S. R. Granade

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.

All-optical production of a degenerate Fermi gas.

We achieve degeneracy in a mixture of the two lowest hyperfine states of 6Li by direct evaporation in a CO2 laser trap, yielding the first all optically produced degenerate Fermi gas. More than 10(5) atoms are confined at temperatures below 4 microK at full trap depth, where the Fermi temperature for each state is 8 microK. This degenerate two-component mixture is ideal for exploring mechanisms of superconductivity ranging from Cooper pairing to Bose-Einstein condensation of strongly bound pairs.

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

Ultrastable CO2 Laser Trapping of Lithium Fermions

We demonstrate an ultrastable CO2 laser trap that provides tight confinement of neutral atoms with negligible optical scattering and minimal laser-noise- induced heating. Using this method, fermionic 6Li atoms are stored in a 0.4 mK deep well with a 1/e trap lifetime of 300 sec, consistent with a background pressure of 10^(-11) Torr. To our knowledge, this is the longest storage time ever achieved with an all-optical trap, comparable to the best reported magnetic traps.

Modeling the evaporative cooling of fermionic atoms in an optical trap

Summary form only given. Evaporative cooling of atoms in an optical trap has recently received renewed attention as a means for obtaining degeneracy in an optically trapped bosonic Cs vapor as well as a fermionic Li gas. We have developed a new model describing the evaporative cooling process for atoms confined in a time-dependent optical potential formed by a single focused Gaussian laser beam. We find that a substantial increase in the phase-space density can be obtained by adiabatically reducing the trap laser power and thereby the trap well depth as a function of time. Since we are specifically interested in the evaporative cooling of fermionic /sup 6/Li atoms, we have included the effect of Fermi statistics in the model. Although the collision rate is suppressed as the temperature T is reduced below the Fermi temperature T/sub F/, we find that values of T/T/sub F//spl Lt/1 can be achieved for suitable initial conditions.

Optically-trapped Fermi gas

Summary form only given. Cold, dense gases of fermions offer exciting new opportunities for fundamental studies of quantum degeneracy, collective behavior, and superfluidity. Fermionic /sup 6/Li has been the subject of numerous theoretical treatments. This is due in part to its anomalously large and negative triplet scattering length, a/sub T/=-2160 a/sub O/, which arises from a near zero energy resonance. The large scattering length leads to two-state mixtures which are predicted to undergo a transition to a superfluid state at a relatively high temperature. To increase our well depth, we retroreflect the CO/sub 2/ beam which forms our optical trap, nearly doubling the intensity of the trapping field and thus the well depth. We will discuss this technique and its effect on evaporative cooling, and report our progress towards producing a degenerate gas of /sup 6/Li.