Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session L45: Strongly Correlated Physics with Atoms and Molecules |
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Sponsoring Units: DAMOP Chair: Joseph Thywissen, University of Toronto Room: A310 |
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L45.00001: Quantum phase transitions in a polarized gas of dipolar molecules forming flexible chain Barbara Capogrosso-Sansone, Anatoly Kuklov We numerically demonstrate the formation of quantum flexible chains in a gas of polar molecules confined into a stack of N 1d or 2d optical lattice layers, and with dipole moment aligned perpendicularly to the layers. Molecules interact via dipole-dipole interaction. Ab initio simulations of a single chain pinned at one end reveal quantum roughening transition. Multi-chain ensemble is studied in the J-current model approximation and chain superfluidity (CSF) is found. Increasing density of the chains leads to quantum phase transition from CSF to N-layered molecular superfluid (N-SF). We discuss the nature of this transition and its dependence on density, and the conditions for experimental realization and detection of the chain soup. [Preview Abstract] |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L45.00002: The Hyperfine Molecular Hubbard Hamiltonian Lincoln D. Carr, Michael L. Wall An ultracold gas of heteronuclear alkali-metal dimer molecules with hyperfine structure loaded into a one-dimensional optical lattice is investigated. The hyperfine molecular Hubbard Hamiltonian (HMHH), an effective low-energy lattice Hamiltonian, is derived from first principles [1]. The large permanent electric dipole moment of these molecules gives rise to long-range dipole-dipole forces in a dc electric field and allows for transitions between rotational states in an ac microwave field. Additionally, a strong magnetic field can be used to control the hyperfine degrees of freedom independently of the rotational degrees of freedom. By tuning the angle between the dc electric and magnetic fields and the strength of the ac field, it is possible to control the number of internal states involved in the dynamics as well as the degree of correlation between the spatial and internal degrees of freedom. The HMHH's unique features have direct experimental consequences such as quantum dephasing, tunable complexity, and the dependence of the phase diagram on the molecular state.\\[4pt] [1] M. L. Wall and L. D. Carr, Phys. Rev. A \textbf{82}, 013611 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 2:54PM - 3:06PM |
L45.00003: The Prediction of a Gapless Topological ``Haldane Liquid" Phase in a One-Dimensional Cold Polar Molecular Lattice Jason Kestner, Bin Wang, Jay Sau, Sankar Das Sarma We show that ultracold two-component fermionic dipolar gases in an optical lattice with strong two-body on-site loss can be used to realize a tunable effective spin-one model. Fermion number conservation provides an unusual constraint that $\sum_i \left(S^z_i\right)^2$ is conserved, leading to a novel topological liquid phase in one dimension which can be thought of as the gapless analog of the Haldane gapped phase of a spin- one Heisenberg chain. The properties of this phase are calculated numerically via the infinite time-evolving block decimation method and analytically via a mapping to a one-mode Luttinger liquid with hidden spin information. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L45.00004: Density wave patterns for fermionic dipolar molecules on a square optical lattice: mean field theory analysis Karlis Mikelsons, Jim Freericks We model a system of ultra cold fermionic dipolar molecules on a two dimensional square lattice. Assuming that the molecules are in their nondegenerate hyperfine ground state, and that the dipole moment is polarized perpendicular to the planes, we approximate these molecules as spinless fermions with long range repulsive dipolar interactions. We use mean field theory to obtain the phase diagram as a function of the filling, the strength of interaction and the temperature. We find a number of ordered density wave phases in the system, as well as phase separation between these phases. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L45.00005: Quantum Phases of Atom-Molecule Mixtures of Fermionic Atoms Nicolas Lopez Nicolas Lopez (University of California, Riverside, USA) Chi-Yong Lin (National Dong Hwa University, Taiwan) Shan-Wen Tsai (University of California, Riverside, USA) Cold atom experiments have realized a variety of multicomponent quantum mixtures, including Bose-Fermi atomic mixtures. Mixtures of fermionic atoms and diatomic molecules, which are boson, have also been obtained by tuning of the interactions with external fields [1]. We study many-body correlations in such a system where the molecules are weakly bound and therefore pairs of fermionic atoms easily convert into and dissociate from the bound molecule state and this exchange mediates a long-range interaction between the fermions. We consider a simple many-body Hamiltonian that includes the destruction of fermionic atom pairs to form single bosonic molecules and vice versa [2]. We employ a functional renomalization-group approach and calculate the renormalized frequency-dependent interaction vertices and fermion self-energies. We find an instability from the disordered quantum liquid phase to a BCS phase and calculate the energy scale for the transition. The unusual frequency-dependence of this mediated interaction leads to strong renormalization of the self-energy, and also affects the couplings in the BCS channel. [1] M. Greiner, C. A. Regal, J. T. Stewart, and D. S. Jin, Phys. Rev. Lett. {\bf 94}, 110401 (2005) [2] E. Timmermans, K. Furuya, P. W. Milonni, and A. K. Kerman, Phys. Lett. A {\bf 285}, 228 (2001) [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L45.00006: Stability and Properties of the Polaron Condensate in a Strongly Interacting Boson-Fermion Mixture Zeng-Qiang Yu, Shizhong Zhang, Hui Zhai In this work we study dilute bosons embedded in a single component Fermi sea across a boson-fermion wide Feshbach resonance using a single channel model. The ground state is a condensation of bosonic polarons, and its stability requires that the interaction strength between bosons exceeds a critical value, which is a universal number at boson-fermion resonance and exhibits a maximum in unitary regime. We calculate the condensate fraction and sound velocity across resonance. The transition from polaron condensate to molecular Fermi gas is also discussed. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L45.00007: Colliding clouds of strongly interacting fermions and out-of-phase spin modes Edward Taylor, William Schneider, Shizhong Zhang, Mohit Randeria Motivated by recent experiments at MIT, we consider the problem of what happens when two Fermi clouds prepared in different hyperfine states collide with each other at low velocities close to a Feshbach scattering resonance. Upon coming into contact with each other, we show that the two clouds evolve preferentially into a metastable upper branch (with amplitude given by the coherent quasiparticle residue Z) where interactions are repulsive, and not the ground state lower branch. As a result, even though the underlying interaction between the fermions is attractive, for sufficiently strong interactions in the unitary region, the clouds will ``bounce" off each other. Using Boltzmann, sum rule and hydrodynamic approaches, we make predictions for the frequency of the bounce mode on the BEC side of resonance, including unitarity, where the scattering length is positive. [Preview Abstract] |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L45.00008: Ferromagnetic ordering in two-component Fermi gas: four particle problem ShengQuan Zhou, David Ceperley, Shiwei Zhang To interpret the experiment of Jo et al. on implementing the Stoner model of itinerant ferromagnetism, we investigate the energy spectrum of a system of four interacting spin-half fermions using exact diagonalization on a finite grid. The formation of molecular bound states and the ferromagnetic transition of the excited scattering states are examined systematically as a function of the interaction coupling constant. If the interaction is modeled by an effective positive scattering length, the transition density to ferromagnetism changes significantly. [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L45.00009: Atom-dimer and dimer-dimer scattering in fermionic mixtures near a narrow Feshbach resonance Jesper Levinsen, Dmitry Petrov We develop a diagrammatic approach for solving few-body problems in heteronuclear fermionic mixtures near a narrow interspecies Feshbach resonance. We calculate s-, p-, and d-wave phaseshifts for the scattering of an atom by a weakly-bound dimer. The fermionic statistics of atoms and the composite nature of the dimer lead to a strong angular momentum dependence of the atom-dimer interaction, which manifests itself in a peculiar interference of the scattered s- and p-waves. This effect strengthens with the mass ratio and is remarkably pronounced in 40K-(40K-6Li) atom-dimer collisions. We discuss the collisional relaxation of the dimers to deeply bound states and evaluate the corresponding rate constant as a function of the detuning and collision energy. Finally, we calculate the scattering length for two dimers formed near a narrow interspecies resonance. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L45.00010: Phases of the attractive Hubbard model in a trap Elias Assmann, George Batrouni, Simone Chiesa, Hans Gerd Evertz, Richard Scalettar We present a quantum Monte Carlo study of the fermion attractive Hubbard model in a quadratic trap. A rather dramatic failure of the local density approximation occurs in the half-filled region where coupling to nearby superfluid domains induces a strong suppression of charge fluctuations. By monitoring the behavior of the equal-time pairing correlations, we show the existence of a low temperature phase consistent with quasi-long-range order. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L45.00011: Thermodynamics of the 3D Hubbard model on approach to the Neel transition Lode Pollet, Sebastian Fuchs, Emanuel Gull, Evgeny Burovksi, Evgeny Kozik, Thomas Pruschke, Matthias Troyer We study the thermodynamic properties of the 3D Hubbard model for temperatures down to the N{\'e}el temperature using cluster dynamical mean-field theory. In particular we calculate the energy, entropy, density, double occupancy and nearest-neighbor spin correlations as a function of chemical potential, temperature and repulsion strength. To make contact with cold-gas experiments, we also compute properties of the system subject to an external trap in the local density approximation. We find that an entropy per particle $S/N \approx 0.65(6)$ at $U/t = 8$ is sufficient to achieve a N{\'e}el state in the center of the trap, substantially higher than the entropy required in a homogeneous system. Precursors to antiferromagnetism can clearly be observed in nearest-neighbor spin correlators. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L45.00012: Superconductivity in strongly repulsive fermions: the role of kinetic-energy frustration Leonid Isaev, Gerardo Ortiz, Cristian Batista We discuss a physical mechanism of a non-BCS nature which can stabilize a superconducting state in a {\it strongly repulsive} electronic system. By considering the 2D Hubbard model with spatially modulated electron hoppings, we demonstrate how kinetic-energy frustration can lead to robust d-wave superconductivity at {\it arbitrarily} large on-site repulsion. This phenomenon should be observable in experiments using fermionic atoms, e.g. ${}^{40}K$, in specially prepared optical lattices. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L45.00013: Non-perturbative predictions for cold atom Bose gases with tunable interactions Bogdan Mihaila, Fred Cooper, Chih-Chun Chien, John F. Dawson, Eddy Timmermans We discuss a theoretical description for dilute Bose gases as a loop expansion in terms of composite-field propagators by rewriting the Lagrangian in terms of auxiliary fields related to the normal and anomalous densities. We demonstrate that already in leading order this non-perturbative approach describes a large interval of coupling-constant values, satisfies Goldstone's theorem, yields a Bose-Einstein transition that is second-order, and is consistent with the critical temperature predicted in the weak-coupling limit by the next-to-leading order large-N expansion. [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L45.00014: Density functional theory for fermionic atom gases Matthias Troyer, Ping Nang Ma, Sebastiano Pilati, Xi Dai We will show how Kohn-Sham density-functional theory (DFT), which forms the basis of most electronic structure calculations in material science, can be applied to ultracold atomic gases in optical lattices. We present the derivation of an exchange correlation functional for atomic gases and show first applications within a local spin density approximation. In particular we will show that the local density approximation in DFT is much more accurate than what is commonly referred to as ``local density approximation'' in the atomic gases community. As an outlook we will discuss how the development of DFT for ultracold atomic gases can form a strong link between materials science and atomic physics. [Preview Abstract] |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L45.00015: Using off-diagonal confinement as a cooling method Valery Rousseau, Kalani Hettiarachchilage, Juana Moreno, Mark Jarrell, Dan Sheehy We show that the recently proposed ``off-diagonal confining" (ODC) method (Phys. Rev. Lett. 104, 167201 (2010)) can lead to temperatures that are smaller than with the conventional ``diagonal confining" (DC) method, depending on the control parameters of the system. We determine these parameters using exact diagonalizations for the hard-core case, then we extend our results to the soft-core case by performing quantum Monte Carlo simulations for both DC and ODC systems at fixed temperatures, and analysing the corresponding entropies. [Preview Abstract] |
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