Repositório Institucional IFSC
URI permanente desta comunidadehttp://143.107.180.6:4000/handle/RIIFSC/1
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Resultados da Pesquisa
Item Excitations in Bose-Einstein condensates: collective modes, quantum turbulence and matter wave statistics(2016-06-20) Tavares, Pedro Ernesto SchiavinattiIn this thesis, we present the generation and studies of a 87Rb Bose-Einstein condensate (BEC) perturbed by an oscillatory excitation. The atoms are trapped in a harmonic magnetic trap where, after an evaporative cooling process, we produce the BEC. In order to study the effect caused by oscillatory excitations, a quadrupole magnetic field time oscillatory is superimposed to the trapping potential. Through this perturbation, collective modes were observed. The dipole mode is excited even for low excitation amplitudes. However, a minimum excitation energy is needed to excite the condensate quadrupole mode. Observing the excited cloud in TOF expansion, we note that for excitation amplitude in which the quadrupole mode is excited, the cloud expands without invert its aspect ratio. By looking these clouds, after long time-of-flight, it was possible to see vortices and, sometimes, a turbulent state in the condensed cloud. We calculated the momentum distribution of the perturbed BECs and a power law behavior, like the law to Kolmogorov turbulence, was observed. Furthermore, we show that using the method that we have developed to calculate the momentum distribution, the distribution curve (including the power law exponent) exhibits a dependence on the quadrupole mode oscillation of the cloud. The randomness distribution of peaks and depletions in density distribution image of an expanded turbulent BEC, remind us to the intensity profile of a speckle light beam. The analogy between matter-wave speckle and light speckle is justified by showing the similarities in the spatial propagation (or time expansion) of the waves. In addition, the second order correlation function is evaluated and the same dependence with distance was observed for the both waves. This creates the possibility to understand the properties of quantum matter in a disordered state. The propagation of a three-dimensional speckle field (as the matter-wave speckle described here) creates an opportunity to investigate the speckle phenomenon existing in dimensions higher than 2D (the case of light speckle).Item Investigation of the momentum distribution of an excited Bose-Einstein condensate: coupling to the normal modes(2015-03-09) Bahrami, AbasaltTurbulence is a young field of research which is characterized by chaotic spinning flow regimes which appears in many important processes in nature. Vorticity, in superfluid systems, may present the simplest form of turbulence, and be a gateway to the study of this phenomenon in quantum gases. A 87Rb Bose condensate was used to observe and investigate the emergence of quantum turbulence, a few years back in our group. The vortices are created on the condensed-thermal interface and propagate across the cloud, setting up the experimental conditions favorable to the emergence of turbulence. Once the turbulent regime is set, the condensate is released and expands under free fall. Then, the atomic density profile is acquired, after some time-of-flight, and used to determine the in situ momentum distribution of the BEC. In this work, we have observed that, the perturbed density profiles are characteristic and different from the standard, non-perturbed ones. We have seen evidences of power law in the studied momentum and energy distributions and also coupling of quadrupolar mode to the momentum distribution of the excited condensate which is the main part of our findings. Additional features of the system, such as the condensates excited collective modes which plays a very important role on the roadmap to the turbulence regime, are discussed. We are currently setting up an experiment to be able to further investigate such features, and also to unfold the effects of interactions on the energy and momentum spectra associated to the density profiles. In doing so, we will further develop the tools and techniques needed to acquire more accurate and reliable results.Item Consequências das excitações oscilatórias em condensados de Bose-Einstein(2012-06-01) Tavares, Pedro Ernesto SchiavinattiNeste 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.