Ultracold Fermi Gases

Our group explores the physics of degenerate 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 ⁴⁰K 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 ⁴⁰K 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.  Our group also explores Bose-Fermi mixtures and ultracold molecules.