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Orientador: Peter Alexander Bleinroth Schulz / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-11T10:20:11Z (GMT). No. of bitstreams: 1 CarrilloNunez_Hamilton_M.pdf: 5573082 bytes, checksum: 82c48bf8cb538a5a4c7444d18080814b (MD5) Previous issue date: 2008 / Resumo: Neste trabalho discutimos e comparamos diferentes definições de localização eletrônica em sistemas quase-1D: o caso estritamente 1D de cadeias de polianilinas e o DNA modelado como uma cadeia dupla. Em ambos os sistemas se estudou o comprimento de localização, obtido da condutância e do numero de participação, tentado estabelecer uma equivalência entre as duas quantidades. Para o sistema 1D a condutância foi obtida pelo método de matriz transferência. Entretanto, para o DNA a condutância se calcula usando o método recursivo das funções de Green, pois o método de matriz transferência para cadeias duplas apresenta instabilidades numéricas. O resultado obtido sugere um novo critério para analisar a extensão da função de onda em sistemas mesoscópicos dentro do regime difusivo de transporte como uma informação complementar para o comprimento de localização / Abstract: In this work we discuss and compare different definitions for localization of electronic states in quase-one-dimensional systems: the 1-D case of polianilines chains and DNA-like molecules. In order to establish ranges of equivalence, the localization length from both, the conductance and participation ratio, is computed. For the 1-D case the con-ductance is obtained by mean of the transfer matrix method, while the conductance for DNA-like double strands are calculated by mean of the recursive Green¿s function method since the transfer matrix method shows numerical instabilities. The final results suggest also criteria to infer the extension of wave function in mesoscopic systems with-in the diffusive transport regime as a complementary information to the localization length / Mestrado / Física da Matéria Condensada / Mestre em Física

In this dissertation, I have studied theoretical problems in statistical physics and electrodynamics of Bose particles, namely, mesoscopic effects in statistics of Bose- Einstein condensate (BEC) of atoms and electromagnetic waveguide effects of planar Bragg structures in Free Electron Lasers. A mesoscopic system of a trapped gas of Bose atoms is the most difficult for the theoretical analysis in quantum statistical physics since it cannot be studied by neither a quantum mechanics of the simple microscopic systems of one or very few atoms nor a standard statistical physics of the macroscopic systems that implies a thermodynamic limit. I present analytical formulas and numerical calculations for the moments and cumulants of BEC fluctuations in both ideal and weakly interacting gas. I analyze the universal scaling and structure of the BEC statistics in a mesoscopic ideal gas in the critical region. I present an exactly solvable Gaussian model of BEC in a degenerate interacting gas and its solution that confirms the universality and constraint-cut-off origin of the strongly non-Gaussian BEC statistics. I consider a two-energy-level trap with arbitrary degeneracy of an upper level and find an analytical solution for the condensate statistics in a mesoscopic ideal gas. I show how to model BEC in real traps by BEC in the two-level or three-level traps. I study wave propagation in the open oversized planar Bragg waveguides, in particular, in a planar metal waveguide with corrugation. I show that a step perturbation in a corrugation phase provides a high selectivity over transverse modes. I present a new Free Electron Laser (FEL) amplifier scheme, in which the radiation is guided by the planar Bragg structure with slightly corrugated walls and a sheet electron beam is traveling at a significant angle to the waveguide axis. By means of nonlinear analysis, I demonstrate that the proposed scheme provides an effective mode filtration and control over the structure of the output radiation and allows one to achieve amplification up to 30 dB in the existing FEL machines.

Submitted by Vasti Diniz (vastijpa@hotmail.com) on 2017-09-11T14:46:48Z No. of bitstreams: 1 arquivototal.pdf: 1205344 bytes, checksum: c7bac6a18f4623c77e994970333f09be (MD5) / Made available in DSpace on 2017-09-11T14:46:48Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 1205344 bytes, checksum: c7bac6a18f4623c77e994970333f09be (MD5) Previous issue date: 2017-02-23 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / The transport of electrons through nanostructures is an important achievement of quantum mechanics. In this dissertation, quantum transport will be explored through the statistic formalism of random scattering matrices, wich will be thoroughly explored in its own chapter. This formalism will then be adequated to be used in computer simulation to achieve some interesting results. We will study Landauer’s formulation for the conductance at low temperatures adapted to scattering matrices. Through this formulation, we will obtain conductance curves, autocorrelation curves, and finally we will analyze the width of the autocorrelation curves for different resonances. We will also study a specific process of generation of entanglement of formation, as defined byWooters, following a model created by Beenakker. Following the same methodology used for the conductance, we will use the scattering matrices formalism to obtain autocorrelation curves for entanglement and conductance. Exclusively for entanglement and concurrence, some new results will be presented through graphs and histograms of probability of entanglement and concurrence for some energies of interest. The results for both the conductance and the entanglement will be obtained through similar systems, consisting of an open quantum dot with two leads and four channels, studied according to the necessity of each formulation. / Uma realizac¸ ˜ao importante da mecˆanica quˆantica ´e o transporte de el´etrons atrav´es de nanoestruturas. Neste trabalho o transporte quˆantico ser´a explorado atrav´es do formalismo estat´ıstico de matrizes de espalhamento aleat´orias, que, por sua vez, ser´a detalhadamente explorado. Este formalismo ser´a ent˜ao adequado a algumas formulac¸ ˜oes para a obtenc¸ ˜ao de resultados de interesse atrav´es de simulac¸ ˜ao computacional. Ser´a estudada a formulac¸ ˜ao de Landauer para a condutˆancia `a baixas temperaturas adaptada para matrizes de espalhamento. Atrav´es desta formulac¸ ˜ao, ser˜ao obtidas curvas de condutˆancia, curvas de autocorrelac¸ ˜ao da condutˆancia e, por fim, ser´a feita uma an´alise da largura das curvas de autocorrelac¸ ˜ao para diferentes resson ˆancias. Ser´a estudado tamb´em um processo espec´ıfico de criac¸ ˜ao de emaranhamento de formac¸ ˜ao definido por Wooters, seguindo um modelo criado por Beenakker. Igualmente, ser´a utilizado o formalismo de matrizes de espalhamento para obtenc¸ ˜ao de curvas de autocorrelac¸ ˜ao do emaranhamento e da concorrˆencia. No caso do emaranhamento e da concorrˆencia ser˜ao apresentados alguns resultados in´editos atrav´es de gr´aficos e histogramas de probabilidade de emaranhamento e concorrˆencia para energias de interesse. Os resultados ser˜ao obtidos atrav´es de sistemas similares, tratando de um ponto quˆantico aberto com dois guias e quatro canais, estudados de acordo com a necessidade de cada formulac¸ ˜ao.

Made available in DSpace on 2015-05-14T12:13:59Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 283471 bytes, checksum: 3c03d229984755f09c77c7e98c3f465e (MD5) Previous issue date: 2011-06-30 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / In this work we present a quantum description of a mesoscopic RLC circuit without source. For this purpose, we model this system for that of a damped harmonic oscillator which is described by the Caldirola-Kanai Hamiltonian. Then, with the aid of the quantum invariant method we solve the Schrödinger equation associated with this Hamiltonian and write the corresponding wave functions in terms of a particular solution of the Milne-Pinney equation. We also construct coherent states for the RLC quantized, and evaluate the quantum fluctuations of the charge and the magnetic flux, as well as the corresponding product of uncertainty. / Nesta dissertação, apresentamos uma descrição quântica de um circuito RLC mesoscópico sem fonte. Com esta finalidade, modelamos este sistema para aquele de um oscilador harmônico amortecido, que é descrito pelo Hamiltoniano de Caldirola-Kanai. Então, com a ajuda do método de invariantes quânticos, resolvemos a equação de Schrödinger para este Hamiltoniano e escrevemos as funções de onda correspondentes em termos da solução particular da equação de Milne-Pinney. Também construímos estados coerentes para o circuito RLC quantizado, e calculamos as flutuações quânticas da carga e do fluxo magnético, bem como o produto de incerteza correspondente.

The study of quantum interference in solid-state systems yields insight in fundamental properties of mesoscopic systems. Electron quantum interference constitutes an important method to explore mesoscopic physics and quantum decoherence. This dissertation focuses on two-dimensional (2D) electron systems in $delta-$Si doped n-type In$_{0.64}$Ga$_{0.36}$As/In$_{0.45}$Al$_{0.55}$As, 2D hole systems in Si-doped p-type GaAs/Al$_{0.35}$Ga$_{0.65}$As and C-doped p-type GaAs/\Al$_{0.24}$Ga$_{0.76}$As heterostructures. The low temperature experiments study the magnetotransport of nano- and micro-scale lithographically defined devices fabricated on the heterostructures. These devices include a single ring interferometer and a ring interferometer array in 2D electron system, Hall bar geometries and narrow wires in 2D hole systems. The single ring interferometer yields pronounced Aharonov-Bohm (AB) oscillations with magnetic flux periodicity of h/e over a wide range of magnetic field. The periodicity was confirmed by Fourier transformation of the oscillations. The AB oscillation amplitude shows a quasi-periodic modulation over applied magnetic field due to local magnetic flux threading through the interferometer arms. Further study of current and temperature dependence of the amplitude of the oscillations indicates that the Thouless energy forms the measure of excitation energies giving quantum decoherence. An in-plane magnetic field was applied to the single ring interferometer to study the Berry's phase and the Aharonov-Casher effect. The ring interferometer array yields both AB oscillations and Altshuler-Aronov-Spivak (AAS) oscillations, the latter with magnetic flux periodicity of h/2e. The AAS oscillations require time-reversal symmetry and hence can be used to qualify time-reversal symmetry breaking. More importantly, the fundamental mesoscopic dephasing length associated with time-reversal symmetry breaking under applied magnetic field, an effective magnetic length, can be obtained by the analysis of the AAS oscillations over magnetic field. A theoretical model for confined ballistic system is confirmed by experimental data fitting. The AAS oscillations are barely resolved above 0.16 T and their amplitude decays with increasing magnetic field. The AB oscillations exist till above 2 T and their amplitude doesn't show the monotonic decay with increasing magnetic field. The different behavior of the AAS and AB oscillations originates in the different symmetries, respectively temporal and spatial, that they are sensitive to. The p-type 2D GaAs system has strong spin-orbit interaction (SOI). Antilocalization in a Hall bar geometry was analyzed by the 2D Hikami-Larkin-Nagaoka (HLN) theory to obtain the spin coherence time and phase coherence time. The 2D hole systems we studied have low density and high mobility, quite different from the 2D electron systems. These high-quality 2D hole systems demonstrate semi-classical ballistic phenomena in mesoscopic structures preferentially to quantum-coherence phenomena. / Ph. D.

Nous avons étudié la formation de nanogaps par électromigration dans des nanofils d'or. Cette technique consiste à provoquer la rupture d'un nanofil en lui appliquant de fortes densités de courant et peut être utilisée pour la caractérisation électrique de nano-objets. L'étude en temps réel du processus d'électromigration par microscopie à force atomique a permis d'apporter un éclairage nouveau de la dynamique du processus. En effet, il apparaît que la structure globale du dispositif est définie dans les premiers temps de l'électromigration et nous avons montré que cette structure est directement liée à la microstructure du film métallique. Pour la première fois, des nanogaps ont été élaborés par électromigration dans des films monocristallins. Malgré l'absence de joints de grain, il est possible de former des nanogaps dans un matériau épitaxié. L'utilisation de ces matériaux permet d'obtenir des nanogaps avec une morphologie plus reproductible. Les propriétés de transports des nanogaps obtenus à partir de films polycristallins ont été caractérisées. Les caractéristiques obtenues présentent toutes des signatures particulières, attribuées à la présence d'agrégats d'or provenant de la procédure d'électromigration et/ou de polymères issus du procédé de nanofabrication. Ces résultats montrent la difficulté à réaliser des mesures à l'échelle de la molécule unique.

<p>This thesis addresses electron spin phenomena in semi-conductor quantum dots/anti-dots from a computational perspective. In the first paper (paper I) we have studied spin-dependent transport through open quantum dots, i.e., dots strongly coupled to their leads, within the Hubbard model. Results in this model were found consistent with experimental data and suggest that spin-degeneracy is lifted inside the dot – even at zero magnetic field.</p><p>Similar systems were also studied with electron-electron effects incorporated via Density Functional Theory (DFT) in paper III. Within DFT we found a significant spin-polarisation in the dot at low electron densities. As the electron density increases the spin polarisation in the dot gradually diminishes. These findings are consistent with available experimental observations. Notably, the polarisation is qualitatively different from the one found in the Hubbard model – this indicates that the simplified approach to electronelectron interaction in the Hubbard model might not always be reliable.</p><p>In paper II we propose a spin-filter device based on resonant backscattering of edge states against a quantum anti-dot embedded in a quantum wire. A magnetic field is applied and the spin up/spin down states are separated through Zeeman splitting. Their respective resonant states may be tuned so that the device can be used to filter either spin in a controlled way.</p> / Report code: LIU-Tek-Lic 2005:65

La supraconductivité est un ordre électronique fascinant dans lequel les électrons s'apparient par le biais d'une interaction attractive et condensent dans un état quantique macroscopique pouvant porter un courant non dissipatif, i. E. Un supercourant. Dans les structures hybrides où des supraconducteurs (S) sont mis en contact avec des matériaux non supraconducteurs (X), les paires se propageant de S " contaminent " X lui conférant des propriétés supraconductrices à proximité de l'interface, parmi lesquels la possibilité de porter un supercourant. La transmission d'un supercourant à travers n'importe quelle structure S-X-S s'explique par l'interférence constructive de paires d'électrons traversant X. En effet, à la manière d'un résonateur Fabry-Perot, une telle interférence a seulement lieu pour certains états électroniques résonants appelés Etats Liés d'Andreev (ELA). Récemment, il est devenu possible de fabriquer une variété de nanostructures dans lesquelles X peut être par exemple un nanofil, un nanotube de carbone ou même une molécule. Ces dispositifs ont en commun que leur X contient seulement quelques électrons de conduction ce qui implique que les ELA sont aussi en petit nombre. Dans ce cas, pour comprendre quantitativement l'effet de proximité dans ces systèmes, il est nécessaire de comprendre en détail la formation des ELA individuellement. Ceci peut être vu comme la question centrale du développement d'électronique supraconductrice à l'échelle nanométrique. Dans cette thèse, nous avons observé des ELA résolus individuellement par spectroscopie tunnel dans un nanotube de carbone.