<p> Semiconductor quantum wire (QWR) structure is a promising candidate for potential applications in long wavelength laser devices. In this thesis, the investigations were focused on the growth and characterization on the structural and optical properties of InAs quantum wires deposited on InGaAlAs lattice matched with InP substrate by gas source molecular beam epitaxy. </p> <P> The practical growth parameters were first determined by studying the samples containing single InAs layer embedded within Ino.s3Gll{)_37Alo.10As barrier layers. These parameters were then employed for fabricating multilayer quantum wires with different (1) spacer layer thicknesses; (2) quantum wire layer thicknesses; and (3) different Al concentrations in the spacer/barrier layer materials. </P> <P>Structural properties of the quantum wires were characterized by (scanning) transmission electron microscopy based techniques. The composition variation, elastic field and the variation of QWR stacking patterns in multilayer samples were qualitatively studied through diffraction contrast imaging. Quantification of the In distribution in individual QWRs and the QWR-induced In composition modulation in barrier layers were obtained by electron energy loss spectrometry and energy dispersive X-ray spectrometry, respectively. These experimentally observed structural features were explained through finite element simulations. </P> <P> The optical properties of the QWR structures were studied by photoluminescence. Optical emission at room temperature was achieved from selected multilayer QWR samples after etching and rapid thermal annealing. The emission wavelength ranging from 1.53 to 1.72 μm makes the QWR structure suitable candidates for laser device applications. </P> / Thesis / Doctor of Philosophy (PhD)

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No description available.

<p>The heteroepitaxial growth of InAs self-assembled quantum wires on vicinal substrates is investigated. InGaAlAs lattice-matched to InP was first deposited onto an InP(001) substrate with and without a 0.9 degree off-cut toward the (110) direction, followed by the deposition of a strained layer of InAs. Dense InAs quantum wires were successfully grown on both nominally flat and vicinal substrates in order to observe the effect of the presence of atomic steps. The off-cut angle was chosen based on the wire spacing on a flat substrate to serve as a template for their nucleation and improve their size distribution for use as 1.55 um wavelength lasers required by the telecommunications industry. The results have shown a modest but statistically significant improvement in the width of their size distribution. In addition, a kinetic Monte Carlo simulation including full strain calculations was developed to further understand the nucleation process. The model developed here disproves the idea that InAs quantum wires are aligned towards the (-110) direction due to diffusion anisotropy. The simulation of the formation of quantum wires similar to those observed experimentally has been achieved and the Stranski-Krastanow growth mode is demonstrated.</p> / Master of Materials Science and Engineering (MMatSE)

<p> The net ecosystem CO2 exchange and Sphagnum net primary production of an abandoned block-cut bog were measured in the field and in the laboratory using gas exchange techniques. Environmental variables were measured concurrently. Seasonal Sphagnum growth and decomposition were determined using crank wires and litter bags.</p> <p> The bog was a net source of CO2 during the summer months. While, Sphagnum did grow over the study period, the Sphagnum net primary production was offset by peat respiration. Sphagnum net primary production and net ecosystem CO2 exchange at the abandoned block-cut bog are within the ranges found for natural peatlands.</p> <p> Sphagnum photosynthesis was greatest at wet sites and at the start and end of the study period when the water table was higher and the temperature was cooler. Ericaceous shrub cover strongly affected Sphagnum growth. Sites with vascular plant cover photosynthesized at approximately twice the rate of sites where cover was removed.</p> <p> Laboratory results indicate that drying and wetting cycles negatively affect Sphagnum net primary production and net ecosystem CO2 exchange. Sphagnum and peat respiration increased 4 to 14 fold upon rewetting whereas Sphagnum photosynthesis did not recover until 20 days of saturation.</p> <p> The results emphasize the importance of stable moisture availability for the development of a new acrotelm. The peatland will likely remain a source of CO2 until the acrotelm is able to counterbalance the peat respiration.</p> / Thesis / Master of Science (MSc)

Seismic design of guyed masts, commonly used in the broadcasting and telecommunications industries, has not been fully addressed in the United States. There is no specific design code, and only a limited amount of research has been reported on the subject. This research investigates the behavior of guyed masts incorporating synthetic ropes as guys, with a particular focus on the effect of snap loads on the mast behavior. This is the third phase of a multi-stage project aimed at analyzing the potential for Snapping-Cable Energy Dissipators (SCEDs) to minimize lateral response in structures. A finite element model of a 120-m-tall guyed mast was developed with the commercial program ABAQUS. The three-dimensional behavior of the mast was observed when subjected to two ground motion records: Northridge and El Centro. Three orthogonal earthquake components were input, two horizontal and one vertical. A series of parametric studies was conducted to determine the sensitivity of the response to guy pretension, which is a measure of the potential slackness in the guys during response. Additionally, the studies examined the effects of guy stiffness, mast properties, and directionality of input motion. Deflections, bending moments, guy tensions, and base shears were examined. The results were used to characterize the trends in the structural response of guyed masts. The level of slackness in the guys changed the behavior, and the lessons learned will be used to continue research on the application of SCEDs in structures. / Master of Science

<p>In this work, we study two important themes in the physics of the interacting one-dimensional (1D) electron gas: the transition from one-dimensional to higher dimensional behavior, and the role of inhomogeneity. The interplay between interactions, reduced dimensionality, and inhomogeneity drives a rich variety of phenomena in mesoscopic physics. In 1D, interactions fundamentally alter the nature of the electron gas, and the homogeneous 1D electron gas is described by Luttinger Liquid theory. We use Quantum Monte Carlo methods to study two situations that are beyond Luttinger Liquid theory --- the quantum phase transition from a linear 1D electron system to a quasi-1D zigzag arrangement, and electron localization in quantum point contacts. </p><p>Since the interacting electron gas has fundamentally different behavior in one dimension than in higher dimensions, the transition from 1D to higher dimensional behavior is of both practical and theoretical interest. We study the first stage in such a transition; the quantum phase transition from a 1D linear arrangement of electrons in a quantum wire to a quasi-1D zigzag configuration, and then to a liquid-like phase at higher densities. As the density increases from its lowest values, first, the electrons form a linear Wigner crystal; then, the symmetry about the axis of the wire is broken as the electrons order in a quasi-1D zigzag phase; and, finally, the electrons form a disordered liquid-like phase. We show that the linear to zigzag phase transition occurs even in narrow wires with strong quantum fluctuations, and that it has characteristics which are qualitatively different from the classical transition.</p><p>Experiments in quantum point contacts (QPC's) show an unexplained feature in the conductance known as the ``0.7 Effect''. The presence of the 0.7 effect is an indication of the rich physics present in inhomogeneous systems, and we study electron localization in quantum point contacts to evaluate several different proposed mechanisms for the 0.7 effect. We show that electrons form a Wigner crystal in a 1D constriction; for sharp constriction potentials the localized electrons are separated from the leads by a gap in the density, while for smoother potentials, the Wigner crystal is smoothly connected to the leads. Isolated bound states can also form in smooth constrictions if they are sufficiently long. We thus show that localization can occur in QPC's for a variety of potential shapes and at a variety of electron densities. These results are consistent with the idea that the 0.7 effect and bound states observed in quantum point contacts are two distinct phenomena.</p> / Dissertation

The active response of a series of polymeric materials was investigated. Both solvent activated and light activated thin films and wire systems show dynamic behaviors when exposed to different stimuli. Solvent mediated fluxional behavior of polymer thin films involved extensive, rapid curling both on infusion and evaporation of good solvents. These films can be either lab-fabricated ones or commercial ones, and the curling behavior can be as fast as seconds. Conditions including polymer materials, chosen solvents, and film geometry can affect the behavior. Methods that allowed for the creation and retention of distorted wire structures were also developed; the asymmetric sputtering of metal components on micron-sized wires permitted for the capture of curled wire components on solvent exposure. The asymmetric metal coated wires which were fabricated within a template of glass capillary arrays (GCA) membrane have shown instant (< 1 s) deformation when exposed to the proper solvents. Deformed shapes can be retained or the original linear shape recovered, depending on the metal film thicknesses. Photostimulation of wires was also investigated and showed a notable dynamic response but not as extensive as with the solvent induced behavior. Micron sized wires made with azobenzene-polyacrylate, exhibited a bending behavior when irradiated with 365 nm UV light and recovered under visible light. The bending behavior can be as slow as several minutes per degree while recovery was relatively faster. Additional efforts with polymers involved the formation of polymeric organic-inorganic hybrids where organic monomers, grafted to perovskite layers, were polymerized within the oxide’s interlayers. Reactions were carried out on protonated perovskite, hydrogen lanthanum niobate (HLaNb2O7). Alcoxyl groups were first grafted to the oxide and the monomer was substituted by exchange reaction with the alcoxyl groups.

Estudamos as propriedades eletrônicas de dois sistemas quase-unidimensionais distintos, resolvendo autoconsistentemente as equações de Schrödinger e Poisson.O método usado para calcular a estrutura eletrônica deste sistema e baseada na solução da equação de Schrödinger dependente do tempo usando a técnica do Split-Operator. No primeiro sistema estudamos os efeitos da corrugação periódica da interface da estrutura n-AlxGa1-xAs/GaAs na densidade eletrônica ao longo desta interface. A forma geométrica desta interface e do tipo dente de serra. Nas camadas de inversão convencionais, os elétrons estão distribuídos uniformemente ao longo da interface plana da heteroestrutura, mas devido à forma dente de serra desta estrutura, os elétrons se distribuem de maneira não uniforme ao longo da interface, produzindo um gás de elétrons quase-unidimensional. A estrutura que investigamos possui um período de 806 ANGSTROM e uma densidade residual uniforme de impurezas aceitadoras da ordem de 1015 cm-3. Calculamos a estrutura eletrônica do gás de elétrons unidimensional confinado na interface corrugada em função da voltagem aplicada ao gate, da densidade de impurezas doadoras e da temperatura. Os resultados obtidos para a densidade eletrônica mostram que, dependendo da densidade de impurezas doadoras, haverá formação de u gás de elétrons quase-unidimensional nos vértices da estrutura dente de serra. O segundo sistema que estudamos é constituído por um gás de elétrons bidimensional, formado na interface de uma camada de Al1-xGa1-xAs com uma camada de GaAs, sobre a qual, temos uma estrutura periódica de \"gates\". Aplicando-se uma voltagem negativa sobre os \"gates\" teremos a formação de fios quânticos nas regiões entre os \"gates\". Neste sistema observamos a transição de um sistema quase-bidimensional para um quase-unidimensional. Investigamos suas propriedades eletrônicas em funçãoo da temperatura, da voltagem aplicada aos \"gates\" e da densidade de impurezas doadoras. / We have studied the electronic properties of two different quasi-one-dimensional systems solving self-consistently the Schrödinger and Poisson equation. The method we use to calculate the electronic levels is based on the solution of the time-dependent Schrödinger equation using the split-operator technique. In the first system we have studied, we present a theoretical calculation of the electronic structure of v-groove quantum wires confined in modulation-doped n-AlxGa1-xAs/GaAs. The system investigated is saw tooth corrugated by bendings with period of 850 ANGSTROM. Results of the electronic structure are obtained as a function of the gate voltage and the donor impurity density. The electronic density shows the existence of a quasi one-dimensional electron gas. The second system studied here is composed by a two-dimensional electron gas confined at the interface of an Al1-xGa1-xAs/GaAs heterostructure, on top of which there is a periodic structure of gates. When a negative voltage is applied to the gates, the regions at the interface beneath them are depleted and quantum wires are formed. We have calculated the electronic structure of subband of that system. We investigated the electronic properties of the quantum wires as a function of gate voltage, from which we determine the threshold between the 2D and ID transitions, the temperature and the ionized donor density.

La thèse, composée de trois parties, est consacrée à des problèmes physiques différents reliés à la Théorie Conforme des Champs (CFT) bidimensionnelle. La première partie s'intéresse aux propriétés de transport hors d'équilibre à travers une jonction de fils quantiques. Trois modèles y sont étudiés. Le premier décrit les fils par un champs bosonique libre compactifié vu comme la bosonisation du liquide de Luttinger d'électrons. La jonction des fils est modélisée par une condition limite assurant la diffusion non triviale des charges entre les fils. Associant la quantification canonique et l'intégrale fonctionnelle, on calcule exactement les fonctions de corrélation des courants dans l'état d'équilibre du modèle, mais aussi dans un état stationnaire hors d'équilibre, ainsi que la statistique complète de comptage pour les transferts de charge et d'énergie entre les fils maintenus en températures et potentiels différents. Les deux autres modèles d'une jonction de fils quantiques sont basés sur la théorie de Wess-Zumino-Witten (WZW). Dans le premier, la jonction est décrite par une "brane cyclique" et dans le deuxième, par une "brane coset". Les résultats dans le premier cas sont aussi complets que pour le champ libre, mais les charges y sont entièrement transmises d'un fils au suivant. Dans le deuxième cas, la diffusion des charges n'est pas triviale, mais le modèle se révèle difficile à résoudre. La deuxième partie de la thèse étudie les anomalies globales de jauge dans les modèles "coset" de CFT réalisés comme la théorie WZW jaugée. La classifications (presque) complète de telles anomalies, lesquelles rendent certains modèles coset inconsistants, est présentée. Elle emploie la classification des sous-algèbres des algèbres de Lie simples due à Dynkin. Finalement, la troisième partie de la thèse décrit la construction géométrique d'indice des familles d'opérateurs unitaires obtenues des projecteurs sur les bandes de valence d'un isolant topologique bidimensionnel invariant par renversement du temps. L'indice construit est relié d'un côté à la racine carrée de l'amplitude de Wess-Zumino d'une telle famille, et, de l'autre, il reproduit l'invariant de Kane-Mele de l'isolant. La dernière identification exige un argument complexe qui exploite une nouvelle anomalie de jauge pour les modèles WZW à bord. Les trois parties de la thèse emploient des outils géométriques de CFT assez semblables, permettant d'obtenir toute une série des résultats originaux. Cette unité de méthode, ainsi que le thème des anomalies, constituent le trait d'union entre les différents composants du manuscrit. / The thesis, consisting of three parts, is focusing on different physical problems that are related to two dimensional Conformal Field Theory (CFT).The first part deals with nonequilibrium transport properties across a junction of quantum wires. Three models are studied. The first one describes the wires by a free compactified bosonic field, seen as the bosonization of the Luttinger liquid of electrons. The junction of the wires is modeled by a boundary condition that ensures nontrivial scattering of the charges between the wires. Combining canonical quantization and functional integral, we compute exactly the current correlation functions in equilibrium, but also in a nonequilibrium stationary state, as well as the full counting statistics of charge and energy between the wires set at different temperatures and potentials. The two other models of quantum wire junction are based on Wess-Zumino-Witten theory (WZW). In the first one, the junction is described by a “cyclic brane” and in the second, by a “coset brane”. The results in the first case are as complete as for the free field, but the charges are fully transmitted from one wire to the next one. In the second case, the scattering is nontrivial, but the model turns out to be difficult to solve.The second part of the thesis studies the global gauge anomalies in “coset” models of CFT, realized as gauged WZW theories. The (almost) complete classification of such anomalies, that lead to some inconsistent coset models, is presented. It is based on Dynkin classification of subalgebras of simple Lie algebras.Finally, the third part of the thesis describes the geometric construction of index from unitary operator families obtained from valence band projectors of a two-dimensional time-reversal invariant topological insulator. The index is related on one hand to the square root of the Wess-Zumino amplitude of such a family, and, on the other hand, it reproduces the Kane-Mele invariant of the insulator. The last identification requires a nontrivial argument that uses a new gauge anomaly of WZW models with boundary.The three parts of the thesis use similar geometrical tool of CFT, that permits to obtain several original results. The unity in the method, as well as the topic of anomalies, builds a bridge between the different components of the manuscript.