Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session B36: Focus Session: Artificial Gauge Fields and Spin Orbit Coupling in Cold Atoms |
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Sponsoring Units: DAMOP Chair: Martin Zwierlein, Massachusetts Institute of Technology Room: 211 |
Monday, March 2, 2015 11:15AM - 11:51AM |
B36.00001: Ultracold atoms in strong synthetic magnetic fields Invited Speaker: Wolfgang Ketterle The Harper Hofstadter Hamiltonian describes charged particles in the lowest band of a lattice at high magnetic fields. This Hamiltonian can be realized with ultracold atoms using laser assisted tunneling which imprints the same phase into the wavefunction of neutral atoms as a magnetic field dose for electrons. I will describe our observation of a bosonic superfluid in a magnetic field with half a flux quantum per lattice unit cell, and discuss new possibilities for implementing spin-orbit coupling. Work done in collaboration with C.J. Kennedy, G.A. Siviloglou, H. Miyake, W.C. Burton, and Woo Chang Chung. [Preview Abstract] |
Monday, March 2, 2015 11:51AM - 12:03PM |
B36.00002: Emergence of Quantum Liquid Crystals of Bosons in Kagome Lattices with Synthetic Gauge Fields Guanyu Zhu, Jens Koch, Ivar Martin We consider a family of tight-binding models based on a kagome lattice with local synthetic gauge flux, which have a lowest flat band in the single particle spectrum. The flat band is spanned by eigenstates forming localized loops on the lattice, with the maximally compact loop states typically breaking the discrete rotational symmetry of the lattice. When populated by locally-interacting particles, the close packing of such maximally compact loop states leads to a nematic loop crystal ground state. If the particles are bosons, we show that mean field theory predicts that increasing filling beyond the close packing filling fraction leads to the formation of quantum liquid crystals including a nematic supersolid and a nematic superfluid phase with broken lattice rotation and $U(1)$ symmetry. [Preview Abstract] |
Monday, March 2, 2015 12:03PM - 12:15PM |
B36.00003: Strongly correlated atoms in artificial gauge fields Ciar\'an Hickey, Pratik Rath, Arun Paramekanti We study ultracold spinor atomic gases in an optical lattice in the presence of artificial gauge fields and a strong Hubbard repulsion. Using a combination of strong coupling approaches and novel numerical techniques, we explore exotic magnetic ground states and their thermal phase transitions induced by the interplay of momentum space topology and real space strong correlation effects. [Preview Abstract] |
Monday, March 2, 2015 12:15PM - 12:27PM |
B36.00004: Realization of BEC on Cylindrical Surfaces with a Landau Gauge Biao Huang, Tin-Lun Ho Landau's famous solution of 2D electron gas on a cylindrical surface with the Landau gauge is one of the most important paradigm in condensed matter physics. Here, we point out the ways to create the Bose analog of this paradigm and discuss the property of a BEC in this setting. The synthetic ``magnetic field'' normal to the cylindrical surface is created through the Berry's phase effects of bosons with hyperfine spins S. As the strength of synthetic field increases, the vortex pattern on the surface undergo a sequence of transitions. These vortex patterns are very different from the triangular lattice array in rotating gases. They have dramatic signatures in time of flight measurements and can be revealed easily. [Preview Abstract] |
Monday, March 2, 2015 12:27PM - 12:39PM |
B36.00005: Synthetic magnetic fields in strained graphene-like optical lattices Binbin Tian, Manuel Endres, David Pekker Integer and fractional quantum hall effects are an area in which ultra cold atoms experiments could address long-standing problems of many-body physics. However there is a missing experimental ingredient: a good way to make ``synthetic magnetic fields'' for the neutral atoms that does not heat the atoms too quickly. Here we present a proposal for a solution by appealing to the physics of graphene. The motion of electrons in graphene is described by the Dirac equation. In the presence of strain the Dirac equation becomes modified as if there is a local magnetic field. We propose to use three laser beams to create a graphene-like optical lattice for ultra cold atoms. By mis-aligning the beams, we can encode a strain into the optical lattice and hence synthesize a uniform ``magnetic'' field. Using a tight binding model, we show that the synthetic magnetic field results in the formation of distinct Landau levels. These levels will persist in presence of a trap potential. The Landau levels can be detected using spectroscopic methods, like Bragg spectroscopy, or alternatively ``kick'' methods, like Bloch oscillations. [Preview Abstract] |
Monday, March 2, 2015 12:39PM - 12:51PM |
B36.00006: Large Artificial Magnetic Fields Realized in a Synthetic Two-Dimensional Lattice Lauren Aycock, Ben Stuhl, Hsin-I Lu, Dina Genkina, Marcell Gall, Ian Spielman We experimentally realize a large artificial magnetic field for a $^{\mathrm{87}}$Rb Bose-Einstein condensate in a synthetic two-dimensional lattice [1]. This lattice combines a 1064nm 1D optical lattice along `x' in real space while the 3 internal states of the manifold F$=$1 define a 3-site wide lattice in a second, synthetic dimension. These internal states are either Rf- or Raman-coupled with a bichromatic light field allowing for tunneling in this synthetic dimension. The finite number of sites in this dimension naturally creates a hard walled potential ideal for studying edge states. The wavelength ratio between the optical lattice potential and the Raman coupling fields imprints a phase around each plaquette, creating a large, artificial magnetic field. We observe cyclotron orbits of the atoms and measure the edge state currents for opposite flux and varying group velocities. [1] A. Celi, P. Massignan, J. Ruseckas, N. Goldman, I. B. Spielman, G. Juzeliunas, and M. Lewenstein$, $Phys. Rev. Lett. 112, 043001 (2014) [Preview Abstract] |
Monday, March 2, 2015 12:51PM - 1:03PM |
B36.00007: Striped Ferronematic ground states in a spin-orbit coupled spin-1 Bose gas Stefan Natu, Xiaopeng Li, William Cole Motivated by recent experiments on spin-orbit coupled quantum gases, and the recent cooling to degeneracy of large spin atoms, we explore the ground state phase diagram of a spin-orbit coupled spin-1 Bose gas. A key new feature of large spin systems is the appearance of liquid crystalline order such as nematic or more exotic platonic solid order, which has no analog in solid state. Here we explore the interplay between spin order, translational symmetry breaking induced by spin-orbit coupling and these liquid crystalline order parameters in the experimentally relevant spin-1 system, finding a rich phase diagram. For repulsive spin-dependent interaction, we find a transition from a uniaxial ferronematic phase with XY spiral spin order but uniform total density to a biaxial ferronematic phase with stripes in the total density. As a function of the quadratic Zeeman shift (q), for attractive spin dependent interactions, we find a transition from a ferromagnetic stripe phase which breaks translational symmetry in real space to a uniform ferromagnet for q>0 and a uniform nematic phase for q<0. We discuss the implications of our predictions to ongoing experiments on spin-orbit coupled large spin quantum gases. [Preview Abstract] |
Monday, March 2, 2015 1:03PM - 1:15PM |
B36.00008: Spin-orbit coupled Fermi gas under sudden quench of Zeeman field Lin Dong, Ying Dong, Ming Gong, Han Pu Motivated by recent efforts to achieve effective spin-orbit coupling in cold degenerate gases, we study the dynamical response of a spin-orbit coupled Fermi gas after a sudden quench of external Zeeman field. By solving the time-dependent Bogoliubov-de Gennes equation self-consistently, we have found three dynamical phases emerging from the time evolution, characterized by the distinctive long time asymptotic behavior of the order parameter. We further map out the phase diagram for the various dynamical states in the parameter space spanned by the initial and final values of the Zeeman field strength. In certain parameter regimes, the dynamical states possess nontrivial topological properties, manifested by the presence of the zero-energy edge state localized at a confining boundary. We present a detailed characterization of these phases. [Preview Abstract] |
Monday, March 2, 2015 1:15PM - 1:27PM |
B36.00009: Bose-Einstein condensates with spin and orbital angular momentum coupling Kuei Sun, Chunlei Qu, Chuanwei Zhang Spin-orbit coupling (SOC) plays a crucial role in many branches of physics. In this context, the recent experimental realization of the coupling between spin and linear momentum of ultra-cold atoms opens a completely new avenue for exploring new spin-related superfluid physics. Here we propose that another important and fundamental SOC, the coupling between spin and orbital angular momentum (SOAM), can be implemented for ultra-cold atoms using higher order Laguerre-Gaussian laser beams to induce Raman coupling between two hyperfine spin states of atoms. We study the ground state phase diagrams of SOAM coupled Bose-Einstein condensates (BECs) on a ring trap and explore their applications in gravitational force detection. We further investigate two-dimensional disk-shaped BECs with focus on the interplay between SOAM coupling, interaction, and external trapping. Our results provide the basis for further investigation of intriguing superfluid physics induced by SOAM coupling. [Preview Abstract] |
Monday, March 2, 2015 1:27PM - 1:39PM |
B36.00010: Berezinskii-Kosterlitz-Thouless Phase Transition in 2D Spin-Orbit Coupled Fulde-Ferrell Superfluids Yong Xu, Chuanwei Zhang The experimental observation of traditional Zeeman-field induced Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superfluids has been hindered by various challenges, in particular, the requirement of low dimension systems. In 2D, it is well known that finite temperature phase fluctuations lead to extremely small Berezinskii-Kosterlitz-Thouless (BKT) transition temperature, raising serious concern regarding the observability of 2D FFLO superfluids. Recently, it was shown that FFLO superfluids can be realized using a Rashba spin-orbit coupled Fermi gas subject to Zeeman fields, which may also support topological excitations such as Majorana fermions in 2D. Here we address the finite temperature BKT transition issue in this system, which may exhibits gapped, gapless, topological, and gapless topological FF phases. We find a large BKT transition temperature due to large effective superfluid densities, making it possible to observe 2D FF superfluids at finite temperature. In addition, we show that gapless FF superfluids can be stable due to their positive superfluid densities. These findings pave the way for the experimental observation of 2D gapped and gapless FF superfluids and their associated topological excitations at finite temperature. [Preview Abstract] |
Monday, March 2, 2015 1:39PM - 1:51PM |
B36.00011: Superfluid Breakdown and Multiple Roton Gaps in Spin-Orbit Coupled Bose-Einstein Condensates on an Optical Lattice Daniele Toniolo, Jacob Linder Based on the results of Phys. Rev. A 89, 061605(R) (2014) I discuss the superfluid phases of a Rashba spin-orbit coupled Bose-Einstein condensate residing on a two dimensional square optical lattice in the presence of an effective Zeeman field $\Omega$. At a critical value $\Omega=\Omega_c$, the single-particle spectrum $ E_{\bf k} $ changes from having a set of four degenerate minima to a single minimum at ${\bf k}=0$, corresponding to condensation at finite or zero momentum, respectively. I describe this quantum phase transition and the symmetry breaking of the condensate phases. The superfluid phase is discussed using the Bogoliubov theory, I present the phase diagram, the excitation spectrum and the sound velocity of the phonon excitations. A novel dynamically unstable superfluid regime occurring when $\Omega$ is close to $\Omega_c$ is analytically identified and the behavior of the condensate quantum depletion is discussed. Moreover, I show that there are two types of roton excitations occurring in the $\Omega<\Omega_c$ regime and obtain explicit values for the corresponding energy gaps. [Preview Abstract] |
Monday, March 2, 2015 1:51PM - 2:03PM |
B36.00012: Fermi Superfluids with Engineered Dispersion: A Consistent Treatment of Fluctuation Effects Brandon Anderson, Chien-Te Wu, Rufus Boyack, Kathyrn Levin Recent experimental advances in cold atoms allows for the possibility of engineered dispersion that give rise to novel physics. For example, in systems with either spin-orbit coupling (SOC) or shaken optical lattices, momentum fluctuations can have energy cost that is quartic instead of quadratic at small momenta. The resulting density of states (DOS) will then more accurately resemble a system with lower dimension, leading to, e.g., enhanced depletion of Bose condensates or enhanced binding energy of Fermions. We consider the effects of the reduced DOS on the stability of fermionic superfluids in the presence of SOC or optical lattice shaking. We establish and characterize fluctuations associated with the standard mean field equations of the superfluid instability. This introduces bosonic degrees of freedom at a level beyond Gaussian fluctuations. Moreover, these bosons must necessarily condense in order for the fermionic superfluid to be stable. This is a non-trivial constraint for fermions, as seen from the observation that, e.g., Rashba SOC destroys a condensate of non-interacting (true) bosons. On this basis we present a phase diagram establishing the regions of fluctuation-stable fermionic superfluidity as a function of scattering length and temperature. [Preview Abstract] |
Monday, March 2, 2015 2:03PM - 2:15PM |
B36.00013: Quantum phase transition in the interacting two-dimensional boson systems with Rashba spin-orbital coupling Congjun Wu, Jianda Wu The two-dimensional free bosons condensate at zero momentum at zero temperature. After turning on the Rashba spin-orbital coupling, the system displays a ring condensation in momentum space with highly degenerate quantum ground states [1]. It is pointed out that, after a mean-field treatment, the ring condensation will disappear when turning on and tuning the Zeeman coupling to a critical value, leading to a novel quantum phase transition in the system [2]. It is of great theoretical and experimental interests for the role the interaction plays in the system. Here we further explore the system via a relatively full treatment of the interaction. We find the presence of the interaction on one hand modifies the mean-field results, and on the other hand also drives the system undergoing quantum phase transition, leading to a new novel phase of ``boson metal.''\\[4pt] [1] C. Wu, I. Mondragon-Shem, and X. F. Zhou, Chin. Phys.. Lett. 28, 097102 (2011).\\[0pt] [2] H. C. Po and Q. Zhou, arXiv:1408.6421. [Preview Abstract] |
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