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Can a gas of spin-up and spin-down fermions become ferromagnetic because of repulsive interactions? We addressed this question, for which there is not yet a definitive theoretical answer, in an experiment with an ultracold two-component Fermi gas. The observation of nonmonotonic behavior of lifetime, kinetic energy, and size for increasing repulsive interactions provides strong evidence for a phase transition to a ferromagnetic state. Our observations imply that itinerant ferromagnetism of delocalized fermions is possible without lattice and band structure, and our data validate the most basic model for ferromagnetism introduced by Stoner., National Science Foundation (U.S.), United States. Office of Naval Research. Multidisciplinary University Research Initiative., United States. Army Research Office (Contract W911NF-07-1-0493), United States. Defense Advanced Research Projects Agency. Optical Lattice Emulator Program

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2011., Cataloged from PDF version of thesis., Includes bibliographical references (p. 218-226)., The thesis presents results from experiments in which ultracold Sodium-6 and Lithium-23 atomic gases were studied near a Feshbach resonance at high magnetic fields. The enhanced interactions between atoms in the presence of a molecular state enhance collisions, leading to inelastic decay and loss, many-body dynamics, novel quantum phases, and molecule formation. Experimental data is presented alongside relevant theory and numerical models. Results are presented for both homonuclear Na 2 and Li 2 molecules, as well as heteronuclear NaLi resonances, although we were unable to isolate and measure NaLi molecules. Furthermore, experiments and theories related to strongly-correlated quantum phases such as Stoner model ferromagnetism, Bose mediated Fermi interactions, and Bose-Fermi mixtures are presented as applicable to Na and Li gases. Conclusions are presented regarding the feasibility of producing deeply bound, dipolar NaLi molecules, as well as future prospects for strongly interacting atomic gases of Na and Li., by Caleb A. Christensen., Ph.D.

Fermi gases with repulsive interactions are characterized by measuring their compressibility as a function of interaction strength. The compressibility is obtained from in-trap density distributions monitored by phase-contrast imaging. For interaction parameters k[subscript F]a>0.25, fast decay of the gas prevents the observation of equilibrium profiles. For smaller interaction parameters, the results are adequately described by first-order perturbation theory. We have developed a phase-contrast imaging method that compensates for dispersive distortions of the images., National Science Foundation (U.S.)., United States. Office of Naval Research, United States. Office of Naval Research. Multidisciplinary University Research Initiative, United States. Army Research Office (grant no. W911NF-07-1-0493)

Fermi gases with repulsive interactions are characterized by measuring their compressibility as a function of interaction strength. The compressibility is obtained from in-trap density distributions monitored by phase-contrast imaging. For interaction parameters k[subscript F]a>0.25, fast decay of the gas prevents the observation of equilibrium profiles. For smaller interaction parameters, the results are adequately described by first-order perturbation theory. We have developed a phase-contrast imaging method that compensates for dispersive distortions of the images., National Science Foundation (U.S.)., United States. Office of Naval Research, United States. Office of Naval Research. Multidisciplinary University Research Initiative, United States. Army Research Office (grant no. W911NF-07-1-0493)

We describe the formation of fermionic NaLi Feshbach molecules from an ultracold mixture of bosonic [superscript 23]Na and fermionic [superscript 6]Li. Precise magnetic field sweeps across a narrow Feshbach resonance at 745 G result in a molecule conversion fraction of 5% for our experimental densities and temperatures, corresponding to a molecule number of 5×10[superscript 4]. The observed molecular decay lifetime is 1.3 ms after removing free Li and Na atoms from the trap. Due to its extremely low reactivity, NaLi molecules in the ground state will have a much longer lifetime than KRb., United States. Air Force Office of Scientific Research. Multidisciplinary University Research Initiative, National Science Foundation (U.S.), United States. Office of Naval Research, United States. Army Research Office (Grant W911NF-07-1-0493), United States. Defense Advanced Research Projects Agency. Optical Lattice Emulator Program, Natural Sciences and Engineering Research Council of Canada (NSERC)

Elongated Bose-Einstein condensates (BECs) exhibit strong spatial phase fluctuations even well below the BEC transition temperature. We demonstrate that atom interferometers using such condensates are robust against phase fluctuations; i.e., the relative phase of the split condensate is reproducible despite axial phase fluctuations. However, larger phase fluctuations limit the coherence time, especially in the presence of some asymmetries in the two wells of the interferometer.

The recombination of two split Bose-Einstein condensates on an atom chip is shown to result in heating which depends on the relative phase of the two condensates. This heating reduces the number of condensate atoms between 10% and 40% and provides a robust way to read out the phase of an atom interferometer without the need for ballistic expansion. The heating may be caused by the dissipation of dark solitons created during the merging of the condensates.