Vol. 57, No. 10 (2008)

Error estimates of element-free Galerkin method for potential problems
Cheng Yu-Min, Cheng Rong-Jun
2008, 57 (10): 6037-6046. doi: 10.7498/aps.57.6037
Abstract +
The error estimates for moving least-square approximation, which is the method for obtaining the shape function in element-free Galerkin method, are presented in Sobolev space Wk,p(Ω) for high dimensional problems. Then on the basis of element-free Galerkin method for potential problems, the error estimates for element-free Galerkin method for potential problems, in which the essential boundary conditions are enforced by penalty methods, are obtained. The error estimates we present in this paper have optimal order when the nodes and shape functions satisfy certain conditions. From the error analysis, it is shown that the error bound of the potential problem is directly related to the radii of the weight functions. Two numerical examples are also given to verify the conclusions in this paper.
Complex variable reproducing kernel particle method for transient heat conduction problems
Cheng Yu-Min, Chen Li
2008, 57 (10): 6047-6055. doi: 10.7498/aps.57.6047
Abstract +
On the basis of reproducing kernel particle method(RKPM), the complex variable reproducing kernel particle method (CVRKPM) is discussed. The advantage of the CVRKPM is that the correction function of a 2-D problem is formed with 1-D basis function when the shape function is obtained. Then, we apply the complex variable method to two-dimensional transient heat conduction problems. In combination with the Galerkin weak form of transient heat conduction problems, the penalty method is employed to enforce the essential boundary conditions, the CVRKPM for transient heat conduction problems is investigated and the corresponding formulae are obtained. Compared with the conventional RKPM, the CVRKPM introduced in this paper has a higher precision and a lower computation cost. Some examples given in this paper verify the effectivity of the proposed method.
The Mei symmetry of discrete difference sequence mechanical system with variable mass
Huang Xiao-Hong, Zhang Xiao-Bo, Shi Shen-Yang
2008, 57 (10): 6056-6062. doi: 10.7498/aps.57.6056
Abstract +
The Mei symmetry and conserved quantity of a discrete difference sequence mechanical system with variable mass are studied in this paper. The form invariance of difference sequence equations for the discrete system under infinitesimal transformation groups is defined as Mei symmetry and the criterion when conserved quantities may be obtained from Mei symmetries is also presented. An example is given to demonstrate the applications of the results.
The relation between force-law and radius in bubble raft
Guo Ping-Bo, Wang Shao-Feng, Wu Xiao-Zhi, Zhang Fu-Zhou, Ye Jin-Qin, Wang Rui
2008, 57 (10): 6063-6067. doi: 10.7498/aps.57.6063
Abstract +
Based on the interaction acting among the bubbles, the relation between the restoring force and the relative displacement is calculated explicitly for the bubble raft. The results indicate that the force-law of the bubble raft strongly depends on the radius. A new form of force-law is proposed to describe the dependence on the bubble radius.
The interaction of ion-acoustic solitary waves in a two-dimensional hot ion plasma
Han Jiu-Ning, Wang Cang-Long, Li Sheng-Chang, Duan Wen-Shan
2008, 57 (10): 6068-6073. doi: 10.7498/aps.57.6068
Abstract +
Using the extended Poincar-Lighthill-Kuo perturbation method, we study the interaction between two ion-acoustic solitary waves with different propagation directions in a two-dimensional hot ion plasma, and obtain two KdV equations for solitary waves in both ξ and η directions. We also obtain the phase shifts and trajectories after the collision of two solitary waves with an arbitrary angle. The effects of the ratio of ion temperatures σ, the ratio of heat capacities γ and the angle α on the phase shifts are studied. The results suggest that these parameters can significantly influence the phase shift of the solitary waves. Moreover, there are compressive solitary waves in such a system.
The equal probability comparison method of determining the equivalent-electron Young basis
Hu Kun-Ming
2008, 57 (10): 6074-6080. doi: 10.7498/aps.57.6074
Abstract +
The concepts of the basic symmetric operator and complete symmetric operator of the equivalent-electron regular Young tableau T[λ]ig are presented, and the concepts of the root state and generative state generated by these symmetric operators acting on each Slater function i are also given. Based on the establishment rules of the vertical permutation operator A[λ]ig of the orthogonal normalization Young tableau T[λ]ie, the symmetric operators in A[λ]ie and the equiprobability comparison method for solving T[λ]ie are presented, which can avoid the complicated algebra involving many operators. Finally, a new method for sloving the Young basis of the electron system with a large N is presented.
Time-delayed Logistic system driven by correlated Gaussian white noises
Guo Yong-Feng, Xu Wei
2008, 57 (10): 6081-6085. doi: 10.7498/aps.57.6081
Abstract +
Time-delayed Logistic system driven by correlated Gaussian white noises could be a more true reflection of the growth of tumor cell population. In this paper, we study the Logistic system through the small time delay approximation to obtain the probability density of the system in its steady state, and further analyze the influences of the additive and multiplicative noise intensity, the noise-correlated time and the time delay on the steady-state probability density. The non-equilibrium phase transition phenomena induced by the noises is also studied.
The existence of two types of generalized synchronization of nonlinear systems
Xu Zhen-Yuan, Guo Liu-Xiao
2008, 57 (10): 6086-6092. doi: 10.7498/aps.57.6086
Abstract +
The existence of two types of generalized synchronization of chaotic nonlinear systems is studied. When the modified system collapses to a stable equilibrium or periodic oscillation, the existence of generalized synchronization can be converted to the problem of compression fixed point under certain conditions. Strict theoretical proofs are given to the exponential attractive property of generalized synchronization manifold. Numerical simulations illustrate the correctness of the present theory.
Generalized projective synchronization of chaotic system based on a single driving variable and its application in secure communication
Li Nong, Li Jian-Fen
2008, 57 (10): 6093-6098. doi: 10.7498/aps.57.6093
Abstract +
A generalized projective synchronization method is proposed based on unidirectional coupled synchronization by only transmitting a single driving variable. The proposed method allows us to arbitrarily direct the scaling factor to a desired value. Since chaos synchronization can be achieved by transmitting a single variable from driving system to response system, this method is more practical. Theoretical deduction and computer simulation show the effectiveness of the proposed method. A scheme for secure communication, which can increase the security of a communication system, is presented based on generalized projective synchronization of unified chaotic system.
Lag synchronization of autocatalytic chemical reaction in continuous stirred tank reactors
Li Yong, Bi Qin-Sheng
2008, 57 (10): 6099-6102. doi: 10.7498/aps.57.6099
Abstract +
The problem of lag synchronization for chemical reaction system composed of two continuous stirred tank reactors is proposed. Based on Lyapunov stability theory, the controller which can make the reaction reach the lag synchronization state is obtained by suitably selecting the output and the synchronous signals. Numerical simulations are performed to demonstrate the effectiveness of the proposed scheme. Finally it is pointed out that there exist two different time scales in the error system of lag synchronization.
Study of attractor based on tangent-delay for elliptic reflecting cavity
Tan Si-Ting, He Yi, Sheng Li-Yuan
2008, 57 (10): 6103-6111. doi: 10.7498/aps.57.6103
Abstract +
By introducing the conversion function on ellipse angle, we use the method of shift matrix to achieve the simplification of ellipse problem. A very simple iterative formula is deduced on the tangent-delay for elliptic reflection, which is very useful for theoretical analysis. There exits a chaotic attractor when tangent delays one unit in TD-ERCS. The origin of the attractor and its stability are analyzed in theory. We find that the attractors in the circular and the elliptic cases are not entirely the same; the ellipse has two immobile lines, but only one of them is steady. We also find that, with the decrease of ellipse compression factor μ, the correlation of nearby iterative data is strengthened when the tangent-delay factor m is arbitrary. It means that in using the system for cryptography, the ellipse compression factor μ can not be too small, and the chaos system requires that it should not be too big, otherwise the degree of safety will be reduced.
Symbolic time series characterization and block entropy analysis of DC-DC converters
Zhang Bo, Qiu Dong-Yuan, Chen Liang-Gang, Wang Xue-Mei
2008, 57 (10): 6112-6119. doi: 10.7498/aps.57.6112
Abstract +
A method based on symbolic time series and entropy theory is proposed to analyse the nonlinear behaviours of DC-DC converters. Firstly, the nonlinear continuous system is described by a discrete time series, which is then transferred to a symbol series composed of simple characters; and the series' block entropy is calculated by means of informatics methodology; consequently, a new quantifiable statistical index is obtained. This study takes a one-order voltage feedback DCM and a two-order current feedback CCM Boost converter as examples, and the results illustrate that the coarse-grained statistical method of block entropy, which can quantify the period-doubling and chaos behaviours in DC-DC converters and precisely confirm the appearance of chaos, is a simple and practical analysis method which has not been used in DC-DC converters yet.
Prediction of the chaotic time series from parameter-varying systems using artificial neural networks
Wang Yong-Sheng, Sun Jin, Wang Chang-Jin, Fan Hong-Da
2008, 57 (10): 6120-6131. doi: 10.7498/aps.57.6120
Abstract +
Prediction of the chaotic time series generated by the complex parameter-varying systems is researched in this paper. The parameter-varying Logistic system is constructed firstly, and the properties of this kind of system are analyzed. These systems, whose parameter values change with time, do not have attractor shape invariable with time evolution because of their continually changing dynamical property. Combining the Takens' embedding theorem and the artificial neural networks (ANN) theory, we interprete the feasible reason that ANN method can be used to predict the chaos systems with the invariable attractor shape, and then discuss the potential problem that will be met when using ANN to predict the parameter-varying system. Experiments of forecasting the chaotic time series from parameter-varying Ikeda system using neural networks have been performed. The previous theoretical analyses are validated by the experiment results. The results also show that if only simply increasing the training data, the neural networks' predicting generalization ability may be reduced, the generalized predicting result on the parameter-varying system is especially seriously affected by the selected training data set. So prediction of the parameter-varying systems must be well resolved before the chaotic time series prediction can be made practical.
Stream cipher design based on inter-perturbations of chaotic systems
Xiang Fei, Qiu Shui-Sheng
2008, 57 (10): 6132-6138. doi: 10.7498/aps.57.6132
Abstract +
A new stream cipher design scheme is proposed, which uses the sequences generated by two chaotic systems to inter-perturb their sequence values and the control parameter, so as to obtain a new sequence as key stream. Inter-perturbed sequence and Logistic sequence are tested by several tests of NIST's (National Institute of Standards and Technology) STS randomness test suite. Simulation results prove that the cryptographical properties of inter-perturbation sequence are better than those of Logistic sequence. k-error approximate entropy is proposed to test the stability of inter-perturbed sequence and Logistic sequence. Simulations indicate that the stability of inter-perturbed sequence is better than that of Logistic sequence. An image is encrypted and decrypted by the inter-perturbed sequence. Simulation results show that the inter-perturbation sequence can cover up plaintext effectively and safely.
Prediction of chaotic time series based on EMD method
Yang Yong-Feng, Ren Xing-Min, Qin Wei-Yang, Wu Ya-Feng, Zhi Xi-Zhe
2008, 57 (10): 6139-6144. doi: 10.7498/aps.57.6139
Abstract +
In order to improve the nonlinear response prediction precision in a long period, the empirical mode decomposition (EMD) method is introduced in the nonlinear prediction. Here, the EMD method is used to decompose the signal, the rosenstein method is used to calculate the largest Lyapunov exponent (LLE), and then the prediction results are obtained on the basis of the LLE. The simulation results of Duffing equation, Lorenz system and cracked rotor system show that the EMD's signals have smaller LLE than the original signal. In this way, the maximum prediction time of a nonlinear signal can be obtained.
Multi-scale recurrence quantification analysis of the dynamic characteristics of two phase flow pattern
Jin Ning-De, Zong Yan-Bo, Wang Zhen-Ya, Dong Fang
2008, 57 (10): 6145-6154. doi: 10.7498/aps.57.6145
Abstract +
Based on the conductance fluctuating signals of the gas/liquid two phase flow measured in a vertical upward pipe, the multi_scale recurrence quantification analysis method is used to study the dynamic characteristics of gas/liquid two phase typical flow patterns. The results show that the low frequency bubble and churn flow patterns take on more developed line texture structure along the diagonal direction of the chaotic recurrence plot, which indicates that the low frequency bubble and churn flow have deterministic motion behavior. However, with increasing motion frequency of bubble and churn flow, the chaotic recurrence characteristics become worse and the motion tends to random gradually. For the slug flow, the intermittent rectangular massive texture appears on chaotic recurrence plot, which indicates that the intermittent motion characteristics of the liquid plug and gas plug appear in the high frequency band, whereas bubble flow motion in slug flow pattern appears in the low frequency band, and with increasing motion frequency of bubble flow, the bubble flow loses the deterministic behavior gradually. It is shown that multi-scale and nonlinear analysis based on conductance fluctuating signals is an efficient approach to understanding and characterizing the dynamic characteristics of gas/liquid two phase flow patterns.
Study of chaos based on single electron device
Kang Qiang, Feng Chao-Wen, Cai Li
2008, 57 (10): 6155-6161. doi: 10.7498/aps.57.6155
Abstract +
Chua′s circuit in structure of cellular neural networks is realized by the hybrid device of single electron transistor and metal oxide semiconductor, named SETMOS. Then single scroll and double scrolls are obtained. SETMOS transconductance amplifier and SETMOS voltage comparator are designed, and the double-scroll-like chaotic circuit built of SETMOS is proposed. The double-scroll-like chaos attractor is verified by simulation. All the simulation results show that the designed circuits have the characteristics of simple structure and low power dissipation, and they can further improve the density of integrated circuits. It also provides a new method for the chaos to be used in engineering.
Anti-control of Hopf bifurcation at zero equilibrium of 4D Qi system
Tang Jia-Shi, Liu Su-Hua
2008, 57 (10): 6162-6168. doi: 10.7498/aps.57.6162
Abstract +
Anti-control of Hopf bifurcation at the zero equilibrium of the 4D Qi system is achieved by the linear and nonlinear state feedback. First, aimed at the emergence of the Hopf bifurcation, linear control terms are determined and the conditions which the linear control gains should satisfy are presented. Then, with respect to the nonlinear control terms, which are associated with the stability of Hopf bifurcation, the nonlinear feedback applied directly to the Jordan form of the Qi system is investigated. By the stability analysis and a proper transformation, the nonlinear control terms which play roles on the stability of Hopf bifurcation are collected and the condition restricting the nonlinear control gain isobtained. The control strategy is different as the bifurcation parameter concerned is altered. Finally, the numerical results are presented that confirm the analytical predictions.
Response analysis of two-degree-of-freedom impact oscillator to random excitation
Wang Liang, Xu Wei, Li Ying
2008, 57 (10): 6169-6173. doi: 10.7498/aps.57.6169
Abstract +
The response of a two-degree-of-freedom impact oscillator to random excitation is investigated. The existence of grazing bifurcation involved in the period-doubling bifurcation is shown clearly. The effects of the random excitation are studied by defining a measure of the stochastic response. The idea that the random factor can change the property of the system in some conditions is proposed. Numerical simulations show that the method is effective.
Low-frequency oscillation in digitally controlled DC-DC Buck converters
Zhang Xiao-Tian, Ma Xi-Kui, Zhang Hao
2008, 57 (10): 6174-6181. doi: 10.7498/aps.57.6174
Abstract +
The z-domain model of digitally controlled Buck converter is derived by only considering the effect of sampler-and-holder. Based on the z-domain model, the cause of the low-frequency bifurcation is analyzed. The location and type of the bifurcation point when a system loses stability is predicted in terms of the system eigenvalues. Moreover, the frequency and the amplitude of the low-frequency oscillation, i.e., the wave equations, are deduced. These theoretical results are verified by PSpice simulations.
Two codimension-3 bifurcations and non-typical routes to chaos of a shaker system
Yu Jian-Ning, Zhang Yong-Xiang, Kong Gui-Qin
2008, 57 (10): 6182-6187. doi: 10.7498/aps.57.6182
Abstract +
The dynamical model and Poincaré maps of a shaker are established. Two types of codimension-3 bifurcations of this system, including Flip-Hopf-Hopf bifurcation and Hopf-Hopf bifurcation in the third order strong resonant case, and three non-typical routes to chaos are investigated by using Poincaré maps. The system exhibits more complicated dynamic behaviors near the points of codimension-3 bifurcation. The results show that near the points of bifurcation there existtriangle attractor, 3T2 torus bifurcation and “pentalpha-like”, “tire-like” attractors in projected Poincaré sections. The routes to chaos via torus explosion, torus-doubling bifurcation and T2 torus bifurcation are analyzed by numerical simulation. The system parameters of shaker may be optimized by studying the stability and bifurcation of periodic motion of the shaker.
Encryption based on inverse iterating composition cellular automata system
Ping Ping, Zhao Xue-Long, Zhang Hong, Liu Feng-Yu
2008, 57 (10): 6188-6195. doi: 10.7498/aps.57.6188
Abstract +
The concepts of cross composition cellular automata and random composition cellular automata are introduced, and their feasibility in application to cryptosystem is analyzed. We use the inverse iteration of cellular automata to encrypt, and construct two encryption systems based on composition cellular automata. The new encryption systems effectively solve the problem of one way error diffusion in a single cellular automata system and acquire large key space with small rule radius. Simulation experiment shows that the diffusion and confusion properties of the new composition system are ideal, it resists brute attack and differential cryptanalysis.
Nuclear reaction stopping of 400MeV/nucleon 56Fe in water
Cheng Jin-Xing, Fang Mei-Hua, Wei Zhi-Yong, Yang Hao
2008, 57 (10): 6196-6201. doi: 10.7498/aps.57.6196
Abstract +
Geant4 toolkit for Monte Carlo simulation is used in high energy heavy ion energy deposition simulation. The energy deposit of relativistic 400MeV/nucleon 56Fe has contributions from three sources, namely electronic stopping, nuclear stopping and nuclear reaction stopping. The electronic stopping and nuclear stopping are well known ionization, excitation and elastic collision of particles, the nuclear reaction stopping comes from the contribution of secondary particles emitted from nuclei during nuclear reaction. Yields of secondary particlesinduced by relativistic 56Fe in water and the energy deposited near the track are calculated.
Error mechanism of light source for synchrotron radiation computed tomography technique
Wang Min, Cen Yu-Wan, Yu Xiao-Liu, Hu Xiao-Fang, Zhu Pei-Ping
2008, 57 (10): 6202-6206. doi: 10.7498/aps.57.6202
Abstract +
Based on the unique advantages of synchrotron radiation X-ray with high resolution and collimation,and the advantage of nondestructive observation of computed-tomography technique,synchrotron radiation computed tomography technique is widely used in many fields. In this paper,the errors caused by light source such as unevenness,supersaturation and super penetration are studied for synchrotron radiation computed tomography technique,and its three basic error forms are given. Numerical simulation results confirm this analysis.
Molecular structure and electronic spectrum of styrene under the external electric field
Luo Wen-Lang, Zhang Li, Zhu Zheng-He, Ruan Wen
2008, 57 (10): 6207-6212. doi: 10.7498/aps.57.6207
Abstract +
The ground states of styrene under different intense electric fields ranging from 0 to 0.05a.u. are optimized using density functional theory DFT/B3P86 at 6-311G basis set level. The excitation energies and oscillator strengths under the applied electric fields are calculated employing the revised hybrid CIS-DFT method. The results show that the electronic state,molecular geometry,total energy,dipole moment,and excitation energy are strongly dependent on the applied electric field. As the electric field changes from 0 to 0.02a.u.,many of the bond lengths of C—H increase,while some of the bond lengths of C—H decrease. However,further increase of the electric field strength results in a increase of them both due to the charge transfer induced by the applied electric field. As to the bond lengths of C—C,some of them increase,some of them decrease and some of them remain unchanged as the electric field changes from 0 to 0.05a.u.. The dipole moment of the ground state increases sharply with the applied field strength. With the electric field increasing,the total energy of the molecule decreases. The excitation energies of the first six excited states of styrene decrease with the increase of the applied electric field,indicating that the molecule is easy to be excited and dissociated under the electric field.
Resonance characteristics of a three-dimensional anisotropic metamaterial bilayer
Meng Fan-Yi, Wu Qun, Fu Jia-Hui, Gu Xue-Mai, Li Le-Wei
2008, 57 (10): 6213-6220. doi: 10.7498/aps.57.6213
Abstract +
The resonance characteristics of a three-dimensional anisotropic metamaterial bilayer is investigated through solving the resonance equation for a rectangular cavity filled with two layers of different isotropic metamaterials. It is found that the solution of subwavelength resonance is highly dependent on the product of the transversal and longitudinal permeabilities,and there exist more than one for the subwavelength resonance in most cases. In addition,it is found that the width of the corresponding waveguide of the rectangular cavity has an essential effect on the number of solutions for the resonance condition equation. These results are of significance in the application of the metamaterials to sub-wavelength resonance structures and miniaturized cavity resonators.
Influence of parameter errors on the temperature measurement of cold atoms via short-distance time-of-flight absorption spectra
He Jun, Wang Jing, Qiu Ying, Wang Yan-Hua, Zhang Tian-Cai, Wang Jun-Min
2008, 57 (10): 6221-6226. doi: 10.7498/aps.57.6221
Abstract +
The basic model and the experimental measurement method of short-distance time-of-flight absorption spectra for determining the temperature of cold atoms are described. Based on the realization of temperature measurement of cold cesium atoms in a magneto-optical trap,the influences of measurement errors of the three experimental parameters,namely the falling distance h,the initial Gaussian radius σ0 of the cold cloud, and the Gaussian radius σp of probe beam, on the determination of temperature have been analyzed and compared.
Loading and cooling of cesium atoms in 3D optical lattice
Qiu Ying, He Jun, Wang Yan-Hua, Wang Jing, Zhang Tian-Cai, Wang Jun-Min
2008, 57 (10): 6227-6232. doi: 10.7498/aps.57.6227
Abstract +
A four-beam 3D near-resonant optical lattice system is implemented and cold atoms are loaded into the optical lattice based on cesium magneto-optical trap and optical molasses. The dependence of the final temperature due to the Sisyphus cooling on the intensity and the frequency detuning of optical lattice are experiemently investigated,and the lifetime of the cold atoms in optical lattice are measured via the short-distance time_of_flight absorption spectra.
2D array of surface optical micro-traps by evanescent wave interference
Yin Jian-Ping, Cao Guo-Rong, Wang Zheng-Ling
2008, 57 (10): 6233-6239. doi: 10.7498/aps.57.6233
Abstract +
We propose a novel scheme to form a 2D surface array of optical micro-traps of cold atoms by using two sets of far red-detuned evanescent wave interference and a blue-detuned evanescent wave. The optical potentials of the 2D surface array of optical micro-traps are high enough to trap cold atoms released from the standard magneto-optical trap,and 87Rb atoms trapped in the array of optical micro-traps can be cooled to 2.56μK with efficient intensity-gradient Sisyphus cooling. Our study shows that the proposed 2D surface array of optical micro-traps can be used in atomic physics,atomic optics, quantum optics and so on.
The interaction of HD+2,H+3 and D+3 with solid foils and the three-body wake effect
Miao Jing-Wei, Yuan Xue-Dong, Shi Mian-Gong, Zhu Zhou-Sen, Liao Xue-Hua
2008, 57 (10): 6240-6248. doi: 10.7498/aps.57.6240
Abstract +
The mechanism to form HD+2,H+3 and D+3 from RF ion source is analyzed by experimental study of the interaction of these ions with solid foils. The structural pattern of HD+2 and the mean internuclear separations are determined using the Coulomb explosion technique. In our experiment,there is no line-shaped structure for HD+2. A theoretical model to deal with the wake effect of three-body Coulomb explosion is proposed and applied to the interaction of HD+2,H+3 and D+3 with solid foils. The model is proved to be successful and the structure of HD+2 is confirmed.
Fully relativistic distorted-wave calculations for electron impact excitation of highly chargednickel-like ions
Xie Lu-You, Zhang Zhi-Yuan, Jiang Jun, Dong Chen-Zhong
2008, 57 (10): 6249-6258. doi: 10.7498/aps.57.6249
Abstract +
A fully relativistic distorted-wave method and the corresponding procedure REIE06 have been developed recently to study the electron impact excitation processes of highly charged ions. In this study,we calculated the collision strengths,cross sections and rate coefficients for electron impact excitation of highly charged Nickel-like Gd36+,Rn58+—U64+(Z=86—92)ions from the ground state to 106 low-lying excitated states. In the calculations,the relativistic effects,electron correlation effects,Breit interactions and high partial wave contributions are considered systematically. Based on the calculations,we studied the contribution of Breit interaction to collision strengths,the cross sections connected with the strongest 4d-4p (J=0—1) X-ray laser transitions along the Ni-like sequence,and the effects of strong configuration interaction on cross sections from the ground state 3d101S0 to the (3d3/24d5/2)1,(3d3/24d3/2)1 and (3p3/24p1/2)1 states. A comparison was made between the present results and previous theoretical calculations for collision strengths of Gd36+ and U64+ ions,and rate coefficients of Gd36+ ion,good agreement was obtained.
Stabilizing the silicon fullerene Si24 by La and Gd encapsulation
Wang Xiao-Qiu, Wang Bao-Lin
2008, 57 (10): 6259-6264. doi: 10.7498/aps.57.6259
Abstract +
Using gradient-corrected spinpolarized density-functional theory (DFT) for electronic structure calculations,we study the fullerene of silicon Si24 with Th and D2d symmetries in the neutral state stabilized by lanthanum and gadolinium encapsulation. The electronic and magnetic properties of La@Si24 and Gd@Si24 are calculated using spin unrestricted DFT. The higher spin magnetic moment is achieved for Gd@Si24 and the magnetic moment of La@Si24 is completely quenched. These results could possibly lead to new phases and derivatives of silicon.
Density functional theory study on the structures and properties of (Ca3N2)n(n=1—4) clusters
Kang Long, Luo Yong-Chun, Zhang Cai-Rong, Yuan Li-Hua, Li Yan-Long, Chen Yu-Hong
2008, 57 (10): 6265-6270. doi: 10.7498/aps.57.6265
Abstract +
Possible geometrical structures and relative stabilities of (Ca3N2)n(n=1—4) clusters are studied by using the hybrid density functional theory (B3LYP) with 6-31G* basis sets. For the most stable isomers of (Ca3N2)n(n=1—4) clusters,the electronic structures,vibrational properties,bond properties,relative stabilities are analyzed. The following tendencies are revealed by the calculated results: the coordination number of N are usually 3—5. The bond lengths are about 0.231—0.251nm for Ca—N and 0.295—0.358nm for Ca—Ca. The population analysis suggests the average natural charges of N atoms are about -1.553e—-2.241e and those of Ca atoms about 1.035e—1.445e,the bonds between Ca and N are strongly ionic. The dynamic stability of Ca3N2 and (Ca3N2)3 clusters are higher than that of other clusters.
Electronic structure and conductance of fullerene C20
Zhang Hong-Yu, Zhang Xiu-Mei, Wang Li-Guang, Yu Ding-Wen, Li Yong
2008, 57 (10): 6271-6276. doi: 10.7498/aps.57.6271
Abstract +
The electronic structure and electronic conductance of C20 molecule bridged between two Au electrodes are studied by the first principles based on the density functional theory and the nonequilibrium Greens function. The three transmission systems based on C20 molecules are modeled,and the transmission spectra and the molecular orbital distributions are obtained. The reasons why the electronic structure and the electronic transmission are formed are discussed. The results show that the electrons are transported through the C20 bridge on the molecular shell. When one K or one Si atom is inlaid inside C20,the conductivities indicate that the electrons can transfer partly inside the C20 molecule,but most electrons ave transported on the C20 fullerene shell.
Influence of rotating magnetic field on the formation of solidification microstructure
Chen Zhao, Chen Chang-Le, Wen Xiao-Li, Wen Jun
2008, 57 (10): 6277-6282. doi: 10.7498/aps.57.6277
Abstract +
The solidification microstructures of Pb-45%Sn hypoeutectic alloy in rotating magnetic field (RMF) were investigated. When the rotating frequency keeps constant,the grain size reduces linearly with the increase of magnetic intensity,and structure of the primary phase changes from the dendrite to spherical crystals. X-ray diffraction (XRD) measurement indicates that the RMF causes the lattice expansion of primary phase (Pb),and the lattice parameter increases at first and then decreases with the increase of RMF intensity. Therefore,there is a critical value of magnetic intensity. Moreover,the Sn content in primary phase (Pb)reduces gradually with the increase of magnetic intensity. The results are analyzed theoretically based on electromagnetism field theory and scattering law. It is revealed that RMF causes severe fluid flow,and accelerates the diffusion of solute atom and heats the melt,which results in the increase of nucleation rate and the decrease of growth velocity of crystal.
New scheme for introducing an oblique incidence plane wave to layered media in finite-difference time-domain
Jiang Yan-Nan, Ge De-Biao
2008, 57 (10): 6283-6289. doi: 10.7498/aps.57.6283
Abstract +
In the application of two-dimensional finite-difference time-domain (2D-FDTD) method to the analysis of scattering by an object embedded in layered media,the obliquely incident wave along the total field-scattered field (TF-SF) contains the incidence pulse and the reflected and multiple-reflected wave,which can not be introduced by use of the traditional method. To solve this problem,a hybrid scheme using different ways to treat the four TF-SF boundaries is presented. The modified 1D-FDTD including oblique incidence angle is used to treat the longitudinal side boundaries,which can produce the reflected and multiple-reflected wave to layered interface automatically. The longitudinal side boundaries are extended downwards,such that the TF-SF lower boundary is located in the perfectly matched layer (PML) completely,thus the transmission wave and scattered wave are both outward-going waves in the lowest PML layer,and are absorbed by PML. The TF-SF upper boundary is located in free space; the incidence wave along it is in fact a properly time-delayed duplication of the waveform at the corner of TF-SF upper longitudinal boundary. The computational results show the reliability and applicability of the presented hybrid scheme.
A general method for finite difference time domain modeling of wave propagation in frequency-dispersive media
Wei Bing, Ge De-Biao, Wang Fei
2008, 57 (10): 6290-6297. doi: 10.7498/aps.57.6290
Abstract +
The analysis of electromagnetic scattering and propagation in dispersive media is complicated in time domain,because its dielectric property is frequency-dependent. A disadvantage of the prevailing algorithms is the need to deduce different formulations for each dispersion model. In this paper,the shift operator finite difference time domain (SO-FDTD) method is developed. First,we prove that the complex permittivity of three kinds of general dispersive media models,i.e. Debye model,the Lorentz model and the Drude model, may be described by rational polynomial functions in jω. By introducing a shift operator zt,the constitutive relation between D and E is derived in discretised time domain. The shift operator method is then applied to the general dispersive medium case. The recursive formulation for D and E available for FDTD computation is obtained. Finally,the scatterings by a dispersive sphere and a PEC object covered with dispersive media are computed. The computed results are in good agreement with the literature and the one obtained by Mies series solution. This illustrates the generality and the feasibility of the presented scheme.
Design of a hybrid infrared apochromatic optical system beyond normal temperature
Song Yan-Feng, Shao Xiao-Peng, Xu Jun
2008, 57 (10): 6298-6303. doi: 10.7498/aps.57.6298
Abstract +
The principle and design method of employing diffractive optical element to athermalize an infrared optical system are discussed. An analysis is made on the diffractive optical elements dispersion characteristics and its superiority in correcting the second spectrum. A hybrid infrared optical system,as a practical application example,which has realized apochromatism and works beyond normal temperature, is presented. The parameters of the system are as follows: the focal length is 100mm,relative aperture is 1/2,field of view is 6°,and the working waveband is 8—11μm. In the system,only two materials,namely silicon and germanium, are used. The image quality of the system achieves the diffractive limit within the temperature range of -80—200℃. At about 0.7 aperture,three longitudinal aberration curves of the system intersect at one point,and the apochromatism of the optical system is achieved.
Study on the critical free electron density for nanosecond laser pulse focusing in air
Feng Guo-Ying, Zhang Qiu-Hui, Jia Jun, Li Gang, Zhou Shou-Huan, Han Jing-Hua, Yang Li-Ming, Zhu Qi-Hua
2008, 57 (10): 6304-6310. doi: 10.7498/aps.57.6304
Abstract +
Experimental study on the energy transmissivity of high -power nanosecond laser pulse focusing in air versus input laser energy has been carried out. There are three stages: when the input laser energy is relatively low,all of the laser energy can pass through the focusing point; when the input laser energy increases gradually,energy transmissivity decreases steeply from nearly 100% to lower values; when the input laser energy keeps on increasing,the transmissivity decreases slowly further. By defining the critical electron density and the corresponding critical time point,successful theoretical analysis has been carried out for the above experimental phenomena. When the free-electron density is less than the critical value,multiphoton ionization plays the dominating role; when the free-electron density is higher than the critical value, the inverse bremsstrahlung process dominates. The critical time point is the time when laser pulse reaches critical electron density in the process of laser pulse passing through the air; its position on the laser pulse is determined by the input laser pulse energy,meanwhile this position determines the energy transmissivity of the laser pulse. So we can calculate the critical electron density by measuring the transmissivity versus input energy and comprehend the characteristics of transmissivity for laser pulse focusing in the air.
Analysis of the transmittance of modified Savart polariscope in the static large field of view polarization interference imaging spectrometer
Du Juan, Zhang Chun-Min, Sun Yao, Zhao Bao-Chang
2008, 57 (10): 6311-6318. doi: 10.7498/aps.57.6311
Abstract +
The principle of beam splitting of modified Savart polariscope in the static large field of view polarization interference imaging spectrometer(SLPIIS) is described. The reflection and refraction in every uniaxial of modified Savart polariscope are analyzed, with the incident plane coincident or perpendicular to the principle section of the left plate,based on the electromagnetic boundary conditions. Formulas for the transmittance of the modified Savart polariscope are given as functions of wavelength of the incident light,angle and thickness of the half-wave plate,which are analyzed through computer simulation with different half-wave plates. It is proved that the modified Savart polariscope has the advantages of having both large field of view and high flux,and can be used as high performance lateral shearing beam splitter in SLPIIS. This theoretical study will provide guidance for the experimental study of new polarization interference imaging spectrometer.
X-ray microscopic imaging of low Z material wrapped by strongly absorbing medium
Shi Shao-Meng, Chen Rong-Chang, Xue Yan-Ling, Ren Yu-Qi, Du Guo-Hao, Deng Biao, Xie Hong-Lan, Xiao Ti-Qiao
2008, 57 (10): 6319-6328. doi: 10.7498/aps.57.6319
Abstract +
It is difficult for the nondestructive detection of inertial fusion capsule. Based on this requirement we constructed a physical model of low Z material wrapped by strongly absorbing medium. By simulation and experiments,we investigate the effect of some factors on phase contrast imaging quality,such as X-ray energy,distance of object to detector, thickness of strongly absorbing materials and so on. We find it feasible to realize the high resolution and nondestructive detection of inertial fusion capsule by X-ray phase contrast imaging.
Design and fabrication of gold transmission phase gratings for soft X-rays
Qiu Ke-Qiang, Xu Xiang-Dong, Liu Ying, Hong Yi-Lin, Fu Shao-Jun
2008, 57 (10): 6329-6334. doi: 10.7498/aps.57.6329
Abstract +
The diffraction efficiency of a gold transmission phase grating is simulated as a function of the depth of grooves and the duty cycle on the basis of the scalar diffraction theories. It is shown that the +1 order diffraction efficiency of a transmission phase grating with appropriate groove depth and duty cycle can beup to 21.9% of the incident light,whereas it is no greater than 10% for a conventional amplitude-mode transmission grating. Transmission gratings with area of 20mm×5mm,period of 1μm,duty cycle of 0.55 and 200nm thick gold bars supported by a membrane of 300nm thick polyimide have been fabricated by combining holographic lithography and electroplating. Its diffraction efficiency has been measured in national synchrotron radiation laboratory,and the maximum efficiency in +1 order is about 16% at λ=7.425nm.
Experimental study of characteristics of high power photonic crystal fiber amplifier
Duan Kai-Liang, Zhao Wei, Wang Yi-Shan, Zhao Zhen-Yu, Wang Jian-Ming
2008, 57 (10): 6335-6339. doi: 10.7498/aps.57.6335
Abstract +
The gain characterization of photonic crystal fiber amplifier is theoretically and experimentally studied by using reverse pumping technique. The gain curves for different pumping powers and different signal lights are obtained experimentally. When the power of the probe light is 6W and the pump power is 160W,the output power up to 104W,has been achieved eventually. It is found in the experiment that when the pump power is increased over a certain value,the output of the amplifier evolves into the unstable state,which in turn degrades the system performance.
Simulation on a new coupling method of laser diode array end-pumped large-aperture amplifiers
Deng Qing-Hua, Sui Zhan, Ding Lei, Li Ming-Zhong, Wang Jian-Jun, Tang Jun, Luo Yi-Ming, Lin Hong-Huan, Zhang Rui, Deng Ying, Lu Zhen-Hua, Peng Han-Sheng
2008, 57 (10): 6340-6347. doi: 10.7498/aps.57.6340
Abstract +
A new coupling method is put forward for high power LDA end-pumped large-aperture amplifiers. The LDA is inscribed on a sphere and then a duct is added adjacent to it. Three-dimensional ray-tracing program is used to simulate the features of the new coupling method. Simulation results show that when the area contraction coefficient is about 30, deposition efficiency in the gain media as high as 93.7% can be achieved; When the emitting area is not less than the aperture of the duct, uniform and top-hat distribution is obtained. When several stacks are spliced to make an inscribing area, amplifiers with larger aperture can be end pumped. When stacks in LDA is placed as m×n(m≠n) arrays, uniform and top-hat distribution is also obtained. In the LDA coupling experiment, the output is top-hat distributed at the end of the duct and the coupling efficiency is 85.4%.
Application of laser-induced breakdown spectroscopy in analyzing microelements in three kinds of fruit samples
Ma Xin-Wen, Zhu Xiao-Long, Zu Kai-Ling, Zhang Da-Cheng, Li Bin
2008, 57 (10): 6348-6353. doi: 10.7498/aps.57.6348
Abstract +
It is a relatively new task to apply the laser-induced breakdown spectrosopy(LIBS) to plant samples. To apply LIBS technique in the field related to foodstuff safety, we have done primary experiments using three kinds of fruit samples prepared by vacuum freeze-dried technique using the technique of discriminated LIBS spectra. The relative contents of six trace elements Ca, Na, K, Fe, Al and Mg in the samples were analyzed. By comparison we found that in apple samples the relative content of Na is the highest and the relative content of Ca is the lowest, and the contents of K, Fe, Mn, Mg are different for different fruit samples. Experiment results showed that LIBS technique is a valid means for measuring and comparing the content of trace elements in plant samples.
Large self-deflection of bright and dark screening photovoltaic solitons in LiNbO3
Lu Ke-Qing, Zhang Yi-Qi, Zhang Lei, Zhang Mei-Zhi, Li Ke-Hao
2008, 57 (10): 6354-6359. doi: 10.7498/aps.57.6354
Abstract +
We demonstrate large self-deflection of both bright and dark screening photovoltaic solitons existing in LiNbO3 crystal under an external applied field with numerical method, and we also demonstrate that the self-deflection is not only connected with the acceptor concentration NA but also with the external applied field E0. Under the same E0, when NA is lower, the self-deflection is more obvious. Under the same NA, when E0 is higher, the self-deflection is more obvious. We also find that self-deflection of the bright screening spatial soliton and that of the dark one are different: the bright screening photovoltaic soliton deflects obviously; while only one side of the dark soliton deflects, its extreme value point and the other side almost do not deflect.
New SBS media——perfluorinated amines
Lü Zhi-Wei, Gong Sheng, He Wei-Ming, Lin Dian-Yang, Zhang Wei, Hasi Wu-Li-Ji
2008, 57 (10): 6360-6364. doi: 10.7498/aps.57.6360
Abstract +
The stimulated Brillouin scattering (SBS) media perfluorinated amines with good SBS characteristics have been discovered based on the dependence of SBS characteristics on the chemical structures. The SBS parameters of perfluorinated amines such as FC-131, FC-3283, FC-40, FC-43 and FC-70 have been measured or calculated. The results demonstrate that their absorption coefficients are below 10-3cm-1 and optical breakdown thresholds are above 100GW/cm2. The perfluorinated amines also exhibit a series of unique physicochemical properties, i.e., non-toxic property, low volatility, and high stability. The discovery not only diversifies the SBS media, but also improves the performance of SBS system, thereby laying a good foundation for the application of SBS phase conjugation mirror in high-power laser systems.
Interaction between spacial optical solitons in sub-strongly non-local nonlinear media
Cao Long-Gui, Lu Da-Quan, Hu Wei, Yang Ping-Bao, Zhu Ye-Qing, Guo Qi
2008, 57 (10): 6365-6372. doi: 10.7498/aps.57.6365
Abstract +
We investigate the interaction between spacial optical solitons in the sub-strongly non-local nonlinear media. From the light-rays equation, we obtained the evolution rule of the beam centers and found that the ratio of initial separation d0 between solitons and the characteristic length wm of the nonlocal nonlinearity have decisive influence on the interaction. We also found the largest angle below which the solitons can be attracted at oblique incidence. Numerical simulations confirm our theoretical results.
Preparation and performance of Cr3+/Tm3+/Ho3+ co-doped fluorophosphate glasses
Lü Jing-Wen, Liu Shuang, Xiao Hong-Liang, Zheng Xiao-Qiu, Li Yue, Li Feng
2008, 57 (10): 6373-6380. doi: 10.7498/aps.57.6373
Abstract +
Fluorophosphate glasses with various Al(PO3)3 contents were prepared,and their structure,thermal properties and spectral properties were studied.The emission properties at around 2.0μm of Cr3+/Tm3+/Ho3+ co-doped fluorophosphate glasses with different mole fractions of Tm3+ and Ho3+were studied. The spectral strength parameters Ωt(t=2,4,6) of those glasses have been calculated. Meanwhile,the spectral parameters, including the oscillator strength,spontaneous emission probability and branching ratios of Ho3+, have been calculated. The results show that the 2.0μm emission intensity gradually grows higher with the increase of the mole fraction of Tm3+. The Tm3+→(3F4) →Ho3+(5I7) energy transfer in (Tm3+/Ho3+) co-doped fluorophosphate glasses is very efficient, and it is dependent on the doping concentration.
Phase properties of photonic crystal heterostructure and its applications
Wu Kai-Shun, Long Xing-Teng, Dong Jian-Wen, Chen Di-Hu, Wang He-Zhou
2008, 57 (10): 6381-6385. doi: 10.7498/aps.57.6381
Abstract +
It is revealed that the reflectance in the whole stop band of a heterostructure, including the defect mode, approaches 1; while the phase-shift depends on the sub-photonic crystal (PC) that faces the incident direction. For every sub-peak in the defect mode or every pass band of the sub-PC facing the incident direction, the phase shift is 2π. These properties are useful for designing optical phase devices several orders more sensitive than those using amplitude-signal. In this paper, a sensitive optical phase logic gate (NAND) is described as an example of optical phase devices. Moreover, these properties are also useful for the study of related physical phenomena.
The design, fabrication and performance of a large area 10000 line/mm metal transmission diffraction gratings for soft X-ray
Shi Pei-Xiong, Zhou Hong-Jun, Huo Tong-Lin, Zhu Xiao-Li, Ma Jie, Xie Chang-Qing, Zhu Wei-Zhong, Wu Yan-Qing, Guo Zhi, Tai Ren-Zhong, Xu Hong-Jie
2008, 57 (10): 6386-6392. doi: 10.7498/aps.57.6386
Abstract +
Based on the rigorous coupled-wave analysis, the optimized design for a transmission two-gratings mask for 13.4nm soft X-ray interference lithography has been accomplished. Then a large area transmission gratings was successfully fabricated by electron beam lithography (EBL), which has an area of 1.5mm×1.5mm, ruling period of 100nm, Cr relief thickness of 50nm, gap/period of 0.6, and Si3N4 substrate thickness of 100nm. Based on the quantitative estimation of the measurement data, the first and second order diffraction efficiencies were determined as 4.41% and 0.49%, respectively, in good agreement with the numerical simulation results. Through comparison between the measurement and the numerical simulation results, it was shown that the relief is entirely vertical and the gap/period was well controlled. This two-grating mask will be used installed on the soft X-ray interference lithography endstation at Shanghai Synchrotron Radiation Facility (SSRF). With its 1st and 2nd order diffraction, 50nm period and 25nm period gratings can be cost-effectively fabricated, respectively.
Phase-modified coupled mode theory for calculation of fiber Bragg grating Fabry-Perot cavity transmission spectrum
Wang Yan-Hua, Ren Wen-Hua, Liu Yan, Tan Zhong-Wei, Jian Shui-Sheng
2008, 57 (10): 6393-6399. doi: 10.7498/aps.57.6393
Abstract +
Based on the influence of reflection phase of fibre Bragg grating on the transmission spectrum of fibre Bragg grating(FBG) Fabry-Perot cavity, the reason for the error of traditional coupled mode theory when calculating the reflection phase of uniform FBG is analyzed. A parameter of initial phase of effective index distribution is introduced to describe the slight offset of the index distribution of FBG in the longitudinal direction, and the real reflection and transmission coefficients are substituted for the simplified coefficients to modify the traditional coupled mode theory by introducing a phase factor, which is related with the initial phase of effective index distribution and independent of wavelength. The phase of transfer matrix method for nonuniform FBG is further modified. The modified fast calculation results are used to calculate the transmission spectrum of FBG F-P cavity and show the influence of initial phase of effective index distribution on the wavelength offset of FBG Fabry-Perot cavity transmission peaks. The results agree with those of Rouard method and experimental values.
Control of dispersion in microring coupled-resonator optical waveguides
Tian He, Zhang Yun-Dong, Wang Hao, Qiu Wei, Wang Nan, Yuan Ping
2008, 57 (10): 6400-6403. doi: 10.7498/aps.57.6400
Abstract +
We calculated the dispersion relation of microring coupled-resonator optical waveguides using transfer matrix method and discussed its dependence on the coupling loss, transmission loss and coupling coefficient. The shape, position and bandwidth of dispersion relation could be controlled by changing the coupling loss, transmission loss and coupling coefficient. The adjustment and control of dispersion relation is important for applications of coupled-resonator optical waveguides in filtering, delay and buffer of optical signal.
Theoretical investigation on the characteristics of transmission spectra of the two-cavity Fabry-Perot structure based on fiber gratings for single-frequency fiber laser
Xu Ou, Lu Shao-Hua, Jian Shui-Sheng
2008, 57 (10): 6404-6411. doi: 10.7498/aps.57.6404
Abstract +
The characteristics of transmission spectra of the all-fiber two-cavity Fabry-Perot (F-P) configuration based on fiber Bragg gratings (FBG) are theoretically analyzed and modeled. The explicit expression of the transmission coefficient for two-cavity F-P structures is derived. When the single resonant transmission peak is produced at the central wavelength in FBG stop band, the general conditions for the lengths of two cavities and reflectivities of FBGs are presented. Based on the theoretical analysis, the transmission spectra of symmetric and asymmetric two-cavity F-P structures are simulated, and the simulation results are discussed and explained qualitatively. The design guidelines of the device, including the choices of cavity length, grating length and index modulation depth, are put forword. The results show that, when the increasing of the cavity length of a single-cavity F-P structure results in multiple resonant peaks in the stop band, the two-cavity F-P structures of the same length can suppress the secondary resonant peaks and keep the main peak unaffected without degrading the performance through appropriately designing the cavity lengths and FBGs.
Whispering-gallery modes based on evanescent field in cylindrical micro-cavity
Yang Rui, Yu Wen-Hua, Bao Yang, Zhang Yuan-Xian, Pu Xiao-Yun
2008, 57 (10): 6412-6418. doi: 10.7498/aps.57.6412
Abstract +
We present the observation of whispering-gallery modes (WGMs) in cylindrical micro-cavity coupled to a fiber taper based on evanescent field with a coupling efficiency of about 10%. All the WGMs spectra of cylindrical micro-cavities with four different diameters are precisely indexed by an explicit asymptotic formula with the positions and separation as parameters. By introducing the parameter “mode field radius" of WGMs, the mode separations calculated by mode field radius agree well with the experimental results. And by determining the difference between experimental resonant wavelengths and the theoretically assigned ones, the dispersive curve of quartz fiber is corrected over the spectrum range of 1295—1320nm and the precision of refractive index determination is as high as 10-5.
Phase locking of three fiber lasers using an all-fiber coupling loop
Lei Bing, Feng Ying, Liu Ze-Jin
2008, 57 (10): 6419-6424. doi: 10.7498/aps.57.6419
Abstract +
An all-fiber coupling loop is constructed by three 2×2 fiber couplers according to certain rules, which connects three independent erbium-doped fiber lasers to form a coupled array. Thanks to the specially designed all-fiber coupling loop, the total loss of the phase locking array is reduced remarkably, and effective mutual injection coupling occurs between the component fiber lasers, so that the phase locking of the array is achieved. The far field interference patterns and output spectrum observed in experiment indicate that the array has achieved phase locking states. When the pump power of three component fiber lasers is 100mW each, the phase locking array gives 94mW stable coherent output.
High energy femtosecond soliton mode-locking laser based on Yb-doped single polarization large-mode-area photonic crystal fiber
Song You-Jian, Hu Ming-Lie, Liu Bo-Wen, Chai Lu, Wang Qing-Yue
2008, 57 (10): 6425-6429. doi: 10.7498/aps.57.6425
Abstract +
A high energy femtosecond fiber laser based on Yb-doped single polarization large-mode-area photonic crystal fiber is demonstrated. A simple linear laser cavity is used. The laser works in soliton mode-locking regime due to the negative dispersion generated by a pair of gratings implemented in the cavity, while the self-starting mode-locking operation is achieved by sermiconductor saturable absorber mirror. The fiber laser directly generates 518 fs laser pulses with the average power of 700 mW at the repetition rate of 47.3MHz (corresponding to a pulse energy of 14.8 nJ). The single pulse energy is two orders higher than that of conventional soliton mode-locking fiber laser.
Equivalent rectangle approximation-staircase concatenation method for wedge shaped fiber
Liu Xu, Chen Lin, Sun Xiao-Han
2008, 57 (10): 6430-6436. doi: 10.7498/aps.57.6430
Abstract +
Wedge shaped fiber (WSF) is the key component to acquire high coupling efficiency between input/output ports of planar lightwave circuit chips and fiber arrays. It is very important to analyze the mode spot and the mode field evolution of the lightwave propagating in WSF using numerical simulation tools. A three_dimensional equivalent rectangle approximation-staircase concatenation method (ERA-SCM) is proposed to fractionize the wedged tip of WSF along the propagating direction and introduce rectangular waveguide approximation so that effective index of the segmentations is obtained and SCM analysis model is established. Lightwave propagation and mode field evolution is analyzed by ERA-SCM and compared with finite difference-beam propagation method (FD-BPM), showing that the former is more precise to solve mode field evolution for asymmetric fiber and waveguide structures. The measurement of the output optical spot verifies the ERA-SCM simulation error is within 1.9%, while that for FD-BPM method is 4.5%. Thus, ERA-SCM is an effective method for analyzing asymmetric fiber and waveguide structures.
Multi-scale simulation of the dynamic behaviors of nano-void in shocked material
Chen Jun, Xu Yun, Chen Dong-Quan, Sun Jin-Shan
2008, 57 (10): 6437-6443. doi: 10.7498/aps.57.6437
Abstract +
In this paper, the multi-scale numerical simulation method was used to simulate the dynamic behaviors of nano-void in metal Al under shock loading. This method couples the molecular dynamics method used in local defect domain and the finite element method used globally by using a bridge-scale function. Based on the calculated strain field and temperature field and atom positions, we reveal that the collapse of nano-void depends on the yield strength of materials and shock loading strength. And compression process of void is the main reason of formation of local hot spot. Meanwhile, the dislocation and hot spot induced by void collapse lead to the easy formation of the local adiabatic shear band.
Application of planar laser induced fluorescence in the investigation of the stagger electrode dielectric barrier discharge plasma
Li Gang, Xu Yan-Ji, Mu Ke-Jin, Nie Chao-Qun, Zhu Jun-Qiang, Zhang Yi, Li Han-Ming
2008, 57 (10): 6444-6449. doi: 10.7498/aps.57.6444
Abstract +
Trace nitric oxide produced by dielectric barrier discharge was detected directly by the planar laser induced fluorescence system. Through numerical simulations, both the experimental results and the mechanism of plasma flow control were discussed briefly. Besides, flow induced by the dielectric barrier discharge plasma was visualized by the planar laser induced fluorescence system.
Two-dimensional particle-in-cell simulation of the ion sheath dynamics in plasma source ion implantation of a hemispherical bowl-shaped target
Wang De-Zhen, Wang Yan-Hui, Liu Cheng-Sen, Liu Tian-Wei
2008, 57 (10): 6450-6456. doi: 10.7498/aps.57.6450
Abstract +
Plasma source ion implantation into a hemispherical bowl-shaped target is simulated by the two-dimensional particle-in-cell method. The numerical procedure is based on solving the Poisson's equation on a grid and tracing the movement of the ions through the grid. The potential and the ion density distributions in the sheath are studied in detail and the trajectories and dynamic states of ions are considered. The implantation dose and impact angle of the ions at different parts of the target surface are obtained. The ion focusing effect due to the nonuniformity of the sheath potential near the brim of the vessel is observed. The results presented here show that the ion focusing causes the nonuniformity of dose on the target surface.


Investigation on the dispersion characteristics of a uniform plasma grating
Yu Lu-Le, Sheng Zheng-Ming, Zhang Jie
2008, 57 (10): 6457-6464. doi: 10.7498/aps.57.6457
Abstract +
Recently, the plasma Bragg gratings induced by two intersecting laser pulses has been predicted theoretically. The dispersion relation of uniform plasma gratings under the oblique incidence of a light wave is investigated with the transfer-matrix method and the coupled-mode theory. Both approaches show that such plasma gratings exhibit a photonic band gap, near which strong dispersion appears. The photonic band gap has different polarization characteristics when the light wave is obliquely incident on the plasma grating. With the increase of the incident angle, the band gap for an S-polarized wave increases gradually, while that for a P-polarized wave decreases first rapidly, then vanishes when it approaches the Brewster angle, and finally becomes wide with the further increase of the incident angle. Since the plasma grating has an ultrabroad photonic band gap and can support an ultrahigh damage threshold of incident light waves, it has the potential to be a novel photonic device to manipulate extremely intense laser pulses.
Characteristics of electron energy distribution function of capacitively coupled plasma excited by 60MHz RF source
Sun Kai, Xin Yu, Huang Xiao-Jiang, Yuan Qiang-Hua, Ning Zhao-Yuan
2008, 57 (10): 6465-6470. doi: 10.7498/aps.57.6465
Abstract +
Capacitively coupled plasma (CCP) source excited by very-high-frequency (VHF) source has attracted much attention in semiconductor industry due to its ability to generate high density plasma with a large area. The electron behavior and discharge properties of capacitively coupled plasma excited by 60MHz RF source were investigated by using current and voltage probe and Langmuir probe techniques. The experimental results show that equivalent resistance or capacitance of capacitively coupled plasma decreases or increases, respectively, with the increment of input RF power. It is also shown that the electron behavior in the plasma is related not only with RF input power but also discharge pressure. Increase in pressure causes the transition of electron energy distribution function from Bi-Maxwellian type to Druyvesteyn type, with its transition pressure much lower than that reported by others, which is due to a great decrease in efficiency of electron bounced resonance heating in CCP driven by 60MHz.
Photon acceleration in the laser wakefield
Cai Da-Feng, Yang Xiang-Dong, Gu Yu-Qiu, Jiao Chun-Ye, Wu Yu-Chi, He Ying-Ling, Teng Jian, Wang Lei, Zhao Zong-Qing, Wang Jian
2008, 57 (10): 6471-6475. doi: 10.7498/aps.57.6471
Abstract +
On the SILEX-Ⅰlaser facility, we measured the transmission spectra after ultrashort and ultraintense laser pulse interaction with tenuous plasma. Due to the existence of the wakefield, the time-dependent refractive index of the plasma can accelerate or decelerate photons of laser pulse when the laser pulse propagates in the plasma. Its main features are the spliting of the fundamental frequency and its evident broadening in the laser transmission spectra.The side_band wave which is often associated with Raman and Self modulation were not observed in our experiments. Utilizing the LPIC++ code, we also simulated the transmission spectrum after the ultrashort_ultrahigh intensity laser pulse interacts with the tenuous plasma. The simulation results are in good agreement with the experiment results.
Structure and physical properties of La0.1Bi0.9-xEuxFeO3 compounds
Liu Yan-Hui, Li Jing-Bo, Luo Jun, Ji Li-Na, Zhang Ji-Ye, Rao Guang-Hui, Liang Jing-Kui
2008, 57 (10): 6476-6481. doi: 10.7498/aps.57.6476
Abstract +
In this work, La0.1Bi0.9-xEuxFeO3 compounds were prepared by solid-state reaction. The phase relation at room temperature was investigated by X-ray powder diffraction. A rhombohedral (R3c) phase exists for x≤0.05; for 0.08≤x≤0.12 a pseudo-R3c phase is found; for x≥0.15 it is an orthorhombic (Pbnm) phase, while the distortion of the Pbnm phase was observed for the composition range of 0.15≤x≤0.20. Magnetic measurements indicate that weak ferromagnetism exists in all the compounds, and for the compounds with x≤0.20 the magnetic moment has the maximum value at x=0.12. The composition dependence of dielectric constant was investigated at room temperature. The relationship between the structure and the physical properties was discussed.
Structure and magnetic properties of the pseudobinary solid solution TbGa1-xGex(0≤x≤0.4)
Zhang Ji-Ye, Luo Jun, Liang Jing-Kui, Ji Li-Na, Liu Yan-Hui, Li Jing-Bo, Rao Guang-Hui
2008, 57 (10): 6482-6487. doi: 10.7498/aps.57.6482
Abstract +
Polycrystalline samples of TbGa1-xGex(x≤0.4) have been prepared by arc-melting. X-ray powder diffraction reveals that all the samples crystallize in a single phase of CrB-type structure and space group Cmcm. The lattice constants of the compounds decrease linearly with the increasing Ge content. The compounds form a solid solution in the Ge content range of 0≤x≤0.4. The asymptotic Curie points and the effective paramagnetic moments of the compounds have been determined by thermomagnetic measurements. The ordering temperatures were derived from ac susceptibility data. The XRD patterns at various temperatures reveal that there is no structure phase transition in the samples with x=0.2 and 0.3 in the temperature range of 113—273K.
Structure and thermoelectric properties of n-type GaxCo4Sb12 skutterudite compounds
Su Xian-Li, Tang Xin-Feng, Li Han, Deng Shu-Kang
2008, 57 (10): 6488-6493. doi: 10.7498/aps.57.6488
Abstract +
Ga-filled GaxCo4Sb12 skutterudite compounds with different Ga contents were synthesized by combining a melting quenching-diffusion-annealing procedure with spark plasma sintering (SPS). The effects of Ga content on thermoelectric properties were investigated. The results of Rietveld refinement indicated that the Ga is located in the 2a void site. The solubility limit of the Ga filling voids in CoSb3 was found to be close to 0.22. The electrical conductivity and the room temperature carrier concentration Np of the samples increase with the increasing Ga content for Ga filled GaxCo4Sb12 skutterudite compounds with x≤0.25, while the Seebeck coefficient decreases with the increasing Ga content. Room temperature Hall measurements show that each Ga atom donates approximately 0.9 electrons, which is significantly less than that of the Ga oxidation state (3+). Ga-filled skutterudites exhibit much lower thermal conductivity and lattice thermal conductivity in comparison with that of other partially filled skutterudites due to the smaller radius of Ga3+ ions compared with that of other filling atoms. The thermal conductivity and lattice thermal conductivity of Ga0.22Co4Sb12 compound are 3.05Wm-1·K-1 and 2.86Wm-1·K-1 respectively. The Ga0.22Co4Sb12 compound possesses the lowest lattice thermal conductivity at 600K. It is as low as 1.83Wm-1·K-1. The maximum Z value of 1.31×10-3K-1 is obtained at 560K for Ga0.22Co4Sb12.
Change of reflectivity of Mo/Si multilayer irradiated by proton
Ni Qi-Liang, Liu Shi-Jie, Wang Xiao-Guang, Chen Bo, Fan Xian-Hong, Li Min
2008, 57 (10): 6494-6499. doi: 10.7498/aps.57.6494
Abstract +
In this paper, we simulated part of the low-Earth orbits space radiation environment to investigate its influence on the reflectivity change of Mo/Si multilayer mirror used in the space solar telescope. The reflectivity was investigated before and after irradiation with proton of different energies and fluences. The results show that the reflectivity decreases when irradiated by proton because of the radiation-induced damage in the multilayer mirror, and it drops more when the energy of the proton decreases while their fluence increases. When the energy of proton E=160keV with fluence =6×1011/mm2,E=100keV with fluence =6×1011/mm2 and E=50keV with fluence =8×1012/mm2,the reflectivity drops about 4.1%, 5.7% and 10.4%, respectively. The roughness measured by atomic force microscopy increases after irradiation, which results in more scattering of incident ray so that the reflectivity of multilayer is reduced.
Interfacial friction calculation based on the coupled-oscillator model
Ding Ling-Yun, Gong Zhong-Liang, Huang Ping
2008, 57 (10): 6500-6506. doi: 10.7498/aps.57.6500
Abstract +
A new model named the coupled-oscillator model, in which the relationship of the lateral force and the normal force are considered, is proposed for studying the interfacial friction. The Maugis-Dugdal model is used to approximately substitute the Lennard-Jones potential of the interfacial friction in the new model. Then, the formulas for frictional force calculation are deduced. Comparison between the theoretical results and the experimental data obtained by an atomic force microscope shows that the model and the formulas are practically feasible. Based on the coupled-oscillator model, it is found that the frictional force increases approximately with the 2/3th power of the normal force for a tip-sample contact system, which is the same as the classical conclusion of nano-tribology.
Structure and photoluminescence of Zn2GeO4 polycrystalline films prepared by radio-frequency magnetron sputtering
Wang Zhen-Ning, Jiang Mei-Fu, Ning Zhao-Yuan, Zhu Li
2008, 57 (10): 6507-6512. doi: 10.7498/aps.57.6507
Abstract +
Films were deposited on Si substrates at different temperatures by RF magnetron sputtering and subsequently annealed in vacuum at 800℃. Well crystallized films were prepared successfully. The influences of substrate temperature on the structure, the componsition and the morphology of Zn2GeO4films were investigated by X-ray diffraction, X-ray photoelectron spectroscopy (XPS) and atom force microscope. The results show that Zn2GeO4 films grow with (220) preferred orientation with the increase of substrate temperature above 400℃, and the substrate temperature between 500 and 600℃ is suitable for GeO2 crystalline growth. XPS spectra show that the films contain four chemical compositions of Zn2GeO4, GeO2, Ge and ZnO. Besides, with the increase of substrates temperature, the size of crystal grains increases, and the surface of films is smooth and continuous. The green emission consisting of two peaks centered at 530 and 550nm, which is attributed to luminescence centers of Ge2+ substituting Zn2+ at different sites.
Electronic structure and optical properties of GaN with Mn-doping
Xing Hai-Ying, Fan Guang-Han, He Miao, Zhang Yong, Zhou Tian-Ming, Zhao De-Gang
2008, 57 (10): 6513-6519. doi: 10.7498/aps.57.6513
Abstract +
Calculations of the electronic structure and the density of states of GaN with Mn are carried out by means of first-principles plane-wave pesudopotential method based on density functional theory. The results reveal a 100% spin polarized impurity band in band structure of Ga1-xMnxN due to hybridization of Mn 3d and N 2p orbitals. The material is half metallic and suited for spin injectors. In addition, a peak of refractive index can be observed near the energy gap. The absorption coefficient increases in the UV region with the increase of the Mn content.
Electronic structure and optical properties of ZnO doped with carbon
Duan Man-Yi, Xu Ming, Zhou Hai-Ping, Chen Qing-Yun, Hu Zhi-Gang, Dong Cheng-Jun
2008, 57 (10): 6520-6525. doi: 10.7498/aps.57.6520
Abstract +
Electronic structure and optical properties of ZnO doped with carbon have been investigated by using density functional theory based on first-principles ultrasoft pseudopotential method. The calculated results show that there is a significant difference in electronic structures between the cases of C substituting O and C substituting Zn in ZnO,which is caused by both the electronic structure of C atom and its interaction with the neighboring atoms. We also find that the optical properties are charged in the low-energy region after doping, while in the high-energy region the optical properties are almost not influenced by doping with C. The changes of optical properties are qualitatively explained in connection with the calculated electronic structure.
Theoretical study on the relative stability of α-Ga and β-Ga nanowires in carbon nanotubes
Pan Bi-Cai, Xu Bo
2008, 57 (10): 6526-6530. doi: 10.7498/aps.57.6526
Abstract +
The stabilities of the gallium nanowires filled in carbon nanotubes (CNTs) have been studied using density functional theory (DFT). The results show that whether in CNTs or in free space, β-Ga nanowires with large sizes are more stable than α-Ga ones. By analyzing the average energies of gallium nanowires as well as the binding energies between gallium nanowires and CNTs, the reasons why not α-Ga but β-Ga is experimentally observed in CNTs are revealed.
Effect of optical phonon-magnon interaction on the magnon life-time at finite temperature
Li Lin, Luo Hong-Chao, Cheng Tai-Min
2008, 57 (10): 6531-6539. doi: 10.7498/aps.57.6531
Abstract +
A magnon-phonon interaction model is set up in a two-dimensional ferromagnetic double square-lattice system. By using the Matsubara Green function theory we have studied the magnon life-time of the system, calculated the magnon damping curves on the main symmetric line in the Brillouin zone, compared the influences of magnetic ion optical-mode phonon and non-magnetic ion optical-mode phonon on the magnon damping of the system, and discussed the influences of the parameter variations and temperature on the magnon damping. The results show that the coupling of the optical-mode phonon and magnon plays the main role in the magnon damping, especially, the longitudinal optical-mode phonon contributes the most for the magnon damping. It is also found that the effect of the nonmagnetic ion on magnon damping is more significant than that of the magnetic ion. According to the expression -ImΣ*(1)(k)=/(2τ), the magnon life-time is discussed.
Effect of thermal initiator concentration on piezoresistivity of polymer-derived amorphous silicon carbonitrides
Wang Wen-Quan, Yuan Zhou, Xu Shi-Feng, Wang Yan-Song, Zhang Li-Gong
2008, 57 (10): 6540-6544. doi: 10.7498/aps.57.6540
Abstract +
Piezoresistivity of polymer-derived silicon carbonitrides made from a polysilazane modified with different amounts of thermal initiator is measured at room temperature. It is found that the thermal initiator has a significant effect on the electric conductivity, which first increases and then decreases with increasing thermal initiator concentration. The highly conductive sample exhibits much higher piezoresistive coefficient as compared with the low conductive samples. The microstructures of the materials are characterized using Raman spectroscopy. Based on these results, the piezoresistive behavior is described using the tunneling-percolation mechanisms, the piezoresistive effect of SiCN ceramics is controlled by the concentration and distribution of free carbon clusters, and the effect of thermal initiator on the formation of free carbon clusters in the materials is discussed.
Multiscale entropy complexity analysis of metallic interconnection electromigration noise
He Liang, Du Lei, Chen Jian-Ping, Zhuang Yi-Qi, Li Wei-Hua
2008, 57 (10): 6545-6550. doi: 10.7498/aps.57.6545
Abstract +
In order to solve the limitation of traditional frequency-domain analysis in Al interconnection electromigration noise, the multiscale entropy (MSE) method is proposed to analyze the electromigration noise time series. The result shows that at the early stage, electromigration noise is irregular, the noise complexity is large, and with the occurrence of nucleation, the electromigration noise becomes more regular, and the complexity decreases obviously. It indicates that the disorder degree of electromigration system decreases continually. Compared with traditional parameters, MSE can be used to successfully characterize electromigration process.
Effect of boron doping on the electrical conduction of tetrahedral amorphous carbon films
Zhu Jia-Qi, Tan Man-Lin, Zhang Hua-Yu, Zhu Zhen-Ye, Han Jie-Cai
2008, 57 (10): 6551-6556. doi: 10.7498/aps.57.6551
Abstract +
Boron doped tetrahedral amorphous carbon films were prepared on a filtered cathodic vacuum arc deposition system by varying the weight percentage of boron in the mixed graphite cathodes. The electrical conductivity versus temperature, I-V characteristic and C-V characteristic for the films were measured by four-probe method, impedance/gain-phase analyzer, and electrochemical interface, respectively. As the boron content increases from 0 to 6.04 at%, the electrical conductivity of the films at room temperature increases gradually and then drops down, while the activation energy varies in the reverse. At the boron content of 2.13 at%, a maximum value of 1.42×10-7 S/cm and a minimum value of 0.1eV were obtained for the above two parameters, respectively. Furthermore, the rectification characteristics in the I-V curve indicated a p-n junction diode was formed for the boron doped tetrahedral amorphous carbon/n-type silicon heterojunction with uniform doping levels in the space at the two ends of the junction.
Linear and nonlinear optical properties of CdSe quantumn dots
Qian Shi-Xiong, Wang Fang-Fang, Zhang Kun, Gu Yu-Zong, Guo Li-Jun, Zhu Bao-Hua, Ma Guo-Hong
2008, 57 (10): 6557-6564. doi: 10.7498/aps.57.6557
Abstract +
On the basis of measurement and theoretical analysis, the linear and nonlinear optical properties of the CdSe quantumn dots with different nanoparticle sizes were investigated. The relation between the nanoparticle size and the absorption peak of CdSe quantumn dots was characterized by UV-VIS-NIR spectroscopy, and the dependence of third-order nonlinear susceptibility of quantumn dots on nanoparticle size was investigated by picosecond Z-scan technique with different excitation wavelengths (532nm and 1064nm,resp.). The analysis of optical properties of the films was carried out using the theory of localized field enhancement. The experimental and theoretical results showed the dependence of third-order nonlinear susceptibilities χ(3) on the CdSe nanoparticle size. With CdSe nanoparticle size around 4.3nm and the excitation wavelength of 532nm, which was close to the resonance, the third-order nonlinear susceptibility χ(3) showed a maximum value of 2.0×10-11 esu.
A new partial SOI high voltage device with double-faced step buried oxide structure
Zhang Bo, Li Zhao-Ji, Li Qi
2008, 57 (10): 6565-6570. doi: 10.7498/aps.57.6565
Abstract +
A novel PSOI (partial silicon_on_insulator) high voltage device with double-faced step buried oxide is proposed, which is called DSB PSOI(PSOI with double_faced step buried_oxide layer). The surface electric field has ideally uniform distribution due to the additive electric field modulation by double step buried oxide. A silicon window underneath the source helps to reduce self-heating.The depletion region spreads into the substrate and the vertical electric field in the buried layer is enhanced, which results in a higher breakdown voltage than that of conventional SOI device. A 2-D quantified optimal relation between the structure parameters is also obtained. The results indicate that the breakdown voltage of DSB PSOI is increased by 58% in comparison with conventional SOI, while maintaining the low on-resistance of the DSB PSOI device.
Research on the temperature stability of triangular ferromagnetic nanowire arrays using the damage spreading method
Guo Zi-Zheng, Xuan Zhi-Guo, Zhang Yuan-Sheng, An Cai-Hong
2008, 57 (10): 6571-6576. doi: 10.7498/aps.57.6571
Abstract +
The temperature stability of the quantum magnetic disk constructed on a triangular manowire array is studied by the damage spreading method. Three aspects are considered in order to make the Ising model more realistic. Firstly, we include the long-range interaction between spins. Secondly, we take into account the disorder of the nanowire lengths. At last, we present an estimation of the spin interaction constant through the system coercive force by deriving the relation between the spin interaction constant and the coercive force. It is shown that the longer the spin interaction acts, the more stable the system becomes. On the other hand, the system has greater temperature instability when the disorder degree of the nanowire lengths is increased.
Electromagnetic and microwave properties of discontinuous CoFeB/MgO soft magnetic multilayer films
Ma Qiang, Jiang Jian-Jun, Bie Shao-Wei, Du Gang, Feng Ze-Kun, He Hua-Hui
2008, 57 (10): 6577-6581. doi: 10.7498/aps.57.6577
Abstract +
A series of discontinuous multilayers of CoFeB-MgO was fabricated by using DC/RF magnetron sputtering, and the electromagnetic and microwave properties were investigated. The results show that the microstructure and electromagnetic properties of the CoFeB-MgO discontinuous multilayer films can be altered by varying the relative content of the magnetic CoFeB phase and the insulating MgO phase. High performance microwave soft magnetic properties along with high resistivity are achieved in the discontinuous [Co64Fe24B12(0.7nm)/MgO(0.4nm)]40 multilayer film. This film has the saturation magnetization of 1.3 T, hard axis coercivity of 130 A/m, and resistivity of 3.4 mΩ·cm. Moreover, the resonance frequency is about 2.1 GHz for this film. Both the real part μ′ and the imaginary part μ″ of the measured permeability are larger than 240 at 1.59 GHz and are both larger than 100 from 0.9 GHz to 2 GHz. So this film has potential for serving as the microwave absorbers and electromagnetic compatibility materials in the GHz frequency range.
Magnetoelectric effect in transition-metal-doped BaTiO3-Tb1-xDyxFe2-y bilayer
Cao Hong-Xia, Zhang Ning
2008, 57 (10): 6582-6586. doi: 10.7498/aps.57.6582
Abstract +
1.0 mol% Mn(Cr,Co)-doped BaTiO3 have been synthesized with sol-gel technique. The doped BaTiO3 were found to have tetragonal structure at room temperature. The phase transition temperatures and the latent heat of ferroelectric to paraelectric transition were observed to decrease with Cr, Mn and Co doping in BaTiO3. Magnetic characterizations including magnetostriction and magnetization were performed for Tb1-xDyxFe2-y(TDF). Bonded bilayer composites Cr∶BTO-TDF, Mn∶BTO-TDF and Co:BTO-TDF have been fabricated and the transverse ME effect of the three bilayers have been investigated. The maximum transverse ME voltage coefficients for Cr∶BTO-TDF bilayer can reach 586mV·cm-1·(80 A·m-1)-1 under a bias magnetic field of 340×80 A·m-1, and that for Mn∶BTO-TDF and Co:BTO-TDF are 480 mV·cm-1·(80 A·m-1)-1 and 445 mV·cm-1·(80 A·m-1)-1 under a bias magnetic field of about 400×80 A·m-1, respectively.
Preparation and properties of GeC thin films deposited by reactive RF magnetron sputtering
Li Yang-Ping, Liu Zheng-Tang, Liu Wen-Ting, Yan Feng, Chen Jing
2008, 57 (10): 6587-6592. doi: 10.7498/aps.57.6587
Abstract +
GeC thin films were deposited on ZnS substrates with reactive RF magnetron sputtering, and the influences of the processing parameters on sputtering of the Ge target and IR transmission properties of the GeC films were studied. At low substrate temperatures, the GeC film contained H atoms in forms of CH2,CH3 and Ge-CH3 groups, which caused IR absorption, whereas the absorption decreased obviously at elevated substrate temperatures. Target-substrate distance, RF power, Ar:CH4 gas flow ratio and total gas pressure had great impacts on the poisoning and sputtering of the Ge target, but their impacts on the IR absorption of the GeC film was small. When the Ge target was poisoned greatly, adhesion of the GeC film was inferior, with the weakening of the target poisoning, adhesion of the GeC film increased. Hydrogen-free GeC film with excellent adhesion was prepared on ZnS substrate under optimized parameters, and its refractive index was about 1.78. The C content in the GeC film was larger than the Ge content, and C and Ge formed mainly C—Ge bonds. GeC/GaP antireflective and protective film system was prepared on ZnS substrate, giving good antireflective effect.
Effect of spin polarization on electron recombination dynamics in bulk intrinsic GaAs
Teng Li-Hua, Yu Hua-Liang, Huang Zhi-Ling, Wen Jin-Hui, Lin Wei-Zhu, Lai Tian-Shu
2008, 57 (10): 6593-6597. doi: 10.7498/aps.57.6593
Abstract +
Time-resolved circularly polarized and linearly polarized pump-probe spectroscopies are used to study the recombination dynamics of spin-polarized and spin-nonpolarized electrons as well as its evolution with photon energy for an intrinsic GaAs at 9.6K. It is found that the spin polarization has a significant influence on the electron recombination dynamics. The spectroscopic measurements give the same recombination lifetime only when measured near the bottom of the conduction band, but different recombination lifetimes were measured in higher excess-energy states. It is pointed out that the recombination lifetime measured by circularly polarized pump-probe spectroscopy, rather than that measured by linearly-polarized pump-probe spectroscopy, should be used to solve for the lifetime of electron-spin coherence from time-resolved Faraday spectroscopic data. The theoretical calculation agrees well with the experimental results.
Energy-dependent evolution of electron spin polarization in bulk intrinsic GaAs
Teng Li-Hua, Yu Hua-Liang, Zuo Fang-Yuan, Wen Jin-Hui, Lin Wei-Zhu, Lai Tian-Shu
2008, 57 (10): 6598-6603. doi: 10.7498/aps.57.6598
Abstract +
Time-resolved circularly polarized pump-probe spectroscopy is used to study the electron spin coherence dynamics in intrinsic GaAs at 9.6K. It is found that the oscillation amplitude of absorption quantum beats reflecting electron spin coherence varies nonmonotonically with photon energy increaing. A circularly dichromatic pump-probe model is developed with both spin-polarized-dependent band filling and band-gap renormalization effects taken into account. The model shows that the oscillation amplitude of quantum beats is dependent on the initial degree of electron spin polarization, spin-detectable sensitivity and band-filling factor whose product results in the non-monotonic variation of the quantum-beat amplitude and agrees very well with our experimental results. The degree of electron-spin polarization involved in energy-split two-level system is defined for the first time. It is found that a degree of electron spin polarization of up to 100% can be photocreated at higher excess-energy levels.
Up-conversion mechanisms of Yb-Er co-doped Al2O3 film and its temperature characteristics
Li Shu-Feng, Ding Jian-Hua, Li Cheng-Ren, Ming Cheng-Guo, Wang Bao-Cheng, Zhang Li
2008, 57 (10): 6604-6608. doi: 10.7498/aps.57.6604
Abstract +
Yb-Er co_doped Al2O3 film was prepared on SiO2/Si substrate using a medium frequency magnetron sputtering system, and the concentrations of dopant Er and Yb ions were 0.3% and 3.6%, respectively. The up-conversion mechanisms of Er3+ photoluminescence at 529 and 549nm were discussed. The fluorescence intensity ratio of the green up-conversion spectra were measured in the temperature range of 291.8—573.3K, and the temperature characteristics were fitted as R=5.37exp(-738/T). At the temperature of 366K, the sensitivity has the maximum value of 0.0039 K-1. The results show that the Yb∶Er∶Al2O3 film is a suitable material for minitype, high-sensitivity and high-temperature optical sensors.
Magnetic field effect of charged excitons in n-doped ZnSe/BeTe type-Ⅱ quantum wells
Akimoto Ryoichi, Takeyama Shojiro, Mino Hirofumi, Oto Kenichi, Muro Kiyofumi, Ji Zi-Wu
2008, 57 (10): 6609-6613. doi: 10.7498/aps.57.6609
Abstract +
We report results of the reflection (type-I, in ZnSe layer) and photoluminescence (PL) spectra (type-Ⅱ) measurements performed on n-doped ZnSe/BeTe/ZnSe type-Ⅱ quantum well structures at low temperature (5—10 K). The reflection spectra show a typical charged exciton (X-) feature. The PL spectra show an asymmetry of the peak, which is a characteristic feature for a charged exciton type transition. The dependence of the PL peak energy on the in-plane magnetic field is considered to be due to the magnetic-field-induced displacement of interlayer negatively charged exciton dispersion in momentum space.
Effect of annealing and Mg content on the microstructure and optical properties of Zn1-xMgxO films
Zhou Jun, Fang Qing-Qing, Wang Bao-Ming, Liu Yan-Mei, Li Mao, Yan Fang-Liang, Wang Sheng-Nan
2008, 57 (10): 6614-6619. doi: 10.7498/aps.57.6614
Abstract +
The MgxZn1-xO(x=0.1,0.2,0.3, 0.4,0.5,0.6 and 0.7)thin films were prepared on glass substrate by the Sol-Gel method. The X-ray diffraction results show that when the value of x is between 0.1 and 0.3, the thin film has the structure of hexagonal wurtzite while the angle of diffraction peak becomes bigger with increasing x, and the MgO impurity phase segregates at x=0.4. The ultraviolet photoluminescence spectra of the films show ultraviolet emission peak at room temperature, which has an increasing blue shift with the increasing content of Mg. The band gap of ZnO broadens with Mg-doping concentration increasing from 0.1 to 0.3. For the sample with x=0.1 annealed at 500℃, the crystal quality of the films is improved with the increase of the rate of temperature rise from 4.5℃/min to 6.0℃/min. For the sample with x=0.2, the crystal quality of the films is improved with the increase in annealing temperature from 500℃ to 560℃. With the annealing temperature increasing above 590 ℃ the crystal quality of the films degenerates.


Electroluminescence of an iridium complex phosphorescent material doped polymeric system
Tang Xiao-Qing, Yu Jun-Sheng, Li Lu, Wang Jun, Jiang Ya-Dong
2008, 57 (10): 6620-6626. doi: 10.7498/aps.57.6620
Abstract +
With a novel iridium complex (pbi)2Ir(acac) doped into carbazole copolymer, polymer doped electrophosphorescent organic light-emitting diodes with the structure of indium-tin oxide(ITO)/poly(N-vinylcarbazole) (PVK):(pbi)2Ir(acac) (x)/2,9-dimethyl-4,7-diphenyl-1,10-phenan throline (BCP) (20nm)/8-Hydroxyquinoline aluminum(Alq3) (10nm)/Mg:Ag were fabricated. The photoluminescent (PL) and electroluminescent (EL) characteristics of the polymer doped system were investigated at the low doping concentrations of 0.1% and 0.5%, respectively. The results demonstrate that the luminescent spectra with different intensities of PVK and (pbi)2Ir (acac) co-existed in the PL and EL spectra of blend system, which was ascribed to an incomplete energy transfer process and direct charge trapping in the electroluminescence process. The 0.1% (pbi)2Ir(acac) doped device achieved a white light emission with the commissions internationale de 1'eclairage (CIE) coordinates of (0.32, 0.38) when the bias voltage was 19V; the 0.5% (pbi)2Ir(acac) doped device had a maximum luminance of 11827 cd·m-2 and a maximum luminance efficiency of 4.13 cd·A-1 corresponding to a bias of 20.6V and 13.4V, respectively.
Exciton tunnelling in (CdZnTe,ZnSeTe)/ZnTe complex quantum wells
Zhang Zhen-Zhong, Zhang Li-Gong, Zheng Zhu-Hong, Shen De-Zhen, Jin Hua, Liu Shu
2008, 57 (10): 6627-6630. doi: 10.7498/aps.57.6627
Abstract +
A new type of (CdZnTe,ZnSeTe)/ZnTe complex quantum wells was designed. Exciton tunneling process was investigated in this complex structure using absorption spectra, photoluminescence (PL) spectra and pump-probe measurements. The exciton decay time was measured in CdZnTe/ZnTe quantum well layers and ZnSeTe/ZnTe quantum well layers, respectively. And a fast exciton tunneling from CdZnTe/ZnTe quantum well to ZnSeTe/ZnTe quantum well was observed by transient differential trans mission. The tunneling time is 5.5ps.
Optical band gap of cubic boron nitride thin films deposited by sputtering
Deng Jin-Xiang, Wang Xu-Yang, Yao Qian, Zhou Tao, Zhang Xiao-Kang
2008, 57 (10): 6631-6635. doi: 10.7498/aps.57.6631
Abstract +
Cubic boron nitride thin films were deposited on silicon (100) substrates by sputtering. The films were characterized by Fourier transform infrared (FTIR) spectroscopy. The reflectance R(λ) of the films was obtained as a function of incident photon wavelengths and the thickness of the films was measured by Alpha-step. Using Kramers-Kronig transform and the reflectance spectrum R(λ), we calculated the absorption coefficient. The optical band gap was found to be 5.38eV for the films containing 55.4% of cubic phase.
Investigation on CNx films deposited by pulsed bias arc ion plating
Li Hong-Kai, Lin Guo-Qiang, Dong Chuang
2008, 57 (10): 6636-6642. doi: 10.7498/aps.57.6636
Abstract +
The CNx films with different nitrogen contents were prepared on single crystal Si(100) substrate under different nitrogen flow rates by pulsed bias arc ion plating. The surface morphology,composition,structure and properties of CNx films are investigated by optical microscope (OM),X-ray photoelectron spectroscopy(XPS), X-ray diffraction(XRD),Raman spectra and Nano-indentation, respectively. The results show that the surface of the films is uniform,smooth and dense. The nitrogen content in the CNx films decreases with the nitrogen flow rate decreasing. The results indicate that the deposited films are amorphous and have the typical characteristic of diamond-like carbon films. As the nitrogen content decreases from 18.9% to 5.3%, the hardness and elastic modulus of the films increase monotonically to a large extent, of which the hardness increases twice from 15.0 to 30.0 GPa. The sp3 content in the CNx films can be sensitively adjusted by controlling the nitrogen flow rate, leading to the changes of hardness and elastic modulus in large ranges.
Numerical simulation of probe induced surface plasmon resonance coupling nanolithography
Tang Xiao-Dong, Hong Xiao-Gang, Xu Wen-Dong, Li Xiao-Gang, Zhao Cheng-Qiang
2008, 57 (10): 6643-6648. doi: 10.7498/aps.57.6643
Abstract +
A new nanolithographic technology, the probe induced surface plasmon resonance coupling nanolithography (PSPRN), is presented and analyzed numerically by using finite difference time domain method for the loss and dispersive materials. The PSPRN uses a fundamental mode Gaussian beam with wavelength of 514.5nm to excite the Kretschmann surface plasmon resonance, and utilizes the metal probe local-field enhancement effect to realize nanolithography. The influences of different distances between probe and recording layer and different sizes of tip on local field enhancement and the distribution of electric field intensity amplitude on the surface of recording layer were investigated. Results show that the local field enhancement effect is most significant and the electric field intensity amplitude contrast ratio is maximal when the probe is in contact with the recording layer. When the distance between tip and recording layer is 5nm, the distribution width of the relative electric field intensity amplitude above the critical value for near-field nanolithography on the surface of recording layer is close to the size of the tip.
Nitrogen doped 4H-SiC homoepitaxial layers grown by CVD
Jia Ren-Xu, Zhang Yi-Men, Zhang Yu-Ming, Wang Yue-Hu
2008, 57 (10): 6649-6653. doi: 10.7498/aps.57.6649
Abstract +
Homoepitaxial growth of 4H-SiC on off-oriented Si-face (0001) 4H-SiC substrates is performed at 1550℃, under the pressure of 100 mbar using the mbar step-controlled technique with rotation in the horizontal low-pressure hot-wall CVD (LP-HW-CVD) system to obtain high quality 4H-SiC epilayers. The surface morphology, structure and optical properties of the epilayers are characterized by SEM, AFM, FTIR and C-V measurement. The 4H-SiC epitaxial layer has a good crystalline structure and mirror-like surface with few surface defects. N type 4H-SiC epilayers are obtained by in-situ doping of N2.The uniformities of thickness are 1.74%, 1.99%, and 1.32%, and the uniformities of doping concentration are tested to be 3.37%, 2.39%, and 2.01%, respectively. The deviations in thickness and concentration between different samples are 1.54% and 3.63% under the same processing conditions, which shows that the process is repeatable and reliable.
The method of decomposing wind field in a limited area and its application to typhoon SAOMEI
Zhou Yu-Shu, Gao Shou-Ting, Cao Jie
2008, 57 (10): 6654-6665. doi: 10.7498/aps.57.6654
Abstract +
The harmonic-cosine series expansion method (H-C method) for horizontal wind is introduced. It divides a function, such as horizontal wind, stream function and velocity potential, into two parts. One is a harmonic function which is the solution of a Laplace equation with nonzero boundary condition, and thus can be called the harmonic part. It is also been called the external part since it is independent of any values inside the limited area. The other is the difference of the original wind and the harmonic part, which is the solution of Poisson equation with homogeneous boundary condition and can be expanded into double Fourierism cosine series. It only depends on vorticity or divergence inside the limited area, so it is also called the inner part. The H-C method uses a spectrum approach in calculating derivatives, so it is more precise than the differential ones. The boundary condition given to solve the harmonic part has definite physical meaning and it is well handled in solving stream function and velocity potential in a limited area. Therefore, problems such as the instability in the calculation, the original wind field not being able to be reversed accurately, and the system deficiencies on the boundary occurring in previous methods, are successfully overcome. Using the NCEP/NCAR 1°×1° analysis data with a time resolution of 6 h, the Regional Spectrum Model (RSM) reanalysis data from Japan Meteorological Agency with a horizontal resolution of 20km and a time interval of 6 h, we decompose the horizontal wind of super-intensity Typhoo SAOMEI (0608) based on the H-C method. The two parts are called the nondivergent and the irrotational components, respectively. Analyses of the two parts indicate that the typhoon center marked by the nondivergent component at lower levels approaches more closely the observed center than the original wind did. In addition, the nondivergent wind can clearly manifest the water transporting passageway. The phenomenon that horizontal winds converge at lower levels and diverge at upper levels is more obvious in irrotational wind field than that in the original one. Convective activities at the South China Sea and in the west of Philippines confront the development of SAOMEI by blocking off its water transport after it landed. It may be a reasonable explanation for the fact that SAOMEI weakened so rapidly despite of its landing as a super-intensity typhoon. Before it landed, the nondivengent and irrotational centers in wind component fields are not always in the same location. It can only be seen by using the decomposing method for horizontal winds. The two components derived from the H-C method may provide more detailed characteristics than the original wind does. Therefore, this approach would be important in understanding the characteristics of typhoon and can be used widely in the future.
Scattering from a diffuse target in the slant atmospheric turbulence
Wei Hong-Yan, Wu Zhen-Sen, Peng Hui
2008, 57 (10): 6666-6672. doi: 10.7498/aps.57.6666
Abstract +
Based on the extended Huygens-Fresnel principle and ITU-R atmospheric turbulence model (the altitude-dependent model proposed by International Telecommunication Union-Recommendation in 2001), the treatment includes the effect of the atmospheric turbulence both on the laser beam as it propagates to the target and on the scattered field as it propagates back to the receiver. The covariance function and variance of the scattering intensity are derived considering the fluctuations of the log-amplitude and phase, and the numerical results are presented. The influences of wavelength, receiver altitude and propagation distance on the covariance function and normalized variance of the scattering are discussed.
Quasi-linear diffusion of the radiation belt energetic electrons by ground-based HF heater-induced ELF/VLF emissions
Ni Bin-Bin, Gu Xu-Dong, Zhao Zheng-Yu, Wang Xiang, Deng Feng
2008, 57 (10): 6673-6682. doi: 10.7498/aps.57.6673
Abstract +
The dynamics of energetic electron fluxes in both the inner and the outer radiation belts is very important with respect to satellite protection, in particular for low and middle-orbit satellites. Based on the quasi-linear theory of gyroresonant wave-particle interaction, we compute the diffusion coefficients and loss timescales for radiation belt energetic electrons due to cyclotron resonance with artificial ELF/VLF emissions that are radiated through modulating the currents in the lower ionosphere by ground-based powerful high-frequency (HF) transmitter. We test the electron pitch-angle scattering in the outer zone, typically at L=4.6 (where the HAARP facility is located) and in the inner zone, typically at L=1.5. The results indicate that the electron loss timescales due to artificial injection of ELF/VLF waves in the inner and the outer radiation belts depend largely on the value of the cold-plasma parameter α*(∝B2/N0, where B is the ambient magnetic field and N0 the electron number density), the properties of wave frequency spectrum, the wave power and the electron energy in resonance with the waves. Generally, relativistic electrons in the outer zone are much easier to be precipitated into the atmosphere by artificial ELF/VLF whistler waves and lower-energy electrons (≤200keV) can undergo pitch-angle scattering more efficiently than higher-energy electrons (≥500keV). Since ELF/VLF waves can experience in situ amplification due to multiple magnetospheric reflections within the magnetospheric cavity, it can be reasonably expected that, under suitable situations, ground-based HF transmitters can provide feasible radiation power into the ionosphere to induce the injection of ELF/VLF waves into the inner magnetosphere, and consequently account for potential rapid removal of outer belt relativistic electrons in a timescale of from 1 to 3 days and of inner belt relativistic electrons that generally have a lifetime of 100 days or more in a timescale of the order of 10 days.
Time offset measurement algorithm based on bispectrum for pulsar integrated pulse profiles
Xie Zhen-Hua, Xu Lu-Ping, Ni Guang-Ren
2008, 57 (10): 6683-6688. doi: 10.7498/aps.57.6683
Abstract +
In order to improve time offset measurement precision of pulsar integrated pulse profile and enhance the capability of noise suppression, a time offset measurement algorithm based on bispectrum for pulsar integrated pulse profile was proposed. Theoretically, bispectrum can completely suppress the additive Gaussian white noise, and the time offset in the third-order cumulants will result in phase rotation in the bispectrum. Time offset measurement algorithm based on bispectrum can be deduced by contrasting self-bispectrum and cross-bispectrum and solving the extremum formula. The results of experiments show that time offset measurement algorithm based on bispectrum can suppress the additive Gaussian white noise, and its precision is higher than that based on power spectrum proposed by Taylor.