Repositório Institucional IFSC

URI permanente desta comunidadehttp://143.107.180.6:4000/handle/RIIFSC/1

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    Study of excitations in a Bose-Einstein condensate
    (2011-11-01) Harutinian, Jorge Amin Seman
    In this work we study a Bose-Einstein condensate of 87Rb under the effects of an oscillatory excitation. The condensate is produced through forced evaporative cooling by radio-frequency in a harmonic magnetic trap. The excitation is generated by an oscillatory quadrupole field superimposed on the trapping potential. For a fixed value of the frequency of the excitation we observe the production of different regimes in the condensate as a function of two parameters of the excitation: the time and the amplitude. For the lowest values of these parameters we observe a bending of the main axis of the condensate. This demonstrates that the excitation is able to transfer angular momentum into the sample. By increasing the time or the amplitude of the excitation we observe the nucleation of an increasing number of quantized vortices. If the value of the parameters of the excitation is increased even further the vortices evolve into a different regime which we have identified as quantum turbulence. In this regime, the vortices are tangled among each other, generating a highly irregular array. For the highest values of the excitation the condensate breaks into pieces surrounded by a thermal cloud. This constitutes a different regime which we have identified as granulation. We present numerical simulations together with other theoretical considerations which allow us to interpret our observations. In this thesis we also describe the construction of a second experimental setup whose objective is to study magnetic properties of a Bose-Einstein condensate of 87Rb. In this new system the condensate is produced in a hybrid trap which combines a magnetic trap with an optical dipole trap. Bose-Einstein condensation has been already achieved in the new apparatus; experiments will be performed in the near future.
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    Investigation with atomic superfluids
    (2015-10-13) Bagnato, Vanderlei Salvador
    This is a cooperative research involving MIT and USP to investigate in Brazil different aspects of quantum Turbulence in samples of BEG, while at MIT we shall cooperate in the investigation of magnetism in samples of quantum fluids of bosons and fermions. The study of vortices in quantum fluids started with the studies of superfluid liquid helium. Those studies has geneerted an incredible amount of physics. The study of vortices in condensates has never suffered for lack of interest. Over the years, studies involving new aspects of both the formation and the dynamics of vortices were performed as well as the development of new techniques of vortex nucleation. Recently, vortices were observed in our laboratory by means of a new technique that consists of introducing off-axis oscillations in the condensate. Besides vortices we also produce antivortices. The existence of vortices and antivortices in the same sample and their mutual interaction and annihilation are new topics never studied before experimentally. This system composed by vortices and anti-vortices evolves in an interesting way in relation to the frequency and amplitude of the excitation. Eventually, these excitations lead the system to a regime called quantum turbulence, where there are tangled vortices in each direction of space. The study of quantum turbulent systems also started with liquid helium. The study of quantum turbulence became interesting as a prototype for the understanding of the statistics as well as the dynamics of the nucleation of vortices in these regimes and possibly extension to classical regimes. Unfortunately, helium is not a good system for the observation and quantitative measurement of features due to the experimental difficulties in the control of the parameters as well as the generation of turbulence. In contrast, Bose-Einstein condensates, where the experimental parameters can be precisely tuned and the addition of angular momentum can be done in a controlled way are the perfect samples for this kind of study. Nevertheless, experimentally and theoretically there have been few studies in this regime until now. Recently, a series of theoretical papers demonstrated several aspects related to quantum turbulence in BEGs. In this project we shall develop experiments to investigate the influence of the temperature on the relaxation of the trubulent state. The transference of vorticity from one specie to a second one will be accomplished in a double specie experiment. Stabilization of a high charged vortex will be investigated using a mixture of two superfluids. Theoretical studies involving turbulence and its control will also be part of this program. While in Brazill we shall investigate quantum turbulence, at MIT we will collaborate with their programs of quantum magnetism which has many aspects. (AU)