bluuebunny/crossref_metadata_2025 huggingface dataset:
Research progress of the control and measurement of the atomic and molecular ultrafast electron dynamics using two-color field
Authors: Liu Can-Dong  , Jia Zheng-Mao  , Zheng Ying-Hui  , Ge Xiao-Chun  , Zeng Zhi-Nan  , Li Ru-Xin  ,   State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China
Published: December 2020

The advent of the ultrafast laser pulse provides the powerful and efficient tool for probing the ultrafast electron dynamics in atoms and molecules. The various nonlinear process induced by the laser-matter interaction allows one to obtain the electron motion information on the sub-femtosecond time scale. A series of the ultrafast spectroscopic technique, such as attosecond streak camera, attosecond transient absorption spectrum, and etc., have been successfully applied to the probe of electron dynamics in atoms, molecules, and solids. Using two-color field is one of the significant methods to achieve the coherent control and exploring of the electron motion. This paper summarizes recent research activities in the field of the atomic and molecular ultrafast process investigated in State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, including the detection of the electron dynamics of the multi-bound states, measurement of the carrier envelope phase (CEP) and the phase of the attosecond pulse, and the ultrafast electron control with the THz/UV and MIR/IR field. To measure the dynamics of the multi-bound states, a broadband attosecond pulse can be used to ionize the electrons after it is excited by the pump laser. By changing the delay between the pump laser and the attosecond pulse, the measured electrons ionized by the broadband xuv attosecond pulse can present the multi-bound states dynamics simultaneously. The XUV/IR scheme is popularly used in attosecond dynamics measurement. But usually, the IR field is not very strong. We find that, if the IR field is strong enough to induce the above threshold ionization (ATI), the interference between the ATI electron and the electron from XUV pulse can be used to measure the CEP of the attosecond XUV pulse. Besides, if the electron ionized by attosecond pulse can be pushed back to the nuclei, the emission from the recombination can be used to determine the spectral phase of the attosecond pulse, which is an all-optical measurement. We also investigate the two color scheme of THz/UV and MIR/IR fields. With THz/UV two color scheme, very high electron localization can be achieved duration molecular dissociation when we use the UV pulse to excite the electron and the THz pulse to control the following electron movement. When we use the MIR/IR field to control the electron motion during the high harmonic generation, the recollision can be greatly decreased and the single attosecond pulse can be produced with multi-cycle MIR laser field.
Strong field photoelectron holography studied by a generalized quantum-trajectory Monte Carlo method
Authors: Lin Cheng  , Zhang Hua-Tang  , Sheng Zhi-Hao  , Yu Xian-Huan  , Liu Peng  , Xu Jing-Wen  , Song Xiao-Hong  , Hu Shi-Lin  , Chen Jing  , Yang Wei-Feng  ,   Department of Physics, College of Science, Shantou University, Shantou 515063, China;,   Institute of Applied Physics and Computational Mathematics, Beijing 100088, China;,   HEDPS, Center for Applied Physics and Technology, Peking University, Beijing 100871, China
Published: December 2020

Strong-field photoelectron holography encodes detailed temporal and spatial information about both theelectron and ion dynamics. Here, we review a series of numerical studies of strong-field photoelectron holographyin atoms and molecules by a generalized quantum-trajectory Monte Carlo method. By comparingthe generalized quantum-trajectory Monte Carlo simulationwiththe numerical solution of thetime-dependent Schrdinger equation, we demonstrate that, in the nonadiabatic tunneling regime, pronounced nonadiabatic effects occur which manifest in the energy cutoff of the holographic interference structure. Moreover, we found that a profound ring-like pattern can be observed in the deep tunneling ionization regime. Theappearance of the ring-like interference pattern masks the holographic interference structure. In contrast to the tunneling regime, the long-range Coulomb potential is found to play an essential role in the formation of the photoelectron holography in the nonadiabatic tunneling regime.
Propagation characteristics of laser-generated surface acoustic waves in composite plate with gradient changes of near-surface viscous moduli
Authors: Guan Yi-Jun  , Sun Hong-Xiang  , Yuan Shou-Qi  , Ge Yong  , Xia Jian-Ping  ,   Faculty of Science, Research Center of Fluid Machinery Engineering and Technology, Jiangsu University, Zhenjiang 212013, China;,   State Key Laboratory of Acoustics, Institute of Acoustics, Chinese Academy of Sciences, Beijing 100190, China
Published: December 2020

Taking into account the gradient change of the near-surface viscous property, we develop a finite element model of laser-generated surface acoustic wave in composite plate. The propagation characteristics of the surface acoustic wave in the composite plate are studied in detail, and the influences of the near-surface viscous modulus, thickness, and Lam constant on the attenuation characteristics of the surface acoustic wave are discussed. In addition, the propagation characteristics of the surface acoustic wave are verified by the theoretical calculations of the dispersion and attenuation curves. The results show that the near-surface viscous modulus and thickness are related to the attenuations of the surface shear wave and the Rayleigh wave, but have no influence on the propagation velocity. Furthermore, the imaginary part of the Lam constant has great influence on the attenuations of the surface shear wave and the Rayleigh wave, whereas the imaginary part of has no effect on the attenuation characteristics of the two waves, which indicates that the attenuation of the surface acoustic wave is related to the near-surface shear viscous modulus. The study gives theoretical basis for evaluating the near-surface mechanical properties of the composite plates by the laser ultrasound technique. Besides, it provides a feasible way to study the surface micro-cracks on the composite plates.
Test and analysis of the dynamic procedure for electrowetting-based liquid lens under alternating current voltage
Authors: Xie Na  , Zhang Ning  , Zhao Rui  , Chen Tao  , Hao Li-Li  , Xu Rong-Qing  ,   School of Physics and Technology, Nanjing Normal University, Nanjing 210023, China;,   Institute of Electronic Science and Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210023, China;,   School of Optoelectronic Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210023, China
Published: December 2020

An experimental setup used to measure the important optical properties of electrowetting liquid lens is proposed. The simple and precise method of measuring dynamic responses and focal lengths of liquid lens under different excitation signals is based on Gaussian beam transmission theory. The measurement method can be widely used in all kinds of zoom lens systems. The device is simple and economical, and also has the advantages of convenient operation, high measurement precision and wide range measurement. This work provides a new way to study the dynamic response of electrowetting liquid lens and the the mechanism of electrowetting liquid lens.
The fabrication process and some relevant noticeable points for the homemade liquid lens are introduced. The testing device of dynamic process of lens consists of a He-Ne laser, an electrowetting lens, a circular diaphragm, a phototube, a digital storage oscilloscope and a computer. The change of the focal length of liquid lens due to the applied voltage will affect the flux detected by the photoelectric receivers. It is proved according to Gaussian beam transmission theory that the light flux received by the phototube changes with time, which represents the relationship between the focal length and time and the dynamic characteristics of the liquid lens. Therefore, the intensity of output signal of photoelectric receiver reflects the focal length of liquid lens.
A dynamic changing process of the focal length of a self-regulating varifocal liquid lens based on electrowetting technology is tested under alternating current signal. It shows that the focal length of the liquid lens changes with the corresponding amplitude and polarity of the sine voltage. In one cycle, 4 peak signals of 50 Hz appear in turn, and the peak amplitude increases with the increase of voltage. Peaks 1 and 2 are caused by the voltage polarity, while peaks 3 and 4 by the oscillation modes. This is due to the fact that the liquid surface changes with time in the spherical shape under low voltage, but it will generate new oscillation mode when the amplitude is high.
Molecular orbital imaging with high spatial and temperal resolutions
Authors: Zhu Xiao-Song  , Zhang Qing-Bin  , Lan Peng-Fei  , Lu Pei-Xiang  ,   School of Physics and Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China;,   Laboratory of Optical Information Technology, Wuhan Institute of Technology, Wuhan 430205, China
Published: December 2020

When atoms and molecules are excited by ultrashort laser pulses, highly nonlinear strong-field processes like above-threshold ionization and high harmonic generation occur. By analyzing the emitted light and electron signals, the atomic and molecular structures and ultrafast dynamics can be detected with a combination of Angstrom spatial resolution and sub-femtosecond temporal resolution, which provides a powerful tool to study the basic structures and physical processes in the microscopic world. The molecular orbital tomography (MOT) developed since 2004 enables one to image the wavefunction of the molecular orbital itself, which will help people gain deeper insight into the chemical reactions. In this paper, the theory of MOT will be introduced, and the progresses of MOT in the past ten years will be reviewed.
Signature of multi-channel interference in high-order harmonic generation from N2 driven by intense mid-infrared pulses
Authors: Li Gui-Hua  , Xie Hong-Qiang  , Yao Jin-Ping  , Chu Wei  , Cheng Ya  , Liu Xiao-Jun  , Chen Jing  , Xie Xin-Hua  ,   School of Science, East China Jiatong University, Nanchang 330013, China;,   School of Science, East China University of Technology, Nanchang 330013, China;,   State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China;,   State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China;,   Institute of Applied Physics and Computational Mathematics, Beijing 100088, China;,   Photonics Institute, Vienna University of Technology, A-1040 Vienna, Austria, EU
Published: December 2020

Recently, three major types of minima (i.e., Cooper-like minimum, two-center interference minimum and multi-channel interference minimum) have been observed in high-order harmonic generation (HHG) spectra. Identification of the origin of the minimum in a HHG spectrum is critical for self-probing of the molecular structures and dynamics, which has been an important subject in attosecond physics. In this paper, we report the investigation of the multi-electron dynamics in HHG from N2 molecules driven by intense mid-infrared laser pulses. Based on a pump-probe experimental setup, clear spectral minima in the cutoff region of high harmonic spectra from N2 molecules are observed in measurements with mid-infrared laser pulses at three wavelengths (i.e., 1300, 1400 and 1500 nm). A systematic investigation has been carried out for clarifying the origin of these minima. We carefully measured the spectral minima under three different experimental conditions:1) different alignment angles of molecules; 2) various peak laser intensities; 3) tunable driving laser wavelengths. Experimental results show that the positions of the spectral minima do not depend on the alignment angles of molecules. In addition, the measured spectral minima shift almost linearly with the laser intensity for all three wavelengths, and the positions of the spectral minima strongly depend on the wavelengths of the driven field. These findings are in conflict with the Cooper-like and two-center interference minima predictions, providing strong evidences on the dynamic multi-channel interference origin of these minima. Besides, we theoretically calculated the positions of multi-channel interference minima by using a classical three-step model and found out perfect agreements between the experimental results and theoretical calculations, which again strongly support the multi-channel interference picture. Moreover, the advantages of the observed dynamic multi-channel interference based on HHG driven by long wavelength lasers are discussed. The long wavelength driver lasers are attractive for not only generating coherent XUV radiation and attosecond pulses, but also investigating structures and dynamics of molecules in strong laser fields.
Directional bond breaking of CO molecules by counter-rotating circularly polarized two-color laser fields
Authors: Lin Kang  , Gong Xiao-Chun  , Song Qi-Ying  , Ji Qin-Ying  , Ma Jun-Yang  , Zhang Wen-Bin  , Lu Pei-Fen  , Zeng He-Ping  , Wu Jian  ,   State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200062, China
Published: December 2020

We experimentally studied the dissociative single and double ionization of CO molecules by counter-rotating circularly polarized two-color (CRTC) laser fields. By coincidently measuring the electrons and the fragmented ions, trefoil asymmetric momentum distributions of C+ in the polarization plane were observed, which are mainly determined by the selective ionization of CO with asymmetric orbitals. The threefold pattern could rotate continuously in the two-dimensional space by finely tuning the relative phase of the CRTC fields, providing a new method to manipulate the directional bond breaking of molecules by strong laser fields.
Blind chaotic signal extraction based on artificial bee colony algorithm
Authors: Li Guang-Ming  , Hu Zhi-Hui  ,   School of Computer Science, Dongguan University of Technology, Dongguan 523808, China;,   School of Electronic and Information Engineering, South China University of Technology, Guangzhou 510641, China
Published: December 2020

This paper is to deal with the blind extraction problem of chaotic signals by using a linear mixing model. In this model, a novel method to describe the distance function in a high dimensional space is proposed which relates the kernel function to objective function. When adopting the artificial bee colony algorithm (ABCA) as an alternative method to solve a multi-modal optimization problem, its analysis under a Markov chain model is also presented. The simulation results show that the objective function of this article has low complexity, and the artificial bee colony algorithm converges to a local minimum quickly. To be specific, the target function is constructed by combining the advantages of the proliferation exponent and the distance kernel function. The proliferation exponent can reflect the chaotic properties of a signal to a large extent, and the distance kernel can help to describe the statistical properties in a higher dimension. Due to the fact that only one frame of time-delay embedded signal is adopted, the computational complexity of our target function is low. The artificial bee colony algorithm is shown to be advantageous over other swarm algorithms. Although adopting ABCA for our evaluation function seems easy, we analyze why this algorithm can work, in contrast to the fact that most literature only runs some simulations to confirm its usefulness. Our analysis is only for a special case when the number of employed bees is set to be 2 and the process of onlooker bees and scouts are temporarily omitted. With smaller complexity than the methods based on proliferation exponents and kurtosises, simulations show that our method can have excellent performance when evaluated by correlation coefficients.
A super-resolution infrared microscopy based on a doughnut pump beam
Authors: Liu Hong-Ji  , Liu Shuang-Long  , Niu Han-Ben  , Chen Dan-Ni  , Liu Wei  ,   College of Opto-Electronic Engineering, Shenzhen University, Shenzhen 518060, China
Published: December 2020

An approach to breaking through the diffraction limitation in infrared microscopies is put forward in this paper. In this method, instead of Gaussian pump beam, an intensive vortex beam is first focused on the sample, leading to the saturation absorption of peripheral molecules in the point spread function (PSF). The vortex beam is followed by a Gaussian probe beam with the same wavelength. Because of the previous saturation absorption, the probe beam can only be absorbed by the molecules near the center, resulting in a shrunk PSF which means super-resolution. Furthermore, the PSF of a system based on this approach is numerically simulated. With a 100 nJ pulse energy vortex beam and a 0.1 nJ pulse energy probe beam, the theoretical resolution FWHM (full width at half maximum) is measured to be about 236 nm which is 14 times better than that of the traditional infrared microscopy.
A first principles study on the active adsorbates on the hydrogenated diamond surface
Authors: Liu Feng-Bin  , Chen Wen-Bin  , Cui Yan  , Qu Min  , Cao Lei-Gang  , Yang Yue  ,   College of Mechanical and Materials Engineering, North China University of Technology, Beijing 100144, China
Published: December 2020

Hydrogenated diamond film exhibits a high surface conductivity, which is very suitable for many in-plane microelectronic and microelectrochemical devices. However, the surface conductivity mechanism of hydrogenated diamond film remains unclear up to now. It inevitably retards its further applications. This work is to elucidate the effects of active adsorbate and water molecule on surface conductivity of hydrogenated diamond film. By the first principles method based on density functional theory, several models corresponding to hydrogenated and oxygenated diamond (100) surfaces physisorbed with various active adsorbates are built up. The adsorbed species include H3O+ ion mixed with H2O molecules with different concentrations. The adsorption energy, equilibrium geometry and density of states corresponding to the adsorption system are investigated. At the same time, the electron populations for different atoms of the physisorbed adsorbates are studied. The results show that the equilibrium geometry of H3O+ ion relaxes significantly after adsorption on hydrogenated diamond (100) surface. In addition, its adsorption energy increases dramatically compared with the system of individual H2O molecule adsorbed on hydrogenated diamond (100) surface. It follows that the strong interactions occur between H3O+ ion and hydrogenated diamond surface. With the concentration of the adsorbed H2O molecules increasing, the adsorption energy between the adsorbate and hydrogenated diamond (100) surface decreases gradually. It indicates that the interactions between H3O+ ion and the substrate weaken as the water molecule concentration increases. Concerning the electronic structure of H3O+ ion adsorbed on hydrogenated diamond (100) surface, shallow acceptors appear near Fermi level, which arises from charge transfer from hydrogenated diamond surface to adsorbed H3O+ ion. Therefore, hydrogenated diamond surface exhibits a p-type conductivity. With regard to the mixed adsorptions of H3O+ ion and H2O molecule, no significant effect on its conductivity is detected, though its surface energy band structure changes. At the same time, the electron transfers from hydrogenated diamond (100) surfaces to the adsorbates are also similar for all the systems with the adsorbates including one H3O+ ion and different H2O molecules. Thus, the adsorbed H2O molecule concentration in this work has no effect on the surface conductivity of hydrogenated diamond surface. However, the adsorbates containing H2O molecules and H3O+ ion physisorbed on oxygenated diamond (100) surfaces do not exist stably. The H3O+ ion will decompose into one H2O molecule and one H atom, which form HO bond with one O atom of oxygenated diamond surface. All the oxygenated diamond surfaces with various adsorbates exhibit an electric insulativity.
Frequency and bias dependent modeling of induced gate noise and cross-correlation noise in 40 nm metal-oxide-semiconductor field-effect transistors
Authors: Wang Jun  , Wang Lin  , Wang Dan-Dan  ,   College of Information Engineering, Southwest University of Science and Technology, Mianyang 621010, China
Published: December 2020

With the development of down-scaling of CMOS technology for low power, mixed-signal, and high frequency applications, the optimal high frequency performance is shown to be shifted from lower moderate inversion toward weak inversion regimes. High-frequency noise model is a prerequisite for designing the radio frequency and millimeter-wave circuits, and is essential for the noise analysis of nanoscale metal-oxide-semiconductor field-effect transistors (MOSFETs). In this paper, based on the physical structure of 40 nm MOSFET and by considering the drift-diffusion equation and charge conservation law, accurate physics-based unified high-frequency noise model is developed for induced gatecurrent noise and its cross-correlation with drain-current noise under different bias conditions, which is used to describe the frequency and bias dependence of 40 nm MOSFET from weak inversion to strong inversion regime. Especially, the effective gate overdrive is explicitly included in unified noise model to offer excellent accuracy, continuity and smoothness, and this makes the proposed analytical models convenient to directly reflect the relationship between the noise model and bias condition. Besides, new analytical model is derived for the induced-gate current noise and its cross-correlation term of weakly inverted MOSFET. These simple expressions not only serve as the asymptotic limit for the validation of the proposed physics-based unified model, but also provide a clearer insight into and better understanding of the gate noise behavior and their cross-correlation in the weak-inversion region. Moreover, in terms of the proposed subthreshold noise model, the charge of weak inversion rather than the normal effective channel thickness approximation is involved. In this way, the model accuracy can be improved. Furthermore, a detailed derivation and discussion are presented by analyzing the physics-based noise generation mechanism of transistor including the channel thermal noise and the shot noise based on the small-signal equivalent circuit of the 40 nm MOSFET device. Using these expressions it is possible to extract the values of all the noise model parameters directly from measurement. The proposed model is demonstrated by using noise data from both measurement and the noise simulation. Excellent agreement between simulated and measured noise data shows that the proposed model can be used for predicting the noise behavior of 40 nm MOSFET under different dimensions and operating conditions. The applicability of reported model for drain-current noise is also verified. As far as small-signal (i.e., linear) bias-dependent operation is concerned, it is shown how most of the findings of this work can also be used to predict the data of long channel devices in the strong-inversion regimes.
Dynamical analysis of the flexible beam-cam oblique-impact system
Authors: Yang Yong-Feng  , Feng Hai-Bo  , Chen Hu  , Wu Min-Juan  ,   Institute of Vibration Engineering, Northwestern Polytechnical University, Xi'an 710072, China;,   AECC Commercial Aircraft Engine CO., LTD., Shanghai 200241, China;,   No. 771 Institute of Aerospace Times Electronics Corporation, Xi'an 710065, China
Published: December 2020

A mechanical system is often modeled as a multi-body system with non-smoothness. Typical examples are the noises and vibrations produced in railway brakes, impact print hammers, or chattering of machine tools. These effects are due to the non-smooth characteristics such as clearance, impact, intermittent contact, dry friction, or a combination of these effects. In a non-smooth system, neither of the time evolutions of the displacements and the velocities are requested to be smooth. Beam-cam device is an important kind of impacting system which has a wide range of applications. The rotation of the cam at some constant speed provides a force to operate the beam. The most common example is the valve trains of internal combustion engines, where the rotation of the cam imparts the proper motion to the engine valves through the follower while a spring provides a restoring force necessary to maintain contact between the components. The impact on beam-cam is a typical oblique-impact. It has been observed that under variations of the cam rotational speed and other parameters, the follower can exhibit a complex behavior including bifurcations and chaos. We study a rigid flexible coupling system, which moves in the horizontal plane, and is composed of the hub, the flexible beam and a cam with constant rotating speed. Considering the second-order coupling of axial displacement which is caused by the transverse deformation of the beam, the kinetic energy and the potential energy of the whole system are calculated. The governing equations of the flexible beam-cam oblique-impact system are derived from Hamilton theory, when taking into account the second-order coupling quantity of axial displacement caused by the transverse displacement of the beam. Hertz contact theory and nonlinear damping theories are used to establish the contact model. By the equivalent conversion method in structural mechanics, the deflection curve of flexible beam is calculated. The acceleration at the contact point of beam and cam is used to judge whether they are separate, or contacted, or impacting. Due to the flexibility of beam, the impact point of beam-cam always changes with time and speed. We propose a method, which is a trial calculation method, to determine the impact point of flexible beam-cam. Simulation results show that there is transverse vibration at the free end of flexible beam. There is inter-coupling among the flexible of beam, the large range of motion, and the impact. After the impact, the rotation angle of the flexible beam changes with time and the angle amplitude mainly decreases with the increase of the time, but the regularity is poor.
Simultaneous measurement on strain and temperature via guided acoustic-wave Brillouin scattering in single mode fibers
Authors: Deng Chun-Yu  , Hou Shang-Lin  , Lei Jing-Li  , Wang Dao-Bin  , Li Xiao-Xiao  ,   School of Science, Lanzhou University of Technology, Lanzhou 730050, China
Published: December 2020

During the last decade, fiber sensor has drawn extensive attention due to its flexible, insulating, and readily operating in most measurement environment. But generally, fiber sensor is sensitive to more than one environmental parameter at the same time, so the cross sensitivity limits the application of the sensor. In the present work, a novel design scheme of sensing simultaneously temperature and strain via guided acoustic-wave Brillouin scattering is proposed for resolving the cross sensitivity induced by temperature and strain in single mode fibers. In the guided acoustic-wave Brillouin scattering which occurs due to the interaction between two optical co-propagating waves and the transverse acoustic wave in optical fiber, multi spectrum peaks appear when the frequencies of pump and Stokes are appropriate. Brillouin frequency shift is dependent on elastic property of fiber material such as sound velocity, density, Young's modulus, etc. and these elastic properties are influenced by the surroundings. So Brillouin spectrum changes with temperature and strain. Because different acoustic modes of guided acoustic-wave Brillouin scattering have different sensitivities to temperature and strain, characteristic frequencies of different acoustic modes shift at different levels. Then the influences of temperature and strain on elastic property of fiber material, and the relationship between material properties and characteristic frequency of each acoustic mode can be worked out, therefore the temperature and strain can be calculated by the different influences of temperature and strain on each acoustic mode. The simulation results indicate that the temperature sensitivity of R02 mode is 0.86% lower than that of TR25 in the SMF-28 fiber, but the strain sensitivity of R02mode is 54.1% higher than that of TR25. Temperature sensitivity of R02 is approximately equal to that of TR25, but strain sensitivity of R02 is obviously diferent from that of TR25. So the influences of temperature and strain on Brillouin frequency shift can be effectively distinguished, thereby simultaneous measurements of temperature and strain can be realized by guided acoustic-wave Brillouin scattering.
Effect of collision parameter on magnetized electronegative plasma sheath structure
Authors: Liu Hui-Ping  , Zou Xiu  , Zou Bin-Yan  , Qiu Ming-Hui  ,   School of Materials Science and Engineering, Dalian Jiaotong University, Dalian 116028, China;,   School of Science, Dalian Jiaotong University, Dalian 116028, China
Published: December 2020

The structure of an electronegative plasma sheath in an oblique magnetic field is investigated. Moreover, the collisions between positive ions and neutral particles are taken into account. It is assumed that the system consists of hot electrons, hot negative ions, and cold positive ions. Also the negative ions and the electrons are assumed to be described by the Boltzmann distributions of their own temperatures, and the accelerated positive ions are treated by the continuity and momentum balance equations through the sheath region. In addition, it is assumed that the collision cross section has a power law dependence on the positive velocity. After theoretical derivation, an exact expression of sheath criterion is obtained. The numerical simulation results include the density distributions of the positive ions for different invariable ion Mach numbers satisfying Bohm criterion, and the comparison of net space charge distribution between variable and invariable ion Mach numbers. Furthermore, three kinds of charged particle densities, the net space charges, and the spatial electric potentials in the sheath are studied numerically for different collision parameters under the condition of the fixed ion Mach number. The results show that the ion Mach number has not only the lower limit but also the upper limit. The ion Mach number affects the sheath structure by influencing the distribution of the positive ion density, and different conclusions can be obtained because ion Mach number is adopted as variable or invariable value when discussing the effects of the other variables which can result in a variety of the ion Mach numbers on the sheath formation. The reason is that the actual sheath structure modification brought on by the variation of a parameter can be divided into two parts. One is the sheath formation change caused directly by the variation of the parameter, and the other is the sheath formation change caused by the Bohm criterion modification which the variation of the parameter results in. Therefore, an identical ion Mach number should be adopted when studying the direct effects of a parameter variety on plasma sheath structure. In addition, it is concluded that the collisions between positive ions and neutral particles make positive ion density curve higher and electron density curve lower than the case without collisions. Negative ion density does not change significantly no matter whether there exists collision. Besides, there is a peak in the profile of the net space charge while in the presence of ion-neutral collision, and the net space charge peak moves toward the sheath edge. The spatial potential increases and the sheath thickness decreases on account of the presence of the collisions between ions and neutral particles.
Instability and turbulent mixing of shocked V shaped interface
Authors: Li Jun-Tao  , Sun Yu-Tao  , Pan Jian-Hua  , Ren Yu-Xin  ,   Graduate School, China Academy of Engineering Physics, Beijing 100088, China;,   Institute of Applied Physics and Computational Mathematics, Beijing 100094, China;,   School of Aerospace Engineering, Tsinghua University, Beijing 100084, China
Published: December 2020

Based on the mass fraction model of multicomponent mixture, the interactions between weak shock wave and V shaped air/SF6 interface with different vertex angles are numerical simulated. The numerical scheme used in the simulation is the high-resolution finite volume method with minimized dispersion and controllable dissipation scheme, in which the dissipation can be adjusted without affecting the already optimized dispersion property of the scheme. The grid sensitivity study is performed to guarantee that the resolution is sufficient in the numerical simulation. After the shock wave interacts with the interface, the baroclinic vorticity is deposited near the interface due to the misalignment of the density and pressure gradient, which is the manifestation of the Richtmyer-Meshkov instability, leading to the vortical structures forming along the interface. The interface perturbations lead to the bubbles and spikes appearing. The predicted leftmost interface displacement and interface width growth rate in the early stage of interface evolution agree well with the experimental results. The process of transition to turbulence at the material interface is studied in detail. The numerical results indicate that with the evolution of the interfacial vortical structure due to Kelvin-Helmholtz instability, the array of vortices begins to merge. As a result, the vortices accumulate in several distinct regions. It is in these regions that the multi-scale structures are generated because of the interaction between vortices. It is shown clearly that in the regions where vortices are accumulated, the fluctuation energy spectrum has many large and smallscale elements, which indicates there may be turbulent structures in these regions. To further examine if there is mixing transition in these regions, the characteristic length scales of the flow fields are calculated. The separation between the Lipemann-Taylor scale and inner viscous scale is observed based on the circulation-based Reynolds number, leading to the appearance of an uncoupled inertial range. The classical Kolmogorov -5/3 power law is also shown in the fluctuation energy spectrum, which means that the inertial range is developed. The appearing of this inertial range confirms that the mixing transition does occur, and the flow field near the material interface will develop into turbulence.
Perpendicular magnetic anisotropy study of CoFeB/Ni multilayers by anomalous Hall effect
Authors: Ju Hai-Lang  , Wang Hong-Xin  , Cheng Peng  , Li Bao-He  , Chen Xiao-Bai  , Liu Shuai  , Yu Guang-Hua  ,   School of Science, Beijing Technology and Business University, Beijing 102488, China;,   Department of Material Physics and Chemistry, University of Science and Technology Beijing, Beijing 100083, China
Published: December 2020

The CoFeB/Ni multilayers with Pt underlayer are prepared by magnetron sputtering technique and the perpendicular magnetic anisotropy (PMA) of each of the samples is studied by anomalous Hall effect (AHE) method. The PMA of CoFeB/Ni multilayer is dependent on the thickness of Pt, Co, CoFeB and the number of CoFeB/Ni bilayers strongly. It is found that the sample structured as Pt(4)/[CoFeB(tCoFeB)/Ni(0.3)]2/Pt(1.0) has a good PMA when the CoFeB thickness is 0.4 nm for the interface anisotropy dominated in the multilayer. So the CoFeB thickness is fixed at 0.4 nm. The effect of Ni thickness on multilayer PMA is also studied. The PMA of the sample is kept relatively well and the Hall resistance (RHall) decreases as the Ni thickness increases. Meanwhile the coercivity (HC) fluctuates in a small range. When the Ni thickness is 0.3 nm, the remanence squareness of the sample is very good and the Hall effect is strongest. The influence of period number n on the sample PMA is significant for it changes the interface of the sample. When n is 3, the sample has a very good remanence squareness, for the interface effect is obvious and the magnetization reversal process is consistent. The Pt underlayer shows a great effect on the PMA performance of the sample, for it can change the (111) texture of the multilayer. The results show that when the Pt thickness is 4 nm, the remanence squareness is good and the sample has a suitable HC. So the optimum CoFeB/Ni multilayer with an excellent performance of PMA is structured as Pt(4)/[CoFeB(0.4)/Ni(0.3)]3/Pt(1.0). Its anisotropy constant Keff is 2.2106 erg/cm3 (1 erg/cm3=10-1 J/m3) which indicates that the sample has an excellent PMA and its interface anisotropy is the main reason for making the Keff have a larger value. The magnetic layer thickness of the optimum sample is 2.1 nm and the total thickness of it is less than 8 nm. The integration with device can be studied further. Furthermore, HC of the CoFeB/Ni multilayer is relatively small and can be increased by inserting the oxidation layer or other ways.
Relationship between dielectric properties and nanoparticle dispersion of nano-SiO2/epoxy composite
Authors: Gao Ming-Ze  , Zhang Pei-Hong  ,   State Key Laboratory Cultivation Base of Dielectrics Engineering, College of Electrical and Electronic Engineering, Harbin University of Science and Technology, Harbin 150080, China
Published: December 2020

Polymer nanocomposites have advantage over traditional materials in electrical properties from the standpoint of dielectrics and electrical insulation. The influences of nanoparticle dispersion in the matrix, which is mainly caused by different preparation methods, on the dielectric properties of composites have been given in the past work. In order to investigate the relationship between the dispersion of nanoparticles in the matrix and the dielectric properties of composites, nano-SiO2/epoxy composites are prepared by different methods. Nano-SiO2 is first modified by silane coupling agent to obtain nano-SiO2 powder and nano-SiO2 dispersing liquid, then unmodified and modified nano-SiO2 powder are mixed into epoxy by mechanical mixing method, and the modified nano-SiO2 dispersing liquid is mixed into epoxy by bubble mixing method to prepare nano-SiO2/epoxy composites. The amounts of nano-SiO2 content in the composites are 2 wt%, 3 wt%, 4 wt%, 5 wt% and 6 wt%, respectively. Breakdown strength and corona-resistance characteristics of the composites are tested. The results show that with the increase of the nano-SiO2 content, the breakdown strength and corona-resistance of nano-SiO2/epoxy composites increase. The maximal breakdown strength appears in the composites with 5 wt% nano-SiO2. This appearance accords with percolation theory. The composites prepared by bubble mixing method have better breakdown strengths and corona-resistances than the composites prepared by mechanical mixing method. The scanning electron microscope images of the nano-SiO2/epoxy composites are analyzed by Image J software to obtain the information about the nanoparticle number in the special grid. Morisita's index is used to characterize the dispersion of nano-SiO2 in the matrix quantitatively. It is concluded that the composites prepared by bubble mixing method have better dispersion than those prepared by mechanical mixing method. Compared with the unmodified nano-SiO2, modified one has good dispersion in the composite because of the improved compatibility between the nanoparticles and the matrix. Based on the role that nano-SiO2 particles block discharge from developing in the composite, the better dispersion means that there are more nanoparticles and more barriers on the discharge path. Meanwhile, the better dispersion also means that more interface areas form between nano-SiO2 and matrix. The shallower traps supplied by the interface area will contribute less energy when current carriers jump into and out off the traps. So the better the dispersion of nano-SiO2 in the matrix, the superior the breakdown strength and corona-resistance of the composites are.
Energy-based hybrid routing strategy for scale-free networks
Authors: Yang Xian-Xia  , Pu Cun-Lai  , Xu Zhong-Qi  , Chen Rong-Bin  , Wu Jie-Xin  , Li Lun-Bo  ,   School of Computer Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
Published: December 2020

The infrastructures such as the internet networks, and phone networks, and their traffic capacity are well discussed in the field of network science. However, there is another type of communication infrastructure, such as the wireless sensor networks, which are usually deployed in tough environments to perform specific tasks. This kind of network usually has limited power supply, and thus the main issue is how to make good use of the energy and prolong the network lifetime. In this paper, we investigate the transport process in power-limited communication networks. We use the complex network models to generate the scale-free networks. We assign each node E0 (a constant) unit of energy and an infinite queue with the first-in-first-out rule for buffering packets. In the traffic model, every node generates packets with a constant rate . The packets' destination nodes are randomly chosen from the network. At each time step, every node delivers at most C packets. If a packet's destination node is among the neighbors of the current node, the packet will be delivered to the destination node directly and then be discarded from the destination node. Otherwise, the packet will be forwarded to a neighbor of the current node with a given routing strategy. In the delivery of a packet, the node consumes a fixed amount of energy, and will die out when it uses up its energy. We propose a hybrid routing strategy for the power-limited scale-free networks based on both the node energy and the shortest path. Specifically, in the routing strategy, we consider the residual energy of neighbor nodes and the shortest path lengths between the neighbor nodes and the destination, and utilize a free parameter to adjust their relative importance. Simulation results demonstrate that there are optimal control parameters which correspond to the maximum network lifetime and the maximum number of delivered packets. According to the proposed routing strategy, we further study the relation between the network topological structure and network lifetime. We find that the more homogeneous the network, the larger the maximum network lifetime is. Moreover, we obtain that the maximum network lifetime gradually increases with the average node degree increasing, but almost decreases linearly with the network scale increasing. In this paper we discuss the network lifetime from the perspective of network science, and give more insights into the transport process on complex networks. In addition, our work provides some clues of how to design the efficient routing strategies for the power-limited communication networks.
Surface morphology improvement of homoepitaxial GaN grown on free-standing GaN substrate by metalorganic chemical vapor deposition
Authors: Li Zhong-Hui  , Luo Wei-Ke  , Yang Qian-Kun  , Li Liang  , Zhou Jian-Jun  , Dong Xun  , Peng Da-Qing  , Zhang Dong-Guo  , Pan Lei  , Li Chuan-Hao  ,   Science and Technology on Monolithic Integrated Circuits and Modules Laboratory, Nanjing Electronic Devices Institute, Nanjing 210016, China
Published: December 2020

Free-standing GaN is generally regarded as an ideal substrate for GaN-based devices due to its advantage of low threading dislocation density (TDD) and good thermal conductivity. However, new surface features such as hillocks and ridges appear on the GaN homoepitaxy films. In this paper, the influences of the intermediate GaN (IM-GaN) layer on the surface defects and crystal quality of GaN homoepitaxy films grown on c-plane GaN substrates by metalorganic chemical vapor deposition are investigated. It is found that hexagonal hillocks and ridges on the surface can be avoided by inserting an IM-GaN layer grown at an intermediate temperature (650850℃), prior to the growth of GaN at 1050℃. The results based on X-ray diffraction (XRD) measurements and differential interference contrast microscopy images demonstrate that the growth temperature of the IM-GaN layer has a significant influence on GaN homoepitaxy layer, which is one of the most critical parameters determining the surface morphology and crystal quality. As the IM-GaN growth temperature decreases from 1050℃ to 650℃, thed densities of hillocks and ridges on the surface reduce gradually. While, the XRD full width at half maximum (FWHM) values of (002) and (102) peaks for the homoepitaxy films are increased rapidly, indicating the adding of the TDD in the films. The atomic force microscopy (AFM) images show that the quasi-step growth mode change into layer-layer growth mode with the growth temperature decreasing from 1050℃ to 650℃ during the IM-GaN layer growing. It is speculated that the growth mode is determined by the diffusion length of adatom on the growing surface, which is proportional to the growth temperature. In the case of IM-GaN grown at low temperature, the formation of hillocks can be suppressed by reducing the adatom diffusion length. Finally, High crystal quality GaN homoepitaxy films (2 m) without hillocks is achieved by optimizing the growth parameters of IM-GaN layer, which is about 150 nm in thickness and grown at 850℃. The crystal quality of GaN homoepitaxy film is assessed by XRD rocking curve measured with double-crystal optics. The FWHMs of the (002) and (102) peaks are 125arcsec and 85arcsec respectively, indicating that rather low TDD is formed in the film. And well defined steps are observed on the image of AFM test, the root-mean square roughness value of the which is only about 0.23 nm for 5 m5 m scan area.
Study on resonance frequency of doping silicon nano-beam by theoretical model and molecular dynamics simulation
Authors: Ma Xia  , Wang Jing  ,   College of Physical Science and Technology, Xinjiang University, Urumqi 830046, China
Published: December 2020

With the rapid development of nanoelectromechanical system technologies, silicon nanostructures have attracted considerable attention for the remarkable mechanical properties. A number of studies have been made on the mechanical properties through theoretical analysis, atomistic or molecular dynamics and experiments. In this paper, the resonance frequency of the doping silicon nano-beam is investigated by a theoretical model based the semi-continuum approach to achieve the goal of accurately capturing the atomistic physics and retaining the efficiency of continuum model. The temperature dependence of the resonance frequency of the nanostructure is important for application design, which is considered by the Keating anharmonic model used to describe the strain energy at finite temperature. The resonance frequencies are also simulated by the molecular dynamics at different temperatures. The studies indicate that the resonance frequency of the P doped silicon nano-beam is influenced by the size, the doping concentration and the temperature. The results show that the resonant frequency decreases with the increase of the length of the beam, and increases with the increase of the doping concentration of the silicon nano-beam. The resonant frequency of silicon nano-beam decreases with the increase of temperature, but the changes of the resonant frequency is not obvious. The doping concentration has a little effect on the resonance frequency of the silicon nano-beam. The conclusion can be drawn that neither the effect of doping concentration nor the effect of temperature on resonant frequency of the silicon nano-beam is obvious, the size is a major factor influencing the resonance frequency of the silicon nano-beam.