The formation of composite bosons by pairing fermions leads to intriguing
phenomena in physics, with superconductivity and 3He superfluidity
being prominent examples. In an ultracold gas of fermionic atoms, the tunable
pairing interaction offers exciting possibilities to explore the crossover from
a molecular Bose-Einstein condensate (BEC) in the strong-coupling regime to a
Cooper-paired superfluid in the weak-coupling regime.
We achieve Bose-Einstein condensation of molecules by first converting fermionic
atoms into bosonic molecules, and then cooling the molecules below the
transition temperature. Remarkably, the molecular BEC has a long lifetime of
>40s and provides a unique opportunity to study the BEC-BCS crossover, as well
as the resonance superfluidity.
We have observed a smooth conversion from a molecular BEC to an degenerate Fermi
gas with temperature T/TF<0.03. In collective modes and excitation
spectra, we identify an abrupt phase transition and the formation of psuedogaps
in the strongly interacting Fermi gas.