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81	Transport phenomena in quasi-one-dimensional heterostructures Dias, Mariama Rebello de Sousa 21 February 2014 (has links) Made available in DSpace on 2016-06-02T20:15:31Z (GMT). No. of bitstreams: 1 5844.pdf: 11430873 bytes, checksum: b80a5790a9ebf6ae63ff48e52968ae60 (MD5) Previous issue date: 2014-02-21 / Universidade Federal de Sao Carlos / O crescimento e caracterização de sistemas de heteroestruturas semicondutoras quasi-unidimensionais têm atraído grande interesse devido à sua potencial de aplicação tecnológica, como foto-detectores, dispositivos opto-eletrônicos assim como seu para o processamento de informação quântica e aplicações em fotônica. O objetivo desta tese é o estudo das propriedades de transporte eletrônico e de spin em sistemas semicondutores quasi-unidimensionais, especificamente trataremos de nanofios (NWs) homogêneos, NWs acoplados, NWs do tipo plano-geminado (TP), diodos de tunelamento ressonante (ETD) e cadeias de pontos quânticos (QDCS). Escolhemos o método k-p, particularmente o Hamiltoniano de Luttinger, para descrever os efeitos de confinamento e tensão biaxial. Este sugeriu uma modulação do caráter do estado fundamental que, complementada com a dinâmica fônons fornecidas pelas simulações da Dinâmica Molecular (MD), permitiu a descrição da modulação da mobilidade de buracos por emissão ou absorção de fônons. Em relação ao sistema de NWs acoplado,estudamos, através do método da matriz de transferência (TMM), as propriedades de transporte de elétrons e spin sob a interação de spin-órbita (SOI) de Eashba, localizada na região de acoplamento entre fios. Foram consideradas várias configurações de tensões de gate (Vg) aplicadas nos fios. Desse modo, compreendemos a modulação do transporte de spin quando esse é projetado no direção-z através da combinação do SOI e das dimensionalidades do sistema. Da mesma forma, a combinação de SOI e da Vg aplicada deu origem a modulação da polarização, quando o spin medido é projetado na mesma direção em que o SOI de Eashba atua, a direção y. Usando o TMM, exploramos as propriedades de transporte de um DBS e o efeito de uma resistência em série com o intuito de provar a natureza da biestabilidade das curvas características I V bem como o aumento de sua área com temperatura, resultados fornecidos por experimentos. O modelo indicou que aumentando da resistência pela diminuição sa temperatura aumenta a área biestável. A presença de uma hetero-junção adicional ao sistema induz uma densidade de carga nas suas interfaces. De acordo com esta configuração, a queda de tensão total do ETDS muda, podendo ser confirmada experimentalmente. A formação dos peculiares campos de deformação e sua influência sobre a estrutura eletrônicas e propriedades de transporte em superredes de TP foi estudada sistematicamente. Assim, as propriedades de transporte, de ambos os elétrons e buracos, pode ser sintonizada eficientemente, mesmo no caso de elétrons r em sistemas de blenda de zinco, contrastando com a prevista transparência de elétrons r em superredes de semicondutores III-V heteroestruturados. Além disso, constatamos que a probabilidade de transmissão para buracos da banda de valência também poderia ser efetivamente modificada através de uma tensão externa.Por fim, colaboradores sintetizaram com sucesso sistemas de QDCs de InGaAs através da epitaxia de feixe molecular e engenharia de tensão. Um comportamento anisotrópico da condutância com a temperatura foi observado em QDCs com diferentes concentrações de dopagem, medida realizada ao longo e entre os QDCs. O modelo teórico 1D de hoppíng desenvolvido mostrou que a presença de estados OD modela a resposta anisotrópica da condutância neste sistemas. / The growth and characterization of semiconductor quasi-one-dimensional heterostructure systems have attracted increasing interest due to their potential technological application, like photo-detectors, optoelectronic devices and their promising features for quantum information processing and photonic applications. The goal of this thesis is the study of electronic and spin transport properties on quasi-one-dimensional semiconductor systems; specifically, homogenous nanowires (NWs), coupled NW s, twin-plane (TP) NWs, resonant tunneling diodes (RTDs), and quantum dot chains (QDCs). The k-p method, in particular the Luttinger Hamiltonian, was chosen to describe the effects of biaxial confinement and strain. This suggested a modulation of the ground state character that, complemented with the phonon dynamics provided by Molecular Dynamics (MD) simulations, allowed the description of the hole mobility modulation by either phonon emission or absorption. Regarding the coupled NW s system, the electron and spin transport properties affected by a Rashba spin-orbit interaction (SOI) at the joined region were unveiled through the Transfer Matrix Method (TMM). Various configurations of gate voltages (Vg), applied on the wire structure, were considered. We were able to understand the modulation of the spin transport projected in the z-direction trough the combination of the SOI and the system dimensionalities. Likewise, the combination of SOI and applied Vg gave rise to a modulation of the polarization, when the measured spin is projected in the same direction where the Rashba SOI acts, the y-direction. The transport properties of a DBS and the effect of a resistance in series was explored within the TMM to prove the nature of a bistability of the I V characteristics and its enhanced area with temperature provided by the experiment. The model indicates that increasing the resistente by decreasing the temperature, the bistable area enhances. The presence of an additional heterojunction induces a sheet charge at its interfaces. Under this configuration, the total voltage drop of the RTD changes and can be confirmed experimentally.The formation of the peculiar strain fields and their influence on the electronic structure and transport properties of a TP superlattice was systematically studied. Hence, the transport properties of both electrons and holes could be effectively tuned even in the case of T-electrons of zincblende systems, contrasting to the predicted transparency of T-electrons in heterolayered III-V semiconductor superlattices. Also, the transmission probability for holes at valence band could also be effectively modified by applying an external stress. Finally, using molecular-beam-epitaxy and skillful strain engineering, systems of In-GaAs QDCs were successfully synthesized by collaborators. The QDCs with different doping concentrations showed an anisotropic behavior of the conductance, measured along and across the QDCs, with temperature. The theoretical ID hopping model developed found that the presence of OD states shapes the anisotropic response of the conductance in this system. Transport phenomena Spin-orbit interaction K-p method Transfer matrix method Electron-phonon interaction Semiconductor nanowires Twin-plane nanowires Resonant tunneling diodes Quantum dot chains Fenômenos de transporte Interação spin-órbita Método de matriz transferência Método k-p Interação elétron-fônon Nanofios semicondutores Nanofios de planos-geminados Diodos de tunelamento ressonante Cadeias de pontos quânticos CIENCIAS EXATAS E DA TERRA::FISICA
82	Polaritons de exciton em super-redes semicondutoras Medeiros, F?bio Ferreira de 03 December 2004 (has links) Made available in DSpace on 2014-12-17T15:15:02Z (GMT). No. of bitstreams: 1 FabioFM.pdf: 1459636 bytes, checksum: 4f2bb557c6a5bddb80e46f325bd5b4dd (MD5) Previous issue date: 2004-12-03 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / In this work we study the spectrum (bulk and surface modes) of exciton-polaritons in infinite and semi-infinite binary superlattices (such as, ???ABABA???), where the semiconductor medium (A), whose dielectric function depends on the frequency and the wavevector, alternating with a standard dielectric medium B. Here the medium A will be modeled by a nitride III-V semiconductor whose main characteristic is a wide-direct energy gap Eg. In particular, we consider the numerical values of gallium nitride (GaN) with a crystal structure wurtzite type. The transfer-matrix formalism is used to find the exciton-polariton dispersion relation. The results are obtained for both s (TE mode: transverse electric) and p (TM mode: transverse magnetic) polarizations, using three diferent kind of additional boundary conditions (ABC1, 2 e 3) besides the standard Maxwell's boundary conditions. Moreover, we investigate the behavior of the exciton-polariton modes for diferent ratios of the thickness of the two alternating materials forming the superlattice. The spectrums shows a confinement of the exciton-polariton modes due to the geometry of the superlattice. The method of Attenuated Total Reflection (ATR) and Raman scattering are the most adequate for probing this excitations / Neste trabalho estudamos o espectro (modos de volume e de superf?cie) dos polaritons de exciton em uma super-rede bin?ria infinita e semi-infinita (tal como, ???ABABA???), onde um meio semicondutor (A), cuja fun??o diel?trica depende da frequ?ncia e do vetor de onda, alterna-se com um diel?trico comum (B). Aqui, o meio A ser? modelado por um semicondutor da fam?lia dos nitretos (semicondutor III-V) que tem como caracter?stica principal um gap de energia (Eg) direto e largo. Em particular, consideramos os valores num?ricos para o nitreto de g?lio (GaN) com uma estrutura cristalina tipo wurtzite. A t?cnica da matriz de transfer?ncia ? utilizada para encontrarmos a rela??o de dispers?o do polariton de exciton. Os resultados s?o obtidos para os modos de polariza??o s (ou modo TE: transversal el?trico) e p (ou modo TM: transversal magn?tico), usando tr?s diferentes condi??es de contorno adicionais (ABC1, 2 e 3), mais as condi??es de contorno padr?es de Maxwell. Al?m disso, investigamos o comportamento dos modos do polariton de exciton para diferentes raz?es entre as espessuras das camadas dos dois materiais que comp?em a super-rede Os espectros encontrados evidenciam um comportamento de confinamento dos polaritons de exciton devido ?s geometrias empregadas. As t?cnicas experimentais ATR ("Attenuated Total Reflection") e o espalhamento Raman s?o as mais adequadas para a caracteriza??o dessas excita??es Exciton de Frenkel Exciton de Wannier-Mott Polaritons de exciton (PE) Super-redes quasiperi?dicas Formalismo da matriz de transfer?ncia Exciton of frenkel Exciton of wannier-mott Exciton-polariton Transfer-matrix formalism Exciton-polariton dispersion relation Nitride CNPQ::CIENCIAS EXATAS E DA TERRA::FISICA
83	Near-infrared plasmonics in planar tunable structures Travkin, Evgenij 21 June 2023 (has links) In dieser Arbeit werden planare plasmonische Schichtsysteme unter Verwendung der transparenten leitfähigen Oxide (TCOs) Zinkgalliumoxid (GZO) und Indiumzinnoxid (ITO) untersucht, die mittels Molekularstrahlepitaxie realisiert werden. Es wird gezeigt, dass solche hochdotierten Schichten aus GZO und ITO sich wie ein Drude-Metall mit einer einstellbaren Plasmafrequenz verhalten und Oberflächenplasmon-Polaritonen (SPPs) aufweisen, die über einen breiten NIR-Spektralbereich abstimmbar sind. Die TCOs können in mehreren Schichten mit unabhängig voneinander einstellbaren Dicken und Dotierungen der einzelnen Schichten gezüchtet werden. Diese abstimmbaren Mehrschichtstrukturen ermöglichen die Realisierung plasmonischer Konfigurationen, die für eine Vielzahl komplexer hybridisierter SPP-Zustände maßgeschneidert sind. Unter anderem wird unter Ausnutzung der Photon-Plasmon-Hybridisierung ein Stopped-Light-Resonator auf Basis von ITO demonstriert. Das Mehrschichtenregime kann zu einem Übergitter aus periodisch abwechselnd dotierten und undotierten TCO-Schichten erweitert werden, das ein hyperbolisches Metamaterial (HMM) darstellt. Die Parameter dieses HMM können nach Bedarf eingestellt werden, was HMMs mit einer maßgeschneiderten Zusammensetzung ihrer einzigartigen spektralen Permittivitätsintervalle ermöglicht. Mithilfe von GZO wird ein HMM in eine planare optische Mikrokavität monolitisch eingebettet. Dieser neuartige NIR-Resonator weist eine anomale Modendispersion auf, einschließlich einem Kontinuum von Moden hoher Ordnung und einer von der Resonatorlänge unabhängigen Mode nullter Ordnung, welche Subwellenlängen-Resonanzen ermöglichen können. Es wird gezeigt, dass die Mode nullter Ordnung bei einer Kavitätslänge deutlich unterhalb ihrer Wellenlänge fortbesteht und ihre Dispersion durch den Füllfaktor des HMM steuerbar ist. Die Ergebnisse stellen somit ein neues allgemeines Konzept für die Realisierung eines Subwellenlängenresonators auf der Basis eines abstimmbaren HMM dar. / In this work, planar, layered plasmonic systems utilizing the transparent conducting oxides (TCOs) zinc gallium oxide (GZO) and indium tin oxide (ITO) facilitated by molecular beam epitaxy are investigated. It is shown that such highly doped layers of GZO and ITO prepared with behave as a Drude metal with a tunable plasma frequency and feature surface plasmon polaritons (SPPs) that are tunable over a broad NIR spectral range. TCOs can be grown in the multilayer regime with independently adjusted thicknesses and doping levels of the individual layers. These tunable multilayer structures allow for the realization of plasmonic configurations tailored to support a variety of intricate hybridized SPP states. Particularly, exploiting photon-plasmon hybridization, a stopped-light cavity is demonstrated using highly doped ITO. The multilayer regime can be extended into a superlattice of periodically alternating doped and undoped TCO layers that constitutes a hyperbolic metamaterial (HMM). The parameters of such an HMM can be set on-demand, thus allowing HMMs with a tailored composition of its unique spectral permittivity intervals. Utilizing GZO, an HMM is embedded in a planar optical microcavity monolithically. This novel type of a NIR optical resonator exhibits an anomalous resonant mode dispersion, including features like a high-order mode continuum and a cavity size independent zeroth-order mode, which can enable subwavelength resonances. It is demonstrated that the zeroth-order mode persists at cavity sizes significantly below its wavelength and its dispersion can be controlled by the fill factor of the HMM. Thus, the results propose a novel general concept for the realization of a subwavelength resonator on the basis of a tunable HMM. Physik Plasmonik Photonik Oberflächenplasmon-Polariton SPP transparente leitfähige Oxide TCO hyperbolisches Metamaterial HMM optische Kavität Subwellenlänge Nahinfrarot NIR FTIR Transfermatrix physics plasmonics photonics surface plasmon polariton SPP TCO hyperbolic metamaterial HMM optical cavity subwavelength near-infrared NIR fourier-transform infrared spectroscopy FTIR transfer matrix transparent conducting oxides 530 Physik ddc:530
84	Models of chromosome architecture and connection with the regulation of genetic expression / Modèles de l'architecture du chromosome et lien avec la régulation de l'expression génétique Le Treut, Guillaume 29 November 2016 (has links) Plusieurs indices suggèrent que le repliement du chromosome et la régulation de l’expression génétique sont étroitement liés. Par exemple, la co-expression d’un grand nombre de gènes est favorisée par leur rapprochement dans l’espace cellulaire. En outre, le repliement du chromosome permet de faire émerger des structures fonctionnelles. Celles-ci peuvent être des amas condensés et fibrillaires, interdisant l’accès à l’ADN, ou au contraire des configurations plus ouvertes de l’ADN avec quelques amas globulaires, comme c’est le cas avec les usines de transcription. Bien que dissemblables au premier abord, de telles structures sont rendues possibles par l’existence de protéines bivalentes, capable d’apparier des régions parfois très éloignées sur la séquence d’ADN. Le système physique ainsi constitué du chromosome et de protéines bivalentes peut être très complexe. C’est pourquoi les mécanismes régissant le repliement du chromosome sont restés majoritairement incompris.Nous avons étudié des modèles d’architecture du chromosome en utilisant le formalisme de la physique statistique. Notre point de départ est la représentation du chromosome sous la forme d’un polymère rigide, pouvant interagir avec une solution de protéines liantes. Les structures résultant de ces interactions ont été caractérisées à l’équilibre thermodynamique. De plus, nous avons utilisé des simulations de dynamique Brownienne en complément des méthodes théoriques, car elles permettent de prendre en considération une plus grande complexité dans les phénomènes biologiques étudiés.Les principaux aboutissements de cette thèse ont été : (i) de fournir un modèle pour l’existence des usines de transcriptions caractérisées in vivo à l’aide de microscopie par fluorescence ; (ii) de proposer une explication physique pour une conjecture portant sur un mécanisme de régulation de la transcription impliquant la formation de boucles d’ADN en tête d’épingle sous l’effet de la protéine H-NS, qui a été émise suite à l’observation de ces boucles au microscope à force atomique ; (iii) de proposer un modèle du chromosome qui reproduise les contacts mesurés à l’aide des techniques Hi-C. Les conséquences de ces mécanismes sur la régulation de la transcription ont été systématiquement discutées. / Increasing evidences suggest that chromosome folding and genetic expression are intimately connected. For example, the co-expression of a large number of genes can benefit from their spatial co-localization in the cellular space. Furthermore, functional structures can result from the particular folding of the chromosome. These can be rather compact bundle-like aggregates that prevent the access to DNA, or in contrast, open coil configurations with several (presumably) globular clusters like transcription factories. Such phenomena have in common to result from the binding of divalent proteins that can bridge regions sometimes far away on the DNA sequence. The physical system consisting of the chromosome interacting with divalent proteins can be very complex. As such, most of the mechanisms responsible for chromosome folding and for the formation of functional structures have remained elusive.Using methods from statistical physics, we investigated models of chromosome architecture. A common denominator of our approach has been to represent the chromosome as a polymer with bending rigidity and consider its interaction with a solution of DNA-binding proteins. Structures entailed by the binding of such proteins were then characterized at the thermodynamical equilibrium. Furthermore, we complemented theoretical results with Brownian dynamics simulations, allowing to reproduce more of the biological complexity.The main contributions of this thesis have been: (i) to provide a model for the existence of transcrip- tion factories characterized in vivo with fluorescence microscopy; (ii) to propose a physical basis for a conjectured regulatory mechanism of the transcription involving the formation of DNA hairpin loops by the H-NS protein as characterized with atomic-force microscopy experiments; (iii) to propose a physical model of the chromosome that reproduces contacts measured in chromosome conformation capture (CCC) experiments. Consequences on the regulation of transcription are discussed in each of these studies. Physique statistique Physique des polymères Chaîne gaussienne Chaîne de Kratky-Porod Théorie de Flory-Huggins Random phase approximation Phases de l’ADN Fonction de structure Matrice de transfert Dynamique browniennne Probabilité de contact Protéine se liant à l’ADN Architecture du chromosome Repliement du chromosome Dynamique du chromosome Co-localisation des gènes Usine à transcription Régulation de la transcription Chromosome conformation capture Statistical physics Polymer physics Gaussian chain Worm-like chain Flory-Huggins theory Random phase approximation DNA phases Structure function Transfer matrix Brownian dynamics Contact probability DNA-binding protein Chromosome architecture Chromosome folding Chromosome dynamics Gene co-localization Transcription factory Transcription regulation Chromosome conformation capture
85	Channel Probing for an Indoor Wireless Communications Channel Hunter, Brandon 13 March 2003 (has links) (PDF) The statistics of the amplitude, time and angle of arrival of multipaths in an indoor environment are all necessary components of multipath models used to simulate the performance of spatial diversity in receive antenna configurations. The model presented by Saleh and Valenzuela, was added to by Spencer et. al., and included all three of these parameters for a 7 GHz channel. A system was built to measure these multipath parameters at 2.4 GHz for multiple locations in an indoor environment. Another system was built to measure the angle of transmission for a 6 GHz channel. The addition of this parameter allows spatial diversity at the transmitter along with the receiver to be simulated. The process of going from raw measurement data to discrete arrivals and then to clustered arrivals is analyzed. Many possible errors associated with discrete arrival processing are discussed along with possible solutions. Four clustering methods are compared and their relative strengths and weaknesses are pointed out. The effects that errors in the clustering process have on parameter estimation and model performance are also simulated. multipath multipath propagation propagation environment indoor propagation indoor multipath indoor multipath propagation channel probing channel probing indoor wireless communications channel indoor wireless communications indoor wireless communications wireless communications wireless communications channel MIMO SISO multiple input multiple output single input single output antenna diversity antenna diversity antenna array channel modeling modeling model wireless propagation clustering methods clustering algorithms clustering parameter estimation parameter channel matrix h matrix ray tracing point spread function psf deconvolution clean algorithm clean algorithm antenna pattern network analyzer temporal pattern 2.4 ghz system 6 ghz system channel capacity capacity estimate channel capacity estimate propagation environment scattering transfer matrix matrix water filling solution water filling method water filling Saleh Valenzuela Saleh and Valenzuela Model arrival time of arrival angle of arrival direction of arrival arrival amplitude arrivals antenna positioner data simulations measurements angle of departure direction of departure correlation convolution errors error CFA CFAD constant false alarm constant false alarm detection signal signal dependent clutter clutter returns transmitter transmit receive receiver transmit antenna receive antenna spatial correlation system PSF system point spread function mapping fuzzy c-means clustering fuzzy clustering c-means clustering c-means fuzzy k-means euclidean distance time amplitude delay performance prediction performance prediction errors fuzzy weighted distance method rectangular method weighted rectangular method statistics statistical statistical model Electrical and Computer Engineering

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