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1
Kozič, Antoni. "Investigation of AIIIBV heterostructures under the action of microwave radiation." Doctoral thesis, Lithuanian Academic Libraries Network (LABT), 2008. http://vddb.library.lt/obj/LT-eLABa-0001:E.02~2008~D_20080723_093503-68603.
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The thesis presents the investigation on how to increase the sensitivity of the narrowed sensors of radiation. Also the thesis also deals with the attempts to analyze the influence of the microwave radiation on to the narrowed semiconductor formations and to reveal the physical nature of the observed effects as well as to determine the influence of structure of the samples on the detected signal magnitude.
The work solves the following major tasks: the characteristics of the narrowed semiconductor heterostructures depending on the quality of the modulation layers and on the parameters of the semiconductor materials as well as the characteristics, depending on the selectively doped structure, on the conductivity of the highly doped semiconductor layer, and on the thickness of the separating layer, and the type of metallization of the gate. In order to achieve the goal there were produced and investigated narrowed different heterostructures (AlGaAs/GaAs, AlGaAs/InGaAs/GaAs) and n-GaAs structures.
The thesis consists of four chapters, the final one is the generalization of the results.
The first chapter (introductory) deals with the actuality of the problem, the aim and the tasks are stated, the novelty of the scientific research is described, the reports of the author are presented together with the publications, and the structure of the thesis.
The second chapter is assigned to the review of the literature. It presents the principals of electromagnetic radiation detection... [to full text]Disertacijoje nagrinėjama, kaip padidinti susiaurintų spinduliuotės jutiklių jautrį. Taip pat disertacijoje siekiama ištirti mikrobangų spinduliuotės poveikį susiaurintiems puslaidininkiniams dariniams ir atskleisti stebimų efektų fizinę prigimtį bei nustatyti bandinių struktūros įtaką detektuojamo signalo dydžiui. Darbe sprendžiami tokie pagrindiniai uždaviniai: tiriamos įvairialyčių susiaurintų puslaidininkinių darinių savybės, priklausančios nuo darinių sluoksnių kokybės ir puslaidininkinių medžiagų parametrų bei analizuojamos savybės, priklausančios nuo stipriai legiruoto puslaidininkinio sluoksnio laidumo, nuo skiriamojo sluoksnio storio ir nuo sklendės pobūdžio metalizacijos. Siekiant užsibrėžto tikslo, buvo gaminami ir tiriami susiaurinti skirtingi įvairialyčiai dariniai (AlGaAs/GaAs, AlGaAs/InGaAs/GaAs) ir n-GaAs dariniai. Disertaciją sudaro penki skyriai, kurių paskutinis – rezultatų apibendrinimas. Pirmajame (įvadiniame) skyriuje nagrinėjamas problemos aktualumas, formuluojamas darbo tikslas bei uždaviniai, aprašomas mokslinis darbo naujumas, pristatomi autoriaus pranešimai, disertacijos struktūra. Antrasis skyrius skirtas literatūros apžvalgai. Jame apžvelgiami elektromagnetinės spinduliuotės detektavimo principai, aptariamos šiluminės ir bigradientinės elektrovaros susidarymo priežastys, AlGaAs/GaAs įvairialytė sandūra, selektyvusis legiravimas bei puslaidininkinių prietaisų fizikinės galimybės. Trečiajame skyriuje pateikta eksperimento tyrimo metodika. Išsamiai... [toliau žr. visą tekstą]
2
Kozič, Antoni. "Įvairialyčių AIIIBV darinių tyrimas mikrobangose." Doctoral thesis, Lithuanian Academic Libraries Network (LABT), 2008. http://vddb.library.lt/obj/LT-eLABa-0001:E.02~2008~D_20081007_193805-87854.
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Disertacijoje nagrinėjama, kaip padidinti susiaurintų spinduliuotės jutiklių jautrį. Taip pat disertacijoje siekiama ištirti mikrobangų spinduliuotės poveikį susiaurintiems puslaidininkiniams dariniams ir atskleisti stebimų efektų fizinę prigimtį bei nustatyti bandinių struktūros įtaką detektuojamo signalo dydžiui.
Darbe sprendžiami tokie pagrindiniai uždaviniai: tiriamos įvairialyčių susiaurintų puslaidininkinių darinių savybės, priklausančios nuo darinių sluoksnių kokybės ir puslaidininkinių medžiagų parametrų bei analizuojamos savybės, priklausančios nuo stipriai legiruoto puslaidininkinio sluoksnio laidumo, nuo skiriamojo sluoksnio storio ir nuo sklendės pobūdžio metalizacijos. Siekiant užsibrėžto tikslo, buvo gaminami ir tiriami susiaurinti skirtingi įvairialyčiai dariniai (AlGaAs/GaAs, AlGaAs/InGaAs/GaAs) ir n-GaAs dariniai.
Disertaciją sudaro penki skyriai, kurių paskutinis – rezultatų apibendrinimas.
Pirmajame (įvadiniame) skyriuje nagrinėjamas problemos aktualumas, formuluojamas darbo tikslas bei uždaviniai, aprašomas mokslinis darbo naujumas, pristatomi autoriaus pranešimai, disertacijos struktūra.
Antrasis skyrius skirtas literatūros apžvalgai. Jame apžvelgiami elektromagnetinės spinduliuotės detektavimo principai, aptariamos šiluminės ir bigradientinės elektrovaros susidarymo priežastys, AlGaAs/GaAs įvairialytė sandūra, selektyvusis legiravimas bei puslaidininkinių prietaisų fizikinės galimybės.
Trečiajame skyriuje pateikta eksperimento tyrimo metodika. Išsamiai... [toliau žr. visą tekstą]The thesis presents the investigation on how to increase the sensitivity of the narrowed sensors of radiation. Also the thesis also deals with the attempts to analyze the influence of the microwave radiation on to the narrowed semiconductor formations and to reveal the physical nature of the observed effects as well as to determine the influence of structure of the samples on the detected signal magnitude. The work solves the following major tasks: the characteristics of the narrowed semiconductor heterostructures depending on the quality of the modulation layers and on the parameters of the semiconductor materials as well as the characteristics, depending on the selectively doped structure, on the conductivity of the highly doped semiconductor layer, and on the thickness of the separating layer, and the type of metallization of the gate. In order to achieve the goal there were produced and investigated narrowed different heterostructures (AlGaAs/GaAs, AlGaAs/InGaAs/GaAs) and n-GaAs structures. The thesis consists of four chapters, the final one is the generalization of the results. The first chapter (introductory) deals with the actuality of the problem, the aim and the tasks are stated, the novelty of the scientific research is described, the reports of the author are presented together with the publications, and the structure of the thesis. The second chapter is assigned to the review of the literature. It presents the principals of electromagnetic radiation detection... [to full text]
3
Jansen, Richard-Jan Engel. "Electron transport and scattering in the 2DEG base hot-electron transistor." Thesis, University of Cambridge, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.389876.
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Wahab, Y. B. "Phonon emission and reflection by a 2DEG studied with superconducting tunnel junctions." Thesis, University of Nottingham, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.235361.
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Benesh, Matthew Joel. "Charge transport dynamics of surface acoustic waves in a GaAs/AlGaAs 2DEG." Thesis, University of Cambridge, 2013. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.608252.
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Zhang, Shiqing. "Novel mechanism of 2DG mediated cancer treatment /Zhang Shiqing." HKBU Institutional Repository, 2016. https://repository.hkbu.edu.hk/etd_oa/353.
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2-deoxy-D-glucose (2DG), a non-metabolizable glucose analog, is currently being used in clinical trials to determine its efficacy in augmenting radiotherapy and chemotherapy of various cancers. It is thought to kill cancer cells by inducing glucose deprivation state. However, 2DG only inhibits glycolysis by 15-40%, not sufficient to simulate glucose deprivation. Further, 2-flour-deoxy-D-glucose (2FDG), which is a more potent inhibitor of glycolysis than 2DG, is less effective than 2DG in killing cancer cells. These observations suggest that glucose deprivation is not the mechanism by which 2DG kills cancer cells. On the other hand, it has been shown that treatment of cancer cells with 2DG leads to increased reactive oxygen species (ROS) production, indicating that cytotoxic effect of 2DG is due to increase ROS. Our lab previously found that inhibition of aldose reductase (AR) activity attenuated 2DG-induced ROS in cardiomyocytes (Tang, et al., 2010). We therefore propose that 2DG-induced ROS increase in cancer cells is a consequence of the depletion of GSH in the process of reduction of 2DG by AR and/or by a related aldose reductase-like enzyme (ARL). These two enzymes are often overexpressed in cancer cells. This proposed project is to test our hypothesis that the cytotoxicity of 2DG is due to the depletion of GSH as a consequence of the reduction of 2DG by AR or ARL. We found that HepG2, SKOV3, HCT116 and CaCo2 cells were sensitive to 2DG, and these cells over-express AR and/or ARL proteins. However, HT29 cells and SW480 cells, which were not sensitive to 2DG, had low level of AR and ARL proteins, indicating that there is a close relationship between sensitivity to 2DG toxicity and the level of AR/ARL in these cells. Further, when AR/ARL activity were inhibited in HepG2, SKOV3, HCT116 and CaCo2 cells by AR/ARL inhibitors fidarestat or tolrestat, the cells were protected against 2DG cytotoxicity. Tolrestat or fidarestat significantly restored the drop of GSH levels in 2DG sensitive cancer cells induced by 2DG. On the other hand, MG-132 and bortezomib, which increased the expression of AR/ARL in HT29 and SW480 cells, made HT29 and SW480 cells more sensitive to 2DG. These experiments confirmed our hypothesis that 2DG toxicity in cancer cells was due to oxidative stress induced by AR/ARL. 2DG is not an efficient substrate for AR/ARL enzymes and it is not very efficient in killing cancer cells. Based on our hypothesis, better AR/ARL substrates should be more toxic to cancer cells that overexpress AR/ARL than 2DG. The cytotoxic effects of glyceraldehyde and diacetyl, which were better substrates for AR/ARL than 2DG, were tested. Both glyceraldehyde and diacetyl were more efficient in killing cancer cells that over-express AR and/or ARL (HepG2, SKOV3, HCT116 and CaCo2) than cancer cells with low levels of AR and ARL proteins (HT29 and SW480). Glyceraldehyde and diacetyl were more efficient in lowering the GSH level in cancer cells that over-express AR and/or ARL. In order to further develop glyceraldehyde and diacetyl as anti-cancer drugs, animal studies were carried out to determine their anti-cancer effects. Both glyceraldehyde and diacetyl significantly inhibited the tumor growth in nude mice tumor xenograft model. In conclusion, this thesis proposed and proved that 2DG kills cancer cells by lowering intracellular GSH levels as a consequence of its reduction by AR/ARL activities, rather than by inhibition of glycolysis. This novel mechanism predicts that better substrates for AR/ARL than 2DG would be more effective in killing cancer cells than 2DG. This was confirmed by using glyceraldehyde and diacetyl. We believe that this would lead to the development of more efficient anti-cancer drugs.
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Newton, M. I. N. "Investigation of the interaction between acoustic phonons and the 2DEG of a silicon MOSFET." Thesis, University of Nottingham, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.380155.
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Kopel, Felipe Macedo. "Influência da dinâmica clássica nos processos de tunelamento em bilhares quânticos baseados no 2DEG." reponame:Repositório Institucional da UFABC, 2012.
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9
Leduc, Vincent Louis Philippe. "Design, fabrication and characterization of a suspended heterostructure." Thesis, Kingston, Ont. : [s.n.], 2007. http://hdl.handle.net/1974/726.
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Sadeghi, Mohammadreza. "Highly sensitive nano Tesla quantum well Hall Effect integrated circuits using GaAs-InGaAs-AlGaAs 2DEG." Thesis, University of Manchester, 2015. https://www.research.manchester.ac.uk/portal/en/theses/highly-sensitive-nano-tesla-quantum-well-hall-effect-integrated-circuits-using-gaasingaasalgaas-2deg(cec2fce1-7cf5-4d36-918d-873e0d38cac0).html.
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Hall Effect integrated circuits are used in a wide range of applications to measure the strength and/or direction of magnetic fields. These sensors play an increasingly significant role in the fields of automation, medical treatment and detection thanks largely to the enormous development of information technologies and electronic industries. Commercial Hall Effect ICs available in the market are all based on silicon technology. These ICs have the advantages of low cost and compatibility with CMOS technology, but suffer from poor sensitivity and detectability, high power consumption and low operating frequency bandwidths. The objective of this work was to develop and fabricate the first fully monolithic GaAs-InGaAs-AlGaAs 2-Dimensional Electron Gas (2DEG) Hall Effect integrated circuits whose performance enhances pre-existing technologies. To fulfil this objective, initially 2 µm gate length pHEMTs and 60/20 µm (L/W) Greek cross Hall Effect sensors were fabricated on optimised GaAs-In.18Ga.82As-Al.35Ga.65As 2DEG structures (XMBE303) suitable for both sensor and integrated circuit designs. The pseudomorphic high electron mobility transistors (pHEMTs) produced state-of-the-art output conductance, providing high intrinsic gain of 405, current cut-off frequency of 4.8 GHz and a low negative threshold voltage of -0.4 V which assisted in designing single supply ICs with high sensitivity and wide dynamic range. These pHEMTs were then accurately modelled for use in the design and simulation of integrated circuits. The corresponding Hall sensor showed a current sensitivity of 0.4 mV/mA.mT and a maximum magnetic DC offset of 0.35 mT at 1 V. DC digital (unipolar) and DC linear Hall Effect integrated circuits were then designed, simulated, fabricated and fully characterised. The DC linear Hall Effect IC provided an overall sensitivity of 8 mV/mT and a power consumption as low as 6.35 mW which, in comparison with commercial Si DC linear Hall ICs, is at least a factor of 2 more power efficient. The DC digital (unipolar) Hall Effect IC demonstrated a switching sensitivity of 6 mT which was at least ~50% more sensitive compared to existing commercial unipolar Si Hall ICs. In addition, a novel low-power GaAs-InGaAs-AlGaAs 2DEG AC linear Hall Effect integrated circuit with unprecedented sensitivity and wide dynamic range was designed, simulated, fabricated and characterised. This IC provided a sensitivity of 533 nV/nT, minimum field detectability of 177 nT (in a 10 Hz bandwidth) at frequencies from 500 Hz up to 200 kHz, consuming only 10.4 mW of power from a single 5 V of supply. In comparison to commercial Si linear Hall ICs, this IC provides an order of magnitude larger sensitivity, a factor of 4 higher detectability, 20 times wider bandwidth and over 20% lower power consumption (10.4 mW vs. 12.5 mW). These represent the first reported monolithic integrated circuits using a CMOS-like technology but in GaAs 2DEG technology and are extremely promising as complements, if not alternatives, to CMOS Si devices in high performance applications (such as high temperatures operations (>150 °C) and radiation hardened environment in the nuclear industry).
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Armaroli, Giovanni. "Caratterizzazione elettrica di eterostrutture basate su nitruri per applicazioni nell'elettronica veloce." Bachelor's thesis, Alma Mater Studiorum - Università di Bologna, 2017. http://amslaurea.unibo.it/14522/.
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In questo lavoro di tesi sono state caratterizzate due eterostrutture basate su nitruri del III gruppo per applicazioni nell’elettronica ad alte frequenze. Su entrambi i campioni sono state effettuate caratterizzazioni corrente - tensione e capacità - tensione tramite contatti Schottky, con l’obiettivo di ricavare dai dati raccolti alcuni parametri fisici utili a comprendere più a fondo le proprietà delle eterostrutture analizzate. In particolare dalle caratteristiche corrente - tensione è stato possibile ricavare la resistenza in serie, il fattore di idealità, la corrente inversa di saturazione e l’altezza di barriera Schottky dei dispositivi. Dalle caratteristiche capacità - tensione è stato possibile risalire al profilo di carica dei campioni e mostrare la presenza di un 2DEG all’interfaccia tra i materiali che compongono queste eterostrutture.
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Saxena, Himanshu. "Tunable Terahertz Detectors Based on Plasmon Exciation in Two Dimensional Electron Gasses in InGaAs/InP and AlGaN/GaN HEMT." Doctoral diss., University of Central Florida, 2009. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/3924.
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The observation of voltage-tunable plasmon resonances in the terahertz range in two dimensional electron gas (2-deg) of a high electron mobility transistor (HEMT) fabricated from the InGaAs/InP and AlGaN/GaN materials systems is reported. The devices were fabricated from a commercial HEMT wafer by depositing source and drain contacts using standard photolithography process and a semi-transparent gate contact that consisted of a 0.5 [micro]m period transmission grating formed by electron-beam lithography. Narrow-band resonant absorption of THz radiation was observed in transmission in the frequency range 10-100 cm-1. The resonance frequency depends on the gate voltage-tuned sheet-charge density of the 2deg. The fundamental and higher resonant harmonics were observed to shift towards lower frequencies with the implementation of negative gate bias. The theory of interaction of sub millimeter waves with 2deg through corrugated structure on top has been applied to calculate and understand the phenomena of resonant plasmon excitations. The observed separation of resonance fundamental from its harmonics and their shift with gate bias follows theory, although the absolute frequencies are lower by about a factor of 2-3 in InGaAs/InP system. However, calculated values match much better with AlGaN/GaN system.Ph.D.
Department of Physics
Sciences
Physics PhD
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Bacchus, Ian Dean. "On the development of a far-infrared bolometric detector using a 2DEG as the absorbing medium." Thesis, Cardiff University, 2008. http://orca.cf.ac.uk/54649/.
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This thesis describes the development of a far-infrared bolometric detector using a two-dimensional electron gas (2DEG) as the detecting medium. The 2DEG is formed at a AlGaAs/GaAs heterojunction made of layers of undoped GaAs and AlGaAs and highly doped (5 x lO126 3) AlGaAs. A 2DEG layer grown in this way in a molecular beam epitaxy (MBE) system is generally within 100 nm of the surface of the wafer and is subsequently patterned by etching away the surrounding wafer material and leaving a mesa containing the buried 2DEG. Ohmic contact to the 2DEG is achieved either through a diffusion of charge carriers into the contact region. Using a 2DEG as the absorbing medium in bolometers will yield detectors that are fast, sensitive, and frequency selective. The low electron densities in 2DEGs (1011 cm
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Yang, Ming. "High magnetic field studies of 2DEG in graphene on SiC and at the LaAlO³/SrTiO³ interface." Thesis, Toulouse, INSA, 2018. http://www.theses.fr/2018ISAT0015/document.
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Cette thèse est dédiée à l'étude des propriétés de magnéto-transport des gaz d'électrons bidimensionnel, et plus spécifiquement du graphène sur carbure de silicium (G/SiC) ainsi qu’à l'interface entre les oxydes complexes LaAlO3 (LAO) et SrTiO3 (STO). Nous exploitons la génération d’un champ magnétique intense (jusqu'à 80 T) et les très basses températures (jusqu'à 40 mK) pour étudier les propriétés de transport quantique, qui sont évocatrices de la structure de bandes électroniques sous-jacente. Dans G/SiC, à la limite du régime d’effet Hall quantique, nous mesurons un plateau de Hall ultra-large quantifié à R=h/2e² couvrant un champ magnétique de plus de 70 T (de 7 T à 80 T). La résistance longitudinale est proche de zéro mais présente, de manière inattendue, de faibles oscillations périodiques avec l’inverse du champ magnétique. Sur la base d’observations microscopiques, ce gaz d’électrons 2D est modélisé par une matrice de graphène ayant une densité de porteurs de charge faible, parsemée d’ilots de taille micrométrique ayant un dopage plus important. Les simulations numériques des propriétés de transport reproduisent bien le plateau de Hall et la présence des oscillations. Au-delà du substrat de SiC qui agit comme un réservoir de charge et stabilise le facteur de remplissage à ν=2, un transfert de charge dépendant du champ magnétique entre les ilots chargés est responsable de la présence des oscillations de la magnétorésistance. Cette étude originale fournit de nouvelles perspectives pour des applications en métrologie. Les propriétés remarquables des gaz d’électrons 2D à l'interface entre les oxydes complexes LAO et STO sont aujourd'hui envisagées pour le développement de futurs dispositifs multifonctionnels. Toutefois, leurs propriétés électroniques sont encore mal connues et nécessitent des recherches plus approfondies. Dans ces systèmes, la magnétorésistance montre des oscillations de Shubnikov-de Haas (SdH) quasi-périodiques et un effet Hall linéaire jusqu'à 55 T à basse température. Nous observons une différence d’un ordre de grandeur entre la densité de porteurs extraite de la période des oscillations SdH et la pente de la résistance de Hall, impliquant la présence de nombreuses sous-bandes à l'énergie de Fermi. Les oscillations quasi-périodiques de la magnétorésistance sont bien reproduites par des simulations numériques prenant en compte l'effet Rashba à l'interface. De plus, à partir de l'évolution des oscillations SdH avec la tension de grille à très basse température (40mK), nous identifions les sous-bandes électroniques contribuant au transport, les orbitales atomiques dont elles dérivent, ainsi que leur localisation spatiale dans la profondeur de l'interfaceThis thesis is devoted to the study of the magneto-transport properties of two dimensional electron gas (2DEG), and more specifically graphene on silicon carbide (G/SiC) as well as the interface between two complex oxides LaAlO3 / SrTiO3 (LAO/STO). We take advantage of very high magnetic field (up to 80 T) and very low temperature (down to 40 mK) to investigate the quantum transport properties, which are evocative of the underlying electronic band-structure. In G/SiC, close to the quantum Hall breakdown regime, we measure an ultra-broad quantum Hall plateau at R=h/2e² covering a magnetic field range of more than 70 T (from 7 T to 80 T). Accordingly, the longitudinal resistance is close to zero, but displays unexpected weak 1/B-periodic oscillations. Based on microscopic observations, this 2DEG is modeled as a low charge carrier density graphene matrix decorated by micrometers-size puddles with larger doping. Numerical simulations of the transport properties reproduce well both the broad Quantum Hall plateau and the presence of the oscillations. Besides the SiC substrate which acts as a charge reservoir and stabilizes the quantum Hall state at filling factor ν=2, a magnetic field dependent transfer of charges involving the puddles is responsible for the presence of the oscillating features. This original study provides new insights for resistance metrology purposes. The 2DEG arising at the interface between the complex oxides LAO and STO is nowadays envisioned for future multi-functional devices. Their electronic properties are still a matter of debate and require further investigations. The high field magneto-resistance of this 2DEG displays quasi-periodic Shubnikov-de Haas Oscillations (SdHO) and a linear Hall effect up to 55 T at low temperature. We observe a large discrepancy between the carrier density extracted from the period of the SdHO and the slope of the Hall resistance, which constitutes a strong evidence for the presence of many sub-bands crossing the Fermi energy. The quasi-periodic oscillations of the magneto-resistance are well reproduced by numerical simulations taking into account the strong Rashba effect at the interface. In addition, from the back-gate voltage evolution of the SdHO at sub-kelvin temperature, we identify the electronic sub-bands contributing to transport, the orbital symmetry from which they derive, as well as their spatial localization along the interface
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Nina, Americo Orccohuarancca. "Controle de ressonâncias de fano na condutância de caixas quânticas bidimensionais baseadas no 2DEG de GaAs." reponame:Repositório Institucional da UFABC, 2014.
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Orientador: Gustavo Michel Mendoza La TorreDissertação (mestrado) - Universidade Federal do ABC, Programa de Pós-Graduação em Física, 2014.
Nesta tese estudamos o transporte eletrônico em sistemas mesoscópicos balísticos. Nós fizemos simulações numéricas de diversos confinamentos de um gás de elétrons bidimensional (2DEG na interface GaAs=AlGaAs) e discutimos as propiedades de transmissão através desses sistemas nanoestruturados. Para nossos cálculos, utilizamos a aproximação de massa efetiva no modelo tight binding para emular o comportamendo da função de onda quando esta interage com um potencial de confinamento, e com alguns potenciais que chamaremos "controladores". Em particular pesquisamos sobre o transporte eletrônico através de confinamentos tipo Open Quantum Dot (OQD). As propiedades de transporte são encontradas utilizando cálculo das funções de Green da rede tight binding para obter as amplitudes de transmissão do sistema e sua aplicação no formalismo Landauer-Büttiker. A pesquisa foi dividida em duas etapas, a primeira engloba um grupo de testes em sistemas conhecidos para ter a certeza de um ótimo funcionamento do programa e para nos familiarizarmos com as características mais fundamentais da transmissão através destes sistemas. Por outro lado, estudamos o efeito que podem ter gates colocados nas posições de máxima intensidade na densidade local de estados. Aqui encontramos deslocamentos dos primeiros estados ressonântes que dependem do potencial aplicado nos controladores. Também encontramos que usando combinações de diferentes gates controladores, podemos mudar a forma de linha das ressonâncias de Fano observadas na conductância, por exemplo, desde um estado ressonânte simétrico para uma ressonância assimétrica, e logo mudar sua forma de linha para um dip o para a forma invertida da ressonância assimétrica mencionada. Observamos aqui também que a varredura de um único gate controlador pode inverter ressonâncias assimétricas de Fano. Em geral, com isto estamos manipulando a localização dos estados da caixa quântica e seu acoplamento com o mar continuo dos leads de entrada e saída. Numa segunda etapa, usamos o nosso método agora aplicado para sistemas de tamanhos realísticos com paredes de potencial de diferentes suavidades. Nos quais, sintonizamos as diferentes formas de linha de Fano (em sistemas realísticos) via a aplicação de gates controladores, emulados por antidots no interior do OQD, usando: paredes parabólicas, paredes tipo soft wall e paredes abruptas. Comparamos as condutâncias, densidades locais de estado, deslocamentos e as formas de linha das ressonâncias de Fano para os OQDs com estes tipos de paredes, na procura do sistema no qual seja possível o melhor controle das ressonâncias de Fano, tanto no iii seu deslocamento como na sua sintonia de formas. Encontramos que a forma do OQD não influi fortemente nos deslocamentos dos estados quase-ligados, mas sim na sintonia das diferentes formas de linha das ressonâncias. Obtivemos que no caso de um OQD abrupto, a sintonia é facilmente atingida, no caso soft wall também atingimos esta sintonia mas com uma pequena diminuição de amplitude nas ressonâncias. Por outro lado com a mesma configuração de gates no caso parabólico, não encontramos esta sintonia tão facilmente. Finalmente, estudamos diferentes configurações de gates controladores para melhorar os nossos resultados, e assim obter um controle optimizado das ressonâncias de Fano.
In this thesis we studied the electronic transport in ballistic mesoscopic systems. We did numeric simulations of various confinements of a Two-Dimensional Electron Gas (2DEG at the interface GaAs/AlGaAs) and discussed the transmission properties through those nanostructured systems. For our calculations, we employed the effective mass approximation in the tight binding model to emulate the wave function¿s behavior when this interact with a confinementpotential, and with other potentials that we will name "drivers". We specifically investigated the electronic transport through Open Quantum Dots (OQDs). The transport properties were found using the calculations of tight binding Lattice Green¿s functions, in order to obtain the system¿s transmission amplitudes and its applications in Landauer-Büttiker formalism. The research was divided in two steps. The first one includes a group of tests in known systems in order to have certainty of an optimal operation of the program and familiarize our selves with the most fundamental characteristics of transmission through these systems. On the other hand, we studied the effect that can result from placing gates on positions of maximum intensity of local density of states (LDOS). Here, we found displacements of first resonant states that depend on the potentials applied at the drivers. Moreover, we found that by using combinations of different gate drivers, we could change the line shape of Fano resonances observed in the conductance, for instance, from a symmetric resonant state to an asymmetric resonance, and then change its line shape to a dip or an inverted form of the asymmetric resonance mentioned. We also observed that the movement of just one gate driver could invert asymmetric Fano resonances. In general, we are manipulating the state localization of the Quantum Dot and its coupling to the continuous sea of leads of entrance and exit. In a second stage, we applied our mentioned method to systems of realistic sizes and potentials walls with different softness. In which, we tune the different Fano line shapes (in realisticsystems) by applying gate drivers, emulated by antidots inside the OQD, using: parabolic walls, "soft walls" and abrupt walls. We compared the conductances, Local Density of States, displacements and Fano line shapes for the OQDs with the walls mentioned, in pursuit of systems in which is possible the best Fano resonances control, on both, their displacements and line shapes. We found that OQD¿s shape does not influence displacements of quasi-bound states in a great way, but affect the tuning of different line shapes resonances. We concluded that in the v case of abrupt OQD, the tuning is easily achieved, and regarding the soft wall we also obtained the tuning but with a small decreasing resonance amplitude. In the other hand, we could not find this tuning as easily in the parabolic case. Finally, we studied different configurations of gate drivers to improve our results, and an optimized control of Fano resonances.
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Park, Danielle. "'Under our protection, that of the church and their own' : papal and secular protection of the families and properties the crusaders left behind, c.1095-1226." Thesis, Royal Holloway, University of London, 2013. http://digirep.rhul.ac.uk/items/fd5ade35-f444-f6ca-2d2e-7724e3040a79/1/.
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‘Ill-defined and incomprehensible to contemporaries': these are two of the charges scholarship has levelled at the papal protection privilege for crusaders. Major innovations in this field have been attributed to Innocent III (1198-1216), yet many of these ideas can be identified as having developed much earlier. This thesis will demonstrate the profound originality of the protection initiated by Urban II in 1095, and discuss the role of the protection in recruitment as an added attraction or, at least, as a way for the pope to negate obstacles to taking the cross. Under Eugenius III (1145-53) this privilege took on a new formula that dominated papal missives beyond Innocent III's pontificate. In essence, crusaders were differentiated from pilgrims, and that protection sharply delineated crusaders' wives, families and possessions from those of the men-at-arms who did not take the cross. During the Second Crusade (1145-49), the metaphor of the two swords of government took on a new centrality within the crusading context. This connection between secular and spiritual authority has not received adequate attention from scholars. Protected status is the starting point of the discussion of papal and secular guardianship over the crusaders' lands and possessions. Crusaders and those remaining in the West were well aware of their status from the outset. This secular experience is determined through detailed discussion of the charters issued by crusade regents. The crusades have been interpreted as ‘windows of opportunity' for wives otherwise excluded from politics, however demonstrably the women chosen for these roles were, in fact, already experienced in government. This thesis also compares and contrasts the effectiveness of papal and secular measures in protecting the crusader's interests, and assesses the political impact of the crusaders' departure on those they left behind. Invasion, rebellion and usurpation could and did occur during the crusaders' long-term absence, but secular and papal protection might, in unison, combat exploitation by the crusaders' enemies or other opportunists.
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Hoeylandt, Pierre van. "Is there a duty of humanitarian intervention? : an empirical study with moral implications." Thesis, University of Oxford, 2001. http://ora.ox.ac.uk/objects/uuid:3289e232-2d4e-4878-8e2f-ba7e667f5b77.
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Large-scale humanitarian crises in foreign countries raise the question of whether or not other countries have a duty to alleviate that suffering. In extreme cases, humanitarian intervention, that is: military intervention for the purpose of alleviating human suffering, is sometimes advocated as the morally required course of action. This thesis suggests that while the international community has a general moral responsibility to prevent and ameliorate humanitarian crises there is no simple duty of military humanitarian intervention. Hitherto, the question has typically been treated as a matter of either moral or legal principle. This thesis argues that empirical factors, which affect the international community's ability to carry out interventions effectively, have not been given their due weight in the debate. On the basis of evaluations of international responses to crises in Somalia and Rwanda, 1992 - 1994, it is suggested that a range of factors undermine the efficacy of humanitarian interventions. These factors include the impact of state interests, the effects of domestic politics in intervening states and, contrary to expectations, the role of humanitarian considerations in decision making on intervention. By showing the limitations of a simplistic view of a duty of humanitarian intervention the thesis seeks to contribute to reconciling idealism with realism in international crisis-responses. Based on sound moral and political judgment military interventions in humanitarian crises would hopefully be less ambitious and ultimately more effective.
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Lukasik, Piotr. "The facultative endosymbionts of grain aphids and the horizontal transfer of ecologically important traits." Thesis, University of Oxford, 2011. http://ora.ox.ac.uk/objects/uuid:6aedd183-2dde-4099-a74a-e3f7c909546a.
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Insects are often infected with facultative endosymbiotic bacteria, which can have a range of important ecological effects. The grain aphid, Sitobion avenae, harbours diverse facultative symbionts, which suggests their importance in grain aphid biology. This thesis attempts to explain the ecological roles of the facultative endosymbionts in S. avenae. It also examines the question of whether the horizontal transmission of symbionts between aphid clones and species can be important for shaping the ecology and evolution of multi-species aphid communities. Novel techniques developed for research with the grain aphid study system are presented. Grain aphid clones vary in their tolerance to low temperatures, but this trait is not affected by their facultative endosymbionts. Strains of a symbiont Hamiltonella defensa do not protect grain aphids from hymenopterous parasitoids, regardless of the host genotype. However, experienced parasitoid females preferentially oviposit in aphids which do not harbour symbionts. Comparison of the fitness consequences of infection with the same Hamiltonella strains in their original and in novel grain aphid host clones reveal no consistent differences. Symbiont strains establish easily following artificial transfer between clones of the grain aphid, but the symbionts transferred from other aphid species form less stable infections. Hamiltonella strains do not affect the fecundity of their grain aphid host clones regardless of their host species of origin, but also do not generally confer protection against parasitoids. There are no clear patterns in the distribution of parasitoid-resistant phenotypes across phylogenetic trees of Hamiltonella and its bacteriophage APSE. Strains of four unrelated species of endosymbionts, Rickettsia, Spiroplasma, Rickettsiella and Regiella, confer the same pathogen-resistant phenotype to a single pea aphid clone. The same symbiont strains can confer resistance to clones of two different aphid species. Some strains in multiple infections may compensate for the costs of infections with other symbionts. The importance of these results for understanding the ecological and evolutionary role of facultative endosymbionts in aphids and other insects are discussed, and directions for further research are proposed.
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Ozlutas, Kezban. "Behaviour of ultra-low density foamed concrete." Thesis, University of Dundee, 2015. https://discovery.dundee.ac.uk/en/studentTheses/ece1c184-2dee-428b-8742-7515c09f230f.
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Kittle, Kevin Jeffrey. "NMR relaxation and self diffusion studies of thallium compounds." Thesis, Royal Holloway, University of London, 1987. http://repository.royalholloway.ac.uk/items/9f3a5efe-2d1e-48be-a2af-3f2bb25509e2/1/.
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205Ti NMR relaxation and self-diffusion measurements were applied to study the probe qualities of thallium cations in sucrose/solvent systems. The value of variable field/variable temperature relaxation measurements in defining relaxation behaviour was demonstrated. The relaxation of the dimethylthallium(III) cation in 60% sucrose/D20 (w/w) solution is dominated by the GSA mechanism at both high and low field. It was necessary to use a temperature dependent Fuoss-Kirkwood distribution of correlation times to rationalise the R1 data and this analysis gave the shielding anisotropy of the cation as 5588 +/- 173ppm, in keeping with previous studies. Similar behaviour was noted for the diethylthallium cation in sucrose/DMSO-d6 solutions (>10% concentration) and a concentration dependence in the distribution width parameter was observed. The 13C relaxation in aqueous sucrose solution at 60% concentration similarly showed complex behaviour. Self-diffusion and relaxation studies on thallium(I) ion in sucrose/D2O solutions showed the cation to be a good diffusional probe in viscous solutions, but a poor probe of reorientational motion. Studies on the self-diffusion of the dimethylthallium(III) cation in D20 were consistent with a solvent structure - limited model for translational motion. Further studies in sucrose/D20 solutions showed the dimethylthallium(III) cation to be limited to studying solutions 30% concentration. Studies of Ti(I)+ ion in aqueous TANOL solutions showed an inverse frequency dependence of R1 and R2 and a dominant electron-nuclear scalar relaxation mechanism was proposed. Finally, 205Ti and 203Ti R1 measurements in neat and diluted thallium(I) ethoxide showed that in the neat solution the rates for both nuclei were equal at high field and were dominated by the GSA mechanism. At low field, 203Ti R1 > 205Ti and a dominant scalar mechanism was proposed. In dilute solution the rates were equal at both high and low field and no GSA contribution was observed.
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Angeletti, Andrea. "Electronic properties of the WO2 (001) polar surface of tungsten trioxide." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2021. http://amslaurea.unibo.it/23141/.
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Tungsten trioxide WO3 shows promising perspectives in many fields of research. Surface states appear to play a fundamental role on the overall functionality of the material. Experimental evidences showed the capability to consistently obtain meta-stable polar surfaces, characterized by large amount of excess charge on the surface. In this study, we investigate the WO2 terminated polar (001) surface, by density functional theory calculations. Our data suggest a tri-dimensional nature of the excess electrons, delocalized across the entire slab, compared to the usually observed two-dimensional electron gas behavior, in the case of common transitional metal oxide. The material is expected to be able to host excess charge also in localized states (namely polarons). Furthermore, this work also suggests a deep coupling between the antiferroelectric distorsions, the oxygen vacancies at the topmost layer, and the dispersion of the excess charge. Our results, constitute overall solid bases to further investigate these connections and a valid starting point towards the opportunity to tune the electronic properties of the WO3 surface.
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Gharbiah, Maey Monir. "Patterning the Mud Snail Ilyanassa obsoleta: The Role of Cell Signaling and Asymmetric Protein Localization." Diss., The University of Arizona, 2009. http://hdl.handle.net/10150/195869.
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The polar lobe of Ilyanassa is asymmetrically partitioned into the D lineage of cells. Two of these cells, 3D and 4d, induce proper axial cell fate patterning in the embryo. Based on known embryological data in Ilyanassa, I hypothesized that Notch signaling would be required for this induction. I found that Notch signaling is required for cell fates induced by 4d and is temporally required well after 4d induction. Based on these results, I hypothesize that Notch signaling is involved in a reciprocal induction between the micromeres and the macromeres (endoderm) resulting in the maintenance of micromere fate induction and endoderm specification.Loss of the polar lobe results in the loss of cell fate induction by 3D/4d. Therefore, I hypothesized that proteins are asymmetrically bequeathed to the inducing D lineage cells by the polar lobe. To test this hypothesis, I compared global protein differences between two cell stage intact embryos, lobeless embryos, and isolated polar lobes by 2-Dimensional Electrophoresis analysis. I found several (12) quantitative differences between these samples including four spots enriched in the polar lobe isolates. I identified voltage-dependent anion-selective channel (VDAC) as one of the candidate proteins enriched in polar lobe isolates. I propose that VDAC is asymmetrically distributed by the polar lobe to the D cell and that it may function in D cell induction and mesendoderm fate specification.Lastly, I identify an acetylated tubulin antigen as a marker for cilia. I describe the pattern of cilia differentiation in the developing larvae that results in the formation of two ciliary bands, the prototroch and the metatroch, required for locomotion and feeding. These ciliary bands are conserved among annelid and mollusc larvae. Interestingly, the metatroch is derived from third quartet derivatives in the annelid Polygordius and from second quartet derivatives in the mollusc Crepidula. I provide evidence that the metatroch is derived from the first quartet derivatives in the mollusc Ilyanassa. Thus while the larval metatroch is conserved, its clonal origin is not. Based on these results, I provide support for the hypothesis that the metatroch is not homologous between annelids and molluscs or even among molluscs.
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Samra, P. K. "A systematic method to develop work-based training for SMEs." Thesis, Coventry University, 2010. http://curve.coventry.ac.uk/open/items/7f1e3a63-2dee-aa3a-8811-6e6c98f0b6d5/1.
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Training plays a key role in bridging employer skills needs and in raising the productivity of employees and individual firms. Many large organisations have long established in-house training programmes and the means for progressive personal development for employees. However, Smaller to Medium sized Enterprises (SMEs) have not mirrored the same. SMEs are less likely to provide off the job training to employees because of operational demands. There is a lack of structured approach to training within a SME. Despite the vast array of training programmes available to SMEs, very little has been done to tailor the training to the specific learning needs of SMEs. Web Based Training Environment (WeBTiE) is a tailored online training solution specific to a SMEs’ training needs. The portal within which online training is provided is the collaborative efforts of the Employer, Employee and Training Vendor. The portal encapsulates not only the learning content, but provides the learning support necessary to guide employees through the training and reinforce work-based practises with the use of a Community of Practice. Unlike many other training models the originality of this model is that it combines Pedagogy principles in the development of the training programme, along with e-learning model, Etivities for Structured On The Job Training. The generic nature of this model allows it to be adaptable for SMEs in a variety of industries and the tailoristic feature provides the flexibility necessary to permit the adaptability. This thesis focuses on the barriers SMEs face when attempting to train employees. By determining employee learning and training needs and establishing a ‘Learning Foundation’, can we move forward with the development of a training programme commissioned for SMEs. The work in this research is underpinned by learning theories and online learning and the application of these (adapted if necessary) in the context of the day-to-day constraints typical of SMEs.
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Müller, Jonas. "Finite frequency dynamics in correlated quantum conductors." Thesis, université Paris-Saclay, 2020. http://www.theses.fr/2020UPASP061.
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Dans ce travail, nous présentons les nouvelles méthodes expérimentales que nous avons développées afin d’étudier la physique fondamentale du transport électronique à travers des conducteurs mésoscopiques en se basant sur la mesure des fluctuations électriques. Dans la première partie de la thèse, nous présentons une nouvelle conception d’un détecteur quantique sans rétroaction pour mesurer séparément la densité spectrale de puissance des fluctuations de courant pour les fréquences positives (bruit d’absorption) et négatives (bruit d’émission). Nous extrayons le bruit d’absorption et d’émission d’une mesure de la puissance échangée entre un conducteur quantique et un résonateur linéaire à fréquence finie, testé pour une jonction SIS couplée à un filtre à cavité. Nos résultats soulignent la signification physique de la formule de Kubo qui, couplée à une description quantique du dispositif de mesure, fournit une version quantique du théorème de Joule. Dans la deuxième partie de la thèse, nous présentons la conception et la construction d’une plate-forme expérimentale pour les mesures RF dépendantes du temps dans les champs magnétiques élevés. L’objectif est de mesurer efficacement un conducteur quantique qui donne lieu à une contre-action de détection sur ses propriétés de transport, connue sous le nom de Blocage de Coulomb Dynamique (DCB). Nous souhaitons étudier de tels effets dans le cas élémentaire d’un canal de conduction unique, avec une transmission arbitraire, interagissant avec un mode électromagnétique unique. Le principal défi consiste à concevoir des résonateurs RF à haute impédance utilisés comme transformateur d’impédance pour coupler efficacement le canal unique à haute impédance (25,8 kΩ) à l’équipement de détection RF désadapté de 50 Ω. Pour notre installation, nous avons d’abord conçu et testé un résonateur tolérant aux champs magnétiques, une bobine métallique plane, qui fournit une impédance caractéristique de 1 kΩ à une fréquence de résonance de 5,4 GHz. En utilisant deux résonateurs en série, il est possible d’obtenir une impédance de détection efficace de 27 kΩ qui assure un couplage suffisant à un seul canal. Avec toutes les méthodologies développées dans cette thèse, il est maintenant possible de réaliser une série remarquable d’expériences diverses dans un futur procheIn this work, we present the new experimental methods that we have developed in order to investigate the fundamental physics of electronic transport across mesoscopic conductors based on the measurement of electrical fluctuations. In the first part of the thesis we present a novel design of a back-action free quantum detector to separately measure the power spectral density of current fluctuations for positive (absorption noise) and negative (emission noise) frequencies. We extract the absorption and emission noise from a measurement of the power exchanged between a quantum conductor and a finite frequency linear resonator, tested for a SIS junction coupled to a cavity filter. Our results stress the physical meaning of the Kubo formula which, coupled to a quantum description of the measurement setup, provides a quantum version of Joule’s theorem. In the second part of the thesis, we present the design and construction of an experimental platform for time dependant RF-measurements in high magnetic fields. The goal is to efficiently measure a quantum conductor that gives rise to a detection back-action on its transport properties, known as Dynamical Coulomb Blockade (DCB). We wish to investigate such effects in the elementary case of a single conduction channel, with arbitrary transmission, interacting with a single electromagnetic mode. The main challenge is the engineering of high impedance RF resonators used as impedance transformer to efficiently couple the high impedance single channel (25.8 kΩ) to the mismatched 50 Ω RF-detection equipment. For our setup we have firstly designed and tested a magnetic field tolerant resonator, a planar metallic coil, that provides a characteristic impedance of 1 kΩ at a resonance frequency of 5.4 GHz. Using two resonators in series, an effective detection impedance of 27 kΩ is achievable that provides sufficient coupling to a single channel. With all the methodologies developed in this thesis, it is now possible to perform an amazing series of various experiments in the near future
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Musa, Ishaq. "Propriétés optiques de nanostructures et composites de polymères à base d'oxyde de zinc." Nantes, 2011. http://archive.bu.univ-nantes.fr/pollux/show.action?id=15e243b4-2dfe-49f7-9172-f8c9e9b62259.
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Cette thèse porte principalement sur l'étude de certaines propriétés optiques de nanostructures d'oxyde de zinc (ZnO) sous forme de nanocristaux, de films minces, et en tant que composant nanométrique dans des composites à base de polymères conducteurs. La première partie du mémoire décrit la synthèse de ces matériaux qui a été effectuée à l'Institut des Matériaux Jean Rouxel, par différentes méthodes. Une étude poussée de la caractérisation morphologique et structurale est effectuée au moyen de la microscopie électronique et de la diffraction des rayons X, appuyée par quelques expériences de diffusion Raman. Dans le chapitre suivant, on découvre que ces nanomatériaux émettent une lumière intense dans l'ultraviolet et dans la région verte du spectre. La preuve du confinement quantique des excitons dans les particules de ZnO est apportée par les techniques d'aborption optique et de photoluminescence suivie par une description succincte des modèles régissant ce confinement. Vient ensuite l'analyse des propriétés émissives, notamment par spectroscopie résolue en temps. Les variations temporelles et spectrales de l'émission sont caractérisées en fonction de la taille et des défauts structurels des nanostructures. Le dernier chapitre aborde la synthèse et l'étude optique de matériaux composites obtenus en mélangeant des nanocristaux de ZnO avec des polymères conjugés tels que MEH-PPV, connus pour leurs propriétés de photoluminescence et leur application aux diodes électroluminescentes. Là encore des propriétés émissives remarquables en fonction de la taille des particules insérées sont mises en évidenceThis thesis presents the synthesis, characterization and optical properties of ZnO nanostructures. In addition, composite thin films made by incorporation of ZnO nanoparticles into conjugated polymer have also been fabricated and studied by optical characterization with special focus on PL measurements. Well-crystallized plate-like and bare ZnO nanoparticles of various sizes (3. 5 - 20 nm) were synthesized by different chemical routes without surface modification. The morphology and structure of the nanoparticles were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, and time-resolved photoluminescence (PL). Strikingly, the intensity of the defect-related emission band is enhanced when the particle size is reduced. In a parallel manner, the energies of near band edge (NBE) UV emission and absorption onsets are blue shifted. The dynamical behavior of exciton confinement is reflected by very a short decay time of the NBE exciton, and by long-lived, multiexponential, intrinsic-defect emission in the green spectral range. This temporal investigation of PL gives strong indication that a quantum confinement effect exists in the electronic structure of ZnO nanoparticles well above the exciton Bohr radius, lasting at subnano/or nanosecond time scales. The observed size dependence of the UV and green emission intensities opens up the possibility of tailoring exciton properties of ZnO nanocrystals for their applications in light emitting diodes or in photovoltaic components. In the same context, the optical properties of ZnO thin films with and without AlN buffer layer will be shortly described as well as those of ZnO coated multi wall carbon nanotubes (MWCNTs). The effect of the various sizes and concentrations of hybrid MEH-PPV/ZnO and PF-oxe/ZnO composites on their optical properties are studied. The PL spectra showed a significant enhancement in intensity in composites when using low nanoparticle concentrations. Additionally, it was also observed that the smaller the size of ZnO nanoparticles the higher the emission efficiency in thin films composites
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Ullah, Md Barkat. "Growth of Zn-polar BeMgZnO/ZnO heterostructure with two dimensional electron gas (2DEG) and fabrication of silver Schottky diode on BeMgZnO/ZnO heterostructure." VCU Scholars Compass, 2017. http://scholarscompass.vcu.edu/etd/4992.
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Title of dissertation: GROWTH OF Zn POLAR BeMgZnO/ZnO HETEROSTRUCTURE WITH TWO DIMENSIONAL ELECTRON GAS (2DEG) AND FABRICATION OF SILVER SCHOTTKY DIODE ON BeMgZnO/ZnO HETEROSTRUCTURE
By Md Barkat Ullah, Ph.D
A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Electrical and Computer Engineering at Virginia Commonwealth University.
Virginia Commonwealth University,2017
Major Director: Dr. Hadis Morkoç, Professor, Electrical and Computer Engineering
This thesis focuses on growth of Zn polar BeMgZnO/ZnO heterostructure on GaN/sapphire template with two dimensional electron gas (2DEG) for the application of UV photodetector/emitter and high speed electronics. The motivation of using BeMgZnO as a barrier layer originates from the need to reach plasmon-LO phonon resonance in order to obtain minimum longitudinal optical (LO) phonon lifetime.
Presence of 2DEG was realized in BeMgZnO/ZnO heterostructure only when the Zn polarity was achieved during the nucleation growth of ZnO on GaN/sapphire template. It was found that, polarity of ZnO on (0001) GaN/sapphire template can be controlled by the oxygen to Zn ratio used during the nucleation growth.
To obtain high structural and optical quality of BeMgZnO quaternary alloy, growth kinetics of BeMgZnO layer has been studied at the temperature range from 450°C-500°C. We have achieved the growth of single crystal Be.03Mg00.15ZnO alloy at 500 °C, more than 100°C higher compared to what reported in literature, on the (0001) GaN/sapphire template through the control of Zn/(Be+Mg) flux ratio. We have also observed a thermodynamic limitation of Mg incorporation into the wurtzite BeMgZnO alloy where the excess Mg adatom accumulated in the growing surface as a MgO rich cluster.
Two dimensional electron gas with high (1.2×1013cm-2) sheet carrier density was achieved at the Be0.03Mg0.41ZnO/ZnO interface through strain engineering by incorporating Be into MgZnO ternary alloy. To obtain the similar sheet carrier density it would require above 60% of Mg in MgZnO/ZnO heterostructure with reduced structural quality. A systematic comparison of sheet carrier density has been made with the already reported results from Zn polar MgZnO/ZnO heterostructure as well as with the theoretical calculation.
Silver Schottky diode on Be0.02Mg0.26ZnO/ZnO heterostructure with barrier height 1.07 eV and ideality factor 1.22 was obtained with 8 order of rectification ratio. The temperature-dependent electrical characteristics were studied by using temperature dependent current-voltage (I-V) measurements. Richardson constant value of 34.8 Acm-2K-2 was found experimentally which was close to the theoretical value of 36 Acm-2K-2 known for Be0.02Mg0.26ZnO alloy.
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Cernicchiaro, Geraldo Roberto Carvalho. "Développement d'un système de mesure d'interférences mésoscopiques : application à l'étude de courants permanents, quantification de conductance et interférences entre canaux dans un anneau 2DEG." Université Joseph Fourier (Grenoble), 1997. http://www.theses.fr/1997GRE10101.
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Cette etude s'interesse au developpement de l'instrumentation et a son application pour la comprehension de certains phenomenes d'interference electronique dans des systemes a l'echelle mesoscopique. Nous montrons de nouvelles mesures de transport quantique et d'aimantation (courants permanents) realisees sur un anneau resistif, lithographie dans un systeme bidimensionnel d'electrons (2deg) a l'interface d'une heterojonction de gaas-gaalas. Nous avons developpe un systeme de detection electronique qui permet l'utilisation de microsquids a micropont hysteretiques comme detecteurs de variation de flux magnetique pour la mesure des courants permanents. Ce systeme permet la mesure de tres petits objets (<100 nm). Il est utilise actuellement dans l'etude de plusiers domaines du micromagnetisme. Nous etudions la dependance de la magnetoconductance d'un anneau 2deg, en fonction d'un champ electrique applique a une grille metallique deposee sur une branche de l'anneau. La figure d'interference resultante presente une serie de changements : alternance de phase, doublement de frequence, variation d'amplitude. Ces mesures suggerent la superposition des effets d'interferences entre les modes propres de propagation (canaux de conduction) et la quantification de la conductance dans une experience de aharonov-bohm, dans le regime quasi balistique. Des simulations numeriques sont en bon accord qualitativement avec les resultats experimentaux.
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Yoon, Hosang. "Two-Dimensional Plasmonics in Massive and Massless Electron Gases." Thesis, Harvard University, 2014. http://nrs.harvard.edu/urn-3:HUL.InstRepos:13070026.
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Plasmonic waves in solid-state are caused by collective oscillation of mobile charges inside or at the surface of conductors. In particular, surface plasmonic waves propagating at the skin of metals have recently attracted interest, as they reduce the wavelength of electromagnetic waves coupled to them by up to ~10 times, allowing one to create miniaturized wave devices at optical frequencies. In contrast, plasmonic waves on two-dimensional (2D) conductors appear at much lower infrared and THz-GHz frequencies, near or in the electronics regime, and can achieve far stronger wavelength reduction factor reaching well above 100. In this thesis, we study the unique machinery of 2D plasmonic waves behind this ultra-subwavelength confinement and explore how it can be used to create various interesting devices.
To this end, we first develop a physically intuitive theoretical formulation of 2D plasmonic waves, whose two main components---the Coulomb restoration force and inertia of the collectively oscillating charges---are combined into a transmission-line-like model. We then use this formulation to create various ultra-subwavelength 2D plasmonic devices. For the 2D conductor, we first choose GaAs/AlGaAs heterostructure---a 2D electron gas consisting of massive (m*>0) electrons---demonstrating plasmonic bandgap crystals, interferometers, and negatively refracting metamaterials. We then examine a 2D plasmonic device based on graphene, a 2D electron gas consisting of effectively massless (m*=0) electrons. We theoretically show and experimentally demonstrate that the massless electrons in graphene can surprisingly exhibit a collective mass when subjected to a collective excitation, providing the inertia that is essential for the propagation of 2D plasmonic waves.
Lastly, we theoretically investigate the thermal current fluctuation behaviors in massive and massless electron gases. While seemingly unrelated on first sight, we show that the thermal current fluctuation is actually intimately linked to the collective mass of the massive or massless electron gas. Thus, we show that the thermal current fluctuation behaviors can also be described by the same theoretical framework introduced earlier, suggesting a possibility to design new concept devices and experiments based on this linkage.Engineering and Applied Sciences
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Liang, Chen-Wei. "A high spatial resolution magnetovision camera using high-sensitivity Quantum Well Hall Effect sensors." Thesis, University of Manchester, 2017. https://www.research.manchester.ac.uk/portal/en/theses/a-high-spatial-resolution-magnetovision-camera-using-highsensitivity-quantum-well-hall-effect-sensors(fbd23629-b9f8-4f1d-9bc7-1ee08bcf79e8).html.
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A systematic and detailed design, building and testing of a high-sensitivity real-time magnetovision imaging system for non-destructive testing (NDT) was the purpose of the research presented here. The magnetic imaging systems developed were all based on an ultra-high sensitivity Quantum Well Hall Effect (QWHE) sensors, denoted as the P2A, which is based on GaAs-InGaAs-AlGaAs 2DEG heterostructures. The research progressed from 0D (single sensor) to 1D (linear array) to 2D (two dimensional arrays) testing modalities. Firstly, the measurement of thermal and shot noises, drift, detection limit, and dynamic offset cancellation of the QWHE sensor were studied in detail to set the framework and limitations of the fundamental QWHE sensors before their eventual use in the imaging systems developed subsequently. The results indicate that the measured data agrees well with calculations for thermal and shot noise when the input bias current is < 3 mA. The measured drift voltages of various QWHE sensors (P2A and P3A) are less than 200 µV when the sensor bias voltage is less than or equal to 2 V. A 4-direction dynamic offset cancellation technique was developed and the results show that the offset equivalent magnetic field of the QWHE sensors can be reduced from ~ 1mT to readings equal to the Earth magnetic field (~ 50 µT). Secondly, a flexible 16 × 1 array and a 32 × 2 staggered array magnetic-field scanners were designed, built, and tested. The QWHE magnetometer had a field strength resolution of 100 nT, and a measurement dynamic range of 138 dB. The flexible 16 × 1 magnetic field scanner can be used to test uneven and/or curved surfaces. This gives the flexible magnetic field scanner better inspection capabilities in both welding hump and circular pipe samples. By the staggered arrangement of two sensor arrays, a 15.4 point per inch horizontal spatial resolution can be achieved for the staggered 32 × 2 magnetic field scanner. Both direct and alternating magnetic flux leakage (DC and AC MFL) tests with the QWHE magnetometer were accomplished to obtain graphical 2-dimensional magnetic field distributions. Both the shape and the location of defects can be identified. The results show that the sensor has high sensitivity and linearity in a wide frequency range which makes it an optimum choice for AC-MFL testing and both ferromagnetic and non-ferromagnetic materials can be investigated. Thirdly, real-time 8 × 8 and 16 16 QWHE array magnetic-field cameras were designed, built, and tested. These prototypes can measure static magnetic field strengths in a 2-dimensional plane. Different shapes of magnets and magnetic field polarities can all be identified by the 8 × 8 magnetic field camera. The camera has a resolution of 3.05 mT, and a dynamic range of 66 dB (the minimum and maximum fields measurable are 3.05 mT and 6.25 mT) and a real time magnetic field measurement rate of 13 frames per second (FPS). By contrast the1616 array magnetic field camera has an improved sampling rate of 600 frame per second and with the use of an interpolation technique, a spatial resolution of 40.6 point per inch can be achieved. The minimum and maximum detectable magnetic field for this magnetic field camera are 1.8 µT and 29.5 mT respectively leading to a record dynamic range of 84 dB for high quality imaging. Finally, a novel, hand held, magnetovision system based on the real-time 16 × 16 QWHE array magnetic-field camera was developed for improved DC and AC electromagnetic NDT testing. The system uses a new super heterodyne technique for data acquisition using the QWHE sensor as a multiplier. This is the first report of such a technique in Hall effect magnetometry. The experimental results of five case studies demonstrate that the defects location and shape can be successfully measured with MFL in DC and AC magnetic field configurations including depth profiling. The major advantages of this real-time magnetic-field camera are: (1) its ease to use as a MFL testing equipment in both DC and AC NDT testing, (2) its ability to provide 2D graphical images similar to Magnetic Particle Inspection (MPI) but without its inherent health and safety drawbacks, (3) its capability to test both ferromagnetic and non-ferromagnetic materials for deep defects below the surface using low frequency alternating magnetic fields, and (4) its ability to identify materials (metals) by alternating external magnetic field illuminations, which has considerable potential in several applications such as security checking and labelling, magnetic markers for analysis, bio-imaging detection, and medical treatments amongst others.
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Gopalakrishnan, Gokul. "Phonon Exchange by Two-Dimensional Electrons in Intermediate Magnetic Fields." The Ohio State University, 2008. http://rave.ohiolink.edu/etdc/view?acc_num=osu1222186784.
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Dwyer, Virginia Michelle Gregory 1955. "A STUDY OF PINEAL GLAND POLYPEPTIDES AND PROTEINS BY POLYACRYLAMIDE GEL ISOELECTRIC FOCUSING (PAG-IEF) AND TWO-DIMENSIONAL ELECTROPHORESIS (2DE) (BRAIN REGIONS)." Thesis, The University of Arizona, 1986. http://hdl.handle.net/10150/276560.
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Nifa, Iliass. "Caractérisation et modélisation du gaz 2D des dispositifs MIS-HEMTs sur GaN." Thesis, Université Grenoble Alpes (ComUE), 2018. http://www.theses.fr/2018GREAT015/document.
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Le travail de thèse effectué porte sur la caractérisation électrique et la modélisation du gaz d’électrons à deux dimensions (2D) dans les dispositifs MOS-HEMT à base de l’hétérojonction AlGaN/AlN/GaN. Ces dispositifs ont un fort potentiel pour les applications d'électronique de puissance. Ce travail de recherche se place en soutien aux efforts de recherche pour l’élaboration des épitaxies GaN sur Si et pour les filières technologiques HEMT sur GaN. Il s'agit de comprendre précisément le fonctionnement du gaz d'électrons 2D et ses propriétés de transport électronique. Une nouvelle méthodologie a été développée pour identifier le dopage résiduel de la couche GaN, lequel est un paramètre important des substrats GaN et était par ailleurs difficile à évaluer. Un deuxième axe de recherche a consisté à proposer des techniques de mesure fiables ainsi qu’une modélisation des propriétés de transport du gaz d'électrons 2D. Dans ce cadre, des mesures split-CV et effet Hall ont été réalisées en fournissant pour chacune d’elles un protocole expérimental adéquat, avec un montage innovant pour les mesures effet Hall. Ce travail expérimental a été enrichi par une modélisation des propriétés du transport du 2DEG basée sur le formalisme de Kubo-Greenwood. Enfin, dans un dernier axe de recherche, un aspect plus général visant la compréhension en profondeur de l’électrostatique de l’empilement de la grille de nos GaN-MOS-HEMT a été proposé. Il est basé sur la caractérisation électrique C-V, la modélisation et l’extraction des paramètres. Le modèle développé a permis de souligner l'impact des charges surfaciques de polarisation et des défauts sur la tension de seuil des MOS-HEMT. Ce modèle a également permis d’estimer une valeur de la déformation dans les couches GaN épitaxiées sur un substrat SiliciumThis thesis aims at studying the electrical characterization and modelling of two-dimensional (2D) electron gas in MOS-HEMT devices based on the hetero-junction AlGaN/AlN/GaN. These devices are very promising candidates for power electronics applications. This research work provides the production team with detailed data on phenomena affecting GaN material. The goal is to understand precisely how 2D electron gas works and evaluate its electronic transport properties. A new methodology has been developed to identify residual doping of the GaN layer. This method was developed in order to answer a real need to know this doping to determine the quality of the epitaxial GaN layer. The second research priority was to provide reliable measurement techniques and modelling of the transport properties of 2D electron gas. Within this framework, the split-CV and Hall effect measurements were carried out by providing for each of them a suitable experimental protocol, with an innovative set-up for Hall effect measurements. In addition, this experimental work was supported by modelling the transport properties of 2DEG based on Kubo-Greenwood's formalism. Finally, a more general aspect aimed at an in-depth understanding of the electrostatic stacking of the GaN-MOS-HEMT gate. It is based on C-V electrical characterization, modelling and parameter extraction. The model developed made it possible to highlight the impact of polarization surface charges and defects on the threshold voltage of MOS-HEMT. This model also contributed to the estimation of the value of deformation in epitaxial GaN layers on a Silicon substrate
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Hoy, Daniel R. "Gallium Nitride and Aluminum Gallium Nitride Heterojunctions for Electronic Spin Injection and Magnetic Gadolinium Doping." The Ohio State University, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=osu1331855661.
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Rodríguez, Falcón Manuel. "Proteómica de expresión diferencial en Acinetobacter baumanii resistente a colistina." Doctoral thesis, Universitat Pompeu Fabra, 2010. http://hdl.handle.net/10803/31820.
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Normally present in water, soil and waste water, Acinetobacter baumannii has become an
important nosocomial pathogen, as causal agent of pneumonias, septicemias and urinary
tract infections, among other complications in compromised patients from hospital’s
intensive care units. One of its last acquired abilities is the resistance to colistin (polymixin
E), the last therapeutic option for its infections. In this thesis, descriptive and quantitative
differential expression proteomics is used in the study of acquired colistin resistance. As
result of this research, 1,097 proteins belonging to the Acinetobacter genus have been
identified by combined application of bidimensional gel electrophoresis (2DE), differential
gel electrophoresis (DIGE), and peptide labeling with stable isobaric isotopes tags
(iTRAQ). Analyses have been performed on the global expressed proteome of a reference,
colistin-sensible strain (A. baumannii ATCC 19606) and, for comparative purposes, on a
derived strain on which colistin resistance has been induced in vitro. The resistant
phenotype shows reduced fitness, with significant differences in expression found in outer
membrane proteins, membrane active transporters, diverse metabolic enzymes (fatty acids,
citrate, phenylacetate, piruvate and nitrogen), proteins involved in stress response and
biofilm formation, as well as in protein synthesis and folding pathways. The work has
allowed to assess the strengths and weaknesses of the different techniques currently used in
this type of proteomic analysis.Acinetobacter baumannii, normalmente aislado en suelos y aguas (corrientes o residuales), se ha convertido en importante patógeno nosocomial, siendo agente causal de, entre otras complicaciones, neumonías, septicemias e infecciones del tracto urinario de pacientes comprometidos en unidades hospitalarias de cuidados intensivos. La más reciente de sus capacidades adquiridas es la resistencia a colistina (polimixina E), antibiótico peptídico considerado la última opción terapéutica en contextos clínicos. Esta tesis doctoral emplea la proteómica descriptiva y de expresión diferencial cuantitativa para investigar la resistencia adquirida por A. baumannii a dicho antibiótico. Los resultados han supuesto la identificación de 1.097 proteínas de Acinetobacter mediante el empleo combinado de electroforesis bidimensional convencional (2DE), 2DE diferencial (DIGE) y marcaje peptídico mediante isótopos isobáricos estables (iTRAQ). Los análisis se han realizado en el proteoma expresado por una cepa de referencia sensible a colistina (A. baumannii ATCC 19606), así como en una cepa derivada de ésta en la que se ha inducido, a efectos comparativos, resistencia a colistina in vitro. El fenotipo resistente manifestó reducida adaptabilidad biológica, encontrándose las principales diferencias en la estructura de la membrana externa, en la expresión de transportadores activos de membrana, en diversos enzimas metabólicos (ácidos grasos, citrato, fenilacetato, piruvato, nitrógeno) y de respuesta a condiciones de estrés, así como en la expresión de proteínas participantes en la formación de biopelículas y en el proceso de síntesis y plegamiento de proteínas. Además, el trabajo ha permitido evaluar los puntos fuertes y débiles de las técnicas empleadas actualmente en este tipo de análisis proteómicos.
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Brun, Boris. "Electron interactions in mesoscopic physics : Scanning Gate Microscopy and interferometry at a quantum point contact." Thesis, Grenoble, 2014. http://www.theses.fr/2014GRENY049/document.
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Au cours de cette thèse nous avons étudié les effets des interactions entre électrons dansles contacts ponctuels quantiques (QPCs). Les contacts ponctuels quantiques sont des petitscanaux quasi-unidimensionnels, définis à partir de gaz électroniques bidimensionnelsde haute mobilité (2DEG). Une tension négative appliquée sur des grilles métalliques audessus de la surface permet d’ouvrir ou fermer le QPC. Lorsqu’un QPC s’ouvre, de plusen plus de modes électroniques peuvent traverser le QPC, et sa conductance augmente parpas discrets, séparés par un quantum de conductance 2e2/h. On peut le comprendre parle transport unidimensionnel d’une seule particule, car chaque mode transverse contribuepour un quantum de conductance.Mais depuis leurs premières réalisations, les QPCs ont montré des déviations par rapportà ce modèle à une particule. Les plus connues sont un épaulement sous le premier plateau,autour de 0.7×2e2/h, appelé "l’anomalie 0.7", et un pic dans la conductance différentiellequi apparaît à basse température: l’anomalie à zéro polarisation (ZBA).L’instrument que nous avons utilisé pour étudier ces effets d’interactions est un microscopeà effet de grille local (SGM). Cette technique consiste à modifier localement le potentield’un dispositif à l’aide d’une pointe de microscope à force atomique (AFM) chargée négativement,et enregistrer les modifications de la conductance en fonction de la position dela pointe. En utilisant cette technique à très basse température, nous avons montré quenous pouvons moduler les anomalies de conductance du QPC. Nous avons interprété nosrésultats comme la signature d’un cristal d’électrons se formant spontanément à bassedensité dans le QPC à cause de la répulsion Coulombienne: un cristal de Wigner. Onpeut modifier le nombre d’électrons cristallisés en approchant la pointe, et obtenir dessignatures de la parité du nombre d’électrons localisés dans le transport électronique.En fonction de cette parité, le cristal de Wigner présente un état de spin différent, etl’écrantage de ce spin par les électrons de conduction au travers d’un mécanisme appeléeffet Kondo donne une anomalie à zéro polarisation formant alternativement un simplepic ou un double pic. Cette découverte apporte une avancée significative à ce domaine,qui a concentré les efforts de plusieurs groupes importants ces 15 dernières années.Nous avons ensuite réalisé des mesures interférométriques à l’aide du microscope SGM,en créant in situ des interféromètres dans le gaz 2D. Nous avons obtenu les signaturesd’un déphasage supplémentaire dans le régime de la ZBA. Nous attribuons cet effet audéphasage universel accumulé par les électrons à la traversée d’un singulet Kondo, ce quirenforce le fait que la ZBA trouve son origine dans les phénomènes Kondo.Enfin, nous avons adapté la technique SGM au transport thermoélectrique dans les QPCs,et avons imagé pour la première fois les interférences d’électrons se déplaçant sous l’effetd’une différence de températureIn this thesis, we studied the effect of electron electron interactions in quantum pointcontacts (QPCs). Quantum point contacts are small quasi-one dimensional channels,designed on a high mobility two-dimensional electron gas (2DEG). A negative voltageapplied on a pair of metallic split gates above the sample surface allows to open or closethe QPC. As a QPC opens, more and more electronic modes are allowed to cross theQPC, and its conductance increases by discrete steps, separated by a conductance quantum2e2/h. This can be understood from a single-particle picture in one-dimensionaltransport, as each transverse mode carries a conductance quantum.But from their first realization 25 years ago, quantum point contacts have shown deviationsfrom this picture, attributed to electron electron interactions. The most well knownare a shoulder below the first plateau, around 0.7×2e2/h, called the "0.7 anomaly", and apeak in the differential conductance that arises at low temperature: the zero bias anomaly(ZBA).The tool we used to study these interaction effects is a scanning gate microscope (SGM).It consists by changing locally the device’s potential with the polarized tip of an atomicforce microscope (AFM), and record the changes in conductance as a function of the tipposition. By performing this technique at very low temperature, we showed that we canmodulate the conductance anomalies of QPCs. We interpret our result as the signatureof a small electrons crystal forming spontaneously at low density in the QPC due to theCoulomb repulsion: a Wigner crystal. We can modify the number of crystallized electronsby approaching the tip, and obtain signatures of the parity of the localized electrons numberin transport features. Depending on this parity, the Wigner crystal has a differentspin state, and screening of this spin by the surrounding electrons through the so-calledKondo effect leads alternatively to a single peak or a split ZBA. This discovery bringsa significant advance in this field, that has attracted research efforts of many importantgroups in the world over the past 15 years.We then performed interferometric measurements thanks to the scanning gate microscopeby creating in-situ interferometers in the 2DEG. We obtained signatures of an additionalphase shift accumulated by the electrons in the ZBA regime. We attribute this effect tothe universal phase shift that electrons accumulate when crossing a Kondo singlet, reinforcingthat the debated origin of the ZBA lies in Kondo physics.Finally, we adapted the SGM technique to the study of thermoelectric transport in QPCs,and for the first time imaged interferences of electrons driven by a temperature difference
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Feist, Timothy Richard. "A grammar of Skolt Saami." Thesis, University of Manchester, 2011. https://www.research.manchester.ac.uk/portal/en/theses/a-grammar-of-skolt-saami(05b6e11a-2dde-494b-9110-21596f1176cd).html.
Full textAbstract:
This thesis is a descriptive grammar of Skolt Saami, a Finno-Ugric language spoken primarily in northeast Finland by less than 400 people. The aim of this thesis is to provide an overview of all the major grammatical aspects of the language. It comprises descriptions of Skolt Saami phonology, morphophonology, morphology, morphosyntax and syntax. A compilation of interlinearised texts is appended. Skolt Saami is a phonologically complex language, displaying contrastive vowel length, consonant gradation, suprasegmental palatalisation and vowel height alternations. It is also well known for being one of the few languages to display three distinctive degrees of quantity; indeed, this very topic has already been the subject of an acoustic analysis (McRobbie-Utasi 1999). Skolt Saami is also a morphologically complex language. Nominals in Skolt Saami belong to twelve different inflectional classes. They inflect for number and nine grammatical cases and may also mark possession, giving rise to over seventy distinct forms. Verbs belong to four different inflectional classes and inflect for person, number, tense and mood. Inflection is marked by suffixes, many of which are fused morphemes. Other theoretically interesting features of the language, which are covered in this thesis, include (i) the existence of distinct predicative and attributive forms of adjectives, (ii) the case-marking of subject and object nominals which have cardinal numerals as determiners, (iii) the marking of negation with a negative auxiliary verb and (iv) the apparent verb-second phenomenon which is observed in clauses displaying an auxiliary verb. Skolt Saami is a seriously endangered language and it is thus hoped that this grammar will serve both as a tool to linguistic researchers and as an impetus to the speech community in any future revitalisation efforts.
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Bryan, Charlotte. "Etude et développement de capteurs thermiques pour composants de puissance." Thesis, Université Grenoble Alpes, 2020. http://www.theses.fr/2020GRALI079.
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Depuis le début du siècle, la demande pour les composants de puissance a fortement augmenté. Ces composants sont principalement utilisés dans les circuits intégrés pour le marché de la communication, tel que celui des portables et des chargeurs, c’est-à-dire des applications nécessitant des fréquences de travail élevées de plusieurs GHz et pour des puissances allant jusqu’à 100 W. Pendant longtemps, ces dispositifs étaient réalisés à base de silicium, mais les limites de ce matériau pour ces composants ont été atteintes et de nouveaux matériaux ont émergé. Dans ce contexte, le nitrure de gallium (GaN) et le nitrure d'aluminium-gallium (AlGaN) ont été développés afin de créer de nouveaux dispositifs tels que les diodes de puissance et les transistors à mobilités électronique élevées (HEMTs). Ces HEMTs délivrent des puissances importantes, cependant, elles sont accompagnées d’une production de chaleur pouvant mener à la dégradation du câblage par fils et des boitiers. La gestion de la thermique dans ces dispositifs est donc une problématique majeure, tout comme dans la microélectronique de manière générale. Des diodes ainsi que des capteurs à base de matériaux thermistants, matériaux présentant des variations importantes de résistance en fonction de la température, sont généralement utilisés pour mesurer ces surchauffes. Cependant, ils nécessitent tous les deux des courants externes pour les faire fonctionner et prennent de la place supplémentaire dans les boitiers.Au cours de cette thèse, des capteurs thermoélectriques ont été développés. Ces capteurs sont basés sur l’effet Seebeck, effet qui convertit directement l’énergie thermique en énergie électrique. La tension de sortie des capteurs thermoélectriques est directement proportionnelle à la différence de température au sein du capteur et ils n’ont pas besoin d’énergie extérieure pour fonctionner. Ces capteurs permettent la lecture aussi bien d’une différence de température que d’un flux thermique. Il s’agit de la première réalisation de ce type de capteur dans les circuits de puissanceDeux types de capteurs ont été réalisés lors de la thèse : le premier est un capteur thermoélectrique « embarqué », il est fabriqué en même temps que le transistor HEMT, lui permettant d’être placé au plus proche de ce dernier pour une mesure en température plus précise. De plus, il est directement intégré dans la puce du transistor HEMT et ne prend donc pas de place supplémentaire dans le boitier. Cette intégration implique néanmoins qu’il doit suivre les règles de dimensionnement et de fabrication des transistors. Ce capteur utilise comme matériau actif le gaz d’électron 2DEG, qui est généré à l’interface de la couche d’AlGaN et de GaN pour le transport d’information électrique.Le deuxième type de capteur est un capteur thermoélectrique « autonome », il est fabriqué indépendamment du HEMT, il a donc moins de contrainte à respecter que les capteurs embarqués. Deux capteurs de ce type ont été fabriqués : un à base du 2DEG et l’autre à base de GaN dopé n. Suite à une étude approfondie effectuée au préalable sur les résistances de contacts et sur les propriétés thermoélectriques de ces deux matériaux, ces capteurs ont été réalisés pour délivrer des performances électriques les plus élevées.Les deux types de capteurs ont été testés pour différentes températures environnantes et sont tous fonctionnels. Dans les deux cas, différentes géométries ont été fabriquées afin de comparer cet effet sur la sensibilité des capteurs. Le capteur embarqué a également été testé lors du fonctionnement d’un transistor HEMT à côté duquel il a été disposé, ce qui constitue un cas réel de fonctionnement et d’utilisation de ces capteurs. Ces capteurs présentent des sensibilités pouvant aller jusqu’à 350 mV/K. De leur côté, les capteurs autonomes ont été caractérisés en utilisant des lignes chauffantes. Ils présentent des valeurs de sensibilité très élevées pouvant monter jusqu’à 14 V/KSince the start of the century, the demand for power components has risen sharply. Power components are used in integrated circuits for applications requiring high frequencies, of several GHz, and powers up to 100 W, mainly for mobile phones and chargers. Materials such as gallium nitride (GaN) and aluminium gallium nitride (AlGaN) have emerged in this field to create new power devices including power diodes and High Electron Mobility Transistors (HEMT), overcoming the limitations of silicon-based devices. HEMTs deliver high power and overheating can occur if they are not well managed, leading to the degradation of its cabling and packaging. Heat management in power circuits, as in electronic circuits in general, is a major issue. Diodes and sensors made from thermistant materials - materials with large variations in resistance as a function of temperature - are used to measure the HEMTs temperature, however, both of these require external currents to operate and use additional space in the device packaging.Thermoelectric sensors for power devices were therefore developed during this research; these sensors are based on the Seebeck effect, which directly converts heat into electrical energy. The output voltage of these thermoelectric sensors is directly proportional to the temperature difference along the sensor so no external energy is required. These sensors can measure a temperature difference and the heat flow can also be deduced. This work describes the first fabrication of such sensors.Two types of sensors were produced: the first is an on-chip sensor; it is fabricated at the same time as the HEMT transistor. This enables it to be placed as close as possible to the transistor for a more accurate temperature measurement. It is also directly integrated onto the HEMT chip so it does not take up additional space in the packaging, which implies that it must follow the same dimensioning and fabrication rules as the transistor. This sensor uses the 2D Electron Gas (2DEG) at the AlGaN and GaN’s interface for electrical transport.The second type of sensor is a stand-alone thermoelectric sensor designed to deliver higher electrical performance. It is fabricated independently, so has fewer constraints than the on-board sensors. Two stand-alone sensors were developed: one using the 2DEG and the other using an n-doped GaN. Their geometry was dimensioned using results from a study carried out beforehand on the contact resistances and on the thermoelectric properties of the two materials.Both types of sensors were tested and verified to be functional. Several geometries were fabricated for each type, and their sensitivities compared. The on-chip sensor was characterised while activating the adjacent transistor, which represents its intended function. The stand-alone sensors were characterised using metallic heat lines to their side. The measurements were taken at a number of different surrounding temperatures in each case. High sensitivities were obtained with these sensors: 350 mV / K for the on-board sensor and 14 V / K for the stand-alone sensor
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De, Liberato Simone. "Cavity quantum electrodynamics and intersubband polaritonics of a two dimensional electron gas." Phd thesis, Université Paris-Diderot - Paris VII, 2009. http://tel.archives-ouvertes.fr/tel-00421386.
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L'électrodynamique quantique en cavité, c'est-à-dire l'étude du couplage lumière-matière en géométries confinées, a permis d'observer, grâce à des cavités de plus en plus performantes, le régime de couplage fort lumière-matière.Dans ce régime, le temps de vie d'un photon est plus long que le temps caractéristique de l'interaction avec la matière ; un seul photon subit donc plusieurs cycles d'absorption et de réémission avant de s'échapper de la cavité.
Les premières expériences dans ce régime, effectuées avec des atomes dans des cavités supraconductrices, ont été suivies par des réalisations en matière condensée, utilisant des excitons dans des microcavités planaires, des boites de Cooper couplées à des résonateurs unidimensionnels ou bien des transitions intersousbandes dans des puits quantiques dopés, couplées à un mode de microcavité. Le couplage fort dans ce dernier système donne naissance à des excitations mixtes, moitié lumière et moitié matière, nommées polaritons intersousbandes.
Ma thèse s'attache à plusieurs aspects de la physique de ces excitations, qui se caractérisent par la force extrême du couplage, qui a poussé les chercheurs à introduire le terme couplage ultra-fort.
Dans la première partie de ma thèse, après avoir donné un aperçu général des différents concepts théoriques engagés, j'étudie les conséquences de ce couplage ultra-fort en présence d'une modulation externe appliquée au système. Je montre, en utilisant une théorie de Langevin quantique, qu'une radiation peut être émise à partir du vide, effet qui rappelle de près l'effet Casimir dynamique. L'intensité de cette radiation est assez forte pour pouvoir être mesurée et je reporte ici les résultats de deux expériences préliminaires menées en vue de l'observation d'un tel effet, auxquelles j'ai participé pour la partie théorique.
J'étudie ensuite la manière dont le couplage fort lumière-matière peut influencer le transport électronique et les expériences d'électroluminescence. Dans ce but j'ai développé des méthodes analytiques et numériques que j'ai exploitées pour montrer qu'il est possible d'augmenter grandement l'efficacité quantique des LEDs basées sur des transitions intersousbandes. J'ai aussi donné une première preuve d'extension de l'effet Purcell au régime de couplage fort.
Enfin, dans ma dernière partie, j'ai développé la théorie du scattering stimulé entre polaritons intersousbandes dû au couplage avec des phonons optiques. Je montre que ce mécanisme peut être exploité afin d'obtenir des lasers sans inversion de population avec un seuil extrêmement bas.
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Ian, Ka Wa. "Routes to cost effective realisation of high performance submicron gate InGaAs/InAlAs/InP pHEMT." Thesis, University of Manchester, 2013. https://www.research.manchester.ac.uk/portal/en/theses/routes-to-cost-effective-relisation-of-high-performance-submicron-gate-ingaasinalasinp-phemt(5aac071c-a4c8-48c7-aadf-e1f3e562e384).html.
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The Square Kilometre Array (SKA) is known to be the most powerful radio telescope of its type. In support of its high observational power, it is estimated that thousands of antenna unit equipped with millions of LNA (low noise amplifier) will be deployed over a large area (radius>3000km). The stringent requirements for high performance and low cost LNA design bring about many challenges in terms of material growth, device fabrication and low noise circuit designs. For the past decade, the Manchester group has been wholeheartedly committed to the research and development of high performance, low cost Monolithic Microwave Integrated Circuit (MMIC) LNA with high breakdown (15V) and low noise characteristics (1.2dB to 1.5dB) for the SKA mid-frequency application (0.4GHz to 1.4GHz). The on-going optimisation of current design is hindered by the restriction of standard i-line 1µm gate lithography. The primary focus of this work is on the design and fabrication of new, submicron gate InGaAs/InAlAs/InP pHEMTs for high frequency applications and future SKA high frequency bands. The study starts with the design and fabrication of InGaAs-InAlAs pHEMT sub-100nm gate structure using E-Beam lithography. To address the problems of short channel effect and parasitic components, devices with 128nm T-gate structure, and with optimised device geometries and enhanced material growth, having fT of 162GHz and fmax of 183GHz are demonstrated, outlining the importance of device scaling for high speed operation. In addition, a gate-sinking technique using Pd/Ti/Au metallisation scheme was investigated to meet the requirement for single voltage supply in circuit design. Device with Pd-buried gate exhibits enhanced DC and RF characteristics and showed no degradation over 5 hours’ annealing at 230˚C. The implementation of this highly thermal stable Pd Schottky gate is key to improving the device’s long-term reliability at high-temperature operation. To solve the problem of low productivity in E-Beam lithography, a simple, low cost, technique termed soft reflow was introduced by utilising the principle of solvent vaporisation in a closed chamber. It provides a hybrid solution for the fabrication of submicron device using low cost i-line lithography. The integration of this new soft reflow process with the Pd-gate sinking technique has enabled the large-scale fabrication of 250nm T-gate pHEMTs, with excellent fT of 108GHz and a fmax of 119GHz and with device yields exceeding 80%. This novel soft reflow technique provides a high yield, fast throughput, solution for the fabrication of submicron gate pHEMT and other ultra-high frequency nanoscale devices.
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Gustafsson, Alexander. "Electron transport in quantum point contacts : A theoretical study." Thesis, Linnéuniversitetet, Institutionen för datavetenskap, fysik och matematik, DFM, 2011. http://urn.kb.se/resolve?urn=urn:nbn:se:lnu:diva-10771.
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Electron transport in mesoscopic systems, such as quantum point contacts and Aharonov-Bohm rings are investigated numerically in a tight-binding language with a recursive Green's function algorithm. The simulation reveals among other things the quantized nature of the conductance in point contacts, the Hall conductance, the decreasing sensitivity to scattering impurities in a magnetic field, and the periodic magnetoconductance in an Aharonov-Bohm ring. Furthermore, the probability density distributions for some different setups are mapped, making the transmission coefficients, the quantum Hall effect, and the cyclotron radius visible, where the latter indicates the correspondance between quantum mechanics and classical physics on the mesoscopic scale.
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Evaldsson, Martin. "Spin splitting in open quantum dots and related systems." Licentiate thesis, Linköping : Linköpings universitet, 2005. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-4939.
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Evaldsson, Martin. "Quantum transport and spin effects in lateral semiconductor nanostructures and graphene." Doctoral thesis, Norrköping : Department of Science and technology, Linköping University, 2008. http://www.bibl.liu.se/liupubl/disp/disp2008/tek1202s.pdf.
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Shoron, Omor Faruk. "Extreme Electron Density Perovskite Oxide Heterostructures for Field Effect Transistors." The Ohio State University, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=osu1429714269.
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Djukic, Uros. "Croissance, structure atomique et propriétés électroniques de couches minces de Bismuth sur InAs(100) et sur InAs(111)." Thesis, Cergy-Pontoise, 2015. http://www.theses.fr/2015CERG0760/document.
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L'émergence d'une une nouvelle classe de matériaux, des isolants topologiques, a stimulé un vaste champ de recherche. Bismuth, un élément du groupe V du tableau périodique, est un des ingrédients clé d'une famille d'isolants topologiques. Pour des applications dans la technologie des composants électroniques, il est essentiel de maîtriser la préparation des matériaux en couches minces. Dans ce travail de thèse, nous avons étudié la croissance et la structure électronique de bismuth sur les surfaces (100) et (111) de semi-conducteur III-V InAs.Déposition de Bi sur la surface InAs(100) résulte en une auto-organisation de Bi qui forme des lignes de taille atomique. On montre que le bismuth interagit extrêmement faiblement avec la surface car la structure d'origine de la surface propre de l'InA(100) reste intacte. L'étude de la bande valence montre la présence d'états résonants fortement dépendants de l'énergie de photons et de la polarisation de la lumière, en cohérence avec la structure quasi unidimensionnelle de la surface.La spécificité de la surface InAs(111) est qu'elle a deux terminaisons différentes: par In, (face A) et par As, (face B). Les deux faces présentent des reconstructions différentes. Par la photoémission des niveaux de coeur nous avons montré une différence de réactivité chimique entre les faces A et B. La croissance de Bi sur la face A résulte en un monocristal de haute qualité pour les films à partir de 10 monocouches. Par contre, lors du dépôt de premières couches, la face B montre une croissance en îlots et un bon monocristal est obtenu seulement pour des films d'au moins de 50 monocouches.Pour la même face, A ou B, nous avons observé des différences de croissance plus subtiles entre les surfaces préparées soit par le bombardement ionique et des recuits soit par l'épitaxie par jets moléculaires.La photoémission résolue en angle a permit de caractériser la dispersion des bandes dans les films de Bi. La dispersion est tout à fait comparable au cristal massif de Bi. La dernière étape consistait à étudier la structure électronique d'un monocristal de Sb déposé sur le film de Bi.Les surfaces propres de InAs(111)A et InAs(111)B présentent une courbure de bande qui résulte en formation d'une couche d'accumulation d'électrons. En déposant le Bi sur ces surfaces, la couche d'accumulation est préservée, elle est même amplifié, car Bi agit comme le donneur dans l'InAs.La couche d'accumulation se traduit par un confinement quantique des électrons, mesurable par la photoémission résolue en angle.Mots clés :Structure électronique de surface, ARPES, semimétal, courbure de bande, Gaz-2D, Bismuth, Sb, InAs(111)A, InAs(111)B, puits quantique, surface Fermi, couches mincesA new class of material is coming up, Topological Insulators, have opened a wide field of research. Bismuth, an element of group V of periodic table, is one of the key ingredient of this Topological Insulators family. With the aim of improving technological applications, especially the electronic compounds, it is of most importance to control the preparation of thin films materials. Within this Phd work, we studied the growth and Bismuth electronic structure on (100) and (111) semiconductor III-V InAs surfaces.Bi deposition on InAs(100) surface result of a Bi self-assembly which forms lines at atomic scale. We show Bi interact extremely weakly with the surface because the beginning structure of clean InAs(100) surface stay unharmed. The study of valence band sheds light on the existence of resonant states strongly photon energy dependent and also depend on the light polarization, consistent with almost one dimensional structure surface.InAs(111) surface specific feature is that it has both surface ending different : In ending, (face A) and As ending, (face B). The both faces pointed out distinguishable reconstructions. By the core-level photoemission we identified a chemical reactivity difference taking place between A and B faces. Bi growth on A-face tend to be a high quality monocrystal for those films from a thickness of 10 monolayers. On the other hand, during the deposition of first layers, the B-face show an island growth and a good monocrystal is obtained only available for films with 50 monolayers at least.For the same face, A or B, we have seen some growth discrepancies more subtle between prepared surfaces either by ionic bombardment and annealing (IBA) either by molecular beam epitaxy (MBE).The angular resolved photoemission allowed to identify the band dispersion inside of this Bi films. The dispersion is absolutely relative to the bulk Bi crystal. The final step involved the study of Sb monocrystal electronic structure deposited onto Bi film.Clean InAs(111)A and InAs(111)B surfaces indicate a band bending which result in the accumulation electron charge formation. With depositing Bi onto these surfaces, the accumulation layer would be kept, it is also increased, given that Bi acts as a donor-like in InAs. The accumulation layer is characterized by an electron quantum confinement, measurable by angle resolved photoemission.Keywords:Electronic structure surface, ARPES, semimetal, band bending effect, 2DEG, Bismuth, Sb, InAs(111)A, InAs(111)B, quatum wells, Fermi surface, thin films
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Bareille, Cédric. "Effets d'une brisure de symétrie sur les stuctures électroniques d'URu2Si2 et de KTaO3." Phd thesis, Université Paris Sud - Paris XI, 2013. http://tel.archives-ouvertes.fr/tel-00937625.
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L'étude des symétries d'un système peut en révéler de nombreuses propriétés physiques. La brisure, spontanée ou non, d'une de ces symétries implique alors d'importantes conséquences sur le comportement du système. On le voit dans la description actuelle de la physique des particules, avec notamment la création de la masse, ou dans la physique des solides, domaine de cette thèse, avec l'apparition de phases aux propriétés diverses, comme le magnétisme ou la supraconductivité. Le présent travail étudie par spectroscopie de photoémission résolue en angle (ARPES) les effets d'une brisure de symétrie dans deux systèmes différents : le système de fermions lourds URu2Si2 et l'oxyde de métal de transition (TMO) KTaO3. Le cristal d'URu2Si2 passe d'une phase paramagnétique pour T>THO, sujette à la cohérence de Kondo, vers la phase dite d'ordre caché pour T
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Xie, Jinqiao. "Low Dislocation Density Gallium Nitride Templates and Their Device Applications." VCU Scholars Compass, 2007. http://scholarscompass.vcu.edu/etd/1286.
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The unique properties, such as large direct bandgap, excellent thermal stability, high μH × ns, of III-nitrides make them ideal candidates for both optoelectronic and high-speed electronic devices. In the past decades, great success has been achieved in commercialization of GaN based light emitting diodes (LEDs) and laser diodes (LDs). However, due to the lack of native substrates, thin films grown on sapphire or SiC substrates have high defect densities that degrade the device performance and reliability. Conventional epitaxy lateral overgrowth (ELO) can reduce dislocation densities down to ∼10-6 cm-2 in the lateral growth area, but requires ex situ photolithography steps. Hence, an in situ method using a SiNx interlayer (nano-scale ELOG) has emerged as a promising technique. The GaN templates prepared by this method exhibit a very low dislocation density (low-10-7 cm-2) and excellent optical and electrical properties. As a cost, such high quality GaN templates containing SiN, nanonetworks are not suitable for heterojunction field effect transistor (HFET) applications due to degenerate GaN:Si layer which serves as parallel conduction channel. This dissertation discusses the growth of low dislocation density GaN templates, by using the in situ SiNx nanonetwork for conductive templates, and the AIN buffer for semi-insulating templates. On SiN x nanonetwork templates, double-barrier RTD and superlattice (SL) exhibited negative differential resistances. Moreover, the injection current of Blue LEDs (450 nm) was improved ∼30%. On semi-insulating GaN templates, nearly lattice matched AlInN/AIN/GaN HFETs were successfully demonstrated and exhibited ∼ 1600 cm2/Vs and 17 600 cm2/Vs Hall mobilities at 300 K and 10 K, respectively. Those mobility values are much higher than literature reports and indicate that high quality HFETs can be realized in lattice matched AlInN/AIN/GaN, thereby solving the strain related issue. The attempt to use InGaN as the 2DEG channel has also been successfully implemented. A Hall mobility (1230 cm2/Vs) was achieved in a 12 nm InGaN channel HFET with AlInGaN barrier, which demonstrates the viability of InGaN channel HFETs.
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Reyes, Ryan. "Sorafenib and 2-Deoxyglucose: The Future of Hepatocellular Carcinoma Therapy." The Ohio State University, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=osu1461275086.
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Beveridge, Rebecca. "Mass spectrometry methods for characterising the dynamic behaviour of proteins and protein complexes." Thesis, University of Manchester, 2016. https://www.research.manchester.ac.uk/portal/en/theses/mass-spectrometry-methods-for-characterising-the-dynamic-behaviour-of-proteins-and-protein-complexes(81961313-2d3e-4ad3-9c6f-6299549e9738).html.
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Research into the relationship between the structure and function of proteins has been ongoing now for several decades. More recently, there has been an explosion in the investigation of the dynamic properties of proteins, and how their dynamic propensity relates to their function. This new direction in protein research requires new techniques to analyse protein dynamics, since most traditional techniques are biased towards a fixed tertiary structure. Mass spectrometry (MS) is emerging as a powerful tool to probe protein dynamics since it can provide information on interconverting conformations and has no preference towards the folded state. Furthermore, its low sample consumption, rapid data acquisition and low data processing positions MS as an attractive tool in protein structure research. The hybrid technique of ion mobility-mass spectrometry provides further insight into the range of conformations adopted by proteins and protein complexes, by providing information on the size in terms of rotationally averaged collision cross section. The work presented in this thesis considers proteins with a range of structural characteristics. We use ion mobility mass spectrometry to investigate proteins of different extents of disorder, protein complexes with dynamic entities and a system that undergoes structural rearrangement upon ligand binding. First, a framework of mass spectrometry experiments is described which allows identification of the extent of structure and disorder within proteins. This framework is tested on a range of different systems throughout the thesis. Differences in the gas-phase properties of two conformationally dynamic proteins which behave similarly in solution are investigated and from this research we postulate a new ionisation mechanism for partially folded proteins. The dynamic propensity of C-terminal p27 is investigated and compared to two permutants which allows us to delineate how the location of charged residues in a primary sequence affects the structure of a protein. We monitor the 'folding-upon-binding' behaviour of p27 upon association with its binding partners, and how this differs with the order of charged residues in the linear sequence. Finally, we describe the structural rearrangement of Fdc1 upon the binding of its cofactor; a prenylated FMN molecule. This thesis demonstrates the suitability of ion mobility-mass spectrometry for the investigation of dynamic properties of proteins and protein complexes.
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Rödel, Tobias. "Two-dimensional electron systems in functional oxides studied by photoemission spectroscopy." Thesis, Université Paris-Saclay (ComUE), 2016. http://www.theses.fr/2016SACLS197/document.
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De nombreux oxydes de métaux de transition (TMOs) possèdent des propriétés physiques complexes (ferroélectricité, magnétisme, supraconductivité à haute Tc ou magnétorésistance colossale). Les différents degrés de liberté (le réseau, la charge, le spin ou l'ordre orbitalaire) interagissent pour donner des phases différentes, très proches en énergie, qui vont former une grande variété d'états fondamentaux accessibles. La possibilité de fabriquer des hétérostructures de TMOs a encore accru la complexité de ces systèmes, de nouveaux phénomènes apparaissant aux interfaces. Un exemple typique est le gaz d'électrons bidimensionnel (2DEG) créé à l'interface entre deux oxydes isolants, LaAlO3 et SrTiO3, qui montre une transition métal-isolant, du magnétisme ou de la supraconductivité (contrôlée par une tension de grille). Le point de départ de cette thèse a été la découverte d'un 2DEG similaire à la surface nue de SrTiO3 fracturée sous vide, rendant possible l'étude de sa structure électronique par photoémission angulaire.Dans cette thèse, l'étude de surfaces préparées, plutôt que de petites facettes fracturées, a permis l'obtention de données spectroscopiques possédant des largeurs de raie proches des valeurs intrinsèques. Il est alors possible d'étudier les effets à N corps comme la renormalisation de la self-énergie due à l'interaction électron-phonon.Ces recherches sur la structure électronique du 2DEG à la surface de SrTiO3 ont pris un tour nouveau lorsqu'une texture de spin complexe y a été mesurée par photoémission résolue en spin. Nous présentons des résultats qui contredisent ces conclusions et nous discutons des raisons pouvant expliquer ce désaccord.Une des motivations de cette thèse était de savoir si la structure électronique et les propriétés du 2DEG pouvaient être contrôlées. L'étude du 2DEG sur des surfaces (110) et (111) de SrTiO3 révèle que sa structure de bandes (ordre orbitalaire, symétrie de la surface de Fermi, masses effectives) peut être ajustée en confinant les électrons sur des surfaces de différentes orientations du même matériau.Un succès majeure est la mise en évidence de 2DEGs à la surface de nombreux autres TMOs (TiO2-anatase, CaTiO3, BaTiO3) ou d'oxydes plus simples utilisés dans les applications (ZnO). Dans tous ces oxydes, nous avons identifié les lacunes en oxygène comme étant à l'origine de la création des 2DEGs.Dans l'anatase, ou d'autres TMOs en configuration électronique initiale d0, les lacunes en oxygène produisent à la fois des électrons localisés ou itinérants (le 2DEG). Il peut être subtile de prévoir quel est le cas est le plus favorable énergétiquement comme le démontre l'étude de deux polymorphes de TiO2, anatase et rutile. Dans CaTiO3, l’octaèdre formé par les atomes d'oxygène autour du Ti est incliné. Cette rupture de symétrie provoque un mélange des orbitales d et modifie le 2DEG. Dans BaTiO3, la création d'un 2DEG entraîne la coexistence de deux phénomènes normalement incompatibles, la ferroélectricité et la métallicité, dans deux zones spatialement distinctes du même matériau. Ce travail démontre qu'un 2DEG existe aussi à la surface de ZnO qui est, contrairement aux oxydes à base de Ti, plutôt un semiconducteur conventionnel, le caractère des orbitales pour les électrons itinérants étant alors de type s et non de type d.Le principal résultat est la mise au point d'une méthode simple et versatile pour la création de 2DEGs en évaporant de l'aluminium sur des surfaces d'oxydes. Une réaction d'oxydo-réduction entre le métal et l'oxyde permet de créer un 2DEG à l'interface entre le métal oxydé et l'oxyde réduit. Dans cette thèse, les 2DEGs ont été étudiés uniquement par photoémission sous ultra-vide. Cette méthode ouvre la possibilité d'étudier ces 2DEGs dans des conditions de pression ambiante en utilisant, par exemple, des techniques de transport, un pas important vers la production de masse et à bas coûts de 2DEGs dans les oxydes pour de futures applicationsMany transition metal oxides (TMOs) show complex physics, ranging from ferroelectricity to magnetism, high-Tc superconductivity and colossal magnetoresistance. The existence of a variety of ground states often occurs as different degrees of freedom (e.g. lattice, charge, spin, orbital) interact to form different competing phases which are quite similar in energy. The capability to epitaxially grow heterostructures of TMOs increased the complexity even more as new phenomena can emerge at the interface. One typical example is the two-dimensional electron system (2DES) at the interface of two insulating oxides, namely LaAlO3/SrTiO3, which shows metal-to-insulator transitions, magnetism or gate-tunable superconductivity. The origin of this thesis was the discovery of a similar 2DES at the bare surface of SrTiO3 fractured in vacuum, making it possible to study its electronic structure by angle-resolved photoemission spectroscopy (ARPES).In this thesis, the study of well-prepared surfaces, instead of small fractured facets, results in spectroscopic data showing line widths approaching the intrinsic value. This approach allows a detailed analysis of many-body phenomena like the renormalization of the self-energy due to electron-phonon interaction.Additionally, the understanding of the electronic structure of the 2DES at the surface of SrTiO3(001) was given an additional turn by the surprising discovery of a complex spin texture measured by spin-ARPES. In this thesis data is presented which contradicts these conclusions and discusses possible reasons for the discrepancy.One major motivation of this thesis was the question if and how the electronic structure and the properties of the 2DES can be changed or controlled. In this context, the study of 2DESs at (110) and (111) surface revealed that the electronic band structure of the 2DES (orbital ordering, symmetry of the Fermi surface, effective masses) can be tuned by confining the electrons at different surface orientations of the same material, namely SrTiO3.A major achievement of this thesis is the generalization of the existence of a 2DES in SrTiO3 to many other surfaces and interfaces of TMOs (TiO2 anatase, CaTiO3, BaTiO3) and even simpler oxides already used in modern applications (ZnO). In all these oxides, we identify oxygen vacancies as the origin for the creation of the 2DESs.In anatase and other doped d0 TMOs, both localized and itinerant electrons (2DES) can exist due to oxygen vacancies. Which of the two cases is energetically favorable depends on subtle differences as demonstrated by studying two polymorphs of the same material (anatase and rutile).In CaTiO3, the oxygen octahedron around the Ti ion is slightly tilted. This symmetry breaking results in the mixing of different d-orbitals demonstrating again why and how the electronic structure of the 2DES can be altered.In BaTiO3, the creation of a 2DES results in the coexistence of the two, usually mutual exclusive, phenomena of ferroelectricity and metallicity in the same material by spatially separating the two.Moreover, this work demonstrates that the 2DES also exists in ZnO which is - compared to the Ti-based oxides - rather a conventional semiconductor as the orbital character of the itinerant electrons is of s and not d-type.The main result of this thesis is the demonstration of a simple and versatile technique for the creation of 2DESs by evaporating Al on oxide surfaces. A redox reaction between metal and oxide results in a 2DES at the interface of the oxidized metal and the reduced oxide. In this thesis the study of such interfacial 2DESs was limited to photoemission studies in ultra high vacuum. However, this technique opens up the possibility to study 2DESs in functional oxides in ambient conditions by e.g. transport techniques, and might be an important step towards cost-efficient mass production of 2DESs in oxides for future applications
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Hosein, Riad. "An investigation of in-shoe plantar pressures and shear stresses with particular reference to diabetic peripheral neuropathy." Thesis, King's College London (University of London), 1996. https://kclpure.kcl.ac.uk/portal/en/theses/an-investigation-of-inshoe-plantar-pressures-and-shear-stresses-with-particular-reference-to-diabetic-peripheral-neuropathy(b0ebff48-2d9e-4fb7-8730-4ae42704ad0b).html.
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