NC State University

Physics Department

Research: Observation of a Strongly Interacting Fermi gas

[Picture of the cooling group] [Picture of the expanding cloud]

In 2002, we produced and studied a strongly-interacting, degenerate Fermi gas of atoms. A cigar-shaped cloud of fermionic 6Li atoms is confined and rapidly cooled to degeneracy in our CO2 laser trap, using a magnetic field to induce strong interactions. Upon abruptly turning off the trap, the gas exhibits a spectacular anisotropic expansion, rapidly moving in the transverse direction while remaining nearly stationary along the cigar axis. By contrast, an ordinary noninteracting gas expands evenly in all directions, quickly assuming a spherical shape. For the conditions of the experiments, the anisotropic expansion may be a signature of superfluidity. However, anisotropic expansion may also arise from collisional hydrodynamics under appropriate conditions. A striking observation is that the energy released by the gas and the initial cloud dimensions correspond to those of a zero-temperature noninteracting gas, while the anisotropy is characteristic of a strongly-interacting system. Since interacting fermions are the building blocks of all matter, these studies impact the development of new nonperturbative theoretical methods in fields of research ranging from condensed matter to nuclear and particle physics.

The interaction strength is controlled using a magnetic field. At 910 G, the gas is very strongly interacting. The figure on the left shows absorption images of the expanding, strongly-interacting gas as a function of time t after release from the trap for t=0.1 ms to t=2.0 ms. As can be seen from the figure, very little motion occurs along the direction that was initially the long axis of the cigar-shaped cloud [Animated GIF (565 kB), 3D JPG (102 kB)]. Most of the energy is released in the transverse dimensions of the cigar, causing the cloud to assume an elliptical shape. In the absence of interactions, the gas would expand with the same speed in all directions, assuming a spherical shape a short time after release. By adjusting the magnetic field to 530 G, the interaction strength can be adjusted to be nearly zero. In that case the gas expands ballistically, assuming a spherical shape as expected.

The production of a strongly-interacting, degenerate Fermi gas was accomplished using the all-optical cooling and trapping methods developed by our group. Beginning in 1997, our group pioneered the development of an ultrastable optical trap for atoms which was first demonstrated in 1999. This trap was used in 2001 to achieve the first all-optical production of a degenerate Fermi gas, which enabled the present experiments.