Dissertations / Theses: 'Electronic transitions'

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Beaton, Sara Armstrong. "High resolution electronic spectroscopy of NcN and CaOCHâ†3 free radicals." Thesis, University of Oxford, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.364147.

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Muthaiah, Janani. "Optimal detection of stochastic state transitions in rechargeable sensor system." Thesis, Wichita State University, 2011. http://hdl.handle.net/10057/3964.

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Wireless sensors are often deployed in remote areas to monitor and detect interesting events. For long-term monitoring of these events, it is necessary for sensors to have perpetual operation. Hence, they are equipped with batteries that recharge using renewable resources. The work in this thesis considered the problem of detecting changes in the state of event process (referred to as state transitions) so that the number of redundant transmissions is reduced. The objective was to maximize the number of transitions detected and transmitted under energy constraints. Two types of transitions were considered: transition transmitted immediately and transition transmitted with a delay. Transitions transmitted immediately reap the maximum reward, while late transmissions are modeled to reap a reward that decreases exponentially with the delay in transmission. The problem was formulated as a partially observable Markov decision process(POMDP), and the optimal policy (maximizes the average reward over time) was evaluated using value iteration. An approximate solution for the optimality equation was formulated, and the applicability of the approximate solution under various state space categories was discussed. Motivated by the structure of the optimal policy, a simple near-optimal policy that is asymptotically optimal was proposed. Thesis (M.S.)--Wichita State University, College of Engineering, Dept. of Electrical Engineering and Computer Science.

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Jackson, A. "Synthesis and properties of materials for use in ferroelectric opto-electronic display devices." Thesis, University of Hull, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.232908.

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Cannon, Caleb C. "Numerical evaluation of fourth-order many-body corrections to transition amplitudes for principal transitions in alkali-metal atoms /." abstract and full text PDF (free order & download UNR users only), 2006. http://0-gateway.proquest.com.innopac.library.unr.edu/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:1440934.

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Thesis (M.S.)--University of Nevada, Reno, 2006. "December, 2006." Includes bibliographical references (leaves 45-46). Online version available on the World Wide Web. Library also has microfilm. Ann Arbor, Mich. : ProQuest Information and Learning Company, [2006]. 1 microfilm reel ; 35 mm.

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Rai, Vidisha. "An ab initio study of electronic states and transitions of tellurium monohalides." [S.l.] : [s.n.], 2002. http://deposit.ddb.de/cgi-bin/dokserv?idn=968337449.

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Piterou, Athina. "A network perspective on sociotechnical transitions : the emergence of the electronic book." Thesis, Brunel University, 2009. http://bura.brunel.ac.uk/handle/2438/4165.

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The sociotechnical system of print-on-paper for the dissemination of textual information prevails despite widespread concerns about its sustainability. On the basis of sociotechnical transitions theory the print-on-paper system is perceived as a regime. Information technology is identified as one of the generic technologies that has the potential to address the unsustainability of the incumbent regime. Its potential effects are examined through the development of the electronic book, which is defined as those IT applications providing an alternative form of textual display to printed paper. Yet, such applications have remained marginal. According to sociotechnical transitions theory the electronic book can be seen as a niche in relation to the print-on-paper regime. An alternative conceptualisation of transitions as a process of network reconfiguration is suggested. On that basis, the electronic book is depicted as a number of emergent innovation networks. Social Network Analysis methods informed by network approaches to innovation theory are applied to visualise and discuss these emergent networks. In one of the representations, the electronic book is mapped as a sociotechnical network including organisations, users and technologies. It emerges that network formation often transgresses a distinct niche-regime divide. Patterns of network interaction are explored and assessed as to whether they represent a sociotechnical transition in progress. The analysis reveals different patterns of network formation which are indicative of prospective sociotechnical trajectories where different concepts of the electronic book are emphasised. It emerges that the discussion of sustainability and the emergence of the electronic book remain largely unlinked.

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Pandey, Saurabh <1987&gt. "Photoinduced electronic transitions and leakage correlation to defects/dislocations in GaN heterostructures." Doctoral thesis, Alma Mater Studiorum - Università di Bologna, 2013. http://amsdottorato.unibo.it/5190/1/Pandey_Saurabh_Tesi.pdf.

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III-nitride materials are very promising for high speed electronics/optical applications but still suffer in performance due to problems during high quality epitaxial growth, evolution of dislocation and defects, less understanding of fundamental physics of materials/processing of devices etc. This thesis mainly focus on GaN based heterostructures to understand the metal-semiconductor interface properties, 2DE(H)G influence on electrical and optical properties, and deep level states in GaN and InAlN, InGaN materials. The detailed electrical characterizations have been employed on Schottky diodes at GaN and InAl(Ga)N/GaN heterostructures in order to understand the metal-semiconductor interface related properties in these materials. I have observed the occurrence of Schottky barrier inhomogenity, role of dislocations in terms of leakage and creating electrically active defect states within energy gap of materials. Deep level transient spectroscopy method is employed on GaN, InAlN and InGaN materials and several defect levels have been observed related to majority and minority carriers. In fact, some defects have been found common in characteristics in ternary layers and GaN layer which indicates that those defect levels are from similar origin, most probably due to Ga/N vacancy in GaN/heterostructures. The role of structural defects, roughness has been extensively understood in terms of enhancing the reverse leakage current, suppressing the mobility in InAlN/AlN/GaN based high electron mobility transistor (HEMT) structures which are identified as key issues for GaN technology. Optical spectroscopy methods have been employed to understand materials quality, sub band and defect related transitions and compared with electrical characterizations. The observation of 2DEG sub band related absorption/emission in optical spectra have been identified and proposed for first time in nitride based polar heterostructures, which is well supported with simulation results. In addition, metal-semiconductor-metal (MSM)-InAl(Ga)N/GaN based photodetector structures have been fabricated and proposed for achieving high efficient optoelectronics devices in future.

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Pandey, Saurabh <1987&gt. "Photoinduced electronic transitions and leakage correlation to defects/dislocations in GaN heterostructures." Doctoral thesis, Alma Mater Studiorum - Università di Bologna, 2013. http://amsdottorato.unibo.it/5190/.

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III-nitride materials are very promising for high speed electronics/optical applications but still suffer in performance due to problems during high quality epitaxial growth, evolution of dislocation and defects, less understanding of fundamental physics of materials/processing of devices etc. This thesis mainly focus on GaN based heterostructures to understand the metal-semiconductor interface properties, 2DE(H)G influence on electrical and optical properties, and deep level states in GaN and InAlN, InGaN materials. The detailed electrical characterizations have been employed on Schottky diodes at GaN and InAl(Ga)N/GaN heterostructures in order to understand the metal-semiconductor interface related properties in these materials. I have observed the occurrence of Schottky barrier inhomogenity, role of dislocations in terms of leakage and creating electrically active defect states within energy gap of materials. Deep level transient spectroscopy method is employed on GaN, InAlN and InGaN materials and several defect levels have been observed related to majority and minority carriers. In fact, some defects have been found common in characteristics in ternary layers and GaN layer which indicates that those defect levels are from similar origin, most probably due to Ga/N vacancy in GaN/heterostructures. The role of structural defects, roughness has been extensively understood in terms of enhancing the reverse leakage current, suppressing the mobility in InAlN/AlN/GaN based high electron mobility transistor (HEMT) structures which are identified as key issues for GaN technology. Optical spectroscopy methods have been employed to understand materials quality, sub band and defect related transitions and compared with electrical characterizations. The observation of 2DEG sub band related absorption/emission in optical spectra have been identified and proposed for first time in nitride based polar heterostructures, which is well supported with simulation results. In addition, metal-semiconductor-metal (MSM)-InAl(Ga)N/GaN based photodetector structures have been fabricated and proposed for achieving high efficient optoelectronics devices in future.

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Shi, Jingming. "Ab initio prediction of crystalline phases and their electronic properties : from ambient to extreme pressures." Thesis, Lyon, 2017. http://www.theses.fr/2017LYSE1110/document.

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Dans cette thèse nous utilisons des méthodes globaux de prédiction des structures cristallographiques combinés à des techniques de grande capacité de traitement de données afin de prédire la structure cristalline de différents systèmes et dans des conditions thermodynamiques variées. Nous avons réalisé des prédictions structurales utilisant l'analyse cristalline par optimisation par essaims particuliers (CALYPSO) combinés avec la Théorie Fonctionnel de la Densité (DFT) ce qui a permis de mettre en évidence la stabilité de plusieurs composés jusqu'à la inconnus dans le digramme de phases du système Ba-Si et dans le système N-H-O. Nous avons également réalisé une étude à haute capacité de traitement de données sur un système ternaire de composition ABX2. Nous avons utilisé la Théorie Fonctionnel de la Densité combinant calculs de prototypes structuraux à partir des prédictions structurelles avec la méthode. Dans les paragraphes suivants nous résumons le contenu de différents chapitres de cette thèse. Le premier chapitre qui constitue une brève introduction au travail de cette thèse est suivi du chapitre 2 présentant les aspects théoriques utilisés dans ce travail. D'abord il est fait une brève introduction à la Théorie Fonctionnel de la Densité. A continuation nous décrivons quelques fonctions d'échange-corrélation choisies qui constituent des approximations rendant l'utilisation de la DFT efficace. Ensuite nous présentons différents procédés de prédiction structurale, et en particulier les algorithmes d'optimisation par essaims particuliers et de « Minima Hopping » qeu nous avons utilisés dans cette thèse. Finalement il est discuté comment doit-on se prendre pour évaluer la stabilité thermodynamique des nouvelles phases identifiées. Dans le chapitre 3, nous considérons le système Ba-Si. A travers l'utilisation d'une recherche structurale non-biaisée basée sur l'algorithme d'optimisation par essaims particuliers combinée avec des calculs DFT, nous faisons une étude systématique de la stabilité des phases et de la diversité structurale du système binaire Ba-Si sous haute pression. Le diagramme de phases résultant est assez complexe avec plusieurs compositions se stabilisant et se déstabilisant en fonction de la pression. En particulier, nous avons identifié des nouvelles phases de stœchiométrie BaSi, BaSi2, BaSi3 et BaSi5 qui devraient pouvoir être synthétisées expérimentalement dans un domaine de pressions étendu. Dans le chapitre 4 est présentée notre étude du diagramme de phases du système N-H-O. S'appuyant sur une recherche structural «évolutive » de type ab initio, nous prédisons deux nouvelles phases du système ternaire N-H-O qui sont NOH4 et HNO3 à de pressions allant jusqu'à 150 GPa. La nouvelle phase de NOH4 est stable entre 71 et 150 GPa, tandis que HNO3 est stable entre 39 et 150 GPa (la pression maximum de cette étude). Ces deux nouvelles phases sont lamellaires. Nous confirmons également que la composition NOH5 perd son stabilité pour des pressions supérieures à 122 GPa se décomposant en NH3 et H2O à cette pression. Le chapitre 5 se focalise sur les électrodes transparentes de type-p à base des chalcogénures ternaires. Nous utilisons une approche à grande capacité de traitement de données basée sur la DFT pour obtenir la delafossite et d'autres phases voisines de composition ABX2. Nous trouvons 79 systèmes qui sont absents de la base de données « Materials project database », qui sont stables du point de vue thermodynamique et qui cristallisent soit dans la structure delafossite, soit dans des structures très proches. Cette caractérisation révèle une grande diversité de propriétés allant depuis les métaux ordinaires aux métaux magnétiques et permettant d'identifier quelques candidats pour des électrodes transparents de type-p. Nous présentons enfin à la fin du manuscrit nos conclusions générales et les perspectives de ce travail In this thesis we use global structural prediction methods (Particle Swarm Optimization and Minima Hopping Method) and high-throughput techniques to predict crystal structures of different systems under different conditions. We performed structural prediction by using the Crystal structure Analysis by Particle Swarm Optimization (CALYPSO) combined with Density Functional Theory (DFT) that made possible to unveil several stable compounds, so far unknown, on the phase diagrams of Ba-Si systerm and N-H-O system. Afterwards, we performed a high-throughput investigation on ternary compounds of composition ABX2, where A and B are elements of the periodic table up to Bi, and X is a chalcogen (O, S, Se, and Te) by using density functional theory and combining calculations of crystal prototypes with structural prediction (Minima Hopping Method). The following paragraphs summarize the content by chapter of this document. Chapter 1 is a short introduction of this thesis. Chapter 2 consists of the basic theory used in this thesis. Firstly, a short introduction of Density Function Theory (DFT) is presented. Then, we describe some approximate exchange- correlation functions that make DFT practical. Next, we introduce different structural prediction algorithms, especially Particle Swarm Optimization and Minima Hopping Method which we used in this thesis. Finally, we discuss the thermodynamic stablility criteria for a new a new structure. In Chapter 3, we first consider Ba–Si system. Using an unbiased structural search based on a particle-swarm optimization algorithm combined with DFT calculations, we investigate systematically the ground-state phase stability and structural diversity of Ba–Si binaries under high pressure. The phase diagram turns out to be quite intricate, with several compositions stabilizing/destabilizing as a function of pressure. In particular, we identify novel phases of BaSi, BaSi2, BaSi3, and BaSi5 that might be synthesizable experimentally over a wide range of pressures. Chapter 4 contains the investigation of the phases diagram of the N–H–O system. By using ab initio evolutionary structural search, we report the prediction of two novel phases of the N–H–O ternary system, namely NOH4 and HNO3 (nitric acid) at pressure up to 150 GPa. Our calculations show that the new C2/m phase of NOH4 is stable under a large range of pressure from 71 GPa to 150 GPa while the P21/m phase of HNO3 (nitric acid) is stable from 39 GPa to 150 GPa (the maximum pressure which we have studied). We also confirmed that the composition NOH5 (NH3H2O) becomes unstable for pressures above 122 GPa. It decomposes into NH3 and H2O at this pressure. Chapter 5 focuses on p-type transparent electrodes of ternary chalcogenides. We use a high-throughput approach based on DFT to find delafossite and related layered phases of composition ABX2, where A and B are elements of the periodic table, and X is a chalcogen (O, S, Se, and Te). From the 15 624 compounds studied in the trigonal delafossite prototype structure, 285 are within 50 meV/atom from the convex hull of stability. These compounds are further investigated using global structural prediction methods to obtain their lowest- energy crystal structure. We find 79 systems not present in the "Materials project database" that are thermodynamically stable and crystallize in the delafossite or in closely related structures. These novel phases are then characterized by calculating their band gaps and hole effective masses. This characterization unveils a large diversity of properties, ranging from normal metals, magnetic metals, and some candidate compounds for p-type transparent electrodes. At the end of the thesis, we give our general conclusion and an outlook

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Campanini, Donato. "Nanocalorimetry of electronic phase transitions in systems with unconventional superconductivity and magnetic ordering." Licentiate thesis, Stockholms universitet, Fysikum, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-116202.

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In this thesis, low temperature specific heat measurements on small (μg) single crystals of different superconducting and magnetic systems are presented. The device used in this work features a combination of high sensitivity and good accuracy over the temperature range 1-400 K and allows measurements in high magnetic fields. It consists of a stack of thin films deposited in the center of a Si3N4 membrane. A batch process for the production of up to 48 calorimeters from a 2" silicon wafer was developed in order to overcome the scarcity of devices and allow systematic investigations. With abundance of calorimeters, single crystals of three different systems were studied. Fe2P is the parent compound of a broad family of magnetocaloric materials. The first-order para- to ferromagnetic phase transition at TC = 216 K was investigated for fields H up to 2 T, applied parallel and perpendicular to the easy axis of magnetization c. Strikingly different phase contours were obtained depending on the field direction. In particular, for H perpendicular to c, two different ferromagnetic phases, with magnetization parallel and perpendicular to c are found. It was also possible to observe the superheating/supercooling states, the latent heat, and the structural change associated to the first-order transition. BaFe2(As1-xPx)2 is a member of the recently discovered iron-based high-temperature superconductors family. Crystals with three different compositions were measured to study the doping dependence of the superconducting properties in the overdoped regime (x > 0.30). The electronic specific heat at low temperatures was analyzed with a two band α model, which allows to extract the gap amplitudes and their weights. The degree of gap anisotropy was investigated from in-field measurements. Additional information on the system was obtained by a combined analysis of the condensation energy and upper critical field. URu2Si2, a heavy fermion material, was studied around and above the hidden-order temperature THO = 17.5 K. The origin of the hidden-order phase is still not understood. High-resolution specific heat data were collected to help clarify if any pseudogap state is seen to exist above THO. We found no evidence for any bulk phase transition above THO.

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Heath, Jennifer Theresa. "Electronic transitions in the bandgap of copper indium gallium diselenide polycrystalline thin films /." view abstract or download file of text, 2002. http://wwwlib.umi.com/cr/uoregon/fullcit?p3072587.

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Thesis (Ph. D.)--University of Oregon, 2002. Typescript. Includes vita and abstract. Includes bibliographical references (leaves 143-148). Also available for download via the World Wide Web; free to University of Oregon users.

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UGENTI, SIMONA. "Electronic correlations: an insight by new models for Auger and Coster-Kronig transitions." Doctoral thesis, Università degli Studi di Roma "Tor Vergata", 2009. http://hdl.handle.net/2108/858.

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Le correlazioni elettroniche hanno sempre svolto un ruolo fondamentale nella Fisica ed in particolare nel campo della materia condensata. Come esempio per quest'ultimo caso basta pensare al condiviso ma non ancora compreso ruolo chiave svolto dalle correlazioni elettroniche nella superconduttività sia da nanotubi di carbonio che da composti di CuO2. A tal fine la nota caratteristica di essere una sonda locale di correlazioni elettroniche definisce la spettroscopia Auger come lo strumento più adatto per estrarre questo genere di informazioni dagli spettri misurati. In particolare gli spettri Auger da solidi con bande non completamente occupate sono promettenti da questo punto di vista a causa del maggiore numero di gradi di libertà disponibili per la delocalizzazione delle buche, il quale garantisce che siano tutti processi profondamente radicati in meccanismi di correlazione. Questo lavoro nasce in tale contesto e il suo obiettivo è quello di pervenire ad una conoscenza più approfondita delle correlazioni elettroniche mediante nuovi modelli che si basano su un approccio di tipo Hubbard e su un modello a due step per la descrizione del decadimento Auger. Il punto di partenza è stato la modifica di modelli per solidi da bande chiuse in modo da applicarli a materiali con bande non completamente occupate. Successivamente abbiamo costruito un approccio completamente ab initio per liberare gli spettri Auger teorici calcolati da parametri determinati dall'esperimento e lo abbiamo applicato a campioni di Zn e Cu metallici, ottenendo nel primo caso un buon accordo tra teoria ed esperimento. Come altro tipo di approccio l'approssimazione di Gutzwiller dipendente dal tempo (TDGA) è stata applicata per la prima volta per modellizzare gli spettri Auger, fornendo sorprendenti risultati quando confrontati con queli della BLA. Piani di CuO2 rappresentano il sistema a cui è stata applicata tale teoria, a causa della loro banda di valenza occupata solo per metà, oltre che della loro importanza per la comprensione della superconduttività ad alta temperatura. Una teoria per studiare i satelliti a tre buche prodotti da transizioni Auger precedute da processi Coster-Kronig è stato l'ultimo modello che abbiamo ideato. Tale approccio può essere utilizzato insieme alla TDGA per modellizzare gli spettri Auger da solidi con bande aperte. Esso è in grado infatti disaccoppiare le strutture dovute ai decadimenti Coster-Kronig da quelle dei corrispondenti decadimenti Auger genitori. Electronic correlations have always played a fundamental role within Physics and in the field of Condensed Matter in particular. As an example for the latter it is enough to think about the agreed but not yet understood key role of correlations in superconductivity both from carbon nanotubes and CuO2 compounds. To this end the well known property of being a local probe for electronic correlations qualifies the Auger spectroscopy as the most suitable instrument to draw out such a kind of information from the measured spectra. In particular Auger spectra from solids with incompletely filled valence bands are promising from this point of view because of the increased number of degrees of freedom available for holes delocalization, which grants they are all deeply rooted in correlation mechanisms. The present work is set within this framework and its task is to gain a deeper view on electronic correlations by means of new models which are based on an Hubbard-like approach and a two-step model to describe the Auger decay. We started by modifying models for closed band solids so as to apply them to materials with incompletely filled valence bands. We built up then an ab initio approach to set free theoretical Auger patterns from experimentally determined parameters and applied it to metallic Zinc and Copper, gaining for the former a good agreement between theory and experiment. As another approach the Time Dependent Gutzwiller Approximation (TDGA) has then been applied to model Auger spectra for the first time, giving surprising results as compared with the ones of the Bare Ladder Approximation. CuO2 planes represent the chosen case of study because of their half filled valence band, besides their relevance for the understanding of high TC superconductivity. A theory to study three-holes satellites which are due to Coster-Kronig preceded Auger transitions is the last new approach we conceived. This approach can be used, together with the TDGA, to model Auger spectra from open bands solids. It makes in fact the user able to disentangle the structures produced by Coster-Kronig transitions from the corresponding parent diagrammatic lines.

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Hullah, Daniel Fearnley. "The electronic spectra of FeH and TeOâ†2." Thesis, University of Oxford, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.301915.

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Coles, Richard A. "Theory of the electronic states of semiconductor heterostructures." Thesis, Durham University, 1999. http://etheses.dur.ac.uk/4495/.

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This thesis is concerned with theoretical calculations of the properties of electronic bound states in semiconductor heterostructures. The complex band structure empirical pseudopotential method (EPM) is used as the foundation of the work. Spin orbit coupling and strain effects (due to lattice mismatch) are included in familiar ways, as is the transfer matrix method, allowing the study of arbitrarily configured heterostructures. These techniques are used to investigate the unusually deep InAs/AlSb conduction band well. The strong possibility of intraband transitions at electro magnetic wavelengths around 1.55µm is predicted, with corresponding enhanced momentum matrix elements and joint density of states over interband transitions. An InAs/GaSb/AlSb asymmetric well is investigated, paying particular attention to the bound states in the vicinity of the InAs/GaSb band overlap. The electron-like states are found to cross with heavy hole and anti-cross with light hole-like states, as a function of heterostructure dimension or applied electrostatic field. This is analogous to the hybridisation of states in the in-plane band structure, except that for zero in-plane wave vector there can be no appreciable hybridisation of electron and heavy hole states. A technique is described that has been developed to extract envelope functions from heterostructure wavefunctions calculated using the realistic complex band structure EPM approach. These envelope functions conform to Burt’s theory (M. G. Burt, J. Phys.: Condens. Matt. 4, 6651 (1992)) in that they are uniquely defined, continuous and smooth over all space. Comparisons with traditional effective mass envelope functions are made. The extracted envelope functions are used to demonstrate conclusively Burt's predictions (M. G. Burt, Superlatt. Mi- crostruct. 17, 335 (1995)) concerning the inadequacy of certain approximations for the calculation of interband dipole matrix elements and charge oscillation. Finally, the issue of k • p operator ordering is convincingly settled, in favour of 'ordered' over 'symmetrised' Hamiltonians, by comparison to EPM calculations, and using EPM derived k • p parameters.

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Durandurdu, Murat. "Polyamorphism in Semiconductors." Ohio University / OhioLINK, 2002. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1040060243.

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Cui, Wenwen. "Ab initio investigation of structural and electronic properties on 1D and 2D nanomaterials." Thesis, Lyon, 2017. http://www.theses.fr/2017LYSE1111/document.

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Le sujet principal de cette thèse est l'utilisation de la Théorie Fonctionnelle de la Densité dans sa variante à liaisons fortes (DFTB) pour l'étude de l'effet de la pression sur des nanotubes de carbone. Nous commençons par étudier l'effondrement radial sous l'effet de la pression de nanotubes de carbone (CNTs) individualisés, soit dans leur forme originale (vides), soit remplis avec des molécules d'eau ou de dioxyde de carbone. Nous étudions à continuation le processus d'effondrement radial de fagots de nanotubes de carbone a faible nombre de parois (double ou triple-parois) en fonction de la pression combinant modélisations et études expérimentales. Finalement nous présentons une étude sur les propriétés électroniques et magnétiques d'une monocouche de MoS2 déposée sur une surface de Ni(111) dans le cadre de laThéorie Fonctionnelle de la Densité incluant des interactions de van der Waals. Le manuscrit est structuré en 7 chapitres. Le chapitre 1 est notre introduction à cette thèse, incluant les motivations, les connaissances préalables sur nos sujets que nous intéressent ici, ainsi que notre contribution et principaux résultats. Le chapitre 2 présente les principaux éléments et définitions sur les CNTs. Nous décrivons ensuite les propriétés électroniques des CNTs et celles du graphène qui constitue un système de référence. Le chapitre 3 contient les éléments théoriques de notre étude. D'abord nous faisons une courte introduction à la Théorie Fonctionnelle de la Densité (DFT). Ensuite nous présentons deux des fonctions d'échange-corrélation les plus utilisées, suivi d'une revue sur les fonctions de van der Waals dont la DFT ordinaire ne peut rendre compte. Finalement, nous faisons une brève introduction à la méthode DFTB que nous utilisons dans nos modélisations des CNTs. Dans le chapitre 4 nous présentons nos modélisations sur l'effondrement radial sous pression hydrostatique de nanotubes de carbone contenant soit de l'eau doit du dioxyde de carbone. Nous montrons que la présence de ces molécules à l'intérieur des tubes modifie la dynamique du processus d'effondrement radial, donnant lieu soit à un support mécanique et repoussant la pression d'effondrement radial soitréduisant leur stabilité mécanique. Pour les CNTs vides, l'effondrement radial est très peu affecté par la nature du milieu transmetteur de pression, mais déterminé par le diamètre des nanotubes de carbone. Nos modélisations avec la méthode DFTB sont en excellent accord avec les modèles de milieux continues surla dépendance de la pression d'effondrement radial avec le diamètre du tube, d, mais montrent égalementune déviation de ce modèle pour les petites valeurs de d, ce qui est dû au moins en partie à la nature atomistique des nanotubes de carbone. Dans le chapitre 5, nous présentons une étude théorique de l'effondrement radial en fonction de la pression pour des nanotubes de carbone à parois simple, double et triple. Nos modélisations sont réalisées par DFTB pour des diamètres internes allant de 0.6 à 3.3 nm. Quand les parois sont séparées par la distance graphitique, nous montrons que la pression d'effondrement radial, Pc, est déterminé par le diamètre du tube interne, din, mais avec un important écart par rapport à une loi à la Lévy-Carrier, Pcdin-3. Nous proposons une expression modifiée, Pcdin3= (1- 2/din2) où  et  sont des paramètres numériques. Dans le chapitre 6 nous étudions par DFT les propriétés électroniques et magnétiques d'une monocouche de MoS2 déposée sur une surface de Ni(111). La prise en compte des interactions de van der Waals s'est avérée essentielle afin de stabiliser la monocouche de MoS2. L'interface est métallique en raisonde la présence sur le niveau de Fermi d'états d du Mo. Elle présente une barrière Schottky de 0.3 eV et une probabilité tunnel pour les électrons élevée. Enfin le dernier chapitre constitue une synthèse des derniers résultats et la présentation de quelques perspectives In this thesis we mainly use the density functional tight-binding method (DFTB) to investigate the effect of high pressure on carbon nanotubes (CNTs). We start by investigating the collapse behavior of individualized CNTs, either empty or filled with water and carbon dioxide molecules. Then we study the collapse process of bundled few-wall (double, triple wall) carbon nanotubes as the function of pressure combining theoretical and experimental studies. Afterwards, we investigate the electronic and magnetic properties of a monolayer MoS2 on the Ni(111) surface with accounting for van der Waals interactions by the density functional theory (DFT). The manuscript is structured in 7 chapters and the following paragraphs summarize the content by chapter of this document.Chapter 1 is our introduction of this thesis, including the motivation and background of our topic as well as our important findings and results. Chapter 2 introduces the main concepts and definitions of CNTs. Then we describe the electronic properties of CNTs as well graphene as a comparison. Chapter 3 consists of the theoretical framework used for our study. Firstly, a short introduction of Density Functional Theory (DFT) is presented. Next we list two mainly used exchange-correlation functions in DFT, then followed by an overview of van der Waals functions which normal DFT cannot account for. Finally, we briefly introduces the Density Functional Tight-Binding method (DFTB) which we use for our CNTs modeling simulation.In chapter 4, we present simulations of the collapse under hydrostatic pressure of carbon nanotubes containing either water or carbon dioxide. We show that the molecules inside the tube alter the dynamics of the collapse process, providing either mechanical support and increasing the collapse pressure, or reducing mechanical stability. At the same time the nanotube acts as a nanoanvil, and the confinement leads to the nanostructuring of the molecules inside the collapsed tube. In this way, depending on the pressure and on the concentration of water or carbon dioxide inside the nanotube, we observe the formation of 1D molecular chains, 2D nanoribbons, and even molecular single and multi-wall nanotubes. For the perfect empty CNTs, collapse behavior theoretically is barely affected by the PTM environment under high pressure but only mainly is determined by the CNTs diameter. Our simulation using DFTB method gives good agreement both for the d dependence predicted by continuum mechanics models and for the deviation at small d which is at least partly due to the atomistic nature of the carbon nanotubes. In chapter 5, we present a theoretical study of the collapse process of single-, double and triple-wall CNTs as a function of pressure. Our theoretical simulations were performed using DFTB for inner tube diameters ranging from 0.6 nm to 3.3 nm. When the walls are separated by the graphitic distance, we show that the radial collapse pressure, Pc, is mainly determined by the diameter of the innermost tube, din and its value significantly deviates from the usual Pcdin-3 Lévy-Carrier law. A modified expression, Pcdin3= (1- 2/din2) with  and  numerical parameters is proposed. In chapter 6 we investigate the electronic and magnetic properties of a monolayer of MoS2 deposited on a Ni(111) surface using DFT method. Accounting for van der Waals interactions is found to be essential to stabilize the chemisorbed MoS2 monolayer. The interface is metallic due to Mo d states positioned at the Fermi energy, with a Schottky barrier of 0.3 eV and a high tunneling probability for electrons. Small magnetic moments are induced on Mo and S atoms, while we measure a significant demagnetization of the Ni layer at the interface. Finally the last chapter synthesizes the main results of this work presenting also some perspectives

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Campbell, Nancy, and University of Lethbridge School of Health Sciences. "Transitions in death : the lived experience of critical care nurses." Thesis, Lethbridge, Alta. : University of Lethbridge, School of Health Sciences, 2008, 2008. http://hdl.handle.net/10133/653.

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Critical care nurses often face the ordeal of witnessing a patient's death in a tense and stressful environment. Anecdotal stories shared among nurses reveal that unusual experiences often occur at the time of or after a patient's death. This hermeneutic phenomenological study explored the meaning of these experiences for critical care nurses. Using Parse's research method, in-depth interviews were conducted with six critical care nurses who described their experiences at the time of a patient's death as well as during the post-death period. These experiences brought a sense of peace and comfort to each individual as well as reinforced their individual belief patterns about life after death. A distinctive sense of nursing knowing at the time of death was also identified. The findings of this study indicate that the experiences of the phenomenon of death by critical care nurses have a significant impact on each individual and that further research and understanding of this impact is needed. ix, 113 leaves ; 29 cm.

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List, Nanna Holmgaard. "Theoretical Description of Electronic Transitions in Large Molecular Systems in the Optical and X-Ray Regions." Doctoral thesis, Department of Physics, Chemistry and Biology, University of Southern Denmark, Denmark, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-201156.

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The size and conformational complexity of proteins and other large systems represent major challenges for today's methods of quantum chemistry.This thesis is centered around the development of new computational tools to gain molecular-level insight into electronic transitions in such systems. To meet this challenge, we focus on the polarizable embedding (PE) model, which takes advantage of the fact that many electronic transitions are localized to a smaller part of the entire system.This motivates a partitioning of the large system into two regions that are treated at different levels of theory:The smaller part directly involved in the electronic process is described using accurate quantum-chemical methods, while the effects of the rest of the system, the environment, are incorporated into the Hamiltonian of the quantum region in an effective manner. This thesis presents extensions of the PE model with theaim of expanding its range of applicability to describe electronic transitions in large molecular systemsin the optical and X-ray regions. The developments cover both improvements with regardto the quantum region as well as the embedding potential representing the environment.Regarding the former, a damped linear response formulation has been implemented to allow for calculations of absorption spectra of large molecular systems acrossthe entire frequency range. A special feature of this development is its abilityto address core excitations that are otherwise not easily accessible.Another important development presented in this thesis is the coupling of the PE model to a multi-configuration self-consistent-field description of the quantum region and its further combination with response theory. In essence, this extends the PE model to the study of electronic transitions in large systems that are prone to static correlation --- a situation that is frequently encountered in biological systems. In addition to the direct environmental effects on the electronic structure of the quantum region, another important component of the description of electronic transitions in large molecular systems is an accurate account of the indirect effects of the environment, i.e., the geometrical distortions in the quantum region imposed by the environment. In thisthesis we have taken the first step toward the inclusion of geometry distortions in the PE frameworkby formulating and implementing molecular gradients for the quantum region. To identify critical points related to the environment description, we perform a theoretical analysis of the PE model starting from a full quantum-mechanicaltreatment of a composite system. Based on this, we present strategies for an accurate yet efficient construction of the embedding potentialcovering both the calculation of ground state and transition properties. The accurate representation of the environment makes it possible to reduce the size of the quantum region without compromising the overall accuracy of the final results. This further enables use of highly accurate quantum-chemical methods despite their unfavorable scaling with the size of the system. Finally, some examples of applications will be presented to demonstrate how the PE model may be applied as a tool to gain insight into and rationalize the factors influencing electronic transitions in large molecular systems of increasing complexity. The dissertation was awarded the best PhD thesis prize 2016 by the Danish Academy of Natural Sciences.QC 20170209

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Liu, Lipeng. "Physics of Electrical Discharge Transitions in Air." Doctoral thesis, KTH, Elektroteknisk teori och konstruktion, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-205401.

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Electrical discharges with a variety of different forms (streamers, glow corona, leaders, etc.) broadly exist in nature and in industrial applications. Under certain conditions, one electrical discharge can be transformed into another form. This thesis is aimed to develop and use numerical simulation models in order to provide a better physical understanding of two of such transitions, namely the glow-to-streamer and the streamer-to-leader transitions in air. In the first part, the thesis includes the two-dimensional simulation of the glow-to-streamer transition under a fast changing background electric field. The simulation is performed with a fluid model taking into account electrons. An efficient semi-Lagrangian algorithm is proposed to solve the convection-dominated continuity equations present in the model. The condition required for the glow-to-streamer transition is evaluated and discussed. In order to enable such simulations for configurations with large interelectrode gaps and long simulation times, an efficient simplified model for glow corona discharges and their transition into streamers is also proposed. The second part of the thesis is dedicated to investigate the dynamics of the streamer-to-leader transition in long air gaps at atmospheric pressure. The transition is studied with a one-dimensional thermo-hydrodynamic model and a detailed kinetic scheme for N2/O2/H2O mixtures. In order to evaluate the effect of humidity, the kinetic scheme includes the most important reactions with the H2O molecule and its derivatives. The analysis includes the simulation of the corresponding streamer bursts, dark periods and aborted leaders that may occur prior to the inception of a stable leader. The comparison between the proposed model and the widely-used model of Gallimberti is also presented. Elektriska urladdningar av olika former (streamers (från engelska), glöd-korona, ledare, etc.) förekommer i stor utsträckning i naturen och i industriella applikationer. Under vissa förhållanden kan en elektrisk urladdning omvandlas till en annan form av elektrisk urladdning. Denna avhandling syftar till att utveckla och använda numeriska simuleringsmodeller för att ge en bättre fysikalisk förståelse av två sådana övergångar, nämligen glöd-till-streamer- och streamer-till-ledar-övergångar, i luft. I den första delen, avhandlas en tvådimensionell simulering av glöd-till-streamer-övergången med ett hastigt föränderligt elektriskt fält i bakgrunden. Simuleringen utförs med en flödesmodell som tar hänsyn till elektronerna. En effektiv semi-Lagrangesk algoritm föreslås för att lösa de konvektionsdominerade kontinuitetsekvationerna i modellen. Vidare utvärderas och diskuteras förutsättningarna för glöd-till-streamer-övergången. För att möjliggöra sådana simuleringar i konfigurationer med stora elektrodavstånd och långa simuleringstider, föreslås också en effektiv och förenklad modell för glöd-korona-urladdningar samt deras övergång till streamers. Den andra delen av avhandlingen är tillägnad att undersöka dynamiken i streamer-till-ledar-övergångar över långa avstånd i luft, under atmosfäriskt tryck. Övergången studeras med en endimensionell termohydrodynamisk modell och en detaljerad kinetisk modell för blandningar av N2/O2/H2O. För att utvärdera effekten av luftfuktighet, innefattar den kinetiska modellen de viktigaste reaktionerna med H2O-molekylen och dess derivat. Analysen innefattar simuleringen av motsvarande streamer-kedjor, mörka perioder och avbrutna ledare som kan förekomma före starten av en stabil ledare. En jämförelse mellan den föreslagna modellen och den allmänt använda modellen av Gallimberti presenteras också. QC 20170418

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Whitaker, Sabrina A. R. A. "The Possibility of Lithophile Elements in Earth's Core: The High Pressure Electronic Transitions of Rubidium and Potassium." The Ohio State University, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=osu1243539591.

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Mishra, Snigdharaj K. "Mean-field and density-functional studies of charge ordering and magnetic transitions in lanthanum manganites /." free to MU campus, to others for purchase, 1997. http://wwwlib.umi.com/cr/mo/fullcit?p9841176.

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Smith, Phillip E. "Atomic diffusion and interface electronic structure of III-V heterojunctions and their dependence on epitaxial growth transitions and annealing." Columbus, Ohio : Ohio State University, 2007. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1177957344.

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Rang, Knut. "A study of the stereochemistry and electronic transitions of cyclic and acyclic thioamdes by crystallography, spectroscopy and computational methods." Lund : Lund University, 1998. http://catalog.hathitrust.org/api/volumes/oclc/40265658.html.

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Rouzhaji, Tuerhong. "Effect of environment on the electronic and magnetic properties of transition metals and rare-earth complexes." Thesis, Strasbourg, 2017. http://www.theses.fr/2017STRAE006/document.

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Cette thèse présente les résultats de mesures expérimentales effectués à basse température par les techniques de microscopie tunnel à balayage et de spectroscopie par tunnel à balayage (STS) sur les métaux de transitions phthalocyanines déposées sur les surfaces de métaux nobles. Les mesures STM/STS ont été effectuées pour les molécules MnPc et CuPc adsorbées sur les surfaces Ag (111) et Au (111) à la température expérimentale de travail de 4,5 K. Ces deux types de molécules présentent une différence substantielle de configurations d'adsorption, des comportements électroniques et magnétiques et des structures vibratoires moléculaire. Les études STM/STS ont principalement porté sur les propriétés magnétiques de ces molécules à travers l’effet Kondo et une attention particulière a été accordée à la molécule de MnPc en raison de son comportement magnétique plus intéressant issu de l'atome Mn central. Particulièrement, nous avons étudié l'évolution spectrale des structures électroniques et magnétiques du MnPc partant d'une molécule unique jusqu'à la structure bicouche ordonnée sur la surface Ag (111). En outre, les études STM/STS ont montré une preuve de couplage magnétique entre les moments magnétiques de l'atome de Co et de la molécule de MnPc ainsi que sa forte dépendance vis-à-vis du site d'adsorption de l'atome de Co. Ces études STM/STS sur ce système nous ont permis de comprendre l'effet des interactions molécule-substrat, molécule-molécule et molécules-atome sur les propriétés électroniques et magnétiques des molécules de MnPc This thesis presents the results of low-temperature scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) studies on transition-metal phthalocyanines molecules on the noble metal surfaces. The STM/STS measurements have been performed for MnPc and CuPc molecules adsorbed on Ag(111) and Au(111) surfaces at the experimental working temperature of 4.5 K. These two types of molecules exhibit substantially different adsorption configurations, the electronic and magnetic behaviors and the molecule vibrational structures. The STM/STS studies have focused mainly on the magnetic properties of these molecules by means of Kondo effect, and special attention has been paid to MnPc molecule due to its more interesting magnetic behavior arising from the central Mn atom. Particularly we investigated the spectral evolution of electronic and magnetic structures of MnPc starting from a single molecule up to the ordered bilayer structures on Ag(111) surface. In addition, the STM/STS investigations showed an evidence of magnetic coupling between the magnetic moments of the Co atom and MnPc molecule and its strong dependence on the adsorption site of Co atom. These STM/STS investigations on this system allowed us to understand the effect of molecule-substrate, molecule-molecule and molecule-atom interactions on the electronic and magnetic properties of MnPc molecules

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Woollacott, Claire. "Electronic and plasmonic properties of real and artificial Dirac materials." Thesis, University of Exeter, 2015. http://hdl.handle.net/10871/18227.

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Inspired by graphene, I investigate the properties of several different real and artificial Dirac materials. Firstly, I consider a two-dimensional honeycomb lattice of metallic nanoparticles, each supporting localised surface plasmons, and study the quantum properties of the collective plasmons resulting from the near field dipolar interaction between the nanoparticles. I analytically investigate the dispersion, the effective Hamiltonian and the eigenstates of the collective plasmons for an arbitrary orientation of the individual dipole moments. When the polarisation points close to normal to the plane the spectrum presents Dirac cones, similar to those present in the electronic band structure of graphene. I derive the effective Dirac Hamiltonian for the collective plasmons and show that the corresponding spinor eigenstates represent chiral Dirac-like massless bosonic excitations that present similar effects to those of electrons in graphene, such as a non-trivial Berry phase and the absence of backscattering from smooth inhomogeneities. I further discuss how one can manipulate the Dirac points in the Brillouin zone and open a gap in the collective plasmon dispersion by modifying the polarisation of the localized surface plasmons, paving the way for a fully tunable plasmonic analogue of graphene. I present a phase diagram of gapless and gapped phases in the collective plasmon dispersion depending on the dipole orientation. When the inversion symmetry of the honeycomb structure is broken, the collective plasmons become gapped chiral Dirac modes with an energy-dependent Berry phase. I show that this concept can be generalised to describe many real and artificial graphene-like systems, labeling them Dirac materials with a linear gapped spectrum. I also show that biased bilayer graphene is another Dirac material with an energy dependent Berry phase, but with a parabolic gapped spectrum. I analyse the relativistic phenomenon of Klein Tunneling in both types of system. The Klein paradox is one of the most counter-intuitive results from quantum electrodynamics but it has been seen experimentally to occur in both monolayer and bilayer graphene, due to the chiral nature of the Dirac quasiparticles in these materials. The non-trivial Berry phase of pi in monolayer graphene leads to remarkable effects in transmission through potential barriers, whereas there is always zero transmission at normal incidence in unbiased bilayer graphene in the npn regime. These, and many other 2D materials have attracted attention due to their possible usefulness for the next generation of nano-electronic devices, but some of their Klein tunneling results may be a hindrance to this application. I will highlight how breaking the inversion symmetry of the system allows for results that are not possible in these system's inversion symmetrical counterparts.

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Crawford, H., and Jean Croce Hemphill. "Bridging the Gap in Care Transitions by Implementing an Electronic Homeless Resource Toolkit for Case Management Personnel: Hospital to Community." Digital Commons @ East Tennessee State University, 2019. https://dc.etsu.edu/etsu-works/7574.

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Jha, Thakur Amitayush. "Relationship between the electronic structure and the functional properties of new quantum materials." Electronic Thesis or Diss., université Paris-Saclay, 2024. http://www.theses.fr/2024UPASP136.

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Cette thèse explore des matériaux novateurs dont les propriétés sont intrinsèquement liées à leur structure électronique. Nous nous concentrons sur l'étude expérimentale de trois matériaux. Premièrement, le CuRhO₂ dopé au Mg, qui présente une valeur élevée du coefficient thermoélectrique en raison de sa structure électronique unique. Deuxièmement, des films minces du métal corrélé CaVO₃, qui subissent une transition métal-isolant (MIT) en dessous d'une épaisseur critique. Enfin, une hétérostructure de dihalogénure de métal de transition magnétique 2D, FeCl₂, déposée sur une surface Au(111), avec des états électroniques uniques près du niveau de Fermi. Le CuRhO₂ est un matériau isolant avec une valeur élevée du coefficient de thermopouvoir. Le dopage en trous du matériau avec du Mg le rend conducteur tout en conservant un thermopouvoir relativement élevé. L'origine de ce thermopouvoir élevé a été théoriquement attribuée à la structure de bande en « moule à pudding » du matériau. Grâce à l'ARPES, nous avons vérifié expérimentalement que les bandes observées dans le CuRhO₂ dopé à 10 % de Mg correspondent aux bandes calculées par DFT et sont de type « moule à pudding ». Nous avons ensuite calculé le coefficient de Seebeck à partir des bandes DFT du matériau dopé (S ∼ 260 µV/K), qui s'avère en accord avec les valeurs expérimentales. Les films minces de CaVO₃ ont été observés comme subissant une MIT en fonction de l'épaisseur du film. Des films minces de CaVO₃ avec des épaisseurs de 90 u.c., 45 u.c., 30 u.c., 20 u.c. et 15 u.c. sur un substrat de SrTiO₃ ont été mesurés par ARPES, et les changements dans la structure électronique ont été suivis. Les changements observés dans la surface de Fermi indiquent une augmentation de la contrainte de traction sur le film à mesure que l'épaisseur diminue. Une réduction de la largeur de bande de conduction et une augmentation des états V³⁺ dans les films plus minces ont été observées, indiquant la localisation des électrons près du niveau de Fermi. Une MIT a été observée lors de la transition d'un film métallique de 20 u.c. à un film isolant de 15 u.c., avec la disparition de la bande de conduction et l'apparition d'une bande plate isolante à ∼ -0,9 eV. Nous concluons que la contrainte épitaxiale accrue, et non la dimensionnalité, est à l'origine de la MIT, en accord avec les études précédentes. Le dihalogénure de métal de transition magnétique 2D FeCl₂ sur Au(111) est observé comme hébergeant des états nouveaux au niveau de Fermi. Une monocouche (ML) de FeCl₂ déposée sur une surface Au(111) a été caractérisée par STM. La ML de FeCl₂ présente une modulation pseudo-périodique sur sa surface, avec une périodicité moyenne de ∼ 2,3 nm, ce qui donne lieu à des points lumineux et sombres dans la densité locale d'états (LDOS) observés à un biais d'échantillon de 1,5 eV. La LDOS a été mesurée à ces sites par spectroscopie tunnel à balayage (STS) et s'est révélée différente aux emplacements lumineux et sombres. Le STS aux points lumineux a révélé la présence de plusieurs états près du niveau de Fermi. Des mesures d'interférence quasi-particulaire (QPI) ont révélé la présence de deux bandes dispersantes avec une masse effective m* similaire à celle des états de surface (SS) d'Au(111). Les deux bandes sont observées comme étant séparées par ∆k ∼ 0,36 nm⁻¹ et ∆E ∼ 0,3 eV. L'origine de ces bandes est hypothétiquement attribuée soit à un dédoublement Zeeman des bandes Rashba de l'Au(111) en raison de la proximité magnétique avec FeCl₂, soit à des états quantifiés dans le puits à l'interface FeCl₂-Au(111) This thesis explores novel materials whose properties are inherently tied to their electronic structure. We focus on the experimental study of three materials. First, Mg-doped CuRhO₂, which displays a high value of the thermoelectric coefficient owing to its unique electronic structure. Second, thin films of the correlated metal CaVO₃, which undergo a metal-to-insulator transition (MIT) below a critical film thickness. Lastly, a heterostructure of 2D magnetic transition metal dihalide FeCl₂ on top of an Au(111) surface, with unique electronic states near the Fermi level. CuRhO₂ is an insulating material with a high value of the thermopower coefficient. Hole doping the material with Mg makes it conductive while still retaining a relatively high thermopower. The origin of the high thermopower has been theoretically proposed as originating from the "pudding-mold" band structure of the material. Through ARPES, we experimentally verify that the observed bands in the 10% Mg-doped CuRhO₂ match the DFT-calculated bands and are of the "pudding-mold" type. We then calculate the Seebeck coefficient from the DFT bands of the doped material (S ∼ 260 μV/K), which is found to be in agreement with the experimental values. Thin films of CaVO₃ are observed to undergo an MIT as a function of film thickness. Thin films of CaVO₃ with thicknesses of 90 u.c., 45 u.c., 30 u.c., 20 u.c., and 15 u.c. on an SrTiO₃ substrate were measured by ARPES, and changes in the electronic structure were tracked. The observed changes in the Fermi surface point towards increased tensile strain on the film as the thickness is decreased. A decrease in conduction band bandwidth and an increase in V³⁺ states in thinner films were observed, indicating the localization of electrons near the Fermi level. An MIT was observed when transitioning from a metallic 20 u.c. film to an insulating 15 u.c. film, with the disappearance of the conduction band and the appearance of an insulating flat band at ∼ -0.9 eV. We conclude that the increased epitaxial strain, rather than dimensionality, is the driver of the MIT, in agreement with previous studies. A 2D magnetic transition metal halide FeCl₂ on Au(111) is observed to host novel states at the Fermi level. A monolayer (ML) of FeCl₂ deposited on an Au(111) surface was characterized by STM. The FeCl₂ ML hosts pseudo-periodic modulation on its surface, with an average periodicity of ∼ 2.3 nm, which gives rise to bright and dark spots in the local density of states (LDOS) observed at 1.5 eV sample bias. The LDOS was measured at these sites through scanning tunneling spectroscopy (STS) and was found to differ at the bright and dark locations. STS at bright spots revealed the presence of multiple states near the Fermi level. Quasi-particle interference (QPI) measurements revealed the presence of two dispersing bands with effective mass m* similar to Au(111) surface states (SS). The two bands are observed to be split by Δk ∼ 0.36 nm⁻¹ and ΔE ∼ 0.3 eV. The origin of these bands is hypothesized to be either Zeeman splitting of Rashba bands of Au(111) due to magnetic proximity with FeCl₂ or from quantized well states at the FeCl₂-Au(111) interface

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Kaneko, Ulisses Ferreira 1984. "Phase transitions and electronic fluctuations in iron-based pnictides = Transições de fase e flutuações eletrônicas em pnictídeos à base de ferro." [s.n.], 2017. http://repositorio.unicamp.br/jspui/handle/REPOSIP/322478.

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Orientador: Eduardo Granado Monteiro da Silva Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin Made available in DSpace on 2018-09-01T17:31:34Z (GMT). No. of bitstreams: 1 Kaneko_UlissesFerreira_D.pdf: 51229533 bytes, checksum: 3d39a6722771458fa1812d75c31661b4 (MD5) Previous issue date: 2017 Resumo: Nesta tese, empregamos espectroscopia Raman para realizar um estudo detalhado dos fônons e das respostas eletrônicas em monocristais de LaFeAsO. Nós observamos as simetrias e frequências dos fônons nas fases tetragonal e ortorrômbica e também a dependência com a temperatura de um pico quase-elástico (do inglês: QEP) na simetria B1g com intensidade máxima em torno da transição magnética TN . A área e a altura do QEP B1g foram atribuídas às flutuações nemáticas de spin, enquanto que a largura do QEP B1g foi relacionada com taxa de relaxação dessas flutuações. Através da análise da largura do QEP B1g propomos que a transição estrutural esté relacionada a um congelamento gradual das flutuaçõees nemáticas de spin, o que deve ser um fenômeno geral presente nos demais supercondutores à base de Fe. Esse estudo foi complementado por medidas de espectroscopia Raman em BaFe2As2 e de difração de raios-X com luz síncrotron em LaFeAsO, BaFe2As2 e SrFe2As2 dopado com Co. Paralelamente, nós estudamos as estruturas cristalinas e magnéticas em compostos da série Ba1?xLaxTi1/2Mn1/2O3 através da técnica de difração de nêutrons Abstract: In this thesis, we employed Raman spectroscopy to perform a detailed study of the phonons and electronic responses in single crystals of the LaFeAsO. We observed the symmetries and frequencies of the phonons in the tetragonal and orthorhombic phases and also the temperature dependence of the quasi-elastic peak (QEP) in the B 1g symmetry with maximum intensity around the magnetic transition T N . The B 1g QEP area and height were ascribed to spin nematic fluctuations, while the B 1g QEP width was related to the relaxation rate of these fluctuations. From the B 1g QEP width analysis we propose that the structural transition is related to a gradual freezing of the spin nematic fluctuations, which may be a general phenomenon present in other Fe-based superconductors. This study was complemented by measurements of Raman spectroscopy in BaFe2As2 and synchrotron X-ray diffraction in LaFeAsO, BaFe2As2 and Co-doped SrFe2As2 . In parallel, we studied the crystal and magnetic structures in compounds of the series Ba1?xLaxTi1/2Mn1/2O3 through the neutron diffraction technique Doutorado Física Doutor em Ciências 140978/2012-7 CNPQ

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Weber, Frank. "Probing electronic phase transitions with phonons via inelastic neutron scattering : superconductivity in borocarbides, charge and magnetic order in manganites$nElektronische Ressource /." [S.l. : s.n.], 2007. http://digbib.ubka.uni-karlsruhe.de/volltexte/1000007516.

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Chandrasekhar, Janani. "Signal to power coupling and noise induced jitter in differential signaling." Thesis, Atlanta, Ga. : Georgia Institute of Technology, 2008. http://hdl.handle.net/1853/24778.

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Thesis (M. S.)--Electrical and Computer Engineering, Georgia Institute of Technology, 2008. Committee Chair: Swaminathan Madhavan; Committee Member: Chatterjee Abhijit; Committee Member: Davis Jeffrey.

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Pentlehner, Dominik [Verfasser], and Alkwin [Akademischer Betreuer] Slenczka. "Perturbations of electronic transitions of organic molecules in helium droplets generated with a new pulsed droplet source / Dominik Pentlehner. Betreuer: Alkwin Slenczka." Regensburg : Universitätsbibliothek Regensburg, 2010. http://d-nb.info/1022872168/34.

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Kasic, Alexander [Verfasser]. "Phonons, free-carrier properties, and electronic interband transitions of binary, ternary, and quaternary group-III nitride layers measured by spectroscopic ellipsometry / Alexander Kasic." Aachen : Shaker, 2003. http://d-nb.info/117054195X/34.

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Gourgout, Adrien. "Instabilités de surface de Fermi avec et sans transitions magnétiques : étude de URhGe, UPd2AI3, UCoGe et CeIrIn5." Thesis, Université Grenoble Alpes (ComUE), 2017. http://www.theses.fr/2017GREAY001/document.

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Dans cette thèse, j'ai étudié l'évolution de la surface de Fermi sous l'influence d'un champ magnétique dans des systèmes massifs facilement polarisables à basse température. La première partie est dévouée aux cas du supraconducteur ferromagnétique UCoGe et du supraconducteur paramagnétique CeIrIn5, où la surface de Fermi peut être modifiée sans transition magnétique. Dans UCoGe, plusieurs anomalies successives ont été détectées dans l'effet Seebeck, la résistivité et l'effet Hall, sans transition nette dans l'aimantation. L'observation d'oscillations quantiques montre que ces anomalies sont reliées à des changements de topologie de la surface de Fermi, aussi appelés transitions de Lifshitz. Dans CeIrIn5, une anomalie est détectée dans l'effet Seebeck à HM = 28 T et les oscillations quantiques observées en magnétométrie torque montrent qu'une transition de Lifshitz à lieu à ce champ.Dans la deuxième partie, j'ai étudié comment varie la surface de Fermi à travers une transition magnétique du premier ordre induite par le champ magnétique dans le supraconducteur ferromagnétique URhGe avec le champ selon l'axe de difficile aimantation b et le supraconducteur antiferromagnétique UPd2Al3 avec le champ dans le plan basal. Dans URhGe, l'effet Seebeck permet d'observer un changement de la surface de Fermi à la transition de réorientation des spins à HR = 11.75 T et avec la résistivité confirme le caractère premier ordre de la transition en plus de fournir la localisation dans le diagramme de phase du point tricritique. Dans UPd2Al3, une nouvelle branche de la surface de Fermi est observée dans les oscillations quantiques de de Haas-van Alphen dans l'état antiferromagnétique et l'effet Seebeck montre que la surface de Fermi change à la transition métamagnétique à HM = 18 T. En outre, quatre nouvelles branches sont observées dans la phase polarisée au delà de HM et qui ne peuvent être associées à celles calculées dans les états paramagnétique et antiferromagnétique In this thesis, we have studied the evolution of the Fermi surface under the influence of a magnetic field in bulk materials that can be easily polarized at low temperature. The first part was devoted to the cases of the ferromagnetic superconductor UCoGe with a magnetic field applied along the easy magnetization c-axis and the paramagnetic superconductor CeIrIn5 with the field along the c-axis. In UCoGe, several successive anomalies were detected in resistivity, Hall effect and thermoelectric power, without any thermodynamic transition being detected in magnetization. The direct observation of quantum oscillations showed that these anomalies are related to topological changes of the Fermi surface, also known as Lifshitz transitions. In CeIrIn5, the thermoelectric power detected an anomaly at HM = 28 T and the quantum oscillations observed in torque magnetometry showed that a Lifshitz transition occurs at this field.In the second part of this thesis, we studied the evolution of the Fermi surface through first order magnetic transitions induced by magnetic field. In the ferromagnetic superconductor URhGe with the field applied along the hard magnetization b-axis and the antiferromagnetic superconductor UPd2Al3 with the field in the basal plane. In URhGe, the thermoelectric power allowed to observe a change in the Fermi surface at the spin reorientation transition at HR = 11.75 T defining the ferromagnetic state and along with resistivity confirmed the first order character of the transition as well as give a location of the tricritical point. In UPd2Al3, a new branch was observed in de Haas-van Alphen experiment in the antiferromagnetic phase and the thermoelectric power showed that the Fermi surface is reconstructed at the metamagnetic transition at HM = 18 T where the antiferromagnetic state is suppressed and could suggest that the Fermi surface changes before this transition. Additionally, four new branches were observed in the polarized paramagnetic phase, above HM, that cannot be associated with calculated branches in the paramagnetic of antiferromagnetic states

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Saroka, Vasil. "Theory of optical and THz transitions in carbon nanotubes, graphene nanoribbons and flat nanoclusters." Thesis, University of Exeter, 2017. http://hdl.handle.net/10871/28863.

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This thesis is devoted to the optical properties of low-dimensional structures based on such two-dimensional materials as graphene, silicene and phosphorene. We investigate optical properties of a variety of quasi-one dimensional and quasi-zero-dimensional structures, which are promising for future optoelectronics. Primarily we focus on their low-energy optical properties and how these properties are influenced by the structures’ geometry, external fields, intrinsic strain and edge disorder. As a consequence of this endeavor, we find several interesting effects such as correlation between the optical properties of tubes and ribbons whose periodic and ‘hard wall’ boundary conditions are matched and a universal value of matrix element in narrow-gap tubes and ribbons characterizing probability of transitions across the band gap opened up by intrinsic strain originating from the tube’s surface curvature or ribbon’s edge relaxation. The analytical study of the gapped 2D Dirac materials such as silicene and germanene, which have some similarity to the aforementioned quasi-one-dimensional systems in terms of physical description, reveals a valley- and polarization-dependent selection rules. It was also found that absorption coefficient should change in gapped materials with increasing frequency and become a half of its value for gap edge transitions when the spectrum is linear. Our analysis of the electronic properties of flat clusters of silicene and phosphorene relates the emergence and the number of the peculiar edge states localized at zero energy, so-called zero-energy states, which are know to be of topological origin, to the cluster’s structural characteristics such as shape and size. This allows to predict the presence and the number of such states avoiding complicated topological arguments and provides a recipes for design of metallic and dielectric clusters. We show that zero-energy states are optically active and can be efficiently manipulated by external electric field. However, the edge disorder is important to take into account. We present a new fractal-based methodology to study the effects of the edge disorder which can be applied also to modeling of composite materials. These finding should be useful in design of optoelectronic devices such as tunable emitters and detectors in a wide region of electromagnetic spectrum ranging form the mid-infrared and THz to the optical frequencies.

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Abushammala, Haneen. "Tuning the multiorbital Mott transition of BaCoS2." Electronic Thesis or Diss., Sorbonne université, 2021. http://www.theses.fr/2021SORUS100.

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Dans cette thèse nous ajustons la phase d’isolant de Mott du BaCoS2, un modèle multiorbital pour l’étude des corrélations électroniques. Nous avons développer une nouvelle technique de self-flux afin de produire des cristaux de qualité et de taille supérieures. Puis nous avons confirmé une phase stable orthorhombique Cmma à l’ambiante où la distorsion orthorhombique est concomitante à l’établissement d’un ordre AFM de longue portée. Ensuite, nous avons exploré la possibilité d’induire un état métallique dans du BaCoS2 via pression chimique et dopage électronique comme paramètres de contrôle. Pour cela, nous avons appliqué des techniques HP pour synthétiser des échantillons de Ba1−xSrxCoS2 et Ba1−xKxCoS2 avec x allant jusqu’à 0,07. On s’attend à ce que la substitution partielle du Ba par du Sr ou du K induise de la pression chimique ou du dopage de trous respectivement, favorisant ainsi une phase métallique dans les deux cas In this thesis we tune the Mott insulating phase of BaCoS2, a playground for the study of electronic correlations in a multiorbital system. We first developed a novel self-flux technique to grow single crystals of improved quality and size. We then confirmed a stable orthorhombic Cmma phase at ambient conditions where the orthorhombic distortion is concomitant to a long-range AFM order, suggesting a scenario of orbital ordering that stabilises this order. Next, we explored the possibility of inducing a metallic state in BaCoS2 using chemical pressure and electronic doping as control parameters. For this goal, we successfully applied HP technique to the synthesis of Ba1−xSrxCoS2 and Ba1−xKxCoS2 samples with x up to 0.07. The partial substitution of Ba for Sr or K is expected to produce chemical pressure or hole-doping, respectively, thus favouring a metallic phase in both cases. We find that both substitutions induce similar structural changes, and a full suppression of the orthorhombic distortion and of the AFM order. However, no indication of metallic properties is found in the Ba1−xSrxCoS2 samples. However, a sizeable value of Sommerfeld coefficient,γ= 5.7mJ mol−1K−2 is found at 7% of K-substitution suggesting a metallic state induced by hole doping

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Rajagopal, Subramanian. "Transition of low-volume complex electronic system industries to lead-free electronics." College Park, Md. : University of Maryland, 2003. http://hdl.handle.net/1903/116.

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Thesis (M.S.) -- University of Maryland, College Park, 2003 Thesis research directed by: Dept. of Mechanical Engineering. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

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Shcheblanov, Nikita. "Numerical study of femtosecond laser interactions with dielectric materials : application to the definition of damage threshold of optical components." Thesis, Saint-Etienne, 2013. http://www.theses.fr/2013STET4029/document.

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Avec l'apparition de nouveaux systèmes laser ultra-courts, des intensités laser extrêmement élevées sont devenues accessibles, permettant ainsi un traitement au laser de pratiquement tous les matériaux. En conséquence, les techniques de traitement extrêmement précises sont en cours de développement étendant considérablement le nombre des applications industrielles et médicales correspondantes. Des progrès dans ce domaine nécessitent une meilleure compréhension des processus fondamentaux impliqués dans les interactions laser. De plus le succès à l’international du développement et de l’utilisation de systèmes laser de forte puissance, dépend de la capacité de la définition minutieuse du seuil d’endommagement de leurs composants optiques. Ces points illustrent l'importance d'une modélisation numérique détaillée des interactions de laser avec des matériaux diélectriques. Sous irradiation laser, des électrons germes apparaissent dans la bande de conduction des matériaux diélectriques en raison de processus de photo-ionisation. En collision avec un troisième corps, ces électrons sont encore chauffés dans le domaine du laser. Lorsque l'énergie des électrons de seuil est atteinte, l'ionisation par impact d'électrons commence. Dans le même temps, les impulsions laser considérées sont si courtes que le sous-système électronique n'a pas le temps d'atteindre un état d'équilibre. Les propriétés optiques résultant sont affectées et la définition du critère de dommages devrait être révisée. Cela représente l'approche proposée pour le non-équilibre et fournit une description détaillée de tous les processus impliqués. En particulier, on considère le processus et l’impact de photo- ionisation, ainsi que électron-électron, électron-phonon et les collisions électron-ion. La distribution d'énergie des électrons et le chauffage de sous-systèmes électroniques et phonons est discutée. Le rôle des paramètres du laser (longueur d'onde, durée d'impulsion, fluence) et les propriétés des matériaux (de l'écart de l'énergie, de la structure de bande) sont étudiées. Le temps de thermalisation est calculé et caractérise l'état de non-équilibre en fonction de la durée d'impulsion du laser. Un nouveau critère thermique est proposé pour la définition des dommages sur la base des énergies d'électrons et phonons. Les seuils d’endommagement calculés sont comparés aux résultats expérimentaux récents. Une analyse d'autres critères (claquage optique classique et thermique) est également effectuée With the appearance of new ultra-short laser systems, extremely high laser intensities became accessible thus allowing laser treatment of practically all materials. As a result, extremely precise processing techniques are under development considerably extending the number of the corresponding industrial and medical applications. Further progress in this field requires a better understanding of fundamental processes involved in the laser interactions. In addition, the success of several national and international involving the development and use of high power laser systems depends on the capacity of careful definition of damage threshold of their optical components. These points illustrate the importance of a detailed numerical modeling of laser interactions with dielectric materials. Under laser irradiation, seed electrons appear in the conduction band of dielectric materials due to photo-ionization process. Colliding with a third-body, these electrons are further heated in laser field. When the threshold electron energy is reached, electron-impact ionization begins. At the same time, the considered laser pulses are so short that electron sub-system has no time to reach an equilibrium state. The resulting optical properties are affected and the definition of the damage criterion should be revised. The proposed approach accounts for the non-equilibrium and provides a detailed description of all the involved processes. In particular, we consider the photo- and impact-ionization processes, as well as electron-electron, electron-phonon and electron-ion collisions. The electron energy distribution and heating of electronic and phonon subsystems is discussed. The role of laser parameters (wavelength, pulse duration, fluence) and material properties (energy gap, band structure) is investigated. The thermalization time is calculated and characterizes the non-equilibrium state as a function of laser pulse duration. A novel thermal criterion is proposed for damage definition based on the electron and phonon energies. The calculated damage thresholds are compared with recent experimental findings. An analysis of other criteria (classical optical breakdown and thermal) is also performed

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Cox, Simon G. "Hyperfine and Zeeman measurements in the infrared spectrum of doubly charged molecule D'3'5 C1'2'+." Thesis, University of Newcastle Upon Tyne, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.366641.

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Kemp, Jeremy. "Electron spectroscopy and electronic structure of first row transition metal oxides." Thesis, University of Oxford, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.257736.

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Arapan, Sergiu. "Understanding Physical Reality via Virtual Experiments." Doctoral thesis, Uppsala universitet, Institutionen för fysik och materialvetenskap, 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-9314.

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In this thesis I have studied some problems of condensed matter at high pressures and temperatures by means of numerical simulations based on Density Functional Theory (DFT). The stability of MgCO3 and CaCO3 carbonates at the Earth's mantle conditions may play an important role in the global carbon cycle through the subduction of the oceanic crust. By performing ab initio electronic structure calculations, we observed a new high-pressure phase transition within the Pmcn structure of CaCO3. This transformation is characterized by the change of the sp-hybridization state of carbon atom and indicates a change to a new crystal-chemical regime. By performing ab initio Molecular Dynamics simulations we show the new phase to be stable at 250 GPa and 1000K. Thus, the formation of sp3 hybridized bonds in carbonates can explain the stability of MaCO3 and CaCO3 at pressures corresponding to the Earth's lower mantle conditions. We have also calculated phase transition sequence in CaCO3, SrCO3 and BaCO3, and have found that, despite the fact that these carbonates are isostructural and undergo the same type of aragonite to post-aragonite transition, their phase transformation sequences are different at high pressures. The continuous improvement of the high-pressure technique led to the discovery of new composite structures at high pressures and complex phases of many elements in the periodic table have been determined as composite host-guest incommensurate structures. We propose a procedure to accurately describe the structural parameters of an incommensurate phase using ab initio methods by approximating it with a set of analogous commensurate supercells and exploiting the fact that the total energy of the system is a function of structural parameters. By applying this method to the Sc-II phase, we have determined the incommensurate ratio, lattice parameters and Wyckoff positions of Sc-II in excellent agreement with the available experimental data. Moreover, we predict the occurrence of an incommensurate high-pressure phase in Ca from first-principle calculations within this approach. The implementation of DFT in modern electronic structure calculation methods proved to be very successful in predicting the physical properties of a solid at low temperature. One can rigorously describe the thermodynamics of a crystal via the collective excitation of the ionic lattice, and the ab initio calculations give an accurate phonon spectra in the quasi-harmonic approximation. Recently an elegant method to calculate phonon spectra at finite temperature in a self-consistent way by using first principles methods has been developed. Within the framework of self-consistent ab initio lattice dynamics approach (SCAILD) it is possible to reproduce the observed stable phonon spectra of high-temperature bcc phase of Ti, Zr and Hf with a good accuracy. We show that this method gives also a good description of the thermodynamics of hcp and bcc phases of Ti, Zr and Hf at high temperatures, and we provide a procedure for the correct estimation of the hcp to bcc phase transition temperature.

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Haupt, Kerstin Anna. "Phase transitions in transition metal dichalcogenides studied by femtosecond electron diffraction." Thesis, Stellenbosch : Stellenbosch University, 2013. http://hdl.handle.net/10019.1/85608.

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Thesis (PhD)--Stellenbosch University, 2013. ENGLISH ABSTRACT: Low-dimensional materials are known to undergo phase transitions to differently or- dered states, when cooled to lower temperatures. These phases often show a periodic modulation of the charge density (called a charge density wave – CDW) coupled with a periodic perturbation of the crystal lattice (called a periodic lattice distortion – PLD). Although many experiments have been performed and much has been learnt about CDW phases in low-dimensional materials, the reasons for their existence are still not fully understood yet. Many processes, involving either strong electron–electron or electron–lattice coupling, have been observed which all might play a role in explaining the formation of different phases under different conditions. With the availability of femtosecond lasers it has become possible to study materials under highly nonequilibrium conditions. By suddenly introducing a known amount of energy into the system, the equilibrium state is disturbed and the subsequent relax- ation processes are then observed on timescales of structural and electronic responses. These experiments can deliver valuable information about the complex interactions between the different constituents of condensed matter, which would be inaccessible under equilibrium conditions. We use time resolved electron diffraction to investigate the behaviour of a CDW system perturbed by a short laser pulse. From the observed changes in the diffraction patterns we can directly deduce changes in the lattice structure of our sample. A femtosecond electron diffraction setup was developed at the Laser Research In- stitute in Stellenbosch, South Africa. Short laser pulses produce photo electrons which are accelerated to an energy of 30 keV. Despite space charge broadening effects, elec- tron pulses shorter than 500 fs at sample position can be achieved. Technical details of this system and its characterisation as well as sample preparation techniques and analysis methods are described in detail in this work. Measurements on two members of the quasi-two-dimensional transition metal di- chalcogenides, namely 4Hb-TaSe2 and 1T-TaS2, are shown and discussed. Both show fast (subpicosecond) changes due to the suppression of the PLD and a rapid heating of the lattice. When the induced temperature rise heats the sample above a phase tran- sition temperature, a complete transformation into the new phase was observed. For 4Hb-TaSe2 we found that the recovery to the original state is significantly slower if the PLD was completely suppressed compared to only disturbing it. On 1T-TaS2 we could not only study the suppression of the original phase but also the formation of the higher energetic CDW phase. Long (100 ps) time constants were found for the tran- sition between the two phases. These suggest the presence of an energy barrier which has to be overcome in order to change the CDW phase. Pinning of the CDW by de- fects in the crystal structure result in such an energy barrier and consequently lead to a phase of domain growth which is considerably slower than pure electron or lattice dynamics. AFRIKAANSE OPSOMMING: Dit is bekend dat lae-dimensionele materie fase oorgange ondergaan na anders ge- ori¨enteerde toestande wanneer afgekoel word tot laer temperature. Hierdie fases toon dikwels ’n periodiese modulasie van die elektron digtheid (genoem ’n “charge density wave” – CDW), tesame met ’n periodiese effek op die kristalrooster (genoem ’n “peri- odic lattice distortion” – PLD). Alhoewel baie eksperimente al uitgevoer is en al baie geleer is oor hierdie CDW fase, is die redes vir hul bestaan nog steeds nie ten volle verstaan nie. Baie prosesse, wat of sterk elektron–elektron of elektron–fonon interaksie toon, is al waargeneem en kan ’n rol speel in die verduideliking van die vorming van die verskillende fases onder verskillende omstandighede. Met die beskikbaarheid van femtosekonde lasers is dit nou moontlik om materie onder hoogs nie-ewewig voorwaardes te bestudeer. Deur skielik ’n bekende hoeveel- heid energie in die stelsel in te voer, word die ewewigstaat versteur en word die daar- opvolgende ontspanning prosesse waargeneem op die tydskaal van atomies struktu- rele en elektroniese bewiging. Hierdie eksperimente kan waardevolle inligting lewer oor die komplekse interaksies tussen die verskillende atomiese komponente van ge- kondenseerde materie, wat ontoeganklik sou wees onder ewewig voorwaardes. Ons gebruik elektrondiffraksie met tyd resolusie van onder ’n pikosekonde om die gedrag van ’n CDW stelsel te ondersoek nadat dit versteur is deur ’n kort laser puls. Van die waargenome veranderinge in die diffraksie patrone kan ons direk aflei watse veranderinge die kristalstruktuur van ons monster ondergaan. ’n Femtosekonde elektronendiffraksie opstelling is ontwikkel by die Lasernavors- ingsinstituut in Stellenbosch, Suid-Afrika. Kort laser pulse produseer foto-elektrone wat dan na ’n energie van 30 keV versnel word. Ten spyte van Coulomb afstoting ef- fekte, kan elektron pulse korter as 500 fs by die monster posisie bereik word. Tegniese besonderhede van hierdie opstelling, tegnieke van die voorbereiding van monsters asook analise metodes word volledig in hierdie tesis beskryf. Metings op twee voorbeelde van kwasi-tweedimensionele semi-metale, naamlik 4Hb-TaSe2 en 1T-TaS2, word gewys en bespreek. Beide wys ’n vinnige (subpikosekon- de) verandering as gevolg van die versteuring van die PLD en ’n vinnige verhitting van die kristalrooster. Wanneer die ge¨ınduseerde temperatuur bo die fase oorgang tempe- ratuur styg, is ’n volledige transformasie na die nuwe fase waargeneem. Vir 4Hb-TaSe2 het ons gevind dat die herstelling na die oorspronklike toestand aansienlik stadiger is as die PLD heeltemal viernietig is in vergelyking met as die PLD net versteur is. Met 1T-TaS2 kon ons nie net alleenlik die vernietiging van die oorspronklike fase sien nie, maar ook die vorming van ’n ho¨er energie CDW fase. Lang (100 ps) tydkonstante is gevind vir die oorgang tussen die twee fases. Hierdie dui op die teenwoordigheid van ’n energie-versperring wat eers oorkom moet word om die CDW fase voledig te ver- ander. Vaspenning van die CDW deur defekte in die kristalstruktuur veroorsaak so’n energie versperring en gevolglik lei dit tot ’n fase van groeiende CDW gebiede wat heelwat stadiger as pure elektron of kritalrooster dinamika is.

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Philoxene, Loic. "Une nouvelle approche pour la détermination des fonctionnalités statiques et dynamiques des systèmes d'électrons fortement corrélés." Electronic Thesis or Diss., Normandie, 2024. http://www.theses.fr/2024NORMC219.

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Dans ce manuscrit, une instance particulière de représentation en bosons esclaves, introduite pour la première fois par Kotliar et Ruckenstein dans le context du modèle de Hubbard, est utilisée dans le but d’analyser les effets des interactions électroniques sur deux classes distinctes de propriétés de systèmes décrits par des modèles de Hubbard étendus.Dans un premier temps, une extension originale du modèle de Hubbard est introduite. Elle tient compte d’un potentiel à une particule modulé dans l’espace, ainsi que d’interactions entre électrons occupant des sites plus proches voisins. Ses propriétés statiques—comme les différents états fondamentaux en compétition dans son diagramme des phases à température nulle, ou encore la structure des bandes de ses quasiparticules—sont étudiées, dans le cas particulier d’une bande demi-remplie, dans l’approximation de point-selle de l’action associée à la représentation de Kotliar et Ruckenstein. Dans le diagramme des phases, une compétition entre deux phases ordonnées est étudiée. Ces phases, harborant toutes deux une modulation alternée de leur distribution de charges, se distinguent par la présence, ou non, d’un ordre antiferromagnétique de leurs spins. Des transitions fortement discontinues entre ces deux phases sont révélées par l’étude des paramètres d’ordre pertinents a différentes échelles de couplage. De larges régions de coexistence, dans lesquelles l’énergie de l’une des phases est légèrement supérieure à celle de l’autre phase, sont cartographiées. Dans ces régions, les différences notables de valeurs du gap de la structure des bandes entre les deux phases sont mises en évidence, donnant alors lieu à de possibles applications dans le domaine des matériaux fonctionnels.Dans un second temps, un modèle de Hubbard, étendu d’un potentiel d’intéraction de Coulomb à longue portée entre les électrons, est analysé à l’aide d’une forme alternativede la représentation de Kotliar et Ruckenstein. Un exemple de propriété dynamique du modèle, son spectre d’excitation de charges à temperature nulle, est calculé sous la forme de la fonction diélectrique dynamique. Plus particulièrement, les fluctuations gaussiennes des champs de bosons esclaves autour de la solution de point-selle paramagnétique sont calculées, permettant ainsi la détermination d’une formule analytique pour la susceptibilité dynamique de charge, et de ce fait pour la fonction diélectrique et la conductivité optique dynamiques. Les spectres, composés d’un continuum d’excitations particule-trou, ainsi que de deux modes collectifs: le plasmon et une signature de la bande de Hubbard supérieure, sont alors comparés avec l’approximation standard de la phase aléatoire, de sorte à mettre en exergue les effets dûs aux fortes corrélations électroniques. Un intérêt particulier est porté à la fréquence plasma, l’energy du mode de plasmon aux grandes longueurs d’ondes, mettant l’accent sur les effets du réseau cristallin et l’importance de la renormalisation de la masse effective inverse due aux interactions entre électrons In this work, a specific instance of a slave boson representation—originally introduced in the context of the Hubbard model by Kotliar and Ruckenstein—is put to use toinvestigate the effect of electronic interactions on two classes of properties of condensed matter systems described by extended Hubbard models.In the first place, an original extension to the Hubbard model, entailing a spatially modulated single-particle potential along with nearest-neighbour electronic interactions, is introduced. Its static properties, namely the competing ground states in its zero-temperature phase diagram, as well as its quasiparticles band structure, are investigated at half filling by means of a saddle-point approximation to the Kotliar-Ruckenstein representation. A competition between two symmetry broken phases, both featuring a checkerboard charge order, and distinguished by the presence or absence of a Néel ordering of the spins, is evidenced. Strongly discontinuous transitions are revealed by an analysis of the relevant order parameters at all coupling scales. Coexistence between both phases, with one phase slightly higher in energy than the ground state, is found in large regions of the parameter space. The band structure of the phases, and more precisely the sizable difference in their band gap, is analyzed, putting forward possible applications of putative experimental realizations of the model as candidate functional materials.In the second place, a Hubbard model, extended by long range Coulomb interactions between electrons, is analyzed by means of an alternative form of the Kotliar-Ruckensteinrepresentation. An instance of dynamical property, namely the model’s zero-temperature charge excitation spectrum, is computed in the form of the electron energy loss function. Specifically, Gaussian fluctuations of the slave boson fields around the paramagnetic saddle-point of the alternative representation are calculated, ultimately allowing for a derivation of an analytical formula for the dynamical charge susceptibility, and thus for the loss function as well as the optical conductivity. The spectrum, generically comprising a particle-hole excitation continuum as well as two collective modes, the plasmon and a signature of the upper Hubbard band, is then compared with standard random phase approximation in order to evidence strong correlation features. Focus is put on the plasma frequency, the energy of the plasmon mode at long wavelengths, emphasizing lattice effects as well as the relevance of the interaction-driven renormalization of the inverse effective mass

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KOUACOU, ABAKA MICHEL. "Apparition du ferromagnetisme itinerant dans des composes de type heusler. Relation avec des transitions isolant-metal." Université Joseph Fourier (Grenoble), 1996. http://www.theses.fr/1996GRE10087.

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L'etude des proprietes magnetiques et de transport de differentes series de solutions solides autour des alliages d'heusler tico#2sn et demi-heusler ticosn montre des caracteristiques classiques du ferromagnetisme itinerant. Dans la serie tico#2sn-tini#2sn, la densite locale est fortement reduite et le critere de stoner n'est plus maintenu pour tini#2sn. Le ferromagnetisme disparait au profit d'un etat paramagnetique de pauli. Vers la limite magnetique-non magnetique, la resistivite et la chaleur specifique montrent de fortes fluctuations de spin. Dans la serie tico#xsn avec 1<x<2, ou le cobalt est dilue par des lacunes, l'etat fondamental du systeme reste ferromagnetique mais les moments et la temperature de curie chutent moins rapidement que dans le cas precedent. Le remplissage de la bande 3d par apport d'un electron (substitution de co par ni) reduit plus efficacement le moment magnetique et la temperature de curie que la simple dilution de co par des lacunes a cause de la diminution de la densite d'etats locale et des correlations sur les sites de cobalt. Ces deux series restent metalliques. Pour les series demi-heusler ticosn-tinisn et ticosn-ticosb, l'apparition du ferromagnetisme itinerant s'accompagne d'une transition semiconducteur-metal. Dans la serie ticosn-tinisn, l'apparition du ferromagnetisme coincide avec la transition semiconducteur-metal. Ce changement de regime provoque la localisation des orbitales de co sans disparition du moment effectif. Les interactions entre moments, du fait de la diminution de la densite d'etats au niveau de fermi, chutent rapidement. L'apparition d'un gap est principalement cause par une hybridation entre les etats p et d de ce systeme, le passage a l'etat metallique est une transition de dopage classique. L'etude de l'effet hall montre qu'un decouplage local de bande se maintient au dessus de t#c. Concernant la serie ticosn-ticosb, la disparition du ferromagnetisme est liee a une forte baisse de la densite d'etats a l'approche de la transition metal-semiconducteur. L'existence probable d'une phase metallique paramagnetique intermediaire est mise en evidence. La configuration non magnetique est probablement stabilisee par l'hybridation co-ti et un gap du aux correlations intra-atomiques de type mott-hubbard

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Bouba, Oumarou. "Théories quantique et semi-classique des intégrales radiales de transitions dipolaires et multipolaires des états excités : Applications au calcul des forces d'oscillateur et des probabilités de transition dans l'approximation à une configuration." Orléans, 1986. http://www.theses.fr/1986ORLE0010.

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Calcul des intégrales radiales à l'approximation quasi classique (JWKB) des fonctions d'onde radiales, complétée par une description en termes de trajectoires elliptiques de l'électron optique. Obtention d'expressions analytiques de ces intégrales dont on simplifie le calcul à l'aide d'une méthode basée sur le développement limite d'une fonction bien adaptée à des calculs par interpolation ou extrapolation : elles s'expriment toutes en termes de deux fonctions fondamentales qui ne dépendent que de la différence entre deux nombres quantiques principaux effectifs des états initial et final. Déduction de forces d'oscillateur et probabilités de transition aussi précises que celles obtenues par quantique.

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Dissertations / Theses on the topic 'Electronic transitions'

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