中国物理B

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A new type of conserved quantity of Mei symmetry for the motion of mechanico–electrical coupling dynamical systems

赵丽, 傅景礼, 陈本永

2011 (4): 40201-040201. doi: 10.1088/1674-1056/20/4/040201

摘要 ( 1699 )

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We obtain a new type of conserved quantity of Mei symmetry for the motion of mechanico-electrical coupling dynamical systems under the infinitesimal transformations. A criterion of Mei symmetry for the mechanico-electrical coupling dynamical systems is given. Simultaneously, the condition of existence of the new conserved quantity of Mei symmetry for mechanico-electrical coupling dynamical systems is obtained. Finally, an example is given to illustrate the application of the results.

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The periodic solutions for coupled integrable dispersionless equations

刘式适, 赵强, 刘式达

2011 (4): 40202-040202. doi: 10.1088/1674-1056/20/4/040202

摘要 ( 1429 )

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By using the Jacobi elliptic-function method, this paper obtains the periodic solutions for coupled integrable dispersionless equations. The periodic solutions include some kink and anti-kink solitons.

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Solitons for a generalized variable-coefficient nonlinear Schrödinger equation

王欢, 李彪

2011 (4): 40203-040203. doi: 10.1088/1674-1056/20/4/040203

摘要 ( 1884 )

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In this paper, we investigate some exact soliton solutions for a generalized variable-coefficients nonlinear Schrödinger equation (NLS) with an arbitrary time-dependent linear potential which describes the dynamics of soliton solutions in quasi-one-dimensional Bose-Einstein condensations. Under some reasonable assumptions, one-soliton and two-soliton solutions are constructed analytically by the Hirota method. From our results, some previous one- and two-soliton solutions for some NLS-type equations can be recovered by some appropriate selection of the various parameters. Some figures are given to demonstrate some properties of the one- and the two-soliton and the discussion about the integrability property and the Hirota method is given finally.

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Linear matrix inequality approach for robust stability analysis for stochastic neural networks with time-varying delay

S. Lakshmanan, P. Balasubramaniam

2011 (4): 40204-040204. doi: 10.1088/1674-1056/20/4/040204

摘要 ( 1337 )

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This paper studies the problem of linear matrix inequality (LMI) approach to robust stability analysis for stochastic neural networks with a time-varying delay. By developing a delay decomposition approach, the information of the delayed plant states can be taken into full consideration. Based on the new Lyapunov-Krasovskii functional, some inequality techniques and stochastic stability theory, new delay-dependent stability criteria are obtained in terms of LMIs. The proposed results prove the less conservatism, which are realized by choosing new Lyapunov matrices in the decomposed integral intervals. Finally, numerical examples are provided to demonstrate the less conservatism and effectiveness of the proposed LMI method.

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Poor–rich demarcation of Matthew effect on scale-free systems and its application

闫栋, 董明, Abdelaziz Bouras, 于随然

2011 (4): 40205-040205. doi: 10.1088/1674-1056/20/4/040205

摘要 ( 1229 )

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In a scale-free network, only a minority of nodes are connected very often, while the majority of nodes are connected rarely. However, what is the ratio of minority nodes to majority nodes resulting from the Matthew effect? In this paper, based on a simple preferential random model, the poor-rich demarcation points are found to vary in a limited range, and form a poor-rich demarcation interval that approximates to k/m ∈ [3,4]. As a result, the (cumulative) degree distribution of a scale-free network can be divided into three intervals: the poor interval, the demarcation interval and the rich interval. The inequality of the degree distribution in each interval is measured. Finally, the Matthew effect is applied to the ABC analysis of project management.

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Chaotic solutions of (2+1)-dimensional Broek–Kaup equation with variable coefficients

杨征, 马松华, 方建平

2011 (4): 40301-040301. doi: 10.1088/1674-1056/20/4/040301

摘要 ( 1426 )

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In this paper, an improved projective approach is used to obtain the variable separation solutions with two arbitrary functions of the (2+1)-dimensional Broek-Kaup equation with variable coefficients (VCBK). Based on the derived solitary wave solution and using a known chaotic system, some novel chaotic solutions are investigated.

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Fidelity susceptibility and geometric phase in critical phenomenon

田立君, 朱长青, 张宏标, 秦立国

2011 (4): 40302-040302. doi: 10.1088/1674-1056/20/4/040302

摘要 ( 1666 )

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Motivated by recent developments in quantum fidelity and fidelity susceptibility, we study relations among Lie algebra, fidelity susceptibility and quantum phase transition for a two-state system and the Lipkin-Meshkov-Glick model. We obtain the fidelity susceptibilities for SU(2) and SU(1,1) algebraic structure models. From this relation, the validity of the fidelity susceptibility to signal for the quantum phase transition is also verified in these two systems. At the same time, we obtain the geometric phases in these two systems by calculating the fidelity susceptibility. In addition, the new method of calculating fidelity susceptibility is used to explore the two-dimensional XXZ model and the Bose-Einstein condensate (BEC).

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Parametric down-conversion with local operation and two-way classical communication

刘强, 谭勇刚

2011 (4): 40303-040303. doi: 10.1088/1674-1056/20/4/040303

摘要 ( 1305 )

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The decoy-state quantum key distribution protocol suggested by Adachi et al. (Phys. Rev. Lett. 99 180503 (2007)) is proven to be secure and feasible with current techniques. It owns two attractive merits, i.e., its longer secure transmission distance and more convenient operation design. In this paper, we first improve the protocol with the aid of local operation and two-way classical communication (2-LOCC). After our modifications, the secure transmission distance is increased by about 20 km, which will make the protocol more practicable.

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Exact propagator for an electron in a quadratic saddle-point potential and a magnetic field

杨涛, 翟智远, 潘孝胤

2011 (4): 40304-040304. doi: 10.1088/1674-1056/20/4/040304

摘要 ( 1374 )

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We study the propagator for an electron moving in a two-dimensional (2D) quadratic saddle-point potential, in the presence of a perpendicular uniform magnetic field. A closed-form expression for the propagator is derived using the Feynmann path integrals.

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Thermal quantum discord in Heisenberg models with Dzyaloshinski–Moriya interaction

王林成, 闫俊彦, 衣学喜

2011 (4): 40305-040305. doi: 10.1088/1674-1056/20/4/040305

摘要 ( 1563 )

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We study the quantum discord of the bipartite Heisenberg model with the Dzyaloshinski-Moriya (DM) interaction in thermal equilibrium state and discuss the effect of the DM interaction on the quantum discord. The quantum entanglement of the system is also discussed and compared with quantum discord. Our results show that the quantum discord may reveal more properties of the system than quantum entanglement and the DM interaction may play an important role in the Heisenberg model.

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Efficient quantum secret sharing scheme with two-particle entangled states

朱珍超, 张玉清, 付安民

2011 (4): 40306-040306. doi: 10.1088/1674-1056/20/4/040306

摘要 ( 1448 )

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This paper proposes a protocol for multi-party quantum secret sharing utilizing four non-orthogonal two-particle entangled states following some ideas in the schemes proposed by Liu et al. (2006 Chin. Phys. Lett. 23 3148) and Zhang et al. (2009 Chin. Phys. B 18 2149) respectively. The theoretical efficiency for qubits of the new protocol is improved from 50% to approaching 100%. All the entangled states can be used for generating the private key except those used for the eavesdropping check. The validity of a probable attack called opaque cheat attack to this kind of protocols is considered in the paper for the first time.

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Controlled quantum teleportation with Bell states

王天银, 温巧燕

2011 (4): 40307-040307. doi: 10.1088/1674-1056/20/4/040307

摘要 ( 1494 )

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We propose a new scheme for controlled quantum teleportation with Bell states in which classical keys for controllers' portion are used. We also discuss the security of the proposed scheme and show that it can satisfy the requirements for controlled quantum teleportation. The comparison between this scheme and the previous ones shows that it is more economical and efficient.

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Feasible schemes for quantum swap gates of optical qubits via cavity QED

唐世清, 张登玉, 汪新文, 谢利军, 高峰

2011 (4): 40308-040308. doi: 10.1088/1674-1056/20/4/040308

摘要 ( 1362 )

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Feasible schemes for implementing quantum swap gates of both coherent-state qubits and photonic qubits are proposed using a Λ-type atomic ensemble trapped in a bimodal optical cavity. In both protocols, the decoherence from atomic spontaneous emission is negligible due to the fact that the excited states of the atoms are adiabatically eliminated under large detuning condition and the swap gates can be created in a single step. In our schemes, the required atoms-cavity interaction time decreases with the increase of the number of atoms, which is very important in view of decoherence. The experimental feasibilities of the schemes are also discussed.

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A quantum search algorithm based on partial adiabatic evolution

张映玉, 胡和平, 路松峰

2011 (4): 40309-040309. doi: 10.1088/1674-1056/20/4/040309

摘要 ( 1341 )

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This paper presents and implements a specified partial adiabatic search algorithm on a quantum circuit. It studies the minimum energy gap between the first excited state and the ground state of the system Hamiltonian and it finds that, in the case of M=1, the algorithm has the same performance as the local adiabatic algorithm. However, the algorithm evolves globally only within a small interval, which implies that it keeps the advantages of global adiabatic algorithms without losing the speedup of the local adiabatic search algorithm.

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Stability of trapped Bose–Einstein condensates in one-dimensional tilted optical lattice potential

方见树, 廖湘萍

2011 (4): 40310-040310. doi: 10.1088/1674-1056/20/4/040310

摘要 ( 1292 )

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Using the direct perturbation technique, this paper obtains a general perturbed solution of the Bose-Einstein condensates trapped in one-dimensional tilted optical lattice potential. We also gave out two necessary and sufficient conditions for boundedness of the perturbed solution. Theoretical analytical results and the corresponding numerical results show that the perturbed solution of the Bose-Einstein condensate system is unbounded in general and indicate that the Bose-Einstein condensates are Lyapunov-unstable. However, when the conditions for boundedness of the perturbed solution are satisfied, then the Bose-Einstein condensates are Lyapunov-stable.

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Determining the long living quasi-normal modes of relativistic stars

吕君丽, 孙纬武

2011 (4): 40401-040401. doi: 10.1088/1674-1056/20/4/040401

摘要 ( 1383 )

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Methods of finding quasi-normal modes of non-rotating relativistic stars have been well established, however, none of the existing treatments which take spacetime and fluid oscillations fully into account can determine modes of long decay time, e.g., the p and g mode series, or the f modes for stars with low compactness ratio (M/R). In this paper we show how the quasi-normal modes of long lifetime can be determined through refinements of a treatment originally due to Detweiler and Lindblom. The determination of the p mode series has been argued in the literature to have implication on the life time of gravitational wave sources and stellar stability. In this paper we 1) provide detailed steps in our treatment to facilitate future effort in this direction; 2) correct mistakes in the literature on the formulation; and 3) analyse the accuracy of the quasi-normal mode frequencies obtained and the limitations of the treatment.

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Pseudo analytical solution to time periodic stiffness systems

王延忠, 周元子

2011 (4): 40501-040501. doi: 10.1088/1674-1056/20/4/040501

摘要 ( 1432 )

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An analytical form of state transition matrix for a system of equations with time periodic stiffness is derived in order to solve the free response and also allow for the determination of system stability and bifurcation. A pseudo-closed form complete solution for parametrically excited systems subjected to inhomogeneous generalized forcing is developed, based on the Fourier expansion of periodic matrices and the substitution of matrix exponential terms via Lagrange-Sylvester theorem. A Mathieu type of equation with large amplitude is presented to demonstrate the method of formulating state transition matrix and Floquet multipliers. A two-degree-of-freedom system with irregular time periodic stiffness characterized by spiral bevel gear mesh vibration is presented to find forced response in stability and instability. The obtained results are presented and discussed.

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Environment-dependent continuous time random walk

林方, 包景东

2011 (4): 40502-040502. doi: 10.1088/1674-1056/20/4/040502

摘要 ( 1405 )

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A generalized continuous time random walk model which is dependent on environmental damping is proposed in which the two key parameters of the usual random walk theory: the jumping distance and the waiting time, are replaced by two new ones: the pulse velocity and the flight time. The anomalous diffusion of a free particle which is characterized by the asymptotical mean square displacement <x²(t)>～t^α is realized numerically and analysed theoretically, where the value of the power index α is in a region of 0 < α < 2. Particularly, the damping leads to a sub-diffusion when the impact velocities are drawn from a Gaussian density function and the super-diffusive effect is related to statistical extremes, which are called rare-though-dominant events.

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Spiral-wave dynamics in excitable medium with excitability modulated by rectangle wave

袁国勇

2011 (4): 40503-040503. doi: 10.1088/1674-1056/20/4/040503

摘要 ( 1439 )

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We numerically study the dynamics of spiral waves in the excitable system with the excitability modulated by a rectangle wave. The tip trajectories and their variations with the modulation period T are explained by the corresponding spectrum analysis. For a large T, the external modulation leads to the occurrence of more frequency peaks and these frequencies change with the modulation period according to their specific rules, respectively. Some of the frequencies and a primary frequency f₁ determine the corresponding curvature periods, which are locked into rational multiplies of the modulation period. These frequency-locking behaviours and the limited life-span of the frequencies in their variations with the modulation period constitute many resonant entrainment bands in the T axis. In the main bands, which follow the relation T/T₁₂=m/n, the size variable R_x of the tip trajectory is a monotonic increasing function of T. The rest of the frequencies are linear combinations of the two ones. Due to the complex dynamics, many unique tip trajectories appear at some certain T. We find also that spiral waves are eliminated when T is chosen from the end of the main resonant bands. This offers a useful method of controling the spiral wave.

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Taming desynchronized bursting with delays in the Macaque cortical network

王青云, Murks Aleksandra, Perc Matjaž, 陆启韶

2011 (4): 40504-040504. doi: 10.1088/1674-1056/20/4/040504

摘要 ( 1345 )

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Inhibitory coupled bursting Hindmarsh-Rose neurons are considered as constitutive units of the Macaque cortical network. In the absence of information transmission delay the bursting activity is desynchronized, giving rise to spatiotemporally disordered dynamics. This paper shows that the introduction of finite delays can lead to the synchronization of bursting and thus to the emergence of coherent propagating fronts of excitation in the space-time domain. Moreover, it shows that the type of synchronous bursting is uniquely determined by the delay length, with the transitions from one type to the other occurring in a step-like manner depending on the delay. Interestingly, as the delay is tuned close to the transition points, the synchronization deteriorates, which implies the coexistence of different bursting attractors. These phenomena can be observed by different but fixed coupling strengths, thus indicating a new role for information transmission delays in realistic neuronal networks.

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Lyapunov exponent calculation of a two-degree-of-freedom vibro-impact system with symmetrical rigid stops

李群宏, 谭洁燕

2011 (4): 40505-040505. doi: 10.1088/1674-1056/20/4/040505

摘要 ( 1418 )

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A two-degree-of-freedom vibro-impact system having symmetrical rigid stops and subjected to periodic excitation is investigated in this paper. By introducing local maps between different stages of motion in the whole impact process, the Poincar'e map of the system is constructed. Using the Poincar'e map and the Gram-Schmidt orthonormalization, a method of calculating the spectrum of Lyapunov exponents of the above vibro-impact system is presented. Then the phase portraits of periodic and chaotic attractors for the system and the corresponding convergence diagrams of the spectrum of Lyapunov exponents are given out through the numerical simulations. To further identify the validity of the aforementioned computation method, the bifurcation diagram of the system with respect to the bifurcation parameter and the corresponding largest Lyapunov exponents are shown.

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Digital image encryption with chaotic map lattices

孙福艳, 吕宗旺

2011 (4): 40506-040506. doi: 10.1088/1674-1056/20/4/040506

摘要 ( 1537 )

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This paper proposes a secure approach for encryption and decryption of digital images with chaotic map lattices. In the proposed encryption process, eight different types of operations are used to encrypt the pixels of an image and one of them will be used for particular pixels decided by the outcome of the chaotic map lattices. To make the cipher more robust against any attacks, the secret key is modified after encrypting each block of sixteen pixels of the image. The experimental results and security analysis show that the proposed image encryption scheme achieves high security and efficiency.

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A new three-dimensional chaotic system and its modified generalized projective synchronization

戴浩, 贾立新, 惠萌, 司刚全

2011 (4): 40507-040507. doi: 10.1088/1674-1056/20/4/040507

摘要 ( 1385 )

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Based on the Chen chaotic system, this paper constructs a new three-dimensional chaotic system with higher order nonlinear term and studies the basic dynamic behaviours of the system. The modified generalized projective synchronization has been observed in the coupled new three-dimensional chaotic system with unknown parameters. Furthermore, based on Lyapunov stability theory, it obtains the control laws and adaptive laws of parameters to make modified generalized projective synchronization of the coupled new three-dimensional chaotic systems. Numerical simulation results are presented to illustrate the effectiveness of this method.

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Ion-acoustic waves in plasma of warm ions and isothermal electrons using time-fractional KdV equation

Sayed A. El-Wakil, Essam M. Abulwafa, Emad K. El-Shewy, Abeer A. Mahmoud

2011 (4): 40508-040508. doi: 10.1088/1674-1056/20/4/040508

摘要 ( 1108 )

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The ion-acoustic solitary wave in collisionless unmagnetized plasma consisting of warm ions-fluid and isothermal electrons is studied using the time fractional KdV equation. The reductive perturbation method has been employed to derive the Korteweg-de Vries equation for small but finite amplitude ion-acoustic wave in warm plasma. The Lagrangian of the time fractional KdV equation is used in a similar form to the Lagrangian of the regular KdV equation with fractional derivative for the time differentiation. The variation of the functional of this Lagrangian leads to the Euler-Lagrange equation that gives the time fractional KdV equation. The variational-iteration method is used to solve the derived time fractional KdV equation. The calculations of the solution are carried out for different values of the time fractional order. These calculations show that the time fractional can be used to modulate the electrostatic potential wave instead of adding a higher order dissipation term to the KdV equation. The results of the present investigation may be applicable to some plasma environments, such as the ionosphere plasma.

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Multirate diversity strategy of fractal modulation

袁勇, 史思红, 罗懋康

2011 (4): 40509-040509. doi: 10.1088/1674-1056/20/4/040509

摘要 ( 1161 )

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Previous analyses of fractal modulation were carried out mostly from a signle perspective or a subband, but the analyses from the perspective of multiscale synthesis have not been found yet; while multiscale synthesis is just the essence of the mutlirate diversity which is the most important characteristic of fractal modulation. As for the mutlirate diversity of fractal modulation, previous studies only dealt with the general outspread of its concept, lacked the thorough and intensive quantitative comparison and analysis. In light of the above fact, from the perspective of multiscale synthesis, in this paper we provide a comprehensive analysis of the multirate diversity of fractal modulation and corresponding quantitative analysis. The results show that mutlirate diversity, which is a fusion of frequency diversity and time diversity, pays an acceptable price in spectral efficiency in exchange for a significant improvement in bit error rate. It makes fractal modulation particularly suitable for the channels whose bandwidth and duration parameters are unknown or cannot be predicted to the transmitter. Surely it is clearly of great significance for reliable communications. Moreover, we also attain the ability to flexibly make various rate-bandwidth tradeoffs between the transmitter and the receiver, to freely select the reception time and to expediently control the total bandwidth. Furthermore, the acquisitions or improvements of these fine features could provide support of the technical feasibility for the electromagnetic spectrum control technology in a complex electromagnetic environment.

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Cascade adaptive control of uncertain unified chaotic systems

魏伟, 李东海, 王京

2011 (4): 40510-040510. doi: 10.1088/1674-1056/20/4/040510

摘要 ( 1465 )

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The chaos control of uncertain unified chaotic systems is considered. Cascade adaptive control approach with only one control input is presented to stabilize states of the uncertain unified chaotic system at the zero equilibrium point. Since an adaptive controller based on dynamic compensation mechanism is employed, the exact model of the unified chaotic system is not necessarily required. By choosing appropriate controller parameters, chaotic phenomenon can be suppressed and the response speed is tunable. Sufficient condition for the asymptotic stability of the approach is derived. Numerical simulation results confirm that the cascade adaptive control approach with only one control signal is valid in chaos control of uncertain unified chaotic systems.

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A weight's agglomerative method for detecting communities in weighted networks based on weight's similarity

沈毅

2011 (4): 40511-040511. doi: 10.1088/1674-1056/20/4/040511

摘要 ( 1391 )

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This paper proposes the new definition of the community structure of the weighted networks that groups of nodes in which the edge's weights distribute uniformly but at random between them. It can describe the steady connections between nodes or some similarity between nodes' functions effectively. In order to detect the community structure efficiently, a threshold coefficient κ to evaluate the equivalence of edges' weights and a new weighted modularity based on the weight's similarity are proposed. Then, constructing the weighted matrix and using the agglomerative mechanism, it presents a weight's agglomerative method based on optimizing the modularity to detect communities. For a network with n nodes, the algorithm can detect the community structure in time O(n²log₂ⁿ). Simulations on networks show that the algorithm has higher accuracy and precision than the existing techniques. Furthermore, with the change of κ the algorithm discovers a special hierarchical organization which can describe the various steady connections between nodes in groups.

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New synchronization analysis for complex networks with variable delays

张化光, 宫大为, 王占山

2011 (4): 40512-040512. doi: 10.1088/1674-1056/20/4/040512

摘要 ( 1396 )

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This paper deals with the issue of synchronization of delayed complex networks. Differing from previous results, the delay interval [0,d(t)] is divided into some variable subintervals by employing a new method of weighting delays. Thus, new synchronization criteria for complex networks with time-varying delays are derived by applying this weighting-delay method and introducing some free weighting matrices. The obtained results have proved to be less conservative than previous results. The sufficient conditions of asymptotical synchronization are derived in the form of linear matrix inequality, which are easy to verify. Finally, several simulation examples are provided to show the effectiveness of the proposed results.

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Pinning impulsive synchronization of stochastic delayed coupled networks

唐漾, 黃偉強, 方建安, 苗清影

2011 (4): 40513-040513. doi: 10.1088/1674-1056/20/4/040513

摘要 ( 1503 )

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In this paper, the pinning synchronization problem of stochastic delayed complex network (SDCN) is investigated by using a novel hybrid pinning controller. The proposed hybrid pinning controller is composed of adaptive controller and impulsive controller, where the two controllers are both added to a fraction of nodes in the network. Using the Lyapunov stability theory and the novel hybrid pinning controller, some sufficient conditions are derived for the exponential synchronization of such dynamical networks in mean square. Two numerical simulation examples are provided to verify the effectiveness of the proposed approach. The simulation results show that the proposed control scheme has a fast convergence rate compared with the conventional adaptive pinning method.

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An improved cellular automaton model considering the effect of traffic lights and driving behaviour

何红弟, 卢伟真, 董力耘

2011 (4): 40514-040514. doi: 10.1088/1674-1056/20/4/040514

摘要 ( 1589 )

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This paper proposes an improved cellular automaton model to describe the urban traffic flow with the consideration of traffic light and driving behaviour effects. Based on the model, the characteristics of the urban traffic flow on a single-lane road are investigated under three different control strategies, i.e., the synchronized, the green wave and the random strategies. The fundamental diagrams and time-space patterns of the traffic flows are provided for these strategies respectively. It finds that the dynamical transition to the congested flow appears when the vehicle density is higher than a critical level. The saturated flow is less dependent on the cycle time and the strategies of the traffic light control, while the critical vehicle density varies with the cycle time and the strategies. Simulated results indicate that the green wave strategy is proven to be the most effective one among the above three control strategies.

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Effect of displacement on resistance and capacitance of polyaniline film

Khasan Sanginovich Karimov, Muhammad Tariq Saeed, Fazal Ahmad Khalid, Syed Abdul Moiz

2011 (4): 40601-040601. doi: 10.1088/1674-1056/20/4/040601

摘要 ( 1424 )

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This paper investigates the properties of displacement sensors based on polyaniline (PANI) films. About 1 wt% of PANI micropowder is mixed and stirred in a solution of 90 wt% water and 10 wt% alcohol at room temperature. The films of PANI are deposited from solution by drop-casting on Ag electrodes, which are preliminary deposited on glass substrates. The thicknesses of the PANI films are in the range of 20 μm-80 μm. A displacement sensor with polyaniline film as an active material is designed and fabricated. The investigations showed that, on average, the AC resistance of the sensor decreases by 2 times and the capacitance accordingly increases by 1.6 times as the displacement changes in the range of 0 mm-0.5 mm. The polyaniline is the only active material of the displacement sensor. The resistance and capacitance of the PANI changes under the pressure of spring and elastic rubber, and this pressure is created by the downward movement of the micrometer.

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Measurements of electron–phonon coupling factor and interfacial thermal resistance of metallic nano-films using a transient thermoreflectance technique

王海东, 马维刚, 过增元, 张兴, 王玮

2011 (4): 40701-040701. doi: 10.1088/1674-1056/20/4/040701

摘要 ( 1366 )

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Using a transient thermoreflectance (TTR) technique, several Au films with different thicknesses on glass and SiC substrates are measured for thermal characterization of metallic nano-films, including the electron-phonon coupling factor G, interfacial thermal resistance R, and thermal conductivity K_s of the substrate. The rear heating-front detecting (RF) method is used to ensure the femtosecond temporal resolution. An intense laser beam is focused on the rear surface to heat the film, and another weak laser beam is focused on the very spot of the front surface to detect the change in the electron temperature. By varying the optical path delay between the two beams, a complete electron temperature profile can be scanned. Different from the normally used single-layer model, the double-layer model involving interfacial thermal resistance is studied here. The electron temperature cooling profile can be affected by the electron energy transfer into the substrate or the electron-phonon interactions in the metallic films. For multiple-target optimization, the genetic algorithm (GA) is used to obtain both G and R. The experimental result gives a deep understanding of the mechanism of ultra-fast heat transfer in metals.

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Measurement of inner surface roughness of capillary by an x-ray reflectivity method

李玉德, 林晓燕, 谭植元, 孙天希, 刘志国

2011 (4): 40702-040702. doi: 10.1088/1674-1056/20/4/040702

摘要 ( 1341 )

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The inner surface roughness of a capillary is investigated by the reflectivity of x-rays penetrating through the capillary. The results are consistent with the data from atomic force microscope (AFM). The roughness measured by this new method can reach the order of angstroms with high quality capillaries.

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Development of x-ray scintillator functioning also as an analyser grating used in grating-based x-ray differential phase contrast imaging

雷耀虎, 刘鑫, 郭金川, 赵志刚, 牛憨笨

2011 (4): 42901-042901. doi: 10.1088/1674-1056/20/4/042901

摘要 ( 1575 )

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In order to push the grating-based phase contrast imaging system to be used in hospital and laboratories, this paper designs and develops a novel structure of x-ray scintillator functioning also as an analyser grating, which has been proposed for grating-based x-ray differential phase contrast imaging. According to this design, the scintillator should have a periodical structure in one dimension with the pitch equaling the period of self-image of the phase grating at the Talbot distance, where one half of the pitch is pixellated and is made of x-ray sensitive fluorescent material, such as CsI(Tl), and the remaining part of the pitch is made of x-ray insensitive material, such as silicon. To realize the design, a deep pore array with a high aspect ratio and specially designed grating pattern are successfully manufactured on 5 inch silicon wafer by the photo-assisted electrochemical etching method. The related other problems, such as oxidation-caused geometrical distortion, the filling of CsI(Tl) into deep pores and the removal of inside bubbles, have been overcome. Its pixel size, depth and grating pitch are 3 μm×7.5 μm, 150 μm and 3 μm, respectively. The microstructure of the scintillator has been examined microscopically and macroscopically by scanning electron microscope and x-ray resolution chart testing, respectively. The preliminary measurements have shown that the proposed scintillator, also functioning as an analyser grating, has been successfully designed and developed.

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boldmath B³Σ_u^-–X³Σ_g^- transition in selenium dimer: ab initio multireference configuration interaction calculations

闫冰, 刘立莉, 魏长立, 郭晶, 张玉娟

2011 (4): 43101-043101. doi: 10.1088/1674-1056/20/4/043101

摘要 ( 1427 )

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Theoretical investigation of low-lying electronic states and B³Σ_u^-–X³Σ_g^- transition properties of selenium dimer using size-extensivity singly and doubly excitation multireference configuration interaction theory with nonrelativistic all-electron basis set and relativistic effective core potential plus its split valence basis set is presented in this paper. The spectroscopic constants of ten low-lying Λ-S bound states have been obtained and compared with experiments. Spin-orbit calculations for coupling between B³Σ_u^- sates and repulsive ¹Π_u, ⁵Π_u states have been made to interpret the predissociation mechanisms of the B³Σ_u^- state. The lifetimes of B³Σ_u^- (v=0～6) have been calculated with scalar relativistic effects included or excluded, respectively, and reasonably agree with experimental values.

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Structural and bonding properties of ScSi_n^- (n=2～6) clusters: photoelectron spectroscopy and density functional calculations

许洪光, 吴苗苗, 张增光, 孙强, 郑卫军

2011 (4): 43102-043102. doi: 10.1088/1674-1056/20/4/043102

摘要 ( 1413 )

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Anion ion photoelectron spectroscopy and density functional theory (DFT) are used to investigate the electronic and structural properties of ScSi_n^- (n=2sim6) clusters and their neutrals. We find that the structures of ScSi_n^- are similar to those of Si_n+1^-. The most stable isomers of ScSi_n^- cluster anions and their neutrals are similar for n=2, 3 and 5 but different for n=4 and 6, indicating that the charge effect on geometry is size dependent for small scandium-silicon clusters. The low electron binding energy (EBE) tails observed in the spectra of ScSi_4,6^- can be explained by the existence of less stable isomers. A comparison between ScSi_n^- and VSi_n^- clusters shows the effects of metal size and electron configuration on cluster geometries.

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Abnormal physics of group-II telluride system: valence contribution of d electrons

段鹤, 董有忠, 黄燕, 陈效双

2011 (4): 43103-043103. doi: 10.1088/1674-1056/20/4/043103

摘要 ( 1304 )

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The physical trend of group-II tellurides is unexpected and contrary to the conventional wisdom. The present first-principles calculations give fundamental insights into the extent to which group-II telluride compounds present special properties upon mixing the d valence character. Our results provide explanations for the unexpected experimental observations based on the abnormal binding ordering of metal d electrons and their strong perturbation to the band edge states. The insights into the binary tellurides are useful for the study and control of the structural and chemical perturbation in their ternary alloys and heterostructures.

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Hydrogen storage capabilities of the most stable isomers of Na_nB_m (m+n=6) clusters

阮文, 谢安东, 余晓光, 伍冬兰

2011 (4): 43104-043104. doi: 10.1088/1674-1056/20/4/043104

摘要 ( 1465 )

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The most stable isomers of Na_{nB_m (m+n=6) clusters and their hydrogen storage properties are investigated by means of density functional theory with a 6-311+G(d) basis set. To study the hydrogen storage properties, all of the stable structures of Na_nB_mH_x (m+n=6) clusters have been optimized. It shows that boron atoms of Na_nB_m are separated from the other boron atoms, and form satellite BH_x (x=3, 4) clusters around the centre, which attach to the system by a bridging bond of a hydrogen atom or an Na atom. Compared with the hydrogen storage capabilities, the Na₃B₃ has the highest hydrogen storage capacity among Na_nB_m clusters. The binding energies, interaction energies of hydrogen atom with Na_nB_m clusters and second difference in energy of Na₃B₃H_x clusters have been calculated. The results show that the stability of the Na_nB_mH_x clusters present an odd-even oscillatory effect, as the number of H atoms increases.}

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MRCI study of spectroscopic and molecular properties of X¹Σ_g⁺ and A¹Π_u electronic states of the C₂ radical

张小妞, 施德恒, 孙金锋, 朱遵略

2011 (4): 43105-043105. doi: 10.1088/1674-1056/20/4/043105

摘要 ( 1573 )

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The potential energy curves (PECs) of X¹Σ_g⁺ and A¹Π_u electronic states of the C₂ radical have been studied using the full valence complete active space self-consistent field (CASSCF) method followed by the highly accurate valence internally contracted multireference configuration interaction (MRCI) approach in conjunction with the aug-cc-pV6Z basis set for internuclear separations from 0.08 nm to 1.66 nm. With these PECs of the C₂ radical, the spectroscopic parameters of three isotopologues (¹²C₂, ¹²C¹³C and ¹³C₂) have been determined. Compared in detail with previous studies reported in the literature, excellent agreement has been found. The complete vibrational levels G(v), inertial rotation constants Bv and centrifugal distortion constants Dv for the ¹²C₂, ¹²C¹³C and ¹³C₂ isotopologues have been calculated for the first time for the X¹Σ_g⁺ and A¹Π_u electronic states when the rotational quantum number J equals zero. The results are in excellent agreement with previous experimental data in the literature, which shows that the presented molecular constants in this paper are reliable and accurate.

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Synchrotron radiation VUV double photoionization of some small molecules

赵玉杰, 单晓斌, 盛六四, 王振亚, 张杰, 余春日

2011 (4): 43201-043201. doi: 10.1088/1674-1056/20/4/043201

摘要 ( 1422 )

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The VUV double photoionizations of small molecules (NO, CO, CO₂, CS₂, OSC and NH₃) were investigated with photoionization mass spectroscopy using synchrotron radiation. The double ionization energies of molecules were determined with photoionization efficiency spectroscopy. The total energies of these molecules and their parent dications (NO²⁺, CO²⁺, CO₂²⁺, CS₂²⁺, OSC²⁺and NH₃²⁺) were calculated using the Gaussian 03 program and Gaussian 2 calculations. Then, the adiabatic double ionization energies of the molecules were predicated by using high accuracy energy mode. The experimental double ionization energies of these small molecules were all in reasonable agreement with their respective calculated adiabatic double ionization energies. The mechanisms of double photoionization of these molecules were discussed based on a comparison of our experimental results with those predicted theoretically. The equilibrium geometries and harmonic vibrational frequencies of molecules and their parent dications were calculated by using the MP2 (full) method. The differences in configurations between these molecules and their parent dications were also discussed on the basis of theoretical calculations.

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The evaluation of temporal electronic structures of nonresonant Raman excited virtual state of thiourea

房超, 孙立风

2011 (4): 43301-043301. doi: 10.1088/1674-1056/20/4/043301

摘要 ( 1227 )

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An algorithm has been introduced to calculate molecular bond polarizabilities of thiourea, which supply essential electronic information about the nonresonant Raman excited virtual states. The main dynamical behaviour of the excited virtual states of thiourea is that the Raman excited electrons tend to flow to the N-H bonds and C-N bonds from the S-C bonds because of the electronic repulsion effect. The difference in Raman excited electron relaxation time of thiourea under 514.5-nm and 325-nm excitations has been observed, which quantitatively shows that the Raman scattering process is dependent on the wavelength of the pumping laser. Finally, the distribution of the electrons at the final stage of relaxation is given out through the comparison between the bond electronic densities of the ground states and the bond polarizabilities after deexcitation.

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Adhesive contact: from atomistic model to continuum model

樊康旗, 贾建援, 朱应敏, 张秀艳

2011 (4): 43401-043401. doi: 10.1088/1674-1056/20/4/043401

摘要 ( 1256 )

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Two types of Lennard-Jones potential are widely used in modeling adhesive contacts. However, the relationships between the parameters of the two types of Lennard-Jones potential are not well defined. This paper employs a self-consistent method to derive the Lennard-Jones surface force law from the interatomic Lennard-Jones potential with emphasis on the relationships between the parameters. The effect of using correct parameters in the adhesion models is demonstrated in single sphere-flat contact via continuum models and an atomistic model. Furthermore, the adhesion hysteresis behaviour is investigated, and the S-shaped force-distance relation is revealed by the atomistic model. It shows that the adhesion hysteresis loop is generated by the jump-to-contact and jump-off-contact, which are illustrated by the S-shaped force-distance curve.

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Effect of reagent vibrational excitation and isotope substitution on the stereo-dynamics of the Ba + HF → BaF + H reaction

赵娟, 罗一

2011 (4): 43402-043402. doi: 10.1088/1674-1056/20/4/043402

摘要 ( 1526 )

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Based on an extended London-Eyring-Polanyi-Sato (LEPS) potential energy surface (PES), the Ba + HF reaction has been studied by the quasi-classical trajectory (QCT) method. The reaction integral cross section as a function of collision energy for the Ba + HF → BaF + H reaction is presented and the influence of isotope substitution on the differential cross sections (DCSs) and alignments of the product's rotational angular momentum have also been studied. The results suggest that the integral cross sections increase with increasing collision energy, and the vibrational excitation of the reagent has great influence on the DCS. In addition, the product's rotational polarization is very strong as a result of heavy-heavy-light (HHL) mass combination, and the distinct effect of isotope substitution on the stereodynamics is also revealed.

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Double-distance propagation of Gaussian beams passing through a tilted cat-eye optical lens in a turbulent atmosphere

赵延仲, 孙华燕, 宋丰华

2011 (4): 44201-044201. doi: 10.1088/1674-1056/20/4/044201

摘要 ( 1442 )

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By using the extended Huygens-Fresnel diffraction integral and the method of expanding the aperture function into a finite sum of complex Gaussian functions, an approximate analytical formula of the double-distance propagation for Gaussian beam passing through a tilted cat-eye optical lens and going back along the entrance way in a turbulent atmosphere has been derived. Through numerical calculation, the effects of incidence angle, propagation distance, and structure constant on the propagation properties of a Gaussian beam in a turbulent atmosphere are studied. It is found that the incidence angle creates an unsymmetrical average intensity distribution pattern, while the propagation distance and the structure constant can each create a smooth and symmetrical average intensity distribution pattern. The average intensity peak gradually deviates from the centre, and the central average intensity value decreases quickly with the increase in incidence angle, while a larger structure constant can bring the average intensity peak back to the centre.

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Double-resonance optical-pumping effect and ladder-type electromagnetically induced transparency signal without Doppler background in cesium atomic vapour cell

杨保东, 高静, 梁强兵, 王杰, 张天才, 王军民

2011 (4): 44202-044202. doi: 10.1088/1674-1056/20/4/044202

摘要 ( 1424 )

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In a Doppler-broadened ladder-type cesium atomic system (6S_1/2-6P_3/2-8S_1/2), this paper characterizes electromagnetically induced transparency (EIT) in two different experimental arrangements, and investigates the influence of the double-resonance optical-pumping (DROP) effect on EIT in both arrangements. When the probe laser is weak, DROP is explicitly suppressed. When the probe laser is moderate, population of the intermediate level (6P_3/2 F'=5) is remarkable, therefore DROP is mixed with EIT. An interesting bimodal spectrum with the broad component due to DROP and the narrow part due to EIT has been clearly observed in cesium 6S_1/2 F=4-6P_3/2 F'=5-8S_1/2 F"=4 transitions.

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Control of slow light in a ring-out-ring structure

王楠, 掌蕴东, 袁萍

2011 (4): 44203-044203. doi: 10.1088/1674-1056/20/4/044203

摘要 ( 1306 )

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This paper proposes a ring-out-ring structure of coupled optical resonators to yield coupled-resonator-induced transparency (CRIT). Considering the insertion loss of the coupler, it theoretically deduces the transmission and the effective phase shift. The influences of the insertion loss of the coupler on the transmittance, the effective phase shift, the group index and the CRIT linewidth are fully studied. We find that the increase in multiple m can effectively enhance the normal dispersion and the group index of the proposed structure. Moreover, the specific expression of the group index at resonance is theoretically deduced and discussed for the proposed structure with two rings. The result shows that the multiple m between the lengths of ring 1 and ring 2 can enhance the group index to m times that of the structure with two equal-sized rings at resonance. The control of slow light in the proposed structure is important for applications of highly sensitivity gyroscopes, optical delay lines and optical buffers, etc.

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Thermal analysis of intense femtosecond laser ablation of aluminum

胡浩丰, 吉扬, 胡阳, 丁晓雁, 刘贤文, 郭静慧, 王晓雷, 翟宏琛

2011 (4): 44204-044204. doi: 10.1088/1674-1056/20/4/044204

摘要 ( 1366 )

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This paper numerically simulates the process of ablation of an aluminum target by an intense femtosecond laser with a fluence of 40 J/cm² based on the two-temperature equation, and obtains the evolution of the free electron temperature and lattice temperature over a large temporal and depth range, for the first time. By investigating the temporal evolution curves of the free electron temperature and lattice temperature at three representative depths of 0, 100 nm and 500 nm, it reveals different characteristics and mechanisms of the free electron temperature evolution at different depths. The results show that, in the intense femtosecond laser ablation of aluminum, the material ablation is mainly induced by the thermal conduction of free electrons, instead of the direct absorption of the laser energy; in addition, the thermal conduction of free electrons and the coupling effect between electrons and lattice will induce the temperature of free electrons deep inside the target to experience a process from increase to decrease and finally to increase again.

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Effect of Lorentz local field correction on propagation of ultrashort laser pulse in one-dimensional para-nitroaniline (PNA) molecules

周勇, 苗泉, 王传奎

2011 (4): 44205-044205. doi: 10.1088/1674-1056/20/4/044205

摘要 ( 1233 )

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This paper investigates the effect of Lorentz local field correction (LFC) on the propagation of ultrashort laser pulses in a para-nitroaniline molecular medium under resonant and nonresonant conditions by solving numerically the full-wave Maxwell-Bloch equations beyond slowly-varying envelope approximation and rotating-wave approximation. The effect of the LFC is considerably obvious when pulses with large areas propagate in the dense molecular medium. In the case of resonance, the group velocity of the sub-pulses split from the incident pulse along propagation is severely decreased by the LFC, especially for the latest sub-pulse. However, in the case of nonresonance, the influence of the LFC on the temporal evolution of the pulse is less obvious and lacks homogeneity with an increase in incident pulse area, propagation distance and molecular density.

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Properties of periodic multicrystal configurations in walk-off-compensating second harmonic generation of ultrashort pulses

黄金哲, 张留洋, 沈涛

2011 (4): 44206-044206. doi: 10.1088/1674-1056/20/4/044206

摘要 ( 1265 )

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This work designs a four-platelet periodic multicrystal configuration in the second harmonic generation of ultrashort pulses as a new walk-off-compensating device. It theoretically investigates a proposed active and a typical passive compensating scheme with the undepleted-pump approximation. The result shows that the angular and spectral bandwidths are proportional to the number of crystal pairs as expected, but the temperature tunability is basically unaltered owing to inter-plate pulse interference. At the same time, an analysis reveals that a misuse of the phase mismatch factor is responsible for a historic controversy about pulse interference. A real design of an ultraviolet second harmonic generation (262.5 nm) is considered in a passive periodic β-Barium Borate-calcite configuration, where the inter-plate pulse interference is found to form an azimuthal tuning restriction and to lower plate length tolerance. A subsequent numerical simulation with pump depletion is in good accordance with theoretical prediction.

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Supercontinuous high harmonic generation from asymmetric molecules in the presence of a terahertz field

杜洪川, 王慧巧, 胡碧涛

2011 (4): 44207-044207. doi: 10.1088/1674-1056/20/4/044207

摘要 ( 1400 )

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We have investigated high-order harmonic generation from asymmetric molecules. It is found that supercontinuous high harmonics, which are produced from asymmetric molecules by significantly steering the ionization, are broken down when the electric field of the 5-fs driving laser pulse is increased to 0.16 a.u. The high harmonic generation from asymmetric molecules with the presence of a terahertz field is also investigated. This reveals that the terahertz controlled laser pulse significantly increases the energy difference between photons, emitted from the ejected electrons, in the first and second halves of the optical cycle at the centre of the driving laser pulse. In this way, a 200-eV broadband supercontinuum can be produced in the plateau, from which a 60-as pulse with a bandwidth of 60 eV can be directly obtained with a minor post-pulse.

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Irradiation effects of CO₂ laser parameters on surface morphology of fused silica

向霞, 郑万国, 袁晓东, 戴威, 蒋勇, 李熙斌, 王海军, 吕海兵, 祖小涛

2011 (4): 44208-044208. doi: 10.1088/1674-1056/20/4/044208

摘要 ( 1547 )

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To understand the surface morphology evolution of fused silica induced by 10.6-μm CO₂ laser irradiation at different parameters, this paper reports that optical microscopy, profilometry, and hydrophilicity tests are utilized to characterize the surface structure and roughness of the laser irradiated area. The results show that three typical surface morphologies and two typical hydrophilicity test images are observed at different laser powers and pulse durations. The correlations between surface temperature and surface morphology as well as hydrophilicity behaviours are presented. The different hydrophilicity behaviours are related to surface structures of the laser-induced crater and thermal diffusion area. The thermal diffusion length monotonously increases with increasing laser power and pulse duration. The crater width is almost determined by the laser beam size. The crater depth is more sensitive to the laser power and pulse duration than the crater width.

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Influence of varied doping structure on photoemissive property of photocathode

牛军, 张益军, 常本康, 熊雅娟

2011 (4): 44209-044209. doi: 10.1088/1674-1056/20/4/044209

摘要 ( 1470 )

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The built-in electric fields within a varied doping GaAs photocathode may promote the transport of electrons from the bulk to the surface, thus the quantum efficiency of the cathode can be enhanced remarkably. But this enhancement, which might be due to the increase in either the number or the energy of electrons reaching the surface, is not clear at present. In this paper, the energy distributions of electrons in a varied doping photocathode and uniform doping photocathode before and after escaping from the cathode surface are analysed, and the number of electrons escaping from the surface in different cases is calculated for the two kinds of photocathodes. The results indicate that the varied doping structure can not only increase the number of electrons reaching the surface but also cause an offset of the electron energy distribution to high energy. That is the root reason for the enhancement of the quantum efficiency of a varied doping GaAs photocathode.

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Structural and thermodynamic properties of OsN₂ from first-principles calculations

刘春梅, 葛妮娜, 付志坚, 程艳, 朱俊

2011 (4): 45101-045101. doi: 10.1088/1674-1056/20/4/045101

摘要 ( 1432 )

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We investigate the structural and thermodynamic properties of OsN₂ by a plane-wave pseudopotential density functional theory method. The obtained lattice constant, bulk modulus and cell volume per unit formula are consistent with the available theoretical data. Moreover, the pressure-induced phase transition of OsN₂ from pyrite structure to fluorite structure has been obtained. It is found that the transition pressure of OsN₂ at zero temperature is 67.2 GPa. The bulk modulus B as well as other thermodynamic quantities of fluorite OsN₂ (including the Grüneisen constant γ and thermal expansion α) on temperatures and pressures have also been obtained.

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Structural characterization of V/Al/V/Au Ohmic contacts to n-type Al_0.4Ga_0.6N

李涛, 秦志新, 许正昱, 沈波, 张国义

2011 (4): 46101-046101. doi: 10.1088/1674-1056/20/4/046101

摘要 ( 1412 )

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This paper investigates the temperature dependence of the specific resistance in annealed V/Al/V/Au (15 nm/85 nm/20 nm/95 nm) contacts on n-Al_0.4Ga_0.6N. Contacts annealed at 700 du and higher temperatures show Ohmic behaviour. Annealing at 800 du produces the lowest contact resistance. Samples annealed at 800 du have been analysed by using cross-sectional transmission electron microscopy and an energy dispersive x-ray spectrum. Limited reaction depths are observed between V-based contacts and n-AlGaN. The VN grains are found to form in the contact layer of the annealed samples, which can be considered as the key to the successful formation of Ohmic contact. The contact layer adjacent to AlGaN material consists of V-Al-Au-N, AlN and AlAu alloys.

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The effect of Si content on the martensitic transfor-mation temperature of Ni_55.5e₁₈Ga_26.5–xSi_x alloys

申华海, 余华军, 付浩, 郭袁俊, 傅永庆, 祖小涛

2011 (4): 46102-046102. doi: 10.1088/1674-1056/20/4/046102

摘要 ( 1319 )

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This paper investigates the effects of substitution of Si for Ga on the martensitic transformation behaviours in Ni-Fe-Ga alloys by using optical metallographic microscope and differential scanning calorimetry (DSC) methods. The structure type of Ni_55.5Fe₁₈Ga_26.5-xSi_x alloys is determined by x-ray diffraction (XRD), and the XRD patterns show the microstructure of Ni-Fe-Ga-Si alloys transformed from body-centred tetragonal martensite (with Si content x = 0) to body-centred cubic austenite (with x = 2) at room temperature. The martensitic transformation temperatures of the Ni_55.5Fe₁₈Ga_26.5-xSi_x alloys decrease almost linearly with increasing Si content in the Si content range of x ≤ 3. Thermal treatment also plays an important role on martensitic transformation temperatures in the Ni-Fe-Ga-Si alloy. The valence electronic concentrations, size factor, L2₁ degree of order and strength of parent phase influence the martensitic transformation temperatures of the Ni-Fe-Ga-Si alloys. An understanding of the relationship between martensitic transformation temperatures and Si content will be significant for designing an appropriate Ni-Fe-Ga-Si alloy for a specific application at a given temperature.

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Theoretical investigation of the thermoelectric transport properties of BaSi₂

彭华, 王春雷, 李吉超, 张睿智, 王洪超, 孙毅

2011 (4): 46103-046103. doi: 10.1088/1674-1056/20/4/046103

摘要 ( 1300 )

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The full-potential linear augmented plane wave method based on density functional theory is employed to investigate the electronic structure of BaSi₂. With the constant relaxation time and rigid band approximation, the electrical conductivity, Seebeck coefficient and figure of merit are calculated by using Boltzmann transport theory, further evaluated as a function of carrier concentration. We find that the Seebeck coefficient is more anisotropic than electrical conductivity. The figure of merit of BaSi₂ is predicted to be quite high at room temperature, implying that optimal doping may be an effective way to improve thermoelectric properties.

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Epidemic thresholds in a heterogenous population with competing strains

吴庆初, 傅新楚, 杨孟

2011 (4): 46401-046401. doi: 10.1088/1674-1056/20/4/046401

摘要 ( 1290 )

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Among many epidemic models, one epidemic disease may transmit with the existence of other pathogens or other strains from the same pathogen. In this paper, we consider the case where all of the strains obey the susceptible-infected-susceptible mechanism and compete with each other at the expense of common susceptible individuals. By using the heterogenous mean-field approach, we discuss the epidemic threshold for one of two strains. We confirm the existence of epidemic threshold in both finite and infinite populations subject to underlying epidemic transmission. Simulations in the Barabasi-Albert (BA) scale-free networks are in good agreement with the analytical results.

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Orientation dependence of structural transition in fcc Al driven under uniaxial compression by atomistic simulations

李莉, 邵建立, 段素青, 梁九卿

2011 (4): 46402-046402. doi: 10.1088/1674-1056/20/4/046402

摘要 ( 1531 )

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By molecular dynamics simulations employing an embedded atom method potential, we have investigated structural transformations in single crystal Al caused by uniaxial strain loading along the [001], [011] and [111] directions. We find that the structural transition is strongly dependent on the crystal orientations. The entire structure phase transition only occurs when loading along the [001] direction, and the increased amplitude of temperature for [001] loading is evidently lower than that for other orientations. The morphology evolutions of the structural transition for [011] and [111] loadings are analysed in detail. The results indicate that only 20% of atoms transit to the hcp phase for [011] and [111] loadings, and the appearance of the hcp phase is due to the partial dislocation moving forward on {111}_fcc family. For [011] loading, the hcp phase grows to form laminar morphology in four planes, which belong to the {111}_fcc family; while for [111] loading, the hcp phase grows into a laminar structure in three planes, which belong to the {111}_fcc family except for the (111) plane. In addition, the phase transition is evaluated by using the radial distribution functions.

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Phase behaviour of polyethylene knotted ring chains

温晓会, 章林溪, 夏阿根, 陈宏平

2011 (4): 46601-046601. doi: 10.1088/1674-1056/20/4/046601

摘要 ( 1249 )

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The phase behaviour of polyethylene knotted ring chains is investigated by using molecular dynamics simulations. In this paper, we focus on the collapse of the polyethylene knotted ring chain, and also present the results of linear and ring chains for comparison. At high temperatures, a fully extensive knot structure is observed. The mean-square radius of gyration per bond <S>² / (Nb²) and the shape factor <δ^*> depend on not only the chain length but also the knot type. With temperature decreasing, chain collapse is observed, and the collapse temperature decreases with the chain length increasing. The actual collapse transition can be determined by the specific heat capacity C_v, and the knotted ring chain undergoes gas-liquid-solid-like transition directly. The phase transition of a knotted ring chain is only one-stage collapse, which is different from the polyethylene linear and ring chains. This investigation can provide some insights into the statistical properties of knotted polymer chains.

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boldmath Study of electronic structures and absorption bands of BaMgF₄ crystal with F colour centre

康玲玲, 刘廷禹, 张启仁, 徐灵芝, 张飞武

2011 (4): 47101-047101. doi: 10.1088/1674-1056/20/4/047101

摘要 ( 1322 )

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The electronic structures of BaMgF₄ crystals containing an F colour centre are studied within the framework of the fully relativistic self-consistent Direc-Slater theory, using a numerically discrete variational (DV-Xα) method. It is concluded from the calculated results that the energy levels of the F colour centre are located in the forbidden band. The optical transition energy from the ground state to the excited state for the F colour centre is about 5.12 eV, which corresponds to the 242-nm absorption band. These calculated results can explain the origin of the absorption bands.

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First-principles study of the structural, mechanical and electronic properties of ZnX₂O₄ (X=Al, Cr and Ga)

张良, 姬广富, 赵峰, 龚自正

2011 (4): 47102-047102. doi: 10.1088/1674-1056/20/4/047102

摘要 ( 1507 )

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This paper performs first-principles calculations to study the structural, mechanical and electronic properties of the spinels ZnAl₂O₄, ZnGa₂O₄ and ZnCr₂O₄, using density functional theory with the plane-wave pseudopotential method. Our calculations are in good agreement with previous theoretical calculations and the available experimental data. The studies in this paper focus on the evolution of the mechanical properties of ZnAl₂O₄, ZnGa₂O₄ and ZnCr₂O₄ under hydrostatic pressure. The results show that the cubic phases of ZnAl₂O₄, ZnGa₂O₄ and ZnCr₂O₄ become unstable at about 50 GPa, 40 GPa and 25 GPa, respectively. From analysis of the band structure of the three compounds at equilibrium volume, it obtains a direct band gap of 4.35 eV for ZnAl₂O₄ and 0.89 eV for ZnCr₂O₄, while ZnGa₂O₄ has an indirect band gap of 2.73 eV.

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First-principles investigation of the electronic, elastic and thermodynamic properties of VC under high pressure

郝爱民, 周铁军, 朱岩, 张新宇, 刘日平

2011 (4): 47103-047103. doi: 10.1088/1674-1056/20/4/047103

摘要 ( 1486 )

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An investigation of the electronic, elastic and thermodynamic properties of VC under high pressure has been conducted using first-principles calculations based on density functional theory (DFT) with the plane-wave basis set, as implemented in the CASTEP code. At elevated pressures, VC is predicted to undergo a structural transition from a relatively open NaCl-type structure to a more dense CsCl-type one. The predicted transition pressure is 520 GPa. The elastic constant, Debye temperature and heat capacity each as a function of pressure and/or temperature of VC are presented for the first time.

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Lasing behaviour from the condensation of polaronic excitons in a ZnO nanowire

刘瑞斌, 邹炳锁

2011 (4): 47104-047104. doi: 10.1088/1674-1056/20/4/047104

摘要 ( 1399 )

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Atoms under optical and magnetic trapping in a limited space at a very low temperature can lead to Bose-Einstein condensation (BEC), even in a one-dimensional (1D) optical lattice. However, can the confinment of dense excitons in a 1D semiconductor microstructure easily reach the excitonic BEC? A lightly Mn(II)-doped ZnO nanowire under a femtosecond laser pulse pump at room temperature produces single-mode lasing from coherent bipolaronic excitons, which is much like a macroscopic quantum state due to the condensation of the bipoaronic excitons if not real BEC. In this process, longitudinal biphonon binding with the exciton plays an important role. We revisit this system and propose possibility of bipolaronic exciton condensation. More studies are needed for this condensation phenomenon in 1D microcavity systems.

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Influence of thermal stress on the characteristic parameters of AlGaN/GaN heterostructure Schottky contacts

吕元杰, 林兆军, 张宇, 孟令国, 曹芝芳, 栾崇彪, 陈弘, 王占国

2011 (4): 47105-047105. doi: 10.1088/1674-1056/20/4/047105

摘要 ( 1342 )

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Ni Schottky contacts on AlGaN/GaN heterostructures have been fabricated. The samples are then thermally treated in a furnace with N₂ ambient at 600 ^circC for different times (0.5, 4.5, 10.5, 18, 33, 48 and 72 h). Current-voltage (I-V) and capacitance-voltage (C-V) relationships are measured, and Schrödinger's and Poisson's equations are self-consistently solved to obtain the characteristic parameters related to AlGaN/GaN heterostructure Schottky contacts: the two-dimensional electron gas (2DEG) sheet density, the polarization sheet charge density, the 2DEG distribution in the triangle quantum well and the Schottky barrier height for each thermal stressing time. Most of the above parameters reduce with the increase of stressing time, only the parameter of the average distance of the 2DEG from the AlGaN/GaN interface increases with the increase of thermal stressing time. The changes of the characteristic parameters can be divided into two stages. In the first stage the strain in the AlGaN barrier layer is present. In this stage the characteristic parameters change rapidly compared with those in the second stage in which the AlGaN barrier layer is relaxed and no strain is present.

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Surface plasmon–polaritons on ultrathin metal films

全军, 田英, 张军, 邵乐喜

2011 (4): 47201-047201. doi: 10.1088/1674-1056/20/4/047201

摘要 ( 1544 )

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We discuss the surface plasmon-polaritons used for ultrathin metal films with the aid of linear response theory and make comparisons with the known result given by Economou E N. In this paper we consider transverse electromagnetic fields and assume that the electromagnetic field in the linear response formula is the induced field due to the current of the electrons. It satisfies the Maxwell equation and thus we replace the current (charge) term in the Maxwell equation with the linear response expectation value. Finally, taking the external field to be zero, we obtain the dispersion relation of the surface plasmons from the eigenvalue equation. In addition, the charge-density and current-density in the z direction on the surface of ultrathin metal films are also calculated. The results may be helpful to the fundamental understanding of the complex phenomenon of surface plasmon-polaritons.

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PEDOT:PSS Schottky contacts on annealed ZnO films

朱亚彬, 胡伟, 纳杰, 何帆, 周岳亮, 陈聪

2011 (4): 47301-047301. doi: 10.1088/1674-1056/20/4/047301

摘要 ( 1305 )

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Polycrystalline ZnO and ITO films on SiO₂ substrates are prepared by radio frequency (RF) reactive magnetron sputtering. Schottky contacts are fabricated on ZnO films by spin coating with a high conducting polymer, poly(3, 4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) as the metal electrodes. The current-voltage measurements for samples on unannealed ZnO films exhibit rectifying behaviours with a barrier height of 0.72 eV (n=1.93). The current for the sample is improved by two orders of magnitude at 1 V after annealing ZnO film at 850 ℃, whose barrier height is 0.75 eV with an ideality factor of 1.12. X-ray diffraction, atomic force microscopy and scanning electron microscopy are used to study the properties of the PEDOT:PSS/ZnO/ITO/SiO₂. The results are useful for applications such as metal-semiconductor field-effect transistors and UV photodetectors.

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Gap opening and tuning in single-layer graphene with combined electric and magnetic field modulation

林鑫, 王海龙, 潘晖, 许怀哲

2011 (4): 47302-047302. doi: 10.1088/1674-1056/20/4/047302

摘要 ( 1542 )

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The energy band structure of single-layer graphene under one-dimensional electric and magnetic field modulation is theoretically investigated. The criterion for bandgap opening at the Dirac point is analytically derived with a two-fold degeneracy second-order perturbation method. It is shown that a direct or an indirect bandgap semiconductor could be realized in a single-layer graphene under some specific configurations of the electric and magnetic field arrangement. Due to the bandgap generated in the single-layer graphene, the Klein tunneling observed in pristine graphene is completely suppressed.

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Characteristic parameters of electromagnetic signals from a human heart system

刘新元, 裴留庆, 王寅, 张素明, 高红蕾, 戴远东

2011 (4): 47401-047401. doi: 10.1088/1674-1056/20/4/047401

摘要 ( 1466 )

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The electromagnetic field of a human heart system is a bioelectromagnetic field. Electrocardiography (ECG) and magnetocardiography (MCG) are both carriers of electromagnetic information about the cardiac system, and they are nonstationary signals. In this study, ECG and MCG data from healthy subjects are acquired; the MCG data are captured using a high-T_c radio frequency superconducting quantum interference device (HTc rf SQUIDs) and the QRS complexes in these data are analysed by the evolutionary spectrum analysis method. The results show that the quality factor Q and the central frequency f_z of the QRS complex evolutionary spectrum are the characteristic parameters (CHPs) of ECG and MCG in the time-frequency domain. The confidence intervals of the mean values of the CHPs are estimated by the Student t distribution method in mathematical statistics. We believe that there are threshold ranges of the mean values of Q and f_z for healthy subjects. We have postulated the following criterion: if the mean values of CHPs are in the proper ranges, the cardiac system is in a normal condition and it possesses the capability of homeostasis. In contrast, if the mean values of the CHPs do not lie in the proper ranges, the homeostasis of the cardiac system is lacking and some cardiac disease may follow. The results and procedure of MCG CHPs in the study afford a technological route for the application of HTc rf SQUIDs in cardiology.

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Nonequilibrium dynamics in two-dimensional Ising spin glass

张开成, 朱岩

2011 (4): 47501-047501. doi: 10.1088/1674-1056/20/4/047501

摘要 ( 1357 )

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This paper investigates the nonequilibrium dynamics of two-dimensional Ising spin glass by dynamical Monte Carlo simulations. A new method is developed to quantitatively measure the age of domain growth. Using this method it investigates how temperature shift affects the effective age of domain growth. It finds that the T-shift dependence of the effective age follows the prediction of the droplet model quite well. It also investigates the overlap length between the spin glass states as well as the correlated flips of spins, which are not consistent with the theoretical predictions. The possible reasons are discussed.

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Determination of the magnetocaloric effect associated with martensitic transition in Ni₄₆Cu₄Mn₃₈Sn₁₂ and Ni₅₀CoMn₃₄In₁₅ Heusler alloys

李哲, 敬超, 张浩雷, 曹世勋, 张金仓

2011 (4): 47502-047502. doi: 10.1088/1674-1056/20/4/047502

摘要 ( 1513 )

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This paper presents a study of the inverse magnetocaloric effect (MCE) corresponding to martensitic transition using various experimental approaches for Ni₄₆Cu₄Mn₃₈Sn₁₂ and Ni₅₀CoMn₃₄In₁₅ Heusler alloy. Through heat capacity measurements, it is found that the "giant inverse MCE" upon martensitic transition evaluated by the Maxwell relation in these alloys are unphysical results. This is due to the coexistence of both martensitic and austenitic phases, as well as thermal hysteresis during martensitic transition. However, careful study indicates that the spurious results during martensitic transition can be removed using a Clausius-Clapeyron equation based on magnetization measurements.

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Microstructural, phase transformation and magnetic properties of Ni–Mn–Ga alloy fabricated by spark plasma sintering

田晓华, 隋解和, 张欣, 冯雪, 蔡伟

2011 (4): 47503-047503. doi: 10.1088/1674-1056/20/4/047503

摘要 ( 1335 )

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The microstructural, phase transformation and magnetic properties of Ni-Mn-Ga alloy fabricated using the spark plasma sintering method have been investigated. The results show that both the as-sintered and annealed sintered specimens exhibit typical martensitic transformation behaviours. The martensite of the sintered specimen after annealing exhibits a ferromagnetic nature. Moreover, study of the fracture surface indicates that the transgranular fracture contributes to the higher ductility of sintered Ni-Mn-Ga alloy. In addition, the transformation strain in sintered Ni-Mn-Ga alloy is studied for the first time.

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All-electric-controlled spin current switching in single-molecule magnet-tunnel junctions

张正中, 沈瑞, 盛利, 王瑞强, 王伯根, 邢定钰

2011 (4): 47504-047504. doi: 10.1088/1674-1056/20/4/047504

摘要 ( 1371 )

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A single-molecule magnet (SMM) coupled to two normal metallic electrodes can both switch spin-up and spin-down electronic currents within two different windows of SMM gate voltage. Such spin current switching in the SMM tunnel junction arises from spin-selected single electron resonant tunneling via the lowest unoccupied molecular orbit of the SMM. Since it is not magnetically controlled but all-electrically controlled, the proposed spin current switching effect may have potential applications in future spintronics.

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Optical and magnetic properties of nitrogen ion implanted MgO single crystal

刘春明, 顾海权, 向霞, 张焱, 蒋勇, 陈猛, 祖小涛

2011 (4): 47505-047505. doi: 10.1088/1674-1056/20/4/047505

摘要 ( 1424 )

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The microstructure, optical property and magnetism of nitrogen ion implanted single MgO crystals are studied. A parallel investigation is also performed in an iron ion implanted single MgO sample as a reference. Large structural, optical and magnetic differences are obtained between the nitrogen and iron implanted samples. Room temperature ferromagnetism with a fairly large coercivity field of 300 Oe (1 Oe=79.5775 A/m), a remanence of 38% and a slightly changed optical absorption is obtained in the sample implanted using nitrogen with a dose of 1×10¹⁸ ions/cm². Transition metal contamination and defects induced magnetism can be excluded when compared with those of the iron ion implanted sample, and the nitrogen doping is considered to be the main origin of ferromagnetism.

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Thickness shear mode quartz crystal resonators with optimized elliptical electrodes

马廷锋, 张超, 江小宁, 冯冠平

2011 (4): 47701-047701. doi: 10.1088/1674-1056/20/4/047701

摘要 ( 1350 )

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Quartz crystal resonators (QCRs) with circular electrodes have been widely used for various liquid and gas sensing applications. In this work, quartz crystal resonators with elliptical electrodes were studied and tested for liquid property measurement. Mindlin's theory was used to optimize the dimension and geometry of the electrodes and a 5-MHz QCR with minimum series resistance and without any spurious modes was obtained. A series of AT-cut QCRs with elliptical electrodes of different sizes were fabricated and their sensing performances were compared to devices with circular electrodes. The experimental result shows that the device with elliptical electrodes can obtain lower resonance impedance and a higher Q factor, which results in a better loading capability. Even though the sensitivities of devices with elliptical and circular electrodes are found to be similar, the sensor with elliptical electrodes has much higher resolution due to a better frequency stability. The study indicates that the performance of QCRs with elliptical electrodes is superior to that of traditional QCRs with circular electrodes.

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Spectral transmittance and module structure fitting for transmission-mode GaAs photocathodes

赵静, 常本康, 熊雅娟, 张益军

2011 (4): 47801-047801. doi: 10.1088/1674-1056/20/4/047801

摘要 ( 1407 )

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A transmission-mode GaAs photocathode includes four layers of glass, Si₃N₄, Ga_1-xAl_xAs and GaAs. A gradient-doping photocathode sample was obtained by molecular beam epitaxy and its transmittance was measured by spectrophotometer from 600 nm to 1100 nm. The theoretical transmittance is derived and simulated based on the matrix formula for thin film optics. The simulation results indicate the influence of the transition layers and the three thin-film layers except glass on the transmittance spectra. In addition, a fitting coefficient needed for error modification enters into the fitted formula. The fitting results show that the relative error in the full spectrum reduces from 19.51% to 4.35% after the formula is modified. The coefficient and the thicknesses are gained corresponding to the minimum relative error, meanwhile each layer and total thin-film thickness deviation in the module can be controlled within 7%. The presence of glass layer roughness, layer interface effects and surface oxides is interpreted on the modification.

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Ellipsometric analysis and optical absorption characterization of gallium phosphide nanoparticulate thin film

章启贤, 魏文生, 阮方平

2011 (4): 47802-047802. doi: 10.1088/1674-1056/20/4/047802

摘要 ( 1240 )

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Gallium phosphide (GaP) nanoparticulate thin films were easily fabricated by colloidal suspension deposition via GaP nanoparticles dispersed in N,N-dimethylformamide. The microstructure of the film was performed by x-ray diffraction, high resolution transmission electron microscopy and field emission scanning electron microscopy. The film was further investigated by spectroscopic ellipsometry. After the model GaP+void|SiO₂ was built and an effective medium approximation was adopted, the values of the refractive index n and the extinction coefficient k were calculated for the energy range of 0.75 eV-4.0 eV using the dispersion formula in DeltaPsi2 software. The absorption coefficient of the film was calculated from its k and its energy gaps were further estimated according to the Tauc equation, which were further verified by its fluorescence spectrum measurement. The structure and optical absorption properties of the nanoparticulate films are promising for their potential applications in hybrid solar cells.

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Electrospun cerium nitrate/polymer composite fibres: synthesis, characterization and fibre-division model

李蒙蒙, 龙云泽, 尹红星, 张志明

2011 (4): 48101-048101. doi: 10.1088/1674-1056/20/4/048101

摘要 ( 1261 )

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Cerium (III) nitrate/poly(vinylpyrrolidone) (Ce(NO₃)₃/PVP) composite fibres have been prepared by electrospinning. After calcining the composite fibres in air at 500 ℃, CeO₂ nanowires were obtained. The characterizations of the as-spun composite fibres and resultant nanowires have been carried out by a scanning electron microscope (SEM), an infrared spectrometer, an x-ray diffractometer and a fluorescence spectrophotometer. Interestingly, some unusual ribbon-like or twin fibres were observed besides the common fibres with circular or elliptic cross sections. We developed a fibre-division model resulting from Coulomb repulsion and solvent vaporization to interpret the formation of the ribbons or twin fibres, which has been confirmed by the SEM studies. Our results also indicate that the formation of the ribbons or twin fibres is less dependent on operation voltage and work distance.

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Effect of Mn-doping on the growth mechanism and electromagnetic properties of chrysanthemum-like ZnO nanowire clusters

闫军锋, 游天桂, 张志勇, 田江晓, 贠江妮, 赵武

2011 (4): 48102-048102. doi: 10.1088/1674-1056/20/4/048102

摘要 ( 1330 )

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Chrysanthemum-like ZnO nanowire clusters with different Mn-doping concentrations are prepared by a hydrothermal process. The microstructure, morphology and electromagnetic properties are characterized by x-ray diffractometer high-resolution transmission electron microscopy (HRTEM), a field emission environment scanning electron microscope (FEESEM) and a microwave vector network analyser respectively. The experimental results indicate that the as-prepared products are Mn-doped ZnO single crystalline with a hexagonal wurtzite structure, that the growth habit changes due to Mn-doping and that a good magnetic loss property is found in the Mn-doped ZnO products, and the average magnetic loss tangent tanδ_m is up to 0.170099 for 3% Mn-doping, while the dielectric loss tangent tanδ_e is weakened, owing to the fact that ions Mn^{2 +}enter the crystal lattice of ZnO.

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Memoryless cooperative graph search based on the simulated annealing algorithm

候健, 颜钢锋, 樊臻

2011 (4): 48103-048103. doi: 10.1088/1674-1056/20/4/048103

摘要 ( 1404 )

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We have studied the problem of reaching a globally optimal segment for a graph-like environment with a single or a group of autonomous mobile agents. Firstly, two efficient simulated-annealing-like algorithms are given for a single agent to solve the problem in a partially known environment and an unknown environment, respectively. It shows that under both proposed control strategies, the agent will eventually converge to a globally optimal segment with probability 1. Secondly, we use multi-agent searching to simultaneously reduce the computation complexity and accelerate convergence based on the algorithms we have given for a single agent. By exploiting graph partition, a gossip-consensus method based scheme is presented to update the key parameter--radius of the graph, ensuring that the agents spend much less time finding a globally optimal segment.

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Comparison of the photoemission behaviour between negative electron affinity GaAs and GaN photocathodes

张益军, 邹继军, 王晓晖, 常本康, 钱芸生, 张俊举, 高频

2011 (4): 48501-048501. doi: 10.1088/1674-1056/20/4/048501

摘要 ( 1382 )

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In view of the important application of GaAs and GaN photocathodes in electron sources, differences in photoemission behaviour, namely the activation process and quantum yield decay, between the two typical types of III-V compound photocathodes have been investigated using a multi-information measurement system. The activation experiment shows that a surface negative electron affinity state for the GaAs photocathode can be achieved by the necessary Cs-O two-step activation and by Cs activation alone for the GaN photocathode. In addition, a quantum yield decay experiment shows that the GaN photocathode exhibits better stability and a longer lifetime in a demountable vacuum system than the GaAs photocathode. The results mean that GaN photocathodes are more promising candidates for electron source emitter use in comparison with GaAs photocathodes.

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Comparison of nitride-based dual-wavelength light- emitting diodes with an InAlN electron-blocking layer and with p-type doped barriers

张运炎, 范广涵

2011 (4): 48502-048502. doi: 10.1088/1674-1056/20/4/048502

摘要 ( 1418 )

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The advantages of nitride-based dual-wavelength light-emitting diodes (LEDs) with an InAlN electron blocking layer (EBL) are studied. The emission spectra, carrier concentration in the quantum wells (QWs), energy band and internal quantum efficiency (IQE) are investigated. The simulation results indicate that an LED with an InAlN EBL performs better over a conventional LED with an AlGaN EBL and an LED with p-type-doped QW barriers. All of the advantages are due to the enhancement of carrier confinement and the lower electron leakage current. The simulation results also show that the efficiency droop is markedly improved and the luminous intensity is greatly enhanced when an InAlN EBL is used.

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boldmath Characterization of the BaBiO₃-doped BaTiO₃ positive temperature coefficient of a resistivity ceramic using impedance spectroscopy with T_c=155℃

袁昌来, 刘心宇, 周昌荣, 许积文, 杨云

2011 (4): 48701-048701. doi: 10.1088/1674-1056/20/4/048701

摘要 ( 1247 )

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BaBiO₃-doped BaTiO₃ (BB-BT) ceramic, as a candidate for lead-free positive temperature coefficient of resistivity (PTCR) materials with a higher Curie temperature, has been synthesized in air by a conventional sintering technique. The temperature dependence of resistivity shows that the phase transition of the PTC thermistor ceramic occurs at the Curie temperature, T_c=155 ℃, which is higher than that of BaTiO₃ ( ≤ 130 ℃). Analysis of ac impedance data using complex impedance spectroscopy gives the alternate current (AC) resistance of the PTCR ceramic. By additional use of the complex electric modulus formalism to analyse the same data, the inhomogeneous nature of the ceramic may be unveiled. The impedance spectra reveal that the grain resistance of the BB-BT sample is slightly influenced by the increase of temperature, indicating that the increase in overall resistivity is entirely due to a grain-boundary effect. Based on the dependence of the extent to which the peaks of the imaginary part of electric modulus and impedance are matched on frequency, the conduction mechanism is also discussed for a BB-BT ceramic system.

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Consensus pursuit of heterogeneous multi-agent systems under a directed acyclic graph

闫敬, 关新平, 罗小元

2011 (4): 48901-048901. doi: 10.1088/1674-1056/20/4/048901

摘要 ( 1421 )

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This paper is concerned with the cooperative target pursuit problem by multiple agents based on directed acyclic graph. The target appears at a random location and moves only when sensed by the agents, and agents will pursue the target once they detect its existence. Since the ability of each agent may be different, we consider the heterogeneous multi-agent systems. According to the topology of the multi-agent systems, a novel consensus-based control law is proposed, where the target and agents are modeled as a leader and followers, respectively. Based on Mason's rule and signal flow graph analysis, the convergence conditions are provided to show that the agents can catch the target in a finite time. Finally, simulation studies are provided to verify the effectiveness of the proposed approach.

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Degree and connectivity of the Internet's scale-free topology

张连明, 邓晓衡, 余建平, 伍祥生

2011 (4): 48902-048902. doi: 10.1088/1674-1056/20/4/048902

摘要 ( 1552 )

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This paper theoretically and empirically studies the degree and connectivity of the Internet's scale-free topology at an autonomous system (AS) level. The basic features of scale-free networks influence the normalization constant of degree distribution p(k). It develops a new mathematic model for describing the power-law relationships of Internet topology. From this model we theoretically obtain formulas to calculate the average degree, the ratios of the k_min-degree (minimum degree) nodes and the k_max-degree (maximum degree) nodes, and the fraction of the degrees (or links) in the hands of the richer (top best-connected) nodes. It finds that the average degree is larger for a smaller power-law exponent λ and a larger minimum or maximum degree. The ratio of the k_min-degree nodes is larger for larger λ and smaller k_min or k_max. The ratio of the k_max-degree ones is larger for smaller λ and k_max or larger k_min. The richer nodes hold most of the total degrees of Internet AS-level topology. In addition, it is revealed that the increased rate of the average degree or the ratio of the k_min-degree nodes has power-law decay with the increase of k_min. The ratio of the k_max-degree nodes has a power-law decay with the increase of k_max, and the fraction of the degrees in the hands of the richer 27% nodes is about 73% (the '73/27 rule'). Finally, empirically calculations are made, based on the empirical data extracted from the Border Gateway Protocol, of the average degree, ratio and fraction using this method and other methods, and find that this method is rigorous and effective for Internet AS-level topology.

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Undetermination of the relation between network synchronizability and betweenness centrality

王圣军, 吴枝喜, 董海荣, 陈关荣

2011 (4): 48903-048903. doi: 10.1088/1674-1056/20/4/048903

摘要 ( 1375 )

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Betweenness centrality is taken as a sensible indicator of the synchronizability of complex networks. To test whether betweenness centrality is a proper measure of the synchronizability in specific realizations of random networks, this paper adds edges to the networks and then evaluates the changes of betweenness centrality and network synchronizability. It finds that the two quantities vary independently.

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In-situ control of oxygen fugacity for laboratory measurements of electrical conductivity of minerals and rocks in a multi-anvil press

代立东, 李和平, 胡海英, 单双明

2011 (4): 49101-049101. doi: 10.1088/1674-1056/20/4/049101

摘要 ( 1341 )

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In this paper, a new of oxygen fugacity controltechnique that can be widely applied to in-situ measurement of the grain interior electrical conductivities of minerals and rocks is presented for high temperature and high pressure. Inside the sample assembly, a metal and corresponding metal oxide form a solid oxygen buffer. The principle of this technique is to randomly monitor and adjust oxygen fugacity in the large-volume multi-anvil press by changing the types of solid oxygen buffer, metal shielding case and electrodes. At a pressure of up to 5.0 GPa and a temperature of up to 1423 K, the electrical conductivities of the dry peridotite are tested under the conditions of different oxygen fugacities. By virtue of this new technique, more and more reasonable and accurate laboratory electrical property data will be successfully obtained under controlled thermodynamic conditions.

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Statistical characteristics of meso-scale vortex effects on the track of a tropical cyclone

罗哲贤, 孙治安, 平凡

2011 (4): 49201-049201. doi: 10.1088/1674-1056/20/4/049201

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This paper examines initial meso-scale vortex effects on the motion of a tropical cyclone (TC) in a system where coexisting two components of TC and meso-scale vortices with a barotropic vorticity equation model. The initial meso-scale vortices are generated stochastically by employing Reinaud's method. The 62 simulations are performed and analysed in order to understand the statistical characteristics of the effects. Results show that the deflection of the TC track at t=24 h induced by the initial meso-scale vortices ranges from 2 km to 37 km with the mean value of 13.4 km. A more significant deflection of the TC track can be reduced when several initial meso-scale vortices simultaneously appear in a smaller TC circulation area. It ranges from 22 km to 37 km with the mean value of 28 km, this fact implies that the initial meso-scale vortices-induced deflection may not be neglected sometimes.

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Hawking effect and quantum nonthermal radiation of an arbitrarily accelerating charged black hole using a new tortoise coordinate transformation

潘伟珍, 杨学军, 谢志堃

2011 (4): 49701-049701. doi: 10.1088/1674-1056/20/4/049701

摘要 ( 1334 )

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Using a new tortoise coordinate transformation, this paper investigates the Hawking effect from an arbitrarily accelerating charged black hole by the improved Damour-Ruffini method. After the tortoise coordinate transformation, the Klein-Gordon equation can be written as the standard form at the event horizon. Then extending the outgoing wave from outside to inside of the horizon analytically, the surface gravity and Hawking temperature can be obtained automatically. It is found that the Hawking temperatures of different points on the surface are different. The quantum nonthermal radiation characteristics of a black hole near the event horizon is also discussed by studying the Hamilton-Jacobi equation in curved spacetime and the maximum overlap of the positive and negative energy levels near the event horizon is given. There is a dimensional problem in the standard tortoise coordinate and the present results may be more reasonable.

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