Stoßlawinen in einem Bose-Einstein-Kondensat

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2002
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Schuster, Johannes
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Collisional Avalanches in a Bose-Einstein Condensate
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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.
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Die vorliegende Arbeit befaßt sich mit elastischen und inelastischen Stößen zwischen ultrakalten Rubidium-Atomen, insbesondere werden Auswirkungen erhöhter Wechselwirkung auf die Stabilität von atomaren Bose-Einstein-Kondensaten untersucht. Dabei gelingt es, einen bisher unberücksichtigten Prozeß zu identifizieren, der die Lebensdauer von Kondensaten auf wesentlich kürzere Zeitskalen beschränken kann, als sie ausgehend von den bekannten Verlustraten durch inelastische Stöße erwartet werden. Er ist auf die Undurchdringbarkeit des Kondensats für niederenergetische Atome zurückzuführen, die mit der Wechselwirkung zunimmt und bedingt, daß die Produkte exothermer Stöße ganze Lawinen elastischer Kollisionen auslösen können. Stoßlawinen werden durch die Isotropie des differentiellen Wirkungsquerschnitts für elastische Stöße im niederenergetischen, sogenannten s-Wellen-Regime begünstigt und treten auf, sobald die Stoßrate in dem System über ein kritisches Maß hinaus erhöht wird. Durch die Kaskade elastischer Stöße kann ein signifikanter Teil der Energie, die bei dem auslösenden inelastischen Stoß freigesetzt wird, im Kondensat dissipiert werden. Da Stoßlawinen nicht allein die Lebensdauer des Gases drastisch reduzieren, sondern auch sein thermodynamisches Gleichgewicht stören, hat ihr Auftreten nachteilige Konsequenzen für das Studium der Wechselwirkung in atomaren Kondensaten. Es wird ein einfaches Modell eingeführt, in dessen Rahmen sich die Auswirkungen von Stoßlawinen quantifizieren lassen und dessen Prognosen gut mit Messungen übereinstimmen. Die Arbeit enthält ferner die Beschreibung und die Charakterisierung einer neuen Apparatur, die es erlaubt, große atomare Kondensate mit hoher Wiederholrate und Genauigkeit zu reproduzieren und damit beste Voraussetzungen für das geplante Studium feldinduzierter elastischer Streuresonanzen bietet.
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530 Physics
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Magnetooptische Falle,Magnetische Speicherung,Verdampfungskühlung,Spinrelaxation,Stoßlawinen,Bose-Einstein condensation,laser cooling and trapping,ultra-cold collisions,collisional avalanches
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ISO 690SCHUSTER, Johannes, 2002. Stoßlawinen in einem Bose-Einstein-Kondensat [Dissertation]. Konstanz: University of Konstanz
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  year={2002},
  title={Stoßlawinen in einem Bose-Einstein-Kondensat},
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  address={Konstanz},
  school={Universität Konstanz}
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