Chin. Phys. Lett.

Volume 24 Issue 7

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Optical, Energetic and Exergetic Analyses of Parabolic Trough Collectors

OZTURK Murat, CICEK BEZIR Nalan, OZEK Nuri

Chin. Phys. Lett. 2007, 24 (7): 1787-1790 .

Abstract

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Parabolic trough collectors generate thermal energy from solar energy. Especially, they are very convenient for applications in high temperature solar power systems. To determine the design parameters, parabolic trough collectors must be analysed with optical analysis. In addition, thermodynamics (energy and exergy) analysis in the development of an energy efficient system must be achieved. Solar radiation passes through Earth's atmosphere until it reaches on Earth's surface and is focused from the parabolic trough collector to the tube receiver with a transparent insulated envelope. All of them constitute a complex mechanism. We investigate the geometry of parabolic trough reflector and characteristics of solar radiation to the reflecting surface through Earth's atmosphere, and calculate the collecting total energy in the receiver. The parabolic trough collector, of which design
parameters are given, is analysed in regard to the energy and exergy
analysis considering the meteorological specification in May, June, July and August in Isparta/Turkey, and the results are presented.

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Noncontinuous Solitary Wave Solutions for Double sine-Gordon Equation

TIAN Li-Xin, YU Shui-Meng, NI Mei-Jie

Chin. Phys. Lett. 2007, 24 (7): 1791-1794 .

Abstract

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Using the tanh method and a variable separated ordinary difference equation method to solve the double sine-Gordon equation, we derive some new exact travelling wave solutions, especially a new type of noncontinuous solitary wave solutions. These noncontinuous solitary wave solutions are verified by using the conservation law theory.

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Robust Generation of Four-Mode Entangled States through Adiabatic Passages

WU Huai-Zhi, YANG Zhen-Biao, ZHENG Shi-Biao

Chin. Phys. Lett. 2007, 24 (7): 1795-1798 .

Abstract

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We propose a robust scheme to generate four-mode entangled states by using the method of adiabatic passage. Our scheme is more insensitive to certain practical sources of noise, such as randomness in the atom's position, atomic spontaneous emission. In addition, the Rabi frequencies of the classical field and interaction time need not to be accurately adjusted as long as the adiabatic condition is fulfilled. The fidelity for the prepared state is higher than 0.97 under current experimental parameters.

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Simple Algorithm for Deterministic Entanglement Concentration

JIN Rui-Bo, CHEN Li-Bing, WANG Fa-Qiang, LU Yi-Qun, LIU Song-Hao

Chin. Phys. Lett. 2007, 24 (7): 1799-1801 .

Abstract

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We first present the general solution and the simplest special solution of the doubly stochastic matrix used in deterministic entanglement concentration. Then, we give a better scheme to realize deterministic entanglement concentration. Under this scheme, the concentration is realized, and more importantly, two partially entangled pairs concentrate to four-particle Greenberger--Horne--Zeilinger (GHZ) states with a certain probability.

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An Optimum Scheme to Generate Entangled Photon Pairs

GAO Xiang, WANG Xiao-Lu, LI Jia-Ming,

Chin. Phys. Lett. 2007, 24 (7): 1802-1804 .

Abstract

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An optimum scheme is proposed to generate a strong entangled photon-pair
light source by combining two incident laser lights. Such entangled photon source should have various potential applications in many fields such as quantum metrology, quantum information, and quantum lithography, etc.

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A Theoretical Scheme for Entanglement Transfer under Intensity-Dependent Couplings

XIONG Heng-Na, GUO Hong

Chin. Phys. Lett. 2007, 24 (7): 1805-1808 .

Abstract

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We consider a theoretical scheme for entanglement transfer between a two-mode squeezed vacuum field and two initially separable atoms through intensity-dependent couplings. We find that the entanglement transfer between the field and the atoms has an exact period for any given squeezing. We also find that the maximum achievable entanglement of the atomic subsystem is a simple increasing function of r. For sufficiently large squeezing parameter r, it is possible for the atoms to be entangled into a
Bell state at half the periodic time points.

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Linear Optical Realization of Qubit Purification with Quantum Amplitude Damping Channel

LIN Qing, LI Jian, GUO Guang-Can

Chin. Phys. Lett. 2007, 24 (7): 1809-1312 .

Abstract

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We provide an experimental scheme which includes the realization of quantum amplitude damping channel and the optimal two-qubit purification. Moreover, we discuss the purification of arbitrary input qubits and arbitrary N qubits. Our scheme only uses linear optical elements and the proposal may be useful in transmission of photons in fibres. This scheme is feasible in the laboratory with the current experimental technology.

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A New Approach of Quantum Mechanics for Neutron Single-Slit Diffraction

WU Xiang-Yao, YANG Jing-Hai, LIU Xiao-Jing, WANG Li, LIU Bing, FAN Xi-Hui, GUO Yi-Qing

Chin. Phys. Lett. 2007, 24 (7): 1813-1816 .

Abstract

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Phenomena of electron, neutron, atomic and molecular diffraction have been studied in many experiments, and these experiments are explained by many theoretical works. We study neutron single-slit diffraction with a quantum mechanical approach. It is found that the obvious diffraction patterns can be obtained when the single-slit width a is in the range of 3λ～60λ. We also find a new quantum effect of the thickness of single-slit which can make a large impact on the diffraction pattern. The new quantum effect predicted in our work can be tested by the neutron single-slit diffraction experiment.

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Dynamical Stability of Gap Soliton of One-Dimensional Condensate in Optical Lattices with Strong Interatomic Interaction

WANG Deng-Long, YAN Xiao-Hong, WANG Feng-Jiao

Chin. Phys. Lett. 2007, 24 (7): 1817-1820 .

Abstract

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By developing the multiple scales method, we analytically study the dynamics
properties of gap soliton of Bose--Einstein condensate in optical lattices. It is shown that the gap soliton will appear at Brillouin zone edge of linear band spectrum of the condensates when the interatomic interaction strength is larger than the lattice depth. Moreover, the density of gap soliton starts to be relatively small, while it increases with time and becomes stable.

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Moller Energy--Momentum Complex in General Relativity for Higher Dimensional Universes

M. Aygun, S. Aygun, .I.Yilmaz, H. Baysal, .I. Tarhan

Chin. Phys. Lett. 2007, 24 (7): 1821-1824 .

Abstract

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Using the Moller energy--momentum definition in general relativity
(GR) we calculate the total energy--momentum distribution associated with (n+2)-dimensional homogeneous and isotropic model of the universe. It is found that total energy of Moller is vanishing in (n+2) dimensions everywhere but n-momentum components of Moller in (n+2) dimensions are different from zero. Also, we evaluate the static Einstein Universe, FRW universe and de Sitter universe in four dimensions by using (n+2)-type metric, then calculate the Moller energy--momentum distribution of these spacetimes. However, our results are consistent with the results of Banerjee and Sen, Xulu, Radinschi,
Vargas, Cooperstock-Israelit, Aygun et al., Rosen, and Johri et al. in four dimensions.

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Ground State Energy of Unitary Fermion Gas with the Thomson Problem Approach

CHEN Ji-Sheng

Chin. Phys. Lett. 2007, 24 (7): 1825-1828 .

Abstract

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The dimensionless universal coefficient ξ defines the ratio of the unitary fermions energy density to that for the ideal non-interacting ones in the non-relativistic limit with T=0. The classical Thomson problem is taken as a nonperturbative quantum many-body arm to address the ground state energy including the low energy nonlinear quantum fluctuation/correlation effects. With the relativistic Dirac continuum field theory formalism, the concise expression for the energy density functional of the strongly interacting limit fermions at both finite temperature and density is obtained. Analytically, the universal factor is calculated to be ξ=4/9. The energy gap is △=5/18k_f²/(2m).

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A Coarse Estimation of Cell Size Region from a Mesoscopic Stochastic Cell Cycle Model

YI Ming, JIA Ya, LIU Quan, ZHU Chun-Lian, YANG Li-Jian

Chin. Phys. Lett. 2007, 24 (7): 1829-1832 .

Abstract

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Based on a deterministic cell cycle model of fission yeast, the effects of the finite cell size on the cell cycle regulation in wee1^- cdc25△ double mutant type are numerically studied by using of the chemical Langevin equations. It is found that at a certain region of cell size, our numerical results from the chemical Langevin equations are in good qualitative agreement with the
experimental observations. The two resettings to the G₂ phase from early stages of mitosis can be induced under the moderate cell size. The quantized cycle times can be observed during such a cell size region. Therefore, a coarse estimation of cell size is obtained from the mesoscopic stochastic cell cycle model.

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Analyses of Optimal Embedding Dimension and Delay for Local Linear Prediction Model

MENG Qing-Fang, PENG Yu-Hua, LIU Yun-Xia, SUN Wei-Feng

Chin. Phys. Lett. 2007, 24 (7): 1833-1836 .

Abstract

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In the reconstructed phase space, a novel local linear prediction model is proposed to predict chaotic time series. The parameters of the proposed model take the values that are different from those of the phase space reconstruction. We propose a criterion based on prediction error to determine the optimal parameters of the proposed model. The simulation results show that the proposed model can effectively make one-step and multi-step prediction for chaotic time series, and the one-step and multi-step prediction accuracy of the proposed model is superior to that of the traditional local linear prediction.

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Dynamic Feedback Controlling Chaos in Current-Mode Boost Converter

LU Wei-Guo, ZHOU Luo-Wei, LUO Quan-Ming

Chin. Phys. Lett. 2007, 24 (7): 1837-1840 .

Abstract

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A method for the control of chaos in the current-mode boost converter is presented by using the first-order dynamic feedback control. The feedback part consists of a resistance and a capacitance in series. The system to be controlled is treated as a third-order model, and then the discrete mapping model is obtained by using the data-sampling method. By analysing the position of the maximum norm eigenvalue, the stable range of feedback gain is ascertained out and its optimization is also carried out. Finally, the results of simulation and experiment confirm the correctness of the theoretical analysis and the validity of the proposed means.

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Correlation between Chaotic Dynamics and Level Spacings: the Lyapunov and Dixon Dip Approaches to Highly Excited Vibration of Deuterium Cyanide

OU Shu-Ching, WU Guo-Zhen

Chin. Phys. Lett. 2007, 24 (7): 1841-1844 .

Abstract

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Chaotic dynamics of highly excited vibration of deuterium cyanide is explored by two independent approaches: (1) the Lyapunov analysis, based on the classical phase space for the levels, and (2) the Dixon dip analysis based on the concepts of pendulum dynamics and quantized levels. The results show that there is evident correlation between these two algorithms. We also propose that the reciprocal of energy difference between two nearby Dixon dips can be taken as a qualitative measure for the degree of dynamical chaos.

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Warp of the Invariant Circle and Onset of Chaos in Josephson Junction Equation

QIAN Min, WANG Jia-Zeng

Chin. Phys. Lett. 2007, 24 (7): 1845-1848 .

Abstract

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The dynamics of the dc and ac driving Josephson junction equation is studied in terms of the two-dimensional Poincare map. The smooth invariant circle on the phase cylinder in over-damped case α>2 loses smoothness as α decreases and becomes a strange attractor eventually. This triggers two kinds of chaos, one occurs in the regions between two Arnold tongues and the other occurs within the tongues.

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Resonance in Defect Turbulence under Periodic External Force

QIAO Chun, FENG Xiao-Bo, WANG Hong-Li, OUYANG Qi

Chin. Phys. Lett. 2007, 24 (7): 1849-1852 .

Abstract

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The periodically forced spatially extended Brusselator is investigated in the chaotic regime. We explore resonant or non-resonant patterns generated under various forcing frequencies and forcing amplitudes. Resonant spatially uniform oscillation and irregular structures are found. Furthermore two types of regular spatial patterns are generated under appropriate parameters. Our
results of numerical simulations demonstrate that periodic force can give rise to resonant patterns in forced systems of spatiotemporal chaos similar to the situation of forced systems of regular oscillations.

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Synchronization of Complex Network with Drivingly Coupled Scheme

CHEN Liang, LU Jun-An

Chin. Phys. Lett. 2007, 24 (7): 1853-1856 .

Abstract

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We present a network model with a new coupled scheme which is the generalization of drive-response systems called a drivingly coupled network. The synchronization of the network is investigated by numerical simulations based on Lorenz systems. By calculating the largest transversal Lyapunov
exponents of such network, the stable and unstable regions of synchronous state for eigenvalues in such network can be obtained and many kinds of drivingly coupled arrays based on Lorenz systems such as all-to-all, star-shape, ring-shape and chain-shape networks are considered.

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Monte Carlo Simulation on Glueball Search at BESIII

QIN Hu, SHEN Xiao-Yan

Chin. Phys. Lett. 2007, 24 (7): 1857-1860 .

Abstract

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The J/ψ radiative decays are suggested as promising modes for glueball search. A full Monte Carlo simulation of J/ψ→γηη and γηη', based on the design of BESIII detector, is performed to study the sensitivity of searching for a possible tensor glueball at BESIII.

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Neutrino Energy Loss by Electron Capture on Nucleus ⁵⁶Mn, ⁵⁶Fe, ⁵⁶Co and ⁵⁶Ni in Strong Electron Screening

LIU Jing-Jing, LUO Zhi-Quan

Chin. Phys. Lett. 2007, 24 (7): 1861-1864 .

Abstract

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Based on the p--f shell model, the neutrino energy loss rates for nucleus 56Mn, 56Fe, 56Co and 56Ni in the electron capture process are canvassed in and not in a strong electron screening. The results show that the neutrino energy loss rates for ⁵⁶Mn, ⁵⁶Fe, 56Co and 56Ni decrease about 15%, 10%, 60% and 1 order of magnitude correspondingly at the temperature T9=15 and even debase 2 orders of magnitude at the temperature T9=1.

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Reflection Asymmetric Relativistic Mean Field Approach and Its Application to the Octupole Deformed Nucleus ²²⁶Ra

GENG Li-Sheng, MENG Jie, , Toki Hiroshi

Chin. Phys. Lett. 2007, 24 (7): 1865-1868 .

Abstract

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A reflection asymmetric relativistic mean field (RAS-RMF) approach is developed by expanding the equations of motion for both the nucleons and
the mesons on the eigenfunctions of the two-centre harmonic-oscillator
potential. The efficiency and reliability of the RAS-RMF approach are demonstrated in its application to the well-known octupole deformed
nucleus ²²⁶Ra and the available data, including the binding energy and the deformation parameters, are well reproduced.

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Global Synchronization of General Delayed Dynamical Networks

LI Zhi

Chin. Phys. Lett. 2007, 24 (7): 1869-1872 .

Abstract

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Global synchronization of general delayed dynamical networks with linear
coupling are investigated. A sufficient condition for the global synchronization is obtained by using the linear matrix inequality and introducing a reference state. This condition is simply given based on the maximum nonzero eigenvalue of the network coupling matrix. Moreover, we show how to construct the coupling matrix to guarantee global synchronization of network, which is very convenient to use. A two-dimension system with delay as a dynamical node in network with global coupling is finally presented to verify the theoretical results of the proposed global synchronization scheme.

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Isoscaling Parameter α as a Possible Probe of Medium Effect of Nucleon--Nucleon Cross Section

LIU Jian-Ye, , HAO Huan-Feng, XING Yong-Zhong, , ZUO Wei, LEE Xi-Guo,

Chin. Phys. Lett. 2007, 24 (7): 1873-1875 .

Abstract

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The medium effect of nucleon--nucleon cross section σ^med_NN(α_m) on the isoscaling parameter α is investigated for two central nuclear reactions ⁴⁰Ca+⁴⁰Ca, ⁶⁰Ca+⁶⁰Ca within isospin-dependent quantum molecular dynamics at beam energies from 40 to 50MeV/nucleon. It is found that there is the very obvious medium effects of nucleon--nucleon cross section σ^med_NN(α_m) on the isoscaling parameters α. In this case the isoscaling parameter α is a possible probe of the medium effect of nucleon--nucleon cross section σ^med_NN(α_m) in the heavy ion collisions. The mechanism of the above-mentioned properties is studied and discussed.

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First Beam Measurements of the S-Band photocathode Radio-Frequency Gun at Tsinghua University

DU Ying-Chao, YAN Li-Xin, DU Qiang, HE Xiao-Zhong, XIANG Dao, TANG Chuan-Xiang, HUANG Wen-Hui, LIN Yu-Zheng

Chin. Phys. Lett. 2007, 24 (7): 1876-1879 .

Abstract

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During the last decades, photocathode rf gun has been proven to be
successful in generating the high brightness electron beam (～1nC,1～πmmmrad,～1ps) which is required by the ILC, XFEL, Thomson scattering x-ray source, etc. A photocathode rf gun system is built to develop electron source for the Thomson scattering x-ray source at Accelerator Laboratory of Tsinghua University. The system consists of a BNL/ATF-type 1.6 cell S-band rf cavity, a solenoid for emittance compensation, a laser system and some simple equipments for beam diagnosis. The first beam measurements of the photocathode rf gun, including the dark current, transverse beam profile, charge and quantum efficiency, are reported.

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Potential Energy Surfaces of Nitrogen Dioxide for the Ground State

SHAO Ju-Xiang, ZHU Zheng-He, CHENG Xin-Lu, YANG Xiang-Dong

Chin. Phys. Lett. 2007, 24 (7): 1879-1882 .

Abstract

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The potential energy function of nitrogen dioxide with the C_2vsymmetry in the ground state is represented using the simplified Sorbie--Murrell many-body expansion function in terms of the symmetry of NO₂. Using the potential energy function, some potential energy surfaces of NO₂(C_2V, X²A₁), such
as the bond stretching contour plot for a fixed equilibrium geometry angle θ and contour for O moving around N--O (R₁), in which R₁ is fixed at the equilibrium bond length, are depicted. The potential energy surfaces are analysed. Moreover, the equilibrium parameters for NO₂ with the C_2v, C_s and D_8hsymmetries, such as equilibrium geometry structures and energies, are
calculated by the ab initio (CBS-Q) method.

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Frequency Stability of Atomic Clocks Based on Coherent Population Trapping Resonance in ⁸⁵Rb

LIU Lu, GUO Tao, DENG Ke, LIU Xin-Yuan, CHEN Xu-Zong, WANG Zhong

Chin. Phys. Lett. 2007, 24 (7): 1883-1885 .

Abstract

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An atomic clock system based on coherent population trapping (CPT) resonance in ⁸⁵Rb is reported, while most past works about the CPT clock are in 87Rb. A new modulation method (full-hyperfine-frequency-splitting modulation) is presented to reduce the effect of light shift to improve the frequency stability of the CPT clock in ⁸⁵Rb. The experimental results show that the short-term frequency stability of the CPT clock in ⁸⁵Rb is in the order of 10^-10/s and the long-term frequency stability can achieve 1.5×10^-11/80000s, which performs as well as ⁸⁷Rb in CPT resonance. This very good frequency stability performance associated with the low-cost and low-power properties of ⁸⁵Rb indicates that an atomic clock based on CPT in ⁸⁵Rb should be a promising candidate for making the chip scale atomic clock.

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Photodetachment of H^- near Elastic Surface in Parallel Electric and Magnetic Fields

WANG De-Hua

Chin. Phys. Lett. 2007, 24 (7): 1886-1889 .

Abstract

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The photodetachment cross section of H^- in parallel electric and magnetic fields near an elastic surface is derived and calculated by using the closed orbit theory. It is found that the elastic surface can produce some interesting effects. Besides the closed orbits previously found by Peters et al. for the H^-in parallel electric and magnetic fields, some additional closed orbits are produced due to the effect of the elastic surface. The results show that the cross section oscillation is much more complicated in comparison with the cross section of H^- in parallel external fields without surface. Each peak in the Fourier transformed cross section corresponds to the period of one detached electron closed orbit. This study provides a new understanding of the photodetachment of negative ions in the presence of external fields and surface.

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Analysis of Aberrations in Laser-Focused Nanofabrication

ZHANG Wen-Tao, ZHANG Bao-Wu, LI Tong-Bao

Chin. Phys. Lett. 2007, 24 (7): 1890-1893 .

Abstract

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Based on the semi-classical model, we analyse the motion equation of chromium atoms in the laser standing wave field under the condition of low intensity light field using fourth-order Adams--Moulton algorithm. The trajectory of the atoms is obtained in the standing wave field by analytical simulation. The image distortion coming from aberrations is analysed and the effects on focal beam features are also discussed. Besides these influences, we also discuss the effects on contrast as well as the feature width of the atomic beam due to laser power and laser beam waist. The simulation results have shown that source imperfection, especially the transverse velocity spread, plays a critical role in broadening the feature width. Based on these analyse, we present some suggestions to minimize these influences.

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Harmonic Enhancement Mechanism of a Superposition State Atom Irradiated by Short Pulses

YANG Yu-Jun, CHEN Ji-Gen, HUANG Yu-Xin, GUO Fu-Ming, ZHANG Hong-Xing, SUN Jia-Zhong, ZHU Hong-Yu, WANG Li, WANG Hui, ZHU Qi-Ren

Chin. Phys. Lett. 2007, 24 (7): 1894-1897 .

Abstract

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We investigate the high-order harmonic generation (HHG) of a model atom
whose initial state is prepared in a superposition of its ground state and an excited state irradiated by different duration laser pulses. Compared to the HHG generated from an atom whose initial state is in its ground state, its conversion efficiency obtains some enhancement. The enhancement originates from the higher ionization rate (rather than the ionization yield) of the atom with superposition initial state.

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Raman Spectrum Analysis on the Solid--Liquid Boundary Layer of BGO Crystal Growth

ZHANG Xia, YIN Shao-Tang, WAN Song-Ming, YOU Jing-Lin, CHEN Hui, ZHAO Si-Jie, ZHANG Qing-Li

Chin. Phys. Lett. 2007, 24 (7): 1898-1900 .

Abstract

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We study the Raman spectra of Bi₄Ge₃O₁₂ crystal at different temperatures, as well as its melt. The structure characters of the single crystal, melt and growth solid--liquid boundary layer of BGO are investigated by their high-temperature Raman spectra for the first time. The rule of structure change of BGO crystal with increasing temperature is analysed. The results show that there exists [GeO₄] polyhedral structure and Bi ion independently in BGO
melt. The bridge bonds Bi--O--Bi and Bi--O--Ge appear in the crystal and at the boundary layer, but disappear in the melt. The structure of the growth solid--liquid boundary layer is similar to that of BGO crystal. In the melt, the long-range order structure of the crystal disappears. The thickness of the growth solid--liquid boundary layer of BGO crystal is about 50μm.

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Relativistic Configuration Interaction Treatment of Generalized Oscillator Strength for Krypton

WANG Huang-Chun, QU Yi-Zhi, LIU Chun-Hua

Chin. Phys. Lett. 2007, 24 (7): 1901-1903 .

Abstract

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A fully relativistic configuration interaction method is developed to investigate the transition energies and general oscillator strengths of the lower lying states of krypton, for both optically allowed and optically forbidden transitions. The calculated results are in agreement with the recent experimental measurements. The calculated transition energies for the 5s and 5s' transitions are 9.970 and 10.717eV, which agree with the experimental data of 10.033 and 10.643eV. The calculated oscillator strengths are 0.211 and 0.170, comparable with the experimental results 0.214 (±0.012) and 0.194 (±0.012), respectively. The momentum transfer positions (K² in a.u.) of the minimum and maximum GOSs in the 4s²4p⁶→o 4s²4p⁵ (5s+5s') transitions are 1.105 and 2.225, comparable with the measurements results 1.24 and 2.97 [Phys. Rev. A 67 (2003) 062708].

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Saturation of Photoassociation in Cs Magneto-optical Trap

MA Jie, WANG Li-Rong, JI Wei-Bang, XIAO Lian-Tuan, JIA Suo-Tang

Chin. Phys. Lett. 2007, 24 (7): 1904-1907 .

Abstract

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An ultrahigh resolution photoassociation spectrum of caesium atoms in a magneto-optical trap is presented. Hyperfine structure of the excited state molecule is obtained by using the lock-in method based on modulated cold atoms in this spectrum. Amplitude of resonant lines related to the rotational levels increases with photoassociation laser intensity, and saturation effect of photoassociation of cold atoms is observed in our experiment. The saturation intensity of photoassociation is deduced by fitting the experimental data to a
saturation model based on scattering theory. Differences among saturation intensities of different rotational progressions in the υ=55 vibrational state of the caesium molecular long-range 0_g^- state have been found.

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Configurations, Electronic Structure and Magnetic Ordering of Small Manganese Clusters

MA Qing-Min, XIE Zun, LIU Ying, LI You-Cheng

Chin. Phys. Lett. 2007, 24 (7): 1908-1911 .

Abstract

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We investigate the configurations, electronic structures, and magnetic ordering of Mn_N (N=2--13) clusters based on all-electron density functional theory. The Jahn--Teller effect plays an important role in determining the
ground state of certain geometries. The magnetic ordering of the Mn_N clusters transits from ferromagnetic ordering for the smallest (N=2, 3) clusters to a near degeneracy state including ferromagnetic and ferrimagnetic ordering in the vicinity of N=4--6 and to a clear ferrimagnetic ordering at N=7 or beyond.
N=6 and 10 are the magic numbers for neutral Mn_N (N=2--13) clusters.

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An Effective Method of Producing Small Neutral Carbon Clusters

XIA Zhu-Hong, CHEN Cheng-Chu, HSU Yen-Chu

Chin. Phys. Lett. 2007, 24 (7): 1912-1914 .

Abstract

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An effective method of producing small neutral carbon clusters C_n(n=1--6) is described. The small carbon clusters (positive or negative charge or neutral) are formed by plasma which are produced by a high power 532nm pulse laser ablating the surface of the metal Mn rod to react with small hydrocarbons supplied by a pulse valve, then the neutral carbon clusters are extracted and photo-ionized by another laser (266nm or 355nm) in the ionization region of a linear time-of-flight mass spectrometer. The distributions of the initial neutral carbon clusters are analysed with the ionic species appeared in mass spectra. It is observed that the yield of small carbon clusters with the present method is about 10 times than that of the traditional widely used technology of laser vaporization of graphite.

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Melting Transition of Small Aluminium Clusters Al_11-20

ZHANG Wei, ZHANG Feng-Shou, ZHU Zhi-Yuan

Chin. Phys. Lett. 2007, 24 (7): 1915-1918 .

Abstract

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Heat capacities of small aluminium clusters Al_11-20 are investigated using MD simulation with empirical many-body Gupta potential. The heat capacities of some clusters Al₁₁, Al₁₂, Al₁₃ and Al₁₉ show well-defined peaks while the
heat capacities of Al_15-18 indicate a gradual melting transition. The spectra of isomers obtained by quenches along the MD trajectory give good interpretation for those results.

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Detection of Isotopes of Mercury Ions by Resonant Ejection in Paul Trap

WANG Wen-Min0g, SHE Lei, LI Jiao-Mei, GAO Ke-Lin

Chin. Phys. Lett. 2007, 24 (7): 1919-1921 .

Abstract

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A simple method to detect mercury ions confined in a Paul trap has been developed by resonant ejection. In this method, frequency of the additional ejection ac voltage is scanned instead of the amplitude of the rf drive voltage in conventional methods. It is possible not only to observe the spectra of the secular oscillation of the trapped ions directly, but also to eject the confined ions from the trap mass-selectively.

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Simplified Model for Analysing Ion/Photoelectron Images

ZHU Jing-Yi, WANG Bing-Xing, GUO Wei, WANG Yan-Qiu, WANG Li

Chin. Phys. Lett. 2007, 24 (7): 1922-1925 .

Abstract

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Based on the Onion--Peeling algorithm (OPA) principle, we present a simplified model for analysing photoion and photoelectron images, which allows the analysis of experimental raw images. A three-dimensional distribution of the nascent charged particles, from which the radial and angular distributions are deduced, can be obtained more easily by this model than by the commonly used procedures. The analysis results of Xe photoelectron images by this model are compared with those from the standard Hankel--Abel inversion. The results imply that this model can be used for complicated (many peaks) and `difficult' (low signal-to-noise) images with cylindrical symmetries, and can provide a reliable reconstruction in some cases when the commonly used Hankel Abel transform method fails.

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Large Positive and Negative Lateral Displacements from Total Internal Reflection Configuration with a Weakly Absorbing Dielectric Film

DING Fang-Gang, CHEN Xi, LI Chun-Fang,

Chin. Phys. Lett. 2007, 24 (7): 1926-1929 .

Abstract

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It is theoretically shown that the simultaneously large positive and negative lateral displacements will appear when the resonant condition is satisfied for a TE-polarized light beam reflected from the total internal reflection configuration with a weakly absorbing dielectric film. Appearance of the enhanced negative lateral displacement is relative to the incidence angle, absorption of the thin film and its thickness. If we select an appropriate weakly absorbing dielectric film and its thickness, the simultaneously enhanced positive and negative lateral displacements will appear at different resonant angles. These phenomena may lead to convenient measurements and interesting applications in optical devices.

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Dynamics of Efficiency Change by Temperature in Diode Pumped Nd:YAG Heat Capacity Laser

WANG Xiao-Jun, TANG Bing, SHU Xiao-Jian

Chin. Phys. Lett. 2007, 24 (7): 1930-1933 .

Abstract

PDF(pc) (258KB) ( 669 )

We investigate the influence of temperature on the efficiency of diode pumped Nd:YAG heat capacity laser is studied. It is shown that the efficiency of such a laser system is greatly reduced at higher temperature. This bad behaviour is mainly caused by the doped-ion redistribution among various Stark levels of the ground state, and by a thermal equilibrium between the upper laser level and the pump level. Meanwhile, the thermal excitations from the ground state to the lower laser level also play a role. We derive a model to describe those effects, with the considerations of emission spectrum of laser diodes, the subtle Stark structures and the linewidth of absorption and of simulated-emission.

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Thermal Performance of Laser Diode Array under Constant Convective Heat Transfer Boundary Condition

YIN Cong, HUANG Lei, HE Fa-Hong, GONG Ma-Li

Chin. Phys. Lett. 2007, 24 (7): 1934-1937 .

Abstract

PDF(pc) (616KB) ( 545 )

Three-dimensional heat transfer model of laser diode array under constant convective heat transfer coefficient boundary condition is established and analytical temperature profiles within its heat sink are obtained by separation of variables. The influences on thermal resistance and maximum temperature variation among emitters from heat sink structure parameters and convective heat transfer coefficient are brought forward. The derived formula enables the thermal optimization of laser diode array.

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An All-Solid-State Tunable Dual-Wavelength Ti:Sapphire Laser with Quasi-Continuous-Wave Outputs

DING Xin, , PANG Ming, , YU Xuan-Yi, WANG Xiao-Heng, ZHANG Shao-Min, , ZHANG Heng, , WANG Rui, , WEN Wu-Qi, , WANG Peng, , YAO Jian-Quan,

Chin. Phys. Lett. 2007, 24 (7): 1938-1940 .

Abstract

PDF(pc) (235KB) ( 639 )

A high power dual-wavelength Ti:sapphire laser system with wide turning range and high efficiency is described, which consists of two prism-dispersed resonators pumped by an all-solid-state frequency-doubled Nd:YAG laser. Tunable dual-wavelength outputs, with one wavelength range from 750nm to 795nm and the other from 800nm to 850nm, have been demonstrated. With a pump power of 23W at 532nm, a repetition rate of 6.5kHz and a pulse width of 67.6ns, the maximum dual-wavelength output power of 5.6W at 785.3nm and 812.1nm, with a pulse width of 17.2ns and a line width of 2nm, has been achieved, leading to an optical-to-optical conversion efficiency of 24.4%.

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Self-Assembled Quasi-Periodic Voids in Glass Induced by a Tightly Focused Femtosecond Laser

DAi Ye, HU Xiao, SONG Juan, YU Bing-Kun, QIU Jian-Rong

Chin. Phys. Lett. 2007, 24 (7): 1941-1944 .

Abstract

PDF(pc) (481KB) ( 769 )

Multiple refocusing of a tightly focused femtosecond laser due to the dynamic transformation between self-focusing and self-defocusing is employed to provide a novel method to produce quasi-periodic voids in glass. It is found that the diameter or the interval of the periodic voids increases with the increasing pulse energy of the laser. The detailed course for producing periodic voids is discussed by analysing the damaged track induced by the tightly focused femtosecond laser pulses. It is suggested that this periodic structure has potential applications in fabrication of three-dimensional optical devices.

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Experimental Identification and Study of Coloured Conical Emission in Quadratic Nonlinear Media

YANG Hua, QIAN Lie-Jia, FU Xi-Quan, LUO Hang, YUAN Peng, ZHU He-Yuan

Chin. Phys. Lett. 2007, 24 (7): 1945-1948 .

Abstract

PDF(pc) (211KB) ( 625 )

We have explicitly identified coloured conical emission (CCE) and noncollinear optical parametric generation (OPG) by spectrum characterizations. With an experimental setup providing different pump pulse durations, CCE and noncollinear OPG are observed both alternatively and simultaneously. Comparisons between CCE and noncollinear OPG are studied. Accumulation behaviour of modulational instabilities is observed in our two-crystal cascaded configuration, which results in enhancement or depression of the CCE formation.

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Period Doubling in a Fabry--Perot Laser Diode Subject to Optical Pulse Injection

ZHAO Yue-Peng, WANG Yun-Cai, ZHANG Ming-Jiang, AN Yi, WANG Ji-Long

Chin. Phys. Lett. 2007, 24 (7): 1949-1952 .

Abstract

PDF(pc) (251KB) ( 693 )

Experimental study and numerical simulations of the period doubling of injected optical pulses in Fabry--Perot laser diodes are presented. In our experiments, the period doubling is achieved within a wide input frequency range and the period doubling of the injected optical pulses with 6.32GHz repetition rate is investigated in detail. The obtained experimental results indicate that period doubling occurs at an appropriate injected optical power level when the bias current of the Fabry--Perot laser diode is located in lower ranges. Moreover, the experimental observed features have been numerically demonstrated by using a coupled rate-equation model. Numerical simulations are consistent with the experimental results.

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A 2.048μm Tm,Ho:GdVO₄ Laser Pumped Doubly Resonant Optical Parametric Oscillator Based on Periodically Poled Lithium LiNbO₃

ZHANG Xing-Bao, YAO Bao-Quan, JU You-Lun, WANG Yue-Zhu

Chin. Phys. Lett. 2007, 24 (7): 1953-1954 .

Abstract

PDF(pc) (204KB) ( 1159 )

A multi-grating periodically poled LiNbO₃ (PPLN) doubly resonant optical parametric oscillator (DROPO) pumped by a 2μm laser is demonstrated experimentally. Employing acousto-optically Q-switched Tm,Ho:GdVO₄ laser with pump pulse of 25ns and repetition rate of 10kHz as pump sources firstly, the noncritically quasi-phase-matched (QPM) tunable mid-IR output in 3.87--4.43μm is produced. Wavelength tuning is achieved with crystal temperature tuning from 50--180°C. When the incident average pump power is 3W at
2.048μm, the total OPO output power of 195mW at wavelength 3.88μm is obtained, corresponding to optical-to-optical conversion efficiency up to 6.5%.

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Er³⁺/Yb³⁺ Codoped Phosphate Glass Fibre with Gain per Unit Length Greater Than 3.0dB/cm

XU Shan-Hui, YANG Zhong-Min, ZHANG Qin-Yuan, LI Zun, DENG Zai-De, XU Wen-Cheng

Chin. Phys. Lett. 2007, 24 (7): 1955-1960 .

Abstract

PDF(pc) (209KB) ( 658 )

We experimentally study a novel fibre with high gain per unit length based on the homemade erbium--ytterbium codoped phosphate glass. The gain and noise characterizations with different pump powers at different wavelengths are investigated. The 1.8-cm-long fibre, dual-pumped by two single mode 980-nm fibre-pigtailed laser diodes, provides a gain per unit length greater than 3.0dB/cm and a noise figure less than 6.5dB. The gain saturation behaviour at 1535nm is obtained and the saturation output power (3dB compression) is greater than 5dBm.

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Near Infrared Broadband Emission from Bismuth--Dysprosium Codoped Chalcohalide Glasses

REN Jing, CHEN Dan-Ping, YANG Guang, XU Yin-Sheng, ZENG Hui-Dan, CHEN Guo-Rong

Chin. Phys. Lett. 2007, 24 (7): 1958-1960 .

Abstract

PDF(pc) (253KB) ( 791 )

We investigate the broadband infrared emission of bismuth doped and bismuth/dysprosium codoped chalcohalide glasses. It is found that the bismuth/dysprosium codoping can drastically enhance the fluorescence as
compared with either bismuth or dysprosium doped glasses. Meanwhile, the full width at half maximum of bismuth/dysprosium codoped glasses is over 170nm, which is the largest value among all the reported rare-earth doped chalcohalide glasses. An ideal way for energy consumption between bismuth and dysprosium ions is supposed. Such improved gain spectra of both bismuth and dysprosium ions may have potential applications in developing broadband fibre amplifiers.

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Polarization Beam Splitter Based on Self-Collimation Effect in Two-Dimensional Photonics Crystal

ZHANG Jie, ZHAO De-Yin, ZHOU Chuan-Hong, JIANG Xun-Ya

Chin. Phys. Lett. 2007, 24 (7): 1961-1963 .

Abstract

PDF(pc) (332KB) ( 661 )

A photonic crystal polarization beam splitter based on the self-collimation effect is proposed. By means of the plane wave expansion method and the finite-difference time-domain method, we analyse the splitting mechanism in two alternative ways: performing a band gap structure analysis and simulating the field distribution. The results indicate that two beams of different polarizations can be split with an extinction ratio of nearly 20dB in a wavelength range of 90nm. The splitter may have practical applications in integrated photonic circuits.

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Comparison of TiO₂ and ZrO₂ Films Deposited by Electron-Beam Evaporation and by Sol-Gel Process

YAO Jian-Ke, LI Hai-Yuan, FAN Zheng-Xiu, TANG Yong-Xing, JIN Yun-Xia, ZHAO Yuan-An, HE Hong-Bo, SHAO Jian-Da

Chin. Phys. Lett. 2007, 24 (7): 1964-1966 .

Abstract

PDF(pc) (375KB) ( 1044 )

TiO₂ and ZrO₂ films are deposited by electron-beam (EB) evaporation and by sol-gel process. The film properties are characterized by visible and Fourier-transform infrared spectrometry, x-ray diffraction analysis, surface roughness measure, absorption and laser-induced damage threshold (LIDT) test. It is found that the sol-gel films have lower refractive index, packing density and
roughness than EB deposited films due to their amorphous structure and high OH group concentration in the film. The high LIDT of sol-gel films is mainly due to their amorphous and porous structure, and low absorption. LIDT of EB deposited film is considerably affected by defects in the film, and LIDT of sol-gel deposited film is mainly effected by residual organic impurities and solvent trapped in the film.

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Polarization State Evolution in Fibre Polarization Transformer Influenced by Phase Difference

SHI Zhi-Dong, JI Min-Ning, BAO Huan-Huan

Chin. Phys. Lett. 2007, 24 (7): 1967-1970 .

Abstract

PDF(pc) (242KB) ( 622 )

In accordance with the intrinsic structure of controllably-spun birefringent-fibre-based fibre polarization transformer (FPT), the Jones vector is calculated from point to point along the polarization transforming fibre by the cascade differential phase retarder model. It is the first time using this concise method to examine the phase-difference effect on the evolution of state of polarization (SOP) inside this special fibre component. Both the extinction ratio and orientation angle of SOP are calculated to give out a whole evolution history from linear polarization light at the slow spun end into circular polarization light at the fast spun end, and vice versa. The influence of phase-difference is discussed on the polarization transforming performance and further referential conclusion is provided for design and test of the FPT component.

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Multi-Edge-Written Long-Period Fibre Gratings with Low PDL by Using High-Frequency CO₂ Laser Pulses

ZHU Tao, RAO Yun-Jiang, WANG Jiu-Ling

Chin. Phys. Lett. 2007, 24 (7): 1971-1972 .

Abstract

PDF(pc) (345KB) ( 589 )

A novel long-period fibre grating (LPFG) with low polarization-dependent loss (PDL) is fabricated by using a multi-edge exposure method with high frequency CO₂ laser pulses. The experimental results show that the PDL of a triple-edge-written LPFG with a peak amplitude of -16.5dB can be as low as 0.22dB. These novel LPFGs can find important applications in optical communication and sensing.

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Design of Infrared Inverted Telephoto-Optical System with Double-Layer Harmonic Diffractive Element

FAN Chang-Jiang, WANG Zhao-Qi, LIN Lie, ZHANG Mei, FAN Hai-Ying

Chin. Phys. Lett. 2007, 24 (7): 1973-1976 .

Abstract

PDF(pc) (267KB) ( 1078 )

A double-layer harmonic diffractive element (HDE) structure is investigated and the optimization procedure is given based on the equation of diffraction efficiency of the double-layer diffractive optical element. A new infrared double-layer inverted telephoto-optical system with an HDE is designed, which can work in the mid- and far-infrared wavebands. The diffraction efficiency of the system at each wavelength in the designed two wavebands is larger than 90%, which improves the image contrast and the imaging quality significantly.

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Geoacoustic Inversion Based on a Vector Hydrophone Array

PENG Han-Shu, LI Feng-Hua

Chin. Phys. Lett. 2007, 24 (7): 1977-1980 .

Abstract

PDF(pc) (421KB) ( 1089 )

We propose a geoacoustic inversion scheme employing a vector hydrophone array based on the fact that vector hydrophone can provide more acoustic field information than traditional pressure hydrophones. Firstly, the transmission loss of particle velocities is discussed. Secondly, the sediment sound speed is acquired by a matched-field processing (MFP) procedure, which is the optimization in combination of the pressure field and vertical particle velocity field. Finally, the bottom attenuation is estimated from the transmission loss difference between the vertical particle velocity and the pressure. The inversion method based on the vector hydrophone array mainly has two advantages: One is that the MFP method based on vector field can decrease the uncertain estimation of the sediment sound speed. The other is that the objective function based on the transmission loss difference has good sensitivity to the sediment attenuation and the inverted sediment
attenuation is independent of source level. The validity of the inverted parameters is examined by comparison of the numerical results with the experimental data.

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Approximate Analytical Solutions for a Class of Laminar Boundary-Layer Equations

Seripah Awang Kechil, Ishak Hashim, Sim Siaw Jiet

Chin. Phys. Lett. 2007, 24 (7): 1981-1984 .

Abstract

PDF(pc) (235KB) ( 611 )

A simple and efficient approximate analytical technique is presented to obtain solutions to a class of two-point boundary value similarity problems in fluid mechanics. This technique is based on the decomposition method which yields a general analytic solution in the form of a convergent infinite series with easily computable terms. Comparative study is carried out to show the accuracy and effectiveness of the technique.

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A Novel Oblique Detonation Structure and Its Stability

TENG Hong-Hui, ZHAO Wei, JIANG Zong-Lin

Chin. Phys. Lett. 2007, 24 (7): 1985-1988 .

Abstract

PDF(pc) (320KB) ( 719 )

Oblique detonation structures induced by the wedge in the supersonic combustible gas mixtures are simulated numerically. The results show that the stationary oblique detonation structures are influenced by the gas flow Mach number, and a novel critical oblique detonation structure, which is characterized by a more complicated wave system, appears in the low Mach number cases. By introducing the inflow disturbance, its nonstationary evolution process is illustrated and its stability is verified.

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MHD Boundary Layer Flow of a Non-Newtonian Fluid on a Moving Surface with a Power-Law Velocity

CHEN Xue-Hui, ZHENG Lian-Cun, ZHANG Xin-Xin

Chin. Phys. Lett. 2007, 24 (7): 1989-1991 .

Abstract

PDF(pc) (227KB) ( 810 )

A theoretical analysis for MHD boundary layer flow on a moving surface with the power-law velocity is presented. An accurate expression of the skin friction coefficient is derived. The analytical approximate solution is obtained by means of Adomian decomposition methods. The reliability and efficiency of the approximate solutions are verified by numerical ones in the literature.

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Concentric-Ring Patterns in a Helium Dielectric Barrier Discharge at Atmospheric Pressure

SHANG Wan-Li, WANG De-Zhen

Chin. Phys. Lett. 2007, 24 (7): 1992-1994 .

Abstract

PDF(pc) (870KB) ( 696 )

We perform the theoretical simulation of the concentric-ring patterns between two parallel electrodes covered with thin dielectric layers within the scope of a two-dimensional diffusion-drift model at atmospheric pressure. The time evolution of the discharge patterns is studied and the concentric-ring patterns with different radii shift alternately. The spatial-temporal evolution of electron density in a cycle at different time scales is performed.

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Measurement of Threshold Pressure Gradient of Microchannels by Static Method

SONG Fu-Quan, JIANG Ren-Jie, BIAN Shu-Li

Chin. Phys. Lett. 2007, 24 (7): 1995-1998 .

Abstract

PDF(pc) (240KB) ( 671 )

The development of oil fields and laboratory experiment present the threshold pressure gradient (TPG) of liquid flow in low permeability porous media, which is called the micro-size effect in porous media. Some micro-size effects in micro-electro-mechanism systems (MEMS) are not always in agreement with each other. We propose an experiment setup to measure the TPG of microchannels by static method in the microchannels with the diameter ranging from 20--320μm. The results present the existence of TPG in microchannel, and show an effect that the TPG of microchannel increases with decreasing hydrodynamic diameter. The relation between TPG and diameter is in agreement with single-log normalization. Additionally, the influence of errors in the experiment shows the data of experiment are valid. Finally, the
mechanism of micro-size effects is discussed by revealing the facial force between liquid and solid and theory of boundary liquid, but the explanation is still not good, and needs further study.

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Ab Initio Calculations of Differential Cross Sections for Single Charge Transfer in ³He²⁺+⁴ He Collisions

WU Yong, YAN Bing, LIU Ling, WANG Jian-Guo,

Chin. Phys. Lett. 2007, 24 (7): 1999-2002 .

Abstract

PDF(pc) (278KB) ( 603 )

The single charge transfer process in ³He²⁺+⁴He collisions is investigated using the quantum-mechanical molecular-orbital close-coupling method, in which the adiabatic potentials and radial couplings are calculated by using the ab initio multireference single- and double-excitation configuration interaction methods. The differential cross sections for the single charge transfer are presented at the laboratorial energies E = 6keV and 10keV for the projectile ³He²⁺. Comparison with the existing data shows that the present results are better in agreement with the experimental measurements than other calculations in the dominant small angle scattering, which is attributed to the accurate calculations of the adiabatic potentials and the radial couplings.

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Oscillations of Magnetized Dust Grains in Plasma Sheath with Negative Ions

GAN Bao-Xia, CHEN Yin-Hua

Chin. Phys. Lett. 2007, 24 (7): 2003-2005 .

Abstract

PDF(pc) (229KB) ( 548 )

The oscillations of a single magnetized dust grain in electronegative plasma sheath are investigated taking into account the existence of an external magnetic field. The influence of the content of negative ions and the magnetic field intensity on the properties of the dust vibration is analysed. The result shows that the existence of the negative ions in plasma reduces the dust oscillation frequency and drops the equilibrium position of dust, whereas the magnetic field raises the equilibrium position and also reduces the dust oscillation frequency on the condition considered.

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Modelling Energetic Electrons by a Kappa-Loss-Cone Distribution at Geostationary Orbit

XIAO Fu-Liang, ZHOU Qing-Hua, HE Hui-Yong

Chin. Phys. Lett. 2007, 24 (7): 2006-2009 .

Abstract

PDF(pc) (252KB) ( 556 )

We adopt a recently developed relativistic kappa-loss-cone (KLC) distribution to model energetic electrons energy spectra observed at the geostationary orbit in the storm of 3--4 November 1993. The KLC distribution is found to fit well with the observed data from four satellites during different universal times. This suggests that the electron flux obeys the power-law not only at the lower energies but also at the relativistic energies, and the KLC distribution may provide a better understanding of environments in those space plasmas where relativistic electrons are present.

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Stochastic Heating of Ions by Linear Polarized Alfvaen Waves

LV Xiang, LI Yi, WANG Shui

Chin. Phys. Lett. 2007, 24 (7): 2010-2013 .

Abstract

PDF(pc) (304KB) ( 582 )

The ion motion in the presence of linear polarized Alfven waves is studied. For a linearly polarized wave, nonlinear resonances can occur when the amplitude of Alfven wave is large enough. Under certain conditions, these resonances can overlap and thus make the ion motion chaotic. In this process, the plasma can be heated without the limitation of cyclotron resonant condition. Taking into account of a spectrum of waves, the stochastic condition can decrease largely. In addition, the preferential heating can be found in the perpendicular
direction.

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Magnetic Fluctuation Measurement in Sino United Spherical Tokamak Plasma

LIU Fei, WANG Wen-Hao, HE Ye-Xi, LIU Jun, TAN Yi, XIE Li-Feng, ZENG Long

Chin. Phys. Lett. 2007, 24 (7): 2014-2017 .

Abstract

PDF(pc) (662KB) ( 733 )

To investigate the magnetic fluctuations and for further transport study, the poloidal and radial magnetic field measurement is conducted on the Sino United Spherical Tokamak (SUNIST). Auto-power spectral density indicates that the magnetic fluctuation energy mainly concentrates in the frequency region lower than 10kHz. The magnetic field oscillations, which are characterized by harmonic frequencies of 40kHz, are observed in the scrape-off layer; by contrast, in the plasma core, the magnetic fluctuations are of Gaussian type. The time-frequency profiles show that the poloidal magnetic fluctuations are temporally intermittent. The autocorrelation calculation indicates that the fluctuations in decorrelation time vary between the core and the edge.

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Elimination of Crystallographic Wing Tilt of Canti-Bridged Epitaxial Laterally Overgrown GaN Films by Optimizing Growth Procedure

YAN Jian-Feng, XING Zhi-Gang, WANG Jing, GUO Li-Wei, ZHU Xue-Liang, PENG Ming-Zeng, YU Nai-Sen, JIA Hai-Qiang, CHEN Hong, ZHOU Jun-Ming

Chin. Phys. Lett. 2007, 24 (7): 2018-2021 .

Abstract

PDF(pc) (676KB) ( 810 )

Canti-bridged epitaxial lateral overgrowth (CBELO) of GaN is performed by metalorganic chemical vapour deposition (MOCVD) on maskless V-grooved sapphire substrates prepared by wet chemical etching with different mesa widths. The wing tilt usually observed in ELO is not found in the CBELO GaN with wide mesa widths, while it can be detected obviously in the GaN with narrow mesa widths. The wing tilt of CBELO GaN grown on a grooved sapphire substrate with narrow mesa can be controlled by adjusting the
thickness of the nucleation layer. The dependence of the wing tilt on the nucleation layer thickness is studied. Cross-sectional scanning electron microscopy is used to characterize the geometry of the wing regions, and double crystal x-ray diffraction is used to analyse the structural characteristics and to measure the magnitude of the crystalline wing tilt. It is found that the crystalline wing tilt can be eliminated completely by first growth of a thin nucleation GaN layer then the CBELO GaN. Possible reason and the origin of the wing tilt in CBELO GaN films are also discussed.

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X-Ray Photoelectron Spectroscopy and Reflection High Energy Electron Diffraction of Epitaxial Growth SiC on Si(100) Using C₆₀ and Si

LIU Yan-Fang, LIU Jin-Feng, XU Peng-Shou, PAN Hai-Bin

Chin. Phys. Lett. 2007, 24 (7): 2022-2024 .

Abstract

PDF(pc) (571KB) ( 711 )

The formation of silicon carbide upon deposition of C₆₀ and Si on Si(100) surface at 850°C is studied via x-ray photoelectron spectroscopy and reflection high energy electron diffraction (RHEED). The C 1s, O 1s and Si 2p core-level spectra and the RHEED patterns indicate the formation of 3C--SiC

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Glass Transition Temperature of Water: from Simulations of Diffusion and Excess Entropy

LIU Jia, WANG Shu-Ying, ZHENG Cai-Ping, XIN Li-Juan, WANG Dan, SUN Min-Hua

Chin. Phys. Lett. 2007, 24 (7): 2025-2027 .

Abstract

PDF(pc) (212KB) ( 776 )

We report a computer simulation study of the glass transition of water with SP2 potential. The temperature dependences of the diffusion coefficient and the excess entropy on cooling process are calculated. It is found that both the diffusion coefficient and the excess entropy show a break point at 160K. Our results support the viewpoint that the glass transition temperature is 160K. According to the calculated viscosity, we obtain a fragility index of water to be 326, which is much larger than the value accepted before.

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Liquid--Liquid Structure Transition in Metallic Melts: Experimental Evidence by Viscosity Measurement

WANG Yu-Qing, WU Yu-Qin, BIAN Xiu-Fang

Chin. Phys. Lett. 2007, 24 (7): 2028-2031 .

Abstract

PDF(pc) (255KB) ( 715 )

Temperature dependence of viscosity for more than ten kinds of metallic melts is analysed based on viscosity measurements. An obvious turning point is observed on the Arrhenius curves. Since viscosity is one of the physical properties sensitive to structure, its discontinuous change with temperature reveals the possible liquid--liquid structure transition in the metallic melts. Furthermore, an integrated liquid structure transition diagram of the Sn--Bi system is presented. The universality of liquid--liquid structure transition is also discussed simply.

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Discrete Element Method Numerical Modelling on Crystallization of Smooth Hard Spheres under Mechanical Vibration

AN Xi-Zhong

Chin. Phys. Lett. 2007, 24 (7): 2032-2035 .

Abstract

PDF(pc) (609KB) ( 844 )

The crystallization, corresponding to the fcc structure (with packing density ρ≈0.74), of smooth equal hard spheres under batch-wised feeding and three-dimensional interval vibration is numerically obtained by using the discrete element method. The numerical experiment shows that the ordered packing can be realized by proper control of the dynamic parameters such as batch of each feeding ξ and vibration amplitude A. The radial distribution function and force network are used to characterize the ordered structure. The defect
formed during vibrated packing is characterized as well. The results in our work fill the gap of getting packing density between random close packing and fcc packing in phase diagram which provides an effective way of theoretically investigating the complex process and mechanism of hard sphere crystallization and its dynamics.

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Mechanical Properties of Single-Walled (5,5) Carbon Nanotubes with Vacancy Defects

YUAN Shi-Jun, KONG Yong, LI Fa-Shen

Chin. Phys. Lett. 2007, 24 (7): 2036-2039 .

Abstract

PDF(pc) (353KB) ( 1220 )

First-principles simulation is used to investigate the structural and mechanical properties of vacancy defective single-walled (5,5) carbon nanotubes. The relations of the defect concentration, distribution and characteristic of defects to Young's modulus of nanotubes are quantitatively studied. It is found that each dangling-bond structure (per supercell) decreases Young's modulus of nanotube by 6.1% for symmetrical distribution cases. However the concentrative vacancy structure with saturated atoms has less influence on carbon nanotubes. It is suggested that the mechanical properties of carbon nanotubes depend strongly upon the structure and relative position of vacancies in a certain defect concentration.

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Critical Exponents for the Re-entrant Phase Transitions in the Three-Dimensional Blume--Emery--Griffiths Model on the Cellular Automaton

N. Seferoglu, B. Kutlu

Chin. Phys. Lett. 2007, 24 (7): 2040-2043 .

Abstract

PDF(pc) (273KB) ( 564 )

The critical behaviour of the three-dimensional Blume--Emery--Griffiths (BEG)
model is investigated at D/J=0, -0.25 and -1 in the range of -1≤K/J≤0 for J=100. The simulations are carried out on a simple cubic lattice using the heating algorithm improved from the Creutz cellular automaton (CCA) under periodic boundary conditions. The universality of the model are obtained for re-entrant and double re-entrant phase transitions which occur at certain D/J and K/J parameters, with J and K representing the nearest-neighbour bilinear and biquadratic interactions, and D being the single-ion anisotropy parameter. The values of static critical exponents β, γ and υ are estimated within the framework of the finite-size scaling theory. The results are compatible with the universal Ising critical behaviour for all continuous phase transitions in these ranges.

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Miscibility and Molecular Orientation of Carbazole in Mixed Langmuir and Langmuir--Blodgett Films

Md. N. Islam, D. Bhattacharjee, S. A. Hussain

Chin. Phys. Lett. 2007, 24 (7): 2044-2047 .

Abstract

PDF(pc) (171KB) ( 676 )

We report the miscibility and molecular orientation of carbazole (CA) molecules in the mixed Langmuir and Langmuir--Blodgett (LB) films of CA
in polymethyl methacrylate (PMMA) and stearic acid (SA) matrices. The π-A isotherm confirms the formation of stable Langmuir films of CA mixed with either PMMA or SA at air--water interface. Characteristics of area per molecule versus molefraction and collapse pressure versus molefraction reveal complete demixing of CA and the matrix PMMA/SA molecules in the mixed films. Absorption spectroscopy certainly confirms the fact that CA molecules have preferred orientation on the substrate of the LB films.

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Electronic Structures of Wurtzite GaN with Ga and N Vacancies

PANG Chao, SHI Jun-Jie, ZHANG Yan, K. S. A. Butcher, T. L. Tansley, J. E.Downes, SHANG Jia-Xiang

Chin. Phys. Lett. 2007, 24 (7): 2048-2051 .

Abstract

PDF(pc) (329KB) ( 829 )

The electronic band structures of wurtzite GaN with Ga and N vacancy defects are investigated by means of the first-principles total energy calculations in the neutral charge state. Our results show that the band structures can be significantly modified by the Ga and N vacancies in the GaN samples. Generally, the width of the valence band is reduced and the band gap is enlarged. The defect-induced bands can be introduced in the band gap of GaN due to the Ga and N vacancies. Moreover, the GaN with high density of N
vacancies becomes an indirect gap semiconductor. Three defect bands due to Ga vacancy defects are created within the band gap and near the top of the valence band. In contrast, the N vacancies introduce four defect bands within the band gap. One is in the vicinity of the top of the valence band, and the others are near the bottom of the conduction band. The physical origin of the defect bands and modification of the band structures due to the Ga and N vacancies are analysed in depth.

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Reduced Magneto-Resistivity of a Rare-Earth Crystalline and the Degeneracy Removals of Its Crystal-Field Levels

LIU Zhao-Sen

Chin. Phys. Lett. 2007, 24 (7): 2052-2055 .

Abstract

PDF(pc) (267KB) ( 581 )

A theoretical approach is generalized and employed to calculate the magneto-resistivity of a rare-earth crystalline (CeAl₂) with degenerate ground crystal-field (CF) level in the presence of external fields. The calculated results show that when a magnetic field is applied in the c-direction, the magneto-resistivity may be reduced by more than 90% in certain cases in comparison with the pure CF contribution at the same temperature, demonstrating the strong effects of the degeneracy removals of the CF levels on the magnetic resistivity.

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Spin-Polarized Transport through the T-Shaped Double Quantum Dots with Fano--Kondo Interaction

YANG Fu-Bin, WU Shao-Quan, SUN Wei-Li

Chin. Phys. Lett. 2007, 24 (7): 2056-2059 .

Abstract

PDF(pc) (265KB) ( 677 )

72.25.-b. 73.63.Kv, 72.15.Qm

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Photovoltaic and Electroluminescence Bifunctional Devices with Starburst Amine and Rare-Earth-Complexes

WEN Fu-Shan, , LI Wen-Lian, WEI Han-Zhi, LIU Yun-Qi, KIM Jin-Hyeok

Chin. Phys. Lett. 2007, 24 (7): 2060-2063 .

Abstract

PDF(pc) (248KB) ( 618 )

We fabricate the organic photovoltaic (PV) devices, in which 4,4',4''-tris-(2-methylphenylphenylamino)triphenylamine (m-MTDATA) and rare earth (RE) (dibenzoylmethanato)₃(bathohenanthroline) (RE(DBM)₃bath) (RE = Nd or Pr) are used as electron donor and acceptor, and investigate their PV properties. The PV diode fabricated in the optimum processing conditions shows the open-circuit voltage of 1.91V, short-circuit current of 0.1mA/cm², fill factor of
0.38, and the overall power conversion efficiency of 1.9% when it is irradiated under UV light (4mW/cm²). The photocurrent density exhibits an increase of 20% at least when a very thin LiF layer is inserted between the RE-complexes and the Al cathode. A strong electroluminescence from the interface is also observed and the maximum luminance of a yellow emission resulted from the exciplex is 580cd/m² at 17V bias.

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Correlation between Light Emissions from Amorphous-Si:H/SiO₂ and nc-Si/SiO₂ Multilayers

MA Zhong-Yuan, HAN Pei-Gao, LI Wei, CHEN De-Yuan, WEI De-Yuan, QIAN Bo, LI Wei, XU Jun, XU Ling, HUANG Xin-Fan, CHEN Kun-Ji, FENG Duan

Chin. Phys. Lett. 2007, 24 (7): 2064-2067 .

Abstract

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We investigate the properties of light emission from amorphous-Si:H/SiO₂ and nc-Si/SiO₂ multilayers (MLs). The size dependence of light emission is well exhibited when the a-Si:H sublayer thickness is thinner than 4nm and the interface states are well passivated by hydrogen. For the nc-Si/SiO₂ MLs, the oxygen modified interface states and nanocrystalline silicon play a predominant role in the properties of light emission. It is found that the light emission from nc-Si/SiO₂ is in agreement with the model of interface state combining with quantum confinement when the size of nc-Si is smaller than 4nm. The role of hydrogen and oxygen is discussed in detail.

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High Field Electrical Conduction in Pre-Formed Al--ZnS--Al Thin Films in Metal--Insulator--Metal Devices

M. Y. Nadee, Nadeem Iqbal, M. F. Wasiq, A. U. Khosa

Chin. Phys. Lett. 2007, 24 (7): 2068-2069 .

Abstract

PDF(pc) (189KB) ( 804 )

The high field electrical conduction mechanism for the widely used ZnS thin films in the microelectronic industry is investigated. Experimental data on the dc conduction as a function of the applied bias for the Al--ZnS--Al devices is carefully compared with the theoretical equations given by Schottky and Poole--Frenkel. The results yield the value of the coefficient of the barrier lowering ?compatible with the Schottky theory rather than the Poole--Frenkel theory, which are also in agreement with the results reported earlier by Maekawa [Phys. Rev. Lett. 24 (1970) 1175]

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Effect of Ultraviolet Light on Hybrid Zinc Oxide Polymer Bulk Heterojunction Solar Cells

LIU Jun-Peng, QU Sheng-Chun, CHEN Yong-Hai, XU Ying, ZENG Xiang-Bo, LIANG Ling-Yan, WANG Zhi-Jie, ZHOU Hui-Ying, WANG Zhan-Guo

Chin. Phys. Lett. 2007, 24 (7): 2070-2073 .

Abstract

PDF(pc) (281KB) ( 831 )

Compared to conjugated polymer poly[2-methoxy-5-(3',7'-dimethyloctyloxy)%
-1,4-phenylenevinylene] (MDMO-PPV) solar cells, bulk heterojunction solar cells composed of zinc oxide (ZnO) nanocrystals and MDMO-PPV have a better energy conversion efficiency. However, ultraviolet (UV) light deteriorates the
performance of solar cells composed of ZnO and MDMO-PPV. We propose a
model to explain the effect of UV illumination on these ZnO:MDMO-PPV solar cells. According to this model, the degradation from UV illumination is due to a decrease of exciton dissociation efficiency. Our model is based on the experimental results such as the measurements of current density versus voltage, photoluminescence, and photocurrent.

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Deposition of MgB₂ Superconducting Films on Different Metal Substrates

CHEN Li-Ping, LI Fen, GUO Tao, ZHUANG ZHeng-Gang, YAO Dan, DING Li-L, ZHANG Kai-Cheng, GAN Zi-Zhao, XIONG Guang-Cheng, FENG Qing-Rong

Chin. Phys. Lett. 2007, 24 (7): 2074-2076 .

Abstract

PDF(pc) (506KB) ( 730 )

By a method of hybrid physical-chemical vapour deposition (HPCVD) on three metal substrates of stainless steel, copper and niobium, we deposit MgB₂ superconducting films over 1μm thickness. All of them have zero resistance temperatures T_C(0)>36K and critical current densities J_C (10K, 0T)
>10⁶A/cm². Meanwhile, in the bending test, all the MgB₂ superconducting films adhere strongly to the metal substrates without peeling off. Therefore, the MgB₂superconducting films supplied by the HPCVD method exhibit preferable electrical, magnetic and mechanical properties, and have potential
applications in future.

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Zn Doping Effect on Magnetic Properties of Zn_xCd_1-xCr₂S₄ Systems by High-Temperature Series Expansions

M. Hamedoun, R. Masrour, K. Bouslykhane, A. Hourmatallah, N. Benzakour, A. Filali

Chin. Phys. Lett. 2007, 24 (7): 2077-2080 .

Abstract

PDF(pc) (229KB) ( 719 )

The effect of Zn doping on the magnetic properties of CdCr₂S₄ systems is studied by mean field theory and high-temperature series expansion (HTSE). The nearest neighbouring and the next-neighbouring super-exchange interactions J₁(x) and J₂(x) are evaluated for the spinel system Zn_xCd_1-xCr₂S₄ in the range 0≤x≤1. The intra-planar and the inter-planar interactions are deduced. The HTSE combined with the Pade approximant method (PA) is applied to the spinel system Zn_xCd_1-xCr₂S₄. The magnetic phase diagram, i.e. T_C versus dilution x, is obtained. The critical exponents associated with the magnetic susceptibility γ and the correlation length ν are deduced. The obtained theoretical results are in agreement with the experimental data obtained by magnetic measurements.

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Characterization and Magnetic Properties of Iron-Based Alloy Antidot Arrays

LIU Qing-Fang, JIANG Chang-Jun, WANG Jian-Bo, FAN Xiao-Long, XUE De-Sheng

Chin. Phys. Lett. 2007, 24 (7): 2081-2084 .

Abstract

PDF(pc) (753KB) ( 707 )

Fe₂₉Co₇₁ and Fe₁₉Ni₈₁ antidot arrays, with different dimensions, are prepared with the rf magnetron sputtering method onto the porous alumina substrate. The size and shape of antidot arrays are characterized by scanning electron microscopy. The glancing angle x-ray diffraction patterns of Fe₂₉Co₇₁ and Fe₁₉Ni₈₁ antidot arrays indicate the bcc and fcc structures, respectively. The coercivities of both the alloys show abnormal thickness dependence, which are discussed qualitatively by considering the pinning and the thickness effect to the films.

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Room-Temperature Ferromagnetic ZnMnO Thin Films Synthesized by Plasma Enhanced Chemical Vapour Deposition Method

LIN Ying-Bin, LU Zhi-Hai, ZOU Wen-Qin, LU Zhong-Lin, XU Jian-Ping, JI Jian-Ti, LIU Xing-Chong, WANG Jian-Feng, LV Li-Ya, ZHANG Feng-Ming, DU You-Wei, HUANG Zhi-Gao, ZHENG Jian-Guo

Chin. Phys. Lett. 2007, 24 (7): 2085-2087 .

Abstract

PDF(pc) (274KB) ( 761 )

Room-temperature ferromagnetic Mn-doped ZnO films are grown on Si (001)
substrates by plasma enhanced chemical vapour deposition (PECVD). X-ray
diffraction measurements reveal that the Zn_1-xMn_xO films have the single-phase wurtzite structure. X-ray photoelectron spectroscopy indicates the existence of Mn²⁺ ions in Mn-doped ZnO films. Furthermore, the decreasing additional Raman peak with increasing Mn-doping is considered to relate to the substitution of Mn ions for the Zn ions in ZnO lattice. Superconducting quantum interference device (SQUID) measurements demonstrate that Mn-doped ZnO films have ferromagnetic behaviour at room temperature.

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Nitrogen-Doped Chemical Vapour Deposited Diamond: a New Material for Room-Temperature Solid State Maser

N. A. Poklonski, N. M. Lapchuk, A. V. Khomich, LU Fan-Xiu, TANG Wei-Zhong, V. G. Ralchenko, I. I. Vlasov, M. V. Chukichev, Sambuu Munkhtsetseg,

Chin. Phys. Lett. 2007, 24 (7): 2088-2090 .

Abstract

PDF(pc) (114KB) ( 676 )

Electron spin resonance (ESR) in polycrystalline diamond films grown by dc arc-jet and microwave plasma chemical vapour deposition is studied. The films with nitrogen impurity concentration up to 8×10¹⁸cm^-3 are also characterized by Raman, cathodoluminescence and optical absorption spectra. The ESR signal from P1 centre with g-factor of 2.0024 (nitrogen impurity atom ccupying C site in diamond lattice) is found to exhibit an inversion with increasing the microwave power in an H₁₀₂ resonator. The spin inversion effect could be of interest for further consideration of N-doped diamonds as a
medium for masers operated at room temperature.

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Negative Goos--Hanchen Effect in Thin-Film Fabry--Perot Filter

LI Ming-Yu, LIU Xu, MA Xin, LI Yi-Yu, GU Pei-Fu

Chin. Phys. Lett. 2007, 24 (7): 2091-2093 .

Abstract

PDF(pc) (388KB) ( 611 )

We investigate the Goos--Hanchen effect of a Gaussian light beam reflected by the thin-film Fabry--Perot filter. It is shown that the Goos--Hanchen shift can be either negative or positive. The Gaussian-beam analysis and stationary phase method are introduced to calculate the lateral shift between the incident beam and the reflected beam at different wavelengths and to analyse the Goos-Hanchen effect in the thin-film Fabry-Perot filter. The effect of the incident beam diameter is also discussed.

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Luminescence and Energy Transfer of Eu²⁺, Mn²⁺ in SrZnP₂O₇

YANG Zhi-Ping, YANG Guang-Wei, WANG Shao-Li, TIAN Jing, GUO Qing-Lin, FU Guang-Sheng

Chin. Phys. Lett. 2007, 24 (7): 2094-2096 .

Abstract

PDF(pc) (237KB) ( 759 )

The SrZnP₂O₇:Eu²⁺, Mn²⁺ phosphor is synthesized by high temperature solid state reaction. The luminescence properties and the energy transfer between Eu²⁺ and Mn²⁺ are investigated. The emission bands of this phosphor peaked at 420nm and 670nm are originated from the 5d→4f transition of Eu²⁺ and from the ⁴T₁ (⁴G) →⁶A₁ (⁶) transition of Mn²⁺, respectively. With the increasing Mn²⁺concentration, the intensity of fixed concentration Eu²⁺ decreases and the intensity of Mn²⁺ also increases. It is suggested that
there is an energy transfer from Eu²⁺ to Mn²⁺ in SrZnP₂O₇ host. According to Dexter's energy transfer formula of multipolar interaction, the energy transfer between Eu²⁺ and Mn²⁺ is due to the electric dipole-quadrupole interaction of the resonance transfer.

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Efficient Top-Emitting Polymer Light-Emitting Diodes Using Chromium as Anode

HUANG Yong, XU Yun-Hua, XU Wei, ZHOU Jian-Gang, PENG Jun-Biao, CAO Yong

Chin. Phys. Lett. 2007, 24 (7): 2097-2100 .

Abstract

PDF(pc) (335KB) ( 642 )

We demonstrate a high efficiency top-emitting polymer light-emitting diode (TPLED) with chromium (Cr) taking as the anode. The TPLED structure is Cr/poly-3, 4-ethylenedioxythiophene (PEDOT:PSS)/poly [2-(4-3',7'-dimethyloctyloxy)-phenyl]-p-phenylenevinylene) (P-PPV)/Ba/Ag. The Cr (100nm) anode is prepared by sputter-depositing in a vacuum chamber. It is found that the device emissive properties are affected dramatically by the thickness of both PEDOT:PSS and the Ag cathode. Optimized thicknesses of PEDOT:PSS and Ag layer are 60nm and 15nm, respectively. The diode exhibits excellent electroluminescence (EL) properties, such as a turn-on voltage of 3.32V, luminous efficiency of 4.41cd/A and luminance of 6989cd/m² at driving
voltage of about 9V.

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Polarization Properties of Quantum-Dot-Based Single Photon Sources

HAN Shuo, HAO Zhi-Biao, LUO Yi

Chin. Phys. Lett. 2007, 24 (7): 2101-2104 .

Abstract

PDF(pc) (246KB) ( 625 )

Polarization properties of single photons emitted by optical pumping from a single quantum dot (QD) are studied by using a four-level system model. The model is capable of explaining the polarization uncertainty observed in single photon emission experiments. It is found that the dependence of photon emission efficiency and polarization visibility on pump power are opposite in general cases. By employing QDs with small size and strong carrier confinement, the photon polarization visibility under high pump power can be improved. In addition, embedding a QD into a well designed microcavity is also found to be favourable, whereas the trade-off between high polarization visibility and multi-photon emission is noted.

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Numerical Analysis of Nano-Aperture Light Source for High-Density Optical Data Storage

XU Ji-Ying, WANG Jia, TIAN Qian, WANG Bo-Xiong

Chin. Phys. Lett. 2007, 24 (7): 2105-2107 .

Abstract

PDF(pc) (380KB) ( 592 )

Two unconventional nano-aperture light sources, an L-shaped nano-aperture source and a 3D nano-aperture source for high-density optical data storage, are numerically investigated. With incidence of a Gaussian beam, the spot size of the Poynting vector coupled into the recording medium is 130×175nm² for the L-aperture and 120×135nm² for the 3D nano-aperture. The quantitative analyses indicate that the unconventional nano-aperture sources can provide enough power density to record marks in the commercial recording medium. It is feasible to use a laser diode with a nano-aperture as an active nanometer light source for high-density optical data storage.

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Investigation of Oxygen Vacancy and Interstitial Oxygen Defects in ZnO Films by Photoluminescence and X-Ray Photoelectron Spectroscopy

FAN Hai-Bo, YANG Shao-Yan, ZHANG Pan-Feng, WEI Hong-Yuan, LIU Xiang-Lin, JIAO Chun-Mei, ZHU Qin-Sheng, CHEN Yong-Hai, WANG Zhan-Guo

Chin. Phys. Lett. 2007, 24 (7): 2108-2111 .

Abstract

PDF(pc) (341KB) ( 6643 )

ZnO films prepared at different temperatures and annealed at 900°C in
oxygen are studied by photoluminescence (PL) and x-ray photoelectron
spectroscopy (XPS). It is observed that in the PL of the as-grown films the green luminescence (GL) and the yellow luminescence (YL) are related, and after annealing the GL is restrained and the YL is enhanced. The O 1s XPS results also show the coexistence of oxygen vacancy (V_O) and interstitial oxygen (O_i) before annealing and the quenching of the V_O after annealing. By combining the two results it is deduced that the GL and YL are related to the V_O and O_i defects, respectively.

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Theoretical Analysis of Current Crowding Effect in Metal/AlGaN/GaN Schottky Diodes and Its Reduction by Using Polysilicon in Anode

CHEN Jia-Rong, CHEN Wen-Jin, WANG Yu-Qi, QIU Kai, LI Xin-Hua, ZHONG Fei, YIN Zhi-Jun, JI Chang-Jian, CAO Xian-Cun, HAN Qi-Feng, DUAN Cheng-Hong, ZHOU Xiu-Ju

Chin. Phys. Lett. 2007, 24 (7): 2112-2114 .

Abstract

PDF(pc) (238KB) ( 1316 )

There exists a current crowding effect in the anode of AlGaN/GaN heterojunction Schottky diodes, causing local overheating when working at
high power density, and undermining their performance. The seriousness
of this effect is illustrated by theoretical analysis. A method of reducing this effect is proposed by depositing a polysilicon layer on the Schottky barrier metal. The effectiveness of this method is provided through computer simulation. Power consumption of the polysilicon layer is also calculated and compared to that of the Schottky junction to ensure the applicability of this method.

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Dependence of Limited Growth Rate of High-Quality Gem Diamond on Growth Conditions

TIAN Yu, MA Hong-An, LI Shang-Sheng, XIAO Hong-Yu, ZHANG Ya-Fei, HUANG Guo-Feng, MA Li-Qiu, JIA Xiao-Peng,

Chin. Phys. Lett. 2007, 24 (7): 2115-2117 .

Abstract

PDF(pc) (576KB) ( 726 )

The growth rate of diamond has been investigated for a long time and researchers have been attempting to enhance the growth rate of high-quality gem diamond infinitely. However, it has been found according to previous research results that the quality of diamond is debased with the increase of growth rate. Thus, under specific conditions, the growth rate of high-quality diamond cannot exceed a limited value that is called the limited growth rate of diamond. We synthesize a series of type Ib gem diamonds by temperature gradient method under high pressure and high temperature (HPHT) using the
as-grown {100} face. The dependence of limited growth rate on growth conditions is studied. The results show that the limited growth rate increases when synthetic temperature decreases, also when growth time is prolonged.

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Cr-Doped InAs Self-Organized Diluted Magnetic Quantum Dots ith Room-Temperature Ferromagnetism

ZHENG Yu-Hong, ZHAO Jian-Hua, BI Jing-Feng, WANG Wei-Zhu, JI Yang, WU Xiao-Guang, XIA Jian-Bai

Chin. Phys. Lett. 2007, 24 (7): 2118-2121 .

Abstract

PDF(pc) (996KB) ( 633 )

Cr-doped InAs self-organized diluted magnetic quantum dots (QDs) are grown by low-temperature molecular-beam epitaxy. Magnetic measurements reveal that the Curie temperature of all the InAs:Cr QDs layers with Cr/In flux ratio changing from 0.026 to 0.18 is beyond 400K. High-resolution cross sectional transmission electron microscopy images indicate that InAs:Cr QDs are of the zincblende structure. Possible origins responsible for the high Curie temperature are discussed.

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A New Empirical Model for Estimation of sp³Fraction in Diamond-Like Carbon Films

DAI Hai-Yang, WANG Li-Wu, JIANG Hui, HUANG Ning-Kang

Chin. Phys. Lett. 2007, 24 (7): 2122-2124 .

Abstract

PDF(pc) (309KB) ( 946 )

A new empirical model to estimate the content of sp³in diamond-like carbon (DLC) films is presented, based on the conventional Raman spectra excited by 488nm or 514nm visible light for different carbons. It is found that bandwidth of the G peak is related to the sp³ fraction. A wider bandwidth of the G peak shows a higher sp³ fraction in DLC films.

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A Base-Emitter Self-Aligned Multi-Finger Si_1-xGe_x/Si Power Heterojunction Bipolar Transistor

XUE Chun-Lai, YAO Fei, SHI Wen-Hua, CHENG Bu-Wen, WANG Hong-Jie, YU Jin-Zhong, WANG Qi-Ming

Chin. Phys. Lett. 2007, 24 (7): 2125-2127 .

Abstract

PDF(pc) (352KB) ( 599 )

With a crystal orientation dependent on the etch rate of Si in KOH-based solution, a base-emitter self-aligned large-area multi-finger configuration power SiGe heterojunction bipolar transistor (HBT) device (with an emitter area of about 880μm²) is fabricated with 2μm double-mesa technology. The maximum dc current gain is 226.1. The collector--emitter junction breakdown voltage BV_CEO is 10V and the collector-base junction breakdown voltage BV_CBO is 16V with collector doping concentration of 1×10¹⁷cm^-3
and thickness of 400nm. The device exhibited a maximum oscillation frequency f_max of 35.5 GHz and a cut-off frequency f_T of 24.9GHz at a dc bias point of I_C=70mA and the voltage between collector and emitter is V_CE=3 V. Load
pull measurements in class-A operation of the SiGe HBT are performed at
1.9GHz with input power ranging from 0dBm to 21dBm. A maximum output
power of 29.9dBm (about 977mW) is obtained at an input power of 18.5dBm
with a gain of 11.47dB. Compared to a non-self-aligned SiGe HBT with the same heterostructure and process, f_max and f_T are improved by about 83.9% and 38.3%, respectively.

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High-Power Electroabsorption Modulator Using Intrastep Quantum Well

CHENG Yuan-Bing, PAN Jiao-Qing, ZHOU Fan, ZHU Hong-Liang, ZHAO Ling-Juan, WANG Wei

Chin. Phys. Lett. 2007, 24 (7): 2128-2130 .

Abstract

PDF(pc) (235KB) ( 1228 )

An electroabsorption modulator using the intrastep quantum well (IQW)
active region is fabricated for optical network systems. The strain-compensated InGaAsP/InGaAsP IQW shows good material quality and improved modulation properties, high extinction ratio efficiency 10dB/V and low capacitance (<0.42pF), with which an ultra high frequency (>15GHz) can be obtained. High-speed measurement under high-power excitation shows no power saturation up to excitation power of 21dBm. To our knowledge, the input optical power is the highest reported for multi-quantum well EAMs without heat sinks.

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Investigation of Chlorophyll Protein 43 and 47 Denaturation by Terahertz Time-Domain Spectroscopy

CHEN Hua, QU Yuan-Gang, PENG Wei-Xian, KUANG Ting-Yun, LI Liang-Bi, WANG Li

Chin. Phys. Lett. 2007, 24 (7): 2131-2134 .

Abstract

PDF(pc) (250KB) ( 749 )

We demonstrate the applications of terahertz time-domain spectroscopy to distinguish conformation changes of the chlorophyll proteins CP43 and CP47 induced by the treatment of guanidine hydrochloride, light irradiation and heating. It is indicated that THz transmission spectroscopy can be used for monitoring protein denaturation and associated conformation change processes in a feasible and effective way.

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Population Growth of Small Harmful Rats in Grassland Subjected to Noise

LIU Xue-Mei, LI Zhi-Bing, XIE Hui-Zhang, AI Bao-Quan, CHENGXiao-Bo, LIU Liang-Gang

Chin. Phys. Lett. 2007, 24 (7): 2135-2137 .

Abstract

PDF(pc) (223KB) ( 678 )

The population growth of small harmful rats in grassland subjected to environment fluctuation has been modelled in a logistic equation. Two correlated random variables responsible to the fluctuation of the genetic factor and the suppression factor are used. A two-peak structure of the steady probability distribution of rate population is observed in the large fluctuation regime of the genetic factor. With the increase of correlation constant λ, the steady probability distribution can change from two peaks to a single peak. The suppression factor μ and its fluctuation also affect the steady
probability distribution and can push it toward a small population.

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A Robustness Model of Complex Networks with Tunable Attack Information Parameter

WU Jun, TAN Yue-Jin, DENG Hong-Zhong, LI Yong

Chin. Phys. Lett. 2007, 24 (7): 2138-2141 .

Abstract

PDF(pc) (229KB) ( 821 )

We introduce a novel model for robustness of complex with a tunable attack information parameter. The random failure and intentional attack known are the two extreme cases of our model. Based on the model, we study the robustness of complex networks under random information and preferential information, respectively. Using the generating function method, we derive the exact value of the critical removal fraction of nodes for the disintegration of networks and the size of the giant component. We show that hiding just a small fraction of nodes randomly can prevent a scale-free network from collapsing and detecting just a small fraction of nodes preferentially can destroy a scale-free network.

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Detrended Fluctuation Analysis of Traffic Data

ZHU Xiao-Yan, LIU Zong-Hua, TANG Ming

Chin. Phys. Lett. 2007, 24 (7): 2142-2145 .

Abstract

PDF(pc) (258KB) ( 920 )

Different routing strategies may result in different behaviour of traffic on internet. We analyse the correlation of traffic data for three typical routing strategies by the detrended fluctuation analysis (DFA) and find that the degree of correlation of the data can be divided into three regions, i.e. weak, medium, and strong correlation. The DFA scalings are constants in both the regions of weak and strong correlations but monotonically increase in the region of medium correlation. We suggest that it is better to consider the traffic on complex network as three phases, i.e. the free, buffer, and congestion phase, than just as two phases believed before, i.e. the free and
congestion phase.

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Response to Disturbance and Abundance of Final State: a Measure for Complexity?

SHEN Dan, WANG Wen-Xiu, JIANG Yu-Mei, HE Yue, HE Da-Ren

Chin. Phys. Lett. 2007, 24 (7): 2146-2148 .

Abstract

PDF(pc) (201KB) ( 654 )

We propose a new definition of complexity. The definition shows that when a system evolves to a final state via a transient state, its complexity depends on the abundance of both the final state and transient state. The abundance of the transient state may be described by the diversity of the response to disturbance. We hope that this definition can describe a clear boundary between simple systems and complex systems by showing that all the simple systems have zero complexity, and all the complex systems have positive complexity. Some examples of the complexity calculations are presented, which supports our hope.

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Multifractal Analysis of Human Heartbeat in Sleep

DING Liang-Jing, PENG Hu, CAI Shi-Min, ZHOU Pei-Ling

Chin. Phys. Lett. 2007, 24 (7): 2149-2152 .

Abstract

PDF(pc) (250KB) ( 598 )

We study the dynamical properties of heart rate variability (HRV) in sleep by analysing the scaling behaviour with the multifractal detrended fluctuation analysis method. It is well known that heart rate is regulated by the interaction of two branches of the autonomic nervous system: the parasympathetic and sympathetic nervous systems. By investigating the multifractal properties of light, deep, rapid-eye-movement (REM) sleep and wake stages, we firstly find an increasing multifractal behaviour during REM sleep which may be caused by augmented sympathetic activities relative to non-REM sleep. In addition, the investigation of long-range correlations of HRV in sleep with second order detrended fluctuation analysis presents irregular phenomena. These findings may be helpful to understand the underlying regulating mechanism of heart rate by autonomic nervous system during wake--sleep transitions.

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Statefinder Diagnostic for Phantom Model with V(<SPAN lang=EN-US style=

CHANG Bao-Rong, LIU Hong-Ya, XU Li-Xin, ZHANG Cheng-Wu

Chin. Phys. Lett. 2007, 24 (7): 2153-2156 .

Abstract

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We investigate the phantom field with potential V(Ф)=V₀(-λФ²) and dark matter in the spatially flat Friedman--Robertson--Walker model. It has been shown by numerical calculation that there is a attractor solution in this model. We also apply the statefinder diagnostic to this phantom model. It is shown that the evolving trajectories of this scenario in the s-r diagram is quite
different from other dark energy models.

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