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arXiv 2019年

作者： Sung, Suk Hyun Schnitzer, Noah Brown, Lola Park, Jiwoong Hovden, Robert Department of Materials Science and Engineering University of Michigan Ann ArborMI48109 United States Intel Electronics Kiryat Gat82109 Israel Department of Chemistry University of Chicago ChicagoIL60637 United States Institute for Molecular Engineering University of Chicago ChicagoIL60637 United States James Franck Institute University of Chicago ChicagoIL60637 United States Applied Physics Program University of Michigan Ann ArborMI48109 United States

The field of two-dimensional (2D) materials has expanded to multilayered systems where electronic, optical, and mechanical properties change—often dramatically—with stacking order, thickness, twist, and interlayer spacing [1–5]. For transition metal dichalcogenides (TMDs), bond coordination within a single van der Waals layer changes the out-of-plane symmetry that can cause metal-insulator transitions [1, 6] or emergent quantum behavior [7]. Discerning these structural order parameters is often difficult using real-space measurements, however, we show 2D materials have distinct, conspicuous three-dimensional (3D) structure in reciprocal space described by near infinite oscillating Bragg rods. Combining electron diffraction and specimen tilt we probe Bragg rods in all three dimensions to identify multilayer structure with sub-Angstrom precision across several 2D materials—including TMDs (MoS2, TaSe2, TaS2) and multilayer graphene. We demonstrate quantitative determination of key structural parameters such as surface roughness, inter- & intra-layer spacings, stacking order, and interlayer twist using a rudimentary transmission electron microscope (TEM). We accurately characterize the full interlayer stacking order of multilayer graphene (1-, 2-, 6-, 12-layers) as well the intralayer structure of MoS2 and extract a chalcogen-chalcogen layer spacing of 3.07 ± 0.11 Å. Furthermore, we demonstrate quick identification of multilayer rhombohedral graphene. Copyright © 2019, The Authors. All rights reserved.

关键词： Molybdenum compounds

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End-to-end symmetry preserving inter-atomic potential energy model for finite and extended systems 18

End-to-end symmetry preserving inter-atomic potential energy...

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Proceedings of the 32nd International Conference on Neural Information Processing Systems

作者： Linfeng Zhang Jiequn Han Han Wang Wissam A. Saidi Roberto Car E. Weinan Program in Applied and Computational Mathematics Princeton University Institute of Applied Physics and Computational Mathematics China and CAEP Software Center for High Performance Numerical Simulation China Department of Mechanical Engineering and Materials Science University of Pittsburgh Program in Applied and Computational Mathematics Princeton University and Department of Chemistry and Department of Physics Princeton University and Princeton Institute for the Science and Technology of Materials Princeton University Program in Applied and Computational Mathematics Princeton University and Department of Mathematics Princeton University and Beijing Institute of Big Data Research China

Machine learning models are changing the paradigm of molecular modeling, which is a fundamental tool for material science, chemistry, and computational biology. Of particular interest is the inter-atomic potential energy surface (PES). Here we develop Deep Potential - Smooth Edition (DeepPot-SE), an end-to-end machine learning-based PES model, which is able to efficiently represent the PES of a wide variety of systems with the accuracy of ab initio quantum mechanics models. By construction, DeepPot-SE is extensive and continuously differentiable, scales linearly with system size, and preserves all the natural symmetries of the system. Further, we show that DeepPot-SE describes finite and extended systems including organic molecules, metals, semiconductors, and insulators with high fidelity.

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Hypostatic jammed packings of frictionless nonspherical particles

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Physical Review E 2018年第1期97卷 012909-012909页

作者： Kyle VanderWerf Weiwei Jin Mark D. Shattuck Corey S. O'Hern Department of Physics Yale University New Haven Connecticut 06520 USA Department of Mechanics and Engineering Science Peking University Beijing 100871 China Department of Mechanical Engineering & Materials Science Yale University New Haven Connecticut 06520 USA Benjamin Levich Institute and Physics Department The City College of New York New York New York 10031 USA Department of Applied Physics Yale University New Haven Connecticut 06520 USA Graduate Program in Computational Biology and Bioinformatics Yale University New Haven Connecticut 06520 USA

We perform computational studies of static packings of a variety of nonspherical particles including circulo-lines, circulo-polygons, ellipses, asymmetric dimers, dumbbells, and others to determine which shapes form packings with fewer contacts than degrees of freedom (hypostatic packings) and which have equal numbers of contacts and degrees of freedom (isostatic packings), and to understand why hypostatic packings of nonspherical particles can be mechanically stable despite having fewer contacts than that predicted from naive constraint counting. To generate highly accurate force- and torque-balanced packings of circulo-lines and cir-polygons, we developed an interparticle potential that gives continuous forces and torques as a function of the particle coordinates. We show that the packing fraction and coordination number at jamming onset obey a masterlike form for all of the nonspherical particle packings we studied when plotted versus the particle asphericity A, which is proportional to the ratio of the squared perimeter to the area of the particle. Further, the eigenvalue spectra of the dynamical matrix for packings of different particle shapes collapse when plotted at the same A. For hypostatic packings of nonspherical particles, we verify that the number of “quartic” modes along which the potential energy increases as the fourth power of the perturbation amplitude matches the number of missing contacts relative to the isostatic value. We show that the fourth derivatives of the total potential energy in the directions of the quartic modes remain nonzero as the pressure of the packings is decreased to zero. In addition, we calculate the principal curvatures of the inequality constraints for each contact in circulo-line packings and identify specific types of contacts with inequality constraints that possess convex curvature. These contacts can constrain multiple degrees of freedom and allow hypostatic packings of nonspherical particles to be mechanically stable.

关键词： Granular materials Granular packing Jamming

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Torsional refrigeration by twisted, coiled, and supercoiled fibers

arXiv

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arXiv 2019年

作者： Wang, Run Fang, Shaoli Xiao, Yicheng Gao, Enlai Jiang, Nan Li, Yaowang Mou, Linlin Shen, Yanan Zhao, Wubin Li, Sitong Fonseca, Alexandre F. Galvão, Douglas S. Chen, Mengmeng He, Wenqian Yu, Kaiqing Lu, Hongbing Wang, Xuemin Qian, Dong Aliev, Ali E. Li, Na Haines, Carter S. Liu, Zhongsheng Mu, Jiuke Wang, Zhong Yin, Shougen Lima, Márcio D. An, Baigang Zhou, Xiang Liu, Zunfeng Baughman, Ray H. State Key Laboratory of Medicinal Chemical Biology College of Pharmacy Key Laboratory of Functional Polymer Materials Nankai University Tianjin300071 China Alan G. MacDiarmid NanoTech Institute University of Texas at Dallas RichardsonTX75080 United States Department of Engineering Mechanics School of Civil Engineering Wuhan University Wuhan Hubei430072 China Shenzhen Geim Graphene Center Tsinghua Shenzhen International Graduate School Shenzhen518055 China School of Life Sciences Tsinghua University Beijing100084 China Applied Physics Department State University of Campinas Campinas SP13081-970 Brazil Department of Mechanical Engineering University of Texas at Dallas RichardsonTX75080 United States Department of Mechanical Engineering Georgia Southern University StatesboroGA30458 United States Materials Science MilliporeSigma MilwaukeeWI53209 United States Institute of Materials Physics Tianjin University of Technology Tianjin300384 China Lintec of America Nano-Science & Technology Center RichardsonTX75081 United States School of Chemical Engineering University of Science and Technology Liaoning Anshan114051 China Department of Science China Pharmaceutical University NanjingJiangsu211198 China

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Microplasticity and yielding in crystals with heterogeneous dislocation distribution

arXiv

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arXiv 2018年

作者： Zhang, Xu Xiong, Jian Fan, Haidong Zaiser, Michael Applied Mechanics and Structure Safety Key Laboratory of Sichuan Province School of Mechanics and Engineering Southwest Jiaotong University Chengdu610031 China Department of Mechanics Sichuan University Chengdu610065 China Institute for Materials Simulation Department of Materials Science and Engineering Friedrich-Alexander Universität Erlangen-Nürnberg Dr.Mack Strasse 77 Fürth90762 Germany

In this study, we use discrete dislocation dynamics (DDD) simulation to investigate the effect of heterogeneous dislocation density on the transition between quasi-elastic deformation and plastic flow in face-centered cubic single crystals. By analyzing the stress-strain curves of samples with an initial, axial dislocation density gradient, we arrive at the following conclusions: (i) in the regime of quasi-elastic deformation before the onset of plastic flow, the effective elastic modulus of the simulated samples falls significantly below the value for a dislocation-free crystal. This modulus reduction increases with decreasing dislocation density gradient: crystals with homogeneous dislocation distribution are thus weakest in the quasi-elastic regime;(ii) the transition towards plastic flow occurs first in regions of reduced dislocation density. Therefore, the overall yield stress decreases with increasing dislocation density gradient;(iii) crystals with dislocation density gradient exhibit a more pronounced hardening stage during which stress is re-distributed onto stronger regions with higher dislocation density until the sample flows at a constant flow stress that is approximately independent on dislocation density gradient. We interpret these findings in terms of a continuum dislocation dynamics inspired model of dislocation density evolution that accounts for inversive dislocation motions. The transition between quasi-elastic and plastic deformation is interpreted as a transition from inversive to non-inversive dislocation motion, and the initial differences in elastic modulus are related to a density dependent polarizability of the dislocation system. The subsequent plastic flow behavior is analyzed in terms of a modified version of Mughrabi's composite model. Copyright © 2018, The Authors. All rights reserved.

关键词： Plastic flow

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Work in progress: Institutional context and the implementation of the red-shirt in engineering model at six universities 125

Work in progress: Institutional context and the implementati...

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125th ASEE Annual Conference and Exposition

作者： Knaphus-Soran, Emily Delaney, Ann Tetrick, Katherine Christine Cunningham, Sonya Cosman, Pamela Ennis, Tanya D. Myers, Beth A. Milford, Jana Llewellyn, Donna C. Riskin, Eve A. Callahan, Janet Pitts, Kevin Ferrez, Michelle United States Boise State University United States Program Washington State University United States University of Washington United States STARS Program Diversity and Access College of Engineering United States University of California San Diego United States University of Colorado College of Engineering and Applied Science United States Boise State University Institute for STEM and Diversity Initiatives United States University of Washington EE Department United States Boise State University Micron School of Materials Science and Engineering United States University of Illinois Urbana-Champaign United States IDEA Engineering Student Center UC San Diego Jacobs School of Engineering United States

Low-income students are underrepresented in engineering and are more likely to struggle in engineering programs. Such students may be academically talented and perform well in high school, but may have relatively weak academic preparation for college compared to students who attended better-resourced schools. Four-year engineering and computer science curricula are designed for students who are calculus-ready, but many students who are eager to become engineers or computer scientists need additional time and support to succeed. The NSF-funded Redshirt in engineering Consortium was formed in 2016 as a collaborative effort to build on the success of three existing "academic Redshirt" programs and expand the model to three new schools. The Consortium takes its name from the practice of Redshirting in college athletics, with the idea of providing an extra year and support to promising engineering students from low-income backgrounds. The goal is to enhance the students' ability to successfully graduate with engineering or computer science degrees. This Work in Progress paper describes the Redshirt programs at each of the six Consortium institutions, providing a variety of models for how an extra preparatory year or other intensive academic preparatory programs can be accommodated. This paper will pay particular attention to the ways that institutional context shapes the implementation of the Redshirt model. For instance, what do the Redshirt admissions and selection processes look like at schools where students are admitted directly to engineering from high school versus schools where students are admitted after one or two years taking engineering prerequisites at the university? What substantive elements of the first-year curriculum are consistent across the consortium? Where variation in curriculum occurs, what are the institutional factors that produce this variation? How does the Redshirt program fit with other pre-existing academic support services on campus, and w

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Nuclear mechanics: Nuclear mechanics within Intact Cells Is Regulated by Cytoskeletal Network and Internal Nanostructures (Small 18/2020)

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Small 2020年第18期16卷

作者： Jitao Zhang Farid Alisafaei Miloš Nikolić Xuefei A. Nou Hanyoup Kim Vivek B. Shenoy Giuliano Scarcelli Fischell Department of Bioengineering University of Maryland College Park MD 20742 USA Department of Materials Science and Engineering School of Engineering and Applied Science University of Pennsylvania Philadelphia PA 19104 USA Center for Engineering Mechanobiology University of Pennsylvania Philadelphia PA 19104 USA Maryland Biophysics Program University of Maryland College Park MD 20742 USA Canon U.S. Life Sciences Inc. 9800 Medical Center Drive Suite C-120 Rockville MD 20850 USA

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Machine learning determination of atomic dynamics at grain boundaries

arXiv

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arXiv 2018年

作者： Sharp, Tristan A. Thomas, Spencer L. Cubuk, Ekin D. Schoenholz, Samuel S. Srolovitz, David J. Liu, Andrea J. Department of Physics and Astronomy University of Pennsylvania PhiladelphiaPA United States Department of Materials Science and Engineering University of Pennsylvania PhiladelphiaPA United States Department of Mechanical Engineering and Applied Mechanics University of Pennsylvania PhiladelphiaPA United States Department of Materials Science Stanford University StanfordCA United States Google Incorporated Mountain ViewCA United States

In polycrystalline materials, grain boundaries are sites of enhanced atomic motion, but the complexity of the atomic structures within a grain boundary network makes it difficult to link the structure and atomic dynamics. Here we use a machine learning technique to establish a connection between local structure and dynamics of these materials. Following previous work on bulk glassy materials, we define a purely structural quantity, softness, that captures the propensity of an atom to rearrange. This approach correctly identifies crystalline regions, stacking faults, and twin boundaries as having low likelihood of atomic rearrangements, while finding a large variability within high-energy grain boundaries. As has been found in glasses [Schoenholz SS, Cubuk ED, Sussman DM, Kaxiras E, and Liu AJ. (2016) Nat Phys, 12(5):469-471;Schoenholz SS, Cubuk ED, Kaxiras E, and Liu AJ. (2017) Proc Natl Academ Sci USA, 114(2):263-267;Sussman DM, Schoenholz SS, Cubuk ED, Liu AJ. (2017) Proc Natl Academ Sci USA, 114(40):10601-10605], the probability that atoms of a given softness will rearrange is nearly Arrhenius. This indicates a well-defined energy barrier as well as a well-defined prefactor for the Arrhenius form for atoms of a given softness. The decrease in the prefactor for low-softness atoms indicates that variations in entropy exhibit a dominant influence on the atomic dynamics in grain boundaries. Copyright © 2018, The Authors. All rights reserved.

关键词： Grain boundaries

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Probing the Band Structure of Topological Silicon Photonic Lattices in the Visible Spectrum

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Physical Review Letters 2019年第11期122卷 117401-117401页

作者： Siying Peng (彭斯颖) Nick J. Schilder Xiang Ni Jorik van de Groep Mark L. Brongersma Andrea Alù Alexander B. Khanikaev Harry A. Atwater Albert Polman Applied Physics California Institute of Technology Pasadena California 91125 USA Center for Nanophotonics AMOLF Science Park 104 1098 XG Amsterdam Netherlands Department of Electrical Engineering City College of City University of New York New York 10031 USA Physics Program The Graduate Center City University of New York 365 Fifth Avenue New York New York 10016 USA Geballe Laboratory for Advanced Materials Stanford University 476 Lomita Mall Stanford California 94305 USA Photonics Initiative Advanced Science Research Center City University of New York 85 St. Nicholas Terrace New York New York 10031 USA

We study two-dimensional hexagonal photonic lattices of silicon Mie resonators with a topological optical band structure in the visible spectral range. We use 30 keV electrons focused to nanoscale spots to map the local optical density of states in topological photonic lattices with deeply subwavelength resolution. By slightly shrinking or expanding the unit cell, we form hexagonal superstructures and observe the opening of a band gap and a splitting of the double-degenerate Dirac cones, which correspond to topologically trivial and nontrivial phases. Optical transmission spectroscopy shows evidence of topological edge states at the domain walls between topological and trivial lattices.

关键词： Nanophotonics Photonic crystals Topological effects in photonic systems

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Raman Spectral Band Oscillations in Large Graphene Bubbles

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Physical Review Letters 2018年第18期120卷 186104-186104页

作者： Yuan Huang Xiao Wang Xu Zhang Xianjue Chen Baowen Li Bin Wang Ming Huang Chongyang Zhu Xuewei Zhang Wolfgang S. Bacsa Feng Ding Rodney S. Ruoff Center for Multidimensional Carbon Materials (CMCM) Institute for Basic Science (IBS) Ulsan 44919 Republic of Korea School of Materials Science and Engineering Ulsan National Institute of Science and Technology Ulsan 44919 Republic of Korea Institute of Applied Mechanics and Key Laboratory of Soft Machines and Smart Devices of Zhejiang Province Zhejiang University Hangzhou 310012 People’s Republic of China CEMES-CNRS and University of Toulouse 29 rue Jeanne Marvig 31055 Toulouse France Department of Chemistry Ulsan National Institute of Science and Technology Ulsan 44919 Republic of Korea

Raman spectra of large graphene bubbles showed size-dependent oscillations in spectral intensity and frequency, which originate from optical standing waves formed in the vicinity of the graphene surface. At a high laser power, local heating can lead to oscillations in the Raman frequency and also create a temperature gradient in the bubble. Based on Raman data, the temperature distribution within the graphene bubble was calculated, and it is shown that the heating effect of the laser is reduced when moving from the center of a bubble to its edge. By studying graphene bubbles, both the thermal conductivity and chemical reactivity of graphene were assessed. When exposed to hydrogen plasma, areas with bubbles are found to be more reactive than flat graphene.

关键词： Adsorption Chemical bonding Surface & interfacial phenomena Thermal conductivity

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