Chin. Phys. Lett.

Volume 25 Issue 4

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Maximum Path Information and Fokker--Planck Equation

LI Wei, Q. A. Wang, A. Le Mehaute

Chin. Phys. Lett. 2008, 25 (4): 1165-1167 .

Abstract

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We present a rigorous method to derive the nonlinear Fokker--Planck (FP) equation of anomalous diffusion directly from a generalization of the principle of least action of Maupertuis proposed by Wang [Chaos, Solitons & Fractals 23(2005)1253] for smooth or quasi-smooth irregular dynamics evolving in Markovian process. The FP equation obtained may take two different but equivalent forms. It was also found that the diffusion constant may depend on both q (the index of Tsallis entropy [J. Stat. Phys. 52(1988)479] and the time t.

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Two Kinds of Square-Conservative Integrators for Nonlinear Evolution Equations

CHEN Jing-Bo, LIU Hong

Chin. Phys. Lett. 2008, 25 (4): 1168-1171 .

Abstract

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Based on the Lie-group and Gauss--Legendre methods, two kinds of square-conservative integrators for square-conservative nonlinear evolution equations are presented. Lie-group based square-conservative integrators are linearly implicit, while Gauss--Legendre based square-conservative integrators are nonlinearly implicit and iterative schemes are needed to solve the corresponding integrators. These two kinds of integrators provide natural candidates for simulating square-conservative nonlinear evolution equations in the sense that these integrators not only preserve the square-conservative properties of the continuous equations but also are nonlinearly stable. Numerical experiments are performed to test the presented integrators.

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Solutions of Three-Dimensional Separable Non-Central Potential

Ozlem Yesiltas

Chin. Phys. Lett. 2008, 25 (4): 1172-1174 .

Abstract

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We show that a wide class of non-central potentials can be analysed via the improved picture of the Nikiforov--Uvarov method [Physica Scripta 75(2007)686]. It has been shown that using the alternative approach, polynomial
solutions of three-dimensional separable non-central potential can be obtained.

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A Measure of Non-Classicality of Even and Odd Coherent States

YE Yong-Hua, LI Zhi-Jian, ZENG Gao-Jian

Chin. Phys. Lett. 2008, 25 (4): 1175-1178 .

Abstract

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A measure of non-classicality of even and odd coherent states is studied. We first calculate the Wigner functions of the even and odd coherent states, which consists of two terms: the positive-definite Gaussian term and the wave term with negativity, and then calculate the integrated value ε_max of the wave term of the Wigner functions of the even and odd coherent states in their area with negativity, and use ε_max to measure non-classicality of the even and odd coherent states. For the even and odd coherent states with certain photon count, it is very convenient for us to use ε_max to measure their non-classicality. The methods of our definition and calculation for ε_max have theoretical reference value.

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Quantum State Transfer between Charge and Flux Qubits in Circuit-QED

WU Qin-Qin, LIAO Jie-Qiao, KUANG Le-Man

Chin. Phys. Lett. 2008, 25 (4): 1179-1182 .

Abstract

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We propose a scheme to implement quantum state transfer in a hybrid circuit quantum electrodynamics (QED) system which consists of a superconducting charge qubit, a flux qubit, and a transmission line resonator (TLR). It is shown
that quantum state transfer between the charge qubit and the flux qubit can be realized by using the TLR as the data bus.

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Intrinsic Decoherence of a Two-Atom System with Dipole--Dipole

QI Lin-Na, ZHU Ai-Dong, ZHANG Shou

Chin. Phys. Lett. 2008, 25 (4): 1183-1186 .

Abstract

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We investigate the effect of dipole--dipole interaction on the intrinsic decoherence of a system which consists of two two-level atoms and an optical cavity. The entanglement of the system is calculated by making use of
concurrence. Our results show that the appropriate choice for the coupling constant Ω of dipole--dipole interaction can restrain the intrinsic decoherence of the system. We also find a special phenomenon. No matter what the value of γ is, the concurrence of system slowly increases and cannot exceed 0.71 when Ω=1.

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Quantum Secret Sharing Protocol between Multiparty and Multiparty with Single Photons and Unitary Transformations

YAN Feng-Li, GAO Ting, LI You-Cheng

Chin. Phys. Lett. 2008, 25 (4): 1187-1190 .

Abstract

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We propose a scheme of quantum secret sharing between Alice's group and Bob's group with single photons and unitary transformations. In the protocol, one member in Alice's group prepares a sequence of single photons in one of four different states, while other members directly encode their information on the sequence of single photons via unitary operations; after that, the last member sends the sequence of single photons to Bob's group. Then Bob's, except for the last one, do work similarly. Finally the last member in Bob's group measures the qubits. If the security of the quantum channel is guaranteed by some tests, then the qubit states sent by the last member of Alice's group can be used as key bits for secret sharing. It is shown that this scheme is safe.

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Optical Realization of Deterministic Entanglement Concentration of Polarized Photons

GU Yong-Jian, XIAN Liang, LI Wen-Dong, MA Li-Zhen

Chin. Phys. Lett. 2008, 25 (4): 1191-1194 .

Abstract

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We propose a scheme for optical realization of deterministic entanglement concentration of polarized photons. To overcome the difficulty due to the lack of sufficiently strong interactions between photons, teleportation is employed to transfer the polarization states of two photons onto the path and
polarization states of a third photon, which is made possible by the recent experimental realization of the deterministic and complete Bell state measurement. Then the required positive operator-valued measurement and further operations can be implemented deterministically by using a linear optical setup. All these are within the reach of current technology.

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Preparation of W State with Superconducting Quantum-Interference Devices in a Cavity via Adiabatic Passage

ZHENG An-Shou, SHEN Xiao-Fang, LIU Ji-Bing, BI Jie, DU Qiu-Jiao,

Chin. Phys. Lett. 2008, 25 (4): 1195-1197 .

Abstract

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We propose an alternative scheme to prepare W state by using superconducting quantum-interference devices (SQIDs) coupled to a largely-detuned cavity. The present scheme is based on evolution by adiabatic passage, where only by tuning adiabatically the Rabi frequencies of the classical microwave pulses we can obtain the standard W state without measurement or any auxiliary SQUIDs. Thus the procedure is simplified and the scheme can be achieved with very high success probability since the errors in dynamical or geometric ways can be avoided. In addition, the SQUID system and the cavity have no probability of being excited state. Thus decoherence caused by the excited-level spontaneous emission or the cavity decay is suppressed.

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Scalable Generation of Cluster State for Multiple Hot Trapped Ions

CHENG Guang-Ling, HU Xiang-Ming

Chin. Phys. Lett. 2008, 25 (4): 1198-1201 .

Abstract

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We propose an alternative scheme for preparing N-qubit cluster state by using a frequency-modulated laser field to simultaneously illuminate the trapped ions. Selecting the index of modulation yields the selective mechanisms of coupling and decoupling between the internal and external
states of the ions. Based on the selective mechanisms, the highly entangled cluster state is achieved. In our scheme, the vibration mode is only virtually excited. Thus the quantum operations are insensitive to the heating and lead to the high-fidelity quantum information processing.

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Moller's Energy of Kerr-NUT Metric

Gamal G. L. Nashed

Chin. Phys. Lett. 2008, 25 (4): 1202-1204 .

Abstract

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The energy distribution of the Kerr-NUT (Newman--Unti--Tamburino) spacetime is calculated using Moller's energy--momentum complex within the framework of the Riemannian geometry.

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Number-Phase Quantization and Deriving Energy-Level Gap of Two LC Circuits with Mutual-Inductance

MENG Xiang-Guo, WANG Ji-Suo, ZHAI Yun, FAN Hong-Yi,

Chin. Phys. Lett. 2008, 25 (4): 1205-1208 .

Abstract

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For two LC circuits with mutual-inductance, we introduce a new quantization scheme in the context of number-phase quantization through the standard Lagrangian formalism. The commutative relation between the charge operator and the magnetic flux operator is derived. Then we use the Heisenberg
equation of motion to obtain the current and voltage equation across the inductance and capacity. The results clearly show how the current and voltage in a single LC circuit are affected by the circuit parameters and inductance coupling coefficient. In addition, adopting invariant eigen-operator method the energy-level gap of the dynamic Hamiltonian which describes two LC circuits with mutual-inductance is obtained.

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Numerical Investigation of Noise Enhanced Stability Phenomenon in a Time-Delayed Metastable System

JIA Zheng-Lin

Chin. Phys. Lett. 2008, 25 (4): 1209-1212 .

Abstract

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The transient properties of a time-delayed metastable system subjected to the additive white noise are investigated by means of the stochastic simulation method. The noise enhanced stability phenomenon (NES) can be
observed in this system and the effect of the delay time on the NES shows a critical behaviour, i.e., there is a critical value of the delay time τ_c≈1, above which the time delay enhances the NES effect with the delay time increasing and below which the time delay weakens the NES effect as the delay time
increases.

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Noise-Induced Phase Transition: Zero-Dimensional Brownian Particles Varying between Ergodicity and Nonergodicity

BAI Zhan-Wu

Chin. Phys. Lett. 2008, 25 (4): 1213-1216 .

Abstract

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We study in phase space a zero-dimensional system of Brownian particles which move in a periodic potential and subject to an internal time derivative
Ornstein--Uhlenbeck noise. To resolve the Fokker--Planck equation in such a case, we propose an approximate analytical method. The theoretical predictions exhibit a second order noise-induced nonequilibrium phase transition, which is confirmed by numerical simulation results. The phase transition brings the system from an ergodicity to a nonergodicity phase as the potential barrier height decreases.

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On Solving the Lorenz System by Differential Transformation Method

M. Mossa Al-Sawalha, M. S. M. Noorani

Chin. Phys. Lett. 2008, 25 (4): 1217-1219 .

Abstract

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The differential transformation method (DTM) is employed to solve a nonlinear differential equation, namely the Lorenz system. Numerical results are compared to those obtained by the Runge--Kutta method to illustrate the preciseness and effectiveness of the proposed method. In particular, we examine the accuracy of the (DTM) as the Lorenz system changes from a
non-chaotic system to a chaotic one. It is shown that the (DTM) is robust, accurate and easy to apply.

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Stochastic Simulation of Turing Patterns

FU Zheng-Ping, XU Xin-Hang, WANG Hong-Li, , OUYANG Qi,

Chin. Phys. Lett. 2008, 25 (4): 1220-1223 .

Abstract

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We investigate the effects of intrinsic noise on Turing pattern formation near the onset of bifurcation from the homogeneous state to Turing pattern in the reaction--diffusion Brusselator. By performing stochastic simulations of the master equation and using Gillespie's algorithm, we check the spatiotemporal
behaviour influenced by internal noises. We demonstrate that the patterns of occurrence frequency for the reaction and diffusion processes are also spatially ordered and temporally stable. Turing patterns are found to be robust against intrinsic fluctuations. Stochastic simulations also reveal that under the influence of intrinsic noises, the onset of Turing instability is advanced in comparison to that predicted deterministically.

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Effects of Polarization-Maintaining Fibre Degrading on Precision of Fibre Optic Gyroscopes in Radiation Environment

XIAO Wen, LIU De-Wen, LIU Yang, YI Xiao-Su, CONG Lin

Chin. Phys. Lett. 2008, 25 (4): 1224-1226 .

Abstract

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In the space environment, the precision of fibre optic gyroscopes (FOGs) degrades because of space radiation. Photonic components of FOGs are affected by radiation, especially the polarization-maintaining (PM) fibre coil. In relation to the space radiation environment characteristic, we have carried
out a series of radiation experiments on a PM fibre coil with ⁶⁰Co radiation source at different dose rates. Based on the experimental results, the formula between the PM-fibre loss and radiation dose rate is built, and the relation between the precision of FOG and radiation dose is obtained accordingly. The results strongly show that the precision of our FOG degrades owing to the
attenuation of the polarization-maintaining fibre, which provides theoretical foundation for the radiation-resistant design of the FOG.

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Gauge Covariant Fermion Propagator in the Presence of Arbitrary External Gauge Field and Its Schwinger--Dyson Equation

ZHANG Ying, WANG Qing

Chin. Phys. Lett. 2008, 25 (4): 1227-1230 .

Abstract

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Gauge covariance for Green's functions of a gauge theory through a fermion propagator in the presence of arbitrary external gauge field is proven and a formalism of gauge and Lorentz covariant Schwinger--Dyson equation for the fermion propagator with external gauge field is built up within ladder approximation.

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Monte Carlo Simulation on Measurements of Branching Fractions for Some Exclusive Semileptonic Decays of Neutral D Mesons on BES-III

LIU Hui-Hui, CHEN Jiang-Chuan, RONG Gang, MA Hai-Long, ZHAO Ming-Gang, LIU Jian

Chin. Phys. Lett. 2008, 25 (4): 1231-1234 .

Abstract

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Using the Monte Carlo samples of about 800 pb^-1 and 500pb^-1 generated at and around 3.773GeV with BOSS (BESIII Offline Software System) 6.0.2 and BOSS 6.1.0, we have studied the branching fractions for D⁰ →K^-l⁺ν_l and D⁰ →πi^-l⁺ν_l (l=e, μ). According to the simulation results, the expected precision of these measurements for 20fb^-1 ψ(3770) data which will be collected on BESIII are well estimated.

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Complete Gluonic Phase in Two-Flavour Colour Superconductivity

WANG Xiao-Ming, ZHOU Bang-Rong

Chin. Phys. Lett. 2008, 25 (4): 1235-1238 .

Abstract

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We study a typical complete gluonic phase (LGP) in two-flavour colour superconductivity (2SC) by calculating the essential cubic and quartic interfering term between the gluonic condensates < A⁸_z> and < A⁶_z> with a gauged NJL model. It is proven that the coefficients of the cubic interfering term and the vacuum contributions of the cubic and quartic interfering term are all equal to zeroes. The coefficients of the quartic interfering term and the
< A⁶_z>'s quartic self-interaction term at stationary points of Larkin--Ovchinnikov--Fulde--Ferrell (LOFF) phase are calculated. Comparisons among the effective potentials of LGP, g2SC and LOFF phase indicate that LGP could be the genuine ground state of 2SC for some reasonable parameters.

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Decay Constants of Vector Mesons

LI Heng-Mei, WAN Shao-Long

Chin. Phys. Lett. 2008, 25 (4): 1239-1242 .

Abstract

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The light vector mesons are studied within the framework of the Bethe--Salpeter equation with the vector--vector-type flat-bottom potential. The Bethe--Salpeter wavefunctions and the decay constants of the vector mesons are obtained. All the obtained results, f_ρ, f_Φ, and f_K*, are in agreement with the experimental values, respectively.

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Neutron and Proton Diffusion in Fusion Reactions for the Synthesis of Superheavy Nuclei

HUANG Ming-Hui, GAN Zai-Guo, FENG Zhao-Qing, ZHOU Xiao-Hong, LI Jun-Qing,

Chin. Phys. Lett. 2008, 25 (4): 1243-1246 .

Abstract

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The restriction of the one dimensional (1D) master equation (ME) with the mass number of the projectile-like fragment as a variable is studied, and a two-dimensional (2D) master equation with the neutron and proton numbers as independent variables is set up, and solved numerically. Our study showed that the 2D ME can describe the fusion process well in all projectile--target
combinations. Therefore the possible channels to synthesize super-heavy nuclei can be studied correctly in wider possibilities. The available condition for employing 1D ME is pointed out.

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Coulomb Potentials between Spherical and Deformed Nuclei

ZHANG Gao-Long, LE Xiao-Yun, LIU Zu-Hua

Chin. Phys. Lett. 2008, 25 (4): 1247-1250 .

Abstract

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Coulomb potentials for spherical-deformed reaction partners are calculated in terms of the double folding model as well as the conventional formulas. Our results show that the Coulomb potentials calculated with different approaches have quite different behaviours in the internal region of the potential. Because fusion process is sensitive to the barrier height and the internal part of the potential, the fusion excitation function, especially the fusion barrier distribution, should provide a strict test of the interaction
potentials. Therefore, we calculate the fusion excitation function and barrier distribution for the ¹⁶O+¹⁵⁴Sm system with different versions of the Coulomb potentials, in comparison with the experimental results. It is found that the fusion excitation function and barrier distribution of ¹⁶O+¹⁵⁴Sm are obviously different for the different versions of the Coulomb potentials. By means of this
comparison, we may conclude that the double folding model with the accurate approximate form can provide rather reasonable Coulomb potentials.

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Cross-Correlation of Excitation Functions for Different Fragments and Different Scattering Angles in ²⁷Al(¹⁹F,x)y Reactions

HAN Jian-Long, WANG Qi, BAI Zhen, DONG Yu-Chuan, LI Song-Lin, DUAN Li-Min, XU Hu-Shan, XU Hua-Gen, CHEN Ruo-Fu, WU He-Yu, LI Zhi-Chang, LU Xiu-Qin, ZHAO Kui, ZHOU Ping, LIU Jian-Cheng, XU Guo-Ji, Sergey Yu Kun,

Chin. Phys. Lett. 2008, 25 (4): 1251-1254 .

Abstract

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Excitation functions have been measured for different projectile-like fragments produced in ²⁷Al¹⁹F,x)y reactions at incident energies from 110.25 to 118.75MeV in 250keV steps. Strong cross section fluctuations of the excitation functions are observed. The cross-correlation coefficients of the excitation functions for different atomic number Z and for different scattering angle θ_cm
have been deduced. These coefficients are much larger than the statistical theoretical calculated ones. This indicates that there are strong correlations between different exit channels in the dissipative heavy ion collision of ²⁷Al(¹⁹F,x)y.

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Stability of Complex-Rotation Method on a Simple Resonant Scattering Problem

SHEN Li, WANG Lei, LIU Xiao-Jun, SHI Ting-Yun, LIU Hong-Ping

Chin. Phys. Lett. 2008, 25 (4): 1255-1258 .

Abstract

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The stability of the complex-rotation method in B-spline basis for a simple atomic resonant scattering problem in free field is investigated. The numerical calculation shows that this method has a feature that the solution will not change in a wide range of rotation angle θ. Our determined scattering resonant energies and widths exactly coincide with the popularly accepted values. A new resonance is identified numerically although it is very broad. The norm of the complex eigenvalue, |E|, is proposed to investigate and to evaluate the stability of the obtained complex eigenvalues.

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Cyclic State Orientation of Polar Molecules Produced by a Train of Half-Cycle Pulse Clusters of a Long Repetition Period

HUANG Yu-Xin, YANG Yu-Jun, WU Bin, GUO Fu-Ming, ZHU Qi-Ren

Chin. Phys. Lett. 2008, 25 (4): 1259-1262 .

Abstract

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Using a variational method, we derive the optimal population distribution of angular momentum eigenstates for any given population range in a rotational wavepacket within the field-free cyclic state orientation framework. Correspondingly, we devise a train of half-cycle pulse clusters to purposively make the structure of the computed wavepacket approach the optimal population distribution, so that we can now utilize much more powerful means to realize an ideal orientation goal.

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Quantum Entanglement and Normal-to-Local Transition in Molecule H₂

LIU Da-Ming, PENG Dong-Ping, HOU Xi-Wen

Chin. Phys. Lett. 2008, 25 (4): 1263-1265 .

Abstract

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Quantum entanglement of two stretches in molecule H₂S is investigated for various initial states in terms of the reduced-density von Neumann entropy and the mean entropy defined by average over time. It is shown that the maximal and the mean entropies of an initial state with local-mode character
are smaller than those with normal-mode character, and the mean entropy displays a maximum near the position of the normal-to-local transition.

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Research on ZrO₂ Thermal Barrier Coatings Modified by High-Intensity Pulsed Ion Beam

WU Di, LIU Chen, ZHU Xiao-Peng, LEI Ming-Kai

Chin. Phys. Lett. 2008, 25 (4): 1266-1269 .

Abstract

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We report a modification method for ZrO₂ thermal barrier coatings (TBCs) by high-intensity pulsed ion beam (HIPIB) irradiation. Based on the temporal and spatial distribution models of the ion beam density detected by Faraday cup
in the chamber and the ions accelerating voltage, the energy deposition of the beam ions in ZrO₂ is calculated by Monte Carlo method. Taking this time-dependent nonlinear deposited energy as the source term of two-dimensional thermal conduction equation, we obtain the temporal and spatial ablation process of ZrO₂ thermal barrier coatings during a pulse time. The top-layer TBC material in thickness of about 0.2μm is ablated by vaporization and the
coating in thickness of 1μm is melted after one shot at the ion current density of 200A/cm₂. This calculation is in reasonable agreement with those measured by HIPIB irradiation experiments. The melted top coat becoming a dense modification layer due to HIPIB irradiation seals the gaps among ZrO₂ crystal clusters, and hence barrels the direct tunnel of oxygen.

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Stability and Growth Modes of Ni--C Clusters: A Study based on All-Electron Density Function Theory

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

Chin. Phys. Lett. 2008, 25 (4): 1270-1273 .

Abstract

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Growth modes of the free-standing NiC_N (N\le 8) and Ni₂C_N (N\le 8) clusters are investigated by the all-electron density functional theory. The results reveal that there are two competing modes for the growth of these clusters: the linear chain and the ring structure without transannular bonds. The lowest-energy geometries of NiC_N (N\le 8) are the linear chains with the Ni atom at one end, except for NiC₂ and NiC₇. The Ni₂C_N (N\le 8) clusters all
prefer to the linear chains with the two Ni atoms at the two ends. Mulliken population analysis indicates that the total spin of the lowest-energy cluster show significant odd--even alternation. The Ni_MC_N (M=1,2) clusters with the even N are one and those with the odd-N are zero.

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Compensation for Radio-over-Fibre Uplink Based on Hybrid Neural Networks

WANG Jian-Ping, ZHOU Xian-Wei, SONG Ya-Li, GUO Wen-Zhe

Chin. Phys. Lett. 2008, 25 (4): 1274-1276 .

Abstract

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The radio-over-fibre (ROF) uplink, which combines the merit of optical fibre with that of microwave technology, can supply the high capacity of communication. However, there are two major issues: nonlinear distortion of the optical link and the multipath dispersion of the wireless channel, affecting the performance of the system. We propose an equalizer based on hybrid
neural networks. The compensation needs no estimation of the channel. The simulated result shows that the ROF uplink can be adequately compensated and the performance of the equalizer depends on the channel noise.

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Improving Visibility of Diffraction Pattern with Pseudo-Thermal Light

GAO Lu, XIONG Jun, ZHANG Shu-Heng, WANG Wei, WANG Kai-Ge

Chin. Phys. Lett. 2008, 25 (4): 1277-1280 .

Abstract

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We report an experimental observation of Poisson's spot with pseudo-thermal light. The experimental results show that the diffraction pattern disappears in the intensity distribution behind the opaque disc but emerges through both auto-correlation and cross-correlation intensity measurements. The auto-correlation scheme can take care of both better visibility and higher resolution of the diffraction pattern under the condition that the thermal light source has a larger spectral bandwidth.

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Continuous-Wave Operation of GaN Based Multi-Quantum-Well Laser Diode at Room Temperature

ZHANG Li-Qun, ZHANG Shu-Ming, YANG Hui, CAO Qing, JI Lian, ZHU Jian-Jun, LIU Zong-Shun, ZHAO De-Gang, JIANG De-Sheng, DUAN Li-Hong, WANG Hai, SHI Yong-Sheng, LIU Su-Ying, CHEN Liang-Hui, LIANG Jun-Wu

Chin. Phys. Lett. 2008, 25 (4): 1281-1283 .

Abstract

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Room-temperature operation of cw GaN based multi-quantum-well laser diodes (LDs) is demonstrated. The LD structure is grown on a sapphire (0001) substrate by metalorganic chemical vapour deposition. A 2.5μm×800μm ridge
waveguide structure is fabricated. The electrical and optical characteristics of the laser diode under direct current injection at room temperature are investigated. The threshold current and voltage of the LD under cw operation are 110mA and 10.5V, respectively. Thermal induced series resistance decrease and emission wavelength red-shift are observed as the injection current is increased. The full width at half maximum for the parallel and perpendicular far field pattern (FFP) are 12°and 32°, respectively.

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A Novel Kind of Transverse Micro-Stack High-Power Diode Bars

ZHANG Lei, CUI Bi-Feng, LI Jian-Jun, GUO Wei-Lling, WANG Zhi-Qun, SHEN Guang-Di

Chin. Phys. Lett. 2008, 25 (4): 1284-1286 .

Abstract

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Novel transverse micro-stack semiconductor laser bars are put forward to improve the output optical power of semiconductor laser bars at low injection current. More importantly, the novel laser bars have a coupled large optical
cavity, which can overcome the problem of catastrophic optical damage and improve light beam quality due to the coherently coupled emitting along the transverse direction. The micro-stack tunnel regeneration tri-active region laser structure was grown by metal organic chemical vapour deposition. For a weakly coupled uncoated device, the optical power exceeds 60W under 50A driving current and the slope efficiency reaches 1.55W/A. Further experiments show that the perpendicular divergence of 23° is achieved from transverse
strongly coupled devices.

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Propagation of Single-Mode Fibre Laser Beams through an Optical ABCD System with Circular Aperture at the Fibre Output End

DUAN Kai-Liang, LI Jian-Feng, ZHAO Wei, WANG Yi-Shan

Chin. Phys. Lett. 2008, 25 (4): 1287-1290 .

Abstract

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Based on the expansion expression of the fundamental mode of a single-mode fibre in terms of Laguerre--Gauss modes, the propagation of a beam of a weakly guiding fibre laser through an optical ABCD system with a circular aperture at the fibre end is studied. The results show that there is much
difference between the propagation of the laser beam described by the
expansion expression and by the Gaussian mode approximation. The depth of focus of the laser beam is longer than that of the Gaussian modes.

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The LD-Cladding-Pumped High Power Tm³⁺-doped Silica Fibre Amplifying at Approximately 1.99μm

ZHOU Hui, JING Tao, ZHANG Yun-Jun

Chin. Phys. Lett. 2008, 25 (4): 1291-1292 .

Abstract

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A laser diode (LD) cladding-pumped cw Tm³⁺-doped silica fibre amplifier is reported that provides up to 3W output with the slope efficiency nearly 30%. The gain fibre is 27.5/400μm D-shaped Tm³⁺-doped silica double-cladding
fibre, and the input signal sources is an LD-pumped Tm³⁺:YAP lasing at 1.99μm. The measured amplified spectrum only stretches a little relative to the input signal spectrum. This is the first report to the authors' knowledge of general experimental investigation of cladding-pumped Tm³⁺-doped fibre amplifier of high power cw at 1.99μm.

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Cr,Er:Y_2.93Sc_1.43Ga_3.64O₁₂ Laser Giant Pulse Generation at 2.79μm Using Electro-Optic Q-Switch

LIU Jin-Sheng, LIU Jing-Jiao,

Chin. Phys. Lett. 2008, 25 (4): 1293-1296 .

Abstract

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We present an electro-optically Q-switched 2.79μm Cr,Er:Y_2.93Sc_1.43Ga_3.64O₁₂ laser using LiNbO₃Pockels cell and undoped Y₃Al₁₅O₁₂ plate polarizer. The optimal delay for triggering the Q-switch with respect to the peak of waveform of the flashlamp is found to be about 120μs. The minimum pump threshold of 13.2J is realized using the output coupler of reflectivity 74%. The maximum outputs of 72.3mJ at 3Hz and 35.4mJ at 10Hz repetition rates are obtained. The minimum pulse width is measured to be 31.9ns. The far-field divergence angle of the laser beam is about 8.8mrad. The influence of repetition rates on the output energy is also discussed.

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Optimized Second Harmonic Generation of Femtosecond Pulse by Phase-Blanking Effect in Aperiodically Optical Superlattice

KONG Yan, CHEN Xian-Feng, XIA Yu-Xing

Chin. Phys. Lett. 2008, 25 (4): 1297-1300 .

Abstract

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In order to minimize the effect of the unconsidered frequency components on the generated compression pulse, the phasing-blanking effect is taken into account of designing the one-dimensional aperiodic domain reversal structure. Hierarchic genetic algorithm for the design of a domain reversal grating to
modulate the spectrum and phase of the generated SH pulse simultaneously are presented. Our simulation shows that the quality of an output pulse is fairly improved.

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Properties of a New Nonlinear Optical Crystal CdZn₂B₂O₆

ZHANG Fan, SHEN De-Zhong, SHEN Guang-Qiu, WANG Xiao-Qing

Chin. Phys. Lett. 2008, 25 (4): 1301-1304 .

Abstract

PDF(pc) (808KB) ( 656 )

Cadmium dizinc diborate (CdZn₂B₂O₆) single crystals have been grown for the first time. The crystal structure of CdZn₂B₂O₆ is the same as that of the Cd₃Zn₃B₄O₁₂. The x-ray diffraction, infrared and Raman spectra, differential scanning calorimetry analysis and density indicate that the physical and chemical properties of both crystals are very similar. Especially, the nonlinear optical coefficients of CdZn₂B₂O₆ and Cd₃Zn₃B₄O₁₂ crystals are 2.6 and 2.4 times as large as that of KH₂PO₄ crystal respectively. Chemical etching
experiments indicated that these crystals are very stable in neutral solution and not hygroscopic in air at room temperature.

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Laser-Duration Dependence of Emission Properties of High-Order Harmonic Generation

GE Yu-Cheng

Chin. Phys. Lett. 2008, 25 (4): 1305-1308 .

Abstract

PDF(pc) (200KB) ( 579 )

Quantitative investigations are made for the laser-duration dependence of the emission properties of high-order harmonic generation (HHG). HHG emission properties produced by few-cycle lasers show some useful characteristics. The cutoff energy is less than that by laser for infinite duration. The single energy distribution pulse decreases much faster than its
duration as the laser duration grows. A two-cycle laser with carrier-envelope phase of 0° can produce a single distribution pulse peaked at the laser carrier phase 1.22rad and spanned 1.18rad with the cutoff energy 2.9U_p+I_p and a bandwidth 0.63U_p, where U_p is the ponderomotive potential of the laser field and I_p is the atomic ionization potential.

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Influence of Misalignment on High-Order Aberration Correction for Normal Human Eyes

ZHAO Hao-Xin, XU Bing, XUE Li-Xia, DAI Yun, LIU Qian, RAO Xue-Jun

Chin. Phys. Lett. 2008, 25 (4): 1309-1312 .

Abstract

PDF(pc) (221KB) ( 573 )

Although a compensation device can correct aberrations of human eyes, the effect will be degraded by its misalignment, especially for high-order aberration correction. We calculate the positioning tolerance of correction device for high-order aberrations, and within what degree the correcting effect is better than low-order aberration (defocus and astigmatism)
correction. With fixed certain misalignment within the positioning tolerance, we calculate the residual wavefront rms aberration of the first-6 to first-35 terms along with the 3rd--5th terms of aberrations corrected, and the combined first-13 terms of aberrations are also studied under the same quantity of misalignment. However, the correction effect of high-order aberrations does not meliorate along with the increase of the high-order terms under some misalignment, moreover, some simple combined terms correction can achieve similar result as complex combinations. These results suggest that it is unnecessary to correct too much the terms of high-order
aberrations which are difficult to accomplish in practice, and gives confidence to correct high-order aberrations out of the laboratory.

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Independent Modulation of Omnidirectional Defect Modes in Single-Negative Materials Photonic Crystal with Multiple Defects

WANG Qiong, YAN Chang-Chun, ZHANG Ling-Ling, CUI Yi-Ping

Chin. Phys. Lett. 2008, 25 (4): 1313-1316 .

Abstract

PDF(pc) (486KB) ( 584 )

Single-negative materials based on photonic crystal with multiple defect layers are designed and the free modulation of defect modes is studied. The results show that the multi-defect structure can avoid the interference between the defect states. Therefore, the designed double defect modes in the zero effective-phase gap can be adjusted independently by changing
the thickness of different defect layers. In addition, the two tunable defect modes have the omnidirectional characteristics. This multi-defect structure with above-mentioned two advantages has potential applications in modern optical devices such as tunable omnidirectional filters.

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A Stable Porous Silicon Dielectric Reflector with a Photonic Band Gap Centred at 10μm

ZHANG Jie, XU Shao-Hui, YANG Shi-Qian, WANG Lian-Wei, CAO Zhi-Shen, ZHAN Peng, WANG Zhen-Lin

Chin. Phys. Lett. 2008, 25 (4): 1317-1320 .

Abstract

PDF(pc) (133KB) ( 439 )

By pulsed anodic etching at low temperature, we prepared a porous silicon reflector with a photonic band gap centred in the long-wavelength infrared spectral region (centred at about 12μm). After proper oxidation process, the
stable reflector structure, which can reflect electromagnetic wave from 8μm to 12μm (centred at 10μm) within wide incidence angles (about 50°), is obtained. The wavelength shift of absorption peak of Si--H and Si--O shows the influence of oxidation process and indicates the stability of oxidized porous silicon dielectric reflector, which offers possible applications for the room temperature infrared sensor.

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Influence of Different Substrates on Laser Induced Damage Thresholds at 1064nm of Ta₂O₅ Films

XU Cheng, MA Jian-Yong, JIN Yun-Xia, HE Hong-Bo, SHAO Jian-Da, FAN Zheng-Xiu

Chin. Phys. Lett. 2008, 25 (4): 1321-1324 .

Abstract

PDF(pc) (2379KB) ( 754 )

Ta₂O₅ films are prepared on Si, BK7, fused silica, antireflection (AR) and high reflector (HR) substrates by electron beam evaporation method, respectively. Both the optical property and laser induced damage thresholds (LIDTs) at 1064nm of Ta₂O₅films on different substrates are investigated before and after annealing at 673K for 12h. It is shown that annealing increases the
refractive index and decreases the extinction index, and improves the O/Ta ratio of the Ta₂O₅ films from 2.42 to 2.50. Moreover, the results show that the LIDTs of the Ta₂O₅films are mainly correlated with three parameters: substrate property, substoichiometry defect in the films and impurity defect at the interface between the substrate and the films. Details of the laser induced damage models in different cases are discusse .

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Generalized Principal-State-of-Polarization Analysis and Matrix Model for Piezoelectric Polarization Controllers

LI Zheng-Yong, WU Chong-Qing, YANG Shuang-Shou, TIAN Chang-Yong, ZHAOShuang, WANG Yong-Jun

Chin. Phys. Lett. 2008, 25 (4): 1325-1328 .

Abstract

PDF(pc) (213KB) ( 876 )

We introduce a generalized concept of principal state of polarization (PSP) to analyse the piezoelectric polarization controller (PPC) and find each PPC unit can be described by a rotation matrix determined by the PSP. Our PPC has
three components, each made of a jaw and a piezoelectric actuator with the squeezing direction tilted 0°, 45° and 0°, which are driven by a tunable power supply. We demonstrate that all the polarization rotation angles are linear to driving voltages and the PSP of unit 2 is nearly orthogonal to others which are almost equal. Taking some approximate treatments we obtain the matrix model of our PPC with respect to three driving voltages. The average error of our theoretical model is 1.51°, and the polarization response time is 50μs, which is promising to realize an open-loop control of polarization.

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A Distributed Fibre Bragg Grating Sensor Interrogator Employing a Reformative Arrayed Waveguide Grating

HUANG Jing-Tang, HUANG Xu-Guang, ZHAO Hua-Wei

Chin. Phys. Lett. 2008, 25 (4): 1329-1332 .

Abstract

PDF(pc) (217KB) ( 694 )

We present an interrogating technique employing a reformative arrayed waveguide grating without output waveguides in combination with a linear charge coupled device. A simple and effective data processing method called area equalized point is used to improve the system resolution. The simulation results show that the wavelength shift of a fibre Bragg grating with temperature can be precisely measured by this interrogation technique. The temperature accuracy and wavelength resolution of the sensor system are smaller than 0.08°C and 0.8pm, respectively. If the FBG 3dB-bandwidth is 0.2nm with the wavelength spacing between two adjacent FBGs of 1.4nm, the temperature and wavelength errors caused by crosstalk are respectively 0.01°C and 0.1pm.

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Design and Fabrication of Polarization-Independent Micro-Ring Resonators

GENG Min-Ming, JIA Lian-Xi, ZHANG Lei, LIU Yu-Liang, YANG Lin, LI Fang

Chin. Phys. Lett. 2008, 25 (4): 1333-1335 .

Abstract

PDF(pc) (501KB) ( 778 )

Origin of polarization sensitivity of photonic wire waveguides (PWWs) is analysed and the effective refractive indices of two different polarization states are calculated by the three-dimensional full-vector beam propagation
method. We find that PWWs are polarization insensitive if the distribution of its refractive index is uniform and the cross section is square. An MRR based on such a polarization-insensitive PWW is fabricated on an 8-inch silicon-on-insulator wafer using 248-nm deep ultraviolet lithography and reactive ion etching. The quasi-TE mode is resonant at 1542.25nm and 1558.90nm, and the quasi-TM mode is resonant at 1542.12nm and 1558.94nm. The corresponding polarization shift is 0.13nm at the shorter wavelength and 0.04nm at the longer wavelength. Thus the fabricated device is polarization independent. The extinction ratio is larger than 10dB. The 3dB bandwidth is about 2.5nm and the Qvalue is about 620 at 1558.9nm.

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Determination of Thickness of an Inaccessible Thin Film under a Multilayered System from Natural Frequencies

ZHOU Chang-Zhi, LI Ming-Xuan, MAO Jie, WANG Xiao-Min

Chin. Phys. Lett. 2008, 25 (4): 1336-1339 .

Abstract

PDF(pc) (190KB) ( 548 )

We investigate the relationship between natural frequencies of a multilayered system of different elastic materials and the thickness of the undermost thin film. The natural frequencies are numerically calculated from the reflection
coefficient of a sample system of ``steel--epoxy resin--aluminium--thin polymer' with normal incidence. Strain energy ratio is defined and calculated to give the physics explanation why some frequencies are sensitive to thickness of the thin film in certain range. Experiments of three specimens indicate that the measured natural frequencies agree well with the theoretical ones. It is found in our experiments that the ratio of the lowest film thickness to wavelength is about 1/5. The average relative errors for the inverted polymer film thicknesses are found to be 11.8%, -4.8% and -1.3%, respectively.

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A Numerical Tackling on Sakiadis Flow with Thermal Radiation

Rafael Cortell

Chin. Phys. Lett. 2008, 25 (4): 1340-1342 .

Abstract

PDF(pc) (108KB) ( 1143 )

Momentum and energy laminar boundary layers of an incompressible fluid with thermal radiation about a moving plate in a quiescent ambient fluid are investigated numerically. Also, it has been underlined that the analysis of the
roles of both velocity and temperature gradient at infinity is of key relevance for our results.

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Essence of Inviscid Shear Instability: a Point View of Vortex Dynamics

SUN Liang

Chin. Phys. Lett. 2008, 25 (4): 1343-1346 .

Abstract

PDF(pc) (110KB) ( 784 )

The essence of shear instability is reviewed both mathematically and physically, which extends the instability theory of a sheet vortex from the viewpoint of vortex dynamics. For this, the Kelvin--Arnol'd theorem is retrieved in linear context, i.e., the stable flow minimizes the kinetic energy
associated with vorticity. Then the mechanism of shear instability is explored by combining the mechanisms of both Kelvin--Helmholtz instability (K-H instability) and resonance of waves. The waves, which have the same phase speed with the concentrated vortex, have interactions with the vortex to trigger the instability. The physical explanation of shear instability is also sketched by extending Batchelor's theory. These results should lead to a more comprehensive understanding on shear instabilities.

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Experimental Study on Liquid Free Surface in Buoyant-Thermocapillary Convection

DUAN Li, KANG Qi, HU Wen-Rui

Chin. Phys. Lett. 2008, 25 (4): 1347-1350 .

Abstract

PDF(pc) (3287KB) ( 552 )

We investigate the surface deformations of buoyant-thermocapillary convection in a rectangular cavity due to gravity and temperature gradient between the two sidewalls. The cavity is 52×42mm in horizontal cross section, the thickness of liquid layer h is changed from 2.5mm to 6.5mm. Surface deformations of h= 3.5mm and 6.0mm are discussed and compared. Temperature difference is increased gradually, and the flow in the liquid layer will change from stable convection to unstable convection. Two kinds of optical diagnostic system with image processor are developed for study of the kinetics of buoyant-thermocapillary convection, they give out the information
of liquid free surface. The quantitative results are calculated by Fourier transform and correlation analysis, respectively. With the increasing temperature gradient, surface deformations calculated are more declining. It is interesting phenomenon that the inclining directions of the convections in thin and thick liquid layers are different. For a thin layer, the convection is mainly controlled by thermocapillary effect. However, for a thick layer, the convection is mainly controlled by buoyancy effect. The surface deformation
theoretically analysed is consistent with our experimental results. The present experiment proves that surface deformation is related to temperature gradient and thickness of the liquid layer. In other words, surface deformation lies on capillary convection and buoyancy convection.

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Role of Convection Flow on the Pattern Formation in the Drying Process of Colloidal Suspension

MA Wen-Jie, WANG Yu-Ren, LAN Ding

Chin. Phys. Lett. 2008, 25 (4): 1351-1354 .

Abstract

PDF(pc) (4773KB) ( 596 )

We study the macroscopic drying patterns of aqueous suspensions of colloidal silica spheres. It was found that convection strength can influence pattern formation. Uniformed films are obtained at weaker convection strength. In
addition, we make clear that it is not reasonable to discuss individually the effect of temperature and humidity on the colloid self-assembly. The physical mechanism is that these factors have relationship with the evaporation rate, which can affect the convection strength.

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Is Brenner's Modification to the Classical Navier--Stokes Equations Able to Describe Sound Propagation in Gases?

W. Marques Jr.

Chin. Phys. Lett. 2008, 25 (4): 1355-1358 .

Abstract

PDF(pc) (108KB) ( 551 )

We analyse the problem concerning the propagation of sound waves in gases by using the modified hydrodynamic theory proposed recently by Brenner for
single-component fluids. The modifications introduced by Brenner are based on his proposal that the translational momentum in fluid motion is not given by the mass flux. Comparison of the sound propagation results derived from Brenner's theory with available experimental data for monatomic gases shows that this modified continuum theory is unable to describe the acoustic measurements not even in the low-frequency limit, a result that from our point of view makes Brenner's proposal questionable.

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Stationary Fluid Dynamic Behaviour of V-Shaped Diffuser/Nozzle Elements for Valveless Micropump

ZHAO Quan-Liang, YUAN Jie, LIU Hong-Mei, DUAN Zhong-Xia, ZHANG Wen-Tong, QIU Cheng-Jun, CAO Mao-Sheng

Chin. Phys. Lett. 2008, 25 (4): 1359-1361 .

Abstract

PDF(pc) (904KB) ( 1062 )

A novel type of V-shaped (isosceles triangle cross-section) diffuser/nozzle element is proposed for use for valveless micropump. Stationary fluid dynamic behaviour of V-shaped diffuser/nozzle elements in a valveless micropump is
investigated by experiment and simulation. Both the results agree well with each other when the Reynolds number is higher than 100 and the pressure loss coefficient ratio of micropump nearly always keeps a constant. For a single V-shaped diffuser, the general trends of variation of pressure loss coefficient with opening angle and Reynolds number are opposite in small and large angles. Compared with conical and flat-wall diffusers, V-shaped diffuser shows the smallest pressure loss when the Reynolds number is 200.

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Experimental Research of Spontaneous Evolution from Ultracold Rydberg Atoms to Plasma

ZHANG Lin-Jie, FENG Zhi-Gang, LI An-Ling, ZHAO Jian-Ming, LI Chang-Yong, JIA Suo-Tang

Chin. Phys. Lett. 2008, 25 (4): 1362-1364 .

Abstract

PDF(pc) (297KB) ( 554 )

The spontaneous evolution from ultracold Rydberg atoms to plasma is investigated in a caesium MOT by using the method of field ionization. The plasma transferred from atoms in different Rydberg states (n=22--32) are obtained experimentally. Dependence of the threshold time of evolving to
plasma and the threshold number of initial Rydberg atoms on the principal quantum number of initial Rydberg states is studied. The experimental results are in agreement with hot--cold Rydberg--Rydberg atom collision ionization theory.

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Influence of Initial Pulse Chirp on Rainbow-Like Supercontinuum Generation from Filamentation in Air

HAO Zuo-Qiang, ZHANG Jie, ZHANG Zhe, LU Xin, JIN Zhan, ZHONGJia-Yong, LIU Yun-Quan, WANG Zhao-Hua

Chin. Phys. Lett. 2008, 25 (4): 1365-1367 .

Abstract

PDF(pc) (686KB) ( 701 )

Supercontinuum (SC) generation from laser filamentation in air is found
to depend strongly on the pulse duration. Rainbow-like SC generation is
observed only for a pulse of appropriate negative chirp that agrees
with the predictions put forward by Golubtsov it et al. [it Quantum Electron. 33 (2003) 525]. The conversion efficiency of an 800-nm laser light to rainbow-like SC is found to be the highest for 257fs pulses with an initial negative chirp. A larger chirp will lead to filamentation surviving at longer distance.

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Vaporization and Plasma Shielding during High Power Nanosecond Laser Ablation of Silicon and Nickel

LIU Dan, ZHANG Duan-Ming

Chin. Phys. Lett. 2008, 25 (4): 1368-1371 .

Abstract

PDF(pc) (249KB) ( 1329 )

A thermal model to describe the high-power nanosecond pulsed laser ablation is presented. It involves the vaporization and the following plasma shielding effect on the whole ablation process. As an example of Si target, we obtain
the time evolution of the calculated surface temperature, ablation rate and ablation depth. It can be seen that plasma shielding plays a more important role in the ablation process with time. At the same time, the ablation depth with laser fluence based on different models is shown. Moreover, we simulate the pulsed laser irradiation Ni target. The evolution of the transmitted intensity and the variation of ablation depth per pulse with laser fluence are performed. Under the same experimental conditions, the numerical results calculated with our thermal model are more in agreement with the experimental data.

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Dust Particle Properties in a Dual-Frequency Driven Sheath

LIU Yu, DAI Zhong-Ling, WANG You-Nian

Chin. Phys. Lett. 2008, 25 (4): 1372-1375 .

Abstract

PDF(pc) (167KB) ( 696 )

We study characteristics of a single dust particle in a dual-frequency capacitively coupled plasma sheath, such as charging and suspending processes, using a collisionless self-consistent model. Also, the movement of the dust grain with time is investigated for the various radii and initial velocities. The numerical results show that, after several microseconds, the charging process of the dust particle reaches equilibrium, and the grain
obtains its equilibrium position, In addition, it is found that the parameters of the low-frequency source impact on the charging and suspending processes of the dust grain significantly.

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A Power Interruption Technique for Investigation of Temperature Difference in Stabilized Low Direct-Current Arc Burning in Pure Argon on Atmospheric Pressure

M. M. Kuzmanovic, J. J. Savovic, D. P. Rankovic, M. Stoiljkovic, A. Antic Jovanovic, M.S. Pavlovic, M. Marinkovic

Chin. Phys. Lett. 2008, 25 (4): 1376-1379 .

Abstract

PDF(pc) (474KB) ( 586 )

Plasma of argon stabilized arc column, in a current range 3--11A, is investigated using emission spectrometric diagnostic techniques. Temperatures are evaluated using several methods: argon line to adjacent recombinational continuum intensity ratio, absolute emissivity of argon line, measurement of electron number density, and power interruption. Electron number density is evaluated from absolute emissivity of recombinational
continuum. The difference between electron T_e and heavy particle T_h temperature ranged from 4500K for 3A to 2300K for 11A arc current. By comparing the present with the previously obtained results, using the same arc device but with the introduction of water aerosol, it is concluded that water aerosol reduces the difference T_e-T_h and brings plasma closer to the partial thermodynamic equilibrium state.

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Thickness Dependence of Resistivity and Optical Reflectance of ITO Films

GAO Mei-Zhen, JOB R, XUE De-Sheng, FAHRNER W R

Chin. Phys. Lett. 2008, 25 (4): 1380-1383 .

Abstract

PDF(pc) (3862KB) ( 4658 )

Indium-tin-oxide (ITO) films deposited on crystalline silicon wafer and Corning glass are prepared by direct-current magnetron sputtering method at room temperature with various thicknesses. The thickness dependences of structure, resistance and optical reflectance of ITO films are characterized.
The results show that when the film thickness is less than 40nm, the resistivity and optical reflectance of the ITO film changes remarkably with thickness. The optoelectrical properties trend to stabilize when the thickness is over 55nm. The GXRD result implies that the ITO film begins to crystallize if only the thickness is large enough.

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Reversibility of Temperature-Induced Liquid Transition in Pb₂₆Sn₄₂Bi₃₂ Melt: Experimental Evidence with Electrical Property

ZU Fang-Qiu, YU Jin, XU Wei, ZHANG Yan, YANG Hui-Zhen

Chin. Phys. Lett. 2008, 25 (4): 1384-1387 .

Abstract

PDF(pc) (243KB) ( 554 )

Exploring nature of liquid structures and properties is becoming more interesting in various fundamental and applied fields. With different resorts including the resistivity method, temperature-induced liquid--liquid structure transitions (TI-LLST) have been suggested and verified to occur in some liquid
alloys, while the reversibility of TI-LLST has rarely been examined as yet. Unlike some other investigated liquid alloys, here we show that electrical resistivity of Pb₂₆Sn₄₂Bi₃₂ melt exhibit an anomalous change in the first heating run and a reversible change in the following cooling and heating cycles. Taking account of the structural sensitivity of electrical resistivity, the abnormal patterns suggest two sorts of TI-LLST that are irreversible and
reversible in the liquid ternary alloy, respectively. This interesting phenomenon together with other growing evidence imply that liquid structures and their change characteristics are multiform and complex.

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Low-Frequency Internal Friction Study on the Structural Changes in Polymer Melts

WU Xue-Bang, XU Qiao-Ling, SHANG Shu-Ying, SHUI Jia-Peng, LIU Chang-Song, ZHU Zhen-Gang

Chin. Phys. Lett. 2008, 25 (4): 1388-1391 .

Abstract

PDF(pc) (149KB) ( 550 )

With the help of the low-frequency internal friction method, we investigate the structural properties of polymer melts, such as amorphous polystyrene (PS), poly(methyl methacrylate) (PMMA), and semi-crystalline poly(ethylene oxide) (PEO). An obvious peak of relaxation type is found in each of the
internal friction curves. The peak temperature T_p follows the relation T_p≈(1.15-1.18)T_g for PS and PMMA melts, while it follows T≈1.22T_m for PEO melt, with T_g being the glass transition temperature and T_m the melting temperature. Based on the analysis of the features of this peak, it is found that this peak is related to the liquid-liquid transition temperature T_ll of
polymer melts. Mechanism of the liquid-liquid transition is suggested to be thermally-activated collective relaxation through cooperation. This finding may be helpful to understand the structural changes in polymer melts. In addition, the internal friction technique proves to be effective in studying dynamics in polymer melts.

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Thermal Conductivity of Carbon Nanotubes Embedded in Solids

CAO Bing-Yang, HOU Quan-Wen

Chin. Phys. Lett. 2008, 25 (4): 1392-1395 .

Abstract

PDF(pc) (1124KB) ( 974 )

A carbon-nanotube-atom fixed and activated scheme of non-equilibrium molecular dynamics simulations is put forward to extract the thermal conductivity of carbon nanotubes (CNTs) embedded in solid argon. Though a 6.5% volume fraction of CNTs increases the composite thermal conductivity to about twice as much as that of the pure basal material, the thermal conductivity of NTs embedded in solids is found to be decreased by 1/8--1/5 with reference to that of pure ones. The decrease of the intrinsic thermal
conductivity of the solid-embedded CNTs and the thermal interface resistance are demonstrated to be responsible for the results.

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Coherent Dynamics of Direct-Current-Driven Quantum-Dot-Array with Two Time-Dependent Embedded Impurities

LI Xiu-Ping, LA Shi-Jiang, WEN Yu-Bing, YAN Wei-Xian

Chin. Phys. Lett. 2008, 25 (4): 1396-1399 .

Abstract

PDF(pc) (378KB) ( 491 )

The coherent dynamics of dc-driven quantum-dot array with two embedded time-dependent impurities is investigated, the exact crossings and avoided crossings in the quasienergies are associated with the evolution dynamics of the electron by the introduction of the envelope lines of the site's maximum returning probability. Through a perturbative scheme, the time-dependent Hamiltonian is replaced by an equivalent static one, whose structure reveals the new quantum tunnelling mechanism in the time-periodic driving system and sheds light on the alternative interpretation of dynamic localization and delocalization.

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Atomic Diffusion in Cu/Si (111) and Cu/SiO₂/Si (111) Systems by Neutral Cluster Beam Deposition

CAO Bo, LI Gong-Ping, CHEN Xi-Meng, CHO Seong-Jin, KIM Hee

Chin. Phys. Lett. 2008, 25 (4): 1400-1402 .

Abstract

PDF(pc) (148KB) ( 1047 )

The Cu films are deposited on two kinds of p-type Si (111) substrates by ionized cluster beam (ICB) technique. The interface reaction and atomic diffusion of Cu/Si (111) and Cu/SiO₂/Si (111) systems are studied at different
annealing temperatures by x-ray diffraction (XRD) and Rutherford backscattering spectrometry (RBS). Some significant results are obtained: For the Cu/Si (111) samples prepared by neutral clusters, the interdiffusion of Cu and Si atoms occurs when annealed at 230°C. The diffusion coefficients of the samples annealed at 230°C and 500°C are 8.5×10^-15cm².s^-1 and 3.0×10^-14cm².s^-1, respectively. The formation of the copper-silicide phase is observed by XRD, and its intensity becomes stronger with the increase of annealing temperature. For the Cu/SiO₂/Si (111) samples prepared by neutral clusters, the interdiffusion of Cu and Si atoms occurs and copper silicides are
formed when annealed at 450°C. The diffusion coefficients of Cu in Si are calculated to be 6.0×10^-16cm².s^-1 at 450°C, due to the fact that the existence of the SiO₂ layer suppresses the interdiffusion of Cu and Si.

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Effect of (2×1) Surface Reconstruction on Elasticity of a Silicon Nano-Plate

WANG Jing, HUANG Qing-An, YU Hong

Chin. Phys. Lett. 2008, 25 (4): 1403-1406 .

Abstract

PDF(pc) (796KB) ( 761 )

A semi-continuum approach is developed to describe the effect of (2×1) surface reconstruction on the elastic modulus of the silicon nano-plate. Young's moduli of a (001) silicon nano-plate along the high-symmetry [100] direction are obtained with and without considering (2×1) surface
reconstruction. The approach predicts that the nano-plate with unreconstructed (001) surface is elastically softer than the bulk while it exhibits the opposite behaviour with (2×1) reconstructed surface. On the (001) surface, the (2×1) reconstructed surface dominates the plate as the thickness of the plate scaling decreases to several tens of nanometre. Whether the nano-plate is softer or stiffer depends on bond loss, bond saturation and direction of bond alignment, which have major impacts on the
mechanics of the nano-plate.

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Direct Observation of Tunnelling through 100-nm-Wide All Metal Magnetic Junction into Si

Nam H. KIM, WANG Ke-Qiang, ZHANG Yu, WANG Jian-Qing

Chin. Phys. Lett. 2008, 25 (4): 1407-1410 .

Abstract

PDF(pc) (674KB) ( 609 )

Nanoscaled spin-dependent tunnelling lines were patterned on doped Si and studied for tunnelling from the SDT ferromagnetic layer through an insulating barrier into Si. The injection contacts have the form of long strips with width and separation, ranging from 100nm to 2μm, and are patterned using e-beam lithography. The measured I-V characteristics versus temperature (80 to 300K) on the 100nm scaled devices between the layered-magnetic metals and the semiconductor clearly showed ballistic tunnelling, with weak dependence on the temperature. This is qualitatively different, at elevated temperatures, from 2-μm-wide scaled-up spin-dependent tunnelling structures, where thermal-ionic emission was observed to dominate carrier transport.

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Annealed Treatment Effect in Poly(3-hexylthiophene):Methanofullerene Solar Cells

YU Huang-Zhong, PENG Jun-Biao

Chin. Phys. Lett. 2008, 25 (4): 1411-1414 .

Abstract

PDF(pc) (1551KB) ( 816 )

Polymer photovoltaic devices based on poly(3-hexylthiophene) (P3HT) : [6,6]-phenyl-C₆₁-butyricacid methyl ester (PCBM) 1:1 weight-ratio blend are reported. The effects of various annealing treatments on the device performance are investigated. Thermal annealing shows significant improvement of the device performances. For devices at 130°C annealing, maximum power conversion efficiency (PCE) of 3.3% and fill factor up to 60.3% is achieved under air mass 1.5, 100mW/cm² illumination. We discuss the effect of thermal annealing by the results of ultraviolet-visible absorption spectroscopy (UV-vis), dark current-voltage curve, atomic force microscopy (AFM).

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Electronic Properties of p-Type δ-Doped GaAs Structure under Electric Field

Emine OZTURK, Ismail SOKMEN

Chin. Phys. Lett. 2008, 25 (4): 1415-1418 .

Abstract

PDF(pc) (144KB) ( 584 )

We theoretically investigate the electronic properties of p-type δ-doped GaAs inserted into a quantum well under the electric field, at T=0K. We will investigate the influence of the electric field on the δ-doping concentration for a uniform distribution. The depth of confining potential, the density profile, the Fermi level, the subband energies and the subband populations calculate by solving the Schrodinger and Poisson equations self consistently. It is found that the changes of the electronic properties are quite sensitive to the applied electric field and the doping concentration. As different from single n-type δ-doped structure, we see a replace between the ground light-hole (lh1) subband and the first excited heavy-hole (hh2) subband whenever the external electric field reaches a critical value. We find the abrupt changing of the subband energies and the subband populations whenever the applied electric field reaches a certain value. Also, it is found that the heavy-hole subbands contain many more energy states than the light-hole ones, the population of the heavy-hole levels represent approximately 91% of all the carriers.

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Cooper-Pair Number--Phase Quantization for Inductance Coupling Circuit Including Josephson Junctions

MENG Xiang-Guo, WANG Ji-Suo, LIANG Bao-Long

Chin. Phys. Lett. 2008, 25 (4): 1419-1422 .

Abstract

PDF(pc) (168KB) ( 888 )

Based on the entangled state representation and a bosonic phase operator formalism, we tackle with Cooper-pair number--phase quantization for the inductance coupling circuit including Josephson junctions, and then investigate how Josephson current equations change due to the presence of the coupling inductance and obtain bosonic operator Faraday formula, as well as the corresponding number--phase uncertainty relation.

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Effects of Contact Geometry on Electron Transport of 1,4-Diaminobenzene

ZHENG Ji-Ming, REN Zhao-Yu, GUO Ping, TIAN Jin-Shou, BAI Jin-Tao

Chin. Phys. Lett. 2008, 25 (4): 1423-1426 .

Abstract

PDF(pc) (4990KB) ( 666 )

The equilibrium electron transport of 1,4-diaminobenzene sandwiched between two Au electrodes is simulated by using a first principles analysis. The results show that equilibrium conductance increases with the molecule-electrode distance decreasing, and a platform occurs at the distance varying from 1.4A to 1.9A, implying the insensitiveness of 1,4-diaminobenzene equilibrium conductance to molecule--electrode distance. This is helpful to understand the improved reliability and reproducibility of conductance measurements using amines.

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SILC during NBTI Stress in PMOSFETs with Ultra-Thin SiON Gate Dielectrics

CAO Yan-Rong, HAO Yue, MA Xiao-Hua, YU Lei, HU Shi-Gang

Chin. Phys. Lett. 2008, 25 (4): 1427-1430 .

Abstract

PDF(pc) (225KB) ( 902 )

Negative bias temperature instability (NBTI) and stress-induced leakage current (SILC) both are more serious due to the aggressive scaling lowering of devices. We investigate the SILC during NBTI stress in PMOSFETs with ultra-thin gate dielectrics. The SILC sensed range from -1V to 1V is divided into four parts: the on-state SILC, the near-zero SILC, the off-state SILC sensed at lower positive voltages and the one sensed at higher positive voltages. We develop a model of tunnelling assisted by interface states and oxide bulk traps to explain the four different parts of SILC during NBTI stress.

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Spin-Filter Effect Induced by Magnetic Edge States of Zigzag Carbon Nanotube

JIANG Zhan-Feng, LI Jian, SHEN Shun-Qing, LIU Wu-Ming

Chin. Phys. Lett. 2008, 25 (4): 1431-1434 .

Abstract

PDF(pc) (424KB) ( 634 )

Spin-filter effect is predicted in a weak coupled junction composed of a nonmagnetic metal electrode and a zigzag carbon nanotube. This effect is induced by the magnetic edge states of the nanotube, and can produce spin-polarized current in the absence of an external magnetic field. We find that the spin polarization of the current changes its sign at the half-filling
point of the nanotube, thus electric field control of spin transport can be realized. Furthermore, we find the coupling strength of the junction may cause a magnetic transition on the edge of the nanotube.

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Composition-Controlled Low Field Magnetostriction of TbFe Amorphous Films

JIANG Hong-Chuan, ZHANG Wan-Li, ZHANG Wen-Xu, PENG Bin

Chin. Phys. Lett. 2008, 25 (4): 1435-1437 .

Abstract

PDF(pc) (153KB) ( 835 )

Amorphous TbFe films are fabricated by dc magnetron sputtering, and their magnetostrictions at low field are examined over a wide range of terbium content (from 32at.% to 70at.%). It is found that the terbium content plays an important role in the magnetic and magnetostrictive properties of TbFe films. TbFe film soft magnetic properties and low field magnetostriction can
be efficiently improved by controlling the terbium at an optimum content. The magnetostriction at lower magnetic field is increased with the increase of terbium content up to 48.2at.%. After reaching the maximum value, further increase of terbium content would result in a great decrease of the low field magnetostriction. By contrast, at higher magnetic field, the magnetostriction is decreased monotonically with the increase of the terbium content.

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Origin of Room-Temperature Ferromagnetism for Cobalt-Doped ZnO Diluted Magnetic Semiconductor

PENG Long, ZHANG Huai-Wu, WEN Qi-Ye, SONG Yuan-Qiang, SU Hua, John Q. Xiao

Chin. Phys. Lett. 2008, 25 (4): 1438-1441 .

Abstract

PDF(pc) (221KB) ( 1000 )

The pure single phase of Zn_0.95Co_0.05O bulks is successfully prepared by solid-state reaction method. The effects of annealing atmosphere on room-temperature ferromagnetic behaviour for the Zn_0.95Co_0.05O bulks are
investigated. The results show that the air-annealed samples has similar weak ferromagnetic behaviour with the as-sintered samples, but the obvious ferromagnetic behaviour is observed for the samples annealed in vacuum or Ar/H₂ gas, indicating that the strong ferromagnetism is associated with high oxygen vacancies density. High saturation magnetization M_s=0.73μ_B/Co and coercivity H_c=233.8Oe are obtained for the Ar/H₂ annealed samples with pure single phase structure. The enhanced room-temperature ferromagnetic behaviour is also found in the samples with high carrier concentration controlled by doping interstitials Zn (Zn_i).

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Ferroelectric Properties of Polycrystalline Ceramics with Dipolar Defect Simulated from the Potts--Ising Model

ZHANG Yan-Fei, WANG Chun-Lei, ZHAO Ming-Lei, LI Ji-Chao, ZHANG Rui-Zhi, LIU Jian, MEI Liang-Mo

Chin. Phys. Lett. 2008, 25 (4): 1442-1445 .

Abstract

PDF(pc) (795KB) ( 875 )

Physical properties of polycrystalline ferroelectrics including the contributions of the fixed dipolar defects and the average grain size in the Potts--Ising model are simulated by using the Monte Carlo method. Domain pattern,
hysteresis loop and switching current of the polarization reversal process are obtained. Two processes are considered in our simulation. In the first one, the grain texture of ferroelectric ceramics are produced from the Potts model, and then the Ising model is implemented in the obtained polycrystalline texture to produce the domain pattern, hysteresis loop and switching current. It is concluded that the defect has the ability to decrease the remnant
polarization P_r as well as the coercive field E_c. The back switching is obviously observed after the electric field is off, and it shows some variation after introducing the fixed dipolar defect. Meanwhile, the spike of the switching current is found to lower with the increasing defect concentration and the decreasing average grain size.

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(Na_0.52K_0.44Li_0.04)Nb_0.9-xSb_xTa_0.1O₃Lead-Free Piezoelectric Ceramics with High Performance and High Curie Temperature

DU Juan, WANG Jin-Feng, ZANG Guo-Zhong, QI Peng, ZHANG Shu-Jun, Thomas R. Shrout

Chin. Phys. Lett. 2008, 25 (4): 1446-1448 .

Abstract

PDF(pc) (156KB) ( 941 )

(Na_0.52K_0.44Li_0.04)Nb_0.9-xSb_xTa_0.1O₃ lead-free piezoelectric ceramics are prepared by a solid-state reaction method. With increasing Sb content, the transition temperature from orthorhombic to tetragonal polymorphic phase decreased. A composition (Na_0.52K_0.44Li_0.04)Nb_0.863Sb_0.037Ta_0.1O₃is found to possess excellent piezoelectric and electromechanical performances (d₃₃=306pC/N, k_p=48%, and k_t=50%), and high Curie temperature (T_C=320°C). These results indicate that (Na_0.52K_0.44Li_0.04)Nb_0.863Sb_0.037Ta_0.1O₃is a promising lead-free piezoceramics replacement for lead zirconate titanate.

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Microstructure and Optical Properties of Ag_xO Prepared by Direct-Current Magnetron-Sputtering Method

GAO Xiao-Yong, LIU Xu-Wei, WANG Song-You, LIU Yu-Fen, LIN Qing-Geng, LU Jing-Xiao

Chin. Phys. Lett. 2008, 25 (4): 1449-1452 .

Abstract

PDF(pc) (1839KB) ( 582 )

Two series of Ag_xO films are prepared on glass substrates by dc magnetron-sputtering method at room temperature and 90°C under different oxygen to argon gas ratio (OAR) conditions. The microstructure is investigated by XRD and SEM in order to obtain the information on the component evolution of
AgO+Ag₂O to Ag₂O. Its optical properties are investigated by reflectance and absorption spectroscopy to extract the information on metallic and dielectric behaviour evolution of Ag₂O, AgO and silver particles and the interband transition. The results indicate that the Ag_xO film prepared at room temperature is mainly made up of AgO and Ag₂O clusters while Ag₂O is the primary component of Ag_xO prepared at 90°C. The Ag_xO film mainly consisting of the primary component shows indirect interband transition
structure occurring at 2.89eV. Combination of increasing OAR and substrate temperature is an effective method to lower the threshold of thermal decomposition temperature of Ag_xO and to deal with the bottleneck of short-wavelength optical and magneto-optical storage.

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Vacuum Ultraviolet Excited Photoluminescence Properties of Novel Na₃Y₉O₃(BO₃)₈:Tb³⁺ Phosphor

ZHANG Jia-Chi, WANG Yu-Hua, ZHANG Zhi-Ya, XIE Peng, LI Hui-Hui, JIANG Yan-Ping

Chin. Phys. Lett. 2008, 25 (4): 1453-1456 .

Abstract

PDF(pc) (474KB) ( 744 )

The novel vacuum ultraviolet (VUV) excited Na₃Y₉O₃(BO₃)₈:Tb³⁺(NYOB:Tb³⁺) green phosphor is prepared. Strong VUV photoluminescence and high quenching concentration of Tb³⁺(20wt%) are observed in NYOB:Tb³⁺ and the strong emission are correlated with the unique layer-type structure of NYOB. All the characteristic 4f-5d transitions of Tb³⁺ and the host absorption band in VUV region are identified in the excitation spectrum. Based on the results, the energy levels scheme of Tb³⁺in NYOB:Tb³⁺ is first established. This newly developed NYOB:Tb³⁺ phosphor shows excellent optical properties when compared with the commercial Zn₂SiO₄:Mn²⁺ and would be a potential VUV-excited green phosphor.

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Efficient Solution-Processed Blue Electrophosphorescent Devices Based on a Novel Small-Molecule Host

HOU Liu-Dong, LI Wei, DUAN Lian, QIU Yong

Chin. Phys. Lett. 2008, 25 (4): 1457-1460 .

Abstract

PDF(pc) (7203KB) ( 960 )

Efficient blue small molecular phosphorescent light-emitting diodes with a blue phosphorescent dyebis(3,5-difluoro-2-(2-pyridyl)-phenyl-(2-carboxypride) iridium (III) (FIrpic) doped into a novel small-molecule host 9,9-bis[4-(3,6-di-tert-butylcarbazol-9-yl)phenyl] fluorene (TBCPF) as the light-emitting layer have been fabricated by spin-coating. The host TBCPF can form homogeneous amorphous films by spin-coating and has triplet energy higher than that of the blue phosphorescent dye FIrpic. All the devices with different FIrpic concentration in the emitting layer give emission from FIrpic indicating complete energy transfer from TBCPF to FIrpic. The device shows the best performance with a peak brightness of 8050cd/m² at 10.2V and the maximum current efficiency up to 3.52cd/A, when the FIrpic doped concentration is as
high as 16%.

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Optical Nonlinear Properties of CdSeS/ZnS Core/Shell Quantum Dots

WU Feng, TIAN Wei, MA i-Na, CHEN Wen-Ju, ZFANG Gui-Lan, ZHAO Guo-Feng, CAO Shi-Dong, XIE Wei

Chin. Phys. Lett. 2008, 25 (4): 1461-1464 .

Abstract

PDF(pc) (2126KB) ( 896 )

The optical nonlinear properties of CdSeS/ZnS quantum dots (QDs) are
investigated by Z-scan technique using fundamental harmonic generation
(1064nm) of mode-locked Nd:YAG laser for the first time. The experimental results show that two photon absorptions (TPA) occur at input intensity up to 12.5GW/cm². CdSeS/ZnS QDs have an average TPA cross section of 13710GM and large nonlinear refractive index on order of 10^-7esu. The large optical nonlinearities perhaps allow the CdSeS/ZnS QDs to be one kind of candidate material for bioimaging and fluorescence label, optical limiting and all-optical switching.

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Optical Properties of Co--BaTiO₃/Mg(100) Nano-Composite Films Grown by Pulsed Laser Deposition Method

WU Wei-Dong, WANG Feng, GE Fang-Fang, BAI Li, LEI Hai-Le, TANG Yong-Jian, JU Xin, CHEN Zheng-Hao, SUN Wei-Guo,

Chin. Phys. Lett. 2008, 25 (4): 1465-1468 .

Abstract

PDF(pc) (139KB) ( 528 )

Co nanoparticles embedded in a BaTiO₃ matrix, namely Co--BaTiO₃ nano-composite films are grown on Mg(100) single crystal substrates by the pulsed laser deposition (PLD) method at 650°C. Optical properties of the Co--BaTiO₃ nano-composite films are examined by absorption spectra (AS) and photoluminescence (PL) spectra. The results indicate that the concentration of Co nano-particles strongly influences the electron transition of the Co--BaTiO₃ nano-composite films. The PL emission band ranging from 1.9 to 2.2eV is reported. The AS and PL spectra suggest that the band gap is in the range of 3.28--3.7eV.

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Dependence of Crystal Quality and β Value on Synthesis Temperature in Growing Gem Diamond Crystals

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

Chin. Phys. Lett. 2008, 25 (4): 1469-1471 .

Abstract

PDF(pc) (2854KB) ( 890 )

High quality Ib gem diamond single crystals were synthesized in cubic anvil high-pressure apparatus (SPD-6×1200) under 5.4GPa and 1230°C--1280°C. The (100) face of seed crystal was used as growth face, and Ni₇₀Mn₂₅Co₅ alloy was used as solvent/catalyst. The dependence of crystal quality and β value (the ratio of height to diameter of diamond crystal) on synthesis temperature was studied. When the synthesis temperature is between 1230°C and 1280°C, the β value of the synthetic high-quality gem diamond crystals is between 0.4 and 0.6. The results show that when the β value is between 0.4 and 0.45, the synthetic diamonds are sheet-shape crystals; however, when the β value is between 0.45 and 0.6, the synthetic diamonds are tower-shape crystals. In addition, when the β value is less than 0.4, skeleton crystals will appear. When the β value is more than 0.6, most of the synthetic diamond crystals are inferior crystals.

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Preparation and Ferroelectric Properties of Double-Scale PZT Composite Piezoelectric Thick Film

DUAN Zhong-Xia, YUAN Jie, ZHAO Quan-Liang, LIU Hong-Mei, LIN Hai-Bo, ZHANG Wen-Tong, CAO Mao-Sheng

Chin. Phys. Lett. 2008, 25 (4): 1472-1475 .

Abstract

PDF(pc) (3066KB) ( 909 )

Dense and crack-free double-scale lead zirconate titanate (Pb(Zr_0.52Ti_0.48)O₃, PZT) composite piezoelectric thick films have been successfully fabricated on Au/Cr/SiO₂/Si substrates by a modified sol-gel method. The XRD analysis indicates that the thick film possesses a single-phase perovskite-type structure. The SEM micrograph shows that the surface is crack-free and the cross section is dense and clear. The thickness of the PZT thick film is about 4μm. It also exhibits good ferroelectric properties, and has high direct current compression resistant properties. At the test frequency of 1kHz, the film has the coercive field of 50kV/cm, the saturation polarization of 54μC/cm² and the remnant polarization of 30μC/cm².

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Optical and Magnetic Properties of Fe-Doped GaN Diluted Magnetic Semiconductors Prepared by MOCVD Method

TAO Zhi-Kuo, ZHANG Rong, CUI Xu-Gao, XIU Xiang-Qian, ZHANG Guo-Yu, XIE Zi-Li, GU Shu-Lin, SHI Yi, ZHENG You-Dou

Chin. Phys. Lett. 2008, 25 (4): 1476-1478 .

Abstract

PDF(pc) (193KB) ( 1399 )

Fe-doped GaN thin films are grown on c-sapphires by metal organic chemical vapour deposition method (MOCVD). Crystalline quality and phase purity are characterized by x-ray diffraction and Raman scattering measurements. There are no detectable second phases formed during growth and no significant
degradation in crystalline quality as Fe ions are doped. Fe-related optical transitions are observed in photoluminescence spectra. Magnetic measurements reveal that the films show room-temperature ferromagnetic behaviour. The ferromagnetism may originate from carrier-mediated Fe-doped GaN diluted magnetic semiconductors or nanoscale iron clusters and Fe--N compounds which we have not detected.

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Monodispersive CoPt Nanoparticles Synthesized Using Chemical Reduction Method

SHEN Cheng-Min, HUI Chao, YANG Tian-Zhong, XIAO Cong-Wen, CHEN Shu-Tang, DING Hao, GAO Hong-Jun

Chin. Phys. Lett. 2008, 25 (4): 1479-1481 .

Abstract

PDF(pc) (2158KB) ( 524 )

Monodispersive CoPt nanoparticles in sizes of about 2.2nm are synthesized by superhydride reduction of CoCl₂ and PtCl₂ in diphenyl ether. The as-prepared nanoparticles show a chemically disordered A₁ structure and are
superparamagnetic. Thermal annealing transforms the A₁ structure into chemically ordered L1₀ structure and the particles are ferromagnetic at room temperature.

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Transient Non-Thermal Mobility of CO for CO--NO Catalytic Reaction on Square Lattice: Monte Carlo Simulation

M. Khalid, A. U. Qaisrani, M. G. Ullah

Chin. Phys. Lett. 2008, 25 (4): 1482-1485 .

Abstract

PDF(pc) (120KB) ( 466 )

We study a model based on precursor mechanism for CO--NO catalytic reaction on square lattice with Monte Carlo simulation. The precursor mechanism clearly demonstrates its impact on the phase diagram. The steady reactive state (SRS) gets established. The width of reactive region increases by increasing the range of precursor mobility. When the precursor mobility is increased to third-nearest neighbourhood, the second-order transition
disappears.

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Water and Ion Permeation through Electrically Charged Nanopore

ZENG Li, ZUO Guang-Hong, GONG Xiao-Jing, LU Hang-Jun, WANGChun-Lei, WU Ke-Fei, WAN Rong-Zheng,

Chin. Phys. Lett. 2008, 25 (4): 1486-1489 .

Abstract

PDF(pc) (112KB) ( 736 )

The behaviour of water and small solutes in confined geometries is important to a variety of chemical and nanofluidic applications. Here we investigate the permeation and distribution of water and ions in electrically charged carbon
cylindrical nanopore during the osmotic process using molecular dynamics simulations. In the simulations, charges are distributed uniformly on the pores with diameter of 0.9nm. For nanopores with no charge or a low charge, ions are difficult to enter. With the increasing of charge densities on the pores, ions will appear inside the nanopores because of the large electronic forces between the ions and the charged pores. Different ion entries induce varying effects on osmotic water flow. Our simulations reveal that the osmotic water
can flow through the negatively charged pore occupied by K⁺ions, while water flux through the positively charged pores will be disrupted by Cl^- ions inside the pores. This may be explained by the different radial distributions of K⁺ ions and Cl^- ions inside the charged nanopores.

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Suprathreshold Stochastic Resonance in Mechanoelectrical Transduction of Hair Cells

ZHAO Xiang-Hui, LONG Zhang-Cai, ZHANG Bing, YANG Nan

Chin. Phys. Lett. 2008, 25 (4): 1490-1493 .

Abstract

PDF(pc) (122KB) ( 578 )

Haircell, the sound signal sensor in the auditory system, works in an environment full of noise. This noise has been demonstrated before to be a factor beneficial to the transfer of subthreshold weak signal. We further theoretically demonstrate that in suprathreshold signal situation, noise can still be a beneficial factor and enhance the haircell nonlinear transfer of suprathreshold signal. This reveals an additional constructive function of noise in hearing and provides theoretic foundation for relevant applications.

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Randomness Effect on Cooperation in Memory-Based Snowdrift Game

ZHANG Ming-Feng, WANG Bing-Hong, WANG Wen-Xu, TANG Chuan-Long, YANG Rui

Chin. Phys. Lett. 2008, 25 (4): 1494-1497 .

Abstract

PDF(pc) (359KB) ( 703 )

A memory-based snowdrift game (MBSG) on spatial small-world networks is investigated. It is found that cooperation rate versus temptation shows some step structures on small-world networks, similar to the case on regular lattices. With the increment of rewiring probability based on four-neighbour
regular lattices, more steps are observable. Interestingly, it is observed that cooperation rate peaks at a specific value of temptation, which indicates that properly encouraging selfish actions may lead to better cooperative behaviours in the MBSG on small-world networks. Memory effects are also discussed for different rewiring probabilities. Furthermore, optimal regions are
found in the parameter planes. The strategy-related average degrees of individuals are helpful to understand the obtained results.

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Subconscious Effect on Pedestrian Counter Flow

KUANG Hua, SONG Tao, LI Xing-Li, DAI Shi-Qiang

Chin. Phys. Lett. 2008, 25 (4): 1498-1501 .

Abstract

PDF(pc) (184KB) ( 655 )

We propose an extended lattice gas model with different maximum velocities to simulate pedestrian counter flow by considering the subconscious behaviour of walkers. Four types of walkers including faster right walkers, slower right walkers, faster left walkers and slower left walkers are involved in the simulation. The simulation results show that our model can capture some essential features of pedestrian counter flows, such as the lane formation, segregation effect and phase separation at higher densities. We also find that the subconscious effect can reduce the occurrence of jam cluster evidently compared with the case of un-subconscious effect. At large maximum velocity, the critical density corresponding to the maximum flow rate of the fundamental diagram is in good agreement with the empirical results.

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Opinion Dynamics on Complex Networks with Communities

WANG Ru, CHI Li-Ping, CAI Xu

Chin. Phys. Lett. 2008, 25 (4): 1502-1505 .

Abstract

PDF(pc) (136KB) ( 895 )

The Ising or Potts models of ferromagnetism have been widely used to describe locally interacting social or economic systems. We consider a related model, introduced by Sznajd to describe the evolution of consensus in the scale-free networks with the tunable strength (noted by Q) of community
structure. In the Sznajd model, the opinion or state of any spins can only be changed by the influence of neighbouring pairs of similar connection spins. Such pairs can polarize their neighbours. Using asynchronous updating, it is found that the smaller the community strength Q, the larger the slope of the exponential relaxation time distribution. Then the effect of the initial up-spin concentration p as a function of the final all up probability E is investigated
by taking different initialization strategies, the random node-chosen
initialization strategy has no difference under different community strengths, while the strategies of community node-chosen initialization and hub node-chosen initialization are different in final probability under different Q, and the latter one is more effective in reaching final state.

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Actual Tropical Waves Identified by the Wavenumber-Frequency Spectrum Analysis during Boreal Summer

MAO Jiang-Yu, WANG Zai-Zhi

Chin. Phys. Lett. 2008, 25 (4): 1506-1509 .

Abstract

PDF(pc) (2203KB) ( 751 )

Based on daily satellite-observed outgoing longwave radiation (OLR) data, the tropical intraseasonal oscillation (ISO) associated with the Asian summer monsoon is found to be dominant during the 2005 summer. Such an ISO along with other tropical waves are identified using a wavenumber-frequency spectrum analysis. Some of significant spectral peaks coincide well with the dispersion curves of the equatorially trapped wave modes from shallow water theory, with various equivalent depths of 12--200m, suggesting that actual tropical waves can be detected from observational data. Although the formation mechanism of the ISO is more complicated than those of other tropical waves with higher frequencies, significant spectral peaks that correspond to the ISO with a period of 30--60 days distinctly appear in the OLR spectra. The selective wavenumber-frequency filtering of OLR data is thus
performed to isolate the ISO, Kelvin and equatorial Rossby waves, and the properties and signatures of such tropical waves that control tropical atmospheric variability for the 2005 summer are clearly identified.

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Global Structures and Multi-Temporal Variabilities of MLT Migrating Diurnal Tide

CHEN Ze-Yu, LU Da-Ren

Chin. Phys. Lett. 2008, 25 (4): 1510-1513 .

Abstract

PDF(pc) (693KB) ( 528 )

Migrating diurnal tide in the MLT region is examined by the application of Hough mode decomposition with the tide delineated from the SABER/TIMED temperatures over 2002--2006. The decomposition results show that in the height range 60--100km, the (1, 1) mode is the most predominant among eight leading Hough modes including four propagating and four trapped modes. It exhibits a sustained maximum at 97km and significant semi-annual oscillation. Additionally, a novel feature of inter-annual variation with period of about two years is clearly seen in the (1, 1) mode, e.g., repeated maxima are seen at the March equinox of 2002, 2004 and 2006, respectively. This feature is further manifested by the tidal amplitudes in the height range 70--100km in the height-time cross-section at the equator. It is likely of the QBO as the height range just coincides to where the zonal mean zonal winds derived by using the UARS data exhibiting the QBO. The other results show that the (1, 2) mode is important at <80km exhibiting comparable amplitude to that of the (1, 1) mode, and in particular the nearly anti-correlation with the (1, 1) mode. The tide at about 85km is suggested of rather complex as the four trapped modes exhibit maximum at these heights, which indicates the presence of local excitations or sources at below.

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Constraints on Slim Accretion Discs

CAI Zhen-Yi, GU Wei-Min, LU Ju-Fu

Chin. Phys. Lett. 2008, 25 (4): 1514-1516 .

Abstract

PDF(pc) (115KB) ( 581 )

We show that when the gravitational force in the vertical direction is correctly calculated, the well-known S-shaped sequence of thermal equilibrium solutions can be constructed only for small radii of black hole accretion flows, such that slim accretion discs can possibly exist only in the inner regions of these flows.

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Limits from Weak Gravity Conjecture on Chaplygin-Gas-Type Models

WU Xing, ZHU Zong-Hong

Chin. Phys. Lett. 2008, 25 (4): 1517-1520 .

Abstract

PDF(pc) (261KB) ( 484 )

The weak gravity conjecture is proposed as a criterion to distinguish the landscape from the swampland in string theory. As an application in cosmology of this conjecture, we use it to impose theoretical constraint on parametersof the Chaplygin-gas-type models. Our analysis indicates that the
Chaplygin-gas-type models realized in quintessence field are in the swampland.

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Comment on `Multi-shock Compressions of Dense Hydrogen--Helium Mixture beyond 100GPa'

TAN Hua, CHEN Qi-Feng

Chin. Phys. Lett. 2008, 25 (4): 1521-1521 .

Abstract

PDF(pc) (98KB) ( 413 )

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Reply to Comment on `Multi-shock Compression of Dense Hydrogen--Helium Mixture beyond 100GPa'

LIU Fusheng-Sheng, ZHANG Ming-Jian, TIAN Chun-Ling

Chin. Phys. Lett. 2008, 25 (4): 1522-1522 .

Abstract

PDF(pc) (97KB) ( 524 )

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