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From Molecules to Cooper Pairs: Experiments in the BEC-BCS Crossover

Dr Markus Bartenstein

Institut für Experimentalphysik, Universität Innsbruck, Austria

Friday 27th May 2005, 3.30PM, Seminar Room AR103, Graduate Research Centre.

We explore the crossover from a molecular Bose-Einstein condensate (BEC) to a Bardeen-Cooper-Schrieffer (BCS) superfluid of “Cooper paired” fermions with an ultracold gas of fermionic 6Li atoms. The crucial parameter in the crossover is the coupling strength between the paired atoms. At sufficiently low temperatures a BEC of tightly bound molecules is formed in the strong coupling limit, while in the weak coupling limit a BCS state of delocalized pairs is created.

A magnetically tunable scattering resonance at a magnetic field of approximately 834 G [1] serves as the experimental key to explore various coupling regimes. Through this Feshbach resonance we control the interactions in the gas and vary the coupling strength over a broad range. The starting point for our experiments is a molecular BEC of tightly bound pairs [2]. Exploiting the Feshbach tuning, we explore the BEC-BCS crossover by studying elementary macroscopic and microscopic properties of the gas.

The analysis of density profiles of the trapped cloud in the BEC-BCS crossover shows that it is smooth and reversible [3]. The investigation of collective modes provides insight into changes of the equation of state and hydrodynamics of the system in the crossover regime [4]. Moreover, spectroscopic measurements of the pairing energy enable the observation of the pairing gap in the strongly interacting Fermi gas [5].

Our experiments open up intriguing prospects for further experiments on the fascinating properties of strongly correlated many-body regimes that are of great relevance for several fields of physics like quantum fluids, neutron stars, and most prominently high Tc superconductors.

[1] M. Bartenstein et al., Phys. Rev. Lett. 94, 103201 (2005).
[2] S. Jochim et al., Science 302, 2101 (2003).
[3] M. Bartenstein et al., Phys. Rev. Lett. 92, 120401 (2004).
[4] M. Bartenstein et al., Phys. Rev. Lett. 92, 203201 (2004).
[5] C. Chin et al., Science 305, 1128 (2004).

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