Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session C52: Vortices, Rotation and Nonlinear Effects in BECs 
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Sponsoring Units: DAMOP Chair: Stephen Eckel, Joint Quantum Institute, University of Maryland Room: Hilton Baltimore Holiday Ballroom 3 
Monday, March 14, 2016 2:30PM  2:42PM 
C52.00001: Melting of Vortex Lattice in BoseEinstein Condensate in Presence of Disorder Bishwajyoti Dey We study the vortex lattice dynamics in BoseEinstein condensate (BEC) in presence of single impurity as well as random impurities or disorder. The single impurity is modeled by a Gaussian function while disorder is introduced in the system by a uniform random potential. Such potentials can be created experimentally by lasers. We solve the timedependent GrossPitaevskii equation in twodimensions using splitstep CrankNicolson method. We first show that a single vortex can be pinned by an impurity. We then show that even a single impurity can distort the vortex lattice. For sufficiently strong impurity potential, the vortex lattice gets pinned to the impurity. We also show that a new type of giant hole with hidden vortices inside it can be created in the vortex lattice by a cluster of impurities. In presence of random impurity potential or disorder, the vortices get pinned at random positions leading to melting of the vortex lattice. We further show that the vortex lattice melting can also be induced by the pseudorandom potential generated by the superposition of two optical lattices. The absence of longrange order in the melted vortex lattice is demonstrated from the structure factor profile and the histogram of the distance between each pair of vortices. [Preview Abstract] 
Monday, March 14, 2016 2:42PM  2:54PM 
C52.00002: From Vortex Rings to Hopfions in 3d BoseEinstein Condensates Panayotis Kevrekidis In this talk we report a number of recent results on threedimensional topological states. Motivated by our earlier work on vortices, we develop a twofold approach for studying vortex rings. We analytically and numerically explore their emergence through an instability from planar or ring dark soliton states in the small amplitude/weak nonlinearity limit. We also analytically and numerically explore the opposite, particle based limit of large density/large nonlinearity in the ThomasFermi regime. We connect these two analytically tractable limits through detailed numerical computations revealing the spectral and nonlinear stability of such states. We also explore a series of other states, including socalled Hopfions, and darksolitonshells examining both their regimes of stability in 3d atomic BECs, as well as their mechanisms and manifestations of dynamical instabilities. [Preview Abstract] 
Monday, March 14, 2016 2:54PM  3:06PM 
C52.00003: Equation of Motion of a Quantum Vortex. Timothy Cox, Philip Stamp Understanding the motion of vortices in quantum fluids is key to understanding the dynamics of such fluids. The motion of quantum vortices has long been understood in terms of the HallVinenIordanski (HVI) equations. A fully quantum mechanical treatment of vortex motion in a twodimensional Bose superfluid[1] leads to a modified version of the HVI equations which include significant history dependent forces and a fluctuating noise force. The dynamics deviates from that described by the HVI equations when the frequency of motion is higher than the temperature. We describe the consequences of the memory and noise for the motion of a single superfluid vortex as well as the circumstances under which their effects should be experimentally observable. [1] Thompson and Stamp, Phys Rev Lett. 108, 184501 (2012) [Preview Abstract] 
Monday, March 14, 2016 3:06PM  3:18PM 
C52.00004: Helicity in superfluids Hridesh Kedia, Dustin Kleckner, Davide Proment, William T.M. Irvine Ideal fluid flow conserves a special quantity known as helicity, in addition to energy, momentum and angular momentum. Helicity can be understood as a measure of the knottedness of vortex lines of the flow, providing an important geometric tool to study diverse physical systems such as turbulent fluids and plasmas. Since superfluids flow without resistance just like ideal (Euler) fluids, a natural question arises: Is there an extra conserved quantity akin to helicity in superfluids? We address the question of a "superfluid helicity" theoretically and examine its consequences in numerical simulations. [Preview Abstract] 
Monday, March 14, 2016 3:18PM  3:30PM 
C52.00005: Finite temperature and density depletion effects on persistent current state transitions and critical velocity of a toroidal BoseEinstein condensate Avinash Kumar, Stephen Eckel, Fred Jendrzejewski, Gretchen Campbell We study the decay of a persistent, quantized current state in a toroidal geometry. Our experiment involves trapping neutral $^{23}$Na atoms in an all optical ``target trap" shaped potential. This potential consists of a disc surrounded by an annular potential. A current in a superfluid can be sustained only below a critical current. This critical current can be tuned by introducing a density perturbation which depletes the local density. The decay time of a persistent current state can also be controlled by enhancing fluctuations of the system thermally. We study the decay at four different temperatures between 30~nK and 190~nK. For each temperature we record the decay at four different perturbation strengths. We find that increasing the magnitude of the density depletion or the temperature leads to a faster decay, and have seen the decay constant change by over two orders of magnitude. We also studied the size of hysteresis loop between different current states as a function of temperature, allowing us to extract a critical velocity. We find that the discrepancies between the experimentally extracted critical velocity and theoretically calculated critical velocity (using localdensity approximation ) decreases as the temperature is decreased. [Preview Abstract] 
Monday, March 14, 2016 3:30PM  3:42PM 
C52.00006: Finitetemperature energy landscapes in rotating ring BECs Brennan Coheleach, Clayton Heller, Mark Edwards, Steve Eckel, Avinash Kumar, Charles Clark, Gretchen Campbell In a recent experiment conducted at NIST a ring BoseEinstein condensate (BEC) was prepared in a unit angular momentum circulation state. A barrier was then slowly raised and left on for a variable hold time and then turned off. The final circulation of the BEC was studied as a function of hold time and barrier energy height. This procedure was carried out for several wellcharacterized nonzero temperatures. We have studied the energetics of this process under the assumption that a vortex is initially present in the center of the ring BEC and then travels out of the ring through the density notch created by the barrier. We have computed the energy per particle of the condensate system for a variable location of the vortex by solving the timedependent Generalized GrossPitaevskii (GGP) equation in imaginary time. To account for finitetemperature we solved selfconsistently for the condensate fraction as a function of temperature in thermal equilibrium for fixed total particle number. This yielded the noncondensate density which appears in the GGP affecting the energy of the vortex. We also modeled the dynamics of the vortex using the ZNG formalism. [Preview Abstract] 
Monday, March 14, 2016 3:42PM  3:54PM 
C52.00007: Cold atoms in onedimensional rings: a Luttinger liquid approach to precision measurement. Stephen Ragole, Jacob Taylor Recent experiments have realized ring shaped traps for ultracold atoms. We consider the onedimensional limit of these ring systems with a moving weak barrier, such as a bluedetuned laser beam. In this limit, we employ Luttinger liquid theory and find an analogy with the superconducting charge qubit. In particular, we find that stronglyinteracting atoms in such a system could be used for precision rotation sensing. We compare the performance of this new sensor to the state of the art noninteracting atom interferometry. [Preview Abstract] 
Monday, March 14, 2016 3:54PM  4:06PM 
C52.00008: Resonant wavepackets and shock waves in an atomtronic SQUID YiHsieh Wang, A. Kumar, F. Jendrzejewski, Ryan M. Wilson, Mark Edwards, S. Eckel, G. K. Campbell, Charles W. Clark The fundamental dynamics of ultracold atomtronic devices are reflected in their phonon modes of excitation. We probe such a spectrum by applying a harmonically driven potential barrier to a $^{23}$Na BoseEinstein condensate in a ringshaped trap \footnote{YiHsieh Wang, A. Kumar, F. Jendrzejewski, Ryan M. Wilson, Mark Edwards, S. Eckel, G. K. Campbell, and Charles W. Clark, arXiv: 1510.02968 (2015)}. This perturbation excites phonon wavepackets. When excited resonantly, these wavepackets display a regular periodic structure. The resonant frequencies depend upon the particular configuration of the barrier, but are commensurate with the orbital frequency of a Bogoliubov sound wave traveling around the ring. Energy transfer to the condensate over many cycles of the periodic wavepacket motion causes enhanced atom loss from the trap at resonant frequencies. Solutions of the timedependent GrossPitaevskii equation exhibit quantitative agreement with the experimental data. We also observe the generation of supersonic shock waves under conditions of strong excitation, and collisions of two shock wavepackets. [Preview Abstract] 

C52.00009: ABSTRACT WITHDRAWN 
Monday, March 14, 2016 4:18PM  4:30PM 
C52.00010: Transport in a capacitive ultracold atomtronic circuit Mark Edwards, Benjamin Eller, Steve Eckel, Charles Clark A recent NIST experiment~\footnote{J.G.\ Lee, et al., arXiv:1506.08413 (2015)} studied the transport of a gaseous BoseEinstein condensate (BEC) confined in an atomtronic ``dumbbell'' circuit. The optically created condensate potential consisted of a tight harmonic potential in the vertical direction confining the BEC to a horizontial plane. The horizontal potential consisted of two cylindrical wells separated by a channel produced by a harmonic oscillator potential transverse to the line joining the wells. The BEC, formed in the ``source'' well, was released to flow toward the ``drain'' well. The evolution of this system was shown to be reproduced by a model electronic circuit consisting of a charged capacitor, $C$, in series with an inductor, $L$, and a parallel combination of a resistor, $R$, and a Josephson junction. We modeled this system with the GrossPitaevskii (GP) equation and found good agreement with the data provided that the confining potential is carefully reproduced. The GP simulations show behavior, not detectable in the experiment, that atoms can jump out of the dumbbell area after filling up the drain well. We also present the dependence of $R$ and $L$ on the channel shape. [Preview Abstract] 
Monday, March 14, 2016 4:30PM  4:42PM 
C52.00011: Collective modes of trapped BoseEinstein condensates undergoing adiabatic deformation from filledsphere to thinshell geometries Courtney Lannert, Kuei Sun, Karmela Padavi'c, Smitha Vishveshwara Collective modes of a trapped BoseEinstein condensate (BEC) are closely related to the groundstate density profile and are experimentally measurable. They are particularly useful for characterizing a BECs threedimensional structure that cannot be well resolved by the twodimensional absorption imaging. In this context, it is essential to understand the signatures of collective modes of a BEC in various typical geometries and how they change with the geometry. Here, we study a BEC confined in a spherical trap that is tunable to shape the BEC to be a filled sphere, a thin shell, or any crossover stage between them. We employ hydrodynamic treatments and realtime simulations of the GrossPitaevskii equation to obtain the collective modes. We find a set of radial modes that can distinguish a sphere from a shell, and an oscillation frequency dip in a crossover region where the central density becomes low. We also explore the angular modes and find a crucial role of the shell BECs inner boundary, which the sphere BEC lacks. Our findings ought to help future experimental investigations on recently realized BECs in bubbletrap potentials. [Preview Abstract] 
Monday, March 14, 2016 4:42PM  4:54PM 
C52.00012: Gravitational Effects on Collective Modes of Superfluid Shells Karmela Padavi\'c, Kuei Sun, Courtney Lannert, Smitha Vishveshwara We study the effects of gravity on collective excitations of shellshaped BoseEinstein condensates (BECs). Superfluid shells are of general interest as examples of hollow geometries that can be produced in ultracold atoms in bubbletrap potentials or optical lattices. Our approach to analyzing superfluid shells is based on a GrossPitaevskii mean field theory and hydrodynamic equations derived from it. Considering a spherically symmetric BEC in general, there are distinct collective excitation spectra for the cases of a fully filled sphere and a very thin shell. Furthermore, an adiabatic change in the potential producing a slow transition from one geometry to the other shows a characteristic evolution. Given that in most realistic experimental conditions gravity cannot be neglected we investigate its effects on the equilibrium profile and the collective modes in the very thin shell limit. We analytically obtain the full excitation spectrum for the thin shell geometry and account for gravity perturbatively at length and energy scales that describe a stable matterwave bubble. We find that gravity breaks spherical symmetry of the equilibrium density profile and affects the collective excitations by coupling adjacent modes in the angular direction. [Preview Abstract] 
Monday, March 14, 2016 4:54PM  5:06PM 
C52.00013: Classical and quantum dissipation of bright solitons in a bosonic superfluid Dmitry K. Efimkin, Johannes B. Hofmann, Victor Galitski We consider the quantum dissipation of a bright soliton in a quasionedimensional bosonic superfluid. The dissipation appears due to interaction of the soliton with Bogoliubov excitations, which act as a bath for the soliton. Using a collective coordinate approach and the Keldysh formalism, we derive a Langevin equation for the soliton motion which contains both a friction and a stochastic force. We argue that due to the integrability of the original problem, Ohmic friction is absent, rendering the dynamics nonMarkovian. We furthermore show that the resulting friction can be interpreted as the backreaction of Bogoliubov quasiparticles emitted by an accelerating soliton, which represents an analogue of the AbrahamLorentz force known in electrodynamic. [Preview Abstract] 
Monday, March 14, 2016 5:06PM  5:18PM 
C52.00014: Interacting multiple zero mode formulation for a dark soliton in a BoseEinstein condensate Junichi Takahashi, Yusuke Nakamura, Yoshiya Yamanaka The system of BoseEinstein condensate (BEC) has a zeromode (ZM) associated with the spontaneous breakdown of the global phase symmetry. However, to formulate the ZMs in quantum field theory for a finitesize system with spontaneous breakdown of symmetries is not trivial, for in the naive Bogoliubov theory one encounters difficulties such as phase diffusion, the absence of a definite criterion for determining the ground state, and infrared divergences. In order to remove this difficulty, we have recently proposed the new treatment of the ZM, which enable us to introduce a unique ground state in the ZM sector\footnote{ J.~Takahashi, Y.~Nakamura, and Y.~Yamanaka, Phys.~Rev.~A {\bf 92}, 023627 (2015). }. Using this ground state, we have evaluated the quantum fluctuation for the phase of condensate. In this presentation, we consider an atomic BEC system with a dark soliton that contains two ZMs corresponding to spontaneous breakdown of the global phase and translational symmetries. In our treatment, the original nonliner interaction of the field operator brings us the interaction between the two ZMs. We evaluate the standard deviations of the ZM operators and see how the mutual interaction between the two ZMs affects them. [Preview Abstract] 
Monday, March 14, 2016 5:18PM  5:30PM 
C52.00015: Anomalous energetics and dynamics of moving vortices Leo Radzihovsky Motivated by the general problem of moving topological defects in an otherwise ordered state and specifically, by the anomalous dynamics observed in vortexantivortex annihilation and coarsening experiments in freelysuspended smecticC films, I study the deformation, energetics and dynamics of moving vortices in an overdamped xymodel and show that their properties are significantly and qualitatively modified by the motion. [Preview Abstract] 
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