Vol. 63, No. 6 (2014)
2014, 63 (6): 060201. doi: 10.7498/aps.63.060201
The vibration spectra of complicated molecule are generally divided into group frequency and ridge frequency. The ridge frequency has a certain symmetry in the vibration spectra of some complicated molecules, such as the ridge frequency of malachite green isothiocyanate (MGITC) molecule which is a molecular probe and has a U(5) symmetry. In this article the U(5) symmetry of molecular vibration spectrum is introduced and tested for its fitting to the low excitation spectrum of MGITC, we find that the theoretical value conforms to the experimental results.
A new method to obtain first order approximate conserved quantities of second-ordinary dynamics system containing nonlinear perturbation terms
2014, 63 (6): 060202. doi: 10.7498/aps.63.060202
We consider the perturbed system as the combination of unperturbed system and perturbed term according to the characteristic of the first order approximate conserved quantities, and we suggest a new method to obtain the first order approximate conserved quantities by three steps: first, we select a suitable method to obtain the conserved quantity I0 of unperturbed system, second, we calculate the influence of perturbed terms on conserved quantity I0, and finally we obtain the first order approximate conserved quantities of the system by using the characteristic of the first order approximate conserved quantities. An actual two-dimensional nonlinear dynamics perturbed system is studied in this paper, and four stable first order approximate conserved quantities are obtained by using this new method. The expressions of first order approximate solution of the system are also obtained by transforming coordinates and using the perturbation method, and four special cases are discussed in this paper.
2014, 63 (6): 060203. doi: 10.7498/aps.63.060203
To cope with the issue of multi-parameter and intricate calculation of classical models in predicting rain attenuation in Ka band, a method is presented to reduce the complexity by utilizing difference stationary time-series. By this method, a prediction model is established based on the difference transform of leader rain attenuation data, and parameters of stationary time-series are estimated to compute the rain attenuation at each frequency point in Ka band for achieving linear prediction. Simulations show that the prediction precision depends on prediction interval, the number of time-series and the frequency of difference. Compared with Dissanayake-Allaut-Haidara model, the proposed model has a prediction error of less than 10-3 when prediction interval is 0.1 GHz and the number of time-series is 20 with quadratic difference. Furthermore, the simulation results also indicate that the proposed method is simple and practical due to the fact that influence of polarization mode on model parameters may be ignored.
2014, 63 (6): 060204. doi: 10.7498/aps.63.060204
The model of nonlinear disturbed mechanism for one-dimensional Fermi gas is investigated. Firstly, the corresponding functional is constructed; secondly, its Lagrange operator is selected; using the modified generalized variational iteration method, the approximate analytic solutions of corresponding path curves are obtained. A simple example is given, and the approximation accuracy obtained by using the modified generalized variational iteration method is shown to be better. The aim of this article is to provide a valid method of solving the nonlinear physical problems.
2014, 63 (6): 060205. doi: 10.7498/aps.63.060205
In this paper, we create an asymptotic method of solving the nonlinear equation for the sea-air oscillator model. And based on a class of oscillator of the sea-air oscillator model. We first expanding the time delay, then discussing the relative values for the small parameters in the case of three copies, and finally, obtain uniformly valid asymptotic expansion by using the theory of the differential inequity.
2014, 63 (6): 060206. doi: 10.7498/aps.63.060206
In the past few years, the balanced sink and source macroscopic open system, which satisfies the parity and time-reversal symmetry, has become a research hot point. We introduce parity and time-reversal (PT) symmetry into fluid system by setting up balanced inflow and outflow in a two-dimensional channel. The flow is governed by Navier-Stokes equation and we use lattice Boltzmann method to solve them. Defining configuration-dependent asymmetric functions in velocity, kinetic energy density, and vorticity fields, we find that the PT function of the flow increases with the increase of the 2th power of Reynolds number i.e., ρn～ Ren. In this work, we use three different velocity profiles to drive the flow. It is demonstrated that in the three driven modes, the power-law schedule holds true. It is concluded that PT asymmetry of the viscous flow is determined by the flow dynamics not by the driven modes, thereby verifies the universality of the power-law scaling in viscous flow with balanced inflow and outflow.
2014, 63 (6): 060301. doi: 10.7498/aps.63.060301
In this paper we consider the case that three identical two-level atoms are trapped in a single-mode cavity which is in a weak coherent state initially, and there is an interaction between two atoms. The tripartite entanglement dynamics among atoms is studied. The influences of atom-atom coupling constant and intensity of the cavity field on the tripartite entanglement among atoms are discussed. The results obtained using the numerical method show that the tripartite entanglement among atoms is strengthened with the increase of intensity of the cavity field; on the other hand, it is weakened with the increase of the coupling constant between atoms.
Quantum dialogue protocols with identification over collection noisy channel without information leakage
2014, 63 (6): 060302. doi: 10.7498/aps.63.060302
Three quantum dialogue protocols with identification are proposed under the condition of collection noise. They are used to resist collective-dephasing noise, collective-rotation noise and both collective noises respectively. The two communication parties encode their own secret information into the quantum states with the generalized unitary transformation. Each communication party can deduce the secret message of his counterpart according to his secret message and the two quantum states (one is quantum state before being encoded, and the other is quantum state after being encoded), to achieve a quantum dialogue. It is important that our protocols all should be able to resist various attacks, such as disturbance attack, Trojan horse attack, intercept-resend attack and entanglement measure attack. Moreover, the efficiency and the information leakage of the proposed protocol are analyzed in detail.
2014, 63 (6): 060303. doi: 10.7498/aps.63.060303
To analyze the trap depth and ion heating rate of a surface ion trap under the influence of substrate power loss and voltage loss, in this paper we proposes analytic expressions of trap depth and ion heating rate. The results show that the voltage loss of Si substrate can reduce the trap depth by 17.19%, and the power loss would accelerate the ion heating rate by 13.37%. In order to reduce the influence of substrate effect, a new surface ion trap with low self-heating and voltage-loss is proposed in this paper, whose substrate is insulated by some vacuum trench to reduce the equivalent conductivity and capacitance. The simulation results illuminate that compared with the surface ion trap with normal Si-SiO2 substrate, the one with vacuum trench insulation exhibits a 20.22% increase in trap depth and a 54.44% reduction in power loss.
2014, 63 (6): 060401. doi: 10.7498/aps.63.060401
An inhomogeneous Benjamin-Davis-Ono-Burgers (BDO-Burgers) equation including underlying surface, slowly changing underlying surface and turbulent dissipation is derived in terms of quasi-geostrophic vorticity equation by employing the singular perturbation method. An inhomogeneous BDO-Burgers equation describing the evolution of the amplitude of solitary Rossby waves is investigated.
2014, 63 (6): 060501. doi: 10.7498/aps.63.060501
A new five-dimensional modified Chua’s system is proposed and its dynamic properties are investigated through numerical simulations, the stabilization of equilibrium points, bifurcation diagrams, and Lyapunov exponent spectrum. The different dynamic behaviors of the new system are analyzed with system parameters changed. Based on the mathematical model of the new system and the digital processing technology, the five-dimensional modified Chua’s system is discretized. According to IEEE-754 standard and module-based design idea, basic floating-point operational modules are designed. Furthermore, the chaotic attractors of the five-dimensional modified Chua’s system are realized by field programmable gate array. The investigation results show that the chaotic system is different from the existing chaotic systems. It also shows a good agreement between numerical simulation and hardware implementation, which proves the existence and realizability of the new chaotic system.
Stochastic response of smooth and discontinuous oscillator under additive and multiplicative Poisson white noise excitation
2014, 63 (6): 060502. doi: 10.7498/aps.63.060502
The stochastic response of the smooth and discontinuous (SD) oscillator under additive and multiplicative Poisson white noise excitation is studied by the generalized cell mapping method. Based on the digraph analysis algorithm, the attractors, basins of attraction, basin boundaries, saddles and invariant manifolds of the SD oscillator can be obtained. The transient and stationary responses of the SD oscillator under Poisson white noise excitation are computed based on the matrix analysis algorithm. It can be found that there is a close relationship between the evolution direction of probability density and the unstable manifold. Monte Carlo results are used to verify the efficiency and accuracy of the matrix analysis algorithm.
2014, 63 (6): 060503. doi: 10.7498/aps.63.060503
In this paper, the spatial-alternated Julia sets are discussed, which are obtained by alternated iteration of quadratic family zm+1,n+azm,n+1=(1+a)2zmn2+ci,i=1,2. The control of spatial-alternated Julia sets is accomplished by using the feedback control. Then the linear generalized synchronization of two different spatial-alternated Julia sets is discussed. The simulations demonstrate the effectiveness of the control methods.
2014, 63 (6): 060504. doi: 10.7498/aps.63.060504
Entropy is an index to reflect the heterogeneity of network structure. By introducing the concept of network flow which comprehensively considers radial measurement and betweenness measurement, we define a new network structure entropy index to solve the problem that classical entropy indices cannot effectively reflect heterogeneity of the global network. Analysis results concerning specific network (e.g. public data set Dolphins network) indicate that this new entropy index can reflect the real topological structure of network, and effectively overcome the shortcomings of other network entropy indices to some extent. The theoretical analyses and simulation experiments on Erdös-Renyi random network, nearest-neighbor coupled network, star network, Barabási-Albert scale-free network, Benchmark network, and the Watts-Strogatz small-world network further prove the effectiveness and applicability of this new network structure entropy index to describe the characteristics of ordinary complex network structures.
2014, 63 (6): 060601. doi: 10.7498/aps.63.060601
To realize metrology of the nanometer thin film thickness with high accuracy, a series of the nanometer film thickness standard samples with single layer is developed which could be measured by contact instruments such as stylus contact surface step profiler and scanning probe microscopy. The measurement and calculation method of grazing incidence X-ray reflectometry (GIXRR) for film thickness are studied. The formula of linear fitting method based on the periodic Kiessig fringes for thickness measurement is presented. A tracing approach of film thickness measurement, which is traceable to the atomic lattice of monocrystalline silicon and national angle standard, is proposed, and a new optics calibration method is presented which can measure angular misalignment of GIXRR apparatus. The relative expanded uncertainty of the nanofilm thickness H measurement is U=0.3 nm+1.5%H with coverage factor k=2.
2014, 63 (6): 060701. doi: 10.7498/aps.63.060701
In this paper we apply the transiently chaotic Hopfield neural networks (TCHNN) to the blind signal detection algorithm with BPSK signals and solve multi-start problem of Hopfield neural networks (HNN). And in this paper we propose an improved algorithm of double sigmoid transiently chaotic Hopfield neural networks (DS-TCHNN) on the basis of TCHNN, construct a new energy function of DS-TCHNN, and prove the stability of DS-TCHNN in asynchronous update mode and synchronous update mode. Simulation results show that TCHNN can skip local minima and has better anti-noise performance than HNN. While, DS-TCHNN inherits all the advantages of TCHNN and its speed of convergence is fast. Besides, DS-TCHNN needs shorter data to reach a global true equilibrium point so that the computational complexity is reduced and the running time is shortened.
2014, 63 (6): 060702. doi: 10.7498/aps.63.060702
All optical solid state streak camera is to carry out the process of deflecting and scanning of the signal light which is coupled into the core of the waveguide, with spatially-modulated pump pulse exciting the light deflector, through precisely controlling the time delay between signal light and pump light. Not only can it solve the problems existing in the traditional photoelectron streak camera such as the decrease of the dynamic range caused by space charge effect and incapability of detecting the infrared light signal for photoelectric cathode, but also it has the advantages of simple structure, systemic stability. And the theoretical temporal resolution can reach up to picosecond scale even sub picosecond scale. For the AlxGa1-xAs/GaAs/AlxGa1-xAs planar waveguide light deflector, we discuss in detail the change of the refractive index of the GaAs with time under the common influences of band filling, band gap shrinkage and free carrier absorption effect; when the change of the refractive index is on the order of 0.01 and the ratio of the signal spot size to the width of the waveguide p=0.5, the theoretical temporal resolution can reach 2 ps; finally, the theoretical spatial resolution is calculated to be 17 lp/mm according to the condition of static experiment, while the experimental results show that spatial resolution is 9 lp/mm.
2014, 63 (6): 060703. doi: 10.7498/aps.63.060703
In single-shot, ultrafast time-resolved measurements, the combination of linear chirp pulse and spectrometer is a common way. In this paper, according to the time-frequency mapping relationship of linear chirp pulse and utilizing an imaging spectrometer as a recording system, we design a chirped pulse velocity interferometer for the ultrafast measurement of free surface velocity. Theoretical research on the principle and applicability shows that picosecond resolution is achievable and the velocity sensitivity is easily adjustable, which greatly widens applications of the velocity interferometer. The numerical simulation of the measurement process shows that reconstructed velocity signal is well consistent with the given velocity, further confirming the feasibility of the chirped pulse velocity interferometer.
ATOMIC AND MOLECULAR PHYSICS
Coupled-cluster single-double theory study on the analytic potential energy function of the SeN2 radicals
2014, 63 (6): 063101. doi: 10.7498/aps.63.063101
The coupled-cluster single-double (CCSD) theory in combination with the quadruple correlation-consistent basis set (cc-pVQZ) of Dunning and co-workers is employed to estimate the equilibrium geometry, dissociation energy and vibrational frequencies of the SeN2 radical. The computational results show that the ground state of SeN2 has C2v symmetry and its ground electronic state is X1A1. The equilibrium parameters of the structure are RSe-N=0.1691 nm, RN-N=0.1970 nm, αN-Se-N =71.289°, and the dissociation energy is De=4.78 eV. The vibrational frequencies are ν1=326.9288 cm-1, ν2=808.0161 cm-1, and ν3=948.3430 cm-1, respectively. The whole potential curves for the ground electronic states of SeN and N2 are further scanned using the above method, the potential energy functions and relevant spectroscopic constants are then obtained by least square fitting to the Murrell-Sorbie function. Compared with other theoretical results and the experimental values, our computational results are very accurate. Then the analytic potential energy function of SeN2 is derived by many-body expansion theory. The potential curves correctly describe the configurations and the dissociation energy for the SeN2 radical.
2014, 63 (6): 063401. doi: 10.7498/aps.63.063401
Based on semiemperical London-Eyring-Polanyi-Sato (LEPS) potential energy surface (PES), stereodynamic properties of the reaction Sr+CH3I→SrI+CH3 in different initial reagent collision energies are studied theoretically by using the quasiclassical trajectory method. The results indicate that the collision energy of reagent has considerable influences on the orientations and alignments of angular momentum of products for the title reaction.
2014, 63 (6): 063402. doi: 10.7498/aps.63.063402
In this paper, the S(3P)+HD→SD+H and SH+D reactions are studied by means of quantum wave packet (QMWP) and quasi-classical trajectory (QCT) methods on a new ab initio 3A" potential energy surface. The reactive probabilities, integral cross sections, intra-molecular isotope parameters and product rotational alignment parameters for both reactive channels are calculated for collision energies in a range between 0.8 and 2.2 eV. The results reveal a pronounced isotopic effect. Plots of the potential energy surface and typical reactive trajectories show the evidence of an additional reaction mechanism for the SD+H product channel. This reaction mechanism, together with mass combination, can explain the isotopic effect for the title reaction.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2014, 63 (6): 064101. doi: 10.7498/aps.63.064101
The effect of plasma on the microwave propagation properties is investigated experimentally by using a high power microwave combiner. The plasma is generated by radio-frequency breakdown on the coupling slit of the combiner through changing the power or pulse width of the prime microwave source. The plasma diffuses from the slit to the high power microwave transmission channel, and induces the absorption of microwave energy and the rotation of microwave polarization. The results show that the spread velocity of plasma is about 1 μs/cm, and the duration is about 5 μs. The polarization rotating angle is determined by electron density and microwave frequency. The maximum rotating angle is 4.1° while the number of electron is about 3.7×1015.
2014, 63 (6): 064201. doi: 10.7498/aps.63.064201
In the field of precision measurement, HeNe laser is the first choice for fabricating laser interferometer. But the frequency difference of Birefringence-Zeeman dual-frequency HeNe laser, which can produce a medium frequency difference of 440 MHz, is randomly from as small as hundreds of kHz to as large as a dozen of MHz. To meet the production requirements for frequency difference of dual-frequency laser interferometer, there are some methods such as elastic strength and hole-drilling stress adjustment. But in the practical application, these methods are not very satisfactory. Laser-micro-engraving method assigns frequency difference by engraving a pattern in the plane mirror, and causes the phase retardation to achieve the purpose of frequency difference assignment. Laser engraving technique has many advantages: harmless to the laser, beauty, low power loss. The output power is close to the original light intensity. The frequency difference can be repeatedly adjusted and is stable. By frequency stabilization, the frequency difference fluctuates in a range of less than 10 kHz per hour.
A method of detecting line spectrum of ship-radiated noise using a new intermittent chaotic oscillator
2014, 63 (6): 064301. doi: 10.7498/aps.63.064301
In order to achieve the effective detection of the line spectrum of ship-radiated noise in low signal-to-noise ratio (SNR), in this paper we improve the conventional intermittent chaotic oscillator series method, and propose a signal detection method based on the adaptive step intermittent chaotic oscillator. Through setting a sequence of the calculating steps which can cover the frequency band of the signal to be measured, the method can use just one Duffing oscillator to accomplish the searching detection for the weak signal with unknown frequency. In order to further improve the weak signal detection performance, we analyze the Holmes Duffing equation’s sensitivity for weak signal detection at different internal frequencies. Through theoretical analysis and simulation study, it is found that the Duffing oscillator has the best weak signal detection performance when its internal frequency is 0.4 rad/s. According to this result we optimize the Duffing oscillator. The simulation result shows that the signal detection performance of the improved Duffing oscillator increases 14 dB. In the end, we use the proposed method to detect a set of actual data which contain the ship radiated line spectrum, the result shows that this method could achieve effective detection of the line spectrum in low SNR.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2014, 63 (6): 066101. doi: 10.7498/aps.63.066101
Crystalline and non-crystalline alloys are characterized by specific nearest-neighbor coordination polyhedral clusters representing the local tomic short-range-order features of their parent phases. In this paper, NiZr, AlZr and BCr crystalline phases are investigated from the view of clusters because they have the same structures described by traditional crystallography. The result is that they have the different local atomic features, which are likely to be due to the slight differences in the size of unit cell and atomic position. Different from traditional crystallography, this description gives the structure information about short-range-order of that phase, so it can provide a new idea for phase classification and material design.
Numerical investigation of non-ionizing energy loss of proton at an energy range of 300 eV to 1 GeV in silicon
2014, 63 (6): 066102. doi: 10.7498/aps.63.066102
The displacement damage due to non-ionizing energy loss (NIEL) is the main reason of photo-electronic device failure in space radiation environment. The basic mechanisms of NIEL are Coulomb and nuclear interactions of silicon atoms with incident protons at energies ranging from threshold to 1 GeV. In the low energy region where the Coulomb interaction is dominant, the NIEL can be calculated by analytical method and TRIM code. MCNPX/HTAPE3X is used to calculate NIEL when the nuclear elastic and non-elastic interactions between proton and target atoms are significant in the high energy range. The results show that it is reasonable to use MCNPX/HTAPE3X to evaluate the NIEL by recoiling nucleus caused by high energy protons. The combination of analytical method and TRIM code can calculate NIEL induced by Coulomb interaction in low energy range, which gives the NIEL of proton in silicon in an energy range from 300 eV to 1 GeV.
Simulation and calculation of the absorbing microwave properties of carbon nanotube composite coating
2014, 63 (6): 066103. doi: 10.7498/aps.63.066103
How to optimize the absorbing microwave properties by using the parameters of carbon nanotube composite coating is a hotspot in the study of electromagnetic shielding. However, the study on the influence of coating parameters on absorbing microwave properties mainly stays in the stage of experimental study, and the influence of structure parameters of carbon nanotube on absorbing microwave properties has not been reported yet. Therefore, it is significant to study the method of optimizing the absorbing microwave properties of materials through micro-structure of the carbon nanotube composite coating. Based on an equivalent circuit of multi-wall carbon nanotubes, using the relationship between the parameters of each element in the equivalent circuit to study the mechanism of carbon nanotube loss microwave, an expression of absorbing microwave properties about carbon nanotube structure parameters is derived. According to the expression, the microwave reflection ratio of coating is calculated and simulated by Matlab software, which is affected by the length, diameter and coating thickness of carbon nanotube and the number of carbon nanotubes. The simulation result shows that the variation of reflection ratio curve of coating with the number of carbon nanotubes is consistent with the experimental results. The number of carbon nanotubes and the thickness of coating are the vital parameters, which influence the absorption intensity and absorption position, while the diameter and length of carbon nanotube are the vital parameters, which influence the absorption intensity.
2014, 63 (6): 066201. doi: 10.7498/aps.63.066201
Digital shearography is proposed to study the out-plane deformation of the Portevin-Le Chatelier (PLC) band during tensile tests in an Al alloy. The geometrical shape and propagation of the PLC bands are visualized in real time through fringe pattern in subtracted images. In 1/15 s, the maximum out-plane displacement is 245 nm within the PLC band. The displacement distribution is asymmetric while in fringe pattern, the white fringe is narrower in the front of the PLC band than in the rear. In addition, the evolution processes of position shifting and inclination transition of the PLC bands are presented in detail.
2014, 63 (6): 066701. doi: 10.7498/aps.63.066701
The density-density correlations of ultracold Bosons released from a two-dimensional square optical lattice are investigated based on the quantum phase field U (1) rotor field. With the effects of the particle number fluctuations and the phase degrees of freedom taken into consideration, the theory can be used to describe the cold atom system with strong interaction. Using this theory, the characteristics of ultracold atom gas in the process in which the superfluid state of optical lattice transits to insulating state, are well described. The results show that in the patterns of the density-density correlations, the continuous diagonal lines gradually transit to scattered peaks. The results match the experimental ones. In addition, the effects of the quantum depletion on the density-density correlation are also included in the result, and the relevant conclusions are also consistent with the existing theoretical and experimental results.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2014, 63 (6): 067101. doi: 10.7498/aps.63.067101
The structural stabilities, electronic and optical properties of SnS bulk, monolayer, and multilayers are systematically studied by using the first-principles calculations within the density-functional theory. Our calculated results indicate that monolayer SnS can be exfoliated from its bulk, and the process is similar to the fabrication of graphene. With the reduction of layer number, the structural stabilities of SnS nanostructures become weak and their band gaps increase due to the quantum confinement effect and the layer interactions. Therefore, the optical properties of SnS can be controlled by adjusting the layer number due to the fact that the optical properties of materials depend on their electronic structures. The main optical absorption peaks of SnS bulk and nanostructures originate from the electron transitions among the orbitals of Sn-5s, 5p and S-2p. Moreover, the optical absorption peaks of SnS show obvious blue shift when SnS structure transforms from its bulk to monolayer. The present study will contribute to the application of SnS materials in the solar cells.
2014, 63 (6): 067102. doi: 10.7498/aps.63.067102
We perform first-principles calculations in the framework of density-functional theory to determine the effects of P doping on the electronic structure and optical properties of single-walled armchair silicon nanotubes. The calculated results indicate that the band-gap of single-walled armchair silicon nanotubes changes from indirect to direct one, with the P element doped. The top of valence band is determined mainly by the Si-3p electrons, and the bottom of conduction band is occupied by the Si-3p electrons and Si-3s electrons. Moreover, the band gap of single-walled armchair silicon nanotubes decreases and the optical absorption is red-shifted, with the P element doped. The results provide useful theoretical guidance for the applications of silicon nanotubes in optical detectors.
2014, 63 (6): 067201. doi: 10.7498/aps.63.067201
Polymer solar cell composed of the blend bulk- heterojunctions is fabricated by the conjugated polymer as donor material and fullerene as acceptor material. P3HT and PCBM are chosen in the present study. The effect of the NiOx layer on the light absorption of the polymer solar cell is investigated via the transfer matrix of the multilayer dielectric film, Maxwell equations and photon absorption equation. NiOx as alternative material of PEDOT:PSS in polymer solar cell can redistribute the electromagetic field of device and effectively improve the light absortption. in the device with structure of ITO/HTL/P3HT:PCBM/LiF:Al, and the effects of PEDOT:PSS and the NiOx on the performance of device are investigated by numerical simulation. The results show that the optimal NiOx layer device with a thickness of 10 nm has a larger short-circuit current, filling factor and energy-conversion efficiency than the 40 mm thick PEDOT:PSS device.
2014, 63 (6): 067202. doi: 10.7498/aps.63.067202
In this paper, resistive switching device based on Cu/SiOx/Al structure is fabricated to examine its resistive switching characteristics and explore its resistive switching mechanisms. By adjusting limiting current, four stable resistance states are obtained. All of the resistive ratios between adjacent resistance states are over than 10. Moreover, the retention data of these four states at room temperature keep stable up to 1000 s. The temperature-dependent measurement and I-V curves fitting results show that the resistive switching mechanisms of the four states are different: resistance states 1 and 2 are due to Ohmic conduction mechanism, resistance state 3 is due to Pool-Frenkel emission, and resistance state 4 is due to Schottky emission mechanism. Subsequently, a resistive switching model for Cu/SiOx/Al structure is proposed.
2014, 63 (6): 067203. doi: 10.7498/aps.63.067203
The Cu elements of Cu (In, Ga) Se2 (CIGS) have very important influences on the electrical properties of CIGS absorber and solar cells. In this paper, Cu-poor and Cu-rich absorber layers (0.7 Cu/(Ga+In) (1.15) and solar cells are prepared by evaporation method. The SEM and Hall measurements reveal that Cu-rich material shows superior structural (larger grain size, better crystalline) and electrical (lower resistivity, higher mobility) properties to Cu-poor material. However, I-V tests show that the efficiency of Cu-poor solar cell is better than that of the Cu-rich device. The temperature-dependent I-V tests indicate that electron loss is mainly due to the bulk recombination in Cu-poor solar cell, and the activation energy of recombination is comparable to the band gap energy of Cu-poor solar cell. In contrast, in the Cu-rich devices the recombination at the heterointerface is dominant, and the activation energy is smaller than the band gap energy of the absorber material, which is an important drawback of open circuit voltage. Finally, Cu-poor surface on Cu-rich absorber is prepared by three-stage evaporation process, which reduces the short-circuit current and open-circuit voltage loss and optimizes the performance of CIGS solar cells. The efficiency of CIGS solar cell is achieved to be as high as more than 15%.
2014, 63 (6): 067301. doi: 10.7498/aps.63.067301
To study the effect of rare earth element doping on the electronic structure of monolayer MoS2, the lattice parameters, band structures, density of states, and electron density differences of La, Ce and Nd doped and intrinsic monolayer MoS2 are calculated, respectively, using first-principles density functional theory based on the plane wave pseudopotential method in this paper. Calculations indicate that variations of bond length near La impurity are maximum, but they are the minimum near Nd impurity. Analysis points out that lattice distortion in doped monolayer of MoS2 is relative to the magnitude of the covalent radius of doping atom. Analysis of band structure shows that La, Ce and Nd doping can induce three, six and four energy levels, respectively, in the forbidden band of MoS2, and that the properties of impurity levels are analyzed. Rare earth doped monolayer MoS2 make change in electron distribution through the analysis of electron density difference, and especially, the existence of f electrons can induce the electron density difference to exhibit a physical image with a great contrast.
2014, 63 (6): 067401. doi: 10.7498/aps.63.067401
There exist two kinds of critical current densities in polycrystalline bulk of MgB2, i.e., the large local critical current density corresponding to the shielding current in inductive measurements, which flows inside the grains, and the small global critical current density that flows through the grains for whole sample. This behavior is considered to be mainly caused by the significant granularity in polycrystalline bulk. In this work, MgB2 superconductors are prepared under different Spark plasma sintering (SPS) heating rates. The microstructures of the samples are investigated, and their critical current densities are measured by Campbell method from the penetrating AC flux profile and the AC magnetic field versus penetration depth. It is found that an extremely high global critical current flows through the whole sample with a bigger grain size, which is prepared by a low heating rate during SPS sintering. That is to say, the grain refinement only increases the local critical current density of the sample. These results imply that the global current is reduced due to the existence of various defects and the poor electrical connectivity in MgB2 sample.
Enhancement of surface photoconductivityin 6H-silicon carbide crystal modified by femtosecond laser pulse irradiation
2014, 63 (6): 067801. doi: 10.7498/aps.63.067801
The photocurrent is very small when the visible light irradiates the semi-insulating silicon carbide. The femtosecond laser pulses are used to modify the silicon carbide surface, and the result shows that the photocurrent is enlarged several times according to the measurement results of photoconductivity. In order to explain the reasons for this change, some characterization means are employed, including the absorption, emission and X-ray photoelectron spectra. There are found some changes in the absorption spectra and emission spectra, and also in the silicon and carbon atom ratio according to the test results of X-ray photoelectron spectrum. We think that the changes of the crystal structure and atom ratio between silicon and carbon lead to the change of electronic energy band structure and the occurrence of many defect states. As a result, the photocurrents are improved in the range of visible light on the surface of 6H-SiC after the femtosecond laser pulses have irradiated the surface.
Ionization parameters of high power microwave flashover on dielectric window surface calculated by particle-in-cell simulation for fluid modeling
2014, 63 (6): 067901. doi: 10.7498/aps.63.067901
The particle-in-cell (PIC) simulation method is used to get the reliable ionization parameters of high power microwave flashover and breakdown on dielectric surface for fluid modeling. Firstly, the PIC method is presented briefly, including dynamic equations, secondary emission and Monte-Carlo collision (MCC) between electron and gas atom. Secondary, the fluid global model (GM) is introduced including continuity and energy conservation functions. Finally, by using a 1D3V PIC-MCC code programmed by the authors, the ionization parameters are calculated under different microwave electric-field values, microwave frequencies, gas types and pressures for fluid modeling, including ionization frequency, breakdown delay time, average electron energy, electron energy distribution function (EEDF). The numerical results could be concluded as follows. Average electron energy is unrelated to EEDF type. At middle and low gas pressures, electron energy satisfies Maxwell distribution, and ionization parameters are unrelated to EEDF type. At middle and high gas pressures, ionization parameter is related to EEDF type, and the relevant coefficient X of EEDF tends to be of high older. Different gases have different EEDF types, and the relevant coefficient X of EEDF should be corrected by PIC simulation. The value of X is also related to microwave electric-field value and frequency, and its value increases with the increase of microwave electric-field value and the decrease of microwave frequency. In a fixed range (microwave electric-field value below 7 MV/m, and microwave frequency below 40 GHz), at middle and low gas pressures, the average electron energy increases with the increase of electric-field value and the decrease of microwave frequency rapidly, and the ionization frequency increases and then decreases with the increase of microwave electric-field value and frequency respectively; at high gas pressure, the average electron energy increases with the increase of electric-field value slowly, the ionization frequency increases with the increase of electric-field value, and the average electron energy and ionization frequency are unrelated to microwave frequency.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2014, 63 (6): 068101. doi: 10.7498/aps.63.068101
In this paper, Invar alloy composition is described by using binary cluster as basic structural unit. We use “cluster resonance” structure model and establish the connection of a known binary clusters with atoms by an atomic ratio of 1:x to describe Invar alloy component empirical criterion. Invar alloy composition formula is given, that is, Invar alloy component=[clusters]1 (glue atom)x. Using this criterion, some of Invar alloy compositions are calculated, and it is confirmed that the experimentally determined typical Invar alloy compositions are in good agreement with the compositions calculated from the formula. This shows that the formation of the rule of the Invar alloy composition is well explained by the composition formulae criterion.
2014, 63 (6): 068102. doi: 10.7498/aps.63.068102
Niobium-doped TiO2 thin films are deposited on strontium titanate substrates by E-beam evaporation deposition. Effects of post-annealing in hydrogen atmosphere on their optoelectrical properties are studied. The results show that the annealing in hydrogen atmosphere can enhance their conductivity values efficiently. The corresponding optium resistivity reaches 5.46×10-3 Ω·cm, and the transmittance values of the thin films are 60%–80%. The improvement in the conductive performance is attributed to the increase of the grain size of polycrystalline thin film, the formation of a lot of oxygen vacancies and H-doping caused by annealing in hydrogen atmosphere.
2014, 63 (6): 068103. doi: 10.7498/aps.63.068103
GaN-based light emitting diodes (LEDs) as the third generation of lighting devices, have been rapidly developed in recent years. Substrate materials, serving as the LED manufacturing basis, have great influences on the production and application of LED. The critical characteristics of substrate affecting the design and fabrication of LED are its crystal structure, thermal expansion coefficient, thermal conductivity, optical transmittance, and electrical conductivity. In this paper, we compare several common substrate materials, namely, sapphire, silicon carbide, silicon, gallium nitride and gallium oxide, review the research progress of the substrate materials in the aspects of high quality epitaxial growths, high performance device designs and preparations of substrates, and comment on their further development.
2014, 63 (6): 068104. doi: 10.7498/aps.63.068104
The lamellar spacing, which is formed by solidified melt of eutectic or near-eutectic composition, plays a very important role in determining the properties of final products. In this study, the lamellar spacing of eutectic growth in steady-state is predicted by the method which is established based on the classical Jackson-Hunt theory, and completed by considering the free energy change during eutectic solidification at small undercooling. The density difference between the solid phases is also considered when calculating the diffusion field in the liquid. It is found that a band of lamellar spacings would be generally selected for a given alloy under fixed growth conditions. In addition, the lamellar spacing can be morphologically stable below the minimum undercooling value, and this overstabilization is only dependent on the intrinsic characteristic properties of a given system at a fixed growth velocity. The analysis results are found to be in reasonable agreement with experimental data of Al-Al2Cu, Sn-Pb and CBr4-C2Cl6 systems available from the literature.
Experimental investigation and numerical simulation of defect elimination by CO2 laser raster scanning on fused silica
2014, 63 (6): 068105. doi: 10.7498/aps.63.068105
Based on the fact that fused silica material can strongly absorb 10.6 μm CO2 laser, a method of using CO2 laser multi-time raster scanning to repair the densely distributed scratches and polishing pits is investigated. The experimental results indicate that the scratches and polishing pits can be fully eliminated under the appropriate parameters. The damage threshold testing results also indicate that the damage threshold for fully eliminating scratches and polishing pits can reach or exceed the damage threshold of substrate. Meanwhile, Combining the simulation results obtained by finite element software-Ansys, the processes of the scratches and polishing pits eliminated by CO2 laser are analyzed. The present work is of significance for the study on how to eliminate the scratches and polishing pits on the surface of component.
2014, 63 (6): 068401. doi: 10.7498/aps.63.068401
On-line identification for low frequency oscillation needs to measure signals which couple with white Gaussian noise from wide area monitoring systems (WAMSs). Processed by low pass filter, white Gaussian noise can turn into colored Gaussian noise, so the accuracy of oscillatory indentification would be reduced. To solve the problem of colored Gaussian noise, in this paper we propose a cross-correlation-function (CCF) method that could reduce the influence of colored Gaussian noise. Combined with TLS-ESPRIT algorithm, CCF-TLS-ESPRIT could identify oscillatory modes in the environment of colored Gaussian noise rapidly. The simulation results show the effectiveness of the proposed method.
Enhancement of performance of P3HT：PCBM based polymer solar cell by Ag2O/PEDOT：PSS composite buffer layer
2014, 63 (6): 068402. doi: 10.7498/aps.63.068402
Ag2O/PEDOT:PSS used as the anode buffer layer is introduced into the P3HT:PCBM based polymer solar cell (PSC). Effect of the Ag2O/PEDOT:PSS composite anode buffer layer on the device performance is investigated. According to the results, we can find that the post-thermal annealing can improve the performance of the PSC with Ag2O/PEDOT:PSS anode buffer layer compare with that without buffer layer. In addition, compared with the devices without such a buffer layer or with only PEDOT:PSS buffer layer, the device with the composite buffer layer can achieve higher Jsc, external quantum efficiency as well as power conversion efficiency. We conclude that the post-thermal annealing can significantly improve the surface morphology which increases the light absorption and the exciton dissociation. The inserted Ag2O together with PEDOT:PSS as the composite buffer layer not only efficiently lowers the hole extraction barrier and improves the hole collection efficiency but also exhibits excellent stability.
2014, 63 (6): 068501. doi: 10.7498/aps.63.068501
Based on the hybrid density functional theory, the relationship between geometric structure of 1,4-butanedithiol molecular junction and the electrodes force and the breaking process of the molecular junction are studied. The electronic transport properties of the molecular junction under different external forces are further investigated using the elastic scattering Green’s function method. The numerical results show that different interface configurations result in different rupture forces. The rupture force is about 1.75 nN when the terminal S atom is sited at the hollow position of Au(111) surface. However, the rupture force is about 1.0 nN when the terminal S atom links with one Au atom which is on the gold surface singly. And with the breakdown of the molecular junction, the single Au atom is pulled away from the gold surface by the terminal S atom. These two results are consistent with different experimental measurements respectively. The molecule is twisted under the electrode pressure and thus further induces the surface Au atom to glide on the gold surface. However, the processes of the molecule twisted by pressure and restored by pulling are two irreversible processes. The stretching force of electrode is 0.7–0.8 nN, and the conductance always shows a minimal value under different interface configurations and twisting states, which is consistent with experimental conclusion. The change of the coupling between the terminal atom and the electrodes induced by the electrode force is the main factor of influencing the conductance of the molecular system. The existence of bimolecular junction results in a small possibility of higher conductance values, which is probed by experiment under a stretching force of about 0.8 nN.
Electronic transport properties of oligophenyleneethynylene molecular junctions in alkaline and acid solutions
2014, 63 (6): 068502. doi: 10.7498/aps.63.068502
Using nonequilibrium Green’s function method combined with density functional theory, we theoretically investigate the influences of alkaline and acid solutions on electronic transport properties in oligophenyleneethynylene molecular junctions with amino and carboxylic groups. The numerical results show that comparing with the case of neutral state, the conductance of the molecular junction is double improved and the rectifying direction is inverted when carboxylic group is deprotonated in the alkaline environment. On the other hand, the rectification ratio is enhanced trebly and the rectifying direction is inverted similarly when amino group is protonated in the acid environment. This theoretical work presents a chemically controllable method of manipulating conductance and rectification of molecular junctions.
2014, 63 (6): 068701. doi: 10.7498/aps.63.068701
Neural mass model (NMM) can generate spontaneous oscillation even in a resting state. However, it remains little known which mechanism is responsible for NMM’s spontaneous oscillation. From dynamical theory, spontaneous oscillation is an intrinsic property of nonlinear system, which means that the sigmoid nonlinear function (S function) of NMM plays a key role in the emergence of its spontaneous oscillation. In this study, describing function approach is employed to analyze the spontaneous oscillation characteristics of a kind of extended NMM. Firstly, the describing function of S function is derived, through which the two S functions in excitatory and inhibitory feedback loop, respectively, are approximated. Secondly, the NMM is transformed into a typical block diagram composed of a nonlinear unit and a linear unit. Thirdly, in the theoretical framework of describing function approach, theoretical analysis of the spontaneous oscillation characteristics of NMM is conducted, and the oscillation frequencies are determined. The simulation results demonstrate that the theoretical results are correct and the employed approach is effective. Since S function exists extensively in neural system, the proposed approach has a potential application in the spontaneous oscillation analysis of other neural model.
2014, 63 (6): 068702. doi: 10.7498/aps.63.068702
The extreme pH-induced lateral reorganization of supported lipid bilayer membranes are studied by fluorescence microscopy. The results show that the fluid dioleoyl-phosphatidylcholine bilayers in extreme acidic or basic solution presents a similar phenomenon to endocytosis and exocytosis, such as rupture, detachment, budding, formation of microtubules etc. In the view of the interaction of polar molecules with H+/H3O+ or OH ions, we conclude that the zwitterionic phospholipid headgroup as core adsorbs the H+/H3O+ or OH ions in electrolyte solution. The asymmetric charge adsorption quantity of the lipid headgroups leads to the effective area discrepancy between the outer and inner leaflets of lipid bilayers. The asymmetric membrane curvatures induce a variety of structures and dynamic responses. The present study helps explain lipid membranes reorganization under extreme pH conditions and provides some guidelines for deformation process of lipid membranes.
2014, 63 (6): 068703. doi: 10.7498/aps.63.068703
In the paper, we present a new method for signal processing in statistical domain. The multiple components obtained from the conventional linear transformation are possibly irrelevant, and usually do not possess the characteristics of statistical independence. Therefore a method of signal decomposition and reconstruction is proposed based on the independent function element. This method not only inherits the advantages of linear transformation, but also has the capability of signal representation in the statistical domain. In this paper, the model, definition and obtaining method of independent function element are discussed, and the applications of heart sounds independent function element in the heart sound signal processing are also analyzed in detail. The validity and practicability of the method are demonstrated through two experiments.
Two-dimensional device simulation and performance optimization of crystalline silicon selective-emitter solar cell
2014, 63 (6): 068801. doi: 10.7498/aps.63.068801
In this paper, device simulation and parameter optimization on crystalline silicon (c-Si) selective-emitter (SE) solar cell are performed by using PC2D two-dimensional simulator. On the basis of achieving perfect fitting to the measured I-V curve of a typical c-Si SE solar cell fabricated by screen printing phosphoric paste method, the effects of physical parameters of gridlines, base, selective emitter and back surface field layer on the optoelectronic performance of the SE solar cell are comprehensively and systematically investigated. Simulation results show that the base minority carrier lifetime, the front surface recombination velocity and the back surface recombination velocity are the three largest efficiency-affecting parameters. In the studied parameter range, when the base minority carrier lifetime rises from 50 s to 600 s, the cell efficiency increaes from 18.53% to 19.27%. Low front surface recombination velocity is the premise of making the optimization of selective emitter sheet resistance meaningful. To obtain an ideal efficiency, the back surface recombination velocity should be controlled to be under 500 cm/s. In addition, under different front surface recombination velocities, the maximum of cell efficiency is always achieved in a range of 5090 / heavily doped region sheet resistance and 110180 / lightly doped region sheet resistance. For different numbers of gridlines, when the radio of heavily doped region width to the gridline pitch equals 32%, the solar cell has the highest efficiency. Moreover, under the condition of low area radio of bas bar, increasing bus bar number appropriately can improve the efficiency. The efficiency of p-type SE solar cell reaches 20.45% after optimization.
2014, 63 (6): 068901. doi: 10.7498/aps.63.068901
Based on the optimal velocity model, with the consideration of the effect of drivers heterogeneity of the disturbance risk appetite, an extended model is proposed. The linear stable judging condition is obtained for the extended model by its stability analysis, which shows the smaller the ratio of the coefficient of the disturbance risk appetite for the former car driver to that for the following-car driver, the larger the stability area of the extended model is. The kink-antikink density wave is attained in an unstable area of the model by nonlinear analysis. Simulation results also show that the smaller the ratio of the coefficient of the disturbance risk appetite for the former car driver to that for the following-car driver, the better it is for the initial traffic flow to inhibit the disturbance propagation, hinder the formation of traffic jams and restore homeostasis. This paper provides a viable idea to reduce traffic congestion, for example, by forming convoys traveling in order from small to big according to the relative size of drivers coefficients of the disturbance risk appetite.
Evolution mechanism of node importance based on the information about cascading failures in complex networks
2014, 63 (6): 068902. doi: 10.7498/aps.63.068902
This paper mainly focuses on the evolution mechanism of node importance based on the information about cascading failures. Firstly, a novel node importance indicator is proposed according to the load turbulence of each node in the redistribution range based on a tunable load redistribution model. The indicator has two characteristics: one is that the failure consequence of the considered node can be clearly pointed out by its value, and the other is that the evolution mechanism of node importance can be analyzed with the factors of load redistribution rule, node capacity, and structural characteristics of the network. Then, an evaluation algorithm is presented. The indicator analytic formulas of Erdös-Rényi networks and Barabási-Albert networks are also presented respectively with the neighbor preferential and global preferential allocation rules. The experiments demonstrate the effectiveness and feasibility of the indicators and its algorithm, with which we also analyze the node importance evolution mechanism in-depth, namely how the not-so-great nodes in structure turns into the critical nodes to trigger cascading failure in complex networks.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2014, 63 (6): 069101. doi: 10.7498/aps.63.069101
A theoretical model of elastic wave propagation in a cracked porous medium is developed in this paper. When elastic wave propagates through the cracked porous medium, the different physical properties and geometries in different pores structures lead to the fluid pressure gradient in cracks and between cracks and pores. The squirt-flow will take place in two mutually-perpendicular directions, thus, it has anisotropic characteristic. The wave respond contains the crack and background medium permeability information simultaneously. Owing to the fluid dynamic flow process, the effective elastic modulus is complex and frequency-dependent. When the wave frequencies are in high and low limit, the porous medium is elastic. The wave attenuation is obvious and the attenuation is frequency-dependent in the middle frequency region. The anisotropic permeability corresponding to anisotropic characteristic times in the cracked porous medium causes the wave propagation to be affected by the crack connectivity. There appears a second attenuation peak for larger thickness value of crack, meanwhile, and the peak of attenuation is influenced by the thickness value and radius of crack.
2014, 63 (6): 069201. doi: 10.7498/aps.63.069201
In this paper, we extend the expression of quasi-geostrophic Q-vector brought forward by Hoskins et al. into the Q-vector equation under the condition of geostrophic momentum approximation, and discuss the method of how to quantitatively estimate vertical motion in synoptic charts with Q-vector expression under geostrophic momentum approximation. Compared with the expression of quasi-geostrophic Q-vector, the Q-vector expression under geostrophic momentum approximation has two more terms which are related to geopotential height extremum and wind velocity extremum. Then, each term of Q-vector under geostrophic momentum approximation is transformed into the expression which is conveniently judged in synoptic charts. The results show that to judge vertical motions in synoptic chart quantitatively, one of the three ways can be used: when contour line presents the form of wave-like trough-ridge and passes through the entrance and exit areas of jet stream, correspondingly use the 1st term of Q-vector under geostrophic momentum approximation (i.e., quasi-geostrophic Q-vector); when there exist a closed pressure system and an angle between contour line and wind velocity, correspondingly use the 2nd term of Q-vector under geostrophic momentum approximation; at the jet streak, correspondingly use the 3rd term of Q-vector under geostrophic momentum approximation.
Test particle simulation of resonant interaction between energetic electrons in the magnetosphere and ELF/VLF waves generated by ionospheric modification
2014, 63 (6): 069401. doi: 10.7498/aps.63.069401
Ionospheric modulation can artificially trigger ELF/VLF whistler waves, which can leak into the inner magnetosphere and contribute to resonant interactions with energetic electrons. Combining the ray tracing method and test particle simulations, we investigate the propagation of these artificially generated ELF/VLF waves through the high ionosphere into the inner magnetosphere, and evaluate the subsequent effects of resonant scattering energetic electrons near the heart of the outer radiation belt. The results show that the artificially triggered ELF/VLF waves become highly oblique in the magnetosphere and their spatial extent of L shell and magnetic latitude can be significantly controlled by the initial launch latitude. Corresponding to the principal first-order resonance, the energetic electrons from ～ 100 keV to 3 MeV can resonate with the artificial VLF waves with frequency above 10 kHz in the inner radiation belt, while in the outer radiation belt these hazardous electrons can resonate with ELF waves from ～100 Hz to 1 kHz. At L=4.5 as the focus in this study, the artificial ELF waves can resonate with 1 MeV electron at the harmonics N=-1, 1, 2. In contrast, the Landau resonance rarely occurs for these energetic electrons. The results of test particle simulations indicate that while wave-induced changes in pitch angle and kinetic energy of a single electron are stochastic, the change averaged over all test electrons increases monotonically within the resonance timescale, which implies that resonant scattering is an overall characteristic of energetic electrons under the influence of the artificial whistler waves. Computed resonant scattering rates based on the test particle simulations indicate that aritificial ELF/VLF waves with an observable in situ wave amplitude of ～ 10 pT can drive efficient local pitch angle scattering of energetic electrons at the magnetic equator, thereby contributing considerably to their precipitation loss and magnetospheric electron dynamics. When the waves become highly oblique during the propagation, besides the fundamental first order resonance, higher order resonances can also drive efficient electron scattering. The results support the feasibility of generating artificially ELF/VLF whistler waves for controlled removal of energetic electrons in the Earth radiation belts.
2014, 63 (6): 069501. doi: 10.7498/aps.63.069501
Owing to its low contrast, the target of low light level (LLL) image is not very salient, and it is difficult to detect automatically. Aimed at this problem, this paper proposes a noise robustness algorithm for computing the local texture coarseness (LTC) of textured images, and provides a texture saliency (TS) calculation method that is applicable to saliency analysis of LLL image. Firstly, we present a novel LTC algorithm, by which the LTC around a pixel using the best size of the pixel. Compared with coarseness measure based on local fractal dimension, the LTC algorithm shows much better noise robustness in the experiments of noised textured images. Then, a TS algorithm is given based on the extraction of texture coarseness feature map. Finally, we apply the TS algorithm to LLL image target detection, which is efficient proved by experimental results.
2014, 63 (6): 069502. doi: 10.7498/aps.63.069502
Based on the most widely used proportional-integrator controller of the tip-tilt correction loop in an adaptive optical system, the relationship between the optimal closed-loop bandwidth and the atmosphere turbulence as well as the sensor noise is analyzed. Numerical simulation is also performed by using the time serial signals that follow the power spectrum density of the global tilt of Kolmogorov turbulence. Results show that with different atmosphere turbulences and sensor noises, the correction capability of the tip-tilt correction loop can be brought into full play by choosing appropriate closed-loop bandwidth and the residual variance can be reduced.
2014, 63 (6): 069601. doi: 10.7498/aps.63.069601
The interaction between lunar plasma and spacecraft may cause surface charging/discharging effects, and degrade the performance of spacecraft. The charging potential is a key factor for discharging process. In order to evaluate charging/discharging effects, it is necessary to obtain the Lunar plasma environment. SWIDA/B on Chang’E-1 are the two scientific instruments of solar wind ion detector which could explore the plasma environment in the 200 km Lunar orbit, thereby deducing the solar wind bulk speed, density and temperature. In this paper we select the data in June 2008 derived from SWIDA sensor. First, we figure out the solar wind ion differential flux and energy spectrum, and then calculate the solar wind parameters such as velocity (300.00–600.00 km/s), plasma density (1–10 cm-3) and plasma temperature (1–20 eV). Finally, we adopt an equivalent circuit model to compute the Lunar spacecraft surface charging voltage which is -7–-70 V.
Effect of stability of X-ray pulsar profiles on range measurement accuracy in X-ray pulsar navigation
2014, 63 (6): 069701. doi: 10.7498/aps.63.069701
The Crab pulsar is an isolated rotation-powered pulsar which emits large X-ray fluxes, making it a candidate source for carrying out the X-ray pulsar navigation (XNAV). The long-term stable profiles are considered as the foundation of XNAV. However, systematic studies of the long-term stability of the X-ray pulsar profile and its effect on range accuracy in XNAV are lacking. In this paper, we use the X-ray band (2–16 keV) data monitored by Rossi X-ray timing explorer (RXTE) spacecraft over the latest 11 years, to first investigate the stability of the Crab pulsar including the profiles after the glitches. Furthermore, some measurements of the long-term profile shape, including Pearson correlation coefficient, standard deviation and spectral entropy, are presented both in the time domain and in the frequency domain. In the data processing, the Fourier analysis and cross-correlation are used to deal with the 191 RXTE data sets. With the help of the Cramer-Rao theory, the effect of the profile variation on the error of the range determination in XNAV is studied. Furthermore, after analyzing those errors, the effect of the stability of the Crab pulsar on range determination is confined to more narrow limits. The results demonstrate that the 2–16 keV profiles are almost constant during the period 2001–2012. The profiles after the glitches show no significant discrepancy. The variation of Crab profile inevitably has an influence on the navigation precision to a certain extent. The calculated range error along the pulsar line-of-sight due to the stability of the pulsar profile is 34 m±25 m.