Optical spin injection in InAs/GaAs quantum dots (QDs) structures under cryogenic temperature has been investigated in this work using continuous-wave optical orientation spectroscopy. Circularly polarized luminescence from trions in the QDs was used as a measure for the degree of spin polarization of the carriers in the QD ground states. The efficiency of spin conservation of the carriers during the injection process into the QDs and also the influence of the nuclear spins in the QDs were studied both under zero and external magnetic field. It was shown in zero magnetic field that the spin states were less conserved during the injection process for correlated excitons and hot free carriers. While under the external magnetic field, measurements were done in Faraday configuration. Confined electron motion yielding the quantized Landau levels in the InGaAs wetting layer (WL) and lifting of the Landau level spin degeneracy was observed. Also possible spin thermalization in the InGaAs WL during spin injection process was found. Finally, the quench of hyperfine induced spin relaxation by dynamic nuclear polarization (DNP) in the QDs was discovered and believed to be a stronger effect under weak/zero magnetic field.

This dissertation discusses the Landau-Zener (LZ) theory and its application in noisy environments and in many-body systems. The first project considers the effect of fast quantum noise on LZ transitions. There are two important time intervals separated by the characteristic LZ time. For each interval we derive and solve the evolution equation, and match the solutions at the boundaries to get a complete solution. Outside the LZ time interval, we derive the master equation, which differs from the classical equation by a quantum commutation term. Inside the LZ time interval, the mixed longitudinal-transverse noise correlation renormalizes the LZ gap and the system evolves according to the renormalized LZ gap. In the extreme quantum regime at zero temperature our theory gives a beautiful result which coincides with that of other authors. Our initial attempts to solve two experimental puzzles - an isotope effect and the quantized hysteresis curve of a single molecular magnet - are also discussed. The second project considers an ultracold dilute Fermi gas in a magnetic field sweeping across the broad Feshbach resonance. The broad resonance condition allows us to use the single mode approximation and to neglect the energy dispersion of the fermions. We then propose the Global Spin Model Hamiltonian, whose ground state we solve exactly, which yields the static limit properties of the BEC-BCS crossover. We also study the dynamics of the Global Spin Model by converting it to a LZ problem. The resulting molecular production from the initial fermions is described by a LZ-like formula with a strongly renormalized LZ gap that is independent of the initial fermion density. We predict that molecular production during a field-sweep strongly depends on the initial value of magnetic field. We predict that in the inverse process of molecular dissociation, immediately after the sweeping stops there appear Cooper pairs with parallel electronic spins and opposite momenta.

In this paper, we mainly discuss the transport properties of the two-dimensional gas of a high-mobility GaAs/AlGaAs semiconductor heterostructure in high magnetic fields and low temperatures. We analyzed the measured longitudinal resistivity and Hall resistivity at the five different temperatures. We observed that the classical Hall effect is valid when the magnetic field is less than 0.25 Tesla; and the quantum Hall plateaux appeared obviously when the magnetic field is larger than 1.6 Tesla. We proceeded to analyze the longitudinal resistivity oscillation occurred in the magnetic fields between 0.477 Tesla and 1.483 Tesla. According to the Lifshitz-Kosevich (LK) formula, we can get the two-dimensional electron concentration, effective mass, and quantum scattering time from the quantum magnetoresistivity oscillation measurement. Our results suggested that the applicable range of the LK formula could be broader than the generally-assumed one. In quantum Hall effect regime at high magnetic field, we can calculate the h/e2 value from the quantum Hall plateaux value. In classical Hall effect regime, the three-dimensional electron concentration and classical mobility (classical scattering time) can be obtained. However, we find out that the zero-field Hall resistivity experimental value is not equal to zero, and this is not conformed to the standard theory. We tried to use the magnetic field shift and Hall resistivity shift to solve the problem, and compared both advantages of them. Finally, we observed the plateau-plateau phase transitions of the two-dimensional electron system

<p>The role of intrinsic hydrodynamic instability of the premixed flame (known as Landau-Darrieus instability) in various flame phenomena is studied by means of direct numerical simulations of the complete system of hydrodynamic equations. Rigorous study of flame dynamics and effect of Landau-Darrieus instability is essential for all premixed combustion problems where multidimensional effects cannot be disregarded.</p><p>The present thesis consists of three parts. The first part deals with the fundamental problem of curved stationary flames propagation in tubes of different widths. It is shown that only simple "single-hump" slanted stationary flames are possible in wide tubes, and "multi-hump" flames in a laminar flow are possible in wide tubes only as a non-stationary mode of flame propagation. The stability limits of curved stationary flames in wider tubes are obtained, together with the dependence of the velocity of the stationary flame on the tube width. The flame dynamics in wider tubes is shown to be governed by a large-scale stability mechanism resulting in a highly slanted flame front.</p><p>The second part of the thesis is dedicated to studies of acceleration and fractal structure of outward freely propagating flames. It is shown that in direct numerical simulation the development of Landau-Darrieus instability results in the formation of fractal-like flame front structure. The fractal excess for radially expanding flames in cylindrical geometry is evaluated. Two-dimensional simulation of radially expanding flames in cylindrical geometry displays a radial growth with 1.25 power law temporal behavior after some transient time. It is shown that the fractal excess for 2D geometry obtained in the numerical simulation is in good agreement with theoretical predictions. The difference in fractal dimension between 2D cylidrical and three-dimensional spherical radially expanding flames is outlined. Extrapolation of the obtained results for the case of spherical expanding flames gives a radial growth power law that is consistent with temporal behavior obtained in the survey of experimental data.</p><p>The last part of the thesis concerns the role of Landau-Darrieus instability in the transition from deflagration to detonation. It is found that in sufficiently wide channels Landau-Darrieus instability may invoke nucleation of hot spots within the folds of the developing wrinkled flame, triggering an abrupt transition from deflagrative to detonative combustion. It is found that the mechanism of the transition is the temperature increase due to the influx of heat from the folded reaction zone, followed by autoignition. The transition occurs when the pressure elevation at the accelerating reaction front becomes high enough to produce a shock capable of supporting detonation.</p>

Nous étudions la régularité des équations cinétiques de type Boltzmann. Nous nous basons essentiellement sur une méthode d'analyse harmonique de type "décomposition de Littlewood-Paley", consistant principalement à travailler avec des couronnes dyadiques. Nous nous intéressons de plus, au cadre homogène où la solution f(t,x,v) dépend uniquement du temps t et de la vitesse v, tout en travaillant avec des sections efficaces réalistes et singulières (non cutoff).<br />Dans une première partie, nous étudions le cas particulier des molécules Maxwelliennes. Sous cette hypothèse, la structure de l'opérateur de Boltzmann et de sa tranformée de Fourier s'expriment de manière simple. Nous montrons ainsi une régularité globale C^\infty.<br />Ensuite, nous traitons le cas des sections efficaces générales avec "potentiel dur". Nous nous intéressons d'abord à l'équation de Landau. C'est une équation limite de l'équation de Boltzmann prenant en compte les collisions rasantes. Nous prouvons que toute solution faible appartient à l'espace de Schwartz S. Nous démontrons ensuite une régularité identique pour le cas de l'équation de Boltzmann. Notons que notre méthode s'applique directement pour toutes les dimensions, en signalant que les preuves sont souvent plus simples comparées à d'autres preuves plus anciennes.<br />Enfin, nous terminons avec l'équation de Boltzmann-Dirac. En particulier, nous adaptons le résultat de régularité obtenu dans le travail de Alexandre, Desvillettes, Wennberg et Villani, en utilisant le taux de dissipation d'entropie relatif à l'équation de Boltzmann-Dirac.

Une étude non-perturbative des corélateurs en QCD est présentée. La méthode principale employée<br />est la simulation numérique sur réseau. Cet outil a été largement utilisé en phénoménologie,<br />mais il peut aussi servir pour étudier les paramètres fondamentaux de la théorie (tels que la<br />constante de couplage) et ses propriétés fondamentales. Ceci est le but principal de la présente<br />thèse. Nous avons étudié les fonctions de corrélation de la théorie Yang-Mills pure en jauge<br />de Landau, notamment les propagateurs du gluon et du fantôme. Nous nous sommes particulièrement<br />intéressés au paramètre LQCD qui est extrait à l'aide des prédictions de la théorie des<br />perturbations (jusqu'à l'ordre NNNLO). Les corrections dominantes en puissance sont aussi considérées,<br />nous montrons qu'elles sont importantes même à des énergies assez grandes (de l'ordre<br />de 10 GeV). Une méthode de soustraction de ces termes correctifs est proposée, ce qui permet<br />une meilleur estimation de LQCD. Notre résultat final est Lambda_nf_MSbar = 269(5)+12−9 MeV. Une autre<br />question que nous considéons est celle du comportement infrarouge des fonctions de Green (aux<br />énergies de l'ordre de ou inférieur à LQCD). A ces énergies le comportement des fonctions de<br />Green change de manière radicale, et cela est probablement lié au confinement. Nous cherchons<br />à clarifier la nature de ces changements afin de comprendre ses origines. Beaucoup de questions<br />se posent: l'ambigu¨té de Gribov, la portée de diverses relations non-perturbatives entre les<br />fonctions de Green, la cohéence de l'approche nuérique aux petites énergies. Les simulations<br />sur réseau permettent de vérifier les prédicitons analytiques, elles donnent accès aux corrélateurs<br />non-perturbatifs. Notre analyse suggère que le propagateur du gluon est fini et non nul dans<br />l'infrarouge, et que le comportement en puissance du propagateur du fanôme est le même que<br />dans le cas libre.

Various properties of linear infinite-dimensional Hamiltonian systems are studied. The structural stability of the Vlasov-Poisson equation linearized around a homogeneous stable equilibrium [mathematical symbol] is investigated in a Banach space setting. It is found that when perturbations of [mathematical symbols] are allowed to live in the space [mathematical symbols], every equilibrium is structurally unstable. When perturbations are restricted to area preserving rearrangements of [mathematical symbol], structural stability exists if and only if there is negative signature in the continuous spectrum. This analogizes Krein's theorem for linear finite-dimensional Hamiltonian systems. The techniques used to prove this theorem are applied to other aspects of the linearized Vlasov-Poisson equation, in particular the energy of discrete modes which are embedded within the continuous spectrum. In the second part, an integral transformation that exactly diagonalizes the Caldeira-Leggett model is presented. The resulting form of the Hamiltonian, derived using canonical transformations, is shown to be identical to that of the linearized Vlasov-Poisson equation. The damping mechanism in the Caldeira-Leggett model is identified with the Landau damping of a plasma. The correspondence between the two systems suggests the presence of an echo effect in the Caldeira-Leggett model. Generalizations of the Caldeira-Leggett model with negative energy are studied and interpreted in the context of Krein's theorem. / text

Ce mémoire de thèse présente une approche semi-analytique des différentes solutions solitons spatio-temporelles de l'équation cubique quintique de Ginzburg-Landau complexe étendue à (3+1)D (GL3D).La méthode semi-analytique choisie est celle des coordonnées collectives qui permet d'approcher le champ exact, dont l'expression analytique est inconnue, par une fonction d'essai, qui comporte un nombre limité de paramètres physiques.En appliquant cette procédure à l'équation GL3D, nous obtenons un système d'équations variationnelles qui gouverne l'évolution des paramètres de la balle de lumière. Nous montrons que cette approche des coordonnées collectives est incomparablement plus rapide que la procédure de résolution directe de l'équation GL3D. cette rapidité permet d'obtenir, en un temps record, une cartographie générale des comportements dynamiques des balles de lumière. Cette cartographie révèle une riche variété d'états dynamiques faite de balles de lumière stationnaires, oscillantes et rotatives.Finalement, les résultats de cette thèse prédisent l'existence de plusieurs familles de balles de lumière, et précisent les domaines respectifs de leurs paramètres physiques. Cette prédiction constitue un pas en avant dans les efforts entrepris ces dernières années en vue d'une démonstration expérimentale de ce type d'impulsions.

Polymers is a class of molecules which can have many different structures due to a large number of degrees of freedom. Many biopolymers, e.g. DNA, but also synthetic macromolecules have special structural features due to their backbone stiffness. Since such structural properties are important for e.g. the biological function, a lot of effort has been put into the investigation of the configurational properties of semi-stiff molecules. A theoretical treatment of these systems is often accompanied by computer simulations. The main idea is to compare theoretically derived models with experimental results for real polymers. Using Monte Carlo simulations, I have investigated how this computational technique can build a bridge between theoretical models and experimentally observed phenomena. The effort was mainly directed to develop sampling techniques, for efficiently exploring the configurational space of semi-stiff chains in a wide range of structures. The work was concentrated on compact conformations, since they, as is well known from previous studies, are difficult to sample using conventional methods. In my studies I have shown that the simple and, at a first glance, time consuming method of bead-by-bead regrow as a way of changing the configuration of a semi-stiff chain gave very promising and encouraging results when combined with modern simulation techniques, like Entropic Sampling with the Wang-Landau algorithm. The resulting simulation package was also suitable for parallelization which resulted in a further speed-up of the calculations. In addition to the more elaborate sampling methods, I also investigated external conditions to induce compaction of a semi-stiff polymer. In the case of a polyampholyte the condensing agent could be a multivalent salt, creating effective attraction between the loops of the chain, while for neutral polymers, an external field and the geometry of the confining volume can induce a compaction.

Submitted by Luciana Sebin (lusebin@ufscar.br) on 2016-10-11T11:52:52Z No. of bitstreams: 1 DissDHGL.pdf: 2463091 bytes, checksum: 8e959cfba32ae62b949f2465a30698d6 (MD5) / Approved for entry into archive by Ronildo Prado (ronisp@ufscar.br) on 2016-10-19T17:29:01Z (GMT) No. of bitstreams: 1 DissDHGL.pdf: 2463091 bytes, checksum: 8e959cfba32ae62b949f2465a30698d6 (MD5) / Approved for entry into archive by Ronildo Prado (ronisp@ufscar.br) on 2016-10-19T17:29:10Z (GMT) No. of bitstreams: 1 DissDHGL.pdf: 2463091 bytes, checksum: 8e959cfba32ae62b949f2465a30698d6 (MD5) / Made available in DSpace on 2016-10-19T17:34:31Z (GMT). No. of bitstreams: 1 DissDHGL.pdf: 2463091 bytes, checksum: 8e959cfba32ae62b949f2465a30698d6 (MD5) Previous issue date: 2016-02-25 / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) / This monograph shows two perturbative methods to determine the time evolution of quantum systems described by time-depependent Hamiltonians. One method is the standard time-dependent perturbation theory usually found in text books of quantum mechanics. The other method is the adiabatic perturbation theory, whose perturbative corrections are obtained from the quantum adiabatic approximation. To compare the efficiency of those methods we apply them to two time-dependent problems. One is the Landau- Zener problem, whose time dependency is linear on time. We also show three analytical solutions to this problem. The other problem is a Hamiltonian which has a cyclic perturbation, whose numerical solution is employed to compare the efficacy of both perturbative methods. The comparison between the two perturbation theories is based on the graphics of transition probability and of the fidelity, where all quantities are computed using the perturbative methods and the exact solutions to the problems. We conclude that the adiabatic perturbation theory improves the adiabatic approximation, is better than the standard theory when we deal with the limited Hamiltonian, and is relatively better than the standard theory for the Landau-Zener model. / Esta dissertação apresenta dois métodos perturbativos para determinar a evolução temporal de sistemas quânticos cujas Hamiltonianas dependam explicitamente do tempo. Um destes métodos é a teoria de perturbação dependente do tempo usualmente encontrada nos livros textos de mecânica quântica. O outro método chama-se teoria de perturbação adiabática, cujas correções perturbativas são obtidas a partir da aproximação adiabática. A fim de comparar a eficiência desses métodos, vamos aplicá-los na solução de dois problemas dependentes do tempo. Um deles será o problema de Landau-Zener com dependência linear no tempo, onde apresentamos três possíveis soluções analíticas entre tantas outras desenvolvidas na literatura. O outro problema será uma Hamiltoniana que varia ciclicamente no tempo, cuja solução numérica será usada para comparar com as teorias perturbativas. Nossa comparação consistiu em analisar os gráficos da probabilidade de transição e da fidelidade, nos quais foram plotados as equações da teoria padrão juntamente com a teoria adiabática e as soluções numéricas/analíticas. Concluímos que a teoria de perturbação adiabática melhora significativamente a aproximação adiabática e é superior a teoria da perturbação temporal no caso da Hamiltoniana limitada e relativamente melhor também no modelo de Landau-Zener.