Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session D16: Focus Session: BEC-BCS Crossover |
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Sponsoring Units: DAMOP Chair: Kathy Levin, University of Chicago Room: 317 |
Monday, March 16, 2009 2:30PM - 3:06PM |
D16.00001: BCS to BEC evolution for mixtures of fermions with unequal masses Invited Speaker: I discuss the zero and finite temperature phase diagrams of a mixture of fermions with unequal masses with and without population imbalance, which may correspond for example to mixtures of $^6$Li and $^{40}$K, $^6$Li and $^{87}$Sr, or $^{40}$K and $^{87}$Sr in the context of ultracold atoms. At zero temperature and when excess fermions are present, at least three phases may occur as the interaction parameter is changed from the BCS to the BEC regime. These phases correspond to normal, phase separation, or superfluid with coexistence between paired and excess fermions. The zero temperature phase diagram of population imbalance versus interaction parameter presents a remarkable asymmetry between the cases involving excess lighter or heavier fermions [1, 2], in sharp contrast with the symmetric phase diagram corresponding to the case of equal masses. At finite temperatures, the phase separation region of the phase diagram competes with superfluid regions possessing gapless elementary excitations [3] for certain ranges of the interaction parameter depending on the mass ratio. Furthermore, a phase transition may take place between two superfluid phases which are topologically distinct. The precise location of such transition is sensitive to the mass ratio between the two species of fermions. Signatures of this possible topological transition are present in the momentum distribution or structure factor, which may be measured experimentally in time-of-flight or through Bragg scattering, respectively. Lastly, throughout the evolution from BCS to BEC, I discuss the critical current and sound velocity for unequal mass systems as a function of interaction parameter and mass ratio. These quantities may also be measured via the same techniques already used in mixtures of fermions with equal masses. \newline [1] M. Iskin, and C. A. R. Sa de Melo, Phys. Rev. Lett. 97, 100404 (2006). \newline [2] M. Iskin and C. A. R. Sa de Melo, Phys. Rev. A 76, 013601 (2007). \newline [3] Li Han, and C. A. R. Sa de Melo, arXiv:0812.xxxx [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D16.00002: Spin-imbalanced atomic Fermi gases in one dimension and the prospects for FFLO superconductivity C.J. Bolech, P. Kakashvili Growing expertise to engineer, manipulate and probe different analogs of condensed matter systems allows to probe properties of exotic pairing states such as the Fulde-Ferrell-Larkin- Ovchinnikov state. Inspired by ongoing experiments at Rice university, we are studying the pairing in spin-imbalanced ultracold atomic system of fermions in one dimension. Calculations are done using the Bethe Ansatz technique and the trap is incorporated into the solution via local density (Thomas-Fermi) approximation. The thermodynamic-Bethe-Ansatz equations are solved numerically and different density profiles (total-, spin- and entropy-densities) are calculated in the trap for different finite temperatures. A scheme to identify the phase diagram using total density profiles in the trap is proposed that would be immediately useful for experimentalists. [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D16.00003: Finite temperature effects of $^{6}$Li-$^{40}$K mixtures in the BCS-BEC crossover Hao Guo, Chihchun Chien, Yan He, Qijin Chen, Kathryn Levin Recent experiments on mixtures of ultra-cold fermions of different species inspire study of pairing between fermions with different masses. We study systematically $^{6}$Li-$^{40}$K mixtures with tunable attractive interactions in the BCS-Bose Einstein condensation crossover. Pairing fluctuations which are important at finite temperatures are included in a consistent fasion. Population imbalance of the two species is also considered. We found an intermediate-temperature superfluid phase which is similar to the one found in polarized Fermi gases with equal mass. We present superfluid transition temperature for a broad range of polarization and interaction strength and analyze stability of possible superfluid phases. Phase diagrams at and near unitarity are presented. Polarized superfluids are shown to be stablized when the light species is the majority. Thus, in contrast to pairing between fermions with equal mass, observation of stable low-temperature polarized superfluids near unitarity is more feasible in $^{6}$Li-$^{40}$K mixtures. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D16.00004: Phenomenology Of Trapped Polarized Fermi Gases Leslie Baksmaty, Hong Lu, Han Pu, Carlos Bolech We discuss recent, apparently contradictory experimental results on trapped, polarized, resonantly interacting interacting fermions. Our analysis occurs in the context of a full 3D Bogoliubov-deGennes analysis and we dwell on the possible roles of the confining geometry in producing density distortions away from the local density approximation with implications for the Superfluid-Normal transition (Clogston limit). [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D16.00005: Rotation induced superfluid-normal phase separation in trapped Fermi gases Menderes Iskin, Eite Tiesinga We use the Bogoliubov-de Gennes formalism to analyze the effects of rotation on the ground state phases of harmonically trapped Fermi gases, under the assumption that quantized vortices are not excited. We find that the rotation breaks Cooper pairs that are located near the trap edge, and that this leads to a phase separation between the nonrotating superfluid (fully paired) atoms located around the trap center and the rigidly rotating normal (nonpaired) atoms located towards the trap edge, with a coexistence (partially paired) region in between. Furthermore, we show that the rotation reveals a topological quantum phase transition: the superfluid phase that occurs in the coexistence region is characterized by a gapless excitation spectrum, and that it is distinct from the gapped phase that occurs near the trap center. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D16.00006: Pairing Instability in Two-Dimensional Rotating Fermion Liquids Near Unitarity Predrag Nikolic Fermionic superfluids can undergo phase transitions into different kinds of normal regimes, characterized by whether Cooper pairs remain locally stable. If the normal phase retains strong pairing fluctuations, it behaves like a liquid of vortices as seen in cuprate superconductors. We argue that analogous strongly correlated normal states exist in two-dimensional neutral fermion liquids near unitarity, where superfluid is destroyed by fast rotation. The formal analysis is based on a model with SP(2N) symmetry which describes the quantum critical region in the vicinity of a broad Feshbach resonance. Assuming that pairing is the only instability in perturbation theory, we map the universal phase diagram in two-dimensions. Such a pairing instability is driven by macroscopically degenerate collective modes, which makes the Abrikosov flux lattice of the superfluid particularly susceptible to quantum melting. Combining this observation with a renormnalization group analysis, we conclude that the unconventional normal states can be expected in the vicinity of the universal pairing instability, especially at low temperatures in the BCS limit. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D16.00007: Number of closed-channel molecules in the BEC-BCS crossover Felix Werner, Leticia Tarruell, Yvan Castin Using a two-channel model, we show that the number of closed-channel molecules in a two-component Fermi gas close to a Feshbach resonance is directly related to the derivative of the energy of the gas with respect to the inverse scattering length. We extract this quantity from the fixed-node Monte~Carlo equation of state and we compare to the number of closed-channel molecules measured in the Rice experiment with lithium [Partridge et al., Phys. Rev. Lett. 95, 020404 (2005)]. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D16.00008: Breathing mode frequencies of a rotating Fermi gas in the BCS-BEC crossover region Theja De Silva We study the breathing mode frequencies of a rotating Fermi gas trapped in a harmonic plus radial quartic potential. We find that as the radial anharmonicity increases, the lowest order radial mode frequency increases while the next lowest order radial mode frequency decreases. Then at a critical anharmonicity, these two modes merge and beyond this merge the cloud is unstable against the oscillations. The critical anharmonicity depends on both rotational frequency and the chemical potential. As a result of the large chemical potential in the BCS regime, even with a weak anharmonicity the lowest order mode frequency increases with decreasing the attractive interaction. For large enough anharmonicities in the weak coupling BCS limit, we find that the excitation of the breathing mode frequencies make the atomic cloud unstable. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D16.00009: Superfluid excitations of dipolar fermi gases Piotr Deuar, Mikhail Baranov, Georgy Shlyapnikov The collective and single-particle excitations of a gas of fermionic dipoles have been determined for the case of a uniform, single-species, fully polarised gas below superfluid (BCS) critical temperature. Its behaviour, especially damping, differs strongly from the s-wave BCS gas due to a node line in its quasiparticle excitation spectrum that resembles that in the hypothetical polar phase of He-3 and exotic superconductors. One finds: (1) Anisotropic damping of collective modes even at T=0. (2) An ``aligned superfluid'' regime with no analogue in the s-wave-interacting gas, for excitations with energy well below kT. Here good quality superfluidity occurs only in directions concentrated broadly around the polarisation, whereas other directions are strongly damped. (3) Current response to external forcing of the gap is anisotropic and at an angle to the applied probe. [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D16.00010: Two-fluid hydrodynamic modes in a strongly interacting Fermi gas Edward Taylor, Hui Hu, Xia-Ji Liu, Sandro Stringari, Allan Griffin Landau's theory of two-fluid hydrodynamics provides an exact description of the low-energy dynamics of all strongly interacting superfluids described by a 2-component order parameter. Extending our recent work, we report on improved variational solutions of the two-fluid hydrodynamic modes in trapped two-component Fermi gases close to unitarity. We show that the two-fluid mode frequencies are identical to the predictions of regular (Euler) hydrodynamics except at certain temperatures where these in-phase modes strongly hybridize with out-of-phase modes. Although two-fluid hydrodynamic modes have been extensively studied in superfluid helium, experiments in trapped quantum gases have yet to detect a clear signal of these modes. We discuss the reasons for this and suggest several experimental signatures of two-fluid behaviour in trapped Fermi superfluids. Measuring the two-fluid mode frequencies is a promising way of testing microscopic theories of the thermodynamic and transport properties at unitarity. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D16.00011: Adiabatic Dynamics of the Superconducting Order Parameter Minxi Jiang, Qian Niu From the time-dependent variational principle and taking into Berry phase effects, we formulate the dynamics of superconducting order parameter in the region where it evolves much slower compared to the timescale of quasi-particles. Collective mode in this region is calculated and compared with previous result obtained from the random-phase approximation which is valid in the opposite limit. We discuss applications to BCS/BEC states of the quantum Fermi gases. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D16.00012: The Efimov Effect and Color Superconductivity in a Three-State Fermi Gas J.R. Williams, J.H. Huckans, E.L. Hazlett, R.W. Stites, Y. Zhang, K.M. O'Hara We have created a quantum degenerate $^6$Li gas with equal populations in the three lowest energy hyperfine states. This three-state Fermi gas is stable against two-body inelastic collisions but decays by three-body recombination. We measure the rate of three-body recombination which can be used as a signature of the Efimov effect and which determines whether conditions are favorable for BCS pairing. The three pairwise $s$-wave scattering lengths exhibit overlapping Feshbach resonances at 690, 810 and 834 Gauss. As we vary the field between 0 and 834 Gauss, we find that the three-body recombination rate constant varies by over four orders of magnitude. High stability is achieved near 0 and 570 Gauss. We observe narrow resonant loss features near 130 and 500 Gauss. Recent calculations indicate that these resonant features arise from Efimov trimer states near threshold[1]. We also report on the rate of three-body recombination between 834 and 2000 Gauss. Our determination of the three-body parameters in this regime will guide future experiments aimed at achieving color superconductivity in this system.\\ \\ E. Braaten, H.-W. Hammer, D. Kang, and L. Platter, arXiv:0806.0587. [Preview Abstract] |
Monday, March 16, 2009 5:18PM - 5:30PM |
D16.00013: Phase diagram, extended domain walls, and soft collective modes in a three-component fermionic superfluid Gianluigi Catelani, Emil Yuzbashyan We study the phase diagram of a three-component Fermi gas with weak attractive interactions, which shows three superfluid and one normal phases. At weak symmetry breaking between the components the existence of domain walls interpolating between two superfluids introduces a new length scale much larger than the coherence length of each superfluid. This, in particular, limits the applicability of the local density approximation in the trapped case, which we also discuss. In the same regime the system hosts soft collective modes with a mass much smaller than the energy gaps of individual superfluids. We derive their dispersion relations at zero and finite temperatures and demonstrate that their presence leads to a significant enhancement of fluctuations near the superfluid-normal transitions. [Preview Abstract] |
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