Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Y35: Collective Modes and Topological Defects in Bose-Einstein Condensates |
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Sponsoring Units: DAMOP Chair: Lauren Aycock, Joint Quantum Institute/ Cornell University Room: 210B |
Friday, March 6, 2015 8:00AM - 8:12AM |
Y35.00001: Deconstruction of excitations in atomtronic systems using phase reference Mark Edwards, Brennan Coheleach, Charles Clark Laboratory atomtronic systems consisting of a Bose--Einstein--condensed gas with strong horizontal confinement and arbitrary planar potential, such as a ring--plus--disk, are now possible~\footnote{S.\ Eckel, et al., {\em Phys.\ Rev.\ X} {\bf 4}, 031052 (2014)}. Perturbing the ring part (e.g., by stirring) can produce excitations such as vortices and solitons. Each excitation uniquely modifies the local condensate phase and these modifications can be probed by overlapping the ring with the unperturbed disk via condensate release. The resulting interference pattern contains signatures of the excitations present at release time. Using the Gross--Pitaevskii equation, we studied whether this interference pattern can be used to determine what excitations were present at release time. We created individual excitations in a ring--plus--disk condensate, released it to see the interference pattern of individual excitations, and created a compendium of these patterns. We also studied whether the individual patterns can be superposed and tested the deconstruction procedure by analyzing the interference of a strongly stirred ring by comparing the deconstruction with the condensate state at release time. [Preview Abstract] |
Friday, March 6, 2015 8:12AM - 8:24AM |
Y35.00002: Thermal and quantum fluctuation effects in rotational hysteresis of ring Bose--Einstein condensates C.W. Clark, Y.-H. Wang, C. Heller, M. Edwards In a recent experment~\footnote{S.\ Eckel, et al., {\em Nature} {\bf 506}, 200 (2014)} a ring Bose--Einstein condensate (BEC) with zero circulation (with winding number $m=0$) and stirred by a barrier jumped to an $m=1$ state when stirred faster than a certain critical speed, $\Omega_{c}^{+}$. Conversely an $m=1$ condensate dropped to $m=0$ when stirred below a critical speed, $\Omega_{c}^{-}$, which was lower than $\Omega_{c}^{+}$. The hysteresis loop areas, $\Omega_{c}^{+}-\Omega_{c}^{-}$, disagreed significantly with the predictions of the zero--temperature Gross--Pitaevskii equation. We report the results of simulating this experiment with both the Zaremba--Nikuni--Griffin (ZNG) theory and the Truncated Wigner Approximation (TWA). The ZNG theory can account for thermal fluctuations while the TWA can also account for quantum fluctations of the gas. We compare the results of these simulations with the experimental data and describe how the dynamics of vortex/antivortex pairs formed in the barrier region during the stirring is modified by the presence of a thermal cloud and by quantum fluctuations beyond the mean field. [Preview Abstract] |
Friday, March 6, 2015 8:24AM - 8:36AM |
Y35.00003: Unusual vortex matter in rotating Bose-Einstein condensates with SU(2) broken symmetry Peder Galteland, Egor Babaev, Asle Sudbo We consider a Ginzburg-Landau model of a rotating two-component Bose-Einstein condensate with SU(2) broken symmetry through the use of numerical Monte Carlo techniques. We include the full spectrum of thermal amplitude- and phase-fluctuations. The model exhibits an unusual state of global phase coherence with no accompanying vortex line lattice. This state has no counterpart in single-component condensates. The conditions for such a state are experimentally realizable in, \textit{e.g.}, homonuclear mixes of atomic gases in separate hyperfine states, tuned to the SU(2) point with Feshbach resonance techniques. [Preview Abstract] |
Friday, March 6, 2015 8:36AM - 8:48AM |
Y35.00004: Vortex annihilation and inverse cascades in two dimensional superfluid turbulence Andrew Lucas, Paul M. Chesler The dynamics of a dilute mixture of vortices and antivortices in a turbulent two-dimensional superfluid at finite temperature is well described by first order Hall-Vinen-Iordanskii equations, or dissipative point vortex dynamics. These equations are governed by a single dimensionless parameter: the ratio of the strength of drag forces to Magnus forces on vortices. When this parameter is small, we demonstrate using numerical simulations that the resulting superfluid enjoys an inverse energy cascade where small scale stirring leads to large scale vortex clustering. We argue analytically and numerically that the vortex annihilation rate in a laminar flow may be parametrically smaller than the rate in a turbulent flow with an inverse cascade. This suggests a new way to detect inverse cascades in experiments on two-dimensional superfluid turbulence using cold atomic gases, where traditional probes of turbulence such as the energy spectrum are not currently accessible. [Preview Abstract] |
Friday, March 6, 2015 8:48AM - 9:00AM |
Y35.00005: Excitation spectrum of a tightly confined spin-2 Bose gas Matjaz Payrits, Ryan Barnett We introduce an effective low-energy action for a spin-2 Bose gas in a tight trap in the presence of a quadratic Zeeman field. We derive the excitation spectrum by expanding the action to second order in deviations from the ground state configuration and subjecting it to a functional integral analysis. This is a generalization of standard Bogoliubov theory as it allows for the study of excitations about fragmented states, occuring ubiquitously in spinor Bose gas systems. It is found that the excitations generally consist of mean-field-like states uniformly integrated over rotations about the direction of the magnetic field (or all of SO(3) in the absence of a magnetic field). This parallels the general observation that states breaking fewer symmetries tend to be lower in energy. Though unstable in the thermodynamic limit, these states are stabilized for finite particle numbers, potentially allowing for a convenient means of studying fragmentation experimentally. For the nematic region of the spin-2 phase diagram, we also show that the quadratic Zeeman dependence of the spectrum smoothly approaches the previously obtained discontinuous mean-field dependence. [Preview Abstract] |
Friday, March 6, 2015 9:00AM - 9:12AM |
Y35.00006: Probing the Collective Modes of Spherical Shell-Shaped Condensates with Quench Numerics Frances Yang, Kuei Sun, Karmela Padavic, Smitha Vishveshwara, Courtney Lannert We explore the collective modes of Bose-Einstein condensates by numerical solution of the Gross-Pitaevskii equation with an external ``bubble trap'' potential ($V_{trap}=\sqrt{(r^2-\Delta)^2/4-\Omega^2}$) that can be continuously tuned between a thin spherical shell-shaped condensate (at large $\Delta$) and an ordinary spherical condensate in a harmonic trap (at $\Delta=\Omega=0$). We excite the condensate's collective modes by making a small sudden change to the trapping potential and analyzing the subsequent time evolution of the condensate wavefunction. We observe the evolution of the frequency of the low-lying collective modes between the limits of a thin-shell condensate and a filled-spherical condensate. [Preview Abstract] |
Friday, March 6, 2015 9:12AM - 9:24AM |
Y35.00007: Microscopic theory of BEC phase transition in a critical region Vitaly Kocharovsky, Vladimir Kocharovsky A microscopic theory, which should connect the asymptotics of the ordered and disordered phases across a critical region, has not been found so far even for anyone of the numerous phase transitions. Here we present such microscopic theory for a phase transition in an interacting Bose gas [Phys. Lett. A 378, n. 49 (2014)]. It allows one to describe formation of a condensate phase from a disordered phase across an entire critical region continuously. We find an exact Hamiltonian for Bose-Einstein condensation (BEC) in a mesoscopic system and derive the exact fundamental equations for the condensate wave function and Green's functions, which are valid both inside and outside critical region. They are reduced to the usual Gross-Pitaevskii and Beliaev-Popov equations in a low-temperature limit outside critical region. The theory is readily extendable to other phase transitions. All these advances come from a correct account, first, of the Noether's symmetry constraints in a many-body Hilbert space and, second, of the related properties of true excitations in a mesoscopic system. In a limit of vanishing interaction, the theory is reduced to a recently found analytical theory of universal critical fluctuations in BEC of an ideal gas [Phys. Rev. A 81, 033615 (2010); 90, 033605 (2014)]. [Preview Abstract] |
Friday, March 6, 2015 9:24AM - 9:36AM |
Y35.00008: Moving solitons in a one-dimensional fermionic superuid - an exact solution Dmitry Efimkin, Victor Galitski A fully analytical theory of a traveling soliton in a one-dimensional fermionic superuid is developed within the framework of time-dependent self-consistent Bogoliubov-de Gennes equations, which are solved exactly. The soliton manifests itself in a kink-like profile of the superconducting order parameter and hosts a pair of Andreev bound states. They adjust to soliton's motion and play an important role in its stabilization. A phase jump across the soliton and its energy decrease with soliton's velocity and vanish at the critical velocity, corresponding to the Landau criterion, where the soliton starts emitting quasiparticles and becomes unstable. The ``inertial'' and ``gravitational'' masses of the soliton are calculated and the former is shown to be orders of magnitude larger than the latter. This results in a slow motion of the soliton in a harmonic trap. \\[4pt] Reference: Dmitry K. Efimkin and Victor Galitski -- ArXiv: 1408.6511 (2014). [Preview Abstract] |
Friday, March 6, 2015 9:36AM - 9:48AM |
Y35.00009: 3D dimeron as a stable topological object Shijie Yang, Yongkai Liu Searching for novel topological objects is always an intriguing task for scientists in various fields. We study a new three-dimensional (3D) topological structure called 3D dimeron in the trapped two-component Bose-Einstein condensates. The 3D dimeron differs to the conventional 3D skyrmion for the condensates hosting two interlocked vortex-rings. We demonstrate that the vortex-rings are connected by a singular string and the complexity constitutes a vortex-molecule. The stability is investigated through numerically evolving the Gross-Pitaevskii equations, giving a coherent Rabi coupling between the two components. Alternatively, we find that the stable 3D dimeron can be naturally generated from a vortex-free Gaussian wave packet via incorporating a synthetic non-Abelian gauge potential into the condensates. [Preview Abstract] |
Friday, March 6, 2015 9:48AM - 10:00AM |
Y35.00010: Black--hole lasing action in laboratory Bose--Einstein condensates Yi-Hsieh Wang, Ted Jacobson, Mark Edwards, Charles W. Clark A recent experiment \footnote{J. Steinhauer, {\em Nature Physics} {\bf 11}, 864 (2014)} infers the the production of Hawking radiation in an analogue black-hole laser, which consists of a Bose-Einstein condensate of about 5,000 $^{87}$Rb atoms in a trap with a translating potential step. In the co-moving reference frame the flow velocity of the condensate exceeds the sound speed in a region confined between two sonic points, the analogue black and white hole horizons. We report simulations of that experiment based on the zero-temperature Gross-Pitaevskii (GP) equation that are consistent with the reported experimental results. The simulations show exponential growth of oscillatory modes trapped between the horizons, with a power spectrum consistent with expectations from the Bogoliubov dispersion relation, which saturates after an initial period. Quantum Hawking radiation occurs spontaneously in the vacuum, but in the presence of a coherent state of phonons it takes on a classical form captured by the zero-temperature GP equation. The growth of the trapped modes results from repeated super-radiant scattering from the black hole horizon, associated with emission of Hawking radiation to the exterior that is not well-resolved in the simulation. [Preview Abstract] |
Friday, March 6, 2015 10:00AM - 10:12AM |
Y35.00011: The fate of a gray soliton in a quenched Bose-Einstein condensate Oleksandr Gamayun, Yulia Bezvershenko, Vadim Cheianov We investigate the destiny of a gray soliton in a repulsive one-dimensional Bose-Einstein condensate undergoing a sudden quench of the non-linearity parameter. The outcome of the quench is found to depend dramatically on the ratio $\eta$ of the final and initial values of the speed of sound. For integer $\eta$ the soliton splits into exactly $2\eta-1$ solitons. For non-integer $\eta$ the soliton decays into multiple solitons and Bogoliubov modes. The case of integer $\eta$ is analyzed in detail. The parameters of solitons in the out-state are found explicitly. Our approach exploits the inverse scattering method and can be easily used for the similar quenches in any classical integrable system. [Preview Abstract] |
Friday, March 6, 2015 10:12AM - 10:24AM |
Y35.00012: Universal dynamics of a soliton after a quantum quench Andrea Trombettoni, Fabio Franchini, Andrey Gromov, Manas Kulkarni In a quantum quench, one prepares a system in an eigenstate of a given Hamiltonian, and then lets it evolve after suddenly changing a control parameter of the Hamiltonian. By observing this evolution, one aims at understanding whether and how a quantum system reaches a (thermal) equilibrium. Typically, the initial state is taken to be the ground state and/or in an extended state: we propose a different experimentally feasible protocol, in which the system is prepared in an excited state corresponding to a collective solitonic excitation. If we are interested only in the single particle density, in the hydrodynamic regime the time evolution can be reduced to a semi-classical non-linear differential equation. The study of such equation shows that the short time dynamics after the quench is universal, and simple analytical predictions can be given for the velocities and profiles. Numerical support for these results is presented using the Calogero model and the non-linear Schrodinger equation (NLSE), relevant for the implementation of the proposed protocol with ultracold bosons. The effect of non-integrable terms (power-law non-linearity and a parabolic potential) in the NLSE is also investigated, and shown to not spoil the universality. [Preview Abstract] |
Friday, March 6, 2015 10:24AM - 10:36AM |
Y35.00013: Conservation of helicity in superfluids Hridesh Kedia, Dustin Kleckner, Davide Proment, William T.M. Irvine Helicity arises as a special conserved quantity in ideal fluids, in addition to energy, momentum and angular momentum. As a measure of the knottedness of vortex lines, Helicity provides an important tool for studying a wide variety of physical systems such as plasmas and turbulent fluids. Superfluids flow without resistance just like ideal (Euler) fluids, making it natural to ask whether their knottedness is similarly preserved. We address the conservation of helicity in superfluids theoretically and examine its consequences in numerical simulations. [Preview Abstract] |
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