Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session A45: Focus Session: Exploring Quantum Phases in Cold Atom Systems |
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Sponsoring Units: DAMOP Chair: Congjun Wu, University of California, San Diego Room: A310 |
Monday, March 21, 2011 8:00AM - 8:12AM |
A45.00001: Correlated phases of bosons in tilted, frustrated lattices Susanne Pielawa, Takuya Kitagawa, Erez Berg, Subir Sachdev The search for correlated quantum phases of cold atoms in optical lattices has focused mainly on entangling the spin degrees of freedom on different lattice sites. We show that there are also rich possibilities for correlated phases in the density sector, and these are likely to be readily accessible by tilting Mott insulators into metastable states. It has been previously shown that a Mott insulator in a potential gradient undergoes an Ising quantum phase transition when the potential drop per lattice spacing is close to the repulsive interaction energy [1]. Here we theoretically study bosons in tilted, frustrated, two-dimensional lattices. The phases we find include phases with charge density order, a sliding Luttinger liquid phase, and a liquid-like ground state with no broken lattice symmetry. \\[4pt] [1] S. Sachdev, K. Sengupta, and S. M. Girvin, Phys. Rev. B 66, 075128 (2002). [Preview Abstract] |
Monday, March 21, 2011 8:12AM - 8:24AM |
A45.00002: Pure Mott Phases in a Trapped 2D Hubbard Model Dave Cone, Valy Rousseau, Simone Chiesa, Richard Scalettar, George Batrouni In this talk, we report on Quantum Monte Carlo simulations of a Hubbard Hamiltonian which incorporates a proposed new method for confining ultracold atoms in an optical lattice. Termed ``Off Diagonal Confinement (ODC),'' this method employs an inhomogeneous array of hopping matrix elements which traps atoms by going to zero at the lattice edges. In contrast, the more conventional diagonal confinement(DC) trap uses a parabolic potential coupled to (diagonal) density operators. ODC has the advantage of producing systems which, while still being inhomogeneous, are entirely in the Mott phase. This makes the insulating behavior and associated antiferromagnetism more apparent, and also allows simulations which are free of the sign problem at low temperatures. We analyze the effects of using different ODC traps and compare results with those from DC traps, for density, spin, and pairing correlation functions, as well as entropy and temperature profiles. Finally, we will discuss the advantages and importance of this new confinement technique for modeling correlated systems, including the potential for reaching lower temperature scales by following constant entropy curves. [Preview Abstract] |
Monday, March 21, 2011 8:24AM - 8:36AM |
A45.00003: Exploring classical and quantum criticality in two-dimensional quantum gases Chen-Lung Hung, Xibo Zhang, Lichung Ha, Shihkuang Tung, Nathan Gemelke, Cheng Chin Continuous phase transitions in two dimensions (2D) are expected to exhibit intriguing universal behaviors near the critical point. Prominent examples include the Berezinsky-Kosterlitz-Thouless (BKT) transition and the superfluid (SF) to Mott insulator (MI) transition described by the Bose-Hubbard model. Both transitions are investigated in our system based on ultracold Bose gases confined in a pancake-like optical trap with or without an optical lattice potential. In this talk, we will present a study of the universal behavior near the BKT transition by probing the density profiles and their fluctuations at various temperatures and atomic interaction strengths. We report the observation of global scale-invariance and universality in scaled thermodynamic observables. Our measurement agrees with the classical field theoretical prediction as well as the Monte Carlo calculations, and shows growing density-density correlations in the critical regime. Further extensions of this work, including exploration of quantum criticality near the SF-MI phase boundary, will be discussed. [Preview Abstract] |
Monday, March 21, 2011 8:36AM - 9:12AM |
A45.00004: Fermions in Optical Lattices: Cooling Protocol to Observe Anti-ferromagnetism Invited Speaker: Experiments on ultracold atoms in optical lattices have the potential of probing the complex phase diagrams arising from simple Hamiltonians. One of the most challenging problems for an optical lattice emulator is that of cooling fermions to observe interesting broken symmetry phases. In this talk I will discuss recent theoretical progress on this question for the simplest model of interacting fermions: the Hubbard model. We determine the equation of state, the density $\rho(\mu,T,U/t)$, and the entropy of the 3D repulsive Hubbard model using exact determinental Quantum Monte Carlo (QMC) simulations. Using the local density approximation (LDA), we calculate the spatial variation of density, entropy density, double-occupancy, local compressibility and local spin correlations for different trap curvatures and interaction strengths $U/t$. In contrast to a homogeneous system, we show that in a trap we can locally squeeze out the entropy from certain regions and observe antiferromagnetic order, even though the total entropy per particle in the cloud is quite high. We show that significant cooling due to entropy redistribution in the trap can be achieved by two mechanisms: (a) by increasing the lattice depth, and (b) by decompressing the cloud. Our calculations can be an important guide in the race to observe antiferromagnetic order in optical lattices. [Preview Abstract] |
Monday, March 21, 2011 9:12AM - 9:24AM |
A45.00005: Criticality in Trapped Atomic Systems Nikolay Prokofiev, Boris Svistunov, Lode Pollet We discuss generic limits posed by the trap in atomic systems on the accurate determination of critical parameters for second-order phase transitions, from which we deduce optimal protocols to extract them. We show that under current experimental conditions the in-situ density profiles are barely suitable for an accurate study of critical points in the strongly correlated regime. Contrary to recent claims, the proper analysis of time-of-fight images yields critical parameters accurately. L. POllet, N. Prokof'ev, and B. Svistunov, Phys. Rev. Lett. 104, 245705 (2010). [Preview Abstract] |
Monday, March 21, 2011 9:24AM - 9:36AM |
A45.00006: Inter-band coupling induced novel condensates in a double-well lattice Qi Zhou, James V. Porto, Sankar Das Sarma We predict novel inter-band physics for bosons in a double-well lattice. An intrinsic coupling between the s and px band due to interaction gives rise to larger Mott regions on the phase diagram at even fillings than the ones at odd fillings. On the other hand, the ground state can form various types of condensates, including a mixture of single-particle condensates of both bands, a mixture of a single-particle condensate of one band and a pair-condensate of the other band, and a pair-condensate composed of one particle from one band and one hole from the other band. The predicted phenomena should be observable in current experiments on double-well optical lattices. [Preview Abstract] |
Monday, March 21, 2011 9:36AM - 9:48AM |
A45.00007: Unconventional Bose-Einstein condensation in high orbital bands Congjun Wu, Zi Cai, Andreas Hemmerich We perform the theoretical study on unconventional Bose-Einstein condensations (UBEC) in higher orbital bands of optical lattices observed by Hemmerich's group. These exotic states of bosons are non-zero condensation wavevectors, and thus beyond the ``no-node'' paradigm. We have studied various effects on UBECs including lattice asymmetry and interactions. The interplay between the kinetic and interaction energies gives rise to two different UBECs with the real and complex-valued condensation wavefunctions, respectively. The latter spontaneously breaks time-reversal symmetry, which is impossible in usual BEC systems. [Preview Abstract] |
Monday, March 21, 2011 9:48AM - 10:00AM |
A45.00008: Topological semimetal: a probable new state of quantum optical lattice gases protected by D$_4$ symmetry Kai Sun, W. Vincent Liu, S. Das Sarma We demonstrate that a novel topological semimetal emerges as a parity-protected critical theory for fermionic atoms loaded in the $p$ and $d$ orbital bands of a two-dimensional optical lattice. The new quantum state is characterized by a parabolic band-degeneracy point with Berry flux $2 \pi$, in sharp contrast to the $\pi$ flux of Dirac points as in graphene. We prove that this topological liquid is a universal property for all lattices of D$_4$ point group symmetry and the band degeneracy is protected by odd parity. Turning on interparticle repulsive interaction, the system undergoes a phase transition to a topological insulator, whose experimental signature includes chiral gapless domain-wall modes, reminiscent of quantum Hall edge states. [Preview Abstract] |
Monday, March 21, 2011 10:00AM - 10:12AM |
A45.00009: Unconventional superfluidity with non-collinear orbital order Hubert Nguyen, Zi Cai, Congjun Wu We propose an unconventional superfluid with spontaneous time-reversal symmetry breaking in p-orbital bands of cubic optical lattices. We find that in contrast to the square lattice which exhibits an antiferromagnetic orbital angular momentum (OAM), quantum fluctuations in the cubic lattice select an exotic superfluid state with non-collinear orderings of OAM moments. The collective excitations and phase transitions in this unconventional superfluid have also been discussed. This exotic superfluid state has no counterpart in solid state systems. [Preview Abstract] |
Monday, March 21, 2011 10:12AM - 10:24AM |
A45.00010: Two distinct Mott-Insulator to Bose-glass transitions and breakdown of self averaging in the disordered Bose-Hubbard model Seungmin Hong, Frank Kruger, Philip Phillips We show that two fixed points govern the Mott insulator to Bose glass transition in the disordered Bose-Hubbard model. At incommensurate fillings, the correlation length and the inverse compressibility diverge with exponents of $\nu=1/D$ and $\gamma=4/D-1$, respectively, $D$ the spatial dimension. We show that it is the breakdown of self-averaging (rare-region Griffiths physics) in the Bose glass that leads to a violation of the bound $\nu\ge 2/D$. At commensurate fillings, the transition is controlled by a different fixed point at which both the disorder and interaction vertices are relevant. [Preview Abstract] |
Monday, March 21, 2011 10:24AM - 10:36AM |
A45.00011: Unbalanced fermion mixtures on an optical lattice Chuntai Shi, Tun Wang, Shan-Wen Tsai We study a two component fermion mixture on a square lattice. We describe such system by a Hubbard model wherein there is only on-site interaction between fermions of different species. Such a model can be realized by loading ultra cold fermions onto an optical lattice and by tuning the interaction strength via Feshbach resonance. We investigate the phase diagram of this system near half filling using the functional renormalization group approach for interacting fermions[1]. We focus on the interesting case where one species is at half filling so that their Fermi surface is nested while the other species is slightly doped so that their Fermi surface is not perfectly nested. We study both the cases with repulsive interaction and the cases with attractive interaction. For the attractive interaction, triplet pairing BCS instability among majority species compete with the singlet s-wave inter-species BCS pairing instability when the populations of two species are different. For the repulsive interaction, fermions with equal population are known to display d-wave singlet BCS pairing when both species are slightly doped away from half filling. When only one species of fermions is doped away from half filling, such d-wave instability is weakened while triplet pairing among majority species becomes possible. \\[0pt] [1]. R. Shankar, Rev. Mod. Phys. 66, 129 (1994). [Preview Abstract] |
Monday, March 21, 2011 10:36AM - 10:48AM |
A45.00012: Bosonic models with Fermi-liquid kinematics: realizations and properties Paul Goldbart, Sarang Gopalakrishnan, Austen Lamacraft We consider models of interacting bosons in which the single-particle kinetic energy achieves its minimum on a surface in momentum space. The kinematics of such models resembles that resulting from Pauli blocking in Fermi liquids; therefore, Shankar's renormalization-group treatment of Fermi liquids [1] can be adapted to investigate phase transitions in these bosonic systems. We explore possible experimental realizations of such models in cold atomic gases: e.g., via spin-orbit coupling [2], multimode-cavity-mediated interactions [3], and Cooper pairing of Fermi gases in spin-dependent lattices. We address the phase structure and critical behavior of the resulting models within the framework of Ref. [1], focusing in particular on Bose-Einstein condensation and on quantum versions of the Brazovskii transition from a superfluid to a supersolid [3]. \\[4pt] [1] R. Shankar, Rev. Mod. Phys. 66, 129-192 (1994) \\[0pt] [2] C. Wang et al., Phys. Rev. Lett. 105, 160403 (2010) \\[0pt] [3] S. Gopalakrishnan, B.L. Lev, and P.M. Goldbart, Nat. Phys. 5, 845-850 (2009) [Preview Abstract] |
Monday, March 21, 2011 10:48AM - 11:00AM |
A45.00013: Pairing and crystallization of one-dimensional atomic mixtures with mass imbalance Tommaso Roscilde, Marcello Dalmonte, Cristian Degli Esposti Boschi We numerically investigate mass-imbalanced binary mixtures of hardcore bosons (or equivalently of fermions) loaded in one-dimensional optical lattices, with special focus on their instabilities towards the loss of first-order (one-body) coherence. We find a fundamental asymmetry between attractive and repulsive interactions. Attraction is found to always lead to pairing, and to pair crystallization for very strong mass imbalance and commensurate fillings. In the repulsive case away from half filling the two atomic components remain instead decoupled (and first-order coherent) over a large parameter range, and undergo crystallization or phase separation only for large mass-imbalance and/or strong interactions. This fundamental asymmetry is at odds with recent theoretical predictions, and can be tested directly via time-of-flight experiments on trapped cold atoms. [Preview Abstract] |
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