Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session A43: Focus Session: Strongly Interacting Fermi Gases and the BCS-BEC Crossover I |
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Sponsoring Units: DAMOP Chair: C. Chin, University of Chicago Room: Baltimore Convention Center 346 |
Monday, March 13, 2006 8:00AM - 8:12AM |
A43.00001: A Two-Channel R-Matrix Analysis of Magnetic Field Induced Feshbach Resonances Paul Julienne, Nicolai Nygaard , Barry Schneider A two channel model of magnetic field induced Feshbach resonances in ultra-low atom-atom collisions is presented. The model uses realistic potentials and is parametrized using available theoretical and experimental data to reproduce the known low-energy scattering properties of the colliding atoms. The coupled equations are solved using the R-Matrix method, expanding the wavefunction in a Finite Element Discrete Variable basis. This basis enables us to capture both the short-range behavior of the molecular vibrational states as well as the long-range scattering states. Results will be presented showing the behavior of the wavefunctions and T- matrices as a function of the external magnetic field for selected alkali atom pairs. [Preview Abstract] |
Monday, March 13, 2006 8:12AM - 8:24AM |
A43.00002: Many-body Feshbach Hamiltonians in the two-body limit. Nicolai Nygaard, James E. Williams, Paul S. Julienne We discuss how the many-body theory of a gas with interactions controlled by a Feshbach resonance can be constructed in a manner, which incorporates the correct two-body physics. This entails the introduction of an energy dependent renormalized coupling constant for atom-molecule conversion that embodies the low energy scattering properties of the entrance channel potential. We demonstrate that with this model the binding energies of the dressed Feshbach molecules may be faithfully reproduced. [Preview Abstract] |
Monday, March 13, 2006 8:24AM - 8:36AM |
A43.00003: Properties of the BCS-BEC condensate in the BEC regime Jesper Levinsen, Victor Gurarie We study a gas of fermions undergoing a wide resonance s-wave BCS-BEC crossover, in the BEC regime at zero temperature. We calculate the chemical potential and the speed of sound of this Bose-condensed gas, as well as the condensate depletion, in the low density approximation. We discuss how higher order terms in the low density expansion can be constructed. We demonstrate that the standard BCS-BEC gap equation is invalid in the BEC regime and is inconsistent with the results obtained here. The low density approximation we employ breaks down in the intermediate BCS-BEC crossover region. Hence our theory is unable to predict how the chemical potential and the speed of sound evolve once the interactions are tuned towards the BCS regime. As a part of our theory, we derive the well known result for the bosonic scattering length diagrammatically and check that there are no bound states of two bosons. [Preview Abstract] |
Monday, March 13, 2006 8:36AM - 9:12AM |
A43.00004: Studying the BEC-BCS crossover with an ultracold gas of $^{6}$Li atoms Invited Speaker: Collective oscillations are a good tool to investigate the properties of an ultracold Fermi gas in the BEC-BCS crossover. We will present new and improved measurements on these collective excitations. Our results help to determine the equation of state of the strongly interacting Fermi gases. [Preview Abstract] |
Monday, March 13, 2006 9:12AM - 9:24AM |
A43.00005: Radio frequency spectroscopy and the pairing gap in trapped Fermi gases Yan He, Qijin Chen, Kathryn Levin We present a theoretical interpretation of radio-frequency (RF) pairing gap experiments in trapped atomic Fermi gases, over the entire range of the BCS-BEC crossover, for temperatures above and below $T_c$. Our calculated RF excitation spectra, as well as the density profiles on which they are based, are in semi-quantitative agreement with experiment. We provide a detailed analysis of the physical origin of the two different peak features seen in RF spectra, one associated with nearly free atoms at the edge of the trap, and the other with (quasi-)bound fermion pairs.\newline Reference: Phys Rev. A 72, 011602(R) (2005). [Preview Abstract] |
Monday, March 13, 2006 9:24AM - 9:36AM |
A43.00006: Dynamical projection of atoms to Feshbach molecules at strong coupling. Roman Barankov, Leonid Levitov An interesting method of atomic state projection to the Feshbach molecules using the magnetic field sweep through the resonance was employed in the recent experiments [1]. The sweep could be made very fast compared to typical fermion time scales, such as the collision frequency or inverse Fermi bandwidth, making the process a ``snapshot probe'' with regard to the collective fermion processes. On a single particle level, broad Feshbach resonances studied in Ref.[1], exhibit strong atom-molecule coupling in a relatively wide detuning range. In this sense, the sweep speed [1] corresponds to essentially adiabatic atom/molecule conversion, slow on the scale of the resonance width. We develop a theory [2] that describes molecules at a sweep fast compared to the elastic collisions, when only the quantum-mechanical processes involving two atoms transition into a molecule are relevant. Our approach accounts for resonance dissociation/association in the presence of time-dependent detuning as well as for fermion pairing correlations in the initial state. An exact solution is found, predicting a $1/3$ power law dependence on the inverse sweep rate for molecule production efficiency at fast sweep. The predicted production efficiency agrees with experimental observations for both condensed and incoherent molecules away from saturation. [1] C. A. Regal, \emph{et al.}, Phys. Rev. Lett. {\bf 92}, 040403 (2004); M. W. Zwierlein, \emph{et al.}, Phys. Rev. Lett. {\bf 92}, 120403 (2004) [2] R. A. Barankov and L. S. Levitov, cond-mat/0506323 [Preview Abstract] |
Monday, March 13, 2006 9:36AM - 9:48AM |
A43.00007: Dressed Feshbach molecules in the BEC-BCS crossover Mathijs Romans, Henk Stoof There has been a lot of interest in Feshbach resonances and the BEC-BCS crossover that is associated with it. We present work that describes the crossover in terms of dressed molecules, and gives a theory to calculate the internal structure of the paired state. We integrate out the fermions exactly, and take into account the fluctuations of the molecular field to determine the probability $Z$ for dressed molecules to be in the closed channel state. Our approach includes the relevant two-body physics completely. Also the density of condensed and noncondensed dressed molecules can be determined for the entire BEC-BCS crossover. We compare our results to experiment. [Preview Abstract] |
Monday, March 13, 2006 9:48AM - 10:00AM |
A43.00008: Two-fluid hydrodynamic modes in a trapped Fermi superfluid gas Edward Taylor, Allan Griffin In the collisional region at finite temperatures, the collective modes of superfluids are described by the Landau two-fluid hydrodynamic equations. This region can now be probed over the entire BCS-BEC crossover in trapped Fermi superfluids with a Feshbach resonance . We have recently developed a variational formulation of the two-fluid hydrodynamic equations that greatly simplifies calculations of the collective modes in trapped superfluid gases. The frequencies are given in terms of effective spring constants involving spatial integrals over position-dependent equilibrium thermodynamic functions. We present results based on this theory using the LDA and thermodyanmic functions for a uniform gas in the BCS-BEC crossover region, with pair fluctuations included using the Nozi\`eres and Schmitt-Rink approximation. The temperature dependent out-of-phase hydrodynamic modes (the analogue of second sound) are of particular interest. [Preview Abstract] |
Monday, March 13, 2006 10:00AM - 10:12AM |
A43.00009: Sound in a strongly-interacting Fermi gas Bason Clancy, Le Luo, J. Kinast, J. Joseph, A. Turlapov, J.E. Thomas Sound propagation in an optically-trapped gas of strongly interacting fermionic $^6$Li atoms is studied. The atoms are prepared as a 50/50 mixture of the two lowest-energy internal states. Strong interactions are achieved by applying a magnetic field in the vicinity of a broad (834 G) s-wave Feshbach resonance. A sound wave is excited by locally applying a pulse of a repulsive blue-light potential. We measure the propagation of the excitation along the axial direction of the cigar-shaped cloud. [Preview Abstract] |
Monday, March 13, 2006 10:12AM - 10:24AM |
A43.00010: Potential-energy (BCS) to kinetic-energy (BEC)-driven pairing in the attractive Hubbard model Bumsoo Kyung, Antoine Georges, Andre-Marie Tremblay The BCS-BEC crossover within the two-dimensional attractive Hubbard model is studied by using the Cellular Dynamical Mean-Field Theory both in the normal and superconducting ground states. Short-range spatial correlations incorporated in this theory remove the normal-state quasiparticle peak and the first-order transition found in the Dynamical Mean-Field Theory, rendering the normal state crossover smooth. For $U$ smaller than the bandwidth, pairing is driven by the potential energy, while in the opposite case it is driven by the kinetic energy, resembling a recent optical conductivity experiment in cuprates. Phase coherence leads to the appearance of a collective Bogoliubov mode in the density-density correlation function and to the sharpening of the spectral function. [Preview Abstract] |
Monday, March 13, 2006 10:24AM - 10:36AM |
A43.00011: Many-body Effects near s- and p-wave Feshbach resonance in BEC-BCS Problem: A Tractable Crossing-symmetric Approach Khandker Quader, Renyuan Liao In the fermion BEC-BCS crossover problem. many-body effects may influence properties, such as scattering parameters, nature of pairing, etc. close to a Feshbach resonance. We study effects such as these using a tractable diagrammatic crossing-symmetric approach. Our method allows us to include quantum fluctuations, such as, density, current, spin, spin-current, and higher-order fluctuations in a self-consistent fashion. The underlying fermionic interaction is reflected in the ``driving'' term. Taking the ``driving'' term to be finite-range, and of arbitrary strength, we perform calculations on the BEC and BCS sides. These are related to the 2-body singlet and triplet scattering parameters, and can be connected with experimental s and p-wave Feshbach resonances. We include the l=0 density and spin fluctuations, as well as the l=1 current and spin-current fluctuatiuons. We obtain renormalized scattering amplitudes, pairing amplitudes and nature of pairing, etc. on the BEC and BCS side. We compare our results with experiment, and with other theory calculations. [Preview Abstract] |
Monday, March 13, 2006 10:36AM - 10:48AM |
A43.00012: Critical Rotating Frequency for Superfluid Fermionic Gases across Feshbach Resonance Hui Zhai, Tin-Lun Ho Vortex lattices have been observed recently in the rotating Fermionic quantum gases near Feshbach resonance. However, a much faster rotation may destroy the superfluid pairing amplitude and reveal the normal state. In this talk, we present the phase diagram for different interaction strength and rotating frequency. We discover that at resonance, pairing is so robust that it can not be destroyed by rotation. In the BCS side close to resonance, we find that the critical frequency as a function of interaction strength shows a series of plateaus as a consequence of quantized Landau levels. In a fast rotating harmonic trap, the superfluid core with vortices is surrounded by a normal cloud, and the area of the superfluid regime gradually shrinks as the increase of the rotating frequency. [Preview Abstract] |
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