Measurement of positive and negative scattering lengths in a Fermi gas of atoms

Abstract

An exotic superfluid phase has been predicted for an ultracold gas of fermionic atoms. This phase requires strong attractive interactions in the gas, or correspondingly atoms with a large, negative s-wave scattering length. Here we report on progress toward realizing this predicted superfluid phase. We present measurements of both large positive and large negative scattering lengths in a quantum degenerate Fermi gas of atoms. Starting with a two-component gas that has been evaporatively cooled to quantum degeneracy, we create controllable, strong interactions between the atoms using a magnetic-field Feshbach resonance. We then employ a novel 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.