Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session T36: Focus Session: Novel Phases With Cold Atoms, Molecules and Ions |
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Sponsoring Units: DAMOP Chair: Ulrich Schneider, Ludwig Maxmillans Universitat Room: 210B |
Thursday, March 5, 2015 11:15AM - 11:27AM |
T36.00001: Chiral and helical superfluid in cold atom system Xiaohui Li, TingPong Choy, Tai-Kai Ng Recently, a chiral spin superfluid has been proposed in weakly interacting boson systems [1]. In this work, we study the properties of bosonic superfluids in a cold atom system with two inequivalent band minima in momentum space related by time reversal symmetry. The system is mapped into an effective spinor-boson model. Without additional symmetries we show that in general there are two possible phases in this model, a ferromagnetic (easy axis) phase and a ``x-y'' (easy plane) phase. In the presence of nonzero k-space Berry curvature at the two band minima points, we show that the ferromagnetic state is ``chiral'' and the x-y state is ``helical.'' The bulk and edge properties of these states are studied where the similarities and differences between the present bosonic superfluids and the corresponding fermionic superconductors are pointed out.\\[4pt] [1] Li, Xiaopeng and Natu, Stefan S and Paramekanti, Arun and Sarma, S Das. ``Chiral spin superfluidity and spontaneous spin Hall effect of interacting bosons.'' \textit{arXiv preprint arXiv:1405.6715}, 2014. [Preview Abstract] |
Thursday, March 5, 2015 11:27AM - 11:39AM |
T36.00002: Interplay between Kondo screening and local singlets in $SU(N)$-symmetric cold atoms Leonid Isaev, Ana Maria Rey We study collective phenomena in strongly interacting fermionic alkaline-earth atoms (AEAs) loaded in an optical lattice. Owing to the strong decoupling between electronic orbital and nuclear-spin degrees of freedom, AEAs prepared in the two lowest electronic states are predicted to obey an accurate $SU(N > 2 I + 1)$ symmetry in their two-body collisions ($I$ is the nuclear spin). The $SU(N)$ symmetric models offer a great opportunity to generate exotic many-body behavior emerging from the increased degeneracy and strict conservation laws. We focus on a parameter regime that realizes an $SU(N > 2)$ (Coqblin-Schrieffer) generalization of the usual Kondo lattice model, and show that for band fillings above one atom per site, the system exhibits a peculiar interplay between Kondo screening and formation of singlets between localized atoms. In the limit of large Kondo coupling, we derive an effective Hamiltonian and determine its phase diagram. Our results can be tested in experiments with ultracold ${}^{173}{\rm Yb}$ or ${}^{87}{\rm Sr}$ atoms and are relevant for the physics of heavy-fermion materials with magnetic frustration. [Preview Abstract] |
Thursday, March 5, 2015 11:39AM - 11:51AM |
T36.00003: Chiral magnetism and spontaneous spin Hall effect of interacting Bose superfluids Xiaopeng Li, Stefan Natu, Arun Paramekanti Recent experiments on ultracold atoms in optical lattices have synthesized a variety of tunable bands with degenerate double-well structures in momentum space. Such degeneracies in the single particle spectrum strongly enhance quantum fluctuations, and may lead to exotic many-body ground states. We consider weakly interacting spinor Bose gases in such bands, and discover a universal quantum ``order by disorder'' phenomenon which selects a novel chiral spin superfluid with remarkable properties such as spontaneous anomalous spin Hall effect and momentum space antiferromagnetism. For bosons in the excited Dirac band of a hexagonal lattice, such a state supports staggered spin loop currents in real space. We show that Bloch oscillations provide a powerful dynamical route to quantum state preparation of such a chiral spin superfluid. Our predictions can be readily tested in spin resolved time-of-flight experiments. [Preview Abstract] |
Thursday, March 5, 2015 11:51AM - 12:27PM |
T36.00004: Engineering Ferromagnetism with Shaken Optical Lattices Invited Speaker: Colin Parker Conventional methods of quantum simulation rely on kinectic energy determined by free particle dispersions or simple sinusoidal optical lattices. Solid state systems, by contrast, exhibit a plethora of band structures which differ quantitatively, qualitatively, and even topologically. To what extent does this variety explain the many electronic phenomena observed in these materials? Here we address this question by subjecting an otherwise simple Bose superfluid to a customized band structure engineered by dynamically phase modulating (shaking) an optical lattice. The engineered dispersion contains two minima which we associate to a pseudospin degree of freedom. Surprisingly, in such a system the Bose superfluid exhibits many new behaviors. The psuedospin develops a ferromagnetic order, which can lead to polarization of the entire sample or to sub-division into polarized domains. The excitations of the system also exhibit the roton-maxon structure associated with strong interactions in superfluid helium. I will also discuss recent progress on engineering further exotic behavior. [Preview Abstract] |
Thursday, March 5, 2015 12:27PM - 12:39PM |
T36.00005: Weyl Superfluidity in a Three-dimensional Dipolar Fermi Gas Bo Liu, Xiaopeng Li, Lan Yin, W. Vincent Liu Weyl superconductivity or superfluidity, a fascinating topological state of matter, features novel phenomena such as emergent Weyl fermionic excitations and anomalies. Here we report that an anisotropic Weyl superfluid state can arise as a low temperature stable phase in a 3D dipolar Fermi gas. A crucial ingredient of our model is a direction-dependent two-body effective attraction generated by a rotating external field. Experimental signatures are predicted for cold gases in radio-frequency spectroscopy. The finite temperature phase diagram of this system is studied and the transition temperature of the Weyl superfluidity is found to be within the experimental scope for atomic dipolar Fermi gases. Work supported in part by U.S. ARO, AFOSR, DARPA-OLE-ARO, Charles E. Kaufman Foundation and The Pittsburgh Foundation, JQI-NSF-PFC, ARO-Atomtronics-MURI, and NSF of China. [Preview Abstract] |
Thursday, March 5, 2015 12:39PM - 12:51PM |
T36.00006: Spin liquid phases of large spin Mott insulating ultracold atoms Todd C. Rutkowski, Michael J. Lawler Understanding exotic forms of magnetism, primarily those driven by large spin fluctuations such as the quantum spin liquid state, is a major goal of condensed matter physics. But, the relatively small number of viable candidate materials poses a difficulty. We believe this problem can be solved by Mott insulating ultracold atoms with large spin moments that interact via whole-atom exchange. The large spin fluctuations of this exchange could stabilize exotic physics similar to condensed matter systems, all in an extremely tunable environment. We have approached the problem by performing a mean field theory for spin-f bosons in an optical lattice which is exact in the large-f limit. This setting is similar to that of SU(N) magnetism proposed for alkali-earth atoms\footnote{A. V. Gorshkov et al., Nature Phys. \textbf{6}, 289-295 (2010)} but without the SU(N) symmetry. We find that states with long-range order, such as the spin nematic phase of f = 1 Na atoms\footnote{A. Imambekov, M. Lukin, and E. Demler, Phy. Rev. A \textbf{ 68}, 063602 (2003)}, become highly entangled spin-liquid-like states for f = 3 Cr atoms. This is evidence that the magnetic phase diagram for Mott insulating atoms at larger spins generically contains exotic forms of magnetism. [Preview Abstract] |
Thursday, March 5, 2015 12:51PM - 1:03PM |
T36.00007: Superfluid-insulator transition in a fermionic optical superlattice Rubem Mondaini, Predrag Nikolic, Marcos Rigol Despite some of the high-temperature superconductivity properties can be exposed in clean and simple cold-atom systems, attempts to simulate the $d$-wave pairing of cuprates with cold atoms is difficult because the required temperatures are much lower than what is experimentally feasible today. However, the ``pseudogap'' physics of $s$-wave pairing is far more accessible. In this paper we consider a simple Mott insulator of tightly bound Cooper pairs as an $s$-wave analogue of a pseudogap state. The XY transition to a superfluid and the crossover to a band-insulator (conventional unpaired state) in the phase diagram are the phenomena that give this Mott insulator a similar role to the pseudogap of cuprates. We numerically investigate this transition of locally attractive fermions at half-filling and $T = 0$ in the presence of a checkerboard potential in two dimensions, using quantum Monte Carlo and exact diagonalization. We can identify that it belongs to (2+1)-XY universality class similarly to the superfluid-normal transition in hard-core bosons. Moreover, we show a crossover of charge excitations, in finite systems, from a fermionic to bosonic character when the attraction between the fermions is increased. [Preview Abstract] |
Thursday, March 5, 2015 1:03PM - 1:15PM |
T36.00008: Spin Liquid Condensate of Spinful Bosons Biao Lian, Shoucheng Zhang We introduce the concept of a bosonic spin liquid condensate (SLC), where spinful bosons in a lattice form a zero-temperature spin disordered charge condensate that preserves the spin rotation symmetry, but breaks the U($1$) symmetry due to a spinless order parameter with charge one. It has an energy gap to all the spin excitations. We show that such SLC states can be realized in a system of spin $S\ge2$ bosons. In particular, we analyze the SLC phase diagram in the spin $2$ case using a mean-field variational wave function method. We show there is a direct analogy between the SLC and the resonating-valence-bond (RVB) state. The existence of SLC reveals the possible existence of a more general new class of superfluid phases in a lattice. [Preview Abstract] |
Thursday, March 5, 2015 1:15PM - 1:27PM |
T36.00009: Ultracold Molecules in Crystals of Light: A Highly Tunable System for Exploring Novel Materials, Quantum Dynamics, and Quantum Complexity Lincoln Carr, Kenji Maeda, Michael L. Wall Ultracold molecules trapped in optical lattices present a new regime of physical chemistry and a new state of matter: \textit{complex dipolar matter}. Such systems open up the prospect of tunable quantum complexity. We present models for the quantum many-body statics and dynamics of present experiments on polar bi-alkali dimer molecules. We are developing Hamiltonians and simulations for upcoming experiments on dimers beyond the alkali metals, including biologically and chemically important naturally occurring free radicals like the hydroxyl free radical (OH), as well as symmetric top polyatomic molecules like methyl fluoride (CH$_3$F). These systems offer surprising opportunities in modeling and design of new materials. For example, symmetric top polyatomics can be used to study quantum molecular magnets and quantum liquid crystals. We use matrix-product-state (MPS) algorithms, supplemented by exact diagonalization, variational, perturbative, and other approaches. MPS algorithms not only produce experimentally measurable quantum phase diagrams but also explore the dynamical interplay between internal and external degrees of freedom inherent in complex dipolar matter. We maintain open source code (openTEBD and openMPS) available freely and used widely. [Preview Abstract] |
Thursday, March 5, 2015 1:27PM - 1:39PM |
T36.00010: Spontaneous quantum Hall effect in an atomic spinor Bose-Fer mi mixture Zhi-Fang Xu, Xiaopeng Li, Peter Zoller, W. Vincent Liu We study a mixture of spin-1 bosonic and spin-1/2 fermionic cold atoms, e.g., Rb-87 and Li-6,confined in a triangular optical lattice. With fermions at 3/4 filling, Fermi surface nesting leads to spontaneous formation of various spin textures of bosons in the ground state, such as collinear, coplanar and even non-coplanar spin orders. The phase diagram is mapped out with varying boson tunneling and Bose-Fermi interactions. Most significantly, in one non-coplanar state the mixture is found to exhibit spontaneous quantum Hall effect in fermions and crystalline superfluidity in bosons, both driven by interaction. [Preview Abstract] |
Thursday, March 5, 2015 1:39PM - 1:51PM |
T36.00011: A density-functional design of an interaction-driven Chern insulator for an optical lattice system Sota Kitamura, Naoto Tsuji, Hideo Aoki One of the most intriguing proposals for many-body effects in topological systems is a possibility of interaction-induced spontaneous symmetry breaking toward topological insulators, which is sometimes called a ``topological Mott insulator (TMI)''. While TMI has been theoretically examined in various tight-binding models such as a checkerborad lattice, condensed-matter realization of the TMI has yet to come. Here we propose to look for a TMI in cold atoms on optical lattices by exploiting their high controllability, and have actually designed an optical lattice for realizing TMI. One of key ingredients in the TMI is large inter-site interaction, which is usually too small in cold-atom systems with short-ranged interactions. We have resolved this by employing a spin-dependent optical lattice potential. Emergence of TMI is then confirmed from first principles for the system in continuous space by extending the existing density-functional theory for cold atom systems to accommodate non-collinear spin structures inherent in topological phases. Namely, the proposed system does indeed exhibit a phase transition from a semimetal to a Chern insulator over a wide region in the phase diagram against the interaction strength and lattice potential parameters. [Preview Abstract] |
Thursday, March 5, 2015 1:51PM - 2:03PM |
T36.00012: Topological phases with long-range interactions Zhexuan Gong, Anzi Hu, Mohammad Maghrebi, Michael Foss-Feig, Alexey Gorshkov Topological phases of matter, including symmetry protected topological phases, typically require the underlying many-body system to posses only short-range interactions, such that the notion of locality is well defined. Whether various topological phases can survive in the presence of long-range interactions, however, is largely unknown. Here we show that a paradigmatic example of a symmetry protected topological system, known as the spin-1 Haldane chain, surprisingly remains in its topological phase under arbitrary algebraically-decaying long range interactions. Our conclusion is supported by strong numeric evidence using variational Matrix Product State (vMPS) algorithms, and is also consistent with analytical calculations using renormalization group theory. The topological phase of this long-range interacting spin-1 chain should be experimentally realizable in a recently developed trapped-ion quantum simulator [1]. Our work will enable further study of various topological phases under the presence of long-range interactions. \\[4pt] [1] C. Senko, P. Richerme, J. Smith, A. Lee, I. Cohen, A. Retzker, and C. Monroe. ``Experimental Realization of a Quantum Integer-Spin Chain with Controllable Interactions.'' arXiv:1410.0937. [Preview Abstract] |
Thursday, March 5, 2015 2:03PM - 2:15PM |
T36.00013: Stoner ferromagnetism in a thermal pseudospin-1/2 Bose gas Juraj Radic, Stefan Natu, Victor Galitski We compute the finite-temperature phase diagram of a pseudospin-$1/2$ Bose gas with contact interactions, using two complementary methods: the random phase approximation and self-consistent Hartree-Fock theory. We show that the spin-dependent interactions, which break the (pseudo) spin-rotational symmetry, generally lead to the appearance of a magnetically ordered phase at temperatures above the superfluid transition. In three dimensions, we predict a normal easy-axis/easy-plane ferromagnet for sufficiently strong repulsive/attractive inter-species interactions respectively. The normal easy-axis ferromagnet is the bosonic analog of Stoner ferromagnetism known in electronic systems. For the case of inter-species attraction, we also discuss the possibility of a bosonic analogue of the Cooper paired phase. This state is shown to significantly lose in energy to the transverse ferromagnet in three dimensions, but is more energetically competitive in lower dimensions. Extending our calculations to a spin-orbit-coupled Bose gas with equal Rashba and Dresselhaus-type couplings (as recently realized in experiment), we investigate the possibility of stripe ordering in the normal phase. Within our approximations however, we do not find an instability towards stripe formation. [Preview Abstract] |
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