Our group explores the physics of Fermi gases at ultracold temperatures and investigates the link between superconductivity and Bose-Einstein condensation (BEC). In late 2003, we made the first direct observation of molecular BEC. We used magnetic-field (Feshbach) scattering resonances to induce pairs of K-40 atoms to form bosonic molecules, which then formed a BEC. In 2004, we demonstrated the first Fermi condensates of correlated atom pairs. Here we used the Feshbach resonance to control atom-atom interactions in a trapped gas of K-40 atoms such that fermions with different spins and opposite momentum became correlated. These correlated atom pairs are closely related to Cooper pairs of electrons in superconductors.

Physicists now believe that the BEC and phenomenon of superconductivity [as described by the Bardeen-Cooper-Schrieffer (BCS) theory] represent two ends of a continuum of quantum mechanical behavior. Since 2004, we have conducted detailed studies of the behavior of atoms in the middle of this continuum, called the BCS-BEC crossover.

The condensation behavior of Fermi systems evolves smoothly from BEC behavior (where fermions are paired in tightly bound molecules) through the BCS- BEC crossover to BCS behavior, where Cooper pairs of atoms form a superfluid, as shown below. In the crossover region, pairs of fermions interact strongly with each other. Depending on experimental conditions (in particular, changes in the interaction strength), Fermi atom pairs can behave more like molecules or more like Cooper pairs.

One of the tools we use to probe the strongly interaction Fermi gas in the BCS-BEC crossover is momentum-resolved rf spectroscopy. This spectroscopy can be used to map out the probability to find a fermion at a particular energy and momentum in the strongly interacting system. For example, we see a quadratic dispersion for weakly interacting fermions, but a negatively dispersing signal with a large energy spread for molecules or pairs.