Bulletin of the American Physical Society
40th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 54, Number 7
Tuesday–Saturday, May 19–23, 2009; Charlottesville, Virginia
Session S3: Optical Lattices II |
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Chair: Dominik Schneble, Stony Brook University Room: Gilmer Hall 190 |
Friday, May 22, 2009 2:00PM - 2:12PM |
S3.00001: Experimental investigation of the Fermi-Hubbard model in a 3D optical lattice of $^6${Li} atoms T.A. Corcovilos, J.M. Hitchcock, P.M. Duarte, R.G. Hulet We present experiments on the three dimensional Fermi-Hubbard model at half-filling in an optical lattice of $^6$Li atoms in an equal mixture of two hyperfine magnetic sublevels. The Fermi-Hubbard model of spins in a lattice predicts magnetically ordered phases and is suspected to support superconductivity in its two-dimensional form. In our experiment, $^6${Li} atoms are cooled to degeneracy in an optical trap and loaded into a simple cubic, far-red detuned optical lattice. By adjusting the $s$-wave scattering length of the atoms (via a magnetic Feshbach resonance) and the depth of the optical lattice, we tune the interaction and hopping terms of the Hubbard hamiltonian. In particular, we search for the Mott insulator phase predicted to occur at low temperature and the antiferromagnetic N\'eel phase predicted to occur at lower temperature and weak interaction strength. An overview of our experimental approach will be presented. In particular, we discuss our methods for cooling the atoms and for detecting the spatial and magnetic ordering of the atoms by Bragg scattering of near-resonant light off of the atoms in the lattice. We also present our progress in achieving the Mott and N\'eel states. [Preview Abstract] |
Friday, May 22, 2009 2:12PM - 2:24PM |
S3.00002: Effects of fermions on the superfluid-insulator phase diagram of the Bose-Hubbard model Roman Lutchyn, Sumanta Tewari, Sankar Das Sarma Building on the work of Fisher et al. [PRB 40, 546 (1989)], we develop a framework for perturbation theory in the Bose-Hubbard model and apply it to calculate the effects of spin-polarized fermions interacting by contact interactions with the constituent bosons. For the single-band Bose-Hubbard model, the only non-trivial effect of the fermions is to induce an effective space- and time-dependent interaction among the bosons. Using a path integral formulation, we develop the appropriate theory describing the perturbative effects of this fermion-mediated interaction on the superfluid-insulator phase diagram. For the single-band Bose-Hubbard model, we find that the net effect of the fermions is to inherently suppress the Mott-insulating lobes and enhance the area occupied by the superfluid phase in the phase diagram. For the more general multi-band Bose-Hubbard model, we find that, in addition to the fermion screening of the boson interactions, the virtual excitations of the bosons to the higher Bloch bands result in an effective increase of the boson on-site repulsion. If this renormalization of the boson on-site potential is dominant over the fermion screening, the area of the Mott insulating lobes of the Bose-Hubbard phase diagram will be enhanced for either sign of the boson-fermion interactions, as seen in recent experiments. [Preview Abstract] |
Friday, May 22, 2009 2:24PM - 2:36PM |
S3.00003: Elastic decay of doublons in the Fermi-Hubbard model Daniel Greif, Niels Strohmaier, Robert Jordens, Leticia Tarruell, Henning Moritz, Tilman Esslinger Investigating the non equilibrium physics of a strongly correlated many-body system is amongst the most challenging tasks in modern physics. In the specific case of the Fermi-Hubbard model, which captures many intriguing phenomena in condensed matter physics such as high-temperature superconductivity, only limited theoretical understanding of the dynamics could be gained so far. Furthermore, coupling to the environment often disguises the main equilibration process. Ultracold atoms in optical lattices offer a very clean approach to study condensed matter systems. We study out-of-equilibrium dynamics of the repulsive Fermi-Hubbard model by exciting particle-hole pairs. The dominant physical mechanism leading to the decay of these doublons is found to be a high order scattering process, where the decay time scales exponentially with the ratio of interaction and kinetic energy. [Preview Abstract] |
Friday, May 22, 2009 2:36PM - 2:48PM |
S3.00004: Direct Observation of Multi-Band Physics using Quantum Phase Diffusion in 3D Optical Lattices Sebastian Will, Thorsten Best, Simon Braun, Ulrich Schneider, Lucia Hackerm\"{u}ller, Dirk-S\"{o}ren L\"{u}hmann, Immanuel Bloch In recent years ultracold atoms in optical lattices have shown their potential to simulate condensed matter quantum systems. A prominent example was the realization of the superfluid to Mott insulator transition, which has theoretically been described by a \emph{single-band} Bose-Hubbard model. Recent theoretical studies, however, have emphasized, that interatomic interactions may bring multi-band effects into play and considerably modify the behaviour of ultracold atomic systems. In our experiment we have trapped a BEC of ${}^{87}$Rb atoms in a 3D optical lattice with minimal underlying harmonic confinement. A rapid increase of the lattice depth allows us to follow the quantum phase diffusion of the macroscopic matterwave field, showing a continuous collapse and revival, whose period is determined by the onsite interaction energy. The observed dynamics give striking evidence of multi-band physics beyond the single-band Hubbard model, our data being in excellent agreement with numerical exact diagonalization. We have extended this experimental method to tunable ${}^{87}$Rb-${}^{40}$K Bose-Fermi mixtures and could elucidate distinct effects of interspecies interactions. [Preview Abstract] |
Friday, May 22, 2009 2:48PM - 3:00PM |
S3.00005: Lattice model for strongly interacting fermions in an optical lattice Jason Kestner, Luming Duan We have numerically treated two distinct fermions across a Feshbach resonance in a few-well trap using a stochastic variational approach with a correlated gaussian wavefunction. We are able to obtain an effective lattice model which reproduces the relevant low-energy physics in the simple cases for which we have performed numerical calculations. However, we argue that this lattice model is, in fact, more generally applicable, and captures the essential physics of a strongly interacting two-component ultracold fermionic gas in an optical lattice with average filling factor less than two. [Preview Abstract] |
Friday, May 22, 2009 3:00PM - 3:12PM |
S3.00006: Effective three-body interactions and decoherence of coherent atom states in optical lattices Philip Johnson, Eite Tiesinga, Trey Porto, Carl Williams We show that effective attractive three-body interactions are generated by the virtual excitations of bosons to higher vibrational states in a three dimensional optical lattice. These processes can quickly decohere non-equilibrium coherent states faster than would be expected from the effects of inhomogenieties, which may explain the surprisingly rapid damping of collapse and revival oscillations seen in some experiments. Using Feshbach resonances, it should be possible to tune the effective three-body interaction strength, creating new opportunities for studying the effects of three-body interactions on the coherence and correlations of both equilibrium and non-equilibrium many-body states. [Preview Abstract] |
Friday, May 22, 2009 3:12PM - 3:24PM |
S3.00007: Soliton in a lattice derived from quantum mechanics Juha Javanainen, Uttam Shrestha Starting from the tenet that measurements are the agent that calls forth nonlinear phenomenology from linear quantum mechanics, we study solitons in an optical lattice carrying a Bose-Einstein condensate. We juxtapose the Bose-Hubbard model and the corresponding classical model. For certain parameter values the stationary lowest-energy state of the classical model is a localized soliton, while the quantum mechanical ground state is translationally invariant. However, if the numbers of the atoms at each lattice site are measured, a distribution of the atoms closely resembling the classical soliton is found. We demonstrate this with a Quantum Monte Carlo (QMC) analysis of the ground state, noting that every sample obtained from our particular QMC method is also a faithful simulation of what a measurement would give for the atom numbers at the lattice sites. [Preview Abstract] |
Friday, May 22, 2009 3:24PM - 3:36PM |
S3.00008: Damping of Dipole Oscillations of a Bose-Einstein Condensate in a Random Potential D. Dries, S.E. Pollack, J. Hitchcock, R.G. Hulet Due to their exquisite controllability, atomic BECs in optical speckle potentials provide unique opportunities to study the interplay of disorder and atomic interactions in a superfluid. We report on the effects of disorder on the collective dipole motion of a BEC of $^7$Li in an optical trap. We observe damping that depends on condensate center of mass velocity $v$, resulting in a non-exponential decay of the oscillation amplitude vs. time. We map out the phase diagram for the damping of the dipole mode as a function of both disorder strength and $v$. The damping peaks at $v\sim c$, where $c$ is the peak speed of sound in the BEC, while for both $v \gg c$ and $v \ll c$, the damping rate tends to zero. By exploiting the extreme tunability of the atomic interactions in $^7$Li, we investigate damping in the regime of weak interactions where $c\rightarrow0$. [Preview Abstract] |
Friday, May 22, 2009 3:36PM - 3:48PM |
S3.00009: BEC in a shaken optical lattice E. Arimondo, D. Ciampini, H. Lignier, O. Morsch, A. Zenesini The formal similarity between matter waves in periodic potential and solid-state physics processes has triggered a large interest on quantum simulation based on Bose/Fermi gases in optical lattices. Here this similarity is extended to matter waves in periodically driven potentials and electrons in oscillating electromagnetic fields. We have demonstrated that the tunneling properties of a Bose-Einstein condensate in spatially ``shaken'' periodic potentials can be precisely controlled. We have taken additional crucial steps towards future applications of this method by proving that the strong shaking of the optical lattice preserves the coherence of the matter wavefunction and that the shaking parameters can be changed adiabatically, even in the presence of interactions. Thus we have reversibly induced the quantum phase transition to the Mott insulator in a driven periodic potential. In addition the exact dynamic localization of a Bose-Einstein condensate in the shaken optical lattice was produce by square-wave forcing. The creation of ``dressed matter waves'' with new properties by shaking a periodic potential appears to be a promising tool to realize band structures for solid-state physics quantum simulations. [Preview Abstract] |
Friday, May 22, 2009 3:48PM - 4:00PM |
S3.00010: Coherent control of Wannier-Stark states via interference between one- and two-phonon excitation Chao Zhuang, Christopher R. Paul, Samansa Maneshi, Luciano S. Cruz, Aephraim M. Steinberg We demonstrate that the control of quantum vibrational states in an optical lattice may be achieved by using interference between two-phonon excitation at $\omega$ and one-phonon excitation at 2$\omega$. We use this technique to improve the ratio of coherent coupling to loss in our system. In our experiment, $^{85}Rb$ atoms are trapped in a vertical optical lattice, leading to a tilted-washboard potential when the effect of gravity is considered. While neighboring Wannier- Stark states may be coherently coupled by sinusoidal drive of the lattice displacement at the secular frequency $\omega$, this also leads to leakage into higher excited states and eventual loss from the lattice. We use coherent control to mitigate this problem, by adding a simultaneous parametric drive at 2$\omega$, directly coupling states of the same parity. The resonant drive corresponds to Raman scattering of laser beams phase-modulated (PM) at $\omega$, while the parametric drive corresponds to Raman scattering of laser beams amplitude-modulated (AM) at 2$\omega$. We demonstrate experimentally that quantum interference between the absorption of two PM quanta and one AM quantum can be used to control the branching ratio, and specifically, to improve the ratio of coherent coupling to loss. [Preview Abstract] |
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