S. L. Hemmer

Observation of a Strongly Interacting Degenerate Fermi Gas of Atoms

We report on the observation of a highly degenerate, strongly interacting Fermi gas of atoms. Fermionic lithium-6 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 direction. 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, whereas superfluidity is plausible.

Evidence for superfluidity in a resonantly interacting Fermi gas.

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.

Measurement of the Zero Crossing in a Feshbach Resonance of Fermionic 6Li

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 CO 2 laser trap as a function of magnetic field B. The temperature decrease and atom loss are minimized for B=52.8′0.4 mT, 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 86 mT, in agreement with previous theoretical predictions. In addition, our theoretical model predicts a second and much narrower Feshbach resonance near 55 mT.

Mechanical stability of a strongly interacting Fermi gas of atoms

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 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

Unitarity-limited elastic collision rate in a harmonically trapped Fermi gas

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 {sigma}(k)=4{pi}/k{sup 2}, with ({Dirac_h}/2{pi})k the relative momentum. The collision rate is given in the form {gamma}={gamma} I(T/T{sub F})--the product of a universal collision rate {gamma}=k{sub B}T{sub F}/(6{pi}({Dirac_h}/2{pi})) and a dimensionless function of the ratio of the temperature T to the Fermi temperature T{sub F}. We find that I has a peak value of {approx_equal}4.6 at T/T{sub F}{approx_equal}0.4, I{approx_equal}82 (T/T{sub F}){sup 2} for T/T{sub F}{ 1.5. We estimate the collision rate for recent experiments on a strongly-interacting degenerate Fermi gas of atoms.

Dynamics of a highly-degenerate, strongly-interacting Fermi gas of atoms

We use all-optical methods to produce a highly-degenerate, Fermi gas of 6Li atoms near a Feshbach resonance, where strong interactions are predicted. In this regime, the zero-energy scattering length is larger than the interparticle spacing, and both the mean field energy and the collision rate take on universal forms as a consequence of unitarity and many-body interactions. Our experiments study universal hydrodynamic expansion of the gas and universal mean field interactions. By measuring the cloud radii of the trapped gas, we determine a universal parameter for strongly interacting two-component Fermi systems, the ratio of the mean field energy to the kinetic energy.

Measurement of the mean-field interaction in a strongly-interacting Fermi gas

Summary form only given. We release a strongly-interacting, two-component Fermi gas from an optical trap. From the release energy, we determine an important, universal many-body parameter: the ratio of the mean-field energy to the local Fermi energy.

Observation of hydrodynamic expansion of a strongly-interacting Fermi gas: signature of superfluidity?

Summary form only given. We observe dramatic anisotropic expansion of a strongly-interacting Fermi gas upon release from an optical trap. The observed hydrodynamic behavior is a possible first signature of the onset of a new type of high-temperature superfluidity.

Stability of a strongly-attractive, two-component Fermi gas

Summary form only given. Using a simple model, we show that a strongly-attractive, two-component Fermi gas of atoms should be mechanically stable as a consequence of unitarity. We demonstrate this stability experimentally in a degenerate gas of /sup 6/Li atoms.