bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamics of slow electrons transmitting through straight glass capillary and tapered glass capillary
Authors: Wan Cheng-Liang  , Li Peng-Fei  , Qian Li-Bing  , Jin Bo  , Song Guang-Yin  , Gao Zhi-Min  , Zhou Li-Hua  , Zhang Qi  , Song Zhang-Yong  , Yang Zhi-Hu  , Shao Jian-Xiong  , Cui Ying  , Reinhold Schuch  , Zhang Hong-Qiang  , Chen Xi-Meng  ,   School of Nuclear Science and Technology, Lanzhou University, Lanzhou 730000, China;,   Nuclear and Radiation Safety Bureau of Gansu Province, Lanzhou 730000, China;,   Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China;,   Physics Department, Stockholm University, S-10691 Stockholm, Sweden
Published: December 2020

It has been found that the transmission rate of the electrons through insulating capillaries as a function of time/incident charge is not the same as that of the ions. The question arises that by using the electrons, if the negative charge patches can be formed to facilitate the transmission of the following electrons, thereby substantiating that the so-called guiding effect works also for electrons. This study aims to observe the time evolutions of the transmission of electrons through a straight glass tube and a tapered glass capillary. This will reveal the details of how and (or) if the negative charge patches can be formed when the electrons transport through them. In this work, a set of MCP/phosphor two-dimensional detection system based on Labview platform is developed to obtain the time evolution of the angular distribution of the transmitted electrons. The pulsed electron beams are obtained to test our detection system. The time evolution of the angular profile of 1.5 keV electrons transmitting through the glass tube/capillary is observed. The transmitted electrons are observed on the detector for a very short time and disappear for a time and then appear again for both the glass tube and tapered glass capillary, leading to an oscillation. The positive charge patches are formed in the insulating glass tube and tapered glass capillary since the secondary electron emission coefficient for the incident energy is larger than 1. It is due to the fact that fast discharge of the deposited charge leads to the increase of the transmission rate, while the fast blocking of the incident electrons due to the deposited positive charge leads to the decrease of the transmission rate. The geometrical configuration of the taper glass capillary tends to make the secondary electrons deposited at the exit part to form the negative patches that facilitate the transmission of electrons. This suggests that if the stable transmission needs to be reached for producing the electron micro-beam by using tapered glass capillaries, the steps must be taken to have the proper grounding and shielding of the glass capillaries and tubes. Our results show a difference in transmission through the insulating capillary between electrons and highly charged ions.
Theoretical model and spectrum characteristics of superimposed photonic crystal fiber grating
Authors: Jiang Peng  , Bi Wei-Hong  , Qi Yue-Feng  , Fu Xing-Hu  , Wu Yang  , Tian Peng-Fei  ,   Institute of Information Science and Engineering, Yanshan University, Qinhuangdao 066004, China;,   Key Laboratory of Especial Optical Fiber and Fiber Sensor(Hebei Province), Qinhuangdao 066004, China
Published: December 2020

The superimposed gratings have attracted considerable interest because they can extend the potential applications of gratings. Superimposed gratings are fabricated by inscribing multiple gratings at the same section of the fiber, and they can demonstrate various features simultaneously. A number of optical devices based on superimposed gratings have been reported, such as multi-wavelength filters, beam shapers, ultrahigh repetition rate optical pulse generators, etc. Photonic crystal fiber (PCF) can bring new optical characteristics by changing the sizes, spacings and arrangements of the air holes in the fiber. In this paper, we present the spectra of the superimposed gratings inscribed in a photonic crystal fiber. A numerical mode is proposed based on the V-I transmission matrices. The traditional cosinoidal variation of refractive index is replaced with a square-type refractive index variation, and the scattering occurs at a localized discrete location. According to the simulations, the reflection spectra and time delays of a superimposed Bragg grating and superimposed chirped Bragg grating are analyzed. A superimposed Bragg grating and a superimposed chirped Bragg grating are fabricated in the single mode photosensitive PCFs under the irradiation of a 193 nm ultraviolet laser. The superimposed Bragg grating is composed of four subgratings with resonance wavelengths at set spacings. And under a phase mask displacement of 1.03 mm, the superimposed chirped Bragg grating has a periodic resonance with a period of 0.82 nm. The results show that the spectrum of superimposed Bragg grating can be flexibly customized by the parameters of each subgrating. Superimposed chirped Bragg gratings have good linear group delays and flat periodic resonance amplitudes, and the resonance period can be adjusted by displacing the phase mask. The grating spectra obtained from experiments are in good agreement with the theoretical analyses. The research results in this paper provide an important basis for designing, fabricating, and applying the superimposed PCF gratings.
Characteristics of flow and thermal boundary layer in turbulent Rayleigh-Bénard convection
Authors: Huang Mao-Jing  , Bao Yun  ,   Department of Applied Mechanics and Engineering, Sun Yat-sen University, Guangzhou 510275, China
Published: December 2020

The parallel direct method of direct numerical simulation (PDM-DNS) for Rayleigh-Bénard (RB) convection is used in this paper. The differences and similarities in flow characteristic between two-dimensional (2D) and three-dimensional (3D) turbulent RB convection are studied using mean field for Ra=109, 1010, 5×1010, and Pr=4.3. Each of 2D and 3D cases has a large-scale circulation and corner rolls. The shape of large-scale circulation becomes round and the size of corner roll turns small as Ra increases. In 2D RB convection, there are four corner vortices at the corner of the square cavity and a stable large-scale circulation which is elliptical. For spanwise averaged 3D RB convection with two corner vortices, large-scale circulation reveals spindle shape. Due to the characteristic of the corner roll, the region plume dominating is wider in 2D RB convection than in the spanwise-averaged 3D case. Further, the Ra-dependence of thermal boundary layer properties is also studied. The thermal boundary layer thickness is scaled with Ra and the scaling exponents of λθ with Ra in the 2D and 3D cases are very similar.
Steady deformation characteristics of double emulsion droplet in shear flow
Authors: Zhang Cheng-Bin  , Yu Cheng  , Liu Xiang-Dong  , Jin Ou  , Chen Yong-Ping  ,   School of Energy and Environment, Southeast University, Nanjing 210096, China;,   School of Hydraulic Energy and Power Engineering, Yangzhou University, Yangzhou 225127, China
Published: December 2020

The manipulation of double emulsion droplet via shear flow field is widely encountered in microfluidic devices. However, the interface evolution and hydrodynamics behavior of double emulsion droplet in shear flow is less understood till now. In this paper, a theoretical model of double emulsion droplet in a Couette flow device is developed and numerically analyzed to characterize the interface behavior of incompressible double emulsion droplet, which is also verified by a visualization experiment. Based on this model, the mechanisms underlying the steady deformation of double emulsion droplet under shear are investigated, and the effects of radius ratio of inner droplet to the outer one and viscosities of working fluids on the steady deformation are discussed. The results indicate that the steady deformation of double emulsion droplet in the shear increases with the rise in capillary number, and the deformation resistance of inner droplet is larger than that of the outer droplet. With increasing the radius ratio of inner droplet to the outer one, the interaction between the inner and outer droplets becomes great and thus the deformation degree of the inner droplet is increased. In addition, the effect of big deformation resistance by the inner droplet tends to be obvious, leading to decreasing the deformation degree of outer droplet. The viscosities of both inner and outer droplets provide a resistance for the deformation of double emulsion droplet. With the rises in viscosities of inner and outer droplets, the deformation degree of integral double emulsion droplet decreases. In addition, the effect of outer droplet viscosity on the steady deformation is more obvious than that of the inner droplet.
Magnetohydrodynamic simulation of conical plasma compression
Authors: Yang Zheng-Quan  , Li Cheng  , Lei Yi-An  ,   School of Physics, Peking University, Beijing 100871, China
Published: December 2020

Low density gas conical implosion can produce high density plasmas with a volume compression ratio of 106 to 109, and a temperature over 10 eV. As the temperature is limited by the plasma-wall interaction, to further increase the density and temperature and confine the plasma energy, we use a theta pinch at the top of the imploding cone. A conical magnetohydrodynamic simulation method is used to calculate the properties of the plasma, in which three cases, i.e., pure conical fluid compression, pure theta pinch, and synergic compression of fluid compressing and theta pinch, are calculated in a two-dimensional conical geometry. Simulation shows that synergic compression can improve the energy confinement and efficiently raise the temperature of the plasma. Different parameter combinations are calculated to find the optimum performance.
Higher-order solitons in an anisotropic Heisenberg spin chain
Authors: Xie Yuan-Dong  ,   School of Physics and Telecommunication Engineering, South China Normal University, Guangzhou 510006, China;,   Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, School of Physics and Telecommunication Engineering, South China Normal University, Guangzhou 510006, China
Published: December 2020

An anisotropic Heisenberg ferromagnetic spin chain model is studied by using Holstain-Primakoff representation. In the semiclassical limit, the exact solutions for bright and dark solitons are found by using the coherent-state method combined with the Holstein-Primakoff bosonic representation of spin operators. These results show that the solutions can be expressed in terms of the elliptic integrals in different parameter regions. Some solutions for dark solitons are the innovation points in this paper.
CaAlSiN3:Eu2+ red phosphors synthesized by atmospheric nitrogen and their luminescence properties
Authors: Liu Wen-Quan  , Chao Ke-Fu  , Wu Wen-Jie  , Bao Fu-Quan  , Zhou Bing-Qing  ,   College of Physics and Electronic Information, Inner Mongolia Normal University, Hohhot 010022, China;,   Inner Mongolia Key Laboratory for Physics and Chemistry of Functional Materials, Hohhot 010022, China
Published: December 2020

In this paper, we propose one-step and two-step process under atmospheric pressure condition for synthesizing the CaAlSiN3:Eu2+ red phosphors by using nano-sized EuB6 and Eu2O3 as raw doping and activator materials. Moreover, the crystal structures, morphologies and luminescence properties of different-doped-Eu-concentration (2%-10%) samples are characterized in detail. According to energy dispersive spectrometer and X-ray diffraction (XRD) results, the cell volume and B content will gradually increase with the increase of the Eu concentration (2%-10%) for the sample prepared by two-step process. In contrast, the cell volume decreases with increasing the Eu concentration for the one-step prepared sample. Meanwhile, B content in the sample is less than that in the sample mentioned above and O content becomes larger. In addition, under the 460-nm blue light excitation, the two-step synthesized samples (nano EuB6 doped) has the highest emission peak in the 652-680 nm range, however, the sample by one-step synthesis (nano Eu2O3 doped) has strong emission peak only in the 630-637 nm range. Moreover, the intensity of fluorescence of the former one is stronger than that of the latter one. Both XRD and fluorescence spectra show that boron element can be introduced into the matrix by using two-step methods under atmospheric nitrogen. The introduction of boron not only reduces the oxygen content in the matrix but also changes the crystal field around Eu ions to adjust CaAlSiN3:Eu2+ phosphor luminescence peak position. Combining XRD and fluorescence spectral analysis, it is believed that boron element is introduced into the host by the two preparation methods of atmospheric nitrogen. The introduction of boron not only reduces the oxygen content in the matrix but also changes the crystal field environment of Eu2+ ions, and thus adjusting the luminescence peak position of Ca0.94AlSiN3:Eu2+ phosphor. Blue LED excitation of combined green-emitting phosphor and Ca0.94AlSiN3:0.06Eu2+ phosphor doped with nano EuB6 can yield white LED device with a color rendering index of 91 at a corresponding color temperature of 3364 K. This work has adopted a simple method to avoid expensive and complex pressure sintering equipment, and also reduces gas sintering equipment. Therefore, it is has a good prospective in industrial application and reducing the production cost.
Theoretical study on the optical response features of silver nanoparticles and arrays
Authors: Jiang Zhi-Yu  , Wang Zi-Yi  , Wang Jin-Jin  , Zhang Rong-Jun  , Zheng Yu-Xiang  , Chen Liang-Yao  , Wang Song-You  ,   Shanghai Engineering Research Center of Ultra-Precision Optical Manufacturing, Department of Optical Science and Engineering, Fudan University, Shanghai 200433, China;,   Key Laboratory for Information Science of Electromagnetic Waves(MoE), Shanghai 200433, China
Published: December 2020

The optical properties of nanoparticles and their array are closely related to their surface plasmon resonance of the particle and periodic structure parameters. In this paper, optical response features of single Ag nanosphere and periodical two-dimensional structure arrays are theoretically studied. The Mie theories and the multipole resonance theory are employed in the simulation. For Ag spheres each with a radius of less than 40 nm, one extinction peak can be observed and attributed to electric dipole resonance. When the radius of Ag sphere is more than 40 nm, apart from the peak contributed by the electric dipole, there is a peak of extinction at short wavelength, caused by resonance of the electric quadrupole. Generally, the frequency of multipole resonance decreases with increasing particle radius. The simulated results are in accord with the experimental data. For an infinite two-dimensional Ag-nanosphere arrays, two resonance peaks come from the dipole resonance of single particle and the Wood-Rayleigh anomalous diffraction. The frequency of multipole resonance can be controlled by tuning the size and the periodicity distribution of arrays. This paper provides a significant method to design advanced nanostructures with particular optical properties.
A method of suppressing vibration for high precision broadband laser frequency scanning interferometry
Authors: Liu Guo-Dong  , Xu Xin-Ke  , Liu Bing-Guo  , Chen Feng-Dong  , Hu Tao  , Lu Cheng  , Gan Yu  ,   School of Electrical Engineering and Automation, Harbin Institute of Technology, Harbin 150001, China
Published: December 2020

In the paper we study the method of reducing environmental influence in broadband laser frequency scanning interferometer. Target displacement caused by vibration will result in Doppler shift in measurement beat frequency. The extent of frequency shift is usually much larger than the actual target displacement. So the direct calculating of the target distance will cause ranging precision to decrease. In this paper, we establish a model for the influence of environmental vibration on the measurement and analyze the influence of the vibration on ranging result. To suppress the vibration effect, the Kalman filter is combined with the overlapping Chirp Z transform to estimate the measured data. The general process is described as follows. Firstly, the tuning nonlinearity will lead to the frequency spectrum broadening, so this paper we use the frequency sampling method to correct the frequency modulation nonlinearity of the laser. The frequency sampling method has the advantages of high speed and high precision. Secondly, the measurement system has the dispersion mismatch effect due to the use of broadband frequency swept laser. To solve this problem, the influence of the dispersion on the measurement is reduced by using the method of dispersion chirp slope calibration. Thirdly, because of the long frequency sweep period of the external cavity swept frequency laser, the vibration process of the target cannot be recorded in real time by single sweep, so in this paper we propose segmenting the measurement signal of single sweep and conducting Chirp Z transform to calculate target distance at different times. Compared with FFT algorithm, Chirp Z transform can achieve arbitrary narrow band spectrum subdivision, with the advantages of high accuracy and fast frequency measurement. Lastly, the Chirp Z ranging result is further combined with the method of Kalman filter to estimate the state of the target distance information. The experimental results indicate that the measurement standard is reduced from 185.4 μm to 9 μm by the proposed method. Without changing the absolute distance measuring device of broadband laser frequency scanning interferometer, this method provides a solution for further improving the ranging accuracy in the vibration environment, and reduces the complexity and cost of the device.
P-bifurcations of a Duffing-Rayleigh vibroimpact system under stochastic parametric excitation
Authors: Xu Wei  , Yang Gui-Dong  , Yue Xiao-Le  ,   Department of Applied Mathematics, Northwestern Polytechnical University, Xi'an 710129, China
Published: December 2020

Vibroimpact dynamics has been widely studied by experts and scholars in the fields of physics, engineering and mathematics. Most of the researches focus on vibroimpact systems under deterministic excitations by using numerical methods. However, random excitation often exists in vibroimpact system, whose roles cannot be neglected, sometimes may be quite important. Stochastic bifurcation is one of the most critical parts of stochastic dynamics, but the relevant researches about vibroimpact system are rarely seen so far due to the fact that the analytical method has its inherent difficulty. This paper aims to investigate the P-bifurcations of a Duffing-Rayleigh vibroimpact system under stochastic parametric excitation based on an equivalent nonlinear system method and the catastrophe theory. Firstly, the original Duffing-Rayleigh vibroimpact system is transformed into a new system without velocity jump by using the nonsmooth transformation method and Dirac function. Then, the equivalent nonlinear system method is introduced to obtain the stationary probability density of the response. Finally, the explicit parameter conditions for stochastic P-bifurcations are derived based on the catastrophe theory. Besides, the effect of stochastic parametric excitation on the system response is also discussed.
Effect of plane target characteristics on ranging distribution for pulse laser detection
Authors: Xu Xiao-Bin  , Zhang He  , Zhang Xiang-Jin  , Chen Shan-Shan  , Zhang Wei  ,   ZNDY of Ministerial Key Laboratory, Nanjing University of Science and Technology, Nanjing 210094, China
Published: December 2020

Laser proximity fuze is a kind of active detecting system which has been extensively equipped in both conventional and guided missile ammunitions. It uses a pulsed laser for detecting and ranging a target, accurately providing information about target angle and distance. The system emits a short pulse of laser beams which come into contact with the target and receives the light scattered back. After transducing light into electric signal, target range could be obtained with proper signal processing technique. The width and amplitude of the echo could vary if targets are of different characteristics, which leads to distributed target ranging results. Therefore, a better understanding of the relationship between target characteristic and ranging distribution can increase the precision of the pulsed laser proximity detecting system.In this paper, the emitted laser is modeled as a Gaussian pulse. The corresponding impulse response equation and echo equation of the pulsed laser for the planer target are derived. Considering the echo broadening effect, relative broadening ratio is determined from the echo equation. Then the relationships between the echo broadening coefficient and the parameters of tilt angle of planar target, laser divergence angle, and laser pulse width are analyzed. The results show that relative broadening ratio increases with the increase of the tilt angle of planar target or laser divergence angle, and decreases as the laser pulse width decreases. Based on the echo equation of pulsed laser and constant threshold leading edge detection, the probability density function of the planar target ranging is derived analytically. The influences of changing parameters of tilt angle of planar target, power of emitted laser, laser divergence angle, and threshold-to-noise ratio（TNR) on statistical ranging distribution are simulated.Monte Carlo simulation is performed for the whole waveform ranging experiment. The pulsed laser ranging platform is built and 20 m ranging experiment is conducted. The result shows that the ranging probability density distribution from Monte Carlo simulation is close to that from the experiment. As the tilting angle of target increases, ranging mean and variance both increase. When the tilting angles are 0, 20, 40 or 60, the signal-to-noise ratio（SNR) is larger than the TNR, and the ranging distribution is Gaussian. When the tilting angle is 70, the SNR is smaller than the TNR, and the ranging distribution is distorted with a longer rising edge and a shorter falling edge from the original Gaussian profile. This study could provide theoretical basis for the research of the ranging distribution of pulsed laser detector with the consideration of target characteristic.
Experimental studies of two sets of four-wave mixing processes in a single-zero-dispersion microstructured fiber by the same pump
Authors: Li Jian-She  , Li Shu-Guang  , Zhao Yuan-Yuan  , Liu Qiang  , Fan Zhen-Kai  , Wang Guang-Yao  ,   State Key Laboratory of Metastable Materials Science and Technology, College of Science, Yanshan University, Qinhuangdao 066004, China
Published: December 2020

A highly nonlinear microstructured fiber with single-zero-dispersion wavelength is designed and drawn by reducing the core area in order to observe two groups of four-wave mixing processes by a single pump. The foundational material of the fiber is silica and its cladding is comprised of seven-layer air holes. The air holes are arranged in a hexagonal lattice and the lattice pitch is =2.5 m. The radius of each of the air holes is r=1.03 m. There is just one zero-dispersion wavelength in our considerable wavelength range for the microstructured fiber and the corresponding wavelength D is nearly 0.85 m（D=0.85 m). The basic properties of the fiber including effective refractive index, dispersion coefficient, and nonlinear coefficient are calculated by the finite element method. The effective mode area is 4.4 m2 and the nonlinear coefficient is 0.057 m-1W-1 for the foundation mode near the wavelength of 0.8 m, and the nonlinear coefficient reaches 0.053 m-1W-1 near the zero dispersion wavelength of 0.85 m. In short, the optical fiber has a stable and high nonlinear coefficient in the whole experimental band（0.80-0.83 m), which provides an important guarantee for the occurrence of four-wave mixing double parameter gain process. In addition, the phase mismatch curve is simulated by using the four-wave mixing phase mismatch formulation. Numerical simulation shows that two sets of four-wave mixing processes can occur in the designed fiber. At the normal dispersion wavelengths of 0.800, 0.810 and 0.820 m with different powers, the experimental result shows a significant feature of four gain wavebands located at both sides of the pump wavelength. By comparing experimental data with the phase mismatch curve, we find that the band generation meets four-wave mixing phase matching condition, thus, the simultaneous occurrence of two groups of four-wave mixing processes observed in the experiment is explained in theory. The experimental results are consistent well with the theoretical predictions. This also proves the theoretical predictions that two sets of parametric gain processes and two pairs of signal and idle frequency waves can be generated in PCF. The four-wave mixing effect occurring in the normal dispersion region can be attributed to the contribution of negative fourth-order dispersion to the phase matching process. The present work can provide valuable reference to designing the microstructure fibers and developing the multi-wavelength conversion technology based on four-wave mixing effect. At the same time, this work can also supply guidance for developing the uncommon waveband lasers and broadband light sources.
Field measurements on flow past a circular cylinder in transcritical Reynolds number regime
Authors: Cheng Xiao-Xiang  , Zhao Lin  , Ge Yao-Jun  ,   State Key Laboratory for Disaster Reduction in Civil Engineering, Tongji University, Shanghai 200092, China;,   College of Civil Engineering, Nanjing Tech University, Nanjing 211816, China
Published: December 2020

Flow around a circular cylinder is a classic scenario which invariably draws the attention of the fluid mechanics circle, because its relevant studies are of both theoretical and practical significances. However, most experiments are conducted below transcritical Reynolds number（Re) regime（Re3.5106) due to the limitations of the wind tunnel modeling technique, which makes the obtained results inapplicable to some full-scale conditions. To this end, the field measurements for wind-induced pressures on a 167-meter high large cooling tower are conducted at Re=6.59107 to enrich the experimental results of flow past a circular cylinder in transcritical Re regime. Besides, the wind effects at low Re（Re=2.1105-4.19105) are also obtained by tests on a 1:200 rigid cooling tower model in a wind tunnel with considering 4 types of wind speeds, 8 types of surface roughness, and 2 flow fields. Employing the data obtained from both field measurements and wind tunnel model tests, the variations of static/dynamic flow characteristics with Re increasing are studied. It is found that 1) with the increase of Re, the drag coefficient for the smooth-walled tower in the uniform flow field decreases dramatically in the critical Re regime and increases slowly in the supercritical regime, which accord with Roshko's and Achenbach's results; 2) for smooth-walled tower, both the base pressure coefficient and pressure coefficient increase significantly with the increase of Re in critical and supercritical regimes, which qualitatively accord with Shih's results; and 3) the finding of the Strouhal number is supportive to Shih's result（i.e., shedding from the rough cylinder persists throughout the Re range tested). More importantly, special attention is paid to the Re-independence phenomenon of fluid flow, which is a typical phenomenon occurring in transcritical Re regime. Results indicate that the Re-independence exists in an Re range from 2105 to 1108 for a circular cylinder with a relative roughness greater than 0.01, and the increased free-stream turbulence can also induce Re-independence which probably exists in a narrow low Re range. Considering the flow mechanism, a reasonable explanation can be found for the Re-independence phenomenon, i.e., the critical and supercritical regimes narrow and move to lower Re range with the increase of surface roughness or the increase of free-stream turbulence, so Re independence can occur at a very low Re.
Research progress in magnetocaloric effect materials
Authors: Zheng Xin-Qi  , Shen Jun  , Hu Feng-Xia  , Sun Ji-Rong  , Shen Bao-Gen  ,   School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China;,   Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China;,   Institute of Physics, Chinese Academy of Sciences, University of Chinese Academy of Sciences, Beijing 100190, China
Published: December 2020

Magnetocaloric effect（MCE) is the intrinsic property of a magnetic material near transition temperature and the magnetic refrigeration based on MCE has been demonstrated as a promising alternative to the conventional gas compression or expansion refrigeration due to its high energy efficiency and environmental friendliness. The development of magnetic refrigeration technology depends on the research progress of magnetic refrigerant materials with large MCEs. Lots of researches of material exploration and material optimization have promoted the progress of magnetic refrigeration technology in recent decades. In this paper, we introduce the basic theory of MCE and the development of refrigeration technology, review the research progress of large MCE materials both in low temperature range and in room temperature range, and specifically focus on the latest progress of some MCE materials. Low temperature MCE materials mainly include those rare earth based materials with low transition temperatures, such as binary alloys（RGa, RNi, RZn, RSi, R3Co and R12Co7), ternary alloys（RTSi, RTAl, RT2Si2, RCo2B2 and RCo3B2), and quaternary alloys（RT2B2C), where R denotes the rare earth and T represents the transition metal. Those materials mainly possess the second-order phase transitions and show good thermal hysteresis, magnetic hysteresis, and thermal conductivities. Room temperature MCE materials are mainly Gd-Si-Ge intermetallic compounds, La-Fe-Si intermetallic compounds, MnAs-based compounds, Mn-based Heusler alloys, Mn-based antiperovskite compounds, Mn-Co-Ge intermetallic compounds, Fe-Rh compounds, and perovskite-type oxides. The above materials usually have the first-order phase transitions and most of these materials possess the large MCEs in room temperature range, therefore they have received much attention home and abroad. Among those room temperature MCE materials, the La-Fe-Si series is considered to be the most promising magnetic refrigerant materials universally and our country has independent intellectual property rights of them. The further development prospects of MCE materials are also discussed at the end of this paper.
Line focal X-ray source imaging
Authors: Liu Xin  , Yi Ming-Hao  , Guo Jin-Chuan  ,   Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China
Published: December 2020

High spatial coherent and bright X-ray beam is necessary condition for acquiring high quality radiography image. However, traditional X-ray tube can only provide high flux X-ray light or high spatial coherent light. In general, X-ray photons are generated by using energetic electrons with several tens or even hundreds keV to hit a target. Unfortunately, over 99% electron energy are converted into heat rather than the energy of X-ray photons. Thus, the heat dissipation of the target restricts the emission power and radiation flux. Increasing the emission area of X-ray can relieve the heat dissipation, but it would bring another serious problemlow spatial coherence that is in inverse proportion to emission area or focal spot. In order to solve the conflict between brightness and spatial coherence, an X-ray source with one-dimensional coherence is proposed in this work. The new X-ray source has a special focal spot where one side is small enough to ensure the spatial coherence and the perpendicular side is big enough to provide sufficient X-ray flux. In the direction of long side, the long size of focal spot will result in losing the image details. Consequently, an algorithm of superposition and rotation, in which many images with different rotation angles are added together, is proposed to retrieve the lost information. On the other hand, the spatial transfer function of superposition is analyzed in the frequency domain, and the result shows that the method of superposition can transfer more components of frequency than single image. Based on a traditional X-ray tube, a line focal spot source is designed and fabricated. Two series of experiments are performed for different destinations. After 17 images of a chip with different rotation angles and the line focal spot are collected, those images are rotated in the reverse direction and added together. The image of superposition clearly presents some details which are invisible in one of 17 images. At the same tube voltage and for the same object, some comparative experiments with micro-focus source, line focal spot source and normal focal spot source are presented. Compared with traditional X-ray tube, line focal spot source can provide high-resolution image. In the aspect of image visibility, the new source has definite advantages compared with micro-focus source, despite no improvement in imaging resolution. Finally, the reason for the difference in imaging resolution is discussed from the aspects of spatial coherence and light flux.
Thermal geometric discords in a two-qubit Heisenberg XY model
Authors: Fan Hong-Rui  , Yuan Ya-Li  , Hou Xi-Wen  ,   Department of Physics, Huazhong Normal University, Wuhan 430079, China
Published: December 2020

We study the influences of the external magnetic field, temperature, and coupling parameter on the thermal quantum correlation in a two-qubit Heisenberg XY model, where the correlation is described in terms of two modified geometric discords and concurrence. It is shown that those three quantities behave similarly in the regions of lower temperature and weak magnetic field. However, the two geometric discords are more robust against temperature and magnetic field than the concurrence. Moreover, two geometric discords exhibit different behaviors under suitable conditions, that is, one discord increases as the magnetic field rises whereas the other discord decreases. In particular, the discord reveals the phenomenon of sudden change, while neither of other discord and concurrence displays such a phenomenon.
Rydberg state excitations and double ionizations of different atoms in strong femtosecond laser field
Authors: Zhao Lei  , Zhang Qi  , Dong Jing-Wei  , Lü Hang  , Xu Hai-Feng  ,   Institute of Atomic and Molecular Physics, Jilin University, Changchun 130012, China
Published: December 2020

We experimentally investigate the Rydberg state excitations (RSEs) of noble gas atoms, He, Ar and Xe, in an 800-nm 50-fs strong laser field, by using the mass resolved pulsed electric field ionization method combined with the time-of-flight mass spectrometer. We measure the yields of the atomic RSE at different laser intensities and ellipticities, and compare the results with those of the nonsequential double ionization (NSDI) in strong laser fields. Our study shows that like that of NSDI, the yield of the atomic RSE increases as the atomic number increases, i.e., RSE yield trend is He Ar Xe. On the other hand, for any of the atoms, the probability of NSDI is lower than that of total RSE at the same laser intensity, which can be understood as that the yield of high energy electrons (for NSDI) is less than that of low energy electrons that can be captured into the Rydberg states. Additionally, our results show that the RSE yield strongly depends on the laser ellipticity, which is completely suppressed by a circularly polarized laser field. The dependence of RSE on laser ellipticity turns weaker as the atomic number increases, and is weaker than that of NSDI for any of the atoms. It is indicated that the atomic RSE in strong laser field can be attibuted to the capture of the low energy electrons after tunneling ionization into Rydberg states by the Coulomb potential at the end of the laser pulse.
Excitations of tearing mode and Kelvin-Helmholtz mode in rotating cylindrical plasmas
Authors: Bi Hai-Liang  , Wei Lai  , Fan Dong-Mei  , Zheng Shu  , Wang Zheng-Xiong  ,   Key Laboratory of Materials Modification by Laser, Ion, and Electron Beams(Ministry of Education), School of Physics and Optoelectronic Technology, Dalian University of Technology, Dalian 116024, China;,   China Aerospace Academy of Systems Science and Engineering, Beijing 100048, China
Published: December 2020

The influences of safety factor q profile and poloidal rotation profile on the q=1 tearing and Kelvin-Helmholtz (K-H) instabilities are investigated numerically by using a magnetohydrodynamic model in cylindrical geometry. With increasing the poloidal rotation, the m/n=1/1 mode is suppressed, while four domains exist for the high-order harmonic modes (such as m/n=2/2, m/n=3/3):the destabilized tearing mode domain, stabilized tearing mode domain, stable-window domain, and unstable K-H mode domain. Further, we find that the growth rate of the m/n=1/1 mode is related to the location of shear layer. Roles of shear flow in the m/n=1/1 mode for the shear layer located t on both the inner and outer sides of rational surface are almost the same, which is different from the scenarios of high-order harmonic modes. In addition, the smaller the magnetic shear on the rational surface, the smaller the growth rate of tearing mode is, and the more easily the K-H instability is excited.
Adsorption and dissociation of water on oxygen pre-covered Cu (110) observed with scanning tunneling microscopy
Authors: Pang Zong-Qiang  , Zhang Yue  , Rong Zhou  , Jiang Bing  , Liu Rui-Lan  , Tang Chao  ,   College of Automation, Nanjing University of Posts and Telecommunications, Nanjing 210023, China;,   Institute of Advanced Materials, Nanjing University of Posts and Telecommunications, Nanjing 210023, China
Published: December 2020

The adsorption and dissociation of water on the oxygen pre-covered Cu(110) surface are studied with scanning tunneling microscopy (STM). At room temperature, oxygen atoms are adsorbed on the Cu(110) surface and self-assembled into ordered (21) Cu-O chains along the[001] direction. The relative proportion of clean and (21) O-strips can be tuned by the sample exposure time to oxygen gas. When the oxygen pre-covered Cu(110) sample is exposed to water molecules at 77 K, the water molecules are adsorbed at the edges and on the top of the Cu-O chains. On the bare Cu(110) surface, we observe the formation of a hexagonal structure right next to the Cu-O stripes at 77 K. This is different from the water molecule adsorption on the clean Cu(110) surface, in which water molecules are adsorbed and self-assembled into ordered zig-zag chains along the[001] direction. While on oxygen pre-covered Cu(110) surface, water molecules prefer to hydrogen bond with oxygen atoms inside the Cu-O chains and then bond with the other water molecules, forming stable hexagonal network. From our earlier STM results, we find that water forms zig-zag chains only when oxygen pre-coverage is lower than 0.125 ML. On the top of hexagonal network, we observe the bright spots and attribute them to the 2nd layer water clusters. The fact that the 2nd layer clusters form on the top of the hexagonal water-hydroxyl regions rather than at the other locations on the Cu(110) surface indicates that the mixed hexagonal network may have more H-dangling bonds that facilitate the 2nd layer growth. In order to remove the upper layer water molecules, we apply a 5 V bias voltage for scanning, for which the tunneling electrons provide enough energy for overcoming the water desorption and dissociation barrier (0.5-0.55 eV at UHV and low temperature). With the excitation of tunneling electrons from the tip, the water molecules in the hexagonal network react with oxygen atoms inside the Cu-O chains (H2O+O2OH). According to Forster proposed Bjerrum defect model, the hexagonal network is formed by water donating hydrogen to hydroxyl, in which two hydrogen atoms are located between two adjacent oxygen atoms. Our results demonstrate that the oxygen atoms pre-adsorbed on the Cu(110) surface act as nucleation centers for water adsorption and catalyze its dissociation, which is important in water gas shift reaction study. However, we still need more X-ray photoelectron spectroscopy experiments to certify whether the water molecules react with the pre-covered oxygen atoms at low temperature (below 100 K).
A 1 k standardresistor device based on quantum Hall array
Authors: Zhong Qing  , Wang Xue-Shen  , Li Jin-Jin  , Lu Yun-Feng  , Li Zheng-Kun  , Wang Wen-Xin  , Sun Qing-Ling  ,   National Institute of Metrology, Beijing 100029, China;,   Institute of Physics, Chinese Academy of Science, Beijing 100190, China
Published: December 2020

Quantum metrological standards based on the fundamental physical constants are the trend of modern metrology because of their attributes such as high accuracy, high stability, and good reproducibility. The quantum Hall effect (QHE), which refers to the electronic charge e and the Planck constant h, is used to define the quantum resistance standard. The quantum Hall resistance (RH) of h/2e2 at the filling factori=2 is used as the standard. It is obvious that the RH is non-integral. However the resistors that need to be calibrated each have a decimal value, such as 1 k, 10 k, 100 k, etc. The calibration chain from the non-integral RH to the real resistor is long. The quantum Hall array resistance standards (QHARSs) are invented to solve this problem. The QHARS which are based on the decimal resistance values can shorten the calibration chain, improve the resistance transfer accuracy, and finally realize the quantization of the whole resistance calibration chain. The QHARS can also replace the traditional physical transfer standard resistor and realize the quantization of the transfer standard resistor. The decimal QHARS devices can be realized by connecting single QHE devices in series or parallel with the multiple link technology.
In this paper we report the design, fabrication and characterization of a 1 k QHARS device based on the GaAs/AlxGa1-xAs heterostructures. In our design, the 1 k array device consists of only 29 Hall bars. The nominal value of the device is 999.9999658 with a relative deviation of -3.4210-8 from 1 k. The ratio between the maximum and minimum current flowing through the Hall bars is as small as 14.5.
The 1 k QHARS devices are measured in the national resistance standard system at a temperature of 1.5 K. The measurement is taken at the central magnetic field of the 2nd quantum Hall plateau. We compare our 1 k QHARS resistor with a 1 k transfer standard resistor using the direct current comparator. The 1 k transfer standard resistor has already been calibrated in advance with our single QHR standard by cryogenic current comparator. Therefore the resistance of our 1 k QHARS resistor can be obtained. The relative deviation between the measured resistance value and the designed value is -1.9610-7with a standard uncertainty of 2.0710-7. The results show that we have realized the 1 k quantum Hall resistance standard device which can be used for the resistance calibration.
Tunneling times in double spin-filter junctions
Authors: Zeng Shao-Long  , Li Ling  , Xie Zheng-Wei  ,   College of Physics and Electronic Engineering, Sichuan Normal University, Chengdu 610068, China
Published: December 2020

Based on the free electronic model and Winful's theory about tunneling times, the dwell times and the phase times in NM/SF/I/SF/NM double spin filter junctions are investigated, where the NM denotes the normal metal, SF the insulator barrier with spin filter effects and I the nonmagnetic insulator barrier. There are three different cases which are analyzed in detail:1) the dependences of dwell time and phase time on the energy of the incident electron; 2) the dependences of dwell time and phase time on the heights of the barrier; 3) the dependences of dwell time and phase time on the width of the barrier and the molecular field in the spin filter layer. The numerical results show that for the first case, when the electrons have low incident energy (smaller than the barrier height), as the influence of the spin-dependent self-interfere term, the phase times are always larger than the dwell times for electrons with different spinorientations. But when the electrons have high incident energy (higher than the barrier heights), the influence of the self-interfere term disappears and the differences between the phase time and dwell time for electrons with different spin orientations disappear also. For case 2, the numerical results show that the variation of nonmagnetic insulator barrier height has little influence on the dwell time and phase time in NM/SF/I/SF/NM double spin filter junctions. But when the nonmagnetic insulator barrier height is lower than the barrier height of spin filter layer, the quantum well will appear and the resonant tunneling can be induced to lead to the peaks in the dependences of dwell and phase times on the insulator barrier height. The variation of spin-filter barrier height has obvious influence on the dwell time and phase time in NM/SF/I/SF/NM double spin filter junction. With increasing the height of spin-filter barrier, the dwell times and phase time both first increase and then decrease. For case 3, the influences of the widths of the nonmagnetic insulator barrier layer and spin filter layer on the dwell time and phase time are little. But when the barrier height of nonmagnetic insulator barrier is lower than that of spin-filter layer, the variation of width of insulator barrier can lead to the resonant tunneling and the peaks in dwell and phase times. Unlike the influence of width of barrier, the influences of molecular field in the spin filter layer on the dwell time and phase time are obvious. For the up-spin electrons, dwell time and phase time decrease with increasing the molecular fields, which is contrary to the scenario for the down-spin electrons.