Vol. 56, No. 1 (2007)

Mei symmetries of a type of dynamical equations
Ge Wei-Kuan
2007, 56 (1): 1-4. doi: 10.7498/aps.56.1
Abstract +
The definition and the criterion of a Mei symmetry for a type of dynamical equations are presented. The Noether conserved quantity can be deduced when the Mei symmetry is a Noether symmetry .The Hojman conserved quantity can be obtained if the Mei symmetry is a Lie symmetry. In addition, a new conserved quantity can be found by the Mei symmetry.
The effect of balanced homodyne detection on continuous variable quantum key distribution
Chen Jin-Jian, Han Zheng-Fu, Zhao Yi-Bo, Gui You-Zhen, Guo Guang-Can
2007, 56 (1): 5-9. doi: 10.7498/aps.56.5
Abstract +
Different from single photon quantum key distribution, continuous variable quantum key distribution uses the balanced homodyne detection. In this paper, the detection error of balanced homodyne caused by vacuum noise of the local light and unbalance of beam splitter is analyzed, the maximal safe distance limited by the electronic noise of balance homodyne detector is given, finally, the relationship between the noise of balanced homodyne detection and key rate are shown.
The generalized uncertainty relation and Dirac field entropy of black hole with an internal global monopole
Yang Shu-Zheng, Han Yi-Wen, Hong Yun
2007, 56 (1): 10-14. doi: 10.7498/aps.56.10
Abstract +
The generalized uncertainty relation is considered in the new equation of state density. Using the WKB approximation, Dirac field entropy of the horizon of the black hole with an internal global monopole is calculated directly. The result shows that the black hole entropy is proportional to the horizon area, which brings to light the relationship between the black hole entropy and the entropy of quantum state near the event horizon. The difference from the brick-wall model is that the present result is convergent without any cutoff. It is indicated that this method can be used to calculate the entropy of the scale field of black hole, and it can be extended to calculate the entropy of Dirac field.
Kinetics theory investigation of holographic polymer dispersed liquid crystal based on acrylate
Liu Yong-Gang, Hu Li-Fa, Xuan Li, Zheng Zhi-Gang, Ma Ji, Song Jing
2007, 56 (1): 15-24. doi: 10.7498/aps.56.15
Abstract +
Holographic polymer-dispersed liquid crystal (HPDLC) is formed by the diffusion of monomers and liquid crystals as a kind of switchable volume holography. The kinetics investigation of HPDLC is very important to improve the diffraction efficiency and optimize the formation conditions of HPDLC. Fourier analysis is widely adopted as analytical method of kinetics. In this paper, we make use of the one-dimensional infinite-length analytical method to develop a kinetics simulation for the real-time formation of the grating. Simultaneously, corresponding diffused kinetics equations and polymeric kinetics equations are given. Then, we find the relationship between these equations and refractive index modulation Δn, and obtain a diffraction efficiency equation which is appropriate for transmission Bragg volume grating. Finally, the ways of increasing diffraction efficiency of HPDLC is discussed. Our theoretical study should provide some useful guidance to improve the properties of HPDLC.
The mean first-passage time for an asymmetric bistable system driven by multiplicative and additive noise with colored correlations
Xu Wei, Ning Li-Juan, Yang Xiao-Li
2007, 56 (1): 25-29. doi: 10.7498/aps.56.25
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In this paper,the effects of colored correlations and asymmetry of the potential on the mean first-passage time(MFPT) are investigated for an asymmetric bistable system driven by multiplicative noise and additive noise.Numerical results show that the intensities of multiplicative and additive noises and the cross-correlation time τ affect the MFPT in the same way,the coupling strength λ and asymmetry r of the potential affected the MFPT in the same way.The increase of τ enhances the escape rate,while increase of λ reduces the escape rate.
Study of the property of the parameters of bistable stochastic resonance
Guo Yan, Leng Yong-Gang, Wang Tai-Yong, Wu Zhen-Yong
2007, 56 (1): 30-35. doi: 10.7498/aps.56.30
Abstract +
Taking a bistable system as a research object, the strategy of choosing parameters for affecting and producing stochastic resonance (SR) is investigated. The system parameters of the bistable system and the frequency parameter of twice sampling are related to each other intrinsically. Their variation can cause changes in the height of the potential barrier, the distance of the potential wells and the transition speed of the particle, causing redistribution of the relative noise energy and leading to different stochastic resonance effects. According to the regularity of the parameter choice for the stochastic resonance, the optimal stochastic resonance can be obtained by choosing these parameters properly, which lays the foundation for further data processing.
Influence of virtual networks to internet collective behavior
Wang Lei, Zhou Shu-Hua, Yuan Jian, Ren Yong, Shan Xiu-Ming
2007, 56 (1): 36-42. doi: 10.7498/aps.56.36
Abstract +
A virtual network is defined as a logical network, in which users exhibit their access behaviors. Virtual networks rely on physical computer networks like the Internet, but have different topologies, and cause significant influence on the physical networks. In this paper, a novel two-tier model is proposed to study influences of virtual networks to Internet collective behavior. It is shown that the queue lengths of the node data packets present phase transition characteristics. Moreover, the phase transition critical point moves to the left and network performance is deteriorated. In a free flow, the nodes are independent of each other or short-range correlative. In the critical state, the nodes are long-range correlative, and there exists a higher power exponent H which means stronger long-range correlation. When the system state is on the right of critical point, virtual network behaviors make the network to present consistent long-range correlative characteristic.
Reservoir neural state reconstruction and chaotic time series prediction
Han Min, Shi Zhi-Wei, Guo Wei
2007, 56 (1): 43-50. doi: 10.7498/aps.56.43
Abstract +
ESN(Echo state network) is a new type of recurrent neural network, which is based on the “reservoir”. ESN has been proved to be significantly efficient to deal with some chaotic time series prediction tasks. This paper makes an analysis of the current iterative prediction method based on “reservoir”, and points out some problems in theory and the obstacles in application. And then, a direct prediction method is proposed, which relates the prediction origin and prediction horizon directly. The direct method does not close the loop in the process of prediction, so there are no such problems as instability and error accumulation. The simulation results show how the reservoir property changes when the loop is closed in the iterative prediction, and then demonstrate the feasibility of the direct prediction method in application to the Mackey-Glass benchmark prediction problem.
Parameter estimation for Lorenz chaotic systems based on chaotic ant swarm algorithm
Li Li-Xiang, Yang Yi-Xian, Peng Hai-Peng, Wang Xiang-Dong
2007, 56 (1): 51-55. doi: 10.7498/aps.56.51
Abstract +
The chaotic ant swarm algorithm is a chaos optimization algorithm based on swarm intelligence theory which was inspired by the chaotic and self-organizing behavior of the ants in nature. It includes both effects of chaotic dynamics and swarm-based search. Through the construction of a suitable fitness function, the problem of parameter estimation of the chaotic system is converted to a problem of parameter optimization which could be solved via chaotic ant swarm algorithm. Chaotic ant swarm algorithm has the ability of global search. A numerical simulation on the well-known Lorenz chaotic system is conducted. Simulation results show that the proposed method is effective in parameter estimation of the chaotic system.
Hop-parameter chaotic synchronization and its applications
Chen Tian-Qi, Chen Bin, Zhang Yong
2007, 56 (1): 56-66. doi: 10.7498/aps.56.56
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In this paper, we build a mathematic model of hop-parameter chaotic synchronization, propose and prove its sufficient condition, then analyze theoretically the condition for its realization. An efficient algorithm is proposed for about the hop-parameter chaotic synchronization and hop-parameter period synchronization. Based on the Chua's system, we have simulated the hop-parameter chaotic synchronization and its secure communication. Finally, the counteraction between the hop-parameter chaotic communication system and the wiretapping system is discussed. The simulation shows the hop-parameter chaotic synchronization and its secure communication have the virtue of easy realization and good security.
Local support vector machine prediction of spatiotemporal chaotic time series
Zhang Jia-Shu, Dang Jian-Liang, Li Heng-Chao
2007, 56 (1): 67-77. doi: 10.7498/aps.56.67
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In this paper, local support vector machine (LSVM), which combins the advantage of traditional local prediction methods and support vector machines, is proposed to make local predictions of spatiotemporal time series. The LSVM is also used to discuss the selection of embedding dimension and the number of nearest neighbours, the coupling-way and the coupling coefficients of spatiotemporal chaotic systems that influence on the local predictions of spatiotemporal chaotic time series. Experimental results show that the LSVM can not only make better predictions of spatiotemporal chaotic time series than that of local zero-order methods and local linear methods and global support vector machine, but the computational complexity can also be reduced greatly compared to the global support vector machine. Moreover, the LSVM is insensitive to the selection of embedding dimension and the number of nearest neighbours. In addition, the local prediction performance of spatiotemporal chaotic time series is influenced by the coupling-way and the coupling coefficients of spatiotemporal chaotic systems.
Differential cryptanalysis of TD-ERCS chaos
Xiao Yan-Yu, Sheng Li-Yuan, Wen Jiang, Cao Li-Ling
2007, 56 (1): 78-83. doi: 10.7498/aps.56.78
Abstract +
Based on the fundamental principle of differential cryptanalysis and corresponding relation between the iteration of chaotic system and the round of block cipher, two important concepts, namely the iterative differential distribution (IDD) and the differential-invalid exponent (DIE) are proposed, which are used in evaluating chaotic system's power of conteracting the differential cryptanalysis. IDD is defined as the distribution of output differential vs. a given input differential at a varied iterative number, and DIE is defined as the minimal iterative number at which both output differential and input differential are independent of each other. By putting chaotic system into the “naked” state and directly analyzing the IDD, the DIE of chaotic system can be gotten. DIE is a universal, measurable and important characteristic exponent of chaos in studying security of chaos. The tests on both TD-ERCS and Logistic systems indicate that in the parameter region of 90%, DIE of DT-ERCS is equal to 2, which is the minimal DIE in theory.In comparison, DIE of Logistic chaotic system equals 55, hence TD-ERCS is a chaotic system with the power of active immunity to differential cryptanalysis.
Delayed fuzzy control of a unified chaotic system
Gao Xin, Liu Xing-Wen
2007, 56 (1): 84-90. doi: 10.7498/aps.56.84
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In this paper, we have studied a new delay-dependent chaos control approach via delayed state feedback. This approach uses Takagi-Sugeno fuzzy model to represent a unified chaotic system and adopts the theory of Lyapunov-Krasovskii stability and linear matrix inequalities (LMIs). Simulation results show that the delay-dependent state feedback controller we designed can stabilize the unified chaotic system to its equilibrium.
Nonfeedback control of Chen’s chaotic system
Chen Long, Wang De-Shi
2007, 56 (1): 91-94. doi: 10.7498/aps.56.91
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Nonfeedback chaos control of Chen's chaotic system is realized with nonresonant parametric drive. The control input is a periodic signal whose frequency is far higher than the system characteristic frequency. The feasibility of this scheme is proved by averaging method and Lyapunov method, and the condition of control parameter is obtained. Numerical researches show that the controlled system can reach stable state rapidly even when the control signal is noisy.
Parameter identification and backstepping design of synchronization controller for uncertain chaotic system
Guo Zhi-An, Lü Ling, Li Yan, Xia Xiao-Lan
2007, 56 (1): 95-100. doi: 10.7498/aps.56.95
Abstract +
A parameter observer is designed to identify Rossler system with unknow parameters. Chaos synchronization between uncertain Rossler system and Coullet system is realized via backstepping method.The synchronization controller is presented based on stability theory, and the area of controlling gain is determined. The simulation results show that all the state variables in the Coullet system can track any desired trajectory in the Rossler system exactly when the parameter observer and backstepping controller are functioning, which shows the method is effective and feasible.
Amplitude control of limit cycle of coupled van der Pol oscillator
Tang Jia-Shi, Xiao Han
2007, 56 (1): 101-105. doi: 10.7498/aps.56.101
Abstract +
Feedback controllers are designed by way of controlling the amplitude of limit cycle of coupled van der Pol oscillators. Bifurcation control equations of weakly nonlinear systems are obtained by using the approximate method, and relationships between the amplitude of limit cycle and the control gain are acquired, hence the amplitude of limit cycle can be controlled effectively. By using numerical method of solving ordinary differential equations, time displacement curves and limit cycle of the uncontrolled or controlled system can be found. The results obtained by approximate method and in numerical calculation are in good agreement. The method may be applied to other coupled van der Pol oscillators.
Empirical study of Chinese city airline network
Liu Hong-Kun, Zhou Tao
2007, 56 (1): 106-112. doi: 10.7498/aps.56.106
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We investigate the empirical properties of Chinese city airline network, where nodes and edges denote cities and direct fights. The present network is a small-world network with short average path length and high degree of clustering, and its degree distribution follows a double power law. The degree-degree correlation of the present network is different from that of the world airport network and the North American air network: The small-degree nodes of the world airport network and the North American air network are positively correlated, whereas those of the present network don't exhibit correlation behavior. And for large-degree nodes, the average neighboring degrees approach a constant in the world airport network, while the present network shows negatively correlated behavior. Previous studies suggested the absence of hierarchical structure in geographical networks, such as the Internet, the electrical power grids, and so on. However, the present network displays an obviously hierarchical property, which provides us an evidence that the geographical ingredient may have no effect on the structural evolution of Chinese city airline network. Furthermore, by setting the number of available seats per week as the edge weight, we investigate the correlation between weight and degree, and find a power-law weight-degree correlation with exponent 1.37.
Study of structure stability and decoration of carbon nanotube under applied electric field
Lin Xi-Wei, Xia Ming-Xia, Yan Ning, Li Hong-Xing, Ning Nai-Dong, Xie Zhong
2007, 56 (1): 113-116. doi: 10.7498/aps.56.113
Abstract +
Structure stability of multi-walled carbon nanotube under applied electric field has been investigated. The results show that anti-bonding between carbon atoms at the tip of nanotube resulting from the local field strength of ~30V/nm leads to structure instability. Using the electron microscope as the nanolaboratory, we have prepared amorphous carbon nanowires on the tip by incorporation of foreign hydrocarbon under the applied electric field, resulting in the nanotube/nanowire structure. Electric measurement indicates that the nanotube/nanowire structure has rectifying characteristics. Consequently, the applied electric field may have potential applications in structure decoration and device fabrication of carbon nanotubes.
Electronic structure and physical properties of γ-Si3N4 under high pressure
Ding Ying-Chun, Pan Hong-Zhe, Shen Yi-Bin, Zhu Wen-Jun, Xu Ming, He Hong-Liang
2007, 56 (1): 117-122. doi: 10.7498/aps.56.117
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The pressure-dependent electronic structure and physical properties of γ- Si3N4 have been calculated by means of plane wave pseudo-potential method (PWP) using GGA-PW91. Based on the calculations, we analyzed the influence of pressure on the optical and mechanical properties γ-Si3N4, which indicates that γ-Si3N4 is quite suitable for applications under high pressure.
Transfer property of electron pulse in a femto-second electron diffraction system
Liu Yun-Quan, Zhang Jie, Tian Jin-Shou, Zhao Bao-Sheng, Wu Jian-Jun, Zhao Wei, Hou Xun
2007, 56 (1): 123-128. doi: 10.7498/aps.56.123
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An ultra-fast electron diffraction system has been designed. The static and dynamic characters of an electron pulse with 150 fs temporal dispersion are studied during its transmission in the whole ultra-fast electron diffraction system, including the size of the electron spot, temporal dispersion, distribution of azimuths and elevation angles. The initial status of the photoelectrons are put down by Monte Carlo method, both the two dimensional and three dimensional electric fields are calculated by finite difference method and the magnetic flux are calculated by finite element method.
Isospin dependence of three-body force rearrangement contribution
Zuo Wei, Xu Zhong-Feng
2007, 56 (1): 129-136. doi: 10.7498/aps.56.129
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Within the isospin-dependent Brueckner framework, we investigate the contribution of three-body force (TBF) rearrangement to isospin symmetry potential as well as its momentum and density dependence.In particular, we investigate the TBF rearrangement effects on the isospin splitting of neutron and proton effective masses in neutron-rich nuclear matter. We show that the rearrangement contribution of TBF to neutron and proton single-particle potentials is repulsive and increases rapidly with increasing density and momentum. At low densities, the influence of the TBF rearrangement on symmetry potential is rather small, and the TBF rearrangement effect becomes more and more pronounced as the density rises. At high densities, the contribution of TBF rearrangement increases considerably the symmetry potential and modifies remarkably the momentum dependence of the symmetry potential. In both cases with and without including the TBF rearrangement contribution, the predicted neutron effective mass in neutron-rich matter is greater than the proton effective mass. The TBF rearrangement effect is to decrease remarkably both the proton and neutron effective masses, and reduce the magnitude of neutron-proton effective mass splitting in neutron-rich matter at high densities.
A study of the cross sections and the spin observables for proton-12C elastic scattering in momentum space in the intermediate energy region
Deng Yi-Bing, Wang Shi-Lai
2007, 56 (1): 137-142. doi: 10.7498/aps.56.137
Abstract +
The differential cross sections and the spin observables of p-12C elastic scattering in the whole intermediate energy region are calculated using the KMT multiple scattering theory with microscopic momentum space optical potential which incorporates the spin dependence of the NN and p-12C interactions. The theory takes into consideration the antisymmetrized NN amplitudes, nucleon recoil and binding energy shifts, Lorentz invariant angle transformations, t matrices with on-shell behavior from NN phase shifts,and off-shell behavior from a realistic separable potential model. Qualitative agreement is found in differential cross sections and polarization data for 200Ep<1000 MeV.
Field emission from single diamond particle
Yuan Guang, Guo Da-Bo, Song Hang, Gu Chang-Zhi, Dou Yan
2007, 56 (1): 143-146. doi: 10.7498/aps.56.143
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The I-V characteristics of a diamond particle deposited by hot-filament chemical vapor deposition on a tungsten tip was measured in a dual-probe scanning electron microscope (DP-SEM) chamber by introducing Mo tips into the chamber as a measurement probe. The I-V characteristic of well crystallized diamond particle follows Ohm's law, and no field electron emission was observed, while the cauliflower-like particle satisfies the Pool-Frenkel (PF) theory and the field emission was observed clearly. These results are contributed to the defects in CVD diamond.
Structure and analytic potential energy function for the ground state of SiO2 molecule
Yu Xiao-Guang, Cheng Xin-Lu, Yang Xiang-Dong, Deng Xiao-Hui, Wu Dong-Lan, Xie An-Dong
2007, 56 (1): 147-151. doi: 10.7498/aps.56.147
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The equilibrium geometry of SiO and SiO2 molecule has been calculated on the computational levels of density functional theory (DFT). The possible electronic state and reasonable dissociation limit for the ground state of SiO and SiO2 X1A1 molecule are determined based on atomic and molecular reaction statics. And Murrell-Sorbie analytic potential energy function of SiO molecule has been derived through the least-square fitting to ab initio data. Similarly, the harmonic frequency has been calculated, and the analytic potential energy function of SiO2 molecule has been derived using many-body expansion theory.
Theoretical study of forbidden M1, M2, E2 transitions for highly charged Ni-like ions
Yan Jun, Wan Jian-Jie, Xie Lu-You, Dong Chen-Zhong, Jiang Jun
2007, 56 (1): 152-159. doi: 10.7498/aps.56.152
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In this paper, the energy structure and the forbidden M1,M2,E2 transition probabilities between the ground state 3s23p63d10 1S0 and the low-lying excited 3s23p63d94l, 3s23p53d104l and 3s3p63d104l (l=s,p,d,f)states have been calculated for Ni-like ions of Z=45—95, by using GRASP92 package and REOS99 code based on the relativistic multi-configuration Dirac-Fock method. By Analyzing the effects of strong configuration interaction (CI) existing in Ni-like sequence, the abnormalies of forbidden transition probabilities existing in several highly charged Ni-like ions are explained,and some general laws for the influence of CI on forbidden transition probabilities are discussed.
A method of acquiring a dark hollow beam of nanometer level
Cui Yi-Ping, Xue Guo-Gang, Liu Cheng, Chen Hao, Yan Chang-Chun
2007, 56 (1): 160-164. doi: 10.7498/aps.56.160
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By using the finite difference time-domain (FDTD) method, the distribution of the modulus of the electric field intensity is obtained in the near field of an optical fiber head.We found that different dimensions of the cylindrical core in a hollow optical fiber had an influence on the electric field intensity. It is concluded that the size of the cylindrical core must be increased to make the hollow beam to cover a larger area. At the same time, after discussing the two situations that the size of the hollow optical fiber either rather large or rather small, we obtained a dark spot in the near field of the fiber that was almost of the same size as the hollow region (10 nanometer level). However, the background light in the dark hollow beam was brighter, and so the hollow region was designed to be a metal (such as silver). We found that the background light in the dark spot became evidently weaker and a cleaner dark hollow beam was acquired in the near field. When the hollow region was made smaller, the dark spot in the near field would become smaller, even reaching the nanometer level, which offered a method of obtaining a dark hollow beam of nanometer level.
A double magneto-optical trap system using a slow atomic beam for atom transfer
Zhou Shu-Yu, Xu Zhen, Zhou Shan-Yu, Wang Yu-Zhu
2007, 56 (1): 165-169. doi: 10.7498/aps.56.165
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A rubidium double magneto-optical trap(MOT) system for Bose-Einstein condensation experiments has been set up. A slow atomic beam has been obtained with a low-velocity intense source (LVIS) and used to transfer atoms into an ultra-high-vacuum (UHV) MOT. By maintaining a pressure difference of 3 orders between LVIS-MOT and UHV-MOT, a lowest pressure of 1×10-9Pa can be achieved in UHV-MOT. The flux of the slow atomic beam is 1×109/s. About 4×108 87Rb atoms can be loaded into UHV-MOT. In this paper, maximum number of atoms captured in MOTs under two typical conditions have also been discussed.
Spectroscopy studies of methyl orange on silver colloids: pH and Cl- effects
Zhang Ai-Ping, Fang Yan
2007, 56 (1): 170-177. doi: 10.7498/aps.56.170
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The UV-Vis absorption and photoluminescence spectral signals of silver colloids and methyl orange (MO) in different pH systems before and after adding Cl- were studied. It was found that the energy transfer efficiency changes with the pH value, i.e., the fluorescence quenching at 428 nm and photoluminescence enhancement at 560 nm was greatly strenthened with the increase of pH of the system. After adding Cl-, dense adsorption layer was formed, leading to stronger spectral signals associated with surface enhancement. On the other hand, because of the decrease of incident light to the inside of Ag-MO complex, energy transfer channel may be suffocated to some extent, the quenching amplitude of 428 nm fluorescence band decreased. Compared with these notable phenomena and discussed with correlative theoretic analysis, it's clear that different molecular structures and adsorption models of MO onto Ag surfaces exist with the changing of pH and the adding of Cl-, which give rise to the different phenomena observed.
Study on the structure and stability of the Al2O3Hx(x=1—3) molecules by density function theory
Gao Tao, Zhu Zheng-He, Chen Xiao-Hong, Luo Shun-Zhong
2007, 56 (1): 178-185. doi: 10.7498/aps.56.178
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The geometric configuration,electronic structures and vibrational frequency of the Al2O3Hx(x=1—3) molecules were calculated using B3LYP method at 6-311++g(d,p) level systematically. The calculations give the total energies, zero point energies, mol heat capacities under constant volume, standard entropies, atomization energies, vertical ionization energies, vertical electron affinity energies of their possible ground state structures. The geometric configurations of possible ground state structures of Al2O3H and Al2O3H2 molecules are planar. The geometric configurations of two possible ground state structures of Al2O3H3 molecule are solid geometric configurations which come from the solid geometry of Al2O3 with D3h symmetry adding three hydrogen atoms to oxygen atoms and aluminum atoms. The configurations of these three molecule clusters with lowest energy are: Al2O3H (2A′)Cs, Al2O3H2(1A′)Cs and Al2O3H3(2A)C1.
Electromagnetic scattering of plane wave/Gaussian beam by parallel cylinders
Wang Yun-Hua, Guo Li-Xin, Wu Zhen-Sen
2007, 56 (1): 186-194. doi: 10.7498/aps.56.186
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Based on the equivalence principle and the reciprocity theorem, the multiple scattering up to Nth-order by N parallel 2-D targets arbitrarily located in a plane wave/Gaussian beam is considered. The first-order solution can easily be obtained by calculating the scattered field from isolated targets. However, due to the difficulty in formulating the couple scattered field, it is almost impossible to find an exact analytical solution for the higher order scattered field. In order to overcome this difficulty, the composite scattering field is studied by employing the technique based on the reciprocity theorem and the equivalence principle and a surface integral solution up to Nth-order is obtained. To evaluate the ith order scattering field,only the i-1th order scattered field from scatterers and the equivalent surface electric and/or magnetic current density induced by the incident wave are required. Using the approach proposed in this paper, the bi- and mono-static scattering width of the incident field by parallel adjacent inhomogeneous plasma-coated conducting cylinders is calculated and the dependence of attenuation of the scattering width on the thickness of the coated layer, the electron number density, the collision frequency and the radar frequency is discussed in detail.
The closed solution and verification of the electromagnetic field of a vertical dipole over the sphere
Wang Hai-Long, Wu Qun, Meng Fan-Yi, Wu Jian, Li Le-Wei
2007, 56 (1): 195-200. doi: 10.7498/aps.56.195
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The closed solution of the electromagnetic filed of the vertical dipole over a perfect conducting sphere is introduced,the problem of discontinuity encountered in the series expression of the direct field is solved. Computation result is compared with the direct summation of the series form solutions of the electromagnetic filed over the sphere. Based on the same electromagnetic model, an approximate method is verified by the closed solution. If the vertical dipole and the observation point are near the surface of the sphere and the frequency is not too high, then the computation results of the two methods are guite the same. If the vertical dipole and the observation point are not near the surface of the sphere and the frequency is high, then the behavior of interference is gradually obtained by the closed solution but not by the approximate method.
Transversal optical vortex in the interference field of two off-axis Gaussian beams
Lü Bai-Da, Wang Xi-Qing, Wang Li, Wang Qing-Feng
2007, 56 (1): 201-207. doi: 10.7498/aps.56.201
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The structure of the wavefront and transversal optical vortices in an interference field of two off-axis Gaussian beams is studied. It is shown that the positions of phase saddles and vortices depend on the off-axis parameter, beam width, phase and relative amplitude of beams. The saddle can be located inside or outside the vortices. For a certain value of the governing parameter, the saddle point coincides with the vortex. For off-axis beams, the encounters of the saddle point and vortex on both sides in the XZ cross-section do not occur at the same governing parameter η. Moreover, the x value corresponding to the encounter of saddle points is not equal to the x value corresponding to the encounter of vortices.
Modulation principles and code design for multilevel optical data storage
Hu Hua, Xiong Jian-Ping, Xu Duan-Yi, Qi Guo-Sheng, Xiao Jia-Xi, Hu Heng
2007, 56 (1): 208-212. doi: 10.7498/aps.56.208
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Multilevel optical data storage is an important technology to improve the recording density of optical disc without changing the optical parameters. One approach is multilevel amplitude modulation, which uses the variance of signal amplitude of length-fixed recorded marks to record information. Another one is multilevel runlength-limited (RLL) modulation, which records information in both the signal amplitude and the length of recorded marks, thus can improve the recording density more effectively than the first way. Multilevel RLL coding is a key technology in multilevel RLL modulation optical recording. In this paper, a new 8-ary (1,3) RLL code is designed with state splitting algorithm. The recording density of this code is as high as 3.0 bits/symbol, and the encoder/decoder is simple and easy to implement. The proposed code can be used for future high density multilevel RLL modulation optical disc systems.
Experimental investigation of interaction of two Laguerre-Gaussian modes carrying opposite topological charges
Zhang Xiao-Bo, Zhang Wei, Shu Fang-Jie, Li Yong-Ping
2007, 56 (1): 213-217. doi: 10.7498/aps.56.213
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The interaction of two Laguerre-Gaussian(LG) modes carrying opposite topological charges in a linear medium is investigated experimentally. The changes of diffractive pattern are observed when these two LG modes propagate, and a dark arc happens by the central dark particles of the two LG modes joining together in case the interaction of two LG modes carrying opposite topological charges propagate in a linear medium. This experimental phenomenon proves that the screw phase dislocations evolve into a new type of dislocation—edge phase dislocation with the optical field propagating.
Three-dimensional surface contouring of reflecting micro-object by digital holography with short-coherence light source
Yuan Cao-Jin, Zhai Hong-Chen, Wang Xiao-Lei, Wu Lan
2007, 56 (1): 218-223. doi: 10.7498/aps.56.218
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In this paper, a new lensless digital holography system with short-coherent light source is designed for recording three-dimensional surface contour of reflecting micro-object. In the experiment, the different layers on the inner wall of a conical pore are recorded by changing the path length of object beams to record a series of sub-holograms for three-dimensional reconstruction. This makes the recording system compact and reduces the additional error in the recording. The measured longitudinal error is about 3.5%,with a lateral error of about 2.6%. Besides, the least squares polynomial fit method is used for the first time to carry out three-dimensional reconstruction with a series of two dimensional intensity images of a micro-object, which not only reduces obviously the complication of the three-dimensional reconstruction, but also can be used to carry out three-dimensional reconstruction of a micro-object with strong laser speckle noise, of which the phase images can not be obtained from the phase-unwrapping process.
Pulse width measurement of ultra-broad-bandwidth Ti: sapphire oscillator using SPIDER technique
Wang Peng, Zhao Huan, Zhao Yan-Ying, Wang Zhao-Hua, Tian Jin-Rong, Li De-Hua, Wei Zhi-Yi
2007, 56 (1): 224-228. doi: 10.7498/aps.56.224
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The pulse width generated by an ultra-broad-bandwidth Ti: sapphire oscillator is measured using SPIDER technique. The measurement result is 7.86 fs, which is, as presently reported in China, the shortest pulse output directly from an oscillator in which only chirped mirrors was used to compensate for the laser dispersion. Layout of the oscillator and the SPIDER setup are introduced in details, and the spectral phase of the laser pulse obtained through calculation is discussed.
Theoretical study on temporal behavior of Brillouin-enhanced four-wave mixing
Zhu Cheng-Yu, Lü Zhi-Wei, He Wei-Ming, Ba De-Xin, Wang Yu-Lei, Gao Wei, Dong Yong-Kang
2007, 56 (1): 229-235. doi: 10.7498/aps.56.229
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The nature of Brillouin-enhanced four-wave mixing (BEFWM) temporal modulating behavior is examined numerically. The evolution of the acoustic wave, which plays a key role in the process of energy inter-coupling, is simulated and analyzed. Based on this, the detailed mechanism of BEFWM temporal characteristics is explained. It is demonstrated that, modulations in the conjugate wave are related with the distribution and fluctuation of the acoustic wave, and influenced by parameters such as phonon lifetime, intensity of the signal and pump wave, effective interaction length, and so on.
Waveguide induced by bright holographic solitons
Zhang Hui-Lan, Zhang Guang-Yong, Wang Cheng, Liu Shi-Xiong, Liu Jin-Song
2007, 56 (1): 236-239. doi: 10.7498/aps.56.236
Abstract +
The properties of waveguides induced by one-dimensional steady-state bright holographic solitons in photorefractive medium are studied numerically. The results show that the number of possible guided modes in such a waveguide depends on the peak intensity of the soliton. The dynamical evolutions of the guided modes of the waveguide are presented. And the wave guiding in such a waveguide is also considered for a probe beam with cosine dependence or sine dependence.
Propagation dynamics of Laue solitons in a resonant photonic crystal
Xue Chun-Hua, Qi Yi-Hong, Zhou Jun
2007, 56 (1): 240-244. doi: 10.7498/aps.56.240
Abstract +
When an incident pulse is input obliquely with certain orientation of the reciprocal lattice vector in a resonant photonic crystal, the so-called Laue diffraction will be created. In this paper, the propagation dynamics of Laue solitons is theoretically researched, and is verified through numerical method. The results show that the incident pulse will give rise to four diffracted modes due to the nonlinear diffraction, in which the propagation dynamics of Laue solitons within the structure is similar to that of the solitons in a normal resonant medium.
Interactions between counter-propagating photovoltaic spatial solitons
Jiang De-Sheng, She Wei-Long
2007, 56 (1): 245-251. doi: 10.7498/aps.56.245
Abstract +
We study the interactions between two counter-propagating photovoltaic spatial solitons with different frequencies. Numerical results show that these two solitons display different dynamic characteristics during spatial evolution, which also differ obviously from the case of co-propagation. These two solitons exhibit different induced focusing or defocusing behavior when they propagate coaxially, which is sensitive to the specimen length. When they propagate in parallel and closely overlapping, the induced focusing or defocusing effects are weakened and the soliton with higher frequency attracts the one with lower frequency markedly. And their evolutions are different from the case of co-propagation.
Investigation of the beam evolution in nonlinear optical lattice
Zhuo Hui, Fu Xi-Quan, Wu Jin-Hua, Wen Shuang-Chun
2007, 56 (1): 252-257. doi: 10.7498/aps.56.252
Abstract +
The paper studies the beam evolution of optical lattice, i.e. media with transverse periodic lattice modulation in the propagation radiation. Using the variational principle,the forms are obtained for the evolution duing propagation of beam width, beam amplitude and frequency chirp. Next, the conditions for soliton formation in nonlinear optical lattice are discussed and the different propagation processes with different beam conditions are obtained. There are analogous characteristics in the actions of periodic lattice potential of optical lattice and nonlinear media. The periodic potential offers a better method to control the lattice soliton formation and propagation.
Top- and side-view method for measuring photorefractive spatial optical solitons
Wang Cheng, Liu Jin-Song, Zhang Guang-Yong, Liu Shi-Xiong, Zhang Hui-Lan
2007, 56 (1): 258-262. doi: 10.7498/aps.56.258
Abstract +
Basing on CCD technology and software processing, a new method, namely top- and side-view method, is presented for the observation of spatial optical solitons in photorefractive crystals. When a laser beam is propagating in a crystal, this method has an ability to show the horizontal and vertical outline of the beam at any location of the crystal very intuitively. By use of this method, we have experimentally observed the two-dimension bright photovoltaic solitons in a KNSBN crystal, and the measured results are qualitatively in agreement with the results of numerical simulation.
Electroluminescence of polymer doped small-molecules light-emitting diodes and the effects of doping trap
Zhang Bo, Tang Xian-Zhong, Nie Hai
2007, 56 (1): 263-267. doi: 10.7498/aps.56.263
Abstract +
Polymer doped small-molecule light-emitting diodes have been fabricated using a novel PTPD (poly-TPD) as the hole transport material and the highly fluorescent rubrene as the dopant. Electroluminescent characteristics are studied at different doped concentrations and different film thicknesses. The results indicate the presence of doping trap effect. It is assumed that the traps are limited at discrete levels. Based upon the double-carrier injection theory, an analytical model of J-V characteristics of doped device is deduced by solving the Poisson equation. Calculated data of this model are consistent with our experimental results.
Dispersion and localization of defect state in one-dimensional photonic crystal consisting of metamaterials
Dong Jian-Wen, Chen Yi-Hang, Wang He-Zhou
2007, 56 (1): 268-273. doi: 10.7498/aps.56.268
Abstract +
The expressions of the dispersion relationship and space localization in the one-dimensional photonic crystal (1DPC) with symmetric semi-infinite period number and a central defect layer are deduced, which are valid for the defect 1DPC consisting of materials with arbitrary frequency-dependence. Based upon them, we found that the eigenfrequency of defect state is red-shifted with the increasing geometry thickness of defect layer in right-handed materials. On the contrary, the frequency is blue-shifted when the defect layer is a left-handed material. Moreover, the defect state is more localized for symmetric semi-infinite period structures consisting of metamaterials than those of conventional right-handed materials.
Multi-wavelength dispersion compensator based on fiber gratings with low crosstalk
Tan Zhong-Wei, Cao Ji-Hong, Chen Yong, Liu Yan, Ning Ti-Gang, Jian Shui-Sheng
2007, 56 (1): 274-279. doi: 10.7498/aps.56.274
Abstract +
A novel method is proposed to suppress the crosstalk of the DWDM system with dispersion compensated by fiber gratings, combining the merits of both concatenated narrowband fiber gratings and sampled fiber gratings. Random time delay is induced only between the adjacent channels to improve the characteristics of fiber gratings' linear crosstalk, with the cross modulation effects and the four wave mixing effects being suppressed simultaneously. Using this method,we can realize the low crosstalk multi-wavelength dispersion compensators based on fiber gratings.
Polaron fluorescence and tomograpy of Ti:LiNbO3 waveguide
Zhang Yun
2007, 56 (1): 280-284. doi: 10.7498/aps.56.280
Abstract +
The small polaron Nb4+Li fluorescence spectrum is in the range of 700—950 nm. Researches of the pure lithium niobate samples with the same composition and different reduction degrees show that the intensity of the fluorescence spectrum reflects the chemical reduction degree of the material. From the shallow level of small polaron, we propose a model of “one center and three energy level” to explain the process of polaron fluorescence. For titanium-diffused lithium niobate (Ti:LiNbO3), homogeneity of the chemical reduction level would be destroyed during fabrication. From the scans of Ti:LiNbO3 waveguide sample, the intensities of fluorescence spectrum on the surface and in the bulk, as well as inside the waveguide and outside the waveguide, are evidently different (intensity on the surface is 6—8 times of that in the bulk). Our researches indicate, using the non-destructive technology of polaron fluorescence spectrum is effective to control the necessary heat treatment in the waveguide fabrication processes. At the same time, the difference in intensity reflects indirectly the profile of the waveguide.
The time-window of pulse injection seeding for effective spectra linewidth narrowing of gain-switched DFB laser diodes
Wang An-Bang, Wang Yun-Cai, Guo Wen-Ge, Zhang Shou-Gang
2007, 56 (1): 285-290. doi: 10.7498/aps.56.285
Abstract +
Spectral linewidth narrowing of gain-switched distributed feedback laser diode with pulse injection seeding is related to injection time, as indicated by experiment and theoretical analysis. The experimental and numerical simulation results show that in an approximately 100 ps time-window before the generation of optical pulse, the spectral linewidth can be narrowed dramatically by injecting pulse seeding, and that spectral linewidth decreases with increasing intensity of injection pulse. Near transform-limit low-chirp ultrashort optical pulses can be obtained with optical pulse injection seeding. A spectral linewidth reduction from 0.46 to 0.08 nm was obtained and the time-bandwidth product of 2.46 lowered down to 0.70 correspondingly in the experiment.
Control of carrier-envelope phase offset in femtosecond laser with PLL and TV-Rb clock
Tong Juan-Juan, Wang Yan-Hui, Dong Tai-Qian, Han Hai-Nian, Zhang Wei, Wang Peng, Wei Zhi-Yi, Li De-Hua, Shen Nai-Chen, Nie Yu-Xin
2007, 56 (1): 291-295. doi: 10.7498/aps.56.291
Abstract +
With the fast development of the cycle-pulse lasers, the control of the carrier-envelope phase offset (CEO) frequency has been an important research subject in such fields as femtosecond optical frequency metrology, strong field physics and high harmonic generation. In this paper, a different measurement method for CEO is demonstrated and subsequently a better beat frequency signal with the signal-to-noise ratio greater than 40 dB is obtained. Based on this optimized beat frequency signal, the CEO of the Ti:sapphire laser is stabilized to the standard signal of 10 MHz of the microwave reference source TV-Rb clock by using the electronic phase-locked loop techniques and the piezo-transducers which are mounted on the mirror inside the laser cavity to change the frequency and phase. The repetition rate of the Ti:sapphire laser is locked to the same TV-Rb clock simultaneously. The experimental results showed that the stability of the repetition rate reached the same level as the TV-Rb clock and that of the CEO frequency was increased by three orders.
Bistability-mapping of L-band dual-wavelength erbium-doped fiber laser based on fiber loop mirror
Mao Qing-He, Feng Su-Juan, Jiang Jian, Zhu Zong-Jiu, Liu Wen-Qing
2007, 56 (1): 296-300. doi: 10.7498/aps.56.296
Abstract +
A technique for controlling and mapping the optical bistability of an L-band dual-wavelength erbium-doped fiber laser (EDFL) with cascaded linearly coupled cavities was presented and demonstrated in this paper. By using a fiber loop mirror (FLM) with a polarization controller (PC) in the loop as the common mirror of the laser, we found that the bistability of the laser may be efficiently controlled by adjusting the reflectivity of the FLM which is determined by the PC state. Moreover, the bistability varies monotonically with the reflectivity of the FLM. By making use of this property, the two stable states and the two potential lasing wavelengths of the laser may be respectively mapped and switched with respect to each other by adjusting the PC for any fixed pump powers, as long as the pump power lies between the first turning point of the bistability for the maximum reflectivity of the FLM and the second turning point of the bistability for the minimum reflectivity of it.
Numerical investigation on single-mode single-polarization photonic crystal fiber using resonant absorption effect
Liu Xiao-Yi, Zhang Fang-Di, Zhang Min, Ye Pei-Da
2007, 56 (1): 301-307. doi: 10.7498/aps.56.301
Abstract +
In this paper, a novel single-mode single-polarization photonic crystal fiber using resonant absorption effect has been put forward for the first time. A full vector finite element method is employed to simulate the fiber properties, including mode field distribution, confinement loss,bandwidth and extinction ratio. Numerical results show that very large bandwidth of single-mode single-polarization operation can be attained and extremely high extinction ratio can be achieved with low loss penalty. The influence of structural parameters on the fiber performance has been investigated. The research results will be instructive for the design and fabrication of new high performance single-mode single-polarization fibers.
A fiber sensor for measuring gas concentration based on laser’s transient regime
Yang Xiu-Feng, Lü Chao, Li Yong-Nan, Tu Cheng-Hou, Lü Fu-Yun, Wang Hong-Jie, Guo Wen-Gang, Luo Shao-Jun, Li En-Bang
2007, 56 (1): 308-312. doi: 10.7498/aps.56.308
Abstract +
A novel fiber sensor for measuring gas concentration is reported, which is based on the transient regime of a Er-doped fiber laser. A Sagnac fiber loop and a FBG that also acts as the sensing element form the linear fiber laser cavity. By adjusting the central wavelength of the grating mirror, the oscillating wavelength is set to 1531.56 nm, which is a strong absorption peakof C2H2. The gas concentration can be obtained by measuring the laser build-up time, which is closely related with the cavity loss and therefore determined by the gas concentration. In the present experiment, the sensor's sensitivity and resolution can be adjusted simply by controlling the pumping level. When the pump pulse is at high level and low level of 25 mW and 5.9 mW, respectively, a sensitivity of 100 ppm/μs and resolution of 20 ppm are achieved.
Study on solid-solid interface waves with laser ultrasonics
Zhu Chang-Ping, Han Qing-Bang, Qian Meng-Lu
2007, 56 (1): 313-320. doi: 10.7498/aps.56.313
Abstract +
The propagation characteristics of interface waves are studied theoretically and experimentally. The problem of solving for roots of characterization equation of interface waves are discussed. It is shown that all roots can be found by means of Riemann sheet analysis. The propagation mechanism of Stoneley wave, Leaky Rayleigh and Leaky interface waves are explored, the wave vector and displacement behaviors of these waves are illustrated to show their differences. At last, the interface waves are measured with laser ultrasonic technique based on photo-elastic effect, and the experimental results are in good agreement with theoretical predictions.
Finite element simulation of laser-generated circumferential waves in hollow cylinder
Zhao Yan, Shen Zhong-Hua, Lu Jian, Ni Xiao-Wu
2007, 56 (1): 321-326. doi: 10.7498/aps.56.321
Abstract +
Based on the thermoelastic mechanism of laser ultrasonic, a finite element model for simulating laser-generated circumferential wave in a hollow cylinder is developed. A control calculation is executed to verify the correctness of the model according to the study of Gao et al (J. Appl. Phys. 91, 6114 (2002)). Our calculated results are in good agreement with the experimental signals obtained by Gao et al. And then, the circumferential waves induced by line laser source in hollow cylinders with different thicknesses and different radii are simulated and typical results are presented. Based on the numerical calculations, the difference between circumferential guided waves in hollow cylinder and Lamb waves in a plate, and the influence of the thickness and radius on the circumferential waves are discussed in detail.
CTMC calculation of O3++He impact processes
Liu Chun-Lei, He Bin, Yan Jun, Wang Jian-Guo
2007, 56 (1): 327-332. doi: 10.7498/aps.56.327
Abstract +
The O3++He collision processes are studied ultilizing a classical trajactory Monte-Carlo (CTMC) method. The initial microcanonical distribution is modified by using eleven different ionization thresholds of electrons in tht target. The total cross sections of all processess as functions of the projectile energy are calculated. For a test, the process of He2++He is calculated and the agreement with experiment is improved by the modified distribution.The total cross sections of O3++He including single- and double-electron ionization and transfer ionization are obtained in the energy range from 3 keV/u to 1 MeV/u, which are found in good agreement with the experiment. But there exists some large difference for the ratio of transfer ionization to single-electron capture, which requires further theoretical and experimental investigations.
Numerical research on intense pulsed ion beam ablation plasma expansion into ambient gases
Gong Ye, Liu Jin-Yuan, Wang Xiao-Gang, Liu Yue, Ma Teng-Cai, Wu Di
2007, 56 (1): 333-337. doi: 10.7498/aps.56.333
Abstract +
The mechanisms are different for intense pulsed ion beam ablation plasma expanding into ambient gas of different pressure or into vacuum. The former is complicated by the interaction between plasma and the ambient gas. The 2D gas dynamical model has been built to describe the process, and the plasma expansion into ambient gas of pressures from 10-6 atmosphere to 1 atmosphere have been calculated. The results show that the phenomena of plasma expanding into ambient gases vary as the pressure changes. It can expand into vacuum freely, but into atmosphere its movement is confined. And the ejection into ambient gas of one thousandth atmosphere or so is special, the ‘snowplow’ phenomenon is observed and the plume plasma splits into fast- and slow-moving parts.
Physical analysis of the certain results in Z-pinch experiments on the “Qiang Guang-I” generator
Ning Cheng, Ding Ning, Yang Zhen-Hua
2007, 56 (1): 338-345. doi: 10.7498/aps.56.338
Abstract +
Starting from the energy necessary for atomic ionization and plasma maintaining, a method is developed to estimate the required energy and implosion velocity for a material to emit a certain shell X-ray radiation in a Z-pinch. The typical load current and voltage in Z-pinch process on “Qiang Guang-I” generator are analyzed to obtain the inductance and the work done by Lorentz force, as well as the effect of load parameters on them. The inductance is not in good agreements with that calculated from the theory based on the shell model of Z-pinch, possibly due to the over simplification of the model and the measurement limitations, such as the measuring points of the current and voltage are always several centimeters away from the load. It is found in the experiments that the inductance increases with the load current until the end of the X-ray radiation pulse, and is saturated at ~100nH. The work done by Lorentz force is maximal when the diameter of tungsten wire array is 8 mm in our experiments. The work done to the load with more wires is greater than that to the load with less wires if the diameters of the wire arrays are the same, say 8 or 10 mm. At the same time, the calculation for optimized masses per unit length of gas-puff and wire array load was carried out for the “Qiang Guang-I” generator on 1.6MA current level. The optimized results show that the tungsten wire array with 8 mm diameter should be the best load for this generator. This is consistent with the experimental result on the “Qiang Guang-I”.
A comparison of energy distribution of hot electrons from the front and the rear sides of targets during the interaction of femtosecond laser with foil targets
Gu Yu-Qiu, Zheng Zhi-Jian, Zhou Wei-Min, Jiao Chun-Ye, Wen Tian-Shu, Chunyu Shu-Tai, Cai Da-Feng
2007, 56 (1): 346-352. doi: 10.7498/aps.56.346
Abstract +
The energy distribution of hot electrons from the front and the rear sides of targets have been measured during the interaction of femtosecond laser with the foil target. The results show that the peaks of energy spectrum of hot electrons from the front and the rear sides of targets are located at 430 keV and 175 keV, respectively. Fitted with Maxwell distribution, the temperatures of hot electrons from the front of targets are 218 keV and 425 keV. Obviously, the energy spectrum of hot electrons from the rear of targets is “softened”, which is due to Debye sheath electric field and self-magnetic field produced by hot electrons transported in the target, and has temperatures of 96 keV and 347 keV, which are lower than those from the front of targets.
Experimental study on Kα X-ray emission from intense femtosecond laser-solid interactions
Chen Li-Ming, Kazuhisa Nakajima, Toshi Tajima, Xu Miao-Hua, Li Yu-Tong, Liu Yun-Quan, Wang Zhao-Hua, Wei Zhi-Yi, Zhang Jie, Yuan Xiao-Hui, Zhao Wei
2007, 56 (1): 353-358. doi: 10.7498/aps.56.353
Abstract +
The characteristics of Kα X-ray sources generated by p-polarized femtosecond laser-solid interactions are experimentally studied in the relativistic regime. By use of knife-edge image technique and a single-photon-counting X-ray CCD camera, we obtaine the source size, the spectrum and the conversion efficiency of the Kα X-ray sources. The experimental results show that the conversion efficiency of Kα photons reaches an optimum value of 7.08×10-6/sr at the laser intensity of 1.6×1018W/cm2, which is different from the Reich's simulation results (Reich et al., 2000 Phys. Rev. Lett. 84 4846). We find that about 10% of laser energy is converted into the forward hot electrons at the laser intensity of 1.6×1018W/cm2.
Infulence of electron temperature on the two peaks of Thomson scattering ion-acoustic waves in laser plasmas
C. Danson, D. Pepler, Jiang Xiao-Hua, Liu Yong-Gang, Huang Li-Zhen, Ding Yong-Kun, Wang Zhe-Bin, Fu Shi-Nian, Li Yu-Tong, Dong Quan-Li, Liu Feng, Zhang Zhe, Zhang Jie, Yu Quan-Zhi
2007, 56 (1): 359-365. doi: 10.7498/aps.56.359
Abstract +
With the effective Thomson scattering diagnostic of the laser plasmas produced under different laser conditions, the distinct asymmetry of the two peaks of the ion-acoustic waves are measured 150 μm in front of the target surface. The more intense peak appears at shorter wavelength when the electron temperature is higher, and switches to longer wavelength when the electron temperature is lower. The two-peak strcture, the asymmetry and the position of the more intensity peak of the ion-acoustic waves can be well interpreted with the Raman scattering effect. This paper, for the first time, establishes the corresponding relationship between Raman scattering of light and Thomson scattering of electons in laser plasmas.
Diagnostic study on the electron density distribution of microwave plasma jet in vacuum environment
Yang Juan, Liu Wen-Yi, Zhu Guo-Qiang, Mao Gen-Wang
2007, 56 (1): 366-370. doi: 10.7498/aps.56.366
Abstract +
In order to diagnose precisely the electron number density of microwave plasma jet in vacuum environment, the integrative probe whose function is the same as that of emissive and Langmiur ones was applied to measure the plasma space potential and its current and voltage property when the probe is charged by the electric power of emissive probe and scanning electric power separately. According to the plasma space potential, saturated current on the current-voltage curve of plasma can be determined, which can be used to estimate the number density of electrons. The experimental results show that the number density of electrons in microwave argon plasma jet in vacuum environment is 1×1016—7.2×1016/m3, when the vacuum pressure is 3.2 and 5.4 Pa, the microwave output power is under 60 W and gas mass flow rate is in the range of 42—106 mg/s.
Microstructures of Al and Cu interconnects
Li Zhi-Guo, Wang Jun-Zhong, Ji Yuan, Liu Zhi-Min, Luo Jun-Feng, Wang Xiao-Dong
2007, 56 (1): 371-375. doi: 10.7498/aps.56.371
Abstract +
The electron backscatter diffraction technique (EBSD) has been used to measure the microstructure of reactive ion etched(RIE) Al and damascene Cu interconnects, including the grain size, grain orientation and grain boundary characteristics. Linewidths of Cu interconnects, as well as the anneal processes of Al and Cu interconnects impacting on the microstructures and causing the electromigration failure were analyzed.
Simulation study of effects of initial melt temperature on microstructure of liquid metal Na during solidification processes
Chen Zhen-Hua, Liu Rang-Su, Wang Xin, Tian Ze-An, Zhou Qun-Yi, Hou Zhao-Yang
2007, 56 (1): 376-383. doi: 10.7498/aps.56.376
Abstract +
Molecular dynamics simulation study has been performed on the effects of thermal history of initial melt temperature on microstructures during solidification of liquid metal Na. The pair distribution function g(r) curves, the bond-type index method and the cluster-type index method have been used to analyze the variations of microstructures during the solidification process. The results show that the solidification structures of liquid metal Na at the cooling rate of 1×1011 K/s with different initial melt temperatures are always crystalline. The 1661 and 1441 bond-types or the bcc basic cluster (14 6 0 8) in the system play the critical role in the microstructure transitions. At the same time, it has been found that the effects of initial melt temperature on solidification microstructures are very remarkable, while they are not obvious in liquid and supercooled states, and the effects can be fully displayed only near the crystallization temperature Tc. The results also demonstrate that the effects of initial melt temperature on the crystallinity of solidification structures are different for different initial melt temperatures. Although the degree of influence is not linearly varying with the decrease of initial melt temperature, it still demonstrate that the influence degree can be controlled through the change of initial melt temperature. The cluster-type index method more clearly describes the cluster configurations in crystal system than the bond-type index method, so it is valuable to deeply investigate the evolution mechanisms of microstructures in liquid metal during solidification processes.
Periodic oscillation of rare-gas atoms in carbon nanotubes
Jin Nian-Qing, Zhang Hai-Yan, Li Shi-Ying, Zeng Xiang-Hua
2007, 56 (1): 384-388. doi: 10.7498/aps.56.384
Abstract +
Based on the classical molecular dynamics method by using TLHT potential, the dynamical process of rare-gas atoms injected into single-walled carbon nanotube (SWCNT) is discussed. Firstly, the minimal diameters of SWCNT absorbing rare-gas atoms He, Ne, Ar, Kr, Xe are obtained as 6.3 ?,7.0 ?, 8.6 ?,8.6 ? and 8.6 ?, respectively. Then, the threshold energies to encapsulate the rare-gas atoms in SWCNT of different diameters are calculated. The interesting result is that the encapsulated rare-gas atoms are constantly oscillating in SWCNT, and the oscillation period bears no relation to the injection energy, while the amplitude depends on the injection energy, that is, the amplitude increases with increasing injection energy. The investigation indicates that for a suitable SWCNT, rare-gas atoms with a relative small kinetic energy will oscillate steadily in SWCNT, and the oscillation frequency can reach GHz order.
A new inversion simulation method for analyzing experimental data of helium bubble growth in materials
Zheng Hui
2007, 56 (1): 389-394. doi: 10.7498/aps.56.389
Abstract +
Using Monte Carlo method, the helium bubble growth model based on the “migration-coalescence” mechanism is studied in this paper. Based on the simulation, an inversion method of analyzing the experimental data is developed and finally applied to palladium.
Atomic-scale study of boron-nitrogen co-doping into diamond
Li Rong-Bin
2007, 56 (1): 395-399. doi: 10.7498/aps.56.395
Abstract +
The structure and lattice damage induced by four boron (B) atoms and eight nitrogen (N) atoms with the energy of 500 eV co-doped into diamond films at room temperature are investigated by molecular dynamics simulation based on Tersoff empirical potential. The results show that the distribution of most of vacancies is nearer to the surface than the interstitials. Interstitials arrange in diamond in tetrahedral type (T-type) configuration or dumbbell type (D-type) configuration. Percentage of boron atoms and nitrogen atoms located in substitutional positions are found to be around 78%. The B-N bond length is shorter than C-C bond length in diamond by 13%. B-N bonding helps to reduce the lattice distortion in diamond.
Effect of bulk inclusion in films on damage probability
Shao Jian-Da, Fan Zheng-Xiu, Xia Zhi-Lin
2007, 56 (1): 400-406. doi: 10.7498/aps.56.400
Abstract +
From the relation between damage probability and density of surface inclusion, combining with the test results of XRD and laser induced damage, we have drawn a conclusion that bulk inclusions dominate the damage in these experiments. Combining thermal and mechanical processes of inclusion absorption and the statistical rule of inclusion density, we have deduced an equation describing the relation between damage probability and density of inclusions, power density of laser pulse, as well as thickness of films. From the model in this paper, we can see that the range of siges in which the inclusions can initialize damage alter with the depth of inclusions. So density of the dangerous inclusions in different depth is different. The results from theory and from experiments are identical. This model can also interpret damage morphology.
Molecular dynamics simulation of the diffusion bonding and tensile behavior of a Cu-Al interface
Liu Hao, Ke Fu-Jiu, Pan Hui, Zhou Min
2007, 56 (1): 407-412. doi: 10.7498/aps.56.407
Abstract +
In this paper, we use molecular dynamics simulations to study the diffusion bonding of a Cu/Al interface. The interface analyzed is that between atomically smooth (001) crystalline planes of a Cu and an Al single crystals. The thickness of the transition layer of the bonded interface is obtained. The structural change of the interfacial region under different cooling rates is analyzed by means of the radial distribution function (RDF) and the pair analysis techniques. The interface assumes an amorphous structure upon initial mechanical pressing and remains amorphous if the subsequent cooling rate is high. At lower cooling rates, however, the interface structure becomes crystalline. The diffusion-bonded Cu/Al specimen is subjected to tensile loading and its strength is compared with those of single-crystalline Cu and single-crystalline Al of the same size along the same direction. Calculated results indicate that the specimen with the interface has a lower tensile strength and a lower ductility than the corresponding single-crystalline Cu and Al specimens.
Molecular dynamics simulation of fracture in α-iron
Cao Li-Xia, Wang Chong-Yu
2007, 56 (1): 413-422. doi: 10.7498/aps.56.413
Abstract +
The deformation process of the crack in α-Fe has been simulated under uniaxial tensile deformation by molecular dynamics (MD) method. The effect of crystal orientations on the deformation mechanisms of the crack has been investigated. Various deformation evidences are clearly observed, such as dislocation nucleation and emission, dislocation movement,stacking faults formation or twinning, and formation and coalescence of voids. The simulated results indicate that the crack propagate by a combination of plastic and elastic processes in which the plastic portion of the crack results from the shear behavior of atoms created by the dislocations that are emitted from the crack tip, and the elastic process occurs as a result of the bond-breaking of the atoms in the dislocation-free zone. It has also been studied the deformation and fracture characteristics of the crack in α-Fe and their dependence on the stress state and the temperature.
A delayed failure of inhomogenous brittle material under shock wave compression
Li Ming-Fa, He Hong-Liang, Chen Deng-Ping, Jing Fu-Qian
2007, 56 (1): 423-428. doi: 10.7498/aps.56.423
Abstract +
The failure property of inhomogenous brittle materials under plate impact experiment has been studied by using AOW (amphibolized olivine websterite) rock as a representative sample. A small recompression signal was observed in the VISAR record of the rear free surface velocity at shock stress much below σHEL, indicating the generation of the failure wave. By using samples with different thickness but impacted at approximately the same shock stress of 3.9 GPa, the trajectory of the propagation of the failure wave through the stressed sample has been determined. The failure wave is generated with a delay time of about 0.5 μs behind the shock wave front, and propagates into the stressed material with a velocity comparable to that of the shock wave front. A physical mechanism involving the “in situ activation and growth” of the originally existing micro-cracks due to the local shear stress under shock wave compression is suggested for these observations.
Influence of ternary mixed crystal defect layer on the localized interface optical-phonon modes in a finite superlattice
Wang Ling-Ling, Huang Wei-Qing, Tang Li-Ming, Zou Bing-Suo, Chen Ke-Qiu, Wang Xin-Jun
2007, 56 (1): 429-436. doi: 10.7498/aps.56.429
Abstract +
Within the framework of dielectric continuum theory, we study the effect of ternary mixed crystal defect layer on the localized interface optical-phonon modes in a finite superlattice by using a transfer matrix method. In such a finite superlattice, the evolution of all the interface modes can be clearly tracked. Our numerical results show there exist two types of localized modes and their macroscopic electrostatic potentials mostly locate in the vicinity of the defect layer or surface layer, respectively. As the relative widths and nature of the structural cell or the defect layer are altered, the localized characteristics of these modes are significantly influenced. Though the conservation of the total number of localized modes is broken, the conservation of the total number of all interface modes including both the extended modes and localized modes is still kept for every value of the wavevector q‖.
Effect of boundary conditions on phonon transmission in a dielectric quantum waveguide
Tang Li-Ming, Wang Yan, Wang Dan, Wang Ling-Ling
2007, 56 (1): 437-442. doi: 10.7498/aps.56.437
Abstract +
We investigate the effect of boundary conditions on the low temperature phonon transport in a quantum waveguide with structural defects by using the scattering-matrix method. A comparison between transmission coefficients is made when Neumann and Dirichlet boundary conditions are applied for acoustic modes, respectively. The result indicates that the behavior of the transmission coefficient versus reduced phonon frequency is qualitatively different for different boundary conditions. The stop-frequency gap and the transmission resonance are observed for certain structural parameters due to mode-mode coupling.
Molecular dynamics simulation of the pressure effect on the formation of glassy Cu
Zhao Jiu-Zhou, Liu Jun, Zhao Yi, Hu Zhuang-Qi
2007, 56 (1): 443-445. doi: 10.7498/aps.56.443
Abstract +
Molecular dynamics simulation has been used to investigate the effect of pressure on the formation of glassy Cu. The results indicate that a higher pressure leads to a strong crystallization tendency during cooling. The glassy Cu formed under high pressures contains more crystal cluster and has a higher degree of order.
Computation of the Ehrlich-Schwoebel barrier to adatom diffusion in heteroepitaxial systems
Li Jia-Yang, Li Rong-Wu, Sun Jun-Dong, Liu Shao-Jun
2007, 56 (1): 446-451. doi: 10.7498/aps.56.446
Abstract +
The Ehrlich-Schwoebel (ES) barriers to the diffusion of adatoms (Pd,Ag and Cu) on different bench terraces of fcc Cu are compared by molecular dynamics computation. The computation reveals that: 1) For both heteroepitaxial and homoepitaxial systems, the ES barriers increase similarly with the increase of bench terrace height until a certain height is reached. 2) The ES barrier of adatom Pd is the biggest, the second one is that of adatom Cu, and the smallest one is that of adatom Ag for the hopping diffusion mechanism, while the ES barrier of adatom Ag is the biggest, the second one is that of adatom Cu, and the smallest one is that of adatom Pd for the exchange diffusion mechanism. 3)The ES barrier of exchange is lower than that of hopping mechanism in most cases of bench terraces.
Study on asymmetricall epitaxial behavior of Cu-Au system by molecular dynamics simulation
Zhang Chao, Meng Yang, Yan Chao, Tang Xin, Wang Yong-Liang, Zhang Qing-Yu
2007, 56 (1): 452-458. doi: 10.7498/aps.56.452
Abstract +
Molecular dynamics simulation has been used to study the heteroepitaxy of Cu/Au(001) and Au/Cu(001) with the embedded atom potentials. It is found that Cu/Au(001) has a different heteroepitaxial behavior from Au/Cu(001). The epi-islands of Cu grow on Au (001) in the lattice of Au (001) without any defect. For the epitaxy of Au/Cu(001), however, mismatching dislocations can be observed in the island larger than 7×7. The asymmetricall epitaxial behavior can be attributed to the difference in the strain state and elastic properties between Cu and Au islands. With the increase of heteroepitaxial islands, the micro-misfit of Cu/Au(001) decreases monotonically, but the micro-misfit of Au/Cu(001) has an oscillating increase. The Cu epi-islands enable the substrate to be deformed at the edge of the islands while the substrate in Au/Cu(001) is deformed at the region under the islands.
Crystallization of Si3N4 and superhardness effect of ZrN/Si3N4 nano-multilayers
Zhao Wen-Ji, Dong Yun-Shan, Yue Jian-Ling, Li Ge-Yang
2007, 56 (1): 459-464. doi: 10.7498/aps.56.459
Abstract +
The crystallization behavior of Si3N4 modulation layers in the multilayers and its influences on the microstructure and mechanical properties of ZrN/Si3N4 multilayers were studied. ZrN/Si3N4 multilayers with different Si3N4 thickness were synthesized by reactive magnetic sputtering. The microstructure of the multilayers was characterized with X-ray diffraction and high-resolution transmission electron microscopy, and a nanoindentor was introduced to measure their mechanical properties. The results show that when the thickness is less than 0.9 nm, Si3N4, normally amorphous in the deposited state, could form a NaCl-type pseudocrystal structure due to the template effect of ZrN crystal layer. Crystallized Si3N4 layers and ZrN template layers grow epitaxialy into columnar crystals. Correspondingly, the hardness of the films was enhanced, showing a superhardness effect. Further increasing Si3N4 layer thickness, the coherent interfaces of the multilayers were damaged and Si3N4 layers become amorphous, accompanied by the decline in the hardness of the films.
First-principles calculation of Be(0001) thin films: quantum size effect and adsorption of atomic hydrogen
Song Hong-Zhou, Zhang Ping, Zhao Xian-Geng
2007, 56 (1): 465-473. doi: 10.7498/aps.56.465
Abstract +
We have carried out first-principles calculations of Be(0001) thin films to study the oscillatory quantum size effects exhibited in the surface energy, work function, and binding energy of the atomic hydrogen monolayer adsorption. The prominent enhancement of the surface density of states at the Fermi level makes Be(0001) thin films more metallic compared to the crystalline Be. As a result, the calculated energetics of Be films and the properties of atomic H adsorption onto Be(0001) surface are featured by a quantum oscillatory behavior. Furthermore, The prominent change in the Be(0001) surface electronic structure by the atomic hydrogen adsorption has also been shown.
The interstitial oxygen atoms and its effects for optical properties of PWO crystals
Chen Teng, Liu Ting-Yu, Zhang Qi-Ren, Tian Dong-Sheng, Zhang Xiu-Yan, Yi Zhi-Jun
2007, 56 (1): 474-479. doi: 10.7498/aps.56.474
Abstract +
The positions of the interstitial oxygen atoms in PWO crystals are numerically simulated by. It was found that the formation energy of isolated point defect is small when oxygen atom exists around the (WO4)2-. Based on the computed results, the total density of states and partial density of states of perfect PWO crystals and PWO crystals with interstitial oxygen atoms were calculated using DV-X α program. It is concluded that interstitial oxygen atoms will combine with oxygen ions of (WO4)2-, forming oxygenic molecule ions which are related to the 350 nm absorption band.
Growth mechanism of diamond-like carbon film based on the simulation model of atomic motion
Ma Tian-Bao, Hu Yuan-Zhong, Wang Hui
2007, 56 (1): 480-486. doi: 10.7498/aps.56.480
Abstract +
Molecular dynamics simulations are performed to study the growth of diamond-like carbon (DLC) films on the atomic scale. The behaviors of incident atoms on the surface are classified into four categories: surface freezing, migration, implantation and rebounding, according to the atomic movement. The behaviors of incident atoms have an important impact on microstructure and growth mode of DLC film. The transverse migration of the atoms is the main route of film relaxation. The competition between atomic implantation and migration determines the growth mode and final structures of the film. Energy dependence of surface behaviors of deposited atoms and film structures is analyzed statistically, which deepens the understanding of DLC film growth mechanism.
Binding energies of excitons in symmetrical GaAs/Al0.3Ga0.7As double quantum wells
Zhang Hong, Liu Lei, Liu Jian-Jun
2007, 56 (1): 487-490. doi: 10.7498/aps.56.487
Abstract +
The binding energies of excitons in symmetrical GaAs/Al0.3Ga0.7As double quantum wells are calculated variationally by using a simple wave function within the effective-mass approximation. The variation of the binding energies as functions of the well and barrier width is studied. It is found that, the changing tendency of the binding energies with the well width is similar to that of the single quantum well. However, we found that the maximum value of the binding energy occurs at well size of about 10?. The well size is smaller than that in single quantum wells. The binding energies have a minimum during the increase of the barrier width, which is related to the penetration into the barrier of the wave function.
Exact diagonalization solution and coherent state variational method of the coupling system of carrier and ferromagnetic material
Huang Shu-Wen, Liu Tao, Fan Yun-Xia, Wang Ke-Lin
2007, 56 (1): 491-499. doi: 10.7498/aps.56.491
Abstract +
The problem of the coupling system of carrier and ferromagnetic material has been transformed into that of the coupling system of carrier and two kinds of Bosons by using Schwinger's oscillator theory of angular momentum. That the existence of bound polaron in ferromagnetic material depends on the existence of impurity potential in this material is solved exactly by diagonalization and the conclusion obtained is in agreement with the general view point of other works. The values of the energy of ground state obtained by the exact solution and coherent state variation method is agree quite well.The qualitative characters of state vector given by both methods are consistent. The results shows that the coherent state variation method is valid and precise enough for this problem. Exact solution method will provide a basis to check the validity of any other approximate methods.
Interface phonons in the active core of a quantum cascade laser
Xu Gang-Yi, Li Ai-Zhen
2007, 56 (1): 500-506. doi: 10.7498/aps.56.500
Abstract +
We consider the propagation of the interface phonons in the active core of a quantum cascade laser. Based on the translation invariance in the interior of the active core, an implicit dispersion relation is derived. Numerical calculations show that the interface phonons in the active core can be classified as bulk phonons and surface phonons. The dispersion curves of bulk phonons form a series of subbands, and their electrostatics potential is distributed in the whole region of the active core, showing the character of Bloch wave. While the dispersion curves of the surface phonons are located in the gap between the subbands of the bulk phonons, and the electrostatic potential is localized near the interface between the active core and the waveguide layer. Our results are helpful to the design of quantum cascade lasers and other intersubband lasers.
Effects of excited states for d3 ions on spin-Hamiltonian parameters of the ground state 4A2 in axial-symmetrical crystal field
Yang Zi-Yuan, Wang Can-Jun, Xu Qi-Ming, Wei Qun
2007, 56 (1): 507-511. doi: 10.7498/aps.56.507
Abstract +
The mechanism of influence of the spin quartets and doublets of d3 ion on the spin-Hamiltonian (SH) parameters(including zero-field splitting(ZFS) and g-factors)of the ground state 4A2(4F) is discussed in terms of complete diagonalization method(CDM) under trigonal symmetry and tetragonal symmetry. And these microcosmic mechanisms, including SO mechanism, SS mechanism, SOO mechanism, SO-SS-SOO combined mechanism have been analyzed. It is found that (1) the contributions to ZFS parameter from the spin quartets and doublets are all important; (2) the contributions to the g-factors of the ground state from the spin quartets are predominant, whereas the contributions from spin doublets are slight. In addition, it is shown that SS mechanism and SOO mechanism are determined by spin quartets in SH parameters calculation.
Fabrication of MgB2 films via multilayer ex-situ annealing
Yu Zeng-Qiang, Wu Ke, Ma Xiao-Bai, Nie Rui-Juan, Wang Fu-Ren
2007, 56 (1): 512-517. doi: 10.7498/aps.56.512
Abstract +
We fabricated superconducting MgB2 films with Mg/B multilayer precursors followed by ex-situ annealing. Different ways of samples' placement have been tried, and we found that turning over the samples can effectively removed the particles of Mg condensed on the film surface. The Tc of the films was about 35K with a transition width of 0.8K. The surface is quite smooth, having a roughness of 10 nm in an area of 5 μm×5 μm. When the thickness of the film is less than 1000?, the Tc is significantly reduced. With the different configurations of the multilayer precursor, the Tc of a 600?-thick film can be optimized to 32K and the Jc is 2.4×106A/cm2 at 20K.
Study on the role of Nb addition in rapid-quenched NdFeB permanent magnets
Zhang Ran, Liu Ying, Li Jun, Ma Yi-Long, Gao Sheng-Ji, Tu Ming-Jing
2007, 56 (1): 518-521. doi: 10.7498/aps.56.518
Abstract +
The effect of Nb addition in rapid-quenched (Nd,Dy)11.5Fe82.4-nNbnB6.1 (n=0,0.5,1,1.5,2) permanent magnets on the magnetic properties, temperature characteristics and microstructure has been investigated. It is found that Hcj increases with increasing Nb content and the irreversible loss of magnetic flux decreases. Nb addition results in smaller grains and more homogeneous and regular grain structure. It is considered that Nb addition may enhance exchange coupling pining field and reduce internal stray magnetic field. As a result, the irreversible loss of magnetic flux decreases notably and the magnetic properties at higher temperature are improved.
Structure and magnetic properties of Co80Cr20/Ti90Cr10 film prepared by pulsed-laser deposition
Zeng Fan-Hao, Zhang Xiao-Zhong
2007, 56 (1): 522-528. doi: 10.7498/aps.56.522
Abstract +
Ti90Cr10 and Co80Cr20/Ti90Cr10 thin films were prepared by pulsed laser deposition on Si (100) substrate at various temperatures. The relations between crystal structure and substrate temperature were investigated by XRD and the result revealed that as the substrate temperature increased, the film changed from amorphous state to crystal state gradually. Furthermore, the grain size and lattice parameters of Ti90Cr10 film were measured by the XRD spectrum. The surface and cross-sectional morphology of Ti90Cr10 film were characterized by TEM. The hardness, as well as boundary adhesion strength of the Ti90Cr10 film was measured by nano-indenter and the result indicates that increase of the substrate temperature is beneficial to increasing the adhesion of the film. The magnetic properties of Co80Cr20/Ti90Cr10 film were studied by VSM and the coercivity was observed to increase with decreasing film thickness. For the Co80Cr20(8nm)/Ti90Cr10(14 nm) film prepared at substrate temperature of 600℃ in vacuum,its coercivity is 65.25 kA/m and the squareness is about 0.86. Finally, the magnetization properties of the Co80Cr20/Ti90Cr10 are also discussed.
Enhanciement of exchange bias with diluted antiferromagnets in FM/AFM bilayers
Ma Mei, Cai Lei, Wang Xing-Fu, Hu Jing-Guo
2007, 56 (1): 529-534. doi: 10.7498/aps.56.529
Abstract +
Based on Monte Carlo method, the characteristic of exchange bias is discussed in diluted antiferromagnets in FM/AFM bilayers by changing the dilute concentration. The simulation results show that the exchange bias in FM/AFM bilayers can be enhanced significantly by introducing nonmagnetic atoms into the AFM layers. And there is a maximum in the diagram of exchange bias as a function of the dilute concentration, i.e., the exchange bias at first increases then decreases with increasing concentration of dilution at the same temperature. Besides, the maximum of the exchange bias occurs at lower concentration for higher temperature. These are consistent with the results of Phys. Rev. Lett. 96 117204 (2006). The cause of these results is that the spin distribution and the magnetic domain structure change greatly when varying dilute concentration. The exchange bias is maximal when the positive and the negative magnetic domains form a connected network. In addition, at the same concentration of dilution, by comparing the results of the random and regular dilution, we find that the exchange bias are more strongly increased by regular dilution than by random dilution for the FM/AFM bilayers, which shows further that the characteristic of exchange bias in FM/AFM bilayers depends on the interface spin-microstructure.
Magetostriction, spin reorientation and M?ssbauer effect studies of Tb0.3Dy0.7-xPrx(Fe0.9Al0.1)1.95 alloys
Zheng Xiao-Ping, Li Fa-Shen, Fan Duo-Wang, Zhang Pei-Feng, Hao Yuan
2007, 56 (1): 535-540. doi: 10.7498/aps.56.535
Abstract +
The effect of Pr substitution for Dy on the structure, magnetostriction, anisotropy and spin reorientation of a series of Tb0.3Dy0.7-xPrx(Fe0.9Al0.1)1.95 alloys (x=0, 0.1, 0.20, 0.25, 0.30, 0.35) at room temperature has been investigated. It was found that the Tb0.3Dy0.7-xPrx(Fe0.9Al0.1)1.95 substantially retained the MgCu2-type C-15 cubic Laves phase structure for compositions with x≤0.1 and the lattice constant increases slowly with increasing x. The magnetostriction of the Tb0.3Dy0.7-xPrx(Fe0.9Al0.1)1.95 alloys decreases drastically with increasing x and the magnetostrictive effect disappears for x>0.2. However, the magnetostriction shows a slightly bigger value at x=0.1 than the free alloys and is saturated more easily with the magnetic field H, showing that a small amount of Pr substitution is beneficial to the decrease in the magnetocrystalline anisotropy. The spontaneous magnetostriction λ111 increases linearly with incresasing x, whereas the spin reorientation temperature increases first and then decreases rapidly. The analysis of the Mssbauer spectra indicated that the easy magnetization direction in the {110} plane deviates slightly from the main axis of symmetry with the increase in Pr concentration x, namely the spin reorientation. Compared with the Al substitution,the effect of Pr substitution for Dy on the spin reorientation is smaller.
Controlling the photonic bandgap structures via manipulation of refractive index of electromagnetically induced transparency vapor
Zhuang Fei, Ye Jun, Shen Jian-Qi
2007, 56 (1): 541-545. doi: 10.7498/aps.56.541
Abstract +
A new application of coherent manipulation of EIT (electromagnetically induced transparency) medium is suggested. A two-dimensional photonic cystal with a triangle lattice of elliptic air cylinders, which are filled with the EIT atomic vapor, in the KAs backgroud material is designed. By manipulating the refractive index of the EIT vapor via the external control field, one can obtain the controllable bandgap structure that is sensitive to the variations of external conditions (e.g. the intensity of the control field). It is shown that a large complete bandgap of about 0.0503ωe in the region of high normalized frequency could be achieved by choosing the specific parameters such as the spontaneous decay rate, dephasing rate, Rabi frequency of control light, atomic number density as well as elliptic geometric parameters.
Luminesce properties of the single white emitting phosphor Eu2+, Mn2+ co-doped Ca2SiO3Cl2
Wang Li-Wei, Yang Zhi-Ping, Xiong Zhi-Jun, Xu Xiao-Ling, Liu Yu-Feng, Yu Quan-Mao
2007, 56 (1): 546-550. doi: 10.7498/aps.56.546
Abstract +
The intense white emitting phosphors Ca2SiO3Cl2:Eu2+, Mn2+ were synthesized by traditional high temperature solid state reaction,and their luminesce properties were studied. The results indicate that the emission band consists of three peaks located at 425 nm,498 nm and 578 nm, respectively. The emission peaks at 425 and 498 nm originate from the transition 5d→4f of Eu2+ ions that occupy the two Ca2+ sites in the crystal of Ca2SiO3Cl2, while the 578 nm emission is attributed to the energy transfer from Eu2+ ions to Mn2+ ions. The white light can be obtained by mixing the three emission colors of blue (425 nm), green(498 nm) and red(578 nm) in the single host. The excitation spectra of the three emission colors all extend from 250 nm to 420 nm, and their peaks positioned around 370 nm, 385 nm, 396 nm and 412 nm, respectively. The phosphor can be excited effectively by InGaN chip in the range of 350—410 nm. The experiment results show that the Ca2SiO3Cl2: xEu2+, yMn2+ is a single host phosphor with superior properties for use in white light emitting diode (w-LED).
Effects of annealing on the photoluminescence of He ion implanted sapphire after 230 MeV Pb ion irradiation
Song Yin, Wang Zhi-Guang, Wei Kong-Fang, Zhang Chong-Hong, Liu Chun-Bao, Zang Hang, Zhou Li-Hong
2007, 56 (1): 551-555. doi: 10.7498/aps.56.551
Abstract +
Single crystal sapphire (Al2O3) samples implanted with 110 keV He and irradiated at 320 K by 208Pb27+ ions with energy of 1.1 MeV/u to the fluences ranging from 1×1012 to 5×1014 ion/cm2 and subsequently annealed at 600, 900 and 1100 K. The obtained PL spectra showed that emission peaks centred at 375, 390, 413, and 450 nm appeared in irradiated samples. The peak of 390 nm became very intense after 600 K annealing. The peak of 390 nm weakened and 510 nm peak started to build up at 900 K annealing, the peak of 390 nm vanished and 510 nm peak increased with the annealing temperature rising to 1100 K. Infrared spectra showed a broadening of the absorption band between 460 cm-1, and 510 cm-1 indicating strongly damaged regions being formed in the Al2O3 samples and position shift of the absorption band at 1000—1300 cm-1 towards higher wavenumber after Pb irradiation.
Effect of annealing temperature on luminescence of nanocrystal ZrO2: Eu3+
Liu Huang-Qing, Wang Ling-Ling, Zou Bing-Suo
2007, 56 (1): 556-560. doi: 10.7498/aps.56.556
Abstract +
Effect of annealing temperature on luminescence of Eu3+ ions was studied in nanocrystal zirconia prepared by co-precipitation. The results reveal that the crystal phase and shape of the samples the tetragonal crystal phase is stable and the content of O2- ions change with annealing temperature increasing. The emission spectra show strong emission at 595 nm and 604 nm. Under UV (394 nm) irradiation the 604 nm emission intensity change from sample to sample. It is related with the difference in surface defects in the surface of the nanocrystalline samples doped with different Eu3+ ion concentrations. In addition, the annealing temperature affects the charge transfer of the samples. The result reveals the change of CTB is also relative to the surface defects of the samples.
The variation of X-ray absorption spectra intensity caused by external magnetic field in drain current mode
Guo Yu-Xian, Wang Jie, Li Hong-Hong, Xu Peng-Shou, Wang Feng, Yan Wen-Sheng
2007, 56 (1): 561-568. doi: 10.7498/aps.56.561
Abstract +
In the measurement of X-ray magnetic circular dichroism (XMCD) spectra, external magnetic field is required during experiment. We have found that, if the X-ray absorption spectra are recorded using drain current, the spectrum intensity is strongly influenced by external magnetic field. The normalized signal intensity changes with the value of the deviation angle between magnetic field and sample surface, the amount of the external magnetic field and the relative position of beam spot on sample surface. In this paper, the K edge absorption spectra of oxygen contained in aluminum foil are measured in different magnetic field configurations. The variation trends of signal intensity with the change in various external parameters are indicated, and reasonable explanations have been given. The experimental data are well accordance with their corresponding fitting curves, suggesting that the model constructed in this paper is applicable for solving this problem. The article provides a basis for analyzing the effects of external field on drain current signal.
Mechanism and characteristics of the new organic nonlinear optical material urea L-malic acid film
Zhu Li, Chen Ding-An, Cui Yi-Ping, Xu Ling-Ling, Yang Wen-Ge, Wang Wen
2007, 56 (1): 569-573. doi: 10.7498/aps.56.569
Abstract +
A new nonlinear optical material urea L-malic acid film (ULMA) was successfully prepared by physical vapour deposition (PVD) at appropriate heating temperature. XRD and FTIR measurements show that the film and the ULMA powder have the same composition and crystal structure. Furthermore, the film keeps the nonlinear properties of the ULMA crystal. Thermal analysis shows that ULMA has good thermal stability with constant melting point. Its evaporation temperature is below the decomposition temperature of its substrates urea and L-malic acid. The difference between evaporation and decomposition temperature is increased in vacuum. Therefore, it is verified that the PVD method is a practical approach to prepare ULMA film if the heating temperature is controlled between the evaporation and decomposition temperatures.
Influence of selenization on the surface morphology and phase structure of CuInSe2 thin films
Li Jian, Zhu Jie
2007, 56 (1): 574-582. doi: 10.7498/aps.56.574
Abstract +
Co-sputtered Cu-In precursors were used as the backing materials, and selenium powder was employed as the raw materials. Five kinds of representative or innovative selenization schemes, including single selenium source, double selenium source, powder spraying, selenizing and annealing step by step or simultaneously, were applied to grow CuInSe2(CIS) thin films. The influence of selenium source location and temperature rising methods on the property of CIS films was evaluated. The similarities and differences in the morphology, composition and phase structure among different methods were compared. The relationship between selenization temperature, anneal temperature, anneal time and the composition of CIS thin films was researched. The rules how element percent alters with the selenization and anneal condition are established, which can provide a reference to better control the composition and phase structure of CIS thin films.
High-sensitivity fast neutron radiography system
Yang Jian-Lun, Li Zheng-Hong, Zhong Yao-Hua, Qin Yi, Chen Fa-Xin, Ying Chun-Tong, Liu Guang-Jun, Zhang Fa-Qiang, Ye Fan
2007, 56 (1): 583-588. doi: 10.7498/aps.56.583
Abstract +
Introduced in this paper is a fast neutron radiography system, which is mainly composed of a scintillation fiber array and a scientific grade optical CCD. The 14 MeV neutrons produced by deuterium-tritium fusion reactions, penetrate through objects and fall into a 50 mm×50 mm scintillation fiber array, and the neutron radiation is converted to green light (496 nm). The array has a length of 100 mm, consisting of 100×100 square scintillation fibers each with a cross-section of 500 μm×500 μm. For 14 MeV neutrons, the detection efficiency is as high as 21.4%. To avoid direct radiation from neutrons, the CCD is coupled to the array by a mirror and lens optics. By taking into account the factors affecting image quality, the resolution is estimated to be 1.5 mm. Several neutron radiographs have been obtained successfully in experiments carried out on K400 neutron generator.
Nonlinear noise reduction for the observation data of climatology based on the searching average method
Feng Guo-Lin, Lian Yi, Hou Wei
2007, 56 (1): 589-596. doi: 10.7498/aps.56.589
Abstract +
The searching average nonlinear noise reduction method, which is based on local linear fit to the nonlinear dynamics, is introduced to reduce the noise in the observation data of climatology. Recurrence plots are used to estimate the size of local neighbors. The noise reduction is improved markedly. In order to show the validity of the program in noise reduction, it is first applied to a noise time series of Henon map contaminated by Gaussian white noise. And then, this noise reduction scheme is applied separately to the observation data of meteorology. The analyses of a nonlinear prediction demonstrate the efficiency of the method for noise reduction.