Vol. 61, No. 14 (2012)
2012, 61 (14): 140201. doi: 10.7498/aps.61.140201
In this paper, the (G'/G)-expansion method is used to solve the two kinds of nonlinear KdV equations with variable coefficients for the first time. And some new exact solutions are obtained successfully. It is proved that the (G'/G)-expansion method is not only appropriate for solving nonlinear evolution equations with constant coefficients but also excellently applicable to the nonlinear evolution equations with variable coefficients, and it also has a broad application prospect.
Stable behavior of solution for the reaction-diffusion system of atmospheric nonlinear dynamics and thermodynamics
2012, 61 (14): 140202. doi: 10.7498/aps.61.140202
A class of nonlinear system is studied. Firstly, the reaction-diffusion system with speed, temperature and humidity for a atmospheric nonlinear force dissipative system is discussed using the atmospheric nonlinear theory of thermodynamics and dynamics. Secondly, the small disturbed solution of atmospheric nonlinear reaction-diffusion system in the neighborhood of homogeneous steady state solution is obtained from the Lyapunov stability theory. Finally, from the variations of the control parameters for transgenic processes, the states of ordered-unordered-ordered processes of atmospheric nonlinear reaction-diffusion system are found. Thus the corresponding local atmospheric nonlinear force dissipative system can be predicated and calculated.
2012, 61 (14): 140203. doi: 10.7498/aps.61.140203
Optical solitons in gaseous atomic media display many striking features under electromagnetically induced transparency (EIT). Study of theoretical model, which describes these features of optical solitons, has important meaning in optical informational process and propagation. Two-dimensional saturated nonlinear Schrdinger equation, which describes the spatial soliton evolution in the three-level gaseous atomic EIT media, is transformed into the Hamilton system with the symplectic structure. The Hamilton system is discretizated by the symplectic method. The corresponding symplectic scheme is obtained. Evolution behaviors of two and four spatial solitons with the same amplitude in a three-level, gaseous atomic EIT media are simulated by the symplectic scheme. Numerical results further show that the phase difference and the direction of the entering gauss beams have an obvious effect on the interaction of multi-solitons. The entering Gauss beam can form the stable optical solitons in a gaseous atomic media.
2012, 61 (14): 140204. doi: 10.7498/aps.61.140204
An infinitesimal canonical transformation and an integral for a generalized Birkhoff system are studied. The equations of the generalized Birkhoff system are established. The results of the infinitesimal canonical transformation and the integral for the Birkhoff system are generalized to the generalized Birkhoff system. An example is given to illustrate the application of the result.
Eigen-vector expansion theorem of a class of operator matrices appearing in elasticity and applications
2012, 61 (14): 140205. doi: 10.7498/aps.61.140205
For a class of operator matrices appearing in elasticity, the eigen-vector expansion theorem is considered. It is shown that the eigen-vectors possess the symplectic orthogonality. And a necessary and sufficient condition for the completeness of the eigen-vector system is further given. As concrete applications, the obtained results are tested for the plate bending equation and the buckling deflection problem of rectangular thin plates.
2012, 61 (14): 140301. doi: 10.7498/aps.61.140301
Using the quadratic form theory, we achieve the decoupling of systematic Hamiltonian of generalization of n-dimensional coupled harmonic oscillators and derive the diagonalized Hamiltonian by three linear transformations with keeping the commutation relations unchanged. The energy eigenvalue and the eigenfunction of the system are also obtained.
2012, 61 (14): 140302. doi: 10.7498/aps.61.140302
In this paper, we handle the Hamiltonian of the particles that are affected by the linear-damping and time-dependent external force given by two different canonical transformations by using the theory of generalized quantum linear transformation. We give the rigorous solution of evolution operator, and the expectation values of coordinate and momentum of the particles quantum fluctuations. Results show that 1) the two regular translations are equivalent; 2) linear damping has a squeezing effect on the momentum of particle, and the deviation of the momentum attenuates with time t according to the rule of negative exponent, and the bigger the damping coefficient, the faster the attenuation is; 3) the expectation values of coordinate and momentum of the particle are equal to their classical values correspondingly.
Study on the phase transitions of ferroelectric systems by Weiss's molecular field theory with an external field
2012, 61 (14): 140501. doi: 10.7498/aps.61.140501
The descriptions of the paramagnetic-ferromagnetic and paraelectric-ferroelectric phase transitions (PTs) by Weiss's molecular field theory (WMFT) are quite successful, and the WMFT is also a theoretical basis of initial analysis of the PTs of structural disorder and complex compositional systems because of its mean-field characteristic. However, there is not any study on the PT behaviors of the WMFT with external field, and only the case of two orientational states of molecules for ferroelectric systems has been investigated by the WMFT. Although the descriptions of the above two kinds of PTs by the WMFT are quite similar, the exact ones and the corresponding results are different more or less due to the difference in microscopic unit between the magnetization and polarization In this work, the exact descriptions of the ferroelectric systems with arbitrary orientational states of molecules by the WMFT are provided, including the temperature dependences of the spontaneous polarization, the internal energy, the specific heat and the static polarizability, and then the evolutions of the PTs with an external field are studied. The obtained results are as follows. 1) Without the external field, the PTs of the systems are of the second order, and the transition temperatures and spontaneous polarizations decrease, which are different from those of the ferromagnetic systems, but the internal energy, the specific heat and the polarizability increase with the increase of the orientational states. 2) The external field drives the second order PT to a diffusive one, and diffusive temperature range becomes wider as the field is increases. The results mentioned above would benefit the deep studies of ferroelectric PT, especially the diffusive one.
Neural network-based backstepping design for the synchronization of cross-strict feedback hyperchaotic systems with unmatched uncertainties
2012, 61 (14): 140502. doi: 10.7498/aps.61.140502
For a class of cross-strict feedback hyperchaotic systems with unmatched uncertainties, a multilayer neural network (MNN) based adaptive backstepping design method is proposed. An MNN is introduced to estimate the uncertainties in systems. Sliding mode and adaptive backstepping control are used to deal with the unmatched uncertainties and the MNN approximation errors. If the virtual control coefficients do not pass through zeros, the proposed method guarantees that the synchronization errors of the systems approach zeros. If the virtual control coefficients pass through zeros, the proposed method guarantees that the synchronization errors of the systems are bounded. Numerical simulations are given to demonstrate the efficiency of the proposed control scheme.
Adaptive modified function projective synchronization of a class of chaotic systems with uncertainties
2012, 61 (14): 140503. doi: 10.7498/aps.61.140503
In this paper, we primarily investigate the adaptive modifying function projective synchronization of a class of chaotic systems with uncertainties. According to the Lyapunov stability theory, the two schemes of obtaining the response system from chaotic system are established, and the controller and the adaptive law are designed. The approach is more robust to noise and external interference, and does not need the upper bound of uncertainties in advance. In addition, it can adjust the system response rapidly by adjusting the accelerated factor. This method is more practical. Numerical simulations of a hyper-chaotic system verify the effectiveness of the proposed method.
2012, 61 (14): 140504. doi: 10.7498/aps.61.140504
Spatiotemporal chaos network synchronization of the laser Maxwell-Bloch equation is studied. The single-mode laser Maxwell-Bloch equation is amended. Then N single-mode laser Maxwell-Bloch equations amended are taken as nodes to constitute a complex network. Considering the fact that the parameters of the spatiotemporal chaos systems taken as nodes may have small deviations from the actual values because of some interference in the network connecting process, the system at the first node is take as a driven system to drive the rest of the N-1 systems in parallel to achieve synchronization. Furthermore, simulation is performed to verify the effectiveness of the method.
2012, 61 (14): 140505. doi: 10.7498/aps.61.140505
With the help of the symbolic computation system Maple, an extended G'/G method and a variable separation method, new exact solutions of the (2+1)-dimensional breaking soliton equation are derived. With the derived solitary wave solutions, we obtain multi Solitoff localized structures and study the soliton evolution with time.
Blind detection of M-ary quaternary phase shift keying signals by a complex Hopfield neural network with amplitude-phase-type hard-multistate-activation-function
2012, 61 (14): 140701. doi: 10.7498/aps.61.140701
Considering the disadvantage of the algorithms based on statistics, a novel algorithm based on complex Hopfield neural network with amplitude-phase-type hard-multistate-activation-function (CHNN-APHM) is proposed to detect M-ary quaternary phase shift keying (MPSK) signals blindly. An amplitude-phase-type hard-multistate-activation-function is constructed. The stabilities of the CHNN-APHM with asynchronous and synchronous operating mode are also analyzed separately. While the weighted matrix of CHNN-APHM is constructed by the complementary projection operator of received signals, the problem of quadratic optimization with integer constraints can be successfully solved with the CHNN-APHM, and the MPSK signals are blindly detected. Simulation results show that the algorithm reaches the real equilibrium points with shorter received signals and if is applicable for channel with common zeros.
Study on chromatic dispersion of beam splitter in spatially modulated Fourier transform spectrometer
2012, 61 (14): 140702. doi: 10.7498/aps.61.140702
Due to the chromatic dispersion of beam splitter, the interferogram units from various wavelengths could shift by different offsets, leading to interferogram aliasing in transverse space. Simultaneously, the interferograms of different wavelengths have different offsets of optical path difference, which makes the interferogram aliasing in vertical space. According to geometric optics principles, the transverse aliasing of the interferogram reduces the area of the interferogram unit, and the vertical aliasing of the interferogram leads to a phase-frequency response which could reduce the spectral line intensity. The calculation and the analysis indicate that the transverse aliasing area is only 3.4% of the total area of the interferogram unit in our study, which could be removed in the data processing; and the phase delay from the vertical aliasing of the interferogram is proportional to the thickness difference between the beam splitter and the compensating plate. The maximal thickness difference is provided when the contrast reversion appears in the interference fringe. Finally, we correct the chromatic dispersion from the aliasing interferogram by solving linear equation set, and recover the ideal spectrum.
Incoherent broadband cavity enhanced absorption spectroscopy for measurements of HONO and NO2 with a LED optical source
2012, 61 (14): 140703. doi: 10.7498/aps.61.140703
An application of incoherent broadband cavity enhanced absorption spectroscopy with a near-ultraviolet LED (peak 372 nm and FWHM is 13 nm) to simultaneously detecting HONO and NO2 is described. The light emitted from the LED is collimated and then coupled into an 70 cm long high finesse cavity formed with two high reflectivity mirrors. The spectra are respectively recorded when the cell is filled with He and then N2, and the mirror reflectivity is determined from the change in transmitted intensity due to the difference in Rayleigh scattering cross-section between He and N2. The maximum of mirror reflectivity is 0.99962 at 390 nm in a spectral region of 360-390 nm, and corresponding maximum of light path length is about 1.71 km when NO2/HONO mixture is measured. The concentrations of HONO and NO2 are obtained using least-squares fit. Detection sensitivity (1) of 0.6 ppbv for HONO and 1.9 ppbv for NO2 are achieved using an acquisition time of 1000 s. The experimental results demonstrate the possible application of this technology to in situ monitoring the trace gases in the atmosphere.
2012, 61 (14): 140704. doi: 10.7498/aps.61.140704
In this paper, we propose a method of measuring a gas by using non-dispersive infrared (NDIR) technique with two analysis channels. The filter parameters of the two analysis channels are calculated by line-by-line integral of the selected absorption spectrum of SO2. The influences of temperature and air pressure on strengths and Lorentzian shape functions are considered accurately in the calculation. The absorption wavelengths at 7.32 μm and 4.0 μm are chosen to detect the SO2 whose concentrations are ≤qslant 280 ppm and > 280 ppm, respectively. The calibration curves of the two analysis channels are obtained by least-squares fitting two 3-order polynomials. The linearity, the sensitivity and the accuracy of the analysis system are analyzed. SO2 concentration with a large range from ～ 5 ppm to 10000 ppm can be retrieved with the measurement linearity > 0.99 and measurement error < 5%. The reasonable tradeoff is made to optimize both sensitivity and measurement range jointly. A fair balance between measurement sensitivity and large span range is obtained. Furthermore, sufficiently good measurement linearity makes cross-interference correction possible in the NDIR multi-gas analyzer.
2012, 61 (14): 140705. doi: 10.7498/aps.61.140705
In this paper, we present a method of measuring atmospheric water vapor concentration by using infrared differential optical absorption spectroscopy (DOAS). The experimental setup is converted from a self-made non-dispersive infrared multi-gas analyzer. In the process of DOAS retrieval, the reference absorption cross section is calculated by applying the Voigt broadening method to the absorption lines from HITRAN database. The influences of temperature, pressure and instrument function are also taken into account in the calculation. A validation study of the water vapor measurement is performed by comparing the results measured by a non-dispersive infrared analyzer. The results show good agreement with each other (correlation coefficient = 0.93347). It indicates that the infrared DOAS technique has the potential applications to other gases measurements which have no or weak absorption within the UV region, e.g. CO2, CH4, CO, N2O, etc.
THE PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
Conformal invariance, Noether symmetry and Lie symmetry for systems with unilateral Chetaev non-holonomic constraints
2012, 61 (14): 141101. doi: 10.7498/aps.61.141101
The conformal invariance of a system with unilateral Chetaev non-holonomic is studied, and its definition is given. The relation between the conformal invariance and the Noether symmetry is discussed. Finally, the relation between the conformal invariance and the Lie symmetry is discussed, and the Hojman conserved quantity due to the conformal invariance of the systems is obtained. In the paper, an example is given to illustrate the application of the results.
2012, 61 (14): 142301. doi: 10.7498/aps.61.142301
Multiferroic BiFe1-xMnxO3 (x= 0, 0.05, 0.10, 0.15, 0.20) (represented as BF1-xMxO) ceramics are prepared by the conventional solid state reaction technique. The effects of Mn4+ doping on density, phase structure, morphology, dielectric and ferroelectric properties are investigated. The X-ray diffraction patterns of the samples indicate that the typical perovskite phase structure of BiFeO3 is formed, and a phase transition starts near x= 0.05, i.e., the phase structure is distorted from rhombohedral to orthorhombic by Mn4+ doping. The dielectric susceptibility of the sample is significantly increased and the dielectric loss is slightly increased with the increase of Mn4+ content. The dielectric constant r of the BiFe0.85Mn0.15O3 ceramic at 10 kHz is as high as 1065, 22 times larger than that for pure BiFeO3. It is suggested by hysteresis loop measurements that the ferroelectric property of the BF1-xMxO ceramics is improved and the remanent polarization is increased by Mn4+ doping. This is probably because Mn4+ is more stable than Fe3+, and the B-site doping with higher valent Mn4+ could reduce the volatilization of Bi3+ and suppress the valence fluctuation of Fe3+, thereby reducing the concentration of oxygen vacancies and the leakage current in the ceramic.
Isospin-dependent nucleon-nucleon cross section and the collective flow of nucleons in heavy-ion collision at intermediate energy
2012, 61 (14): 142501. doi: 10.7498/aps.61.142501
In this paper, the influences of the different forms of nucleon-nucleon (NN) cross sections on the rapidity distributions and the in-plane flows of nucleons in reactions 60Ca+60Ca and 60Ni+60Ni are studied by using IBUU04 transport model with the moment-dependent symmetry potential. The calculation results indicate that (1) the discrepancies of the rapidity distributions and in-plane flows corresponding to different forms of NN cross sections are large, (2) the sensitivity of the in-plane flow to the NN cross section still holds when the new form of moment-dependent symmetry energy is adopted, and (3) the influence of the N/P ratio of reaction systems on the transverse flow calculated with a certain NN cross section is more obvious when the incident energy is near the balance energy.
ATOMIC AND MOLECULAR PHYSICS
2012, 61 (14): 143201. doi: 10.7498/aps.61.143201
We theoretically investigate the high-order harmonic and attosecond pulse generation by numerically solving the one-dimensional time-dependent Schrödinger equation from a hydrogen atom in a two-color laser field, which is synthesized by adding a suitable multicycle infrared pulse to a multicycle 800 nm fundamental pulse. Our results clearly show that when there are 12 Ti: sapphire optical cycles in the pulse envelope, by adding a suitable second optical field, the electric field of the synthesized pulse presents three-segments, and only amplitude of the electric field in middle segment makes a major contribution to the plateau and cutoff region of the harmonic spectrum. By analyzing the compression mechanism of two-color electric field, we further enlarge the duration of the synthesized pulse to 60 fs, and obtain a single 160 as short pulse. This has been the longest pulse duration used for obtaining single attosecond pulse so far. Here the effect of synthesized pulses is similar to the effect of single 5 fs ultrashort pulse. This scheme greatly reduces the requirements for the pump laser system used traditionally for generating an isolated attosecond pulse, and it allows us to use a well-established conventional high-power pumping laser.
2012, 61 (14): 143401. doi: 10.7498/aps.61.143401
The triple differential cross sections for electron impact ionization helium (e, 2e) reactions are calculated by use of the BBK and modified BBK model at different gun angles of incident electron energy 64.6 eV. The structures of the cross sections and the physical natures of these structures are discussed in detail. It is found that for the non-coplanar geometry, cross sections are determined by a variety of effects together and these effects are converted to each other under different conditions. Moreover, strong dynamical screening effect is observed in the final state of the perpendicular plane symmetric geometry.
2012, 61 (14): 143601. doi: 10.7498/aps.61.143601
In this paper, a simulation discussing the cause inducing the anisotropy of hydrogen cluster expansion is implemented by using LAMMPS tool for the molecule dynamics simulations. Through analyzing the behavior of electrons contained in the cluster and the variations of distance between outermost protons of all directions and cluster center with time, we clearly find that the expansion of hydrogen cluster is anisotropic, which is due mainly to the anisotropies of the quiver and escaping of electrons. Then we study the evolutions of proton energy component and anisotropic degree, and find that the anisotropic degree first increases with laser electric field increasing, then decreases gradually to a stable value greater than one. Additionally, we analyze the relationship between observation angle and average proton energy from hydrogen cluster irradiated by ultreshort laser pulse, and find that our simulation results accord with the experimental results qualitatively.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
Investigation of electromagnetic scattering interaction between the column with rectangular cross-section and rough land surface covered with snow using finite difference time domain method
2012, 61 (14): 144101. doi: 10.7498/aps.61.144101
The dielectric properties of the actual land surface and snow deposit are described using the four-component model of dielectric constant of soil and the dielectric constant model of snow respectively. The actual rough land surface is simulated with the model of exponential type distribution rough surface and Monte Carlo method, the composite electromagnetic scatterings from the rough land surface covered with snow and the column with rectangular cross-section above it are studied using the finite-difference time-domain method, the curves of angular distribution of composite scattering coefficient are obtained, the bistatic composite scattering coefficient varying with the roughness parameter and dielectric parameter of soil and snow, the geometric parameter and dielectric parameter of the column with rectangular cross-section are calculated in detail. The characteristics of the composite scattering coefficient from rough land surface covered with snow and the column with rectangular cross-section above it are also obtained.
2012, 61 (14): 144102. doi: 10.7498/aps.61.144102
Firstly, based on the Gaussian-schell model source theory and the propagation of cross-spectral density function in free space, the spatial coherence of undulator source (BL15U) in Shanghai Synchrotron Radiation Facility is studied. Secondly, the influences of pre-focusing mirror and the monochromator on the beam spatial coherence are discussed. Finally, the spatial coherent length at mono slit S2 is measured. The spatial coherent length at S2 theoretically is 66.5 μm, but experimentally is 27 μm. This difference is due to the high frequency vibration of pre-focusing mirror or monochromato. Nevertheless, high coherent hard X ray beams can be obtained at the location of experimental sample and many coherent experiments can be fulfilled on this beamline.
A new technique for generating non-diffracting beam with long propagation distance using two axicons
2012, 61 (14): 144201. doi: 10.7498/aps.61.144201
A new technique for generating quasi non-diffracting beam with long propagation distance by using simple optical elements including the convergent lens and axicons is proposed. The theory of generating such a beam is studied with geometrical optics and diffraction theory. The formation process of the beam is simulated, and the transverse intensity distributions at various distances are obtained. The simulation results show that the transverse intensity distribution at long-distance accords with Bessel distribution. A comparison between the quasi non-diffracting beam, which is obtained by our experiment, and that in the literature (Belyi et al. 2010 Opt. Exp. 18 1966) shows that its propagation distance is more than 50 m longer, and the beam divergence angle is compressed by 22 times. In the experiment, the beam patterns are captured at different propagation positions, and the obtained results are in good agreement with the theoretical analyses.
Effective radius of curvature of spatially partially coherent beams propagating through non-Kolmogorov turbulence
2012, 61 (14): 144202. doi: 10.7498/aps.61.144202
The laser beam propagation through atmospheric turbulence is of importance for both theoretical study and practical applications. Taking the Gaussian Schell-model (GSM) beam as a typical example of spatially partially coherent beams,based on the non-Kolmogorov spectrum and generalized Huygens-Fresnel principle, the analytical expression for the effective radius of curvature of GSM beams propagating through non-Kolmogorov turbulence is derived. The effects of turbulence parameters (including generalized exponent parameter, inner scale l0, and outer scale L0) and propagation distance z on the effective radius of curvature of GSM beams are stressed. It is shown that the effective radius of curvature of GSM beams increases with outer scale L0 decreasing for 3.6 4 and the inner scale l0 increasing, but dose not monotonically vary with the increases of exponent parameter and propagation distance z. The results are explained physically.
2012, 61 (14): 144203. doi: 10.7498/aps.61.144203
We present a perfect metamaterial abosorber composed of metallic leaf-shaped cells, dielectric substrate and metallic film. Based on the metallic Drude principle, this structure can realize an perfect absorptivity of 99.5% at the infrared communication frequencies with appropriate geometric parameters. Moreover, this structure can simultaneously achieve perfect absorptions at two infrared frequencies with the maximal magnitudes of 99.67% and 97.13% respectively, which effectively broadens the bandwidth of absorptivity, thereby benefiting the design and the application of absorber at the infrared frequencies. Afterward, the effects of the neck width on the position of the double absorptivity peak are investigated, and expected to be used in frequency modulation. The model proposed in this paper has a series of advantages such as simple structure, high absorptivity and broad operating bandwidth.
Quantitative infrared prediction method for defect depth in carbon fiber reinforced plastics composite
2012, 61 (14): 144204. doi: 10.7498/aps.61.144204
In this work, we present a quantitative method of predicting defect depth within carbon fiber reinforced plastics (CFRP) composite with unknown thermal property of specimen. The method discussed in this paper is derived from theoretical model of ideal one-dimensional heat conduction in a flat plate sample. The principle of depth prediction using pulsed infrared thermography is presented; the time based on the peak second-derivative of the temperature difference in the logarithmic scale is selected as the characteristic time; the calibration curve based on data of single defect depth can be easily affected by random error. The calibration curve is put forward in the paper and obtained by designing a stepped reference sample, and it can be obtained by a curve fit of different depths and corresponding characteristic time using polynomial function. The polyfitting curve is used as the calibration curve for depth prediction when the relative error square is minimum. The experimental results show that the depth measured by the calibration curve based on many reference defect depths is more accurate than by single reference defect depth. Defect depth in CFRP composite can be measured by the method without knowing thermal property of specimen.
2012, 61 (14): 144205. doi: 10.7498/aps.61.144205
In the case of incoherent pumping, laser without population inversion in a resonant three-level Δ -configuration system is investigated. By taking the strong coupling field limit, we obtain the approximate steady-state analytical solution of populations and imaginary part of coherences within the dressed-state regime, and discuss the condition of the generating of laser without inversion and the dependences of population distribution and system gain on Rabi frequency of the probe and coherence fields. The results show that the resonant three-level Δ-configuration system is always in the state of no population inversion, and laser without inversion occurs if one of the two fields is strong. When one of the two fields is much stronger than the other one, the gain without inversion is independent of the two Rabi frequencies.
2012, 61 (14): 144206. doi: 10.7498/aps.61.144206
The Yb3+ doped double clad photonic crystal fiber (PCF) with a large core of around 90 m in diameter is prepared from SiO2-Al2O3-P2O5 core glass of an optical fiber preform through a conventional modified chemical-vapor deposition technique, solution doping and gas doping. The double clad PCF has a mode area of about 1330 m2. The core and inner cladding have numerical apertures of about 0.065 and 0.5, respectively. 1 m long photonic crystal fiber laser generates up to 102 W output power with a slope efficiency of 76%. Such lasers are potentially scalable to high power.
Research on the influence of impurity particles on the optical breakdown threshold value of stimulated Brillouin scattering medium
2012, 61 (14): 144207. doi: 10.7498/aps.61.144207
In this paper, we establish the physical model of hot damage to the stimulated Brillouin scattering (SBS) medium, and with numerical simulation method the curve of the temperature of impurity particles versus the radius is investigated. The result indicates that impurity particle has a maximal thermal effect radius and when the sizes of particles contained in the medium are near the radius, the optical breakdown phenomenon is most likely to appear, and at this point the optical breakdown threshold is lowest. We use FC-3283, GF-180 and HFE-7100 as the SBS medium in Continuum Nd: YAG laser system. The SBS medium passes through the filter with different diameters. We study the optical breakdown thresholds and energy reflectivities before and after filtration. It is demonstrated that with the filter diameter decreasing, the optical breakdown threshold increases gradually and the energy reflectivity is significantly improved through filter. In addition, a method of using He-Ne laser transmission light spot size to judge whether optical breakdown phenomenon happens, is also developed. This method is convenient and accurate, and can effectively reduce the error caused by naked eye observation.
The propagation characteristics of a Gaussian beam passing through the thick nonlinear medium with defects
2012, 61 (14): 144208. doi: 10.7498/aps.61.144208
The defects unavoidably existing on the optical component will modulate the phase and the amplitude of the beam propagation in the system. Based on the Fresnel diffractiond and split-step Fourier method, the two-dimensional intensity distributions of the Gaussion beam propagation in the nonlinear medium and the free space are simulated. And the intensity distributions of the beam propagation in the medium and the free space are also studied in the case where there exist defects on the front surface of thick medium. It is shown that the thicker the medium and the lager the nonlinear index of refraction, the stronger the beam focus and the closer to the medium surface the focal position is. The defects on the medium surface will lead to a strong beam intensity near the rear surface of the medium. And the intensity generated by the phase modulated defects is stronger than by the amplitude modulated defects.
The influences of Sr or Ba substitution on the structure and luminescence properties for Ca2.955Si2O7: 0.045Eu2+ phosphor
2012, 61 (14): 144209. doi: 10.7498/aps.61.144209
A series of Ca2.955-xMxSi2O7: 0.045Eu2+ (M= Sr, Ba, x= 0.10.5) phosphors is prepared by solid-state reaction method. The influences of Sr or Ba with larger radius substitution on the structure and luminescence properties for Ca2.955Si2O7: 0.045Eu2 + phosphor are investigated. The XRD results show that a small quantity of Sr or Ba substitution of Ca does not change the structure of Ca3Si2O7 host which has a monoclinic crystal structure. Before the substitution, the emission peak is at about 574 nm. If the Ca2+ ions are substituted by Sr2+ or Ba2+ ions, the emission peak is blue shifted when the Sr or Ba concentration is 0.5. In addition, for the same doping contents of Sr and Ba, the Ba doped phosphors have stronger emission intensity.
2012, 61 (14): 144210. doi: 10.7498/aps.61.144210
The manipulation of the Goos-Hnchen shifts in the reflected and transmitted light beam through a cavity containing four-level atomic medium is investigated by modifying the dispersion-absorption properties of the intracavity medium. Via two external coherent control fields, the Goos-Hnchen shifts of the reflected and transmitted beam can be easily controlled. It is found that around the transparent region of the medium, the Goos-Hnchen shifts are very sensitive and can be enhanced as compared with strong absorption and amplification.
2012, 61 (14): 144301. doi: 10.7498/aps.61.144301
According to the ultrasonic propagation characteristics in the multi-medium, a control method of ultrasonic focus in the multi-medium is presented through introducing the phase compensation factors and using genetic algorithm. The patterns of multi-focus ultrasonic field generated by two-dimensional phased array consisting of 1616 elements are simulated in multi-biological tissue, and the ultrasonic fields are calculated under the various thicknesses and the various absorption coefficients of medium layer. The results show that this approach can optimize the patterns of multi-focus ultrasonic field and restrain side-lobe, which can improve the ultrasonic intensity gain and make the maximal ultrasonic intensity focus on the target area. When the thickness and the absorption coefficient of biological tissue are changed, the focus position remains unchanged, but the ultrasonic intensity of the focus area will change relatively.
2012, 61 (14): 144501. doi: 10.7498/aps.61.144501
In this paper the experiment to study the features of pedestrians' walking preference is designed. Then the cellular automata model which considering pedestrians' walking preference features is built, in which the forward-parameter, right-parameter, surpass-parameter and the correction-parameters are included to mend the probability of the pedestrian getting to each neighboring cell. Based on this model and k-nearest-neighbor interaction pattern, the complex network of pedestrians is modeled. The simulation results obtained from the model well illustrate the density-speed curve and density-volume curve. Meanwhile the self-organization phenomena of the bi-direction pedestrian flow can be observed from the model simulation. In the further analysis of the pedestrian flow's basic parameter and the main feature parameters of pedestrians' complex network, it is found out that the average speed and the average path length are changed with the state of the flow. Finally it can be concluded that there is a linear negative correlation between these two parameters by fitting the data; in other words, pedestrian flow with shorter average-path length has a higher average speed.
2012, 61 (14): 144502. doi: 10.7498/aps.61.144502
There are differences in efficiency when pedestrian flows with various directions pass through the bottleneck of a subway in different manners, and the mechanisms of congestion at the bottleneck are distinct as well. The weaving motion of pedestrian flows with various directions in subways is simplified into two crowds with different ODs passing through the bottleneck which connects two parallel channels. The lattice gas model is improved by introducing the floor field so that it is suitable for the description of pedestrian flow under complex situations. It is shown by experiments that different ways lead to the difference in total time when the crowds pass through the field of interest. Then the total time of crowds passing through the field in different ways is investigated numerically, and the effect of bottleneck width is taken into account as well. Numerical simulations confirm the findings from experiments. It is found that the diagonal movement of a pedestrian should be included in the model in order to give a better description of real pedestrian traffic. And the mechanism of congestion near the bottleneck is discussed in detail.
Characteristic analyses of high birefringence and two zero dispersion points photonic crystal fiber with octagonal lattices
2012, 61 (14): 144601. doi: 10.7498/aps.61.144601
Aiming to the requirements of high birefringence and multiple zero dispersion points of optic-fiber sensing and communication systems, a new type of photonic crystal fiber (PCF) is proposed. The cladding is arrayed by octagonal air holes, and two big elliptical air holes are added in inner cladding to increase the birefringence. Numeral results show that this type of PCF exhibits high birefringence with a level of 10-3 when wavelength ranges from 0.8-2 m, which fulfils the requirement of high birefringence, and two zero dispersion points are obtained after optimization. In addition, high nonlinear coefficient with a level of 10-2 m-1W-1 is obtained, which could be used in the case where the nonlinearity is highly required.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2012, 61 (14): 145201. doi: 10.7498/aps.61.145201
"Plasma driven micro-particle accelerator" is a ground device for simulating impact effects of small debris in space. The particle velocity is determined mainly by axial velocity of plasma in a coaxial gun. Emission spectrometry is used to study the plasma axial velocity at different voltages and gas pressures. The experimental results indicate that axial velocity increases with the increase of discharging voltage, and doesn't change significantly with the pressure of working gas, which is consistent with the result of numerical simulation. This result is useful to improve the plasma axial velocity further, and provides an experimental basis for optimizing the accelerator.
2012, 61 (14): 145202. doi: 10.7498/aps.61.145202
In this paper, a flat-type surface wave plasma (SWP) source generated by microwave discharg is introduced systematically. The principle of the surface wave plasma is analyzed and the energy absorption mechanism of the surface wave plasma discharge is explored. A novel wave-mode converter composed of the single-mode resonator array, sub-wavelength diffraction grating and a new type of slot antenna array is introduced. The research findings, such as the mechanism of the generation, the realization, the characteristics of plasma parameters and the numerical simulation of the new SWP sources are beneficial to industrial applications, will promote the effectiveness of the microelectronics industry and obtain a new breakthrough.
Behaviors of ion and electron temperatures on EAST with lower hybrid current drive and lithium wall coating
2012, 61 (14): 145203. doi: 10.7498/aps.61.145203
It is observed that the profiles of ion and electron temperatures become broader within a region of r/a 0.6 on experimental advanced superconducting tokamak with high-power lower hybrid wave heating and lithium wall coating. It is found that the above phenomena are related to the low recycling at the first wall as a result of the lithium wall coating. The lithium wall coating affects the plasma particles coming from the plasma boundary to the first wall, thereby causing a reduction in recycling. The low recycling causes the temperature profiles to change. It is also found that the electron and ion temperatures approach to each other as a result of high collision rate between electrons and ions when the plasma density increases.
Properties and fabrication status of capsules for ignition targets in inertial confinement fusion experiments
2012, 61 (14): 145204. doi: 10.7498/aps.61.145204
The inertial confinement fusion program has proposed a laser capable of producing ignition and gain as the next step. Several choices exist in the design and production of capsules. In this paper the important features of each ablator material and the status of production are summarized. The design consists of ablators made of germanium-doped carbon hydrogen (CH), beryllium doped copper, polyimide, B4C and diamond. The CH and beryllium capsules are two of the most important choices. Compared with the beryllium shell, the CH shell has no microstructure and has a transparent wall that allows optical characterization of the fuel ice layer. The CH shell has the advantage that the specification can be easy to satisfy the ignition acquirements. The current ignition point has been designed in USA since 2010. The ignition target design has a series of demands for the capsule, such as capsule dimensions, coating density, void defects and volume, surface roughness, uniformity, doping and impurity levels. Now, the CH capsule can meet ignition requirements in USA, while the relevant work has just started in China.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2012, 61 (14): 146101. doi: 10.7498/aps.61.146101
We investigate the structural and dynamic properties of isothermal crystallization of Cu nanocluster which contains 500 Cu atoms (Cu500), according to the embedded atom model, using molecular dynamics simulations. We calculate the Honeycutt-Anderson bond-type index, the inherent structure (IS) and the revisionary mean-square displacement of Cu nanocluster in crystallization process. All analyses suggest that the crystallization time of Cu500 is dependent on temperature. At high temperature, the crystallization time is well represented by a Gaussian distribution, which is not observed at low temperature. Cu500 displays multi-step crystallization at low temperature. On the other hand, we note that the influence of initial configuration on isothermal crystallization is significant. For the same thermodynamic state, especially at low temperature, the lower the IS of initial configuration, the longer the crystallization time is.
2012, 61 (14): 146102. doi: 10.7498/aps.61.146102
Steered molecular dynamics (SMD) simulations are performed to study the peeling of a single wall carbon nanotube (8, 8) from a silicon surface at room temperature. There is a regular relationship between the average force probed by the ideal spring and the peeling distance when the carbon nanotube (CNT) is peeled from the silicon substrate. A large positive and a large negative peak value can be found in the peeling process. The average force for varying peeling velocities is investigated and their peak values are fitted to a function of the peeling velocity. The SMD simulation results show that there is a linear relationship between the peak value and the peeling velocity, which is consistent well with some biophysics peeling experiments. Compared with macromolecules, the CNT has a strong adhesion to the silicon surface. The influences of both radius and length as well as the defects of the CNT on the peeling process are also examined. The numerical results indicate that the peak value of the peeling force is independent of the length of the CNT but increases linearly with the radius of the CNT increasing. The peak value of the peeling force is almost independent of the 5-7-7-5 defect in the CNT but critically weakened by the radius defect of the CNT. The suggested method provides a theoretical prediction for the future experiment at atomic scale, which is helpful for the potential application of the CNT in the silicon-based microelectronics industry.
First principles investigations of the structural stability and thermal dynamical properties of metal Ba under high pressure
2012, 61 (14): 146301. doi: 10.7498/aps.61.146301
The stabilities and the thermal dynamical properties of the three high-pressure phases of Ba (Ba-I, Ba-Ⅱ and Ba-V ) are investigated systemically by first principles method. Our results show that all the three phase meet the criteria of mechanical and dynamical stabilities at 0 K. With pressure increasing, the phonon frequencies in Ba-I and Ba-Ⅱ phases become softened, whereas the Ba-V phase exhibits the hardening of phonon frequencies. Although both Ba-Ⅱ and Ba-V phase are the same hcp structures, they show different elastic anisotropies under high pressure. We also find that Ba-Ⅱ phase at higher pressure still meets the mechanical stability criterion, but does not meet the dynamical stability criterion. The absence of dynamical stability may be the reason for the transition from Ba-Ⅱ phase to Ba-I!V phase. We also calculate and make comparisons of sound velocity, Debye temperature, bulk modulus and shear modulus etc between Ba-Ⅱ and Ba-V phases, showing that Ba possesses the thermal dynamical properties under high pressure.
2012, 61 (14): 146401. doi: 10.7498/aps.61.146401
Homogenerous nucleation process induced by the colored noise is simulated by the phase field crystal method. The results indicate that the noise strenghth within a certain range almost has no influence on the parameters of equilibrium thermodynamics of the nucleation system, such as nucleation engergy barrier and size of crtical nucleus; however, it will strongly affect the dynamics of the system. The incubation time of nucleation decreases exponenially with the increase of the noise strength. Further analysis shows that this is attributed to the fact that the colored noise can affect the atom mobility in the nucleation process.
Dual drag reduction mechanism of water-based dispersion with hydrophobic nanoparticles in core microchannel and experimental verification
2012, 61 (14): 146801. doi: 10.7498/aps.61.146801
Dual drag reduction mechanisms of water-based dispersion with nanoparticle is proposed. A contrastive study is take to verify the mechanism, in which the changes of surface microstructure and wettabilities of the core slices take place before and after treating by dispersion with hydrophobic nanoparticles and scouring by water. The results show that the surface of core slice which is treated by water-based dispersion with hydrophobic nanoparticles has strong hydrophilic property, and a compact nanoparticle adsorption layer forms on it. The nanoparticle adsorption layer still exists after scouring, but the core slice surface is changed into strong/super hydrophobic, reflecting that the surfactants which are adsorbed on the nanoparticles adsorption layer surface are gradually cleaned. The water-based dispersion with hydrophobic nanoparticles are mainly manifested as the chemical surfactant drag reduction effect during initial injection. With the injection continued, the mechanical drag reduction induced by the slip effect of super hydrophobic surface is reflected mainly. Core displacement results show that the water-phase effective permeability could increase about 84.3% on average. The results strongly confirm the dual drag reduction mechanism of the water-based dispersion with hydrophobic nanoparticles.
2012, 61 (14): 146802. doi: 10.7498/aps.61.146802
The physisorption properties of CH4 in MOR and MFI zeolites are studied using the grand canonical Monte Carlo method. Some important physical quantities are obtained, such as adsorption amounts, isosteric heats, interaction energies of zeolite, CH4 molecules, etc. The results show that the physisorption properties of MOR zeolite are superior to those of MFI zeolite in all conditions. The reasons causing such different results between them are analyzed from the interaction energies between zeolites and CH4 molecules, the relationships between isosteric heats and adsorption amounts, etc.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2012, 61 (14): 147101. doi: 10.7498/aps.61.147101
By using the first-principles ultrasoft pseudo-potential approach of the plane wave based on the density functional theory, the electronic structures and their changes, the bandstructures, the densities of states, and the charge densities of pure Be-doped and Be-O codoped wurtzite AlN are calculated. The calculation results reveal that Be-doped wurtzite AlN gives rise to the formation of deep Be acceptor levels in the band gap and the carriers (hole) are localized near the top of the valence band, and the introduction of activated donor O atoms makes the acceptor level wider and the non-local characteristics distinctive, and can cause the primary level to shift toward the low energy, forming a shallow acceptor level, thereby enhancing the doping concentration of Be atoms and the stability of the system. Be, O co-doped p-type is more conducive to obtaining AlN.
2012, 61 (14): 147301. doi: 10.7498/aps.61.147301
Electric-pulse-induced resistances (EPIRs) and I-V characteristics of polycrystalline Nd1-xAxMnO3 (A = Ca, Ba, Sr, x = 0-0.9) ceramics synthesized by solid state reaction are investigated. The results show that similar to Nd0.7Sr0.3MnO3, compounds Nd0.7Ba0.3MnO3 and Nd0.7Ca0.3MnO3, with the same doped concentration as that of Nd0.7Sr0.3MnO3, can also exhibit a nonlinear I-V behaviour and a stable EPIR effect at room temperature. Further studies on the Nd1-xSrxMnO3 series indicate that the stability of EPIR is closely correlated with the Sr doped concentration. Around the half doping x= 0.5, the EPIR effect can be observed stably. With Sr concentration increasing or decreasing, however, the EPIR becomes weaker gradually and disappears completely if Sr concentration further increases or decreases. The redistribution of various defects between the electrode and bulk interface with polar pulses is proposed to explain the unique transport behaviour.
2012, 61 (14): 147302. doi: 10.7498/aps.61.147302
The resistively detected nuclear magnetic resonance (RDNMR), a high-sensitivity NMR technique developed by Klaus von Klitzing's group in 1988, is used to investigate exotic electron and nuclear spin properties in GaAs two-dimensional electron gases (2DEGs). Because the dynamic nuclear polarization (DNP) approach required for the RDNMR demonstration is strongly dependent on unique material properties of GaAs, this highly-sensitive technique has not yet been applied to 2DEGs confined in other host semiconductors. More recently, we have developed a novel DNP method for demonstration of RDNMR in a 2DEG within the typical narrow-gap semiconductor InSb. In this article, we focus on the discussion of our newly-developed DNP method, experimental details and results as well as future prospects after some preliminary remarks on the principles of RDNMR and DNP.
2012, 61 (14): 147303. doi: 10.7498/aps.61.147303
As energy crisis is aggravated, solar cell, as a common form of the development and utilization of solar energy, has attracted more and more attention all over the world. With solar cells developing towards the direction of high efficiency, thin film, non-toxic and rich raw materials, the pure silicon solar cell could not meet these requirements, so the new material and process are imminently required. This paper deals with the photovoltaic effect of the carbon material based on the silicon heterostructure, and its possible application to solar cells. Co2-C98/Al2O3/Si heterostructure with a 4 nm-thick Al2O3 layer shows the best photovoltaic effect performance with a short-current density of 18.75 mA/cm2, an open-circuit voltage of 0.447 V and a power conversion efficiency of 3.27% with AM1.5 illumination, which is much better than Co2-C98/Si heterostructure without the Al2O3 layer. The effect of Al2O3 layer is attributed to the reduction of the interface defects, the suppression of the surface recombination and the enhancement of barrier height, which are proved by the capacitance-voltage and current-voltage measurements under dark condition. This work may shed light on the carbon/silicon based solar cells.
2012, 61 (14): 147304. doi: 10.7498/aps.61.147304
The structural, the magnetic, the transport and the half-metallic properties of quaternary Heusler alloy Fe2Co1-xCrxSi are investigated. The studies of X-ray diffraction and temperature dependence of magnetization reveal that Fe2Co1-xCrxSi alloy always maintains a high degree of order, while the Curie temperature decreases gradually with the increase of Cr concentration x. Importantly, the lattice constant and the saturation magnetic moment of Fe2Co1-xCrxSi alloy follow the Vegard law and half-metallic Slater-Pauling rule, respectively. Based on the band structure calculation, Fe2Co1-xCrxSi alloy keeps a 100% spin polarization and Fermi level moves from the top of valence band to the bottom of conduction band, depending on Cr concentration x. Our results indicate that quaternary Fe2Co1-xCrxSi Heusler alloy is a promising robust half-metallic candidate for spintronics applications.
Preliminary investigation on the method of determining electron mobility of tris (8-hydroxyquinolinato) aluminum by space charge limited current
2012, 61 (14): 147801. doi: 10.7498/aps.61.147801
The charge-carrier mobility of an organic semiconducting material determines the material potential applications in devices. The investigation on mobility of organic material plays a significant role in improving the performance of organic device, such as organic light emitting diode, organic solar cell and organic thin film transistor. In this paper, we employ the space charge limited current (SCLC) method to evaluate the electron mobility of the controlled device based on tris (8-hydroxyquinolinato) aluminum (Alq3). The zero-field mobilities and field-dependent factors of the four devices are fitted respectively. The results show that depositing Al as top-electrode onto buffer layer LiF (1 nm) and Alq3 (100 nm) can significantly improve the the zero-field mobility and field-dependent factor of Alq3. The reason for that is that LiF could strengthen the complex reaction between Al and Alq3 to form Li+1Alq-1 particles, which leads to the enhanced ohmic injection and electron injection.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
Effects of annealing time on the microstructural and electrochemical properties of B-doped nanocrystalline diamond films
2012, 61 (14): 148101. doi: 10.7498/aps.61.148101
The effects of annealing time under 1000 ℃ on the microstructural and the electrochemical properties of boron-doped nanocrystalline diamond (BDND) films are investigated by HRTEM, UV and visible Raman spectroscopy, and cyclic voltammetry measurements. The results show that the size of nano-diamond grain in the film decreases with annealing time increasing. When the annealing time is 0.5 h, the grain size decreases from about 15 nm in the unannealed sample to about 8 nm and the content of diamond phase increases. When the annealing time increases to 2.0 h, the diamond grain size decreases to 2-3 nm, and the content of diamond phase decreases with the grain boundary increasing. In the case of annealing time of 2.5 h, the grain size of nano-diamond and the content of diamond phase increase slightly. The variations of nano-diamond grain size and the content of diamond phase indicate that the transformation between the diamond phase and the amorphous carbon occurs under the annealing with different times. The visible Raman spectra show that the G-peak position and the ID/IG value exhibit similar variations with annealing time increasing, revealing that the ordering of the amorphous graphite phase is improved when sp2 carbon cluster increases in number or size. The reactions on the electrode surface are quasi-reversible when the annealing times are 0.5, 1.0, 1.5 and 2.0 h. On the contrary, the reactions are irreversible when the sample is unannealed or annealed for 2.5 h. It is observed that the annealing treatment is beneficial to the improvement of the electrode mass transfer efficiency of BDND film. When the annealing time is 0.5 h, the electrode mass transfer efficiency as well as the ability of catalytic oxidation of BDND film is best. The results suggest that the smaller size of nano-diamond grain, the higher content of diamond phase and the uniform distribution of the nanocrystalline diamond grains are conducible to the improvement of the reaction reversibility on the electrode surface and the ability of catalytic oxidation of BDND films.
Study on the optical characteristic of black silicon antireflection coating prepared by plasma immersion ion implantation
2012, 61 (14): 148102. doi: 10.7498/aps.61.148102
Surface texturing is an effective method to reduce surface reflectance and improve the efficiency of silicon solar cell. In this paper, the black silicon antireflection coating is fabricated by using plasma immersion ion implantation. The surface morphology and reflectance are investigated by atomic force microscope and UV-VIS-NIR spectrophotometer, respectively. Results show that mountain-like structure with a depth of 0550 nm is fabricated on black silicon surface. Both the fractional area occupied by silicon and refractive index decrease smoothly with the increase of depth across the antireflection coating. The weighted average reflectance of black silicon is as low as 6.0% in a wavelength range of 3001000 nm. The depression mechanism of the optical reflectance is analyzed by simulating the structure with the transfer matrix method, and the simulation result fits the measured spectrum well.
Study of characteristics of plume based on hue-saturation-intensity during high-power disk laser welding
2012, 61 (14): 148103. doi: 10.7498/aps.61.148103
During high-power disk laser welding, the surface of the weldment is gasified strongly by the laser irradiation. Meanwhile, the metal vapor plume is generated and changed into the plasma state. This plume can transmit through the laser beam reversely and has a significant shielding effect on the laser beam. Therefore, the laser power which should be transferred inside the weldment is decreased, the welding efficiency and welding quality are seriously affected. By studying and exploring the relation between the characteristics and variation rules of plume and the welding quality, the state of laser welding can be monitored in real-time by the plume characteristics. In the high-power disk laser bead on plate welding of a type 304 austenitic stainless steel plate at a continuous laser power of 10 kW, an extraviolet and visible sensitive high speed color camera is used to capture the metal vapor plume dynamic images. These digital images are transfered to the hue-saturation-intensity color spaces from the red-green-blue color spaces. Also, the area of metal vapor plume and the path length which the laser beam runs through the metal vapor plume are segmented and defined as the plume eigenvalues. The fluctuation of weld bead width is used to evaluate the stability of welding quality. By analyzing the statistics of the variance and mean values of plume eigenvalues, the experimental results confirm that the stability of the welding could be evaluated by the plume characteristics dynamically.
Effect of crystallographic orientation on instability behavior of planar interface in directional solidification
2012, 61 (14): 148104. doi: 10.7498/aps.61.148104
The instability process of planar interface in directional solidification with respect to the crystallographic orientation is studied using a transparent model alloysuccinonitrile-acetone. Three typical crystal grains which have preferred dendrite, tilted dendrite and seaweed patterns at rapid pulling velocity respectively are chosen in our experiment. The experimental results show that the preferred dendrite grain has the shortest incubation time and the smallest initial perturbation wavelength of planar interface instability, the tilted dendrite grain has the largest ones and the seaweed grain has median ones. These results accord qualitatively with previous analytical results and phase-field simulation results. It is also found that the interfacial non-steady-state evolution behaviors of the preferred dendrite grain and the tilted dendrite grain are significantly different from that of the seaweed grain, suggesting that the non-steady-state evolution behavior of planar interface instability is closely related to the crystallographic orientation.
2012, 61 (14): 148105. doi: 10.7498/aps.61.148105
The structural behaviors of IrTi are studied using first-principles density-functional theory with pseudopotentials and a plane-wave basis. Phonon calculations indicate that the tetragonal (L10) structure is dynamically unstable. We obtain the orthorhombic structure (Cmmm) which is shown to be a global energy minimum by the frozen phonon method. The resulting structure is mechanically and dynamically stable and its lattice constant is similar to the experimentally observed lattice constant of low-temperature structure, which demonstrates that the low-temperature phase of IrTi is the orthorhombic structure (Cmmm). Thus, we put an end to the experimental debate regarding the low-temperature phase: whether it is orthorhombic or monoclinic, and demonstrate theoretically that the IrTi alloys will undergo a cubic→tetragonal→orthorhombic transformation.
2012, 61 (14): 148106. doi: 10.7498/aps.61.148106
The silicon nitride films are prepared on c-Si substrates by plasma enhanced chemical deposition (PECVD) with silane as the silicon source in mixture gas (N2/NH3) as the nitrogen source. We prepare different kinds of films at different flow rates of the nitrogen with other conditions kept the same. X-ray diffraction (XRD) is employed to analyze the crystal structure, and the existence of the silicon nanoparticles embedded in the silicon nitride film is verified according to the caculation of the lattice size. Fourier transform infrared spectra are employed to probe the concentration evolutions of various chemical bonds with the flow rate of the nitrogen, with which by combining the chemical reaction process, the formation mechanism of the silicon nano-clusters embedded in silicon nitride is investigated. The results show the initial positions of silicon nanoparticles are conducible to the formation of silicon nanoparticles when the chemical reaction proceeds towards the direction in which the SiSi bonds form. In addition, XRD analysis and photoluminescence characteristics show that the size and the concentration of the embedded nanoparticles increase with the flow rate of the nitrogen increasing.
2012, 61 (14): 148201. doi: 10.7498/aps.61.148201
Gel propellant has the advantage of controllable flux as liquid propellant and long-term reservation as solid propellant, however, the evaporation and combustion problem of gel spray droplet bores with the gel propellant development and combustor design all the time, and hampers gel propellant practical engineering applications. In this paper, the gel single droplet combustion experiment system is designed and constructed, and then the evaporation and combustion mechanism is explored deeply based on the experimental phenomena of organic gel unsymmetrical dimethylhydrazine (UDMH) single droplet burning in nitrogen tetroxide. The organic gel spray droplet multi-component evaporation model is developed for three different evaporation phases of the gel layer, i.e. the forming, expanding and bursting of the gel layer based on the single droplet evaporation characteristics in experiment, and then the gel single droplet vaporization in high temperature gas phase is numerically simulated and compared with the result of conventional liquid droplet using the elementary model parameters and physic property parameters. The result shows that the gel content on the droplet surface increases slowly at the beginning of evaporation, however it would increase rapidly to a mass fraction of 95% and form the gel layer in a limited time after exceeding a critical evaporation time, which results in surface mass flux dropping to 0 and surface temperature reaching the UDMH boil point rapidly. After the gel layer forming, the droplet radius and surface UDMH vapor mass fraction exhibit oscillation as the swelling-bursting phenomena in experiment. The gel droplet surface temperature holds above the boil point and the mass flux of gel droplet inner boiling evaporation is stronger than the conventional liquid droplet surface steady evaporation which makes the life time of gel droplet much shorter.
2012, 61 (14): 148401. doi: 10.7498/aps.61.148401
In this paper, we present a model of calculating the electron beam space-charge field using the partial Galerkin moments method. The space-charge field can be expanded as a series of different modes and thus using the Galerkin series. So, the space-charge field can be obtained finally through the solving the Galerkin coefficient equations set. A 2.5-dimensinal computer program of the beam-wave interaction in a klystron is compiled based on this model. Using the results from the program, the characteristic of the beam space-charge field is analyzed and the effects of the parameter in the simulation on the calculation result of the space-charge field are studied. This model uses the particle in cell method and it can be used in the 2- or 3-dimensional calculation model of the beam-wave interaction in a klystron.
The model for parameter adaptation problem in green cognitive radio and a particle swarm optimization based solution are proposed. Simulations are conducted to show the performance of the proposed method. Results show that given different quality of service (QoS) requirements, the proposed method can reduce the total transmit power of the system while maintaining the QoS requirements. Thus the goal of reducing energy consumption is reached.
2012, 61 (14): 148403. doi: 10.7498/aps.61.148403
An ocean surface wind direction retrieval method from multi-polarization airborne synthetic aperture radar images is presented in order to retrieve high precision wind direction SAR image and improve the accuracy of ocean surface wind. The method relies on the ability of two-dimensional (2D) continuous wavelet technique with better time-frequency than Fourier transform and local gradient, which combines 2D continuous wavelet and FFT to exact the ocean surface wind direction from airborne SAR images. The proposed method is executed by using several kinds of mother wavelet functions for the C-band co-polarization and cross-polarization airborne sounding images, and the retrieved ocean surface wind direction is compared with NCEP data and buoy data. The verification results show that the wavelet based ocean surface wind direction retrieval algorithm is suited to retrieve wind direction from airborne SAR sounding data. The choice of mother wavelet function has a certain influence on the results, and 2D Mexican-Hat is the best mother wavelet for wind retrieval. Both co-polarization and cross-polarization airborne sounding images are suited to retrieve ocean wind direction.
Radiometric calibration accuracy of airborne synthetic aperture radar in ocean surface wind field sounding
2012, 61 (14): 148404. doi: 10.7498/aps.61.148404
Sounding accuracy is difficult to study systematically and effectively because of complex airborne synthetic aperture radar system. In this paper, according to the role of radiometric calibration accuracy in ocean surface wind field sounding process of airborne synthetic aperture radar, a method is proposed to investigate the system sounding accuracy by radiometric calibration accuracy. The parameter relationship of CMOD4 geophysical model function is simulated. The radiometric calibration accuracy of airborne synthetic aperture radar ocean surface wind field sounding at different incidence angles and in wind directions is ascertained. The results show that the radiometric calibration accuracy decreases with incidence angle increasing and changes periodically with wind direction changing. The law is accordance with the change of backscattering cross-section, cased by wind speed in CMOD4 geophysical model function. According to this law, we find that the accuracy of ocean surface wind field retrieval can be improved by using active calibration scale, designing carrier aircraft crosswind flight and selecting the large incidence angle sounding imagery as retrieval area.
The application of phase contrast imaging to implosion capsule diagnose in high energy density physics environment
2012, 61 (14): 148701. doi: 10.7498/aps.61.148701
The variation of multi-layer shell capsule in implosion process is the most important part of inertial confinement fusion. Phase contrast imaging which relies on gradients of the refractive index and wave interference is proposed to characterize the typical implosion capsule. The experiments are performed on the Shenguang Ⅱ laser facility. The point-like X-ray source at 4.75 keV can be efficiently produced from laser interactions with Ti target and observed by pinhole- point backlight technique. The phase contrast images obtained with point-like X-ray source provide complementary information about the multi-layer shell capsule, and the spatial resolution is better than 10 μm. The X-ray phase contrast imaging shows more detailed features than absorb imaging, and good agreement with one-dimensional numerical simulations.
2012, 61 (14): 148801. doi: 10.7498/aps.61.148801
Using the density functional theory of first principles, we investigate the binding mechanism of a single transition metal atom-titanium adsorbing hydrogen molecules. We find that a single titanium atom can absorb eight hydrogen molecules. The hydrogen molecules around Ti atom form two symmetrical pyramid-like structures with an average adsorption energy of -0.28 eV. By calculating the orbital energie and the distribution of differential charge density, we analyse the intrinsic physical mechanism of determining adsorption structure, adsorption energy and hydrogen storage capacity. The results show that a 4s electron of a titanium atom transfers to the 3d orbit, which can produce a strong polarization electric field, resulting in polarization of the hydrogen molecules. Therefore, the titanium atom adsorbs hydrogen molecules by electrostatic polarization. Our results will present a guidance for designing high-density hydrogen storage materials.
2012, 61 (14): 148901. doi: 10.7498/aps.61.148901
We propose a method of estimating complex network topology with a noisy environment. Our method can estimate not only dynamical equation of the chaotic system and its parameters but also topology, the dynamical equation of each node, all the parameters, coupling direction and coupling strength of complex dynamical network composed of coupled unknown chaotic systems using only noisy time series. Estimating the system structure and parameter is regard as estimating the linear regression coefficients by reconstructing system with universal polynomial structure. Reconstruction algorithm of Bayesian compressive sensing is used for estimating the coefficients of regression polynomial. For the reconstruction from noisy time series we adopt relevance vector machine, namely we use sparse Bayesian learning to solve sparse undetermined linear equation to obtain the objects mentioned above. The Lorenz system and a scale free network composed of 200 Lorenz systems are provided to illustrate the efficiency. Simulation results show that our method improves the robust to noise compared with the compressive sensing and has fast convergence speed and tiny steady state error compared with the least square strategy.
2012, 61 (14): 148902. doi: 10.7498/aps.61.148902
In reality many complex networks present modules or community structures obviously. Modularity is a benefit function used in quantifying the quality of a division of a network into communities. And it usually can be used as a basis for optimization methods of detecting community structure in networks. But the most popular modularity which is proposed by M. E. J. Newman and M. Girvan has the resolution limit in community detection. Multi-resolution modularity cannot overcome the misclassifications caused by merging and splitting the communities either. In this paper, we propose a multi-resolution density modularity based on the network density. The proposed function is tested on the artificial networks. Computational results show that it can reduce the rate of misclassification considerably. And the systematicness of the community structures can be demonstrated by the multi-resolution density modularity.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
Viscoelastic relaxation attenuation property for saturated sandstones and corresponding investigation of micro-scale mechanism
2012, 61 (14): 149101. doi: 10.7498/aps.61.149101
Attenuation experiments are performed by Metravib dynamic mechanical analyzer with sine wave loading style to study the viscoelastic relaxation property of pump-oil saturated and glycerol saturated Pengshan sandstone and Zigong arkoses with three kinds of porosities. Based on the thermal relaxation regularities, the activation energies and the atomic vibration frequencies of relaxation attenuation peaks for three kinds of saturated sandstones are evaluated. The results show that activation energies and atomic vibrations frequencies of sandstone samples are lower than those of interstitial atoms. The overall vibration behavior of atomic cluster with defects can be used to explain why vibration frequencies are low in samples. Besides the solid atoms, the gas and liquid atoms filled in defects contribute greatly to overall vibration of sample. Saturated sandstone, cemented by a combination of mineral crystals, is a polycrystalline, multiphase solid with internal complex structure and widespread defects, and it easily takes on thermal relaxation property under sine wave loading. Such flaws and defects as point defects, dislocation and grain boundary in samples and their interaction interaction can produce relaxation attenuation peak. To explain the relaxation mechanism by saturated liquid and internal structure of the sandstone, it is natural to relate the attenuation characteristics to its macro-meso-structure. It is notable that when taking the defects, multiple phase boundary into consideration, a new interesting phenomenon appears, there produces multi relaxation with broader peak and more widely distributed parameter. This investigation is helpful to study theoretical model and seismic data interpretation.
The development of atmosphere-current-wave fully coupled model and its application during a typhoon process
2012, 61 (14): 149201. doi: 10.7498/aps.61.149201
The shared memory and semaphores of inter-process communication in Linux system are used as the pipeline to couple the WRF, POM and WW3 models. The forcing from atmosphere to sea and wave, the changes in atmospheric underlying surface induced by ocean and the wave-induced mixing are all involved in the high-resolution fully coupled typhoon model. The typhoon ‘KAEMI’ is taken as a simulated example and the outcomes are compared with the available observation data to verify the good performance of the fully coupled model. The results show that after the spin-up, the coupled model could reproduce the typhoon, and the simulated surface lowest pressure, wind speed and track are all consistent with the observations. Meanwhile, the model can also have a good performance about the sea surface temperature cooling and the significant wave height. The simulated errors are related to the microphysical process parameterizations, the large initial fields, the construction of bogus typhoon and the physical process parameterization in upper ocean.
2012, 61 (14): 149202. doi: 10.7498/aps.61.149202
It is viewed traditionally that the attenuation and scattering of rain have no effect on C-band scatterometer because its wavelength is greater than the diameter of raindrops, so rain effects on C-band scatterometer wind measurement are often ignored. According to the attenuation and the volume backscatter of the scatterometer signal by raindrops and the perturbation of the water surface by rain, in this paper, we derive the radar equation of the rainfall, collect the data from ASCAT backscatter, rain data from PR and wind field from the European Centre for Medium-Rang Weather Foremasts in 2010, and quantitatively analyze the influence of rainfall on the normalized radar backscattering cross section of the C-band scatterometer. Our results show that the attenuation increases as the rain rate and the incidence angle increase, the volume backscatter and the perturbation of the water surface increase with the increase of rain rate and decrease with the increase of incidence angle, and the influence of the perturbation on the wind measurement of the scattermeter is greater than the volume backscatter. In addition, we establish the C-band active microwave radiative transfer model for the rainfall by the radar equation and the collected data. The experimental results indicate that the new model can improve the C-band scatterometer wind measurement accuracy under rainfall conditions.
2012, 61 (14): 149203. doi: 10.7498/aps.61.149203
In the traditional implementations of four-dimensional variational data assimilation (4dvar for short), it is assumed that the model used is perfect. However the model error in the model can directly affect the accuracy of data assimilation. The weak constraint 4dvar is an effective way of correcting and estimating the model error in 4dvar. In this paper, an approach to weak constraint 4dvar with model error forcing control variable is studied and implemented in the one-dimensional shallow water equations. The results show that when the model error cannot be ignored, the prediction error with the weak constraint 4dvar is smaller than with the traditional 4dvar in both the assimilation window and the prediction period, and the improvement with weak constraint 4dvar is more obvious in the condition with large model error. Also, the weak constraint 4dvar approach to estimating model error captures some basic features of model error including the magnitude and the characteristic of distribution.
A new method of analoguedynamical prediction of monsoon precipitation based on analogue prediction principal components of model errors
2012, 61 (14): 149204. doi: 10.7498/aps.61.149204
To correct the model errors in analogue-dynamical prediction, a new idea of using the analogue prediction of principal components of model errors, instead of analogue prediction of model error directly, is proposed. By decomposing the empirical orthogonal function, the principal components of the model errors are divided into two parts subjectively: predictable and unpredictable. For the predictable part, it is analogically predicted by the scheme of dynamical and optimal configuration of multiple predictors; while for the unpredictable part, it is estimated by average of the system. Based on the National Climate Center (NCC) of China operational seasonal prediction model results for the period 1983-2010 and the US National Weather Service Climate Prediction Center merged analysis of precipitation in the same period, together with the 74 circulation indices of NCC Climate System Diagnostic Division and 40 climate indices of NOAA of US during 1951-2010, the method is implemented in objective and quantitative prediction of monsoon precipitation in Northeast China. The independent sample validation shows that this technique has effectively improved the monsoon precipitation prediction skill during 2005-2010, for which the averaged anomaly correlation coefficients and the system correct of errors are 0.29 and 0.04 respectively. This study demonstrates that the analogue-dynamical approach can enhance the prediction level of NCC operational seasonal forecast model obviously.
The research on the sensitivity of climate change in China in recent 50 years based on composite index
2012, 61 (14): 149205. doi: 10.7498/aps.61.149205
Climate change index is one of advanced issues in climate change research. There exist many specific indices in climate change research in China and other countries, but comprehensive indexes are very rare. So in this paper, a comprehensive climate change index (CCI) is defined based on single factor of temperature and precipitation index to assess the sensitivity of climate change, and the comprehensive information about climate change is obtained. Because the index size represents the difference in frequency between before and after extreme climate events around abrupt climate change, reflecting the ability for one region to respond to climate change and the sensitivity to the climate change, the index indicates a variety of information about climate change and can provide a certain judgment basis to better deal with extreme climate events. According to the CCI, the climate change and its regional sensitivity in China in recent 50 years are discussed. The results show that Inner Mongolia, northeast central, northwest and central Yunnan have higher CCI indexes, which indicates that the extreme climate events in these regions happen more frequently after the abrupt climate change. The mean CCI is computed of all stations in each province in China, showing that South China and east part of Southwest China each have a minimal index, indicating that these areas are not sensitive to climate change; in the North and Northeast China extreme events happen frequently. Climate change is obvious in high latitude and tropical and subtropical regions, the North and Southwest China are more sensitive, while the South of the Yellow River is less sensitive. The coastal areas with relatively high CCI have strong sensitivities due to the heavy rainfall influence from monsoon and typhoon.
2012, 61 (14): 149301. doi: 10.7498/aps.61.149301
The parameters retrieved from GPS occultation data can be assimilated into numerical weather prediction model. The accuracy of excess phase is affected by the noise from ionosphere and L2 frequency, especially, when L2 signal is encrypted. In order to improve the accuracy of excess phase and remove the noise by the single Difference (SD) method from reference link into occulted link, we use the updated single difference (USD) method to compute atmospheric excess phase. The combined frequency LC data from the reference link are used and smoothed in an interval time window of 2 s. The antenna phase center correction and relativity correction are estimated. The USD method is validated by utilizing COSMIC data. The case analyses of representative rising and setting occultation events indicate that the excess phases from USD and COSMIC are in good agreement. The excess phase of globally distributed 440 occultation events is also computed. The results show that USD method can obviously improve the distribution of mean bias and standard deviation of bias, which demonstrates the reliability of our USD algorithm. The USD method can be applied to the data processing for FY-3 GNOS (GNss Occultation Sounder) occultation mission which uses semi-codeless technique to track L2 signal.
2012, 61 (14): 149302. doi: 10.7498/aps.61.149302
Optimal design of nuclear magnetic resonance (NMR) logging device sensor can enhance its detection performance and improve signal-to-noise ratio, and the accuracy of results from numerical method is absolutely critical for optimal design. In this paper, an eccentric NMR logging device static field distribution and radio frequency field distribution are simulated by the electromagnetic finite element method, and the influences of model shape, model size and element shape on simulation results are analyzed. A comparison between the measurements and simulation data indicates that they are in good agreement with each other. In the design process of NMR logging device sensor, by choosing a circinal model which is similar to the shape of borehole, setting model size to be 10-15 times the sensor diameter, and adopting triangle element, the accuracy of numerical simulation can be improved and the reliability of optimization design can also be enhanced.