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Volume 20 Issue 6
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Original Articles
Peakon and Foldon Excitations In a (2+1)-Dimensional Breaking Soliton System
ZHENG Chun-Long, , ZHANG Jie-Fang, HUANG Wen-Hua, CHEN Li-Qun
Chin. Phys. Lett. 2003, 20 (6): 783-786 .  
Abstract   PDF(pc) (726KB) ( 789 )
Starting from the standard truncated Painlevé expansion and a variable separation approach, a general variable separation solution of the breaking soliton system is derived. In addition to the usual localized coherent soliton excitations like dromions, lumps, rings, breathers, instantons, oscillating soliton excitations, and previously revealed chaotic and fractal localized solutions, some new types of excitations, peakons and foldons, are obtained by introducing appropriate lower dimensional piecewise smooth functions and multiple valued functions.
Effect of Noise on Berry’s Phase for Quantum Computing
JING Hui, GE Mo-Lin
Chin. Phys. Lett. 2003, 20 (6): 787-789 .  
Abstract   PDF(pc) (211KB) ( 409 )
Based on a new type of two-level-system reservoir, we investigate the impact of dissipation on Berry’s phase of a spin trapped in a periodical external field. It is found that the existence of environmental noise could lead to a decaying term in the matrix of Berry’s phase as the sign of a decoherence process, which is in agreement with the result of Nazir et al. (Phys. Rev. A 65 (2002) 042303). Particularly, in comparison with a specialized case of the traditional Leggett dissipation model, we only shows the dependence of time but not temperature in decaying term. A concrete case is exhibited by using the one-dimensional Ohmic function.
Stimulated Emission of an Atom in Circularly Polarized Light
LI Jin-Hui, ZENG Gao-Jian, YE Yong-Hua
Chin. Phys. Lett. 2003, 20 (6): 790-792 .  
Abstract   PDF(pc) (228KB) ( 357 )
We study the stimulated emission of a two-level atom in an electromagnetic wave of circular polarization. The correlation function G(r1t, r2t) = < E--E+ > of atom radiation fields at dipole approximation are computed. Under the resonance condition, the atom stimulated emission is influenced by the circularly polarized electromagnetic wave discussed. We have found that the time-averaged value of energy density does not depend on the initial conditions. we have also deduced the relation between the emission power of an atom and the Rabi frequency Ω.

Geometric Phase for Mixed States
TONG Dian-Min, CHEN Jing-Ling, DU Jiang-Feng
Chin. Phys. Lett. 2003, 20 (6): 793-795 .  
Abstract   PDF(pc) (200KB) ( 417 )
The geometric phase of mixed states with non-degenerate eigenvalues is investigated. A general formula of geometric phase for mixed state under unitary evolution is given. In particular, we also furnish an expression of Hamiltonians for equivalent evolutions, by which one can understand what kind of evolutional operator U(t) (or Hamiltonian) is related to zero instantaneous dynamic phase. Moreover, the geometric phase and related Hamiltonians in the spin-half case are provided as an explicit example.

Solution to the Master Equation of a Free Damped Harmonic Oscillator with Linear Driving
YANG Jie, LU Huai-Xin, ZHAO Bo, ZHAO Mei-Sheng, ZHANG Yong-De
Chin. Phys. Lett. 2003, 20 (6): 796-798 .  
Abstract   PDF(pc) (203KB) ( 819 )
We use the Lie algebra representation theory for superoperators to solve the master equation for a harmonic oscillator with a linear driving term in a squeezed thermal reservoir. By using the quantum displacement transformation and squeeze transformation, we show that the master equation has an su(1,1) Lie algebra structure, with which we obtain the explicit solution to the master equation. A simple but typical example is given to illustrate our method.
Evidence for a Bose-Einstein Condensate in Dilute Rb Gas by Absorption Image in a Quadrupole and Ioffe Configuration Trap
WANG Yu-Zhu, ZHOU Shu-Yu, LONG Quan, ZHOU Shan-Yu, FU Hai-Xiang
Chin. Phys. Lett. 2003, 20 (6): 799-801 .  
Abstract   PDF(pc) (282KB) ( 682 )
We report the realization of Bose-Einstein condensation (BEC) in dilute rubidium gas. The BEC was achieved in a quadrupole and Ioffe configuration trap. The number of condensed atoms is around 4 x 104 in total 5 x 105, and the transition temperature is 250 nK. We have studied the light scattering of the atom cloud and the condensate in a tightly confined magnetic trap. We show that it is possible to use the diffraction patterns in the near-resonant imaging of the trapped cold atomic samples to give the information of the BEC phase transition.
(2+1)-Dimensional Envelope Solitons in a Disk-Shaped Bose-Einstein Condensate
HUANG Guo-Xiang
Chin. Phys. Lett. 2003, 20 (6): 802-805 .  
Abstract   PDF(pc) (227KB) ( 419 )
We show that, due to the nonlinear coupling between a wavepacket superposed by short-wavelength collective modes and a long-wavelength mean field generated by the self-interaction of the wavepacket, the (2+1)-dimensional envelope solitons decaying in all spatial directions, i.e., dromions, are possible nonlinear excitations in a disk-shaped Bose-Einstein condensate with a repulsive interatomic interaction.
Ultra High Energy Cosmic Rays: Strangelets?
XU Ren-Xin, WU Fei
Chin. Phys. Lett. 2003, 20 (6): 806-809 .  
Abstract   PDF(pc) (314KB) ( 329 )
The conjecture that ultra high energy cosmic rays (UHECRs) are actually strangelets is discussed. Besides the reason that strangelets can do as cosmic rays beyond the Greisen-Zatsepin-Kuzmin-cutoff, another argument to support the conjecture is addressed by the study of formation of TeV-scale microscopic black holes when UHECRs bombarding bare strange stars. It is proposed that the exotic quark surface of a bare strange star could be an effective astro-laboratory in the investigations of the extra dimensions and of the detection of ultra-high energy neutrino fluxes. The flux of neutrinos (and other point-like particles) with energy larger than 2.3 x 1020 eV could be expected to be smaller than 10-26cm-2 s-1 if there are two extra spatial dimensions.

Stochastic Resonance and Ratchets: Two Closely Related Phenomena in Brownian Motors
QIAN Min, WANG Yan, ZHANG Xue-Juan,
Chin. Phys. Lett. 2003, 20 (6): 810-812 .  
Abstract   PDF(pc) (333KB) ( 643 )
Ratchet effect as well as the phenomenon of stochastic resonance (SR) is investigated in a simple flashing ratchet model. Numerical simulations show that both the power spectrum amplitude and the mean mobility of the system vary non-monotonically in similar manners with the increase of the noise intensity and both reach their highest point nearly at the same noise level. We point out that the consistency of these two phenomena are just due to the existence of circular flux in non-equilibrium state.

Mean First-Passage Time of a Bistable Kinetic Model Driven by Multiplicative Coloured Noise and Additive White Noise
XIE Chong-Wei, MEI Dong-Cheng
Chin. Phys. Lett. 2003, 20 (6): 813-816 .  
Abstract   PDF(pc) (446KB) ( 579 )
We study the transient properties of a bistable kinetic system driven by correlated noises for the cases of multiplicative coloured noise and additive white noise. The mean first-passage time (MFPT) of the system is calculated. From numerical computations we find that: (i) The MFPT of the bistable system are affected by the correlation time of multiplicative coloured noise τ and the cross-correlation strength between noises λ and, τ and λ play the same roles in the MFPT. (ii) The MFPT corresponding to weakly correlated noises and strongly correlated noises exhibits the very different behaviour and there is a one-peak structure in the MFPT for strongly correlated noises. The peak grows highly as τincreases, which means that the noisy colour causes the suppression effect of the escape rate to become more pronounced.

A Complexity View of Acid Deposition
ZHU Jian-Lin, FANG Jian-Shu
Chin. Phys. Lett. 2003, 20 (6): 817-820 .  
Abstract   PDF(pc) (449KB) ( 390 )
We show that acid deposition is analogous to complex systems composed of a series of interconnected components. Frequency-size distributions of weekly hydrogen deposition (WHD) of precipitation are consistent with double power-law in two different regimes separated by a crossover WHD. The distribution of variations in acid deposition over a week interval is remarkably symmetrical, with long tail extending over eight orders of magnitude. The acid deposition fluctuations exhibit fractal Brown motion with two different temporal scaling regimes and long-range correlation exists in the series. The power-laws in the acid deposition dynamics are considered to be indicators of self-organization of atmosphere under environmental pollution stress.
Parton Distributions of Pions in the Valon Model
YANG Chun-Bin
Chin. Phys. Lett. 2003, 20 (6): 821-824 .  
Abstract   PDF(pc) (271KB) ( 455 )
Based on the universality of the parton distributions inside valons, parton distributions of pions are investigated and parameterized from the experimentally given parton distributions of proton within the valon model. The structure function F2(x) is given at fixed scale Q20 = 1 GeV2. Due to the abundance of the pair numbers of sea quarks at a very small x region, the structure function decreases rapidly for small x to a local minimum. Then from the contribution of the valence quarks, F2(x) increases to a local maximum at x 0.5.
Indication of Negative Triaxial Deformation in the Very Neutron-Deficient Odd-A Re Isotopes
ZHOU Xiao-Hong, XU Fu-Rong, ZHENG Yong, ZHANG Yu-Hu
Chin. Phys. Lett. 2003, 20 (6): 825-828 .  
Abstract   PDF(pc) (1106KB) ( 409 )
For the 9/2-[514] bands in light odd-A Re isotopes, the energy signature splitting and its relation with the signature dependence of M1 transition matrix elements are investigated in connection with the deviation of nuclear shape from axial symmetry. By comparing the energy signature splittings and relative magnetic transition rates between the experimental values and the theoretical calculations assuming axially symmetric shapes, it is found that discrepancies increase with the decreasing neutron number. These discrepancies strongly suggest an appreciable negative γ deformation for the very neutron-deficient odd-A Re isotopes.
Measurement of Change of 7Be Decay Rate in Be and Au
LIU Zhi-Yi, LI Cheng-Bo, WANG Si-Guang, ZHOU Jing, MENG Qiu-Ying, LU Shao-Jun, ZHOU Shu-Hua
Chin. Phys. Lett. 2003, 20 (6): 829-831 .  
Abstract   PDF(pc) (299KB) ( 521 )
We have measured the possible change of the decay rate of 7Be implanted into hosts of natural beryllium and natural gold. No difference between the 7Be decay rates in the two hosts is observed within the experimental precision of 0.12%. This result implies that change of the decay rate of 7Be implanted in different materials cannot be simply expected from the electron affinity difference consideration lonely and the lattice structure of the host materials should be taken into account.

Medium Influence of the Nucleon-Nucleon Cross Section on the Fragmentation
LIU Jian-Ye, , XING Yong-Zhong, GUO Wen-Jun
Chin. Phys. Lett. 2003, 20 (6): 832-835 .  
Abstract   PDF(pc) (487KB) ( 373 )
Based on an isospin-dependent quantum molecular dynamics model we studied the influence of a medium correction of an isospin-dependent nucleon-nucleon cross section on the fragmentation at the intermediate energy heavy ion collisions. We found that the medium correction from an isospin-dependent nucleon-nucleon cross section increases the dependence of the fragmentation on the isospin effect of in-medium nucleon-nucleon cross section, at the same time, the momentum-dependent interaction also produces an important role for enhancing the influence of the medium correction on the isospin effect of two-body collisions in the fragmentation process.

Thermal Charmed Quark Contribution to Dileptons in Chemically Equilibrating Quark-Gluon Matter
HE Ze-Jun, , LONG Jia-Li, LU Zhao-Hui, MA Yu-Gang, LIU Bo
Chin. Phys. Lett. 2003, 20 (6): 836-838 .  
Abstract   PDF(pc) (307KB) ( 755 )
We find that in a chemically equilibrating baryon-rich quark-gluon matter, due to the slow cooling rate, high initial temperature, large gluon density as well as large fusion cross section of gg → cc in the intermediate mass region, the gluon fusion gg → cc provides a dominant contribution to dileptons with intermediate masses, resulting in the significant enhancement of intermediate mass dileptons.

Energy and Auger Widths of Triply Excited 3p3p3p 2Po State of Lithium
GOU Bing-Cong, YU Kai-Zhi
Chin. Phys. Lett. 2003, 20 (6): 839-840 .  
Abstract   PDF(pc) (175KB) ( 396 )
The high-lying triply excited 3p3p3p 2Po state of the double hollow lithium atom is studied by using the saddle-point complex-rotation method. The energy and Auger widths of this resonance are calculated. The relativistic corrections and mass polarization are included. The total Auger width is obtained by coupling the important open channels and summing over the other channels. The oscillator strength is also calculated. The results are compared with other theoretical and experimental data in the literature.

Low-Lying Resonance States of Slow Electron Collisions With Atomic Oxygen
WU Jian-Hua, YUAN Jian-Min
Chin. Phys. Lett. 2003, 20 (6): 841-844 .  
Abstract   PDF(pc) (309KB) ( 346 )
A 39-target state close-coupling calculation of low-energy electron scattering from atomic oxygen is carried out with core-valence electron correlation by using R-matrix method. It is shown that the elastic cross section has a huge and sharp increase with the electron energy going down below 1 eV. This remarkable structure is attributed to a few very low-lying potential resonances and the features of these resonances are given with partial cross sections. It is also shown that after considering excitations of two electrons from 2s shell, the three lowest atomic energy levels are in agreement with experimental results better than that just considering excitations of two electrons from the 2p shell as well as only one electron from the 2s shell. Elastic and two excitation (3P → 1D and 3P → 1S) cross sections are given and compared with the other theoretical and experimental results.

Generation of Ultrashort Light Bullets in Dispersive Kerr Media Using the Fourth-Order Dispersion-Dependent Spatiotemporal Instability
WEN Shuang-Chun, QIAN Lie-Jia, FAN Dian-Yuan
Chin. Phys. Lett. 2003, 20 (6): 845-847 .  
Abstract   PDF(pc) (395KB) ( 391 )
A new method for generation of a train of ultrashort pulses or a sequence of ultrashort light bullets is proposed. This method is based on the fourth-order dispersion-dependent spatiotemporal instability in dispersive Kerr media. The repetition-rate of the generated bullets can be made quite large by increasing the corresponding spatial modulation frequency locating in the new instability region resulted from fourth-order dispersion.

High-Order Harmonic Generation by Two Non-collinear Coherent Femtosecond Laser Pulses
LU Wei-Xin, LIU Ting-Ting, YANG Hong, SUN Tao-Heng, GONG Qi-Huang
Chin. Phys. Lett. 2003, 20 (6): 848-851 .  
Abstract   PDF(pc) (458KB) ( 567 )
We have studied the high-order harmonic generated by two coherent pulses in argon gas produced by a gas jet. A loop in the relationship of the harmonic intensity versus the absolute values of relative phase difference was observed for non-collinear arrangement. Compared with the collinear arrangement, increase of 10 times of the conversion efficiency for 17th-order harmonic generation at an appropriate relative phase difference was obtained. The calculation of the intensity and phase for the laser field near the focus gives a simple reason for these phenomena.

Influence of Higher-Order Dispersions and Raman Delayed Response on Modulation Instability in Microstructured Fibres
WEN Shuang-Chun, SU Wen-Hua, Zhang Hua, FU Xi-Quan, QIAN Lie-Jia, FAN Dian-Yuan,
Chin. Phys. Lett. 2003, 20 (6): 852-854 .  
Abstract   PDF(pc) (275KB) ( 735 )
Modulation instability (MI) in microstructured photonic crystal fibres is studied by using an extended nonlinear Schrödinger equation. The roles of arbitrary higher-order dispersions and Raman delayed response in MI are identified. It is shown that all the odd-order dispersions contribute nothing to MI, whereas all the even-order dispersions not only affect the conventional instability regions but also may lead to the appearance of new MI regions. Raman delayed response makes MI occur in both the normal and anomalous dispersive regimes and exerts different influences on the Stokes and anti-Stokes components to some extent.

Fracture Criterion for Fracture Mechanics of Magnets
PAN Hao, YANG Wen-Tao
Chin. Phys. Lett. 2003, 20 (6): 855-857 .  
Abstract   PDF(pc) (154KB) ( 428 )
The applicability and limitation of some fracture criteria in the fracture mechanics of magnets are studied. It is shown that the magnetic field intensity factor can be used as a fracture criterion when the crack in a magnet is only affected by a magnetic field. For some magnetostrictive materials in which the components of magnetostriction strain do not satisfy the compatibility equation of deformation, the stress intensity factor can no longer be effectively applicable as a fracture criterion when the crack in a magnet is affected by a magnetic field and mechanical loads simultaneously.

Drag Force of Non-newtonian Fluid on a Continuous Moving Surface with Strong Suction/Blowing
ZHENG Lian-Cun, ZHANG Xin-Xin, HE Ji-Cheng
Chin. Phys. Lett. 2003, 20 (6): 858-861 .  
Abstract   PDF(pc) (479KB) ( 509 )
A theoretical analysis for the laminar boundary layer flow of a non-Newtonian fluid on a continuous moving flat plate with surface strong suction/blowing is made. The types of potential flows necessary for similar solutions to the boundary layer are determined and both analytical and numerical solutions are presented. It is shown that the solution of the boundary layer problem depends not only on the ratio of the velocity of the plate to the velocity of the free stream, but also on the suction/blowing parameter. The skin friction decreases with the increase of the parameters of power law and blowing. In the case of existing suction, the shear force decreases with the increases of tangential velocity, the largest shear force occurs at wall and the smallest shear force occurs at the edge of the boundary layer. However, in the case of existing surface blowing, the shear force initially increases with tangential velocity and the biggest shear force occurs at the interior of the boundary layer, the skin friction approaches to zero as the blowing rate approaches the critical value.
Temperature Diagnostics for Iron Plasmas by Means of Transmission Spectrum Obtained by Accurate Atomic Data
ZENG Jiao-Long, JIN Feng-Tao, ZHAO Gang, YUAN Jian-Min
Chin. Phys. Lett. 2003, 20 (6): 862-864 .  
Abstract   PDF(pc) (340KB) ( 533 )
Accurate atomic data such as fine-structure energy levels and oscillator strengths are calculated to obtain the radiative opacity and transmission of iron plasmas. At a temperature of 22 eV and a density of 0.01g/cm3, the calculated transmission spectrum is in better agreement with the experiment than that by other opacity codes such as OPAL. The transmission spectrum is very sensitive to the temperature of plasmas, meaning that it is an ideal diagnostic tool for the temperature of plasmas. Practical calculations show that the uncertainty of the temperature diagnostics obtained by theoretical predictions is better than ±2 eV.

Modification of Boundary Fluctuations by LHCD in the HT-7 Tokamak
SONG Mei, WAN Bao-Nian, XU Guo-Sheng, LING Bi-Li
Chin. Phys. Lett. 2003, 20 (6): 865-868 .  
Abstract   PDF(pc) (651KB) ( 372 )
Measurements of boundary fluctuations and fluctuation driven electron fluxes have been performed in ohmic and lower hybrid current drive enhanced confinement plasma using a graphite Langmuir probe array on HT-7 tokamak. The fluctuations are significantly suppressed and the turbulent fluxes are remarkably depressed in the enhanced plasma. We characterized the statistical properties of fluctuations and the particle flux and found a non-Gaussian character in the whole scrap-off layer with minimum deviations from Gaussian in the proximity of the velocity shear layer in ohmic plasma. In the enhanced plasma the deviations in the boundary region are all reduces obviously. The fluctuations and induced electron fluxes show sporadic bursts asymmetric in time and the asymmetry is remarkably weakened in the lower hybrid current driving (LHCD) phase. The results suggest a coupling between the statistical behaviour of fluctuations and the turbulent flow.
Statistical Characterization of Boundary Fluctuations in the HT-7 Tokamak
SONG Mei, WAN Bao-Nian, XU Guo-Sheng, LING Bi-Li
Chin. Phys. Lett. 2003, 20 (6): 869-872 .  
Abstract   PDF(pc) (460KB) ( 377 )
The statistical properties of plasma fluctuations are characterized in the boundary region of HT-7 tokamak. A non-Gaussian feature is observed in fluctuations of ion saturation current and floating potential in most of the scrape-off layer regions. The statistical properties of fluctuations have a clear radial dependence, showing a near-Gaussian character in the proximity of the velocity shear layer location and another region where the poloidal velocity has a trend to zero. Fluctuations show a bursty character with pulses asymmetric in time and the time asymmetry reaches the minimum around the shear layer. From the results, we can see an obvious coupling of the pulses and the poloidal flow.
Effects of Wall Ablation on the Trigger Process in Two-Gap Capillary Plasma Generator
XIA Sheng-Guo, LIU Ke-Fu, PAN Yuan, HU Xi-Wei
Chin. Phys. Lett. 2003, 20 (6): 873-876 .  
Abstract   PDF(pc) (1086KB) ( 373 )
Two-gap capillary plasma generator is a realistic alternative to the conventional capillary in electrothermal launchers. It is designed to accomplish two goals: repetitive operation and a compact power supply system in which capillary itself serves as a closing switch. A theoretical model is proposed to investigate the trigger process of this kind of generator. Both ablation case and no ablation case are considered. Trigger delay time is also theoretically analysed. The calculation shows that capillary wall ablation can greatly change the behaviour of trigger discharge arc and plasma flow generated. Trigger delay time can be drastically shortened.

Experimental Scaling for Mass Ablation Rate in Planar Targets Irradiated by Smoothed 0.351μm Laser Beam
YANG Jia-Min, DING Yao-Nan, ZHANG Bao-Han, YANG Guo-Hong, ZHENG Zhi-Jian, ZHANG Wen-Hai, HU Xin, WANG Yao-Mei, ZHANG Ji-Yan, LI Jun
Chin. Phys. Lett. 2003, 20 (6): 877-880 .  
Abstract   PDF(pc) (327KB) ( 401 )
Layered planar targets, consists of Al and Au layers, were irradiated by smoothed 0.351-μm-high power laser with relatively large focus spot on Xingguang II laser facility. The burnthrough time of laser ablation to Al sample has been obtained by measuring the delay time of Au N-Band x-ray emission with respect to the Al K-Band x-ray emission. The scaling law of mass ablation rate with the laser flux has been obtained by only varying the laser energy on the target surface on different shots. The experimental scaling law is greatly different from the results of Key et al. [Phys. Fluids 23(1983) 2011] and is in good agreement with the analytical model.

Improved Edge Confinement due to Ion Cyclotron Radio Frequency Boronization in the HT-7 Tokamak
XU Guo-Sheng, WAN Bao-Nian, SONG-Mei, LING Bi-Li
Chin. Phys. Lett. 2003, 20 (6): 881-884 .  
Abstract   PDF(pc) (511KB) ( 736 )
The phenomena of improved edge confinement due to ion cyclotron radio frequency boronization were observed with a fast reciprocating Langmuir probe in the HT-7 tokamak. A strong shear layer of radial electric field was produced in the plasma edge region, which resulted in the formation of an edge transport barrier. As a consequence, the edge profiles of electron density were steepened, and both the absolute and relative fluctuation levels were suppressed by the shearing E x B flows. Concomitant reduction of the coherence between electron density and poloidal electric field fluctuations and the change of their cross-phase resulted in turbulent particle flux dropping by more than a half at the plasma edge. This demonstrates the de-correlation effect of turbulence and its contribution to edge transport. The results presented here suggest a link between wall conditions and boundary plasma physics, especially an interaction between atomic processes and turbulence through the formation of radial electric field shear at the plasma edge.

Experiment and Simulation of Atmospheric Pressure Glow Surface Discharge
JIANG Zhong-He, HU Xi-Wei, LIU Ming-Hai, GU Cheng-Lin, PAN Yuan
Chin. Phys. Lett. 2003, 20 (6): 885-887 .  
Abstract   PDF(pc) (423KB) ( 502 )
Atmospheric pressure glow discharge was observed in a surface discharge generator. The frequency of ac power supply is more than 9 kHz and the sinusoidal peak-to-peak applied voltage is 9 kV. The electric field intensity in a kind of surface discharge generators is calculated with the boundary element method. Then a two-dimensional fluid model was used to simulate the ion trapping and electron trapping in a surface discharge just before the breakdown. The simulation results are in good agreement with our observation.

Determination of Inelastic Mean Free Path by Electron Holography Along with Electron Dynamic Calculation
WANG Yan-Guo, LIU Hong-Rong, YANG Qi-Bin, ZHANG Ze
Chin. Phys. Lett. 2003, 20 (6): 888-890 .  
Abstract   PDF(pc) (278KB) ( 484 )
Off-axis electron holography in a field emission gun transmission-electron microscope and electron dynamic calculation are used to determine the absorption coefficient and inelastic mean free path (IMFP) of copper. Dependence of the phase shift of the exit electron wave on the specimen thickness is established by electron dynamic simulation. The established relationship makes it possible to determine the specimen thickness with the calculated phase shift by match of the phase shift measured in the reconstructed phase image. Based on the measured amplitudes in reconstructed exit electron wave and reference wave in the vacuum, the examined IMFP of electron with energy of 200kV in Cu is obtained to be 96 nm.

Bergman Clusters Related to Bulk Amorphous Alloys and Quasi-crystals
WANG Xiao-Dong, QI Min, DONG Chuang,
Chin. Phys. Lett. 2003, 20 (6): 891-893 .  
Abstract   PDF(pc) (723KB) ( 513 )
The fcc Zr2Ni and MgCu2-type phases, as primary crystallization phases from some bulk amorphous alloys, have similar Bergman clusters in the first-two-shell atomic arrangement, which also appears in the hp MgZn2-type Laves phase coexisting with icosahedral quasicrystals in the TiZrNi system. Therefore, Bergman cluster may be a bridge linking bulk amorphous alloys and quasicrystals. The stability of the Bergman clusters Zr31Ni14, Mg31Cu14 and Mg18Zn27 can be reflected by density of states at the Fermi level and distribution of charge density on the plane of (100) calculated by local density functional theory.
Analysis of Thermodynamics of Liquid d- and f-Shell Metals with the Variational Approach
J. K. Baria
Chin. Phys. Lett. 2003, 20 (6): 894-896 .  
Abstract   PDF(pc) (185KB) ( 480 )
We use recently proposed potential to calculate internal energy (enthalpy), entropy and Helmholtz free energy of liquid d- and f-Shell metals with the variational approach. The parameter of the potential is determined with the standard zero pressure condition along with well established Taylor screening function for exchange and correlation effects to the liquid d- and f-Shell metals. Here we clearly mention that the parameter of the potential is independent of any fitting procedure either with any experimental data or any theoretical values of any physical properties. The structure factor derived by Percus-Yevick solution for hard sphere fluids, which is characterized by hard sphere diameter, is used. A good agreement between theoretical investigations and experimental findings has confirmed the ability of the model potential to the liquid d- and f-Shell metals.
Mesoscopic Transport Characteristics of a Normal-Metal-Superconducting-Grain- Superconductor System
FENG Jin-Fu, XIONG Shi-Jie
Chin. Phys. Lett. 2003, 20 (6): 897-900 .  
Abstract   PDF(pc) (446KB) ( 792 )
We investigate transport properties of a normal-metal-superconducting-grain-superconductor system by the use of the equivalent single-particle multi-channel networks, taking into account the multi-level structure, the Coulomb interaction, and the pair potential on the grain. The dependence of the current on the gate voltage shows oscillating behaviour with a period related to 2e of the charge on the grain, reflecting the charge transfer in units of Cooper pairs. The conductance can be enhanced when the pairing parameter is near the Coulomb energy e2/2C, due to the resonance of the Andreev reflection through the grain. The magnitude of the Andreev reflection as a function of the bias voltage exhibits complicated structures, reflecting the multiple levels, the spin orientations, and the interaction energy on the grain.

Electronic Transport for a Quantum Wire Partly Irradiated under THz Electromagnetic Wave
YANG Mou, ZHOU Guang-Hui,
Chin. Phys. Lett. 2003, 20 (6): 901-904 .  
Abstract   PDF(pc) (295KB) ( 411 )
We study the electronic transport of a quantum wire partly irradiated under an external terahertz (THz) electromagnetic field. Using the free-electron model and scattering matrix approach we demonstrate that although the electrons in a ballistic quantum wire only suffer from lateral collision with photons, the reflection of electrons also takes place. More interestingly there is a sharp step-structure in the transmission probability as the total energy of electron increases to a threshold value when the frequency of electromagnetic field is resonant with the separation of lateral levels of the wire. The interference structure of transmission for the system apparently appears when the field only irradiates the middle part of the wire.

Spectral Properties of Erbium-Doped Oxyfluoride Silicate Glasses for Broadband Optical Amplifiers
XU Shi-Qing, YANG Zhong-Min, DAI Shi-Xun, YANG Jian-Hu, WEN Lei, HU Li-Li, JIANG Zhong-Hong
Chin. Phys. Lett. 2003, 20 (6): 905-908 .  
Abstract   PDF(pc) (445KB) ( 608 )
The new oxyfluoride silicate glasses of Er3+-doped 50SiO2-(50-x)PbO-xPbF2 were prepared. With increasing PbF2 content in the glass composition, the fluorescence full width at half max maximum and lifetimes of the 4I13/2 level of Er3+ increase, while the refractive indices and densities decrease. Er3+-doped 50SiO2-50PbF2 glass showed broad fluorescence spectra of 1.55μm with a large stimulated emission cross-section and long lifetimes of 4I13/2 level of Er3+. Compared with other glass hosts, the gain bandwidth properties of Er3+-doped 50SiO2-50PbF2 glass are close to those of tellurite and bismuth glasses, and have advantages over those of silicate, phosphate and germante glasses. The broad and flat 4I13/24I15/2 emission of Er3+ around 1.55μm can be used as host material for potential broadband optical amplifier in wavelength-division-multiplexing network system.
Effects of Sm Substitution on Magnetic and Magneto-Optical Properties of TbCo/Cr Films
LI Zuo-Yi, HUANG Zhi-Xin, LIN Gen-Qi, JIN Fang, LI Zhen
Chin. Phys. Lett. 2003, 20 (6): 909-911 .  
Abstract   PDF(pc) (368KB) ( 479 )
The effects of Sm substitution on magnetic and magneto-optical properties of TbCo/Cr films have been investigated. It was found that partial Tb substituted by Sm will directly affect the magnitude of saturation magnetization Ms and the Kerr rotation angle. These results are explained by the ferri-magnetical structure of the rare earth-transition metal alloy. When the magnetic layer composition was (Sm0.343Tb0.657)31Co69, values of magnetization as high as 385 emu/cm3 and coercivity above 4.7 kOe at room temperature were obtained.
Magnetic Transitions and Magnetoresistance of Y1-xDyxMn6Sn6 ( x = 0.2 and 0.3)
ZHANG Shao-Ying, YAO Jin-Lei, ZHANG Li-Gang, LI Yun-Bao, ZHAO Tong-Yun, SHEN Bao-Gen
Chin. Phys. Lett. 2003, 20 (6): 912-914 .  
Abstract   PDF(pc) (462KB) ( 774 )
Magnetic transitions and magnetoresistance of HfFe6Ge6-type Y1-xDyxMn6Sn6 ( x = 0.2 and 0.3) have been investigated in the temperature range of 5-380 K. It was found that the strong exchange interaction between the Dy and Mn sublattices results in incomplete ferrimagnetism at low temperatures. At higher temperatures, the metamagnetic transition from an antiferromagnetic state to a ferrimagnetic state can be induced by a fairly small threshold field or by increasing temperature. The magnetic transition is accompanied by a large magnetoresistance effect of about -29% and -16% at 5 K for x = 0.2 and 0.3, respectively.
Magnetic Properties and Magnetoresistance of HfFe6Ge6-Type Er1-xGdxMn6Ge6 and Ho1-xGdxMn6Ge6 (0.2 ≤ x ≤ 0.9) Compounds
ZHANG Li-Gang, WANG Ru-Wu, LI-Lu, ZHANG Shao-Ying, YAO Jin-Lei, SHEN Bao-Gen
Chin. Phys. Lett. 2003, 20 (6): 915-918 .  
Abstract   PDF(pc) (642KB) ( 516 )
Magnetic properties and magnetoresistance effects of the Er1-xGdxMn6Ge6 and Ho1-xGdxMn6Ge6 (0.2 ≤ x ≤ 0.9) compounds have been studied by magnetic property and resistivity measurements in applied magnetic field up to 5 T. For increasing Gd content, two series display a transition from the antiferromagnetic state to the ferrimagnetic state. The Er1-xGdxMn6Ge6(x = 0.2 and 0.5) and Ho1-xGdxMn6Ge6 (x = 0.2 and 0.4) compounds order antiferromagnetically at 430, 432, 423 and 425 K, respectively. The Er1-xGdxMn6Ge6 (x = 0.8 and 0.9) and Ho1-xGdxMn6Ge6 (x = 0.7, 0.8 and 0.9) compounds order ferrimagnetically at 462, 471, 450, 463 and 470 K, respectively. The Er1-xGdxMn6Ge6 and Ho1-xGdxMn6Ge6 compounds undergo the second transitions below 71 and 82 K, respectively. The magnetoresistance curves of the Er0.1Gd0.9Mn6Ge6 and Ho0.1Gd0.9Mn6Ge6 compounds in a field of 5 T are presented and the magnetoresistance effects are related to the metamagnetic transitions.
Giant Positive Magnetoresistance in Magnetic Multilayer Film Prepared by Ion-Beam Sputtering
ZHANG Dong-Jie, DU You-Wei
Chin. Phys. Lett. 2003, 20 (6): 919-920 .  
Abstract   PDF(pc) (296KB) ( 368 )
The magnetic multilayers Ni78Co22//Cu//Ni78Co22/Ni78Co22O//Ta were fabricated by ion-beam sputtering through applied magnetic field and treatment under high vacuum. Resistance against applied magnetic field was measured by the standard four-point probe method at room temperature. The giant positive magnetoresistance has been observed. A maximum positive magnetoresistance at room temperature was obtained to be 280%.

Vibrational Modes Analysis of Poly (Vinylidene Fluoride-Trifluoroethylene)
LI Ji-Chao, WANG Chun-Lei, ZHONG Wei-Lie, LI Zheng-Fa
Chin. Phys. Lett. 2003, 20 (6): 921-923 .  
Abstract   PDF(pc) (455KB) ( 1482 )
The ground state geometry and vibrational modes of poly (vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] molecular chains have been investigated by ab initio molecular orbital calculation. The gauche bonds in P(VDF-TrFE) are introduced by TrFE monomers. The vibrational modes are localized within all-trans segments. These results cause the infrared absorption weaker and diffused, and may be responsible for dielectric relaxation of the copolymers.

Enhanced Nonlinear Optical Properties of Laser Deposited Ag/BaTiO3 Nanocomposite Films
YANG Guang, WANG Wei-Tian, YANG Guo-Zhen, CHEN Zheng-Hao
Chin. Phys. Lett. 2003, 20 (6): 924-927 .  
Abstract   PDF(pc) (352KB) ( 678 )
The composite films containing Ag particles embedded in BaTiO3 matrix were deposited on MgO (100) substrates by using the pulsed laser deposition technique and characterized by x-ray diffraction and x-ray photoelectron spectroscopy. The absorption peak resulting from the surface plasmon resonance of Ag particles is found at the wavelength of 430 nm. The third-order nonlinear optical characteristics of the films are determined by z-scan technique using a nanosecond laser. The results show that the Ag/BaTiO3 nanocomposite films exhibit great optical nonlinearities and the real and imaginary parts of the third-order nonlinear susceptibility X(3) are calculated to be 4.544 x 10-6 esu and 2.352 x 10-7 esu, respectively. The enhancement mechanism of X(3) is discussed.

Synthesis and Strong Blue-Green Emission Properties of ZnO Nanowires
LIU Dong-Fang, TANG Dong-Sheng, CI Li-Jie, YAN Xiao-Qin, LIANG Ying-Xin, ZHOU Zhen-Ping, YUAN Hua-Jun, ZHOU Wei-Ya, WANG Gang
Chin. Phys. Lett. 2003, 20 (6): 928-931 .  
Abstract   PDF(pc) (426KB) ( 592 )
ZnO nanowires were catalytically grown on Au-coated silicon substrates by the carbon thermal reduction method. The process involved addition of a low partial pressure of hydrogen sulfide to the argon carrier flow. The addition of H2S led to the higher yield and longer nanowires without any morphology change, and no sulfuric content was observed by the energy dispersive x-ray spectroscopy in the resulting nanowires. The nanowires exhibited strong blue-green emission at room temperature and an increasing intensity when the partial pressure of H2S was raised. The temperature-dependent photoluminescence spectra show that intensity of the blue-green emission, almost without shift, decreases slowly with increasing temperature. Heat treatments indicated that quenching resulted in a higher ratio of blue-green emission to ultraviolet emission.
Influence of Femtosecond Laser Irradiation and Heat Treatment on Precipitation of Silver Nanoparticles in Glass
ZENG Hui-Dan, QIU Jian-Rong, JIANG Xiong-Wei, QU Shi-Liang, ZHU Cong-Shan, GAN Fu-Xi
Chin. Phys. Lett. 2003, 20 (6): 932-934 .  
Abstract   PDF(pc) (411KB) ( 524 )
Silver nanoparticles were precipitated inside an Ag2O-doped glass by femtosecond laser irradiation and successive heat treatment. The influence of heat treatment temperature on the precipitation of silver nanoparticles was investigated. Absorption spectra show that the femtosecond laser irradiation results in an apparent decrease of the treatment temperature for the precipitation of Ag nanoparticles. We demonstrate the control of precipitation, dissolution and growth of silver nanoparticles inside glass by changing the heat treatment temperature or using further femtosecond laser irradiation.
Red Light-Emitting-Diode Based on an Organic Salt
MENG Rui-Ping, XU Hong-Guang, XU Chun-Xiang, ZHANG Jun-Xiang, HE Guo-Hua, CUI Yi-Ping
Chin. Phys. Lett. 2003, 20 (6): 935-937 .  
Abstract   PDF(pc) (412KB) ( 382 )
A novel organic salt trans-4-[P-(N-ethyl-N-(hydroxylethyl)-amino) styryl]-N-methylpyridinium tetraphenylborate (abbreviated as ASPT) has been employed as an active layer in an organic electroluminscent device. Bright red emission with high quantum efficiency has been obtained. The brightness of the ASPT device is one order magnitude higher than that of Alq3 devices at about 12 V. The device shows high thermal stability because of the ionic interaction within the organic salt molecules. It is assumed that the high performance of such a device is related to the formation of dipole moments in the ASPT layer.

Improved Quantum Efficiency of Organic Light Emitting Diodes with Gradiently Doped Double Emitting Zone
GAO Wen-Bao, JIANG Wen-Long, SUN Jia-Xin, FENG Jing, HOU Jing-Ying, LIU Shi-Yong
Chin. Phys. Lett. 2003, 20 (6): 938-941 .  
Abstract   PDF(pc) (508KB) ( 534 )
We investigate electroluminescent characteristics of gradiently doped organic light-emitting diodes, which were gradiently doped in both the hole and the electron-transporting layer to form a double emitting zone. The device structure was ITO/(15 nm) CuPc/(60 nm)NPB:rubrene/(30 nm)Alq3:rubrene (20 nm)Alq3/(0.5 nm)LiF/Al. We observed that charge carriers were well trapped by the dopant molecules and the main emitting zone was localized at the NPB:rubrene side close to the interface of NPB:rubrene/Alq3:rubrene. The quantum efficiency (cd/A) was enhanced to 5.89 cd/A at 6 V. We attributed this improvement to the charge carriers trapping and the emitting of the double emitting zone.
Structural and Optical Characterization of Zn1-xCdxO Thin Films Deposited by dc Reactive Magnetron Sputtering
MA De-Wei, YE Zhi-Zhen, HUANG Jing-Yun, ZHAO Bing-Hui, WAN Shou-Ke, SUN Xue-Hao, WANG Zhan-Guo
Chin. Phys. Lett. 2003, 20 (6): 942-943 .  
Abstract   PDF(pc) (284KB) ( 730 )
Zn1-xCdxO crystal thin films with different compositions were prepared on silicon and sapphire substrates by the dc reactive magnetron sputtering technique. X-ray diffraction measurements show that the Zn1-xCdxO films are of completely (002)-preferred orientation for x ≤ 0.6. For x = 0.8, the film is a mixture of ZnO hexagonal wurtzite crystals and CdO cubic crystals. For pure CdO, it is highly (200) preferential-oriented. Photoluminescence spectrum measurement shows that the Zn1-xCdxO (x = 0.2) thin film has a red-shift of 0.14 eV from that of ZnO reported previously.
Field Emission from Amorphous Carbon Nitride Films Deposited on Silicon Tip Arrays
LI Jun-Jie, ZHENG Wei-Tao, SUN Long, BIAN Hai-Jiao, JIN Zeng-Sun, ZHAO Hai-Feng, SONG-Hang, MENG Song-He, HE Xiao-Dong, HAN Jie-Cai
Chin. Phys. Lett. 2003, 20 (6): 944-946 .  
Abstract   PDF(pc) (366KB) ( 444 )
Amorphous carbon nitride films (a-CNx) were deposited on silicon tip arrays by rf magnetron sputtering in pure nitrogen atmosphere. The field emission property of carbon nitride films on Si tips was compared with that of carbon nitride on silicon wafer. The results show that field emission property of carbon nitride films deposited on silicon tips can be improved significantly in contrast with that on wafer. It can be explained that field emission is sensitive to the local curvature and geometry, thus silicon tips can effectively promote field emission property of a-CNx films. In addition, the films deposited on silicon tips have a smaller effective work function (F = 0.024 eV) of electron field emission than that on silicon wafer (F = 0.060 eV), which indicates a significant enhancement of the ability of electron field emission from a-CNx films.
Influence of Thickness on Field Emission Characteristics of Nanometer Boron Nitride Thin Films
GU Guang-Rui, LI Ying-Ai, TAO Yan-Chun, HE Zhi, LI Jun-Jie, YIN Hong, LI Wei-Qing, ZHAO Yong-Nian,
Chin. Phys. Lett. 2003, 20 (6): 947-949 .  
Abstract   PDF(pc) (331KB) ( 529 )
Nanometer boron nitride (BN) thin films with various thickness (54-135 nm) were prepared on Si(100) by rf magnetic sputtering physical vapor deposition. The field emission characteristics of the BN thin films were measured in an ultrahigh vacuum system. A threshold electric field of 11 V/μm and the highest emission current density of 24μA/cm2 at a electric field of 23 V/μm were obtained for the about 54-nm-thick BN film. The threshold electric field increases with the increase of the thickness in the nanometer range. The Fowler-Nordheim plots show that electrons were emitted from BN to vacuum by tunneling through the potential barrier at the surface of BN thin films.

Displacement and Force Measurements with Quadrant Photodetector in Optical Tweezers
GUO Hong-Lian, LIU Chun-Xiang, LI Zhao-Lin, DUAN Jian-Fa, HAN Xue-Hai, CHENG Bing-Ying, ZHANG Dao-Zhong
Chin. Phys. Lett. 2003, 20 (6): 950-952 .  
Abstract   PDF(pc) (374KB) ( 1096 )
A technique of displacement and force measurements with a photodiode quadrant detector in an optical tweezers system is presented. The stiffness of optical trap is calibrated and the leukemia cell membrane tension is measured. The results show that the optical tweezers combined with the quadrant detector is a very useful tool for detecting the displacement and force with a millisecond-order response.

Optimum Nanoporous TiO2 Film and Its Application to Dye-sensitized Solar Cell
DAI Song-Yuan, WANG Kong-Jia
Chin. Phys. Lett. 2003, 20 (6): 953-955 .  
Abstract   PDF(pc) (371KB) ( 1545 )
Properties of TiO2 nanoporous films, which are one of the crucial technologies in dye-sensitized solar cell, are investigated. The nanocrystalline TiO2 films were prepared with the sol-gel method at different pH in precursor and treatment temperature in autoclave for their application to dye-sensitized solar cells. The thickness of the TiO2 film is very important to the transfer of photoelectron as well as adsorption of dye, it is also known as one of the source to the dark current. The results show that the TiO2 films, such as different particle sizes of TiO2, different pH in precursor and treatment temperature in autoclave, have a strong influence on the photoelectrochemical properties of the dye-sensitized solar cells. We give the optimum TiO2 film thickness and morphology for the application to dye-sensitized solar cells.

High-Efficiency Organic Double-Quantum-Well Light-Emitting Devices Using 5,6,11,12-Tetraphenylnaphthacene Sub-monolayer as Potential Well
XIE Wen-Fa, LI Chuan-Nan, LIU Shi-Yong
Chin. Phys. Lett. 2003, 20 (6): 956-958 .  
Abstract   PDF(pc) (480KB) ( 597 )
The double-quantum-well organic light-emitting devices of indium-tin-oxide (ITO)/NPB (50 nm)/rubrene (0.05 nm)/ NPB(4 nm)/rubrene (0.05 nm)/Alq3 (50 nm)/LiF (0.5 nm)/Al were fabricated, in which N,N-bis-(1-naphthyl)-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine (NPB) is used as a barrier potential or hole transport layer, tris (8-hydroxyquinoline) aluminum (Alq3) used as electron transport layer, and 5,6,11,12-tetra\-pheny\-lnaphthacene(rubrene) as a potential well and emitter. The brightness can reach 18610 cd/m2 at 13 V. The maximum electroluminescent efficiency of the device was 6.61 cd/A at 7 V, which was higher than that of common dope-type devices. In addition, the electroluminscence efficiency is relatively independent of the drive voltage in the range from 5 to 13 V.

A Model for Light-Driven Proton Pump and Transfer in Bacteriorhodopsin
HUANG Yu-Hua, ZHAO You-Yuan, OU-YANG Xiao-Ping, LI Fu-Ming, DING Jian-Dong, LI Qing-Guo
Chin. Phys. Lett. 2003, 20 (6): 959-961 .  
Abstract   PDF(pc) (413KB) ( 496 )
A theoretical model was assumed to describe the kinetic processes of light-driven proton pump and motion. The calculation shows that the photovoltage signals are greatly affected by experimental conditions including pulse width of excitation and impedance of measurement system. At the same excitation, only a negative photovoltage signal can be observed with small impedance and both the negative and positive signals can be observed with large impedance. With the same impedance, shorter response time of the pulsed photoelectric signals is responsible for shorter pulsed excitation. All these phenomena are in good agreement with the experimental results.

Abundance Distribution of Slow-Process Main Heavy Elements in AGB Stars
ZHANG Miao-Jing, , ZHANG Bo, , LI Guang-Lie,
Chin. Phys. Lett. 2003, 20 (6): 962-964 .  
Abstract   PDF(pc) (507KB) ( 435 )
By re-analysing the results of the theoretical abundance of asymptotic giant branch (AGB) stellar models and the observed abundances of 51 AGB samples, we find that the abundance distribution of s-process main heavy (SMH) elements of any AGB star is similar to the scaled s-process main component of the solar system. This means that superposition of the SMH element abundance distributions of AGB stars should be similar to the scaled solar s-process main component. As a conclusion, the abundance pattern of the solar SMH elements is not only an average result of a complex chemical evolution of galaxy, but also a typical one that can be used as a standard in abundance investigation.
A Toy Model for Advection Dominated Accretion Flows
WANG Ding-Xiong, LEI Wei-Hua, XIAO Kan
Chin. Phys. Lett. 2003, 20 (6): 965-968 .  
Abstract   PDF(pc) (365KB) ( 375 )
A toy disk model with advection dominated accretion onto a black hole is presented. The advection dominated accretion flows (ADAFs) are assumed to exist in the inner thick disk with rin < r < rout, and the disk is assumed to be geometrically thin for r > rout. Compared with Paczynski's toy model the thick disk is not limited to be 100% advective. It turns out that the inner radius rin depends not only on the outer radius rout but also on the ADAF parameters f and ε. The effects of the inner thick disk on the radiation efficiency and the temperature profile of the outer thin disk are discussed in details.

Unusual Polarization Properties of Supernova Remnant G4.8+6.2 at 1400 MHz
ZHANG Xi-Zhen, R. G. Strom, W. Reich
Chin. Phys. Lett. 2003, 20 (6): 969-971 .  
Abstract   PDF(pc) (323KB) ( 405 )
Unusual polarization properties of the high galactic latitude supernova remnant (SNR) G4.8+6.2 are reported. The percentage polarization is larger than 70% over the two main shells of G4.8+6.2 at 1400 MHz. It is the second SNR with such high polarization at such a low frequency among the known 225 SNRs. We show that this is very unusual for an SNR. Its morphology and polarization are very similar to those of the supernova remnant DA530 (G93.0+6.5), which is another high Galactic latitude SNR.
Chaos in Einstein-Maxwell-Dust System
CHEN Ju-Hua, WANG Yong-Jiu
Chin. Phys. Lett. 2003, 20 (6): 972-974 .  
Abstract   PDF(pc) (402KB) ( 436 )
We study the dynamics of the Einstein-Maxwell-dust system and find that the system presents a critical point of a center-center-type structure in the phase space. By performing the numerical experiments, we find that the dynamics of the Einstein-Maxwell-dust system is unstable and its chaotic behaviour is obvious when the Hamiltonian becomes smaller.

57 articles