Bulletin of the American Physical Society
38th Annual Meeting of the Division of Atomic, Molecular, and Optical Physics
Volume 52, Number 7
Tuesday–Saturday, June 5–9, 2007; Calgary, Alberta, Canada
Session G4: Strongly Interacting Ultracold Gases |
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Chair: R. Hulet, Rice University Room: TELUS Convention Centre Glen 206 |
Thursday, June 7, 2007 8:00AM - 8:12AM |
G4.00001: Phase Diagram of a Polarized Fermi Gas Across a Feshbach Resonance Wenhui Li, Yean-an Liao, Guthrie Partridge, Randy Hulet We investigate a Fermi gas of $^6$Li atoms with unbalanced populations in two spin states, whose interactions are tuned by a Feshbach resonance. At the unitarity limit, we observe three distinct phases connected by a tricritical point on a polarization vs. temperature (\textit{P}-\textit{T}) phase diagram: a phase-separated state at low \textit{T}, a polarized superfluid and a polarized normal gas at higher \textit{T}. \footnote{G.B. Partridge {\it et al.}, {\it Science} {\bf 311}, 503 (2006).}$^,$\footnote{G.B. Partridge {\it et al.}, {\it Phys. Rev. Lett.} {\bf 97}, 190407 (2006).} We are currently mapping out the phase diagram as a function of \textit{P}, \textit{T} and interaction. At \textit{T} = 0, as the interaction strength is tuned toward the BEC side of the resonance, we expect to encounter a phase boundary between the phase-separated state and the polarized superfluid. Conversely, on the BCS side, for finite \textit{P}, a transition to the polarized normal gas is expected. We will present our latest results. [Preview Abstract] |
Thursday, June 7, 2007 8:12AM - 8:24AM |
G4.00002: Spatially resolved RF spectroscopy of a strongly interacting trapped Fermi gas Yong-il Shin, Christian Schunck, Andre Schirotzek, Wolfgang Ketterle RF spectroscopy has been used to study unitary limited interactions and pairing in a strongly interacting Fermi mixture. However, most of the spectroscopy experiments have been performed with inhomogeneous samples confined in trapping potentials, eventually limited by the density broadening effects. We developed spatially resolved RF spectroscopy based on the \textit{in situ} imaging of the trapped samples. We obtained a density-broadening-free RF spectrum and clearly identified a gap in the RF spectrum of strongly interacting Fermi mixtures. In this talk, we will present a measurement of the paring gap energy of a Fermi mixuture at the unitarity limit. [Preview Abstract] |
Thursday, June 7, 2007 8:24AM - 8:36AM |
G4.00003: Scissors mode oscillations for a finite-temperature strongly-interacting ultracold Fermi gas. M.J. Wright, S. Riedl, A. Altmeyer, C. Kohstall, E. Sanchez, J. Hecker Denschlag, R. Grimm We investigate the finite-temperature phase diagram for a strongly-interacting Fermi gas. This system consists of two distinct regimes, hydrodynamic and collisionless, which have different collisional properties and depend on interaction strength and temperature. We study the transition from hydrodynamic and collisionless behavior throughout the crossover by examining the scissors mode. This mode proves to be an excellent candidate for finite-temperature measurements as the oscillation is independent of the equation of state and has a low damping term, which provides better resolution of the mode's frequency characteristics. [Preview Abstract] |
Thursday, June 7, 2007 8:36AM - 8:48AM |
G4.00004: Paired Phases of a Trapped Fermi Gas with Unequal Spin Populations G.B. Partridge, Wenhui Li, Y.A. Liao, R.G. Hulet A strongly interacting ultra-cold gas of fermionic $^6$Li with unequal numbers of two spin components exhibits two distinct low temperature states.\footnote{G. B. Partridge, Wenhui Li, Y. A. Liao, R. G. Hulet, M. Haque, and H. T. C. Stoof. \emph{Phys. Rev. Lett.} \textbf{97}, 190407 (2006) } Phase separation is observed at the lowest temperatures, where a uniformly paired core is maintained in the center of the trap, up to large number imbalance, by the expulsion of excess unpaired atoms. Sharp boundaries between the core and the unpaired atoms are consistent with a first-order phase transition. In addition, the spatial distribution of the superfluid core deforms, in violation of the local density approximation. At higher temperature, the core remains unpolarized up to a critical polarization before going normal, but does not deform. The boundaries are not sharp in this case, indicating a partially polarized shell between the core and the unpaired atoms. The temperature dependence is consistent with a tri-critical point in the phase diagram. [Preview Abstract] |
Thursday, June 7, 2007 8:48AM - 9:00AM |
G4.00005: Photoassociation Rate of a $^{7}$Li Bose-Einstein Condensate near a Feshbach Resonance M. Junker, D. Dries, Y.P. Chen, C. Welford, J. Hitchcock, R.G. Hulet In a photoassociation (PA) process a pair of atoms collide in the presence of a resonant light field creating an excited molecule. For ultracold atoms, the rate of PA depends on the s-wave scattering length, $\textit{a}_{s}$, which determines the wavefunction overlap between the collisional ground state and the excited molecular bound state. We investigate this dependence experimentally using a pure Bose condensate of $^{7}$Li in the \textit{F}=1, \textit{m}$_{F}$=1 hyperfine state confined in an optical dipole trap. We vary $\textit{a}_{s}$ below the 730 G Feshbach resonance and measure the rate of loss from the condensate due to a PA pulse which couples atoms to the $v' = 83$ vibrational level of the $1^{3}\Sigma_{g}^{+}$ molecular state. The measured loss rate varies by more than a factor of 30 over the magnetic field range of 660 - 730 G. At 710 G the rate approaches zero, which we attribute to a node in the ground state wavefunction. We also compare the PA loss rate in a BEC to a thermal gas over this same range of magnetic fields. [Preview Abstract] |
Thursday, June 7, 2007 9:00AM - 9:12AM |
G4.00006: Bragg spectroscopy of a strongly interacting BEC S.B. Papp, J.M. Pino II, R.J. Wild, D.S. Jin, C.E. Wieman, E.A. Cornell The ability to tune the scattering length in ultracold atomic gases has enabled access to strongly interacting Bose-Einstein condensates (BECs). Particularly intriguing physics is expected to occur when the scattering length is increased to on order of the interatomic spacing; in this case many-body interactions are important. The capability to probe the excitation spectrum of the BEC will be important in understanding this experimental regime. We will report initial Bragg spectra of a strongly interacting BEC. Using a Feshbach resonance, we vary the scattering length in a $^{85}$Rb BEC to study the Bragg spectra over a wide range of interaction strength. [Preview Abstract] |
Thursday, June 7, 2007 9:12AM - 9:24AM |
G4.00007: A Simple Mean-Field Model of Steady-State Magnetoassociation of an Atomic BEC in a Feschbach Resonance Andrew Carmichael, Juha Javanainen We investigate a simple mean-field model describing magnetoassociation of a single species atomic Bose-Einstein condensate in the presence of a Feschbach resonance. The Hamiltonian, which allows for the creation and destruction of Bose- condensed molecules, leads to Heisenberg equations of motion which are solved analytically in the steady state for the classical quantities of the occupancies of the atomic and molecular condensates and the anaomalous pairing ampitudes. Approximations include the elimination of non-condensed molecules. [Preview Abstract] |
Thursday, June 7, 2007 9:24AM - 9:36AM |
G4.00008: Coherent and adiabatic photoassociation in a Bose-Einstein condensate Pascal Naidon, Eite Tiesinga, Francoise Masnou-Seeuws, Paul Julienne Photoassociation is a process by which two colliding atoms associate into an excited bound state by absorbing a resonant photon. This excited state subsequently decays by spontaneous emission. The atom loss rate in a gas photoassociated by a laser can be calculated from the two-body theory. It is linear with the laser intensity for small intensities, then saturates and decreases at higher intensities, in accordance with the two-body unitary limit. Many-body models have predicted that the rate of photoassociation in a Bose-Einstein condensate may saturate before the unitary limit. However this rate limit has not been observed. We revisit the many-body theory of photoassociation in a condensate and explain why the rate limit has not been seen. In particular we identify two different regimes, the coherent and the adiabatic regimes. All photoassociation experiments up to now have been performed in the adiabatic regime, for which the limitation on the rate occurs at larger intensities than initially predicted by many-body models. We also find that this limitation on the rate is essentially a two-body effect rather than a many-body effect. Finally we investigate the possibility to explore the coherent regime using Bose-Einstein condensate of alkaline-earth metals. [Preview Abstract] |
Thursday, June 7, 2007 9:36AM - 9:48AM |
G4.00009: Rogue decoherence in the formation of a macroscopic atom-molecule superposition Matt Mackie, Olavi Dannenberg We theoretically examine two-color photoassociation of a Bose-Einstein condensate, focusing on the role of rogue decoherence in the formation of macroscopic atom-molecule superpositions. Rogue dissociation occurs when two zero-momentum condensate atoms are photoassociated into a molecule, which then dissociates into a pair of atoms of equal-and-opposite momentum, instead of dissociating back to the zero-momentum condensate. As a source of decoherence that may damp quantum correlations in the condensates, rogue dissociation is an obstacle to the formation of a macroscopic atom-molecule superposition. We study rogue decoherence in a setup which, without decoherence, would yield a macroscopic atom-molecule superposition, and find that the most favorable conditions for said superpositions are a density $\rho\sim10^{12}\,{\rm cm}^{-3}$ and temperature $T\sim0.1$~nK. [Preview Abstract] |
Thursday, June 7, 2007 9:48AM - 10:00AM |
G4.00010: Enhanced Sensitivity to Fundamental Constants In Ultracold Atoms and Molecules near Feshbach Resonances Cheng Chin, Victor Flambaum Scattering length, which can be measured in Bose-Einstein condensate and Feshbach molecule experiments, is extremely sensitive to the variation of fundamental constants, in particular, the electron-to-proton mass ratio ($m_{e}$/$m_{p}$ or $m_{e}$/$\Lambda _{QCD}$, where $\Lambda_{QCD}$ is the QCD scale). Based on single- and two-channel scattering models, we show how the variation of the mass ratio propagates to the scattering length. Our results suggest that variation of $m_{e}$/$m_{p}$ on the level of 10$^{-11}\sim $10$^{-14}$ can be detected near a narrow Feshbach resonance by monitoring the scattering length on the 1{\%} level. In this talk, we will present evidences that demonstrate the ultrahigh high sensitivity on atomic mass and suggest possible experiment approaches to precisely determine the scattering lengths. [Preview Abstract] |
Thursday, June 7, 2007 10:00AM - 10:12AM |
G4.00011: Nonequilibrium free-fermion pair correlations in molecular dissociation. Karen Kheruntsyan, Matthew Davis, Murray Olsen We analyze the pair correlations of fermionic atoms formed through the dissociation of a Bose-Einstein condensate of molecular dimers.~With bosonic atomic constituents, this would be a direct atom optics analog of optical parametric down-conversion. Known as the best source of squeezed light and entangled photon pairs, down-conversion has led to a number important applications, such as precision measurements and fundamental tests of quantum theory. We envisage that molecular dissociation will play a similarly important role in quantum-atom optics in the near future, with the different possible quantum statistics of the constituent atoms revealing new physics. Here we discuss how the fermionic statistics leads to the new paradigm of fermionic quantum-atom optics. Using a simple theoretical model for molecular dissociation, we analyze the pair correlations between the fermionic atoms in two spin states and quantify the strength of correlations via number-difference squeezing [1]. This is the first step towards a quantitative theoretical analysis of recent experimental measurements of atom correlations in dissociation of potassium dimers performed at JILA [2]. [1] K.V. Kheruntsyan, Phys. Rev. Lett. 96, 110401 (2006). [2] M. Greiner et al., Phys. Rev. Lett. 94, 110401 (2005). [Preview Abstract] |
Thursday, June 7, 2007 10:12AM - 10:24AM |
G4.00012: Equation of state of Bose and Fermi systems beyond s-wave determined by the lowest order constrained variational method: large scattering length limit Ryan Kalas, Doerte Blume Dilute Fermi systems with large $s$-wave scattering length $a_s$ exhibit universal properties if the interparticle spacing $r_o$ greatly exceeds the range of the underlying two-body interaction potential. In this regime, $r_o$ is the only relevant length scale and observables such as the energy per particle depend only on $r_0$ (or, equivalently, the energy $E_{FG}$ of the free Fermi gas). We investigate Bose and Fermi systems with non-vanishing angular momentum using the lowest order constrained variational (LOCV) method. We focus on the regime where the generalized scattering length becomes large and determine the relevant length scales at unitarity. We obtain simple expressions for the energy per particle in terms of a combined $l$-dependent length scale $\xi_l$. For example, within the LOCV framework the energy per particle of $p$-wave and $d$-wave interacting fermions depends not only on $E_{FG}$, as in the case of $s$-wave fermions, but also on an energy scale that depends on the range of the underlying two-body potential. Furthermore, we investigate the behaviors of $s$-wave interacting Bose and Fermi systems in the non-universal, density-dependent regime. *Supported by NSF. [Preview Abstract] |
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