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LINSCAN - A Linearity Based Clustering Algorithm

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arXiv 2024年

作者： Dennehy, Andrew Zou, Xiaoyu Semnani, Shabnam J. Fialko, Yuri Cloninger, Alexander Committee on Computational and Applied Mathematics University of Chicago ChicagoIL United States Institute of Geophysics and Planetary Physics Scripps Institution of Oceanography University of California La Jolla San DiegoCA92093 United States Department of Structural Engineering University of California La Jolla San DiegoCA92093 United States Department of Mathematics Halcioglu Data Science Institute University of California La Jolla San DiegoCA92093 United States

DBSCAN and OPTICS are powerful algorithms for identifying clusters of points in domains where few assumptions can be made about the structure of the data. In this paper, we leverage these strengths and introduce a new algorithm, LINSCAN, designed to seek lineated clusters that are difficult to find and isolate with existing methods. In particular, by embedding points as normal distributions approximating their local neighborhoods and leveraging a distance function derived from the Kullback Leibler Divergence, LINSCAN can detect and distinguish lineated clusters that are spatially close but have orthogonal covariances. We demonstrate how LINSCAN can be applied to seismic data to identify active faults, including intersecting faults, and determine their orientation. Finally, we discuss the properties a generalization of DBSCAN and OPTICS must have in order to retain the stability benefits of these algorithms. © 2024, CC BY.

关键词： Normal distribution

在天文学中基于机器学习的流体力学问题加速求解

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在天文学中基于机器学习的流体力学问题加速求解

第十三届全国流体力学学术会议

作者：毛顺元董若冰王炜琦 Kwang Moo Yi 陆路王思凡 Paris Perdikaris Department of Physics and Astronomy University of Victoria Department of Computer Science University of British Columbia Department of Statistics and Data Science Yale University Graduate Group in Applied Mathematics and Computational Science University of Pennsylvania Department ofMechanical Engineering and Applied Mechanics University of Pennsylvania

探索行星形成的过程不仅揭示了地球及其生命的起源,还为寻找地外生命提供了关键线索。行星在原行星盘中形成。原行星盘是由气体和尘埃构成的扁平盘状系统,围绕在新生恒星的周围,其跨度可达数千亿公里。近年来,通过分析行星对原行星盘的... 详细信息

探索行星形成的过程不仅揭示了地球及其生命的起源,还为寻找地外生命提供了关键线索。行星在原行星盘中形成。原行星盘是由气体和尘埃构成的扁平盘状系统,围绕在新生恒星的周围,其跨度可达数千亿公里。近年来,通过分析行星对原行星盘的引力扰动,并比较观测和数值模拟,天文学家得以推断正在盘中形成的行星的质量和轨道。然而,这种方法依赖于复杂的流体力学数值模拟,因计算成本高昂而难以普及。为了克服这一挑战,我们开发了一款基于机器学习的工具。该工具应用数据驱动的深度算子网络(DeepONet),将系统的参数映射到流体力学偏微分方程的解,从而无需昂贵的数值模拟即可实现两项功能:1)快速求解正问题,即从行星参数预测盘中物质分布和速度场;2)高效求解反问题,即从观测到的扰动图像中推断行星参数。我们展示了该工具在求解速度、精确性和鲁棒性方面的显著优势,并讨论了其在理论研究和观测中的潜在应用。我们还探索了物理信息神经网络(PINN)在这一问题的应用,特别是针对原初行星盘中旋转吸积流的时间依赖性二维可压缩Navier-Stokes方程。行星引力扰动引入显著的时间依赖性空间特征,使问题复杂化。通过对物理定律而非标注数据的训练,PINN在捕捉系统演化的空间高频特征方面展现潜力。标准PINN在此任务中存在收敛困难、预测误差随时间累积、长时间解偏离物理定律以及捕捉空间高频特征能力不足的问题。为此,我们的研究引入了自适应激活函数、时间推进策略和网络权重继承方法,以提升PINN性能。改进后的PINN模型在与标准PINN的比较中表现出显著更低的预测误差,成功再现了大部分空间高频特征,无需标注数据。这项研究为将PINN应用于类似的时间依赖性流体力学问题提供了重要参考。

关键词：天文行星形成深度学习深度算子网络物理信息神经网络

Good plasmons in a bad metal

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arXiv 2024年

作者： Ruta, Francesco L. Shao, Yinming Acharya, Swagata Mu, Anqi Jo, Na Hyun Ryu, Sae Hee Balatsky, Daria Pashov, Dimitar Kim, Brian S.Y. Katsnelson, Mikhail I. Analytis, James G. Rotenberg, Eli Millis, Andrew J. van Schilfgaarde, Mark Basov, D.N. Department of Physics Columbia University New York United States Department of Applied Physics and Applied Mathematics Columbia University New York United States National Renewable Energy Laboratory GoldenCO United States Advanced Light Source E.O. Lawrence Berkeley National Laboratory BerkeleyCA United States Department of Chemistry University of California BerkeleyCA United States Department of Physics University of California BerkeleyCA United States Theory and Simulation of Condensed Matter King’s College London London United Kingdom Department of Mechanical Engineering Columbia University New York United States Institute for Molecules and Materials Radboud University Nijmegen Netherlands Center for Computational Quantum Physics Flatiron Institute New York United States

Correlated materials may exhibit unusually high resistivity increasing linearly in temperature (1), breaking through the Mott-Ioffe-Regel bound, above which coherent quasiparticles are destroyed (2). The fate of collective charge excitations, or plasmons, in these systems is a subject of debate. Several studies suggest plasmons are overdamped (3-5) while others detect unrenormalized plasmons (6-8). Here, we present direct optical images of low-loss hyperbolic plasmon polaritons (HPPs) in the correlated van der Waals metal MoOCl2 (9). HPPs are plasmon-photon modes that waveguide through extremely anisotropic media (10-15), and are remarkably long-lived in MoOCl2. Many-body theory supported by photoemission results reveals that MoOCl2 is in an orbital-selective and highly incoherent Peierls phase. Different orbitals acquire markedly different bonding-antibonding character (16), producing a highly-anisotropic, isolated Fermi surface (17-18). The Fermi surface is further reconstructed and made partly incoherent by electronic interactions, renormalizing the plasma frequency. HPPs remain long-lived in spite of this, allowing us to uncover previously-unseen imprints of electronic correlations on plasmonic collective modes. © 2024, CC BY.

关键词： Van der Waals forces

Multiple scales analysis of a nonlinear timestepping instability in simulations of solitons

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arXiv 2024年

作者： Hyatt, Benjamin A. Lecoanet, Daniel Anders, Evan H. Burns, Keaton J. Department of Engineering Sciences and Applied Mathematics Northwestern University EvanstonIL60208 United States Center for Interdisciplinary Exploration and Research in Astrophysics Northwestern University EvanstonIL60201 United States Kavli Institute for Theoretical Physics University of California Santa Barbara Santa BarbaraCA93106 United States Department of Mathematics Massachusetts Institute of Technology CambridgeMA02139 United States Center for Computational Astrophysics Flatiron Institute New YorkNY10010 United States

The susceptibility of timestepping algorithms to numerical instabilities is an important consideration when simulating partial differential equations (PDEs). Here we identify and analyze a pernicious numerical instability arising in pseudospectral simulations of nonlinear wave propagation resulting in finite-time blow-up. The blow-up time scale is independent of the spatial resolution and spectral basis but sensitive to the timestepping scheme and the timestep size. The instability appears in multi-step and multi-stage implicit-explicit (IMEX) timestepping schemes of different orders of accuracy and has been found to manifest in simulations of soliton solutions of the Korteweg-de Vries (KdV) equation and traveling wave solutions of a nonlinear generalized Klein-Gordon equation. Focusing on the case of KdV solitons, we show that modal predictions from linear stability theory are unable to explain the instability because the spurious growth from linear dispersion is small and nonlinear sources of error growth converge too slowly in the limit of small timestep size. We then develop a novel multi-scale asymptotic framework that captures the slow, nonlinear accumulation of timestepping errors. The framework allows the solution to vary with respect to multiple time scales related to the timestep size and thus recovers the instability as a function of a slow time scale dictated by the order of accuracy of the timestepping scheme. We show that this approach correctly describes our simulations of solitons by making accurate predictions of the blow-up time scale and transient features of the instability. Our work demonstrates that studies of long-time simulations of nonlinear waves should exercise caution when validating their timestepping schemes. © 2024, CC BY-NC-ND.

关键词： Solitons

Quantum embedding methods for correlated excited states of point defects: Case studies and challenges

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Physical Review B 2022年第23期105卷 235104-235104页

作者： Lukas Muechler Danis I. Badrtdinov Alexander Hampel Jennifer Cano Malte Rösner Cyrus E. Dreyer Center for Computational Quantum Physics Flatiron Institute 162 5th Avenue New York New York 10010 USA Theoretical Physics and Applied Mathematics Department Ural Federal University 620002 Yekaterinburg Russia Department of Physics and Astronomy Stony Brook University Stony Brook New York 11794-3800 USA Institute for Molecules and Materials Radboud University Heijendaalseweg 135 6525 AJ Nijmegen Netherlands

A quantitative description of the excited electronic states of point defects and impurities is crucial for understanding materials properties, and possible applications of defects in quantum technologies. This is a considerable challenge for computational methods, since Kohn-Sham density functional theory (DFT) is inherently a ground-state theory, while higher-level methods are often too computationally expensive for defect systems. Recently, embedding approaches have been applied that treat defect states with many-body methods, while using DFT to describe the bulk host material. We implement such an embedding method, based on Wannierization of defect orbitals and the constrained random-phase approximation approach, and perform systematic characterization of the method for three distinct systems with current technological relevance: a carbon dimer replacing a B and N pair in bulk hexagonal BN (CBCN), the negatively charged nitrogen-vacancy center in diamond (NV−), and an Fe impurity on the Al site in wurtzite AlN (FeAl). In the context of these test-case defects, we demonstrate that crucial considerations of the methodology include convergence of the bulk screening of the active-space Coulomb interaction, the choice of exchange-correlation functional for the initial DFT calculation, and the treatment of the “double-counting” correction. For CBCN we show that the embedding approach gives many-body states in agreement with analytical results on the Hubbard dimer model, which allows us to elucidate the effects of the DFT functional and double-counting correction. For the NV− center, our method demonstrates good quantitative agreement with experiments for the zero-phonon line of the triplet-triplet transition. Finally, we illustrate challenges associated with this method for determining the energies and orderings of the complex spin multiplets in FeAl.

关键词： First-principles calculations Nitrogen vacancy centers in diamond Point defects Wide band gap systems Density functional theory Exact diagonalization Wannier function methods

Magnetic octupole moment of Yb173 using collinear laser spectroscopy

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Physical Review A 2021年第3期103卷 032826-032826页

作者： R. P. de Groote S. Kujanpää Á. Koszorús J. G. Li I. D. Moore Department of Physics University of Jyväskylä PB 35(YFL) FIN-40351 Jyväskylä Finland Department of Physics University of Liverpool Liverpool L69 7ZE United Kingdom Institute of Applied Physics and Computational Mathematics Beijing 100088 China

The hyperfine constants of the 4f146s6pP2o3 state in neutral Yb have been measured using three different dipole transitions. This state was recently shown to have a comparatively large hyperfine magnetic octupole splitting, and thus a puzzlingly large magnetic octupole moment. The measurement is performed using collinear laser spectroscopy on a fast atomic beam, which provides a straightforward route to probing long-lived metastable atomic states with high resolution. From the combined analysis of all three lines we find no significant evidence for a nonzero octupole moment in Yb173.

关键词： Fine & hyperfine structure Laser spectroscopy 150 ≤ A ≤ 189 Magnetic moment

A self-induced mechanism of large-scale helical structures in compressible turbulent flows

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arXiv 2024年

作者： Yan, Zheng Wang, Jianchun Wang, Lifeng Lei, Zhu Wu, Junfeng Duan, Junyi Tong, Fulin Li, Xinliang Yu, Changping Institute of Applied Physics and Computational Mathematics Beijing100094 China Department of Mechanics and Aerospace Engineering Southern University of Science and Technology Guangdong Shenzhen518055 China Center for Applied Physics and Technology HEDPS Peking University Beijing100871 China LHD Institute of Mechanics Chinese Academy of Sciences Beijing100190 China School of Engineering Science University of Chinese Academy of Sciences Beijing100049 China State Key Laboratory of Aerodynamics China Aerodynamics Research and Development Center Mianyang621000 China

A novel self-sustaining mechanism is proposed for large-scale helical structures in compressible turbulent flows. The existence of two channels of subgrid-scale and viscosity terms for large-scale helicity evolution is confirmed for the first time, through selecting a physical definition of the large-scale helicity in compressible turbulence. Under the influence of the fluid element expansion, it is found that the helicity is generated at small scales via the second-channel viscosity, and the inverse cross-scale helicity transfers at inertial scales through the second-channel helicity flux. Together, they form a self-induced mechanism, which provides a physical insight into the long-period characteristic of large-scale helical structures in the evolution of compressible flow systems. © 2024, CC0.

关键词： Turbulent flow

Atomic Rydberg-state excitation in strong spatially inhomogeneous fields

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Physical Review A 2023年第3期107卷 033104-033104页

作者： Shilin Hu Xiaopeng Yi Li Guo Chengrui Bi Jing Chen Guangdong Provincial Key Laboratory of Quantum Metrology and Sensing & School of Physics and Astronomy Sun Yat-Sen University (Zhuhai Campus) Zhuhai 519082 China Institute of Applied Physics and Computational Mathematics P. O. Box 8090 Beijing 100088 China Department of Physics Shanghai Normal University Shanghai 200234 China Department of Modern Physics and Hefei National Research Center for Physical Sciences at the Microscale and School of Physical Sciences University of Science and Technology of China Hefei 230026 China Shenzhen Key Laboratory of Ultraintense Laser and Advanced Material Technology Center for Advanced Material Diagnostic Technology and College of Engineering Physics Shenzhen Technology University Shenzhen 518118 China

We investigated the Rydberg-state excitation process of a hydrogen atom subjected to spatially homogeneous and inhomogeneous laser fields by means of ab initio calculations. It is found that, comparing with atoms exposed to spatially homogeneous laser fields, the excitation probability decreases and the electron tends to occupy the states with lower principal quantum numbers and angular quantum numbers for atoms in spatially inhomogeneous laser pulses. Furthermore, calculations of a quantum model without taking into account ionization of the electron after it is coherently captured by the Rydberg state are inconsistent with the above-stated findings by ab initio calculations. Analysis indicates that the aforementioned intriguing features can be attributed to the enhanced ionization of the Rydberg states by inhomogeneous laser fields since the distributions of Rydberg states of high principal quantum numbers and angular quantum numbers locate far away from the core where the inhomogeneous electric fields become significant.

关键词： Atomic & molecular processes in external fields Electronic excitation & ionization Ultrafast phenomena Atoms Rydberg atoms & molecules Schroedinger equation

Emergent conformal boundaries from finite-entanglement scaling in matrix product states

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arXiv 2023年

作者： Huang, Rui-Zhen Zhang, Long Läuchli, Andreas M. Haegeman, Jutho Verstraete, Frank Vanderstraeten, Laurens Department of Physics and Astronomy University of Ghent Belgium Kavli Institute for Theoretical Sciences CAS Center for Excellence in Topological Quantum Computation University of Chinese Academy of Sciences Beijing100190 China Laboratory for Theoretical and Computational Physics Paul Scherrer Institute Villigen Switzerland Lausanne Switzerland Department of Applied Mathematics and Theoretical Physics University of Cambridge United Kingdom Center for Nonlinear Phenomena and Complex Systems Université Libre de Bruxelles Belgium

The use of finite entanglement scaling with matrix product states (MPS) has become a crucial tool for studying one-dimensional critical lattice theories, especially those with emergent conformal symmetry. We argue that finite entanglement introduces a relevant deformation in the critical theory. As a result, the bipartite entanglement Hamiltonian defined from the MPS can be understood as a boundary conformal field theory with a physical and an entanglement boundary. We are able to exploit the symmetry properties of the MPS to engineer the physical conformal boundary condition. The entanglement boundary, on the other hand, is related to the concrete lattice model and remains invariant under this relevant perturbation. Using critical lattice models described by the Ising, Potts, and free compact boson CFTs, we illustrate the influence of the symmetry and the relevant deformation on the conformal boundaries in the entanglement spectrum. Copyright © 2023, The Authors. All rights reserved.

关键词： Lattice theory