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Ring geometries provide natural arenas for probing transport properties of superfluids. Metastable states of quantized superfluid flow ---persistent currents--- exhibit remarkable properties, and the manner in which they form is an incredibly rich subject. Studies of quenched superfluids demonstrate that persistent currents can form from fragments of spontaneous symmetry breaking as second-order phase transitions are crossed at finite rates. The extent of these fragments of the higher-symmetry phase can in some limits be predicted by the Kibble-Zurek mechanism (KZM), which is fundamentally tied to the universal properties characterizing the transition. Thus, studies of spontaneous currents in superfluid rings can shed light on universality classes that microscopically distinct systems fall into.  

 This thesis describes the experimental results of two separate yet complimentary investigations of the physics of ultracold $^6$Li atoms confined to ring geometries. The subject of the first investigation is the heating of degenerate fermionic rings subject to collisions with background molecules. The most important result of this study was that the heating due to these ever-present collisions can be substantially reduced by maintaining a reservoir of non-degenerate fermions in contact with the deeply-degenerate atoms in the ring. These findings permit the possibility to perform seconds-long experiments that require maintaining low temperatures. The second part of the thesis describes the first experimental studies of the KZM ever conducted with ultracold fermions in ring-shaped traps. By exploiting long lifetimes offered by the trapping potential utilized in the aforementioned heating studies, we reveal two distinct regimes of quench dynamics. The fast-quench regime agrees with KZM predictions, while the slow-quench regime governed by finite-size effects follows a different trend. Our KZM studies should be readily extendable to scenarios that include current biases, inhomogeneities and disorder, where these controls can be employed to obtain additional information about the phase transition.

Graduate Advisor: Professor Kevin Wright

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