The aim of this work is to evaluate ride quality of a typical passenger car. This requires both identifying the excitation resources, which result to undesired noise inside the vehicle, and studying human reaction t applied vibration. Driveline linear torsional vibration will be modelled by a 14-degress of freedom system while engine cylinder pressure torques are considered as an input force for the structure. The results show good agreement with the corresponding reference output responses which proves the accuracy of the numerical approach fourth order Runge-kutta. An eighteen-degree of freedom model is then used to investigate coupled motion of driveline and the tire/suspension assembly in order to attain vehicle body longitudinal acceleration subject to engine excitations. Road surface irregularities is simulated as a stationary random process and further vertical acceleration of the vehicle body will be obtained by considering the well-known quarter-car model including suspension/tire mechanisms and road input force. Finally, ISO diagrams are utilized to compare RMS vertical and lateral accelerations of the car body with the fatigue-decreased proficiency boundaries and to determine harmful frequency regions. According to the results, passive suspension system is not functional enough since its behaviour depends on frequency content of the input and it provides good isolation only when the car is subjected to a high frequency excitation. Although longitudinal RMS acceleration of the vehicle body due to engine force is not too significant, driveline torsional vibration itself has to be studied in order to avoid any dangerous damages for each component by recognizing resonance frequencies of the system. The report will come to an end by explaining different issues which are not investigated in this thesis and may be considered as future works.

The discovery of carbon nanotubes by Iijima in 1991 has created a torrent of new research activities. Research on carbon nanotubes ranges from studying their fundamental properties, such as their electron band structure and plasma frequencies, to developing new applications, such as self-assembled nano-circuits and field emission displays. Robust models are now needed to enable a better understanding of the electronic response of carbon nanotubes. We use time-dependent density functional theory to derive a two-fluid two-dimensional (2D) hydrodynamic model describing the collective response of a multiwalled carbon nanotube with dielectric media embedded inside or surrounding the nanotube. We study plasmon hybridization of the nanotube system in the UV range, the stopping force for ion channelling, the dynamical image potential for fast ions, channelled diclusters and point dipoles, and the energy loss for ions with oblique trajectories. Comparisons are made of results obtained from the 2D hydrodynamic model with those obtained from an extension of the 3D Kitagawa model to cylindrical geometries.

Des boîtes quantiques à fort confinement tridimensionnel et de très basse densité (106cm-2) ont été démontrées dans des structures qui ont été originellement développées comme des double puits quantique de GaAs/AlAs avec des barrières de GaAlAs. Le fait que ces structures soient de type II permet de détecter les boîtes quantiques très facilement grâce à la très longue durée de vie des excitons indirects (de l'ordre de quelques millisecondes) et à leur capacité à diffuser efficacement (jusqu'à 100mm) dans les pièges zéro dimensionnels. Cet effet est généralement difficile à obtenir dans les structures directes. Les boîtes quantiques peuvent donc être facilement remplies par des excitons, provoquant la formation, non seulement d'excitons, mais aussi d'excitons chargés et de biexcitons, et également de molécules excitoniques plus complexes montrant un caractère zéro dimensionnel.<br />Ce travail est consacré à l'étude des complexes excitoniques fortement confinés, à leur nature et aux processus permettant leur formation.<br />Nous présentons des études spectroscopiques de l'émission d'une boîte quantique unique sous différentes conditions d'excitation et détectée de différentes manières.<br /> La possibilité de contrôler optiquement le nombre d'électrons et de trous qui occupent les niveaux discrets des boîtes quantiques nous a permis d'étudier la formation de complexes multi-excitoniques en fonction de la densités d'excitons. Les effets observés sont décrits par le modèle de la normalisation des bandes d'énergie comprenant les effets multi-corps, les interactions d'échange et les effets de corrélation.<br />L'influence d'un champ magnétique sur les complexes multi-excitoniques est d'abord discutée. De manière générale, il est montré comment l'application d'un champ magnétique modifie la structure énergétique des transitions observées. Des propriétés typiques de boîtes quantiques telles que l'effet Zeeman, décalage diamagnétique et l'énergie de liaison excitonique sont discutées. Ces études ont permis une analyse de la symétrie et de la taille du potentiel de confinement des boîtes.<br />Ensuite, les mécanismes de capture d'excitons dans les boîtes sont considérés. Le rôle important des processus de diffusion contribuant au temps de relaxation de l'émission des boîtes quantiques uniques est discuté sur la base d'expériences de spectroscopie résolue en temps.<br />Le rôle des processus radiatifs et non radiatifs dans l'émission de complexes multi-excitoniques est montré dans l'émission thermiquement activée de boîtes quantiques uniques.<br />Les mesures de corrélation de photon ont permis la classification des différentes lignes d'émission des complexes multi-excitoniques, d'étude du caractère de mécanisme de capture des porteurs photo-créés et la dynamique des fluctuations de charge caractéristiques d'une boîte quantique unique.<br />L'approche expérimentale à un problème de boîte quantique unique est largement discutée et des modèles théoriques sont appliqués pour décrire les effets observés.

Quelques aspects de la physique des mésons B ont été étudiés par la<br />simulation numérique de la Chromodynamique Quantique sur réseau, qui est une approche non perturbative - basée sur les principes premiers de la Théorie Quantique des Champs - de calculer les fonctions de Green de la théorie. <br />Les couplages $\hat{g}$ et $\tilde{g}$ paramétrant le Lagrangien chiral effectif qui décrit les interactions entre mésons lourd-légers et pions mous ont été calculés au-delà de l'approximation quenched (à N_f=2). L'opérateur<br />$\bar{q}\gamma_\m \gc q$ a été renormalisé non perturbativement en utilisant les identités de Ward chirales. On trouve <br />$\hat{g}=0.4\div 0.6$ et $\tilde{g}=-0.1\div-0.3$. <br />La masse du quark étrange a été estimée par<br />une simulation unquenched (à N_f=2): elle a été renormalisée<br />dans le schéma non perturbatif RI-MOM. On obtient en faisant le raccordement avec le schéma MSbar m_s^{MSbar}(2 GeV)=101\pm 8^{+25}_{-0} MeV. <br />Une méthode a été proposée pour évaluer sur le réseau les facteurs de<br />forme associés aux transitions semileptoniques à recul nul <br />B \to D^{**} décrites dans le cadre de la Théorie Effective des Quarks Lourds. La constante de renormalisation de l'opérateur $\bar{h}\gamma_i \gc D_j h$ a été calculée à l'ordre d'une boucle de la théorie des perturbations.<br />On trouve \tau_1/2(1)=0.3\div 0.5 et \tau_3/2(1)=0.5\div 0.7. <br />Enfin le paramètre de sac associé à l'amplitude de mélange B_s-anti B_s a été évalué, en choisissant une action pour le quark étrange qui vérifie la symétrie chirale à maille a du réseau finie. De cette manière les erreurs systématiques, provenant des mélanges de chiralité, sont nettement réduites lors de la renormalisation de l'opérateur à 4 fermions $\bar{h}\gamma_{\m L}q\bar{h}\gamma_{\m L}q$. On obtient dans l'approximation quenched $B_{B_s}=0.92(3)$.

A theoretical framework describing the carbon nanotubes interaction, involving two distinct approaches, is presented. Based on the results obtained practical applications using carbon nanotubes are further proposed. First a classical approach is employed for different geometrical configurations, such as parallel or concentric carbon nanotubes. For all the cases analytical expressions for the systems potential energies are derived. The results obtained using the classical approach are used to propose a few practical applications. These applications include a non-contact device for profiling surfaces and a custom telescopic double wall carbon nanotube for nanolithography applications. It is expected that such devices can be effectively used with major advantages. Next the interaction between nanotubes is considered using a quantum electrodynamics approach suitable for dispersing and absorbing media. Each carbon nanotube is characterized by its individual full dielectric response. The method also allows taking into account the full carbon nanotube cylindrical geometry by imposing the appropriate boundary conditions at the nanotubes surfaces. It is found that at small nanotube separations, similar to their equilibrium distances, the interaction is dominated by the collective excitations in the electron energy loss spectra originating from interband transitions. Furthermore, it is shown that the collective surface excitations and their chirality dependent characteristics play a profound role in the interaction strength in double wall carbon nanotube systems. The obtained results are in good agreement with experimental measurements on determining the chirality of individual double wall carbon nanotubes

Neste projeto de doutorado estudamos dois aspectos em condensados de Bose-Einstein de gases alcalinos diluídos: (i) a expansão auto-similar de um superfluido turbulento, e (ii) a física dos pólarons no contexto de misturas de superfluidos e redes de vórtices. Ambas as análises estão relacionadas com nossas tendências experimentais em átomos frios. Na primeira etapa generalizamos as equações hidrodinâmicas dos superfluidos para descrever a expansão anômala de uma nuvem condensada turbulenta. A física por detrás dessa assinatura característica da natureza turbulenta da nuvem pôde ser compreendida através das equações derivadas em nosso modelo, que considerou a energia cinética advinda de uma configuração de vórtices enovelados. Na segunda parte do trabalho abordamos a física do pólaron, analisando as propriedades de uma impureza neutra acoplada com os modos Tkachenkos de um condensado de Bose-Einstein contendo uma rede de vórtices. Através da função espectral da impureza, pudemos acompanhar a evolução das propriedades de quase-partícula em função da magnitude do parâmetro de interação, à medida que caminhávamos em direção ao regime de baixas energias do sistema. A função espectral apresentou inicialmente um alargamento do seu perfil Lorentziano para baixos valores dos momentos da impureza e das excitações, mesmo a temperatura zero. Ao atingir a proximidade de um ponto fixo de baixas energias, porém, o espectro passa a adquirir um perfil de decaimento com lei de potência. Trata-se de uma assinatura do fenômeno da catástrofe de ortogonalidade, com a quebra da natureza de quase-particula do sistema. Aplicamos uma transformação canônica com operadores unitários e técnicas de grupo de renormalização para avaliar o fluxo das constantes da teoria à medida que diminuíamos as escalas de energia características do nosso sistema. Na etapa final apresentamos alguns resultados preliminares sobre o sistema de duas espécies de condensado sobrepostas, uma delas contendo a rede de vórtices. Por meio de uma analogia com superfluidos em redes ópticas, mapeamos nosso Hamiltoniano em um modelo Bose-Hubbard e variamos o comprimento de espalhamento atômico das espécies envolvidas para induzir a transição de fase quântica naquela aprisionada na rede. Mostramos que essa nossa nova configuração quântica de rede permite investigações que vão além daquelas estudadas com redes ópticas estáticas. / In this thesis we studied two aspects of Bose-Einstein condensation in dilute gases: (i) the self-similar expansion of a turbulent superfluidity, and (ii) the polaron physics in the context of the superfluid mixtures and vortex lattices. Both analyses are closely related to our experimental trends. Concerning the first subject, we generalized the superfluid hydrodynamic equations to describe the anomalous expansion of a turbulent condensate cloud. The physics behind this characteristic signature of the turbulence could be clarified through the expressions derived in our model, that considered the kinetic energy associated with a tangled vortex configuration. As for the second item, we present the polaron physics of a neutral impurity coupled with the Tkachenko modes of a vortex lattice Bose-Einstein condensate. Through the impurity spectral function, we tracked how the quasiparticle properties varied as a function of the interaction strength toward the lower energy regimes. The spectral function exhibits a Lorentzian broadening for small wave vectors, even at zero temperature, until it starts to reach the low energy fixed point, where it acquires a power law decay. That is the signature of orthogonality catastrophe phenomena, with the breakdown of the quasiparticle picture. We applied canonical unitary transform and renormalization group equations to evaluate the flow of the theory parameters as we go further down in the characteristic energy scales. Finally, we provide preliminary results on the calculation of a system composed of two condensate species, one immersed in a second containing an array of vortices. Making an analogy with superfluids in an optical lattice, we map our Hamiltonian onto a Bose-Hubbard type model and tune the atomic scattering length of the two species to induce a quantum phase transition in the confined cloud. This is a new quantum system which allows investigation beyond the present studies with static optical lattices.

Espectros Raman de super-redes &#948-dopadas, na geometria de retro¬espalhamento, mostram uma estrutura na região do fônon LO que é alargada e deslocada em relação à correspondente linha observada no material intrínseco. Neste trabalho, este fenômeno é teoricamen¬te explicado através do acoplamento de um fônon LO de momento q, propagando-se ao longo da super-rede, com as excitações eletrônicas desse sistema. O deslocamento e o alargamento da linha são obtidos através do cálculo da auto-energia desse fônon, a qual pode ser expressa em termos da função resposta densidade-densidade do gás de elétrons modulado. Efeitos da interação elétron-elétron são tratados através da aproximação das fases aleatórias (RPA). / The Raman spectra of &#948-doping superlattice, in backscattering ge¬ometry, show a structure in the region of the LO phonon which is broadened and shifted in comparison whit the same structure in the intrinsic material. In this work, this phenomena is interpreted as consequence of the coupling of LO xne-phonon of momentum q, along the axis of the superlattice, whit the excitations of the modulate electron gas, that exists in this system. The shift and the broadening of the phonon are calculated as the real and imagi¬nary parts of the phonon self-energy, which in turn is related to the density-density response function of modulate electron gas. Effects of electron-electron interaction are calculated within the Random Phase Approximation (RPA).

Neste trabalho, estudamos as consequências causadas em um condensado de Bose-Einstein de átomos de 87Rb quando sujeito a uma excitação oscilatória externa. Para a produção do condensado utilizamos técnicas de resfriamento atômico, como o resfriamento a laser e o resfriamento evaporativo, em átomos aprisionados em uma armadilha magnética harmônica. A excitação externa é produzida através de um campo magnético quadrupolar oscilatório que é sobreposto ao campo de aprisionamento. Para uma forma fixa dessa excitação, observamos que podemos excitar modos coletivos de oscilação na amostra de condensado, em especial analisamos os modos breathing, dipolar, quadrupolar e o scissor mode. Durante o movimento dipolar do condensado na armadilha magnética, identificamos que há um escoamento contrapropagante do condensado em relação à nuvem térmica que o permeia. Esse escoamento é analisado como um movimento de dois fluidos, onde o condensado desempenha o papel de um superfluido e a nuvem térmica o de um fluido normal. Irregularidades na superfície do condensado são observadas e seus comprimentos característicos crescem na região dos pontos de retorno do movimento dipolar do condensado. Nesses pontos, a forma das irregularidades sugerem estar relacionadas com o processo de geração de vórtices na amostra superfluida. Com base nestas análises, determinamos o valor da velocidade relativa dos fluidos e qual deve ser, tipicamente, o valor da velocidade relativa crítica para que as irregularidades se tornam maiores, possibilitando a geração de vórtices. As análises apresentadas neste trabalho são dedicadas a entender os mecanismos que possibilitaram, em nosso sistema, a observação de vórtices, turbulência quântica e a granulação do condensado, em 2009. / In this work we have studied the effects caused by an external oscillatory excitation in a Bose-Einstein condensate of 87Rb. The condensate is produced through by atomic laser cooling techniques, as laser cooling and evaporative cooling, for trapped atoms in a harmonic magnetic trap. The external excitation is generated by an oscillating magnetic quadrupole field superimposed to the trapping field. For a fixed type of excitation, we observe that collective modes of oscillation are excited in the condensate sample, particularly we analyze the dipole mode, quadrupole and scissor mode. During the motion of the condensate in dipolar mode inside the magnetic trap, we have identified a counterflow, i. e. a relative motion between the condensate and the thermal cloud. This flow is analyzed as a two fluids motion, where the condensate plays the role of a superfluid and the thermal cloud a normal fluid. Irregularities on the condensate surface are observed and their characteristic lengths grow in the turning point regions of this dipolar motion. At these points, the shape of this irregularities seems to be related to the vortices generation process in a superfluid sample. Based on this analysis, we determine the velocity of the counterflow and the critical velocity for the irregularities to become larger, allowing the generation of vortices. The analyses presented in this work are dedicated to understand the mechanisms that allowed in our system the observation of vortices, quantum turbulence and the condensate granulation, in 2009.

We investigate the quantum dynamics of the spins on different Heisenberg antiferromagnetic spin lattice systems. Firstly, we applied the coupled-cluster method to the spin-1/2 antiferromagnetic XXZ model on a square lattice by employing an approximation which contains two-body long-range correlations and high-order four-body local correlations. Improvement is found for the ground-state energy, sublattice magnetization, and the critical anisotropy when comparing with the approximation including the two-body correlations alone. We also obtain the full excitation spectrum which is in good agreement with the quantum Monte Carlo results and the high-order spin-wave theory. Secondly, we study the longitudinal excitations of quantum antiferromagnets on a triangular lattice by a recently proposed microscopic many-body approach based on magnon-density waves. We calculate the full longitudinal excitation spectra of the antiferromagnetic Heisenberg model for a general spin quantum number in the isotropic limit. Similar to the square lattice model, we find that, at the center of the first hexagonal Brillouin zone Γ(q=0) and at the magnetic ordering wavevectors ±[Q= (4π/3,0)], the excitation spectra become gapless in the thermodynamic limit, due to the slow, logarithmic divergence of the structure factor. However, these longitudinal modes on two-dimensional models may be considered as quasi-gapped, as any finite-size effect or small anisotropy will induce a large energy gap, when compared with the counterpart of the transverse spin-wave excitations. We have also investigated the excited states of the quasi-one-dimensional quantum antiferromagnets on hexagonal lattices, including the longitudinal modes based on the magnon-density waves. A model Hamiltonian with a uniaxial single-ion anisotropy is first studied by a spin-wave theory based on the one-boson method; the ground state thus obtained is employed for the study of the longitudinal modes. The full energy spectra of both the transverse modes (i.e., magnons) and the longitudinal modes are obtained as functions of the nearest-neighbor coupling and the anisotropy constants. We have found two longitudinal modes due to the non-collinear nature of the triangular antiferromagnetic order, similar to that of the phenomenological field theory approach by Affleck. The excitation energy gaps due to the anisotropy and the energy gaps of the longitudinal modes without anisotropy are then investigated. We then compares our results for the longitudinal energy gaps at the magnetic wavevectors with the experimental results for several antiferromagnetic compounds with both integer and non-integer spin quantum numbers, and we find good agreements after the higher-order contributions are included in our calculations.

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