Bulletin of the American Physical Society
41st Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 55, Number 5
Tuesday–Saturday, May 25–29, 2010; Houston, Texas
Session L1: Focus Session: Many-Body Physics with Ultracold Atoms |
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Chair: Brian DeMarco, University of Illinois at Urbana-Champaign Room: Imperial East |
Thursday, May 27, 2010 2:00PM - 2:30PM |
L1.00001: Towards quantum magnetism with ultracold atoms Invited Speaker: Spin ordering or quantum magnetism can be realized with both ultracold bosons and fermions. We have studied ultracold fermions with strong repulsive interactions. When a Feshbach resonance is approached, a two-component Fermi gas shows non- monotonic behavior of lifetime, kinetic energy and size. This provides strong evidence for a phase-transition to a ferromagnetic state, or a state with strong ferromagnetic fluctuations. A two component Mott insulator of bosons in an optical lattice should show spin ordering at sub-nanokelvin temperatures. As a step towards this goal, we have developed a novel method of thermometry, spin gradient thermometry. In this method, the two states are pulled towards opposite sides of the trapped sample by a magnetic field gradient. The width of the domain wall is proportional to the temperature. Using this method, we optimized the temperature in a Mott insulator to 1 nK, the lowest measured temperature in a lattice, indicating that the system has reached the quantum regime, where insulating shells are separated by superfluid layers. [Preview Abstract] |
Thursday, May 27, 2010 2:30PM - 3:00PM |
L1.00002: Ultracold Quantum Gases in Hexagonal Optical Lattices Invited Speaker: Hexagonal structures occur in a vast variety of systems, ranging from honeycombs of bees in life sciences to carbon nanotubes in material sciences. The latter, in particular its unfolded two-dimensional layer -- Graphene -- has rapidly grown to one of the most discussed topics in condensed-matter physics. Not only does it show proximity to various carbon-based materials but also exceptional properties owing to its unusual energy spectrum. In quantum optics, ultracold quantum gases confined in periodic light fields have shown to be very general and versatile instruments to mimic solid state systems. However, so far nearly all experiments were performed in cubic lattice geometries only. Here we report on the first experimental realization of ultracold quantum gases in a state-dependent, two-dimensional, Graphene-like optical lattice with hexagonal symmetry. The lattice is realized via a spin-dependent optical lattice structure with alternating $\sigma ^{+}$ and $\sigma ^{-}$ -sites and thus constitutes a so called `magnetic'-lattice with `antiferromagnetic'-structure. Atoms with different spin orientation can be loaded to specific lattice sites or -- depending on the parameters -- to the whole lattice. As a consequence e.g. superpositions of a superfluid spin component with a different spin component in the Mott-insulating phase can be realized as well as spin-dependent transport properties, disorder etc. After preparing an antiferromagnetically ordered state we e.g. measure sustainable changes of the transport properties of the atoms. This manifests in a significant reduction of the tunneling as compared to a single-component system. We attribute this observation to a partial tunneling blockade for one spin component induced by population in another spin component localized at alternating lattice sites. Within a Gutzwiller-Ansatz we calculate the phase diagrams for the mixed spin-states and find very good agreement with our experimental results. Moreover, by state-resolved recording of the position of the atoms we observe a profound redistribution of the atoms in the lattice due to interstate interactions. [Preview Abstract] |
Thursday, May 27, 2010 3:00PM - 3:12PM |
L1.00003: Phase Diagram of a One-Dimensional Spin-Imbalanced Fermi Gas Ann Sophie Rittner, Yean-an Liao, Tobias Paprotta, Wenhui Li, Guthrie Partridge, Randy Hulet We report experimental measurements of spin and density profiles of a two spin mixture of ultracold $^{6}$Li atoms trapped in an array of one dimensional (1D) tubes\footnote{Y. Liao {\it et al.}, Submitted; in collaboration with S. K. Baur and E. J. Mueller.}, a system analogous to electrons in 1D wires. Compared to the three-dimensional case\footnote{G. B. Partridge \textit{et al.}, \textit{Science} 311, 503-505 (2006); G. B. Partridge {\it et al.}, {\it Phys. Rev. Lett.} {\bf 97}, 190407 (2006).}, at finite spin imbalance the 1D system shows an inverted phase separation: a partially polarized core surrounded by wings composed of either a completely paired BCS superfluid or a fully polarized Fermi gas, depending on the degree of polarization. The observations are in quantitative agreement with the Bethe ansatz and numerous other theoretical calculations\footnote{G. Orso, Phys. Rev. Lett. 98, 070402 (2007); H. Hu et. al, Phys. Rev. Lett. 98, 070403 (2007).}, in which the partially polarized phase is found to be a 1D analogue of the FFLO state, a superfluid state with spatially modulated magnetic order. [Preview Abstract] |
Thursday, May 27, 2010 3:12PM - 3:24PM |
L1.00004: The BCS-BEC crossover and the disappearance of FFLO-correlations in a spin-imbalanced, 1D Fermi gas Fabian Heidrich-Meisner, Adrian Feiguin, Ulrich Schollwoeck, Wilhelm Zwerger We present a numerically exact study of the one-dimensional BCS-BEC crossover of a spin-imbalanced Fermi gas, using a two-channel model. Specifically, the crossover is described by the Bose-Fermi resonance model in a real space representation. Our main interest is in the behavior of the pair correlations, which, in the BCS limit, are of the Fulde-Ferrell-Larkin-Ovchinnikov type, while in the BEC limit, a superfluid of diatomic molecules forms that exhibits quasi-condensation at zero momentum. We use the density matrix renormalization group method to compute the phase diagram as a function of the detuning of the molecular level and the polarization. As a main result, we show that, for sufficiently large densities, FFLO-like correlations disappear well below full polarization close to the resonance, and on the BCS side. The critical polarization depends on both the detuning and the filling. F. Heidrich-Meisner, A. Feiguin, U. Schollwoeck, W. Zwerger, Phys. Rev. A, in press, arXiv:0908.3074 [Preview Abstract] |
Thursday, May 27, 2010 3:24PM - 3:36PM |
L1.00005: The Imbalanced Antiferromagnet Henk Stoof We study the rich properties of the imbalanced antiferromagnet in an optical lattice. We present its phase diagram, discuss spin waves and explore the emergence of topological excitations in two dimensions, known as merons, which are responsible for a Kosterlitz-Thouless transition that has never unambiguously been observed. [Preview Abstract] |
Thursday, May 27, 2010 3:36PM - 3:48PM |
L1.00006: Quantum phase transition in space in a ferromagnetic spin-1 Bose-Einstein condensate Bogdan Damski, Wojciech Zurek A quantum phase transition between the symmetric phase and the phase with broken symmetry can be induced in a ferromagnetic spin-1 Bose-Einstein condensate by an inhomogeneous magnetic field. We consider such a phase transition and show that the transition region in the vicinity of the critical point exhibits scalings that reflect a compromise between the rate at which the transition is imposed (i.e., the gradient of the control parameter) and the scaling of the divergent healing length in the critical region. Our results explore analogies between dynamics of quantum phase transitions induced by time-dependent (Kibble-Zurek mechanism, etc.) and position-dependent variations of the driving field. They also suggest a new method for the direct measurement of the scaling exponent $\nu $. This work is summarized in: B. Damski and W.H. Zurek, New J. Phys. \textbf{11}, 063014 (2009). [Preview Abstract] |
Thursday, May 27, 2010 3:48PM - 4:00PM |
L1.00007: Anomalous and Spin Hall Insulator in A Fermionic Cold Atom Optical Lattice Chuanwei Zhang, Junren Shi, Qian Niu We construct a band insulator in a two dimensional fermionic cold atom optical lattice. The insulator has quantized anomalous Hall conductance and non-vanishing spin Hall coefficient. Although the spin Hall coefficient in the insulator is not quantized in general, it is extremely robust against disorder. [Preview Abstract] |
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