B. Clancy

Measurement of the entropy and critical temperature of a strongly interacting Fermi gas.

We report a model-independent measurement of the entropy, energy, and critical temperature of a degenerate, strongly interacting Fermi gas of atoms. The total energy is determined from the mean square cloud size in the strongly interacting regime, where the gas exhibits universal behavior. The entropy is measured by sweeping a bias magnetic field to adiabatically tune the gas from the strongly interacting regime to a weakly interacting regime, where the entropy is known from the cloud size after the sweep. The dependence of the entropy on the total energy quantitatively tests predictions of the finite-temperature thermodynamics.

Is a Gas of Strongly Interacting Atomic Fermions a Nearly Perfect Fluid

Abstract We use all-optical methods to produce a highly-degenerate Fermi gas of spin-1/2 6Li atoms. A magnetic field tunes the gas near a collisional (Feshbach) resonance, producing strong interactions between spin-up and spin-down atoms. We have measured properties of a breathing mode over a wide range of temperatures. As the temperature is increased from below the superfluid transition to above, the frequency of the mode is always close to the hydrodynamic value, while the damping rate increases. A complete explanation of both the frequency and the damping rate in the normal collisional regime has not been achieved. Our measurements of the damping rate as a function of the energy of the gas are used to estimate an upper bound on the viscosity. Using our new measurements of the entropy of the gas, we estimate the ratio of the shear viscosity to the entropy density and compare the result with a recent string theory conjecture for the minimum viscosity of a perfect quantum fluid.

Observation of anomalous spin segregation in a trapped Fermi gas.

We report the observation of spin segregation, i.e., time-dependent separation of the spin density profiles of two spin states, in a trapped, coherently prepared Fermi gas of 6Li with a magnetically tunable scattering length a12 close to zero. For |a12| approximately = 5 bohr, as the cloud profiles evolve, the measured difference in the densities at the cloud center increases in 200 ms from 0 to approximately = 60% of the initial mean density and changes sign with a12. The data are in disagreement in both amplitude and temporal evolution with a spin-wave theory for a Fermi gas. In contrast, for a Bose gas, an analogous theory has successfully described previous observations of spin segregation. The observed segregated atomic density profiles are far from equilibrium, yet they persist for approximately = 5 s, long compared to the axial trapping period of 6.9 ms. We find the zero crossing in a12=0, where spin segregation ceases, at 527.5+/-0.2 G.

Observation of nearly perfect irrotational flow in normal and superfluid strongly interacting fermi gases

Strongly interacting Fermi gases [1] provide a unique paradigm for exploring strongly interacting nearly perfect fluids in nature, from high temperature superfluids and superconductors to exotic normal fluids, such as the quark-gluon plasma of the Big Bang [2, 3], or low viscosity quantum fields [4]. Rotating superfluids require irrotational flow, which quenches the moment of inertia or produces a vortex lattice [5]. Recently, the observation of vortices has directly demonstrated superfluidity in a strongly interacting Fermi gas [6]. However, the moment of inertia has not been measured and the rotational properties have not been characterized in the normal regime, where irrotational flow is not required.

Evaporative cooling of unitary Fermi gas mixtures in optical traps

We measure the scaling laws for the number of atoms and the cloud size as a function of trap depth for evaporative cooling of a unitary Fermi gas in an optical trap. A unitary Fermi gas comprises a trapped mixture of atoms in two hyperfine states which is tuned to a collisional (Feshbach) resonance using a bias magnetic field. Near resonance, the zero energy s-wave scattering length diverges, and the s-wave scattering cross-section is limited by unitarity to be 4?/k2, where k is the relative wavevector of the colliding particles. In this case, the collision cross-section for evaporation scales inversely with the trap depth, enabling runaway evaporation under certain conditions. We demonstrate high evaporation efficiency, which is achieved by maintaining a high ratio ? of trap depth to thermal energy as the trap depth is lowered. We derive and demonstrate a trap lowering curve which maintains ? constant for a unitary gas. This evaporation curve yields a quantum degenerate sample from a classical gas in a fraction of a second, with only a factor of three loss in atom number.

Optical trapping and fundamental studies of atomic fermi gases

Optical traps provide tight confinement and very long storage times for atomic gases. Using a single focused beam from a CO 2 laser, we confine a mixture of spin-up and spin-down fermionic 6 Li atoms, achieving storage times of ten minutes, and evaporative cooling to quantum degeneracy in seconds. A bias magnetic field tunes the gas to a collisional (Feshbach) resonance, producing extremely strong spin-pairing. This system now tests current many-body predictions for high-temperature superconductors, universal interactions in neutron stars, and hydrodynamic flow of quark-gluon plasmas, a state of matter that existed microseconds after the Big Bang.Optical traps provide tight confinement and very long storage times for atomic gases. Using a single focused beam from a CO2 laser, we confine a mixture of spin-up and spin-down fermionic 6Li atoms, achieving storage times of ten minutes, and evaporative cooling to quantum degeneracy in seconds. A bias magnetic field tunes the gas to a collisional (Feshbach) resonance, producing extremely strong spin-pairing. This system now tests current many-body predictions for high-temperature superconductors, universal interactions in neutron stars, and hydrodynamic flow of quark-gluon plasmas, a state of matter that existed microseconds after the Big Bang.