bluuebunny/crossref_metadata_2025 huggingface dataset:
A blind beamforming algorithm for multitarget signals based on time-frequency analysis
Authors: Liu Ya-Qi  , Liu Cheng-Cheng  , Zhao Yong-Jun  , Zhu Jian-Dong  ,   The PLA Information Engineering University, Zhengzhou 450001, China;,   Unit 63880, Luoyang 471003, China
Published: December 2020

The existing blind beamforming methods are effective only under the condition that the source signals have some special statistical or structural characteristics. Additionally, the structure of cascade model is complicated and the stability of parallel model is poor when dealing with multi-target signals. To address these problems, a novel blind beamforming algorithm for multi-target signals based on time-frequency (TF) analysis is proposed in this paper. The received array signals are first transformed into time-frequency domain by using quadratic time-frequency distributions (TFDs). Then, the single-source auto-term TF points which show energy concentration at a single signal are extracted through three operations:(i) removing noise points by setting a reasonable threshold, (ii) separating auto-term TF points from cross-term points, and (iii) selecting the single-source auto-term TF points from the auto-term ones. Moreover, these single-source auto-term TF points are classified by the principal eigenvector of their spatial time-frequency distribution matrixes. For each class of TF points, the uncertain set of signal steering vector is given, whose radius is defined as the ultimate range between the center and the elements in the class. Within the uncertain set, an optimization algorithm is provided to get the optimal estimation of the signal steering vector. Finally, the blind beamforming for multi-target signals is achieved based on the Capon method, which can enhance the desired signals and suppress the noise and interference signals. In addition, the influence of parameters selection, the clustering method of unknown source number, and the computational complexity of the proposed algorithm are analyzed. The proposed algorithm can achieve parallel output of multi-target signals under the condition that the array manifold and the direction of arrival (DOA) are unknown. Also, the complex iterative solving processing may be avoided and special limitations on signal characteristics are unnecessary. As a result, it has wide applicability and superior stability compared with the existing blind beamforming methods. Simulations illustrate that the proposed algorithm can well separate multi-target signals which are TF-nondisjoint to a certain extent. It can achieve a higher output signal to interference plus noise ratio (SINR) compared with the constant modulus algorithm (CMA), the independent component analysis (ICA) algorithm, and the joint approximate diagolization of eigenmald (JADE) algorithm. Furthermore, the output performance of the proposed algorithm is close to the optimal Capon beamformer.
Numerical simulation on stirring motion and mixing characteristics of ellipsoid particles
Authors: Liu Yang  , Han Yan-Long  , Jia Fu-Guo  , Yao Li-Na  , Wang Hui  , Shi Yu-Fei  ,   Department of Engineering, Northeast Agricultural University, Harbin 150030, China
Published: December 2020

To investigate the motion characteristics and the law of identical property for particles obtained under segregation to uniform distribution conditions in forced agitation mixing, the mixing process of the same sized ellipsoidal particles at different rotating speeds in a U-tank is simulated using three-dimensional discrete element method. Macroscopic mixing law and partial mixing characteristics in particle mixing process are analysed in the view of single particle random motion trajectory and motion vector diagram of macroscopic particle flow. And the mathematical relation between mixability and revolutions of agitating blades is described quantitatively. Results show that convective mixing and four partial mixing characteristics control the mixing homogeneity process of identical property of segregation particles in forced agitation mixing. Mixability of segregation particles is independent of rotating speed of the agitating shaft, but has a direct correlation with revolutions. The relation between mixability and revolutions agrees with the exponential growth model. Research results can provide the basis and reference for equipment improvement and operating control of bulk material in the industry of the augmenting of mix.
Study on electrical properties of ion-beam-etched HgCdTe crystal
Authors: Xu Guo-Qing  , Liu Xiang-Yang  , Zhang Ke-Feng  , Du Yun-Chen  , Li Xiang-Yang  ,   State Key Laboratory of Transducer Technology, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China;,   Key Laboratory of Infrared Imaging Material and Detectors, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China;,   Graduate School of the Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

In this paper, we study the electrical properties of ion-beam-etched Hg1-xCdxTe (x=0.236) crystal with the help of mobility spectrum analysis technique. In step-by-step chemical etching, it is shown that the p-HgCdTe is completely converted to the n-type one which includes a damaged surface electron layer with a low mobility and a bulk electron layer with a higher mobility after ion etching. The mobility spectra at different temperatures show that the mobility of the surface electrons is independent of temperature in the measurement temperature range while the bulk electrons exhibit a classical behavior of n-HgCdTe with characteristics that are strongly dependent on temperature. Hall data for different thicknesses show that the electrical properties of the bulk layer are uniform. Otherwise, the surface electron layer may be found to consist of a concentration about 2-3 order of magnitude higher than the bulk electron layer.
Simulations of the size effect on the elastic properties and the inherent mechanism of metallic nanowire
Authors: Yang Xi-Yuan  , Quan Jun  ,   Physics Science and Technology School, Lingnan Normal University, Zhanjiang 524048, China
Published: December 2020

In this paper molecular dynamics (MD) method and the modified analytical embedded atom model (MAEAM) are used to investigate the size effect on the elastic properties of Ni, Al and V nanowires and the role the free surface plays. For convenience of comparison, the elastic properties of these corresponding perfect bulk materials are also studied. Results obtained indicate that the calculated values of the elastic properties of these perfect materials are in good agreement with those previously given theoretical and experimental ones. But the calculated bulk moduli of the nanowires, which are lower than those of the prefect materials, increase exponentially with increasing size of the nanowire and are nearly close to a constant (180.20 GPa for the Ni nanowire, 83.98 GPa for the Al nanowire and 162.48 GPa for the V nanowire). Meanwhile, the surface energy of the nanowire decreases exponentially with the increase of its size and reaches a minimal value (1.84 J·m-2 for the Ni nanowire, 0.77 J·m-2 for the Al nanowire, and 1.71 J·m-2 for the V nanowire), which is consistent with the corresponding bulk material. And the critical value of the size, which has a distinct effect on the elastic properties and the surface energy, is about 5.0 nm for all nanowires. On this basis, the free surface dependence of the elastic properties of these metallic nanowires and the inherent mechanisms are further discussed by exploring the size effect on the surface energies of Ni, Al and V nanowires and their distribution characteristics, showing that the free surface plays a more and more important role in the diminution of the elastic properties of nanowires as the size decreases. The mode of the surface impacting on the elastic properties of nanowire is described as follows:The surface first reduces the compressional stress of the internal core region of nanowires and then the reduced compressional stress results further in the decrease in the elastic properties of nanowires.
First-principles calculation for hydrogen-doped hematite
Authors: Shi Yu  , Bai Yang  , Mo Li-Bin  , Xiang Qing-Yun  , Huang Ya-Li  , Cao Jiang-Li  ,   Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China
Published: December 2020

Hexagonal α-Fe2O3 is one of the most common functional material used as magnetic semiconductor, and plays an important part in various applications, such as electronic devices etc. Based on the density functional theory, the lattice parameters, density of states and Bader charge analysis of α-Fe2O3 have been calculated using the first-principles calculation with GGA+U method. As Fe is a transition metal element, the value of U can be more accurate by considering the influence of the strong on-site Coulomb interaction between 3d electrons. First, the crystal equilibrium volume, the magnetic moment of Fe atom, and the band gap value of α-Fe2O3 are synthetically researched and compared with those with different U. Results indicate that the calculation model of α-Fe2O3 are in good agreement with the experimental model when the value of U is 6 eV. These parameters can also be adapted to the following doping calculaton. The α-Fe2O3 unit cell has both tetrahedral and octahedral interstitial sites. The calculation of doping formation energy shows that the α-Fe2O3 system is most stable when the doped hydrogen atom is in the tetrahedral interstitial site. The density of states show that the valence band and conduction band compositions are similar for the bulk and hydrogen-doped α-Fe2O3. That is, the valence bands are dominated mainly by both O 2p and Fe 3d orbitals with the O 2p orbitals playing a leading role, while the conduction band is dominated by Fe 3d orbitals. The band gap of α-Fe2O3 decreases from 2.2 to 1.63 eV after hydrogen doping. Also, a strong hybrid peak occurs near the Fermi level after hydrogen doping, which is chiefly composed of Fe 3d orbital, and the O 2p orbital also has a small contribution. The H 1s orbital is mainly in the lower level below the top valence band. Results of the Bader charge analysis and the density of states calculation for partial correlated atoms suggest that the new hybrid peak is chiefly caused by Fe atom which is closest to the hydrogen atom in the crystal cell. In this process, H atom loses electrons, and the nearest neighbors of H atom, i.e. O and Fe atoms, almost obtain all the electrons H atom loses, so H and O atoms are bonded together strongly, causing the hybrid peak, to expand the width of the top valence band and shift down the bottom of the conduction band, so that the band gap decreases and the electrical conductivity increases. Hydrogen doping is suggested to be an effective method to modify the band.
Hall effect of different textured CVD diamond films
Authors: Su Qing-Feng  , Liu Chang-Zhu  , Wang Lin-Jun  , Xia Yi-Ben  ,   Institute of New Energy, Shanghai Lianfu New Energy S&T Group Co., Ltd, Shanghai 201201, China;,   School of Materials Science and Engineering Shanghai University, Shanghai 200444, China
Published: December 2020

Due to its smoothest surface, fewer defects, and better crystal quality, [100] textured diamond film is well suited for the application of optoelectronic and microelectronic devices. Carrier concentration and mobility are very important parameters of semiconductor materials. In order to further broadening the application of diamond films in optoelectronics and microelectronics, it is necessary to made a research on Hall effect characteristics of [100] textured and [111] textured films. In this paper, different textured polycrystalline diamond films are deposited on silicon substrates by hot filament chemical vapor deposition (HFCVD) method under different conditions. Microstructures of diamond films are characterized by X-ray diffraction (XRD). High quality [100] textured and [111] textured diamond films are obtained. Dark current-voltage (I-V) characteristics of different-oriented films after annealing are investigated at room temperature. The carrier concentration and mobility of diamond films are measured by Hall effect test system as the temperature changing from 100 to 500 K. Results indicate that the textures of diamond films affect the value of carrier mobility:carrier concentration increases and mobility decreases with the decrease of temperature; and the deposited films are of p-type materials. The carrier concentration and mobility of polycrystalline [100]-textured diamond films at room temperature are 4.3×104 cm-3 and 76.5 cm2/V·s, respectively.
Investigation on the flux pinning force and flux pinning mechanism in Ba1-xKxFe2As2 single crystal with Tc = 38.5 K
Authors: Wang Chun-Lei  , Yi Xiao-Lei  , Yao Chao  , Zhang Qian-Jun  , Lin He  , Zhang Xian-Ping  , Wang Dong-Liang  , Ma Yan-Wei  ,   Key Laboratory of Advanced Micro/Nano Functional Materials of Henan Province, School of Physics and Electronic Engineering, Xinyang Normal University, Xinyang 464000, China;,   Key laboratory of applied superconductivity, Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, China
Published: December 2020

The discovery of superconductivity in iron-based superconductors by Professor Hosono in Japan in 2008 has triggered off an enormous group of researches the world wide. The iron-based superconductors are regarded as another kind of high-Tc superconductors, which possess lots of merits, such as very high upper critical field (Hc2), high critical current density (Jc), and small crystal anisotropy (), are promising for high field applications. Ba1-xKxFe2As2, as a typical FeAs-122 superconductor, is focused on by both theoretical physicists and material scientists since its discovery. In this paper, we first successfully fabricate Ba1-xKxFe2As2 single crystal. It has an onset transition temperature up to 38.5 K, while its zero resistivity temperature reaches 37.2 K. Both the R-T and M-T data of it show very sharp superconducting transition, and its critical current density at 5 K and self field is over 106 Acm-2 and almost field independent. The flux pinning force and the relative pinning mechanisms in Ba1-xKxFe2As2 are discussed by analyzing the data obtained from the measurements about the R-T and M-H under different conditions. Results indicate that the Ba1- xKxFe2As2 superconductors have very strong intrinsic vortex pinning force, and the vortex potentials (U0) under 9 T field are 5800 K and 8100 K for the H//c and H//ab, respectively. Furthermore, the vortex pinning mechanism is also investigated by analyzing the relationship Jc-B. According to the present results, the flux pinning mechanism should be (l) pinning because of the change of mean free path for electrons induced by nano-size crystallographic defects in Ba1-xKxFe2As2.
Magnetic property and electronic structure of BaFe4-xTi2+xO11
Authors: Hu Ya-Ya  , Zhu Yuan-Yuan  , Zhou Bei-Bei  , Liu Shuo  , Liu Yong  , Xiong Rui  , Shi Jing  ,   School of Physics and Technology, Wuhan University, Wuhan 430072, China
Published: December 2020

In this paper, polycrystalline BaFe4-xTi2+xO11 (x=0, 0.25, 0.5, 0.75, 1) samples have been synthesized by the conventional solid-state reaction method. X-ray diffraction (XRD) patterns of all the samples show that the diffraction peaks correspond to that of an R-type hexagonal ferrite structure, and no trace of second phase is detected. Measurement of X-ray photoelectron spectroscopy (XPS) reveals that most of the Fe ions in BaFe4Ti2O11 are trivalent and the fitting of two peaks in Fe 2p spectrum corresponding to different Fe ion sites, while the amount of Fe2+ ions increases with the increase of Ti ions in BaFe4-xTi2+xO11. The spectroscopy of Ti ions confirms that the valence of Ti in BaFe4-xTi2+xO11 are tetravalent. Magnetic susceptibility of BaFe4-xTi2+xO11 (x= 0, 0.25, 0.5, 0.75, 1) reveals two magnetic transitions at T1～250 K and T2～83 K, which indicate a complex magnetic order driven by competing interactions on a frustrated lattice with a noncentrosymmetric structure. For all the samples, the magnetic susceptibility obeys Curie-Weiss law above T1, and M-H curves exhibit a linear variation with magnetic field in this temperature range, which is consistent with the paramagnetic behavior. A decrease of the effective magnetic moment is due to the increase of Fe2+ ions with the increase of Ti content in BaFe4-xTi2+xO11. Below T1, the magnetization curve as a function of temperature (M-T) and the magnetization versus magnetic field (M-H) at different temperatures imply its characteristic of a typical canted antiferromagnetic or ferrimagnetic state. Meanwhile, the transition temperature T2 drops gradually with the increase in Ti content, which might be related to the change of occupying of Fe ions in the kagome layers.
Crossing behavior of pedestrians and vehicles at unsignalized intersection based on dirty faces game
Authors: Wang Lu  , Ye Shun-Qiang  , Xie Neng-Gang  ,   School of Mechanical Engineering, Anhui University of Technology, Maanshan 243002, China
Published: December 2020

The conflicts between pedestrians and vehicles at unsignalized intersections are rather dangerous. Besides the motion features, pedestrians and vehicle drivers also have behavioral features, so modelling the crossing behaviors of pedestrians and vehicles from the sociological aspect is necessary. Through an in-depth analysis of the interference mechanism between pedestrians and vehicles at unsignalized intersections, in this paper, a dirty faces game to analyze the crossing behaviors of pedestrians and vehicles as “rational man” is proposed. The formation process of common knowledge, the producing mechanism of dominant strategy and the kinetic process of dirty faces game are analyzed. Under the assumption of probability of the dominant strategy and distribution of crossing times, theoretical derivations of the benefits gained by pedestrians and vehicles as well as the conflict probability are given. Results show that a collision is more likely to occur when the theoretical time of pedestrian passing through is closer to that of the vehicle driver, and this is consistent with the actual situation. Furthermore, the influences of vehicle type, waiting time and heterogeneity of pedestrians on the conflict probability are also considered.
Transport properties of fractional coupled Brownian motors in ratchet potential with feedback
Authors: Qin Tian-Qi  , Wang Fei  , Yang Bo  , Luo Mao-Kang  ,   College of Mathematics, Sichuan University, Chengdu 610065, China
Published: December 2020

Based on the theory of fractional integration, direct transport behaviors of coupled Brownian motors with feedback control in viscoelastic media are investigated. The mathematical model of fractional overdamped coupled Brownian motors is established by adopting the power function as damping kernel function of general Langevin equation due to the power-law memory characteristics of cytosol in biological cells. Numerical solution is observed by fractional difference method and the influence of model parameters on cooperative direct transport of the coupled Brownian motors is discussed in detail by numerical simulation. The research shows that the memory of the fractional dynamical system can affect the direct transport phenomenon of the coupled Brownian motors through changing the on-off switching frequency of the ratchet potential with feedback control. To be more specific, in a proper range of the fractional order, the memory of the dynamical system can increase the on-off switching frequency of the ratchet potential, which can lead to the velocity increase of the direct transport. Furthermore, in the case of small fractional order, since the coupled Brownian motors move under the competition between the damping force with memory and the potential force with feedback control, the resultant force exerted on the coupled particles is always positive when the ratchet potential with feedback control is on although the fractional damping force is large, which leads to the result that the coupled Brownian motors move in the positive direction in the mass. On the contrary, in the case of large fractional order, the on-off switching frequency of potential with feedback control becomes small, as a result of which the main influential factor of the direct transport becomes the potential depth. Therefore the coupled Brownian motors are more likely to stay in the potential wells for a long time because the probability that describes the possibility that the coupled Brownian motors surmount the potential barriers becomes small. Finally, with the parameters of the fractional dynamical system (e.g. potential depth, noise intensity) fixed, the direct transport velocity of the coupled Brownian motors shows the generalized stochastic resonant phenomenon while the fractional order varies.
Generation of arbitrary vector beam based on optical holography
Authors: Xi Si-Xing  , Wang Xiao-Lei  , Huang Shuai  , Chang Sheng-Jiang  , Lin Lie  ,   Key Laboratory of Optical Information Science and Technology, Ministry of Education, Institute of Modern Optics, Nankai University, Tianjin 300071, China
Published: December 2020

According to the phase and amplitude modulation of the spatial light modulator (SLM) loading the phase distribution for generating arbitrary vector beams, we present a method of generating arbitrary vector beams based on the optical holography with angle multiplexing. First of all, we use the optical holography to record the special phase distribution on the SLM, and so an optical holographic grating is obtained. In the reproduction process, the two conjugate reference beams with the same incident angle illuminate the holographic grating and the superposition of the two reproduced beams is achieved, thus the arbitrary vector beams are obtained. This method can avoid the emergence of complex polarization distribution, and has advantages such as simple optical setup, convenient operation, and higher polarization purity of generated arbitrary vector beams. Good results of the arbitrary vector beams are also obtained by computer simulation.
Correction to near-range multiwavelength lidar optical parameter based on the measurements of particle size distribution
Authors: Zhao Hu  , Hua Deng-Xin  , Mao Jian-Dong  , Zhou Chun-Yan  ,   School of Mechanical and Precision Instrument Engineering, Xi’an University of Technology, Xi’an 710048, China;,   College of Electrical and Information Engineering, Beifang University of Nationalities, Yinchuan 750021, China
Published: December 2020

The lidar observations of aerosols under near range condition are distorted because of insufficient overlapping between the transmitting laser beam and the field of view of receiving telescope in non-coaxial lidar especially in multiwavelength lidar. However, the near-range atmosphere is closely related with human’s activity. So it is important to correct the near-range signal of multiwavelength lidar. This paper presents a novel method of correcting the near-range optical parameter of multiwavelength lidar based on aerosol particle size distribution measurements and Mie scattering theory. The near-range aerosol extinction profiles on fine day, cloudy day and foggy day are corrected. The results show that the method is convenient and feasible compared with the traditional methods. This method reduces the uncertainty of inversion and correction because the wavelength dependences of lidar ratio and the weather correlation of aerosol size distribution are taken into account. This method realizes the direct correction to near-range optical parameter of multiwavelength lidar. It is favorable and convenient to study physical and optical properties of boundary layer atmosphere by using this method. It will have a broad application and prospect.
Absorption spectrum analysis and crystal-field calculation of Nd3+ doped in GdTaO4 crystal
Authors: Gao Jin-Yun  , Zhang Qing-Li  , Wang Xiao-Fei  , Liu Wen-Peng  , Sun Gui-Hua  , Sun Dun-Lu  , Yin Shao-Tang  ,   Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China
Published: December 2020

In this paper, Nd3+-doped GdTaO4 laser crystal for scintillator with high density is successfully grown by the Czochralski method, and the absorption spectra are measured along the a, b and c directions in a wide wavelength range of 260-2000 nm. The experimental energy levels for Nd3+ are analyzed and identified. The free-ions and crystal-field parameters are fitted by the experimental energy levels with the root mean square deviation of 12.66 cm-1, and 102 Stark energy levels for Nd3+ in GdTaO4 host crystal are assigned. The fitting results of free-ions and crystal-field parameters are compared with those already reported for Nd3+:GdxLu1-xTaO4 (x=0.85) crystal. It indicates that the fitting results of Stark energy levels agree well with the experimental spectra.
Two-tone optical heterodyning method for the self-calibrated measurement of half-wave voltage of electrooptic phase modulator
Authors: Wang Heng  , Zhang Shang-Jian  , Zou Xin-Hai  , Liu Jun-Wei  , Zhang Ya-Li  , Li He-Ping  , Liu Yong  ,   State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Information, University of Electronic Science and Technology of China, Chengdu 610054, China
Published: December 2020

High speed electrooptic phase modulators play very important roles in the high-speed optical fiber communication system, microwave photonic system, and coherent optical communication system, due to their advantages of bias voltage free and linear modulation. As an intrinsic parameter, the half-wave voltage of an electrooptic phase modulator has been characterized by using an electrical spectrum method and an optical spectrum method in the last two decades. The optical spectrum method is generally limited by the line-width of the laser source and the resolution of the available optical spectrum analyzer, while the electrical spectrum method requires the conversion from phase modulation to intensity modulation before photodetection, since a phase modulator generates a phase modulated optical signal with constant envelope. The major difficulty in the electrical spectrum method lies in the extra calibration for the responsivity fluctuation in the photodetection. In this paper, a novel self-calibrated measurement of half-wave voltage of electrooptic phase modulators is carried out based on the optical heterodyning between the two-tone phase modulated sidebands and the frequency-shifted carrier. The method achieves a self-calibration measurement, and avoids the effect of the responsivity fluctuation in the photodetection by setting a specific frequency relationship between the two-tone microwave signals. Moreover, it extends the measuring frequency range to the double bandwidth of photodetection and spectrum analysis. Compared with the optical spectrum method, the proposed method achieves very high frequency resolution measurement, and simultaneously avoids the line-width influence of laser source by use of two-tone heterodyning. Compared with the traditional electrical spectrum method, our method works under no small-signal assumption nor photodetection calibration, and eliminates the limits of electrical driving amplitude and operating wavelength. Moreover, it decreases by at least half bandwidth requirement for the photodetector and spectrum analyzer. Our experimental demonstration shows that the measured half-wave voltages of the electrooptic phase modulator obtained by our method agree well with the data measured by the optical spectrum method, and the two-tone heterodyning method greatly improves the measurement range and frequency resolution. The proposed measurement method provides a very simple analysis method for the microwave characterization of high-speed electrooptic phase modulators, which is also a reference for other optoelectronic devices.
Cycle response characteristics research on a class of piecewise nonlinear elastic and damping double constraint system
Authors: Liu Fei  , Liu Bin  , Liu Hao-Ran  ,   School of Information Science and Engineering, Yanshan University, Qinhuangdao 066004, China;,   Key Laboratory for Special Fiber and Fiber Sensor of Hebei Province, Yanshan University, Qinhuangdao 066004, China
Published: December 2020

Piecewise nonlinear constraint exists in various fields and it always affects the stability of a system. In order to realize the dynamic characteristic of the system constrained by these nonlinearity, we consider two kinds of typical piecewise nonlinear constraints under the dynamic conditions, and establish a dynamic model with double piecewise nonlinear constraint of elasticity and damping, according to the generalized dissipation Lagrange principle. An average method is used to solve the amplitude and frequency response of the system under a periodic external incentive. By a numerical simulation, we compare the time domain responses under different piecewise nonlinear elastic constraints. The results show that the stronger the piecewise nonlinear elastic constraint, the more obvious the piecewise nonlinear damping constraint is. We also compare the bifurcation responses under different piecewise nonlinear damping constraints, the results show that the chaos state will emerge in an enlarged scope with the increase of the piecewise nonlinear damping coefficient, and threaten the stability of the system. The dynamic evolution process of the system is shown by the phase diagrams and Poincaré sections under the corresponding constraint conditions. By comparing the amplitude-frequency characteristics of the system under different constraint conditions, we obtain the response characteristic of the system and its change rule with the piecewise nonlinear constraints. By comparing and analyzing the amplitude-frequency characteristics under the piecewise nonlinear elastic and piecewise nonlinear damping constraint, we obtain the law of system stability influenced by different nonlinear factors, and the interaction relationship between the two piecewise nonlinear constraints.
Effect of direct current degradation on dielectric property of CaCu3Ti4O12 ceramic
Authors: Zhao Xue-Tong  , Liao Rui-Jin  , Li Jian-Ying  , Wang Fei-Peng  ,   State Key Laboratory of Power Transmission Equipment and System Security and New Technology, Chongqing University, Chongqing 400044, China;,   State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University, Xi’an 710049, China
Published: December 2020

CaCu3Ti4O12 ceramic has drawn much attention due to its stable colossal dielectric permittivity and pronounced nonlinear electrical characteristics. In this work, the effects of direct current degradation on the dielectric response and electrical property of CaCu3Ti4O12 ceramic aged for 60 h under 3.5 kV/cm are investigated. The results of J-E characteristic analysis show that the breakdown field E1mA decreases from 216 V/mm to 144 V/mm and nonlinear coefficient η decreases from 4.1 to 2.1. The barrier heights of CaCu3Ti4O12 ceramics are calculated to be in a range of 293-368 K, based on the J-E curves, which decrease from 0.57 eV to 0.31 eV. It is found that the dielectric constant and dielectric loss at low frequencies are significantly increased. Based on Debye function, it is indicated that the dielectric loss is composed of direct current conductance loss and relaxation loss, especially the direct current conductance loss is enhanced by the direct current degradation. At 233 K, two relaxation peaks whose activation energies are 0.10 eV and 0.50 eV can be found, which are considered to be related to grain and domain boundary and not vary with direct current degradation. Electric modulus spectra are used to characterize the role of direct current degradation in the relaxation process of CaCu3Ti4O12 ceramic. The results show that the variation of interfacial space charges caused by direct current degradation obeys the Maxwell-Wagner polarization. It may be a key factor to lead to the increase of dielectric permittivity below 10 Hz, and a new corresponding relaxation peak θ can be observed in the modulus plot at low frequency. In the impedance spectra in 323-473 K, the relaxation peaks of grain boundary shift toward high frequency after direct current degradation. The results from the complex impedance plane show that the resistance of the grain boundary decreases by about two orders of magnitude and its activation energy drops off from 1.23 eV to 0.72 eV, while the resistance of grain decreases a little and its activation energy has no obvious variation. Therefore, it is proposed that direct current degradation should play an important role in grain boundary and affect its electrical property and dielectric response. An RC circuit model is proposed to elucidate the correlation between dielectric relaxation and electrical property of CaCu3Ti4O12 ceramic.
Crystal growth and characterization of hydrogen-doped single diamond with Fe(C5H5)2 additive
Authors: Fang Chao  , Jia Xiao-Peng  , Chen Ning  , Zhou Zhen-Xiang  , Li Ya-Dong  , Li Yong  , Ma Hong-An  ,   State Key of Laboratory of Superhard Materials, Jilin University, Changchun 130012, China;,   Tongren University, Tongren 554300, China
Published: December 2020

In this paper, a series of high-quality hydrogen-doped diamonds is successfully synthesized in Ni70Mn25Co5-C system by using Fe(C5H5)2 as hydrogen source at pressures ranging from 5.5 GPa to 6.0 GPa and temperatures of 1280-1400 ℃. We find that both pressure and temperature conditions strengthen with adding the Fe(C5H5)2. Scanning electron microscope micrographs show that the obtained diamonds at low levels of Fe(C5H5)2 additive have smooth surfaces. However, many defects are found and some pores appear on the diamond surface with increasing the Fe(C5H5)2 additive in the system. From the obtained Fourier transform infrared (IR) spectrum, we notice that there is no significant change of nitrogen concentration in the synthesized diamond with the Fe(C5H5)2 additive lower than 0.3 wt%, while the nitrogen concentration gradually decreases with the further increase of Fe(C5H5)2 additive. In the system with 0.5 wt% Fe(C5H5)2 additive, the nitrogen concentration in synthesized diamond is only half that of system without Fe(C5H5)2 additive. Meanwhile, the hydrogen associated IR peaks of 2850 cm-1 and 2920 cm-1 are gradually enhanced with the increase of Fe(C5H5)2 additive in the system, indicating that most of the hydrogen atoms in the synthesized diamond are incorporated into the crystal structure as sp3-CH2-symmetric (2850 cm-1) and sp3 CH2-antisymmetric (2920 cm-1) vibrations. From the obtained Raman spectrum, we find the incorporation of hydrogen impurity leads to a significant shift of the Raman peak towards higher frequencies from 1333.90 cm-1 to 1334.42 cm-1 with increasing the concentration of Fe(C5H5)2 additive from 0.1 wt% to 0.5 wt%, thereby giving rise to some compressive stress in the diamond crystal lattice. This is the first time that the gem-grade hydrogen-doped diamond single crystal, with size up to 3.5 mm has been successfully synthesized by using new hydrogen source Fe(C5H5)2 additive. We believe that our work can provide a new method to study the influence of hydrogen impurity on diamond synthesis and it will help us to further understand the genesis of natural diamond in the future.
Selective area growth of Ge film on Si
Authors: Wang Jian-Yuan  , Wang Chen  , Li Cheng  , Chen Song-Yan  ,   Semiconductor Photonics Research Center, Department of Physics, Xiamen University, Xiamen 361005, China
Published: December 2020

According to low temperature Ge buffer layer and selective area epitaxy technology, we selectively grow Ge film on patterned Si/SiO2 substrate using ultra-high vacuum chemical vapor deposition. By using X-ray diffraction (XRD), scanning electron microscope, atomic force microscopy and Raman scattering spectrum, we obtain its crystal quality and the laws of stress and other parameters varying with shape size. The results show that threading dislocation density decreases with shape size decreasing. Moreover, the tensile strain of Ge layer first increases and then turns stable with the increase of shape size, which can be attributed to the formation of (113) facet during Ge selective area growth. The formation of (113) facet reduces the strain energy of epitaxial material system, and the reduction of strain energy per unit volume decreases with increasing the shape size. The root-mean-square surface roughness of the Ge epilayer with a thickness of 380 nm is about 0.2 nm, the full-width-at-half maximum of the Ge peak of the XRD profile is about 678". It is indicated that the selective area epitaxial Ge layer is of good quality and will be a promising material for Si-based optoelectronic integration.
Recognition of epilepsy electroencephalography based on AdaBoost algorithm
Authors: Zhang Tao  , Chen Wan-Zhong  , Li Ming-Yang  ,   Department of Communication Engineerings, Jilin University, Changchun 130012, China
Published: December 2020

Automatic recognition of epilepsy electroencephalography (EEG) signal has become a research focus because of its high efficiency, and many algorithms have been put forward to achieve it. As one of the classic algorithms of boosting algorithm, AdaBoost algorithm has been widely used in face detection and target tracking fields, but the algorithm also has a disadvantage that is its degradation. In order to solve this problem, this paper puts forward three measures to optimize the algorithm by filtering the weak classifiers whose recognition rates are low, introducing the smoothing factor and a weighted correction function. In order to verify the robustness of optimized algorithm, we choose three main parameters, i.e., the number of weak classifier, which is denoted by T; the base of logarithmic function, which is denoted by α; the threshold of weight, which is denoted by β. The experimental results of optimized AdaBoost show that it has good robustness and high recognition rate. #br#In this paper, we divide the whole process into three steps. The first step is to use the Butterworth digital low-pass filter in which the cutoff frequency of pass band is 40 Hz to filter noise whose frequency is above 40 Hz. The second step is to do feature extraction with the help of wavelet packet decomposition. The third step is to compute the sum of absolute value which are the wavelet packet coefficients of fourth layer, the wavelet package entropy and the sum of signal amplitude square and combine them together to form the feature vector of each EEG. Because the wavelet package entropy is far less than the sum of absolute value and the sum of signal amplitude square, in order to make sure that the entropy reacts in the third step, we use one thousandth of the sum of absolute wavelet packet coefficients, one hundredth of the sum of signal amplitude square and the wavelet package entropy as the weighted feature vector. Finally, we succeed in distinguishing EEGs between epilepsy and normal by using the optimized AdaBoost whose input is the weighted feature vector. The result shows that the presented method has a high recognition rate, it can identify 96.11% epilepsy EEGs and 99.51% normal EEGs, thus it provides an effective solution for the correct diagnosis of epilepsy.
Numerical investigations of dynamic behaviors of the restricted solid-on-solid model for Koch fractal substrates
Authors: Yang Yi  , Tang Gang  , Zhang Zhe  , Xun Zhi-Peng  , Song Li-Jian  , Han Kui  ,   Department of Physics, China University of Mining and Technology, Xuzhou 221116, China
Published: December 2020

In order to investigate the influence of structures of substrates on the dynamic properties of a discrete growth model, the restricted solid-on-solid model for Koch lattice and Koch curve fractal substrates, which have different fractal dimensions and spectrum dimensions but the same walk dimensions, is studied by means of numerical simulations. Surface width and distribution of the extremal height of the saturated surface are calculated. Results show that the random walk exponent plays the determinative part in the saturated regime. Although the fractal substrates have different fractal dimensions and spectral dimensions, the value of roughness exponents for the two substrates are almost the same within the error. The data of maximal height distributions (minmal height distribution) on the width of the saturated surface for the two fractal substrates can be well collapsed together and fitted by Asym2Sig distribution.
Numerical modeling and research on nonlinear dynamic behaviors of two-stage photovoltaic grid-connected inverter
Authors: Liao Zhi-Xian  , Luo Xiao-Shu  , Huang Guo-Xian  ,   College of Electronic Engineering, Guangxi Normal University, Guilin 541004, China
Published: December 2020

In this paper, the piecewise smooth state equation of a two-stage photovoltaic grid-connected (TPG) inverter is established and studied; based on the solution to the piecewise smooth state equation of the TPG inverter, effects of the photovoltaic array voltage on nonlinear dynamical behaviors of the TPG inverter are analyzed by using bifurcation diagram, folded diagram, 3D phase diagram, and Poincaré section. Then the nonlinear dynamical behaviors of TPG inverter are compared with the conventional one. And a strategy of expanding the input voltage range for the TPG inverter is explored. Finally, the nonlinear dynamical behaviors in it caused by the variation of main circuit parameters: such as the output inductance and capacitance of the front-stage, as well as those of the second-stage, are discussed through slow-scale bifurcation diagrams. Studies have found that it is effective to expand the input voltage range of the TPG inverter by segment control of the photovoltaic array voltage, and the chaotic phenomena in the TPG inverter can be avoided by increasing the parameter values of inertial devices such as output inductance and capacitance in the front-stage appropriately, but the values of output inductance and capacitance in the second-stage should be away from the multiple noncontiguous region, since it can cause chaotic behavior. The above work may have important guiding significance and application for improving the stability and efficiency of two-stage photovoltaic grid-connected inverter based photovoltaic power generation system.
Experimental research on loading strontium bosons into the optical lattice operating at the “magic” wavelength
Authors: Tian Xiao  , Wang Ye-Bing  , Lu Ben-Quan  , Liu Hui  , Xu Qin-Fang  , Ren Jie  , Yin Mo-Juan  , Kong De-Huan  , Chang Hong  , Zhang Shou-Gang  ,   Key Laboratory of Time and Frequency Primary Standards of Chinese Academy of Sciences, National Time Service Center, Xi'an 710600, China;,   University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

The optical lattice clock with neutral atoms occupies an outstanding position in the research field of atomic clocks, demonstrating the great potential of its performance (like the uncertainty and the stability). At present, the optical lattice clock has realized a 10-18 level of its uncertainty. In this paper, we present the realization of loading bosonic atoms 88Sr (strontium, alkaline-earth metals) into a one-dimensional (1D) optical lattice in our laboratory. The optical lattice where the atoms are trapped can make the energy level shift, called Stark shift. But there is the special optical lattice operating at the “magic” wavelength for clock transitions (5s2) 1S0-(5s5p) 3P0, which can make the same Stark light-shift for both of them, indicating a zero light-shift relative to the clock. In our experiment, Sr atoms are cooled in a two-stage cooling and its temperature can be as low as 2 μK. Then these cold atoms are confined in the Lamb-Dicke region by the lattice laser output from an amplified diode laser operating at the “magic” wavelength, 813 nm. Experimentally, it is straightforward to provide 850 mW of lattice power focused to a 38 μm beam radius. After the cold atoms have trapped in the optical lattice, the lifetime of atoms in 1D optical lattice is measured to be 270 ms. The temperature and the number are about 3.5 μK and 1.2×105 respectively. Besides, effects of the power of the lattice laser on both the number and temperature are analyzed. The number changes linearly with the laser power, while there is no obvious influence on the temperature by the power. This original and special approach for atoms trapped in the optical lattice can provide a long interrogation time for probing the clock transition. Furthermore, it may be the foundation for developing our optical lattice clock of strontium atoms.
Design and optimization of off-beam NO2 QEPAS sensor by use of E-MOCAM with a high power blue laser diode
Authors: Yin Xu-Kun  , Zheng Hua-Dan  , Dong Lei  , Wu Hong-Peng  , Liu Xiao-Li  , Ma Wei-Guang  , Zhang Lei  , Yin Wang-Bao  , Jia Suo-Tang  ,   State Key Laboratory of Quantum Optics and Quantum Optic Devices, Institute of Laser Spectroscopy, Shanxi University, Taiyuan 030006, China
Published: December 2020

A highly sensitive NO2 optical sensor has been designed by means of combining the electrical modulation cancellation method (E-MOCAM) and off-beam quartz enhanced photoacoustic spectroscopy (QEPAS). A high power multimode blue laser diode emitting at around 450 nm is used as the excitation light source of the photoacoustic signal. In the E-MOCAM, the balance signal is generated from a dual-channel function generator and introduced to the pin of the quartz tuning fork (QTF) to balance out the huge background noise. The principle of the E-MOCAM is explained in detail from the perspective of equivalent circuit of QTF, and the background noise of the high power LD-based QEPAS sensor is analyzed. Results show that stray light noises coming from the LD beam and blocked by the resonator and the photoacoustic cell are dominated in all the noises. Gas flow noise of QEPAS sensor is also estimated, and excessive noise could be introduced by the gas flow even at a rate below 200 sccm. The gas flow noise is measured at different gas flow rate, from 60 to 200 sccm. Compared with the QEPAS sensor based on wavelength modulation, the sensor based on amplitude modulation, especially in the case of high power light source, is more sensitive to the gas flow. The ultimate background noise of the off-beam QEPAS sensor can be reduced by 269 times after the E-MOCAM is applied. The performance of the NO2 QEPAS sensor is evaluated in the NO2/N2 mixtures of different concentrations, ranging from ppb to ppm levels. In the case of the 2.85 ppm NO2 measurement, the SNR of 630 is achieved. A linear fitting is implemented to evaluate the response of the sensor, resulting in an R square value of 0.999. Allan plot is used to investigate the long term stability of the sensor. The original background noise produced from the off-beam QEPAS configuration is less than that from the on-beam QEPAS configuration, thus the combination of off-beam QEPAS configuration and E-MOCAM shows a better stability. A detection limit of 0.34 ppb (1, 46 s integration time) for NO2 in N2 at atmospheric pressure can be achieved, which corresponds to a normalized noise equivalent absorption coefficient of 2.210-8 cm-1W/Hz1/2.