Vol. 59, No. 12 (2010)
2010, 59 (12): 8499-8511. doi: 10.7498/aps.59.8499
Digital holographic tomography by a few projections is developed. A section of fiber with symmetrical structure and a plaster head model with nonsymmetrical structure are used as test samples. Both simulation analysis and experimental results are analyzed. Simulation analysis shows that the weighted coefficientwij and the relaxation factor λ affect evidently the quality of the reconstructed image, in addition, the relaxation factor λ also influences the computer speed. Thus the weighted coefficient wij is evaluated by using a nonlinear method and the relaxation factor λ is selected appropriately by comparing several simulation reconstructions. Experimental results reveal the feasibility and the availability of digital holographic tomography by a few projections for symmetrical samples and nonsymmetrical one. In the next work, the real time digital holographic tomography based on three projections and single hologram will be developed to reconstruct the multi-refractive index of biological samples.
2010, 59 (12): 8801-8806. doi: 10.7498/aps.59.8801
The compressive behavior is one of most basic mechanical properties for engineering materials, and the struts of porous materials under compression may display the buckling phenomena when the porous bodies are under compressive loadings. In the present paper, a failure model of simplified structures is established for isotropic three-dimensional reticulated porous materials, and the failure modes resulting from the strut bucklings are analyzed for these materials under compressive loadings. The mathematical relationships between nominal main stresses and porosities are systemically derived for this buckling failures of these materials under uniaxial compression, biaxial compression and triaxial compression. Based on these results, the relevant loading conditions resulting in the strut buckling are further obtained for these materials under compressive loadings.
Effects of AlGaN layer parameter on ultraviolet response of n+-GaN/i-AlxGa1-xN/n+-GaN structure ultraviolet-infrared photodetector
2010, 59 (12): 8903-8909. doi: 10.7498/aps.59.8903
We have investigated the effect of AlGaN layer parameter on the ultraviolet response of n+-GaN/i-AlxGa1－xN/n+-GaN structure ultraviolet-infrared photodetector and its physical mechanism. Through the simulation, it is found that the decrease of AlGaN background concentration has a positive effect on device’s ultraviolet quantum efficiency. When AlGaN layer background concentration cannot be reduced, the decrease of its thickness can ensure the efficiency. Besides, interfical state should be minimized during materials growth and device fabrication. In addition, small reverse bias voltage can greatly increase ultraviolet quantum efficiency. All these phenomena may be mainly attributed to the existence of the back-to-back heterojunction and the opposite electrical field. It is suggested that we need to adjust structural parameters to obtain high quantum efficiency according to the materials quality in device design.
2010, 59 (12): 8311-8315. doi: 10.7498/aps.59.8311
A class of generalized (3+1)-dimensional nonlinear Burgers system is studied. Using the homotopic mapping method, the corresponding mapping expansions are constructed and using the iteration method the series solution of travelling wave for a solitary wave is obtained.
In order to solve the problems of particle degeneration and lackness of diversity of particle filter, a new particle filter based on Stiefel manifold (SMPF) is proposed in this paper. In the SMPF the system model is based on Stiefel manifold, Langevin distribution is used as a prior density, the matrix normal distribution serves a as likelihood function, and particle is sampled on the manifold distribution. First, manifold is embedded in Euclidean space, then the mean of particles is calculated in Euclidean space and its result is projected back to embedded manifold. So the influence on variance of particle weight caused by statistic characteristics of noise is removed, and a kind of universal selecting scheme of important probability density function is acquired which is hardly restrained to system state model. The simulation results based on univariate nonstationary growth model nonlinear system indicate that the SMPF works much better than scentless particle filter in real-time performance, robustness, filtering precision and filtering efficiency.
Generalized Pfaff-Birkhoff-d’Alembert principle and form invariance of generalized Birkhoff’s equations
2010, 59 (12): 8322-8325. doi: 10.7498/aps.59.8322
The generalized Pfaff-Birkhoff-dAlembert principle and the form invariance of the generalized Birkhoff s equations under the infinitesimal transformation of group are studied. The criterion of the form invariance is given and the Noether conserved quantity and a new conserved quantity are obtained by the invariance.
A Birkhoffian representation of Whittaker equations is introduced. A Hamiltonian corresponding to the Birkhoffian representation is constructed. The Whittaker equations are solved by using the Hamilton-Poisson method. The difference between the above Hamiltonian and the Hamiltonians of traditional analytical mechanics is pointed.
2010, 59 (12): 8330-8334. doi: 10.7498/aps.59.8330
Greenhill formula for Kirchhoff elastic rod is extended to that of exact model of the rod. Under the assumption of the plane cross section, the configuration of an extensible and shearable elastic rod is expressed as a history of the cross section with arc coordinate. A special solution which describes equilibrium in straight line state of the rod is obtained from a differential equilibrium equation. A linear perturbation equation is derived and its general solution is obtained in which the integral constants are determined by constrained conditions at two ends of the rod. The condition for a non zero solution of the integral constants to exist leads to the Greenhill formula of exact elastic rod model, which shows that the boundary of stable area of the force screw is a closed curve and of symmetry and the inference of extensible and shearable to stability of the rod is dependent on three factors: the difference in flexibility between shear and extension of a section of the rod, the bending stiffness, and the length of the rod.
2010, 59 (12): 8335-8342. doi: 10.7498/aps.59.8335
Experimental investigation on the effect of using spanwise oscillating Lorentz force to control near-wall turbulence is presented in this paper. The effects of spanwise oscillating Lorentz force on macroscopic flow field, near-wall coherent structure and skin friction are discussed. Direct numerical simulation is carried out to verify the experimental results. The results from experiment and numerical simulation suggest that the spanwise oscillating Lorentz force can make the macroscopic flow field oscillating periodically, the streak structures are twisted into the spanwise direction and the skin friction is reduced.
2010, 59 (12): 8343-8347. doi: 10.7498/aps.59.8343
By applying the bifurcation method of dynamical systems to a class of nonlinear dispersive Boussinesq equations, the analytic expressions of implicit solitary wave solutions are obtained under different parameter conditions. Numerical simulations are given to show the correctness of our results.
Quantized energy spectrum and exact wave function of three-dimensional anisotropic coupled harmonic oscillator
2010, 59 (12): 8348-8358. doi: 10.7498/aps.59.8348
By the quadratic form theory, we succeed in diagonalizing the Hamiltonian for three-dimensional anisotropic coupled harmonic oscillators, through three symplectic transformations keeping the commutation relations unchanged. The quantized energy spectrum and the exact wave function of the system are given.
Entanglement properties of two-atom inside cavities controlled by manipulating the atom outside the cavity
2010, 59 (12): 8359-8364. doi: 10.7498/aps.59.8359
Considering three two-level atoms initially in the W state, then two of them are placed into two initially empty cavities respectively and made resonantly interacted. The two-atom entanglement evolution inside cavities is investigated. The effects of rotational manipulation and state-selective measurement of the atoms outside the cavities on the two-atom entanglement evolution inside cavities are discussed. The results obtained using the numerical method show that the two-atom entanglement inside cavities is controlled by manipulating the atom outside the cavity.
2010, 59 (12): 8365-8370. doi: 10.7498/aps.59.8365
We consider a model consisting of three two-level atoms in a heavily damped cavity. We show that the quantum-jump-based feedback can be used to generate a steady entangled state of three atoms against decoherence. When the feedback acts on just one of the atoms, it can protect a maximally entangled state of other two atoms. When the feedback acts on three atoms, by choosing appropriate parameters we can obtain a decoherence-free subspace spanned by two vectors, and by using quantum trajectory Monte Carlo wave function method we find that the maximally entangled state of three atoms in this decoherence-free subspace can be obtained for some specific initial conditions.
2010, 59 (12): 8371-8376. doi: 10.7498/aps.59.8371
To study the surface hydrophobicity of solid materials with the lattice Boltzmann method, a rough surface is approximately treated as simple periodic rectangular, saw-tooth and semicircular protrusion structures separately. A two-phase lattice Boltzmann model is used to simulate the change of surface hydrophobicity by calculating the contact angle of a droplet on the rough surface. Finally, the fluid flow in a nano-channel with rough walls is studied.
2010, 59 (12): 8377-8384. doi: 10.7498/aps.59.8377
In this paper, an exciton-photon model is created in an optic microcavity, and then in Bose condensation (BC), the variations of chemical potential range and number density of particles with temperature and position are studied in cases: constant width and varying width. Taking a semiconductor optic microcavity GaAs as example, the influence of temperature on BC is analyzed. The result shows that the range of chemical potential is related to dielectric function and microcavity width, while the number densities of photons and excitons and the sum of both particle numbers are related not only to them but also to temperature. The theoretical temperature of BC of GaAs is close to the experimental value. The densities of photons and excitons are almost equal, and their distributions are restricted to r=0 when BC occurs. With the reduction of temperature the number densities of both particles increase and their distributions expand, and the number of photons is more than that of excitons no matter how the width of optic microcavity changes.
2010, 59 (12): 8385-8392. doi: 10.7498/aps.59.8385
To study the characteristics of the chaotic systems and their applications, an electronic circuit of simplified Lorenz chaotic system with one parameter is designed and experimented with discrete components. The system parameters correspond to the circuit element parameters. By regulating the variable resistor in the circuit, dynamic behaviors including limit cycle, pitchfork bifurcation, period-doubling bifurcation, chaos, and route to chaos by period-doubling bifurcation, are observed. The necessary condition for the existence of chaos in the fractional-order simplified Lorenz system is deduced. The lowest order of the fractional-order simplified Lorenz system and the variation law of the lowest order with system parameters are determined. Circuit simulations and experiments show that the simplified Lorenz system has rich dynamic characteristics, and that theoretical analysis and circuit experiment are accordant with each other.
Study on border collision and bifurcation of two-dimensional piecewise smooth systems in current mode controlled Buck converter
2010, 59 (12): 8393-8406. doi: 10.7498/aps.59.8393
Border collision bifurcations have received much attention in recent years. A class of piecewise smooth maps with two border and three zones is derived to describe the dynamics of a current-programmed Buck converter operating in a discontinuous mode. The numerical simulation is carried out and the bifurcation diagrams with the reference current as a parameter are obtained. Then, a concrete analysis of the stable existence condition of the fixed point is made, and the structure of bifurcation diagrams and a change in operation mode of the converter at the border collision point are studied. Finally, simulation and experimental results show that the theoretical analysis is correct.
2010, 59 (12): 8407-8413. doi: 10.7498/aps.59.8407
A new switched four-scroll hyperchaotic system with two subsystems which are interconnected and can be switched to each other, is generated. The two subsystems are analyzed in detail by presenting the bifurcation diagram, the feature of equilibrium, the phase portraits of chaotic attractor, the Lyapunov exponent and the evolution course of the dynamical action. A practical circuit is designed to conveniently realize two subsystems for the hyperchaotic systems each with only smooth quadratic nonlinearity. Based on time-dependent switching law and state-dependent switching law, the two subsystems can be switched from one to another either randomly or independently, just by using this special circuit.
Controlling hyperchaos and periodic state synchronization of degenerate optical parametric oscillators
2010, 59 (12): 8414-8419. doi: 10.7498/aps.59.8414
Hyperchaotic control and periodic synchronization between two degenerate optical parametric oscillators are presented by mutual coupling parameter modulation, based on the nonlinear kinetic characteristic of degenerate optical parametric oscillator. Theoretical results show that the two degenerate optical parametric oscillators in hyperchaotic state can be controlled into periodic output by mutual couple, no matter whether the two degenerate optical parametric oscillators are identical. It is also shown that the periodic states can result in identical synchronization or anti-synchronization only in the case where the largest Lyapunov exponent of the system is negative. Thus synchronization type and evolution process of synchronization are determined by modulating coefficient and initial conditions.
Characteristic of hybrid single electron transistor and metal oxide semiconductor structure in Chua’s circuit
2010, 59 (12): 8420-8425. doi: 10.7498/aps.59.8420
The negative differential resistance (NDR) characteristic equation of single electron transistor and metal oxide semiconductor (SETMOS) hybrid structure is simplified by a fitting method. And a new approach to designing multi-scroll chaotic circuit with SETMOS is proposed. The effect of NDR characteristic on the equilibrium point of multi-scroll Chua’s circuit is analyzed both qualitatively and quantitatively. The results show that the unidirectional motions of radial contract and axial tension occur in the negative sections of multi-scroll Chua’s circuit, whereas in the positive sections appear the scroll motions of radial tension and axial contract. The result provides theoretical basis for the construction of multi-scroll chaotic circuits and the further study of their complex dynamical behaviors.
Implementation of multi-scroll Chua’s circuit by hybrid single electron transistor and metal oxide semiconductor structure
2010, 59 (12): 8426-8431. doi: 10.7498/aps.59.8426
Based on the structure of cellular neural network, multi-scroll Chua’s circuit is implemented by the nanoelectronic device of hybrid single electron transistor and metal oxide semiconductor (SETMOS) structure with its negative differential resistance characteristic. The basic dynamical properties, including phase portrait, bifurcation diagram, Lyapunov exponent spectrum, Poincaré mapping and power spectrum are studied by theoretic analysis and numerical simulation. The validity and the feasibility of three-order Chua’s circuit with four scrolls are further confirmed by the circuit simulation experiment. Finally, the results show that the negative differential resistance characteristic of SETMOS determines complex dynamical behaviors of multi-scroll Chua’s circuit. Also, the designed circuit has simple structure and is easy to realize.
2010, 59 (12): 8432-8443. doi: 10.7498/aps.59.8432
Based on the autonomous character of one-cycle control, the average model of one-cycle controlled Cuk power factor correction converter can be derived. Subsequently, the approximate analytical expressions of periodic solutions are obtained by using the harmonic balance method. Furthermore, through analyzing the changing trend of Floquet multipliers, the location and the type of the first bifurcation point are predicted. Thus, the intrinsic mechanism of such an instability can also be explained. The results show that the intermediate-scale instability is due to the loss of stability of periodic solutions, which is named the Neimark-Sacker bifurcation in the circuit system. Finally, experimental results are presented for verification purpose. Our work has revealed the salient feature of the intermediate-scale instability, and provided a theoretical basis for facilitating design of the converter.
Effect of frictional force on subharmonic bifurcations of a completely inelastic ball bouncing on a vibrating table
2010, 59 (12): 8444-8450. doi: 10.7498/aps.59.8444
The behavior of a completely inelastic ball bouncing on a vertically vibrating table in the presence of frictional force is investigated. The frictional force is assumed to be constant. It is found that the sequence of bifurcation, controlled solely by the normalized vibration acceleration Γ, is the same as that in the absence of frictional force, but the value of each bifurcation point becomes larger. In the bifurcation diagram of ball flight time, the structure consisting of an infinity of bifurcation cascades in a narrow range of Γ is observed. Compared with that of no frictional force, it is longitudinally compressed and transversely stretched, and has a different fractal property. A comparison with the bifurcations observed in vertically vibrated granular beds is also made. When the fractional force is set to be 20%—30% of the whole weight of the particles, the results from the bouncing ball model are in good agreement with experimental observations.
2010, 59 (12): 8451-8458. doi: 10.7498/aps.59.8451
Detection of buried trash in scattering media is a very important research project. Detection efficiency is influenced by several imaging parameters, such as incident light intensity, trash buried depth, medium refractivity, medium scattering coefficient and medium anisotropic factor. Firstly, current Monte Carlo algorithm is revised. Secondly, influential laws of those parameters are researched by simulating a great number of photons propagating in a scattering medium. The results indicate that light intensity, trash buried depth and medium refractivity greatly affect both reflection and transmission imaging results; and their influential laws are similar. Detection efficiency of reflection imaging can be improved by increasing light intensity, reducing trash buried depth or medium refractivity. Detection efficiency of transmission imaging can be improved by increasing light intensity, reducing medium refractivity and scattering coefficient, or increasing anisotropic factor. These conclusions have universal significance of guidance in the detection of buried trash in scattering media.
2010, 59 (12): 8459-8464. doi: 10.7498/aps.59.8459
The subharmonic heterodyne combined with cut-off waveguide method is proposed to measure the frequency of 0.14 THz high-power nanosecond pulse in this paper. Using the cut-off waveguide method, the frequency range of the terahertz pulse is determined, in which two non-standard rectangular waveguides with cut-off frequencies of 0.125 and 0.15 THz are separately used as a receiver. According to the preliminary result, the subharmonic order is chosen to be 8, while the frequency of local oscillator is set to be in a range of 15—20 GHz. A series of tests on the 8th harmonic mixer is done using the pulses in Ka-band and the 0.14 THz continuous wave source, separately. Experimental results indicate that the mixer can be used in the heterodyne measurement of 0.14 THz nanosecond pulse. Finally, the heterodyne measurement is carried out on the high-power terahertz puls source, and an accurate frequency of 0.1465 THz is determined in this experiment. This novel method, which greatly reduces the frequency requirement of the local oscillator, is proved to have a good performance in the frequency measurement of 0.14 THz nanosecond pulse, and probably provides a new idea for frequency measurement of pulses in the long-wavelength terahertz-band.
2010, 59 (12): 8465-8470. doi: 10.7498/aps.59.8465
In this paper the 178Hfm2 isomer prepared through the bombardment of 27MeV α particles on natural metallic Yb foil in the CS30 cyclotron has been studied. Analysising the γ spectrum of sample irradiated by α particles, the amount of178Hfm2 isomer can be determined to be about 1.5×1011 nuclei, the main long-lived isomers of sample are identified and the possible reaction paths of these isomers are deduced. By monitoring the radioactivity dose of sample, the cooling time for chemical separation is also obtained. The preliminary results are conducive to further developing the method of 178Hfm2 isomer production.
ATOMIC AND MOLECULAR PHYSICS
2010, 59 (12): 8471-8477. doi: 10.7498/aps.59.8471
The velocity distribution of emission electrons in the transfer ionization process of 70keV He2+ colliding on He has been studied with a reaction microscope. These distributions show that the electrons lie mainly in the scattering plane which are observed to be emitted preferentially in the forward direction, lying between 0 and projectile velocity Vp. The distributions also display a typical two-fingered structure with a local minimum on internuclear axis. This characteristic can be qualitatively explained as being due to the interference between σ amplitude and π amplitude of electrons final wavefunction. It depends also on the impact parameters; the π amplitude contribution is dominant at small impact parameters, which leads to a symmetric velocity distribution around the internuclear axis. However, at large impact parameters the σ amplitude contribution increases relatively obviously, resulting in an asymmetric electrons velocity distribution around the internuclear axis.
CLASSICAL AREA OF PHENOMENOLOGY
2010, 59 (12): 8478-8483. doi: 10.7498/aps.59.8478
A time-dependent theory for helix traveling wave tubes in beam-wave interaction is presented. The effect of wave on electrons is described by radio frequency (RF) field equations and space charge (SC) field equations, while the effect of electrons on waves is described by electron dynamic equations. The RF field equations are achieved from Ampere’s law and Faraday law, combined with sheath helix model RF field. The SC field equations are achieved from a space charge wave model. The electron dynamic equations are achieved by substituting the RF field equations and the SC field equations into the Lorenz force equation. Using coupling impedance to treat exciting sources of RF field equations, the RF and the SC field equations can be solved with the help of high frequency simulation software, such as HFSS or HFCS, which makes this time-dependent theory more flexible. The feasibility of this theory is proved by numerical simulation.
Physics and simulation of terahertz folded waveguide traveling wave tube regenerative feedback oscillators
2010, 59 (12): 8484-8489. doi: 10.7498/aps.59.8484
Traveling wave tube (TWT) regenerative feedback oscillators have been demonstrated to be very attractive terahertz source devices. The principle and the physical model of this kind of device are analyzed. To see more detailed oscillation process, a 560 GHz folded waveguide TWT oscillator is presented. The results show steady-state oscillation frequencies exist between 550—600 GHz, and a single-frequency oscillation is found near 560 GHz. An oscillation frequency step-tuning behavior with the continuous variation of the beam voltage is discussed.
Measurement of orbital angular momentum of Laguerre-Gaussian beam by using phase vortices of interference fields
2010, 59 (12): 8490-8498. doi: 10.7498/aps.59.8490
The phase and the zero-contour of the real part and the imaginary part of the interference field on a far-field plane generated by multi-aperture diffraction screen are simulated. It is found that when the orbital angular momentum quantum number of incident beam is equal to zero, at the center of interference field the zero-lines cannot cross each other, therefore, thereby the phase vortices cannot form. When the orbital angular momentum quantum numbers of incident beam are opposite to each other in sign, namely -1 and +1 at the center of interference field the zero-lines are perpendicular to and cross each other, the signs of the phase vortices at the corresponding positions in interference fields are also opposite to each other. When the orbital angular momentum quantum numbers of incident beam are equal to ±2 and ±3, there are four zero-lines that cross each other at the center of interference fields, where the topological charge values of phase vortices are just equal to the orbital angular momentum quantum numbers of the Laguerre-Gaussian beam. Therefore, these results can be used to measure the orbital angular momentum of optical vortex beam.
2010, 59 (12): 8512-8517. doi: 10.7498/aps.59.8512
In this paper, we have calculated the absorption of the probing field in a N-type four-level atomic system with three dipole transitions using density matrix equation. The results indicate that the absorption or amplification of the probing field depends on the Rabi phases of the control field and signal field under introducing laser field Rabi phase, and the absorption or amplification periodically changes with the Rabi phases of the control field and the signal field at a period of 2π. The Rabi phase change of the probe field has no effect on the absorption. In addition, the effect of the signal field Rabi phase on absorption is the same as that of the control field Rabi phase on absorption. Rabi phase has an effect mainly on atomic coherence but little on atomic population.
Entanglement and preparation for an atom interacting with two-mode coherent strong field via intensity-dependent coupling multiphoton processes
2010, 59 (12): 8518-8525. doi: 10.7498/aps.59.8518
In this paper, effective Hamiltonian of the two-mode multiphoton Jaynes-Cummings model is given via the intensity-dependent coupling in the rotating wave approximation. Under strong field, the atom-field entanglement and the entanglement between two modes of coherent field, according to the above model, are investigated by using the quantum reduced entropy and the quantum relative entropy, respectively. It is showed that properties of these two types of entanglements are considerably relevant to the absorption or emission photon number k, per atomic transition. Different properties of entanglements in the processes of two-photon (k=1) and mutiphoton (k>2) are revealed respectively. The preparation of entangled states is discussed. The Einstein-Podolsky-Rosen states of atom-field irrelevant to time, as well as entangled states between two models of the coherent field are prepared, respectively.
2010, 59 (12): 8526-8534. doi: 10.7498/aps.59.8526
We investigate quantum nonlocality dynamics of a three-level atom and a class of two-mode non-classical states (including the entangled coherent state, the pair coherent state and the two-mode squeezed state) in the non-degenerate two-photon Jaynes-Cummings model. It is shown that the time evolution of the Bell nonlocality for the case of the entangled coherent state is closely related to the average number of photons in the two-mode field. For the cases of the pair coherent state and the two-mode squeezed state, the effect of Bell nonlocality sudden death is demonstrated, and the absolute value of the 〈BCHSH〉 exhibits exact periodic collapses and revivals.
Analysis on thermal effect of laser-diode array end-pumped composite rod laser by finite element method
2010, 59 (12): 8535-8541. doi: 10.7498/aps.59.8535
A numerical model of the rod laser medium pumped by laser-diode array (LDA) end is set up. Considering the influence of temperature correlation of the thermodynamic parameters of the material and heat transfer coefficient between air and medium, using the thermal conduction equations and the thermal-elastic equations, the transient distributions of temperature and thermal stress and strain in the composite rod medium and un-composite rod medium are calculated by the finite element analysis method. The effects of pump power and heat transfer coefficient and time on transient distributions of temperature and thermal stress and strain in the medium are analyzed. The results indicate that the positions of maximal temperature maximal tensile stress and maximal axial strain of the composite rod are different from those of the un-composite rod. And the maximal temperature, maximal tensile stress and maximal axial strain of the composite rod are respectively 73 per cent, 60 per cemt, and 33 per cent of the corresponding values of the un-composite rod. It is obvious that the thermal effect of the composite rod is greatly reduced. The theoretical results provide theoretical reference for the design of solid laser pumped by LDA and experimental study.
2010, 59 (12): 8542-8547. doi: 10.7498/aps.59.8542
The distribution of injected current in the active region of external cavity oxide-confined photonic crystal vertical cavity surface emitting lasers is studied extensively. An advanced three-dimensional model of current distribution is used to analyse the effects of photonic crystal structures on current density distribution and series resistance of the device. It is found that the deeper the photonic crystal holes are, the worse the circular symmetry of the current density distribution is and the higher the series resistance is,especially when the ethching depths of holes are larger than 2 μm. Different patterns of photonic crystal structures have a great imfluence on current density distribution and circular symmetry. The results are beneficial to the research and the design of external cavity oxide-confined photonic crystal vertical cavity surface emitting lasers.
2010, 59 (12): 8548-8553. doi: 10.7498/aps.59.8548
Inspired by the idea of the stero-coupling, we propose a new sandwich-like photonic crystal microcavity which is composed of double layer photonic crystal slabs H1 (DLPCS-H1) cavity with an air layer in between. We calculate the electromagnetic field distribution and the quality factor of the dipole mode by the three-dimensional finite-difference time-domain method and the Padé approximation method. Through carefully analyzing the effect of the air layer height on the quality factor of the dipole mode, we obtain an optimized DLPCS-H1 cavity in which the height of intermediate air layer is about 0.5a (a is the lattice constant, a=420 nm). In this cavity, the quality factor of the dipole mode is 4 times as large as that of the conventional single layer photonic crystal slab H1 cavity. Furthermore, we study the three-layer photonic crystal slabs H1 cavity, and the quality factor of its dipole mode is increased over 7 times.
Investigation on stimulated Brillouin scattering medium——perfluoropolyether at high and low temperatures
2010, 59 (12): 8554-8558. doi: 10.7498/aps.59.8554
In this paper, the stimulated Brillouin scattering (SBS) media perfluoropolyether (PFPE) with unique physicochemical properties and SBS properties are researched. The PFPE has a small absorption coefficient and can be used to increase the load-ability of SBS system. The PFPE with a high boiling point and a low pouring point can operate at high and low temperatures, respectively. The SBS properties of PFPE have been analyzed in detail and validated in Q-switched Nd:YAG laser system. The experimental results indicate that the media not only have good SBS characteristics but also can be used in a wide temperature range, thereby has great advantage for experimental investigation on SBS phase-conjugated mirror at different temperatures.
Optimization of attosecond pulses from the interaction of ultrarelativistic laser with overdense plasma
2010, 59 (12): 8559-8565. doi: 10.7498/aps.59.8559
Using one-dimensional particle-in-cell simulations, the generation of attosecond pulses is studied due to the interaction of a short ultrarelativistic laser pulse with overdense plasma. According to the ultrarelativistic similarity theory, we analyze the motion of the electrons and the generation of high-order harmonics. We find that when the plasma density is constant and the dimensionless similarity parameter S decreases, the conversion efficiency of attosecond pulses first increases and then decreases. So we can choose a laser pulse with an appropriate intensity to obtain an attosecond pulse with a high conversion efficiency. Furthermore, when S is fixed, with the increase of the plasma density, the conversion efficiency of attosecond pulses shows an upward tendency. This implies that we can obtain a higher attosecond pulse when a laser with an appropriate intensity is incident on a denser plasma.
2010, 59 (12): 8566-8573. doi: 10.7498/aps.59.8566
In this paper, based on the equivalent transmission line model a method of designing left-handed metamaterials micro-structures is proposed, verified and analyzed. Firstly, according to the basic transmission line model with left and right-handed characteristics, we build a new left-handed model by analogy. Then according to the qualitative analysis of the corresponding relationship of each component in the transmission line network to the shape and size of continuous medium micro-structure configuration, the specific size of model is inferred, and thus a reasonable left-handed metamaterials micro-structure configuration and the corresponding transmission line model are established. We have conducted a series of tests including numerical simulation, which verifies that there does exist the left-handed characteristic in a certain frequency band. Finally based on the numerical simulation and experiment the effects each micro-structural parameter on left-handed frequency position and bandwidth are analyzed, and the size parameter that affects the left hand characteristic most seriously is determined.
2010, 59 (12): 8574-8578. doi: 10.7498/aps.59.8574
We study the tunable resonance characteristics of double split ring resonator (DSRR) at microwave band numerically. For the incident electromagnetic wave parallel to the incident plane, the DSRR structure shows magnetic and electric resonances at different frequencies respectively. When the E polarization along the two splits of the DSRR, the magnetic and the electric resonances have the strongest resonant intensities. As the DSRR metamaterial rotates along the H field, the frequencies of magnetic and electric resonances are kept unchanged, however, the resonant intensities decrease rapidly. The tunable metamaterial proposed in this paper only need rotate the metamaterial without structure changes or additional applied field, and has potential applications in electromagnetic switch and phase modulation. This simple tunable method could be used for higher frequency metamaterial, and expend the applications at terahertz and optical frequencies.
2010, 59 (12): 8579-8583. doi: 10.7498/aps.59.8579
The magnetic plasmon (MP) modes in metal-dielectric-metal nanosandwich structures are investigated numerically using finite-difference time-domain method. We demonstrate the law of the quality factor in this kind of resonator.According to the MP modes in the nanosandwich structures, we design a series of resonators of this kind in order to investigate how to tune the resonante frequencies of MP modes through tuning the geometrical parameters and the dielectric layer refractive index of resonators, and the obtained result is well consonant with our analysis by the equivalent inductance capacitance circuit. We also investigate the electromagnetic energy confinement of the nanosandwich resonators, and analyse the Q factors of these structures from thermal and radiant points of view, which will guide one in designing new waveguides and lasers based on MP modes.
2010, 59 (12): 8584-8590. doi: 10.7498/aps.59.8584
Negative permittivity can be realized from the periodic unit cells of the metallic bar structure, and the effects of the length and the width of the metallic bar on negative permittivity is discussed. Through arraying two unit cells along the direction of the wave vector k , the negative permeability can be achieved from a strong resonance response of the two metallic bars to external magnetic field, so a one-dimensional left-handed metamaterials is obtained. The left-handed properties can also be obtained if magnetic boundary conditions of the two unit cells are interchanged with those of wave ports, but electrical boundary conditions are unchanged, i.e., the wave vector k rotates by 90°. So a two-dimensional left-handed metamaterials is achieved.
Preliminary investigation on the design and experiment of a spatial filter for dual band high power microwave
2010, 59 (12): 8591-8596. doi: 10.7498/aps.59.8591
A spatial filter/diplexer for gigawatt level high power microwave is theoretically and experimentally investigated in this paper. The spatial filter consists of several cylindrical rods which are located in the near field of the applied radiating antenna. The S/X dual band microwaves are reflected and transmitted by the spatial filter in the same propagation directions. In the low power experiments, the transmitted and the reflected patterns are very close to that of the applied antenna. The measured reflection and transmission efficiencies, calculated from the power densities in the far-field region, are about 96% and 97% respectively. In addition, the spatial filter has the isolation level higher than 25 dB and the cross polar level much lower than -25 dB. Under the illumination by S/X band high power microwave with a pulse magnitude of about 1.5 GW and duration of about 100 ns, no microwave breakdown is observed.
2010, 59 (12): 8597-8606. doi: 10.7498/aps.59.8597
A novel theory, namely, Fourier mode coupling (FMC) theory for fiber Bragg gratings (FBGs) is proposed in this paper. During analyzing coupled modes of FBGs, the Fourier transform relations among the amplitude coefficients of coupled modes are found for the first time. The general expressions of reflective and transmissive spectra of FBGs are deduced from the combination of Fourier transform with the well-known coupled-mode theory. In the proposed FMC theory, the spectral characteristics of the FBG are achieved by the calculation of coupled modes in the spatial domain spectrum, which is the Fourier transform result of refractive index perturbation in the FBG. The FBG spectrum based on the FMC theory is simulated here, and compared with those obtained from the coupled mode theory and pure Fourier transform. The comparison shows that the FMC theory for and the derived spactra of FBGs are in accordance with the coupled mode theory and the practical spectra of the FBG respectively. The FMC theory has many features, these being simple, clear, direct, accurate and fast, which could be used as a universal tool for fast spectrum analysis of any FBG with an arbitrary distribution of refractive index perturbation along the fiber axis.
2010, 59 (12): 8607-8614. doi: 10.7498/aps.59.8607
A novel theory, namely, Fourier mode coupling (FMC) theory for long-period fiber gratings (LPFGs) is proposed in this paper. During analyzing the co-propagating coupling between the core mode and cladding modes in LPFGs, the Fourier transform relations among the amplitude coefficients of co-propagating coupled-modes are found for the first time, to the best of authors’ knowledge. The general expressions of the coupling and transmission spectra of LPFGs are also deduced from the combination of Fourier transform with the well-known coupled-mode theory. In the proposed FMC theory, the spectral characteristics of the LPFGs without over-coupling are derived from the calculation of co-propagating mode coupling in the spatial domain spectrum, which is the Fourier transform result of refractive index perturbation in the LPFG. According to the FMC theory, the spectra of the LPFGs in different perturbation amplitudes and lengths are numerically simulated here. A measured transmission spectrum is also compared with the calculated transmission spectra based on the FMC theory and the coupled mode theory, respectively. The comparison shows that the FMC theory and the derived spectra for LPFGs are in consistance with the coupled-mode theory and the practical spectra of LPFGs respectively. The FMC has many features, these being simple, fast and accurate, which could be employed for spectrum analysis of any LPFG with an arbitrary distribution of refractive index perturbation along the fiber axis.
2010, 59 (12): 8615-8624. doi: 10.7498/aps.59.8615
Fast and efficient access to spectral characteristics of multi-cascaded fiber grating Fabry-Perot (F-P) cavity, is an important foundation and premise to ensure optimizing the design of cascade multi-wavelength lasers, amplifiers and other optical devices, and complex distributed sensing network based on the F-P structure. V-I transmission matrix method is presented first in this paper to analyze the reflection spectral characteristics of fiber grating F-P cavity and then the V-I transmission matrix model is established. Spectral characteristics of three different structures of fiber grating F-P cavity at different parameters are analyzed based on the V-I transmission matrix model. Compared with multi-layer method, V-I transmission matrix method can save the computation time apparently under the premise of ensuring analysis accuracy. The experimental results show that for analyzing the spectral characteristics of fiber grating F-P cavity, the V-I transmission matrix method is more accurate than the coupled-mode method.
2010, 59 (12): 8625-8631. doi: 10.7498/aps.59.8625
The coupled-mode equations in photonics crystal fiber with triangular structure triple-core (TTC-PCF) are achieved based on the coupled-mode theory, thereby the directional coupling between the cores in this structure is numerically studied in detail. The influences of the fiber structure and the incident wavelength on the coupling coefficient, and the effect of the amplitude ratio of the inject beams at the input end on the transfer of energy between the cores are analyzed. From the results, it can follow that continuous modulation of the coupling intensity between the cores can be achieved by changing the amplitude ratio of the inject beams. The coupled-mode theory and the beam propagation method present the results well consistent with each other. The unique coupling transmission performance of the TTC-PCF shows its possible applications in the design and the preparation of coupling strength continuously tunable fiber directional coupler and large mode field fiber lasers.
2010, 59 (12): 8632-8639. doi: 10.7498/aps.59.8632
A novel rectangular lattice photonic crystal fiber is proposed which is composed of a central defect core and a cladding with square mesh structure by introducing another air hole row between two air hole rows for every other line into a conventional rectangular lattice photonic crystal fiber. Its dispersion, birefringence and confinement loss are numerically investigated by full vector finite element method with anisotropic perfectly matched layers. Numerical results indicate that the proposed fiber shows higher birefringence negative dispersion effect and stronger confinement ability of guided mode, in which the confinement loss is lower than 10-2 dB ·m-1. The wavelength for high birefringence negative dispersion can be optimized by adjusting the parameters of proposed fiber, such as Λ and d/Λ. The dispersion and the dispersion slope are both negative, the birefringence is higher than 10-2, and nonlinear parameter is close to 55 km-1W-1 over C band (i.e. 1.53—1.565 μm) under the condition of Λ=2.0 μm and d/Λ=0.4. This fiber will have important applications in the fields of polarization maintaining transmission system and dispersion compensation, and also in the design of widely tunable wavelength converter based on four-wave mixing.
2010, 59 (12): 8640-8645. doi: 10.7498/aps.59.8640
Bragg fiber is one kind of microstructure fiber with one-dimensional photonic crystal layer in its fiber cladding. In this paper, the hollow-core Bragg fiber for trace gas detection, with a transmission band in the mid-infrared band, is designed and fabricated based on semiconductor glass and polymer materials. The measurements of the fiber sample demonstrate its bandgap-guided properties, showing two obvious bandgap guided transmission bands. The central wavelength of the lowest transmission band is 4.4 μm.
2010, 59 (12): 8646-8654. doi: 10.7498/aps.59.8646
Patch nearfield acoustic holography based on the distributed source boundary point method (DSBPM) is proposed to reduce an error of the reconstructed result obtained from the pressure measured on a small aperture. The extrapolated data on a larger holographic surface is obtained through the measured data on the small holographic surface using the DSBPM, then the extrapolated data is used to reconstruct sound source. The simulation results and the experimental results show that the method can provide a good reconstructed result under the condition of the small holographic aperture.
2010, 59 (12): 8655-8661. doi: 10.7498/aps.59.8655
Based on Biot-Tsiklauri model and the combined Biot/squirt model, a universal acoustic model for non-Newtonian fluid (Maxwell fluid) saturated porous medium with an arbitrary pore size distribution is presented, in which the squirt-flow mechanism has been taken into account. The influences of non-Newtonian effect of Maxwell fluid on the attenuation and the dispersion characteristics of elastic waves propagating in such a porous medium are investigated. It shows that the non-Newtonian effect and the squirt-flow effect are the important factors for elastic wave dispersion and attenuation. When the squirt flow mechanism occurs, the squirt flow mechanism dominates the contribution to the energy loss of compressional waves at low frequencies. It shows that the squirt flow only affects the dispersion and the attenuation characteristics of two compressional waves, while the non-Newtonian effect of Maxwell fluid not only affects the dispersion and the attenuation characteristics of the compressional waves, but also influences the dispersion and the attenuation characteristics of shear waves.
2010, 59 (12): 8662-8668. doi: 10.7498/aps.59.8662
The difference wave equation is obtained by the discretization of nonlinear acoustic wave equation in atmosphere with in the second-order miniterm approximation based on the finite-difference time-domain method. And the two-dimensional field distribution of continuous sinusoidal wave vertically or obliquely radiated by a linear array of five monopole sound sources is numerically simulated at the different initial acoustic pressures. A comparison between nonlinear simulation and linear one show that weak nonlinearity has definite and apparent effects on the distribution of acoustic field and focused gain of array, leading the distribution of waveform to be closer to the array and the focusing gain to deteriorate, and that the strong nonlinearity has a strong waveform aberration, because the other waves of non-fundamental frequency appear. No evident distinction is found in the effect between the oblique propagation and the vertical propagation, though less focusing gain and influence caused by strong nonlinearity produced by the acoustic geometric spreading result in more attenuation of axial pressure in the case of oblique propagation.
2010, 59 (12): 8669-8675. doi: 10.7498/aps.59.8669
This paper deals with an experimental investigation of two-phase frictional pressure drop behavior of 1,1-difluoroethane in an 8 mm inside-diameter smooth horizontal tube. Pressure drop characteristics are measured in a pressure range of 0.19—0.41 MPa, heat flux range of 14—62 kW/m2, and mass flux range of 128—200 kg/m2s. The effects of experimental parameters on pressure drop are analyzed. It is found that with the increases of mass flow and vapor quality, the frictional pressure drop increases. The proportion of momentum pressure drop in the total pressure drop increases slightly as heat flux increases, and accordingly the proportion of the frictional pressure drop decreases. The frictional pressure drop increases with saturation pressure decreasing. Experimental results are compared with the calculations from the two extensively used correlation formulae. Our investigations show that the Friedel model has a relatively large deviation, and the Müller-Steinhagen-Heck model accords well with the experimental results.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2010, 59 (12): 8676-8684. doi: 10.7498/aps.59.8676
It is found that the vacuum backpressure affects the flow of background gas into a Hall thruster channel. The flow of background gas can affect the ionization of propellants and the conductivity of electrons, and thereby affect the physical process and discharge characteristic of Hall thrusters. In order to investigate these effects, the atomic and the ionic emission spectra in the discharge channel and the ions energy distribution are measured under different vacuum backpressures. The experimental results indicate that the increase of vacuum backpressure can reduce the electron temperature and ionization efficiency of propellants and a new ionization region appears near exit plane in the channel; and moreover, it is found that the new ionization region penetrates into the channel more deeply under the higher vacuum backpressure case.
2010, 59 (12): 8685-8691. doi: 10.7498/aps.59.8685
Planar wire arrays have been widely studied as a kind of non-cylindrical wire array load in recent years. In this paper the magnetostatic simulation of individual wire in a planar wire array is performed using a radial motion equation in order to understand the patterns of the current partition through the wires, the force distribution on the individual wire, the magnetic field distribution on the planar wire array, and the variations of the total load inductance and the kinetic energy of the wire array during the implosion phase. The model has also been used to calculate the implosion trace and time of the planar wire array used on "Qiangguang-Ⅰ" generator. The calculation results are compared with the experimental results and it is concluded that the model gives an implosion time with an error of about 10%. All the simulation results mentioned above can help one further understand the process of the planar wire array Z-pinch and design the load parameters.
2010, 59 (12): 8692-8700. doi: 10.7498/aps.59.8692
Characteristics of dust particulates (charging, movement and temperature) in magnetic fusion devices with different plasma parameters are discussed. It is shown that the charge quantity of dust particulates is obviously influenced by the ratio between electron and ion temperatures; secondary electron emission reduces the charge quantity; the charging relaxation time of dust particulates in fusion plasma is much shorter than in laboratory plasma; dust particulates speed reach hundreds of meters per second under the action of ion drag force; the lifetime of carbon dust in fusion plasma is on the millisecond scale. The results are consistent with experimental observations.
Numerical simulation on the plasma field within discharge chamber of electron cyclotron resonance ion thruster
2010, 59 (12): 8701-8706. doi: 10.7498/aps.59.8701
The discharge chamber is a key component of electron cyclotron resonance ion thruster. The plasma inside it is generated through a electron cyclotron resonance process. The drift and diffusion model of plasma inside the discharge chamber for numerical simulation is developed in this paper. The time dependent parameters of plasma are obtained by solving this model with upwind difference scheme. The results can provide useful information for the design and also the experiment of electron cyclotron resonance ion thruster.
2010, 59 (12): 8707-8716. doi: 10.7498/aps.59.8707
To develop reliable numerical simulation tools is very important for the theoretical research, the experimental analysis and the load designing of Z-pinch implosions. A dedicated Z-pinch implosion physical scheme of the two-dimensional numerical simulation of three-temperature radiation magnetohydrodynamics MARED code is introduced. The results of its one-dimensional test demonstrate that the MARED code is suited for simulating implosions on different devices and in a wide range of load parameters. A combination between the simulations and the measurements of the wire-array Z-pinch experiment shows that under the same load conditions, the X-ray radiation power produced by the tungsten wire-array implosion is much higher than that generated by the aluminum wire array. With the same load current, a greater load mass gains a lower X-ray power. However, the X-ray radiation power increases with the load current. The MARED code is found to be able to reproduce the primary dynamic characteristics of the Z-pinch implosions, and the development of the instability qualitatively aecords with the simplified instability theoretical analysis and experimental results. It is also used to simulate the radiation field formation of the wire-array with filling column at the axis, and its preliminary results are qualitatively consistent with the simulation results from the Sandia laboratory.
2010, 59 (12): 8717-8724. doi: 10.7498/aps.59.8717
The linear theory and the post process program LIP are developed to describe the stimulated scattering in laser-plasma-interaction. The two damping rates, especially the collision damping rate, are included in the program LIP. The effects of the laser intensity and electron temperature are investigated in an ignition scale plasma. The importance of the collision damping is revealed, and it is found that the growth rate of the stimulated scattering is effectively reduced by the collision damping when the plasma density is close to one-quarter of critical density. Whereas, the electron temperature is responsible for the increase of the growth rate on the specific density scale. The results may provide the reference for the ignition design in reducing the reflectivity.
2010, 59 (12): 8725-8732. doi: 10.7498/aps.59.8725
By time division multiplexing (TDM) technique, an all-solid-state all-fiber multibeam optical pulse generation system used for an inertial confinement fusion driver is developed. The continuous wave signal is provided by a single longitudinal mode oscillator. The TDM technique and he high speed electro-optic modulation technique are utilized to realize the train pulse and arbitrary pulse-shaping. The polarization independent acoustooptic modulation technique is adopted to independently output the multibeam. Eight sub-pulses which may be shaped arbitrarily are included in each train pulse, and the time delay between adjacent sub-pulses is 120 ns. A single pulse selection module is used so that each beam turns into an independent output beam and then each beam is amplified and transmitted to output. Finally, it is successfully verified that the train pulse can be produced by the TDM technique, and its sub-pulse can be shaped arbitrarily and independently. The each beam energy is 1.275 μJ.
Experimental generation of 58 MeV quasi-monoenergetic electron beam by ultra-intense femto-second laser wakefield
2010, 59 (12): 8733-8738. doi: 10.7498/aps.59.8733
Monoenergetic electron bunches can be generated by the interaction between the ultra-intense laser and underdense plasma on the mm scale. In the experiment conducted on the SILEX-Ⅰ, by the interaction between an ultra-intense femtosecond laser pulse and 2.7 mm supersonic He gas jet, a 58 MeV quasi-monoenergetic electron beam with 15.5% energy spread and 15 mrad beam divergence is produced. The total charge of the electron beam is about 15.4 nC when the laser power is 70 TW. In this paper the experimental conditions, the method and main results are presented.
Influences of gas flow on gas temperature and discharge mode in dielectric barrier discharge of nitrogen at atmospheric pressure
2010, 59 (12): 8739-8746. doi: 10.7498/aps.59.8739
Dielectric barrier discharge in nitrogen at atmospheric pressure is studied with the spectroscopy and the fast photography of the discharge. By the introduction of a nitrogen flow into the discharge gap, the homogeneous discharge in a 2 mm gap can be maintained. Based on the waveform of the discharge current characterized by a current pulse per half cycle of the applied voltage and the 1 μs exposure discharge photograph showing a luminous layer covering the entire surface of the anode, the homogeneous discharge is identified with a Townsend discharge. The instrumental broadening of the spectrometer used in the experiment is calibrated with a helium-neon laser. The data relevant to the instrumental broadening are input into a code called Specair for calculating the spectrum profiles of 0—2 band in the second positive system of nitrogen molecules at different gas temperatures. By fitting the calculated spectrum profiles to the experimental one, the rotational temperature of the nitrogen molecules is determined. The results show that the dielectric barrier Townsend discharge in nitrogen at atmospheric pressure cannot heat the nitrogen to a high temperature (ΔTg≤50 K) and the small rising in temperature does not induce the thermal instability that leads to the transition of the Townsend discharge to a filamentary discharge. By the addition of a gas flow into the discharge gap, the nitrogen is indeed cooled down to a lower temperature. However, it is not the reason for the Townsend discharge to be maintained. By comparing the discharge spectra with and without the gas flow, it could be concluded that the gas flow much reduces the density of the impurity oxygen desorbed from the dielectric by the discharge and makes it possible for more nitrogen metastables to survive to the beginning time of the next discharge and to provide sufficient seed electrons which are necessary for Townsend discharge.
2010, 59 (12): 8747-8754. doi: 10.7498/aps.59.8747
A self-consistent one-dimensional fluid coupled model is built to describe the low pressure xenon dielectric barrier discharge (DBD). And the finite-element method is employed to investigate gas voltages, discharge currents and the time evolutions of surface charges on dielectric barrier under different applied voltage amplitudes and frequencies. The spatial and temporal distributions of electrons, ions, excited, resonance, metastable particles and spatial electrical field are also achieved. The simulation results show that the surface charges accumulated on the dielectric barriers play a key role in the ignition and the extinguishment of the discharge. And based on the variation of gas voltage, the surface charging can be divided into six stages in one discharge cycle. With the increase of applied voltage amplitude, the gas gap breakdown moves ahead of the zero-crossing point of applied voltage gradually, and the discharge becomes more and more intense. Furthermore, with the increase of applied voltage frequency, the gas voltage decreases gradually, gas gap tends to breakdown, and discharge becomes uniform. Finally, spatiotemporal distributions of particles and electric field indicate that the xenon DBD is a typical glow discharge.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
High pressure phase transition and phonon-dispersion relations of BeO calculated by first-principles method
2010, 59 (12): 8755-8761. doi: 10.7498/aps.59.8755
The high pressure phase transition at zero temperature and the phonon-dispersion relations at zero temperatue and zero pressue of BeO have been studied by a first-principles method. The results show that a phase transition from wurtzite structure (B4) to cubic sodium chloride structure (B1) happens at about 122 GPa and the zinc blende phase (B3) is of a meta-stable structure at zero temperature and zero pressure. The phonon-dispersion relations of B1, B3 and B4 phase BeO at zero temperature and zero pressure are investigated by the frozen phonon method. The calculations show that at zero temperature and zero pressure B1 phase is an unstable phase and B4 and B3 phases are of two very simliar structure, but they are still distinguishable from each other by their phonon-dispersion relations. Finally, the phase diagrams of BeO at high temperature and high pressure are studied.
High luminance carbon nanotube field emission cold cathode based on indium tin oxide/Ti composite electrode
2010, 59 (12): 8762-8769. doi: 10.7498/aps.59.8762
Carbon nanotube (CNT) cathode with an indium tin oxide (ITO)/Ti composite electrode is successfully fabricated using both magnetron sputtering technology and screen-printed technology which can improve adhesive performance between electrode and CNT cathode of transplanted-type CNT cold cathode, thus eliminating the effects of interface barrier and non-ohmic contact on field emission uniformity and stability of CNT cathode. Microstrcture of ITO/Ti-based CNT cathode is studied by X-ray diffraction and field emission scanning electron microscopy. The results show that TiC phase forms in ITO/Ti-based CNT cathode, thereby a strong interaction system is created between CNT and Ti substrate which reduces, or even eliminates the interface barrier between electrode and CNT, and increases the probability of forming ohmic contact. The resistivity measurement by four probe technology shows that the ITO/Ti-based CNT cathode has performance of resistances in parallel and electric conductivity of CNT cathode increases. Characteristic test of ITO/Ti-based CNT cathode shows that field emission current reaches 384 μA/cm2 which significantly increases compared with that of ITO-based CNT cathode, and that the tested anode can be induced to emit stable, uniform and high luminance. So the ITO/Ti composite electrode is an effective way to make a CNT cathode with stable and uniform field emission and low power.
Mechanism of large grain polycrystalline Si preparation by aluminum induced crystallization with temperature gradient profile
2010, 59 (12): 8770-8775. doi: 10.7498/aps.59.8770
In order to shorten processing time for large grain polycrystalline Si thin films prepared by aluminum induced crystallization, the stack of a-Si/SiO2/Al is deposited by radio frequency magnetron sputtering and annealed at two different irregular temperature profiles. The influence of irregular temperature profile on the processing of amorphous silicon prepared by the aluminum induced crystallization is investigated and the condition is discussed under which whether new nucleation appears when the annealing temperature increases. It turns out that the formation of nucleation is governed by the relationship among the grain radium at low temperature, the distance of depletion region, and the distance of adjacent grains.
2010, 59 (12): 8776-8782. doi: 10.7498/aps.59.8776
We investigate the structure stability, stretching, compressing, shearing, random shifting and X-ray diffraction of Ni based superconductive material EuNi2Si2 with different space group numbers based on inversed interatomic potentials obtained with Chen-Mbius lattice-inversion technique. It is found that the space group number of 139 has the lowest binding energy and the structure is the most stable. Furthermore, the phonon density and the thermodynamic properties of the stable structure are calculated and discussed. The phonon density of states shows that the low frequency range is dominated by the rare-earth element Eu with larger atomic mass. While with frequency increasing, the Si atoms with smaller atomic mass become more and more prominent. For the specific heat and the vibrational entropy, Eu and Ni contribute more in the low temperature range, Si becomes more and more prominent with temperature increasing.
Effects of Si content on phase composition and thermoelectric properties of higher manganese silicide
2010, 59 (12): 8783-8788. doi: 10.7498/aps.59.8783
MnSi1.70+x(x=0, 0.05, 0.1, 0.15) compounds have been prepared by induction melting-annealing procedure combined with spark plasma sintering method. The phase composition and the thermoelectric properties of higher manganese silicide (HMS) with different Si contents are investigated. The results indicate that the samples with x＜0.1 include HMS phase and MnSi phase, and the relative content of MnSi phase decreases with x value increasing. The sample with x=0.1 is single phase HMS. The phase compositions of the sample with x＞0.1 are HMS and Si. As x value inereases the electrical conductivity of the sample gradually decreases, while the Seebeck coefficient increases because metallic MnSi phase decreases. The carrier concentration and the effective mass of the sample at room temperature decrease and the carrier mobility increases with x value increasing. In the sample with x=0.1, the impurity phase content is the least, which results in the lowest thermal conductivity and a minimum value of 2.25 W ·m-1K-1 at 800 K. The maximum ZT value of 0.45 is obtained at 850 K for MnSi1.80.
2010, 59 (12): 8789-8793. doi: 10.7498/aps.59.8789
Using the first-principles method based on density functional theory, we investigate the structural properties of seven different phases of BiFeO3 including R3c, R3m, P4mm, Cm, Pm3 m, R3 m and R3 c and structural transition. The results show that the ground state has the R3c phase, and that phase transitions can occur among these phases, which may be characterized by two types of structural transitions. One is the relative displacement between octahedral FeO6 and Bi3+, and the other is the rotation of octahedral FeO6 along the  axis. Furthermore, the BiFeO3 film is found to be able to change from R3c phase to P4mm phase, owing to the substrate effect.
Study on thermal conductivity of porous silicon thermal isolation layer based on micro-structure mathematical model
2010, 59 (12): 8794-8800. doi: 10.7498/aps.59.8794
Porous silicon can be used as a thermal isolation layer because of its low thermal conductivity. Different from other models based on the mechanism that the thermal conductivity of porous silicon is attributed to complex microcosmic thermal conductivity through such as boundary scattering, the model used in this paper is based on the mechanism that the low conductivity of the porous silicon material is due to its structure factors, such as the existence and distribution of pores and porous silicon is viewed as a compound microstructure piece, which is constructed by both silicon continuous material and pore continuous material medium, connecting in parallel and series patterns. Therefore, the authors give a more understandable and simpler reason why the conductivity of such a material is at such a low level. It is pointed out that the influence of porosity on equivalent thermal conductivity can be divided into two parts: vertical and horizontal, thereby giving semi-quantitative relationships between the porosity and equivalent thermal conductivity for different pore structures and distributions. A comparison between the calculated results and experimental results shows the validity of this model, thus supporting the reason why the thermal conductivity of porous silicon material is relatively low.
2010, 59 (12): 8807-8813. doi: 10.7498/aps.59.8807
The low-energy bombardments of noble metal atoms (Ni, Pd, Pt, Cu, Ag, Au) on Pt (111) surface are studied by molecular dynamics (MD) simulations. The atomic interaction potential with embedded atom is used in the simulation. The incident-energy effects on adatom yields, sputtering yields, and vacancy yields for different projectiles have been observed and summarized. When the incident energy Ein varies from 0.1 to 200 eV, surface atoms transfer layer by layer and the incident energy dependent transition occurs when the incident energy values are about 5 and 70 eV. When the incident energy is lower than 5 eV, projetiles are deposited as adatom and the value of defect yield is 0. While 70 eV > Ein > 5 eV, no atoms can be implanted into the depth beyond the second layer and the vacancy yield in the third layer is about 0. For the case of Ein > 70 eV, deposited atoms enter into the third layer. And then, vacancy occurs. Furthermore, defect yield sharply increases with the increase of incident energy. Based on the result of our MD simulations, a guide to the choice of optimum deposition parameters is suggested.
Effects of synthesis parameters on the growth of magnesium oxide nanowries by vapor-liquid-solid mechanism at low temperatures
2010, 59 (12): 8814-8819. doi: 10.7498/aps.59.8814
MgO nanowires are synthesized at a lower temperature (T=600 ℃) by pulsed liquid injection metal-organic chemical vapor deposition with Mg(C11H19O2)2 (magnesium bis (2, 2, 6, 6-tetramethyl-3, 5-heptanedionate)) as precursor. The MgO nanowires grow along the  direction with gold nanoparticles on the tips, which leads to the vapor-liquid-solid growth mechanism. The growth mode of nanowires (vertical growth to the substrate or parallel growth to the substrate) can be controlled by adjusting the injection period or the injection mass/period.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2010, 59 (12): 8820-8828. doi: 10.7498/aps.59.8820
Using the first-principles method, we have studied the structure transition of two-dimensional hexagonal boron nitride (2D h-BN) under large uniaxial strain. The strain is applied by changing the values of Lx and Ly, which correspond to the lengths of primitive cell in the directions perpendicular and parallel to B—N bonds, respectively. For the large asymmetrical tensile strain perpendicular to B—N bonds, the rhombic structure is stable when Lx≤0.3388 nm. As the strain increases, the system transforms from the rhombic structure to a rectangular structure consisting of interlaced interacting BN chains, which becomes stable when Lx≥0.3488 nm. When the strain further increases, the system finally changes into the one comprised of isolated BN chains. For the 2D h-BN with large asymmetrical tensile strain distribution parallel to B—N bonds, there is no stable rectangular structure and the system becomes the one composed of isolated BN chains when Ly>0.571 nm.
2010, 59 (12): 8829-8835. doi: 10.7498/aps.59.8829
Effects of Al and N codoping on the optical properties of Zn1-xMgxO in ultraviolet and visual spectral regions have been systematically investigated using density functional theory based on the first-principles method. It is found that the optical properties of Zn1-xMgxO are caused by the codoped Al and N to vary mainly in a low energy region, whereas they remain almost unchanged in a high energy region. The detailed calculations, including the imaginary part of the dielectric constant, absorption spectrum and extinction coefficient, indicate that due to the codoped Al and N, the optical absorption of Zn1-xMgxO shifts toward lower energy and the absorption of the ultraviolet and the visible absorption increase. Study on the real part of the dielectric constant and reflection spectrum of Zn1-xMgxO shows that the codoped Al and N are responsible for the increased reflection peak intensity, and also for the static dielectric constant increasing from 2.64 to 3.23. In addition, the calculations from the current electron energy loss function indicate that the codoped Al and N lead to the enhanced amplitude and the blue shift of the plasma resonant frequency of Zn1-xMgxO.
Plastic deformation of single-crystalline copper films with surface orientation  : molecular dynamics simulations
2010, 59 (12): 8836-8842. doi: 10.7498/aps.59.8836
Molecular dynamics simulations are performed to study the plastic deformation behavior of single-crystalline copper film subjected to biaxial tensile strain. The unltrathin film is oriented normal to the  crystallographic direction. Beyond a critical value, the film deforms plastically through nucleation, motion and interaction of dislocations. The progress of twinning formed by the motion of Shockley partial dislocations in successive atomic planes is analyzed. Additional stacking faults bounded by twin boundaries are formed in some large newborn twins to release residual strain.
2010, 59 (12): 8843-8849. doi: 10.7498/aps.59.8843
The effects of the impurity on the energy-optical spectrum and the Aharonov-Bohm oscillation of the negatively charged exciton X－ on the quantum ring are discussed. In the present case, the total orbital angular momentum is not conserved. To find the wave function, a set of basic functions is formed according to their total orbital angular momenta. Then the typical diagonalization method is used to find the eigenvalues and the eigen functions of the system. The calculated results are satisfactory. When the impurities are not in the x-y plane, the calculation skill using the equivalent charge transformation formula becomes simple and charts are also discussed.
2010, 59 (12): 8850-8855. doi: 10.7498/aps.59.8850
The free-carrier concentration in Si-doped AlxGa1-xAs has been calculated by grand-canonical-ensemble statistics without any fitting parameters. Our results are quantitatively in agreement with various experimental data in the temperature range 77—300 K, which indicates that the physical picture of the ground state of DX center (DX-) is of an electronic bipolaron due to the interaction between excess electrons and lattice. When exposed to light, one bipolaron can turn into a polaron, meantime release one electron to the conduction band accompanied by lattice relaxations. Our calculations also prove that DX0 is unstable at thermal equilibrium, which further confirms our bipolaron mechanism.
2010, 59 (12): 8856-8861. doi: 10.7498/aps.59.8856
Theoretically we have studied the current spin polarization in the structure of ferromagnetic/organic semiconductor under Schottky contact and discussed its variations with potential barrier height, the special carriers in organic semiconductor layer and the its mobilities, doping concentration near the interface. The calculations show that the high mobilities of the carriers in organic semiconductors are conducive to the spin injection. We also find that a significant depletion region at Schottky contact is highly undesirable for spin injection. For an efficient spin injection, the depletion region near the interface should be heavily doped and the effective barrier height should be restricted wichin certain range.
2010, 59 (12): 8862-8869. doi: 10.7498/aps.59.8862
Considering the coexisting of electron resonant tunneling and miniband transport processes in a split-level energy system, and the effect of photogenerated carrier, we propose an optoelectronic transport theoretical model for the nanosilicon structure. We employ this model to calculate current density, electric field and electron density distribution under illumination, and the results show that resonant tunneling plays a major role in transporting the photogenerated electrons in a nanosilicon structure. Furthermore, we study the relationship between the photocurrent and the absorption coefficients, the applied bias, and the number of nanolayers. It is found that under certain conditions, hopping phenomenon occurs with photocurrent increasing, which is due to the redistribution of electric field inside the nanosilicon structure.
Effects of substrate resistivity and interface defect density on performance of solar cell with silicon heterojunctions
2010, 59 (12): 8870-8876. doi: 10.7498/aps.59.8870
For silicon heterojunction solar cell with p-type a-Si:H back surface field, the effects of substrate resistivity on the performance of solar cell with different defect densities on the front and the rear surfaces of the p-type c-Si wafer are investigated numerically by computer simulation. The results indicate that the optimized resistivity of the substrate (ρop) is related to the interface defect density on the front surface of c-Si wafer (Dit1), and ρopincreases with the increase of Dit1.The value scale of resistivity of substrate is influenced greatly by the interface defect density on the rear surface of c-Si wafer (Dit2) for ρ>ρop, and the larger the value of Dit2, the smaller will the range of acceptable ρ value be.
Threshold voltage analytic model for strained SiGe-on-insulator p-channel metal-oxide-semiconductor-field-effect-transistor
2010, 59 (12): 8877-8882. doi: 10.7498/aps.59.8877
This paper investigates the threshold voltage analytic model of strained SiGe-on-insulator p-channel metal-oxide-semiconductor-field-effect-transistor (SGOI pMOSFET), revises the energy band model of strained-silicon, and extracts the main physical parameters of strained-SiGe devices. These parameters include the energy gap, electron affinity, build-up potential, etc. In this paper, the two-dimensional Possions equation of build-in potential in strained silicon SGOI pMOSFET is also presented. By using the boundary conditions to solve these equations, an accurate threshold voltage analytic model is proposed and its validity is verified.
2010, 59 (12): 8883-8889. doi: 10.7498/aps.59.8883
The polycrystalline samples of La0.45Ca0.55Mn1-xVxO3(x=0.00, 0.10) have been prepared by the solid-phase reaction. Efects of V5+ substitution for Mn3+/Mn4+ on charge ordering and spin-glass state are studied by X-ray diffraction spectrum, temperature dependence of magnetization, and electron spin resonance spectra. The results indicate that charge ordering of the original system is almost destroyed, and spin-glass state at about 40 K is melted by 10% of V substitution for Mn. The charge ordering phase is destroyed mainly because of V5+ ions substitution for Mn3+/Mn4+, which increases the ratio of Mn3+ to Mn4+ and so causes ferromagnetic double-exchange to be superior to antiferromagnetic super-exchange. In addition, the spin-glass state is melted because V substitution for Mn destroys the formation condition of spin-glass state that a small quantity of ferromagnetic components exist under the antiferromagnetic backgrornd.
2010, 59 (12): 8890-8896. doi: 10.7498/aps.59.8890
The ferroelectric hysteresis loops of 63PbTiO3-37BiScO3 ceramics are measured under sinusoidal electric field varying from 5 to 55 kV/cm in a frequency range from 0.1 to 100 Hz. The fitting results show that the logarithm of remanent polarization and the logarithm of coercive electric field are both linearly related to electric field in the first and third field regions, but not in the second region. The three-stage behavior is distinct from the existing two-stage behavior, and it can be attributed to the dependence of ferroelectric polarization behavior on field stage.
Porosity effects on depoling characteristics of Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 ferroelectric ceramics under shock wave load
2010, 59 (12): 8897-8902. doi: 10.7498/aps.59.8897
Four kinds of Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 ferroelectric ceramics with different porosities are prepared, and the porosity effects on the depoling characteristics of those samples under shock wave load are investigated. The results show that under short circuit condition, the releasing current waveforms are in the form of square pulse for all samples. The amplitude of the current pulse decreases, but the width increases with the porosity increasing. The releasing charge decreases with the porosity increasing, which is consistent with the measurement by P-E loop. Porous ceramics has lower shock impedance, which improves the impedance match to the encapsulation medium. The depoling characteristics of those samples under high resistance load are simulated well by Lysne model. The results reveal that the dielectric constant of the sample is 4—5 times larger than that under static state, moreover, it decreases with porosity increasing. After the shock wave passing through the sample, the discharging time constant increases with porosity increasing. As the load increases, the rising edge becomes less steep for the restrain of the ferroelectric phase-antiferroelectric phase transformation under high electric field, and the breakdown happens to the dense ceramics.
2010, 59 (12): 8910-8914. doi: 10.7498/aps.59.8910
The synthetic process of MgxZn1-xO (0.30xxZn1-xO (x=0.4, 0.5, 0.6) are fabricated reproducibly by sintering mixture of ZnO and MgO powders under pressures of 4—5.6 GPa and temperatures of 1000—2000 ℃, and the problem of phase separation is solved. The MgxZn1-xO sample is characterized through several measurements of X-ray diffraction, scanning electron microscope, etc. The synthetic mechanism of cubic MgxZn1-xO is clarified and the phase diagram between temperature and components of MgxZn1-xO is obtained based on the experimental results.
2010, 59 (12): 8915-8919. doi: 10.7498/aps.59.8915
Porous silicon (PS) treated by water vapor annealing and vacuum annealing has been studied by positron annihilation lifetime spectroscopy and age-momentum correlation measurement. It is found that after water vapor annealing, non-radiative defects are reduced and defects dominating light source appear. These two types of defects change the lifetime and the S parameter of the positron annihilation and cause a drastic enhancement in the photoluminescence (PL) efficiency. Defects that cause the PL of PS show no obvious change after annealing at 300 ℃ in vacuum, therefore the PL of the sample is not influenced.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
Frictional properties of fluorinated diamond-like carbon films prepared by radio frequency reactive magnetron sputtering
2010, 59 (12): 8920-8926. doi: 10.7498/aps.59.8920
The fluorinated diamond-like carbon (F-DLC) films are prepared by radio frequency reactive magnetron sputtering with different flow ratios of CHF3 and Ar as a source gas and pure graphite as a target. Surface morphology, hardness, bonding configuration and tribological properties are investigated by atomic force microscope, nanoindenter, Raman spectra, Fourier transform infrared (FTIR) spectra and a ball-on-disk test rig, respectively. The results show that the F-DLC films are distributed compactly and homogeneously and exhibit good friction-reducing behaviors. The minimum of friction coefficient reaches about 0.42 at r=1 ∶6 while the hardness of films is highest. Raman spectra and FTIR spectra reveal that with the increase of r, the fluorine content gradually increases. However, the intensity ratio ID/IG of Raman bands of disordered graphite (D-band) and graphite (G-band) of F-DLC films decreases, which is indicative of the decrease of the fraction of aromatic ring. The results also show that the F content is another significant factor which affects the friction coefficient. The weakening of —CF2 asymmetric stretch vibration intensity and the formation of CFH in C C chains may result in a lower friction coefficient of F-DLC films.
Microstructures and thermoelectric properties of n-type melting spun(Bi0.85Sb0.15)2(Te1-xSex)3 compounds
2010, 59 (12): 8927-8933. doi: 10.7498/aps.59.8927
The single phase n-type (Bi0.85Sb0.15)2(Te1-xSex)3(x=0.15, 0.17, 0.19, 0.21)compounds have been synthesized by melt-spinning combined with subsequent spark plasma sintering technique, and the microstructures and thermoelectric transport properties of the bulk materials have been systematically investigated. The results of field emitted scanning electron microscopy images show that the bulk materials possess refined crystalline and a large number of layered structures with the sizes from 10 nm to 100 nm, and their differences in composition and phase are detected neither from the back scattering image nor from element face distributing images of polishing surface. With the increase of content of selenium, the electrical conductivity and the thermal conductivity increase but the Seebeck coefficient decreases. Comparing with the traditional zone melted material, the samples with higher selenium content possesse higher thermoelectric optimum value ZT after 420 K and the highest ZT of the sample (Bi0.85Sb0.15)2(Te0.83Se0.17)3 can reach 0.96 at 360 K, whose ZT increases by 48% at 500 K correspondingly. In addition, the temperature of the peak ZT can be adjusted by varying the content of selenium, which is meaningful for the design and the fabrication of multi-scale or grade thermoelectric device.
2010, 59 (12): 8934-8940. doi: 10.7498/aps.59.8934
The propagation character of shock wave in SiO2 aerogel under explosive loading is studied using a polyvinylidene fluoride piezoelectric sensor. The propagation character of shock wave in aluminum foam is also determined for comparison. The results show that the peak stress of the shock wave follows the exponential attenuation relation with the propagation distance of the shock wave in SiO2 aerogel. The shock wave attenuates more obviously in SiO2 aerogel than in aluminum foam. The special nano-porous network structure of SiO2 aerogel results in better attenuation effect of shock wave in aerogel. The shock wave velocity in SiO2 aerogel is extremly low, thus the chase of the unloading wave leads to the further attenuation of shock wave.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2010, 59 (12): 8941-8953. doi: 10.7498/aps.59.8941
The fourth generation of comprehensive model of geomagnetic field CM4 is a numerical model in which earths magnetic field can completely be divided into internal and external parts. In this paper we have calculated and plotted the grid values of internal and external fields of Chinese mainland in every 10 a from 1960 to 2000 by combining CM4 with Taylor polynomial model based on all survey data during 1960—2000, and made more efforts to analyze the whole distribution of crustal anomaly field. Results show that internal field value of northern component X decreased with time. The amplitude was about 750 nT in the period from 1960 to 2000, the trend of external field variation increased in 1960, then gradually decreased about 32 nT until 2000. The internal field value of eastern component Y decreased first, then continuously increased, and its intensities decreased about 40 nT from 1960 to 2000. The trend of external field variation was increased, decreased and increased as time went on, and the external field increased up to about 3.8 nT since 1960. The internal field value of vertical component Z was similar to that of Y, its intensities totally increased about 600 nT. The trend of external field variation decreased and then increased with time, amplitude of which was about 4.6 nT. As for anomalous field, the distributions of components X and Z, total intensity F, and horizontal component H were all negative, the intensities of X and H decreased with longitude while those of Z and F increased. The distributions of Y and declination D were very similar and both of them had positive values in west-central region of China. Their intensities decreased with longitude. The distribution of inclination I was mainly negative, and its intensity increased with longitude.
2010, 59 (12): 8954-8967. doi: 10.7498/aps.59.8954
The closed physical constraint equations of sand-dust atmosphere are established based on both the interaction of wind-blown-sand two-phase flows and the mass change. Ideal analysis of the equations shows the following findings: (1) the density of the sand-dust atmosphere is bigger than that of the pure atmosphere, which will reduce the velocities of air parcels of the former to some extent; (2) velocity difference between sand-dust particles and air can make the fine particles located in a high-speed region and the coarse particles located in a low-speed region; (3) temperature difference between sand-dust particles and air can make the particles tend to enhance convection through acting as a heat source in updraft and a cold source in downdraft; (4) the constant-volume mass-specific heat capacity of the sand-dust atmosphere can give rise to the generation of new temperature gradient on an isobaric surface and then enhance the entrainment at the boundary of a sand-dust cloud; (5) the gas constant of the sand-dust atmosphere can lead to the generation of new pressure gradient on an isothermal surface and also enhance the entrainment at the boundary of a sand-dust cloud; (6) mass change can largely affect density, velocity and temperature of the sand-dust atmosphere. In brief, compared with the sand-dust cloud given by equations based on passive scalar, the real sand-dust cloud is high and great. Besides this, its inner convection and its entrainment at the boundary are active, and its horizontal motion is slow.
A new approach to adjusting sea surface wind using altimeter wind data by variational regularization method
2010, 59 (12): 8968-8977. doi: 10.7498/aps.59.8968
To adjust non-divergent and divergent background wind fields by altimeter wind data, a novel approach, the variational regularization method, is presented. Numerical simulation results show that it is positive to use altimeter wind speed to adjust the background wind field, particularly effective in the altimeter track region. At the same time, the radar backscattering cross-section sensitivity is evaluated, when the backscattering cross-section contains noise, and the result proves that the above method can be used to suppress the noise. Finally, a real case shows that the above method is feasible and effective. This approach is an effective and reliable method of using the altimeter wind data to adjust the sea surface wind.
2010, 59 (12): 8978-8985. doi: 10.7498/aps.59.8978
In order to find out the distribution of lightning electromagnetic pulse (LEMP) fields under the ground, the two-dimensional finite-difference time-domain method is used to calculate LEMP under the ground. The distributions of LEMP under the ground are calculated in the cases of different distances from the lightning channel, different ground conductivities, different ground permittivities and different depths. The attenuations of lightning fields in the ground are compared with those of other high-power electromagnetic environments. The calculated results show the following points: the LEMP dramatically attenuates as the distance increases; the attenuation of the electric component is significant when the ground conductivity is reasonably high; the change of ground permittivity mainly causes the change of vertical electric field component, which decreases as the ground permittivity increases; the attenuation of the high frequency electric component increases and that of the low frequency electric component invisibly changes as the depth increases.
2010, 59 (12): 8986-8993. doi: 10.7498/aps.59.8986
We develop a method to compute the segment size in the detrended fluctuation analysis (DFA), which is based on the basic concept of the information theory, and verify the method effectiveness by numerical experiment. This method is freed from the problem of subjectivity in the former process to choose the segment size which usually leads to false result. We Change the length of sequence with dynamics being the same, the results remain stable. The results indicate that when the length of sequence is too short, even the optimal selection of segment size is not enough for the portrait of the overall dynamic system, thus the DFA cannot be used in this circumstance. The method we developed in this paper can enhance the reliability of DFA results by judging whether the sequences analyzed meet the requirements of DFA. We also obtain the DFA index from 1961 to 2000 of China through DFA method and analyze its spatial characteristics of distribution.
2010, 59 (12): 8994-8998. doi: 10.7498/aps.59.8994
Considering the fact that there is a correlation between the black hole horizon and cosmological horizon, we show that the thermodynamic entropy of de Sitter spacetime is the sum of thermodynamic entropies of the black hole horizon and cosmological horizon. The thermodynamic properties of de Sitter spacetime are also derived. It is shown that the upper limit energy of de Sitter spacetime is equal to the energy of pure de Sitter spacetime, The thermal capacity of de Sitter spacetime is negative, so de Sitter spacetime is mechanically unstable.