Evidence for Superfluidity in a Resonantly Interacting Fermi Gas

We observe collective oscillations of a trapped, degenerate Fermi gas of ${}^6\mathrm{Li}$ atoms at a magnetic field just above a Feshbach resonance, where the two-body physics does not support a bound state. The gas exhibits a radial breathing mode at a frequency of 2837(05) Hz, in excellent agreement with the frequency of ${\nu }_H\equiv \sqrt{10{\nu }_x{\nu }_y/3}=2830(20)\mathrm{Hz}$ predicted for a hydrodynamic Fermi gas with unitarity-limited interactions. The measured damping times and frequencies are inconsistent with predictions for both the collisionless mean field regime and for collisional hydrodynamics. These observations provide the first evidence for superfluid hydrodynamics in a resonantly interacting Fermi gas.

Figure 1

Excitation of the breathing mode in a noninteracting Fermi gas of ${}^6\mathrm{Li}$ at 526 G. Error bars $=68\%$ confidence interval.

Figure 2

Excitation of the breathing mode in a strongly interacting Fermi gas of ${}^6\mathrm{Li}$ at 870 G. (a) $T/T_F=0.50$. (b) $T/T_F=0.33$. (c) $T/T_F=0.17$. Error bars $=68\%$ confidence interval.

Figure 3

Product of the damping time and the measured breathing mode frequency versus temperature. The open circle shows the result when the trap depth is increased by a factor of 4.1.