bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurement and improvement of rubidium spin noise spectroscopy
Authors: Shang Ya-Xuan  , Ma Jian  , Shi Ping  , Qian Xuan  , Li Wei  , Ji Yang  ,   State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Acadamy of Sciences, Beijing 100083, China;,   College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, China;,   School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China;,   Department of Ocean Operations and Civil Engineering, Norwegian University of Science and Technology, Aalesund 6025, Norway
Published: December 2020

Spin noise spectroscopy (SNS) is a new kind of Faraday rotation technique, which does not need spin injection to generate polarized spin. This method uses a linearly polarized laser to detect the spontaneous spin fluctuation in a thermal equilibrium state. However, the signal of spontaneous spin fluctuation is so weak (~V) in the thermal equilibrium system that a big signal-noise ratio (SNR) is often demanded. Here, we report on the build-up and improvement of a spin noise spectrum measurement system. A home-made field-programmable gate array (FPGA) based data-acquisition card with real-time fast Fourier transform (DAC-FFT) is used to improve the SNR of the SNS measurement system. The reduction of intrinsic noise in the experimental system is discussed in detail. Both the dependence of background noise and the dependence of spin noise on the intensity of probe laser are analyzed. We find that the background noise is proportional to the intensity of the probe laser, while the spin noise signal shows square dependence on probe laser intensity. The spin noise indeed comes from the spontaneous spin fluctuation as experimentally confirmed via an acousto-optic modulator (AOM) inserted in the measurement system. The measurement performances of two FPGA based DAC-FFTs (the 8-bit FFTsDAC1 and the 12-bit FFTsDAC2, respectively) are compared. Several factors are found to affect the SNR of the system, including the measurement efficiency and the acquisition resolution. The FFTsDAC2 has longer single acquisition time and faster data transmission speed (with USB 3.0) than the FFTsDAC1, when the total measurement time is set to be the same, the effective measurement time realized in FFTsDAC2 is longer than in FFTsDAC1. With better measurement efficiency and sampling depth and longer single acquisition time, the FFTsDAC2 has a better SNR and finer frequency resolution with a much narrower full width at half maximum (FWHM) value. Moreover, the simulations of the measurement process show the effect of the single acquisition time on the FWHM of spin noise peak, further clarifying the reason why the spin noise spectrum measured by FFTsDAC2 is more accurate.
Influences of hydroxyl groups on friction behavior and energy dissipation of carbon nanotube
Authors: Wang Shi-Wei  , Zhu Peng-Zhe  , Li Rui  ,   School of Mechanical Engineering, University of Science & Technology Beijing, Beijing 100083, China;,   School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, China
Published: December 2020

In this paper, the influences of hydroxyl groups between interfaces on friction and energy dissipation are investigated by molecular dynamics simulations. The simulation systems include horizontal oriented carbon nanotube and Si substrate. The hydroxyl groups are grafted only on the substrates or between interfaces in different cases. The simulation procedure is as follows. First, the structure of the simulation system is optimized through energy minimization. Then the relaxation is conducted to ensure the the system reaches an equilibrium state. Finally, carbon nanotube moves at a constant speed along the x direction on the Si substrate. The results show that the average friction on carbon nanotube increases significantly due to the formation of hydrogen bonds between interfaces. The number of hydrogen bonds between interfaces increases with hydroxyl group ratio increasing, which is similar to the trend of friction. The chiral angle of carbon nanotube has a certain effect on friction. The friction on the armchair carbon nanotube is larger than on other types of carbon nanotubes. The diameter has an obvious influence on friction. The friction between the interfaces increases with the diameter of carbon nanotube increasing. The reason is that carbon nanotube with a large diameter becomes flattened at the bottom, which leads to the increase of contact area between interfaces. New peaks appear in the phonon state density of simulation system due to the introduction of hydroxyl groups. With the increase of hydroxyl groups ratio, the values of corresponding peaks of hydroxyl groups in the phonon state density become higher, which indicates that the vibration of hydroxyl groups plays a more important role in energy dissipation. When the hydroxyl group ratio on the carbon nanotube and Si substrate reach 10% and 20% respectively, most energy dissipates through the vibration of hydroxyl groups rather than the vibration of the carbon nanotube and Si substrate. The total energy of the system increases with hydroxyl group ratio increasing, and the potential energy of carbon nanotube also increases with the augment of hydroxyl group ratio on the carbon nanotube. However, when the hydroxyl group ratio on the carbon nanotube remains constant, the potential energy of carbon nanotube decreases with the increase of hydroxyl group ratio on Si substrate. This phenomenon becomes obvious when the hydroxyl group ratio is high. The reason can be attributed to the larger interaction between the carbon nanotube and Si substrate. In general, the energy dissipation of the system is related to the total energy, but the energy dissipating through the carbon nanotube may become less with the increase of total energy.
Study of pedestrian flow on stairs with a cellular transmission model
Authors: Jin Hui , Guo Ren-Yong ,   College of Computer Science, Inner Mongolia University, Hohhot 010021, China
Published: December 2020

The aim of this study is to address the following issues: 1) revealing the typical behaviors and properties of pedestrian movement when going upstairs and downstairs; 2) constructing a pedestrian evacuation model to formulate the walking process of pedestrians in stair area; 3) verifying that the cell transmission model widely used in the two-dimensional walking space can also be applied to the three-dimensional staircase area. Firstly, an observation experiment is carried out to gain the pedestrian movement data in the process of going upstairs and downstairs. By collating the data, the relation between density and flow in the unidirectional process of going upstairs or going downstairs, and in the bi-directional process of going upstairs and downstairs, are drawn respectively. Then, by analyzing the fundamental diagrams, several characteristics of pedestrian movement in stair area are revealed. Based on these characteristics, an extended cell transmission model is proposed. In this model, a potential correction coefficient is introduced to change the route choice of pedestrians by using the influence of different directional pedestrians on the potential; a flow modification coefficient is introduced to describe the effect of physical parameters on the maximum flow at the boundary between two neighboring cells; and an offset coefficient is introduced to correct movement rules and strengthen the influence of preferential direction on pedestrian route choice. Further, simulations relied on the proposed model are conducted. By comparing the simulation results with the experimental data, the model is calibrated. Then the calibrated model is employed to formulate the pedestrian movement in stair area, and the sensitivity of the potential correction parameter is also discussed. The simulation results indicate that the proposed model can successfully reproduce the movement of pedestrians on stair. Moreover, the route-choice behaviors of pedestrians can be directed by varying the values of the potential correction coefficient, which can present important information about optimizing the evacuation process of pedestrians on stair, thereby reducing the risk of an accident, such as congesting and treading.
Excitonic magnetic polarons and their luminescence in II-VI diluted magnetic semiconductor micro-nanostructures
Authors: Zou Shuang-Yang  , Muhammad Arshad  , Yang Gao-Ling  , Liu Rui-Bin  , Shi Li-Jie  , Zhang Yong-You  , Jia Bao-Hua  , Zhong Hai-Zheng  , Zou Bing-Suo  ,   1. School of Materials Science and Engineering, Beijing Key Laboratory of Nanophotonics and Ultrafine Optoelectronic Systems, Beijing Institute of Technology, Beijing 100081, China;,   2. Department of Physics, College of Science Majmaah University, Al-Zulfi 11932, Saudi Arabia;,   3. Department of Physics of Complex Systems, Faculty of Physics, Weizmann Institute of Science, Rehovot 7610001, Israel;,   4. Centre for Micro-Photonics, Faculty of Science, Engineering and Technology, Swinburne University of Technology, Victoria 3122, Australia
Published: December 2020

Spin is an intrinsic nature of the angular momentum of elementary particle like electron and photon. Currently the collective spin behaviors of the multi-electrons in condensed matter, such as GMR, CMR and topological insulator which are the behaviors of ground state, have been a research focus in the condensed matter physics, due to the fact that the collective spin is related to electronic transports. Exciton is another type of bosonic quasiparticle, an excited state of electronhole pair in solid, which has a short lifetime and can recombine to emit light. Whether excitons can also exhibit the spin-polarized dominance before they recombine, has not been understood yet. It is proposed that excitons form condensate by themselves or light binding. Can coupled spins conduce to the formation of the exciton condensate in solid? Excitonic magnetic polaron (EMP) is the composite exciton of ferromagnetically coupled spins and free excitons in magnetic semiconductors, which may lead to ferromagnetic Bose-Einstein condensate (BEC) due to the binding of collective spins in a microstructure, like the photon binding excitons (exciton polaritons) in an optical cavity However, this subject has not been a research focus yet. Here in this paper, we review the progress of the EMP formation, its dynamic behaviors and spin polarized collective EMP emission and lasing in Ⅱ-VI dilute magnetic semiconductor micro-structures in our group Besides, we also present some expectations for the applications or advances in the quantum phenomena such as spin-related emission and lasing, spin induced BEC, photon induced magnetism and Hall effect, etc. Even more achievements of EMP could be expected in the future.
Rayleigh-Bénard-Marangoni convection characteristics during mass transfer between liquid layers
Authors: Chen Jun , Shen Chao-Qun , Wang He , Zhang Cheng-Bin ,   School of Energy and Environment, Southeast University, Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, Nanjing 210096, China,   School of Hydraulic, Energy and Power Engineering, Yangzhou University, Yangzhou 225127, China
Published: December 2020

Rayleigh-Bénard-Marangoni convection (RBM convection) induced by the mass transfer has a great influence on the performance of real chemical engineering process. However, the researches of RBM convection characteristics during mass transfer across the interface in liquid-liquid system and their influence on the interface morphology are still limited. In this research, a visualization experiment via the amplified shadowgraph method is conducted to investigate the mass transfer in water-toluene-acetone system in a vertical slit. The convective structure of RBM and its evolution are visually observed. The effects of the initial acetone concentration of aqueous phase and toluene phase, and the thickness of toluene layer on the RBM characteristics and the morphology of the liquid-liquid interface are investigated. The experimental results show that these structures are induced by the interface tension difference along the interface and the vertical density difference caused by non-uniform mass transfer at the interface. As a result of the mass transfer at the interface, the density stratification occurs at the top of the aqueous phase, where the light liquid layer supports heavy one. In addition, non-uniform mass transfer produces perturbation at the top of the aqueous phase, which induces the Rayleigh-Taylor instability at the " interface” between the heavy and light liquid layer. Consequently, a wave-shaped-mound " interface” in the upper aqueous phase is formed as the heavy liquid comes down into the light one, and it can be further evolved into a plume flow with the enhancement of the imbalance between density and pressure at the " interface”. Due to the difference in mass transfer characteristic caused by different concentration gradients in the plume " interface”, the plumes can also evolve into weak plumes and strong plumes. Under the large acetone concentration gradient, a number of RBM convective structures are generated near the interface in a short time and the convective cloud is formed due to the dramatic interaction and coalescence between these structures. With the weakening of mass transfer, the convective cloud disappears and the strong plume is gradually formed. In addition, the strength of RBM convection is demonstrated to be positively correlated with the acetone concentration gradient across the aqueous solution- toluene interface. In addition, the roughness of the interface and its unsteady fluctuation grow up with the increase of acetone concentration gradient across the interface.
Mechanism of soil environmental regulation by aerated drip irrigation
Authors: Yang Hai-Jun  , Wu Feng  , Fang Hai-Ping  , Hu Jun  , Hou Zheng-Chi  ,   1. Division of Interfacial Water & CAS Key Laboratory of Interfacial Physics and Technology, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China;,   2. School of water conservancy, North China University of Water Resource and Electric Power, Zhengzhou 450046, China
Published: December 2020

Soil is the foundation of food security, water safety and wider ecosystem security. China's water resources is featured by its poverty and uneven distribution. Flood irrigation in traditional agriculture not only uses large amount of water, but also destroys soil aggregate structure, resulting in soil degradation, such as soil compaction and soil salinization. Underground drip irrigation have obvious water saving efficiency with the effective utilization rate of water larger than 95%, but it will also destroy the soil structure to a certain extent. It has been reported in many researches that using aerated water drip irrigation can not only increase crop yields, but also improve crop quality. The influence of several factors such as the burial depth of drop head, the frequency of dripping, the amount of irrigation, the growth period of plant, the mode of aerating and the equipment and so on, and the effects of the aerated drip irrigation on the water environment, the air environment, the microbial environment, the nutrient environment and the mineral environment of soil are summarized. And the regulation mechanism of soil environment by the aerated drip irrigation is put forward. The changes in water, gas, microorganism, nutrition and minerals are the result of the change of soil structure. The experimental results of in situ synchrotron radiation X-ray computed tomography confirmed that aerated drip irrigation can change the structure of soil.
Network heterogeneity based on K-order structure entropy
Authors: Huang Li-Ya  , Huo You-Liang  , Wang Qing  , Cheng Xie-Feng  ,   College of Electronic and Optical Engineering and College of Microelectronics, Nanjing University of Posts and Telecommunications, Nanjing 210023, China
Published: December 2020

Structure entropy can evaluate the heterogeneity of complex networks, but traditional structure entropy has deficiencies in comprehensively reflecting the global and local network features. In this paper, we define a new structure entropy based on the number of the K-order neighbor nodes which refer to those nodes which a node can reach within K steps. It can be supposed that the more K-order neighbors a node has, the more important role the node plays in the network structure. Combining the formula of Shannon entropy, the K-order structure entropy can be defined and figured out to explain the differences among the relative importance among nodes. Meanwhile, the new structure entropy can describe the network heterogeneity from the following three aspects. The first aspect is the change tendency of structure entropy with the value of K. The second aspect is the structure entropy under a maximum influence scale K. The last aspect is the minimum value of the K-order structure entropy. The simulation compares the heterogeneities of five classic networks from the above three aspects, and the result shows that the heterogeneity strengthens in the from-weak-to -strong sequence:regular network, random network, WS (Watts-Strogatz) small-world network, BA (Barabási-Albert) scale-free network and star network. This conclusion is consistent with the previous theoretical research result, but hard to obtain from the traditional structure entropy. It is remarkable that the K-order structure entropy can better evaluate the heterogeneity of WS small-world networks and suggests that the greater small-world coefficients a network has, the stronger heterogeneity the network has. Besides, the K-order structure entropy can fully reflect the heterogeneity variation of star networks with network size, and reasonably explain the heterogeneity of regular networks with additional isolated nodes. It suggests that when i additional isolated nodes are added to a regular network with n nodes, the new network has weaker heterogeneity than the old one, but has stronger heterogeneity than the regular network with n+i nodes. Finally, the validity of the K-order structure entropy is further confirmed by simulations for the western power grid of the United States. Based on the minimum value of the K-order structure entropy, the heterogeneity of the western power grid is the closest to that of WS small-world networks.
Molecular dynamics study of release mechanism of stress at Si/Ge interface on a nanoscale
Authors: Chen Xian , Zhang Jing , Tang Zhao-Huan ,   Science and Technology on Analog Integrated Circuit Laboratory, Chongqing 400060, China
Published: December 2020

In this paper, the stress distribution of Si/Ge interface and the mechanism of the point defect buffer layer in the stress release process of Si/Ge structure on a nanoscale are studied by the molecular dynamics method. The results show that in a one-dimensional interface model, the stress relaxation at the Si/Ge interface is closely related to the size of Ge of the simulation, and there is an approximately linear relationship between the decrease rate of interface stress and the size of Ge. The vacancy defect is introduced into the Ge film near the Si/Ge interface, and a point defect rich buffer layer forms, reducing the stress at the Si/Ge interface significantly. As the defect density in the buffer layer increases, the interface stress decreases step by step. Moreover, in the paper examined also is the mechanism by which the buffer layer affects the interface stress. The relationship between the stress variation caused by the point defect in the pure Ge structure and the defect density is compared and analyzed. The introduction of the defect buffer layer and the increase of the defect density can accelerate the release of the Si/Ge interface stress. The internal mechanism of the stress reduction would be that the defect introduces the compressive stress, which can offset the tensile stress generated by the lattice mismatch of the Si and Ge structure. Then the Si/Ge interface stress is reduced. Besides, as the defect density increases, the compressive stress introduced by the defect increases and the interface stress decreases. The examination shows that the introduction of the defect buffer layer into the growth of the pure Ge film on silicon can reduce the probability of the dislocation defect by structuring the buffer layer to reduce interface stress. And this method is indirectly confirmed by preliminary study of silicon-based Ge film growth with low-temperature buffer layer method.
Scanning broadband optical parametric chirped pulse amplification based on optical beam deflection
Authors: Ye Rong , Zhong Zhe-Qiang , Wu Xian-Yun ,   College of Physics and Engineering, Chengdu Normal University, Chengdu 611130, China,   College of Electronics and Information Engineering, Sichuan University, Chengdu 610064, China
Published: December 2020

One of the goals pursued in laser pulse is to achieve a laser with a shorter duration and higher intensity. In the past two decades, the laser pulse duration has been shortened by more than 7 orders of magnitude due to the development of Q-switched, Mode-locked and pulse compression technology. The peak power of laser pulse has been increased to PW, even EW and ZW from initial MW with the development of pulse amplification technology, whose focused intensity can reach to 1023 W/cm2. Thus, it provides unprecedented extreme conditions, and speeds up the laser applications in ultrafast nonlinear optics, strong field physics, fast ignition of laser nuclear fusion, optic communication, etc. The optical parametric chirped pulse amplification (OPCPA) is one of the important technologies in ultra-short laser pulse field. It is of great significance to increase the gain bandwidth for improving the conversion efficiency of OPCPA and achieving broadband optical parametric amplification. Combining the optical beam deflection and non-collinear OPCPA, a novel scanning broadband OPCPA model is proposed based on the optical beam deflection. The basic principle of increasing the gain bandwidth for the scanning broadband OPCPA is analyzed theoretically, which ensures the phase matching of each frequency component of signal by optical beam deflecting to change the non-collinear angle constantly. Namely, the non-collinear angles of incident frequency components of signal are different from each other, which, however, makes the whole phase matching of signal, i.e. momentum conservation in optics. The optical parametric amplification of signal pulse with 800 nm central wavelength and almost 100 nm bandwidth is simulated numerically by the proposed scanning broadband OPCPA. The results show that the bandwidth after being amplified is almost the same as before and there is no spectral narrowing, and the scanning broadband OPCPA increases the gain bandwidth and conversion efficiency greatly compared with the amplification with a constant given non-collinear angle, which leads to broadband optical parametric amplification. Finally, it is necessary to make sure that the on-load voltage to the KTN crystal matches with the frequency of signal pulse in time and reduces the unfavorable voltage deviation and time-delay for the maximizing gain bandwidth and conversion efficiency and ensuring the phase matching of each signal frequency component. The results of this paper not only provide an approach to increasing the gain bandwidth of OPCPA, but also supply some theoretical references and the basis for the experimental work of OPCPA in ultra-short laser pulse system.
An effective reduction on secondary electron emission yield of gold coated surfaces by laser etching
Authors: Wang Dan , Ye Ming , Feng Peng , He Yong-Ning , Cui Wan-Zhao ,   School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, China,   Science and Technology on Space Microwave Laboratory, China Academy of Space Technology (Xi’an), Xi’an 710100, China
Published: December 2020

Multipactor is a frequent discharging phenomenon for space high-power microwave components, and this detrimental effect is mainly induced by secondary electron emission (SEE) and electron resonance in vacuum. Plenty of researches have verified that suppressing SEE is an efficient approach to mitigate the multipactor. Therefore, low SEE yield surfaces are urgently needed for mitigating the multipactor in the field of space science. In the past few decades, a number of technics have been developed to acquire low SEE yield surfaces, including surface coating, surfaces roughening, depositing coessential nanostructure, etc. Laser etching has been partly reported to be an advisable way to construct micro- or nano-structure on some materials’ surfaces, and able to further suppress the SEE yield. Whereas, employing laser etching to obtain the SEE yield reduction on gold coated surfaces is rarely investigated. In this work, by using the laser etching technic, we fabricate four micro hole arrays and three orthogonal groove arrays with various porosities and aspect ratios, and we also characterize their three-dimensional and accurate two-dimensional morphologies. In addition, we investigate the dependence of SEE yield on surface morphology. Experimental results indicate that the laser etched microstructures can effectively suppress the SEE yield from gold coated surfaces, and the suppression levels on SEE yield of these samples are superior to those of many other low SEE yield technics. Furthermore, experiments reveal that the ability to suppress the SEE yield is positively related to the porosity and aspect ratio, as well as that the porosity influences SEE yield more strongly than the aspect ratio does. To theoretically verify the experimental phenomena, we utilize the Monte Carlo method combining with the SEE phenomenological model and the electron trajectory tracking algorithm, to simulate the SEE characteristics of the fabricated microstructures. And the simulation results can qualitatively explain the experimental phenomena. This work digs out an advisable method to sharply reduce the SEE yield from gold coated surfaces by laser etching, which is of considerable importance for exploiting the low SEE yield surface engineering in space microwave systems, and for improving the performance of the space microwave components with gold coated surface.
Enhanced stimulated Raman scattering by suppressing stimulated Brillouin scattering in liquid water
Authors: Shi Jiu-Lin , Xu Jin , Luo Ning-Ning , Wang Qing , Zhang Yu-Bao , Zhang Wei-Wei , He Xing-Dao ,   Jiangxi Engineering Laboratory for Optoelectronics Testing Technology, Nanchang Hangkong University, Nanchang 330063, China,   Key Laboratory of Nondestructive Test (Ministry of Education), Nanchang Hangkong University, Nanchang 330063, China
Published: December 2020

Stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) are two kinds of emblematic inelastic scattering processes resulting from the interaction of high-intensity laser with matter. Generally, competition between SBS and SRS is a common phenomenon in many substances. In liquid or high-pressure gas, if a single longitudinal mode laser is used as a pump source, both SBS and SRS can be excited, but the SBS will become very strong due to higher gain and optical phase conjugation. In comparison, the SRS gain is typically 2 orders of magnitude smaller than the SBS gain so that most of the pump laser energy is spent on the SBS and the SRS is greatly suppressed. To improve the output energy of SRS in liquid medium, a method of suppressing the SBS process by controlling temperature of medium is proposed. The SRS generation system using broadband pulse laser of 532 nm in wavelength as a pumping source is designed, the output energy of forward SRS (FSRS) and backward SBS (BSBS) in water with different temperatures are measured, and the physical mechanisms of the influences of water temperature, pumping linewidth and thermal defocusing on the output energy of SRS are analyzed. The experimental results indicate that by reducing the water temperature, the SBS process can be significantly suppressed, and the beam distortion caused by thermal defocusing effect can be reduced, thus effectively improving the output energy of SRS. Unlike the single longitudinal mode laser, when the pump source is handled in multiple longitudinal modes with a wide linewidth, the gain of FSRS is higher than that of the backward SRS (BSRS). Meanwhile, since the SBS gain coefficient is restricted by the linewidth of the pump laser, the FSRS process is dominant and both backward SBS and BSRS are significantly suppressed. It is necessary to state that none of the influence of backward SRS, self-focusing, optical breakdown and other non-linear effects on the output energy of SRS is considered in this paper, and only the effectiveness of reducing temperature to improve the energy output of forward SRS is verified from the perspective of temperature change. The results are of great significance for the multi-wavelength conversion of SRS in liquid medium.
Mathematical model of memristor with sensory memory
Authors: Shao Nan  , Zhang Sheng-Bing  , Shao Shu-Yuan  ,   1. School of Computer Science and Engineering, Northwestern Polytechnical University, Xi'an 710072, China;,   2. School of Electronics and Information, Northwestern Polytechnical University, Xi'an 710072, China
Published: December 2020

In Atkinson-Shiffrin model, the formation of human memory includes three stages:sensory memory (SM), short-term memory (STM), and long-term memory (LTM). A similar memory formation process has been observed and reported in the experimental studies of memristors fabricated by different materials. In these reported experiments, the increase and decrease of the memristance (resistance of a memristor) would normally be regarded as the loss and formation of the memory of the device. These memristors can be divided into two types based on the memory formation process. The memory formation of some memristors consists of only STM and LTM, and these memristors in this paper are called STM → LTM memristors; the memory formation of other memristors contains all three stages like human memory, and these memristors here are named SM → STM → LTM memristors. The existing mathematical model of this kind of memristor can only describe the STM → LTM memristor. Three state variables are included in this model:w describes the memory of the device, wmin describes the long-term memory, and τw0 is the time constant of the forgetting curve of the short-term memory. In this paper, a phenomenological memristor model is proposed for SM → STM → LTM memristors. The model is designed by redefining a+, a constant in the existing STM → LTM memristor model, as a state variable, and the design of corresponding state equation is based on the reported experimentally observed behaviors of SM → STM → LTM memristors during the SM period. Simulations of the proposed model show its ability to describe the behavior of SM → STM → LTM memristors. Stimulated by repeated positive pulses starting from the high-memristance state, the memristor stays in the SM state during the stimulation of first several pulses, and no obvious memory is formed during this period; STM and LTM would be gradually formed when the following pulses are applied. A faster memory formation speed can be achieved by applying pulses with longer duration, shorter interval, or higher amplitude. The formation and annihilation of the conductive channel between two electrodes of a memristor is a commonly used explanation for the change of the memristance. In this model, w can be understood as the normalized area index of the conductive channel, wmin is the normalized area index of the stable part of the conductive channel, τw0 describes the amount of time taken by the annihilation of the unstable part, and a+ determines the variation of the conductive channel when different positive voltages are applied.
Cellular automaton simulation of molten pool migration due to temperature gradient zone melting
Authors: Fang Hui , Xue Hua , Tang Qian-Yu , Zhang Qing-Yu , Pan Shi-Yan , Zhu Ming-Fang ,   Jiangsu Key Laboratory of Advanced Metallic Materials, School of Materials Science and Engineering, Southeast University, Nanjing 211189, China,   School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
Published: December 2020

Directional solidification is a common and important process in both scientific research and industrial practice. Owing to the presence of temperature gradients during directional solidification, local remelting and solidification in the mushy zone occurs, resulting in some typical phenomena such as temperature gradient zone melting (TGZM). The TGZM influences the solidifying microstructure and microsegregation significantly. In the present work, a two-dimensional (2D) cellular automaton (CA) model involving the mechanism of both solidification and melting is adopted to investigate the migration phenomena of molten liquid pools in the mushy zone due to the TGZM. The effect of pulling velocity, initial liquid pool position, temperature gradient, and alloy composition on the TGZM kinetics are studied. The simulation results are compared with the analytical predictions, and good agreement between two models is obtained. It is found that under a temperature gradient, the liquid pool always migrates towards the high temperature direction. When the pulling velocity is lower than the critical velocity, the liquid pool migrates through the liquidus into the bulk liquid and the time required for a liquid pool to reach the liquidus increases with pulling velocity increasing. On the other hand, when a pulling velocity higher than the critical value is adopted, the liquid pool moves towards the solidus and the time required for migrating liquid pool to reach the solidus decreases with pulling velocity increasing. For a given pulling velocity, the liquid pools located above the critical position move towards the liquidus, while the others gradually approach to the solidus. When a molten liquid pool migrates towards the liquidus, the migration velocity and liquid pool thickness are found to gradually increase, while the liquid pool composition decreases with time. Inversely, for the molten liquid pool that moves towards the solidus, the migration velocity and liquid pool thickness gradually decrease, while the liquid pool composition increases with time going by. The average migration velocity of liquid pool caused by the TGZM effect increases with temperature gradient increasing and alloy composition decreasing. The CA simulations provide an insight into the complicated interactions among the local temperature, solute distribution and diffusion, and the kinetics of local remelting and solidification in the TGZM process.
Recent progress of solidification of suspensions
Authors: You Jia-Xue  , Wang Jin-Cheng  , Wang Li-Lin  , Wang Zhi-Jun  , Li Jun-Jie  , Lin Xin  ,   State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an 710072, China
Published: December 2020

Suspensions include solvent and uniformly dispersed particles. Solidification of suspensions is to freeze the solvent while numerous particles disturb the pattern formation during the growth of the solid/liquid interface. It is a new interdisciplinary subject, involving the fields of freeze-casting porous materials, frost heaving, sea ice and biological tissue engineering and so on. Especially in recent years, many advanced materials with excellent properties were developed based on the processing of suspension solidification. Experimental phenomenon in suspension solidification is different from that in alloy solidification, such as the close-packed particle layer and self assembly, the ice lamellae structure and the periodic ice lenses and so on. Up to now, the formation mechanisms of these microstructures are still unclear. In this paper, we first review the historical development of suspension solidification in theory and in experiment. Then we demonstrate some recent progress of microstructural evolution and dynamical particle packing of suspension solidification. Finally, the outlooks of the future study on solidification of suspensions are also presented.
Laser cooling and trapping of polar molecules
Authors: Chen Tao , Yan Bo ,   Department of Physics, State Key Laboratory of Modern Optical Instrumentation, Zhejiang University, Hangzhou 310027, China
Published: December 2020

Different from atoms, molecules have unique properties, and play an important role in the research of atomic, molecular and optical physics. Cold molecules have important applications in science and have been studied for more than 20 years. But traditional methods, such as the Stark decelerator, have hit a bottleneck: it is hard to increase the phase space density of molecules. Extending the direct laser-cooling technique to new molecular species has recently been a hot topic and also a big challenge. In this review paper, on one hand, we make a brief review to recent progresses on the direct laser cooling of polar molecules. On the other hand, a demonstration on the feasibility of laser cooling BaF molecule has been experimentally illustrated, including the analysis on the molecular energy levels, measurements of the high-resolution spectroscopy, efficient pre-cooling and state preparation via buffer-gas cooling and detailed investigations on the molcule-light interactions. All these results not only pave the way for future laser-cooling and -trapping experiments, but also serve as a reference for the laser-cooling explorations on new molecular species.
Single-shot optical speckle imaging based on pseudothermal illumination
Authors: Xiao Xiao , Du Shu-Man , Zhao Fu , Wang Jing , Liu Jun , 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,   University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

Scattering in medium is a serious problem that limits the imaging depth or imaging distance. According to the absorption and scattering of light in biological tissues, it is difficult for both excited light and signal light to penetrate biological tissues, and the scattering effect in biological tissues will destroy the phase information of signal light, so it is difficult to directly carry out high resolution imaging in deep biological tissues. In the recent studies it is surprisingly found that two-dimensional image information of an object can be directly recovered from the disordered speckle pattern with pseudothermal light sources based on the optical memory effect (ME) and autocorrelation (AC) method. In this paper, we study a speckle imaging method based on pseudothermal illumination, where the Gerchberg-Saxton algorithm is used to perform the phase recovery of the object. Here, the advantages and disadvantages of HIO&ER algorithm and ping-pang (PP) algorithm based on the ME and AC method for imaging through random scattering medium are compared by using numerical simulation. By comparing the recovery effects and the numbers of iterations between HIO&ER algorithm and PP algorithm, it is found that PP algorithm has a fast running speed when a higher recovery quality is maintained. In addition, a continuous He-Ne laser and rotating ground glass are used to produce a pseudothermal light source. And a single frame imaging of different shape objects, which are a few millimeters away from random scattering medium, is carried out by objective lens. Then PP algorithm is adopted to recover the actual image of micron object. Furthermore, we experimentally find that the magnification, resolution and image intensity, which are qualitatively studied, are seriously affected by the distance between the focal plane of the object lens and scattering medium. We find that with the increase of the distance, the obtained autocorrelation graph and retrieval graph have corresponding amplification and the object sampling point information collected on sCOMS increases, which improves its resolution. However, the scattered light intensity collected by objective lens decreases after passing through the scattering medium, making the intensity of recovered image weaken. The results of this study will further promote the application of ME and AC method in the study of deep tissue medical imaging.
Kelvin-Helmholtz instability in anisotropic viscous magnetized fluid
Authors: Liu Ying , Chen Zhi-Hua , Zheng Chun ,   Key Laboratory of Transient Physics, Nanjing University of Science and Technology, Nanjing 210094, China
Published: December 2020

Kelvin-Helmholtz instability in anisotropic viscous fluid with uniform density in the presence of magnetic field is simulated through solving the non-ideal magnetohydrodynamic equations. The magnetic field is uniform and parallel to the stream. The magnetohydrodynamic equations are solved by corner transport upwind algorithm and constrained transport algorithm. In this paper, the influence of viscous anisotropy on Kelvin-Helmholtz instability is studied. The viscous anisotropy is embodied in the direction of the magnetic field, that is, viscosity parallel to the direction of the magnetic field line is much larger than that in the other directions. The results in the isotropic and the anisotropic viscous cases are compared from the aspects of flow structure, vortex evolution, and magnetic field distribution. It shows that the viscous anisotropy is more advantageous to the stability in a magnetized shear layer than the viscous isotropy. The flow structure evolves similarly in large scales but vortices evolve differently in small scales. Due to the decrease of the shear rate in the direction of the magnetic field lines, the rolling-up degree of interface and the number of laps decrease; the multiplication and merging of small vortices in the rolled-up structure destroy the regular growth of the vortex, which contributes to the stability of the flow. The increase of the magnetic energy at the sheared interface slows down effectively by the viscous anisotropy, which weakens the growth of the transverse magnetic pressure and anti-bending magnetic tension. However, viscous anisotropy shows much greater influence on the transverse magnetic pressure than on the anti-bending magnetic tension. The total enstrophy decreases slowly in viscous isotropy and anisotropy case. It increases quickly in late time in the former case, but is heavily suppressed in the latter case.
Influence of surface duct on the vertical spatial characteristics of wind-generated noise in deep ocean
Authors: Jiang Guang-Yu , Sun Chao , Xie Lei , Liu Xiong-Hou ,   School of Marine Science and Technology, Northwestern Polytechnical University, Xi'an 710072, China,   Key Laboratory of Ocean Acoustics and Sensing, Ministry of Industry and Information Technology, Xi'an 710072, China,   State Key Laboratory of Acoustics, Institute of Acoustics, Chinese Academy of Sciences, Beijing 100190, China
Published: December 2020

Vertical spatial characteristics of the wind-generated noise include the noise vertical directionality and the noise vertical coherence, which seriously affect the performance of sonar devices and play an important role in ocean environment parameters inversion. In this paper, we investigate the influence of surface duct on the noise vertical spatial characteristics at the depths below the duct. The Kuperman-Ingenito (K/I) model is employed to describe the distribution of the noise sources, and Pekeris-branch-cut-based normal modes are used to represent the Green's functions between the noise sources and the receivers. Both the noise vertical directionality and the noise vertical coherence are expressed as a function of the normal modes, so that we can investigate the physical reason for the variance of the noise vertical spatial characteristics by analyzing the variance of the normal modes. The numerical simulations on the noise vertical spatial characteristics show that the influence of surface duct above the critical depth is different from that below the critical depth. Above the critical depth, there exists a peak in the noise vertical directionality at the edge of the horizontal notch close to the bottom side. In the presence of surface duct, this peak significantly rises up, and the noise vertical coherence deviates from that in the absence of surface duct and tends to be perfect positive coherence and perfect negative coherence periodically as the vertical distance between the two receivers increases. On the other hand, below the critical depth, the noise power comes from the horizontal direction becomes stronger and the noise vertical coherence tends to be perfect positive coherence in the presence of surface duct as compared with the case in the absence of surface duct. Moreover, the influence will become severer as the thickness of the surface duct increases, while almost keep unchanged when the sound speed gradient in the surface duct varies. The modal analysis indicates that the noise source excites more refracted normal modes with stronger modal intensity in the presence of surface duct, and the excited refracted normal modes become more and stronger if the surface duct is thicker. As the result, the increase of the refracted mode number and the enhancement of their modal intensity cause the vertical spatial characteristics of noise to change.
Performance optimization of three down-sampling imaging strategies and their comparison with the conventional Fourier telescope
Authors: Dong Lei , Lu Zhen-Wu , Liu Xin-Yue , Li Zheng-Wei ,   Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, China,   University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

In order to obtain a new imaging strategy of the Fourier telescope (FT) with a better imaging quality and a less imaging time, we optimize and compare three down-sampling imaging strategies in this paper: the compressed sensing method (CS), the low-frequency full sampling method (LF) and the variable-density random sampling method (VD), which are different from the traditional Fourier telescope in both of the image quality and the imaging time. The analytical methods are as follows: based on the target’s spectral data obtained from the field experiment of traditional FT, three down-sampling methods (LF, VD and CS) are used to reconstruct the target’s images according to their own sampling modes and reconstruction methods, respectively; the differences between the three down-sampling methods and the traditional FT regarding the image quality are compared by the instinctive observation and the Strehl ratio; based on the analysis of the imaging time, the differences between the three down-sampling methods and the traditional FT regarding the imaging time are preliminarily compared. The analysis shows that: 1) the image quality of the compressed sensing method is better than that of the other two down-sampling methods (LF and VD), slightly lower than that of the traditional imaging; 2) although the image quality of the compressed sensing method is slightly lower than that of the traditional FT, its imaging time is much lower than that of the traditional FT; 3) the field data used in the analysis contain noises, which means that the reconstruction methods of the above three down-sampling strategies have a better robustness to the noises. Based on the above results, it can be seen that the Fourier telescope based on compressed sensing (CS-FT) is an excellent imaging strategy which can greatly reduce the imaging time in the condition with actual noises.