Degenerate, Strongly-Interacting Fermi Gas of Atoms
Atomic Physics Division, Laser Cooling and Trapping Group, NIST, Gaithersburg, MD 20899-8910
Staci Hemmer, Michael Gehm, Stephen Granade, and John Thomas
Physics Department, Duke University, Durham, NC 27708
Interacting fermions are the building blocks of all matter. When the interactions are strong, in the sense that the interaction length scale is larger than the interparticle spacing, all energy scales are proportional to the Fermi energy and the system exhibits universal behavior. Hence, tabletop experiments with strongly-interacting Fermi gases can be used to examine new theories of strong interactions in systems ranging from superconductors to nuclear matter.
We have produced a highly-degenerate Fermi gas of atoms near a magnetically tunable Feshbach resonance, providing a test bed for nonperturbative theories of strongly-interacting Fermi systems. Temperatures as low as 0.1 T_F are achieved via rapid forced evaporative cooling in an optical trap. We find that the gas is stable at a density far exceeding that predicted previously for the onset of mechanical instability. Upon release from the trap, we observe dramatic anisotropic expansion (see figure), a possible first signature of the onset of a new type of high-temperature superfluidity. From the release energy, we determine an important, universal many-body parameter: the ratio of the mean-field energy to the local Fermi energy.