bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamic impact strength of diamond-SiC superhard composite
Authors: Li Yuan-Yuan , Yu Yin , Meng Chuan-Min , Zhang Lu , Wang Tao , Li Yong-Qiang , He Hong-Liang , He Duan-Wei ,   Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610065, China,   National Key Laboratory of Shock Wave and Detonation Physics, Institute of Fluid Physics, CAEP, Mianyang 621900, China,   School of Sciences, Northeastern University, Shenyang 110819, China
Published: December 2020

Unlike the ductile materials, the failure seriously limits the strength of the brittle medium. To understand the mechanism of controlling the dynamic impact strength of diamond-SiC superhard composite under shock wave compression, the numerical simulation is conducted with a lattice-spring model that can describe the mechanical properties of diamond-SiC superhard composite quantitatively. For the simulation, the diamond-SiC superhard composite is constructed by different volume content of diamond and SiC particles. The obtainted shock wave profiles indicate that the dynamic impact strength first increases and then decreases with the increase of diamond content in the sample. The analysis based on the meso-scale damage pattern reveals that such a variation of dynamic impact strength corresponds to three damage evolution modes. When the diamond content increases to a value between 10%–50% in volume percentage, the long slip bands are first dominated, and then becomes short slip bands when the diamond content is 70%, and damage happens mainly in SiC matrix whereas most of the diamond particles are not damaged. When the diamond content is above a critical value of 70% in volume percentage, even the short slip bands are limited heavily, which makes it difficult to relax the shear stress on diamond particles and causes serious damage to diamond particles, finally results in the reduction of dynamic strength.
Pressure of active system under the electric double layer interaction
Authors: Jin Kang , Jing Guang-Yin ,   School of Physics, Northwest University, Xi'an 710127, China
Published: December 2020

Self-driven particle systems consist of particles that can extract energy from the environment and transform into active motion, and thus are significantly different from the classical passive particle systems. For such an active system, the question of whether there is a classical equation of state (EOS) has caused spreading concern. Recent studies analyzed the validity of the EOS of an active system under the harmonic potential (Solon et. al, 2015 Nature Physics, 11 673). In contrast, this paper explores the conditions for and the specific forms of the EOS of an active system under electric double-layer interaction between the wall and the particles. The results show that the wall pressure is related to the shape of the active particles. When a wall exerts a moment on the active particles, the particles orientation turns to the equilibrium state parallel to the wall surface under the action of the moment, and the increase of the wall-particle interaction strength enhances the parallel-orientation trend, which reduces the system pressure. The association of pressure and wall means that the active system does not have a general equation of state. In the case where the wall-particle interaction intensity is extremely small or extremely large, by defining the effective temperature, the active system has an equation of state similar to that of the ideal gas. In addition, it is found that the extent of the shape of particles deviating from the rotational symmetry is a key factor affecting the pressure of active particles. The research results provide a reference for the study of the current active system equilibrium properties, and provide a basis for studying the thermodynamic properties of active systems under more complex interaction potentials.
Modeling of three-dimensional sound propagation through solitary internal waves
Authors: Zhang Ze-Zhong , Luo Wen-Yu , Pang Zhe , Zhou Yi-Qing ,   State Key Laboratory of Acoustics, Institute of Acoustics, Chinese Academy of Sciences, Beijing 100190, China,   University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

An accurate and numerically efficient numerical model is very important for studying the effect of internal wave on underwawter sound propagation. A full-wave, three-dimensional (3D) coupled-mode model is able to deal with the internal wave problem with satisfactory accuracy, but such a model is in general numerically inefficient. A numerically efficient 3D model is presented for sound propagation in a range-dependent waveguide in the presence of solitary internal waves in this work. The present model is a forward-marching model that neglects backscattering. In this 3D model, an efficient two-dimensional (2D) coupled-mode model, C-SNAP, is adopted to compute 2D acoustic field solutions excited by a line source. The C-SNAP is a 2D forward-marching model, which uses an energy-conserving matching condition to preserve accuracy. An appealing aspect of C-SNAP is that its efficiency is competitive with that of the existing parabolic equation model. The integral transform technique is used to extend C-SNAP to a 3D model, where a complex integration contour is used for evaluating the wavenumber integral. A brief review of C-SNAP and formulation of the present 3D model are given. The forward-marching models are primarily suitable for treating the range-dependent problems with weak backscattering, such as with a slowly varying bathymetry. Since in general the backscattering from internal wave is weak, which is also validated numerically in this work, the present model is able to address the problem of sound propagation through internal wave with satisfactory accuracy. At the same time, it achieves an efficiency gain of at least an order of magnitude over that of full two-way, 3D model. In addition to the internal wave, the present model is also suitable for solving the general range-dependent problems where backscattering is weak, such as in the presence of a bottom ridge of a small height. Numerical simulations are also provided to validate the present model, where a two-way, 3D model serves as the benchmark. The numerical results show that the effect of the internal wave on the acoustic field is negligible for the region between the source and the internal wave. However, the effect is significant on the other side of the internal wave. A more interesting observation is the angular dependence of the interference pattern induced by the internal wave.
Design and fabrication of low-noise superconducting quantum interference device magnetometer
Authors: Han Hao-Xuan , Zhang Guo-Feng , Zhang Xue , Liang Tian-Tian , Ying Li-Liang , Wang Yong-Liang , Peng Wei , Wang Zhen ,   State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China,   Chinese Academy of Sciences, Center for Excellence in Superconducting Electronics, Shanghai 200050, China,   University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

Superconducting quantum interference device (SQUID) is the most sensitive magnetic flux sensor known, which is widely used in biomagnetism, low-field nuclear magnetic resonance, geophysics, etc. In this paper, we introduce a high-sensitivity SQUID magnetometer, which consists of an SQUID and a flux transformer. The SQUID is first-order gradiometer configuration, which is insensitive to interference noise. The flux transformer includes a multi-turn spiral input coil and a large-sized pickup coil. And the input coil is inductively coupled to the SQUID through mutual inductance. We present an SQUID magnetometer fabricated with Nb/Al-AlOx/Nb Josephson junction technology on a 4-inch silicon wafer at our superconducting electronics facilities. We develop a fabrication process based on selective niobium etching process consisting of five mask levels. In the first two mask levels, the trilayer is patterned by a dry etch to define base electrode, contact pads, and interconnects. The shunt resistor and a dielectric insulating layer are then deposited and patterned by using lift-off and dry etchant, respectively. Finally, the niobium wiring layer is deposited and patterned by using reactive ion etching to define input, pickup and feedback coils. The measurement of the SQUID magnetometer is performed inside a magnetically shielded room. The operating temperature is realized by immersing the SQUID into the liquid helium (4.2 K). Moreover, a superconducting niobium tube is employed to protect the SQUID from being disturbed by external environments. A homemade readout electronics instrument with low input voltage noise is used to characterize the SQUID magnetometer. The results of low-temperature measurements indicate that the magnetometer has a magnetic field sensitivity of 0.36 nT/Φ0 and a white flux noise of 8 μΦ0/√Hz,corresponding to a white field noise of 2.88 fT/√Hz. This kind of SQUID magnetometer is suitable for multi-channel systems, e.g., magnetocardiography, magnetoencephalography, etc. Although the SQUID process development benefits from the rapid advance of semiconductor process technology, the uniformity of the SQUID on one wafer is fluctuated due to the film deposition. Now, we have realized a best SQUID yield of 50% on a 4-inch wafer. In the future, the SQUID chip yield should be improved by well controlling the optimizing process. The device yield is expected to reach as high as 80%.
Resonant X-ray diffraction studies on modulation structures of high temperature superconducting sample Sr2CuO3.4
Authors: Wang Hai-Bo , Luo Zhen-Lin , Liu Qing-Qing , Jin Chang-Qing , Gao Chen , Zhang Li ,   Tonghua Normal University, Tonghua 134002, China,   University of Science and Technology of China, Hefei 230026, China,   Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China
Published: December 2020

Sr2CuO3+δ is cuprate, a high temperature superconducting (HTS) material that has a single copper oxide layer and a relatively high critical temperature. Its structure is simple and contains fewer atoms, but there are many modulation structures in it. A lot of studies have pointed out that the modulation structure is related to its superconductivity. In order to further study the relationship between modulated structure and superconductivity in Sr2CuO3.4 HTS sample, a new explanation for the formation mechanism of modulation structure is proposed in this paper.

The synchrotron radiation resonant X-ray diffraction (RXD) technique is used to detect the variation of modulation structure near the absorption edge of Cu atom. Cu2+, Cu3+ valence order is detected and used to explain the formation mechanism of modulation structure in Sr2CuO3.4 high temperature superconducting sample. The energy values of incident light are selected to be 8.52, 8.95, 8.98, 9.05, 9.5, and 10.0 keV near the edge of Cu K. The energy resolution is about 1.5 eV. The detector used in the experiment is Mar165 CCD surface detector. The distance from the detector to the sample is about 315 mm. The two-dimensional diffraction pattern recorded by the CCD plane detector is processed by Fit2D software to obtain the diffraction integral intensity. In addition, the energy calibration for each of the copper foil samples is carried out prior to the start of the experiment and in the process of varying energy value.

The experimental results show that the Bragg diffraction peaks corresponding to Tc = 48 K and the modulation structures of Fmmm and Pmmm are visible and calibrated. The intensity of the corresponding (2/5, 4/5, 0) diffraction peak of Fmmm is energy-dependent near the Cu K edge and first increases and then decreases abruptly near the absorption edge. This indicates that a stable ordered arrangement structure of Cu2+ and Cu3+ is formed at this time. The weak diffraction signal of this ordered arrangement structure confirms the fact that the copper-O bonding is stronger.

The experiments indicate that oxygen vacancies occupy both the apical position and the CuO2 plane. The ordering arrangement of oxygen vacancies results in the ordering of copper ions with variable valence. The Cu2+, Cu3+ valence order is related to the superconductivity of Sr2CuO3.4.
Efficient third harmonic generation of 355 nm picosecond laser pulse
Authors: Cheng Meng-Yao , Wang Zhao-Hua , He Hui-Jun , Wang Xian-Zhi , Zhu Jiang-Feng , Wei Zhi-Yi ,   School of Physics and Optoelectronic Engineering, Xidian University, Xi’an 710071, China,   Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
Published: December 2020

Picosecond laser with high-repetition-rate and high pulse energy is widely favorite in many scientific and industrial applications. Some nonlinear crystals can be used to efficiently convert a near-infrared laser into a green laser or an ultraviolet laser which has a higher photon energy and a smaller focal area. Especially for high-quality and high-speed transparent hard material fabrication, green or ultraviolet picosecond laser has been found to possess unique advantages. In this paper, the experiments on high-efficiency second-harmonic-generation (SHG) and third-harmonic-generation (THG) by using a home-made all-solid-state picosecond laser amplifier and an LBO crystal are reported. The picosecond laser amplifier consists of a seed source, a regenerative amplifier and a two-stage single-pass amplifier. The seed source is a commercial all-solid-state picosecond oscillator with a pulse duration of 8.3 ps and a repetition rate of 68 MHz. The repetition rate is reduced from 68 MHz to 500 kHz by an electro-optic Pockels cell (PC), and the period doubling bifurcation is minimized by reducing the duration of high voltage in PC. Both the regenerative amplifier and the two-stage single-pass amplifier are pumped by three 30-W continuous-wave fiber-coupled laser diodes. After the regenerative amplifier, the seed laser is amplified to 4.86 W with a repetition rate of 500 kHz at 1064 nm. Then the laser power is increased to 23.2 W by a two-stage single-pass amplifier, and the M2 value of the amplified laser in the X direction and in the Y direction are 1.330 and 1.235, respectively. The final pulse duration is 13.4 ps, which is slightly stretched in the amplification chain compared with the seed pulse duration (8.3 ps). For high-efficiency SHG and THG from near-infrared to green and ultraviolet, we carefully study the optical characteristics of some nonlinear crystals, such as LBO, BBO, BIBO, CLBO, etc., and we find that the LBO crystal, which has a high damage threshold, small walk-off and high nonlinear coefficient, is the best choice for both SHG and THG. Then the parameters of the two crystals for SHG and THG are specially designed according to the phase matching condition, the walk-off and the laser parameter. As a result, a 4-mm-long type-I phase matching LBO with cutting angle of θ = 90° and φ = 11.6° is used for SHG, and a 3-mm-long type-II phase matching LBO with cutting angle of θ = 42.2° and φ = 90° is used for THG. Finally, we realize high-efficiency frequency conversion with SHG power of 12.7 W at 532 nm and THG power of 9.25 W at 355 nm. The corresponding optical-optical conversion efficiencies reach 54.7% and 39.6%, respectively.
Research progress of plasmonic nanofocusing
Authors: Li Pan ,   Beijing Key Laboratory for Nano-Photonics and Nano-Structure (NPNS), Department of Physics, Capital Normal University, Beijing 100048, China
Published: December 2020

Surface plasmons (SPs) are the surface waves of collective oscillations of free electrons at metal-dielectric interface, which have the ability to overcome the diffraction limit and to enhance the giant near-field. Tapered metallic nanostructures that support surface plasmons’ propagation are highly attractive to nanophotonic applications because of their waveguiding and field-focusing properties. This distinct morphologic feature enables the functionality known as nanofocusing. As a result, the plasmons can be guided in these nanostructures and finally focused on the sharp apex to greatly enhance the local field. This attractive effect can be widely used for effective remote-excitation detection/sensing. In this paper, we review various types of plasmonic nanofocusing structures operating in the visible and infrared region. We focus on their fundamentals, fabrications, and applications. Firstly, we discuss the mechanisms of the plasmonic nanofocusing. Then, the characteristics of various tapered metallic nanostructures of SPs are reviewed, including on-chip waveguides, metal tips and bottom-up fabricated nanowires. For applications, some prototypes of plasmonic nanofocusing for bio/chemo sensing are demonstrated. Finally, a summary and outlook of plasmonic waveguides are given.
Simulation of static and dynamic mechanical characteristics of carbon nanotubes and carbon nano-peapods with defects
Authors: Wang Lei , Zhang Ran-Ran , Fang Wei ,   College of Mechanics and Materials, Hohai University, Nanjing 211100, China
Published: December 2020

The static and dynamic mechanical characteristics of carbon nanotubes with double and multiple vacancy defects are simulated by the molecular dynamics method. Firstly, the effects of diatomic and polyatomic vacancy defects on the quasi-static mechanical properties of carbon nanotubes are discussed. Then, the effects of defects and axial pre-stress on the dynamics of C60 molecular oscillation in carbon nano-peapods are discussed. The results show that the ultimate stress, ultimate strain and elastic modulus of carbon nanotube containing different types of diatomic vacancies are significantly reduced as compared with those of non-defective carbon nanotubes. When the carbon nanotubes have many defective atoms and the defects are connected together to form a crack, the axial compressive properties of the carbon nanotubes are greatly reduced. Compared with the circumferential development of cracks, the cracks along the axis greatly reduce the compressive capacity of carbon nanotubes, which is similar to that of shell models with cracks. The oscillation frequency of C60 molecular in defective carbon nano-peapods is affected by the number of missing atoms. The single vacancy defect increases the oscillation frequency of C60 molecule, while with the further increase of vacancy number, the oscillation frequency of C60 molecule decreases gradually. When the defective carbon nano-peapod has axial tensile or compressive pre-stress, the oscillation of the C60 molecule is affected not only by the defects, but also by the axial pre-stress, which makes the oscillation of C60 molecule more complicated.
Effect of concentration-dependent diffusion coefficient on dendrite growth in directional solidification
Authors: Chu Shuo , Guo Chun-Wen , Wang Zhi-Jun , Li Jun-Jie , Wang Jin-Cheng ,   State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China,   Faculty of Mechanical Engineering, Kyoto Institute of Technology, Kyoto 606-8585, Japan,   National Defense Technology Industry Precision Forging and Technology Innovation Center, Northwestern Polytechnical University, Xi’an 710072, China
Published: December 2020

Solute diffusion is an important process that determines the dendrite growth during solidification. The theoretical model generally simplifies the solute diffusion coefficient in liquid phase into a constant. Nevertheless, the composition of the boundary layer changes greatly in the solidification process, the diffusion coefficient will no longer be a constant and is dependent on concentration. In this paper, the quantitative phase field model is used to simulate the effect of concentration-dependent diffusion coefficient on dendrite growth in directional solidification. In the model, the concentration-dependent diffusion process is investigated by coupling the concentration-dependent diffusion coefficient in the liquid solute diffusion equation. A series of simulation results confirms that the concentration-dependent diffusion process has a significant effect on the dendrite growth. The results show that the increase of the coupling intensity of solute concentration will enhance the diffusion of solute in the mushy zone between primary dendrites to the dendrite tip, resulting in the increase of solute enrichment at the dendrite tip, thereby increasing the tip undercooling. The variation of diffusion coefficient in liquid phase has little effect on the tip radius of dendrite, and the simulation results are in good agreement with those from the theoretical model. Moreover, the amplitude of dendritic side branches decreases with the increase of solute diffusion coefficient. In the study of dendrite arrays, it is found that the concentration-dependent diffusion coefficient increases the primary spacing and reduce the tip position. The results of this study indicate that for a system with a concentration-dependent coefficient significantly, the effect of concentration-dependent diffusion on tip undercooling and side branches should be considered in the quantitative and experimental verification of the existing model.
Effect of equilibrium distribution positivity on numerical performance of lattice Boltzmann method
Authors: Ye Huan-Feng , Jin Di , Kuang Bo , Yang Yan-Hua ,   School of Nuclear Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China,   State Power Investment Central Research Institute Nuclear Power Software Development Center, Beijing 100029, China
Published: December 2020

Traditionally, the numerical performance of the lattice Boltzmann method is mainly determined by the moment degree of a discrete equilibrium distribution. The equilibrium distribution positivity is merely considered as an ancillary property which is used to constrict the numerical configuration. With the newly-developed partial Gaussian-Hermite quadrature scheme, the positivity of equilibrium distribution is validated as an independent property like moment degree which can be adjusted by discrete velocities. Researchers speculated that the positivity should also be significant for the numerical performance by the lattice Boltzmann method and can be used to improve the performance. Comparing with the classical improvement through moment degree, the positivity approach will not bring additional computation. However, due to the lack of boundary treatment, the speculation has not been validated by detailed numerical simulations. In this paper, through employing a periodic case, the Taylor-Green vortex, to avoid the boundary issue, we in depth analyze the numerical effect of the model positivity, including the numerical accuracy in the model positive range, the influence of positivity on the numerical performance, and the significance comparison between positivity and moment degree. The results show that for a given model, the numerical accuracy is not consistent in the whole positive range. As the configuration is close to the border of positive range, the accuracy will degrade though it is still acceptable. The numerical performance of a model depends on both moment degree and positivity. The role that the moment degree plays lies mainly in the qualification of a model on Galilean invariance. Once a model fulfills the Galilean invariance, its numerical performance is solely dependent on the positivity. Hence, the improvement approach through modifying the model positivity is a viable solution, and a Galilean invariant model with wider positive range does possess a better numerical performance regardless of its moment degree. Furthermore, based on the numerical results in this paper, all DnHm models with high moment degree are better than the classical D2Q9 model. Of the above models, the D2H3-2 model has the best performance and deserves to be further studied
A three-dimensional simplified simulation model based on charge conservation law for internal charging in spacecraft
Authors: Yuan Qing-Yun , Sun Yong-Wei , Zhang Xi-Jun ,   National Key Laboratory of Electromagnetic Environment Effect, Army Engineering University of PLA, Shijiazhuang 050003, China
Published: December 2020

The simulation is one of the important methods to evaluate the internal charging risk in spacecraft. In this paper, based on the charge conservation law, a three-dimensional calculation model of the potential and electric field of internal charging is established, and the one-dimensional steady state and transient solution algorithm and the two-dimensional and three-dimensional solution scheme of the model are given. An interative algorithm is designed to solve the required conductivity and the electric field intensity, and the convergence of the interative algorithm is analyzed. Using the finite element algorithm and the local mesh refinement, the model has the advantage of easily investigating the electric field distortion at key points. Comparing with the existing radiation-induced conductivity (RIC) model, due to the fact that the internal charging time constant is much higher than the charge capture time and the trap density in the dielectric is much higher than the charge density after the charge balance, the free charge will be rapidly converted into the captured charge. Therefore, it is unnecessary to consider the charge capture mechanism in the RIC model. The CCL model can be used to evaluate the internal charging and has higher computational efficiency. Comparing with the experimental data, the correctness of the three-dimensional calculation model is verified. It provides a means to evaluate the dielectric internal charging in spacecraft.
Recognition of adsorption phase transition of polymer on surface by neural network
Authors: Sun Li-Wang , Li Hong , Wang Peng-Jun , Gao He-Bei , Luo Meng-Bo ,   College of Mathematics, Physics and Electronic Information Engineering, Wenzhou University, Wenzhou 325035, China,   Department of Information, Wenzhou Vocational and Technical College, Wenzhou 325035, China,   Department of Physics, Zhejiang University, Hangzhou 310027, China
Published: December 2020

Traditional Monte Carlo simulation requires a large number of samples to be employed for calculating various physical parameters, which needs much time and computer resources due to inefficient statistical cases rather than mining data features for each example. Here, we introduce a technique for digging information characteristics to study the phase transition of polymer generated by Monte Carlo method. Convolutional neural network (CNN) and fully connected neural network (FCN) are performed to study the critical adsorption phase transition of polymer adsorbed on the homogeneous cover and stripe surface. The data set (conformations of the polymer) is generated by the Monte Carlo method, the annealing algorithm (including 48 temperatures ranging from T = 8.0 to T = 0.05) and the Metropolis sampling method, which is marked by the state labeling method and the temperature labeling method and used for training and testing of the CNN and the FCN. The CNN and the FCN network can not only recognize the desorption state and adsorption state of the polymer on the homogeneous surface (the critical phase transition temperature TC = 1.5, which is close to the critical phase transition temperature TC = 1.625 of the infinite chain length of polymer adsorbed on the homogeneous surface regardless of the size effect), but also recognize the desorption state, the single-stripe adsorption state and the multi-stripe adsorption state of polymer on the stripe surface(the critical phase transition temperature T1 = 0.55 and T2 = 1.1, which are consistent respectively with T1 = 0.58 and T2 = 1.05 of polymer adsorbed on the stripe-patterned surface derived from existing research results). We obtain almost the same critical adsorption temperature by two different labeling methods. Through the study of the relationship between the size of the training set and the recognition rate of the neural network, it is found that the deep neural network can well recognize the conformational state of polymer on homogeneous surface and stripe surface of a small set of training samples (when the number of samples at each temperature is greater than 24, the recognition rate of the polymer is larger than 95.5%). Therefore, the deep neural network provides a new calculation method for polymer simulation research with the Monte Carlo method.
Complex network centrality method based on multi-order K-shell vector
Authors: Wang Kai-Li , Wu Chun-Xue , Ai Jun , Su Zhan ,   School of Optical-Electrical and Computer Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China
Published: December 2020

The K-shell has important theoretical significance and application value in measuring the importance of nodes in complex networks. However, in the K-shell method, most of nodes possess an identical K-shell value so that the relative importance of the identical K-shell nodes cannot be further compared with each other. Therefore, based on the K-shell value of nodes in the complex network and the K-shell values of multi-order neighbors in complex networks, in this paper we use the vectors to represent the relative importance of node in each of complex networks, which is named multi-order K-shell vector. Multi-order K-shell vector centrality defines a vector indicating the number of multi-order neighbors with different K-shells and groups them into elements of the vector. Each vector infers to not only the original K-shell of the given node but also the number of its multi-order neighbors and their K-shell values, which indicates the propagation capability of the given node. An approach to comparing two multi-order K-shell vectors is also presented, which is used to sort the vectors to evaluate the node importance. The method is explored by comparing several existing centrality methods. Through the experiments of SI propagation and static attack experiments in seven real-world networks, it is found that multi-order K-shell vector centrality provides low computational complexity, effectively evaluates nodes with high propagation capability, which confirms the improved performance in susceptible infected model propagation experiments. On the other hand, the static attack experiments show that the multi-order K-shell vector tends to preferentially select the core structure with powerful propagation capability in the network. The multi-order K-shell vector greatly improves the difference rate of node centrality under the premise of preserving the K-shell structure information, as well as balancing the importance measure of nodes in the complex network and the structure evaluation of propagation capability. The multi-order K-shell vector is not appropriate for all types of networks when considering the result of network attacking. For the networks with low clustering coefficients and high average path lengths, multi-order K-shell vector method is dominant and the effect is relatively obvious. By contrast, multi-order K-shell vector surpasses most of centrality approaches when spreading information is our priority. In a few networks, eigenvector centrality presents a slightly better performance with a larger computational complexity. The proposed centrality measure is therefore of great theoretical and practical importance.
Rashba effect in perovskites and its influences on carrier recombination
Authors: Wei Ying-Qiang , Xu Lei , Peng Qi-Ming , Wang Jian-Pu ,   Key Laboratory of Flexible Electronics (KLOFE) and Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), Nanjing 211816, China
Published: December 2020

When there is a strong spin-orbit coupling in some direct semiconductor with an inversion-asymmetric structure, the Rashba effect will exist, splitting the spin-degenerated bands into two sub-bands with opposite spin states. These two sub-bands will deviate from the symmetry center of the Brillouin zone, making the semiconductor an indirect band gap semiconductor. Metal halide perovskites exhibit strong spin-orbit coupling and possess an inversion-asymmetric crystal structure, showing great potential in Rashba effect research. In this review, we systematically review the Rashba effects in perovskites, including the theoretical and experimental studies for demonstrating the Rashba effect in perovskites, the influence of Rashba effect on the carrier recombination, and the current debates concerning the Rashba effect in perovskites. Then, several problems that need to be solved urgently are proposed,they being 1) whether there exists the Rashba effect in the perovskite, 2) whether the Rashba effect can exert a significant influence on carrier recombination, and 3) what the relationship between the Rashba effect and the perovskite stucture is. The prospects are also given for the future research including the study of the Rashba effect in perovskites by various spectral methods and the applications of the Rashba effect in optical-electronic-magnetic devices.
Automatic contour extraction algorithm for swept-source optical coherence tomography cornea image
Authors: Wang Yi , Liu Shan-Shan , Zhang Wei-Qian , Cai Huai-Yu , Chen Xiao-Dong ,   Key Laboratory of Opto-Electronics Information Technology of Ministry of Education, College of Precision Instrument and Opto-Electronic Engineering, Tianjin University, Tianjin 300072, China
Published: December 2020

In a swept source-optical coherence tomography system, the telecentric scanning mode gives rise to central saturation artifacts,partial structural loss, and low SNR (signal-to-noise ratio) area in the corneal image, which affects the accuracy of corneal contour extraction. In order to solve this problem, in this paper we propose an automatic extraction algorithm for corneal image of low quality. This algorithm divides the image into high and low SNR region according to the standard deviation distribution of the cornea image. For the high SNR region, we localize the peak point to extract the contour. For the low SNR region, image enhancement is achieved by the registration and superposition of successive frames, which provides reference contour points for low SNR areas. Then corneal contour localization is achieved by weighing the advantages and disadvantages of reference contour points and local line fitting results. Finally, global polynomial fitting is used to achieve the whole corneal contour information. Experiments on the optical eye model show that comparing with the existing algorithms, the accuracy of corneal contour extraction is improved by 4.9% on average.
First-principles study of magnetism of TM@Cu12N12 nanoclusters
Authors: Yin Min , Zhang Min , Lü Jin , Wu Hai-Shun ,   The School of Chemical and Material Science, Key Laboratory of Magnetic Molecules, Magnetic Information Materials Ministry of Education, Shanxi Normal University, Linfen 041004, China
Published: December 2020

The stability of structure, spin, orbital magnetic moment and magnetic anisotropy energy of TM@Cu12N12 (TM = Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Ir, Pt) are systematically investigated within the framework of the generalized gradient approximation with on-site coulomb repulsion density-functional theory (DFT-GGA+U). In the orbital moment and magnetic anisotropy energy (MAE) computation procedure, the spin-orbit coupling is considered and implemented. In this article, we mainly focus on the structure stability and tunable magnetism of the TM atom (Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Ir, Pt) substituting the centre atom of icosahedron (ICO) Cu13N12 cluster, finally disclose the physics origin of the structure stability, change magnetism and larger MAE. The results show that the different TM atom doping makes the ICO structure of Cu13N12 cluster appears a tiny deformation. The stabilities of the clusters are evidently enhanced due to the formation of Cu—N and Cu—TM bond. In addition, the N-capped clusters more prefer to present a larger magnetic moment than the pure Cu13 one. The magnetic environment of clusters is improved to varying degrees by doping different TM (TM = Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Ir, Pt) atoms, which endows TM@Cu12N12 clusters with various magnetic properties. For instance, the doping of 3d atoms further enhances the spin magnetic moment of the clusters, the Mn, Fe and Co atoms replacing the centre atom of the ICO Cu13N12 generate 35, 32 and 33 giant moments, respectively. In light of the doping of 4d, 5d transition metal atoms, the orbital moments of the TM@Cu12N12 clusters do not increase evidently, but the MAE remarkably strengthens for the doping of Rh and Pt atoms, the MAE values reach to 15.34 meV/atom and 6.76 meV/atom for Rh@Cu12N12 and Pt@Cu12N12, respectively. The tunable magnetism of TM@Cu12N12 cluster provides promising applications in spintronics.
Measurement of gravity acceleration by cold atoms in a harmonic trap using Kapitza-Dirac pulses
Authors: He Tian-Chen , Li Ji ,   Department of Physics, Taiyuan Normal University, Jinzhong 030619, China
Published: December 2020

The interferometry of two Kapitza-Dirac (KD) pulses acting on cold atoms in a harmonic oscillator potential well is investigated by Feynman path integral method. The wave function and density distribution function of the system at a given time are calculated analytically by using the propagator under the action of an external field. The first KD pulse acts on cold atoms to produce a large number of modes in the harmonic oscillator potential well. The maximum value of wave packet of mode 0 is larger than those of other modes. These modes evolve along different paths. The external field changes the phase of each mode and makes the evolution path of the mode deviate from that without the external field. When the second KD pulse is added, it splits the mode of the first KD pulse, and thus generates more modes. These modes will evolve along different paths under the action of external field and harmonic potential well, and interfere with each other. At the moment of measurement, all the wave packets are separated without overlapping. The effect of the external field does not change the magnitude of the density distribution at the time of measurement, but makes the wave packet of each mode shift. The phase difference between adjacent modes decreases linearly with the increase of field intensity. When the external field is a gravity field, we calculate the Fisher information and the Cramer-Rao lowér bound. The Fisher information is proportional to the mass of atoms and inversely to the third power of harmonic potential well frequency. We can improve the measurement accuracy of interferometer by reducing the frequency of harmonic potential well and increasing atomic mass. When the initial state is the ground state of the harmonic potential well, the accuracy of the gravity acceleration measured by the interference device can be obtained to be 10–9 by using the experimental parameters. The initial state is the ground state of the harmonic potential well and the external field, and the calculation result indicates that the measurement accuracy will decrease. At the same time, the enhancement of inter-atomic repulsion and attraction interaction will also lead the measurement accuracy to increase.
Effect of surfactants on thin film spreading under influence of surface acoustic wave
Authors: Li Chun-Xi , Shi Zhi-Xian , Zhuang Li-Yu , Ye Xue-Min ,   Department of Power Engineering, North China Electric Power University, Baoding 071003, China
Published: December 2020

For the spreading of thin and free film of a partially wetting liquid with insoluble surfactant under the influence of surface acoustic wave, the dimensionless evolution equations governing the spreading dynamics are derived. The evolution equations contain the film thickness and the surface concentration of insoluble surfactant. Assuming that the thickness of the thin film is much smaller than the wavelength of sound in the liquid, the sound leaking off the surface acoustic wave cannot be sustained in the liquid film, and the acoustic radiation pressure and attenuation of the acoustic wave in the solid are both weak. Then the films spreading under different physical mechanisms are observed by numerical simulation. The results show that the surface acoustic wave drives the liquid film to spread and move. When the capillary stress is weak and the liquid film spreading is mainly controlled by the drift induced by surface acoustic wave, the spreading process consists of rapid spreading stage and balancing stage, and the Marangoni effect caused by uneven distribution of surfactant makes the liquid film spread faster in the first stage. When the capillary stress and the drift jointly dominate film spreading, the spreading process contains three stages, i.e. spreading stage, contracting stage and balancing stage. The effect of surfactant accelerates the spreading process, but the existence of contracting stage makes it take longer for the film to reach equilibrium. In addition, the disjoining pressure used in this paper promotes the liquid film spreading, as well as the Marangoni effect. As the correlation coefficient between disjoining pressure and surfactant concentration, α, and the Marangoni number, M, increase, the maximum thickness and the spreading radius of liquid film change faster.
Polarization state of second harmonic generation in split ring resonator based meta-molecule
Authors: Ma Ming-Yu , Wu Han , Chen Zhuo ,   School of Physics, Nanjing University, Nanjing 210093, China
Published: December 2020

In this paper, we study the second harmonic generation (SHG) from the stero-stacked meta-molecules consisting of two vertically stacked split ring resonators (SRRs) that resonate at the fundamental wavelength. When pumped by the linearly polarized incident wave with the electric field direction along one of the SRRs’ arms, the meta-molecules emit the SHG that can have two non-zero orthogonal electric field components, provided that the top SRR and the bottom SRR are not arranged in mutually parallel or anti-parallel manner. Due to the strong coupling between the two SRRs, the plasmonic properties of the stero-stacked meta-molecules could be tuned by varying the twist angle between the two SRRs. In this process, we demonstrate that the amplitudes of the two orthogonal SHG field components, and the phase difference between these two components can be varied with changing the twist angle between two SRRs. Based on the concept of the light polarization, different polarization states can be achieved by changing the differences in phase and amplitude between the orthogonal field components. Therefore, the twist angle dependent amplitudes of and phase difference between two orthogonal SHG field components can be used to manipulate the polarization states of the emitted SHG. For the stero-stacked meta-molecules with a fixed twist angle of 60°, elliptically, near-circularly andnear-linearly polarized SHG emission can be obtained at different fundamental wavelengths. In addition, for the fundamental wave with a fixed wavelength of 1500 nm, the stero-stacked meta-molecules with different twist angles are demonstrated to be able to emit SHG with elliptical andnear-linear polarization states.
Recent progress of ion migration in organometal halide perovskite
Authors: Wang Ji-Fei , Lin Dong-Xu , Yuan Yong-Bo ,   Hunan Key Laboratory of Super Microstructure and Ultrafast Process, School of Physics and Electronics, Central South University, Changsha 410083, China,   School of Physics and Electronic Sciences, Hunan Institute of Science and Technology, Yueyang 414006, China
Published: December 2020

In recent years, metal halide perovskite solar cells have attracted widespread attention due to their unique technological superiority. Remarkable progress of device performance has been achieved in last few years. The certified efficiency has reached 24.2%, which is close to the efficiency of the commercial crystalline silicon solar cells. Halide perovskite is a kind of semiconductor and ionic conductor material, which not only has the ability to transfer charges, but also exhibits the phenomenon of ion migration under an external electric field. Ion migration can directly change the local chemical ratio of perovskite crystals, and can also greatly affect the electrical properties of materials and the working mechanisms of corresponding devices. In this review, the formation mechanism, basic characteristics and effects of ion migration on the working mechanism of the device (such as giant switchable photovoltaic phenomenon, current hysteresis, etc.) are summarized, and then some recent advances in the suppression of ion migration are introduced. Since there exist still many doubts about ion migration in perovskite materials, it is very important to understand the phenomenon of ion migration in perovskite materials in order to promote the development and application of perovskite solar cells.
Ultraviolet picosecond conversion efficiency improvement system at 355 nm based on fundamental frequency laser amplified
Authors: Zhang Xu-Dong , Chu Yu-Xi , Jia Wei , Hu Ming-Lie ,   Ultrafast Laser Laboratory, College of Precision Instrument and Optoelectronics Engineering, Key Laboratory of Opto-Electronic Information Technology (Ministry of Education), Tianjin University, Tianjin 300072, China
Published: December 2020

In recent years, picosecond laser in ultraviolet (UV) has manifested great importance for applications both in science and industry, such as biomedical research, micro machining, etc. Now, the well proven approach to generating ultra-short UV pulses is extra-cavity frequency conversion based on nonlinear optical (NLO) crystal, due to the lack of suitable laser sources directly generating UV laser. In this process of harmonic generation, the length of nonlinear crystal is an important factor affecting the conversion efficiency and beam-quality. The optimal length of the nonlinear crystal is influenced by incident laser parameters and crystal absorption coefficient. At present, for the UV 355 nm picosecond laser generated from extra-cavity sum frequency, published are few reports about detailed analysis and research on the influence of photon ratio of the incident laser beams and nonlinear crystal absorption on optimal length of sum frequency crystal. In this paper, the steady-state solutions with the highest conversion efficiency under different incident conditions are obtained by theoretical analysis and numerical simulation of the three waves coupling equations. The effects of different photon ratios and absorption effect of the sum frequency crystal on the optimum crystal length are analyzed. We propose a solution based on the fundamental frequency laser amplified to shorten crystal length and improve conversion efficiency. In this scheme, the 532 nm second harmonic laser with a high conversion efficiency over 65% can be achieved by LiB3O5 crystal. After that, the 1064 nm fundamental frequency laser is separated from the second harmonic laser, and then it is amplified by the Nd:YVO4 laser crystal pumped by an 808 nm laser diode. Finally, the ultraviolet 355 nm picosecond laser is obtained by combining the 1064 nm fundamental frequency laser with the 532 nm second harmonic laser in the LiB3O5 crystal. The simulation results show that the incident photon ratio of the sum frequency reaction can be changed by amplifying the residual fundamental frequency laser, and the optimum length of the sum-frequency crystal corresponding to the highest conversion efficiency can be shortened. Meanwhile, the absorption and walk-away effect of the sum frequency crystal can be also reduced. The final 355 nm laser output power can be increased more than 40 percent compared with the traditional scheme of early reports. In consequence, the high sum frequency conversion efficiency of the UV 355 nm picosecond laser can be obtained by changing the photo ratio of the incident laser beams through amplifying the fundamental frequency laser.