16,931 research outputs found

    Fermionic matter-wave quantum optics with cold-atom impurity models

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    Motivated by recent cold-atom realisations of matter-wave waveguide QED, we study simple fermionic impurity models and discuss fermionic analogues of several paradigmatic phenomena in quantum optics, including formation of non-trivial bound states, (matter-wave) emission dynamics, and collective dissipation. For a single impurity, we highlight interesting ground-state features, focusing in particular on real-space signatures of an emergent length scale associated with an impurity screening cloud. We also present novel non-Markovian many-body effects in the quench dynamics of single- and multiple-impurity systems, including fractional decay around the Fermi level and multi-excitation population trapping due to bound states in the continuum.Comment: 24 pages, 7 figures, comments welcom

    Neutron scattering studies of heterogeneous catalysis

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    Understanding the structural dynamics/evolution of catalysts and the related surface chemistry is essential for establishing structure–catalysis relationships, where spectroscopic and scattering tools play a crucial role. Among many such tools, neutron scattering, though less-known, has a unique power for investigating catalytic phenomena. Since neutrons interact with the nuclei of matter, the neutron–nucleon interaction provides unique information on light elements (mainly hydrogen), neighboring elements, and isotopes, which are complementary to X-ray and photon-based techniques. Neutron vibrational spectroscopy has been the most utilized neutron scattering approach for heterogeneous catalysis research by providing chemical information on surface/bulk species (mostly H-containing) and reaction chemistry. Neutron diffraction and quasielastic neutron scattering can also supply important information on catalyst structures and dynamics of surface species. Other neutron approaches, such as small angle neutron scattering and neutron imaging, have been much less used but still give distinctive catalytic information. This review provides a comprehensive overview of recent advances in neutron scattering investigations of heterogeneous catalysis, focusing on surface adsorbates, reaction mechanisms, and catalyst structural changes revealed by neutron spectroscopy, diffraction, quasielastic neutron scattering, and other neutron techniques. Perspectives are also provided on the challenges and future opportunities in neutron scattering studies of heterogeneous catalysis

    Beam scanning by liquid-crystal biasing in a modified SIW structure

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    A fixed-frequency beam-scanning 1D antenna based on Liquid Crystals (LCs) is designed for application in 2D scanning with lateral alignment. The 2D array environment imposes full decoupling of adjacent 1D antennas, which often conflicts with the LC requirement of DC biasing: the proposed design accommodates both. The LC medium is placed inside a Substrate Integrated Waveguide (SIW) modified to work as a Groove Gap Waveguide, with radiating slots etched on the upper broad wall, that radiates as a Leaky-Wave Antenna (LWA). This allows effective application of the DC bias voltage needed for tuning the LCs. At the same time, the RF field remains laterally confined, enabling the possibility to lay several antennas in parallel and achieve 2D beam scanning. The design is validated by simulation employing the actual properties of a commercial LC medium

    Mechanical Response of Lattice Structures under High Strain-Rate and Shock Loading

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    Lattice structures are a class of architected cellular materials composed of similar unit cells with structural components of rods, plates, or sheets. Current additive manufacturing (AM) techniques allow control and tunability of unit cell geometries, which enable lattice structures to demonstrate exceptional mechanical properties such as high stiffness- and strength-to-mass ratios and energy absorption. Lattice structures exist on two length scales corresponding to the unit cell and continuum material, and therefore demonstrate mechanical behavior dependent on structural geometry and base material. These effects extend to the dynamic regime where lattice structures demonstrate distinct deformation modes under varying strain-rate loading. Experimental investigation of the dynamic and shock compression behavior of lattice structures remains largely unstudied and is the central focus of this thesis where the high strain-rate, transient dynamic, and shock compression behaviors of different topologies of lattice materials are explored. The first part of this thesis investigates the high strain-rate behavior of lattice structures via polymeric Kelvin lattices with rod- and plate-based geometries and relative densities of 15-30%. High strain-rate behavior is characterized by deformation modes similar to that of low strain-rate behavior. High strain-rate experiments (1000/s) are performed and validated using a viscoelastic polycarbonate split-Hopkinson (Kolsky) pressure bar system coupled with high-speed imaging. Both low and high strain-rate experiments show the formation of a localized deformation band which initiates in the middle of the specimen. Strain-rate effects of lattice specimens are observed to correlate with effects of the base polymer material and mechanical properties depend strongly on the relative density of the lattice specimen and exhibit distinct scaling with geometry type (rod, plate) and loading rate despite a similar unit cell shape. Explicit finite element simulations with a tensile failure material model are then used to validate deformation modes and scaling/property trends, and match those observed in experiments. The second part of this thesis explores the transient dynamic and transition to shock compression behavior of lattice structures using polymeric lattices with cubic, Kelvin, and octet-truss topologies with relative densities of about 8%. Transient dynamic behavior is characterized by a compaction wave initiating at an impact surface and additional deformation bands with modes similar to low strain-rate modes of deformation. Dynamic testing is conducted through gas gun direct impact experiments (25 - 70 m/s) with high-speed imaging coupled with digital image correlation (DIC) and a polycarbonate Hopkinson pressure bar. Full-field DIC measurements are used to characterize distinct mechanical behaviors induced by topology such as elastic wave speeds, deformation modes, and particle velocities. At lower impact velocities, a transient dynamic response is observed. At higher impact velocities, shock compression behavior occurs and is characterized by a sole compaction wave initiating and propagating from the impact surface of the lattice. One-dimensional continuum shock theory with Eulerian forms of the Rankine-Hugoniot jump conditions is used with full-field measurements to quantify a non-steady shock response and the varied effect of topology on material behaviors. The final part of this thesis examines the steady-state shock compression behavior of lattice structures through stainless steel 316L (SS316L) octet-truss lattices with relative densities of 10-30%. Powder gun plate impact experiments (270 - 390 m/s) with high-speed imaging and DIC are conducted and reveal a two-wave structure consisting of an elastic precursor wave and a planar compaction (shock) wave. Local shock parameters of lattice structures are defined using full-field DIC measurements and a linear shock velocity (us) versus particle velocity (up) relation is found to approximate measurements with a unit slope and linear fit constant equal to the crushing speed. One-dimensional continuum shock analysis is again performed using Eulerian forms of the Rankine-Hugoniot jump conditions to extract relevant mechanical quantities. Explicit finite element simulations of the lattice specimens using the Johnson-Cook constitutive model exhibit similar shock behavior to experiments. The simulations reveal a linear us-up relation and corresponding Hugoniot calculations agree with experimental trends. Notably, 1D shock theory is applied to simulations without resorting to a us-up relation for the base material, which characterizes this deformation regime and compaction wave as a `structural shock.' Major contributions of this thesis include experimental demonstration of ranged strain-rate behaviors for lattice structures of various base materials and topologies including low strain-rate, high strain-rate, transient dynamic, and shock compression regimes; use of full-field quantitative visualization techniques for local mechanical behavior and shock analysis; and finally, characterization of a 'structural' shock compression regime in lattice structures.</p

    The scalar curvature in wedge spaces: existence and obstructions

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    We study the scalar curvature of incomplete wedge metrics in certain stratified spaces with a single singular stratum (wedge spaces). Building upon several well established technical tools for this category of spaces (the corresponding Yamabe, elliptic and index theories) we provide existence and obstruction results for such metrics under suitable positivity assumptions on the underlying geometry. This is meant to be a follow-up to a previous paper of ours (AGAG, 2022), where the case of spaces with an isolated conical singularity was considered.Comment: 25 pages; minor modifications to improve the presentation; a few references added; submitted versio

    Physics-guided adversarial networks for artificial digital image correlation data generation

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    Digital image correlation (DIC) has become a valuable tool in the evaluation of mechanical experiments, particularly fatigue crack growth experiments. The evaluation requires accurate information of the crack path and crack tip position, which is difficult to obtain due to inherent noise and artefacts. Machine learning models have been extremely successful in recognizing this relevant information given labelled DIC displacement data. For the training of robust models, which generalize well, big data is needed. However, data is typically scarce in the field of material science and engineering because experiments are expensive and time-consuming. We present a method to generate synthetic DIC displacement data using generative adversarial networks with a physics-guided discriminator. To decide whether data samples are real or fake, this discriminator additionally receives the derived von Mises equivalent strain. We show that this physics-guided approach leads to improved results in terms of visual quality of samples, sliced Wasserstein distance, and geometry score

    Microstructure Design of Multifunctional Particulate Composite Materials using Conditional Diffusion Models

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    This paper presents a novel modeling framework to generate an optimal microstructure having ultimate multifunctionality using a diffusion-based generative model. In computational material science, generating microstructure is a crucial step in understanding the relationship between the microstructure and properties. However, using finite element (FE)-based direct numerical simulation (DNS) of microstructure for multiscale analysis is extremely resource-intensive, particularly in iterative calculations. To address this time-consuming issue, this study employs a diffusion-based generative model as a replacement for computational analysis in design optimization. The model learns the geometry of microstructure and corresponding stress contours, allowing for the prediction of microstructural behavior based solely on geometry, without the need for additional analysis. The focus on this work is on mechanoluminescence (ML) particulate composites made with europium ions and dysprosium ions. Multi-objective optimization is conducted based on the generative diffusion model to improve light sensitivity and fracture toughness. The results show multiple candidates of microstructure that meet the design requirements. Furthermore, the designed microstructure is not present in the training data but generates new morphology following the characteristics of particulate composites. The proposed approach provides a new way to characterize a performance-based microstructure of composite materials

    Atomistically-informed continuum modeling and isogeometric analysis of 2D materials over holey substrates

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    This work develops, discretizes, and validates a continuum model of a molybdenum disulfide (MoS2) monolayer interacting with a periodic holey silicon nitride (Si3N4) substrate via van der Waals (vdW) forces. The MoS2 layer is modeled as a geometrically nonlinear Kirchhoff–Love shell, and vdW forces are modeled by a Lennard-Jones (LJ) potential, simplified using approximations for a smooth substrate topography. Both the shell model and LJ interactions include novel extensions informed by close comparison with fully-atomistic calculations. The material parameters of the shell model are calibrated by comparing small-strain tensile and bending tests with atomistic simulations. This model is efficiently discretized using isogeometric analysis (IGA) for the shell structure and a pseudo-time continuation method for energy minimization. The IGA shell model is validated against fully-atomistic calculations for several benchmark problems with different substrate geometries. Agreement with atomistic results depends on geometric nonlinearity in some cases, but a simple isotropic St.Venant–Kirchhoff model is found to be sufficient to represent material behavior. We find that the IGA discretization of the continuum model has a much lower computational cost than atomistic simulations, and expect that it will enable efficient design space exploration in strain engineering applications. This is demonstrated by studying the dependence of strain and curvature in MoS2 over a holey substrate as a function of the hole spacing on scales inaccessible to atomistic calculations. The results show an unexpected qualitative change in the deformation pattern below a critical hole separation

    DSG-Net: Learning disentangled structure and geometry for 3D shape generation

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    3D shape generation is a fundamental operation in computer graphics. While significant progress has been made, especially with recent deep generative models, it remains a challenge to synthesize high-quality shapes with rich geometric details and complex structures, in a controllable manner. To tackle this, we introduce DSG-Net, a deep neural network that learns a disentangled structured & geometric mesh representation for 3D shapes, where two key aspects of shapes, geometry and structure, are encoded in a synergistic manner to ensure plausibility of the generated shapes, while also being disentangled as much as possible. This supports a range of novel shape generation applications with disentangled control, such as interpolation of structure (geometry) while keeping geometry (structure) unchanged. To achieve this, we simultaneously learn structure and geometry through variational autoencoders (VAEs) in a hierarchical manner for both, with bijective mappings at each level. In this manner, we effectively encode geometry and structure in separate latent spaces, while ensuring their compatibility: the structure is used to guide the geometry and vice versa. At the leaf level, the part geometry is represented using a conditional part VAE, to encode high-quality geometric details, guided by the structure context as the condition. Our method not only supports controllable generation applications, but also produces high-quality synthesized shapes, outperforming state-of-the-art methods

    Proceedings Of The East Asia Joint Symposium On Fields And Strings 2021

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    This volume contains the proceedings of the East Asia Joint Symposium on Fields and Strings 2021, held at the Media Center of Osaka City University on November 22-27, 2021. About 160 physicists from all over East Asia attended physically or joined online this symposium and more than 50 researchers presented their results in the invited lectures, the short talks or the poster session. Quantum field theory and string theory in the context of several exciting developments were discussed, which include frontiers of supersymmetric gauge theory, anomalies and higher form symmetries, and several issues on quantum gravity and black holes
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