Bulletin of the American Physical Society
50th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics APS Meeting
Volume 64, Number 4
Monday–Friday, May 27–31, 2019; Milwaukee, Wisconsin
Session D07: Excitations in Degenerate Gases |
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Chair: Peter Schauss, University of Virginia Room: Wisconsin Center 103AB |
Tuesday, May 28, 2019 2:00PM - 2:12PM |
D07.00001: The Structure of Polarized Vortices in the Unitary Fermi Gas Chunde Huang, Michael Forbes, Gabriel Wlazłowski, Piotr Magierski, Konrad Kobuszewski Fermionic superfluids do not generally support polarization, and the nature of the ground state of a slightly polarized unitary Fermi gas remains an open question. However, vortices naturally support polarization since the pairing gap vanishes in the core of superfluid vortices. The structure of a polarized vortex is not well understood, and may have some interesting properties. To study the microscopic structure of a vortex, we use a density functional theory called the asymmetric superfluid local density approximation (ASLDA) to simulate how vortexes interact, evolve and how energy transfers between paired and unpaired particles. In this talk, I will discuss the structure and properties of polarized vortices using the ASLDA, and how these related to polarized phases through the Thomas-Fermi (TF) approximation [Preview Abstract] |
Tuesday, May 28, 2019 2:12PM - 2:24PM |
D07.00002: Dynamics of Fermionic Vortices Saptarshi Sarkar, Michael Forbes, Khalid Hossain, Konrad Kobuszewski, Piotr Magierski, Gabriel Wlazłowski Studying the dynamics of superfluid fermionic vortices can be computationally expensive. In this talk, I will discuss how one can simulate the dynamics of fermionic vortices as a gas of bosonic dimers representing the Cooper pairs. This significantly reduces the computational cost, allowing one to study macroscopic systems. In this talk, I will discuss how polarization affects the properties and dynamics of fermionic vortices, and to what extent these properties can be captured by bosonic simulations. [Preview Abstract] |
Tuesday, May 28, 2019 2:24PM - 2:36PM |
D07.00003: Giant vortex clusters in a two-dimensional superfluid Tyler W. Neely, Guillaume Gauthier, Matthew T. Reeves, Kwan Goddard Lee, Xiaoquan Yu, Ashton Bradley, Mark Baker, Thomas A. Bell, Halina Rubinsztein-Dunlop, Matthew J. Davis As first recognized by Onsager, a high energy closed system of 2D point vortices exhibits equilibria characterized by concentrated vortex clusters. Onsager's theory has proved highly-influential, providing understanding of diverse quasi-2D systems such as turbulent soap films and guiding-center plasmas. However, experimental systems demonstrating Onsager's point-vortex statistical mechanics have remained elusive. We report our observation of negative-temperature vortex clusters injected directly into a uniform elliptical BEC, though stirring the condensate with a pair of elliptical barriers. We find that over many seconds of hold time vortex annihilation is suppressed and the clustered fraction is stable, while exhibiting energy loss (cooling) with increasing hold time. We characterize the cooling rate in response to variable non-uniformity of the BEC density and finite temperature. We further characterize axisymmetric and non-axisymmetric equilibria of a single-sign (chiral) vortex gas. We find that an initial nonequilibrium configuration can rapidly thermalize, resulting in an equilibrium state predicted by the point-vortex angular momentum and energy. [Preview Abstract] |
Tuesday, May 28, 2019 2:36PM - 2:48PM |
D07.00004: Vortex-Induced Dissipation across a Josephson Junction. Klejdja Xhani, Elettra Neri, Matteo Zaccanti, Kean Loon Lee, Luca Galantucci, Andrea Trombettoni, Francesco Scazza, Alessia Burchianti, Giacomo Roati, Nikolaos Proukakis The transition from Josephson oscillations to a dissipative regime was experimentally observed (Science 350,1505(2015)), due to the generation of vortices inside a thin barrier coupling two parts of a superfluid from the BEC to the BCS limit. We model this experiment in the molecular BEC regime by performing 3D numerical simulations at both zero and finite temperatures. The onset of the dissipative regime depends on the barrier height and it is characterised in terms of the critical initial population imbalance between the two wells and the maximum superfluid current. We show that the vortices generated inside the barrier are vortex rings (VR) and give a full characterisation of their dynamics (position, radius, and shape oscillations). For the first VR we show the dependence of its lifetime and velocity on the initial population imbalance $z_0$. Experimentally the VRs are observed in time of flight images after gradually removing the barrier. We consider this effect and show that the barrier removal strongly enhances the VR stability, thus facilitating its experimental observation, even in the presence of a small thermal cloud. [Preview Abstract] |
Tuesday, May 28, 2019 2:48PM - 3:00PM |
D07.00005: Observation of First and Second Sound in a Homogeneous Bose Gas Timon Hilker, Christoph Eigen, Lena Bartha, Jake Glidden, Robert Smith, Zoran Hadzibabic The existence of two distinct sound velocities is one of the hallmarks of superfluids. We present the first observation of both sound modes in a moderately interacting ultracold Bose gas. Using a magnetic field gradient, we excite center-of-mass oscillations of a homogeneous K-39 Bose gas in a three-dimensional box trap, revealing distinct oscillations of both the condensed and the thermal component. As predicted by the two-fluid model, we find that the slower mode (second sound) is predominantly associated with the BEC component, while the faster mode (first sound) is linked to the thermal component. We study the speed and damping of both modes for various interaction strengths and temperatures, including temperatures above $T_c$ for the first sound. [Preview Abstract] |
Tuesday, May 28, 2019 3:00PM - 3:12PM |
D07.00006: Creation and Observation of second sound in Homogenous Unitary Fermi gases Zhenjie Yan, Parth Patel, Biswaroop Mukherjee, Airlia Shaffer-Moag, Cedric Wilson, Richard Fletcher, Martin Zwierlein Second sound is a distinct excitation mode which exists in superfluids. In contrast to the first sound which is a density wave, the second sound mode is a temperature wave caused by the out-of-phase oscillation between the normal and superfluid components. Using a resonant oscillating gradient potential, we create stable standing waves of second sound in a uniform trap. In order to observe these temperature waves, we apply an off-resonant rf pulse, which is sensitive to the pair-breaking excitations, as a local temperature probe. Using the aforementioned methods, we measure the speed and decay rate of the second sound at various temperatures. Using the properties of second sound, we obtain the superfluid fraction, thermal diffusivity and heat capacity ratio of a unitary Fermi superfluid. [Preview Abstract] |
Tuesday, May 28, 2019 3:12PM - 3:24PM |
D07.00007: Observation of quantum-critical breakdown of the Bose Polaron Zoe Yan, Yiqi Ni, Alexander Chuang, Carsten Robens, Martin Zwierlein Strongly-coupled Bose polarons are created in equilibrium by immersing 40K impurities in a 23Na Bose-Einstein condensate (BEC). We perform locally-resolved radiofrequency ejection spectroscopy by transferring the impurities from a strongly-interacting into a non-interacting internal state. Our spectra reveal the energy, lifetime, and short-range correlations of the strongly-coupled impurity state as a function of temperature, from low temperatures in which the polaronic quasiparticle is well-defined to the regime near the BEC phase transition. At low temperatures, the impurity behaves as a localized Bose polaron with its spatial extent given by the interboson distance. Approaching the critical BEC temperature, the impurity's spectral width increases to several times the measured binding energy, signaling a breakdown of a simple quasiparticle picture in the vicinity of the quantum-critical point. [Preview Abstract] |
Tuesday, May 28, 2019 3:24PM - 3:36PM |
D07.00008: Dynamical variational approach to finite-temperature Bose polarons David Dzsotjan, Richard Schmidt, Michael Fleischhauer We discuss finite-temperature effects on the interaction of a mobile impurity with a Bose-Einstein condensate using a dynamical variational approach based on coherent state wave functions. Taking into account the thermal occupation of Bogoliubov excitations by averaging over initial coherent states with Gaussian random amplitudes, we predict the finite-temperature polaron absorption spectrum from a calculation of the the time-dependent Ramsey signal. In the limit of infinite impurity mass, the variational problem can be solved exactly, while in the general case we rely on a mean-field decoupling of the auxiliary random fields. Different from previous predictions [1], we do not find a thermally-induced splitting of the polaron peak, but solely temperature-induced broadening and shifts. We compare our results with recent experiments on Bose polarons where temperature effects are relevant [2]. $\text{Ref.}$: [1] N.-E. Guenther, et al., Phys. Rev. Lett. 120, 050405 (2018), [2] N.B. Jorgensen, et al., Phys. Rev. Lett. 117, 055302 (2016) [Preview Abstract] |
Tuesday, May 28, 2019 3:36PM - 3:48PM |
D07.00009: Driving Correlated Quantum Fluctuations from a Bose-Einstein Condensate Liang-Ying Chih, Murray Holland In a recent experiment that observed a Bose firework pattern by Clark et al. in Nature 551, 356 (2017), a Bose-Einstein condensate emitted high momentum atoms in jets when driven by a periodic modulation of the scattering length. This experiment demonstrated the capability to selectively amplify quantum fluctuations with a specific momentum that could be spectroscopically tuned. We use the generalized Hartree-Fock-Bogoliubov theory to quantify the initial quantum fluctuations in a stationary condensate, and then numerically simulate the resulting dynamics of the system when the scattering length is modulated. The classical external field that excites pairs of particles with the same energy but opposite momenta resembles the coherently driven nonlinearity in a parametric amplifier, and generates a squeezed matter-wave state in the quasiparticle mode on resonance with the modulation frequency. Since this is a highly coherent process with automatic phase matching, we observe monotonic growth in the pair correlation amplitude. We propose applying Ramsey interferometry to experimentally probe the pair correlations in a future experiment. [Preview Abstract] |
Tuesday, May 28, 2019 3:48PM - 4:00PM |
D07.00010: Dynamical interplay of few-body correlations in a thermal unitary Bose gas Victor Colussi, Bjorn van Zwol, Jose D'Incao, Servaas Kokkelmans We study the growth of few-body correlations in an ultracold Bose gas quenched to unitarity. This is encoded in the dynamics of the two- and three-body contacts. By connecting many-body correlations dynamics with few-body models\footnote{V. E. Colussi, J. P. Corson, and J. P. D'Incao, PRL 120 (10), 100401 (2018)}, we map out signatures of the Efimov effect. For the thermal resonantly interacting Bose gas where the contact dynamics have been measured experimentally\footnote{R. J. Fletcher, R. Lopes, J. Man, N. Navon, R. P. Smith, M. W. Zwierlein, and Z. Hadzibabic, Science 355, 377 (2017)}, we find that atom-bunching leads to an enhanced growth of few-body correlations. These atom-bunching effects also highlight the interplay between few-body correlations that occurs before genuine many-body effects enter on Fermi timescales. [Preview Abstract] |
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