Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session K43: Trapped Fermi Gases |
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Sponsoring Units: DAMOP Chair: C. Sa de Melo, Georgia Institute of Technology Room: Baltimore Convention Center 346 |
Tuesday, March 14, 2006 2:30PM - 2:42PM |
K43.00001: Population of closed-channel molecules in trapped Fermi gases with broad Feshbach resonances Qijin Chen, K. Levin We compute the fraction of closed-channel molecules in trapped atomic Fermi gases, over the entire range of accessible fields and temperatures. We use a two-channel model of BCS--Bose-Einstein condensation (BEC) crossover theory at general temperature $T$, and show that this fraction provides a measure of the $T$ dependent pairing gap. Our calculations, containing no free parameters, are in good quantitative agreement with recent low $T$ measurements in $^6$Li. We present readily testable predictions for the dependencies of the closed-channel fraction on temperature and Fermi momentum. \newline Reference: arXiv:cond-mat/0505689; Phys. Rev Lett. 95, Dec 31, 2005. [Preview Abstract] |
Tuesday, March 14, 2006 2:42PM - 2:54PM |
K43.00002: Quantum Monte Carlo Simulations of Dilute Fermion Gases at Finite Temperature Vamsi Akkineni, David Ceperley, Nandini Trivedi In a system of fermions with attractive interactions at low temperature, the crossover from the BCS regime to a molecular BEC, with increasing interaction strengh, is a problem of particular interest in many-body physics. The development of successful experimental techniques to trap and cool dilute gases of fermionic alkali atoms, and to tune the effective inter-atomic interaction over a wide range, have opened exciting possibilities for studying the physics of interacting fermions in this crossover region. The composition of the ground state, nature of fermion pairs, nature of excitations, pairing energy scale, and the pairing and condensation transitions are all important considerations in this crossover regime. Ab initio quantum simulations are invaluable tools for obtaining accurate values of various physical quantities of interest. We utilize the Restricted Path Integral Monte Carlo (RPIMC) technique to calculate to the energy, pairing energy, and the superfluid density at the unitarity point. We present these results along with an estimate of the critical temperature for the superfluid transition. [Preview Abstract] |
Tuesday, March 14, 2006 2:54PM - 3:06PM |
K43.00003: Evolution from BCS to BEC superfluidity in p-wave Fermi gases Menderes Iskin, Carlos Sa de Melo We consider the evolution of superfluid properties of a three dimensional $p$-wave Fermi gas from weak (BCS) to strong (BEC) coupling as a function of scattering volume. We analyse the order parameter, quasi-particle excitation spectrum, chemical potential, average Cooper pair size and the momentum distribution in the ground state ($T = 0$). We also discuss the critical temperature $T_{\rm c}$, chemical potential and number of unbound, scattering and bound fermions in the normal state ($T = T_{\rm c}$). Lastly, we derive the time-dependent Ginzburg-Landau equation for $T \approx T_{\rm c}$ and extract the Ginzburg-Landau coherence length. [Preview Abstract] |
Tuesday, March 14, 2006 3:06PM - 3:18PM |
K43.00004: Many body exchange effects close to the s-wave Feshbach resonance in two component Fermi systems: is a triplet superfluid possible? Sergio Gaudio, Jason Jackiewicz, Kevin Bedell We suggest that the fluctuations close to a Feshbach resonance in a two component Fermi gas can result in an effective p-wave attractive interaction. On the BCS side of the resonance, the magnitude of this effective interaction is comparable to the s-wave interaction, therefore leading to the possibility of a spin-triplet superfluid in the range of temperatures of actual experiments. We compare the results for our effective scattering length to the mean field result and to the observed binding energy. Finally, we show that by including particle-hole exchange in the fluctuations, the divergence of the effective scattering length disappears. [Preview Abstract] |
Tuesday, March 14, 2006 3:18PM - 3:30PM |
K43.00005: Spin noise spectroscopy to probe quantum states of ultracold fermionic atom gases Bogdan Mihaila, Scott Crooker, Krastan Blagoev, Dwight Rickel, Peter Littlewood, Darryl Smith Physical systems are often studied by measuring their response to an external perturbation. Measuring the intrinsic noise of a physical system can provide the same information as measuring its response to a perturbation, but noise spectroscopy measurements often disturb the physical system less strongly and scale more favorably with system size reduction. For quantum systems at very low temperature, noise from quantum fluctuations in the ground state of an observable that does not commute with the Hamiltonian can be used as a probe of the system properties. We describe the use of electron spin noise spectroscopy to probe the quantum states of ultracold fermionic atomic gases. The electron spin is not a good quantum number of the atomic gases and fluctuations of electron spin can be measured using optical Faraday rotation. We show that electron spin noise spectroscopy gives unique signatures for different models describing the interatomic interactions in the ultracold fermionic gases, can be used to probe the BCS/BEC crossover in these systems, and weakly disturbs the atomic gases. Measurements of spin noise in classical Boltzmann gases of alkali atoms are used to estimate the expected signal strength for spin noise measurements in ultracold atom systems and to show that these measurements are tractable. [Preview Abstract] |
Tuesday, March 14, 2006 3:30PM - 3:42PM |
K43.00006: Atomic Fermi gases with unequal spin populations Meera Parish We investigate the properties of a gas of fermionic atoms where the two spin populations are unequal. By considering how the ground state evolves as a function of inter-atomic interaction and population imbalance, we will determine what the measurable differences are between the various theoretical models of the BCS-BEC crossover in atomic gases. [Preview Abstract] |
Tuesday, March 14, 2006 3:42PM - 3:54PM |
K43.00007: Spontaneous Vortices in Imbalance Populated Fermion Gas, Finite Size System Jung-Jung Su, Yun-pil Shim , Rembert Duine, 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] A. Sedrakian et al.,cond-mat/0404577 [3] R. Hulet, preprint. [Preview Abstract] |
Tuesday, March 14, 2006 3:54PM - 4:06PM |
K43.00008: Superfluid properties across the BEC/BCS transition Jacques Tempere, Jozef Devreese The superfluid properties of a Bose-Einstein condensate (BEC) and of the Bardeen-Cooper-Schrieffer (BCS) state are reasonably well understood. Recent experimental breakthroughs allow investigating the transition region between the BEC and BCS limits. In this contribution, we apply the path-integral formalism to study how the vortex core size, the superfluid critical velocity and the pair-pair scattering length vary in the crossover region. These quantities depend on the interaction strength of the atoms and change quantitatively from BEC to BCS side. In particular we discuss how the pair-pair scattering length obtained here relates to the mean-field results and to the four-body scattering result. [Preview Abstract] |
Tuesday, March 14, 2006 4:06PM - 4:18PM |
K43.00009: Relaxation and persistent oscillations of the order parameter in the non-stationary BCS theory Emil Yuzbashyan, Oleksandr Tsyplyatyev, Boris Altshuler We determine the limiting dynamics of a fermionic condensate following a sudden perturbation for various initial conditions. We demonstrate that possible initial states of the condensate fall into two classes. In the first case, the order parameter asymptotes to a {\it constant} value. The approach to a constant is oscillatory with an inverse square root decay. This happens, e.g., when the strength of pairing is abruptly changed while the system is in the paired ground state and more generally for any {\it nonequilibrium} state that is in the same class as the ground state. In the second case, the order parameter exhibits persistent oscillations with several frequencies. This is realized for nonequilibrium states that belong to the same class as excited stationary states. Our {\it classification} of initial states extends the concept of excitation spectrum to nonequilibrium regime and allows one to predict the evolution without solving equations of motion. [Preview Abstract] |
Tuesday, March 14, 2006 4:18PM - 4:30PM |
K43.00010: Dynamical properties of the pairing gap in ultra-cold Fermi gases Maxim Dzero, Emil Yuzbashyan We study the dynamics of a condensate of an atomic ultra-cold Fermi gas following an abrupt change in the pairing strength. At long times the order parameter asymptotes to a constant value, which we determine exactly. Under certain conditions, the system reaches a novel steady state with peculiar properties. [Preview Abstract] |
Tuesday, March 14, 2006 4:30PM - 4:42PM |
K43.00011: Exploring the Possibility of Collapse in a Degenerate Fermi Gas J. von Stecher, S. T. Rittenhouse, C. H. Greene, M. J. Cavagnero Trapped ultracold atomic gases with tunable attractive interactions can, in principle, collapse to a deep many-body bound state. Mean-field theories have predicted collapse of Bose-Einstein condensates with negative scattering length $a$ [1], which has been confirmed experimentally [2]. Mean field treatments of degenerate Fermi gases have also predicted collapse [3]. In a recent study, we proposed [4] a hyperspherical coordinate treatment of a two-component degenerate Fermi gas in an isotropic oscillator trap, for which the atoms interact through a zero-range pseudopotential. Ref.[4] predicts a ($T=0)$ collapse at $k_{f }$\textit{a = -1.21}. Using Monte Carlo methods, we study variationally the collapse for different two body finite range interactions. We introduce Jastrow-type correlations in the many-body wave function, which allows us to test the validity of the mean-field approach and the zero-range interaction. [1] E. V. Shuryak, Phys. Rev. A 54, 3151 (1996). [2] E. A. Donley et al, Nature 412, 295-299 (2001). [3] M. Houbiers et al, Phys. Rev. A 56, 4864 (1997). [4] S. T. Rittenhouse et al, arXiv:cond-mat/0510454. [Preview Abstract] |
Tuesday, March 14, 2006 4:42PM - 4:54PM |
K43.00012: Viscous relaxation and transport in a 2d Fermi liquid Dmitry Novikov Long-wavelength transport and relaxation properties of interacting systems are naturally described in terms of hydrodynamic modes. Here we focus on the viscosity of the two-dimensional (2d) interacting fermions. It is well-known that the viscosity of a clean Fermi liquid in three dimensions is proportional to the quasiparticle lifetime, scaling as $E_F/T^2$. We find that for the 2d fermions, the viscosity is logarithmically enhanced by the factor of $\ln(E_F/T)$, that arises from the phase-space restrictions for quasiparticle scattering in two dimensions. In particular, these restrictions enforce the dominant contributions of the collisions between quasiparticles from opposite sides of the Fermi surface, the effect specific to the 2d geometry. We discuss the effects of viscous modes that can manifest themselves in the electron transport in the metallic phase, and in the momentum relaxation of cold fermion gases. [Preview Abstract] |
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