bluuebunny/crossref_metadata_2025 huggingface dataset:
Focusing effect of channel target on ultra-intense laser-accelerated proton beam
Authors: Yang Si-Qian  , Zhou Wei-Min  , Wang Si-Ming  , Jiao Jin-Long  , Zhang Zhi-Meng  , Cao Lei-Feng  , Gu Yu-Qiu  , Zhang Bao-Han  ,   Science and Technology on Plasma Physics Laboratory, Laser Fusion Research Center, China Academy of Engineering Physics, Mianyang 621900, China;,   IFSA Collaborative Innovation Center, Shanghai Jiao Tong University, Shanghai 200240, China
Published: December 2020

In laser proton acceleration, the inevitable transverse divergence of proton beam restricts its applications in many fields. In this paper, a structured target with a properly wide channel attached to the backside of a foil is proposed, and the interaction of the ultra-short laser pulse with the structured channel target is investigated via two-dimensional particle-in-cell simulation. The simulations show that for the structured channel target, electrons on the front surface are heated by the incident high-intensity laser pulse and then the induced hot electrons transport through the target to the rear surface, building an electrostatic field in the longitudinal direction to accelerate the protons to high energies as the typical target normal sheath acceleration scheme. In the case of the structured channel target, the simulation results indicate that a strong transverse electrostatic field is created by charge separation along the inner surface of the channel while hot electrons propagate along the channel side walls under the guidance of self-induced magnetic and electric fields, which can focus the emitted proton beam transversely, leading to a smaller divergence. By comparing the channel target case with the traditional foil target case under the same conditions, it is found that the divergence angle of the proton beam from the channel target is reduced significantly. Protons with energies above 3 MeV have a divergence angle of 5.3° at the time of 500 fs in the channel target case, while the value is 17.1° in the foil case for a laser intensity of 5.4×1019 W/cm2. Additionally, the effect of the channel target on the maximum proton energy is considered. The simulation results of the energy spectra reveal that the maximum proton cut-off energy of the channel target is about 1 MeV lower than that of the foil target. This small energy loss is due to the refluxing of the cold electrons on the channel walls, which suppresses the increasing of the sheath potential. Therefore, it is concluded that the focusing electric field can work on the proton beam effectively, leading to a better collimation with conserving the proton energy by using the proposed channel target. Especially when the inner diameter of the channel target is comparable to the laser focal spot size, the proton beam can be confined to a small divergence, and a relatively higher laser energy conversion efficiency can be ensured as well.
Yield behavior of amorphous alloy based on percolation theory
Authors: Ping Zhi-Hai  , Zhong Ming  , Long Zhi-Lin  ,   College of Civil Engineering and Mechanics, Xiangtan University, Xiangtan 411105, China
Published: December 2020

According to the microstructure of amorphous crystal, the percolation theory, which is a theoretical approach to dealing with the inhomogeneous physical systems or random fractals, is used to describe the plastic flows of amorphous alloys under shear yielding. In order to understand in depth the critical problems about the shear band initiations in amorphous alloys, a percolation model for shear transformations of these alloys is established by combining with the existing free volume model and shear transformation zone model. Taking the binary amorphous alloy Cu25Zr75 for example, the percolation threshold for the shearing instability of the atomic clusters prone to producing plastic flows in the shear transformation zone is calculated when a shear band comes into being. In addition, the size of the above-mentioned cluster is also roughly estimated. The calculated results show that the percolation threshold of the shearing instability is similar to the critical reduced free volume value (xC) of~2.4% for the onset of yielding in amorphous alloy although this threshold is closely related to the dispersity of free volume. The present study may provide a new idea and method of studying the ductile-brittle transition in amorphous alloy.
Joint estimation of frequency and direction of arrival under the single-and-parallel spatial-temporal undersampling condition
Authors: Huang Xiang-Dong  , Liu Ming-Zhuo  , Yang Lin  , Liu Kun  , Liu Tie-Gen  ,   School of Electrical and Information Engineering, Tianjin University, Tianjin 300072, China;,   School of Precision Instrument and Opto-electronics Engineering, Tianjin University, Tianjin 300072, China
Published: December 2020

As the application frequency is increasingly high, it becomes difficult to design joint estimators for the frequencies and directions of arrival (DOAs) under the spatial-temporal undersampling condition. Specifically, on one hand, the temporal Nyquist theorem requires that the sampling rate be at least twice the highest frequency, which is unfordable for the existing analog-to-digital converters; on the other hand, the spatial Nyquist theorem also requires that each inter-element spacing be less than or equal to half the wavelength, which inevitably results in severe mutual coupling among sensors. To solve these intractable problems, in this paper, we propose a joint estimator based on a co-prime sparse array. Firstly, based on the combination of this sparse array and the closed-form robust Chinese remainder theorem (CRT), the theoretical model for the proposed frequency and DOA joint estimator is built up. Secondly, at each sensor, a frequency estimate for the source object can be calculated through implementing the closed-form robust CRT on two frequency remainders, which are generated by applying the Tsui spectrum correction to the discrete Fourier transform results of two receiver sequences. Then, averaging these estimates at all sensors yields the final frequency estimate. Lastly, on the basis of frequency estimation, the final DOA estimate can be calculated through implementing the closed-form robust CRT on all phase-difference remainders, which are also derived from the Tsui spectrum correction. It needs to be emphasized that the proposed joint estimator possesses two attractive merits. One merit is that due to the fact that the proposed array allows a high sparsity of element-spacings, both the hardware cost and the mutual coupling among sensors can be considerably reduced; the other merit is that compared with the existing estimators, the proposed joint estimator achieves high estimation precision even in the single-and-parallel undersampling condition (i.e., multi-time undersampling is bypassed in each sensor element, leading to a high data processing efficiency). In particular, this high accuracy attributes to two aspects:1) the Tsui spectum corrector incorporated in the proposed joint estimator can provide high-accuracy remainders for the CRT recovery; 2) the closed-form robust CRT itself is unbiased and thus the CRT recovery brings no extra system errors. Numerical results verify that the proposed joint estimator possesses both strong noise robustness and high estimation accuracy, which presents a vast potential application in several passive sensing fields such as radar and remote sensing.
Influences of Doppler broadening of absorption lines on a multi-step photoionization process of atoms
Authors: Lu Xiao-Yong  , Zhang Xiao-Zhang  , Zhang Zhi-Zhong  ,   Department Engineering of Physics, Tsinghua University, Beijing 100084, China;,   Research Institute of Physics and Chemical Engineering of Nuclear Industry, Tianjin 300180, China}
Published: December 2020

Photoexcitation and photoionization of atoms, the central part of atom vapor laser isotope separation (AVLIS), relate to the ionization yield and isotope selectivity directly. Doppler broadening of absorption lines is one of the parameters that influence the photoexcitation and photoionization process of atoms. When evaporation temperature is high or beam equipment is absent, Doppler broadening has obvious influence on the ionization yield because most of atoms are non-resonantly excited. In this paper, we study the influences of Doppler broadening of absorption lines on a multi-step photoexcitation and photoionization process of atoms according to the facts of AVLIS. A Doppler-broadened three-level atom system with two resonant lasers is investigated. The interaction between laser field and atoms is described by a density matrix, which is calculated by fourth-order Runge-Kutta numerical method with variable steps. The results show that the ionization yield of atoms decreases with the increase of Doppler broadening of absorption lines under the same laser parameters. At a constant laser power, the ionization yield reaches its maximum value at the best laser bandwidths, which is different from that obtained without Doppler broadening, as reported in published papers. Meanwhile, the best laser bandwidth increases with the increase of Rabi frequency and Doppler broadening when other parameters are constant. Moreover, the best bandwidth of the second laser is smaller than that of the first laser in a multi-step photoionization process of atoms. Therefore, lasers should work at the best bandwidths in AVLIS for highest ionization yield. It is advantageous to make laser bandwidths slightly bigger than the best bandwidths technically for smaller fluctuation of ionization yield, owing to incoercible stochastic volatility in laser bandwidths. The ionization yield increases with the decrease of Doppler broadening, especially at the best laser bandwidths. Therefore, it is necessary to reduce Doppler broadening of atom vapor in laser ionization zone.
Analysis of wavelength dependence of mode in high power fiber laser
Authors: Liang Jing-Chuan  , Feng Guo-Ying  , Zhang Shu-Lin  , Lan Bin  , Zhou Shou-Huan  ,   Institute of Laser and Micro/Nano Engineering, College of Electronics and Information Engineering, Sichuan University, Chengdu 610064, China;,   North China Research Institute of Electro-Optics, Beijing 100015, China}
Published: December 2020

High power fiber lasers and amplifiers are widely used in the scientific and industrial field. In order to meet the requirements for high output powers the effective area of fibers becomes larger and larger to reduce optical nonlinearities. With the increase of effective area, the number of high-order modes will increase. In the case of high output power, the spectral shift and broadening of the optical fiber will also affect the modal number and content. The number and content of fiber modes affect the pointing stablity and quality of the laser beam. The M2-parameter is commonly used to define the quality of the laser beam, but a small M2 number is not guaranteed for single mode operation. Therefore, the relationship between wavelength and transmission mode in fiber transmission is studied in this paper. We use the spatial and spectral Fourier transform (F2) method to establish a theoretical-experimental method of describing the relationship between wavelength and mode. This method can directly give out the modal content of optical fibers without any priori parameter such as the properties of fiber and requirement for setup accuracy. On the one hand, the theoretical modeling of wavelength affects modal content. In the simulation, the sources with the same wavelength bandwidth and different central wavelengths are used to test the fiber. The results show that the modal content and number of the fiber change with the wavelength bandwidth and center wavelength. The mode components of the corresponding optical fiber will change after changing the central wavelength. As the spectral width of the light source increases, the number of high-order modes increases. On the other hand, in order to further verify the relationship between wavelength and mode of fiber, the F2 method is used to measure the optical fiber modal content with different wavelengths. The final experimental results are in agreement with the theoretical results. The experimental and simulation results show that the mode field distribution of each mode varies with wavelength:the longer the wavelength, the larger the mode field is. The beam quality has little change with the wavelength except for those positions with frequency near the cutoff frequency, and the power ratio of each mode relates to the wavelength.
Avalanche effect in plasma under high-power microwave irradiation
Authors: Li Zhi-Gang  , Cheng Li  , Yuan Zhong-Cai  , Wang Jia-Chun  , Shi Jia-Ming  ,   State Key Laboratory of Pulsed Power Laser Technology, National University of Defense Technology, Hefei 230037, China
Published: December 2020

High-power microwave (HPM) weapon, which is destructive to electronic systems, has developed rapidly due to the great progress of HPM devices and technologies. Plasma with distinctive electromagnetic characteristics is under advisement as one of potentially effective protection materials. Therefore, research on avalanche ionization effect in plasma caused by the interaction between HPM and plasma is of significance for its HPM protection performance. Based on the method of fluid approximation, the wave equation, the electron drift diffusion equation and the heavy species transport equation, explaining the propagation of microwave and the change of the charged particles inside plasma, are established to study the avalanche ionization effect under the HPM radiation. A two-dimensional physical model is built with the help of software COMSOL according to the plasma protection array designed to disturb the propagation of the HPM pulses. It can be shown that the emergence of avalanche effect is greatly affected by the incident power of microwave, and the generation time would be influenced by the initial electron density. Moreover, it can be observed that the avalanche effect appears only when the plasma array is irradiated for a period of time, which means that the performance of HPM is presented as gathering effect, and a large amount of energy is needed to change the internal particle balance in plasma. In addition, the electron density inside the plasma changes rapidly and complicatedly while the avalanche effect comes into being. Besides, the cutoff frequency of the plasma exceeds the frequency of the incident wave with the increase of electron density, which leads to that the electromagnetic wave cannot propagate in the plasma, so that the plasma can be used to protect the HPM irradiation.
Attosecond X-ray generation driven by the relativistic laser pulse based on the semi-analytical self-consistent theory
Authors: Wang Shao-Yi  , Tan Fang  , Wu Yu-Chi  , Fan Quan-Ping  , Jiao Jin-Long  , Dong Ke-Gong  , Qian Feng  , Cao Lei-Feng  , Gu Yu-Qiu  ,   Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, China;,   Key Laboratory of Sciences and Technology on Plasma Physics, China Academy of Engineering Physics, Mianyang 621900, China;,   IFSA Collaborative Innovation Center, Shanghai Jiao Tong University, Shanghai 200240, China
Published: December 2020

A semi-analytical theory of the interaction between a relativistic laser pulse and the overdense plasma to generate an attosecond X-ray source is presented.The physical parameters such as plasma oscillation trajectory,surface electric field and magnetic field can be given by this model,and the high-order harmonic spectrum is also calculated accurately from the solution of the plasma surface oscillations,the obtained result is consistent with the result from the PIC simulation program.This model can be valid for arbitrary laser duration,solid densities,and a large set of laser peak intensities (1018-1021 W/cm2).In addition,the model is not applicable for the small laser focal spots (less than ten times the laser wavelength),although two-dimensional effects such as the pulse finite size may significantly change the movement progress of the electrons,the laser spot can be larger than ten times the laser wavelength under the general laboratory conditions. In this model,the laser energy absorption is small,and the electron kinetic pressure is also small.Due to the radiation pressure of the laser pulse,the electrons are pushed into the solid,forming a very steep density profile.As a result,the relevant forces makes the electrons ponderomotive and the longitudinal electric field is caused by the strong electric charge separation effect.This semi-analytical self-consistent theory can give us a reasonable physical description, and the momentum equation and the continuity equation of the electric and magnetic field at the boundary allow us to determine the plasma surface oscillations.The spatiotemporal characteristics of the reflected magnetic and electric field at the boundary can allow us to determine the emitting characteristics of the high order harmonic. Our results show that the radiation of the attosecond X-ray source is dependent on the plasma surface oscillation. The plasma surface oscillates with a duration about twice the laser optical cycle,and the high-order harmonics also emit twice the laser optical cycle,thus an attosecond pulse train driven by the multi-cycle laser pulse can be formed.By using a few-cycle laser field,the smooth high-order harmonics can be obtained,which leads to a single attosecond pulse with high signal-to-noise ratio.In a word,our calculation results show that the time evolution progress of plasma surface can be controlled by changing the carrier envelope phase of the few-cycle laser pulse,and then the radiation progress of the high-order harmonics can be influenced as result of a single attosecond X-ray pulse.
Stability analysis of interfacial Richtmyer-Meshkov flow of explosion-driven copper interface
Authors: Yin Jian-Wei  , Pan Hao  , Wu Zi-Hui  , Hao Peng-Cheng  , Duan Zhuo-Ping  , Hu Xiao-Mian  ,   School of Mechatronical Engineering, Beijing Institute of Technology, Beijing 100081, China;,   Institute of Applied Physics and Computational Mathematics, Beijing 100094, China;,   Graduate School of China Academy Engineering Physics, Beijing 100088, China
Published: December 2020

In this paper, a stability analysis is given to study the unstable mechanism of the Richtmyer-Meshkov flow of explosion-driven copper interface. The Richtmyer-Meshkov flow refers as an interfacial instability growth under shockwave incident loading. Numerical investigations are performed to check the applicability of the two-dimensional hydrocode, which is named AFE2D, and the physical models of detonation waves propagating in the high explosives, equations of state and the constitutive behaviors of solids in the analysis of Richtmyer-Meshkov flow problems. Here we theoretically analyze the two key issues of the unstable mechanism in Richtmyer-Meshkov flow in solids. The unstable mechanism includes temperature related melting mechanism and the plastic evolution related tensile fracture mechanism. In the analysis of the temperature related unstable mechanisms, the calculated temperature increase during the shockwave compression from the shock Hugoniot data in the shockwave physics is not enough to melt the material near the perturbed interface. On the other hand, the temperature increase from the translation of plastic work during perturbation growth which relats to the distribution of the cumulative effective plastic strain is also not enough to supply the thermal energy which is needed to melt the crystal lattice of solid, either. Therefore, the temperature related melting mechanism is not the main factor of the unstable growth of copper interface under explosion driven. In the analysis of the plastic tensile fracture related unstable mechanism, a scaling law between the maximum cumulative effective plastic strain and the scaled maximum amplitude of spikes is proposed to describe the relationship between the plastic deformation of material and the perturbation growth of interface. Combined with a critical plastic strain fracture criterion, the unstable condition of the scaled maximum amplitude of spikes is given. If the spikes grow sufficiently to meet the unstable condition, the interfacial growth will be unstable. Numerical simulations with varying initial configurations of perturbation and yield strength of materials show good agreement with the theoretical stability analysis. Finally, a criterion to judging whether the growth is stable is discussed in the form of competition between the temperature related unstable mechanism and the tensile fracture unstable mechanism.
Intercalation and its mechanism of high quality large area graphene on metal substrate
Authors: Guo Hui  , Lu Hong-Liang  , Huang Li  , Wang Xue-Yan  , Lin Xiao  , Wang Ye-Liang  , Du Shi-Xuan  , Gao Hong-Jun  ,   Institute of Physics and University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100190, China
Published: December 2020

Graphene, a two-dimensional material with honeycomb lattice, has attracted great attention from the communities of fundamental research and industry, due to novel phenomena such as quantum Hall effect at room temperature, Berry phase, and Klein tunneling, and excellent properties including extremely high carrier mobility, high Young's modulus, high thermal conductivity and high flexibility. Some key issues hinder graphene from being used in electronics, including how to integrate it with Si, since Si based technology is widely used in modern microelectronics, and how to place high-quality large area graphene on semiconducting or insulating substrates. A well-known method of generating large-area and high-quality graphene is to epitaxially grow it on a single crystal metal substrate. However, due to the strong interaction between graphene and metal substrate, the intrinsic electronic structure is greatly changed and the conducting substrate also prevents it from being directly used in electronics. Recently, we have developed a technique, which intercalates silicon between epitaxial graphene and metal substrate such as Ru (0001) and Ir (111). Experimental results from Raman, angle-resolved photoemission spectroscopy, and scanning tunneling spectroscopy confirm that the intercalation layer decouples the interaction between graphene and metal substrate, which results in the recovery of its intrinsic band structure. Furthermore, we can use this technique to intercalate thick Si beyond one layer and intercalate Si between graphene and metal film, which indicates the possibility of integrating both graphene and Si device and vast potential applications in industry by reducing its cost. Besides Si, many other metal elements including Hf, Pb, Pt, Pd, Ni, Co, Au, In, and Ce can also be intercalated between graphene and metal substrate, implying the universality of this technique. Considering the versatility of these elements, we can expect this intercalation technique to have wide applications in tuning graphene properties. We also investigate the intercalation mechanism in detail experimentally and theoretically, and find that the intercalation process is composed of four steps:creation of defects, migration of heteroatoms, self-repairing of graphene, and growth of intercalation layers. The intercalation of versatile elements with different structures by this technique provides a new route to the construction of graphene heterostructures, espectially van der Waals heterostructure such as graphene/silicene and graphene/hafnene, and also opens the way for placing graphene on insulating substrate for electronic applications if the intercalation layer can be oxidized by further oxygen intercalation.
Three-dimensional magnetization dynamics in majority gate studied by using multiferroic nanomagnet
Authors: Wei Bo  , Cai Li  , Yang Xiao-Kuo  , Li Cheng  ,   Science College, Air Force Engineering University, Xi'an 710051, China
Published: December 2020

The scaling of traditional complementary metal oxide semiconductor (CMOS) device is reaching its physical limit, and alternative emerging devices are being explored as possible CMOS substitutes. One of the most promising device technologies is nano-magnetic logic (NML), which is an energy-efficient computing paradigm. The inherent nonvolatility and low energy consumption make NML device possess wide application perspectives. The basic element of multiferroic NML is a sub-100 nm sized single domain magnet. Generally, the x-y plane determines the in-plane dimension, while the z direction indicates the thickness of nanomagnet. Classical binary logic states 0 and 1 are encoded in two stable magnetization orientations along the easy axis (major axis) of the elliptical nanomagnet, while the hard axis (minor axis) refers to null logic. In order to propagate logic bits between the neighbor nanomagnets, one requires a clock that periodically flips every magnet's magnetization along the hard axis simultaneously, and the dipole-dipole interaction between the neighbors will force the magnet into the correct orientation along the easy axis, and thus the logic bit propagates unidirectionally. In multiferroic NML, the majority gate is a basic element of nanomagnet logcal circuit. In this paper, the three-dimensional switching dynamic model of a multiferroic nanomagnetic majority gate is established, and its magnetization dynamics is simulated by solving the Landau-Lifshitz-Gilbert equation with considering the thermal fluctuation effects. The majority gate is implemented with dipole-coupled two-phase (magnetostrictive/piezoelectric) multiferroic elements and is simulated by using different strain clocks and changing the input. It is found that the majority gate works efficiently and correctly when receiving new input. It is also found that the optimal time interval of stress releasing between central nanomagnet and output nanomagnet is 0.1-0.2 ns. Removing stress earlier will reduce the success rate of the majority gate operation while the work frequency increases. The reason behind the phenomenon may be that removing stress earlier results in weak dipole-coupled interaction, which cannot overcome the shape anisotropy. These findings are beneficial to the design of multiferroic logic circuit.
Topological phase transitions in core-shell gyromagnetic photonic crystal
Authors: Shen Qing-Wei  , Xu Lin  , Jiang Jian-Hua  ,   Department of Physics, Optoelectronics and Energy, Soochow University, Suzhou 215006, China;,   Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006, China
Published: December 2020

We found that core-shell gyromagnetic photonic crystal of two-dimensional triangular lattice exhibit topological phases. In a certain parameters and magnetic field, those phases could be a conventional insulator phase, a quantum spin Hall phase, and a quantum anomalous Hall phase. Different from the structure of Ref[1], phase transitions in our core-shell structure does not involve changes of space symmetry, which depend on parameters of our structure and the broken of time reverse symmetry. Our result shows the variety of topological phase transitions in photonic crystals.
The topological phase transition and the tunable interface states in granular crystal
Authors: Wang Qing-Hai  , Li Feng  , Huang Xue-Qin  , Lu Jiu-Yang  , Liu Zheng-You  ,   Department of Physics, South China University of Technology, Guangzhou 510640, China;,   Department of Physics, Wuhan University, Wuhan 430072, China
Published: December 2020

Topological phase is a new degree of freedom to describe the state of matter in condensed matter physics. One could predict the existence of the interface state between two topological different phononic crystals. The band structures of phononic crystal depend on the characteristics of their composite and their combination, such as geometry, filling fraction, and stiffness. However, after the phononic crystal is fabricated out, it is relatively difficult to tune their band structure and its topology. In order to broaden the application scope of phononic crystals, different kinds of tunable phononic crystals have been proposed. One method to achieve this tunability is to introduce nonlinearity into the phononic crystals. Granular crystals is one type of tunable nonlinear material, whose nonlinearity stems from nonlinear Hertzian contact. By changing the static precompression, the dispersion of granular crystals can be tuned. In this paper, by combining topology with nonlinear we create a new type of interface state switch without changing the experimental setup. Based on the Su-Schrieffer-Heeger (SSH) model–an example of a one dimensional (1D) topological insulator, we present a 1D nonlinear granular crystal, to realize the topological transition by precompression. First, we construct a 1D mechanical structure, which is made up of nonlinear granular crystal and linear phononic crystal. The 1D nonlinear granular crystal is simplified as a “mass-spring” model with tunable elastic constant and invariable elastic constant. By calculating the band topology–the Zak phase, we found that the Zak phase of the two bands can switch from π to 0. There exist a critical precompression F0, when F F0 the Zak phase of the band is π, when F > F0 the Zak phase is 0. The granular crystal vary from nontrivial bandgap to trivial one as precompression gradually increase. This effect enables us to design interface state switch at the interface between granular crystals with trivial and nontrivial band gap. Furthermore, when F F0, we find that the localization of interface state decreases as the applied precompression increases. Thus, we investigate existence of the interface state under different precompression and found that the interface state can be controlled freely. We anticipate these results to enable the creation of novel tunable acoustic devices.
High-efficiency on-line measurement of junction temperature based on bipolar transistors in accelerated experiment
Authors: Guo Chun-Sheng  , Ding Yan  , Jiang Bo-Yang  , Liao Zhi-Heng  , Su Ya  , Feng Shi-Wei  ,   Deputy Dean, College of Electronic Science and Technology, Faculty of Information Technology, Beijing University of Technology, Beijing 100124, China
Published: December 2020

Junction temperature is an important factor affecting the reliabilities of semiconductor devices. Usually, the method of measuring the junction temperature is not tested on-line. However, due to the fact that neither contact thermal resistance nor thermal resistance varying with temperature is taken into account, there exists an error in the off-line measurement. A way to solve the problem of off-line measurement is to measure the junction temperature on-line. In this paper, we propose an electrical method of measuring the temperature rise of high-power bipolar transistor in the working condition. The measurement method is based on a good linear relationship between base-emitter voltage (Vbe) and temperature during the steady-state. Taking the model 2N3055 of bipolar high power transistor for example, in this paper we study the relationship between base-emitter voltage (Vbe) and temperature under the conditions of constant collector-emitter voltage (Vce) and collector-current (Ice). During the experiment, the device is placed in a thermostat. A voltage is applied to the device collector, a current is applied to the base, and the emitter is earthed. Before the device is measured, we set different temperatures and make sure that the equipment is in a steady state. In order to avoid the effect of self-heating, the pulse current is used in the experiment. The pulse width and the period are 500 μups and 1 ms, respectively.The research result shows that the base-emitter voltage (Vbe) decreases linearly with temperature increasing and the base-emitter current is changed below 4% when the temperature is in a range of 40 ℃-140 ℃. In this paper we also deduce the mathematical expressions for base-emitter voltage (Vbe) and temperature under a steady state. It is proved that the Vbe-temperatrue curve is linear and temperature error is less than 0.5 ℃ when the changes of base current value does not exceed 4%. Therefore, in this paper we deduce a new method of testing the junction temperature in the speeding up measurement experiment. By measuring any of the reference points on the calibration curve under certain experimental conditions, the junction temperature can be calculated quickly according to the proposed formula.Finally, the phase 11 is used to verify the proposed method. We measure the thermal resistance upper the case for the junction of model 2N3055 and the thermal resistance under the case for the junction of model 2SD1047. The measurement results of phase11are compared with the junction temperature calculated using the test formula. The results show that the error of junction temperature between the two methods is less than 0.7%, which is corresponding with the needs of practical application. It proves the correctness and feasibility of the method.
Electron-theoretical study on the influences of torsional deformation on electrical and optical properties of O atom absorbed graphene
Authors: Fan Da-Zhi  , Liu Gui-Li  , Wei Lin  ,   College of Constructional Engineering, Shenyang University of Technology, Shenyang 110870, China
Published: December 2020

The effects of torsional deformation on the structural stability, the electronic structures and the optical properties, including adsorption energy, band gap, absorption coefficient and reflectivity of O atom adsorbed graphene are studied by using the first-principles calculations. Our results indicate that the C atom closest to O atom is pulled up, causing the graphene plane to be distorted after the O atom has been adsorbed. The adsorption energy calculations show that due to the adsorption of O atom, the structural stability of graphene system decreases, but the degree of torsion has a weak effect on the structural stability. The analysis of band structure shows that the adsorption of O atom causes the graphene to convert into a semiconductor from a metal. Torsional deformation makes it change from a semiconductor to a metal, and to a semiconductor. The O atom adsorption system with a torsion angle of 12° has an indirect band gap but the band gaps of other systems are all direct bandgaps. Compared with the intrinsic graphene torsion system, the adsorbed O atom system has an electronic structure that is less sensitive to torsional deformation. When the torsion angle changes from 10° to 16°, the bandgap is always stable at around 0.11 eV. And the adsorption system always corresponds to a narrow bandgap semiconductor in this torsion angle range. For optical properties, comparing with the O atoms adsorbed on graphene with the 0° torsion angle, the peaks of the absorption coefficient and the reflectivity of the system are reduced, and have a transform of red shift into blue shift in a torsion angle ranging from 2° to 20°.
Electron screening lengths and plasma spectrum in single layer MoS2
Authors: Tao Ze-Hua  , Dong Hai-Ming  ,   School of Physical Science and Technology, China University of Mining and Technology, Xuzhou 221116, China
Published: December 2020

We obtain the energy eigenvalues and wave functions of the single layer molybdenum disulfide by using an effective Hamiltonian. Moreover, the density of states and high electron-electron screening length up to 108 cm-1 are also evaluated based on the dielectric function of MoS2. It is shown that the quasi-linear energy bands split off due to the spin-orbit couplings. Plasmons in such a system are investigated theoretically within diagrammatic self-consistent field theory. In the random phase approximation, it is found that two plasma spectra can be produced via intra band transitions induced in conduction bands in monolayer MoS2 because of splitting off. The plasma spectrum frequency increases with increasing wave-vector q and electron density. It is found that the two plasmon modes induced by the spin intra-subband transitions are acoustic-like and depend strongly on wave-vector q. We find that the plasma spectrum is very different from those of graphene and two-dimensional electron gas due to the quasi-linear dispersion and spin-orbit couplings in single layer MoS2. Moreover, the plasmon frequency can be effectively controlled through changing the doping electron density. Our results exhibit some interesting features which can be utilized to realize the plasmonic devices based on the single layer MoS2.
An assessment method for aviation network optimization based on time-varying small world model
Authors: Han Ding-Ding  , Yao Qing-Qing  , Chen Qu  , Qian Jiang-Hai  ,   Shanghai Key Laboratory of Multidimensional Information Processing, School of Information Science and Technology, East China Normal University, Shanghai 200241, China;,   School of Mathematics and Physics, Shanghai University of Electric Power, Shanghai 200090, China
Published: December 2020

The optimization of aviation networks is of great significance for optimizing the allocation of resources, improving transport efficiency, and enhancing the competitiveness among airline companies. There have been a lot of researches which combine the theory of complex network and the actual situations to analyze the air transportation system. The present work provides a certain theoretical basis for the plan of airline schedule. Firstly, we regard an airport as a node, flight frequency as a link weight, and build a heterogeneous network. Through empirical analysis, we find that the aviation network has small-world and scale-free properties. In addition, considering that the instant network consists of current flights changing over time, time-varying is another important characteristic of aviation network. Also, a spatiotemporal correspondence between the flight frequency and route geometric distance is demonstrated to be τij~rij-C. Secondly, by Monte Carlo simulation, we know that the time-ordered topologies influence the optimal navigation structure and make it different from those from traditional static models. Specially, we can obtain a unique restriction between C and optimal structural exponent α, which unveils a new optimization principle in route design and schedule arrangement. Applying these features to the cost-minimized optimization model, a method to evaluate the optimization of network is proposed, by which we can directly predict the overall optimal distribution of flight distances and corresponding flight frequencies only based on the information about the passenger flow assignment. Thirdly, China aviation network data from 2001 to 2010 are used for empirical study. It is found that the predictions consist with the actual data. Compared with traditional optimization methods, it can simplify the computational complexity, and therefore it takes full advantage of the structural convenience and provides a new perspective for the overall scheduling of air transportation system. In this case, companies are able to estimate route adjustments easily to see whether they are reasonable and analyze the development trend of network to provide suggestions for future optimization.
First-principle calculation on electronic structures and optical properties of hybrid graphene and BiOI nanosheets
Authors: Wang Yi-Fei  , Li Xiao-Wei  ,   National Laboratory of Mineral Materials, Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, School of Materials Science and Technology, China University of Geosciences(Beijing), Beijing 100083, China
Published: December 2020

Photocatalytic technology has wide potential applications in the fields of energy generation and pollutant purification due to its advantages of low cost and environmental friendliness. Besides traditional photocatalysts of TiO2 and ZnO, the developing of new photocatalyst with novel properties of strong oxidation, reduction ability, and visible light response has received more attention. Bismuth compounds, such as BiOX (X=Cl, Br, I), exhibit highly efficient photocatalytic activity because of its layered structure and electronic characteristics. The special layered structure, resulting in built-in-field, is favorable for the separation and migration of photogenerated electrons and holes. Among BiOX compounds, BiOI has the best optical absorption characteristics in the range of visible light, and also has the best photocatalytic activity for the degradation of organic pollutants under visible light irradiation. Graphene is an ideal two-dimensional crystal with zero band gap and a high specific surface area. Many researches have shown that graphene can effectively reduce recombination probability of hole and electron because of its unique electron transport property, and it can improve the photocatalytic activity and light stability of the composite catalytic materials.
In this paper, by constructing BiOI nanosheets and hybrid graphene/BiOI, the nanocomposite photocatalytic materials each with a high specific surface area and good photocatalytic activity are obtained. First-principle calculation based on density functional theory is used to investigate the electronic and optical properties of single/double layer BiOI nanosheets and their nanocomposites with graphene. Three kinds of vacancy defects, such as Bi, O and I in BiOI, are also considered. The calculated results show that the spontaneous charge transfer from graphene to BiOI takes place, forming electron-hole puddle because of the interface interaction between graphene and BiOI. Additionally, the hybrid graphene/BiOI complex displays an enhanced optical absorption behavior in the visible light region, improving its photocatalytic activity. The calculated results about the vacancy defects show that the Bi vacancy enhances the charge transfer between BiOI and graphene and forms more electron-hole puddles. In contrast, O and I defects restrain the charge separation between two layers and reduce the formation of electron-hole puddles.
Focusing properties of Lucas sieves
Authors: Xie Jing  , Zhang Jun-Yong  , Yue Yang  , Zhang Yan-Li  ,   National Laboratory on High Power Laser and Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China
Published: December 2020

A kind of optical diffractive element named photon sieve, which is essentially Fresnel zone plate in which the transmissive rings are replaced with a large number of randomly distributed isolated pinholes, can be used to focus X-ray and extreme ultraviolet lithography spectrum into spots with sizes smaller than the diameter of the smallest circular pinhole. However, both the traditional photon sieves and Fibonacci sieves have no more than two axial foci. In order to break this limitation, the Lucas sequence is introduced into the design of photon sieves, and thus producing four axial foci. With respect to the previous Fibonacci sequence, Lucas sequence has the same recursion relation as well as the same eigenvalue of golden mean =(1 + 5)/2. The only difference between them is the first two initial seeds. Based on Fresnel-Kichhoff diffraction theory, the simulation results show that there exist four focal spots with approximately equal intensity along the optical axis on condition that the hole diameters are set to be 1.16 times the underlying Fresnel zone width. Then in order to verify the validity of our proposed model, a Lucas sieve of diameter 12.11 mm and referred focal length 180 mm is fabricated by photolithography and its focusing properties are precisely measured by the in-line phase-shifting digital holography. In experiment, a quarter wave plate is used to realize two-step phase-shift interferences, and obtain the quad-focal length by auto-focusing algorithm in holography. Meanwhile, the quad-focal spots can also be calculated through the diffraction propagation of reconstructed object wave. Compared with the theoretical values, the measurement results indicate that the maximum deviation of quad-focal lengths is less than 0.9%, and the relative errors of the full width at half maximum of four Airy spots are all less than 5%. The experimental results agree well with the theoretical analysis results. Owing to the advantages of small volume, little weight and easy processing, Lucas sieve has great potential in X-ray microscopy, array imaging for living biological cell and especially in the next generation of synchrotron light sources.
Preliminary study on numerical solver of inhomogeneous burnup equations
Authors: Fu Yuan-Guang  , Deng Li  , Li Gang  ,   Institute of Applied Physics and Computational Mathematics, Beijing 100088, China;,   CAEP Software Center for High Performance Numerical Simulation, Beijing 100088, China
Published: December 2020

The inhomogeneous burnup equation is often used for describing the time evolution of nuclides' depletion in nuclear systems which have a significant nuclide migration effect. However, lots of burnup calculations codes only deal with the homogeneous cases instead of the inhomogeneous ones, among them there are a few codes that can work only when the inhomogeneous term of the equation is constant. Based on the condition that the inhomogeneous term can be approximated by finite-order Taylor expansion, two methods are introduced to solve the inhomogeneous burnup equation whose inhomogeneous term is time dependent. For the first method, the transmutation trajectory analysis method is used to decompose the connections between nuclides into linear chains, for one chain the analytical solution is derived strictly by using the Laplace transform. For the second method, a solution of the inhomogeneous equation in the form of summation of infinite matrix series is first derived, and then the sum function of the series is found. Furthermore, the different-order nearly-best rational approximation function of the sum function is found by using Carathéodory-Fejér method. The error between the sum function and the rational function fluctuates in a certain range without exceeding a limit value, while the maximum error decreases exponentially with the order of rational function increasing. By adopting the nearly-best rational approximation, the summation of infinite matrix series converts into a finite expansion of matrix fraction, which is much easier to deal with. These two methods are implemented in the burnup calculation code JBURN and numerical tests are done through using two examples. The first example is a small-scale matrix example and the result shows that the results from the two methods agree well in at least 6 decimal precision together with the results from the reference solution. The second example is a large-scale problem based on real nuclides' reaction database, and the result shows that less than 1% among all nuclides have a deviation larger than 10% between two methods, while about 8% nuclides have a deviation larger than 0.01% and the remaining ones have a deviation smaller than 0.01%. These results validate the correctness and accuracy for each of the two methods. Finally, this paper provides a possible implementation process for solving inhomogeneous burnup equations which have other time-dependent forms of inhomogeneous term.