From molecules to Cooper pairs

This Ph. D. thesis reports on the first experiments with an ultracold gas of fermionic 6Li atoms in the crossover from a molecular Bose-Einstein condensate (BEC) to a Bardeen-Cooper-Schrieffer (BCS) superfluid of „Cooper paired“ fermions. 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 serves as the experimental key to explore various coupling regimes. Starting from a molecular BEC in the strong coupling limit we exploit the tunability of the coupling strength to explore the BEC-BCS crossover by studying elementary macroscopic and microscopic properties of the gas.