Ulm, Univ., Diss., 2007.

Zugl.: Stuttgart, Univ., Diss., 2006

In this thesis I present a nonequilibrium quantum field theory that describes the production of molecular dimers from a two-component quantum-degenerate atomic Fermi gas, via a linear downward sweep of a magnetic field across an s-wave Feshbach resonance. This problem raises interest because it is presently unclear as to why deviations from the universal Landau-Zener formula for the transition probability at two-level crossing are observed in the experimentally measured production efficiencies. The approach is based on evaluating real-time Green functions within the Keldysh- Schwinger formalism. The effects of quantum statistics associated with Pauli blocking for fermions and induced emission for bosons, characteristic of particle scattering in a quantum-degenerate many-body medium, are fully accounted for. I show that the molecular conversion efficiency is represented by a power series in terms of a dimensionless parameter which, in the zero-temperature limit, depends solely on the initial gas density and the Landau-Zener parameter. This result reveals a hindrance of the canonical Landau-Zener transition probability due to many-body effects, and presents an explanation for the experimentally observed deviations. A second topic treated in this thesis concerns the study of non-adiabatic transitions in N-state Landau-Zener systems. In connection to this, I provide a proof of the conjecture put forth by Brundobler and Elser, regarding the survival probability on the diabatic levels with maximum/minimum slope.

Clustering effects are studied experimentally in 24Mg and are searched for in 28Si and 56Ni. The mechanisms of reactions that populate fissioning states are also studied. The angular distribution of the 12C(24Mg,12C12C)12C reaction at 170 MeV has been measured near 180° in the centre of mass frame by reversing the kinematics. Compared to previously measured forward angle data, the backward angle cross section is a factor of 45 lower. This is interpreted as a strong indication that the reaction mechanism is direct. Measurements of the 12C(16O,12C12C)alpha reaction at beam energies between 67 and 77 MeV reveal symmetrically fissioning states in 24Mg with spins of 6-12h and excitation energies of 22-32 MeV. Two states at 23.7 MeV (Jpi=(6,8)+) and 25.1 MeV (Jpi=10+) have not been observed in previous data at 99 and 113 MeV. The observed states show no clear correlation with 12C+12C scattering resonances. The variation of yield with bombarding energy supports previous indications of a compound reaction mechanism. Measurements of the 40Ca+16O reaction at a 40Ca beam energy of 190 MeV, previously proposed as a molecular resonance energy in this channel, show no yield into the sequential breakup channel 16O(40Ca,12C16O)28Si. This is apparently at variance with the predictions of a widely successful model for cluster reactions (the Harvey model). Measurements of other exit channels, in particular the 28Si+28Si, 24Mg+ 32S and 20Ne+36Ar channels, agree with statistical model Hauser-Feshbach calculations. Tests have been performed on the gas cells of the hybrid detectors used in these experiments. Optimum values for the operating parameters are determined. A systematic variation of detector signal with position is observed, which can be corrected in analysis. It is shown that two gridded electrodes of 100 mum wires can be replaced with a single 20 mum grid increasing the two-fold coincidence efficiency from 64% to 96%.

Orientador: Marco Aurelio Pinheiro Lima / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-06T20:07:28Z (GMT). No. of bitstreams: 1 Oliveira_ElianeMarquesde_M.pdf: 1735401 bytes, checksum: 96cad8d360043e7a2e9b4a6b65936027 (MD5) Previous issue date: 2006 / Resumo: A relação entre ressonâncias de Feshbach e excitações eletrônicas é bem conhecida no caso de espalhamento de elétrons por moléculas de H2, mas ainda pouco explorada no espalhamento de pósitrons, tornando interessante, portanto, um estudo comparativo. Porém, antes de pensarmos em comparações, nos preocupamos em tentar entender se, de fato, uma ressonância de Feshbach se manifesta no espalhamento de pósitrons por moléculas de H2. Esta preocupação foi motivada devido à não observação experimental desta ressonância em investigação realizada logo após nosso grupo de pesquisa ter publicado um estudo no qual havia evidências para a existência de uma ressonância de Feshbach em colisões pósitron-H2, obtida num cálculo realizado na aproximação de núcleos fixos para a distância internuclear de equilíbrio Ro=1.4ao. Nossa estratégia foi calcular seções de choque, ainda na aproximação de núcleos fixos, para diversas distâncias internucleares em torno de Ro. Os resultados obtidos sugerem que a ausência experimental da ressonância esteja ligada à abertura de um novo canal eletrônico para R > Ro. Para esses cálculos utilizamos o Método Multicanal de Schwinger (SMC), para obtenção dos parâmetros de colisão (posição e largura da ressonância), em combinação com a aproximação do Potencial Complexo Local (também conhecido por Boomemng Model), para inclusão do acoplamento dos graus de liberdade vibracionais. A ausência da ressonância para algumas distâncias internucleares no espalhamento de pósitrons impede o cálculo de seções de choque vibracionalmente resolvidas através da aproximação local. Por outro lado, para o espalhamento de elétrons, que é um desafio como problema teórico, realizamos os cálculos e encontramos acordo razoável com os dados experimentais disponíveis / Abstract: The relationship between Feshbach resonances and electronic excitation is well known in electron scattering by Hydrogen molecules, but still little exploited in the positron scattering case, making interesting, a comparative study. However, before thinking about comparisons, we have to address the question if, in fact, a Feshbach resonance manifests itself in the positron scattering by Hydrogen molecules. This issue is motivated by the fact that recent measured cross sections have not shown the same features presented by our earlier calculated Feshbach resonance in positron-Hydrogen collisions, carried out in the fixed-nuclei approach for the equilibrium internuclear distance, Ro=1.4ao. Our strategy was to calculate cross sections, still in the fixed nuclei approach, for several internuclear distances around Ro. The obtained results suggest that the experimental absence of the resonance is related to the opening of a new electronic channel for R > Ro. For these calculations we have used the Schwinger Multichannel Method (SMC) to obtain the collision parameters (position and width of the resonances) in combination with the Local Complex Potential approach (also known for Boomerang Model), for inclusion of the coupling of the vibrational degrees of freedom. The absence of the resonance for some internuclear distantes prevents the calculation of vibrationally resolved cross sections by positron impact using the local approach. On the other hand, for the electron scattering case, which is also a challange as a theoretical problem, we have carried out the calculation and have found good agreement with available experimental data / Mestrado / Física Atômica e Molecular / Mestre em Física

No description available.

Ce manuscrit est motivé par la possibilité d’explorer de nombreux régimes quantiques grâce aux atomes froids : l’utilisation de résonances de diffusion rend possible un contrôle très fin des effets des interactions et l’ajout de pièges extérieurs permet de s’approcher de régimes de dimension réduite. Nous étudions ici quelques propriétés d’atomes froids piégés dans des guides d’onde uni- ou bidimensionnels au voisinage de résonances de Feshbach. En jouant sur l’intensité du confinement, on peut étudier la transition entre un système tridimensionnel et un système en dimension réduite. Nous modélisons les interactions par un modèle à deux voies qui inclut le couplage cohérent entre atomes et molécules de Feshbach. Nous mettons en évidence l’existence d’un régime unidimensionnel, que l’on peut décrire par un modèle de contact, et dans lequel la portée effective est un paramètre essentiel. Nous examinons alors le problème à trois corps dans ce régime pour des bosons ainsi que dans le régime équivalent pour des fermions totalement polarisés, en particulier leurs propriétés d’intégrabilité. Enfin, nous étudions le développement du viriel d’un gaz d’atomes froids. Nous démontrons, grâce à une approche diagrammatique, une généralisation de la formule de Beth et Uhlenbeck qui prend en compte à la fois les molécules de Feshbach et l’existence d’un guide d’onde. / This manuscript is motivated by the possibility of exploring many quantum regimes using cold atoms: interactions can be tuned very precisely by using Feshbach resonances and low-dimensional regimes can be approached with external potentials. Here, we study some properties of cold atoms trapped into uni- or bidimensional waveguides in the vicinity of a Feshbach resonance. The transition from a tridimensionnal to a low-dimensional system can then be studied by varying the intensity of the confinement. Interactions are described with a two-channel model that includes the coherent coupling between atoms and Feshbach molecules. We highlight a unidimensional regime, that can be described with a contact model and in which the effective range parameter is essential. Thus we investigate the three-body problem in this regime for bosons and also in the equivalent regime for totally polarized fermions, in particular their integrability properties. Finally, we study the virial expansion of a gas of cold atoms. Using a diagrammatic approach, we derive a generalization of the Beth and Uhlenbeck formula, that takes into account both the Feshbach molecules and the existence of a waveguide.

Les gaz d'atomes ultra-froids sont des systèmes à N-corps quantiques extrêmement propres et versatiles qui permettent de revisiter dans un environnement contrôlé des concepts fondamentaux souvent issus de la matière condensée. Dans notre système expérimental, nous travaillons avec des gaz d'atomes de potassium 39, qui sont des bosons et qui offrent la possibilité de modifier à loisir les interactions inter-atomiques grâce à des résonances magnétiques de diffusion, ou résonances de Feshbach. Notre équipe s'intéresse particulièrement à la physique des gaz quantiques en basses dimensions et en présence de désordre. Dans un premier temps, nous présentons l’observation et l’étude de la propagation d'un soliton brillant, une onde de matière 1D en interaction attractive, à travers un potentiel désordonné créé à partir d’une figure de speckle optique. Ce travail constitue la première mise en évidence d’effets non-linéaires sur la propagation d’atomes dans un milieu désordonné. Les limites de l’expérience, notamment en terme d’imagerie et de contrôle des champs magnétiques, ont motivé le design et la construction d’une nouvelle enceinte à vide. La suite du manuscrit est dédiée à la description et la caractérisation du nouveau dispositif expérimental, de sa construction à son utilisation pour la production de condensats de Bose-Einstein. La dernière partie de cette thèse est consacrée à l’étude de la localisation d’Anderson d’atomes-froids en présence d’une force constante. La localisation d’Anderson est caractérisée par une suppression de la conductivité sous l’effet du désordre. Dans le cadre des atomes-froids, elle s’explique par la prise en compte de la nature ondulatoire des atomes pendant les processus de diffusion dans le milieu désordonné. Bien qu’à 1D la localisation soit toujours présente, des études théoriques prédisent qu’une force constante appliquée au système modifie de manière drastique les signatures de la localisation (décroissance algébrique de la fonction d’onde) et peut conduire à une délocalisation complète du système. L’étude expérimentale que nous avons menée confirme les prédictions théoriques. / Ultracold atoms gases are quantum many-body systems very clean and versatile which allow to study basic concepts of condensed matter in controlled media. In our experimental system, we work with 39 potassium atoms which are bosons and allow us to modify the interactions between atoms using magnetic resonances of diffusion also called Feshbach resonances. Our team is particulary interested in the physics of quantum gases in low dimension in the presence of disorder. First, we present the observation and study of the propagation of bright solitons, which are 1D matter wave with attractive interactions, through a disordered potentiel made from a speckle pattern of light. This study led to the first observation of nonlinear effects over the propagation of cold atoms in disorder. The limits of the experiment, especially in terms of imaging and magnetic field control, motivated the design and construction of a new vacuum chamber. The next part of the manuscript is dedicated to the description and characterization of the new experimental device, from its construction to its use for the production of Bose-Einstein condensates. The last part of this thesis is devoted to the study of Anderson localization of cold atoms in the presence of a constant bias force. Anderson localization is characterized by a suppression of conductivity under the effect of disorder. In the context of the cold atoms, it is explained by taking into account the wave nature of the atoms during the diffusion processes in the disordered medium. Even if localization is always present in 1D systems, theoretical studies predict that a constant force applied to the system drastically modifies the signatures of the localization (algebraic decay of the wave function) and can lead to a complete delocalization of the system. Our experimental study confirms the theoretical predictions.

D’immenses progrès ont été réalisés ces dernières années dans le champ de production de condensats de Bose-Einstein moléculaires pour l’obtention et le piégeage de molécules ultrafroides ioniques et neutres. Ces échantillons de molécules ultrafroides trouvent des applications dans des domaines très variés tels que les mesures pectroscopiques de haute précision ou bien encore le stockage de l’information quantique. La volonté d’optimiser les procédés de refroidissement et des techniques de piégeage a stimulé un grand nombre d’études théoriques. La plupart de ces études sont dédiées au refroidissement des molécules en collision avec un gaz tampon (3 He). La surface d’énergie potentielle de l’état fondamental du complexe de van der Waals He–MnH(X7Σ+) est présentée, suivie du calcul des états liés correspondant au complexe ainsi que l’étude de la relaxation Zeeman associée au système en fonction du champ magnétique appliqué. Dans le domaine de l’astrochimie, l’étude de la désexcitation rotationnelle de CH+ en collision avec l’hélium est également présentée. / Tremendous progress in experimental production and trapping of ultracold neutral and ionic molecules has been achieved over the past few years which even allowed the production of molecular Bose–Einstein condensates. These ultracold molecules samples have potential applications in many different fields, such as precision spectroscopic measurements or quantum information storage and processing. The optimization of the cooling processes and the trapping techniques also stimulated a great number of theoretical studies. Many of them are dedicated to inelastic scattering of molecules in collisions with 3He atoms (buffer gas cooling method). The potential energy surface of the ground state of the He–MnH(X7Σ+) van der Waals complex is presented, followed by the calculations of the bound states of this system and the Zeeman relaxation in function of the magnetic field. In the field of astrochemistry, a quantum mechanical investigation of rotational energy transfer in cold collisions of CH+ with 4He atoms is presented.

Die ultrakalte Atom- und Molekülephysik, zu welcher man zum Beispiel bei der Bose-Einstein-Kondensation von verdünnten Gasen Zugang hat, wurde untersucht. In solchen Systemen dominieren Zwei-Körper-Stöße und ihre detaillierte Untersuchung ist eines der zentralen Themen dieser Arbeit. Diese wurden durchgeführt unter Berücksichtigung von elementaren chemischen Reaktionen, Photoassoziation und magnetischen Feshbach-Resonanzen. Weiterhin wurden Untersuchungen von Atomen in optischen Gittern durchgeführt. Die Viel-Teilchen-Systeme wurden nicht nur mit dem üblichen mean-field Ansatz behandelt, sondern auch darüber hinausgehend, um die voll korrelierte Bewegung zu simulieren. / The ultracold atomic and molecular physics as it is accessible, e.g., in Bose-Einstein condensates of dilute gases was investigated. In such systems two-body collisions are dominant and their detailed study is one of the central topics of this work. They were done considering elementary chemical reactions as photoassociation, and magnetic Feshbach resonances. Additionally, studies of atoms in optical lattice sites were carried out. The many-body systems were not only considered within the usually adopted mean-field approach but also beyond that in order to simulate the fully correlated motion.