Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session B4: Population Imbalanced Superfluid Fermi Gases |
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Sponsoring Units: DAMOP Chair: Allan Griffin, University of Toronto Room: Colorado Convention Center Korbel 2B-3B |
Monday, March 5, 2007 11:15AM - 11:51AM |
B4.00001: Phase-Separation in a Polarized Fermi Gas Invited Speaker: In fermionic systems, the formation of pairs between two constitute components is the essential ingredient of superfluidity and superconductivity. While many advances have been made in understanding pairing between components with equal chemical potentials, the possible pairing mechanisms and corresponding phases in systems with mismatched chemical potentials are topics of active debate. In contrast to the difficulties in generating magnetized superconductors, mismatched Fermi surfaces can be readily realized by creating an imbalance between the populations of two spin components in a gas of ultracold fermionic atoms. Exotic new states of matter are predicted for the unbalanced systems that, if realized, may have important implications for our understanding of nuclei, compact stars, and quantum chromodynamics. We investigate a strongly interacting Fermi gas of $^6$Li atoms with unbalanced populations by {\it in-situ} imaging of real-space density distributions \footnote{G.B. Partridge {\it et al.}, {\it Science} {\bf 311}, 503 (2006).}, \footnote{G.B. Partridge {\it et al.}, {\it Phys. Rev. Lett.} {\bf 97}, 190407 (2006).}. We observe two low-temperature regimes, both with an evenly paired core. At the lowest temperatures, an unpolarized core separates from the excess unpaired atoms by a sharp boundary, which is consistent with a phase separation driven by a first-order phase transition. Moreover, the unpolarized core deforms with increasing polarization, presumably due to surface tension at the superfluid/normal boundary. At higher but still degenerate temperatures, an unpolarized central core remains up to a critical polarization, but does not deform. In this case, the boundaries are not sharp, indicating a partially-polarized shell between the core and the unpaired atoms, consistent with a second-order phase boundary. The observed temperature dependence supports a phase diagram with a tricritical point. The phase- separated phase is only possible for temperatures below the tricritical point, while the higher temperature phase is a polarized superfluid. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:27PM |
B4.00002: Superfluidity in a Strongly Interacting Polarized Fermi Gas Invited Speaker: Cooper pairing is the underlying mechanism for the Bardeen-Cooper-Schrieffer superfluid state of equal mixture of two fermionic components. An interesting situation arises when the symmetry between the two components is broken, such as mass, density, or chemical potential. Is the pairing mechanism robust enough to overcome an asymmetric stress and keep driving superfluidity? Does a new form of superfluidity emerge out of two mismatched Fermi seas? We experimentally study these questions in an unequal mixture of strongly interacting ultracold fermionic atoms trapped in a three dimensional harmonic potential. We observe that due to strong interaction, the system maintains superfluidity up to a critical population imbalance, showing Pauli limit of superfluidity [1]. By correlating condensation fraction and in-situ density distribution, we identify that a superfluid has equal densities of two components and spatially separates from a normal gas of unequal densities [2]. Recent experimental results will be discussed. [1] M.W. Zwierlein et al., Science 311, 492 (2006). [2] Y. Shin et al., Phys. Rev. Lett 97, 030401 (2006). [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 1:03PM |
B4.00003: The Low-Temperature Phases of Polarized Fermionic Superfluids Invited Speaker: A number of recent experiments have achieved paired superfluidity of trapped fermionic atomic gases. Such pairing, occurring between two atomic hyperfine-state species (forming a pseudo-spin $1/2$ system), is possible due to the strong attractive interactions provided by a magnetic field tuned Feshbach resonance (FR). At equal populations, the superfluidity of resonantly interacting Fermi gases undergoes the well-studied crossover between Bardeen-Cooper-Schrieffer (BCS) pairing and Bose-Einstein condensation (BEC) as a function of FR detuning (or interaction strength). I will discuss recent work [1] aimed at understanding the case of {\it unequal\/} populations (i.e., imposed spin polarization), an easily controllable experimental knob that is predicted to interrupt the continuous equal-population BCS-BEC crossover, yielding a variety of distinct phenomena including regions of singlet paired superfluid, unpaired polarized normal Fermi liquid, polarized Fulde-Ferrell-Larkin-Ovchinnikov superfluid, polarized magnetic superfluid, and phase-separated mixtures of these uniform states. I will describe the low-temperature phase diagram of such polarized fermionic superfluids, focusing particularly on experimental signatures of the various phases in the inhomogeneous environment of the trap. [1] D.E. Sheehy and L. Radzihovsky, Phys. Rev. Lett. {\bf 96}, 060401 (2006); cond-mat/0607803 (Annals of Physics, in press). [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:39PM |
B4.00004: Surface tension and collective modes in population imbalanced Fermi gases in the BCS-BEC crossover regime Invited Speaker: Motivated by the striking experiments from MIT and Rice University, we study population imbalanced Fermi gases in the BCS-BEC crossover regime. We have calculated the surface tension in the boundary separating superfluid and polarized normal regions in a trap. We show how this surface tension can explain apparent inconsistencies between the two groups. Using several candidate equations of state, we calculate frequencies of the breathing mode, finding that collective mode measurements can distinguish between the various possibilities. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 2:15PM |
B4.00005: The important role of temperature in BCS--Bose-Einstein condensation crossover phenomena with population imbalance Invited Speaker: Any comparison between theory and experiment in the cold Fermi gases requires that one include the effects of non-zero temperature $T$. In this talk we show how to include finite $T$ in a way which is compatible with the generalized BCS-like ground state, assumed in essentially all $T=0$ calculations of gases with population imbalance. We use a pairing fluctuation theory of BCS--Bose-Einstein condensation (BEC) based on a $T$-matrix formalism. Distinguishing this theory from strict mean-field theories is our self-consistent treatment of incoherent, finite-momentum pairs along with single fermions. This leads to a pseudogap in the fermion excitation spectrum at finite $T$ which is necessary in order to arrive at physically meaningful transition temperatures $T_c(p)$, where $p$ is the polarization. We present phase diagrams in the $p$-$T$ plane with variable scattering length, $1/k_Fa$, and identify the regions where bulk superfluidity, normal phases and phase separation appears. For the trapped Fermi gases, we present particle density profiles for general $1/k_Fa$ as well as a detailed comparison with recent measurements at both MIT and Rice University. We find reasonably good agreement with these experimental data. \\ 1. C.-C. Chien, Q.J. Chen, Y. He, and K. Levin, \textit{Intermediate temperature superfluidity in an atomic Fermi gas with population imbalance}, Phys. Rev. Lett. 97, 090402 (2006). \\ 2. Q.J. Chen, Y. He, C.-C. Chien, and K. Levin, \textit{Stability conditions and phase diagrams for two component Fermi gases with population imbalance}, cond-mat/0608454; Phys. Rev. A 74, 06xxxx (2006). \\ 3. C.-C. Chien, Q.J. Chen, Y. He, and K. Levin, \textit{Finite temperature effects in trapped Fermi gases with population imbalance}, Phys. Rev. A 74, 021602(R) 2006. [Preview Abstract] |
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