Vol. 64, No. 10 (2015)
2015-05-20
GENERAL
2015, 64 (10): 100201.
doi: 10.7498/aps.64.100201
Abstract +
Extracting the harmonic signal from the chaotic interference background is very important for theory and practical application. The wavelet transform and empirical mode decomposition (EMD) have been widely applied to harmonic extraction from chaotic interference, but because the wavelet and EMD both present the mode mixing and are sensitive to noise, the harmonic signal often cannot be precisely separated out. The synchrosqueezing wavelet transform (SST) is based on the continuous wavelet transform, through compressing the time-frequency map of wavelet transform in the frequency domain, the highly accurate time-frequency curve is obtained. The time-frequency curve of SST which does not exist between cross terms, can better improve the mode mixing. The SST has also good robustness against noise. When the signal is a mixed strong noise, the SST can still obtain the clear time-frequency curve and approximate invariant decomposition results. In this paper, the SST is applied to the multiple harmonic signal extraction from chaotic interference background, and a new harmonic extracting method is proposed based on the SST. First, the signal obtained by mixing chaotic and harmonic signals is decomposed into intrinsic mode type function (IMTF) by the SST. Then using the Hilbert transform the frequency of each IMTF is analyzed, and the harmonic signals are separated from the mixed signal. Selecting the Duffing signal as the chaotic interference signal, the extracting ability of the proposed method for multiple harmonic signals is analyzed. The different harmonic extraction experiments are conducted by using the proposed SST method for different frequency intervals and different noise intensity multiple harmonic signals. And the experimental results are compared with those from the classical EMD method. When the chaotic interference signal is not contained by noise, the harmonic signal extraction effect is seriously affected by the frequency interval between harmonic signals. If the harmonic frequency interval between harmonic signals is relatively narrow, each harmonic signal cannot be accurately extracted by the EMD method. However, the harmonic extraction precision of SST method is not seriously influenced by the change of harmonic frequency interval, and when the frequency interval between harmonic signals is small the SST method can still accurately extract each harmonic signal from chaotic interference. When the noise contains a chaotic interference signal, the harmonic extraction effect of EMD method significantly decreases with noise intensity increasing. When the noise level reaches 80%, the extracted harmonic signal from the EMD method is seriously distorted, the correlation coefficient of the extracted harmonic signal with original harmonic signal is only about 0.6. With the increase of noise intensity, the harmonic extraction effect of SST method has also a declining trend. But as the noise intensity is within 120%, the harmonic extraction effect of SST method does not greatly change and the extracted harmonic signal precision is still higher, which shows that the harmonic extraction method based on the SST has good robustness against noise. The comprehensive experimental results show that the proposed SST method has high extracting precision for multiple harmonic signals of different frequency intervals, and the SST method has better robustness against Gauss white noise. The extracted results of harmonic signal are better than those from the classical empirical mode decomposition method.
2015, 64 (10): 100301.
doi: 10.7498/aps.64.100301
Abstract +
We study the problem of canonical quantization of classical scalar and Dirac field theories in the finite volumes respectively in this paper. Unlike previous studies, we work in a completely discrete version. We discretize both the space and time variables in variable steps and use the difference discrete variational principle with variable steps to obtain the equations of motion and boundary conditions as well as the conservation of energy in discrete form. For the case of classical scalar field, the quantization procedure is simpler since it does not contain any intrinsic constraint. We take the boundary conditions as primary Dirac constraints and use the Dirac theory to construct Dirac brackets directly. However, for the case of classical Dirac field in a finite volume, things are complex since, besides boundary conditions, it contains intrinsic constraints which are introduced by the singularity of the Lagrangian. Furthermore, these two kinds of constraints are entangled at the spatial boundaries. In order to simplify the process of calculation, we calculate the final Dirac brackets in two steps. We calculate the intermediate Dirac brackets by using intrinsic constraints. And then, we obtain the final Dirac brackets by bracketing the boundary conditions. Our studies show that we can not only construct well-defined Dirac brackets at each discrete space-time lattice but also keep the conservation of energy discretely at the same time.
2015, 64 (10): 100701.
doi: 10.7498/aps.64.100701
Abstract +
According to irradiation-reflection model, by combining the generalized bounded operation model with guide filter, the problem of enhancement for multispectral degraded images with blurred details can be effectively solved and the contrast and low signal-to-noise ratio can be improved. The multi-scale reflection component image, i.e., final enhanced image is obtained through the following procedures: using the adaptive different scales of guide filter function as surround function estimate reaction; separating out the high-low-frequency information; obtaining the different scales Irradiation images which react the overall structure of the image; using the bounded generalized logarithmic ratio (GLR) model addition to replace the Retinex logarithmic transformation; taking a similar logarithmic transformation to the original image to improve the contrast of the image and make the dark area of image details enhanced; again using GLR model subtraction to remove illuminate components from the original image to segment the different scales of the reflection image, thereby avoiding the loss of small details and the big details caused halo effect and noise interference. With four direction Sobel gradient image which reflects the comprehensive edge details of image information the adaptive gain function can be obtained. To avoid the smooth area noise amplification, by using the GLR model multiplication and addition to fuse the effective information of different scales images, the multi-scale reflection image, namely the final enhanced image are obtained. The effective suppression of the emergence of halo effect and computing overflow, which can retain a large number of image details; the comparision of subjective visual effect and the quantitative parameter analysis of the visible low illumination image, haze image, infrared image and X-ray medical images (a total of four groups of multispectral degraded images), the use of the contrast and entropy as evaluation indices, qualitative and quantitative comparison with a variety of image enhancement algorithms, show that the proposed algorithm strengthens and keeps the details of the image texture and edge, realizes the image contrast enhancement and the effective dynamic range compression, has a strong anti-noise ability, and can meet a variety of practical engineering image enhancement needs. The results of the study has been used in the infrared thermal imager, and good results have been achieved. The proposed algorithm is only for 8-bit grayscale image enhancement, and the color image enhancement will be studied in the future.
2015, 64 (10): 100202.
doi: 10.7498/aps.64.100202
Abstract +
The cohesiveness of autonomous cooperative system is the basis of achieving the effective communication and cooperation among the multiple vehicles. Therefore, the biosphere mechanism of queen mandibular pheromone is used for reference in this paper, to abstract the macro motion characteristic of multiple unmanned aerial vehicles autonomous cooperative tracking system. Then, some Lyapunov functions are constructed to analyze the stability of this system, thus obtaining its judgment criterion of stability. Finally, the simulation is given to verify the effectiveness of the proposed stable mechanism. The results show that the proposed stable mechanism not only can ensure the stability of autonomous cooperative system, but also can control the scale of this system effectively by adapting some relevant control parameters.
2015, 64 (10): 100203.
doi: 10.7498/aps.64.100203
Abstract +
Based on the standardized precipitation index data of 89 meteorological stations in southwest China (Sichuan Province, Yunnan Province, Guizhou Province, Chongqing) during 1961-2010, probability model containing drought duration and drought severity is established by using the theory of run and the Copula function. The influences of the drought sample number on the distribution parameters, the probability and drought return period are discussed. The result shows that the stability of distribution parameters needs larger sample number. The sample number is greater than 50 in some regions and the requirements for sample number of each parameter is not consistent. The sample number of severity distribution parameters is largest. The probability and return period obtained in the case where the sample number is about 10 have no significant difference (the significant level is 0.05) from those in the case where the sample number is 40 in most of region. With the results used as the standard, statistical model can greatly reduce the requirements for the sample number. And then it demonstrates that the distribution function of drought duration and drought severity can still be established in the lack of measurement data and the inconsistency between starting and ending time. Climate warming has no influence on the minimum of sample number. The fluctuation is mostly between -5 to 5. Statistical model has a certain stability. Meanwhile, the division of climate state reduces the need for distribution test sample number and makes it easier to build model.
2015, 64 (10): 100204.
doi: 10.7498/aps.64.100204
Abstract +
For the mechanical analyses of the axisymmetric structures in civil and mechanical engineering, combining the interpolating reproducing kernel particle method and the principle of minimum potential energy of space axisymmetrical elastic problems, the interpolating particle method for space axisymmetrical problem of elasticty is presented. And the corresponding matrix equations are deduced. This method employs the shape function with interpolating properties of scatter points and forms the displacement trial function to get rid of dependence on meshes, so it has an advantage that it can directly exert boundary conditions and can increase the computation efficiency. This method can obtain the global continuous stress field directly and avoid the fitting calculation error of stress in the post-processing of finite element method, then it is a high-precision numerical simulation method. Numerical examples are given to show the validity of the new mesh-less method in the paper.
2015, 64 (10): 100205.
doi: 10.7498/aps.64.100205
Abstract +
Radio acoustic sounding system (RASS) is a detection technique using the interaction between radio wave and acoustic wave to remotely measure vertical profiles of the atmospheric temperature, and usually composed of a Doppler radar with fixed beam (monostatic or bistatic) and an acoustic source with high power. By combining acoustic propagation equation and radio wave propagation equation in a disturbance medium and using a finite-difference time-domain method, a numerical model describing the interaction between acoustic wave and electric wave is constructed, and the model is used to analyze the effects of wind and temperature on detection height of RASS. In the atmospheric temperature background, the propagations of a single frequency acoustic wave packet under different wind conditions are simulated, and the scattering propagation of electric wave packets corresponding to the acoustic scatterer are analyzed and compared. Besides, the entire physical process are described from the angle of energy density. The numerical simulation results show that the propagation trajectories of both acoustic wave and radio wave backscattering echo are changed due to the existence of wind field and temperature profile. The presence of wind field results in an offset of acoustic wave front, reducing the strength and changing the trajectory of radio wave backscattering echo, so that the detection height is limited due to the reduction of receiving data. The simulation results of the acoustic wave reveal that the temperature profile mainly affects the propagation velocity of acoustic wave, while the presence of wind field may result in shifts of propagation trajectory and acoustic wave front, and the greater the wind speed, the more the horizontal shift of acoustic wave front is. The numerical analyses of scattering propagations of radio wave with the acoustic scatterer at the same height under different background atmospheric conditions manifest that the stronger the wind speed, the more the deviation of electric wave echo from the receive antenna is, and the smaller the echo intensity is when the scattering echo propagates to the same position. The theoretical calculations with the acoustic wave scatterer at different heights under the same atmospheric wind field (strong wind) background demonstrate that if the height of scattering point is reduced, the offset of the scattering echo “bunching point” at the same altitude will be greatly improved and the intensity will be enhanced, but it also means the decline of detection height. In order to improve the detection height under the background of wind field, some methods are adopted, such as using a bistatic radar antenna or increasing the reception antenna area.
2015, 64 (10): 100501.
doi: 10.7498/aps.64.100501
Abstract +
Noise, which is ubiquitous in real systems, has been the subject of various and extensive studies in nonlinear dynamical systems. In general, noise is regarded as an obstacle. However, counterintuitive effects of noise on nonlinear systems have recently been recognized, such as noise suppressing chaos and stochastic resonance. Although the noise suppressing chaos and stochastic resonance have been studied extensively, little is reported about their relation under coexistent condition. In this paper by using Lyapunov exponent, Poincaré section, time history and power spectrum, the effect of random phase on chaotic Duffing system is investigated. It is found that as the intensity of random phase increases the chaotic behavior is suppressed and the power response amplitude passes through a maximum at an optimal noise intensity, which implies that the coexistence phenomenon of noise suppressing chaos and stochastic resonance occurs. Furthermore, an interesting phenomenon is that the optimal noise intensity at the SR curve is just the critical point from chaos to non-chaos. The average effect analysis of harmonic excitation with random phase and the system’s bifurcation diagram shows that the increasing of random phase intensity is in general equivalent to the decreasing of harmonic excitation amplitude of the original deterministic system. So there exists the critical noise intensity where the chaotic motion of large range disintegrates and non-chaotic motion of small scope appears, which implies the enhancing of the regularity of system motion and the increasing of the response amplitude at the input signal frequency. After that, the excess noise will not change the stability of the system any more, but will increase the degree of random fluctuation near the stable motion, resulting in the decreasing of the response amplitude. Therefore, the formation of stochastic resonance is due to the dynamical mechanism of random phase suppressing chaos.
NUCLEAR PHYSICS
2015, 64 (10): 102801.
doi: 10.7498/aps.64.102801
Abstract +
The traditional digital imaging of neutron radiography is based on neutron scintillation screen cooperated with charge coupled device (CCD) camera, whose spatial resolution and neutron detection efficiency are contradictory. Neutron detection method based on microchannel plates (MCP) could solve the problem appearing in traditional method. It could supply high spatial resolution, high neutron detection efficiency and high time resolution. It is of benefit to high-resolution neutron radiography and neutron energy choice imaging. Tremsin et al. [Tremsin A S, Feller W B, Downing R G, Mildner D R 2004 U. S. Government Work not Protected by U. S. Copyright p340] calculated the detection efficiency of thermal neutron sensitivity MCP in 2004. Then his team fabricated a prototype of neutron detection system based on MCP and carried out the neutron imaging experiments on several neutron sources. The experimental results show that spatial resolution is nearly 15 μm and neutron detection efficiency for cold neutron is more than 70%. In China, Yang Y G et al.[15] from Tsinghua University developed a neutron detection system based on MCP, and preliminary neutron experimental results indicate that spatial resolution is about 200 μm.#br#In order to find the optimal structure of MCP, in this paper we calculate the detection efficiency of thermal neutron sensitive MCP doped (or coated) by boron and gadolinium with Monte-Carlo method. The neutron detection efficiency P is determined by three terms P1, P2 and P3, which are related by P=P1× P2× P3. Here, P1 is the possibility that the neutrons are absorbed by MCP solid parts, P2 is the possibility that the secondary particle escapes into MCP channel and generates an electron avalanche, and P3 is the possibility that the electron avalanch is recorded by readout system. Theoretical analysis indicates that more solid parts of MCP can make P1 higher and increase the difficulty for secondary particle to escape, and make P2 lower. There may be an optimal geometry to make the total P maximal. This paper gives the calculation method of P1 and P2, and approximates P3 to 1. #br#The calculation results show that the neutron detection efficiency depends on channel diameter (or coated thickness) and material, but not on the structure of MCP. When the thickness of MCP is 0.4 mm, the pixel of MCP is 15 μm, and the neutron sensitivity material is 10B2O3, the optimal thermal neutron detection efficiency is more than 40% with a channel diameter of 8.0 μm for the doped MCP, and it is nearly 60% with a coated thickness of 1.5 μm for the coated MCP. With the same geometry parameters and the neutron sensitive material such as natural Gd2O3, the optimal thermal neutron detection efficiency is more than 30% with a channel diameter of 9.0 μm for the doped MCP, and it is more than 50% with a coated thickness of 0.5 μm for the coated MCP.
2015, 64 (10): 102901.
doi: 10.7498/aps.64.102901
Abstract +
Fusion reactor is considered as one of the solutions for the sustaining development of nuclear energy. International Thermonuclear Experimental Reactor (ITER) is the biggest fusion reactor research plan in the world. High-intensity D-T fusion neutron generator can generate 14 MeV neutrons, and it matches the neutrons generated in ITER and be competently used for imitating the neutron environment in nuclear fusion reactor, which is important for the relevant experimental researches of blanket materials of fusion reactors. It can also be used for validating the correctness and reliability of the simulations and analyses in fusion basic studies, and can guide the subsequent material improvement and innovation of calculation methodology. A rotating tritium target system for D-T fusion neutron generator with a neutron yield of 1012 n·-1, i.e., a high intensity D-T fusion neutron generator, is proposed in this paper and the design, main parameters, technical difficulties and heat transfer enhancement method are introduced. The key and innovative technology of this rotating target system is the integration of the sprayed water cooling, mechanical seal and magnetic fluid seal technologies, which focuses on the heat transfer of the high heat power density in the target system. The most important technical index is that the maximum temperature on the target should not be above 200 ℃ as the tritium ions run away heavily from the tritium target when the target temperature is bigger than 200 ℃. To investigate the heat transfer characteristics of this rotating target system, the effects of water layer thickness, water flow rate and rotating speed on the heat transfer of this rotating target system are analyzed by computational fluid dynamics method. And the heat transfer processes of the target system under different heat power densities are also simulated and studied. The analysis results show that big water layer thickness, big water flow rate and high rotating speed are good for the heat transfer enhancement of the rotating target system, but the effects of the changes of the water layer thickness and water flow rate on the heat transfer process are both very small. Due to the design index, the heat power density on the target should be under a limit value, which is about 12 kW·cm-2 in the calculation results of this paper.
ATOMIC AND MOLECULAR PHYSICS
2015, 64 (10): 103402.
doi: 10.7498/aps.64.103402
Abstract +
Sympathetic cooling is one of the most promising techniques for producing ultracold molecules from precooled molecules. The previous work has shown that it is inadequate to use the ultracold alkali-metal atoms as coolant for sympathetic cooling. Whether the ultracold alkali-earth-metal atoms can be used as coolant deserves to be investigated. In this paper, the cold collision dynamic behaviors for Mg atom and CO molecule are investigated by quantum scattering calculations. The influences of electric field on the elastic and inelastic collision cross sections of low field seeking state within cold and ultracold temperature are explored. The results show that sympathetic cooling CO molecule with ultracold Mg atom might be difficult to perform.
2015, 64 (10): 103401.
doi: 10.7498/aps.64.103401
Abstract +
For α-boron, R3m group, Lennard-Jones (L-J) pair potential function is fitted, and a pairwise many-body potential is constructed. For constructing both interatomic potentials, only the atomic average cohesive energy and geometric information are needed. And the cohesive energy and geometry of α-boron crystal are calculated by first-principles code Castep. The fitting procedure for the potentials is as follows. For L-J potential, the minimum of the function is set to be located at the nearest neighbors. For the pairwise many-body potential, L-J potential is minimal, and the form of the function is chosen as a piecewise function, which consists of the L-J function and polynomial function. The minima of L-J potential are located at the distances between the different neighbors of atoms, and the potential barriers are at the midpoints of the distances of the two neighbor minima. L-J potential, L-J pair potential, and Tersoff potential for boron are tested and compared with each other, by energy minimization method in molecular dynamics (MD) simulation. The radial distribution function is used to analyze the structure obtained from the simulation results obtained by using different potentials. The results show that the structure after minimization deviates significantly from the initial crystal of α-boron by L-J potential, and final structure is consistent well with the initial ideal crystal, with L-J potential used. The NVT ensemble is used in MD simulation, where the temperature is set to be 2000 K, and the α-boron crystal experiences the thermodynamic evolutions for 10 fs and 100 fs, to obtain the deviated initial structures. Then the minimization by MD simulation is made to test the three potentials, which also shows that the L-J potential can give the much better result than the other two potentials.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2015, 64 (10): 104201.
doi: 10.7498/aps.64.104201
Abstract +
The underwater visibility is very important in underwater vision research and target detection. However, most underwater vision systems cannot guarantee to possess the performance under complex water conditions. This is because underwater images are usually degraded by light-water interactions of absorption and scattering. The ambient light is scattered into the cameras line of sight by water molecules and suspended particles in the water medium, which adds a layer of haze to the image and reduces the contrast of the image. This part of scattered light is usually called background light, which is the main reason for underwater image degradation. In this paper, the formations of background light in underwater imaging under two different lighting conditions: natural illumination and artificial lighting, are analyzed by setting up physical models. The models developed include the parameters such as camera parameters, light source parameters, inherent optical properties, and camera-source-object geometry. Based on the models, the relationship between the background light and the above parameters is studied. Computer analysis shows that the global background light under two illumination conditions has a close relationship between the inherent optical properties of water medium and camera parameters. The global background light under natural illumination is proportional to the scattering coefficient and inversely proportional to the attenuation coefficient. The background light under the two illumination conditions both can be described in simple exponential falloff expressions of the global background light. The simple expression greatly reduces the computational complexity of simulations. The intensity of background light mainly depends on the inherent optical properties, camera-scene distance, camera-source distance and cameras imaging angle. The relationship between the global background light and the inherent optical properties can be used to estimate the attenuation coefficient, scattering coefficient and scene depth information. The result of this paper can be very useful for designing and improving the underwater imaging systems.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2015, 64 (10): 105101.
doi: 10.7498/aps.64.105101
Abstract +
To obtain the base data for insulation design of the electrical equipment used in the high temperature gas cooled reactor nuclear power plant, an experimental apparatus for testing helium insulation property under high pressure is designed. The apparatus is composed of a pressure vessel, a heating system, an electrical penetration assembly, a vacuum pump, a pressure gauge, a safety pressure valve, a release valve, and a helium bottle. The highest pressure that the vessel can hold is 10 MPa, and in this experiment the safety pressure valve is set to be 8 MPa. The temperature inside the vessel can be heated to 200 ℃ by a heating system. The resolution of the pressure gauge is 1 kPa, and the highest pressure that the gauge can measure is 9.999 MPa. The purity of the helium used in this experiment is 99.999%. The breakdown voltage of helium gas between two parallel plane electrodes is measured by the apparatus under the conditions of 15-20 ℃ and 0.1-7 MPa. The electrodes are made of copper, and their diameters are both 100 mm. The distances between the parallel electrodes are 0.25 mm, 0.35 mm and 0.5 mm respectively. The error of the distance is less than 0.01 mm. The DC voltage between the electrodes is supplied by GPI-735A, a withstand voltage and insulation tester produced by GW Instek corporation. The voltage increases slowly from 0 to 6000 V (highest), until the current more than 0.1 mA is detected. The highest voltage recorded is the breakdown voltage. With other researchers’ experimental data under low pressure, the Paschen equation for helium gas is obtained. It is found that the calculated breakdown voltage for this equation is larger than the experimental result in this paper under high pressure. And the deviation becomes larger as the product of the pressure and the distance increases. Firstly, it is because the ionization coefficient γ in the equation is influenced by the gas pressure. Secondly, because of larger areas of electrodes, worse surface roughness and less electrode distance, under the same product of the pressure and the distance, the breakdown voltages in this paper are less than the ones in other researches, which are the base of the calculated values. The Paschen equation is modified to accord with the values under high pressure. Under high pressure, the Paschen curve is almost a straight line. A linear equation is presented for calculating the breakdown voltages of helium gas under 0.1-7 MPa. And an equation is presented to calculate the slope of the line. The slope is influenced by the collision cross section, the ionization energy and the temperature of the helium. The experimental data also show that the breakdown voltage of helium is far lower than that of air under the same condition. As the pressure increases, the breakdown voltage of helium increases. The value of helium under 3 MPa is equal to the one of air under atmosphere, and the value of helium under 7 MPa is about twice as high as that of helium under 3 MPa. So, it is possible to replace some experiments or tests under high pressure helium by the same operations under atmospheric air.
2015, 64 (10): 105201.
doi: 10.7498/aps.64.105201
Abstract +
Backward Raman amplification (BRA) in plasma can be used for generating ultra-powerful laser pulses. In this paper, the plasma density effect on backward Raman laser amplification is studied by using particle-in-cell method. It is found that using a low plasma density can lead to the premature Langmuir wave breaking and thus result in a small energy-transfer efficiency. On the other hand, using a high plasma density will enhance the developments of unwanted instabilities, which rapidly disturb the Raman amplification, thus limiting the interaction length and output power. Therefore, an optimal plasma density for BRA is near the threshold of Langmuir wave breaking in order to achieve both high efficiency and large energy flux. The space frequency spectrum analysis shows that the saturated intensity of amplified pulses is limited mainly by the self-phase modulation instability. By using a 1013 W·cm-2 pump pulse, our simulation results show that the initial 1013 W·cm-2 seed pulse can be well be well amplified into a pulse with an energy power of 1017 W·cm-2, a duration of 40 fs, and and an energy conversion efficiency of up to 58%.
EDITOR'S SUGGESTION
2015, 64 (10): 105202.
doi: 10.7498/aps.64.105202
Abstract +
Shock wave is a common phenomenon in astrophysics. Shock wave acceleration has been regarded as a source of high-energy cosmic rays. Very strong magnetic field exists in the surrounding of the shock wave at the edge of the supernova remnants. But the mechanisms of generation and amplification of such a strong magnetic field are not clear yet. In this paper, the properties of shock wave driven by the laser irradiating on un-magnetized and magnetized plasmas are investigated using two-dimensional particle-in-cell (PIC) simulations. It is found that very strong spontaneous magnetic field can be generated around the laser-driven shock front in the un-magnetized plasma. The spontaneous magnetic field can store energy and accelerate electrons further. When an external magnetic field is introduced, the electrons and ions are accelerated more efficiently by the shock wave than in the un-magnetized plasma. The external magnetic field can transfer its energy to electrons and ions, and strengthen the shock wave. In simulations, the introduced external magnetic field has three different strengths: 1072 MG, 107.2 MG and 10.72 MG, which determine the shock structures through the driven currents. There are two single-polar magnetic arcs that constitute the shock structure when the external magnetic field is 1072 MG, i.e., one is the shock itself and the other is actually the reverse shock, whereas only one magnetic arc is produced but with a bipolar structure in the direction perpendicular to the shock propagation when the externally added magnetic fields are much lower (107.2 MG and 10.72 MG). The two bipolar magnetic structures will evolve into a single-polar arc when the externally added magnetic field is 107.2 MG, but they are kept for all the time when the external magnetic field is 10.72 MG. It can be explained by taking the Larmor radius into the consideration. That the amplification ratio of the magnetic field decreases as the introduced external magnetic field increases implies that the magnetic amplification in the space is possibly due to the local field generation rather than the field compression. An amplification ratio of tens of the external magnetic field is achieved due to the pseudo Rayleigh-Taylor instability, but still much smaller than that around the astrophysical shock front, indicating that other efficient mechanisms are responsible for the observed magnetic amplification around shocks in the supernova remnants.
2015, 64 (10): 105203.
doi: 10.7498/aps.64.105203
Abstract +
In this paper, the interaction between the low energy proton ring-beam with an initial velocity perpendicular to the background magnetic field, and the background plasma is studied by one-dimensional (1D) hybrid simulations. In the initial stage, the excited plasma waves experience a fast growth exponentially, which is consistent with the linear theory. After that, three non-linear stages, including the saturation process, the fast damping process and the relatively stable stage, follow in sequence. In the linear stage, the mode-resonance damps with the pitch angle scattering of the injected protons, and the plasma oscillation reaches the peak quickly. The continuing pitch angle scattering makes the velocity distributions of the proton beam and the background ions uniformly distributed. Meanwhile, the initially excited right-handed resonant instability decreased, with only the Alfven waves left in the stable stage. The results also show that the effective heating of the background plasma is achieved after the linear stage, instead from the very beginning of the injection of the protons. This demonstrates that the excited plasma waves lead to the energy transferring from the injected proton beams to the background plasma.
2015, 64 (10): 105204.
doi: 10.7498/aps.64.105204
Abstract +
Air corona discharge ionic wind exciter can generate driving force without any rotating component, which makes it commonly used in aviation and aerospace field. Although there are many explanations of the thrust generating mechanism of the air corona discharge ionic wind exciter, no existing theories can unify the experiment results obtained under various conditions. A further study is still needed. The paper focuses on the characteristics of wire-aluminum foil exciter. The experiments show that the electrostatic force acting on the wire-aluminum foil is asymmetric and the variations of the height in lengthways aluminum foil and the air pressure can change the electrostatic force. Meanwhile, with the theoretical analysis the calculation model of the force of the wire-aluminum foil exciter’s corona discharge is established by taking the influences of corona layer and space charge into consideration. The calculation fits the measured value. By combining with the theoretical analysis, the thrust of wire-aluminum foil electrode corona discharge exciter is proved to come from the space charge produced by wire electrode corona discharge, which exerts an asymmetric electrostatic force on the electrode system and generates a net electrostatic force for the exciter.
2015, 64 (10): 105205.
doi: 10.7498/aps.64.105205
Abstract +
With the slow effect of electric field of thundercloud, a kind of positive glow corona without streamers is initiated from the surface of object near the ground, and a large number of positive space charges are injected into the surrounding space, consequently, lighting targets selected by the lighting leader can be changed. In this paper, a numerical simulation of positive glow corona discharge initiated from the long ground wire with the effect of the electric field of thundercloud is presented. In consideration of the attachment and collision effects between positive ions and other ions, an accurate two-dimensional positive glow corona model is established. Meanwhile, a high-voltage corona discharge experiment is done in the laboratory to measure the corona current in different background electric fields, and the results are compared with the simulation results in order to verify the correctness of the model established in this paper. According to the established model, the initiation and development progress of glow corona with the effect of thundercloud are simulated and the corona current, laws of positive ion density distribution and migration are revealed. Results show that positive ions generated from the glow corona discharge present a circular symmetric distribution in the plane perpendicular to the ground wire at their early stage of migration, but the distribution is shaped as an elongated oval later when the ions move farther from the ground wire for the effect of electric field of thundercloud, that is to say, the overwhelming majority of the ions will be finally distributed in the upper area of the ground wire and gradually migrate towards the thundercloud. Due to the accumulation effects of positive ions in the upper migration area near the ground wire, the positive space charge background is formed, which has a damping effect on the electron beam. Thus the formation of electron avalanche is suppressed and the probability for electron avalanche to be converted into streamer is reduced. Meanwhile, the positive space charge background improves the collision surface of the gas and increases the compound probability between positive ions and electrons. Therefore, the conversion processes from electron avalanche and streamer to upward leader are impeded and the initiation of upward leader is suppressed.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2015, 64 (10): 106101.
doi: 10.7498/aps.64.106101
Abstract +
The thermal-stability and melting mechanism of the Ag nanocrystalline in the process of high-temperature relaxation are investigated with embedded atomic method and molecular dynamics simulations. The dynamic evolution of the crystalline’s morphology is revealed based on the analyses of mean square displacement and lifetime of the stability. It is concluded that there are obviously anisotropic behaviors in the process of high-temperature relaxation in the quasi-cubic nanocrystal which is cut along the inter-perpendicular facet of {110}, {211} and {111}. The thermal-stability decreases in the sequence of facet (111), facet (110), and facet (112). The lifetimes of the first outmost and the second outmost atoms in those three different facets are extremely short and show no evidently difference from each other. However considering the facets with the same crystal plane indices, the lifetimes are longer within the third atom layer and subtly increase with the increase of the number of atom layers. However, the lifetimes are distinctly different from each other among the three different facets within the third atom layer.
2015, 64 (10): 106103.
doi: 10.7498/aps.64.106103
Abstract +
Spray forming is a kind of near-net-shaped rapid solidification process based on powder metallurgy gas atomization technology. In this work, the FGH4095M is fabricated by spray forming. The pre-alloy is prepared by vacuum induction melting and vacuum arc remelting techniques. Then the alloy is sprayed by SK2 facility with atomization gas nitrogen at University of Bremen in Germany. In this paper we study the density and microstructure of the spray-formed billet, especially the special morphology of γ’ phase. The results show that density is associated with different parts of the deposited billet. The relative density of the bottom part is higher (99.63%) than those in the other parts. The relative density of top part (98.91%) is lowest. After hot isostatic pressing, the relative density can be up to 100%. Uniform and fine equiaxed grains are the remarkable morphology of spray-formed alloy without prior particle boundary. The sizes of grains are in a range of about l0-40 μm and the grains at bottom part of billet are finest. The grain sizes of primary γ’ phase are in a range of about 0.6-0.8 μm, and the grain sizes of secondary γ’ phase in a range of about 0.1-0.5 μm as well as dispersed spherical tertiary γ’ particles with the sizes of 10-20 nm. The special morphology of secondary γ’ phase occurs with the splitting of γ’ particle, which is related to the low cooling rate of the depositing process. The splitting behavior reduces the total energy of γ’ particle. Total energy of γ’ particle includes elastic interaction energy, elastic strain energy and surface energy, among which the elastic strain energy is invariable. The surface energy increases with the splitting process and the elastic interaction energy reduces. The effect of elastic interaction energy on particles is the major reason why the total energy is reduced. The trend of splitting behavior is analyzed by calculating the equivalent diameter of splitting γ’ particle. It indicates that when the equivalent diameter is over 0.40 μm, there is the possibility to split. Subsequently, spray-formed FGH4095M billet is treated by hot isostatic pressing, isothermal forging and heat treatment process to obtain the FGH4095M alloy turbine disk. The research of tensile property of FGH4095M alloy turbine disk shows an excellent property either at room temperature or at high temperature for the optimized alloy. The relationship between special morphology of γ’ phase and excellent property needs further investigating.
2015, 64 (10): 106401.
doi: 10.7498/aps.64.106401
Abstract +
Shock and release experiments are performed on the porous Sn with sub-micropores with porosity m=1.01. Time-resolved interfacial velocities between the porous Sn and LiF window are measured with Doppler pins system under seven pressure points from 31.8 GPa to 66.1 GPa. From the interfacial velocity, the Euler longitudinal sound velocities and the bulk sound velocities are obtained. The corresponding Poisson ratio and shear modulus are determined, too. From the transition of longitudinal sound velocity to bulk sound velocity at high pressures, the shock-induced melting of Sn with porosity 1.01 occurs at about 49.1 GPa. With the Euler longitudinal sound velocities, the bulk sound velocities and the shear moduluses of porous and dense Sn, the melting pressure zone of dense Sn can be determined to be between 53.5 GPa and 62.3 GPa. Comparing the melting zone of porous Sn and that of dense Sn, micropores in the material reduce the the shock melting pressure obviously. The Exact shock melting pressure of dense Sn needs further experimental data in the corresponding pressure zone. From the longitudinal velocity of porous Sn in the measured solid zone, no bcc phase transition takes place for this material. This may relate with the micropores in the material or the difference in material component, which needs further investigating.
EDITOR'S SUGGESTION
2015, 64 (10): 106102.
doi: 10.7498/aps.64.106102
Abstract +
A coaxial cylindrical heterojunction of carbon tubes, which consists of alternant bands of 5- and 7-membered rings, can be formed by one armchair (n, n) carbon nanotube and one zigzag (2n, 0) carbon nanotube. The torsional mechanical properties of this kind of (n, n)-(2n, 0) heterojunction constructed by the same length of armchair and zigzag nanotubes are studied by using molecular dynamics method. In order to make a comparison, the relations of the torque and axial stress to torsional angle of (n, n) and (2n, 0) carbon tubes are also systemically calculated. Moreover, the transfer process of torsional stress in the (n, n)-(2n, 0) heterojunction is analyzed. Some important conclusions are obtained. Firstly, the torsional angle corresponding to the buckling point of carbon nanotubes is closely related to their torsional stiffness. The buckling angle decreases monotonically with torsional stiffness. Secondly, as the torsion develops, the torsional stress appears from the joint position due to the fact that the junction part in the (n, n)-(2n, 0) heterojunction has the smallest torsional stiffness and then transfers from the joint position to both ends. The propagation velocity of the torsional stress in (n, n) nanotube which has smaller stiffness is faster than that in (2n, 0) nanotube with bigger stiffness. Finally, for the process of torsion within the elastic limit, no axial stress is produced in (n, n)-(2n, 0) heterojunction during the torsion. This effect is of great significance for designing the carbon nanotube-based nano-oscillator devices.
2015, 64 (10): 106104.
doi: 10.7498/aps.64.106104
Abstract +
Structural kinetics in crystalline solids is driven heterogeneously at an atomic level by localized defects, which in turn drive mesoscopic and macroscopic phenomena such as structural phase transformation, fracture, and other forms of plastic flows. A complete description of such processes therefore requires a multiscale approach. Existing modeling methods typically operate exclusively either on an atomic scale or on a mesoscopic scale and macroscopic scale. Phase-field-crystal model, on the other hand, provides a framework that combines atomic length scale and mesoacpoic/diffusive time scale, with the potential reaching a mesoacpoic length through systemic multiscale expansion method. In order to study the dislocation movement under shear strain, the free energy density functional including the exerting shear force term is constructed and also the phase field crystal model for system of shear stain is established. The climb and glide of single dislocation in two-grain system are simulated, and the glide velocity of dislocation and the Peierls potential for dislocation gliding are calculated. The results show that the energy curve changing with time are monotonically smooth under a greater shear strain rate, which corresponds to dislocation movement at a constant speed, which is of rigorous characteristic; while under less shear strain rate, the energy change curve of system presents a periodic wave feature and the dislocation movement in the style of periodic “jerky” for gliding with the stick-slip characteristic. There is a critical potential for dislocation starting movement. The Peierls potential wall for climbing movement is many times as high as that for gliding movement. The results in these simulations are in a good agreement with the experimental ones.
2015, 64 (10): 106105.
doi: 10.7498/aps.64.106105
Abstract +
Grain boundary (GB) research is always the most fundamental and active study field in interface science. Grain boundary premelting (GBPM) is induced as a consequence of local inner strain around defects in material at high temperature. When GB premelting is under an external stress, it is referred to as stress induced GBPM (SIGBPM). Owing to the fact that the width of a GB usually is a few atoms thick, it is difficult to observe the GBPM directly in experiment, thus the development of computational simulation experiment can make up for the shortcomings in experiment. For this reason, a new method which is named phase field crystal (PFC) model based on density functional theory is proposed. Because the method can be used to simulate the evolution of macroscopic structure of polycrystalline material on a diffusive time and atomic scale, therefore, PFC has a great advantage in simulating the evolution of microstructure. In this paper, PFC method is used to investigate the annihilation process of dislocation pairs of premelted grain boundary under strain at high temperature. Simulated results show that the essence of separation process of sub-GB (SGB) from original GB is that sub-grain structures are generated. The SGB migration is the process of the new grain swallowing up the old one. The annihilation process of GBPM under applied strain at high temperature can be divided into two stage features. The first stage is the stage of system energy increasing, which is corresponding to the process of SGB migration, dislocation gliding; the second stage is the energy decreasing, which corresponds to the interaction of SGBs and annihilation of dislocations, while the speed of annihilation in this process is slow and the peak of energy curve is wide and smooth. According to the changing process of the atomic density distribution projected along the directions of x and y axis with strain increasing, we can reveal that the nature of annihilation of double dislocation pairs at high temperature is the process of two-step annihilations, of which the detailed process is not easy to observe at low temperature due to its fast annihilating speed of dislocation pairs.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2015, 64 (10): 107302.
doi: 10.7498/aps.64.107302
Abstract +
In a semi-infinite crystal, the periodic potential is destroyed at the surface, and the electronic wave functions exponentially decay from the surface to both sides. Such localized electronic states in the vicinity of the surface are known as Tamm surface states. In analogy to the electronic Tamm states, in recent years, optical Tamm states have been found at the surface of the truncated photonic crystal composed of two kinds of dielectrics. Very recently, novel types of optical Tamm states including backward Tamm states in which the phase velocity and the group velocity of optical waves are in the opposite direction have been discovered in the photonic structures containing metamaterials. In fact, the concepts in electronic field and photonic field can inspire each other. Many unique phenomena in photonic systems can also be mapped to the electronic systems. In this paper, we study the novel types of electronic Tamm states in electronic systems, inspired by the novel types of optical Tamm states in photonic structures. #br#At first, comparing Maxwell equations with Schrodinger equations, one can see a correspondence between the parameters in electromagnetic system and the parameters in the electronic system. In particular, Hg1-xCdxTe semiconductors with special electronic band structures can realize various electronic materials in analogy to the optical metamaterials with various values of permittivity and permeability. By tuning the parameter x of Hg1-xCdxTe, we obtain a variety of metamaterial-like electronic materials, in analogy to the single-negative metamaterials, the double-negative metamaterials and the near-zero-index metamaterials in optical systems. Then, inspired by the one-dimensional heterostructures with metamaterials that generate optical Tamm states, we design a one-dimensional electronic heterostructure consisting of Hg0.847Cd0.153Te and CdTe/HgTe superlattice. When Hg0.847Cd0.153Te is analogous to the double-negative metamaterial, we find the backward electronic Tamm states in which the phase velocity and the group velocity of electronic waves are in the opposite directions. When Hg0.847Cd0.153Te is analogous to the near-zero-index metamaterial, we find a novel electronic Tamm states in which the amplitude of the electronic probability decays very slowly in Hg0.847Cd0.153Te. The discovery of these new types of electronic Tamm states enlarges our knowledge of electronic surface states.
2015, 64 (10): 107501.
doi: 10.7498/aps.64.107501
Abstract +
Recently, magnonic crystals which are the magnetic counterparts of photonic crystals or phononic crystals are becoming a hot area of research. In this paper, band structure of two-dimensional magnotic crystal composed of square rods triangularly arranged are calculated by using the plane-wave expansion method. Spin-wave band structures of two-dimensional magnonic crystal composed of Fe triangularly arranged Fe in an EuO matrix. The results show that when the filling ratio f=0.4, only two absolute band gaps can be found in the case of θ=0°. The first gap appears between the first band and the second band, the second gap between the sixth band and the seventh band. However, the number of band gaps can be improved by rotating the square rods through θ=25°, there are eight absolute band gaps that can be found. The first gap appears between the first band and the second band, the fifth gap between the sixth band and the seventh band. The new band gaps can be found, the second gap appears between the third band and the fourth band, the third gap between the fourth band and the fifth band, the fourth gap between the fifth band and the sixth band, the sixth gap between the seventh band and the eighth band, the seventh gap between the eighth band and the ninth band, the eighth gap between the ninth band and the tenth band. These results show that it is possible to create spin-wave gaps by rotating square rods in a two-dimensional magnotic crystal. The numerical results of the normalized gap width ΔΩ/Ωg of the first gap between the first band and the second band always changes with filling fraction f and rotational angles θ. When f=0.6 we calculated the first normalized gap width ΔΩ/Ωg. when f=0.6 and θ=0°, the first gap width ΔΩ=0.812(μ0ω/g) and the normalized gap width ΔΩ/Ωg=0.9187. The results show that from the first normalized gap widths the largest one can be found when f=0.6 and θ=5°, the first gap width ΔΩ=0.937(μ0ω/g) and the normalized gap width ΔΩ/Ωg=0.9591. The results show that the numerical, rotating square rods can make the low frequency band gap widen in the triangular lattice of two-dimensional magnonic crystal.
2015, 64 (10): 107502.
doi: 10.7498/aps.64.107502
Abstract +
In order to study how the ingredient, sintering temperature, oxygen, doping and other conditions affect magnetic properties of strontium ferrite powder, a strontium ferrite powder is prepared by sol-gel method, and a new method of studying magnetic properties of strontium ferrite powder based on an electron paramagnetic resonance (EPR) is established in this paper. The sintered samples are tested by electron paramagnetic resonance spectrometer. Results show that -Fe2O3, a paramagnetic intermediate, is most compared with other ratios under calcined at 400 ℃ and the strontium iron mole ratio of 1:9; while at the other temperatures it decreases and the ferromagnetic phase increases; the optimum calcination temperature is between 800 ℃ and 900 ℃. These facts are caused by both external magnetic field and other magnetic fields, thus resulting in some new stronger magnetic moment interactions. Results also show that a large quantity of paramagnetic -Fe2O3 is found under hypoxic annealing environment, which is not conducible to generating the ferrimagnetic phase; X-ray diffraction (XRD) analysis shows that the others are paramagnetic and ferrimagnetic phase except a bit of other phases; both EPR spectra and XRD spectra show that the paramagnetic phase is least, and ferrimagnetic phase is most in the sintering sample when strontium iron mole ratio is 1:9, so the sample owns the strongest magnetism. The sample remanence experiment by milli-tesla meter also confirms these results. It is also found that paramagnetic phase can effectively decrease and ferrimagnetism is enhanced when samples are doped by lanthanum ion accounting for 20%-30% of the total number of moles of strontium lanthanum.
2015, 64 (10): 107801.
doi: 10.7498/aps.64.107801
Abstract +
Many GaInN light-emitting diodes (LEDs) are subjected to a great temperature variation during their serving. In these applications, it is advantageous that GaInN LEDs have a weak temperature dependence of forward voltage. However, the factors determining the exact temperature dependence of the forward voltage characteristics are not fully understood. In this paper, two series of GaInN LEDs are prepared for investigating the correlation between the epitaxial structural and the temperature dependence of the forward voltage characteristics. The forward voltage characteristics of samples are studied in a temperature range from 100 K to 350 K. The curves of forward voltage versus temperature (dV/dT) are compared and analyzed. For the three samples in series I, according to the barrier thickness and emitting wavelength, they are designated as blue multiquantum well (MQW) with thin barrier (sample A), blue MQW with thick barrier (sample B), and green barrier with thick barrier (sample C) respectively. Their structures of active region including the insertion layer between n-GaN and MQW, the MQW, and the emitting wavelength are different from each other. However, the same slopes of dV/dT at room temperature (300 K± 50 K) are observed in the samples. Moreover, samples B and C with the same p-type layer design also have the same slopes of dV/dT at cryogenic temperatures. Sample A with a much thinner p-type layer shows a lower slope than samples B and C. Based on the these experimental data, it is deduced that the intrinsic physic properties of active region such as structure and emission wavelength have a little influence on the variation of the slope of dV/dT either at room temperature or at cryogenic temperatures. Moreover, the Mg concentration of the p-GaN main region determines the slope of dV/dT at cryogenic temperatures. Low doping concentration leads to a high slope of dV/dT.#br#In order to find the decisive factor determining the slope of dV/dT at room temperature, three samples in series II are grown. For sample E, at the MQW-EBL (electron blocking layer) interface, the Mg concentration increases very slowly while an abruptly varying doping profile is observed for samples D and F. The slopes of samples D and F are both -1.3 mV·K-1. This is very close to the calculation value of the lower bond for the change in forward voltage (-1.2 mV·K-1). Meanwhile, the slope of sample E is -2.5 mV·K-1, which is much higher than those of samples D and F. Thus, it is suggested that the major factor influencing the slope of dV/dT at room temperature is the Mg doping profile of the initial growth stage of the p-AlGaN electron blocking layer. These phenomena are mainly attributed to the changes of the activation energy of p-AlGaN and p-GaN, since it relies on the doping concentration and temperature. Our findings clarify the roles of active region, p-AlGaN and p-GaN in the temperature dependence of the forward voltage characteristic. More importantly, the results obtained in this study are helpful for optimizing the growth parameters to achieve LED devices with forward voltage that has a low sensitivity to temperature.
2015, 64 (10): 107101.
doi: 10.7498/aps.64.107101
Abstract +
We use a density-matrix renormalization group method to study quantitatively the phase diagram of the half-filled one-dimensional (1D) extended Hubbard model in the presence of a staggered ionic potential Δ. An extensive finite-size scaling analysis is carried out on the relevant structure factors and localization operator to characterize the Mott-insulator (MI)-bond-ordered insulator (BOI)-band-insulator (BI) transitions. The intermediate BOI phase occupies a small region of the phase diagram, and this region is enlarged in the presence of Δ. In addition, the phase diagram of ionic Hubbard (the nearest-neighbor electron-electron interaction V=0) is also given.
2015, 64 (10): 107201.
doi: 10.7498/aps.64.107201
Abstract +
We theoretically investigate the effects of electronic correlations (including spin and Coulomb correlations) on the magnetotransport through a parallel double quantum dot (DQD) coupled to ferromagnetic leads. Two dots couple coherently through electron correlations, rather than tunneling directly between two dots, and each dot is coupled to two semi-infinite ferromagnetic leads. We assume that the intradot Coulomb repulsion is much larger than the interdot Coulomb repulsion U. Thus, only the zero, one and two-particle DQD states are relevant to transport. Because of interdot electron correlation, the I-V characteristics of each dot is sensitive to the change in the state of the other dot. This work focuses on the effects of electron spin correlation and electron Coulomb correlation on magnetotransport through this system. In order to determine the transport properties of the system, we use the generalized master equation method. This method is based on the reduced density operator defined by averaging the statistical operator of the total system over the states of all leads. With the framework of the generalized master equation and the sequential tunneling approximation, we calculate the current, differential conductance and tunnel magnetoresistance (TMR) in the dot 1 as a function of bias for different spin correlations and Coulomb correlations. Our results reveal that the magnetotransport through this system is more sensitive to Coulomb correlation than to spin correlation; when Coulomb correlation equals zero, the spin correlation can induce a giant tunnel magnetoresistance, which is further larger than the Jullieres value of TMR; when Coulomb correlation occurs, the giant tunnel magnetoresistance disappears; when Coulomb correlation is equal to or larger than spin correlation, Coulomb correlation can suppress spin correlation; while the coexistence of Coulomb correlation and asymmetry of the DQD system can result in dynamical channel blockade, which can lead to the occurrence of negative tunnel magetoresistance and negative differential conductance. These novel properties lead to the potential applications in nanoelectronics, and relevant underlying physics of this problem is discussed.
2015, 64 (10): 107301.
doi: 10.7498/aps.64.107301
Abstract +
Since the topological insulator was discovered, the investigation of topological properties has become the hot spot in condensed matter physics. In this paper, we study topological properties of chalcogenide compounds Ge2X2Te5 (X=Sb, Bi) crystals and their monolayer and bilayer films as well as the vertical uniaxial pressure induced topological quantum phase transitions in monolayer and bilayer films. The results show that for A-type crystal, the bulk structures of these two compounds are topological insulators, the monolayer structures of these two compounds are conventional metals, and bilayer structures are topological metals. There is no topological quantum phase transition in monolayer nor bilayer film under the uniaxial compression. While for B-type crystal, the bulk structures of these two compounds are conventional insulators, the monolayer Ge2Sb2Te5 is conventional metal, its bilayer structure as well as monolayer and bilayer of Ge2Bi2Te5 films is conventional insulator. All the B-type monolayer and bilayer films each undergo a topological quantum phase transition to the topological metals under the uniaxial compression.
2015, 64 (10): 107303.
doi: 10.7498/aps.64.107303
Abstract +
First-principles calculation is a quite powerful tool for explaining experimental phenomena and predicting the properties of new materials. Based on the first-principles calculation within the density functional theory, the energetic stabilities and electronic properties of Mg and Si doped GaN/AlN superlattices with wurtzite and zinc-blende structures are investigated. The results show that there is no variation in formation energy if the doping position is changed when the impurities are doped in the well (GaN) region, and the same situation also happens in the barriers (AlN) region. Thus it is equivalent for dopants to replace Ga atoms in the cation site of wells or Al atoms in the cation site of barrers. However, the formation energies of these dopants in the well region and the barrier region are different. Compared with the formation energy in the barrier region, it is much lower in the well region. That is to say, the impurities in the cation site (MgGa, MgAl, SiGa and SiAl) present lower formation energies in the wells of GaN/AlN SLs with wurtzite and zinc-blende structures. In addition, the impurities in zinc-blende GaN/AlN superlattices present lower formation energy than in the wurtzite structure. The negative formation energy illustrates that the defects are spontaneously formed if Mg-atom is mixed into the wells of the zinc-blende structure. Therefore, in experiment, for the zinc-blende superlattice structure, preparing p-type semiconductor needs less energy than preparing n-type semiconductor. And for the wurtzite superlattice structure, preparing p-type semiconductor needs the same energy as preparing n-type semiconductor. Furthermore, the relationships between the distribution of the electronic states and their structures are analyzed. It is found that the different kinds of dopants lead to different band bendings, owing to the modified polarization fields. The spatial distributions of electrons and holes, plotted by the partial charge densities, reveal that electrons and holes experience redistributions by Si or Mg dopants in different phases. The band gap of doped GaN/AlN superlattice decreases and the projected density of states also accounts for the change of defect formation energy. The calculated results provide a new reference for the fabrication of modulation-doping GaN/AlN SL under desired control, which could be considered to control phase.
2015, 64 (10): 107701.
doi: 10.7498/aps.64.107701
Abstract +
The development of high-performance integrated circuit chips and the shrinkage of feature sizes according to Moore’s law bring forward continuously the requirements for low dielectric constant (low-k) materials with various excellent properties in the back-end-of-the-line (BEOL) interconnect. Porous SiCOH films prepared by plasma enhanced chemical vapor deposition (PECVD) through a porogen approach are widely applied to industry and extensively studied. Thermal annealing is an important process for fabricating the porous low-k films, which has a great influence on film structure as well as properties. SiCOH films are deposited by PECVD using tetraethoxysilane and limonene as precursors, and annealed at 450 ℃ for 1.5 h under nitrogen atmosphere. The evolutions of film structure and properties during thermal annealing are revealed, and the reaction mechanism for structure change is also proposed. Fourier transform inferred spectroscopy and solid state nuclear magnetic resonance results show that the as-deposited film is an organic-inorganic hybrid film composed of various kinds of Si-O-Si, -CHx, Si-O-CH2CH3, etc. The organic component is removed almost completely during thermal annealing, making a porous film with a Si-O-Si inorganic skeleton. The skeleton is also rearranged at the same time. Deconvolution of the Si-O-Si absorption band of the FTIR spectrum reveals that the cage-like Si-O-Si occupies the major part for both as-deposited and annealed films, while the amount of silicon suboxide Si-O-Si decreases and that of network Si-O-Si increases during thermal annealing, making the film more robust. More C=C and Si-C are formed through chemical reactions between Si-H, -CHx and Si-O-CH2-CH3, and crosslinking is further enhanced. Nitrogen adsorption/desorption isothermal measurement reveals that a large number of micropores with diameter less than 2-3 nm are created during thermal annealing, which is consistent with the removal of organic groups and the existence of cage-like Si-O-Si. As a result, both the refractive index and dielectric constant decrease significantly from 1.476 (λ =630 nm) and 3.45 to 1.365 and 2.60, respectively. Because of the increase of C=C after annealing, extinction coefficient and leakage current density increase. Although there is a shrinkage of 14.7% in film thickness and a reduction of mechanical properties after annealing, the Young’s modulus is still larger than 4 GPa. Considering all kinds of properties, the obtained film appears to be a competitive candidate as inter layer dielectrics in the BEOL interconnect of integrated circuits.
2015, 64 (10): 107802.
doi: 10.7498/aps.64.107802
Abstract +
In order to optimize the performance of the coupled resonator optical waveguide (CROW) gyroscope, a well-designed structure by optimizing the ring number and the transmission coefficient of the coupler is used as the core component of the planar waveguide optical gyroscope. The structure with double coupled ring resonator may possess large effective group refractive index to enhance angular sensing sensitivity was proposed. The concept of the Sagnac effect in this new double coupled ring resonator structure with a large effective group refractive index is investigated, it’s found that the rotation-induced phase shift is proportional to the effective group refractive index. On the basis of this effect, we calculate the general relation expression of theoretical rotation sensitivity and the effective group refractive index for this two-ring bidirectional CROW gyroscope by numerically simulation. Based on the relation, the phase shift characteristics of double coupled ring resonator and single ring resonator was analyzed. And the changing characteristics of the transmission coefficient of the couplers and the effective group refractive index was discussed based on the double cascaded ring resonator coupling mode theory. In the case of R1=R2=100 μm and ring transmission loss coefficient t1=t2=0.95, the generating condition of the largest effective group refractive index was obtained, according to the different effects of the couplers between rings and waveguide on the effective group refractive index. By using the parameters of R=100 μm and t=0.95, the sensitivity of a single ring resonator gyroscope is (104-105) °/h, and the sensitivity of double coupled ring resonator gyroscope can reach to 10 °/h. In summary, we show that the theoretical sensitivity of the double coupled ring gyroscope and single ring gyroscope are comparable when both have the same parameters. Using numerical and analytical methods, we demonstrated that coupling multiple resonators together can enhance rotation sensitivity. This research is important for applications of coupled ring resonator in optical angular velocity detection, and a promising regime to realize highly compact optical gyroscope.
2015, 64 (10): 107803.
doi: 10.7498/aps.64.107803
Abstract +
Eu3+ doped CaMoO4 micron phosphors of different concentrations were prepared by chemical precipitation method. The photoluminescence properties were studied in detail. The X-ray diffraction measurements indicate that the samples are scheelite structure, and doping Eu3+ enlarge the lattice parameter of host material. The scanning electron microscope images show that the particle morphology is near-spherical and the size is 4-5 μm. The excitation and emission spectra of CaMoO4:Eu show that CaMoO4:Eu can be effectively excited by blue light and near UV-light, and the red light emission of high colour purity can be realized. The electron-phonon coupling properties were also studied. The results indicate that the magnitude of Huang-Rhys factor is 10-2, so the CaMoO4:Eu is a weak electron-phonon coupling material. The results also indicate that the Huang-Rhys factor increases with the increase of concentration. It is probably because that increasing the doping concentration enhances the lattice relaxation. The transition intensity parameter Ω2 of Eu3+ enlarges while the concentration is increasing. Because Ω2 and the luminescent center environment are closely related, the numerical value of Ω2 enlarges with the increase of degree of environmental disorder. But Ω4 doesn't have obvious change. It can be explained that the environmental sensitivity level of Eu3+5D0→7F2 transition is higher than that of 5D0→7F4 transition. The quantum efficiency of Eu3+ 5D0 energy level decreases with the increase of doping concentration. This can be attributed that the enhancement of Eu3+ doping concentration enlarges the energy transfer rate between luminescent centers, so the energy of excited electrons can be transferred to quenching center more easily and then the nonradiative relaxation rate of excited electrons increases. The best doping concentration of Eu3+ is 25% by drawing the concentration quenching curve. Furthermore, the energy transfer type of Eu3+ in CaMoO4 host is confirmed to be exchange interaction and the critical distance is calculated to be 8.4 Å. The chromaticity coordinate of CaMoO4:Eu phosphors is (0.654, 0.334), so the samples have high colour purity. The study indicates that CaMoO4:Eu micron phosphor prepared by chemical precipitation method is a red phosphor material with excellent property.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2015, 64 (10): 108101.
doi: 10.7498/aps.64.108101
Abstract +
Precious metals exhibit fascinating properties and extensive applications in chemical engineering, high-temperature measurement, and electronic industry. The microstructures of these metals are generally polycrystalline and the precious metals like Ir and Ru with polycrystalline microstructures are difficult to deform at room temperature. However, the single crystal of precious metal can be well deformed to the final product, and it can be effectively used as a material. In this paper, electron beam floating zone method (EBFZM) is employed to prepare single crystals of precious metals, due to the fact that precious metals, e. g. Ir and Ru have high melting points of 2443 ℃ and 2310 ℃ respectively, and no crucible can be used for this processing. Considering the fact that the height of floating zone plays a key role in EBFZM, we deduce the expression for height of floating zone in EBFZM based on pedestal growth and zone melting techniques. The effects of crystal growth angle, interface growth mechanism, and solidification rate on the height of floating zone are discussed. The results show that the heights of floating zone for six precious metals are in a sequence order of Ru >Pd >Ir >Pt >Ag >Au. The crystal growth angles of these metals are calculated in a range of 8.4°-10.7°. For the same growth angle, the heights of floating zone, calculated by the Pedestal growth, zone melting and Czochralski-like growth techniques, are close to each other. But for different growth angles, the height of floating zone increases with increasing the growth angle for pedestal growth and Czochralski-like growth techniques, different from zone melting technique. Meanwhile, the height of floating zone changes with interface growth mechanism and solidification rate. For the pedestal growth technique, the height of floating zone is low for continuous growth mechanism, and for zone melting technique, its height of floating zone, calculated from continuous growth mechanism, is larger than those from the dislocation and faceting growth mechanisms. Furthermore, it reveals that the growth angle and height of floating zone vary slightly with continuous growth mechanism. In addition, a predicted solidification rate of 2.4 mm/min, available for single-crystal growth of precious metals, is in agreement with the previous experimental results of single crystals Ir and Ru prepared by EBFZM.
2015, 64 (10): 108103.
doi: 10.7498/aps.64.108103
Abstract +
The substrate of as-cast Ni-28 wt% Sn hypoeutectic alloy immersed in liquid nitrogen is rapidly remolten and solidified by laser surface remelting with a scanning velocity of 10 mm/s and the laser power of 1950 W. The microstructure of the substrate and its effect on the microstructure of the molten pool are investigated by scanning electron microscope carefully. It is found that the substrate of the Ni-28 wt%Sn ingot is composed of coarse primary α-Ni dendrites and the interdendritic (α-Ni+Ni3Sn) eutectic. The growth orientations of α-Ni dendrites and the interdendritic eutectic are distributed nearly randomly in the as-cast substrate. There are three kinds of microstructure characterstic zones from the top to the bottom of melted pool. The growth directions of α-Ni dendrites with the primary dendritic spacings ranging from 4.19 to 6.91 μm are approximately parallel to the laser scanning direction at the top of the molten pool due to the fact that the temperature gradient at the interface between the molten pool and substrate tends to be parallel to the laser scanning direction. In the middle of the molten pool, the epitaxial α-Ni columnar dendrites are found to be inclined to grow in the direction vertical to the bottom of the molten pool due to the fact that the temperature gradients in most zones of the molten pool are perpendicular to the bottom of the molten pool. The formation of new primary dendrites by the growth of the tertiary arm results in the decrease of primary dendritic spacing in comparison with that at the bottom of the molten pool. There are a small quantity of residual α-Ni primary phase and a large amount of (α-Ni+Ni3Sn) eutectic at the bottom of the molten pool. The microstructure of laser remolten zone is greatly influenced by the substrate microstructure, and the growth direction of the α-Ni dendrite in the molten pool is also affected remarkably by both the heat flux and the preferred crystal orientations for dendritic growth. Compared with the mixed lamella, rod and divorced (α-Ni+Ni3Sn) eutectic microstructures in the substrate, the eutectic structure in the molten pool is completely composed of the refined lamellar eutectic, which grows epitaxially in the direction perpendicular to the interface between the molten pool and the substrate at the bottom of molten pool. The eutectic lamellar spacing increases from the top (0.23 μm± 0.01 μm) to the bottom (0.42 μm± 0.02 μm) of the molten pool due to the interface growth velocity decreasing from the top to the bottom. The Kurz-Giovanola-Trivedi model for rapid dendritic growth and the Trivedi-Magnin-Kurz model for eutectic growth are used to estimate the growth undercooling of the microstructure in the molten pool respectively. It is found that the growth undercooling of dendrites and the eutectic in the molten pool should be between 50.4 K and 112.5 K, which is much larger than the critical undercooling for anomalous eutectic growth found in the high undercooled solidification in the previous researches. This phenomenon means that the critical undercooling for anomalous eutectic growth reported in the previous literature may be not the sufficient condition for generating the anomalous eutectic.
2015, 64 (10): 108202.
doi: 10.7498/aps.64.108202
Abstract +
Li-ion battery is a complicated distributed parameter system that can be described precisely by field theory and partial differential equations. In order to reduce the calculation amount and the solution difficulty, a distributed parameter system is often described by ordinary differential equation model during the design and the analysis. As a result, systemic error is caused, and the reliability of the system model is reduced. The rechargeable Li-ion batteries are widely used in many fields because of their excellent properties. The research on the modeling and failure monitor of Li-ion battery can evaluate its working state, and improve the security during its servicing. Li-ion battery system is regarded as a distributed parameter system in this paper. Single particle model is a simplification of a Li-ion battery under a few assumptions. According to the measured data, single particle model can be used for estimating the parameter at a fast simulation speed. Li-ion battery model based on partial difference equations and single particle model is proposed to detect the failure and evaluate the working state of Li-ion battery system. Lithium ion concentration is an unmeasurable distributed variable in the anode of Li-ion battery. The failure monitor system can track the real-time Li ion concentration in the anode of Li-ion battery, calculate the residual which is the difference between the measured value and the ideal value. A failure can be judged when the residual is beyond a predefined failure threshold. A simulation example verifies that the accuracy and the effectiveness of Li-ion battery failure monitor system based on parabolic partial difference equations and single particle model is reliable.
2015, 64 (10): 108301.
doi: 10.7498/aps.64.108301
Abstract +
Based on the standard linear solid model, the solutions in Laplace domain, such as particle velocity v, particle displacement u, radial stress σr, tangential stress σθ, radial strain εr, tangential strain εθ, reduced velocity potential γ (RVP), and reduced displacement potential ψ (RDP), are derived from the spherical wave equations. The propagating characteristics of these physical quantities, as mentioned above, are calculated by using Crump algorithm for inverse Laplace transformation. The numerical inversion results reveal that the initial response to strong discontinuity spherical stress wave in viscoelastic material is purely elastic response. The strong discontinuities, such as σr, σθ, εr, εθ and v, contain geometrical attenuation and viscoelastic damping in the process of wave propagation. The variables, such as σr, σθ, εr, εθ,u and ψ, converge to steady values as time approaches to infinity. The peak values of RVP γ and RDP ψ, which are constant in a purely elastic material, are steadily reduced with the spreading distance increasing in viscoelastic material. The steady values of ψ are in inverse relation to the static shear modulus Ga, and directly proportional to the steady cavity pressure and the cube of the cavity radius r.
2015, 64 (10): 108401.
doi: 10.7498/aps.64.108401
Abstract +
Phase-shift full-bridge (PSFB) DC-DC converter benefits from high efficiency by zero-voltage switching turn-on of all switches without any additional auxiliary circuit, and PSFB DC-DC converter has been widely used in high power applications. In this paper, the operating mode of PSFB DC-DC converter is studied, and the energy iteration model of PSFB DC-DC converter is established. The discrete phase shift (DPS) control technique for PSFB DC-DC converter is proposed and discussed. Unlike the conventional PWM PSFB control technique, the DPS control technique uses two preset phase shift times tpsH and tpsL as control variables where 0tpsHtpsL ≤Tw with Tw being the switching period. When output voltage is lower than the reference voltage, phase shift time tpsH is selected, and a large duty cycle DH is obtained on the secondary side, which makes output voltage increase. Similarly, when output voltage is higher than the reference voltage, phase shift time tpsL is selected, and a small duty cycle DL is obtained on the secondary side, which makes output voltage decrease. With the energy iteration model, the energy iteration process is clearly revealed, steady-state and transient performances are studied. From the analysis results it can be known that the DPS controlled PSFB DC-DC converter always operates in a multi-periodic state. The simulation reasults show that the proposed control technique has an advantage over the conventional PWM PSFB control technique in simple design, great robust and excellent transient performance.
2015, 64 (10): 108404.
doi: 10.7498/aps.64.108404
Abstract +
The sheet beam klystron is a kind of novel powerful microwave and millimeter-wave vacuum electron device, in which used is a thin rectangular sheet beam with high aspect ratio in order to improve beam-wave interaction efficiency by improving space-charge-limiting current of electron beam and obtaining big electric current, and it has many actual and potential applications. Based on the motion of the single electron under the small signal condition, the transit-time effect of electron beam in 2π-mode standing wave electric field in a multiple-cavity resonator is investigated, the expression of electron load conductance in a multiple-cavity resonator is presented, and the influence of the cavity number N on transit-time effect in a multiple-cavity resonator is discussed. The high frequency characteristics of the three-gap extended cavity are studied. The abilities for the single-gap cavity and three-gap cavity to modulate the sheet beam are compared by 3D PIC simulation. The simulation result shows that the three-gap extended interaction cavity operating at 2πmode is better than the single-gap cavity. The electron load conductance is derived and corrected based on the theory of relativity, by which a more accurate relation of electron load conductance to transmit angle can be obtained. In order to improve the output power and electron efficiency, the three-gap extended output cavity is used in the relativistic klystron to replace the single gap output cavity. By using the electromagnetic simulation software and 3D PIC code, a Ka-band sheet beam relativistic extended interaction klystron amplifie is designed. A sheet electron beam with a width-to-height ratio of 30 is adopted to reduce the space charge effect. In the PIC simulation, when the beam voltage is 500 kV and current is 1 kA, the device can generate a 190 MW output power at 40 GHz with an efficiency of 38% and a gain of 69 dB. The 3 dB bandwidth of the EISBK is about 150 MHz. Meanwhile, the output microwave is without the clutter jamming, which makes the contribution avoid the shrinkage of output microwave impulse. This study is of great importance for the physical design and process in engineering of the Ka-band sheet beam extended interaction relativistic klystron amplifier.
2015, 64 (10): 108503.
doi: 10.7498/aps.64.108503
Abstract +
Due to the excellent characteristics of field-effect transistor and its high absorption coefficient in the visible region, pentacene has been widely used in phototransistors. The channel length of the vertical transistor could be designed to be very short (on the order of nanometers). In this way, the device performances and its working frequency can be effectively improved, and the energy consumption can be reduced simultaneously. In this paper, we fabricate a kind of low-voltage pentacene photodetector ITO(S)/Pentacene/Al(G)/Pentacene/Au(D), based on the vertical transistor configuration. The threshold voltage and “on/off” current ratio are -0.9 V and 104 at a low working-voltage of -3 V, respectively. The pentacene photodetector ITO/Pentacene(80 nm)/Al(15 nm)/Pentacene (80 nm)/Au exhibits a good p-type transistor behavior and low-voltage-controlling performance. The photosensitivity and responsivity vary with incident monochromatic light from 350 nm to 750 nm, and the photosensitivity peak of 308 is obtained at 350 nm with a responsivity of 219 mA·W-1, which is even higher than that of the standard Si-based photodetector under 350 nm incident light. Therefore, this work provides an easy way to fabricate a high sensitivity all-organic photodetector working at low voltages.
2015, 64 (10): 108701.
doi: 10.7498/aps.64.108701
Abstract +
Cortical cortex is mainly composed of excitatory and inhibitory neurons. Balance between excitation and inhibition is a ubiquitous experimental phenomenon in brain. On the one hand, balanced excitation and inhibition plays a crucial role in maintaining normal brain functions; on the other hand, the loss of balance between the two opposing forces will cause neural diseases, such as epilepsy, Parkinson, schizophrenia, etc. Thus the research on balance between excitation and inhibition increasingly focuses on the field of neuroscience. Feedback neural circuit with recurrent excitatory and inhibitory connections is ubiquitous in cortical cortex. However, it is still little known how to achieve and maintain the balance between excitation and inhibition in feedback neural circuit. In this study it is proposed that inhibitory synaptic plasticity should play a key role in regulating the balance between excitation and inhibition. Firstly, the feedback neural circuit model is constructed using leaky integrate-and-fire neuron model, mainly composed of excitatory feed-forward loop, and excitatory and inhibitory recurrent connections. The proposed inhibitory synaptic model is incorporated into the feedback neural circuit model, and whose mathematical formulation is presented in detail. Secondly, the excitatory and inhibitory synaptic currents are obtained through numerical simulations, which demonstrate that the precise balance between excitation and inhibition is achieved under the regulation of inhibitory synaptic plasticity. Furthermore, the research results show that this balance is robust to the fluctuation inputs and disturbances. Thirdly, the balance mechanism underlined by inhibitory synaptic plasticity is elucidated through theoretical and simulation analysis, separately, which provides a clear explanation and an insight into how to achieve and maintain the balance between excitation and inhibition in a feedback neural circuit. Finally, the numerical results reveal that the neuron numbers in excitatory and inhibitory feedback loop exert an influence on the balance, and the larger number can enhance the balance between excitation and inhibition, which explains, to some extent, why there are dense connections between neurons in brain. The results in this study shed light on the balance mechanism of feedback neural circuit, and provide some clues for understanding the mechanism of balance between excitation and inhibition in the brain area.
2015, 64 (10): 108801.
doi: 10.7498/aps.64.108801
Abstract +
In this work, the composite anode of BCP/Ag replaces the composite anode of Ca/Al, and the PTB7:PC71BM acts an as active layer for polymer solar cells. Calcium (Ca) is not a desirable candidate as electron extraction layer (EEL) for long-term stability polymer solar cells (PSCs) on account of its nature of active metal. And then, due to the poor stability of Al, which is not a desirable candidate as electrode, the bathocuproine (BCP) layer acts as an exciton blocking layer in organic device such OLEDs and small molecule solar cells, which has a k value that is close to zero for a broad range of wavelengths. The Ag has the nature of better chemical stability and conductivity than Al. In the device architecture described below, we replace the typical back metal electrode composed of a thin Ca layer and a thicker Al electrode by a few nanometer thick bathocuproine (BCP) layer and a thick 150 nm Ag layer. We investigate the effects of BCP thickness on the power conversion efficiency (PCE) and stability. The results reveal that the photovoltaic performances are improved, and a PCE of 6.82% at the 5 nm of BCP thickness, higher than the PCE of Ca/Al acted composite anode, is achieved. The substitution of BCP for Ca, can largely enhance light harvesting and exhibits an optimal light absorption by the active layer. This enhanced reflectivity of the buffer layer/electrode back contact results in an increase of the short circuit current. Compared with the devices of Ca/Al composite anode, it increases Jsc and external quantum efficiency with BCP/Ag composite anode. At the same time, it has the better stability of BCP/Ag composite anode of device, and almost the same PCE decrease ratio as free BCP devices and significantly improves the stability compared with Ca/Al composite anode. The stability test shows the better stability of BCP/Ag as composite anode than that of Ca/Al composite anode. The PCE of the device with Ca/Al as composite anode rapidly decreases by about 70% after 50 hour servicing due to the poor stabilities of Ca and Al. The device with BCP/Ag as composite anode shows favorable stability, owing to the PCE moderate decrease by less than 30% after the same story time. Our results indicate that substitution of BCP/Ag for Ca/Al composite anode is an alternative candidate for high performance and longterm photo stability PSCs.
2015, 64 (10): 108802.
doi: 10.7498/aps.64.108802
Abstract +
In this paper, the basic laws of spectral response, open-circuit voltage and short circuit current of GaAs/Ge solar cells are obtained by ground simulation irradiation test under the different-energy electrons’ irradiations, such as 1, 1.8 and 10 MeV. The carriers’ transport mechanism in cells is analyzed using the PC1 D simulation program. The variations of the majority carriers’ concentration and the minority carriers’ diffusion length with the irradiation particle fluence are obtained in GaAs/Ge solar cells under different-energy electrons’ irradiation. Majority carriers’ removal rate and minority carriers’ diffusion length damage coefficient are calculated under different-energy electron irradiations. The results show that majority carriers’ concentration and minority carriers’ diffusion length decrease with increasing the incident electron fluence. The majority carriers’ removal rate and the damage coefficient of minority carriers’ diffusion length increase with increasing the electrons energy. The majority carriers’ removal effect and the minority carriers’ diffusion length shortened are the main reasons of open-circuit voltage and short circuit current degradation of the solar cells, respectively.
2015, 64 (10): 108102.
doi: 10.7498/aps.64.108102
Abstract +
A three-dimensional (3D) numerical analysis model of tungsten inert gas welding arc interacting with an anode material is presented based on the local thermodynamic equilibrium assumption and taking the behavior of metal vapor into account. The thermodynamic parameters and transport coefficients of plasma arc are dependent on the local temperature and metal vapor concentration. A second viscosity approximation is used to express the diffusion coefficient which describes the metal vapor diffuse in the argon plasma. The weld pool dynamic is described by taking into account the buoyancy, Lorentz force, surface tension, and plasma drag force. The temperature coefficient of the surface tension at the weld pool surface is considered in two ways: one is taken as a function of temperature with only oxygen being the active component, and the other is taken as a constant value. The distributions of temperature field and velocity field of arc plasma and weld pool, metal vapor concentration and current density in the arc plasma are investigated by solving the Maxwell equations, continuity equation, momentum conservation equation, energy conservation equation and the components of the transport equation. The influence of metal vapor on arc plasma behavior and that of arc plasma on the weld pool are studied and compared with the non-metal vapor results. It is shown that the distribution of Fe vapor concentrates around the weld pool surface. Metal vapor has obvious shrinkage effect on arc plasma, and weak influences on velocity and potential of the arc plasma. In addition, the metal vapor has a weak effect on the distributions of velocity and shear force on the weld pool surface and no obvious influence on the molten pool shape. We test two different methods to illustrate this point in the case with or without metal vapor. The method used for a variable temperature coefficient of surface tension allows the prediction of a depth-to-width ratio and weld pool shape in agreement with experimental result when taking the behavior of metal vapor into account. The results in this paper, obtained by simulation are in good agreement with experimental results and also with the simulation results by some other authors.
2015, 64 (10): 108104.
doi: 10.7498/aps.64.108104
Abstract +
This study deals with the case of multiple internal heat source inversion problem of steady-state based on nondestructive infrared detection. We construct homogeneous and heterogeneous steady heat conduction models of different shapes. Neither the number of heat sources, nor their locations, nor their sizes nor their intensities are known. We use the finite element method (FEM) based on numerical algorithm to analyze the two-dimensional model discretely. The internal heat conduction process of model is analyzed. The resultant temperature field can be decomposed into the temperature field caused by the ambient temperature and those given by internal heat sources. We simplify the finite element matrix equation according to decomposition process above. Finally, the problem boils down to solving the highly underdetermined matrix equation of Ax = b. The unknown x item corresponds to internal thermal heat source field. The piecewise polynomial spectral truncated singular value decomposition (PPTSVD) is applied for the first time to the inverse heat source problem. Its regular operator matrix is changed from the original more order differential operator matrix to regional node weighted matrix. After replacement this solution improves the effect of heat source field tending to the boundary. Results of the solution confirms a real heat source field when there are less heat sources or different heat sources are far from each other. But there also exits a serious superimposed effect between neighboring heat sources. We improve the algorithm to study this problem through using the iterative elimination process which complies with the idea of spreading heat source field and then gathering. The iteration tolerance and number of times belonging to one single PPTSVD solving process are reduced. Through iterating the multiple PPTSVD solving process and reconstructing matrixes A and b in each iteration, we obtain the scatter heat source field distribution surrounding real field. Finally, this scattered distribution solution is gathered again. According to the heat source parameter calculated by algorithm, the temperature field of whole model can be reconstructed using FEM. Comsol numerical simulation and real physical experiments are performed to verify the validity and accuracy of the algorithm in different heat conduction models. The results demonstrate that the algorithm can access each parameter of multiple heat sources. Even in the heterogeneous model, it can still obtain accurate results and reconstruct the temperature field of the two-dimensional model. The algorithm can be applied to non-destructive material detection and human infrared medical imaging fields.
2015, 64 (10): 108201.
doi: 10.7498/aps.64.108201
Abstract +
Fluorescence resonance energy transfer (FRET) is non-radiation energy transfer that occurs between a donor (D) molecule in an excited state and an acceptor (A) molecule in a ground state by dipole-dipole interactions. The efficiency of FRET is dependent on the extent of spectral overlap between the donor photoluminescence peak and the absorption spectrum of acceptor, the quantum yield of the donor, and the distance between the donor and acceptor molecules. Currently, FRET is commonly used for determining the metal ion, analyzing the protein, biological molecular fluorescence probe, etc. In this study, the FRET between CdTe quantum dots (QDs) with different sizes and Rhodamine B (RhB) in aqueous solution is investigated by using the time-resolved fluorescence test system under two-photon excitation. In this two-photon FRET aqueous system, QD is used as donor while RhB as acceptor. The time resolved two-photon photoluminescence and fluorescence lifetime measurements are performed for analyzing the two-photon-excited luminescence by using a titanium sapphire femtosecond laser with a wavelength of 800 nm, pulse width of 130 fs, repetition frequency of 76 MHz, with the power fixed at 500 mW. The fluorescence spectrum is measured by fluorescence spectrometer and the fluorescence decay curves are recorded by single photon counter. Besides, the steady state photoluminescence is also studied with a JASCO FP-6500 Fluorescence Spectrometer. The result shows that with the increase of spectral overlap of the CdTe emission spectrum and the Rhodamine B absorption spectrum, the FRET efficiency of the QDs-RhB system becomes higher. Specifically, the fluorescence intensity of QDs decreases and the lifetime of QDs becomes shorter while RhB shows the opposite tendency. By means of the Förster theory of energy transfer, the spectral overlap integral J(λ), Foster radius R0 and the FRET efficiency E are calculated and the FRET characteristics of QD-RhB system is characterized. Theoretical analysis reveals that the physical source is the increase of the sample’s Forster radius. Moreover, the relationship between the ratio of acceptor/donor concentration and the FRET efficiency is investigated experimentally. When the ratio of acceptor/donor concentration increases, the lifetime of QDs turns shorter, and the FRET efficiency of the QDs-RhB system becomes higher. The two-photon excited FRET efficiency can reach 40.1% when the concentration of RhB is 3.0×10-5 mol·L-1. This study shows a brighter future in biological and optoelectronic applications.
2015, 64 (10): 108402.
doi: 10.7498/aps.64.108402
Abstract +
A class of single tooth-shaped plasmonic filter based on graphene nanoribbon is proposed in this paper, and the structure is numerically analysed by using finite-difference time-domain method. The tooth-shaped structure of graphene nanoribbon can induce a sharp band-stop effect in the transmission spectrum, and the filtering characteristics can be analysed by the scattering matrix method. The effective refractive index of the plasmonic waveguide mode in the graphene nanoribbon is analysed numerically, and it is found that the effective refractive index is influenced by both the chemical potential and the width of the nanoribbon, and when the width is narrower than 30 nm, the higher order mode disappears and the ribbon becomes a single mode waveguide. According to the scattering matrix method, the central frequencies of the transmission dips can be changed by changing the length and the width of the tooth. Flexible electrical tunability of this kind of filter by tiny change of the chemical potential of the graphene through electrical gating is also validated. In addition, transmission spectrum of multi-teeth shaped plasmonic filter is also studied. This kind of structure can possess the broad band-stop filtering property. The influences of tooth number and tooth period on transmission spectrum are investigated. We find that the transmission value can be reduced down to almost zero by adjusting the number of the teeth, also the tooth period can influence the central frequency of the stop band because of the coupling effects between each other. Like the single-tooth filter based on graphene nanoribbon, the multi-tooth broad band-stop filter can also be flexibly tuned by the geometric parameters of the structure and the chemical potential of the graphene. This work provides an effective method of designing graphene based ultra-compact tunable devices, and has extensive potential for designing all-optical integrated architectures for optical networks, communication and computing devices.
2015, 64 (10): 108403.
doi: 10.7498/aps.64.108403
Abstract +
Cognitive radio can significantly improve spectrum efficiency by temporarily sharing under-utilized licensed frequency with primary users. Its spectrum management framework consists of four parts: spectrum sensing, spectrum decision, spectrum sharing and spectrum handoff. The last part is what we focus on in this paper. Spectrum handoff, which aims at guaranteeing requirement for service of secondary users and shortening time delay produced by interruption from primary users, is an important functionality of cognitive radio networks. For solving the problem of optimizing the extended data delivery time, a spectrum handoff model is proposed based on the preemptive resume priority M/G/m queuing theory. In order to minimize the extended data delivery time, the queuing method with mixed queuing and parallel service is adopted. In this model, each channel has its own high-priority queue and there is only one low-priority queue for all secondary users. The primary and secondary users respectively enter into the high-priority and low-priority queue to establish corresponding primary connections and secondary connections and execute corresponding data transmission. On the above basis, secondary users’ channel usage behaviors are thoroughly analyzed in the cases of multiple secondary users, multiple licensed channels and multiple spectrum handoffs. In this process, when multiple interruptions occur, the secondary user will stay on the current channel and suspend data transmission until primary users finish their data transmission, otherwise the secondary user will switch from the current channel to the predetermined target channel to resume his unfinished data transmission. The target channel is sequentially obtained from the target channel sequence, which is determined by channel parameter estimation algorithm. Based on the analysis of channel usage behaviors for secondary users, the total time delay caused by spectrum handoffs within the whole data transmission process is derived first. The total time delay can be deduced from two scenarios. One is that the target channel is the current channel. For this reason, the total time delay equals transmission time of primary users in high-priority queue. Obviously, the other is that the target channel is not the current channel. Thus, the total time delay equals the sum of transmission times of primary users in high-priority and secondary users ahead in low-priority. In addition, appearance of new primary users should also be considered in the data transmission process. Then, expressions of the extended data delivery time in two different cases (i. e. always-staying strategy and always-changing strategy) are respectively derived. Furthermore, the adaptive spectrum handoff strategy is finally discussed, which is to choose the optimal scheme from always-staying and always-changing strategy when a spectrum handoff happens. Simulation results verify that this model can not only describe handoff behaviors of secondary users more perfectly, but also can make the transmission time delay smaller and make the extended data delivery time shorter than the existing spectrum handoff model. Especially, with the increase of service intensity of primary users, the advantages of the proposed spectrum handoff model are more outstanding. In addition, the allowable secondary user service intensity is improved and the receptive number of secondary user is increased in cognitive radio networks. All in all, the proposed spectrum handoff model improves the performance of spectrum handoff, increases the capacity of cognitive radio networks and optimally realizes spectrum sharing between secondary users and primary users.
2015, 64 (10): 108501.
doi: 10.7498/aps.64.108501
Abstract +
Low frequency noise in the partially depleted silicon-on-insulator (SOI) NMOS device is investigated in this paper. The experimental results show low frequency noise behaviors are in good consistence with classical noise model. Based on McWhorter model, the low frequency noise in the SOI device results from the exchange of carriers between channel and oxide. The densities of trapped charges in the front gate oxide and buried oxide are extracted. Due to the difference between manufacture processes, the extracted density of trapped charges in the buried oxide (Nt=8×1017 eV-1·cm-3) is larger than that in the gate oxide (Nt=2.767×1017 eV-1·cm-3), and the result is in good agreement with testing result of transfer characteristics in part 2. Based on the charge tunneling mechanism, the spatial distribution of trapped charges in the gate oxide and buried oxide are extracted by using the tunneling attenuation coefficient (λ=0.1 nm for SiO2) and time constant (τ0=10-10 s), and the result also proves that the trap in buried oxide is larger than that in gate oxide. In addition, the influence of channel length on the low frequency noise in the SOI device is discussed. The variations of normalized channel current noise power spectral density with channel length are investigated at four frequencies(10 Hz, 25 Hz, 50 Hz, and 100 Hz). The experimental results show that the normalized noise power spectral density decreases linearly with the increase of channel length, which indicates the low frequency noise of SOI device is mainly caused by the flicker noise in the channel, and the contribution of source/drain contact and parasitic resistances could be ignored. Finally, the dependences of back gate voltage on the front gate threshold voltage, front channel current and front channel noise are discussed by considering the charge coupling effect. The experimental results show the measured channel current and channel noise with applying front gate voltage and back gate voltage simultaneously are larger than those with applying the front gate voltage and back gate voltage separately.
2015, 64 (10): 108502.
doi: 10.7498/aps.64.108502
Abstract +
According to the completeness theory of circuit, Chua [Chua L O 1971 IEEE Trans. Circ. Theor. 1971 18 507] put forward the fourth basic circuit element memristor besides resistor, inductor and capacitor in 1971. And memorisistance is defined as the ratio of the flux to the charge passing through it. With the emergence and development of nanoscale semiconductor technology, HP laboratoty successfully fabricated a physical memristor in 2008. The successful fabrication of memristive device caused a stir in the whole electronic field and thus a vast number of researchers were involved in the research, owing to its superior properties, i.e., nanoscale dimension, continuous input and output property, switching characteristics and unique non-volatile memory capacity. With all these extraordinary properties, memristors possess many possibilities for the development of future integrated circuits and analog computer. With the gradually in-depth study of memristor, memristor is being microsized, and its internal structure, motion law among its internal particles and influences resulting from the parameters are further explored. In recent years, the memristor has made significant achievement in the areas of non-volatile solid-state memory, intelligent storage, very-large-scale integrated circuitry, programmable analog circuits, and artificial neural networks. So far, the influence of size parameter on the memristor has been little studied, although the size parameter is one of the key factors in the memristor fabrication technology, which severely restricts the memristor development and its practical application. In the paper, we theoretically analyze the influences of size parameter on two practical memristor models (i.e., the HP memristor and spintronic memristor) in detail based on the Ohm’s law. On this basis, a series of circuit experimental simulation is carried out, and the corresponding memristor characteristic curves are thus obtained. Furthermore, we choose 4 most representative experimental results, and make specific analysis on them. Those results indicate that the optimal length of HP memristor is in a range from 8 nm to 12 nm, while the proper range of spintronic memristor is from 500 nm to 600 nm. The final results can not only contribute to memristor physical implementation and its applications, but also provide theoretical references and reliable experiment basis for the further development of the titanium oxide memristor devices and the relevant research.
REVIEW
EDITOR'S SUGGESTION
2015, 64 (10): 106801.
doi: 10.7498/aps.64.106801
Abstract +
Graphene plasmons have aroused a great deal of research interest in recent years due to their unique features such as electrical tunability, ultra-strong field confinement and relatively low intrinsic damping. In this review paper, we summarize the fundamental optical properties of localized and propagating plasmons supported by graphene, and the experimental techniques for excitation and detection of them, with focusing on their dispersion relations and plasmon-phonon coupling mechanism. In general, the dispersion of graphene plasmons is affected by the Fermi level of graphene and the dielectric environment. The graphene plasmons can exist in a broad spectrum range from mid-infrared to terahertz. This has been experimentally verified for both the localized and propagation plasmons in graphene. On the one hand, the excitation frequency and confinement of localized plasmons supported by graphene micro/nano-structures are constrained by the structural geometry. Additionally, influenced from the tunability of the optical conductivity of graphene, the excitation frequency of graphene plasmons can be tuned by electrostatic or chemical doping. On the other hand, propagating plasmons have been launched and detected by using scattering-type scanning near-field optical microscopy. This technique provides the real-space imaging of the electromagnetic fields of plasmons, thereby directly confirming the existence of the graphene plasmons and verifying their properties predicted theoretically. In a similar regime, the launching and controlling of the propagating plasmons have also been demonstrated by using resonant metal antennas. Compared to metal plasmons, graphene plasmons are much more easily affected by the surroundings due to their scattering from impurity charges and coupling with substrate phonons. In particular, graphene plasmons can hybridize strongly with substrate phonons and there are a series of effects on plasmon properties such as resonance frequency, intensity and plasmon lifetime. The designing of the dielectric surrounding can effectively manipulate the graphene plasmons. Finally, we review the emerging applications of graphene plasmon in the mid-infrared and terahertz, such as electro-optical modulators and enhanced mid-infrared spectroscopy.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2015, 64 (10): 109101.
doi: 10.7498/aps.64.109101
Abstract +
The effect of gas bubble on acoustic characteristic of sediment is important for ocean science, ocean geology, ocean geophysics, etc. Twenty five samples of ocean bottom sediments are extracted through gravity sampling equipment from the East China Sea and are sealed in PVC pipes for storage in order to study the effect of gas bubble on acoustic characteristic of sediment. In order to obtain the gas content of sediment, in this the paper the Micro-CT scanning technology is introduced into sediment measuring method. The different X ray absorption rates of water, gas and solid particles in sediment samples are obtained through Micro-CT scanning using Siemens Micro-CT scanner. The gas volume content and water volume content in sediment can be obtained according to CT number distribution. The acoustic measurement is carried out in laboratory using intelligent nonmetal ultrasonic detector and the 40 kHz waves are launched from one side of the sediment sample and obtained from another side. The acoustic attenuation can be obtained according to the amplitudes of launched and received waves and the acoustic velocity can be obtained according to travelling time when acoustic wave goes through the sediment. The attenuation of sediment sample is about a few to twenty and the velocity is about 1100 to 1700 m-1. By mean of analysis of regression, the correlations are obtained among gas content, fluid content, acoustic velocity, attenuation and power function, which better match the measuring data. The result of study indicates that slight augment of gas content can cause sharp decrease of acoustic velocity and rapid increase of acoustic attenuation. The increment and decrement decrease obviously when the gas content exceeds 10%. The result in this paper is useful to explore oil and gas seismic.
