Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session V31: Focus Session: Strongly Interacting Quantum Gases |
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Sponsoring Units: DAMOP Chair: David Pekker, Harvard University Room: E141 |
Thursday, March 18, 2010 8:00AM - 8:36AM |
V31.00001: Metallic and Insulating Phases of Interacting Fermions in a 3D Optical Lattice Invited Speaker: Ultracold fermions in optical lattices are a promising tool to simulate solid state physics, since they represent an ideal and highly tunable implementation of the Hubbard Hamiltonian. A proof of principle is to demonstrate a Mott insulating state, where repulsive interactions between the atoms lead to an insulating behavior in a half-filled conduction band. In our experiments we study repulsively and attractively interacting $^{40}$K atoms within the combination of a red-detuned dipole trap and a blue detuned lattice. This setup allows us to gradually transform the system from metallic to Mott-insulating and band insulating states. We measure the phase of the system by analyzing the system size and the number of doubly occupied sites and compare our findings to DMFT theory. In addition we investigate the dynamical behavior of interacting fermionic mixtures. We prepare a band insulating system and suddenly release it into a homogenous lattice. We detect a symmetric behavior from a ballistic expansion for non-interacting clouds to a strongly suppressed expansion due to the formation of attractively or repulsively bound pairs. This experiment allows us to study transport properties of the Hubbard model. This work was done together with U.Schneider, S. Will, Th. Best, S. Braun, I. Bloch and with theoretical support from T.A. Costi, R.W. Helmes, D. Rasch, A.Rosch, B. Paredes, M. Moreno-Cardoner, T. Kitagawa, E.Demler. [Preview Abstract] |
Thursday, March 18, 2010 8:36AM - 8:48AM |
V31.00002: Equation of State of a Strongly Interacting Atomic Fermi Gas Andre Schirotzek, Ariel Sommer, Mark Ku, Martin Zwierlein We present absorptive in-situ imaging of an ultracold Fermi gas of Li-6 in the unitary regime. The low noise density distribution in an external potential directly reveals the Equation of State under the local density approximation. Regions of low density allow us to extract the chemical potential and the temperature using the virial expansion of the equation of state. The experimental results are compared to recent Monte-Carlo calculations. [Preview Abstract] |
Thursday, March 18, 2010 8:48AM - 9:00AM |
V31.00003: Observing quantum criticality in cold atoms Kaden Hazzard, Erich Mueller At finite temperature, near a quantum critical point, thermodynamic response functions obey scaling relations. These scaling relations hold even in the absence of well defined quasiparticles. Although this structure is hidden in current visualizations of the data, I discuss how this structure can be probed in a cold gas experiment. I calculate the role of finite size effects, showing that a trapped gas can give information about bulk quantum criticality. I consider readily accessible systems, such as bosons in an optical lattice, and discuss both static observables (such as trap profiles) as well as finite-frequency probes such as RF spectroscopy. [Preview Abstract] |
Thursday, March 18, 2010 9:00AM - 9:36AM |
V31.00004: Computations of the Entanglement Entropy Area Law and Strongly Correlated Systems Invited Speaker: |
Thursday, March 18, 2010 9:36AM - 9:48AM |
V31.00005: Ground State Properties of Cold Bosonic Atoms At Large Scattering Lengths Junliang Song, Fei Zhou In this work, we study bosonic atoms at large scattering lengths using a variational method where the condensation amplitude is a variational parameter. We further examine momentum distribution functions, chemical potentials and speed of sound, and spatial density profiles of cold bosonic atoms in a trap in this limit. The later two properties turn out to bear similarities of those of Fermi gases. Estimates obtained here are applicable near Feshbach resonances, particularly when the fraction of atoms forming three-body structures is small and can be tested in future cold atom experiments. [Preview Abstract] |
Thursday, March 18, 2010 9:48AM - 10:00AM |
V31.00006: Repulsive Fermions in the Optical Lattices: Phase separation or Coexistence of Antiferromagnetism and d-Superfluidity? S. Y. Chang, S. Pathak, M. Baranov, P. Zoller, N. Trivedi We investigate the system of the strong and repulsively interacting fermions in a two-dimensional square optical lattice by using a variational quantum Monte Carlo method. We show that the phase diagram at small hole dopings away from the half filling results from the competition of the homogeneous phase with both antiferromagnetic(AF) and d-wave superfluid(SF$_d$) order parameters and the mixed phase of pure AF and SF$_d$. We provide the thermodynamic constraints for the phase separation and the phase coexistence. We also establish a connection to the conventional {\it t-J} model and the resonant fermions in the optical lattice where the multi-band effects can be accounted by an effective model. [Preview Abstract] |
Thursday, March 18, 2010 10:00AM - 10:12AM |
V31.00007: Anomalous suppression of the Bose glass at commensurate fillings in the disordered Bose-Hubbard model Frank Kruger, Jiansheng Wu, Philip Phillips We study the weakly disordered Bose-Hubbard model on a cubic lattice through a renormalization group analysis of the corresponding effective field theory which is explicitly derived by combining a strong-coupling expansion with a replica disorder average. The method is applied not only to generic uncorrelated on-site disorder but also to simultaneous correlated hopping disorder as induced by fine-grained optical speckle potentials in optical lattice experiments. As a result of strong coupling, the strength of the disorder vertex, responsible for the emergence of a Bose glass, crucially depends on the chemical potential and the Hubbard repulsion and vanishes to leading order in the disorder at commensurate boson fillings. As a consequence, at such fillings a direct transition between the Mott-insulator and the superfluid in the presence of disorder cannot be excluded on the basis of a one-loop calculation. At incommensurate fillings, at a certain length scale, the Mott insulator will eventually become unstable towards the formation of a Bose glass. Phase diagrams as a function of the microscopic parameters are presented and the finite-size crossover between the Mott-insulating state and the Bose glass is analyzed. [Preview Abstract] |
Thursday, March 18, 2010 10:12AM - 10:24AM |
V31.00008: Bose - Einstein condensation in quantum glasses Giuseppe Carleo, Marco Tarzia, Francesco Zamponi A ``superglass'' is a phase of matter which is characterized at the same time by superfluidity and a frozen amorphous structure. Latest experimental and numerical evidence of a metastable superglass Helium 4 has stimulated research in the direction of a satisfactory microscopical characterization of this novel phase. I will review a recent theoretical approach to model the problem [Carleo,Tarzia and Zamponi arXiv:0909.2328, in press on PRL], where we have shown that geometrical frustration plays a prominent role in the stabilisation of Bose-Einstein condensation in quantum glasses. I will describe the physical insights offered by a model of strongly interacting bosons on a frustrated lattice, showing the main differences with the otherwise insulating ``Bose glasses.'' The solution of the model by means of recently introduced methods, namely the Quantum Cavity Method and the Canonical Worm Algorithm, will also be discussed. In the last part, I will focus on possible routes to new experiments with cold atoms. [Preview Abstract] |
Thursday, March 18, 2010 10:24AM - 10:36AM |
V31.00009: Instabilities in fermions and BEC mixtures Shan-Wen Tsai, Ryan M. Kalas, Eddy Timmermans We study instabilities in a mixture of interacting fermionic and bosonic ultra-cold atoms. We focus on BCS transitions of the fermions that can be generated from attractive interactions mediated by bosons that are in a BEC phase. We study the p-wave instability [1] for indistinguishable (single spin) fermions in detail, taking into account the dynamical part of the mediated interaction. We employ a functional renormalization-group approach that takes retardation effects into account [2], calculate the renormalized interaction vertices and self-energies for this system, and obtain the phase diagram, sub-dominant instabilities, and transition temperatures, giving estimates for realistic parameters. We also investigate what happens in this system close to the phase-separation transition [3], and explore other possible fermionic phases, including fermion BCS pairings with other pairing symmetries. \\[4pt] [1] D. V. Efremov and L. Viverit, Phys. Rev. B {\bf 65}, 134519 (2002)\\[0pt] [2] S.-W. Tsai {\it et al.}, Phys. Rev. B {\bf 72}, 054531 (2005)\\[0pt] [3] D. H. Santamore and E. Timmermans, Phys. Rev. A {\bf 78}, 013619 (2008) [Preview Abstract] |
Thursday, March 18, 2010 10:36AM - 10:48AM |
V31.00010: Disordered commensurate bosons: 20-year-old conjectures are now theorems Boris Svistunov, Victor Gurarie, Lode Pollet, Nikolay Prokof'ev, Matthias Troyer We prove the theorem of inclusions stating that in the presence of generic, bounded disorder there exist rare, but arbitrarily large, regions of the competing phase across the generic transition line. We argue that the only non-generic phase transitions are the Griffiths-type ones, driven by rare regions in which disorder emulates some regular external perturbation. An immediate implication of the theorem of inclusions is the absence of direct superfluid-to-gapped-insulator quantum phase transitions, in the presence of this type of disorder, no matter how weak. With an enhanced version of the theorem of inclusions we prove finite compressibility (and thus relevance of disorder) on the critical line of superfluid to Bose glass transition. The exceptional role of the Griffiths-type transitions implies that the transition from Mott insulator to any gapless insulator should be inevitably of this type. [Preview Abstract] |
Thursday, March 18, 2010 10:48AM - 11:00AM |
V31.00011: The inverted kagome lattice: frustrated bosons without superexchange Sebastian Huber, Ehud Altman The route to quantum magnetism in ultracold atom systems is obstructed by the difficulties of reaching low enough temperatures of the order of the superexchange coupling $J$. The prospect of simulating the square lattice antiferromagnet, and its expected descendant, the $d$-wave superconductor arouse a lot of current interest. Here we want to show how a frustrated ``quantum magnet'' can be obtained with bosons in an inverted kagome lattice without the need for temperatures that are much smaller than the hopping $t$. We discuss a possible experimental setup and the rich zoo of phases which can be expected in this system: ranging from glassy arrangements of localized states over a coexisting density-wave--superfluid phase to the $\exp(3 i \varphi)$ condensate. [Preview Abstract] |
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