Investigation of quantum fluids specially excited for the generation of vortices, turbulences and other states

Abstract

The main topic of the collaboration will be the investigation of the properties of strongly nonequilibrium Bose-Einstein condensates of trapped atoms. The generation of strongly nonequilibrium condensates is produced experimentally in the group of Prof. V.S. Bagnato bymodulating the trapping potential with an external alternating field. We shall give theoretical interpretations for these experimental results and will compare them with numerical simulations. The aim of the studies is to form the detailed understanding of the intricate behavior of Bose condensates under the action of strong external perturbation. Exposing a trapped Bose-Einstein condensate to external perturbations can produce different spatial structures. First, there appear quantum vortices. Increasing the amount of the injected energy leads to the formation of vortex tangles representing quantum turbulence. With increasing further the amount of injected energy, in a strongly nonequilibrium Bose-condensed system of trapped atoms, vortices become destroyed and there develops a new kind of spatial structures exhibiting essentially heterogeneous spatial density. These structures consist of the high-density droplets, or grains, surrounded by the regions of low density. The droplets are randomly distributed in space, where they move. The grains live sufficiently long time to be treated as a type of metastable creatures. Such structures have been observed in nonequilibrium trapped Bose gases of 87Rb subject to the action of alternating fields. Perturbing the system even stronger transforms the granular structure into wave turbulence, where Bose condensate is destroyed. This complicated picture requires a thorough theoretical explanation and has to be compared with numerical simulations of the nonequilibrium processes employing the realistic parameters corresponding to the experimental setup. (AU)