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2014年, 第23卷, 第1期　刊出日期：2014-01-12 上一期    下一期

GENERAL

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Single-photon modulation spectrum 刘岩, 于波, 何博, 张国峰, 肖连团, 贾锁堂 2014 (1):  10101-010101.  doi: 10.1088/1674-1056/23/1/010101 摘要 ( 646 )   PDF(299KB) ( 594 )   We present a robust method of single-photon modulation by directly modulating the single photons and observe its frequency spectrum. Compared with conventional photon counting technique, the single-photon modulation spectrum shows that the method could not only realize high-frequency modulation but also obtain higher signal-to-noise ratio. Moreover, the theoretical calculations show good agreement with the experimental results. 参考文献 | 相关文章 | 计量指标

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Symmetries and conservation laws of one Blaszak–Marciniak four-field lattice equation 王鑫, 陈勇, 董仲周 2014 (1):  10201-010201.  doi: 10.1088/1674-1056/23/1/010201 摘要 ( 702 )   PDF(266KB) ( 428 )   In this paper, by using the classical Lie symmetry approach, Lie point symmetries and reductions of one Blaszak–Marciniak (BM) four-field lattice equation are obtained. Two kinds of exact solutions of a rational form and an exponential form are given. Moreover, we show that the equation has a sequence of generalized symmetries and conservation laws of polynomial form, which further confirms the integrability of the BM system. 参考文献 | 相关文章 | 计量指标

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Control of epileptiform spikes based on nonlinear unscented Kalman filter 刘仙, 高庆, 李小俚 2014 (1):  10202-010202.  doi: 10.1088/1674-1056/23/1/010202 摘要 ( 475 )   PDF(480KB) ( 535 )   A new control strategy based on nonlinear unscented Kalman filter (UKF) is proposed for a neural mass model that serves as a model for simulating real epileptiform stereo-electroencephalographic (SEEG) signals. The UKF is used as an observer to estimate the state from the noisy measurement because it has been proved to be effective for state estimation of nonlinear systems. A UKF controller is constructed via the estimated state and is illustrated to be effective for epileptiform spikes suppression of aforementioned model by numerical simulations. 参考文献 | 相关文章 | 计量指标

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Nonlocal symmetry, optimal systems, and explicit solutions of the mKdV equation 辛祥鹏, 苗倩, 陈勇 2014 (1):  10203-010203.  doi: 10.1088/1674-1056/23/1/010203 摘要 ( 561 )   PDF(361KB) ( 646 )   The nonlocal symmetry of the mKdV equation is obtained from the known Lax pair; it is successfully localized to Lie point symmetries in the enlarged space by introducing suitable auxiliary dependent variables. For the closed prolongation of the nonlocal symmetry, the details of the construction for a one-dimensional optimal system are presented. Furthermore, using the associated vector fields of the obtained symmetry, we give the reductions by the one-dimensional sub-algebras and the explicit analytic interaction solutions between cnoidal waves and kink solitary waves, which provide a way to study the interactions among these types of ocean waves. For some of the interesting solutions, the figures are given to show their properties. 参考文献 | 相关文章 | 计量指标

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Implementation of a one-dimensional quantum walk in both position and phase spaces 秦豪, 薛鹏 2014 (1):  10301-010301.  doi: 10.1088/1674-1056/23/1/010301 摘要 ( 724 )   PDF(262KB) ( 584 )   Quantum walks have been investigated as they have remarkably different features in contrast to classical random walks. We present a quantum walk in a one-dimensional architecture, consisting of two coins and a walker whose evolution is in both position and phase spaces alternately controlled by the two coins respectively. By analyzing the dynamics evolution of the walker in both the position and phase spaces, we observe an influence on the quantum walk in one space from that in the other space, which behaves like decoherence. We propose an implementation of the two-coin quantum walk in both position and phase spaces via cavity quantum electrodynamics (QED). 参考文献 | 相关文章 | 计量指标

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Monogamy of quantum correlations in the one-dimensional anisotropic XY model 徐帅, 宋学科, 叶柳 2014 (1):  10302-010302.  doi: 10.1088/1674-1056/23/1/010302 摘要 ( 462 )   PDF(1900KB) ( 431 )   In this paper, the monogamy properties of some quantum correlations, including the geometric quantum discord, concurrence, entanglement of formation and entropy quantum discord, in the anisotropic spin-1/2 XY model with staggered Dzyaloshinskii–Moriya (DM) interaction have been investigated using the quantum renormalization group (QRG) method. We summarize the monogamy relation for different quantum correlation measures and make an explicit comparison. Through mathematical calculations and analysis, we obtain that no matter whether the QRG steps are carried out, the monogamy of the given states are always unaltered. Moreover, we conclude that the geometric quantum discord and concurrence obey the monogamy property while other quantum correlation measures, such as entanglement of formation and quantum discord, violate it for this given model. 参考文献 | 相关文章 | 计量指标

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Distributed wireless quantum communication networks with partially entangled pairs 余旭涛, 张在琛, 徐进 2014 (1):  10303-010303.  doi: 10.1088/1674-1056/23/1/010303 摘要 ( 478 )   PDF(323KB) ( 671 )   Wireless quantum communication networks transfer quantum state by teleportation. Existing research focuses on maximal entangled pairs. In this paper, we analyse the distributed wireless quantum communication networks with partially entangled pairs. A quantum routing scheme with multi-hop teleportation is proposed. With the proposed scheme, is not necessary for the quantum path to be consistent with the classical path. The quantum path and its associated classical path are established in a distributed way. Direct multi-hop teleportation is conducted on the selected path to transfer a quantum state from the source to the destination. Based on the feature of multi-hop teleportation using partially entangled pairs, if the node number of the quantum path is even, the destination node will add another teleportation at itself. We simulated the performance of distributed wireless quantum communication networks with a partially entangled state. The probability of transferring the quantum state successfully is statistically analyzed. Our work shows that multi-hop teleportation on distributed wireless quantum networks with partially entangled pairs is feasible. 参考文献 | 相关文章 | 计量指标

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Hierarchical and probabilistic quantum state sharing via a non-maximally entangled |χ> state 彭家寅, 柏明强, 莫智文 2014 (1):  10304-010304.  doi: 10.1088/1674-1056/23/1/010304 摘要 ( 469 )   PDF(223KB) ( 485 )   A scheme that probabilistically realizes hierarchical quantum state sharing of an arbitrary unknown qubit state with a four-qubit non-maximally entangled |χ> state is presented in this paper. In the scheme, the sender Alice distributes a quantum secret with a Bell-state measurement and publishes her measurement outcomes via a classical channel to three agents who are divided into two grades. One agent is in the upper grade, while the other two agents are in the lower grade. Then by introducing an ancillary qubit, the agent of the upper grade only needs the assistance of any one of the other two agents for probabilistically obtaining the secret, while an agent of the lower grade needs the help of both the other two agents by using a controlled-NOT operation and a proper positive operator-valued measurement instead of the usual projective measurement. In other words, the agents of two different grades have different authorities to reconstruct Alice’s secret in a probabilistic manner. The scheme can also be modified to implement the threshold-controlled teleportation. 参考文献 | 相关文章 | 计量指标

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Quantum broadcast communication and authentication protocol with a quantum one-time pad 昌燕, 许春香, 张仕斌, 闫丽丽 2014 (1):  10305-010305.  doi: 10.1088/1674-1056/23/1/010305 摘要 ( 531 )   PDF(200KB) ( 595 )   A quantum broadcast communication and authentication protocol with a quantum one-time pad based on the Greenberger–Horne–Zeilinger state is proposed. A binary string is used to express the identity of the receiver, which is encoded as a single sequence of photons. The encoded photon sequence acts as a detection sequence and implements authentication. An XOR operation serves as a one-time pad and is used to ensure the security of the protocol. The binary string is reused even in a noisy channel and proves to be unconditionally secure. In contrast with the protocols proposed by Wang et al. [Chin. Phys. 16 1868 (2007)] and Yang et al. [Chin. Phys. B 19 070304 (2010)], the protocol in this study implements the identity authentication with a reusable binary string; no hash function or local unitary operation is used. The protocol in this study is also easier to implement and highly efficient without losing security. 参考文献 | 相关文章 | 计量指标

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Quantum electrodynamics in a laser and the electron laser collision 张启仁 2014 (1):  10306-010306.  doi: 10.1088/1674-1056/23/1/010306 摘要 ( 556 )   PDF(353KB) ( 357 )   Quantum electrodynamics in a laser is formulated, in which the electron–laser interaction is exactly considered, while the interaction of an electron and a single photon is considered by perturbation. The formulation is applied to the electron–laser collisions. The effect of coherence between photons in the laser is therefore fully considered in these collisions. The possibility of γ-ray laser generation by use of this kind of collision is discussed. 参考文献 | 相关文章 | 计量指标

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Oscillating multidromion excitations in higher-dimensional nonlinear lattice with intersite and external on-site potentials using symbolic computation B. Srividya, L. Kavitha, R. Ravichandran, D. Gopi 2014 (1):  10307-010307.  doi: 10.1088/1674-1056/23/1/010307 摘要 ( 495 )   PDF(2202KB) ( 595 )   We show by an extensive method of quasi-discrete multiple-scale approximation that nonlinear multi-dimensional lattice waves subjected to intersite and external on-site potentials are found to be governed by (N+1)-dimensional nonlinear Schrödinger (NLS) equation. In particular, the resonant mode interaction of (2+1)-dimensional NLS equation has been identified and the theory allows the inclusion of transverse effect. We apply the exponential function method to the (2+1)-dimensional NLS equation and obtain the class of soliton solutions with a purely algebraic computational method. Notably, we discuss in detail the effects of the external on-site potentials on the explicit form of the soliton solution generated recursively. Under the action of the external on-site potentials, the model presents a rich variety of oscillating multidromion patterns propagating in the system. 参考文献 | 相关文章 | 计量指标

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Production and detection of ultracold Cs2 molecules via four-photon adiabatic passage 李健, 刘勇, 丛书林 2014 (1):  10308-010308.  doi: 10.1088/1674-1056/23/1/010308 摘要 ( 464 )   PDF(381KB) ( 274 )   We study theoretically how to produce and detect the ultracold ground-state Cs2 molecule from Feshbach state. Numerical calculations are performed by solving the quantum Liouville equation based on multilevel Bloch model. The producing efficiency reaches 55% and the detecting efficiency is 31%. The producing and detecting efficiencies are closely related to the Rabi frequencies of laser pulses. The decay of relevant electronic and vibrational states obviously reduces the producing and detecting efficiencies. 参考文献 | 相关文章 | 计量指标

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Bifurcation analysis of the logistic map via two periodic impulsive forces 姜海波, 李涛, 曾小亮, 张丽萍 2014 (1):  10501-010501.  doi: 10.1088/1674-1056/23/1/010501 摘要 ( 597 )   PDF(17093KB) ( 351 )   The complex dynamics of the logistic map via two periodic impulsive forces is investigated in this paper. The influences of the system parameter and the impulsive forces on the dynamics of the system are studied respectively. With the parameter varying, the system produces the phenomenon such as periodic solutions, chaotic solutions, and chaotic crisis. Furthermore, the system can evolve to chaos by a cascading of period-doubling bifurcations. The Poincaré map of the logistic map via two periodic impulsive forces is constructed and its bifurcation is analyzed. Finally, the Floquet theory is extended to explore the bifurcation mechanism for the periodic solutions of this non-smooth map. 参考文献 | 相关文章 | 计量指标

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Stochastic resonance for a metapopulation system driven by multiplicative and additive colored noises 王康康, 刘先斌 2014 (1):  10502-010502.  doi: 10.1088/1674-1056/23/1/010502 摘要 ( 621 )   PDF(321KB) ( 393 )   We investigate the stochastic resonance (SR) phenomenon induced by the periodic signal in a metapopulation system with colored noises. The analytical expression of signal-to-noise is derived in the adiabatic limit. By numerical calculation, the effects of the addictive noise intensity, the multiplicative noise intensity and two noise self-correlation times on SNR are respectively discussed. It shows that: (i) in the case that the addictive noise intensity M takes a small value, a SR phenomenon for the curve of SNR appears; however, when M takes a large value, SNR turns into a monotonic function on the multiplicative noise intensity Q. (ii) The resonance peaks in the plots of the multiplicative noise intensity Q versus its self-correlation time τ1 and the addictive noise intensity M versus its self-correlation time τ2 translate in parallel. Meanwhile, a parallel translation also appears in the plots of τ1 versus Q and τ2 versus M. (iii) The interactive effects between self-correlation times τ1 and τ2 are opposite. 参考文献 | 相关文章 | 计量指标

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Surface structures of equilibrium restricted curvature model on two fractal substrates 宋丽建, 唐刚, 张永伟, 韩奎, 寻之朋, 夏辉, 郝大鹏, 李炎 2014 (1):  10503-010503.  doi: 10.1088/1674-1056/23/1/010503 摘要 ( 521 )   PDF(351KB) ( 457 )   With the aim to probe the effects of the microscopic details of fractal substrates on the scaling of discrete growth models, the surface structures of the equilibrium restricted curvature (ERC) model on Sierpinski arrowhead and crab substrates are analyzed by means of Monte Carlo simulations. These two fractal substrates have the same fractal dimension df, but possess different dynamic exponents of random walk zrw. The results show that the surface structure of the ERC model on fractal substrates are related to not only the fractal dimension df, but also to the microscopic structures of the substrates expressed by the dynamic exponent of random walk zrw. The ERC model growing on the two substrates follows the well-known Family–Vicsek scaling law and satisfies the scaling relations 2α+df ≈ z ≈ 2zrw. In addition, the values of the scaling exponents are in good agreement with the analytical prediction of the fractional Mullins–Herring equation. 参考文献 | 相关文章 | 计量指标

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Robust networked H∞ synchronization of nonidentical chaotic Lur’e systems 杨德东 2014 (1):  10504-010504.  doi: 10.1088/1674-1056/23/1/010504 摘要 ( 584 )   PDF(707KB) ( 362 )   We mainly investigate the robust networked H∞ synchronization problem of nonidentical chaotic Lur’e systems. In the design of the synchronization scheme, some network characteristics, such as nonuniform sampling, transmission-induced delays, and data packet dropouts, are considered. The parameters of master–slave chaotic Lur’e systems often allow differences. The sufficient condition in terms of linear matrix inequality (LMI) is obtained to guarantee the dissipative synchronization of nonidentical chaotic Lur’e systems in network environments. A numerical example is given to illustrate the validity of the proposed method. 参考文献 | 相关文章 | 计量指标

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Lyapunov function as potential function：A dynamical equivalence 袁若石, 马易安, 苑波, 敖平 2014 (1):  10505-010505.  doi: 10.1088/1674-1056/23/1/010505 摘要 ( 817 )   PDF(275KB) ( 882 )   For a physical system, regardless of time reversal symmetry, a potential function serves also as a Lyapunov function, providing convergence and stability information. In this paper, the converse is constructively proved that any dynamics with a Lyapunov function has a corresponding physical realization: a friction force, a Lorentz force, and a potential function. Such construction establishes a set of equations with physical meaning for Lyapunov function and suggests new approaches on the significant unsolved problem namely to construct Lyapunov functions for general nonlinear systems. In addition, a connection is found that the Lyapunov equation is a reduced form of a generalized Einstein relation for linear systems, revealing further insights of the construction. 参考文献 | 相关文章 | 计量指标

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Gradient method for blind chaotic signal separation based on proliferation exponent 吕善翔, 王兆山, 胡志辉, 冯久超 2014 (1):  10506-010506.  doi: 10.1088/1674-1056/23/1/010506 摘要 ( 501 )   PDF(635KB) ( 363 )   A new method to perform blind separation of chaotic signals is articulated in this paper, which takes advantage of the underlying features in the phase space for identifying various chaotic sources. Without incorporating any prior information about the source equations, the proposed algorithm can not only separate the mixed signals in just a few iterations, but also outperforms the fast independent component analysis (FastICA) method when noise contamination is considerable. 参考文献 | 相关文章 | 计量指标

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Control schemes for synchronizing two subnetworks with weak couplings 张建宝, 马忠军, 张刚 2014 (1):  10507-010507.  doi: 10.1088/1674-1056/23/1/010507 摘要 ( 499 )   PDF(444KB) ( 353 )   In this paper, we propose a well-designed network model with a parameter and study full and partial synchronization of the network model based on the stability analysis. The network model is composed of a star-coupled subnetwork and a globally coupled subnetwork. By analyzing the special coupling configuration, three control schemes are obtained for synchronizing the network model. Further analysis indicates that even if the inner couplings in each subnetwork are very weak, two of the control schemes are still valid. In particular, if the outer coupling weight parameter θ is larger than (n2-2n)/4, or the subnetwork size n is larger than θ2, the two subnetworks with weak inner couplings can achieve synchronization. In addition, the synchronizability is independent of the network size in case of 0< θ < n/(n+1). Finally, we carry out some numerical simulations to confirm the validity of the obtained control schemes. It is worth noting that the main idea of this paper also applies to any network consisting of a dense subnetwork and a sparse network. 参考文献 | 相关文章 | 计量指标

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Tracking of a third-order maneuvering target under an arbitrary topology 董立静, 柴森春, 张百海 2014 (1):  10508-010508.  doi: 10.1088/1674-1056/23/1/010508 摘要 ( 521 )   PDF(236KB) ( 398 )   We explore the tracking problem of a maneuvering target. Tracking agents with third-order kinematics can communicate with each other via wireless network. The communication network topology is arbitrary rather than switches among several fixed topologies. The information sharing and interaction among agents are position, velocity, and acceleration. Some sufficient conditions of tracking strategy have been proposed. Finally, a numerical example is employed to demonstrate the effectiveness of proposed tracking strategy. 参考文献 | 相关文章 | 计量指标

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Fast filtering algorithm based on vibration systems and neural information exchange and its application to micro motion robot 高娃, 查富生, 宋宝玉, 李满天 2014 (1):  10701-010701.  doi: 10.1088/1674-1056/23/1/010701 摘要 ( 897 )   PDF(893KB) ( 439 )   This paper develops a fast filtering algorithm based on vibration systems theory and neural information exchange approach. The characters, including the derivation process and parameter analysis, are discussed and the feasibility and the effectiveness are testified by the filtering performance compared with various filtering methods, such as the fast wavelet transform algorithm, the particle filtering method and our previously developed single degree of freedom vibration system filtering algorithm, according to simulation and practical approaches. Meanwhile, the comparisons indicate that a significant advantage of the proposed fast filtering algorithm is its extremely fast filtering speed with good filtering performance. Further, the developed fast filtering algorithm is applied to the navigation and positioning system of the micro motion robot, which is a high real-time requirement for the signals preprocessing. Then, the preprocessing data is used to estimate the heading angle error and the attitude angle error of the micro motion robot. The estimation experiments illustrate the high practicality of the proposed fast filtering algorithm. 参考文献 | 相关文章 | 计量指标

ATOMIC AND MOLECULAR PHYSICS

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Simulating electron momentum spectra of iso-C2H2Cl2：A study of the core electronic structure 黄艳茹, 陈明明 2014 (1):  13101-013101.  doi: 10.1088/1674-1056/23/1/013101 摘要 ( 693 )   PDF(912KB) ( 370 )   Electron momentum spectroscopy (EMS) has been used for the first time to study core electronic structure of iso-C2H2Cl2. In the present work, the pronounced difference between ionization energies of two C1s core orbitals (2A1 and 3A1) is seen as a chemical shift of 3 eV, which is due to different chemical environments of the related carbon atoms. Both the calculated spherically averaged core electron momentum distributions (MDs) and three-dimensional electron momentum density maps show that these core molecular orbitals (MOs) 2A1 and 3A1 exhibit strong atomic orbital characteristics in real and momentum space. However, the core states 2B2 and 4A1, which are almost degenerate and related to two equivalent atoms, exhibit notable differences between the momentum and position depictions. In contrast to the position space, the momentum density maps of these two core MOs highlight the interference effects which are due to the nuclear positions. The 2B2 orbital of iso-C2H2Cl2 is the antisymmetric counterpart of the 4A1 core orbital in real space. However, it relates to the 4A1 orbital by an exchange of maxima and minima in momentum space. Due to interference effects between electrons scattered from different atomic centers, modulations with a periodicity of 1.12 a.u. can be seen in the computed momentum densities, which tend to decay with increasing electron momenta. Accordingly, the EMS can not only effectively image the electronic structure of compounds by studying valence orbitals, but also provides direct information on the nature of the nuclear geometry by investigating the core states. 参考文献 | 相关文章 | 计量指标

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Effects of pressure and gas-jet thickness on the generation of attosecond pulse 李小勇, 王国利, 周效信 2014 (1):  13102-013102.  doi: 10.1088/1674-1056/23/1/013102 摘要 ( 545 )   PDF(243KB) ( 426 )   We investigate how the intensity and duration of an attosecond pulse generated from high-order harmonic generation are affected by the pressure and thickness of the gas jet by taking into account the macroscopic propagation of both fundamental and harmonic fields. Our simulations show that, limited by the propagation effects, especially the absorption of harmonics, the intensity of an attosecond pulse cannot be improved by just independently increasing the gas pressure or the medium length. On the other hand, due to good phase-matching conditions, the duration of a generated attosecond pulse can be improved by changing the gas pressure. 参考文献 | 相关文章 | 计量指标

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Density functional theory study of Mg2Nin (n=1–8) clusters 张建婷, 李晶, 盛勇 2014 (1):  13103-013103.  doi: 10.1088/1674-1056/23/1/013103 摘要 ( 561 )   PDF(3342KB) ( 618 )   The density functional theory B3PW91 with LANL2DZ basis sets has been used to study the possible geometries of Mg2Nin (n=1–8) clusters. For the lowest energy structures of the clusters, stabilities, electronic properties, and natural bond orbital (NBO) are calculated and discussed. The results show that the doped Mg atoms reduce the stabilities of pure Ni clusters. The Mg2Ni2, Mg2Ni4, and Mg2Ni6 clusters are more stable than neighboring clusters. The system appears magic number characteristics. In addition, the hybridization phenomenon occurs, owing to the interaction of Mg and Ni. The result of charge transfer is that Ni atom is negative and the Mg atom is positive. We also conclude that the 3p and 4d orbitals of the Ni atom have an effect on the stabilities of the clusters. 参考文献 | 相关文章 | 计量指标

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Ionization energies and term energies of the ground states 1s22s of lithium-like systems 李金英, 王治文 2014 (1):  13201-013201.  doi: 10.1088/1674-1056/23/1/013201 摘要 ( 586 )   PDF(197KB) ( 580 )   We extend the Hamiltonian method of the full-core plus correlation (FCPC) by minimizing the expectation value to calculate the non-relativistic energies and the wave functions of 1s22s states for the lithium-like systems from Z=41 to 50. The mass-polarization and the relativistic corrections including the kinetic-energy correction, the Darwin term, the electron–electron contact term, and the orbit–orbit interaction are calculated perturbatively as first-order correction. The contribution from quantum electrodynamic (QED) is also explored by using the effective nuclear charge formula. The ionization potential and term energies of the ground states 1s22s are derived and compared with other theoretical calculation results. It is shown that the FCPC methods are also effective for theoretical calculation of the ionic structure for high nuclear ion of lithium-like systems. 参考文献 | 相关文章 | 计量指标

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Theoretical study on isotope separation of an ytterbium atomic beam by laser deflection 周敏, 徐信业 2014 (1):  13202-013202.  doi: 10.1088/1674-1056/23/1/013202 摘要 ( 599 )   PDF(2799KB) ( 408 )   Isotope separation by laser deflecting an atomic beam is analyzed theoretically. Interacting with a tilted one-dimensional optical molasses, an ytterbium atomic beam is split into multi-beams with different isotopes like 172Yb, 173Yb, and 174Yb. By using the numerical calculation, the dependences of the splitting angle on the molasses laser intensity and detuning are studied, and the optimal parameters for the isotope separation are also investigated. Furthermore, the isotope separation efficiency and purity are estimated. Finally a new scheme for the efficient isotope separation is proposed. These findings will give a guideline for simply obtaining pure isotopes of various elements. 参考文献 | 相关文章 | 计量指标

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High-resolution photoassociation spectroscopy of ultracold Cs2 long-range 0g- state：The external well potential depth 刘文良, 武寄洲, 马杰, 肖连团, 贾锁堂 2014 (1):  13301-013301.  doi: 10.1088/1674-1056/23/1/013301 摘要 ( 601 )   PDF(375KB) ( 340 )   High-resolution photoassociation spectroscopy is reported using a modulation spectroscopy technology in a cesium atomic magneto-optical trap. The two lowest vibrational levels have been experimentally observed which have been theoretically predicted in [Phys. Rev. A 75 052501 (2007)]. A new potential curve is obtained by using the Rydberg–Klein–Ress method with a well depth of -82.384±0.026 cm-1, which is deeper than the result of previous experiment (-77.909 cm-1) and the theoretical prediction (-81.6445 cm-1). 参考文献 | 相关文章 | 计量指标

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Field-free molecular orientation induced by combined femtosecond single- and dual-color laser pulses：The role of delay time and quantum interference 秦朝朝, 贾光瑞, 张现周, 刘玉芳, 龙金友, 张冰 2014 (1):  13302-013302.  doi: 10.1088/1674-1056/23/1/013302 摘要 ( 458 )   PDF(13719KB) ( 361 )   The coherent control of field-free molecular orientation of CO with combined femtosecond single- and dual-color laser pulses has been theoretically studied. The effect of the delay time between the femtosecond single- and dual-color laser pulses is discussed, and the physical mechanism of the enhancement of molecular orientation with pre-alignment of the molecule is investigated. It is found that the basic mechanism is based on the creation of a rotational wave packet by the femtosecond single-color laser pulse. Furthermore, we investigate the interference between multiple rotational excitation pathways following pre-alignment with femtosecond single-color laser pulse. It is shown that such interference can lead to an enhancement of the orientation of CO molecule by a factor of 1.6. 参考文献 | 相关文章 | 计量指标

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Mαβ X-ray production cross sections of Pb and Bi by 9–40 keV electron impact 吴英, 王冠鹰, 穆强, 赵强 2014 (1):  13401-013401.  doi: 10.1088/1674-1056/23/1/013401 摘要 ( 527 )   PDF(264KB) ( 380 )   The experimental data of Mαβ X-ray production cross sections for Pb and Bi by 9–40 keV electron impact have been given. Thin films with thick carbon substrates are used in the experiment. The effects of target structure on the Mαβ X-ray production cross sections are corrected by using the Monte Carlo method. The corrected experimental data are compared with calculated cross sections in terms of the distorted-wave Born approximation (DWBA) theory. The measured Mαβ X-ray production cross sections for Pb and Bi are lower than the DWBA calculations. The atomic relaxation parameters used in comparing the DWBA values with experimental results affect the degree of difference. 参考文献 | 相关文章 | 计量指标

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Radio-frequency spectroscopy of weakly bound molecules in ultracold Fermi gas 黄良辉, 王鹏军, 付正坤, 张靖 2014 (1):  13402-013402.  doi: 10.1088/1674-1056/23/1/013402 摘要 ( 532 )   PDF(379KB) ( 390 )   We create weakly bound Feshbach molecules in ultracold Fermi gas 40K by sweeping a magnetic field across a broad Feshbach resonance point 202.2 G with a rate of 20 ms/G and perform the dissociation process using radio-frequency (RF) technology. From RF spectroscopy, we obtain the binding energy of the weakly bound molecules in the vicinity of Feshbach resonance. Our measurement also shows that the number of atoms generated from the dissociation process is different at various magnetic fields with the same RF amplitude, which gives us a deeper understanding of weakly bound Feshbach molecules. 参考文献 | 相关文章 | 计量指标

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Structure, stability and electronic properties of SrSin (n=1–12) clusters：Density-functional theory investigation 张帅, 秦怡, 马毛粉, 卢成, 李根全 2014 (1):  13601-013601.  doi: 10.1088/1674-1056/23/1/013601 摘要 ( 409 )   PDF(1577KB) ( 452 )   Geometric structures, stabilities, and electronic properties of SrSin (n=1–12) clusters have been investigated using the density-functional theory within the generalized gradient approximation. The optimized geometries indicate that one Si atom capped on SrSin-1 structure and Sr atom capped Sin structure for difference SrSin clusters in size are two dominant growth patterns. The calculated average binding energy, fragmentation energy, second-order energy difference, the highest occupied molecular orbital, and the lowest unoccupied molecular orbital (HOMO–LUMO) gaps show that the doping of Sr atom can enhance the chemical activity of the silicon framework. The relative stability of SrSi9 is the strongest among the SrSin clusters. According to the mulliken population and natural population analysis, it is found that the charge in SrSin clusters transfer from Sr atom to the Sin host. In addition, the vertical ionization potential, vertical electron affinity, and chemical hardness are also discussed and compared. 参考文献 | 相关文章 | 计量指标

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Electrostatic surface trap for cold polar molecules on a chip 王琴, 李胜强, 侯顺永, 夏勇, 汪海玲, 印建平 2014 (1):  13701-013701.  doi: 10.1088/1674-1056/23/1/013701 摘要 ( 572 )   PDF(1226KB) ( 485 )   We propose a simple scheme for trapping cold polar molecules in low-field seeking states on the surface of a chip by using a grounded metal plate and two finite-length charged wires that half embanked in an insulating substrate, calculate the electric field distributions generated by our charged-wire layout in free space and the corresponding Stark potentials for ND3 molecules, and analyze the dependence of the trapping center position on the geometric parameters. Moreover, the loading and trapping processes of cold ND3 molecules are studied by using the Monte Carlo method. Our study shows that the loading efficiency of the trap scheme can reach 11.5%, and the corresponding temperature of the trapped cold molecules is about 26.4 mK. 参考文献 | 相关文章 | 计量指标

ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS

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Extraordinary terahertz transmission through subwavelength spindle-like apertures in NbN film 郑小睿, 程飞, 吴敬波, 金飚兵, 朱北沂 2014 (1):  14101-014101.  doi: 10.1088/1674-1056/23/1/014101 摘要 ( 472 )   PDF(375KB) ( 452 )   We studied numerically the temperature dependent extraordinary terahertz transmission through niobium nitride (NbN) film perforated with subwavelength spindle-like apertures. Both the resonant frequency and intensity of extraordinary terahertz transmission peaks can be greatly modified by the transition of NbN film from the normal state to the superconducting state. An enhancement of the (±1,0) NbN/magnesium oxide (MgO) peak intensity as high as 200% is demonstrated due to the combined contribution of both the superconducting transition and the excitation of localized surface plasmons (LSPs) around the apertures. The extraordinary terahertz transmission through spindle-like hole arrays patterned on the NbN film can pave the way for us to explore novel active tuning devices. 参考文献 | 相关文章 | 计量指标

RAPID COMMUNICATION

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Low temperature enhancement of alignment-induced spectral broadening of femtosecond laser pulses 王翡, 蒋红兵, 龚旗煌 2014 (1):  14201-014201.  doi: 10.1088/1674-1056/23/1/014201 摘要 ( 518 )   PDF(325KB) ( 368 )   The propagation of femtosecond laser pulses in N2-filled hollow fibers is studied both theoretically and experimentally. The laser pulse aligns the N2 molecules and changes the refractive index, which meanwhile modulates the spectrum of the pulse in turn. The dependence of the spectral modulation on the gas temperature is investigated. We find that both spectral broadening and frequency red-shift are enhanced at low temperature. The degree of enhancement is found to be dependent on the pulse duration. Based on our findings, we propose a method for femtosecond pulse spectral broadening and few-cycle pulse generation via the molecular alignment. 参考文献 | 相关文章 | 计量指标

ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS

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Third-order aberration analysis of tilted-pupil optical systems 王钰, 张新, 王灵杰, 王超 2014 (1):  14202-014202.  doi: 10.1088/1674-1056/23/1/014202 摘要 ( 549 )   PDF(326KB) ( 992 )   The aberration field of an optical system with a tilted pupil is explored through expanding the vector expressions of the third-order wavefront aberrations. First, the vector forms of the wavefront aberrations are modified to obtain the aberration expressions with the pupil tilted; full field displays of coma and astigmatism in this situation are given. Then, the third-order aberration formulas with the pupil decentered and tilted simultaneously are derived and discussed. Finally, an example is taken to certify the validity of aberration distribution properties. 参考文献 | 相关文章 | 计量指标

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Optical properties of ytterbium-doped tandem-pumped fiber oscillator 郝金坪, 闫平, 肖起榕, 李丹, 巩马理 2014 (1):  14203-014203.  doi: 10.1088/1674-1056/23/1/014203 摘要 ( 725 )   PDF(385KB) ( 1099 )   With the rapid development of ytterbium-doped fiber lasers, some obtrusive limitations on power scaling appeared. In order to avoid these problems, a scheme called tandem pumping is introduced into the fiber laser field. In this paper, the optical properties of an ytterbium-doped tandem-pumped fiber oscillator are presented. According to the oscillator profile, the proper gain fiber type and pump wavelength range are picked out, under the comprehensive consideration of laser conversion efficiency and beam quality. In addition, the photodarkening performances of tandem pumping lasers and conventional ones are compared based on practical application, with all possible impact parameters taken into account. Moreover, an all-fibered tandem-pumped oscillator centered at 1079.5 nm is built, in the way of clad pumping by a 1030-nm fiber laser. The laser power of the oscillator reaches 7 W, with an opto-optic efficiency of 82.4%. 参考文献 | 相关文章 | 计量指标

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Nonvolatile photorefractive properties in triply doped stoichiometric Mg：Fe：Mn：LiTaO3 crystals 孙婷, 张晓东, 孙亮, 王锐 2014 (1):  14204-014204.  doi: 10.1088/1674-1056/23/1/014204 摘要 ( 466 )   PDF(336KB) ( 383 )   We have grown triply doped Mg:Fe:Mn:LiTaO3 crystals with near stoichiometry using the top seeded solution growth technique. The defect structure was investigated by infrared absorption spectra and Curie temperature. Using a blue laser as the source, excellent photorefractive properties were obtained. Nonvolatile holographic storage properties were investigated using the dual wavelength technique. We got a very high fixed diffraction efficiency and nonvolatile holographic storage sensitivity. The blue light has more than enough energy to excite holes of deep (Mn) and shallow (Fe) trap centers with the same phase, which enhance dramatically the blue photorefractive properties and the nonvolatile holographic storage. Mg2+ ion is no longer damage resistant at blue laser, but enhances photorefractive characteristics. 参考文献 | 相关文章 | 计量指标

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Controllable beating signal using stored light pulse 王磊, 杨庆禹, 王潇潇, 罗梦希, 范云飞, 康智慧, 戴天缘, 毕升, 王海华, 吴金辉, 高锦岳 2014 (1):  14205-014205.  doi: 10.1088/1674-1056/23/1/014205 摘要 ( 520 )   PDF(415KB) ( 374 )   We experimentally study the controllable generation of a beating signal using stored light pulses based on electromagnetically induced transparency (EIT) in a solid medium. The beating signal relies on an asymmetric procedure of light storage and retrieval. After storing the probe pulse into the spin coherence under the EIT condition, two-color control fields with opposite detunings instead of the initial control field are used to scatter the stored spin coherence. The controllable beating signal is generated due to alternative constructive and destructive interferences in the retrieved signal intensities. The beating of the two-color control fields is mapped into the beating of weak probe fields by using atomic spin coherence. This beating signal will be important in precise atomic spectroscopy and fast quantum limited measurements. 参考文献 | 相关文章 | 计量指标

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Irradiation effect on strain sensitivity coefficient of strain sensing fiber Bragg gratings 金靖, 林松, 宋凝芳 2014 (1):  14206-014206.  doi: 10.1088/1674-1056/23/1/014206 摘要 ( 597 )   PDF(415KB) ( 364 )   The effect of irradiation on the strain sensitivity coefficient of strain sensing fiber Bragg gratings (FBGs) has been investigated through experiments. FBGs were fabricated in single mode fibers with 3 mol% Ge-concentration in the core and with a H2-loading treatment. In experiments, the FBGs were subjected to γ-radiation exposures using a Co60 source at a dose-rate of 25 Gy/min up to a total dose of 10.5 kGy. The GeO defect in fiber absorbs photons to form a GeE’ defect; the interaction with H2 is a probable reason for the γ-radiation sensitivity of gratings written in hydrogen loaded fibres. The effect mechanism of radiation on the strain sensitivity coefficient is similar to that of radiation on the temperature sensitivity coefficient. Radiation affects the effective index neff, which results in the change of the thermo-optic coefficient and the strain-optic coefficient. Irradiation can change the strain sensitivity coefficient of FBGs by 1.48%–2.71%, as well as changing the Bragg wavelength shift (BWS) by 22 pm–25 pm under a total dose of 10.5 kGy. Our research demonstrates that the effect of irradiation on the strain sensitivity coefficient of FBG is small and that strain sensing FBGs can work well in the radiation environment. 参考文献 | 相关文章 | 计量指标

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Experimental demonstration of single-mode fiber coupling using adaptive fiber coupler 罗文, 耿超, 武云云, 谭毅, 罗奇, 刘红梅, 李新阳 2014 (1):  14207-014207.  doi: 10.1088/1674-1056/23/1/014207 摘要 ( 679 )   PDF(402KB) ( 1386 )   Coupling plane wave into a single-mode fiber (SMF) with high and steady coupling efficiency is crucial for fiber-based free-space laser systems, where random angular jitters are the main influencing factors of fiber coupling. In this paper, we verified a new adaptive-optic device named adaptive fiber coupler (AFC) which could compensate angular jitters and improve the SMF coupling efficiency in some degree. Experiments of SMF coupling under the angular jitter situation using AFC have been achieved. Stochastic parallel gradient descent (SPGD) algorithm is employed as the control strategy, of which the iteration rate is 625 Hz. In closed loop, the coupling efficiency keeps above 65% when angular errors are below 80 μrad. The compensation bandwidth is 35 Hz at sine-jitter of 15 μrad amplitude with average coupling efficiency of above 60%. Also, experiments with simulated turbulence have been studied. The average coupling efficiency increases from 31.97% in open loop to 61.33% in closed loop, and mean square error (MSE) of coupling efficiency drops from 7.43% to 1.75%. 参考文献 | 相关文章 | 计量指标

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Seismoelectric waves in a borehole excited by an external explosive source 周久光, 崔志文, 吕伟国, 张玉君, 王克协 2014 (1):  14301-014301.  doi: 10.1088/1674-1056/23/1/014301 摘要 ( 569 )   PDF(472KB) ( 416 )   The conversion of energy between seismic and electromagnetic wave fields has been described by Pride’s coupled equations in porous media. In this paper, the seismoelectric field excited by the explosive point source located at the outside of the borehole is studied. The scattering fields inside and outside a borehole are analyzed and deduced under the boundary conditions at the interface between fluid and porous media. The influences of the distance of the point source, multipole components of the eccentric explosive source, and the receiving position along the axis of vertical borehole, on the converted waves inside the borehole are all investigated. When the distance from the acoustic source to the axis of a borehole is far enough, the longitudinal and coseismic longitudinal wave packets dominate the acoustic and electric field, respectively. The three components of both electric field and magnetic field can be detected, and the radial electric field is mainly excited and converted by the dipole component. Owing to the existence of borehole, the electric fields and magnetic fields in the borehole are azimuthal. The distance from the point where the maximum amplitude of the axial components of electric field is recorded, to the origin of coordinate indicates the horizontal distance from the explosive source to the axis of vertical borehole. 参考文献 | 相关文章 | 计量指标

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Longitudinal and transverse structure functions in decaying nearly homogeneous and isotropic turbulence Imtiaz Ahmad, 卢志明, 刘宇陆 2014 (1):  14701-014701.  doi: 10.1088/1674-1056/23/1/014701 摘要 ( 521 )   PDF(486KB) ( 450 )   Streamwise evolution of longitudinal and transverse velocity structure functions in a decaying homogeneous and nearly isotropic turbulence is reported for Reynolds numbers Reλ up to 720. First, two theoretical relations between longitudinal and transverse structure functions are examined in the light of recently derived relations and the results show that the low-order transverse structure functions can be well approximated by longitudinal ones within the sub-inertial range. Reconstruction of fourth-order transverse structure functions with a recently proposed relation by Grauer et al. is comparatively less valid than the relation already proposed by Antonia et al. Secondly, extended self-similarity methods are used to measure the scaling exponents up to order eight and the streamwise evolution of scaling exponents is explored. The scaling exponents of longitudinal structure functions are, at first location, close to Zybin’s model, and at the fourth location, close to She–Leveque model. No obvious trend is found for the streamwise evolution of longitudinal scaling exponents, whereas, on the contrary, transverse scaling exponents become slightly smaller with the development of a steamwise direction. Finally, the stremwise variation of the order-dependent isotropy ratio indicates the turbulence at the last location is closer to isotropic than the other three locations. 参考文献 | 相关文章 | 计量指标

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Dual solutions in boundary layer flow of a moving fluid over a moving permeable surface in presence of prescribed surface temperature and thermal radiation Swati Mukhopadhyay 2014 (1):  14702-014702.  doi: 10.1088/1674-1056/23/1/014702 摘要 ( 499 )   PDF(471KB) ( 373 )   An analysis of the heat transfer for a boundary layer forced convective flow past a moving permeable flat surface parallel to a moving fluid is presented. Prescribed surface temperature at the boundary is considered. A thermal radiation term in the energy equation is considered. The similarity solutions for the problem are obtained and the reduced ordinary differential equations are solved numerically. To support the validity of the numerical results, a comparison is made with the available results for some particular cases of this study. Dual solutions exist when the surface and the fluid move in the opposite directions. 参考文献 | 相关文章 | 计量指标

PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES

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A growth kinetics model of rate decomposition for Si1-xGex alloy based on dimer theory 戴显英, 吉瑶, 郝跃 2014 (1):  15101-015101.  doi: 10.1088/1674-1056/23/1/015101 摘要 ( 623 )   PDF(296KB) ( 430 )   According to the dimer theory on semiconductor surface and chemical vapor deposition(CVD) growth characteristics of Si1-xGex, two mechanisms of rate decomposition and discrete flow density are proposed. Based on these two mechanisms, the Grove theory and Fick’s first law, a CVD growth kinetics model of Si1-xGex alloy is established. In order to make the model more accurate, two growth control mechanisms of vapor transport and surface reaction are taken into account. The paper also considers the influence of the dimer structure on the growth rate. The results show that the model calculated value is consistent with the experimental values at different temperatures. 参考文献 | 相关文章 | 计量指标

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Exploration of the Townsend regime by discharge light emission in a gas discharge device Hilal Yucel Kurt 2014 (1):  15201-015201.  doi: 10.1088/1674-1056/23/1/015201 摘要 ( 482 )   PDF(1285KB) ( 535 )   The Townsend discharge mechanism has been explored in a planar microelectronic gas discharge device (MGDD) with different applied voltages U and interelectrode distance d under various pressures in air. The anode and the cathode of the MGDD are formed by a transparent SnO2 covered glass and a GaAs semiconductor, respectively. In the experiments, the discharge is found to be unstable just below the breakdown voltage Ub, whereas the discharge passes through a homogeneous stable Townsend mode beyond the breakdown voltage. The measurements are made by an electrical circuit and a CCD camera by recording the currents and light emission (LE) intensities. The intensity profiles, which are converted from the 3D light emission images along the semiconductor diameter, have been analysed for different system parameters. Different instantaneous conductivity σt regimes are found below and beyond the Townsend region. These regimes govern the current and spatio-temporal LE stabilities in the plasma system. It has been proven that the stable LE region increases up to 550 Torr as a function of pressure for small d. If the active area of the semiconductor becomes larger and the interlectrode distance d becomes smaller, the stable LE region stays nearly constant with pressure. 参考文献 | 相关文章 | 计量指标

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Growth rate of peeling mode in the near separatrix region of diverted tokamak plasma 石秉仁 2014 (1):  15202-015202.  doi: 10.1088/1674-1056/23/1/015202 摘要 ( 714 )   PDF(197KB) ( 328 )   An analytical expression of the peeling mode in the near separatrix region of diverted tokamak plasma is derived. It is shown that in diverted plasmas both with single and double X points, though the perturbed potential energy of the unstable peeling mode tends to be large, its growth rate becomes very small due to the even larger kinetic energy. Compared to some recent studies that give qualitatively correct results about this growth rate, our result is directly related with the diverted equilibrium quantities suitable for application to realistic experiments. 参考文献 | 相关文章 | 计量指标

CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES

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Quasi-homoepitaxial GaN-based blue light emitting diode on thick GaN template 李俊泽, 陶岳彬, 陈志忠, 姜显哲, 付星星, 姜爽, 焦倩倩, 于彤军, 张国义 2014 (1):  16101-016101.  doi: 10.1088/1674-1056/23/1/016101 摘要 ( 529 )   PDF(533KB) ( 568 )   The high power GaN-based blue light emitting diode (LED) on an 80-μ-thick GaN template is proposed and even realized by several technical methods like metal organic chemical vapor deposition (MOCVD), hydride vapor-phase epitaxial (HVPE), and laser lift-off (LLO). Its advantages are demonstrated from material quality and chip processing. It is investigated by high resolution X-ray diffraction (XRD), high resolution transmission electron microscope (HRTEM), Rutherford back-scattering (RBS), photoluminescence, current-voltage and light output-current measurements. The width of (0002) reflection in XRD rocking curve, which reaches 173" for the thick GaN template LED, is less than that for the conventional one, which reaches 258". The HRTEM images show that the multiple quantum wells (MQWs) in 80-μm-thick GaN template LED have a generally higher crystal quality. The light output at 350 mA from the thick GaN template LED is doubled compared to traditional LEDs and the forward bias is also substantially reduced. The high performance of 80-μm-thick GaN template LED depends on the high crystal quality. However, although the intensity of MQWs emission in PL spectra is doubled, both the wavelength and the width of the emission from thick GaN template LED are increased. This is due to the strain relaxation on the surface of 80-μm-thick GaN template, which changes the strain in InGaN QWs and leads to InGaN phase separation. 参考文献 | 相关文章 | 计量指标

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Structural stability and electrical properties of AlB2-type MnB2 under high pressure 孟祥旭, 范靖, 包括, 李芳菲, 黄晓丽, 李岩, 田夫波, 段德芳, 靳锡联, 朱品文, 何志, 周强, 高春晓, 刘冰冰, 崔田 2014 (1):  16102-016102.  doi: 10.1088/1674-1056/23/1/016102 摘要 ( 579 )   PDF(379KB) ( 768 )   The structural stability and electrical properties of AlB2-type MnB2 were studied based on high pressure angle-dispersive x-ray diffraction, in situ electrical resistivity measured in a diamond anvil cell (DAC) and first-principles calculations under high pressure. The x-ray diffraction results show that the structure of AlB2-type MnB2 remains stable up to 42.6 GPa. From the equation of state of MnB2, we obtained a bulk modulus value of 169.9±3.7 GPa with a fixed pressure derivative of 4, which indicates that AlB2-type MnB2 is a hard and incompressible material. The electrical resistance undergoes a transition at about 19.3 GPa, which can be explained by a transition of manganese 3d electrons from localization to delocalization under high pressure. 参考文献 | 相关文章 | 计量指标

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Structural and electron charge density studies of a nonlinear optical compound 4,4 di-methyl amino cyano biphenyl Naima Boubegra, Abdelkader Chouaih, Mokhtaria Drissi, Fodil Hamzaoui 2014 (1):  16103-016103.  doi: 10.1088/1674-1056/23/1/016103 摘要 ( 560 )   PDF(724KB) ( 368 )   The 4,4 dimethyl amino cyano biphenyl crystal (DMACB) is characterized by its nonlinear activity. The intra molecular charge transfer of this molecule results mainly from the electronic transmission of the electro-acceptor (cyano) and electro-donor (di-methyl-amino) groups. An accurate electron density distribution around the molecule has been calculated based on a high-resolution X-ray diffraction study. The data were collected at 123 K using graphite-monochromated Mo Kα radiation to sin(θ)/λ=1.24 Å-1. The integrated intensities of 13796 reflections were measured and reduced to 6501 independent reflections with I≥3σ(I). The crystal structure was refined using the experimental model of Hansen and Coppens (1978). The crystal structure has been validated and deposited at the Cambridge Crystallographic Data Centre with the deposition number CCDC 876507. In this article, we present the thermal motion and the structural analysis obtained from the least-square refinement based on F2 and the electron density distribution obtained from the multipolar model. 参考文献 | 相关文章 | 计量指标

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Structure of Lennard–Jones nanowires encapsulated by carbon nanotubes 吴雯倩, 田明丽, 陈航燕, 袁清红, 孙得彦 2014 (1):  16104-016104.  doi: 10.1088/1674-1056/23/1/016104 摘要 ( 449 )   PDF(986KB) ( 512 )   Molecular dynamics simulations have been performed to investigate the structures of Lennard–Jones (LJ) nanowires (NWs) encapsulated in carbon nanotubes (CNTs). We find that the structures of NWs in a small CNT only adopt multi-shell motifs, while the structures of NWs in a larger CNT tend to adopt various motifs. Among these structures, three of them have not been reported previously. The phase boundaries among these structures are obtained regarding filling fractions, as well as the interaction between NWs and CNTs. 参考文献 | 相关文章 | 计量指标

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First-principles calculations of 5d atoms doped hexagonal-AlN sheets：Geometry, magnetic property and the influence of symmetry and symmetry-breaking on the electronic structure 张召富, 周铁戈, 赵海洋, 卫湘蕾 2014 (1):  16801-016801.  doi: 10.1088/1674-1056/23/1/016801 摘要 ( 592 )   PDF(834KB) ( 785 )   The geometry, electronic structure and magnetic property of the hexagonal AlN (h-AlN) sheet doped by 5d atoms (Lu, Hf, Ta, W, Re, Os, Ir, Pt, Au and Hg) are investigated by first-principles calculations based on the density functional theory. The influence of symmetry and symmetry-breaking is also studied. There are two types of local symmetries of the doped systems: C3v and D3h. The symmetry will deviate from exact C3v and D3h for some particular dopants after optimization. The total magnetic moments of the doped systems are 0μB for Lu, Ta and Ir; 1μB for Hf, W, Pt and Hg; 2μB for Re and Au; and 3μB for Os and Al-vacancy. The total densities of state are presented, where impurity energy levels exist. The impurity energy levels and total magnetic moments can be explained by the splitting of 5d orbitals or molecular orbitals under different symmetries. 参考文献 | 相关文章 | 计量指标

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Kinetic Monte Carlo simulations of three-dimensional self-assembled quantum dot islands 宋鑫, 冯昊, 刘玉敏, 俞重远, 尹昊智 2014 (1):  16802-016802.  doi: 10.1088/1674-1056/23/1/016802 摘要 ( 525 )   PDF(1208KB) ( 380 )   By three-dimensional kinetic Monte Carlo simulations, the effects of the temperature, the flux rate, the total coverage and the interruption time on the distribution and the number of self-assembled InAs/GaAs (001) quantum dot (QD) islands are studied, which shows that a higher temperature, a lower flux rate and a longer growth time correspond to a better island distribution. The relations between the number of islands and the temperature and the flux rate are also successfully simulated. It is observed that for the total coverage lower than 0.5 ML, the number of islands decreases with the temperature increasing and other growth parameters fixed and the number of islands increases with the flux rate increasing when the deposition is lower than 0.6 ML and the other parameters are fixed. 参考文献 | 相关文章 | 计量指标

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Superamphiphobic, light-trapping FeSe2 particles with a micro-nano hierarchical structure obtained by an improved solvothermal method 郁菁, 王会杰, 邵伟佳, 许小亮 2014 (1):  16803-016803.  doi: 10.1088/1674-1056/23/1/016803 摘要 ( 509 )   PDF(724KB) ( 726 )   Wettability and the light-trapping effect of FeSe2 particles with a micro-nano hierarchical structure have been investigated. Particles are synthesized by an improved solvothermal method, wherein hexadecyl trimethyl ammonium bromide (CTAB) is employed as a surfactant. After modifying the particles with heptadecafluorodecyltrimethoxy-silane (HTMS), we find that the water contact angle (WCA) of the FeSe2 particles increases by 6.1° and the water sliding angle (WSA) decreases by 2.5° respectively, and the diffuse reflectivity decreases 29.4% compared with similar FeSe2 particles synthesized by the conventional method. The growth process of the particles is analyzed and a growth scenario is given. Upon altering the PH values of the water, we observe that the superhydrophobic property is maintained quite consistently across a wide PH range of 1–14. Moreover, the modified particles were also found to be superoleophobic. To the best of our knowledge, there is no systematic research on the wettability of FeSe2 particles, so our research provides a reference for other researchers. 参考文献 | 相关文章 | 计量指标

CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES

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Phase transition and thermodynamic properties of ThO2：Quasi-harmonic approximation calculations and anharmonic effects 李强, 杨俊升, 黄多辉, 曹启龙, 王藩侯 2014 (1):  17101-017101.  doi: 10.1088/1674-1056/23/1/017101 摘要 ( 570 )   PDF(478KB) ( 457 )   The thermodynamic properties and the phase transition of ThO2 from the cubic structure to the orthorhombic structure are investigated using the first-principles projector-augmented wave method. The vibrational contribution to Helmholtz free energy is evaluated from the first-principles phonon calculations. The anharmonic contribution to quasi-harmonic free energy is accounted for by using an effective method (2010 Phys. Rev. B 81 172301). The results reveal that at ambient temperature, the phase transition from the cubic phase to the orthorhombic phase occurs at 26.45 GPa, which is consistent with the experimental and theoretical data. With increasing temperature, the transition pressure decreases almost linearly. By comparing the experimental results with the calculation results, it is shown that the thermodynamic properties of ThO2 at high temperature improve substantially after including the anharmonic correction to quasi-harmonic free energy. 参考文献 | 相关文章 | 计量指标

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Spin texturing in a parabolically confined quantum wire with Rashba and Dresselhaus spin–orbit interactions S. Saríkurt, S. Şakiroğlu, K. Akgüngör, İ. Sökmen 2014 (1):  17102-017102.  doi: 10.1088/1674-1056/23/1/017102 摘要 ( 513 )   PDF(914KB) ( 627 )   In this study, we investigate theoretically the effect of spin–orbit coupling on the energy level spectrum and spin texturing of a quantum wire with a parabolic confining potential subjected to the perpendicular magnetic field. Highly accurate numerical calculations have been carried out using a finite element method. Our results reveal that the interplay between the spin–orbit interaction and the effective magnetic field significantly modifies the band structure, producing additional subband extrema and energy gaps. Competing effects between external field and spin–orbit interactions introduce complex features in spin texturing owing to the couplings in energy subbands. We obtain that spatial modulation of the spin density along the wire width can be considerably modified by the spin–orbit coupling strength, magnetic field and charge carrier concentration. 参考文献 | 相关文章 | 计量指标

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Effects of the 3d transition metal doping on the structural, electronic, and magnetic properties of BeO nanotubes 张建民, 宋婉婷, 李欢欢, 徐可为, Ji Vincent 2014 (1):  17103-017103.  doi: 10.1088/1674-1056/23/1/017103 摘要 ( 659 )   PDF(421KB) ( 462 )   First-principles calculations have been performed on the structural, electronic, and magnetic properties of seven 3d transition-metal (TM) impurities (V, Cr, Mn, Fe, Co, Ni, and Cu) doped armchair (5,5) and zigzag (8,0) beryllium oxide nanotubes (BeONTs). The results show that there exists a structural distortion around the 3d TM impurities with respect to the pristine BeONTs. The magnetic moment increases for V- and Cr-doped BeONTs and reaches a maximum for Mn-doped BeONT, and then decreases for Fe-, Co-, Ni-, and Cu-doped BeONTs successively, consistent with the predicted trend of Hund’s rule to maximize the magnetic moments of the doped TM ions. However, the values of the magnetic moments are smaller than the predicted values of Hund’s rule due to the strong hybridization between the 2p orbitals of the near O and Be ions of BeONTs and the 3d orbitals of the TM ions. Furthermore, the V-, Co-, and Ni-doped (5,5) and (8,0) BeONTs with half-metal ferromagnetism and thus 100% spin polarization character are good candidates for spintronic applications. 参考文献 | 相关文章 | 计量指标

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Thickness dependence of the anomalous Hall effect in disordered face-centered cubic FePt alloy films 陈明, 何攀, 周仕明, 时钟 2014 (1):  17104-017104.  doi: 10.1088/1674-1056/23/1/017104 摘要 ( 527 )   PDF(343KB) ( 389 )   The anomalous Hall effect in disordered face-centered cubic (fcc) FePt alloy films is experimentally studied. The longitudinal resistivity independent term of the anomalous Hall conductivity (AHC) increases and approaches saturation with increasing film thickness. The contribution of side jump scattering is suggested to decrease monotonically with increasing film thickness, which can be ascribed to the variation of the surface scattering with the film thickness. The sign of the skew scattering contribution to the AHC is opposite to that of the intrinsic contribution in the system. 参考文献 | 相关文章 | 计量指标

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Theoretical study of thermoelectric properties of MoS2 郭怀红, 杨腾, 陶鹏, 张志东 2014 (1):  17201-017201.  doi: 10.1088/1674-1056/23/1/017201 摘要 ( 1116 )   PDF(882KB) ( 1600 )   We systematically studied the thermoelectric properties of MoS2 with doping based on the Boltzmann transport theory and first-principles calculations. We obtained an optimal doping region (around 1019 cm-3) for thermoelectric properties along in-plane and cross-plane directions. MoS2 in the optimal doping region has a vanishingly small anisotropy of thermopower possibly due to the decoupling of in-plane and cross-plane conduction channels, but big anisotropies of electrical conductivity σ and electronic thermal conductivity κe arising from the anisotropic electronic scattering time. The κe is comparable to the lattice counterpart κl in the plane, while κl dominates over κe across the plane. The figure of merit ZT can reach 0.1 at around 700 K with in-plane direction preferred by doping. 参考文献 | 相关文章 | 计量指标

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Resonant tunneling through double-barrier structures on graphene 邓伟胤, 朱瑞, 肖运昌, 邓文基 2014 (1):  17202-017202.  doi: 10.1088/1674-1056/23/1/017202 摘要 ( 599 )   PDF(1057KB) ( 655 )   Quantum resonant tunneling behaviors of double-barrier structures on graphene are investigated under the tight-binding approximation. The Klein tunneling and resonant tunneling are demonstrated for the quasiparticles with energy close to the Dirac points. The Klein tunneling vanishes by increasing the height of the potential barriers to more than 300 meV. The Dirac transport properties continuously change to the Schrödinger ones. It is found that the peaks of resonant tunneling approximate to the eigen-levels of graphene nanoribbons under appropriate boundary conditions. A comparison between the zigzag- and armchair-edge barriers is given. 参考文献 | 相关文章 | 计量指标

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Spin and valley half metal induced by staggered potential and magnetization in silicene 汪萨克, 田宏玉, 杨永宏, 汪军 2014 (1):  17203-017203.  doi: 10.1088/1674-1056/23/1/017203 摘要 ( 131681 )   PDF(639KB) ( 132429 )   We investigate the electron transport in silicene with both staggered electric potential and magnetization; the latter comes from the magnetic proximity effect by depositing silicene on a magnetic insulator. It is shown that the silicene could be a spin and valley half metal under appropriate parameters when the spin–orbit interaction is considered; further, the filtered spin and valley could be controlled by modulating the staggered potential or magnetization. It is also found that in the spin-valve structure of silicene, not only can the antiparallel magnetization configuration significantly reduce the valve-structure conductance, but the reversing staggered electric potential can cause a high-performance magnetoresistance due to the spin and valley blocking effects. Our findings show that the silicene might be an ideal basis for the spin and valley filter analyzer devices. 参考文献 | 相关文章 | 计量指标

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Electronic structure and magnetic properties of substitutional transition-metal atoms in GaN nanotubes 张敏, 史俊杰 2014 (1):  17301-017301.  doi: 10.1088/1674-1056/23/1/017301 摘要 ( 598 )   PDF(540KB) ( 685 )   The electronic structure and magnetic properties of the transition-metal (TM) atoms (Sc–Zn, Pt and Au) doped zigzag GaN single-walled nanotubes (NTs) are investigated using first-principles spin-polarized density functional calculations. Our results show that the bindings of all TM atoms are stable with the binding energy in the range of 6–16 eV. The Sc- and V-doped GaN NTs exhibit a nonmagnetic behavior. The GaN NTs doped with Ti, Mn, Ni, Cu and Pt are antiferromagnetic. On the contrary, the Cr-, Fe-, Co-, Zn- and Au-doped GaN NTs show the ferromagnetic characteristics. The Mn- and Co-doped GaN NTs induce the largest local moment of 4μB among these TM atoms. The local magnetic moment is dominated by the contribution from the substitutional TM atom and the N atoms bonded with it. 参考文献 | 相关文章 | 计量指标

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Optical binding forces between plasmonic nanocubes：A numerical study based on discrete-dipole approximation 张小明, 肖君军, 张强 2014 (1):  17302-017302.  doi: 10.1088/1674-1056/23/1/017302 摘要 ( 534 )   PDF(493KB) ( 405 )   Plasmonic nanocubes are ideal candidates in realizing controllable reflectance surfaces, unidirectional nanoantennas and other plasmon-associated applications. In this work, we perform full-wave calculations of the optical forces in three-dimensional gold nanocube dimers. For a fixed center-to-center separation, the rotation of the plasmonic nanocube leads to a slight shift of the plasmonic resonance wavelength and a strong change in the optical binding forces. The effective gap and the near field distribution between the two nanocubes are shown to be crucial to this force variation. We further find that the optical binding force is dominated by the scattering process while the optical forces in the wavevector direction are affected by both scattering and absorption, making the former relatively more sensitive to the rotation of (an effective gap between) the nanocubes. Our results would be useful for building all-optically controllable meta-surfaces. 参考文献 | 相关文章 | 计量指标

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Time-dependent degradation of threshold voltage in AlGaN/GaN high electron mobility transistors 马晓华, 姜元祺, 王鑫华, 吕敏, 张霍, 陈伟伟, 刘新宇 2014 (1):  17303-017303.  doi: 10.1088/1674-1056/23/1/017303 摘要 ( 554 )   PDF(318KB) ( 744 )   This paper gives a detailed analysis of the time-dependent degradation of the threshold voltage in AlGaN/GaN high electron mobility transistors (HEMTs) submitted to off-state stress. The threshold voltage shows a positive shift in the early stress, then turns to a negative shift. The negative shift of the threshold voltage seems to have a long recovery time. A model related with the balance of electron trapping and detrapping induced by shallow donors and deep acceptors is proposed to explain this degradation mode. 参考文献 | 相关文章 | 计量指标

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Ab initio study of structural, electronic and optical properties of ternary CdO1-xSex alloys using special quasi-random structures Muhammad Rashid, Fayyaz Hussain, Muhammad Imran, S. A. Ahmad, N. A. Noor 2014 (1):  17304-017304.  doi: 10.1088/1674-1056/23/1/017304 摘要 ( 573 )   PDF(3487KB) ( 634 )   The structural, electronic, and optical properties of binary CdO, CdSe, and their ternary CdO1-xSex alloys (0≤ x ≤ 1) in the rock salt and zinc blend phases have been studied by the special quasi-random structure (SQS) method. All the calculations are performed using full-potential linearized augmented plane wave plus local orbital’s (FP-LAPW+lo) method within the framework of density function theory (DFT). We use Wu–Cohen (WC) generalized gradient approximation (GGA) to calculate structural parameters, whereas both Wu–Cohen and Engel–Vosko (EV) GGA have been applied to calculate electronic structure of the materials. Our predicted results of lattice constant and bulk modulus show only a slight deviation from Vegard’s law for the whole concentrations. The obtained band structure indicates that for the rock-salt phase, the ternary alloys present semi-metallic behavior, while for the zinc blend phase, semiconductor behavior with direct bandgap is observed with decreasing order of x except for CdSe. Finally, by incorporating the basic optical properties, we discuss the dielectric function, refractive index, optical reflectivity, the absorption coefficient, and optical conductivity in terms of incident photon energy up to 14 eV. The calculated results of both binaries are in agreement with existing experimental and theoretical values. 参考文献 | 相关文章 | 计量指标

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Mobility limited by cluster scattering in ternary alloy quantum wires 张恒, 杨少延, 刘贵鹏, 王建霞, 金东东, 李辉杰, 刘祥林, 朱勤生, 王占国 2014 (1):  17305-017305.  doi: 10.1088/1674-1056/23/1/017305 摘要 ( 506 )   PDF(344KB) ( 435 )   The mobility limited by cluster scattering in ternary alloy semiconductor quantum wire (QWR) is theoretically investigated under Born approximation. We calculate the screened mobility due to clusters (high indium composition InGaN) scattering in the InxGa1-xN QWR structure. The characteristics of the cluster scattering mechanism are discussed in terms of the indium composition of clusters, the one-dimensional electron gas (1DEG) concentration, and the radius of QWR. We find that the density, breadth of cluster, and the correlation length have a strong effect on the electron mobility due to cluster scattering. Finally, a comparison of the cluster scattering is made with the alloy-disorder scattering. It is found that the cluster scattering acts as a significant scattering event to impact the resultant electron mobility in ternary alloy QWR. 参考文献 | 相关文章 | 计量指标

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Electronic structures of halogen-doped Cu2O based on DFT calculations 赵宗彦, 易娟, 周大成 2014 (1):  17401-017401.  doi: 10.1088/1674-1056/23/1/017401 摘要 ( 460 )   PDF(521KB) ( 757 )   In order to construct p–n homojunction of Cu2O-based thin film solar cells that may increase its conversion efficiency, to synthesize n-type Cu2O with high conductivity is extremely crucial, and considered as a challenge in the near future. The doping effects of halogen on electronic structure of Cu2O have been investigated by density function theory calculations in the present work. Halogen dopants form donor levels below the bottom of conduction band through gaining or losing electrons, suggesting that halogen doping could make Cu2O have n-type conductivity. The lattice distortion, the impurity formation energy, the position, and the band width of donor level of Cu2O1-xHx (H=F, Cl, Br, I) increase with the halogen atomic number. Based on the calculated results, chlorine doping is an effective n-type dopant for Cu2O, owing to the lower impurity formation energy and suitable donor level. 参考文献 | 相关文章 | 计量指标

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Application of longitudinal generalized magneto-optical ellipsometry in magnetic ultrathin films 王晓, 连洁, 张福军, 高尚, 陈延学, 于晓红, 李萍, 王英顺, 孙兆宗 2014 (1):  17501-017501.  doi: 10.1088/1674-1056/23/1/017501 摘要 ( 421 )   PDF(416KB) ( 316 )   The longitudinal generalized magneto-optical ellipsometry (GME) method is extended to the measurement of three-layer ultrathin magnetic films. In this work, the theory of the reflection matrix is introduced into the GME measurement to obtain the reflective matrix parameters of ultrathin multilayer magnetic films with different thicknesses. After that, a spectroscopic ellipsometry is used to determine the optical parameter and the thickness of every layer of these samples, then the magneto-optical coupling constant of the multilayer magnetic ultrathin film can be obtained. After measurements of a series of ultrathin Fe films, the results show that the magneto-optical coupling constant Q is independent of the thickness of the magnetic film. The magneto-optical Kerr rotations and ellipticity are measured to confirm the validity of this experiment. Combined with the optical constants and the Q constant, the Kerr rotations and ellipticity are calculated in theory. The results show that the theoretical curve fits very well with the experimental data. 参考文献 | 相关文章 | 计量指标

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Determination of the magnetic anisotropy constant of Cu/Fe/SiO2/Si by a magneto-optical Kerr effect susceptometer 贾义娇, 何为, 叶军, 胡泊, 陈子瑜, 高有辉, 张向群, 杨海涛, 成昭华 2014 (1):  17502-017502.  doi: 10.1088/1674-1056/23/1/017502 摘要 ( 816 )   PDF(444KB) ( 817 )   The magneto-optical Kerr effect susceptometry technique is proposed to determine the uniaxial magnetic anisotropy (UMA) constant Ku. The magnetic properties of Cu/Fe/SiO2/Si grown by dc magnetron sputtering were investigated. The in-plane uniaxial magnetic anisotropy was probed by the magneto-optical Kerr effect (MOKE). The value of UMA, Ku=2.5×103 J/m3, was simulated from the field dependence of ac susceptibility along the hard axis according to the Stoner–Wohlfarth (S–W) model, which is consistent with Ku=2.7×103 J/m3 calculated from the magnetic hysteresis loops. Our results show that the magneto-optical Kerr effect susceptometry can be employed to determine the magnetic anisotropy constant owing to its high sensitivity. 参考文献 | 相关文章 | 计量指标

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Interfacial and electrical properties of HfAlO/GaSb metal-oxide-semiconductor capacitors with sulfur passivation 谭桢, 赵连锋, 王敬, 许军 2014 (1):  17701-017701.  doi: 10.1088/1674-1056/23/1/017701 摘要 ( 593 )   PDF(520KB) ( 540 )   Interfacial and electrical properties of HfAlO/GaSb metal-oxide-semiconductor capacitors (MOSCAPs) with sulfur passivation were investigated and the chemical mechanisms of the sulfur passivation process were carefully studied. It was shown that the sulfur passivation treatment could reduce the interface trap density Dit of the HfAlO/GaSb interface by 35% and reduce the equivalent oxide thickness (EOT) from 8 nm to 4 nm. The improved properties are due to the removal of the native oxide layer, as was proven by x-ray photoelectron spectroscopy measurements and high-resolution cross-sectional transmission electron microscopy (HRXTEM) results. It was also found that GaSb-based MOSCAPs with HfAlO gate dielectrics have interfacial properties superior to those using HfO2 or Al2O3 dielectric layers. 参考文献 | 相关文章 | 计量指标

SPECIAL TOPIC --- Non-equilibrium phenomena in soft matters

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Energy band alignment at ferroelectric/electrode interface determined by photoelectron spectroscopy 陈峰, 吴文彬, 李舜怡, Andreas Klein 2014 (1):  17702-017702.  doi: 10.1088/1674-1056/23/1/017702 摘要 ( 470 )   PDF(3124KB) ( 1705 )   The most important interface-related quantities determined by band alignment are the barrier heights for charge transport, given by the Fermi level position at the interface. Taking Pb(Zr,Ti)O3 (PZT) as a typical ferroelectric material and applying X-ray photoelectron spectroscopy (XPS), we briefly review the interface formation and barrier heights at the interfaces between PZT and electrodes made of various metals or conductive oxides. Polarization dependence of the Schottky barrier height at a ferroelectric/electrode interface is also directly observed using XPS. 参考文献 | 相关文章 | 计量指标

CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES

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Numerical investigation of the enhanced unidirectional surface plasmon polaritons generator 张志东, 王红艳, 张中月, 王辉 2014 (1):  17801-017801.  doi: 10.1088/1674-1056/23/1/017801 摘要 ( 697 )   PDF(1397KB) ( 480 )   A unidirectional surface plasmon polaritons (SPPs) generator with greatly enhanced generation efficiency is proposed. The SPPs generator consists of an asymmetric single nanoslit coated with a polyviny alcohol (PVA) film and a silver rectangle block. The generation efficiency of this SPPs generator is investigated using the finite difference time domain method. Due to the presence of the silver rectangle block, the SPPs generation efficiency of the asymmetric single nanoslit with PVA film can be greatly enhanced and the corresponding wavelength with the maximum enhancement factor can be tuned flexibly. The influence of the structural parameters on the generation efficiency is also investigated for the enhanced unidirectional SPPs generator. 参考文献 | 相关文章 | 计量指标

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Multi-band microwave metamaterial absorber based on coplanar Jerusalem crosses 王国栋, 刘明海, 胡希伟, 孔令华, 程莉莉, 陈兆权 2014 (1):  17802-017802.  doi: 10.1088/1674-1056/23/1/017802 摘要 ( 759 )   PDF(1169KB) ( 1018 )   The influence of the gap on the absorption performance of the conventional split ring resonator (SRR) absorber is investigated at microwave frequencies. Our simulated results reveal that the geometry of the square SRR can be equivalent to a Jerusalem cross (JC) resonator and its corresponding metamaterial absorber (MA) is changed to a JC absorber. The JC MA exhibits an experimental absorption peak of 99.1% at 8.72 GHz, which shows an excellent agreement with our simulated results. By simply assembling several JCs with slightly different geometric parameters next to each other into a unit cell, a perfect multi-band absorption can be effectively obtained. The experimental results show that the MA has four distinct and strong absorption peaks at 8.32 GHz, 9.8 GHz, 11.52 GHz and 13.24 GHz. Finally, the multi-reflection interference theory is introduced to interpret the absorption mechanism. 参考文献 | 相关文章 | 计量指标

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Tandem white organic light-emitting diodes adopting a C60：rubrene charge generation layer 毕文涛, 吴晓明, 华玉林, 孙金娥, 肖志慧, 王丽, 印寿根 2014 (1):  17803-017803.  doi: 10.1088/1674-1056/23/1/017803 摘要 ( 520 )   PDF(337KB) ( 1315 )   Organic bulk heterojunction fullerence (C60) doped 5, 6, 11, 12-tetraphenylnaphthacene (rubrene) as the high quality charge generation layer (CGL) with high transparency and superior charge generating capability for tandem organic light emitting diodes (OLEDs) is developed. This CGL shows excellent optical transparency about 90%, which can reduce the optical interference effect formed in tandem OLEDs. There is a stable white light emission including 468 nm and 500 nm peaks from the blue emitting layer and 620 nm peak from the red emitting layer in tandem white OLEDs. A high efficiency of about 17.4 cd/A and CIE coordinates of (0.40, 0.35) at 100 cd/m2 and (0.36, 0.34) at 1000 cd/m2 have been demonstrated by employing the developed CGL, respectively. 参考文献 | 相关文章 | 计量指标

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Vibration analysis of a new polymer quartz piezoelectric crystal sensor for detecting characteristic materials of volatility liquid 谷宇, 李强, 许保军, 赵喆 2014 (1):  17804-017804.  doi: 10.1088/1674-1056/23/1/017804 摘要 ( 642 )   PDF(643KB) ( 552 )   We present a new polymer quartz piezoelectric crystal sensor that takes a quartz piezoelectric crystal as the basal material and a nanometer nonmetallic polymer thin film as the surface coating based on the principle of quartz crystal microbalance (QCM). The new sensor can be used to detect the characteristic materials of a volatile liquid. A mechanical model of the new sensor was built, whose structure was a thin circle plate composing of polytef/quartz piezoelectric/polytef. The mechanical model had a diameter of 8 mm and a thickness of 170 μ. The vibration state of the model was simulated by software ANSYS after the physical parameters and the boundary condition of the new sensor were set. According to the results of experiments, we set up a frequency range from 9.995850 MHz to 9.997225 MHz, 17 kinds of frequencies and modes of vibration were obtained within this range. We found a special frequency fsp of 9.996358 MHz. When the resonant frequency of the new sensor’s mechanical model reached the special frequency, a special phenomenon occurred. In this case, the amplitude of the center point O on the mechanical model reached the maximum value. At the same time, the minimum absolute difference between the simulated frequency based on the ANSYS software and the experimental measured stable frequency was reached. The research showed that the design of the new polymer quartz piezoelectric crystal sensor perfectly conforms to the principle of QCM. A special frequency value fsp was found and subsequently became one of the most important parameters in the new sensor design. 参考文献 | 相关文章 | 计量指标

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High quality above 3-μm mid-infrared InGaAsSb/AlGaInAsSb multiple-quantum well grown by molecular beam epitaxy 邢军亮, 张宇, 徐应强, 王国伟, 王娟, 向伟, 倪海桥, 任正伟, 贺振宏, 牛智川 2014 (1):  17805-017805.  doi: 10.1088/1674-1056/23/1/017805 摘要 ( 559 )   PDF(1617KB) ( 534 )   The GaSb-based laser shows its superiority in the 3–4 μm wavelength range. However, for a quantum well (QW) laser structure of InGaAsSb/AlGaInAsSb multiple-quantum well (MQW) grown on GaSb, uniform content and high compressive strain in InGaAsSb/AlGaInAsSb are not easy to control. In this paper, the influences of the growth temperature and compressive strain on the photoluminescence (PL) property of a 3.0-μm InGaAsSb/AlGaInAsSb MQW sample are analyzed to optimize the growth parameters. Comparisons among the PL spectra of the samples indicate that the In0.485GaAs0.184Sb/Al0.3Ga0.45In0.25As0.22Sb0.78 MQW with 1.72% compressive strain grown at 460 ℃ posseses the optimum optical property. Moreover, the wavelength range of the MQW structure is extended to 3.83 μm by optimizing the parameters. 参考文献 | 相关文章 | 计量指标

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Spin–orbit coupling effects on the in-plane optical anisotropy of semiconductor quantum wells 俞金玲, 陈涌海, 赖云锋, 程树英 2014 (1):  17806-017806.  doi: 10.1088/1674-1056/23/1/017806 摘要 ( 577 )   PDF(359KB) ( 426 )   We theoretically study the influence of the spin–orbit coupling (SOC) on the in-plane optical anisotropy (IPOA) induced by in-plane uniaxial strain and interface asymmetry in (001) GaAs/AlGaAs quantum wells (QWs) with different well width. It is found that the SOC has more significant impact on the IPOA for the transition of the first valence subband of heavy hole to the first conduction band (1H1E) than that of 1L1E. The reason has been discussed. The IPOA of (001) InGaAs/InP QWs has been measured by reflectance difference spectroscopy, whose amplitude is about one order larger than that of GaAs/AlGaAs QWs. The anisotropic interface potential parameters of InGaAs/InP QWs are also determined. The influence of the SOC effect on the IPOA of InGaAs/InP QWs when the QWs are under tensile, compressive or zero biaxial strain are also investigated in theory. Our results demonstrate that the SOC has significant effect on the IPOA especially for semiconductor QWs with small well width, and therefore cannot be ignored. 参考文献 | 相关文章 | 计量指标

INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY

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Thermal stability and high-temperature shape memory characteristics of Ti-20Zr-10Ta alloy 郑晓航, 隋解和, 张欣, 杨哲一, 蔡伟 2014 (1):  18101-018101.  doi: 10.1088/1674-1056/23/1/018101 摘要 ( 570 )   PDF(373KB) ( 577 )   The microstructure, martensite transformation behavior, thermal stability, and shape memory behavior of Ti-20Zr-10Ta high temperature shape memory alloy were investigated. The Ti-20Zr-10Ta alloy exhibited the reversible transformation with the high martensite transformation temperature of 500℃ and good thermal stability. The alloy displayed the elongation of 15% and a maximum recovery stain of 5.5% with 8% pre-strain. 参考文献 | 相关文章 | 计量指标

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Tunable ultra-wideband terahertz filter based on three-dimensional arrays of H-shaped plasmonic crystals 袁偲, 徐世林, 姚建铨, 赵晓蕾, 曹小龙, 吴亮 2014 (1):  18102-018102.  doi: 10.1088/1674-1056/23/1/018102 摘要 ( 419 )   PDF(1040KB) ( 764 )   A face-to-face system of double-layer three-dimensional arrays of H-shaped plasmonic crystals is proposed, and its transmission and filtering properties are investigated in the terahertz regime. Simulation results show that our design has excellent filtering properties. It has an ultra-wide bandgap and passband with steep band-edges, and the transmittance of the passband and the forbidden band are very close to 1 and 0, respectively. As the distance between the two face-to-face plates increases, the resonance frequency exhibits a gradual blueshift from 0.88 THz to 1.30 THz. Therefore, we can dynamically control the bandwidths of bandgap and passband by adding a piezoelectric ceramic plate between the two crystal plates. Furthermore, the dispersion relations of modes and electric field distributions are presented to analyze the generation mechanisms of bandgaps and to explain the location of bandgaps and the frequency shift phenomenon. Due to the fact that our design can provide many resonant modes, the bandwidth of the bandgaps can be greatly broadened. This paper can serve as a valuable reference for the design of terahertz functional devices and three-dimensional terahertz metamaterials. 参考文献 | 相关文章 | 计量指标

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First-principles investigation of chemical modification on two-dimensional iron–phthalocyanine sheet 王洪博, 苏燕, 陈刚 2014 (1):  18103-018103.  doi: 10.1088/1674-1056/23/1/018103 摘要 ( 531 )   PDF(851KB) ( 424 )   Successful synthesis of single iron–phthalocyanie (FePc) framework layer on substrate and its transferrable properties open the door for decorating the separately distributed transition metals for exploring the diverse properties. We have studied the effects of chemical modification on two-dimensional FePc organometallic framework with density functional theory. For simplicity, the non-metal atoms with variant valence electrons are used as prototypes to estimate the effects from chemical modifications with different functional groups. The thermo-stabilities of the non-metal atom decorated complex sheet materials have been estimated by the first-principles constant energy molecular dynamic simulations. Upon the non-metal atom adsorption, the magnetic moment could be changed from 2 μB to 0, 1, 2, and 3 μB per unit cell for the case of tetra-, penta-, hexa-, and hepta-valent non-metal modifications, respectively, showing interesting promise to tailor its magnetic properties for potential applications. 参考文献 | 相关文章 | 计量指标

SPECIAL TOPIC --- Non-equilibrium phenomena in soft matters

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The basis of organic spintronics：Fabrication of organic spin valves 陈彬彬, 姜生伟, 丁海峰, 蒋正生, 吴镝 2014 (1):  18104-018104.  doi: 10.1088/1674-1056/23/1/018104 摘要 ( 618 )   PDF(757KB) ( 910 )   Organic spintronics focuses on utilizing the spin degree of freedom in organic materials because of the long spin relaxation time. The vertical organic spin valve (OSV) is a typical sample structure used to study the spin transport phenomena. However, the fabrication of high quality OSVs is difficult, which results in controversial experiment results and hence hinders the development of organic spintronics. In this work, we describe our recent study on the fabrication of typical vertical organic spin valves, La0.67Sr0.33MnO3 (LSMO)/Alq3/Co. The LSMO bottom electrodes are annealed to obtain an atomically smooth surface and improved magnetic properties. The top Co electrodes are deposited by an indirect deposition method to reduce the interfusion between Co and Alq3. The controlled fabrication process provides much better performance and sample yield of OSVs. 参考文献 | 相关文章 | 计量指标

INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY

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Effect of metal oxide arrester on the chaotic oscillations in the voltage transformer with nonlinear core loss model using chaos theory Hamid Reza Abbasi, Ahmad Gholami, Seyyed Hamid Fathi, Ataollah Abbasi 2014 (1):  18201-018201.  doi: 10.1088/1674-1056/23/1/018201 摘要 ( 752 )   PDF(1895KB) ( 1212 )   In this paper, controlling chaos when chaotic ferroresonant oscillations occur in a voltage transformer with nonlinear core loss model is performed. The effect of a parallel metal oxide surge arrester on the ferroresonance oscillations of voltage transformers is studied. The metal oxide arrester (MOA) is found to be effective in reducing ferroresonance chaotic oscillations. Also the multiple scales method is used to analyze the chaotic behavior and different types of fixed points in ferroresonance of voltage transformers considering core loss. This phenomenon has nonlinear chaotic dynamics and includes sub-harmonic, quasi-periodic, and also chaotic oscillations. In this paper, the chaotic behavior and various ferroresonant oscillation modes of the voltage transformer is studied. This phenomenon consists of different types of bifurcations such as period doubling bifurcation (PDB), saddle node bifurcation (SNB), Hopf bifurcation (HB), and chaos. The dynamic analysis of ferroresonant circuit is based on bifurcation theory. The bifurcation and phase plane diagrams are illustrated using a continuous method and linear and nonlinear models of core loss. To analyze ferroresonance phenomenon, the Lyapunov exponents are calculated via the multiple scales method to obtain Feigenbaum numbers. The bifurcation diagrams illustrate the variation of the control parameter. Therefore, the chaos is created and increased in the system. 参考文献 | 相关文章 | 计量指标

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Stereodynamics in reaction O(1D)+CH4→OH+CH3 沙广燕, 袁久闯, 孟长功, 陈茂笃 2014 (1):  18202-018202.  doi: 10.1088/1674-1056/23/1/018202 摘要 ( 390 )   PDF(874KB) ( 333 )   A theoretical study of the stereodynamics for reaction O(1D)+CH4→OH+CH3 has been carried out using the quasiclassical trajectory method (QCT) on a potential energy surface structured by Gonzalez et al. The integral cross sections (ICSs), differential cross sections (DCSs) and product rotational angular momentum polarization have been calculated. With the collision energy increasing, the ICS decreases. There is no threshold energy, because no barrier is found on the minimum energy path. The DCS results show that the backward and forward scatterings exist at the same time. With the collision energy increasing, the dominant rotation of the product changes from the right-handed direction to the left-handed direction in planes parallel to the scattering plane. In the isotopic effect study, the decrease of the mass factor weakens the polarization degree of the rotational angular momentum vectors of the products. 参考文献 | 相关文章 | 计量指标

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Quasi-two-dimensional threshold voltage model for junctionless cylindrical surrounding gate metal-oxide-semiconductor field-effect transistor with dual-material gate 李聪, 庄奕琪, 张丽, 靳刚 2014 (1):  18501-018501.  doi: 10.1088/1674-1056/23/1/018501 摘要 ( 536 )   PDF(358KB) ( 551 )   Based on the quasi-two-dimensional (2D) solution of Poisson’s equation in two continuous channel regions, an analytical threshold voltage model for short-channel junctionless dual-material cylindrical surrounding-gate (JLDMCSG) metal-oxide-semiconductor field-effect transistor (MOSFET) is developed. Using the derived model, channel potential distribution, horizontal electrical field distribution, and threshold voltage roll-off of JLDMCSG MOSFET are investigated. Compared with junctionless single-material CSG (JLSGCSG) MOSFET, JLDMCSG MOSFET can effectively suppress short-channel effects and simultaneously improve carrier transport efficiency. It is also revealed that threshold voltage roll-off of JLDMCSG can be significantly reduced by adopting both a small oxide thickness and a small silicon channel radius. The model is verified by comparing its calculated results with that obtained from three-dimensional (3D) numerical device simulator ISE. 参考文献 | 相关文章 | 计量指标

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Enhancing light extraction of GaN-based blue light-emitting diodes by a tuned nanopillar array 陈湛旭, 任远, 肖国辉, 李俊韬, 陈夏, 王雪华, 金崇君, 张佰君 2014 (1):  18502-018502.  doi: 10.1088/1674-1056/23/1/018502 摘要 ( 541 )   PDF(3067KB) ( 552 )   Surface patterning of p-GaN to improve the light extraction efficiency of GaN-based blue light-emitting diodes (LEDs) has been investigated. Periodic nanopillar arrays on p-GaN have been fabricated by polystyrene (PS) nanosphere lithography; the diameter of the nanopillars can be tuned to optimize the electrical and optical properties of the LEDs. The electroluminescence intensity of the nanopillar-patterned LEDs is better than that of conventional LEDs; the greatest enhancement increased the intensity by a factor of 1.41 at a 20 mA injection current. The enhancements can be explained by a model of bilayer film on a GaN substrate. This method may serve as a practical approach to improve the efficiency of light extraction from LEDs. 参考文献 | 相关文章 | 计量指标

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Spin pumping at the Co2FeAl0.5Si0.5/Pt interface 吴勇, 赵月雷, 熊强, 徐晓光, 孙阳, 张十庆, 姜勇 2014 (1):  18503-018503.  doi: 10.1088/1674-1056/23/1/018503 摘要 ( 514 )   PDF(449KB) ( 672 )   Spin pumping at the Co2FeAl0.5Si0.5/Pt and Pt/Co2FeAl0.5Si0.5 interfaces has been studied by ferromagnetic resonance technology (FMR). The spin mixing conductance of the Co2FeAl0.5Si0.5/Pt and Pt/Co2FeAl0.5Si0.5 interfaces was determined to be 3.7×1019 m-2 and 2.1×1019 m-2 by comparing the Gilbert damping in a Co2FeAl0.5Si0.5 single film, Co2FeAl0.5Si0.5/Pt bilayer film and a Pt/Co2FeAl0.5Si0.5/Pt trilayer film. Spin pumping is more efficient in the Co2FeAl0.5Si0.5/Pt bilayer film than in permalloy/Pt bilayer film. 参考文献 | 相关文章 | 计量指标

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Chemical synthesis of zinc oxide nanorods for enhanced hydrogen gas sensing Musarrat Jabeen, Muhammad Azhar Iqbal, R Vasant Kumar,Mansoor Ahmed, Muhammad Tayyeb Javed 2014 (1):  18504-018504.  doi: 10.1088/1674-1056/23/1/018504 摘要 ( 524 )   PDF(488KB) ( 1752 )   Zinc oxide (ZnO) nanorods are prepared using equimolar solution of zinc nitrate ((Zn(NO3)2) and hexamethylenetetramine (C6H12N4) by the hydrothermal technique at 80 ℃ for 12 h. Epitaxial growth is explored by X-ray diffraction (XRD) patterns, revealing that the ZnO nanorods have a hexagonal (wurtzite) structure. Absorption spectra of ZnO are measured by UV–visible spectrometer. The surface morphology is investigated by field emission scanning electron microscopy (FESEM). The synthesized ZnO nanorods are used for detecting the 150 ℃ hydrogen gas with a concentration over 1000 ppm. The obtained results show a reversible response. The influence of operating temperature on hydrogen gas detecting characteristic of ZnO nanorods is also investigated. 参考文献 | 相关文章 | 计量指标

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A snapback suppressed reverse-conducting IGBT with uniform temperature distribution 陈伟中, 李泽宏, 张波, 任敏, 张金平, 刘永, 李肇基 2014 (1):  18505-018505.  doi: 10.1088/1674-1056/23/1/018505 摘要 ( 666 )   PDF(880KB) ( 787 )   A novel reverse-conducting insulated-gate bipolar transistor (RC-IGBT) featuring a floating P-plug is proposed. The P-plug is embedded in the n-buffer layer to obstruct the electron current from flowing directly to the n-collector, which achieves the hole emission from the p-collector at a small collector size and suppresses the snapback effectively. Moreover, the current is uniformly distributed in the whole wafer at both IGBT mode and diode mode, which ensures the high temperature reliability of the RC-IGBT. Additionally, the P-plug acts as the base of the N-buffer/P-float/N-buffer transistor, which can be activated to extract the excessive carriers at the turn-off process. As the the simulation results show, for the proposed RC-IGBT, it achieves almost snapback-free output characteristics with a uniform current density and a uniform temperature distribution, which can greatly increase the reliability of the device. 参考文献 | 相关文章 | 计量指标

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Electrical and dielectric properties of Al/p-Si and Al/perylene/p-Si type diodes in a wide frequency range Ahmet Kaya, Sedat Zeyrek, Sait Eren San, Şmsettin Altindal 2014 (1):  18506-018506.  doi: 10.1088/1674-1056/23/1/018506 摘要 ( 545 )   PDF(1510KB) ( 495 )   The perylene (C20H12) layer effect on the electrical and dielectric properties of Al/p-Si (MS) and Al/perylene/p-Si (MPS) diodes have been investigated and compared in the frequency range of 0.7 kHz–2 MHz. Experimental results show that C–V characteristics give an anomalous peak for two structures at low frequencies due to interface states (Nss) and series resistance (Rs). The increases in C and G/ω at low frequencies confirm that the charges at interface can easily follow an ac signal and yield excess capacitance and conductance. The frequency-dependent dielectric constant (ε’) and dielectric loss (ε") are subtracted using C and G/ω data at 1.5 V. The ε’ and ε" values are found to be strongly dependent on frequency and voltage, and their large values at low frequencies can be attributed to the excess polarization coming from charges at traps. Plots of ln(σ ac)–ln(ω) for two structures have two linear regions, with slopes of 0.369 and 1.166 for MS, and of 0.077 and 1.061 for MPS, respectively. From the C-2–V characteristics, the doping acceptor atom concentration (NA) and barrier height (ΦB) for Schottky barrier diodes (SBDs) of MS and MPS types are also obtained to be 1.484×10^15 and 1.303×1015 cm-3, and 1.10 and 1.13 eV, respectively. 参考文献 | 相关文章 | 计量指标

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Position difference regularity of corresponding R-wave peaks for maternal ECG components from different abdominal points 张洁敏, 管群, 汤黎明, 刘铁兵, 刘红星, 黄晓林, 司峻峰 2014 (1):  18701-018701.  doi: 10.1088/1674-1056/23/1/018701 摘要 ( 572 )   PDF(571KB) ( 370 )   We collected 343 groups of abdominal electrocardiogram (ECG) data from 78 pregnant women and deleted the channels unable for experts to determine R-wave peaks from them; then, based on these filtered data, the statistics of position difference of corresponding R-wave peaks for different maternal ECG components from different points were studied. The resultant statistics showed the regularity that the position difference of corresponding maternal R-wave peaks between different abdominal points does not exceed the range of 30 ms. The regularity was also proved using the fECG data from MIT–BIH PhysioBank. Additionally, the paper applied the obtained regularity, the range of position differences of the corresponding maternal R-wave peaks, to accomplish the automatic detection of maternal R-wave peaks in the recorded all initial 343 groups of abdominal signals, including the ones with the largest fetal ECG components, and all 55 groups of ECG data from MIT–BIH PhysioBank, achieving the successful separation of the maternal ECGs. 参考文献 | 相关文章 | 计量指标

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A local-world evolving hypernetwork model 杨光勇, 刘建国 2014 (1):  18901-018901.  doi: 10.1088/1674-1056/23/1/018901 摘要 ( 616 )   PDF(675KB) ( 513 )   Complex hypernetworks are ubiquitous in the real system. It is very important to investigate the evolution mechanisms. In this paper, we present a local-world evolving hypernetwork model by taking into account the hyperedge growth and local-world hyperedge preferential attachment mechanisms. At each time step, a newly added hyperedge encircles a new coming node and a number of nodes from a randomly selected local world. The number of the selected nodes from the local world obeys the uniform distribution and its mean value is m. The analytical and simulation results show that the hyperdegree approximately obeys the power-law form and the exponent of hyperdegree distribution is γ=2+1/m. Furthermore, we numerically investigate the node degree, hyperedge degree, clustering coefficient, as well as the average distance, and find that the hypernetwork model shares the scale-free and small-world properties, which shed some light for deeply understanding the evolution mechanism of the real systems. 参考文献 | 相关文章 | 计量指标

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Output regulation for linear multi-agent systems with unmeasurable nodes 梁洪晶, 张化光, 王占山, 王军义 2014 (1):  18902-018902.  doi: 10.1088/1674-1056/23/1/018902 摘要 ( 545 )   PDF(322KB) ( 468 )   The output regulation of linear multi-agent systems with partial unmeasurable agents is investigated in this paper. All the agents except the exosystem can be classified into two groups. Agents in the first group can be measured by themselves and their neighbors. State variables are not fully accessible for direct communication and full order Luenberger observers are constructed for the unmeasurable agents. We give a state feedback control law to solve the output regulation problem under the communication topologies based on both measurable and unmeasurable agents. The heterogeneous agents’ synchronization problem is a general case of our results. Finally, examples are utilized to show the effectiveness of the obtained results. 参考文献 | 相关文章 | 计量指标

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Unsupervised neural networks for solving Troesch’s problem Muhammad Asif Zahoor Raja 2014 (1):  18903-018903.  doi: 10.1088/1674-1056/23/1/018903 摘要 ( 524 )   PDF(464KB) ( 620 )   In this study, stochastic computational intelligence techniques are presented for the solution of Troesch’s boundary value problem. The proposed stochastic solvers use the competency of a feed-forward artificial neural network for mathematical modeling of the problem in an unsupervised manner, whereas the learning of unknown parameters is made with local and global optimization methods as well as their combinations. Genetic algorithm (GA) and pattern search (PS) techniques are used as the global search methods and the interior point method (IPM) is used for an efficient local search. The combination of techniques like GA hybridized with IPM (GA-IPM) and PS hybridized with IPM (PS-IPM) are also applied to solve different forms of the equation. A comparison of the proposed results obtained from GA, PS, IPM, PS-IPM and GA-IPM has been made with the standard solutions including well known analytic techniques of the Adomian decomposition method, the variational iterational method and the homotopy perturbation method. The reliability and effectiveness of the proposed schemes, in term of accuracy and convergence, are evaluated from the results of statistical analysis based on sufficiently large independent runs. 参考文献 | 相关文章 | 计量指标

GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS

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Continuity and momentum equations for moist atmospheres 冉令坤, 高守亭, 曹洁 2014 (1):  19201-019201.  doi: 10.1088/1674-1056/23/1/019201 摘要 ( 661 )   PDF(241KB) ( 657 )   The moist atmosphere with occurring precipitation is considered to be a multiphase fluid composed of dry air, water vapor and hydrometeors. These compositions move with different velocities: they take a macroscopic motion with a reference velocity and a relative motion with a velocity deviated from the reference velocity. The reference velocity can be chosen as the velocities of dry air, a gas mixture and the total air mixture. The budget equations of continuity and momentum are formulated in the three reference-velocity frames. It is shown that the resulting equations are dependent on the chosen reference velocity. The diffusive flux due to compositions moving with velocities deviated from the reference velocity and the internal sources due to the phase transitions of water substances result in additional source terms in continuity and momentum equations. A continuity equation of the total mass is conserved and free of diffusive flux divergence if the reference velocity is referred to the velocity of the total air mixture. However, continuity equations in the dry-air and gas-mixture frames are not conserved due to the mass diffusive flux divergence. The diffusive flux introduces additional source terms in the momentum equation. In the dry-air frame, the diffusive flux of water substances and the phase transitions of water substances contribute to the change of the total momentum. The additional sources of total momentum in the frame of a gas mixture are associated with the diffusive flux of hydrometeors, the phase transitions of hydrometeors and the gas-mixture diffusive flux. In the frame of total air mixture, the contribution to the total momentum comes from the diffusive flux of all atmospheric compositions instead of the phase transitions. The continuity and momentum equations derived here are more complicated than the traditional model equations. With increasing computing power, it becomes possible to simulate atmospheric processes with these sophisticated equations. It is helpful to the improvement of precipitation forecast. 参考文献 | 相关文章 | 计量指标