中国物理B

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Nonlocal symmetries and negative hierarchies related to bilinear Bäcklund transformation

胡晓瑞, 陈勇

2015 (3): 30201-030201. doi: 10.1088/1674-1056/24/3/030201

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In this paper, nonlocal symmetries defined by bilinear Bäcklund transformation for bilinear potential KdV (pKdV) equation are obtained. By introducing an auxiliary variable which just satisfies the Schwartzian form of KdV (SKdV) equation, the nonlocal symmetry is localized and the Levi transformation is presented. Besides, based on three different types of nonlocal symmetries for potential KdV equation, three sets of negative pKdV hierarchies along with their bilinear forms are constructed. An impressive result is that the coefficients of the third type of (bilinear) negative pKdV hierarchy (N>0) are variable, which are obtained via introducing an arbitrary parameter by considering the translation invariance of the pKdV equation.

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Explicit solutions from residual symmetry of the Boussinesq equation

刘希忠, 俞军, 任博

2015 (3): 30202-030202. doi: 10.1088/1674-1056/24/3/030202

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The Bäcklund transformation related symmetry is nonlocal, which is hard to be applied in constructing solutions for nonlinear equations. In this paper, the residual symmetry of the Boussinesq equation is localized to Lie point symmetry by introducing multiple new variables. By applying the general Lie point method, two main results are obtained: a new type of Bäcklund transformation is derived, from which new solutions can be generated from old ones; the similarity reduction solutions as well as corresponding reduction equations are found. The localization procedure provides an effective way to investigate interaction solutions between nonlinear waves and solitons.

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Controllability of fractional-order Chua's circuit

张浩, 陈帝伊, 周坤, 王一琛

2015 (3): 30203-030203. doi: 10.1088/1674-1056/24/3/030203

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The ultimate proof of our understanding of nature and engineering systems is reflected in our ability to control them. Since fractional calculus is more universal, we bring attention to the controllability of fractional order systems. First, we extend the conventional controllability theorem to the fractional domain. Strictly mathematical analysis and proof are presented. Because Chua's circuit is a typical representative of nonlinear circuits, we study the controllability of the fractional order Chua's circuit in detail using the presented theorem. Numerical simulations and theoretical analysis are both presented, which are in agreement with each other.

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Hybrid natural element method for large deformation elastoplasticity problems

马永其, 周延凯

2015 (3): 30204-030204. doi: 10.1088/1674-1056/24/3/030204

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We present the hybrid natural element method (HNEM) for two-dimensional elastoplastic large deformation problems. Sibson interpolation is adopted to construct the shape functions of nodal incremental displacements and incremental stresses. The incremental form of Hellinger-Reissner variational principle for elastoplastic large deformation problems is deduced to obtain the equation system. The total Lagrangian formulation is used to describe the discrete equation system. Compared with the natural element method (NEM), the HNEM has higher computational precision and efficiency in solving elastoplastic large deformation problems. Some numerical examples are selected to demonstrate the advantage of the HNEM for large deformation elastoplasticity problems.

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Inverse problem of pulsed eddy current field of ferromagnetic plates

陈兴乐, 雷银照

2015 (3): 30301-030301. doi: 10.1088/1674-1056/24/3/030301

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To determine the wall thickness, conductivity and permeability of a ferromagnetic plate, an inverse problem is established with measured values and calculated values of time-domain induced voltage in pulsed eddy current testing on the plate. From time-domain analytical expressions of the partial derivatives of induced voltage with respect to parameters, it is deduced that the partial derivatives are approximately linearly dependent. Then the constraints of these parameters are obtained by solving a partial linear differential equation. It is indicated that only the product of conductivity and wall thickness, and the product of relative permeability and wall thickness can be determined accurately through the inverse problem with time-domain induced voltage. In the practical testing, supposing the conductivity of the ferromagnetic plate under test is a fixed value, and then the relative variation of wall thickness between two testing points can be calculated via the ratio of the corresponding inversion results of the product of conductivity and wall thickness. Finally, this method for wall thickness measurement is verified by the experiment results of a carbon steel plate.

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Approximate analytical solution of the Dirac equation with q-deformed hyperbolic Pöschl-Teller potential and trigonometric Scarf Ⅱ non-central potential

Ade Kurniawan, A. Suparmi, C. Cari

2015 (3): 30302-030302. doi: 10.1088/1674-1056/24/3/030302

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An approximate solution of the Dirac equation for a spin-1/2 particle under the influence of q-deformed hyperbolic Pöschl-Teller potential combined with trigonometric Scarf Ⅱ non-central potential is studied analytically. It is assumed that the scalar potential equals the vector potential in order to obtain analytical solutions. Both radial and angular parts of the Dirac equation are solved using the Nikiforov-Uvarov method. A relativistic energy spectrum and the relation between quantum numbers can be obtained using this method. Several quantum wave functions corresponding to several states are also presented in terms of the Jacobi Polynomials.

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Spin transport properties of a Dresselhaus-polygonal quantum ring

唐翰昭, 翟利学, 沈曼, 刘建军

2015 (3): 30303-030303. doi: 10.1088/1674-1056/24/3/030303

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We propose a theoretical method to investigate the effect of the Dresselhaus spin-orbit coupling (DSOC) on the spin transport properties of a regular polygonal quantum ring with an arbitrary number of segments. We find that the DSOC can break the time reversal symmetry of the spin conductance in a polygonal ring and that this property can be used to reverse the spin direction of electrons in the polygon with the result that a pure spin up or pure spin down conductance can be obtained by exchanging the source and the drain. When the DSOC is considered in a polygonal ring with Rashba spin-orbit coupling (RSOC) with symmetric attachment of the leads, the total conductance is independent of the number of segments when both of the two types of spin-orbit coupling (SOC) have the same value. However, the interaction of the two types of SOC results in an anisotropic and shape-dependent conductance in a polygonal ring with asymmetric attachment of the leads. The method we proposed to solve for the spin conductance of a polygon can be generalized to the circular model.

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Measurement-induced nonlocality in the W and Greenberger-Horne-Zeilinger superposition states

林秦, 白彦魁, 叶明勇, 林秀敏

2015 (3): 30304-030304. doi: 10.1088/1674-1056/24/3/030304

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Measurement-induced nonlocality (MIN) is a newly defined quantity to measure correlations in bipartite quantum states [Luo S and Fu S 2011 Phys. Rev. Lett. 106 120401]. MIN in the n-qubit W and Greenberger-Horne-Zeilinger (GHZ) superposition states is considered. It is revealed that n=3 and n≥ 4 states have very different characteristics, especially the monogamy relation about MIN, and the monogamy equality of MIN is held in all n-qubit W states (n≥ 3).

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Topological phase transitions driven by next-nearest-neighbor hopping in noncentrosymmetric cold Fermi gases

王瑞, 张存喜, 计青山

2015 (3): 30305-030305. doi: 10.1088/1674-1056/24/3/030305

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We investigate the topological phase marked by the Thouless-Kohmoto-Nightingale-Nijs (TKNN) number and the phase transitions driven by the next nearest neighbor (NNN) hopping in noncentrosymmetric cold Fermi gases, both spin-singlet pairing and spin-triplet pairing are considered. There exists a critical t'_c for the NNN hopping, at which the quantum phase transition occurs, and the system changes from an Abelian (non-Abelian) phase to a non-Abelian (Abelian) one. By numerically diagonalizing the Hamiltonian in the real space, the energy spectra with edge states for different topological phases and the Majorana zero modes are discussed. Although the spin-triplet pairing does not contribute to the gap closing and the phase diagram, it induces gapless states in the presence of a magnetic field, and the TKNN number in this region is still zero.

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Influence of the environmental noise on determining the period of a torsion pendulum

罗杰, 田苑, 邵成刚, 王典洪

2015 (3): 30401-030401. doi: 10.1088/1674-1056/24/3/030401

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The environmental noise can restrict the accuracy of period estimation since the torsion pendulum is sensitive to weak forces. Two typical models for the environmental noise are proposed to make an evaluation. Generally, the stationary environmental noise is modeled as a white noise, and contributes to the period uncertainty as a function of the initial amplitude, the quality factor, the variance of noise and the time length. As to a sudden sharp disturbance acting on the pendulum, a narrow impulse model is constructed. It results in a sharp jump in the phase difference, which can be excluded with the 3σ criterion for a correction. An experimental data analysis for the measurement of the gravitational constant G with the time-of-swing method shows that the period uncertainty due to the environmental noise is about one and a half times the fundamental thermal noise limit. Though this result is dependent on the ambient environment, the analysis is instructive to improve the measurement accuracy of experiments.

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Bifurcation behavior and coexisting motions in a time-delayed power system

马美玲, 闵富红

2015 (3): 30501-030501. doi: 10.1088/1674-1056/24/3/030501

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With the increase of system scale, time delays have become unavoidable in nonlinear power systems, which add the complexity of system dynamics and induce chaotic oscillation and even voltage collapse events. In this paper, coexisting phenomenon in a fourth-order time-delayed power system is investigated for the first time with different initial conditions. With the mechanical power, generator damping factor, exciter gain, and time delay varying, the specific characteristic of the time-delayed system, including a discontinuous “jump” bifurcation behavior is analyzed by bifurcation diagrams, phase portraits, Poincaré maps, and power spectrums. Moreover, the coexistence of two different periodic orbits and chaotic attractors with periodic orbits are observed in the power system, respectively. The production condition and existent domain of the coexistence phenomenon are helpful to avoid undesirable behavior in time-delayed power systems.

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Policy iteration optimal tracking control for chaotic systems by using an adaptive dynamic programming approach

魏庆来, 刘德荣, 徐延才

2015 (3): 30502-030502. doi: 10.1088/1674-1056/24/3/030502

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A policy iteration algorithm of adaptive dynamic programming (ADP) is developed to solve the optimal tracking control for a class of discrete-time chaotic systems. By system transformations, the optimal tracking problem is transformed into an optimal regulation one. The policy iteration algorithm for discrete-time chaotic systems is first described. Then, the convergence and admissibility properties of the developed policy iteration algorithm are presented, which show that the transformed chaotic system can be stabilized under an arbitrary iterative control law and the iterative performance index function simultaneously converges to the optimum. By implementing the policy iteration algorithm via neural networks, the developed optimal tracking control scheme for chaotic systems is verified by a simulation.

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Observer of a class of chaotic systems: An application to Hindmarsh-Rose neuronal model

罗润梓, 张春华

2015 (3): 30503-030503. doi: 10.1088/1674-1056/24/3/030503

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This paper first investigates the observer of a class of chaotic systems, and then discusses the synchronization between two identical Hindmarsh-Rose (HR) neuronal chaotic systems. Both the drive and response systems are assumed to have only one state variable available. By constructing proper observers, some novel criteria for synchronization are proposed via a scalar input. Numerical simulations are given to demonstrate the efficiency of the proposed approach.

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Robust sliding mode control for fractional-order chaotic economical system with parameter uncertainty and external disturbance

周柯, 王智慧, 高立克, 孙跃, 马铁东

2015 (3): 30504-030504. doi: 10.1088/1674-1056/24/3/030504

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This paper presents a modified sliding mode control for fractional-order chaotic economical systems with parameter uncertainty and external disturbance. By constructing the suitable sliding mode surface with fractional-order integral, the effective sliding mode controller is designed to realize the asymptotical stability of fractional-order chaotic economical systems. Comparing with the existing results, the main results in this paper are more practical and rigorous. Simulation results show the effectiveness and feasibility of the proposed sliding mode control method.

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Infrared transparent frequency selective surface based on iterative metallic meshes

于淼, 徐念喜, 高劲松

2015 (3): 30701-030701. doi: 10.1088/1674-1056/24/3/030701

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In this paper, we present an infrared transparent frequency selective surface (ITFSS) based on iterative metallic meshes, which possesses the properties of high transmittance in infrared band and band-pass effect in millimeter wave band. Cross-slot units are designed on the iterative metallic meshes, which is composed of two same square metallic meshes with a misplaced overlap. In the infrared band of 3-5 μm, the ITFSS has an average transmittance of 80% with a MgF₂ substrate. In the millimeter wave band, a transmittance of -0.74 dB at the resonance frequency of 39.4 GHz is obtained. Moreover, theoretical simulations of the ITFSS diffractive characteristics and transmittance response are also investigated in detail. This ITFSS may be an efficient way to achieve the metamaterial millimeter wave/infrared functional film.

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Precision calculation of fine structure in helium and Li⁺

张佩佩, 钟振祥, 严宗朝, 史庭云

2015 (3): 33101-033101. doi: 10.1088/1674-1056/24/3/033101

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The fine structure constant α can be extracted from high-precision spectroscopy of the 2³P_J fine structure splittings in helium and light helium-like ions. In this work, the 2³P_J fine structure splittings of helium and Li⁺ ion are calculated, including relativistic and QED corrections of order mα⁴, mα⁴(m/M), mα⁵, mα⁵(m/M), and Douglas-Kroll operators of mα⁶ and mα⁶(m/M), which provide an independent verification for the previous calculations performed by Drake [Can. J. Phys. 80 1195 (2002)] and by Pachucki and Yerokhin [Phys. Rev. A 79 062516 (2009); Phys. Rev. Lett. 104 070403 (2010); Can. J. Phys. 89 1139 (2011)]. The results of the three groups agree with each other.

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Compton profile of molecular hydrogen

赵小利, 杨科, 徐龙泉, 马永朋, 闫帅, 倪冬冬, 康旭, 刘亚伟, 朱林繁

2015 (3): 33301-033301. doi: 10.1088/1674-1056/24/3/033301

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The Compton profile of molecular hydrogen has been determined at an incident photon energy of 20 keV based on the third generation synchrotron radiation, and the statistical accuracy of 0.2% is achieved at p_z=0. Different theoretical methods, i.e., the density functional method, and the Hartree-Fock method, were used to calculate the Compton profiles of hydrogen with different basis sets, and the theoretical calculations are in agreement with the experimental observation in the whole p_z region. Compared with the HF calculation, the DFT-B3LYP ones are in better agreement with the present experiment, which indicates the electron correlation effect is very important to describe the wavefunction in the ground state of hydrogen.

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Ellipticity-dependent ionization/dissociation of carbon dioxide in strong laser fields

张军峰, 马日, 左万龙, 吕航, 黄红卫, 徐海峰, 金明星, 丁大军

2015 (3): 33302-033302. doi: 10.1088/1674-1056/24/3/033302

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Ionization and dissociation of linear triatomic molecules, carbon dioxide, are studied in 50-fs 800-nm strong laser fields using time-of-flight mass spectrometer. The yields of double charged ions CO₂²⁺ and various fragment ions (CO⁺, Oⁿ⁺, and Cⁿ⁺ (n=1, 2)) are measured as a function of ellipticity of laser polarization in the intensity range from 5.0× 10¹³ W/cm² to 6.0× 10¹⁴ W/cm². The results demonstrate that non-sequential double ionization, which is induced by laser-driven electron recollision, dominates double ionization of CO₂ in the strong IR laser field with intensity lower than 2.0×10¹⁴ W/cm². The electron recollision could also have contribution in strong-field multiple ionization and formation of fragments of CO₂ molecules. The present study indicates that the intensity and ellipticity dependence of ions yields can be used to probe the complex dynamics of strong-field ionization/dissociation of polyatomic molecules.

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Projectile angular-differential cross sections for single electron transfer in fast He⁺-He collisions

Ebrahim Ghanbari-Adivi, Hoda Ghavaminia

2015 (3): 33401-033401. doi: 10.1088/1674-1056/24/3/033401

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The four-body Coulomb-Born distorted wave approximation is applied to investigate the integral as well as projectile angular-differential cross sections for single-electron capture in the collision of energetic singly positive charged helium ions with helium atoms in their ground states. The formalism satisfies the correct boundary conditions. The influence of the dynamic electron correlations on the cross sections is studied by considering the inter electronic interactions in the complete perturbation potentials in post form. Also, the sensitivity of the cross sections to the static electronic correlations is studied by using the single-zeta and the highly correlated Byron-Joachain wave functions to describe the initial bound state of the active electrons. The obtained results for the energy range of 40-5000 keV/amu are reported and compared with other three- and four-body theoretical data and available experimental measurements. The comparison leads us to discuss the validity of the applied approach and survey the interaction effects on the cross sections by recognizing the electron- electron interaction. Particularly, for differential cross sections, the comparison of the present four-body method with the experiment shows that the agreement is not as good as that for its three-body version.

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Formation of hydrogen atom in 2s state in proton-sodium inelastic scattering

Sabbah A. Elkilany

2015 (3): 33402-033402. doi: 10.1088/1674-1056/24/3/033402

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The inelastic collision of protons with sodium atoms are treated for the first time within the framework of the coupled-static and frozen core approximations. The method is used for calculating partial and total cross-sections with the assumption that only two channels (elastic and hydrogen formation in 2s state) are open. In each case, the calculations are carried out for seven values of the total angular momentum l (0 ≤ l ≤ 6). The target is described using the Clementi Roetti wave functions within the framework of the one valence electron model. We use Lipmann-Swinger equation to solve the derived equations of the problem, then apply an iterative numerical method to obtain the code of computer to calculate iterative partial cross-sections. This can be done through calculating the reactance matrix at different values of considered energies to obtain the transition matrix that gives partial and total cross sections. The present results for total hydrogen (2s state) formation cross sections are in agreement with results of other available ones in wide range of incident energy.

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Fast-electron-impact ionization process by 3p of hydrogen-like ions in Debye plasmas

祁月盈, 叶丹丹, 王建国, 屈一至

2015 (3): 33403-033403. doi: 10.1088/1674-1056/24/3/033403

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The plasma screening of fast-electron-impact-ionization by excited state (3p) of Hydrogen-like ions was investigated in the first Born approximation with a plasma screening length δ varying from 1000a₀ to 10a₀. The generalized oscillator strength densities showed dramatic changes: some accessional minima occurred along with a remarkable enhancement in certain continuum-energy domains. The double-differential cross sections exhibit not only the same structures as the Bethe surface for moderate and large momentum transfers, but also a broadened enhancement for small momentum transfers. The single-differential cross sections exhibit a near-zero-energy-enhancement and prodigious multiple-shape resonances, depending on the continuum energy and the plasma screening length. These features are analogous to those of the photo-ionization cross section. These findings, for both types of cross section, can be explained by processes associated with continuum electrons, as long as the potential has a short-range character.

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Morphology and structural stability of Pt-Pd bimetallic nanoparticles

刘暾东, 郑骥文, 邵桂芳, 范天娥, 文玉华

2015 (3): 33601-033601. doi: 10.1088/1674-1056/24/3/033601

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The morphologies and structures of Pt-Pd bimetallic nanoparticles determine their chemical and physical properties. Therefore, a fundamental understanding of their morphologies and structural stabilities is of crucial importance to their applications. In this article, we have performed Monte Carlo simulations to systematically explore the structural stability and structural features of Pt-Pd alloy nanoparticles. Different Pt/Pd ratios, and particle sizes and shapes were considered. The simulated results reveal that the truncated octahedron, which has the remarkably lowest energy among all the considered shapes, exhibits the best structural stability while the tetrahedron has the worst invariably. Furthermore, all the structures of Pt-Pd alloy nanoparticles present Pd-rich in the outmost layer but Pt-rich in the sub-outmost layer. Especially, atomic distribution and chemical short-range order parameter were applied to further characterize the structural features of Pt-Pd alloy nanoparticles. This study provides a significant insight not only into the structural stability of Pt-Pd alloy nanoparticles with different compositions, and particle sizes and shapes but also to the design of bimetallic nanoparticles.

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Implementation of ternary Shor's algorithm based on vibrational states of an ion in anharmonic potential

刘威, 陈书明, 张见, 吴春旺, 吴伟, 陈平形

2015 (3): 33701-033701. doi: 10.1088/1674-1056/24/3/033701

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It is widely believed that Shor's factoring algorithm provides a driving force to boost the quantum computing research. However, a serious obstacle to its binary implementation is the large number of quantum gates. Non-binary quantum computing is an efficient way to reduce the required number of elemental gates. Here, we propose optimization schemes for Shor's algorithm implementation and take a ternary version for factorizing 21 as an example. The optimized factorization is achieved by a two-qutrit quantum circuit, which consists of only two single qutrit gates and one ternary controlled-NOT gate. This two-qutrit quantum circuit is then encoded into the nine lower vibrational states of an ion trapped in a weakly anharmonic potential. Optimal control theory (OCT) is employed to derive the manipulation electric field for transferring the encoded states. The ternary Shor's algorithm can be implemented in one single step. Numerical simulation results show that the accuracy of the state transformations is about 0.9919.

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Drift effect on vacuum birefringence in a strong electric and magnetic field

黄永盛, 王乃彦, 汤秀章

2015 (3): 34201-034201. doi: 10.1088/1674-1056/24/3/034201

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As an important QED effect to detect the vacuum polarization, birefringence in the presence of a strong electric and magnetic field, E₀⊥B₀,E₀≤cB₀, is considered. The directional dependence of birefringence is obtained. In two special cases: E₀ = 0 and E₀ = cB₀, our results are consistent with the previous ones. The refractive index of the probe wave propagating in the E₀×B₀ direction decreases with E₀/cB₀, while that in the -E₀×B₀ direction increases with E₀/cB₀. The physics of the direction dependence of birefringence maybe the E₀×B₀ drift velocity of the virtual electrons and positrons.

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Spontaneous emission of “polarized” V-type three-level atoms strongly coupled with an optical cavity

薛艳丽, 朱诗灯, 李家方, 丁伟, 冯宝华, 李志远

2015 (3): 34202-034202. doi: 10.1088/1674-1056/24/3/034202

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Polarization, an intrinsic ingredient of photon, plays a critical role in its interaction with matter. A general polarization state can be an appropriate superposition of two basic polarization states, say, the vertical and horizontal linear polarized state. Here we study spontaneous emission of a V-type three-level atom (with two upper states close in energy level) strongly coupled with a single-mode damped optical cavity. By defining a general polarization state of atom as a specific superposition of the two upper quantum states, we can prepare atoms with linear polarization at arbitrary direction, left and right circular polarization, and left and right elliptical polarization, similar to photons. We find that the spontaneous emission of light from these “polarized” three-level atoms shows very different profiles of side and axis spectra. This means that the polarization state of three-level atoms can become an active ingredient to manipulate its interaction with light and control the quantum interference effect. Exploitation of the coherent superposition and interference of quantum states in “polarized” atoms would allow one to deeply explore new frontiers of light-matter interaction.

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Preparation of SiO₂@Au nanorod array as novel surface enhanced Raman substrate for trace pollutants detection

侯孟婧, 张弦, 崔肖阳, 刘灿, 李正操, 张政军

2015 (3): 34203-034203. doi: 10.1088/1674-1056/24/3/034203

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An effective surface enhanced Raman scattering (SERS) substrate is designed and fabricated by synthesis of SiO₂ nanorods array via glancing angle deposition, followed by coating Au nanoparticles onto SiO₂ surface in order to create numerous “hot spots”. The detecting sensitivity of such substrate could be optimized by simply adjusting the deposition time of Au. Thus, it can be used for detection of Rhodamine 6G at concentration as low as 10^-9 M. Furthermore, our SERS substrate is applied to detect 5 μg/g polychlorinated biphenyls in soil sample, which proves its potential for trace environmental pollutants detection.

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Sensitive absorption measurements of hydrogen sulfide at 1.578 μm using wavelength modulation spectroscopy

夏滑, 董凤忠, 吴边, 张志荣, 庞涛, 孙鹏帅, 崔小娟, 韩荦, 王煜

2015 (3): 34204-034204. doi: 10.1088/1674-1056/24/3/034204

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Sensitive detection of hydrogen sulfide (H₂S) has been performed by means of wavelength modulation spectroscopy (WMS) near 1.578 μm. With the scan amplitude and the stability of the background baseline taken into account, the response time is 4 s for a 0.8 L multi-pass cell with a 56.7 m effective optical path length. Moreover, the linearity has been tested in the 0-50 ppmv range. The detection limit achievable by the Allan variance is 224 ppb within 24 s under room temperature and ambient pressure conditions. This tunable diode laser absorption spectroscopy (TDLAS) system for H₂S detection has the feasibility of real-time online monitoring in many applications.

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First-principles study on linear and nonlinear optical properties of ZnGeP₂

肖瑞春, 吴海信, 倪友保, 王振友, 黄昌保, 程旭东

2015 (3): 34205-034205. doi: 10.1088/1674-1056/24/3/034205

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The study of the linear and nonlinear optical properties of ZnGeP₂ based on density functional theory has been carried out. In order to get a more physical picture in the infrared region, terms which are considered as the phonon effect were added to the calculated refractive dispersion curves. The phonon-corrected calculation curves show excellent agreement with experimental refractive indexes, which gives a better comprehension of the linear optical proprieties in the transparent region. The static nonlinear optical susceptibility was investigated using approaches based on the “sum over states” and the 2n+1 theorem methods. Both of the results of these two methods reasonably coincided with the experimental results.

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Attosecond pulse generation from two-electron harmonic emission spectrum

冯立强, 刘航

2015 (3): 34206-034206. doi: 10.1088/1674-1056/24/3/034206

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In this paper, we theoretically investigate the high-order harmonic generation and attosecond pulse generation when a two-electron He atom is exposed to the intense laser pulse. It shows that due to the two-electron double recombination mechanism, an extended plateau beyond the classical single-electron harmonic has been obtained on the two-electron harmonic spectrum. Further by using this two-electron harmonic extension scheme combined with the two-color field, two supercontinuum bandwidths with 200 eV have been obtained. As a result, a series of sub-60 as extreme ultraviolet (XUV) pulses have been directly generated.

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Enhancement of third harmonic generation in air filamentation using obstacles

刘晓龙, 鲁欣, 杜志贵, 马景龙, 李玉同, 张杰

2015 (3): 34207-034207. doi: 10.1088/1674-1056/24/3/034207

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The intensity of third harmonic emission in air filamentation disturbed by copper fibers and alcohol droplets has been investigated experimentally. Enhancement of the third harmonic emission up to more than one order of magnitude has been observed. The physical mechanism of third harmonic enhancement is attributed to suppression of the destructive interference by comparison of the experimental results and it is closely related to the type, size, and relative position of the obstacles.

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Non-Gaussian quantum states generation and robust quantum non-Gaussianity via squeezing field

唐绪兵, 高放, 王耀雄, 匡森, 双丰

2015 (3): 34208-034208. doi: 10.1088/1674-1056/24/3/034208

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Recent studies show that quantum non-Gaussian states or using non-Gaussian operations can improve entanglement distillation, quantum swapping, teleportation, and cloning. In this work, employing a strategy of non-Gaussian operations (namely subtracting and adding a single photon), we propose a scheme to generate non-Gaussian quantum states named single-photon-added and -subtracted coherent (SPASC) superposition states by implementing Bell measurements, and then investigate the corresponding nonclassical features. By squeezed the input field, we demonstrate that robustness of non- Gaussianity can be improved. Controllable phase space distribution offers the possibility to approximately generate a displaced coherent superposition states (DCSS). The fidelity can reach up to F≥0.98 and F ≥ 0.90 for size of amplitude z = 1.53 and 2.36, respectively.

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Frequency detection of self-adaption control based on chaotic theory

徐艳春, 瞿晓东, 李振兴

2015 (3): 34301-034301. doi: 10.1088/1674-1056/24/3/034301

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Low-order Duffing and high-order Rössler chaotic oscillator are connected together and new self-adaption frequency detection method is presented. The frequency difference control between unknown signal and the periodic driving force is realized in this paper and the self-adaption is obtained. Thus, the detection precision and speed are promoted. The limitation that there are too many chaotic oscillators in Duffing system is broken. Meanwhile the disadvantage that the detection speed is lower in Rössler chaotic control is overcome. The self-adaption choice of frequency difference control is realized using the Duffing and Rössler different chaotic oscillators to obtain unknown signal frequency. The simulation results show that the presented method is feasible and effective.

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Time fractional dual-phase-lag heat conduction equation

续焕英, 蒋晓芸

2015 (3): 34401-034401. doi: 10.1088/1674-1056/24/3/034401

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We build a fractional dual-phase-lag model and the corresponding bioheat transfer equation, which we use to interpret the experiment results for processed meat that have been explained by applying the hyperbolic conduction. Analytical solutions expressed by H-functions are obtained by using the Laplace and Fourier transforms method. The inverse fractional dual-phase-lag heat conduction problem for the simultaneous estimation of two relaxation times and orders of fractionality is solved by applying the nonlinear least-square method. The estimated model parameters are given. Finally, the measured and the calculated temperatures versus time are compared and discussed. Some numerical examples are also given and discussed.

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Enhanced thermoelectric performance of TiO₂-based hybrid materials by incorporating conducting polymer

吴子华, 谢华清, 翟永彪, 甘良华, 刘俊

2015 (3): 34402-034402. doi: 10.1088/1674-1056/24/3/034402

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In order to study the thermoelectric properties of TiO₂-based hybrid materials, TiO₂/polyparaphenylene (PPP) nanocomposites are fabricated by spark plasma sintering (SPS). The results show that the electrical conductivity follow percolation theory is enhanced due to the electron transfer highway provided by the conducting PPP phase. Furthermore, the thermal conductivity is reduced due to the drastic difference of vibrational spectra between organic and inorganic components. As a result, the greatest ZT=0.24 is obtained for TiO₂/0.75 wt% PPP sample, which is 15-fold higher than pure TiO₂ (ZT=0.016).

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High coercivity in large exchange-bias Co/CoO-MgO nano-granular films

葛传楠, 万贤纲, Eric Pellegrin, 胡志伟, Wen-I Liang, Michael Bruns, 邹文琴, 都有为

2015 (3): 34501-034501. doi: 10.1088/1674-1056/24/3/034501

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We present a detailed study on the magnetic coercivity of Co/CoO-MgO core-shell systems, which exhibits a large exchange bias due to an increase of the uncompensated spin density at the interface between the CoO shell and the metallic Co core by replacing Co by Mg within the CoO shell. We find a large magnetic coercivity of 7120 Oe around the electrical percolation threshold of the Co/CoO core/shell particles, while samples with a smaller or larger Co metal volume fraction show a considerably smaller coercivity. Thus, this study may lead to a route to improving the magnetic properties of artificial magnetic material in view of potential applications.

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Experimental study and analysis on the rising motion of grains in a vertically-vibrated pipe

刘煜, 赵俊红

2015 (3): 34502-034502. doi: 10.1088/1674-1056/24/3/034502

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Previous experimental investigations have shown that when a narrow pipe is inserted into a granular bed and is vibrated vertically but the granular bed is kept still, the grains in the bed can enter the pipe and rise against gravity along the pipe and finally stabilized at a certain height. The growth velocity and final stable height of the grain column inside the pipe can be controlled by varying the vibration conditions. In this paper, we discuss those experimental findings. We establish a mathematic relation between the grain column height (h) and time (t), and by using the relation we discuss the change of the growth velocity (dh/dt) and acceleration (d²h/dt²) with t and h, respectively. We also analyze the mechanism of the rising motion of the grains during vibration. Furthermore, we derive a theoretical expression for describing the final stable height (d_st), which shows that the main factors influencing the height are vibration strength (Γ), bulk density of grains, inner diameter of the pipe, and vibration frequency, and that h_st increases nonlinearly in the presence of air and linearly in a vacuum environment with increasing Γ.

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Frequency-dependent friction in pipelines

蒋丹, 李松晶, 杨平, 赵天扬

2015 (3): 34701-034701. doi: 10.1088/1674-1056/24/3/034701

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A comprehensive study of modeling the frequency-dependent friction in a pipeline during pressure transients following a sudden cut-off of the flow is presented. A new method using genetic algorithms (GAs) for parameter identification of the weighting function coefficients of the frequency-dependent friction model is described. The number of weighting terms required in the friction model is obtained. Comparisons between simulation results and experimental data of transient pressure pulsations close to the valve in horizontal upstream and downstream pipelines are carried out respectively. The validity of the parameter identification method for weighting function coefficients and the calculation method for the number of weighting terms in the friction model is confirmed.

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Modeling of nonlinear envelope solitons in strongly coupled dusty plasmas: Instability and collision

S. K. El-Labany, E. F. El-Shamy, W. F. El-Taibany, N. A. Zedan

2015 (3): 35201-035201. doi: 10.1088/1674-1056/24/3/035201

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Modeling of instability and collision of nonlinear dust-acoustic (NDA) envelope solitons in strongly coupled dusty plasmas (SCDPs) is theoretically investigated. The SCDPs consists of strongly correlated negatively variable-charged dust grains and weakly correlated Boltzmann electrons and ions. Using the derivative expansion perturbation technique, a nonlinear Schrödinger-type (NLST) equation for describing the propagation of NDA envelope solitons is derived. Moreover, the extended Poincaré-Lighthill-Kuo (EPLK) method is employed to deduce the analytical phase shifts and the trajectories after the collision of NDA envelope solitons. In detail, the results show that both modulation instability and phase shift after collision of NDA envelope solitons will modify with the increase in the effects of the viscosity, the relaxation time, and the dust charge fluctuation. Crucially, the modeling of dust-acoustic envelope solitons collision, as reported here, is helpful for understanding the propagation of NDA envelope solitons in strongly coupled dusty plasmas.

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Start-up phase plasma discharge design of a tokamak via control parameterization method

郭珊, 许珂, 许超, 任志刚, 肖炳甲

2015 (3): 35202-035202. doi: 10.1088/1674-1056/24/3/035202

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The tokamak start-up is a very important phase during the process to obtain a suitable equalizing plasma, and its governing model can be described as a set of nonlinear ordinary differential equations (ODEs). In this paper, we first estimate the parameters in the original model and set up an accurate model to express how the variables change during the start-up phase, especially how the plasma current changes with respect to time and the loop voltage. Then, we apply the control parameterization method to obtain an approximate optimal parameters selection problem for the loop voltage design to achieve a desired plasma current target. Computational optimal control techniques such as the variational method and the costate method are employed to solve the problem, respectively. Finally, numerical simulations are performed and the results obtained via different methods are compared. Our numerical parameterization method and optimization procedure turn out to be effective.

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A tunable magnetically insulated transmission line oscillator

樊玉伟, 王晓玉, 赫亮, 钟辉煌, 张建德

2015 (3): 35203-035203. doi: 10.1088/1674-1056/24/3/035203

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A tunable magnetically insulated transmission line oscillator (MILO) is put forward and simulated. When the MILO is driven by a 430 kV, 40.6 kA electron beam, high-power microwave is generated with a peak output power of 3.0 GW and frequency of 1.51 GHz, and the relevant power conversion efficiency is 17.2%. The 3-dB tunable frequency range (the relative output power is above half of the peak output power) is 2.25-0.825 GHz when the outer radius of the slow-wave structure (SWS) vanes ranges from 77 mm to 155 mm, and the 3-dB tuning bandwidth is 92%, which is sufficient for the aim of large-scale tuning and high power output.

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Synthesis and performance of Zn-Ni-P thin films

V. Soare, M. Burada, I. Constantin, M. Ghita, V. Constantin, F. Miculescu, A. M. Popescu

2015 (3): 36101-036101. doi: 10.1088/1674-1056/24/3/036101

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The electroplating of Zn-Ni-P thin film alloys from a sulfate bath containing phosphoric and phosphorous acid was investigated. The bath composition and the deposition parameters were optimized through Hull cell experiments, and the optimum experimental conditions were determined (pH = 2, temperature = 298-313 K, zinc sulfate concentration = 30 g· L^-1, EDTA concentration = 15 g· L^-1, and current density=1.0-2.0 A· dm^-2). The SEM analysis of the coating deposited from the optimum bath revealed fine-grained deposits of the alloy in the presence of EDTA. Optical microscopy analysis indicated an electrodeposited thin film with uniform thickness and good adhesion to the steel substrate. The good adherence of the coatings was also demonstrated by the scratch tests that were performed, with a maximum determined value of 25 N for the critical load. Corrosion resistance tests revealed good protection of the steel substrate by the obtained Zn-Ni-P coatings, with values up to 85.89% for samples with Ni contents higher than 76%. The surface analysis of the thin film samples before and after corrosion was performed by X-ray photoelectron spectroscopy (XPS).

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Flexural wave band-gaps in phononic metamaterial beam with hybrid shunting circuits

张浩, 温激鸿, 陈圣兵, 王刚, 温熙森

2015 (3): 36201-036201. doi: 10.1088/1674-1056/24/3/036201

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Periodic arrays of hybrid-shunted piezoelectric patches are used to control the band-gaps of phononic metamaterial beams. Passive resistive-inductive (RL) shunting circuits can produce a narrow resonant band-gap (RG), and active negative capacitive (NC) shunting circuits can broaden the Bragg band-gaps (BGs). In this article, active NC shunting circuits and passive resonant RL shunting circuits are connected to the same piezoelectric patches in parallel, which are usually called hybrid shunting circuits, to control the location and the extent of the band-gaps. A super-wide coupled band-gap is generated when the coupling between RG and the BG occurs. The attenuation constant of the infinite periodic structure is predicted by the transfer matrix method, which is compared with the vibration transmittance of a finite periodic structure calculated by the finite element method. Numerical results show that the hybrid-shunting circuits can make the band-gaps wider by appropriately selecting the inductances, negative capacitances, and resistances.

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Two-dimensional arsenic monolayer sheet predicted from first-principles

濮春英, 叶小涛, 蒋华龙, 张飞武, 卢志文, 何俊宝, 周大伟

2015 (3): 36301-036301. doi: 10.1088/1674-1056/24/3/036301

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Using first-principles calculations, we investigate the two-dimensional arsenic nanosheet isolated from bulk gray arsenic. Its dynamical stability is confirmed by phonon calculations and molecular dynamics analyzing. The arsenic sheet is an indirect band gap semiconductor with a band gap of 2.21 eV in the hybrid HSE06 functional calculations. The valence band maximum (VBM) and the conduction band minimum (CBM) are mainly occupied by the 4p orbitals of arsenic atoms, which is consistent with the partial charge densities of VBM and CBM. The charge density of the VBM G point has the character of a π bond, which originates from p orbitals. Furthermore, tensile and compressive strains are applied in the armchair and zigzag directions, related to the tensile deformations of zigzag and armchair nanotubes, respectively. We find that the ultimate strain in zigzag deformation is 0.13, smaller than 0.18 of armchair deformation. The limit compressive stresses of single-layer arsenic along armchair and zigzag directions are -4.83 GPa and -4.76 GPa with corresponding strains of -0.15 and -0.14, respectively.

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Structural phase transitions of tellurium nanoplates under pressure

李超, Arfan Bukhtiar, 沈希, 孔盼盼, 王伟鹏, 赵豪飞, 姚湲, 邹炳锁, 李延春, 李晓东, 刘景, 靳常青, 禹日成

2015 (3): 36401-036401. doi: 10.1088/1674-1056/24/3/036401

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In situ high-pressure angle dispersive x-ray diffraction experiments using synchrotron radiation on Te nanoplates were carried out with a diamond anvil cell at room temperature. The results show that Te-I with a trigonal structure transforms to triclinic Te-Ⅱ at about 4.9 GPa, Te-Ⅱ transforms to monoclinic Te-Ⅲ at about 8.0 GPa, Te-Ⅲ turns to rhombohedral Te-IV at about 23.8 GPa, and Te-IV changes to body centered cubic Te-V at 27.6 GPa. The bulk moduli B₀ of Te nanoplates are higher than those of Te bulk materials.

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Strain analysis of free-standing strained silicon-on-insulator nanomembrane

孙高迪, 董林玺, 薛忠营, 陈达, 郭庆磊, 母志强

2015 (3): 36801-036801. doi: 10.1088/1674-1056/24/3/036801

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Based on the ultra-thin strained silicon-on-insulator (sSOI) technology, by creatively using a hydrofluoric acid (HF) vapor corrosion system to dry etch the SiO₂ layer, a large area of suspended strained silicon (sSi) nanomembrane with uniform strain distribution is fabricated. The strain state in the implemented nanomembrane is comprehensively analyzed by using an UV-Raman spectrometer with different laser powers. The results show that the inherent strain is preserved while there are artificial Raman shifts induced by the heat effect, which is proportional to the laser power. The suspended sSOI nanomembrane will be an important material for future novel high-performance devices.

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Water-assisted highly enhanced crystallographic etching of graphene by iron catalysts

薛磊江, 余芳, 周海青, 孙连峰

2015 (3): 36802-036802. doi: 10.1088/1674-1056/24/3/036802

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We report the assisted role of water vapor in crystallographic cutting of graphene via iron catalysts in reduced atmosphere. Without water, graphene can be tailored with smooth trenches composed of straight lines with angles of 60° or 120° between two adjacent trenches. After the addition of water, new chacteristics are found: such as almost no iron particles can be detected along the trenches; each trench becomes longer and lots of graphene nanoribbons can be generated. The underlying mechanism is proposed and discussed, which is attributed to stimulating and lengthening of the catalytic activity of iron particles by water vapor.

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Dynamic resistive switching in a three-terminal device based on phase separated manganites

王志强, 颜志波, 秦明辉, 高兴森, 刘俊明

2015 (3): 37101-037101. doi: 10.1088/1674-1056/24/3/037101

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A three-terminal device based on electronic phase separated manganites is suggested to produce high performance resistive switching. Our Monte Carlo simulations reveal that the conductive filaments can be formed/annihilated by reshaping the ferromagnetic metal phase domains with two cross-oriented switching voltages. Besides, by controlling the high resistance state (HRS) to a stable state that just after the filament is ruptured, the resistive switching remains stable and reversible, while the switching voltage and the switching time can be greatly reduced.

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Electrical bistable devices using composites of zinc sulfide nanoparticles and poly-(N-vinylcarbazole)

曹亚鹏, 胡煜峰, 李剑焘, 叶海航, 吕龙锋, 宁宇, 鲁启鹏, 唐爱伟, 娄志东, 侯延冰, 滕枫

2015 (3): 37201-037201. doi: 10.1088/1674-1056/24/3/037201

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N-dodecanethiol capped zinc sulfide (ZnS) nanocrystals were synthesized by the one-pot approach and blended with poly (N-vinylcarbazole) (PVK) to fabricate electrical bistable devices. The corresponding devices did exhibit electrical bistability and negative differential resistance (NDR) effects. A large ON/OFF current ratio of 10⁴ at negative voltages was obtained by applying different amplitudes of sweeping voltage. The observed conductance switching and the negative differential resistance are attributed to the electric-field-induced charge transfer between the nanocrystals and the polymer, and the charge trapping/detrapping in the nanocrystals.

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Spin and valley filter in strain engineered silicene

汪萨克, 汪军

2015 (3): 37202-037202. doi: 10.1088/1674-1056/24/3/037202

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The realization of a perfect spin or valley filtering effect in two-dimensional graphene-like materials is one of the fundamental objectives in spintronics and valleytronics. For this purpose, we study spin- and valley-dependent transport in a silicene system with spatially alternative strains. It is found that due to the valley-opposite gauge field induced by the strain, the strained silicene with a superlattice structure exhibits an angle-resolved valley and spin filtering effect when the spin-orbit interaction is considered. When the interaction that breaks the time reversal symmetry is introduced, such as the spin or valley dependent staggered magnetization, the system is shown to be a perfect spin and valley half metal in which only one spin and valley species is allowed to transport. Our findings are helpful to design both spintronic and valleytronic devices based on silicene.

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New SOI power device with multi-region high-concentration fixed interface charge and the model of breakdown voltage

李琦, 李海鸥, 唐宁, 翟江辉, 宋树祥

2015 (3): 37203-037203. doi: 10.1088/1674-1056/24/3/037203

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A new SOI power device with multi-region high-concentration fixed charge (MHFC) is reported. The MHFC is formed through implanting Cs or I ion into the buried oxide layer (BOX), by which the high-concentration dynamic electrons and holes are induced at the top and bottom interfaces of BOX. The inversion holes can enhance the vertical electric field and raise the breakdown voltage since the drain bias is mainly generated from the BOX. A model of breakdown voltage is developed, from which the optimal spacing has also been obtained. The numerical results indicate that the breakdown voltage of device proposed is increased by 287% in comparison to that of conventional LDMOS.

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Two-dimensional metallic behavior at polar MgO/BaTiO₃ (110) interfaces

杜颜伶, 王春雷, 李吉超, 张新华, 王芙凝, 刘剑, 祝元虎, 尹娜, 梅良模

2015 (3): 37301-037301. doi: 10.1088/1674-1056/24/3/037301

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The first-principles calculations are employed to investigate the electrical properties of polar MgO/BaTiO₃ (110) interfaces. Both n-type and p-type polar interfaces show a two-dimensional metallic behavior. For the n-type polar interface, the interface Ti 3d electrons are the origin of the metallic and magnetic properties. Varying the thickness of BaTiO₃ may induce an insulator-metal transition, and the critical thickness is 4 unit cells. For the p-type polar interface, holes preferentially occupy the interface O 2p_y state, resulting in a conducting interface. The unbalance of the spin splitting of the O 2p states in the interface MgO layer leads to a magnetic moment of about 0.25μ_B per O atom at the interface. These results further demonstrate that other polar interfaces, besides LaAlO₃/SrTiO₃, can show a two-dimensional metallic behavior. It is helpful to fully understand the role of polar discontinuity on the properties of the interface, which widens the field of polar-nonpolar interfaces.

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Majorana fermion realization and relevant transport processes in a triple-quantum dot system

邓明勋, 郑诗菡, 杨谋, 胡梁宾, 王瑞强

2015 (3): 37302-037302. doi: 10.1088/1674-1056/24/3/037302

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Nonequilibrium electronic transports through a system hosting three quantum dots hybridized with superconductors are investigated. By tuning the relative positions of the dot levels, we illustrate the existence of Majorana fermions and show that the Majorana feimions will either survive separately on single dots or distribute themselves among different dots with tunable probabilities. As a result, different physical mechanisms appear, including local Andreev reflection (LAR), cross Andreev reflection (CAR), and cross resonant tunneling (CRT). The resulting characteristics may be used to reveal the unique properties of Majorana fermions. In addition, we discuss the spin-polarized transports and find a pure spin current and a spin filter effect due to the joint effect of CRT and CAR, which is important for designing spintronic devices.

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Influences of fringing capacitance on threshold voltage and subthreshold swing of a GeOI metal-oxide-semiconductor field-effect transistor

范敏敏, 徐静平, 刘璐, 白玉蓉, 黄勇

2015 (3): 37303-037303. doi: 10.1088/1674-1056/24/3/037303

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Models of threshold voltage and subthreshold swing, including the fringing-capacitance effects between the gate electrode and the surface of the source/drain region, are proposed. The validity of the proposed models is confirmed by the good agreement between the simulated results and the experimental data. Based on the models, some factors impacting the threshold voltage and subthreshold swing of a GeOI metal-oxide-semiconductor field-effect transistor (MOSFET) are discussed in detail and it is found that there is an optimum thickness of gate oxide for definite dielectric constant of gate oxide to obtain the minimum subthreshold swing. As a result, it is shown that the fringing-capacitance effect of a short-channel GeOI MOSFET cannot be ignored in calculating the threshold voltage and subthreshold swing.

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Improvement of the off-state breakdown voltage with field plate and low-density drain in AlGaN/GaN high-electron mobility transistors

张鹏, 赵胜雷, 侯斌, 王冲, 郑雪峰, 马晓华, 张进成, 郝跃

2015 (3): 37304-037304. doi: 10.1088/1674-1056/24/3/037304

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We present an AlGaN/GaN high-electron mobility transistor (HEMT) device with both field plate (FP) and low-density drain (LDD). The LDD is realized by the injection of negatively charged fluorine (F^-) ions under low power in the space between the gate and the drain electrodes. With a small-size FP and a LDD length equal to only 31% of the gate-drain spacing, the device effectively modifies the electric field distribution and achieves a breakdown voltage enhancement up to two times when compared with a device with only FP.

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Low field induced giant anisotropic magnetocaloric effect in DyFeO₃ single crystal

柯亚娇, 张向群, 葛恒, 马跃, 成昭华

2015 (3): 37501-037501. doi: 10.1088/1674-1056/24/3/037501

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We have investigated the anisotropic magnetocaloric effect and the rotating field magnetic entropy in DyFeO₃ single crystal. A giant rotating field entropy change of -ΔS_M^R=16.62 J/kg·K was achieved from b axis to c axis in bc plane at 5 K for a low field change of 20 kOe. The large anisotropic magnetic entropy change is mainly accounted for the 4f electron of rare-earth Dy³⁺ ion. The large value of rotating field entropy change, together with large refrigeration capacity and negligible hysteresis, suggests that the multiferroic ferrite DyFeO₃ singlecrystal could be a potential material for anisotropic magnetic refrigeration at low field, which can be realized in the practical application around liquid helium temperature region.

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Double spin-glass-like behavior and antiferromagnetic superexchange interaction between Fe³⁺ ions in α-Ga_2-xFe_xO₃ (0 ≤ x ≤ 0.4)

吕益飞, 向建勇, 温福昇, 吕伟明, 胡文涛, 柳忠元

2015 (3): 37502-037502. doi: 10.1088/1674-1056/24/3/037502

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Single phase of Fe³⁺-doped α-Ga_2-xFe_xO₃ (α -GF_xO, x=0.1, 0.2, 0.3, 0.4) is synthesized by treating the β -Ga_2-xFe_xO₃ (β -GF_xO) precursors at high temperatures and high pressures. Rietveld refinements of the X-ray diffraction data show that the lattice constants increase monotonically with the increase of Fe³⁺ content. Calorimetric measurements show that the temperature of the phase transition from α -GF_xO to β -GF_xO increases, while the associated enthalpy change decreases upon increasing Fe³⁺ content. The optical energy gap deduced from the reflectance measurement is found to decrease monotonically with the increase in Fe³⁺ content. From the measurements of magnetic field-dependent magnetization and temperature-dependent inverse molar susceptibility, we find that the superexchange interaction between Fe³⁺ ions is antiferromagnetic. Remnant magnetization is observed in the Fe³⁺-doped α -GF_xO and is attributed to the spin glass in the magnetic sublattice. At high Fe³⁺ doping level (x=0.4), two evident peaks are observed in the image part of the AC susceptibility χ" _ac. The frequency dependence in intensity of these two peaks as well as two spin freezing temperatures observed in the DC magnetization measurements of α -GF_0.4O is suggested to be the behavior of two spin glasses.

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Magnetic properties and magnetocaloric effects in HoPd intermetallic

莫兆军, 沈俊, 高新强, 刘瑶, 吴剑峰, 沈保根, 孙继荣

2015 (3): 37503-037503. doi: 10.1088/1674-1056/24/3/037503

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A large reversible magnetocaloric effect accompanied by a second order magnetic phase transition from PM to FM is observed in the HoPd compound. Under the magnetic field change of 0-5 T, the magnetic entropy change -ΔS_M^max and the refrigerant capacity RC for the compound are evaluated to be 20 J/(kg · K) and 342 J/kg, respectively. In particular, large -ΔS_M^max (11.3 J/(kg · K)) and RC (142 J/kg) are achieved under a low magnetic field change of 0-2 T with no thermal hysteresis and magnetic hysteresis loss. The large reversible magnetocaloric effect (both the large -ΔS_M and the high RC) indicates that HoPd is a promising material for magnetic refrigeration at low temperature.

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Reduction of defect-induced ferromagnetic stability in passivated ZnO nanowires

吴芳, 孟培雯, 罗康, 刘云飞, 阚二军

2015 (3): 37504-037504. doi: 10.1088/1674-1056/24/3/037504

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First-principles calculations are performed to study the electronic structures and magnetic properties of ZnO nanowires (NM). Our results indicate that the single Zn defect can induce large local magnetic moment (～ 2μ_B) in the ZnO NWs, regardless of the surface modification. Interestingly, we find that local magnetic defects have strong spin interaction, and favor room-temperature ferromagnetism in bared ZnO NW. On the other hand, although H passivation does not destroy the local magnetic moment of Zn vacancy, it does greatly reduce the spin interaction between magnetic defects. Therefore, our results indicate that H passivation should be avoided in the process of experiments to maintain the room-temperature ferromagnetism.

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Tailoring the structural and magnetic properties of Cu-doped ZnO by c-axis pressure

巩纪军, 陈继培, 张飞, 吴昊, 秦明辉, 曾敏, 高兴森, 刘俊明

2015 (3): 37505-037505. doi: 10.1088/1674-1056/24/3/037505

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The structural and magnetic properties of the Cu-doped ZnO (ZnO:Cu) under c-axis pressure were studied using first-principle calculations. It was found that the ZnO:Cu undergoes a structural transition from Wurtzite to Graphite-like structure at a c-axis pressure of 7-8 GPa. This is accompanied by an apparent loss of ferromagnetic stability, indicating a magnetic transformation from a ferromagnetic state to a paramagnetic-like state. Further studies revealed that the magnetic instability is closely related to the variation in crystalline field originated from the structural transition, which is in association with the overlapping of spin-charge density between the Cu²⁺ and adjacent O^2-.

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Effect of optimized aging processing on properties of the sintered Dy-doped Nd-Fe-B permanent magnet

王晓丽, 赵利娜, 丁开鸿, 崔胜利, 孙永聪, 李木森

2015 (3): 37506-037506. doi: 10.1088/1674-1056/24/3/037506

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We investigate the effect of the optimized aging processing on magnetism and mechanical property of the sintered Dy-doped Nd-Fe-B permanent magnet. The experimental results show that the magnetism, especially intrinsic coercivity, of the optimized aged Dy-doped Nd-Fe-B magnet is more excellent than that of the sintered one, but the former's strength and hardness are lower than that of the latter. It was observed that the optimized aged Dy-doped Nd-Fe-B magnet have more uniform grain size, thinner (Nd, Dy)-rich boundary phase. By means of the EBSD technology, the number of larger angle grain boundaries in the optimized aged Dy-doped Nd-Fe-B magnet is more than that of the sintered one. The reasons for the increased intrinsic coercivity and decreased mechanical properties of the optimized aged Dy-doped Nd-Fe-B magnet are also discussed.

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Fabrication and magnetocrystalline anisotropy of NiCo(002) films

樊维佳, 马丽, 时钟, 周仕明

2015 (3): 37507-037507. doi: 10.1088/1674-1056/24/3/037507

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A series of 30-nm-thick epitaxial Ni_xCo_1-x (002) alloy films are fabricated by DC magnetron sputtering. MgO (002) and SrTiO₃ (002) single substrates are used for x>0.5 and x<0.5, respectively. The magnetocrystalline anisotropy of Ni_xCo_1-x (002) alloy films is studied in the entire composition region for 0≤x≤ 1.0. When x decreases, the cubic magnetic anisotropy constant K₁ changes sign from negative to positive at x=0.96 and becomes negative again at x= 0.79. It becomes more negative as x decreases from 0.79 to 0. The uniaxial anisotropy K_u is smaller than the K₁ by a factor of two orders.

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Current-induced magnetic soliton solutions in a perpendicular ferromagnetic anisotropy nanowire

李秋艳, 赵飞, 贺鹏斌, 李再东

2015 (3): 37508-037508. doi: 10.1088/1674-1056/24/3/037508

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Magnon density distribution can be affected by the spin-transfer torque in a perpendicular ferromagnetic anisotropy nanowire. We obtain the analytical expression for the critical current condition. For the cases of below and above the critical value, the magnon density distribution admits bright and dark soliton states, respectively. Moreover, we discuss two-soliton collision properties that are modulated by the current. Each magnetic soliton exhibits no changes in both velocity and width before and after the collision.

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Quantitative calculations of polarizations arising from the symmetric and antisymmetric exchange strictions in Tm-doped GdMnO₃

秦明辉, 林林, 李林, 贾兴涛, 刘俊明

2015 (3): 37509-037509. doi: 10.1088/1674-1056/24/3/037509

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The ferroelectric polarization and phase diagram in Tm-doped GdMnO₃ are studied by means of Monte Carlo simulation based on the Mochizuki-Furukawa model. Our work well reproduces the low temperature polarization at various substitution levels observed experimentally. It is demonstrated that the Tm-doping can control the multiferroic behaviors through modulating the spin structures, resulting in the flop of the electric polarization. In addition, the polarization in the ab-plane cycloidal spin phase arises from comparable contributions of the symmetric exchange striction and antisymmetric exchange striction, leading to much bigger polarization than that in the bc-plane cycloidal spin phase where only the contribution of the latter striction is available. The phase diagram obtained in our simulation is helpful for clarifying the multiferroic properties in doped manganite systems and other related multiferroics.

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Theoretical study of mutual control mechanism between magnetization and polarization in multiferroic materials

刘宇, 翟良君, 王怀玉

2015 (3): 37510-037510. doi: 10.1088/1674-1056/24/3/037510

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The mutual control mechanism between magnetization and polarization in multiferroic materials is studied. The system contains a ferromagnetic sublattice and a ferroelectric sublattice. To describe the magneto-electric coupling, we propose a linear coupling Hamiltonian between ferromagnetism and ferroelectricity without microscopic derivation. This coupling enables one to retrieve the hysteresis loops measured experimentally. The thermodynamic properties of the system are calculated, such as the temperature dependences of the magnetization, polarization, internal energy and free energy. The ferromagnetic and ferroelectric hysteresis loops driven by either a magnetic or an electric field are calculated, and the magnetic spin and pseudo-spin are always flipped synchronously under the external magnetic and electric field. Our theoretical results are in agreement with the experiments.

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Influence of strain distribution on the morphology evolution of a Ge/GeO₂ core/shell nanoparticle confined in ultrathin Al₂O₃ thinfilm by surface oxidation

章英, 黄红华, 刘晓山, 骆兴芳, 袁彩雷, 叶双莉

2015 (3): 37701-037701. doi: 10.1088/1674-1056/24/3/037701

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The influence of strain distribution on morphology evolution of Ge/GeO₂ core/shell nanoparticle confined in ultrathin Al₂O₃ thin film by surface oxidation is investigated. A finite-element simulation is performed to simulate the morphology evolution of the confined Ge/GeO₂ core/shell nanoparticle under the influence of the local strain distribution. It indicates that the resultant oxidation-related morphology of Ge/GeO₂ core/shell nanoparticle confined in ultrathin film is strongly dependent on the local strain distribution. On the other hand, the strain gradients applied on the confined GeO₂ shell can be modified by the formation of polycrystalline GeO₂ shell, which has potential application in tailoring the microstructure and morphology evolution of the Ge/GeO₂ core/shell nanoparticle.

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Phase transformation and morphology tuning of β-NaYF₄: Yb³⁺, Er³⁺ nanocrystals through K⁺ ions codoping

梁志琴, 赵谡玲, 崔越, 田丽娇, 张俊杰, 徐征

2015 (3): 37801-037801. doi: 10.1088/1674-1056/24/3/037801

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In this work, a simple method to modulate the crystal phase and morphology with a large amount of K⁺ ions codoping is proposed. The phase changes to the mixture of β-NaYF₄ and β -KYF₄ with increasing the content of K⁺ ions to 80 mol%. When it exceeds 80 mol%, β -NaYF₄ disappears gradually and β -KYF₄ dominates with a poor crystalline. In addition, the morphology changes from nanosphere to nanoplate, and then to nanoprism, which indicates that a higher content of K⁺ ions favors the growth rates along [0001] than the [10-10] of the nanocrystals. Additionally, the upconversion (UC) luminescence properties and the ratio of red/green (R/G) UC intensity of samples with different phases and morphologies are detected, which makes it possible to tune the UC fluorescence by varying the concentration of K⁺ ions.

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Improvement of electron injection of organic light-emitting devices by inserting a thin aluminum layer into cesium carbonate injection layer

辛利文, 吴晓明, 华玉林, 肖志慧, 王丽, 张欣, 印寿根

2015 (3): 37802-037802. doi: 10.1088/1674-1056/24/3/037802

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We investigate the electron injection effect of inserting a thin aluminum (Al) layer into cesium carbonate (Cs₂CO₃) injection layer. Two groups of organic light-emitting devices (OLEDs) are fabricated. For the first group of devices based on Alq₃, we insert a thin Al layer of different thickness into Cs₂CO₃ injection layer, and the device's maximum current efficiency of 6.5 cd/A is obtained when the thickness of the thin Al layer is 0.4 nm. However, when the thickness of Al layer is 0.8 nm, the capacity of electron injection is the strongest. To validate the universality of this approach, then we fabricate another group of devices based on another blue emitting material. The maximum current efficiency of the device without and with a thin Al layer is 4.51 cd/A and 4.84 cd/A, respectively. Inserting a thin Al layer of an appropriate thickness into Cs₂CO₃ layer can result in the reduction of electron injection barrier, enhancement of the electron injection, and improvement of the performance of OLEDs. This can be attributed to the mechanism that thermally evaporated Cs₂CO₃ decomposes into cesium oxides, the thin Al layer reacts with cesium oxides to form Al-O-Cs complex, and the amount of the Al-O-Cs complex can be controlled by adjusting the thickness of the thin Al layer.

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Effect of helium implantation on SiC and graphite

郭洪燕, 葛昌纯, 夏敏, 郭立平, 陈济鸿, 燕青芝

2015 (3): 37803-037803. doi: 10.1088/1674-1056/24/3/037803

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Effects of helium implantation on silicon carbide (SiC) and graphite were studied to reveal the possibility of SiC replacing graphite as plasma facing materials. Pressureless sintered SiC and graphite SMF-800 were implanted with He⁺ ions of 20 keV and 100 keV at different temperatures and different fluences. The He⁺ irradiation induced microstructure changes were studied by field-emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM).

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Effects of FeNi-phosphorus-carbon system on crystal growth of diamond under high pressure and high temperature conditions

胡美华, 毕宁, 李尚升, 宿太超, 周爱国, 胡强, 贾晓鹏, 马红安

2015 (3): 38101-038101. doi: 10.1088/1674-1056/24/3/038101

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This paper reports the crystal growth of diamond from the FeNi-Carbon system with additive phosphorus at high pressures and high temperatures of 5.4-5.8 GPa and 1280-1360℃. Attributed to the presence of additive phosphorus, the pressure and temperature condition, morphology, and color of diamond crystals change obviously. The pressure and temperature condition of diamond growth increases evidently with the increase of additive phosphorus content and results in the moving up of the V-shape region. The surfaces of the diamonds also become coarse as the additive phosphorus added in the growth system. Raman spectra indicate that diamonds grown from the FeNi-phosphorus-carbon system have more crystal defects and impurities. This work provides a new way to enrich the doping of diamond and improve the experimental exploration for future material applications.

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Modeling and experiments of N-doped vanadium oxide prepared by a reactive sputtering process

王涛, 于贺, 董翔, 蒋亚东, 胡锐麟

2015 (3): 38102-038102. doi: 10.1088/1674-1056/24/3/038102

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An original numerical model, based on the standard Berg model, is used to simulate the growth mechanism of N-doped VO_x deposited with changing oxygen flow in the reactive gas mixture. In order to compare with the numerical model, N-doped VO_x films are prepared by reactive magnetron sputtering from a metallic vanadium target immersed in a reactive gas mixture of Ar+O₂+N₂. Both experimental and numerical results show that the addition of N₂ to the process alleviates the hysteresis effect with respect to the oxygen supply. Film compositions obtained from the XPS analysis are compared to the numerical results and the agreement is satisfactory. The results also show that the compound of VN is only found at very low O concentration because of the replacement reaction of VN by O₂ atoms with higher oxygen flow rate.

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Energy-band alignment of atomic layer deposited (HfO₂)_x(Al₂O₃)_1-x gate dielectrics on 4H-SiC

贾仁需, 董林鹏, 钮应喜, 李诚瞻, 宋庆文, 汤晓燕, 杨霏, 张玉明

2015 (3): 38103-038103. doi: 10.1088/1674-1056/24/3/038103

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We study a series of (HfO₂)_x(Al₂O₃)_1-x/4H-SiC MOS capacitors. It is shown that the conduction band offset of HfO₂ is 0.5 eV and the conduction band offset of HfAlO is 1.11-1.72 eV. The conduction band offsets of (HfO₂)_x(Al₂O₃)_1-x are increased with the increase of the Al composition, and the (HfO₂)_x(Al₂O₃)_1-x offer acceptable barrier heights (> 1 eV) for both electrons and holes. With a higher conduction band offset, (HfO₂)_x(Al₂O₃)_1-x/4H-SiC MOS capacitors result in a ～ 3 orders of magnitude lower gate leakage current at an effective electric field of 15 MV/cm and roughly the same effective breakdown field of ～ 25 MV/cm compared to HfO₂. Considering the tradeoff among the band gap, the band offset, and the dielectric constant, we conclude that the optimum Al₂O₃ concentration is about 30% for an alternative gate dielectric in 4H-SiC power MOS-based transistors.

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Theoretical prediction of energy dependence for D+BrO→DBr+O reaction: The rate constant and product rotational polarization

张莹莹, 解廷献, 李泽瑞, 石英, 金明星

2015 (3): 38201-038201. doi: 10.1088/1674-1056/24/3/038201

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A quasi-classical trajectory (QCT) calculation is used to investigate the vector and scalar properties of the D+BrO→DBr+O reaction based on an ab initio potential energy surface (X¹A' state) with collision energy ranging from 0.1 kcal/mol to 6 kcal/mol. The reaction probability, the cross section, and the rate constant are studied. The probability and the cross section show decreasing behaviors as the collision energy increases. The distribution of the rate constant indicates that the reaction favorably occurs in a relatively low-temperature region (T < 100K). Meanwhile, three product angular distributions P(θ_r), P(ø_r), and P(θ_r,ø_r) are presented, which reflect the positive effect on the rotational angular momentum j' polarization of the DBr product molecule. In addition, two of the polarization-dependent generalized differential cross sections (PDDCSs), PDDCS₀₀ and PDDCS₂₀, are computed as well. Our results demonstrate that both vector and scalar properties have strong energy dependence.

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New layered metal oxides as positive electrode materials for room-temperature sodium-ion batteries

穆林沁, 胡勇胜, 陈立泉

2015 (3): 38202-038202. doi: 10.1088/1674-1056/24/3/038202

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In order to achieve better Na storage performance, most layered oxide positive electrode materials contain toxic and expensive transition metals Ni and/or Co, which are also widely used for lithium-ion batteries. Here we report a new quaternary layered oxide consisting of Cu, Fe, Mn, and Ti transition metals with O3-type oxygen stacking as a positive electrode for room-temperature sodium-ion batteries. The material can be simply prepared by a high-temperature solidstate reaction route and delivers a reversible capacity of 94 mAh/g with an average storage voltage of 3.2 V. This paves the way for cheaper and non-toxic batteries with high Na storage performance.

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A novel integrated ultraviolet photodetector based on standard CMOS process

汪涵, 金湘亮, 陈长平, 田满芳, 朱柯翰

2015 (3): 38501-038501. doi: 10.1088/1674-1056/24/3/038501

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A novel integrated ultraviolet (UV) photodetector has been proposed, which realizes a high UV selectivity by combining a conventional UV-selective photodiode with an extra infrared (IR) photodiode. The IR photodiode is designed for compensating the photocurrent response of the UV photodiode in the infrared band and is 15 times smaller than the UV one. The integrated photodetector has been fabricated in a 0.35 μm standard CMOS technology. Some critical performance indices of this new structure photodetector, such as spectral responsivity, breakdown voltage, quenching waveform, and transient response, are measured and analyzed. Test results show that the complementary UV-IR photodetector has a maximum spectral responsivity of 0.27 A · W^-1 at the wavelength of 400 nm. The device has a high UV selectivity of 3000, which is much higher than that of the single UV photodiode.

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High performance silicon waveguide germanium photodetector

李冲, 薛春来, 李亚明, 李传波, 成步文, 王启明

2015 (3): 38502-038502. doi: 10.1088/1674-1056/24/3/038502

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High-performance Ge-on-SOI p-i-n waveguide photodetectors with different sizes were fabricated. The performances, in terms of dark-current, photo current responsivity and 3-dB bandwidth, were well studied. A responsivity of 0.842 A/W at 1550 nm and dark current of 70 nA was measured from this detector at -1 V. The detector with a size of 4 μm× 10 μm demonstrated an optical band width of 19 GHz at -5 V for 1550 nm. Both the experimental results and the finite-difference time domain simulation show that, when the device size is above a certain threshold, the absorption is not sensitively dependent on such designing parameters as the width and length of the photodetector.

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Performance improvement of GaN-based light-emitting diodes transferred from Si (111) substrate onto electroplating Cu submount with embedded wide p-electrodes

柳铭岗, 王云茜, 杨亿斌, 林秀其, 向鹏, 陈伟杰, 韩小标, 臧文杰, 廖强, 林佳利, 罗慧, 吴志盛, 刘扬, 张佰君

2015 (3): 38503-038503. doi: 10.1088/1674-1056/24/3/038503

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Crack-free GaN/InGaN multiple quantum wells (MQWs) light-emitting diodes (LEDs) are transferred from Si substrate onto electroplating Cu submount with embedded wide p-electrodes. The vertical-conducting n-side-up configuration of the LED is achieved by using the through-hole structure. The widened embedded p-electrode covers almost the whole transparent conductive layer (TCL), which could not be applied in the conventional p-side-up LEDs due to the electrode-shading effect. Therefore, the widened p-electrode improves the current spreading property and the uniformity of luminescence. The working voltage and series resistance are thereby reduced. The light output of embedded wide p-electrode LEDs on Cu is enhanced by 147% at a driving current of 350 mA, in comparison to conventional LEDs on Si.

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Closed-loop control of epileptiform activities in a neural population model using a proportional-derivative controller

王俊松, 王美丽, 李小俚, Ernst Niebur

2015 (3): 38701-038701. doi: 10.1088/1674-1056/24/3/038701

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Epilepsy is believed to be caused by a lack of balance between excitation and inhibitation in the brain. A promising strategy for the control of the disease is closed-loop brain stimulation. How to determine the stimulation control parameters for effective and safe treatment protocols remains, however, an unsolved question. To constrain the complex dynamics of the biological brain, we use a neural population model (NPM). We propose that a proportional-derivative (PD) type closed-loop control can successfully suppress epileptiform activities. First, we determine the stability of root loci, which reveals that the dynamical mechanism underlying epilepsy in the NPM is the loss of homeostatic control caused by the lack of balance between excitation and inhibition. Then, we design a PD type closed-loop controller to stabilize the unstable NPM such that the homeostatic equilibriums are maintained; we show that epileptiform activities are successfully suppressed. A graphical approach is employed to determine the stabilizing region of the PD controller in the parameter space, providing a theoretical guideline for the selection of the PD control parameters. Furthermore, we establish the relationship between the control parameters and the model parameters in the form of stabilizing regions to help understand the mechanism of suppressing epileptiform activities in the NPM. Simulations show that the PD-type closed-loop control strategy can effectively suppress epileptiform activities in the NPM.

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Some regularity on how to locate electrodes for higher fECG SNRs

张洁敏, 黄晓林, 管群, 刘铁兵, 李萍, 肇莹, 刘红星

2015 (3): 38702-038702. doi: 10.1088/1674-1056/24/3/038702

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The electrocardiogram (ECG) recorded from the abdominal surface of a pregnant woman is a composite of maternal ECG, fetal ECG (fECG) and other noises, while only the fECG component is always needed by us. With different locations of electrode pairs on the maternal abdominal surface to measure fECGs, the signal-to-noise ratios (SNRs) of the recorded abdominal ECGs are also correspondingly different. Some regularity on how to locate electrodes to obtain higher fECG SNRs is needed practically. In this paper, 343 groups of abdominal ECG records were acquired from 78 pregnant women with different electrode pairs locating, and an appropriate extended research database is formed. Then the regularity on fECG SNRs corresponding to different electrode pairs locating was studied. Based on statistical analysis, it is shown that the fECG SNRs are significantly higher in certain locations than others. Reasonable explanation is also provided to the statistical result using the theories of the fetal cardiac electrical axis and the signal phase delay.

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Boron implanted emitter for n-type silicon solar cell

梁鹏, 韩培德, 范玉洁, 邢宇鹏

2015 (3): 38801-038801. doi: 10.1088/1674-1056/24/3/038801

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The effects of ion doses on the properties of boron implanted Si for n-type solar cell application were investigated with doses ranging from 5× 10¹⁴ cm^-2 to 2× 10¹⁵ cm^-2 and a subsequent two-step annealing process in a tube furnace. With the help of the TCAD process simulation tool, knowledge on diffusion kinetics of dopants and damage evolution was obtained by fitting SIMS measured boron profiles. Due to insufficient elimination of the residual damage, the implanted emitter was found to have a higher saturation current density (J_0e) and a poorer crystallographic quality. Consistent with this observation, V_oc, J_sc, and the efficiency of the all-implanted p⁺-n-n⁺ solar cells followed a decreasing trend with an increase of the implantation dose. The obtained maximum efficiency was 19.59% at a low dose of 5× 10¹⁴ cm^-2. The main efficiency loss under high doses came not only from increased recombination of carriers in the space charge region revealed by double-diode parameters of dark I-V curves, but also from the degraded minority carrier diffusion length in the emitter and base evidenced by IQE data. These experimental results indicated that clusters and dislocation loops had appeared at high implantation doses, which acted as effective recombination centers for photogenerated carriers.

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A new collision avoidance model for pedestrian dynamics

王千龄, 陈姚, 董海荣, 周敏, 宁滨

2015 (3): 38901-038901. doi: 10.1088/1674-1056/24/3/038901

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The pedestrians can only avoid collisions passively under the action of forces during simulations using the social force model, which may lead to unnatural behaviors. This paper proposes an optimization-based model for the avoidance of collisions, where the social repulsive force is removed in favor of a search for the quickest path to destination in the pedestrian's vision field. In this way, the behaviors of pedestrians are governed by changing their desired walking direction and desired speed. By combining the critical factors of pedestrian movement, such as positions of the exit and obstacles and velocities of the neighbors, the choice of desired velocity has been rendered to a discrete optimization problem. Therefore, it is the self-driven force that leads pedestrians to a free path rather than the repulsive force, which means the pedestrians can actively avoid collisions. The new model is verified by comparing with the fundamental diagram and actual data. The simulation results of individual avoidance trajectories and crowd avoidance behaviors demonstrate the reasonability of the proposed model.

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A new cellular automaton for signal controlled traffic flow based on driving behaviors

王扬, 陈艳艳

2015 (3): 38902-038902. doi: 10.1088/1674-1056/24/3/038902

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The complexity of signal controlled traffic largely stems from the various driving behaviors developed in response to the traffic signal. However, the existing models take a few driving behaviors into account and consequently the traffic dynamics has not been completely explored. Therefore, a new cellular automaton model, which incorporates the driving behaviors typically manifesting during the different stages when the vehicles are moving toward a traffic light, is proposed in this paper. Numerical simulations have demonstrated that the proposed model can produce the spontaneous traffic breakdown and the dissolution of the over-saturated traffic phenomena. Furthermore, the simulation results indicate that the slow-to-start behavior and the inch-forward behavior can foster the traffic breakdown. Particularly, it has been discovered that the over-saturated traffic can be revised to be an under-saturated state when the slow-down behavior is activated after the spontaneous breakdown. Finally, the contributions of the driving behaviors on the traffic breakdown have been examined.

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Correlation between the magic wavelengths and the polarization direction of the linearly polarized laser in the Ca⁺ optical clock

刘培亮, 黄垚, 边武, 邵虎, 钱源, 管桦, 唐丽艳, 高克林

2015 (3): 39501-039501. doi: 10.1088/1674-1056/24/3/039501

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The magic wavelengths for different Zeeman components are measured based on the ⁴⁰Ca⁺ optical clock. The dynamic dipole polarizability of a non-zero angular moment level has correlation with the polarization direction of the linearly polarized laser beam, and we show that the four hyperfine structure levels of 4s_{1/2, m = ± 1/2} and 3d_{5/2, m = ± 1/2} for ⁴⁰Ca⁺ have the same dynamic dipole polarizability at the magic wavelength and a certain polarization direction. In addition, the existence of a specific direction of polarization may provide a new idea for improving the precision of magic wavelength measurement in experiment.

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