5,628 research outputs found

    Beam scanning by liquid-crystal biasing in a modified SIW structure

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    A fixed-frequency beam-scanning 1D antenna based on Liquid Crystals (LCs) is designed for application in 2D scanning with lateral alignment. The 2D array environment imposes full decoupling of adjacent 1D antennas, which often conflicts with the LC requirement of DC biasing: the proposed design accommodates both. The LC medium is placed inside a Substrate Integrated Waveguide (SIW) modified to work as a Groove Gap Waveguide, with radiating slots etched on the upper broad wall, that radiates as a Leaky-Wave Antenna (LWA). This allows effective application of the DC bias voltage needed for tuning the LCs. At the same time, the RF field remains laterally confined, enabling the possibility to lay several antennas in parallel and achieve 2D beam scanning. The design is validated by simulation employing the actual properties of a commercial LC medium

    Ultrafast Optical Control of Order Parameters in Quantum Materials

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    Developing protocols to realize quantum phases that are not accessible thermally and to manipulate material properties on demand is one of the central problems of modern condensed matter physics. Impulsive electromagnetic stimulus provides an extensive playground not only to exert desired control over the material macroscopic properties but also to optically detect the underlying microscopic mechanisms. Two indispensable components form the cornerstone to realize these goals: a meticulous comprehension of light-induced phenomena and a suitable and versatile platform. Abundant photoinduced phenomena emerge upon light irradiation. A collective oscillation of order parameter can be launched and probed in the weak perturbation regime; further increasing light intensity can transiently modulate the free-energy landscape, inducing a suppression, enhancement, reversal, and switch of order parameters; in the strong non-perturbative excitation regime, the system can be driven nonlinearly with microscopic coupling parameters modified. Understanding these light driven emergent phenomena lays the foundation of optical control and novel functionalities. Quantum materials, embodying a large portfolio of topological and strongly correlated compounds, afford an exceptional venue to realize optical control. Owing to the complex interplay between the charge, spin, orbital, and lattice degrees of freedom, a rich phase diagram can be generated with various phases that are selectively and independently accessible via optical perturbations. They hence offer a wealth of opportunities to not only improve our comprehension of the underlying physics but also develop the next generation of ultrafast technologies. In Chapter I of this thesis, I will first cover a multitude of light-induced emergent phenomena in quantum materials under the framework of time-dependent Landau theory, Keldysh theory, and Floquet theory, and then introduce several canonical microscopic models to quantitatively rationalize the intra- and interactions between different degrees of freedom in quantum materials. As the necessary theoretical background is established, three main experimental techniques that have been extensively utilized in my research: time-resolved reflectivity and Kerr effect, time-resolved second harmonic generation rotational anisotropy, and coherent phonon spectroscopy will be introduced in Chapter II. In Chapter III, I will demonstrate that a light-induced topological phase transition can be engendered concomitant with an inverse-Peierls structural phase transition in elemental Te. In Chapter IV, I will describe signatures of ultrafast reversal of excitonic order in excitonic insulator candidate Ta2NiSe5 and substantiate a manipulation of the reversal as well as the Higgs mode with tailored light pulses. In Chapter V, a light-induced switch of spin-orbit-coupled quadrupolar order in multiband Mott insulator Ca2RuO4 will be introduced. In Chapter VI, a Keldysh tuning of nonlinear carrier excitation and Floquet bandwidth renormalization in strongly driven Ca2RuO4 will be covered.</p

    Advances in Micro- and Nanomechanics

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    This book focuses on recent advances in both theoretical and experimental studies of material behaviour at the micro- and nano-scales. Special attention is given to experimental studies of nanofilms, nanoparticles and nanocomposites as well as tooth defects. Various experimental techniques were used. Magneto- and thermoelastic coupling were considered, as were nonlocal models of thin structures

    Applications and Properties of Magnetic Nanoparticles

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    This Special Issue aimed to cover the new developments in the synthesis and characterization of magnetic nanoconstructs ranging from conventional metal oxide nanoparticles to novel molecule-based or hybrid multifunctional nano-objects. At the same time, the focus was on the potential of these novel magnetic nanoconstructs in several possible applications, e.g. sensing, energy storage, and nanomedicine

    Modeling and Simulation in Engineering

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    The Special Issue Modeling and Simulation in Engineering, belonging to the section Engineering Mathematics of the Journal Mathematics, publishes original research papers dealing with advanced simulation and modeling techniques. The present book, “Modeling and Simulation in Engineering I, 2022”, contains 14 papers accepted after peer review by recognized specialists in the field. The papers address different topics occurring in engineering, such as ferrofluid transport in magnetic fields, non-fractal signal analysis, fractional derivatives, applications of swarm algorithms and evolutionary algorithms (genetic algorithms), inverse methods for inverse problems, numerical analysis of heat and mass transfer, numerical solutions for fractional differential equations, Kriging modelling, theory of the modelling methodology, and artificial neural networks for fault diagnosis in electric circuits. It is hoped that the papers selected for this issue will attract a significant audience in the scientific community and will further stimulate research involving modelling and simulation in mathematical physics and in engineering

    Electrical and Optical Modeling of Thin-Film Photovoltaic Modules

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    Heutzutage ist durch viele wissenschaftliche Studien nachgewiesen, dass die Erde lĂ€ngst dem Klimawandel unterworfen ist. Daher muss die gesamte Menschheit vereint handeln, um die schlimmsten Katastrophenszenarien zu verhindern. Ein vielversprechender Ansatz - wenn nicht sogar der vielversprechendste ĂŒberhaupt - um diese angesprochene, grĂ¶ĂŸte Herausforderung in der Geschichte der Menschheit zu bewĂ€ltigen, ist es, den Energiehunger der Menschheit durch die Erzeugung erneuerbarer und unerschöpflicher Energie zu sĂ€ttigen. Die Photovoltaik (PV)-Technologie ist ein vielversprechender AnwĂ€rter, die leistungsstĂ€rkste erneuerbare Energiequelle zu stellen, und spielt aufgrund ihrer direkten Umwandlung des Sonnenlichtes und ihrer skalierbaren Anwendbarkeit in Form von großflĂ€chigen Solarmodulen bereits jetzt eine große Rolle bei der Erzeugung erneuerbarer Energie. Im PV-Sektor sind Solarmodule aus Siliziumwafern die derzeit vorherrschende Technologie. Neu aufkommende PV-Technologien wie die DĂŒnnschichttechnologie haben jedoch vorteilhafte Eigenschaften wie einen sehr geringen Kohlenstoffdioxid (CO2)-Fußabdruck, eine kurze energetische Amortisierungszeit und das Potenzial fĂŒr eine kostengĂŒnstige monolithische Massenproduktion, obwohl diese derzeit noch nicht final ausgereift ist. Um die DĂŒnnschichttechnologie jedoch gezielt in Richtung einer breiten Marktreife zu entwickeln, sind numerische Simulationen eine wichtige SĂ€ule fĂŒr das wissenschaftliche VerstĂ€ndnis und die technologische Optimierung. WĂ€hrend sich traditionelle Simulationsliteratur hĂ€ufig mit materialspezifischen Herausforderungen befasst, konzentriert sich diese Arbeit auf industrieorientierte Herausforderungen auf Modulebene, ohne die zugrundeliegenden Materialparameter zu verĂ€ndern. Um ein allumfassendes, digitales Modell eines Solarmoduls zu erstellen, werden in dieser Arbeit mehrere SimulationsansĂ€tze aus verschiedenen physikalischen Bereichen kombiniert. Zur Abbildung elektrischer Effekte, einschließlich der rĂ€umlichen Spannungsvariation innerhalb des Moduls, wird eine Finite Elemente Methode (FEM) zur Lösung der rĂ€umlich quantisierten Poisson-Gleichung verwendet. Um optische Effekte zu berĂŒcksichtigen, wird eine generalisierte Transfermatrix-Methode (TMM) verwendet. Alle Simulationsmethoden sind in dieser Arbeit von Grund auf neu programmiert worden, um eine VerknĂŒpfung aller Simulationsebenen mit dem höchstmöglichen Grad an Anpassung und VerknĂŒpfung zu ermöglichen. Die Simulation und die Korrektheit der Parameter wird durch externe Quanteneffizienz (EQE)-Messungen, experimentelle Reflexionsdaten und gemessene Strom-Spannungs (I-U)-Kennlinien verifiziert. Der Kernpunkt der Vorgehensweise dieser Arbeit ist eine ganzheitliche Simulationsmethodik auf Modulebene. Dies ermöglicht es, die LĂŒcke zwischen der Simulation auf Materialebene ĂŒber die Berechnung von Laborwirkungsgraden bis hin zur Bestimmung der von zahlreichen Umweltfaktoren beeinflusste Leistung der Module im Freifeld zu ĂŒberbrĂŒcken. Durch diese VerknĂŒpfung von Zellsimulation und Systemdesign ist es lediglich aus Laboreigenschaften möglich, das Freifeldverhalten von Solarmodulen zu prognostizieren. Sogar das ZurĂŒckrechnen von experimentellen Messungen zu Materialparameter ist mittels des in dieser Arbeit entwickelten Verfahrens des Reverse Engineering Fittings (REF) möglich. Das in dieser Arbeit entwickelte numerische Verfahren kann fĂŒr mehrere Anwendungen genutzt werden. ZunĂ€chst können durch die Kombination von elektrischen und optischen Simulationen ganzheitliche Top-Down-Verlustanalysen durchgefĂŒhrt werden. Dies ermöglicht eine wissenschaftliche Einordnung und einen quantitativen Vergleich aller Verlustleistungsmechanismen auf einen Blick, was die zukĂŒnftige Forschung und Entwicklung in Richtung von technologischen Schwachstellen von Solarmodulen lenkt. DarĂŒber hinaus ermöglicht die Kombination von Elektrik und Optik die Detektion von Verlusten, die auf dem nichtlinearen Zusammenspiel dieser beiden Ebenen beruhen und auf eine rĂ€umliche Spannungsverteilung im Solarmodul zurĂŒckzufĂŒhren sind. Diese Arbeit verwendet die entwickelten numerischen Modelle ebenfalls fĂŒr Optimierungsprobleme, die an digitalen Modellen realer Solarmodule durchgefĂŒhrt werden. HĂ€ufig auftretende Fragestellungen bei der Entwicklung von Solarmodulen sind beispielsweise die Schichtdicke des vorderen optisch transparenten, elektrisch leitfĂ€higen Oxids (TCO) oder die Breite von monolithisch verschalteten Zellen. Die Bestimmung des Optimums dieser mehrdimensionalen AbwĂ€gungen zwischen optischer Transparenz, elektrischer LeitfĂ€higkeit und geometrisch inaktiver FlĂ€che zwischen den einzelnen Zellen ist ein Hauptmerkmal der Methodik dieser Arbeit. Mittels des FEM-Ansatzes dieser Arbeit ist es möglich, alle gegenseitigen Wechselwirkungen ĂŒber verschiedene physikalische Ebenen hinweg zu berĂŒcksichtigen und ein ganzheitlich optimiertes Moduldesign zu finden. Auch topologisch komplexere Probleme, wie das Finden eines geeigneten Designs fĂŒr das Metallisierungsgitter, können auf Grundlage der Simulation mittels der Methode der Topologie-Optimierung (TO) gelöst werden. In dieser Arbeit wurde das TO-Verfahren zum ersten Mal fĂŒr monolithisch integrierte Zellen eingesetzt. DarĂŒber hinaus wurde gezeigt, dass sowohl einfache Optimierungen der TCO-Schichtdicken als auch Topologie-Optimierungen stark von den vorherrschenden BeleuchtungsverhĂ€ltnissen abhĂ€ngen. Daher ist eine Optimierung auf den Jahresertrag anstelle des Laborwirkungsgrades fĂŒr industrienahe Anwendungen wesentlich sinnvoller, da die mittleren Jahreseinstrahlungen deutlich von den Laborbedingungen abweichen. Mit Hilfe dieser Ertragsoptimierung wurde in dieser Arbeit fĂŒr die Kupfer-Indium-Gallium-Diselenid CuIn1−x_{1-x}Gax_xSe2_2 (CIGS)-Technologie ein Leistungsgewinn von ĂŒber 1 % im Ertrag fĂŒr einige geografische Standorte und gleichzeitig eine Materialeinsparung fĂŒr die Metallisierungs- und TCO-Schicht von bis zu 50 % errechnet. Mit Hilfe der numerischen Simulationen dieser Arbeit können alle denkbaren technologischen Verbesserungen auf Modulebene in das Modell eingebracht werden. Auf diese Weise wurde das aktuelle technologische Limit fĂŒr CIGS-DĂŒnnschicht-Solarmodule berechnet. Unter Verwendung der Randbedingungen der derzeit verfĂŒgbaren Materialien, Technologie- und Fertigungstoleranzen und des derzeit besten in der Literatur veröffentlichten CIGS-Materials ergibt sich ein theoretisches Wirkungsgradmaximum von 24 % auf Modulebene. Das derzeit beste veröffentlichte Modul mit den gegebenen Restriktionen weist einen Wirkungsgrad von 19,2 % auf [1]. Verbessert sich der CIGS-Absorber vergleichbar mit jenem von Galliumarsenid (GaAs) im Hinblick auf dessen Rekombinationsrate, ergibt sich ein erhöhtes Wirkungsgradlimit von etwa 28 %. Im Falle eines idealen CIGS-Absorbers ohne intrinsische Rekombinationsverluste wird in dieser Arbeit eine maximale Effizienzobergrenze von 29 % berechnet

    Simulating the gravimetric detection of submarines, calculating high-accuracy terrain corrections using LIDAR elevation data and performing a microgravity survey in the Campsie Fells

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    The work in this thesis relates to the field of gravimetry, the measurement of gravitational fields and their variations, which is carried out using highly-sensitive accelerometers known as gravimeters. By using gravimeters to measure the changes in gravitational field strength from place to place, it is possible to detect differences in the concentration of mass around the gravimeter and this has historically been used to monitor geophysical activity (such as variations in groundwater, volcanic activity or glacial mass), for geological exploration (such as searching for mineral or hydrocarbon resources) and many other applications. This work covers a range of topics in gravimetry, starting with the use of computer programs to simulate the gravitational fields that would be generated when a submarine travelled past a stationary gravimeter, or array of gravimeters, situated underwater. This is done with the aim of estimating the efficacy of a new gravimeter known as the ‘Wee-g’ under development at the University of Glasgow at the time of writing and also has applications to the gravitational detection of submarines more generally. The gravitational field of a 100m-long submarine is simulated, using a simplified one-dimensional density profile approximating the real density variations along the length of a large submarine. The simulated gravity field is then compared to the sensitivity of a prototype Wee-g gravimeter of 5”Gal/ √ Hz to give an initial estimate of the maximum detection range of such a signal by the Wee-g, which is found to be approximately 20m. Then, synthetic noisy signals are made by combining the simulated gravity signals with real Wee-g sensor noise data and digital signal processing methods are used to try and recover the corrupted signal from the noise in a way that maximises the detection range. Matched filtering is applied which uses foreknowledge of the signal being searched for to significantly increase the signal to noise ratio (SNR) in the noisy data by an order of magnitude, which increases the Wee-g’s detection range of the modelled submarine to ∌ 30m. In addition, computer programs are made that determine a quantity known as the terrain correction at a given gravity survey point using digitised elevation data describing the surrounding topography. Terrain correction is the effect that the presence of surrounding hills and valleys has on the gravitational field strength at a location and, if it is not accounted for, substantial variations in gravity (and hence, potentially useful information) can be partially or completely obscured. Methods already exist to calculate the terrain correction but these are either slow and laborious, inaccurate (in comparison to contemporary gravimeter performance) or both, while the program presented in this work makes use of modern computing speed and high-accuracy elevation maps to improve on these. The terrain correction program presented here analyses terrain out to a distance of 166.735km from the survey point, using 1m-resolution LiDAR elevation data to describe the nearest 2km2 , and can calculate terrain correction values in approximately 9s when run on a computer with 8GB of RAM. Terrain at all distances from the survey point is modelled using many flat-topped rectangular prisms and the gravitational field strength due to each prism is calculated using an already existing analytic solution. An in-depth analysis of the terrain correction computation of the innermost 2km is carried out to compare the accuracy of the method used with simple analytic solutions. This analysis concludes that terrain corrections can be calculated with an uncertainty of 2”Gal or less when using 1m2 -resolution elevation data, provided the terrain immediately around the survey point has an incline of less than 10◩ . Finally, two gravity surveys carried out in January of 2020 by the author with a Scintrex CG-5 commercial gravimeter are described: one in the Campsie Fells — a range of hills roughly 10km north of Glasgow — and the second in the cloisters of the Gilbert-Scott building on the University of Glasgow campus. The Campsies survey is compared with a gravimeter survey of the same region carried out in 1969 and discrepancies of up to a few mGal are observed, understood to be due to terrain correction inaccuracies in the older survey. Results from the survey in the cloisters are compared to the gravitational field due to underfloor air ducts described by plans of the building but little correlation is found. This is suspected to be the result of either inaccuracies in the building plans or the impact of environmental noise on the measurements

    Elements of Ion Linear Accelerators, Calm in The Resonances, Other_Tales

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    The main part of this book, Elements of Linear Accelerators, outlines in Part 1 a framework for non-relativistic linear accelerator focusing and accelerating channel design, simulation, optimization and analysis where space charge is an important factor. Part 1 is the most important part of the book; grasping the framework is essential to fully understand and appreciate the elements within it, and the myriad application details of the following Parts. The treatment concentrates on all linacs, large or small, intended for high-intensity, very low beam loss, factory-type application. The Radio-Frequency-Quadrupole (RFQ) is especially developed as a representative and the most complicated linac form (from dc to bunched and accelerated beam), extending to practical design of long, high energy linacs, including space charge resonances and beam halo formation, and some challenges for future work. Also a practical method is presented for designing Alternating-Phase- Focused (APF) linacs with long sequences and high energy gain. Full open-source software is available. The following part, Calm in the Resonances and Other Tales, contains eyewitness accounts of nearly 60 years of participation in accelerator technology. (September 2023) The LINACS codes are released at no cost and, as always,with fully open-source coding. (p.2 & Ch 19.10)Comment: 652 pages. Some hundreds of figures - all images, there is no data in the figures. (September 2023) The LINACS codes are released at no cost and, as always,with fully open-source coding. (p.2 & Ch 19.10

    Nanoskalige Spinstrukturen an OberflÀchen aufgrund von Austauschfrustration und Wechselwirkungstermen höherer Ordnung

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    In this thesis, density functional theory (DFT) calculations using the full potential linearized augmented planewave and the projector augmented wave method in combination with an atomistic spin model are employed to explore the effect of higher-order exchange interactions on the magnetic ground state of ultrathin transition metal (TM) films on surfaces. First, a systematic study of higher-order interactions (HOI) beyond the pairwise Heisenberg exchange in magnetic trilayers comprising a single hexagonal Fe or Co layer sandwiched between 4d and 5d TM layers is presented. While for Fe based trilayers HOI terms can range on the same order of magnitude as the Heisenberg exchange interaction, they turn out relatively small for Co based systems. The trends obtained for freestanding trilayers serve as a basis to understand HOI in ultrathin films on surfaces that are amenable to experiments. Further DFT calculations show that HOI can induce spontaneous nanoscale two-dimensional multi-Q states with collinear spin structure in hexagonal Fe/Rh films with different stacking sequence and thickness on the Ir(111) surface studied by experimental collaborators via spin-polarized scanning tunneling microscopy (SP-STM). DFT combined with an atomistic spin model elucidates a competition of frustrated Heisenberg exchange and HOI as the driving mechanism for the formation of the observed complex spin structures while the DMI is weak. The same approach further demonstrates that the interplay of pairwise Heisenberg exchange, HOI and the DMI is responsible for the symmetry and collinearity of spin lattices in Fe monolayers in direct contact with Ir(111): while for fcc-Fe the well-known square nanoskyrmion lattice is obtained as the magnetic ground state, a hexagonal multi-Q state with nearly collinear magnetic moments occurs in hcp-Fe similar to the above-mentioned hexagonal spin structures in Fe/Rh bilayers
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