title: Transition from collisionless to hydrodynamic behavior in an ultracold Fermi gas.
abstract: Relative motion in a two-component, trapped atomic gas provides a sensitive probe of interactions. By studying the lowest frequency excitations of a two-spin-state gas confined in a magnetic trap, we have explored the transition from the collisionless to the hydrodynamic regime. As a function of collision rate, we observe frequency shifts as large as 6% as well as a dramatic, nonmonotonic dependence of the damping rate. The measurements agree qualitatively with expectations for behavior in the collisionless and hydrodynamic limits and are quantitatively compared to a classical kinetic model.
authors.affiliation: JILA, Quantum Physics Division, National Institute of Standards and Technology, University of Colorado, Boulder, Colorado 80309, USA.
authors.forename: S D
authors.initials: SD
authors.lastname: Gensemer
authors.forename: D S
authors.initials: DS
authors.lastname: Jin