Vol. 63, No. 21 (2014)
2014-11-05
GENERAL
2014, 63 (21): 210201.
doi: 10.7498/aps.63.210201
Abstract +
A class of solar-forced recharge oscillator model for the El Niño /Southern Oscillation (ENSO) is considered. By transforming the ENSO model equations into the van der Pol-Duffing oscillator with periodic forcing, we obtain the Hopf bifurcation conditions by the harmonic balance method. Numerical simulations also show that the 11-yr solar-cycle forced ENSO system undergoes a transition from the quasi-periodicity route to chaos as the solar-cycle forcing is increased.
2014, 63 (21): 210301.
doi: 10.7498/aps.63.210301
Abstract +
Recognition of micro-motion characteristics has important research values in target detection field, which can be realized by employing the dynamic speckle technology. Based on the rough surface scattering theory, this paper studies the dynamic speckle time correlation function for rotating rough convex targets, and provides the dynamic speckle time correlation function for rotating cones. Comparison of the simulation results with those obtained under the experimental conditions confirms the validity of target time correlation length for cone targets. The angular velocity and line-of-sight angle for rotating cones are inversed by adopting particle group algorithm. Results show that this method is capable to identify the line-of-sight angle within 2090 and the angular velocity within 0.56 r/min for rotating cones, thus providing theoretical and experimental bases for recognition of micro-motion characteristics.
2014, 63 (21): 210701.
doi: 10.7498/aps.63.210701
Abstract +
This paper presents a digital image tamper detection and recovery algorithm, which uses the dither and chaos technology. Certification of generated image and the repair information gained by low frequency sub-diagram and dither technology after wavelet transform can effectively reduce the amount of data. At the same time, the watermark is embedded in high frequency sub-diagram, so as to make the watermark invisible. Chaos technology is used to complete watermark embedding and encryption; and combined with the Chinese remainder theorem, the impact on the image quality can further be reduced. Experimental results show that the algorithm's watermark has high invisibility and robustness, and also can repair tampered images. So it has a high practical significance in the image authentication and recovery.
2014, 63 (21): 210502.
doi: 10.7498/aps.63.210502
Abstract +
This paper presents a class of quadratic nonlinear system by introducing a linear term x of the third equation into the second equation of a chaotic system based on analyzing and studying some chaos. Using nonlinear dynamics method we analyze the steady, quasi-periodic and chaotic transition process when the system parameter varies. Experiment results are in good agreement with the Matlab simulation results. The Lyapunov exponent of the system with absolute value operation is larger than the original system, and the absolute value operation makes the wing of the original system doubled. Based on Takagi-Sugeno (T-S) fuzzy model and linear matrix inequality, a robust fuzzy controller is designed for the double-wing chaotic system being in asymptotical stability. Simulation results are provided to illustrate the effectiveness of the proposed scheme.
2014, 63 (21): 210501.
doi: 10.7498/aps.63.210501
Abstract +
In this paper, we study the stochastic resonance in a piecewise nonlinear system driven by a periodic signal and colored noises, which is described by multiplicative and additive colored noises with colored cross-correlation. Using the two-state theory and the unified colored approximation, we can derive the analytical expressions of the steady-state probability density and the signal-to-noise ratio (SNR). Effects of colored noises and the periodic signal on SNR are presented. It is found that the conventional stochastic resonance and bona-fide stochastic resonance may exist in this system. Moreover, the value of the SNR peak decreases with increasing correlation time and correlation between the additive and multiplicative noises.
2014, 63 (21): 210702.
doi: 10.7498/aps.63.210702
Abstract +
The coded aperture imaging spectrometer system based on the double-Amici-prism is designed in this paper, which includes the telescope objective, the coded template, the double-Amici-prism, the collimator lens and the imaging lens. This optical system can obtain a high diffraction efficiency. Compared to the imaging spectrometer system with a slit for this kind of system, the field of view is a two-dimensional spatial expansion, increasing the difficulty of design. For the subsequent data inversion algorithm, perfect imaging is needed, and based on this, the optical aberration of the system should be corrected carefully wildly. In this paper, we design and analyse the features of the imaging spectrometer system based on the double-Amici-prism, then the comple imaging spectrometer system. Telescope objective is designed to be telecentric, its MTF at 39 line pairs is 0.8, implying that good images can be obtained. Innovatively, an inverted telescope objective is used as a collimation system.The imaging spectrometer system's MTF at 39 line pairs is higher than 0.65. The imaging of the outdoor target scene, obtained by the coded aperture imaging spectrometer, proves that the design principle is feasible,the system is of high diffraction efficiency, the full-field imaging is of good quality, and the full spectrum of data is creditabe.
2014, 63 (21): 210203.
doi: 10.7498/aps.63.210203
Abstract +
In this paper, a corrected smoothed particle hydrodynamics (SPH) method is proposed to solve the problems of non-isothermal non-Newtonian viscous fluid. The proposed particle method is based on the corrected kernel derivative scheme under no kernel derivative and incompressible conditions, which possesses higher accuracy and better stability than the traditional SPH method. Meanwhile, a temperature-discretization scheme is deduced by the concept of SPH method for the purpose of precisely describing the evolutionary process of the temperature field. Reliability of the corrected SPH method for simulating the non-Newtonian viscous fluid flow is demonstrated by simulating the isothermal Poiseuille flow and the jet fluid of filling process; and the validity and accuracy of the proposed SPH discrete scheme in a temperature model for solving the non-isothermal fluid flow are tested by solving the non-isothermal Couette flow and 4:1 contraction flow. Subsequently, the proposed corrected SPH method combined with the SPH temperature-discretization scheme is tentatively extended to include the simulation of the non-isothermal non-Newtonian viscous free-surface flows in the ring-shaped and C-shaped cavities. Especially, the convergence of numerical simulations is analyzed, and the influences of heat flow parameters on the temperature and fluid flow at different positions are discussed.
2014, 63 (21): 210202.
doi: 10.7498/aps.63.210202
Abstract +
The following steps are given to search for new solutions to equations of sine-Gordon type. Step one, according to function transformation, the solving of sine-Gordon equation and sinh-Gordon equation is changed into the solving of two kinds of nonlinear ordinary differential equations. Step two, two kinds of nonlinear ordinary differential equations and quasi-Bcklund transformation of the first kind of elliptic equation are obtained. Finally, new infinite sequence solutions to equations of sine-Gordon type are constructed by applying Bcklund transformation and new solutions of the first kind of elliptic equation.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2014, 63 (21): 215201.
doi: 10.7498/aps.63.215201
Abstract +
According to the charging equation and charge conservation, the charging frequency (electric relaxation rate) is deduced and the formulation of charging electric current of the weakly ionized dust plasma is given according to the double Maxwellian distribution. It is shown that the directional speed have an influence on charging frequency; the charging frequency will decrease with the increase of directional speed; the charging frequency expression are the same as that in the literature when the directional speed is much smaller than the electron thermal velocity.
2014, 63 (21): 215202.
doi: 10.7498/aps.63.215202
Abstract +
Polarization smoothing is a technique for reducing speckle pattern contröst on a target by overlaying two uncorrelated speckle patterns with orthogonal polarizations, and it can reduce focal spot contröst by a factor of 1/√2. Improvement of focal spot contröst by using traditional polarization wedge for polarization smoothing is usually concentrated at some special spatial frequency and is lack of effect in physical experiments. To improve the spatial spectrum of polarization smoothing, a new method is proposed, in which two orthogonal polarization states are separated by the angle distribution differences of the beam direction angle and the uniaxial crystal optic axis; the angle can induce the optical phase differences between “o” and “e” light. Theoretical analysis and numerical simulation are carried out to analyze the new method. Results show that based on the viable control of beam random polarization state at near field, besides the reduction of the focal spot contröst by 1/√2, the new method can improve the whole spatial spectrum at the focal spot. The boundary conditions that continuous phase plate is used as the way to induce the been direction angle for polarization smoothing is obtained.
2014, 63 (21): 215203.
doi: 10.7498/aps.63.215203
Abstract +
It is important to diagnose electron density of a plasma irradiated by lasers for inertial confinement fusion, in high energy density physics and related fields, especially for measuring high-Z plasma near the interface. Use of soft X-ray laser as a probe is an important method in diagnosis of plasma electron density distribution. However, it is difficult to carry out the research in high-Z laser plasma, because of the problem of excessive plasma spontaneous radiation. In view of the characteristics of soft X-ray laser, several specific experimental techniques have been developed. By using these techniques, which can greatly suppress effects of spontaneous radiation, diagnosis of high-Z plasma with soft X-ray laser probe method becomes possible. As a typical example, an experiment of diagnosing gold plasma is performed and clear images are obtained, indicating that the techniques are effective and feasible.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2014, 63 (21): 219701.
doi: 10.7498/aps.63.219701
Abstract +
Since X-ray pulsar signals cannot be detected on the ground, the pulsar signals need be simulated using the ground experiments. In this paper,two new simulation methods to obtain the time of arrival are put forward. The first method is based on a statistical physics model, which should overcome the shortcomings of narrow adaptation, low speed, and low time resolution in normal simulation method. This method has a high time-resolution and rapid simulation for any random pulse signals. 23 Another one is for the optimization of statistical models that satisfy signals in non-homogeneous Poisson process. The is second method has an improved simulation speed at least 30 times higher than the common Poisson model, even achieves four orders of magnitude greater than that for low flow pulsars, and this method also achieves a nanosecond time resolution simulation.
EDITOR'S SUGGESTION
2014, 63 (21): 219101.
doi: 10.7498/aps.63.219101
Abstract +
The physical mechanism of earthquake remains a challenging issue to be clarified. Seismologists used to attribute shallow earthquake to the elastic rebound of crustal rocks. The seismic energy calculated following the elastic rebound theory and on the basis of experimental results of rocks, however, shows a large discrepancy with measurementa fact that has been dubbed the heat flow paradox. For the intermediate-focus and deep-focus earthquakes, both occurring in the region of the mantle, there is not any reasonable explanation yet. The current article will discuss the physical mechanism of earthquake from a new perspective, starting from the fact that both the crust and the mantle are discrete collective systems of matters with slow dynamics, as well as from the basic principles of physics, especially some new concepts of condensed matter physics emerging in recent years. 1. Strss distribution in earth's crust: Without taking the tectonic force into account, according to the rheological principle that everything flows, the vertical and the horizontal strsses must be in balance due to the effect of gravitational pressure over a long period of time, thus no differential strss in the original crustal rocks is to be expected. The tectonic force is successively transferred and accumulated via stick-slip motions of rocky blocks to squeeze the fault gouges, and then applied to other rocky blocks. The superposition of such additional horizontal tectonic force and the original strss gives rise to the real-time strss in crustal rocks. The mechanical characteristics of fault gouge are different from rocks as it consists of granular matters. Thus the elastic modulus of the fault gouge is much lower than that of rocks, and will become larger with increasing pressure. This character of the fault gouge leads to a tectonic force that increases with depth in a nonlinear fashion. The distribution and variation of tectonic strss in the crust are then specified. 2. Strength of crust rocks: The gravitational pressure can initiate the transition from elasticity to plasticity in crust rocks. A method for calculating the depth dependence of elasticity-plasticity transition is formulated, and demonstrated by exemplar systems. According to the actual situation analysis the behaviors of crust rocks fall into three typical zones: elastic, partially plastic and fully plastic. As the proportion of plastic parts in the partially plastic zone reaches about 10%, plastic interconnection may occur and the variation of shear strength of rocks is mainly characterized by plastic behavior. The equivalent coefficient of friction for the plastic slip is smaller by an order of magnitude, or even less, than that for brittle fracture, thus the shear strength of the rocks for plastic sliding is much less than that for brittle breaking. Moreover, with increasing depth a number of other factors can further reduce the shear yield strength of rocks. On the other hand, since earthquake is a large-scale damage, the rock breaking must occur along a weakest path. Therefore, the actual fracture strength of rocks in a shallow earthquake is assuredly lower than the normally observed average shear strength of rocks. The typical distributions of averaged strength and actual fracture strength in crustal rocks varying with depth are schematically illustrated in the paper. 3. Conditions and mechanisms of earthquake: An earthquake will lead to large volume expansion, and the expansion must break through the obstacles. The condition for an earthquake to occur may be as follows: the tectonic force should exceed the sum of (a) the fracture strength of rocks, (b) the friction force of fault boundary, and (c) the resistance from obstacles. Therefore, the shallow earthquake is characterized by plastic sliding of rocks that break through the obstacles. Accordingly, four possible patterns for shallow earthquakes are put forward. Deep-focus earthquakes are believed to result from a wide-range rock flow that breaks the jam. Both shallow earthquakes and deep-focus earthquakes are the slip or flow of rocks following a jamming-unjamming transition. 4. Energetics and precursors of earthquake: The energy of earthquake is the kinetic energy released from the jamming-unjamming transition. Calculation shows that the kinetic energy of seismic rock sliding is comparable to the total work for rocks' shear failure and for overcoming the frictional resistance. There will be no heat flow paradox. More importantly, some valuable seismic precursors are likely to be identified by observing the accumulation of additional tectonic forces, local geological changes, as well as the effect of rock state changes, etc.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2014, 63 (21): 214103.
doi: 10.7498/aps.63.214103
Abstract +
An analytical formulation has been developed for the electromagnetic leakage from an apertured rectangular cavity excited internally by an electric dipole. The leakage fields are represented by the equivalent electric and magnetic dipoles located at the aperture center with their dipole moments related to the “closed cavity” field within the framework of the Bethe's small aperture coupling theory. The “closed cavity” field is obtained by using the mode-expansion method. In this formulation, the leakage field can be expressed as a function of the frequency, the source point, the field point, and the position of the aperture. The formulation then is employed to analyze the influences of the above factors on the shielding effectiveness and the corresponding physical mechanisms are also illuminated. Comparison with the full wave simulation software CST has verified the formulation over a very broad frequency range. It is shown that the near-field shielding effectiveness is smaller than the far-field one, and the electric shielding effectiveness is different from the magnetic one in the near-field zone.
2014, 63 (21): 214102.
doi: 10.7498/aps.63.214102
Abstract +
Based on terahertz time-domain spectrscopy (THz-TDS) technology, a broad-band time domain terahertz radar system can be used to do research on scattering characteristics of objects. At present, the optical structure and mechanism of this system-showing the radar detection principle and imaging mechanism-has attracted a lot of interest in the terahertz research field. Based on the femtosecond Ti: sapphire osillators pumped terahertz time-domain spectrscopy system, this paper constructs the first terahertz radar system in this country (0.11.3 THz). System calibration is carried out by measurements on standard metallic spheres. Three military scale models are measured by the Radar system. The shapes of the models are retrieved by the improved back projection algorithm, which verifies the new imaging mechanism based on time domain scattering signal. With high frequency broad band spectrum and high imaging resolution, the terahertz radar would make great contribution to stealth military units design and become a new research platform on terahertz scattering characteristics.
2014, 63 (21): 214201.
doi: 10.7498/aps.63.214201
Abstract +
These years, with the development of material fabrication technology, some new synthetic materials that have unique electromagnetic properties have come out, one of which, the gradient meta-surface constructed into a special topological structure, gains rising popularity among experts. This paper establishes a refractive index gradient meta-surface and discusses its physical mechanism in optics. Normal reflection, anomalous reflection, and transformation from plane wave to medium wave can be realized by using this meta-surface. If the refractive index is larger than some threshold, the sine value of the reflection angle will vary linearly with that of the incidence angle, implying that parallel incident waves will become parallel emergent waves. If refraction index gradient is larger than some threshold, the plane wave at any incidence angle will give out a total reflection inside the medium, in other words, it is bound within the medium. Therefore the control of direction and intensity of beams can be achieved by adjusting parameters, providing a new engineering implementation method of artificial controllable full-medium gradient meta-surface.
2014, 63 (21): 214202.
doi: 10.7498/aps.63.214202
Abstract +
The properties of off-center Gaussian vortex beams focused by a high numerical aperture are investigated on the basis of vector Debye integral. A complex amplitude of off-center Gaussian vortex beams through a high numerical aperture objective is derived and numerical calculation is performed to analyze the intensity and phase distributions of the beams in the focal plane. It is shown that the intensity and phase distributions change significantly with the variation of off-axial distance; the intensity distribution in the focal plane is different in the direction of y-axis which is enhanced with the increase of off-axial distances; the sign of off-axial distance determines the direction in which the intensity is concentrated. On the other hand, different from the 1st order off-axial vortex beam, high-order off-axial vortex will split in the tightly focused field. Multiple phase singularities will appear and the number of singularities is equal to the topological charge of the original beam. Besides, the split singularities are symmetric, obviously. It is found that the splitting of high-order vortex is due to the tight focusing.
2014, 63 (21): 214206.
doi: 10.7498/aps.63.214206
Abstract +
We report a wave-locked 878.6 nm diode-laser-pumped multi-segmented Nd:YVO4 laser operating at 1064 nm, which is compared with the high doping concentration and the low doping concentration monolithic Nd:YVO4 lasers. Experimental results show that the configuration of the wave-locked 878.6 nm diode-laser-pumped multi-segmented crystals not only can reduce thermal effects of the laser but also can improve the optical-to-optical conversion efficiency. We have achieved an output power of 28.2 W at 1064 nm with an incidence pump power of 40 W, corresponding to the optical-to-optical efficiency of 70.5%, slope efficiency of 70.6%. For absorbed pump power, the optical-to-optical efficiency is 76% and the slope efficiency is 76.4%. The laser also has an excellent output stability while the temperature is varied from 10 ℃ to 40 ℃.
2014, 63 (21): 214601.
doi: 10.7498/aps.63.214601
Abstract +
Profile of a dark hollow beam in sub-diffraction -limit imaging is of crucial importance for its spatial resolution when using the coherent anti-Stokes Raman scattering microscopy, as far as the imaging quality is concerned. Therefore, the generation of dark hollow beams through a vortex phase plate will be theoretically analyzed based on the Fresnel diffraction theory. Influences of different incidence conditions on the intensity distribution of the generated dark hollow beams are also investigated. And it is shown that a perfect dark hollow beam could be produced when a Gaussian beam is vertically incident upon a first-order vortex phase plate, with the incident light wavelength equal to that of the phase plate. However, both the circular symmetry of the incident beam's intensity distribution and the alignment between the centers of Gaussian beam and phase plate may affect the intensity distribution of the dark hollow beam, which will almost be in circular symmetry though it may shift some distance from the image center when at a small incident angle. Furthermore, the dark hollow beam's intensity distribution will scarcely change when the central wavelength deviation is very small from the incidence light and the phase plate. These results may be of great value in generation of perfect dark hollow beams in sub-diffraction –limit imaging by coherent anti-Stokes Raman scattering microscopy.
2014, 63 (21): 214204.
doi: 10.7498/aps.63.214204
Abstract +
Oxygen A-band is an ideal inversion channel. Absorption coefficient is one of the important parameters, its precision determines the accuracy of inversion result. The influence factor for the absorption of oxygen A-band is analyzed using HITRAN2012 database and temperature profiles of atmosphere. The temperature dependence is deduced for each influence factor, and then for the absorption coefficient. It is found that the influence of temperature on the coefficient is poor for HWHM (half width at half maximum) of the spectral line, but the HWHM of the line is greatly influenced by the temperature. The linetype function has two changes during the variation of temperature: The function value decreases with increasing temperature beyond the HWHM; it, however, slowly increases from the center frequency to HWHM of the line. The line intensity is strongly dependent on the temperature. Using the line by line integral algorithm, the absorption of oxygen A-band is calculated. The temperature dependences are considered to come from the pressure broadening effect, spectral line intensity, and HWHM. A conclusion is given that the temperature dependence of absorption of oxygen A-band comes from line intensity, and especially the center frequency. While, the temperature dependence of the linetype function with Lorentzian is not obvious. Finally, the absorption of oxygen A-band is measured at 63m using BRUKER spectrometer with 1 cm-1. The error is less than 0.83% as compared with that in theoretical model under the same condition. The correctness of the temperature calibration model is thus verified.
2014, 63 (21): 214205.
doi: 10.7498/aps.63.214205
Abstract +
The distribution of population inversion has been obtained by solving the rate equations of a Tm and Ho codoped laser system. A giant pulse laser output of 88.4 mJ with pulse duration of 426 ns has been predicted at 0.1% duty cycle of LD pump source. In a 2 m ring resonator, the Tm, Ho:LuLF laser material has been side-pumped from three directions and Q-switched for acoustic-optic device. The slope efficiency is 6.36% in free running mode and 2.9% in Q-switching mode. When 3.25 J pump energy in 1 Hz repetition frequency is injected, it has been demonstrated that the laser pulse energy of 103.2 mJ with an optical-to-optical efficiency of 3.17% in the free running mode and 30.3 mJ with 0.93% optical-to-optical efficiency. When 3.5 J pump energy is injected, it has been obtained that the pulse energies of 129.3 mJ and 35.9 mJ in the two modes with optical-to-optical efficiencies of 3.69% and 1.02% respectively. The greatest dynamic to static ratio is 32.8%. The pulse duration is 417.2 ns when 3.25J pump energy is injected. If the ring resonator operates unidirectionally, the laser energy and the corresponding optical efficiency will be doubled, the result being in agreement with the theoretical analysis of rate equations.
2014, 63 (21): 214207.
doi: 10.7498/aps.63.214207
Abstract +
Under the condition that the light pulses meet the slowly varying function pulses, the higher-order nonlinear Schrödinger equation has been deduced by taking into consideration the Raman gain. The linear operator and nonlinear operator specific expressions are obtained using split-step Fourier numerical method. The Raman gain on the self-steepening of the Gaussian pulse has been simulated and then the result is compared with the self-steepening effect without taking into consideration the Raman gain when the pulse propagate in the isotropic optical fiber. Raman gain specific impact on the self-steepening of the Gaussian pulse has been obtained under different conditions. Results show that the Raman gain may affect the Gaussian pulse broadening, pulse peak attenuation as well as the oscillation of the edge. These influences depend on the parameters of self-steepening, input power, and dispersion coefficient.
2014, 63 (21): 214101.
doi: 10.7498/aps.63.214101
Abstract +
This paper utilizes a kinetic theory model to gain the accurate electromagnetic (EM) wave characteristics in the plasma region, based on the solution of Maxwell-Boltzmann (MB) equation system. The system is solved by finite-difference time-domain (FDTD) algorithm, which gives the results of the electric field intensity and particle velocity distribution function. Furthermore, the validity and effectiveness of the proposed method is verified by comparing the results of reflection coefficient and transmission coefficient of the wave that impinges on plasma plate region with that of the analytic solution.
2014, 63 (21): 214401.
doi: 10.7498/aps.63.214401
Abstract +
Saturated fluid convective heat transfer in ordered three-dimensional porous media has been investigated numerically using a CFD software. Spherical particles with diameters dp equal to respectively 14, 9.4, 7 mm have orderly arrangements constituting the porous media skeleton. There is a copper plate of constant heat flux density above the porous skeleton. The distribution of temperature and local convective heat transfer coefficient in the channel, as well as factors affecting the convective heat transfer are investigated by the method of fluid-solid coupling. Results show that the thickness of the thermal penetration and the thermal boundary layer increase in the flow direction, and decrease with the flow rate. When the thermal conductivity of the skeleton is relatively high, the convective heat transfer increases slightly with decreasing particle diameter. The convective heat transfer coefficient decreases with the increase in mass concentration of polyacrylamide solution, because the viscous dissipation weakens convection heat transfer.
2014, 63 (21): 214701.
doi: 10.7498/aps.63.214701
Abstract +
Under the steady condition, the spherical geostrophic wind relationship in spherical coordinates (λ,φ,r) is applied to illustrate that when isobars take the simplest zonal distribution, the Antarctic is at a low ambient pressure, while the Arctic is at high. However, when friction is incorporated into the geostrophic wind relationship, the closed vortex over Antarctic and Arctic will turn to a spiral vortex, and there will exist a three-dimensional heteroclinic orbit between Antarctic and Arctic on the spherical surface.
2014, 63 (21): 214104.
doi: 10.7498/aps.63.214104
Abstract +
The traditional method of calculating electromagnetic scatterings from the rough sea surface, which does not need a geometrical sample of sea surface, considers mainly the total scattering echoes by average assembling of different incident parameters. With the development of synthetic aperture radar and radar imaging, it is important to fully describe the sea surface at every point, and to obtain the result of electromagnetic scatterings from the facets of the rough sea surface. Meanwhile, with increasing wind speed above the sea surface, the influence of the sea foam layer on the scatterings will increase. In this paper, the facet-model is considered, and taking into account the impact of the foam layer on the scatterings at a large angle incidence, the backscattering coefficient is calculated for different wind speeds; and the result of numerical simulation is compared with the experimental data to verify the accuracy of our method.
2014, 63 (21): 214301.
doi: 10.7498/aps.63.214301
Abstract +
Acoustic scattering from the submerged finite periodically ribbed two concentric cylindrical shells, insonified by an incident plane wave, is studied. Motion of the shell is described by Donnell equations, while the motion of the disks is described by thin plate motion equations and plane ströss equations ignoring the effects of axial forces on the disks. Through angle-frequency spectra, besides the elastic waves of the shells, the contributions of the fluid between the inner and outer shells and the interaction of the inner and outer shells with the disks play an important role in the far scattering field. There are fine echo waves such as Bragg scattering and fluid-added waves excited on the angle-frequency spectra. Finally the theories are verified by experiments.
2014, 63 (21): 214302.
doi: 10.7498/aps.63.214302
Abstract +
The mode coupling and energy transfer are studied by considering the influences of variation in topography on sound energy transmission and structures of interference in a range-dependent waveguide. A larger level-stepped coupled mode model and a three-dimensional coupled mode model for the wedge bottom are obtained such that the mode coupling and energy transfer may be analyzed efficiently and rapidly. According to the coupled mode models, the transfer of energy is expounded for the forward pressure field in the waveguide with varying topography. Meanwhile, the mechanism is explained by the ray-mode theory for variation of energy distribution caused by variation of topography. Numerical simulations show that the coupling between normal modes and the energy transfer may occur remarkably when the imaginary parts of eigenvalues take on a huge modification, and the propagation direction of sound field will be changed to the increasing direction of sea depth due to variation of topography. In the energy transfer and the modification of propagation direction, the energy of sound field tends to remain in the waveguide, rather than to leak to the seafloor. Meanwhile, the energy distribution will be affected by the compression or sparseness so that interference structures such as ellipse, will be produced.
2014, 63 (21): 214303.
doi: 10.7498/aps.63.214303
Abstract +
Matched-field replica vector should be calculated using parabolic equation in a range-dependent waveguide, this means that the matched-field localization is too computationally intensive, hence its engineering application is seriously hindered. A virtual time-reversal method for passive source localization for a range-dependent waveguide is presented. The number of parabolic equation computational grids in virtual time-reversal method is much smaller than that in matched-field processing for a range-dependent waveguide. Thus, the computational cost can be greatly reduced. Different from the matched-field processing, the virtual time-reversal method is a back-propagation process, which explores and utilizes the properties of reciprocity and superposition. It can be realized by weighting the replica surface with the complex conjugate of the data received on the corresponding element, followed by a summation of the processed received data. This performance of virtual time-reversal method for source localization is validated through numerical simulations and data from the Mediterranean Sea.
2014, 63 (21): 214304.
doi: 10.7498/aps.63.214304
Abstract +
In engineering applications, many measurements of physical quantities can be converted into the problems of frequency estimation. The current frequency estimators are mainly divided into two categories: iterative approaches and direct approaches. However, iterative approaches are not suitable for rapid physical measurement occasions due to its complicated process. But most of the direct approaches are the biased estimators, which are incapable of providing quantitative estimates of variance expression. To enhance the accuracy of the direct frequency estimator and derive the closed-form theoretic expression of the estimated error variance, this paper proposes a novel phase difference frequency estimator based on the forward and backward sub-segmenting. This estimator implements forward and backward fast Fourier transform (FFT) on the given samples separately, and then extracts the phase difference from the peak FFT bins of these two sub-segments to estimate the frequency. And it is emphasized that the proposed method is an unbiased frequency estimator, whose closed-form theoretic expression of the frequency estimated error variance is also derived. Moreover, simulation not only verifies the correctness of this closed-form expression but also proves that the proposed frequency estimator's mean square error is closer to the Cramer-Rao lower bound than that of apFFT/FFT phase difference estimator and Candan estimator. In conclusion, the proposed estimator has higher accuracy in measuring frequencies and has a wide application prospect.
2014, 63 (21): 214602.
doi: 10.7498/aps.63.214602
Abstract +
Granular coal is a kind of multilayer and structured solid matter, which consists of a large assemblage of individual granular solids and has dual characters of coal and granular materials. Its crack propagation law can be studied using the coal mechanics and multi-scale characteristics. First of all, the linear elastic material may be damaged under compression; and the cause of crack propagation is explained from the viewpoint of energy that the dynamics of crack propagation is due to the release of strain energy. And then, further study on the granular coal crack propagation under uniaxial compression from macroscopic and microscopic raspects is carried out by physical experiment and numerical simulation. Results show that a cone is formed at the bottom of the heap, the crack propagation is slowed down with the decrease of coal particle size, mutation points in crack propagation will appear, and the crack will not be smooth after the granular coal is damaged completely. Besides, due to the influence of granular coal particle size, uniformity has an important effect on crack propagation. The greater the homogeneity coefficient, the later the fractures occur. The acoustic emission energy release becomes frequent and severe in the mild, moderate and deep stages of crack propagation. These results will be conducive to further study on the law of geotechnical granular materials crack propagation under compression.
2014, 63 (21): 214703.
doi: 10.7498/aps.63.214703
Abstract +
In this paper, the microscale non-equilibrium gas flow, and the oscillating Couette and Poiseuille flows, have been investigated by an effective MRT-LBM. The Knudsen layer model is introduced into lattice Boltzmann method (LBM) for the relaxation time correction. In the simulations the plate or external force oscillates in the form of sine curve, and the Couette flow contains a singular oscillation and a double-plate oscillation. It is revealed that the corrected MRT-LBM model can well handle the simulation of microscale non-equilibrium gas flow. For the Couette flow, the wall slip phenomenon is obvious for a larger Kn number, and the streamwise velocity profiles appear to be of a nonliner character when St number increases. When the two plates oscillate, the streamwise velocity profiles almost overlap with each other at small Kn and St. In the Poiseuille flow case, the extent of phase lag decreases as St exceeds a certain value. Compared to the Kn number, St has a bigger impact on the emerging of phase lag in the oscillating Couette and Poiseuille flows.
2014, 63 (21): 214704.
doi: 10.7498/aps.63.214704
Abstract +
This paper investigates the capillary flows in variable interior corners along the axis when a microgravity environment and the Concus-Finn condition are satisfied. The governing equation for capillary-driven flows in variable interior corners is established, and an approximately analytic solution is obtained. Then we compare the approximately analytic solution with the numerical simulation results obtained using the software FLOW-3D. Results show that the relative error between the approximately analytical and numerical solutions is getting smaller and smaller with increasing time, and it will be less than 5% after 6 s. The influence of different parameters on the interior corner flow is studied using a set of typical parameters. Numerical results show that the liquid-front position and meniscus height decrease with increasing interior angles, contact angle, slope, and power exponent. The liquid meniscus height increases with the duration at different times. But it remains constant at the initial time. The conclusion of this paper can be applied when designing containers and choosing the suitable solution in the space fluid management.
2014, 63 (21): 214706.
doi: 10.7498/aps.63.214706
Abstract +
Fluid flow and heat transfer in a microstructure may depart from the traditional behavior due to the scale effect, and its velocity slip and temperature jump will occur at the fluid-solid interface. A molecular dynamics model of coupled fluid flow and heat transfer in rough nanochannels is developed to investigate the effect of surface roughness on nanoscale fluid flow and heat transfer, as well as velocity slip and temperature jump at the fluid-solid interface. The fluid microscopic structure, velocity and temperature distributions, interfacial velocity slip and temperature jump in a rough nanochannel are evaluated and compared with the corresponding smooth nanochannel. Effects of solid-liquid interaction and wall stiffness on the velocity slip and temperature jump are analyzed. Results indicate that the velocity of the fluid flow under an external force in a nanochannel in a bulk region is of a parabolic distribution, and the viscous dissipation due to shear flow induces the fourth-order temperature profile in the nanochannel. And the velocity slip and temperature jump will occur at the fluid-solid interface. The presence of roughness may introduce an extra viscous dissipation in shear flow, leading to a reduction of overall velocity and an increase in temperature in the nanochannel when compared with the smooth nanochannel. In addition, the degree of velocity slip and temperature jump at a rough liquid-solid interface is smaller than that at a smooth interface. In particular, the increase in fluid-solid interaction strength and reduction in wall stiffness will lead to a small velocity slip and temperature jump.
2014, 63 (21): 214702.
doi: 10.7498/aps.63.214702
Abstract +
Based on the immersed boundary concept that the border may be constructed by feedback force, a numerical simulation is carried out by modifying previous inner fluid treatment and incorporating it with non-equidistant grid. Flow around two elongated rectangles in tandem arrangement is computed in the range of Reynolds numbers from 200 to 103. Results indicate that when the Reynolds number is in the range 200–300, a vortex shedding of front rectangle is under control of two separated shear layers. The vortex between the two rectangles belongs to Karman type, which is hindered by small spacings thus symmetric vortices are formed. Shielding effects is mainly reflected by the phenomenon that mean drag coefficients of the rear rectangle is smaller than the front one. At the critical spacing ratio, a vortex sheet between the two rectangles is fully established. The mean drag coefficient also has a jump at this spacing ratio, which is still less than that of the front rectangle. In this phase, as Reynolds number increases, the vortex regime, the jump and the critical spacing all become minimized. When Re=400, the vortex shedding of front rectangle is characterized by an impinging-shear-layer, and thw drag coefficient is no longer a regular oscillation. After that as Reynolds number rises, an impinging-shear-layer is established gradually. More vortices on the surface are produced by flow separation of the front rectangle, which leads to a less magnitude of wake vortex. Shielding effect will disappear at this time. A fluctuation impact on the rear rectangle is induced by drastic vortex shedding from the front rectangle. But proper spacing between the two rectangles can make the drag coefficient of the rear rectangle jump, which is larger than that of the front rectangle.
2014, 63 (21): 214705.
doi: 10.7498/aps.63.214705
Abstract +
We have studied the effective diffusion coefficient of permeable particles with different radii at different permeability and volume fractions by using the numerical simulation results of short-time diffusion dynamics of permeable particles in concentrated suspensions and the combination of Cohen-de Schepper and Percus-Yevick approximations. As a result, the diffusivity of particles having the same radius will increase monotonically with increasing permeability to a certain volume fraction, and decrease linearly with increasing volume fraction to a certain permeability. While the effect of particle radius on the measured effective diffusion coefficients for the permeable particles with larger wave-numbers at the same ratio of particle radius to the hydrodynamic penetration depth may be neglected.
2014, 63 (21): 214203.
doi: 10.7498/aps.63.214203
Abstract +
We demonstrate an efficient tunable phase-stabilized near-infrared optical parametric amplifier (OPA) made from a BBO (β -BaB2O4) single crystal in this paper. By using the seeded white-light continuum produced by CEP(carrier envelop phase)-stabilized femtosecond laser amplifier system which is seeded into the two stages of a type II OPA system, the pump-to-signal conversion efficiency of 34% can be achieved at 1350 nm. The CEP jitter of amplified pulses measured by an f–2f interferometer is 137 mrad in 30 minutes. This paper demonstrates a simple, feasible and efficient way to produce tunable femtosecond pulses with CEP control.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
EDITOR'S SUGGESTION
2014, 63 (21): 217202.
doi: 10.7498/aps.63.217202
Abstract +
Considering Rashba spin orbit interaction and spin quantum transport in the spin field effect transistor, we study the influence of the barrier strength on the spin coherence transport in spin field effect transistors. It is found that when the barrier strength is weak, the tunneling junction conductance exhibits oscillatory phenomenon obviously with increasing Rashba spin orbit interaction strength. The conductance exhibites barrier-dependent conductive switching effect as the barrier strength increases. When the barrier strength gradually increases, parallel conductance exhibits a monotonicall decreasing trend, while the anti-parallel conductance fluctuates, and such a fluctuation leading to the tunneling magnetoresistance also exhibits oscillatory phenomenon with the variation of barrier strength . For a suitable thickness of quasi one-dimensional electron gas, the tunneling magnetoresistance value can produce positive and negative inversion, and the effect will shed light on the application of spin information storage electronic device.
2014, 63 (21): 217201.
doi: 10.7498/aps.63.217201
Abstract +
The freewheeling diode in power electronic converters may generate a voltage peak on the load during the reverse recovery process, and the peak voltage becomes larger when the forward conduction time is smaller, which very likely induces the over-voltage failure of the power devices. To effectively guide the reliability design of power electronic devices, the switching transition mechanism of the PIN freewheeling diode is discussed thoroughly based on semiconductor physics and the essential structure of power diodes. The law of reverse recovery voltage peak variation with switching transition time is deduced by methods of stored charge analysis, which shows that the peak voltage is larger for shorter conduction time and decreases abruptly as the transient conduction time increases. Experiments are carried out using the two-level half-bridge inverter unit with insulated-gate bipolar transistors and PIN diodes. Results show that the reverse recovery voltage peak decreases with the increase of the transition time, following an exponential rule, and tends to be constant after the freewheeling current becomes stable and finally approaches a steady state as the steady forward conduction current vanishes, thus proving the correctness of the presented analysis. This paper shows the theoretical and application values in the optimization of the reverse recovery mechanism of power diodes and the reliability improvement of power converters.
EDITOR'S SUGGESTION
2014, 63 (21): 217804.
doi: 10.7498/aps.63.217804
Abstract +
Positron annihilation lifetime spectroscopy, which takes positron lifetime as a detected object, has been widely applied to the study on micro-defects of semiconductors and other materials, and is very sensitive to the type of crystal structure, defect types, and temperature, and so on. Therefore, the analysis of fast and accurate calculation of positron lifetime theoretically combined with the experimental data is particularly necessary. In this paper, the superposed neutral atom model, the pseudo-potential model, and the full-potential model are used to deal with the positron local potential. While the positron wave function is solved self-consistently by the finite difference method, the positron-electron correlation potential and its enhancement factor are handled within the frame work of the local density approximation and the generalized gradient approximation. We have respectively calculated the positron bulk lifetime of three kinds of single crystal solid: the alpha iron of a body-centered cubic structure, the aluminum of a face-centered cubic structure, and the silicon of a double face-centered cubic structure. Calculation results agree well with the published experimental data. At the same time, the impact on positron bulk lifetime due to electron density grid point accuracy, positron-electron correlation potential and enhancement factor is analyzed carefully. Finally, we discuss the advantages and disadvantages of the three methods for calculating the positron bulk lifetime. In summary, an effective and reasonable calculation for the positron bulk lifetime should take into account the electron density, positron-electron correlation potential, and enhancement factor, etc. especially the enhancement factor.
2014, 63 (21): 217801.
doi: 10.7498/aps.63.217801
Abstract +
The organic phosphorescent OLED (PhOLED) has been widely studied because its inner quantum efficiency can reach 100%, but there has been much debate about the internal luminescence mechanism and process, mainly because they are explained using the luminescence theory of inorganic LED. In this paper, we set up a transient electroluminescence (EL) and delay EL measurement system, and for the first time asfar as we know use this measurement system to study the internal luminescence mechanism and process of PhOLED. In these studies we first fabricate a PhOLED which uses a kind of new efficient red-emitting iridium(III) complexes (Bis[2-(9, 9-dimethyl-9H-flouren-2-yl) benzothiazolato-N, C2'] iridium(III) (acetylacetonate)) doped with TAZ as the emitting layer. From the results, we find that there exists an overshoot at the end edge of the driving pulse; through the research we find that this is reasonable for the holes and electrons to accumulate in the object materials Irf and host TAZ, respectively. We also find that at the interface between host transfer layer and emission layer there exist a large number of holes. Through the delay luminescence measurement, we have proved that the emission of this doped system mainly comes from the directly trapped holes and electrons in Irf, and then excitons are formed.
2014, 63 (21): 217101.
doi: 10.7498/aps.63.217101
Abstract +
Structure and stability of the Li-adsorbed silicene under a biaxial strain are studied by using the first-principles plane-wave pseudopotential method based on the density functional theory. Results show that Li-adsorbed silicenes keep their original configurations basically when the tensile and certain compressive strains are applied, while the silicene plane bulges towards the Li atom when a larger compressive strain is applied, and the total energy of the corresponding system becomes distinctly lower. We also calculate the phonon spectra of the silicene under various strains, and analyze the reason why the Li-adsorbed silicene is unstable under the compressive strain.
2014, 63 (21): 217802.
doi: 10.7498/aps.63.217802
Abstract +
Molybdenum disulphide (MoS2), a layered quasi-two dimensional (2D) chalcogenide material, is a subject of intense research because of its electronic, optical, mechanical and physicochemical properties. Since the monolayer MoS2 is a direct-gap seminconductor, it is widely used in the field of light-emitting area. However, its photoluminescence (PL) efficiency is very low due to excessive doping in monolayer MoS2 and rich non-radiative centers. In this letter, we reportits synthesis using the gold nanoparticles which have a resonance absorption peak around 514 nm. The gold nanoparticles are dispersed on the surface of the MoS2 samples by means of spin-coating. Then, we measure the photoluminescence (PL) of the monolayer, bilayer and multilayer samples before and after the spin-coating, and find a great enhancement in the PL intensity of the monolayer sample. Also the PL intensities of bi-layer and multiple layer MoS2 samples are slightly enhanced. Our work shows that gold nanoparticles may impose an obvious p-doping effect to the monolayer and bi-layer MoS2 samples to enhance the PL, and a surface plasmon polariton effect of the gold nanoparticles is also a positive factor for the enhancement.
2014, 63 (21): 217803.
doi: 10.7498/aps.63.217803
Abstract +
Nano-silicon carbide/silicone rubber composites with nonlinear conductivity characteristics have been made by mixing nano-sized silicon carbide (SiC) into silicone rubber. In this paper, the nonlinear conductivity characteristics of composites made by mixing 5 wt%, 15 wt%, 30 wt% and 45 wt% of nano-SiC and silicone rubber are investigated. The relationship between conductivity and electric field strength is obtained by experiments. Nonlinear conductivity mechanisms of the composites are analyzed, and their breakdown properties and dielectric spectröscopy characteristics of composites are studied. In order to know the effect of nonlinear SiC/silicone rubber composites on homogenizing electric field, the electric ströss distribution of cables termination and composite insulators are simulated by the COMSOL Multiphysics software. Results show that the maximum electric field strength at cable termination and the end of the composite insulators decreases efficiently when the nano-SiC/silicone rubber composites are used.
2014, 63 (21): 217805.
doi: 10.7498/aps.63.217805
Abstract +
Cooperative luminescence, down-conversion laser oscillation and cascaded Raman laser in Yb3+-doped 55.93P2O5-3.57Al2O3-15Na2CO3-20SiO2 phosphosilicate glass are studied in the biconical fiber-microsphere coupling system in this paper. A single-mode semiconducter laser with the center wavelength at 976 nm and line-width 0.15 nm is used as a pump source. The blue-shifted cooperative luminescence centered at 476.1 nm is obtained in the Yb3+-doped phosphosilicate microsphere. And a suitable model may be applied to explain the reason for the 11.9 nm blue-shift in this process. Meanwhile, the single-mode laser (at 1058.26 nm) and multimode laser oscillations (from 1060.02 to 1126.08 nm) have also been obtained as the result of resonant oscillation in microsphere cavity. In addition, for the first time so far as we know a self-stimulated cascaded Raman laser is observed in the same phosphosilicate microsphere, which is generated by the down-conversion laser of Yb3+. When the pump power is 8.53 mW, a two-order cascade Raman laser is generated, which extends the laser wavelength to near 1300 nm.
2014, 63 (21): 217806.
doi: 10.7498/aps.63.217806
Abstract +
GaN-based light-emitting diode (LED) thin films grown on Si(111) substrates are successfully detached and transferred to copper and silicon submounts, and then become 40mil high power vertical structure LED chips. Electroluminescence properties of the two kinds of chips with the same expitaxial structure are investigated at different forward current densities and ambient temperatures. The obtained results are as follows. 1) at the same temperature, the EL peak wavelength of the chip with copper submount is longer than that of the chip with silicon submount. Under 13 K, the EL peak wavelength of the chip with copper submount is about 6 nm longer than that of chip with silicon submount as the driving current increases from 0.01 mA to 400 mA. While under 300 K, the difference in EL peak wavelength between the two kinds of chips at 0.01 mA is only about 3 nm; as the current increases to 400 mA, the difference will tend to zero and the spectra will coincide. 2) At the same current density, as the temperature increases from 13 K to 320 K, the EL peak wavelengths of the two kinds of chips are S-shaped, and the spectra tend to coincide. 3)When the temperature is below 100 K, the current density droop effect of the chips with copper submount is more abvious than that of chips with silicon submount, while above 100 K, the results are just inverse. Perhaps, it is due to the fact that the differences in thermal expansion coefficient and thermal conductivity between the two kinds of submounts lead to the diffrent EL properties.
2014, 63 (21): 217601.
doi: 10.7498/aps.63.217601
Abstract +
A theoretical study is presented on the magnetic-field-excited and adjusted ferromagnetic resonance in the spin valve structures with perpendicular anisotropy. Through linearizing the Landau-Lifshitz-Gilbert equation including the spin-transfer torque term, the magnetic-field-excited and adjusted ferromagnetic resonant spectra are obtained. The dependences of the resonant linewidth, resonant frequency and resonant magnetic field on the magnitude and direction of dc current density and dc magnetic field are shown. The effective damping of the system can be minimized through adjusting the magnitude and the direction of current density.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2014, 63 (21): 218902.
doi: 10.7498/aps.63.218902
Abstract +
Epidemics and cascades are independently studied topics in network science, but in practice, there are cases where they interact and neither of their effects can be ignored, e.g., when a digital virus spreads in a communication network that is transferring data. We have built a model to study their interplay in previous works. Here we present the epidemic threshold criteria of this model. When the infectivity is fixed, the tolerance parameter , in capturing the capacity of nodes, must be larger than a critical value to fulfill the criteria, and at equilibrium the fraction of nodes both uninfected and un-failed is the largest at this critical point. So the the presentation of the epidemic threshold criteria is of significance.
2014, 63 (21): 218401.
doi: 10.7498/aps.63.218401
Abstract +
Technical parameter design is an effective approach to improve marine environment sounding capability for fully polarimetric synthetie apertuer radar (SAR). By analyzing geophysical contact between noise-equated backscattering coefficient and marine environment sounding, and that between radiometric resolution and marine environment sounding, we present the key technical parameters design method for fully polarimetric SAR. We first calculate radar backscattering coefficient in different marine environments by using the fully polarimetric ocean surface scattering model, and accordingly determine the noise-equated backscattering coefficient of fully polarimetric SAR ocean sounding. Then the noise-equated backscattering coefficient is used as an input parameter in SAR equation, and the function relationship between radiometric resolution and signal to noise ratio is used as a constraint condition, we thus can carry out technical parameter design, such as for signal to noise ratio, radiometric resolution and system power aperture product. By simulation calculation of fully polarimetric ocean surface scattering, we find that the noise-equated backscattering coefficient of ocean sounding is designed to -35.0 dB, which can meet the needs of fully polarimetric SAR sounding in different marine environments. Studying the function relationship between radiometric resolution and signal to noise ratio, we find that the optimal signal to noise ratio of ocean sounding is 8.0 dB. Results of C-band airborne fully polarimetric SAR design show that the above method can make technical parameter of fully polarimetric SAR meet both the need for marine environment application and system design, because of taking into account the needs of marine environment sounding.
2014, 63 (21): 218701.
doi: 10.7498/aps.63.218701
Abstract +
EEG (electroencephalogram) is generated by the brain activity and is present in the central nervous system of spontaneous electrical activity, which is an important biological signal. EEG is a very weak and nonlinear as well as irreversible signal. This paper presents a new method to describe it based on the relative entropy of transition probability for the forward and reverse sequences. Besides, we may apply this method to study the normal EEG and epileptic EEG irreversibility, and the experimental results show that the EEG irreversibility of patients who suffer from epilepsy is significantly less than that of normal people. This shows that the relative transfer entropy can be used as aparameter to detect the irreversible degree of EEG for recognizing whether a patient is suffering from epilepsy or not, which may be a positive index for clinical diagnosis.
2014, 63 (21): 218901.
doi: 10.7498/aps.63.218901
Abstract +
Based on the condition of inhibiting traffic flow instability, this paper studies the effects of comprehensive information of the nearest following vehicle obtained by drivers' rear view behavior on traffic flow instability. Research shows that in the real practice the probability of paying attention to the information of the preceding vehicle is greater than that of paying attention to the information of the following vehicle, and the drivers' sensitive coefficient is assumed to be greater than 0, many conclusions such as the following can be obtained: 1) the information of the nearest following vehicle headway may reduce traffic flow instability, and the larger the attention probability, the smaller the traffic flow instability; on the contrary, the information of the velocity difference between the vehicle and the nearest following vehicle may increase the traffic flow instability, so the larger the attention probability, the bigger the traffic flow instability; 2) the instability reduction effect due to the comprehensive information from the nearest following vehicle is greater than the increasing effect on the traffic flow instability; 3) the bigger the driver's sensitive coefficient of the distance difference, the bigger the reducing effect of the information of the nearest following vehicle on traffic flow instability; 4) the bigger the driver's sensitive coefficient of the velocity difference, the bigger the increasing effect of the information of the nearest following vehicle on traffic flow instability.
2014, 63 (21): 218801.
doi: 10.7498/aps.63.218801
Abstract +
A tandem solar cell, composed of a MEH-PPV:PCBM bulk heterojunction front sub-cell and a CuPc/PCBM small molecule back sub-cell, is fabricated by spin-coating and vacuum evaporating methods. Measurement results show that an optimal tandem solar cell with 50 nm MEH-PPV:PCBM active layer thickness and 0.5 nm Ag inter-layer thickness can be obtained with a power conversion efficiency of 1.86%
2014, 63 (21): 218102.
doi: 10.7498/aps.63.218102
Abstract +
Ca0.64WO4:Eu0.24 also crystalizes in the phase of CaWO4 with about 12 mol% of Schottky defects on Ca2+-sites in the crystal lattice of CaWO4. The question whether such a phase is well stable at high temperatures remains to be studied, so the impacts of over-sintering on the structure of Ca0.64WO4:Eu0.24 ceramics are examined. The probable origins resulting in the phase transition at high temperatures are discussed, and the influences of such a phase transition on the luminescence properties are also studied. Observations reveal that some oxygen ions bonded to Schottky defects may be released when the sintering temperature is over 1100 ℃. This leads to the shortage of oxygen element for the bulk Ca0.64WO4:Eu0.24 ceramics, and a phase transition in CaWO4 may have occurred. A monoclinic phase of the formula Eu2 WO6 is generated. It is found that the distance between crystal planes in CaWO4 becomes larger after the phase transition. This may be one of the primary reasons accounting for the sharp decrease of luminescent intensities of Ca0.64WO4:Eu0.24 ceramics.
2014, 63 (21): 218101.
doi: 10.7498/aps.63.218101
Abstract +
Vanadium dioxide (VO2) thin film have been fabricated on the ZnO-doped Al conductive glass (AZO) substrates by DC magnetron sputtering and after thermal annealing. Effect of different annealing temperature and time olunation on the VO2/AZO composite films has been studied, then the structure, components and optical-electrical properties of the composite films are tested and analyzed by suitable instruments. Results show that the AZO film deposited on the conductive glass substrate dose not change the preferred orientation growth of the VO2 thin film, but its surface morphology characteristics are changed. Compared with those fabricated on ordinary glass substrates by the same processes and conditions, the phase transition temperature of the VO2/AZO composite film is decreased by about 25 ℃, and the width of thermal hysteresis is narrowed to about 6 ℃. Before and after phase transition, the visible light transmittance remains higher than 50%, and the infrared transmittances at a wavelength of 1500nm are 55% and 21% respectively. Furthermore, the resistivity rangeability is also up to three orders of magnitude before and after phase transition. In conclusion, the VO2/AZO composite films are easy to be fabricated and have a high degree of stability, smooth ness and compact surface morphology, thus they may be used to make new photoelectric devices
ATOMIC AND MOLECULAR PHYSICS
2014, 63 (21): 213101.
doi: 10.7498/aps.63.213101
Abstract +
Among the three methods (B3LYP, BP86 and B3LYP*) in density functional theory (DFT), the best tools for predicting the ground state of metal hydride, the B3LYP method for predicting the harmonic frequencies and geometric parameters of the ground state of FeH2 gives result in good accordance with the experimental data; so it is employed to optimize the structure of molecules FeH and FeH2 in possible geometries and multiplicities based on 6-311++g(d,p) level in searching of the structure with the lowest energy. Results show that their electronic states in the ground states are FeH(4Δ) and FeH2(5A1), supposing that the two molecules have three and four unpaired electrons respectively, with spin polarization effect, and they are paramagnetic substances, and the stable structure of molecule FeH2 is of C2v symmetry. The Murrell-Sorbie potential energy function-the sufficient analytical potential function form for biatomic molecules-with 4 parameters in molecule FeH is derived via the least square method. Their spectra data and force constants are deduced according to the results. The analytical potential energy function of FeH2 is also obtained from the many-body expansion theory, which gives the analytical potential function of triatom molecules of the single-value potential surface consisting of three parts with single body terms, two body terms, and three body terms. The deduced analytical functions for FeH2 in this paper predict successfully a global minimum stable structure of quintet FeH2 with a 4.68 eV depth potential trap, and other higher energy stable and saddle structures. This potential function predicts the balanced ground structure and the second derivative force constants of this molecule. According to the potential function of FeH2(C2v), when it is formed from H and FeH, a potential trap with its depth being 4.68 eV is excited and the complex molecule of H–Fe–H is easily formed. The reaction of Fe+H2 → HFeH is exothermic with ΔH=-0.08305 eV.
2014, 63 (21): 213701.
doi: 10.7498/aps.63.213701
Abstract +
We propose a new scheme for molecular mirror with a blue-detuned surface plasmon ploariton field excited on a microstructural metal film and study the dynamic process of reflection of cold molecules by Monte Carlo simulation. Our study shows that this mirror can realize a reflection of cold iodine molecular beam with a longitudinal temperature of 10 mK and a transverse temperature of 1 mK with a reflectivity of 55.89% when the incident laser is of a 10 ns pulse width and its intensity is I0=1.0 109 W/cm2; and the molecular reflectivity increases with increasing incident laser intensity.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2014, 63 (21): 216101.
doi: 10.7498/aps.63.216101
Abstract +
Specimens of 6H-SiC were irradiated by 300keV He ions at temperatures of RT, 450, 600 and 750 ℃ with fluences ranging from 11015 to 11017 cm-2. Post-irradiation, virgin and irradiated 6H-SiC specimens are measured and studied by microscopic laser confocal Raman spectrometer and UV-visible transmission apparatus. Analyses of both experimental results shown that production and recovery of defects caused by irradiation are directly related to the fluences and temperatures. Amorphization of 6H-SiC irradiated at RT occurrs, which is reflected by the disappearance of the Raman peaks and the saturation of the relative Raman intensity(simultaneously a strong Si-Si peak appears). Recovery of defects may exist in high-temperature irradiation, when helium bubbles do not exist, so that irradiation-induced defects can be easily recovered during irradiation process at elevated temperatures; but when helium bubbles are present, they can inhibit defects to recover, as shown in the trend of slopes of curves representing the relative Raman intensity and the relative absorption coefficients. This paper mainly focuses on the effects of helium bubbles on defect accumulation and recovery under the condition of high temperature irradiation, and then the comparison with the results of 6H-SiC irradiated by Si ions at elevated temperatures.
2014, 63 (21): 216102.
doi: 10.7498/aps.63.216102
Abstract +
This article uses the Miller model to simulate the ferroelectric polarization of the metal-ferroelectrics-insulator-substrate (MFIS) structured ferroelectric field effect transistor (FeFET), interfacial charge concentration, and charge migration rate under ionizing radiation. The capacitance and source-drain current at different total dose and different dose rate are calculated. Results show that the total dose of 0.1 MGy changes slightly the source leakage current and capacitance of the FeFET, and the total dose of 1 MGy leads to a larger variation in these quantities. When the radiation dose rate is varied, the minimal changes in the drain-source current and capacitance are observed. These results suggest that FeFET has a relatively large radiation hardness.
2014, 63 (21): 216401.
doi: 10.7498/aps.63.216401
Abstract +
The pressure dependence of electrical properties of ZnSe was observed by means of in situ high pressure DC electrical resistivity measurement and AC impedance spectrum methods in a range of 0–35 GPa. Two structure phase transitions have been observed corresponding to the wurtzite-cinnabar-rocksalt transitions. The temperature dependence of the electrical resistivity of ZnSe is measured under different pressures. Results show that ZnSe undergoes a semiconductor to metal transition at 12 GPa. AC data also proves the above results. The pressure dependence of grain and grain boundary resistances indicates that the cinnabar is close to an isotropic material.
2014, 63 (21): 216801.
doi: 10.7498/aps.63.216801
Abstract +
We have studied the vibration behavior of a water droplet vibrated on a superhydrophobic surface via a high-speed camera. The resonance frequencies of the droplet satisfy the Rayleigh equation, suggesting that the droplet on a superhydrophobic surface can be regarded as a free droplet. Its real oscillation frequency is half of the driving frequency when it is vibrated at low frequencies(<200 Hz). It shows large shape deformation from a compressed puddle to a stretched spheroid. The three-phase contact line exhibits a stick-slip behavior. However, when the droplet is vibrated at frequencies greater than 200 Hz, the three-phase contact line is pinned to the substrate and the droplet is vibrated at the same frequencies as the external driving frequencies. It is found that the oscillation of the contact line and the large shape deformation of the droplet are responsible for the distinct behavior at low frequency.
