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Calibration source for OH radical based on synchronous photolysis
Abstract +
OH radical is the most important oxidant in the atmosphere, and control the tropospheric concentration of tropospheric trace gases such as CO, SO2, NO2, CH4 and other volatile organic compounds (VOCs). Accurate measurement of the concentration of OH radical in troposphere is the key to clarify the formation mechanism of secondary pollution in China. In this paper, a portable calibration method of OH radicals based on simultaneous photolysis is introduced. The synthetic air with a certain water vapor concentration is radiated in laminar flow by 185 nm light of mercury lamp and the photolysis of water vapor and O2 produce OH, HO2 radicals and O3. To reduce the uncertainty of the calibration method, the accurate measurement of ozone concentration, profile distribution factor P and oxygen absorption cross-section is carried out, and the portable calibration device is established by establishing the corresponding relationship between ozone concentration and light intensity. By changing the concentration of water vapor in the flow tube, the OH radicals of 3 × 10^8 cm^-3 ~ 2.8 × 10^9 cm^-3 concentration were produced, which were used to calibrate the atmospheric OH radicals measurement instrument based on Laser-Induced Fluorescence technique (LIF). The fluorescence signal had a good correlation with the concentration of OH. The calibration device of OH radical was applied to calibrate LIF system during “A comprehensive STudy of the Ozone foRmation Mechanism in Shenzhen” (STORM) field observation in autumn 2018. The calibration results under the field conditions show that the calibration uncertainty of the calibration device for LIF instrument is 13.0%, which has good stability and accuracy.
The influence of the secondary electron emission characteristic of dielectric materials on the microwave breakdown
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The physical mechanisms of the microwave breakdown in low pressure air and multipactor breakdown in vacuum for parallel plate system filled with a dielectric layer were researched in this paper. In order to study the effect of dielectric materials properties on the breakdown threshold, the secondary electron yield and the secondary electron energy spectrum of seven kinds of dielectric materials were measured. According to the positive charged stability condition in the process of secondary electron emission, the relationships among the steady state surface potential of dielectric materials and the secondary electron yield and the spectrum parameter were calculated. After the equivalent DC electric field corresponding to surface potential of the dielectric layer was introduced in the parallel plate system, the effects of steady state surface potential on the microwave breakdown threshold were studied based on the electron diffusion model and the electronic resonant conditions in low pressure discharge and multipactor respectively. The results show that, the steady state surface potential and the microwave breakdown threshold were increased with the increase of secondary electron emission yield or secondary electron energy spectrum parameters. The conclusions may be used in the choice of filling dielectric materials.
Anisotropic Dissipation in a Dipolar Bose-Einstein Condensate
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The ability to support frictionless motion is one of the manifestations of superfluidity. An impurity immersed in a superfluid can move without dissipation below the critical velocity, which, according to the Landau criterion, is related to the elementary excitation spectrum of the system. In quantum gases of the ultracold atoms, the critical velocity can be measured by stirring a laser beam through the atomic cloud, and the emergence of dissipation can be observed via the heating effect above the threshold stirring speed. Recently, such technique is exploited to study the superfluidity of the Bose-Einstein condensate (BEC) of 162Dy atoms with dipole-dipole interactions. It is shown that both the critical velocity and the heating rate reflect the anisotropy of the underlying dipolar excitation spectrum. In this work, we theoretically investigate the anisotropic dissipation of a point-like impurity moving through a dipolar BEC. For the motion along the principal axes, the dissipation rate above the critical velocity is analytically derived according to the linear response theory. At a given reduced velocity, we find the dissipation rate is of a higher value in the direction parallel to the dipole moment, which qualitatively explains the recent experimental observation in dysprosium atoms. Moreover, in the moving direction away from the principal axes, asymptotic s for the dissipation rate are obtained in the high-speed limit, as well as the regime close to the dissipation threshold. By combining these analytical results with numerical simulations, we conclude that, in a dipolar BEC, the angular dependence of the dissipation rate always shows the same anisotropy as the critical velocity. Our predictions can be examined in the current experiments with cold atoms, and the results presented here may be also helpful to understand the anisotropic superfluidity in other systems.
Analysis of Coherent Combination Characteristics of Beam Array via Tight Focusing
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In order to obtain focal spot with high power and spot size comparable with wavelength scale, a novel approach to achieve the coherent combination of beam array by tight focusing has been proposed. The physical model of coherent beam combination of beam array via tight focusing was built up by the use the vector diffraction integral. On the basis, the influences of beam configuration, polarization state, beam width, beam interval and numerical aperture of the tight focusing system have been discussed in detail. The results indicate that, the coherent combination of beam array with linear and circular polarization via tight focusing is the first rate, and with the radial polarization is the second rate, but with the azimuthal polarization is the worst. The beam array of linear and circular polarization with rectangle configuration can be tight focused into center point, while the beam array with hexagon is focused into two points. In addition, by enlarging the beam width and the beam interval to a certain extent, the combination efficiency can be increased. By optimizing the beam configuration, beam width and interval, and selecting rational numerical aperture of the tight focusing geometry, the focal spot with high energy concentration can be obtained with high beam quality and combination efficiency.
Identifying two different configurations of the H32+ by the direct above-threshold ionization spectrum in two-color laser field
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Since imaging the geometrical structure of molecules can help to understand the microscopic world intuitively, and thereby to promote the development of physics, chemistry, material science and biological science, it has long been an important subject for scientists to probe the molecular internal structure. Generally, however, because of the relative complexity of the molecular internal structure, it is difficult to obtain the relevant information by ordinary experimental means. With the development of
Measurement of Magnetically Insensitive State Coherent Time in Blue Dipole Trap
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Qubit coded in single neutral atoms is a crucial experimental platform to research quantum computation, quantum information processing and quantum simulation. Based on the single neutral atom captured in blued detuned dipole trap, we have studied the coherence of qubit, which was coded in magnetically insensitive ground state of cesium atom |6S_(1/2),F=3,m_F=-1> and |6S_(1/2),F=4,m_F=+1>, in the “magic” magnetic field condition. Adopting two photon resonant excitation, we have obtained Raman spectrum in different magnetic field. The “magic” magnetic field was determined by the minimum differential energy shift. Finally, applying spin-echo technique Ramsey spectrum, we prolong the qubit dephasing time to 1s. The realization of magic magnetic field restrains the effect of magnetic fluctuation on coherence time efficiently.
Internal dynamic detection of soliton molecules in a Ti: sapphire femtosecond laser
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Soliton is a basic nonlinear wave in nature, which can maintain its shape during propagating. Due to its special properties, solitons are widely used in plasma physics, high energy electromagnetics, hydrodynamics and nonlinear optics. The interaction between solitons can reflect the multi-body dynamic in complex nonlinear system, which has significant basic research value. Passive mode-locked laser is an ideal platform for studying soliton interaction. The attraction and repulsion between optical solitons can form soliton molecules. In the past, limited by the bandwidth and sampling speed, the optical spectrum analyzer cannot record the real-time dynamics of the solitons in the cavity. Time-stretched dispersive Fourier transformation (TS-DFT) is a powerful measurement technology, which can map the spectrum of an optical pulse to a temporal waveform under sufficient dispersion. TS-DFT makes it possible to detect the dynamics of solitons in real time. Based on TS-DFT, the internal dynamics of the solitons in Ti: sapphire femtosecond laser are studied experimentally. By changing the pump power, stable soliton molecules with a separation of 180 fs and weak phase oscillatory soliton molecules with a separation of 105 fs are observed. The vibration amplitude of the latter is only 0.05 rad. It is found that under the influence of environment, soliton molecules in stable state can transform into phase sliding state. These optical soliton molecules with spacing of 100 femtoseconds are of great significance for the study of short-range nonlinear interactions of solitons.
Current Phases in Hofstadter Ladder with Staggered Hopping
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Hofstadter ladder describes a Boson ladder under a uniform magnetic field and supports nontrivial energy band and fractional quantum Hall states. Staggered hopping is illuminated from the SSH model and proved to have non-trivial effects on current phases. We introduce staggered hopping on Hofstadter ladder to study the novel current phases. Exact diagonalization (ED) and density matrix renormalization group (DMRG) methods have been employed to study the current phases of the ladder in noninteraction and strong interaction (hard core boson) cases. By observing energy singularities and the new flux patterns when increasing the staggered hopping strength, we extend Meissner and vortex phase to horizontal current phase, vertical current phase and vortex phase. The horizontal current phase has stronger chiral currents in horizontal direction, which is the long direction of the ladder. The vertical current phase has stronger chiral currents in vertical direction. The above two phases do not break translational invariance while the vortex phase does. The current patterns of horizontal current phase are proved to be continuously deformed form the Meissner phase, and the vortex phase has similar signatures. The vertical current phase is only visible when the hopping is staggered. These phases generally exist in noninteraction regimes and interacting superfluid regimes. We have defined new quantities (i.e. current inhomogeneity and nearest overlap) to characterize different quantum phases. In noninteraction case, the horizontal current phase go through the vortex phase to enter the vertical current phase by second order phase transitions, but in strong interaction case such a change can be directly made in a first order phase transition. The direct transition is made in higher fillings with almost identical flux. Surprisingly, the three phases turn into only two phases in Mott regimes, and the phase transition between the horizontal current phase and the vertical current phase has disappeared. We call the new phase as Mott-homogenous phase. The staggered hopping has exotic effects in strong interaction case. For filling, the staggered hopping shrinks the region of vortex phases and produces Mott-SF transition. When the staggered hopping is weak, the system achieves Mott-SF transition just by varying the flux. This research can enrich current phases in lattice systems and illuminate further studies on chiral currents.
Thermodynamics of Laser Plasma Removal of Micro and Nano Particles
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Micro-impurity pollution has always been one of the key factors affecting the quality and service life of precision devices. Micro-nano impurity particles are difficult to remove by traditional cleaning methods (Ultrasonic cleaning, etc.) and low removal efficiency by laser cleaning methods (Dry Laser cleaning, etc.). The laser plasma shock wave has high pressure and high temperature characteristics, which can remove nano-scale impurity particles, and has great application potential. This article mainly studies the thermodynamic effect of the laser plasma in the process of removing micro-nano particles. In this work, the Al particles on the Si substrate were removed by laser plasma shock wave, and the transformation of the particle state was discussed through the changes of the experimental sample morphology after different pulse effects. The experimental results show that: with the increase of the pulse number, the micro-nano particle residues gradually decrease. At the same time, on the surface of the sample after removal, it can be found that large particles break up and change into small particles, and some particles will change into smooth spheres when they reach the melting point. These phenomena are the result of the interaction of the thermodynamic effect of the plasma. In order to study the transformation process of particle state, based on the study of plasma shock wave propagation theory, the evolution law of pressure and temperature characteristics
The physics-based model of AlGaN/GaN high electron mobility transistor outer fringing capacitances
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With the development of the application of AlGaN/GaN high electron mobility transistors in the radio frequency field, a capacitance model that can accurately describe the C-V characteristics of the device has become an important research topic. The gate capacitance of GaN HEMT can be divided into two parts: intrinsic capacitance and fringing capacitance related to two-dimensional electronic gas (2DEG) electrode. The fringing capacitance plays an important part in the switching device. The outer fringing capacitance Cofs/d dominates the fringing capacitance and is affected by the bias applied, especially the drain outer fringing capacitance Cofd. In order to establish the Cofd model which is related to the bias condition, the physics-based model of Cofd is established based on the conformal mapping, including the drain channel length variable. Since the drain channel length is related to the bias applied, the channel length modulation effect can be used to study how bias apllied effect the channel, and the relationship between Cofd and the bias condition is obtained. In addition, the threshold voltage variable is introduced when the channel length modulation effect is considered, and the threshold voltage drift caused by changes in the internal parameters and temperature of the device is studied using the threshold voltage variable in the model, and the relationship between Cofd and threshold voltage and temperature under different bias was obtained. It is found from the results of the study that as drain bias increases from zero, the channel length modulation effect keeps Cofd unchanged at lower drain bias. When the drain bias continues to increase, Cofd begins to decay again, and its decay rate slows down with the increase of gate bias. The decrease of donor impurity concentration and Al component in AlGaN barrier layer may increase the threshold voltage, which will strengthen the channel length modulation effect on Cofd, resulting in linear attenuation of Cofd. With the increasing of drain bias, the influence of threshold voltage shift on Cofd is enhanced, and the change of device operating temperature will enhance the threshold voltage shift and cause the deviation of Cofd. Moreover, with the continuous increase of drain bias, Cofd becomes more sensitive to the temperature variation.
Control of spiral waves in excitable media under polarized electric fields
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Spiral waves are ubiquitous in diverse physical, chemical, and biological systems. Periodic external fields, such as polarized electric fields, especially circularly polarized electric fields which possess rotation symmetry may have significant effects on spiral wave dynamics. In this paper, control of spiral waves in excitable media under polarized electric fields is reviewed, including resonant drift, synchronization, chiral symmetry breaking, stabilization of multiarmed spiral waves, spiral waves in subexcitable media, control of scroll wave turbulence, unpinning of spiral waves in cardiac tissues, control of spiral wave turbulence in cardiac tissues, and so on.
Preparing GaN nanowires on Al2O3 substrate without catalyst and its optical property research
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A green and low-cost method to prepare high-quality GaN (gallium nitride) nanowires is important for GaN-based devices Large-scale application. In our works, high quality GaN nanowires were successfully prepared by a green plasma enhanced chemical vapor deposition method without catalyst, using Al2O3 as substrate, metal Ga as gallium source and N2 as nitrogen source, respectively. The obtained GaN nanomaterials were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, and photoluminescence (PL) spectroscopy. XRD result demonstrated that hexagonal-wurtzite GaN was obtained and no other phased existed. SEM result showed that GaN nanowires and hexagonal GaN microsheets were obtained at different temperature. When the growth temperature was at 950 ℃ (reaction time for 2 h), hexagonal GaN microsheets with the size of 15 μm were obtained. When the growth temperature was at 1000 ℃(reaction time for 2 h), GaN nanowires with the length of 10-20 μm were obtained. With the reaction temperature increased from 0.5 h to 2 h, the length of GaN nanowires increased. TEM result suggested that GaN nanowires had high crystallinity and growth direction of GaN nanowires was [0001]. Raman result indicated that the compressive stress presented in GaN nanowires and the stress was 0.84 GPa. Meanwhile, the growth mechanism of GaN nanowires also was proposed. The morphology of GaN nanomaterials were tailed by the growth temperature, which may be caused by Ga atomic surface diffusion. Ga atoms have low diffusion energy and small diffusion length at 950 ℃. They gathered in the non-polar m plane. (0001) plane with the lowest energy began to grow. then, hexagonal GaN microsheets were obtained. When reaction temperature was at 1000 ℃, the diffusion length of Ga atoms increased. Ga atoms enable to diffuse to (0001) plane. In order to maintain the lowest surface energy, GaN nanowires grew along [0001]. PL result indicated that the obtained GaN nanowires just had an intrinsic and sharp luminescence peak at 360 nm, which had promisingly applications in photoelectric devices such as ultraviolet laser emitter. Our research will also provide a low-cost and green technical method for the fabrication of new photoelectric devices.
High-speed and large-scale light-sheet microscopy with electrically tunable lens
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Fluorescence microscopic imaging technology realizes specific imaging by labeling biological tissue with fluorescence molecules. Which has a high signal-to-noise ratio and has been widely used in the field of medical biology research. Some typical fluorescence microscopy techniques, such as confocal microscopy or two-photon microscopy, have high fluorescence intensity, long exposures can cause phototoxicity and photobleaching of biological tissues, which are difficult to meet the demands of long-time observation or noninvasive imaging. Then, light sheet fluorescence microscopy (LSFM) has become hot topics in fluorescence micro-imaging in recent years due to its fast speed, high resolution, low photobleaching and low phototoxicity. The imaging speed of a typical light sheet microscopy is not fast enough to observe fast biological activities such as transmission of neural signals, blood flow, heart beats. At present, many reported light-sheet fluorescence microscopies still have problems such as fixed imaging surface, slow imaging speed, small imaging depth or residual artifacts. Therefore, in this paper, a rapid light-sheet fluorescence microscopy based on electrically tunable lens is built. To achieve rapid movement of the focal plane of the detection objective lens, the electrically tunable lens is introduced to meet the fast changing of the diopter. Similarly, rapid movement of light sheet is achieved by introducing one-dimensional galvanometer to change the rotation angle. Fast imaging requires keeping the light sheet and focal plane overlapping in real time. Which is then combined with a high-speed sCMOS receiving fluorescence to complete the whole imaging. In the experiment, the vertical depth is significantly increased by modifying the optical path, and the LABVIEW programming is used to coordinate and improve the dynamic imaging quality, which effectively reduced the artifacts generated in rapid imaging. Finally, an imaging speed of 275 frames/s with a lateral resolution of ~0.73 μm, vertical resolution of ~5.5 μm, and an imaging depth of ~138 μm was achieved. This is of significance for the development of real-time and non-invasive imaging of living biological tissues.
Study on vibrational characteristics of surface mode for W (100) surface by modified analytic embedded atomic method
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In the framework of surface lattice dynamics theory, the surface phonon spectrums along three symmetry directions are simulated for the clean W(100) surface by using the modified analytic embedded atom method. According to the judgment bases and marking method of surface mode, the surface modes are drawn along different directions, and the distribution range and mode coupling of surface modes are discussed. Using the phonon frequencies as the input parameter, the surface vibration states is constructed for the atomic layers in the vicinity of the surface. And then the polarization ways and location characteristics of the surface modes in different symmetry directions are analyzed. Based on polarization vector, the coupling phenomena between surface modes, such as avoid crossing and independence crossing is displayed visually.
Effect of Swift Heavy Ions Irradiation on the Microstructure and Current-Carrying Capability in YBa2Cu3O7-δ High Temperature Superconductor Films
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YBa2Cu3O7-δ(YBCO) high temperature superconductor materials have many promising applications in energy, transportation and so on. Nonetheless, the application of YBCO in high magnetic field was limited because of low critical current. One-dimensional latent tracks produced by swift heavy ions irradiation can be effective pinning centers, thus enhancing superconductivity in external field. YBCO high temperature superconducting films were irradiated with 1.9 GeV Ta ions at room temperature and vacuum condition. Structure damages in irradiated samples were characterized by transmission electron microscopy (TEM). Continuous amorphous latent tracks, with diameter from 5 nm to 15 nm, throughout the whole superconducting layer can be observed from TEM images. Physical property measurement system (PPMS) was used to measure superconducting properties of samples before and after irradiation. When irradiated at optimal fluence of 8×1010 ions/cm2, critical current reaches its maximum value and pinning force was twice of unirradiated sample, while critical temperature almost unchanged. The analysis of experimental results shows that latent tracks produced by swift heavy ion irradiation can enhance in-field current-carrying capability, without decreasing critical temperature. In the power-law regime values of ɑ decreased with the increasing of fluence, indicating a weaker magnetic field dependence of critical current. ɑ reaches its lowest value 0.375 when irradiated at a fluence of 5.0×1011 ions/cm2, corresponding to a lowest in-field Jc. This result may be a combination of increasing pinning centers and decreasing superconductor volumes that work together. Normalized pinning force fp=Fp/Fp,max of sample irradiated with different fluence as a function of magnetic field h=H/Hmax was analyzed using Higuchi model. Fitting results show that planar defects are main source of pinning when h>1, independent of irradiation. Whereas, dominate pinning centers shifting from surface pinning to isotropic normal point pinning with increasing fluence when h<1. Given that latent tracks produced by Ta ions irradiation act as strong anisotropic pinning centers, the reason of the dominate pinning centers changes with increasing fluence remains to be further studied.
Measurement of time-varying electron density of air spark shock wave plasma jet by the method of microwave Rayleigh scattering
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It is difficult in measuring the electron density of an atmospheric air spark shock wave plasma jet, due to its variation on the time scale of sub-microseconds. In this paper, the time-varying electron density of air spark shock wave plasma jet is measured, based on the principle of microwave Rayleigh scattering. The system constant A is determined by using calibration of materials with known properties; the results show that the system constant is obtained as A= . According to the principle of microwave Rayleigh scattering, the electron density of the plasma jet is related to its radius and length of the plasma jet plume. Combined with the discharge image captured by ICCD camera, it is observed that the plasma jet plumes are with irregular patterns. In order to facilitate the calculation, the plasma jet plumes are replaced by cylinders with the same volume as the original shapes. Thus, the equivalent radius and length of the plasma jet plume are obtained. According to the known data, the electron density is determined to be in the order of 1020m-3; its value increases rapidly to the peak value, and after then exponential attenuates along with time. In addition, the effect of different equivalent dimensions of the plasma jet plume on the measurement results is also discussed. It is shown that the calculation result with the time-varying equivalent radius and the time-varying equivalent length is the most effective one. In addition, the first fast peak is caused by the ionization wave of the photo ionization. The actual ionization process is that the air discharge in the cathode cavity releases a large number of high energy photons, which pass through the cathode nozzle and project into the region outside the nozzle; and then the O2 molecule in the ambient air are ionized by those high energy photons to form the plasma jet plume at the time of 1μs.
Efficient Angle and Polarization Parameter Estimaiton for electromagnetic vector sensors Multiple-Input Multiple-Output radar by using Sparse Array
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In this paper, a new sparse transmitting and receiving array is designed to improve the joint angle and polarization parameter estimation performance for bistatic EMVS-MIMO radar. Firstly, large array aperture can be obtained with the aid of the sparse transmitting and receiving array. Then, an effective third-order tensor model is constructed in order to make full use of the multidimensional space-time characteristics of output data after matching filtering. And, the PARAFAC-TALS algorithm is proposed to deal with the constructed third-order tensor model, which can yield closed-form automatically paired 2D-DOD and 2D-DOA estimation without additional angle pair matching process. Furthermore, two sets of high accuracy rotation invariance relationships corresponding to transmit elevation angle and receive elevation angle can be achieved by using the estimated transmit steering vector matrix and receive steering vector matrix. After the accuracy transmit elevation angle and receive angle are obtained, the corresponding transmitting and receiving azimuth angle, polarization angle and polarization phase difference can be accurately estimated by using the vector-cross-product algorithm. Compared with existing algorithms, the proposed algorithm can avoid high dimensional eigenvalue decomposition and additional parameter matching process. Moreover, the high estimation performance of the proposed can be further guaranteed by using the designed sparse array. Finally, simulation results demonstrate the effectiveness and superiority of the proposed method when compared to the state-of-the-art methods in terms of estimation accuracy and angle resolution.
Simulation and Architectural Design for Schottky Structure Perovskite Solar Cells
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In this paper, the wx-AMPS simulation software was used to model and simulate the Schottky perovskite thin film solar cells. Front and back electrodes with different work functions were applied to the Schottky perovskite solar cells to study the effect of band structure changes on the performance of solar cells. The results show that within 3.8eV to 4.4eV, as the work function of the front electrode decreases, the conversion efficiency of the Schottky solar cell gradually increases. When the work function of the front electrode is low, the electric field strength is large, which facilitates the transport of carriers in the light-absorbing layer of the perovskite and reduces the carrier recombination rate of
Investigate the effect of source-drain conduction in single-event transient on nanoscale bulk fin field effect transistor
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Fin Field Effect Transistor (FinFET) is the most widely used structure when the field effect transistor scaling below 30 nm. And there are few studies on single-event transient of FinFET devices with gate length below 30nm. Since single-event-transient on FinFET with gate length below 30 nm is worthy of study. The single-event-transient response of bulk FinFET with 30 nm,40 nm,60 nm and 100 nm gate length is examined using pulsed laser and technology computer aided design (TCAD) simulation in this article. First, we use the pulsed laser to ionize the gate of the FinFET device and detect the drain current transient of the FinFET device. The experimental results show that there are obvious platforms for the drain current transient tails of different gate length FinFET, and the platform current increases as the gate length of FinFET becomes shorter. The charges collected in the platform of FinFET devices with gate lengths of 100 nm, 60 nm, 40 nm, and 30 nm are 34%, 40%, 51%, and 65% of the total charge collected in drain current transient, respectively. Therefore, when the FinFET device with the gate length below 100nm, the platform current will seriously affect the device performance. Second, we use TCAD to simulate the heavy ion single-event effect of FinFET devices and study the generation mechanism of platform region in drain current transient. TCAD simulation explains the mechanism of this. Laser or heavy ions ionize high concentration electron-hole pairs in the device. The holes are quickly collected and the high concentration electrons are left under the FinFET channel. High concentration electrons conduct source and drain, generating the source-to-drain current at the tail of the drain current transient. Moreover source-drain conduction enhances the electrostatic potential below the FinFET channel and suppresses high-concentration electron diffusion, making source-to-drain current reduce slow and form the platform. Drain current transient tail has a long duration and a large amount of collected charge, which seriously affects FinFET performance. This is a problem that needs to be focused on in the single-event effect of FinFET devices. It’s also a difficult problem to be overcome when applying FinFET devices to spacecraft. And the generation mechanism of the drain current transient plateau region of FinFET devices can provide theoretical guidance for solving these problems.
Mechanism of laser intensity-dependent below-threshold harmonic generation
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High-order-harmonic generation (HHG) is a fundamental atomic and molecular process in strong laser fields and plays a crucial role in the development of ultrafast science and technology. The essential features in HHG, such as the above-threshold harmonic plateau and cutoffs, can be well understood by the semiclassical three-step model. The HHG cutoff occurs approximately at the energy, where is the atomic ionization potential, and is the ponderomotive potential. In the past, most studies focused on the HHG above the ionization threshold, and the general pattern of the HHG spectrum can be qualitatively explained by means of the strong field approximation (SFA) and the quantum treatment of three-dimensional time-dependent Schr?dinger equation (TDSE). However, the SFA results in inadequate description for the process in the harmonic generation below the ionization threshold since it neglects the Coulomb potential and the detailed electronic structure of atoms. Recently, as a promising method to produce vacuum-ultraviolet frequency comb, the HHG in the near- and below-ionization threshold has been increased considerably. However, the dynamical origin of in these lower harmonics is less understood and largely unexplored. Here we perform an ab initio quantum study of the near- and below-threshold harmonic generation of hydrogen atom by means of the time-dependent generalized pseudospectral method. We study the intensity dependence of the harmonic spectra below the ionization threshold of hydrogen atom in the intense laser field. The high-order harmonic spectra are calculated by the Fourier transform of the atom induced dipole moment in the laser field. The below-threshold harmonic spectra yield is scaled as a function of the laser peak intensity. We find that the spectra yield in below-threshold harmonic generation (BTHG) dependents on the light intensity in the multiphoton ionization regime. And the laser intensity plays an important role in the channel selection process for BTHG. There are mainly two kinds of quantum channels to be responsible for the BTHG. Namely, the generalized short
Electron transport through a quantum-dot-Su-Schrieffer-Heeger-chain system
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The Su-Schrieffer-Heeger (SSH) is a typical one-dimensional system with topological edge states, which has been experimentally realized in the photon and cold atom systems. Therefore, how to confirm the existence of the edge states from theoretical and experimental has become one of the most important topics in condensed matter physics. In this paper, using the tight-binding approximation and transfer-matrix method, we have studied the transport signatures of electron through a quantum dot-SSH chain hybrid system. Here, the two quantum dots play a role in modulating the tunneling coupling strength between the SSH chain and the two electrodes. When the quantum dots are weakly coupled to the SSH chain, the quadruple-degenerate edge states of the quantum dot-SSH chain hybrid system correspond to that the SSH chain has two degenerate zero-energy edge states; whereas the twofold-degenerate ones correspond to that the SSH chain has no edge states. While the quantum dots are strongly coupled to the SSH chain, the edge states only exist when the intra-cell hopping amplitude is larger than the inter-cell hopping amplitude. In this situation, however, there is no edge states in the SSH chain. In particular, when the quantum dot-SSH chain hybrid system is strongly coupled to the two external electrodes, the number of transmission resonance peaks of the edge states of the quantum dot-SSH chain hybrid system will be reduced by 2. For example, in the case of the quadruple-degenerate edge states, the number of transmission resonance peaks will be two; whereas in the case of twofold-degenerate ones, that will disappear. Therefore, by modulating the tunneling coupling strength between the quantum dots and the SSH chain and that between the quantum dots and the two external electrodes, we can observe the variation of the number of transmission resonance peaks of edge states to detect whether the SSH chain is in the nontrivial topological state or not.
Avalanche photodiode single-photon detector with high time stability
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Avalanche photodiode single-photon detector is one of the ultra-sensitivity photoelectric detector, which has important applications in the fields of long-distance laser ranging, laser imaging, and quantum communication. However, due to the high temperature sensitivity of the avalanche voltage, the avalanche photodiode single-photon detector is prone to fluctuation of the avalanche gain when it works in the field environment, which leads to the delay drift and seriously reduces the time stability. In this paper, we proposed a method of stabilizing the delay of the single-photon detector. An embedded system was used to control avalanche photodiode at constant low temperature and compensate the delay drift of the detection circuit caused by the change of environment temperature in real time. A high time stability avalanche photodiode single-photon detector was realized by this method. In the experiment, the environment temperature changed from 16 ℃ to 36 ℃, and the avalanche photodiode was controlled at 15 ℃. After compensation, the delay drift of the avalanche photodiode single-photon detector was within ± 1 ps, and the time deviation was 0.15 ps @ 100 s. This work is expected to provide an effective solution for the application of high-stability single-photon detector in the field and space environment.
Link local connectivity to atomic-scale relaxation dynamics in metallic glass-forming systems
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For a long time, it has been well recognized the existence of a deep link between the fast vibrational excitations and the slow diffusive dynamics in glass-forming systems. However, it remains as an open question whether and how the short-time scale dynamics associated with vibrational intrabasin excitations is related to the long-time dynamics associated to diffusive interbasin hoppings. In this paper we briefly review the research progress that addresses this challenge. By identifying a structural order parameter–local connectivity of a particle which is defined as the number of nearest neighbors having the same local spatial symmetry, it is found that the local connectivity can tune and modulate both the short-time vibrational dynamics and the long-time relaxation dynamics of the studied particles in a model of metallic supercooled liquid. Furthermore, it reveals that the local connectivity leads to the change in the long-time decay of the correlation functions from stretched exponentials to compressed ones, denoting a dynamic crossover from diffusive to hyperdiffusive motions. This is the first time to report that in supercooled liquids particles with particular spatial symmetry can present a faster-than-exponential relaxation that has so far only been reported in out-of-equilibrium materials. The recent results suggest a structural bridge to link the fast vibrational dynamics and the slow structural relaxation in glass-forming systems and extends the compressed exponential relaxation phenomenon from earlier reported out-of-equilibrium materials to the metastable supercooled liquids.
Topological quantum phase transitions in one-dimensional $p$-wave superconductors with modulated chemical potentials
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We consider a one-dimensional $p$-wave superconducting quantum wire with the modulated chemical potential, which is described by $\hat{H}=\sum_{i}\left[ \left( -t\hat{c}_{i}^{\dagger }\hat{c}_{i+1}+\Delta \hat{c}_{i}\hat{c}_{i+1}+ h.c.\right) +V_{i}\hat{n}_{i}\right]$, $V_{i}=V\frac{\cos \left( 2\pi i\alpha + \delta \right) }{1-b\cos \left( 2\pi i\alpha+\delta \right) }$ and can be solved by the Bogoliubov-de Gennes method. When $b=0$, $\alpha$ is a rational number, the system undergoes a transition from topologically nontrivial phase to topologically trivial phase which is accompanied by the disappearance of the Majorana fermions and the changing of the $Z_2$ topological invariant of the bulk system. We find the phase transition strongly depends on the strength of potential $V$ and the phase shift $\delta$. For some certain special parameters $\alpha$ and $\delta$, the critical strength of the phase transition is infinity. For the incommensurate case, \emph{i.e.} $\alpha=(\sqrt{5}-1)/2$, the phase diagram is identified by analyzing the low-energy spectrum, the amplitudes of the lowest excitation states, the $Z_2$ topological invariant and the inverse participation ratio (IPR) which characterizes the localization of the wave functions. Three phases emerge in such case for $\delta=0$, topologically nontrivial superconductor, topologically trivial superconductor and topologically trivial Anderson insulator. For a topologically nontrivial superconductor, it displays zero-energy Majorana fermions with a $Z_2$ topological invariant. By calculating the IPR, we find the lowest excitation states of the topologically trivial superconductor and topologically trivial Anderson insulator show different scaling features. For a topologically trivial superconductor, the IPR of the lowest excitation state tends to zero with the increase of the size, while it keeps a finite value for different sizes in the trivial Anderson localization phase.
High Precision Calibration of spectral parameters of CO at 1567 nm based on wavelength modulation - direct absorption spectroscopy (WM-DAS)
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Direct absorption spectrum (DAS) can measure the molecular absorptivity function and determine the spectral parameters of the gas by fitting the measured absorptivity function. Wavelength modulation-direct absorption spectroscopy (WM-DAS) is based on DAS and combines the idea of harmonic analysis in wavelength modulation spectrum (WMS). The measurement accuracy of absorptivity function can be effectively improved by using Fourier transform. In this paper, the absorptivity function of CO R5-R11 near infrared weak absorption line at 1567nm were accurately reproduced by using WM-DAS method combined with long optical path gas absorption cell at room temperature and low pressure. The standard deviation of the fitting residual reached 3×10-5, and then the spectral parameters such as collision broadening, Dicke narrowing and speed-dependent collision broadening coefficients were measured in high precision. These parameters were compared with the results of high sensitivity continuous wave cavity ring down spectroscopy (CW-CRDS). The experimental results showed that the signal-to-noise ratio of the absorptivity function measured by CW-CRDS was about 2.5 times that of the long-path WM-DAS, and the measured spectral parameters were highly consistent. The relative errors of the collision broadening coefficients using Voigt profile of the two methods were less than 1%. At the same time, the detection limit of CO at 1567nm based on WM-DAS method was about 80ppb, and the corresponding absorption coefficient was 2×10-10cm-1, which was slightly higher than CW-CRDS. However, WM-DAS has the advantages of fast measurement speed, simple system and low cost, and is expected to provide a new measurement method for the measurement of weak absorption lines.
Research on few-mode PAM regenerator based on nonlinear optical fiber loop mirror
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In recent years, the demand for network bandwidth has increased significantly, and the capacity of wave division multiplexing (WDM) systems has reached the nonlinear Shannon limit. In order to adapt to the development of future networks, space division multiplexing (SDM) technology attracts more and more attention. In this paper, we put forward a novel structure of pulse amplitude modulation (PAM) regenerator based on few-mode nonlinear optical fiber loop mirror (FM-NOLM) for the first time, and theoretically analyze the working principle for few-mode reshaping. It can regenerate degraded PAM signals and improve transmission performance in SDM system. The detailed design steps of the regenerator are given, in which the sulfide highly nonlinear fiber and multimode coupler are used to build up the FM-NOLM and their mode characteristics are simulated by COMSOL software. The parameters of the regenerator are determined by taking the few-mode optical fiber supporting LP01, LP11, and LP21 modes as an example, and then the power transfer function (PTF) curve of each mode for PAM signals is calculated. We simulate and analyze the noise reduction ratio (NRR) performance of the few-mode regenerator for PAM-4 signals, and compare with the case of single mode fiber. Our simulation shows that, (1) for each spatial mode of PAM signal, all regenerative levels have the same consistent power transfer performance; (2) for the input signal-to-noise ratio (SNR) greater than 20 dB, the NRR for each mode can exceed 3 dB, and increase with the input SNR at the slope of about 1.2; (3) the NRR difference between the three modes is less than 1.1 dB for the same input signal-to-noise ratio (SNR). In order to illustrate the reshaping function of the regenerator, we also present the power distribution histograms for PAM-4 signals before and after regeneration when the input SNR is 25 dB. This scheme proposed here has some advantages over the existing regenerators in the applicability for the long-haul SDM system with high spectral efficiency and regeneration of any level number of PAM signals in theory due to its uniform multi-level regeneration function, but also is capable of being extended to the wavelength domain for higher transmission capacity.
Sound focusing in inhomogeneous waveguides
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The method for studying sound focusing analytically in inhomogeneous waveguides is presented. From the viewpoint of acquiring the maximum acoustic pressure at an arbitrary position with normalized energy flux injection, optimal incident waves can be derived based on the multimodal admittance method. The first step is to expand the wave solution onto a complete orthogonal basis set so that the Helmholtz equation can be transformed into two sets of first-order coupled differential equations in modal domain. The second step is to solve the coupled equations numerically using admittance matrix, which can give stable numerical results. Eventually the mapping between the acoustic pressure at an arbitrary position and that of the incident wave can be obtained, and the optimal incident waves that generate wave focusing are able to be derived together with the focused fields. We study the sound focusing in waveguides with varying cross-sections, scatterers or sound-speed profile, the results show that the optimal incident waves will take full advantage of wave scattering during propagation to achieve the maximum pressure at foci, leading to good sound single-point and multi-point focusing performance. We believe that our research can provide guidance on designing acoustic lenses or metamaterials to focus sound waves in complex media, and can offer inspiration in wave communications, imagings and non-destructive testing.
Diagnosis of capacitively coupled plasma driven by pulse-modulated 27.12 MHz by using an emissive probe
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There are several methods for the diagnosis of capacitively coupled discharges, such as microwave resonance probe, Langmuir probe, and so on, but methods like microwave resonance probe are mainly used in the determination of electron density. Moreover, in diagnosing plasma potential, the emissive probe has a higher accuracy compared with the traditional electrostatic probes, and it can directly monitor the potential in real time. However, in the existing work, emissive probe is mostly applied to the diagnosis of plasmas with high density or plasmas modulated by pulsed dual frequency (one of the radio frequency sources is modulated), experiment of the emissive probe diagnosis for plasmas excited by a pulsed single frequency is quite rare. In this paper, by using an emissive probe operated in floating point mode, the temporal evolution of the plasma potential and electron temperature with input power and pressure in a pulsed 27.12 MHz capacitively coupled argon plasma has been investigated. The plasma potential is obtained by measuring emissive probe potential under a strongly heated condition, while the electron temperature is estimated from the potential difference between the emissive probe under strongly heated and cold conditions. The measurements show that as the pulse is set on, the plasma potential will rise rapidly and become saturated within 300 microseconds due to the requirement of neutrality condition; while the pulse is set off, the plasma potential undergoes a rapid decline and then stabilizes. An overshoot for the electron temperature occurs as the onset of the pulse, because of the influence of radio frequency electric field and residual electrons from the last pulse; during the pulse off time, rapid loss of high-energy electrons causes the electron temperature to rapidly drops to 0.45 eV within 300 microseconds, then it rises slightly, which is related to the electrons emitted by the probe. The plasma potential basically has a linear dependence on the change of input power and pressure for the pulse on and off time; and the input power has a greater impact on the difference between the overshoot electron temperature and the steady state electron temperature during the pulse on time. Corresponding explanations are given for the temporal evolution of plasma potential and electron temperature in different pulse stages and under different discharge conditions.
Bifurcation and nonlinear evolution of convection in binary fluid mixtures with weak Soret effect
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Rayleigh-Bénard (RB) convection in binary fluid mixtures, which shows rich and interesting pattern formation behavior, is a paradigm for understanding instabilities, bifurcations, self-organization with complex spatiotemporal behavior and turbulence, with many applications in atmospheric and environmental physics, astrophysics, and process technology. In this paper, by using a high-order compact finite difference method to solve the full hydrodynamic field equations, we study numerically the RB convection in binary fluid mixtures such as ethanol-water with a very weak Soret effect (separation ratio $\psi=-0.02$) in a rectangular container heated uniformly from below. The direct numerical simulations are conducted in the rectangular container with aspect ratio of $\Gamma=12$ and with four no-slip and impermeable boundaries, isothermal horizontal and perfectly insulated vertical boundaries. The bifurcation and the origin and evolution of pattern in RB convection for the considered physical parameters are studied, and the bifurcation diagram is presented. By performing two-dimensional simulations, we observe three stable states of Blinking state, localized traveling wave and stationary overturning convection (SOC) state, and discuss the transitions between them. The results show that there is a hysteresis in the transition from the Blinking state to the localized
Simulation of the nonlinear cahn-hilliard equation based onthe local refinement pure meshless method
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The phase separation phenomenon between different matters plays an important role in many science fields. And the high order nonlinear Cahn-Hilliard (C-H) equation is often used to describe the phase separation phenomenon between different matters. However, it is difficult for the theorical methods and the grid-based methods to solve the high-order nonlinear C-H equations. Therefore, this work turns to the meshless methods, and proposes a local refinement finite pointset method (LR-FPM) to numerically investigate the high-order nonlinear C-H equations with different boundary conditions. Its constructive process is: (1) the fourth derivative is decomposed into two second derivatives, and then the spatial derivative is discretized by FPM which is based on the Taylor series expansion and weighted least square method; (2) the local refinement and quintic spline kernel function are employed to improve the numerical accuracy; (3) the Neumann boundary condition with high-order derivatives is accurately enforced when solving the local linear equation sets. The 1D/2D C-H equations with different boundary conditions are first solved to show the ability of the LR-FPM, and the analytical solutions are available for comparison. Meanwhile, we also investigate the numerical error and convergence order of LR-FPM with uniform/non-uniform particle distribution and local refinement. Finally, 1D/2D C-H equation without analytical solution is predicted using by LR-FPM, and compared with the FDM result. The numerical results show that the implement of the
Research on NH3 Aliasing Absorption Spectra at 1103.4cm-1 Based on Continuous Quantum Cascade Laser
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Due to the important role of NH3 in atmospheric aerosol chemistry, rapid and accurate inversion of ammonia concentration is very important for environmental issues. In this paper, a 9.05 μm room temperature continuous QCL laser is used as the light source, and the wavelength scanning direct absorption tunable diode laser absorption spectroscopy (TDLAS) technology is used to study the spectral characteristics of the QCL laser at 1103.4 cm-1. A low-pressure experimental platform based on two-level temperature control was designed to measure the six aliasing absorption lines of ammonia at 1103.4cm-1. The spectrum broadening becomes smaller under the condition of reducing the pressure, and the aliasing spectra are separated. The line strength of each absorption line further analyzes the measurement uncertainty. A method for accurate inversion of single-spectrum gas concentration by low-pressure separation was proposed for severely aliased spectra, and experimental verification was performed. By comparing the results with the HITRAN database, it is concluded that the experimental measurement line strength value of ammonia at 1103.4cm-1 and the database deviation are 2.71%-4.71%, and the uncertainty of the experimental measurement line strength value is in the range of 2.42%-8.92%. The deviation between the inversion concentration and the actual value is between 1% and 3%. The study of this method provides reference for future inversion of trace gas concentrations in the atmospheric environment.
Adhesion and nanotribological properties of folded graphene prepared by mechanical exfoliation
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Atomic force microscope was used to study the adhesion of mechanical exfoliated graphene under air and nitrogen conditions. It was found that the adhesion under nitrogen was smaller. The adhesion of graphene edge was larger than that of the inner region. The relationship between the adhesion of folded graphene and the number of layers along with its frictional properties were investigated under nitrogen atmosphere. The results showed that the adhesion was independent of the number of folded graphene layers. The frictional properties of each area of the folded graphene were far beyond the SiO2 substrate. The friction coefficients of the single layer, the fold on single layer, the double layers and the fold on double layers regions were successively decreased, which were 0.049, 0.031, 0.023 and 0.021 respectively. The friction forces were successively decreased as well. The frictional property of the folded graphene was weaker than the unfolded graphene of same number of layers due to the weaker bonding force between the layers. When measuring the adhesion with a sharp tip or a ball tip, the measurement history of adhesion had little influence on subsequent adhesion. Studies on freshly folded graphene in the air showed that the friction force of the folded region was significantly higher than that of the unfolded region.
Time-resolved ultrafast dynamics in triple degenerate topological semimetal MoP
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We employ the time resolved pump probe experiment to investigate the ultrafast dynamics in a topological semimetal molybdenum phosphide (MoP), which exhibits triple degenerate points in the momentum space. Two relaxation processes with the lifetime of 0.3 ps and 150 ps have been observed. We attribute the fast component to the electron-phonon scattering and the slow component to the phonon-phonon scattering, respectively. Temperature dependence investigation shows that both the lifetimes of the fast and slow components enhance slightly with increasing temperature. We also successfully generate and detect a thermal-stress-induced coherent acoustic phonon mode with a frequency of 0.033 THz, which does not vary with temperature. Our ultrafast spectroscopy investigation of the quasiparticle dynamics and the coherent phonon in MoP provides useful experimental facts and information about the overall excited state dynamics and the temperature dependence of electron-phonon coupling.
Theoretical calculating on the photoelectron angular distribution of the sequential two-photon double ionization for Ar atom
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The for the photoelectron angular distributions in a sequential two-photon process have been given based on the multi-configuration Dirac-Fock method and the density matrix theory. And then, the relativistic calculation program for photoelectron angular distribution is further developed with the help of the program packages GRASP2K and RATIP which are based on the multi-configuration Dirac-Fock method. By using this code, the sequential two-photon double ionization of the 3p shell in atomic argon is studied theoretically. The cross sections, magnetic cross sections, alignment of residual ions and the asymmetry parameters of the photoelectron angular distribution for the first and the second step of sequential two-photon double ionization of argon as a function of photon energy have been present. The calculations predict that the alignment have a maximum value and the asymmetry parameters have a minimum value in the region of the Cooper minimum. The angular distribution of the first step ionization for Ar atom and the second step ionization for Ar+ ion have been given at 33.94eV and 55.34eV photon energy. In addition, the difference properties have been discussed between the angular distributions of the first photoelectron in sequential two-photon double ionization and in conventional one-photon single ionization. The present calculated results are compared with other available results, showing that they are in good agreement with each other. The results of this paper will be helpful in studying nonlinear processes in the XUV range.
Compound relaxation oscillations connected by pulse-shaped explosion
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Relaxation oscillations are ubiquitous in various fields of natural science and engineering technology. Exploring possible routes to relaxation oscillations is one of the important issues in the study of relaxation oscillations. Recently, the pulse-shaped explosion (PSE), a novel mechanism which can lead to relaxation oscillations, has been reported. The PSE means pulse-shaped sharp quantitative changes related the variation of system parameters in branches of equilibrium points and limit cycles, which leads the system’s trajectory to undertake sharp transitions and further induces relaxation oscillations. Regarding externally and parametrically excited nonlinear systems with different frequency ratios, some work on PSE has been reported. The present paper focuses on the PSE and the related relaxation oscillations in a externally and parametrically excited Mathieu-van der Pol-Duffing system. We show that if there is an initial phase difference between the slow excitations with the same frequency ratio, the fast subsystem may compose of two parts with different s, each of which determines a different vector field. As a result, the bistable behaviors are observed in the system. In particular, one of the vector fields exhibits two groups of bifurcation behaviors, which are symmetric with respect to the positive and negative PSE, and each can induce a cluster in the relaxation oscillations. Based on this, we present several routes to compound relaxation oscillations, and obtain new types of relaxation oscillations connected by pulse-shaped explosion, which we call compound “subHopf/fold-cycle” relaxation oscillations and compound “supHopf/supHopf” relaxation oscillations, respectively. Our results show that the two clusters in the resultant relaxation oscillations are connected by the PSE, and the initial phase difference plays an important role in transitions to different attractors and the generation of relaxation oscillations. Although the research in this paper is based on a specific nonlinear system, we would like to point out that the bistable behaviors, the PSE and the resultant compound relaxation oscillations can also be observed in other dynamical systems. The reason is that the fast subsystem composes of two different vector fields induced by the initial phase difference, which dose not rely on a specific system. The results of this paper deepen the research about PSE as well as the complex dynamics of relaxation oscillations.
First-principles study of Ca5N4 at high pressure
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Recent studies have shown that introducing metal elements into nitrogen matrix can induce more stable poly-nitrogen structures compared with the pure nitrogen phase, due to the ionic interaction between metal elements and nitrogen matrix. Many types of poly-nitrogen structures have been reported by applying the alkaline earth metal elements (M = Be, Mg, Ca, Sr, Ba) as the coordinate elements. For example, the 1D infinite armchair poly-nitrogen chains (N∞) structures and N6 ring structures are obtained for the MN4 and MN3 chemical stoichiometries, respectively. Interestingly, the stability of theses MNx structures has been enhanced 2-3 times compared with the pure nitrogen. Therefore, exploring the novel and stable poly-nitrogen structure by introducing alkaline earth metal elements under high pressure is a great significant work. As an alkaline earth element, Ca is abundant in the earth. Its ionization energy (I1=590 KJ/mol) is far lower than that of Be (900KJ/mol) and Mg (738KJ/mol), which means Ca can form calcium nitrides easier. Peng et.al has proposed that Ca-N system can obtain poly-nitrogen structures under high pressure, such as CaN4 structure with armchair nitrogen chain, CaN5 and CaN3 consisting of pentazolate “N5” and benzene-like “N6” anions. These poly-nitrogen structures have potential application prospects in the field of high energy density materials. Here, we report the prediction of Ca-N system at 100GPa by using particle swarm optimization (PSO) algorithm technique for crystal structure prediction. A new thermal stable phase with P21/c-Ca5N4 space group is found at 100GPa, which enriched the phase of Ca-N system under high pressure. The dynamic stability and mechanical stability of new phase are confirmed by phono dispersion spectrum and elastic constants calculations. ELF analysis shows that the nitrogen atoms in P21/c-Ca5N4 are bonded by N-N single bond and electron transfer from Ca atom to N atom enables Ca5N4 as an ionic-bonding interaction structure. Band structure calculation shows that Ca5N4 is a semiconductor structure with a 1.447eV direct band gap. PDOS calculation shows the valence band near Fermi energy is mainly contributed by N_p electrons, while the conduction band is mainly contributed by Ca_d electrons, indicating that electrons are transferred from Ca atom to N atom. Bader calculation shows that each N atom obtains 1.26e from Ca atom in P21/c-Ca5N4. The Raman spectrum is calculated and detailed Raman vibration modes are identified, which provides theoretical guidance for experimental synthesis.
First-principles study of Cu:Fe:Mg:LiNbO3 crystals
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In this paper the electronic structures and optical properties of Cu:Fe:Mg:LiNbO3 crystals and their comparative group are investigated by first-principles based on the density functional theory, to explore the characteristics of charge transfer in crystals, and to analyse the parameters of the two-colour holographic storage technology base on optical properties of crystals. The basic crystal model is built as a supercell structure 2×2×1 of near-stoichiometric pure LiNbO3 crystal with 120 atoms, including 24 Li atoms, 24 Nb atoms and 72 O atoms. Above that five doped crystal models are established as follows, the copper doped LiNbO3 crystal (Cu:LiNbO3), the ferri doped LiNbO3 crystal (Fe:LiNbO3), the copper and ferri co-doped LiNbO3 crystal (Cu:Fe:LiNbO3), the copper, ferri and magnesium tri-doped LiNbO3 crystal (Cu:Fe:Mg:LiNbO3) with doping ions at Li sites, and the other copper, ferri and magnesium tri-doped LiNbO3 crystal (Cu:Fe:Mg(E):LiNbO3) with ferri ions at Nb sites and magnesium ions at both Li sites and Nb sites. The last two models represent the concentration of Mg ions below the threshold (~6.0 mol%) and over the threshold respectively. The charge compensation forms are taken successively as Cu+ Li—V— Li, Fe2+ Li—2V— Li, Fe2+ Li—Cu+ Li—3V— Li, Mg+ Li—Fe2+ Li—Cu+ Li—4V— Li and 3Mg+ Li—Mg3- Nb—Fe2- Nb—2Cu+ Li in
Comparison of electrical wire explosion characteristics of single wire and wire array in air
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In this paper, discharge characteristics of a planar copper wire array explosion driven by a microsecond pulsed current source (500 J stored energy) in atmospheric air medium were studied. Meanwhile, controlled experiments were performed with single wire loads. At 2cm distance between electrodes, 2-16 copper wires with diameter of 100μm were selected to form planar copper wire arrays, and single copper wire with diameter of 50-400μm was selected for comparison. Load voltage, circuit current and light radiation intensity were measured, and electric power, deposited energy calculated. The experimental results show that with the increase of mass, the process of vaporization and ionization become slower, manifested as a delay of the voltage peak and an increase of the full width half maximum (FWHM) of the voltage pulse from 0.07μs to 0.64μs. In contrast, although the explosion time of wire array load was delayed with the increase of mass, the duration of vaporization and ionization did not change significantly with a FWHM of 0.11±0.01μm. In addition, the deposited energy of wire array load before breakdown was lower than that of single wire load with the same mass. As for the optical radiation intensity, under three cases with the same mass, the optical radiation intensity of wire array explosion is about 28%, 49% and 52% higher than that of single wire explosion. There may be two reasons which cause the difference between the single wire load and wire array load. First, the larger specific surface area of the wire array load makes faster phase transitions. Second, the development of thermal or magnetohydrodynamics for the two kinds of loads was different, which should be responsible for the differences in energy deposition and optical emission.
Properties of one-way propagation in the magneto-optical planar waveguide
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Surface magnetoplasmons (SMPs) is a kind of near-field electromagnetic wave, which propagates at the interface of dielectric and magneto-optical material under the action of biased magnetic field. In this paper, a structure of three-layer planar waveguide with silver, silicon, and magneto-optical material is studied. It is found that both the fundamental mode and the higher-order mode of SMPs have one-way propagation characteristics in forward or backward direction within a specific frequency range. The dispersion equations of the planar waveguides with gyromagnetic material and gyroelectric material are calculated respectively. As a result, the thickness of silicon layer and the external magnetic field have significant influence on bulk mode and the one-way propagation region of SMPs. By increasing the thickness of the silicon layer or increasing the intensity of the magnetic field, the higher-order mode can appear at the lower frequency region, thus compressing the one-way propagation region or even losing the one-way propagation mode. The one-way propagation bandwidth of planar waveguides with gyromagnetic material YIG and gyroelectric material InSb are calculated. The colormap of YIG waveguide and InSb waveguide under 400~2000Oe magnetic field and 0.1~1T magnetic field are obtained. As a result, the one-way mode of YIG waveguide appears in GHz band, and the maximum one-way propagation bandwidth can reach 2.45 GHz. While, the one-way mode of InSb waveguide appears in THz band with a maximum one-way propagation bandwidth of 3.9 THz.
Thermal characteristics of surface liquid crystal vertical cavity surface emitting laser arrays
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The combination of liquid crystal and vertical cavity surface emitting semiconductor laser (VCSELs) array can realize wavelength tunability, precise polarization control, etc. at the same time, the introduction of liquid crystal will also change the thermal characteristics of VCSELs array, Therefore, in this paper, the experimental research on the thermal characteristics of VCSELs array structure is carried out. The effect of nematic liquid crystal layer on the thermal characteristics of VCSEL array is compared and analyzed. The experimental results show that the threshold current temperature change rate of 1 × 1, 2 × 2 and 3 × 3 surface liquid crystal VCSEL array can be reduced by 23.6% and the thermal resistance can be reduced by 26.75%. At the same time, the uniformity of the light-emitting units of the laser array is significantly improved, and the temperature difference between the optical outlet and the surrounding is less than 0.5 ℃. To sum up, the introduction of liquid crystal layer in VCSEL array not only greatly accelerates the thermal diffusion of laser array elements, but also reduces the junction temperature of active region and improves the thermal characteristics of VCSELs laser array.
Application Research of Ant Colony Cellular Optimization Algorithm in Population Evacuation Path Planning
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Aiming at the problem of path planning, the ant colony cellular optimization (ACCO) algorithm is proposed as the path planning model based on the grid map. First, to unify simulation time step, a grid map based on hexagonal cells is established. Then the heuristic function is optimized by using the static field, and the update method of pheromone is optimized by using the segment rule. Finally, the parameters of ACCO algorithm are optimized through the particle swarm optimization (PSO) and the optimal combination of parameters is solved. The simulation results show that when the ACCO algorithm is used for path planning, the search speed is acceletated and the understanding space is increased, which can increased the search ability to effectively avoid the local optimal solution.
Investigation of interaction between α-Fe metal and H atom by ab-initio method
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Hydrogen-induced cracking(HIC) is key problem restricting the application of ultra-high strength steel. It is necessary to analyze distribution of diffusible hydrogen to reveal the mechanism law of HIC. The site occupation tendency of H in interstitial and vacancy positions are investigated by ab-initio and the stable configuration and steady state energy was obtained, solution tendency of H atom in interstitial and vacancy positions is analyzed based on the aforementioned results. Specifically, The Mulliken population, density of states, charge density difference are calculated and helped clarify the interaction between α-Fe metal and H atom. The results showed that the dissolved hydrogen tended to occupy tetrahedral sites of the body-centered cubicαody-centered cuthe weak hybridization interaction between interstitial hydrogen and nearest neighbour Fe atom is contributed by the H 1s orbital and Fe 4s orbital. Vacancies can capture H atom easily and H atom tended to occupy isoelectric surface near the inwall of vacancies. A vacancy defect can hold up to 3 H atoms and H atoms are difficult combined with others by covalent bond to form H2 molecule. H atoms in vacancies and interstitial positions change charge distribution, weaken the bonds between atoms totally, and form anti-bonding orbital. The proposed thermodynamical model allows the determination of the equilibrium vacancy and the dissolved hydrogen concentration for a given temperature and H chemical potential in the reservoir, the calculated results are in good agreement with the actual results.
Research on the static measurement of the absolute gravity in a truck based on atomic gravimeter
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Currently, most of the experimental apparatuses of atomic gravimeters are complex in architecture, large in size, and poor in environmental adaptability, so that they could not be operated in field for the measurement of absolute gravity. Thus, the application areas of atomic gravimeters have been limited. In this paper, we have integrated a system of absolute gravity measurement on a truck based on a compact homemade atomic gravimeter. This system is consist of atomic gravimeter for gravity measurement, passive isolation platform for vibration suppression, the posture platform for tilt adjustment, differential GPS for altitude measurement, UPS for power supply, the air-conditioned truck for temperature control and transportation. At first, we have estimated the performance of environmental adaptability for this measurement system on the truck, and it is found that the system still can work even in a high field temperature of 40 centigrade and a big tilt angle of 8° for the road. Besides, the experimental procedure of absolute gravity measurement and the processing methods of the measured data have been introduced, and the dependence of measured gravity on the orientation of the truck has been measured and analyzed. Finally, repeated line measurements have been performed on a flat field road, and the accuracy of self-coincidence for absolute gravity measurement has been evaluated to be 30 μGal. Besides, we have obtained the vertical gravity gradient of the earth by measuring the absolute gravity values of different altitude sites on a slope road, and the value is estimated to be -231(36) μGal/m. The presented results could provide the basic reference for the field absolute gravity survey.
Numerical Simulation Analysis of Gas-liquid Two-phase Flow in Gas Lift System
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The gas lift system has a large of significant advantages in sewage treatment, deep well oil recovery, liquid metal cooled reactor and magneto hydrodynamic power converters. The densities of different liquid mediums and gas mediums have great influences on the performance of gas lift system. Therefore, based on Fluent simulation software, using Euler model and k-omega SST turbulence model, the gas-liquid two-phase flow behaviors in nitrogen-water, nitrogen-kerosene, nitrogen-mercury and air-water, argon-water, nitrogen-water of gas lift systems were studied. The rules of void fraction and liquid flow rate at lifting pipe, liquid radial velocity at lifting pipe outlet, promoting efficiency were analyzed. The results show that: (1) In the nitrogen-water, nitrogen-kerosene and nitrogen-mercury systems, the higher the density of liquid medium, the smaller the void fraction in the lifting pipe, the greater the flow rate of lifting liquid and the higher the promoting efficiency. (2) In the air-water, argon-water and nitrogen-water systems, the higher the density of gas medium, the smaller the void fraction in the lifting pipe, the larger the flow rate of lifting liquid, and the smaller the peak value of promoting efficiency. (3) With the increase of gas flow rate, the liquid radial velocity at the lifting pipe outlet increases with overall fluctuation. Finally, the liquid velocity near the center of pipe axis is large, near the pipe wall is small. The research provide the scientific theoretical basis for the optimization of gas lifting technology in applications such as sewage treatment, deep well oil recovery, liquid metal cooled reactor and magneto hydrodynamic power converters.
The Effects of Salt Concentrations and Pore Surface Structure on the Water Flow through Rock Nanopores
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The surface dissolution of rock nanopores caused by the acidic environment increases the salt concentration of water solution flowing in the nanopores and destroys the surface structure of the rocks, which can be found in CO2 geological sequestration and crude oil and shale gas exploration. In this paper, the molecular dynamics method was adopted to study flow characteristics of water solution in the forsterite (Mg2SiO4) slit nanopores, by which the effects of salt concentrations and structure destruction of pore surface on the velocity profiles of water solution confined in nanopores were systematically analyzed. The hydrogen bond density, radial distribution function (RDF) and water density distribution were calculated to interpret the changes in viscosity, velocity profiles and interaction between water and nanopore surface. The results show that as the salt concentration increases, the water solution flow in the rock nanopore obeys the Hagen-Poiseuille equation, considering the velocity profiles of water solution remain parabolic shape. However, the hydrogen bond network among water molecules becomes denser with the higher salt concentration, which is accounting for the linear increase in the viscosity of water solution. Besides, the higher salt concentration leads to the larger water flow resistance from the pore surface. As a result, with the salt concentration increasing, the maximum of water velocity decreases and the curvature radius of the parabolic velocity profile curve becomes bigger. Moreover, the surface structure destruction in rock nanopores changes the roughness of surface in the flow channel, which enhances the attraction of nanopore surface to H2O. As the structure destruction of nanopore surface deteriorates, the water density near the rough surface moves upward, whereas the velocity of water near the rough surface declines obviously. Interestingly, when the degree of surface structure destruction reaches 50%, a significant negative boundary slip near the rough surface appears.
Progress in Non-classical Microwave States Preparation Based on Cavity Optomechanical System
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As a novel hybrid quantum system, cavity optomechanical system shows super strong coupling strength, extremely low noise level under superconducting condition and considerable coherent time when quantum phenomena are operated on this platform. In this paper, we briefly introduce basic principles of cavity optomechanics and cavity optomechanical systems. Meanwhile, we also classify widely studied cavity optomechanical systems into five categories in aspect of their materials and structures. Furthermore, we introduce the progress in non-classical microwave quantum states preparation utilizing generalized cavity optomechanical systems in detail, and we also analyze performance merits and remaining problems of this preparation method. In the end, we summarize the application cases at present and look forward to the potential application scenarios in the future.
Theoretical Analysis of Wide-angle Metamaterial Absorbers Based on Equivalent Medium Theory
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In the past decade, most of researchers have been devoted to broadening the bandwidth of absorber. There are few literatures on how to achieve wide-angle absorbing materials with detailed theoretical analysis and design guidance. It is still difficult to design wide-angle absorbing materials. In this paper, based on the equivalent medium theory, the reflectivity of the metamaterial absorber with a single layer medium backed with metal reflector is analyzed in detail. Starting from the basic electromagnetic theory, the reflection coefficients of the absorber under TE and TM wave irradiation are derived. And the equivalent electromagnetic parameters of realizing the wide-angle absorbing effect are analyzed, which provide a theoretical basis for the design of wide-angle metamaterial absorber. In addition, this paper also theoretically analyzes the values of the equivalent electromagnetic parameters for ahcieving wide-band and wide-angle absorbing materials, and makes a theoretical verification. The results show that the wide-band and wide-angle absorbers can be achieved theoretically, while the equivalent electromagnetic parameters of the medium vary with frequency according to some special curves.
Review of Pedestrian Tracking: Algorithms andApplications
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Pedestrian tracking is a hotspot and a difficult point in computer vision research. Through the tracking of pedestrians in video materials, trajectories can be extracted to support the analysis of individual or collected behavior dynamics. In this review, we first discusses the difference between pedestrian tracking and pedestrian detection. Then we summarize the development of traditional tracking algorithms and deep learning-based tracking algorithms, and introduce classic pedestrian dynamic models. In the end, typical applications, including intelligent monitoring, congestion analysis, and anomaly detection are introduced systematically. With the rising use of big data and deep learning techniques in the area of computer vision, the research on pedestrian tracking has made a leap forward, which can support more accurate, timely extraction of behavior patterns and then to facilitate large-scale dynamic analysis of individual or crowd behavior.
Theoretical study on the thermodynamic properties of NO gas
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Nitric oxide (NO) is one of atmospheric interest molecules and has attracted considerable attention due to its important role in the chemical processes taking place in flow field of hypersonic vehicle, in which the thermodynamic properties are required in the calculation of the aerothermodynamic flow field. Moreover, the total internal partition function is key to the calculation of the thermodynamic properties of high-temperature gases. For diatomic molecules, according to the product approximation, the total internal partition function is split into three contributions: electronic, vibration and rotation partition functions. In this paper, by using the quantum statistical ensemble theory based on some classical thermodynamic and statistical formulae, the thermodynamic properties of NO are analyzed and discussed. Firstly, in order to obtain an accurate energy of molecule, the Variational Algebraic Method (VAM) is employed to carry out the full vibrational energies that are in good agreement with the experimental results and yield realistic predictions of the unobserved higher vibrational energies that converge to the dissociation limit. Secondly, an attempt has been made to use the full VAM vibrational energies, the Rydberg-Klein-Rees (RKR) vibrational energies, the Simple Harmonic Oscillator (SHO) model and the quantum-mechanical vibrational energies obtained by the Multiconfiguration Self-consistent-field (MCSCF) to calculate the vibrational partition function. Then, with the rotational contributions from the Müller- McDowell formula, the internal partition function can be determined by combining the product of electronic, vibration and rotation partition functions. Thirdly, according to the thermodynamic and statistical formulae, it is easy to calculate the internal energies, entropies and heat capacities for the NO molecule in the range of 1000-5000K. Comparison of different calculated heat capacities with the experimental ones reveals that the heat capacities of which vibrational contributions determined by the full VAM vibrational energies agree better with the experimental ones, with the maximum relative error being no more than 2.4%, whereas it can be seen that those thermodynamic results evaluated from the Simple Harmonic Oscillator (SHO) model attest to a failure for the summation of infinite vibrational energies. The thermodynamic results of NO may give proper applications in areas where cases can be of great importance in theoretical and(or) experimental aspects.
Exploring the roots of social gravity law
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Many spatial mobility of people, goods and information, such as human travel, population migration, commodity trade, information communication, social interaction and scientific cooperation, follow a law similar to Newton's law of universal gravitation. This law, named social gravity law, is that the flow between two locations is directly proportional to the product of the vitality of these two locations, and inversely proportional to a power function of their distance. The gravity model established by analogy with the gravity law has also been widely used to predict trip distribution, population migration, interregional trade flows and so on. But why many complex social systems have such a simple law? It is an interesting and valuable issue. This paper reviews the research on exploring the roots of the social gravity law from various perspectives, including statistical physics, microeconomics, and game theory.
Discrete modulation continuous-variable quantum key distribution based on quantum catalysis
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Compared to discrete variable quantum key distribution (DVQKD), continuous variable (CV) QKD has higher security bit rate and other advantages, but it is slightly insufficient in secure transmission distance. In addition, the application of quantum catalysis has significantly improved the performance of Gaussian modulated (GM) CVQKD, especially in secure transmission distance.Recently, the application of quantum catalysis has significantly improved the performance of GM- CVQKD. However, whether it can be used to improve the performance of discrete modulated (DM) CVQKD protocols is still ambiguous. Therefore, a scheme of DM CVQKD protocol based on quantum catalysis is proposed in this paper to further boost the performance of the proposed protocol in terms of secure key rate, secure transmission distance and maximum tolerable noise. Our results shows that under the same parameters, when the transmittance T introduced by quantum catalysis is optimized, the proposed scheme can effectively further improve the performance of QKD systems, compared with the original four-state modulation CVQKD scheme. In particular, when the tolerable excess noise is 0.002, the use of quantum catalysis can break the safe communication distance of 300 km with a key rate of 10-8 bits/pulse. However, if this noise is too large, the improvement effect of quantum catalysis on protocol performance will be restrained. In addition, in order to highlight the advantages of the use of quantum catalysis, the ultimate limit PLOB bound of point-to-point quantum communication is given in this paper. The simulation results indicate that although neither the original scheme nor the proposed scheme can break the bound, compared with the former, the latter can be closer to the boundary in long-distance transmission. These results provide theoretical basis for achieving the ultimate goal of global quantum security communication.
The application of electrical action for design and analysis of magnetically driven solid liner implosion
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As a typical cylindrical-convergent drive technique, magnetically driven solid liner implosion could compress interior substance with a shock or quasi-isentropic manner, which has been widely applied in hydrodynamic behavior, the dynamic characteristics of material and fusion energy and so on. For specific facility, the implosion parameters depend on material, radius and thickness of the liner, and the ablation of liner restrict the optional parameters. The concept of electrical action is introduced via thin shell model, which not only is the representation of states for conductive metal, but also indicates the change of liner velocity on the condition of thin shell hypothesis. The result shows that the outer velocity of liner increases linearly as electrical action and is directly proportional to liner thickness but inversely proportional to liner density. The incompressible zero-dimensional model is used to calculate the dynamic parameters of thin shell liner including the implosion time, the outer interface velocity, the implosion kinetic energy and the electrical action on the condition of low linear current density. There exist optimal radius and thickness which can achieve the maximum velocity, momentum, kinetic energy. The aluminum is suitable for reaching higher velocity and the copper can obtain higher pressure according to a proportionality coefficient Qb/ρ which is an intrinsic quality of metal. A one-dimensional elastic plastic magnetic hydrodynamic code which is called SOL1D is developed to simulate liner implosion behavior. The modified relationship between resistivity and electrical action is introduced to SOL1D, which can adapt higher hydrodynamic pressure. According to current waves, the one-dimensional code can be used to simulate liner implosion behavior for all kinds of current densities. The 1-D simulation liner velocity is agree with both the experimental results and the electrical action model for liner implosion experiment on FP-1 facility. Simulation of isentropic compression experiment at ZR facility shows that the magnetic diffusion process is suppressed at extra high current density and hydrodynamic pressure, and the electrical action is larger than the experimental value of wire electrical explosion. The 0-D and 1-D simulation show that estimating the liner velocity and liner phase changing via the electrical action are suitable when thin shell hypothesis and low current density assumption are satisfied.
Interaction between monodisperse fine particles in a standing wave acoustic field
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The external acoustic field can be used to promote the interactions between fine particles suspended in the gas phase. Due to the particle interaction, collision and agglomeration between fine particles occur, causing the average particle size to increase and the particle number concentration to decrease. This offers an important technical route to control the emissions of fine particles. However, the interaction behaviors between the fine particles under the acoustic field are still not well understood, which severely hinders the development of technology for fine particle emission control using acoustic agglomeration. In order to reveal the interaction between monodisperse fine particles in a standing wave acoustic field, a particle interaction model with consideration of the drag force, gravity and acoustic wake effect was developed. The particle motion equations in the model were solved using the classical Runge-Kutta method combined with the second-order implicit Adams interpolation method. The particle velocities due to acoustic entrainment and the interaction processes between particles obtained from the numerical simulation were compared with the corresponding analytical solutions and experimental results to validate the accuracy of model prediction. Good agreements were found, which indicates that the model and the numerical method are capable of accurately predicting the interaction between fine particles in the standing wave acoustic field. On this basis, the effects of initial conditions and diameters of particles on the interaction behaviors were explored. The results show that when the initial particle centerline is closer to the acoustic wave direction or the initial particle position is closer to the wave antinode, the acoustic wake effect between the particles becomes stronger, and shorter time is required for particles to collide. It is also found that the influence of particle diameter on particle interaction depends on the initial deviation of particle centerline from the acoustic wave direction. When the deviation is small, the larger the particle diameter, the shorter the time required for particles to collide. When the deviation is large, the collision between particles with smaller diameters occurs, while the collision between particles with larger diameters may not occur.
Coherence and path information
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Complementarity, or more specifically, the wave-particle duality, and quantum coherence are two fundamental concepts in quantum mechanics. Recently, motivated by the progress of the quantification of quantum coherence, the complementary relation between coherence and path information is investigated by many authors, and various duality relations between them are established. Such relations not only provide insights into fundamental problems of quantum mechanics, such as the understanding of quantum coherence and wave-particle duality; but also are important in applications of quantum technologies. In this paper, based on the Bures distance and unambiguous quantum state discrimination, systematic analysis of the complementarity between the quantum coherence and path information in two path interferometers is carried out. Similarly as other related works, the wave aspect, or the visibility of the interferometer, is quantified by the $l_1$-norm measure of quantum coherence, and the path information is considered via unambiguous quantum state discrimination. In this way, a novel duality relation in two path interferometers is obtained. Compared with known results, our work considers mixed states as well as pure states; considers the path predictability resulting from the intrinsic path asymmetry of the quantum state, as well as the path distinguishability resulting from the use of a which-path detector. Therefore, our work systematically generalizes known results in two path interferometers by removing all the unnecessary restrictions. Specifically, the most general form of quantum states in two path interferometers is considered and the duality relation between quantum coherence and path information is proved based on the positivity property of density matrices. The cases of path predictability and path distinguishability are considered separately. For path predictability, the proof is straightforward; whereas some advanced mathematical techniques, such as the Schur-Weyl inequality, properties of the fidelity and properties of positive matrices, are required in order to give a rigorous proof of the duality relation between coherence and path distinguishability. Concrete examples are provided to illustrate the abstract method and results. Our work concerns about two path interferometers exclusively and depends heavily that the dimensionality is two, therefore it would be an interesting task to generalize the results in this paper to $n$-path interferometers.
Numerical Simulation of Shear-thinning Droplet Impact on Surfaces with Different Wettability
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Controlling impact dynamics of droplet on solid surfaces is a significant problem in a variety of applications,such as inkjet printing,spray cooling and coating et al. Most fluids used in industries always contain various kinds of additives such as surfactants,polymers and particles,therefore these fluids exhibit non-Newtonian behaviors,including yield-stress,viscoelastic,shear-thickening and shear-thinning. The impact dynamics of Newtonian droplet on solid surfaces has been extensively investigated. However,the number of works about fluids with non-Newtonian properties is comparatively very small. In this work,the finite element scheme coupled with level set method was applied to simulate the impact process of droplet on solid surfaces. By introducing a limiting low shear rate to avoid an unrealistic high viscosity in very low shear rate regimes,a truncated power-law model was employed to express the shear-rate dependent viscosity. The numerical results are found to show good agreement with numerical and experiment data in the literature. By performing extensive numerical simulations varying the rheological parameters and surface wettability,the influence of these parameters on the impact dynamics are evaluated,and the dominant effects that govern the spreading and receding process are determined. The results show that,for droplet impact on hydrophilic surface,the spreading became stronger with decreasing power-law index as evidenced by larger shape deformation and faster interface moving speed. As m decreases,we expect an increase in the maximum dimensionless diameter and a decrease in the minimum dimensionless height during inertial spreading. For droplet with lower power-law index (m=0.85 and 0.80),which indicates lower viscous dissipation during impact,the dimensionless parameters have significant differences. After first receding,the impacting droplet is not balanced yet and it starts to spread again until its kinetic energy is completely damped by fluid viscous dissipation. For case of droplet (m=0.80) impact on hydrophilic surface,center breakage can be observed during droplet spreading,which results from the effect of strong shear-thinning property. When a shear-thinng droplet impact on hydrophobic surface,the oscillation behavior can be observed and the oscillation amplitude increases as the power law index decreases. Bouncing phenomenon can be observed when a droplet impact on superhydrophobic surface,regardless of fluid property. Finally,an empirical model was proposed to predict the maximum dimensionless diameter of shear-thinning droplet impact on hydrophilic surface as a function of nondimensional parameters.
A Symmetrical Wedge-To-Wedge THz Hybrid SPPs Waveguide with Low Propagation Loss
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A symmetrical wedge-to-wedge THz hybrid SPPs waveguide (WWTHSW) with low propagation loss is investigated. The WWTHSW consists of two identical dielectric wedge waveguides symmetrically placed on each side of a micro wedge-patterned thin metal film. The mode characteristics of the WWTHSW, such as the propagation length (Lp), the normalized effective mode area (A) and the figure of merit (FOM) are analyzed by using the Finite Element Method (FEM). Firstly, the influences of the height of Si micro wedge waveguide (H) and the gap between two wedges (g) on Lp and A are studied. For the same g, A first decreases and then increases with the increase of H. A achieves a minimum at an H of ~40 μm. However, Lp monotonically increases as H increases. The change of Lp slows down when H is greater than 40 μm. At a fixed H, Lp slightly increases with the increase of g. But A achieves a minimum when g is ~50 nm. Secondly, the dependencies of the mode
Review of the research on nano-structure used as light harvesting in perovskite solar cells
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In recent years, nano-structures used as light harvesting have been widely used in perovskite cells to enhance the photon absorption of cells. The introduction of trapping structures in perovskite cells can change the photon propagation in the cell and the photon energy absorbed by the cell. The nano-structure used in different interfaces of perovskite cells can increase the absorption of light by the device to different degrees, and ultimately improve the efficiency of the solar cell. Therefore, the effective light trapping structure has become trending in the application of perovskite cells. How to effectively apply the such nano-structure is the key to improve the power conversion efficiency(PCE) of perovskite cells. This paper starts from the description of the perovskite cell with different nano-structures, comparing and summarizing the different structures, and analyzes the advantages and disadvantages.
Macroscopic basic characteristics of a road network under the influence of traffic generation and attraction source agglomeration
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The macroscopic fundamental diagram (MFD), which can describe the macroscopic state of a regional network intuitively, describes a unimodal, statistical and reproducible relationship between accumulation and the trip completion flow of a region. Existing researches have proved that using MFD characteristics can ‘metering’ the boundary flow and relieve the traffic congestion problem effectively. As the foundation of traffic control, existing studies on the characteristic of MFD have proved that external origin to destination demand does not influence the MFD distribution. However, the traffic generation and attraction sources in the regional road network will changes the distribution of traffic density of the road network, thus affecting the characteristic of MFD. However, to date, no related analysis explored the influence of the traffic generation and attraction sources in the regional road network. To solve this problem, according to the spatial and temporal distribution of traffic generation and attraction sources in the regional road network, this paper puts forward an aggregation degree index analysis model of traffic generation and attraction sources, based on the dynamic parameters of traffic generation and attraction sources, and section traffic impedance. Taking a square-format road network as the target, nine groups of schemes for different traffic generation and attraction sources are designed. Two conclusions can be drawn after comparing the MFD curve of the road network under different aggregation degree index of traffic generation and attraction sources: (1) when the traffic flow of the road network is in the state of free flow or critical flow, the aggregation effect of traffic generation and attraction sources has no effect on MFD distribution; (2) when the traffic flow of the road network is in the congestion flow state, the aggregation effect of traffic generation and attraction sources influences MFD distribution. Moreover, under the same road network flow conditions, the aggregation degree of traffic generation and attraction sources is lower in the road network (the distribution of traffic generation and attraction volume is more balanced), the trip completion flow of road network will be higher. Otherwise, the aggregation degree of traffic generation and attraction sources is higher in the road network (the
The Enhanced Optical Absorption of Graphene by Plasmon
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The plasmons in grapheme has the superior properties than metal surface plasmons, such as high field confinement, low Ohmic loss and long wave propagation, highly tunable via electrostatic. More importantly, the frequency of plasmons range from terahertz to infrared which indicates graphene an ideal candidate for terahertz plamsonics. On the other hand, the strong coupling between incident photons and plasmons in grapheme can lead to the enhancement of optical absorption. However, it is difficult for light couple directly with plasmons in grapheme for that the momentum of incident photons can not match the plasmons in grapheme. A metal grating can be used to compensate the momentum of photons so that it can match that of plasmons in grapheme. In this work, we theoretically investigate the effect of plasmons on the terahertz optical absorption of graphene with grating based on Finite Difference Time Domain. A great enhancement of electric field component of light field can be obtained near the gold grating strip in the sheet of graphene. Thus, the photons, of which the momentum is compensated by the grating, can strongly couple with plasmons in graphene. An obviously decrease of the transmission of the graphene structure can be seen at the resonant frequency. The transmission peak corresponds to the resonant frequency splits into two peaks due to that two plasmon polariton modes formed by the coupling of photons and palsmons. So we also studied the plasmon polariton modes made by the coupling of photon and palsmon based on the many body self-consistent method. Two plasmon polariton modes are obtained and an obviously splitting at the resonant frequency can be seen due to the coupling between photons and plasmons. The work is pertinent to help gaining deep understanding of the photoelectric properties of graphene and to the terahertz plasmonics based on graphene.
Numerical simulation of beam deflection for smoothed laser beams
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When it reaches high energy density state, new features of laser propagation in plasma arises in the contrast to that of research field in classical optics. Such as beam deflection, a laser beam can change its propagation direction while it comes across a transverse plasma flow. On the other hand, employment of all sorts of smoothed laser beams becomes very common in high power laser facilities for high energy density physics experiments. Therefore, on what condition beam deflection comes into play for smoothed beams are necessary to be investigated. This paper presents numerical simulation results for that, which is performed by laser plasma interaction code LAP3D. It is a three dimensional massively parallel code, including a laser paraxial envelope solver and a nonlinear Eulerian hydrodynamics package, and models for filamentation, stimulated Raman scattering and stimulated Brillouin scattering, with beam smoothed by continuous phase plate (CPP), spectral
Dense Light Field Reconstruction Algorithm Based on Dictionary Learning
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The camera array is an important tools to obtain the light field of targets in space. The method of obtaining high angular resolution light field by large-scale dense camera array increases the difficulty of sampling and the cost of equipments. At the same time, the demand of synchronization and transmission of a large number of data also limits the sampling rate of light field. In order to complete the dense reconstruction of sparse sampling of light field, we analyze the correlation and redundancy of multi view images in the same scene based on sparse light field data, then establishe an effective mathematical model of light field dictionary learning and sparse coding. The trained light field atoms can sparsly express the local spatial-angular consistency of light field, The 4D light field patches can be reconstructed from a 2D local image pacth centered around each pixel in the sensor. The global and local constraints of the four-dimensional light field are mapped into the low-dimensional space by the dictionary. These constraints are shown as the sparsity of each vector in the sparse representation domain, the constraints between the positions of non-zero elements and their values. According to the constraints among sparse encoding elements, we establishe the sparse encoding recovering model of virtual angular image, and propose the sparse encoding recovering method in transform domain. The atoms of light field in dictionary are screened and the patches of light field are represented linearly by the sparse representation matrix of the virtual angular image. At the end, the virtual angular images are constructed by image fusion after sparse inverse transform. According to multi scene dense reconstruction experiments, the effectiveness of the proposed method is verified. The experimental results show that the proposed method can recover the occlusion, shadow and complex illumination in satisfying quality. That is to say, it can be used for dense reconstruction of sparse light field in complex scenes. In our study, dense reconstruction of linear sparse light field is achieved. In the future, dense reconstruction of nonlinear sparse light field will be studied to promote the application of light field imaging in practical.
Molecular dynamics study on structural characteristics of Lennard-Jones supercritical fluids
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Supercritical fluids (SCF) have been widely utilized in the industrial processes, such as extraction, cleaning, drying, foaming and power generation driven by primary energy. Therefore, SCF have attracted more and more attention in recent years. At supercritical state, liquid, and gas phase are not clearly distinguished, but the thermal-physical properties of fluid show an interesting characteristic, especially near the pseudo-critical temperature. Thus, it is of great significant to study the structure and density time series evolution of SCF. Due to high pressure and temperature for SCF, it can be challenging to collect experimental data of SCF. However, the advantage of molecular dynamics simulation in convenience, safty and cost over experiments. Therefore, in this paper, molecular dynamics simulation was performed to investigate the fluid structure and density series fluctuation curves at supercritical state, and the influence of parameters varitation including pressure and temperature on structural characteristics was analyzed. In the simulation system, more than 104 atoms and simple Lennard-Jones (LJ) supercritical fluids were contained. The radial distribution function (RDF), coordination number (CN), density time series curve and permutation entropy of fluids at different pressures and temperatures were calculated. At specified pressure, the position of the first peak value of RDF gradually moves to the right with the increase of temperature, and the trend weakens with the increase of pressure. CN shows a downward trend with the increase of pressure and the CN difference at different temperatures gradually decreases. Simultaneously, the CN distribution area becomes narrow with the increase of pressure. The high/low density region calibrated by CN is stable, concentrated and large area distribution at low pressure, and the average density region is small, with the increase of pressure, the area of high/low density region is only a size of a few molecular and fluctuates sharply with time, and the area of average region is constantly expanding. At relatively low pressure, the density time series curve shows the characteristic that both the fluctuation range and quasi-period are large at pseudo-critical temperature. Simultaneously, the permutation entropy obtained from the time series curve shows three cases: (i) at low pressure ( ), the minimum permutation entropy is obtained under the temperature that is lower than pseudo-critical temperature, and the system has higher orderliness; (ii) at moderate pressure ( and ), the state points corresponding to minimum permutation entropy is consistent with that corresponding to the maximum of isothermal compression coefficient and (iii) at high pressure ( ), the permutation entropy curve fluctuates slightly and remains basically on the horizontal line. The results provide reliable support for revealing the characteristics of SCF from the microscale, and also provide useful inspiration for the practical application of SCF.
Pulsar candidate selection based on self-normalizing neural networks
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Pulsar candidate selection is an important step in the pulsars search task. The traditional candidate selection is heavily dependent on human inspection. However, the human inspection is a subjective, time consuming, and error prone process. One modern radio telescopes pulsar survey project can produce totally millions of candidates, so the manual selection becomes extremely difficult and inefficient due to the large amount of candidates. Therefore, this study has focused on machine learning in recent years. In order to improve the efficiency of pulsar candidates selection, we propose a candidate selection method based on self-normalizing neural networks. This method uses three techniques: self-normalizing neural networks, genetic algorithm and synthetic minority over-sampling technique. Self-normalizing neural networks can improve the identification accuracy by applying deep neural networks to pulsar candidate selection. At the same time, it overcomes the problem of gradient disappearance and explosion in the training process of deep neural networks by using its self-normalizing property, which greatly accelerates the training process. In addition, in order to eliminate the redundancy of the sample data, we use genetic algorithm to choose sample features of pulsar candidates. The genetic algorithm for feature selection can be summarized into three steps: initializing the population, assessing population fitness, and generating new populations. Decoding the individual with the largest fitness value in the last generation population, we can get the best subset of features. Due to radio frequency interference or noise, there are a large number of non-pulsar signals in candidates, and only a few real pulsar signals. Aiming at solving the severe class imbalance problem, we use the synthetic minority over-sampling technique to increase the pulsar candidates (minority class) and reduce the imbalance degree of data. By using k-nearest neighbor and linear interpolation to insert a new sample between two minority classes of samples that are close to each other according to certain rules, we can prevent the classifier from becoming biased towards the abundant non-pulsar class (majority class). Experimental results on three pulsar candidate datasets show that the self-normalizing neural network has higher accuracy and faster convergence speed than the traditional artificial neural network in the deep structure, By using genetic algorithm and synthetic minority over-sampling technique can effectively improve the selection performance of pulsar candidates.
Comparing Rank Aggregation Alrogithms in Aggregating a Small Number of Long Rank Lists
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Rank aggregation aims to combine multiple rank lists into a single one, which has wide applications in recommender systems, link prediction, metasearch, proposal selection, and more. Some existing works have summarized and compared different rank aggregation algorithms, but most of them cover only a few algorithms, the data used to test algorithm does not have a clear statistical property, and the metric used to quantify aggregated results has certain limitations. Moreover, while different algorithms all claim to be superior to existing ones when proposed, the baseline algorithms, the testing samples, and the application scenario are all different from case to case. Therefore, it is still unclear which algorithm is better for a certain task. Here we review 9 rank aggregation algorithms and compare their performance in aggregating a small number of long rank lists. We propose an algorithm to generate different types of rank lists with known statistical properties and apply a more reliable metric to quantify the aggregation results. We find that despite the simplicity of heuristic algorithms, they work pretty well when the rank lists are full and with high similarities. In some cases, they can reach or even surpass the performance of optimization-based algorithms. The number of ties in the list will decrease the quality of the consensus rank and increase fluctuations. The quality of aggregated rank changes non-monotonically with the number of rank lists that need to combine. Overall, the algorithm FAST outperforms all others in 3 different rank types, which can sufficiently complete the task of aggregating a small number of long rank lists.
Monitoring of ambient methane and carbon dioxide concentrations based on wavelength modulation - direct absorption spectroscopy (WM-DAS)
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Wavelength modulation - direct absorption spectroscopy (WM-DAS) combines the advantages of measuring absolute absorbance profile from calibration-free direct absorption spectrum (DAS) with the enhanced noise rejection and high sensitivity of wavelength modulation spectrum (WMS). This method can be used to precisely recover the crucial absorbance profile via extraction of the characteristic frequencies of the modulated transmitted light. In this paper, the WM-DAS method with non-calibration and high signal-to-noise ratio is combined with a Herriott cell (about 128m). Under the condition of atmospheric pressure and room temperature, the absorptance functions of two spectral lines of CO2 (6330.821 cm-1) and CH4 (6046.964 cm-1) in air were measured, and the standard deviation of spectral fitting residual was 5.6 × 10-5 and 7 × 10-5, respectively. Subsequently, the concentrations of CO2 and CH4 in air were monitored on-line by WM-DAS method combined with the Herriott cell, and compared with the highly sensitive continuous wave cavity ring down spectroscopy (CW-CRDS). The experimental results showed that the measured results of the long optical path WM-DAS method were consistent with those of the CW-CRDS method, and the linear correlations between the two methods were above 0.99. The detection limits of CO2 and CH4 based on WM-DAS method were 170ppb and 1.5ppb respectively, which is slightly higher than those of CW-CRDS. However, the measurement speed of WM-DAS is much higher than that of CW-CRDS, and has the advantages of simpler operation, lower environmental requirements, long-term stability and so on.
Research on microstructure and thermoelectric property of (Bi1-xTbx)2(Te0.9Se0.1)3 fabricated by high pressure sintering technique
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n-type Bi2Te2.7Se0.3 nanocrystalline bulk materials doped with Tb were fabricated by high pressure sintering (HPS) technique. The HPS samples were then annealed for 36 h in a vacuum at 633 K. The effects of Tb contents on crystal structure and thermoelectric properties of the samples were studied. The results show that HPS samples are consisted of nanoparticles. For the HPS samples with Tb doping, the cell volume bigger. Besides, the power factor increase but thermal conductivity decrease through Tb doping, thus the ZT value increase. Tb doping amount of x=0.004 is the optimal doping amount. The maximum thermoelectric figure of merit (ZT) of 0.99 is achieved for annealed sample with x = 0.004 at 373 K. Furthermore, it is worthwhile to note that this annealed sample possesses a ZT value larger than 0.8 when the temperature is higher than 323K. These results can be applied to the aspect of thermoelectric power generation.
Study on AlGaN/GaN HEMT devices ionizing radiation damage mechanism and bias correlation
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In this paper, an experimental study on the total dose effect of 60Co γ ionization irradiation on AlGaN/GaN high electron mobility transistor (HEMT) devices under different biases was carried out. The experimental results were measured and analyzed by 1/f noise combined with DC electrical characteristic parameters. The analysis results showed that, influenced by the irradiation induced oxide trap charge and the interface state, when the total irradiation dose reached 1 Mrad(Si), the
Phase Transition, magnetic properties and exchange bias of Heusler alloy Mn50-xCrxNi42Sn8
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In this paper, phase transition, magnetic properties and exchange bias of Mn50-xCrxNi42Sn8 (x=0,0.4,0.6,0.8) polycrystalline samples have been investigated. It is found that all the alloys have non-modulated tetragonal martensite structure at room temperature. The transformation temperature decreases with the increase of Cr content. The M-T curves under the effect of 20 kOe show that the magnetic properties of these alloys are weak. The maximum magnetization difference between martensite and austenite phases is ?M=7.61 emu/g. The change of magnetic properties is mainly related to the change of Mn-Mn distance and the hybridization strength between Ni (A) -Mn (D). At low temperatures, the magnetization of the martensite phase increases with the increase of Cr content. The exchange bias field of up to 2624 Oe was observed in Mn50Ni42Sn8 alloy after cooling from room temperature to 5 K in 500 Oe magnetic field. Along with the increase of Cr content, the exchange bias field decreases gradually. When the content of Cr is x=0.8, the exchange bias field increases first and then followed by a decrease with the increase of the cooling field. The exchange bias field is the largest when the cooling field is 500 Oe. This is mainly attributed to the change of the interface exchange coupling between the spin glass state and antiferromagnetic region. The results show that these alloys have potential applications in magnetic memory devices and spin valves.
Experimental Research on the Influence of Turbine Guide Vane on the Propagation Characteristics of Rotating Detonation Wave
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Detonation is a combustion wave driven by an exothermic reaction that is sustained by a shock wave. Compared with deflagration, detonation has higher thermodynamic efficiency and faster heat release rate. The traditional propulsion system based on the isostatic combustion has been relatively mature, and it is very difficult to improve its performance substantially. Therefore, the use of detonation for propulsion is expected to greatly improve the performance of the propulsion system. Replacing the isobaric combustion chamber of turbojet engine with rotating detonation combustor can not only improve the combustion chamber efficiency, but also reduce the number of compressor stages, reduce the weight of engine and simplify the structure of engine. In order to study the operation characteristics of rotating detonation combustor with a turbine guide vane, a series of experiments are conducted with hydrogen as fuel and air as oxidant at different equivalence ratios. Based on the signals of high frequency pressure sensor and static pressure sensor, the operation mode of rotating detonation combustor with a turbine guide vane and the effect of a turbine guide vane on the non-uniform and unstable detonation products are analyzed in detail. The experimental results show that when the equivalent ratio is low, the rotating detonation combustor operates in a rapid deflagration mode and the pressure of combustion wave is about 0.6bar. The rotating detonation combustor begins to operate in an unstable rotating detonation mode when the equivalence ratio increases to 0.6, the pressure of detonation wave is about 6bar. The rotating detonation combustor operates in a stable rotating detonation mode when the equivalence ratio reaches 0.88, the pressure of detonation wave is about 16.3bar which is far greater than the pressure of rapid deflagration mode. In addition, the propagation velocity gradually increases and stability improves with the increase of the equivalence ratio. The oblique shock wave downstream of detonation wave interacts with the turbine guide vane. Part of the shock waves will be reflected back to combustor, which causes some small pressure fluctuations in combustor. The turbine guide vane can obviously suppress the amplitude of pressure oscillation, but has little effect on the frequency of pressure oscillation. With the increase of equivalence ratio, the static pressure of both upstream and downstream of the turbine guide vane increases simultaneously, and the static pressure of combustion products decreases obviously after passing through the turbine guide vane.
Ground state cooling of the mechanical resonator in a double optical cavity
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The ground state cooling of mechanical resonator is one of the fundamental problems in cavity quantum photomechanics. The ground state cooling is to make the number of steady-state phonons of the mechanical resonator less than one. In this paper, we first propose an electromagnetically-induced-transparency-like cooling mechanism in a double-cavity optomechanical system to cool a mechanical resonator. In the double-optical cavity optomechanical system, the right additional cavity, which is directly coupled to the standard optomechanical system, contains an ultra-cold two-level atomic ensemble. By selecting the optimal parameters to meet that the cooling process of the mechanical resonator corresponds to the maximum value of the optical fluctuation spectrum and the heating process of the mechanical resonator corresponds to the minimum value of the optical fluctuation spectrum, the mechanical resonator can be cooled by observing the phonon number. We also give the atomic additional cavity’s effects on the quantum Langevin equations and optical fluctuation spectrum. We find the atomic double-cavity system may have a good ground-state cooling than the double-cavity in certain parameters. To date, the researchers have proposed a number of theoretical cooling schemes in order to achieve ground-state cooling of mechanical resonator. As far as know, the sideband cooling for just a standard optomechanical system is most famous scheme and the mechanical resonator is coupled to the optical field via radiation pressure force. By means of quantum theory of mechanical resonator’s sideband cooling, the optical fluctuation spectrum determines the transition rates of both cooling and heating processes of the mechanical resonator. That’s to say that the optical fluctuation spectrum at the mechanical resonator frequency ω_m is corresponding to the cooling transition, whereas the optical fluctuation spectrum at 〖-ω〗_m is corresponding to the heating transition. The physical interpretation is respectively for anti-Stokes and Stokes effects. Under resolvable sideband conditions, the optical field’s decay rate (the half-width of the single Lorentzian peak of optical fluctuation spectrum) is less than the frequency of the mechanical resonator. So, the ground-state cooling of the mechanical resonator can be obtained by making the maximum and minimum value of the optical fluctuation spectrum respectively correspond to the cooling anti-Stokes process and heating Stokes process. In this paper, we mainly observe the electromagnetically-induced-transparency-like ground-state cooling in a double-cavity optomechanical system with an ensemble of two-level atoms. By adjusting the maximum and minimum value of the optical fluctuation spectrum in the position of ω=ω_m and ω=-ω_m, the mechanical resonator could be cooled to be closed to the ground state. Even when there exists an ensemble of two-level atoms in the right additional cavity, the mechanical resonator could be better cooled than just a cavity. These results may benefit the ground-state cooling of the mechanical resonator in future experiment.
Research on spatial resolution of a novel liquid scintillating capillaries array
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Scintillating array image plates allow for high resolution through a thicker detector which increases quantum efficiency without degrading the imaging resolution substantially. Due to limitations imposed by process capability, scintillator fiber array with pixel diameter less than 0.2 mm is hardly manufactured to improve performance. Therefore, a liquid scintillator capillaries array with 0.1 mm pixel was developed to improve the detection efficiency and spatial resolution of image plate for low intensity radiation imaging. Its performances were studied and tested by simulation and experiment, and were compared with that of scintillating fiber array. Especially in order to gain high fidelity representation of modulation transfer function of these array image plates, a method of slanted knife edge response simulation and measurement and iterative algorithm was introduced. For 14 MeV neutron and 1.25 MeV gamma, the slanted knife edge response of these array image plates with pixel dimension from 0.1 mm to 0.5 mm was respectively simulated by MCNPx program and the modulation transfer functions(MTFs) were obtained. Simulation results show that compared with scintillating fiber array, the liquid scintillator capillaries array has obvious merit on spatial resolution because of greater stopping power for secondary charged particle in the capillary quartz glass wall with 0.02 mm thick. Its ultimate resolution can reach to 1.8 lp/mm for 14 MeV neutron by simulation. At the 4000 Ci 60Co facility, A 5 cm thick tungsten bar, one side of it had a curvature with 0.1 radian to minimize misalignment effects, was made as a knife edge. MTFs of the scintillating fiber array with 0.3 mm and 0.5 mm pixel and newly developed liquid scintillator capillaries array were measured through this tungsten knife edge. Experimental measurement results have also verified that liquid scintillator capillaries array performs well on spatial resolution and luminescent uniformity for 1.25 MeV gamma. The ultimate spatial resolution, 0.9 lp/mm was gained, and that of other scintillating fiber arrays was less than 0.5 lp/mm. Moreover, experimental test validated the simulating method and simulated results, although the measured value was slight less than the simulated value because of the effect of dimension of 60Co source.
Transient Characteristics of Electron Beam Induced Current in Dielectric/Semiconductor Samples
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The electron beam induced current (EBIC) characteristics of dielectric/semiconductor thin films under the electron beam (e-beam) irradiation is the important means in the electron microscopic detection. The transient EBIC characteristics of the SiO2/Si thin film irradiated by high-energy e-beam are investigated, by combining the numerical simulation and the experimental measurement. The scattering process of electrons is simulated by the Rutherford scattering model and the fast secondary electron model, and the charge transport, trapping and the recombination processes are calculated by the current continuity equation and the Poisson equation. The transient charge distribution, EBIC and the transmission current are obtained, and influences of the beam current and the beam energy on them are analyzed. The results show that, due to the electron scattering effect, the free electron density decreases gradually along the incident direction. The net charge density near the surface is positive and negative along the incident direction because of secondary electrons (SEs) emission from the surface, and therefore the electric field intensity is positive near the surface and negative inside sample, which causes some electrons transport to the substrate and some SEs return to the surface. The negative charge density at the SiO2/Si interface is higher than that in the nearby region because some electrons are trapped by the interface trap. With the decrease of the net charge density with e-beam irradiation, the charging intensity decreases gradually. Meanwhile, electrons gradually transport to the substrate, and consequently EBIC and the sample current increase and the electric field intensity decreases with e-beam irradiation. However, due to the weak charging intensity, the surface emission current and the transmission current remain almost invariant with e-beam irradiation. The EBIC, the transmission current and the surface emission current are approximately proportional to the beam current. For the SiO2/Si thin film in this work, the transmission current increases gradually to the beam current value with the increase of the beam energy, and the EBIC presents a maximum at the beam energy of about 15 keV.
A control method of adaptive optical system combined with image restoration technology
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In the field of astronomical high resolution imaging, adaptive optical correction and image restoration are necessary, and these two techniques can be used either alone or in combination to improve the quality of observed images. However, for a long time, adaptive optics and image restoration technology have been developing independently because they belong to different research fields, and even the combined approach is a simple splicing of the two technologies, with no crossover between the two. Such control method results in adaptive optical correction producing only the best possible intermediate result—optical imaging, but is out of control for the final result—restored image. Therefore, it is of great significance to study a control method that combines the two methods in order to obtain the high quality restored image. In this paper, the traditional hybrid method (adaptive optics + image post-deconvolution) is analyzed and its defects are expounded. The idea of combining adaptive optics and image restoration for system analysis is proposed for the first time, and the concept of correction degree of deformable mirror (the scaling ratio of control voltage of deformable mirror relative to that of traditional control voltage) is proposed. By changing the degree of correction, the correction residual of the deformation mirror and the detection error of the wavefront sensor can be adjusted. It is proved that there exists an optimal value of the quality of the reconstructed image in the direction of decreasing correction degree, and a new control method is obtained by using the optimal correction degree to correct the control voltage of the deformation mirror. For the application in point target imaging, the simulation is carried out with 37-element and 61-element deformable mirrors under several typical wavefront aberrations, and the results show that this method can get better restoration image than traditional methods. This method has more application potential for adaptive optical systems with large fitting residuals. The idea, which considers adaptive optics and image restoration as a whole, has not been reported in the literature before, so the work of this paper provides a new way of thinking for the research in related fields.
Theoretical studies on the driving-responding relationship for opinion particles in a bistable potential field
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From the perspective of physics, evolution of group opinion can be regarded as the collective effect of the state change of view particles. In this paper, we calculate the time correlation function and the relaxation time descripting driveresponse relationship by the Laguerre complete orthogonal function method and investigate the state transition of noise-induced opinion particles in bistable potential.The results indicate that there is a critical value Dc for noise intensity.When the noise intensity is greater than Dc, the time correlation function will increase exponentially with correlation time τ . There also are two points at which the dependence of the relaxation time on the noise intensity/aspect ratio of the energy barrier are divergent. The divergence implies that the state transition of opinion particles can not be achieved. The linear relationship between relaxation time that and that between coefficient C and aspect ratio of energy barrier means that the driving-responding relationship for opinion particles in the bistable potential field just like the Newton’s second law, in which the time relaxation plays the role of quasi-inertia.
Fabrication and Ultraviolet-Visible-Near Infrared Absorption Properties of Silver Nano arrays based on Aluminum
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Highly ordered periodic silver nanosphere arrays were fabricated by vacuum evaporation based on anodic aluminum oxide (AAO) template. Different sizes of silver nanosphere arrays were adjusted just by controlling the thickness of evaporation, which can modulate effectively absorption peaks and bandwidths in ultraviolet-visible-near infrared (UV-Vis-NIR) regions. The spectra measures show that nano-arrays have obvious electromagnetic wave absorption characteristics in the ultraviolet, visible and near infrared bands, respectively. On the basis of this, FDTD (finite-difference time-domain) theoretical simulation combined with experiments to analyze the physical mechanism of light absorption characteristics at different wavebands, the ultraviolet strong absorption is Fano resonance induced by asymmetric dielectric environment of silver and aluminum; visible absorption originates from local surface Plasmon resonance of silver nanoparticles and near infrared strong absorption is surface lattice resonance of silver nano sphere arrays.
Effect of buoyancy and acceleration for heat transfer to supercritical fuids flowing in tubes
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Based on the concept of single?phase fluid, the abnormal heat transfer behavior of supercritical fluid has been investigated for many years. However, there is no unified cognition about the mechanism of flow and heat transfer. This paper firstly reviews the public literatures on the investigations of buoyancy and acceleration effect on supercritical fluids, and examines the effect of buoyancy and acceleration on the flow structure and heat transfer through a heated tube upward vertically by theoretical analysis and experiments. Results show that there is no conclusive experimental evidence that the abnormal heat transfer behavior of supercritical fluid is directly related to buoyancy and flow acceleration, and the existing criterias for estimating buoyancy and acceleration effect are based on the constant physical fluid and a lot of assumptions are draw, and researchers drawn different conclusions using the same predict method. Finally, we investigate heat transfer deterioration of supercritical fluids based on pseudo-boiling theory, and supercritical -boiling-number distinguish the normal heat transfer (NHT) and heat transfer deterioration (HTD) of supercritical fluid. Our work paves a new way to understand the supercritical fluids flow and heat transfer mechanism. The supercritical-boiling-number is important for establishing the optimum operating conditions for power cycles using supercritical fluids for different technologies.
A channel thermal noise model of the nanoscale Metal-Oxide-Semiconductor Field-Effect Transistor
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With the development of the integrated circuit manufacturing process, the device dimensions have been on the nanoscale, while the device performance, such as the mobility and thermal noise, is significantly affected by the hot-carrier effect, which further affects the channel thermal noise of the device. However, the thermal noise model based on the existing electron temperature does not take into account the impact of the temperature gradient on the electron temperature when it deals with the impact of the hot carrier effect. As the size of the device decreases, the thermal noise model based on the existing electron temperature underestimates the impact of the hot carrier effect and the channel thermal noise cannot be accurately predicted with this . In this paper, the of the channel transverse electric field is derived based on the channel potential equation and the boundary condition of the channel electric field. By combining the distribution of the temperature gradient and the of the transverse electric field, the energy balance equation is solved considering the impact of the temperature gradient, and then the electron temperature is obtained. The electron temperature shows the distribution of the electron temperature along the channel. By utilizing the derived electron temperature with the drain current , a channel thermal noise model is established. The hot carrier effect is taken into account in the thermal noise model by utilizing the proposed electron temperature . Meanwhile in calculating the thermal noise, the impact of the electron temperature on mobility degradation and the influence of the temperature gradient on thermal noise are also involved. The results show that the temperature gradient has a significant impact on the electron temperature with the reduction of the device size, which further increases the impact of the hot carrier effect, resulting in the increase of the thermal noise caused by the hot carrier effect exceeding the decrease of the thermal noise caused by the mobility degradation, thus ultimately leading to an increase in thermal noise. The impact of the hot carrier effect on the channel thermal noise also increases significantly with the increase of the bias. The channel thermal noise model proposed in this paper can be applied to the noise performance analysis and modeling of nano-sized MOSFET devices.
Experiments and analytical solutions of light driven flow in nanofluid droplets
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Adding nanoparticles with high light response characteristics to a light-transmitting fluid medium can form a light-driven nanofluid and achieve efficient use of light energy. This paper conducts experimental observation and theoretical analysis of the optical drive nanofluid flow behavior, which is the theoretical basis for achieving precise control of optical drive nanofluid. To realize the efficient conversion of light energy into kinetic energy, here, the motion of Fe3O4 particles with a diameter of 300nm in droplets induced by the Marangoni effect is studied under different light sources by using the particle image velocimetry (PIV). The experimental results show that when the number density of particles is higher than the critical value, the vertical vortices with symmetrical structure can be induced. At the bottom of the droplet, the particles move from the edge of droplet to the center, and at the top of the droplet, the particles move from the center to the edge of droplet. In addition, the frequency of light source and the number density of particles are the dominant factors in this process. Subsequently, for the optical drive nanofluid experiment in this paper, the analytical solution of the flow field distribution is achieved by using the Stokes equation and the surface tension gradient boundary conditions. The analytical solution of the flow field distribution obtained here is consistent with the experimental results, confirming the validity of the quantitative theory. Finally, the correlation between driving modes that introduce surface tension and other types of driving modes is discussed. This research provides theoretical support for the precise regulation of flow behavior and efficient conversion of light energy in the optical microfluidic system.
The spring oscillator model degenerated into the coupled-mode theory by using secular perturbation theory
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In the past few decades, although coupled-mode theory (CMT) has been extensively studied in quantum system, atomic system, plasmon system, circuit system, and so on, the theoretical origin is still plaguing many researchers. In the book of waves and fields in optoelectronics, the second-order differential equations of the simplest LC simple harmonic vibration circuit was turned into the first-order differential equation using the method of variable substitution by Haus. However, there is not loss in the simplest LC simple harmonic vibration circuit, loss term is introduced by qualitative analysis. Although this method of dealing with problems has no problems from a physical point of view, it is not rigorous enough from a mathematical point of view. In this paper, based on the secular perturbation theory, the well-known spring oscillator model is degenerated into two-mode CMT. Starting from the second-order differential equations of the spring oscillator model, the secular perturbation theory is used to obtain first order differential equations of two-mode CMT. The results show the relationships between each term’s coefficients in two-mode CMT and the physical quantities in Classical Mechanics are established by using the secular perturbation theory. Through solving two-mode coupled-mode equations, the energy transfer efficiency has been obtained. To verify the correctness of two-mode CMT, we design a coupled tuning fork mechanical vibration system, which consists of two experimental instruments to provide driving force and receive signals, two tuning forks and springs. The amplitude spectra are measured by an experimental instrument of forced vibration and resonance (HZDH4615), which provides a periodic driving signal for the tuning fork. To clarify the mechanism of the spectra, the numerical fitting has been performed by mathematica software based on the energy transfer efficiency. Theoretically, the obtained fitting parameters can also evaluate some important attributes of the system. The theoretical results are in close correspondence with the experiment. That is to say, two-mode CMT is suitable for classical vibration system. This study provides a more rigorous derivation for each term’s origin in two-mode CMT, and has guiding significance in the theoretical research of linear coupled vibration system.
Design of polarization-insensitive 1×2 multimode interference demultiplexer based on Si3N4/SiNx/Si3N4 sandwiched structure
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An ultra-compact 1*2 demultiplexer based on multimode interference (MMI) waveguide is proposed to separate the 1310nm and 1550nm wavelengths, which uses Si3N4/SiNx/Si3N4 sandwiched structure to realize polarization insensitivity. Firstly, how to use Si3N4/SiNx/Si3N4 sandwich structure to achieve polarization-independent is discussed. Keeping the width of MMI waveguide unchanged, the beat lengths of two orthogonal polarization states at same wavelength versus refractive indexes of SiNx are calculated. Similar simulation curves with different and wavelength are also provided. The result shows that there are crossing points in the beat lengths curves. It means that the beat lengths for the two orthogonal polarization states at same wavelength can be identical by choosing the proper refractive index of the SiNx. More importantly, under exactly the same premise, for the two wavelengths, their crossing points are almost identical. Then, how to realize the function of wavelength separation is studied. A variable called the beat length ratio is given, which is defined as the beat length ratio of two working wavelengths under the same polarization state. When the beat length ratio equals an even number divided by an odd number, it means that one wavelength
Optical Resonance Enhanced Cs Activated Nano-structured Ag Photocathode
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Metallic photocathodes have drawn attention due to their outstanding performances of ultrafast photoelectric response and long operational lifetime. However, due to their high work function and high number of scattering events, metallic photocathodes typically were driven by ultraviolet laser pulses and were characterized by low intrinsic quantum efficiency (QE). In this work, a new type of Mie-type silver (Ag) nano-sphere resonant structure fabricated on Ag/ITO composite substrate was used to enhance the photocathode QE, where Mie scattering resonance was used to enhance the local density of optical state and then to improve the light absorption and electron transporting efficiency in Ag nano-spheres. The Cesium (Cs) activation layer was also used to lower the electron work function and then to excite photoemission in the visible waveband for Ag photocathode. The optical characteristics of Ag nano-sphere arrays were analyzed using finite difference time domain method. For the investigated Ag nano-sphere array, theoretical results show that Mie-type electric dipole resonance modes can be obtained over the 400 ~ 600 nm waveband by adjusting sphere diameter, and the large resonance-enhanced absorption can be achieved in nanospheres at the resonance wavelengths. Ag nano-spheres were fabricated on Ag/ITO substrate by magnetron sputtering and annealing processes, then the Cs activation layer was deposited on surface, and finally QE was measured in an ultra-high vacuum test apparatus. Experimental results show that over 0.35% QE was obtained for Ag nano-sphere particle (with diameter of 150nm) at the wavelength of 425nm, and the wavelength positions of QE maxima are in agreement with Mie resonance for corresponding geometry predicted from the computational model. Given these unique optoelectronic properties, Ag nanophotonic resonance structured photocathodes represent a very promising alternative to photocathodes with flat surfaces that are widely used in many applications today.
Length-controllable picking method and conductivity analysis of carbon nanotubes
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In this paper, a length-controllable picking-up method of carbon nanotubes (CNTs) is proposed and the electrical performance data utilized for the conductivity analysis of CNT is also obtained. The micro-nano-operation system inside scanning electron microscope (SEM) are assembled into 4 manipulation units with 3 DOFs individually, which is driven by piezoelectric ceramics and flexure hinges. In this micro manipulation system, an atomic force microscope (AFM) probe is used as the end effector to adjust the spatial pose of the CNT based on van der Waals force and two tungsten needles are used to cut the CNT from the target length and to measure the I-V characteristic data simultaneously. At first, the AFM probe is moved in the z direction to approach CNT until the end of the CNT is adsorbed onto the surface of the AFM probe. And then the AFM probe moved alternately in the x and z direction in order to stretch the CNT in a horizontal straight line form, only in this way can the length of the CNT be measured accurately and can the cutting position be determined. Two cleaned tungsten needles by using hydrofluoric acid to remove the oxide layer were controlled to contact both sides of the cutting position on CNT and connected to the TECK 2280S power supply through the electric cabinet to apply a gradually increasing DC voltage, and the current in the circuit was measured and recorded by the TECK DMM7510 until the current abruptly changes to zero which indicates that the CNT between the tungsten needles has been cut off. The stress of the CNT in contact with the tungsten needles and the AFM probe were analyzed. The modeling of van der Waals force between AFM probe and CNT which can influence the pick-up length error caused by the deformation of CNT under the force of tungsten needles was completed. It is found that the contact length of them and the pick-up length error reduce while the van der Waals force between the AFM probe and CNT increases. The circuit models for contact between the tungsten needles and three operating objects, such as semiconducting CNT, metallic CNT and CNT bundle, were also established. In addition, the I-V characteristic equations of circuit models which could be used to fit the I-V data were derived separately. The CNT pick-up experiment is carried out and results demonstrate that the proposed picking method can not only control the length of CNT effectively, but the conductivity of CNT can also be judged by fitting the I-V obtained the experiment data through the derived I-V characteristic equations.
Atomic Simulation of SiyHx structure configuration in a-Si:H thin Films
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The hydrogenated amorphous silicon (a-Si:H) film is the core structure of hetero junction with intrinsic thin layer solar cell. Its quality determinates the photoelectric conversion efficiency of this solar cell directly. The configuration of SiyHx is an important structure characteristic of a-Si:H films, and it can influence on the quality of a-Si:H thin films and their application properties. However, it is difficult to study them in depth and detail by the existing analytical and testing methods. In this paper, the structure configuration of SiyHx in a-Si:H /c-Si thin films and the effect of substrate temperature on its content have been simulated and analyzed by molecular dynamics method. A modified Tersoff potential developed by Murty was used to calculate the inter-atomic forces. The results showed that the SiyHx structure in a-Si:H thin films can be summarized into six configurations. Three traditional configurations, including SiH, SiH2 and SiH3, can be referred to as SiHx configurations. The other three nove configurations, including HSi2(s), HSi2(l) and HSi3, can be referred to as HSiy configurations. The main differences between the configurations of HSi2(l) and HSi2(s) are the longer Si-H bonds and bigger bond angle in HSi2(l) configuration than those in HSi2(s) configuration. All of the Si-H bonds in SiHx configurations are strong chemical bonds, while the Si-H bonds in HSiy configurations are weak physical bonds. The further calculations of the Si-H bond energies in six configurations have been carried out by the first principle method. According the bond energies results, we can deduce that the order of the stability of six configurations from high to low is SiH > SiH2 >SiH3 >HSi2(s) >HSi2(l) >HSi3. Comparing the Si-H bond energies of the six configurations with the solar energy, it is found that the Si-H bond energy in HSiy configuration is in the range of visible and infrared light in solar light. Si-H physical bonds are easy to fracture in HSiy configuration caused by solar light. This may be the main mechanism of producing Steabler-Wronski (SW) effect in amorphous silicon thin film cells. In addition, the rise of substrate temperature in the deposition process of a-Si:H films will lead to a significant decrease in the configuration content of all kinds of SiyHx configurations.
Research on Dynamic Model of High Speed Following Traffic Flow
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For the physical phenomenon of high speed car-following in the road traffic flow, the vehicles with small spacing are not all running at low speed. The speed of the vehicles is significantly higher than that of they normally should be when they are in the density. There are more than 7% of high-speed following vehicles in the measured data. At present, the traditional traffic flow model cannot simulate the phenomenon of high-speed car following, so a new nonlinear dynamic mathematical model is needed to describe and analyze the physical phenomenon. In order to study the physical phenomenon of high-speed car-following in road traffic, a traffic flow dynamics model for simulating the phenomena is proposed, which combines prospect theory and takes into account the factors such as driver's decision-making behavior and randomization. It is called HCCA model. In the model, the prospect theory is used to analyze the driver's lane changing behavior under the uncertain conditions. Combined with the characteristics of the radical driver, the dynamic prediction speed is considered for the front car followed by the radical driver, and the HCCA dynamics rules of high-speed following traffic flow mechanics are defined. By means of computer numerical simulation, the evolution mechanism and the characteristics of high-speed car-following flow are studied. The results show that compared with the two-lane STCA dynamic model, the HCCA dynamic model established in this paper can simulate abundant traffic physical phenomena, and reproduce complex traffic phenomena such as free flow, synchronous flow and wide-range motion congestion. Finally, the phenomenon of high-speed car-following is simulated and the results of high-speed car-following rate over 7% with small spacing are in good agreement with the measured results. It overcomes the shortage that traditional STCA model can't simulate the synchronous flow. It is found that the larger the proportion of radical drivers is, the larger the high-speed car-following rate and traffic flow with small spacing are under the same road density. The high speed car-following traffic flow mechanics model proposed in this paper has certain reference significance and practical value for analyzing the physical phenomenon of high speed car-following and enriching the traffic flow theory.
Design of a second harmonic terahertz gyrotron cavity based on double confocal waveguide
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Quasi-optical confocal cylindrical waveguide performs a lot of good characters, such as big power capacity and low mode density, which can suppress the mode competition in beam-wave interaction. So quasi-optical waveguides have a great advantage in the design of high harmonic terahertz gyrotrons. For the reason that a part of electron beams located in regions of weak field intensity play a limited role in beam-wave interactions, the beam-wave interaction is not efficient in confocal cavity. Motivated by enhancing the beam-wave interaction efficiency of quasi-optical gyrotron, a novel terahertz harmonic gyrotron cavity with double confocal waveguide is proposed in this paper. The transverse field distribution and the mode spectrum in double confocal waveguide are analyzed and presented. A 330 GHz second harmonic gyrotron with double confocal cavity is designed, theoretically analyzed and simulated by a Particle-In-Cell (PIC) code. Results obtained for double confocal cavity are compared with the results for single confocal cavity, the physical mechanism of beam-wave interaction enhancement in double confocal cavity is discussed. Theoretical results show that double confocal cavity is able to increase the coupling strength of beam-wave interaction, thus, to improve the output power and the interaction efficiency of quasi-optical gyrotron. PIC simulation results suggest that a high-order waveguide mode in double confocal cavity can steady interact with the high harmonic cyclotron mode of electron beam without mode competition. Driven by a 40 kV, 2 A electron beam with guiding center radius of 1.65 mm and velocity ratio equal to 1.5, an output power of 9.9kW at 328.93 GHz can be generated in the designed double confocal cavity. The beam-wave interaction efficiency increases from 5.3% in single focal cavity to 12.4% in dual confocal cavity under the same operation parameters. The double confocal cavity has great application potentiality in terahertz band. Moreover, this study indicates that the eigen mode in double confocal waveguide is a kind of hybrid mode superimposed of two independent single confocal waveguide modes. This mode characteristic will be beneficial to design a multifrequency gyrotron oscillator operated in two modes and two cyclotron harmonics, simultaneously, with a single electron beam, which provides a new possibility to develop the novel terahertz radiation source.
Time Domain Hybrid Method for the Coupling Analysis of Multi-conductor Transmission Lines on the Lossy Dielectric Layer Excited by Ambient Wave
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At present, the numerical methods applied for the coupling analysis of transmission lines on the lossy dielectric layer excited by ambient wave are still rare in the literature. As a temptation to fill this gap, a novel time domain hybrid method is proposed, in which the modified transmission line (TL) equations, finite-difference time-domain (FDTD) method, and some interpolation schemes are organically combined together. It can overcome the modeling difficulty of ambient wave coupling with transmission lines on the lossy dielectric layer greatly. In this method, the modified transmission line (TL) equations suitable for the coupling analysis of multi-conductor transmission lines (MTLs) on the lossy dielectric layer are derived from the traditional TL equations firstly. Compared with the traditional TL equations, the electromagnetic fields in the lossy dielectric layer are introduced into the equivalent distribution sources of modified TL equations. Generally, the precision of TL equations is depending on the accuracy of equivalent distribution sources, which are obtained from the incident electric fields along and perpendicular to the MTLs. Therefore, the FDTD method is utilized to model the structure of lossy dielectric layer to calculate the electromagnetic field distribution surrounding the MTLs and in the dielectric layer. Since the heights and distances of MTLs can be arbitrary values, the electric fields along and perpendicular to the MTLs may be not on the edges of FDTD grids absolutely, which should be computed via some interpolation schemes. Then the modified TL equations are established, which should be solved by the central difference scheme of FDTD method to obtain the voltages and currents on the MTLs and terminal loads. The significant feature of this proposed method is that it can realize the synchronous calculations of electromagnetic field radiation and transient responses on the MTLs. Finally, numerical simulations of single and multiconductor transmission lines on the lossy dielectric layer excited by ambient wave with different incident angles are employed to exhibit the accuracy and efficiency of the proposed method by comparing with the simulation software CST. Because the structures of MTLs do not need to be meshed, the proposed method outperforms the simulation software CST in both memory usage and computation time.
1/2 Sub-harmonic Resonance in Bistable Structure and Its Effect on Vibration Isolation Characteristics
Abstract +
Expansive modern applications need the low-frequency and heavy-load isolators to reduce the vibration transmissions. The unique properties of nonlinear systems, such as jumping, bifurcation and chaos, provide new ideas for the design of new functional structures. Bistable system is a typical non-linear system, features highly static and low dynamic stiffness, which is promising to realize a low-frequency isolator with ensuring heavy load capacity. However, more studies are necessary to clarify the sub-harmonic resonances and its generation process, parameters influences, vibration isolation characteristics of the bistable structure. By adopting the equivalent, analytical, numerical and experimental methods, the 1/2 sub-harmonic resonances, evolution process and its influence on the vibration isolation characteristics of the bistable structure are studied in this paper. When the amplitude or nonlinear stiffness coefficient kn increases to a certain extent, 1/2 sub-harmonic resonance appears, where the response contains high-amplitude ω/2 component under the excitation frequency ω, so energy is transferred from high frequency to low frequency. We study the bifurcation and varying process of the fundamental transmission and 1/2 sub-harmonic transmission by increasing the amplitude. At critical bifurcation amplitude, the sub-harmonic transmission rapidly increases from 0 to a large peak value. And then, it reduces gradually when damping is absent. However, the peak value of 1/2 harmonic does not cause a sudden change of the transmission of fundamental wave. When there are considerable damping, along with increasing the amplitude, 1/2 harmonic does not always exist, instead, it follows an interesting "generation-enhancement-degeneration- disappearance" process. This process offers great significance in applying the 1/2 harmonic in vibration manipulation or avoiding the resonant enhancement induced by it. Moreover, in this process, both the peak frequency and the peak transmission of the bistable isolation system descend first. The optimal combination of the parameters can reduce the resonance frequency by 17.8% through increasing the driving amplitude. However, they jump to large values when 1/2 harmonic plays a dominant role. Additionally, the negative stiffness k0 has a significant effect on the primary resonance characteristics: As |k0| increases under a specified excitation amplitude, the system resonance peak shifts to higher frequency and the transmission increases. Besides the main effect on the sub-harmonic resonances and the equilibrium points, the nonlinear coefficient kn also effects the peak and resonance frequency of the system, but its effect is much less than the impact caused by k0. Furthermore, the sub-harmonic resonances, bifurcations and vibration isolation characteristics of the bistable bulking beam structure are demonstrated in experiments. Our experiment results show that: i) the 1/2 sub-harmonic resonances can appear under the excitation of a certain bandwidth; ii) increasing the driving amplitude reduces the transmission of the fundamental wave; iii) the transmission of 1/2 harmonic jumps from 0 upward to a large value at a critical amplitude, and then it decreases gradually. The experimental results are consistent with the analytical and numerical results. The experiment also demonstrates the laws of frequency shift and transmission reduction of peak values, so properly increasing the amplitude improves the vibration isolation capacity. However, sub-harmonic resonance will reduce the isolation effect. In practical engineering, strong sub-harmonic resonance should be avoided in the nonlinear vibration isolation system.
Untrafast Smoothing Scheme based on Dynamic Interference Structures between Beamlets of A Laser Quad
Abstract +
Aimed at the high requirement of illumination uniformity on the target in laser-driven inertial confinement fusion (ICF) facilities, an ultrafast smoothing method based on dynamic interference structures between beamlets of a laser quad is proposed. The basic principle of this scheme is to use a conjugate phase plate array to add the conjugate phase modulation to the multiple beamlets of a laser quad with certain wavelength differences. Consequently, each two beamlets are coherently superposed in the far field to generate a dynamic interference pattern, resulting in the fast redistribution of the speckles introduced by continuous phase plate (CPP) inside the focal spot and further improving the illumination uniformity on the target in the picosecond timescale. The coherent beamlets with certain wavelength difference can be generated by using a broadband seed laser. Taking the laser quad of the typical ICF facilities as an example, the physical model of the ultrafast smoothing method based on dynamic interference structures of beamlets is built up. The influences of the phase-plate types, the peak-to-valley values of the phase modulation and the wavelength differences between the beamlets are analyzed quantitatively, and the smoothing characteristics of the focal spot are discussed in detail and compared with the traditional temporal smoothing scheme such as smoothing by spectral dispersion (SSD). The results indicate that the directions of the moving speckles in the focal spot are determined by the phase-plate type. However, the required time to achieve stable illumination uniformity, i.e, the decay time, is determined by the wavelength differences between the beamlets. Moreover, the illumination uniformity on the target becomes better with the increasing peak-to-valley value of the phase modulation at first and then remains almost the same. Thus, the ultrafast smoothing method based on dynamic interference structures with well-designed phase arrays and wavelength combinations of the beamlets can realize the multi-directional and multi-dimensional speckle sweeping inside the focal spot, and further improving the irradiation uniformity on the target within several picoseconds or sub-picoseconds. By combining with the traditional beam smoothing scheme, better illumination uniformity can be achieved in an ultrashort timescale. This novel scheme can be used as an effective supplement to the existing temporal beam smoothing techniques.
Hydrogen storage capacity of alkali metal atoms decorated porous graphene
Abstract +
Porous graphene (PG), graphene-related materials with nanopores in the graphene plane, exhibits novel properties different from those of pristine graphene, leading to its potential application in numerous fields. Owing to naturally existing periodic nanopores in the two-dimensional layer, PG can be used as an ideal candidate for hydrogen storage material. High hydrogen storage capacity of Li-decorated PG has been reported in theoretical investigations, but the effect of temperature on the stability of the H2 adsorbed on Li-PG has been not discussed. In this paper, using first-principles method, hydrogen storage capacity on alkaline metal atoms (Li、Na、K) decorated porous graphene has been investigated deeply with generalized gradient approximation, and the effect of the temperature on the stability of the hydrogen adsorption system has been elucidated by the ab initio molecular-dynamics simulation. The results show that most favorable adsorption site of Li, Na and K are the hollow center site of the C hexagon, and four alkaline metal atoms can be adsorbed stably on double sides of PG unit cell without clustering. Alkaline metal adatoms adsorbed on PG become positively charged by transferring charge to PG and adsorbed H2 molecules, and three H2 can be adsorbed around each alkaline metal atom. By analyzing Mulliken atomic populations, charge density differences and density of states of H2 adsorbed on Li-PG system, attractive interaction between positively charged alkaline metal adatoms and negatively charged H and weak van der Waals interaction are responsible for the H2 molecules adsorbed on alkaline metal atoms decorated
Surface enhanced Raman scattering characteristics of three-dimensional pyramid stereo composite substrate
Wu Mei-Mei, Zhang Chao, Zhang Can, Sun Qian-Qian, Liu Mei
Abstract +
Surface enhanced Raman scattering (SERS) is a highly sensitive spectroscopy technique, which is widely used in chemical reaction detecting, medical diagnostics, and food analysis. The construction of the substrate structure has a very important influence on enhancing the SERS signal of the probe molecule. In this paper, a three-dimensional (3D) pyramid stereo composite SERS substrate is prepared by using polymethyl methacrylate (PMMA) to encapsulate silver nanoparticles, which achieves high sensitivity detection of Rhodamine 6G (R6G) molecules. By adjusting the dispersion density of silver nanoparticles in thePMMA acetone solution, the effective oscillation of light in the pyramid valley is realized, which not only ensures the high-density "hot spot" effect of the 3D structure, but also avoids deforming the adsorption probe molecules caused by the metal-molecule interaction. It also effectively prevents the silver nanoparticles from being oxidized and provides a larger range of electromagnetic enhancement for probe molecules, resulting in a stable output of the enhanced Raman signal. This research result provides an effective strategy for designing a high performance and reusable SERS substrate, meanwhile, it has important guiding significance for further designing an SERS substrate with improved 3D structure in the future study.
Crystal-orientation effects of the optical extinction in shocked Al2O3: a first-principles investigation
Li Tian-Jing, Cao Xiu-Xia, Tang Shi-Hui, He Lin, Meng Chuan-Min
Abstract +
$V_O^{ + 2}$ (the +2 charged O vacancy) defects at the pressure range of 131-255 GPa. It is found that: 1) there are obvious crystal-orientation effects of the extinction in shocked Al2O3 at high-pressure region, and they strengthen with increasing pressure; 2) shock-induced $V_O^{ + 2}$ defects could play an important role in determining these crystal-orientation effects, but the influences of pressure and temperature factors on them are relatively weak. A further analysis shows that, at the wavelength range adopted in shock experiments, the extinction of a-orientation is the weakest (the best transparency), the extinction of c-orientation is the strongest (the worst transparency), and the extinction of s-orientation is between them; at the same time, the extinction of m-orientation is similar to that of a-orientation, the extinction of r-, n- and d-orientations is close to that of c-orientation, and the extinction of g-orientation is weaker than that of s-orientation. In view of this, we suggest that the a- or m-oriented Al2O3 is chosen as an optical window in shock-wave experiments of the high-pressure region. Our predictions could be not only helpful to understand further the optical properties of Al2O3 at extreme conditions, but also important for future experimental study.">Sapphires (Al2O3) is an important ceramic material with extensive applications in high-pressure technology and geoscience. For instance, it is often used as a window material in shock-wave experiments. Consequently, understanding the behavior of its transparency change under shock compression is crucial for correctly interpreting the experimental data. Sapphire has excellent transparency at ambient conditions, but its transparency is reduced under shock loading. This shock-induced optical extinction phenomenon in Al2O3 has been studied experimentally and theoretically a lot at present, but the knowledge on the crystal-orientation effects of the extinction is still insufficient. the experimental investigations at low-pressure region (within 86 GPa) have indicated that the shock-induced extinction in Al2O3 is related to its crystal orientation, but it is not clear whether the correlation also exists at high-pressure region (~131–255 GPa). Here, to investigate this question, we have performed first principles calculations of the optical absorption properties of a-, c-, d-, r-, n-, s-, g- and m-oriented Al2O3 crystals without and with $V_O^{ + 2}$ (the +2 charged O vacancy) defects at the pressure range of 131-255 GPa. It is found that: 1) there are obvious crystal-orientation effects of the extinction in shocked Al2O3 at high-pressure region, and they strengthen with increasing pressure; 2) shock-induced $V_O^{ + 2}$ defects could play an important role in determining these crystal-orientation effects, but the influences of pressure and temperature factors on them are relatively weak. A further analysis shows that, at the wavelength range adopted in shock experiments, the extinction of a-orientation is the weakest (the best transparency), the extinction of c-orientation is the strongest (the worst transparency), and the extinction of s-orientation is between them; at the same time, the extinction of m-orientation is similar to that of a-orientation, the extinction of r-, n- and d-orientations is close to that of c-orientation, and the extinction of g-orientation is weaker than that of s-orientation. In view of this, we suggest that the a- or m-oriented Al2O3 is chosen as an optical window in shock-wave experiments of the high-pressure region. Our predictions could be not only helpful to understand further the optical properties of Al2O3 at extreme conditions, but also important for future experimental study.