Vol. 63, No. 20 (2014)
2014-10-20
GENERAL
2014, 63 (20): 200201.
doi: 10.7498/aps.63.200201
Abstract +
Yb-doped double-clad fibers are prepared through a conventional modified chemical-vapor deposition technique and solution doping method: each fiber contains a core of around 10 μm in diameter. These fibers are divided into groups under 60Co γ radiations of different doses, and we investigate the fiber absorption spectra and laser spectral properties before and after irradiation. Experimental results show that with increasing the irradiation dose the absorption of the fiber after irradiation increases significantly in the visible region, we believe that the enhancement of optical fiber absorption in the visible region may be due to the color center defects existing already in the fiber (such ODC (Ⅱ)) and the color center defects produced by the irradiation (E'center, POR and Yb2+ions). We also analyze slope efficiencies, bare efficiencies, and transmission characteristics of the fiber before and after laser irradiation. Finally, we use the power-law to fit the radiation-induced losses of the fiber under different radiation doses, and the results obtained in this paper provide a theoretical basis for studying the anti-radiation of optical fibers.
2014, 63 (20): 200302.
doi: 10.7498/aps.63.200302
Abstract +
Cold polar molecules have long coherence time and strong dipole-dipole interaction and thus are regarded as a promising quantum carrier for quantum information processing. In this paper, by employing the pendular states of polar molecules as qubit, we investigate the properties of three types of tripartite quantum correlations for three linear polar molecules and numerically analyze the relations of tripartite negativity, measurement-induced disturbance (MID), and tripartite quantum discord (TQD) to three dimensionless reduced variables that relate to external field strength, dipole moment, rotational constant, dipole-dipole coupling, and temperature. The result shows that if the values of the other parameters are fixed, the three quantum correlations decrease with the increase of the field strength, and the three quantum correlations first increase to their respective maxima and then diminish gradually as the dipole-dipole coupling becomes larger. Moreover, as the temperature increases, both tripartite negativity and TQD become small, but with the variation of temperature there exhibit different evolution tendencies for MID between the influence of the electric field strength and that of the dipole-dipole coupling. In addition, the three quantum correlations of polar molecules in pendular state can be manipulated by tuning the external electric field strength, dipole-dipole coupling, and temperature.
2014, 63 (20): 200501.
doi: 10.7498/aps.63.200501
Abstract +
In this paper we study the thermodynamic performance of a Brownian heat engine, which is driven by temperature difference. Brownian particles move in a periodic single-barrier sawtooth potential under the action of an external load force, and contact an alternating hot and cold reservoir. The kinetic energy change of the Brownian particles and the heat leak between hot and cold reservoir are considered simultaneously. The dynamics of the Brownian particle is governed by the Smoluchowski equation. The expressions for heat flow, power output and efficiency are derived analytically. The influences of the height of barrier, the external load force and the heat reservoir boundary on efficiency and power output are discussed by numerical calculation. It is shown that the Brownian heat engine is irreversible when the irreversible heat flow caused by the kinetic energy change and the heat leak exist. The curve of the power output versus the efficiency is a loop-shaped one. When the barrier boundary coincides with the heat reservoir boundary, the heat engine has a maximum power output. The efficiency of the heat engine is enhanced, but the power output is reduced when the heat reservoir boundary is changed.
2014, 63 (20): 200304.
doi: 10.7498/aps.63.200304
Abstract +
Measurement-device-independent quantum key distribution (MDI-QKD) is immune to all the detection attacks, thus when it is combined with the decoy state method, the final key is unconditionally safe. In this paper, we propose to perform MDI-QKD with odd coherent state (OCS) and compare the results with weak coherent source scenario. Our simulation indicates that both the secure key rate and transmission distance can be improved evidently with OCS owing to the lower probability of multi-photon events of the OCS. Furthermore, we apply the finite key analysis to the decoy state MDI-QKD with OCS and obtain a practical key rate.
2014, 63 (20): 200502.
doi: 10.7498/aps.63.200502
Abstract +
Based on the fact that the inductor and the capacitor are of fractional order in nature, the mathematical model of the fractional order boost converter in pseudo continuous conduction mode is established by using the fractional order calculus theory. Due to the affine non-linear nature in this mathematical model and a similar Lyapunov stability theorem of the fractional order system, a fractional order non-linear controller is designed. On the basis of chain fractance and the improved Oustaloup algorithm, the circuit model of fractional order inductor and capacitor are built. The fractional order controller is verified by the Matlab/Simulink software. Simulation results show that the controller enhances the dynamic and steady-state performance, so as to realize the stability and achieve good dynamic performance of the fractional order system during large fluctuation of power supply and load disturbance.
2014, 63 (20): 200503.
doi: 10.7498/aps.63.200503
Abstract +
The characteristics of classical Chua's circuit variables proposed by Leon O. Chua are the voltage on capacitor, the current in inductor and its normalized equation of state. Based on these characteristics, in this paper, three optimized and scaled circuit design methods are proposed. One is the Chua's circuit with five operational amplifiers used for limiting power amplitude for integration. The other two are Chua's function equivalent circuits with nonlinear diode for cellular neural network system integration. At the same time, an optimized cubic form of Chua's diffeomorphism circuit is given. The methods of designing these circuits are applicable to the cubic random circuits dominated by three broken lines. The universality and practical application of design method are demonstrated in the experiment on chaos secure communication.
2014, 63 (20): 200505.
doi: 10.7498/aps.63.200505
Abstract +
Extreme learning machine (ELM) is a recently proposed learning algorithm for single-hidden-layer feedforward neural networks, which has a fast learning speed while avoiding the problem of local optimal solution. However, the performance of ELM may be affected due to the random determination of the input weights and hidden biases. In this paper, a multi-objective optimized extreme learning machine (MO-ELM) is proposed to solve this problem. The algorithm uses the no-dominated sorting genetic algorithm II algorithm to select input weights and hidden biases. Both the learning errors and the mean square value of output weights are used as optimization objects. The MO-ELM algorithm is used in the multi-step forecast of irregular complex flow oscillations of natural circulation system in rolling motion, and the influences of learning errors and output weights on forecast results are analyzed. Experimental results show that MO-ELM can achieve good generalization performance with much more compact networks and provide a relatively accurate forecast method of flow rate, and the forecast results can be used as reference to nuclear power system operators.
2014, 63 (20): 200508.
doi: 10.7498/aps.63.200508
Abstract +
The velocity of lithium iron across the entrance of carbon nanotube VLi is an important factor for the charge performance of lithium iron battery. The molecular dynamics simulation is adopted to evaluate the effects of control factors which include electric strength, functional group type, the diameter of carbon nanotube and temperature. By the L16(45) orthogonal array method, the simulations are carried out. The order of influences of control factors is electric filed intensity > functional group > diameter > temperature. Within the ranges of the control factors studied in this work, VLi increases with increasing the diameter of carbon nanotube and electric field strength. VLi decreases with successively modifying the functional groups at the end of carbon nanotube into -H, -OH, -NH2 and -COOH. With the increasing of temperature, VLi first increases then decreases, but on the whole its change is not big.
2014, 63 (20): 200204.
doi: 10.7498/aps.63.200204
Abstract +
In order to well understand the probability hypothesis density, according to the physical-space model given by Ozgur Erdin, we deduce the probability hypothesis density filter function with the Bayes theorem and the total probability theorem. The derivation result is identical to the result in the literature, and the derivation process is described in detail. The results in this paper will provide a theoretical basis for solving the target-death problem.
Analysis and relief method of reentry aerodynamic load based on matched asymptotic expansions method
2014, 63 (20): 200401.
doi: 10.7498/aps.63.200401
Abstract +
Reentry velocity of lunar module reaches the second cosmic velocity, which could make the aerodynamic environment insupportable. So it is essential to analysis the reentry aerodynamic load. The equation of motion for reentry vehicle is a group of ordinary differential equations, and numerical methods are inadequate for online mission because their computation amount is too large. An analytical method of solving the reentry equation of motion is proposed in this paper to analyze the reentry aerodynamic load. First, matched asymptotic method is used to obtain solutions of longitudinal equation of motion in outer and inner region independently and combine them to obtain a unified closed-form solution. Reentry aerodynamic load has been analyzed in three fragments using the closed-form solution, and approximate solution of load is compared with the exact solution. Second, suppositional initial conditions are obtained by solving the closed-form solution using current state, then an analytical method of predicting the first load peak is proposed, and the relative prediction error is analyzed for different bank angles. Third, the load relief method based on load peak prediction is proposed, which can redistribute the total dissipated energy in the whole reentry process, and the validity of the method is verified by Monte Carlo simulation.
2014, 63 (20): 200504.
doi: 10.7498/aps.63.200504
Abstract +
Scale-less range is an interval of measurement of time series in which fractional self-similarity exists statistically. In order to solve the problem of the lack of necessary steps to calculate fractal range in multi-fractal detrended fluctuation analysis algorithm (MF-DFA) in traffic flow, a new scale-less identification method based on MF-DFA is proposed through analyzing the characteristics of the mutation point in logistic curve of traffic flow wave function in steps of MF-DFA and the principles of the traditional fractal scale-less range identification method. Beijing's road network is taken for example to investigate the fractal scale-less range. Analysis results show that the identification method based on MF-DFA algorithm is valid, automatic and steady in identifying the fractal scale-less range in Beijing's traffic flow. Further, the reason why the scale-less range in traffic is limited is that small traffic flow waves account for a bigger percentage in scale-less range while big wave is bigger so that it is out of the scale-less range.
2014, 63 (20): 200301.
doi: 10.7498/aps.63.200301
Abstract +
According to small size and easy-to-realize features of visible light radiator, a new idea of adopting light wave produced by light radiator models to measure an object microwave radar cross section (RCS) at big scale factor is presented. The similitude of object RCS in inequal electrical length condition is discussed. The constrained condition and compensation process for such a model measurement are also provided.
2014, 63 (20): 200507.
doi: 10.7498/aps.63.200507
Abstract +
Considering the interspecies and intraspecies interactions as being repulsive, we study the dynamic behaviors of the gray-gray and black-black solitons in two-species Bose-Einstein condensates trapped in the harmonic external potentials. The results show that in the absence of the axial trapping frequencies, both the gray-gray and black-black solitons can keep localization stable. In the presence of the axial trapping frequencies, the gray-gray can be converted into bright-bright solitons.
2014, 63 (20): 200202.
doi: 10.7498/aps.63.200202
Abstract +
The periodic solution problem of a relative rotation nonlinear system is considered. Firstly, the relative rotation nonlinear dynamic system is established, which contains nonlinear elastic force, commonly damped force and forcing periodic force. Secondly, the result about the nonexistence of periodic solution of the corresponding autonomous system is obtained, and some results about the existence of periodic solutions of the system are obtained by using the continuation theorem of coincidence degree theory. The significance is that we generalize the existing results of the literature. Finally an example is given to illustrate that our results are right.
2014, 63 (20): 200506.
doi: 10.7498/aps.63.200506
Abstract +
A missing data recovery method based on compressed sensing is proposed for vibration data of rotating machinery. Firstly, the incomplete signal is transformed into lossy signal by setting the data values corresponding to the time without input as zeros. According to the indices of zero elements in lossy signal, the observation matrix in the frame of compressed sensing is constructed based on identity matrix. Secondly, the dictionary matrix with which the vibration signal can be represented sparsely is chosen or constructed according to the signal needed to be recovered and other prior knowledge. Finally, the original complete signal is recovered based on the lossy signal, observation matrix and dictionary matrix by using an effective and steady pursuit algorithm. The efficiency of the proposed method is validated with simulation data and practical bearing vibration data. Recovery results are discussed by comparing the characteristic values corresponding to the complete signal, lossy signal and recovered signal in time domain and frequency domain. The test results show that the proposed method can well achieve the missing data recovery, and from the view of statistical characteristics, the recovery signal can describe the complete vibration signal more accurately than the lossy signal.
2014, 63 (20): 200701.
doi: 10.7498/aps.63.200701
Abstract +
An adaptive compressed sensing architecture based on selective measure is proposed in this paper, in order to reduce the effects of non-sparse noise component on the performance of existing compressed sensing reconstruction algorithm. Firstly, in this paper we analyze and deduces the statistics characteristic of the measured noise and its influence on the reconstruction performance; then we propose a compressive-domain projection filter combined with iterative noise detector method to obtain the location information of noise subspace based on minimal output energy criteria; thirdly, we measure matrix adaptively with the location information, and focus on the signal subspace directly without sensing the noise component in analog part. Simulation results show that compared with the existing compressed sensing procedures, our method can obviously improve the performance of reconstruction of signals with noise, and can be used to perform the front-end spectrum analysis of absorbing materials and better detect the active channels in cognitive radio.
2014, 63 (20): 200702.
doi: 10.7498/aps.63.200702
Abstract +
A novel time-difference fluxgate sensor with weak sinusoidal and Gauss noise excitation is presented in this paper. Expression of the transition rate is derived according to the bistable peculiarity and the Fokker-Planck equation of the soft magnetic material. Relationships among transition rate, external magnetic field, amplitude of the excitation field, and noise intensity are discussed through numerical simulation. By converting the periodic transition rate signal to the square signal, the time difference between the high level and the low level of the square signal can be related with the external magnetic field. And the expression of the sensor's sensitivity is derived. Simulation results show that the sensitivity is inversely proportional to the amplitude and frequency of the excitation magnetic field, and the measuring range is proportional to the amplitude of the excitation magnetic field. Experiments have been carried out on a fluxgate prototype with a measuring range of ±10.7 A/m. The minimum sensitivity could achieve 9.8696 ms/(A/m), and the novel time-difference fluxgate can be of great interest in the detection of quasi-static extremely weak magnetic field.
2014, 63 (20): 200203.
doi: 10.7498/aps.63.200203
Abstract +
An equivalence transformation of Whitham-Broer-Kaup equations with variable coefficients (VCWBK) is obtainedby using modified Clarkson-Kruskal direct method. Further, the relationship between the solutions of VCWBK equationsand ones of the corresponding WBK equations with constant coefficients is obtained. In addition, by applying directsymmetry method, some symmetries and similarity reductions of the corresponding WBK equations with constantcoefficients are derived. Using an auxiliary function to solve some special cases, we obtain some new exact solutionsof VCWBK equations, including rational solutions, hyperbolic function solutions, trigonometric function solutions, andJacobi elliptic function solutions.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2014, 63 (20): 207303.
doi: 10.7498/aps.63.207303
Abstract +
A structural stable crystalline analogue of α-graphyne is predicted by an efficient semi-empirical Hamiltonian scheme based on quantum mechanics. This analogue can be derived by substituting carbon atoms in six corners of α-graphyne with Ge atoms, which is referred to as α-CGeyne. We investigate the structure stability, electronic and thermodynamic properties of this analogue, and the calculations indicate that the most stable structure is a hexagonal honeycomb planar structure with a lattice constant of 8.686 Å. This material is a semiconductor with a band gap of 1.078 eV and it can keep intact until 2280 K and recovers to its initial structure through quenching.
2014, 63 (20): 207101.
doi: 10.7498/aps.63.207101
Abstract +
The geometry and band structures as well as the density of states of Ti-doped nonstoichiometric W18O49 nanowire are studied by employing the ab-initio plane-wave ultra-soft pseudo potential technique based on the density functional theory. Meanwhile, the adsorption and NO2-sensing properties of the doped nanowire are analyzed by further calculating the adsorption energy, planar averaged charge density difference and atomic Mulliken charge population of the NO2/Ti-W18O49 nanowire adsorption system. The results reveal that Ti-doping modifies the electronic structure and then the gas sensitivity of W18O49 nanowire obviously. After Ti-doping, new electronic states are introduced and the band structure near Fermi level (EF) is changed obviously, resulting in the variation of the band gap and EF position and then the increase of electronic conductivity. The adsorbed NO2 molecule acts as a charge accepter to extract electrons from the conduction band of W18O49 nanowire, causing the gas-sensing response due to the conductivity change of the nanowire. NO2 adsorption on Ti-doped W18O49 nanowire can cause more electrons to transfer from nanowire to NO2 molecule than the case on pure W18O49 nanowire, theoretically suggesting the validity of Ti-doping that can improve the sensitivity of W18O49 nanowire. The population calculations on different gas molecules adsorbed on Ti-doped W18O49 nanowire further indicate the much good sensitivity and selectivity of the doped nanowire to NO2 gas.
2014, 63 (20): 207304.
doi: 10.7498/aps.63.207304
Abstract +
In this paper, InGaN/GaN multiple quantum-well blue light emitting diodes (LEDs) are successfully grown on Si(110) and Si(111) substrates. The micro-structural properties of the LEDs are characterized by means of high-resolution X-ray diffraction, atomic force microscopy, Raman spectra, and temperature dependent photoluminescence measurements. The results show that the sample on Si(110) substrate exhibits the high crystal quality, weak tensile strain, and large internal quantum efficiency. The optoelectronic properties of the LED devices are also investigated. The I-V curves indicate that the LED devices fabricated on Si(110) and Si(111) substrates have similar series resistances and low reverse leakage currents, but the LED devices fabricated on Si(110) substrate possess lower turn-on voltages. The relationship between light output intensity and injection current suggests that the LED device fabricated on Si(110) substrate has a strong light output efficiency. The study on the variation of spectral peak energy with injection current of the LED device reveals that LED device on Si(110) substrate presents a smaller spectral shift range when increasing the injection current. And the smaller spectral shift range reflects the weak quantum-confined Stark effect in the device, which can be attributed to the high crystal quality and weak strain between well layer and barrier film in the LED sample grown on Si(110).
2014, 63 (20): 207801.
doi: 10.7498/aps.63.207801
Abstract +
Based on the surface plasmon primitives, subtractive color filters fabricated from ultrathin metallic film nanogratings, have many advantages, such as reliability, easy fabrication and high transmittance. In this work, color filter behaviors of films of four metals, i.e., silver, copper, aluminum and nickel, with one-dimensional nanogratings are systematically studied. We obtain their transmission spectra, the chromaticity coordinates and the empirical formulas of the relationship of the wavelength of transmission minimum with film thickness and period. It is found that nickel has a poor ability to select waves, which is not an ideal material for color filter. Silver, copper and aluminum perform better in selecting the wavelength. They have different transmission spectra, specifically, silver has a wide range of wavelength selection, copper is appropriate for the long wavelength filtering, and aluminum for short wavelength filtering. To verify the simulation results, we fabricate some one-dimensional nanogratings with different materials by using a direct-current magnetron sputtering system and focused ion beam machine. The experimentally observed colors for all fabricated Ag, Cu, and Al film nanogratings are consistent with their simulation results. Both simulation and experimental results reveal that different metal materials and different nanostructures exhibit lager differences in color filter behavior. The obtained results here are useful for designing desirable color filters by selecting the appropriate material and nanostructure.
2014, 63 (20): 207302.
doi: 10.7498/aps.63.207302
Abstract +
The formation energy and electronic structure of iodine (I)-doped rutile TiO2(110) surface are investigated using the first-principles method based on the density functional theory. The results indicate that I prefers to replace the five-coordinated Ti in the oxidation environment and the bridging O could be replaced by I preferentially in the reducing environment. Whether I replaces O or Ti can reduce the band gap and cause the red shift of the absorption band edge or produce the absorption in the visible light. The band gap narrows most obviously when I replaces the bridging O. The absorption spectrum shows that I doping could not only improve its visible light response but also enhance its absorption peak of UV-light, leading to the improvement in photocatalytic performance under visible and UV light.
2014, 63 (20): 207102.
doi: 10.7498/aps.63.207102
Abstract +
In this paper, ZnO nanowire (ZnO NW) array is prepared based on microfluidic technology. The crystalline structures and morphologies of as-synthesized ZnO NWs are characterized by X-ray diffraction and scanning electron microscopy. The results show that ZnO NW is high-quality crystalline and c-axis oriented. At the same time, the gas-sensing properties of ZnO NWs are investigated for different gases, such as acetone, methanol and ethanol. The measured results prove that ZnO NW shows a sensitivity of 8.26 at 475 ℃, and the response and recovery times can reach 9 and 5 s separately, when exposed to 200 ppm (1 ppm=10-6) acetone. Compared with the method of conventional hydrothermal technology, the ZnO NWs based on microfluidic technology shows high sensitivity and fast recovery time. Finally, the gas sensing mechanism of ZnO NWs is also discussed from the aspect of gain and lose electron of oxygen molecules on material surface.
2014, 63 (20): 207301.
doi: 10.7498/aps.63.207301
Abstract +
Using first-principles calculations based on density functional theory and plane pseudo-potential method, we investigate the geometric structures, magnetic properties and electronic structures of phosphorene with phosphorus substituted by carbon (CP), oxygen (OP) and sulfur (SP). The results show that the phosphorene with SP presents a less geometric distortion than with OP and CN. Although the pure phosphorene is non-magnetic, all the doped systems are magnetic, each with a magnetization value of 1 μB per cell. The doped systems with stable ferromagnetism play an important role in developing spin electric devices.
2014, 63 (20): 207501.
doi: 10.7498/aps.63.207501
Abstract +
A low frequency magnetoelectric (ME) response model of magnetostrictive/piezoelectric laminate composite is presented based on energy conversion principle, and ME response characteristics of different laminate structures are compared in this paper. In this model it is assumed that the energy transfer between the layers of the composite laminates is achieved by the interlayer shear force. The stresses and strains of the magnetostrictive and piezoelectric layers are analyzed by the stress function method. While the strain and stored magnetic energy of magnetostrictive layers and the strain and electric field energy of piezoelectric layers are solved. Under open-circuit conditions, the interlayer shear force and the low frequency ME response model of laminate composites are obtained by using Hamilton principle of minimum energy. The theoretical results show that the ME voltage coefficient is related to the Poisson ratio, magnetic permeability, magnetomechanical coupling coefficient of magnetostrictive material, Poisson ratio, and electromechanical coupling coefficient of piezoelectric material. The influences of these parameters are analyzed. The magnetoelectric characteristics of two- and three-tier laminated structures are compared in this paper, showing that different laminated structures have different formulas for ME coefficient and calculation errors will be smaller when the corresponding ME coefficient formula is used. The experimental results show that the analytical error is smaller than 6% and the model can better describe the low frequency ME response characteristics of laminated magnetoelectric materials.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2014, 63 (20): 204202.
doi: 10.7498/aps.63.204202
Abstract +
Through the electro-optic modulation on photons with different polarization modes from quantum dot entanglement sources, the way to eliminate the photon frequency difference is analyzed systematically. We design an electro-optic modulation scheme with Pockels cell, to which a rising or falling voltage of 8 V·ns-1 is applied, thereby realizing a photon frequency shift with a bandwidth of 18 MHz. Simply increasing the modulation voltage slope would realize over 1 GHz frequency shift effectively, which provides a practical basis for realizing the future quantum dot entanglement sources with a perfect certainty.
2014, 63 (20): 204702.
doi: 10.7498/aps.63.204702
Abstract +
Based on hydrodynamics, the pressure of the liquid outside an aspheric encapsulated bubble driven by ultrasound is studied, and its analytical expression is derived. Numerical simulation shows that 1) the aspheric shape of an encapsulated bubble makes little influence on the pressure of the liquid far away from the bubble; 2) the pressure is extremely high at some local places of the liquid near an aspheric encapsulated bubble, and the pressure values at these places are apparently larger than those for a spherical encapsulated bubble at the same conditions. This phenomenon is of significance in the applications such as high intensity ultrasound therapy, drug delivery, cell membrane perforation, etc. As the ultrasound frequency shifts to the resonance frequency of an encapsulated bubble, or bubble shape deviates from sphericity, the localized high pressure becomes even greater.
2014, 63 (20): 204201.
doi: 10.7498/aps.63.204201
Abstract +
We study the properties of optomechanically induced transparency and normal mode splitting in an optical parametric amplifier (OPA) cavity. We find that the value of phase of the field driving the OPA and the nonlinear gain of the OPA will evidently affect the width of the optomechanically induced transparency window and the property of the normal mode splitting. In particular, when the coupling field works at the mechanical red sideband, narrower transparency window than the case of empty cavity (no OPA) can be realized by adjusting the value of phase of the field driving the OPA and the nonlinear gain of the OPA, accompanied by the steep dispersion curve. These results may be applied to the realization of fast/slow light, and light storage in optomechanical system.
2014, 63 (20): 204301.
doi: 10.7498/aps.63.204301
Abstract +
In order to obtain water-vapor profiles in the lower troposphere with a high accuracy and a temporal-spatial resolution, an improved ground-based differential absorption lidar system for water-vapor measurements is demonstrated. The key components of the system and the errors possibly existing in the system are described in detail. A real-time differential absorption cross-section measurement device that can compensate for measuring error caused by laser transmitter is presented. Combined with the water-vapor concentrations of different seasons in Shanghai, the differential optical thickness, the relationship between the gain of avalanche photo-diodes and the signal to noise ratio of received signal, and the statistical error in detection of four absorption lines in the 935 nm H2O absorption band are simulated in detail. The result shows that a particular absorption line can be selected, which depends on season and meteorological condition, to achieve the best outcome. With a time resolution of 5 min and a vertical resolution of 300 m, the statistical error of water vapor concentration is no more than 18% in a range of 300-5000 m.
2014, 63 (20): 204701.
doi: 10.7498/aps.63.204701
Abstract +
The behavior of reacting shock wave in supersonic premixed flow with the effect of incoming boundary layer is investigated experimentally. A supersonic premixed flow at a Mach number of 3 encounters a ramp, and an oblique detonation wave (ODW) is produced. Four ramp angles (θ) are designed from 36° to 45° (interval of 3 degree) and the equivalence ratio (Φ) can be varied. At a lower equivalence ratio, the ODW cannot be initiated and instead the shock-induced combustion (SIC) comes into being. It is discovered that the overall flow field presents more significant unsteadiness for SIC than for inert shock wave because the separation region is greatly enlarged for SIC due to heat release by chemical reactions in the separation region. As for the ODW, it is prone to propagating upstream after initiated for current experimental conditions. For 39° ramp, the separation region of boundary layer is relatively small, and the ODW presents an abrupt pattern for which a transverse wave exists. However, larger separation region for 42° ramp and its unsteadiness make the transverse wave intermittently appear. For 45° ramp, the even larger separation region makes the transverse wave thoroughly disappear and the ODW presents a smooth pattern.
2014, 63 (20): 204101.
doi: 10.7498/aps.63.204101
Abstract +
The degree of polarization of partially coherent Airy beams propagating through atmospheric turbulence is studied in this paper. Polarization fidelity of the beams is used to judge the propagation effect. It is shown that the degree of polarization of partially coherent Airy beams propagating through atmospheric turbulence tends to its initial value after it has propagated over a sufficient distance, and in free space the degree of polarization retains a certain value. The polarization distribution alone the symmetry axis is an Airy function if the propagation distance is not so long and evolves into a Gaussian-like profile when propagating at sufficient long distance in atmospheric turbulence. The polarization fidelity of the beams with larger waist width ω0 and larger coherence length σ0 is better. There is an exponential truncation factor which badly degrades the polarization fidelity. The results obtained in this paper are very useful for communication of Airy beams.
2014, 63 (20): 204302.
doi: 10.7498/aps.63.204302
Abstract +
The generalized likelihood ratio test (GLRT) detector and its theoretical detection performance for an underwater narrowband source with an unknown position are both given in this paper. Via the eigenvalue decomposition of the mode correlation matrix (MCM), the GLRT detector is decomposed into different spectrum components corresponding to the eigenvalues of the MCM. Based on the derived statistical property of each component, the spatial processing gain of each spectrum component with respect to the input signal is obtained, which is proportional to the corresponding eigenvalue. As there are several eigenvalues of the MCM approaching to zero when the modal information is incompletely sampled, the components corresponding to these small eigenvalues contribute much less spatial processing gain to the input signal than other components. By discarding the components corresponding to these small eigenvalues, the effective spectrum detector (ESD) is proposed, of which the target signal component in the output is approximately identical to that of the GLRT detector, and the noise in the ESD output is much less. Therefore, a much more robust detection performance is obtained by ESD than by the GLRT detector. Numerical simulations in a typical shallow water environment demonstrate that 1) the theoretical analyses, derivations and the effectiveness of the proposed ESD are verified; 2) the more incomplete the modal information sampling is, the more significant performance improvement of ESD over the GLRT detector can be acquired; 3) the numerical stability of the ESD is better than that of the GLRT detector.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2014, 63 (20): 206501.
doi: 10.7498/aps.63.206501
Abstract +
The electronic structures, elastic properties and thermodynamic properties of three different proportions of copper tungsten alloys (Cu3W, CuW, CuW3) at ground state, high temperature and high pressure are investigated by using the combined method of the first-principles calculation method based on density function theory. The calculated elastic constants indicate that Cu3W has an unstable structure and the CuW and CuW3 have the stable structures, which are in agreement with the phonon spectrum result. The metallic bond increases and the curve of the density of states moves toward the deep level with the increase of pressure. The quasi harmonic Debye model and the quasi harmonic approximation model are employed to calculate the bulk moduli, thermal expansion coefficients, Debye temperatures and specific heats of the Cu1-xWx alloys at different temperatures and different pressures.
ATOMIC AND MOLECULAR PHYSICS
2014, 63 (20): 203601.
doi: 10.7498/aps.63.203601
Abstract +
The influence of static vacuum on the cluster size and density of the supersonic gas jet is studied by Raleigh scattering method. It is found that in a range from 3×10-4 Pa to 9.9×104 Pa, the static vacuum has very little influence on the cluster size and desnity. The self-limiting effect which was found for hydrogen cluster at liquid nitrogen temperature is also found for argon cluster at positions far from the nozzle, and it is found to play an important role in the cluster generation and transporation. The result is very valuable for the construction of the desktop neutron source based on laser fusion method, since the pumps and device to supply the vacuum can be reduced and simplified.
2014, 63 (20): 203201.
doi: 10.7498/aps.63.203201
Abstract +
We have investigated the characteristics of high harmonic and attosecond pulse generated by helium atom exposed to the combined field of two laser pulses with the same color and a midinfrared laser pulse in nano-structure plasmon by solving the one-dimensional time-dependent Schrödinger equation. It is shown that the cut-off position of the harmonics is obviously extended in the combined field, however, the plasmon structure determined effect of absorbing the electron in the continuum states has an important influence on the cut-off position of the harmonics. It is found that the contribution from some single trajectory of electron is enhanced and other trajectories are suppressed by changing the plasmon position, so a single isolated attosecond pulse is obtained. Comparison with the scenario of atoms driven in homogeneous field, the width of the pulse decreases greatly, and an isolated pulse is achieved to be as short as 28 attoseconds.
2014, 63 (20): 203101.
doi: 10.7498/aps.63.203101
Abstract +
In this paper, charge trapping memory (CTM) is studied for analyzing the over-erase phenomenon, based on the first principles and VASP package. The nitrogen vacancy (VN) in Si3N4 and the interstitial oxygen (IO) in HfO2 are selected as model, because of the formation energy. The result about trapping energy shows that the electrons are trapped more easily than holes in these models, so the electrons are selected as programming/erase object. The energy after programming/erase operation, Bader charge analysis, different charge densities, adsorption energy and density of states are all studied to explain the over-erase micro change. The energy and electron change show that HfO2 as trapping layer makes CTM more reliable than Si3N4 as trapping layer; and after a programming/erase cycle, electrons in Si3N4 are erased more than programming ones; and the result of adsorption energy shows that the electrons can exchange more easily in Si3N4 than in HfO2. Finally, the research on the density of states shows that Si3N4 has shallow trapping energy level, HfO2 has deep trapping energy level. In conclusion, the essence of the over-erase in Si3N4 is that the atoms near the defect have weaker localized action on the electrons, resulting in the instinct electrons that are erased in erase operation. The over-erase essence is revealed, which is of benefit to improving the reliability and retention.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2014, 63 (20): 209203.
doi: 10.7498/aps.63.209203
Abstract +
Scaling exponent is a valid nonlinear dynamics index, and can effectively detect the dynamic structure mutations in the correlation time series by means of moving a fixed widow and the moving cut a fixed window technique. When there is short-term correlation, sequence with short-term correlation and the calculation result of scaling exponent with rescaled range will be influenced to a certain degree, resulting in a certain deviation for the scaling exponent calculation of a moving cut window sequence, and some false mutations point and mutations range for abrupt change detection. In view of this, we present a new method of the dynamic structure mutation detection-moving cut data-rescaled variance analysis. The numerical testing of ideal time series shows that the moving cut data-rescaled variance analysis has strong stability and accuracy, which is much better than the moving rescaled variance and the moving approximate entropy. The test results have not false mutation point and interval when the moving window is small. The further application to practically measured data validates the reliability of the new method.
2014, 63 (20): 209401.
doi: 10.7498/aps.63.209401
Abstract +
Deep-layer dielectric charge and discharge in insulating material irradiated by energetic electrons are one of the major factors causing spacecraft anomalies. In this paper we establish a two-dimensional physical model of deep-layer dielectric charging, based on charge distribution and energy deposition of incident electrons and conductivity properties. The model is accomplished by finite element method, and the deep-layer dielectric charging characteristics of polytetrafluoroethene irradiated by energetic electrons are calculated. The calculation results show that in the vacuum environment, in the surface of the dielectric there exists a weak reverse electric field, and it first decreases to zero and then increases with the increase of depth. The maximum electric field appears near the ground, but the electric field presents a slight reduction at the position of ground point. Space-time evolution characteristics of the maximum potential and maximum electric field in different radiation times (one hour, one day, ten days and 30 days) within dielectric are analyzed. With the increase of radiation time, the maximum potential increases from -128 V to -7.9× 104 V, and the maximum electric field increases from 2.83×105 V·m-1 to 1.76×108 V·m-1. Finally, the influence of electron-beam density on the maximum electric field is discussed. In a typical space environment (1×10-10 A·m-2), the maximum electric field reaches 2.95×106 V/m·m-1 for ten days. However, in severe space environment (2×10-8 A·m-2, the maximum electric field rapidly reaches 108 V/m for 42 hours, exceeding the breakdown threshold (about 108 V·m-1), which may easily cause electrostatic discharge). The physical model and numerical method can be used as a research basis of multi-dimension electric filed simulation of spacecraft complex parts.
2014, 63 (20): 209201.
doi: 10.7498/aps.63.209201
Abstract +
The traditional evaluation of simulation perfomance for numerical model is based on the differences in average, trend, probability density distribution, extreme value, variance and self-correlation between observational data and the simulated data by numerical model. These evaluation methods mainly depend on the comparison of statistical difference between the simulation and observation, but ignore the comparison in the dynamical characteristics of climate system. In view of this, in the present paper we use detrended fluctuation analysis (DFA) method to analyze the scaling characteristics of daily temperature from observational data and the corresponding simulated data by climate model, and investigate whether the long-range correlation of meteorological elements can exist in the simulated data of climate model. If so, we can assess the performance of climate model based on the comparison of scaling index between the observational data and the simulated data. As an example, the performances of Beijing Climate Center Climate System Model (BCC_CSM1.1(m)) model, including simulated daily average temperature, daily maximum temperature and daily minimum temperature, are assessed by using DFA. The results indicate that the long-range correlation characteristics of temperature in China can be simulated by BCC_CSM1.1(m). But the simulation errors are relatively large in Qinghai-Tibet Plateau, most West China, North China, and Huanghuai district. Thereinto, and the performance of BCC_CSM1.1(m) in Qinghai-Tibet Plateau and western West China is worst in China.
2014, 63 (20): 209202.
doi: 10.7498/aps.63.209202
Abstract +
Owing to the fact that the objective identification methods of rainy season are not totally identical in the three provinces of Northeast China, no unified monitoring criterion of rainy season is available in this area. In order to implement the regional key climate monitoring and to meet the needs of the national weather service, from the angle of the whole region, based on the daily precipitation data of 102 observational stations, by comprehensively analyzing the daily precipitation series disposed by 5-point moving average, the objective identification method of midsummer rainy season in the northeast China is presented in this paper. Then based on the daily reanalysis data of wind field and geopotential height field provided by NCEP/NCAR, the atmospheric circulation fields of different levels before, during and after the midsummer rainy season are contrastively analyzed, and the rationality of the objective identification method is validated. Finally, the partition criterion of the beginning and ending dates are determined. Results show that: the average beginning and ending dates of midsummer rainy season in the northeast China respectively are June 26th and August 30th. The partition criterion of midsummer rainy season in the northeast China by using the objective identification method is reasonable.
2014, 63 (20): 209204.
doi: 10.7498/aps.63.209204
Abstract +
Based on the analyses of monthly precipitation, National Centers Environmental Prediction/University Corporation for Atmospheric Research reanalysis dataset, and 2°×2° sea surface temperature (sst) data of extended reconstructed sea surface temperature during 1983-2011, we discuss the interdecadal variations of precipition around late-1990s in East Asia, the contributions of the atmospheric internal dynamic processes, and the extra-forcing factors such as sst that presents interdecal characteristics corresponding to the results obtained by using regress analysis. The results show that the interdecadal variations of the summer rainfall over Northern East Asia around late-1990s were more obvious than over Southern East Asia, and the summer rainfall over Southern East Asia experienced a notable interdecadal change around early-1990s and early-2000s. The summer geopotential height at 500 hPa, the summer wind at 850 hPa, the summer U-wind at 200 hPa, the water vapor transportation field, the index of EAP and EASM all have the notable characteristics of the interdecadal variations, which further validate the interdecadal change of precipition of late-1990s from the angle of the atmospheric internal dynamic process. At the same time the sea surface temperature interdecadal anomalies of North Pacific and West Pacific may also be one of the important reasons for causing the interdecadal change over East Asia of the late-1990s.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2014, 63 (20): 208502.
doi: 10.7498/aps.63.208502
Abstract +
In this paper, an InP-based mesa-structure uni-traveling-carrier photodetector is designed. By adopting Gaussian doping scheme in the absorption layer and incorporating an appropriate cliff layer, high speed and high saturation current characteristics are both achieved simultaneously. For the device with a 14 μm2 active area, the simulated results indicate that the bandwidth reaches 58 GHz and DC saturation current increases up to 158 mA at a reverse bias of 2 V. Under high optical injection, the bandwidth degradation and current saturation are studied, which are caused by energy band shift and electric field collapse.
2014, 63 (20): 208501.
doi: 10.7498/aps.63.208501
Abstract +
SiC semi-superjunction vertical double diffused MOS (VDMOSFET) has higher breakdown voltage than conventional SiC VDMOSFET with the same on-resistance. The ion implantation to form p pillar region on N-type epilayer is a key process to form semi-superjunction stucture. The influences of charge imbalance induced by ion implantation on breakdown voltages of 4H-SiC superjunction and semi-superjunction VDMOSFET are investigated through two-dimensional numerical simulation, and the largest breakdown voltage is obtained when charge imbalance is 30%. With the same structure parameters of devices, when breakdown voltage decreases by 15% due to the deviation of doping concentration in P pillars, the tolerance of doping concentration for the semi-superjunction VDMOSFET is 69.5% higher than for superjunction VDMOSFET which means that less precise process control of ion implantation for semi-superjunction VDMOSFET, will be required with less difficulty in the manufacture of pillars.
2014, 63 (20): 208201.
doi: 10.7498/aps.63.208201
Abstract +
The dynamics of spiral waves in a two-layer coupled excitable medium is studied by using the Bär-Eiswirth model. The two layers of medium are connected via network, i.e., a excitable unit is selected in each column of an excitable medium as a central point, and all excitable units in the same column of a layer medium are connected only with the corresponding central point and its eight neighbors in the opposite medium. The numerical results show that when the coupling strength is appropriately small, the two coupled spiral waves via local coupling can achieve their synchronization. Increasing coupling strength will induce meandering and drifting of spiral waves, leading to a desynchrony between the coupled spiral waves. The spiral wave with the resting state, low frequency plan wave and irregular pattern are observed to coexist. The coupled spiral waves via the transformation of spiral wave to synchronous plane wave disappear if the coupling strength is appropriately chosen. The physical mechanisms behind these phenomena are discussed.
2014, 63 (20): 208902.
doi: 10.7498/aps.63.208902
Abstract +
Microblog provides convenience to the society, but at the same time, it also brings some adverse effects. To obtain the propagation mechanism of microblog rumor, and then take effective measures to prevent its spread, according to the complex network theory, in this paper we investigate the internal characteristics of microblog user relationship networks, and present a microblog user relationship network evolution model. By using the mean-field theory, the topological statistical property of our evolution model, and the dynamical behaviors of rumor spreading on such a model are analyzed. Theoretical analysis and simulation results show that such an evolving network exhibits a scale-free property. The degree distribution exponent not only is related to the reverse connection probability, but also depends on the node attraction degree distribution. It is also found that when the node attraction degree follows a power-law distribution, the steady-state rumor prevalence is great compared with the exponential distribution and uniform distribution. Moreover, as the reverse connection probability or the number of node initial edges increases, the probability of rumor outbreak and the number of nodes finally infected by the rumor will also increase.
2014, 63 (20): 208401.
doi: 10.7498/aps.63.208401
Abstract +
In this paper, a multimode steady-state theory for Gyro-TWT is established. Effects of the input power and guiding center radius on backward wave oscillation in a TE11 Gyro-TWT are studies by this theory. The result shows that either of the two effects can reduce the output power of the backward wave oscillation to a certain extent, while both of them simultaneously can more effectively improve the output characteristics and the stability of the TE11 mode.
2014, 63 (20): 208901.
doi: 10.7498/aps.63.208901
Abstract +
In this paper, we propose a hypernetwork model with a nonlinear preferential attachment, and study the evolving mechanism and topological properties of the hypernetwork. We analyze the model by using a Poisson process theory and a continuous technique, and give a characteristic equation of hyperdegrees. We obtain the stationary average hyperdegree distribution of the hypernetwork by the characteristic equation. The analytical result shows that the hypernetwork has a phenomenon of the rich get richer, and it accords well with the simulation. It is shown in this paper that the hyperdegree distribution of the dynamic model exhibits a stretched exponential distribution with the increase of the hypernetwork size. It proves that the rich get richer does not necessarily induce a power-law distribution.
2014, 63 (20): 208705.
doi: 10.7498/aps.63.208705
Abstract +
In this study, we propose a kendall rank correlation based synchronous algorithm inverse rank correlation (IRC). The kendall rank correlation is a generalized algorithm of nonlinear dynamics analysis which can effectively measure nonlinear correlations between variables. The study of complex networks has gradually penetrated into various fields of the social sciences. We use our algorithm to construct functional brain networks based on the data from electroencephalogram (EEG). The average node degree of complex brain networks is analyzed to investigate whether epileptic functional brain networks are distinctly different from normal brain networks. Results show that our method can distinguish between epileptic and normal functional brain networks and needs to record a very small number of EEG data. Experimental data show that our method suited to distinguish between epilepsy and normal brain node degree, which may contribute to further deepening the study of the brain neural dynamic behaviors, and provide an effective tool for clinical diagnosis.
2014, 63 (20): 208402.
doi: 10.7498/aps.63.208402
Abstract +
Complexity of power system energy flow is mainly reflected in the features of dynamic behavior such as real-time, nonlinear, uncertainty, etc. The analysis of dynamics on network is a critical means. The potential energy function models of system and branches are established, which are based on power system dynamic equilibrium equation to extract the system energy information after the fault duration. And the complexity of the system energy flow evolution is analyzed by using the multi-scale entropy method. The analysis results show that the system complexity under steady operation is much low. In addition, the complexity of the fault system increases with the increase of fault duration. Moreover, the system complexity under unstable operation shows a strong uncertainty in a small time scale and obvious regularity in a large time scale. And the most important is that the system complexities of the critical stable state and the critical unstable state are clearly distinct in different time scales. Such a difference can be used as a positive reference to the identification of the critical point in the dynamic state. Research results reveal the evolution of power energy flow in physical dynamic process. This can provide new ideas and methods for analyzing the dynamics behavior of power system.
2014, 63 (20): 208701.
doi: 10.7498/aps.63.208701
Abstract +
Mass detection in mammograms usually has high false positive (FP) rate. Content based mass retrieval can effectively reduce the FP rate by comparing the image which is to be determined with mass images which have already been diagnosed. In this paper, a method combining discriminating anchor graph hashing (DAGH) and linear neighborhood propagation (LNP) is proposed for mammogram mass retrieval. Original AGH image representation does not consider pathological relevance in defining image similarity. To solve this problem, DAGH is put forward as a new image representation, which introduces the pathological class into image similarity. Furthermore, LNP is employed as a relevance feedback technique. Finally, interactive retrieval for mammogram masses is implemented based on the learning strategy between the underlying features and high-level semantic for images. Mammograms provided by the Breast Center of Peking University People's Hospital (BCPKUPH) are used to test the proposed method. Experimental results show that the DAGH image representation introducing pathological class is superior to original AGH in analyzing the similarity of mass images. Compared with existing methods, the proposed method shows obvious improvement in mass retrieval performance.
EDITOR'S SUGGESTION
2014, 63 (20): 208703.
doi: 10.7498/aps.63.208703
Abstract +
X-ray imaging based on variable energy can expand the dynamic range of the imaging system and perfectly show the structure information of the detection objects, by acquiring and fusing the image sequences. However, the fusion method is ordinarily based on image quality optimization, and neglects the gray mapping accuracy of the actual high dynamic imaging. It cannot guarantee the physical matching between the image information and actual structure information. Therefore, in this paper we propose an X-ray image gray characterization algorithm of high dynamic fusion based on variable energy. First, take a standard wedge block as test object, and use the fusion image of low dynamic image sequences as input data. The output data are the actual high dynamic image. Then establish the X-ray imaging gray characterization model by neural network training. At the same time, because the attenuation coefficients of different heterogeneous materials are different, a modified model of physical characterization is established to achieve a correct characterization of real object. Finally, experiments by 12 bit and 16 bit imaging systems acquire the variable voltage image sequences using 12 bit detector. After image fusion, image mapping and gray level correction, the output image not only achieves the same effect of 16 bit detector, but also satisfies the gray relation. Also this method can effectively expand the dynamic range of the imaging system.
EDITOR'S SUGGESTION
2014, 63 (20): 208704.
doi: 10.7498/aps.63.208704
Abstract +
To investigate the influence of pain exposure on autonomic nervous system of newborns, and develop a detection model based on heart rate variability (HRV) indexes, 40 newborns are recruited in the study and short-term HRV analyses are performed on electrocardiogram before and after pain exposure using time-domain, frequency domain and nonlinear methods. Wilcoxon signed rank test is adopted for statistical comparison, and the support vector machine (SVM) is used for developing a detection model. The results demonstrate that 3 linear indexes such as the mean of RR intervals aRR, absolute powers of low frequency band LF and absolute powers of high frequency band HF, and 9 nonlinear indexes such as approximate entropy ApEn, sample entropy SampEn, and determinism DET before pain exposure are significantly different from after pain exposure; and that a detection accuracy of 83.75% could be achieved by the model based on the combination of 5 indexes, i.e., aRR, proportion of adjacent intervals greater than 50 ms pNN50, ApEn, correlation dimension D2 and recurrence rate REC, and SVM. It suggests that HRV indexes can reveal the response of autonomous nervous system to pain exposure of newborns, and the model based on HRV indexes and SVM could be employed for the detection of pain.
REVIEW
EDITOR'S SUGGESTION
2014, 63 (20): 200303.
doi: 10.7498/aps.63.200303
Abstract +
Over the past decade, there have been increasing demand on and interest in high performance single photon detectors in many new applications in quantum information technology. Compared with conventional single photon detectors, superconducting transition edge sensor (TES) based single photon detectors have great advantages in the performance of detection efficiency, energy resolution, photon number resolving capability, dark count rate, etc. They have been successfully used in quantum optics experiments and quantum key distribution systems, and they will play a more important role in the research field of quantum information science and technology. In this paper we review the operating principle, fabrication process, performance, measurement system and the research progress on superconducting TES based single photon detectors.
2014, 63 (20): 208702.
doi: 10.7498/aps.63.208702
Abstract +
Computed tomography (CT) is a technology widely used in medicine and industrial non-destructive testing, and the image reconstruction algorithm is a core technology of CT. Now, the image reconstruction from few-view projections is a hot point in the study of reconstruction algorithm. With the advancements in theories and algorithms, the sparse optimization has recently been applied to few-view reconstruction for CT image, and shown to have a good performance in both accuracy and speed. In this paper, basic conclusions and classical algorithms in sparse optimization are introduced. Furthermore, the spare optimization based few-view reconstruction algorithms for CT image, in particular the main results and the values of spare optimization, are summarized. Finally, the future research direction of sparse optimization based few-view reconstruction for CT image is discussed.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2014, 63 (20): 205202.
doi: 10.7498/aps.63.205202
Abstract +
In this paper, a three-layer composite absorber with a split-spiral frequency selective surface layer clamped by two traditional absorber layers, is designed. The 1.4, 1.7 and 2.0 mm thick composites reach, respectively, the bandwidth values of 9.29, 6.69 and 7.11 GHz at a reflectivity lower than the -10 dB level, whose operating bandwidths are increased by 159.5%, 69.3% and 129.4% compared with those of the traditional absorbing sheet without frequency selective surface (under otherwise identical parameters). Further, the paper shows that the thinner composite sheet can have more excellent performance in reflection than the absorb sheets. The split-spiral frequency selective surfaces generated multiple resonance can be used to generate additional absorption band. The simulation results show that the embedded frequency selective surface improves an impedance matching between absorber with the free space, which changes the reflection of the absorbing sheet.
2014, 63 (20): 205204.
doi: 10.7498/aps.63.205204
Abstract +
532 nm pulse beam is used to induce the stimulated Raman scatting of water molecules. Different Stocks and anti-Stocks characteristic peaks of OH stretching vibrational from water molecules are obtained when the position of the focal spot is changed. The unique ± 3400 cm-1 Stokes characteristic peak with its anti-Stokes is present when the distance between focal spot and interface is fixed to be over 20 mm; both ±3000 cm-1 and ±3400 cm-1 with their anti-Stokes exist when the distance is reduced under 20 mm; both characteristic peaks are enhanced especially the 3000 cm-1 peak whose change is more remarkable when the spot continues to move up. Experimental results indicate that laser-induced plasma generated from water molecules enhances the stimulated Raman scattering of OH stretching vibrational from water molecules by affecting hydrogen bonds between water molecules and OH from single water molecules.
2014, 63 (20): 205205.
doi: 10.7498/aps.63.205205
Abstract +
The time-resolved spectra of the air-breakdown plasma generated by the nanosecond pulsed 1064 nm beam of a Nd:YAG laser are investigated by using a PI-MAX-II ICCD with nano-time-resolved features. The results show that the laser-induced plasma spectrum in atmospheric environment is widely distributed in a range of 300-900 nm, and it is superimposed by continuous spectra and sharp line spectra. The spectral lines of oxygen, nitrogen and hydrogen are identified according to the National Institute of Standards and Technology database of atomic emission. The intuitive figures of laser-induced breakdown atmospheric plasma spectra with time resolution are given. The electron temperature and electron density are calculated according to the emission spectra. The results have scientific significance for improving the accuracy and precision of the on-line measurement in the atmospheric environment.
2014, 63 (20): 205201.
doi: 10.7498/aps.63.205201
Abstract +
The viscosities, thermal conductivities and electrical conductivities of xenon plasma are obtained using Chapman-Enskog method expanded up to a higher approximation in a computation range from 300 to 40000 K under different pressures. In the local thermodynamic equilibrium regime, the results are compared with published experimental and computational results, showing that they are in good agreement with each other, which validates the accuracy of the computational method. The transport properties of xenon plasma are further obtained under the chemical equilibrium and thermal nonequilibrium, in which the electron temperature Te is different from that of heavy species Th. The evolutions of the transport properties with pressure and thermal nonequilibrium parameters (θ=Te/Th) are presented and analyzed.
2014, 63 (20): 205203.
doi: 10.7498/aps.63.205203
Abstract +
The experiments on nanosecond electrical explosion of twisted aluminum wires with different wavelengths (λt=0.37, 0.5, 0.75, 1.0 mm) are carried out. The experimental results indicate that a specific wavelength can strongly affect the energy deposition, expansion velocity, and radiation intensity. The energy deposition is about 3.3 times the atomic enthalpy of aluminum when the twisted wavelength is 0.5 mm. While for the other three twisted wavelengths, the energy depositions are all about 1.8 times the atomic enthalpy. The expansion velocity is about 3.8×103 m·s-1 for the wavelength 0.5 mm, and the optical radiation intensity is also strongest for this wavelength. The initial twisted structure is strongly imprinted in the freely expanding aluminum column after the electrical explosion. In the experiments for the wavelength 0.5 mm, a neural particle column with a diameter of 1.6 mm is formed and its density is about 1019 cm-3 at t=246 ns. A periodic structure with the wavelength 0.5 mm and the amplitude 0.3 mm is observed on the surface of this column.
NUCLEAR PHYSICS
2014, 63 (20): 202901.
doi: 10.7498/aps.63.202901
Abstract +
A real-time α/γ pulse shape discriminator based on CsI(Tl) scintillators is developed. The charge comparison method is adopted as an inserted algorithm. In this paper, the design scheme of the system is introduced and the real-time system is tested with a 60Co-γ source and a 241Am-α source. The influence of parameter setting on the discrimination result is investigated and the best parameter setting is provided. The results show that the designed real-time system has a small size. The best figure of merits is above 1.4. The system can discriminate α/γ events exactly and its counting rate of events can achieve 3×105 per second.
