Codimension-two bifurcation of axial loaded beam bridge subjected to an infinite series of moving loads

A novel model is proposed which comprises of a beam bridge subjected to an axial load and an infinite series of moving loads. The moving loads, whose distance between the neighbouring ones is the length of the beam bridge, coupled with the axial force can lead the vibration of the beam bridge to codimension-two bifurcation. Of particular concern is a parameter regime where non-persistence set regions undergo a transition to persistence regions. The boundary of each stripe represents a bifurcation which can drive the system off a kind of dynamics and jump to another one, causing damage due to the resulting amplitude jumps. The Galerkin method, averaging method, invertible linear transformation, and near identity nonlinear transformations are used to obtain the universal unfolding for the codimension-two bifurcation of the mid-span deflection. The efficiency of the theoretical analysis obtained in this paper is verified via numerical simulations.     

Lower Exciton Number Strong Light Matter Interaction in Plasmonic Tweezers

The plasmonic nanocavity is an excellent platform for the study of light matter interaction within a sub-diffraction volume under ambient conditions.We design a structure of plasmonic tweezers,which can trap molecular Jaggregates and also serve as a plasmonic cavity with which to investigate strong light matter interaction.The optical response of the cavity is calculated via finite-difference time-domain methods,and the optical force is evaluated based on the Maxwell stress tensor method.With the help of the coupled oscillator model and virtual exciton theory,we investigate the strong coupling progress at the lower level of excitons,finding that a Rabi splitting of 230 meV can be obtained in a single exciton system.We further analyze the relationship between optical force and model volume in the coupling system.The proposed method offers a way to locate molecular J-aggregates in plasmonic tweezers for investigating optical force performance and strong light matter interaction.     

Dynamic characteristics of nanoindentation in Ni:A molecular dynamics simulation study

In this work,three-dimensional molecular dynamics simulation is carried out to elucidate the nanoindentation behaviour of single crystal Ni.The substrate indenter system is modelled using hybrid interatomic potentials including the manybody potential(embedded atom method) and two-body Morse potential.The spherical indenter is chosen,and the simulation is performed for different loading rates from 10 m/s to 200 m/s.Results show that the maximum indentation load and hardness of the system increase with the increase of velocity.The effect of indenter size on the nanoindentation response is also analysed.It is found that the maximum indentation load is higher for the large indenter whereas the hardness is higher for the smaller indenter.Dynamic nanoindentation is carried out to investigate the behaviour of Ni substrate to multiple loading-unloading cycles.It is observed from the results that the increase in the number of loading unloading cycles reduces the maximum load and hardness of the Ni substrate.This is attributed to the decrease in recovery force due to defects and dislocations produced after each indentation cycle.     

Near-Field Birefringence Response of Liquid Crystal Molecules in Thickness Direction of Liquid Crystal Thin Film Orientated by Shear Force

Information of molecular orientation in nematic liquid crystal （LC） is attractive and important for applications in the field of display devices. We demonstrate a novel method using a birefringence scanning near-field optical microscope （Bi-SNOM） with a probe which is inserted into the LC thin film to detect the molecular orientation from its birefringence responses in the thickness direction of the LC thin film. The probe is laterally vibrated when going forward into the LC thin film, and the retardation and azimuth angle are recorded as the probe going down. Firstly, the thickness of the LC thin film is measured by the shear force detection. Since the shear force acts as a stimulation to reorientate the LC molecules above the substrate surface, we can detect the molecular orientation caused by a polyimide alignment substrate and the effect to molecular orientation caused by vibration of fibre probe. As a result, the orientation profiling of the LC film in depth direction is obtained in both the cases that the direction of probe vibrating is vertical/parallel to the rubbing direction of the alignment film. Furthermore, the thickness of completely orientated layers just above the substrate surface can also be obtained by either vibrating probe or no-vibrating probe. Ultimately, the LC thin film can be modelled in thickness direction from all the results using this method.     

Quantum dynamics on a lossy non-Hermitian lattice

We investigate quantum dynamics of a quantum walker on a finite bipartite non-Hermitian lattice,in which the particle can leak out with certain rate whenever it visits one of the two sublattices.Quantum walker initially located on one of the non-leaky sites will finally totally disappear after a length of evolution time and the distribution of decay probability on each unit cell is obtained.In one regime,the resultant distribution shows an expected decreasing behavior as the distance from the initial site increases.However,in the other regime,we find that the resultant distribution of local decay probability is very counterintuitive,in which a relatively high population of decay probability appears on the edge unit cell which is the farthest from the starting point of the quantum walker.We then analyze the energy spectrum of the non-Hermitian lattice with pure loss,and find that the intriguing behavior of the resultant decay probability distribution is intimately related to the existence and specific property of the edge states,which are topologically protected and can be well predicted by the non-Bloch winding number.The exotic dynamics may be observed experimentally with arrays of coupled resonator optical waveguides.     

Roles of V/III ratio and mixture degree in GaN growth: CFD and MD simulation study

To understand the mechanism of Gallium nitride (GaN) film growth is of great importance for their potential applica- tions. In this paper, we investigate the growth behavior of the GaN film by combining computational fluid dynamics (CFD) and molecular dynamics (MD) simulations. Both of the simulations show that V/III mixture degree can have important impacts on the deposition behavior, and it is found that the more uniform the mixture is, the better the growth is. Besides, by using MD simulations, we illustrate the whole process of the GaN growth. Furthermore, we also find that the V/III ratio can affect the final roughness of the GaN film. When the V/III ratio is high, the surface of final GaN film is smooth. The present study provides insights into GaN growth from the macroscopic and microscopic views, which may provide some suggestions on better experimental GaN preparation.     

Casimir Force on a Piston in Randall-Sundrum Models

The Casimir effect of a piston for massless scalar fields which satisfy Dirichlet boundary conditions in the context of five-dimensional Randall- Sundrum models is studied. In these scenarios we derive and calculate the expression for the Casimir force on the piston. We also discuss the Casimir force in the limit that one outer plate is moved to the remote place to show that the nature of the reduced force between the parallel plates left. In the Randall~undrum model involving two branes the two plates attract each other when they locate very closely, but the reduced Casimir force turns to be repulsive as the plates separation is not extremely tiny, which is against the experimental phenomena, meaning that the RSI model can not be acceptable. In the case of one brane model the shape of the reduced Casimir force is similar to that of the standard two-parallel-system in the four-dimensional fiat spacetimes while the sign of force remains negative.     

Unveiling the early stage evolution of local atomic structures in the crystallization process of a metallic glass

The early stage evolution of local atomic structures in a multicomponent metallic glass during its crystallization process has been investigated via molecular dynamics simulation. It is found that the initial thermal stability and earliest stage evolution of the local atomic clusters show no strong correlation with their initial short-range orders, and this leads to an observation of a novel symmetry convergence phenomenon, which can be understood as an atomic structure manifestation of the ergo...     

Empirical Analysis and Modeling of the Global Economic System

In the global economic system, each economy stimulates the growth of its gross domestic products （GDP） by increasing its international trade. Using a fluctuation analysis of the flux data of GDP and foreign trade, we find that both GDP and foreign trade are dominated by external force and driven by each other. By excluding the impact of the associated trade dependency degree, GDP and the total volume of foreign trade collapse well into a power-law function. The economy＇s total trade volume scales with the number of trade partners, and it is distributed among its trade partners in an exponential form. The model which incorporated these empirical results can integrate the growth dynamics of GDP and the interplay dynamics between GDP and weighted international trade networks simultaneously.     

Monte Carlo Simulation on Coordinated Movement of Kinesin and Dynein Motors

Kinesin and dynein are two important classes of molecular motors which are responsible for active organelle trafficking and cell division. They can work together to carry a cargo, moving along the microtubule in a coordinated way. We use Monte Carlo method to simulate the dynamics of this coordinated movement. Based on four essential assumptions, our simulations reproduce some features of the recent in vivo experiments. The fast moving speed of the cargo is simulated and the speed distribution is presented.     

Joint UAVs’ Load Balancing and UEs’ Data Rate Fairness Optimization by Diffusion UAV Deployment Algorithm in Multi-UAV Networks

Unmanned aerial vehicles (UAVs) can be deployed as base stations (BSs) for emergency communications of user equipments (UEs) in 5G/6G networks. In multi-UAV communication networks, UAVs’ load balancing and UEs’ data rate fairness are two challenging problems and can be optimized by UAV deployment strategies. In this work, we found that these two problems are related by the same performance metric, which makes it possible to optimize the two problems simultaneously. To solve this joint optimization problem, we propose a UAV diffusion deployment algorithm based on the virtual force field method. Firstly, according to the unique performance metric, we define two new virtual forces, which are the UAV-UAV force and UE-UAV force defined by FU and FV, respectively. FV is the main contributor to load balancing and UEs’ data rate fairness, and FU contributes to fine tuning the UEs’ data rate fairness performance. Secondly, we propose a diffusion control stratedy to the update UAV-UAV force, which optimizes FV in a distributed manner. In this diffusion strategy, each UAV optimizes the local parameter by exchanging information with neighbor UAVs, which achieve global load balancing in a distributed manner. Thirdly, we adopt the successive convex optimization method to update FU, which is a non-convex problem. The resultant force of FV and FU is used to control the UAVs’ motion. Simulation results show that the proposed algorithm outperforms the baseline algorithm on UAVs’ load balancing and UEs’ data rate fairness.     

Dynamics of load balancing with constraints

In this paper, we consider a centralized strategy for scheduling charging patterns of electrical vehicles and other batteries in power grids. We formulate it as a load balancing problem with constraints, which tries to distribute the charging loads both spatially and temporally. We show that a variant of herding system can be applied to load balancing.     

Ordering effects in disordered systems: the Au-Si system

We have characterized the short-range order in the liquid and undercooled states of Au-Si alloy at the eutectic composition using molecular dynamics simulations. The interactions are described via a modified embedded-atom model refined to take into account the liquid properties. For the eutectic liquid, the local structure is characterized by a strong Au-Si affinity, namely a well-pronounced chemical short-range order which leads to the slowing down of the formation of icosahedral local motifs in the undercooled regime. Moreover we discuss the influence of this peculiar local structure on the dynamic and thermodynamic properties of the liquid phase and compare our results with available experimental data.     

基于内存约束的一维负载平衡方法及其应用

在内存约束的条件下,提出一种新的一维负载平衡方法.该方法是一个迭代法,严格的理论证明表明,它可以线性收敛到最优剖分.针对典型的负载不平衡模型和三维分子动力学问题,1024个处理器上的实际应用表明,该方法可以取得较好的效果.     

泡泡布点方法及其并行性

针对无网格和有限元方法中的布点需求,借鉴分子动力学方法和泡泡网格化方法的思想和处理技术,提出一种单纯的布点方法--泡泡布点方法.将区域内的点看作有相互作用力的泡泡,通过动态模拟得到高质量的点集,模拟中不需要网格连接.算例显示,生成的均匀点集和非均匀点集都具有很好的结构和渐进性,并对复杂区域具有很好的适应性.同时由于相互作用力的局部性,该布点方法类似于短程作用的分子动力学模拟,拥有潜在的并行性.通过初步的并行算例,验证了其并行化的可行性.     

In situ observations of compressive behaviour of aluminium foams by local tomography using high-resolution X-rays

We have investigated the compressive behaviour of closed-cell aluminium foams using a high-resolution X-ray CT. The microstructures of cell walls or Plateau borders in the foams were visualized in 3D using the local tomography technique which is a high-resolution CT method to reconstruct a region of interest within a large sample. The shapes and the 3D distribution of micropores, particles, and regions of solute segregation in the foams are evaluated, comparing the cell walls with the Plateau borders. Under compressive loads, the damage behaviour of such microstructures has been observed using an in situ test rig. It is found that the microcracks were mainly initiated from the cell walls and the micropores with large diameters were also damaged. The crack initiation sites are classified from the results. In addition, a method for non-destructive characterization of elastic and plastic deformation in the foams, which is called a 3D microstructure gauge (MG) method, is presented. Thousands of micropores as markers on each load were automatically matched by the information of those volumes and surface areas. The local strain mapping by the MG indicates that the edges of the micropores with large diameters have large strain under compression and this is consistent with the crack analyses.     

Symmetry-adapted graph neural networks for constructing molecular dynamics force fields

Molecular dynamics is a powerful simulation tool to explore material properties. Most realistic material systems are too large to be simulated using first-principles molecular dynamics. Classical molecular dynamics has a lower computational cost but requires accurate force fields to achieve chemical accuracy. In this work, we develop a symmetry-adapted graph neural network framework called the molecular dynamics graph neural network(MDGNN) to construct force fields automatically for molecular dynamics simulations for both molecules and crystals. This architecture consistently preserves translation, rotation, and permutation invariance in the simulations. We also propose a new feature engineering method that includes high-order terms of interatomic distances and demonstrate that the MDGNN accurately reproduces the results of both classical and first-principles molecular dynamics. In addition, we demonstrate that force fields constructed by the proposed model have good transferability.The MDGNN is thus an efficient and promising option for performing molecular dynamics simulations of large-scale systems with high accuracy.     

网络竞价系统中负载均衡技术优化

在网络系统正常运行中,如何确保数据准确实时高效的传输变得越来越重要,在一些高并发、高吞吐的服务中,确保每个事务功能正常运转越来越重要;系统的负载是否均衡是确保整个系统能否完整工作的关键,现在有很多负载均衡算法旨在使得每个服务器的负载趋于相等,可以归纳为静态均衡方法和动态均衡方法;考虑到影响服务器的负载有多重因数,结合贪心算法作出相对合理的负载均衡策略,应用到本次竞价系统中对比以前的平衡策略,通过比较可以得出多衡量指标的综合考虑会提高负载均衡的性能,对网络竞价系统能够起到更优化的作用。     

非绝热分子动力学的量子路径模拟

基于近期发展的经典-量子混合模拟非绝热分子动力学的量子路径方案,本文对5个典型势能面模型进行了模拟,包括单交叉模型、双交叉模型、拓展耦合模型、哑铃模型以及双弓模型.由于难以在严格意义上得到退相干速率,数值模拟中,我们比较了三个不同的退相干速率公式,包括冻结高斯波包近似退相干速率、能量分辨速率以及力分辨速率.在模拟过程中,我们恰当地处理了势能面跳跃时的能量守恒和力的反向问题.通过与全量子动力学模拟的精确结果进行对比发现,对于结构较简单的势能面模型,三种退相干速率都能得到较好的结果;然而对于较复杂的势能面模型,由于复杂量子干涉的原因,与其他混合经典-量子动力学方案类似,量子路径方案仍然难以得到较准确的结果.如何发展更加有效的混合经典-量子模拟方案,是未来研究的重要课题.