Strongly-interacting fermions in an optical lattice

Abstract

Two sets of studies are described in this thesis. The first describes studies conducted with sodium Bose-Einstein condensates (BEC) while the second focuses on the pairing of fermionic lithium-6 pairs in an optical lattice within the strongly interacting BEC-BCS regime. Common to both sets of studies is the use of a magnetically tunable Feshbach resonance to manipulate interactions between the atoms. In the first experiment, we destabilize a sodium BEC by switching its interactions from repulsive to attractive and studied the resulting dynamics. A local amplification of low momentum energetic instabilities was observed and the measured rate of amplification agreed well with theoretical predictions. For large condensates, this process depleted the condensate faster than the global inward collapse. Subsequently, I describe the major construction effort that was undertaken to convert our BEC machine to a two-species machine capable of cooling fermionic lithium-6. Upon its completion, we obtained a resonance superfluid of loosely bound 6Li pairs in the BECBCS crossover. When placed in a shallow optical lattice, long range phase coherence of this resonance superfluid was inferred from the presence of sharp interference peaks after ballistic expansion. With this observation we have obtained the first evidence of superfluidity of fermions in an optical lattice. A loss in phase coherence occurred when the lattice depth was increased past a critical value, possibly signaling a transition to an insulating state. Further preliminary explorations of this novel system is described followed by an outline of its potential for studying condensed matter phenomena like high temperature superconductivity.