Vol. 52, No. 7 (2003)

Form invariance and Noether symmetry of a relativistic mechanical system
Fang Jian-Hui, Yan Xiang-Hong, Chen Pei-Sheng
2003, 52 (7): 1561-1564. doi: 10.7498/aps.52.1561
Abstract +
In this paper,we study the form invariance and Noether symmetry of a relativistic mechanical system-Firstly,we give the Noether theorem of a relativistic mechanical system,and the definition and criterion and conserved quantity of the form invariance in the system-Next,the relation between the form invariance and Noether symmetry of the system is obtained-Finally,we give an example to illustrate the application of the result-
Multisoliton-like solutions for (2+1)-dimensional dispersive long wave equations and (2+1)-dimensional Broer-Kaup equations
Wang Ke-Xie, Naranmandula
2003, 52 (7): 1565-1568. doi: 10.7498/aps.52.1565
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By extending homogeneous balance method and improving some of its procedures, we have obtained new multi-soliton-like solutions for (2+1)-dimensional dispersive long-wave equations and (2+1)-dimensional Broer-Kaup equations- The solution we obtained presents generality, because it contain some multisoliton-like solutio ns which are given in other papers-
Improved tanh-function method and the new exact solutions for the general variab le coefficient KdV equation and MKdV equation
Zhang Hong-Qing, Li De-Sheng
2003, 52 (7): 1569-1573. doi: 10.7498/aps.52.1569
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In this paper, by using the extended tanh-function method,the general variable coefficient KdV and MKdV equations are reduced to first-order variable coefficient nonlinear ordinary differential equations,and then the new exact solutions for these equations,which include exact soliton-like,rational formal and triangle function solutions,are obtained through solving these ordinary differential equations-
Exact solutions to two nonlinear equations
Wang Yue-Ming, Wang Ming-Liang, Zhang Jin-Liang, Fang Zong -De
2003, 52 (7): 1574-1578. doi: 10.7498/aps.52.1574
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By using the traveling wave reduction method, the exact solutions to the (1+1)-d imensional Zakharov equation, Korteweg-de Vries equation with variable coefficie nts are obtained with the aid of exact solutions to the cubic nonlinear Klein-Go rdon equation-
Bound states of the Klein-Gordon equation with n-dimensional scalar and vect or hydrogen atom-type potentials
Chen Chang-Yuan, Liu Cheng-Lin, Lu Fa-Lin, Sun Dong-Sheng
2003, 52 (7): 1579-1584. doi: 10.7498/aps.52.1579
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Characteristics of the bound states of Klein-Gordon equation with n-dimensional scalar and vector hydrogen atom-type potentials have been studied,the exact solutions of bound states are obtained-The exact energy expressions and the normalized analytically wave functions for bound states are presented-Two recurrence formulas and some explicit expressions for lower power radial average values are also derived-
Inquiry about the exact interior solution to Einstein field equation for a perfe ct fluid sphere
Zhong Ming-Qian
2003, 52 (7): 1585-1588. doi: 10.7498/aps.52.1585
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When the density of a static spherically symmetric perfect fluid is a function of the radial coordinate, the Oppenheimer-Volkoff (OV) equation turns into a Riccati equation- If a particular solution of the OV equation is given, it can be transformed into an integrable Bernoulli equation, we can obtain a general exact solution and an other particular solution of the OV equation- Further more, the exact interior solutions of Einstein field equation for the perfect fluid sphere are also obtained, i-e- the analytical expressions of the metric compone nts-
Experimental study for impulsive synchronization of a discrete chaotic system
Zhang Ru-Yuan, Chen Ju-Fang, Peng Jian-Hua
2003, 52 (7): 1589-1594. doi: 10.7498/aps.52.1589
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A two-dimensional discrete system with chaotic and hyperchaotic characteristics has been designed and its dynamic behaviour has been analyzed, and the impulsiv e synchronization conditions have been obtained by mathematical analysis- In our experiment, impulsive synchronization of the circuit system has been realiz ed- The experimental data are in good agreement with the results of theoretical analysis and numerical calculation- In addition, the experimental results demons trate the effectiveness and robustness to noise by means of the above method-
Projective synchronization in a unified chaotic system and its control
Chen Shi-Hua, Lu Jun-An, Liu Jie
2003, 52 (7): 1595-1599. doi: 10.7498/aps.52.1595
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Projective synchronization phenomenon in unidirectionally coupled unified systems is studied- A scheme is proposed to control the scaling factor formed in the projective synchronization- Under the control strategy, one can arbitrarily mani pulate the scaling factor to a desired value- This special phenomenon and the co ntrol scheme can be used in some secure communication schemes- Illustrations ar e also given to show the rightness of the theoretical analysis and the effective ness of the method proposed-
Global stability of bidirectional associative memory neural networks with dela ys
Xu Jin, Zhang Qiang, Gao Lin, Wang Chao
2003, 52 (7): 1600-1605. doi: 10.7498/aps.52.1600
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The stability of the equilibrium point of bidirectional associative memory (BAM) neural networks with delays is studied by constructing a suitable Lyapunov functional and combining with some inequality analysis techniques- On the assumption that the activation functions of neurons are less restrictive than those in t he literature (which may not satisfy Lipschitz condition), a new sufficient cond ition ensuring the global asymptotic stability of BAM neural networks with delay s is derived- The results presented here can be applied to the design of a wider class of neural networks including non-Lipschitz activation functions of neuron s-
Study of the dynamics of a first-order cellular neural networks with delay
Xu Jin, Zhang Qiang, Gao Lin, Wang Chao, Yuan Tao
2003, 52 (7): 1606-1610. doi: 10.7498/aps.52.1606
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The dynamics of a first-order cellular neural network with delay has been studied in this paper- By using the delay as a bifurcation parameter, the Hopf bifurc ations are proved- Furthermore, by choosing suitable parameters and delay, chaot ic behaviour has been observed- Computer simulations, as well as Lyapunov ex ponent, are presented to support our results- Due to the simple structure of thi s network, it can serve as a chaos-generator in practice-
A study of the bifurcation effect of closed orbits on the recurrence spectra of Li Rydberg atoms in an external electric field
Song Xiao-Hong, Lin Sheng-Lu
2003, 52 (7): 1611-1616. doi: 10.7498/aps.52.1611
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The semiclassical closed-orbit theory begins to fail when the returning wave bec omes infinite near the core, and this is the typical situation when bifurcations of closed-orbits occur- We describe the phenomena of bifurcations and calculate the recurrence spectra of Li Rydberg atoms in the vicinity of the bifurcation e nergy using the uniform approximation method- The spectra with a constant energy ε=-2-94, and a constant scaled electric field 135-86-1/4<160 -11 are recorded and the effect of the core-scattering is taken into account- T he recurrence spectra are compared with those obtained by the primary extended c losed-orbit theory and those of hydrogen atoms- The results display the importan t contributions of bifurcations and core-scattering to the recurrence spectra of Li Rydberg atoms-
The thermodynamical calculations of uranium-water vapor system
Xue Wei-Dong, Zhu Zheng-He, Jiang Gang, Wang Hong-Yan, Xiong Bi-Tao, Meng Da-Qiao
2003, 52 (7): 1617-1623. doi: 10.7498/aps.52.1617
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In this paper, we study the relative stable molecular structures of uranium-wate r vapor system- For the uranium atom were used the relativistic effective core p otential and contracted valued basis sets (6s5p2d4f)/[3s3p2d2f], and for oxygen and hydrogen atoms were used 6-311G** basis sets- We obtain the elect ronic state, geometric structure, energy, harmonic frequency, mechanical property, et c-,of these six quintuple relative stable structures- It indicated that the first step of uranium-water vapor reaction is the interaction of uranium and oxyg en atoms- The thermodynamical stability of UOH2(5A1) was calculated an d its disassociation Gibbs free energy ΔG°decreased with increasing temper ature -The result showed that low temperature favoured its existence-
Investigations of concentration effects of La on the infrared optical properties of PLZT thin films
Hu Zhi-Gao, Shi Fu-Wen, Huang Zhi-Ming, Wang Gen-Shui, Meng Xiang-Jian, Lin Tie, Chu Jun-Hao
2003, 52 (7): 1624-1629. doi: 10.7498/aps.52.1624
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Lead lanthanum zirconate titanate (PLZT) thin films with different La concentrations (x), whose composition is x/40/60, have been grown directly on Pt/Ti/SiO2/Si substrates by a modified sol-gel method. x-ray diffraction analysis shows that the PLZT thin films are polycrystalline with a single perovskite p hase. The infrared optical properties of the PLZT thin films are investigated us ing the infrared spectroscopic ellipsometry in the spectral range of 2.5—12.6μ m. By fitting the measured ellipsometric parameters, and a classical dispers ion relation for the thin films, the optical constants and thickness of the thin films have been obtained. The refractive index of the PLZT thin films decreases with increasing La concentration; however, the extinction coefficient increases with increasing La concentration except for the PLZT (4%) thin films. It is bel ieved that the decrease in the refractive index for the PLZT thin films with inc reasing La concentration is mainly due to the crystallinity and the electronic b and structure of the PLZT thin films. The absorption coefficient of the PLZT thi n films is larger than that of the PZT thin films. The values of the effective s tatic charge in the PLZT thin films, which are smaller than those of the purely ionic materials for the PLZT thin films, decrease with increasing La concentrati on. This indicates that PLZT belongs to a mixed ionic-covalent compound.
Properties of the injected atomic inversion in the micromaser
Wu Shu-Dong, Chen Ai-Xi, Jin Li-Xia
2003, 52 (7): 1630-1634. doi: 10.7498/aps.52.1630
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In this paper, we establish the expression of photon statistics in the steady state in the micromaser with Kerr medium cavity, study the properties of the atomi c inversion in a stationary state when the atom is in thermal-atom, critical or ultracold-atom regimes in a micromaser. The results show that the properties of the atomic inversion in different regimes are very distinct. In the thermal-atom regime, there are some regions of the cavity length L where the atoms are i nversive, with the increase of the atomic injection rate r, the regions of a tomic inversion and the probability of the atomic inversion increase. In the cri tical regime, atomic inversion exhibits the collapse and revival phenomena. Howe ver, in ultracold-atom regime, there are all regions of the cavity length L where the atoms are not inversive. Kerr effect and the detuning make the probabi lity of the atomic inversion smaller.
Parametric study of plasma resistance in gas pulsed discharges
Chen Gang, Pan Bai-Liang, Yao Zhi-Xin
2003, 52 (7): 1635-1639. doi: 10.7498/aps.52.1635
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The dependences of dynamic plasma resistances on discharge voltage, repetitive frequency and buffer gas pressure are studied and measured. It is found that the plasma resistance reduced with the increase of discharge voltage and repetit ive frequency, but increased with enhancing buffer gas pressure. The experimenta l formula of plasma resistance depending on discharge voltage, repetitive freque ncy and buffer gas pressure was proposed and the pulsed discharge current was si mulated using the experimental formula of plasma resistance.
A study on multiple filamentation of locally modulated laser beams
Qian Lie-Jia, Fan Dian-Yuan, Wen Shuang-Chun
2003, 52 (7): 1640-1644. doi: 10.7498/aps.52.1640
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Based on the nonlinear Schrdinger equation,we have simulated the multiple filamentation of locally modulated laser beams,and found that for B integral smaller than some values the local modulations grow to form filaments mainly by drawing energy from that part of background beam;the evolution of two parts of local modulation are basically independent if they are sufficiently far away from each other.
Analysis of the existence of one-dimensional dark and gray spatial solitons in l ogarithmically nonlinear media
Wei Qing, Wang Qi, Shi Jie-Long
2003, 52 (7): 1645-1649. doi: 10.7498/aps.52.1645
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The existence of dark and gray spatial solitons in logarithmically nonlinear media is investigated. It is shown that both dark and gray solitons are possible in corresponding nonlinear media. The spatial width for dark solitons is analyzed .It is revealed as follows: the beam width decreases dramatically with increasing power when the peak power is much lower, exhibiting the good nonlinear effects in such cases. The variation of beam width is slow at first and then becomes constant when the peak power continuously increases .
Numerical studies of neon gas-puff Z-pinch dynamic processes
Ning Cheng, Yang Zhen-Hua, Ding Ning
2003, 52 (7): 1650-1655. doi: 10.7498/aps.52.1650
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Dynamic processes of neon gas-puff Z-pinch are studied numerically in this paper.A high temperature plasma with a high density can be generated in the process.Based on some physical analysis and assumption,a set of equations of one-dimensional Lagrangian radiation magneto-hydrodynamic (MHD) and its code are developed to solve the problem.Spatio-temporal distributions of plasma parameters in the processes are obtained,and their dynamic variations show that the major results are self-consistent.The duration for the plasma pinched to centre,as well as the width and the total energy of the x-ray pulse caused by the Z-pinch are in reasonable agreement with experimental results of GAMBLE-Ⅱ.A zipping effect is also clearly shown in the simulation.
Particle simulation method for the electron temperature gradient instability in toroidal plasmas
Jian Guang-De, Dong Jia-Qi
2003, 52 (7): 1656-1662. doi: 10.7498/aps.52.1656
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The gyrokinetic equations for the electrostatic electron-temperature gradient mo des in toroidal plasmas are solved with particle simulation method. An axisymmet ric geometry with circular flux surfaces is employed. The full kinetics, includi ng finite Larmor radius effects, transit k∥v∥, and toro idal (curvature and magnetic gradient) drift motion, is retained. The basic method of the parti cle simulation is described briefly. The fourth-order adaptive stepsize scheme i s adopted, that saves computer time and is simple. The basic characteristics of the modes are discussed. The scaling of the critical gradient with respect to to roidicity and to the ratio of electron temperature over ion temperature is given . Comparison with experiments are made and the theoretical results are close to the experimental observations.
Design and optimization of wave impedance distribution for flyer materials
Zhang Lian-Meng, Wang Chuan-Bin, Hua Jin-Song, Tan Hua, Shen Qiang, Jing Fu-Qian
2003, 52 (7): 1663-1667. doi: 10.7498/aps.52.1663
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Flyers with wave impedance gradient can be regarded as a new kind of functionally graded material, which has come to show great potential for the application in dynamic high-pressure technology. In the present paper, a theoretical model of generating quasi-isentropic compression via the W-Mo-Ti system flyer was established, and the numerical simulation has been carried out. The results show that the design for flyers with graded wave impedance should conform to the parabolic or cubic function relationship of the change in the thickness. The thickness of target and the impact velocity should also be well controlled for creating a be tter quasi-isentropic compressive wave profile.
Filamentation instability of intense laser in partially ionized plasma
Zhang Jia-Tai, Liu Song-Fen, Hu Bei-Lai
2003, 52 (7): 1668-1671. doi: 10.7498/aps.52.1668
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The difference between fully ionized and partially ionized plasmas lies in the presence of bound electrons. From this difference the propagation of intense laser pulse and the refraction index in a partially ionized plasma are analyzed. An atomic filamentation instability (AFI) can be significantly enhanced by bound electrons. The AFI in Au plasma-produced 0.35 and 0.53 μm wavelength laser light to irradiate golden target are analyzed and calculated. The results show that the growth rate of the AFI is substantially higher than that of the relativistic filamentation instability.
Absorption of femtosecond laser pulses in metallic and dielectric slabs
Chen Zheng-Lin, Zhang Jie, Chen Li-Ming, Teng Hao, Dong Quan-Li, Zhao Li-Zeng, Wei Zhi-Yi
2003, 52 (7): 1672-1675. doi: 10.7498/aps.52.1672
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Laser absorption, and hot electron emission and its energy spectrum are studied in the interaction between P-polarized femtosecond laser pulses at 8×1015 W/cm2 and solid-density plasmas. Comparison is made between th e different b ehaviours of metallic and dielectric slabs irradiated by femtosecond laser pulse s. It appears that there are less laser absorption, fewer hot electrons and high er hot electron temperature from dielectric slabs compared with metallic ones. T hese are concluded to be due to the lower conductivity and consequently lower ch arge separation potential of the dielectric slabs.
Study of the hollow cathode plasma electron-gun
Zhang Yong-Hui, Jiang Jin-Sheng, Chang An-Bi
2003, 52 (7): 1676-1681. doi: 10.7498/aps.52.1676
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For developing a novel high-current, long pulsewidth electron source, the theoretics and mechanism of the hollow cathode plasma electron-gun are analyzed in detail in this paper, the structure and the physical process of hollow cathode pl asma electron-gun are also studied. This gun overcomes the limitations of most high-power microwave tubes, which employ either thermionic cathodes that produce low current-density beams because of the limitation of the space charge, or f ield-emission cathodes that offer high current density but provide only short pu lsewidth because of plasma closure of the accelerating gap. In the theories stud ying on hollow cathode plasma electron-gun, the characteristic of the hollow-cat hode discharge is introduced, the action during the forming of plasma of the sti mulating electrode and the modulating anode are discussed, the movement of elect rons and ions and the primary parameters are analyzed, and the formulas of the e lectric field, beam current density and the stabilization conditions of the beam current are also presented in this paper. The numerical simulation is carried o ut based on Poisson's equation, and the equations of current continuity and move ment. And the optimized result is reported. On this basis, we have designed a ho llow-cathode-plasma electron-gun, whose output pulse current is 2kA, and pulse w idth is 1 microsecond.
Propagation of x-ray in the laser plasma and its effect in the diagnosis of elec tric density
Guo Hong, Fu Xi-Quan
2003, 52 (7): 1682-1687. doi: 10.7498/aps.52.1682
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In this paper, by introducing an error estimate parameter η and analyzing the magnitude of each term of the propagation equation, the propagation equation s of amplitude and phase of x-rays can be obtained in the laser-produced plasma. It is found that the change of phase is only dependent on the electron density , and the change of amplitude is directly dependent on phase. Moreover, only if η<1, by using iterative approximation, the approximate solutions of phase and amplitude can be obtained. Finally, by computer simulation these solutions a re confirmed.
Nonlinear threshold of two-dimensional Rayleigh-Taylor growth for incompressible liquid
Zhang Wei-Yan, ]Ye Wen-Hua, He Xian-Tu, Wu Jun-Feng
2003, 52 (7): 1688-1693. doi: 10.7498/aps.52.1688
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Nonlinear threshold of two-dimensional Rayleigh-Taylor(RT) instability is analyzed in planar and cylindrical and spherical geometries. Density amplitude is defined relating to instable interface and formulas of nonlinear threshold values for RT instability in three kinds of geometries are given, then high-order algorithm is used to simulate two-dimensional RT instability in these geometries, and t he simulation results agree well with the formulas.
Numerical simulation of dielectric-barrier-controlled glow discharge at atmosphe ric pressure
Wang Yan-Hui, Wang De-Zhen
2003, 52 (7): 1694-1700. doi: 10.7498/aps.52.1694
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The space and time distributions of the electric field and the electron and ion densities, as well as the time evolutions of the discharge current density and the surface charge density of the dielectric layer in He dielectric-barrier-controlled glow discharge at atmospheric pressure are calculated by solving the on e-dimensional continuity and momentum equations for electrons and ions, coupled to the current continuity equation. The properties of uniform atmospheric pressu re glow discharge under the conditions of different driving frequency, voltage o r dielectric layer are discussed and analyzed. When the driving frequency is hig h enough, a large number of ions are trapped and the induced space charge field makes a great many of electrons stay in the discharge volume. These seed electro ns lead to a Townsend discharge at atmosphere pressure. The structure of this di scharge is similar to that of low-pressure glow discharge, i.e. there exist four specific regions: the cathode fall, the negative glow, the Faraday dark space a nd the positive column. The discharge current becomes small with decreasing voltage amplitude applied. The secondary electron emission from the dielectric layer makes the discharge current increase. The thicker the dielectric layer is or the smaller the permittivity, the smaller the discharge current is.
PIC simulation of the transverse wave-breaking in laser wake-field
Xu Han, Chang Wen-Wei, Yin Yan, Zhuo Hong-Bin, Ma Yan-Yun
2003, 52 (7): 1701-1706. doi: 10.7498/aps.52.1701
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This paper focuses on the influence upon the structure of the wake-field and the production of hot electrons due to a finite-width laser of high intensity and ultra-short duration propagation through an underdense plasma. Under the action of lognitudinal and transverse pondermotive forces, the plasma density forms a horseshoe cavity which acts as a moving convex to make the laser pulse self-focusing. The wake wave front curvature increases with time until a transverse wav e breaking occurs which depresses the wave breaking limit of the electrostatic f ield. The decrease of transverse wave breaking pushes more electrons into the ac celerating phase region to be trapped by wake wave and decreases the maximum ele ctronic kinetic energy.
Deposition of vinyl acetic acid through the pulsed rf plasma polymerization
Zhang Jing, Feng Xiang-Fen, Xie Han-Kun, Shi Yun-Cheng, Pu Tian-Shu
2003, 52 (7): 1707-1713. doi: 10.7498/aps.52.1707
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In this paper, pulsed rf plasma polymerization using vinyl acetic acid was investigated to obtain plasma polymers with less cross-linked structure and high-degr ee retention of the starting monomer groups. The chemical structure and the surf ace morphology of the polymerized vinyl acetic acid were characterized by Fourie r transform infrared ( FTIR) spectroscopy, x-ray photoelectron spectroscope (XPS ) and scanning electron microscope (SEM). The duty cycles were observed to be on e of the key parameters to change the film structure. The FTIR results showed th at more carboxylic groups could be “retained” with the decrease of the duty cy cles. The XPS results were consistent with the FTIR measurements. Surface energy measurements indicated that the plasma films were hydrophilic. So controlling t he chemical composition with some special surface functional groups being “rete nted” or “tailored” was available by pulsed plasma technologies. Some regular graft “lighting” network patterns were interestingly found in the plasma fil ms kept at room temperature for some time by SEM. It was estimated that the plas ma polymerization proceeded differently along the “lighting” network and on th e valley of the pattern. The details need to be further studied.
Experimental investigation of bublles by underwater wire exploding
Zhang Chi, Gao Bo, Zhang Han-Hong
2003, 52 (7): 1714-1719. doi: 10.7498/aps.52.1714
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The pulsation of bubbles generated by underwater wire exploding is studied by means of high-speed photography. We discovered that mechanical parameters of the b ubbles are mainly determined by the total discharge energy and related not much to the voltage and the capacitance. Boundary influence is also not too large in the experiment. The energy of pulsation occupies 20%—40% of the total discharg e energy, and the proportion between the pulsation energies of the first time an d the second time is about this value. The bursting forth phenomenon is the main reason of the decline of bubble's energy.
Phonon spectrum of single-walled carbon nanotubes
Xiao Yang, Yan Xiao-Hong, Cao Jue-Xian, Ding Jian-Wen
2003, 52 (7): 1720-1725. doi: 10.7498/aps.52.1720
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The phonon spectra are calculated within the frame of force constant model. By virtue of five rotation operations determined by the coordinates of carbon atoms, it is convenient for us to obtain the force matrix. The calculations show that there are only 12 non-degenerate phonon branches and 6(n-1) degenerate branc hes for armchair (n, n) tube and zigzag (n, 0) tube.
Thermodynamic calculation of the oriented internal stress in single crystals of Ni52Mn24Ga24
Meng Fan-Bin, Qu Jing-Ping, Li Yang-Xian, Liu Zhu-Hong, Wang Wen-Hong, Zhang Ming, Chen Jing-Lan, Wu Guang-Heng, Cui Yu-Ting, Wang Wan-Lu
2003, 52 (7): 1726-1731. doi: 10.7498/aps.52.1726
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The internal stress in single crystals of Ni52Mn24Ga2 4 has been calculated based on the balance equation and the experimental data on transformation strains with or without bias field. It was found that the residual oriented internal stress caused by the directional solidification during the crystal growth plays an important role in achieving the reorientation of the variants, and resulting in a large transformation strain.Furthermore, according to Maxwell equation and Ullako's thermal dynamic model, the theoretical value of magnetic-field-induced strains and internal stress at different temperatures in martensite was calculated based on a best fitting of the strain curves versus the bias field. Compared with the experimental observation, the availability of various models has been discussed.
First-principles calculation on the formation energies oflithium insertion in In Sb
Liu Hui-Ying, Yang Yong, Hou Zhu-Feng, Zhu Zi-Zhong, Huang Mei-Chun
2003, 52 (7): 1732-1736. doi: 10.7498/aps.52.1732
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InSb as an anode material in lithium batteries has attracted considerable attention in recent investigations. An ab initio method with norm-conserving non-local pseudopotentials based on the local density functional theory has been used to investigate the non-carbon-bearing anode material InSb for lithium batteries. The formation energies of lithium intercalation and their electronic structures have been calculated. The changes of volume, band structures, electronic density of states and charge density contour plots for lithium intercalation in InSb are also discussed. We found that the formation energies of Li insertion in InSb are all about 2.2 eV per Li atom.
Simulating the three-dimensional dendritic growth of Al alloy using the phase-fi eld method
Zhao Dai-Ping, Jing Tao, Liu Bai-Cheng
2003, 52 (7): 1737-1742. doi: 10.7498/aps.52.1737
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Numerical simulation of three-dimensional dendritic growth is performed by using a macro-micro coupled method and a capturing liquid method. The growth of the dendritic is controlled by the solution of the phase field equation. Since it is difficult to compute the microstructure of a whole casting, a scheme is adopted that the temperature field is calculated on the whole casting while the microstructure computation is carried out by selecting a macro-cell in the casting samples. Under the condition of not changing the phase-field model, a calculating method is advanced, which captures the liquid cells into the interface ones. In that method, every calculated micro-cell is endowed with a variable which identifies whether the cell is in the interface region and the phase field equation is only solved for those cells in that region. When the grain grows, the method captures the liquid cells into the interface ones and pushes the interface regi on forward. For the captured liquid cells, the values of the phase variable are modified. We apply the calculating method to improve the calculating efficiency, so the simulation of three-dimensional single grain and multiple grains growth for aluminum alloy samples, can be processed. The simulation results are compared with those obtained experimentally.
Optimal photoluminescence spectrum from Ti-doped ZnTe
Shao Jun
2003, 52 (7): 1743-1747. doi: 10.7498/aps.52.1743
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In this paper, experimental conditions are carefully optimized of excitation energy, power and degree of focus, water disturbance and temperature to overcome th e difficulties involved in photoluminescence (PL) study of ZnTe:Ti. It is illust rated that:(i)Higher PL intensity is accessible with excitation energy just belo w the band-gap energy of ZnTe.(ii)Water disturbance not only changes the relative intensities of the PL lines but also increases the measuring error of the transition energies. (iii) Excitation with relatively stronger focus produces better PL spectrum. Two zero-phonon lines (ZPLs) are unambiguously identified at 3903.5 and 3905.9 cm-1, respectively. They are ascribed to be due to Ti3+ in ZnTe with the aid of the crystal field theory, the transition energy, the energy difference of the two ZPLs and the temperature dependence of the relative intensity of the ZPLs. It is the first evidence from PL study for the Ti3+ in Ⅱ-Ⅵ compound semiconductors.
Effects of annealing on the structure and photoluminescence of ZnO films
Fang Ze-Bo, Gong Heng-Xiang, Liu Xue-Qin, Xu Da-Yin, Huang Chun-Ming, Wang Yin-Yue
2003, 52 (7): 1748-1751. doi: 10.7498/aps.52.1748
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Highly oriented polycrystalline ZnO films have been prepared by rf actively sputtering technique. We have investigated the structural and optical properties of ZnO films. x-ray diffraction was employed to analyze the influence of the post-treatment on the properties of ZnO thin films. The grain size increases with annealing temperature. The shift of the diffraction peak position from its normal powder value was observed. The photoluminescence (PL) spectra of these samples consist of one emission peak centred at 2.9eV.The intensities of PL peaks decrease with increasing annealing temperature. We propose that the emission comes from the interstitial Zn.
Nonlinear electrical properties of (Sr, Co, Nb)-doped SnO2 varistors
Qi Peng, Wang Jin-Feng, Chen Hong-Cun, Su Wen-Bin, Wang Wen-Xin, Zang Guo-Zhong, Wang Chun-Ming
2003, 52 (7): 1752-1755. doi: 10.7498/aps.52.1752
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The effect of Sr on the microstructure and nonlinear electrical properties of the (Co, Nb)-doped SnO2 varistors was investigated. The breakdown volta ge of the SnO2-based varistors increased significantly from 240 to 1482V/mm w ith increasing SrCO3 concentration from 0 to 1.50 mol%. Measurement of the ba rrier height at grain boundaries reveal that the significant decrease of the SnO2 grain size with increasing SrCO3 concentration from 0 to 1.50 mol%, is the reason of the enhancement of the breakdown voltage. The origin for the reduction of SnO2 grain size with increasing SrCO3 concentration was ex plained. The 1.50 mol% SrCO3-doped SnO2 varistor with ultrahigh breakdown vo ltage (1482V/mm) and larger nonlinear coefficient α(21.4) is a candidate used in the ultrahigh voltage protection system.
Influnce of Al-content on the property of the two-dimensional electron gases in AlxGa1-xN/GaN heterostructures
Kong Yue-Chan, Zheng You-Dou, Chu Rong-Ming, Gu Shu-Lin
2003, 52 (7): 1756-1760. doi: 10.7498/aps.52.1756
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By self-consistently sovling the coupled Schrdinger and Poisson equations, we have investingated the property of the two-dimensional electron gases (2DEG) in AlxGa1-xN/GaN heterostructures. We demonstrate the depen dence of the density, the distribution, and the subband occupation of the 2DEG on the Al-content of the AlGaN barrier. Band offset and mechanism of spontaneous and piezoelectric polarization were concerned to discuss our results.
Study of optical properties in GaAs1-xSbx/GaAssingle quant um wells
Luo Hai-Lin, Wang Yu-Qi, Wang Jian-Nong, Ge Wei-Kun, Luo Xiang-Dong, Bian Li-Feng, Xu Zhong-Ying
2003, 52 (7): 1761-1765. doi: 10.7498/aps.52.1761
Abstract +
GaAsSb/GaAs single quantum wells (SQWs) grown by molecular beam epitaxy are studied by selectively-excited photoluminescence (SEPL) measurement. For the first time, we have simultaneously observed the PL from both type Ⅰ and type Ⅱ transitions in GaAsSb/GaAs heterostructure in the SEPL. The two transitions exhibit different PL behaviours under different excitation energy. As expected, the peak energy of type I emission remains constant in the whole excitation energy range we used, while type Ⅱ transition shows a significant blue shift with increasing excitation energy.The observed blue shift is well explained in terms of electron-hole charge separation model at the interface. Time-resolved(TR) PL exhibits more type Ⅱ characteristic of GaAsSb/GaAs QW. Moreover, the results of the excitation-power-dependent PL and TRPL provide more direct information on the type-Ⅱ nature of the band alignment in GaAsSb/GaAs quantum-well structures. By combining the experimental results with some simple calculations, we have obtained the strained and unstrained valence band offsets of Qv=1.145 and Q0v=0.76 in our samples, respectively.
Multipeak negative-differential-resistance device by combining SET and MOSFET
Zhang Zhi-Yong, Wang Tai-Hong
2003, 52 (7): 1766-1770. doi: 10.7498/aps.52.1766
Abstract +
A multipeak negative-differential-resistance(NDR) device which comprises a singl e-electron transistor (SET) and a metal-oxide-semiconductor field-effect-transis tor (MOSFET) can achieve infinite number of peaks in principle. The MOS device e liminates the large SD voltage dependence of the peak and volley currents of the SET. The multipeak NDR device can be widely used in multiple-valued logics, and analog-to-digital converters (ADCs). We obtain a multiple-valued memory unit th rough the multipeak NDR device. And by using the folding I-V characteristic, a four-bit ADC is achieved. Compared with the traditional flash ADC, the SET-MO SFET ADC is very simple.
The reversal of spontaneous magnetization in high Tc superconducting π rings
Ling Jian, Wang Ke, Xie Fei-Xiang, Ma Ping, Yang Tao, Wang Fu-Ren, Dai Yuan-Dong
2003, 52 (7): 1771-1775. doi: 10.7498/aps.52.1771
Abstract +
The reversal of spontaneous magnetization of a superconducting π ring under a pulse current injection has been thoroughly analyzed with the fourth-ordered Runge-Kutta method. We find that the reversals from negative to positive and from positive to negative will produce different pulse output with a periodic pulse injection and some certain parameters which can give a checkout of directions of the spontaneous magnetization as well as the phenomena of reversal under pulse injection.
Phase transitions of XY model in ultra-thin magnetic film with a triangular latt ice
Wang Yi, Han Ru-Qi, Liu Xiao-Yan, Horiguchi Tsuyoshi
2003, 52 (7): 1776-1782. doi: 10.7498/aps.52.1776
Abstract +
Phase transitions and critical phenomena have been clarified for an XY model on the triangular lattice of ultra-thin magnetic films. We have calculated the ther modynamic quantities such as the internal energy, the specific heat, the chirali ty and so on by using Monte-Carlo simulations. The critical behaviours have been clarified by using finite-size scaling analyses. We have found a new critical b ehaviour due to a steep increase of the chirality as a function of temperature f or the XY model on a layered triangular lattice with antiferromagnetic layers an d ferromagnetic layers in some range of interaction parameters, besides the Kost erlize-Thouless transition and the usual chirality transition. In the new chiral ity transition, the value of a critical exponent has not been observed over any thickness dependence of this value as far as the layered systems are concerned. In the other chirality transition, the value of a critical exponent has been obs erved over the thickness.
SFM investigation of nanoscale domain structure in ferroelectric PZT thin films
Zeng Hua-Rong, Li Guo-Rong, Yin Qing-Rui, Tang Xin-Gui
2003, 52 (7): 1783-1787. doi: 10.7498/aps.52.1783
Abstract +
Nanoscale domain structures and polarization reversal behaviour in (111)-oriented PZT60/40 thin film were investigated in-situ with scanning force microscopy p iezoresponse mode. Complex domain contrast is related to the arrangement of doma ins in grains and to the orientation of the grains in the film. The step structu re of ~ 30 nm in width was directly observed, which was formed during the polar ization reversal process. The presence of the step structures reveals that the f orward domain growth mechanism prevails in the polarization switching process of PZT60/40 thin films.
Structural properties of GaN1-xPx films
Chen Dun-Jun, Shen Bo, Zhang Kai-Xiao, Deng Yong-Zhen, Fan Jie, Zhang Rong, Shi Yi, Zheng You-Dou
2003, 52 (7): 1788-1791. doi: 10.7498/aps.52.1788
Abstract +
X-ray diffraction (XRD) and Raman spectra for a series of high-phosphorus-content GaN1-xPx films, with phosphorus content up to 15%, gr own by means of light-radiation heating, low-pressure metal-organic chemical vapor deposition have been investigated. XRD results show that (0002) peaks of GaN1-xPx samples have shifted to smaller angles compared with t hat of undoped GaN sample. The Raman spectra of GaN1-xPx films exh ibit four new vibrational modes compared with undoped GaN sample. These modes are assigned to gap modes related to the Ga—P bond vibrations, local vibrational mode related to the phosphorus clusters, and disorder-activated scattering, respectively. The frequency of the A1(LO) mode is found to decrease with incre asing x. This redshift is attributed to the effects of alloying and strain.
The photoluminescence characterization of rare earths (Tb, Gd) embedded into por ous silicon
Peng Ai-Hua, Xie Er-Qing, Jiang Ning, Zhang Zhi-Min, Li Peng, He De-Yan
2003, 52 (7): 1792-1796. doi: 10.7498/aps.52.1792
Abstract +
Rare earth (Tb,Gd) ions were embedded into porous silicon films by electrochemical method. Fluorescence photospectrometer and scanning electron microscope were employed to characterize the photoluminescence and surface morphology of samples. The distribution of rare earth ions embedded into porous silicon films was observed by Rutherford backscattering spectrometry. The luminescence intensity of porous silicon after doping is greatly increased. Blue shift of luminescence peak was observed also. It is attributed to the transition luminescence of transitions between 4f energy levels of Tb3+, such as 5D4 —7F3,5D4—7F2 and 5D 4—5F0. Intense blue luminescence was observed after doping with Gd. The luminescence mechanism of porous silicon doped with rare earths was discussed also.
The effect of annealing on the field emission properties of amorphous CNx films
Zheng Wei-Tao, Bian Hai-Jiao, Lü Xian-Yi, Jiang Zhi-Gang, Bai Yi-Zhen, Jin Zeng-Sun, Zhao Yong-Nian, Li Jun-Jie
2003, 52 (7): 1797-1801. doi: 10.7498/aps.52.1797
Abstract +
The carbon nitride films deposited by r.f. magnetron sputtering in pure N2 discharge were annealed in vacuum up to 900 ℃. The chemical composition and bonding structure of the films were studied using x-ray photoelectron spectroscopy, Fourier Transform Infrared. The effects of thermal annealing on the bonding structure and the electron field emission characteristics of CNx films wer e investigated. It is found that the sp2 bonds and N content in CNx films are closely related to the filed emission of CNx films. The results show that thermal annealing treatment causes a great loss of N content and a larger formation of sp2 bonds in CNx films, which would influence sign ificantly the field emission properties for the CNx films. The CNx films annealed at 750 ℃ show the optimal electron emission properties. Besides, the correlation between the chemical bonding structures and electron emission properties for the CNx films was also discussed.
Effect of grid and bias on the characteristic of CHF3 electron cyclot ron resonance discharge plasma
Ye Chao, Du Wei, Ning Zhao-Yuan, Cheng Shan-Hua
2003, 52 (7): 1802-1807. doi: 10.7498/aps.52.1802
Abstract +
The characteristics of CHF3 electron cyclotron resonance(ECR) plasma, which is formed in the case of a floating grid in an ECR-chemical vapor deposition system and +60V or -60V biased at grid, were investigated by an actinometric optical emission spectroscopy. It is found that the effects of grid and biasing on the distribution of radicals occur mainly at a low microwave input power. It is considered that the distribution of radicals at a low power is controlled by ele ctron collision and sheath potential together due to low electron temperature. H owever, the effect of sheath potential at a high power decreases due to increasi ng electron temperature. As a result, the distribution of radicals at a high pow er is dominated by electron collision.
The effect of different catalysts on the growth of boron carbonitride nanotubes by thermal decomposition
Guo Xin-Yong, Du Zu-Liang, Zhang Hong-Rui, Ding Pei, Lia ng Er-Jun
2003, 52 (7): 1808-1811. doi: 10.7498/aps.52.1808
Abstract +
The effect of cobalt, nickel, cobalt/nickel, cobalt/ferrocene, nickel/ferrocene and ferrocene catalysts on the morphology and yield of boron carbonitride(BCN) nanotubes produced by thermal decomposition at 860℃ was studied. It is found that the catalysts have a strong effect on the growth of BCN nanotubes. Transmission electron microscopy images reveal that bamboo-shaped thinner wall nanotubes with a higher yield are produced with nickel/ferrocene and cobalt/ferrocene as catalysts, whereas thicker wall nanotubes with a lower yield are generated with cobalt, nickel and cobalt/nickel as catalysts. BCN nanotubes cannot be produced with ferrocene alone as catalyst. Catalyst particles were found together with the BCN nanotubes. The morphology and yield of BCN nanotubes depend on the catalysts in the following order: nickel/ferrocene≈cobalt/ferrocene>cobalt≈nickel>cobalt/nickelferrocene. Their Raman spectroscopies were also studied.
Localization of current dipoles in a realistic head shape model by a magnetoence phalogram-multiple signal classification algorithm
Zhu Hong-Yi, Li Jun, Shen Jian-Qi, He Sai-Ling
2003, 52 (7): 1812-1817. doi: 10.7498/aps.52.1812
Abstract +
It has been proved that the magnetoencephalogram-multiple signal classification algorithm has many advantages over the general global optimization methods in localizing current dipoles in a spherically symmetrical conductor head model. Making use of this method, we can tell the total number of current dipoles conveniently and localize those dipoles one by one in a shorter time, simply by calculating generalized eigenvalues of a two- or three-dimensional matrix. In this paper, this method is used to localize the current dipoles in a realistic head shap e model. Numerical simulation demonstrates that this algorithm is effective and has great advantages in the localization of multiple dipoles.
Investigation on magnetic pulse compressor used for copper halide laser
Liu Cai-Ming
2003, 52 (7): 1818-1821. doi: 10.7498/aps.52.1818
Abstract +
In copper halide lasers, the excitation of copper atoms is produced through the discharge of a storage capacitor. Performances of it were essentially linked to the type of switch used and also to its mode of operation. Thyratrons are the most common switches. Nevertheless, their technological limitations do not allow a high repetition rate and high rate of rise of current. But these limitations can be surpassed by combining the thyratron to magnetic pulse compression (MPC). The MPC driver can improve the laser excitation pulse rise time and increase the repetition rate. A single stage MPC system for CuBr laser has been developed in this paper. With this MPC, the power of laser output was increased more than 20%. The mechanism of MPC performance was analyzed. The design advisements and parameter choices of MPC were presented particularly.
The statistical entropy of Dirac field in spherically symmetric non-static black hole
Meng Qing-Miao, Su Jiu-Qing, Li Chuan-An
2003, 52 (7): 1822-1826. doi: 10.7498/aps.52.1822
Abstract +
In terms of the improved brick-wall model, the statistical entropy of Dirac field in the most general spherically symmetric non-static black hole is calculated. It is shown that the entropy of the black hole is proportional to the area of event horizon at any time. It is noteworthy that the calculating formula for the dynamic proportional coefficient is obtained in this paper. Through calculating these dynamic proportional coefficients, the statistical entropies of the Dirac field in all kinds of the spherically symmetric non-static black hole can be obtained directly.