Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session J14: Focus Session: Berezinskii-Kosterlitz-Thouless Regime and Rotating Quantum Gases |
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Sponsoring Units: DAMOP Chair: Hui Zhai, University of California, Berkeley Room: Morial Convention Center 205 |
Tuesday, March 11, 2008 11:15AM - 11:27AM |
J14.00001: Kosterlitz-Thouless Transition in Finite-Size BEC Systems Gary Williams The Kosterlitz-Thouless transition in two-dimensional BEC condensates is calculated taking into account the fact that these are finite-size systems. The outer boundaries of the condensate effectively act as hard walls, and this has a polarizing effect on the vortex pairs. As a consequence the superfluid fraction becomes strongly anisotropic, with the tensor component perpendicular to a wall falling to zero there, while the parallel component remains finite. The decreased pair energy near the walls results in an enhanced vortex density near the boundaries. Possible experiments are proposed to probe the anisotropic properties of the superfluid density, including an unusual sharp dip in the superfluid density that is predicted to occur down the middle of a long superfluid strip. [Preview Abstract] |
Tuesday, March 11, 2008 11:27AM - 11:39AM |
J14.00002: Interference of a stack of coupled two-dimensional BEC pancakes David Pekker, Vladimir Gritsev, Eugene Demler We study the superfluid-normal transition in a stack of Josephson coupled two-dimensional BEC pancakes. Using a combination of the Renormalization Group and the self-consistent harmonic approximation we look at the transition from Kosterlitz-Thouless type behavior to 3D XY type behavior in this finite sized system. We compute the form of the interference patterns that can be observed experimentally if the gas is allowed to expand. In particular, we concentrating on the amplitude modulations in the direction normal to the two-dimensional pancakes, i.e. the direction of fastest expansion. [Preview Abstract] |
Tuesday, March 11, 2008 11:39AM - 11:51AM |
J14.00003: Evolution from BCS to BKT superfluidity in one-dimensional optical lattices Menderes Iskin, Carlos A.R. Sa de Melo We analyze the finite temperature phase diagram of Fermi-Fermi mixtures in one-dimensional optical lattices as a function of fermion-fermion interaction strength. At low temperatures, the Fermi-Fermi mixture evolves from a three-dimensional (3D) Bardeen-Cooper-Schrieffer (BCS) to a two-dimensional (2D) Berezinskii-Kosterlitz-Thouless (BKT) superfluid as the interaction strength increases. We show that the Ginzburg- Landau-Wilson action near the critical temperature is of the Lawrence-Doniach type for all interaction strengths, and explore the phase space of interaction strength versus hopping (or lattice depth) to determine the characteristic line where the behavior of mixture changes from 3D to 2D. Furthermore, we discuss the existence of vortex loop excitations, and how they evolve as a function of interaction strength. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:27PM |
J14.00004: Berezinskii-Kosterlitz-Thouless Crossover in a Trapped Atomic Gas Invited Speaker: Physics of a Bose gas in 2D is quite different from the usual 3D situation. In a homogeneous 2D fluid of identical bosons long-range order is always destroyed by long wavelength thermal fluctuations and this system cannot undergo conventional Bose-Einstein condensation. Nevertheless, it can become superfluid at a finite critical temperature. This phase transition does not involve any symmetry breaking and in the Berezinskii-Kosterlitz-Thouless (BKT) paradigm it is explained in terms of binding and unbinding of pairs of vortices with opposite circulations. Above the critical temperature, proliferation of unbound vortices is expected. Using optical lattice potentials we can create two parallel, independent 2D atomic clouds with similar temperatures and chemical potentials. When the clouds are suddenly released from the trapping potential and allowed to freely expand, they overlap and interfere. This realizes a matter wave heterodyning experiment which gives direct access to several features of the phase distributions in the two planes. Long wavelength phase fluctuations create a smooth and random variation of the interference fringes and free vortices appear as sharp dislocations in the interference pattern. Both the temperature study of these effects and the measurements of the critical point support the BKT picture of the development of quasi-long-range coherence in these systems. \newline \newline [1] Zoran Hadzibabic, Peter Kruger, Marc Cheneau, Baptiste Battelier, and Jean Dalibard, Nature 441, 1118 (2006). \newline [2] Peter Kruger, Zoran Hadzibabic, and Jean Dalibard, Phys. Rev. Lett. 99, 040402 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J14.00005: Density Matrix Renormalization Group study of rapidly-rotating two-dimensional bosons Dmitry Kovrizhin We study a system of two-dimensional bosons with contact interactions in a rapidly-rotating anisotropic trap. The ground state phase diagram and the excitation spectra are investigated using the Density Matrix Renormalization group (DMRG) and the exact diagonalization methods. [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J14.00006: A vortex dipole in a trapped 2D Bose condensate Weibin Li, Masud Haque, Stavros Komineas We study the dynamics and the stationary states of a vortex-antivortex pair in a two-dimensional Bose condensate in a circular trap. The dynamics of this system turns out to be surprisingly complicated, reflecting the nonlinearity of the underlying Gross-Pitaevskii equation. We use a combination of methods --- a time-dependent variational calculation, explicit numerical solutions of the time-dependent and time-independent Gross-Pitaevskii equations, and an exact solution of the non-interacting case --- to uncover the physics of the vortex dipole system. [Preview Abstract] |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J14.00007: Vortex lattices of bosons in deep rotating lattices Daniel Goldbaum, Erich Mueller We study vortex-lattice phases for a Bose gas trapped in a rotating optical lattice near the Mott-Hubbard transition. Unlike the case of shallow lattices, the physics in this regime is dominated by the strong on-site interaction between bosons. We find a series of first-order structural transitions between square lattices where vortices are pinned with their cores on plaquettes/sites. We discuss connections between these vortex structures and the Hofstadter butterfly spectrum of free particles on a rotating lattice. We also investigate vortex configurations in a harmonic trap, where superfluid and Mott phases can coexist in a shell structure. \newline \newline [1] D. Goldbaum and E. Mueller, \emph{Vortex lattices of bosons in deep rotating lattices}, arXiv.org:0710.1090 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J14.00008: Trapped Fermi Gases in Rotating Optical Lattices: Realization and Detection of the Topological Hofstadter Insulator Mehmet Oktel, Hui Zhai, Onur Umucalilar We consider a gas of non-interacting spinless fermions in a rotating optical lattice and calculate the density profile of the gas in an external confinement potential. The density profile exhibits distinct plateaus, which correspond to gaps in the single particle spectrum known as the Hofstadter butterfly. The plateaus result from insulating behavior whenever the Fermi energy lies within a gap. We discuss the necessary conditions to realize the Hofstadter insulator in a cold atom setup and show how the quantized Hall conductance can be measured from density profiles using the Streda formula. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J14.00009: Phase transitions in rotating Bose-Einstein condensates Mahir Hussein, Piet Van Isacker, Klaus Bartschat, Oleg Vorov The transition to the Abrikosov state has been observed in cold atomic gases. Such critical behavior is very sensitive [1] to the interaction between the particles in the condensate [2,3]. We give an analytic description [1] of the phase-transition point and classify the types of the corresponding instabilities that depend on the interaction. This toy model of a continuous phase transition predicts the same behavior patterns for all systems governed by a similar energy functional. [1] O. K. Vorov, P. Van Isacker, M. S. Hussein and K. Bartschat, Phys. Rev. Lett. 95, 230406 (2005). [2] O. K. Vorov, M. S. Hussein and P. Van Isacker, Phys. Rev. Lett. 90, 200402 (2003). [3] O. K. Vorov, P. Van Isacker, M. S. Hussein and K. Bartschat, to be submitted to Nature (2007). [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J14.00010: Schrodinger Leopards in Bose-Einstein Condensates Lincoln D. Carr, Dimitri R. Dounas-Frazer We present the complex quantum dynamics of vortices in Bose-Einstein condensates in a double well via exact diagonalization of a discretized Hamiltonian. When the barrier is high, vortices evolve into macroscopic superposition (NOON) states of a vortex in either well -- a Schrodinger cat with spots. Such {\it Schrodinger leopard} states are more robust than previously proposed NOON states, which only use two single particle modes of the double well potential. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J14.00011: Vortex lattice transitions in cyclic spinor condensates Subroto Mukerjee, Ryan Barnett, Joel Moore We study the energetics of vortices and vortex lattices produced by rotation in the cyclic phase of $F=2$ spinor Bose condensates. In addition to the familiar triangular lattice predicted by Tkachenko for $^4$He, many more complex lattices appear in this system as a result of the spin degree of freedom. In particular, we predict a magnetic-field-driven transition from a triangular lattice to a honeycomb lattice. Other transitions and lattice geometries are driven at constant field by changes in the temperature-dependent ratio of charge and spin stiffnesses, including a transition through an aperiodic vortex structure. [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J14.00012: Condition for the existence of complex modes in a trapped Bose-Einstein condensate with a highly quantized vortex Makoto Mine, Eriko Fukuyama, Masahiko Okumura, Tomoka Sunaga, Yoshiya Yamanaka We consider a trapped Bose-Einstein condensate (BEC) with a highly quantized vortex. For the BEC with a doubly, triply, or quadruply quantized vortex, the numerical calculations have shown that the Bogoliubov-de Gennes equations, which describe the fluctuation of the condensate, have complex eigenvalues. In this talk, we show the analytic expression of the condition for the existence of complex modes, using the method developed by Rossignoli and Kowalski$^1$ for the small coupling constant. To derive it, we make the two-mode approximation. With the derived analytic formula, we can identify the quantum numbers of the complex modes for each winding number of the vortex$^2$. Our result is consistent with those obtained by the numerical calculation in the case that the winding number is two, three, or four. We prove that the complex modes always exist when the condensate has a highly quantized vortex$^2$. \\ $^1$ R. Rossignoli and A. M. Kowalski, Phys. Rev. A {\bf 72}, 032101 (2005). \\ $^2$ E. Fukuyama, M. Mine, M. Okumura, T. Sunaga and Y. Yamanaka, Phys. Rev. A {\bf 76}, 043608 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 2:03PM - 2:15PM |
J14.00013: Control of atomic currents using a quantum stirring device Moritz Hiller, Tsampikos Kottos, Doron Cohen We propose a BEC stirring device which can be regarded as the incorporation of a quantum pump into a closed circuit. It produces a DC circulating atomic current in response to a cyclic adiabatic change of the on-site potentials and the tunneling rates between adjunct sites of an optical trap. We show that the nature of the transport process depends crucially on the sign and on the strength of the interatomic interactions, ranging from a one-by-one transport of atoms (for strong repulsive interaction) to a regime where the particles are are glued together and behave like a huge classical ball that rolls from site to site (for strong attractive interaction). We demonstrate the feasibility of this concept and point out that such device can be utilized in order to probe the interatomic interactions. [Preview Abstract] |
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