Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session Q14: Bosons in Optical Lattices |
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Sponsoring Units: DAMOP Chair: Theja De Silva, State University of New York, Binghamton Room: Morial Convention Center 205 |
Wednesday, March 12, 2008 11:15AM - 11:27AM |
Q14.00001: Control of Decoherence of Many-Body Excitations in a Bose-Einstein Condensate Nir Bar-Gill, Eitan Rowen, Nir Davidson In this work we discuss collective, many-body excitations of a BEC, and their decay into the condensate continuum. We measure the excitation spectrum of a BEC loaded into a 1D optical lattice, and the decoherence of these excitations due to Beliaev damping, as a function of the lattice depth. The cause for decoherence is the Beliaev decay of Bogoliubov quasi-particles, both of which (the excitation and decay products) are observable experimentally. The unique structure of the BEC reservoir, which is composed of a continuum of initially unoccupied quasi-particle modes, results from the quantum interference between the hole and particle amplitudes of these modes. This structure can be modified in a well- controlled manner by adiabatically loading the BEC into and optical lattice. Therefore, by changing the depth of the lattice, we can control the decoherence rate of the excitations. Our experimental results are compared to 1D calculations of the Bloch-Bogoliubov theory. We find that the rate of decay is either enhanced or suppressed as a function of lattice depth. These results can be explained in the framework of the general formula for decay, which also accounts for both the quantum zeno and anti-zeno effects. Such control of the coherence time could allow further studies of dynamics and phase fluctuations of this many-body system. [Preview Abstract] |
Wednesday, March 12, 2008 11:27AM - 11:39AM |
Q14.00002: Investigating Universal Few-body Physics based on Bose-Einstein Condensates of Cesium Atoms in Optical Lattices Nathan Gemelke, Chen-Lung Hung, Xibo Zhang, Cheng Chin We present progress on an experiment designed to investigate universality in few-body systems based on Bose-condensed atoms in optical lattices. In particular, we describe how collapse and revival of matter wave coherence may be used as a sensitive probe for non-binary collisions and correlations. We discuss limitations on the technique due to lattice inhomogeneity and hydrodynamic time-of-flight, and suggest remedies based on Feshbach-mediated control of interactions, and application of spin-echo techniques to matter-wave revival. To reach this goal, we have recently demonstrated Bose-Einstein condensation of $3\times 10^5$ cesium atoms in a crossed-beam dipole trap. A novel trapping geometry is adopted which allows us to smoothly convert the crossed-beam trap to a single layer of two-dimensional (2D) lattices. The single-layer 2D lattice will allow us to directly monitor the density distribution of atoms and identify domains with different atomic populations. [Preview Abstract] |
Wednesday, March 12, 2008 11:39AM - 11:51AM |
Q14.00003: Temperature effects when adiabatically ramping up the optical lattice Lode Pollet, Corinna Kollath, Kris Van Houcke, Matthias Troyer When atoms are loaded into an optical lattice, the process of gradually turning on the lattice is almost adiabatic. We calculate the entropy in the single band Bose- Hubbard model for various densities, interaction strengths and temperatures using Quantum Monte Carlo. This allows us to draw equi-entropy lines in order to study the adiabatic heating when going from the gapless superfluid phase to the gapped Mott phase. We present results in one and two dimensions for homogeneous and trapped systems. We find that current experiments remain in the quantum degenerate regime, and our theory can reproduce experimental time-of-flight images. [Preview Abstract] |
Wednesday, March 12, 2008 11:51AM - 12:03PM |
Q14.00004: Bose-Einstein condensation and superfluidity in optical lattices and periodic porous media; a path integral Monte Carlo study Ali Shams, Henry Glyde We evaluate the Bose-Einstein condensate density and the superfluid fraction of bosons in a periodic external potential using Path-Integral Monte Carlo (PIMC) methods. A unit cell containing a potential well is replicated into a lattice along 1D using periodic boundary conditions. The aim is to describe bosons in a 1D optical lattice or helium confined in a periodic porous medium. The One-Body Density Matrix (OBDM) is evaluated and diagonalized to obtain the single boson natural orbitals (e.g. the condensate orbital) and the occupation of these orbitals (e.g. the condensate fraction). The superfluid density is obtained from the winding number. We investigate (1) the impact of the periodic external potential on the spatial distribution of the condensate, and (2) the correlation between localizing the condensate into separated parts and the loss of superflow along the lattice. For strongly interaction Bosons, as the well depth increases, the condensate becomes depleted in the wells (depletion by interaction). For weakly interacting bosons, as the well depth increases, the BEC is localized at the center of the wells (tight binding). In both cases, the localization of the condensate suppresses superflow leading to a superfluid-insulator cross-over. The temperature dependence is investigated and comparison with Hubbard models and experiment is made. [Preview Abstract] |
Wednesday, March 12, 2008 12:03PM - 12:15PM |
Q14.00005: Superfluid-Insulator and Roughening Transitions in Domain Walls Sebnem Gunes Soyler, Barbara Capogrosso-Sansone, Nikolay Prokof'ev, Boris Svistunov We have investigated superfluid behavior of one and two dimensional interfaces separating solid domains. The system is described by the hard-core Bose-Hubbard Hamiltonian with nearest-neighbor interaction. We present the analysis of superfluid-insulator transition of the interface based on our quntantum Monte Carlo simulations. We also show that, in one dimension the transition is accompanied by the roughening transition, driven by proliferation of charge-1/2 quasiparticles. [Preview Abstract] |
Wednesday, March 12, 2008 12:15PM - 12:27PM |
Q14.00006: Ground state properties of one- and two-component Bose-Hubbard model Barbara Capogrosso-Sansone, Sebnem Gunes Soyler, Nikolay Prokof'ev, Boris Svistunov We report results for the ground state properties of the single component Bose-Hubbard model and preliminary results for the two-component system at total unity filling factor. Our study is based on exact quantum Monte Carlo simulations by worm algorithm. We compare our answers with those of existing analytic calculations based on strong coupling expansion and mean field theory. [Preview Abstract] |
Wednesday, March 12, 2008 12:27PM - 12:39PM |
Q14.00007: Mott-insulator mediated Josephson physics Courtney Lannert, Smitha Vishveshwara We investigate the phenomenon of Josephson tunneling between superfluid regions separated by Mott insulating regions in the context of the Bose-Hubbard model. Such systems can be realized when bosons on a lattice close to a commensurate filling are subject to a random potential or when bosons of sufficiently small hopping strength are trapped in an applied confining potential (i.e. in cold-atomic systems). Using a pseudospin approach valid at small t/U (large interaction strength or small hopping), we derive the equations of motion of the system and show that they lead to Josephson coupling between disjoint superfluid regions. We find expressions for the bulk energy and the Josephson tunneling energy and evaluate them numerically for realistic experimental parameters in a radially-symmetric parabolically-confined cold atom system. [Preview Abstract] |
Wednesday, March 12, 2008 12:39PM - 12:51PM |
Q14.00008: Radio frequency spectra of interacting bosons in an optical lattice trap Kuei Sun, Smitha Vishveshwara, Courtney Lannert We study the inhomogeneous system of interacting bosons in an optical lattice trap. We focus on the weak tunneling region wherein a condensate is predicted to exist between two Mott insulating phases and we consider the effect of applying a radio- frequency (RF) magnetic field in this region. We find that the RF spectrum for driving transitions from one hyperfine species of bosons to another is markedly different between the Mott insulating phase and the condensed phase. In particular, the former has one resonant peak, while the latter has two peaks which show shifts of the order of the tunneling strength between lattice sites. Our results and analyses provide a means of verifying the existence of the condensate. [Preview Abstract] |
Wednesday, March 12, 2008 12:51PM - 1:03PM |
Q14.00009: Bosons with three-body interactions on optical lattices Stefan Wessel Motivated by a recent proposal on using polar molecules in optical lattices driven by microwave fields to induce strong three-body interactions (H. P. B\"uchler et al., Nature Physics 3, 726 (2007)), we study the quantum phase diagram of the boson Hubbard model with nearest neighbor three-body repulsion using quantum Monte Carlo simulations. In particular, we consider the case of a one-dimensional system in the hard-core limit, and assess the nature of the phases that appear in this regime. Our exact numerical results are compared to analytical findings based on a bosonization approach to the same model. Extensions to higher-dimensional systems are mentioned. [Preview Abstract] |
Wednesday, March 12, 2008 1:03PM - 1:15PM |
Q14.00010: Supersolidity from defect-condensation in the extended boson Hubbard model Yu-Chun Chen, Roger G. Melko, Stefan Wessel, Ying-Jer Kao We study the ground state phase diagram of the hard-core extended boson Hubbard model on the square lattice with both nearest- (nn) and next-nearest-neighbor (nnn) hopping and repulsion, using Gutzwiller mean field theory and quantum Monte Carlo simulations. We observe the formation of supersolid states with checkerboard, striped, and quarter-filled crystal structures, when the system is doped away from commensurate fillings. In the striped supersolid phase, a strong anisotropy in the superfluid density is obtained from the simulations; however, the transverse component remains finite, indicating a true two-dimensional superflow. We find that upon doping, the striped supersolid transitions directly into the supersolid with quarter-filled crystal structure, via a first-order stripe melting transition. [Preview Abstract] |
Wednesday, March 12, 2008 1:15PM - 1:27PM |
Q14.00011: Bose Hubbard model in the presence of Ohmic dissipation Denis Dalidovich, Malcolm Kennett We study the zero temperature mean-field phase diagram of the Bose-Hubbard model in the presence of local interactions between the bosons and an external bath. We consider a coupling that conserves the on-site occupation number, preserving the robustness of the Mott and superfluid phases. We show that interaction with the bath shrinks the size of the Mott lobes, leading to superfluidity around the points where $\mu/U$ is integer, even in the absence of hopping between the sites. It also imposes an upper limit on the possible occupation numbers in the Mott phase, $n_{\rm max}$ which is relatively small. We discuss the role that such a bath coupling may play in experiments that probe the formation of the insulator-superfluid shell structure in systems of trapped atoms. [Preview Abstract] |
Wednesday, March 12, 2008 1:27PM - 1:39PM |
Q14.00012: Incommensurate superfluidity of bosons in the optical lattice of double-well potentials Vladimir M. Stojanovic, Congjun Wu, W. Vincent Liu, Sankar Das Sarma We study the first excited band of the Bose-Hubbard model in a double-well optical lattice, a setup recently experimentally realized by a group at NIST. A unique feature of this system is the two lowest bands being far separated from the higher bands, which leads to a greatly reduced phase space for the decay of bosons initially occupying the first excited band. By calculating the parameters of the Bose-Hubbard model based on the nonseparable optical lattice potential used in the NIST experiments, we estimate that in the most favorable situations the lifetime of bosons in the first excited band can be several orders of magnitude longer than the characteristic time scales associated with nearest-neighbor tunneling. An additional novel feature of this system is that the band-minima of the excited band occur at an incommensurate finite crystal momentum, suggesting a new superfluid state of circulating currents that spontaneously breaks the time-reversal, rotational, and translational symmetries. We discuss possible physical consequences of this unconventional state. [Preview Abstract] |
Wednesday, March 12, 2008 1:39PM - 1:51PM |
Q14.00013: Superfluid-insulator transition in Fermi-Bose mixtures and the orthogonality catastrophe Gil Refael, Eugene Demler The superfluid-insulator transition of bosons is strongly modified by the presence of Fermions. Through an imaginary-time path integral approach, we account for the statical as well as the dynamical screening effects of the Fermions on the boson's superfluid transition line. We find that an effect akin to the fermionic orthogonality catastrophy, arising from the fermionic screening fluctuations, suppresses superfluidity. We analyze this effect for various mixture parameters and temperatures, and consider possible signatures of the orthogonality catastrophe effect in other measurables of the mixture. [Preview Abstract] |
Wednesday, March 12, 2008 1:51PM - 2:03PM |
Q14.00014: Novel few- and many-body lattice methods for cold atoms Dean Lee We discuss general methods for measuring scattering phase shifts, spin-orbit coupling, and mixing angles for quantum particles on a lattice. We also present many-body lattice results for ground state properties at unitarity and deviations due to finite S-wave scattering length, S-wave effective range, and P-wave scattering volumes. [Preview Abstract] |
Wednesday, March 12, 2008 2:03PM - 2:15PM |
Q14.00015: Cat state production with ultracold bosons in rotating ring superlattices Andreas Nunnenkamp, Ana Maria Rey, Keith Burnett Ultracold bosons in rotating ring lattices have previously been shown to form cat-like superpositions of different quasi-momentum states. We demonstrate that cat state production in slightly non-uniform ring lattices has several advantages: the energy gap decreases less severely with the number of particles, the sensitivity to detunings from the critical rotation frequency is reduced, and the scheme is not limited to commensurate filling. We show that different quasi-momentum states can be distinguished in time-of-flight absorption imaging and propose to probe cat-like correlations via the many-body oscillations induced by a sudden change in the rotation frequency. [Preview Abstract] |
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