中国物理B

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Homotopic mapping solution of an oscillator for the El Niño La Niña-southern oscillation

周先春, 林一骅, 林万涛, 莫嘉琪

2009 (11): 4603-4607. doi: 10.1088/1674-1056/18/11/001

摘要 ( 1885 )

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This paper considers a class of oscillator for the El Ni?o/La Ni?a-southern oscillation (ENSO) model. By using the homotopic mapping method, it obtains approximations of the solution for the ENSO model.

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Approximation of the soliton solution for the generalized Vakhnenko equation

莫嘉琪

2009 (11): 4608-4612. doi: 10.1088/1674-1056/18/11/002

摘要 ( 1824 )

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A class of generalized Vakhnemko equation is considered. First, we solve the nonlinear differential equation by the homotopic mapping method. Then, an approximate soliton solution for the original generalized Vakhnemko equation is obtained. By this method an arbitrary order approximation can be easily obtained and, similarly, approximate soliton solutions of other nonlinear equations can be acquired.

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Zone inhomogeneity with the random asymmetric simple exclusion process in a one-lane system

肖松, 蔡九菊, 刘飞

2009 (11): 4613-4621. doi: 10.1088/1674-1056/18/11/003

摘要 ( 1575 )

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In this paper we use theoretical analysis and extensive simulations to study zone inhomogeneity with the random asymmetric simple exclusion process (ASEP). In the inhomogeneous zone, the hopping probability is less than 1. Two typical lattice geometries are investigated here. In case A, the lattice includes two equal segments. The hopping probability in the left segment is equal to 1, and in the right segment it is equal to p, which is less than 1. In case B, there are three equal segments in the system; the hopping probabilities in the left and right segments are equal to 1, and in the middle segment it is equal to p, which is less than 1. Through theoretical analysis, we can discover the effect on these systems when p is changed.

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Variable coefficient nonlinear systems derived from an atmospheric dynamical system

唐晓艳, 高原, 黄菲, 楼森岳

2009 (11): 4622-4635. doi: 10.1088/1674-1056/18/11/004

摘要 ( 2034 )

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Variable coefficient nonlinear systems, the Korteweg de Vries (KdV), the modified KdV (mKdV) and the nonlinear Schr?dinger (NLS) type equations, are derived from the nonlinear inviscid barotropic nondivergent vorticity equation in a beta-plane by means of the multi-scale expansion method in two different ways, with and without the so-called y-average trick. The non-auto-B\"acklund transformations are found to transform the derived variable coefficient equations to the corresponding standard KdV, mKdV and NLS equations. Thus, many possible exact solutions can be obtained by taking advantage of the known solutions of these standard equations. Further, many approximate solutions of the original model are ready to be yielded which might be applied to explain some real atmospheric phenomena, such as atmospheric blocking episodes.

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Conformal invariance and Noether symmetry, Lie symmetry of holonomic mechanical systems in event space

张毅

2009 (11): 4636-4642. doi: 10.1088/1674-1056/18/11/005

摘要 ( 1902 )

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This paper is devoted to studying the conformal invariance and Noether symmetry and Lie symmetry of a holonomic mechanical system in event space. The definition of the conformal invariance and the corresponding conformal factors of the holonomic system in event space are given. By investigating the relation between the conformal invariance and the Noether symmetry and the Lie symmetry, expressions of conformal factors of the system under these circumstances are obtained, and the Noether conserved quantity and the Hojman conserved quantity directly derived from the conformal invariance are given. Two examples are given to illustrate the application of the results.

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Conformal invariance and generalized Hojman conserved quantities of mechanico-electrical systems

李元成, 夏丽莉, 王小明

2009 (11): 4643-4649. doi: 10.1088/1674-1056/18/11/006

摘要 ( 1924 )

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This paper studies conformal invariance and generalized Hojman conserved quantities of mechanico-electrical systems. The definition and the determining equation of conformal invariance for mechanico-electrical systems are provided. The conformal factor expression is deduced from conformal invariance and Lie symmetry under the infinitesimal single-parameter transformation group. The generalized Hojman conserved quantities from the conformal invariance of the system are given. An example is given to illustrate the application of the result.

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Conformal invariance and conserved quantity of third-order Lagrange equations for non-conserved mechanical systems

张明江, 方建会, 路凯, 张克军, 李燕

2009 (11): 4650-4656. doi: 10.1088/1674-1056/18/11/007

摘要 ( 1803 )

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This paper studies conformal invariance and conserved quantity of third-order Lagrange equations for non-conserved mechanical systems. Third-order Lagrange equations, the definition and a determining equation of conformal invariance of the system are presented. The conformal factor expression is deduced from conformal invariance and Lie symmetry. The necessary and sufficient condition that conformal invariance of the system would have Lie symmetry under single-parameter infinitesimal transformations is obtained. The corresponding conserved quantity of conformal invariance is derived with the aid of a structure equation. Lastly, an example is given to illustrate the application of the results.

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Wigner function and density operator of the photon-subtracted squeezed thermal state

胡利云, 范洪义

2009 (11): 4657-4661. doi: 10.1088/1674-1056/18/11/008

摘要 ( 2105 )

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By using the technique of integration within an ordered product of operators, the normal ordered density operator of the photon-subtracted squeezed thermal state (PSSTS) is derived. Then the corresponding Wigner function is presented by using the coherent state representation of the Wigner operator. The nonclassical properties of the PSSTS are discussed based on the negativity of the Wigner function.

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Entanglement evolution of a two-qubit system with decay beyond the rotating-wave approximation

杨青, 杨名, 李大创, 曹卓良

2009 (11): 4662-4666. doi: 10.1088/1674-1056/18/11/009

摘要 ( 1462 )

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The entanglement property of two identical atoms, initially entangled in Bell states, coupled to a single-mode cavity is considered. Based on the reduced non-perturbative quantum master equation method, the entanglement evolution of the two atoms with decay is investigated beyond the conventional rotating-wave approximation. We show that the counter-rotating wave terms, usually neglected, have a great influence on the disentanglement behaviour of the system. The phenomena of entanglement sudden death and entanglement sudden birth will occur. In addition, we show that the entanglement can be strengthened by introducing the dipole--dipole interaction of the two atoms.

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Strain distributions and electronic structure of three-dimensional InAs/GaAs quantum rings

刘玉敏, 俞重远, 贾博雍, 徐子欢, 姚文杰, 陈智辉, 芦鹏飞, 韩利红

2009 (11): 4667-4675. doi: 10.1088/1674-1056/18/11/010

摘要 ( 1552 )

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This paper presents a finite element calculation for the electronic structure and strain distribution of self-organized InAs/GaAs quantum rings. The strain distribution calculations are based on the continuum elastic theory. An ideal three-dimensional circular quantum ring model is adopted in this work. The electron and heavy-hole energy levels of the InAs/GaAs quantum rings are calculated by solving the three-dimensional effective mass Schr?dinger equation including the deformation potential and piezoelectric potential up to the second order induced by the strain. The calculated results show the importance of strain and piezoelectric effects, and these effects should be taken into consideration in analysis of the optoelectronic characteristics of strain quantum rings.

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Entanglement sudden death of two atoms interacting with a cavity via the two-photon process in a strong-driving-assisted system

陈丽, 邵晓强, 张寿

2009 (11): 4676-4682. doi: 10.1088/1674-1056/18/11/011

摘要 ( 1641 )

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We examine the entanglement dynamics between two strongly driven atoms off-resonantly coupled with a single-mode cavity via the two-photon process with the help of negativity in two different types of initial states. The results show that entanglement sudden death may occur under both the above conditions and the sudden death effect can be monitored by modulating the atom--cavity detunings. Furthermore, we also find an atomic decoherence-free subspace so that the initial entanglement between two atoms remains invariable in application.

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Generation of an N-qubit phase gate via atom--cavity nonidentical coupling

张英俏, 张寿

2009 (11): 4683-4689. doi: 10.1088/1674-1056/18/11/012

摘要 ( 1476 )

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A scheme for approximate generation of an N-qubit phase gate is proposed in cavity QED based on nonidentical coupling between the atoms and the cavity. The atoms interact with a highly detuned cavity-field mode, but quantum information does not transfer between the atoms and cavity field, and thus the cavity decay is negligible. The gate time does not rise with an increase in the number of qubits. With the choice of a smaller odd number l (related to atom--cavity coupling constants), the phase gate can be generated with a higher fidelity and a higher success probability in a shorter time (the gate time is much shorter than the atomic radiative lifetime and photon lifetime). When the number of qubits N exceeds certain small values, the fidelity and success probability rise slowly with an increase in the number of qubits N. When N\rightarrow\infty, the fidelity and success probability infinitely approach 1, but never exceed 1.

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High-capacity three-party quantum secret sharing with superdense coding

顾斌, 李传起, 徐飞, 陈玉林

2009 (11): 4690-4694. doi: 10.1088/1674-1056/18/11/013

摘要 ( 1686 )

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This paper presents a scheme for high-capacity three-party quantum secret sharing with quantum superdense coding, following some ideas in the work by Liu et al (2002 Phys. Rev. A 65 022304) and the quantum secret sharing scheme by Deng et al (2008 Phys. Lett. A 372 1957). Instead of using two sets of nonorthogonal states, the boss Alice needs only to prepare a sequence of Einstein--Podolsky--Rosen pairs in d-dimension. The two agents Bob and Charlie encode their information with dense coding unitary operations, and security is checked by inserting decoy photons. The scheme has a high capacity and intrinsic efficiency as each pair can carry 2lbd bits of information, and almost all the pairs can be used for carrying useful information.

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Entanglement transfer between atoms in two distant cavities via an optical fibre

肖兴, 方卯发

2009 (11): 4695-4701. doi: 10.1088/1674-1056/18/11/014

摘要 ( 1642 )

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This paper presents a treatment of the entanglement transfer between atoms in two distant cavities coupled by an optical fibre. If the atoms resonantly and collectively interact with the local single-mode cavity fields and the dipole--dipole interaction between the atoms is neglected, then it shows that a complete transfer of entanglement from one pair of atoms to another can be deterministically realized. Furthermore, it also investigates the effects of dipole--dipole interaction on entanglement transfer on the condition that the interaction between the atoms and the cavity is much weaker than the coupling between the cavity and the fibre.

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Stokes identification in an atomic ensemble using a filtering system

罗霄鸣, 宁波, 陈丽清, 周玥, 钟志萍, 蒋硕

2009 (11): 4702-4706. doi: 10.1088/1674-1056/18/11/015

摘要 ( 1727 )

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Polarization filtering and atomic cell filtering are applied in the identification of Stokes signals in an atomic ensemble, and reduce the noise to a level of 10^ - 5 and 10^ - 4 respectively. Good Stokes signals are then obtained. In this article the two filtering systems and the final Stokes output are presented, and the optimization of the polarization filtering system is highlighted.

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Fidelity of an electron in one-dimensional determined potentials

宋文广, 童培庆

2009 (11): 4707-4710. doi: 10.1088/1674-1056/18/11/016

摘要 ( 1554 )

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We numerically study the fidelity of an electron in the one-dimensional Harper model and in the one-dimensional slowly varying potential model. Our results show that many properties of the two models can be well reflected by the fidelity: (i) the mobility edge and metal--insulator transition can be characterized by the static fidelity; (ii) the extended state and localized state can be identified by the dynamic fidelity. Therefore, it may broaden the applied areas of the fidelity.

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Preservation of quantum states via a super-Zeno effect on ensemble quantum computers

任婷婷, 罗军, 孙献平, 詹明生

2009 (11): 4711-4715. doi: 10.1088/1674-1056/18/11/017

摘要 ( 1582 )

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Following a recent proposal by Dhar et al (2006 Phys. Rev. Lett. 96 100405), we demonstrate experimentally the preservation of quantum states in a two-qubit system based on a super-Zeno effect using liquid-state nuclear magnetic resonance techniques. Using inverting radiofrequency pulses and delicately selecting time intervals between two pulses, we suppress the effect of decoherence of quantum states. We observe that preservation of the quantum state |11\rg with the super-Zeno effect is three times more efficient than the ordinary one with the standard Zeno effect.

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On particles tunneling from the Taub-NUT-AdS black hole

曾晓雄, 李强

2009 (11): 4716-4720. doi: 10.1088/1674-1056/18/11/018

摘要 ( 1762 )

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This paper discusses tunneling of scalar particles and Dirac particles from the Taub-NUT-AdS black hole by the Hamilton--Jacobi equation, initially used by Angheben et al, and the Dirac equation, recently proposed by Kerner and Mann. This is performed in the dragging coordinate frame so as to avoid the ergosphere dragging effect. A general form is obtained for the temperature of scalar and Dirac particles tunneling from the Taub-NUT-Ads black hole, which is commensurate with other methods as expected.

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Fermion tunneling of charged particles from a non-static black hole in de Sitter space

李慧玲, 杨树政

2009 (11): 4721-4725. doi: 10.1088/1674-1056/18/11/019

摘要 ( 1513 )

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Introducing a new coordinate system and choosing a set of appropriate matrices γ^μ , this paper attempts to investigate the fermion tunneling of charged particles across the event horizon from the Vaidya--Bonner de Sitter black hole. The result shows that the tunneling rate of the non-static black hole is related not only to the change of Bekenstein--Hawking entropy but also to the integral of the changing horizon, which violates unitary theory and is different from the stationary case.

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Effects of periodic optical lattice potential on dark solitons in a Bose Einstein condensate

李宏, 王东宁

2009 (11): 4726-4733. doi: 10.1088/1674-1056/18/11/020

摘要 ( 1585 )

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This paper investigates the dynamics of dark solitons in a Bose--Einstein condensate with a magnetic trap and an optical lattice (OL) trap, and analyses the effects of the periodic OL potential on the dynamics by applying the variational approach based on the renormalized integrals of motion. The results show that the dark soliton becomes only a standing-wave and free propagation of the dark soliton is not possible when the periodic length of the OL potential is approximately equal to the effective width of the dark soliton. When the periodic length is very small or very large, the effects of the OL potential on the dark soliton will be sharply reduced. Finally, the numerical results confirm these theoretical findings.

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Quantum backreaction of quantum fluid in Bose Einstein condensates

徐岩, 熊祖周, 陈兵, 李照鑫, 谭磊

2009 (11): 4734-4737. doi: 10.1088/1674-1056/18/11/021

摘要 ( 1593 )

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In this paper, with the full field operator \hat ψ expressed in terms of a particle-number-conserving mean-field ansatz, we investigate the dynamical behaviour of Bose--Einstein condensates from microscopic physics. Including the first-order term correction from single-particle excitation and the remaining higher-order term correction from collective excitations simultaneously, we obtain the formulation for a closed local expression of quantum backreaction Q, and discuss the influence on static Bose--Einstein condensates. Even though the quantum backreaction is small, it still has some influence on its dynamics.

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The nonequilibrium phase transition and the symmetry revival induced by time delay in an asymmetric bistable system with correlated noises

龙飞, 杜鲁春, 梅冬成

2009 (11): 4738-4741. doi: 10.1088/1674-1056/18/11/022

摘要 ( 1877 )

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The nonequilibrium phase transition and the symmetry revival induced by time delay in a bistable system are investigated. The stationary probability distribution function (SPDF) of the bistable system with time delay and correlated noises are calculated by an analytical method and stochastic simulation respectively. The analytical and simulative results indicate that: (1) There is a certain value of λ (λ denotes the strength of correlations between the multiplicative and additive noises) to make the SPDF symmetric under some time delay; however, above or below the given value, the symmetry will be broken; (2) With the monotonic change of λ , the unimodal peak structure of SPDF becomes bimodal at the beginning, then it becomes unimodal again; this means that there is a reentrance phenomenon in the process; (3) There is a critical value of delay time, which makes the lower peak of SPDF equal to the higher one under the critical condition. This means that the symmetry revival phenomenon emerges.

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Mode shift and stability control of a current mode controlled buck-boost converter operating in discontinuous conduction mode with ramp compensation

包伯成, 许建平, 刘中

2009 (11): 4742-4747. doi: 10.1088/1674-1056/18/11/023

摘要 ( 1870 )

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By establishing the discrete iterative mapping model of a current mode controlled buck-boost converter, this paper studies the mechanism of mode shift and stability control of the buck-boost converter operating in discontinuous conduction mode with a ramp compensation current. With the bifurcation diagram, Lyapunov exponent spectrum, time-domain waveform and parameter space map, the performance of the buck-boost converter circuit utilizing a compensating ramp current has been analysed. The obtained results indicate that the system trajectory is weakly chaotic and strongly intermittent under discontinuous conduction mode. By using ramp compensation, the buck-boost converter can shift from discontinuous conduction mode to continuous conduction mode, and effectively operates in the stable period-one region.

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Amplitude control of limit cycle in a van der Pol Duffing system

欧阳克俭, 唐驾时, 梁翠香

2009 (11): 4748-4753. doi: 10.1088/1674-1056/18/11/024

摘要 ( 1797 )

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This paper applies washout filter technology to amplitude control of limit cycles emerging from Hopf bifurcation of the van der Pol--Duffing system. The controlling parameters for the appearance of Hopf bifurcation are given by the Routh--Hurwitz criteria. Noticeably, numerical simulation indicates that the controllers control the amplitude of limit cycles not only of the weakly nonlinear van der Pol--Duffing system but also of the strongly nonlinear van der Pol--Duffing system. In particular, the emergence of Hopf bifurcation can be controlled by a suitable choice of controlling parameters. Gain-amplitude curves of controlled systems are also drawn.

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Cascading failures in congested complex networks with feedback

郑建风, 高自友, 傅白白, 李峰

2009 (11): 4754-4759. doi: 10.1088/1674-1056/18/11/025

摘要 ( 1579 )

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In this article, we investigate cascading failures in complex networks by introducing a feedback. To characterize the effect of the feedback, we define a procedure that involves a self-organization of trip distribution during the process of cascading failures. For this purpose, user equilibrium with variable demand is used as an alternative way to determine the traffic flow pattern throughout the network. Under the attack, cost function dynamics are introduced to discuss edge overload in complex networks, where each edge is assigned a finite capacity (controlled by parameter α). We find that scale-free networks without considering the effect of the feedback are expected to be very sensitive to α as compared with random networks, while this situation is largely improved after introducing the feedback.

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Network traffic prediction by a wavelet-based combined model

孙韩林, 金跃辉, 崔毅东, 程时端

2009 (11): 4760-4768. doi: 10.1088/1674-1056/18/11/026

摘要 ( 1657 )

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Network traffic prediction models can be grouped into two types, single models and combined ones. Combined models integrate several single models and thus can improve prediction accuracy. Based on wavelet transform, grey theory, and chaos theory, this paper proposes a novel combined model, wavelet--grey--chaos (WGC), for network traffic prediction. In the WGC model, we develop a time series decomposition method without the boundary problem by modifying the standard \grave\rm a trous algorithm, decompose the network traffic into two parts, the residual part and the burst part to alleviate the accumulated error problem, and employ the grey model GM(1,1) and chaos model to predict the residual part and the burst part respectively. Simulation results on real network traffic show that the WGC model does improve prediction accuracy.

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An efficient numerical approach to electrostatic microelectromechanical system simulation

李普

2009 (11): 4769-4776. doi: 10.1088/1674-1056/18/11/027

摘要 ( 1498 )

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Computational analysis of electrostatic microelectromechanical systems (MEMS) requires an electrostatic analysis to compute the electrostatic forces acting on micromechanical structures and a mechanical analysis to compute the deformation of micromechanical structures. Typically, the mechanical analysis is performed on an undeformed geometry. However, the electrostatic analysis is performed on the deformed position of microstructures. In this paper, a new efficient approach to self-consistent analysis of electrostatic MEMS in the small deformation case is presented. In this approach, when the microstructures undergo small deformations, the surface charge densities on the deformed geometry can be computed without updating the geometry of the microstructures. This algorithm is based on the linear mode shapes of a microstructure as basis functions. A boundary integral equation for the electrostatic problem is expanded into a Taylor series around the undeformed configuration, and a new coupled-field equation is presented. This approach is validated by comparing its results with the results available in the literature and ANSYS solutions, and shows attractive features comparable to ANSYS.

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Design and gamma sensitivity measurement of a novel dual-emitter vacuum Compton detector

韩和同, 王群书, 夏良斌, 管兴胤, 张子川

2009 (11): 4777-4780. doi: 10.1088/1674-1056/18/11/028

摘要 ( 1644 )

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A novel dual-emitter vacuum Compton detector (D-VCD) with higher gamma ray detecting efficiency is proposed. The emitters are made of Ta--Al clad metal. The gamma ray sensitivity is studied by Monte Carlo simulation using the MCNP code. A comparison between calculations and results measured by using the 1.25~MeV gamma ray of Co-60 is also performed. Experimental sensitivities for two sample D-VCDs with the same materials and structures are 1.92×10^ - 20 and 2.02×10^ - 20~C.cm²/MeV separately, which are consistent with the simulation result of 1.98×10^ - 20~C.cm²/MeV and are 4 times higher than that of VCD with a single Fe emitter. According to the simulation results, in a gamma energy range from 0.5 to 3~MeV, the maximum sensitivity variance for the D-VCD is less than 15%, and less than 5% in a range from 1 to 2~MeV in particular. The novel D-VCD is applicable to the detection of intense pulse gamma rays.

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Disappearance of isospin effect in projectile fragmentation at intermediate energy

马春旺, 魏慧玲, 王俊阳, 刘高杰

2009 (11): 4781-4785. doi: 10.1088/1674-1056/18/11/029

摘要 ( 1577 )

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The 140~MeV/u ^40,48Ca+^9Be and ^58,64Ni+^9Be reactions are simulated by the statistical abrasion ablation model, and the simulation results are compared to the National Superconducting Cyclotron Laboratory (NSCL) experimental data. By comparing the fragment isotopic distributions of ^40,48Ca and ^58,64Ni, we study the isospin effect in the projectile fragmentation induced by the neutron-rich nuclei at intermediate energy experimentally and theoretically. It is found that the isospin effect in projectile fragmentation decreases and even disappears as the violence of the collision increases.

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Effects of bulk viscosity on hadron spectra and the Hanbury-Brown Twiss radius by causal viscous hydrodynamics

李建伟, 马余刚, 马国亮

2009 (11): 4786-4790. doi: 10.1088/1674-1056/18/11/030

摘要 ( 1503 )

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The particle spectra and Hanbury-Brown Twiss (HBT) radius of Au+Au collisions at RHIC energy are investigated by a hydrodynamical expanding source with both shear and bulk viscosities ($\zeta$). With a large width of the ratio of $\zeta$ to entropy density s, both the particle transverse momentum spectra and the ratio R_\rm out/R_\rm side of HBT radii in the direction of the total transverse momentum of detected two particles (R_\rm out) and perpendicular to both this direction and the beam direction (R_\rm side) become a little steeper.

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Scaled-energy spectroscopy of a M= 1 Rydberg barium atom in an electric field

汪磊, 全威, 沈礼, 杨海峰, 史庭云, 柳晓军, 刘红平, 詹明生

2009 (11): 4791-4796. doi: 10.1088/1674-1056/18/11/031

摘要 ( 1480 )

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We observe strong energy-dependent quantum defects in the scaled-energy Stark spectra for |M|=1 Rydberg states of barium atoms at three scaled energies: ε=-2.000, ε=-2.500 and ε=-3.000. In an attempt to explain the observations, theoretical calculations of closed orbit theory based on a model potential including core effect are performed for non-hydrogenic atoms. While such a potential has been uniformly successful for alkali atoms with a single valence electron, it fails to match experimental results for barium atoms in the 6sNp Rydberg states with two valence electrons. Our study points out that this discrepancy is due to the strong perturbation from the 5d8p state, which voids the simple approximation for constant quantum defects of principle quantum number N.

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Apparent diffusion behaviour of intermolecular double-quantum coherence modulated by a distant dipolar field in solution NMR

沈桂平, 蔡聪波, 蔡淑惠, 陈忠

2009 (11): 4797-4806. doi: 10.1088/1674-1056/18/11/032

摘要 ( 1665 )

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A modified correlated spectroscopy (COSY) revamped with asymmetric Z-gradient echo detection sequence was designed to investigate the influence of diffusion behaviour on intermolecular double-quantum coherence signal attenuation during the pre-acquisition period. Theoretical formulas were deduced and experimental measurements and simulations were performed. It is found that the diffusion behaviour of intermolecular double-quantum coherence in the pre-acquisition period may be different from that of conventional single-quantum coherence, depending on the relative orientation of diffusion weighting gradients to coherence selection gradients. When the orientation of the diffusion weighting gradients is parallel or anti-parallel to the orientation of the coherence selection gradients, the diffusion is modulated by the distant dipolar field. This study is helpful for understanding the signal properties in intermolecular double-quantum coherence magnetic resonance imaging.

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Ionization of atoms by chirped attosecond pulses

谭放, 彭良友, 龚旗煌

2009 (11): 4807-4814. doi: 10.1088/1674-1056/18/11/033

摘要 ( 1762 )

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We investigate the ionization dynamics of atoms by chirped attosecond pulses using the strong field approximation method. The pulse parameters are carefully chosen in the regime where the strong field approximation method is valid. We analyse the effects of the chirp of attosecond pulses on the energy distributions and the corresponding left-right asymmetry of the ionized electrons. For a single chirped attosecond pulse, the ionized electrons can be redistributed and the left-right asymmetry shows oscillations because of the introduction of the chirp. For time-delayed double attosecond pulses at different intensities with the weaker one chirped, exchanging the order of the two pulses shows a relative shift of the energy spectra, which can be explained by the different effective time delays of different frequency components because of the chirp.

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Jet-like structures in photoelectron angular distributions

王懿, 张敬涛, 任向河, 徐至展

2009 (11): 4815-4822. doi: 10.1088/1674-1056/18/11/034

摘要 ( 1543 )

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The photoelectron angular distributions (PADs) of hydrogen atoms in an intense laser field of linear polarization are studied using the S-matrix theory in the length gauge. The PADs show main lobes along the laser polarization and jet-like structures sticking from the waist of main lobes. Our previous prediction, based on a nonperturbative scattering theory of photoionization developed by Guo et al, showing that the number of jets on one side of PADs may increase by one, three, or other odd numbers and may decrease by one when one more photon is absorbed, is confirmed by this treatment. Within the strong-field approximation, good agreement is obtained between these two quite different treatments. We further study the influence of the Coulomb attraction to PADs, by taking a Coulomb--Volkov state as the continuum state of photoelectrons. We find that under the influence of the Coulomb attraction, the PADs change greatly but the predicted phenomena still appear. This study verifies that the jet-like structures have no relation with the angular momentum of photoelectrons.

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Dynamic splitting and merging of an atom cloud on an atom chip

柯敏, 颜波, 程峰, 王育竹

2009 (11): 4823-4828. doi: 10.1088/1674-1056/18/11/035

摘要 ( 1672 )

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Chip-based atom interferometers bring together the advantages of atom chips and Bose--Einstein condensates. Their central prerequisite is that a condensate can be coherently split into two halves with a determined relative phase. This paper demonstrates the dynamical splitting and merging of an atom cloud with two U-wires on an atom chip. Symmetrical and asymmetrical splittings are realized by applying a bias field with dif\/ferent directions and magnitudes. The trajectories of the splitting are consistent with theoretical calculations. The atom chip is a good candidate for constructing an atom interferometer.

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M??ssbauer study of the field induced uniaxial anisotropy in electro-deposited FeCo alloy films

李志伟, 杨旭, 王海波, 刘忻, 李发伸

2009 (11): 4829-4833. doi: 10.1088/1674-1056/18/11/036

摘要 ( 1539 )

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Thin ferromagnetic films with in-plane magnetic anisotropy are promising materials for obtaining high microwave permeability. The paper reports a M?ssbauer study of the field induced in-plane uniaxial anisotropy in electro-deposited FeCo alloy films. The FeCo alloy films were prepared by the electro-deposition method with and without an external magnetic field applied parallel to the film plane during deposition. Vibrating sample magnetometry and M?ssbauer spectroscopy measurements at room temperature indicate that the film deposited in external field shows an in-plane uniaxial anisotropy with an easy direction coinciding with the external field direction and a hard direction perpendicular to the field direction, whereas the film deposited without external field does not show any in-plane anisotropy. M?ssbauer spectra taken in three geometric arrangements show that the magnetic moments are almost constrained in the film plane for the film deposited with applied magnetic field. Also, the magnetic moments tend to align in the direction of the applied external magnetic field during deposition, indicating that the observed anisotropy should be attributed to directional ordering of atomic pairs.

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Interference of dissociating wave packets in the I₂ molecule driven by femtosecond laser pulses

韩永昌, 胡文辉, 于杰, 丛书林

2009 (11): 4834-4839. doi: 10.1088/1674-1056/18/11/037

摘要 ( 1589 )

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The interference between two dissociating wave packets of the I₂ molecule driven by femtosecond laser pulses is theoretically studied by using the time-dependent quantum wave packet method. Both the internuclear distance- and velocity-dependent density functions are calculated and discussed. It is demonstrated that the interference pattern is determined by the phase difference and the delay time between two pump pulses. With two identical pulses with a delay time of 305~fs and a FWHM of 20~fs, more interference fringes can be observed, while with two pump pulses with a delay time of 80~fs and a FWHM of 20~fs, only a few interference fringes can be observed.

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Triatomic wake effect and the determination of the molecular structure of HD₂⁺ from the Coulomb explosion

朱洲森, 缪竞威, 廖雪花, 缪蕾, 袁学东, 师免恭

2009 (11): 4840-4845. doi: 10.1088/1674-1056/18/11/038

摘要 ( 1679 )

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A new theoretical model of the triatomic molecular wake effect is proposed and applied to molecular ions D₃⁺ and HD₂⁺ while passing through a solid. The wake effects resulting from the reactions of the two similar ions with thin carbon foil are also investigated by using the Coulomb explosion technique. The experimental results are in good agreement with theoretical estimates and the molecular structure of HD₂⁺ is determined by using the model.

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High order Correlation polarization potential for vibrational excitation scattering of diatomic molecules by low-energy electrons

冯灏, 孙卫国, 曾阳阳

2009 (11): 4846-4851. doi: 10.1088/1674-1056/18/11/039

摘要 ( 1615 )

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This paper introduces a correlation--polarization potential with high order terms for vibrational excitation in electron--molecule scattering. The new polarization potential generalizes the two-term approximation so that it can better reflect the dependence of correlation and polarization effects on the position coordinate of the scattering electron. It applies the new potential on the vibrational excitation scattering from N₂ in an energy range which includes the ²П_g shape resonance. The good agreement of theoretical resonant peaks with experiments shows that polarization potentials with high order terms are important and should be included in vibrational excitation scattering.

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Modelling of a DNA packaging motor

钱钧, 谢平, 薛晓光, 王鹏业

2009 (11): 4852-4864. doi: 10.1088/1674-1056/18/11/040

摘要 ( 1781 )

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During the assembly of many viruses, a powerful molecular motor packages the genome into a preassembled capsid. The Bacillus subtilis phage φ 29 is an excellent model system to investigate the DNA packaging mechanism because of its highly efficient \textitin vitro DNA packaging activity and the development of a single-molecule packaging assay. Here we make use of structural and biochemical experimental data to build a physical model of DNA packaging by the φ 29 DNA packaging motor. Based on the model, various dynamic behaviours such as the packaging rate, pause frequency and slip frequency under different ATP concentrations, ADP concentrations, external loads as well as capsid fillings are studied by using Monte Carlo simulation. Good agreement is obtained between the simulated and available experimental results. Moreover, we make testable predictions that should guide future experiments related to motor function.

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Tapered dielectric structure in metal as a wavelength-selective surface plasmon polariton focuser

张洋, 赵清, 廖志敏, 俞大鹏

2009 (11): 4865-4869. doi: 10.1088/1674-1056/18/11/041

摘要 ( 1577 )

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Symmetric tapered dielectric structures in metal have demonstrated applications such as the nanofocusing of surface plasmon polaritons, as well as the waveguiding of V-channel polaritons. Yet the fabrication of smooth-surfaced tapered structure remains an obstacle to most researchers. We have successfully developed a handy method to fabricate metal-sandwiched tapered nanostructures simply with electron beam lithography. Though these structures are slightly different from conventional symmetric V-shaped structures, systematic simulations show that similar functionality of surface plasmon polariton nanofocusing can still be achieved. When parameters are properly selected, wavelength-selective nanofocusing of surface plasmon polaritons can be obtained.

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Analysis of waveguide structure for surface plasmon polariton interference

王景全, 梁慧敏, 方亮, 李敏, 牛晓云, 杜惊雷

2009 (11): 4870-4874. doi: 10.1088/1674-1056/18/11/042

摘要 ( 1813 )

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This paper describes a multi-reflected mode based on a narrow waveguide to enlarge the interferential area of surface plasmon polaritons (SPPs). A reasonable thickness of metal film is coated under the waveguide, the incident angle and the waveguide thickness are optimized in order to effectively increase interferential area. This is a key point for research into the Goos--H\"anchen shift to optimize the waveguide thickness. Finally, the SPP interferential field is simulated with the finite-difference time-domain (FDTD) technique to prove the optimized results, and indicates that not only is the interferential area enlarged, but the high contrast is also maintained. Furthermore, the mode can fabricate some specific interferential patterns by adding some modulating techniques to the waveguide. So the mode has potential application in the fabrication of sub-wavelength patterns.

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Investigation of the limit of lateral beam shifts on a symmetrical metal-cladding waveguide

陈麟, 朱亦鸣, 张大伟, 曹庄琪, 庄松林

2009 (11): 4875-4880. doi: 10.1088/1674-1056/18/11/043

摘要 ( 1702 )

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This paper reports that Goos--H\"anchen (GH) shifts occurring on a symmetrical metal-cladding waveguide are experimentally identified. It was found that there exists a critical thickness of the upper metal layer, h_cr, above which negative shift is observed and, reversely, positive shift occurs. Both positive and negative GH shifts near the critical thickness do not vary dramatically and can achieve a maximum on the submillimeter scale, which is different from simulated results using the stationary-phase method. It also shows that this critical thickness, h_cr, can be obtained at the position for zero reflectivity by setting the intrinsic damping to be the same as the radiative damping. The GH effects observed near the critical thickness are produced by extreme distortion of the reflected beam profiles, which limits the amplitude of the GH shift and, further, the sensitivity of the GH optical sensor based on the symmetrical metal-cladding waveguide.

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Measurement of a discontinuous object based on a dual-frequency grating

乔闹生, 蔡新华, 姚春梅

2009 (11): 4881-4886. doi: 10.1088/1674-1056/18/11/044

摘要 ( 1537 )

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The dual-frequency grating measurement theory is proposed in order to carry out the measurement of a discontinuous object. Firstly, the reason why frequency spectra are produced by low frequency gratings and high frequency gratings in the field of frequency is analysed, and the relationship between the wrapped-phase and the unwrapping-phase is discussed. Secondly, a method to combine the advantages of the two kinds of gratings is proposed: one stripe is produced in the mutation part of the object measured by a suitable low frequency grating designed by MATLAB, then the phase produced by the low frequency grating need not be unfolded. The integer series of stripes is produced by a high frequency grating designed by MATLAB based on the frequency ratio of the two kinds of gratings and the high frequency wrapped-phase, and the high frequency unwrapping-phase is then obtained. In order to verify the correctness of the theoretical analysis, a steep discontinuous object of 600×600 pixels and 10.00~mm in height is simulated and a discontinuous object of ladder shape which is 32.00~mm in height is used in experiment. Both the simulation and the experiment can restore the discontinuous object height accurately by using the dual-frequency grating measurement theory.

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Photon-added and photon-subtracted displaced Fock states and their non-classical properties

司坤, 张淼, 贾焕玉

2009 (11): 4887-4892. doi: 10.1088/1674-1056/18/11/045

摘要 ( 1511 )

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A new kind of quantum optical state, photon-added and -subtracted displaced Fock states, is introduced by applying the inverse of bosonic creation and annihilation operators to displaced Fock states. The quantum statistical properties of these states are investigated by numerical methods. Numerical results indicate that these states reveal some interesting non-classical properties, such as anti-bunching effects, sub-Poisson distributions and negativities of their Wigner functions.

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Implementation of a many-qubit Grover search by cavity QED

范浩权, 杨万里, 黄学人, 冯芒

2009 (11): 4893-4900. doi: 10.1088/1674-1056/18/11/046

摘要 ( 1589 )

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We explore the possibility of an N-qubit (N>3) Grover search in cavity QED, based on a fast operation of an N-qubit controlled phase-flip with atoms in resonance with the cavity mode. We demonstrate both analytically and numerically that our scheme can be achieved efficiently to find a marked state with high fidelity and high success probability. As an example, a ten-qubit Grover search is simulated specifically under the discussion of experimental feasibility and challenge. We argue that our scheme is applicable to the case involving an arbitrary number of qubits. As cavity decay is involved in our quantum trajectory treatment, we can analytically understand the implementation of a Grover search subject to dissipation, which will be very helpful for relevant experiments.

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Sub-natural linewidth of the probe absorption spectrum in a cold Cs atom molecule system

冯志芳, 李卫东, 肖连团, 贾锁堂

2009 (11): 4901-4905. doi: 10.1088/1674-1056/18/11/047

摘要 ( 1626 )

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We theoretically analysed the linewidth of the probe absorption spectrum in a cold Cs atom--molecule system. The tunnelling coupling between the two excited molecular states, especially for the Cs atom--molecule system, plays an important role in obtaining the sub-natural linewidth of the probe absorption spectrum. For example, when the tunnelling couple strength fulfils \sigma _12=10γ _ab₁, the linewidth is only about 0.66~MHz. Moreover, since the linewidth of interest is dominated by the tunnelling coupling, the absorption peak becomes very narrow even in the case of large pump laser intensities.

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Photoluminescence evolution in self-ion-implanted and annealed silicon

杨宇, 王茺, 杨瑞东, 李亮, 熊飞

2009 (11): 4906-4911. doi: 10.1088/1674-1056/18/11/048

摘要 ( 1660 )

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Si⁺ ion-implanted silicon wafers are annealed at different temperatures from room temperature to 950~℃ and then characterized by using the photoluminescence (PL) technique at different recorded temperatures (RETs). Plentiful optical features are observed and identified clearly in these PL curves. The PL spectra of these samples annealed in different temperature ranges are correspondingly dominated by different emission peaks. Several characteristic features, such as an R line, S bands, a W line, the phonon-assistant W^\rm TA and Si^\rm TO peaks, can be detected in the PL spectra of samples annealed at different temperatures. For the samples annealed at 800~\du, emission peaks from the dislocations bounded at the deep energy levels of the forbidden band, such as D_1 and D₂ bands, can be observed at a temperature as high as 280~K. These data strongly indicate that a severe transformation of defect structures could be manipulated by the annealing and recorded temperatures. The deactivation energies of the main optical features are extracted from the PL data at different temperatures.

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Analytical modelling of end thermal coupling in a solid-state laser longitudinally bonded by a vertical-cavity top-emitting laser diode

吴坚, H.D.Summers

2009 (11): 4912-4918. doi: 10.1088/1674-1056/18/11/049

摘要 ( 1775 )

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The intrinsic features involving a circularly symmetric beam profile with low divergence, planar geometry as well as the increasingly enhanced power of vertical-cavity surface-emitting lasers (VCSELs) have made the VCSEL a promising pump source in direct end bonding to a solid-state laser medium to form the minimized, on-wafer integrated laser system. This scheme will generate a surface contact pump configuration and thus additional end thermal coupling to the laser medium through the joint interface of both materials, apart from pump beam heating. This paper analytically models temperature distributions in both VCSEL and the laser medium from the end thermal coupling regarding surface contact pump configuration using a top-emitting VCSEL as the pump source for the first time. The analytical solutions are derived by introducing relative temperature and mean temperature expressions. The results show that the end contact heating by the VCSEL could lead to considerable temperature variations associated with thermal phase shift and thermal lensing in the laser medium. However, if the central temperature of the interface is increased by less than 20~K, the end contact heating does not have a significant thermal influence on the laser medium. In this case, the thermal effect should be dominated by pump beam heating. This work provides useful analytical results for further analysis of hybrid thermal effects on those lasers pumped by a direct VCSEL bond.

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Surface superlattice solitons in nonlocal nonlinear media

黄惠昌, 何影记, 汪河洲

2009 (11): 4919-4923. doi: 10.1088/1674-1056/18/11/050

摘要 ( 1662 )

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We analyse surface solitons at the interface between a one-dimensional photonic superlattice and a uniform medium with weak nonlocal nonlinearity. We demonstrate that in deep lattices there exist three kinds of surface solitons when the propagation constant exceeds a critical value, including two on-site solitons and one off-site soliton. These three kinds of surface solitons have unique dynamical properties. If the relative depth of the superlattice is low, there is only one kind of off-site soliton; however, the solitons of this kind can propagate stably, unlike their deep superlattice counterparts. Dipole surface solitons are also investigated, and the stable domain is given.

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Bloch oscillation and Landau\ben Zener tunnelling in a modulated optical lattice in a photovoltaic photorefractive crystal

张冰志, 崔虎, 李湘衡, 佘卫龙

2009 (11): 4924-4931. doi: 10.1088/1674-1056/18/11/051

摘要 ( 1452 )

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We theoretically study the beam dynamical behaviour in a modulated optical lattice with a quadratic potential in a photovoltaic photorefractive crystal. We find that two different Bloch oscillation patterns appear for the excitation of both broad and narrow light beams. One kind of optical Landau--Zener tunnelling also appears upon the Bloch oscillation and can be controlled by adjusting the parameter of the optical lattice. Unlike the case of linear potential, the energy radiation due to Landau--Zener tunnelling can be confined in modulated lattices of this kind. For high input intensity levels, the Landau--Zener tunnelling is suppressed by the photovoltaic photorefractive nonlinearity and a symmetry breaking of beam propagation from the modulational instability appears.

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Influence of the abdominal wall on the nonlinear propagation of focused therapeutic ultrasound

刘振波, 范庭波, 章东, 龚秀芬

2009 (11): 4932-4937. doi: 10.1088/1674-1056/18/11/052

摘要 ( 1746 )

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This article theoretically studies the influence of inhomogeneous abdominal walls on focused therapeutic ultrasound based on the phase screen model. An inhomogeneous tissue is considered as a combination of a homogeneous medium and a phase aberration screen. Variations of acoustic parameters such as peak positive pressure, peak negative pressure, and acoustic intensity are discussed with respect to the phase screen statistics of human abdominal walls. Results indicate that the abdominal wall can result in energy loss of the sound in the focal plane. For a typical human abdominal wall with correlation length of 7.9~mm and variance of 0.36, the peak acoustic intensity radiated from a 1~MHz transmitter with a radius of 30~mm can be reduced by about 14% at the focal plane.

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Dispersion relations of lattice waves in three-dimensional strongly coupled complex plasma crystals

杨雪峰, 王晓钢, 刘悦

2009 (11): 4938-4943. doi: 10.1088/1674-1056/18/11/053

摘要 ( 1673 )

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Dispersion relation matrices, with the screened Coulomb interaction between a charged dust particle and all other particles taken into account, are derived for waves in body centred cubic (bcc) and face centred cubic (fcc) lattices in three-dimensional strongly coupled complex plasma crystals separately. The matrices are then calculated in characteristic directions to obtain the longitudinal and transverse eigenmodes. The longitudinal and transverse waves for these cases are discussed separately.

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Crystal structure and magnetic properties of Nd(Mn_1-xFe_x)₂Si₂ compounds

陈业青, 骆军, 梁敬魁, 李静波, 饶光辉

2009 (11): 4944-4948. doi: 10.1088/1674-1056/18/11/054

摘要 ( 1692 )

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X-ray powder diffraction, resistivity and magnetization studies have been performed on polycrystalline Nd(Fe_xMn_1 - x)₂Si₂ (0 ≤ x ≤ 1) compounds which crystallize in a ThCr₂Si₂-type structure with the space group I4/mmm. The field-cooled temperature dependence of the magnetization curves shows that, at low temperatures, NdFe₂Si₂ is antiferromagnetic, while the other compounds show ferromagnetic behaviour. The substitution of Fe for Mn leads to a decrease in lattice parameters a, c and unit-cell volume V. The Curie temperature of the compounds first increases, reaches a maximum around x=0.7, then decreases with Fe content. However, the saturation magnetization decreases monotonically with increasing Fe content. This Fe concentration dependent magnetization of Nd(Fe_xMn_1-x)₂Si₂ compounds can be well explained by taking into account the complex effect on magnetic properties due to the substitution of Mn by Fe. The temperature's square dependence on electrical resistivity indicates that the curve of Nd(Fe_0.6Mn_0.4)₂Si₂ has a quasi-linear character above its Curie temperature, which is typical of simple metals.

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Reverse Monte Carlo study on structural order in liquid and glassy AlFe alloys

张景祥, 李辉, 张洁, 宋西贵, 边秀房

2009 (11): 4949-4954. doi: 10.1088/1674-1056/18/11/055

摘要 ( 1739 )

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This paper reports that anomalous local order in liquid and glassy AlFeCe alloy has been detected by x-ray diffraction measurements. The addition of the element Ce has a great effect on this local structural order. The element Ce favours interpenetration of the icosahedra by sharing a common face and edges. It argues that frustration between this short-range order and the long-range crystalline order controls the glass-forming ability of these liquids. The obtained results suggest that a system having a stronger tendency to show local icosahedral order should be a better glass-former. This scenario also naturally explains the close relationship between the local icosahedral order in a liquid, glass-forming ability, and the nucleation barrier. Such topological local order has also been analysed directly using the reverse Monte Carlo method. It also estimated the fraction of local ordered and disordered structural units in a glassy AlFeCe alloy.

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Surface energetic and bonding characteristics of tetrahexahedral platinum nanocrystals enclosed by high-index facets

文玉华, 张杨, 朱梓忠, 孙世刚

2009 (11): 4955-4959. doi: 10.1088/1674-1056/18/11/056

摘要 ( 1721 )

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This paper uses a molecular static approach with a many-body potential to investigate the surface energetic and bonding characteristics of tetrahexahedral platinum nanocrystals enclosed by high-index facets such as 210, 310, 410, 520 and 730. It mainly focuses on the effect of crystal size and surface Miller index on these characteristics. The results show that the surface energy and dangling bond density increase with decreasing diameter of tetrahexahedral nanocrystals and generally show an order of 210 > 730 > 520 > 310 > 410. However, this order is not valid at crystal sizes below 7~nm or so. The results of corresponding surfaces are also presented for comparison.

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Fabrication and magnetic properties of Ni_0.5_0.5Zn_0.5Fe₂O₄ nanofibres by electrospinning

向军, 沈湘黔, 宋福展, 刘明权

2009 (11): 4960-4965. doi: 10.1088/1674-1056/18/11/057

摘要 ( 1689 )

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NiZn ferrite/polyvinylpyrrolidone composite fibres were prepared by sol--gel assisted electrospinning. Ni_0.5Zn_0.5Fe₂O₄ nanofibres with a pure cubic spinel structure were obtained subsequently by calcination of the composite fibres at high temperatures. This paper investigates the thermal decomposition process, structures and morphologies of the electrospun composite fibres and the calcined Ni_0.5Zn_0.5Fe₂O₄ nanofibres at different temperatures by thermo-gravimetric and differential thermal analysis, x-ray diffraction, Fourier transform infrared spectroscopy and field emission scanning electron microscopy. The magnetic behaviour of the resultant nanofibres was studied by a vibrating sample magnetometer. It is found that the grain sizes of the nanofibres increase significantly and the nanofibre morphology gradually transforms from a porous structure to a necklace-like nanostructure with the increase of calcination temperature. The Ni_0.5Zn_0.5Fe₂O₄ nanofibres obtained at 1000~℃ for 2~h are characterized by a necklace-like morphology and diameters of 100--200~nm. The saturation magnetization of the random Ni_0.5Zn_0.5Fe₂O₄ nanofibres increases from 46.5 to 90.2~emu/g when the calcination temperature increases from 450 to 1000~\du. The coercivity reaches a maximum value of 11.0~kA/m at a calcination temperature of 600~\du. Due to the shape anisotropy, the aligned Ni_0.5Zn_0.5Fe₂O₄ nanofibres exhibit an obvious magnetic anisotropy and the ease magnetizing direction is parallel to the nanofibre axis.

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SiC based Si/SiC heterojunction and its rectifying characteristics

朱峰, 陈治明, 李连碧, 赵顺峰, 林涛

2009 (11): 4966-4969. doi: 10.1088/1674-1056/18/11/058

摘要 ( 1818 )

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The Si on SiC heterojunction is still poorly understood, although it has a number of potential applications in electronic and optoelectronic devices, for example, light-activated SiC power switches where Si may play the role of an light absorbing layer. This paper reports on Si films heteroepitaxially grown on the Si face of (0001) n-type 6H-SiC substrates and the use of B₂H_6 as a dopant for p-Si grown at temperatures in a range of 700--950~\du. X-ray diffraction (XRD) analysis and transmission electron microscopy (TEM) tests have demonstrated that the samples prepared at the temperatures ranged from 850~℃ to 900~℃ are characterized as monocrystalline silicon. The rocking XRD curves show a well symmetry with FWHM of 0.4339° Omega. Twin crystals and stacking faults observed in the epitaxial layers might be responsible for widening of the rocking curves. Dependence of the crystal structure and surface topography on growth temperature is discussed based on the experimental results. The energy band structure and rectifying characteristics of the Si/SiC heterojunctions are also preliminarily tested.

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Influence of dislocations in the GaN layer on the electrical properties of an AlGaN/GaN heterostructure

高志远, 郝跃, 张进城, 李培咸, 谷文萍

2009 (11): 4970-4975. doi: 10.1088/1674-1056/18/11/059

摘要 ( 1628 )

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This paper reports on a comparative study of the spatial distributions of the electrical, optical, and structural properties in an AlGaN/GaN heterostructure. Edge dislocation density in the GaN template layer is shown to decrease in the regions of the wafer where the heterostructure sheet resistance increases and the GaN photoluminescence band-edge energy peak shifts to a high wavelength. This phenomenon is found to be attributed to the local compressive strain surrounding edge dislocation, which will generate a local piezoelectric polarization field in the GaN layer in the opposite direction to the piezoelectric polarization field in the AlGaN layer and thus help to increase the two-dimensional electron gas concentration.

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Numerical study on the thermo-stress of ZrO₂ thermal and barrier coatings by high-intensity pulsed ion beam irradiation

吴迪, 刘臣, 朱小鹏, 雷明凯

2009 (11): 4976-4980. doi: 10.1088/1674-1056/18/11/060

摘要 ( 1715 )

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This paper studies numerically the thermo-mechanical effects of ZrO₂ thermal barrier coatings (TBCs) irradiated by a high-intensity pulsed ion beam in consideration of the surface structure. Taking the deposited energy of ion beams in TBCs as the source term in the thermal conduction equation, the distribution of temperature in TBCs was simulated. Then, based on the distribution, the evolution of thermal stress was calculated by the finite element method. The results show that tensile radial stress formed at the valley of TBC surfaces after irradiation by HIPIB. Therefore, if cracks happen, they must be at valleys instead of peaks. As for the stress waves, no matter whether through peak or valley position, tensile and compressive stresses are present alternately inside TBCs along the depth direction, and the strength of stress decreases with time.

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Electronic and thermodynamic and elastic properties of pyrite RuO₂

杨则金, 郭云东, 王光昶, 李劲, 戴伟, 刘锦超, 程新路, 杨向东

2009 (11): 4981-4987. doi: 10.1088/1674-1056/18/11/061

摘要 ( 1764 )

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This paper calculates the elastic, thermodynamic and electronic properties of pyrite (Pa\bar 3) RuO₂ by the plane-wave pseudopotential density functional theory (DFT) method. The lattice parameters, normalized elastic constants, Cauchy pressure, brittle--ductile relations, heat capacity and Debye temperature are successfully obtained. The Murnaghan equation of state shows that pyrite RuO₂ is a potential superhard material. Internal coordinate parameter increases with pressure, which disagrees with experimental data. An analysis based on electronic structure and the pseudogap reveals that the bonding nature in RuO₂ is a combination of covalent, ionic and metallic bonding. A study of the elastic properties indicates that the pyrite phase is isotropic under usual conditions. The relationship between brittleness and ductility shows that pyrite RuO₂ behaves in a ductile matter at zero pressure and the degree of ductility increases with pressure.

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First-principles calculations on the electronic and vibrational properties of β-V₂O₅

周波, 苏庆, 贺德衍

2009 (11): 4988-4994. doi: 10.1088/1674-1056/18/11/062

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Using a first-principles approach based on density functional theory, this paper studies the electronic and dynamical properties of β-V₂O_5. A smaller band gap and much wider split-off bands have been observed in comparison with α -V₂O_5. The Raman- and infrared-active modes at the \varGamma point of the Brillouin zone are evaluated with LO/TO splitting, where the symbol denotes the longitudinal and transverse optical model. The nonresonant Raman spectrum of a β-V₂O_5 powder sample is also computed, providing benchmark theoretical results for the assignment of the experimental spectrum. The computed spectrum agrees with the available experimental data very well. This calculation helps to gain a better understanding of the transition from α - to β-V₂O_5.

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A novel analytical thermal model for multilevel nano-scale interconnects considering the via effect

朱樟明, 李儒, 郝报田, 杨银堂

2009 (11): 4995-5000. doi: 10.1088/1674-1056/18/11/063

摘要 ( 1525 )

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Based on the heat diffusion equation of multilevel interconnects, a novel analytical thermal model for multilevel nano-scale interconnects considering the via effect is presented, which can compute quickly the temperature of multilevel interconnects, with substrate temperature given. Based on the proposed model and the 65~nm complementary metal oxide semiconductor (CMOS) process parameter, the temperature of nano-scale interconnects is computed. The computed results show that the via effect has a great effect on local interconnects, but the reduction of thermal conductivity has little effect on local interconnects. With the reduction of thermal conductivity or the increase of current density, however, the temperature of global interconnects rises greatly, which can result in a great deterioration in their performance. The proposed model can be applied to computer aided design (CAD) of very large-scale integrated circuits (VLSIs) in nano-scale technologies.

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Effect of pressure on the thermal expansion of MgO up to 200~GPa

孙小伟, 刘子江, 陈其峰, 宋婷, 王成伟

2009 (11): 5001-5007. doi: 10.1088/1674-1056/18/11/064

摘要 ( 1558 )

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Constant temperature and pressure molecular dynamics (MD) simulations are performed to investigate the thermal expansivity of MgO at high pressure, by using effective pair-wise potentials which consist of Coulomb, dispersion, and repulsion interactions that include polarization effects through the shell model (SM). In order to take into account non-central forces in crystals, the breathing shell model (BSM) is also introduced into the MD simulation. We present a comparison between the volume thermal expansion coefficient α dependences of pressure P at 300 and 2000~K that are obtained from the SM and BSM potentials and those derived from other experimental and theoretical methods in the case of MgO. Compared with the results obtained by using the SM potentials, the MD results obtained by using BSM potentials are more compressible. In an extended pressure and temperature range, the α value is also predicted. The properties of MgO in a pressure range of 0--200~GPa at temperatures up to 3500~K are summarized.

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The structural and electronic and magnetic properties of SrFeO_n_n (n=2 2.5): a GGA+U study

吴海平, 邓开明, 谭伟石, 肖传云, 胡凤兰, 李群祥

2009 (11): 5008-5014. doi: 10.1088/1674-1056/18/11/065

摘要 ( 1521 )

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Using density-functional calculations within the generalized gradient approximation (GGA)+U framework, we investigate the structural, electronic, and magnetic properties of the ground states of SrFeO_n (n=2 and 2.5). The magnetism calculations show that the ground states of both SrFeO₂ and SrFeO_2.5 have G type antiferromagnetic ordering, with indirect band gaps of 0.89 and 0.79~eV, respectively. The electronic structure calculations demonstrate that Fe cations are in the high-spin state of (d_z²)²(d_xz,d_yz)²(d_xy)¹(d_x²-y²)¹(S=2), unlike the previous prediction of (d_xz,d_yz)³(d_xy)¹(d_z²)¹(d_x²-y²)¹(S=2) for SrFeO₂, and in the high-spin state of (d_xy,d_xz,d_yz,d_x²-y²,d_z²)^5(S=5/2) for SrFeO_2.5.

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Effects of electron radiation on shielded space and triple-junction GaAs solar cells

高欣, 杨生胜, 薛玉雄, 李凯, 李丹明, 王鹢, 王云飞, 冯展祖

2009 (11): 5015-5019. doi: 10.1088/1674-1056/18/11/066

摘要 ( 1536 )

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The displacement damage dose methodology for analysing and modelling the performance of triple-junction InGaP₂/GaAs/Ge solar cells in an electron radiation environment is presented. Degradations at different electron energies are correlated with displacement damage dose (D_\rm d). One particular electron radiation environment, relative to a geosynchronous earth orbit (GEO), is chosen to calculate the total D_\rm d behind the different thicknesses coverglasses to predict the performance degradation at the end of the 15-year mission.

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Characterization of ZnO nanowire field-effect transistors and exposed to ultraviolet radiation

黎明, 张海英, 郭常新, 徐静波, 付晓君, 陈普锋

2009 (11): 5020-5023. doi: 10.1088/1674-1056/18/11/067

摘要 ( 1565 )

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A ZnO nanowire (NW) field-effect transistor (FET) is fabricated and characterized, and its characterization of ultraviolet radiation is also investigated. On the one hand, when the radiation time is 5~min, the radiation intensity increases to 5.1~μ W/cm², while the saturation drain current (I_\rm dss) of the nanowire FET decreases sharply from 560 to 320~nA. The field effect mobility (μ ) of the ZnO nanowire FET drops from 50.17 to 23.82~cm²/(V.s) at V_\rm DS=2.5~V, and the channel resistivity of the FET increases by a factor of 2. On the other hand, when the radiation intensity is 2.5~μ W/cm^2 , the DC performance of the FET does not change significantly with irradiation time (its performances at irradiation times of 5 and 20~min are almost the same); in particular, the I_\rm dss of NW FET only reduces by about 50~nA. Research is underway to reveal the intrinsic properties of suspended ZnO nanowires and to explore their device applications.

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Annealing effect on structure and green emission of ZnO nanopowder by decomposing precursors

钟红梅, 刘茜, 孙艳, 陆卫

2009 (11): 5024-5028. doi: 10.1088/1674-1056/18/11/068

摘要 ( 1547 )

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ZnO nanopowder is successfully synthesized by annealing the precursors in oxygen gas using the chemical precipitation method. Structural and optical properties of thus synthesized ZnO nanopowder are characterized by scanning electron microscopy (SEM) and photoluminescence (PL). The morphology of ZnO nanopowders evolves from nanorod to cobble as annealing temperature increases from 500 to 1000~\du, while spiral structures are observed in the samples annealed at 900 and 1000~\du. The PL spectra of ZnO nanopowder consist of largely green and yellow emission bands. The green emission from ZnO nanopowder depends strongly on the annealing temperature with a peak intensity at a temperature lower than 800~℃ while the yellow emission is associated with interstitial oxygen \rm O_\i.

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High-temperature current conduction through three kinds of Schottky diodes

李菲, 张小玲, 段毅, 谢雪松, 吕长志

2009 (11): 5029-5033. doi: 10.1088/1674-1056/18/11/069

摘要 ( 1614 )

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Fundamentals of the Schottky contacts and the high-temperature current conduction through three kinds of Schottky diodes are studied. N-Si Schottky diodes, GaN Schottky diodes and AlGaN/GaN Schottky diodes are investigated by I--V--T measurements ranging from 300 to 523~K. For these Schottky diodes, a rise in temperature is accompanied with an increase in barrier height and a reduction in ideality factor. Mechanisms are suggested, including thermionic emission, field emission, trap-assisted tunnelling and so on. The most remarkable finding in the present paper is that these three kinds of Schottky diodes are revealed to have different behaviours of high-temperature reverse currents. For the n-Si Schottky diode, a rise in temperature is accompanied by an increase in reverse current. The reverse current of the GaN Schottky diode decreases first and then increases with rising temperature. The AlGaN/GaN Schottky diode has a trend opposite to that of the GaN Schottky diode, and the dominant mechanisms are the effects of the piezoelectric polarization field and variation of two-dimensional electron gas charge density.

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Fe-doping induced Griffiths-like phase in La_0.7Ba_0.3CoO₃

黄万国, 张向群, 李国科, 孙阳, 李庆安, 成昭华

2009 (11): 5034-5037. doi: 10.1088/1674-1056/18/11/070

摘要 ( 1766 )

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The effect of Fe-doping on the magnetic properties of the ABO₃-type perovskite cobaltites La_0.7Ba_0.3Co_1 - yFe_yO₃ (0 ≤ y ≤ 0.80) is reported. With no apparent structural change in any doped sample, the Curie temperature (T_C) and the magnetization (M) are greatly suppressed for y ≤ 0.30 samples, while a distinct increase in T_C for the y=0.40 sample is observed. With the further increase of Fe concentration, T_C increases monotonically. Griffiths-like phases in 0.40 ≤ y ≤ 0.60 samples are confirmed. The formation of the Griffiths-like phase is ascribed to B-site disordering induced isolation of ferromagnetic (FM) clusters above T_C.

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Coherent incoherent transition in the spin boson model with a super-ohmic bath

谢友兰, 陈芝得

2009 (11): 5038-5043. doi: 10.1088/1674-1056/18/11/071

摘要 ( 2041 )

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The temperature effect on tunnelling splitting in the spin--boson model with a super-ohmic bath is studied by the small polaron theory. The coherent--incoherent transition temperature is calculated and its dependence on dissipation strength and bare tunnelling splitting is analysed. In additional to the traditional transition point described in textbooks, a new kind of transition is found in the low dissipation region, showing different temperature dependence in the transition. The relation to the corresponding transition in the polaron--phonon system is also discussed.

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Nb/Al-AlO_x/Nb junctions with controllable critical current density for qubit application

曹文会, 于海峰, 田野, 于洪伟, 任育峰, 陈赓华, 赵士平

2009 (11): 5044-5046. doi: 10.1088/1674-1056/18/11/072

摘要 ( 1668 )

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Nb/Al-AlO_x/Nb tunnel junctions with controllable critical current density J_c are fabricated using the standard selective Nb etching process. Tunnel barriers are formed in different oxygen exposure conditions (oxygen pressure P and oxidation time t), giving rise to J_c ranging from 100~A/cm² to above 2000 A/cm². J_c shows a familiar linear dependence on P×t in logarithmic scales. We calculate the energy levels of the phase- and flux-type qubits using the achievable junction parameters and show that the fabricated Nb/Al-AlO_x/Nb tunnel junctions can be used conveniently for quantum computation applications in the future.

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A Green's function model for ferromagnetism and spin excitations of (Ga Mn)As diluted magnetic semiconductors

刘贵斌, 刘邦贵

2009 (11): 5047-5054. doi: 10.1088/1674-1056/18/11/073

摘要 ( 1681 )

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We study (Ga, Mn)As diluted magnetic semiconductors in terms of the Ruderman--Kittel--Kasuya--Yosida quantum spin model in Green's function approach. Random distributions of the magnetic atoms are treated by using an analytical average of magnetic configurations. Average magnetic moments and spin excitation spectra as functions of temperature can be obtained by solving self-consistent equations, and the Curie temperature T_C is given explicitly. T_C is proportional to magnetic atomic concentration, and there exists a maximum for T_C as a function of carrier concentration. Applied to (Ga, Mn)As, the theoretical results are consistent with experiment and the experimental T_C can be obtained with reasonable parameters. This modelling can also be applied to other diluted magnetic semiconductors.

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Structures and magnetic anisotropy of β-Mn₂V₂O₇ crystals synthesized by the molten salt method

周传仓, 刘发民, 丁芃

2009 (11): 5055-5060. doi: 10.1088/1674-1056/18/11/074

摘要 ( 1661 )

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β-Mn₂V₂O_7 crystals with strip shape are successfully prepared by the molten salt method in a closed crucible, and are characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area of electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM). The results indicate that the sample is of the β -Mn₂V₂O₇ crystal with monoclinic symmetry, level natural cleavage facets and directional growth. Magnetic properties are measured by vibration sample magnetometry (VSM) at room temperature, and the magnetic hysteresis loop indicates that the β-Mn₂V₂O₇ has anti-ferromagnetic properties with low coercive force and remnant magnetization. The magnetic measurement results in different directions exhibit that the β -Mn₂V₂O₇ has magnetic anisotropy, which is due to the fact that the magnetic interaction energy of the β -Mn₂V₂O₇ is lowest only when the electron configuration is in a certain direction.

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The effects of current-path patterns on magnetotransport in spatially-confined structures by Monte Carlo simulation

伍建春, 孙华, 李振亚

2009 (11): 5061-5065. doi: 10.1088/1674-1056/18/11/075

摘要 ( 1546 )

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Simulations are performed on clusters of finite size to study the effects of size and current-path structure on magnetotransport in spatially-confined samples. Magnetotransport networks are established and calculated based on fractal structures including Koch curves and percolation backbones extracted from regular lattices. The structure pattern of clusters is shown to play an important role in the magnetotransport behaviours by affecting the magnetoresistance fluctuations due to spin disorder in the systems of small size, which suggests the possibility of controlling the magnetotransport by the design of current-path configurations.

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Localized electronic states in gaps on hole-net structures of silicon

黄伟其, 吕泉, 张荣涛, 王晓允, 于示强

2009 (11): 5066-5071. doi: 10.1088/1674-1056/18/11/076

摘要 ( 1710 )

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Hole-net structure silicon is fabricated by laser irradiation and annealing, on which a photoluminescence (PL) band in a the region of 650--750~nm is pinned and its intensity increases obviously after oxidation. It is found that the PL intensity changes with both laser irradiation time and annealing time. Calculations show that some localized states appear in the band gap of the smaller nanocrystal when Si=O bonds or Si--O--Si bonds are passivated on the surface. It is discovered that the density and the number of Si=O bonds or Si--O--Si bonds related to both the irradiation time and the annealing time obviously affect the generation of the localized gap states of hole-net silicon, by which the production of stimulated emission through controlling oxidation time can be explained.

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Analysis and finite element simulation of electromagnetic heating in the nitride MOCVD reactor

李志明, 郝跃, 张进成, 许晟瑞, 倪金玉, 周小伟

2009 (11): 5072-5077. doi: 10.1088/1674-1056/18/11/077

摘要 ( 1643 )

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Electromagnetic field distribution in the vertical metal organic chemical vapour deposition (MOCVD) reactor is simulated by using the finite element method (FEM). The effects of alternating current frequency, intensity, coil turn number and the distance between the coil turns on the distribution of the Joule heat are analysed separately, and their relations to the value of Joule heat are also investigated. The temperature distribution on the susceptor is also obtained. It is observed that the results of the simulation are in good agreement with previous measurements.

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Vacuum relaxation and annealing-induced enhancement of mobility of regioregular poly(3-hexylthiophene) field-effect transistors

田雪雁, 徐征, 赵谡玲, 张福俊, 徐叙瑢, 袁广才, 李婧, 孙钦军, 王赟

2009 (11): 5078-5083. doi: 10.1088/1674-1056/18/11/078

摘要 ( 1382 )

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In order to enhance the performance of regioregular poly(3-hexylthiophene) (RR-P3HT) field-effect transistors (FETs), RR-P3HT FETs are prepared by the spin-coating method followed by vacuum placement and annealing. This paper reports that the crystal structure, the molecule interconnection, the surface morphology, and the charge carrier mobility of RR-P3HT films are affected by vacuum relaxation and annealing. The results reveal that the field-effect mobility of RR-P3HT FETs can reach 4.17×10^ - 2~m²/(V.s) by vacuum relaxation at room temperature due to an enhanced local self-organization. Furthermore, it reports that an appropriate annealing temperature can facilitate the crystal structure, the orientation and the interconnection of polymer molecules. These results show that the field-effect mobility of device annealed at 150~℃ for 10 minutes in vacuum at atmosphere and followed by placement for 20 hours in vacuum at room temperature is enhanced dramatically to 9.00×10^ - 2 ~cm²/(V.s).

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Refractivity estimation from radar sea clutter

黄思训, 赵小峰, 盛峥

2009 (11): 5084-5090. doi: 10.1088/1674-1056/18/11/079

摘要 ( 1809 )

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This paper addresses the problem of estimating lower atmospheric refractivity under the nonstandard propagation conditions frequently encountered in low altitude maritime radar applications. The vertical structure of the refractive environment is modeled by using a five-parameter model, and the horizontal structure is modeled as range-independent. The electromagnetic propagation in the troposphere is simulated by using a split-step fast Fourier transform based on parabolic approximation to the wave equation. A global search marked as a modified genetic algorithm (MGA) for the 5 environmental parameters is performed by using a genetic algorithm (GA) integrated with a simulated annealing technique. The retrieved results from simulated runs demonstrate the ability of this method to make atmospheric refractivity estimations. A comparison with the classical GA and the Bayesian Markov Chain Monte Carlo (Bayesian-MCMC) technique shows that the MGA can not only shorten the inverse time but also improve the inverse precision. For real data cases, the inversion values do not match the reference data very well. The inverted profile, however, can be used to synoptically describe the real refractive structure.

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A preliminary investigation on the topology of Chinese and climate networks

王革丽, Anastasios A Tsonis

2009 (11): 5091-5096. doi: 10.1088/1674-1056/18/11/080

摘要 ( 1504 )

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Complex networks have been studied across many fields of science in recent years. In this paper, we give a brief introduction of networks, then follow the original works by Tsonis et al (2004, 2006) starting with data of the surface temperature from 160 Chinese weather observations to investigate the topology of Chinese climate networks. Results show that the Chinese climate network exhibits a characteristic of regular, almost fully connected networks, which means that most nodes in this case have the same number of links, and so-called super nodes with a very large number of links do not exist there. In other words, though former results show that nodes in the extratropical region provide a property of scale-free networks, they still have other different local fine structures inside. We also detect the community of the Chinese climate network by using a Bayesian technique; the effective number of communities of the Chinese climate network is about four in this network. More importantly, this technique approaches results in divisions which have connections with physics and dynamics; the division into communities may highlight the aspects of the dynamics of climate variability.

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