Thermodynamics and superfluidity of a strongly interacting Fermi gas

Strongly interacting two-component Fermi gases are prototypes for other exotic systems in nature, including high temperature superconductors, quark-gluon plasmas and neutron stars. Interactions between spin components can be varied widely, permitting exploration of the crossover from Bose to Fermi statistics. In the strongly interacting limit, the behavior of these systems becomes independent of the microscopic details of their interactions. Hence, strongly interacting Fermi gases provide a testing ground for many-body nonperturbative calculations of strongly interacting matter.; In this dissertation, I describe the first thermodynamic study of a strongly interacting degenerate Fermi gas. This study of the heat capacity shows a transition in behavior at T/TF = 0.27(0.02), which is interpreted as a superfluid phase transition by Kathy Levin's pseudogap theory. This is the first direct measurement of the superfluid transition temperature in the strongly interacting regime. I also describe the first measurement of the temperature dependence of the radial breathing mode in a strongly interacting Fermi gas. As the temperature of the gas is lowered, we find an increase in the lifetime of the breathing mode oscillation at the hydrodynamic frequency. This is inconsistent with expectations for a collisional system, and provides evidence for a superfluid state. As the temperature increases, we observe a transition in behavior at T/TF = 0.35. In the high temperature regime, an abrupt increase in the damping rate is interpreted as the breaking of noncondensed atom pairs. Further, I describe studies of the magnetic field dependence of the radial breathing mode in a low temperature system, which test the best current many-body predictions for the equation of state of the gas. An increase in the damping rate above the center of a broad Feshbach resonance is interpreted as the breaking of atom pairs. I describe our techniques for producing degenerate, stable, strongly interacting two-component mixtures of 6Li confined in an optical trap. Evaporative cooling yields 2.0(0.2) x 105 atoms at temperatures as low as T/TF = 0.06. Starting from these cold atom samples, I describe the techniques used to study the heat capacity and radial breathing mode in a strongly interacting Fermi gas.