Jul 2014

Jul 14-Jul 18, Jinlong Yu, Jul 21-Jun 25, Zhifang Xu, July 28-Aug 1, Jiyao chen

Aug 1
1. arXiv:1407.7904 (cross-list from physics.atom-ph) [pdf, other]
Anomalous Behavior of Dark States in Quantum Gases of 6Li
Mariusz Semczuk, Will Gunton, William Bowden, Kirk W.~Madison
Comments: 5 pages, 3 figures
Journal-ref: Phys. Rev. Lett. 113, 055302(2014)
Subjects: Atomic Physics (physics.atom-ph); Quantum Gases (cond-mat.quant-gas)
We create atom-molecule dark states in a degenerate Fermi gas of 6Li in both weakly and strongly interacting regimes using two-photon Raman scattering to couple fermion pairs to bound molecular states in the ground singlet and triplet potential. Near the unitarity point in the BEC-BCS crossover regime, the atom number revival height associated with the dark state abruptly and unexpectedly decreases and remains low for magnetic fields below the Feshbach resonance center at 832.2~G. With a weakly interacting Fermi gas at 0~G we perform precision dark-state spectroscopy of the least-bound vibrational levels of the lowest singlet and triplet potentials. From these spectra, we obtain binding energies of the v′′=9, N′′=0 level of the a(13Σ+u)potential and the v′′=38, N′′=0 level of the X(11Σ+g) potential with absolute uncertainty as low as 20 kHz. For the triplet potential we resolve the molecular hyperfine structure.


Jul 31
1. arXiv:1407.8081 [pdf, ps, other]
On the momentum of solitons and vortex rings in a superfluid
L.P. Pitaevskii
Comments: Paper accepted for publication in the special issue of JETP devoted to 75-th anniversary of A.F. Andreev, 10 pages
Subjects: Quantum Gases (cond-mat.quant-gas)
This paper is devoted to the calculation of the momentum of localized excitations, such as solitons and vortex rings, moving in a superfluid. The direct calculation of the momentum by integration of the mass flux density results in a badly-converging integral. I suggest a method for the renormalization of the integral with the explicit separation of a term related to the vortex line. This term can be calculated explicitly and gives the main contribution for the rings whose size is large compared to the healing length. I compare my method with the Jones and Roberts prescription for the renormalization. I investigate the case of a uniform superfluid, and that of a superfluid in a cylindrical trap. I discuss the calculation of the jump in the phase of the order parameter and obtain a simple estimate for this jump for a large ring in the trap.

2. arXiv:1407.8021 [pdf, other]
Fulde-Ferrell-Larkin-Ovchinnikov state of two-dimensional imbalanced Fermi gases
Daniel E. Sheehy
Comments: 5 pages plus 12 pages of supplementary material. 12 Figures filesSubjects: Quantum Gases (cond-mat.quant-gas)
The ground-state phase diagram of attractively-interacting Fermi gases in two dimensions with a population imbalance is investigated. We find the regime of stability for the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase, in which pairing occurs at finite wave vector, and determine the magnitude of the pairing amplitude Δ and FFLO wavevector q in the ordered phase, finding that Δ can be of the order of the two-body binding energy. Our results rely on a careful analysis of the zero temperature gap equation for the FFLO state, which possesses nonanalyticities as a function of Δ and q, invalidating a Ginzburg-Landau expansion in small Δ.


Jul 30
1. arXiv:1407.7572 [pdf, ps, other]
Low Temperature Quasi-Particle Transport in Bosonic and Fermionic Superfluids
Rufus Boyack, K. Levin
Comments: 5 pages, 1 figure
Subjects: Quantum Gases (cond-mat.quant-gas)
In this paper we use a Kubo approach to address low temperature transport associated with the quasi-particles in bosonic and fermionic superfluids. Our analysis for bosonic superfluids utilizes the framework of the one-loop Bogoliubov approximation, whereas for fermionic superfluids we apply strict BCS theory. Interestingly we find that the transport properties of these two different systems have very similar structure, albeit with different quasi-particle dispersions and statistics. Our focus is on thermoelectric transport and the shear viscosity, η, which are accessible (and in some instances already measured) in low T experiments on trapped gases. While the different quasi-particle dispersions lead to rather dramatic contrasts between power law and exponential temperature dependence for η alone, the ratio of shear viscosity to entropy density is more similar. It involves the same linear dependence on the ratio of temperature to inverse quasi-particle lifetime.


Jul 29
1.arXiv:1407.7469 [pdf, ps, other]
Birth of a quasi-stationary black hole in an outcoupled Bose-Einstein condensate
J. R. M. de Nova, D. Guéry-Odelin, F. Sols, I. Zapata
Comments: 19 pages, 12 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
We study the evolution of an initially confined atom condensate which is progressively outcoupled by gradually lowering the confining barrier on one side. The goal is to identify protocols that best lead to a quasi-stationary sonic black hole separating regions of subsonic and supersonic flow. An optical lattice is found to be more efficient than a single barrier in yielding a long-time stationary flow. This is best achieved if the final conduction band is broad and its minimum not much lower than the initial chemical potential. An optical lattice with a realistic Gaussian envelope yields similar results. We analytically prove and numerically check that, within a spatially coarse-grained description, the sonic horizon is bound to lie right at the envelope maximum. We derive an analytical formula for the Hawking temperature in that setup.


2.arXiv:1407.7157 [pdf, other]
Roton-maxon excitation spectrum of Bose condensates in a shaken optical lattice
Li-Chung Ha, Logan W. Clark, Colin V. Parker, Brandon M. Anderson, Cheng Chin
Comments: 9 pages, 7 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
We present experimental evidence showing that an interacting Bose condensate in a shaken optical lattice develops a roton-maxon excitation spectrum, a feature normally associated with superfluid helium. The roton-maxon feature originates from the double-well dispersion in the shaken lattice, and can be controlled by both the atomic interaction and the lattice shaking amplitude. We determine the excitation spectrum using Bragg spectroscopy and measure the critical velocity by dragging a weak speckle potential through the condensate - both techniques are based on a digital micromirror device. Our dispersion measurements are in good agreement with a modified-Bogoliubov model.

3.arXiv:1407.7336 (cross-list from quant-ph) [pdf, other]
Subwavelength vacuum lattices and atom-atom interactions in photonic crystals
A. González-Tudela, C.-L. Hung, D. E. Chang, J. I. Cirac, H. J. Kimble
Comments: 16 pages, 10 figuresSubjects: Quantum Physics (quant-ph); Quantum Gases (cond-mat.quant-gas)
We propose the use of photonic crystal structures to design subwavelength optical lattices in two dimensions for ultracold atoms by using both Guided Modes and Casimir-Polder forces. We further show how to use Guided Modes for photon-induced large and strongly long-range interactions between trapped atoms. Finally, we analyze the prospects of this scheme to implement spin models for quantum simulation.

4. arXiv:1407.7034 (cross-list from cond-mat.str-el) [pdf, other]
Continuous Preparation of a Fractional Chern Insulator
M. Barkeshli, N.Y. Yao, C.R. Laumann
Comments: 9 pages, 5 figuresSubjects: Strongly Correlated Electrons (cond-mat.str-el); Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)
We present evidence of a direct, continuous quantum phase transition between a Bose superfluid and the 1/2 Laughlin state in a microscopic lattice model of optically dressed spin defects. In the process, we develop a detailed field theoretic description of this transition in terms of the low energy vortex dynamics, which takes into account the role of lattice symmetry breaking and half-filling. The continuity of this transition enables the quasi-adiabatic preparation and study of the topological state by optical techniques.


Jul 28
1. arXiv:1407.6934 [pdf, other]
Far-from-equilibrium spin transport in Heisenberg quantum magnets
Sebastian Hild, Takeshi Fukuhara, Peter Schauß, Johannes Zeiher, Michael Knap, Eugene Demler, Immanuel Bloch, Christian Gross
Subjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el)
We study experimentally the far-from-equilibrium dynamics in ferromagnetic Heisenberg quantum magnets realized with ultracold atoms in an optical lattice. After controlled imprinting of a spin spiral pattern with adjustable wave vector, we measure the decay of the initial spin correlations through single-site resolved detection. On the experimentally accessible timescale of several exchange times we find a profound dependence of the decay rate on the wave vector. In one-dimensional systems we observe diffusion-like spin transport with a dimensionless diffusion coefficient of 0.22(1). We show how this behavior emerges from the microscopic properties of the closed quantum system. In contrast to the one-dimensional case, our transport measurements for two-dimensional Heisenberg systems indicate anomalous super-diffusion.


Jul 25
1. arXiv:1407.6533 [pdf, other]
Rice-Mele model with topological solitons in an optical lattice
Anna Przysiezna, Omjyoti Dutta, Jakub Zakrzewski
Comments: 7pp, 4figs comments welcome
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)

Attractive ultra-cold fermions trapped in a one-dimensional periodically shaken opticla lattices are considered. For an appropriate resonant shaking the system realizes paradigmatic dimes physics described by Rice-Mele model. The important feature of our system is the possible presence of controlled defects. They result in the creation of topologically protected loclaized modes carrying fractional particle number. Their possible experimental signatures are discussed.

Jul 24
1. arXiv:1407.6196 [pdf, ps, other]
Exploring unconventional Hubbard models with doubly-modulated lattice gases
Sebastian Greschner, Luis Santos, Dario Poletti
Comments: 5 pages, 3 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

Recent experiments have shown that periodic modulations of cold atoms in optical lattices may be used to engineer and explore interesting models. We show that double modulation, combining lattice shaking and modulated interactions, allow for the engineering of a much broader class of lattice models with correlated hopping, which we study for the particular case of one-dimensional systems. We show in particular that using this double modulation it is possible to study Hubbard models with asymmetric hopping, which, contrary to the standard Hubbard model, present insulating phases with both parity and string order. Moreover, double modulation allows for the simulation of lattice models in unconventional parameter regimes, as we illustrate for the case of the spin-1/2 Fermi-Hubbard model with correlated hopping, a relevant model for cuprate superconductors.

2. arXiv:1407.6037 [pdf, other]
Relaxation dynamics of a Fermi gas in an optical superlattice
D. Pertot, A. Sheikhan, E. Cocchi, L. A. Miller, J. E. Bohn, M. Koschorreck, M. Köhl, C. Kollath
Subjects: Quantum Gases (cond-mat.quant-gas)

This paper comprises an experimental and theoretical investigation of the time evolution of a Fermi gas following fast and slow quenches of a one-dimensional optical double-well superlattice potential. We investigate both the local tunneling in the connected double wells and the global dynamics towards a steady state. The local observables in the steady-state resemble those of an equilibrium state, whereas the global properties indicate a strong non-equilibrium situation.
Jul 23
1. arXiv:1407.5932 [pdf, other]
Observation of antiferromagnetic correlations in the Hubbard model with ultracold atoms
Russell A. Hart, Pedro M. Duarte, Tsung-Lin Yang, Xinxing Liu, Thereza Paiva, Ehsan Khatami, Richard T. Scalettar, Nandini Trivedi, David A. Huse, Randall G. Hulet
Subjects: Quantum Gases (cond-mat.quant-gas)

Ultracold atoms in optical lattices are a versatile platform for creating quantum many-body states of matter. These systems may be able to address some of the most important issues in many-body physics, such as high-temperature (high-T_c) superconductivity. The Hubbard model describes many of the features shared by the cuprate superconductors, including an interaction-driven Mott insulating state and antiferromagnetism (AFM). Optical lattices filled with a two-spin-component Fermi gas of ultracold atoms can faithfully realize the Hubbard model with readily tunable parameters, and thus provide a platform for its systematic exploration. Realization of strongly correlated phases in optical lattices, however, has been hindered by the need to cool the atoms to temperatures as low as the magnetic exchange energy, and also by the lack of reliable thermometry. Here we demonstrate spin-sensitive Bragg scattering of light, in analogy to neutron scattering in condensed matter, and use it to measure the spin correlations at temperatures down to 1.4 times that of the AFM phase transition. We achieve these low temperatures using a novel compensated lattice to flatten the confining potential, tune the density, and mitigate heating in the lattice. We deduce the temperature of the atoms in the lattice by comparing the light scattering to determinantal quantum Monte Carlo (DQMC) and numerical linked-cluster expansion (NLCE) calculations, using the local density approximation (LDA) to account for the inhomogeneity of the density. Further refinement of the compensated lattice may produce even lower temperatures which, along with light scattering thermometry, has important implications for achieving other novel quantum states and addressing the role of the Hubbard model in cuprate superconductivity.

2. arXiv:1407.5635 [pdf, other]
An Aharonov-Bohm interferometer for determining Bloch band topology
Lucia Duca, Tracy Li, Martin Reitter, Immanuel Bloch, Monika Schleier-Smith, Ulrich Schneider
Comments: 5+5 pages
Subjects: Quantum Gases (cond-mat.quant-gas); Other Condensed Matter (cond-mat.other); Strongly Correlated Electrons (cond-mat.str-el)

The geometric structure of an energy band in a solid is fundamental for a wide range of many-body phenomena in condensed matter and is uniquely characterized by the distribution of Berry curvature over the Brillouin zone. In analogy to an Aharonov-Bohm interferometer that measures the magnetic flux penetrating a given area in real space, we realize an atomic interferometer to measure Berry flux in momentum space. We demonstrate the interferometer for a graphene-type hexagonal lattice, where it has allowed us to directly detect the singular \pi Berry flux localized at each Dirac point. We show that the interferometer enables one to determine the distribution of Berry curvature with high momentum resolution. Our work forms the basis for a general framework to fully characterize topological band structures and can also facilitate holonomic quantum computing through controlled exploitation of the geometry of Hilbert space.

3. arXiv:1407.5637 (cross-list from cond-mat.str-el) [pdf, other]
Condensation of Lattice Defects and Melting Transitions in Quantum Hall phases
Gil Young Cho, Onkar Parrikar, Yizhi You, Robert G. Leigh, Taylor L. Hughes
Comments: 11 pages, 2 figuresSubjects: Strongly Correlated Electrons (cond-mat.str-el); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Gases (cond-mat.quant-gas)

Motivated by recent progress in understanding the interplay between lattice and electronic topological phases, we consider quantum-melting transitions of {\it weak} quantum liquid crystals, a crystal and a nematic phase, in which electrons form a quantum Hall state. In certain classes of Chern band insulators and quantum Hall phases, it has been previously demonstrated that there are topological Chern-Simons terms such as a Hall viscosity term and a gravitational Chern-Simons term for local lattice deformations. The Chern-Simons terms can induce anyonic statistics for the topological lattice defects and furthermore dress the defects with certain symmetry quantum numbers. On the other hand, the melting transitions of such liquid-crystalline orders are driven by the condensation of lattice defects. Based on these observations, we show how the topological terms can change the nature of the proximate disordered phases of the quantum liquid crystalline phases. We derive and study the effective dual field theories for the liquid crystalline phases with the geometric Chern-Simons terms, and carefully examine the symmetry quantum numbers and statistics of defects. We show that a crystal may go through a continuous phase transition into another crystal with the new discrete translational symmetries because the dislocation, the topological defect in the crystal, carries non-zero crystal momentum due to the Hall viscosity term. For the nematic phase, the disclination will condense at the phase transition to the isotropic phase, and we show that the isotropic phase may support a deconfined fractionally charged excitation due to the Wen-Zee term, and thus the isotropic phase and the nematic phase have different electromagnetic Hall responses.

Jul 22
1.arXiv:1407.5469 [pdf, ps, other]
Beyond mean-field study of a binary bosonic mixture in a state-dependent honeycomb lattice
Lushuai Cao, Sven Krönke, Jan Stockhofe, Peter Schmelcher, Juliette Simonet, Klaus Sengstock, Dirk-Sören Lühmann
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)

We investigate a binary mixture of bosonic atoms loaded into a state-dependent honeycomb lattice. For this system, the emergence of a so-called twisted-superfluid ground state was experimentally observed in [Soltan-Panahi et al., Nat. Phys. 8, 71 (2012)]. Theoretically, the origin of this effect is not understood. We perform numerical simulations of an extended Bose-Hubbard model adapted to the experimental parameters employing the Multi-Layer Multi-Configuration Time-Dependent Hartree method for Bosons. Our results confirm the overall applicability of mean-field theory within the relevant parameter range. Beyond this, we provide a detailed analysis of correlation effects correcting the mean-field result. These have the potential to induce asymmetries in single shot time-of-flight measurements, but we find no indication of the patterns characteristic of the twisted superfluid. We comment on the restrictions of our model and possible extensions.

2.arXiv:1407.5354 [pdf, other]
Phase diagram of the Kondo lattice model on the Kagome lattice
Shivam Ghosh, Patrick O' Brien, Michael J. Lawler, Christopher L. Henley
Comments: 4.5 pages + 6.5 pages Supplementary Information. 11 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el)

We investigate complex magnetic orders arising in a Kondo Lattice Model on the Kagome lattice, i.e. a model of (classical) local moments "Kondo" coupled to itinerant electrons, deriving the first picture of the full phase diagram, as a function of the filling and the coupling strength. This phase diagram is dominated by \emph{non-coplanar} states, both commensurate (e.g having 12 spin directions aligned with the corners of a cuboctahedron) and incommensurate multiple ordering vector modulated spirals. We show how (nesting of) the Fermi surface geometry determines the ordering wave vectors and how the latter evolve as the filling is increased. Additionally, we identify a new regime of the phase diagram at intermediate coupling which is not an interpolation between the small and large coupling regimes: commensurate states increase in importance, but incommensurate non-coplanar states make new appearances.
Jul 21
1.arXiv:1407.5087 [pdf, other]
Collisions of matter-wave solitons
Jason H.V. Nguyen, Paul Dyke, De Luo, Boris A. Malomed, Randall G. Hulet
Subjects: Quantum Gases (cond-mat.quant-gas); Pattern Formation and Solitons (nlin.PS)

Solitons are localised wave disturbances that propagate without changing shape, a result of a nonlinear interaction which compensates for wave packet dispersion. Individual solitons may collide, but a defining feature is that they pass through one another and emerge from the collision unaltered in shape, amplitude, or velocity. This remarkable property is mathematically a consequence of the underlying integrability of the one-dimensional (1D) equations, such as the nonlinear Schr\"odinger equation, that describe solitons in a variety of wave contexts, including matter-waves^{1,2}. Here we explore the nature of soliton collisions using Bose-Einstein condensates of atoms with attractive interactions confined to a quasi-one-dimensional waveguide. We show by real-time imaging that a collision between solitons is a complex event that differs markedly depending on the relative phase between the solitons. Yet, they emerge from the collision unaltered in shape or amplitude, but with a new trajectory reflecting a discontinuous jump. By controlling the strength of the nonlinearity we shed new light on these fundamental features of soliton collisional dynamics, and explore the implications of collisions that bring the wave packets out of the realm of integrability, where they may undergo catastrophic collapse.
1. Zabusky, N.J. & Kruskal, M.D. Interaction of "solitons" in a collisionless plasma and the recurrence of initial states. Phys. Rev. Lett. 15, 240 (1965).
2. Zakharov, V.E. & Shabat, A.B. Exact theory of two-dimensional self-focusing and one-dimensional self-moduation of waves in nonlinear media. Sov. Phys. JEPT. 34, 62 (1972).

2. arXiv:1407.4920 [pdf, ps, other]
Excitation picture of an interacting Bose gas
M. Kira
Comments: 58 pages, 6 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

Atomic Bose-Einstein condensates (BECs) can be viewed as macroscopic objects where atoms form correlated atom clusters to all orders. Therefore, the presence of a BEC makes the direct use of the cluster-expansion approach --- lucrative e.g. in semiconductor quantum optics --- inefficient when solving the many-body kinetics of a strongly interacting Bose. An excitation picture is introduced with a nonunitary transformation that exclusively describes the system in terms of atom clusters within the normal component alone. The nontrivial properties of this transformation are systematically studied, which yields a cluster-expansion friendly formalism for a strongly interacting Bose gas. Its connections and corrections to the standard Hartree-Fock Bogoliubov approach are discussed and the role of the order parameter and the Bogoliubov excitations are identified. The resulting interaction effects are shown to visibly modify number fluctuations of the BEC. Even when the BEC has a nearly perfect second-order coherence, the BEC number fluctuations can still resolve interaction-generated non-Poissonian fluctuations.

3. arXiv:1407.4803 [pdf, other]
Universal High-Frequency Behavior of Periodically Driven Systems: from Dynamical Stabilization to Floquet Engineering
Marin Bukov, Luca D'Alessio, Anatoli Polkovnikov
Comments: 35 pages, 16 figures. Comments are welcome
Subjects: Quantum Gases (cond-mat.quant-gas); Statistical Mechanics (cond-mat.stat-mech)

We give a general overview of the high-frequency regime in periodically driven systems, and identify three distinct classes of driving protocols, in which the infinite-frequency Floquet Hamiltonian is not equal to the time-averaged Hamiltonian. These classes cover systems, such as the Kapitza pendulum, the Harper-Hofstadter model of neutral bosons in a magnetic field, the Haldane Floquet Chern insulator, and others. In all setups considered, we discuss both the infinite-frequency limit, and the leading finite-frequency corrections to the Floquet Hamiltonian, using the Magnus expansion. We also provide a short overview of Floquet theory, focusing on the gauge structure associated with the choice of stroboscopic frame, and the differences between stroboscopic and non-stroboscopic dynamics. In the latter case, the evolution is still governed solely by the Floquet Hamiltonian, but one has to work with a properly dressed initial density matrix and dressed observables. We derive perturbative expressions in the driving period for the dressed operators using the Magnus expansion. We also comment on the application of the Magnus series to systems described by static Hamiltonians with well-separated energy scales and, in particular, discuss parallels between the Magnus expansion and the Schrieffer-Wolff transformation.

4. arXiv:1407.4954 [pdf, other]
Steering matter wave superradiance with an ultra-narrowband optical cavity
H. Keßler, J. Klinder, M. Wolke, A. Hemmerich
Comments: Article & Supplement, 4 figures, to appear in Phys. Rev. Lett. (2014)
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)

A superfluid atomic gas is prepared inside an optical resonator with an ultra-narrow band width on the order of the single photon recoil energy. When a monochromatic off-resonant laser beam irradiates the atoms, above a critical intensity the cavity emits superradiant light pulses with a duration on the order of its photon storage time. The atoms are collectively scattered into coherent superpositions of discrete momentum states, which can be precisely controlled by adjusting the cavity resonance frequency. With appropriate pulse sequences the entire atomic sample can be collectively accelerated or decelerated by multiples of two recoil momenta. The instability boundary for the onset of matter wave superradiance is recorded and its main features are explained by a mean field model.

Jul 18
1. arXiv:1407.4710 [pdf, other]
Temperature driven crossover in the Lieb-Liniger model
Andreas Klumper, Ovidiu I. Patu
The large-distance behavior of the density-density correlation function in the Lieb-Liniger model at finite temperature is investigated by means of the recently derived nonlinear integral equations characterizing the correlation lengths. We present extensive numerical results covering all the physical regimes fromweak to strong interaction and all temperatures. We find that the leading term of the asymptotic expansion becomes oscillatory at a critical temperature which decreases with the strength of the interaction. As we approach the Tonks-Girardeau limit the asymptotic behavior becomes more complex with a double crossover of the largest and next-largest correlation lengths. The crossovers exist only for intermediate couplings and vanish for γ=0 and γ=∞.

Jul 17
1. arXiv:1407.4205 [pdf, other]
Revealing the topology of Hofstadter bands with ultracold bosonic atoms
M. Aidelsburger, M. Lohse, C. Schweizer, M. Atala, J. T. Barreiro, S. Nascimbène, N. R. Cooper, I. Bloch, N. Goldman
Sixty years ago, Karplus and Luttinger pointed out that quantum particles moving on a lattice could acquire an anomalous transverse velocity in response to a longitudinal force, providing a natural explanation for the unusual Hall effect observed in ferromagnetic metals. Since then, the anomalous Hall velocity has found a modern interpretation through the geometric structure of Bloch bands, where it is captured by the Berry curvature. Remarkably, this phenomenon provides the most fundamental explanation for the celebrated integer quantum Hall effect, where the quantized values of the Hall conductivity were shown to be determined by the Chern number, an integer topological invariant obtained by integrating the Berry curvature over the Brillouin zone. Here we provide direct measurements of the transverse Hall deflection of ultracold bosonic atoms in artificially generated Hofstadter bands, which correspond to a magnetic flux Φ=π/2 per lattice unit cell. The Hofstadter model is a particularly intriguing case for the study of artificial gauge fields, as it directly provides very flat and topologically nontrivial bands. By exposing the atoms to a linear optical gradient, we observe a transverse spatial deflection of the atomic cloud. This displacement is reversed by changing the direction of the artificial magnetic flux or by reversing the optical gradient, in good agreement with theory. In contrast, exposing the atoms to zero or staggered flux configurations leads to no deflection of the atomic cloud. Combining these measurements with a novel band-mapping technique, which enables us to determine the populations of the different Hofstadter bands, we obtain an experimental value for the Chern number of the lowest band νexp=0.99(5). Our measurements are facilitated by a new all-optical artificial gauge field scheme, which enables flux rectification in optical superlattices.

Jul 16
1. arXiv:1407.3842 [pdf, other]
Fermionic suppression of dipolar relaxation
Nathaniel Q. Burdick, Kristian Baumann, Yijun Tang, Mingwu Lu, Benjamin L. Lev
We observe the suppression of inelastic dipolar scattering due to Fermi statistics in ultracold gases of the highly magnetic atom, dysprosium. Inelastic dipolar scattering in non-zero magnetic fields leads to heating or to loss of the trapped population, both detrimental to experiments intended to study quantum many-body physics with strongly dipolar gases. Fermi statistics, however, should lead to a kinematic suppression of these harmful reactions. Indeed, we observe a 120-fold suppression of dipolar relaxation in fermionic versus bosonic Dy, as expected from theory describing universal inelastic dipolar scattering. Similarly low inelastic cross sections are observed in spin mixtures, also with striking correspondence to theory predictions. The suppression of relaxation opens the possibility of employing fermionic dipolar species---atoms or molecules---in studies of quantum many-body physics involving, e.g., synthetic gauge fields and pairing.

Jul 15
1. arXiv:1407.3720 [pdf, ps, other]
Spontaneous quantum Hall effect in an atomic spinor Bose-Fermi 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., 87Rb and 6Li, 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 a spontaneous quantum Hall effect in fermions and crystalline superfluidity in bosons, both driven by interaction.

2. arXiv:1407.3483 [pdf, ps, other]
Berezinskii-Kosterlitz-Thouless Phase Transition in 2D Spin-Orbit Coupled FF Superfluids
Yong Xu, Chuanwei Zhang
We investigate the Berezinskii-Kosterlitz-Thouless (BKT) phase transition for Fulde-Ferrell (FF) superfluids (including gapped, gapless, topological, and gapless topological FF phases) in a two-dimensional (2D) Rashba spin-orbit coupled Fermi gas subject to both in-plane and our-of-plane Zeeman fields. Our main findings are 1) The superfluid densities in both directions are nonzero at finite temperature for such spin-orbit coupled FF superfluids, in contrast to zero transverse superfluid density in traditional Zeeman field induced FF superfluids. This leads to a finite BKT transition temperature, making it possible to observe FF superfluids at finite temperature in 2D. 2) Gapless FF superfluids have positive superfluid densities, thus are stable. 3) There exists an inflection point for the BKT temperature as a function of Zeeman fields, reflecting the gap close of quasi-particle excitations at zero momentum. 4) The anisotropic superfluid densities lead to anisotropic vortex-antivortex pairs below the BKT temperature.


Jul 14
1. arXiv:1407.2949 [pdf, ps, other]
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 rotating external field that generates a direction-dependent two-body effective attraction. 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.

2. arXiv:1407.3203 [pdf, ps, other]
Thermodynamic properties of Rashba Spin-Orbit Coupled Fermi Gas
Zhen Zheng, Han Pu, Xubo Zou, Guangcan Guo
We investigate the thermodynamic properties of a superfluid Fermi gas subject to Rashba spin-orbit coupling and effective Zeeman field. We adopt a T-matrix scheme that takes beyond-mean-field effects --- which are important for strongly interacting systems --- into account. We focus on the calculation of two important quantities: the superfluid transition temperature and the isothermal compressibility. Our calculation shows very distinct influences of the out-of-plane and the in-plane Zeeman field on the Fermi gas. We also confirm that the in-plane Zeeman field induces a Fulde-Ferrell superfluid below the critical temperature and an exotic finite-momentum pseudo-gap phase above the critical termperature.

3. arXiv:1407.3212 [pdf, ps, other]
High-fidelity rapid ground-state loading of an ultracold gas into an optical lattice
Shumpei Masuda, Katsuhiro Nakamura, Adolfo del Campo
A protocol is proposed for the rapid coherent loading of a Bose-Einstein condensate into the ground state of an optical lattice, without residual excitation associated with the breakdown of adiabaticity. The driving potential required to assist the rapid loading is derived using the fast forward technique, and generates the ground state in any desired short time. We propose an experimentally feasible loading scheme using a bichromatic lattice potential, which approximates the fast-forward driving potential with high fidelity.

4. arXiv:1407.2972 [pdf, ps, other]
Supersolid with nontrivial topological spin textures in spin-orbit coupled Bose gases
Wei Han, Gediminas Juzeliūnas, Wei Zhang, Wu-Ming Liu
Supersolid is a long-sought exotic phase of matter, which is characterized by the co-existence of a diagonal long-range order of solid and an off-diagonal long-range order of superfluid. Possible candidates to realize such a phase have been considered, including hard-core bosons with long-range interaction and soft-core bosons. Here we demonstrate that an ultracold atomic condensate of hard-core bosons with contact interaction can establish a supersolid phase when simultaneously subjected to spin-orbit coupling and a spin-dependent periodic potential. This supersolid phase is accompanied by topologically nontrivial spin textures, and is signaled by the separation of momentum distribution peaks, which can be observed via time-of-flight measurements.
Jul 7-Jul 11, Bo Liu
Jul 11

1. arXiv:1407.2852 [pdf, ps, other]
Spectral and Entanglement Properties of the Bosonic Haldane Insulator
Satoshi Ejima, Florian Lange, Holger Fehske
We discuss the existence of a nontrivial topological phase in one-dimensional interacting systems described by the extended Bose-Hubbard model with a mean filling of one boson per site. Performing large-scale density-matrix renormalization group calculations we show that the presence of nearest-neighbor repulsion enriches the ground-state phase diagram of the paradigmatic Bose-Hubbard model by stabilizing a novel gapped insulating state, the so-called Haldane insulator, which, embedded into superfluid, Mott insulator, and density wave phases, is protected by the lattice inversion symmetry. The quantum phase transitions between the different insulating phases were determined from the central charge via the von Neumann entropy. The Haldane phase reveals a characteristic fourfold degeneracy of the entanglement spectrum. We finally demonstrate that the intensity maximum of the dynamical charge structure factor, accessible by Bragg spectroscopy, features the gapped dispersion known from the spin-1 Heisenberg chain.
2.arXiv:1407.2804 [pdf, other]
Non-equilibrium relaxation transport of ultracold atoms
Fernando Gallego-Marcos, Christian Nietner, Gernot Schaller, Gloria Platero, Tobias Brandes
We analyze the equilibration process between two either fermionic or bosonic reservoirs containing ultracold atoms with a fixed total number of particles that are weakly connected via a few-level quantum system. We allow for both the temperatures and particle densities of the reservoirs to evolve in time. Subsequently, linearizing the resulting equations enables us to characterize the equilibration process and its time scales in terms of equilibrium reservoir properties and linear response transport coefficients. Additionally, we investigate the use of such a device as particle transistor or particle capacitor and analyze its efficiency.




Jul 10


1. arXiv:1407.2530 [pdf, other]
Quantum phase transitions in networks of Lipkin-Meshkov-Glick models
A. V. Sorokin, V. M. Bastidas, T. Brandes
We study the quantum critical behavior of networks consisting of Lipkin-Meshkov-Glick models with an anisotropic ferromagnetic coupling. We focus on the low-energy properties of the system within a mean-field approach and the quantum corrections around the mean-field solution. Our results show that the weak-coupling regime corresponds to the paramagnetic phase when the local field dominates the dynamics, but the local anisotropy leads to the existence of an exponentially-degenerate ground state. In the strong-coupling regime, the ground state is twofold degenerate and possesses long-range magnetic ordering. Analytical results for a network with the ring topology are obtained.




Jul 9


1.arXiv:1407.1953 [pdf, other]
Spin Orbit Coupling in Periodically Driven Optical Lattices
Julian Struck, Juliette Simonet, Klaus Sengstock
We propose a method for the emulation of artificial spin orbit coupling in a system of ultracold, neutral atoms trapped in a tight-binding lattice. This scheme does not involve near-resonant laser fields, avoiding the heating processes connected to the spontaneous emission of photons. In our case, the necessary spin dependent tunnel matrix elements are generated by a rapid, spin dependent, periodic force, which can be described in the framework of an effective, time averaged Hamiltonian. An additional radio frequency coupling between the spin states leads to a mixing of the spin bands.



Jul 8


1.arXiv:1407.1321 [pdf, other]
Exact Solutions of Fractional Chern Insulators: Interacting Particles in the Hofstadter Model at Finite Size
Thomas Scaffidi, Steven H. Simon
We show that all the bands of the Hofstadter model on the torus have an exactly flat dispersion and Berry curvature when a special system size is chosen. This result holds for any hopping and Chern number. Our analysis therefore provides a simple rule for choosing a particularly advantageous system size when designing a Hofstadter system whose size is controllable, like a qubit lattice or an optical cavity array. The density operators projected onto the flat bands obey exactly the Girvin-MacDonald-Platzman algebra, like for Landau levels in the continuum in the case of C=1, or obey its straightforward generalization in the case of C>1. This allows a mapping between density-density interaction Hamiltonians for particles in the Hofstatder model and in a continuum Landau level. By using the well-known pseudopotential construction in the latter case, we obtain fractional Chern insulator phases, the lattice counterpart of fractional quantum Hall phases, that are exact zero-energy ground states of the Hofstadter model with certain interactions. Finally, the addition of a harmonic trapping potential is shown to lead to an appealingly symmetric description in which a new Hofstadter model appears in momentum space.




Jul 7


1. arXiv:1407.1146 [pdf, other]
Direct Probe of Topological Order for Cold Atoms
Deng Dong-Ling, Wang Shengtao, Lu-Ming Duan
Cold-atom experiments in optical lattices offer a versatile platform to realize various topological quantum phases. A key challenge in those experiments is to unambiguously probe the topological order. We propose a method to directly measure the characteristic topological invariants (order) based on the time-of-flight imaging of cold atoms. The method is generally applicable to detection of topological band insulators in one, two, or three dimensions characterized by integer topological invariants. Using detection of the Chern number for the 2D anomalous quantum Hall states and the Chern-Simons term for the 3D\ chiral topological insulators as examples, we show that the proposed detection method is practical, robust to typical experimental imperfections such as limited imaging resolution, inhomogeneous trapping potential, and disorder in the system.

Jun 30-Jul 4, Ahmet Keles,
July 4

[1] arXiv:1407.0704 [pdf, other]
Pairing Glue in the Two Dimensional Hubbard ModelEmanuel Gull, Andrew J. MillisCluster dynamical mean field calculations are used to construct the superconducting gap function of the two dimensional Hubbard model. The frequency dependence of the imaginary part of the gap function indicates that the pairing is dominated by fluctuations at two characteristic frequencies: one at the scale of the hopping matrix element $t$ and one at a much lower scale. The lower frequency component becomes more important as the doping is reduced into the pseudogap regime. Comparison to available information on the spin fluctuation spectrum of the model suggests that the superconductivity arises from exchange of spin fluctuations. The inferred pairing glue function is in remarkable qualitative consistency with the pairing function inferred from time-resolved optical conductivity data.

[2] arXiv:1407.0796 [pdf, other]
Violation of the Spin Statistics Theorem and the Bose-Einstein Condensation of Particles with Half Integer SpinH. D. Scammell, O. P. SushkovWe consider the Bose condensation of particles with spin 1/2. The condensation is driven by an external magnetic field. Our work is motivated by ideas of quantum critical deconfinement and bosonic spinons in spin liquid states. We show that both the nature of the novel Bose condensate and the excitation spectrum are fundamentally different from that in the usual integer spin case. We predict two massive ("Higgs") excitations and two massless Goldstone excitations. One of the Goldstone excitations has a linear excitation spectrum and another has quadratic spectrum. This implies that the Bose condensate does not support superfluidity, the Landau criterion is essentially violated. We formulate a "smoking gun" criterion for searches of the novel Bose condensation.
[3] arXiv:1407.0826 [pdf, other]Equilibration of a finite temperature binary Bose gas formed by population transferR. W. Pattinson, N. P. Proukakis, N. G. ParkerWe consider an equilibrium single-species Bose gas from which a proportion of the atoms are instantaneously and coherently transferred to a second species, thereby forming a binary Bose gas in a non-equilibrium initial state. We study the ensuing evolution towards a new equilibrium, mapping the dynamics and final equilibrium state out as a function of the population transfer and the interspecies interactions, for a homogeneous system and by means of classical field methods. While in certain regimes, the condensate fractions are largely unaffected by the population transfer process, in others, particularly for immiscible interactions, one or both condensate fractions are vastly reduced to a new equilibrium value.

[4] arXiv:1407.0851 [pdf, ps, other]Polaron formation in the vicinity of a narrow Feshbach resonance in atomic and exciton-polariton condensatesWim Casteels, Michiel WoutersThe polaronic system consisting of an impurity in a dilute Bose-Einstein condensate is considered in the presence of a narrow Feshbach resonance. For this purpose a coupled-channel model is used, which at the mean field level predicts the formation of quasiparticles that are a superposition of the impurity and the molecular states. The impurity-boson interactions and the coupling between the open and closed channels are then considered weak and a perturbative calculation of the corrections to the mean field results is presented. This allows to examine the properties of the quasiparticles, such as the lifetime and the effective mass. The model is applied to two physical systems: an impurity atom in a Bose-condensed atomic gas in 3D and a spin down lower polariton in a Bose-Einstein condensate of spin up lower polaritons in 2D. The model parameters are linked to the physical parameters by identifying the low energy T-matrix and applying a proper renormalization scheme.
[5] arXiv:1407.0869 [pdf, other]Design local spin models for Gutzwiller-projected parton wave functionsJia-Wei Mei, Xiao-Gang WenWe introduce a method to design a local spin Hamiltonian to realize a Gutzwiller-projected parton wave functions (GPWF) as its ground state. For example, the Dirac spin liquid (DSL) state is quite close to the true ground state of the spin-1/2 Heisenberg model on kagome lattice. We examine what kind of perturbations we should add in order to drive the DSL to more stable chiral spin liquid (CSL), valence bond solid (VBS) or Gutzwiller-projected spin Hall (GSH) states. We compute the two-body reduced-density-matrices (2-RDM) of GPWFs for those target states, and compare them to the 2-RDM of the DSL. This allows us to design local spin models with only two-body interactions that may realize those interesting target states. Our results agree very well with recent numerical calculations for CSL on kagome lattice. We also study spin-1 systems on kagome lattice, and design local spin models that may realize CSL, VBS and symmetry-protected topological (SPT) states. Our work establishes a directional guide for further numerical simulations.

[6] arXiv:1407.0990 [pdf, other]
Equation of state and phase transition in spin-orbit coupled Bose gases at finite temperature: a perturbation approach
Zeng-Qiang Yu

We study two-component Bose gases with Raman induced spin-orbit coupling via a perturbation approach at finite temperature. For weak coupling, free energy is expanded in terms of Raman coupling strength up to the second order, where the coefficient (referred to as Raman susceptibility) is determined according to linear response theory. The equation of state for the stripe phase and the plane-wave phase are obtained in Popov approximation, and the first order transition between these two phases is investigated. As temperature increases, we find the phase boundary bends toward the stripe phase side in the most temperature region, which implies the ferromagnetic order is more robust than the crystalline order in presence of thermal fluctuations. Our results qualitatively agree with the recent experimental observation in rubidium atomic gases. A method to measure the Raman susceptibility through the two-photon Bragg scattering experiment is also discussed.

[7] arXiv:1407.1000 [pdf, other]
Robust finite-temperature disordered Mott insulating phases in inhomogeneous Fermi-Fermi mixtures with density and mass imbalance
Anzi Hu, M. M. Maśka, Charles W. Clark, J. K. Freericks

Ultracold mixtures of different atomic species have great promise for realizing novel many-body phenomena. In a binary mixture of femions with a large mass difference and repulsive interspecies interactions, a disordered Mott insulator phase can occur. This phase displays an incompressible total density, although the relative density remain compressible. We use strong-coupling and Monte Carlo calculations to show that this phase exists for a broad parameter region for ultracold gases confined in a harmonic trap on a three-dimensional optical lattice, for experimentally accessible values of the trap parameters.

July 3
[1] arXiv:1407.0431 [pdf, ps, other]
Radio-Frequency Manipulation of Fano-Feshbach Resonances in an Ultracold Fermi Gas of $^{40}$KLianghui Huang, Pengjun Wang, B. P. Ruzic, Zhengkun Fu, Zengming Meng, Peng Peng, J. L. Bohn, Jing ZhangExperimental control of magnetic Fano-Feshbach resonances in ultracold $^{40}$K Fermi gases, using radio-frequency (RF) fields, is demonstrated. Spectroscopic measurements are made of three molecular levels within 50 MHz of the atomic continuum, along with their variation with magnetic field. Modifying the scattering properties by an RF field is shown by measuring the loss profile versus magnetic field. This work provides the high accuracy locations of ground molecular states near the s-wave Fano-Feshbach resonance, which can be used to study the crossover regime from a Bose-Einstein condensate to a Bardeen-Cooper-Schrieffer superfluid in presence of an RF field.
[2] arXiv:1407.0605 [pdf, other]Detecting Floquet resonances with directed transport of ultracold atomsChristopher Grossert, Martin Leder, Sergey Denisov, Peter H?anggi, Martin WeitzCoherent directed quantum transport of ultracold rubidium atoms in an optical realization of a quantum rocking ratchet is investigated experimentally. By changing parameters of the underlying periodic modulations we resolve transport resonances in the mean momentum of a dilute atomic Bose-Einstein condensate. These resonances are attributed to the avoided crossings between Floquet eigenstates which are widely separated on the energy scale. By increasing the amplitude of the drive we observe a theoretically predicted bifurcation of a single-peak resonance into a doublet. Our results prove the feasibility of fine experimental control over nonequlibrium, coherent quantum transport in ac-driven optical lattices
July 2

[1] arXiv:1407.0368 [pdf, other]
Phase separation in doped Mott insulatorsChuck-Hou Yee, Leon BalentsMotivated by the commonplace observation of Mott insulators away from integer filling, we construct a simple thermodynamic argument for phase separation in first-order doping-driven Mott transitions. We show how to compute the critical dopings required to drive the Mott transition using electronic structure calculations for the titanate family of perovskites, finding good agreement with experiment. The theory predicts the transition is percolative and should exhibit Coulomb frustration.

[2] arXiv:1407.0329 [pdf, ps, other]
Correlations and entanglement in flat band models with variable Chern numbersMasafumi Udagawa, Emil J. BergholtzWe discuss a number of illuminating results for tight binding models supporting a band with variable Chern number, and illustrate them explicitly for a simple class of two-banded models. First, for models with a fixed number of bands, we show that the minimal hopping range needed to achieve a given Chern number C is increasing with C, and that the band flattening requires an exponential tail of long-range processes. We further verify that the entanglement spectrum corresponding to a real-space partitioning contains C chiral modes and thereby complies with the archetypal correspondence between the bulk entanglement and the edge energetics. Finally, we address the issue of interactions and study the problem of two interacting particles projected to the flattened band as a function of the Chern number. Our results provide valuable insights for the full interacting problem of a partially filled Chern band at variable filling fractions and Chern numbers.
July 1

[1] arXiv:1406.7332 [pdf, ps, other]
Dynamic Hubbard model for solids with hydrogen-like atoms
J. E. Hirsch

We discuss how to construct a tight binding model Hamiltonan for the simplest possible solid, composed of hydrogen-like atoms. A single orbital per atom is not sufficient because the on-site electron-electron repulsion mixes in higher energy orbitals. The essential physics is captured by a dynamic Hubbard model with one electronic orbital and an auxiliary spin degree of freedom per site. We point out that this physics can lead to a substantial shift in the position and width of electronic energy bands relative to what is predicted by conventional band structure calculations.

[2] arXiv:1406.7396 [pdf, other]
Dimensional crossover and cold-atom realization of topological Mott insulators
Mathias S. Scheurer, Stephan Rachel, Peter P. Orth

We propose a cold-atom setup which allows for a dimensional crossover from a two-dimensional quantum spin Hall insulating phase to a three-dimensional strong topological insulator by tuning the hopping between the layers. We further show that additional Hubbard onsite interactions can give rise to spin liquid-like phases: weak and strong topological Mott insulators. They represent the celebrated paradigm of a quantum state of matter which merely exists because of the interplay of the non-trivial topology of the band structure and strong interactions. While the theoretical understanding of this phase has remained elusive, our proposal shall help to shed some light on this exotic state of matter by paving the way for a controlled experimental investigation in optical lattices.

[3] arXiv:1406.7477 [pdf, other]
Entanglement Entropies of the quarter filled Hubbard model
Pasquale Calabrese, Fabian H. L. Essler, Andreas M. Lauchli

We study Renyi and von Neumann entanglement entropies in the ground state of the one dimensional quarter-filled Hubbard model with periodic boundary conditions. We show that they exhibit an unexpected dependence on system size: for L=4 mod 8 the results are in agreement with expectations based on conformal field theory, while for L=0 mod 8 additional contributions arise. We show that these can be understood in terms of a "shell-filling" effect, and we develop a conformal field theory approach to calculate the additional contributions to the entropies. These analytic results are found to be in excellent agreement with density matrix renormalisation group computations for weak Hubbard interactions. We argue that for larger interactions the presence of a marginal irrelevant operator in the spin sector strongly affects the entropies at the finite sizes accessible numerically, and we present an effective way to take them into account.

[4] arXiv:1406.7761 [pdf, ps, other]
Pairing effects in the normal phase of a two-dimensional Fermi gas
F. Marsiglio, P. Pieri, A. Perali, F. Palestini, G. C. Strinati

In a recent experiment, a pairing gap was detected in a two-dimensional (2D) Fermi gas with attractive interaction at temperatures where superfluidity does not occur. The question remains open whether this gap is a pseudo-gap phenomenon or is due to a molecular state. In this paper, by using a t-matrix approach we reproduce quite well the experimental data for a 2D Fermi gas, and set the boundary between the pseudo-gap and molecular regimes. We also show that pseudo-gap phenomena occurring in 2D and 3D can be related through a variable spanning the BCS-BEC crossover in a universal way.
Jun 30

[1] arXiv:1406.7004 [pdf, ps, other]
Josephson and Persistent Spin Currents in Bose-Einstein Condensates of Magnons
Kouki Nakata, Kevin A. van Hoogdalem, Pascal Simon, Daniel Loss
Using the Aharonov-Casher (A-C) phase, we present a microscopic theory of the Josephson and persistent spin currents in quasi-equilibrium Bose-Einstein condensates (BECs) of magnons in ferromagnetic insulators. Starting from a microscopic spin model that we map onto a Gross-Pitaevskii Hamiltonian, we derive a two-state model for the Josephson junction between the weakly coupled magnon-BECs. We then show how to obtain the alternating-current (ac) Josephson effect with magnons as well as macroscopic quantum self-trapping in a magnon-BEC. We next propose how to control the direct-current (dc) Josephson effect electrically using the A-C phase, which is the geometric phase acquired by magnons moving in an electric field. Finally, we introduce a magnon-BEC ring and show that persistent magnon-BEC currents flow due to the A-C phase. Focusing on the feature that the persistent magnon-BEC current is a steady flow of magnetic dipoles that produces an electric field, we propose a method to directly measure it experimentally.

[2] arXiv:1406.7056 [pdf, other]
Stable knots in the trapped Bose-Einstein condensates
Yong-Kai Liu, Shiping Feng, Shi-Jie Yang
The knot of spin texture is studied within the two-component Bose-Einstein condensates which are described by the nonlinear Gross-Pitaevskii equations. We start from the non-interacting equations including an axisymmetric harmonic trap to obtain an exact solution, which exhibits a non-trivial topological structure. The spin-texture is a knot with an integral Hopf invariant. The stability of the knot is verified by numerically evolving the nonlinear Gross-Pitaevskii equations along imaginary time.

[3] arXiv:1406.7067 [pdf, ps, other]
Phasons and excitations in skyrmion lattice
Gen Tatara, Hidetoshi Fukuyama
Excitations of two-dimensional skyrmion lattice are theoretically studied based on a collective coordinate description. Starting from the representation of skyrmion lattice in terms of three helices, we identify the canonical coordinates describing low energy excitations as phasons. The phason excitation spectra turn out to have one gapless mode with a quadratic dispersion and one massive mode, in agreement with previous studies. We will show that there is another collective mode governing the topological nature and the stability of skyrmion lattice and that the fluctuation of this mode leads to a screening of the topological charge of the lattice. Experimental implications of the screening effect in microwave absorption, topological Hall effect and depinning threshold current in metals are discussed.

[4] arXiv:1406.7278 [pdf, ps, other]
Short-range entanglement and invertible field theories
Daniel S. Freed
Quantum field theories with an energy gap can be approximated at long-range by topological quantum field theories. The same should be true for suitable condensed matter systems. For those with short range entanglement (SRE) the effective topological theory is invertible, and so amenable to study via stable homotopy theory. This leads to concrete topological invariants of gapped SRE phases which are finer than existing invariants. Computations in examples demonstrate their effectiveness.