Table of contents

16 May 2003, Volume 12 Issue 5 Previous issue    Next issue

GENERAL

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Canonical symmetry properties of the constrained singular generalized mechanical system Li Ai-Min (李爱民), Jiang Jin-Huan (江金环), Li Zi-Ping (李子平) Chinese Physics, 2003, 12 (5):  467-471.  DOI: 10.1088/1009-1963/12/5/301 Abstract ( 995 )   PDF (210KB) ( 564 )   Based on generalized Apell－Chetaev constraint conditions and to take the inherent constrains for singular Lagrangian into account, the generalized canonical equations for a general mechanical system with a singular higher-order Lagrangian and subsidiary constrains are formulated. The canonical symmetries in phase space for such a system are studied and Noether theorem and its inversion theorem in the generalized canonical formalism have been established.

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Solitons in a generalized (2+1)-dimensional Ablowitz－Kaup－Newell－Segur system Zheng Chun-Long (郑春龙), Zhang Jie-Fang (张解放), Wu Feng-Min (吴锋民), Sheng Zheng-Mao (盛正卯), Chen Li-Qun (陈立群) Chinese Physics, 2003, 12 (5):  472-478.  DOI: 10.1088/1009-1963/12/5/302 Abstract ( 1323 )   PDF (411KB) ( 716 )   In the previous Letter (Zheng C L and Zhang J F 2002 Chin. Phys. Lett. 19 1399), a localized excitation of the generalized Ablowitz－Kaup－Newell－Segur (GAKNS) system was obtained via the standard Painlevé truncated expansion and a special variable separation approach. In this work, starting from a new variable separation approach, a more general variable separation excitation of this system is derived. The abundance of the localized coherent soliton excitations like dromions, lumps, rings, peakons and oscillating soliton excitations can be constructed by introducing appropriate lower-dimensional soliton patterns. Meanwhile we discuss two kinds of interactions of solitons. One is the interaction between the travelling peakon type soliton excitations, which is not completely elastic. The other is the interaction between the travelling ring type soliton excitations, which is completely elastic.

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The effects of dust temperature and the dust charge variation in a dusty plasma with vortex-like ion distribution Duan Wen-Shan (段文山) Chinese Physics, 2003, 12 (5):  479-482.  DOI: 10.1088/1009-1963/12/5/303 Abstract ( 1478 )   PDF (188KB) ( 740 )   By considering both the dust temperature and the dust charge variation in dusty plasma with vortex-like ion distribution, we obtained a modified Korteweg－de Vries equation. It indicates that the effect of dust charge variation can cause the one-dimensional soliton amplitude to become larger, and the dust temperature can cause the soliton amplitude to become larger as well. Moreover, as the dust temperature increases, the soliton amplitude will increase.

NUCLEAR PHYSICS

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Experimental study on the activation process of GaAs spin-polarized electron source Ruan Cun-Jun (阮存军) Chinese Physics, 2003, 12 (5):  483-487.  DOI: 10.1088/1009-1963/12/5/304 Abstract ( 1098 )   PDF (215KB) ( 493 )   GaAs spin-polarized electron source is a new kind of electron source, where the GaAs semiconductor crystal is used as a photocathode under the irradiation of helicity light. In this paper the activation process of the GaAs spin-polarized electron source is investigated experimentally in detail, during which the negative electron affinity of the photo cathode should be achieved more carefully by absorbing the caesium and oxygen on the surface of the GaAs crystal under ultrahigh vacuum conditions. Besides the different activation processes, the important physical parameters are studied to achieve the optimum activation results. At the same time the stability and lifetime of the polarized electron beam are explored for future experiments. Some important experimental data have been acquired.

ATOMIC AND MOLECULAR PHYSICS

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The excitation energies and term energies of the excited states 1s2ns (n=3,4,5) and 1s2nf (n=4,5) of lithium-like systems of Z=11－20 Ge Zi-Ming (葛自明), Wang Zhi-Wen (王治文), Zhou Ya-Jun (周雅君), He Li-Ming (贺黎明), Liu Guo-Guang (刘国光) Chinese Physics, 2003, 12 (5):  488-494.  DOI: 10.1088/1009-1963/12/5/305 Abstract ( 1263 )   PDF (242KB) ( 454 )   In this paper, the full-core plus correlation (FCPC) and the Ritz method is extended to calculate the non-relativistic energies of 1s2ns (n=3,4,5) and 1s2nf (n=4,5) states and the wavefunctions of the lithium-like systems from Z=11－20. The mass-polarization and the relativistic correction including the kinetic-energy correction, the Darwin term, the electron－electron contact term, and the orbit－orbit interaction are evaluated perturbatively as the first-order correction. The contribution from quantum electrodynamic is also included by using the effective nuclear charge formula. The excited energies, the term-energy and fine structure, are given and compared with the other theoretical calculation and experimental results. It is shown that the correlative wave in the FCPC method embodies well the strong correlation between the 1s2 core and the valence electron.

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Effects of discharge current and voltage on the high density of metastable helium atoms Feng Xian-Ping (冯贤平), D Andruczyk, B W James, K Takiyama, S Namba, T Oda Chinese Physics, 2003, 12 (5):  495-501.  DOI: 10.1088/1009-1963/12/5/306 Abstract ( 1002 )   PDF (274KB) ( 559 )   Both hollow-cathode and Penning-type discharges were adopted to excite helium atoms to a metastable state. Experimental data indicate that Penning discharge is more suitable for generating high fractions of metastables in a low-density helium beam for laser-induced fluorescence technique in measuring electric fields at the edge of a plasma. The metastable density increases with increasing helium gas pressure in the range of 1.33×10-2－66.7Pa. The highest metastable density of 3.8×1016m-3 is observed at a static gas pressure of 66.7Pa. An approximately linear relationship between the density of metastable helium atoms and the plasma discharge current is observed. Magnetic field plays a very important role in producing a high density of metastable atoms in Penning discharge.

CLASSICAL AREAS OF PHENOMENOLOGY

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A modified split-step Fourier method for optical pulse propagation with polarization mode dispersion Rao Min (饶敏), Sun Xiao-Han (孙小菡), Zhang Ming-De (张明德) Chinese Physics, 2003, 12 (5):  502-506.  DOI: 10.1088/1009-1963/12/5/307 Abstract ( 1389 )   PDF (275KB) ( 706 )   A modified split-step Fourier method (SSFM) is presented to solve the coupled nonlinear Schr?dinger equation (CNLS) that can be used to model high-speed pulse propagation in optical fibres with polarization mode dispersion (PMD). We compare our approach with the SSFM and demonstrate that our approach is much faster with no loss of accuracy. We discuss the pulse distortion and system Q-factor of non-return-to-zero (NRZ), return-to-zero (RZ) and pre-chirped RZ (CRZ) formats in the presence of high PMD through this approach. The simulation results show that CRZ pulses are the most tolerant to high PMD values and the extinct ratio has a great impact on the transmission performance.

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Optical limiting effect based on stimulated Brillouin scattering in CCl4 Lü Zhi-Wei (吕志伟), Lü Yue-Lan (吕月兰), Yang Jun (杨珺) Chinese Physics, 2003, 12 (5):  507-513.  DOI: 10.1088/1009-1963/12/5/308 Abstract ( 1393 )   PDF (261KB) ( 439 )   The optical limiting effect based on stimulated Brillouin scattering (SBS) in a nonlinear medium was investigated. We numerically treated the nonlinear propagation process with a theoretical model, which includes the spontaneous nature of the initiation of SBS, and obtained optical limiting effect in the process. Energy limiting, pulse reshaping and stabilization have been demonstrated on SBS mechanism with the nonlinear medium CCl4. The input optical signals were Nd:YAG nanosecond laser pulses with width varying from 16ns to 7ns then to 2ns, the relationship between the transmitted signal and launched pump signal was shown. In the experimental regime, the most stable pulse and a superior energy stabilization of the transmitted pulse were obtained when the laser pulse-width became as short as 2ns. For the energy variation of laser pulses in a wide range of 14－88mJ, the output energy was limited in a quite narrow range 4.5－5.5mJ.

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Period and temperature tuning of cascaded optical parametric oscillator based on periodically poled LiNbO3 Lin Xue-Chun (林学春), Li Rui-Ning (李瑞宁), Yao Ai-Yun (姚爱云), Bi Yong (毕勇), Cui Da-Fu (崔大复), Xu Zu-Yan (许祖彦) Chinese Physics, 2003, 12 (5):  514-517.  DOI: 10.1088/1009-1963/12/5/309 Abstract ( 1375 )   PDF (216KB) ( 477 )   We report the broadly tunable source by a cascaded optical parametric oscillator in the periodically poled LiNbO3 (PPLN) with domain grating period and temperature tuning. The optical parametric oscillator was pumped by a passive Q-switched Nd:YVO4 laser. Multi-wavelength outputs from visible to infrared were obtained. The temperature of the PPLN crystal changed within the range of 70－150℃ with different periods of PPLN. The tunable range covered from 433 to 1657nm.

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Effects of transverse profile of pump field on second harmonic generation in periodic nonlinear materials Zhang Han-Zhuang (张汉壮), Yang Jian-Bing (杨建冰), Gao Jin-Yue (高锦岳) Chinese Physics, 2003, 12 (5):  518-523.  DOI: 10.1088/1009-1963/12/5/310 Abstract ( 1314 )   PDF (343KB) ( 424 )   We report on a theoretical analysis of the effects of a converging pump field of Gaussian transverse profile on second harmonic generation in a periodic nonlinear material with quasi-phase-matching. The outputs of the centre intensity and the intensity flux for second harmonic generation are derived by simulation, based on the parameters of quasi-phase-mismatch, the waist and focus positions of the input pump beam. The results show that when the transverse profile of the pump field is taken into account, the quasi-phase-match value and focus position of input beam for maximal second harmonic generation follow new criteria.

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Experimental study of conversion from atomic high-order harmonics to x-ray emissions Wang Qi (王骐), Chen Jian-Xin (陈建新), Xia Yuan-Qin (夏元钦), Chen De-Ying (陈德应) Chinese Physics, 2003, 12 (5):  524-527.  DOI: 10.1088/1009-1963/12/5/311 Abstract ( 1117 )   PDF (228KB) ( 389 )   There are two physical phenomena in a strong laser intensity. One is the high-order harmonic emission; the other is x-ray emission from optical-field ionized plasmas. The experiment of conversion from high-order harmonics to x-ray emissions was given with a 105fs Ti:sapphire laser by adjusting laser intensities. The ingredient in plasma was investigated by the numerical simulations. Our experimental results suggested that the free electrons have detrimental effects on harmonic generation but are favourable for x-ray emission from optical-field ionized plasmas. If we want to obtain more intense harmonic signals as a coherent light source in the soft x-ray region, we must avoid the production of free electrons in plasmas. At the same time, if we want to observe x-rays for the development of high-repetition-rate table-top soft x-ray lasers, we should strip all atoms in the plasmas to a necessary ionized stage by the optical-field-ionization in the field of a high-intensity laser pulse.

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Single mode rate equations for two sections self-pulsating DFB laser Wang Chun-Lin (王春林), Wu Jian (伍剑), Lin Jin-Tong (林金桐) Chinese Physics, 2003, 12 (5):  528-531.  DOI: 10.1088/1009-1963/12/5/312 Abstract ( 1260 )   PDF (181KB) ( 450 )   We propose a set of single mode rate equations for multi-section distributed feedback (DFB) lasers. On the basis of the rate equations, the self-pulsation in the two-section DFB lasers with asymmetrical injection current was explained. It was found that the dynamic distribution of the power in two sections play key role in the self-pulsation running.

PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES

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“Cold” dispersion relation of corrugated waveguide filled with plasma immersed in a finite magnetic field Li Wei (李伟), Wei Yan-Yu (魏彦玉), Xie Hong-Quan (谢鸿全), Liu Sheng-Gang (刘盛纲), Gong Ma-Li (巩马理) Chinese Physics, 2003, 12 (5):  532-537.  DOI: 10.1088/1009-1963/12/5/313 Abstract ( 1237 )   PDF (256KB) ( 708 )   A general dispersion equation of a partially filled plasma corrugated waveguide immersed in a finite magnetic field is presented. When the guiding magnet $B_0 \to \infty$ or 0, this equation can be reduced to the results obtained in previous works.

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Effect of dust charge variation on dust-acoustic solitary waves in a magnetized two-ion-temperature dusty plasma Xue Ju-Kui (薛具奎), Lang He (郎和) Chinese Physics, 2003, 12 (5):  538-541.  DOI: 10.1088/1009-1963/12/5/314 Abstract ( 1341 )   PDF (209KB) ( 545 )   The effect of dust charge variation on the dust-acoustic solitary structures is investigated in a warm magnetized two-ion-temperature dusty plasma consisting of a negatively and variably charged extremely massive dust fluid and ions of two different temperatures. It is shown that the dust charge variation as well as the presence of a second component of ions would modify the properties of the dust-acoustic solitary structures and may excite both dust-acoustic solitary holes (soliton waves with a density dip) and positive solitons (soliton waves with a density hump).

CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES

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Dependence of in-tube doping on the radius and helicity of single-wall carbon nanotubes Liu Hong (刘红), Dong Jin-Ming (董锦明), Qian Mei-Chun (钱湄杶), Wan Xian-Gang (万贤纲) Chinese Physics, 2003, 12 (5):  542-547.  DOI: 10.1088/1009-1963/12/5/315 Abstract ( 1274 )   PDF (233KB) ( 387 )   Using the Lennard－Jones interaction potential between the impurity atom and carbon atom, we have studied the dependence of in-tube impurity doping on the radius of a single-wall carbon nanotube (SWNT), as well as its helicity. The obtained results show that the radius of the most stably doped SWNT is different for different kinds of impurity atoms. This is useful for producing the required doped SWNT. In addition, it is found that the helicity of tube has a strong effect on the potential energy of the atoms doped in the SWNT.

CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES

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Electronic confinement in quasi-one-dimensional triblock copolymers Liu De-Sheng (刘德胜), Wang Lu-Xia (王鹿霞), Xie Shi-Jie (解士杰), Han Sheng-Hao (韩圣浩), Mei Liang-Mo (梅良模) Chinese Physics, 2003, 12 (5):  548-552.  DOI: 10.1088/1009-1963/12/5/316 Abstract ( 1147 )   PDF (240KB) ( 568 )   A tight-binding calculation to describe the triblock copolymer xPA(polyacetylene)/nPPP (poly(p-phenylene))/yPA or xPPP/nPA/yPPP is presented. The interfacial coupling between homopolymer segments is attributed to the hopping of $\pi$-electrons and the coupling of $\sigma$-bonds. The dependence of the band gap of triblock copolymers on the interfacial couplings or on the composite segment lengths is studied. The influence of composite segment lengths on the electron density is also studied. For nPPP/xPA/nPPP structures, the band gap varies with PA segment length over a wide range of 1.32－2.74eV. For nPA/xPPP/nPA structures,the band gap is invariant with PPP segment length. It is found that a spontaneous tunnelling phenomenon could take place in nPA/xPPP/nPA structures. Furthermore, the polaron caused by doping an electron into nPA/xPPP/nPA will tend to be confined in one of the well (PA) parts. This kind of confinement may increase the electron－hole recombination probability.

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The frequency dependence of AC transport losses in stacked Bi-2223/Ag superconducting tapes Zhang Guo-Min (张国民), Lin Liang-Zhen (林良真), Xiao Li-Ye (肖立业), Qiu Ming (丘明), Yu Yun-Jia (余运佳), Hui Dong (惠东) Chinese Physics, 2003, 12 (5):  553-556.  DOI: 10.1088/1009-1963/12/5/317 Abstract ( 1051 )   PDF (223KB) ( 399 )   AC transport losses in a single superconducting tape, double- and triple-stacked Bi-2223/Ag superconducting tapes were measured by use of electrical method. The measurements were carried out at 77K with the frequency of AC transport currents ranging from 50 to 1000Hz. The dependence of AC losses on frequency and the number of tapes in the stack were presented and analysed.

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Bifurcation, chaotic phenomena and control of chaos in a one-dimensional discrete Josephson lattice Du Hao-Chen (杜浩晨), Liao Hong-Yin (廖红印), Zhou Shi-Ping (周世平) Chinese Physics, 2003, 12 (5):  557-561.  DOI: 10.1088/1009-1963/12/5/318 Abstract ( 1161 )   PDF (270KB) ( 483 )   We have investigated the fluxon dynamical behaviour in a one-dimensional parallel array of small Josephson junctions in the presence of an externally applied magnetic field. In the case of high damping, the system is in stable states. On the contrary, in the case of low damping, bifurcation and chaotic phenomena have been observed. Control of chaos is achieved by a delayed feedback mechanism, which drives the chaotic system into a selected unstable periodic orbit embedded within the associated strange attractor. It is attractive to control chaos to a periodic state, rather than operating always outside the device parameter space where chaos dominates.

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Magnetic properties and magnetoresistance of HfFe2Ge2-type Dy1-xGdxMn6Ge6 (x=0.1－0.6) compounds Zhang Li-Gang (张立刚), Wang Ru-Wu (汪汝武), Li Lu (李路), Zhang Shao-Ying (张绍英), Yao Jin-Lei (姚金雷), Shen Bao-Gen (沈保根) Chinese Physics, 2003, 12 (5):  562-565.  DOI: 10.1088/1009-1963/12/5/319 Abstract ( 1351 )   PDF (227KB) ( 459 )   The magnetic properties and magnetoresistance effects of Dy$_{1-x}$Gd$_x$Mn$_6$Ge$_6$ (x=0.1－0.6) compounds have been studied by magnetic properties and resistivity measurements in applied magnetic fields up to 5T. The compounds with x=0.1, 0.2, 0.4 and 0.5 order antiferromagnetically at 425, 428, 430 and 432K, respectively, and there are second magnetic phase transitions below 100K. The compound with x=0.6 exhibits a transition from ferrimagnetic to antiferromagnetic, then to ferrimagnetic state again with decreasing temperature. Furthermore, it displays a field-induced metamagnetic transition, and its threshold field decreases with increasing temperature. The magnetoresistance curve of the compound with x=0.6 in applied magnetic fields up to 5T is presented and the magnetoresistance effects are related to the metamagnetic transitions.