Breakdown of hydrodynamics in the radial breathing mode of a strongly interacting Fermi gas

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 7700.5em0exto0.5em0ex9100.3em0exG, 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 9250.3em0exG, the measured frequency begins to decrease below predictions. For fields near 10800.3em0exG, 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 9100.3em0exG.