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 770 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.