Bulletin of the American Physical Society
51st Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 65, Number 4
Monday–Friday, June 1–5, 2020; Portland, Oregon
Session M09: Fermi GasesLive
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Chair: Joseph Thywissen, University of Toronto Room: Portland 256 |
Thursday, June 4, 2020 8:00AM - 8:12AM Live |
M09.00001: An ideal Josephson junction in an ultracold two-dimensional Fermi gas Niclas Luick, Lennart Sobirey, Markus Bohlen, Vijay Pal Singh, Ludwig Mathey, Thomas Lompe, Henning Moritz Two-dimensional structures are present in almost all known superconductors with high critical temperatures, but the role of the reduced dimensionality is still under debate. Recently, ultracold atoms have emerged as an ideal model system to study such strongly correlated 2D systems. Here, we present our realisation of a Josephson junction in an ultracold 2D Fermi gas. We measure the frequency of Josephson oscillations as a function of the phase difference across the junction and find excellent agreement with the sinusoidal current phase relation of an ideal Josephson junction. Furthermore, we determine the critical current of our junction in the crossover from tightly bound molecules to weakly bound Cooper pairs. Our measurements clearly demonstrate phase coherence and provide strong evidence for superfluidity in a strongly interacting 2D Fermi gas. [Preview Abstract] |
Thursday, June 4, 2020 8:12AM - 8:24AM Live |
M09.00002: Direct observation of superfluidity in an ultracold two-dimensional Fermi gas Lennart Sobirey, Markus Bohlen, Niclas Luick, Hauke Biss, Henning Moritz, Thomas Lompe Understanding the mechanism for superfluidity in low dimensional systems with strong correlations is one of the major unsolved problems of condensed matter physics. Ultracold two-dimensional Fermi gases model these systems in a clean and controllable way, but so far, superfluidity has not been directly observed. Here, we present direct evidence of superfluidity in a strongly interacting 2D Fermi gas. We drag an optical lattice through a homogeneous 2D Fermi gas and observe no dissipation below a critical velocity and temperature, in excellent agreement with the Landau criterion. We find evidence for superfluidity across a wide range of interaction strengths in the BEC-BCS crossover. [Preview Abstract] |
Thursday, June 4, 2020 8:24AM - 8:36AM Live |
M09.00003: Observation of heat transport in Unitary Fermi gases Zhenjie Yan, Parth Patel, Biswaroop Mukherjee, Cedric Wilson, Airlia Shaffer-Moag, Richard Fletcher, Martin Zwierlein Understanding heat transport in strongly correlated quantum matter has proven to be both experimentally and theoretically a challenging task as it involves detecting local temperature variations. Here, we demonstrate experimentally a method to create and probe a local temperature disparity in a unitary Fermi gas. In order to create a local hot spot, we modulate a spatially periodic optical potential on part of the atomic cloud to create fast decaying high energy phonons. Using a radio frequency pulse as a temperature sensitive probe allows us to map out the spatial temperature distribution of the atomic gas. We observe two distinctive modes for the transfer of heat in unitary fermi gases: heat propagates diffusively above the superfluid transition, while a wave-like motion happens below the critical temperature. Measuring the time scale of thermal evolution processes allows us to extract the thermal diffusivity as well as the superfluid fraction below transition temperature to superfluid. [Preview Abstract] |
Thursday, June 4, 2020 8:36AM - 8:48AM Live |
M09.00004: Coherent control in a driven Fermi-Hubbard system Anne-Sophie Walter, Konrad Viebahn, Joaquin Minguzzi, Kilian Sandholzer, Frederik Goerg, Tilman Esslinger Coherent control is a widely applied technique in fields ranging from chemistry to ultracold atoms. It aims at steering quantum dynamics by controlling the relative phase between external light fields. In the context of Floquet engineering in periodically driven optical lattices, the drive can resonantly couple to higher Bloch bands and subsequently lead to dissipation. To overcome this problem we report on a coherent control scheme which allows for a wider range of possible driving frequencies. \\In our experiment, we periodically modulate the potential depth of a 3D optical lattice at a frequency that excites atoms to a higher band. We apply coherent control by tuning the phase of an additional drive at twice the fundamental frequency which destructively interferes with the first. Through this technique we preserve both the band population as well as the fraction of double occupancies for more than two orders of magnitude longer compared to the single-frequency case. We find this technique to be effective even at strong Hubbard interactions. Strikingly, the lifetime of spin correlations, which are highly susceptible to heating, is also improved by more than two orders of magnitude and comparable to the static value. [Preview Abstract] |
Thursday, June 4, 2020 8:48AM - 9:00AM On Demand |
M09.00005: Precision Spectroscopy of an Interacting Ytterbium Fermi-Fermi Mixture Benjamin Abeln, Marcel Diem, Koen Sponselee, Nejira Pintul, Klaus Sengstock, Christoph Becker We study an ultracold interacting Fermi-Fermi mixture of $^{171}$Yb and $^{173}$Yb. The Yb Fermi-Fermi mixture in the ground state is characterized by strongly attractive inter-species interactions, while the intra-species interactions are vanishing for $^{171}$Yb and repulsive for $^{173}$Yb. Performing precision spectroscopy on the $^{1}$S$_{0}$ to $^{3}$P$_{0}$ clock transition we find and characterize a SU($\it{2}$) $\times$ SU($\it{6}$) symmetric interspecies interorbital interaction. We discuss prospects of spectroscopic methods to gain information on the many-body state of the system. [Preview Abstract] |
Thursday, June 4, 2020 9:00AM - 9:12AM On Demand |
M09.00006: Rapid preparation and characterization of low entropy SU(N) Fermi gases Lindsay Sonderhouse, Christian Sanner, Ross B. Hutson, Akihisa Goban, Lingfeng Yan, William R. Milner, Thomas Bilitewski, Ana Maria Rey, Jun Ye We use $^{87}$Sr atoms to study a deeply degenerate Fermi gas under SU(N) symmetry, where N can be as large as 10. We demonstrate enhanced interactions in a 3D $^{87}$Sr degenerate gas. Using all 10 spin states during evaporation allows us to have efficient sample preparation while reaching deep degeneracy, with ${T/T_F = 0.07}$ in under 3 s. We also characterize the SU(N) gas by observing anisotropic expansion in time-of-flight and by examining density fluctuations in the gas. We show that SU(N) $\-$ symmetric interactions significantly modify the behavior of a non- interacting Fermi gas and need to be accurately accounted for to extract thermodynamic properties. [Preview Abstract] |
Thursday, June 4, 2020 9:12AM - 9:24AM Not Participating |
M09.00007: High Frequency Sound in a Unitary Fermi Gas Sascha Hoinka, Carlos Kuhn, Ivan Herrera, Paul Dyke, Jami Kinnunen, Georg Bruun, Chris Vale We present an experimental and theoretical study of the phonon mode in a unitary Fermi gas. Using two-photon Bragg spectroscopy, we measure excitation spectra at a momentum of approximately half the Fermi momentum, both above and below the superfluid critical temperature T$_{c}$. Below T$_{c}$, the dominant excitation is the Bogoliubov-Anderson (BA) phonon mode, driven by gradients in the phase of the superfluid order parameter. The temperature dependence of the BA phonon is consistent with a theoretical model based on the quasiparticle random phase approximation in which the dominant damping mechanism is via collisions with thermally excited quasiparticles. As the temperature is increased above T$_{c}$, the phonon evolves into a strongly damped collisional mode, accompanied by an abrupt increase in spectral width. Our study reveals strong similarities between sound propagation in the unitary Fermi gas and liquid helium. [Preview Abstract] |
Thursday, June 4, 2020 9:24AM - 9:36AM Not Participating |
M09.00008: Transport of Nambu-Goldstone modes in a fermionic superfluid point contact Shun Uchino In fermionic superfluids, Nambu-Goldstone (NG) modes emerge as a result of spontaneous symmetry breaking. Here, we discuss DC transport of such NG modes through a quantum point contact. We show that contrary to a naive view that enhancement of the phase stiffness may suppress transport of the NG modes, there must be an anomalous contribution that survives at low temperature. We discuss that the anomalous contribution is enhanced in the good-contact limit and strong interaction. [Preview Abstract] |
Thursday, June 4, 2020 9:36AM - 9:48AM |
M09.00009: Hydrodynamic expansion of a strongly interacting Fermi-Fermi mixture of $^{161}$Dy and $^{40}$K Marian Kreyer, Cornelis Ravensbergen, Elisa Soave, Vincent Corre, Jeong Ho Han, Emil Kirilov, Rudolf Grimm We study the hydrodynamic expansion of a near-degenerate Fermi-Fermi mixture in the strongly interacting regime of an interspecies Feshbach resonance [1]. While the $^{161}$Dy majority component is only weakly affected in the width of its Gaussian distribution, the $^{40}$K minority component features a strong effect of the mass imbalance, manifesting itself in pronounced side wings in the expansion profiles. We reproduce this unexpected behavior in a classical Monte Carlo simulation, taking into account the resonant cross section between the two species. The simulation shows the same dependence on interaction strength as the experimental data and can be extended to study other phenomena such as collective modes in the collisionally hydrodynamic regime. \newline \newline [1] C. Ravensbergen et al., arXiv:1909.03424. \newline \newline Supported by the Austrian Science Fund (FWF) within projects P32153-N36 and W1259-N27. [Preview Abstract] |
Thursday, June 4, 2020 9:48AM - 10:00AM |
M09.00010: Quenched Fermi gases Paul Dyke, Andrew Hogan, Ivan Herrera, Sascha Hoinka, Carlos Kuhn, Chris Vale We present studies of two-component Fermi gas following a rapid quench from weak to strong interactions. Starting with a spin-balanced weakly attractive gas of lithium-6 atoms in the normal phase, we ramp the interactions to the unitarity limit at a range of sweep rates to investigate the dynamics of the pair condensate formation. By fitting the equation of state for the unitary gas we can determine the amount of energy and entropy added as a function of the sweep rate and investigate the timescales for the formation of the superfluid and pair correlations in the strongly interacting regime. [Preview Abstract] |
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