Bulletin of the American Physical Society
39th Annual Meeting of the APS Division of Atomic, Molecular, and Optical Physics
Volume 53, Number 7
Tuesday–Saturday, May 27–31, 2008; State College, Pennsylvania
Session J1: New Physics with Strongly Interacting Fermions |
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Chair: Ana Maria Rey, ITAMP, Harvard University Room: Nittany Lion Inn Ballroom CDE |
Thursday, May 29, 2008 11:00AM - 11:36AM |
J1.00001: Mixtures of Ultracold Fermions with Unequal Masses Invited Speaker: The quantum phases of ultracold fermions with unequal masses are discussed in continuum and lattice models for a wide variety of mixtures which exhibit Feshbach resonances, e.g., mixtures of $^6$Li and $^{40}$K. The evolution of superfluidity from the Bardeen-Cooper-Schrieffer (BCS) to the Bose-Einstein condensation (BEC) regime in the continuum is analyzed as a function of scattering parameter, population imbalance and mass anisotropy. In the continuum case, regions corresponding to normal, phase-separated or coexisting uniform-superfluid/excess-fermion phases are identified and the possibility of topological phase transitions is discussed [1]. For optical lattices, the phase diagrams as a function of interaction strength, population imbalance, filling fraction and tunneling parameters are presented [2]. In addition to the characteristic phases of the continuum, a series of insulating phases emerge in the phase diagrams of optical lattices, including a Bose-Mott insulator (BMI), a Fermi-Pauli insulator (FPI), a phase-separated BMI/FPI mixture, and a Bose-Fermi checkerboard (BFC) phase. Lastly, the effects of harmonic traps and the emergence of unusual shell structures are discussed for mixtures of fermions with unequal masses. \newline [1] M. Iskin, and C. A. R. S{\' a} de Melo, Phys. Rev. Lett {\bf 97}, 100404 (2006); \newline [2] M. Iskin, and C. A. R. S{\' a} de Melo, Phys. Rev. Lett. {\bf 99}, 080403 (2007). [Preview Abstract] |
Thursday, May 29, 2008 11:36AM - 12:12PM |
J1.00002: Exploring an ultracold Fermi-Fermi mixture of $^{6}$Li and $^{40}$K atoms Invited Speaker: All experiments in the prospering field of strongly interacting Fermi gases have so far been restricted to spin mixtures of either $^{6}$Li or $^{40}$K atoms. Many new opportunities are offered by mixtures of different species, with the combination of $^{6}$Li and $^{40}$K being the obvious prime candidate for a Fermi-Fermi mixture. Such systems promise new experimental model systems, e.g., for superfluid regimes with pairing of particles with different masses or novel quantum phases in optical lattices. Essential for further progress in this field is to understand the elementary interaction properties of such a mixture. We have realized an optically trapped mixture of $^{6}$Li and $^{40}$K and identified a number of Feshbach resonances in various combinations of spin states. We have interpreted our data using a simple asymptotic bound state model and full coupled channels calculations. This unambiguously assigns the observed resonances in terms of various s- and p-wave molecular states and fully characterizes the ground-state scattering properties in any combination of spin states. We find a triplet scattering length of +63.5(1)a$_{0}$ and a singlet scattering length of +52.1(3)a$_{0}$, where a$_{0}$ is Bohr's radius. All identified s-wave Feshbach resonances are rather narrow and closed-channel dominated. This finding is important for further experiments, e.g. for generalized BEC-BCS crossover studies in an ultracold Fermi-Fermi system involving different masses. Work performed in collaboration with E. Wille, F. Spiegelhalder, G. Kerner, D. Naik, A. Trenkwalder, G. Hendl, F. Schreck (IQOQI and Univ. Innsbruck); T. Tiecke, J. Walraven (Univ. Amsterdam); S. Kokkelmans (Univ. Eindhoven); E. Tiesinga, P. Julienne (JQI, NIST and Univ. Maryland). [Preview Abstract] |
Thursday, May 29, 2008 12:12PM - 12:48PM |
J1.00003: Small Mass- and Trap-Imbalanced Two-Component Fermi Gases Invited Speaker: Motivated by the prospect of optical lattice experiments with two-component Fermi gases consisting of different atomic species such as Li and K, we calculate the energies for N fermions under harmonic confinement as a function of the mass- and trap-imbalance, i.e., the ratio between the masses and frequencies of species one and two, using microscopic approaches. Our energies for N=2 through 6 can be used to determine the energetically most favorable configuration for a given number of atoms per species of a deep lattice in which each lattice site is approximately harmonic and in which tunneling between neighboring sites can be neglected. Extending the calculations for equal trapping lengths to up to N=20, we determine and interpret the excitation gap for unequal-mass systems with equal oscillator lengths. [Preview Abstract] |
Thursday, May 29, 2008 12:48PM - 1:24PM |
J1.00004: Measurements of the Paired Fraction in the BEC-BCS Crossover Invited Speaker: Pairing in fermionic systems is the essential ingredient of superfluidity and superconductivity. Feshbach resonances in atomic gases allow the interactions between atoms to be tuned continuously from weak to strong, causing the condensate of paired atoms to change from a BCS-like superfluid to a Bose-Einstein condensate of molecules. I will report quantitative measurements of the paired fraction of a two-spin Fermi gas of $^6$Li atoms across the broad Feshbach resonance as a function of temperature. The paired fraction is determined by tuning a laser probe to resonance between the paired state and an excited molecular triplet level. A transition to the molecular state leads to a detectable loss of atoms, as in a previous experiment where the closed-channel fraction was measured by driving transitions to a molecular singlet level \footnote{G.B. Partridge, K.E. Strecker, R.I. Kamar, M.W. Jack, and R.G. Hulet, \textit{Phys. Rev. Lett.} \textbf{95} 020404 (2005).}. Atoms in a pair correlated state, either molecules on the BEC side of resonance or correlated pairs on the BCS side, will be excited at a rate independent of density, while the rate of excitation of unpaired atoms (photoassociation) is density-dependent. Depletion of pairs occurs rapidly, and is easily distinguished from photoassociation of unpaired atoms, enabling the determination of the paired fraction. By driving the dominant triplet transition, the rate of excitation can be much faster than pair reformation. This method may be used to quantitatively explore ``preformed'' pairing that occurs above T$_c$ in high-temperature superconductors. [Preview Abstract] |
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