title: Measurement of positive and negative scattering lengths in a Fermi gas of atoms.
abstract: We report on progress toward realizing a predicted superfluid phase in a Fermi gas of atoms. We present measurements of both large positive and large negative scattering lengths in a quantum degenerate Fermi gas of atoms near a magnetic-field Feshbach resonance. We employ an rf spectroscopy technique to directly measure the mean-field interaction energy, which is proportional to the s-wave scattering length. Near the peak of the resonance we observe a saturation of the interaction energy; it is in this strongly interacting regime that superfluidity is predicted to occur. We have also observed anisotropic expansion of the gas, which has recently been suggested as a signature of superfluidity. However, we find that this can be attributed to a purely collisional effect.
authors.affiliation: JILA, National Institute of Standards and Technology and Department of Physics, University of Colorado, Boulder, CO 80309-0440, USA.
authors.forename: C A
authors.initials: CA
authors.lastname: Regal
authors.forename: D S
authors.initials: DS
authors.lastname: Jin