Many-Body Physics with Ultracold Atomic Fermions and Bosons

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Gases of atoms cooled to the ultra-low temperatures of 100 billionths of a degree above absolute zero have emerged as a highly versatile platform for the study of collective many-body behavior (the physics of how a collection of objects behaves in ways that is different from what one would expect from knowledge of how the objects behave individually). Such collective phenomena are usually associated with electrons in solid matter, where details about the crystal structure, strong interactions between electrons, and the underlying theory in physics which describes the system (quantum mechanics) can combine to create materials with highly unusual and sometimes very practical properties. The best known examples of such materials are high-temperature superconductors, in which electron currents flow without any resistance. Surprisingly, nearly 30 years after their discovery, physicists still do not understand the underlying mechanisms that create high-temperature superconductors; nor do they know if there is an upper bound on the superconducting temperature. Experiments with ultra-cold atoms on a so-called "optical lattice" created with light waves may help solve this mystery, thereby facilitating the development of practical applications of high temperature superconductors, including the efficient transmission of electrical energy and more cost-effective medical imaging. In this experiment, lithium atoms, which obey the same basic laws of physics as electrons, will act as stand-ins for the electrons in real materials. The atomic system is much cleaner than the real material, since there are no impurities, defects, or lattice dislocations. Furthermore, the parameters of the atomic system, including the atom-atom interaction strength, density, and the lattice parameters, are highly tunable. Ultra-cold lithium atoms will be used to create the most promising model and determine whether or not it contains the essence of superconductivity. The group will exploit the broad Feshbach resonances in the lithium isotopes, the fermion Li-6 and the boson Li-7, to explore many-body physics, in and out of lattices, in contexts that have both fundamental and practical implications. Realization of the full potential of ultracold atoms in optical lattices has been impeded by an inability to cool to sufficiently low temperatures. The group has recently demonstrated a method to evaporatively cool in optical lattices which resulted in the observation of antiferromagnetic correlations in the Fermi-Hubbard model, an archetypal model of condensed matter physics and the most prominent model of high-temperature superconductivity. Even lower temperatures are needed to explore the most novel strongly correlated phenomena. The group proposes to refine the cooling method, using Bragg scattering from magnetic correlations as a sensitive in-situ thermometer. With this system, the group plans to study the phase diagram of the Hubbard model for both repulsive and attractive interactions. Theory suggests that an attractive interaction combined with spin polarization (population imbalance) offers an exciting opportunity to observe the exotic FFLO pairing mechanism. Finally, for the bosonic isotope, the group will conduct a fundamental investigation of matter-wave tunneling for non-interacting and interacting Bose condensates to explore the role of the nonlinearity, including the regime of solitons. The barrier forms a beam splitter, which in a one-dimensional geometry constitutes a Mach-Zehnder interferometer. Its coherence properties and hence, its suitability as a matter-wave interferometer, will be explored.