Breakdown of Hydrodynamics in the Radial Breathing Mode of a Strongly-Interacting Fermi Gas

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We measure the magnetic field dependence of the frequency and damping time for the radial breathing mode of an optically trapped, Fermi gas of ⁶Li atoms near a Feshbach resonance. The measurements address the apparent discrepancy between the results of Kinast et al., [Phys. Rev. Lett. 92, 150402 (2004)] and those of Bartenstein et al., [Phys. Rev. Lett. 92, 203201 (2004)]. Over the range of magnetic field from 770 G to 910 G, the measurements confirm the results of Kinast et al. Close to resonance, the measured frequencies are in excellent agreement with predictions for a unitary hydrodynamic gas. At a field of 925 G, the measured frequency begins to decrease below predictions. For fields near 1080 G, we observe a breakdown of hydrodynamic behavior, which is manifested by a sharp increase in frequency and damping rate. The observed breakdown is in qualitative agreement with the sharp transition observed by Bartenstein et al., at 910 G.