Vol. 61, No. 23 (2012)
Laser intensity dependence of nonsequential double ionization of argon atoms by elliptically polarized laser pulses
2012, 61 (23): 233202. doi: 10.7498/aps.61.233202
With a classical ensemble model, we investigate the intensity dependence of nonsequential double ionization (NSDI) of argon atoms by elliptically polarized laser pulses. The results show that the correlated behavior of two electrons along the long axis of the laser polarization plane depends on the laser intensity. At the relatively high intensity, the correlated electron momentum spectrum displays a correlated behavior and shows V-like structures in the first and third quadrants. At the relatively low intensity, the correlated electron momentum spectrum shows a strongly anticorrelated behavior. For the different laser intensities, the momentum spectra of two electrons along the short axis of the laser polarization plane all show strongly anticorrelated behaviors. The analysis of the classical trajectories of NSDI shows that the final-state electron repulsion plays a decisive role in both the V-like shape along the long axis of the laser polarization plane and the anticorrelated behavior along the short axis of the laser polarization plane.
2012, 61 (23): 234501. doi: 10.7498/aps.61.234501
Acoustic signal is used to study the mechanical properties of dense granular system under direct shear. In the process of direct shearing, it is found that the velocity of the sound wave decreases rapidly with the increase of shear stress. After the yield point of stress, with the increase of strain, the velocity of the propagating wave decreases gradually and attains a constant. The net reduction in velocity is found to be less than 20%, which differs from the fact that acoustic wave (especially the transverse wave) could not propagate through the shear band. This is because the direct shear strain rate is far smaller than the frequency of sound wave, and particles under shear can be regarded as quasi-static elastic-plastic solid. Effective medium theory and granular dlastic theory are employed for explaining these results.
Positron annihilation spectrum study in non-ferroelectric piezoelectricity SrTiO3-Bi12TiO20 (ST-BT) composite ceramics
2012, 61 (23): 237805. doi: 10.7498/aps.61.237805
The positron annihilation lifetime spectroscopy, combined with X-ray diffraction and scanning electron microscopy is used to measure a new type of non-ferroelectric piezoelectricity SrTiO3-Bi12TiO20 (ST-BT) composite ceramic, which is prepared by conventional solid-state reaction in Shandong University. The structural characteristics and sinter process for ST-BT composite ceramic are systematically studied. A large number of crystal defects are observed in ST-BT composite ceramic with the sintering temperature being between 860 to 940 ℃. The large scale defects are formed due to the Bi12TiO20 phase decomposed when the sintering temperature rises to 980 ℃. The experimental results indicate that the structural characteristic and piezoelectricity of ST-BT composite ceramic shows good stability with the sintering temperature being between 920 to 940 ℃.
The Borkhoffian expression of Boltzmann-Hamel equation of nonholonomic system and its generalized symplectic geometric algorithm
2012, 61 (23): 230201. doi: 10.7498/aps.61.230201
For a Boltzmann-Hamel equation of nonholonomic mechanical system, when it meets certain conditions, the Boltzmann-Hamel equation can be transformed into a Birkhoffian system. By constructing the generating function, the system is investigated numerically using the generalized symplectic geometric algorithm of the nonautonomous Birkhoffian system. Compared with the above-mentioned algorithm with the classical Runge-Kutta method, Birkhoffian symplectic scheme is very accurate in a long-term tracing.
Analysis on positive effect of time-delay on a class of second-order oscillatory systems with unit negative feedback
2012, 61 (23): 230202. doi: 10.7498/aps.61.230202
The stability analysis on a class of oscillatory systems whose controlled object has a standard form of second-order systems is presented. The effect of the signal transmission delay on the performance of the closed loop control system with unit negative feedback is discussed. The relationship between the time-delay and the stability of closed loop control system is obtained via drawing and analyzing the Nyquist plots in the different regions of the relative damping coefficient. The simulation example of unit step response for each case is processed and the simulation results show that the analysis is correct.
The image quality of the imaging system is often reduced by the wave-front aberrations which arises from a variety of sources including atmospheric turbulence, system aberration, and so on. Phase diversity image restoration is an image restoration technique that mainly aims to solve the problem of wave-front aberrations. This technique needs neither reference object nor the degraded function and has a good convergence. It can enhance the resolution and sharpness, increase the information content and improve the visual effect of the image, and has broad application prospect in the field of image restoration. In this paper, the phase diversity image restoration technique is studied, and its performance is analyzed in virtue of the concept of the optical transfer function, modulation transfer function and point spread function. Simulation and experimental results both show that phase diversity image restoration technique can not only reduce the influence of wavefront aberrations but also increase the information of low frequency within the cutoff frequency, and improve the contrast. So, the image acquired by phase diversity image restoration can be better than the image acquired by the diffraction limited system.
The element-free Galerkin method based on the shifted basis for solving the Kuramoto- Sivashinsky equation
2012, 61 (23): 230204. doi: 10.7498/aps.61.230204
The Kuramoto-Sivashinsky equation is a kind of high-order nonlinear evolution equation which can describe complicated chaotic nature. Due to the existence of high-order derivatives in the equation, the shape functions violate the consistency conditions when using traditional element-free Galerkin method which adopts high-order polynomial basis functions to construct the shape functions. In order to solve the problems encountered in the traditional element-free Galerkin method, a kind of element-free Galerkin method adopting the shifted polynomial basis functions is presented in this paper. Compared with the traditional element-free Galerkin method, the Galerkin principle is still used to discrete the equation in this method, but the shape functions are constructed by moving least squares based on the shifted polynomial basis functions. Numerical results for the Kuramoto-Sivashinsky equation having traveling wave solution and chaotic nature prove the validity of the presented method.
Manipulation of entanglement and preparation of quantum states for moving two-atom and the light field via intensity-dependent coupling
2012, 61 (23): 230301. doi: 10.7498/aps.61.230301
In this paper, the Tavis-Cummings model is generalized to simultaneously consider the atomic motion and the field via intensity dependent coupling. Under the conditions of vacuum field, weakly and strongly coherent field, the entanglement evolution properties of two-atom-field and two-atom-two-atom are investigated using atomic reduced entropy and concurrence, respectively. According to evolution characteristics above, we prepare the W-class states of two-atom-field, two-atom Bell state, fidelity state of Bell-state atoms, single-photon state, two-photon state and stable number-states of field by selecting the interaction time of the two-atom-field, selecting the entanglement factor of the two-atom, regulating the field-mode structure parameter, controlling the probability amplitude of pure state of the system and selective measurement. The manipulation of two-atom Bell state sudden generation and its maintenance in a limited time, the formation of periodic quantum echo of Bell-state atoms and the continuous fidelity of Bell-state atomic information are achieved. The results show that the system has a powerful function of quantum information, and provide the physical carrier and theoretical parameters for experimental implementation of quantum information processing.
2012, 61 (23): 230302. doi: 10.7498/aps.61.230302
Establishing three different environment models (i.e., single reservoir model, common reservoir model and independent reservoir model), we investigate the evolutional characteristic of the entangled system initially in Bell-like state by the pseudomode method. Through comparing the entanglement decays under the three different environment models, we find that the entangled Bell-like state cos θ |ee> + sin θ |gg> will be kept in the single reservoir model for a time longer than in the other two models. However, the entanglement decay behavior of Bell-like state cos θ |eg> + sin θ |ge> is dependent on the specific environment model. Meanwhile, the comparison of entanglement decay between the above two Bell-like states at the same environment models is performed.
2012, 61 (23): 230303. doi: 10.7498/aps.61.230303
The influence of initial entanglement on entanglement evolution under phase damping channel in noninertial frame is investigated. Compared with the scenario of amplitude damping channel, when the state parameter α has different values with the same intial entanglement, the evolution curves are identical. When the Unruh single particle state has right and left components, there is more serious quantum decoherence than the case where the Unruh single particle state has only right component with the sudden death of entanglement taking place early, and the time of sudden death of entanglement is independent of initial entanglement.
2012, 61 (23): 230304. doi: 10.7498/aps.61.230304
We study bosonic atoms trapped in a triple-well with synthetic gauge field. The energy spectrum of the system can be tuned by an effective magnetic field. For a non-interacting system, eigen-states with different momenta will be the ground state with the magnetic field varying. In the full quantum scheme, we find the critical ratio between the tunneling and interaction strength for localization of system under the repulsive interaction. We also discuss the macro-rotation effect induced by the magnetic field and the boundary effect of the temporal evolution for a non-interacting system.
2012, 61 (23): 230501. doi: 10.7498/aps.61.230501
For any digital chaotic sequence generator, evaluating periodic characteristics which exist in only part of domains of binary chaotic sequences is extremely difficult. In this paper, we present a method which we name the binary sequence period detection (BSPD). The BSPD is a novel detection which evaluates the periodicity in a binary chaotic sequence, by which both the accurate-periodic phenomena and periodic phenomena in part of domains can be detected. Moreover, any periodic phenomenon pattern of a binary sequence can be located by the BSPD method. The experimental results show that the BSPD can detect and extract the periodic phenomena of the classical Logistic chaotic sequence generators.
Large parameter stochastic resonance of two-dimensional Duffing oscillator and its application on weak signal detection
2012, 61 (23): 230502. doi: 10.7498/aps.61.230502
In this paper, the stochastic resonance of two-dimensional Duffing oscillator under the adiabatic assumption is studied. For the large parameter condition, we propose the large parameter stochastic resonance of two-dimensional Duffing oscillator, and discuss the relationship between the scale transformation stochastic resonance and the parameter adjustment stochastic resonance. Then we reveal the mechanism of signal detection by Duffing oscillator stochastic resonance in large parameter condition, and extend its application to weak signal detection.
Control and modeling of chaotic dynamics for a free-floating rigid-flexible coupling space manipulator based on minimal joint torque's optimization
2012, 61 (23): 230503. doi: 10.7498/aps.61.230503
The dynamic model of a planar free-floating rigid-flexible redundant space manipulator with tree joints is derived by the assumed mode method, Lagrange principle and momentum conservation. Based on minimal joint torque optimization, the state equations of chaos motion for the free-floating redundant space manipulator are built. Then chaotic numerical method is used to analyze chaotic dynamic characteristics of the model. The robust Proportional-Derivative(PD) compensatory control, the delayed feedback control and the optimal control are designed in work space, joint space and mode space respectively. The composite control can assure the trajectory tracking, chaotic suppression and vibration control. The experimental results show the effectiveness of the presented methods.
Experimental confirmation on period-doubling bifurcation and chaos in dielectric barrier discharge in helium at atmospheric pressure
2012, 61 (23): 230504. doi: 10.7498/aps.61.230504
A series of dielectric barrier discharge (DBD) experiments in helium at atmospheric pressure is carried out under a parallel electrode configuration. The discharge waveforms are observed in time-domain and analyzed in frequency-domain. Results show that under a certain condition the discharge can experience period-one, period-two and period-four discharges and finally enters into chaotic discharge as the amplitude of the driving voltage is increased. Our work verifies that the period-doubling route into chaos in DBD at atmospheric helium pressure can observed in experiment, beside in numerical simulations.
2012, 61 (23): 230505. doi: 10.7498/aps.61.230505
In this paper, we investigate the signal tracking problem of the uncertain time-varying delay chaotic system with control input nonlinearity and the fuzzy adaptive variable structure control method is proposed. The fuzzy logic system approximates the nonlinearity of the system. The unknown time-varying delay uncertainties are compensated using an appropriate Lyapunov-Krasovskii functional. The proposed controller can make the tracking errors converge to an adjustable region and all signals of the closed system is bounded. The designed control method can have a certain robustness against the disturbance of the system. Finally, taking the simulations on a multi-scroll chaotic system for example, the effectiveness of the proposed method is illustrated.
2012, 61 (23): 230506. doi: 10.7498/aps.61.230506
A novel chaotic one-way hash function based on spatial expansion construction with controllable parameter is presented which combines with the advantages of both chaotic system and parallel hash function. In the proposed approach, the hash model of message block is determined by chaotic dynamic parameter. The new method improves the security of hash function and avoids degrading the system performance at the same time. Theoretical and experimental results show that the proposed method has high performance in parallel algorithm, nearly uniform distribution and desired diffusion and confusion properties.
2012, 61 (23): 230507. doi: 10.7498/aps.61.230507
We investigate the dynamic characteristics of vertical upward water-dominant oil-water two-phase flow by multiscale permutation entropy algorithm. We first measure the two-phase flow conductance fluctuation signals in a 20 mm inner-diameter pipe. Then we calculate the multiscale permutation entropies for different water-dominant flow patterns. The results indicate that multiscale permutation entropy rate and average of mutiscale permutation entropy can be used as quantitative parameters for measuring complexity of water dominant oil-water two-phase flow. In addition, we propose a new approach to identify different water-dominant flow patterns by using the joint distribution of multiscale permutation entropy rate and cumulant of increment series.
Oscillation-localization transition behavior of bright-bright solitons in two species Bose-Einstein condensates
2012, 61 (23): 230508. doi: 10.7498/aps.61.230508
Considering the time-dependent interspecies interactions, we study the dynamic behaviors of bright-bright solitons in two species Bose-Einstein condensates in the harmonic external potentials. In the presence of unchanged attractive interspecies and intraspecies interactions, bright-bright solitons exhibit periodic oscillations. When the attractive intraspecies interactions are fixed but the attractive interspecies interactions are increased exponentially, interestingly, a transition behavior from oscillation to localization is observed. Meanwhile, this transition behavior can be controlled by tuning the transverse trapping frequency of harmonic potential. Furthermore, an experimental protocol is designed to observe the transition behavior in the current experimental conditions.
2012, 61 (23): 230509. doi: 10.7498/aps.61.230509
In this paper, we present a method that unlike-pair interactions are determinated by explosive detonation data. The unlike-pair interaction parameters kN2-H2O= 1.03, kN2-CO2= 1.035, kH 2O-CO2= 0.96 are deduced from the calculated results of explosive RX-23-AB,HNB and PETN with our statistical mechanical chemical equilibrium code CHEQ-like. At the same time, the theoretical Hugoniots of explosive PBX9404 are calculated with these parameters. The results in better agreement with the experimental data show that the parameters and method are reliable.
2012, 61 (23): 230601. doi: 10.7498/aps.61.230601
The national standard densitometer can be used for calibrating photographic film and visual photo. The development of biomedical image and space technology need large measurement range and high uncertainty of visual density. A newly designed equipment using hemispherical emitter with optic fiber can enhance the diffuse coefficient of the light to 0.945 which is higher than the other national standards (0.02-0.03). Thus the measurement range can be increased to 0-6.0 and the combined uncertainties are 0.0015 (0 D D < 6.0). On the other hand, the structure design for the whole equipment can realize the reciprocal fixing between the light module and the detect module. So the validity of Helmholtz reciprocity for transmittance measurement can be examined and proved on this new equipment.
2012, 61 (23): 230701. doi: 10.7498/aps.61.230701
With the combination of pupil division, Wollaston prism angle shear and Savart plate lateral shear interference, a new technology that can aquire the message of imaging, polarization and spectrum simultaneously is presented. The system works at the pushing mode with neither mechanical movable nor electrically tunable device, and four quarters of the single charge-coupled device can get four interferograms of different polarization states (two polarization-difference spectral imagings). The device is discussed in detail, and the relationship between azimuth of the prism principal cross section and the output light intensity is analyzed, and then system parameters are optimized. In the paper, we derive the interference fringe intensity distribution formula, use computer numerical simulation to verify the feasibility and accuracy of this method, and thus provide an important theory basis and the practice instruction for designing a new type of imaging spectrometer and its engineering applications.
THE PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
2012, 61 (23): 231401. doi: 10.7498/aps.61.231401
In the view of the experimentally confirmed existence of the a2(1325), f2(1525), f2(1270) and K2*(1430), which are the ground tensor meson 13P2 members, through combining the Regge trajectory and meson mass mixing matrix, we investigate the assignment of the first radial excitation of tensor meson. We suggest that the states f2(1810) and f2(2010) should be the candidates of this assignment. Moreover, some information about decaying of f2(1810) and f2(2010), which serve as pseudoscalar mesons, are presented. The results should be tested in the experiment in the future.
2012, 61 (23): 232501. doi: 10.7498/aps.61.232501
The geometric configurations, vibration frequencies and thermodynamics properties of Al2O3X2 (X= H, D, T) molecular clusters with lower energy are optimized using the B3LYP/6-311++g (d, p) method. The changes of entropy, enthalpy and Gibbs free energy of the reactions between Al2O3 and hydrogen (deuterium or tritium) gas are calculated by the solid electron-vibration approximate method and thermodynamic formulae at temperatures of 298, 398, 498, 598, 698, 798, 898, 998 and 1098 K, and then the relationships between the equilibrium pressure of hydrogen (deuterium or tritium) gas and temperature in these reactions are obtained. The results show that the ground state of the gaseous Al2O3H2 is Al2O3X2 (X= H, D, T) (1A') Cs. Hydrogen can be displaced by deuterium; deuterium can be displaced by tritium in the reactions between Al2O3 and X2 with the productions of solid Al2O3X2 which relates to ground gaseous Al2O3X2 with Cs symmetry. This displacement sequence is opposite to that in the reactions between titanium and X2. These displacement effects are very weak, and they are weaker and weaker as the temperature increases.
2012, 61 (23): 232502. doi: 10.7498/aps.61.232502
A new neutron detector is designed. Two 235UO2 target foils are used as radiators, and the 4He gas is used as scintillator in the detector. The detector combines the features of 235U and 4He fully, so it has some excellent performances, such as flat energy response to neutrons, better n/γ discrimination, etc. It can be used well to detect neutrons in a pulsed mixture radiation field. In this paper, the principle and structure of the detector are introduced. The energy depositions of different energies of neutrons and γ -rays in 4He gas are calculated. The neutron sensitivity, γ ray sensitivity, n/γ discrimination, and time response of the detector are investigated theoretically and experimentally. The neutron sensitivity is about 10-15 C·cm2, γ ray sensitivity is about 10-17 C·cm2, and time response is about 33.1 ns.
ATOMIC AND MOLECULAR PHYSICS
2012, 61 (23): 233201. doi: 10.7498/aps.61.233201
According to the Λ -type three-level system consisting of cesium 6S1/2 (F=3 and F=4) long-lived ground states and 6P3/2 (F'=4) excited state, we experimentally investigate and theoretically analyze the parameters of coherent population trapping (CPT) spectra in a cesium atomic vapor cell with or without neon as buffer gas. The CPT of the full width at half maximum (FWHM), the signal amplitude as a function of optical intensity of the two phased-locked laser beams with a frequency difference of 9.19263177 GHz (the hyperfine splitting in the cesium ground state 6S1/2), relative intensity ratio, beam spot size, residual magnetic field of the magnetic shielding, and the influence of with and without the cesium vapor cell containing neon as buffer gas are investigated experimentally. With the optimized parameters, we obtain the CPT signal FWHM to be as narrow as ～ 340 Hz.
2012, 61 (23): 233203. doi: 10.7498/aps.61.233203
We theoretically study the high-order harmonic generation (HHG) of a model He atom exposed to a combined field of 1600 nm and 800 nm laser pulses. The changing of rising rate and falling rate with time for the combined filed during main HHG, can present the different characteristics of high-order harmonic generation from the short electron trajectories and the long electron trajectories. By superimposing several harmonics generated from He atom driven by the combined field, we could obtain shorter attosecond pulses. Finally, by adjusting the time delay between two laser pulses, we can effectively suppress the contribution of the short electron trajectories to HHG, and obtain an isolated attosecond pulse with duration 33.7 as.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2012, 61 (23): 234101. doi: 10.7498/aps.61.234101
As one of strong scattering sources of planes, assessing accurately the characteristic of radar cross-section (RCS) about inlets is still hot in investigation into computational electromagnetics. An iterative physical optics method, which an impedance boundary condition and forward-backward method are added to, is studied and a box-segmentation method of blanking off is proposed. It can compute accurately the electric magnetic current distribution on the wall and the RCS. What is more, by developing a kind of software which can be verified by the relevant examples, the RCS characteristic of any metal or dielectric cavity can be evaluated. On this basis, using the software, the numerical simulation about the metal/coated medium S-diffuser with rectangular shape is carried out, and the RCS variational regulation of diffuser under an incident wave length in 0.03 m is obtained. The improved iterative physical optics method is applicable to the design of any cavity and the study of coating absorbing materials, therefore the design cycle is shortened.
2012, 61 (23): 234102. doi: 10.7498/aps.61.234102
Laser accelerated high-energy electron beam has the properties of small source size, quasi-monoenergetic, and short duration. In this paper the radiography by the electron beam is simulated using a Monte-Carlo code. Various radiographies are simulated, such as a step target and a thick iron block of inside cracks by a collimated 200-MeV beam, a modelled inertial-confinement-fusion target by a 11-MeV point-like beam, and the deflectometry of a 70-MeV electron beam by a laser plasma generated magnetic field. The obtained results indicate that the radiography by a laser-accelerated electron beam is of high spatial resolution and sensitivity in the detection of the inside defects of material, identification of the interface of thin material, and diagnosis of electromagnetic field.
Vectorial structure of a hollow Gaussian beam diffracted by a circular phase aperture in the far field
2012, 61 (23): 234201. doi: 10.7498/aps.61.234201
Based on the vector angular spectrum representation of the Maxwell equations and the method of stationary phase, only with the far field approximation, the analytical expressions of energy flux density of a hollow Gaussian beam diffracted by a circular phase aperture is presented. It is shown that the total energy flux of the beam can be expressed as the sum of the unapertured term and the additional term, where the additional term is the effect of the phase aperture on the beam; the total energy flux, the unapertured one and the additional one each can be expressed as the sum of TE term and TM term. The distributions of the total energy flux, the unapertuered one, the additional one, and their TE terms and TM terms are depicted in the far field. The influences of truncation parameter and the phase delay on the longitudinal total energy flux distributions are discussed.
2012, 61 (23): 234202. doi: 10.7498/aps.61.234202
Normally, Strehl ratio, resolution, and focus depth, the parameters that are used to evaluate the performance of an optical imaging system, are always calculated by scalar diffraction method. Considering the vectorial properties of light, especially when the system numerical aperture (NA) is high, the accuracy of scalar method can no longer fulfill the requirement of analysis. The variations of properties of these parameters accompanied by using of vectorial method are discussed. In order to describe the effects of polarization clearly, assuming a system with identical polarization state within the entrance pupil, by formula deduction and numerical simulation, we conclude that due to the influence of the polarization, the maximum intensity of system diffraction spot decreases, the resolution between image plane and parallel polarized orientation of incident light drops down, then the diffraction spot loses its circular symmetry, the focus depth changes slightly, and these effects becomes more apparent with the increase of system NA. Finally, the method of calculating the off-axis images is discussed in the paper.
Nonlinear dynamics of 1550 nm vertical-cavity surface-emitting laser with polarization- preserved optical feedback
2012, 61 (23): 234203. doi: 10.7498/aps.61.234203
Based on the framework of the spin-flip model, the nonlinear dynamics of 1550 nm vertical-cavity surface-emitting laser (VCSEL) subject to polarization-preserved optical feedback is theoretically investigated. The results show that for a free-running 1550 nm-VCSEL, the current value for polarization switching (PS) is affected seriously by the internal parameters; with the increase of the gain anisotropy coefficient γa, the corresponding current for PS increases. Due to the introduction of the polarization-preserved optical feedback, the dominant polarized mode (Y polarized mode) will display different dynamical states for different injection currents while the other mode (X polarized mode) may be excited. As a result, the average output powers of the two polarized modes increase with fluctuation with the increase of current. For different feedback times, the dynamic states of 1550 nm-VCSEL with polarization- preserved optical feedback maybe route into chaos via different evolution paths such as period doubling bifurcation, quasi-periodic bifurcation and intermittent chaos.
2012, 61 (23): 234204. doi: 10.7498/aps.61.234204
The thermometry based on absorption spectroscopy and developing of a temperature sensor is introduced, in which temperature is retrieved by measuring line strength ratio between a pair of absorptions of water vapor near 1.4 μm. The temperature sensor is first calibrated on a tubular furnace and then used to verify measurement results with the heating tube whose both sides are open, an average fluctuation of about 50 K is obtained at each temperature setpoint. Some new absorption lines of H2O near one absorption (centered at 7153.7 cm -1) of the selected line pair are observed during demonstration measurements for CH4/air flat flame burner. These absorption lines overlapping with 7153.7 cm-1, are not listed in HITRAN08 database while HITEMP data give absorption information about these wave number positions. But line center of the other absorption (centered at 7154.354 cm-1) in HITEMP seems different from our measurement and HITRAN database. A combination of our calibrated results from tubular furnace and line parameters of new features from HITEMP is chosen and some results measured at different equivalence ratios on the burner are shown here.
2012, 61 (23): 234205. doi: 10.7498/aps.61.234205
The study of gas spectrum characteristics at high pressure is the foundation for tunable diode absorption spectroscopy technique used in pulsed detonation engines and other high-pressure combustion environments. To understand variations of gas spectrum characteristics with pressure, especially the absorption spectrum characteristics at high pressures, gas absorption spectrum characteristics at high pressure are studied by tunable diode laser absorption spectroscopy in this paper, a method to calculate gas concentration at high pressure is also presented. Within a pressure range of 1-10.13×105 Pa, CO2 absorption spectrum in a near-infrared band of 1.58 μm is simulated. Gas online measurement at high pressure is set up. The CO2 absorption spectrum is measured by a direct absorption method near 1578.0-1579.7 nm. The concentration at high-pressure environment is calculated by linear fitting method and compared with theoretical absorption spectrum. The results show that the absorption spectrum obtained experimentally is in good agreement with simulations result. In high-pressure environment, maximum error of linear fitting method to calculate gas concentration at high pressure is 5.5%, and the average error is 2.6%.
2012, 61 (23): 234206. doi: 10.7498/aps.61.234206
High efficiency broadband second-harmonic conversion plays an important role in communication, signal processing, spectroscopy and so on. In general, the study of high efficiency broadband second-harmonic conversion focuses on a few of wavelengths. For obtaining tunable high efficiency broadband second-harmonic conversion in quasi-phase matching, the group-velocity and quasi-phase matched condition are analyzed. The temperature effect on high-efficiency broadband second-harmonic conversion in types 0 and Ⅰ quasi-phase matched condition for 5 mol% periodically poled LiNbO3 and periodically poled LiNbO3 is studied. The results show that 15 nm and 341 nm tunable high efficiency broadband second-harmonic conversions are obtained in types 0 and Ⅰ quasi-phase matched conditions for 5 mol% periodically poled LiNbO3; 44 nm and 98 nm tunable high efficiency broadband second-harmonic conversions are obtained in types 0 and Ⅰ quasi-phase matched condition for periodically poled LiNbO3. The range of high efficiency broadband second-harmonic conversion wavelength is expanded.
2012, 61 (23): 234207. doi: 10.7498/aps.61.234207
In this paper, Fe-doped lithium niobate crystals with different Li compositions are prepared. The composition and temperature dependence of the light-induced scattering in Fe-doped lithium niobate are studied. The results show that the crystals with the compositions lower than 49.0 mol% suffer from the serious light-induced scattering, but when the composition increases to 49.3 mol% the light-induced scattering is suppressed significantly, in particular at a composition of 49.8 mol% the light-induced scattering disappears completely. It is also found that the light-induced scattering of the crystals with lower compositions can be suppressed completely by elevating the temperature to 150 ℃. However the temperature for the complete suppression of the light-induced scattering in the crystal with a composition of 49.3 mol% is only 80 ℃. Based on these results, it is suggested that a critical Li composition may exist in a range from 48.9 mol% to 49.3 mol% and beyond this composition the suppression and temperature dependence of the light-induced scattering will change significantly.
Light propagation characteristics of one-dimensional photonic crystal with double-barrier quantum well
2012, 61 (23): 234208. doi: 10.7498/aps.61.234208
The light transfer characteristics of one-dimensional photonic crystal with single and double-barrier quantum well are studied by transfer matrix method. The results show that when the refractive index of the barrier layer is high, the transmission peaks in single-barrier quantum well of photonic crystal will be narrower and the inner localized field will be stronger, that the peak in the double-barrier is narrower than the one in the single-barrier, and also the inner localized field is stronger in the double-barrier, that with the number of period layer in the photonic crystal increasing, the inner localized field in the double-barrier well is enhanced, furthermore, the bigger the refractive index ratio between barrier and dielectric layers of well, the stronger the inner localized field in the photonic crystal quantum well is. In addition, when the periodicity of the barrier layer in the photonic crystal with a thicker refractive index increases, the inner localized field will strengthen faster, and accordingly, the transmittance of the transmission peak will decrease more quickly. Both of the strengthening and decreasing will work at the top speed when all periods in different barriers increase at the same time. While the period number of the photonic crystal in well layers increases, the inner localized fields in both single and double-barrier will increase, but their transmittances of the transmission peak will keep the same. The characteristics above can provide guidance for designing new high-quality quantum optical devices.
Radiation from the unsymmetrical modes of the periodical waveguide structure excited by eccentric electron beam
2012, 61 (23): 234209. doi: 10.7498/aps.61.234209
The special kind of diffraction radiation from the uniformly moving electron beam in a periodical waveguide structure has great potential applications in developing the tunable terahertz radiation sources. Rigorous theoretical analyses and detailed computer simulation on the diffraction radiation from the periodical waveguide structure excited by the eccentric electron beam are carried out. Our results show that the eccentric beam can primarily excite the axial symmetrical modes (TM0 modes) and axial unsymmetrical modes with axial variation numbers of 1 and 2. The energies of the unsymmetrical modes increase with the distance of electron beam to the axis. For each mode, the radiation intensity changes with beam location as the square of modified Bessel function and in direct proportion with the square of the charge quantity. The results of theoretical analyses and computer simulations are in good agreement with each other. These results are of significance for developing this kind of radiation source.
High birefringence and negative dispersion effect of hexagonal honeycomb lattice photonic crystal fiber
2012, 61 (23): 234210. doi: 10.7498/aps.61.234210
A novel hexagonal honeycomb lattice photonic crystal fiber is proposed, which is composed of a central defect core, a cladding with elliptical air-hole and small round air-holes. Based on the full vector finite element method with anisotropic perfectly matched layers, its birefringence, dispersion, nonlinearity, leakage loss and mode field are numerically investigated. We compare hexagonal honeycomb lattice photonic crystal fiber and hexagonal elliptical lattice photonic crystal fiber, both of which have the same structure parameters. Numerical results indicate that the proposed fiber shows high birefringence and negative dispersion effect. The birefringence is 1.02× 10-2, both its dispersion and dispersion slope are negative, the dispersion slope values are between -0.132- -0.121 ps·km-1·nm-2 over C band, the leakage loss is close to 102 dB·m-1 and the non-linear coefficient is 45.7 km-1·W-1 at a wavelength of 1.55 μm, if the parameter is selected as Λ =1.15 μm, η =0.5, f=0.48, and d1=0.4 μm. It is found that the hexagonal honeycomb lattice photonic crystal fiber easily obtains high birefringence, large negative dispersion and low non-linear coefficient. It is demonstrated that the hexagonal honeycomb lattice photonic crystal fiber has huge potential in designing dispersion compensation photonic crystal fiber.
2012, 61 (23): 234301. doi: 10.7498/aps.61.234301
Aiming at the low data rate of traditional direct sequence spread spectrum, M-ary spread spectrum and code shift keying underwater acoustic spread spectrum communication, a new combine method of M-ary code shift keying underwater acoustic spread spectrum communication is proposed based on the small Kasami sequence with good self and mutual correlation. Different sequence information and the same sequence code phase information are used in this method. The influences of self and mutual correlation function on M-ary code shift keying underwater acoustic spread spectrum communication over gaussian and fading channel are analyzed. The performances under the two kinds of channels are simulated. The simulation shows that the ability for M-ary code shift spread spectrum to overcome the noise is better than for the direct sequence spread spectrum, M-ary spread spectrum and code shift keying spread spectrum. The comparative experiment is conducted to compare the M-ary spread spectrum, code shift keying spread spectrum and M-ary code shift keying spread spectrum in the pool. The M-ary code shift keying communication rate of 256.3 bps with no transmitting error in 104 bit data volume is realized.
The two-dimensional phononic crystal band gaps tuned by the symmetry and orientation of the additional rods in the center of unit cell
2012, 61 (23): 234302. doi: 10.7498/aps.61.234302
The effects of symmetry and orientation of the additional steel rods on the band gaps of two-dimensional phononic crystals with steel-air system are numerically investigated by using the plane wave expansion method. The original steel rods of the phononic crystals are of columns in square and hexagonal lattices, and the additional steel rods are of regular square, hexagon, octagon prisms and columns, which are placed, respectively, in the center of each unit cell of the two kinds of lattices. The gap maps are introduced to illustrate the influences of the filling fraction and orientation of the additional rods on band gaps. It is found that in the case of the additional rods with low filling fraction, the band gaps can be obtained most easily because the degeneracy of bands is lifted when the cross section of additional rod has the same shape as that of lattice, but the widest band gaps appear under the condition of the additional rods with highest symmetry and largest filling fraction. The influence of orientation on band gap in square lattice is more obvious than that in hexagonal lattice. If the column lattice points are changed by square prisms in simple square lattice, the lower and wider band gaps can be produced by rotating the square prisms, which is contrary to the scenario that emerges in square lattice with additional rods at the center of unit cell. Using one of the methods of adding additional rods or rotating the original prisms is more beneficial to the generation of band gaps than combining effect of these two means in simple lattices. Furthermore, the mechanisms of above results are analyzed.
2012, 61 (23): 234601. doi: 10.7498/aps.61.234601
A lagging response in time exists between the propagation of heat flux and the establishment of temperature gradient and it is affected by the space effect during the heat conduction with the micro-scale property. Based on the general heat conduction law of thermal mass, the dynamic model of generalized thermoelasticity is established by Clausius inequality and Helmholtz free energy, where the inertia effect on the time and space of heat flux and temperature is involved. The guiding equations are derived and given for the isotropic and homogeneous materials. By comparison with the existing models of generalized thermoelasticity, the guiding equations can reduce to the L-S, G-L and G-N models when the heat flux is not very high, so that the inertia effect on space of heat flux and temperature can be ignored. For micro-scale heat conduction, the heat flux may be very high and the inertial force due to the spatial velocity variation cannot be ignored, the non-Fourier phenomenon will take place even under steady state condition. In such cases, the thermal conductivity is affected by the inertia effect of the space, which can be explained by the model established in the paper. Meanwhile, the physically impossible phenomenon that thermal conductivity changes with structure size induced by existing generalized model can also be eliminated.
2012, 61 (23): 234602. doi: 10.7498/aps.61.234602
Flow induced residual stress is the major reason for stress cracking and warping of plastic products, the study on which is significant to overcome the flaws of products. In this paper, the energy equation based on Extended Pom-Pom constitutive relationship is deduced. A non-isothermal viscoelastic-Newtonian two-phase fluid model for mold filling process of viscoelastic materials is set up. The conservative interface capturing technique and the flow field solving method are coupled to perform a dynamic simulation. The distribution of the frozen skin layer and the shear rates are given. The flow induced residual stress is predicted and analyzed. The numerical results show that the thickness of the frozen skin layer is dependent on the injection velocity and a higher injection velocity corresponds to a thin frozen skin layer. Near the walls of the product, the shear rate and the residual stress are almost zero. At the position of subsurface, the shear rate and the residual stress reach their largest values. At the positions far away from the walls of the product, the shear rate and the residual stress are small.
2012, 61 (23): 234701. doi: 10.7498/aps.61.234701
According to smoothed particle hydrodynamics, we investigate numerically the micro-jets from grooved surfaces of different metals, where the velocities of jet head and material sources are discussed in detail. Our simulation results suggest that the jetting factor reaches its maximum at the half angle 45 degree, and the jetting factor reduces with the increase or decrease of the groove angle; also, the maximum velocity of jet shows a linear reduction with the increase of groove angel. Those results are consistent with the corresponding experimental results. The jetting material source and its dynamical process are analyzed. It is shown that with the increase of groove angel, the jet material sources transfer to the bottom of groove from two-side layer, while at the groove angel near 45 degree, a homogenous source layer throughout the groove comes into being. Finally, we further explain the jetting dynamics from different grooves by particle trajectory and its mechanical quantity history.
A quantitative model and experimental investigations of wall shear stress between solid and gaseous fluid using liquid crystal coating
2012, 61 (23): 234702. doi: 10.7498/aps.61.234702
Measurement of wall shear stress (WSS) has important significance in many fields, such as spacecraft surface flow visualization and drag reduction of solid-liquid interface. Various measuring methods have been developed with the rapid development of micro and nano technology. This paper focuses on the measurement of WSS by using nematic liquid crystal coating. A quantitative model is built to describe the relationship between the variations of director orientation and the WSS based on the curvature elasticity theory of liquid crystal. Then an experiment setup is built to verify this model using 5 CB and 7 CB through the optical method. There is good consistence between the theoretical model and experiment results. Finally, the WSS measuring limits and influencing factors are discussed such as the thickness of the liquid crystal layer, the elastic coefficient of the molecule and measurement resolution.
2012, 61 (23): 234703. doi: 10.7498/aps.61.234703
Ejecta mixing takes place at the interface between metal and gas under shock loading, i.e., the transport process of ejecta from metal surface appears in the gas. In this paper, adopting disperse particles instead of the initial ejecta, we simulate the ejection mixing process according to two-phase flow of gas and particle. We give the numerical results of the evolution process of the mixing, and analyze the effects of initial gas pressure and particle size on the mixing zone. The pneumatic break is observed from the numerical simulations, which can lead to evident reduction of the particle and then become an important factor affecting the evolution of mixture; also, our simulations are consistent with the corresponding measurements, showing that the gas and particle two-phase flow model is an effective method to simulate the ejection mixing.
The largest Lyapunov exponent and the turbulent fluctuation of the time series from air turbulent jets
2012, 61 (23): 234704. doi: 10.7498/aps.61.234704
The velocity time series of round air jets are acquired with the hot-wire anemometer. We compute the largest Lyapunov exponent and the turbulent incoherent perturbation of the velocity signals at 939 ≤ Re ≤ 3758 using an analytical method of chaotic time series which is based on the invariant of the largest Lyapunov exponent. The results show that the largest Lyapunov exponent increases with exit Reynolds number, and decreases with the distance away from nozzle exit. The reciprocal of the largest Lyapunov exponent is positively correlated with correlation time. The incoherent perturbation of turbulence increases either with exit Reynolds number or with the distance away from nozzle exit. The incoherent perturbation of turbulence is negatively correlated with Kolmogorov scale.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
Numerical simulation of energy deposition and extraction efficiency of the volume produced negative hydrogen ions
2012, 61 (23): 235201. doi: 10.7498/aps.61.235201
In this paper, the negative hydrogen ion transportation and energy deposition mechanism are investigated deeply, and the collisions between negative hydrogen ions with other particles are treated by the three-dimensional Monte Carlo method. The storage of the particles and the constraint function of magnetic field are disposed by particle-in-cell (PIC) technology under the CHIPIC software. Based on this, negative hydrogen ion transportation and scattering case in the JAERI 10 A multicusp ion source are simulated. The influences of pressure and filter magnetic field on negative hydrogen ion transportation and extraction characteristics are analyzed and discussed. Results show that the greater the pressure, the bigger the collision loss of the volume produced negative hydrogen ions, and also the smaller the number of negative hydrogen ions reaching the wall and extracted is; in the lower discharge case, the more intensively the negative hydrogen ion extraction is influenced by the filter magnetic field, and also the stronger the filter magnetic field, the worse the extraction effectiveness is.
Effects of pressure and incident power on self-organization pattern structure during microwave breakdown in high pressure air
2012, 61 (23): 235202. doi: 10.7498/aps.61.235202
Pressure and microwave power are the most important parameters during microwave breakdown in air and affect the self-organization plasma pattern structure and its propagation directly. In order to study the effects of pressure and microwave power, an effective-diffusion fluid plasma equation is solved together with Maxwell's equations, and the double grid method is also used to meet the different grid size requirement of plasma equation and finite-difference-time-domain for Maxwell's equations. The numerical results show that with lower pressure the plasma behaves as a more diffuse plasmoid instead of a well defined plasma pattern structure under higher pressure, and the increase of incident microwave power will lead to a rapid growth of the front propagation velocity and a well separated and sharp pattern structure, and the higher incident power also results in jump-like front propagation.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2012, 61 (23): 236101. doi: 10.7498/aps.61.236101
Vanadium oxide nanotubes are prepared on a Si substrate with hydrothermal method in this experiment with dodecylamine serving as template. X-ray diffraction, Raman spectroscopy and scanning electron microscope (SEM) are employed to characterize the structural and electronic properties of these nanotubes. The results show that the growth of VOX nanotubes and their gas sensing properties are affected by the hydrothermal reaction time rather than mixing time. With longer reaction time, the VOX nanotubes display better properties with smoother walls, stronger gas sensitivity and faster response time (15 s), than with shorter reaction time. SEM results reveal that VOX nanotubes have inner diameters between 25-35 nm and outer diameters between 65-100 nm. The sample inner diameters vary between 25 and 35 nm. The outer diameters are also quite similar in all tubes and lie between 65 and 100 nm.
Effect of point defects on copper precipitation in heavily boron-doped Czochralski silicon p/p+ epitaxial wafer
2012, 61 (23): 236102. doi: 10.7498/aps.61.236102
The effect of point defects on copper precipitation in heavily boron-doped Czochralski silicon epitaxial wafer is systematically investigated by optical microscopy. The samples first experience the rapid thermal processing (RTP) in different atmospheres, and then are Cu-contaminated at different temperatures. It is found that RTP in O2 ambient leads to a low density of copper precipitation in the p+ substrate, however, high density of copper precipitation is observed in the sample subjected to RTP in Ar or N2 ambient. Additionally, in all of the samples, no defects are found in the epitaxial layer. On the basis of the experiments, it can be concluded that interstitial silicon will prevent the process of copper precipitation while vacancy has an opposite effect. Furthermore, p/p+ epitaxial structure could absorb copper impurities, thereby keeping the epitaxial layer free of defect.
Quantitative separation of radiation induced charges for gate controlled later PNP bipolar transistors
2012, 61 (23): 236103. doi: 10.7498/aps.61.236103
A new test structure of gate controlled lateral PNP bipolar transistors designed and fabricated. An independent gate terminal is patterned on the oxide layer above the active base region of normal lateral PNP bipolar transistors. According to the gate sweep technique, by sweeping the voltage applied to the gate terminal, one can obtain the characteristic of base current versus gate voltage. The quantitative variations of oxide trapped charges and interface traps are analytically estimated and numerically calculated, and the radiation induced defects in the gate controlled lateral PNP bipolar transistors during 60Co-γ irradiation and annealing at room temperature are separated independently. The test structures and measurements of the bipolar transistors used in the experiment are introduced in detail in this paper.
Dynamic response of metal to shock loading plays an improtant role in the fields of civil engineering, aeronautics, etc. The forming process of ejection of metal under shock loading, including micro-jet, fragmentation and micro-spall remain to be studied. The present work is devoted to laser-shock experiments in metal ejection on the SGIII laser facility. And the fragment recovery and post-test evaluation of the fragment-size distribution are achieved.
2012, 61 (23): 236301. doi: 10.7498/aps.61.236301
Using the first principles calculations based on the density-functional theory, we study the geometric structures, magnetisms, and electronic structures of h-BN monolayer with group IA/I!IA elements (Li, Na, K, B, Mg and Ca) replacing B as impurities. It is shown that the nonmagnetic substitutional impurities can induce spin polarization in nonmagnetic BN monolayer. For Li, Na and K impurities, the total magnetic moment of the supercell is 2 μB; for Mg and Ca, it is 1 μB. The magnetic moments are mainly localized on the nearest neighbor N atoms. The magnetic moment of the supercell with Be impurity is 0.705 μB, distributed over all N atoms. Spin polarized densities of states are presented, including total density of states and orbital-projected partial density of states. The origin of local magnetic moments and impurity energy level are explained. It is also found that the Mg and Ca doped systems are half metallic.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2012, 61 (23): 237101. doi: 10.7498/aps.61.237101
The lattice constants, energy band properties and phase stabilities of wurtzite (WZ) and rocksalt (RS) Be1-xMgxO alloys are investigated by the plan-wave pseudopotential method in the generalized gradient approximation based on the density functional theory. The theoretical results show that the lattice constants of WZ and RS Be1-xMgxO alloys increase and their bandgaps decrease as the content x of Mg increases from 0 to 1. For the same Mg content values, the bandgap of RS Be1-xMgxO alloy is greater than that of WZ Be1-xMgxO alloy. The WZ phase will transit to the RS phase when the Mg content is about 0.89. In order to obtain the theoretical values in accordance with the experimental results, the bandgaps of WZ and RS Be1-xMgxO alloys are corrected and the values of bandgap bowing parameter b are 3.451 eV and 4.96 eV for WZ Be1-xMgxO and RS Be1-xMgxO respectively. The reason of large band gap bowing parameter b is attributed to a large difference in ionic radius between Be and Mg. Besides, the relations among energy bandgap, bowing parameter and lattice constant of wurtzite BeO-ZnO-CdO ternary alloy are analyzed.
2012, 61 (23): 237102. doi: 10.7498/aps.61.237102
In this paper, the hole effective mass along arbitrarily k wavevector direction and the hole isotropic effective masses in strained Ge/(001)(101)(111)Si1-xGex are obtained with in the frame work of kp theory. It is found that the hole effective mass of the top valence band along  wave vector decreases obviously with stress increasing and its absolute value is smallest. The hole effective mass of the second valence band tends to gently decrease with stress increasing, and is not significant in magnitude. Compared with the existing isotropic effective quality, the result obtained in this paper is proved to be correct.
First principles calculation of electronic structure for Al-doped 3C-SiC and its microwave dielectric properties
2012, 61 (23): 237103. doi: 10.7498/aps.61.237103
The electronic structure and permittivity of Al-doped 3C-SiC are studied by using the first principles plane-wave pseudopotential method based on the density functional theory, and compared with those of undoped 3C-SiC. Results show that the Fermi energy level introduced into valence band and band gap is slightly widened through Al doping for 3C-SiC, and that the permittivity is greatly improved in a frequency range of 8.2-12.4 GHz. Al doped 3C-SiC powder absorber is prepared by combustion synthesis, and the permittivities of the samples are measured in the frequency range of 8.2-12.4 GHz by vector network analyzer, which validates the results of theoretical calculation. The mechanism of microwave loss is discussed.
2012, 61 (23): 237104. doi: 10.7498/aps.61.237104
In this paper, based on the surface potential model, taking into account both the deep and tail state distributions simultaneously, and using the simplified Fermi-Dirac function, a unified local-state model is obtained. Using the effective characteristic temperature, the unified current-voltage (I-V) model for a-Si:H thin-film transistor a-Si:H TFT is developed. This model can describes all operating regions including subthreshold region, linear area and saturated zone through a single equation. By comparison with the experimental data, it is shown that this model can accurately describe the current voltage characteristic of the a-Si:H TFT.
2012, 61 (23): 237105. doi: 10.7498/aps.61.237105
Au nanowire arrays with high aspect ratios are prepared in anode aluminum oxide templates by electrochemical deposition. The obtained structures are investigated by scanning Auger microscopy. Surface plasmon resonances of Au nanowire arrays induce a transverse mode (T mode) and a longitudinal mode (L mode) in the optical absorption, which indicates the strong anisotropy of the Au nanowires. The L mode intensity is related to the angle and polarization of the incident light. The L mode position shows a shift with the increase of aspect ratio of the nanowires. The plasmon coupling between Au nanowires is compared with the H-aggregation of organic chromophores. The blue shift of the L mode in the arrays compared with a single nanowire is induced by multi-coupling of the electromagnetic field between neighbouring nanowires. A finite element method is used to simulate the electric field distributions of a single Au nanowire and an array of plasmonically coupled wires.
The enhancement of current in superconductor wires by modifying and changing the surface region microstructure
2012, 61 (23): 237106. doi: 10.7498/aps.61.237106
The electromagnetic characteristics of superconductor are briefly analyzed and described, when a magnetic field is applied to the superconductor, flux lines penetrate it from the surface. The flux lines are pinned by pinning centers on the surface of the superconductor, and cannot penetrate deeply from the surface and the density of the flux lines will be higher near the surface and lower in the inner region, with the participation of surface barrier effect and Meissner effect, which indicates the enhancement of pining in the core of the superconducting wire is useless and blocks the transport current. Based on this knowledge, the new multilayer structure wire is design, and the core of the wire is the high-density superconductor crystal covered by doping or changing microstructure layer with thickness as the penetration depth, outer part of the superconducting region is modified, by nanoparticle surface decorating, electron and heavy ion irradiation and extension to surface. Furthermore the Columnar defects, located near the surface, which suppress the surface barrier but create gigantic surface pinning, and the other pining centers have not this property. Then the superconducting region is wrapt by the normal metal.
First principles calculations of the electronic structure and optical properties of pure and (Nb, N) co-doped anatase
2012, 61 (23): 237107. doi: 10.7498/aps.61.237107
Titanium dioxide (TiO2), as a kind of excellent photocatalyst, has been widely researched and applied. The geometry structures, band structures, densities of states and absorption coefficients of three possible (Nb, N) co-doped anatase are studied by the density functional theory based on GGA+U method, and the results are compared with those of the single doped (Nb/N) anatase. It is shown that the introduction of dopants leads to the lattice distortion. Besides, compared with undoped TiO2, (Nb, N) co-doped TiO2 has small band gap and absorption edge, and the level of N 2p state changes from partially occupied to occupied, which reduces the recombination of electron-hole pairs. In addition, the smaller formation energy of co-doped anatase indicates that it is more stable than the N doped anatase. Therefore, (Nb, N) co-doping anatase is a kind of stable photocatalyst and it has better photocatalytic performance in visible light band than pure TiO2.
2012, 61 (23): 237201. doi: 10.7498/aps.61.237201
SiGe, as a reliable and most efficient high-temperature thermoelectrics, has been utilized in special fields for many years, but there is no large-scale commercial application due to its high cost and low efficiency. Therefore, it is necessary to improve the dimensionless figure of merit ZT of a Si-based system, free of Ge which is expensive and rare earth, thereby becoming competitive in cost and efficiency for the commercial application. Since pure silicon possesses rather low ZT, for example 0.01 at room temperature, we have developed doped and nano-structured Si100P2.5 (GaP)1.5 bulk material and obtained ZT 0.47. In this work, a new approach to inducing random pores with four size distributions of 50 nm, 100 nm, 300 nm, and 1-2 μm is applied to the Si100P2.5 (GaP)1.5 bulk material, and ZT is improved by 32%. The increase of ZT can be attributed to the enhancement of the electrical conductivity and the Seebeck coefficient, and the reduction of the lattice thermal conductivity. The enhancement of electrical conductivity is ascribed to the doping effect of a small amount of Sb, while the increase of Seebeck coefficients stems mainly from the filter of low-energy carriers, and the reduction of lattice thermal conductivity arises mainly from phonons scattering. It is proved in this work that inducing random pores is an effective approach to improving the figure of merit of Si-based system.
Investigation on influence of antisite defects on electronic structure and optical properties of silicon carbide nanotube
2012, 61 (23): 237301. doi: 10.7498/aps.61.237301
Electronic structure and optical properties of a (5, 5) single-walled silicon carbide nanotube are studied with first principles calculation based on density functional theory. Depression and salient are formed near CSi defect and SiC defect in the surface of the nanotube. Defect energy levels are formed near the bottom of conduction band, which results in an n-type conductivity for nanotubes with antisite defects. In dielectric functions parallel and perpendicular to the axis of the nanotube, novel resonance peak is formed from transitions between top of the conduction band and the defect energy level.
2012, 61 (23): 237302. doi: 10.7498/aps.61.237302
The magnetotransport measurement is performed on a GaN/AlxGa1-xN heterostructure sample in a low temperature range of 1.4-25 K and at magnetic fields ranging from 0 T up to 13 T. Magnetoresistance of a two-dimensional electron gas confined in the heterostructure is investigated. The negative magnetoresistivity in the whole magnetic field range originates from the electron-electron interactions (EEIs), while the positive magnetoresistivity in the high field range results from the parallel conductance. The EEI correction terms, as well as the concentration and mobility of the parallel channel are obtained by fitting the experimental data. Furthermore, another method of calculation is used to check their accuracy.
2012, 61 (23): 237801. doi: 10.7498/aps.61.237801
In this paper, we give the expressions of transmissivity, reflectivity and the distribution of election field in a photon crystal at an arbitrary incidence angle. We investigate the field intensity when angle frequency of incidence light is odd or even times the fundamental frequency in photon crystal. Besides, we study the effects of incidence angle, the incident direction of light and defect layer on election field distribution, and obtain some new and valuable results. We expect the research results to be helpful for designing photonic crystals.
2012, 61 (23): 237802. doi: 10.7498/aps.61.237802
Oxygen-chlorine tellurite glasses: (90-x)TeO2-10Nb2O5-xBaCl2 (x=10, 20, 30) are prepared using the conventional melting method. The influences of component on the density, structure, optical properties, refractive index and dispersion of the glasses are studied using density specific gravity balance, Raman spectroscopy, infrared-visible-UV spectrophotometer and Prism Coupler. The results show that the glass density first increases and then decreases with the increase of content of BaCl2. Furthermore, the structural units are converted from [TeO4] trigonal bipyramid into [TeO3] trigonal pyramid. Moreover, glass refractive index gradually decreases. In addition, new tellurite glass has high linear refractive index n (2.02259), nonlinear refractive index N2 (6.8× 10-12 esu) and third-order nonlinearpolarizability χ(3)(3.7× 10-13 esu). The new tellurite glass exhibits a high transmittance in the visible and infrared spectral region and possesses an cut-off edge in the near UV spectral region, and they each undergo an obvious blue shift with the increase of content of BaCl2. The direct and indirect transition optical band gaps of the samples are calculated according to the Tauc equation, and they increase with the content of BaCl2 increasing.
2012, 61 (23): 237803. doi: 10.7498/aps.61.237803
We present a new glass able to produce ultra-broadband yellow fluorescence. The luminescence system is Ce3+ doped low silica calcium aluminosilica glass excited by blue light emitting diode (LED). The glass is prepared by the conventional melting process. We analyze the luminescence properties of the samples by measuring their absorption spectra, excitation spectra, fluorescence spectra. Furthermore, we study the influence of excitation wavelength on the fluorescence intensity and peak wavelength. The doped glass combined with blue LED to obtain the white lighting is achieved. Besides we calculate the Commission Internationale de I'Eclairage chromaticity coordinate. Therefore these glasses have large potentials to be used as white lighting phosphor materials.
2012, 61 (23): 237804. doi: 10.7498/aps.61.237804
In this article we report on the green-light wavelength InGaN/AlN quantum dots (QDs) grown by molecular beam epitaxy, and propose a method to determine the composition of the InGaN QDs by combining reflection high-energy electron diffraction in-situ measurement and photoluminescence measurement, in which the strain relaxation and the influences of strain and quantum-confined Stark effect on the exciton energy are taken into consideration.
2012, 61 (23): 237901. doi: 10.7498/aps.61.237901
We investigate the effect of preparation parameters on the formation of microstructures created on a single crystal silicon wafer covered with a polyethylene terephthalate (PET) transparent film by femtosecond pulse laser with a central wavelength of 800 nm. The results reveal that formation of the cone microstructures depends on laser fluence, and the cone microstructures cannot be created by lower laser fluence or can be destroyed by higher laser fluence. The laser scanning speed can directly affect the quality of the cone microstructures, and lower speed will destroy the cone, while higher speed will create indistinct cone microstructures each with smaller depth. Some idealized structures are obtained by optimizing the experimental parameters. Finally, we find that the cone microstructures on silicon wafer covered with a film are caused by both laser ablation and oxidation, and laser ablation plays a major role.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
Temperature-dependant growth and properties of W-doped ZnO thin films deposited by reactive magnetron sputtering
2012, 61 (23): 238101. doi: 10.7498/aps.61.238101
W-doped ZnO (WZO) thin films for thin film solar cells have been deposited by pulsed direct-current reactive magnetron sputtering. The microstructures, surface morphologies, optical and electrical properties of WZO thin films are investigated at different substrate temperatures. The experimental results indicate that a proper substrate temperature is the key factor for fabricating high-quality WZO thin films. The surface roughness of WZO thin films increases firstly from 15.65 nm to 37.60 nm, and then decreases from 37.60 nm to 11.07 nm with the increase of substrate temperature. Higher Hall mobility deposited at the higher temperatures is attributed to the compact structure and good crystallization quality. The WZO thin film prepared at the temperature of 325 ℃ presents excellent optical and electrical properties with an average transmittance of 85.7% in the wavelength range from 400 nm to 1500 nm, a low resistivity of 9.25× 10-3 Ω·cm, a sheet resistance of 56.24 Ω /sq and a high Hall mobility of 11.8 cm2·V-1·s-1.
2012, 61 (23): 238102. doi: 10.7498/aps.61.238102
The Bi2Fe4O9 fibrous precursor is successfully synthesized through electrospinning technique, and the single-phase claviform Bi2Fe4O9 is obtained after calcining the precursor. The UV-vis absorption spectrum shows that the absorption range of the as-prepared Bi2Fe4O9 is wide, which indicates that the as-prepared Bi2Fe4O9 could be a photocatalyst with visible-light-driven photocatalytic activity. In addition,the result of magnetic hysteresis loop for Bi2Fe4O9 shows weak ferromagnetism and good soft magnetic properties, which indicate that the as-prepared Bi2Fe4O9 has a good application in soft magnetic material.
2012, 61 (23): 238401. doi: 10.7498/aps.61.238401
Based on the principle of planar near-field measurement and theory on the spatial pattern of planar antenna arrays, a comprehensive investigation on the spatial pattern of planar antenna arrays tilting away from the planar near-field scanning plane is performed. An efficient computational algorithm on the near-field-far-field pattern in the plane determined by the rotation axis and normal line of the tilted array is developed. The parametric equations on the intersecting curve locus between the plane and unit directional sphere of the planar near-field scanning are thus derived. Consequently, the spatial pattern of the tilted antenna array based on the near-field data is calculated by fast Fourier transform technique, and at the same time a two-dimensional interpolating scheme to evaluate the pattern in the plane is developed. The capability, efficiency, and feasibility of this new algorithm are demonstrated by the good consistency of the calculated results with the numerical simulations and experimental measurements.
2012, 61 (23): 238402. doi: 10.7498/aps.61.238402
The advantages of the tri-axial relativistic klystron amplifier and the multi-beam klystron amplifier are analyzed, and then an X-band coaxial multi-beam relativistic klystron amplifier is designed in order to increase the output microwave power and frequency. Based on the results from partial-in-cell simulation, the experiment is performed on the Sinus accelerator. In experiment, the transmission and bunching of electron beam are analyzed, and then the amplifier is driven by a beam of 5.3 kA at 670 kV, and the maximum output power is 420 MW when input power is 30 kW, and frequency is 9.375 GHz. The experiment proves that a ten kW-level input power can drive the X-band coaxial multi-beam relativistic klystron amplifier to generate more than one hundred MW-level output power.
The effect of electromagnetic compound field on the solidified microstructure of Zn-10 wt%Bi hyper-monotectic alloy
2012, 61 (23): 238501. doi: 10.7498/aps.61.238501
The effects of different electro-magnetic body forces on the microstructure and solidifying process of Zn-10 wt%Bi hyper-monotectic alloy under the condition of high magnetic field are investigated in this paper. The result indicate, on the one hand, that the solidified structure is wanted most and the diameter of the second phase particle is minimum when the alternative current (50 Hz) increases to a certain value. On the other hand, the degree of segregation, dispersion and size of the second Bi phase particles in solidified structure of Zn-10 wt%Bi hyper-monotectic alloy are refined significantly with the increase of magnetic induction intensity under a fixed alternative current (50 Hz) value. The analysis result indicates that both the magnetic induction intensity and alternative current intensity have a significant effect on solidified structure so that we can obtain a more competitive solidified structure of monotectic alloy by controlling the intensity of magnetic induction and alternative current.
2012, 61 (23): 238502. doi: 10.7498/aps.61.238502
The paper deals with the design optimization of SiGe heterojunction bipolar transistor (HBT) on thin film silicon-on-insulator (SOI). The basic DC and AC current characteristics are obtained and the differences between the SOI and bulk SiGe HBT are analyzed. As the incorporation of SOI substrate makes the SOI SiGe HBT a four-terminal device, the influences of the substrate bias on Gummel plot, output current and avalanche current are studied emphatically. Finally, the physical parameters of material and geometric parameters of the device are discussed by changing the frequency characteristics. Compared with the bulk counterpart, the SOI SiGe HBT is designed and fabricated with a great degree of freedom for better performance. This systematic analysis of SOI SiGe HBT provides a valuable reference for the SOI SiGe BiCMOS circuit design and simulation.
2012, 61 (23): 238701. doi: 10.7498/aps.61.238701
In this paper, a new rumor spreading SEIR model is proposed with considering both the spreading characteristics of rumors in the real online social network and the epidemic dynamics models with latency. First, the dynamical evolution equations based on the SEIR model are established. Then an efficient immunization strategy—important acquaintance immunization strategy, is given for the inhibition of rumor spreading. Finally, based on the user datasets of Facebook, the SEIR model and its dynamical evolution equations with various immunization strategies including important acquaintance immunization strategy are utilized in the computer simulation for the evolution process of rumor spreading before and after immunization. Simulation results show that SEIR model conforms to propagation characteristics of the real online social network, and the important acquaintance immunization strategy is an optimal scheme to solve the inhibition of rumor spreading in the online social network.
2012, 61 (23): 238901. doi: 10.7498/aps.61.238901
Based on the research of the human mobilety patterns, a random-walker network model is used to explain the empirical results on the networks of mobile agents, and shows a possible evolution mechanism of these networks. The simulation results show that the degree distribution of this network takes Poisson distribution, and it exhibits the small-world behavior. Moreover, we present a numerical investigation on the displacements of the mobile agents, and find that the distribution of displacements takes power-law. Our numerical results are in good agreement with the empirical results on the human travel from the trajectories of human bank notes.
2012, 61 (23): 238902. doi: 10.7498/aps.61.238902
Based on a general dynamical model for disaster spreading, in different network structures, i.e. in Erdos-Renyi network, scale-free network and small world network, the influences of network centrality on the speed and trend of disaster spreading are analyzed by simulation. By changing the initial spreading condition, the influence of initial state on the spreading efficiency is analyzed. In this paper the differences between final disaster spreading state are mainly discussed based on four initial vertex-choosing strategies. For the four strategies, it is shown that there are apparent differences. Complex network has a strong ability to resist random attacks but is is fragile to resist intentional attacks. However, the three networks show different degrees of fragility. And then, the theoretical analysis results are verified in an actual network.