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1
Göres, Jörn. "Correlation effects in 2-dimensional electron systems composite fermions and electron liquid crystals /." [S.l. : s.n.], 2004. http://www.bsz-bw.de/cgi-bin/xvms.cgi?SWB11612017.
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Geisler, Martin C. "The Hofstadter butterfly and quantum interferences in modulated 2-dimensional electron systems." [S.l. : s.n.], 2005. http://nbn-resolving.de/urn:nbn:de:bsz:93-opus-26027.
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Rosner, Helge. "Electronic structure and exchange integrals of low-dimensional cuprates." [S.l. : s.n.], 1999. http://www.bsz-bw.de/cgi-bin/xvms.cgi?SWB9470690.
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Nübler, Johannes [Verfasser], and David [Akademischer Betreuer] Wharam. "Density Dependence of the v=5/2 Fractional Quantum Hall Effect - Compressibility of a Two-dimensional Electron System under Microwave Irradiation / Johannes Nübler ; Betreuer: David Wharam." Tübingen : Universitätsbibliothek Tübingen, 2011. http://d-nb.info/1162699248/34.
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Magyar, Peter. "Quelques aspects du transport électronique bidimensionnel : études théoriques en champ magnétique faible et fort." Université Joseph Fourier (Grenoble ; 1971-2015), 1997. http://www.theses.fr/1997GRE10025.
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Ce travail presente deux etudes de transport des systemes d'electrons bidimensionnels dans les limites du champ magnetique faible et fort. Premierement, nous avons analyse l'utilite d'une nouvelle approche theorique, destinee a calculer la mobilite limitee par (a) des impuretes ionisees ou par (b) des dislocations. Ce traitement a pour but d'ameliorer la description de l'effet d'ecran. Notre etude a revele que la methode ne represente que des avantages tres limites dans le cas (a), tandis que dans le cas (b) les courbes calculees en fonction de la temperature et de la largeur du puits sont en accord avec les mesures. Deuxiemement, l'effet hall quantique entier a ete etudie dans un potentiel de desordre modele. Le systeme a ete couple a un bain thermique. A partir de l'etat stationnaire d'une equation de boltzmann quantique, les conductivites de hall et dissipative ont ete obtenues en fonction de la temperature pour tous les facteurs de remplissage. Nos resultats sont en accord qualitatif avec les experiences. En outre, ils montrent que, du aux phonons, les plateaux quantifies de la conductivite de hall sont sensiblement plus larges a tres basses temperatures que ceux de la conductivite dissipative.
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Carter, Paul James Anthony. "Acoustic phonon scattering by a 2 dimensional electron gas." Thesis, University of Nottingham, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.329844.
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Wan, Zhong. "Induced Superconductivity in Two Dimensional Electron Gas System." Thesis, Purdue University, 2018. http://pqdtopen.proquest.com/#viewpdf?dispub=10830652.
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Recently, interest in superconductor-semiconductor interfaces was renewed by the search for non-Abelian states. One of the possible platform is proximity induce superconductivity into an 1D semiconductor system with strong spin orbit (SO) interaction, such system is predicted to support Majorana excitation. Another candidate is superconductivity coupled to the edge of fractional quantum Hall state, in such system, higher order of non-Abelian statistics is predicted. With such non-Abelian states, topological quantum computing can be realized. In this thesis, I will discuss the approach made by us to investigate such system.
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Ford, E. M. "Single electron conduction in 2-dimensional arrays of gold nanodots." Thesis, University of Cambridge, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.599113.
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This dissertation considers single electron conduction through arrays of nanometer-size metallic islands fabricated by Focused Ion Beam (FIB) lithography. Control of the array geometry and the corresponding electrical characteristics were achieved by variation of the FIB deposition conditions. Scanning electron microscopy was used to examine FIB depositions on Si/SiO2 substrates. For more accurate examination transmission electron microscopy was used to image arrays deposited on carbon membranes. Image analysis software revealed a general increase in island diameter and reduction of island density with increasing FIB dose. The average island size was found to decrease with increasing FIB landing energy, although the uniformity of the deposition was reduced. The process of island formation is discussed in relation to the trends observed. Devices were fabricated with a planar structure, consisting of closely-spaced (~10 nm) source and drain electrodes situated within a large area array of islands. The devices were created on silicon substrates with a top layer of thermally grown silicon dioxide. A chromium oxide film was deposited by R.F. sputtering and the electrodes were then defined by high-resolution electron beam lithography. Electrical measurements were carried out at temperatures between 4.2K and 300K. Characteristics typical of single-electron conduction were observed, including the Coulomb gap and staircase, and the reduction of the Coulomb gap width with increasing temperature. The width of the low-temperature Coulomb gap was found to diminish with increasing dose, while devices fabricated with higher landing energies have lower operating temperatures. A novel device structure was also fabricated, where a small number of gold islands were isolated on the surface of a silicon nanowire. Silicon wires of length ~1μm and width >10nm were fabricated reproducibly using a sacrificial metallic etch mask. The electrical characteristics exhibited Coulomb gaps consistent with single electron transport.
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Martin, Theodore Peyton. "Low-dimensional electron transport in mesoscopic semiconductor devices /." view abstract or download file of text, 2006. http://proquest.umi.com/pqdweb?did=1280149921&sid=2&Fmt=2&clientId=11238&RQT=309&VName=PQD.
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Thesis (Ph. D.)--University of Oregon, 2006.Typescript. Includes vita and abstract. Includes bibliographical references (leaves 187-196). Also available for download via the World Wide Web; free to University of Oregon users.
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McEnaney, Kevin Bernard. "Magneto-absorption of surface acoustic waves by a 2-dimensional electron gas." Thesis, University of Nottingham, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.293651.
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Lee, Sungkoo. "A constraint-based 2-dimensional object display system." Case Western Reserve University School of Graduate Studies / OhioLINK, 1991. http://rave.ohiolink.edu/etdc/view?acc_num=case1055342896.
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Merkt, Rainer. "Density of states and delocalization in low dimensional disordered electron systems." [S.l. : s.n.], 2000. http://deposit.ddb.de/cgi-bin/dokserv?idn=960264817.
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Teske, Ekkehard. "Coulomb effects in magnetotransport of a nondegenerate two-dimensional electron system." [S.l.] : [s.n.], 2004. http://deposit.ddb.de/cgi-bin/dokserv?idn=97109361X.
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Wright, Philippa Anne. "Radiofrequency and optical studies of a quantum two dimensional electron system." Thesis, University of Oxford, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.308672.
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An, Sanghun. "Emerging Phenomena in Two-dimensional Electron System in Intermediate Magnetic Fields." The Ohio State University, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=osu1230923979.
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Kellogg, Melinda Jane Eisenstein James P. "Evidence for excitonic superfluidity in a bilayer two-dimensional electron system /." Diss., Pasadena, Calif. : California Institute of Technology, 2004. http://resolver.caltech.edu/CaltechETD:etd-08102004-204105.
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Salameh, Belal. "Electron spin resonance investigations of organic spin chains and two-dimensional organic conductors." [S.l. : s.n.], 2005. http://www.bsz-bw.de/cgi-bin/xvms.cgi?SWB11947828.
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Klironomos, Filippos. "Tunneling between two dimensional electron systems in a high magnetic field and crystalline phases of a two dimensional electron system in a magnetic field." [Gainesville, Fla.] : University of Florida, 2005. http://purl.fcla.edu/fcla/etd/UFE0009801.
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Bhatti, Arshad Saleem. "Light scattering studies of the plasmon modes of two-dimensional electron gases." Thesis, University of Cambridge, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.388821.
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Brownlie, Matthew. "Theoretical studies of the two-dimensional interacting electron system in high magnetic field." Thesis, University of Nottingham, 2013. http://eprints.nottingham.ac.uk/13810/.
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This is a mathematical study of certain aspects of the interacting electron system in very high perpendicular magnetic field. We analyse restrictions imposed upon the density correlation functions of this system and propose a set of sum rules which they must obey. We study the possibility of building a bosonisation scheme for the projected density operators in the lowest Landau level. We suggest a second order bosonisation, along with an approximation scheme, which may be useful for carrying out calculations in the lowest Landau level. We analyse the possible ground states of the system. We suggest a set of variational wavefunctions which can have lower energy than the Laughlin state for sufficiently soft interaction potentials. We study the collective excitations of the system, paying particular attention to its symmetries. We suggest a set of variational excited states and discuss their applicability to finite as well as infinite systems.
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Scheid, Matthias. "Tailoring semiconductor spintronics devices : tools for the creation and control of spins in two-dimensional electron gases." Regensburg Univ.-Verl. Regensburg, 2010. http://epub.uni-regensburg.de/14048/.
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Hu, Xuelong. "A study of a two-dimensional electron system in the variable-range hopping regime." Case Western Reserve University School of Graduate Studies / OhioLINK, 1994. http://rave.ohiolink.edu/etdc/view?acc_num=case1061225773.
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Amriou, Samia. "Synthesis and electrochemical properties of new l,3-dithiol-2-ylidene derivatives." Thesis, Durham University, 2003. http://etheses.dur.ac.uk/4007/.
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The π-electron donor system 9-(l,3-dithiol-2-ylidene)thioxanthene has been synthesised via Homer-Wadsworth-Emmons reactions of l,3-dithioI-2-phosphonate ester reagents with thioxanthen-9-one. The electrochemistry of a range of derivatives shows a single, two-electron oxidation wave, which is quasi-reversible. X-Ray crystal analysis reveals a folded structure. Compound 33c has proved to be a key building block which via lithiation chemistry has provided the hydroxymethyl derivative 37 and hence the synthesis of derivatives with covalently attached fluorenone, dicyanomethylenefluorene and PTM (4-[bis(2,3,4,5,6-pentachlorophenyl)methyl]-2,3,5,6-tetrachlorobenzoyl chloride radical) as electron acceptor substituents has been achieved. Electrochemical, spectroscopic and spectroelectrochemical studies have revealed the presence of intramolecular interactions in these compounds. 9-(4-Iodo-5-methyl-1,3-dithiol-2-ylidene)thioxanthene has been synthesised and shown to undergo clean metal-catalysed cross-coupling reactions under Suzuki and Sonogashira conditions, which is unusual for l,3-dithiol-2-ylidene derivatives. Aldehyde functionality has been attached to the 9-(l,3-dithiol-2-ylidene)thioxanthene core and reacting these aldehyde reagents with C(_60) and sarcosine gave covalent 9-(l,3-dithiol-2-ylidene)thioxanthene- C(_60) 1:1 diad molecules, and the analogous 2:1 adducts. A second series of compounds, with more extension of the π-system, eg compound 66a, has been synthesised and characterised by X-ray crystallography, a range of spectroscopic studies and by epr and electrochemistry.
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Zakaria, Marwan F. "An automated vision system using a fast 2-dimensional moment invariants algorithm /." Thesis, McGill University, 1987. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=66244.
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Wüster, Wolf [Verfasser]. "Cavity quantum electrodynamics with many-body states of a two-dimensional electron system / Wolf Wüster." München : Verlag Dr. Hut, 2015. http://d-nb.info/1079768408/34.
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Lesne, Edouard. "Non-Equilibrium Spin Accumulation Phenomena at the LaAlO3/SrTiO3(001) Quasi-Two-Dimensional Electron System." Thesis, Paris 6, 2015. http://www.theses.fr/2015PA066417/document.
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Nous avons étudié la création, manipulation et détection d’accumulation de spin hors-équilibre dans l’hétérostructure d’oxydes non-magnétique LaAlO3/SrTiO3 (LAO/STO), hôte d’un système d’électron quasi-bidimensionnel (q2DES). Combiné à des expériences d’injection de spin, nous employons l’effet Hanle (à 3 terminaux) pour sonder l’amplitude de l’accumulation de spin dans des jonctions tunnel Co/LAO/STO. Nous observons une large amplification du signal de spin, attribuée à des processus d’effet tunnel séquentiel préservant le spin via des états localisés avec des longs temps de vie de spin. Une importante modulation du signal de spin par effets de champ électrique atteste de la création d’accumulation de spin au sein même du q2DES. Nous avons utilisé la technique de pompage de spin en cavité, induite par résonance ferromagnétique d’une couche de permalloy, pour générer un courant de spin à l’interface LAO/STO, lequel est converti en un large courant de charge au sein du q2DES. Nous l’attribuons à un effet Edelstein inverse, dérivant d’une interaction spin-orbite de type Rashba. Lesquels sont efficacement modulés par effets de champ. Ainsi, nos résultats permettent d’étendre le champ d’intérêt depuis le transport de charge planaire vers l’exploration de phénomènes dépendant du spin dans un canal conducteur prototypique d’oxydes non-magnétique. Nous avons par ailleurs démontré que l’épaisseur critique pour l’observation d’un q2DES à l’interface LAO/STO peut être réduite à une monocouche de LAO en recourant à une variété de couches métalliques. Cela ouvre un nouveau champ d’investigation pour tenter d’identifier les potentiels mécanismes à l’origine de la formation du q2DESWe investigated the generation, manipulation, and detection of non-equilibrium spin accumulation in the nonmagnetic LaAlO3/SrTiO3 (LAO/STO) oxide heterostructure, which is the host of a quasi-two-dimensional electron system (q2DES). In electrical tunneling spin injection experiments, we made use of the (three-terminal) Hanle effect to probe the magnitude of spin accumulation at Co/LAO/STO interfaces. We report on large amplification effects of the spin signal, ascribed to spin-conserving sequential tunneling processes via localized electronic states of enhanced spin lifetimes. A substantial modulation of the spin signal, by electrostatic field-effect, evidences the successful generation of spin accumulation inside the q2DES. We further resorted to ferromagnetic resonance experiments in a cavity to adiabatically pump a spin current from a permalloy layer toward the LAO/STO interface. We find that the generated spin current is converted into a sizeable planar charge current within the q2DES. This is attributed to an inverse Edelstein effect deriving from a Rashba-like spin-orbit interaction, both of which are efficiently modulated by electrostatic field-effect. Hence, our findings expand the general field of interest from planar charge transport to the exploration of spin-dependent phenomena in a prototypical nonmagnetic conducting oxide channel. Additionally, we have also demonstrated that the critical thickness threshold for the onset of a q2DES at LAO/STO interfaces can be reduced to a single unit cell of LAO when resorting to various metal capping layers. It opens up a new field of investigation to tentatively identify the potential mechanisms driving the formation of the q2DES
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Digby, Julian Edward. "Phonon absorption by a 2-dimensional carrier system in the quantised Hall regime." Thesis, University of Nottingham, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.363907.
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Rödel, Tobias. "Two-dimensional electron systems in functional oxides studied by photoemission spectroscopy." Thesis, Université Paris-Saclay (ComUE), 2016. http://www.theses.fr/2016SACLS197/document.
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De nombreux oxydes de métaux de transition (TMOs) possèdent des propriétés physiques complexes (ferroélectricité, magnétisme, supraconductivité à haute Tc ou magnétorésistance colossale). Les différents degrés de liberté (le réseau, la charge, le spin ou l'ordre orbitalaire) interagissent pour donner des phases différentes, très proches en énergie, qui vont former une grande variété d'états fondamentaux accessibles. La possibilité de fabriquer des hétérostructures de TMOs a encore accru la complexité de ces systèmes, de nouveaux phénomènes apparaissant aux interfaces. Un exemple typique est le gaz d'électrons bidimensionnel (2DEG) créé à l'interface entre deux oxydes isolants, LaAlO3 et SrTiO3, qui montre une transition métal-isolant, du magnétisme ou de la supraconductivité (contrôlée par une tension de grille). Le point de départ de cette thèse a été la découverte d'un 2DEG similaire à la surface nue de SrTiO3 fracturée sous vide, rendant possible l'étude de sa structure électronique par photoémission angulaire.Dans cette thèse, l'étude de surfaces préparées, plutôt que de petites facettes fracturées, a permis l'obtention de données spectroscopiques possédant des largeurs de raie proches des valeurs intrinsèques. Il est alors possible d'étudier les effets à N corps comme la renormalisation de la self-énergie due à l'interaction électron-phonon.Ces recherches sur la structure électronique du 2DEG à la surface de SrTiO3 ont pris un tour nouveau lorsqu'une texture de spin complexe y a été mesurée par photoémission résolue en spin. Nous présentons des résultats qui contredisent ces conclusions et nous discutons des raisons pouvant expliquer ce désaccord.Une des motivations de cette thèse était de savoir si la structure électronique et les propriétés du 2DEG pouvaient être contrôlées. L'étude du 2DEG sur des surfaces (110) et (111) de SrTiO3 révèle que sa structure de bandes (ordre orbitalaire, symétrie de la surface de Fermi, masses effectives) peut être ajustée en confinant les électrons sur des surfaces de différentes orientations du même matériau.Un succès majeure est la mise en évidence de 2DEGs à la surface de nombreux autres TMOs (TiO2-anatase, CaTiO3, BaTiO3) ou d'oxydes plus simples utilisés dans les applications (ZnO). Dans tous ces oxydes, nous avons identifié les lacunes en oxygène comme étant à l'origine de la création des 2DEGs.Dans l'anatase, ou d'autres TMOs en configuration électronique initiale d0, les lacunes en oxygène produisent à la fois des électrons localisés ou itinérants (le 2DEG). Il peut être subtile de prévoir quel est le cas est le plus favorable énergétiquement comme le démontre l'étude de deux polymorphes de TiO2, anatase et rutile. Dans CaTiO3, l’octaèdre formé par les atomes d'oxygène autour du Ti est incliné. Cette rupture de symétrie provoque un mélange des orbitales d et modifie le 2DEG. Dans BaTiO3, la création d'un 2DEG entraîne la coexistence de deux phénomènes normalement incompatibles, la ferroélectricité et la métallicité, dans deux zones spatialement distinctes du même matériau. Ce travail démontre qu'un 2DEG existe aussi à la surface de ZnO qui est, contrairement aux oxydes à base de Ti, plutôt un semiconducteur conventionnel, le caractère des orbitales pour les électrons itinérants étant alors de type s et non de type d.Le principal résultat est la mise au point d'une méthode simple et versatile pour la création de 2DEGs en évaporant de l'aluminium sur des surfaces d'oxydes. Une réaction d'oxydo-réduction entre le métal et l'oxyde permet de créer un 2DEG à l'interface entre le métal oxydé et l'oxyde réduit. Dans cette thèse, les 2DEGs ont été étudiés uniquement par photoémission sous ultra-vide. Cette méthode ouvre la possibilité d'étudier ces 2DEGs dans des conditions de pression ambiante en utilisant, par exemple, des techniques de transport, un pas important vers la production de masse et à bas coûts de 2DEGs dans les oxydes pour de futures applicationsMany transition metal oxides (TMOs) show complex physics, ranging from ferroelectricity to magnetism, high-Tc superconductivity and colossal magnetoresistance. The existence of a variety of ground states often occurs as different degrees of freedom (e.g. lattice, charge, spin, orbital) interact to form different competing phases which are quite similar in energy. The capability to epitaxially grow heterostructures of TMOs increased the complexity even more as new phenomena can emerge at the interface. One typical example is the two-dimensional electron system (2DES) at the interface of two insulating oxides, namely LaAlO3/SrTiO3, which shows metal-to-insulator transitions, magnetism or gate-tunable superconductivity. The origin of this thesis was the discovery of a similar 2DES at the bare surface of SrTiO3 fractured in vacuum, making it possible to study its electronic structure by angle-resolved photoemission spectroscopy (ARPES).In this thesis, the study of well-prepared surfaces, instead of small fractured facets, results in spectroscopic data showing line widths approaching the intrinsic value. This approach allows a detailed analysis of many-body phenomena like the renormalization of the self-energy due to electron-phonon interaction.Additionally, the understanding of the electronic structure of the 2DES at the surface of SrTiO3(001) was given an additional turn by the surprising discovery of a complex spin texture measured by spin-ARPES. In this thesis data is presented which contradicts these conclusions and discusses possible reasons for the discrepancy.One major motivation of this thesis was the question if and how the electronic structure and the properties of the 2DES can be changed or controlled. In this context, the study of 2DESs at (110) and (111) surface revealed that the electronic band structure of the 2DES (orbital ordering, symmetry of the Fermi surface, effective masses) can be tuned by confining the electrons at different surface orientations of the same material, namely SrTiO3.A major achievement of this thesis is the generalization of the existence of a 2DES in SrTiO3 to many other surfaces and interfaces of TMOs (TiO2 anatase, CaTiO3, BaTiO3) and even simpler oxides already used in modern applications (ZnO). In all these oxides, we identify oxygen vacancies as the origin for the creation of the 2DESs.In anatase and other doped d0 TMOs, both localized and itinerant electrons (2DES) can exist due to oxygen vacancies. Which of the two cases is energetically favorable depends on subtle differences as demonstrated by studying two polymorphs of the same material (anatase and rutile).In CaTiO3, the oxygen octahedron around the Ti ion is slightly tilted. This symmetry breaking results in the mixing of different d-orbitals demonstrating again why and how the electronic structure of the 2DES can be altered.In BaTiO3, the creation of a 2DES results in the coexistence of the two, usually mutual exclusive, phenomena of ferroelectricity and metallicity in the same material by spatially separating the two.Moreover, this work demonstrates that the 2DES also exists in ZnO which is - compared to the Ti-based oxides - rather a conventional semiconductor as the orbital character of the itinerant electrons is of s and not d-type.The main result of this thesis is the demonstration of a simple and versatile technique for the creation of 2DESs by evaporating Al on oxide surfaces. A redox reaction between metal and oxide results in a 2DES at the interface of the oxidized metal and the reduced oxide. In this thesis the study of such interfacial 2DESs was limited to photoemission studies in ultra high vacuum. However, this technique opens up the possibility to study 2DESs in functional oxides in ambient conditions by e.g. transport techniques, and might be an important step towards cost-efficient mass production of 2DESs in oxides for future applications
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Araki, Moto. "Steplike electric conduction in a classical two-dimensional electron system through a narrow constriction in a microchannel." 京都大学 (Kyoto University), 2013. http://hdl.handle.net/2433/175099.
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PETIT, PIERRE. "Magnetisme et proprietes de conduction des derives cristallins et liquides cristallins de la bisphtalocyanine de lutecium : effet de dimensionnalite." Université Louis Pasteur (Strasbourg) (1971-2008), 1987. http://www.theses.fr/1987STR13163.
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Etude parrpe et par des mesures de la conductivite dc et de la reponse dielectrique ac hyperfrequence. Mise en evidence de la nature radicalaire de type pi du systeme, a l'etat cristallin comme en solution; effet du solvant et de la temperature; observation de sauts de l'electron non apparie d'un macrocycle a l'autre a une frequence voisine de 1 mhz. Mise en evidence de la possibilite de formation de deux systemes cristallins (empilement de chaines ou de plans paralleles) et de leur caractere 1d ou 2d au moyen du modele de richards. La susceptibilite du systeme mesomorphe indique un desordre magnetique comparable a celui de la phase liquide; la conductivite est celle des systemes desordonnes. Les cristaux solvates sont des semiconducteurs moleculaires intrinseques
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Zhang, Yao. "Experimental Measurements by Antilocalization of the Interactions between Two-Dimensional Electron Systems and Magnetic Surface Species." Diss., Virginia Tech, 2014. http://hdl.handle.net/10919/49020.
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Low-temperature weak-localization (WL) and antilocalization (AL) magnetotransport measurements are sensitive to electron interference, and thus can be used as a probe of quantum states. The spin-dependent interactions between controllable surface magnetism and itinerant electrons in a non-magnetic host provide insight for spin-based technologies, magnetic data storage and quantum information processing. This dissertation studies two different host systems, an In$_{0.53}$Ga$_{0.47}$As quantum well at a distance from the surface of a heterostructure, and an accumulation layer on an InAs surface. Both the systems are two-dimensional electron systems (2DESs), and possess prominent Rashba spin-orbit interaction caused by structural inversion asymmetry, which meets the prerequisites for AL. The surface local moments influence the surrounding electrons in two ways, increasing their spin-orbit scattering, and inducing magnetic spin-flip scattering, which carries information about magnetic interactions. The two effects modify the AL signals in opposing directions: the spin-flip scattering of electrons shrinks the signal, and requires a close proximity to the species, whereas the increase of spin-orbit scattering broadens and increases the signal. Accordingly, we only observe an increase in spin-orbit scattering in the study of the interactions between ferromagnetic Co$_{0.6}$Fe$_{0.4}$ nanopillars and the relatively distant InGaAs quantum well. With these CoFe nanopillars, a decrease in spin decoherence time is observed, attributed to the spatially varying magnetic field from the local moments. A good agreement between the data and a theoretical calculation suggests that the CoFe nanopillars also generate an appreciable average magnetic field normal to the surface, of value $\sim$ 35 G. We also performed a series of comparative AL measurements to experimentally investigate the interactions and spin-exchange between InAs surface accumulation electrons and local magnetic moments of rare earth ions Sm$^{3+}$, Gd$^{3+}$, Ho$^{3+}$, of transition metal ions Ni$^{2+}$, Co$^{2+}$, and Fe$^{3+}$, and of Ni$^{2+}$-, Co$^{2+}$-, and Fe$^{3+}$-phthalocyanines deposited on the surface. The deposited species generate magnetic scattering with magnitude dependent on their electron configurations and effective moments. Particularly for Fe$^{3+}$, the significant spin-flip scattering due to the outermost 3d shell and the fairly high magnetic moments modifies the AL signal into a WL signal. Experiments indicate a temperature-independent magnetic spin-flip scattering for most of the species except for Ho$^{3+}$ and Co$^{2+}$. Ho$^{3+}$ yields electron spin-flip rates proportional to the square root of temperature, resulting from transitions between closely spaced energy levels of spin-orbit multiplets. In the case of Co$^{2+}$, either a spin crossover or a spin-glass system forms, and hence spin-flip rates transit between two saturation regions as temperature varies. Concerning the spin-orbit scattering rate, we observe an increase for all the species, and the increase is correlated with the effective electric fields produced by the species. In both 2DESs, the inelastic time is inversely proportional to temperature, consistent with phase decoherence via the Nyquist mechanism. Our method provides a controlled way to probe the quantum spin interactions of 2DESs, either in a quantum well, or on the surface of InAs.Ph. D.
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Mausser, Marcel D. [Verfasser], and Klaus von [Akademischer Betreuer] Klitzing. "Scanning single-electron transistor array microscope to probe a two-dimensional electron system under quantum Hall conditions below 40 milli-Kelvin / Marcel D. Mausser ; Betreuer: Klaus von Klitzing." Stuttgart : Universitätsbibliothek der Universität Stuttgart, 2017. http://d-nb.info/112981615X/34.
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Berciu, Mona Inesa. "A microscopic model for non-fermi-liquid behavior and charge carrier pairing in a purely repulsive two-dimensional electron system." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 2000. http://www.collectionscanada.ca/obj/s4/f2/dsk2/ftp03/NQ49975.pdf.
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Wolff, Karsten [Verfasser], and Hilbert von [Akademischer Betreuer] Löhneysen. "Anisotropic Electronic Transport of the Two-Dimensional Electron System in Al₂O₃ /SrTiO₃ Heterostructures / Karsten Wolff ; Betreuer: Hilbert von Löhneysen." Karlsruhe : KIT-Bibliothek, 2018. http://d-nb.info/1164080946/34.
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Hakimi, Shirin. "Exact Diagonalization of Few-electron Quantum Dots." Thesis, Linnaeus University, School of Computer Science, Physics and Mathematics, 2009. http://urn.kb.se/resolve?urn=urn:nbn:se:lnu:diva-2550.
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We consider a system of few electrons trapped in a two-dimensional circularquantum dot with harmonic confinement and in the presence of ahomogeneous magnetic field, with focus on the role of e-e interaction. Byperforming the exact diagonalization of the Hamiltonian in second quantization,the low-lying energy levels for spin polarized system are obtained. The singlet-triplet oscillation in the ground state of the two-electron system showing up inthe result is explained due to the role of Coulomb interaction. The splitting ofthe lowest Landau level is another effect of the e-e interaction, which is alsoobserved in the results.
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Lipps, Ferdinand. "Electron spins in reduced dimensions: ESR spectroscopy on semiconductor heterostructures and spin chain compounds." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2011. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-74470.
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Spatial confinement of electrons and their interactions as well as confinement of the spin dimensionality often yield drastic changes of the electronic and magnetic properties of solids. Novel quantum transport and optical phenomena, involving electronic spin degrees of freedom in semiconductor heterostructures, as well as a rich variety of exotic quantum ground states and magnetic excitations in complex transition metal oxides that arise upon such confinements, belong therefore to topical problems of contemporary condensed matter physics.
In this work electron spin systems in reduced dimensions are studied with Electron Spin Resonance (ESR) spectroscopy, a method which can provide important information on the energy spectrum of the spin states, spin dynamics, and magnetic correlations. The studied systems include quasi onedimensional spin chain materials based on transition metals Cu and Ni. Another class of materials are semiconductor heterostructures made of Si and Ge.
Part I deals with the theoretical background of ESR and the description of the experimental ESR setups used which have been optimized for the purposes of the present work. In particular, the development and implementation of axial and transverse cylindrical resonant cavities for high-field highfrequency ESR experiments is discussed. The high quality factors of these cavities allow for sensitive measurements on μm-sized samples. They are used for the investigations on the spin-chain materials. The implementation and characterization of a setup for electrical detected magnetic resonance is presented.
In Part II ESR studies and complementary results of other experimental techniques on two spin chain materials are presented. The Cu-based material Linarite is investigated in the paramagnetic regime above T > 2.8 K. This natural crystal constitutes a highly frustrated spin 1/2 Heisenberg chain with ferromagnetic nearest-neighbor and antiferromagnetic next-nearestneighbor interactions. The ESR data reveals that the significant magnetic anisotropy is due to anisotropy of the g-factor. Quantitative analysis of the critical broadening of the linewidth suggest appreciable interchain and interlayer spin correlations well above the ordering temperature. The Ni-based system is an organic-anorganic hybrid material where the Ni2+ ions possessing the integer spin S = 1 are magnetically coupled along one spatial direction. Indeed, the ESR study reveals an isotropic spin-1 Heisenberg chain in this system which unlike the Cu half integer spin-1/2 chain is expected to possess a qualitatively different non-magnetic singlet ground state separated from an excited magnetic state by a so-called Haldane gap. Surprisingly, in contrast to the expected Haldane behavior a competition between a magnetically ordered ground state and a potentially gapped state is revealed.
In Part III investigations on SiGe/Si quantum dot structures are presented. The ESR investigations reveal narrowlines close to the free electron g-factor associated with electrons on the quantum dots. Their dephasing and relaxation times are determined. Manipulations with sub-bandgap light allow to change the relative population between the observed states. On the basis of extensive characterizations, strain, electronic structure and confined states on the Si-based structures are modeled with the program nextnano3. A qualitative model, explaining the energy spectrum of the spin states is proposed.
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Klijn, Jan [Verfasser]. "Local density of states of the adsorbate-induced two-dimensional electron system studied at zero and strong magnetic fields / Jan Klijn." Aachen : Shaker, 2003. http://d-nb.info/1170541305/34.
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Ong, Seow Meng. "A Mission Planning Expert System with Three-Dimensional Path Optimization for the NPS Model 2 Autonomous Underwater Vehicle." Thesis, Monterey, California. Naval Postgraduate School, 1990. http://hdl.handle.net/10945/23457.
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Approved for public release; distribution is unlimitedUnmanned vehicle technology has matured significantly over the last two decades. This is evidenced by its widespread use in industrial and military applications ranging from deep-ocean exploration to anti-submarine warefare. Indeed, the feasiblity of short-range, special-purpose vehicles (whether aunonomous or remotely operated) is no longer in question. The research efforts have now begun to shift their focus on development of reliable, longer-range, high-endurance and fully autonomous systems. One of the major underlying technologies required to realize this goal is Artificial Intelligence (AI). The latter offers great potential to endow vehicles with the intelligence needed for full autonomy and extended range capability; this involves the increased application of AI technologies to support mission planning and execution, navigation and contingency planning. This thesis addresses two issues associated with the above goal for Autonomous Underwater Vehicles (AUV's). Firstly, a new approach is proposed for path planning in underwater environments that is capable of dealing with uncharted obstacles and which requires significantly less planning time and computer memory. Secondly, it explores the use of expert system technology in the planning of AUV missions.
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Moji, Kabelo McDonald. "Comparison of measured photon and electron beam dose distributions between 3D water phanton and profiler 2 scanning system, South Africa." Thesis, University of Limpopo (Medunsa Campus), 2013. http://hdl.handle.net/10386/1086.
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Thesis ( MSc ( Physics) ) -- University of Limpopo, 2013.Background and Objectives: To establish whether the profiler 2 scanning system can be used as a substitute for the 3D-water phantom, by comparing the percentage depth doses and beam profiles for both the photons and electron beams, and validating the results using CMS XiO treatment planning system. Methods: Beam data (profiles, percentage depth doses and absolute dosimetry) were acquired for the two systems: (3D-water phantom and profiler 2 scanning system) for beam energies 6 MV and 15 MV photon beams, and 4, 6, 8, 10, 12 and 15 MeV electron beams generated by the Elekta Synergy linear accelerator (linac) for the field sizes of 6 × 6 cm2, 10 × 10 cm2, 14 × 14 cm2, 20 × 20 cm2, and 25 × 25 cm2 at depths of 0.5 cm, 1.0 cm, 2.0 cm, and 5.0 cm respectively. These measurements were acquired using ionization chambers in water and diode detectors in Perspex. The acquired data was sent to CMS XiO treatment planning system for validation. Results: In general, the dose distributions for both systems compared very well with uncertainties within recommended limits. The largest maximum difference in symmetry was 1.6 % for a 6 MV photon beam defined at 25 × 25 cm2 field size. The largest maximum difference in flatness was 2.77 % for a 4 MeV electron beam defined at 10 × 10 cm2 applicator size. The penumbra largest maximum difference was 1.708 cm for 8 MeV electron beam defined at 25 × 25 cm2 applicator size, which was outside the recommended limit of 1.2 cm. The largest maximum difference in field size was 2.388 cm for a 6 MeV electron beam defined at 20 × 20 cm2 applicator size, which was outside the recommended limit of 0.4 cm. The largest maximum difference in percentage depth dose at 10 cm depth was 1.69 % for the 6 MV photon beam. The absolute dose output measurements showed a very good agreement between the two systems to a maximum percentage difference and highest standard deviation of -0.99 % and 0.69 % respectively for the 6 MV photon beam. Validation measurements showed an agreement to less than 1 % and 2 mm for percentage depth doses and beam profiles respectively. Conclusion and recommendation: From the results obtained, it is evident that the profiler 2 scanning system can be used as a substitute for the 3D-water phantom beam data acquisitions during linear accelerator commissioning. The future work based on this study could be to study the limitations involved with the profiler 2 scanning system when used during measurements for commissioning of a linear accelerator. Limitations like field size (maximum field size of 20 × 30 cm2 at SSD = 100 cm), number of Perspex slabs to be used on top of the profiler 2 scanning system and diagonal profile measurements.
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Huang, Ran. "Thermodynamics and Ideal Glass Transition on the Surface of a Monatomic System Modeled as Quasi "2-Dimensional" Recursive Lattices." University of Akron / OhioLINK, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=akron1342379960.
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Negri, Carlotta. "Controlling electron transport : quantum pumping and single-electron tunneling oscillations." Thesis, Bordeaux 1, 2012. http://www.theses.fr/2012BOR14670/document.
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Exploiter des effets dépendants du temps pour induire et contrôler des courants à travers des conducteurs mésoscopiques et nanoscopiques est un enjeu majeur dans le domaine du transport quantique. Dans cette thèse, nous considérons deux systèmes de taille nanométrique pour lesquels un courant est induit grâce au couplage entre champs extérieurs dépendants du temps et le transport d'électrons. Nous étudions d'abord un problème de pompage quantique au sein d'un système à trois sites en configuration d'anneau, en considérant la possibilité d'induire un courant continu par modulation temporelle des paramètres de contrôle. Nous nous intéressons en particulier à la transition entre régime adiabatique et antiadiabatique en présence d'un mécanisme de dissipation modélisé par un couplage entre le système et un bain extérieur.Nous montrons que le modèle dissipatif admet une solution analytique complète valable pour la composante DC du courant à fréquence arbitraire. Ceci nous permet de bien comprendre comment le courant induit dépend de la fréquence de pompage. Nous nous concentrons ensuite sur un autre système de contrôle du courant exploitant le phénomène des oscillations tunnel à un électron (SETOs). Contrairement au cas précédent, ici la circulation d'un courant continu à travers un circuit comportant une jonction tunnel produit, pour le régime approprié, un courant quasi-périodique d'électrons. On étudie le spectre de bruit à température nulle d'une jonction tunnel dans différents environnements résistifs dans le but de déterminer les limites du régime des SETOs et de quantifier leur degré de périodicité. Nous généralisons par la suite les résultats à température finie et discutons des effets des fluctuations quantiquesExploiting time-dependent effects to induce and control currents through mesoscopic and nano\-scopic conductors is a major challenge in the field of quantum transport. In this dissertation we consider two nanoscale systems in which a current can be induced through intriguing mechanisms of coupling between excitations by external fields and electron transport.We first study a quantum pumping problem, analyzing the possibility to induce a DC response to an AC parametric driving through a three-site system in a ring configuration. We are interested in particular in the crossover between adiabatic and antiadiabatic driving regimes and in the presence of dissipation, which is accounted for by coupling with an external bath. We show that for a clever choice of this coupling the dissipative model admits a full analytical solution for the steady state current valid at arbitrary frequency, which allows us to fully understand the pumping-frequency dependence of the induced current. We then focus on a different current-controlling scheme exploiting the phenomenon of single-electron tunneling oscillations (SETOs). In this case, opposite to what happens for pumping, an AC effect, an almost periodic current of single electrons, arises through a tunnel junction circuit as a consequence of a DC bias. We study the zero-temperature noise spectrum of a tunnel junction in different resistive environments with the aim to determine the boundaries of the SETOs regime and quantify their quality in terms of periodicity. We then discuss the finite-temperature generalization and the possibility to account for the effects of quantum fluctuations
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Lee, Paul Chong Chan. "A QUALITATIVE AND QUANTITATIVE ANALYSIS OF SOFT TISSUE CHANGE EVALUATION BY ORTHODONTISTS IN CLASS II NON EXTRACTION ORTHODONTIC TREATMENT USING THE 3dMD SYSTEM." Master's thesis, Temple University Libraries, 2013. http://cdm16002.contentdm.oclc.org/cdm/ref/collection/p245801coll10/id/217032.
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Oral BiologyM.S.
With the advent of cephalometrics in the 1930s, numerous studies have focused on the profile of a face to achieve a more esthetic orthodontic treatment outcome. With such heavy emphasis on facial esthetics, a shift in focus from the profile view to the oblique view has become necessary as the smile in the oblique view is what the general public evaluates. The purpose of this pilot study was to determine whether the current tools for diagnosis and treatment evaluation are sufficient. Currently, 2-dimensional composite photographs are utilized in evaluating the soft tissue. At Temple University, 3-dimensional images, which show all sides of the patient's face, are used adjunctively to 2-dimensional composite photographs. In this study, faculty members at the Temple University Department of Orthodontics were asked to complete surveys after viewing two different image modalities, 2-dimensional images and a 3-dimensional video of the same patient. They were asked to fill out the soft tissue goals for specific facial landmarks. Patient photos were in the smiling view as current literature lacks studies on this view. Faculty members' responses from analyzing the 2-dimensional images and 3-dimensional video for each patient were compared to determine which areas had frequent discrepancies from using two different image modalities. During the survey, a voice recorder captured any comments regarding the images. The ultimate goal of this qualitative pilot study was to identify when 3-dimensional imaging is necessary in treatment planning and evaluation, with an added hope to further advance research in 3-dimensional imaging and its vast possibilities to advance the field of orthodontics. Based on the data collected, the following conclusions were made: 1. The qualitative data highlighted that 3-dimensional imaging would be necessary in cases with skeletal deformities. 2. In the oblique view, 3-dimensional imaging is superior than 2-dimensional imaging by showing more accurate shadow, contour, and depth of the soft tissue. 3. Further improvement is necessary to create a virtual patient with treatment simulation abilities. 4. The comfort level among orthodontists of 2-dimensional imaging was higher than 3-dimensional imaging. With more widespread use of 3-dimensional imaging, more orthodontists may gradually reach a higher comfort level in using this relatively new technology. 5. Faculty members expressed high willingness to use 3-dimensional imaging if improvement in new technology could allow for more manipulation and accurate soft tissue prediction. 6. 3-dimensional imaging is superior in its efficiency, quick capture time, and lack of need for multiple images. Implementation of 3-dimensional imaging could streamline the records process and help with practice efficiency without compromising the image quality. 7. Both patients and orthodontists may benefit from using 3-dimensional imaging. Patients can see an accurate representation of themselves and possibly view their own treatment simulation upon further improvement in current technology. Orthodontists would benefit with much more accurate images that may serve as the virtual patient. 8. Besides the exorbitantly high cost, faculty members thought that more advances were needed and the current benefit was not great enough to justify the investment. The results were consistent with other studies that used the oblique view in that the 2-dimensional oblique view lacks depth and does not provide adequate information. With further improvement in current 3-dimensional imaging, this technology can benefit orthodontists in visualizing their patients. In addition, patients can benefit by hopefully seeing a live and accurate simulation of themselves instantly as a virtual patient. With these benefits of 3-dimensional imaging, it may one day be the new standard in patient records in the field of orthodontics.
Temple University--Theses
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Wang, Yin-Yu. "Réseaux supraconducteurs : température critique et états mixtes." Grenoble 1, 1987. http://www.theses.fr/1987GRE10060.
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Les effets de quantification du flux magnetique sur des reseaux supraconducteurs bidimensionnels sont etudies dans ce memoire. Du point de vue theorique, la temperature de transition supraconductrice en fonction du champ magnetique applique est determinee en resolvant les equations d'alexander pour divers reseaux : periodiques, quasiperiodiques et fractals. Quelques un de ces reseaux submicroniques ont ete realises par lithographie electronique. Leurs temperatures critiques sont mesurees et l'accord entre la theorie et l'experience est discute. A l'interieur de la phase supraconductrice, les vortex entrent dans le reseau et induisent des supercourants diamagnetiques. L'etat d'equilibre est traite d'une maniere analogue au traitement de l'etat mixte d'un supraconducteur massif d'abrikosov : on determine l'amplitude du parametre d'ordre en tenant compte du terme non-lineaire de l'equation de ginzburg-landau par une methode de perturbation. On en deduit les quantites thermodynamiques, en particulier l'energie libre et l'aimantation du reseau, qui sont des fonctions de la difference entre la temperature et la temperature critique t - t::(c), ainsi que du parametre d'abrikosov beta ::(a). Ce formalisme a ete applique au reseau carre pour lequel les configurations du parametre d'ordre et les distributions du courant diamagnetique ont ete determinees pour quelques valeurs simples du champ magnetique. D'autre part, la derivee de la ligne critique par rapport au champ magnetique, qui s'identifie a la derivee du bord du spectre d'hofstadter a ete calculee a l'aide d'une approximation continue. Cette derivee contient un terme anormal que l'on peut relier a la phase de berry. Elle donne directement la derivee de l'aimantation par rapport a la temperature d rond m/d rond t. Nous donnons egalement quelques resultats preliminaires sur l'etat mixte du reseau supraconducteur en presence d'un courant exterieur. Le courant critique au-dessus duquel cet etat n'existe plus est obtenu pour quelques cas simples (boucles, reseau carre). Quelques mesures preliminaires de ce courant ont ete effectuees sur un reseau carre
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Madhavi, S. "Carrier Mobility And High Field Transport in Modulation Doped p-Type Ge/Si1-xGex And n-Type Si/Si1-xGex Heterostructures." Thesis, Indian Institute of Science, 2000. http://hdl.handle.net/2005/294.
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Modulation doped heterostructures have revolutionized the operation of field effect devices by increasing the speed of operation. One of the factors that affects the speed of operation of these devices is the mobility of the carriers, which is intrinsic to the material used. Mobility of electrons in silicon based devices has improved drastically over the years, reaching as high as 50.000cm2/Vs at 4.2K and 2600cm2/Vs at room temperature. However, the mobility of holes in p-type silicon devices still remains comparatively lesser than the electron mobility because of large effective masses and complicated valence band structure involved. Germanium is known to have the largest hole mobility of all the known semiconductors and is considered most suitable to fabricate high speed p-type devices. Moreover, it is also possible to integrate germanium and its alloy (Si1_zGex ) into the existing silicon technology.
With the use of sophisticated growth techniques it has been possible to grow epitaxial layers of silicon and germanium on Si1_zGex alloy layers grown on silicon substrates. In tills thesis we investigate in detail the electrical properties of p-type germanium and n-type silicon thin films grown by these techniques. It is important to do a comparative study of transport in these two systems not only to understand the physics involved but also to study their compatibility in complementary field effect devices (cMODFET).
The studies reported in this thesis lay emphasis both on the low and high field transport properties of these systems. We report experimental data for the maximum room temperature mobility of holes achieved m germanium thin films grown on Si1_zGex layers that is comparable to the mobility of electrons in silicon films. We also report experiments performed to study the high field degradation of carrier mobility due to
"carrier heating" in these systems. We also report studies on the effect of lattice heating on mobility of carriers as a function of applied electric field.
To understand the physics behind the observed phenomenon, we model our data based on the existing theories for low and high field transport. We report complete numerical calculations based on these theories to explain the observed qualitative difference in the transport properties of p-type germanium and ii-type silicon systems. The consistency between the experimental data and theoretical modeling reported in this work is very satisfactory.
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Blauch, David N. Anson Fred C. Anson Fred C. "Part I. Improvements in the rotation-rate step experiment for the evaluation of diffusion coefficients at rotating disk electrodes. : Part II. Ion-pairing and electric field effects on electron hopping in the Nafion-tris(2,2'-bipyridine)osmium(3+/2+) system /." Diss., Pasadena, Calif. : California Institute of Technology, 1991. http://resolver.caltech.edu/CaltechETD:etd-06152007-081035.
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Ahmadouche, Ahmed Abdelkader. "Modélisation électrique des interconnexions multiniveaux en présence de diélectriques stratifiés." Grenoble INPG, 1987. http://www.theses.fr/1987INPG0140.
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Modelisation des lignes d'interconnexions dans un circuit logique a partir de matrices capacite, inductance resistance et conductance. Le probleme est traite en deux ou trois dimensions suivant la configuration envisagee. Presentation des resultats theoriques obtenus sur des dispositifs realises en technologie silicium et arseniure de gallium et en technologie hybride et comparaison a ceux issus du test
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Le, Touze Christophe. "Etude des propriétés associées aux ondes de densité de charge dans les bronzes oxygénés quasi-bidimensionnels (PO2)4(WO3)2m." Université Joseph Fourier (Grenoble), 1996. http://www.theses.fr/1996GRE10183.
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Nous avons etudie les proprietes associees aux ondes de densite de charge dans les bronzes quasi-bidimensionnels (po#2)#4(wo#3)#2#m (m=4, 5, 6, 7, 10, 11 et 12). Ces composes subissent des transitions de peierls a des temperatures t#p vers un etat d'onde de densite de charge caracterise par une distorsion periodique de reseau couplee a une modulation de la densite electronique. La chaleur specifique a ete mesuree pour les composes 4m entre 1,7k et 300k. Nous avons obtenu des termes lineaires en temperature anormalement eleves et qui augmentent avec m. Les temperatures de debye se situent autour de 250k et diminuent lorsque m augmente. Ce comportement pourrait etre du a la diminution de la liaison metallique ou (et) a l'augmentation du caractere bidimensionnel quand le parametre m croit. Des oscillations quantiques shubnikov-de haas et de haas-van alphen ont ete observees dans les composes m=4 et m=6 lors de mesures de magnetoresistance (m=4 et m=6) et de couple magnetique (m=6) effectuees a tres basse temperature et sous fort champ magnetique. Nous avons attribue ces phenomenes a l'existence de petites poches cylindriques subsistant sur la surface de fermi dans l'etat onde de densite de charge, plus petites pour le compose m=6 que pour le compose m=4: la surface de fermi est plus detruite dans le compose m=6 que dans le compose m=4. Le caractere de basse dimensionnalite pourrait donc augmenter avec m. Enfin, nous avons mesure la resistivite electrique des composes 10m2 en fonction de la temperature entre 4k et 800k. Cette etude nous a permis de differencier les composes a petit m des composes a grand m: contrairement aux premiers, dans le domaine des basses temperatures, les seconds ont une resistivite qui decroit a temperature croissante. Nous avons attribue l'augmentation en fonction de m de la resistivite a 300k (p(300k)) et du rapport de resistivite p(300k)/p(4k) a l'augmentation des interactions electrons-electrons via la diminution du nombre moyen d'electrons de conduction par atome de tungstene. D'autre part, nous pensons que l'absence d'anomalie tres marquee, caracteristique d'une transition de peierls, pour la plupart des composes a grand m, est due a des phenomenes de desordre empechant l'apparition d'un ordre a grande distance dans l'etat onde de densite de charge
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Utard, Christian. "Les oscillateurs microondes faible bruit de fond a base de mesfet gaas, tegfet gaalas et transistor bipolaire silicium : modelisation, caracterisation et comparaison." Toulouse 3, 1988. http://www.theses.fr/1988TOU30078.
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On presente une methode simplifiee de modelisation fort signal des mesfet et tegfet et transistors bipolaires. Ces trois types de transistors sont utilises dans un montage oscillateur dont les caracteristiques ont ete determines de trois facons differentes, analytique par simulation electrique temporelle et par mesures experimentales. On presente enfin une etude en bruit bf et bruit mf des transistors et des oscillateurs afin de determiner le composant le plus performant. Nous proposons trois facons de determiner le coefficient de conversion bruit bf - bruit mf, par des mesures directes; indirectes et par simulation temporelle
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Atzkern, Stefan. "Elektronen-Energieverlustspektroskopie von quasi-eindimensionalen Kupraten und Vanadaten." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2002. http://nbn-resolving.de/urn:nbn:de:swb:14-1014207158671-42512.
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This work presents a joint theoretical and experimental investigation of the electronic structure of quasi one-dimensional cuprates and vanadates. Electron energy-loss spectroscopy in transmission was employed to measure the momentum-dependent loss function of Li2CuO2, CuGeO3, V2O5 and NaV2O5. The comparison between the experimental data and the results from bandstructure as well as cluster calculations allows an explanation of the mobility and correlations of the electrons in these systems. The investigation of the electronic structure of the structurally related cuprates Li2CuO2 and CuGeO3 is exemplary for the study of the transition from a quasi zero-dimensional to a quasi one-dimensional system. In contrast to Li2CuO2 where the electron transitions are strongly localized, the excited states in CuGeO3 can be assigned to the electron hopping to the nearest-neighboured CuO4 plaquettes. The shift of spectral weight from the high energy to the low energy region with increasing coupling between the plaquettes, observed in edge-sharing CuO2 chains, is confirmed by the applied cluster modell. The momentum dependent loss functions of NaV2O5 deliver information about the mobility and correlations of electrons in a quarter-filled ladder system which determine the transition from the charge ordered state into the unordered state at 34 K. Thcontributions of the 3d electrons to the EELS spectra of NaV2O5 are filtered by comparing these spectra with the loss functions of the structurally related V2O5 (d0 configuration). For NaV2O5 the picture of linear chains of V-O-V rungs containing a single d electron in a molecular orbital-like state is confirmed. The comparison of the experimentally determined optical conductivities and those derived from the bandstructrure calculations yield a good agreement upon adoption of an on-site Coulomb interaction U = 2-3 eV. In contrast to the strongly anisotropic hopping within the ladder plane the intersite Coulomb interactions V are about the same size. These interactions are the driving force for the transition from an unordered state at room temperature into a zigzag ordered state observed at low temperaturesIn einer Kombination aus experimentellen und theoretischen Methoden wurden in dieser Arbeit die Elektronenstrukturen von quasi-eindimensionalen Kupraten und Vanadaten untersucht. Dazu wurde die impulsabhängige Verlustfunktion mit Hilfe der Elektronen-Energieverlustspektroskopie in Transmission an Einkristallen von Li2CuO2, CuGeO3, V2O5 und NaV2O5 gemessen. Der Vergleich der experimentellen Daten mit Ergebnissen aus Bandstruktur- und Cluster-Rechnungen erlaubte Rückschlüsse auf die Beweglichkeit und Korrelationen der Elektronen in diesen Systemen. Die Untersuchung der elektronischen Anregungen in den strukturell sehr ähnlichen Kupraten Li2CuO2 und CuGeO3 ist beispielhaft für das Studium des Übergangs von einem quasi-nulldimensionalen zu einem quasi-eindimensionalen System. In Li2CuO2 finden die elektronischen Übergänge vorwiegend lokal auf der CuO4-Plakette statt. Dagegen findet man in CuGeO3 angeregte Zustände, die als das Hüpfen der Elektronen auf benachbarte Plaketten interpretiert werden können. Das angewandte Cluster-Modell bestätigt für eine zunehmende Kopplung zwischen den Plaketten die in eckenverbundenen Kupratketten beobachtete Verschiebung des spektralen Gewichts vom hoch- zum niederenergetischen Bereich. Die Verlustfunktionen von NaV2O5 liefern wertvolle Informationen über die Freiheitsgrade und Korrelationen der Elektronen in einem viertelgefüllten Leitersystem, die wesentlich den Phasenübergang zwischen geordneter und ungeordneter Ladung bei 34 K bestimmen. Die Beiträge der 3d-Elektronen von NaV2O5 zu den EELS-Spektren konnten durch eine vergleichende Studie der Verlustfunktionen des strukturell verwandten V2O5, das keine d-Elektronen besitzt, separiert werden. Die Beschreibbarkeit der Elektronenstruktur in NaV2O5 durch ein effektives Modell einfach besetzter, molekülähnlicher V-O-V-Sprossen wird bestätigt. Die Coulomb-Wechselwirkung U kann in diesem Modell auf den Wertebereich zwischen 2 und 3 eV eingeschränkt werden. Im Gegensatz zu den stark anisotropen Hüpfwahrscheinlichkeiten in der Leiterebene sind die Coulomb-Wechselwirkungen V zwischen Elektronen auf benachbarten Vanadiumplätzen nahezu von gleicher Größe. Diese Wechselwirkungen sind die treibende Kraft für den Übergang von einem ungeordneten Zustand bei Raumtemperatur in einen zickzackgeordneten Grundzustand bei tiefen Temperaturen
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Atzkern, Stefan. "Elektronen-Energieverlustspektroskopie von quasi-eindimensionalen Kupraten und Vanadaten." Doctoral thesis, Technische Universität Dresden, 2001. https://tud.qucosa.de/id/qucosa%3A24117.
Full textAbstract:
This work presents a joint theoretical and experimental investigation of the electronic structure of quasi one-dimensional cuprates and vanadates. Electron energy-loss spectroscopy in transmission was employed to measure the momentum-dependent loss function of Li2CuO2, CuGeO3, V2O5 and NaV2O5. The comparison between the experimental data and the results from bandstructure as well as cluster calculations allows an explanation of the mobility and correlations of the electrons in these systems. The investigation of the electronic structure of the structurally related cuprates Li2CuO2 and CuGeO3 is exemplary for the study of the transition from a quasi zero-dimensional to a quasi one-dimensional system. In contrast to Li2CuO2 where the electron transitions are strongly localized, the excited states in CuGeO3 can be assigned to the electron hopping to the nearest-neighboured CuO4 plaquettes. The shift of spectral weight from the high energy to the low energy region with increasing coupling between the plaquettes, observed in edge-sharing CuO2 chains, is confirmed by the applied cluster modell. The momentum dependent loss functions of NaV2O5 deliver information about the mobility and correlations of electrons in a quarter-filled ladder system which determine the transition from the charge ordered state into the unordered state at 34 K. Thcontributions of the 3d electrons to the EELS spectra of NaV2O5 are filtered by comparing these spectra with the loss functions of the structurally related V2O5 (d0 configuration). For NaV2O5 the picture of linear chains of V-O-V rungs containing a single d electron in a molecular orbital-like state is confirmed. The comparison of the experimentally determined optical conductivities and those derived from the bandstructrure calculations yield a good agreement upon adoption of an on-site Coulomb interaction U = 2-3 eV. In contrast to the strongly anisotropic hopping within the ladder plane the intersite Coulomb interactions V are about the same size. These interactions are the driving force for the transition from an unordered state at room temperature into a zigzag ordered state observed at low temperatures.In einer Kombination aus experimentellen und theoretischen Methoden wurden in dieser Arbeit die Elektronenstrukturen von quasi-eindimensionalen Kupraten und Vanadaten untersucht. Dazu wurde die impulsabhängige Verlustfunktion mit Hilfe der Elektronen-Energieverlustspektroskopie in Transmission an Einkristallen von Li2CuO2, CuGeO3, V2O5 und NaV2O5 gemessen. Der Vergleich der experimentellen Daten mit Ergebnissen aus Bandstruktur- und Cluster-Rechnungen erlaubte Rückschlüsse auf die Beweglichkeit und Korrelationen der Elektronen in diesen Systemen. Die Untersuchung der elektronischen Anregungen in den strukturell sehr ähnlichen Kupraten Li2CuO2 und CuGeO3 ist beispielhaft für das Studium des Übergangs von einem quasi-nulldimensionalen zu einem quasi-eindimensionalen System. In Li2CuO2 finden die elektronischen Übergänge vorwiegend lokal auf der CuO4-Plakette statt. Dagegen findet man in CuGeO3 angeregte Zustände, die als das Hüpfen der Elektronen auf benachbarte Plaketten interpretiert werden können. Das angewandte Cluster-Modell bestätigt für eine zunehmende Kopplung zwischen den Plaketten die in eckenverbundenen Kupratketten beobachtete Verschiebung des spektralen Gewichts vom hoch- zum niederenergetischen Bereich. Die Verlustfunktionen von NaV2O5 liefern wertvolle Informationen über die Freiheitsgrade und Korrelationen der Elektronen in einem viertelgefüllten Leitersystem, die wesentlich den Phasenübergang zwischen geordneter und ungeordneter Ladung bei 34 K bestimmen. Die Beiträge der 3d-Elektronen von NaV2O5 zu den EELS-Spektren konnten durch eine vergleichende Studie der Verlustfunktionen des strukturell verwandten V2O5, das keine d-Elektronen besitzt, separiert werden. Die Beschreibbarkeit der Elektronenstruktur in NaV2O5 durch ein effektives Modell einfach besetzter, molekülähnlicher V-O-V-Sprossen wird bestätigt. Die Coulomb-Wechselwirkung U kann in diesem Modell auf den Wertebereich zwischen 2 und 3 eV eingeschränkt werden. Im Gegensatz zu den stark anisotropen Hüpfwahrscheinlichkeiten in der Leiterebene sind die Coulomb-Wechselwirkungen V zwischen Elektronen auf benachbarten Vanadiumplätzen nahezu von gleicher Größe. Diese Wechselwirkungen sind die treibende Kraft für den Übergang von einem ungeordneten Zustand bei Raumtemperatur in einen zickzackgeordneten Grundzustand bei tiefen Temperaturen.