Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session V50: Quantum Gases in Optical Lattices |
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Sponsoring Units: DAMOP Chair: B. Svistunov, University of Massachusetts Amherst Room: Hilton Baltimore Holiday Ballroom 1 |
Thursday, March 17, 2016 2:30PM - 2:42PM |
V50.00001: Phase diagram of ferromagnetic spinor bosons in an optical lattice under an external magnetic field Kohaku H. Z. So, Masahito Ueda Recently, cold atoms with spin degrees of freedom have attracted considerable interest because of the possibility they offer of modelling quantum magnetism and exploring the interplay between spatial and spin degrees of freedom. While spinor bosons with antiferromagnetic interaction loaded in optical lattices have been widely studied in this context because of their properties such as an even-odd effect in the superfluid to Mott-insulator transition, those with ferromagnetic interaction has not been studied extensively. However, mean-field analysis in the continuum systems suggests that the competition between an external magnetic field and the ferromagnetic interaction could give rise to new and rich phases. We have studied ferromagnetic spinor bosons in an optical lattice under an external magnetic field. Using the decoupling mean-field approximation, we have obtained a rich ground-state phase diagram, in which, in addition to the well-known Mott-insulator and superfluid phases, polar and broken-axisymmetry superfluid phases arise. We also found that the transition between broken-axisymmetry superfluid phase and other phases is a first-order one across some part of the phase boundary, in remarkable contrast to the case without external magnetic fields. [Preview Abstract] |
Thursday, March 17, 2016 2:42PM - 2:54PM |
V50.00002: Unconventional Bose-Einstein Condensations of Two-species Bosons in the $p$-orbital Bands in Optical Lattice Jhih-Shih You, I-Kang Liu, Daw-Wei Wang, Shih-Chuan Gou, Congjun Wu We investigate the unconventional Bose-Einstein condensations of two-species mixture with $p$-wave symmetry in the second band of a bipartite optical lattice. Different from the single-species case, the two-species boson mixture exhibits two non-equivalent complex BECs in the intraspecies-interaction-dominating regime, with one breaking time-reversal symmetry while the other not. When the interspecies interaction is tuned across the SU(2) invariant point, the system undergoes a quantum phase transition toward a real-valued checkerboard state characterized by a staggered spin density structure. An experimental scheme for phase measurement is presented. Finally, we will show strong coupling analysis on anti-Hund'srule, Mott-insulating states and the superfluid. [Preview Abstract] |
Thursday, March 17, 2016 2:54PM - 3:06PM |
V50.00003: Phase diagram of strongly attractive p-orbital fermions on optical lattices Theja De Silva We examine a system of doubly degenerate p-orbital polarized fermions on a two-dimensional square lattice with a strong on-site interaction. For strong attractive interactions at the half filling density limit, a four-site square plaquette interaction term is generated from the directional tunneling dependence of p-orbitals. By treating both on-site interaction and the four-site square plaquette interaction term in fourth order perturbation theory, we derive an effective Hamiltonian for the system. We then map the resulting effective particle Hamiltonian into an effective spin-Hamiltonian and study the phase diagram of the system by using a variational mean field approach and a linear spin-wave theory. Further, we discuss the experimental signatures of the resulting phases within the context of current cold-atom experimental techniques. [Preview Abstract] |
Thursday, March 17, 2016 3:06PM - 3:18PM |
V50.00004: Superfluidity of ultracold atomic gases of Fermi-Fermi mixtures on an optical lattice Jibiao Wang, Qijin Chen Superfluidity of ultracold atomic gases of Fermi-Fermi mixtures has been under active investigation recently. Experimentally, mixtures of $^6$Li-$^{40}$K, $^{171}$Yb-$^{173}$Yb and $^6$Li-$^{173}$Yb, for example, have been prepared and cooled down to the quantum degeneracy regime, making the superfluid phase accessible in the near future. In this talk, we will address the superfluidity of ultracold Fermi-Fermi mixtures on 1D through 3D optical lattices, with varying mass and population imbalances and different densities, as they undergo BCS-BEC crossover, within a pairing fluctuation theory which includes self-consistently the important pseudogap effects at finite temperatures. We will present various phase diagrams and show the dramatic combined effects of mass and population imbalances and lattice periodicity. Implications for future experiment will be discussed. References: [1] Q. J. Chen, I. Kosztin, B. Janko, and K. Levin, Phys. Rev. B 59, 7083 (1999). [2] C. -C. Chien, Y. He, Q. J. Chen, and K. Levin, Phys. Rev. A 77, 011601(R) (2008). [3] C. -C. Chien, Q. J. Chen, and K. Levin, Phys. Rev. A 78, 043612 (2008). [4] Q. J. Chen, Phys. Rev. A 86, 023610 (2012). [Preview Abstract] |
Thursday, March 17, 2016 3:18PM - 3:30PM |
V50.00005: Magnetic-field-tunable Kondo effect in alkaline-earth cold atoms Leonid Isaev, Ana Maria Rey We study quantum magnetism and emergent Kondo physics in strongly interacting fermionic alkaline-earth atoms in an optical lattice with two Bloch bands: one localized and one itinerant. For a fully filled narrow band (two atoms per lattice site) we demonstrate that an applied magnetic field provides an efficient control of the ground state degeneracy due to the field-induced crossing of singlet and triplet state of the localized atomic pairs. We exploit this singlet-triplet resonance, as well as magnetically tunable interactions of atoms in different electronic states via the recently-discovered inter-orbital Feshbach resonance, and demonstrate that the system exhibits a magnetic field-induced Kondo phase characterized by delocalization of local singlets and a large Fermi surface. We also determine the phase diagram of the system within an effective low-energy model that incorporates the above magnetic-field effect as well as atomic interactions in the two optical lattice bands. Our results can be tested with ultracold ${}^{173}{\rm Yb}$, and provide a model for the magnetic field-induced heavy-fermion state in filled skutterudites such as ${\rm Pr Os_4 Sb_{12}}$. [Preview Abstract] |
Thursday, March 17, 2016 3:30PM - 3:42PM |
V50.00006: Cooling a Band Insulator with a Metal: Fermionic Superfluid in a Dimerized Holographic Lattice Arijit Haldar, Vijay B. Shenoy A cold atomic realization of a quantum correlated state of many fermions on a lattice, eg.~superfluid, has eluded experimental realization due to the entropy problem. Here we propose a route to realize such a state using holographic lattice and confining potentials. The potentials are designed to produces a \textit{band insulating} state (low heat capacity) at the trap center, and a metallic state (high heat capacity) at the periphery. The metal ``cools'' the central band insulator by extracting out the excess entropy. The central band insulator can be turned into a superfluid by tuning an attractive interaction between the fermions. Crucially, the holographic lattice allows the emergent superfluid to have a \textit{high transition temperature} -- even twice that of the effective trap temperature. The scheme provides a promising route to a laboratory realization of a fermionic lattice superfluid, even while being adaptable to simulate other many body states. Reference: Scientific Reports {\bf 4}, 6665 (2014). [Preview Abstract] |
Thursday, March 17, 2016 3:42PM - 3:54PM |
V50.00007: Dirac and Weyl Rings in Three Dimensional Cold Atom Optical Lattices. Yong Xu, Chuanwei Zhang Recently three dimensional topological quantum materials with gapless energy spectra have attracted considerable interests in many branches of physics. Besides the celebrated example, Dirac and Weyl points which possess gapless point structures in the underlying energy dispersion, the topologically protected gapless spectrum can also occur along a ring, named Dirac and Weyl nodal rings. Ultra-cold atomic gases provide an ideal platform for exploring new topological materials with designed symmetries. However, whether Dirac and Weyl rings can exist in the single-particle spectrum of cold atoms remains elusive. Here we propose a realistic model for realizing Dirac and Weyl rings in the single-particle band dispersion of a cold atom optical lattice. Our scheme is based on previously experimentally already implemented Raman coupling setup for realizing spin-orbit coupling. Without the Zeeman field, the model preserves both pseudo-time-reversal and inversion symmetries, allowing Dirac rings. The Dirac rings split into Weyl rings with a Zeeman field that breaks the pseudo-time-reversal symmetry. We examine the superfluidity of attractive Fermi gases in this model and also find Dirac and Weyl rings in the quasiparticle spectrum. [Preview Abstract] |
Thursday, March 17, 2016 3:54PM - 4:06PM |
V50.00008: The Detection of Massive Goldenstone (Higgs) Mode in Two-Dimensional Ultra-cold Atomic Lattice Systems Kun Chen, Longxiang Liu, Youjin Deng, Manuel Endres, Lode Pollet, Nikolay Prokof'ev We discuss how to reveal the massive Goldstone mode, often referred to as the Higgs amplitude mode, near the Superfluid-to-Insulator quantum critical point (QCP) in a system of two-dimensional ultra-cold bosonic atoms in optical lattices. The spectral function of the amplitude response is obtained by analytic continuation of the kinetic energy correlation function calculated by Monte Carlo methods. Our results enable a direct comparison with the recent experiment [M. Endres, T. Fukuhara, D. Pekker, M. Cheneau, P. Schau{\ss}, C. Gross, E. Demler, S. Kuhr, and I. Bloch, Nature {\bf 487}, 454-458 (2012)], and demonstrate a good agreement for temperature shifts induced by lattice modulation. Based on our numerical analysis, we formulate the necessary conditions in terms of homogeneity, detuning from the QCP and temperature in order to reveal the massive Goldstone resonance peak in spectral functions experimentally. We also propose to apply a local modulation at the trap center to overcome the inhomogeneous broadening caused by the parabolic trap confinement. [Preview Abstract] |
Thursday, March 17, 2016 4:06PM - 4:18PM |
V50.00009: Evolution of the Hofstadter butterfly in a tunable optical lattice Mehmet O. Oktel, Nur Unal, Firat Yilmaz Advances in realizing artificial gauge fields on optical lattices promise experimental detection of topologically non-trivial energy spectra. Self-similar fractal energy structures, known as Hofstadter butterflies, depend sensitively on the geometry of the lattice, as well as the applied magnetic field [1]. The recent demonstration of an adjustable lattice geometry [L. Tarruell et al., Nature 483, 302 (2012)] presents a unique opportunity to study this dependence. We calculate the Hofstadter butterflies that can be obtained in such an adjustable lattice and find three qualitatively different regimes. We show that the existence of Dirac points at zero magnetic field does not imply the topological equivalence of spectra at finite field. As the real-space structure evolves from the checkerboard to the honeycomb lattice, two square lattice Hofstadter butterflies merge to form a honeycomb lattice butterfly in a topologically non-trivial way, as it is accomplished by sequential closing of infinitely many gaps. We discuss the evolution of topological properties with underlying lattice geometry by calculating the Chern numbers and comment on the validity of simulating graphene in such an adjustable lattice. [1] F. Yilmaz, F. Nur Unal, and M. O. Oktel, Phys. Rev. A 91, 063628 (2015). [Preview Abstract] |
Thursday, March 17, 2016 4:18PM - 4:30PM |
V50.00010: Proposals for quantum simulating simple lattice gauge theory models using optical lattices Jin Zhang, Judah Unmuth-Yockey, Alexei Bazavov, Yannick Meurice, Shan-Wen Tsai We derive an effective spin Hamiltonian for the (1$+$1)-dimensional Abelian Higgs model in the strongly coupled region by integrating out the link variables. With finite spin truncations, the Hamiltonian can be matched with a 1-dimensional two-species Bose Hubbard model in the strong-coupling limit that can be implemented with cold atoms on an optical lattice. We study the phase diagram of the original Abelian Higgs model with Monte Carlo simulation and Tensor Renormalization Group methods. The results show a crossover line which terminates near the Kosterlitz-Thouless transition point. The effective quantum Hamiltonian is also studied with the DMRG method, and we find that they have a similar behavior. We discuss practical experimental implementations for our quantum simulator. Species-dependent optical lattices and ladder systems with double-well potentials are considered. We show how to obtain each of the interaction parameters required in the Bose-Hubbard model that we obtained, and confirm the possibility of tuning these interactions to the region in which our mapping is valid. We emphasize that this proposal for quantum simulating a gauge theory uses a manifestly gauge-invariant formulation and Gauss's Law is therefore automatically satisfied. [Preview Abstract] |
Thursday, March 17, 2016 4:30PM - 4:42PM |
V50.00011: Topological Nodal-Line Superfluid in Spin-Orbit Coupled Cold Atomic Systems Wen-Yu He, Dong-Hui Xu, Tong Zhou, K. T. Law Topological nodal line superconductivity or superfluidity is a fascinating topological gapless phase which hosts bulk Weyl ring degeneracy in the quasiparticle excitation spectrum and supports Majorana zero bound modes with a large density of states at the edge. In this work, based on the experimental realized 1D spin orbit coupling, we show the emergence of topological nodal line superfluid phase in Fermionic atoms trapped in 3D cubic optical lattice when the s wave pairing field is introduced through Feshbach resonance between the two atomic hyperfine spin states. The nodal line degeneracy is further found to evolve into Weyl nodes once another component of spin orbit coupling field enters to break the chiral symmetry. The momentum resolved radio frequency spectroscopy is suggested to manifest the topological nodal line superfluid phase. [Preview Abstract] |
Thursday, March 17, 2016 4:42PM - 4:54PM |
V50.00012: Vacancy dynamics in the paramagnetic environment Johan Carlstrom, Boris Svistunov, Nikolay Prokofiev We consider the motion of a single vacancy in a magnetic spin-1/2 lattice. Examples of realisations of this model include vacancies in a He3 solid or holes in the t-J model with J=0. This motion is ballistic when spins exhibit ferromagnetic order, with $<|r(t)|>$ growing linearly with time. However in the disordered paramagnet, the motion of the vacancy becomes highly nontrivial, and no exact solution to this outstanding problem is known. This scenario is known as the Brinkman-Rice problem. We report here numerical results for the spatial probability distribution of the vacancy obtained by integrating the time evolution operator through Monte Carlo and averaging over a large number of random spin realisations. We find highly counterintuitive results, with the probability of finding the vacancy at a given site oscillating in time. These results can be verified in experiments with ultra-cold fermions in optical lattice at a temperature high enough that the spins are disordered. [Preview Abstract] |
Thursday, March 17, 2016 4:54PM - 5:06PM |
V50.00013: Scratched-XY Universality and Phase Diagram of Disordered 1D Bosons in Optical Lattice Zhiyuan Yao, Lode Pollet, Nikolay Prokof'ev, Boris Svistunov The superfluid-insulator quantum phase transition in a 1D system with weak links belongs to the so-called scratched-XY universality class, provided the irrenormalizable exponent $\zeta$ characterizing the distribution of weak links is smaller than $2/3$. With a combination of worm-algorithm Monte Carlo simulations and asymptotically exact analytics, we accurately trace the position of the scratched-XY critical line on the ground-state phase diagram of bosonic Hubbard model at unity filling. In particular, we reveal the location of the tricritical point separating the scratched-XY criticality from the Giamarchi-Schulz one. [Preview Abstract] |
Thursday, March 17, 2016 5:06PM - 5:18PM |
V50.00014: Application of the DMRG in two dimensions: a parallel tempering algorithm Shijie Hu, Jize Zhao, Xuefeng Zhang, Sebastian Eggert The Density Matrix Renormalization Group (DMRG) is known to be a powerful algorithm for treating one-dimensional systems. When the DMRG is applied in two dimensions, however, the convergence becomes much less reliable and typically "metastable states" may appear, which are unfortunately quite robust even when keeping a very high number of DMRG states. To overcome this problem we have now successfully developed a parallel tempering DMRG algorithm. Similar to parallel tempering in quantum Monte Carlo, this algorithm allows the systematic switching of DMRG states between different model parameters, which is very efficient for solving convergence problems. Using this method we have figured out the phase diagram of the xxz model on the anisotropic triangular lattice which can be realized by hardcore bosons in optical lattices. [Preview Abstract] |
Thursday, March 17, 2016 5:18PM - 5:30PM |
V50.00015: Many--body physics of ultracold doublet sigma molecules in optical lattices Gavriil Shchedrin, Daniel Jaschke, Wei Han, Lincoln Carr, Dermot G. Green, Jesus Aldegunde, Jeremy M. Hutson The creation of ultracold polar molecules provides a unique opportunity to discover and explore new regimes in strongly interacting many--body quantum systems. Polar molecules have strong long--range dipole--dipole interactions that allow one to realize exotic phenomena such as topological phases and quantum magnetism. We explore quantum many--body systems formed by molecules in doublet sigma ($^{2}\Sigma$) states, with both electric dipole moments and electron spin $S=1/2$, but without electronic orbital momentum. The Hamiltonian for doublet sigma molecules includes molecular rotation terms, spin--rotation interaction, hyperfine terms including both spin--spin and nuclear electric quadrupole interactions, and molecular dipole--dipole interactions. The complete control of the molecular quantum states can be accomplished by applying electric and magnetic fields to molecules trapped in optical lattices. We provide the complete theoretical treatment for experimentally relevant doublet sigma molecules such as SrF and CaF and discuss the associated single--body and many--body physics. [Preview Abstract] |
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