Vol. 63, No. 4 (2014)
Application of the symmetry classification to the boundary value problem of nonlinear partial differential equations
2014, 63 (4): 040201. doi: 10.7498/aps.63.040201
In this paper, we study the application of the symmetry classification to the boundary value problem of nonli-near partial differential equations. Firstly, by using differential characteristic set algorithm for the complete symmetry classification of partial differential equations, the complete symmetry classification of a given boundary value problem of nonlinear partial differential equations is proposed. Secondly, by using an extended symmetry, the boundary value problem of nonlinear partial differential equations is reduced to an initial value problem of the original differential equations. Finally, we numerically solve the initial value problem of the original differential equations by using Runge-Kutta method.
2014, 63 (4): 040202. doi: 10.7498/aps.63.040202
In recent years, there has been the increasing interest in the research on the consensus of multi-agent systems because of the need of application and the development of technology. In communication networks and engineering applications, due to signal propagation delays, time delay may exist in the consensus state of systems. In this paper, a concept on delay consensus is first defined, and then the problem of first-order delay consensus in leader-following multi-agent systems is discussed. By designing an appropriate control protocol and using the graph theory, matrix theory and Lyapunov stability theory, a sufficient condition is given to realize delay consensus of the multi-agent systems. Numerical simulations are given to verify the theoretical results.
2014, 63 (4): 040501. doi: 10.7498/aps.63.040501
In this paper we study the theory of nonrelativistic weakly interacting Bose gas from the point of view of effective field theory. Firstly, the nonrenormalizability of the theory is briefly discussed. Then, starting from the effective Lagrangian, the lowest order contributions of Feynman diagrams are calculated for the parameters in the effective Lagrangian. These illustrate the running coupling constant phenomenon. After that, the differential align of the parameters in the effective Lagrangian is obtained. We show that the intrinsic energy scale of this effective theory can be removed, while it is not possible to do so for relativistic Bose gas. Our results can help to study the critical behavior of weakly interacting Bose gas.
2014, 63 (4): 040502. doi: 10.7498/aps.63.040502
Based on the piecewise smooth model, the smooth model and the discrete iterative model of proportion-integration (PI)-based voltage-mode Buck converter are derived. In this paper, it is proved that the chaotic attractor moves on the load line and is controlled by duty cycle, and that the manifold of the model moves around the chaotic attractor accompanied by the occurrences of period 1, period 2 and chaos phenomenon. The linear relationship between output voltage of the PI-controller and output voltage of Buck converter is derived, and then reveals that the proportional factor is a dominant one in PI controller. The period-doubling bifurcation, border collision and chaos are analyzed, and the state transfer process is exhibited. Experimental results verify that the theoretical modeling analysis and the simulation are correct.
2014, 63 (4): 040503. doi: 10.7498/aps.63.040503
To obtain compound attractors between different chaotic systems, based on theoretical analysis, numerical simulation, and circuit simulation methods, compound attractors between different 2-scroll systems, between different multi-scroll chaotic systems, between 2-scroll system and 2-wing system and between multi-scroll system and multi-wing system, are designed via switching control. Dynamical characteristics of the system are analyzed by observing the attractor phase diagram, the largest Lyapunov exponent and the Poincaré section. A circuit for a compound multiple scroll-multiple wing chaotic attractor is designed and simulated. Numerical simulation and circuit simulation are consistent with each other. It shows that the method of obtaining compound attractors between different chaotic systems via switching control is correct.
2014, 63 (4): 040504. doi: 10.7498/aps.63.040504
Heart rate variability (HRV) has the characteristics of non stationary and transient fluctuation. In this paper, we propose using the autoregressive conditional heteroskedasticity (ARCH) to analyze such fluctuations. Analytical methods include using auto-regressive moving average model to eliminate sequence trends and correlativity, using the F-test method to determine whether there is ARCH effect in residual sequence, while giving an acceptance probability of ARCH effect. The fluctuations of HRV in congestive heart failure (CHF) patients and normal people are analyzed in this paper. The large difference in statistics value of acceptance probability between CHF patients and normal people indicates that the method can well distinguish between different groups. It provides a new method for the study of ECG information.
2014, 63 (4): 040505. doi: 10.7498/aps.63.040505
In order to reveal the internal dynamic property of short-term traffic flow, the nonlinear analysis method is used to identify the chaotic property of traffic flow which is the basis for the prediction of the traffic flow time series. Traffic flow time series is reconstructed in phase-space based on chaos theory. The embedding dimension and delay time are first calculated via the C-C method. The correlative dimension of attractor is then calculated with the Grassberger-Procaccia method. The largest Lyapunov exponent of traffic flow set is calculated on the basis of the improved small data set method to verify the presence of the chaos in traffic flow time series. A novel multi-step adaptive prediction method is proposed to solve the problem of adjusting the filter parameters of the chaos local adaptive prediction method during traffic flow multi-step prediction. The traffic flow time series are found to have chaotic properties in different statistical scales of 2, 4, and 5 min and show that the improved small data set method can accurately evaluate the chaotic property for traffic flow time series, and that the multi-step adaptive prediction method is capable of effectively predicting its fluctuation, which provides a useful reference for traffic guidance and control.
Phase synchrony in the cerebral infarction electroencephalogram based on the degree of network-links
2014, 63 (4): 040506. doi: 10.7498/aps.63.040506
Recently, there has been increasing interest in applying graph theory to the quantitative analysis of brain functional networks, while phase synchronization (PS) analysis has been demonstrated to be a useful method to infer functional connectivity with multichannel neural signals, e.g., electroencephalogram (EEG). In this paper, we focus on the case that the number of channels in EEG data is not adequate for the use of graph theory analysis. The degree of network-links (DNLs), an index based on the PS analysis of all the EEG wave pairs, is proposed to study the relevant and the overall characteristics of the brain. With the help of a novel division to the frequency range 0.5–30 Hz, we analyze the DNLs in different frequency bands of the EEG signals. As a comparison, a frequency band analysis of the relative power spectrum is conducted. The results demonstrate that when the cerebral infarction (CI) patients and normal control people are analyzed, there is a need for the reasonable length of EEG data to quantify the differences between different dynamical systems; under a reasonable data length, the frequency band (19–24 Hz) yields the best accuracy for diagnosing CI, which lies within the classical beta band (13–30 Hz); furthermore, only in the 19–24 Hz band, as for the values of relative power spectrum, in each EEG channel, there presents a similar relationship between the CI group and control group. The experimental results suggest that 19–24 Hz should be the optimal range for the diagnosis of CI, further the DNLs calculated within this band serve as an assist indicator in the CI diagnosis.
Analysis of time-varying channel effects on transmission performance of deep-sea abandoned measuring instrument
2014, 63 (4): 040507. doi: 10.7498/aps.63.040507
Because of the environmental requirements, deep-sea abandoned measuring instrument mostly uses the cable channel. However, the stability of the channel transmission is seriously affected by length change in electrical impedance of the channel. In this paper, a new method is presented, which is used for dynamically analysing the effects of the cable channel on the amplitude and phase of the transmission signal by using XCTD (expendable conductivity, temperature and depth) profiler transmission channel. Firstly, regulations about the parameters of each circuit varying with the channel length is proposed. Meanwhile, the simplified transmission circuit model and its transfer function are established. Through qualitative analysis we can conclude that the transmission frequency, distributed capacitance and winding inductance have an influence on the signal phase. Finally, in this paper we analyse and discusse how the cable influences the transmission of signal, and present the reason of the influences with MATLAB, which can further guide the design of the XCTD measuring system and improve the accuracy of the measurements and the stability of signal transmission.
New exact solutions and complex wave excitations for the (2+1)-dimensional dispersive long wave equation
2014, 63 (4): 040508. doi: 10.7498/aps.63.040508
By the Riccati equation expansion method and a variable separation method, a series of variable separation solutions of the (2+1)-dimensional dispersive long wave equation is derived. According to the derived solitary wave solution, we obtain some nove complex wave localized structures and study the time evolutions of complex waves.
2014, 63 (4): 040509. doi: 10.7498/aps.63.040509
Because of the limited energy of wireless sensor network, the optimization of network lifetime becomes a primary problem in the design of the network. Through analyzing the node residual energy and the load effect on the node lifetime, the fault tolerant topology evolution model in wireless sensor networks is put forward for extending lifetime. And the scope of load adjustment coefficient is obtained under the condition of network survival time. The simulation results show that the topological structure based on the evolution of the scale free network has the good fault tolerance characteristics. And it can also balance the network node energy consumption and the lifetime extension of the network.
ATOMIC AND MOLECULAR PHYSICS
2014, 63 (4): 043101. doi: 10.7498/aps.63.043101
The ground states of BN molecule under different strong electric fields ranging from -0.06 a.u. to 0.06 a.u. are optimized using density functional method B3LYP at 6-311++g(d.p) level. Optimized parameters, dipole moment, charge distribution. HOMO energy, LUMO energy, energy gaps, infrared spectrum are obtained. The result shows that with the increasing of the external electric field, the correlation between molecular structure parameters and the electric field intensity becomes obvious and presents the asymmetry. In addition, TDDFT method at the same level is used to study the influence of external electric field on BN molecular excitation energy and oscillator strength, and the result shows that the electron transition spectrum is blue-shifted with the increase of the external electric field, but the intensity of the vibrator has a more complicated change, showing that the spectral intensity is affected by the electric field.
2014, 63 (4): 043102. doi: 10.7498/aps.63.043102
The analytical potential energy curves of stable diatomic electronic states are investigated based on the algebraic method (AM) and energy consistent method (ECM). The AM-ECM method is applied to 4 electronic states of 7Li2-23∑g+, KH-X1∑+, NaLi-X1∑+ and NaLi-A1∑+. The results show that the AM-ECM potentials are superior to some other analytical potentials, and can generate accurate analytical potential expressions of these electronic states in the molecular asymptotic and dissociation region.
Squeezing properties of atom laser from two-level atomic Bose-Einstein condensate interacting with two-mode light field
2014, 63 (4): 043201. doi: 10.7498/aps.63.043201
The Hamiltonian operator of a system of two-level atomic Bose-Einstein condensate interacting with two-mode squeezed coherent light field is improved in terms of the lattice-liquid model. The squeezing properties of atom laser in this system are studied. Results show that the two quadrature components of the atom laser can be squeezed periodically, and the maximum depth depends greatly on the initial squeezing factor of the light field. The collapse-revival oscillatory frequency of the atom laser and the quantum Rabi frequency are largely determined by the interaction intensity between the light field and the atoms and by the circle frequency of the light field, respectively.
Squeezing properties of two-mode atom laser in a system of Ξ-type three-level atomic Bose-Einstein condensate interacting with single-mode light field
2014, 63 (4): 043202. doi: 10.7498/aps.63.043202
The squeezing properties of the two-mode atom laser in a system of Ξ-type three-level atomic Bose-Einstein condensate interacting with single-mode squeezed coherent light field are studied. The results show that the two-mode atom laser can be squeezed periodically, and has two types of oscillations: quantum Rabi oscillation and collapse-revival oscillation. The maximum depth of squeezing and the collapse-revival frequency of the atom laser depend greatly on the interaction intensity between the light field and atoms, and the quantum Rabi frequency is largely determined by the frequency of the light field.
2014, 63 (4): 043301. doi: 10.7498/aps.63.043301
A method of detecting ethanol vapor concentration using a narrow bandwidth tunable diode laser is studied. The relatively narrow absorption feature in the vicinity of 7180 cm-1 is used as an identification of ethanol vapor. To eliminate the interference of water vapor, multidimensional linear regression analysis is used to solve the problem of multiple molecule absorptions. The detection limit is obtained to be 25 ppm·m experimentally.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2014, 63 (4): 044101. doi: 10.7498/aps.63.044101
A theoretical analysis of the high frequency characteristics for an all-metal slow-wave system (SWS), i.e., a sine waveguide, is performed in this paper. The continuous profile of the cosine groove is approximated by a series of connected rectangular steps, and the SWS is divided into several regions reasonably, then the expressions of the field in all the regions are given. The dispersion equation and coupling impedance formula of the SWS are obtained by using the field matching theory combined with the matching of the admittance. The high frequency characteristics for a sine waveguide used in a 220 GHz TWT are discussed, and the calculation results are in good agreement with those obtained from three-dimensional electromagnetic simulation software CST-MWS. The effects of the geometrical parameters on high frequency characteristic of the sine waveguide are also investigated in detail.
2014, 63 (4): 044102. doi: 10.7498/aps.63.044102
Microwave tomography is enhanced by using an inhomogeneous background. In this paper, the measurement region is located in a circular perfect electrical conductor (PEC) chamber where a known object is placed inside the imaging domain as an inhomogeneous background. This can not only make use of the prior information about the background, but also increase the equivalent radiation source for the target detection. The Green function of a circular PEC chamber with inhomogeneous background is obtained using the method of moments. Based on the Green functions for both homogeneous and inhomogeneous background in circular PEC chamber, the properties of the radiation operators are analyzed by comparing the condition numbers and the singular value spectra. Simulations are carried out in homogeneous, lossless inhomogeneous and lossy inhomogeneous backgrounds respectively, and the relative errors are discussed. The results show that using inhomogeneous background can improve the convergence rate and accuracy, and the lossy inhomogeneous background produces better results than the lossless one. In addition, it can enhance the inversion results without changing the microwave tomography system, which can be used in the medical imaging and industrial nondestructive detection.
Characteristics of electromagnetic scattering from the vehicle-mounted ground penetrating radar in layered media
2014, 63 (4): 044201. doi: 10.7498/aps.63.044201
A physical model for the electromagnetic scattering of layered media is established. Radar equation and Snell's law are adopted to calculate the electromagnetic scattering field of multiple layer media. The effects of the electrical properties of the layer, i.e., permittivity and loss tangent on the detection range of the radar system and scattering field are studied. The vehicle-mounted ground penetrating radar (GPR) for the exploration of lunar subsurface is taken as the background. The finite difference time domain method is used to model and calculate the whole vehicle-mounted GPR, and the intensity of the scattering echoes is studied. The simulation results are compared with the theoretical results, and they are found to be in good agreement with each other. Finally some analyses of the results are presented.
2014, 63 (4): 044202. doi: 10.7498/aps.63.044202
Aiming at the problems of low-resolution and poor visual quality of infrared images, a locality-constrained group sparsity based infrared image super-resolution algorithm is proposed. Firstly with considering the texture self-similarity of infrared images and group structural sparsity of atom coefficients, a locality-constrained group sparse (LCGS) model is proposed. Secondly, under LCGS and K-singular value decomposition, a pair of group structural dictionaries is learned. The dictionary pair can well capture and preserve the intrinsic geometrical manifold of low and high resolution data. Finally, the high-resolution infrared images are recovered by the high-resolution dictionary and the corresponding low-resolution group sparse coefficients. Experimental results show that the proposed method obtains excellent performance in objective evaluation and subjective visual effect.
Characteristics of the slow and fast light in the band-edge of gain spectrum of the fiber-optic parametric amplification
2014, 63 (4): 044203. doi: 10.7498/aps.63.044203
Compared to the large slope of the gain spectrum, in this paper, the characteristics of the slow and fast light on the band-edge of gain spectrum of the narrow-band fiber-optic parametric amplification (FOPA) are analyzed. Firstly, we derive the analytical solution of the signal delay under the condition of small-signal gain. Secondly, the characteristics of 10 Gb/s pseudo-random binary sequence pulse signal delay are investigated experimentally under the condition of low pump power (<1 W). Finally, the factors that influence the signal delay, such as pump power, dispersion, nonlinear coefficient, etc. are analyzed. The results obtained in this paper provide a detailed theoretical reference for designing slow and fast delay line in band-edge of gain spectrum of FOPA.
2014, 63 (4): 044204. doi: 10.7498/aps.63.044204
Phase modulation is an important method of designing accelerating optical beams. In this paper, we present new self-accelerating non-diffracting Bessel-like Hermite-Gaussian beams based on our previous research on Bessel-like beams. The evolutions of the beams along different trajectories are studied numerically and experimentally. These beams are designed by modulating the phase of the initial Hermite-Gaussian beams. With the split-step beam propagation method, we show numerically that (0,1), (1,0), (1,1) and (1,2) modes of the Bessel-like Hermite-Gaussian beams can propagate along different predesigned trajectories. With the computer-generated hologram and spatial light modulator, we observe the propagating behaviors of the Bessel-like Hermite-Gaussian beams along different trajectories, including parabolic, hyperbolic, hyperbolic secant and 3D trajectories. Experimental results show good agreement with the theoretical prediction. It is also demonstrated that the peculiar profile of Bessel-like Hermite-Gaussian beams exhibits nondiffracting and self-healing properties propagating along the tunable trajectories. These beams generalize the concept of Bessel-like beams, suggesting that more exotic optical beams can be obtained.
2014, 63 (4): 044301. doi: 10.7498/aps.63.044301
Spherical cavity transducer can generate high acoustic pressure in a narrow focal region at a sub-wavelength level, which suggests a great potential application in high intensity focused ultrasound (HIFU) treatment. In this paper, tissue lesion induced by a spherical cavity transducer is investigated based on the nonlinear acoustics theory and bio-heat transfer theory. A 430 kHz spherical cavity transducer with a diameter of 240 mm is used to generate tissue lesion for liver tissues. The result shows that the tissue lesion with a size less than a wavelength can be formed under the 2 s HIFU exposure at a acoustic pressure of 53 kPa. The numerical simulations coincide well with the experimental measurements, and the optimization of the design of the spherical cavity transducer is also discussed. This work demonstrates that the spherical cavity transducer can be used in the accurate HIFU tumor treatment.
Chirp spread spectrum of orthogonal frequency division multiplexing underwater acoustic communication system based on multi-path diversity receive
2014, 63 (4): 044302. doi: 10.7498/aps.63.044302
Aiming at the problem of traditional orthogonal frequency division multiplexing (OFDM) communication system performance degradation when the channel is subjected to complex multi-path and frequency-selective deeply fading, a chirp spread spectrum (CSS) of OFDM underwater acoustic communication system called CSS-OFDM is proposed in this paper. The CSS-OFDM system spreads the spectrum of traditional OFDM signals, whose sub-carriers are modulated into the same chirp rate, different center frequency orthogonal chirp signals with overlapped bandwidth. Dispreading after underwater acoustic coherent multi-path channel, at the receiving end, the multi-path signals will be expanded from each other in the frequency domain. Applying virtual time reversal mirror technology, the energies of the multi-path signals are focused to complete the multi-path channel diversity receiving. The system performance is improved not only by suppressing the frequency selective fading, but also taking full advantage of multi-path energy of the channel. The effectiveness and reliability of this system are verified through a number of simulations and lake trials.
Modal characteristic frequency in a range-dependent shallow-water waveguide and its application to passive source range estimation
2014, 63 (4): 044303. doi: 10.7498/aps.63.044303
In order to implement robust passive source ranging in a range-dependent shallow-water waveguide, the modal characteristic frequencies are theoretically analyzed based on warping transform. Formulae of the modal spatiotemporal arrival structure and instantaneous phase are derived in the waveguide with an absolute hard bottom. The relationship between the characteristic frequency and the source-receiver range is developed. With the relationship, a modified passive source-range estimation approach is presented in a range-dependent shallow-water waveguide. Simulation and experiment verify the theory and the approach.
2014, 63 (4): 044501. doi: 10.7498/aps.63.044501
In order to analyze the lane-changing behavior on the upstream of the intersections, based on the cellular automata traffic theory, a comprehensive lane-changing model for the traffic flow near the approach is proposed by combining the improved mandatory lane-changing model and discretionary lane-changing model. And the charts describing the scenarios of the space-time and traffic density for the traffic flow near the approach are obtained through simulation based on the comprehensive lane-changing model. Results show that the lane-changing rate at the approach is higher when the traffic density is in the ranges of [0, 0.2] and [0.4, 0.6]. The average vehicle speed is gradually reduced as lane-changing rate increases under median low vehicle stream density, but the variation of vehicle speed with lane-changing rate is not significant under high vehicle stream density.
2014, 63 (4): 044502. doi: 10.7498/aps.63.044502
By using numerical simulation, the kinetic energy/temperature, the energy dissipation and the volume fraction in a 1D vertical vibrational bed are studied. Discrete element simulation shows that the granular bed moves up and down as an ensemble and the kinetic energy of the particles increases along the bed height when the bed bottom vibrates at a low-frequency and low-amplitude. For high-frequency vibrations, the particles in the bed move randomly and their kinetic energy decreases along the bed height. The energy dissipation and the volume fraction of the particles are also influenced by the vibrations frequency obviously, and they show different distributions at the high and low frequencies. In addition, we have compared the result of the discrete element simulation with that of the hydrodynamic simulation. When the bed bottom vibrates at high frequency, the two simulation methods can get the similar results. However, for the low-frequency and low-amplitude vibrations, the computed results are opposite to each other. Since the particles in the bed do not move and collide randomly, the application of the hydrodynamic simulation to the bed with low-frequency and low-amplitude vibrations should be investigated and discussed further.
Numerical analysis for the characteristics of flow control around a circular cylinder with a turbulent boundary layer separation using the electromagnetic force
2014, 63 (4): 044701. doi: 10.7498/aps.63.044701
A detached eddy simulation method with the turbulent separation is presented to simulate and analyze the characteristics for the flow around a circular cylinder with a turbulent boundary layer separation and its lift/drag coefficients in a weakly conductive fluid at a high subcritical Reynolds number 1.4×105 under an electromagnetic force. The results show that the electromagnetic force can increase the fluid kinetic energy near the turbulent boundary layer, delay the turbulent boundary layer separation of the flow around the circular cylinder, and weaken the intensity of the large-scale eddy in the turbulent flow around the circular cylinder in the streamwise and spanwise directions, reduce the time-average drag, and inhibit the lift fluctuation amplitude on the circular cylinder. Moreover, after the electromagnetic force parameter reaches a certain critical value, the turbulent boundary layer separation disappears and the jet phenomenon appears in the wake of the circular cylinder, so that the electromagnetic force produces the thrust action on the circular cylinder and the negative drag occurs, and the lift fluctuation amplitude declines to almost zero and the lift phenomenon on the circular cylinder disappears.
2014, 63 (4): 044205. doi: 10.7498/aps.63.044205
A 104.2 W supercontinuum spanning from 750 nm to beyond 1700 nm is generated by pumping a piece of home-made seven-core photonic crystal fiber with a 141.6 W picosecond fiber laser. The evolution of the output spectrum is investigated in detail with the increase of the picosecond laser power. The feasibilities of broadening the spectral range and scaling the output power for the supercontinuum source based on the seven-core photonic crystal fiber are analyzed and discussed. The research results will make a contribution to the further development of high-power supercontinuum source.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
Investigation of C atom doped armchair (n, n) single walled BN nanotubes with density functional theory
2014, 63 (4): 046101. doi: 10.7498/aps.63.046101
Structures and properties of single walled (n,n) BN nanotubes doped with the C atom at different positions are studied by the DFT B3LYP/3-21G(d) theoretical method combined with the one-dimensional (1D) periodic boundary conditions. Their structure parameters, energies, stabilities, band structures and the energy gaps are explored. For the BN nanotubes doped with the C atom at different positions, the C atom concentrations x=1/4n (n=3–9) are examined. It is found that the N site tubes are almost undistorted and more stable. The band gaps are within 1.054–2.411 eV for the C atom doped at the B sites, and those are narrower and within 0.252–1.207 eV for the N sites. All of the doped tubes are shown to be semiconducting and have direct gaps.
2014, 63 (4): 046102. doi: 10.7498/aps.63.046102
The magnetic and electronic properties of fluorographene doped with M (M=B, N, P, Si) atoms are studied by employing the first principles calculation based on the spin-polarized density functional theory. The results show that the fluorographene doped with B (or P) atoms can cause the semiconductor-to-metal transitions and the fluorographene with doped N (or Si) atoms is still the semiconductor; the substitutional B, P, and N atoms induce magnetic moments of adjacent carbon atoms. For Si atoms doped fluorographene sheet, semiconductor properties keep unchanged, but the band gap changes.
2014, 63 (4): 046103. doi: 10.7498/aps.63.046103
We investigate the effects of niobium (Nb) on helium (He) dissolution and diffusion behaviors in tungsten (W) using a first-principles method. The results show that Nb can effectively reduce the solution energy of He in the bulk W, which is because of the charge density redistribution. This leads to a strong attraction between He and Nb in W, and thus Nb can serve as a He trapping centre in W. The trapping energy of He is 0.37 eV at the most stable site surrounding Nb atom. It is demonstrated that the diffusion barrier of He towards Nb will decrease with the decreasing of He-Nb distance. This suggests that the kinetic process in which He is trapped by Nb is feasible. Therefore, the presence of Nb will facilitate the nucleating and blistering of He in W.
Molecular dynamics simulation of effect of tilt angle on mechanical property of magnesium bicrystals
2014, 63 (4): 046201. doi: 10.7498/aps.63.046201
The effects of grain boundary on the deformation behavior of magnesium bicrystals under tensile loading are investigated using molecular dynamics simulation method. Deformation mechanisms of both asymmetric and symmetric magnesium bicrystals are studied. The results show that the twist angle could exert a significant influence on the flow stress in the plastic stage. It is found that the plastic property of symmetric magnesium bicrystal is much better than that of asymmetric counterpart. In addition, for bicrystalline metal with large twist angle, its plastic response is better than that with small angle, owing to the dislocation nucleation activity in the grain boundary region.
2014, 63 (4): 046301. doi: 10.7498/aps.63.046301
The electronic structures and optical properties of intrinsic, Cu, Co doped and Cu-Co codoped ZnO compounds are calculated using first-principles plane-wave ultrasoft pseudopotential method based on the the density functional theory. The results show that the conductivity of ZnO can be improved by doping Cu and Co because of the increase of the carrier concentration under the order of magnitude of doping concentration in this paper. Cu-Co codoping leads to the degeneration and makes ZnO metallic. Thses three kinds of dopings can cause light absorption enhancement phenomenon in the visible and near ultrasoft regions, in which Cu-Co codoping greatly increases the absorption of solar light due to the synergistic effect between Cu ions and Co ions, which can be used to prepare the high efficiency solar cells.
2014, 63 (4): 046401. doi: 10.7498/aps.63.46401
Molecular dynamics simulation based on the newly developed embedded atom method has been performed to explore the microstructure of liquid Fe50Cu50 alloy. The results show that coordination numbers (CNs) of Fe-Fe and Cu-Cu for Fe50Cu50 melt gradually increase with relaxation time increasing, and they are 9.9 and 9.3 respectively as the liquid is in an equilibrium state; while the CN of heterogeneous atomic pairs Fe-Cu gradually decreases, and it is about 4.6. The correlation length (CL) extracted from Bhatia-Thornton (B-T) structure factor increases with relaxation time increasing. Both CN and CL indicate that the Fe50Cu50 melt exhibits liquid-liquid (L-L) phase separation. The interconnected type of structure can be observed in the Fe50Cu50 melt at the early stage, then the heterogeneous atomic pairs separate gradually with time going by, the Fe-rich and Cu-rich structure are formed, which shows the characteristics of spinodal decomposition. By comparison, the atom snapshot of Fe75Cu25 melt is also visualized in the paper, and the finding indicates that the smaller number difference between Fe atom and Cu atom may lead to the stronger L-L phase separation, as a result of shorter time to reach stable layer-like structure. Our studies mentioned above characterize L-L phase separation of metallic liquid on the atomic scale.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
Phase transition, electronic and optical properties of Si3N4 new phases at high pressure with density functional theory
2014, 63 (4): 047101. doi: 10.7498/aps.63.047101
Characteristics of the hexagonal polymorph Si3N4 i.e., phase transition, electronic and optical properties (band structure, dielectric function, reflectivity and energy loss function) are investigated by the first-principles pseudo-potential method. The results suggest that it is feasible that the P6 transition takes place at room temperature. The critical pressures of the P6 and P6 transformations are 42.9 and 47.7~GPa, respectively. The phase transition from P6 is accompanied by the volume shrinkage. The calculated results also show that the P6 and P6' phases belong to direct bandgap and indirect bandgap semiconductors, respectively. The calculated band gaps are 4.98 and 4.01 eV for the P6 and P6' phases, respectively. Besides, the static dielectric constants are also obtained. The reflectivity shows that the two phases can serve as the shielding and detecting devices for ultraviolet radiation and they have optical transparent behaviors in the visible light region.
Impedance and dielectric spectroscopy analysis of high TC lead-free BaTiO3-(Bi1/2Na1/2)TiO3 positive temperature coefficient resistivity ceramics
2014, 63 (4): 047102. doi: 10.7498/aps.63.047102
92 mol%BaTiO3-8 mol% (Bi1/2Na1/2)TiO3 (BBNT8) lead-free positive temperature coefficient resistivity (PTCR) ceramics doped with Y2O3 are prepared by the solid state reaction method. The microstructure of the sample is observed by transmission electron microscope. Results show that the microstructure of the sample mainly consists of grain and grain boundary, and no obvious shell structure is found. The electrical properties of the sample are further analyzed using the impedance spectroscopy. It is found that the total resistance of BBNT8 is composed of the resistances of grain and grain boundary. The resistance of grain is low and slightly changes with temperature when the temperature is above the Curie temperature. The PTCR effect of the material is mainly dominated by the grain boundary resistance. As the temperature increases up to above the Curie temperature, the grain boundary permittivity decreases, leading to the increase of the potential barrier height and the resistivity of the grain boundary. As a result, the distinct PTCR effect takes place. Finally, the room-temperature resistivity of the BBNT8 is studied by measuring the permittivity-frequency characteristics.
2014, 63 (4): 047201. doi: 10.7498/aps.63.047201
One of the traditional methods of preparing Cu (In, Ga) Se2 (CIGS) is the three-step co-evaporation. Using this technique, four elements of Cu, In, Ga, and Se form a parabolic Ga gradient distribution through their inter-diffusion and interaction. In this paper, CIGS films and solar cells with different Ga gradient distributions are prepared through adjusting the temperature of Ga source. The effects of different Ga gradients on structural and electrical properties of CIGS film surface and back surface are studied through a variety of measurement methods. The influences of surface conduction band offset value and back surface electric field on the performance of solar cell are investigated and analysed. Finally, an optimal Ga gradient distribution is obtained, where the spectral response is improved, and the solar cell shows better performance.
Effect of neutron irradiation on the electrical properties of AlGaN/GaN high electron mobility transistors
2014, 63 (4): 047202. doi: 10.7498/aps.63.047202
SiN-passivated AlGaN/GaN high electron mobility transistors (HEMTs) are exposed to 1 MeV neutron at fluences up to 1015 cm-2. The device shows a negligible degradation at neutron fluences below 1014 cm-2, while the gate leakage current (Ig) slightly changes (the forward IF increases, the reverse IR decreases.) at low fluencies and the IR degrades dramatically at fluences higher than 1014 cm-2. Moreover, near the knee voltage, the transconductance decreases at fluences up to 1015 cm-2, but the Schottky characteristicis become degraded after neutron irradiation. And the 20-hour annealing results do not show any significant annealing recovery effect at room temperature, while the parameters also continues to degrade a little. Therefore, the drain current (near the knee voltage) and the IF degradation of SiN-passivated AlGaN/GaN HEMT can be attributed to the irradiation induced defects in SiN passivation layers, demonstrating that the effectiveness of the SiN layer in passivating surface state in the source-gate spacer and gate-drain spacer is undiminished by neutron irradiation. And the Ohmic contact is so relatively robust to neutron, but the Schottky characteristics degrade obviously. The annealing results prove that the damage induced by neutron may be recovered more difficultly. SiN-passivated AlGaN/GaN HEMT appear to be an attractive candidate for space and terrestrial applications where resistance to displacement damage is required.
Influences of post-heat treatment on microstructures, optical and magnetic properties of unintentionally doped GaN epilayers implanted with Mn ions
2014, 63 (4): 047501. doi: 10.7498/aps.63.047501
In this study, GaN:Mn thin films are fabricated by implementing Mn ions into the undoped GaN material. The effects of annealing temperature on microstructures, optical and magnetic properties of the thin films are investigated. The Raman spectra measured from Mn-implanted GaN samples at different annealing temperatures show that new phonon modes, which are related to macroscopic disorder or vacancy-related defects caused by Mn-ion implantation and the local vibrational mode of Mn atoms in the (Ga, Mn)N, are created. The results of photoluminescence measurement show that new peaks appear at 2.16, 2.53, and 2.92 eV. Among these, the new emission around 2.16 eV, besides some contributions from optical transitions from the conduction band or shallow donor to a deep acceptor, cannot exclude the contribution from optical transitions of free electrons in the conduction band to Mn acceptor level. The Hall test shows that the annealed samples are of n type. Ferromagnetism is observed in the Mn doped GaN thin film at 300 K and found to be sensitive to the density of holes that mediate the Mn-Mn magnetic exchange interaction in this Mn-related impurity band.
2014, 63 (4): 047502. doi: 10.7498/aps.63.047502
The superconducting magnet is used to provide magnetic field in a magnetic property measurement system (SQUID-VSM, Quantum Design), since it can provide high magnetic field up to 7 T. Due to the pinned magnetic flux, there is residual magnetic field as the magnetic field is set to be 0 from high field. There appears an error between the reported filed and the real filed. Sometimes, the residual magnetic field can be more than 30 Oe. It is so large that it may provide incorrect experiment data, such as Hc, Mr and inverted hysteresis loops. The effect of residual magnetic field on the soft magnetic material measurements should not be neglected. The residual magnetic field is dependent on initial magnetic field. The experimental data must be corrected by measuring the standard sample. In the paper we investigate the origin and the regular pattern of the residual magnetic field. The effects on the measurement results and the correction method are presented.
MgO polycrystal is found to be weakly magnetic experimentally, although its single crystal is non magnetic. In this work, the magnetic properties of surfaces of crystal and nano-particles of MgO are studied by the first-principles density functional theory. The obtained results show that there are the oxygen-rich regions in all the magnetic surfaces discussed in this work, especially in the (111) surface with pure oxygen layer and the (114) surface with pure oxygen chains. Other surfaces with high Miller indices generally have the oxygen-rich regions. For MgO nano-particles, the facets with high Miller indices and the edges and vertexes formed by different orientation surfaces are oxygen-rich possibly and have strong magnetism. The itinerant magnetism is indentified for the magnetism on the surfaces of MgO crystal and the surfaces of MgO nano-particles. That the special MgO ∑ 7 grain boundary is not magnetic means that the magnetism of MgO grain boundary is weak if the chemical composition in grain-boundary region is slightly different from that in the crystal. It can be inferred that the magnetism of MgO polycrystal is mainly contributed by the polycrystal surface, the micro-pores, micro-voids and micro-cracks.
Research on polarization effect for relaxor ferroelectrics by spherical random bond-random field model
2014, 63 (4): 047701. doi: 10.7498/aps.63.047701
Based on spherical random bond-random field model, micro-macro domain mechanism under electric field, and the fuzzy domain boundary with fractal dimension of electric field, the mechanism of polarization effect is analyzed. The results show that the effect of electric field on polarization effect of increment of domain dipoles leads to an unsaturated electric hysteresis loop and its associated big electrostrictive effect. But the variation of binding energy, when dipole couples to the increment of domain dipoles induced by electric field, has a little influence on electric hysteresis loop in a low electric field, and has almost no influence in a high electric field. The initial size of the micro domain is very important for electric hysteresis loop: small micro domain can lead to a long electric hysteresis loop and better linear relationship between electric field and electrostriction.
Investigation on the electron dynamics of periodic nano ripple formation on fused silica induced by temporally shaped femtosecond laser
2014, 63 (4): 047901. doi: 10.7498/aps.63.047901
We investigate the electron dynamic process of periodic nano ripples formation on fused silica induced by temporally shaped femtosecond laser. The dynamic model for predictions of ripple period is proposed by introducing the mechanisms of non-linear ionization and surface plasmon polaritons (SPPs) interaction. The quantitative relation between nano ripple period and temporally shaped pulse separation is obtained by using the proposed model. It is revealed that the nano ripple period can be manipulated by adjusting the temporal intervals of the temporally shaped pulses due to the transient adjustment of the wavevector matching process during ripple formation via temporally shaped pulses. The theoretical predictions of the ripple period match well with the experimental data. This study provides the fundamental understanding of transient formation mechanism of ripples, and the precise manipulating of ripple period.
A fast single particle Monte-Carlo method of computing the breakdown threshold of multipactor in microwave device
2014, 63 (4): 047902. doi: 10.7498/aps.63.047902
To compute the breakdown thresholds of multipactor in microwave devices, a fast single particle Monte-Carlo (SP-MC) method is presented, which considers the random nature of secondary electrons and their initial energies, phases and angles. With Runge-Kutta method and Furman model, the motion of the electron and the secondary electron yield (SEY) of the wall of the device are computed. An effective SEY is regarded as a criterion to estimate whether multipactor occurs, which is computed by averaging the SEYs for all impacts. As an example, the multipactor in a transmission line composed of parallel plates is investigated with the presented SP-MC method, traditional Monte-Carlo method, statistical theory method and particle-in-cell method separately. The results obtained from the SP-MC method accord well with those from the statistical theory method and particle-in-cell method, including the results of the susceptibility zones, break thresholds on specific products of frequency and gap space. Moreover, the SP-MC method is more adaptive than the statistical theory method, more stable than the traditional Monte-Carlo method and much more efficient than the particle-in-cell method.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
Characterization of typical infrared characteristic peaks of hydrogen in nitrogen and hydrogen co-doped diamond crystals
2014, 63 (4): 048101. doi: 10.7498/aps.63.048101
The 3107 cm-1 peak is observed in the infrared absorption spectra of all types of Ia diamonds, but it has not been observed in the iron-based catalyst. A series of nitrogen and hydrogen-doped diamond crystals is successfully synthesized using P3N5 as the nitrogen source in a catalyst-carbon system at a lower pressure and temperature (6.3 GPa, 1500 ℃). Fourier transform infrared micro-spectroscopy reveals that the hydrogen atoms existing in the synthesized diamond are in two forms. The one is attributed to the CH bond stretching (3107 cm-1) and bending (1405 cm-1) vibrations of the vinylidene group (C＝CH2). The other is due to sp3 hybridization CH bond symmetric (2850 cm-1) and anti-symmetric (2920 cm-1) vibrations. According to our result, we find that the 3107 cm-1 hydrogen absorption peak is related to the aggregated nitrogen in synthetic diamond. The 3107 cm-1 peak could not be observed in synthetic diamond without aggregated nitrogen, even if it has a high nitrogen concentration. And the hydrogen absorption peaks at 2920 and 2850 cm-1 are more widespread than the absorption peak at 3107 cm-1, this suggests that the sp3 CH bond more widely exists in diamond than the vinylidene group (C＝CH2). Infrared spectra analysis indicates that the hydrogen impurity mainly exists in the natural diamond as vinylidene group as seen from the absorption peak intensity. We believe that our results provide a new way to study the formation mechanism of the natural diamond. Moreover, the ideal synthesis condition in our system supplies a possible way for us to design n-type diamond semiconductor.
Preparation of boron nanowires using AuPd nanoparticles as catalyst and their field emission behavios
2014, 63 (4): 048102. doi: 10.7498/aps.63.048102
Large-area boron nanowires are successfully prepared by chemical vapor deposition using different compositions of AuPd bimetal nanoparticles as catalysts. The lengths of the boron nanowires are in a range of 5–10 μm and their average diameter is 50 nm. Structural and morphology analysis indicate that these nanowires are single crystalline with a β-rhombohedral structure. The diameters of nanowires gradually decrease with the increase of the concentration of Pd in bimetal nanoparticles. Field emission results show that the field emission properties of boron nanowires can be tuned through using different diameters and densities of boron nanowires.
2014, 63 (4): 048201. doi: 10.7498/aps.63.048201
With the constant development of the microelectronics industry, the etching scale has come up to nanoscale, which makes the plasma etching mechanism attract more and more attention. The profile surface simulation is one of the most significant technologies for the study of ion etching. In the process of ion etching surface simulation, the ion etching yield model serves as an important model for the study of etching mechanism as well as the basic foundation of some simulations such as cellular automata. In order to solve the problem that it is difficult to achieve accurate parameters of etching yield model by adopting the traditional method, the paper proposes an optimization method for ion etching yield modeling based on etching velocity matching. Aiming at reducing the mean square error between the simulated etching velocity and the real etching velocity, it optimizes the parameters of ion etching yield modeling by using the decomposition-based multi-object evolution algorithm, which then is applied to etching simulation process on the basis of cellular automata. And the validity of the proposed method was verified by the experimental results.
Simulated annealing reconstruction of LiCoO2 cathode microstructure and prediction of its effective transport properties
2014, 63 (4): 048202. doi: 10.7498/aps.63.048202
Reconstruction and characterization of the porous composite electrode via experimental and numerical approaches is one of the most important ingredients of mesoscopic modeling. It is also the basis and prerequisite for bottom-to-up design and optimization of electrode microstructure. In the present work, a simulated annealing approach is employed to reconstruct the LiCoO2 cathode of a commercial Li-ion battery. Important statistical characteristic parameters of the real LiCoO2 cathode, such as porosity or component volume fraction, the real size distribution curve of LiCoO2 particles, which are taken from experimental data or extracted from the source materials used to fabricate the cathode, are used to regulate the reconstruction process. The reconstructed electrode evidently distinguishes the three individual phases: LiCoO2 as active material, pores or electrolyte, and additives. An extensive characterization is subsequently performed, which calculates some important structural and transport properties, including the geometrical connectivity of an individual phase, the specific surface area, etc. Particularly, a self-developed D3Q15 LB (lattice Boltzmann) model is utilized to calculate the effective thermal (or electric) conductivity and the effective species diffusivity in electrolyte (or solid) phase, and the tortuosity of an individual phase. The LB model predictions indicate that the effective transport coefficients are closely related to the micro-morphology in electrodes and the tortuosity values assessed by LBM are more reliable than those predicted by random walk simulation or the Bruggeman equation.
2014, 63 (4): 048401. doi: 10.7498/aps.63.048401
The conduction of nano-scale titanium oxide memristor exhibits complex characteristics, owing to the change of self-parameters and the coexistence of different conductive mechanisms. However, there has been no detailed discussion about the influence of the cross section area change on the conductive characteristics of memristor. Based on dopant drift and tunnel barrier mechanisms, the conductive process of memristor is analysed, and the relevance between cross section area and key physical factors of the conductive process is studied, then the influences of the changes of titanium oxide and tunnel barrier cross section area on conductive characteristics of memristors are studied, respectively. The differences and connections between the two cases are analysed. In the case of the coexistence of those two mechanisms, compared with the change of titanium oxide cross section area, the change of tunnel barrier cross section area is proved to be the chief factor which causes changes of memristor conductive characteristics, it is also a possible factor causing the change of non-ideal conductive characteristics of memristor. The research results contribute to further explaining the complexity of memristor conductions and providing basis for optimizing memristor modeling.
Mechanism and characteristic analysis and optimization of SiGeC heterojunction bipolar transistor with super junction
2014, 63 (4): 048501. doi: 10.7498/aps.63.048501
A novel SiGeC heterojunction bipolar transistor (HTB) with super junction is presented. The effects of SiGeC base and super junction on device performance are analyzed in detail, and current transport mechanism of novel device is studied. Based on SiGeC/Si heterojunction technology, the high frequency characteristic of the novel device can be excellent. The breakdown voltage of device is improved greatly, because of two-dimensional direction of the electric field distribution in the collector region. The results show that the breakdown voltage of SiGeC HBT with super junction is increased by 48.8%, compared with that without super junction. More importantly, the introduction of super junction changes neither the high current gain nor the high frequency characteristics of SiGeC HBT. Compared with the Si bipolar transistor (BJT) with the same parameters, the novel device has a current gain that increases 10.7 times, and its cutoff frequency and maximum oscillation frequency are also improved greatly. A good trade-off is achieved among high current gain, high frequency and high breakdown voltage, in the novel SiGeC HBT with super junction. The layers and width of column region are designed to be optimal. With the increase in the number of column region layers, the breakdown voltage of the novel device is increased significantly, the current gain is improved somewhat, and the cutoff frequency and maximum oscillation frequency are reduced slightly. Taken together, the pnpn four-layer structure of super junction region is reasonable.
2014, 63 (4): 048701. doi: 10.7498/aps.63.048701
To further study the properties of P53 suppressor gene, the 3-base periodicity of P53 coding sequence (CDS) and the corresponding mRNA are analyzed. And the codon biases of P53 CDS and mRNA are discussed through analyzing their relative synonymous codon usage and quasi relative synonymous codon usage values. The results show that the CDS of P53 exhibits 3-base periodicity, whereas the corresponding mRNA of P53 does not, and that the P53 CDS has a stronger bias towards C and G ending codons than the mRNA. This suggests that the 3-base periodicity is closely related to the codon usage bias of P53 gene, and the degree of codon bias has an effect on the 3-base periodicity, which further explains the difference in 3-base periodicity between P53 CDS and mRNA from the point of view of biology. This characteristic of P53 gene may be useful in increasing the correct rate of gene recognition and the extensive investigation of P53 gene.
2014, 63 (4): 048702. doi: 10.7498/aps.63.048702
In this paper we investigate a generalized three-dimensional (3D) scattering channel model for microcell environments, which idealizes a directional antenna at the center of the semi-spheroid. The joint and marginal PDFs of angle of arrival and Doppler spectra are derived. According to the theory of the relative motion, with the movement at the mobile station, the base station is relatively moving, which will also produce Doppler spectra. Comparisons between our theoretical calculations and customary 3D results show that the analyses are correct and applicable to microcell environments, which promotes the research of the 3D scattering channel models.
2014, 63 (4): 048703. doi: 10.7498/aps.63.048703
In the paper we study the effects of laser wavelength on capture and delivery of polystyrene microspheres using a nanofiber. Theoretical analysis shows that when the fiber diameter and the power of the laser are fixed, with the increase of the laser wavelength, the range and intensity of evanescent wave outside optical fiber become large and light gradient and scattering forces exerted on spheres also increase, which means that the ability to capture and transportation of spheres using nanofiber increases with the laser wavelength. The experimental phenomena and theoretical predictions are completely consistent with each other, when the lasers with three wavelengths are separately injected into the fiber with a diameter of 600 nm, with the increase of the wavelength, the critical power becomes small, and when the laser power is constant, with the increase of the wavelength, velocities of spheres also increase, showing that the ability to capture and transport the microsheres is enhanced.
An optimal direction strategy of diffusion sensitive gradient mangnetic fields in magnetic resonance diffusion tensor imaging based on generalized Fibonacci sequence
2014, 63 (4): 048704. doi: 10.7498/aps.63.048704
In order to accurately investigate the directionally anisotropic diffusion information of water molecule in tissue, the diffusion sensitive gradient fields need to be applied alone many directions in order to obtain corresponding diffusion coefficients in diffusion tensor imaging (DTI) and high angular resolution diffusion imaging (HARDI) experiments. The problems facing to current diffusion sensitive gradient magnetic fields encoding schemes include the spatial uniformity of directions needs to be improved, there is no general direction design for arbitrary number of directions, flaw in any directions will cause failure or defect of the whole dataset. In this paper, we provide a generalized Fibonacci number based direction encoding scheme. This scheme can generate nearly uniform distribution for arbitrary number of directions and satisfy the spatial uniformity using partial directions from one raw data set. Besides, the diffusion sensitive gradients of neighboring directions are nearly opposite, which will reduce eddy current induced by rapid varying gradient magnetic fields.
2014, 63 (4): 048801. doi: 10.7498/aps.63.048801
In the paper, a series of bulk-heterojunction polymer solar cells using poly(3-hexyl-thiophene) (P3HT) as a donor and indene-C60 bisadduct (ICBA) as an accepter is fabricated. Effect of annealing process on the cell characteristics is investigated. It is found that the cell having a multiple temperature-step annealing can distinctly improve its photovoltaic performance. At the same time the multiple temperature-step annealing process can also significantly improve the life time of the device. The device maintains a stable performance without being encapsulated.
Correlation of the holding behaviour of the holding-based network of Chinese fund management companies based on the node topological characteristics
2014, 63 (4): 048901. doi: 10.7498/aps.63.048901
The data in this paper was collected from the semi-annual reports between 2003 and 2012 which disclosed the Chinese fund management companies' shareholdings in different listed companies. The holdings-based network of Chinese fund management companies is constructed by taking fund management companies as the nodes, the holding of stock of the same listed company at the same period as the edges, and the number of the listed companies holding at the same time as the weight of the edges. Based on the methods such as statistical physics, etc., the stability of the networks at different times is analyzed, and then the correlation of the holding behavior between the nodes with different topological characteristics and three different sets of nodes are calculated and analyzed. Three different sets are the set of nodes in the full graph with a given stock at t-1 (the first type of nodes), the set of nodes in the holing-based network without holding a given stock at t-1 (the second type of nodes), and the set of the new nodes which appear at t (the third type of nodes). The result shows as follows. Firstly, the correlation coefficient of the holding behavior between the node and the second type of nodes rises with the node clustering coefficient increasing; secondly, the node holding behavior is highly correlated with the second type of nodes and with the third type of nodes only when the values of the node average degree and strength are high; finally, the node holding behavior is not related to the third type of nodes at all. This paper propose a new method to study the correlation of stock market, and it is a basis for the further investigation on the structure equivalence network in stock market and also the differences of the importance between the nodes.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2014, 63 (4): 049201. doi: 10.7498/aps.63.049201
Based on the index of atmospheric teleconnection and the key areas of the sea, taking the relationship between the average of the height fields of the key areas and the average of the key areas of SST as boundary condition, the associated network that embodies the interaction between atmospheric teleconnection areas and the key areas of SST is constructed. Then the image of the interaction between atmospheric and ocean is given. Based on the structural characteristics of network, the basic nature of the network and the key nodes of the interaction network are studied and the influence of the key nodes on network is analysed. The Results show that the associated network belongs to the small world that has a smaller average path length and a lager clustering coefficient. Besides, the average path length has a stable value in the threshold less than 0.4. In other words, in this case the characteristics of network remain unchanged, and then it is proved that using the correlation 0.19 as the condition of edge connection is effective. The single node characteristic is analysed in the network and the results show that the areas of the East Asia-Pacific 1 and 2 and the Pacific/North American pattern 1 and 4 have lager node degrees. That is to say, these four atmosphere regions have stronger effect than other regions in the atmosphere and sea, the North Pacific Oscillation 1 and the East Asia-Pacific 3 have low node degrees, so these two regions have low effects on the region interaction. In addition, it is proved that the network has certain robustness by removing nodes in turn. The study is of significance to grasp some important areas of the teleconnection and the sea, and also it provides a new method to study air-sea interaction.
Variability periodicity and chaotic property of quasars based on ensemble empirical mode decomposition
2014, 63 (4): 049701. doi: 10.7498/aps.63.049701
The radio fluxes of the quasars 3C 345, 3C 279 and 3C 273 at a frequency of 8.0 GHz were collected in a time period from 1965 to 2012 based on the database of University of Michigan Radio Astronomy Observatory. The time series of the quasar variability is decomposed into trend components, periodic components and high frequency components by the ensemble empirical mode decomposition. Then chaotic characteristic quantities of the high frequency component is analyzed, that is, saturation correlation dimension, maximum Lyapunov exponent and Kolmogorov entropy. The results show that the variability of quasars has the chaotic characteristic as well as the characteristic of periodicity. The variability of quasars is due to the comprehensive consequence of a physical mechanism of the periodic phenomenon combined with the nonlinear mechanism of the chaos phenomenon.
2014, 63 (4): 049801. doi: 10.7498/aps.63.049801
Long-term observations show that blazar object S5 0716 +714 has complicated behavious on multiband. In this paper, the light curves in the radio, V-band and X-ray bands of four outbursts are studied by the chaos theory. We calculate their correlation dimensions and Lyapunov exponents and find that 1) these two values are in a range between a stochastic system and a pure chaotic system; 2) the phase locus of S5 0716 +714 is a chaotic system similar to the Rossler attractor. Various internal or external factors may produce noises, resulting in the fact that the correlation dimension and Lyapunov exponent may be higher than those of the Rossler attractor and the phase locus might be orderless. According to our analysis, we conclude that the light curves of S5 0714+716 show some chaotic features and the variability of this source may be a chaotic phenomenon.