Bulletin of the American Physical Society
2005 36th Meeting of the Division of Atomic, Molecular and Optical Physics
Tuesday–Saturday, May 17–21, 2005; Lincoln, Nebraska
Session F5: Degenerate Fermi Gases |
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Chair: Deborah Jin, NIST, JILA, and University of Colorado Room: Burnham Yates Conference Center Arbor |
Thursday, May 19, 2005 10:30AM - 10:42AM |
F5.00001: Thermodynamics of a Strongly-Interacting Fermi Gas Joseph Kinast, Andrey Turlapov, John Thomas The study of Fermi gases near a Feshbach resonance provides a convenient proving ground for theories of strongly-interacting exotic systems ranging from nuclear matter to high temperature superconductors. Here, we report on the measurement of basic thermodynamic quantities in a strongly-interacting Fermi gas from the highly degenerate regime to the classical limit. An equal mixture of two spin states of $^6$Li is evaporatively cooled well into the degenerate regime in a stable optical trap. We measure the volume of the gas versus energy and its heat capacity, using novel energy input and thermometry techniques. We find very good quantitative agreement between our measurements and a recent pseudogap theory. [Preview Abstract] |
Thursday, May 19, 2005 10:42AM - 10:54AM |
F5.00002: Opposite-spin Fermion pairs in the strongly-interacting regime Javier von Stecher, Chris H. Greene We propose a new method to study pair formation in the BCS-BEC crossover regime. The method considers a pair of opposite spin fermions that interact with a degenerate Fermi gas trapped in a spherical oscillator potential. In the vicinity of a Feshbach resonance, we magnetically vary the S-wave scattering length in the negative region. This produces an attractive interaction between fermions which is not strong enough to bind them. We construct an antisymmetric wavefunction formed by the spin-orbitals of the Fermi gas and a pair function of opposite spin fermions. We study variationally the ground state energy of the composite system of this pair in the presence of a degenerate Fermi gas, as a function of the magnetic field. In particular, we consider the region close to the collapse of the Fermi gas, in the strongly interacting regime. [Preview Abstract] |
Thursday, May 19, 2005 10:54AM - 11:06AM |
F5.00003: A simple approach to BEC-BCS crossover at a Feshbach resonance in a degenerate Fermi gas Juha Javanainen, Andrew Carmichael We present a simplified theoretical approach to the Feshbach resonance in a degenerate (effectively zero temperature) two-component Fermi gas. The key technical idea is to treat the bosonic molecules as a classical field, whereupon the hierarchy of the equations of motion for field correlation functions closes at the level of pair correlations. Topics to discuss include the shift of the Feshbach resonance induced by the Fermi sea, the effects of a nontrivial energy dependence of atom-molecule coupling, and BEC-BCS crossover in the limits of both weak and strong coupling. [Preview Abstract] |
Thursday, May 19, 2005 11:06AM - 11:18AM |
F5.00004: Molecular Fraction of the BEC-BCS Crossover State G. B. Partridge, R. I. Kamar, K. E. Strecker, M. W. Jack, R. G. Hulet We have created a molecular Bose-Einstein Condensate (BEC) from pairs of fermionic $^6$Li atoms. This BEC is used as a starting point to probe the many body wavefunction of the molecules/pairs in the BEC-BCS crossover regime in proximity to the broad Feshbach resonance near 837 G. These molecules/pairs are superpositions of atoms in an electronic triplet state and deeply-bound singlet molecules. A laser tuned to an excited singlet molecular state projects out the singlet component, which results in a loss. This loss is measured for fields between 680 G and 850 G. We find that the molecular contribution is more than $\sim$10$\%$ inside the strongly interacting regime. This is orders of magnitude larger than that expected from two-body physics. We have also observed coherent oscillations between atoms and molecules. [Preview Abstract] |
Thursday, May 19, 2005 11:18AM - 11:30AM |
F5.00005: Collective Excitations of Trapped Fermi Gas in the BCS-BEC Crossover Alexander L. Zubarev, Yeong E. Kim The zero-temperature properties of a dilute two-component Fermi gas in the BCS-BEC crossover are investigated. On the basis of a generalization of the Hylleraas-Undheim method we construct rigorous upper bounds to the collective frequencies for the radial and the axial breathing mode of the Fermi gas under harmonic confinement in the framework of the hydrodynamic theory. The bounds are compared to experimental data on confined vapors of Li(6) atoms. Note that we do not use the scaling approximation of Refs.[1], the polytropic approximation of Refs.[2] and the first order perturbation approximation of Ref.[3]. [1] Y.E. Kim and A.L. Zubarev, Phys. Rev. A 70, 033612 (2004); Phys. Lett. A 327, 397 (2004). [2] H. Heiselberg, cond-mat/0403041; H. Hu et al, Phys. Rev. Lett. 93, 190 403 (2004); N. Manini and L. Salasnich, cond-mat/0407039. [3] A. Bulgac and G.F. Bertsch, cond-mat/0404687. [Preview Abstract] |
Thursday, May 19, 2005 11:30AM - 11:42AM |
F5.00006: Many body effects close to the s-wave Feshbach resonance in two-component Fermi systems: Is a triplet superfluid possible? Sergio Gaudio, Jason Jackiewicz, Kevin Bedell We investigate the p-wave correlations at the BEC-BCS crossover in ultracold fermionic atom gases. We use a generalization of the $s$-wave approximation for the mean field properties of a multi-state fermionic Hamiltonian in the HFB framework. We explore the p-wave contributions to the ground state correlations, and the corresponding density and spin response functions. The simulations are carried out for a short-range attractive interaction, by keeping fixed the particle density number of the system, and varying the scattering of the interaction. The p-wave contributions are expected to be significant at large densities, and vanish in the dilute limit. [Preview Abstract] |
Thursday, May 19, 2005 11:42AM - 11:54AM |
F5.00007: Feshbach Resonances in Lithium-6 Christian H. Schunck, Martin W. Zwierlein, Claudiu A. Stan, Sebastian M.F. Raupach, Wolfgang Ketterle, Andrea Simoni, Eite Tiesinga, Carl J. Williams, Paul S. Julienne Feshbach resonances in lithium-6 were experimentally studied and theoretically analyzed. In addition to two previously known s-wave resonances, we found three p-wave resonances. Four of these resonances are narrow and yield a precise value of the singlet scattering length, but do not allow us to accurately predict the location of the broad resonance near 83 mT. Its position was previously measured in a molecule-dissociation experiment for which we will discuss systematic shifts. \newline [Preview Abstract] |
Thursday, May 19, 2005 11:54AM - 12:06PM |
F5.00008: Towards ultra-cold Bose-Fermi mixtures in a micro-magnetic trap S. Aubin, M. Extavour, S. Myrskog, L. LeBlanc, A. Stummer, J. H. Thywissen We present progress on producing quantum degenerate fermionic potassium ($^{40}$K) and bosonic rubidium ($^{87}$Rb) gases in a micro-magnetic chip trap. The two atomic species are cooled and trapped simultaneously in a vapor loaded magneto-optical trap (MOT). The cold two-species atomic cloud is transported in a quadrupole magnetic trap to the surface of a chip, where it is loaded into a micro-magnetic trap. In optimizing the loading process, we developed an optical probe with high signal-to-noise for mapping out the minima of the micro-magnetic field by creating a 1D MOT at the surface of the chip. We have seen evidence of RF evaporation and are working towards quantum degeneracy. This approach to degeneracy requires only a single chamber, because the rapid evaporative cooling due to the tight confinement of the chip trap relaxes the stringent vacuum requirements of a traditional magnetic trap. In describing our experimental approach, we address the experimental challenges related to microtrapping fermions and future studies of cold Fermi gases. Work supported by NSERC, CFI, PRO, and OIT. [Preview Abstract] |
Thursday, May 19, 2005 12:06PM - 12:18PM |
F5.00009: Dimer Photodissociation in the Presence of a Degenerate Fermi Gas Josh W. Dunn, Chris H. Greene We explore the effects of the presence of a degenerate Fermi gas on the photodissociation spectrum of a weakly bound dimer. For a dimer made up of fermionic atoms identical to those in the Fermi gas, Pauli-blocking effects reduce the available decay states for the dimer. This can lead to a narrowing of the width of the dimer photodissociation spectrum. We calculate such dissociation spectra and discuss applications to current experiments. [Preview Abstract] |
Thursday, May 19, 2005 12:18PM - 12:30PM |
F5.00010: Scattering properties of weakly bound dimers of fermionic atoms Dmitry Petrov, Christophe Salomon, Georgy Shlyapnikov We discuss the behavior of weakly bound homonuclear and heteronuclear bosonic molecules formed in a two-component Fermi gas with a large positive scattering length for the interspecies interaction. We present a theoretical approach for solving a few-body scattering problem and describe the physics of dimer-dimer elastic and inelastic scattering. We explain why these diatomic molecules, while in the highest ro-vibrational level, are characterized by remarkable collisional stability. [Preview Abstract] |
Thursday, May 19, 2005 12:30PM - 12:42PM |
F5.00011: Formation Time of a Fermion Pair Condensate Martin W. Zwierlein, Christian H. Schunck, Claudiu A. Stan, Sebastian M.F. Raupach, Wolfgang Ketterle Atomic Fermi gases close to a Feshbach resonance offer the unique possibility to study a strongly interacting many-body system with tunable interactions. At ultracold temperatures, fermions form bosonic pairs which can condense into the ground state of the confining potential. The nature of the pairs depends on the interaction strength: The atoms can be either tightly bound into a small molecule or they can form long-range pairs, whose size can become comparable to or even larger than the interparticle spacing. We studied the formation time of a condensate of fermionic atom pairs close to a Feshbach resonance. This was done using a phase-shift method in which the delayed response of the many-body system to a modulation of the interaction strength was recorded. The observable was the fraction of condensed molecules in the cloud after a rapid magnetic field ramp across the Feshbach resonance. The measured response time was slow compared to the rapid ramp, which provides final proof that the molecular condensates reflect the presence of fermion pair condensates before the ramp. [Preview Abstract] |
Thursday, May 19, 2005 12:42PM - 12:54PM |
F5.00012: Feshbach-Resonant Interactions in $^{40}$K and $^6$Li Degenerate Fermi Gases Matt Mackie, Jyrki Piilo We theoretically examine a system of Fermi degenerate atoms coupled to bosonic molecules by a Feshbach resonance, focusing on the superfluid transition to a molecular Bose-Einstein condensate dressed by Cooper pairs of atoms. This problem raises an interest because it is unclear at present whether bimodal density distributions observed recently in $^{40}$K and $^6$Li are due to a condensate of bosonic molecules or fermionic atom pairs. As opposed to $^{40}$K, we find that any measurable fraction of above-threshold bosonic molecules are necessarily absent for the $^6$Li system in question, which strongly implicates Cooper pairs as the culprit behind its bimodal distributions. [Preview Abstract] |
Thursday, May 19, 2005 12:54PM - 1:06PM |
F5.00013: Experimental study of Lithium-6 close to Feshbach resonances Frederic Chevy, J. McKeever, J. Zhang, E.G.M. van Kempen, T. Bourdel, L. Khaykovich, J. Cubizolles, M. Teichmann, L. Tarruell, S.J.J.M.F. Kokkelmans, C. Salomon We discuss recent experiments performed on ultra-cold gases of fermionic $^6\mathrm{Li}$ in a crossed-beam dipole trap. This system enables the investigation of the so-called BEC-BCS crossover region. Using a Feshbach resonance, the magnitude of the atomic interactions can be tuned between the strongly interacting regime, where the ground state is a Bose-Einstein condensate (BEC) of molecules, to the weakly interacting regime, where the BCS (Bardeen-Cooper-Schrieffer) theory of superconductivity is more appropriate. A broad s-wave resonance was used to form a BEC of weakly bound $^6\mathrm{Li}_2$ molecules in the optical trap. The molecule- molecule scattering length and the expansion energy of the cloud in the crossover region were measured. Furthermore, we have studied the scattering properties of the gas close to several p-wave Feshbach resonances, whose positions are in very good agreement with theory. We have also experimentally shown the existence of new s-wave resonances in the heteronuclear $^6 \mathrm{Li}$-$^7\mathrm{Li}$ mixture. [Preview Abstract] |
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F5.00014: Zero-Temperature Theory of Collisionless Rapid Adiabatic Passage from a Fermi Degenerate Gas of Atoms to a Bose-Einstein Condensate of Molecules Matt Mackie, Olavi Dannenberg We theoretically examine a zero-temperature system of Fermi degenerate atoms coupled to bosonic molecules via collisionless rapid adiabatic passage across a Feshbach resonance, focusing on saturation of the molecular conversion efficiency at the slowest magnetic-field sweep rates. Borrowing a novel {\em many-fermion} Fock-state theory, we find that a proper model of the magnetic-field sweep can systematically remove saturation. We also debunk the common misconception that many-body effects are responsible for molecules existing above the two-body threshold. [Preview Abstract] |
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