Bulletin of the American Physical Society
43rd Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 57, Number 5
Monday–Friday, June 4–8, 2012; Orange County, California
Session M2: Strongly Interacting Fermions |
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Chair: Elise Novitski, Harvard University Room: Grand Ballroom GF |
Thursday, June 7, 2012 8:00AM - 8:12AM |
M2.00001: Compressibility of a Repulsive Fermi Gas of $^6$ Li Myoung-Sun Heo, Yeryoung Lee, Tout T. Wang, Timur M. Rvachov, Wolfgang Ketterle, David E. Pritchard We studied the equilibrium properties of a two-component repulsive Fermi gas of $^6$ Li by measuring the compressibility at various interaction strengths near a Feshbach resonance at 834G. The compressibility was extracted from in-trap atomic density profiles, and agrees with first order perturbation theory. Our experiment uses phase-contrast imaging to probe atomic clouds with large optical density, requiring compensation of dispersive effects. The feasibility window for such experiments is limited by decay to a near-resonant molecular bound state, and we explore the limits of this window by measuring decay rates at various interaction strengths. [Preview Abstract] |
Thursday, June 7, 2012 8:12AM - 8:24AM |
M2.00002: Revealing the Superfluid Lambda Transition in the Universal Thermodynamics of a Unitary Fermi Gas Mark Ku, Ariel Sommer, Lawrence Cheuk, Martin Zwierlein We have observed the superfluid phase transition in a strongly interacting Fermi gas via high-precision measurements of the local compressibility, density and pressure down to near-zero entropy. We perform the measurements by in-situ imaging of ultracold $^6$Li at a Feshbach resonance. Our data completely determine the universal thermodynamics of strongly interacting fermions without any fit or external thermometer. The onset of superfluidity is observed in the compressibility, the chemical potential, the entropy, and the heat capacity. In particular, the heat capacity displays a characteristic lambda-like feature at the critical temperature of $T_c/T_F = 0.167(13)$. This is the first direct thermodynamic signature of the superfluid transition in a spin-balanced atomic Fermi gas. We measure the ground-state energy of the superfluid to be $3/5\xi NE_F$, with $\xi=0.376(4)$. The experimental results are compared to recent Monte-Carlo calculations. Our measurements provide a benchmark for many-body theories on strongly interacting fermions, relevant for problems ranging from high-temperature superconductivity to the equation of state of neutron stars. [Preview Abstract] |
Thursday, June 7, 2012 8:24AM - 8:36AM |
M2.00003: Pseudogap Effects in Rotating Fermi Gases from BCS to BEC Peter Scherpelz, Vivek Mishra, Dan Wulin, Kathryn Levin, Attipat Rajagopal In this talk we focus on pseudogap effects present in a rotating, ultracold Fermi gas as the system is tuned from a BCS regime to a BEC regime. Such pseudogap effects are expected to be present away from the BCS regime [1]. Importantly, no theory of rotating Fermi gases has yet incorporated these non-condensed pair effects. Our work is based on reformulating the Gor'kov equations, with the inclusion of a pseudogap, into a Landau level basis [2]. With this reformulation we can calculate quantities including the local density of states in the presence of vortices, and the upper critical rotation frequency. In a related way we present linear response calculations in the presence of a pseudogap which include the shear viscosity and moment of inertia. We show that finite size effects give rise to a non-classical moment of inertia even in the normal state. Testable predictions resulting from this theory, as well as connections with high temperature superconductors, will also be discussed.\\[4pt] [1] Stajic \textit{et al.} Phys. Rev. A \textbf{69} 063610 (2004).\\[0pt] [2] Scherpelz \textit{et al.} arXiv: 1112.1112. [Preview Abstract] |
Thursday, June 7, 2012 8:36AM - 8:48AM |
M2.00004: Repulsive polarons in a strongly interacting Fermi mixture Christoph Kohstall, Matteo Zaccanti, Michael Jag, Andreas Trenkwalder, Florian Schreck, Rudolf Grimm, Pietro Massignan, Georg Bruun Ultracold Fermi gases with tunable interactions represent a unique test bed to explore the many-body physics of strongly interacting quantum systems. In the past decade, experiments have mainly focused on the ground-state properties but metastable states in Fermi gases with strong repulsive interactions represent an exciting new frontier in the field. Here, we exploit radio-frequency spectroscopy to measure the complete excitation spectrum of fermionic $^{40}$K impurities resonantly interacting with a Fermi sea of $^{6}$Li atoms. In particular, we show that a well-defined quasiparticle exists for strongly repulsive interactions. For this ``repulsive polaron'' we measure its energy and its lifetime against decay. We also probe its coherence properties by measuring the quasiparticle residue. The results are well described by a theoretical approach that takes into account the finite effective range of the interaction in our system. We find that a non-zero range of the order of the interparticle spacing results in a substantial lifetime increase. This major benefit for the stability of the repulsive branch opens up new perspectives for investigating novel phenomena in metastable, repulsively interacting fermion systems. [Preview Abstract] |
Thursday, June 7, 2012 8:48AM - 9:00AM |
M2.00005: Dynamics of a Repulsive Fermi-Fermi Mixture Marko Cetina, Matteo Zaccanti, Michael Jag, Christoph Kohstall, Andreas Trenkwalder, Florian Schreck, Rudolf Grimm There is a growing interest in repulsively interacting Fermi gas mixtures, which could enable investigations of correlated quantum systems. As repulsive interactions in Fermi gases near a Feshbach resonance arise from the presence of a loosely bound molecular state, they are always associated with decay into bosonic molecules [1]. The interplay between repulsion and decay has made the realization and understanding of repulsive Fermi systems challenging [2,3]. We investigate the dynamics of a strongly interacting $^{40}$K-$^{6}$Li Fermi-Fermi mixture on the repulsive side of an interspecies Feshbach resonance. For this purpose, we employ magnetic field ramps, RF spectroscopy, in-situ and time-of-flight imaging. Close to the resonance, we observe a static behavior of the K atomic population, redistribution of the Li density away from the K cloud and an absence of the interaction-induced shifts in the dissociation RF spectra in the strongly interacting repulsive regime. In spite of a strong initial atom loss, our results suggest that repulsive interactions are crucial for the dynamics of our system.\\[4pt][1] D. Pekker, et al., {\sl Phys. Rev. Lett.} {\bf 106}, 050402 (2011).\\[0pt][2] G. Jo, et al., {\sl Science} {\bf 325}, 5947 (2009).\\[0pt][3] C. Sanner et al., arXiv:1108.2017 [Preview Abstract] |
Thursday, June 7, 2012 9:00AM - 9:12AM |
M2.00006: Highly Polarized Fermi Gases across a Narrow Feshbach Resonance Ran Qi, Hui Zhai We address the phase of highly polarized Fermi gases across a narrow Feshbach resonance starting from the problem of a single down spin fermion immersed in a Fermi sea of up spins. Both polaron and pairing states are considered using the variational wave function approach, and we find that the polaron to pairing transition will take place at the BCS side of the resonance, strongly in contrast to a wide resonance where the transition is located at the BEC side. For pairing phase, we find out the critical strength of repulsive interaction between pairs above which the mixture of pairs and fermions will not phase separate. Therefore, nearby a narrow resonance, it is quite likely that magnetism can coexist with s-wave BCS superfluidity at large Zeeman field, which is a remarkable property absent in conventional BCS superconductors (or fermion pair superfluids). [Preview Abstract] |
Thursday, June 7, 2012 9:12AM - 9:24AM |
M2.00007: Towards Probing Homogeneous Strongly Interacting Fermi Gas Yoav Sagi, Tara Drake, Rabin Paudel, John Stewart, John Gaebler, Deborah Jin Superfluidity in strongly correlated matter is still an open question after many years of research. High Tc superconductors, for example, are known to exhibit a pseudo-gap phase above the critical temperature - a phase in which pairing exists in spite of the absence of long range order. An ultra-cold strongly interacting Fermi gas realizes a clean and controllable model system for studying these questions. It was shown recently that the normal state of a strongly interacting Fermi gas has a spectral behavior typical to a pseudo-gap phase [1]. One of the problems, however, which complicates the interpretation of these experiments is the inherent density inhomogeneity of the gas which arises due to the harmonic confinement. Here we present a technique to overcome this difficulty by spatially selecting only part of the cloud for interrogation while still retaining momentum resolution. We present measurements of the momentum distribution of a degenerate gas of 40K atoms revealing for the first time a truly sharp Fermi surface. We further extend the use of this technique to momentum resolved photo-emission spectroscopy and contact measurements. \\[4pt] [1] J. P. Gaebler, J. T. Stewart, T. E. Drake, D. S. Jin, A. Perali, P. Pieri, and G. C. Strinati, Nature Physics 6, 569-573 (2010). [Preview Abstract] |
Thursday, June 7, 2012 9:24AM - 9:36AM |
M2.00008: Realization of a Resonant Fermi Gas with a Large Effective Range Y. Zhang, E.L. Hazlett, R.W. Stites, K.M. O'Hara We have measured the interaction energy and three-body recombination rate for a two-component Fermi gas near a narrow Feshbach resonance and found both to be strongly energy dependent. Even though the deBroglie wavelength greatly exceeds the van der Waals length scale for all cases studied, the behavior of the interaction energy as a function of temperature cannot be described by atoms interacting via a contact potential. Rather, energy dependent corrections beyond the scattering length approximation are required, indicating a resonance with an anomalously large effective range. This narrow resonance can be used to study strongly correlated Fermi gases that simultaneously have a sizeable effective range and a large scattering length. Resonant Fermi gases with energy-dependent two-body interactions accurately describe dilute neutron matter found at densities in the interior of neutron stars, may exhibit extraordinarily high superfluid critical temperatures, and could enable the observation of exotic new forms of matter such as the breached-pair superfluid phase in a polarized Fermi gas. [Preview Abstract] |
Thursday, June 7, 2012 9:36AM - 9:48AM |
M2.00009: Degeneracies in trapped two-component Fermi gases D. Blume, K.M. Daily, D. Rakshit We report on previously unobserved degeneracies in two-component equal-mass Fermi gases with zero-range interactions under isotropic harmonic confinement. Over the past 10 years or so, two-component Fermi gases with zero-range interactions have become a paradigm for modeling condensed matter systems, nuclear matter and neutron matter. We provide strong evidence that the eigen energies of the $(3,1)$ system consisting of three spin-up atoms and one spin-down atom are degenerate with the eigen energies of the $(2,2)$ system consisting of two spin-up atoms and two spin-down atoms for any $s$-wave scattering length $a_s$, including infinitely large, positive and negative $a_s$. Evidence for the existence of analogous degeneracies for larger systems is presented. The degeneracies evidenced in our study introduce a new class of highly-correlated systems for which such degeneracies, and thus an underlying symmetry, exist. [Preview Abstract] |
Thursday, June 7, 2012 9:48AM - 10:00AM |
M2.00010: Expansion of 1D polarized superfluids Hong Lu, L.O. Baksmaty, C.J. Bolech, Han Pu We study the axial expansion dynamics of a one dimensional polarized Fermi gas after suddenly released from the confining trap. We perform our comparative studies using both mean-field Bogoliubov-de Gennes (BdG) and the numerically exact Time-Evolving Block Decimation (TEBD) methods. Our results show that strong spin density modulations, which are manifesting signatures of FFLO (Fulde-Ferrel-Larkin-Ovchinnikov) state and can be readily detected in experiment, develop during the expansion of the cloud, giving incontrovertible evidence to the FFLO state. [Preview Abstract] |
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