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arXiv 2023年

作者： Trotta, Domenico Pezzi, Oreste Burgess, David Preisser, Luis Blanco-Cano, Xochitl Kajdic, Primoz Hietala, Heli Horbury, Timothy S. Vainio, Rami Dresing, Nina Retino', Alessandro Marcucci, Maria Federica Sorriso-Valvo, Luca Servidio, Sergio Valentini, Francesco The Blackett Laboratory Department of Physics Imperial College London LondonSW7 2AZ United Kingdom Consiglio Nazionale delle Ricerche Via Amendola 122/D BariI-70126 Italy Department of Physics and Astronomy Queen Mary University of London E1 4NS United Kingdom Space Research Institute Austrian Academy of Sciences Graz Austria Departamento de Ciencias Espaciales Instituto de Geofísica Universidad Nacional Autónoma de México Ciudad Universitaria Ciudad de México Mexico Department of Physics and Astronomy University of Turku TurkuFI-20014 Finland LPP Ecole Polytechnique CNRS UPMC Université Paris Sud Palaiseau France INAF-Istituto di Astrofisica e Planetologia Spaziali Rome Italy Space and Plasma Physics School of Electrical Engineering and Computer Science KTH Royal Institute of Technology Stockholm Sweden Ångström Laboratory Lägerhyddsvägen 1 Uppsala75121 Sweden Dipartimento di Fisica Università della Calabria Rende87036 Italy

The complex interaction between shocks and plasma turbulence is extremely important to address crucial features of energy conversion in a broad range of astrophysical systems. We study the interaction between a supercritical, perpendicular shock and pre-existing, fully-developed plasma turbulence, employing a novel combination of magnetohydrodynamic (MHD) and small-scale, hybrid-kinetic simulations where a shock is propagating through a turbulent medium. The variability of the shock front in the unperturbed case and for two levels of upstream fluctuations is addressed. We find that the behaviour of shock ripples, i.e., shock surface fluctuations with short (a few ion skin depths, ) wavelengths, is modified by the presence of pre-existing turbulence, which also induces strong corrugations of the shock front at larger scales. We link this complex behaviour of the shock front and the shock downstream structuring with the proton temperature anisotropies produced in the shock-turbulence system. Finally, we put our modelling effort in the context of spacecraft observations, elucidating the role of novel cross-scale, multi-spacecraft measurements in resolving shock front irregularities at different scales. These results are relevant for a broad range of astrophysical systems characterised by the presence of shock waves interacting with plasma turbulence. © 2023, CC BY.

关键词： Shock waves

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A van der Waals interface hosting two groups of magnetic skyrmions

arXiv

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arXiv 2021年

作者： Wu, Yingying Francisco, Brian Wang, Wei Zhang, Yu Wan, Caihua Han, Xiufen Chi, Hang Hou, Yasen Lodesani, Alessandro Cui, Yong-Tao Wang, Kang L. Moodera, Jagadeesh S. Francis Bitter Magnet Laboratory Plasma Science and Fusion Center Massachusetts Institute of Technology CambridgeMA02139 United States Department of Physics and Astronomy University of California RiversideCA92521 United States Key Laboratory of Flexible Electronics & Institute of Advanced Materials Jiangsu National Synergetic Innovation Center for Advanced Materials Nanjing Tech University Nanjing211816 China Institute of Physics Chinese Academy of Sciences Beijing100190 China U.S. Army CCDC Army Research Laboratory AdelphiMD20783 United States Department of Electrical and Computer Engineering University of California Los Angeles Los AngelesCA90095 United States Physics Department Massachusetts Institute of Technology CambridgeMA02139 United States

Multiple magnetic skyrmion phases add an additional degree of freedom for skyrmion based ultrahigh-density spin memory devices. Extending the field to two-dimensional van der Waals magnets is a rewarding challenge, where the realizable degree of freedoms (e.g. thickness, twisting angle and electrical gating) and high skyrmion density result in intriguing new properties and enhanced functionality. We report a van der Waals interface, formed by two 2D ferromagnets Cr2Ge2Te6 and Fe3GeTe2 with a Curie temperature of ∼ 65 K and ∼ 205 K, respectively, hosting two groups of magnetic skyrmions. Two sets of topological Hall effect are observed below 60 K when Cr2Ge2Te6 is magnetically ordered. These two groups of skyrmions are directly imaged using magnetic force microscopy. Interestingly, the magnetic skyrmions persist in the heterostructure in the remanent state with zero applied magnetic field. Our results are promising for the realization of skyrmionic devices based on van der Waals heterostructures hosting multiple skyrmion phases. © 2021, CC BY.

关键词： Van der Waals forces

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Solder joint image adaptive block compressive sensing with convex optimisation and Gini index

Solder joint image adaptive block compressive sensing with c...

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7th International Symposium on Test Automation and Instrumentation, ISTAI 2018

作者： Zhao, Huihuang Wang, Yaonan Zheng, Jinhua Qiao, Zhijun Zhang, Yun Department of Computer Science and Technology College of Computer Science and Technology Hengyang Normal University Hengyang Hunan China Hunan Provincial Key Laboratory of Intelligent Information Processing and Application Hengyang China Department of Electrical and Engineering College of Electrical and Information Engineering Hunan University Changsha China School of Mathematical and Statistical Sciences University of Texas Rio Grande ValleyTX United States Institute of Radio and TV Technology Communication University of Zhejiang Hangzhou310018 China

This study aims to improve the performance in solder joint image compression and reconstruction. A novel adaptive block compressive sensing with convex optimisation and Gini index (Ad_BCSGB_Gini) methodology for solder joint image compression and reconstruction is proposed. At first, the image is split into square blocks and each block is resized into a row which consists of a new image. Then, the new image is transformed into a sparse signal by an orthogonal basis matrix, and the image reconstruction is handled as a convex optimisation problem. Moreover, a gradient-based method which has fast computational speed is used to reconstruct image. There is a control factor which controls a norm l1 in the optimisation problem. To achieve the best performance, at last, the proposed method adaptively selects the best result by comparing Gini index of the reconstruction results based on different control factor values. Experimental results with different methods indicate that the Ad_BCSGB_Gini method is able to achieve the best performance in quantisation comparison than several classical algorithms, and Ad_BCSGB_Gini has a good robustness. © 2019 Institution of engineering and technology. All rights reserved.

关键词： Convex optimization

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Quantum engineering with hybrid magnonics systems and materials

arXiv

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arXiv 2021年

作者： Awschalom, D.D. Du, C.H.R. He, R. Heremans, F.J. Hoffmann, A. Hou, J.T. Kurebayashi, H. Li, Y. Liu, L. Novosad, V. Sklenar, J. Sullivan, S.E. Sun, D. Tang, H. Tiberkevich, V. Trevillian, C. Tsen, A.W. Weiss, L.R. Zhang, W. Zhang, X. Zhao, L. Zollitsch, C.W. Pritzker School of Molecular Engineering University of Chicago ChicagoIL United States Materials Science Division Center for Molecular Engineering Argonne National Laboratory LemontIL United States Department of Physics University of California San Diego San DiegoCA92093 United States Department of Electrical and Computer Engineering Texas Tech University LubbockTX79409 United States Materials Research Laboratory Department of Materials Science and Engineering University of Illinois at Urbana-Champaign UrbanaIL61801 United States Electrical Engineering and Computer Science Massachusetts Institute of Technology CambridgeMA United States London Centre for Nanotechnology University College London 17-19 Gordon Street LondonWCH1 0AH United Kingdom Materials Science Division Argonne National Laboratory LemontIL United States Department of Physics and Astronomy Wayne State University DetroitMI48201 United States Department of Physics North Carolina State University RaleighNC27695 United States Department of Electrical Engineering Yale University New HavenCT United States Department of Physics Oakland University MI48309 United States Institute for Quantum Computing Department of Chemistry University of Waterloo WaterlooON Canada Center for Nanoscale Materials Argonne National Laboratory LemontIL United States Department of Physics University of Michigan Ann ArborMI48109 United States

Quantum technology has made tremendous strides over the past two decades with remarkable advances in materials engineering, circuit design and dynamic operation. In particular, the integration of different quantum modules has benefited from hybrid quantum systems, which provide an important pathway for harnessing the different natural advantages of complementary quantum systems and for engineering new functionalities. This review focuses on the current frontiers with respect to utilizing magnetic excitatons or magnons for novel quantum functionality. Magnons are the fundamental excitations of magnetically ordered solid-state materials and provide great tunability and flexibility for interacting with various quantum modules for integration in diverse quantum systems. The concomitant rich variety of physics and material selections enable exploration of novel quantum phenomena in materials science and engineering. In addition, the relative ease of generating strong coupling and forming hybrid dynamic systems with other excitations makes hybrid magnonics a unique platform for quantum engineering. We start our discussion with circuit-based hybrid magnonic systems, which are coupled with microwave photons and acoustic phonons. Subsequently, we are focusing on the recent progress of magnon-magnon coupling within confined magnetic systems. Next we highlight new opportunities for understanding the interactions between magnons and nitrogen-vacancy centers for quantum sensing and implementing quantum interconnects. Lastly, we focus on the spin excitations and magnon spectra of novel quantum materials investigated with advanced optical characterization. Copyright © 2021, The Authors. All rights reserved.

关键词： Quantum optics

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Learning data-adaptive non-parametric kernels

The Journal of Machine Learning Research

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The Journal of Machine Learning Research 2020年第1期21卷 8590-8628页

作者： Fanghui Liu Xiaolin Huang Chen Gong Jie Yang Li Li Department of Electrical Engineering ESAT-STADIUS KU Leuven Belgium Institute of Image Processing and Pattern Recognition Institute of Medical Robotics Shanghai Jiao Tong University Shanghai China PCA Lab Key Laboratory of Intelligent Perception and Systems for High-Dimensional Information of Ministry of Education School of Computer Science and Engineering Nanjing University of Science and Technology China and Department of Computing Hong Kong Polytechnic University Hong Kong SAR China Department of Automation BNRist Tsinghua University China

In this paper, we propose a data-adaptive non-parametric kernel learning framework in margin based kernel methods. In model formulation, given an initial kernel matrix, a data-adaptive matrix with two constraints is imposed in an entry-wise scheme. Learning this data-adaptive matrix in a formulation-free strategy enlarges the margin between classes and thus improves the model flexibility. The introduced two constraints are imposed either exactly (on small data sets) or approximately (on large data sets) in our model, which provides a controllable trade-off between model flexibility and complexity with theoretical demonstration. In algorithm optimization, the objective function of our learning framework is proven to be gradient-Lipschitz continuous. Thereby, kernel and classifier/regressor learning can be efficiently optimized in a unified framework via Nesterov's acceleration. For the scalability issue, we study a decomposition-based approach to our model in the large sample case. The effectiveness of this approximation is illustrated by both empirical studies and theoretical guarantees. Experimental results on various classification and regression benchmark data sets demonstrate that our non-parametric kernel learning framework achieves good performance when compared with other representative kernel learning based algorithms.

关键词： support vector machines non-parametric kernel learning gradient-Lipschitz continuous

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Opacity of Nondeterministic Transition Systems: A (Bi)Simulation Relation Approach

Opacity of Nondeterministic Transition Systems: A (Bi)Simula...

引用

作者： Zhang, Kuize Yin, Xiang Zamani, Majid ACCESS Linnaeus Center School of Electrical Engineering KTH Royal Institute of Technology Stockholm10044 Sweden Department of Automation Key Laboratory of System Control and Information Processing Shanghai Jiao Tong University Shanghai200240 China Computer Science Department University of Colorado Boulder BoulderCO80309 United States

In this paper, we propose several opacity-preserving (bi)simulation relations for nondeterministic transition systems (NTSs) in terms of initial-state opacity, current-state opacity, K-step opacity, and infinite-step opacity. We also show how one can leverage quotient constructions to compute such relations. As a result, although the opacity verification problem for infinite NTSs is generally undecidable, if one can find such an opacity-preserving relation from an infinite NTS to a finite one, the (lack of) opacity of the infinite NTS can be easily verified over the finite one, which is decidable. © 1963-2012 IEEE.

关键词： Opacity

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laboratory realization of relativistic pair-plasma beams

arXiv

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arXiv 2023年

作者： Arrowsmith, C.D. Simon, P. Bilbao, P. Bott, A.F.A. Burger, S. Chen, H. Cruz, F.D. Davenne, T. Efthymiopoulos, I. Froula, D.H. Goillot, A.M. Gudmundsson, J.T. Haberberger, D. Halliday, J. Hodge, T. Huffman, B.T. Iaquinta, S. Miniati, F. Reville, B. Sarkar, S. Schekochihin, A.A. Silva, L.O. Simpson, R. Stergiou, V. Trines, R.M.G.M. Vieu, T. Charitonidis, N. Bingham, R. Gregori, G. Department of Physics University of Oxford Parks Road OxfordOX1 3PU United Kingdom Geneva 23CH-1211 Switzerland GSI Helmholtzzentrum für Schwerionenforschung GmbH Planckstraße 1 Darmstadt64291 Germany GoLP Instituto de Plasmas e Fusão Nuclear Instituto Superior Técnico Universidade de Lisboa Lisboa1049-001 Portugal Lawrence Livermore National Laboratory 7000 East Ave LivermoreCA94550 United States Rutherford Appleton Laboratory Chilton DidcotOX11 0QX United Kingdom University of Rochester Laboratory for Laser Energetics RochesterNY14623 United States Science Institute University of Iceland Dunhaga 3 ReykjavikIS-107 Iceland School of Electrical Engineering and Computer Science KTH Royal Institute of Technology StockholmSE-100 44 Sweden AWE Aldermaston Reading BerkshireRG7 4PR United Kingdom Max-Planck-Institut für Kernphysik Saupfercheckweg 1 HeidelbergD-69117 Germany School of Applied Mathematics and Physical Sciences National Technical University of Athens Athens157 72 Greece Department of Physics University of Strathclyde GlasgowG4 0NG United Kingdom

Relativistic electron-positron (e±) plasmas are ubiquitous in extreme astrophysical environments such as black holes and neutron star magnetospheres, where accretion-powered jets and pulsar winds are expected to be enriched with such pair plasmas. Their behaviour is quite different from typical electron-ion plasmas due to the matter-antimatter symmetry of the charged components and their role in the dynamics of such compact objects is believed to be fundamental. So far, our experimental inability to produce large yields of positrons in quasi-neutral beams has restricted the understanding of electron-positron pair plasmas to simple numerical and analytical studies which are rather limited. We present first experimental results confirming the generation of high-density, quasi-neutral, relativistic electron-positron pair beams using the 440 GeV/c beam at CERN's Super Proton Synchrotron (SPS) accelerator. The produced pair beams have a volume that fills multiple Debye spheres and are thus able to sustain collective plasma oscillations. Our work opens up the possibility of directly probing the microphysics of pair plasmas beyond quasi-linear evolution into regimes that are challenging to simulate or measure via astronomical observations. © 2023, CC BY.

关键词： Magnetosphere

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Boosting Electrocatalytic Nitrogen Reduction on Cobalt-Based Perovskite via Regulating Reaction Pathway Through Donation-Back-Donation Modulation

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Angewandte Chemie 2025年第25期137卷

作者： Ning Han Wei Zhang Jianxiang Wu Kaibin Chu Shihui Feng Shuo Wang Alain R. Puente-Santiago Jinlin Long Bo Weng Bao-Lian Su The Edward S. Rogers Department of Electrical and Computer Engineering University of Toronto Ontario ON M5S 1A4 Canada These authors are cofirst authors. Institute of Energy Materials Science University of Shanghai for Science and Technology Shanghai 200093 China Shanghai Key Laboratory of Materials Protection and Advanced Materials Electric Power Shanghai University of Electric Power Shanghai 200090 China School of Materials Science and Engineering Linyi University Linyi 276000 China Department of Materials and Environmental Chemistry Arrhenius Laboratory Stockholm University Stockholm 10 691 Sweden State Key Laboratory of Chemistry for NBC Hazards Protection State Key Laboratory of Photocatalysis on Energy and Environment College of Chemistry Fuzhou University Fuzhou 350116 China Department of Chemistry and Biochemistry Florida International University Miami FL USA State Key Laboratory of Advanced Environmental Technology Institute of Urban Environment Chinese Academy of Sciences Xiamen 361021 China State Key Laboratory of Advanced Technology for Materials Synthesis and Processing Wuhan University of Technology Wuhan 430070 China Laboratory of Inorganic Materials Chemistry (CMI) University of Namur 61 rue de Bruxelles Namur 5000 Belgium

The electrocatalytic approach of combining N 2 and H 2 O to produce ammonia, known as the electrocatalytic N 2 reduction reaction (eNRR), has garnered significant attention due to its environmental benefits and potential for supporting a decentralized agricultural economy. However, the underlying chemistry governing the reaction pathways remains poorly understood, hindering the design of low-cost and efficient eNRR catalysts. Here we report the enhancement of the electrocatalytic eNRR activity of perovskite oxides by tuning the reaction pathway through a “donation-back-donation” mechanism. This is achieved by controlling the spin state via adjusting the distribution of d orbital electrons in low-cost transition metals, such as cobalt. Specifically, the cobalt in perovskite SrCoO 3 (SC) with a low-spin state demonstrates an 18 times higher ammonia yield rate compared to that in Co 3 O 4 and 1.5 times higher than cobalt in perovskite LaCoO 3 (LC). The low spin states of cobalt in SC enable better control of the eNRR reaction pathway over the transformation of *N 2 H to *NHNH or *NNH 2 , resulting in alternating hydrogenation in SC rather than distal hydrogenation in LC with a high spin state. The unprecedented improvement in eNRR by regulating the spin state of Co demonstrates the bright of low-cost Co-based electrocatalysts for ammonia production.

关键词： D orbital electron distribution Nitrogen reduction reaction Perovskite Reaction pathway Regulation of spin state

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Synthesis of Honeycomb-Structured Beryllium Oxide via Graphene Liquid Cells

引用

Angewandte Chemie 2020年第36期132卷 15864-15870页

作者： Wang L. Liu L. Chen J. Mohsin A. Yum J.H. Hudnall T.W. Bielawski C.W. Rajh T. Bai X. Gao S.-P. Gu G. Beijing National Laboratory for Condensed Matter Physics Institute of Physics Chinese Academy of Sciences Beijing 100190 China Department of Electrical Engineering and Computer Science University of Tennessee Knoxville TN 37996 United States Department of Materials Science and Engineering College of Engineering Peking University Beijing 100871 China Department of Physics and Astronomy London Centre for Nanotechnology Thomas Young Centre University College London London WC1H 0AJ United Kingdom School of physics Peking University Beijing 100871 China Max Planck Institute for Solid State Research Stuttgart 70569 Germany Center for Multidimensional Carbon Materials (CMCM) Institute for Basic Science (IBS) Ulsan 44919 South Korea Department of Chemistry and Department of Engineering Ulsan National Institute of Science and Technology (UNIST) Ulsan 44919 South Korea Department of Chemistry and Biochemistry Texas State University San Marcos TX 78666 United States Center for Nanoscale Materials Argonne National Laboratory Lemont IL 60439 United States Department of Materials Science Fudan University Shanghai 200433 China

Using high-resolution transmission electron microscopy and electron energy-loss spectroscopy, we show that beryllium oxide crystallizes in the planar hexagonal structure in a graphene liquid cell by a wet-chemistry approach. These liquid cells can feature van-der-Waals pressures up to 1 GPa, producing a miniaturized high-pressure container for the crystallization in solution. The thickness of as-received crystals is beyond the thermodynamic ultra-thin limit above which the wurtzite phase is energetically more favorable according to the theoretical prediction. The crystallization of the planar phase is ascribed to the near-free-standing condition afforded by the graphene surface. Our calculations show that the energy barrier of the phase transition is responsible for the observed thickness beyond the previously predicted limit. These findings open a new door for exploring aqueous-solution approaches of more metal-oxide semiconductors with exotic phase structures and properties in graphene-encapsulated confined cells. © 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

关键词： aqueous-solution synthesis beryllium oxide graphene liquid cells high-resolution transmission electron microscopy structural phase transition thermodynamic ultra-thin limit

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Observation of Nonlinear Standing Waves Excited by plasma-Series-Resonance-Enhanced Harmonics in Capacitive Discharges

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Physical Review Letters 2019年第18期122卷 185002-185002页

作者： Kai Zhao De-Qi Wen Yong-Xin Liu Michael A. Lieberman Demetre J. Economou You-Nian Wang Key Laboratory of Materials Modification by Laser Ion and Electron Beams (Ministry of Education) School of Physics Dalian University of Technology Dalian 116024 China Plasma Processing Laboratory Department of Chemical and Biomolecular Engineering University of Houston Houston Texas 77204-4004 USA Department of Electrical Engineering University of California Berkeley California 94720 USA

We report the first experimental observation of nonlinear standing waves excited by plasma-series-resonance-enhanced harmonics in low pressure, very high frequency, parallel plate, capacitively coupled plasmas. Spatial structures of the harmonics of the magnetic field, measured by a magnetic probe, are in very good agreement with simulations based on a nonlinear electromagnetics model. At relatively low pressure, the nonlinear sheath motion generates high-order harmonics that can be strongly enhanced near the series resonance frequencies. Satisfying certain conditions, such nonlinear harmonics induce radial standing waves, with voltage and current maxima on axis, resulting in center-high plasma density. Excitation of higher harmonics is suppressed at higher pressures.

关键词： High-frequency & RF discharges in plasmas Magnetic field generation & plasma dynamo plasma discharges in vacuum plasma sources Wave-wave, wave-particle interactions

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