Mar 2016

Feb 29-Mar 4 Bo Liu, Mar 7-Mar 11 Max, Mar 14-Mar 18 Haiyuan Zou, Mar 21-Mar 25 Ahmet Kel

Mar 18
arXiv:1603.05312 (cross-list from quant-ph) [pdf, other]
Anomalous edge state in a non-Hermitian lattice
Tony E. Lee
Comments: 5 pages + appendix. To appear in PRL
Subjects: Quantum Physics (quant-ph); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Gases (cond-mat.quant-gas); Optics (physics.optics)
We show that the bulk-boundary correspondence for topological insulators can be modified in the presence of non-Hermiticity. We consider a one-dimensional tight-binding model with gain and loss as well as long-range hopping. The system is described by a non-Hermitian Hamiltonian that encircles an exceptional point in momentum space. The winding number has a fractional value of 1/2. There is only one dynamically stable zero-energy edge state due to the defectiveness of the Hamiltonian. This edge state is robust to disorder due to protection by a chiral symmetry. We also discuss experimental realization with arrays of coupled resonator optical waveguides.

Mar 17
arXiv:1603.04898 [pdf, ps, other]
Inflationary quasiparticle creation and thermalization dynamics in coupled Bose-Einstein condensates
Anna Posazhennikova, Mauricio Trujillo-Martinez, Johann Kroha
Comments: 9 pages (including Supplementary Material), 5 Figures
Subjects: Quantum Gases (cond-mat.quant-gas)

A bosonic Josephson junction is a prototype system to investigate how (if at all) an isolated quantum system driven out of equilibrium reaches a steady and possibly thermalized state for long times. The relaxation happens because of the quasiparticle excitations induced by the initial quenching of the Josephson coupling. We describe such a coupled BEC and incoherent excitations system in terms of the quantum kinetic equations and analyse in detail system's nonequilibrium dynamics. We find, interestingly, that the nonequilibrium dynamics is governed by three different time scales. After an initial period $\tau_c$ of undamped Josephson oscillations, quasiparticles are created in an avalanche manner due to a dynamically generated parametric resonance between the Josephson frequency and the quasiparticle excitation energies. This leads to a fast depletion as well as damping of the BEC amplitude. When the final number of quasiparticles, allowed by total energy conservation, is reached, however, the quasiparticle system effectively decouples from the BEC oscillations (freeze-out time of the BEC $\tau_f$), and the total quasiparticle number becomes nearly conserved. Under this approximate conservation law the system enters into a quasi-hydrodynamical regime which is characterized by a slow, exponential relaxation of the quasiparticle system into a thermalized state with thermalization time $\tau_{th}$. We prove this behavior by a detailed Fourier analysis of the oscillatory behavior in the different time regimes.
Mar 15
arXiv:1603.04254 [pdf, other]
Dual gauge field theory of quantum liquid crystals in two dimensions
Aron J. Beekman, Jaakko Nissinen, Kai Wu, Ke Liu, Robert-Jan Slager, Zohar Nussinov, Vladimir Cvetkovic, Jan Zaanen
Comments: Review article, 101 pages, 32 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Superconductivity (cond-mat.supr-con)

We present a self-contained review of the theory of dislocation-mediated quantum melting at zero temperature in two spatial dimensions. The theory describes the liquid-crystalline phases with spatial symmetries in between a quantum crystalline solid and an isotropic superfluid: quantum nematics and smectics. It is based on an Abelian-Higgs-type duality mapping of phonons onto gauge bosons ("stress photons"), which encode for the capacity of the crystal to propagate stresses. Dislocations and disclinations, the topological defects of the crystal, are sources for the gauge fields and the melting of the crystal can be understood as the proliferation (condensation) of these defects, giving rise to the Anderson-Higgs mechanism on the dual side. For the liquid crystal phases, the shear sector of the gauge bosons becomes massive signaling that shear rigidity is lost. Resting on symmetry principles, we derive the phenomenological imaginary time actions of quantum nematics and smectics and analyze the full spectrum of collective modes. The quantum nematic is a superfluid having a true rotational Goldstone mode due to rotational symmetry breaking, and the origin of this 'deconfined' mode is traced back to the crystalline phase. The two-dimensional quantum smectic turns out to be a dizzyingly anisotropic phase with the collective modes interpolating between the solid and nematic in a non-trivial way. We also consider electrically charged bosonic crystals and liquid crystals, and carefully analyze the electromagnetic response of the quantum liquid crystal phases. In particular, the quantum nematic is a real superconductor and shows the Meissner effect. Their special properties inherited from spatial symmetry breaking show up mostly at finite momentum, and should be accessible by momentum-sensitive spectroscopy.

Mar 14
arXiv:1603.03439 [pdf, other]
Evidence for a fermionic symmetry-protected topological phase in a two-dimensional Hubbard model
Cheng-Chien Chen, Lukas Muechler, Roberto Car, Titus Neupert, Joseph Maciejko
Comments: 5+3 pages, 4+2 figures, including the supplemental material
Subjects: Strongly Correlated Electrons (cond-mat.str-el)

We study the two-dimensional (2D) Hubbard model using exact diagonalization for spin-1/2 fermions on the triangular and honeycomb lattices decorated with a single hexagon per site. In certain parameter ranges, the Hubbard model maps to a quantum compass model on those lattices. On the triangular lattice, the compass model exhibits collinear stripe antiferromagnetism, implying $d$-density wave charge order in the original Hubbard model. On the honeycomb lattice, the compass model has a unique, quantum disordered ground state that transforms nontrivially under lattice reflection. The ground state of the Hubbard model on the decorated honeycomb lattice is thus a 2D fermionic symmetry-protected topological phase. This state -- protected by time-reversal and reflection symmetries -- cannot be connected adiabatically to a free-fermion topological phase.

Mar 4
Anchor
 Anchor1. arXiv:1603.01044 (cross-list from quant-ph) [pdf, ps, other]
Topological Phase Transitions and Thouless Pumping of Light in Photonic Lattices
Yongguan Ke, Xizhou Qin, Feng Mei, Honghua Zhong, Yuri S. Kivshar, Chaohong Lee
Photonic lattices provide an excellent platform for simulating conventional topological systems, and they can also be explored for the study of novel topological phases. However, a direct measurement of bulk topological invariants remains a great challenge. Here we study topological features of generalized commensurate Aubry-Andre-Harper (AAH) photonic lattices and construct a topological phase diagram by calculating all bulk Chern numbers, and then explore the bulk-edge correspondence by analyzing the topological edge states and their winding numbers. In contrast to incommensurate AAH models, we find that diagonal and off-diagonal commensurate AAH models are not topologically equivalent. In particular, nontrivial topological phases with large Chern numbers and topological phase transitions are possible. By implementing Thouless pumping of light in photonic lattices, we propose a simple scheme to measure the bulk Chern numbers.

Mar 3
Anchor
 Anchor1. arXiv:1603.00513 [pdf, other]
Dynamical Buildup of a Quantized Hall Response from Non-Topological States
Ying Hu, Peter Zoller, Jan Carl Budich
Motivated by the current interest in dynamically preparing topological states in ultracold atomic gases, we consider a two-dimensional system initialized in a topologically trivial state before its Hamiltonian is ramped into a Chern-insulator phase. Under coherent dynamics, the non-equilibrium Hall response is found to approach a topologically quantized time averaged value in the limit of slow parameter ramps, even though the Chern number of the state is constrained to remain trivial. Quite remarkably, the destruction of quantum coherence by dephasing stabilizes the Hall response towards its asymptotically quantized mean value by damping its oscillations. We demonstrate how this phenomenology generically arises from the interplay of Landau-Zener dynamics and dephasing. In the limit of a fast ramp (quench), we show how the presence of a cooling quantum bath enables a dynamical transition of the state's Chern number from trivial to non-trivial, accompanied by the onset of a quantized Hall response.

Mar 2
Anchor1. arXiv:1603.00451 [pdf, ps, other]
Global quantum phase diagram of strongly interacting spinor bosons with generic 2 dimensional spin-orbital couplings in a square lattice
Fadi Sun, Jinwu Ye, Wu-Ming Liu
Comments: 8 pages main text with 3 figures + 4 pages SM with 2 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
Recently, there are ground breaking experimental advances in generating 2 dimensional spin-orbit coupling (SOC) for cold atoms in both continuum and optical lattices. The possible heating issues in these experiments are well under control, novel magnetic phenomena due to the interplay between SOC and strong interactions are ready to be investigated. One typical experiment set-up is to load spinor bosons at integer fillings in an optical lattice subject to a 2d SOC. In the strong coupling limit, it leads to the Rotated Ferromagnetic Heisenberg model (RFHM) which is a new class of quantum spin models to describe quantum magnetisms in cold atoms or some materials with strong SOC. In a previous work, we investigate various quantum phenomena of the RFHM along a solvable line in the SOC parameter space. In this paper, starting from the results achieved along the solvable line, we study the RFHM in the whole SOC parameter space. Its global phase diagram displays many novel quantum phenomena such as masses generated from "order from disorder " mechanism, quantum commensurate ( C ) and In-commensurate ( IC ) skyrmion phases, quantum Lifshitz C-IC transitions, spiral phases, metastable states, hysteresis, devil staircases and fractals, etc. Connections to the classical Frenkel-Kontorowa (FK) model are explored. Implications to cold atom systems and so called Kitaev materials with SOC are discussed. Various intriguing perspectives are outlined.


Mar 1

Anchor
 Anchor1. arXiv:1602.08992 (cross-list from quant-ph) [pdf, other]
Dynamics of atomic and molecular solitary waves inatom-molecular hybrid Bose-Einstein condensates coupled by Magnetic Feshbach Resonance: Role of induced decays of Feshbach Molecules
Krishna Rai Dastidar, Deb Shankar Ray
Comments: 17 pages, 7 figures
Subjects: Quantum Physics (quant-ph); Quantum Gases (cond-mat.quant-gas)

Dynamics of atomic and molecular bright solitons in a hybrid atom-molecular BEC (Bose Einstein Condensate) system of 85Rb coupled through Magnetic Feshbach Resonance (MFR) has been investigated. By solving the time independent atom-molecular coupled equations, the initial atomic and molecular wavefunctions were obtained and the dynamics of the initial atomic and molecular waves in a spherical one-dimensional trap were studied by solving the time-dependent atom-molecular coupled equations. During evolution two types of induced or stimulated decays of the feshbach molecules were switched on. These two types of stimulated decays of the feshbach molecules to the two-atom continuum and to the bound level of the lowest hyperfinee state of the molecule were induced by laser or Radio Frequency fields. Hence the strength of these two induced decays can be controlled by varying the laser or RF field parameters e.g. intensity, detuning etc. It is found that depending on the relative strength of these two types of stimulated or induced decays initial atomic and molecular waves assume solitonic nature as bright solitons during evolution and the stability of these solitonic waves can be achieved by controlling the relative strength of induced decays in two different channels. It is shown that these two induced decays lead to the formation of stable atomic and molecular solitons by suppressing the initial oscillations and instability in the atom-molecular coupled system of 85Rb atoms.