Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session K27: Topological Physics in AMO Systems III 
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Sponsoring Units: DCMP DAMOP Chair: Gediminas Juzeliunas, Vilnius University Room: LACC 404B 
Wednesday, March 7, 2018 8:00AM  8:12AM 
K27.00001: Mott Transition in the SquareLattice SU(4) Fermionic Hubbard Model with a πFlux Gauge Field Zhichao Zhou, Congjun Wu, Yu Wang Inspired by recent ultracold atom experiments in both the synthetic gauge field and the SU(2N) symmetry, by employing the largescale projector quantum Monte Carlo simulations, we investigate the ground state properties of the squarelattice SU(4) fermionic Hubbard model with a πflux gauge field. In the noninteracting limit, the ground state is the gapless Dirac semimetal. With the increase of repulsive interaction, we show that, in the presence of a πflux gauge field per plaquette, a Mott transition occurs from the semimetal to the valence bond solid phase, accompanied by the breaking of Z_{4} discrete symmetry. The phase transition point and the critical exponent have also been estimated by the finite size scaling of numerical data. Interestingly, in the strong coupling limit we have analyzed the role of ring exchange terms for the order formation of the ground state by perturbation theory. 
Wednesday, March 7, 2018 8:12AM  8:24AM 
K27.00002: SpinOrbit Coupled Bosons in One Dimension: Emergent Gauge Field and Lifshitz Transition William Cole, Junhyun Lee, Khan Mahmud, Yahya Alavirad, Ian Spielman, Jay Sau We describe a twocomponent Bose liquid in onedimension (1D) in terms of scalar bosons coupled to an emergent dynamical gauge field. In the presence of strong spinindependent interactions and spinorbit coupling, the system undergoes an interaction (or density) tuned quantum phase transition. Although the order parameter describes a broken Z_{2} spin symmetry, the associated phase is qualitatively distinct from the Ising phase as it is accompanied by a nonvanishing momentum which is generated by the gauge fluctuations at the phase transition. This quantum phase transition is distinct from the conventional Ising transition and has a dynamical critical exponent z ≈ 2, typical of a Lifshitz transition. Our model describes 1D ultracold atoms with Ramaninduced spinorbit coupling, and provides a route to quantum emulation of a model dynamical gauge theory as well as exotic critical behavior. 
Wednesday, March 7, 2018 8:24AM  8:36AM 
K27.00003: Color superfluidity of neutral ultracold fermions in the presence of colorflip and colororbit fields Doga Kurkcuoglu, Carlos A Sa de Melo

Wednesday, March 7, 2018 8:36AM  8:48AM 
K27.00004: Topological Phase Transitions in Finitesize Periodically Driven Translationally Invariant Systems Yang Ge, Marcos Rigol In the thermodynamic limit, the Chern number of a translationally invariant system cannot change under unitary time evolutions that are smooth in momentum space. But the Bott index, a realspace counterpart of the Chern number, has been shown to change in periodically driven systems with open boundary conditions. Using the Bott index, we show that, in finitesize translationally invariant systems, a Fermi sea under a periodic drive that is turned on slowly can acquire a nontrivial topology. This can happen provided that the gapclosing points in the thermodynamic limit are absent in the discrete Brillouin zone of the finite system, which allows the topological charge enclosed by the system to "leak out". Hence a periodic drive can be used to dynamically prepare topologically nontrivial states starting from trivial ones in finitesize systems, that are either translationally invariant or with open boundary conditions. 
Wednesday, March 7, 2018 8:48AM  9:00AM 
K27.00005: Spin Pumping and Entanglement Spectrum of Quantum Spin Hall States in a πflux Model Jiaxin Wu, YuanMing Lu, TinLun Ho We propose a new way of realizing quantum spin Hall effect in cold atoms by shaking an optical lattice, which produces an effective πflux per plaquette on the square lattice. We demonstrate a spin pumping process during flux insertion in this quantum spin hall state. We also study the evolution of entanglement spectrum under the flux insertion, which provides a sharp signature for the quantum spin Hall effects. 
Wednesday, March 7, 2018 9:00AM  9:12AM 
K27.00006: Setting Boundaries with Memory: Generation of Topological Boundary States in FloquetInduced Synthetic Crystals Yuval Baum, Gil Refael When a ddimensional quantum system is subjected to a periodic drive, it may be treated as a (d+1)dimensional system, where the extra dimension is a synthetic one. In this work, we take these ideas to the next level by showing that nonuniform potentials, and particularly edges, in the synthetic dimension are created whenever the dynamics of system has a memory component. We demonstrate that topological states appear on the edges of these synthetic dimensions and can be used as a basis for a wave packet construction. Such systems may act as an optical isolator which allows transmission of light in a directional way. We supplement our ideas by an example of a physical system that shows this type of physics. 
Wednesday, March 7, 2018 9:12AM  9:24AM 
K27.00007: Properties of the oneparticle density matrix in an interacting Chern insulator Fabian HeidrichMeisner, Andrew Hayward, Marie Piraud The notion of a topological insulator is rooted in the physics of noninteracting 
Wednesday, March 7, 2018 9:24AM  9:36AM 
K27.00008: Entanglement Properties of Degenerate Eigenstates in fSUSY Cheryne Jonay, Timothy Hsieh It was recently shown [T. Iadecola, T.H. Hsieh] that a driven (Floquet) system endowed with time reflection symmetry can exhibit an exponentially large degeneracy of the $E=0,\pi$ eigenspaces. In addition to being exponentially large, the degeneracy of the $0,\Pi$ quasienergies is robust to disorder interactions which respect time reflection symmetry. Here, we study the nature of the degenerate states, asking if they be distinguished from the rest of the eigenstates by looking at their entanglement scaling. We do so in the simplest variant of this model— a onedimensional spin 1/2 chain with a two part drive. 
Wednesday, March 7, 2018 9:36AM  9:48AM 
K27.00009: NonEquilibrium Dynamics of Chern Insulators Marcello Davide Caio, Nigel Cooper, Joe Bhaseen We investigate the nonequilibrium properties of Chern insulators following a quantum quench between topological and nontopological phases. We show that this yields an excited state for which the Chern number is preserved under unitary evolution [1]. However, this does not imply the preservation of other physical observables, such as the Hall response [2]. We discuss the time scale at which the Chern number preservation ultimately breaks down. 
Wednesday, March 7, 2018 9:48AM  10:00AM 
K27.00010: pOrbital Superfluid with S^{5} Manifold Simon Lieu, Andrew Ho, Derek Lee, Piers Coleman We study a continuum model of the weakly interacting Bose gas in the presence of an external field with minima forming a triangular lattice. The second lowest band of the singleparticle spectrum (pband) has three minima at nonzero momenta. We consider a metastable Bose condensate at these momenta and find that, in the presence of interactions that vary slowly over the lattice spacing, the order parameter space is isomorphic to S^{5}. The enlarged symmetry manifests itself in an additional number of gapless excitation modes, as well as the loss of topologically stable vortices. We expect that this nonAbelian condensate is a “failed superfluid” that does not undergo a BerezinskiiKosterlitzThouless (BKT) transition. Orderbydisorder splitting appears highly suppressed, implying that signatures of the S^{5} manifold ought to be observable at small temperatures. 
Wednesday, March 7, 2018 10:00AM  10:12AM 
K27.00011: Accessing 1D phases of matter via magnetic noise spectroscopy with single spins Joaquin Rodriguez Nieva, Kartiek Agarwal, Thierry Giamarchi, Bertrand Halperin, Mikhail Lukin, Eugene Demler The study of exotic 1D states, particularly those featuring strong spinorbit coupling at the edges of topological materials, demand experimental probes that can access the interplay between charge and spin degrees of freedom. One potential candidate is a single spin probe, which has recently emerged as a versatile tool to probe nanoscale systems in a noninvasive fashion. Here we present a theory to describe noise magnetometry of 1D systems using single spin probes, and outline protocols to access correlations which are not available to other probes. One key observation is that, by exploiting the spin degree of freedom of the probe, it is possible to measure independently local charge and spin fluctuations in a variety of 1D systems. We describe how experimentally tunable parameters, such as temperature and probetosample distance, can be used to study charge and spin excitations, e.g. whether they propagate ballistically or are pinned by disorder. We discuss some applications in the context of quantum spin Hall states and quantum Hall effects in graphene. The ability to sample both charge and spin fluctuations in a wide range of legthscales opens new pathways to bridge the large gap between atomic scale resolution of tunneling probes and global transport measurements. 
Wednesday, March 7, 2018 10:12AM  10:24AM 
K27.00012: Topological Band Theory for NonHermitian Hamiltonians Huitao Shen, Bo Zhen, Liang Fu We develop the topological band theory for systems described by nonHermitian Hamiltonians, whose energy spectra are generally complex. After generalizing the notion of gapped band structures to the nonHermitian case, we classify "gapped" bands in one and two dimensions by explicitly finding their topological invariants. We find nontrivial generalizations of the Chern number in two dimensions, and a new classification in one dimension, whose topology is determined by the energy dispersion rather than the energy eigenstates. We then study the bulkedge correspondence and the topological phase transition in two dimensions. Different from the Hermitian case, the transition generically involves an extended intermediate phase with complexenergy band degeneracies at isolated "exceptional points" in momentum space. We also systematically classify all types of band degeneracies. 
Wednesday, March 7, 2018 10:24AM  10:36AM 
K27.00013: Superfluidquasicrystal in a BoseEinstein condensate Junpeng Hou, Haiping Hu, Kuei Sun, Chuanwei Zhang Quasicrystal is a class of ordered structures defying conventional classification of solid crystals and may carry classically forbidden (e.g., 5fold) rotational symmetries. In view of longsought supersolids, a natural question is whether a superfluid can spontaneously form quasicrystalline order that is not possessed by the underlying Hamiltonian, forming ``superfluidquasicrystals". Here we show that a superfluidquasicrystal stripe state with the minimal 5fold rotational symmetry can be realized as the ground state of a BoseEinstein condensate within a practical experimental scheme. There exists a rich phase diagram consisting of various superfluidquasicrystal, supersolid, and planewave phases. Our scheme can be generalized for generating other higherorder (e.g., 7fold) quasicrystal states, and provides a platform for investigating such new exotic quantum matter. 
Wednesday, March 7, 2018 10:36AM  10:48AM 
K27.00014: Topological Structures and Corresponding Accurate Ground State Wave Functions for ν=2+½ Fractional Quantum Hall Effect Sudipto Das, Sutirtha Mukherjee, SUDHANSU MANDAL The fractional quantum Hall state at ν=2+½ remains enigmatic because of the lack of accurate ground state wave function for Coulomb interaction. It is believed that the most probable ground state wave function is one of those two associated with incompressible state occuring in spherical geometry for the Pfaffian and antiPfaffian flux shifts 3 and 1, given by the respective total number of flux quanta, N_{Φ}=2N3 and 2N+1. Although the well known MooreRead wave function and its particlehole conjugate partner belong to these shifts, these most likely do not describe true topology of the Coulomb state. Starting with these flux and particle relations, we determine corresponding topological structures, i.e., number of zeros felt by an electron at the positions of other N1 electrons. Using these, we determine all possible linearly independent antysymmetric functions in Jastrow form for small systems. A linear conbination of these antysymmetric functions represents the actual ground state wave function for Coulomb interaction . We obtain these wave functions for N=10 and 8 respectively for Pfaffian and antiPfaffian shifts. The character of these wave functions suggest the physics of 2+½ state is beyond the weak coupling regime of cooper pairs of composite fermions. 
Wednesday, March 7, 2018 10:48AM  11:00AM 
K27.00015: Exact Treatment of SpinOrbit Coupled Bosons in an Optical Ring Cavity Sebastian Garcia, David Feder Confining ultracold atoms in optical ring cavities can yield synthetic magnetic fields and spinorbit coupling (SOC). Previous meanfield calculations showed that the interplay of the cavitymediated longranged interactions and SOC can stabilize novel states, such as a stripe phase, but the strength of the SOC is limited by the usual continuum atomic recoil momentum. We investigate this system for small numbers of atoms and photons using exact diagonalization. We find that the ground state is significantly different from the meanfield approximation, in particular characterized by large entanglement. Furthermore, the system can be constructed so that the effective SOC persists for much higher atomphoton coupling strengths than can be obtained in the continuum. 
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