Spinor Bose gases form a family of quantum fluids manifesting both magnetic order and superfluidity. This article reviews experimental and theoretical progress in understanding the static and dynamic properties of these fluids. The connection between system properties and the rotational symmetry properties of the atomic states and their interactions are investigated. Following a review of the experimental techniques used for characterizing spinor gases, their mean-field and many-body ground states, both in isolation and under the application of symmetry-breaking external fields, are discussed. These states serve as the starting point for understanding low-energy dynamics, spin textures and topological defects, effects of magnetic dipole interactions, and various non-equilibrium collective spin-mixing phenomena. The paper aims to form connections and establish coherence among the vast range of works on spinor Bose gases, so as to point to open questions and future research opportunities.

Spinor Bose gases: Symmetries, magnetism, and quantum dynamics

This monograph, written by an outstanding expert in time resolved spectroscopy of semiconductors, presents the most striking recent advances in the field of ultrafast spectroscopy of semiconductors and their nanostructures. The book contains 8 chapters with in total 1160 references and 186 figures, preface and subject index. It begins with an introductory chapter on basic concepts of semiconductor physics and ultrafast spectroscopic techniques. The following five chapters are arranged in the order of occurrence of events in a homogeneous semiconductor following photoexcitation by an ultrashort pulse. These events comprise four temporally-overlapping regimes:

Ultrafast Spectroscopy of Semiconductors and Semiconductor Nanostructures

Storing hydrogen in materials is based on the observation that metals can reversibly absorb hydrogen. However, the practical application of such a finding is found to be rather challenging especially for vehicular applications. The ideal material should reversibly store a significant amount of hydrogen under moderate conditions of pressures and temperatures. To date, such a material does not exist, and the high expectations of achieving the scientific discovery of a suitable material simultaneously with engineering innovations are out of reach. Of course, major breakthroughs have been achieved in the field, but the most promising materials still bind hydrogen too strongly and often suffer from poor hydrogen kinetics and/or lack of reversibility. Clearly, new approaches have to be explored, and the knowledge gained with high-energy ball milling needs to be exploited, i.e. size does matter! Herein, progress made towards the practical use of magnesium as a hydrogen store and the barriers still remaining are reviewed. In this context, the new approach of tailoring the properties of metal hydrides through size restriction at the nanoscale is discussed. Such an approach already shows great promise in leading to further breakthroughs because both thermodynamics and kinetics can be effectively controlled at molecular levels.

Hydrogen in magnesium: new perspectives toward functional stores

In this work, we describe in detail the chopped random basis (CRAB) optimal control technique recently introduced to optimize time-dependent density matrix renormalization group simulations [P. Doria, T. Calarco, and S. Montangero, Phys. Rev. Lett. 106, 190501 (2011)]. Here, we study the efficiency of this control technique in optimizing different quantum processes and we show that in the considered cases we obtain results equivalent to those obtained via different optimal control methods while using less resources. We propose the CRAB optimization as a general and versatile optimal control technique.

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Chopped random-basis quantum optimization

This paper reports an experimental investigation of the near wake of a finite-length square cylinder, with one end mounted on a flat plate and the other free. The cylinder aspect ratio or height-to-width ratio H/d ranges from 3 to 7. Measurements were carried out mainly in a closed-loop low-speed wind tunnel at a Reynolds number Red, based on d and the free-stream velocity of 9300 using hot-wire anemometry, laser Doppler anemometry and particle image velocimetry (PIV). The planar PIV measurements were performed in the three orthogonal planes of the three-dimensional cylinder wake, along with flow visualization conducted simultaneously in two orthogonal planes (Red = 221). Three types of vortices, i.e. the tip, base and spanwise vortices were observed and the near wake is characterized by the interactions of these vortices. Both flow visualization and two-point correlation point to an inherent connection between the three types of vortices. A model is proposed for the three-dimensional flow structure based on the present measurements, which is distinct from previously proposed models. The instantaneous flow structure around the cylinder is arch-type, regardless of H/d, consisting of two spanwise vortical ‘legs’, one on each side of the cylinder, and their connection or ‘bridge’ near the free end. Both tip and base vortices are the streamwise projections of the arch-type structure in the (y, z) plane, associated with the free-end downwash flow and upwash flow from the wall, respectively. Other issues such as the topological characteristics, spatial arrangement and interactions among the vortical structures are also addressed.

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The finite-length square cylinder near wake

The effects of initial conditions on interaction between a boundary layer over a flat plate and flow around a wall-mounted finite-length cylinder were experimentally investigated. A square cylinder with a characteristic width (d) of 20mm and a length of H=5d was vertically mounted on a horizontal flat plate. Three different boundary layers were investigated, their momentum thickness being 0.07d, 0.13d, and 0.245d, respectively, measured at the cylinder axis in the absence of the cylinder. All the experiments were carried out in a closed-loop water tunnel at a Reynolds numbers of 11 500 based on d and the free-stream velocity U∞. It is found that initial boundary layer conditions have a profound effect on the near wake, including the flow near the cylinder free end that is well beyond the boundary layer. With increasing boundary layer thickness, the base vortex is enhanced, inducing a stronger upwash flow from the cylinder base, which acts to weaken the downwash free-end shear layer and the tip vortex. Con...

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Effect of initial conditions on interaction between a boundary layer and a wall-mounted finite-length-cylinder wake

A Large Eddy Simulation (LES) model capable of accurately representing finite-rate chemistry effects in turbulent premixed combustion is presented. The LES computations use finite-rate chemistry and implicit LES combustion modelling to simulate an experimentally well-documented lean-premixed jet flame stabilized by a stoichiometric pilot. The validity of the implicit LES assumption is discussed and criteria are expressed in terms of subgrid scale Damkohler and Karlovitz numbers. Simulation results are compared to experimental data for velocity, temperature and species mass fractions of CH4, CO and OH. The simulation results highlight the validity and capability of the present approach for the flame and in general the combustion regime examined. A sensitivity analysis to the choice of the finite-rate chemistry mechanism is reported, this analysis indicates that the one and two-step global reaction mechanisms evaluated fail to capture the reaction layer with sufficient accuracy, while a 20-species skeletal ...

Large Eddy Simulations of a piloted lean premix jet flame using finite-rate chemistry

In this work discrete element modeling (DEM) was applied to the flow of granular jets against a target. The resulting sheetlike or conelike formations under different conditions are described and explained by means of kinetic analysis. A qualitative and quantitative comparison with experimental results [Cheng et al., Phys. Rev. Lett. 93, 188001 (2007)] provides interesting insights in the theoretical treatment of the head-on collision of granular jets. Results presented in this paper provide a theoretical description of this type of physical system. However, there still exist obstacles in obtaining quantitative results.

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Granular jet impingement on a fixed target.

We have carried out first-principles calculation of Mg(0 0 0 1) free-standing thin films to study the oscillatory quantum size effect exhibited in the surface energy, work function, interlayer relaxation, and adsorption energy of the atomic hydrogen adsorbate. The quantum well states have been shown. The calculated energetics and interlayer relaxation of clean and H-adsorbed Mg films are clearly featured by quantum oscillations as a function of the thickness of the film, with oscillation period of about eight monolayers, consistent with recent experiments. The calculated quantum size effect in H adsorption can be verified by observing the dependence of H coverage on the thickness of Mg(0 0 0 1) thin films gown on Si(1 1 1) or W(1 1 0) substrate which has been experimentally accessible.

First-principles calculation of Mg(0 0 0 1) thin films : quantum size effect and adsorption of atomic hydrogen

Dynamic response of the F=2 spinor Bose-Einstein condensate (BEC) under the influence of external magnetic fields is studied. A general formula is given for the oscillation period to describe population transfer from the initial polar state to other spin states. We show that when the frequency and the reduced amplitude of the longitudinal magnetic field are related in a specific manner, the population of the initial spin-0 state will be dynamically localized during time evolution. The effects of external noise and nonlinear spin-exchange interaction on the dynamics of the spinor BEC are studied. We show that while the external noise may eventually destroy the Rabi oscillations and dynamic spin localization, these coherent phenomena are robust against the nonlinear atomic interaction.

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Cheong-ki Chan

Papers

Effect of initial conditions on interaction between a boundary layer and a wall-mounted finite-length-cylinder wake

Large Eddy Simulations of a piloted lean premix jet flame using finite-rate chemistry

Granular jet impingement on a fixed target.

First-principles calculation of Mg(0 0 0 1) thin films : quantum size effect and adsorption of atomic hydrogen

Dynamics of the spin-2 Bose condensate driven by external magnetic fields