We report on the observation of a highly-degenerate, strongly-interacting Fermi gas of atoms. Fermionic 6 Li atoms in an optical trap are evaporatively cooled to degeneracy using a magnetic field to induce strong, resonant interactions. Upon abruptly releasing the cloud from the trap, the gas is observed to expand rapidly in the transverse direction while remaining nearly stationary in the axial. We interpret the expansion dynamics in terms of collisionless superfluid and collisional hydrodynamics. For the data taken at the longest evaporation times, we find that collisional hydrodynamics does not provide a satisfactory explanation, while superfluidity is plausible.As the fundamental constituents of matter are interacting fermions, the experimental study of stronglyinteracting, degenerate Fermi gases will impact theories in fields from particle physics to materials science. Although the interactions between fermions are understood when they are weak (e.g., quantum electrodynamics), the treatment of very strong interactions requires the development of new theoretical approaches. To test these new approaches, there is a need for experimental systems with widely tunable interaction strengths, densities, and temperatures. Ultracold atomic Fermi gases have exactly these properties, and thus enable tests of calculational techniques for fundamental systems ranging from quarks in nuclear matter to electrons in high temperature superconductors [1,2, 3,4,5,6,7,8,9,10,11]. For this reason, a number of groups are developing methods for creating and exploring ultracold atomic Fermi gases [12,13,14,15,16,17]. We report on the study of a strongly-interacting, degenerate Fermi gas. In contrast to the isotropic expansion previously observed for a noninteracting degenerate Fermi gas [12], we observe anisotropic expansion when the gas is released from an optical trap.An exciting feature of strongly-interacting atomic Fermi gases is the possibility of high-temperature superfluids that are analogs of very high temperature superconductors [8,9,10,11]. Our experiments produce the conditions predicted for this type of superfluid transition. Further, the anisotropic expansion we observe has been suggested as a signature of the onset of superfluidity in a Fermi gas [18]. We interpret the observed anisotropic expansion in terms of both collisionless superfluid hydrodynamics [18] and a new form of collisional hydrodynamics.Strong, magnetically-tunable interactions are achieved in our experiments by employing a Fermi gas comprising a 50-50 mixture of the two lowest hyperfine states of 6 Li, i.e., the |F = 1/2, M = ±1/2 states in the low-magnetic-field basis. This mixture has a predicted broad Feshbach resonance near an applied magnetic field of 860 G [19,20], where the energy of a bound 6 Li-6 Li molecular state is tuned into coincidence with the total energy of the colliding atoms. This enables the interaction strength to be widely varied [19,20,21,22]. It has also been suggested that interactions between fermions can be modified by immers...

We observe collective oscillations of a trapped, degenerate Fermi gas of 6Li 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) identical with sqrt[10nu(x)nu(y)/3]=2830(20) 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.

A strongly-attractive, two-component Fermi gas of atoms exhibits universal behavior and should be mechanically stable as a consequence of the quantum mechanical requirement of unitarity. This requirement limits the maximum attractive force to a value smaller than that of the outward Fermi pressure. To experimentally demonstrate this stability, we use all-optical methods to produce a highly degenerate, two-component gas of 6 Li atoms in an applied magnetic field near a Feshbach resonance, where strong interactions are observed. We find that the gas is stable at densities far exceeding that predicted previously for the onset of mechanical instability. Further, we provide a temperature-corrected measurement of an important, universal, many-body parameter which determines the stability-the mean field contribution to the chemical potential in units of the local Fermi energy.

We measure a zero crossing in the scattering length of a mixture of the two lowest hyperfine states of 6 Li. To locate the zero crossing, we monitor the decrease in temperature and atom number arising from evaporation in a CO2 laser trap as a function of magnetic field B. The temperature decrease and atom loss are minimized for B = 528 ± 4 G (1 G=10 −4 T), consistent with no evaporation. We also present preliminary calculations using potentials that have been constrained by the measured zero crossing and locate a broad Feshbach resonance at approximately 860 G, in agreement with previous theoretical predictions. In addition, our theoretical model predicts a second and much narrower Feshbach resonance near 550 G.

We derive the elastic collision rate for a harmonically trapped Fermi gas in the extreme unitarity limit where the s-wave scattering cross section is (k)ϭ4/k 2 , with បk the relative momentum. The collision rate is given in the form ⌫ϭ␥ I(T/T F )-the product of a universal collision rate ␥ϭk B T F /(6ប) and a dimensionless function of the ratio of the temperature T to the Fermi temperature T F . We find that I has a peak value of Ӎ4.