Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session D32: Focus Session: Fermi Gases with Unequal Spin Populations or Masses |
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Sponsoring Units: DAMOP Chair: Leo Radzihovsky, University of Colorado Room: Colorado Convention Center 402 |
Monday, March 5, 2007 2:30PM - 2:42PM |
D32.00001: Polarized Fermi gases in an axially symmetric trap: A Bogoliubov-deGennes analysis William Schneider, Rajdeep Sensarma, Mohit Randeria We study the T=0 Fermi gas with an unequal population of up and down spins in an axially symmetric three-dimensional trap. Our motivation is to understand the differences in the experimental data from the MIT and Rice groups, which might arise from the rather different asymmetries of the trapping potentials. Using a fully self-consistent numerical solution of the Bogoliubov deGennes equations, we address the question of the validity of the local density approximation (LDA) as a function of asymmetry. We will present results for the spatial variations of the up and down densities and the superfluid order parameter as a function of polarization, trap asymmetry and interaction strength in the vicinity of unitarity. [Preview Abstract] |
Monday, March 5, 2007 2:42PM - 2:54PM |
D32.00002: Single-plane-wave Larkin-Ovchinnikov-Fulde-Ferrell state in BCS--Bose-Einstein condensation crossover Yan He, Chih-Chun Chien, Qijin Chen, Kathy Levin We study the single-plane-wave Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) states for BCS--Bose-Einstein condensation (BEC) crossover at general temperatures $T$. Because we include the important effects of noncondensed pairs, our $T \neq 0$ phase diagrams are different from those reported in earlier work. We find that generalized LOFF phases may be the ground state for a wide range of (weak through moderately strong) interactions, including the unitary regime. However, these LOFF phases are readily destroyed by non-zero $T$. We also explore the competition between LOFF phases and phase separated states. In the cold gases, phase separation is generally the more stable, although in QCD and other applications, of LOFF physics, phase separation is not always a physical option. \\ cond-mat/0610274 \\ cond-mat/0608662 [Preview Abstract] |
Monday, March 5, 2007 2:54PM - 3:06PM |
D32.00003: Pairing and superfluid properties of dilute fermions with unequal masses Chien-Hua Pao, Shin-Tza Wu, Sungkit Yip We study the pairing between Fermions of different masses in a harmonic trap potential. Within the mean field theory, we calculate the density profiles systemically for the weak coupling BCS, the unitary limit, and the strong coupling BEC regimes. For a system with spin population imbalance, we found that the system is phase separated into concentric shells with the superfluid in the core surrounded by the normal fermion gas in both the weak-coupling BCS side and at unitary limit. In the strong-coupling BEC side, the composite bosons and left-over fermions can be mixed. The density profiles for unequal mass Fermions can be drastically different from their equal-mass counterparts in the unitary limit. We will discuss some possible experiments with different mass ratios which exhibit different ground state properties compared to the equal masses cases. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:42PM |
D32.00004: Universal phase diagram of a strongly interacting polarized Fermi gas Invited Speaker: The recent combined theoretical and experimental breakthroughs in the field of ultra-cold Fermi systems have permitted the clarification of the ground state properties of an ensemble of attractive fermions with equal spin populations. However, many open questions remain concerning the behavior of polarized systems, where the different spin states are unequally populated. The various theoretical models imply a wide range of different scenarios and phase digram, while two recent experiments performed at Rice and MIT present contradictory results. We will present an analysis of the ground state of an ensemble of fermions with unequal spin population in the regime of infinite scattering length. To address this problem, we will make use of universality which is characteristic of this strongly interacting regime and leads to simple scaling laws for the different physical quantities describing the system. We will in particular show that this problem is closely related to the study of an impurity imbedded in a non interacting Fermi sea of polarized atoms. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 3:54PM |
D32.00005: Finite Temperature Effects in Trapped Unitary Fermi Gases with Population Imbalance Chih-Chun Chien, Qijin Chen, Yan He, Katheryn Levin We study the finite temperature $T$ behavior of trapped Fermi gases in the unitary regime and in the presence of a population imbalance with polarization $p$. We obtain a phase diagram in the $p$ - $T$ plane, which establishes various superfluid and normal phases. Our theory, which is consistent with the standard $T=0$ calculations in the literature, incorporates the important effect of non-condensed pairs. These are essential in order to arrive at physically meaningful transition temperatures $T_c(p)$. Moreover, as a result of these non-condensed pairs our $ T \leq T_c$ profiles evolve from the well documented featureless behavior at $p=0$ to behavior which shows clear indications of the presence of a condensate at $p \neq 0$. We also show profiles and central densities in different regimes of the phase diagram, and detailed comparisons with recent experiments are presented, \\ 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. 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] |
Monday, March 5, 2007 3:54PM - 4:06PM |
D32.00006: Unconventional interaction between vortices in a polarized Fermi gas Vladimir M. Stojanovic, W. Vincent Liu, Yong Baek Kim Using effective field theory approach we study a homogeneous superfluid state with a single (gapless) Fermi surface, recently suggested as a possible phase for an ultracold Fermi gas with spin-population imbalance. We find an unconventional form of the interaction between vortices. The presence of gapless fermions gives rise to an attractive long-range potential oscillating in space, analogous to the RKKY magnetic interaction in metals. Our study then leads to an interesting question as to the nature of the vortex lattice in the presence of the competition between the usual repulsive logarithmic Coulomb and the fermion-induced attractive oscillatory interactions. [Preview Abstract] |
Monday, March 5, 2007 4:06PM - 4:18PM |
D32.00007: Two-component Fermi gas with unequal masses at unitarity: A diffusion Monte Carlo study D. Blume, G.E. Astrakharchik, S. Giorgini Two-component Fermi gases with varying interaction strengths have been realized in the laboratory using ultracold atoms in two different hyperfine states. In view of experimental efforts to simultaneously cool and trap two fermionic species with different masses, such as Li and K, we investigate the behavior of two-component Fermi gases with unequal masses in the strongly-interacting regime using the diffusion Monte Carlo technique. We consider mass ratios ranging from one to 100, and determine the equation of state at unitarity for a gas with identical number of ``spin up'' and ``spin down'' atoms. Furthermore, we determine the pairing gap of the system and interpret our findings. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D32.00008: The effective Bose-Fermi scattering length in spin-polarized Fermi superfluids Edward Taylor, Allan Griffin, Yoji Ohashi The analysis of experiments done on the BEC side of a Feshbach resonance for spin-polarized Fermi superfluids is greatly simplified by realizing that the system can be described by a Hamiltonian for a Bose-Fermi mixture, where the bosons are diatomic molecules and the fermions are the remaining unpaired atoms. To do this, however, one needs an expression for the effective boson-fermion scattering length $a_{BF}$ that includes many-body effects which become important close to unitarity. For two-body scattering {\it in vacuo}, Skorniakov and Ter-Martirosian (STM) showed in 1957 that the exact value is $a_{BF} = 1.18a_F$, a result also obtained recently by Brodsky and coworkers using a diagrammatic approach. We derive an expression for $a_{BF}$ in the BEC region of a spin-polarized Fermi superfluid using an alternative path-integral treatment of quartic fluctuations, which gives the essential physics of $a_{BF}$ is a simple manner and also allows us to include many-body effects. In the experimentally relevant regime outside the extreme BEC limit, we find corrections to the STM value arising from the fact that scattering occurs in a background gas of condensed Cooper pair bosons, and not in the vacuum. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D32.00009: Pairing in Asymmetrical Fermi Systems with Intra- and Inter-Species Correlations Renyuan Liao, Khandker Quader Motivated by ultracold fermions, we study pairing in two-species Fermi systems with unequal population. We include both inter-species ``singlet'' and intra-species ``triplet'' pairing interactions. Using the equation of motion method, we derive two-point correlation functions, from which various physical quantities can be extracted. We self-consistently solve the resulting coupled mean-field equations for superfluid gap functions and chemical potentials, and study the effects of ``triplet'' correlations on various quantities at T=0 and finite-T. By imposing stability conditions, we construct a phase diagram across the BEC-BCS regimes; it is dramatically different from that without triplet correlations: the BCS singlet superfluid state can sustain a finite polarization, P. For larger P, we find phase separation in BCS and BEC regimes. A superfluid phase exists for all P deep in the BEC regime. [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D32.00010: FFLO phase in one-dimensional polarized Fermi gases Xia-Ji Liu, Hui Hu, Peter Drummond Based on the integrable Gaudin model and local density approximation, we discuss the phase structure of one- dimensional trapped Fermi gases with imbalanced spin populations for arbitrary attractive interactions. A phase separation state, with a polarized superfluid core immersed in an unpolarized superfluid shell, emerges below a critical spin polarization. Above it, a coexistence of polarized superfluid matter and a fully polarized normal gas is favored. These two exotic states could be realized experimentally in highly elongated atomic traps, and diagnosed by measuring the lowest density compressional mode. We identify the polarized superfluid as having an FFLO structure, and predict the resulting mode frequency as a function of the spin polarization. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D32.00011: Nonuniform mixed-parity superfluid state in Fermi gases M. S. Mar'enko, K. V. Samokhin We study the effects of dipole interaction on the superfluidity in a homogeneous Fermi gas with population imbalance. We show that the Fulde-Ferrell-Larkin-Ovchinnikov phase is replaced by another nonuniform superfluid phase, in which the order parameter has a nonzero triplet component induced by the dipole interaction. [Preview Abstract] |
Monday, March 5, 2007 5:06PM - 5:18PM |
D32.00012: Polarised Fermi condensates with equal and unequal masses Francesca Maria Marchetti, Meera Parish, Austen Lamacraft, Ben Simons We consider a two-spin atomic Fermi gas, where both the mass and population of each component are unequal. We show that the finite temperature phase diagram contains a region of phase separation between the superfluid and normal states that touches the boundary of second-order superfluid transitions at a tricritical point, reminiscent of the phase diagram of $^3$He-$^4$He mixtures. We discuss the implications of our findings for a trapped gas at finite temperature and the different topologies of phase separation by changing the mass ratio. Finally we find that the interior gap state is always unstable to phase separation, while the breached pair state with one Fermi surface for the excess fermions exhibits differences in its DoSs and pair correlation functions depending on which side of the resonance it lies. [Preview Abstract] |
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