From Molecules to Cooper Pairs: Experiments in the BEC-BCS Crossover
Dr Markus Bartenstein
Institut f�r Experimentalphysik, Universit�t 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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