Bulletin of the American Physical Society
2006 37th Meeting of the Division of Atomic, Molecular and Optical Physics
Tuesday–Saturday, May 16–20, 2006; Knoxville, TN
Session Z3: Fermionic Superfluidity in Ultracold Gases |
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Chair: Doerte Blume, Washington State University Room: Knoxville Convention Center 301C |
Saturday, May 20, 2006 9:00AM - 9:12AM |
Z3.00001: Fermionic Superfluidity with Imbalanced Spin Populations and the Quantum Phase Transition to the Normal State Martin W. Zwierlein, Andre Schirotzek, Christian H. Schunck, Wolfgang Ketterle Whether it occurs in superconductors, helium-3 or inside a neutron star, fermionic superfluidity requires pairing of fermions, particles with half-integer spin. For an equal mixture of two states of fermions (``spin up'' and ``spin down''), pairing can be complete and the entire system will become superfluid. When the two populations of fermions are unequal, not every particle can find a partner. Will the system nevertheless stay superfluid? In this talk we present our study of this intriguing question in an unequal mixture of strongly interacting ultracold fermionic atoms. The superfluid region vs population imbalance is mapped out by employing two complementary indicators: The presence or absence of vortices in a rotating mixture, as well as the fraction of condensed fermion pairs in the gas. Due to the strong interactions near a Feshbach resonance, the superfluid state is remarkably stable in response to population imbalance. The final breakdown of superfluidity marks a new quantum phase transition, the Pauli limit of superfluidity. [Preview Abstract] |
Saturday, May 20, 2006 9:12AM - 9:24AM |
Z3.00002: Pairing and Phase Separation in a Polarized Fermi Gas Guthrie Partridge, Wenhui Li, Ramsey Kamar, Yean-an Liao, Randall Hulet We have prepared an ultra-cold gas of atomic fermions ($^6$Li) with unequal numbers in two spin components. A Feshbach resonance is used to control the interaction strength between these components. In the unitarity regime within the Feshbach resonance, our data shows a critical polarization, beyond which the gas separates into a phase that is consistent with a superfluid paired core surrounded by a shell of normal unpaired fermions.\footnote{G. B. Partridge, W. Li, R. I. Kamar, Y. Liao, R. G. Hulet, \emph{Science}, \textbf{311}, 503 (2006); published online 22 December 2005 (10.1126/science.1122876) } For near zero polarization, we measure the parameter $\beta = -0.54 \pm 0.05$ describing the universal energy of a strongly interacting paired Fermi gas, and find good agreement with recent theory. We also find that the critical polarization diminishes with decreasing attractive interaction strength on the BCS side of the Feshbach resonance. [Preview Abstract] |
Saturday, May 20, 2006 9:24AM - 9:36AM |
Z3.00003: Direct Observation of Resonance Condensation in Imbalanced Fermi Mixtures Martin W. Zwierlein, Andre Schirotzek, Christian H. Schunck, Wolfgang Ketterle We directly observe pair condensation in an unequal mixture of resonantly interacting fermionic $^6$Li atoms. Condensation is revealed by the sudden appearance of a bimodal density distribution in the minority spin component below a critical temperature. Already above the critical temperature for condensation, strong interactions between the two spin states are manifest in the deformed density distribution of the larger cloud. Temperatures can be directly determined from the non- interacting wings of the majority component. Beyond a critical population imbalance of $71(3)\%$ on resonance, no condensates are observed, in agreement with our earlier observation of the Pauli limit of superfluidity. We show that for higher than critical imbalance, the central densities of the two components are no longer matched. [Preview Abstract] |
Saturday, May 20, 2006 9:36AM - 9:48AM |
Z3.00004: Pairing in a Strongly Interacting Polarized Fermi Gas Wenhui Li, Guthrie Partridge, Ramsey Kamar, Yean-an Liao, Duong Nguyen, Randall Hulet We have observed pairing in an atomic Fermi gas of $^6$Li atoms with unequal numbers of two components.\footnote{G. B. Partridge \textit{et al.}, \emph {Science}, \textbf{311}, 503 (2006).} Beyond a critical polarization, the gas separates into paired core surrounded by a shell of normal unpaired fermions. At polarization smaller than this critical value, however, the atoms of two spins are able to spatially coexist with mismatched Fermi surfaces. The nature of such a system has been a topic of debate for decades. In these experiments, we investigate possible pairing mechanisms in this regime. The results are relevant to predictions of exotic new phases of quark matter and of strongly magnetized superconductors. [Preview Abstract] |
Saturday, May 20, 2006 9:48AM - 10:00AM |
Z3.00005: Trapped Fermions across a Feshbach resonance with population imbalance Wei Yi, Luming Duan We investigate the phase separation of resonantly interacting fermions in a trap with imbalanced spin populations, both at zero and at finite temperatures. We directly minimize the thermodynamical potential under the local density approximation instead of using the gap equation, as the latter may give unstable solutions. At zero temperature, on the BEC side of the resonance, one may cross three different phases from the trap center to the edge: the superfluid phase (SF), where all particles are paired and there is no population imbalance; the breached gap phase (BP), where superfluid and polarized fermions coexist; and the normal Fermi sea with different Fermi surfaces for different spin components. On the BCS side or at resonance, typically only the SF and the normal phase show up. At finite temperature, we show that there exist fermionic excitations even in the superfluid phase, which carry population imbalance of the spin components. As a result, it becomes easier to satisfy the population imbalance constraint, which helps to stabilize the superfluid phase. Because of the fermionic excitations at finite temperature, the boundary between the BP phase and the SF phase becomes obscure. The phase separation between the paired phase (SF/BP) and the normal phase however, is marked by a peak in the population difference. We compare our results with a recent experiment (M.W. Zwierlein, et. al., cond-mat/0511197), and the agreement is remarkable. [Preview Abstract] |
Saturday, May 20, 2006 10:00AM - 10:12AM |
Z3.00006: Spontaneous Vortices in Imbalance Populated Fermion Gas, Finite Size System Rembert Duine, Jung-Jung Su, Yun-pil Shim, Allan H MacDonald Atomic Fermion gases with mismatched densities have attracted much interest recently both experimentally and theoretically. These gases are related to superconductors in a magnetic field, to color superconductivity in high density QCD and to other systems. The main focus of recent research is on the possibility of unusual pairing states, the Larkin-Ovchinnikov-Fulde-Ferrel(LOFF)[1] phase, the Deformed Fermi surface(DFS)[2] and other states have been suggested in the past few years. We work specifically on two-dimensional systems with circular hard walls which contain atoms with two different hyperfine states and different populations. In addition to phase separation, a phenomenon that has already been observed[3], we consider the possibility of the spontaneous formation of vortices and giant vortices in some regions of parameter space. [1] Qinghong Cui, C.-R. Hu, J.Y.T. Wei, and Kun Yang, cond-mat/0510717 [2] Armen Sedrakian, Jordi Mur-Petit, Artur Polls, Herbert M\"{u}ther , cond-mat/0404577 [3] Guthrie B. Partridge, Wenhui Li, Ramsey I. Kamar, Yean-an Liao, Randall G. Hulet, cond-mat/0511752 [Preview Abstract] |
Saturday, May 20, 2006 10:12AM - 10:24AM |
Z3.00007: First and second sound in the BCS-BEC crossover in uniform Fermi superfluid gases Allan Griffin, Edward Taylor . The Landau two-fluid hydrodynamic equations are valid in the BCS-BEC crossover when the scattering length is very large. The frequencies of the in-phase and out-phase hydrodynamic modes at finite temperatures are determined by the equilibrium thermodynamic functions, including the temperature-dependent superfluid density. In turn , these depend on the Fermionic and Bosonic thermal excitations. We have calculated all these functions at finite temperatures in a uniform superfluid Fermi gas, including the particle--particle fluctuations using the well-known Nozieres-Schmitt-Rink approximation. These results will be extended to calculate the analogous hydrodynamic modes in a trapped superfluid gas, working within the LDA. [Preview Abstract] |
Saturday, May 20, 2006 10:24AM - 10:36AM |
Z3.00008: Light scattering of degenerate fermions S. Aubin, L. J. LeBlanc, S. Myrskog, M. H. T. Extavour, D. McKay, A. Stummer, J. H. Thywissen We report on progress in measuring the suppression of resonant light scattering in a gas of degenerate fermions. A gas of trapped degenerate fermions is expected to exhibit narrower optical linewidths and longer excited state lifetimes than single atoms when the Fermi energy is larger than the photon recoil energy [1-3]. In this case, the number of available states into which a scattered atom can recoil is significantly reduced due to the filling of the Fermi sea. We produce a degenerate gas of 4$\times $10$^{4}$ ultra-cold fermionic $^{40}$K atoms by sympathetic cooling with bosonic $^{87}$Rb in a micro-magnetic chip trap. The atoms can then be loaded into a tight dipole trap just above the surface of the chip and probed with a near resonance laser pulse. [1] Th. Busch, J. R. Anglin, J. I. Cirac, and P. Zoller, \textit{Europhys. Lett.} \textbf{44}, 1 (1998). [2] B. DeMarco and D. S. Jin, \textit{Phys. Rev. A} \textbf{58}, R4267 (1998). [3] J. Javanainen and J. Ruostekosky, \textit{Phys. Rev. A} \textbf{52}, 3033 (1995). Work supported by NSERC, CFI, OIT, Research Corporation, and PRO. [Preview Abstract] |
Saturday, May 20, 2006 10:36AM - 10:48AM |
Z3.00009: Interaction Energy in a Paired Two-Component Degenerate Fermi Gas in the BEC-BCS Crossover R.I. Kamar, G.B. Partridge, W. Li, R.G. Hulet We have measured the interaction energy of the ground-state of a paired Fermi gas in the BEC-BCS crossover. The crossover describes the smooth transition from a BEC of tightly bound dimers to that of a paired BCS superfluid. A Feshbach resonance is used to tune the atomic scattering length \textit{a}. The interaction energy is extracted from \textit{in-situ} images of a trapped two component gas of $^{6}$Li atoms near $T=0$. Near the peak of the Feshbach resonance the interaction energy is expected to be parameterized by a single universal parameter $\beta$ describing a strongly interacting, paired superfluid. We find that $\beta = -0.54$ $\pm$ $0.05$ in good agreement with recent Monte Carlo calculations giving $\beta = -0.58$. On the BEC side of resonance, we find that the molecular scattering length is $0.6 a$, in agreement with recent theory, but only for fields not too close or too far from resonance. [Preview Abstract] |
Saturday, May 20, 2006 10:48AM - 11:00AM |
Z3.00010: Superfluid Expansion of a Rotating Fermi Gas Christian Schunck, Martin Zwierlein, Andre Schirotzek, Wolfgang Ketterle Pairing and superfluidity in strongly interacting Fermi gases are pure many-body effects: Fermion pairs exist only due to the stabilizing presence of the surrounding atoms. The pairs are thus dependent on density and can break as the density is decreased. In contrast to a Bose-Einstein Condensate (BEC) an expanding superfluid Fermi gas therefore eventually undergoes the phase transition to the normal state. Here we observe the expansion of a rotating, superfluid Fermi gas. As superfluids can contain angular momentum only in the form of vortices, the presence and absence of vortices in the gas is used to distinguish superfluid and normal parts of the expanding cloud. We find that superfluid Fermion pairs survive during expansion. Breakdown of superfluid flow is observed as the density, and hence the binding energy, decreases below a critical value. [Preview Abstract] |
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