Vol. 63, No. 16 (2014)
Anti-control of Hopf bifurcation in the new chaotic system with piecewise system and exponential system
2014, 63 (16): 160505. doi: 10.7498/aps.63.160505
In order to understand the complex three-dimensional dynamical system with the unstable nodes, we propose a nonlinear controller. The corresponding controlling system makes the codimension one, two, and three Hopf bifurcations happen. The mathematical deduction demonstrates that the system can be controlled to produce the degenerate Hopf bifurcation at desired location and stability of controllable bifurcation.
2014, 63 (16): 160503. doi: 10.7498/aps.63.160503
In this paper, a spatiotemporally asymmetric fractional Langevin-like ratchet is constructed for the operation of a one-dimensional linear molecular motor subjected to both temporally asymmetric unbiased Langevin-like noise generated by the Logistic mapping and spatially asymmetric periodic potential. In this ratchet, the Langevin-like noise is used to describe fluctuations of intracellular surrounding, and the fractional order is responsible for the effect of the non-ideal intracellular surrounding. Then, by deducing the corresponding discrete mapping, dependance of ratchet effect on parameters are numerically investigated. Numerical results show that both the temporal asymmetry of noise and the spatial asymmetry of potential are crucial to the directed-transport of the ratchet, and competitive spatially asymmetric potential can even reverse the unidirected transport generated by the temporally asymmetric noise at suitable parameters.
Synchronizing a class of uncertain and variable time-delay fractional-order hyper-chaotic systems by adaptive sliding robust mode control
2014, 63 (16): 160507. doi: 10.7498/aps.63.160507
In view of a class of synchronization problems about uncertain and variable time-delay systems, this paper puts forward a method of adaptive sliding robust control. Based on the Lyapunov stability theory and adaptive sliding mode control methods, the adaptive sliding robust controllers and the parameter adaptive rate are designed. A single controller designed by the synchronous control method is applicable to the synchronizing of a class of fractional-order hyper-chaotic systems, and it has a great ability to resist noise-perturbed. What is more, it can also well control the time-varying time-delay systems. So the controller is of highly practical value. Furthermore, by introducing a certain amount of compensation into the system, the influences of the uncertainty and the noise-disturbance can be eliminated, thus the synchronization of the uncertainty fractional-order hyper-chaotic system is realized. In addition, the control of the synchronous errors of the systems can be stable in arbitrarily small domain. Finally, time-varying and time-delay fractional-order Chen's hyper-chaotic systems with the external noisy disturbances and uncertain parameters are numerically simulated, and the effectiveness of the proposed control method is verified.
2014, 63 (16): 160504. doi: 10.7498/aps.63.160504
In order to enhance the usefulness of the theory of stochastic resonance in the areas of weak signal detection, a new method based on quantum particle swarm optimization is proposed to conquer with the problem of adaptive stochastic resonance. First, the problem of adaptive stochastic resonance is converted into the problem of multi-parameter optimization. Then simulation experiments are conducted respectively under a Langevin system and Duffing oscillator system. At the same time, Point detection method is chosen as the comparative test in the Langevin system. While in the Duffing system, the optimization results are compared with those from the Langevin system directly. Results show that the method based on quantum particle swarm optimization is obviously superior to the point detection method and optimization result in the Duffing oscillator is better than that from Langevin system under the same condition. Besides, it is also found that the lower the SNR of input signal, the more effective the quantum particle swarm optimization is. Finally, the regularity of optimization results of the stochastic resonance system parameters is summarized.
2014, 63 (16): 160501. doi: 10.7498/aps.63.160501
With the development of network services, social networking has become a new medium of information dissemination. Thus, many scholars pay close attention to the evolution of network public opinion which has great significance and practical value. In this paper, an opinion evolution model of social network based on information entropy is proposed, which is helpful to investigate the evolution of network public opinion. The model has two characteristics: firstly, it can reflect the psychological process when an individual makes a choice facing with two opposite views; secondly, it can reflect the influence of subjective and objective factors when the individual forms new ideas. In the simulation, some issues are discussed which includes the effects of the public opinion environment on the individual opinion evolution, the influences of the initial opinion and self-confidence on the individual opinion evolution, and the effects of leaders' opinions on the opinion evolution of populations. From the experimental results it follows that the model works well and can reflect the characteristics of individuals in real social networks. For example, the formation of the individual's opinion will be affected by public opinion environment, and an individual will not accept the opinions from others if he is very confident, and the opinion evolution of populations will be influenced when there exist the leader's opinions in network.
2014, 63 (16): 160502. doi: 10.7498/aps.63.160502
Based on the research on transport phenomenon of fractional Brownian motor, a systematic parameter (i.e. symmetry parameter) which describes the asymmetry of the periodic potential field is introduced, and the influences of the symmetry parameter and the memory parameter (i.e. the fractional order) on the transport behavior are also investigated. The numerical results show that the combined effect of fractional order and symmetry parameter can result in the reverse flow of Brownian particle's transport, and the fractional order corresponding to the maximal averaged velocity is irrelevant to the frequency of the external periodic force, but it will still increase monotonically as the symmetry parameter increases.
2014, 63 (16): 160506. doi: 10.7498/aps.63.160506
In this paper, we investigate the influence of each parameter of Willis chaotic system on the system, and calculate the Lyapunov exponent of Willis chaotic system containing chaotic phase. The analysis of the influence of chaotic phase on the Willis system shows that the random phase can control chaos. We also analyze the differences in intensity among various noises. Finally, the effectiveness of the above method is verified through the analysis of integrating phase portraits and Poincaré surface.
2014, 63 (16): 160701. doi: 10.7498/aps.63.160701
Using X-ray as carrier signal to realize the high rate information transmission in a distant space is attracting the attention of researchers. The development of this technology has a positive significance for broadening the scope of use of the electromagnetic spectrum. In this paper, the novel grid control X-ray source, which consists of a traditional X-ray tube and a signal control grid, is designed to meet the requirements of X-ray communication in the simulation vacuum experimental system by means of three-dimensional electromagnetic simulation software CST particle studio. The tube potential distribution, electron trajectory, actual focal spot and the number of electrons at the anode are simulated by the computer simulation software. It works by changing the grid voltage to control the X-ray pulse emission. The data of the simulation are as follows. The actual focal spot size is 0.4 mm×4 mm, the effective gate-on voltage is 0 V, and the gate-off voltage is-10 V. X-ray tube grid-controlled characteristics are tested in experiment. The test results are well consistent with the simulation results. Finally, the digital signal transmission is successfully implemented in the X-ray vacuum experiment system.
2014, 63 (16): 160508. doi: 10.7498/aps.63.160508
Considering the problem that least squares support vector machine prediction model with single kernel function cannot significantly improve the prediction accuracy of chaotic time series, a combination kernel function least squares support vector machine prediction model is proposed. The model uses a polynomial function and radial basis function to construct the kernel function of least squares support vector machine. An improved genetic algorithm with better convergence speed and precision is proposed for parameter optimization of prediction model. The simulation experimental results of Lorenz, Mackey-Glass, Sunspot-Runoff in the Yellow River and chaotic network traffic time series demonstrate the effectiveness and characteristics of the proposed model.
2014, 63 (16): 160201. doi: 10.7498/aps.63.160201
A function transformation is presented to change a kind of nonlinear coupled system into a set of two elliptic equations of the first kind. Then new infinite sequence complexion two-soliton solutions to a kind of nonlinear coupled system are constructed by new solutions and Bäcklund transformation of elliptic equation of the first kind.
2014, 63 (16): 162301. doi: 10.7498/aps.63.162301
Based on the exponential formula of β--decay half-lives for nuclei far from stable line, the half-lives of nuclei around N=82 (R-process waiting point nuclei) are calculated. The results are compared with recent theoretical and experimental data. It is shown that compared with the complicated and time-consuming microscopic calculation, the exponential formula including the shell effect can give the results of β--decay half-lives for R-process waiting point nuclei quicker and better. The results can be used as reliable inputs for the network calculation for nuclei synthesis in cosmos.
2014, 63 (16): 162501. doi: 10.7498/aps.63.162501
Thorium fission reaction rate is an important datum in uranium-thorium fuel cycle. In order to measure the thorium fission rate on the thorium sample equipment which is set up by the conceptual design of the subcritical reactor and to check the thorium data, the off-line activation γ measurement method of thorium fission rate is developed. Combined with thorium fission yield data of 85mKr, the 232Th fission reaction rate distribution in thorium sample device can be obtained by measuring the 151.16 keV feature gamma rays emitted by fission fragment 85mKr. Details of the principles and factors of this method are discussed, and the verification experiment is carried out on a depleted uranium shell of R13.1/30.0 cm with D-T neutrons. The relative uncertainty of experiment is 5.3%-5.5% for thorium fission reaction rates. The experiment is simulated using MCNP5 with ENDF/B-VI and ENDF/B-VII libraries, simulation results and experimental results accord well with each other within the experimental uncertainty, showing that this approach developed in this paper can work well for determining the thorium fission rate.
Determination of 232Th(n, γ) reaction rate induced by D-T neutrons in one-dimensional alternate depleted uranium/polyethylene shells
2014, 63 (16): 162801. doi: 10.7498/aps.63.162801
A series of neutron integral fundamental researches of thorium nuclear data in set-ups containing thorium samples is carried out. One-dimensional alternate depleted uranium/polyethylene shells containing thorium samples are constructed by referring to the conceptual design of fusion-fission hybrid reactor, where a D-T neutron source driven by accelerator is used to simulate the fusion core of the reactor. 232Th (n, γ ) reaction rates in samples located at different positions in the shells are measured in 5% uncertainty by using activated thorium sample decay γ-ray off-line measurement technique. The results show that the moderation of polyethylene to 14.1 MeV neutron will efficiently increase the capture rate of thorium, and the depleted uranium is also conducible to this increase obviously. The comparison between our measured data and the results available from mainstream nuclear data bank shows that the calculation results from ENDF/B-VI.6 and JENDL-3.3 are around 6% higher than the experimental results, while the newer ENDF/B-VII.0 will achieve better results, around 4% higher than the experimental results. We recommend the ENDF/B-VII.0 to be used in one-dimensional alternate depleted uranium/polyethylene shells related conceptual design when calculating the 232Th (n, γ) reaction rate.
Isomer 178m2Hf prepared by using α particles to bombard a natural ytterbium target and analysis ofhigh purity Ge γ spectrum
2014, 63 (16): 162303. doi: 10.7498/aps.63.162303
178m2Hf isomer is of great importance in science and application. Using natural ytterbium bombarded by α particles to produce 178m2Hf through 176Yb (α, 2n) 178m2Hf nuclear reaction may be a low-cost way. Three natural ytterbium targets of about 200 μ m in thickness are fabricated by magnetron sputtering of natural Yb onto copper substrates, and then irradiated with 30 MeV, 50 μ A α beam in the CS-30 cyclotron for about 50 hours. After cooling, Hf is isolated from one target through chemical isolation. Gamma spectra of two targets and the isolated Hf sample are measured with high purity Ge detector. Overlapping peaks of the complex spectrum are separated by using Gaussian multi-peak fitting. The content of 178m2Hf nuclide is identified to be 1012 orders of magnitude in each target, and 1011 orders of magnitude in the purified sample from chemical isolation. The content values of other radioactive nuclides (175Hf, 172Lu, 172Hf, 177Lu, 173Lu ) in targets are also measured.
Systematic calculations on cluster radioactivity half-lives of trans-lead nuclei with effective liquid drop model
2014, 63 (16): 162302. doi: 10.7498/aps.63.162302
The cluster radioactivities of trans-lead nuclei are systematically investigated by using the effective liquid drop description with the varying mass asymmetry shape and effective inertial coefficient. An effective nuclear radius constant formula is used instead of the original empirical formula in calculation. The calculated half-lives are in good agreement with the available experimental data. The root-mean-square deviation between the calculated logarithmic half-lives and the experimental ones is only 0.895. From the plots of the calculated lgT1/2 values versus the neutron (or proton) number of daughter, the shell effect of neutron magic number N=126 (or proton magic number Z=82) can be clearly seen. The odd-even-stagger can be clearly seen in the odd clusters 25Ne, 29Mg. The calculated half-lives conform to the Geiger-Nuttall law. We obtain some important conclusions about the Geiger-Nuttall law from the calculated results.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
Optimization of light spot intensity and coverage to a triple-junction solar cell under non-uniform illumination
2014, 63 (16): 168801. doi: 10.7498/aps.63.168801
The cells in high concentrated photovoltaic module are usually high efficiency triple-junction solar cells. Due to the non-ideal concentrators, the light intensity distribution on a solar cell is highly non-uniform, so the appropriate increase of the ratio between light spot size and cell area is a method to reduce the influence of non-uniform illumination on the electrical performance of the solar cell. The circuit network model is used to calculate the influences of light spot intensity distribution and size on a triple-junction solar cell. The light spot intensities and sizes, the cell efficiencies, and the temperature distributions of the cell under four design schemes (uniform illumination, non-uniform illumination, maximum cell efficiency, and maximum module efficiency) are compared. The results show that the cell efficiency in the maximum module efficiency design is not the maximum cell efficiency under the standard testing condition. The design to make the cell achieve the maximum efficiency obtains the minimum module efficiency. The design to achieve maximum module efficiency has a smaller size of concentrator, so the cost of the module goes up. The design to achieve the maximum cell efficiency has a bigger size of concentrator and a lowest cell temperature, so the cost of the module will reduce and the reliability will improve. Above all, the requirement of electricity quantity should be fully considered in the module design, in which an appropriate geometric concentration ratio and light spot coverage to solar cells should be chosen.
Influences of the substrate position on the morphology and characterization of phosphorus doped ZnO nanomaterial
2014, 63 (16): 168101. doi: 10.7498/aps.63.168101
One-dimensional phosphorus doped ZnO nanowires and nanonails are prepared on Si substrate without employing any metal catalyst by chemical vapor deposition method. Field-emission scanning electron microscopy shows that the samples located downstream 1.5 cm away from the source material are of nanowire structure and located 1 cm above source materials of nanonail structure, and the growth mechanisms of phosphorus doped ZnO nanostructures with different morphologies are discussed. The photoluminescence properties of phosphorus doped ZnO nanowires and nanonails are studied at a temperature of 10 K. The phosphorus related acceptor emissions are observed. Furthermore, the current-voltage (I-V) measurement based on the ZnO nanostructures/Si heterojunctions shows a typical semiconductor rectification characteristic with positive open electric fields being 4.8 and 3.2 V, respectively.
Lithium intercalation properties of SnSb/C composite in carbonthermal reduction as the anode material for lithium ion battery
2014, 63 (16): 168201. doi: 10.7498/aps.63.168201
The SnSb/C composite material is prepared by using the carbonthermal reduction to deal with the mixture of SnO2 and SbO3, respectively with different carbon reductant-glucose and mesocarbon microbead (MCMB). The morphologies and electrochemical properties of two kinds of structures of SnSb/C composite are compared. To characterize the phase and morphology of the composite material, X-ray diffraction, Raman spectra and scanning electron microscope are used. The current charge and discharge, cyclic voltammograms and AC impedancetests are also used to test the electrochemical performance of SnSb/C. The experimental results show that a kind of core-shell structure, of which the alloy particle serves as the core and the pyrolytic carbon as the outside shell, is formed when the glucose is used as the reducing agent. The first discharge specific capacity is 793.379 mA·h/g and it is still kept at 449.987 mA·h/g after 50 cycles. However, when the MCMB is used as the reducing agent, there are only a few of alloy particles attaching to the surface of MCMB, and it is not a kind of core-shell structure but a mixture of alloy particles and MCMB spheres. Its initial discharge specific capacity is 1164.938 mA·h/g, and after 50 cycles it is only 290.807 mA·h/g.
Cyclical game coupling with Levy flight and Brownian motion and stable coexistence conditions of species
2014, 63 (16): 168701. doi: 10.7498/aps.63.168701
Cyclical game is often used to study the biodiversity in ecosystem. However, the interaction distance mode considered in previous studies of cyclical game is only the interaction between nearest neighbors, a fixed distance, or a random value of fixed distance among the individuals of species. This is not consistent with the actual situation. In this paper, considering the fact that Levy flight and Brownian motion widespreadly exist in ecosystem, and comprehensively considering the nearest-neighbor-interaction and long-range-interaction given by Levy flight and Brownian motion, the cyclical game and conditions of maintaining biodiversity are investigated. The critical relation of maximal step length of flight versus choosing probability is presented, including Logistic and exponent relations. Further the critical relation between power-law exponent and choosing probability is found. The condition of maintaining species coexistence is also found.
2014, 63 (16): 168702. doi: 10.7498/aps.63.168702
Quantum noise is an important factor influencing the quality of the images obtained by X-ray phase contrast imaging because it induces a random fluctuation on the intensity. In this paper, a method is reported to analyze the noise characteristics of hard X-ray differential phase contrast imaging, and the relationship between the quantum noise and images is determined by numerical simulation. The results show that the mean square deviations of the refractive and scattering images are inversely proportional to the visibility of moir fringe. That is to say, the higher visibility of moir fringe leads to the lower mean square deviations of the refractive and scattering images. Therefore, in the case of ensuring an enough X-ray detection efficiency, the improvement in the visibility of moir fringe will be beneficial to obtaining high-quality phase contrast images and the effective reduction of X-ray exposure dose.
ATOMIC AND MOLECULAR PHYSICS
2014, 63 (16): 163202. doi: 10.7498/aps.63.163202
The EuLX-ray emission from the impact of Eu20+ ions of 3.0-6.0 MeV kinetic energies on Au target is detected. We obtain the experimental relationship of single ion X-ray yield with kinetic energy of ions. Considering the ion stopping in the target, the yield of the single ionLshell holes produced by the collision of Eu20+ ion with Au atom is calculated. According to the fluorescence yield of EuLshell holes, we deduce the theoretical relationship of X-ray yields with kinetic energy of ion. The results show that the theoretical curve is well consistent with the experimental data.
Density functional theory study of structure characteristics and stabilities of Al2Sn(n=2-10) clusters
2014, 63 (16): 163601. doi: 10.7498/aps.63.163601
The geometric configurations and electronic structures of the Al2Sn (n=2-10) clusters are calculated by the B3LYP (density functional theory) method at 6-311G** level. The variations of the ground state structure, charge transfer and bonding characteristic of the aluminum-sulfur doped clusters are discussed in detail. The results show that the structures of pure Sn clusters are fundamentally changed due to the doping of the Al atoms. Our work exhibits that the all the ground states of Al2Sn clusters share the same four-member Al2S2 ring, and from single ring to three rings plane and three-dimensional structure structures are formed by inserting one S atom or S clusters. The stability of cluster structure is influenced by both the chemical bond structure and number. The stability information of Al2Sn clusters is obtained by analyzing the dissociation energies and the second-order difference energies of the ground state structures.
2014, 63 (16): 163101. doi: 10.7498/aps.63.163101
According to density functional first-principles calculations, we study the substitutional doping of Co atoms in core-shell silicon nanowires. By comparing the formation energies, we find that all the doping configurations obtained from shell-chain doping, core doping, and whole shell doping are stable, and core-shell doping silicon nanowire has the highest structural stability. All the doped configurations show metallic property, and the conductance channels increase with the increasing of doping concentration. Co-doped silicon nanowires show ferromagnetic, possessing magnetic moment. Bader charge analysis shows that charge is transferred from Si atoms to Co atoms in doped silicon nanowires. In transition metal Co atom, charge is transferred from 4s orbital to 3d and 4p orbital. The reducing of unpaired electron in 3d orbital and part of charge transferring from up-spin to down-spin in 4s, 3d and 4p orbital, makes magnetic moments in Co atom reduced.
2014, 63 (16): 163201. doi: 10.7498/aps.63.163201
The characteristic X-ray spectra produced by the impact of 129Xe26+ with kinetic energies from 350 to 600 keV and from 1.8 to 3.9 MeV on Au surface are measured. It is found that 129Xe26+ with kinetic energies from 350 to 600 keV can excite only the characteristic X-ray spectra of Mα of Au, but 129Xe26+ with kinetic energies from 1.8 to 3.9 MeV can excite the characteristic X-ray spectra of Mζ, Mα, Mγ and Mδ. The relation between the characteristic X-ray intensity, the ratio of X-ray yield and the ion kinetic energy is analyzed. The kinetic energy threshold of L-X-ray of Xe emitted by Xe26+ is estimated.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2014, 63 (16): 164203. doi: 10.7498/aps.63.164203
According to the stochastic behavior of the continuous phase plate (CPP) surface shape distribution, the autocorrelation function and the correlation length are used to analyze its surface shape characteristics. A Gaussian random distribution function is used to derive the analytic relation between the correlation length and the far-field distribution of CPP. Numerical algorithm is used to calculate the variance and the energy usage rate of CPP's far-field distribution, and also the specific influence of correlation length on far-field distribution of CPP. The conclusion is that the smaller the correlation length, the better the homogeneity of far-field distribution is, and the closer to the target spot the shape of the focal spot will be, and at the same time the higher the energy usage rate will be.
Fast computation of electromagnetic scattering characteristics from conducting targets using modified-physical optics and graphical electromagnetic computing
2014, 63 (16): 164202. doi: 10.7498/aps.63.164202
A novel high-frequency radar cross section prediction method is introduced to solve the scattering from electrically large conductive target. It can be realized by combining a modified-vector physical optics algorithm and a graphical electromagnetic computing algorithm. In consideration of the traditional physical optics algorithm which needs to compute the edge diffraction, the radar cross section of the target cannot be calculated directly, but it is needed to first calculate the diffraction contribution, then to add the scattering contribution, therefore the radar cross section of the target can be finally obtained. By using the improved graphical electromagnetic computing, the surface normals, thus the surface current, can be directly corrected. Therefore the diffraction at the edges is taken into account, thereby improving the efficiency of the algorithm. The central processing unit running time involves only the time needed to compute the electromagnetic part, with leaving the geometric part to the graphics hardware. The line integrals are computed over an arbitrary shape in real time. Numerical results demonstrate the good accuracy and efficiency of the modified method.
Effects of laser wavelength on both water temperature measurement precision and detection depth of Raman scattering lidar system
2014, 63 (16): 164209. doi: 10.7498/aps.63.164209
Airborne Raman scattering laser lidar technology can measure the three-dimensional (3D) distribution of subsurface seawater temperature rapidly, and it has important social and economic values. In this paper, firstly, the relationship between Raman stretching vibration spectrum peak position and excitation wavelength, and the relationship between the full width half maximum (FWHM) of Raman stretching vibration spectrum and excitation wavelength are analyzed theoretically. The results show that as the excitation wavelength increases, Raman scattering peak gradually shifts toward longer wavelength and the Raman spectrum FWHM increases noticeably. Secondly, to verify the theoretical results, the Raman spectra at different water temperatures excited by 450 nm and 532 nm lasers are measured experimentally, and the fitting analyses of them by single Gauss peak fitting method are made, the relationship between Gauss peak wavelength and temperature is obtained, and the effect of laser wavelength on the temperature measurement precision is analyzed. It is found that larger excitation wavelength can increase Raman spectrum measurement accuracy, thereby improving the temperature measurement precision. Finally, the Raman scattering lidar equation is established, the Raman scattering coefficients and attenuation coefficients of different wavelength lasers are analyzed, and the corresponding effects of laser wavelength on the lidar system detection depth are studied. Results show that the lidar system detection depth is greatly influenced by the laser wavelength, lidar system with laser wavelength below 480 nm has a good detection ability, and large wavelength laser greatly reduces lidar system detection depth. The effects of laser wavelength on both temperature measurement precision and detection depth should be considered in the desigin of Raman scattering lidar system.
2014, 63 (16): 164204. doi: 10.7498/aps.63.164204
We propose a new ptychographical imaging algorithm based on illuminating beam cooperating with rotational phase encoding. Compared with the traditional ptychographical imaging, the proposed algorithm has a diffractive wave front that is encoded with the uniform phase modulator in a block. It leads to the faster convergent speed for the iterative algorithm of the ptychographical imaging. The illuminating method can present fantastic robustness to resist bad influences brought by the noise and transverse shift of probes. Compared with the random phase modulator, the rotated phase modulator introduced into our algorithm is convenient to be fabricated and located into the real configuration. Therefore, the proposed algorithm has a good value in practical applications such as real-time microscopy and superresolution imaging.
2014, 63 (16): 164302. doi: 10.7498/aps.63.164302
To generalize the harmonic potential of the linear random vibration system, a more general power type potential is presented, and the corresponding power function type nonlinear single-well random vibration system is obtained. The first moment of the system steady-state response and the stationary variance of the system response, which are influenced by noise strength, parameters of the potential and the periodic excitation, are studied by using the second order stochastic Runge-Kutta algorithm. The parameter b, which determines the shape of the potential, goes through b b > 2 and b=2 (harmonic potential), and it is shown that varying the noise strength, if b b=2 (harmonic potential) or b > 2, this phenomenon does not occur; varying the parameters of the potential, the first moment of the system steady-state response and the stationary variance of the system response can also be non-monotonic.
Experimental research of four-wave mixing and soliton effects in a photonic crystal fiber pumped femtosecond pulses at the wavelength located normal dispersion regime away from the zero dispersion point
2014, 63 (16): 164206. doi: 10.7498/aps.63.164206
In this paper, we show experimentally that a pair of signal wave and idler wave bands is generated by four-wave mixing, and a pair of optical solitons and dispersion wave bands is induced by intrapulse Raman scattering and non-soliton radiation, and we also observe the power saturation phenomenon of the photonic crystal fiber (PCF) by using the incident femtosecond pulses in normal dispersion region away from the zero-dispersion wavelength of the PCF. The dispersion and nonlinear characteristics of the PCF are studied by the finite element method. The possible positions of the signal wave band and the idler wave band that is generated in the fiber satisfying the phase-matching condition are simulated by four-wave mixing phase matching, the results are in good agreement with the experimental results and show that even the pump laser pulse in the normal dispersion region can also produce the PCF four-wave mixing and optical solitons effects. In the present study it also revealed that the four-wave mixing experiment is generated by the fourth-order dispersion parameter β4, and the origins of optical solitons and dispersion wave bands generation are further explained.
Spectral phase measurement and compensation of femtosecond laser pulse based on multi-photon intra-pulse interference phase scan
2014, 63 (16): 164207. doi: 10.7498/aps.63.164207
In this paper, we develop a system that can be used to measure and compensate spectral phase of femtosecond laser pulses based on the multi-photon intra-pulse interference phase scan (MIIPS). In the system, a liquid crystal spatial light modulator (LC-SLM) is used to impose phase scan on the femtosecond laser pulse, while a spectrometer is used to record the MIIPS trace. Both the LC-SLM and the spectrometer are driven by home-developed LabVIEW programs. By analyzing the MIIPS trace, we obtain the spectral phase of the pre-chirped femtosecond laser pulse with only 3-times iteration. The femtosecond laser pulse is centered at 810 nm and has a repetition of 1 kHz. The accuracy of our measurement is less than ±0.1 rad. The measured phases are compensated by the LC-SLM, then we obtain a femtosecond laser pulse which is almost in the transform limit state. The device will be useful in many fields, for example, multi-photon microscopy, pulse shaping, and femtosecond laser spectroscopy etc.
Effects of turbulent medium refractive index structure constant Cn2 on the propagation characteristics of double-half hollow Gaussian beams
2014, 63 (16): 164208. doi: 10.7498/aps.63.164208
Based on the Rayleigh-Sommerfeld diffraction integral theory, the propagation analytical expression of double-half Gaussian hollow beams in turbulent atmosphere is derived by using the cross spectral density function, and the effects of turbulent medium refractive index structure constant Cn2 on the propagation characteristics of double half a hollow Gaussian beams are studied and the intensity distributions under the different conditions are obtained as well. The research results show that the increased Cn2 can exacerbate the near-field diffraction effects of double-half Gaussian hollow beams, which not only shortens the propagation distance of hollow beams fully evolved into the Gaussian beams, but also increases the extent of the Gaussian beam extend outward.
Analytical calculation and evolutionary regression method for isentropic exponent of hydrogen gas at the throat of critical nozzle
2014, 63 (16): 164701. doi: 10.7498/aps.63.164701
As one of the most promising renewable energy resources, the hydrogen has been used in the fields such as aerospace, industry, and fuel cells. Critical nozzles are widely used for mass flow-rate measurement of high hydrogen gas, since the flow measurement process is not affected by its downstream flow disturbance. The flow rule of real hydrogen gas through a critical nozzle is complicated and the thermophysical property of hydrogen at the nozzle throat is vital to the accurate measurement of hydrogen flow. In this paper, based on explicit Helmholtz energy and entropy-enthalpy equations, the basic flow parameter and isentropic volume change exponent are analytically calculated. In addition, an accurate explicit equation is determined by the nonlinear regression analysis where the ways of selection, exchange and mutation derived from evolutionary algorithm are introduced to search for optimal population individual. The regression standard deviation is 0.0089%, mean residual deviation is 0.0285%, and maximum residual deviation is 0.1781%. The result shows that it not only can rapidly find the optimal solution which has the lowest number of equation items and the great overfitting suppression capability, but also has a high computation accuracy. This algorithm can also be applied to modeling flow characteristic parameters for every other flow device.
2014, 63 (16): 164702. doi: 10.7498/aps.63.164702
Biofiltration technology has received much attention because of its effectiveness, economy and environmentally friendly property, which can filter the odors caused chiefly by H2S via the biological treatments. In this study, the lattice Boltzmann method is adopted to numerically investigate the heterogeneous flow in three porous biofilter models. The numerical results indicate that the property of porous media and the inlet flow condition have significant influence on the value of critical Rayleigh number. With the increase of Darcy number and porosity, the critical Rayleigh number will gradually decrease; however, it will steadily increase with the augment of inlet Reynolds number. The present study is helpful to provide a rational theoretical guidance for the optimized design of biofilters.
2014, 63 (16): 164102. doi: 10.7498/aps.63.164102
The presence of sea spikes can cause the radar false alarm probability rise and performance degradation of multi-target environment detection. Therefore, study of the phenomenon of sea spikes is of great significance. HH polarization scattering intensity close to or even greater than the VV polarization scattering intensity is an important feature of sea spike phenomenon. Overturning wave crest is considered to be one of the reasons of generating sea spike. In this paper, overturning wave crest model is introduced with the consideration of the wind speed, and the method of moment is used for studying HH and VV backward scattering coefficient for different incident frequencies, incident angles, wind speeds, and wind directions. It is found that super phenomenon (HH scattering intensity is greater than VV polarization scattering intensity) is more obvious in the cases of low grazing and large wind speed, thus it is deduced that the sea spike phenomenon occurs with a high probability in the case of low grazing angle. Moreover, the distributions of sea clutter amplitude and Doppler spectra are also examined for the overturning wave crest model with low grazing incidence.
Propagation characteristics of small-bandwidth pulsed beams with smoothing by spectral dispersion in high power laser system
2014, 63 (16): 164201. doi: 10.7498/aps.63.164201
In this paper, we numerically study the propagation characteristics through spatial filter and free space of small-bandwidth pulsed beams with smoothing by spectral dispersion (SSD) in high power solid-state laser system. The numerical simulation with time-, space-and spectrum-resolution is achieved by using laser propagation and amplification software developed by Research Center of the Laser Fusion. The graphs of corresponding time-, space-and spectrum-distribution are presented, and the propagation characteristics between the SSD and non-SSD beam are compared. Besides, the effects of some SSD parameters on the diffraction propagation are further discussed. The results can help to choose the proper beam divergence of SSD which matches with the size of the spatial filter hole, to design the appropriate modulation frequency and the color circle number and so on. Moreover, it could guide the design of the light path, especially the design of free space propagation of small-diameter laser.
2014, 63 (16): 164205. doi: 10.7498/aps.63.164205
To reconstruct the target shape distribution in the distance, full waveform analysis algorithm is utilized by extracting and analyzing the number of the peaks, the time of the peak maximum and other parameters. A novel fast full waveform analysis algorithm (simulated tempering Markov chain Monte Carlo algorithm, STMCMC) is proposed, which is able to process the waveform data automatically. For the different types of the parameters, simulated tempering strategy and the Metropolis strategy are presented. In simulated tempering strategy, due to the demand of speed or accuracy, active intervention tempering is used to control the process of solving the vector parameters. On the other hand, the Metropolis strategy is adopted for non-vector parameters to reduce computation amount. Both the strategies are based on Markov chain algorithm, and meanwhile can hold the convergence of the Markov chain, which makes the STMCMC algorithm robust.
2014, 63 (16): 164301. doi: 10.7498/aps.63.164301
The axisymmetric multiple-relaxation-time lattice Boltzmann (LB) method is used to study the acoustic levitation of a rigid disk sample in a closed cylindrical resonant chamber. The simulation results show that the resonant cavity length L is equal to 0.499λ for (001) mode, and the resonance shift δL is approximately equal to-0.9 with a disk sample located in the chamber center, which accord with the analytical results derived from linear acoustics. The LB method naturally includes the viscosity and resonance shift during the simulation of acoustic levitation force on the disk sample, which gives the results not only consistent with the theory in magnitude, but also coherent with the experiments in more details. Some of the nonlinear effects associated with acoustic levitation, such as waveform distortion, acoustic streaming, and radiation pressure, are also revealed by the LB simulation.
Newmark algorithm in the finite-difference time-domain analysis of ferrite magnetized in an arbitrary direction
2014, 63 (16): 164101. doi: 10.7498/aps.63.164101
The permeability tensor of saturated magnetized ferrite in frequency domain in the laboratory coordinate system is obtained by using the transformation matrix between the principal and laboratory system. The constitutive relation in time domain, which is a kind of second order differential equation, is derived by employing the transformation from the frequency domain jω to time domain ∂/∂t and solved by utilizing the Newmark algorithm. Consequently, a Newmark finite-difference time-domain method is proposed to deal with the problem of electromagnetic scattering by ferrite which is subjected to an external magnetic field in an arbitrary direction. The electromagnetic scattering by a magnetized ferrite layer and a sphere is simulated, and the numerical results demonstrate the validity and feasibility of the proposed method.
Conformal invariance and conserved quantity of Mei symmetry for Appell equations in nonholonomic systems of Chetaev’s type with variable mass
2014, 63 (16): 164501. doi: 10.7498/aps.63.164501
Conformal invariance and conserved quantity of Mei symmetry for Appell equations of nonholonomic system of Chetaev's type with variable mass are studied. The conformal invariance and Mei symmetry for Appell equations of nonholonomic systems of Chetaev's type with variable mass are defined under the infinitesimal transformation of group, and the determining equations of conformal invariance of Mei symmetry for the system are given. Then, the expression of the corresponding conserved quantity of the system is derived. Finally, an example is given to illustrate the application of the results.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
The full orbit of energetic ion in tokamak is simulated by strictly solving Hamiltonian equations in guiding center coordinate system, and the semi-trapped orbit which is predicted in a recent theoretical research and the stagnation orbit are confirmed by the presently developed full orbit theory. The prompt loss of the energetic particle in international thermonuclear experimental reactor is calculated according to the present full orbit theory. It is found that the prompt loss rate of energetic ion in the full orbit theory can be larger than 14% the result in gyro-averaged theory, and the detailed analysis is carried out and physical understanding is presented in this paper.
2014, 63 (16): 165202. doi: 10.7498/aps.63.165202
Magnetized target fusion (MTF) is an alternative approach to fusion, of which the plasma lifetime and density are those between inertial confinement fusion and magnetic confinement fusion. Field-reversed configuration (FRC) is a candidate target plasma of MTF. In this paper, a two-dimensional magneto-hydrodynamic code MPF-2D is developed, and it is used to simulate the formation process of FRC on experimental devices FRX series at Los Alamos National Laboratory. In addition, design parameters of FRC on Yingguang-1 device are also evaluated, which will be constructed in 2015 at the Institute of Fluid Physics, China Academy of Engineering Physics.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
Electronic structures and phase instabilities of cubic Na1/2Bi1/2TiO3 and K1/2Bi1/2TiO3： a first-principles comparative study
2014, 63 (16): 167101. doi: 10.7498/aps.63.167101
The electronic structures, potential energy surfaces for the displacement of ions along the tetragonal  and Γ phonon properties of Na1/2Bi1/2TiO3 and K1/2Bi1/2TiO3 are investigated by employing the first-principles method based on density functional theory. The results indicate that the electronic structures of Na1/2Bi1/2TiO3 and K1/2Bi1/2TiO3 are very similar. The valence band is dominated by O 2p states with an admixture of Ti 3d and Bi 6p states. The lower energy region of the conduction band is mainly composed of Ti 3d orbitals. The bond strengths of Ti–O and Bi–O increase when Na is substituted by K. Moreover, minor differences are observed from the potential energy surface for the displacement of ions along the tetragonal , which indicates that the phase instabilities of the two compounds mainly come from the displacement of O ions, which plays a more important role when Na is substituted by K. There are three soft modes for Na1/2Bi1/2TiO3 and K1/2Bi1/2TiO3, which mainly originate from the vibration of O6 group. The A2u soft mode becomes harder when Na is substituted by K.
2014, 63 (16): 167503. doi: 10.7498/aps.63.167503
The effect of orientation degree on high-frequency magnetic properties of planar anisotropy carbonyl-iron particles is studied. We build the samples for planar carbonyl-iron particles, whose magnetic moments obey Gaussian distribution. Meanwhile, we obtain a new method about complex permeability by utilizing Landau-Lifshitz-Gilbert equation. Then the frequency-dependent complex permeability of sample is calculated using our method. The result shows that with the increase of orientation , the standard deviation of Gaussian distribution decreases and the orientation degree f increases. Furthermore, the real part of initial permeability of the planar carbonyl-iron sample becomes higher with the increase of orientation degree f. For comparison, we measured the orientation degree and complex permeability of the sample by Mssbauer spectroscopy and vector network analyzer respectively. It is found that our experimental data accord well with the simulation results.
Micromagnetic simulation and analysis of Nd2Fe14B/Fe65Co35 magnetic bilayered thin films with different orientations of the easy axis
2014, 63 (16): 167505. doi: 10.7498/aps.63.167505
The hysteresis loops, angular distribution, nucleation field, coercivity and energy product are calculated by three-dimensional micromagnetic method for Nd2Fe14B/Fe65Co35 bilayers with a deviation of the easy axis, and the results are compared with the experimental results. The results show that obvious nucleation can be observed only when the β between the easy axis and the applied field is equal to 0°, and the nucleation field and the coercivity decrease as the thickness of the soft phase Ls increases. The remanence decreases and the squareness of the hysteresis loop weakens as β increases, leading to the decrease of the energy product while the largest maximum energy product (561.61 kJ/m3) occurs at Ls=1 nm and β=0°. The shapes of the hysteresis loops, the remanence and the coercivity obtained from calculations and experiments are close to each other.
2014, 63 (16): 167801. doi: 10.7498/aps.63.167801
Solvent effect plays an important role in determining electron-vibration coupling constant, however, the physical properties of solvent effects and the relationship between them still need to be investigated. We measure the UV-visible absorption and Raman spectra of β-carotene in 10 typical solvents. The results show that polarizability and dielectric constant of solvent have a significant effect on the electron-vibration coupling constant of β-carotene. With the increase of polarizability, no matter whether the solvent is of polarity, Huang-Ryes factor and the electron-vibration coupling constant of β-carotene decrease and the Raman scattering cross section of β-carotene increases. As to nonpolar solvent, Huang-Ryes factor and the electron-vibration coupling constant of β-carotene decrease with increasing the dielectric constant of the solvent. For polar solvent, no good regularity is obtained. In this paper, the regularity of solvent effect on the electron-vibration coupling constant is presented, which can provide a reference for how to choose the solvent in studying the electron-vibration coupling constant.
2014, 63 (16): 167802. doi: 10.7498/aps.63.167802
Here we report a red organic electroluminescent device (OLED) with an emission wavelength at 705 nm in which PCDTBT is used as an emitting layer with the structure ITO/PEDOT:PSS/PCDTBT/BCP/LiF/Al. The device shows good performances such as an onset voltage of 2 V, a maximum brightness of 29000 cd/m2 at 9 V and a maximum current efficiency of 3.5 cd/A. The effects of annealing on the luminescent property of the device are studied at different temperatures. The experimental results suggest that the device presents the best performance at an annealing temperature of 50℃, then the performance decreases with annealing temperature increasing. We find that relatively low temperature annealing is beneficial to the solvent evaporation, and the high temperature annealing is conducible to reducing the coherence length of the π-π stacking, which results in the degradation of the performance of the device.
2014, 63 (16): 167902. doi: 10.7498/aps.63.167902
The modulation transfer function (MTF) of graded band-gap AlGaAs/GaAs transmission-mode photocathodes is numerically solved from the two-dimensional continuity equations. According to the MTF model, we calculate the theoretical MTF of graded band-gap and uniform band-gap transmission-mode photocathodes, and analyze the effects of Al composition, wavelength of incident photon, and thickness values of AlGaAs and GaAs layer on the resolution. The calculated results show that compared with the uniform band-gap photocathode, the graded band-gap structure can increase the resolution of photocathode evidently. If the spatial frequency f ranges from 100 to 500 lp·mm-1, the increase of resolution is more pronounced. Let f=200 lp·mm-1, the resolution of graded band-gap photocathode generally increases 150%-260%. The resolution improvement of graded band-gap photocathode is attributed to the built-in electric field. While too high built-in electric field will influence the spectral response of long-wavelength photons due to higher Al composition in the AlGaAs/GaAs photocathodes.
Dependence of annealing temperatures on the optimized resistive switching behavior from SiOx (x=1.3) films
2014, 63 (16): 167201. doi: 10.7498/aps.63.167201
SiOx films (x=1.3) are deposited on the silicon substrates by electron beam evaporation. The resistive switching behaviors from the device consisting of indium tin oxide (ITO)/SiOx/Si/Al with annealed SiOx layer as the resistive layer are investigated. It is found that on/off ratio of the device increases with the annealing temperature rising. The maximum on/off ratio reaches 109. The analyses of X-ray photoelectron spectrum and electron paramagnetic resonance spectrum reveal that the silicon dangling bonds in different valence states can be formed at different annealing temperatures, which is the main source of the conducting filament pathway. The result of ellipsometer indicates that the increase of refractive index of annealed SiOx film leads to the increase of the resistance of high resistance state.
2014, 63 (16): 167504. doi: 10.7498/aps.63.167504
The L10-ordered FePt films are promising materials for ultra high density magnetic recording media due to their high magnetic anisotropies. In this work, the L10-ordered FePt thin films are prepared by magnetron sputtering on CrW underlayer. A three-dimensional micromagnetic model, based on the symmetry of the L10 phase, is set up for FePt perpendicular media. According to the mismatch between the underlayer and FePt magnetic layer, a residual tensile stress is applied in the film plane. The simulated M-H loops accord well with the experimental results. The tetragonal crystalline anisotropy, especially high in-plane anisotropy, could enlarge the in-plane coercivity. The simulated perpendicular and longitudinal loops each have an open up in the tail, which is mainly due to the magnetostriction of the L10 phase.
2014, 63 (16): 167903. doi: 10.7498/aps.63.167903
Molecular packing and interfacial electronic properties of well-ordered organic semiconductor, copper phthalocyanine, thin films grown on MoS2(0001) are studied with low energy electron diffraction (LEED) optics, atomic force microscope (AFM) and photoelectron spectroscopy (PES). The band structure of MoS2(0001) around the Γ point of the surface Brillouin zone is given by angle-resolved photoelectron spectroscopy. The LEED patterns indicate that three equivalent well-ordered two-dimensional square lattices are formed in CuPc monolayer thin film along three surface crystalline axes (,  and ) of MoS2 (0001) substrate, respectively. The AFM measurements show that the growth of CuPc on MoS2 (0001) occurs in a Stranski-Krastanov mode. The CuPc molecule can be flat-laying on MoS2(0001) at low coverage (～0.3 nm), but form strip-like crystals along the surface crystal axes of MoS2 (0001) at high coverage (>2.4 nm). The CuPc molecule shows obvious anisotropy, indicating that the molecular plane is not parallel to the MoS2 surface. The PES measurements show there is no charge transfer process at the interface, indicating weak van der Waals interaction between CuPc and MoS2(0001).
2014, 63 (16): 167302. doi: 10.7498/aps.63.167302
The fluorescence blinking characteristics of the single CdSe/ZnS core/shell quantum dots (QDs) absorbed on the cover glass surface, indium-tin oxide (ITO) nanoparticles, and polymethyl methacrylate (PMMA) film surface are measured by a laser scanning confocal fluorescence microscopy. It is found that all the distributions of bright state duration time of QDs on the three different interfaces can be described by a truncated power law P(t)∝ t-αexp(-t/μ). The statistical on-time durations of single QDs absorbed on the ITO nanoparticles is shorter than on the glass. In addition, the on-time duration with single QDs absorbed on the PMMA is longer than on the others. These differences can be attributed to the diverse interfacial electron transfers between QD and different materials.
2014, 63 (16): 167501. doi: 10.7498/aps.63.167501
According to the shock wave experiment on the Nd2Fe14B ferromagnet, the relationship between pressure and temperature on the shock front is calculated in a pressure range from 3.3 GPa to 7.2 GPa. In order to analyze the magnetic transition mechanism of Nd2Fe14B under different temperatures and applied pressures, the equivalent pressure field is introduced to improve the two-sublattice model based on the molecular field theory. The pressure dependence of magnetostriction coefficient, susceptibility, magnetization, and Curie temperature of Nd2Fe14B are calculated. The criteria of the ferromagnetic-paramagnetic phase transition occurring in Nd2Fe14B at different temperatures and pressures are obtained. The results indicate that the Curie temperature of Nd2Fe14B decreases as pressure increases. The Curie temperature reduces from 584 K at 0 GPa to 298 K at 1.142 GPa. With the increasing of pressure, the magnetization of Nd2Fe14B declines. The critical demagnetization pressure of Nd2Fe14B also decreases with the increasing of temperature. In a pressure region from 3.3 GPa to 7.2 GPa, there appears the pressure induced ferromagnetic-paramagnetic phase transition of Nd2Fe14B.
Effects of Co doping on the magnetic properties, entropy change, and magnetocaloric effect in Mn3Sn1-xCoxC1.1compounds
2014, 63 (16): 167502. doi: 10.7498/aps.63.167502
The Mn3Sn1-xCoxC1.1 compounds are synthesized by a solid-state reaction method. The effects of Co doping on the magnetic properties, phase transition and entropy change are investigated in Mn3Sn1-xCoxC1.1 compounds. The Curie temperature first decreases from 283 K to 212 K (Mn3Sn0.9Co0.1C1.1) with increasing the Co concentration, and then increases to 332 K (Mn3Sn0.2Co0.8C1.1) with further increasing the Co concentration in Mn3Sn1-xCoxC1.1. The first-order transition of Mn3Sn1-xCoxC1.1 gradually changes into the second-order transition, in the mean time, the entropy change decreases and the phase transition region broadens from 9 K to 300 K with increasing the Co content. Both the magnetic entropy change and broadening the transition temperature span can influence the relative cooling power R. Finally we obtain the large R=103 J/kg (H=1.6 MA/m) in Mn3Sn0.8Co0.2C1.1, which could be used as the room-temperature magnetic refrigerant materials.
Band offsets of ZnO/PbTe heterostructure determined by synchrotron radiation photoelectron spectroscopy
2014, 63 (16): 167301. doi: 10.7498/aps.63.167301
Accurate determination of the band offsets of a heterostructure is essential to its study and application. In this paper, we use synchrotron radiation photoelectron spectroscopy to determine the band offset of ZnO/PbTe heterostructure. The valence band offset is 2.56 eV, and the conduction band offset is 0.49 eV, which indicates that the heterostructure has a type-I band alignment. By performing the depth scanning measurement, we find there are two bonding structures at the interface of ZnO/PbTe heterostructure, corresponding to Pb-O bonding (low energy side) and Pb-Te bonding (high energy side). At the interface of ZnO/PbTe heterostructure, the conduction band offset is much smaller than the valence band offset which is conducive to the transportation of excited electrons in PbTe source layer to ZnO electrode. Due to the unique band structure the ZnO/PbTe heterostructure has potential applications in the fabrication of high efficiency solar cells, mid infrared detectors and lasers.
Influence of vacancy on spontaneous polarization of wurtzite AlN： a maximally localized Wannierfunction study
2014, 63 (16): 167701. doi: 10.7498/aps.63.167701
By using first-principles plane-wave ultrasoft pseudopotential method based on the density functional theory, the maximally localized Wannier functions of N vacancy and Al vacancy with different charge states in wurtzite AlN are calculated. With the shape and center of the computed Wannier function, the electronic structure and the spontaneous polarization of vacancy are studied respectively. The results show that N–Al bond possesses a large ionicity. It is found that the electrons of the dangling bonds displace to the positions of vacancies in N vacancy structure, while in Al vacancy structure the electrons of dangling bonds keep away from vacancies and move to the other side of N atoms. Since the vacancy breaks the centrosymmetry of the  and  orientations, the polarization will be induced in these two directions, and the polarization will increase as the charge increases. In the  orientation, the spontaneous polarization in N vacancy will reverse as charge increases because of the dramatic variation of the electronic structure. Contrary to N vacancy, the reversion does not occur although the spontaneous polarization of Al vacancy increases as charge increases.
2014, 63 (16): 167901. doi: 10.7498/aps.63.167901
The multipactor phenomenon between metal wall and dielectric window disk of pill-box window can behave as double surface multipactor, which is affected by the normal electric field of TM11 mode. The Monte Carlo code is used to build up simulation model, calculate the multipactor susceptive curve, threshold voltage, evolution of particle number, and the trajectory of particle motion under the action of double surface normal field between alumina window and copper wall. Through investigating the behavior of secondary electrons, the regularity of normal field double surface resonant multipactor and non-resonant multiapctor is achieved. Besides, the feature of the transform from double-surface multipactor to single-surface multipactor is also obtained. This research can provide a theoretical basis for window breakdown mechanism analysis.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
Effect of predeformation on the transition from hexagonal phase to square phase near the melting point using phase field crystal method
2014, 63 (16): 166401. doi: 10.7498/aps.63.166401
The two-mode phase field crystal (PFC) method is used to calculate the phase diagram. And in this paper it is used to simulate the effects of predeformation degree and isothermal temperature on the hexagonal grain boundary evolution and on the hexagonal/square phase transition. Results show that when there is no predeformation in the initial phase, the grain boundary defect causes the pre-melting around the melting point; predeformation increases and the interaction between deformation and defects induces the pre-melting around the melting point; and the predeformation further increases, deformation induces liquid phase and square phase simultaneously at the distortion place. The bigger the predeformation and the closer to melting point the maintained temperature, the more obvious the growth of liquid phase is; on the contrary, the square phase grows obviously. The distortion energy is released with time and the phase of grain finally becomes square phase. It can be concluded that keeping the hexagonal phase isothermal near the melting temperature, the liquid phase appears at the grain boundary or at the other defects because the predeformation leads to the increase of atom activity, thus increasing atom disorder degree. Then with the release of distortion energy, the grain phase finally transforms into an equilibrium square phase. In this way the hexagonal/square transition time is extended.
2014, 63 (16): 166802. doi: 10.7498/aps.63.166802
ZnO thin films are prepared by sol-gel method on Si substrates. The structural and optical properties of the films annealed at different temperatures are analyzed by X-ray diffraction (XRD), scanning electron microscopy, X-ray photoelectron spectroscopy and photoluminescence. The results of XRD show that each of all the ZnO thin films has a wurtzite phase and is preferentially oriented along the c-axis direction. The sample annealed at 900℃ exhibits a better crystalline quality. Bright and stable structured green luminescence is achieved from the Cu-doped ZnO thin film. The intensity of the green emission increases significantly after annealing at 800℃, while starts to decrease with further increasing temperature. Green luminescence is correlated with the creation of Zn vacancies. Green emission peaks are found to be dependent on the relative concentration of defect centers. The substitution of Cu2+ by Cu+ will increase concentration of defects in the Cu:ZnO thin film and result in very strong green emission.
Epitaxial growth micro-structure and magnetic studies of FePt nanoparticles：MgO multi-layer composite thin films
2014, 63 (16): 166801. doi: 10.7498/aps.63.166801
The FePt：MgO multi-layer nanocomposite thin films are deposited on MgO (100) substrates by using pulsed laser deposition method. The composition of FePt is Fe48Pt52. FePt nanoparticles (NPs) are embedded in MgO epitaxial layer periodically. The results of in-situ reflection high energy electron diffraction show that MgO epitaxial layers grow into the layer-by-layer mode and FePt NPs grow into the island mode. The alternation of the two growth modes is achieved in the whole deposition process. The high resolution transmission electron microscope results show that the phase of FePt is converted from the disordered face-centered cubic structure into the ordered face-centered tetragonal L10 structure after annealing. The perfect crystalline FePt NPs (around 5 nm in diameter) are flat-hexagonal in shape and array layer by layer on MgO matrix. The M-H loop of the film shows that the ordering degree and magnetism are enhanced after annealing.