Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session L13: New Phenomena in 2D Fermi GasesInvited
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Sponsoring Units: DAMOP Chair: Cheng Chin, University of Chicago Room: 309 |
Wednesday, March 16, 2016 11:15AM - 11:51AM |
L13.00001: A strongly interacting two-dimensional Fermi gas Invited Speaker: Selim Jochim We will present our progress realizing two-dimensional supefluids with ultracold fermionic lithium atoms confined in a quasi two-dimensional potential. In this setting, a generic two-dimensional Fermi gas is realized with interactions tunable to any value within the short-range limit. We measured the phase diagram for this system and found the coherence properties to be decaying algebraically, a signature of the Beresinskii-Kosterlitz-Thouless phase which is expected for a homogeneous two-dimensional superfluid. We furthermore extracted from our data a the equation of state, which will be an important benchmark for many body theories. We are currently working to transfer our fermionic atoms into an optical lattice potential. Our vision to realize exotic superfluidity in this system will be discussed. [Preview Abstract] |
Wednesday, March 16, 2016 11:51AM - 12:27PM |
L13.00002: Experimental studies of spin-imbalanced Fermi gases in 2D geometries Invited Speaker: John Thomas We study the thermodynamics of a quasi-two-dimensional Fermi gas, which is not quite two-dimensional (2D), but far from three dimensional (3D). This system offers opportunities to test predictions that cross interdisciplinary boundaries, such as enhanced superfluid transition temperatures in spin-imbalanced quasi-2D superconductors, and provides important benchmarks for calculations of the phase diagrams. In the experiments, an ultra-cold Fermi gas is confined in an infrared CO$_{2}$ laser standing-wave, which produces periodic pancake-shaped potential wells, separated by 5.3 $\mu $m. To study the thermodynamics, we load an ultra-cold mixture of N$_{1} \quad =$ 800 spin \textonehalf -up and N$_{2}$ \textless N$_{1}$ spin \textonehalf -down $^{6}$Li atoms into each well and image the individual cloud profiles as a function of interaction strength and spin imbalance N$_{2}$/N$_{1}$. The measured properties are in disagreement with 2D-BCS theory, but can be fit by a 2D-polaron gas model, where each atom is surrounded by a cloud of particle-hole pairs of the opposite spin. However, this model fails to predict a transition to a spin-balanced central region as N$_{2}$/N$_{1\, }$is increased. [Preview Abstract] |
Wednesday, March 16, 2016 12:27PM - 1:03PM |
L13.00003: Fermi-to-Bose crossover in a trapped quasi-2D gas of fermionic atoms Invited Speaker: Andrey Turlapov Neither long-range order nor Bose condensation may appear in uniform 2D systems at finite temperature. Despite that, 2D superconductors, such as cuprates, are among the systems with highest critical temperatures. 2D quantum systems remain intriguing, and their understanding is incomplete despite huge progress seen in the recent decades. Ultracold atoms are a platform for studying 2D physics. Using tunability of atomic gases, we have realized a crossover between a 2D gas of Fermi atoms and a 2D gas of weakly-bound diatomic Bose molecules by varying s-wave interactions in the gas. Between these two asymptotic states, there is a regime of strong interactions, whose quantitative description is challenging, e.~g., a mean field of Cooper pairs fails to describe the crossover even qualitatively, unlike in 3D gases. At the lowest achievable temperatures, $\sim10\%$ of the Fermi energy, the pressure is measured in the whole Fermi-to-Bose crossover and compared with the available theoretical models, including those which appeared over the last year. In the Fermi regime of weak interactions, the pressure is systematically above a Fermi-liquid-theory prediction, which maybe due to mesoscopic effects. Alternatively, this upshift is partially reproduced within a recent mean-field theory supplemented with fluctuations. On the Bose side of the crossover, the molecules easily condense, which is found in interferometric measurements. On one hand, such condensation is expected because the gas is held in a nearly harmonic trap, which favors condensation unlike the uniforms space. On the other hand, each molecule is locally in a flat potential, which is the sum of the trap and the strong repulsive mean field, and this should inhibit the condensation. [Preview Abstract] |
Wednesday, March 16, 2016 1:03PM - 1:39PM |
L13.00004: Quasi-condensation in trapped two-dimensional Fermi gases Invited Speaker: Brandon Anderson It is well known that the Mermin-Wagner theorem prohibits true long range order 2D systems. Nevertheless, recent experiments [1,2] provide strong evidence that 2D Fermi gases undergo a form of pair condensation, along with aspects of BKT physics. In this talk we apply a BCS-BEC theory (which is compatible with the Mermin-Wagner theorem) to characterize the nature of pair (quasi-) condensation in 2D Fermi gases. Here we follow the same analysis and protocols of these recent experiments. We find a strong zero momentum peak in the pair momentum distribution which importantly occurs at a reasonably well defined onset temperature. We demonstrate that the resulting phase diagram, pair momentum distribution, and algebraic power law decay are compatible with these experiments throughout the continuum from BEC to BCS. Finally, we present sharp qualitative experimental signatures to test this physical picture. [1] Phys. Rev. Lett. 114, 230401 (2015) [2] Phys. Rev. Lett. 115, 010401 (2015) [3] Phys. Rev. Lett. (To be published.) [Preview Abstract] |
Wednesday, March 16, 2016 1:39PM - 2:15PM |
L13.00005: BKT physics in trapped 2D Bose and Fermi gases Invited Speaker: Markus Holzmann I will discuss superfluid and spatial coherence properties of two-dimensional trapped Fermi gases in the BEC-BCS crossover regime [1]. On the bosonic side, experimental data are in quantitative agreement with path-integral quantum Monte Carlo calculations of point like molecules up to large values of the interaction. Algebraic correlations in the first-order correlation function characterize the phase below the Kosterlitz-Thouless transition temperature. Whereas the inhomogeneous trapping potential introduces important quantitative modifications, the effective exponent of the power-law decay at the superfluid transition remains approximately constant for all interaction strengths in the BEC-BCS crossover regime. \\ [1] P.A. Murthy, I. Boettcher, L. Bayha, M. Holzmann, D. Kedar, M. Neidig, M.G. Ries, A.N. Wenz, G. Zuern, and S. Jochim, Phys. Rev. Lett. 115, 010401 (2015). [Preview Abstract] |
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