Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session V45: Rotation and Artificial Gauge Fields: Vortices and Quantum Hall Physics |
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Sponsoring Units: DAMOP Chair: Hui Zhai, Tsinghua University Room: A310 |
Thursday, March 24, 2011 8:00AM - 8:12AM |
V45.00001: Acoustic Hawking radiation in dynamically expanding Bose-Einstein condensate Takao Morinari Black holes are not just an absorver but emit radiation. Verification of this phenomenon, called Hawking radiation, for real black holes is almost hopeless because the characteristic temperature is much lower than the cosmic microwave background radiation. There are attempts to verify Hawking radiation physics using acoustic black holes. In this paper, I will present a numerical simulation result for a dynamically expanding Bose-Einstein condensate of cold atoms. The result shows that the radiation spectrum obeys the Planck distribution function with the temperature on the order of 0.1nK and the particle creation occurs near the horizon. I will discuss the result from the view point of the phase coherence of Bose-Einstein condensate. [Preview Abstract] |
Thursday, March 24, 2011 8:12AM - 8:24AM |
V45.00002: Interferometry with Synthetic Gauge Fields Brandon Anderson, Jacob Taylor, Victor Galitski We propose a compact atom interferometry scheme for measuring weak, time-dependent accelerations. Our proposal uses an ensemble of dilute trapped bosons with two internal states that couple to a synthetic gauge field with opposite charges. The trapped gauge field couples spin to momentum to allow time dependent accelerations to be continuously imparted on the internal states. We generalize this system to reduce noise and estimate the sensitivity of such a system to be $S\sim 10^{-7} \frac{\textrm{m} / \textrm{s}^2}{\sqrt{\textrm{Hz}}}$. [Preview Abstract] |
Thursday, March 24, 2011 8:24AM - 8:36AM |
V45.00003: Pulling Fluxes Away from Particles in Quantum Hall States of Atomic Gases Weiran Li, Tin-Lun Ho Quantum Hall states are often described as states with magnetic fluxes attached to the particles. In the case of rapidly rotating atomic gases, we show that by deforming the trapping potential of a rotating cluster, one can in fact pull the fluxes away from the particles in their quantum Hall state, as a consequence of the balance between rotational energy and interaction energy. This phenomenon can be revealed clearly from the density profile of the clusters after releasing the atoms from the trap, as well as from photoassociation experiments which measure the short range correlations. [Preview Abstract] |
Thursday, March 24, 2011 8:36AM - 8:48AM |
V45.00004: Lattice Quantum Hall Effect Layla Hormozi, Gunnar Moller, Steven Simon We study the groundstate of a two-dimensional system of interacting ultra-cold atoms (bosons and fermions), trapped in the periodic potential of an optical lattice, under the influence of a strong synthetic magnetic field. In the absence of inter-particle interactions, the energy spectrum is depicted by the Hofstadter butterfly --- a fractal structure seemingly very different from the Landau levels in the continuum. However, when the number of flux quanta per lattice cell is close to a rational fraction, the energy splittings in the Hofstadter butterfly resemble Landau levels. Inspired by this similarity we establish a mapping between the wavefunctions of the non-interacting system in the lattice near rational fractions and the corresponding wavefunctions in the continuum. Using these single-particle wavefunctions we calculate pseudopotential coefficients for the interacting system. These effective interaction potentials can then be used to construct trial wavefunctions for the groundstate of the interacting system on a lattice. For the case of bosons with contact interaction, in addition to the interaction obtained by Palmer et al. [1], we find anomalous terms in the pseudopotential coefficients resembling the umklapp process.\\[0pt] [1] R. N. Palmer, A. Klein and D. Jaksch, Phys. Rev. A 78, 013609 (2008). [Preview Abstract] |
Thursday, March 24, 2011 8:48AM - 9:00AM |
V45.00005: Fractionalization via $Z_{2}$ Gauge Fields at a Cold Atom Quantum Hall Transition Yafis Barlas, Kun Yang We study a single species of fermionic atoms in an ``effective'' magnetic field at total filling factor $\nu_{f}=1$, interacting through a p-wave Feshbach resonance, and show that the system undergoes a quantum phase transition from a $\nu_{f} =1 $ fermionic integer Quantum Hall state to $\nu_{b} =1/4 $ bosonic fractional Hall state as a function of detuning. The transition is in the $(2+1)$D-Ising universality class. We formulate a dual theory in terms of quasiparticles interacting with a $Z_{2}$ gauge field, and show that charge fractionalization follows from this topological quantum phase transition. The resultant effective theory contains the lattice $Z_{2}$ gauge theory action along with a ``Hopf'' term which encodes the quasiparticle statistics. The transition occurs in the $Z_{2}$ sector and is a confinement-deconfinement transition for the quasiparticles. [Preview Abstract] |
Thursday, March 24, 2011 9:00AM - 9:12AM |
V45.00006: Bogoliubov theory of interacting bosons on a lattice in a synthetic magnetic field Stephen Powell, Ryan Barnett, Rajdeep Sensarma, Sankar Das Sarma We present a theoretical study of the effect of a magnetic field on a bosonic superfluid in a tight-binding lattice, motivated by advances in the synthesis of gauge potentials for ultracold atoms. An analysis based on the magnetic symmetry group shows that the superfluid has simultaneous spatial order, and that this depends on commensuration between the magnetic field and lattice. We predict clear signatures of many-body effects in time-of-flight images, and use a Bogoliubov expansion to calculate quasiparticle spectra that may be measured using Bragg spectroscopy. This work has been supported by JQI-NSF-PFC, ARO-DARPA-OLE, and Atomtronics-ARO-MURI. [Preview Abstract] |
Thursday, March 24, 2011 9:12AM - 9:24AM |
V45.00007: How does a synthetic non-Abelian gauge field influence the bound states of two spin-$1/2$ fermions? Jayantha Vyasanakere, Vijay Shenoy We study the bound states of two spin-$1/2$ fermions interacting via a contact attraction (characterized by the scattering length) in the singlet channel in $3D$ space in presence of a uniform non-Abelian gauge field. The configuration of the gauge field that generates a Rashba type spin-orbit interaction is described by three coupling parameters $(\lambda_x, \lambda_y, \lambda_z)$. For a generic gauge field configuration, the critical scattering length required for the formation of a bound state is {\em negative}, i.e., shifts to the ``BCS side'' of the resonance. Interestingly, we find that there are special high-symmetry configurations (e.g., $\lambda_x = \lambda_y = \lambda_z$) for which there is a two body bound state for {\em any} scattering length however small and negative. Our results show that the BCS-BEC crossover is drastically affected by the presence of a non-Abelian gauge field. We discuss possible experimental signatures of our findings both at high and low temperatures. [Preview Abstract] |
Thursday, March 24, 2011 9:24AM - 9:36AM |
V45.00008: Dynamic vortex unbinding following a quantum quench in bosonic mixtures Ludwig Mathey, Kenneth Guenter, Jean Dalibard, Anatoli Polkovnikov, Charles Clark We study the many-body dynamics of a mixture of two hyperfine states of bosonic atoms in 2D, following a pi/2-pulse. Using both a numerical implementation of the Truncated Wigner approximation and an analytical approach, we find that a dynamic phase transition can be triggered, in which the system relaxes from a superfluid to a disordered state via vortex unbinding. This process can be dynamically suppressed, which creates a long-lived metastable supercritical state. We discuss the realization and detection of these effects. [Preview Abstract] |
Thursday, March 24, 2011 9:36AM - 9:48AM |
V45.00009: Macroscopic superposition states of cold bosons in an asymmetric double well with Orbital Degrees of freedom Miguel-Angel Garcia-March, Lincoln D. Carr We study the dynamics of ultracold bosons in three-dimensional double wells when they are allowed either to condense in single-particle ground states or to occupy excited states. On the one hand, the introduction of second level single-particle states opens a range of new dynamical regimes. On the other, since the second level eigenstates can carry angular momentum, NOON-like macroscopic superposition (MS) states of atoms with non-zero angular momentum can be obtained. This leads to the study of the dynamics of atoms carrying vorticity while tunneling between wells. We obtain new tunneling processes, like vortex hopping and vortex-antivortex pair superposition along with the sloshing of atoms between both wells. The resulting vortex MS states are much more robust against decoherence than the usual NOON states, as all atoms in the vortex core region must be resolved, not just a single atom. [Preview Abstract] |
Thursday, March 24, 2011 9:48AM - 10:00AM |
V45.00010: Optical Lattice Hamiltonians for Relativistic Quantum Electrodynamics Eliot Kapit, Erich Mueller We show how interpenetrating optical lattices containing Bose-Fermi mixtures can be constructed to emulate the thermodynamics of 2+1d quantum electrodynamics (QED3). We present a model of neutral atoms on planar lattices whose low energy effective action reduces to that of photons coupled to Dirac fermions. We overview the properties of QED3 and discuss how two of its most interesting features, chiral symmetry breaking and Chern-Simons physics, could be observed experimentally in our cold atom system. [Preview Abstract] |
Thursday, March 24, 2011 10:00AM - 10:12AM |
V45.00011: Vortex Dynamics and Hall Conductivity of Hard Core Bosons Assa Auerbach, Netanel Lindner, Daniel Arovas Magneto-transport of hard core bosons (HCB) is studied using an XXZ quantum spin model representation, appropriately gauged on the torus to allow for an external magnetic field. We find strong lattice effects near half filling. An effective quantum mechanical description of the vortex degrees of freedom is derived. Using semiclassical and numerical analysis we compute the vortex hopping energy, which at half filling is close to magnitude of the boson hopping energy. The critical quantum melting density of the vortex lattice is estimated at 6.5x10-5 vortices per unit cell. The Hall conductance is computed from the Chern numbers of the low energy eigenstates. At zero temperature, it reverses sign abruptly at half filling. At precisely half filling, all eigenstates are doubly degenerate for any odd number of flux quanta. We prove the exact degeneracies on the torus by constructing an SU(2) algebra of point-group symmetries, associated with the center of vorticity. This result is interpreted as if each vortex carries an internal spin-half degree of freedom ('vspin'), which can manifest itself as a charge density modulation in its core. Our findings suggest interesting experimental implications for vortex motion of cold atoms in optical lattices, and magnet-transport of short coherence length superconductors. [Preview Abstract] |
Thursday, March 24, 2011 10:12AM - 10:24AM |
V45.00012: Birefringent break up of Dirac fermions in a square optical lattice Nazanin Komeilizadeh, Kamran Kaveh, Peter M. Smith, Malcolm P. Kennett We introduce a model of spinless fermions on a square lattice in a spatially periodic magnetic field. This model has Dirac points in its spectrum when there is an average flux of half a flux quantum per plaquette. The dispersion in the vicinity of these Dirac points has the unusual feature that the double degeneracy of Dirac cones is broken. This corresponds to a situation in which the low energy excitations have two different ``speeds of light.'' This is a consequence of broken chiral symmetry in the model, which occurs in the kinetic energy term, and hence leaves the spectrum gapless in the vicinity of the Dirac points. This chiral symmetry breaking is fundamentally different from spontaneous chiral symmetry breaking that leads to mass generation in field theoretic models. We investigate the effects of several perturbations on the spectrum such as staggered potentials, nearest neighbor interactions, and domain wall topological defects. We provide a physical setting in which this model might be realized, namely for fermions in an optical lattice in an artificial magnetic field. [Preview Abstract] |
Thursday, March 24, 2011 10:24AM - 10:36AM |
V45.00013: Generation and Characterization of Free Electron Vortices Benjamin McMorran, Amit Agrawal, Ian Anderson, Gregg Gallatin, Henri Lezec, Jabez McClelland, John Unguris Free electron vortex beams -- composed of electron wavefunctions imprinted with a helical phase -- are remarkable for their unique topology, quantized orbital angular momentum, and magnetic moment. We recently produced free electron vortex beams in a transmission electron microscope (TEM) using nanofabricated diffraction holograms. We used this technique to generate well-defined free electron vortices in various orbital states, demonstrating beams with up to 100 $\hbar $ of orbital angular momentum per electron.The helical phase of the electrons was measured directly using interferometric techniques. The orbital magnetic moment of the electron vortex scales with the topological charge of the vortex, and leads to interesting behavior in magnetic fields. As one example of several immediate applications for the electron vortex beam, we discuss how these beams can provide elementally sensitive magnetic imaging capabilities in a TEM by using the transfer of quantized orbital angular momentum to induce preferred atomic excitations in a sample. [Preview Abstract] |
Thursday, March 24, 2011 10:36AM - 10:48AM |
V45.00014: Vortex structures in ultra-rapidly rotating two-component Bose-Einstein condensates C.-H. Hsueh, I.-G. Liu, S.-C. Gou, W.C. Wu We investigate the vortex structures in rotating two-component Bose-Einstein condensates with a rotating frequency larger than the harmonic trapping frequency. Representative cases for the three phases, miscible, symmetric phase-separated, and asymmetric phase-separated, are studied. It is shown that the three different phases are manifested in the vortex structures to which at the cannular region around the center, vortices of each component form an annular structure and interlace with those of the other component. To determine the vortex structure in an authentic equilibrium state, the result obtained via imaginary-time propagating method is used as the initial state of the stochastic Gross-Pitaevskii equation and one keeps it propagating until the density profile saturates. [Preview Abstract] |
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