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Stable phases of freestanding monolayer TiO2: Emergence of out-of-plane ferroelectricity

Physical Review B 2024年第16期109卷 165435-165435页

作者： Xiangtian Bu Haitao Liu Yuanchang Li Key Lab of Advanced Optoelectronic Quantum Architecture and Measurement (MOE) and Advanced Research Institute of Multidisciplinary Science Beijing Institute of Technology Beijing 100081 China Institute of Applied Physics and Computational Mathematics Beijing 100088 China National Key Laboratory of Computational Physics Beijing 100088 China

Despite being successfully synthesized [Zhang et al., Nat. Mater. 20, 1073 (2021)], the monolayer structure of stable hexagonal TiO2 is unknown, and it is not even clear whether it can exist in a freestanding form. Through first-principles calculations, we have identified two previously uncharted stable structures, namely, distorted 1×3 1T−TiO2 and 3×3 1T−TiO2, both of which are energetically more favorable than commonly adopted 1H and 1T phases. Here, structural distortions are characterized by the out-of-plane shifts of Ti atoms due to the pseudo-Jahn-Teller interactions, which break one and all two inversion symmetries of 1T configuration. As a consequence, the 1×3 1T remains centrosymmetric, while the 3×3 1T exhibits out-of-plane ferroelectricity. Electronic structure calculations show that both are wide-band-gap semiconductors with band gaps larger than their bulk counterparts. In this paper, we not only deepen the understanding of structural instability in wide-gap semiconductors but also add a member to the rare family of two-dimensional out-of-plane ferroelectrics.

关键词： Crystal structure Ferroelectricity Pseudo Jahn-Teller effect Monolayer films First-principles calculations

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The effect of external magnetic field on projectile in the stopping power of warm dense deuterium for electrons

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The European Physical Journal Special Topics 2025年 1-16页

作者： Chen, Yuzhu Mo, Chongjie Xue, Quanxi Fu, Zhenguo Liu, Hao Department of Applied Physics School of Physics and Electronics Hunan University Changsha China Beijing Computational Science Research Center Beijing China State Key Laboratory of Intense Pulsed Radiation Simulation and Effect Northwest Institute of Nuclear Technology Xi’an China Institute of Applied Physics and Computational Mathematics Beijing China

The stopping power of warm dense plasmas for electrons is a critical aspect in the study of hot electron transport. An externally applied strong magnetic field can significantly influence electron transport behavior due to various factors. However, the impact of external magnetic fields on the motion of incident particles is often overlooked. Through molecular dynamics simulations using the electron force field (eFF) method, this study investigates the stopping process of individual hot electrons in warm dense deuterium plasma under an applied longitudinal magnetic field. Results show that, at typical laboratory magnetic field intensities, the magnetic field significantly alters electron trajectories without notable effects on average stopping power, trajectory length, or scattering angle. Even with increased magnetic field intensity beyond 500 kT, it doesn’t affect the total kinetic energy loss of incident electrons but reduces stopping power by compressing the scattering angle distribution width. Due to the increase in the scattering angle distribution width with intensified fluctuations in high-temperature targets, the impact of the additional magnetic field on stopping power becomes more pronounced with an increase in target temperature.

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Effect of ablation on the nonlinear spike growth for the single-mode ablative Rayleigh-Taylor instability

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Matter and Radiation at Extremes 2023年第1期8卷 53-62页

作者： J.Y.Fu H.S.Zhang H.B.Cai P.L.Yao S.P.Zhu Institute of Applied Physics and Computational Mathematics Beijing 100088People’s Republic of China Laboratory of Computational Physics Institute of Applied Physics and Computational MathematicsBeijing 100088People’s Republic of China Graduate School China Academy of Engineering PhysicsP.O.Box 2101Beijing 100088People’s Republic of China Science and Technology on Plasma Physics Laboratory Laser Fusion Research CenterCAEPMianyang 621900People’s Republic of China

The effect of ablation on the nonlinear spike growth of single-mode ablative Rayleigh–Taylor instability(RTI)is studied by two-dimensional numerical *** is shown that the ablation can reduce the quasi-constant velocity and significantly suppress the reacceleration of the spike in the nonlinear *** is also shown that the spike growth can affect the ablation-generated vorticity inside the bubble,which further affects the nonlinear bubble *** vorticity evolution is found to be correlated with the mixing width(i.e.,the sum of the bubble and spike growths)for a given wave number and ablation *** considering the effects of mass ablation and vorticity,an analytical model for the nonlinear bubble and spike growth of single-mode ablative RTI is developed in this *** is found that the nonlinear growth of the mixing width,induced by the single mode,is dominated by the bubble growth for small-scale ablative RTI,whereas it is dominated by the spike growth for classical RTI.

关键词： phase nonlinear bubble

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CONVERGENCE OF DISCONTINUOUS GALERKIN METHODS FOR QUASICONVEX AND RELAXED VARIATIONAL PROBLEMS

arXiv

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arXiv 2025年

作者： Grekas, Georgios Koumatos, Konstantinos Makridakis, Charalambo Vikelis, Andreas Computer Electrical Mathematical Sciences and Engineering Division KAUST Saudi Arabia Department of Mathematics University of Sussex United Kingdom Institute of Applied & Computational Mathematics Foundation for Research & Technology-Hellas Greece Department of Mathematics and Applied Mathematics University of Crete Greece Faculty of Civil Engineering Czech Technical University of Prague Czech Republic

In this work, we establish that discontinuous Galerkin methods are capable of producing reliable approximations for a broad class of nonlinear variational problems. In particular, we demonstrate that these schemes provide essential flexibility by removing inter-element continuity while also guaranteeing convergent approximations in the quasiconvex case. Notably, quasiconvexity is the weakest form of convexity pertinent to elasticity. Furthermore, we show that in the non-convex case discrete minimisers converge to minimisers of the relaxed problem. In this case, the minimisation problem corresponds to the energy defined by the quasiconvex envelope of the original energy. Our approach covers all discontinuous Galerkin formulations known to converge for convex energies. This work addresses an open challenge in the vectorial calculus of variations: developing and rigorously justifying numerical schemes capable of reliably approximating nonlinear energy minimization problems with potentially singular solutions, which are frequently encountered in materials science. © 2025, CC BY.

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Vortex inverted pin beams: Mitigation of scintillations in strong atmospheric turbulence

arXiv

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arXiv 2024年

作者： Droulias, Sotiris Loulakis, Michalis Papazoglou, Dimitris G. Tzortzakis, Stelios Chen, Zhigang Efremidis, Nikolaos K. Institute of Applied and Computational Mathematics Foundation for Research and Technology - Hellas Crete Heraklion70013 Greece Institute of Electronic Structure and Laser Foundation for Research and Technology - Hellas Crete Heraklion70013 Greece Department of Materials Science and Technology University of Crete Crete Heraklion70013 Greece MOE Key Laboratory of Weak-Light Nonlinear Photonics TEDA Applied Physics Institute School of Physics Nankai University Tianjin300457 China Department of Mathematics and Applied Mathematics University of Crete Crete Heraklion70013 Greece

We recently introduced a new class of optical beams with a Bessel-like transverse profile and increasing beam width during propagation, akin to an "inverted pin". Owing to their specially engineered distribution, these beams have shown remarkable performance in atmospheric turbulence. Specifically, inverted pin beams were found to have reduced scintillation index as compared to collimated or focused Gaussian beams as well as other types of pin beams especially in moderate to strong turbulence. In this work, we demonstrate that inverted pin beams carrying orbital angular momentum can further suppress intensity scintillations in moderate to strong irradiance fluctuation conditions. Our results can be useful in improving the performance and link availability of free-space optical communication systems. Copyright © 2024, The Authors. All rights reserved.

关键词： Atmospheric turbulence

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Multi-phase-field simulation of austenite peritectic solidification based on a ferrite grain

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Chinese Physics B 2021年第1期30卷 572-577页

作者： Chao Yang Jing Wang Junsheng Wang Yu Liu Guomin Han Haifeng Song Houbing Huang School of Materials Science and Engineering Beijing Institute of TechnologyBeijing 100081China Advanced Research Institute of Multidisciplinary Science Beijing Institute of TechnologyBeijing 100081China Laboratory of Computational Physics Institute of Applied Physics and Computational MathematicsBeijing 100088China Software Center for High Performance Numerical Simulation China Academy of Engineering PhysicsBeijing 100088China

A multi-phase-field model is implemented to investigate the peritectic solidification of Fe-C alloy. The nucleation mode of austenite is based on the local driving force, and two different thicknesses of the primary austenite on the surface of the ferrite equiaxed crystal grain are used as the initial conditions. The simulation shows the multiple interactions of ferrite, austenite, and liquid phases, and the effects of carbon diffusion, which presents the non-equilibrium dynamic process during Fe-C peritectic solidification at the mesoscopic scale. This work not only reveals the influence of the austenite nucleation position, but also clarifies the formation mechanism of liquid phase channels and molten pools. Therefore, the present study contributes to the understanding of the micro-morphology and micro-segregation evolution mechanisms of Fe-C alloy during peritectic solidification.

关键词： multi-phase-field simulation morphology evolution peritectic solidification carbon diffusion Fe-C alloy

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Quasi-Lindblad pseudomode theory for open quantum systems

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Physical Review B 2024年第19期110卷 195148-195148页

作者： Gunhee Park (박건희) Zhen Huang Yuanran Zhu Chao Yang Garnet Kin-Lic Chan Lin Lin Division of Engineering and Applied Science California Institute of Technology Pasadena California 91125 USA Department of Mathematics University of California Berkeley California 94720 USA Applied Mathematics and Computational Research Division Lawrence Berkeley National Laboratory Berkeley California 94720 USA Division of Chemistry and Chemical Engineering California Institute of Technology Pasadena California 91125 USA

We introduce a new framework to study the dynamics of open quantum systems with linearly coupled Gaussian baths. Our approach replaces the continuous bath with an auxiliary discrete set of pseudomodes with dissipative dynamics, but we further relax the complete positivity requirement in the Lindblad master equation and formulate a quasi-Lindblad pseudomode theory. We show that this quasi-Lindblad pseudomode formulation directly leads to a representation of the bath correlation function in terms of a complex weighted sum of complex exponentials, an expansion that is known to be rapidly convergent in practice and thus leads to a compact set of pseudomodes. The pseudomode representation is not unique and can differ by a gauge choice. When the global dynamics can be simulated exactly, the system dynamics is unique and independent of the specific pseudomode representation. However, the gauge choice may affect the stability of the global dynamics, and we provide an analysis of why and when the global dynamics can retain stability despite losing positivity. We showcase the performance of this formulation across various spectral densities in both bosonic and fermionic problems, finding significant improvements over conventional pseudomode formulations.

关键词： Open quantum systems Open quantum systems & decoherence Strongly correlated systems Anderson impurity model Lindblad equation Non-Markovian processes Nonperturbative methods Spin-boson model

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Unified gas-kinetic wave-particle methods IV: multi-species gas mixture and plasma transport

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Advances in Aerodynamics 2021年第1期3卷 186-216页

作者： Chang Liu Kun Xu Department of Mathematics Hong Kong University of Science and TechnologyHong KongChina Institute of Applied Physics and Computational Mathematics Beijing 100088China Shenzhen Research Institute Hong Kong University of Science and TechnologyShenzhen 518057China.

In this paper,we extend the unified gas-kinetic wave-particle(UGKWP)methods to the multi-species gas mixture and multiscale plasma *** construction of the scheme is based on the direct modeling on the mesh size and time step scales,and the local cell’s Knudsen number determines the flow *** proposed scheme has the multiscale and asymptotic complexity diminishing *** multiscale property means that according to the cell’s Knudsen number the scheme can capture the non-equilibrium flow physics when the cell size is on the kinetic mean free path scale,and preserve the asymptotic Euler,Navier-Stokes,and magnetohydrodynamics(MHD)when the cell size is on the hydrodynamic scale and is much larger than the particle mean free *** asymptotic complexity diminishing property means that the total degrees of freedom of the scheme reduce automatically with the decreasing of the cell’s Knudsen *** the continuum regime,the scheme automatically degenerates from a kinetic solver to a hydrodynamic *** the UGKWP,the evolution of microscopic velocity distribution is coupled with the evolution of macroscopic variables,and the particle evolution as well as the macroscopic fluxes is modeled from a time accumulating solution of kinetic scale particle transport and collision up to a time step *** plasma transport,the current scheme provides a smooth transition from particle-in-cell(PIC)method in the rarefied regime to the magnetohydrodynamic solver in the continuum *** the continuum limit,the cell size and time step of the UGKWP method are not restricted by the particle mean free path and mean collision *** the highly magnetized regime,the cell size and time step are not restricted by the Debye length and plasma cyclotron *** multiscale and asymptotic complexity diminishing properties of the scheme are verified by numerical tests in multiple flow regimes.

关键词： Unified gas-kinetic wave-particle method Multiscale modeling Gas mixture Plasma transport

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On the optimally controlled stochastic shallow lake

arXiv

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arXiv 2023年

作者： Koutsimpela, Angeliki Loulakis, Michail School of Applied Mathematical and Physical Sciences National Technical University of Athens Athens15780 Greece Institute of Applied and Computational Mathematics Foundation of Research and Technology Hellas Crete Heraklion70013 Greece

We consider the stochastic control problem of the shallow lake and continue the work of G. T. Kossioris, Loulakis, and Souganidis (2019) in three directions. First, we generalise the characterisation of the value function as the viscosity solution of a well-posed problem to include more general recycling rates. Then, we prove approximate optimality under bounded controls and we establish quantitative estimates. Finally, we implement a convergent and stable numerical scheme for the computation of the value function to investigate properties of the optimally controlled stochastic shallow lake. This approach permits to derive tails asymptotics for the invariant distribution and to extend results of Grass, Kiseleva, and Wagener (2015) beyond the small noise *** Codes 93E20, 60H30, 49L25 Copyright © 2023, The Authors. All rights reserved.

关键词： Stochastic systems

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TensorMD: Molecular Dynamics Simulation with Ab Initio Accuracy of 50 Billion Atoms 25

TensorMD: Molecular Dynamics Simulation with Ab Initio Accur...

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Proceedings of the 30th ACM SIGPLAN Annual Symposium on Principles and Practice of Parallel Programming

作者： Yucheng Ouyang Ying Liu Honghui Shang Zhenchuan Chen Jiahao Shan Huimin Cui Xiaobing Feng Xin Chen Xingyu Gao Lifang Wang Haifeng Song Rongfen Lin Fang Li Institute of Computing Technology Chinese Academy of Sciences Beijing China National Research Center of Parallel Computer Engineering and Technology Beijing China Institute of Applied Physics and Computational Mathematics Beijing China

ISBN: (纸本)9798400714436

Molecular dynamics simulation emerges as an important area that HPC+AI helps to investigate the physical properties, with machine-learning interatomic potentials (MLIPs) being used. General-purpose machine-learning (ML) tools have been leveraged in MLIPs, but they are not perfectly matched with each other, since many optimization opportunities in MLIPs have been missed by ML tools. This inefficiency arises from the fact that HPC+AI applications work with far more computational complexity compared with pure AI scenarios. This paper has developed an MLIP, named TensorMD, independently from any ML tool. TensorMD has been evaluated on two supercomputers and scaled to 51.8 billion atoms, i.e., ~ 3× compared with state-of-the-art.

关键词： GPU

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