Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session S35: Vortices, Solitons, and Driven Systems |
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Sponsoring Units: DAMOP Chair: Tom Purdy, JILA Room: 702 |
Thursday, March 6, 2014 8:00AM - 8:12AM |
S35.00001: Josephson junction in the double-well potential with a fast-oscillating barrier Aydin Cem Keser, Juraj Radic, Victor Galitski We present an analysis of the Bose gas in a double-well potential with a fast-oscillating barrier. We study the Floquet spectrum of the system and find the effective time-independent Hamiltonian where the tunneling coefficient gets modified due to the periodic driving. The system realizes a Josephson junction with a high control of the tunneling coefficient (the coefficient can now change sign, which is impossible in the stationary double-well potential). We connect the corresponding Josephson equations with equations of motion for Kapitsa's pendulum and study the ways to dynamically stabilize certain states of the system. [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S35.00002: Finite--temperature effects in rotational hysteresis of ring Bose--Einstein condensates N. Murray, C. Lanier, M. Edwards, Y.-H. Wang, C.W. Clark, S. Eckel, F. Jendrzejewski, G.K. Campbell A ring Bose--Einstein condensate (BEC) with zero circulation ($m=0$) stirred by a barrier will eventually jump to an $m=1$ state when stirred faster than a certain critical speed, $\Omega_{c}^{+}$. A ring BEC with $m=1$ will drop to $m=0$ when stirred at a critical speed, $\Omega_{c}^{-}$, which is lower than $\Omega_{c}^{+}$. The loop areas, $\Omega_{c}^{+}-\Omega_{c}^{-}$, of this hysteretic response of the BEC to stirring predicted by zero--temperature Gross--Pitaevskii equation (GPE) disagreed significantly with the results of a recent experiment. In the work reported here, we simulated this experiment with the phenomenologically damped GPE, [S. Choi, S. A. Morgan, and K. Burnett, Phys.\ Rev.\ A {\bf 57}, 4057 (1999)], and with the Zaremba--Nikuni--Griffin (ZNG) theory. The ZNG theory can account for finite--T, non--equilibrium dynamics. We compare the results of these simulations with the experimental data. The simulations show that a vortex/antivortex pair forms in the barrier region during the stirring and that this drives the hysteresis. We also show how the presence of an interacting, thermal cloud affects the dynamics of these pairs. [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S35.00003: Persistent Currents in Bosonic Mixtures in the Ring Geometry Zhigang Wu, Eugene Zaremba We investigate the possibility of bosonic mixtures supporting persistent currents in the ring geometry. Our analysis is based on an approach developed by F. Bloch which focuses on the ground state energy of the condensate as a function of its angular momentum $L$, the so-called yrast spectrum. According to this approach, persistent currents are stable if the energy exhibits a local minimum at some non-zero angular momentum. We extend Bloch's analysis to a two-component mixture containing $N_A$ atoms of species $A$ and $N_B$ atoms of species $B$, with masses $M_A$ and $M_B$, respectively. For the special case of $M_A=M_B$ and equal interaction strengths between all the species, we use analytic soliton solutions of a two-component Bose gas in the ring geometry to analyze the mean-field yrast spectrum of the system. We find that the spectrum exhibits a surprisingly rich structure as a result of an intricate interplay of interparticle interactions and population imbalance. We discuss the implication of these results in regard to the possibility of persistent currents at higher angular momenta. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S35.00004: Oscillation and Instability of a Soliton in Superfluid Atomic Gas Liangsheng Zhang, David Huse We use superfluid hydrodynamics and force equations to phenomenologically investigate the oscillation of a soliton in harmonic trap and the ``snake'' instability of a soliton in a uniform background. The results obtained are functions of missing mass $m_s$ which characterizes the missing number of atoms inside the soliton and a ``mobility'' parameter $C$ which determines the relation between the soliton velocity and the phase difference across it to leading order. It is found that by making $|m_s|$ and $C$ small, the soliton will have a slower oscillation and tend to be more stable, as is seen in recent MIT experiment on the unitary Fermi gas [T. Yefsah, A. T. Sommer, M. J. H. Ku, L. W. Cheuk, W. Ji, W. S. Bakr, and M. W. Zwierlein, Nature 499, 426 (2013)]. We also use the hydrodynamic equations with perturbation theory to approximately solve Gross Pitaevskii equation and then use the solution to test our hydrodynamic approach to oscillation and instability in the case of Bose Einstein condensation with weak interactions. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S35.00005: Bright-like dark solitons and current-phase characteristics of superfluid Bose mixtures near the first-order Mott transition Ippei Danshita, Daisuke Yamamoto, Yasuyuki Kato We consider a superfluid phase of binary Bose mixtures in an optical lattice. It is well known that the superfluid-Mott insulator transition in this system is of first order when the filling factor is even and the inter-species repulsion is smaller than but close to the intra-species repulsion. We show that in the vicinity of the first-order boundaries to the Mott insulators the superfluid order parameters obey the nonlinear Schr\"{o}dinger equation (NLSE) with not only cubic but also quintic nonlinearity. We analytically solve the cubic-quintic NLSE to obtain soliton solutions. In particular, when the superfluid state changes from a ground state to a metastable one, a standard dark soliton turns into a bright-like dark soliton, which has a non-vanishing density dip and no $\pi$ phase kink even in the case of a standing soliton. In the presence of a potential barrier, we find the critical barrier strength above which there is no superfluid solution and unconventional current-phase characteristics, owing to the bright-like dark soliton. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S35.00006: Emergence of Reflectionless Scattering from Linearizations of Physically Relevant Integrable PDEs around Solitons Andrew Koller, Zaijong Hwang, Maxim Olshanii We present four examples of integrable partial differential equations (PDEs) of mathematical physics, that when linearized around a localized stationary solution, exhibit scattering without reflection---at {\it all} energies. Starting from the most well-known and the most empirically relevant phenomenon of the transparency of one-dimensional bright bosonic solitons to Bogoliubov excitations, we proceed to the sine-Gordon, Korteweg-de Vries, and Liouville's equations whose stationary solitons also support our assertion. The proposed connection between integrability and reflectionless scattering seems to span two distinct integrability mechanisms: Lax pairing in the first three cases, and a nonlinear differential map to a linear PDE in the last one. We argue that the transparency shown by linearized integrable PDEs is necessary to ensure that they can support the transparency of stationary solitons at the level of the original nonlinear PDE. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S35.00007: Topological solitons in scalar field theory Aliaksei Halavanau Over last 40 years the topological solitons, the localized, lump-like, finite-energy field configuration which appear in non-linear theories in various space-time dimensions have been intensively studied in various frameworks. We present a numerical study of the process of the kink-antikink collisions in three one-dimensional potential models, such as $\phi^4$ (double well), coupled $\phi^4$ and $\phi^6$ (triple well). We also take into consideration the case of real scalar field in 3 spacial dimensions, where there are simple theories from the Skyrme family with soliton solutions. Different types of field configurations are discussed. Our results reveal new types of soliton solutions in coupled $\phi^4$ model along with new high charge and massive configurations in Faddeev-Skyrme model. Extensive study of $\phi^4$ potential is presented. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S35.00008: Knotting of vortex tangle in three-dimensional random waves Alexander Taylor, Mark Dennis Quantised vortices are fundamental to the description of disordered 3D complex scalar fields such as turbulent superfluids or BECs, but also a wide range of other phenomena including optical volume speckle, the quantum eigenfunctions of chaotic 3D cavities, and liquid crystal phases. These systems all exhibit statistically random large scale vortex tangles that are difficult to describe analytically, but certain properties appear universal despite the physically different origin of complexity. We track vortex tangle in numerical simulations of the random wave model of chaotic eigenfunctions [1], where the waves are linear, but the zeros themselves are very nonlinear features forming a dense tangle of filaments whose geometry and topology we analyse numerically. We observe that while many standard quantities reveal only a common statistical scaling on the large scale, the topology - particularly the occurrence of knots in vortex loops - discriminates between tangles with different origins. In fact, knotting is surprisingly rare when compared to standard random walk models. \newline [1] M V Berry and M R Dennis, \emph{Proc R Soc A} \textbf{456}, 2059-79 (2000) [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S35.00009: Fractional Vortices in $J = 2$ Condensates David Ferguson, James Sauls We consider the possible ground-states and topologically stable line defects in BCS and BEC condensates with total spin $J=2$, including spinor BECs, as well BCS condensates with total angular momentum $J=2$. For cold Fermi gases it may be possible to realize $^{1}D_{2}$ or $^{3}P_{2}$ condensates of BCS pairs described by a symmetric and traceless matrix, $A_{\mu\nu}$, for the $2J+1=5$ complex amplitudes that transform as a rank 2 tensor under joint spin and orbital rotations. Condensates with $J=2$ have a rich phase diagram. We discuss the residual symmetry and fundamental group of $J=2$ condensates exhibiting \emph{complex, bi-axial} order, $A_{\mu\nu}=\Delta\,e^{i\varphi} \left[u_{\mu}u_{\nu}+\epsilon v_{\mu} v_{\nu}+\epsilon^2 w_{\mu} w_{\nu}\right]$, where $\epsilon=e^{i\,2\pi/3}$ and $u,v,w$ are an othogonal triad. This remarkable phase has tetrahedral point symmetry and is described by a non-abelian fundamental group $\pi_{1}(G/H)$. We classify the topologically stable line defects and show that conventional $U(1)$ phase vortices can dissociate into \emph{fractional} vortices with $2 \pi/3$ phase winding combined with tetrahedral rotations, indexed by the conjugacy classes of the non-abelian isotropy subgroup $H$, and consider associated fermionic bound states. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S35.00010: Rotational properties of a rapidly rotating two-component Bose gas Elife Karabulut, Francesc Malet, Georgios Kavoulakis, Stephanie Reimann One of the hallmarks of a superfluid is its response to rotation. Bose-Einstein condensates (BECs) of ultra-cold atoms are ideal systems for exploring this problem. In such systems, the presence and properties of the quantized vortex states are strongly influenced by the form of the confinement. Several experimental and theoretical studies report that confining potentials rising more steeply than quadratically introduce many novel phases, where the picture becomes more interesting in the case of a multi-component BEC. We investigate the rotational properties of a two-component BEC confined in an anharmonic trapping potential using both numerical and analytic methods. More specifically, with the use of a variational approach we derive analytically the phase diagram of the system as a function of the rotational frequency of the trap and of the coupling constant for sufficiently weak values of the anharmonicity and of the coupling. The more general structure of the phase diagram is investigated numerically. We compare our results with the ones of (i) a single-component BEC confined in an anharmonic potential and (ii) a two-component BEC, which is confined in a harmonic trapping potential. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S35.00011: Annular Bose metal from interacting Rashba Bosons Ashvin Vishwanath, Andreas Ruegg There has been much recent interest in realizing ultracold atoms with spin-orbit coupling. Here we study bosons in 2$+$1 dimensions with Rashba spin-orbit interactions. The dispersion minimum occurs along a circle in momentum space that frustrates Bose condensation and raises the possibility of a novel phase. Here we propose a`Bose metal' ground state, for which we construct a wavefunction and evaluate its properties using Variational Monte Carlo calculations. We show that this is an uncondensed state with a hidden Fermi surface and a ring like momentum distribution function -- hence the name annular Bose metal. We also discuss the competition with an ordinary Bose condensate and related states. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S35.00012: Imaging and manipulating effective ferromagnetism in a shaken optical Colin Parker, Li-Chung Ha, Karina Jim{\'e}nez-Garc{\'i}a, Cheng Chin Recentely, we have developed a powerful lattice shaking technique to introduce long-range itinerant ferromagnetic order in cold atomic gases, using only one atomic internal state[1]. By using near-resonant lattice shaking we can engineer a band with two minima, which we label as spin-up and spin-down. Here we extend this technique to shaking in two directions. The resulting band has four minima and thus permits four types of domains, allowing for new possibilities such as ``Y'' or ``X'' type boundaries. I will discuss the prospects for mapping this domain structure, as well as for using a superlattice to manipulate domains. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S35.00013: Non-adiabatic dynamics of strongly paired fermions across a Feshbach resonance Maxim Dzero, Emil Yuzbashyan, Victor Gurarie We present a theory of far-from-equilibrium degenerate Fermi gas interacting through a diatomic Feshbach resonance. The basis of our theory is a two-channel model which describes strongly interacting fermionic and bosonic (molecular) degrees of freedom. We employ integrability of the two-channel model to describe the limiting dynamics of the pairing amplitude as well as the steady state wave function for sudden changes of detuning frequency across the BCS-BEC crossover. In collisionless regime, on a time scale larger then the order parameter relaxation time condensate reaches a steady state. We find the following three steady states for an arbitrary strength of the perturbation: (i) gapless steady state; (ii) steady state with constant value of the order parameter; (iii) steady state with the periodic order parameter and determine the asymptotic behavior of the order parameter in each of these regimes exactly. We also discuss the features of the superfluid steady-state dynamics which would allow experimental verifications of our results. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S35.00014: Quantum kinetic description of attractive two-component fermions from the weak-coupling Fermi liquid to the strong-coupling Bose liquid regime Mehrtash Babadi, Eugene Demler We derive a set of quantum kinetic equations governing the non-equilibrium dynamics of two-component fermions with short-range attractive interactions from the leading order large-N expansion of the effective action of an Sp(N)-symmetric Fermi gas. The derived kinetic equations reduce to the Boltzmann equation describing the evolution of the occupation of fermionic quasiparticles and long-lived composite bosons in the weak- and strong-coupling limits, respectively, while providing a smooth interpolation of the two limits for the intermediate pairing pseudogap regime. The obtained formalism successfully explains the findings of a recent experiment with two-dimensional ultracold Fermi gases. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S35.00015: Ferromagnetic response of a ``high-temperature'' quantum antiferromagnet Xin Wang We study the antiferromagnetic phase of the ionic Hubbard model at finite temperature using dynamical mean-field theory. We find that the ionic potential plays a dual role in determining the antiferromagnetic order. A small ionic potential (compared to the Hubbard repulsion) increases the super-exchange coupling, thereby implying an increase of the Neel temperature of the system, which should facilitate observation of antiferromagnetic ordering experimentally. On the other hand, for large ionic potential, the antiferromagnetic ordering is killed and the system becomes a charge density wave with electron occupancies alternating between 0 and 2. This novel way of degrading antiferromagnetism leads to spin polarization of the low energy single particle density of states. The dynamic response of the system thus mimics ferromagnetic behavior, although the system is still an antiferromagnet in terms of the static spin order [1].\\[4pt] [1] X. Wang, R. Sensarma, and S. Das Sarma, arXiv:1308.1091 [Preview Abstract] |
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