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 K1: Focus Session: Strongly Interacting Fermi Gases I |
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Chair: Sophie Rittner, Rice University Room: Imperial East |
Thursday, May 27, 2010 10:30AM - 11:00AM |
K1.00001: Interacting Fermions in Optical Lattices: Entropy and Temperature Invited Speaker: The challenge for the research field of quantum gases is to gain distinctive and new insights into quantum many-body physics - and, if possible, to answer long-standing questions of an underlying model. The combination of quantum degenerate Fermi gases with optical lattice potentials allows the study of a centerpiece of condensed matter physics, the Fermi-Hubbard model, and thereby provides a new route to open questions in quantum magnetism. The talk will report on quantitative experiments with an interacting Fermi gas of potassium atoms inside an optical lattice. From a direct comparison between measurements and theoretical calculations we quantify the entropy and temperature in the approach to magnetic order. Furthermore, a new method will be presented in which the linear response to a periodic perturbation is used for thermometry of the Fermi gas inside the optical lattice. [Preview Abstract] |
Thursday, May 27, 2010 11:00AM - 11:30AM |
K1.00002: Dissipative Fluid Dynamics for the Dilute Fermi Gas at Unitarity Invited Speaker: We summarize recent attempts to extract transport coefficients of the dilute Fermi gas at unitarity from experiments using scaling flows. We analyze data from collective mode, elliptic flow, and rotational flow experiments. We compare with expectations from kinetic theory, and discuss alternative strategies for extracting transport properties. [Preview Abstract] |
Thursday, May 27, 2010 11:30AM - 11:42AM |
K1.00003: Spin Transport in a Strongly Interacting Fermi Gas Ariel Sommer, Andre Schirotzek, Mark Ku, Martin Zwierlein We study the collision and subsequent diffusion of the two spin components of a strongly interacting Fermi gas. The spin components are initially fully separated via a Stern-Gerlach gradient pulse. By quickly turning on an external magnetic field in the vicinity of a Feshbach resonance we control the interaction between the two spin states in the subsequent collision. From our measurements, we find the spin diffusion coefficient as a function of the interaction strength between the two components. It attains a minimum at the Feshbach resonance on the order of h/m, where h is Planck's constant and m is the atomic mass. At positive scattering length, atoms in the two spin states form molecules as they mix. We observe the formation of molecules and the evolution of the atomic and molecular populations using spatially resolved RF spectroscopy. [Preview Abstract] |
Thursday, May 27, 2010 11:42AM - 11:54AM |
K1.00004: Ferromagnetism versus Correlation in Strongly Repulsively Interacting Fermi Gases Hui Zhai, Xiaoling Cui Whether a spin-1/2 Fermi gas with strong repulsive interaction will become ferromagnetic is a long-standing controversial issue. Recently this problem has been studied experimentally by Jo et al., Science, 325, 1521 (2009) on fermi gases with large positive scattering length, and they attribute the observation to Stoner ferromagnetism. We first point out that the experiment evidence itself can hardly rule out the possibility of a non-magnetic but strongly short-range correlated state. We then construct variational wave function to study whether a fully polarized state is energetically stable against a single spin flip. Our variational wave function contains sufficient short-range correlation at least to the same level as Gutzwiller's projected wave function. For Hubbard lattice model and continuum model with pure repulsive interaction, we show a fully polarized Fermi gas is generally unstable even when the repulsive strength becomes infinite. While for a resonance model, it shows that ferromagnetic state is possible if the scattering length is positive and sufficient large, and the system is prepared in scattering state orthogonal to molecular bound state. However, we can not rule out the possibility that more exotic correlation can make the ferromagnetic state unstable. [Preview Abstract] |
Thursday, May 27, 2010 11:54AM - 12:06PM |
K1.00005: Measurement of the ``Contact'' and the Paired Fraction of Ultracold Atoms in the BEC-BCS Crossover Yean-an Liao, Wenhui Li, T. Paprotta, A.S.C. Rittner, R.G. Hulet We use a photoassociation technique to probe small-length scale correlations of a two-spin mixture of $^{6}$Li atoms as a function of interaction strength and temperature. The interaction strength is tuned from the molecular BEC regime to the BCS regime using a Feshbach resonance. The fraction of atoms that are paired is measured by photo-exciting them to a spatially small vibrational level of an electronically excited state of the Li$_{2}$ diatomic molecule$^{2}$. The measured paired fraction is independent of of the photo-excitation laser intensity, as long as it is fast compared to the pair reformation time. At sufficiently low temperature, we find the paired fraction varies from near unity at the BEC limit, to below our limits of detectability on the BCS side. At unitarity, we observe preformed pairs above \textit{T}$_{c}$, a phenomenon shared with high-temperature superconductors. The previously measured closed-channel fraction\footnote{G. B. Partridge {\it et al.}, {\it Phys. Rev. Lett.} {\bf 95}, 020404 (2005).} is directly related to the ``contact''\footnote{F. Werner, L. Tarruell, \& Y. Castin, {\it Eur. Phys. J. B} {\bf 68}, 401 (2009).}, a universal thermodynamic quantity. [Preview Abstract] |
Thursday, May 27, 2010 12:06PM - 12:18PM |
K1.00006: Unitary Superfluidity Of Polarized Fermionic Gases In Highly Elongated Traps L. Baksmaty, H. Lu, C. Bolech, H. Pu Recent groundbreaking experiments on resonantly interacting fermionic superfluids encountered qualitative and quantitative discrepancies which seem to be a function of the confining geometry. Despite long familiarity with BCS (Bardeen-Cooper-Schrieffer) superfluids in a wide range of physical systems such as nuclear matter, QCD, Astrophysics and Condensed Matter, these observations have defied theoretical explanation. Mindful of quantum rigidity and motivated by this impasse, we study the solution space for 3-dimensional fully self-consistent mean field formulation. Relying on numerical algorithms specifically developed for this purpose, we study realistic systems with up to 10$^{5}$ atoms. We find that for a large enough sample in a cigar-shaped trap, there are typically three types of solutions which are almost degenerate and have the ff. properties: (i) There is a solution very similar to the local density approximation (LDA) which is consistently the lowest in energy. (ii) However one of the other two solutions, connected by a smooth transition, and which are more consistent with experiment at high aspect ratio, supports a state very similar to the long sought FFLO (Fulde Ferrel Larkin Ovchinnikov) state. We submit that these solutions are relevant false vacua because, given high energy barriers and near degeneracy of the obtained solutions, the actual states observed in an experiment could be a strong function of the experimental procedure. Darpa OLE grant, ARO Grant no. W911NF-07-1-0464, Welch foundation (C-1669, C-1681) and NSF. [Preview Abstract] |
Thursday, May 27, 2010 12:18PM - 12:30PM |
K1.00007: Measurement of Universal Thermodynamic Functions for a Unitary Fermi Gas Munekazu Horikoshi, Shuta Nakajima, Masahito Ueda, Takashi Mukaiyama We measured universal thermodynamic functions for a Fermi gas at the unitarity limit where the s-wave scattering length diverges, by using ultracold $^{6}$Li Fermi atoms confined in an optical dipole trap and its Feshbach resonance. At the unitarity limit, thermodynamics is expressed with a universal form that depends only on the particle density and temperature. We measured the universal function of the internal energy as a function of these parameters by using only the general form of the equation of state and the equation of force balance. The validity of the measured thermodynamic function was confirmed in two ways, i.e., the measurement of energy and speed of sound. Other thermodynamic functions such as the free energy, chemical potential, and entropy were derived from that of the internal energy through general thermodynamic relations. The critical point for the superfluid transition was determined by detecting the emergence of the zero center-of-mass momentum component of the paired fermions by using of a rapid field-sweep technique. The measured critical temperature, internal energy, chemical potential, and entropy at the unitarity limit for a homogeneous case are $T$/$T_{F}$=0.17(1), $E$/\textit{N$\varepsilon $}$_{F}$=0.34(2), \textit{$\mu $}/\textit{$\varepsilon $}$_{F}$=0.43(1), and $S$/\textit{Nk}$_{B}$=0.8(3), respectively. [Preview Abstract] |
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