bluuebunny/crossref_metadata_2025 huggingface dataset:
Fiber modal content analysis based on spatial and spectral Fourier transform
Authors: Zhang Shu-Lin  , Feng Guo-Ying  , Zhou Shou-Huan  ,   Institute of Laser and Micro/Nano Engineering, College of Electronics and Information Engineering, Sichuan University, Chengdu 610064, China;,   North China Research Institute of Electro-optics, Beijing 100015, China
Published: December 2020

As is well known, a typical measure of the quality of an optical beam is the M2 parameter, but characterizing the beam quality only by M2 is insufficient. A low value of M2 is generally considered to be equivalent to the single-mode operation with a stable beam. However, even when a large amount of power is contained in high-order modes, the existence of a low value of M2is still possible. Hence, a low value of M2 does not guarantee the single-mode operation. Therefore, a new measurement technique, which aims at measuring modal content of high power fiber laser, is proposed and demonstrated in this paper. This method is named spatial and spectral Fourier transform, or F2 transform in short, and it is based on measuring Fourier transform of both spatial domain and spectral domain of output laser. The experimental set is simple in structure and high in robustness. Another advantage of the method is that it requires no prior detailed knowledge of the fiber properties. In this paper, the patterns of the high-order modes between and after Fourier transform are simulated. From the graph it is evident that the energy of spot diffuses outward and is convenient to measure. We also simulate and compare the group delay difference curve of F2 with existing S2, which are well matched with each other. Experimentally, the high-order modes are stimulated by extruding the fiber periodically, which ensures that we can measure it. Firstly, by scanning two-dimensional (2D) pattern of beam after spatial domain Fourier transform and recording the experimental data, and then through the Fourier transform of data in spectral domain, the group delay differences between the high-order modes and the fundamental mode can be obtained. Finally, different modes in spatial domain are reconstructed and the relative power of every mode is calculated. Additionally, we set up an automatic measuring device to verify the effectiveness of the method. The reconstructed modal patterns are presented in the final section of this paper. We can clearly identify the fundamental mode and the high-order modes, such as LP01, LP02, LP03, LP21, LP11, LP12, LP13 and LP14. It reconfirms that this method is feasible. Compared with the S2 method, this method reduces the requirement for precision of mobile platform greatly and thus it is suited to measure the modal content of high power fiber laser output beam. This technique can be effectively applied to a wide variety of measurements, such as dispersion compensator of large-mode-area fiber, bend loss measurement of the high-order modes, refractive index profiles measurement of fiber and mode convertor fiber.
Numerical simulation of a class of FitzHugh-Nagumo systems based on the lattice Boltzmann method
Authors: He Yu-Bo  , Tang Xian-Hua  , Lin Xiao-Yan  ,   School of Mathematics and Statistics, Central South University, Changsha 410083, China;,   Department of Mathematics and Applied Mathematics, Huaihua University, Huaihua 418008, China
Published: December 2020

The lattice Boltzmann method (LBM) was proposed as a novel mesoscopic numerical method, and is widely used to simulate complex nonlinear fluid systems. In this paper, we develop a lattice Boltzmann model with amending function and source term to solve a class of initial value problems of the FitzHugh Nagumo systems, which arises in the periodic oscillations of neuronal action potential under constant current stimulation higher than the threshold value. Firstly, we construct a non-standard lattice Boltzmann model with the proper amending function and source term. For different evolution equations, local equilibrium distribution functions and amending function are selected, and the nonlinear FitzHugh Nagumo systems can be recovered correctly by using the Chapman Enskog multi-scale analysis. Secondly, through the integral technique, we obtain a new method on how to construct the amending function. In order to guarantee the stability of the present model, the L stability of the lattice Boltzmann model is analyzed by using the extremum principle, and we get a sufficient condition for the stability that is the initial value u0(x) must satisfy |u0(x)|1 and the parameters must satisfy i-(1+)(t)/(x), (i=1-4). Thirdly, based on the results of the grid independent analysis and numerical simulation, it can be concluded that the present model is convergent with two order space accuracy. Finally, some initial boundary value problems with analytical solutions are simulated to verify the effectiveness of the present model. The results are compared with the analytical solutions and numerical solutions obtained by the modified finite difference method (MFDM). It is shown that the numerical solutions agree well with the analytical solutions and the global relative errors obtained by the present model are smaller than the MFDM. Furthermore, some test problems without analytical solutions are numerically studied by the present model and the MFDM. The results show that the numerical solutions obtained by the present model are in good agreement with those obtained by the MFDM, which can validate the effectiveness and stability of the LBM. In conclusion, our model not only can enrich the applications of the lattice Boltzmann model in simulating nonlinear partial difference equations, but also help to provide valuable references for solving more complicated nonlinear partial difference systems. Therefore, this research has important theoretical significance and application value.
Photoelectric properties of Ti doped ZnO: First principles calculation
Authors: Qu Ling-Feng  , Hou Qing-Yu  , Xu Zhen-Chao  , Zhao Chun-Wang  ,   College of Science, Inner Mongolia University of Technology, Hohhot 010051, China;,   College of Arts and Sciences, Shanghai Maritime University, Shanghai 201306, China;,   Inner Mongolia Key Laboratory of Thin Film and Coatings, Hohhot 010051, China
Published: December 2020

Nowadays, the studies on absorption spectra and conductivities of Ti doped ZnO systems have presented distinctly different experimental results when the atom fraction of impurity increases in a range from 1.04 at% to 1.39 at% To solve this contradiction, all calculations in this paper are carried out by the CASTEP tool in the Materials Studio software based on the first-principals generalized gradient approximation (GGA) plane wave ultra-soft pseudopotential method of the density functional theory. The supercell geometric structures of ZnO, Zn0.9792Ti0.208O and Zn0.9722Ti0.278O systems are used as the calculation models. For all the geometry optimization models, the band structures, densities of states, electron density differences, population and absorption spectra are calculated by the method of GGA+U. The results show that with the Ti doping amount increasing from 1.04 at% to 1.39 at%, the lattice parameters and also the volume of the doping system increase. The higher the total energy of the doping system, the higher the formation energy of the doping system is, thereby making doping difficult and lower stability of the doping system. The increase of Ti-doping concentration weakens the covalent bond, but strengthens the ionic bond. As the Ti substitutional doping concentration increases, the Mulliken bond populations decrease, but bond lengths of Ti-O increase for the doping system Meanwhile, the higher the Ti doping content, with all the doping systems converted into n-type degenerate semiconductor the wider the band gap of the doping system will be and the more significant the blue shift of absorption spectra of Ti-doped ZnO systems. In this paper the mechanism of band gap widening is reasonably explained. In addition, the higher the Ti doping content, the higher the electronic effective mass of doping systems is The higher the electronic concentration of doping systems, the lower the electronic mobility of doping systems is. The lower the electronic conductivity of doping systems, the worse the doping systems conductivity is. The calculation results of absorption spectrum and conductivity of Ti-doped ZnO system are consistent with the experimental data. And the contradiction between absorption spectrum and conductivity of Ti-doped ZnO system in experiment is explained reasonably by temperature effect. In this paper, the comprehensive optical and electrical properties of Ti-doped ZnO systems are calculated by first-principals GGA+U method. And these results may improve the design and the preparation of photoelectric functional materials for Ti-doped ZnO at quite a low temperature.
An improved algorithm for prediction of protein loop structure based on position specificity of amino acids
Authors: Yuan Fei  , Zhang Chuan-Biao  , Zhou Xin  , Li Ming  ,   School of Physical Science, University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

Loop region is necessary structural element of protein molecule, and plays significant roles in protein functioning, e.g., in signaling, ligand recognition. Unlike the well-defined secondary structures (i.e., helix, sheet), however, loop regions vary in structure and some of them are even not able to be measured by ordinary experimental methods. For these reasons, computer-aided prediction of loop structure became a hotspot in bioinformatics and biophysics. Sorts of algorithms have been developed for this purpose. So far, however, the prediction of long loop is still a challenge. Among all the common algorithms, LEAP algorithm achieves the highest precision on long loop prediction. Our investigation on a test data set with LEAP algorithm reveals that the ultimate loop structure predicted by LEAP is almost entirely determined by the initial sampling of the conformation of the loop backbone. If all the backbone conformations in the initial sampling are quite distant from the real (native) conformation, the ultimately predicted structure is also distant from the native conformation, and the prediction accuracy cannot be improved obviously only by increasing the computation time. In the original LEAP, the initial sampling is based on the rough distribution of the backbone torsion angle (Ramachandran plot, R-plot) which doesn't consider the sequence information of the loop region. Many conformations which are far from the native conformation are most likely generated in the sampling. So there raises the open question, is it possible to enhance the initial sampling to be more targeted to the native conformation? In this paper, we suggest an approach to introduce the position-specific amino-acid sequence information into the initial sampling of the backbone conformation, which may generate more targeted initial decoys. An algorithm of protein secondary structure prediction, SPINE X, is used to generate rough but reasonable estimates of torsion angles of each amino acid of the loop backbone in sequence-dependent way. We then combine these values with the original R-plot to reconstruct a new R-plot for each amino acid in the loop, and the initial sampling is performed according to the new R-plot. We applied this new algorithm to a test set of loops (generated from single-chain proteins in CASP 10), and found the medians/means of RMSDs can reduce about 0.12 /0.13 , 0.25 /0.27 , 0.47 /0.27 for loop sets of length 10, 11, 12, respectively. Comparing to the original LEAP algorithm, the probability of making more accurate predictions is almost doubled when using the refined algorithm. The logic of our approach is not limited to LEAP, and can be extended to other algorithms which are also significantly dependent on initial sampling.
Modeling information diffusion on microblog networks based on users' behaviors
Authors: Liu Hong-Li  , Huang Ya-Li  , Luo Chun-Hai  , Hu Hai-Bo  ,   Department of Management Science and Engineering, East China University of Science and Technology, Shanghai 200237, China
Published: December 2020

Online social networks, such as Facebook, Twitter and YouTube, play a vital role in information sharing and diffusion, and recently many dynamics models on social networks have been proposed to model information diffusion. However most models are theoretical, their parameters do not come from realistic data and their validity and reliability have not been evaluated empirically. In the paper we first analyze the users' behaviors of reading and reposting microblog in Sina Weibo, a Twitter-like website in China, and find that users' number of fans, the average reposted number of users' microblog, the intensity of users' interaction and the similarity between microblog topics and users' topic interests can significantly influence reposting behavior. Then we propose an information diffusion model Susceptible-Infected-Recovered based on Users' Behaviors (SIRUB) on microblog networks, compute the users' probability of reading microblog in the model according to the probability of their logging on microblog in a day, and obtain the reposting probability utilizing the logistic regression which considers 16 possible factors influencing users' reposting behavior. The 16 factors can be divided into three categories: the characteristics of microblog publishers, microblog text features and social relationship characteristics. We utilize the beginning 2/3 microblog data to obtain model parameters and logistic regression coefficients, and the remaining 1/3 data to examine the validity of the model. The experiments on Sina Weibo network show that the model can predict users' reposting behavior accurately only when it considers both reading and reposting probabilities. F-score which considers precision and recall is used to assess prediction effect of the model. The highest F-score for the prediction of SIRUB model on users' reposting behavior is 0.228 which is much larger than those of classical Susceptible-Infected-Recovered (SIR, F-score=0.039) and Susceptible-Infected-Contacted-Recovered (SICR, F-score=0.037) models. The prediction on the spreading scope of microblog for SIR and SICR models is related with users' number of fans while for SIRUB model not. For SIRUB model the mean and standard deviation of the errors of prediction on spreading scope are smaller than those of SIR and SICR models. These results indicate that users' behaviors of reading and reposting microblog should be appropriately taken in account when modeling information diffusion on microblog networks, and that, in general, the prediction performance of the data-driven SIRUB model proposed in the paper is better than those of SIR and SICR models regardless of the prediction of users' reposting behavior or diffusion scope of microblog.
Molecular dynamics study on relaxation properties of monolayer MoS2 nanoribbons
Authors: Wang Wei-Dong  , Li Long-Long  , Yang Chen-Guang  , Li Ming-Lin  ,   School of Mechano-Electronic Engineering, Xidian University, Xi’an 710071, China;,   Department of Mechanical and Biomedical Engineering, City University of Hong Kong, Hong Kong 999077, China;,   Fuzhou University, School of Mechanical Engineering and Automation, Fuzhou 350116, China
Published: December 2020

In order to study the essential structural characteristics of monolayer MoS2 nanoribbons in natural state, and also the effects of the aspect ratio and the ambient temperature on the relaxation properties of the nanoribbons, the relaxation properties of monolayer MoS2 nanoribbons with different aspect ratios are simulated by molecular dynamics (MD) method based on REBO potential functions at different thermodynamic temperatures from 0.01 K to 1500 K. The energy curves and surface morphologies of monolayer MoS2 nanoribbon are obtained, and the dynamic equilibrium process of the monolayer MoS2 nanoribbons is also discussed in all the simulation process. The simulation results show that the monolayer MoS2 nanoribbons do not generate a fluctuation at the ideal temperature (0.01 K) for the reason that the kinetic energy of the monolayer MoS2 nanoribbons is almost zero and the vibration amplitude is small. However, a certain degree of fluctuations occurs at the edges and inside of the monolayer MoS2 nanoribbons at the room temperature or high temperature. The fluctuation height and the fluctuation degree also increase with increasing the ambient temperature and the aspect ratio of the MoS2 nanoribbon, even a high aspect ratio monolayer MoS2 nanoribbon exhibits a surface curved fluctuation, which is perpendicular to the surface of the MoS2 nanoribbons under high temperature condition. Finally, the influences of chirality on relaxation property under different temperature conditions are studied in this paper further, the results show that unlike the armchair structure, the zigzag monolayer MoS2 nanoribbons not only present a surface fluctuation, but also exhibit an obvious bending phenomenon along the width direction simultaneously. Like the armchair nanoribbons, the surface fluctuation height and the surface fluctuation degree of the zigzag nanoribbons also increase with increasing both the ambient temperature and the aspect ratio of the MoS2 nanoribbons. It is also observed that the armchair and zigzag monolayer MoS2 nanoribbons with a similar aspect ratio have a similar surface fluctuation degree at the same ambient temperature. Unlike the armchair nanoribbons, the bending phenomenon along the width direction of the zigzag nanoribbons is more significant, and the bending width and the bending degree increase with increasing the ambient temperature and the aspect ratio of the MoS2 nanoribbons. Although the bending degree of the zigzag nanoribbons becomes larger with the increase of temperature, the increasing rate of the bending degree will become smaller and smaller until the bending degree reaches a maximum value.
Dynamic behaviors of spreading in generalized Fibonacci time quasiperiodic quantum walks
Authors: Wang Wen-Juan  , Tong Pei-Qing  ,   School of Physical and Mathematical Sciences, Nanjing Tech University, Nanjing 211800, China;,   Department of Physics and Institute of Theoretical Physics, Nanjing Normal University, Nanjing 210023, China;,   Jiangsu Key Laboratory for Numerical Simulation of Large Scale Complex Systems, Nanjing Normal University, Nanjing 210023, China
Published: December 2020

Quantum walk (QW), the quantum mechanical counterpart of classical random walk, has recently been studied in various fields. The evolution of the discrete time quantum walk can be described as follows: the walker changes its spin state by the coin operator C, then takes one step left or right according to its spin state. For homogeneous quantum walk, the coin operator is independent of time and the standard deviation of the position grows linearly in time. It is quadratically faster than that in the classical random walk. In this work, we numerically study the dynamical behaviors of spreading in a one-dimensional discrete time quasiperiodic quantum walk (DTQQW). The DTQQW is that the coin operator is dependent on time and takes two different coins C() and C() arranged in generalized Fibonacci (GF) sequences. The GF sequences are constructed from A by the recursion relation: AAmBn, BA, for m, n are positive integers. They can be classified into two classes according to the wandering exponent . For 0, they belong to the first class, and for 0, they belong to the second class. For one dimensional system, the behaviors of two classes of GF systems are different either for the electronic spectrum of an electron in quasiperiodic potentials or for the quantum phase transitions of the quasiperiodic spin chains. In this paper, we discuss the cases of two different C operators (C();C()) arranged in GF sequences and find that the spreading behaviors are superdiffusion (the standard deviation of the position ~t; 0:5 1) for the two classes of GF DTQQW. For the second class of GF DTQQW, the exponent values are larger than those of the first class of GF DTQQW in the case of two identical C operators. By exploring the probability distribution in the real space, we find that for the first class of GF DTQQW, the probability distributions are almost the same for different initial states and are similar to the classical Gaussian distribution. For the probability distributions of the second class of GF DTQQW, there are two peaks at the two edges and the height of the two peaks can be different for different initial states. They are similar to the ballistic distribution of the homogeneous quantum walk. Therefore, we conclude that for the first class of GF DTQQW, the spreading behaviors are close to those of the classical random walk ( = 0:5) while for the second class of GF DTQQW, they are close to those of the homogeneous quantum walk ( = 1). This result is quite different from the characteristics of the quantum phase transitions in two classes of GF quasiperiodic quantum spin chains.
Calculations on rovibrational spectra of two lowest electronic states in sulfur monoxide molecule by explicitly correlated approach
Authors: Wei Chang-Li  , Liang Gui-Ying  , Liu Xiao-Ting  , Yan Pei-Yuan  , Yan Bing  ,   Institute of Atomic and Molecular Physics, Jilin University, Changchun 130012, China
Published: December 2020

Accurate spectroscopic constants and rovibrational level information are of great importance in molecular physics and astrophysical field. In this work, a new computational scheme is presented to further improve the accuracy of spectroscopic constants, and the two lowest electronic states of sulfur monoxide molecule are investigated as a typical case study. High-level ab initio calculations are carried out to compute the potential energy curves (PECs) of the lowest bound states, the ground triplet states X3Σg-, and the first excited singlet states a1Δg of SO molecule. The explicitly correlated multi-reference configuration interaction method (MRCI-F12) and cc-pCVQZ-F12 basis set are adopted in the electronic structure computations. The Davidson correction is taken into account to eliminate the size-consistency error. The core-valence (CV) electron correlations of the n=2 shell of S atom and n=1 shell of O atom are estimated by the MRCI-F12 method, whereas only the 1 s orbital of S atom is excluded in the CV calculations. Moreover, we introduce the scalar relativistic effect into our study by utilizing the second-order Douglas-Kroll and Hess (DKH) one-electron integrals by the MRCI method through combining with the uncontracted cc-pCVQZ-F12 basis set. On the basis of the PECs of the SO dimer, the spectroscopic constants (Te, Re, ωe, ωeχe, Be, αe and De) of the two electronic states are determined by solving the one-dimensional nuclear rovibrational Schrödinger equations. Our spectroscopic constants are found to be in excellent agreement with previous experimental and theoretical values. Furthermore, the detailed information about the vibrational energy levels and rotation spectroscopic constants (Bν, Dν) of the two states is also presented with a deviation of 0.1% order of magnitude from the available experimental results. Our present computational work is valuable for future experimental studies on the rovibrational energy levels for the SO molecule and also indicates that the MRCI-F12 approach is cheap and accurate and expected to have wide applications in the PECs of other small molecular systems.
Properties of free radical BeH in external electric field
Authors: Xu Mei  , Linghu Rong-Feng  , Zhi Qi-Jun  , Yang Xiang-Dong  , Wu Wei-Wei  ,   School of Physics and Electronic Science, Guizhou Normal University, Guiyang 550001, China;,   School of Physics and Electronic Science, Guizhou Normal College, Guiyang 550018, China;,   Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610065, China;,   School of Physics and Electronic Engineering, Kaili University, Kaili 556011, China
Published: December 2020

In this paper, the QCISD(T), CCSD(T), B3PW91 and B3LYP methods and the basis sets of aug-cc-pVTZ, 6-311G are used to calculate the structure of the ground state of free radical BeH molecule. The equilibrium distance and the energy of the molecule are optimized. The calculated results are compared with the experimental data, and the B3LYP method with the basis sets 6-311G is found to be able to provide the results that are the closest to the experimental values. So, in this paper the density function B3LYP method and the basis sets 6-311G are chosen and used to optimize the geometric structures of the ground state of free radical molecule of BeH in electric fields ranging from -0:02 to 0:02 a.u. The effects of external electric field on bond distance, system energy, charge distribution, energy levels, dipole moment, HOMO-LUMO gap, and infrared spectrum are studied．The results show that the molecular bond distance, the total atomic charge, the dipole moment, and the IR intensity decrease gradually with the increase of the external electric field along the molecular axis H→Be. At the same time, the total energy, the HOMO-LUMO gap, and the frequencies increase. The total energy increases sharply while the reverse electric field Be→H increases.
Double reflection of spin-orbit-coupled cold atoms
Authors: Huang Zhen  , Zeng Wen  , Gu Yi  , Liu Li  , Zhou Lu  , Zhang Wei-Ping  ,   School of Physics and Material Science, East China Normal University, Shanghai 200241, China;,   Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China
Published: December 2020

Artificial spin-orbit coupling in neutral cold atom have been experimentally implemented in alkali-metal atoms. Nowadays people begin to explore its possible applications. One of the most interesting applications is the atomic mirror, which is a key element in atom optics. And spin-orbit coupling provides the atomic beam with the possibility that the atomic spin can flip during its propagation, thus can be used to prepare the quantum-state-selective atomic mirror. In 2008, Juzeliūnas, et al. [Juzeliūnas G, et al. 2008 Phys. Rev. Lett. 100 200405] studied a spin-orbit-coupled matter wave packet of cold atom gas impinging on an infinite step potential created by the optical light field. Results showed that there is not only ordinary specular reflection, but also non-specular one. The reflected atoms split into two beams and double reflection takes place. Based on the previous study, here we consider a matter wave packet of spin-orbit-coupled cold atom gas impinging on a finite step potential created by the optical light field. Due to the effect of the spin-orbit coupling, in addition to the propagating state, the eigenstates of cold atoms include evanescent state and oscillating evanescent state. Under suitable conditions double reflection will take place. If there are just evanescent waves in the step potential, total internal reflection will take place. In other words, when there is propagating wave in the step potential, partial reflection will take place. By taking into account both the total internal reflection and partial reflection, we study not only the polarization rate but also the reflectivity each as a function of incident energy, incident angle and spin-orbit coupling strength. The properties different from those of previous studies are found. In the case of total internal reflection, we find that the polarization rate of the reflected atoms is sensitive to incident angle instead of the spin-orbit coupling strength and incident energy. While in the case of partial reflection, all these factors strongly affect the polarization rate and reflectivity. We carefully study these properties and find that, on one hand, high efficiency atomic mirror can be acquired in the case of total internal reflection, and on the other hand, we can acquire the different polarization rates by adjusting the incident angle, the spin-orbit coupling strength and incident energy in the case of partial reflection.
Blind source extraction based on time-frequency characteristics for underwater object acoustic scattering
Authors: Yang Yang  , Li Xiu-Kun  ,   Acoustic Science and Technology Laboratory, Harbin Engineering University, Harbin 150001, China;,   College of Underwater Acoustic Engineering, Harbin Engineering University, Harbin 150001, China
Published: December 2020

The physical mechanism and signal characteristics of acoustic scattering are the vital basis for target recognition. But underwater target acoustic scattering components are aliasing in time-frequency (TF) domain, for which the target elastic acoustic scattering characteristics are difficult to detect. Additionally, the existing blind source separation methods are effective only on condition that the number of array elements is equal to or greater than the number of the source signals. To address these problems, a novel TF domain blind source extraction method of separating target acoustic scattering components is proposed in this paper. The method only uses the TF energy characteristic differences among the target acoustic scattering components, and special limitations on target echo structures are unnecessary. Image morphology filter is used to remove the cross-term interference from time-frequency distribution (TFD) of the received array signals. Then, the single source which shows maximum energy concentration at the corresponding auto-term TF points is extracted through three operations: i) selecting the single source auto-term TF points from the auto-term ones; ii) constructing the spatial TFD matrix according to the selected single source auto-term TF points; iii) obtaining the single source by decomposing the eigenvalue of their spatial TFD matrix. Finally, the extracted single signal is excluded by the tightening process from the received array signals, and each single signal is separated successively by repeating the above steps. In addition, a signal processing model which can describe the physical characteristics of the target echoes is established based on the separated signal components. Simulations illustrate that the image morphological filter can remove the cross-term interference and improve the TF resolution of the Wigner-Ville distribution. Anechoic pool experimental results show that the TF domain blind source extraction algorithm can well separate each target acoustic scattering component, it can also achieve a higher output signal-to-noise ratio. Furthermore, the separated elastic acoustic scattering components are in good agreement with the results computed by the surface wave generating theory, so the method can provide the robust and reliable feature for underwater target recognition.
Morphology simulation and mechanical analysis of primary dendrites for continuously cast low carbon steel
Authors: Zuo Xiao-Jing  , Meng Xiang-Ning  , Huang Shuo  , Wang Xin  , Zhu Miao-Yong  ,   School of Metallurgy, Northeastern University, Shenyang 110819, China
Published: December 2020

The initial growing dendrite is influenced significantly by the complicated solidification conditions in continuously oscillating mold. The uneven growth of dendrite causes some defects seen commonly such as internal crack, subsurface porosity, subsurface inclusion and other defects of continuous casting billet. The induced initial defects in mold can be expanded and propagated in the following process such as secondary cooling, straightening, rolling and other subsequent handling procedure and then evolve into serious defects that can restrict the development and the quality refinement of final steel products. The mechanical stress caused by mold oscillation and the melt flowing is a crucial factor that leads to the uneven microstructure growth of initial solidifying shell in continuous casting mold.
In this work, we simulate the growth and the morphology evolution of primary dendrites in mold area by using the cellular automaton (CA) method in combination with the actual conditions for continuously cast low carbon billet (Fe-0.6 wt.%C). Further, the mechanical state of initial dendrite is analyzed by regarding primary dendrite as a cantilever beam and its mechanical stress is calculated by combining thermo-physical properties and flow rate of steel based on the principle of materials mechanics to shed light on the formation of initial defects formation in mold area of continuous casting process. The results show that the solute concentration of initial dendrite tip gradually increases with undercooling from 2 to 10 K, and the maximum concentration rises by 0.07% when the increment of undercooling is 2 K. The length of dendrite arm increases significantly with undercooling from 2 to 6 K. However, the length of dendrite arm remains steady in a stable growth rate of 0.08 mms-1 when the undercooling is enhanced from 6 to 10 K. The increase of undercooling reduces the bending stress at dendrite root when the flow rate of molten steel is improved from 0.13 to 0.33 ms-1, while the mechanical stress continuously increases with the growth of primary dendrite at a constant undercooling. The bending stress of dendrite root has a high possibility to exceed its critical fracture strength under the condition of undercooling below 6 K or dendrite grow up more than 1 s. The primary dendrite is likely to be fractured and form initial defects of billet.
Pressure dependence of refractive index of Ge near the absorption edge
Authors: Shi Wen-Jun  , Yi Ying-Yan  , Li Min  ,   Department of Physics, Wuhan University of Technologe, Wuhan 430070, China
Published: December 2020

Pressure-dependent refractive index of semiconductor germanium (Ge) near the absorption edge has not been well studied theoretically and experimentally to date. In this paper, we present a pressure-dependent refractive index of Ge film near its absorption threshold (about 1550 nm), deduced from the reflectivity of high crystalline Ge film coated on a fiber end. The thin Ge layer is deposited on one end of an optical fiber by using an E-beam evaporation machine equipped with a substrate heater of a quartz halogen lamp. In order to obtain high crystalline film, the quartz halogen lamp heater provides a constant substrate temperature of 450 ℃ during film deposition. After the film forming, the sample is transferred into a muffle furnace with a nitrogen atmosphere and annealed at 600 ℃ for 20 h to guarantee the formation of higher crystalline film. The process of light propagating through the optical fiber and reflecting from the Ge thin-film involves multi-beam interference. An abnormal dispersion is observed in the refractive index spectra of the polycrystalline Ge near the absorption edge. A comparison shows that the refractive index spectrum of the amorphous Ge is normal dispersion. Unlike previously reported results that the pressure-dependent refractive index had a negative value, in our experiment it is observed to be a positive value near the absorption edge. To better understand this phenomenon, we use a critical point model including the pressure effect to successfully fit the experimental data. We obtain an abnormal dispersion range of 1505-1585 nm and a range of negative value of pressure-dependent refractive index of 1500-1580 nm from the critical point model. In this paper, we adopt the method of high crystalline Ge film coated on a fiber end. This method has the advantages of small volume, high precision and strong stability, which can be used to measure the optical properties of many thin film materials under the different conditions (temperature, electric field or magnetic field, etc.). In this work, we obtain the refractive index of the Ge film and its pressure dependence near the C-band of the optical fiber communication, and these results are conducive to optimized design of the Ge-based optical systems near the C-band of the optical fiber communication.
Nondestructive detecting method for metal material defects based on multimodal signals
Authors: Sun Ming-Jian  , Liu Ting  , Cheng Xing-Zhen  , Chen De-Ying  , Yan Feng-Gang  , Feng Nai-Zhang  ,   School of Information and Electrical Engineering, Harbin Institute of Technology at Weihai, Weihai 264209, China;,   School of Astronautics, Harbin Institute of Technology, Harbin 150001, China
Published: December 2020

Metal materials play an important role in many domains, which are significant to the national economy. However, different kinds of metal defects, such as cracks, contraction cavities, impurities, will be generated in the process of production and service. These defects will affect the metal service life and mechanical properties directly, and even cause serious economic loss. Therefore, it is vital to detect the metal defects.
Numerous nondestructive testing (NDT) methods have been proposed for detecting metal defects, such as ultrasonic (US) testing, eddy current testing, photoacoustic (PA) testing, magnetic particle testing, etc. However, each of them uses a single modal signal, which leads to a limited detection range. A nondestructive detecting method for metal material defects based on multimodal signals is proposed to expand the scope of detection and obtain more complete information. Specifically, optical signal, PA signal and US signal are combined together in this method, with the consideration of their complementarities.
Simulation and experiments are conducted to validate the effectiveness of the proposed method. Firstly, finite element simulation is employed to analyze the relationship between material parameters and the absorption of laser energy. Meanwhile, the influence of defect size on PA surface wave is simulated and analyzed. Then, a multimodal NDT platform is established to collect and process optical, PA and US signals of the metal defects. These three modal signals contain information about metal surface, shallow surface and internal defects respectively. Eventually, the information, including the location, appearance on the surface, depth, extension path in the material, is obtained.
As demonstrated in the results, the nondestructive detecting method based on multimodal signals can detect the metal defects accurately and comprehensively. This method improves the existing methods in terms of detection range and quantitative detection. Additionally, it provides a new way for the quantitative detection and comprehensive diagnosis of metal defects.
High power microwave damage mechanism on high electron mobility transistor
Authors: Li Zhi-Peng  , Li Jing  , Sun Jing  , Liu Yang  , Fang Jin-Yong  ,   Xi’an Branch, China Academy of Space Technology, Xi’an 710000, China;,   MCCC of China Wenchang Space Center, Wenchang 571300, China;,   School of Microelectronics, Xidian University, Key Laboratory of Wide Band-Gap Semiconductor Materials and Devices, Xi’an 710071, China
Published: December 2020

In this paper, the damage process and mechanism of the typical high electron mobility transistor by injecting high power microwave signals are studied by simulation and experiment methods. By using the device simulator software Sentaurus-TCAD, a typical two-dimensional electro-thermal model of high electron mobility transistor is established with considering the high-field saturation mobility, Shockley-Read-Hall generation-recombination and avalanche breakdown. The simulation is carried out by injecting the 14.9 GHz, 20 V equivalent voltage signals into the gate electrode. Then, the distributions of the space charge density, electric field, current density and temperature with time are analyzed.
During the positive half cycle, a conduction channel appears beneath the gate electrode near the source side within device. It is found that the electric field is extremely strong and the current density is very large. Therefore, the temperature increases mainly occurs beneath the gate electrode near the source side. During the negative half cycle, because of the concentration of the large number of carriers induced by avalanche breakdown, the electric field is stronger than that in the positive half cycle. But the current density is lower than that in positive half cycle. Therefore, the increase of temperature is dominated by the electric field. With the effects of both strong electric field and high current density, the temperature of the transistor rises in the whole signal cycle. In addition, temperature in the positive half-cycle rises faster than that in the negative half-cycle.Furthermore, the peak temperature appears at the location beneath gate electrode near the source side because the electric field and current density are strongest in this area. When the temperature within the device is higher than 750 K, intrinsic breakdown occurs in GaAs material, so the heating process becomes quicker. With the temperature increases, the GaAs reaches its melting point, and the device fails permanently.
Furthermore, taking the original phase of 0 and for example, we discuss the influences of different original phases on damage process. It is shown that when original phase is zero, the temperature increase rate is faster, and the burn-out time is shorter.Failure analysis of high electron mobility transistor devices damaged by microwaves is carried out with scanning electron microscope, and the simulation results are well consistent with the experimental results. The conclusion may provide guidance for studying high power microwave defense of low noise amplifier and rugged design of high electron mobility transistor in fabrication technology.
An efficient node influence metric based on triangle in complex networks
Authors: Han Zhong-Ming  , Chen Yan  , Li Meng-Qi  , Liu Wen  , Yang Wei-Jie  ,   Beijing Technology and Business University, Beijing 100048, China;,   Beijing Key Laboratory of Big Data Technology for Food Safety, Beijing 100048, China
Published: December 2020

Influential nodes in large-scale complex networks are very important for accelerating information propagation, understanding hierarchical community structure and controlling rumors spreading. Classic centralities such as degree, betweenness and closeness, can be used to measure the node influence. Other systemic metrics, such as k-shell and H-index, take network structure into account to identify influential nodes. However, these methods suffer some drawbacks. For example, betweenness is an effective index to identify influential nodes. However, computing betweenness is a high time complexity task and some nodes with high degree are not highly influential nodes. Presented in this paper is a simple and effective node influence measure index model based on a triangular structure between a node and its neighbor nodes (local triangle centrality (LTC)). The model considers not only the triangle structure between nodes, but also the degree of the surrounding neighbor nodes. However, in complex networks the numbers of triangles for a pair of nodes are extremely unbalanced, a sigmoid function is introduced to bound the number of triangles for each pair of nodes between 0 and 1. The LTC model is very flexible and can be used to measure the node influence on weighted complex networks. We detailedly compare the influential nodes produced by different approaches in Karata network. Results show that LTC can effectively identify the influential nodes. Comprehensive experiments are conducted based on six real complex networks with different network scales. We select highly influential nodes produced by five benchmark approaches and LTC model to run spreading processes by the SIR model, thus we can evaluate the efficacies of different approaches. The experimental results of the SIR model show that LTC metric can more accurately identify highly influential nodes in most real complex networks than other indicators. We also conduct network robustness experiment on four selected networks by computing the ratio of nodes in giant component to remaining nodes after removing highly influential nodes. The experimental results also show that LTC model outperforms other methods.
Colloids in the study of fundamental physics
Authors: Zhang Tian-Hui  , Cao Jing-Sheng  , Liang Ying  , Liu Xiang-Yang  ,   Center for Soft Condensed Matter Physics and Interdisciplinary Research; College of Physics, Optoelectronics and Energy Physics, Soochow University, Suzhou 215006, China;,   Research Institute for Biomimetics and Soft Matter, Xiamen University, Xiamen 361005, China
Published: December 2020

Colloidal particles in solution exhibit rich phase behaviors and behavior like big-atom. In the past decades, as modelling systems, colloids have been widely employed in the study of nucleation, crystallization, glass transition and melting. A number of advances have been achieved. These advances to a large extent extend and complete the understanding of various phase transitions. Recently, a number of active fields are emerging with colloidal model systems. In this review, the advances and the emerging fields are summarized. At the end, the potential directions and the challenges for future studies are suggested.
Vibration energy harvesting with uni-polar electret film
Authors: Ma Xing-Chen  , Ye Rui-Feng  , Zhang Tian-Le  , Zhang Xiao-Qing  ,   Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology, School of Physics Science and Engineering, Tongji University, Shanghai 200092, China
Published: December 2020

Electronic devices are highly demanded commodities and will continue increasing in popularity in the near future, all of which require powers in one way or another. A challenge that arises in remote or inconvenient locations is access to reliable power sources. Energy harvesting technology is critical in the development of self-powered electronic devices. In this paper we present a novel approach to vibration energy harvesting, which is based on uni-polar electret film. Uni-polar electret film is of a flexible polymeric material which can exhibit permanent polarization and induce durable electric filed. In this study, real charge electret films are prepared by using the negative corona charging one-side metalized irradiation cross-linked polypropylene (IXPP) films. Vibration energy harvesters based on such electret films are designed and fabricated. The charge stability in IXPP electret film is investigated by measuring the surface potential of sample. The electromechanical properties of the energy harvester sample are tested by measuring quasi-static and dynamic sensitivities. The energy harvesting from vibrations by using the energy harvester sample, at various vibration frequencies, load resistances, and seismic mass values, is also studied. The results show that as the IXPP film is charged with a corona voltage of -13 kV, grid voltage of -2.0 kV and charging time of 60 s, the stable surface potential of -680 V is obtained after 15-day storage in the laboratory environment. The quasi-static sensitivity of energy harvester sample is 1800 pC/N at a pressure of 1.3 kPa. At an optimum load resistance of 80 M and a resonance frequency of 70 Hz, a maximum output power of 5 W is obtained for an energy harvester sample with an effective area of 13 cm2 and a seismic mass value of 42.2 g.