Vol. 63, No. 7 (2014)
2014, 63 (7): 070201. doi: 10.7498/aps.63.070201
The damped brachistochrone problem and that with non-zero initial velocity are studied. Based on the discussion of these problems, one may take theorems of motion as constraints for some systems, and whether the constraints are holonomic or nonholonomic is related to the fact that the differential theorems of motion are integrable or non-integrable.
2014, 63 (7): 070202. doi: 10.7498/aps.63.070202
Optimal control of train movement is an important way to reduce transport cost, enhance service level, and realize sustainable development. In this paper, based on traditional optimal velocity car-following model, an improved simulation model is presented, it is used to optimize the velocity control of train movement in urban railway system. The proposed model is established by introducing a new function of objective optimal velocity into the classical optimal velocity model (See Phys. Rev. E 51, 1035, Bando et al, 1995) to realize the optimal control of train movement in complicated conditions. Numerical simulation takes the Beijing City Metro Yi Zhuang line as an example. Here some reality measurement data is used. Results show that the proposed model can well describe the dynamic characteristics of train movement under the complex limited condition. Simulation results are close to reality measurement data. This demonstrates that the proposed model is valid. Further, by analyzing the space-time graph, the change of train velocity and travel time, the evolution characters of train flow under complex conditions are discussed.
2014, 63 (7): 070203. doi: 10.7498/aps.63.070203
Combined with the Lossy Drude-Lorentz dispersive model, a symplectic finite-difference time-domain (SFDTD) algorithm is proposed to deal with the double dispersive model. Based on matrix splitting, symplectic integrator propagator and the auxiliary differential equation (ADE) technique, with the rigorous and artful formula derivation, the algorithm is constructed, and detailed formulations are provided. Excellent agreement is achieved between the SFDTD-calculated and exact theoretical results when transmittance coefficient in simulation of double dispersive film in one dimension is calculated. As to numerical results for a more realistic structure in three dimensions, the simulation of periodic arrays of silver split-ring resonators using the Drude dispersion model are also included. The transmittance, reflectance, and absorptance of the structure are presented to test the efficiency of the proposed method. Our method can be used as an efficiency simulation tool for checking the experimental data.
2014, 63 (7): 070204. doi: 10.7498/aps.63.070204
In order to enhance the performance of AlGaN/GaN high electron mobility transistor (HEMT) biosensor, millimeter grade AlGaN/GaN HEMT structure have been designed and successfully fabricated. Factors influencing the capability of the AlGaN/GaN HEMT biosensor are analyzed. UV/ozone is used to oxidize GaN surface and then 3-aminopropyl trimethoxysilane (APTES) self-assembled monolayer can be bound to the sensing region. This serves as a binding layer in the attachment of prostate specific antibody (anti-PSA) for prostate specific antigen detection. The millimeter grade biomolecule-gated GaN/AlGaN HEMT sensor shows a quick response when the target prostate specific antigen in a buffer solution is added to the antibody-immobilized sensing area. The detection capability of this biomolecule-gate sensor estimated to be below 0.1 pg/ml level using a 21.5 mm2 sensing area, which is the best result of GaN/AlGaN HEMT biosensor for PSA detection till now. The electrical result of the biomolecule-gated GaN/AlGaN HEMT biosensor suggests that this biosensor might be a useful tool for the prostate cancer screening.
Stochastic resonance in an overdamped monostable system with multiplicative and additive α stable noise
2014, 63 (7): 070501. doi: 10.7498/aps.63.070501
In this paper we combine α stable noise with a monostable stochastic resonance (SR) system to investigate the overdamped monostable SR phenomenon with multiplicative and additive α stable noise, and explore the action laws of the stability index α (0 α ≤ 2) and skewness parameter β (-1 ≤ β ≤ 1) of the α stable noise, the monostable system parameter a, and the amplification factor D of the multiplicative α stable noise against the resonance output effect. Results show that for different distributions of α stable noise, the single or multiple low-and high-frequency weak signals detection can be realized by adjusting the parameter a or D within a certain range. For a or D, respectively, there is an optimal value which can make the system produce the best SR effect. Different α or β can regularly change the system resonance output effect. Moreover, when α or β is given different values, the evolution laws in the monostable SR system excited by low-and high-frequency weak signals are the same. The conclusions drawn for the study of single-and multi-frequency monostable SR with α stable noise are also the same. These results will be the foundation for realizing the adaptive parameter adjustment in the monostable SR system with α stable noise.
Modeling and simulation analysis of fractional-order Boost converter in pseudo-continuous conduction mode
2014, 63 (7): 070502. doi: 10.7498/aps.63.070502
Based on the fact that the inductor and the capacitor are fractional in nature, the fractional order mathematical model of the Boost converter in pseudo-continuous conduction mode is established by using fractional order calculus theory. According to the state average modeling method, the fractional order state average model of Boost converter in pseudo-continuous conduction mode is built. In view of the mathematical model, the inductor current and the output voltage are analyzed and the transfer functions are derived. Then the differences between the integer order and the fractional order mathematical models are analyzed. On the basis of the improved Oustaloup fractional order calculus for filter approximation algorithm and the model of fractional order inductance and capacitance, the simulation results have been compared between the mathematical model and circuit model with Matlab/Simulink software; the origins of model error are analyzed and the correctness of the modeling in fractional order and the theoretical analysis is verified. Finally, the differences and the relations of Boost converter among the continuous conduction mode, the discontinuous conduction mode, and the pseudo-continuous conduction mode are indicated.
2014, 63 (7): 070503. doi: 10.7498/aps.63.070503
Chaotic SPWM control has attracted much interests due to its effectiveness for EMI suppression in power converters. However, most researches focus on the simulation and experiment of power converter under chaotic SPWM control, which is lacking a quantitative method. Based on double Fourier series this paper provides a spectrum calculation method for multi-period SPWM or quasi-random SPWM signals firstly, and the related spectrum calculation and simulation for multi-period SPWM are given to verify the accuracy of the spectrum calculation method; then the calculation method is extended to the spectral analysis of chaotic SPWM signals. To observe the impact on the spectrum of chaotic SPWM signals generated by different mappings and in different variation ranges of carrier period, a spectrum comparison between the Tent and Chebyshev mappings is conducted, in which results indicate that the variation range of the carrier period and the selection of mappings have a great influence on spectrum distribution; in the long term, probability density distribution of chaotic mapping will certainly affect the spectrum, and in the short term the initial value of the mapping will also affect the spread spectrum distribution. In summary, the proposed spectrum calculation method in this paper provides a theoretical foundation for the spread spectrum principle of chaotic SPWM control and for the design reference in practical engineering application.
Manipulation of the complete chaos synchronization in dual-channel encryption system based on polarization-division-multiplexing
2014, 63 (7): 070504. doi: 10.7498/aps.63.070504
For the dual-channel encryption system, based on polarization-division-multiplexing, we put forward a new control scheme for complete chaos synchronization by means of linear electro-optic (EO) effect. In the scheme, the chaotic synchronization quality of each linear polarization (LP) mode component varies periodically with the applied electric field. The variation regulation is as follows: Complete chaos synchronization ↔ acute oscillation. With the applied electric field fixed at a certain value, the robustness of the complete chaotic synchronization quality due to the bias current and the feedback strength is improved greatly by EO modulation. Each LP mode can obtain the complete chaos synchronization in a large range of the bias current and the feedback strength. And the encoding message modulated to each LP mode can be almost re-established.
2014, 63 (7): 070702. doi: 10.7498/aps.63.070702
A new method of background elimination and baseline correction is proposed, since there are background signal and larger baseline signal in the first harmonic (1f) of the tunable diode laser absorption spectroscopy (TDLAS). The laser-associated intensity modulation signal, electronic noise, and optical interference fringes of the 1f background are analyzed. Harmonic detection in none absorption spectral region (HDINASR) is used to eliminate the background signal. Then the relationship curve between current and intensity is given in different operating temperatures to design a remaining baseline correction method after eliminating the background. The principle of background signal searching and the LabView software flow chart are also given. The TDLAS experimental system is designed to detect hydrogen fluoride (HF) gas. According to spectral line selection principle, the absorption line -1312.59 nm is selected, whose operating temperature is set at 27.0 ℃ and the background temperature is set at 30.2 ℃. After eliminating the background and correcting the baseline, signal distortion is significantly improved and baseline is corrected. Then it is verified that the method is valid at other operating temperature of the laser (26.7-27.2). And the improvement of HF gas concentration is quantitatively analyzed. It is convenient for the subsequent processing of 1f signal.
2014, 63 (7): 070701. doi: 10.7498/aps.63.070701
Since the last decades, superconducting single-photon technology has been extensively used in the quantum security communication and the linear-optic quantum computing fields. Especially, the device based on the coplanar waveguide resonator has attracted substantial interests due to its evident advantages, including the relatively simple structure, the sufficiently high detection efficiency, and the photon-resolving capability, etc. With the profound investigation in optimizing the depositing methods and the material selections, as well as the the development of the relevant theories, the technology of single photon detection based on the coplanar waveguide resonator has obtained a breakthrough. In this review paper we begin from the basic principle of the coplanar waveguide detector, then interpret the relevant theory and some design details of the devices. Finally, based on some of the recent experimental results measured with the low-temperature devices in our lab, we give a brief perspective on the future development of the superconducting coplanar waveguide single photon detectors.
ATOMIC AND MOLECULAR PHYSICS
2014, 63 (7): 073201. doi: 10.7498/aps.63.073201
We use the Lewenstein model to study the high harmonic generated for a μp atom exposed to two-color XUV pulses. Calculated results show a super continuum plateau in high harmonic spectrum which is formed when the time delay is 0 and XUV frequencies are 5 and 2.5. By synthesizing the continuous high harmonic spectra, a pulse as short as 130 zeptosecond is obtained. Such a single zeptosecond pulse may work as an ultrafast camera to capture ultrafast processes occurring inside nuclei.
Theoretical and experimental study on the multi-color broadband coherent anti-Stokes Raman scattering processes
2014, 63 (7): 073301. doi: 10.7498/aps.63.073301
In order to exactly distinguish and quantitatively analyze the different or unknown components in a mixture, the global molecular CARS spectra information should be obtained simultaneously with a broad-band coherent anti-Stokes Raman scattering (CARS) spectroscopy in supercontinuum. In a broad-band CARS spectroscopy, two-and three-color CARS processes are generated due to different functions of effective spectroscopic components in supercontinuum. Firstly, we theoretically analyzed the generation conditions of CARS signals and the relationships between their intensity and power of excitation lights in the two types of CARS process with the broad-band excitation. On this basis, the two types of CARS process are achieved with a home-built broad-band CARS spectroscopic system, respectively. Using the functional fitting analysis of the obtained CARS spectral signals of benzonitrile, the relationships between CARS signals and excitation lights are experimentally verified in two different kinds of CARS process. Further optimizations of broad-band time-resolved CARS spectroscopic and microscopic systems, for simultaneously obtaining the global CARS spectral signals of samples, can be achieved under the guidance of theoretical and experimental results.
2014, 63 (7): 073302. doi: 10.7498/aps.63.073302
A high-precision quantum chemistry ab initio multi-reference configuration interaction method with aug-cc-pVQZ basis sets has been used to calculate the four states of BP molecule. The four -S states are X3, 3-, 5 and 5-, which are correlated to the lowest dissociation limit of B(2Pu)+P(4Su). Analysis of the electronic structures of -S states shows that the -S electronic states are essentially multi-configurational. We take the spin-orbit interaction into account for the first time so far as we know, which makes the four -S states split into fifteen states. 30+ state is confirmed to be the ground state. The SOC effect is essential for the BP molecule, which leads to the avoided crossings for 0+ and 1 states from X3 and 3-. Based on the PECs of -S and states, the accurate spectroscopic constants are obtained by solving the radial Schrdinger equation. The spectroscopic results may be conducive to further research on BP molecule in experiment and theory.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2014, 63 (7): 074201. doi: 10.7498/aps.63.074201
A new method for generating a single bottle beam directly by light emitting diode (LED) with a secondary optical lens is proposed for the first time, so far as we know. Firstly, in the aspect of geometrical optics, we analyze the principle of generation of a single bottle beam by the LED spot light with a secondary optical lens. Then, we calculate the expression of the length and the radius of the biggest dark region of the bottle beam. After that, a new type of a secondary optical lens is calculated numerically and simulated by numerical recipes software Matlab, three-dimensional modeling software Solidworks and optical simulation software Tracepro. Meanwhile, the minimum size of the bottle beam and the scattering force for trapping particles are calculated. The result shows that the designed secondary optical lens can produce a single bottle beam, the length and the radius of the biggest dark region of the generated bottle beam are in accordance with the theoretical calculations. This result offers a practical and available method for generating a bottle beam with light emitting diode at a low cost.
2014, 63 (7): 074202. doi: 10.7498/aps.63.074202
The accurate background correction can determine the minimum limit of trace mercury measurement in atmosphere by the cold vapor atomic absorption method. This paper studies a new method of mercury detection using the common mercury lamp as sources which correct the background according to the transverse Zeeman effect. The resonance spectral line (253.65 nm) of the meccury lamp generates σ-, σ+, and π linear polarized light in the vertical direction of the magnetic field. This study obtains mercury absorbance of σ-, σ+, and π light in different magnetic field intensity by using ultra-high resolution spectrometer, then gets the minimum field intensity of the method. We discuss the existing possible interference caused by benzene with narrow-band absorption and acetone with broadband absorption under 1.78 T magnetic field intensity. Taking σ- and σ+ as background light, and π as absorption light, we quantify the saturated mercury vapor cell with different lengths. With the accurate background correction, the R value of absorption fitting curve can achieve 0.99. Results indicate that the method can accomplish the job of accurate background correction and can be applied to trace mercury measurement in atmosphere.
2014, 63 (7): 074203. doi: 10.7498/aps.63.074203
In recent years, Er3+ doped CdF2-CdCl2-NaF-BaF2-BaCl2-ZnF2 (CNBZN) glass has become one of the new materials in the field of laser cooling of solids. In this paper, using the theory of laser output and standing wave resonance, intracavity-and extracavity-enhanced laser cooling of Er3+-doped CNBZN glass are theoretically analyzed. Calculated results show that enhancement factor can achieve tens to hundreds of times. Moreover, two schemes are compared with each other, and the results show that for low material absorption, especially when the sample length is less than 0.3 mm, intracavity configuration has the advantage of high pumping power and high absorption. However, for high material absorption, especially when the sample length is longer than 3 mm, the extracavity configuration becomes a more efficient means for laser cooling. Finally, according to the operating wavelength and power requirements of Er3+-doped material, cavity enhancement can be realized experimentally using semiconductor diode laser.
2014, 63 (7): 074204. doi: 10.7498/aps.63.074204
The shot-to-shot variation of laser fluence spatial distribution on a large-aperture high-power laser facility is statistically analyzed. Statistical results show that the maximum fluence spatial distribution to which any location in the optic beam will be exposed after N shots, can be described by Gaussian function, and the average fluence across the beam increases with laser shots while standard deviation is relatively constant, independent of laser shots. This is due to the fact that laser fluence spatial distribution possesses similarity over the whole beam and dissimilarity at local positions for different laser shots.
2014, 63 (7): 074205. doi: 10.7498/aps.63.074205
We proposed the fabrication of gold micro-electrode and grating electrode through laser assembling of gold nanoparticles and realized the electrical interconnection of the single carbon nanotube and gold nanolines, which can decrease the damage of the functional unit to a great extent. This method can also solve the problem of inadequate mass transport in the fabrication of ions. The microstructure could keep unoxidized in the atomasphere with excellent continuity, integrity, and electrical properties, which made this technique have wide application prospects.
2014, 63 (7): 074206. doi: 10.7498/aps.63.074206
Based on a 529-actuator adaptive optic (AO) system, the sparseness of slope response matrix from deformable mirror to Hartmann wavefront sensor and the sparseness of iterative matrix in wavefront reconstruction are analyzed. The influence of actuator coupling coefficient on the slope response matrix sparseness, the iterative matrix sparseness, and the AO system correction quality are also studied under the condition of constant actuator spacing. Larger coupling coefficient results in a lower sparseness of slope response matrix and an iterative matrix. Too large or too small coupling coefficient will lead to lower stability and correction quality of AO system. Finally, the optimal range of coupling coefficient is provided by the balancing correction quality, sparseness of slope response matrix, and stability.
2014, 63 (7): 074207. doi: 10.7498/aps.63.074207
In the applications of two external fields, such as stresses and electric fields, the optical modulation properties of some crystals are theoretically analyzed using the method of index ellipsoid. Simple mathematical formulas for the calculations of the field-induced principal refractive indexes of some crystals and corresponding azimuthal angles of their principal axes can be deduced from the equation of index ellipsoid if there exists only one nonzero cross term in the equation, e.g. x1x2. According to these simple formulas, we can find out some crystals exhibiting dual transverse electrooptic effect, e.g. crystals of the 6 symmetry point group. Under two simultaneously applied external stresses, elastooptic birefringence of a crystal is proportional to the difference between the two external stresses, and the orientations of their birefringent axes are unchanged. When a stress and an electric field are simultaneously and perpendicularly applied to some crystals such as cubic crystals of 43m point group, the field-induced birefringence of the crystal is proportional to the weighted geometric mean of the applied stress and electric field, and the orientations of their birefringent axes only depend on the ratio of the applied electric field and stress. The above electrooptic and elastooptic modulation properties are useful to the design of novel optical modulators and sensors.
2014, 63 (7): 074208. doi: 10.7498/aps.63.074208
A new block-based recursive moving-target indication algorithm in velocity domain is proposed to solve the problem which is the rapid detection of dim and small target for infrared search and tracking system. Firstly, the two-dimensional least mean square filter is adopted to filter the infrared image sequence, which extracts small targets and residual errors of image sequence. Then, block-based recursive moving-target indication algorithm is adopted to accumulate small target in image block sequence for the enhancement of small target velocity in velocity domain. Finally, resulting image is obtained by using classical recursive moving-target indication algorithm and target velocity for small target detection. Compared with classical method, the proposed method requires less running time, and can be used to detect dim small target image effectively as demonstrated by several groups of experimental results.
Analysis of electron momentum relaxation time in fused silica using a tightly focused femtosecond laser pulse
2014, 63 (7): 074209. doi: 10.7498/aps.63.074209
The electron momentum relaxation time is studied systematically in order to understand its effect during the excited nonlinear ionization process in fused silica with an irradiation of tightly focused femtosecond laser pulses. According to the analysis of a (3+1)-dimensional extended general nonlinear Schrödinger equation, the electron momentum relaxation time shows a huge effect on peak intensity, free electron density, and fluence distributions in the focal region of the incident pulse, meanwhile a value of 1.27 fs is thought to meet the present experimental result based on the theoretical model. Further research indicates that the change of electron momentum relaxation time can have significant difference among several nonlinear mechanisms, such as the laser-induced avalanche ionization, reverse bremsstrahlung, self-defocusing of plasma, etc. Results show that the electron momentum relaxation time plays an important role in the process of femtosecond laser pulses interaction with materials.
2014, 63 (7): 074210. doi: 10.7498/aps.63.074210
All-solid photonic bandgap fiber shave attracted great attention of researchers due to their particular band gap and dispersion character as well as the merit of easily splicing the traditional optical fiber. We have fabricated the all-solid photonic bandgap fibers using the plasma chemical vapor deposition (PCVD) and a modified tack and draw technique, and the loss and dispersion characteristics were simulated by the finite-difference frequency-domain (FDFD) method. The fiber obtained by this method has a low-loss region at around 1550 nm and can be used as single-mode; its effective model field area and the dispersion of the fiber at 1550 nm are 191.81 μm2 and 16.418 ps/(km·nm), respectively. Combined with the experimental results, the fiber parameters are further optimized by simulation.
2014, 63 (7): 074211. doi: 10.7498/aps.63.074211
Combining depth information and color image, D-RGB cameras provide a ready detection of human and associated 3D skeleton joints data, facilitating, if not revolutionizing, conventional image centric researches in, among others, computer vision, surveillance, and human activity analysis. Applicability of a D-RBG camera, however, is restricted by its limited range of frustum of depth in the range of 0.8 to 4 meters. Although a D-RGB camera network, constructed by deployment of several D-RGB cameras at various locations, could extend the range of coverage, it requires precise localization of the camera network: relative location and orientation of neighboring cameras. By introducing a skeleton-based viewpoint invariant transformation (SVIT), which derives the relative location and orientation of a detected humans upper torso to a D-RGB camera, this paper presents a reliable automatic localization technique without the need for additional instrument or human intervention. By respectively applying SVIT to two neighboring D-RGB cameras on a commonly observed skeleton, the respective relative position and orientation of the detected humans skeleton for these two cameras can be obtained before being combined to yield the relative position and orientation of these two cameras, thus solving the localization problem. Experiments have been conducted in which two Kinects are situated with bearing differences of about 45 degrees and 90 degrees; the coverage can be extended by up to 70% with the installment of an additional Kinect. The same localization technique can be applied repeatedly to a larger number of D-RGB cameras, thus extending the applicability of D-RGB cameras to camera networks in making human behavior analysis and context-aware service in a larger surveillance area.
2014, 63 (7): 074401. doi: 10.7498/aps.63.074401
Direct non-equilibrium molecular dynamics (NEMD) was used to simulate the thermal conductivities of the monolayer and the bilayer silicon functionalized graphenes along the length direction respectively, with the Tersoff potential and the Lennard-Jones potential, based on the velocity Verlet time stepping algorithm and the Fourier law. Simulation results indicate that the thermal conductivity of the monolayer silicon functionalized graphene decreases rapidly with increasing amount of silicon atoms. This phenomenon could be primarily attributed to the changes of graphene phonon modes, mean free path, and motion speed after silicon atoms are embedded in the graphene layer. Meanwhile, the thermal conductivity of the monolayer graphene is declined in the temperature range from 300 to 1000 K. As for the bilayer silicon functionalized graphene, its thermal conductivity increases as a few silicon atoms are inserted into the layer, but decreases when the number of silicon atoms reaches a certain value.
Bifurcation and chaos of some strongly nonlinear relative rotation system with time-varying clearance
2014, 63 (7): 074501. doi: 10.7498/aps.63.074501
The dynamic equation for the relative rotation nonlinear dynamic system with time-varying clearance is investigated. Firstly, transformation parameter is deduced by using the method of MLP; the bifurcation response equations of 1/2 harmonic resonance then are generated by the method of multiple scales, while singularity analysis is employed to obtain the transition set of steady motion; further more the bifurcation characteristic and the bifurcation of the system under the situation of non-autonomy are analyzed. Finally, numerical simulation exhibits many different motions, such as periodic motion, period-doubling motion, and chaos. It is shown that the change of clearance and damp parameters may influence the motion state of the system.
2014, 63 (7): 074701. doi: 10.7498/aps.63.074701
Intermittent turbulence means that the turbulence eddies do not fill the space completely, so the dimension of an intermittent turbulence takes the values between 2 and 3. Turbulence diffusion is a super-diffusion, and the probability of density function is fat-tailed. In this paper, the viscosity term in the Navier-Stokes equation will be denoted as a fractional derivative of Laplatian operator. Dimensionless analysis shows that the order of the fractional derivative α is closely related to the dimension of intermittent turbulence D. For the homogeneous isotropic Kolmogorov turbulence, the order of the fractional derivatives α=2, i.e. the turbulence can be modeled by the integer order of Navier-Stokes equation. However, the intermittent turbulence must be modeled by the fractional derivative of Navier-Stokes equation. For the Kolmogorov turbulence, diffusion displacement is proportional to t3, i.e. Richardson diffusion, but for the intermittent turbulence, diffusion displacement is stronger than Richardson diffusion.
2014, 63 (7): 074702. doi: 10.7498/aps.63.074702
Low fluid friction is difficult to obtain on super-hydrophobic surfaces for a large flow velocity, because the entrapped gas within the surface is weakened substantially. Once the gas removed, the friction of the fluid increases markedly due to its own surface roughness. In this study, a hydrophobic transverse microgrooved surface is designed to sustain the air pockets in the valleys for a long time. Direct optical measurements are conducted to observe the entrapped gas when water flows past the surface in the perpendicular direction of grating patterns. More importantly, this hydrophobic transverse micro-grooved surface has been determined to have the capability of automatic formation of gas. Some of the gas is continually carried away from the surface and new gas is continually generated to substitute the lost gas. And the stable slippages at the surface are achieved corresponding to the relative stable gas on this designed surface.
2014, 63 (7): 074703. doi: 10.7498/aps.63.074703
A suitable arbitrarily complex boundary condition treatment using the lattice Boltzmann sheme is developed in the fluid-solid coupling field. The new method is based on a half-way bounce back model. A virtual boundary layer is built with the fluid-solid coupling, and all the properties used on the virtual boundary are inter-/extrapolated from the surrounding nodes combining with the finite difference method. The improved method ensures that the particles bounce the same location as that of the macro-speed sampling point, and considers the offset effect on the accuracy of the calculated results when the actual physical borders and the grid lines do not coincide. And its scope is extended to any static or mobile, straight or curved boundary. The processing power of the method under the classic conditions, such as the Poiseuille flow, the flow around a circular cylinder, the Couette flow, etc. is studied. Results prove that the theoretically calculated values agree well with the experimental data. Compared with the results published in the literature, this method has a greater precision when the actual physical borders and gridlines do not coincide.
Investigation of electromagnetic hydrodynamics propulsion and vector control by surfaces based on a rotational navigation body
2014, 63 (7): 074704. doi: 10.7498/aps.63.074704
Realization of electromagnetic hydrodynamics (MHD) propulsion by surfaces needs an electromagnetic body force generated in a conductive fluid (such as seawater and plasma, etc.) around the navigation body. Furthermore, the reaction force against the electromagnetic body force could be used to propel. Based on the basic control equations of electromagnetic field and fluid mechanics, the vector control effect has been analyzed by virtue of field intensity and force distribution characteristic on the rotational navigation body, under two different force action areas. Results show that the navigation attitude adjustment could be realized by this control method without changing attacks and propulsion directions. An upward force moment could be achieved by the control model A. Accordingly, both of the pitching moment and yaw moment could be changed by the control model B. Thus, as a new way of propulsion, the MHD propulsion by surfaces offers several advantages, such as high speed, high efficiency, easy operation, high payload etc. Additionally, in this paper, the vector propulsion has been proved to be one of the remarkable advantages for MHD propulsion by surface.
Study on the gain characteristics of terahertz surface plasma in optically pumped graphene multi-layer structures
2014, 63 (7): 075201. doi: 10.7498/aps.63.075201
Based on the developed optically pumped graphene multilayer terahertz surface plasma structures, this paper calculates the real part of propagation index and amplification coefficient in optically pumped graphene multilayer structures, discusses the inluences of momentum relaxation time, temperature, numbers of grapheme layers, and the quasi-Fermi energy in the topmost grapheme layer on the real part of propagation index and amplification coefficient. It is shown that when the real part of dynamic conductivity becomes negative in the terahertz range of frequencies in the optically pumped graphene multilayer structures, the surface plasma of graphene layers can achieve gain. By comparing the peeling-graphene-structure with the graphene structure that has a high conducting bottom graphene layer in optically pumped scheme, it can be said that the surface plasma of the peeling-graphene-structure can get a high efficient amplification. Meanwhile, the structure having properly numbers of graphene layers can get a larger amplification than the simple graphene structure in an optically pumped scheme at low temperatures.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
Three-dimensional modelling and numerical simulation on segregation during Fe-Pb alloy solidification in a multiphase system
2014, 63 (7): 076101. doi: 10.7498/aps.63.076101
The three-dimensional mathematical model for a three-phase flow during its horizontai solidification is studied using fluid dynamics method based on Eulerian-Eulerian and volume of fraction methods, in which the mass, momentum, species, and enthalpy conservation equations of the Fe-Pb alloy solidification process are solved simultaneously. Effects of Pb area quadratic gradient (∇ (∇SPb)) and Pb concentration quadratic gradient (∇ (∇CPb)) on the segregation formation are investigated. Results show that the segregation mode is manifested as X-segregates in the upper and V-segregates in the lower part during flow-solidification of liquid phase and gas phase. The X-segregates result from the phase transformation driving force of gas phase and “scattering” is due to the orientation of phase transition. When t >tc the lower ∇ (∇SPb) and ∇ (∇CPb) curves cause a larger yielding rate of Pb with a larger down angle of X-segregates and a smaller up angle of X-segregates and V-segregates. All these are favorable for the formation of a well-dispersed microstructure. In addition, the gas-liquid two-phase flow interaction term has an effect on channel segregation, showing that channels occur only in the region where the flow-phase transition interaction term (ul·∇cl and ug·∇cg) is negative. With a negative flow-phase transition interaction term the increase in flow velocity due to the flow perturbation and flow-phase transition interaction becomes more negative, thus the channel continues to grow and tends to be stable. Calculated results show good agreement with experimental data.
2014, 63 (7): 076102. doi: 10.7498/aps.63.076102
By using the density functional theory with B3LYP/6-31G+(d) we compute the optimization, vibration frequencies, electron structures of gg conformation of oligochitosans, and study the average binding energies and the zero-point energy corrections using WB97XD method. We also analyze the thermodynamic properties of oligochitosans. Results show that the hydrogen-bond makes the oligochitosan become spiral; average binding energies tend to decrease and stability tends to improve with the increasing degree of polymerization (DP); the water degradation of oligochitosan is an exothermic reaction, so it is feasible to reduce the temperature to improve the degradation yield in experiment; in addition, the energy gap of oligochitosan quickly converges to 6.99 eV with the increase of DP; furthermore, the value of DP7 oligochitosan is in accordance with the convergence value. The HOMO and LUMO of oligochitosan show that chemical activity is mainly distributed in C2 amino, C6 hydroxyl groups, and both ends of oligochitosan chain. These results have instructive significance on the modeling, and can provide a theoritical basis for degradation process, chemical activity position, and size-dependence in physical chemical properties of oligochitosan.
2014, 63 (7): 076103. doi: 10.7498/aps.63.076103
The local strain fields around an extended edge dislocation in copper are studied via the quasicontinuum multiscale simulation method combined with the virial strain calculation techniques. Results show that in the regions, tens of nanometers away from the dislocation, atoms are experiencing infinitesimal strain; virial strain calculation results are consistent with the predictions from elastic theory very well. In the regions near the dislocation, the virial strain fields can outline the core areas of Shockley partial dislocations precisely, which are in the shape of ellipse with a longer axis 7b1 and a shorter axis 3b1, where b1 is the length of burgers vector of the partial dislocation.
Characterization of thermal conductivity for GNR based on nonequilibrium molecular dynamics simulation combined with quantum correction
2014, 63 (7): 076501. doi: 10.7498/aps.63.076501
A nonequilibrium molecular dynamics model combined with quantum correction is presented for characterizing the thermal conductivity of graphene nanoribbons (GNR). Temperature effect on graphene nanoribbon thermal conductivity is revealed based on this model. It is shown that different from the decreasing dependence in classical nonequilibrium molecular dynamics simulations, an “anomaly” is revealed at low temperatures using quantum correction. Besides, the conductivity of GNR shows obvious edge and scale effects: The zigzag GNR have higher thermal conductivity than the zigzag GNR. The whole temperature range of thermal conductivity and the slope of thermal conductivity at low temperatures both show an increasing dependence of width. Boltzmann-Peierls phonon transport equation is used to explain the temperature and scale effects at low temperatures, indicating that the model constructed is suitable for a wide temperature range of accurate calculation for thermal conductivity of different chirality and width. Research provides a possible theoretical and computational basis for heat transfer and dissipation applications of GNR.
2014, 63 (7): 076801. doi: 10.7498/aps.63.076801
Hydrogenated microcrystalline silicon germanium (μc-Si1-xGex:H) thin films have been prepared by radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) using a mixture of SiH4 and GeH4 as the reactive gases. Effects of electrode separation on the structural properties of μc-Si1-xGex:H thin films have been investigated. Results show that reduction of the electrode separation can increase the Ge content in the films. Moreover, μc-Si1-xGex:H thin film deposited at a lower electrode separation of 7 mm possesses not only a stronger (220) orientation and a larger grain size, but also a lower microstructural factor. Then, the decomposition characteristics of the reactive gases are analyzed according to the variation of the structural properties of the μc-Si1-xGex:H thin films. It is found that the increase of the Ge content is due to the decrease of the SiH4 decomposition rate in the plasma. While the better film quality obtained at the lower electrode separation is attributed to the enhancement of the diffusibility of the Ge precursors caused by improving the proportion of GeH3 radicals
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2014, 63 (7): 077101. doi: 10.7498/aps.63.077101
The first-principles calculations based on the density functional theory have been performed to investigate the doping behaviors of Na and F dopants in ZnO. It turns out from the calculated results of the band structure, density of states, and effective masses that in the F mono-doping case, the impurity states are localized and the formation energy is up as high as 4.59 eV. In the Na mono-doping case, the impurity states are delocalized and the formation energy decreases as low as -3.01 eV. One cannot obtain p-type ZnO in both instances On the contrary, in the Na-F codoping case, especially when the ratio of F and Na is 1:2, the Fermi-level shifts to the valence bands, the corresponding effective masses are small (0.7m0) and the formation energy is the lowest (-3.55 eV). These may indicate the formation of p-type ZnO having a good conductivity.
2014, 63 (7): 077102. doi: 10.7498/aps.63.077102
Four Zn0.97Cr0.03O films were deposited on quartz wafers in various oxygen environment (0, 0.05, 0.15 and 0.2 Pa) using pulsed laser deposition (PLD). The films were characterized by XRD, PL, XPS, magnetic and electrical properties. Experimental results indicate that: (1) All the films are well crystallized and display a pure orientation. (2) All the films have ferromagnetism, and the film deposited at 0.15 Pa has the biggest Ms. (3) There exist VZn, Oi, Zni, VZn- and VO defects in the four films above, and the percentage of resonance peak area for VZn to the total area of all defects as a function of oxygen pressure is similar to Ms, which means that the magnetizations of the samples are closely related to Zn vacancy VZn. There is a Cr3+ state in the four films when the content of Cr3+ is the largest at 0.15 Pa. To sum up, the experimental results indicate that the substitutive Cr in the oxidation state of t3 and the neutral Zn vacancy in the Zn0.97Cr0.03O films is the most favorable defect complex to maintain a high stability of ferromagnetic order, which is consistent with the calculated results by the first-principle calculations.
2014, 63 (7): 077201. doi: 10.7498/aps.63.077201
On the basis of mixer circuit model of light, the terahertz power generated by the carbon nanotubes (CNT) photomixer is analyzed. By simulating mixer conductance, impedance of the antenna, and light plus paranoid voltage, it is shown that the improved mixer conductance, antenna impedance and light plus paranoid voltage can improve the output power of terahertz waves. The output power can reach dozens of microwatt level in the small-signal limit.
One-dimensional photonic crystal(1D PC)-based back reflectors for amorphous silicon thin film solar cell
2014, 63 (7): 077301. doi: 10.7498/aps.63.077301
New-type back reflectors based on one-dimensional photonic crystal (1D PC) for amorphous silicon thin film solar cells have been investigated, designed and fabricated. These 1D PCs consist of alternating amorphous Si (a-Si) and silicon dioxide (SiOx), of which the deposition process is compatible with current silicon thin film solar cells technology. Results indicate that the total reflectance of 1D PCs increases with the increase of period number. An average reflectance over 96% can be achieved in the range from 500 to 750 nm with 4 periods or more. Applying the 4-period 1D PC as back reflector in NIP amorphous silicon thin film solar cell with device-configuration of glass/1D PC/AZO/NIP a-Si:H/ITO, a conversion efficiency of 7.9% can be obtained, which is comparable to the AZO/Ag-based solar cell of 7.7% and is much better than the SS-based solar cell of 6.9% (a relative enhancement of 14.5%).
2014, 63 (7): 077302. doi: 10.7498/aps.63.077302
The transport properties of a single wall carbon tube with transition metal atoms embedded in it are studied by using the first principles method based on the density functional theory and the nonequilibrium Green’s function. Different transition metal atoms filled in the carbon tube are investigated, and the respective charge and spin transport properties are studied. The conductance of the nanotube is found to be distinctive for different metal elements encapsulated, and quantized reductions of conductance can be seen by a quantum unit (2e2/h). In particular, nanotubes with two iron atoms encapsulated in display different I-V curves when the spins of the two iron atoms are in parallel and antiparallel states respectively. These results can be explained by spin-dependent scattering and charge transfer. The encapsulation may tailor the doping and add magnetic behavior to the carbon nanotubes, which would provide a new and promising approach to detect nanoscale magnetic activity.
Analyses of wavelength dependence of the electro-optic overlap integral factor for LiNbO3 channel waveguides
2014, 63 (7): 077801. doi: 10.7498/aps.63.077801
Wavelength dependence of the electro-optic overlap integral factor (Γ) for a single-mode LiNbO3 (LN) channel waveguide was analyzed experimentally and theoretically. By measuring the half-wave voltage (Vπ) of the LN waveguide at different wavelengths and then substituting the measured values into a formula that describes the relationship between Vπ and Γ, the quantitative dependence of Γ on wavelength was obtained; and it showed that Γ rapidly decreases with increasing wavelength. On the other hand, numerical simulations of the modulating electric field distribution, the modal field distribution, and Γ at different wavelengths were carried out; the calculated relationship between Γ and wavelength is in good agreement with the measured results. Further simulations indicate that as the wavelength increases, the center of the modal field profile gradually moves toward the weak electric field side from the waveguide surface, thus leading to a smaller Γ at a longer wavelength. Such a relationship between Γ and wavelength is partially responsible for the nonlinear dependence of Vπ on wavelength obtained experimentally. This would be useful for designing and optimization of LN waveguide-based devices.
2014, 63 (7): 077802. doi: 10.7498/aps.63.077802
A blue emitting phosphor Ba2Ca(PO4)2:Eu2+ is synthesized by a high temperature solid state method. Effect of the conditions is inverstigated, such as preparation temperature and time, the ratio of Ba/Ca, and Eu2+ concentration, on the phase and luminescent property. Results show that Ba2Ca(PO4)2 and Ba2Ca(PO4)2:Eu2+ have been achieved by selecting the appropriate conditions, such as the temperature 900/1200 ℃ and the time 4 h. The compound Ba2Ca(PO4)2:Eu2+ produces an asymmetric emission band centered at 454 nm under 343 nm UV excitation. For the 454 nm emission, the excitation spectrum extends from 200 to 450 nm with a peak at 343 nm, and has an obvious excitation band in the range of 350–410 nm. With increasing Eu2+ concentration, there occur the concentration quenching effect and redshift phenomenon. With decreasing ratio of Ba/Ca, there has an obvious enhancement in the green region, and the emission color gradually turns from blue to cyan. It is shown that the Eu2+ ion not only can occupy the Ba2+ site but also the Ca2+ site. Therefore, different luminescence centers of Eu2+ can exist in Ba2Ca(PO4)2, and affect its luminescence.
Luminescence property of Ce3+-Tb3+-Sm3+ co-doped borosilicate glass under various ultraviolet excitations
2014, 63 (7): 077803. doi: 10.7498/aps.63.077803
Ce3+-Tb3+-Sm3+ co-doped white light emitting borosilicate glasses were fabricated by high-temperature melting technique. In this paper, the excitation spectra and the emission spectra of Ce3+, Tb3+ and Sm3+ ions-doped and co-doped samples were measured and the energy transfer mechanism of Ce3+, Tb3+, and Sm3+ were studied by analyzing the fluorescence lifetime of single-doped and co-doped samples. The color coordinate, rendering index, and color temperature of the emission spectra can be adjusted by changing the excitation wavelength of ultraviolet LED. Finally, we have obtained the white light which fits for life, study, and work.
Bluish-green high-brightness long persistent luminescence materials Ba4(Si3O8)2：Eu2+Pr3+, and the afterglow mechanism
2014, 63 (7): 077804. doi: 10.7498/aps.63.077804
A bluish-green long persistent luminescence material Ba4(Si3O8)2:Eu2+, Pr3+, was synthesized by traditional solid state method in a reductive atmosphere According to the photoluminescence and afterglow spectra measurement, the emission center is the cation Eu2+ in the photoluminescence and afterglow procedure. The Pr3+ co-doped sample forms new defects which could capture current carriers after excitation. On the basis of thermoluminescence and afterglow decay measurement, the afterglow intensity of Pr3+ co-doped sample sharply enhances as compared with Eu2+ doped one, the reason is that the lower depth traps are generated in the shallow trap areas (T1 region). At the same time, the Pr3+ co-doped sample have longer afterglow decay than that doped with only Eu2+; the reason is that the deep traps concentration decreases in the deep trap areas (T2 region). The afterglow mechanism of Pr3+ co-doped sample have two different excitation paths, path 1: the electron of the host is directly projected to traops at 268 nm excitation; path 2 the electron of the Eu2+ corresponds to the transitions from the ground state to the 5d excited state at 330 nm excitation. Then the different afterglow mechanism of phosphor was produced.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2014, 63 (7): 078101. doi: 10.7498/aps.63.078101
The uniform and compact Fe-based amorphous alloy coating was prepared by active combustion high velocity air fuel (AC-HVAF) spray method. By tuning the parameters of AC-HVAF spray process, the influence of the spraying gun length, spraying distance, and powder feed rate on non-crystallization has been studied carefully. Results indicate that spraying gun length is the key factor in forming perfect amorphous coating. Spraying distance and powder feed rate may determine the thickness and formation rate of the coating. The prepared coatings have a tight adhesion with the substrate, low porosity, and good non-crystallization, which would effectively maintain the excellent mechanical properties of the Fe-based amorphous alloy. The coating can provide a good protection for the substrate material.
Nematic ordering pattern formation in the process of self-organization of microtubules in a gravitational field
2014, 63 (7): 078201. doi: 10.7498/aps.63.078201
By virtue of a functional scaling, the free energy for cytoskeletal microtubule (MT) solution system in the gravitational field has been proposed theoretically, and on this basis the influence of gravitational field on MT’s self-organization process is studied. A concentration gradient coupled with orientational order characteristic of nematic ordering pattern formation is the new feature emerging in the presence of gravity. Theoretical calculation results show that gravity facilitates the isotropic to nematic phase transition, which is reflected in a significantly broader transition region and the phase coexistence region increases with increasing g or MT concentration. We also discuss the numerical results obtained due to local MT concentration changing with the height of the vessel and some phase transition properties.
2014, 63 (7): 078401. doi: 10.7498/aps.63.078401
This paper presents a double-threshold cooperative spectrum sensing algorithm which is based on trust and satisfies both reliability and efficiency. The cognitive nodes that satisfy the request of double-threshold have the priority to participate in cooperative sensing and that satisfy the requirement of trust parameters may participate in cooperative sensing if only the number of the former is smaller than a preset value. The fusion center stores the sensing record of each cognitive node and sets the fusion weights according to the partial detected results. Theoretical analysis and simulation show that the bandwidth required for transmitting the sensing parameters decreases, and the detection performance improves because the unreliable users are reduced. Additionally, the algorithm can be made to adapt to different wireless service by adjusting the parameter nt.
2014, 63 (7): 078402. doi: 10.7498/aps.63.078402
Frequency selective radome is one of the most important applications of frequency selective surface (FSS). In order to obtain better stealth performance, a novel element FSS, based on a regular slot element FSS, is presented in this paper. The novel element consists of a slot element in the center and at least two slot strips placed on the periodic boundary. We call such FSS the “hybrid-element type FSS” because it exhibits characteristics of both slot type and patch type FSS. Simulation and optimization work is carried out by using a period moment method and a discrete particle swarm optimization method based on the application requirements of a missile radome. Simulation results show that the hybrid-element type FSS has much steeper transition section between pass-band and stop-band, and much lower transmittance in stop-band when compared with the corresponding slot type FSS. The new FSS also has much lower insertion loss in pass-band, much thinner thickness, much simple structure and fabrication process when compared with the ordinary two-layer FSS. Equivalent sample plate is fabricated using printed circuit method and tested using the free space method. Good fit between simulation and testing results verify the accuracy and feasibility of this novel FSS design. The hybrid-element type FSS is especially suitable for the stealth radome when woking frequencies of both sides are very close. It provides a simple and feasible approach for developing frequency selective radome.
2014, 63 (7): 078403. doi: 10.7498/aps.63.078403
The ordinary SAR imaging algorithms are inapplicable to missile-bo-rne SAR due to the high speed dive and high-squint of missile. Aiming at this problem, this paper firstly sets up the model of echo and analyses of two-dimensional spectrum. By using azimuth nonlinear chirp scaling (NLCS) based on variable decoupling, an imaging algorithm for missile-borne SAR is proposed.It can effectively compensate the scene with longitudinal and transverse Doppler shift and improve the focusing quality while it can also simplify the geometric image correction operation. Simulation results are provided to confirm the effectiveness of the proposed algorithm.
Synthesis of nanoparticles in SiO2 by implantation of Cu and Zn ions and their thermal stability in oxygen atmoshphere
2014, 63 (7): 078501. doi: 10.7498/aps.63.078501
Cu nanoparticles (NPs) embedded in silica were synthesized by implantation of 45 keV Cu ions at a fluence of 1.01017 cm-2, and then subjected to post irradiation with 50 keV Zn ions at fluences of 0.51017 cm-2 and 1.01017 cm-2, respectively. Zn post ion implantation induced modifications in structures, optical absorption properties of Cu NPs as well as their thermal stability in oxygen ambient have been investigated in detail. Results clearly show that Cu-Zn alloy NPs could be formed in the Cu pre-implanted silica followed by Zn ion irradiation at a fluence of 0.51017 cm-2, which causes an unique surface plasmon resonance (SPR) absorption peak at about 516 nm. Subsequent annealing in oxygen atmosphere results in the decomposition of Cu-Zn alloy NPs, at 450 ℃, and thus, ZnO and Cu NPs appear in the substrate. Further increase of annealing temperature to 550 ℃ could transform all the Zn and Cu into ZnO and CuO. Moreover, results also demonstrate that introduction of Zn into SiO2 substrate could effectively suppress the oxidation of Cu NPs, meanwhile, the existence of Cu could promote thermal diffusion of Zn towards substrate surface, which enhances the oxidation of Zn. The underlying mechanism has been discussed.
2014, 63 (7): 078701. doi: 10.7498/aps.63.078701
In this paper, we derive the response function of angle signal in a two-dimensional X-ray grating interferometry system under the condition of parallel coherent light, and depict the surface of the function with Matlab.Although there are four kinds of commonly used beam splitter gratings and three kinds of analyzer gratings, and there are still different compound modes between them, we may find that the ultimate surface of the response function of angle signal can only be of three kinds: the peak type, the valley type, and the peak-valley symmetry type of shifting surfaces. As there is a numerical complementary ralationship between the peak type and the valley type of shifting surfaces, we can take the two kinds as one; and finally we only need to consider two kinds of shifting surface.This conclusion simplies the common understanding of the two-dimensional X-ray grating interferometry method, and lays the foundation for the research of quantitatively extracting the two-demensional signal in the future.
2014, 63 (7): 078702. doi: 10.7498/aps.63.078702
In this paper, a method that uses magnetic extreme signals for equivalent source reconstruction is presented. Through simulation of specific current dipoles given as the sources of magnetic field signals, the feasibility of a multi-chamber heart model is investigated and the accuracy analysis of equivalent source reconstruction in inhomogeneous media is conducted. The magnitude of the magnetic extreme signals is indicative of the influence of volume conductor on the cardiac magnetic field is analyzed. The method is compared with other four methods which are the method of magnetic gradient extreme signals, the Nelder-Mead algorithm, the trust region reflective algorithm, and the particle swarm optimization algorithm against the criteria in terms of accuracy of source reconstruction and computation time of the algorithm. Results show that the method is practically useful for solving inverse cardiac magnetic field problems.
2014, 63 (7): 078703. doi: 10.7498/aps.63.078703
Mass segmentation plays an important role in computer-aided diagnosis (CAD) system. The segmentation result seriously affects classifying mass as benign and malignant. By combining the simplified pulse coupled neural network (SPCNN) and the improved vector active contour without edge (vector-CV), a novel method of mass segmentation in mammogram is proposed in this paper. First, the parameters and termination conditions of SPCNN are obtained through mathematical analysis and the initial contour is segmented by SPCNN. Then, the vector CV model is accordingly modified to overcome the shortcomings of traditional CV model. Finally, combined with the initial contour, the improved vector-CV is used to segment the mass contour. The experiments implemented on the public digital database for screening mammography (DDSM) and the clinical images which are provided by the Center of Breast Disease of Peking University People’s Hospital indicate that the proposed method is better than the existing methods, especially when dealing with the dense breasts of Oriental female.
2014, 63 (7): 078704. doi: 10.7498/aps.63.078704
We carried out a detailed analysis and a comparison between normal and epileptic electroencephalogram (EEG) based on multiscale permutation entropy. The relationship between multiscale permutation entropy values of EEG and age, and the effect of scale factor on multiscale permutation entropy value were also discussed. By analyzing normal and epileptic EEG based on multiscale permutation entropy, we found that, at the same age, multiscale permutation entropy value of the normal group’s EEG is higher than that of the epileptic group by an average of 0.19, about 7.9%. In addition, for people of age 3 to 35, their multiscale permutation entropies are clearly maximum. When scale factor is smaller than 15, the value of their entropy would reduce no matter whether the age increases or decreases. The results indicate that multiscale permutation entropy can distinguish between normal and epileptic EEG and reflect the general process of human brain development.
Effects of NPB anode buffer layer on the performances of inverted bulk heterojunction polymer solar cells
2014, 63 (7): 078801. doi: 10.7498/aps.63.078801
Inverted configuration bulk heterojunction polymer solar cells based on ITO/ZnO/P3HT:PCBM/NPB/Ag were fabricated, with the donor material being poly(3-hexylthiophene)(P3HT), and the acceptor material being [6, 6]-phenyl-C60-butyric acid methyl ester(PCBM). N, N’-diphenyl-N, N’-bis(1-naphthyl)-1, 1’-biphenyl-4, 4’-diamine(NPB) thin anode buffer layers with different thicknesses, which were used to improve the performances of the devices; and the effects of NPB anode buffer were investigated. The insertion of 1 nm thick NPB improves charge collection of the device, both of the short circuit current and open circuit voltage are enhanced. When the thickness of NPB reaches 25 nm, the series resistances are significantly increased, leading to reduced device performances. Effects of different thicknesses of NPB on charge injection and collection are investigated by capacitance-voltage measurements. NPB with 1 nm thickness improves charge collection of the device but without improving charge injection, and the charge recombination mechanism is dominant if the NPB layer is too thick. NPB thin layer with appropriate thickness could be used to enhance the performances of bulk heterojunction polymer solar cells.
Study on cascading invulnerability of multi-coupling-links coupled networks based on time-delay coupled map lattices model
2014, 63 (7): 078901. doi: 10.7498/aps.63.078901
The couplings among different networks facilitate their communications, while at the same time they also bring the risk of enhancing the wide spread of cascading failures to the coupled networks. Given that there is usually the time-delay during the spread of failures and more than one coupling link a node might possess, a cascading failure model for scale-free multi-coupling-link coupled networks is built in this paper, based on time-delay coupled map lattices (CML) model, which may be wider representative than previous models. Our research shows that in BA (Barabási-Albert) scale-free coupled networks, there is a threshold hT ≈ 3: when the coupling strength is bellow this threshold, the stronger coupling strength corresponds to a lower invulnerability; and vice versa, the stronger coupling strength would bring a higher invulnerability. In addition, our studies show that the presence of time-delay not only prolongs the failure spreading time during which measures can be taken to suppress cascading failures, but also has a significant influence on the eventual cascading size, for detail, if intra-layer time-delay τ1 and inter-layer time-delay τ2 can have any values, then the multiples of the two numbers will cause larger cascading size. We hope our research can provide a reference for building high-invulnerable coupled networks or the increase of the invulnerability of the coupled networks.
2014, 63 (7): 078902. doi: 10.7498/aps.63.078902
In this paper, a virtual trajectory planning method for vehicle lane changing in automated highway system is studied, and a trajectory model for lane changing on variable curvature road is established with odd-order polynomial constraints. Assuming that the starting lane and the target lane have the same instantaneous center, the motion for lane changing of vehicle on the curved road can be decomposed into a linear centripetal motion and a circular motion around the instantaneous centre of the curved road. If the centripetal motion displacement and the rotational angular displacement meet the requirement of odd-order polynomial constraints, the boundary condition of the above two kinds of motion may be obtained from the constraints, such as time, location, and desired state of vehicle at the start and end of the lane changing behavior. By applying the boundary conditions, the polynomial coefficient is deduced, and the mathematical model of virtual trajectory for lane changing can be designed based on the polynomial models of centripetal displacement and angular displacement. Compared with the existing trajectory planning method for lane changing on curved road, the curvature change has been taken into consideration, and the trajectory model for lane changing has been generalized. Simulation results verify the feasibility of the trajectory planning method proposed in this paper for lane changing on a curved road with variable curvature.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2014, 63 (7): 079201. doi: 10.7498/aps.63.079201
The ensemble variational data assimilation method may be subject to significant uncertainties due to the size of forecast ensemble. We found that this problem occurs because the analysis increment of this method is expressed as a linear combination of ensemble perturbation vectors or expansion of the orthogonal basis vectors. Though this method avoids introducing adjoint model while calculating the gradient of object function, the number of physical control variables is much larger than the sample size of forecast ensemble, which causes the assimilation results to be sensitive to the number of ensemble members. For this reason, the regional successive analysis scheme of ensemble variational method is proposed. By this scheme, the ratio between the number of physical control variables in analysis region and the sample size is decreased, so that it is expected that the problem can be solved. The results of numerical experiments using shallow water model show that the regional successive analysis scheme can give better assimilation results than traditional method, and the analysis precision is improved appreciably.
Relationship between the quasi-linear diffusion coefficients and the key parameters of spatial energetic electrons
2014, 63 (7): 079401. doi: 10.7498/aps.63.079401
It has been proved that the ground-based electromagnetic wave can transfer into ionosphere and interact with high-energy particles. By changing the pitch angle and momentum, the particles are imposed to enter the bounce loss cone and drift loss cone, then electron precipitation takes place and the particle bursts form. In recent decades, the relationship has been observed among electromagnetic disturbance and particle bursts and seismic activity based on satellite data. Here, by wave-particle cyclotron resonant interaction combined with the observation range of LEO satellite (about 350–1000 km), the evolvement trend of the pitch angle quasi-linear diffusion coefficients induced by field-aligned electromagnetic waves, is studied with the change of VLF electromagnetic wave frequency, band width, energies of electron (0.1–20 MeV) and L shell (L=1.1–3). We also show the relationship between VLF electromagnetic wave frequency and minimum energy of precipitation electron induced by it, under certain pitch angle value. The relationship among these quantities may be used to provide theoretical explanation for satellite observations of energetic particle precipitation examples, to provide guidance for extracting information associated with earthquakes from the detection of high-energy particles on the satellite, and to lay the foundation on the data analysis of China seismo-electromagnetic satellite planned to launch at about the end of 2016.