Chin. Phys. B

The principle of the Internet evolving and the conjecture of the optimal structure of Internet

Li Ying(李旲), Cao Hong-Duo(曹宏铎), Shan Xiu-Ming(山秀明), Ren Yong(任勇), and Yuan Jian(袁坚)

Chin. Phys. B, 2009, 18 (5): 1721-1724. DOI: 10.1088/1674-1056/18/5/001

Abstract ( 1511 )

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In this paper we will give the statistical characteristics and general principles of an optimal structure of the Internet, which is a scale-free network. Since the purpose of the Internet is to allow fast and easy communication, the average path length is used to measure the performance of the network, and the number of edges of the network is used as a metric of its cost. Based on this, the goal of this Internet optimization problem is to obtain the highest performance with the lowest cost. A multi goal optimization problem is proposed to model this problem. By using two empirical formulas of $\langle k\rangle $ and $\langle l\rangle$, we are able to find the statistical characteristics of the optimal structure. There is a critical power law exponent $\alpha _{\rm c}$ for the Internet with power law degree distribution, at which the Internet can obtain a relatively good performance with a low cost. We find that this $\alpha _{\rm c}$ is approximately 2.1.

Control of stochastic resonance in bistable systems by using periodic signals

Lin Min(林敏), Fang Li-Min(方利民), and Zheng Yong-Jun(郑永军)

Chin. Phys. B, 2009, 18 (5): 1725-1730. DOI: 10.1088/1674-1056/18/5/002

Abstract ( 1301 )

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According to the characteristic structure of double wells in bistable systems, this paper analyses stochastic fluctuations in the single potential well and probability transitions between the two potential wells and proposes a method of controlling stochastic resonance by using a periodic signal. Results of theoretical analysis and numerical simulation show that the phenomenon of stochastic resonance happens when the time scales of the periodic signal and the noise-induced probability transitions between the two potential wells achieve stochastic synchronization. By adding a bistable system with a controllable periodic signal, fluctuations in the single potential well can be effectively controlled, thus affecting the probability transitions between the two potential wells. In this way, an effective control can be achieved which allows one to either enhance or realize stochastic resonance.

Mei conserved quantity of the Nielsen equation for a non-Chetaev-type non-holonomic system

Cui Jin-Chao(崔金超), Zhang Yao-Yu(张耀宇) , and Jia Li-Qun(贾利群)

Chin. Phys. B, 2009, 18 (5): 1731-1736. DOI: 10.1088/1674-1056/18/5/003

Abstract ( 1466 )

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The Mei symmetry and Mei conserved quantity of the Nielsen equation for a non-Chetaev-type non-holonomic non-conservative system are studied. The differential equations of motion of the Nielsen equation for the system, the definition and the criterion of Mei symmetry and the condition and the form of Mei conserved quantities deduced directly from the Mei symmetry for the system are obtained. Finally, an example is given to illustrate the application of the results.

Gain/loss effect on a bright solitary wave in a cigar-shaped attractive condensate in the presence of an expulsive parabolic potential

Wang Feng-Jiao(王凤姣), Ding Jian-Wen(丁建文), Yan Xiao-Hong(颜晓红), and Wang Deng-Long(王登龙)

Chin. Phys. B, 2009, 18 (5): 1737-1741. DOI: 10.1088/1674-1056/18/5/004

Abstract ( 1309 )

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Taking into account both gain/loss and time-dependent atomic scattering length, this paper analytically derives an exact bright solitary wave in a cigar-shaped attractive condensate in the presence of an expulsive parabolic potential. Due to the balance of the scattering length and gain/loss, the bright solitary wave is shown to have constant amplitude. Especially, it is found that the bright solitary wave is accelerated by expulsive force, whose velocity can be modulated by changing the axial and transverse angular frequencies. The results are in good agreement with the experimental observations by Khaykovich et al (2002 Science 296 1290).

Teleportation of tripartite entangled coherent states

Sun Yong(孙勇), Man Zhong-Xiao(满忠晓), and Xia Yun-Jie(夏云杰)

Chin. Phys. B, 2009, 18 (5): 1742-1748. DOI: 10.1088/1674-1056/18/5/005

Abstract ( 1377 )

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This paper proposes a feasible scheme for the quantum teleportation of tripartite entangled coherent states by using linear optical devices such as beam splitters, phase shifters and photo detectors. The scheme is based on the bipartite maximally entangled coherent state and the tripartite entangled coherent state with bipartite maximal entanglement as quantum channels. It shows that when the mean number of photons is equal to 2, the total minimum of the average fidelity for an arbitrary tripartite entangled state is 1-0.67×10 ^-3.

Entropy of a rotating and charged black string to all orders in the Planck length

Zhao Ren(赵仁), Wu Yue-Qin (武月琴), and Zhang Li-Chun(张丽春)

Chin. Phys. B, 2009, 18 (5): 1749-1754. DOI: 10.1088/1674-1056/18/5/006

Abstract ( 1240 )

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By using the entanglement entropy method, this paper calculates the statistical entropy of the Bose and Fermi fields in thin films, and derives the Bekenstein--Hawking entropy and its correction term on the background of a rotating and charged black string. Here, the quantum field is entangled with quantum states in the black string and thin film to the event horizon from outside the rotating and charged black string. Taking into account the effect of the generalized uncertainty principle on quantum state density, it removes the difficulty of the divergence of state density near the event horizon in the brick-wall model. These calculations and discussions imply that high density quantum states near the event horizon of a black string are strongly correlated with the quantum states in a black string and that black string entropy is a quantum effect. The ultraviolet cut-off in the brick-wall model is not reasonable. The generalized uncertainty principle should be considered in the high energy quantum field near the event horizon. From the viewpoint of quantum statistical mechanics, the correction value of Bekenstein--Hawking entropy is obtained. This allows the fundamental recognition of the correction value of black string entropy at nonspherical coordinates.

Stochastic resonance driven by time-modulated correlated coloured noise sources in a single-mode laser

Chen De-Yi(陈德彝) and Zhang Li(张莉)

Chin. Phys. B, 2009, 18 (5): 1755-1760. DOI: 10.1088/1674-1056/18/5/007

Abstract ( 1296 )

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This paper investigates the phenomenon of stochastic resonance in a single-mode laser driven by time-modulated correlated coloured noise sources. The power spectrum and signal-to-noise ratio R of the laser intensity are calculated by the linear approximation. The effects caused by noise self-correlation time $\tau_1$, $\tau_2$ and cross-correlated time $\tau_3$ for stochastic resonance are analysed in two ways: $\tau_1$, $\tau_2$ and $\tau_3$ are taken to be the independent variables and the parameters respectively. The effects of the gain coefficient $\varGamma$ and loss coefficient K on the stochastic resonance are also discussed. It is found that besides the presence of the standard form and the broad sense of stochastic resonance, the number of extrema in the curve of R versus K is reduced with the increase of the gain coefficient $\varGamma$.

A method of estimating initial conditions of coupled map lattices based on time-varying symbolic dynamics

Shen Min-Fen(沈民奋), Liu Ying(刘英), and Lin Lan-Xin(林兰馨)

Chin. Phys. B, 2009, 18 (5): 1761-1768. DOI: 10.1088/1674-1056/18/5/008

Abstract ( 1361 )

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A novel computationally efficient algorithm in terms of the time-varying symbolic dynamic method is proposed to estimate the unknown initial conditions of coupled map lattices (CMLs). The presented method combines symbolic dynamics with time-varying control parameters to develop a time-varying scheme for estimating the initial condition of multi-dimensional spatiotemporal chaotic signals. The performances of the presented time-varying estimator in both noiseless and noisy environments are analysed and compared with the common time-invariant estimator. Simulations are carried out and the obtained results show that the proposed method provides an efficient estimation of the initial condition of each lattice in the coupled system. The algorithm cannot yield an asymptotically unbiased estimation due to the effect of the coupling term, but the estimation with the time-varying algorithm is closer to the Cramer--Rao lower bound (CRLB) than that with the time-invariant estimation method, especially at high signal-to-noise ratios (SNRs).

Chaotification for a class of nonlinear systems

Liu Na(刘娜) and Guan Zhi-Hong(关治洪)

Chin. Phys. B, 2009, 18 (5): 1769-1773. DOI: 10.1088/1674-1056/18/5/009

Abstract ( 1486 )

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More and more attention has been focused on effectively generating chaos via simple physical devices. The problem of creating chaotic attractors is considered for a class of nonlinear systems with backlash function in this paper. By utilizing the Silnikov heteroclinic and homoclinic theorems, some sufficient conditions are established to guarantee that the nonlinear system has horseshoe-type chaos. Examples and simulations are given to verify the effectiveness of the theoretical results.

Fuzzy modeling and impulsive control of hyperchaotic Lü system

Zhang Xiao-Hong(张小洪) and Li Dong(李东)

Chin. Phys. B, 2009, 18 (5): 1774-1779. DOI: 10.1088/1674-1056/18/5/010

Abstract ( 1833 )

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This paper presents a novel approach to hyperchaos control of hyperchaotic systems based on impulsive control and the Takagi--Sugeno (T--S) fuzzy model. In this study, the hyperchaotic Lü system is exactly represented by the T--S fuzzy model and an impulsive control framework is proposed for stabilizing the hyperchaotic Lü system, which is also suitable for classes of T--S fuzzy hyperchaotic systems, such as the hyperchaotic R?ssler, Chen, Chua systems and so on. Sufficient conditions for achieving stability in impulsive T--S fuzzy hyperchaotic systems are derived by using Lyapunov stability theory in the form of the linear matrix inequality, and are less conservative in comparison with existing results. Numerical simulations are given to demonstrate the effectiveness of the proposed method.

N-soliton solution of a coupled integrable dispersionless equation

Zhaqilao(扎其劳), Zhao Yin-Long(赵银龙), and Li Zhi-Bin(李志斌)

Chin. Phys. B, 2009, 18 (5): 1780-1786. DOI: 10.1088/1674-1056/18/5/011

Abstract ( 1313 )

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A new coupled integrable dispersionless equation is presented by considering a spectral problem. A Darboux transformation for the resulting coupled integrable dispersionless equation is constructed with the help of spectral problems. As an application, the N-soliton solution of the coupled integrable dispersionless equation is explicitly given.

Analysis of ferroresonance in neutral grounding system with nonlinear core loss

Hui Meng(惠萌), Zhang Yan-Bin(张彦斌), and Liu Chong-Xin(刘崇新)

Chin. Phys. B, 2009, 18 (5): 1787-1791. DOI: 10.1088/1674-1056/18/5/012

Abstract ( 1414 )

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The chaotic behaviour exhibited by a typical ferroresonant circuit in a neutral grounding system is investigated in this paper. In most earlier ferroresonance studies the core loss of the power transformer was neglected or represented by a linear resistance. However, this is not always true. In this paper the core loss of the power transformer is modelled by a third order series in voltage and the magnetization characteristics of the transformer are modelled by an 11th order two-term polynomial. Extensive simulations are carried out to analyse the effect of nonlinear core loss on transformer ferroresonance. A detailed analysis of simulation results demonstrates that, with the nonlinear core loss model used, the onset of chaos appears at a larger source voltage and the transient duration is shorter.

Circuit implementation and multiform intermittency in a hyper-chaotic model extended from the Lorenz system

Cang Shi-Jian(仓诗建), Chen Zeng-Qiang(陈增强), and Wu Wen-Juan(吴文娟)

Chin. Phys. B, 2009, 18 (5): 1792-1800. DOI: 10.1088/1674-1056/18/5/013

Abstract ( 1447 )

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This paper presents a non-autonomous hyper-chaotic system, which is formed by adding a periodic driving signal to a four-dimensional chaotic model extended from the Lorenz system. The resulting non-autonomous hyper-chaotic system can display any dynamic behaviour among the periodic orbits, intermittency, chaos and hyper-chaos by controlling the frequency of the periodic signal. The phenomenon has been well demonstrated by numerical simulations, bifurcation analysis and electronic circuit realization. Moreover, the system is concrete evidence for the presence of Pomeau--Manneville Type-I intermittency and crisis-induced intermittency. The emergence of a different type of intermittency is similarly subjected to the frequency of periodic forcing. By statistical analysis, power scaling laws consisting in different intermittency are obtained for the lifetime in the laminar state between burst states.

Terahertz radiation from armchair carbon nanotube dipole antenna

Wang Yue(王玥), Wu Qun(吴群), He Xun-Jun(贺训军), Zhang Shao-Qing(张少卿), and Zhuang Lei-Lei(庄蕾蕾)

Chin. Phys. B, 2009, 18 (5): 1801-1806. DOI: 10.1088/1674-1056/18/5/014

Abstract ( 1544 )

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This paper investigates the electromagnetic radiation characteristics of a metallic, large aspect ratio single walled carbon nanotube antenna in the terahertz frequency region below 12.5 THz. The key features of terahertz pulse have been revealed on the carbon nanotube antenna in comparison with conventional photoconductive switching. The terahertz waveforms, radiation power and their field distributions have been evaluated and are analysed. The Fourier transformed spectra over the whole frequency range demonstrate that the carbon nanotube antenna can be used as radiation source for broadband terahertz applications.

Studying a kind of portable ultra-bright microfocus x-raysource

Wang Kai-Ge(王凯歌), Wang Lei(王雷), and Niu Han-Ben(牛憨笨)

Chin. Phys. B, 2009, 18 (5): 1807-1813. DOI: 10.1088/1674-1056/18/5/015

Abstract ( 1592 )

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This paper studies the properties of a kind of portable ultra-bright microfocus x-ray source with the Monte-Carlo method in detail. The new x-ray source consists of an electron-emission system, an electrostatic focusing system and a metal target. A crystal Lanthanum Hexaboride cathode, a Wehnelt grid and an extracted electrode compose the triode electrode electron-gun system. Two equal radius cylinder electrodes form the focusing system. The key factors determining the focus properties of the electron beam such as the ratio D_w/H, grid bias V_g, and the properties of the extracted electrode are numerically studied. The calculated results reveal that when D_w/H, V_g, the length of the extracted electrode, and the distance between the grid and the extracted electrode equals 5, --0.6 kV, 10 mm, and 8 mm respectively, the electron beam focal spot can be concentrated down to 9 μm in radius and a reasonable focal length about 72.5 mm can be achieved, at the same time, the cathode emission currents can be as high as 30 mA.

Knot solitons in AFZ model

Ren Ji-Rong(任继荣), Mo Shu-Fan(墨淑凡), and Zhu Tao(朱涛)

Chin. Phys. B, 2009, 18 (5): 1814-1820. DOI: 10.1088/1674-1056/18/5/016

Abstract ( 1219 )

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This paper studies the topological properties of knotted solitons in the (3+1)-dimensional Aratyn--Ferreira--Zimerman (AFZ) model. Topologically, these solitons are characterized by the Hopf invariant I, which is an integral class in the homotopy group $\pi$₃(S³)=Z. By making use of the decomposition of U(1) gauge potential theory and Duan's topological current theory, it is shown that the invariant is just the total sum of all the self-linking and linking numbers of the knot family while only linking numbers are considered in other papers. Furthermore, it is pointed out that this invariant is preserved in the branch processes (splitting, merging and intersection) of these knot vortex lines.

Infinite series symmetry reduction solutions to the modified KdV--Burgers equation

Yao Ruo-Xia(姚若侠), Jiao Xiao-Yu(焦小玉), and Lou Sen-Yue(楼森岳)

Chin. Phys. B, 2009, 18 (5): 1821-1827. DOI: 10.1088/1674-1056/18/5/017

Abstract ( 1284 )

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From the point of view of approximate symmetry, the modified Korteweg--de Vries--Burgers (mKdV--Burgers) equation with weak dissipation is investigated. The symmetry of a system of the corresponding partial differential equations which approximate the perturbed mKdV--Burgers equation is constructed and the corresponding general approximate symmetry reduction is derived; thereby infinite series solutions and general formulae can be obtained. The obtained result shows that the zero-order similarity solution to the mKdV--Burgers equation satisfies the Painlevé II equation. Also, at the level of travelling wave reduction, the general solution formulae are given for any travelling wave solution of an unperturbed mKdV equation. As an illustrative example, when the zero-order tanh profile solution is chosen as an initial approximate solution, physically approximate similarity solutions are obtained recursively under the appropriate choice of parameters occurring during computation.

Regarding the 2¹S₀ and 2³S₁ multiplets

Liu Yun-Hu(刘云虎)

Chin. Phys. B, 2009, 18 (5): 1828-1832. DOI: 10.1088/1674-1056/18/5/018

Abstract ( 1538 )

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This paper discusses the masses and possible members of $2^1S_0$ and $2^3S_1$ meson multiplets in the framework of meson--meson mixing and Regge phenomenology. For the $2^1S_0$ meson nonet, $K^*(1460)$ may be the observed state of the $2^1S_0$ isodoublet with a mass of 1400~MeV and the mass of $\omega(1420)$ is 1412 MeV. For the $2^3S_1$ meson nonet, $K^*(1410)$ seems too light to be the $2^3S_1$ $n\overline{s}$ member. The fact that the agreement between the present findings and those given by experiments and other different approaches is satisfactory implies that the assignments in this paper may be reasonable.

Investigation of autoionization spectra of Sm atoms using an isolated-core excitation method

Qin Wen-Jie(秦文杰), Dai Chang-Jian(戴长建), Xiao Ying(肖颖), and Zhao Hong-Ying(赵洪英)

Chin. Phys. B, 2009, 18 (5): 1833-1837. DOI: 10.1088/1674-1056/18/5/019

Abstract ( 1298 )

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Using the isolated-core-excitation scheme and three-step laser resonance ionization spectroscopy approach, this paper, for the first time, has systematically investigated the autoionization spectra of atomic Sm, belonging to the 4 f⁶6pnl and 4f⁵5d6snl (l=0, 2) configurations. In the experiment, the first two tunable dye lasers are employed to excite the Sm atom from its initial state to the different 4f⁶6snl bound Rydberg states, then the third dye laser is scanned to drive the atom to the doubly-excited autoionizing states. With the above excitation scheme, the measured transition profiles of the autoionizing states are nearly symmetric, from which the level energies and widths can be easily obtained.

Measurement of quantum defect of nS and nD states using field ionization spectroscopy in ultracold cesium atoms

Zhang Lin-Jie(张临杰), Feng Zhi-Gang(冯志刚), Li An-Ling(李安玲), Zhao Jian-Ming(赵建明), Li Chang-Yong(李昌勇), and Jia Suo-Tang(贾锁堂)

Chin. Phys. B, 2009, 18 (5): 1838-1842. DOI: 10.1088/1674-1056/18/5/020

Abstract ( 1599 )

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This paper reports that ultracold atoms are populated into different nS and nD Rydberg states (n=25～52) by two-photon excitation. The ionization spectrum of an ultracold Rydberg atom is acquired in a cesium magneto-optical trap by using the method of pulse field ionization. This denotes nS and nD states in the ionization spectrum and fits the data of energy levels of different Rydberg states to obtain quantum defects of nS and nD states.

Evolution of three-shell onion-like and core-shell structures in (AgCo)₂₀₁ bimetallic clusters

Wang Qiang(王强), Li Guo-Jian(李国建), Li Dong-Gang(李东刚), Lü Xiao(吕逍), and He Ji-Cheng(赫冀成)

Chin. Phys. B, 2009, 18 (5): 1843-1849. DOI: 10.1088/1674-1056/18/5/021

Abstract ( 1364 )

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This paper studies the structural evolution of (AgCo)₂₀₁ clusters with different Co concentrations under various temperature conditions by using molecular dynamics with the embedded atom method. The most stable position for Co atoms in the cluster is the subsurface layer at low temperature (lower than 200 K for the Ag₂₀₀Co₁ cluster). The position changes to the core layer with the increase of temperature, but there is an energy barrier in the middle layer. This makes the Ag--Co cluster form an Ag--Co--Ag three-shell onion-like configuration. When the temperature is high enough [higher than 800 K for (AgCo)₂₀₁ clusters with 50% Co], Co atoms can obtain enough energy to overcome the energy barrier and the cluster forms an Ag--Co core-shell configuration. Amorphization for the onion-like and core-shell clusters is induced by the large lattice misfit at Ag--Co interfaces. The structural evolution in the Ag--Co cluster is related to the release of excess energy.

The simplified material parameter equation for elliptical cylinder cloaks

Ma Hua(马华), Qu Shao-Bo(屈绍波), Xu Zhuo(徐卓), Zhang Jie-Qiu(张介秋), and Wang Jia-Fu(王甲富)

Chin. Phys. B, 2009, 18 (5): 1850-1852. DOI: 10.1088/1674-1056/18/5/022

Abstract ( 1873 )

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We simplify the material parameter equation for elliptical cylinder cloaks under transverse-electric and transverse-magnetic models, respectively, and confirm these simplified equations by numerical simulations. As a result, the number of the component parameters is reduced from three to two, which simplifies the design of meta-materials and thus opens up the possibility of achieving elliptical cylinder cloaks in an easy way.

Nonparaxial propagation of a vectorial apertured off-axis Lorentz beam

Zhou Guo-Quan(周国泉)

Chin. Phys. B, 2009, 18 (5): 1853-1890. DOI: 10.1088/1674-1056/18/5/023

Abstract ( 1551 )

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Based on the vectorial Rayleigh--Sommerfeld integral formula and the complex Gaussian expansion of the hard-edge aperture function, an analytical propagation expression for a nonparaxial vectorial off-axis Lorentz beam passing through a rectangular aperture is derived. The unapertured case, the far field expression and the scalar paraxial result are also presented as special cases of the general formulae, respectively. Some numerical examples are also given to show the propagation characteristics of a nonparaxial vectorial off-axis Lorentz beam through a rectangular aperture. It is indicated that the f parameter, the off-axis displacement and the truncation parameter all play an important role in determining nonparaxial propagation behaviour.

Investigation of the influence of key parameters on the system performance in all-ptical label switching based on FSK/ASK orthogonal modulation format

Wei Lai(魏莱), Xin Xiang-Jun(忻向军), Ma Jian-Xin(马建新), Zhang Qi(张琦), Wang Kui-Ru(王葵如), Yu Chong-Xiu(余重秀), and Liu Bo(刘博)

Chin. Phys. B, 2009, 18 (5): 1861-1866. DOI: 10.1088/1674-1056/18/5/024

Abstract ( 1398 )

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The transmission characteristics of the optical label switching system based on the FSK/ASK orthogonal modulation format is investigated. The factors that affect the transmission performance, such as the FSK tone space, dispersion compensation and coupler split ratio, are studied by numerical simulation. The proposed scheme is also experimentally demonstrated with a transmission of 155 Mbit/s FSK label combined with 10 Gbit/s ASK payload.

Scheme for implementing quantum logic gates for two atoms trapped in different cavities

Lin Li-Hua(林丽华)

Chin. Phys. B, 2009, 18 (5): 1867-1871. DOI: 10.1088/1674-1056/18/5/025

Abstract ( 1472 )

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A scheme is presented for realizing quantum logic gates for two atoms localized in two distant optical cavities. Our scheme works in a regime in which the atom--cavity coupling strength is smaller than the cavity decay rate. Thus the requirement on the quality factor of the cavities is greatly relaxed. Furthermore, the fidelity of our scheme is not affected by detection inefficiency and atomic decay. These advantages are important in view of experiment.

A solvable operator motion equation of an excited-atom coupled leaky cavity in single-photon generation

Chai Jin-Hua(柴金华) and Han Zheng-Fu(韩正甫)

Chin. Phys. B, 2009, 18 (5): 1872-1876. DOI: 10.1088/1674-1056/18/5/026

Abstract ( 1326 )

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A model for an excited-atom coupled leaky cavity in single-photon generation is proposed based on universal modes. Solvable motion equations of the atomic operators are obtained under the single-photon condition by adopting the Lorentzian line type of the universal modes.

Mechanism of laser-induced plasma shock wave evolution in air

Zhao Rui(赵瑞), Liang Zhong-Cheng(梁忠诚), Han Bing(韩冰), Zhang Hong-Chao(张宏超), Xu Rong-Qing(徐荣青), Lu Jian(陆建), and Ni Xiao-Wu(倪晓武)

Chin. Phys. B, 2009, 18 (5): 1877-1883. DOI: 10.1088/1674-1056/18/5/027

Abstract ( 1576 )

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A theoretical model is proposed to describe the mechanism of laser-induced plasma shock wave evolution in air. To verify the validity of the theoretical model, an optical beam deflection technique is employed to track the plasma shock wave evolution process. The theoretical model and the experimental signals are found to be in good agreement with each other. It is shown that the laser-induced plasma shock wave undergoes formation, increase and decay processes; the increase and the decay processes of the laser-induced plasma shock wave result from the overlapping of the compression wave and the rarefaction wave, respectively. In addition, the laser-induced plasma shock wave speed and pressure distributions, both a function of distance, are presented.

Simulation analysis of evolution of hot-images induced by coplanar multi-scatterers

Peng Tao(彭涛), Zhao Jian-Lin(赵建林), Li Dong(李东), Ye Zhi-Jun(叶知隽), and Xie Liang-Ping(谢良平)

Chin. Phys. B, 2009, 18 (5): 1884-1890. DOI: 10.1088/1674-1056/18/5/028

Abstract ( 1283 )

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Based on the diffraction theory model for hot-image formation, the evolution of hot-images induced by multi-scatterers located in the same plane perpendicular to the propagation axis is numerically simulated. The simulation results show that hot-images induced by coplanar multi-scatterers are also coplanar no matter whether they exist simultaneously or severally. However, if they exist simultaneously the peak intensity of the primary hot-images is weaker than if they exist severally. The unequal competition for energy between the scattered beams from the scatterers leads to the fact that part of the corresponding hot-images are relatively enhanced and the others are restrained. The results show that the hot-images of certain scatterers become stronger when any of these parameters, i.e. amplitude modulation coefficient, phase modulation coefficient and size of the surrounding scatterer, decrease.

Transverse effects in photorefractive two-wave mixing

Cai Xin(蔡欣), Liu Jin-Song(刘劲松), Wang Sheng-Lie(汪盛烈), and Liu Shi-Xiong(刘时雄)

Chin. Phys. B, 2009, 18 (5): 1891-1897. DOI: 10.1088/1674-1056/18/5/029

Abstract ( 1304 )

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In a biased photorefractive crystal, the process of two one-dimensional waves mixing, i.e., the dynamical evolution of both pump beam and signal beam, is traced by numerically solving the coupled-wave equation. Direct simulations show that the propagation and stability of the two beams are completely determined by the system parameters, such as the external bias field, the intensity and the beam waist of the pump beam. By adjusting these parameters, one can control the state of two Gaussian waves mixing. The numerical results are helpful for performing a two-wave mixing experiment.

Concrete damage diagnosed using the non-classical nonlinear acoustic method

Zhou Dao(周到), Liu Xiao-Zhou(刘晓宙), Gong Xiu-Fen(龚秀芬), Nazarov V E, and Ma Li(马力)

Chin. Phys. B, 2009, 18 (5): 1898-1905. DOI: 10.1088/1674-1056/18/5/030

Abstract ( 1486 )

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It is known that the strength of concrete is seriously affected by damage and cracking. In this paper, six concrete samples under different damage levels are studied. The experimental results show a linear dependence of the resonance frequency shift on strain amplitude at the fundamental frequency, and approximate quadratic dependence of the amplitudes of the second and third harmonics on strain amplitude at the fundamental frequency as well. In addition, the amplitude of the third harmonics is shown to increase with the increase of damage level, which is even higher than that of the second harmonics in samples with higher damage levels. These are three properties of non-classical nonlinear acoustics. The nonlinear parameters increase from 10⁶ to 10⁸ with damage level, and are more sensitive to the damage level of the concrete than the linear parameters obtained by using traditional acoustics methods. So, this method based on non-classical nonlinear acoustics may provide a better means of non-destructive testing (NDT) of concrete and other porous materials.

Third-order Stokes wave solutions for interfacial internal waves in a three-layer dendity-stratified fluid

Chen Xiao-Gang(陈小刚), Guo Zhi-Ping(郭志萍), Song Jin-Bao(宋金宝), He Xiao-Dong(赫晓东), Guo Jun-Ming(郭军明), Bao Shu-Hong(包曙红), and Cui Wei(崔巍)

Chin. Phys. B, 2009, 18 (5): 1906-1916. DOI: 10.1088/1674-1056/18/5/031

Abstract ( 1476 )

PDF (276KB) ( 1733 )

Interfacial internal waves in a three-layer density-stratified fluid are investigated using a singular perturbation method, and third-order asymptotic solutions of the velocity potentials and third-order Stokes wave solutions of the associated elevations of the interfacial waves are presented based on the small amplitude wave theory. As expected, the third-order solutions describe the third-order nonlinear modification and the third-order nonlinear interactions between the interfacial waves. The wave velocity depends on not only the wave number and the depth of each layer but also on the wave amplitude.

Formation mechanism of anomalous eutectics in highly undercooled Ag--39.9 at%Cu alloys

Zhao Su(赵素), Li Jin-Fu(李金富), Liu Li(刘礼), and Zhou Yao-He(周尧和)

Chin. Phys. B, 2009, 18 (5): 1917-1922. DOI: 10.1088/1674-1056/18/5/032

Abstract ( 1357 )

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This paper investigates the solidification behaviour of the Ag--Cu eutectic alloy melt undercooled up to 100K. It is revealed that lamellar eutectics grow in a dendritic form in the Ag--Cu eutectic melt with undercooling equal to or greater than 76K. As undercooling increases, the remelted fraction of the primary eutectics during recalescence rises. The severe remelting and the subsequent ripening of the primary eutectic dendrites lead to the formation of anomalous eutectics.

Study of theoretical tensile strength of Fe by a first-principles computational tensile test

Liu Yue-Lin(刘悦林), Zhang Ying(张颖), Hong Rong-Jie(洪荣杰), and Lu Guang-Hong(吕广宏)

Chin. Phys. B, 2009, 18 (5): 1923-1930. DOI: 10.1088/1674-1056/18/5/033

Abstract ( 1301 )

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This paper employs a first-principles total-energy method to investigate the theoretical tensile strengths of bcc and fcc Fe systemically. It indicates that the theoretical tensile strengths are shown to be 12.4, 32.7, 27.5 GPa for bcc Fe, and 48.1, 34.6, 51.2 GPa for fcc Fe in the [001], [110] and [111] directions, respectively. For bcc Fe, the [001] direction is shown to be the weakest direction due to the occurrence of a phase transition from ferromagnetic bcc Fe to high spin ferromagnetic fcc Fe. For fcc Fe, the [110] direction is the weakest direction due to the formation of an instable saddle-point `bct structure' in the tensile process. Furthermore, it demonstrates that a magnetic instability will occur under a tensile strain of 14%, characterized by the transition of ferromagnetic bcc Fe to paramagnetic fcc Fe. The results provide a good reference to understand the intrinsic mechanical properties of Fe as a potential structural material in the nuclear fusion Tokamak.

High energy electron radiation effect on Ni and Ti/4H-SiC Schottky barrier diode at room temperature

Zhang Lin(张林), Zhang Yi-Men(张义门), Zhang Yu-Ming(张玉明), Han Chao(韩超), and Ma Yong-Ji(马永吉)

Chin. Phys. B, 2009, 18 (5): 1931-1934. DOI: 10.1088/1674-1056/18/5/034

Abstract ( 1428 )

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This paper reports that Ni and Ti/4H-SiC Schottky barrier diodes (SBDs) were fabricated and irradiated with 1 MeV electrons up to a dose of 3.43×10¹⁴e/cm². After radiation, the Schottky barrier height $\phi _{\rm B} $ of the Ni/4H-SiC SBD increased from 1.20 eV to 1.21 eV, but decreased from 0.95 eV to 0.94 eV for the Ti/4H-SiC SBD. The degradation of $\phi _{\rm B} $ could be explained by interface states of changed Schottky contacts. The on-state resistance R_S of both diodes increased with the dose, which can be ascribed to the radiation defects. The reverse current of the Ni/4H-SiC SBD slightly increased, but for the Ti/4H-SiC SBD it basically remained the same. At room temperature, $\phi _{\rm B} $ of the diodes recovered completely after one week, and the R_S partly recovered.

Bound polaron in quantum dot quantum well structures

Xing Yan(邢雁), Wang Zhi-Ping(王志平), and Wang Xu(王旭)

Chin. Phys. B, 2009, 18 (5): 1935-1941. DOI: 10.1088/1674-1056/18/5/035

Abstract ( 1554 )

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The problem of bound polarons in quantum dot quantum well (QDQW) structures is studied theoretically. The eigenfrequencies of bulk longitudinal optical (LO) and surface optical (SO) modes are derived in the framework of the dielectric continuum approximation. The electron--phonon interaction Hamiltonian for QDQW structures is obtained and the exchange interaction between impurity and LO-phonons is discussed. The binding energy and the trapping energy of the bound polaron in CdS/HgS QDQW structures are calculated. The numerical results reveal that there exist three branches of eigenfrequencies of surface optical vibration in the CdS/HgS QDQW structure. It is also shown that the binding energy and the trapping energy increase as the inner radius of the QDQW structure decreases, with the outer radius fixed, and the trapping energy takes a major part of the binding energy when the inner radius is very small.

ZnO micro-nano composite hydrophobic film prepared by the three-step method

Ma Kai(马恺), Li Hua(李华), Zhang Han(张晗), Xu Xiao-Liang(许小亮), Gong Mao-Gang(公茂刚), and Yang Zhou(杨周)

Chin. Phys. B, 2009, 18 (5): 1942-1946. DOI: 10.1088/1674-1056/18/5/036

Abstract ( 1514 )

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The hydrophobicity of the lotus leaf is mainly due to its surface micro--nano composite structure. In order to mimic the lotus structure, ZnO micro--nano composite hydrophobic films were prepared via the three-step method. On thin buffer films of SiO₂, which were first fabricated on glass substrates by the sol--gel dip-coating method, a ZnO seed layer was deposited via RF magnetron sputtering. Then two different ZnO films, micro--nano and micro-only flower-like structures, were grown by the hydrothermal method. The prepared films have different hydrophobic properties after surface modification. The structures of the obtained ZnO films were characterized using x-ray diffraction and field-emission scanning electron microscopy. A conclusion that a micro--nano composite structure is more beneficial to hydrophobicity than a micro-only structure was obtained through research into the effect of structure on hydrophobic properties.

Monte Carlo simulation of ramified aggregates on hetero-substrates

Qian Chang-Ji(钱昌吉), Li Hong(李洪), Zhong Rui(钟瑞), Luo Meng-Bo(罗孟波), and Ye Gao-Xiang(叶高翔)

Chin. Phys. B, 2009, 18 (5): 1947-1954. DOI: 10.1088/1674-1056/18/5/037

Abstract ( 1289 )

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We have studied the aggregation of particles on a hetero-substrate consisting of two different substrates A and B with finite surface barriers $E_{\rm AB}$ and $E_{\rm BA}$ between the AB and BA boundaries, respectively. With the diffusion energy limited aggregation (DELA) model, we find that the number of clusters and the mean radius of gyration of the clusters are dependent on the surface barriers $E_{\rm AB}$ and $E_{\rm BA}$. For the case with a constant of $E_{\rm BA}$, a series of minima are summarized as $E_{\rm AB }= (E_{0 }- k_{\rm BA }E_{\rm BA })/ k_{\rm AB}$ with $k_{\rm AB}$ and $k_{\rm BA}$ being two integers, for main minima ($k_{\rm BA}=k_{\rm AB}- 1$) and two local minima ($k_{\rm BA}=k_{\rm AB}$ and $k_{\rm BA}=k_{\rm AB} + 1$) between two neighbouring main minima.

Guiding of 150keV O⁶⁺ ions through nanocapillaries in an uncoated Al₂O₃ membrane: special time dependence of the transmission profile width

Chen Xi-Meng(陈熙萌), Xi Fa-Yuan(席发元), Qiu Xi-Yu(邱玺玉), Shao Jian-Xiong(邵剑雄), Xiao Guo-Qing(肖国青), Cui Ying(崔莹), Sun Guang-Zhi(孙光智), Wang Jun(王俊), Chen Yi-Feng(陈益峰), Liu Hui-Ping(刘会平), Yin Yong-Zhi(尹永智), Wang Yu-Yu(王瑜玉), Li De-Hui(李德慧), Lou Feng-Jun(娄凤君), Wang Xing-An(王兴安), Xu Jun-Kui(徐俊奎), and Zhou Chun-Lin(周春林)

Chin. Phys. B, 2009, 18 (5): 1955-1960. DOI: 10.1088/1674-1056/18/5/038

Abstract ( 1326 )

PDF (361KB) ( 463 )

This paper reports that the transmission of O$^{6 + }$ ions with energy of 150keV through capillaries in an uncoated Al$_{2}$O$_{3}$ membrane was measured, and agreements with previously reported results in general angular distribution of the transmitted ions and the transmission fractions as a function of the tilt angle well fitted to Gaussian-like functions were observed. Due to using an uncoated capillary membrane, our $\psi _{\rm c} $ is larger than that using a gold-coated one with a smaller value of $\frac{E_{\rm p}}{q}$, which suggests a larger equilibrium charge $Q_\infty $ in our experiment. The observed special width variation with time and a larger width than that using a smaller $\frac{E_{\rm p} }{q}$ were qualitatively explained by using mean-field classical transport theory based on a classical-trajectory Monte Carlo simulation.

4f^N-15d energy levels of lanthanides: a quasi-angular-momentum approach and its application to Cs₂NaYF₆:Er³⁺

Ma Chong-Geng(马崇庚), Jiang Sha(江莎), Duan Chang-Kui(段昌奎), Yin Min(尹民), and Xia Shang-Da(夏上达)

Chin. Phys. B, 2009, 18 (5): 1961-1967. DOI: 10.1088/1674-1056/18/5/039

Abstract ( 1318 )

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The spin--orbit interaction of the 5d electron needs to be taken into account to give the proper energy structure for the 4f^N-15d configuration of heavy lanthanide ions occupying a site with ligands forming an octahedron. This paper derives theoretical results for the energy structure by treating the t₂ orbitals as quasi p orbitals and then using angular-momentum coupling techniques. An analytic expression for the electric dipole absorption line strengths between 4f^N multiplets and 4f^N-15d states is given in terms of various angular-momentum quantum numbers and re-coupling coefficients. The result is then applied to interpret the excitation spectrum of Cs₂NaYF₆:Er³⁺. The high-spin and low-spin states of Cs₂NaYF₆:Er³⁺ are discussed in terms of the wavefunctions obtained by using the developed theoretical model.

Simulation of water potential for the electronic structure of serine

Wang Xing-Rong(王兴荣) and Zheng Hao-Ping(郑浩平)

Chin. Phys. B, 2009, 18 (5): 1968-1978. DOI: 10.1088/1674-1056/18/5/040

Abstract ( 1220 )

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First-principles, all-electron, ab initio calculations have been performed to construct an equivalent water potential for the electronic structure of serine (Ser) in solution. The calculation is composed of three steps. The first step is to search for the configuration of the Ser + nH₂O system with a minimum energy. The second step is to calculate the electronic structure of Ser with the water molecule potential via the self-consistent cluster-embedding method (SCCE), based on the result obtained in the first step. The last step is to calculate the electronic structure of Ser with the dipole potential after replacing the water molecules with dipoles. The results show that the occupied states of Ser are raised by about 0.017~Ry on average due to the effect of water. The water effect can be successfully simulated by using the dipole potential. The obtained equivalent potential can be applied directly to the electronic structure calculation of protein in solution by using the SCCE method.

Elastic constants and thermodynamic properties of Mg₂Si_xSn_1-x from first-principles calculations

Liu Na-Na(刘娜娜), Song Ren-Bo(宋仁伯), and Du Da-Wei(杜大伟)

Chin. Phys. B, 2009, 18 (5): 1979-1984. DOI: 10.1088/1674-1056/18/5/041

Abstract ( 1403 )

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This paper stuides the elastic constants and some thermodynamic properties of Mg₂Si_xSn_1-x (x=0, 0.25, 0.5, 0.75, 1) compounds by first-principles total energy calculations using the pseudo-potential plane-waves approach based on density functional theory, within the generalized gradient approximation for the exchange and correlation potential. The elastic constants of Mg₂Si_xSn_1-x were calculated. It shows that, at 273 K, the elastic constants of Mg₂Si and Mg₂Sn are well consistent with previous experimental data. The isotropy decreases with increasing Sn content. The dependences of the elastic constants, the bulk modulus, the shear modulus and the Debye temperatures of Mg₂Si and Mg₂Si$_{0.5}$Sn$_{0.5}$ on pressure were discussed. Through the quasi-harmonic Debye model, in which phononic effects were considered, the specific heat capacities of Mg₂Si_xSn_1-x at constant volume and constant pressure were calculated. The calculated specific heat capacities are well consistent with the previous experimental data.

Phase--field model of isothermal solidification with multiple grains growth

Feng Li(冯力), Wang Zhi-Ping(王智平), Zhu Chang-Sheng(朱昌盛), and Lu Yang(路阳)

Chin. Phys. B, 2009, 18 (5): 1985-1990. DOI: 10.1088/1674-1056/18/5/042

Abstract ( 1295 )

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This paper develops a new phase--field model for equiaxed dendrite growth of multiple grains in multicomponent alloys based on the Ginzberg--Landau theory and phase--field model of a single grain. Taking Al--Cu and Al--Cu--Mg alloys for example, it couples the concentration field and simulates the dendrite growth process of multiple grains during isothermal solidification. The result of the simulation shows dendrite competitive growth of multiple grains, and is reapplied to the process of dendrite growth in practical solidification.

The stabilization effect of substituted atoms and magnetism for intermetallic compounds YFe_12-xCr_x

Ma Huan-Feng(麻焕锋), Huang Zheng(黄整), Chen Bo(陈波), Qiang Wei-Rong(强伟荣), and Pan Min(潘敏)

Chin. Phys. B, 2009, 18 (5): 1991-1995. DOI: 10.1088/1674-1056/18/5/043

Abstract ( 1349 )

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Based on density functional theory, using the scheme of the linearized augmented plane wave and the improved local orbital (APW+lo), this paper analyses and calculates the lattice parameters, formation energy and magnetism of the rare-earth permanent magnetic materials YFe_12-xCr_x (x=0～ 5). The stabilization effect and the possible positions of the substituted atom Cr are analysed. Magnetic moments for the system and the atoms are obtained.

Efficient frequency upshift of 10fs laser pulses propagating in a 5-cm length birefringent photonic crystal fibre

Li Shu-Guang(李曙光), Zhang Wei(张炜), Wei Zhi-Yi(魏志义), Zhou Gui-Yao(周桂耀), and Hou Lan-Tian(侯蓝田)

Chin. Phys. B, 2009, 18 (5): 1996-2001. DOI: 10.1088/1674-1056/18/5/044

Abstract ( 1245 )

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This paper reports that a 5-cm length birefringent photonic crystal fibre is used to tune the output frequency of unamplified 10-fs Ti:sapphire pulses. The zero dispersion of the fibre is at 823 nm and 800 nm for slow and fast fundamental modes, respectively. It is demonstrated that efficient upshift of the output frequency can be achieved when the pumped radiation is polarized along the slow axis of the fibre. When the average input power reaches 320 mW, about 60% of the output energy is located in one peak at 600 nm and is accompanied by depletion of the pulse inside the anomalous dispersion region.

Scattering behaviour of a two-dimensional electron gas induced by Al composition fluctuation in Al_xGa_1-xNbarrier in Al_xGa_1-xN/GaN heterostructures

Wang Yan(王彦), Shen Bo(沈波), Xu Fu-Jun(许福军), Huang Sen(黄森), Miao Zhen-Lin(苗振林), Lin Fang(林芳), Yang Zhi-Jian(杨志坚), and Zhang Guo-Yi(张国义)

Chin. Phys. B, 2009, 18 (5): 2002-2005. DOI: 10.1088/1674-1056/18/5/045

Abstract ( 1267 )

PDF (647KB) ( 544 )

This paper reports that cathodoluminescence (CL) measurements have been done to study the alloy fluctuation of the Al_0.3Ga_0.7N layer in Al_0.3Ga_0.7N/GaN heterostructures. The CL images and linescanning results demonstrate the existence of compositional fluctuation of Al in the Al_0.3Ga_0.7N barrier. A model using a $\delta$-shape perturbation Hamilton function has been proposed to simulate the scattering probability of the two dimensional electron gases (2DEG) induced by Al composition fluctuation. Two factors, including conduction band fluctuation and polarization electric field variation, induced by the Al composition fluctuation have been taken into account. The scattering relaxation time induced by both factors has been estimated to be 0.31 ns and 0.0078 ns, respectively, indicating that the variation of the piezoelectric field is dominant in the scattering of the 2DEG induced by Al fluctuation.

Symmetry ensemble theory of the spin wave emitting effect driven by current in nanoscale magnetic multilayer

Ren Min(任敏), Zhang Lei(张磊), Hu Jiu-Ning(胡九宁), Dong Hao(董浩), Deng Ning(邓宁), and Chen Pei-Yi(陈培毅)

Chin. Phys. B, 2009, 18 (5): 2006-2011. DOI: 10.1088/1674-1056/18/5/046

Abstract ( 1540 )

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This paper proposes a symmetry ensemble model for the magnetic dynamics caused by spin transfer torque in nanoscale pseudo-spin-valves, in which individual spin moments in the free layer are considered as subsystems to form a spinor ensemble. The magnetization dynamics equation of the ensemble was developed. By analytically investigating the equation, many magnetization dynamics properties excited by polarized current reported in experiments, such as double spin wave modes and the abrupt frequency jump, can be successfully explained. It is pointed out that an external field is not necessary for spin wave emitting (SWE) and a novel perpendicular configuration structure can provide much higher SWE efficiency in zero magnetic field.

Raman scattering of polycrystalline GaSb thin films grown by the co-evapouration process

Qiao Zai-Xiang(乔在祥), Sun Yun(孙云), He Wei-Yu(何炜瑜), Liu Wei(刘玮), He Qing(何青), and Li Chang-Jian(李长健)

Chin. Phys. B, 2009, 18 (5): 2012-2015. DOI: 10.1088/1674-1056/18/5/047

Abstract ( 1423 )

PDF (299KB) ( 878 )

This paper reports that GaSb thin films have been co-deposited on soda-lime glass substrates. The GaSb thin film structural properties are characterized by Raman spectroscopy. The Sb--A_1g/GaSb--TO ratio decreases rapidly with the increase of substrate temperature, which suggests a small amount of crystalline Sb in the GaSb thin film and suggests that Sb atoms in the thin film decrease. In Raman spectra, the transverse optical (TO) mode intensity is stronger than that of the longitudinal optical (LO) mode, which indicates that all the samples are disordered. The LO/TO intensity ratio increases with increasing substrate temperature which suggests the improved polycrystalline quality of the GaSb thin film. A downshift of the TO and LO frequencies of the polycrystalline GaSb thin film to single crystalline bulk GaSb Raman spectra is also observed. The uniaxial stress in GaSb thin film is calculated and the value is around 1.0 GPa. The uniaxial stress decreases with increasing substrate temperature. These results suggest that a higher substrate temperature is beneficial in relaxing the stress in GaSb thin film.

Controlled growth and field emission of vertically aligned AlN nanostructures with different morphologies

Liu Fei(刘飞), Su Zan-Jia(苏赞加), Liang Wei-Jie(梁炜杰), Mo Fu-Yao(莫富尧), Li Li(李力), Deng Shao-Zhi(邓少芝), Chen Jun(陈军), and Xu Ning-Sheng(许宁生)

Chin. Phys. B, 2009, 18 (5): 2016-2023. DOI: 10.1088/1674-1056/18/5/048

Abstract ( 1419 )

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The controllable growth of three different morphologies of AlN nanostructures (nanorod, nanotip and nanocrater) arrays are successfully realized by using chemical vapour deposition (CVD) technology. All three nanostructures are of single crystal h-AlN with a growth orientation of [001]. Their growth is attributed to the vapour-liquid-solid (VLS) mechanism. To investigate the factors affecting field emission (FE) properties of AlN nanostructures, we compare their FE behaviours in several aspects. Experimental results show that AlN nanocrater arrays possess the best FE properties, such as a threshold field of 7.2~V/μm and an emission current fluctuation lower than 4%. Moreover, the three AlN nanostructures all have good field emission properties compared with a number of other excellent cathode nanomaterials, which suggests that they are future promising FE nanomaterials.

Thermal effect and energy-level transition rule for a mesoscopic LC circuit with inductance--capacitance coupling

Su Jie(苏杰), Wang Ji-Suo(王继锁), Liang Bao-Long(梁宝龙), and Zhang Xiao-Yan(张晓燕)

Chin. Phys. B, 2009, 18 (5): 2024-2029. DOI: 10.1088/1674-1056/18/5/049

Abstract ( 1355 )

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This paper reports that the mesoscopic inductance and capacitance coupling LC circuit is quantized by means of the canonical quantization method. Using the 'invariant eigen-operator' method, it deduces the energy-level transition rule when the system is disturbed by an external electromagnetic field. At the same time, the quantum fluctuations for the system at finite temperature are examined by virtue of the generalized Hellmann--Feynman theorem.

Persistent currents in three-dimensional shell-doped nanorings

Xu Ning(徐宁), Ding Jian-Wen(丁建文), Chen Hong-Bo(陈宏波), and Ma Ming-Ming(马明明)

Chin. Phys. B, 2009, 18 (5): 2030-2034. DOI: 10.1088/1674-1056/18/5/050

Abstract ( 1214 )

PDF (608KB) ( 409 )

The persistent current in three-dimensional ($P\times N^2)$ nanorings as a function of the unit cell number ($P)$, the channel number ($M = N^2)$, surface disorder ($\xi )$, and temperature ($T)$ is theoretically investigated in terms of rotational symmetry. On the whole, the typical current increases linearly with $\sqrt M $ but decreases exponentially with $P$, while wide fluctuations exist therein. In the presence of surface disorder, the persistent current decreases with $\xi $ in the regime of weak disorder but increases in the regime of strong disorder. In addition, it is found that the persistent current in perfect rings decreases exponentially with temperature even at $T < T^*$, while in most disorder rings, the typical current decreases slightly with temperature at $T < T^*$.

Different effects of grain boundary scattering on charge and heat transport in polycrystalline platinum and gold nanofilms ^Hot!

Ma Wei-Gang(马维刚), Wang Hai-Dong(王海东), Zhang Xing(张兴), and Takahashi Koji

Chin. Phys. B, 2009, 18 (5): 2035-2040. DOI: 10.1088/1674-1056/18/5/051

Abstract ( 1315 )

PDF (547KB) ( 723 )

The in-plane electrical and thermal conductivities of several polycrystalline platinum and gold nanofilms with different thicknesses are measured in a temperature range between the boiling point of liquid nitrogen (77K) and room temperature by using the direct current heating method. The result shows that both the electrical and thermal conductivities of the nanofilms reduce greatly compared with their corresponding bulk values. However, the electrical conductivity drop is considerably greater than the thermal conductivity drop, which indicates that the influence of the internal grain boundary on heat transport is different from that of charge transport, hence leading to the violation of the Wiedemann--Franz law. We build an electron relaxation model based on Matthiessen's rule to analyse the thermal conductivity and employ the Mayadas & Shatzkes theory to analyse the electrical conductivity. Moreover, a modified Wiedemann--Franz law is provided in this paper, the obtained results from which are in good agreement with the experimental data.

Electronic properties of hydrogen- and oxygen-terminated diamond surfaces exposed to the air

Liu Feng-Bin(刘峰斌), Wang Jia-Dao(汪家道), Chen Da-Rong(陈大融), and Yan Da-Yun(颜大运)

Chin. Phys. B, 2009, 18 (5): 2041-2047. DOI: 10.1088/1674-1056/18/5/052

Abstract ( 1382 )

PDF (844KB) ( 1790 )

The electronic properties of hydrogen- and oxygen-terminated diamond surfaces exposed to the air are investigated by scanning probe microscopy (SPM). The results indicate that for the hydrogen-terminated diamond surface a shallow acceptor above the valence-band-maximum (VBM) appears in the band gap. However, the oxygen-terminated diamond film exhibits a high resistivity with a wide band gap. Based on the density-functional-theory, the densities of states, corresponding to molecular adsorbate in hydrogenated and oxygenated diamond (100) surfaces, are studied. The results show that the shallow acceptor in the band gap for the hydrogen-terminated diamond film can be attributed to the interaction between the surface C--H bonding orbitals and the adsorbate molecules, while for the oxygen-terminated diamond film, the interaction between the surface C--O bonding orbitals and the adsorbate molecules can induce occupied states in the valence-band.

Monte Carlo study of the antiferromagnetical Ising model on a centred honeycomb lattice

Wang Zhou-Fei(王宙斐) and Chen Li(陈莉)

Chin. Phys. B, 2009, 18 (5): 2048-2053. DOI: 10.1088/1674-1056/18/5/053

Abstract ( 1528 )

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We use the Monte Carlo method to study an antiferromagnetical Ising spin system on a centred honeycomb lattice, which is composed of two kinds of 1/2 spin particles A and B. There exist two different bond energies $J_{\rm A - A}$ and $J_{\rm A - B}$ in this lattice. Our study is focused on how the ratio of $J_{\rm A - B}$ to $J_{\rm A - A }$ influences the critical behaviour of this system by analysing the physical quantities, such as the energy, the order parameter, the specific heat, susceptibility, {etc} each as a function of temperature for a given ratio of $J_{\rm A - B}$ to $J_{\rm A - A}$. Using these results together with the finite-size scaling method, we obtain a phase diagram for the ratio $J_{\rm A - B}$ / $J_{\rm A - A}$. This work is helpful for studying the phase transition problem of crystals composed of compounds.

Growth and transport features of electron-doped superconductor Pr_1-xLaCe_xCuO_4-$\delta$ thin films ^Hot!

Gao Li-Juan(高丽娟), Jin Kui(金魁), Zhao Li(赵力), Wu Bin-Xin(吴彬新), Li Wei-Yong(李位勇), Zhu Bei-Yi(朱北沂), Cao Li-Xin(曹立新), Xu Bo(许波), Qiu Xiang-Gang(邱详冈), and Zhao Bai-Ru(赵柏儒)

Chin. Phys. B, 2009, 18 (5): 2054-2057. DOI: 10.1088/1674-1056/18/5/054

Abstract ( 1440 )

PDF (429KB) ( 553 )

T$'$-phase electron-doped superconductor Pr$_{1 -x}$LaCe$_{x}$CuO$_{4 - \delta }$ (PLCCO) thin films are successfully prepared on SrTiO$_{3}$ (100) substrates by using the dc magnetron sputtering method. It is found that the films each have a highly oriented structure along the $c$-axis. For optimally doped films with $x \approx 0.10$, the superconducting transition temperature $T_{\rm c}$ is 23.5 K, which is similar to that of a single crystal. The quadratic temperature dependence of the resistivity is observed when $T>T_{\rm c}$, which can be attributed to the two-dimensional Fermi liquid behaviour. Besides, the optimal conditions for preparing the T$'$-phase PLCCO thin films are also discussed in detail.

Effect of R substitution on magnetic properties and magnetocaloric effects of La_1-xR_xFe_11.5Si_1.5 compounds with R=Ce, Pr and Nd

Shen Jun(沈俊), Li Yang-Xian(李养贤), Sun Ji-Rong(孙继荣), and Shen Bao-Gen(沈保根)

Chin. Phys. B, 2009, 18 (5): 2058-2062. DOI: 10.1088/1674-1056/18/5/055

Abstract ( 1556 )

PDF (246KB) ( 713 )

Magnetic properties and magnetocaloric effects of La$_{1 - x}R_{x}$Fe$_{11.5}$Si$_{1.5}$ ($R$=Pr, ($0 \le x \le 0.5$); $R$ = Ce and Nd, ($0 \le x \le 0.3$)) compounds are investigated. Partially replacing La with $R$ = Ce, Pr and Nd in La$_{1 - x}R_{x}$Fe$_{11.5}$Si$_{1.5}$ leads to a reduction in Curie temperature due to the lattice contraction. The substitution of $R$ for La causes an enhancement in field-induced itinerant electron metamagnetic transition, which leads to a remarkable increase in magnetic entropy change $\Delta S_{\rm m}$ and also in hysteresis loss. However, a high effective refrigerant capacity $RC_{\rm eff}$ is still maintained in La$_{1 - x}R_{x}$Fe$_{11.5}$Si$_{1.5}$. In the present samples, a large $\Delta S_{\rm m}$ and a high $RC_{\rm eff}$ have been achieved simultaneously.

Microwave properties and surface topography of soft magnetic films with high resistivity

Ma Qiang(马强), Jiang Jian-Jun(江建军), Bie Shao-Wei(别少伟), Tian Bin(田斌), Liang Pei(梁培), and He Hua-Hui(何华辉)

Chin. Phys. B, 2009, 18 (5): 2063-2067. DOI: 10.1088/1674-1056/18/5/056

Abstract ( 1215 )

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A single layer of CoFeB and a multilayer of CoFeB--MgO films are prepared by means of DC/RF magnetron sputter deposition. The excellent microwave properties and high electrical resistivity are simultaneously achieved in the discontinuous multilayer structure of [Co$_{44}$Fe$_{44}$B$_{12}$(0.7 nm)/MgO(0.4 nm)]$_{40}$ film. This film has a high permeability ($\mu \prime $) (larger than 100 below 2.1 GHz), a high magnetic loss ($\mu''$) (larger than 100 in a range from 1.5 to 3.3 GHz), a resistivity of 3.3$\times 10^{3}\mu \Omega \cdot$ cm, a saturation magnetization of 1.2 T, and an in-plane uniaxial anisotropy field of 5.5 kA/m. The microstructure and the surface topography of the film are also analysed. The relatively large surface roughness for the discontinuous film is responsible for the wide frequency band of magnetic loss. The magnetic loss and the high resistivity indicate that the discontinuous CoFeB--MgO multilayer film has potential applications in microwave absorbers and electromagnetic interference (EMI) shielding materials in a GHz frequency range.

Luminescence characteristics of Eu²⁺ activated Ca₂SiO₄ Sr₂SiO₄ and Ba₂SiO₄ phosphorsfor white LEDs

Wang Zhi-Jun(王志军), Yang Zhi-Ping(杨志平), Guo Qing-Lin(郭庆林), Li Pan-Lai(李盼来), and Fu Guang-Sheng(傅广生)

Chin. Phys. B, 2009, 18 (5): 2068-2071. DOI: 10.1088/1674-1056/18/5/057

Abstract ( 1679 )

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This paper investigates the luminescence characteristics of Eu²⁺ activated Ca₂SiO₄, Sr₂SiO₄ and Ba₂SiO₄ phosphors. Two emission bands are assigned to the f--d transitions of Eu²⁺ ions doped into two different cation sites in host lattices, and show different emission colour variation caused by substituting M²⁺ cations for smaller cations. This behaviour is discussed in terms of two competing factors of the crystal field strength and covalence. These phosphors with maximum excitation of around 370nm can be applied as a colour-tunable phosphor for light-emitting diodes (LEDs) based on ultraviolet chip/phosphor technology.

Evolution of native point defects in ZnO bulk probed by positron annihilation spectroscopy

Peng Cheng-Xiao(彭成晓), Wang Ke-Fan(王科范), Zhang Yang(张杨), Guo Feng-Li(郭凤丽), Weng Hui-Min(翁惠民), and Ye Bang-Jiao(叶邦角)

Chin. Phys. B, 2009, 18 (5): 2072-2077. DOI: 10.1088/1674-1056/18/5/058

Abstract ( 1314 )

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This paper studies the evolution of native point defects with temperature in ZnO single crystals by positron lifetime and coincidence Doppler broadening (CDB) spectroscopy, combined with the calculated results of positron lifetime and electron momentum distribution. The calculated and experimental results of the positron lifetime in ZnO bulk ensure the presence of zinc monovacancy, and zinc monovacancy concentration begins to decrease above 600℃ annealing treatment. CDB is an effective method to distinguish the elemental species, here we combine this technique with calculated electron momentum distribution to determine the oxygen vacancies, which do not trap positrons due to their positive charge. The CDB spectra show that oxygen vacancies do not appear until 600℃ annealing treatment, and increase with the increase of annealing temperature. This study supports the idea that green luminescence has a close relation with oxygen vacancies.

Field electron emission from bunchy flake-like nano-carbon films

Wang Xiao-Ping(王小平), Wang Li-Jun(王丽军), Duan Xin-Chao(段新超), Wang Long-Yang(王隆洋), Zhang Lei(张雷), Lv Cheng-Rui (吕承瑞), and Lei Tong(雷通)

Chin. Phys. B, 2009, 18 (5): 2078-2081. DOI: 10.1088/1674-1056/18/5/059

Abstract ( 1161 )

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This paper reports that bunchy flake-like nano-graphite crystallite films (BNGCFs) were deposited on Si substrates by using the microwave chemical vapour deposition technique. Furthermore the BNGCFs were characterized by x-ray diffraction spectra, scanning electron microscopy, Raman spectra and field emission (FE) $I$--$V$ measurements, and a lowest turn-on field of 1.5 V/$\mu $m, and a high average emission current density of 30 mA/cm$^{2}$ at a macroscopic electric field of 8.0 V/$\mu $m were obtained. The $J$--$E$ data did not follow the original Fowler--Nordheim (F--N) relation since they were not well represented in the F--N plot by a straight line. A model considering the F--N mechanism, and the statistic effects of FE tip structures has been applied successfully to explain all the FE data observed for $E<8.8$ V/$\mu $m.

Stochasticity in the yeast mating pathway

Wang Hong-Li(王宏利), Fu Zheng-Ping(傅正平), Xu Xin-Hang(徐昕航), and Ouyang Qi(欧阳颀)

Chin. Phys. B, 2009, 18 (5): 2082-2095. DOI: 10.1088/1674-1056/18/5/060

Abstract ( 1331 )

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We report stochastic simulations of the yeast mating signal transduction pathway. The effects of intrinsic and external noise, the influence of cell-to-cell difference in the pathway capacity, and noise propagation in the pathway have been examined. The stochastic temporal behaviour of the pathway is found to be robust to the influence of inherent fluctuations, and intrinsic noise propagates in the pathway in a uniform pattern when the yeasts are treated with pheromones of different stimulus strengths and of varied fluctuations. In agreement with recent experimental findings, extrinsic noise is found to play a more prominent role than intrinsic noise in the variability of proteins. The occurrence frequency for the reactions in the pathway are also examined and a more compact network is obtained by dropping most of the reactions of least occurrence.

Effects of optical interference and annealing on the performances of poly (3-hexylthiophene): fullerene based solar cells

You Hai-Long(游海龙) and Zhang Chun-Fu(张春福)

Chin. Phys. B, 2009, 18 (5): 2096-2100. DOI: 10.1088/1674-1056/18/5/061

Abstract ( 1449 )

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In this paper, the effects of optical interference and annealing on the performance of P3HT:PCBM based organic solar cells are studied in detail. Due to the optical interference effect, short circuit current density (J_SC) shows obvious oscillatory behaviour with the variation of active layer thickness. With the help of the simulated results, the devices are optimized around the first two optical interference peaks. It is found that the optimized thicknesses are 80 and 208 nm. The study on the effect of annealing on the performance indicates that post-annealing is more favourable than pre-annealing. Based on post-annealing, different annealing temperatures are tested. The optimized annealing condition is 160$^{o}$C for 10 min in a nitrogen atmosphere. The device shows that the open circuit voltage V_OC achieves about 0.65V and the power conversion efficiency is as high as 4.0% around the second interference peak.

Ion pickup by the intrinsic low-frequency Alfvén waves with a spectrum

Lu Quan-Ming(陆全明), Li Xing(李醒), and Dong Chuan-Fei(董川飞)

Chin. Phys. B, 2009, 18 (5): 2101-. DOI: 10.1088/1674-1056/18/5/062

Abstract ( 1243 )

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Ion pickup by a monochromatic low-frequency Alfvén wave, which propagates along the background magnetic field, has recently been investigated in a low beta plasma (Lu and Li 2007 Phys. Plasmas 14 042303). In this paper, the monochromatic Alfvén wave is generalized to a spectrum of Alfvén waves with random phase. It finds that the process of ion pickup can be divided into two stages. First, ions are picked up in the transverse direction, and then phase difference (randomization) between ions due to their different parallel thermal motions leads to heating of the ions. The heating is dominant in the direction perpendicular to the background magnetic field. The temperatures of the ions at the asymptotic stage do not depend on individual waves in the spectrum, but are determined by the total amplitude of the waves. The effect of the initial ion bulk flow in the parallel direction on the heating is also considered in this paper.

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