Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session H43: Focus Session: Strongly Interacting Fermi Gases and the BCS-BEC Crossover II |
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Sponsoring Units: DAMOP Chair: Murray Holland, University of Colorado Room: Baltimore Convention Center 346 |
Tuesday, March 14, 2006 11:15AM - 11:27AM |
H43.00001: Understanding the Superfluid phase diagram in trapped Fermi gases Kathy Levin, Qijin Chen We address previous experimental observations of the condensation of fermionic atom pairs involving trapped Fermi gases which can be tuned from the BCS to BEC regime, with the application of magnetic fields. In the intermediate regime, condensation is demonstrated experimentally by a sweep technique that pairwise projects fermionic atoms onto molecules. While the condensate fraction is measured after a fast sweep to BEC, the temperature is measured by a slow adiabatic sweep to the Fermi gas regime. In this paper we compare the normal-superfluid phase boundary in this temperature-magnetic field plane as obtained in this way for $^{40}$K and computed theoretically. We demonstrate good agreement between the two.\newline References: arXiv:cond-mat/0411090; Phys. Rev. Lett., in production. [Preview Abstract] |
Tuesday, March 14, 2006 11:27AM - 11:39AM |
H43.00002: Critical Temperature and Thermodynamic Properties of Attractive Fermions Evgeni Burovski, Nikolay Prokof'ev, Boris Svistunov, Matthias Troyer The unitarity regime of the BCS-BEC crossover can be realized by diluting a system of two-component lattice fermions with an on-site attractive interaction. We perform a systematic-error- free finite-temperature simulations of this system by diagrammatic determinant Monte Carlo. We report the data obtained on the Cray X1E ``Phoenix'' of the Oak Ridge National Laboratory. The critical temperature in units of Fermi energy is found to be $T_c / E_F = 0.152(7)$. We also report the behaviour of the thermodynamic functions, and discuss the issues of thermomentry of ultracold Fermi gases. [Preview Abstract] |
Tuesday, March 14, 2006 11:39AM - 11:51AM |
H43.00003: Dynamical Mean-Field Equations for Strongly Interacting Fermi Gas in a Trap Wei Yi, Luming Duan We derive the time evolution equations at zero temperature for the wavefunctions of the molecular bosons and the fermion pairs in a trapped Fermi gas near a wide Feshbach resonance. The derivation of the equations is based on the variational principle and the BCS-like ansatz state: $|\Phi\rangle=\mathcal{N}e^{\int \phi_b(\mathbf{r})\Psi_b^{\dag}(\mathbf{r})d^3\mathbf{r}} e^ {\int \rho(\mathbf{r},\mathbf{r'})\Psi_{\uparrow}^{\dag}(\mathbf{r}) \Psi_{\downarrow}^{\dag}(\mathbf{r'})d^3\mathbf{r}d^3\mathbf {r'}}|0\rangle$. In deriving the equations, we have assumed that the external trapping potential and the wavefunction of the molecular bosons are spatially slow-varying on the length scale of the size of the fermionic atom pairs, which should be valid over a wide range on the BEC side of resonance, including the resonance point. In the bosonic region ($\mu\le0$, where $\mu$ is the chemical potential), the equations will reduce to one that resembles a Gross- Pitaevskii (GP) equation. We solve the the stationary ground state of the system at different detunings near the crossover region and self-consistently checked our assumptions. The time evolution equations provide macroscopic description for the wavefunctions of the molecular bosons and of the fermion pairs near the interesting BCS-BEC crossover region. In future studies, these equations can be used to analyze the interesting physics of vortices or the excitation spectrum in the Fermi condensate. [Preview Abstract] |
Tuesday, March 14, 2006 11:51AM - 12:27PM |
H43.00004: Studying the BCS-BEC crossover regime with a Fermi gas of $^{40}$K atoms Invited Speaker: Recent years have seen the emergence of an intriguing Fermi system achieved with ultracold atomic gases. With these systems it is possible to widely tune the s-wave interatomic interaction strength using a Feshbach resonance. Of particular interest is the strongly interacting regime ($-1 < 1/k_Fa < 1$) where a crossover between BCS theory of superconductivity and Bose-Einstein condensation (BEC) of molecules occurs. Recently experiments with $^6$Li and $^{40}$K have succeeded in studying many aspects of this superfluid Fermi system. In my talk I will discuss recent experiments performed at JILA on this Fermi system using $^{40}$K. [Preview Abstract] |
Tuesday, March 14, 2006 12:27PM - 12:39PM |
H43.00005: Counting Statistics of Density Fluctuations in a Quantum Gas Wolfgang Belzig, Christian Schroll, Christoph Bruder We investigate the statistics of density fluctuations in a coherent ensemble of interacting fermionic atoms around the BEC-BCS crossover. Adapting the concept of full counting statistics, well-known from quantum optics and mesoscopic electron transport, we study second-order as well as higher-order correlators of density fluctuations. This method is applied to the crossover from a molecular BEC state to a fermionic BCS state and yields a transition from Poissonian statistics to binomial statistics. The large Poissonian fluctuations confirm the picture of independent molecules on the BEC side of the transition. Strongly suppressed fluctuations on the BCS side reflect the correlation among Cooper pairs and the inertness of a Fermi sea. In the transition region the third cumulant is a measure of the particle hole symmetry of the distribution function. The statistics can thus be used as an experimental tool to gain information on the many-body ground states. [Preview Abstract] |
Tuesday, March 14, 2006 12:39PM - 12:51PM |
H43.00006: A mean-field model for condensates in the BEC-BCS crossover regime Cheng Chin We present a new mean-field model to describe Fermionic condensates in the BEC-BCS crossover regime. By introducing an effective mean-field potential, this approach allows us to analytically evaluate the chemical potential, the equation of states, and the pair wave function. The results agree surprisingly well with recent quantum Monte Carlo calculations. Density profiles and the collective mode frequencies of a trapped gas can thus be analytically determined. In particular, the pair wave function can be expressed in terms of Airy function. We show that this wave function naturally approaches molecular wave function in the BEC limit. In the presence of strong interaction, the wave function is compressed to a smaller size, which implies an upshift of the pairing energy. This effect can be understood based on the repulsive interaction between two pairs (Petrov et al.) and has been observed in rf spectroscopy experiment. Although the mean-field approach does not explicitly include anti-symmetric fermionic correlations, it does provide a complementary picture to view the crossover physics. The validity of this approach is based on the fact that a degenerate Fermi gas constitutes the same quantum phase as that of a condensate of pairs. Both descriptions should therefore be similarly effective in the crossover. One advantage of the bosonic approach is that the molecular mean-field term can conveniently capsulate four-fermion interactions. It is, however, more difficult to do so in the BCS-type calculation. [Preview Abstract] |
Tuesday, March 14, 2006 12:51PM - 1:03PM |
H43.00007: FFLO State in a Rotating Cold Fermionic Atom System Yun-pil Shim, Rembert Duine, Allan H. MacDonald Superconductors with a large Zeeman splitting are expected to have an inhomogeneous order parameter. This Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state was proposed in the early 1960's, and recent experiments in various solid state systems have shown some progress in realizing this state. We study the FFLO state for rotating fermionic atom systems consisting of two hyperfine species with different populations. The fermi surface mismatch due to the population imbalance effectively plays the role of Zeeman splitting. In recent experiments[1,2], vortex structures were observed in rotating cold fermion systems over the whole range of the BEC-BCS crossover, but the exotic vortex structures expected for the FFLO state have yet to be observed. We use a fully quantum mechanical approach to include the Landau level quantization effect due to the rotation and present a phase diagram for superconducting phase transition with center of mass (COM) motion of the paring atoms in different Landau levels. The FFLO state is expected over small range of parameters and as the population imbalance increases, the paring condensation occurs at higher COM Landau level. [1] M. W. Zwierlein et al, Nature 435, 1047 (2005) [2] M. W. Zwierlein et al, cond-mat/0511197 [Preview Abstract] |
Tuesday, March 14, 2006 1:03PM - 1:15PM |
H43.00008: Spin-polarized fermionic superfluidity in a trap Daniel E. Sheehy, Leo Radzihovsky Recent research on Feshbach resonantly paired superfluids has focused on the fate of such superfluidity upon changing the relative population of the two hyperfine states undergoing pairing. I will discuss recent work[1] showing that such imposed spin polarization frustrates pairing and leads to a rich phase diagram consisting of a polarized superfluid, phase separation, and Fulde-Ferrell-Larkin-Ovchinnikov phases. In the spatially inhomogeneous environment of a trap, the local spin polarization (magnetization) is also inhomogeneous, providing a direct signature of these phases. I will discuss this and other signatures of such phases in degenerate atom experiments. This work was supported by NSF DMR-0321848 and the Packard Foundation. [1] D.E. Sheehy and L. Radzihovsky, cond-mat/0508430. [Preview Abstract] |
Tuesday, March 14, 2006 1:15PM - 1:27PM |
H43.00009: Density profile and collective modes of the unpolarized and partially polarized trapped Fermi gas in the BCS-BEC crossover region. Theja De Silva, Erich Mueller We study the zero temperature BCS-BEC crossover physics of both the unpolarized and partially polarized 2-component trapped Fermi gas near a Feshbach resonance. We investigate the spatial distribution of the partially polarized atomic system; studying superfluid and normal fluid phase separation and coexistence within a local density approximation. We calculate collective mode frequencies as a function of the interaction strength, finding quantitative agreements with experiments. [Preview Abstract] |
Tuesday, March 14, 2006 1:27PM - 1:39PM |
H43.00010: Pairing in Feshbach-resonant trapped fermionic atom gases with unequal species populations Masudul Haque, Henk Stoof Interest in BCS-like pairing with unequal Fermi surfaces has recently been revived due to the possibility of its realization in ultracold atomic gases, and the first experimental measurements have appeared in the past few months. We briefly review zero-momentum pairing with unequal chemical potentials for the two species. We then describe effects of an external trapping potential on the pairing process, and the resulting shell structure of different phases in the trap. [Preview Abstract] |
Tuesday, March 14, 2006 1:39PM - 1:51PM |
H43.00011: Strong coupling theory for the superfluidity of Bose-Fermi mixtures Daw-Wei Wang We develop the strong coupling theory for the superfluidity of fermion $s$-wave pairing state in a Bose-Fermi mixture. Dynamical screening, self-energy renormalization, and pairing gap function are included self-consistently in the regime where the phonon velocity is smaller than the Fermi velocity. Analytical form for the transition temperature ($T_c$) is derived within reasonable approximations. In typical $^{40}$K-$^{87}$Rb mixtures, the obtained $T_c$ is several times larger than that in the weak coupling theory and can be as high as several percents of Fermi temperature. [Preview Abstract] |
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