Stoßlawinen in einem Bose-Einstein-Kondensat

Abstract

This thesis deals with elastic and inelastic collisions among ultra cold rubidium atoms. In particular, the impact of increased interaction on the stability of Bose-Einstein condensates is investigated both, experimentally and theoretically. A new decay mechanism is identified that significantly reduces the life time of condensates with respect to the expectation based on the known inelastic collision rates. Along with the interaction also the column density of the condensate is increased so that it eventually becomes impenetrable to products of exoergic collisions. These products then trigger avalanches of elastic collisions in which a significant part of the energy released in the initiatory exoergic collision is distributed among condensate atoms. Collisional avalanches are favoured by the isotropy of the differential scattering cross section in the low energy (s-wave) limit and occur as soon as the elastic collision rate exceeds a critical value. Since avalanches not only reduce the life time of the condensate but also disturb its thermodynamic equilibrium, their occurrence restricts the possibilities to explore the strongly interacting regime. A simple model is introduced to quantify the impact of avalanches on the decay and agrees well with experimental data. The thesis further contains the description and the characterisation of a new experimental apparatus which allows to create large atomic condensates with excellent reproducibility and high repetition rate complying with the requirements for the investigation of field-induced elastic scattering resonances.