Vol. 64, No. 8 (2015)

Energy balance and robustness adjustable topology control algorithm for wireless sensor networks
Hao Xiao-Chen, Liu Wei-Jing, Xin Min-Jie, Yao Ning, Ru Xiao-Yue
2015, 64 (8): 080101. doi: 10.7498/aps.64.080101
Abstract +
In wireless sensor networks, the interference around the application environment may cause the actual distance between any pair of nodes to fail to be measured accurately. Enclosure graph (EG) model uses this distance between nodes as its weight to construct the topology, which does not fully consider the interference. Consequently it will lead to a large amount of energy consumption induced by the application environment. Even it shortens the survival time. According to the feature of network energy inequality in a wireless sensor network and the defect of EG, we first introduce the adjustable factor of node degree, establish a model of communication metric and a model for the node actual survival time. Then according to the demand of network energy equalization and maximum network lifetime, we quantitatively analyze the network node degree, and achieve its regular pattern. In accordance with this regular pattern and sufficient conditions of function extremum, the maximum node energy consumption and the maximum node actual survival time are deduced. And the corresponding optimal node degree is achieved. Finally, according to the above model, in this paper we propose an energy balance and robustness adjustable topology control algorithm for wireless sensor networks. Theoretical analyses show that this algorithm can guarantee that the network is connected and the link of the network is bi-directionally connected. Experiments show that the network takes advantage of this optimal node degree to obtain the high robustness, thus guaranteeing that the information can be transferred unfailingly. This algorithm can effectively balance the node energy, improve the node survival time, enhance the network robustness, and prolong the network's lifetime.
A new noise covariance matrix estimation method of Kalman filter for satellite clock errors
Lin Xu, Luo Zhi-Cai
2015, 64 (8): 080201. doi: 10.7498/aps.64.080201
Abstract +
The satellite clock plays a key role in the global navigation satellite system (GNSS). The accuracy of GNSS and its applications depend on the quality of the satellite clock. Therefore, precisely estimating and predicting the satellite clock is an important issue in the fields of GNSS and its application. As an optimal estimation algorithm, Kalman filter has been used to estimate and predict the satellite clock. However, in a conventional Kalman filter algorithm, the noise covariance matrices of satellite clock need to be predetermined, which restricts its further applications since the noise covariance matrices, especially the process noise covariance matrix, are usually unknown in the real cases. With inappropriate noise covariance matrices, the state estimation of conventional Kalman filter is suboptimal. To cope with this problem, a new noise covariance matrix estimation method of Kalman filter is proposed, and then we apply it to the problem of satellite clock estimation and prediction. Considering the fact that the process noise covariance matrix depends on the unknown noise parameters, the problem of estimating process noise covariance matrix can be solved by estimating the unknown noise parameters. First, the correlation between the Kalman innovations is used to establish a linear relationship with the unknown noise parameters. Then the unknown parameters can be estimated by least-squares estimation. Finally, the satellite clock can be estimated and predicted with the estimated noise parameters. In the new method, no prior information about the noise parameters is needed. Even with some extreme prior noise parameters, the new method can also work very well and has good convergence properties. For comparison, we conduct two experiments using the new method and the adaptively robust Kalman filter with classified adaptive factors based on opening windows separately, both results are consistent with each other very well, which verifies the correctness and effectiveness of this new method.
Simulation and prediction of the weld-line in the filling process using a corrected smoothed particle hydrodynamics method
Ren Jin-Lian, Lu Wei-Gang, Jiang Tao
2015, 64 (8): 080202. doi: 10.7498/aps.64.080202
Abstract +
A corrected smoothed particle hydrodynamics (SPH) method for viscoelastic fluid is proposed and used to tentatively simulate and predict the behavior of the molecule near the weld line in the filling process of the FENE-P fluid in this paper. And the corrected SPH scheme for the viscoelastic fluid is simultaneously presented. Firstly, a coupled macro-micro model based on the SPH method for the viscoelatic fluid is set up. Then, some benchmarks, such as the flow behavior of the periodic cylinders of FENE-P fluid and the non-isothermal Poiseuille flow based on the Oldroyd-B model which is a simplified model of FENE-P model, are simulated to verify their validity and the convergence of the corrected SPH method of solving the coupled macro-micro problem of the polymer and the discrete SPH temperature model for viscoelastic fluid. Finally, the filling process of the viscoelastic fluid based on the FENE-P model in a ring-shaped mold is simulated, and the behavior of the micro molecules in the filling process is tentatively shown by orientation ellipse. Meanwhile the non-isothermal filling process of the FENE-P fluid is also implemented. The numerical results show clearly the behavior of the molecules in the filling process of the FENE-P fluid, the weld line is indeed observed in the filling process of the FENE-P fluid in the ring-shaped mold, and the non-isothermal filling process can improve the weld line to some extent. In order to further discuss the improvement of the weld line, the filling processes of different cases are simulated in the multiple-gating C-shaped mold by using the pattern of the hot runner and valve gates, and the obtained results are compared with other available data. Moreover, the effect of the delay time needed for the fluid to be injected on the flow is also investigated. The numerical results show that the pattern of the hot runner and valve gates can improve and even remove the weld-line in the filling process of the polymer melt, especially for the big-sized product, and the shorter the delay time is, the faster the flow is, and the bigger the appearance probability of the weld line is.
Near classical states of three-dimensional isotropic harmonic oscillator in spherical coordinate system
Li Xing-Hua, Yang Ya-Tian
2015, 64 (8): 080301. doi: 10.7498/aps.64.080301
Abstract +
One can easily understand the transition from special relativity to Newton mechanics under the condition of v/c 1. But it is not so easy to understand the transition from quantum representation to classical representation from the point of view of wave mechanics. We define such a quantum state as near classical state (NCS), in which the mean value of coordinates equals the classical solution on a macroscopic scale. We take the NCS for three-dimensional isotropic harmonic oscillator in a spherical coordinate system for example. We take and choose cnl =(1/(2N+1))(1/(2lM+1)). The mean values of coordinates are r2 =(Ecl)/(2)(1+1-((2Lcl2)/(Ecl2)cos(2t)) and tg = (Ecl/lcl)[1-1-((Lcl)/(Ecl)2]tg(t)) in this NCS, which are in agreement with the classical solution on a macroscopic scale, where N/N1, lM/lM1. N and lM are determined by the macroscopic state. N =[(Ecl)/(ħ)], Ecl = 1/22(a2+ b2) , lM= [Lcl}/ħ], and Lcl = ab. Here , Ecl and Lcl respectively denote the mass, the energy and the angular momentum of harmonic oscillator. And the bracket [c] means taking the integer part of the number c, for example [2.78]=2. It is also emphasized that for a definite macro state, there are many NCS corresponding to a macro state; just like the case in statistical physics, many micro dynamical states correspond to a macro thermodynamic state. Thus the transition from quantum representation to classical representation is a coarse-graining process and also an information losing process.


Effects of Dzyaloshinskii-Moriya interaction and intrinsic decoherence on quantum dense coding via a two-qubit Heisenberg spin system
Zou Qin, Hu Xiao-Mian, Liu Jin-Ming
2015, 64 (8): 080302. doi: 10.7498/aps.64.080302
Abstract +
By solving the Milburn equation, we investigate the properties of optimal channel capacity for the quantum dense coding via a two-qubit Heisenberg spin system with Dzyaloshinskii-Moriya (DM) interaction in the presence of intrinsic decoherence. The influences of different DM interactions, different initial states, anisotropic coupling parameters, and intrinsic decoherence on optimal coding capacity are analyzed in detail. It is found that the initial state of the system affects optimal coding capacity greatly, whose dependent parameters are not identical for different types of initial states. When the system is initially in the form of the nonmaximally entangled state cft| {01} ightangle + dft| {10} ightangle , a weak z-component DM interaction can enhance the value of optimal coding capacity as compared with the value without DM interaction, and the phase decoherence effect can suppress the oscillation of optimal coding capacity and make the capacity decrease to a stable value for the long-time evolution. It is also found that under the influence of intrinsic decoherence, the optimal transmission capacity of dense coding can keep an ideal maximal value of 2 by choosing the proper initial maximally entangled state. Moreover, no matter from which direction the DM interaction is introduced, the optimal coding capacity via the two-qubit Heisenberg spin system is always larger than the transmission capacity of any classical communication.
Statistical properties of binomial and negative-binomial combinational optical field state and its generation in quantum diffusion channel
Fan Hong-Yi, Wu Ze
2015, 64 (8): 080303. doi: 10.7498/aps.64.080303
Abstract +
According to the combinational binomial-negative-binomial distribution, we propose a binomial-negative-binomial combinational optical field state, which can be generated in the process of a Fock state |m>m| passing through a quantum-mechanical diffusion channel. We derive the second-order coherence degree formula, g(2)(t) =2-((m2+m)/(m+κt2)), which is the diffusion constant. We find that in the process of the Fock state undergoing quantum diffusion and becoming classical, the corresponding photon statistics evolves from sub-Poissonian distribution to Poisson distribution and finally goes to a chaotic state. We also find that the more photons in the initial Fock state, the longer time is needed for quantum decoherence.
Effect of anomalous magnetic moment on thermodynamic properties of weakly interacting Fermi gas in weak magnetic field
Chen Xin-Long, Men Fu-Dian, Tian Qing-Song
2015, 64 (8): 080501. doi: 10.7498/aps.64.080501
Abstract +
Taking the anomalous magnetic moment into consideration, the analytical expression of the free energy for a weakly interacting Fermi gas in a weak magnetic field is derived by using the pseudopotential method and thermodynamic theory, therefore the thermodynamic properties can be studied. Based on the derived expression, the thermodynamic properties of this system at both high and low temperatures are given, and the effect of anomalous magnetic moment on thermodynamic properties can be analyzed. The effect of anomalous magnetic moment on the thermodynamic properties is related to temperature, and with the rise of temperature this effect increases in the low temperature zone, but decreases in the high temperature zone. For the chemical potential and internal energy of the system, the anomalous magnetic moment strengthens the influence of the magnetic field, but weakens the influence of the interaction. Under the influence of anomalous magnetic moment, the heat capacity of the system decreases in the low temperature zone, but increases in the high temperature zone.
Thermodynamic properties of Fermi gas under generalized uncertainty principle
Li He-Ling, Wang Juan-Juan, Yang Bin, Wang Ya-Ni, Shen Hong-Jun
2015, 64 (8): 080502. doi: 10.7498/aps.64.080502
Abstract +
When taking into account the generalized uncertainty principle in statistical physics, the density of states must make a correction, which causes all the results of traditional statistical physics to have different degrees of correction. In high-energy or high-temperature conditions, this amendment can subvert the traditional concept and there are also some certain amendments at low temperatures. In this paper we study the thermodynamic properties of the ideal and weakly interacting Fermi gas in low temperature conditions when the generalized uncertainty principle is taken into account. Firstly, analytical expressions of chemical potential, internal energy and heat capacity at constant volume of ideal or weakly interacting Fermi gas are given. Then the properties of copper electron gas are computed as an example, showing that when the generalized uncertainty principle is taken into account the chemical potential, Fermi energy and the ground state energy increase with the increase of temperature, while the heat capacity decreases. When the temperature is lower than 0.3 times TF0, the internal energy increases with the increase of temperature, but becomes decreased when temperature is high than 0.3 times TF0. These amendments are mostly dependent on particle density, which becomes bigger and bigger with particle density increasing.
Broadband energy harvesting from coherence resonance of a piezoelectric bistable system and its experimental validation
Lan Chun-Bo, Qin Wei-Yang, Li Hai-Tao
2015, 64 (8): 080503. doi: 10.7498/aps.64.080503
Abstract +
Piezoelectric effect is an effective way of harvesting energy from the environmental broadband vibration. In this paper, we investigate the coherence resonance of a piezoelectric bistable vibration energy harvester theoretically and experimentally. The device is comprised of a cantilever beam with magnetic repulsive force. Firstly, the electromechanical coupled equation is derived based on the Euler-Bernoulli beam theory. Then, analyzing the potential shapes, we learn that when the system oscillates between the two potential wells, it will produce a large voltage generally. And the beam dynamic response under the random excitation is simulated by Euler-Maruyama method. The results of simulations and experiments show that there is a coherence resonance threshold in the Duffing type piezoelectric bistable energy harvester. When the standard deviation of the random excitation is less than the threshold, the motion state of the system will be trapped in a single potential well, which results in a low average output power. And when the excitation standard deviation is larger than the threshold, the system stochastic stability will change. The dynamic displacement and strain clearly show that the system can exhibit large oscillation between the two potential wells. Then, Kramers rate is used to explain the coherence resonance threshold of the bistable system under the broadband random excitation. The experimental results show that when the coherence resonance takes place, the beam will oscillate between the two potential wells more frequently, and the broadband vibration energy can be transformed into large amplitude narrow band low-frequency oscillation response, which can greatly improve the harvesting effectiveness of broadband vibration energy.
Magneto-hydrodynamic calculation of magnetic flux compression with explosion driven solid liners and analysis of quasi-isentropic process
Zhao Ji-Bo, Sun Cheng-Wei, Gu Zhuo-Wei, Zhao Jian-Heng, Luo Hao
2015, 64 (8): 080701. doi: 10.7498/aps.64.080701
Abstract +
Magnetic cumulative generator (MC-1) is a kind of high energy density dynamic device. A liner is driven by a cylinderical explosive implosion to compress the magnetic flux preset in the cavity. Then the chemical energy is converted into magnetic one, which is cumulated nearby the axis to form ultra-intense magnetic field used to load sample in non-touch manner. This loading technique can bring higher pressure and relatively low elevated temperature in the sample and has a very high-degree isentropy in the course of compression. The configuration magneto-hydrodynamic code SSS/MHD is used to develop one-dimensional magneto-hydrodynamic calculation of magnetic flux compression with explosion driven solid liner. The calculation results of magnetic field in cavity and velocity of inner wall of sample tube are obtained and accord with the magnetic field measured by probe and the velocity measured by laser interference. The buckling and Bell-Plesset instabilization produced by linerly compressing magnetic field are shown through frame photography. The change laws of magnetic diffusion, eddy current and magnetic pressure in liner and sample tube are analyzed, which show that the magnetic field and pressure and eddy near to cavity in the sample tube are all higher than the ones in the liner with the same distance to cavity. The balance between the electromagnetism force and implosion action and the difference between sample tube and liner velocities are the main reasons under imploding movement. The change of isentropic increment with compression degree at the same location, whose distance is 0.05 mm to magnetic cavity in the sample tube, is discussed. The result indicates that the ratio of the maximum increment to specific heat of sample tube material is about 10%, which shows that the process of compression magnetic flux with explosion is quasi-isentropic. In general, SSS/MHD code can reveal in depth the physic images which are difficult to measure or observe in the magneto-hydrodynamics experiment.
Simulation and experiment for electrode capacitance based on Vernier anode photon counting detector
Xing Yan, Chen Bo, Wang Hai-Feng, Zhang Hong-Ji, He Ling-Ping, Jin Fang-Yuan
2015, 64 (8): 080702. doi: 10.7498/aps.64.080702
Abstract +
According to the principle of Vernier anode photon counting detector, in this paper we discuss a decoding algorithm of calculating centroid location and interelectrode capacitance between electrodes, which has a close relationship with preamplifier circuit noise. Based on the Poisson's equation, the theoretical model of Vernier anode is established. The method of calculating the irregular shape uniplanar self-capacitance and interelectrode capacitance between electrodes using ANSYS finite element analysis is introduced. In addition, a Vernier anode plate is manufactured with picosencond laser micromachining process on a 1.5 mm thick quartz substrate with gold film as conductors. The Vernier anode pattern has a picth of 9.9 mm, an active area of 19.8 mm×19.8 mm, insulation channel depth of 10 μm, and insulation channel width of 30 μup$m. Comparing the simulated capacitances with the measured capacitances, the validity of the three-dimensional finite-element method is proved. A simulation study on the effects of substrate permittivity, insulation channel width and depth on capacitance change is carried out. The simulation result provides a basis for structure design of Vernier anodes.
Calibration system for OH radicals based on differential optical absorption spectroscopy
Zhu Guo-Liang, Hu Ren-Zhi, Xie Pin-Hua, Chen Hao, Qin Min, Fang Wu, Wang Dan, Xing Xing-Biao
2015, 64 (8): 080703. doi: 10.7498/aps.64.080703
Abstract +
In the present paper, we describe a calibration system for OH radicals based on differential optical absorption spectroscopy (DOAS). In the system OH radicals can be produced by photolysis of H2O which is irradiated by the 185 nm light in a cavity. The produced OH radicals with a certain concentration can be detected exactly. The system consists of a xenon lamp as light source in which the light has been collimated, a 1.25 m multiple-reflection cell in which the light can reflect 60 times to achieve 75.0 m whole path-length, and a double pass high resolution echelle spectrometer that is suitable for the measurement of OH radicals (best resolution: 3.3 pm). Utilizing the system the measurement spectra and lamp spectra can be obtained for OH concentration retrieval. OH concentration can be calculated by DOAS retrieval and during the DOAS retrieval the reference absorption cross section is obtained by applying the Voigt broadening method to the absorption lines. By changing water vapor concentration, the system accurately detects OH concentration ranging from 5×108 molecules/cm3 to 1.8×1010 molecules/cm3. In the concentration range, OH concentration fluctuation is very small. For example, when the volume ratio between water vapor and pure N2 reaches 0.3 L:24.7 L, the fluctuation is just ± 4%. Taking into account the effects of absorption cross section, gas pressure in the cavity and other factors, the total systematic error of the instrument is less than 7.3%. According to the results in the paper, the system can be used for the fluorescence assay by gas expansion technology calibration in field experiments.
Modified method for diffraction aberration of one-dimensional wire chamber
Wen Zhi-Wen, Qi Hui-Rong, Dai Hong-Liang, Zhang Yu-Lian, Zhang Jian, Wei Kun, Ouyang Qun, Shao Jian-Xiong
2015, 64 (8): 082901. doi: 10.7498/aps.64.082901
Abstract +
The one-dimensional position sensitive wire gaseous detector is developed for the synchrotron radiation diffraction, which consists of a single wire of gold-plated tungsten and 200 cathode strips as the readout. The induced signal is produced by the several adjacent cathode strips when X-ray is incident on anode wire with high voltage. Using the center gravity method to analyze the adjacent signals in one dimension, the primary ionization position of the X-ray can be obtained and the position resolution is 160 μm (FWHM). In Beijing Synchrotron Radiation Facility, the diffraction test is done at the experimental station. When the X-ray irradiates the crystal sample of SiO2, the different sizes of the diffraction rings can be produced. The diffraction angles are measured to be 11.148° and 14.201°, and the two-dimensional diffraction rings are reconstructed when the detector is moved and scanned with the several steps in the diffraction ring range. The diffraction aberration of one-dimensional wire chamber is very obvious. In the paper, the relevant influence factors of the detector construct are discussed. The thickness of the working gas and the width of active area window can give rise to the diffraction ring aberration. The theoretical calculation value of the diffraction aberration is larger than the position resolution value of the gaseous detector. The correction method is established based on the corresponding physical analysis, and by this method the value of the diffraction ring point is calculated. The relative aberration of diffraction position is improved by up to 7% with using the method, and the two-dimensional diffraction ring with no aberration can be reconstructed.
Molecular dynamics simulation of the thermophysical properties and phase change behaviors of aluminum nanoparticles
Lin Chang-Peng, Liu Xin-Jian, Rao Zhong-Hao
2015, 64 (8): 083601. doi: 10.7498/aps.64.083601
Abstract +
With the development of energy storage technology, phase change materials which can be used to store thermal energy have received much attention in recent years. The nano-metallic materials are universally used as phase change materials due to their many desirable thermophysical properites. In this paper, the molecular dynamics simulation method is adopted to simulate the variations of melting point, density and phonon thermal conductivity of the nano aluminum with grain size ranging from 0.8 nm to 3.2 nm. The variations of density, specific heat capacity and phonon thermal conductivity with temperature of aluminum nanoparticles at a grain size of 1.6 nm are also studied. By using the embedded-atom potential, the thermophysical properties and phase change behaviors of aluminum nanoparticles are stimulated. The phase transition temperature of aluminum nanoparticles is studied based on the energy-temperature curve and the specific heat capacity-temperature curve. The surface energy theory and the size effect theory are applied to the analysis of the variation of the melting point of the aluminum nanoparticles, and the results show that the melting point increases as grain size augments, and it increases slowly when its grain size is between 2.2 nm and 3.2 nm but still holds the trend of increase. In order to obtain accurate thermal conductivity, the Green-Kubo method is adopted to calculate the phonon thermal conductivity of aluminum nanoparticle. As the grain size of aluminum nanoparticles increases, its density monotonically decreases, and the thermal conductivity monotonically increases linearly, which is in line with the theory of phonon. Similarly, with the increase of temperature, the density and thermal conductivity of aluminum nanoparticles of 1.6 nm in grain size both decrease. Moreover, the density of aluminum nanoparticle is generally lower than that of its bulk material. The study also shows that the heat transfer manner of aluminum nanoparticle is based on ballistic-diffusive heat conduction instead of the traditional diffusive heat conduction when it is in a nanoscale. The simulation studies the thermophysical properties of nanoparticles from the atomic perspective, and is of significance for guiding the design of the phase change materials based on the aluminum nanoparticles for thermal energy storage.
Radar cross section reduction of microstrip antenna based on wide-band metamaterial absorber
Li Wen-Hui, Zhang Jie-Qiu, Qu Shao-Bo, Yuan Hang-Ying, Shen Yang, Wang Dong-Jun, Guo Meng-Chao
2015, 64 (8): 084101. doi: 10.7498/aps.64.084101
Abstract +
We design a wide-band metamaterial absorber (WBMA) with stable polarization and wide incident angle by loading a lumped resistance. The full-width at half-maximum of the absorber is 98%, the bandwidth of absorbing rate of more than 90% reaches 10.42 GHz, and the peak absorbing rate is 99.9% in the normal wave incidence. Loading the WBMA around the microstrip antenna by sharing the same substrate and ground plane, we fabricate the WBMA antenna, whose radar cross section (RCS) sharply decreases in the wide frequency range. Simulation and experimental results show that the antenna's radiation pattern is almost unchanged: just only the former gain improves 0.53 dB after loading the WBMA. Under different polarized waves, the antenna's monostatic RCS reduction is more than 3 dB within the working frequency band and beyond the working frequency band (6.95-17.91 GHz), the maximum value is 21.2 dB. The bistatic RCS decreases significantly from -48° to 48° at the middle working frequency (8 GHz), which well achieves the antenna stealth at the wide-band frequency and wide angle.
Simulation and experimental study of phase characteristics in high-gain relativistic klystron amplifier
Wu Yang, Xu Zhou, Xie Hong-Quan, Li Zheng-Hong, Ma Qiao-Sheng
2015, 64 (8): 084102. doi: 10.7498/aps.64.084102
Abstract +
In order to realize the combination of the coherent high power microwave, an S-band high gain relativistic klystron amplifier with high phase stability is presented and studied theoretically and experimentally. The phase characteristics of the output microwave are explored in particle-in-cell code and experiment. The experimental results accord well with the simulation results, which indicates that the parasitic oscillation excited by retrograde electrons is the main cause of phase variation in the development of high-gain relativistic klystron with high phase stability. When the input RF power is 10 kW, a microwave pulse with about ± 10° in relative phase difference fluctuation between output and input is obtained in experiment. And the corresponding locking duration is about 90 ns. Accordingly, the expectation of power combination efficiency of 10 high-gain relativistic klystron amplifiers can be achieved to be 99% with continuous uniform distribution of random relative phase difference.
Reconstruction and calibration on aero-optical wavefront aberration based on Background oriented schlieren based wavefront sensing
Zhang Tian-Tian, Yi Shi-He, Zhu Yang-Zhu, He Lin
2015, 64 (8): 084201. doi: 10.7498/aps.64.084201
Abstract +
Background oriented schlieren based wavefront sensing (BOS-WS) is a new experimental technique for measuring the two-dimensional distribution of optical wavefronts and the optical path differences (OPDs) induced by the flow-field density variations. Background oriented schlieren (BOS) is traditionally used to test the flow-field density distribution, which restricts the obtaining of useful information since the obtained density information is integrated over the optical path. The OPD is very important for predicting the optical distortion when light travels through the flow field and it is tested by BOS-WS. In order to obtain the optical distortion generated by aero-optic effect, and restore the original image from the distortion known information so as to explore a new kind of supersonic imaging guidance method, theory analysis, numerical simulation and experimental methods are used based on BOS-WS. Through theoretical analysis, the wavefront measurement method based on BOS is verified and the calculation methods of using wavefront information known to predict distortion displacement field and using known displacement field to reconstruct wavefront are explored. By numerical simulation, the error sizes and the result rationalities of one stepped integral algorithm and Southwell method on the wavefront reconstruction are compared, and through the error analysis it is proved that the Southwell method is more accurate and reasonable. By a wavefront aberration experiment carried out in the flow field above the candle flame and a lens perturbation experiment, the methods of using OPD known to reconstruct distorted displacement field and correcting image distortion by the field are creatively explored. The verification experiments show the effectiveness of the correction method.
Study of positive and negative gradient refractive index alternant surface
Liu Xiao-Bo, Liu Ming-Li, Chen Jian-Zhong, Shi Hong-Yu, Chen Bo, Jiang Yan-Sheng, Xu Zhuo, Zhang An-Xue
2015, 64 (8): 084202. doi: 10.7498/aps.64.084202
Abstract +
In this paper, we design a kind of positive and negative gradient refractive index alternating surface and discuss its physical mechanism by the geometrical optics method and the numerical simulation of electromagnetic field. This structure can control the propagation of electromagnetic waves by adjusting some parameters such as refractive gradient. Under certain parameters, electromagnetic waves from space can be confined mainly in the media all the time, or are released into the space after propagating a certain distance in the media. This structure is polarization-independent and wide-band. It means that this structure can be used as a stealth surface by reducing the scattering cross section. Finally, the characteristics of the structure are verified by the numerical simulation.
Experimental study on multi-wavelength ptychographic imaging in visible light band
Wang Dong, Ma Ying-Jun, Liu Quan, Shi Yi-Shi
2015, 64 (8): 084203. doi: 10.7498/aps.64.084203
Abstract +
We perform a series of computer simulations and optical experiments for multiple-wavelength ptychographic imaging to analyze the relationship between the imaging quality and the magnitude of wavelength. Two kinds of samples including the grating-like and the biological sample are tested. Our experimental results are highly consistent with simulations, demonstrating the feasibility and effectiveness of the multiple-wavelength ptychography. Compared with the single-wavelength ptychographic imaging, it can achieve very good imaging quality with a fast speed of iterative convergence and a high robustness to the noises in the case of multiple-wavelengh ptychography. In addition, optical experiments also reveal that with the magnitude of wavelength increasing, the complexity of the ptychographic system is grown up within increased noises and errors, which causes the imaging quality to keep no enhancement or even to get worse. For our concrete configuration in this paper, with a simple digital procedure for noise depressing, the best results may be obtained for the case of dual-wavelength. Furthermore, it implies that there is an optimized condition for multiple-wavelength ptychography. We find that it requires mainly analyzing the balance between the multiple-wavelength-benefited noise-resistance and the systematic complexity for the optimized condition, which may be really important and meaningful for the practical utilizing of multiple-wavelength ptychography.
Influence of feedback strength on the characteristics of the random number sequence extracted from an external-cavity feedback semiconductor laser
Yang Hai-Bo, Wu Zheng-Mao, Tang Xi, Wu Jia-Gui, Xia Guang-Qiong
2015, 64 (8): 084204. doi: 10.7498/aps.64.084204
Abstract +
Under proper feedback strength, an external-cavity feedback semiconductor laser can operate at a chaos state, and its chaotic output can be used as a physical entropy source to generate a physical random number sequence. In this paper, we focus on the influence of feedback strength on the randomness of the obtained binary code sequence. The simulation results show that with the increase of feedback strength, the time delay characteristic peak of the chaotic signal from an external-cavity feedback semiconductor laser first decreases and then increases gradually, meanwhile, the permutation entropy characteristic value of chaotic signal first increases and then decreases gradually, namely, there exists an optimized feedback strength for obtaining the chaotic signal with the weakest time delay signature and high complexity. The randomness of binary code sequences, generated by the chaotic signal from the external-cavity feedback semiconductor laser under different feedback strengths, is tested by NIST Special Publication 800-22, and the influence of feedback strength on the test results is also discussed.
300 W all-fiber amplifier with distributed side-coupled pump configuration
Dong Fan-Long, Ge Ting-Wu, Zhang Xue-Xia, Tan Qi-Rui, Wang Zhi-Yong
2015, 64 (8): 084205. doi: 10.7498/aps.64.084205
Abstract +
The angle-polished side-pump coupler is fabricated by fused splicing angle-polished multimode fibers and 20/400 μm double-clad Yb-doped large mode area fiber together. The coupling efficiency of side-pumped coupler for 975 nm pumping light can reach up to 97%, and the leakage ratio of 1080 nm signal light is less than 2%. We analyze the performance of a single side-pumped coupler and also the influence of the distribution of cascade stage couplers on the efficiency of fiber amplifier. Using self-developed side-pumped couplers, we build a distributed side-pumped, ytterbium-doped double-clad all-fiber master oscillator amplifier, and an continuous and single mode laser of 1080 nm wavelength with a 303 W maximum output power is obtained. Finally, increasing the number of the side couplers to improve pump power, the output power of the amplifier becomes higher.


Single-mode narrow linewidth random fiber laser with enhanced feedback from Rayleigh scattering
Li Yang, Liu Yan, Liu Zhi-Bo, Jian Shui-Sheng
2015, 64 (8): 084206. doi: 10.7498/aps.64.084206
Abstract +
The origin of Rayleigh scattering in fiber waveguides is numerically demonstrated, which indicates that the inhomogeneous doping and diameter variations during drawing are the two dominant reasons. And the scattering fiber with a loss as high as 0.54 dB/km is successfully fabricated based on such principles. The overall Rayleigh backscattering intensity of 5 km scattering fiber is 5 dB higher than that of SMF-28 with the same length in telecommunication window. The principle of single-mode random fiber laser is also studied. The emission spectrum is the superposition of a large number of random modes with arbitrary amplitudes and phases, among which only the highest gain modes can lasing through gain competition. In experiment, a single-mode erbium-doped fiber linear laser with a narrow linewidth of 3.5 kHz and a high contrast of 50 dB is achieved by combining with 500 m scattering fiber as the random feedback. The threshold pump current is reduced by 80 mA and the max output power is increased by 3 dBm for the proposed laser compared with those of the laser with 500 m SMF-28 as the feedback. The tunabiltiy of the proposed laser is determined mainly by the fiber Bragg grating.
High-efficiency intracavity 2 μm degenerate optical parametric oscillator
Guo Jing, He Guang-Yuan, Jiao Zhong-Xing, Wang Biao
2015, 64 (8): 084207. doi: 10.7498/aps.64.084207
Abstract +
A high-efficiency 2 μm optical parametric oscillator based on MgO-doped periodically poled LiNbO3 intracavity pumped by a 1.064 μm Q-switched Nd:YVO4 laser is reported. With the intense fluence inside the laser cavity, a maximum 2 μm average power of 3.5 W is obtained at 15 kHz repetition rate in a degenerate state. A maximum optical-to-optical conversion efficiency of 17.5% with a slope efficiency of 25% is achieved when the laser diode power is 20 W. To the best of our knowledge, the efficiency is the highest ever achieved from an intracavity degenerate optical parametric oscillator in 2 μm region. The pulse duration of 2 μm is 1.4 ns, which is much shorter than that of 1.064 μm. A bandwidth of 30 nm is obtained at a degenerate wavelength of 2128 nm. The M2 values for the 2 μm beam are 3.47 and 3.54 in the horizontal and vertical directions, respectively. The standard deviation of the power fluctuation is ~ 2% at the maximum power in half an hour.
Correction of error induced by nonlinear movement of feedback mirror in laser feedback stress measurment system
Niu Hai-Sha, Niu Yan-Xiong, Liu Ning, Liu Wen-Wen, Wang Cai-Li
2015, 64 (8): 084208. doi: 10.7498/aps.64.084208
Abstract +
Stress distribution of optical materials can be measured by using the laser feedback effect. Owing to the non-linear movement of the feedback mirror, the result accuracy of the system will decrease. In this work, we measure the displacement of the feedback mirror by using a high precision quasi-common-path laser feedback interferometry. The displacement-time function is obtained by a high-end fitting method. The stress measurement error induced by the non-linear movement of the feedback mirror is calculated according to the displacement-time function and a three-mirror cavity equivalent model, and the correction for the system accuracy is achieved. The results show that the different movement direction of the feedback mirror gives rise to an opposite variation trend of error. Measurement error can be reduced by averaging the results in different directions. In the study we analyze the influence of the non-linear movement of the feedback mirror on the measurement accuracy, and a method of improving the error is proposed. This method is significant for correcting the measurement results and improving the accuracy.
Dark signal degradation in proton-irradiated complementary metal oxide semiconductor active pixel sensor
Wang Bo, Li Yu-Dong, Guo Qi, Liu Chang-Ju, Wen Lin, Ren Di-Yuan, Zeng Jun-Zhe, Ma Li-Ya
2015, 64 (8): 084209. doi: 10.7498/aps.64.084209
Abstract +
In this paper, we discuss the dark signal increase in complementary metal oxide semiconductor (CMOS) active pixel sensor due to proton-induced damage, and present the basic mechanism that may cause failure. When the fluence of protons reaches a predetermined point, the change of dark signal of the device is measured offline. The experimental result shows that as the fluence of protons increases, mean dark signal increases rapidly. The main reason for dark signal degradation is: 1) the ionizing damage causes a build-up of oxide trapped charge and interface state at the Si-SiO2 interface. The creation of the interface traps (with energy levels within the silicon bandgap), which can communicate with carriers in the silicon, gives rise to the thermal generation of the electron-hole pairs and, hence increasing the dark signals; 2) when protons pass through the sensor, there is a possibility of collisions with silicon lattice atoms in the bulk silicon. In these collisions, atoms can be displaced from their lattice sites and defects are formed. These resulting defects can give rise to states with energy levels within the forbidden bandgap. The increasing of dark signal is therefore one of the prominent consequences of bulk displacement. We use multi-layered shielding simulation software to calculate the ionization damage dose and displacement damage dose. Based on the comparison of the test data of gamma radiation, combined with the device structure and process parameters, a theoretical model for separation proton-induced ionization and displacement damage effects on CMOS active pixel is constructed, and the degradation mechanism of the mean dark signal is investigated. The result shows that the contribution of ionization effect induced surface dark signal and the contribution of displacement damage induced bulk dark signal to dark signal degradation of the whole device are roughly equal in this domestic CMOS active pixel.


New features of the speckle phase singularity produced in large angle scattering
Song Hong-Sheng, Liu Gui-Yuan, Zhang Ning-Yu, Zhuang Qiao, Cheng Chuan-Fu
2015, 64 (8): 084210. doi: 10.7498/aps.64.084210
Abstract +
Based on the interference patterns of the speckle field and the reference beam recorded by the charge-coupled device, and the digital Fourier transform technique, the complex amplitudes and phases of speckle field produced at different scattering angles are extracted. The phase distribution and the statistical properties at the singular point, such as the angle between two zero-contour lines of real part and imaginary part of the complex amplitude, the eccentricity of the intensity contours, etc. are studied. We find that there are some special properties of phase singularity when the scattering angle is large enough. With the increase of the scattering angle, great changes have taken place in the spatial distributions of the amplitude and the phase, and the probability of the angle between two zero-contour lines of real part and imaginary part is close to a smaller value, and the average eccentricity of the intensity contours around the phase singularity gradually increase. Moreover, the most interesting thing is that the eccentricity is probably greater than 1 in large anger scattering. The phase singular line as a new kind of phase singularity is found at a large scattering angle; the phase mutation rules and the vortex distribution characteristics of the phase on both sides of the phase singular line are investigated, and the hyperbolic or parabolic shape intensity contour around the phase singular line is found.
Adaptive compressive sensing toward low signal-to-noise ratio scene
Wen Fang-Qing, Zhang Gong, Tao Yu, Liu Su, Feng Jun-Jie
2015, 64 (8): 084301. doi: 10.7498/aps.64.084301
Abstract +
As an alternative paradigm to the Shannon-Nyquist sampling theorem, compressive sensing enables sparse signals to be acquired by sub-Nyquist analog-to-digital converters thus may launch a revolution in signal collection, transmission and processing. In the practical compressive sensing applications, the sparse signal is always affected by noise and interference, and therefore the recovery performance reduces based on the conventional compressive sensing, especially in the low signal-to-noise scene, the sparse recovery is usually unavailable. In this paper, the influence of noise on recovery performance is analyzed, so as to provide the theoretical basis for the noise folding phenomenon in compressive sensing. From the analysis, we find that the expected noise gain in the random measure process is closely related to the row and column of the measurement matrix. However, only those columns corresponding to the support for the sparse signal contribute to the sparse vector. In the traditional Shannon-Nyquist sampling system, an antialiasing filter is applied before the sampling process, so as to filter the noise beyond the passband of interest. Inspired by the necessity of antialiasing filtering in bandpass signal sampling, we propose a selective measurement scheme, namely adapted compressive sensing, whose measurement matrix can be updated according to the noise information fed back by the processing center. The measurement matrix is specially designed, and the sensing matrix has directivity so that the signal noise can be suppressed. The measurement matrix senses only the spectrum of interest, where the sparse spectrum is most likely to lie. Moreover, we compare the recovery performance of the proposed adaptive scheme with those of the non-adaptive orthogonal matching pursuit algorithm, FOCal underdetermined system solver algorithm, and sarse Bayesian learning algorithm. Extensive numerical experiments show that the proposed scheme has a better improvement in the performance of the sparse signal recovery. From the viewpoint of implementation, the measurement noise should be taken into consideration in the system, and more efficient algorithms will be developed for source pre-estimation at lower signal-to-noise ratio.
Research and design of directional heat transmission structure based on metamaterial
Sun Liang-Kui, Yu Zhe-Feng, Huang Jie
2015, 64 (8): 084401. doi: 10.7498/aps.64.084401
Abstract +
Based on the research of thermal cloak, directional heat transmission structure is proposed in this paper. On the basis of transformation thermodynamics, the thermal conductivity expression for directional heat transmission structure is derived by the oblique coordinate transformation. The results from the numeric calculation indicate that the heat flux flows to the designed high temperature side while the low temperature side remains at low temperature. Furthermore, rotational transformation is performed on the basis of oblique coordinate transformation. The derived thermal conductivity expression has two vertical segments. The calculation results display that with the increase of the thermal conductivity along the normal of the high temperature side, the heat transmission efficiency is improved greatly. Moreover, the temperature difference between the high and low temperature side increases after the rotational transformation. Directional heat transmission has potential applications in infrared stealth and heat protection.
Dolphin-leap of the torpedo and the selection of the initial rotational angular velocity
Xu Bao-Wei, Feng Jin-Fu, Hu Jun-Hua, Liu An, Cheng Xiang-Dong
2015, 64 (8): 084501. doi: 10.7498/aps.64.084501
Abstract +
The process which the torpedo, with relying on its inertia, jumps out of water in a certain gesture and falls into water, is called dolphin-leap. According to the dolphin-leap, the torpedo is required to get into the water at its zero angle of attack, but this gesture cannot be controlled during the torpedo moving in air. In order to solve the problem, a solution to control the initial water-exit rotational angular velocity is developed according to the proposed dolphin-leap model. The variables like added mass, buoyancy, buoyant center, wetted area, wetted volume, etc. are dependent on water-exit gesture and process. The derivative term of each physical quantity is fully considered in the dolphin-leap model, and the relationship between torpedo's hydrodynamic drag coefficient and attack angle is analyzed, then the motion model is built and the torpedo's dolphin-leap law is obtained. The optimal search algorithm is used to obtain the initial rotational angular velocity which makes the torpedo dolphin-leap fall into the water at its zero angle of attack. Simulation results show the validity of the proposed model and the solution for controlling the initial rotational angular velocity.
Theoretical study of the microwave air breakdown at dielectric surface
Zhou Qian-Hong, Dong Ye, Dong Zhi-Wei, Zhou Hai-Jing
2015, 64 (8): 085201. doi: 10.7498/aps.64.085201
Abstract +
Microwave air breakdown at dielectric surface is investigated by numerically solving the fluid-based plasma equations coupled with the Maxwell equations. The plasma formation and microwave scattering and absorption by plasma are investigated by one-dimensional (1D) and two-dimensional (2D) models. In the 1D model, it is found that at the initial stage of microwave breakdown, the plasma develops in the whole plasma region. As time increases, the plasma in the upstream grows much faster than in the downstream. Although the electron density distributions for ne = 0 and j = 0 are different, the microwave reflection, absorption and transmission are almost the same. It is found that the electron number density in the upstream region for 20 mm is larger than for 5 mm. In the 2D model, it is found for TE10 mode that the plasmoid first grows in the middle of waveguide until its density becomes large enough to diffract the incident field, then the plasma region moves along the surface to both sides. The plasma region cannot reach the wall of waveguide, where the electric field is smaller than the breakdown threshold. After comparison between the computational and experimental results, it is found that the simulated absorbed power is larger than the measured one, and the transmitted power is smaller than than measured one. The reason is that the initial electron densities in 1D and 2D simulation are both assumed to cover the whole dielectric surface, but the plasma in experiment develops in a very small region.
Quasi-isentropic experiment based on Shen Guang-III prototype laser facility with laser direct drive illumination
Wang Feng, Peng Xiao-Shi, Xue Quan-Xi, Xu Tao, Wei Hui-Yue
2015, 64 (8): 085202. doi: 10.7498/aps.64.085202
Abstract +
Quasi-isentropic compression technique is very useful for new material, shock wave physics, and earth physics. With shaping pulse laser, the quasi-isentropic compression technique is provided. For the designed experimental condition, the high energy density of shaping lasers can be used to generate shockless loading on the solid material to reach a high compression rate state with low temperature, which cannot be obtained with shock compression and isentropic compression technique. Then a new way to study the material can be provided. In this paper, the isentropic compression experiment with laser direct-drive illumination based on Shen Guang-III prototype laser facility is conducted. The theoretical model, target designing, experimental results, key technique, experimental characteristics and experimental data are analyzed in detail. The compression pressure above 400 GPa on the loading surface is obtained with experimental data and processing program, which is the highest pressure achieved to date. After comparing the apparent particle velocity with the true particle velocity, the dynamic correction curve can be obtained to achieve the real particle velocity, which is more accurate. The improving direction is provided, which will provide the important information. The experimental data and design will give the valuable reference for the study in this field.
Numerical simulation research of plasma characteristics in a multi-cusp proton source based on magnets layout
Yang Chao, Yin Mao-Wei, Shang Li-Ping, Wang Wei, Liu Yi, Xia Lian-Sheng, Deng Jian-Jun
2015, 64 (8): 085203. doi: 10.7498/aps.64.085203
Abstract +
It is significant to study negative hydrogen ion source for the construction of Chinese national spallation neutron source (CSNS) and the implementation of the international thermonuclear experimental reactor (ITER) project. Numerical simulation is an indispensable research measure due to the physical characteristics of ion source. In view of the facts above, in this paper we first elaborate the self-developed three-dimensional particle-in-cell/Monte Carlo collisions (PIC/MCC) algorithm and then describe the mechanism of negative hydrogen ion (H-) volume production. Based on these, the multi-cusp proton source of the Chinese atomic energy research center is systematically simulated. In the cases of the same polarities and the opposite polarities of extraction magnetic fields, multi-cusp proton source discharge characteristics are discussed and analyzed respectively. The result shows that in the case of the opposite polarities of the two extraction magnets, magnetic drift directions near the two extraction magnets are the same and have great values, namely intense magnetic drift, which causes the total number of electrons to be big, and induces the high-energy electrons to become active in a specific area. And so, the volume production rate of H- ions is higher, that is to say, H- ions present Y drift. On the contrary, in the case of the same polarities of the two extraction magnets, the binding effect of electron is worse and the volume production rate of H- ions is lower, but spatial distribution of volume production H- is uniform.
Reliability of partially-depleted silicon-on-insulator n-channel metal-oxide-semiconductor field-effect transistor under the ionizing radiation environment
Zhou Hang, Cui Jiang-Wei, Zheng Qi-Wen, Guo Qi, Ren Di-Yuan, Yu Xue-Feng
2015, 64 (8): 086101. doi: 10.7498/aps.64.086101
Abstract +
With the development of semiconductor technology, the small size silicon-on-insulator metal-oxide-semiconductor field-effect transistor devices start to be applied to the aerospace field, which makes the device in use face dual challenges of the deep space radiation environment and conventional reliability. The small size device reliability test under ionizing radiation environment is conducible to the assessing of the comprehensive reliability of the device. With reference to the national standard GB2689.1-81 constant stress life test and accelerated life test method for the general electric stress, the conventional reliability of the sub-micron type partially-depleted silicon-on-insulator n-channel metal-oxide-semiconductor is studied under the ionizing radiation environment. The experiment is divided into three groups marked by A, B and C. For all the experimental devices, the gate oxide tox=12.5 nm, channel length L=0.8 μm and width W=8 μm, and nominal operating voltage V=3.5 V. We carry out the electrical stress test on A group after irradiation with γ -ray dose up to 1×104 Gy (Si) under the bias condition. Before group B is tested, it has been irradiated by the same dose γ -ray and annealed for one week. Group C is not irradiated by γ -ray before the electric stress test. After irradiation we measure the DC characteristics of the devices: the drain current versus gate voltage (IDS-VGS) and the drain current versus drain voltage (IDS-VDS). The hot carrier injection (HCI) experiment is periodically interrupted to measure the DC characteristics of the device. The sensitive parameters of HCI and irradiation are VT, GM and IDlin, and after HCI stress, all parameters are degenerated. Through the contrast test, we qualitatively analyze the influences of the oxide trap charge and interface state on the sensitive parameters. We obtain the curve of the oxide trap charge and interface state versus time, and the influences of the different stages on device parameters. The results show that the combination of the total dose radiation environment and electrical stress causes the sensitive parameters of the device to rapidly degrade, this combination of these two factors gives rise to bigger effect than a single influence factor.
Universal function of Grneisen γ and the complete equation of state
Chen Jun-Xiang, Yu Ji-Dong, Li Ping, He Hong-Liang
2015, 64 (8): 086401. doi: 10.7498/aps.64.086401
Abstract +
The existing Grneisen coefficient γ expressions and the experimental data fitting relations consider only the γ data fitting, rather than the change rule of γ. In this paper, the universal function of Grneisen γ is established according to the property of Grneisen γ function and the high pressure characteristics of thermodynamic γ, such as γ changing quickly at low pressure but slowly at high pressure. This universal function is substituted into the thermodynamic function γ(v, T) to obtain the isentropic temperature Ts(v), and then the Hgoniot temperature is deduced by using the relationship between isentropic temperature and Hgoniot temperature, thus Hgoniot equation becomes the complete equation of state. All the thermodynamic state variables can be calculated by the thermodynamic relations. The universal function of Grneisen γ is applied to several metals, such as Al, Ta and Cu, and the Hgoniot equation is deduced according to the isothermal equation, or the isothermal equation is calculated by the Hgoniot equation. The results are in good agreement with experimental data. There are good compatibility between the universal Grneisen γ and the heat capacity Cv. It is shown that the proposed universal function of Grneisen γ can reasonably describe the thermodynamic properties of many metals at high pressure and high temperature.
Surface potential dynamic characteristics of the insulating sample under high-energy electron irradiation
Li Wei-Qin, Hao Jie, Zhang Hai-Bo
2015, 64 (8): 086801. doi: 10.7498/aps.64.086801
Abstract +
The surface potential and electron yield dynamic characteristics of an insulating thick sample under high-energy electron beam irradiation are obtained by combining the numerical simulation and experimental measurement. The numerical model takes into account the electron scattering, charge trapping, and charge transport. The results show that due to the electron scattering and transport, the space charge is weakly positive in the near surface and strongly negative inside sample; along the depth direction, the space potential decreases to a minimum value slowly, and then increases gradually and finally tends to zero; with the electron beam irradiation, the surface potential decreases to the negative kV magnitude gradually, and the total electron yield gradually increases to a stable value that is slightly less than unity. After stopping irradiation, the surface potential increases gradually, but charges are not eliminated completely. The surface potential decreases linearly with the increase of the beam energy, and increases with the increase of the incident angle, however it decreases slightly with the increase of the sample thickness.
First-principles study of electronic structures and electrochemical properties for Al, Fe and Mg doped Li2MnSiO4
Jia Ming-Zhen, Wang Hong-Yan, Chen Yuan-Zheng, Ma Cun-Liang, Wang Hui
2015, 64 (8): 087101. doi: 10.7498/aps.64.087101
Abstract +
Li2MnSiO4 is one of the potential cathode materials for lithium batteries due to its high capacities, but the poor conductivity hinders its further development. The cycling performance and electrochemical property of Li2MnSiO4 cathode material can be improved by doping metal cation. Twelve structures LixMn1-yMySiO4 (x=2, 1, 0; y=0.5, 1; M= Al, Fe, Mg) by doping Al, Fe and Mg are constructed in this paper, and their structures, electronic properties and delithiation process are studied by using the density functional theory of first principles within the GGA+U scheme. The best doping site and delithiated structure are found by comparing their energies. The results show that Al-doping is the best way to improve the conductivity and cyclability of the cathode material Li2MnSiO4. The pure Li2MnSiO4 has a low conductivity because of its large band gap (3.41 eV), while Al-doping Li2MnSiO4 crystal has metallic characteristics due to its electron densities of state with up-spin and down-spin cross through the Fermi level. The band gap is also reduced when it is Fe-doped, which improves the conductivity of Li2MnSiO4. Among the delithiated structures LixMnSiO4 (x=1, 0), Al-doping enhances the structural stability because of the lowest formation energy and its cyclability is improved by reducing the volume change. Within the lithium ion extraction from the Li2MnSiO4 and Li2Mn0.5M0.5SiO4 (M=Al, Fe, Mg), the Mn-O and M-O bonding have much more ionic features, while the covalent bonding feature between Si and O is almost unchanged. And the fully delithiated MnSiO4 and Mn0.5M0.5SiO4 show semic-metallic properties depending on the density of states of configuration. The delithiated voltages for the first Li extraction process decrease when Al and Fe are doped. Therefore the Al-doping in the Li2MnSiO4 is expected to be an effective way to improve the cycling performance and electrochemical property for Li-ion battery cathode material.
Charge distribution of Li-doped few-layer MoS2 and comparison to graphene and BN
Chen Xin, Yan Xiao-Hong, Xiao Yang
2015, 64 (8): 087102. doi: 10.7498/aps.64.087102
Abstract +
According to first-principles calculation, we study the charge distribution of Li-doped few-layer (1-3 layers) MoS2 and compare it with the results of graphene and BN. It is found that the stable adsorption sites of Li are the top (Mo) site for MoS2 layer, and the hexagonal center for graphene and BN layers. Band structures of pristine MoS2 show that single-layer MoS2 is a direct band gap semiconductor while few-layer MoS2 is an indirect one. As MoS2 is doped, the Fermi level will shift to the conduction band, indicating a charge transfer between Li and MoS2. The charge transfer takes place mostly between Li and the topmost MoS2 layer, which is very similar to that happening between graphene and BN. However, the second and third layer of MoS2, which are far from Li, can acquire about 10% of transferred charges. In contrast, the second and third layer obtain no more than 2% of charges for graphene and BN. Based on the electrostatic theory, we derive for both double and triple layers the formulas of electrostatic energy, which show clearly that only charge transfer between Li and the topmost layer will give the lowest electrostatic energy. Moreover, we calculate the work functions of pristine MoS2, graphene and BN, and find that, despite similar work functions of MoS2 and BN, the larger band gap of BN will make charge transfer between Li and BN harder. The analyses of electrostatic energy and work function show that the charge distribution is dominated by both interlayer electrostatic interaction and work function of material. It is expected that the above results could be helpful for doping layered structures and designing devices.
Pressure-induced structural transition and thermodynamic properties of NbSi2 from first-principles calculations
Pu Chun-Ying, Wang Li, Lü Lin-Xia, Yu Rong-Mei, He Chao-Zheng, Lu Zhi-Wen, Zhou Da-Wei
2015, 64 (8): 087103. doi: 10.7498/aps.64.087103
Abstract +
Applying the particle swarm optimization algorithm to the crystal structure prediction, we first predict a novel high pressure phase of NbSi2 with Si tetrahedron embedded diamond structure of Nb. NbSi2 alloy undergoes a first-order phase transition from hexagonal phase to cubic phase at about 175 GPa with a volume collapse of 4.1%, indicating the first-order transition. New predicted NbSi2 phase is dynamically stable in the absence of any imaginary phonon frequency in the whole Brillouin zone of phonon spectrum. The calculations of total and partial density of states indicate that the NbSi2 is in hexagonal phase at 0 GPa and it is in cubic structure at 180 GPa, both of which exhibit metal behaviors, which is dominated by Nb atom. There exists obviously the p-d hybridization between Nb and Si, and more charges accumulate in Si tetrahedron. Based on the “stress-strain” method, elastic constants, bulk modulus, shear modulus, Young's modulus, and Debye temperature of NbSi2 in two phases under pressure are systematically investigated using first principles calculations combined with the quasi-harmonic Debye model. To evaluate the ductile and brittle characteristics of NbSi2 alloy, pressure dependence of G/B ratio is investigated. Furthermore, the values of hardness and percent anisotropy AB and AG and the universal anisotropic index AU (inset) for NbSi2 alloy in hexagonal and cubic structures are also calculated. Our results show that external pressure has different effects on the values of ductility and hardness and anisotropy of the two phases. External pressure can improve the ductility of hexagonal phase, while it has a negligible effect on that of cubic phase. The hardness values of two phases of NbSi2 are analyzed in detail by using the G/B ratio. As pressure increases, the elastic anisotropy of hexagonal phase increases rapidly, while that of cubic phase remains unchanged.
Mechanism of high pulsed magnetic field treatment of the plasticity of aluminum matrix composites
Wang Hong-Ming, Li Pei-Si, Zheng Rui, Li Gui-Rong, Yuan Xue-Ting
2015, 64 (8): 087104. doi: 10.7498/aps.64.087104
Abstract +
For aluminum matrix composite, the introduced particles will strengthen the matrix, but as the obstacles, the heterogeneous particles will hinder the dislocation movement, generate uneven material structure, and may become a source of stress concentration. Therefore, they are detrimental severely to the elongation and plasticity of composite. It is known that dislocations exhibit a paramagnetic behavior because they contain paramagnetic centers including localized electrons, holes, triplet excitons, ion radicals, etc. The initial radical pair of the dislocation-obstacle S (spin angular momentum) = ± 1/2 is in a singlet state, and the total spin of the radical pair is 0 and in the antiparallel spin direction, offsetting a magnetism of the radical pair. The magnetic field can change the spin direction from singlet state to triplet state. In the triplet state the electron spin is 1 and in the same spin direction. A strong bond of the dislocation-obstacle is formed only in the singlet state when the spins of the two electrons are antiparallel. So an obstacle is able to pin a dislocation only if the radical pair is in the singlet state. Under the condition of high pulsed magnetic field treatment (HPMFT) the conversion of electronic spin will be a fundamental cause of dislocation motion along a glide plane. The movement of pinned dislocations will change the material microstructure and influence the performance of material. By comparing the microstructural evolutions and the residual stresses of samples subjected to HPMFT with different values of magnetic induced density (B), the positive influence of magnetoplastic effect on the plasticity of aluminum matrix composite is investigated in this paper. The results show that the dislocation density is significantly increased when B changes from 2 T to 4 T. When B=4 T the dislocation density is enhanced by 3.1 times compared with that of the sample without HPMFT. Moreover, the residual stress is reduced apparently from 41 MPa (B=0) to -1 MPa (B=3 T). In the view of atomic scale, the high magnetic field leads to a magnetoplastic effect which contributes to the dislocation movement and promotes the dislocation depinning, thereafter, the number of movable dislocations increases up. From the viewing of the internal structure of composite, the magnetic field accelerates the releasing rate of internal stress and lowers the residual stress in material, which is beneficial to improving the plasticity of aluminum matrix composite.
Suppression of metal grating to surface plasma radiation
Wang Ping, Hu De-Jiao, Xiao Yu-Fei, Pang Lin
2015, 64 (8): 087301. doi: 10.7498/aps.64.087301
Abstract +
Surface plasmon polaritons (SPP) are widely investigated in many fields because of the surface confinement of their electrocmagnetic field. Grating coupling is one of the methods to achieve the momentum match between light in free space and the surface plasmon to excite SPP. Because of the nature of the grating coupling, its parameters will greatly affect the coupling efficiency. Varying the grating modulation depth but keeping other parameters unchanged, we investigate the reflection spectra of onedimensional rectangle metallic grating by rigorous coupled-wave theory under the irradiation of incident light of 780 and 1500 nm in wavelength, respectively. According to Fano theory, the reflectance of metallic grating is the result of interference of two components, i.e., a directly reflected mode from the metal surface and a resonance radiation mode coupled out by the SPP propagating along the grating surface. We derive the Fano-type expression to describe the reflection spectra, and explain the contributions of directly reflected mode, SPP resonance radiation mode and the interference between these two effects. Near-filed electromagnetic distribution on metallic grating surface proves that the Fano-type expression is accurate enough to reflect the nature of the interference between the direct and radiation modes. Most importantly, our results from the expressions suggest that in some special grating condition, the metallic grating almost completely suppresses the SPP radiation propagating from grating to free space, which means that the energy of light can be completely trapped inside the grating. The phenomenon can be employed in designing light trapping device.
Electric field controlled energy gap and Landau levels in silicene
Ji Qing-Shan, Hao Hong-Yan, Zhang Cun-Xi, Wang Rui
2015, 64 (8): 087302. doi: 10.7498/aps.64.087302
Abstract +
We investigate the electric field controlled energy gap and the Landau levels in silicene in detail. The energy gap at different Dirac points has different closing and reopening conditions and the 2-fold degeneracy induced by the K-K' symmetry is resolved. An externally applied electric field gives rise to two Rashba spin-orbit-couplings between the nearest neighbour and the next nearest neighbour in silicene. Both these couplings can resolve the spin degeneracy at some isolated values of the electric field, where the crossover of the successive Landau levels become anti-crossover. Except some special values of the electric field, the 4-fold degeneracy of energy levels associated with the K-K' symmetry and spin symmetry is completely resolved in silicene, each level has a definite spin polarization, which correspond to the quantum Hall plateaux with filling factor ν=0, ±1, ±2,….
Theoretical and experimental investigations on wavefront distortion and thermal-stress induced birefringence in a laser diode pumped helium gas-cooled multislab Nd:glass laser amplifier
Huang Wen-Fa, Li Xue-Chun, Wang Jiang-Feng, Lu Xing-Hua, Zhang Yu-Qi, Fan Wei, Lin Zun-Qi
2015, 64 (8): 087801. doi: 10.7498/aps.64.087801
Abstract +
Thermal effect is still the most serious problem: it restricts the high power and high beam quality of solid laser to be further enhanced. The efficient thermal management is an important approach to suppress the thermal effect. In this paper, the thermal effect in a gas-cooled laser diode pumped multislab Nd:glass amplifier operating at a repetition rate is investigated in detail both theoretically and experimentally. The three-dimensional distributions of temperature, stress, strain, and birefringence are calculated by a finite element analysis. Based on these data, the thermally induced wavefront distortions and depolarization losses are determined with considering six slabs and one laser head. It is revealed that the theoretical data are in good agreement with the experimental results: the total wavefront distortion is 6.77 and a depolarization loss of more than 90% accumulates over six slabs when the heat deposition is 0.7 W/cm3.
Experimental study of ultrafast carrier dynamics in polycrystalline ZnTe nanofilm
Jia Lin, Tang Da-Wei, Zhang Xing
2015, 64 (8): 087802. doi: 10.7498/aps.64.087802
Abstract +
Zinc telluride, due to its direct band gap and broadband light absorption, has the good application prospects in terahertz devices, solar cells, waveguide devices, and green light emitting diodes. In the photovoltaic field, it is possible to further improve the photoelectron conversion efficiency of multi-junction tandem solar cells by combining zinc telluride with III-V semiconductors. Ultrafast photo-excited carrier dynamics is fundamental to understand photoelectron conversion process of nanofilm solar cells. In this study, the ultrafast energy carrier dynamics of N-doped polycrystalline zinc telluride is investigated by using the femtosecond laser two-color pump-probe method at room temperature. The polycrystalline zinc telluride nanofilm is grown on a 500 μm GaAs (001) substrate via molecular beam epitaxy and doped by using a nitrogen ratio frequency plasma cell. The laser pulses with a central wavelength of 800 nm are divided into pump beam and probe beam by a beam splitter, after which the pump beam passes through a bismuth triborate crystal and its frequency is doubled to 400 nm. The 400 nm pump beam and 800 nm probe beam are focused on the sample collinearly through the same objective lens. Photo-excited carriers will be generated since the excitation photon energy of 400 nm pump beam (3.1 eV) is higher than the band gap of zinc telluride (~ 2.39 eV). The experimental data are analyzed by using the theoretical fitting model which includes energy relaxation processes of electrons and lattice, and the theoretical curves are consistent well with the experimental data. The fitted results show that the three dominated relaxation processes which affect the initial reflectivity recovery are in sub-picosecond time regime. The positive amplitude electron relaxation process is attributed to inter-band carrier cooling and carrier diffusion through electron-photon interactions, and the deduced decay time of this positive amplitude electron relaxation process is about 0.75 ps. The negative amplitude electron relaxation process is characterized as a photo-generated carrier trapping process induced by defects, and the decay time of this process is about 0.61 ps. The lattice heating process is realized through electron-phonon coupling process, and the calculated time constant of the lattice heating is about 0.86 ps.
Effects of growth conditions on the microstructures and photoluminescence properties of Ga-doped ZnO films
Zhou Xiao-Hong, Yang Qing, Zou Jun-Tao, Liang Shu-Hua
2015, 64 (8): 087803. doi: 10.7498/aps.64.087803
Abstract +
ZnO has a wide direct band gap of 3.37 eV and a large exciton binding energy of 60 meV at room temperature, which is recognized as one of the promising semiconductors for optoelectronic device applications. However, ZnO generally displays visible defect-related deep-level emission and/or UV near-band-edge emission, which is strongly dependent on the growth method and condition. It has been reported that doping with IIIA elements can improve the optical properties of ZnO. Among them, Ga doping is considered not to induce large lattice distortion of ZnO due to the fact that the bonding lengths of Ga-O and Zn-O are similar and ionic radii of Ga3+ and Zn2+ are also similar. The gallium related compounds such as triethylgallium, gallium nitrate and gallium oxide are used as the Ga doping sources. It has been proved that ZnO film can be grown directly by the thermal oxidation of ZnS substrate. In this research, the Ga doping is adopted in the growth of ZnO film by applying the molten gallium to the surface of ZnS substrate and performing the subsequent thermal oxidation in the air at 650 and 700 °C for 3 and 8 h, respectively. The effects of growth condition on the microstructures and photoluminescence properties of the Ga-doped ZnO film are investigated by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and photoluminescence at room temperature. In addition, the relationship among the oxidation temperature, oxidation time, Ga doping content and photoluminescence properties is discussed. The results show that the Ga-doped ZnO films grown under different growth conditions exhibit various amounts of Ga content and the gallium is present in the ZnO matrix as Ga3+ by partially substituting Zn2+. The Ga doping affects the microstructure and photoluminescence property by changing the defect type and level, stoichiometric ratio, and crystal quality of ZnO film. As the oxidation temperature increases, the amount of Ga doping content increases. In addition, the grain size of the Ga-doped ZnO film increases and becomes uniform, and the ratio of ultraviolet emission intensity to visible emission intensity increases. However, as the oxidation time increases, the amount of Ga doping content decreases, the grain size of the Ga-doped ZnO film becomes non-uniform, and the ratio of ultraviolet emission intensity to visible emission intensity decreases.
Experimental study on scan imaging of silicon wafer by laser-induced photocarrier radiometry
Liu Jun-Yan, Song Peng, Qin Lei, Wang Fei, Wang Yang
2015, 64 (8): 087804. doi: 10.7498/aps.64.087804
Abstract +
In this paper, the modulated laser-induced excess minority carrier density wave (CDW) of bipolar semiconductor is developed. An analytical expression of CDW in frequency domain is introduced. The numerical simulations are carried out to analyze the effects of minority carrier transport parameters (minority carrier lifetime, diffusion coefficient, and two surfaces recombination velocities) on response of laser-induced photocarrier radiometry (PCR) signal to frequency for semiconductor silicon wafer. The PCR amplitude increases with increasing the minority carrier lifetime, and decreased with increasing the carrier diffusion coefficient and surface recombination velocity. In contrast, the PCR phase lag decreases with increasing the minority carrier lifetime, and increases with increasing the carrier diffusion coefficient and surface recombination velocity. The silicon (Si) wafer with an artificial defect (mechanical scratch) on the surface is experimentally investigated by PCR scanning image system. The distribution maps of the minority carrier transport parameters are obtained by best-fitting method which is based on the carrier density wave analytical expression, and the influences of the artificial defect on carrier transport parameters are discussed in detail. The experimental results indicate that the surface recombination velocity and carrier diffusivity at artificial damaged location are dramatically increased compared with those in the healthy region. The carrier bulk lifetime of whole Si wafer is obtained to be about 38.33 μs by PCR scanning image measurements. Simultaneously, quasi steady-state photoconductance (QSSPC) method is used to measure the carrier effective lifetime of Si wafer, and it is about 33.85 μs. Therefore, the carrier bulk lifetime of Si wafer by PCR scanning image measurement is in good agreement with the QSSPC measurement. However, QSSPC measurement could obtain only the carrier effective lifetime of Si wafer. Furthermore, PCR scanning image measurement can be employed to measure the carrier transport parameters with high resolution in comparison with QSSPC measurement, and to evaluate the localized imperfection.
Isoelectronic traps and thermoluminescence characteristics in MSO4:Eu2+ (M =Mg, Ca, Sr, Ba)
Luo Da-Ling, Tang Qiang, Guo Jing-Yuan, Zhang Chun-Xiang
2015, 64 (8): 087805. doi: 10.7498/aps.64.087805
Abstract +
A series of alkaline earth sulphate phosphors MSO4:Eu2+ (M =Mg, Ca, Sr, Ba) is obtained in doping experiments. It is discovered that these phosphors doped with Eu2+ ions have the thermoluminescence (TL) characteristics which are quite different from those in the alkaline earth sulphate phosphors doped with trivalent rare earth ions RE3+ (RE= Dy, Tm, Eu). It is also observed that there is only one glow peak in the three-dimensional emission spectrum and the radiation dose response of the glow peak is linear-sublinear in the series of phosphors MSO4:Eu2+ (M =Mg, Ca, Sr, Ba). However, quite a lot of experimental results show that there are several glow peaks in the three-dimensional emission spectrum, and the TL radiation dose responses are linear-supralinear in the series of phosphors MSO4:RE3+ (M =Mg, Ca, Sr, Ba and RE= Dy, Tm, Eu). The reason lies in the structures of defect complexes which are formed in the course of preparation of these phosphors and include intrinsic imperfects and dopants. These defect complexes can be regarded as basic elements in the TL multi-stage process. In the series of phosphors MSO4:Eu2+ (M =Mg, Ca, Sr, Ba), the isoelectronic traps produced by doping Eu2+ ions which have the same valences as superseded alkaline earth ions are very localized traps to form the defect complexes (Eu2+ isoelectronic trap-SO42-) that are basic elements in the TL multi-stage process, in which there are one-hit TL events basically. However, in the MSO4:RE3+ (M =Mg, Ca, Sr, Ba) phosphors, the defect complexes (RE3+-SO42--cation vacancy VM) are basic elements in the TL multi-stage process, in which there are two-hit TL events basically. It is clear that the Eu2+ isoelectronic trap phosphors play key roles in TL Characteristics in MSO4:Eu2+ (M = Mg, Ca, Sr, Ba). In addition, it has been observed that the wave length at the single TL peak in each of the three-dimensional emission spectra of the series of phosphors MSO4:Eu2+ (M =Mg, Ca, Sr, Ba) is related to the substrate of each of these phosphors, such as the wave lengths at the TL peaks are 440, 385, 375 and 375 nm for MgSO4:Eu2+, CaSO4:Eu2+, SrSO4:Eu2+, and BaSO4:Eu2+ respectively. The experimental results display the characteristics of Eu2+ isoelectronic traps formed by substituting alkaline earth ions which have different cationic radii and electronegativities. The SrSO4:Eu2+ phosphor can be called typical isoelectronic trap phosphor which has the higher TL and optical stimulated luminescence efficiency.
Preparation and exchange bias effects of Bi0.8Ba0.2FeO3/La0.7Sr0.3MnO3 heterostructures
Wei Ji-Zhou, Zhang Ming, Deng Hao-Liang, Chu Shang-Jie, Du Min-Yong, Yan Hui
2015, 64 (8): 088101. doi: 10.7498/aps.64.088101
Abstract +
Bi0.8Ba0.2FeO3/La0.7Sr0.3MnO3 multiferroic heterostructures are successfully synthesized on single crystal LaAlO3(100) substrates by pulsed laser deposition via adjusting the parameters of laser energy, laser frequency, substrate temperature, oxygen pressure, distance between substrate and target, etc. The pure phase with perovskite structure is confirmed by the X-ray diffraction measurements. Using high-resolution transmission electron microscopy and energy dispersive X-Ray spectroscopy, we find that all the layers show preferential (00l) orientation, suggesting the epitaxial growth of the multilayered structure. Isothermal (7 K) M-H curves measured on sample after cooling the sample down to lower than ± 1 T fields reveal a shift in M-H loop. The strong temperature dependence of HEB is likely to be directly related to an electronic orbital reconstruction at the interface and complex interplay between orbital and spin degrees of freedom.
Ordered structures of nanorods induced by the helixes of semiflexible polymer chains
Hua Yun-Feng, Zhang Dong, Zhang Lin-Xi
2015, 64 (8): 088201. doi: 10.7498/aps.64.088201
Abstract +
Self-assembly of nanoparticles, such as nanospheres, nanorods (NRs), and nanotubes, in polymer systems is one of the most prominent and promising candidates for the development of novel materials with high mechanical, optical, and electrical performances. A most concerned topic on the nanoparticle/polymer composites is the spatial arrangement and distribution of nanoparticles in the nanocomposites, which is controlled by the competition between the entropic packing constraints related to the incompatibility between species with different sizes and geometries, and the enthalpic consequences of a variety of polymer-nanoparticle interactions. The studies on the nonspherical nanoparticles, such as NRs, are of more challenging than on spherical nanoparticles, because both positional and orientational ordering of anisotropic nanoinclusion have an important influence on the morphology of nanocomposition, while those studies are necessary for applications of nanoscopic anisotropic objects in photovoltaic and filled emission devices. When low-volume fractions of NRs are immersed in a binary, phase-separating blend, the rods can self-assemble into needle-like, percolating networks and this special structure can enhance the macroscopic electrical conductivity and mechanical property of the material. When an electric field is applied, the phase separations of ligand-functionalized NRs in a polymer matrix and densely packed hexagonal arrays of NRs are produced. In this paper, by employing the coarse-grained model and molecular dynamics simulation, we explore the structures of nanocomposites in which a small number of NRs bind with semiflexible polymer chain. The morphology of NRs/polymer mixture is greatly affected by the bending energy b of semiflexible polymer and the binding energy D0 between NRs and semiflexible polymer. If the binding energy D0 is less than 1.1kBT, the NRs are almost free and a gas-like phase is observed. For a suitably large value of D0, three completely different morphologies of NRs/polymer mixtures are identified, namely, the side-to-side parallel aggregation of NRs, the end-to-end parallel aggregation of NRs, and the dispersion of NRs. For the flexible polymer chain (i.e., small bending energy b), the sideto- side parallel aggregation structure of NRs and the disordered conformation of adsorbed polymer chain are observed. In general, a typical equilibrium conformation of free flexible polymer chain is random coil, the binding energy between NRs and polymer can lead to the collapse of a random coil for flexible polymer chain, and the NRs aggregate in the manner of the side-to-side parallel to each other because the enthalpy is maximized through sharing the more polymer monomers between neighbor NRs. That is to say, the local aggregation of NRs can be found because the orientational entropy can make the aggregated NRs arrange in the side-to-side parallel manner. In the rigid polymer chain limit (very large bending energy), the rigid polymer chain is stretched and the NRs are well dispersed. As the rigid polymer holds a long persistence length, the NRs can move freely along the stretched polymer chain, and the dispersed conformation of NRs is formed. For the semiflexible polymer chain with a moderate bending energy, the NRs are aggregated in the end-to-end parallel arrangement. Meanwhile, the polymer monomers wrap around those NRs in a well-defined helical structure. The above discussion indicates that the morphologies of NRs are closely related to the conformations of polymer chains. In fact, when a semiflexible polymer chain binds with a large rigid surface, such as nanotube, the helical structure will be formed and it is driven by entropy. The formation of helical structures for a semiflexible polymer chain can induce NRs to form an end-to-end parallel aggregation. The formation of end-to-end parallel arrangement of NR aggregation is driven by the helical structure of semiflexible polymer chain. For the moderate binding energy, the entropy can drive the semiflexible polymer chain to form local helical structure around the NRs. When more NRs are added to the semiflexble polymer chain/NR mixtures, more local helical structures around NRs are formed. Because the movements of NRs binding with the semiflexible chain are nearly free and an end-to-end parallel arrangement of NRs can form more helical structures than the dispersed NRs, the self-assembly of NRs into an end-to-end parallel structure is expected. That is to say, the formation of end-to-end parallel aggregation of NRs is induced by the helix of semiflexible polymers because it can gain more entropies. The self-assembly of a small number of NRs can be well controlled by varying the stiffness of adsorbed polymer chain. This investigation may provide a new pathway to develop smart medium to manipulate the aggreagtion behavior of a few NRs and to construct novel materials with high performance.
Effect of ZnO electron-transport layer on light-soaking issue in inverted polymer solar cells
Li Chang, Xue Wei, Han Chang-Feng, Qian Lei, Zhao Su-Ling, Yu Zhi-Nong, Zhang Ting, Wang Ling-Xue
2015, 64 (8): 088401. doi: 10.7498/aps.64.088401
Abstract +
A common phenomenon of polymer solar cells with metal oxide electron-transport layers (ETLs), known as “light-soaking” issue, is that the as-prepared device exhibits an anomalous S-shaped J-V characteristic, resulting in an extremely low fill factor (FF) and thus a poor power conversion efficiency. However, the S-shape disappears upon white light illumination with UV spectral components, meanwhile the performance parameters of the device recover the normal values eventually. This behavior appears to be of general validity for various metal oxide layers regardless of the synthesis and fabricating processes. Its origin is still under debate, while the ETL interface problems have generally been claimed to be the underlying reason so far. In this paper, both conventional and inverted cells with using ZnO nanoparticles (NPs) as ETL are fabricated to clarify the interface effect of the ETL on the light soaking procedure. The inverted device shows a typical light-soaking issue with an initial FF less than 20% as expected, whereas the J-V curves of the conventional cell remain regular shapes throughout the test. This result indicates that the ITO/ZnO interface is a key reason of S-shaped J-V characteristics, which is further verified via the use of Cs2CO3/ZnO ETL. The insert of Cs2CO3 layer isolates the ITO electrode from contacting with ZnO layer, and the kink disappears in the as-prepared device with this bi-layered ETL inverted structure. Our explanation for the result above is that the oxygen impurities absorbed onto the surface of ZnO NPs during fabrication process, behave as strong electron traps, and thus increasing the width of the energy barrier (EB) at the interface of ITO/ZnO. Subsequently, photogenerated electrons accumulate in the ZnO layer adjacent to the interface, resulting in extremely poor performance. Upon white light illumination, however, the trap sites are filled by photogenerated carriers within the ZnO layer, and therefore narrowing the EB. As the barrier width becomes thin enough to be freely tunneled through, a good selectivity behavior of ZnO ETL is reached, leading to a fully remarkable recovery in device performances.
Transfer entropy analysis of electroencephalogram based on adaptive template method
Wang Ying, Hou Feng-Zhen, Dai Jia-Fei, Liu Xin-Feng, Li Jin, Wang Jun
2015, 64 (8): 088701. doi: 10.7498/aps.64.088701
Abstract +
Electroencephalogram (EEG) is a very weak random signal with complex mechanism, which comprehensively reflects the activities and the functional states of brain tissue. Due to the weak characteristic of EEG, the traditional basic template method is a good tool for EEG analysis. In order to further enhance the performance of this method, we propose a new transfer entropy method based on adaptive template. The method improves the symbolization of time series based on the original basic template method. Numerical experiments show that the improved adaptive template method can obtain better dynamic characteristics, and also has better ability to distinguish the results in the analysis of time series. We use the transfer-entropy-based adaptive template method to analyze adolescent and adult EEG. We also study the relationship of the transfer-entropy-based adaptive template method to the total data length L and the data length l of the divided cells. Numerical results show that the transfer entropy value of adult EEG based on adaptive template is significantly higher than that of teenager EEG. This indicates that adult has a more significantly mental activity and the functional status of the brain is more complex. We then apply this method to human EEG signals and investigate their statistical properties. The results show that compared with the result of the basic method, the transfer-entropy-based adaptive template method can significantly show the EEG coupling for adolescents and adults EEG, which has a better discrimination and can better capture dynamic information and the change of the system dynamic complexity. At the same time, it will be more conducive to clinical diagnosis and provides a new and better method to judge whether brain is in a pathological state.
Solar cell temperature prediction model of support vector machine optimized by particle swarm optimization algorithm
Zhao Zhi-Gang, Zhang Chun-Jie, Gou Xiang-Feng, Sang Hu-Tang
2015, 64 (8): 088801. doi: 10.7498/aps.64.088801
Abstract +
Establishing a general and precise solar cell temperature model is of crucial importance for photovoltaic system modeling, the loss analysis of output power, and conversion efficiency. According to the complex mechanism of solar cell temperature, in this paper we study the steady state thermal model (SSTM) of solar cell temperature and accurate prediction model of method of support vector machine (SVM). Firstly, based on the approximate linear relationship among air temperature, solar radiation intensity, wind speed and solar cell temperature, the polynomial model of solar cell temperature is established and the unknown parameters of the model are extracted with the improved differential evolution algorithm. Secondly, in order to improve the accuracy of SVM prediction model, the particle swarm optimization algorithm is adopted to optimize the parameters (including kernel parameter g and penalty factor C from the radial basis function kernel) of SVM. After the input/output sample set is determined and the training set and test set are classified, a prediction model of solar cell temperature based on particle swarm optimization support vector machine is established. Finally, experimental acquisition platform is built to reduce the influences of air humidity, solar incidence angle, and thermal hysteresis effects on PV cell temperature. Through contrasting experiments, it is shown that the established fitting of the SSTM is better than the models given in other literature, and the prediction model is reliable, comprehensive and simple. The selected parameter optimization algorithm is superior to genetic algorithm and cross-validation method established on the optimization performance, and the accuracy of prediction model is superior to the prediction performance of back propagation neural network and identified SSTM.
Evaluating influential spreaders in complex networks by extension of degree
Min Lei, Liu Zhi, Tang Xiang-Yang, Chen Mao, Liu San-Ya
2015, 64 (8): 088901. doi: 10.7498/aps.64.088901
Abstract +
Evaluating influential spreaders in networks is of great significance for promoting the dissemination of beneficial information or inhibiting the spreading of harmful information. Currently, there are some central indices that can be used to evaluate spreading influence of {nodes}. However, most of them ignore the spreading probability and take into consideration only the network topology or the location of source node, so the excellent results can be achieved only when the spreading probability is in a specified range. For example, the degree centrality is appropriate for a minor spreading probability, but to ensure the accuracy, semi-local and closeness centralities are more suitable for a slightly larger one. To solve the sensitivity problem of spreading probability, a novel algorithm is proposed based on the extension of degree. In this algorithm, the coverage area of degree is recursively extended by the overlapping of degree of neighbors, which makes different extension levels correspond to different spreading probabilities. For a certain spreading probability, the proper level index is calculated by finding the most correlate ranking sequences of sampling {nodes}, which is obtained by matching the results of different spreading levels and SIR simulation. In this paper, the relationship between extension level and spreading probability is explained by the theory of fitting the weight and infected possibility of {nodes}, and the feasibility of the sampling method is verified by the computational experiments. The experimental results on both real and computer-generated datasets show that the proposed algorithm can effectively evaluate the spreading influences of {nodes} under different spreading probabilities, and the performance is close or even superior to that evaluated by using other central indices.
Correlative analysis between the changes of surface solar radiation and its relationship with air pollution, as well as meteorological factor in eastern and western China in recent 50 years
Qi Yue, Fang Shi-Bo, Zhou Wen-Zuo
2015, 64 (8): 089201. doi: 10.7498/aps.64.089201
Abstract +
Based on the solar radiation information and climatic observation data from 1961 to 2011 in both East and West China, the interannual trends of surface solar radiation are investigated. By combining with cloud cover, sunshine percentage, sunshine duration, wind speed, atmospheric turbidity, and relative humidity, the causes affecting the variations of the total surface solar radiation in East China and West China are analyzed. The results show that the surface solar radiation decreased from 1961 to 2010 in both East and West China, but there was an abrupt change, which occurred in the early 1990s, followed by sustained increase. The surface solar radiation decreased from 1961 to 2010 in East China, and the decrease of surface solar radiation in the eastern region was greater than in the western region. The surface solar radiation is significantly correlated with sunshine duration in both East and West China. In East China, the surface solar radiation is significantly positively correlated with the percentage of sunshine. It indicates that the percentage of sunshine duration is the main factor affecting the total surface solar radiation in East China. The decrease of the total surface solar radiation is influenced by many factors. In East China, the percentage of sunshine and the atmospheric turbidity factor are the main factors affecting the surface solar radiation reduction. There is no correlation between cloud and ground solar total radiation. However, the percentage of sunshine is affected mainly by cloud cover and air pollution. In West China, no significant correlation is found between surface solar radiation and cloud cover, but significant correlation is detected between surface solar radiation and sunshine percentage. These indicate that the sunshine percentage could be a main factor affecting surface solar radiation. Wind becomes an important factor affecting the total surface solar radiation, since wind is a main factor expelling troposphere air pollution such as smog. A comparison of the variation in ground solar radiation between the East and West China shows that the aerosol particles of air pollution are the main factor affecting the reduction of total surface solar radiation in East China. Many scholars have found that the significant reduction of the surface solar radiation is mainly due to the absorption and scattering atmospheric aerosols. All these findings suggest that the air pollution has become an important factor affecting the surface solar total radiation, especially in East China.
For the first time fengyun3 C satellite-global navigation satellite system occultation sounder achieved spaceborne Bei Dou system radio occultation
Wang Shu-Zhi, Zhu Guang-Wu, Bai Wei-Hua, Liu Cong-Liang, Sun Yue-Qiang, Du Qi-Fei, Wang Xian-Yi, Meng Xiang-Guang, Yang Guang-Lin, Yang Zhong-Dong, Zhang Xiao-Xin, Bi Yan-Meng, Wang Dong-Wei, Xia Jun-Ming, Wu Di, Cai Yue-Rong, Han Ying
2015, 64 (8): 089301. doi: 10.7498/aps.64.089301
Abstract +
The radio occultation (RO) technique using signals from the global navigation satellite system, is widely used to observe the atmosphere for applications such as numerical weather prediction (NWP) and global climate monitoring. Since 1995, there have been turborogue sounder on board global positioning system/meteorology, black jack sounder on board challenging minisatellite payload and gravity recovery and climate experiment, IGOR sounder on board constellation observing system for meteorology, ionosphere and climate, GRAS on board meteorological operational, which have been recieving a large number of RO data, but their observed signals come only from global positioning system (GPS). These RO data have been wildly used in NWP and climate monitoring, however they cannot meet the requirements for high accuracy and real time atmosphere observation, in this case compatible RO sounder to obtain more RO observations is significant. Global navigation satellite system occultation sounder (GNOS) on board the fengyun3 C (FY3 C) satellite, which is the first Bei Dou system (BDS)/GPS compatible RO sounder in the world, was launched on 23 September 2013. Up to now, lots of RO observations have been obtained. In this study, the components of GNOS are introduced; one-day GNOS RO events and their global distribution are analyzed; compared with the GPS RO observations, the accuracy and consistency of BDS real-time positioning results and BDS RO products are analyzed. The preliminary results show that the BDS can enhance the number of RO events by 33.3%; the average deviation and standard deviation of BDS real time positioning results are 6 m and 7 m, respectively; the BDS/GPS difference standard deviation of refrectivity, temperature, humidity, pressure and ionospheric electron density are lower than 2%, 2 K, 1.5 g/kg, 2%, and 15.6%, respectively. The BDS observations/products are consistent with those of GPS, therefore BDS RO products can bring benefit to numerical wheather prediction and global chlimate change analysis.
Theoretical model of the rotationally and tidally distorted binaries
Zhan Qiong, Song Han-Feng, Tai Li-Ting, Wang Jiang-Tao
2015, 64 (8): 089701. doi: 10.7498/aps.64.089701
Abstract +
Rotation and tide are two important factors which have an influence on the stellar structure and evolution. They cannot be neglected. According to the observation data of the massive binary system V478 Cyg, we test the theoretical model including the deformation which is induced by rotation and tide (our model). We compare our model with Kähler-Eggleton (KE) model, and the distorted model is more consistent with observations than the traditional model (KE model). Besides, it is found that great deformation occurs in the outer envelope, where its density is lower than the mean density. Rotation and tide can cause the gravity at the two polar points to increase and the gravity in the equatorial plane to decrease. Therefore, the radiative flux, which depends on the local effective gravity, is not constant on the equipotentials any more. The poles which become hotter, experience a high mass loss than the equator, which becomes cooler. Furthermore, the two components in our model have bigger radii, equatorial velocities and central compactness and low H-energy production rate. The bigger mean radius of the distorted star produces a smaller temperature gradient inside the star, resulting in a lower energy transport. The lower energy generation rate inside the distorted model will widen the main sequence and increase the stellar lifetime. Stellar temperature and luminosity of the distorted model are shifted toward lower value. The tidal distortion inside the secondary star plays a most important role in the rate of the apsidal motion because of lower compactness. The apsidal motion derived from rotation is larger than the one derived from the general relativity.