
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Cousins, Christopher Stanley George. "Inner elasticity and the higher-order elasticity of some diamond and graphite allotropes". Thesis, University of Exeter, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.342008.
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Morrison, Craig Neil. "Lattice-modelling of nuclear graphite for improved understanding of fracture processes". Thesis, University of Manchester, 2016. https://www.research.manchester.ac.uk/portal/en/theses/latticemodelling-of-nuclear-graphite-for-improved-understanding-of-fracture-processes(10b302d1-88fb-466b-9030-d34b4fc33293).html.
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The integrity of graphite components is critical for their fitness for purpose. Since graphite is a quasi-brittle material the dominant mechanism for loss of integrity is cracking, most specifically the interaction and coalescence of micro-cracks into a critically sized flaw. Including mechanistic understanding at the length scale of local features (meso-scale) can help capture the dependence on microstructure of graphites macro-scale integrity. Lattice models are a branch of discrete, local approach models consisting of nodes connected into a lattice through discrete elements, including springs and beams. Element properties allow the construction of a micro-mechanically based material constitutive law, which will generate the expected non-linear quasi-brittle response. This research focuses on the development of the Site-Bond lattice model, which is constructed from a regular tessellation of truncated octahedral cells. The aim of this research is to explore the Site-Bond model with a view to increasing understanding of deformation and fracture behaviour of nuclear graphite at the length scale of micro-structural features. The methodology (choice of element, appropriate meso length-scale, calibration of bond stiffness constants, microstructure mapping) and results, which include studies on fracture energy and damage evolution, are presented through a portfolio of published work.
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Dutreix, Clément. "Impurity and boundary modes in the honeycomb lattice". Thesis, Paris 11, 2014. http://www.theses.fr/2014PA112217/document.
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La présente thèse s’articule autour de deux sujets. Le premier concerne la localisation des électrons en présence d’impuretés ou d’interfaces dans le réseau hexagonal. Le deuxième, en revanche, traite de l’accumulation de spin dans un supraconducteur hors-Équilibre de type s.Le graphène est la principale motivation de la première partie. Ce matériau bidimensionnel consiste en un feuillet d’atomes de carbones et peut être décrit comme un réseau hexagonal, c’est-à-dire un réseau de Bravais triangulaire avec un motif diatomique. La structure de bande électronique révèle alors l’existence d’électrons de Dirac sans masse et chiraux à basse énergie.D’une part, il est possible d’annihiler ces fermions chiraux en étirant de façon uni-Axiale le matériau. Pour une valeur seuil de l’étirement, les électrons deviennent massiques et non-Relativistes, ce qui définit une transition de phase dite de Lifshitz. Afin de caractériser cette transition, nous étudions la diffusion des électrons sur des impuretés en fonction de l’étirement. Une impureté localisée induit des interférences quantiques dans la densité électronique, connues sous le nom d’oscillations de Friedel. Etant sensibles à la nature chirale des électrons, nous montrons que ces oscillations décroissent selon des lois de puissances qui permettent de caractériser chacune des phases de la transition. La même étude est réalisée dans le cas limite où le diffuseur est une lacune.D’autre part, le motif diatomique du réseau hexagonal propose aussi une incursion dans le monde des isolants et supraconducteurs topologiques. Pour ces systèmes, la caractérisation topologique de la structure de bande électronique permet de prédire l’existence d’états de bord aux interfaces. Nous développons notamment un modèle de supraconducteur topologique basé sur le réseau hexagonal du graphène, en présence de supraconductivité de type singulet (s ou d). Lorsque la symétrie par renversement du temps est brisée par un champ Zeeman, et en présence de couplage spin-Orbit Rashba, nous donnons une prescription qui permet de caractériser les différentes phases topologiques possibles et de prédire l’apparition d’états de bord (états de Majorana) dans des nano-Rubans de graphène.La seconde partie discute l’accumulation de spin dans un supraconducteur hors-Équilibre, joint à un ferromagnétique. Lorsqu’il est à l’équilibre, le supraconducteur est composé de quasiparticules et d’un condensat. L’injection de particules polarisées en charge et en spin, à savoir des électrons polarisés en spin, induit une accumulation de spin et de charge à l’intérieur du supraconducteur. Si l’injection cesse, les populations de spin et de charge vont relaxer vers l’équilibre, mais pas nécessairement sur des échelles de temps identiques. Récemment, la réalisation d’une expérience a mis en évidence que le la charge pouvait relaxer bien plus rapidement que le spin. Afin de confirmer cet effet, une nouvelle expérience a été réalisée grâce à des mesures établies dans le domaine fréquentiel. Ici, nous adressons un model relatif à cette dernière expérience, dans le but d’extraire le temps caractéristique de relaxation du spin qui s’avère être de l’ordre de quelques nanosecondesTwo fields of research define the framework in which the present thesis can be apprehended. The first one deals with impurity and boundary modes in the hexagonal lattice. The second one concerns a spin accumulation in an out-Of-Equilibrium superconductor.Two fields of research define the framework in which the present thesis can be apprehended. The first one deals with impurity and boundary modes in the hexagonal lattice. The second one concerns a spin accumulation in an out-Of-Equilibrium superconductor.Graphene is the main motivation of the first part. From a crystallographic perspective, the carbon atoms in graphene, a graphite layer, design a triangular Bravais lattice with a diatomic pattern. This gives rise to an extra degree of freedom in the electronic band structure that crucially reveals chiral massless Dirac electrons at low-Energy. First of all, it is possible to make these chiral fermions annihilate when a uniaxial strain stretches the graphene layer. For a critical value of the strain, all the fermions become massive and nonrelativistic, which defines a Lifshitz transition. We study the impurity scattering as a function of the strain magnitude. A localised impurity yields quantum interferences in the local density of states that are known as Friedel oscillations. Because they are affected by the chiral nature of the electrons, we show that the decaying laws of these oscillations are specific to the phase the system belongs to. Thus, the impurity scattering offers the possibility to fully characterise the transition.Second, the diatomic pattern of the graphene lattice can also be considered as an invitation to the world of topological insulators and superconductors. The existence of edge states in such systems relies on the topological characterization of the band structure. Here we especially introduce a model of topological superconductor based on the honeycomb lattice with induces spin-Singlet superconductivity. When a Zeeman field breaks the time-Reversal invariance, and in the presence of Rashba spin-Orbit interactions, we give a prescription to describe the topological phases of the system and predict the emergence of Majorana modes (edge states) in strained and doped nanoribbons.The second part discusses the study of a spin accumulation in an out-Of-Equilibrium s-Wave superconductor. At the equilibrium, the superconductor is made of particles coupled by a s-Wave pairing, as well as unpaired quasiparticles. Injecting spin-Polarised electrons into the superconductor induces charge and spin imbalances. When the injection stops, it may happen that charge and spin do not relax over the same time-Scale. The first experiment that points out such a spin-Charge decoupling has recently been realised. In order to confirm this chargeless spin-Relaxation time, a new experiment has been developed [96], based on measurements in the frequency domain. Here, we address a model that fits the experimental data and thus enables the extraction of this characteristic time that is of the order of a few nanoseconds
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Clough, Duncan. "Lattice Boltzmann liquid simulations on graphics hardware". Master's thesis, University of Cape Town, 2014. http://hdl.handle.net/11427/9206.
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Includes bibliographical referencesFluid simulation is widely used in the visual effects industry. The high level of detail required to produce realistic visual effects requires significant computation. Usually, expensive computer clusters are used in order to reduce the time required. However, general purpose Graphics Processing Unit (GPU) computing has potential as a relatively inexpensive way to reduce these simulation times. In recent years, GPUs have been used to achieve enormous speedups via their massively parallel architectures. Within the field of fluid simulation, the Lattice Boltzmann Method (LBM) stands out as a candidate for GPU execution because its grid-based structure is a natural fit for GPU parallelism. This thesis describes the design and implementation of a GPU-based free-surface LBM fluid simulation. Broadly, our approach is to ensure that the steps that perform most of the work in the LBM (the stream and collide steps) make efficient use of GPU resources. We achieve this by removing complexity from the core stream and collide steps and handling interactions with obstacles and tracking of the fluid interface in separate GPU kernels. To determine the efficiency of our design, we perform separate, detailed analyses of the performance of the kernels associated with the stream and collide steps of the LBM. We demonstrate that these kernels make efficient use of GPU resources and achieve speedups of 29.6_ and 223.7_, respectively. Our analysis of the overall performance of all kernels shows that significant time is spent performing obstacle adjustment and interface movement as a result of limitations associated with GPU memory accesses. Lastly, we compare our GPU LBM implementation with a single-core CPU LBM implementation. Our results show speedups of up to 81.6_ with no significant differences in output from the simulations on both platforms. We conclude that order of magnitude speedups are possible using GPUs to perform free-surface LBM fluid simulations, and that GPUs can, therefore, significantly reduce the cost of performing high-detail fluid simulations for visual effects.
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Lewis, Robert R. "Three dimensional texturing using lattices /". Full text open access at:, 1988. http://content.ohsu.edu/u?/etd,179.
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Wu, Di, i 吳迪. "Theoretical studies of electronic tunneling properties in monolayer and bilayer graphene lattices". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2008. http://hub.hku.hk/bib/B40887960.
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Wu, Di. "Theoretical studies of electronic tunneling properties in monolayer and bilayer graphene lattices". Click to view the E-thesis via HKUTO, 2008. http://sunzi.lib.hku.hk/hkuto/record/B40887960.
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Rutter, Gregory Michael. "Atomic scale properties of epitaxial graphene grown on sic(0001)". Diss., Georgia Institute of Technology, 2008. http://hdl.handle.net/1853/26570.
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Graphene, a honeycomb lattice of sp2-bonded carbon atoms, has received considerable attention in the scientific community due to its unique electronic properties. Distinct symmetries of the graphene wave functions lead to unusual quantum properties, such as a unique half-integer quantum Hall effect. As an added consequence of these symmetries, back-scattering in graphene is strongly prohibited leading to long coherence lengths of carriers. These charge carriers at low energy exhibit linear energy-momentum dispersion, much like neutrinos. Thus, carriers in graphene can be described as massless Dirac fermions. Graphene grown epitaxially on semiconducting substrates offers the possibility of large-scale production and deterministic patterning of graphene for nanoelectronics.
In this work, epitaxial graphene is created on SiC(0001) by annealing in vacuum. Sequential scanning tunneling microscopy (STM) and spectroscopy (STS) are performed in ultrahigh vacuum at a temperature of 4.2 K and 300 K. These atomic-scale studies address the growth, interfacial properties, stacking order, and quasiparticle coherence in epitaxial graphene. STM topographic images show the atomic structure of successive graphene layers on the SiC substrate, as well as the character of defects and adatoms within and below the graphene plane. STS differential conductance (dI/dV) maps provide spatially and energy resolved snapshots of the local density of states. Such maps clearly show that scattering from atomic defects in graphene gives rise to energy-dependent standing wave patterns. We derive the carrier energy dispersion of epitaxial graphene from these data sets by quantifying the dominant wave vectors of the standing waves for each tunneling bias.
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Foulger, Iain. "Quantum walks and quantum search on graphene lattices". Thesis, University of Nottingham, 2014. http://eprints.nottingham.ac.uk/27717/.
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This thesis details research I have carried out in the field of quantum walks, which are the quantum analogue of classical random walks. Quantum walks have been shown to offer a significant speed-up compared to classical random walks for certain tasks and for this reason there has been considerable interest in their use in algorithmic settings, as well as in experimental demonstrations of such phenomena. One of the most interesting developments in quantum walk research is their application to spatial searches, where one searches for a particular site of some network or lattice structure. There has been much work done on the creation of discrete- and continuous-time quantum walk search algorithms on various lattice types. However, it has remained an issue that continuous-time searches on two-dimensional lattices have required the inclusion of additional memory in order to be effective, memory which takes the form of extra internal degrees of freedom for the walker. In this work, we describe how the need for extra degrees of freedom can be negated by utilising a graphene lattice, demonstrating that a continuous-time quantum search in the experimentally relevant regime of two-dimensions is possible. This is achieved through alternative methods of marking a particular site to previous searches, creating a quantum search protocol at the Dirac point in graphene. We demonstrate that this search mechanism can also be adapted to allow state transfer across the lattice. These two processes offer new methods for channelling information across lattices between specific sites and supports the possibility of graphene devices which operate at a single-atom level. Recent experiments on microwave analogues of graphene that adapt these ideas, which we will detail, demonstrate the feasibility of realising the quantum search and transfer mechanisms on graphene.
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Huder, Loïc. "Lien entre structure et propriétés électroniques des moirés de graphène étudié par microscopie à effet tunnel". Thesis, Université Grenoble Alpes (ComUE), 2017. http://www.theses.fr/2017GREAY083/document.
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Les dernières années ont vu l'avènement des couches cristallines bidimensionnelles, appelées matériaux 2D. L'exemple le plus connu est le graphène, d'autres étant le nitrure de bore hexagonal isolant et le diséléniure de niobium supraconducteur. Ces matériaux 2D peuvent être empilés de manière contrôlée sous la forme d'hétérostructures de van der Waals pour obtenir les propriétés électroniques désirées. L’une des plus simples hétérostructures de van der Waals est l'empilement de deux couches de graphène tournées. Cet empilement donne naissance à un moiré qui peut être vu comme un potentiel superpériodique dépendant de l'angle entre les deux couches. Les propriétés électroniques des couches tournées de graphène sont intimement liées à ce moiré.Le sujet de cette thèse est l'étude expérimentale du lien entre la structure et les propriétés électroniques des couches tournées de graphène par Microscopie et Spectroscopie à effet tunnel à basse température.Alors que l'effet de l'angle entre les couches sur les propriétés électroniques a déjà été étudié en détail, la modification de celles-ci par une déformation des couches n'a été envisagée que récemment. La première partie de ce travail expérimental étudie la modification par la déformation des propriétés électroniques de couches de graphène tournées d'un angle de 1.26° crûes sur carbure de silicium. La déformation en question est différente dans les deux couches et son effet apparait clairement dans la densité locale d'états électroniques du moiré. Contrairement à une déformation appliquée identiquement aux deux couches, une différence de déformations entre les couches (déformation relative) modifie fortement la structure de bandes même à faibles valeurs de déformations. Alors que la déformation relative était spontanément présente, la deuxième partie de cette thèse s'intéresse à l'effet d'une déformation appliquée directement aux couches de graphène. Cette déformation vient d'une interaction induite par l'approche de la pointe STM vers la surface de graphène. La modification active de la densité d'états qui en résulte dépend de la position de la pointe dans le moiré avec l'apparition d'instabilités périodiques lorsque la distance entre la pointe et l'échantillon est très faible.La troisième partie de cette thèse concerne l'étude d'un autre type de modification des propriétés électroniques consistant en l'induction de supraconductivité dans les couches de graphène. Cette modification est effectuée par une croissance du graphène en une seule étape sur du carbure de tantale supraconducteur. Les résultats montrent la formation d'une couche de carbure de tantale de grande qualité sur laquelle les couches de graphène forment des moirés. La mesure à basse température de la densité d'états de ces moirés montre la présence d'un effet de proximité supraconducteur induit par le carbure de tantaleRecent years have seen the emergence of two-dimensional crystalline layers, called 2D materials. Examples include the well-known graphene, insulating hexagonal boron nitride and superconducting niobium diselenide. The stacking of these 2D materials can be controlled to achieve desirable electronic properties under the form of van der Waals heterostructures. One of the simplest van der Waals heterostructures is the misaligned stacking of two graphene layers. Twisted graphene layers show a moiré pattern which can be viewed as a superperiodic potential that depends on the twist angle. The electronic properties of the twisted graphene layers are strongly linked to this moiré pattern.The subject of the present thesis is the experimental study of the link between the structural and the electronic properties of twisted graphene layers by means of low-temperature Scanning Tunneling Microscopy and Spectroscopy (STM/STS).While the effect of the twist angle has already been studied in great details, the modulation of the electronic properties by the deformation of the layers has been explored only recently. In the first part of this experimental work, a strain-driven modification of the electronic properties is probed in graphene layers with a twist angle of 1.26° grown on silicon carbide. The determined strain is found to be different in the two layers leading to a clear signature in the local electronic density of states of the moiré even at low strain magnitudes. Contrary to a strain applied in the two layers, this difference of strain between the layers (relative strain) modifies strongly the electronic band structure even at low strain magnitudes. While this relative strain is natively present, the second part of the work explores the effect of an applied strain in the layers. This is realized by approaching the STM tip to the graphene surface to trigger an interaction between the two. The resulting active modification of the density of states is shown to depend on the position on the moiré, leading to periodic instabilities at very low tip-sample distances.In the third part of the work, another type of modification of the electronic properties is studied when superconductivity was induced in the graphene layers. This is done by growing graphene on superconducting tantalum carbide in a single-step annealing. The results show the formation of a high-quality tantalum carbide layer on which graphene layers form moiré patterns. The low-temperature density of states of these moirés show evidence of a superconducting proximity effect induced by the tantalum carbide
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Chu, Siu-Hang. "Making digital painting organic /". View abstract or full-text, 2007. http://library.ust.hk/cgi/db/thesis.pl?CSED%202007%20CHU.
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Gurrieri, Maria Vittoria. "Twisted bilayer graphene: effective model, topological states and relevant symmetries". Master's thesis, Alma Mater Studiorum - Università di Bologna, 2021. http://amslaurea.unibo.it/23177/.
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Correlated Mott-insulating states alternated with unconventional superconductivity have been lately observed in magic-angle graphene superlattices. For small angles of rotation the band structure of twisted bilayer graphene (TBG) exhibits isolated nearly flat bands, which are responsible for such correlated behaviour. At first, we describe TBG effective model in order to construct a topological phase diagram for the mini-bands near charge neutrality. Then, through symmetry analysis, we try to unravel and understand the limit of such model, when one tries to revert back to localised low-energy orbitals in real space (Wannier functions) for Hubbard-like descriptions.
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Haughey, Kyle J. "Boundless Fluids Using the Lattice-Boltzmann Method". DigitalCommons@CalPoly, 2009. https://digitalcommons.calpoly.edu/theses/117.
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Computer-generated imagery is ubiquitous in today's society, appearing in advertisements, video games, and computer-animated movies among other places. Much of this imagery needs to be as realistic as possible, and animators have turned to techniques such as fluid simulation to create scenes involving substances like smoke, fire, and water. The Lattice-Boltzmann Method (LBM) is one fluid simulation technique that has gained recent popularity due to its relatively simple basic algorithm and the ease with which it can be distributed across multiple processors. Unfortunately, current LBM simulations also suffer from high memory usage and restrict free surface fluids to domains of fixed size. This thesis modifies the LBM to utilize a recursive run-length-encoded (RLE) grid data structure instead of the standard fixed array of grid cells, which reduces the amount of memory required for LBM simulations as well as allowing the domain to grow and shrink as necessary to accomodate a liquid surface. The modified LBM is implemented within the open-source 3D animation package Blender and compared to Blender's current LBM simulator using the metrics of memory usage and time required to complete a given simulation. Results show that, although the RLE-based simulator can take several times longer than the current simulator to complete a given simulation, the memory usage is significantly reduced, making an RLE-based simulation preferable in a few specific circumstances.
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Stanek, Lucas James. "Deformation of a Graphene Sheet Driven by Lattice Mismatch with a Supporting Substrate". University of Akron / OhioLINK, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=akron1493999094753307.
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Siegel, Jakob. "A CUDA optimized Lattice Boltzmann method implementation using control-structure splitting techniques". Access to citation, abstract and download form provided by ProQuest Information and Learning Company; downloadable PDF file, 69 p, 2009. http://proquest.umi.com/pqdweb?did=1885754631&sid=4&Fmt=2&clientId=8331&RQT=309&VName=PQD.
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Hass, Joanna R. "Structural characterization of epitaxial graphene on silicon carbide". Diss., Atlanta, Ga. : Georgia Institute of Technology, 2008. http://hdl.handle.net/1853/26654.
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Thesis (Ph.D)--Physics, Georgia Institute of Technology, 2009.Committee Co-Chair: Conrad, Edward; Committee Co-Chair: First, Phillip; Committee Member: Carter, Brent; Committee Member: de Heer, Walter; Committee Member: Zangwill, Andrew. Part of the SMARTech Electronic Thesis and Dissertation Collection.
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Tuesday, Andrew J. "Modeling Atomic Defects in a Two-Dimensional Lennard-Jones Lattice Using Molecular Dynamics Simulations". University of Akron / OhioLINK, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=akron1334579948.
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Milicevic, Marijana. "Manipulation of Dirac Cones and Edge states in Polariton Honeycomb Lattices". Electronic Thesis or Diss., Sorbonne université, 2018. http://www.theses.fr/2018SORUS515.
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Les polaritons de cavité sont des particules mixtes lumière-matière résultant du couplage fort entre excitons de puits quantiques et photons de cavité dans des microcavités semi-conductrices. Ils s'avèrent être une plate-forme extraordinaire pour émuler des Hamiltoniens 1D et 2D. Cela est dû au contrôle de site unique lors de la fabrication de réseaux de polaritons, ainsi qu'à la possibilité de visualiser directement la dispersion, les fonctions propres résolus en espace et la propagation des polaritons via des expériences de photoluminescence. En gravant une microcavité à base de GaAs, un réseau en nid d'abeille pour polaritons a été fabriqué. Les deux bandes d’énergie les plus basses de cette structure émulent pour les photons les bandes π et π * du graphène. Remarquablement, le système permet également d'explorer des degrés de liberté orbitaux, inaccessibles dans le graphène réel. Dans la première partie de cette thèse, cet émulateur à polaritons est utilisé pour aborder la physique des états de bord dans un réseau en nid d'abeille. De nouveaux états de bord, à caractère plat et dispersif, ont été découverts et visualisés dans le graphène orbital. Dans la deuxième partie de la thèse, nous démontrons expérimentalement une méthode pour adapter la dispersion de Dirac pour les photons. En mettant en œuvre une déformation uniaxiale dans le réseau en nid d'abeille, des photons de Dirac qui combinent des masses effectives nulle, finie et infinie sont créés. Les résultats présentés ouvrent de nouvelles perspectives pour l'ingénierie d’interfaces entre différents types de dispersions de Dirac. De plus, la partie excitonique des polaritons assure une sensibilité au champ magnétique, créant la possibilité de briser la symétrie d'inversion temporelle du système et d'étudier les états de bord topologiques photoniques dans des cônes de Dirac exotiquesExciton polaritons are mixed light-matter particles arising from strong coupling of quantum well excitons and cavity photons in semiconductor microcavities. They prove to be an extraordinary platform to emulate 1D and 2D Hamiltonians. This is due to the single site control when fabricating polariton lattices as well as to the possibility to directly visualize dispersion, spatial eigenfunctions and propagation of polaritons in photoluminescence experiments. By etching GaAs-based microcavity a honeycomb lattice for polaritons has been fabricated. The lowest two bands of this structure emulate for photons the π and π* bands of graphene. Remarkably, the system also permits exploring orbital degrees of freedom, inaccessible in actual graphene. In the first part of this thesis polariton emulator is used to address the physics of edge states in honeycomb lattice. New edge states, with flat and dispersive character have been discovered and visualised in orbital graphene. In the second part of the thesis we demonstrate experimentally a method to tailor the Dirac dispersion for photons. By implementing uni-axial strain in the honeycomb lattice Dirac photons that combine zero, finite and infinite effective masses are created. Presented results open new perspectives for the engineering of interfaces between various types of Dirac dispersions. Furthermore, the excitonic part of polaritons assures sensitivity to the magnetic field, creating the possibility to break the time reversal symmetry of the system and study photonic topological edge states in exotic Dirac cones. Finally, nonlinear Dirac physics can be probed in this system owing to polariton-polariton interactions
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Kleeberg, Katja [Verfasser]. "Dyson-Schwinger Equations for strongly interacting fermions on the hexagonal graphene lattice / Katja Kleeberg". Gießen : Universitätsbibliothek, 2019. http://d-nb.info/1200352521/34.
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Delbosc, Nicolas. "Real-time simulation of indoor air flow using the lattice Boltzmann method on graphics processing unit". Thesis, University of Leeds, 2015. http://etheses.whiterose.ac.uk/13546/.
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This thesis investigates the usability of the lattice Boltzmann method (LBM) for the simulation of indoor air flows in real-time. It describes the work undertaken during the three years of a Ph.D. study in the School of Mechanical Engineering at the University of Leeds, England. Real-time fluid simulation, i.e. the ability to simulate a virtual system as fast as the real system would evolve, can benefit to many engineering application such as the optimisation of the ventilation system design in data centres or the simulation of pollutant transport in hospitals. And although real-time fluid simulation is an active field of research in computer graphics, these are generally focused on creating visually appealing animation rather than aiming for physical accuracy. The approach taken for this thesis is different as it starts from a physics based model, the lattice Boltzmann method, and takes advantage of the computational power of a graphics processing unit (GPU) to achieve real-time compute capability while maintaining good physical accuracy. The lattice Boltzmann method is reviewed and detailed references are given a variety of models. Particular attention is given to turbulence modelling using the Smagorinsky model in LBM for the simulation of high Reynolds number flow and the coupling of two LBM simulations to simulate thermal flows under the Boussinesq approximation. A detailed analysis of the implementation of the LBM on GPU is conducted. A special attention is given to the optimisation of the algorithm, and the program kernel is shown to achieve a performance of up to 1.5 billion lattice node updates per second, which is found to be sufficient for coarse real-time simulations. Additionally, a review of the real-time visualisation integrated within the program is presented and some of the techniques for automated code generation are introduced. The resulting software is validated against benchmark flows, using their analytical solutions whenever possible, or against other simulation results obtained using accepted method from classical computational fluid dynamics (CFD) either as published in the literature or simulated in-house. The LBM is shown to resolve the flow with similar accuracy and in less time.
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Lapitski, Denis. "Development of the Quantum Lattice Boltzmann method for simulation of quantum electrodynamics with applications to graphene". Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:e89cd11b-da2c-4c34-be9f-7b3d711e2e64.
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We investigate the simulations of the the Schrödinger equation using the onedimensional quantum lattice Boltzmann (QLB) scheme and the irregular behaviour of solution. We isolate error due to approximation of the Schrödinger solution with the non-relativistic limit of the Dirac equation and numerical error in solving the Dirac equation. Detailed analysis of the original scheme showed it to be first order accurate. By discretizing the Dirac equation consistently on both sides we derive a second order accurate QLB scheme with the same evolution algorithm as the original and requiring only a one-time unitary transformation of the initial conditions and final output. We show that initializing the scheme in a way that is consistent with the non-relativistic limit supresses the oscillations around the Schrödinger solution. However, we find the QLB scheme better suited to simulation of relativistic quantum systems governed by the Dirac equation and apply it to the Klein paradox. We reproduce the quantum tunnelling results of previous research and show second order convergence to the theoretical wave packet transmission probability. After identifying and correcting the error in the multidimensional extension of the original QLB scheme that produced asymmetric solutions, we expand our second order QLB scheme to multiple dimensions. Next we use the QLB scheme to simulate Klein tunnelling of massless charge carriers in graphene, compare with theoretical solutions and study the dependence of charge transmission on the incidence angle, wave packet and potential barrier shape. To do this we derive a representation of the Dirac-like equation governing charge carriers in graphene for the one-dimensional QLB scheme, and derive a two-dimensional second order graphene QLB scheme for more accurate simulation of wave packets. We demonstrate charge confinement in a graphene device using a configuration of multiple smooth potential barriers, thereby achieving a high ratio of on/off current with potential application in graphene field effect transistors for logic devices. To allow simulation in magnetic or pseudo-magnetic fields created by deformation of graphene, we expand the scheme to include vector potentials. In addition, we derive QLB schemes for bilayer graphene and the non-linear Dirac equation governing Bose-Einstein condensates in hexagonal optical lattices.
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Pedersen, Daniel. "Development of a Kinetic Monte Carlo Code". Thesis, Uppsala universitet, Materialteori, 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-202711.
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A framework for constructing kinetic monte carlo (KMC) simulations of diffusive events on a lattice was developed. This code was then tested by running simulations of Fe adatom diffusion on graphene and graphene-boron nitride surfaces. The results from these simulations was then used to show that the modeled diffusion adheres to the laws of brownian motion and generates results similar to recent research findings.
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Ravaioli, Giacomo. "Simulazione di fluidi in Computer Graphics". Bachelor's thesis, Alma Mater Studiorum - Università di Bologna, 2016. http://amslaurea.unibo.it/10296/.
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Questa tesi si focalizza sullo studio dei modelli fisico-matematici attualmente in uso per la simulazione di fluidi al calcolatore con l’obiettivo di fornire nozioni di base e avanzate sull’utilizzo di tali metodi. La trattazione ha lo scopo di facilitare la comprensione dei principi su cui si fonda la simulazione di fluidi e rappresenta una base per la creazione di un proprio simulatore. E’ possibile studiare le caratteristiche di un fluido in movimento mediante due approcci diversi, l’approccio lagrangiano e l’approccio euleriano. Mentre l’approccio lagrangiano ha lo scopo di conoscere il valore, nel tempo, di una qualsiasi proprietà di ciascuna particella che compone il fluido, l’approccio euleriano, fissato uno o più punti del volume di spazio occupato da quest’ultimo, vuole studiare quello che accade, nel tempo, in quei punti. In particolare, questa tesi approfondisce lo studio delle equazioni di Navier-Stokes, approcciandosi al problema in maniera euleriana. La soluzione numerica del sistema di equazioni differenziali alle derivate parziali derivante dalle equazioni sopracitate, approssima la velocità del fluido, a partire dalla quale è possibile risalire a tutte le grandezze che lo caratterizzano. Attenzione viene riservata anche ad un modello facente parte dell’approccio semi-lagrangiano, il Lattice Boltzmann, considerato una via di mezzo tra i metodi puramente euleriani e quelli lagrangiani, che si basa sulla soluzione dell’equazione di Boltzmann mediante modelli di collisione di particelle. Infine, analogamente al metodo di Lattice Boltzmann, viene trattato il metodo Smoothed Particles Hydrodynamics, tipicamente lagrangiano, secondo il quale solo le proprietà delle particelle comprese dentro il raggio di una funzione kernel, centrata nella particella di interesse, influenzano il valore della particella stessa. Un resoconto pratico della teoria trattata viene dato mediante delle simulazioni realizzate tramite il software Blender 2.76b.
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Vinogradov, Nikolay. "Controlling Electronic and Geometrical Structure of Honeycomb-Lattice Materials Supported on Metal Substrates : Graphene and Hexagonal Boron Nitride". Doctoral thesis, Uppsala universitet, Institutionen för fysik och astronomi, 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-194089.
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The present thesis is focused on various methods of controlling electronic and geometrical structure of two-dimensional overlayers adsorbed on metal surfaces exemplified by graphene and hexagonal boron nitride (h-BN) grown on transition metal (TM) substrates. Combining synchrotron-radiation-based spectroscopic and various microscopic techniques with in situ sample preparation, we are able to trace the evolution of overlayer electronic and geometrical properties in overlayer/substrate systems, as well as changes of interfacial interaction in the latter.It is shown that hydrogen uptake by graphene/TM substrate strongly depends on the interfacial interaction between substrate and graphene, and on the geometrical structure of graphene. An energy gap opening in the electronic structure of graphene on TM substrates upon patterned adsorption of atomic species is demonstrated for the case of atomic oxygen adsorption on graphene/TM’s (≥0.35 eV for graphene/Ir(111)). A non-uniform character of adsorption in this case – patterned adsorption of atomic oxygen on graphene/Ir(111) due to the graphene height modulation is verified. A moderate oxidation of graphene/Ir(111) is found largely reversible. Contrary, oxidation of h-BN/Ir(111) results in replacing nitrogen atoms in the h-BN lattice with oxygen and irreversible formation of the B2O3 oxide-like structure. Pronounced hole doping (p-doping) of graphene upon intercalation with active agents – halogens or halides – is demonstrated, the level of the doping is dependent on the agent electronegativity. Hole concentration in graphene on Ir(111) intercalated with Cl and Br/AlBr3 is as high as ~2×1013 cm-2 and ~9×1012 cm-2, respectively. Unusual periodic wavy structures are reported for h-BN and graphene grown on Fe(110) surface. The h-BN monolayer on Fe(110) is periodically corrugated in a wavy fashion with an astonishing degree of long-range order, periodicity of 2.6 nm, and the corrugation amplitude of ~0.8 Å. The wavy pattern results from a strong chemical bonding between h-BN and Fe in combination with a lattice mismatch in either [11 ̅1] or [111 ̅] direction of the Fe(110) surface. Two primary orientations of h-BN on Fe(110) can be observed corresponding to the possible directions of lattice match between h-BN and Fe(110). Chemical vapor deposition (CVD) formation of graphene on iron is a formidable task because of high carbon solubility in iron and pronounced reactivity of the latter, favoring iron carbide formation. However, growth of graphene on epitaxial iron films can be realized by CVD at relatively low temperatures, and the formation of carbides can be avoided in excess of the carbon-containing precursors. The resulting graphene monolayer creates a periodically corrugated pattern on Fe(110): it is modulated in one dimension forming long waves with a period of ~4 nm parallel to the [001] direction of the substrate, with an additional height modulation along the wave crests. The novel 1D templates based on h-BN and graphene adsorbed on iron can possibly find an application in 1D nanopatterning. The possibility for growing high-quality graphene on iron substrate can be useful for the low-cost industrial-scale graphene production.
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Szulakowska, Ludmila. "Electron-electron Interactions and Optical Properties of Two-dimensional Nanocrystals". Thesis, Université d'Ottawa / University of Ottawa, 2020. http://hdl.handle.net/10393/40983.
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This thesis presents a theory of electron-electron interaction effects and optical properties
of nanostructures of two-dimensional (2D) honeycomb crystals - graphene and transition metal dichalcogenides (TMDC). Graphene, a semimetallic hexagonal lattice of carbon atoms can be described by a massless Dirac fermion model, with the conduction band (CB) and valence band (VB) touching in the corners of a hexagonal Brillouin zone, valleys K and -K. TMDC crystals sites host either a transition metal atom or a chalcogen dimer, which opens the energy gap and allows for describing their low-energy nature with massive Dirac fermion (mDf) model. The metal atom in TMDC crystals causes strong spin-orbit (SO) coupling, resulting in large SO splitting in bands at both valleys. For TMDCs it is possible to excite carriers in each valley with oppositely circularly polarised light, which offers promising prospects for devices based on electrons valley index, i.e. valleytronic devices. Additionally, the optical response of TMDCs is enhanced by the presence of secondary CB minima, at Q-points.
The dimensionality of 2D crystals can be further reduced to form quantum dots (QDs) - nanostructures con ned in all dimensions. This thesis first discusses hexagonal graphene QDs, which exhibit energy gap oscillation as a function of size, due to the edge type: zigzag or armchair. These QDs are divided into concentric rings, analysed with tight-binding (TB) model. An armchair edged QD is built from a zigzag edged QD by adding a 1D Lieb lattice of carbon atoms on its edge. The energy gap is formed differently for both edges: from the outer ring states for zigzag edge and from the 1D Lieb lattice zero-energy states for armchair edge, which causes the energy gap. The remaining portion of the thesis focuses on TMDC materials. First a TB model is presented for a member of TMDC group, MoS2, using three d orbitals of Mo atom and three p orbitals of the S2 dimers. The tunneling matrix elements between nearest-neighbor and next-nearest-neighbour sites are explicitly derived at K and -K to form a six band TB Hamiltonian. Its solutions are fitted to the bands obtained from the density functional theory ab initio calculations to obtain the correct behaviour of bands around K and additional minima at Q-points, which explains the role of d orbitals in TMDCs. Close to K the TB model is reduced to mDf model, which
is then studied in response to light, yielding the valley-dependent selection rules for
absorption. The interaction of mDf with light is further studied in the presence of strong external magnetic eld, which leads to the formation of Landau levels (LLs), asymmetric
between both valleys, and valley Zeeman splitting. These LLs are populated with electrons
to form a Hartree-Fock ground state (GS), which can exhibit valley polarisation due to the LL asymmetry. Quasi-electron-hole excitations out of the GS are then formed and their self-energy, vertex corrections and scattering energy is calculated. The effect of electron-electron interactions on valley Zeeman splitting is demonstrated and the Bethe-Salpeter equation is numerically solved to give magnetoexciton spectrum for both valleys. The results include a valley-dependent absorption spectrum for mDf magnetoexcitons that vary with the valley polarisation.
The final part of this thesis discusses the single particle and interacting effects in gated MoS2 QDs. First, I perform a single electron atomistic calculation for a million-atom computational box with periodic boundary conditions based on a TB model developed from ab initio methods for bulk MoS2. Electrons are then con ned with a parabolic electrostatic potential from top metallic gates. They exhibit twofold degenerate harmonic oscillator energy spectrum with shell spacing ω associated with valleys K as well as a sixfold degenerate energy spectrum derived from the Q-points. The degeneracy of electronic shells is broken due to valley contrasting Berry curvature,which acts as an effective magnetic eld splitting opposite angular momentum states in both valleys. I populate up to ve K-derived harmonic oscillator shells with up to six electrons and turn on the electron-electron interactions. The resulting GS phases form two regimes dependent on ω, which are dominated each by a broken-symmetry phase, i.e. valley and spin polarised GS for low ω and valley and spin unpolarised but spin intervalley antiferromagnetic GS for higher ω. This behaviour is explained as an effect of the strong SO splitting, weak intervalley exchange interaction and strong correlations. Means of detecting these effects in experiment based on the spin and valley blockade are proposed. These results advance the understanding of interaction-driven breaking of symmetry for valley systems, crucial for designing of valleytronic devices in the future.
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Tonnoir, Charlène. "Spectroscopie tunnel de graphène épitaxié sur du rhénium supraconducteur". Thesis, Grenoble, 2013. http://www.theses.fr/2013GRENY043/document.
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Obtenir une interface transparente entre le graphène et un supraconducteur s'est révélé être difficile et pourtant essentiel pour induire des corrélations supraconductrices dans le graphène par effet de proximité. Cette thèse présente une étude par spectroscopie tunnel (STS) à très basse température (50 mK) d'un système nouveau qui réalise ce bon couplage électronique en faisant croitre du graphène par épitaxie sur du rhénium supraconducteur. La fabrication et sélection des films minces de rhénium de haute qualité cristalline sont brièvement expliquées, suivies par le procédé de croissance CVD du graphène sur divers métaux et en particulier sur du rhénium. Les images topographiques obtenues par STM révèlent un moiré qui résulte de la différence de paramètre de maille entre le graphène et le rhénium. Nous identifions ce système à une monocouche de graphène en forte interaction avec le substrat, résultat corroboré par des calculs DFT. Des analyses STS dans une gamme d'énergie de plusieurs centaines de meV montrent une modulation spatiale de la densité d'états (DOS) à l'échelle du moiré, indiquant différentes forces de couplage entre les ‘collines' et les ‘vallées' du moiré. Les propriétés supraconductrices de l'échantillon en volume sont sondées par des mesures de transport, desquelles nous extrayons la température de transition Tc~2K et la longueur de cohérence supraconductrice ξ=18nm. Le gap supraconducteur est extrait de la DOS mesurée par STS à 50 mK (Δ=330µeV) et trouvé homogène à l'échelle du moiré. L'état mixte supraconducteur est étudié sous champ magnétique et un réseau de vortex d'Abrikosov est mis à jour. Enfin, une étude sur diverses morphologies de surface présente un effet de proximité supraconducteur latéral anormal, en contradiction avec les modèles existantsObtaining a transparent interface between graphene and a superconductor has proved to be very challenging and yet essential to induce superconducting correlations in graphene via the so-called proximity effect. This thesis presents a scanning tunneling spectroscopy (STS) study at very low temperature (50 mK) of a novel system achieving such a good electronic contact by the growth of epitaxial graphene on superconducting rhenium. The fabrication and selection of high-crystallographic quality rhenium thin films are briefly explained, followed by the CVD growth process of graphene on various metal substrates and in particular rhenium. STM topographic images reveal a moiré pattern due to the lattice mismatch between graphene and rhenium. We identify this system to a graphene monolayer in strong interaction with the underlying substrate, as corroborated by DFT calculations. STS analyses in the hundreds-meV energy range show a spatial modulation of the density of states (DOS) at the moiré scale, indicating different coupling strengths between ‘hills' and ‘valleys' regions. The bulk superconducting properties are probed by transport measurements, from which we extract the transition temperature Tc~2K and a superconducting coherence length ξ=18nm. The superconducting gap is extracted from the DOS at 50 mK (Δ=330µeV) and found homogeneous at the moiré scale. The superconducting mixed state is studied under magnetic field and an Abrikosov vortex-lattice is uncovered. Finally, a study on various surface morphologies exhibits an anomalous lateral superconducting proximity effect in contradiction with the existing models
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Bach, Matthias [Verfasser], Volker [Akademischer Betreuer] Lindenstruth, Owe [Akademischer Betreuer] Phlipsen i Tilo [Akademischer Betreuer] Wettig. "Energy- and cost-efficient Lattice-QCD computations using graphics processing units / Matthias Bach. Gutachter: Volker Lindenstruth ; Owe Phlipsen ; Tilo Wettig. Betreuer: Volker Lindenstruth". Frankfurt am Main : Univ.-Bibliothek Frankfurt am Main, 2015. http://d-nb.info/1067918248/34.
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Sjölund, Johannes. "Real-time Thermal Flow Predictions for Data Centers : Using the Lattice Boltzmann Method on Graphics Processing Units for Predicting Thermal Flow in Data Centers". Thesis, Luleå tekniska universitet, Institutionen för system- och rymdteknik, 2018. http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-70530.
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The purpose of this master thesis is to investigate the usage of the Lattice Boltzmann Method (LBM) of Computational Fluid Dynamics (CFD) for real-time prediction of indoor air flows inside a data center module. Thermal prediction is useful in data centers for evaluating the placement of heat-generating equipment and air conditioning. To perform the simulation a program called RAFSINE was used, written by Nicholas Delbosc at the University of Leeds, which implemented LBM on Graphics Processing Units (GPUs) using NVIDIA CUDA. The program used the LBM model called Bhatnagar-Gross-Krook (BGK) on a 3D lattice and had the capability of executing thermal simulations in real-time or faster than real-time. This fast rate of execution means a future application for this simulation could be as a predictive input for automated air conditioning control systems, or for fast generation of training data sets for automatic fault detection systems using machine learning. In order to use the LBM CFD program even from hardware not equipped with NVIDIA GPUs it was deployed on a remote networked server accessed through Virtual Network Computing (VNC). Since RAFSINE featured interactive OpenGL based 3D visualization of thermal evolution, accessing it through VNC required use of the VirtualGL toolkit which allowed fast streaming of visualization data over the network. A simulation model was developed describing the geometry, temperatures and air flows of an experimental data center module at RISE SICS North in Luleå, Sweden, based on measurements and equipment specifications. It was then validated by comparing it with temperatures recorded from sensors mounted in the data center. The thermal prediction was found to be accurate on a room-level within ±1° C when measured as the average temperature of the air returning to the cooling units, with a maximum error of ±2° C on an individual basis. Accuracy at the front of the server racks varied depending on the height above the floor, with the lowest points having an average accuracy of ±1° C, while the middle and topmost points had an accuracy of ±2° C and ±4° C respectively. While the model had a higher error rate than the ±0.5° C accuracy of the experimental measurements, further improvements could allow it to be used as a testing ground for air conditioning control or automatic fault detection systems.
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Fielding, Andrew L. "Final state effects in neutron Compton scattering measurements". Thesis, University of Portsmouth, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.310441.
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Judice, Sicilia Ferreira Ponce Pasini. "Modelagem e simulação de fluidos via técnicas de sketching, modelos de difusão-reação e método de lattice Boltzmann". Laboratório Nacional de Computação Científica, 2017. https://tede.lncc.br/handle/tede/241.
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Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)
This work presents a methodology involving two-dimensional fluids initialization through sketching techniques (free draw on tablets, for example) along with diffusion-reaction equations and fluid simulation using Lattice Boltzmann Method (LBM). In the initial stage, the framework converts the drawings made by the users into streamlines. From these lines it is generated a partial velocity field, which is widespread throughout the fluid domain through diffusion-reaction techniques. The velocity field obtained at the end of the process is used to initialize the fluid simulation via the LBM method. We tested two diffusion-reaction techniques, one based on differential equations (Gradient Vector Flow) and another in LBM models. We discussed aspects of vector fields topology and fluid dynamics involved in the sketching conversion into streamlines, as well as the preservation of singularities by the diffusion-reaction methods analyzed. The aim of our application is in the areas of visual effects and computer graphics. In this sense, we presented visual results involving the theoretical concepts which demonstrate the potential of the methodology presented for computational fluid animation .
Neste trabalho apresentamos uma metodologia envolvendo inicialização de fluidos bidimensionais usando técnicas de sketching (desenho livre em tablets, por exemplo) juntamente com equações do tipo difusão-reação e simulação do fluido usando método Lattice Boltzmann (LBM). Na etapa inicial, o framework converte os desenhos feitos pelo usuário em linhas de corrente. A partir destas linhas é gerado um campo de velocidades parcial, que é difundido por todo o domı́nio do fluido via técnicas de difusão-reação. O campo de velocidades obtido no final deste processo é usado para inicializar a simulação do fluido via método LBM. Foram testadas duas técnicas de difusão-reação, uma baseada em equações diferenciais (Gradient Vector Flow ) e outra em modelos do tipo LBM. São discutidos aspectos de topologia de campos vetoriais e dinâmica de fluidos envolvidos na conversão do sketching em linhas de corrente, bem como na preservação de singularidades pelos métodos de difusão-reação analisados. Nosso foco de aplicação está nas áreas de efeitos visuais e computação gráfica. Neste sentido, são apresentados resultados visuais envolvendo os conceitos teóricos os quais demonstram a potencialidade da metodologia apresentada para animação computacional de fluidos.
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Obrecht, Christian. "High performance lattice Boltzmann solvers on massively parallel architectures with applications to building aeraulics". Phd thesis, INSA de Lyon, 2012. http://tel.archives-ouvertes.fr/tel-00776986.
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With the advent of low-energy buildings, the need for accurate building performance simulations has significantly increased. However, for the time being, the thermo-aeraulic effects are often taken into account through simplified or even empirical models, which fail to provide the expected accuracy. Resorting to computational fluid dynamics seems therefore unavoidable, but the required computational effort is in general prohibitive. The joint use of innovative approaches such as the lattice Boltzmann method (LBM) and massively parallel computing devices such as graphics processing units (GPUs) could help to overcome these limits. The present research work is devoted to explore the potential of such a strategy. The lattice Boltzmann method, which is based on a discretised version of the Boltzmann equation, is an explicit approach offering numerous attractive features: accuracy, stability, ability to handle complex geometries, etc. It is therefore an interesting alternative to the direct solving of the Navier-Stokes equations using classic numerical analysis. From an algorithmic standpoint, the LBM is well-suited for parallel implementations. The use of graphics processors to perform general purpose computations is increasingly widespread in high performance computing. These massively parallel circuits provide up to now unrivalled performance at a rather moderate cost. Yet, due to numerous hardware induced constraints, GPU programming is quite complex and the possible benefits in performance depend strongly on the algorithmic nature of the targeted application. For LBM, GPU implementations currently provide performance two orders of magnitude higher than a weakly optimised sequential CPU implementation. The present thesis consists of a collection of nine articles published in international journals and proceedings of international conferences (the last one being under review). These contributions address the issues related to single-GPU implementations of the LBM and the optimisation of memory accesses, as well as multi-GPU implementations and the modelling of inter-GPU and internode communication. In addition, we outline several extensions to the LBM, which appear essential to perform actual building thermo-aeraulic simulations. The test cases we used to validate our codes account for the strong potential of GPU LBM solvers in practice.
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Hagan, Aaron M. "PARALLEL 3D IMAGE SEGMENTATION BY GPU-AMENABLE LEVEL SET SOLUTION". Kent State University / OhioLINK, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=kent1242262556.
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Morvan, Alexis. "Honeycomb lattices of superconducting microwave resonators : Observation of topological Semenoff edge states". Thesis, Université Paris-Saclay (ComUE), 2019. http://www.theses.fr/2019SACLS037/document.
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Cette thèse décrit la réalisation et l’étude de réseaux bidimensionnels de résonateurs supraconducteurs en nid d’abeille. Ce travail constitue un premier pas vers la simulation de systèmes de la matière condensée avec des circuits supraconducteurs. Ces réseaux sont micro-fabriqués et sont constitués de plusieurs centaines de sites. Afin d’observer les modes propres qui y apparaissent dans une gamme de fréquence entre 4 et 8 GHz, nous avons mis au point une technique d’imagerie. Celle-ci utilise la dissipation locale créée par un laser avec lequel nous pouvons adresser chaque site du réseau. Nous avons ainsi pu mesurer la structure de bande et caractériser les états de bord de nos réseaux. En particulier, nous avons observé les états localisés qui apparaissent à l'interface entre deux isolants de Semenoff ayant des masses opposées. Ces états, dits de Semenoff, sont d'origine topologique. Nos observations sont en excellent accord avec des simulations électromagnétiques ab initioThis thesis describes the realization and study of honeycomb lattices of superconducting resonators. This work is a first step towards the simulation of condensed matter systems with superconducting circuits. Our lattices are micro-fabricated and typically contains a few hundred sites. In order to observe the eigen-modes that appear between 4 and 8 GHz, we have developed a mode imaging technique based on the local dissipation introduced by a laser spot that we can move across the lattice. We have been able to measure the band structure and to characterize the edge states of our lattices. In particular, we observe localized states that appear at the interface between two Semenoff insulators with opposite masses. These states, called Semenoff states, have a topological origin. Our observations are in good agreement with ab initio electromagnetic simulations
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Carrega, Matteo. "Coulomb drag and Dirac plasmons in novel 2D electron systems". Doctoral thesis, Scuola Normale Superiore, 2014. http://hdl.handle.net/11384/85870.
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[from the introduction]: This Thesis focusses on
the physics of e-e interactions in single-layer graphene and on the role of interlayer e-e
interactions in vertical heterostructures comprised of two closely spaced graphene sheets.
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Schumacher, Stefan [Verfasser], Thomas [Akademischer Betreuer] Michely, Tim O. [Akademischer Betreuer] Wehling i Heiko [Akademischer Betreuer] Wende. "Structure, Magnetism, and Binding of Novel Two-Dimensional Materials: Europium-Intercalated Graphene, Cluster Lattices, and Polar Oxide Bilayers / Stefan Schumacher. Gutachter: Thomas Michely ; Tim O. Wehling ; Heiko Wende". Köln : Universitäts- und Stadtbibliothek Köln, 2014. http://d-nb.info/1058946226/34.
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Oliveira, Fabíola Martins Campos de 1988. "GPU implementation of a fluid dynamics interactive simulator based on the Lattice Boltzmann method = Implementação em GPU de um simulador interativo de fluidodinâmica com o método das Redes de Boltzmann". [s.n.], 2015. http://repositorio.unicamp.br/jspui/handle/REPOSIP/265825.
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Orientador: Luiz Otávio Saraiva FerreiraDissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica
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Resumo: Recentes avanços na tecnologia de processadores multinúcleos e vários-núcleos popularizaram a computação paralela, acelerando a execução de programas e possibilitando a simulação de domínios maiores. Dentre os problemas complexos que requerem alta velocidade de processamento, os problemas de fluidodinâmica computacional se destacam, já que suas simulações tendem a ter um alto custo computacional e exigem grandes domínios de simulação. O método baseado nas Redes de Boltzmann é um método de fluidodinâmica computacional apropriado para o uso de paralelismo que vem ganhando destaque na comunidade científica. Embora haja trabalhos que explorem o paralelismo em GPU nesse método, um simulador eficiente na execução e visualização interativa ainda não foi explorado adequadamente. Assim, a proposta deste trabalho é implementar em GPU um simulador interativo de fluidodinâmica com o método das Redes de Boltzmann. Inicialmente, o simulador foi desenvolvido em linguagem C e foi paralelizado em CPU usando MPI. Em seguida, foi paralelizado em GPU usando CUDA e convertido para linguagem orientada a objetos em C++. Depois, a visualização interativa foi acrescentada utilizando técnicas como interoperabilidade entre CUDA e OpenGL, texturização 3D, fluxo programável da GPU, além de funções de interação com o usuário. O simulador foi validado para casos 2D e 3D em fluxos monocomponentes monofásicos. Além disso, para demonstrar o ganho de desempenho em velocidade de processamento de problemas paralelizados em relação a execuções sequenciais, um conjunto de testes com tamanhos crescentes de domínio foi desenvolvido. O resultado dessa comparação indicou que o simulador implementado em GPU com visualização interativa teve desempenho 71.3 vezes maior em relação à versão sequencial em CPU sem visualização interativa. Dessa forma, a abordagem de paralelização em GPU com visualização interativa mostrou-se muito adequada à execução de simulações fluidodinâmicas, sendo uma ferramenta útil no estudo de escoamentos fluídicos, capaz de executar inúmeros cálculos e lidar com a grande quantidade de memória exigida por simulações fluídicas
Abstract: Recent advances on multicore and many-core processor technology have popularized the parallel computing, accelerating program execution and enabling the simulation of larger domains. Within the complex problems that require a high processing speed, the computational fluid dynamics problems stand out, since their simulations tend to have high computational cost and demand large simulation domains. The method based on the Lattice Boltzmann is an appropriate computational fluid dynamics algorithm to explore parallelism that has been noteworthy in scientific community. Although there are several works that approach GPU parallelism in this method, an efficient simulator implementation and interactive visualization have not been explored adequately. Thus, the purpose of this work is to implement in GPU an interactive fluid dynamics simulator based on the Lattice Boltzmann method. First, the simulator was developed in C language and was parallelized in CPU using MPI. Next, it was parallelized in GPU using CUDA and converted into C++ object-oriented language. Then, the interactive visualization was added using techniques such as CUDA-OpenGL interoperability, 3D texturing, GPU programmable pipeline, and interaction features. The simulator was validated for 2D and 3D cases in single component, single phase flows. Besides that, to show the performance gain in processing velocity of parallelized problems in relation to sequential executions, a test set with increasing domain sizes was developed. This comparison result indicated the GPU-implemented interactive simulator was 71.3 times faster in relation to sequential CPU version without interactive visualization. Thereby, the GPU parallelization approach with interactive visualization showed to be very adequate to fluid dynamics simulations, being a useful tool in fluid flow study, capable of simulating numerous calculations and dealing with the large amount of memory required in fluidic simulations
Mestrado
Mecanica dos Sólidos e Projeto Mecanico
Mestra em Engenharia Mecânica
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Pogorelcnik, Romain. "Decomposition by complete minimum separators and applications". Thesis, Clermont-Ferrand 2, 2012. http://www.theses.fr/2012CLF22301/document.
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Nous avons utilisé la décomposition par séparateurs minimaux complets. Pour décomposer un graphe G, il est nécessaire de trouver les séparateurs minimaux dans le graphe triangulé H correspondant. Dans ce contexte, nos premiers efforts se sont tournés vers la détection de séparateurs minimaux dans un graphe triangulé. Nous avons défini une structure, que nous avons nommée 'atom tree'. Cette dernière est inspirée du 'clique tree' et permet d'obtenir et de représenter les atomes qui sont les produits de la décomposition. Lors de la manipulation de données à l'aide de treillis de Galois, nous avons remarqué que la décomposition par séparateurs minimaux permettait une approche de type `Diviser pour régner' pour les treillis de Galois. La détection des gènes fusionnés, qui est une étape importante pour la compréhension de l'évolution des espèces, nous a permis d'appliquer nos algorithmes de détection de séparateurs minimaux complets, qui nous a permis de détecter et regrouper de manière efficace les gènes fusionnés. Une autre application biologique fut la détection de familles de gènes d'intérêts à partir de données de niveaux d'expression de gènes. La structure de `l'atom tree' nous a permis d'avoir un bon outils de visualisation et de gérer des volumes de données importantesWe worked on clique minimal separator decomposition. In order to compute this decomposition on a graph G we need to compute the minimal separators of its triangulation H. In this context, the first efforts were on finding a clique minimal separators in a chordal graph. We defined a structure called atom tree inspired from the clique tree to compute and represent the final products of the decomposition, called atoms. The purpose of this thesis was to apply this technique on biological data. While we were manipulating this data using Galois lattices, we noticed that the clique minimal separator decomposition allows a divide and conquer approach on Galois lattices. One biological application of this thesis was the detection of fused genes which are important evolutionary events. Using algorithms we produced in the course of along our work we implemented a program called MosaicFinder that allows an efficient detection of this fusion event and their pooling. Another biological application was the extraction of genes of interest using expression level data. The atom tree structure allowed us to have a good visualization of the data and to be able to compute large datasets
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Gastineau, Nicolas. "Partitionnement, recouvrement et colorabilité dans les graphes". Thesis, Dijon, 2014. http://www.theses.fr/2014DIJOS014/document.
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Nos recherches traitent de coloration de graphes avec des contraintes de distance (coloration de packing) ou des contraintes sur le voisinage (coloration de Grundy). Soit S={si| i in N*} une série croissante d’entiers. Une S -coloration de packing est une coloration propre de sommets telle que tout ensemble coloré i est un si-packing (un ensemble où tous les sommets sont à distance mutuelle supérieure à si). Un graphe G est (s1,... ,sk)-colorable si il existe une S -coloration de packing de G avec les couleurs 1, ...,,k. Une coloration de Grundy est une coloration propre de sommets telle que pour tout sommet u coloré i, u est adjacent à un sommet coloré j, pour chaque ji. Ces résultats nous permettent de déterminer des S-colorations de packings de ces grilles pour plusieurs séries d’entiers. Nous examinons une classe de graphe jamais étudiée en ce qui concerne la S -coloration de packing: les graphes subcubiques. Nous déterminons que tous les graphes subcubiques sont (1,2,2,2,2,2,2)-colorables et (1,1,2,2,3)-colorables. Un certain nombre de résultats sont prouvés pour certaines sous-classes des graphes subcubiques. Pour finir, nous nous intéressons au nombre de Grundy des graphes réguliers. Nous déterminons une caractérisation des graphes cubiques avec un nombre de Grundy de 4. De plus, nous prouvons que tous les graphes r-réguliers sans carré induit ont pour nombre de Grundy de r+1, pour r<5Our research are about graph coloring with distance constraints (packing coloring) or neighborhood constraints (Grundy coloring). Let S={si| i in N*} be a non decreasing sequence of integers. An S-packing coloring is a proper coloring such that every set of color i is an si-packing (a set of vertices at pairwise distance greater than si). A graph G is (s1,... ,sk)-colorable if there exists a packing coloring of G with colors 1,... ,k. A Grundy coloring is a proper vertex coloring such that for every vertex of color i, u is adjacent to a vertex of color j, for each ji. These results allow us to determine S-packing coloring of these lattices for several sequences of integers. We examine a class of graph that has never been studied for S-packing coloring: the subcubic graphs. We determine that every subcubic graph is (1,2,2,2,2,2,2)-colorable and (1,1,2,2,3)-colorable. Few results are proven about some subclasses. Finally, we study the Grundy number of regular graphs. We determine a characterization of the cubic graphs with Grundy number 4. Moreover, we prove that every r-regular graph without induced square has Grundy number r+1, for r<5
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Dao, Ngoc Bich. "Réduction de dimension de sac de mots visuels grâce à l’analyse formelle de concepts". Thesis, La Rochelle, 2017. http://www.theses.fr/2017LAROS010/document.
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La réduction des informations redondantes et/ou non-pertinentes dans la description de données est une étape importante dans plusieurs domaines scientifiques comme les statistiques, la vision par ordinateur, la fouille de données ou l’apprentissage automatique. Dans ce manuscrit, nous abordons la réduction de la taille des signatures des images par une méthode issue de l’Analyse Formelle de Concepts (AFC), qui repose sur la structure du treillis des concepts et la théorie des treillis. Les modèles de sac de mots visuels consistent à décrire une image sous forme d’un ensemble de mots visuels obtenus par clustering. La réduction de la taille des signatures des images consiste donc à sélectionner certains de ces mots visuels. Dans cette thèse, nous proposons deux algorithmes de sélection d’attributs (mots visuels) qui sont utilisables pour l’apprentissage supervisé ou non. Le premier algorithme, RedAttSansPerte, ne retient que les attributs qui correspondent aux irréductibles du treillis. En effet, le théorème fondamental de la théorie des treillis garantit que la structure du treillis des concepts est maintenue en ne conservant que les irréductibles. Notre algorithme utilise un graphe d’attributs, le graphe de précédence, où deux attributs sont en relation lorsque les ensembles d’objets à qui ils appartiennent sont inclus l’un dans l’autre. Nous montrons par des expérimentations que la réduction par l’algorithme RedAttsSansPerte permet de diminuer le nombre d’attributs tout en conservant de bonnes performances de classification. Le deuxième algorithme, RedAttsFloue, est une extension de l’algorithme RedAttsSansPerte. Il repose sur une version approximative du graphe de précédence. Il s’agit de supprimer les attributs selon le même principe que l’algorithme précédent, mais en utilisant ce graphe flou. Un seuil de flexibilité élevé du graphe flou entraîne mécaniquement une perte d’information et de ce fait une baisse de performance de la classification. Nous montrons par des expérimentations que la réduction par l’algorithme RedAttsFloue permet de diminuer davantage l’ensemble des attributs sans diminuer de manière significative les performances de classificationIn several scientific fields such as statistics, computer vision and machine learning, redundant and/or irrelevant information reduction in the data description (dimension reduction) is an important step. This process contains two different categories : feature extraction and feature selection, of which feature selection in unsupervised learning is hitherto an open question. In this manuscript, we discussed about feature selection on image datasets using the Formal Concept Analysis (FCA), with focus on lattice structure and lattice theory. The images in a dataset were described as a set of visual words by the bag of visual words model. Two algorithms were proposed in this thesis to select relevant features and they can be used in both unsupervised learning and supervised learning. The first algorithm was the RedAttSansPerte, which based on lattice structure and lattice theory, to ensure its ability to remove redundant features using the precedence graph. The formal definition of precedence graph was given in this thesis. We also demonstrated their properties and the relationship between this graph and the AC-poset. Results from experiments indicated that the RedAttsSansPerte algorithm reduced the size of feature set while maintaining their performance against the evaluation by classification. Secondly, the RedAttsFloue algorithm, an extension of the RedAttsSansPerte algorithm, was also proposed. This extension used the fuzzy precedence graph. The formal definition and the properties of this graph were demonstrated in this manuscript. The RedAttsFloue algorithm removed redundant and irrelevant features while retaining relevant information according to the flexibility threshold of the fuzzy precedence graph. The quality of relevant information was evaluated by the classification. The RedAttsFloue algorithm is suggested to be more robust than the RedAttsSansPerte algorithm in terms of reduction
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Mohanta, Zinia. "Engineering Graphene Oxide Surface Chemistry and Investigating Its Effect on The Physicochemical and Biological Properties". Thesis, 2021. https://etd.iisc.ac.in/handle/2005/4902.
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In the current study, systematic tuning of oxidation degree of GO has been explored and a two-step methodology for engineering GO surface chemistry was successfully developed. The investigation of the effects of oxidation degree of GO on its physicochemical properties such as nuclear relaxivity and magnetic resonance imaging (MRI) contrast enhancement and biological properties such as cytotoxicity, drug release kinetics and biomolecule conjugation efficacy were undertaken.
Defect density in the graphite lattice increases with ball-milling time and these induced defects increase the reactivity of the ball-milled graphite. Guided by this, we designed a methodology involving mechanical milling of high purity graphite for different times followed by oxidation of the ball-milled graphite, for synergistically tuning the total oxygen content, relative abundance of different oxygen moieties and size of the resultant GO sheets. The work showcases extensive characterization of the various properties of GO and their interrelation. It also demonstrates how the nature of defects of graphitic precursor can affect the morphology and oxygen content of GO. GO has been used as a nanoplatform for inorganic nanoparticles for designing MRI contrast agents, however the effects of GO on relaxation property of water remains scarcely explored. In this work, the relaxation behavior of GO was probed using nuclear magnetic resonance (NMR) and MRI techniques. Interestingly, the GO samples produced form graphite ball-milled for different times exhibited different relaxivities and different MRI contrast enhancement. The various physicochemical properties responsible for relaxation behavior of GO was investigated and a strong combinatorial effect of manganese paramagnetic ions and oxidation degree of GO on the corresponding relaxivities was noticed. Further, the toxicological effect of the GO series (GO_0, GO_10, GO_20, GO_30_GO_50, GO_80, GO_100) was studied. All GO were loaded with doxorubicin as model drug and the drug release kinetics were observed at different pH. The different GO samples were covalently conjugated with Hyaluronic acid (HA), the model biomolecule taken for the study, and the conjugation efficacy of GO was inspected. Studying these properties can facilitate development of GO based multifunctional agents for cancer cell imaging, targeting and therapy.
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Sio, Kuan-Un, i 蕭君源. "Multi-Functional Light Emitter Based on Band-Edge Modes Near Γ-Point in Graphite Lattice Photonic Crystal". Thesis, 2011. http://ndltd.ncl.edu.tw/handle/60622431264128165064.
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碩士國立交通大學
光電工程學系
99
Photonic band-edge mode utilizes the flat photonic band near lattice symmetry point, enhances light-gain interaction and achieves lasing operation due to the slow light effect. Among those symmetry points, band-edge mode near Γ-point with almost zero in-plane wave vector component favor for directional surface emission. In this thesis, we successfully demonstrated a multi-functional light emitter based on band-edge modes near Γ-point of graphite lattice in two-dimensional photonic crystal slab. Compare to typical triangular or square lattice, graphite lattice with reduced air hole density leads to larger gain volume and better heat dissipation. At first, the photonic band diagram and related band-edge mode profiles of graphite lattice are simulated and characterized by 3D plane-wave-expansion method. And real devices with different lattice constant are fabricated by series of semiconductor fabrication process. By measuring the optical properties of band-edge modes near different Γ-point, including the monopole (Γ2), dipole (Γ4,5), and quadrupole (Γ6,7) modes. We observed single mode lasing action from monopole and quadrupole modes at room temperature respectively. The monopole mode with high side-mode suppression-ratio of 30 dB shows its ability for optical micro-laser application. And quadrupole mode exhibit high index sensitivity of 375 nm/RIU by the measuring lasing spectra with different index match liquid, show its potential in refractive index sensor for biochemical detection. Besides these two lasing modes, over five-fold photoluminescence enhancement with broad band-width of 100 nm from the dipole mode is also observed, which is promising in realize high brightness light-emitting-diodes.
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Sanchez-Cortezon, Emilio [Verfasser]. "The role of lattice defects in the catalytic oxidation of methanol over graphite catalysts / vorgelegt von Emilio Sanchez-Cortezon". 2003. http://d-nb.info/967765366/34.
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Clough, Duncan. "Lattice Boltzmann Liquid Simulations on Graphics Hardware". Thesis, 2013. http://pubs.cs.uct.ac.za/archive/00000951/.
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Fluid simulation is widely used in the visual effects industry. The high level of detail required to produce realistic visual effects requires significant computation. Usually, expensive computer clusters are used in order to reduce the time required. However, general purpose Graphics Processing Unit (GPU) computing has potential as a relatively inexpensive
way to reduce these simulation times. In recent years, GPUs have been used to achieve enormous speedups via their massively parallel architectures. Within the field of fluid simulation, the Lattice Boltzmann Method (LBM) stands out as a candidate for GPU execution because its grid-based structure is a natural fit for GPU parallelism.
This thesis describes the design and implementation of a GPU-based free-surface LBM fluid simulation. Broadly, our approach is to ensure that the steps that perform most of the work in the LBM (the stream and collide steps) make efficient use of GPU resources. We achieve this by removing complexity from the core stream and collide steps and handling interactions with obstacles and tracking of the fluid interface in separate GPU kernels.
To determine the efficiency of our design, we perform separate, detailed analyses of the performance of the kernels associated with the stream and collide steps of the LBM. We demonstrate that these kernels make efficient use of GPU resources and achieve speedups of 29.6 and 223.7, respectively. Our analysis of the overall performance of all kernels
shows that significant time is spent performing obstacle adjustment and interface movement as a result of limitations associated with GPU memory accesses. Lastly, we compare our GPU LBM implementation with a single-core CPU LBM implementation. Our results show speedups of up to 81.6 with no significant differences in output from the simulations
on both platforms.
We conclude that order of magnitude speedups are possible using GPUs to perform free-surface LBM fluid simulations, and that GPUs can, therefore, significantly reduce the cost of performing high-detail fluid simulations for visual effects.
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Dammertz, Sabrina [Verfasser]. "Rank-1 lattices in computer graphics / Sabrina Dammertz". 2009. http://d-nb.info/997297174/34.
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Lin, Bo-sing, i 林柏興. "Graphene Oxide Cathode of Field Emission Emitter with Square Lattice Island Array". Thesis, 2016. http://ndltd.ncl.edu.tw/handle/58838607041257048340.
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碩士國立中山大學
電機工程學系研究所
104
Graphene has attracted the attention of most scholars for its unique properties, such as high electron mobility, high thermal conductivity, low resistivity and other physical characteristics which was discovered by A.K Geim since 2004. Graphene can be widely applied such as transistor, sensor, field emission device and other research. This thesis is divided into three parts: First of all, the graphene oxide is made by electrochemical exfoliation. Secondly, to make graphene oxide as square lattice island array is using screen printing method. Lastly, the field emission of graphene oxide patterns is measured in different diameter. Graphene oxide is made from highly oriented pyrolytic graphite by electrochemical exfoliation. Before the screen printing process, graphene oxide is analyzed with Raman spectroscopy, XRD, and XPS, which makes sure the quality and purity. In the research, use the AutoCAD software to design the mask in different diameter for the screen printing. In OM and SEM images, the screen printed graphene oxide with the island patterns constructing perpendicular edge shows the best aspect ratio, and then all of them has become square lattice array emitter. When the screen-printed graphene oxide in different diameter is measured, it’s found that the best performance is the diameter of 150μm. The emission behavior of the diameter in 150μm is 1.65 V⁄μm with the maximum emission current of 20.8μA and the field enhancement factor of 15060. It''s worth noting that the diameter in 100μm shows the worst emission behavior because of the screening effect. The luminance measurement of the best condition coating the phosphor shows the maximum amount of green light with the wavelength at 562 nm and the lumens of 5.04 when the emission current is up to 20.8μA.
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Carlstrand, Harald, Björn Rosén i Helena Andersson. "Elastic properties of Heusler alloys by ab initio theory and lattice dynamics of graphene". Thesis, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-226508.
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This study investigates elastic properties and lattice dynamics of promising compounds such as Heusler alloys and 2D graphene using Density Functional Theory (DFT). Simulations have been done in the software Quantum Espresso (Q.E) to produce a good model for the different solids. The lattice dynamics and elastic properties have been compared to experimental data where possible. In conclusion, the systems are stable and provide good candidates for future applications.
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Kitt, Alexander. "Manipulating graphene's lattice to create pseudovector potentials, discover anomalous friction, and measure strain dependent thermal conductivity". Thesis, 2014. https://hdl.handle.net/2144/15101.
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Graphene is a single atomic sheet of graphite that exhibits a diverse range of unique properties. The electrons in intrinsic graphene behave like relativistic Dirac fermions; graphene has a record high Young's modulus but extremely low bending rigidity; and suspended graphene exhibits very high thermal conductivity. These properties are made more intriguing because with a thickness of only a single atomic layer, graphene is both especially affected by its environment and readily manipulated. In this dissertation the interaction between graphene and its environment as well as the exciting new physics realized by manipulating graphene's lattice are investigated.
Lattice manipulations in the form of strain cause alterations in graphene's electrical dispersion mathematically analogous to the vector potential associated with a magnetic field. We complete the standard description of the strain-induced vector potential by explicitly including the lattice deformations and find new, leading order terms. Additionally, a strain engineered device with large, localized, plasmonically enhanced pseudomagnetic fields is proposed to couple light to pseudomagnetic fields.
Accurate strain engineering requires a complete understanding of the interactions between a two dimensional material and its environment, particularly the adhesion and friction between graphene and its supporting substrate. We measure the load dependent sliding friction between mono-, bi-, and trilayer graphene and the commonly used silicon dioxide substrate by analyzing Raman spectra of circular, graphene sealed microchambers under variable external pressure. We find that the sliding friction for trilayer graphene behaves normally, scaling with the applied load, whereas the friction for monolayer and bilayer graphene is anomalous, scaling with the inverse of the strain in the graphene.
Both strain and graphene's environment are expected to affect the quadratically dispersed out of plane acoustic phonon. Although this phonon is believed to provide the majority of graphene's very high thermal conductivity, its contributions have never been isolated. By measuring strain and pressure dependent thermal conductivity, we gain insight into the mechanism of graphene's thermal transport.
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Wang, Sheng. "Artificial Graphene in Nano-patterned GaAs Quantum Wells and Graphene Growth by Molecular Beam Epitaxy". Thesis, 2016. https://doi.org/10.7916/D8DF6RGW.
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In this dissertation I present advances in the studies of artificial lattices with honeycomb topology, called artificial graphene (AG), in nano-patterned GaAs quantum wells (QWs). AG lattices with very small lattice constants as low as 40 nm are achieved for the first time in GaAs. The high quality AG lattices are created by optimized electron-beam (E-beam) lithography followed by inductively coupled plasma reactive-ion etching (ICP-RIE) process. E-beam lithography is used to define a honeycomb lattice etch mask on the surface of the GaAs QW sample and the optimized anisotropic ICP-RIE process is developed to transfer the pattern into the sample and create the AG lattices. Such high-resolution AG lattices with small lattice constants are essential to form AG miniband structures and create well-developed Dirac cones.
Characterization of electron states in the nanofabricated artificial lattices is by optical experiments. Optical emission (photoluminescence) yields a determination of the Fermi energy of the electrons. A significant reduction of the Fermi energy is due to the nano-patterning process. Resonant inelastic light scattering (RILS) spectra reveal novel transitions related to the electron band structures of the AG lattices. These transitions exhibit a remarkable agreement with the predicted joint density of states (JDOS) based on the band structure calculation for the honeycomb topology.
I calculate the electron band structures of AG lattices in nano-patterned GaAs QWs using a periodic muffin-tin potential model. The evaluations predict linear energy-momentum dispersion and Dirac cones, where the massless Dirac fermions (MDFs) appear, occur in the band structures. Requirements of the parameters of the AG potential to achieve isolated and well-developed Dirac cones are discussed. Density of states (DOS) and JDOS from AG band structures are calculated, which provide a basis to interpret quantitatively observed transitions of electrons involving AG bands.
RILS of intersubband transitions reveal intriguing satellite peaks that are not present in the as-grown QWs. These additional peaks are interpreted as combined intersubband transitions with simultaneous change of QW subband and AG band index. The calculated JDOS for the electron transitions within the AG lattice model provide a remarkably accurate description of the combined intersubband excitations.
Novel low-lying excitation peaks in RILS spectra, interpreted as direct transitions between AG bands without change in QW subband, provide a more direct insight on the AG band structures. We discovered that RILS transitions around the Dirac cones are resonantly enhanced by varying the incident photon energies. The spectral lineshape of these transitions provides insights into the formation of Dirac cones that are characteristic of the honeycomb symmetry of the AG lattices. The results confirm the formation of AG miniband structures and well-developed Dirac cones. The realization of AG lattices in a nanofabricated high mobility semiconductor offers the advantage of tunability through methods suitable for device scalability and integration.
The last part of this thesis describes the growth of nanocrystalline single layer and bilayer graphene on sapphire substrates by molecular beam epitaxy (MBE) with a solid carbon source. Raman spectroscopy reveals that fabrication of single layer, bilayer or multilayer graphene crucially depends on MBE growth conditions. Etch pits revealed by atomic force microscopy indicate a removal mechanism of carbon by reduction of sapphire. Tuning the interplay between carbon deposition and its removal, by varying the incident carbon flux and substrate temperature, should enable the growth of high quality graphene layers on large area sapphire substrates.
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"Quantum Monte Carlo study of frustrated systems". Thesis, 2010. http://library.cuhk.edu.hk/record=b6074995.
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In the chapter 3, we study ferromagnetic fluctuations on two types of bilayer triangular lattices by the single-band Hubbard model. We start from the tight-binding model to obtain energy spectrum, the density of sates, and the spin susceptibility. With finite Coulomb interaction turned on, we apply the random phase approximation and use the determinant quantum Monte Carlo method to study spin susceptibility for the two bilayer triangular lattices and make comparisons of their magnetic properties. The effects of the interlayer coupling is also examined in detail.In the chapter 4, we addresses the issue of the ferromagnetism in graphene-based samples. To study magnetic correlations in graphene, we systematically carry out quantum Monte Carlo simulations of the Hubbard model on a honeycomb lattice. In the filling region below the Van Hove singularity, the system shows a short-range ferromagnetic correlation, which is slightly strengthened by the on-site Coulomb interaction and markedly by the next-nearest-neighbor hopping integral. The ferromagnetic properties depend on the electron filling strongly, which may be manipulated by the electric gate. Due to its resultant high controllability of ferromagnetism, graphene-based samples may facilitate the new development of many applications.
In the chapter 5, we examined theoretically the magnetism of impurity adatoms in graphene by quantum Monte Carlo simulation technique based on Hirsch-Fye algorithm. When tuning the Fermi energy of graphene by gate voltage with available experiments, the values of occupancy and local moment for impurity can be changed. Furthermore, with medium and large hybridizations between impurity and graphene atoms, the local moment can be switched on and off by Kondo effects. We also use maximum entropy method to study the spectral density from Green's function for impurity, and we find very unconventional behaviors which are absolutely different from the cases in the normal metal. These signatures of spectral density enlarge the possibility for controlling the impurity magnetism by gate voltage.
In this research thesis, we mainly study three strongly correlated systems: Hubbard model in bilayer triangular lattice which corresponds to the real material of NaxCoO2 · yH 2O, strong-interaction electrons in graphene system and Anderson impurity in graphene. Our numerical method is determinant quantum Monte Carlo method which will be introduced in the chapter 2.
Hu, Feiming = 阻錯系統的量子門特卡洛研究 / 胡飛鳴.
Adviser: Lin Hai-Qing.
Source: Dissertation Abstracts International, Volume: 73-01, Section: B, page: .
Thesis (Ph.D.)--Chinese University of Hong Kong, 2010.
Includes bibliographical references (leaves 107-126).
Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web.
Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [201-] System requirements: Adobe Acrobat Reader. Available via World Wide Web.
Abstract also in Chinese.
Hu, Feiming = Zu cuo xi tong de liang zi Mente Kaluo yan jiu / Hu Feiming.