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Acta Mechanica Sinica 2021年第9期37卷 1371-1377,I0001页

作者： Zengyao Lv Peng Liu Yuanshuai Ding Hangyu Li Yongmao Pei Department of Engineering Mechanics Center for Flexible Electronics TechnologyApplied Mechanics LaboratoryTsinghua UniversityBeijing100084China State Key Laboratory for Turbulence and Complex Systems Department of Mechanics and Engineering ScienceCollege of EngineeringPeking UniversityBeijing100871China Locomotive and Car Research Institute China Academy of Railway Sciences Corporation LimitedBeijing100081China

Metamaterials can control incident waves in the sub-wavelength range through the design of artificial structures, and realize the functions that natural materials cannot achieve. The study of metamaterials has important theoretical value and application prospects. In recent years, the proposal of computational metamaterials has opened up a brand-new direction for analog computing, providing high-throughput, energy-free computing methods for special computing tasks. However, the development of acoustic computing metamaterials is relatively preliminary, and it is necessary to develop design theories. There is no work to solve partial differential equations and realize fractional Fourier transform in spatial domain acoustic computing metamaterials. In this paper, the acoustic wave computational metamaterial is designed, and the simulation realizes the spatial domain fractional Fourier transform and partial differential equation calculation. It is expected that acoustic computational metamaterials will enable new capabilities in signal acquisition and processing, network computing, and drive new applications of sound wave.

关键词： Fractional Fourier transform Partial differential equations Acoustic computational metamaterials

A STOCHASTIC TRUST-REGION FRAMEWORK FOR POLICY OPTIMIZATION

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Journal of Computational Mathematics 2022年第6期40卷 1004-1030页

作者： Mingming Zhao Yongfeng Li Zaiwen Wen Beijing International Center for Mathematical Research Peking UniversityBeijing 100871China

In this paper,we study a few challenging theoretical and numerical issues on the well known trust region policy optimization for deep reinforcement *** goal is to find a policy that maximizes the total expected reward when the agent acts according to the *** trust region subproblem is constructed with a surrogate function coherent to the total expected reward and a general distance constraint around the latest *** solve the subproblem using a preconditioned stochastic gradient method with a line search scheme to ensure that each step promotes the model function and stays in the trust *** overcome the bias caused by sampling to the function estimations under the random settings,we add the empirical standard deviation of the total expected reward to the predicted increase in a ratio in order to update the trust region radius and decide whether the trial point is ***,for a Gaussian policy which is commonly used for continuous action space,the maximization with respect to the mean and covariance is performed separately to control the entropy *** theoretical analysis shows that the deterministic version of the proposed algorithm tends to generate a monotonic improvement of the total expected reward and the global convergence is guaranteed under moderate *** with the state-of-the–art methods demonstrate the effectiveness and robustness of our method over robotic controls and game playings from OpenAI Gym.

关键词： Deep reinforcement learning Stochastic trust region method Policy optimization Global convergence Entropy control

Synthesis and structural analysis of titanium-μ-dinitrogen complex supported by di-anionic guanidinate ligands

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Science China Chemistry 2023年第3期66卷 755-759页

作者： Botao Wu Rui Feng Zhu-Bao Yin Haihan Yan Xueli Wang Gao-Xiang Wang Junnian Wei Zhenfeng Xi Key Laboratory of Bioorganic Chemistry and Molecular Engineering Ministry of Education and Beijing National Laboratory for Molecular ScienceCollege of ChemistryPekingUniversityBeijing100871China

The synthesis,characterization,and theoretical studies of titanium-μ-N_(2) complexes with di-anionic guanidinate ligands were reported as the first example of its ***,with(Me_(3)Si)_(2)N-guanidinate ligands,the mono-anionic guanidinate-supported titanium-μ-N2complex 1 was ***,reduction of 1 with potassium afforded the di-anionic guanidinate-supported titanium-μ-N_(2) complex 2 via cleavage of one N–Si bond of the(Me3Si)2N substituents in 1,changing the guanidinate ligands automatically from mono-anionic to di-anionic and remarkably lengthening the bond length of theμ-N_(2).Characteristic studies and DFT calculations were performed to reveal that the di-anionic guanidinate ligands stabilized the geometry of 2 and increased the charge density on the bridging dinitrogen.

关键词： dinitrogen complex di-anionic guanidinate ligand titanium

Implementing fractional Fourier transform using SH_(0) wave computational metamaterials in space domain

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Science China(Technological Sciences) 2021年第11期64卷 2560-2565页

作者： LV ZengYao DING YuanShuai LI HangYu PEI YongMao Department of Engineering Mechanics Center for Flexible Electronics TechnologyApplied Mechanics LaboratotyTsinghua University'Beijing 100084China State Key Laboratory for Turbulence and Complex Systems Department of Mechanics and Engineering ScienceCollege of EngineeringPeking UniversityBeijing 100871China

Through the design of artificial structures, metamaterials can control the incident wave in the sub-wavelength range to achieve functions that natural materials cannot achieve, such as high absorption, negative refraction, cloaking [1–3],asymmetric propagation [4,5], and holography [6–8]. In recent years, the new concept of computational metamaterials[9–12] has opened up a new direction for analog computing。

关键词： wave fractional refraction

Design method and machine learning application of acoustic holographic computational metamaterials

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Science China(Technological Sciences) 2022年第1期65卷 238-243页

作者： LV ZengYao LI JianQing HOU ZeWei DING YuanShuai XU WangDong PEI YongMao Department of Engineering Mechanics/Center for Flexible Electronics Technology Applied Mechanics LaboratoryTsinghua UniversityBeijing 100084China State Key Laboratory for Turbulence and Complex Systems Department of Mechanics and Engineering ScienceCollege of EngineeringPeking UniversityBeijing 100871China

Acoustic holographic method has been used in imaging,particle manipulation,material preparation and other *** acoustics,it is important to design the implementation of any acoustic wave formation surface[1-5].Traditio... 详细信息

关键词： holographic method acoustics

Isotope effect on the Casimir force

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Science China(Physics,Mechanics & Astronomy) 2023年第10期66卷 131-137页

作者： Lanyi Xie Fuwei Yang Bai Song Department of Energy and Resources Engineering Peking UniversityBejing 100871China Center for Nano and Micro Mechanics Tsinghua UniversityBejing 100084China Department of Advanced Manufacturing and Robotics Peking UniversityBejing 100871China Beijing Innovation Center for Engineering Science and Advanced Technology Peking UniversityBeijing 100871China

Isotopic dependence of the Casimir force is key to probing new physics and pushing novel technologies at the micro and nanoscale, but is largely unexplored. In 2002, an isotope effect of 10^(-4) was estimated for metals—orders of magnitude beyond the experimental resolution. Here, by employing the Lifshitz theory, we reveal a significant isotope effect of over 10^(-1) for polar dielectrics. This effect arises from the isotope-mass-induced line shift of the zone-center optical phonons and is insensitive to the linewidth. We perform numerical analyses on both the imaginary and real-frequency axes, and derive analytical formulas for predicting the isotope effect. The three-orders-of-magnitude difference between polar dielectrics and metals arises from the distinct isotopic dependence of the phonon and plasma frequencies. Our work opens up a new avenue for engineering forces at small scales and may also facilitate the quest for the fifth force of nature.

关键词： isotope effect Casimir force polar dielectric phonon line shift plasma frequency

Dynamic inversion of the rupture parameters on fault system with complex geometry:A GPU parallel genetic algorithm based on BIEM

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Earthquake Science 2019年第5期32卷 187-196页

作者： Feng Qian Haiming Zhang Department of Geophysics School of Earth and Space SciencesPeking UniversityBeijing 100871China

In this study,we attempted to perform an earthquake source dynamic inversion to obtain dynamic parameters on fault system with complex *** forward modeling of the spontaneous rupture process is carried out using a boundary integral equation method(BIEM)based on unstructured meshing,and the inversion method is implemented by a genetic algorithm based on a parallel acceleration of the *** source model in this study is a branched fault,which is described by two physical parameters,the initial stress T0 and the critical slip-weakening distance *** investigated the effect of the inherited parameters on the accuracy and convergence of the inversion *** results showed that if a set of parameters are assigned properly,the inversion of rupture parameters is accurate and converges *** is easy to converge to a local optimal solution during the inversion process if inappropriate inherited parameters are *** with T0,D。has better convergence and accuracy *** inversion process.

关键词： dynamic inversion genetic algorithm initial stress critical slip-weakening distance

Synthesis and reactivity of cobalt dinitrogen complex supported by nonsymmetrical pincer ligand

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Chinese Chemical Letters 2024年第7期35卷 259-263页

作者： Yuanjin Chen Xianghui Shi Dajiang Huang Junnian Wei Zhenfeng Xi Beijing National Laboratory for Molecular Sciences(BNLMS) Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of EducationCollege of ChemistryPeking UniversityBeijing 100871China State Key Laboratory of Organometallic Chemistry Shanghai Institute of Organic ChemistryShanghai 200032China

A nonsymmetrical PNN pincer ligand[6-(^(t)Bu_(2)PNH)C_(5)H_(4)N-2-(3-Mes)C_(3)H_(2)N_(2)]and its corresponding cobalt-N_(2)complex were synthesized and *** the stoichiometric reaction of the PNN ligand lithium salt with CoCl_(2),the complex 3,(PNN)CoCl,was ***,reduction of 3 with NaBHEt3under a dinitrogen atmosphere yielded complex 5,(PNN)Co(Ⅰ)(η^(1)-N_(2)).Single-crystal X-ray analysis,IR spectrum,and DFT calculations revealed that the dinitrogen in 5 was only weakly reduced by the cobalt *** reactions of 5 with carbon monoxide and 2,6-dimethylphenyl isocyanide gave carbonyl and isocyanide complexes 6 and 7 with the release of N_(2),***,these cobalt complexes,especially complex 5,demonstrated the capacity to convert dinitrogen to N(TMS)_(3)with moderate efficiency.

关键词： Dinitrogen fixation Dinitrogen transformation Nonsymmetrical pincer ligand Cobalt complex Catalytic silylamine formation

Spontaneous T-symmetry breaking and exceptional points in cavity quantum electrodynamics systems

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Science Bulletin 2018年第17期63卷 1096-1100页

作者： Yu-Kun Lu Pai Peng Qi-Tao Cao Da Xu Jan wiersig Qihuang Gong Yun-Feng Xiao State Key Laboratory for Mesoscopic Physics and Collaborative Innovation Center of Quantum Matter School of Physics Peking University Beijing 100871 China Collaborative Innovation Center of Extreme Optics Shanxi University Taiyuan 030006 China Department of Electrical Engineering and Computer Science Massachusetts Institute of Technology Cambridge MA 02139 USA Institut für Physik Otto-von-Guericke-Universitat Magdeburg Postfach 4120 D-39016 Magdeburg Germany

Spontaneous symmetry breaking has revolutionized the understanding in numerous fields of modern physics. Here, we theoretically demonstrate the spontaneous time-reversal symmetry breaking in a cavity quantum electrodynamics system in which an atomic ensemble interacts coherently with a single resonant cavity mode. The interacting system can be effectively described by two coupled oscillators with positive and negative mass, when the two-level atoms are prepared in their excited states. The occurrence of symmetry breaking is controlled by the atomic detuning and the coupling to the cavity mode,which naturally divides the parameter space into the symmetry broken and symmetry unbroken *** two phases are separated by a spectral singularity, a so-called exceptional point, where the eigenstates of the Hamiltonian coalesce. When encircling the singularity in the parameter space, the quasiadiabatic dynamics shows chiral mode switching which enables topological manipulation of quantum states.

关键词： Exceptional point Spontaneous symmetry breaking Topological singularity Cavity QED

Light-triggered fluorochromic cholesteric liquid crystal elastomer with hydrogen-bonded fluorescent switch: dualmodal-switchable circularly polarized luminescence

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Science China Chemistry 2024年第8期67卷 2719-2727页

作者： Siyang Lin Tianqi Ren Xianyu Meng Wenxin Kang Jinbao Guo Key Laboratory of Carbon Fibers and Functional Polymers Ministry of EducationCollege of Materials Science and EngineeringBeijing University of Chemical TechnologyBeijing 100029China

Solid materials with dynamically tunable circularly polarized luminescence(CPL) feature higher security levels and devicefriendly characteristics, showing great superiority in the field of information technology and anti-counterfeiting. To address the limited photoisomerization of fluorescent photoswitch in CPL-active solid materials, here a cholesteric liquid crystal elastomer(CLCE) containing a hydrogen-bonded(H-bonded) polymerizable fluorescent switch AHBA-PSD is prepared. Owing to the good flexibility and low glass-transition temperature of CLCE, AHBA-PSD shows a fast and fully reversible photocyclization/cycloreversion with fluorochromic behavior in CLCE. Further, by controlling the spectral overlapping area between the emission and reflection bands, the CLCE exhibits a strong CPL with a g_(lum) value of up to 0.76 accompanied by a reversible phototunable CPL signal. Meanwhile, the H-bond helps to stabilize the mechanical property, and the force-induced switching-off of CPL signal could be achieved due to the destroyed helical structures by external force. Finally, an automatically-recognized identification card with abundant chiroptical information is demonstrated.

关键词： cholesteric liquid crystal elastomer circularly polarized luminescence fluorescent photoswitch diarylethene photoismerization