Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session J36: Trapped Fermi Gases and Feshbach Resonances |
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Sponsoring Units: DAMOP Chair: Lincoln Carr, JILA, Boulder, CO Room: LACC 510 |
Tuesday, March 22, 2005 11:15AM - 11:27AM |
J36.00001: Superfluid Density of an inhomogeneous Fermi Gas Nicolai Nygaard, Charles W. Clark We present a microscopic calculation of the superfluid density for a Fermi gas with a spatially varying density profile. By imposing an infinitesimal twist on the phase of the order parameter and calculating the resulting strain energy perturbatively, we find the helicity modulus, which is directly tied to the density of the superfluid component. By self consistently solving the Bogoliubov-de Gennes equations for a harmonically trapped gas we compare the exact expression to that obtained by using the homogeneous gas result in a local density approximation. In addition, it is shown that as the critical temperature is approached the superfluid density acquires the same spatial profile as the squared norm of the order parameter. [Preview Abstract] |
Tuesday, March 22, 2005 11:27AM - 11:39AM |
J36.00002: Quench Dynamics of a Superfluid Fermi Gas Geoffrey Warner, Anthony Leggett With an eye toward the interpretation of so-called `cosmological' experiments performed on the low temperature phases of $^{3}$He, in which regions of the superfluid are destroyed by local heating with neutron radiation, we have studied the behavior of a Fermi gas subjected to uniform variations of an attractive BCS interaction parameter $\lambda$. In $^{3}$He the quenches induced by the rapid cooling of the hot spots back through the transition may lead to the formation of vortex loops via the Kibble-Zurek mechanism. A consideration of the free energy available in the quenched region for the production of such vortices reveals that the Kibble-Zurek scaling law gives at best a lower bound on the defect spacing. Further, for quenches which fall far outside the Ginzburg-Landau regime, the dynamics on the pair subspace, as initiated by quantum fluctuations, tends irreversibly to a self-driven steady-state with a gap $\Delta_{\infty} = \epsilon_{C}(e^{2/N(0)\lambda} - 1)^{- 1/2}$. In weak coupling this is only half the BCS gap, the extra energy being taken up by residual collective motion of the pairs. [Preview Abstract] |
Tuesday, March 22, 2005 11:39AM - 11:51AM |
J36.00003: Density and spin response functions in ultracold fermionic gases Bogdan Mihaila, Sergio Gaudio, Krastan Blagoev, Alexander Balatsky, Peter Littlewood, Darryl Smith We study the two-body correlation functions in a two-component ultracold fermionic gas governed by an attractive short-range interaction. Based on a zero-temperature mean-field analysis we suggest that considerable insight in the properties of the ground-state can be gained by measuring the density and spin response functions, and predict differences between the properties of a $^{40}$K ultracold fermionic gas and the properties of a $^6$Li ultracold fermionic gas. [Preview Abstract] |
Tuesday, March 22, 2005 11:51AM - 12:03PM |
J36.00004: Ferromagnetism in Fermi Gases Rembert Duine, Allan MacDonald We investigate the possibility of itinerant ferromagnetism in a two-component Fermi gas with strongly repulsive short-range inter-particle interactions. Interestingly, unlike the case of an electron gas with long-range Coulomb interactions, we find that the Hartree-Fock theory underestimates the tendency towards ferromagnetism. A Fermi gas with strongly respulsive interactions may be realized in experiments with ultracold fermionic alkali atoms near a Feshbach resonance. We investigate the prospects for observing the ferromagnetic state in these cold atoms systems. One possible detection scheme is based on a spectroscopic method for measuring the pseudospin susceptibility which we expect to be strongly enhanced when repulsive interactions are strong. The same method can be used to look for supressed pseudospin susceptibility when the singlet superconductor state is approached on the attractive interaction side of the Feshbach resonance. [Preview Abstract] |
Tuesday, March 22, 2005 12:03PM - 12:15PM |
J36.00005: Fast rotating clusters of fermions with general half integer spin Nedeltcho Zahariev, Tin-Lun Ho We have studied the ground states of fast rotating Fermi gases with half integer spins. Remarkable correlations between spin and orbital angular momentum are found. Because of the similarity of scattering lengths in different angular momentum channels, density-density interaction dominates and the system has close to SU$(2f+1)$ symmetry. We have found general solution to the SU$(2f+1)$ symmetry case for both repulsive and attractive density interaction, and we have constructed simple ``Hund's Rules" to describe the correlation between spin and orbital angular momentum in the ground state. Residual interactions split the degeneracy of the SU$(2f+1)$ symmetry ground state which leads to fine structures with regular patterns. [Preview Abstract] |
Tuesday, March 22, 2005 12:15PM - 12:27PM |
J36.00006: Acoustic attenuation in atom traps Sergio Gaudio, K.B. Blagoev, K.S. Bedell, Eddy Timmermans We study the damping rate of a BEC phonon excitation in a Bose-Fermi mixute gas. We show how the damping varies in the presence of the BCS superfluid phase in the Fermi gas. We, furthmore, point out some substantial differences with the condensed matter analogue in normal superconductors. [Preview Abstract] |
Tuesday, March 22, 2005 12:27PM - 12:39PM |
J36.00007: Ultracold fermion cooling cycle using heteronuclear Feshbach resonances M.A. Morales, N. Nygaard, J.E. Williams, Charles W. Clark Ideal gas models have given much insight into the physics of dilute fermion gases that can form into bosonic molecules via a Feshbach resonance, and have even given good quantitative agreement with the molecular and Bose-Einstein condensate fractions observed in recent experiments. [1] We develop such a model for a harmonically -trapped ideal gas with three components: bosonic atoms, fermionic atoms, and a fermionic diatomic molecule produced by a Feshbach resonance involving the two atomic species. Such systems have been produced in recent experiments. [2,3] We map out the phase diagram for this three-component mixture in chemical and thermal equilibrium. Considering adiabatic association and dissociation of the molecules, we identify a possible cooling cycle, which in ideal circumstances can yield an exponential increase of the phase-space density. \newline \newline [1] J. E. Williams, N. Nygaard and C. W. Clark, {\it New J. Phys.} {\bf 6}, 123 (2004) \newline [2] C. A. Stan {\it et al., Phys. Rev. Lett.} {\bf 93}, 143001 (2004) \newline [3] S. Inouye {\it et al., Phys. Rev. Lett.} {\bf 93}, 183201 (2004) [Preview Abstract] |
Tuesday, March 22, 2005 12:39PM - 12:51PM |
J36.00008: Nonequilibrium response of a molecular Bose-Einstein condensate near a Feshbach resonance. Masudul Haque, Henk Stoof We investigate the response of a molecular Bose-Einstein condensate near a Feshbach resonance to magnetic-field sweeps and resonant laser probes. For magnetic-field sweeps across the resonance, we study the process of dissociation and determine the energy distribution of the atoms produced after the sweep. We present both exact and numerical results. For laser couplings to excited molecular states, we explore the possibility of probing features of the molecular density of states that would not be accessible in equilibrium or in linear-response situations. [Preview Abstract] |
Tuesday, March 22, 2005 12:51PM - 1:03PM |
J36.00009: Decoupling transition in a one-dimensional fermionic gas interacting via a Feshbach resonance Leo Radzihovsky, Daniel Sheehy We study a fermionic atomic gas confined to move in one dimension and interacting via an s-wave Feshbach resonance. At low energies the system is characterized by a model of two Josephson-coupled Luttinger liquids, corresponding to fermionic atoms and their diatomic molecules. In contrast to higher dimensions, we find that this system exhibits a quantum phase transition from a phase in which the two superfluids are strongly coupled to one where the Feshbach resonance coupling becomes irrelevant and the two types of superfluid decouple. [Preview Abstract] |
Tuesday, March 22, 2005 1:03PM - 1:15PM |
J36.00010: Damping of amplitude modes in a neutral BCS superfluid Austen Lamacraft In contrast to Bose-Einstein condensates, fermionic superfluids exhibit an amplitude collective mode related to the internal dynamics of fermion pairs, as well as the phase -- or Bogoliubov -- mode. Recent work has proposed the existence of a collisionless regime in which highly nonlinear dynamics of the amplitude mode may be observed in systems of fermionic atoms pairing via a Feshbach resonance. Here we examine the damping of this dynamics by the phase mode and the general issue of collisionless dynamics versus the time-dependent Ginzburg-Landau description. [Preview Abstract] |
Tuesday, March 22, 2005 1:15PM - 1:27PM |
J36.00011: Phase slip in a superfluid Fermi gas near a Feshbach resonance Lan Yin, Ping Ao The properties of a phase slip in a superfluid Fermi gas is studied near a Feshbach resonance. The phase slip can be generated by the phase imprinting method. Below the superfluid transition temperature, it appears as a dip in the density profile, and becomes more pronounced when the temperature is lowered. Therefore the phase slip can provide a direct evidence of the superfluid state. The condensation energy of the superfluid state can be extracted from the density profile of the phase slip, due to the unitary properties of the Fermi gas near the resonance. The width of the phase slip is proportional to the inverse of the square root of the difference between the transition temperature and the temperature. The signature of the phase slip in the density profile becomes more robust across the BCS-BEC crossover. [Preview Abstract] |
Tuesday, March 22, 2005 1:27PM - 1:39PM |
J36.00012: Bosonic and Fermionic Cold atoms in optical lattices. Miguel A. Cazalilla, Andrew F. Ho, Thierry Giamarchi We examine the properties of coupled one dimensional tubes of Bosons or Fermions, in an optical lattice. For bosons, we find that the intertube coupling induces a deconfinement transition where the system goes from a one dimensional Mott insulator to a three dimensional superfluid. We compute the phase diagram and physical properties and discuss the results in connection with experiments on cold atoms. For fermions, the presence of the optical lattice allows for a richer phase diagram than for standard interacting fermions in one dimension. We analyse the resulting phases and discuss how to observe them for cold atoms [Preview Abstract] |
Tuesday, March 22, 2005 1:39PM - 1:51PM |
J36.00013: Engineering Superfluidity in Bose-Fermi Mixtures of Ultracold Atoms D.-W. Wang, M.D. Lukin, E. Demler We investigate many-body phase diagrams of atomic boson-fermion mixtures loaded in the two-dimensional optical lattice. Bosons mediate an attractive, finite-range interaction between fermions, leading to fermion pairing phases of different orbital symmetries. Specifically, we show that by properly tuning atomic and lattice parameters it is possible to create superfluids with $s$-, $p$-, and d-wave pairing symmetry as well as spin and charge density wave phases. These phases and their stability are analyzed within the mean-field approximation for systems of $^{40}$K-$^{87}$Rb and $^{40}$K-$^{23}$Na mixtures. For the experimentally accessible regime of parameters, superfluids with unconventional fermion pairing have transition temperature around a percent of the Fermi energy. [Preview Abstract] |
Tuesday, March 22, 2005 1:51PM - 2:03PM |
J36.00014: Dirac Fermions in Optical Lattices Thomas Davis, Marcel Franz Two dimensional interacting Dirac fermions arise in many different contexts in condensed matter physics and are simmultaneously of great interest in elementary particle physics. We propose methods of constructing Dirac fermions in atomic gas systems in the presence of optical lattices. At the mean field level, the effective Hamiltonian admits a `chiral symmetry breaking' phase transition between a gapped antiferromagnet and a gapless semimetal when the on-site Hubbard interaction is varied. We show that this transition will have an epxerimental signature in the density-density correlation spectrum. Close to the criticality, the nontrivial exponents of this quantum phase transition can be experimentally probed. [Preview Abstract] |
Tuesday, March 22, 2005 2:03PM - 2:15PM |
J36.00015: Exotic $p$-wave superfluidity of single hyperfine state Fermi gases in optical lattices Menderes Iskin, Carlos Sa de Melo We consider $p$-wave pairing of single hyperfine state ultracold atomic gases trapped in quasi-two-dimensional optical lattices. We discuss superfluid $p$-wave (triplet) states that break time reversal, spin and orbital symmetries, but preserve total spin-orbit symmetry. We calculate the atomic compressibility, and spin susceptibility as a function of band filling for tetragonal (e.g., trapping potentials with same field intensity and same wavelengths) and orthorhombic (e.g., trapping potentials with same field intensities but different wavelenths) optical lattices. In the case of tetragonal lattices, we show that the atomic compressibility (or spin susceptibility) has a peak at low temperatures exactly at a half-filling, but this peak splits into two as the wavelength of the optical lattice is changed in one direction. These peaks reflect the $p$-wave structure of the order parameter for superfluidity and they disappear as the critical temperature is approached from below. We also calculate the superfluid density tensor, and show that for the orthorhombic case there is no off-diagonal component, however in the tetragonal case an off- diagonal component develops below the superfluid critical temperature, and becomes a key-signature of the exotic $p$-wave state. [Preview Abstract] |
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J36.00016: Adiabatic construction of d-wave resonating valence bond states of ultracold fermionic atoms in optical lattices Simon Trebst, Matthias Troyer, Peter Zoller, Ulrich Schollw\"ock We discuss a controlled experimental setup to adiabatically construct superconducting $d$-wave resonating valence bond (RVB) states of fermionic atoms confined in a 2D optical lattice. The key idea is to start from an already cold initial state, in our approach from fully filled 1D tubes of atoms. The adiabatic transformation then allows to reach ultralow temperatures of a few percent of the Fermi temperature which is required to observe the $d$-wave RVB states. We discuss hole doping techniques and describe a simple experimental measurement to study the $d$-wave pairing of such vacancies. Our experimental setup can be used to effectively probe ground state properties of the doped and undoped half-filled Hubbard model on (coupled) plaquettes, ladders and the 2D lattice. [Preview Abstract] |
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