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Chin. Phys. B  
  Chin. Phys. B--2007, Vol.16, No.2
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Eavesdropping on the `ping-pong' quantum communication protocol freely in a noise channel

Deng Fu-Guo, Li Xi-Han, Li Chun-Yan, Zhou Ping, Zhou Hong-Yu
Chin. Phys., 2007, 16 (2): 277-281 doi: 10.1088/1009-1963/16/2/001
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We introduce an attack scheme for eavesdropping freely the ping-pong quantum communication protocol proposed by Bostr\"{o} m and Felbinger [Phys. Rev. Lett. 89, 187902 (2002)] in a noise channel. The vicious eavesdropper, Eve, intercepts and measures the travel photon transmitted between the sender and the receiver. Then she replaces the quantum signal with a multi-photon signal in the same state, and measures the returned photons with the measuring basis, with which Eve prepares the fake signal except for one photon. This attack increases neither the quantum channel losses nor the error rate in the sampling instances for eavesdropping check. It works for eavesdropping the secret message transmitted with the ping-pong protocol. Finally, we propose a way for improving the security of the ping-pong protocol.

A novel configuration model for random graphs with given degree sequence

Xu Xin-Ping, Liu Feng
Chin. Phys., 2007, 16 (2): 282-286 doi: 10.1088/1009-1963/16/2/002
Full Text: [PDF 123 KB] (Downloads:561)
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Recently, random graphs in which vertices are characterized by hidden variables controlling the establishment of edges between pairs of vertices have attracted much attention. This paper presents a specific realization of a class of random network models in which the connection probability between two vertices (i,j) is a specific function of degrees ki and kj. In the framework of the configuration model of random graphs, we find the analytical expressions for the degree correlation and clustering as a function of the variance of the desired degree distribution. The obtained expressions are checked by means of numerical simulations. Possible applications of our model are discussed.

A realistic model for complex networks with local interaction,self-organization and order

Chen Fei, Chen Zeng-Qiang, Yuan Zhu-Zhi
Chin. Phys., 2007, 16 (2): 287-291 doi: 10.1088/1009-1963/16/2/003
Full Text: [PDF 333 KB] (Downloads:545)
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In this paper, a new mechanism for the emergence of scale-free distribution is proposed. It is more realistic than the existing mechanism. Based on our mechanism, a model responsible for the scale-free distribution with an exponent in a range of 3-to-5 is given. Moreover, this model could also reproduce the exponential distribution that is discovered in some real networks. Finally, the analytical result of the model is given and the simulation shows the validity of our result.

A Birkhoff-Noether method of solving differential equations

Shang Mei, Guo Yong-Xin, Mei Feng-Xiang
Chin. Phys., 2007, 16 (2): 292-295 doi: 10.1088/1009-1963/16/2/004
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In this paper, a Birkhoff--Noether method of solving ordinary differential equations is presented. The differential equations can be expressed in terms of Birkhoff's equations. The first integrals for differential equations can be found by using the Noether theory for Birkhoffian systems. Two examples are given to illustrate the application of the method.

A new high-order spectral problem of the mKdV and its associated integrable decomposition

Ji Jie, Yao Yu-Qin, Yu Jing, Liu Yu-Qing
Chin. Phys., 2007, 16 (2): 296-302 doi: 10.1088/1009-1963/16/2/005
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A new approach to formulizing a new high-order matrix spectral problem from a normal 2× 2 matrix modified Korteweg--de Vries (mKdV) spectral problem is presented. It is found that the isospectral evolution equation hierarchy of this new higher-order matrix spectral problem turns out to be the well-known mKdV equation hierarchy. By using the binary nonlinearization method, a new integrable decomposition of the mKdV equation is obtained in the sense of Liouville. The proof of the integrability shows that r-matrix structure is very interesting.

Lie symmetry analysis and reduction of a new integrable coupled KdV system

Qian Su-Ping, Tian Li-Xin
Chin. Phys., 2007, 16 (2): 303-309 doi: 10.1088/1009-1963/16/2/006
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In this paper, Lie symmetry is investigated for a new integrable coupled Korteweg--de Vries (KdV) equation system. Using some symmetry subalgebra of the equation system, we obtain five types of the significant similarity reductions. Abundant solutions of the coupled KdV equation system, such as the solitary wave solution, exponential solution, rational solution and polynomial solution, etc. are obtained from the reduced equations. Especially, one type of group-invariant solution of reduced equations can be acquired by means of the Painlev\'e I transcendent function.

New travelling wave solutions for combined KdV-mKdV equation and (2+1)-dimensional Broer-Kaup-Kupershmidt system

Yang Xian-Lin, Tang Jia-Shi
Chin. Phys., 2007, 16 (2): 310-317 doi: 10.1088/1009-1963/16/2/007
Full Text: [PDF 114 KB] (Downloads:777)
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Some new exact solutions of an auxiliary ordinary differential equation are obtained, which were neglected by Sirendaoreji {\it et al in their auxiliary equation method. By using this method and these new solutions the combined Korteweg--de Vries (KdV) and modified KdV (mKdV) equation and (2+1)-dimensional Broer--Kaup--Kupershmidt system are investigated and abundant exact travelling wave solutions are obtained that include new solitary wave solutions and triangular periodic wave solutions.

Entanglement-enhanced classical communication through generalized amplitude damping channel

Hou Li-Zhen, Fang Mao-Fa
Chin. Phys., 2007, 16 (2): 318-323 doi: 10.1088/1009-1963/16/2/008
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The problem of sending a single classical bit through a generalized amplitude damping channel is considered. When two transmissions through the channel are available as a resource, we find that two entangled transmissions can enhance the capability of receiver's judging information correctly under certain conditions compared with two product-state transmissions. In addition, we find a special case in which the two entangled transmissions can always make a classical bit more effectively disable the noise influence.

Ground-state entanglement in a three-spin transverse Ising model with energy current

Zhang Yong, Liu Dan, Long Gui-Lu
Chin. Phys., 2007, 16 (2): 324-328 doi: 10.1088/1009-1963/16/2/009
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The ground-state entanglement associated with a three-spin transverse Ising model is studied. By introducing an energy current into the system, a quantum phase transition to energy-current phase may be presented with the variation of external magnetic field; and the ground-state entanglement varies suddenly at the critical point of quantum phase transition. In our model, the introduction of energy current makes the entanglement between any two qubits become maximally robust.

Teleportation of two-atom entangled state in resonant cavity quantum electrodynamics

Yang Zhen-Biao
Chin. Phys., 2007, 16 (2): 329-334 doi: 10.1088/1009-1963/16/2/010
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An alternative scheme is presented for teleportation of a two-atom entangled state in cavity quantum electrodynamics (QED). It is based on the resonant atom--cavity field interaction. In the scheme, only one cavity is involved, and the number of the atoms needed to be detected is decreased compared with the previous scheme. Since the resonant atom--cavity field interaction greatly reduces the interaction time, the decoherence effect can be effectively suppressed during the teleportation process. The experimental feasibility of the scheme is discussed. The scheme can easily be generalized to the teleportation of N-atom Greeninger--Horne--Zeilinger (GHZ) entangled states. The number of atoms needed to be detected does not increase as the number of the atoms in the GHZ state increases.

The structure and dynamics of water inside armchair carbon nanotube

Zhou Xiao-Yan, Lu Hang-Jun
Chin. Phys., 2007, 16 (2): 335-339 doi: 10.1088/1009-1963/16/2/011
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In this paper we present some simulation results about the behaviour of water molecules inside a single wall carbon nanotube (SWNT). We find that the confinement of water in an SWNT can induce a wave-like pattern distribution along the channel axis, similar phenomena are also observed in biological water channels. Carbon nanotubes(CNTs) can serve as simple nonpolar water channels. Molecular transport through narrow CNTs is highly collective because of tight hydrogen bonds in the protective environment of the pore. The hydrogen bond net is important for proton and other signal transports. The average dipoles of water molecules inside CNTs (7,7), (8,8) and (9,9) are discussed in detail. Simulation results indicate that the states of dipole are affected by the diameter of SWNT. The number of hydrogen bonds, the water--water interaction and water--CNT interaction are also studied in this paper.

Evidence of parameter-induced aperiodic stochastic resonance with fixed noise

Li Jian-Long
Chin. Phys., 2007, 16 (2): 340-345 doi: 10.1088/1009-1963/16/2/012
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Stochastic resonance (SR) is based on the cooperative effect between the stochastic dynamical system and the external forcing. As is well known, the cooperative effect is produced by adding noises. In this paper, we show the evidence that by changing the system parameters and the signal intensity, a nonlinear system in the presence of an input aperiodic signal can yield the cooperative effect, with the noise fixed. To quantify the nonlinear system output, we determine the theoretical bit error rate (BER). By numerical simulation, the validity of the theoretical derivation is checked. Besides, we show that parameter-induced SR is more realizable than SR via adding noises, especially when the noise intensity exceeds the resonance level, or when the characteristic of the noise is not known.

Synchronization of chaotic systems with different orders

Lü Ling, Luan Ling, Guo Zhi-An
Chin. Phys., 2007, 16 (2): 346-351 doi: 10.1088/1009-1963/16/2/013
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A controller is designed to realize the synchronization between chaotic systems with different orders. The structure of the controller, the error equations and the Lyapunov functions are determined based on stability theory. Hyperchaotic Chen system and Rossler system are taken for example to demonstrate the method to be effective and feasible. Simulation results show that all the state variables of Rossler system can be synchronized with those of hyperchaotic Chen system by using only one controller, and the error signals approach zero smoothly and quickly.

Least mean square error difference minimum criterion for adaptive chaotic noise canceller

Zhang Jia-Shu
Chin. Phys., 2007, 16 (2): 352-358 doi: 10.1088/1009-1963/16/2/014
Full Text: [PDF 295 KB] (Downloads:580)
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The least mean square error difference (LMS-ED) minimum criterion for an adaptive chaotic noise canceller is proposed in this paper. Different from traditional least mean square error minimum criterion in which the error is uncorrelated with the input vector, the proposed LMS-ED minimum criterion tries to minimize the correlation between the error difference and input vector difference. The novel adaptive LMS-ED algorithm is then derived to update the weights of adaptive noise canceller. A comparison between cancelling performances of adaptive least mean square (LMS), normalized LMS (NLMS) and proposed LMS-ED algorithms is simulated by using three kinds of chaotic noises. The simulation results clearly show that the proposed algorithm outperforms the LMS and NLMS algorithms in achieving small values of steady-state excess mean square error. Moreover, the computational complexity of the proposed LMS-ED algorithm is the same as that of the standard LMS algorithms.

Energy-optimal control model for train movements

Li Ke-Ping, Gao Zi-You, Mao Bao-Hua
Chin. Phys., 2007, 16 (2): 359-364 doi: 10.1088/1009-1963/16/2/015
Full Text: [PDF 184 KB] (Downloads:1128)
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In this paper, we propose a new cellular automaton (CA) model for train movement simulations under mixed traffic conditions. A kind of control strategy is employed for trains to reduce energy consumption. In the proposed CA model, the driver controls the train movements by using some updated rules. In order to obtain a good insight into the evolution behaviours of the rail traffic flow, we investigate the space--time diagram of the rail traffic flow and the trajectories of the train movements. The numerical simulation results demonstrate that the proposed CA model can well describe the dynamic behaviours of the train movements. Some complex phenomena of train movements can be reproduced, such as the train delay propagations, etc.

Multi-fractal analysis of highway traffic data

Shang Peng-Jian, Shen Jin-Sheng
Chin. Phys., 2007, 16 (2): 365-373 doi: 10.1088/1009-1963/16/2/016
Full Text: [PDF 241 KB] (Downloads:605)
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The purpose of the present study is to investigate the presence of multi-fractal behaviours in the traffic time series not only by statistical approaches but also by geometrical approaches. The pointwise H\"{o} lder exponent of a function is calculated by developing an algorithm for the numerical evaluation of H\"{o} lder exponent of time series. The traffic time series observed on the Beijing Yuquanying highway are analysed. The results from all these methods indicate that the traffic data exhibit the multi-fractal behaviour.

Coherence vortices of partially coherent beams in the far field

Liu Pu-Sheng, Lü Bai-Da
Chin. Phys., 2007, 16 (2): 411-415 doi: 10.1088/1009-1963/16/2/021
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Based on the propagation law of cross-spectral density function, studied in this paper are the coherence vortices of partially coherent, quasi-monochromatic singular beams with Gaussian envelope and Schell-model correlator in the far field, where our main attention is paid to the evolution of far-field coherence vortices into intensity vortices of fully coherent beams. The results show that, although there are usually no zeros of intensity in partially coherent beams with Gaussian envelope and Schell-model correlator, zeros of spectral degree of coherence exist. The coherence vortices of spectral degree of coherence depend on the relative coherence length, mode index and positions of pairs of points. If a point and mode index are kept fixed, the position of coherence vortices changes with the increase of the relative coherence length. For the low coherent case there is a circular phase dislocation. In the coherent limit coherence vortices become intensity vortices of fully coherent Laguerre--Gaussian beams.

Quantum state transfer via adiabatic passage

Chen Mei-Ying, Lin Xiu-Min, Chen Zhi-Hua
Chin. Phys., 2007, 16 (2): 416-419 doi: 10.1088/1009-1963/16/2/022
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Based on adiabatic passage, we propose a scheme for implementing the quantum transfer of an unknown atomic state. In our scheme, we utilize photons for ideal quantum transmission between two cavities with the successful probability being about 1. Meanwhile, the scheme is robust against the effects of atomic spontaneous emission. It may be useful for transferring quantum information among spatially distant atoms.

Entropy squeezing of a moving atom and control of noise of the quantum mechanical channel via the two-photon process

Zhou Bing-Ju, Liu Xiao-Juan, Zhou Qing-Ping, Liu Ming-Wei
Chin. Phys., 2007, 16 (2): 420-428 doi: 10.1088/1009-1963/16/2/023
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Based on the quantum information theory, we have investigated the entropy squeezing of a moving two-level atom interacting with the coherent field via the quantum mechanical channel of the two-photon process. The results are compared with those of atomic squeezing based on the Heisenberg uncertainty relation. The influences of the atomic motion and field-mode structure parameter on the atomic entropy squeezing and on the control of noise of the quantum mechanical channel via the two-photon process are examined. Our results show that the squeezed period, duration of optimal entropy squeezing of a two-level atom and the noise of the quantum mechanical channel can be controlled by appropriately choosing the atomic motion and the field-mode structure parameter, respectively. The quantum mechanical channel of two-photon process is an ideal channel for quantum information (atomic quantum state) transmission. Quantum information entropy is a remarkably accurate measure of the atomic squeezing.

Enhancement of Kerr nonlinearity with vanishing absorption in a tripod scheme

Sun Hui, Niu Yue-Ping, Gong Shang-Qing
Chin. Phys., 2007, 16 (2): 429-434 doi: 10.1088/1009-1963/16/2/024
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The giant enhancement of Kerr nonlinearity in a four-level tripod type system is investigated theoretically. By tuning the value of the Rabi frequency of the coherent control field, owing to the double dark resonances, the giant-enhanced Kerr nonlinearity can be achieved within the right transparency window. The influence of Doppler broadening is also discussed.

Engineering three-dimensional maximally entangled states for two modes in a bimodal cavity

Yang Zhen-Biao, Su Wan-Jun
Chin. Phys., 2007, 16 (2): 435-440 doi: 10.1088/1009-1963/16/2/025
Full Text: [PDF 118 KB] (Downloads:398)
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An alternative scheme is proposed for engineering three-dimensional maximally entangled states for two modes of a superconducting microwave cavity. In this scheme, an appropriately prepared four-level atom is sent through a bimodal cavity. During its passing through the cavity, the atom is coupled resonantly with two cavity modes simultaneously and addressed by a classical microwave pulse tuned to the required transition. Then the atomic states are detected to collapse two modes onto a three-dimensional maximally entangled state. The scheme is different from the previous one in which two nonlocal cavities are used. A comparison between them is also made.

Phase-dependent inversionless gain in a four-level atomic system with a closed interaction loop

Xu Wei-Hua, Wu Jin-Hui, Gao Jin-Yue
Chin. Phys., 2007, 16 (2): 441-448 doi: 10.1088/1009-1963/16/2/026
Full Text: [PDF 181 KB] (Downloads:544)
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A four-level atomic system with a closed interaction loop connected by two coherent driving fields and a microwave field is investigated. The results show that inversionless gain can be achieved on a higher frequency transition outside the closed interaction loop, and the gain behaviour can be modulated by the phase of the closed loop as well as the amplitude of the microwave field. The phase sensitivity property in such a scheme is similar to that in an analogous configuration with spontaneously generated coherence, but it is beyond the rigorous condition of near-degenerate levels with non-orthogonal dipole moments. Therefore this scheme is much more convenient in experimental realization.

Generation and propagation of subpicosecond pulse train

Zhang Hua-Feng, Wang Juan-Fen, Li Lu, Jia Suo-Tang, Zhou Guo-Sheng
Chin. Phys., 2007, 16 (2): 449-455 doi: 10.1088/1009-1963/16/2/027
Full Text: [PDF 290 KB] (Downloads:505)
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Higher-order nonlinear Schr\"{o}dinger equation with the Hirota constraint conditions is considered, and an analytic solution, which can describe the modulational instability process, is presented. Based on the solution, a new pulse train without continuous wave (CW) background is generated in quadratures and the propagation of the pulse train is discussed in detail by simulating numerically. The results show that, unlike the propagation of the picosecond pulse train, under the effects of the higher-order terms, the pulse train cannot propagate along the fibre when the energy is very high; however, for some medium energy the pulse train can stably propagate. We also investigate the stability of the pulse train against violation of the Hirota conditions, and the results show that the pulse train can still propagate stably when the Hirota conditions are broken.

Intense laser beam guiding in self-induced electron cavitation channel in underdense plasmas

Cang Yu, Yu Wei, Wu Hui-Chun, Xu Han, Tian You-Wei
Chin. Phys., 2007, 16 (2): 456-462 doi: 10.1088/1009-1963/16/2/028
Full Text: [PDF 179 KB] (Downloads:546)
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In underdense plasmas, the transverse ponderomotive force of an intense laser beam with Gaussian transverse profile expels electrons radially, and it can lead to an electron cavitation. An improved cavitation model with charge conservation constraint is applied to the determination of the width of the electron cavity. The envelope equation for laser spot size derived by using source-dependent expansion method is extended to including the electron cavity. The condition for self-guiding is given and illuminated by an effective potential for the laser spot size. The effects of the laser power, plasma density and energy dissipation on the self-guiding condition are discussed.

Chaos synchronization in injection-locked semiconductor lasers with optical feedback

Liu Yu-Jin, Zhang Sheng-Hai, Qian Xing-Zhong
Chin. Phys., 2007, 16 (2): 463-467 doi: 10.1088/1009-1963/16/2/029
Full Text: [PDF 273 KB] (Downloads:716)
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Based on the rate equations, we have investigated three types of chaos synchronizations in injection-locked semiconductor lasers with optical feedback. Numerical simulation shows that the synchronization can be realized by the symmetric or asymmetric laser systems. Also, the influence of parameter mismatches on chaos synchronization is investigated, and the results imply that these two lasers can achieve good synchronization, with smaller tolerance of parameter mismatch existing.

Improving nucleation in the fabrication of high-quality 3D macro-porous copper film through the surface-modification of a polystyrene colloid-assembled template

Lan Ding, Wang Yu-Ren, Yu Yong, Ma Wen-Jie, Li Cheng
Chin. Phys., 2007, 16 (2): 468-471 doi: 10.1088/1009-1963/16/2/030
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A new approach is developed to the fabrication of high-quality three-dimensional macro-porous copper films. A highly-ordered macroporous copper film is successfully produced on a polystyrene sphere (PS) template that has been modified by sodium dodecyl sulfate (SDS). It is shown that this procedure can change a hydrophobic surface of PS template into a hydrophilic surface. The present study is devoted to the influence of the electrolyte solution transport on the nucleation process. It is demonstrated that the permeability of the electrolyte solution in the nanochannels of the PS template plays an important role in the chemical electrodeposition of high-quality macroporous copper film. The permeability is drastically enhanced in our experiment through the surface modification of the PS templates. The method could be used to homogeneously produce a large number of nucleations on a substrate, which is a key factor for the fabrication of the high-quality macroporous copper film.

Accurate determination of the absolute phase and temporal-pulse phase of few-cycle laser pulses

Xia Ke-Yu, Gong Shang-Qing, Niu Yue-Ping, Li Ru-Xin, Xu Zhi-Zhan
Chin. Phys., 2007, 16 (2): 472-477 doi: 10.1088/1009-1963/16/2/031
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A Fourier analysis method is used to accurately determine not only the absolute phase but also the temporal-pulse phase of an isolated few-cycle (chirped) laser pulse. This method is independent of the pulse shape and can fully characterize the light wave even though only a few samples per optical cycle are available. It paves the way for investigating the absolute phase-dependent extreme nonlinear optics, and the evolutions of the absolute phase and the temporal-pulse phase of few-cycle laser pulses.

Design and fabrication of Panda-type erbium-doped polarization-maintaining fibres

Zheng Kai, Chang De-Yuan, Fu Yong-Jun, Wei Huai, Wei Yan, Yan Feng-Ping, Lou Shu-Qin, Ning Ti-Gang, Jian Wei, Jian Shui-Sheng
Chin. Phys., 2007, 16 (2): 478-484 doi: 10.1088/1009-1963/16/2/032
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To obtain the stable operation of erbium-doped fibre laser, the simple and ideal technology is adopted by use of the erbium doped polarization maintaining fibre (EDPMF). The design criteria of the Panda-type EDPMF are presented, which take into account the cutoff wavelength, mode field diameter, modal birefringence and background loss. Four groups of optimum structural parameter combinations are determined in terms of the design criteria. Two kinds of the Panda-type EDPMFs are selected to be fabricated. The fabrication process and the parameter control of the Panda-type EDPMFs are presented in detail. Their refractive index profiles, birefringence and absorption spectra are experimentally investigated. The absorption coefficient of the EDPMF, whose core is co-doped with Bi, Ga, Al and Ge, is about 57.9dB/m at 1.53\mum. Co-doping Bi, Ga and Al can greatly increase the erbium concentration in the silica-based fibre. The high birefringence is obtained for the Panda-type EDPMF. The group birefringence of the EDPMF, whose outer cladding diameter is 125\mum, is about 4.8\ti10-4.

Radiative decay from doubly to singly excited states of He via generalization of Laguerre-type orbitals: A non-orthogonal formalism

Xiong Zhuang, Bacalis N C
Chin. Phys., 2007, 16 (2): 374-381 doi: 10.1088/1009-1963/16/2/017
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By taking full account of the non-orthogonality of the orbitals between the low-lying doubly excited states 1Po and the singly excited states 1Se and 12De of He, the corresponding radiative decay rates have been investigated theoretically via analytic generalized Laguerre-type atomic orbitals at a nearly numerical multi-configuration self-consistent field accuracy in a general non-orthogonal configuration interaction scheme. From these rates, we calculate the VUV photon emission and metastable atom spectra, and both are found to be in good qualitative agreement with recent excellent measurements. We obtain, successfully, the enhancement of the VUV photon spectrum, experimentally observed at the energy of (2s4p-4s2p)/(2p,3d) 1Po as compared with other nearby lying states. The mechanism proposed by Odling-Smee et al is verified, implying that taking appropriate account of the overlap existing between orbitals of the low-lying doubly excited and singly excited states (especially important for the compact orbitals) can reveal basic physical dominant mechanism and is crucial in understanding these spectra.

Saturation effects on Ba 6pnl (l=0,2) and 6pnk (|M|=0,1) autoionization spectra

Li Shi-Ben, Dai Chang-Jian
Chin. Phys., 2007, 16 (2): 382-391 doi: 10.1088/1009-1963/16/2/018
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Using a three-step laser saturation excitation technique, the saturation effects on the Ba 6pns (J=1) and 6pnd (J=1, 3) autoionization spectra are observed systemically in zero field. These saturation spectra are introduced to determine the high n members of 6pnl (l=0, 2) autoionizing series and are used to analyse the channel interactions among the autoionizing series in zero field. Furthermore, the saturation excitation technique is applied to the electric field case, in which the saturation spectra of Ba 6pnk (|M|=0, 1) autoionizing Stark states are measured. Most of these saturation spectra are observed for the first time so far as we know, which indicate the mixing of the autoionizing states in the electric fields.

Protein structural codes and nucleation sites for protein folding

Jiang Fan, Li Nan
Chin. Phys., 2007, 16 (2): 392-404 doi: 10.1088/1009-1963/16/2/019
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One of the long-standing controversial arguments in protein folding is Levinthal's paradox. We have recently proposed a new nucleation hypothesis and shown that the nucleation residues are the most conserved sequences in protein. To avoid the complicated effect of tertiary interactions, we limit our search for structural codes to the nucleation residues. Starting with the hypotheses of secondary structure nucleation and conservation of residues important for folding, we have analysed 762 folds classified as unique by SCOP. Segments of 17 residues around the top 20% conserved amino acids are analysed, resulting in approximately 100 clusters each for the main secondary structure classes of helix, sheet and coil. Helical clusters have the longest correlation range, coils the shortest (four residues). Strong specific sequence-structure correlation is observed for coil but not for helix and sheet, suggesting a mapping relationship between the sequence and the structure for coil. We propose that the central sequences in these clusters form `structural codes', a useful basis set for identifying nucleation sites, protein fragments stable in isolation, and secondary structural patterns in proteins (particularly turns and loops).

Molecular dynamics simulation of thermodynamical properties of copper clusters

Wu Zhi-Min, Wang Xin-Qiang, Yang Yuan-Yuan
Chin. Phys., 2007, 16 (2): 405-410 doi: 10.1088/1009-1963/16/2/020
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The melting and freezing processes of CuN (N=180, 256, 360, 408, 500, 628 and 736) nanoclusters are simulated by using micro-canonical molecular dynamics simulation technique. The potential energies and the heat capacities as a function of temperature are obtained. The results reveal that the melting and freezing points increase almost linearly with the atom number in the cluster increasing. All copper nanoclusters have negative heat capacity around the melting and freezing points, and hysteresis effect in the melting/freezing transition is derived in CuN nanoclusters for the first time.

Effect of RF power on the properties of transparent conducting zirconium-doped zinc oxide films prepared by RF magnetron sputtering

Lü Mao-Shui, Pang Zhi-Yong, Xiu Xian-Wu, Dai Ying, Han Sheng-Hao
Chin. Phys., 2007, 16 (2): 548-552 doi: 10.1088/1009-1963/16/2/044
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Transparent and conducting zirconium-doped zinc oxide films with high transparency and relatively low resistivity have been successfully prepared by radio frequency (RF) magnetron sputtering at room temperature. The RF power is varied from 75 to 150W. At first the crystallinity and conductivity of the film are improved and then both of them show deterioration with the increase of the RF power. The lowest resistivity achieved is 2.07×10-3\Omegacm at an RF power of 100W with a Hall mobility of 16cm2V-1s-1 and a carrier concentration of 1.95×1020cm-3. The films obtained are polycrystalline with a hexagonal structure and a preferred orientation along the c-axis. All the films have a high transmittance of approximately 92% in the visible range. The optical band gap is about 3.33eV for the films deposited at different RF powers.

Formation mechanism of incubation layers in the initial stage of microcrystalline silicon growth by PECVD

Hou Guo-Fu, Xue Jun-Ming, Guo Qun-Chao, Sun Jian, Zhao Ying, Geng Xin-Hua, Li Yi-Gang
Chin. Phys., 2007, 16 (2): 553-557 doi: 10.1088/1009-1963/16/2/045
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The incubation layers in microcrystalline silicon films (\muc-Si:H) are studied in detail. The incubation layers in \muc-Si:H films are investigated by bifacial Raman spectra, and the results indicate that either decreasing silane concentration (SC) or increasing plasma power can reduce the thickness of incubation layer. The analysis of the in-situ diagnosis by plasma optical emission spectrum (OES) shows that the emission intensities of the SiH*(412\,nm) and H_\al (656nm) lines are time-dependent, thus SiH*/H_\al ratio is of temporal evolution. The variation of SiH*/H_\al ratio can indicate the variation in relative concentration of precursor and atomic hydrogen in the plasma. And the atomic hydrogen plays a crucial role in the formation of \muc-Si:H; thus, with the plasma excited, the temporal-evolution SiH*/H_\al ratio has a great influence on the formation of an incubation layer in the initial growth stage. The fact that decreasing the SC or increasing the plasma power can decrease the SiH*/H_\al ratio is used to explain why the thickness of incubation layer can reduce with decreasing the SC or increasing the plasma power.

Simulation of radio-frequency atmospheric pressure glow discharge in γ mode

Shang Wan-Li, Wang De-Zhen, Michael G. Kong
Chin. Phys., 2007, 16 (2): 485-492 doi: 10.1088/1009-1963/16/2/033
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The existence of two different discharge modes has been verified in an rf (radio-frequency) atmospheric pressure glow discharge (APGD) by Shi [J. Appl. Phys. 97, 023306 (2005)]. In the first mode, referred to as \al mode, the discharge current density is relatively low and the bulk plasma electrons acquire the energy due to the sheath expansion. In the second mode, termed \ga mode, the discharge current density is relatively high, the secondary electrons emitted by cathode under ion bombardment in the cathode sheath region play an important role in sustaining the discharge. In this paper, a one-dimensional self-consistent fluid model for rf APGDs is used to simulate the discharge mechanisms in the \ga mode in helium discharge between two parallel metallic planar electrodes. The results show that as the applied voltage increases, the discharge current becomes greater and the plasma density correspondingly increases, consequentially the discharge transits from the \al mode into the \ga mode. The high collisionality of the APGD plasma results in significant drop of discharge potential across the sheath region, and the electron Joule heating and the electron collisional energy loss reach their maxima in the region. The validity of the simulation is checked with the available experimental and numerical data.

Anchoring properties of substrate with a grating surface

Ye Wen-Jiang, Xing Hong-Yu, Yang Guo-Chen
Chin. Phys., 2007, 16 (2): 493-498 doi: 10.1088/1009-1963/16/2/034
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The anchoring properties of substrate with a grating surface are investigated analytically. The alignment of nematic liquid crystal (NLC) in a grating surface originates from two mechanisms, thus the anchoring energy consists of two parts. One originates from the interaction potential between NLC molecules and the molecules on the substrate surface, and the other stems from the increased elastic strain energy. Based on the two mechanisms, the expression of anchoring energy per unit area of a projected plane of this grating surface is deduced and called the equivalent anchoring energy formula. Both the strength and the easy direction of equivalent anchoring energy are a function of the geometrical parameters (amplitude and pitch) of a grating surface. By using this formula, the grating surface can be replaced by its projected plane and its anchoring properties can be described by the equivalent anchoring energy formula.

Thermoelasticity of CaO from first principles

Liu Zi-Jiang, Qi Jian-Hong, Guo Yuan, Chen Qi-Feng, Cai Ling-Cang, Yang Xiang-Dong
Chin. Phys., 2007, 16 (2): 499-505 doi: 10.1088/1009-1963/16/2/035
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The thermoelastic properties of CaO over a wide range of pressure and temperature are studied using density functional theory in the generalized gradient approximation. The transition pressure taken from the enthalpy calculations is 66.7GPa for CaO, which accords with the experimental result very well. The athermal elastic moduli of the two phases of CaO are calculated as a function of pressure up to 200GPa. The calculated results are in excellent agreement with existing experimental data at ambient pressure and compared favourably with other pseudopotential predictions over the pressure regime studied. It is also found that the degree of the anisotropy rapidly decreases with pressure increasing in the B1 phase, whereas it strongly increases as the pressure increases in the B2 phase. The thermodynamic properties of the B1 phase of CaO are predicted using the quasi-harmonic Debye model; the heat capacity and entropy are consistent with other previous results at zero pressure.

Electronic structures and magnetism of Fe nanowires on Cu(001) and Ag(001): A first-principles study

Jin Ying-Jiu, Lin Jing-Bo, Lee Jae Il
Chin. Phys., 2007, 16 (2): 506-510 doi: 10.1088/1009-1963/16/2/036
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The electronic structures and magnetism of Fe nanowires along the [110] direction on Cu(001) and Ag(001) [Fe(nw)/Cu(001) and Fe(nw)/Ag(001)] are investigated by using the all-electron full-potential linearized augmented plane wave method in the generalized gradient approximation. It is found that the magnetic moment of Fe atom for the Fe(nw)/Cu(001) is 2.99\muB, which is slightly smaller than that (3.02\muB) for the Fe(nw)/Ag(001) but much larger than that (2.22\muB) for the bcc iron. The great enhancement of magnetic moment in the Fe nanowires can be explained by the Fe d-band narrowing and enhancement of the spin-splitting due to a reduction in coordination number. From the calculated spin-polarized layer-projected density of states, it is found that the Fe 3d-states are strongly hybridized with the adjacent Cu 3d-states in the Fe(nw)/Cu(001), and there exists a strong hybridization between the Fe sp- and the adjacent Ag 4d-states in the Fe(nw)/Ag(001).

A resistance formula for coherent multi-barrier structures

Zhang Zhi-Chen, Yang Jian-Hong
Chin. Phys., 2007, 16 (2): 511-516 doi: 10.1088/1009-1963/16/2/037
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Using the Landauer formula and the quantum S-matrix scattering theory, we derive a resistance formula for multi-barrier structure under phase coherent transmission condition. This formula shows that when the transport is coherent, the potential wells of the structure are just like conductors contributing to the overall resistance. And because the resistance formula is derived based on the scattering theory, the barrier resistance will change with the number of scattering centres (i.e. the number of barriers) in the structure.

Anomalous Hall effect of heavy holes in III-V semiconductor quantum wells

Wang Zhi-Gang, Zhang Ping
Chin. Phys., 2007, 16 (2): 517-523 doi: 10.1088/1009-1963/16/2/038
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The anomalous Hall effect of heavy holes in semiconductor quantum wells is studied in the intrinsic transport regime, where the Berry curvature governs the Hall current properties. Based on the first--order perturbation of wave function the expression of the Hall conductivity the same as that from the semiclassical equation of motion of the Bloch particles is derived. The dependence of Hall conductivity on the system parameters is shown. The amplitude of Hall conductivity is found to be balanced by a competition between the Zeeman splitting and the spin--orbit splitting.

Improvement of electrical properties of Cu/SiCOH low-k film integrated system by O2 plasma treatment

Qian Xiao-Mei, Wei Yong-Xia, Yu Xiao-Zhu, Ye Chao, Ning Zhao-Yuan, Liang Rong-Qing
Chin. Phys., 2007, 16 (2): 524-528 doi: 10.1088/1009-1963/16/2/039
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This paper investigates the effect of O2 plasma treatment on the electric property of Cu/SiCOH low dielectric constant (low-k) film integrated structure. The results show that the leakage current of Cu/SiCOH low-k integrated structure can be reduced obviously at the expense of a slight increase in dielectric constant k of SiCOH films. By the Fourier transform infrared (FTIR) analysis on the bonding configurations of SiCOH films treated by O2 plasma, it is found that the decrease of leakage current is related to the increase of Si-O cages originating from the linkage of Si dangling bonds through O, which makes the open pores sealed and reduces the diffusion of Cu to pores.

Electrical properties and reliability of HfO2 gate-dielectric MOS capacitors with trichloroethylene surface pretreatment

Xu Jing-Ping, Chen Wei-Bing, Lai Pui-To, Li Yan-Ping, Chan Chu-Lok
Chin. Phys., 2007, 16 (2): 529-532 doi: 10.1088/1009-1963/16/2/040
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Trichloroethylene (TCE) pretreatment of Si surface prior to HfO2 deposition is employed to fabricate HfO2 gate-dielectric MOS capacitors. Influence of this processing procedure on interlayer growth, HfO2/Si interface properties, gate-oxide leakage and device reliability is investigated. Among the surface pretreatments in NH3, NO, N2O and TCE ambients, the TCE pretreatment gives the least interlayer growth, the lowest interface-state density, the smallest gate leakage and the highest reliability. All these improvements should be ascribed to the passivation effects of Cl2 and HCl on the structural defects in the interlayer and at the interface, and also their gettering effects on the ion contamination in the gate dielectric.

Persistent spin current in a quantum wire with weak Dresselhaus spin--orbit coupling

Sheng Wei, Wang Yi, Zhou Guang-Hui
Chin. Phys., 2007, 16 (2): 533-536 doi: 10.1088/1009-1963/16/2/041
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The spin current in a parabolically confined semiconductor heterojunction quantum wire with Dresselhaus spin--orbit coupling is theoretically studied by using the perturbation method. The formulae of the elements for linear and angular spin current densities are derived by using the recent definition for spin current based on spin continuity equation. It is found that the spin current in this Dresselhaus spin--orbit coupling quantum wire is antisymmetrical, which is different from that in Rashba model due to the difference in symmetry between these two models. Some numerical examples for the result are also demonstrated and discussed.

Investigation of gate current in nano-scale MOSFETs by Monte Carlo solution of quantum Boltzmann equation

Xia Zhi-Liang, Du Gang, Liu Xiao-Yan, Kang Jin-Feng, Han Ru-Qi
Chin. Phys., 2007, 16 (2): 537-541 doi: 10.1088/1009-1963/16/2/042
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This paper investigates gate current through ultra-thin gate oxide of nano-scale metal oxide semiconductor field effect transistors (MOSFETs), using two-dimensional (2D) full-band self-consistent ensemble Monte Carlo method based on solving quantum Boltzmann equation. Direct tunnelling, Fowler--Nordheim tunnelling and thermionic emission currents have been taken into account for the calculation of total gate current. The 2D effect on the gate current is investigated by including the details of the energy distribution for electron tunnelling through the barrier. In order to investigate the properties of nano scale MOSFETs, it is necessary to simulate gate tunnelling current in 2D including non-equilibrium transport.

Effect of annealing treatments on the microwave electromagnetic properties of amorphous FeCuNbSiB microwires

Liang Di-Fei, Han Man-Gui, Yan Bo, Deng Long-Jiang
Chin. Phys., 2007, 16 (2): 542-547 doi: 10.1088/1009-1963/16/2/043
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The amorphous FeCuNbSiB microwires are fabricated by using the melt extraction method and annealed separately at temperatures T = 573, 673, 723 and 773K for 1h. The effect of annealing treatment on the microwave electromagnetic properties of FeCuNbSiB wires/wax composites has been investigated for the first time. It is found that in a frequency range of 0.5--4.0GHz, the complex permittivity, permeability, magnetic and electric loss tangents of FeCuNbSiB wires/wax composites are strongly dependent on the annealing temperature and frequency. For T = 573, 723 and 773K, two resonance peaks are found at frequency f = 1.2 and 3.3GHz. However, for T = 673K, only one resonance peak occurs at f = 3.3GHz. The resonance peak at f = 1.2GHz is believed to be due to the stress-induced anisotropy, while the resonance peak at f = 3.3GHz is attributed to the random anisotropy.

On the ponderomotive force and the effect of loss reaction on parametric instability

Wu Jun, Wu Jian, Cesar La Hoz
Chin. Phys., 2007, 16 (2): 558-563 doi: 10.1088/1009-1963/16/2/046
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In this paper, the growth rate, ponderomotive force and the exciting condition for parametric instability are derived by considering the loss reaction using a new method. On the basis of the hydrodynamic equations, we take the production and loss reactions in plasma into account to derive the coupling equations for the electron plasma oscillation and ion acoustic oscillation, and obtain the growth rate for the parametric instability, the ponderomotive force and the exciting condition. The result shows that (a) the production reaction has no effect on the parametric instability, and the effect of loss reaction on the parametric instability is a damping one, (b) the more intensive the external field or pump is, the larger the growth rate is, (c) there exist two modes of the ponderomotive force, i.e.\ the high frequency mode and the low frequency mode, and (d) when ponderomotive force counteracts the damping force, the oscillations become non-damping and non-driving. The ratio of the electron plasma oscillation to ion acoustic oscillation is independent of the loss reaction and the external field.
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