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Volume 25 Issue 5
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Original Articles
Conformal Invariance and Conserved Quantities of General Holonomic Systems
CAI Jian-Le
Chin. Phys. Lett. 2008, 25 (5): 1523-1526 .  
Abstract   PDF(pc) (148KB) ( 788 )
Conformal invariance and conserved quantities of general holonomic systems are studied. A one-parameter infinitesimal transformation group and its infinitesimal transformation vector of generators are described. The definition of conformal invariance and determining equation for the system are provided. The conformal factor expression is deduced from conformal invariance and Lie symmetry. The necessary and sufficient condition, that conformal invariance of the system would be Lie symmetry, is obtained under the infinitesimal one-parameter transformation group. The corresponding conserved quantity is derived with the aid of a structure equation. Lastly, an example is given to demonstrate the application of the result.
Complexiton Solutions of a Special Coupled mKdV System
YANG Jian-Rong, MAO Jie-Jian
Chin. Phys. Lett. 2008, 25 (5): 1527-1530 .  
Abstract   PDF(pc) (1209KB) ( 927 )
For a special coupled mKdV system, which can be derived from a two-layer
fluid model, Hirota's bilinear direct method is used to construct and yield the complexiton solutions. The detailed physical properties of complexitons are further illustrated graphically.
Exact Periodic Solitary-Wave Solution for KdV Equation
DAI Zheng-De, LIU Zhen-Jiang, LI Dong-Long
Chin. Phys. Lett. 2008, 25 (5): 1531-1533 .  
Abstract   PDF(pc) (608KB) ( 1227 )
A new technique, the extended homoclinic test technique, is proposed to seek periodic solitary wave solutions of integrable systems. Exact periodic solitary-wave solutions for classical KdV equation are obtained using this
technique. This result shows that it is entirely possible for the (1+1)-dimensional integrable equation that there exists a periodic solitary-wave.
Topological Quantization of Instantons in SU(2) Yang--Mills Theory
ZHONG Wo-Jun, DUAN Yi-Shi
Chin. Phys. Lett. 2008, 25 (5): 1534-1537 .  
Abstract   PDF(pc) (119KB) ( 558 )
By decomposing SU(2) gauge potential in four-dimensional Euclidean SU(2) Yang--Mills theory in a new way, we find that the instanton number related to the isospin defects of a doublet order parameter can be topologically quantized by the Hopf index and Brouwer degree. It is also shown that the instanton number is just the sum of the topological charges of the isospin
defects in the non-trivial sector of Yang--Mills theory.
Two New Simple Multi-Symplectic Schemes for the Nonlinear Schrodinger Equation
WANG Yu-Shun, LI Qing-Hong, SONG Yong-Zhong
Chin. Phys. Lett. 2008, 25 (5): 1538-1540 .  
Abstract   PDF(pc) (134KB) ( 647 )
We investigate the multi-symplectic Euler-box scheme for the nonlinear Schrodinger equation. Two new simple semi-explicit scheme are derived. A composition scheme based on the new derived schemes is also discussed. Some numerical results are reported to illustrate the efficiency of the new
schemes.
Numerical Simulation and Experimental Analysis of Photonic Band Gap in Hollow-Core Photonic Crystal Fibres
YUAN Jin-Hui, HOU Lan-Tian, ZHOU Gui-Yao, WEI Dong-Bin, CHEN Chao, WANG Qing-Yue, HU Ming-Lie, LIU Bo-Wen
Chin. Phys. Lett. 2008, 25 (5): 1541-1544 .  
Abstract   PDF(pc) (1234KB) ( 616 )
Based on the full-vector plane-wave method (FVPWM), a hollow-core photonic crystal fibre (HC-PCF) fabricated by using the improved stack-and-draw technique is simulated. Under given propagation constants β, several
effective photonic band gaps with different sizes emerge within the visible wavelength range from 575 to 720nm. The fundamental mode and second-order mode lying in a part of PBGs are investigated. In the transmission spectrum tested, the positions of PBGs are discovered to be shifting to shorter wavelengths. The main reason is the existence of interstitial holes at nodes in the cladding region. In the later experiment, green light is observed propagating in the air-core region, and the result is more consistent with our
theoretical simulation.
Solution of the Fermi--Ulam Model in the Case of Periodic Perturbation
LI Chao, WU Jun-Fang, XU Wen-Cheng
Chin. Phys. Lett. 2008, 25 (5): 1545-1548 .  
Abstract   PDF(pc) (175KB) ( 495 )
We discuss the evolution of the state and the average energy of the Fermi--Ulam model in the case of periodic perturbation. By a perturbation technique, the time-dependent Schrodinger equation is solved and it is found that the particle will continuously absorb or radiate energy if the frequency of the oscillating wall meets the resonance condition. Usually, these two states cannot exist together at a certain frequency. However, there is an exception if the frequency is at some special values. We find these values and reveal that the energy for transmission has the minimum equivalent unit, which is in the form of a harmonic oscillator.
Semiclassical Calculation of Conductance Transmission through an Open Equilateral Triangular Billiards
XU Xue-You, ZHANG Yan-Hui, LI Hong-Yun, GAO Song, LIN Sheng-Lu
Chin. Phys. Lett. 2008, 25 (5): 1549-1552 .  
Abstract   PDF(pc) (212KB) ( 518 )
We study the transport property passing through a weakly open equilateral triangular billiards by using the semiclassical method. We extend the Green function and the transport matrix theory to include the multiple scattering effect at the boundary and the diffractions of the pair of the lead apertures. For analysing the structure of semiclassical pseudo path kinks, the geometric and the special dynamical symmetries of the system are simultaneously taken into account. The conductance is calculated by Landauer formula as a function of the electron's Fermi wave number. Its Fourier transformation, the quantum path-length spectrum, is qualitatively in accordance with the results of the classical trajectories, which indicates that such approach provides an obvious
improvement of the semiclassical description.
Quantum Nonlocality and Entanglement of Pair Cat States with Phase Decoherence
XU Hang-Shi, XU Jing-Bo
Chin. Phys. Lett. 2008, 25 (5): 1553-1556 .  
Abstract   PDF(pc) (1026KB) ( 520 )
We investigate the entanglement of pair cat states in the phase damping channel by adopting the log-negativity and then study the possible violations of Bell's inequalities for the pair cat states in terms of the Wigner
representation in phase space based upon parity measurement and
displacement operation.
Entanglement in Ising Chain with Inhomogeneous Magnetic Field
Cenk Akyuz, Ekrem Aydiner
Chin. Phys. Lett. 2008, 25 (5): 1557-1560 .  
Abstract   PDF(pc) (582KB) ( 447 )
We have numerically calculated the thermal entanglement of a two-qubit system at low temperatures in a isotropic Ising chain under an inhomogeneous magnetic field. It is shown that in the homogeneous magnetic field, the two-qubit system has entangled states. It is concluded that the presence of the inhomogeneity in the magnetic field plays an effective role on the entangled states. Finally, it is suggested that the
inhomogeneity in the magnetic field can be used to create two separated entangled formations in a two-qubit system.
A Special Eavesdropping on One-Sender Versus N-Receiver QSDC Protocol
GAO Fei, LIN Song, WEN Qiao-Yan, ZHU Fu-Chen
Chin. Phys. Lett. 2008, 25 (5): 1561-1563 .  
Abstract   PDF(pc) (141KB) ( 661 )
We analyse the security of a quantum secure direct communication (QSDC)
protocol and find that an eavesdropper can utilize a special property of GHZ states to elicit all or part of the transmitted secrets without being detected. The particular attack strategy is presented in detail. We give an improved version of this protocol so that it can resist this attack.
Nonlinear Landau--Zener Tunnelling with Two and Three-Body Interactions
WEI Xiu-Fang, TANG ong-An, YONG Wen-Mei, XUE Ju-Kui
Chin. Phys. Lett. 2008, 25 (5): 1564-1567 .  
Abstract   PDF(pc) (156KB) ( 567 )
We investigate the nonlinear Landau--Zener tunnelling of Bose--Einstein condensate (BEC) in an accelerating optical lattice with two- and three-body interactions between the particles. The influence of the three-body interaction on the eigenstates and the transition probability are discussed both
analytically and numerically. The analytical eigenstates and the tunnelling probability are obtained, which are verified by numerical methods. It is shown that the eigenstates and the tunnelling probability are modified dramatically by three-body interaction.
Non-Spherical Gravitational Collapse of Strange Quark Matter
Zade S S, Patil K D, Mulkalwar P N
Chin. Phys. Lett. 2008, 25 (5): 1568-1570 .  
Abstract   PDF(pc) (154KB) ( 505 )
We study the non-spherical gravitational collapse of the strange quark null fluid. The interesting feature which emerges is that the non-spherical collapse of charged strange quark matter leads to a naked singularity whereas the gravitational collapse of neutral quark matter proceeds to form a black hole. We extend the earlier work of Harko and Cheng [Phys. Lett. A 266(2000)249] to the non-spherical case.
Inhomogeneous Vacuum: An Alternative Interpretation of Curved Spacetime
YE Xing-Hao, LIN Qiang
Chin. Phys. Lett. 2008, 25 (5): 1571-1574 .  
Abstract   PDF(pc) (1143KB) ( 680 )
The strong similarities between the light propagation in a curved spacetime and that in a medium with graded refractive index are found. It is pointed out that a curved spacetime is equivalent to an inhomogeneous vacuum for light propagation. The corresponding graded refractive index of the vacuum in a static spherically symmetrical gravitational field is derived. This result provides a simple and convenient way to analyse the gravitational lensing in astrophysics.
Accelerated Multi-Level Atoms in an Electromagnetic Vacuum and Fulling--Davies--Unruh Effect
ZHU Zhi-Ying, YU Hong-Wei
Chin. Phys. Lett. 2008, 25 (5): 1575-1578 .  
Abstract   PDF(pc) (110KB) ( 484 )
We consider, from the point of view of a coaccelerated frame, a uniformly accelerated multi-level atom in interaction with vacuum quantum electromagnetic fields in the multi-polar coupling scheme, and calculate the rate of change of the atom's energy assuming a thermal bath at a finite temperature T in the Rindler wedge. Comparison with the spontaneous excitation rate of the atom calculated in the instantaneous inertial frame of the atom shows that both the inertial and coaccelerated observer would
agree with each other only when the temperature of the thermal bath equals the FDU value TFDU=a/2π.
Tunnelling Effect of Two Horizons from a Gibbons--Maeda Black Hole
REN Jun
Chin. Phys. Lett. 2008, 25 (5): 1579-1582 .  
Abstract   PDF(pc) (110KB) ( 637 )
We study the thermal characters of the inner horizon of a Gibbons--Maeda black hole. In order to satisfy the Nernst theorem of the third law, the entropy of the black hole with two horizons must depend not only on the area of the outer horizon but also on the area of the inner horizon. Then the temperature of the inner horizon is calculated. Lastly, the tunnelling effect including the inner horizon of a Gibbons--Maeda black hole is investigated. We also calculate the tunnelling rate of the outer horizon Γ+ and the inner horizon Γ-. The total tunnelling rate Γ should be the product of the rates of the outer and inner horizon, Γ=Γ+-. It is found that the total tunnelling rate is in agreement with the Parikh's standard result, Γ→exp(∆SBH), and there is no information loss.
Effects of Fractal Size Distributions on Velocity Distributions and Correlations of a Polydisperse Granular Gas
CHEN Zhi-Yuan, ZHANG Duan-Ming
Chin. Phys. Lett. 2008, 25 (5): 1583-1586 .  
Abstract   PDF(pc) (143KB) ( 442 )
By the Monte Carlo method, the effect of dispersion of disc size distribution on the velocity distributions and correlations of a polydisperse granular gas with fractal size distribution is investigated in the same inelasticity. The dispersion can be described by a fractal dimension D, and the smooth hard
discs are engaged in a two-dimensional horizontal rectangular box, colliding inelastically with each other and driven by a homogeneous heat bath. In the steady state, the tails of the velocity distribution functions rise more significantly above a Gaussian as D increases, but the non-Gaussian velocity distribution functions do not demonstrate any apparent universal form for any value of D. The spatial velocity correlations are apparently stronger with the
increase of D. The perpendicular correlations are about half the parallel correlations, and the two correlations are a power-law decay function of dimensionless distance and are of a long range. Moreover, the parallel velocity correlations of postcollisional state at contact are more than twice as large as the precollisional correlations, and both of them show almost linear behaviour of the fractal dimension D.
Stochastic Resonance in a Gene Transcriptional Regulatory System with Time Delay
ZENG Chun-Hua, XIE Chong-Wei
Chin. Phys. Lett. 2008, 25 (5): 1587-1590 .  
Abstract   PDF(pc) (153KB) ( 669 )
We study the stochastic resonance (SR) phenomenon in a time-delayed gene transcriptional regulatory system under the simultaneous action of a multiplicative noise and an additive noise and a weak periodic signal. The expression of the signal-to-noise ratio RSNR is obtained by applying the
two-state theory in adiabatic limit under the condition of small delay time. The effects of delay time and intensity of the correlation between multiplicative and additive noise on RSNR are discussed. It is found that the delay time τ enhances the SR of the system. The correlation intensity λ enhances the SR in the RSNR-D plot (D denotes the multiplicative noise intensity), but weakens the SR in the RSNR-α plot (α denotes the additive noise intensity). 
Amplified Signal Response by Neuronal Diversity on Complex Networks
SHEN Chuan-Sheng, CHEN Han-Shuang, ZHANG Ji-Qian
Chin. Phys. Lett. 2008, 25 (5): 1591-1594 .  
Abstract   PDF(pc) (316KB) ( 855 )
The effect of diversity on dynamics of coupled FitzHugh--Nagumo neurons on complex networks is numerically investigated, where each neuron is subjected to an external subthreshold signal. With the diversity the network is a mixture of excitable and oscillatory neurons, and the diversity is determined by the variance of the system's parameter. The complex network is constructed by randomly adding long-range connections (shortcuts) on a nearest-neighbouring coupled one-dimensional chain. Numerical results show that external signals are maximally magnified at an intermediate value of the diversity, as in the case of well-known stochastic resonance. Furthermore, the effects of the number of shortcuts and coupled strength on the diversity-induced phenomena are also discussed. These findings exhibit that the diversity may play a constructive role in response to external signal, and highlight the importance of the diversity on such complex networks.
Detecting Determinism in Firing Activities of Retinal Ganglion Cells during Response to Complex Stimuli
CAI Chao-Feng, ZHANG Ying-Ying, LIU Xue, LIANG Pei-Ji, ZHANG Pu-Ming
Chin. Phys. Lett. 2008, 25 (5): 1595-1598 .  
Abstract   PDF(pc) (879KB) ( 514 )
Complex stimuli are used to probe the response properties of the chicken's retinal ganglion cells (GCs). The correlation dimension method and the nonlinear forecasting method are applied to detect the determinism in the firing activities of the retinal GCs during response to complex stimuli. The inter-spike interval (ISI) series and the first difference of the ISI (DISI) series are analysed. Two conclusions are drawn. Firstly, the first difference operation of the ISI series makes it comparatively easier for determinism detection in the firing activities of retinal GCs. Secondly, the nonlinear forecasting method is more efficient and reliable than the correlation dimension method for determinism detection.
Painleve Property and New Analytic Solutions for a Variable-Coefficient Kadomtsev--Petviashvili Equation with Symbolic Computation
WEI Guang-Mei, GAO Yi-Tian, XU Tao, MENG Xiang-Hua, ZHANG Chun-Yi
Chin. Phys. Lett. 2008, 25 (5): 1599-1602 .  
Abstract   PDF(pc) (1839KB) ( 620 )
A variable-coefficient Kadomtsev--Petviashvili equation is investigated. The Painleve analysis leads to its explicit Painleve-integrable conditions. An auto-Backlund transformation and the bilinear form are presented via the truncated Painleve expansion and symbolic computation. Several families of new analytic
solutions are presented, including the soliton-like and periodic solutions.
High-Pressure and High-Temperature Behaviour of Gallium Oxide
MA Yan-Mei, CHEN Hai-Yong, YANG Kai-Feng, LI Min, CUI Qi-Liang, LIU Jing, ZOU Guang-Tian
Chin. Phys. Lett. 2008, 25 (5): 1603-1605 .  
Abstract   PDF(pc) (620KB) ( 1017 )
High-temperature and high-pressure behaviours of β-Ga2O3 powder are studied by energy-dispersive x-ray diffraction in a diamond anvil cell (DAC). It is found that the phase transition from the monoclinic β-Ga2O3 to the trigonal α-Ga2O3 occurs at around 19.2GPa under cold compression. By heating the powder to 2000K at 30GPa, we confirm that α-Ga2O3 is the most stable structure at the high pressure. Furthermore, the structural transition from β-Ga2O3 to α-Ga2O3 is irreversible. After laser heating, the recrystallized Ga2O3
has a preferable (012) orientation. This interesting behaviour is also discussed.
Experimental Investigation on a Fibre-Optic Hydrophone with a Cylindrical Helmholtz Resonator
WANG Ze-Feng, HUNG Yong-Ming, MENG Zhou, NI Ming
Chin. Phys. Lett. 2008, 25 (5): 1606-1608 .  
Abstract   PDF(pc) (168KB) ( 798 )
A novel mechanical anti-aliasing filtering fibre-optic hydrophone with a cylindrical Helmholtz resonator is constructed and tested. The experimental results show that the hydrophone has a function of low-pass filtering. The low
frequency acoustic sensitivity is about -160dB (1radμPa), and the response curve has a resonance determined by the Helmholtz resonator. Theoretical and experimental results both show that the resonant frequency moves towards high frequency with the increasing orifice diameters. The sensitivity attenuation of high frequency is larger than 10dB. This new fibre-optic
hydrophone is a prototype device for a class of sensors used to eliminate the aliasing in future sonar systems.
Chiral Suppression and SU(3) Symmetry in Scalar Glueball Decays
JIN Hong-Ying, LIU Shao-Min, ZHANG Zhu-Feng, LI Xue-Qian
Chin. Phys. Lett. 2008, 25 (5): 1609-1612 .  
Abstract   PDF(pc) (231KB) ( 447 )
We theoretically evaluate the decay rates of a scalar glueball up to the order O(αs), where only two Feynman diagrams contribute and a special attention is paid on possible flavour SU(3) symmetry breaking in the process. It is
concluded that the SU(3) flavour symmetry may be respected in any case. However, due to chiral suppression in G0→qq, Γ(G0→qq qq) very likely is larger than Γ(G0→qq). These results are supported by the experimental data on the decays of χc0(1P) (0++). Based on this result, we propose a criterion to identify the scalar glueball.
Single Production of Heavy Top Quark from Three-Site Higgsless Model
YUE Chong-Xing, WANG Li-Hong, WEN Jia
Chin. Phys. Lett. 2008, 25 (5): 1613-1616 .  
Abstract   PDF(pc) (154KB) ( 372 )
We consider single production of the heavy top quark T predicted by the three-site Higgsless model in future high energy collider experiments, such as the high energy linear e+e- collider (ILC), the linear-ring type ep collider (THERA), and the CERN Large Hadron Collider (LHC). Our numerical results show that the possible signals of the heavy top quark T might be detected via the subprocess qbi→q'T at the LHC.
Influence of Spin--Orbit Force on Nucleon--Nucleon Scattering in the Quark Delocalization Colour Screening Model
HUANG Hong-Xia, CHEN Ling-Zhi, PANG Hou-Rong, PING Jia-Lun, WANG Fan
Chin. Phys. Lett. 2008, 25 (5): 1617-1620 .  
Abstract   PDF(pc) (164KB) ( 463 )
The symmetric spin--orbit interactions of one-gluon-exchange and confinement are included in the nucleon--nucleon phase shift calculation in the framework of quark delocalization colour screening model. The spin--orbit interaction has little influence on D wave phase shift. For the triplet P waves, 3PT is in good agreement with the experimental data and 3PLS is attractive but not strong enough, whereas 3PC is too strongly repulsive. Our results indicate that the symmetric spin--orbit interaction of one-gluon-exchange and
confinement potential cannot give a good description of the triplet P wave phase shifts. More sophisticated considerations, the delocalization depending on the relative orientation between two cluster, might be needed to improve the description of P-wave NN interaction.
Triaxiality of High-K Isomers in A~130 Region
LIU Hong-Liang, XU Fu-Rong,
Chin. Phys. Lett. 2008, 25 (5): 1621-1624 .  
Abstract   PDF(pc) (201KB) ( 472 )
Remarkable triaxiality is found in the high-K isomeric states of A ~ 130 nuclei using the configuration-constrained calculations of potential-energy surfaces.
The triaxiality gives the further understanding of the decay properties of the isomers. The calculation with triaxiality can well explain the measured quadrupole moment of the Kπ=7- isomer in130Ce.
Symmetry Energy and Isovector Giant Dipole Resonance in Finite Nuclei
CAO Li-Gang, MA Zhong-Yu,
Chin. Phys. Lett. 2008, 25 (5): 1625-1628 .  
Abstract   PDF(pc) (121KB) ( 545 )
We study the relationship between the properties of the isovector giant dipole resonance of finite nuclei and the symmetry energy in the framework of the relativistic mean field theory with six different parameter sets of nonlinear
effective Lagrangian. A strong linear correlation of excited energies of the dipole resonance in finite nuclei and symmetry energy at and below the saturation density is found. This linear correlation leads to the symmetry energy at the saturation density at the interval 33.0MeV ≤ S(ρ0)≤37.0,MeV. The comparison to the present experimental data in the soft dipole mode
of 132Sn constrains approximately the symmetry energy at ρ= 0.1fm-3 at the interval 21.2MeV~22.5MeV. It is proposed that a precise measurement of the soft dipole mode in neutron rich nuclei could set up an important constraint on the equation of state for asymmetric nuclear matter.
Core Polarization and Tensor Coupling Effects on Magnetic Moments of Hypernuclei
YAO Jiang-Ming, LU Hong-Feng, Hillhouse Greg, MENG Jie, ,
Chin. Phys. Lett. 2008, 25 (5): 1629-1632 .  
Abstract   PDF(pc) (200KB) ( 420 )
Effects of core polarization and tensor coupling on the magnetic moments in 13ΛC, 17Λ, and 41ΛCa Λ-hypernuclei are studied by employing the Dirac equation with scalar, vector and tensor potentials. It is found that the effect of core polarization on the magnetic moments is suppressed by Λ tensor coupling. The Λ tensor potential reduces the spin--orbit splitting of pΛ states considerably. However, almost the same magnetic moments are obtained using the hyperon wavefunction obtained via the Dirac equation either with or without the Λ tensor potential in the electromagnetic current vertex. The deviations of magnetic moments for pΛ states from the Schmidt values are found to increase with nuclear mass number.
Prolate and Oblate Shape Coexistence in 188Pt
LIU Yuan, ZHOU Xiao-Hong, ZHANG Yu-Hu, ZHENG-Yong, LIU Min-Liang, GUO Ying-Xiang, M. Oshima, Y. Toh, M. Koizumi, A. Osa, Y. Hatsukawa, SUN Yang
Chin. Phys. Lett. 2008, 25 (5): 1633-1635 .  
Abstract   PDF(pc) (422KB) ( 560 )
A standard in-beam γ-spectroscopy experiment for 188Pt is performed via the
176Yb(18O, 6n) reaction at beam energies of 88 and 95MeV, and the level scheme for 188Pt is established. Prolate and oblate shape coexistence has been demonstrated to occur in 188Pt by applying the projected shell model. The rotation alignment of i13/2 neutrons drives the yrast sequence changing
suddenly from prolate to oblate shape at angular momentum 10h, indicating likely a new type of shape phase transition along the yrast line in 188Pt.
BEPCII Injector Linac Upgrade and Beam Instabilities
WANG Shu-Hong, PEI Guo-Xi, CHI Yun-Long, CHEN Yan-Wei, CAO Jian-She, KONG Xiang-Cheng, ZHAO Feng-Li, HOU Mi, LIU Wei-Bin, GENG Zhe-Qiao, PEI Shi-Lun, DENG Bing-Lin, CHEN Zhi-Bi
Chin. Phys. Lett. 2008, 25 (5): 1636-1639 .  
Abstract   PDF(pc) (314KB) ( 742 )
The upgrade project of the Beijing Electron Positron Collider (BEPCII) and its injector linac is working well. The linac upgrade aims at a higher injection rate of 50mA/min into the storage ring, which requires an injected beam with low
emittance, low energy spread and high beam orbit and energy stabilities. This goal is finally reached recently by upgrading the linac components and by dealing with rich and practical beam physics, which are described in this study.
Multislit-Based Emittance Measurement of Electron Beam from a Photocathode Radio-Frequency Gun
YAN Li-Xin, DU Ying-Chao, XIANG Dao, LI Ren-Kai, QIAN Hou-Jun, HUANG Wen-Hui, TANG Chuan-Xiang, LIN Yu-Zheng, CHENG Jian-Ping
Chin. Phys. Lett. 2008, 25 (5): 1640-1643 .  
Abstract   PDF(pc) (6251KB) ( 761 )
Measurement of emittance for a space-charge dominated electron beam from a photocathode rf gun is performed by employing the multislit-based method at Accelerator Laboratory of Tsinghua University. We present the design considerations on the multislit system and the experimental results, with special attention to the study of space charge induced emittance growth. The experimental results are in reasonable agreement with the PARMELA simulations.
Electronic Curves Crossing in Methyl Iodide by Spin--Orbit Ab Initio Calculation
LI Rui, YAN Bing, ZHAO Shu-Tao, GUO Qing-Qun, LIAN Ke-Yan, TIAN Chuan-Jin, PAN Shou-Fu
Chin. Phys. Lett. 2008, 25 (5): 1644-1645 .  
Abstract   PDF(pc) (131KB) ( 456 )
An ab initio investigation of electronic curve crossing in a methyl iodide molecule is carried out using spin--orbit multiconfigurational quasidegenerate perturbation theory. The one-dimensional rigid potential curves and optimized
effective curves of low-lying states, including spin--orbit coupling and relativistic effects, are calculated. The spin--orbit electronic curve crossing between 3Q 0+ and 1Q1, and the shadow minimum in potential energy curve of 3Q0+ at large internuclear distance are found in both sets of the curves according to the present calculations. The crossing position is in the range of
RC-I=0.2370±0.0001nm. Comparisons with other reports are presented.
Ultra-Slow Atomic Beam Generation by Velocity Selective Resonance
MA Hong-Yu, CHENG Hua-Dong, WANG Yu-Zhu, LIU Liang, METCALF Harold
Chin. Phys. Lett. 2008, 25 (5): 1646-1649 .  
Abstract   PDF(pc) (130KB) ( 461 )
We describe a method to generate an ultra-slow atomic beam by velocity selective resonance (VSR). A VSR experiment on a metastable helium beam in a magnetic field is presented and the results show that the transverse velocity of the deflected beam can be cooled and precisely controlled to less than the recoil velocity, depending on the magnitude of the magnetic field. We extend this idea to a cold atomic cloud to produce an ultra-slow 87Rb beam that can be used as a source of an atomic fountain clock or a space clock.
L-Shell X-Ray Yields and Production Cross Sections of Zr and Mo Bombarded by Slow Highly Charged Ar16+ Ions
XU Jin-Zhang, DU Juan, CHEN Xi-Meng, SHAO Jian-Xiong, YANG Zhi-Hu, CUI Ying, GAO Zhi-Min, LIU Yu-Wen
Chin. Phys. Lett. 2008, 25 (5): 1649-1652 .  
Abstract   PDF(pc) (207KB) ( 470 )
The L-shell x-ray yields of Zr and Mo bombarded by slow Ar16+ ions are measured. The energy of the Ar16+ ions ranges from about 150keV to 350keV. The L-shell x-ray production cross sections of Zr and Mo are extracted from these yields data. The explanation of these experimental results is in the framework of the adiabatic direct-ionization and the binding energy modified BEA approximation. We consider, in the slow asymmetric collisions such as Ar and Mo/Zr, the transient united atoms (UA) are formed during the ion-surface interaction and the direct-ionization is the main mechanism for the inner-shell vacancy production. Generally, the theoretical
results are in good agreement with the experimental data.
Fluorescence Detection and Buffer Gas Cooling of Trapped Mercury Ions in Paul Trap
SHE Lei, , WANG Wen-Ming, , BAI Lei, , SUN Huan-Yao, , ZHU Xi-Wen, LI Jiao-Mei, GAO Ke-Lin,
Chin. Phys. Lett. 2008, 25 (5): 1653-1656 .  
Abstract   PDF(pc) (218KB) ( 566 )
Hg ions are confined in a hyperboloid ion trap. With the rf discharge 202Hg isotope lamp, the fluorescence signal of trapped Hg ions is observed. By means of buffer gas cooling, the ionic temperature is reduced. As a result, the trapping time is increased and the signal-to-noise ratio (SNR) of the fluorescent signal is improved. The temperature of ion cloud is estimated by measuring the space charge shift.
Electromagnetic Invisibility of Elliptic Cylinder Cloaks
YAO Kan, LI Chao, LI Fang
Chin. Phys. Lett. 2008, 25 (5): 1657-1660 .  
Abstract   PDF(pc) (2965KB) ( 673 )
Structures with unique electromagnetic properties are designed based on the approach of spatial coordinate transformations of Maxwell's equations. This
approach is applied to scheme out invisible elliptic cylinder cloaks, which provide more feasibility for cloaking arbitrarily shaped objects. The transformation expressions for the anisotropic material parameters and the field distribution are derived. The cloaking performances of ideal and lossy elliptic cylinder cloaks are investigated by finite element simulations. It is found that the cloaking performance will degrade in the forward direction with
increasing loss.
Bend-Induced Distortion in Large Mode Area Holey Fibre
TAN Xiao-Ling, GENG You-Fu, ZHANG Tie-Li, WANG Wei-Neng, WANG Peng, YAOJian-Quan
Chin. Phys. Lett. 2008, 25 (5): 1661-1663 .  
Abstract   PDF(pc) (389KB) ( 504 )
A simplified scheme of bend-induced mode distortion is introduced into bent holey fibres, the distorted mode distribution and mode effective area reduction are investigated using the finite difference method. Numerical results show that the modes of bent holey fibres with small bend radius shift away from the core and are deformed greatly, and the mode areas drop significantly as the bend radius decreases, which severely affects the fibre laser performance. The propagation characteristics of bent holey fibres at
given wavelength are determined by fill factor and normalized bend radius. Finally, the transition normalized bend radius that represents the location of the mode area beginning to fall off is obtained.
Tight Focusing of Radially and Azimuthally Polarized Vortex Beams through a Dielectric Interface
ZHANG Zhi-Ming, PU Ji-Xiong, WANG Xi-Qing
Chin. Phys. Lett. 2008, 25 (5): 1664-1667 .  
Abstract   PDF(pc) (835KB) ( 703 )
Based on vectorial Debye theory, tight focusing of radially and azimuthally polarized vortex beams passing through a dielectric interface are studied. The intensity distribution in the focal region is illustrated by numerical calculations. We show the influence of numerical-aperture (NA) on the full-width at half maximum (FWHM) of the focal spot or the focal hole. It has been found that compared with the azimuthally polarized Bessel--Gaussian (BG) beams, the longitudinal component in the z direction of the radially polarized BG beams has no influence on the FWHM of the focal spot and hole, but enhances the total light intensity.
High Sensitivity Transmission-Type SPR Sensor by Using Metallic--Dielectric Mixed Gratings
WU Bin, WANG Qing-Kang
Chin. Phys. Lett. 2008, 25 (5): 1668-1672 .  
Abstract   PDF(pc) (781KB) ( 787 )
We theoretically investigate transmission-type SPR sensors with novel metallic--dielectric mixed gratings by rigorous coupled-wave analysis (RCWA), compared to the conventional dielectric gratings based structure. It is found that the transmittance efficiency and the full width at half-maximum (FWHM) of the transmission curve can be modulated by increasing or decreasing the metallic part. Therefore, appropriate proportion of metal part will induce enhancement factor of sensor merit. Furthermore, this novel structure will also bring enhancement of resonant angle shift, which can be explained by plasmonic interpretation based on a surface limited increase of interaction
area and excitation of localized surface plasmons (LSPs). The proposed configuration has a wide range of potential applications not only as sensor but also other optical devices.
Debye Series of Scattering by a Multi-Layered Cylinder in an Off-Axis 2D Gaussian Beam
WU Zhen-Sen, LI Hai-Ying
Chin. Phys. Lett. 2008, 25 (5): 1672-1675 .  
Abstract   PDF(pc) (178KB) ( 633 )
The Debye series of light scattering by an infinite multi-layered cylinder in an off-axis 2D Gaussian beam is studied. A simplified but rigorous iterative formula for scattering coefficients is presented. The numerical calculations of
scattering intensity by a cylinder in on-axis and off-axis beams are developed. It is indicated that the results of Debye series reach an agreement with those of generalized Lorenz--Mie theory and the off-axis distances vary the results to a great extent. The Debye series components of a two-layered cylinder are further discussed. The relations between them with rainbow phenomena are analysed.
Study of Dark-Hollow Beams Generated with Different Multimode Fibres
ZHAO Cheng-Liang, LU Xuan-Hui, CHEN He
Chin. Phys. Lett. 2008, 25 (5): 1676-1679 .  
Abstract   PDF(pc) (367KB) ( 445 )
A dark-hollow beam (DHB) is generated by a coupling of a single fundamental mode He--Ne laser beam with a misalignment multimode fibre (MMF) in a special way. Effects of the misalignment angle, diameter and length of the MMF are studied. The generated DHBs can be used for guiding and trapping of atoms, manipulating particles, or as optical tweezers.
All-Optical Clock Recovery from NRZ-DPSK Signals at Flexible Bit Rates
YU Yu, ZHANG Xin-Liang, DONG Jian-Ji, HUANG De-Xiu
Chin. Phys. Lett. 2008, 25 (5): 1680-1683 .  
Abstract   PDF(pc) (589KB) ( 471 )
We propose and demonstrate all-optical clock recovery (CR) from nonreturn-to-zero differential phase-shift-keying (NRZ-DPSK) signals at different bit rates
theoretically and experimentally. By pre-processing with a single optical filter, clock component can be enhanced significantly and thus clock signal can be extracted from the preprocessed signals, by cascading a CR unit with a semiconductor optical amplifier based fibre ring laser. Compared with the previous preprocessing schemes, the single filter is simple and suitable for different bit rates. The clock signals can be achieved with extinction ratio over 10dB and rms timing jitter of 0.86 and 0.9 at 10 and 20Gb/s, respectively. The output performances related to the bandwidth and the detuning of the filter are analysed. By simply using a filter with larger bandwidth, much higher operation can be achieved easily.
Design of High-Efficiency Diffraction Gratings Based on Rigorous Coupled-Wave Analysis for 800nm Wavelength
KONG Wei-Jin, YUN Mao-Jin, LIU Shi-Jie, JIN Yun-Xia, FAN Zheng-Xiu, SHAO Jian-Da
Chin. Phys. Lett. 2008, 25 (5): 1684-1686 .  
Abstract   PDF(pc) (331KB) ( 689 )
We report on the design of a high diffraction efficiency multi-layer dielectric grating with wide incident angle and broad bandwidth for 800nm. The optimized grating can achieve >95% diffraction efficiency in the first order at an incident angle of 5° from Littrow and a wavelength from 770nm to 830nm, with peak diffraction efficiency of >99.5% at 800nm. The electric field distribution of the optimized multi-layer dielectric grating within the gratings ridge is 1.3 times enhancement of the incidence light, which presents
potential high laser resistance ability. Because of its high-efficiency, wide incident, broad bandwidth and potential high resistance ability, the multi-layer dielectric grating should have practical application in Ti:sapphire laser systems.
Teleportation of a Superposition of Three Orthogonal States of an Atom without Bell-State Measurement
ZHONG Zhi-Rong
Chin. Phys. Lett. 2008, 25 (5): 1687-1689 .  
Abstract   PDF(pc) (91KB) ( 670 )
A scheme to teleport a superposition of three orthogonal states of an atom without Bell-state measurement in cavity QED is proposed. The scheme based on the resonant interaction of two λ-type three-level atoms with a bimodal
cavity. The detection of atom a collapses atom b to the initial state of atom a with cavity mode left in two-mode vacuum state. The probability of success and the fidelity of this scheme are 0.112 and 0.999, respectively.
Low-Threshold Conjugated Polymer Distributed Feedback Lasers on InP Substrate
ZHANG Su-Mei, ZHANG Ding-Ke, MA Dong-Ge
Chin. Phys. Lett. 2008, 25 (5): 1690-1692 .  
Abstract   PDF(pc) (753KB) ( 468 )
We demonstrate a low threshold polymer solid state thin-film distributed feedback (DFB) laser on an InP substrate with the DFB structure. The used gain medium is conjugated polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,
4-phenylenevinylene] (MEH-PPV) doped polystyrene (PS) and formed by drop-coating method. The second order Bragg scattering region on the InP substrate gave rise to strong feedback, thus a lasing emission at 638.9nm with a line width of 1.2nm is realized when pumped by a 532nm frequency-doubled Nd:YAG pulsed laser. The devices show a laser threshold as low as 7nJ/pulse.
High Efficient Continuous-Wave Ho:YAG Laser Pumped by a Diode-pumped Tm:YLF Laser at Room Temperature
DUAN Xiao-Ming, YAO Bao-Quan, ZHANG Yun-Jun, SONG Cheng-Wei, GAO Jing, JU You-Lun, WANG Yue-Zhu
Chin. Phys. Lett. 2008, 25 (5): 1693-1696 .  
Abstract   PDF(pc) (336KB) ( 576 )
We present a high efficient continuous wave Ho:YAG laser pumped by a diode-pumped Tm:YLF laser with a Fabry--Perot etalon tuning at 1.91μm. The maximum output power reaches 7.2W when the absorbed pump power is 10.8W. The slope efficiency (relative to the absorbed power) is 74.1%, and the Tm:YLF to Ho:YAG optical conversion efficiency of 60%, then the diode-to-Holmium optical conversion efficiency achieved is 21.0%. The wavelength is 2090nm when the transmission of output coupler is larger than 20%. The beam quality factor is M2~1.15 measured by the travelling knife-edge method.
Amplitude Regenerative Characteristics of RZ-DPSK Wavelength Converter Based on Four-Wave Mixing in SOA
JIANG Huan, WEN He, HAN Liu-Yan, GUO Yi-Li, ZHANG Han-Yi
Chin. Phys. Lett. 2008, 25 (5): 1697-1700 .  
Abstract   PDF(pc) (1338KB) ( 792 )
We investigate the phase-preserving amplitude regenerative characteristics of the return-to-zero (RZ) differential-phase-shift-keying (DPSK) wavelength conversion based on four-wave mixing (FWM) in a semiconductor optical amplifier (SOA). The Q-factor and the optical signal-to-noise ratio (OSNR)
before and after conversion are experimentally obtained and analysed in different input noise power levels. In both the continuous-wave and synchronous clock pumping cases, we find that there is amplitude clamping in the FWM conversion due to the gain saturation of SOA, which can suppress the amplitude fluctuation of the converted DPSK signal before and after demodulation. We have achieved 2-dB Q penalty improvement in our experiment demonstration of 10Gbit/s RZ-DPSK signal with OSNR lower than 19dB.
Generation of 60-GHz Millimetre Wave with 20-GHz DSB Optical
MA Jian-Xin, YU Chong-Xiu, XIN Xiang-Jun, HUANG Hui-Ying, RAO Lan, LIU Kun
Chin. Phys. Lett. 2008, 25 (5): 1701-1704 .  
Abstract   PDF(pc) (377KB) ( 523 )
A new scheme to generate a 60-GHz millimetre (mm) wave by tripling the frequency of the 20-GHz DSB optical mm wave is proposed. According to our analysis and numerical simulation of transmission along a fibre, this scheme can not only eliminate code form distortion, but also reduces the influence of
fading effect as it transmits along the fibre. Therefore, the radio-over-fibre link based on the generated optical mm-wave signal has better performance.
Single-SOA-Based Ultrahigh-Speed All-Optical Half Subtracter with PolSK Modulated Signals
LI Pei-Li, HUANG De-Xiu, ZHANG Xin-Liang, WANG Yang
Chin. Phys. Lett. 2008, 25 (5): 1705-1708 .  
Abstract   PDF(pc) (241KB) ( 515 )
A novel ultrahigh-speed all-optical half subtracter based on four-wave mixing (FWM) in a single semiconductor optical amplifier (SOA) is proposed. This scheme only requires a single SOA and two input signals without additional light source, so it is quite simple and compact. Due to the polarization-shift-keying (PolSK) modulated signals being used in this scheme, pattern-dependent degradation can be avoided. By numerical simulation, dependence of the critical factors of the logic gate performance, e.g., the output power of logic 1 and extinction ratio (ER), on two input signals power is investigated. In addition, the effect of the gain polarization dependence of SOA is analysed.
Experiments of Multi-Level Read-Only Recording Using Readout Signal Wave-Shape Modulation
TANG Yi, PEI Jing, PAN Long-Fa, NI Yi, HU Hua, ZHANG Bu-Qing
Chin. Phys. Lett. 2008, 25 (5): 1709-1712 .  
Abstract   PDF(pc) (787KB) ( 530 )
An innovative multilevel read-only recording method is proposed. In this method, a short pit/land is deliberately inserted to the original land/pit. This modifies the wave-shape of readout signal. Taking the wave-shape as the symbol of level detection, a signal wave-shape modulation (SWSM) multilevel method is realized. This method is carried out and validated on the DVD read-only manufacture and readout system. A capacity of 15GB can be expected, and a bit error rate of 10-4 is achieved. The capacity can meet the demand of high definition movie publication. This method also provides a potential multi-level solution for other storage formats and systems.
Fibre Optical Parametric Amplification in Defect Bragg Fibres with Zero Dispersion Slow Light Effect
XIAO Li, ZHANG Wei, HUANG Yi-Dong, PENG Jiang-De, ZHAO Hong, YANG Ke-Wu
Chin. Phys. Lett. 2008, 25 (5): 1713-1716 .  
Abstract   PDF(pc) (386KB) ( 506 )
Nonlinearity enhancement by slow light effect and strong light confinement in defect Bragg fibres is demonstrated and analysed in applications of fibre optical parametric amplifiers. Broadband low group velocity and zero dispersion as well as the strong light confinement by band gap enhances the nonlinear coefficient up to more than one order than the conventional high
nonlinear fibres. Moreover, the zero dispersion wavelength of coupled core mode can be designed arbitrarily, under which the phase-matching bandwidth of the nonlinear process can be extended.
Fabrication of Triplexers Based on Flattop SOI AWG
AN Jun-Ming, WU Yuan-Da, LI Jian, LI Jian-Guang, WANG Hong-Jie, LI Jun-Yi, HU Xiong-Wei
Chin. Phys. Lett. 2008, 25 (5): 1717-1719 .  
Abstract   PDF(pc) (883KB) ( 772 )
A triplexer is fabricated based on SOI arrayed waveguide gratings (AWGs). Three wavelengths of the triplexer operate at different diffraction orders of an arrayed waveguide grating. The signals of 1490nm and 1550nm, which are
input from central input waveguide of an AWG, are demultiplexed and the signal of 1310nm, which is input from central output waveguide of an AWG, is uploaded. The tested results show that the downloaded and uploaded signals have flat-top response. The insertion loss is 9dB on chip, the nonadjacent crosstalk is less than -30dB for 1490nm and 1301nm, and is less than -25dB for 1550nm, the 3dB bandwidth equates that of the input light source.
Hamiltonian Formulation of Singular Lagrangians on Time Scales
JARAD Fahd, BALEANU Dumitru, MARAABA Abdeljawad Thabet
Chin. Phys. Lett. 2008, 25 (5): 1720-1723 .  
Abstract   PDF(pc) (156KB) ( 604 )
The Hamiltonian formulation of Lagrangian on time scale isinvestigated and the equivalence of Hamilton and Euler--Lagrange equations is obtained. The role of Lagrange multipliers is discussed.
Numerical Simulation of Random Close Packing with Tetrahedra
LI Shui-Xiang, ZHAO Jian, ZHOU Xuan
Chin. Phys. Lett. 2008, 25 (5): 1724-1726 .  
Abstract   PDF(pc) (250KB) ( 1008 )
The densest packing of tetrahedra is still an unsolved problem. Numerical simulations of random close packing of tetrahedra are carried out with a sphere assembly model and improved relaxation algorithm. The packing density and average contact number obtained for random close packing of regular tetrahedra is 0.6817 and 7.21 respectively, while the values of spheres are 0.6435 and 5.95. The simulation demonstrates that tetrahedra can be randomly packed denser than spheres. Random close packings of tetrahedra with a range of height are simulated as well. We find that the regular tetrahedron might be the optimal shape which gives the highest packing density of tetrahedra.
Pattern Formation in a Vibrated Granular Layer on an Inclined Base
SHI Xiao-Dong, MIAO Guo-Qing
Chin. Phys. Lett. 2008, 25 (5): 1727-1730 .  
Abstract   PDF(pc) (347KB) ( 516 )
We carry out the simulations of pattern formation in a two-dimensional vibrated granular layer on an inclined base by molecular dynamics. It is found that the maximum amplitude of the pattern is greater at the lower part than at the higher part of the base, and is proportional to the thickness of the layer. Meanwhile, the wavelength varies non-monotonically as the inclined
angle of the base is increased.
Wavelet Cross-Spectrum Analysis of Multi-Scale Disturbance Instability and Transition on Sharp Cone Hypersonic Boundary Layer
HAN Jian, JIANG Nan .
Chin. Phys. Lett. 2008, 25 (5): 1731-1733 .  
Abstract   PDF(pc) (2130KB) ( 577 )
Experimental measurement of hypersonic boundary layer stability and transition on a sharp cone with a half angle of 5° is carried out at free-coming stream Mach number 6 in a hypersonic wind tunnel. Mean and
fluctuation surface-thermal-flux characteristics of the hypersonic boundary layer flow are measured by Pt-thin-film thermocouple temperature sensors installed at 28 stations on the cone surface along longitudinal direction. At hypersonic speeds, the dominant flow instabilities demonstrate that the growth rate of the second mode tends to exceed that of the low-frequency mode. Wavelet-based cross-spectrum technique is introduced to obtain the multi-scale cross-spectral characteristics of the fluctuating signals in the
frequency range of the second mode. Nonlinear interactions both of the second mode disturbance and the first mode disturbance are demonstrated to be dominant instabilities in the initial stage of laminar-turbulence transition for hypersonic shear flow.
Experimental Investigation of Influence of Interfacial Tension on Convection of Two-Layer Immiscible Liquid
LI Lu-Jun, DUAN Li, HU Liang, KANG Qi
Chin. Phys. Lett. 2008, 25 (5): 1734-1737 .  
Abstract   PDF(pc) (280KB) ( 782 )
Béenard--Marangoni convections of two-layer fluids heated from the bottom are investigated experimentally with a particle imagine velocimetry. The flows are visualized from the side, and various velocity fields near the onset of convection, such as three-layer vortex convective patterns, are observed when the depth ratio varies in a wide range. A new classification of the convective patterns is proposed with more detail than in previous studies. The analysis of the results indicates that the interface tension greatly influences the motion intensities of the bottom and top layers. The dimensionless wave number increases with the Bond number when the motion in the top layer is not more intense than that in the bottom layer, which agrees with the theoretical prediction.
Frequency Response of Near-Wall Coherent Structures to Localized Periodic Blowing and Suction in Turbulent Boundary Layer
LIU Jian-Hua, JIANG Nan
Chin. Phys. Lett. 2008, 25 (5): 1738-1741 .  
Abstract   PDF(pc) (219KB) ( 820 )
We experimentally investigate the frequency response of near-wall coherent structures to localized periodic blowing and suction through a spanwise slot in a turbulent boundary layer by changing the frequency of periodic disturbance at similar velocities of free stream. The effects of blowing and suction disturbance on energy redistribution, turbulent intensity u'+rms over y+ and waveforms of phase-averaged velocity during sweeping process are respectively discussed under three frequencies of periodic blowing and suction in near-wall region of turbulent boundary layer, compared with those in a standard turbulent boundary layer. The most effective disturbance frequency is figured out in this system.
Effects of Buoyancy on Langmuir Circulation
SONG Jun, SONG Jin-Bao
Chin. Phys. Lett. 2008, 25 (5): 1742-1745 .  
Abstract   PDF(pc) (149KB) ( 534 )
Based on the Navier--Stokes equation, an equation describing the Langmuir circulation is derived by a perturbation method when the influences of Coriolis
force and buoyancy force are both considered. The approach used in the analysis is similar to the works carried out by Craik and Leibovich [J. Fluid Mech. 73(1976)401], Leibovich [J. Fluid Mech. 79(1977)715] and Huang [J. Fluid Mech. 91(1979)191]. Potential applications of the equation proposed are discussed in the area of Antarctic circumpolar current.
Sprout Branching of Tumour Capillary Network Growth: Fractal Dimension and Multifractal Structure
KOU Jian-Long, LU Hang-Jun, WU Feng-Min, XU You-Sheng
Chin. Phys. Lett. 2008, 25 (5): 1746-1749 .  
Abstract   PDF(pc) (131KB) ( 683 )
A tumour vascular network, characterized as an irregularly stochastic growth, is different from the normal vascular network. We systematically analyse the
dependence of the branching. It is found that anastomosis of tumour on time is according to a number of tumour images, and both the fractal dimensions and multifractal spectra of the tumours are obtained. In the cases studied, the fractal dimensions of the tumour vascular network increase with time and the multifractal spectrum not only rises entirely but also shifts right. In addition, the best drug delivery stage is discussed according to the difference of the singularity exponent δα(δα=αmax}-αmin), which shows some change in the growth process of the tumour vascular network. A common underlying
principle is obtained from our analysis along with previous results.
Investigation on Mechanisms of Polymer Enhanced Oil Recovery by Nuclear Magnetic Resonance and Microscopic Theoretical Analysis
ZHANG Ji-Cheng, SONG Kao-Ping, LIU Li, YANG Er-Long
Chin. Phys. Lett. 2008, 25 (5): 1750-1752 .  
Abstract   PDF(pc) (768KB) ( 2036 )
Polymer flooding is an efficient technique to enhance oil recovery over water flooding. There are lots of discussions regarding the mechanisms for polymer flooding enhancing oil recovery. The main focus is whether polymer flooding
can increase sweep efficiency alone, or can increase both of sweep efficiency and displacement efficiency. We present a study on this problem. Oil displacement experiments on 4 natural cores show that polymer flooding can increase oil recovery efficiency by more than 12% over water. Moreover, photos are taken by the nuclear magnetic resonance (NMR) method both after water flooding and after polymer flooding, which show remaining oil saturation distribution at the middle cross section and the central longitudinal section. Analyses of these photos demonstrate that polymer flooding can increase both sweep efficiency and displacement efficiency.
Silicon Dioxide Coating Deposited by PDPs on PET Films and Influence on Oxygen Transmission Rate
SUN Yun-Jin, FU Ya-Bo, CHEN Qiang, ZHANG Chun-Mei, SANG Li-Jun, ZHANG Yue-Fei
Chin. Phys. Lett. 2008, 25 (5): 1753-1756 .  
Abstract   PDF(pc) (5539KB) ( 1276 )
A silicon dioxide film is deposited on the polyethyleneterephtalate (PET) by a penning discharge plasma source at ambient temperature in a high vacuum chamber. Hexamethyldisiloxane and oxygen are adopted as precursor and reactive reagent to grow a nano-scale silicon dioxide layer on polymer
surfaces. For the chemical structure analysis x-ray photoelectron spectroscopy is performed to demonstrate the content of Si, O and C elements. It is noticed that a higher silicon concentration is contained if Ar plasma is used for pretreatment. X-ray diffraction analysis shows that a micro-crystal silicon dioxide is formed by peak patterns at 25.84° and 21.8°. The barrier properties examined by oxygen transmission rate show that the permeation
parameter of the 12-μm-thick PET film drastically decreases from 135cc/m2 per day for the control one to 0.713cc/m2 per day for the as-deposited one after Ar plasma treatment. The surface morphology related to the barrier properties of SiOx-coated polymers os also investigated by scanning electron microscopy and atomic force microscopy.
Structures and Equation of State of ε-Fe under High Pressure
ZENG Zhao-Yi, LIU Zhong-Li, CHEN Xiang-Rong, CAI Ling-Cang, JINGFu-Qian,
Chin. Phys. Lett. 2008, 25 (5): 1757-1760 .  
Abstract   PDF(pc) (206KB) ( 512 )
The equation of state (EOS) and the axial ratio c/a of ε-Fe at high pressures are investigated by using the generalized gradient approximation (GGA) within the plane-wave pseudopotential density functional theory (DFT). The
results show that at the lower pressure, the EOS of ferromagnetic ε-Fe is consistent with the experimental result. While at higher pressure, the EOS of the nonmagnetic ε-Fe is in good agreement with the experimental result. Meanwhile, we find an obvious increase of the axial ratio c/a with pressure, and there is only a small increase with increasing temperature at high pressure.
Large Volume and High Density Surface Wave Plasmas Sustained by Two Microwave Launchers
LIANG Yi-Zi, OU Qiong-Rong, LIANG Bo, LIANG Rong-Qing
Chin. Phys. Lett. 2008, 25 (5): 1761-1763 .  
Abstract   PDF(pc) (224KB) ( 1081 )
Surface wave plasma (SWP) is an electromagnetic excitation along the planar interface between a dielectric and plasma medium when plasma density is so large that its permittivity becomes negative. An experiment SWP system consisting of two microwave launchers (upper and side microwave launcher) has been developed for producing large volume surface wave plasmas in our
laboratory. The experimental investigation shows that comparable uniformity plasma with not only large volume but also high density properties has been obtained by the two launchers.
Reconnection Rate in Collisionless Magnetic Reconnection under Open Boundary Conditions
HUANG Jun, MA Zhi-Wei
Chin. Phys. Lett. 2008, 25 (5): 1764-1767 .  
Abstract   PDF(pc) (668KB) ( 1081 )
Collisionless magnetic reconnection is studied by using two-dimensional
Darwin particle-in-cell simulations with different types of open boundary conditions. The simulation results indicate that reconnection rates are strongly dependent on the imposed boundary conditions of the magnetic field Bx in the inward side. Under the zero-gradient Bx boundary condition, the reconnection rate quickly decreases after reaching its maximum and no steady-state is found. Under both electromagnetic and magnetosonic boundary conditions, the system can reach a quasi-steady state. However, the reconnection rate Er≈0.08 under the electromagnetic boundary condition
is weaker than Er≈0.13 under the magnetosonic boundary condition.
Detailed Characteristics of Expansion Velocity of Si from Laser Ablated SiC
CHEN Ming, LIU Xiang-Dong, SUN Yu-Ming, YANG Xin-Mei, ZHAO Ming-Wen, QI Huan-Jun, CHEN Xiu-Fang, XU Xian-Gang
Chin. Phys. Lett. 2008, 25 (5): 1768-1771 .  
Abstract   PDF(pc) (189KB) ( 797 )
Optical emission of plasma is used to investigate the characteristics of dynamics distribution in the plume generated by ablation of a SiC sample using Nd:YAG laser. The plume expansion dynamics is characterized by time-of-flight measurement. We find that the profiles of Si (I) (390.55nm) split
into two components and the Si (II) (634.71nm) spectra show two distinct expansion dynamics regions. The time-of-flight measurement of Si(II) (634.71nm) under different laser irradiance conditions, from 0.236GW/cm2 to 1.667GW/cm2, are presented and discussed.
An Accurate Image Simulation Method for High-Order Laue Zone Effects
CAI Can-Ying, ZENG Song-Jun, LIU Hong-Rong, YANG Qi-Bin,
Chin. Phys. Lett. 2008, 25 (5): 1772-1775 .  
Abstract   PDF(pc) (3651KB) ( 683 )
A completely different formulation for simulation of the high order Laue zone (HOLZ) diffractions is derived. It refers to the new method, i.e. the Taylor series (TS) method. To check the validity and accuracy of the TS method, we take polyvinglidene fluoride (PVDF) crystal as an example to calculate the exit wavefunction by the conventional multi-slice (CMS) method and the TS method. The calculated results show that the TS method is much more accurate than the CMS method and is independent of the slice thicknesses. Moreover, the pure first order Laue zone wavefunction by the TS method can reflect the major potential distribution of the first reciprocal plane.
Surface Plasmon Resonance and Field Enhancement of Au/Ag Alloyed Hollow Nanoshells
ZHOU Li, YU Xue-Feng, FU Xiao-Feng, HAO Zhong-Hua, LI Kai-Yang
Chin. Phys. Lett. 2008, 25 (5): 1776-1779 .  
Abstract   PDF(pc) (1597KB) ( 745 )
We investigate the nanostructure, surface plasmon resonance (SPR) absorption and nonlinear enhancement of Au/Ag alloyed hollow nanoshells prepared by the replacement reaction of Ag nanoparticles in a HAuCl4 aqueous solution. As the volume of HAuCl4 increases from 0mL to 0.5mL, the SPR band of the Au/Ag alloyed nanoshells is tuned from 430nm to 780nm, and
the third-order nonlinear optical susceptibility is enhanced nearly by an order of magnitude, which indicates a large enhancement of local field in the Au/Ag alloyed hollow nanoshells with hole defects.
Synthesis of [100] Wurtzite InN Nanowires and [011] Zinc-Blende InN Nanorods
NIE Chao, ZHANG Rong, XIE Zi-Li, XIU Xiang-Qiang, LIU Bin, FU De-Yi, LIU Qi-Jia, HAN Ping, GU Shu-Lin, SHI Yi, ZHENG You-Dou
Chin. Phys. Lett. 2008, 25 (5): 1780-1783 .  
Abstract   PDF(pc) (7607KB) ( 528 )
One-dimensional wurtzite InN nanowires and zincblende InN nanorods are prepared by chemical vapour deposition (CVD) method on natural cleavage plane (110) of GaAs. The growth direction of InN nanowires is [100], with wurtzite structure. The stable crystal structure of InN is wurtzite (w-InN), zincblende structure (z-InN) is only reported for 2D InN crystals before.
However, in this work, the zincblende InN nanorods [011] are synthesized and characterized. The SEM and TEM images show that every nanorod shapes a conical tip, which can be explained by the anisotropy of growth process and the theory of Ehrlich--Schwoebel barrier.
Molecular Dynamics Simulations of Helium Behaviour in Titanium Crystals
SUN Tie-Ying, LONG Xing-Gui, WANG Jun, HOU Qing, WU Zhong-Cheng, PENG Shu-Ming, LUO Shun-Zhong
Chin. Phys. Lett. 2008, 25 (5): 1784-1787 .  
Abstract   PDF(pc) (500KB) ( 822 )
Molecular dynamics simulations are performed to investigate the behaviour of helium atoms in titanium at a temperature of 300K. The nucleation and growth of helium bubble has been simulated up to 50 helium atoms. The approach to simulate the bubble growth is to add helium atoms one by one to the bubble and let the system evolve. The titanium cohesion is based on the tight binding scheme derived from the embedded atom method, and the
helium--titanium interaction is characterized by fitted potential in the form of a Lennard-Jones function. The pressure in small helium bubbles is approximately calculated. The simulation results show that the pressure will decrease with the increasing bubble size, while increase with the increasing helium atoms. An analytic function about the quantitative relationship of the pressure with the bubble size and number of helium atoms is also fitted.
Dynamic Simulation for Hysteresis in Shape Memory Alloy under Tension
WANG Ping, ZHAO Jian-Bo, TANG Shao-Qiang
Chin. Phys. Lett. 2008, 25 (5): 1788-1791 .  
Abstract   PDF(pc) (145KB) ( 737 )
We demonstrate that the Suliciu model is capable to model the hysteresis phenomenon observed experimentally in NiTi shape memory alloy micro-tubes. This model allows a class of stationary phase interfaces. By a series of fully dynamic numerical simulations that mimic quasi-static loading and unloading, the nominal stress--strain curve exhibits a big hysteresis loop, which quantitatively agrees with the experimental results.
Variational Approach for the Bose--Hubbard Model
YU De-Shui, CHEN Jing-Biao
Chin. Phys. Lett. 2008, 25 (5): 1792-1794 .  
Abstract   PDF(pc) (146KB) ( 423 )
The phase diagram of the one-dimensional Bose--Hubbard model describing interacting bosons in optical lattice is investigated with the variational approach. This method can also be generalized to the two-dimensional case.
Capillary Interactions between a Probe Tip and a Nanoparticle
SUN Li-Ning, WANG Le-Feng, RONG Wei-Bin
Chin. Phys. Lett. 2008, 25 (5): 1795-1798 .  
Abstract   PDF(pc) (185KB) ( 560 )
To understand capillary interactions between probe tips and nanoparticles under ambient conditions, a theoretical model of capillary forces between them is developed based on the geometric relations. It is found that the contribution of surface tension force to the total capillary force attains to similar order of magnitude as the capillary pressure force in many cases. It is also shown that the tip shape and the radial distance of the meniscus have great influence on the capillary force. The capillary force decreases with the
increasing separation distances, and the variance of the contact angles may change the magnitudes of capillary forces several times at large radial distances. The applicability of the symmetric meniscus approximation is discussed.
Reliable Lateral Manipulation of Single Adatom on Metal fcc(111) Surfaces with a Single-Atom Tip
XIE Yi-Qun, LIU Qing-Wei, ZHANG Peng, GAN Fu-Xi, ZHUANG Jun
Chin. Phys. Lett. 2008, 25 (5): 1799-1802 .  
Abstract   PDF(pc) (255KB) ( 837 )
Using molecular statistics simulations based on the embedded atom method potential, we investigate the reliability of the lateral manipulation of single Pt
adatom on Pt(111) surface with a single-atom tip for different tip heights (tip-surface distance) and tip orientations. In the higher tip-height range, tip orientation has little influence on the reliability of the manipulation, and there is an optimal manipulation reliability in this range. In the lower tip-height
range the reliability is sensitive to the tip orientation, suggesting that we can obtain a better manipulation reliability with a proper tip orientation. These results can also be extended to the lateral manipulation of Pd adatom on Pd(111) surface.
High-Rate Growth and Nitrogen Distribution in Homoepitaxial Chemical Vapour Deposited Single-crystal Diamond
LI Hong-Dong, ZOU Guang-Tian, WANG Qi-Liang, CHENG Shao-Heng, LI Bo, Lü Jian-Nan, Lü Xian-Yi, JIN Zeng-Sun
Chin. Phys. Lett. 2008, 25 (5): 1803-1806 .  
Abstract   PDF(pc) (2054KB) ( 863 )
High rate (>50μm/h) growth of homoepitaxial single-crystal diamond (SCD) is carried out by microwave plasma chemical vapour deposition (MPCVD) with added nitrogen in the reactant gases of methane and hydrogen, using a
polycrystalline-CVD-diamond-film-made seed holder. Photoluminescence results indicate that the nitrogen concentration is spatially inhomogeneous in a large scale, either on the top surface or in the bulk of those as-grown SCDs. The presence of N-distribution is attributed to the facts: (i) a difference in N-incorporation efficiency and (ii) N-diffusion, resulting from the local growth
temperatures changed during the high-rate deposition process. In addition, the formed nitrogen-vacancy centres play a crucial role in N-diffusion through the growing crystal. Based on the N-distribution observed in the as-grown crystals, we propose a simple method to distinguish natural diamonds and man-made CVD SCDs. Finally, the disappearance of void defect on the top surface of SCDs is discussed to be related to a filling-in mechanism.
Theoretical Prediction for Structural Stabilities and Optical Properties of SrS, SrSe and SrTe under High Pressure
YANG Xiao-Cui, HAO Ai-Min, YANG Jie, HAN Yong-Hao, PENG Gang, GAO hun-Xiao, ZOU Guang-Tian
Chin. Phys. Lett. 2008, 25 (5): 1807-1810 .  
Abstract   PDF(pc) (186KB) ( 737 )
An investigation on the structural stabilities and electronic properties of SrX (X=S, Se and Te) under high pressure is conducted using the first-principles
calculation based on density functional theory (DFT) with the plane wave basis set as implemented in the CASTEP code. Our results demonstrate that the sequence of the pressure-induced phase transition of the three compounds is the NaCl-type (B1) structure (Fm3m) to the CsCl-type (B2) structure (Pm3m). The phase transition and the metallization pressures are determined theoretically. The pressure effect on the optical properties is
discussed. The results are compared with the previous calculations and experimental data.
Room-Temperature Ferromagnetism in Semiconducting TiO2-δ Nanoparticles
ZHAO Qian, WU Ping, LI Bao-Ling, LU Zun-Ming, JIANG En-Yong,
Chin. Phys. Lett. 2008, 25 (5): 1811-1814 .  
Abstract   PDF(pc) (443KB) ( 712 )
TiO2-δ nanoparticles are synthesized by the sol-gel method and annealed under different reducing atmosphere. The x-ray diffraction patterns show that
anatase is the dominant phase with small amounts of the rutile phase of TiO2-δ for all the samples. Magnetic measurements indicate that the samples annealed in reducing atmosphere exhibit inprecedented room-temperature ferromagnetism, in particular, the saturation magnetization Ms is up to about 8.6×10-3emu/g for the sample annealed in H2/Ar mixture. Analysis of the x-ray photoelectron spectroscopy spectra for the samples processed under
different conditions indicates that the amounts of Ti3+ or Ti2+ cations, namely, the concentration of oxygen vacancies, increase with intensifying reducing atmosphere during processing, which shows that ferromagnetism in this material strongly depends on the concentration of oxygen vacancies. The relationships between the ferromagnetism and the crystal structure as well as the grain size in this material are also discussed.
Comparison of Gain Properties with Electron--Electron and Electron--LO-Phonon Interactions in Quantum Cascade Structure
CHEN Gui-Chu, FAN Guang-Han
Chin. Phys. Lett. 2008, 25 (5): 1815-1817 .  
Abstract   PDF(pc) (127KB) ( 435 )
The gain properties of (AlN)m/(GaN)n superlattice-based quantum cascade structure are investigated by using a nonequilibrium Green's function (NGF) theory. In this theory, the electron--electron interaction and electron--LO-phonon interaction are both considered. The gain spectra of QCL are calculated from some current-driven items, which are derived from these two interactions. The results show that the effect of the electron--electron interaction is notable in the low-photon-energy range and the electron--LO-phonon interaction only takes effect in the high-photon-energy range, where photon energy is close to or larger than LO-phonon energy of GaN materials.
Sinusoidal Steady State Analysis on 4H--SiC Buried Channel MOSFETs
ZHANG Tao, LU Hong-Liang, ZHANG Yi-Men, ZHANG Yu-Ming, YE Li-Hui
Chin. Phys. Lett. 2008, 25 (5): 1818-1821 .  
Abstract   PDF(pc) (328KB) ( 629 )
With the combined use of the drift-diffusion (DD) model, experimental measured parameters and small-signal sinusoidal steady-state analysis, we extract the Y-parameters for 4H--SiC buried-channel metal oxide semiconductor field effect transistors (BCMOSFETs). Output short-circuit current gain G and Mason's invariant U are calculated for extrapolating unity current gain frequency in the common-source configuration fT and the maximum frequency of oscillation fmax, respectively. Here fT=800MHz and fmax=5GHz are extracted for the 4H--SiC BCMOSFETs, while the field effect mobility reaches its peak value 87cm2/Vs when VGS=4.5V. Simulation results clearly show that the characteristic frequency of 4H--SiC BCMOSFETs and field effect mobility are superior, due to the novel structure, compared with conventional MOSFETs.
Gap Caused by Strong Pairing in the Ladder Model of DNA Molecules
HU Dong-Sheng, ZHU Chen-Ping, ZHANG Long-Qiang, HE Da-Ren, WANG Bing-Hong
Chin. Phys. Lett. 2008, 25 (5): 1822-1824 .  
Abstract   PDF(pc) (140KB) ( 413 )
By directly diagonalizing the Hamiltonian of the ladder model of deoxyribonucleic acid (DNA) molecules, the density of states is obtained. It is found that DNA behaves as a conductor when the interchain hopping is smaller than twice the intrachain one, otherwise, DNA behaves as a semiconductor.
Au5Si2/Si Heterojunction Nanowires Formed by Combining SiO Evaporation with Vapour--Liquid--Solid Mechanism
PAN Guo-Wei, YING Guo-Liang, YONG Ben-Shou, WAN Yu-Ting, ZENG Yue-Wu, SU Zi-Xue
Chin. Phys. Lett. 2008, 25 (5): 1825-1828 .  
Abstract   PDF(pc) (3235KB) ( 443 )
Crystalline Au5Si2/Si heterojunction nanowires (Au5Si2/SiNWs) are obtained by thermal evaporating SiO powders on thick gold-coated silicon substrates in a
low vacuum system. Structure analysis of the produced Au5Si2/Si heterojunctions is performed by employing a transmission electron microscope (TEM) and a selected area electric diffractometer. The chemical compositions are studied by a energy-dispersive x-ray spectroscope attached to the TEM. A two-step growth model is proposed to describe the formation of the Au5Si2/SiNWs. During the first step, crystalline SiNWs are formed via a growth mechanism combining the oxide-assisted growth process with the vapour-liquid-solid model at relatively high temperature. In the second step, the temperature decreases and one segment of the preformed SiNWs reacts with the remnant Au to form single crystalline Au5Si2 nanowires by a solid-liquid-solid process. The present work should be useful for the future synthesis and research of high-quality gold silicide nanowires and microelectronic devices based on the nanowires.
Dependence of Photovoltaic Property of ZnO/Si Heterojunction Solar Cell on Thickness of ZnO Films
ZHANG Wei-Ying, ZHONG Sheng, SUN Li-Jie, FU Zhu-Xi
Chin. Phys. Lett. 2008, 25 (5): 1829-1831 .  
Abstract   PDF(pc) (170KB) ( 1704 )
N-ZnO/p-Si heterojunctions are prepared by sputtering deposition of intrinsic ZnO films on p-Si substrates. Thicknesses of ZnO films are altered by varying the deposition time from 1h to 3h. The electrical properties of these structures are analysed from capacitance--voltage (C--V) and current--voltage (I--V) characteristics performed in a dark room. The results demonstrated that all the samples show strong rectifying behaviour. Photovoltaic property for the samples with different thicknesses of ZnO films are investigated by measuring open circuit voltage and short circuit current. It is found that photovoltages are kept to be almost constant of 320mV along with the thickness while
photocurrents changing a lot. The variation mechanism of the photovoltaic effect as a function of thickness of ZnO films is investigated.N-ZnO/p-Si heterojunctions are prepared by sputtering deposition of intrinsic ZnO films on p-Si substrates. Thicknesses of ZnO films are altered by varying the deposition time from 1h to 3h. The electrical properties of these structures are analysed from capacitance--voltage (C--V) and current--voltage (I--V) characteristics performed in a dark room. The results demonstrated that all the samples show strong rectifying behaviour. Photovoltaic property for the samples with different thicknesses of ZnO films are investigated by measuring open circuit voltage and short circuit current. It is found that photovoltages are kept to be almost constant of 320mV along with the thickness while
photocurrents changing a lot. The variation mechanism of the photovoltaic effect as a function of thickness of ZnO films is investigated.
Organic Light Emitting Diodes Using Doped Alq3 as the Hole-transport Layer
LIANG Chun-Jun, WANG Yang, YI Li-Xin
Chin. Phys. Lett. 2008, 25 (5): 1832-1835 .  
Abstract   PDF(pc) (190KB) ( 1511 )
Effects of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ)
doping on the hole conductivity of Alq3 layer are measured. In the hole-only device of Alq3, the current densities increase in 1--3 orders of magnitude upon doping with F4TCNQ, suggesting that the doping can effectively enhance the hole-injection and hole-transport ability of Alq3. An organic
light-emitting device using an F4TCNQ doped Alq3 layer as the hole-injection and hole-transport layer, and pristine Alq3 as the electron-transport and emitting layer is fabricated and characterized. Bright emission is achieved in the simple OLED with p-doped Alq3 as the hole-transport layer and the intrinsic Alq3 as the electron-transport and emitting layer. The emitting efficiency and brightness of the device are further improved by inserting a thin electron block layer to confine the carrier recombination zone in the middle of the organic layers.
Photon-Assisted Electron Transport for a λ-shaped Carbon Nanotube Junction
JIANG Lian-Jun, LIAO Wen-Hu, ZHOU Guang-Hui
Chin. Phys. Lett. 2008, 25 (5): 1836-1839 .  
Abstract   PDF(pc) (1010KB) ( 378 )
We theoretically study the electron transport properties for two coupled single-walled carbon nanotube quantum dots connected to metallic electrodes under the irradiation of an external electromagnetic field at low temperatures. Using the standard nonequilibrium Green's function techniques, we examine the time-averaged transmission coefficient and linear conductance. It is shown that by some numerical examples, the photon-assisted inter-dot coupling causes Fano resonance and the conductance of the system is sensitive to the external field parameters. The transport dependence on the external field parameters may be used to detect the high-frequency microwave irradiation.
Effects of Contact Atomic Structure on Electronic Transport in Molecular Junction
XIA Cai-Juan, FANG Chang-Feng, HU Gui-Chao, ZHAO Peng, WANG Yi-Ming, XIE Shi-Jie, LIU De-Sheng,
Chin. Phys. Lett. 2008, 25 (5): 1840-1843 .  
Abstract   PDF(pc) (1105KB) ( 704 )
Based on nonequilibrium Green's function and first-principles calculations, we investigate the change in molecular conductance caused by different adsorption sites with the presence of additional Au atom around the metal-molecule contact in the system that benzene sandwiched between two Au(111) leads. The motivation is the variable situations that may arise in break junction experiments. Numerical results show that the enhancement of conductance induced by the presence of additional Au is dependent on the adsorption sites of anchoring atom. When molecule is located on top site with the presence of additional Au atoms, it can increase molecular conductance remarkably and present negative differential resistance under applied bias which cannot be found in bridge and hollow sites. Furthermore, the effects of
different distance between additional Au and sulfur atoms in these three adsorption sites are also discussed.
Persistent Spin and Charge Currents in Open Conducting Ring Subjected to Rashba Spin--Orbit Coupling
ZHANG Xi-Hua, XIONG Shi-Jie
Chin. Phys. Lett. 2008, 25 (5): 1844-1847 .  
Abstract   PDF(pc) (666KB) ( 592 )
We investigate persistent charge and spin currents of a one-dimensional ring with Rashba spin--orbit coupling and connected asymmetrically to two external leads spanned with angle φ0. Because of the asymmetry of the structure and the spin-reflection, the persistent charge and spin currents can
be induced. The magnification of persistent currents can be obtained when tuning the energy of incident electron to the sharp zero and sharp resonance of transmission depending on the Aharonov--Casher (AC) phase due to the spin--orbit coupling and the angle spanned by two leads φ0. The general dependence of the charge and spin persistent currents on these parameters is obtained. This suggests a possible method of controlling the magnitude and direction of persistent currents by tuning the AC phase and φ0, without the electromagnetic flux though the ring.
Switching Characteristics of Phase Change Memory Cell Integrated with Metal-Oxide Semiconductor Field Effect Transistor
XU Cheng, LIU Bo, CHEN Yi-Feng, LIANG Shuang, SONG Zhi-Tang, FENG Song-Lin, WAN Xu-Dong, YANG Zuo-Ya, XIE Joseph, CHEN Bomy
Chin. Phys. Lett. 2008, 25 (5): 1848-1849 .  
Abstract   PDF(pc) (801KB) ( 516 )
A Ge2Sb2Te5 based phase change memory device cell integrated with metal-oxide semiconductor field effect transistor (MOSFET) is fabricated using standard 0.18μm complementary metal-oxide semiconductor process
technology. It shows steady switching characteristics in the dc current-voltage measurement. The phase changing phenomenon from crystalline state to amorphous state with a voltage pulse altitude of 2.0V and pulse width of 50ns is also obtained. These results show the feasibility of integrating phase change memory cell with MOSFET.
Influence of Strain-Reducing Layer on Strain Distribution of Self-Organized InAs/GaAs Quantum Dot and Redshift of Photoluminescence Wavelength
LIU Yu-Min, YU Zhong-Yuan, REN Xiao-Min
Chin. Phys. Lett. 2008, 25 (5): 1850-1853 .  
Abstract   PDF(pc) (458KB) ( 778 )
A systematic investigation about the strain distributions around the InAs/GaAs quantum dots using the finite element method is presented. A special attention is paid to influence of an In0.2Ga0.8As strain reducing layer. The numerical results show that the horizontal- and vertical-strain components and the biaxial strain are reinforced in the InAs quantum
dot due to the strain-reducing layer. However, the hydrostatic strain in the quantum dot is reduced. In the framework of eight-band k∙p theory, we study the band edge modifications due to the presence of a strain reducing layer. The results demonstrate that the strain reducing layer yields the decreasing band gap, i.e., the redshift phenomenon is observed in experiments. Our calculated results show that degree of the redshift will increase with the
increasing thickness of the strain-reducing layer. The calculated results can explain the experimental results in the literature, and further confirm that the long wavelength emission used for optical fibre communication is realizable by adjusting the dependent parameters. However, based on the calculated electronic and heavy-hole wave function distributions, we find that the intensity of photoluminescence will exhibits some variations with the
increasing thickness of the strain-reducing layer.
Spin Pumping from a Quantum Dot in the Presence of Decoherence
XIONG Yong-Jian, GE Sen-Quan
Chin. Phys. Lett. 2008, 25 (5): 1854-1856 .  
Abstract   PDF(pc) (212KB) ( 492 )
We study the pumped spin current of an interacting quantum dot tunnel
coupled to a single lead in the presence of electron spin resonance (ESR) field. The spin decoherence in the dot is included by the Buttiker approach. Using the nonequilibrium Green's function technique, we show that ESR-induced spin flip can generate finite spin current with no charge transport. Both the Coulomb interaction and spin decoherence decrease the amplitude of spin current. The dependence of pumped spin current on the intensity and frequency of ESR field, and the spin decoherence is discussed.
Metal--Insulator Transition in Ca-Doped Sr14-xCaxCu24O41 Systems Probed by Thermopower Measurements
WANG Qing-Bo, XU Xiang-Fan, TAO Qian, WANG Hong-Tao, XU Zhu-An
Chin. Phys. Lett. 2008, 25 (5): 1857-1860 .  
Abstract   PDF(pc) (219KB) ( 468 )
High quality Sr14-xCaxCu24O41 single-crystals are successfully grown by floating-zone technique, and the transport properties are studied. The temperature dependence of resistivity along the c-axis direction is semiconductor-like for x≤10 and it can be fitted by the thermal activation equation ρ=ρ0exp(Δ/kBT) with k_B being the Boltzmann constant and Δ the activation energy. A break in the slope of thermopower (S) versus the inverse
temperature (1/T) corresponding to the formation of charge-density waves (CDW) is first observed for x≤6. The temperature dependence of thermopower becomes metallic for x≥8 while the resistivity is still semiconductor-like. We propose that the insulation behaviour of the resistivity in the Ca doping range 8≤ x≤11 could result from the localization of the charge carriers due to the disorder induced by Ca doping and a revised electronic phase diagram is derived based on our observations.
Photovoltaic Characteristic of La0.7Sr0.3MnO3/ZnO p-n Heterojunction
SUN Zhi-Hui, NING Ting-Yin, ZHOU Yue-Liang, ZHAO Song-Qing, CAO Ling-Zhu
Chin. Phys. Lett. 2008, 25 (5): 1861-1864 .  
Abstract   PDF(pc) (197KB) ( 727 )
We report on the photovoltaic properties of La0.7Sr0.3MnO3/ZnO heterojunction fabricated by pulsed laser deposition methods. Nanosecond
photovoltaic pulses are observed in this junction in the wavelength range from ultraviolet--visible to infrared. A qualitative explanation is presented, based on an analysis of the photovoltaic signals of p-n heterojunction.
Fabrication and Mossbauer Study of FeCo Alloy Nanotube Array
ZHOU Dong, LI Zhi-Wei, YANG Xu, WEN Fu-Sheng, LI Fa-Shen
Chin. Phys. Lett. 2008, 25 (5): 1865-1867 .  
Abstract   PDF(pc) (824KB) ( 668 )
Arrays of FeCo nanotubes are fabricated in the pores of porous anodic aluminium oxide templates. Transmission electron microscopic result shows that the nanotubes are regular and uniform. Magnetic hysteresis loops measured at room temperature are different from those of nanowires with the same composition, which are caused by the unique shape of nanotubes. The Mossbauer spectra show that the hyperfine field is smaller than that of the bulk's and increases with decrease of measuring temperature. However, the areas of the doublets appeared in Mossbauer spectra decrease with
decrease of measuring temperature.
THz Generation by Optical Rectification and Competition with Other Nonlinear Processes
ZHAO Zhen-Yu, HAMEAU Sophie, TIGNON Jerome
Chin. Phys. Lett. 2008, 25 (5): 1868-1870 .  
Abstract   PDF(pc) (120KB) ( 545 )
We present a study of the competition between tera-hertz (THz) generation by optical rectification in <110> ZnTe crystals, two-photon absorption, second
harmonic generation and free-carrier absorption. The two-photon nonlinear absorption coefficient, second harmonic generation efficiency and free-carrier absorption coefficient in the THz range are measured independently. The incident pump field is shown to be depleted by two-photon absorption and the THz radiation is shown to be reduced, upon focusing, by free-carrier absorption. The reduction of the generated THz radiation upon tight focusing is explained, provided that one also takes into account diffraction effects from
the sub-wavelength THz source.
Artificial Modulation of Ferroelectric Thin Films into Antiferroelectric through H+ Implantation for High-Density Charge Storage
ZHANG Yan-Jun, FEI Jin-Wen, TANG Ting-Ao, JIANG An-Quan
Chin. Phys. Lett. 2008, 25 (5): 1871-1874 .  
Abstract   PDF(pc) (190KB) ( 575 )
Hydrogen ions are implanted into Pb(Zr0.3Ti0.7)O3 thin films at the energy of 40keV with a flux of 5×1014ions/cm2. Pseudo-antiferroelectric behaviour in the implanted thin films is observed, as confirmed by the measurements of polarization versus electric hysteresis loops and capacitance versus voltage curves. X-ray diffraction patterns show the film structures before and after H+ implantation both to be perovskite of a tetragonal symmetry. These findings indicate that hydrogen ions exist as stable dopants within the films. It is believed that the dopants change domain-switching behaviour via the boundary charge compensation. Meanwhile, time dependence of leakage current density after time longer than 10s indicates the enhancement of the leakage current nearly in one order for the implanted film, but the current at time shorter than 1s is mostly the same as that of the original film without the ionic implantation. The artificial tailoring of the antiferroelectric behaviour through H+ implantation in ferroelectric thin films is finally proven to be achievable for the device application of high-density charge storage.
Raman Spectroscopic Studies of Pressure-Induced Phase Transitions on 1-Dodecene
DU Zhi-Xue, ZHENG Hai-Fei
Chin. Phys. Lett. 2008, 25 (5): 1875-1877 .  
Abstract   PDF(pc) (135KB) ( 566 )
Raman spectroscopic features of 1-dodecene are studied in a moissanite anvil cell up to 3.0GPa at 21°C. Our data indicate that 1-dodecene is chemically stable under the experimental condition because no new Raman peaks can be observed. However, two significant discontinuities in the plots of Raman shift versus pressure indicate two phase transitions of 1-dodecene. One is liquid--solid transition at pressure of about 500MPa, the other is solid--solid phase transition at pressure from 1300 to 1550MPa. The latter is considered to be related to the orientational change of the plane structure of ethylene. A
rudimentary phase diagrams for 1-dodecene, n-pentane, n-hexane are
proposed based on the results and previous data.
Effects of Coupling Lens on Optical Refrigeration of Semiconductors
DING Kai, ZENG Yi-Ping
Chin. Phys. Lett. 2008, 25 (5): 1878-1880 .  
Abstract   PDF(pc) (114KB) ( 550 )
Optical refrigeration of semiconductors is encountering efficiency difficulties caused by nonradiative recombination and luminescence trapping. A commonly used approach for enhancing luminescence efficiency of a semiconductor device is coupling a lens with the device. We quantitatively study the effects of a coupling lens on optical refrigeration based on rate equations and photon recycling, and calculated cooling efficiencies of different coupling mechanisms and of different lens materials. A GaAs/GaInP heterostructure coupled with a homo-epitaxial GaInP hemispherical lens is recommended.
Solid State Dye Lasers Based on Coumarin 440 and Pyrromethene 567 Codoped Polymethyl Methacrylate
FAN Rong-Wei, LI Xiao-Hui, XIA Yuan-Qin, JIANG Yu-Gang, HE Wei-Ming, CHEN De-Ying
Chin. Phys. Lett. 2008, 25 (5): 1881-1883 .  
Abstract   PDF(pc) (186KB) ( 900 )
Laser dye coumarin 440(C440) is codoped with pyrromethene 567 (PM567) into polymethyl methacrylate (PMMA). The effects of C440 concentration on the performance of the solid state dye medium, including spectra property, slope efficiency and photostability, are studied. When C440 is codoped with PM567 at the same concentration 1×10-4mol/L, the highest efficiency and
photostability can be obtained. Compared with the medium based on pure PM567 doped PMMA, about 50% increase in slope efficiency and at least five-fold enhancement in the photostability are observed.
Concentration and Temperature Dependences of YBO3:Bi3+ Luminescence under Vacuum Ultraviolet Excitation
CHEN Lei, JIANG Yang, ZHANG Guo-Bin, WU Can, YANG Guang-Tao, WANG Chun, LI Guo-Hua
Chin. Phys. Lett. 2008, 25 (5): 1884-1887 .  
Abstract   PDF(pc) (184KB) ( 786 )
Bi3+ doped YBO3 phosphors are prepared by solid state reaction and their luminescent properties are investigated by using synchrotron radiation instrument. Concentration and temperature dependences of YBO3:Bi3+ luminescence under VUV/UV excitation is observed. The emission and excitation spectra are assigned, and the mechanism for these phenomena is explored, which result from the energy transfer between Bi3+ ions occupying
different sites in YBO3 crystal lattice.
Eigenmode Splitting in all Hydrogenated Amorphous Silicon Nitride Coupled Microcavity
ZHANG Xian-Gao, CHEN Kun-Ji, QIAN Bo, CHEN San, DING Hong-Lin, LIUKui, WANG Xiang, XU Jun, LI Wei, HUANG Xin-Fan
Chin. Phys. Lett. 2008, 25 (5): 1888-1890 .  
Abstract   PDF(pc) (1023KB) ( 941 )
Hydrogenated amorphous silicon nitride based coupled optical microcavity is investigated theoretically and experimentally. The theoretical calculation of the transmittance spectra of optical microcavity with one cavity and coupled
microcavity with two-cavity is performed. The optical eigenmode splitting for coupled microcavity is found due to the interaction between the neighbouring localized cavities. Experimentally, the coupled cavity samples are prepared by plasma enhanced chemical vapour deposition and characterized by photoluminescence measurements. It is found that the photoluminescence peak wavelength agrees well with the cavity mode in the calculated transmittance spectra. This eigenmode splitting is analogous to the electron state energy splitting in diatom molecules.
Broadband Infrared Luminescence from Bismuth-Doped GeS2--Ga2S3 Chalcogenide Glasses
DONG Guo-Ping, XIAO Xiu-Di, REN Jin-Jun, RUAN Jian, LIU Xiao-Feng, QIU Jian-Rong, LIN Chang-Gui, TAO Hai-Zheng, ZHAO Xiu-Jian
Chin. Phys. Lett. 2008, 25 (5): 1891-1894 .  
Abstract   PDF(pc) (180KB) ( 730 )
Near-infrared luminescence is observed from bismuth-doped GeS2--Ga2S3 chalcogenide glasses excited by an 808nm laser diode. The emission peak with a maximum at about 1260nm is observed in 80GeS2--20Ga2S3:0.5Bi
glass and it shifts toward the long wavelength with the addition of Bi gradually. The full width of half maximum (FWHM) is about 200nm. The broadband infrared luminescence of Bi-doped GeS2--Ga2S3 chalcogenide glasses may be predominantly originated from the low valence state of Bi, such as Bi+. Raman scattering is also conducted to clarify the structure of glasses. These Bi-doped GeS2--Ga2S3 chalcogenide glasses can be applied potentially in novel broadband optical fibre amplifiers and broadly tunable laser in optical communication system.
Simulation of Photocurrents of Terahertz Quantum-Well Photodetectors
XIONG Feng, GUO Xu-Guang, CAO Jun-Cheng
Chin. Phys. Lett. 2008, 25 (5): 1895-1897 .  
Abstract   PDF(pc) (135KB) ( 524 )
We theoretically simulate the influence of the scattering processes of the electrons in the continuum states on the shapes of the photocurrent spectra of THz quantum well photodetectors. The width of the photocurrent peak
should be wider according to the uncertainty relation when these scattering processes lead to shorter relaxation time. We take a simple approximation model to include this influence and calculate the photocurrent spectra, which are in agreement with experimental results qualitatively.
Reactive Mechanical Alloying Synthesis of Nanocrystalline Cubic Zirconium Nitride
QIU Li-Xia, YAO Bin, DING Zhan-Hui, ZHAO Xu-Dong, JI Hong, DU Xiao-Bo, JIA Xiao-Peng, ZHENG Wei-Tao
Chin. Phys. Lett. 2008, 25 (5): 1898-1901 .  
Abstract   PDF(pc) (480KB) ( 812 )
Zirconium nitride powders with rock salt structure (γ-ZrNx) are prepared by mechanical milling of a mixture of Zirconium and hexagonal boron nitride
(h-BN) powders. The products are analysed by x-ray diffraction (XRD), canning electron microscopy (SEM), and Raman spectroscopy (RS). The formation mechanism of γ-ZrNx by ball milling technique is investigated in detail. N atoms diffuse from amorphous BN (a-BN) into Zr to form Zr(N) solid solution alloy, then the Zr(N) solid solution alloy decomposes into γ-ZrNx. No
ZrB2 is observed in the as-milled samples or the samples annealed at 1050°C for 2h.
Mechanism of Enhanced Dielectric Properties of SiC/Ni Nanocomposites
ZHOU Yan, KANG Yu-Qing, FANG Xiao-Yong, YUAN Jie, SHI Xiao-Ling, SONG Wei-Li, CAO Mao-Sheng
Chin. Phys. Lett. 2008, 25 (5): 1902-1904 .  
Abstract   PDF(pc) (881KB) ( 637 )
Dielectric properties of SiC/Ni nanocomposites prepared by a simple and facile electroless plating approach at X band are investigated. Compared to the original SiC nanoparticles (SiCP), the real part of the permittivity, ε', and the dielectric loss tangent tan δe of SiC/Ni nanocomposites are clearly enhanced by about 31% and 33%, respectively. The effective equations for complex permittivity of SiC/Ni nanocomposites are proposed. We also calculate ε' and tan δe of SiC/Ni nanocomposites and the calculated results are well consistent with the measured data.
Heteroclinic Bifurcation of Strongly Nonlinear Oscillator
ZHANG Qi-Chang, WANG Wei, LI Wei-Yi
Chin. Phys. Lett. 2008, 25 (5): 1905-1907 .  
Abstract   PDF(pc) (172KB) ( 594 )
Analytical prediction of heteroclinic bifurcation of the strongly nonlinear oscillator is presented by using the extended normal form method. We consider the approximate periodic solution of the system subject to the quintic nonlinearity by introducing the undetermined fundamental frequency. For the occurrence of heteroclinicity, the bifurcation criterion is accomplished. It depends on the contact of the limit cycle with the saddle equilibrium. As is illustrated, the explicit application shows that the new results coincide very well with the results of numerical simulation when disturbing parameter is of
arbitrary magnitude.
Memory Effect of Metal--Insulator--Silicon Capacitors with SiO2/HfO2/Al2O3 Dielectrics
LIAO Zhong-Wei, HUANG Yue, ZHANG Min, SUN Qing-Qing, DING Shi-Jin, ZHANG Wei
Chin. Phys. Lett. 2008, 25 (5): 1908-1911 .  
Abstract   PDF(pc) (213KB) ( 595 )
Charge trapping characteristics of the metal--insulator--silicon (MIS) capacitors with SiO2/HfO2/Al2O3 stacked dielectrics are investigated for memory applications. A capacitance-voltage hysteresis memory window as large as 7.3V is achieved for the gate voltage sweeping of ±12V, and a flat-band voltage shift of 1.5V is observed in terms of programming under 5V and 1ms.Furthermore, the time- and voltage-dependent charge trapping
characteristics are also demonstrated, the former is related to charge trapping saturation and the latter is ascribed to variable tunnelling barriers for electron injecting and discharging under different voltages.
A Non-Stationary Poisson Model for the Scaling of Urban Traffic Fluctuations
CHEN Yu-Dong, LI Li, ZHANG Yi, JIN Xue-Xiang
Chin. Phys. Lett. 2008, 25 (5): 1912-1915 .  
Abstract   PDF(pc) (200KB) ( 562 )
We investigate the traffic flow volume data on the time dependent activity of Beijing's urban road network. The couplings between the average flux and the fluctuations on individual links are shown to follow certain scaling laws and yield a wide variety of scaling exponents between 1/2 and 1. To quantitatively explain this interesting phenomenon, a non-stationary Poisson arriving model is proposed. The scaling property is interpreted as the result of the time-variation of the arriving rate of flux over the network, which nicely explicates the effect of aggregation windows, and provides a concise model for the dependence of scaling exponent on the external/internal force ratio.
Interhemispheric Comparison of Dipole Tilt Angle Effects on Latitude of Mid-Altitude Cusp
GUO Jian-Guang, SHI Jian-Kui, ZHANG Tie-Long, LIU Zhen-Xing
Chin. Phys. Lett. 2008, 25 (5): 1916-1918 .  
Abstract   PDF(pc) (125KB) ( 632 )
A statistical study of interhemispheric comparison of dipole tilt angle effect on the latitude of the mid-altitude cusp is preformed by a data set of the Cluster cusp crossings over a 5-year period. The result shows that the dipole tilt angle has a clear control of the cusp latitudinal location. When the dipole tilts sunwards, the cusp is shifted poleward. The northern cusp moves 1° ILAT for every 15.4° increase in the dipole tilt angle, while the southern cusp moves 1° ILAT for every 20.8° increase in the dipole tilt angle. This suggests that an interhemispheric difference appears in the dependence of cusp latitudinal location on the dipole tilt angle.
Nonlinear Local Lyapunov Exponent and Quantification of Local Predictability
DING Rui-Qiang, LI Jian-Ping, HA Kyung-Ja
Chin. Phys. Lett. 2008, 25 (5): 1919-1922 .  
Abstract   PDF(pc) (217KB) ( 3893 )
Nonlinear local Lyapunov exponent (NLLE) is applied to quantitatively determine the local predictability limit of chaotic systems. As an example, we find that the local predictability limit of Henon attractor varies considerably with time, and some underlying phase-spatial structure does not appear. The local predictability limit of initially adjacent points in phase space may be completely different. This will cause difficulties in making the long-time analogue forecast.
109 articles