Experiments on Molecular Bose-Einstein Condensates

This research proposal is for the continuation of the previous Lise-Meitner research project on ultracold molecules at Universität Innsbruck. In November 2003, the applicant and co-workers in Univ.-Prof. Rudolf Grimm`s group in Innsbruck demonstrated the formation and Bose-Einstein condensation of lithium-6 molecules. This breakthrough provided us with an excellent starting point and opens up unique perspectives to explore the long-sought crossover phenomena between Bose-Einstein condensation and Cooper pairing in Bardeen-Cooper-Schreiffer state (BEC- BCS crossover). Subsequent studies on the conversion process, collective excitation modes and pairing gaps provide a strong case for the existence of superfluidity in the ultracold Fermi gas. In this project, the applicant proposes to further explore the superfludity in the crossover regime by the creation of vortices and by the introduction of optical lattices: 1. Production of vortices in the BEC-BCS crossover regime The formation of vortices will provide direct and unambiguous evidence of superfluidity. By first creating a vortex in the BEC and then tuning the system into the crossover regime, the persistence of the vortex will not only confirm the smooth conversion from bosonic superfluid to fermionic superfluid, but also reveal the essential properties of the fermionic superfluidity in the strongly coupling regime. 2. Coherence of atomic Cooper pairs in optical lattices For atomic condensate, the diffraction phenomenon of matter waves in optical lattices is well understood. In the crossover regime, the interplay between the long-range pairs and the optical lattices will provide direct evidence of the superfluid coherence and a quantitative measure of the condensate fraction. Furthermore, Bragg diffractions induced by two-photon Raman transitions can reveal both the static and the dynamic structure factors of the superfluid. To summarize, the goal of this project is to explore the superfluidity in the BEC-BCS crossover regime. The experiments will provide the essential information of the coherence properties of a fermionic system and uncover the underlying pairing mechanism in the strongly interacting regime. We expect that new phenomena and physics discovered in this project can lead to a deeper understanding of other superfluid systems, eg., high-Tc superconductor, He-3 and He-4 superfluids and neutron star.