42 research outputs found

    Advances in the physics studies for the JT-60SA tokamak exploitation and research plan

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    JT-60SA, the largest tokamak that will operate before ITER, has been designed and built jointly by Japan and Europe, and is due to start operation in 2020. Its main missions are to support ITER exploitation and to contribute to the demonstration fusion reactor machine and scenario design. Peculiar properties of JT-60SA are its capability to produce long-pulse, high-ß, and highly shaped plasmas. The preparation of the JT-60SA Research Plan, plasma scenarios, and exploitation are producing physics results that are not only relevant to future JT-60SA experiments, but often constitute original contributions to plasma physics and fusion research. Results of this kind are presented in this paper, in particular in the areas of fast ion physics, high-beta plasma properties and control, and non-linear edge localised mode stability studies.Postprint (published version

    Nanofabrication

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    We face many challenges in the 21st century, such as sustainably meeting the world's growing demand for energy and consumer goods. I believe that new developments in science and technology will help solve many of these problems. Nanofabrication is one of the keys to the development of novel materials, devices and systems. Precise control of nanomaterials, nanostructures, nanodevices and their performances is essential for future innovations in technology. The book "Nanofabrication" provides the latest research developments in nanofabrication of organic and inorganic materials, biomaterials and hybrid materials. I hope that "Nanofabrication" will contribute to creating a brighter future for the next generation

    Modélisation Mathématique et Simulation Numérique de Systèmes Fluides Quantiques

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    The PhD thesis is concerned with the study of a new class of quantum transport models: the quantum fluid models derived from the entropy principle. These models have been derived in two articles published in 2003 and 2005 by Degond, Méhats and Ringhofer in the Journal of Statistical Physics, by adapting to the quantum framework the moment method developed by Levermore in the classical framework. This method consists in taking the moments of the Quantum Liouville equation and closing this system by a local equilibrium (or quantum Maxwellian) defined as the minimizer of a quantum entropy with constraints on some physical quantities such as the mass, current, and energy. The main interest of such macroscopic models is their low cost in terms of numerical implementation compared to microscopic models such as the Schrödinger equation or the Wigner equation. Moreover, such models take implicitly into account collisions which are much more difficult to handle with quantum microscopic models. The goal of this thesis is thus to propose numerical methods to implement these models and to test them on some physical devices.We have started in chapter I by proposing a discretization for the most simple of these models which is the Quantum Drift-Diffusion model on a closed domain. We have then decided in chapter II and III to apply this model to electron transport in semiconductors by choosing as open device the resonant tunneling diode. We have then studied in chapter IV the Isothermal Quantum Euler model, before considering in chapter V the study of non isothermal models such as the Quantum Hydrodynamic and the Quantum Energy Transport models. Finally, chapter VI is concerned with a slightly different problem which is the implementation of an asymptotically stable scheme in the semiclassical limit for the fluid formulation of the Schrödinger equation: the Madelung system.Le sujet de la thèse porte sur l'étude d'une nouvelle classe de modèles de transport quantique: les modèles fluides quantiques issus du principe de minimisation d'entropie. Ces modèles ont été dérivés dans deux articles publiés en 2003 et 2005 par Degond, Méhats et Ringhofer dans Journal of Statistical Physics en adaptant au cadre de la théorie quantique la méthode des moments développée par Levermore dans le cadre classique. Cette méthode consiste à prendre les moments de l'équation de Liouville quantique et à fermer ce système par un équilibre local (ou Maxwellienne quantique) défini comme minimiseur d'une certaine entropie quantique sous contrainte de conservation de certaines quantités physiques comme la masse, le courant, et l'énergie. Le principal intérêt des modèles quantiques ainsi obtenus provient du fait qu'étant macroscopiques, ils sont biens moins coûteux numériquement que des modèles microscopiques comme l'équation de Schrödinger ou l'équation de Wigner, et de plus, ils prennent en compte implicitement des effets de collision bien plus difficiles à modéliser à un niveau microscopique. Le but de cette thèse est donc de proposer des méthodes numériques pour implémenter ces modèles et de les tester sur des dispositifs physiques adéquats.Nous avons donc commencé dans le chapitre I par proposer une discrétisation du plus simple de ces modèles qu'est le modèle de Dérive-Diffusion Quantique sur un domaine fermé. Puis nous avons décidé dans le chapitre II et III d'appliquer ce modèle au transport d'électrons dans les semiconducteurs en choisissant comme dispositif ouvert la diode à effet tunnel résonnant. Ensuite nous nous sommes intéressés au chapitre IV à l'étude et l'implémentation du modèle d'Euler Quantique Isotherme, avant de s'attaquer aux modèles non isothermes dans le chapitre V avec l'étude des modèles d'Hydrodynamique Quantique et de Transport d'Énergie Quantique. Enfin, le chapitre VI s'intéresse à un problème un petit peu différent en proposant un schéma asymptotiquement stable dans la limite semi-classique pour l'équation de Schrödinger écrite dans sa formulation fluide: le système de Madelung

    Quantum transport and spin-orbit coupling in semiconductor nanostructures

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    Semiconductor nanostructures present a promising path to enhance current computing technology and develop quantum information technologies. The electronic structure of these nanostructures has direct implications on the transport of electrons through them and their potential device applications. This thesis explores the electronic structures of various semiconducting nanostructures and their dependence on electromagnetic fields, dopant atoms, spin-orbit coupling and atomistic features through experiment and modelling. A form of quantum transport, known as single electron charging, is demonstrated through the electron-bound states of dopant atoms in a silicon nanostructure. A portion of a recently published work is presented which models the response of valley states in a gate-defined quantum dot within a silicon quantum well to an interface step and applied electric fields. The main work of this thesis employs an atomistic tight-binding model to determine the effective g-factor anisotropy of InAs nanowires under various atomistic and electromagnetic conditions. The spin-orbit interactions present in the nanowires are extracted from the effective g-factor anisotropy with an effective model. The modelling results provide insights for InAs nanowire applications in fields such as Majorana zero mode research, spintronic devices and quantum information technology.Thesis (MPhil) -- University of Adelaide, School of Physics, Chemistry and Earth Sciences, 202

    Accurate quantum transport modelling and epitaxial structure design of high-speed and high-power In0.53Ga0.47As/AlAs double-barrier resonant tunnelling diodes for 300-GHz oscillator sources

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    Terahertz (THz) wave technology is envisioned as an appealing and conceivable solution in the context of several potential high-impact applications, including sixth generation (6G) and beyond consumer-oriented ultra-broadband multi-gigabit wireless data-links, as well as highresolution imaging, radar, and spectroscopy apparatuses employable in biomedicine, industrial processes, security/defence, and material science. Despite the technological challenges posed by the THz gap, recent scientific advancements suggest the practical viability of THz systems. However, the development of transmitters (Tx) and receivers (Rx) based on compact semiconductor devices operating at THz frequencies is urgently demanded to meet the performance requirements calling from emerging THz applications. Although several are the promising candidates, including high-speed III-V transistors and photo-diodes, resonant tunnelling diode (RTD) technology offers a compact and high performance option in many practical scenarios. However, the main weakness of the technology is currently represented by the low output power capability of RTD THz Tx, which is mainly caused by the underdeveloped and non-optimal device, as well as circuit, design implementation approaches. Indeed, indium phosphide (InP) RTD devices can nowadays deliver only up to around 1 mW of radio-frequency (RF) power at around 300 GHz. In the context of THz wireless data-links, this severely impacts the Tx performance, limiting communication distance and data transfer capabilities which, at the current time, are of the order of few tens of gigabit per second below around 1 m. However, recent research studies suggest that several milliwatt of output power are required to achieve bit-rate capabilities of several tens of gigabits per second and beyond, and to reach several metres of communication distance in common operating conditions. Currently, the shortterm target is set to 5−10 mW of output power at around 300 GHz carrier waves, which would allow bit-rates in excess of 100 Gb/s, as well as wireless communications well above 5 m distance, in first-stage short-range scenarios. In order to reach it, maximisation of the RTD highfrequency RF power capability is of utmost importance. Despite that, reliable epitaxial structure design approaches, as well as accurate physical-based numerical simulation tools, aimed at RF power maximisation in the 300 GHz-band are lacking at the current time. This work aims at proposing practical solutions to address the aforementioned issues. First, a physical-based simulation methodology was developed to accurately and reliably simulate the static current-voltage (IV ) characteristic of indium gallium arsenide/aluminium arsenide (In-GaAs/AlAs) double-barrier RTD devices. The approach relies on the non-equilibrium Green’s function (NEGF) formalism implemented in Silvaco Atlas technology computer-aided design (TCAD) simulation package, requires low computational budget, and allows to correctly model In0.53Ga0.47As/AlAs RTD devices, which are pseudomorphically-grown on lattice-matched to InP substrates, and are commonly employed in oscillators working at around 300 GHz. By selecting the appropriate physical models, and by retrieving the correct materials parameters, together with a suitable discretisation of the associated heterostructure spatial domain through finite-elements, it is shown, by comparing simulation data with experimental results, that the developed numerical approach can reliably compute several quantities of interest that characterise the DC IV curve negative differential resistance (NDR) region, including peak current, peak voltage, and voltage swing, all of which are key parameters in RTD oscillator design. The demonstrated simulation approach was then used to study the impact of epitaxial structure design parameters, including those characterising the double-barrier quantum well, as well as emitter and collector regions, on the electrical properties of the RTD device. In particular, a comprehensive simulation analysis was conducted, and the retrieved output trends discussed based on the heterostructure band diagram, transmission coefficient energy spectrum, charge distribution, and DC current-density voltage (JV) curve. General design guidelines aimed at enhancing the RTD device maximum RF power gain capability are then deduced and discussed. To validate the proposed epitaxial design approach, an In0.53Ga0.47As/AlAs double-barrier RTD epitaxial structure providing several milliwatt of RF power was designed by employing the developed simulation methodology, and experimentally-investigated through the microfabrication of RTD devices and subsequent high-frequency characterisation up to 110 GHz. The analysis, which included fabrication optimisation, reveals an expected RF power performance of up to around 5 mW and 10 mW at 300 GHz for 25 μm2 and 49 μm2-large RTD devices, respectively, which is up to five times higher compared to the current state-of-the-art. Finally, in order to prove the practical employability of the proposed RTDs in oscillator circuits realised employing low-cost photo-lithography, both coplanar waveguide and microstrip inductive stubs are designed through a full three-dimensional electromagnetic simulation analysis. In summary, this work makes and important contribution to the rapidly evolving field of THz RTD technology, and demonstrates the practical feasibility of 300-GHz high-power RTD devices realisation, which will underpin the future development of Tx systems capable of the power levels required in the forthcoming THz applications

    Online learning of physics during a pandemic: A report from an academic experience in Italy

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    The arrival of the Sars-Cov II has opened a new window on teaching physics in academia. Frontal lectures have left space for online teaching, teachers have been faced with a new way of spreading knowledge, adapting contents and modalities of their courses. Students have faced up with a new way of learning physics, which relies on free access to materials and their informatics knowledge. We decided to investigate how online didactics has influenced students’ assessments, motivation, and satisfaction in learning physics during the pandemic in 2020. The research has involved bachelor (n = 53) and master (n = 27) students of the Physics Department at the University of Cagliari (N = 80, 47 male; 33 female). The MANOVA supported significant mean differences about gender and university level with higher values for girls and master students in almost all variables investigated. The path analysis showed that student-student, student-teacher interaction, and the organization of the courses significantly influenced satisfaction and motivation in learning physics. The results of this study can be used to improve the standards of teaching in physics at the University of Cagliar
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