Bulletin of the American Physical Society
2013 Joint Meeting of the APS Division of Atomic, Molecular & Optical Physics and the CAP Division of Atomic, Molecular & Optical Physics, Canada
Volume 58, Number 6
Monday–Friday, June 3–7, 2013; Quebec City, Canada
Session C1: Vortices in Bose Gases |
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Chair: Brian Anderson, University of Arizona Room: 200A |
Tuesday, June 4, 2013 2:00PM - 2:12PM |
C1.00001: Studies of a toroidal superfluid Bose-Einstein condensate with a weak link S. Eckel, J.G. Lee, F. Jendrzejewski, A. Kumar, K.C Wright, W.D. Phillips, C.J. Lobb, G.K. Campbell We have created a toroidally-shaped BEC of $^{23}Na$ mechanically stirred with a rotating barrier potential. In this system, we have observed hysteresis in the persistent current state, which is a common feature of superfluid and superconducting systems with Josephson junctions. The barrier, which is modeled as a weak link, induces phase slips in the superfluid between well-defined persistent current states. The rotation frequency at which these phase slips occur differ, depending on whether the phase slip results in an increase or decrease of the persistent current. In addition to the observed hysteresis, we will discuss recent progress toward measuring the current-phase relationship of our weak link. With both hysteresis and the appropriate current-phase relation, it is possible that our toroidal BEC could be used as a sensitive rotation sensor, in a manner analogous to an RF-SQUID's sensitivity to magnetic field. [Preview Abstract] |
Tuesday, June 4, 2013 2:12PM - 2:24PM |
C1.00002: Stirring a ring Bose-Einstein condensate: vortices and overall circulation Noel Murray, Mark Edwards, Charles W. Clark We have studied the process whereby stirring a superfluid Bose--Einstein condensate confined in a ring-shaped potential leads to an overall circulation. We solved the time-dependent Gross--Pitaevskii equation under conditions chosen to match those of an experiment recently conducted at NIST. Briefly, 500,000 Na atoms where confined at the ring-shaped intersection of a red-detuned horizontal light sheet and a vertically propagating Laguerre--Gauss beam. Stirring was carried via a blue--detuned gaussian beam. We found that, at first, the stirring spawned a number of vortex--antivortex pairs and then stopped. These vortices displayed a complicated dynamical behavior which slowly reduced the number of vortices pairwise via annihilation and singly via diffusion into surface modes of the condensate. At the end of this dynamics, the set of vortices was replaced by an overall circulation of atoms around the ring. We present examples of this behavior, give a simple model of vortex motion and vortex-vortex interaction, and show how the production and annihilation of vortices gets turned into a overall circulation of the ring Bose--Einstein condensate. [Preview Abstract] |
Tuesday, June 4, 2013 2:24PM - 2:36PM |
C1.00003: In-situ imaging of vortex distributions in Bose-Einstein condensates. Kali Wilson, Zachary Newman, Joseph Lowney, Brian P. Anderson Experimental measurements of vortex dynamics in Bose-Einstein condensates (BEC) are essential for the development of a clear understanding of quantum fluid turbulence in BECs. One approach towards this goal involves obtaining multiple in-situ images of vortex distributions in a single trapped BEC. As a first step, we have implemented an imaging method capable of in-situ detection of two-dimensional vortex distributions. We have experimentally confirmed that our method can resolve vortex distributions in a single-component BEC held in a magnetic trap. In this talk we will discuss our imaging methods, present results demonstrating in-situ vortex imaging, and discuss prospects for using these methods in measurements of vortex dynamics. [Preview Abstract] |
Tuesday, June 4, 2013 2:36PM - 2:48PM |
C1.00004: Nucleation of Quantized Vortices in an Ultracold Atomic Gas Michael Ray, Emine Altunta\c{s}, Thomas Langin, David Hall In analogy with the rotating bucket experiment of liquid helium, we create vortices in a trapped Bose-Einstein condensate by cooling the atomic sample through the phase transition in the presence of a rotating magnetic trapping potential. The thermal cloud remains in quasi-equilibrium during the cooling, ultimately producing rotating condensates in the ground state. The method presents a way to consistently create a set number of vortices ($N_V < 10$). We show that the trap rotation frequency at which a vortex first appears agrees closely with theoretical predictions, and that the number of vortices within the condensate is established by the rotation frequency at the phase transition. Once the condensate has started to form, the number of vortices in the condensate is stable against changes in the frequency. Images of the condensate taken during evaporation suggest that the vortex spatial configuration is similarly determined early on in the growth of the condensate. We use this nucleation method to explore the angular momentum of a condensate with vortices. [Preview Abstract] |
Tuesday, June 4, 2013 2:48PM - 3:00PM |
C1.00005: Route to Quantum Turbulence in Trapped Bose-Einstein Condensates A. Joy Allen, Nick G. Parker, Nick P. Proukakis, Carlo F. Barenghi Turbulence in superfluid Helium has been the subject of many experimental and theoretical investigations (for review see e.g. L. Skrbek and K.R. Sreenivasan, Phys. of Fluids 24, 011301 (2012)) and recently, experimentalists have been able to visualize vortex lines, reconnection events and Kelvin waves (E. Fonda \emph{et al.} arXiv:1210.5194). Weakly interacting Bose-Einstein condensates however, present a unique opportunity to resolve the structure of vortices and in turn study the dynamics of a vortex tangle (as has recently been created in an atomic cloud E.A.L. Henn \emph{et al.} Phys. Rev. Lett 103, 04301 (2009)). We investigate ways of generating turbulence in atomic systems by numerically stirring the condensate using a Gaussian 'spoon' (analogous to a laser beam in the experiments), and study the isotropy of the resulting vortex tangle depending on whether the path the spoon stirs is circular or random. We model the system using the Gross-Pitaevskii Equation and extend our analysis to finite temperature using the Zaremba-Nikuni-Griffin (ZNG) formalism (E. Zaremba \emph{et al.} Jour. Low Temp. Phys. 116, 277 (1999)), whereby the full dynamics of the noncondensate atoms are described by a semiclassical Boltzmann equation. [Preview Abstract] |
Tuesday, June 4, 2013 3:00PM - 3:12PM |
C1.00006: Measures of turbulence in Bose-Einstein condensates Angela White, Nick Proukakis, Carlo Barenghi Turbulence, a dynamically reconnecting tangle of vortices, is ubiquitous throughout nature and can be found in both classical and quantum fluids. Trapped atomic condensates are an ideal test-bed for investigating the small-scale properties of turbulent tangles of vortices, due to the simple quantized nature of vortices in quantum fluids and the experimental control over condensate dimensionality and vortex dynamics.The quest to understand and study quantum turbulence in trapped atomic Bose-Einstein condensates raises unique challenges. In particular, it is important to understand how to create and characterize turbulent tangles. Motivated by work in classical fluids, we investigate if methods that are known to efficiently mix classical fluids, known as pseudo-Anosov stirring protocols, also efficiently mix trapped atomic condensates. In order to characterize turbulence, we develop some measures that are experimentally accessible, based on the density and distribution of vortices in trapped atomic condensates. At scales larger than the vortex core size, we describe how the momentum spectrum of vortices scales with vortex number. We also investigate velocity correlations as a measure of turbulence and vortex distribution in two-dimensional condensates. [Preview Abstract] |
Tuesday, June 4, 2013 3:12PM - 3:24PM |
C1.00007: Singular sound in a trapped quantum gas driven by two phase-conjugated optical vortices Alexey Okulov The trapped atomic cloud irradiated by two counter-propagating $\delta \omega$ frequency detuned Laguerre-Gaussian optical vortices with opposite angular momenta $\pm \ell \hbar$ is considered.\footnote{F. Dalfovo et al. Rev.Mod.Phys., \textbf{71}, 463 (1999).}$^,$\footnote{A.Yu.Okulov. J.Low Temp.Phys., in press (2013).} When period of spatial modulation $\lambda/2$ and LG carrier frequency detuning $\delta \omega$ \footnote{A.Yu.Okulov. J. Phys. B , \textbf{41}, 101001 (2008).} are in resonance with dispersion curve $\epsilon(p)$ \footnote{A.Yu.Okulov. Phys.Lett.A, \textbf{376}, 650-655 (2012).} the acoustical vortices carrying orbital angular momentum\footnote{J.L.Thomas and R.Marchiano. Phys.Rev.Lett., \textbf{91},244302 (2003).} are expected to occur.\footnote{A.Yu.Okulov. JETP.Lett., \textbf{88}, 487 (2008).}$^,$\footnote{Fam Le Kien,V.I.Balykin,K.Hakuta.Phys.Rev.A, \textbf{73},053823 (2006).} The experimentally accessible range of $\lambda$ and $\delta \omega$ \footnote{A.Yu.Okulov. J. Opt. Soc. Am. B, \textbf{29}, 714-718 (2012).} is analyzed from the point view of direct measurements of the excitation spectrum $\epsilon(p)$\footnote{R.P.Feynman. {\itshape ``Statistical mechanics''}, Ch.11(1972).}$^,$\footnote{M. R. Andrews et al. Phys.Rev.Lett.} [Preview Abstract] |
Tuesday, June 4, 2013 3:24PM - 3:36PM |
C1.00008: Probing phase fluctuations in a quasi-2D Bose gas by free expansion Jae-yoon Choi, Sang Won Seo, Woo Jin Kwon, Yong-il Shin We measure the power spectrum of the density distribution of a freely expanding quasi-2D Bose gas, where irregular density modulations gradually develop due to initial phase fluctuations in the sample [1]. The spectrum has an oscillatory shape, where the peak positions are found to be independent of temperature and show scaling behavior in the course of expansion. The relative intensity of phase fluctuations is estimated from the normalized spectral peak strength and observed to decrease at lower temperatures, confirming the thermal nature of the phase fluctuations. Using the time evolution of the power spectrum, we investigate the relaxation dynamics of a sample prepared in a nonequilibrium state. We will also discuss our recent experiments where we detect thermally activated vortices with clear density-depleted cores and reveal the pairing of vortices in a 2D degenerate Bose gas [2]. \\[4pt] [1] J. Choi, S. W. Seo, W. J. Kwon, and Y. Shin, Phys. Rev. Lett. 109, 125301 (2012).\\[0pt] [2] J. Choi, S. W. Seo and Y. Shin, arXiv:1211.5649 (2012). [Preview Abstract] |
Tuesday, June 4, 2013 3:36PM - 3:48PM |
C1.00009: Equations of state and superfluid transition of a two-dimensional Bose gas. Lauriane Chomaz, R\'emi Desbuquois, Tarik Yefsah, Christof Weitenberg, J\'er\^ome Beugnon, Jean Dalibard Two-dimensional (2D) systems cannot undergo phase transitions associated with continuous symmetry breaking. Nevertheless, they may exhibit a continuous transition to a superfluid phase with quasi-long range order, via the Berezinskii-Kosterlitz-Thouless (BKT) mechanism. I will present our experimental results in which we characterized the thermodynamical and dynamical properties of the 2D gas in the normal, in the critical and in the superfluid regions. Using local density approximation (LDA) and density measurement in a trapped quasi-2D gas, we infer equations of state for the homogeneous 2D gas and compare them to theories. We confirm the predicted scale invariance and identify a critical region in terms of the unique dimensionless parameter $\mu/k_B T$, ratio of the chemical potential $\mu$ on the temperature $T$. We also probe the transport properties of our gases according to $mu/k_B T$ by stirring a micron-sized obstacle on circular trajectories centered on the cloud. We measure the heating generated by the stirring at the radius $r$ and using LDA, deduce the heating behavior of the homogeneous gas at the corresponding chemical potential $\mu(r)$. We identify a transition from a normal dissipative regime to a superfluid frictionless behavior at critical $mu/k_B T |_c$. [Preview Abstract] |
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