Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session U32: Focus Session: Novel Phases in Quantum Gases |
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Sponsoring Units: DAMOP Chair: Charles Clark, National Institute of Standards and Technology Room: Colorado Convention Center 402 |
Thursday, March 8, 2007 8:00AM - 8:12AM |
U32.00001: Cold Atoms on Frustrating Lattices Dagim Tilahun, Allan MacDonald Ultracold atoms in optical lattices undergo a quantum phase transition from a superfluid to a Mott insulator as the lattice potential depth is increased. We present a theory of the ground state and the elementary excitations of cold atoms in which the potential $\Sigma_i$ which induces coherence between different number states on a given site is elevated from a variational parameter to a quantum degree of freedom. In this approach mean-field theory is equivalent to minimizing the energy with respect to the $\Sigma_i$. The theory is applied to the Boson Hubbard model of optical lattice systems, to frustrated lattice models for rotating atoms, and to inhomogenous systems with a harmonic trapping potential superimposed on the lattice potential. [Preview Abstract] |
Thursday, March 8, 2007 8:12AM - 8:24AM |
U32.00002: Atomtronics: Ultracold atom analogs of electronic circuits and devices Ronald Pepino, Brian Seaman, Murray Holland Atomtronics focuses on creating an analogy between electronic devices and circuits with ultracold atoms. Such an analogy can come from the Mott-insulator characteristic of ultracold gases trapped in optical lattices. The highly tunable parameters of optical lattices allow one to construct and precisely manipulate them. This lets one to create conditions that cause atoms in lattices to exhibit the same behavior as electrons moving through solid state media. We present our model and show how the atomtronic diode and the field effect transistor can be realized. These fundamental components can lead to the construction of other atomtronic devices such as the bipolar junction transistor and possibly amplifiers and switches. Besides the similarities to condensed matter systems, there are also differences that can be explored: atomtronic current carriers can be either bosons or fermions having spin not equal to 1/2. Also, there are no thermal fluctuations or phonon modes associated with the lattice itself. [Preview Abstract] |
Thursday, March 8, 2007 8:24AM - 8:36AM |
U32.00003: Reversible quantum phase dispersion in two-component quantum gases A. Widera, S. Trotzky, P. Cheinet, S. F\"olling, F. Gerbier, I. Bloch Controlling fundamental interactions on an atomic scale has offered the unique possibility to engineer strongly correlated quantum states in ultracold atomic samples during recent years. In particular, controlling the interactions of an ensemble of particles implies the possibility of pushing into the intriguing regime of coherent many-body physics. Here we report on the controlled manipulation of a quantum many-body state in a 2D-array of mesoscopic spinor gases. Starting from a coherent spin-state, controllable interatomic interactions close to a Feshbach resonance are used to induce a dynamics which changes the distribution of intrinsic spin fluctuations. The resulting phase dispersion is detected by monitoring the decay of coherence through Ramsey spectroscopy. We demonstrate the coherent nature of this interaction effect by time-reversal of the dynamics, observing a substantial revival of coherence in the system. These results have implications not only on our understanding of decoherence in ultracold atomic systems but also point towards the possibility of dynamically creating correlated spin states or even maximally entangled mesoscopic Schr\"odinger cats. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 9:12AM |
U32.00004: Finite quantal systems -- from semiconductor quantum dots to cold atoms in traps Invited Speaker: Many-body systems that are set rotating may form vortices, characterized by rotating motion around a central cavity. This is familiar to us from every-day life: you can observe vortices while stirring your coffee, or watching a hurricane. In quantum physics, vortices are known to occur in superconducting films and rotating bosonic He-4 or fermionic He-3 liquids, and recently became a hot topic in the research on cold atoms in traps. Here we show that the rotation of trapped particles with a repulsive interaction may lead to vortex formation regardless of whether the particles are bosons or fermions. The exact many-particle wave function provides evidence that the mechanism is very similar in both cases. We discuss the close relation between rotating BECs and quantum dots at strong magnetic fields. The vortices can stick to particles to form composite particles, but also occur without association to any particular particle. In quantum dots we find off-electron vortices that are localized, giving rise to charge deficiency or holes in the density, with rotating currents around them. The vortex formation is observable in the energetics of the system. ``Giant vortices'' may form in anharmonic potentials. Here, the vortices accumulate at the trap center, leading to large cores in the electron and current densities. Turning from single traps to periodic lattices, we comment upon the analogies between optical lattices with cold fermionic atoms, and regular arrays of few-electron quantum dots. Trapping a few (N $<$ 12) fermions in each of the single minima of the lattice, we find that the shell structure in the quantum wells determines the magnetism, leading to a systematic sequence of non-magnetic, ferromagnetic and antiferromagnetic states. \newline M. Toreblad \textit{et al}., Phys. Rev. Lett. 93, 090407 (2004); \newline H. Saarikoski, \textit{et al}., Phys. Rev. Lett. 93, 116802 (2004) , Phys. Rev. B 71, 035421 (2005); \newline M. Manninen, \textit{et al.}, Phys. Rev. Lett. 94, 106405 (2005); \newline E. R\"{a}s\"{a}nen, \textit{et al., }Phys. Rev. B 73, 235324 (2006); \newline M. Koskinen, \textit{et al.}, Phys. Rev. Lett. 90, 066802 (2003) ; \newline K. Karkkainen, \textit{et al.,} to be published (2006) [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U32.00005: Thermal Fluctuations of Vortex Matter in Trapped Bose-Einstein Condensates Steinar Kragset, Egor Babaev, Asle Sudbo We perform Monte Carlo studies of vortices in three dimensions in a cylindrical confinement, with uniform and nonuniform density. The former is relevant to rotating 4He, the latter is relevant to a rotating trapped Bose--Einstein condensate. In the former case we find dominant angular thermal vortex fluctuations close to the cylinder wall. For the latter case, a novel effect is that at low temperatures the vortex solid close to the center of the trap crosses directly over to a tension-less vortex tangle near the edge of the trap. At higher temperatures an intermediate tensionful vortex liquid located between the vortex solid and the vortex tangle, may exist. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U32.00006: Radial and angular rotons in trapped dipolar gases Shai Ronen, Daniele Bortolotti, John Bohn We study Bose-Einstein condensates with purely dipolar interactions in oblate (pancake) traps. We find that the condensate always becomes unstable to collapse when the number of particles is sufficiently large. We analyze the instability, and find that it is the trapped-gas analogue of the ``roton- maxon'' instability previously reported for a gas that is unconfined in two dimensions. In addition, we find that under certain circumstances, the condensate wave function attains a biconcave shape (like a red-blood cell), with its maximum density away from the center of the gas. These biconcave condensates become unstable due to azimuthal excitation - an angular roton. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U32.00007: Cold atoms in time dependent optical lattices I. B. Spielman, B. Brown, P. Lee, N. Lundblad, J. V. Porto, W. D. Phillips Cold atoms in optical lattices provide new avenues for studying iconic condensed matter problems. Using an initially Bose condensed sample of $^87$Rb atoms, we first implement the Bose-Hubbard model (the intensity of the static lattice potential determine the constants in the Bose-Hubbard model). This ``native'' Hamiltonian, with only on-site interactions, exhibits just two phases of matter: insulator and superfluid. Additional phases, such as a supersolid and density wave, are expected when nearest-neighbor interactions are added. Here we show preliminary results where we extend the ``native'' Bose-Hubbard Hamiltonian by rapidly varying the lattice potential. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U32.00008: Topological defects and the 2D superfluid transition in $S=1$ spinor condensates Subroto Mukerjee, Cenke Xu, Joel Moore Condensates of non-zero spin have recently attracted a lot of interest both theoretically and experimetally. The spin degree of freedom can give rise to interesting magnetically ordered phases. This talk will focus on condensates of Spin-1 atoms ($^{23}$Na, $^{87}$Rb). These will be shown to have intresting ground state manifolds and topological defects. The toplogical defects play an important role in the superfluid transition in two dimensions. The low temperature phase of $^{23}$Na will be shown to be a spin disordered nematic superfluid of boson pairs. The superfluid transition is of the Kosterlitz-Thouless type but mediated by half vortices. Extensions of these ideas to higher spin systems will be discussed. Journal Ref: Phys. Rev. Lett. 97, 120406 (2006) [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U32.00009: Kosterlitz-Thouless physics in a one-dimensional optical lattice Anibal Iucci, Miguel A. Cazalilla, Thierry Giamarchi We study a system of quasi two-dimensional Bose gases formed in the nodes of a one-dimensional optical lattice potential. We focus on the effect of the tunneling of the atoms between adjacent planes on the Kosterlitz-Thouless crossover recently observed in the experiments of the Paris group [Z. Hadzibabic \emph{et al}., Nature (London) \textbf{441}, 1118 (2006)]. We compute the contrast of the interference pattern between two condensates, finding a behavior different from the one observed in the Kosterlitz-Thouless crossover. Finally, we consider the stack of a large number of pancakes. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U32.00010: Superfluid, Supersolid, and Phase Separation in Strongly Correlated Square Lattice Bosons Liang Ren, Ziqiang Wang The Bose-Hubbard model and its mapping onto the quantum spin-1/2 XXZ model have played an important role in the understanding of the possible phases of strongly correlated lattice bosons. We present here a different mapping to the quantum spin-1/2 model in the hard-core limit and a mean field solution that accounts for both the direct and the exchange correlations on equal footing. We discuss the possible phase structure as a function of boson density, involving superfluid, supersolid, Neel solid, and phase separation, and make comparisons to the available quantum Monte Carlo simulations. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U32.00011: Exotic Superconducting Phases of Ultracold Atom Mixtures on Triangular Lattices Shan-Wen Tsai, Ludwig Mathey, Antonio H. Castro Neto We study two-dimensional Bose-Fermi mixtures of ultracold atoms on a triangular optical lattice, in the limit when the velocity of bosonic condensate fluctuations is much larger than the Fermi velocity$^1$. Interactions, lattice geometry and frustration lead to a rich phase diagram in this system. Using functional renormalization group techniques we show that this phase diagram contains exotic superconducting and spin-density wave phases. For spin-1/2 fermions on an isotropic lattice we find a competition of $s$-, $p$-, extended $d$-, and $f$-wave symmetry, as well as antiferromagnetic order. For an anisotropic lattice, we further find an extended $p$-wave phase. A Bose-Fermi mixture with spinless fermions on an isotropic lattice shows a competition between $p$- and $f$-wave symmetry.\\ $^1$ L. Mathey, S.-W. Tsai, A.H. Castro Neto, cond-mat/0609212 [Preview Abstract] |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U32.00012: Temperature scale and Adiabatic Processes of Bosons in Optical Lattices Qi Zhou, Tin-Lun Ho We show that as the optical lattice is ramped up adiabatically in a Bose gas, the temperature first decreases in the superfluid regime due to kinetic effects, but eventually increases in the Mott regime due to interaction effects. We also show that in the Mott regime, the density profile of superfluid between Mott steps can be used as a temperature scale. [Preview Abstract] |
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