Vol. 62, No. 5 (2013)
2013, 62 (5): 053201. doi: 10.7498/aps.62.053201
The photoionization detection method is employed to systematically study the odd-parity bound-excited states of Sm atom. Three different excitation paths are designed to carry out three-step excitation and photoionization processes for the Sm atom in the same energy region. By scanning the wavelength of third-step dye laser not only the level energies of a large number of odd-parity bound-excited states are determined, but also the information about relative intensity of the corresponding transition is obtained. Comparison of the three groups of spectra corresponding to the three paths enables us to assign the J-value, the total angular momentum of the excited states of Sm atom uniquely. In addition, a small number of level energies measured with different methods previously are also confirmed in this work.
The chiral asymmetry revealed by the Raman differential bond polarizability of (2R, 3R)-(-)- 2, 3-butanediol
2013, 62 (5): 053301. doi: 10.7498/aps.62.053301
The Raman optical activity (ROA) of (2R, 3R)-2, 3-butanediol is investigated through the intensities of Raman spectrum and ROA spectrum, bond polarizability and differential bond polarizability. In view of the two possible optimized structures, i.e., C1 and C2 group, we obtain the conclusion that ample information concerning the physical pictures of this chiral system does not depend on the two variant structures.Based on the analysis of bond polarizabilities, the charge moves from the periphery to the skeleton structure in the Raman relaxation process. The analysis of differential bond polarizabilities shows that the signs of differential bond polarizabilities on the both sides of the plane associated with the asymmetric C and H atoms are opposite. This means that the chiral asymmetry of this molecule is rather complete.Also, it is observed that bond polarizabilities for the symmetric modes are larger than for the antisymmetric modes, while for the differential bond polarizabilities, the situation is just reverse.
2013, 62 (5): 058503. doi: 10.7498/aps.62.058503
Wireless energy transfer has broad prospective applications. Current researches focus on electromagnetic induction and magnetic resonance. The former approach is sensitive to position and the latter has larger size, both of which affect the broad application of wireless energy transfer. Two layers of magnetostrictive effect materials and one layer of piezoelectric effect material are bound by epoxy resin, which generates magnetoelectric laminated composite. It is the first time that the output voltage, current and magnetoelectric factor have been deduced without DC magnetic bias. Three samples are implemented and the wireless energy transfer system based on them is developed. The tests on the samples verify the correctness of the theoretic analysis. Further experiments illustrate that there are double frequency characteristics for the magnetoelectric laminated composites; the resonant frequency is proportional to the reciprocal of the length of the composite; the open circuit voltage of the composite could reach 100 V (rms) under a magnetic field of 20 Oe; the maximum energy transferred is 520 mW, which is the highest record reported up to now, with the energy density 1.21W/cm3 and maximum transfer efficiency 35%; the rotation less than 30° has little effect on the output of the composites. Theoretical analyses and experimental results suggest that the magnetoelectric laminated composite based on Metglas/PFC is a very interesting approach to small volume and small power wireless energy transfer applications.
Chaotic signal denoising method based on independent component analysis and empirical mode decomposition
2013, 62 (5): 050201. doi: 10.7498/aps.62.050201
According to the characteristics of empirical mode decomposition and denoise of independent component analysis, an adaptive denoising method of chaotic signal is proposed based on independent component analysis and empirical mode decomposition. First, the chaotic signal is decomposed into a set of intrinsic mode functions by empirical mode decomposition; then, the multi-dimensional input vectors are constructed based on the translation invariant empirical mode decomposition, and the noise of each intrinsic mode function is removed through the constructed multi-dimensional input vectors and the independent component analysis; finally, the denoisied chaotic signal is obtained by accumulating and reconstructing all the processed intrinsic mode functions. Both the chaotic signal generated by Lorenz map with different level Gaussian noises, and the observed monthly series of sunspots are respectively used for noise reduction using the proposed method. The results of numerical experiments show that the proposed method is efficient. It can better correct the positions of data points in phase space and approximate the real chaotic attractor trajectories more closely．
Application of extended Fourier amplitude sensitivity test (EFAST) method in land surface parameter sensitivity analysis
2013, 62 (5): 050202. doi: 10.7498/aps.62.050202
In this paper, we explore the application of extended Fourier amplitude sensitivity test (EFAST) to land surface parameter sensitivity analysis. Based on observations of Tongyu site (Jilin province) at a degraded grassland in 2008, the EFAST method is verified and applied. The sensible heat flux and latent heat flux are used as the test variables. With the full consideration of the sensitivity of individual parameters, the coupling between the parameters is taken into account. We explore the influence on the nonlinear system under the constraints of the interaction of multiple land surface parameters with a quantitative analysis. The results show that sand content in the soil (sand) and minimal porosity of permeable water are the key factors which affect the heat sensible flux and latent heat flux significantly, which is consistent with the existing result. The sensitivity result confirms the feasibility of the EFAST method. The results of this paper are expected to guide people in designing the field observation and development of parameterization schemes in land surface model.
2013, 62 (5): 050501. doi: 10.7498/aps.62.050501
By numerical simulations we find that the Duffing state transition from big-cycle motion to chaotic motion is more suited to weak signal detection. Based on this, in this paper, we propose a new weak periodic signal detection method from the angle of theoretical analysis. The principle of the proposed method is introduced and its feasibility is analyzed. Then, the proposed method and the traditional method are compared and analyzed both in transition zone effect and detection probability, and then a contrast simulation is carried out. The analysis and simulation results both indicate, the Duffing oscillator state transition from big-cycle motion to chaotic motion is affected less by transition zone under the same condition, based on which the proposed detection method possesses better detection capability. The simulation data also show that weak signal detection with Duffing oscillator must be based on unilateral state transition. The frequency difference detection based on intermittent chaos is suited only for the case where the signal-to-noise ratio of the detected signal is relatively high.
Effects of time-periodic modulation of cross-correlation intensity between noises on stochastic resonance of over-damped linear system
2013, 62 (5): 050502. doi: 10.7498/aps.62.050502
For an over-damped linear system subjected to correlated additive and multiplicative noise and a periodic signal, when the cross-correlation intensity between noises is a time-periodic function, the analytical expression of the output signal-to-noise ratio (SNR) is derived by means of the stochastic averaging method. It is found that this system has richer dynamic behaviors than the linear systems in which cross-correlation intensity between noises is constant, that the evolution of the output SNR with the cross-correlation modulation frequency presents periodic oscillation, and that the parameters of cross-correlation intensity between noises diversify the stochastic resonance phenomena.The time-periodic modulation of cross-correlation intensity between noises is conductive to enhancing the sensitivity to weak periodic signal detection and implementing the frequency estimation of weak periodic signal.
2013, 62 (5): 050503. doi: 10.7498/aps.62.050503
An improved discrete-time model for a digital controlled single-phase full-bridge voltage inverter is proposed in this paper. Based on state-space averaging in every switching cycle, the improved discrete-time model combines the advantages of the average model and the traditional discrete-time model, which can accurately analyze the digital control delay and sample-and-hold process inherently in digital controlled system. Consequently, under the accuracy premise, the improved discrete-time model can effectively simplify the traditional discrete-time modeling. As an example, an LC filter capacitor-voltage and inductor-current feed-back plus voltage-reference feed-forward control algorithm is analyzed based on the improved discrete-time model. The stability boundary and oscillation frequency are accurately predicted. Finally, theoretical results are verified by simulations and experiments.
2013, 62 (5): 050504. doi: 10.7498/aps.62.050504
For the noisy chaotic series prediction problem, traditional methods are quite empirical, and are lacking in the analysis of the composition of the prediction error, thereby ignoring the the difference between chaotic dynamics reconstruction and prediction model. Based on the composition of actual prediction error, the predictor bias error and input disturbance error are defined in this paper and two kinds of global forecasts, ensemble least-square method and regularization method are analysed. It is shown that the ensemble least-square method is suitable for the reconstruction of chaotic dynamics, but has a greater influence on the predictor error. On the other hand, the regularization method can improve the sensitivity of the predictor, but it can be influenced by the input perturbation error. Two simulation examples are used to demonstrate the difference between the chaotic dynamical reconstruction and the establishment of prediction model, and to compare the ensamble least-square method and the regularization method, and at the same time indicate that the actual prediction error is influenced both by the input disturbance error and by the predictor error. In practice, a balance should be stricken between the two, in order to optimize the model prediction accuracy.
Chaotic time series prediction model of wind power based on ensemble empirical mode decomposition-approximate entropy and reservoir
2013, 62 (5): 050505. doi: 10.7498/aps.62.050505
According to the chaotic feature of wind power time series, a combined short-term wind power forecasting approach based on ensemble empirical mode decomposition (EEMD)-approximate entropy and echo state network (ESN) is proposed. Firstly, in order to reduce the calculation scale of partial analysis for wind power and improve the wind power prediction accuracy, the wind power time series is decomposed into a series of wind power subsequences with obvious differences in complex degree by using EEMD-approximate entropy. Then, the forecasting model of each subsequence is created with least squares support vector machine (LSSVM), ESN and EEMD-ESN improved with the regularized high frequency parts. Finally, the simulation is performed by using the real data collected from a certain wind farm, the results show that the EEMD-ESN model is better in the training speed and forecasting accuracy, than those obtained from the least square support vector machine (LSSVM) model, which provides a new useful reference for the short-term forecasting of wind power in online engineering application.
2013, 62 (5): 050506. doi: 10.7498/aps.62.050506
A microscopic pedestrian model based on cellular automata is proposed and three groups of experiments on pedestrian evacuation from a double-exit room are conducted to investigate the route choice of pedestrians during evacuation. In the model, the route-choice behavior of pedestrians is affected by the route distance from the pedestrians to the exit, the capacity of the frontal route, and the repulsive force between pedestrians. Through the analysis of the video recordings, certain conclusions related to the route choice of pedestrians in these experiments are obtained. Model parameters are calibrated by the experimental data. Simulation results indicate that the model can reproduce the evacuation behavior of pedestrians in the room effectively and evacuation time is an increasing linear function of student number. The present study is helpful for devising evacuation strategies and schemes in buildings that are similar to the room.
2013, 62 (5): 050507. doi: 10.7498/aps.62.050507
The two-mode phase-field-crystal (PFC) method is used to calculate the phase diagram and to simulate the transformation of hexagonal to square structure in two dimensions. The nucleation, grain growth and dynamic feature of the phase structure transformation show that square phase prefers to be present at the juncture place of the three hexagonal grains, and swallows the hexagonal phase at grain boundary. The square grains grow and push the boundary of hexagonal grain toward the inside of hexagonal grain and then the square grains grow up and extend the area of square phase. The orientations of new square grains due to the structure transformation are nearly randomly distributed, and have no relation to those of hexagonal grains. The dynamic curve of area fraction of square phase shows the typical S shape with time increasing. The Avrami index curve shows that there are two stages in the transformation. The Avrami index n of second satge in simulation is in a range from 2.0 to 3.0, which is in good agreement with the value from the JMAK theory.
Adaptive synchronization and parameter identification for Lorenz chaotic system with stochastic perturbations
2013, 62 (5): 050508. doi: 10.7498/aps.62.050508
In this paper, Lorenz chaotic system with stochastic perturbation and unknown parameters is investigated, in which the stochastic perturbations is one-dimensional random process of the standard Wiener. Based on stochastic Lyapunov stability theory, It formula and adaptive control method combined with three adaptive control laws and two adaptive control laws respectively, two mean square Asymptotic adaptive synchronization standards are put forward theoretically. These new standards are in a simple form and easy to deal with. Moreover, with these standards, not only drive system with stochastic perturbations can be synchronized with the respond system, but also unknown parameters in the system can be identified. Finally, the Matlab numerical simulations confirm that the proposed results are correct and effective.
ATOMIC AND MOLECULAR PHYSICS
2013, 62 (5): 053101. doi: 10.7498/aps.62.053101
Based on DFT-GGA calculations, we systematically investigate the structures, electronic and magnetic properties of ConAl (n= 18) clusters. The results indicate that the aluminum prefers to maximize the number of Co-Al bonds by selecting the site which increases the coordination of cobalt atoms with Al. The doped Al makes the stability of ConAl clusters weakened and the magnetism decreased as compared with that of Con+1 clusters. The reduction magnitude of magnetism of the doping Al accords well with recent Stern-Gerlach experimental result for larger ConAlM clusters. In all of the ConAl alloy clusters, the Al atom is found to be aligned antiferromagnetically with its neighbor Co atoms except for Co4Al. As compared with the magnetism of pure Co cluster, the magnetism of ConAl cluster is reduced, which is attributed mainly to nor-magnetism Al element embeding and the weakening of spin polarization of the Co atoms.
2013, 62 (5): 053102. doi: 10.7498/aps.62.053102
The stable chemical potential phases of BiXO3 (X= Cr, Mn, Fe, Ni) are studied by density functional theory with the consideration of thermodynamics equilibrium conditions. It is found that the BiFeO3 and BiCrO3 have stable chemical potential regions and are expected to be synthesized, under thermodynamic equilibrium conditions. On the contrary, no stable regions are found for BiMnO3 and BiNiO3, indicating that they are hard to synthesize. Therefore the approaches to their preparation under non-thermodynamic equilibrium conditions should be considered.
2013, 62 (5): 053103. doi: 10.7498/aps.62.053103
The structures and the hydrogen storage capacities of the B6 clusters and the lithium decorated B6 clusters are investigated by using the density functional theory. The results show that the hydrogen is adsorbed in the atomic form by chemical bonds in the three possible structures of the B6 cluster. The lithium atoms do not cluster on the surface of decorated B6 cluster. Every lithium atom, as hydrogen molecules are adsorbed on the surface of lithium atoms decorated B6 clusters, can adsorb several intact hydrogen molecules. Of the lithium decorated B6 clusters the B6 cage cluster which is decorated by two lithium atoms can most adsorb the intact hydrogen molecules. The calculated gravimetric density and the average adsorption energy of hydrogen molecule are 20.38% and 1.683 kcal/mol, respectively, which are suitable for reversible hydrogen storage under the ambient condition of the normal temperature and pressure.
2013, 62 (5): 053601. doi: 10.7498/aps.62.053601
With the accurate ab initio method, the adsorption and dissociation process of H2 molecule on Al7- cluster anion are investigated. The stable structures of molecular adsorption and dissociative adsorption are confirmed. The photoelectron spectra of different structures are further analyzed. The calculations indicate that the adsorption of H2 on Al7- is weak physical adsorption with the adsorption energy about 0.02 eV. The investigation of the dissociation process shows that the energy barrier of dissociation is about 0.75 eV. The densities of states of the Al7- cluster and the dissociative adsorption complex Al7H2- are in good agreement with those obtained by the photoelectron spectroscopy. It suggests that H2 can be dissociated when it is absorbed on Al7- anions produced by laser ablation.
2013, 62 (5): 053602. doi: 10.7498/aps.62.053602
The photoionization mass spectra and photoionization efficiency curves of ArCO clusters are obtained with synchrotron radiation mass spectrometry. By comparison with absolute photoabsorption spectra of CO, the photoionization efficiency curve of ArCO clusters in an energy region from 13.9 to 14.6 eV reflects mainly the properties of Rydberg series converging to the X2+ (v+= 1, 2 and 3) of CO+, and these of n= 3 vibration sequence of the series converging to the A2 state of CO+. In the energy region from 14.6 to 15.75 eV, the curve reflects mainly the absorption property of CO, but its five strong peaks shift toward blue due to the interaction between Ar and CO. In an energy region from 15.75 to 15.80 eV, the curve reflects mainly the absorption properties of Ar and CO. At the same time, ionization energy of ArCO, and dissociation energies of ArCO and ArCO + are also calculated using the theory of quantum chemistry.
2013, 62 (5): 053603. doi: 10.7498/aps.62.053603
CO molecules adsorbed on the Wn clusters are systematically investigated by using density functional theory at the B3LYP/LANL2DZ level.The result indicates that the ground state structures of WnCO clusters are generated when CO molecules are adsorbed on Wn clusters or anionic cluster. We find that among the molecular adsorption states exists mainly the form of end-on type geometry, and that the bridge site adsorption type geometry plays a supplementary role. On the face, the adsorption is a non-dissociative adsorption. The CO bond length increases 0.1200.123 nm in WnCO cluster (compared with 0.116 nm in free CO molecule), which demonstrates that the CO molecules are activated. The stability analysis shows that W3CO and W5CO clusters are more stable than other clusters; natural bond orbital (NBO) analysis indicate that the interaction between W atom and CO molecule is primarily contributed by hybridization of molecular orbits within CO and 6s, 5d, 6p and 6d orbits of W atoms.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2013, 62 (5): 054101. doi: 10.7498/aps.62.054101
In order to calculate the equivalent EM parameter of mixed medium, in this paper, we proposes a new method of impedance simulation based on an equivalent circuit. First, the relationship between electromagnetic parameters and capacitance, inductance of material is set up, and then the model that can describe real composite accurately is established. The equivalent EM parameter could be obtained by calculating the equivalent impedance of mixed medium. Comparing this method with the classical formulae and finite-difference numerical method, it is proved that it can be used to calculate the equivalent EM parameter of material. In addition, introducing thin films to simulate the factor such as surface effect, the result of calculation would match with experimental results perfectly. This method has more advantages than classical formulae and numerical method.
2013, 62 (5): 054202. doi: 10.7498/aps.62.054202
Single-layer graphene film is fabricated on the copper foil by using chemical vapor deposition method, and the corresponding Raman spectrum is measured. The graphene membrane is transferred to the facet of zirconium oxide of a fiber jumper. The mode-lock Er-doped fiber laser with few-layer graphene membrane as saturable absorber is experimentally studied, which has a ring structure and uses a 10% portion of a fiber coupler as the laser output. The laser generates a pulse train at a 7.69 MHz repetition rate, with a full width at half maximum of 58.8 ps. The corresponding time-bandwidth product is 1.98, indicating that the pulses are chirped. The peak wavelength and 3 dB bandwidth of the laser are 1560.1 nm and 0.27 nm respectively. Through changing the air gap between two the fiber ends, 4 nm wavelength tuning is observed.
2013, 62 (5): 054203. doi: 10.7498/aps.62.054203
To investigate the energy scaling level of large-mode-area photonic crystal fiber-based dissipative soliton mode-locked fiber oscillators under limited pump power, a multipass cell is inserted in the cavity to lower the repetition rate of the system, and thus higher single energy level can be mapped under the same average power level. High energy mode-locked fiber lasers based on two spectral filters with different bandwidths are demonstrated both working in the all-normal dispersion regime at a repetition rate of 15.58 MHz. Employment of filters with FWHMs of 6nm and 12 nm can achieve stable mode-locked pulses with average powers of 3.73 W and 4.9 W, corresponding to single pulse energies as high as 239 nJ and 314 nJ, respectively. The FWHM durations of the dechirped pulses by a transmission grating pair can reach 56 fs and 75 fs, which can generate pulses with peak powers exceeding 3MW in both cases.
2013, 62 (5): 054204. doi: 10.7498/aps.62.054204
With a new scheme of effective roughness length for heterogeneous terrain, based on the atmospheric boundary layer Monin-Obukhov similarity theory as well as flux and mass conservation principles, the statistical features of effective roughness length and its sensitivity to atmospheric stratification stability and roughness step for three surface category case are investigated. The results show that the effective roughness length is greater than the area-weighted logarithmic average one and the effective drag coefficient is more than 10% greater than the average one in most cases. The effective roughness length is much more sensitive to the roughness step, though it is dependent on the atmospheric stratification stability, and the relative percentage of effective roughness length and the effective drag coefficient will be 4 times and 3 times, respectively, for the double roughness step case. Therefore, the area-weighted average roughness length should be replaced by the effective one when the surface heterogeneity is considered in numerical models, which can represent the integrated effect of heterogeneous terrain.
2013, 62 (5): 054205. doi: 10.7498/aps.62.054205
Disturbance by ambient magnetic field is an important factor, which leads to measurement error of the laser gyro. In order to reduce the magnetic sensitivity, the polarization eigenstate in ring cavity is analyzed with matrix perturbation method, considering factors of small nonplanarity, mirror anisotropy and stress birefringence, gain and so on. The main factors affecting the magnetic sensitivity of the laser gyro are discussed. Ellipticities of eigen modes in clockwise and anti-clockwise direction caused by nonplanarity are identical and both are proportional to magnetic sensitivity. The influence of stress birefringence on ellipticity is related to mirror position, propagating direction of eigenmodes and main axis of stress. Nonplanarity is zero when ellipticities of eigen modes in clockwise and anti-clockwise direction are equal without stress birefringence in the passive cavity. A lower cavity loss is better to reduce the magnetic sensitivity of the laser gyro. The minimum magnetic sensitivity is not identical with peak gain for the laser gyro. Large phase and amplitude anisotropies of mirrors are useful to reduce magnetic sensitivity and ellipticity. These findings are significant for the reduction of magnetic sensitivity in ring laser gyros.
2013, 62 (5): 054206. doi: 10.7498/aps.62.054206
In September 2011, we used 3 ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) instruments of different designs and operation protocols to measure tropospheric NO2 for about 20 days, at the Station of Atmospheric Comprehensive Observatory, Institute of Atmospheric Physics, Chinese Academy of Sciences (Xianghe 117.0N, 39.77E). All instruments are oriented to an azimuth angle of 270 (north), in a common wavelength range and with a set of cross sections for the inversion of NO2 slant column in visible and UV wavelength range respectively. Intercomparison of NO2 slant columns among three MAX-DOAS is introduced. The results obtained from the different instruments are in good accordance with each other, and the correlation coefficients are all higher than 0.95, but systematical errors exist. Daily average errors of three MAX-DOAS instruments are almost below 6%, showing that the instruments work steadily and the data are cogent. The UV results are smaller than those in the visible range, especially on the overcast days, related to the wavelength dependence of Rayleigh and Mie scattering. After the correction of systematical errors, there is better consistency among different results, which indicates that the three MAX-DOAS instruments have a capability to validate the atmospheric component products of satellite.
2013, 62 (5): 054207. doi: 10.7498/aps.62.054207
The ultra-short pulse equation in a metamaterial is solved by the extended F-expansion method. The new phenomena and characteristics of solitons, caused by the anomalous self-steepening effect and the second-order nonlinear dispersion in metamaterials, are discussed. The results show that the second-order nonlinear dispersion in the positive-index region may take the place of the linear dispersion to form the bright and dark solitons. Due to the switchable sign of the anomalous self-steepening effect in the positive-index and negative-index regions, the bright and dark solitons separately exist in the anomalous and normal dispersion regions under some specific conditions. The moving directions of the centers of bright and dark solitons can be controlled by the sign of the anomalous self-steepening effect or by the combination of the anomalous self-steepening effect and third-order linear dispersion.
2013, 62 (5): 054208. doi: 10.7498/aps.62.054208
Based on the analysis of the recording and reconstructing of hologram and the point spread function of pre-magnification digital micro-holographic system (PMDMHS), the performances of the six common PMDMHSs are compared in imaging resolution, imaging quality and conveniences, for the first time to our knowledge. The results show that the digital image-plane holography (DIPH) has the highest imaging resolution and the best imaging quality; the resolution of DIPH is independent of the photosensitive surface size of the recording device; this system can completely record the information about the object transmitted through the micro-objective (MO); it is not needed to consider the size of illuminated region of object. Moreover, the reconstruction process of DIPH is very simple. DIPH is an optimized digital holographic imaging system. The DIPH with equal curvature of reference wavefront and object wavefront is very conducive to phase unwrapping and phase aberration compensation; therefore DIPH is more suitable for phase microscopy. The experimental results demonstrate the validation of the theoretical analysis.
2013, 62 (5): 054301. doi: 10.7498/aps.62.054301
The acoustic energy distribution of high intensity focused ultrasound (HIFU) is influenced by the attenuation and the dispersion of the biological tissue. In this paper we modify the KZK equation according to the fractional wave equation, in order to accurately describe the sound field of HIFU. The theoretical and experimental studies of frequency dependences of attenuation and sound speed examine the validity of the fractional wave equation. Furthermore, the numerical simulation of HIFU field is performed using the modified KZK equation. The results demonstrate that the introduction of the fractional derivative equation could solve the problems of the attenuation and sound dispersion, leading to the accuracy improvement of HIFU therapy.
2013, 62 (5): 054501. doi: 10.7498/aps.62.054501
Using the Lagrange principle of dissipative system, the nonlinear dynamic equation of a relative rotation with combined harmonic excitation is established, which contains nonlinear stiffness and nonlinear damping. The stability and bifurcation characteristics of autonomous system are analyzed by constructing Lyapunov function. Bifurcation response equation of non-autonomous system under the combined harmonic excitation is obtained by the method of multiple scale. Finally, numerical method is employed to analyze the effects of external excitation, system damping and nonlinear stiffness on the process that the system enter into chaos motion via period-doubling bifurcation by bifurcation diagram, time domain waveform, phase trajectory and Poincaré map.
2013, 62 (5): 054701. doi: 10.7498/aps.62.054701
We design three types of groove structures which are arranged in closely-packedarry (space free), periodic and quasiperiodic orders. The drag reduction properties of these structures are studied by numerical simulations and experimental shear stress measurements. Particularly, the effect of groove arrangement on the drag reduction is elucidated. Based on both the numerical and experimental results, it is found that the quasiperiodic arrangement can obtain more effective drag reduction than the close-packed groove structure and periodic structure. The underlying mechanism of the drag reduction is analyzed by vortex redistribution caused by the groove structures. The high-speed flow can be modulated by the disturbance wave resulting from the quasi-periodic groove structure, forming stripe-like flow patterns arranged in quasiperiodic style. This restrains the formation of big vortex in both the spanwise and the streamwise directions, hence leading to substantial drag reduction. Furthermore, the modulation effect on the streamwise vortex is more remarkable than on spanwise vortex, suggesting that the modulation of streamwise vortex plays a more important role in the drag reduction.
COMPREHENSIVE SURVEY FOR THE FRONTIER DISCIPLINES
2013, 62 (5): 054201. doi: 10.7498/aps.62.054201
Cylindrical vector beams are spatially inhomogeneously polarized, whose intensity in the center is zero, and can produce special field components in the vicinity of focus when they are focused by an objective lens. In the case of optical system with high apodization factor, radial polarization can achieve tight focus by adapting pupil filtering and image restoration technology compared with linear and circular polarization. The properties of cylindrical vector beams are introduced. Based on electric dipole radiation model and vector diffraction theory, focal field properties for cylindrical vector beams focused by a high-NA objective lens are discussed. The method to achieve tight focus by cylindrical vector beams is presented. Furthermore, we consider it feasible for the super resolution laser polarized differential confocal microscopy by adapting the differential confocal microscopy, and put forward the prospective development.
2013, 62 (5): 056801. doi: 10.7498/aps.62.056801
After several decade developments the critical dimension of an integrated circuit will reach its limit value in the next 10-15 years, and the substitute materials been to be researched. Graphene has beed considered the most likely candidate, however, pristine graphene does not have a bandgap, a property that is essential for many application, including transistors. The two-dimensional layer of molybdenum disulfide (MoS2) has recently attracted much attention due to its excellent semiconductor property and potential applications in nanoelectronics. The device preparation, two-dimensional material research and property analysis of MoS2 are summarized and the trend for future research on large sigle-layer MoS2 crystal is presented.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
High power microwave breakdown in gas using the fluid model with non-equilibrium electron energy distribution function
2013, 62 (5): 055101. doi: 10.7498/aps.62.055101
The electron energy distribution function (EEDF) is usually assumed to be of the Maxwellian distribution in the fluid model in the simulation of high power microwave breakdown in gas. However, this assumption may lead to some large errors in the simulations. In this paper we compute the non-equilibrium EEDF via solving the Boltzmann equation directly, and incorporate it into the fluid model for argon breakdown. Numerical simulations show that the breakdown time obtained by the fluid model with the non-equilibrium EEDF accords well with the Particle-in-cell-Monte Carlo collision simulation result, while the Maxwellian EEDF has higher energy tail and results in faster breakdown time at low mean electron energy. Based on the non-equilibrium EEDF, the dependence of the breakdown threshold on the pressure predicted by the fluid model accord well with the argon breakdown experimental result.
2013, 62 (5): 055201. doi: 10.7498/aps.62.055201
In order to solve the interaction problem between the wavefront distortion correction and optical alignment in a high-power laser system, especially in a multi-pass amplification system, based on fresnel diffraction integral theory, the beam propagation model of 310 mm×310 mm aperture beam in four-pass amplifier is built by analyzing the properties of this system. The problem of excessive amount of calculation, caused by high resolution in traditional calculation, is solved by coordinate transformation method, Using this model, the differences of far-field accuracy among centroid method, circle fitting and pure phase matching filter in beam with wavefront distortion are discussed; what influence the wavefront correction has on focal spot position is analyzed. This article provides an important basis for optimizing collimation adjustment programs in four-pass amplifier.
Effect of wall secondary electron emission on the characteristics of double sheath near the dielectric wall in Hall thruster
2013, 62 (5): 055202. doi: 10.7498/aps.62.055202
To further reveal the characteristics of sheath near the dielectric wall in Hall thruster discharge channel, a one-dimensional fluid sheath model combined with the velocity distribution function of electron emitted from wall is used to study the influence of secondary electron emission yield (SEEy) σ on the characteristics of double sheath near wall. Analytic results show that because of the contribution of secondary electron flux to the density of sheath electron, the sheath presents single-layer positive ion sheath formation when σ is lower than a critical SEEy σdc, and also presents double-layers formation that joins with positive ion sheath and electron sheath when σ>σdc. However, when σ further increases to 0.999, the sheath presents the formation of three-layers that are alternated by positive ion sheath, electron sheath and positive ion sheath. Numerical results also indicate that with the increase of σ, the joining point between positive ion sheath and electron sheath moves away from wall, and the thickness of electron sheath increases obviously.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2013, 62 (5): 056101. doi: 10.7498/aps.62.056101
In this article. We report on the study of mosaic structures of different thick GaN films grown on sapphire (0001) by metalorganic chemical vapor deposition (MOCVD), using high resolution x-ray diffraction. The result from the symmetrical reflections show that the mosaic vertical and lateral correlation lengths that are calculated by two methods increase with film thickness increasing, and the vertical correlation lengths are close to the film thickness, and the same trend in the lateral correlation lengths derived from the reciprocal space maps. By the help of asymmetrical reflections and Williamson-Hall extrapolation method, the tilt and twist mosaic drop with thickness increasing at different rates. All this shows that the increase in thickness lads to the more uniform and neat grain arrangement and the higher-quality epitaxial wafers.
Molecular dynamics simulation of the critical and subcritical nuclei during solidification of nickel melt
2013, 62 (5): 056102. doi: 10.7498/aps.62.056102
The microstructures of nickel solidified at different cooling rates are studied by using molecular dynamics simulation and the critical condition for nickel melt to form ideal metallic glass is calculated. The simulation results show that the crystal structure is obtained after the nickel melt has been solidified at a cooling rate that is lower than 1011 K/s; while a mixture is composed of crystal structure and amorphous structure when the cooling rate is in a region from 1011 K/s to 1014.5 K/s. The solidified crystal of nickel is of fcc structure when the cooling rate is lower than 1010 K/s, while it changes into crystal structure composed of fcc and hcp when the cooling rate is between 1010 K/s and 1014.5 K/s. By analyzing the calculation and simulation results, it is determined that the critical cooling rate for nickel melt to form ideal metallic glass is 1014.5 K/s. Moreover, it is found that the structures of the subcritical nuclei (the cooling rate is higher than 1014.5 K/s), critical nuclei (the cooling rate is 1014.5 K/s), and the growing crystal (the cooling rate is lower than 1014.5 K/s) are the lamellar structures composed of fcc and hcp atoms, which indicates that the subcritical nuclei, critical nuclei and the growing crystal have the same structures.
2013, 62 (5): 056103. doi: 10.7498/aps.62.056103
Graphene nanoribbons (GNRs) are important nanomaterials. A carbon nanotube can be viewed as a GNR rolled into a seamless cylinder. By using the first-principles method based on the density-functional theory, the rolling deformation-dependent electronic characteristics of GNRs, including the band structure (particularly the bandgap), density of states (DOS), and transmission spectrum, are studied systematically. It is found that before all types of GNRs are rolled into carbon nanotubes, they are not sensitive to the rolling deformations, which means that for electronic structures and transport properties, GNRs have a very strong ability to resist the rolling deformations. After GNRs are rolled into nanotubes, zigzag-edge GNRs (ZGNRs) and armchair-edge GNRs (AGNRs) present distinct differences in property, ZGNRs almost maintain unchanged metallic behaviors or become quasi-metallic. But for AGNRs, their electronic characteristics experience large variations, and transformations occur between the quasi-metal and semiconductor with various bandgaps, which might be closely related to the periodical boundary conduction along the direction of tubular circumference of a carbon nanotube and variation of quantum confinement. These studies presented here are of significance for understanding the rolling effects on electronic characteristic and relationship of electronic characteristics between GNRs and carbon nanotubes (structure-property relationship).
2013, 62 (5): 056104. doi: 10.7498/aps.62.056104
Ag-doped ZnO nanorods with different morphologies and optical properties are synthesized by hydrothermal method on the DC magnetron-sputtered Al-doped ZnO (AZO) seed layers. The influences of the molar ratio of Ag ions to Zn ions (RAg/Zn) and the AZO seed layer on the structural and optical properties of the Ag-doped ZnO nanorods are carefully studied by using X-ray diffractometry, scanning electron microscopy, spectrophotometry, EDS spectrum, etc. The changes in the microstructure and optical property of Ag-doped ZnO nanorods are closely related to the change in the average head-face dimension induced by Ag doping as RAg/Zn increases, owing to the different relative proportions of Ag ions doped in ZnO nanorods resulting from the different particle sizes and densities of the seed layers. The photoluminescence intensity in the visible region for the ZnO nanorods growing on the 15 min-sputtered AZO is stronger than that of the ZnO nanorods growing on the 10 min-sputtered AZO seed layer at the same RAg/Zn, which results from the increased defects in ZnO. More point defects caused by Ag doping are produced as RAg/Zn increases, resulting in the broadening of PL envelope in the visible region. The microstructure of pure ZnO nanorod is related to the seed layer thickness-related degree of crystallinity and particle size.
2013, 62 (5): 056105. doi: 10.7498/aps.62.056105
By using first principles calculation based on density functional theory, band structures, densities of states and electron density differences for an ideal (6, 0) ZnO nanotube (ZnONT), Al doped, N doped and Al, N co-doped nanotubes are investigated. The calculated results reveal that the doped nitrogen atom results in the formation of acceptor level in the band gap of the ZnONT, which indicates that the doped nanotube has the characteristic of a p-type semiconductor. While the high locality of the acceptor level leads to a lower solubility for the doped nitrogen atoms, the acceptor level is broadened and shows delocalizing characteristics in nanotube with Al, N co-doped. This co-doping may be an efficient method of preparing p-type ZnONTs.
2013, 62 (5): 056401. doi: 10.7498/aps.62.056401
We perform ab initio calculations on two different transition mechanisms of the bcc-to-hcp phase transition in Fe under pressure distinguished by the occurrence of the metastable fcc intermediate phase on the transition path, that is, the bcc-hcp and the bcc-fcc-hcp. The calculated results indicate that the occurrence of the fcc intermediate state during the transition is energetically unfavorable, which is consistent with the recent in situ XRD experiments. The enthalpy barrier of the fcc-hcp increases with pressure increasing, which indicates that the pressure tends to impede the transformation from fcc to hcp phase in Fe. The details of the structural and magnetic behaviors of the intermediate states during the transition are investigated, which indicates that there are complex magnetism transitions during the phase transition. The physical origins of the influence of magnetism on the phase transition are discussed. Moreover, the origin of the occurrence and evolution of the fcc metastable structure during the transition in the MD simulations are also discussed.
2013, 62 (5): 056601. doi: 10.7498/aps.62.056601
The phase change material (PCM), which is based on straight-chain alkanes, has received more and more attention for thermal management. To explore the mechanism of the thermal property of PCM from the microscopic scale, the molecular model of the PCM which consists of n-dodecane with amorphous structure is established in this study. The molecular dynamics (MD) simulation is performed with periodic boundary conditions and COMPASS force field. The melting temperature of n-dodecane is also determined by differential scanning calorimeter (DSC). The results show that the deviation of the isobaric heat capacity of the n-dodecane based PCM by MD method from the perviously reported value is 6.5%. The deviation of the simulated melting temperature of the PCM from the value from the DSC analysis is 7.6%. The simulated thermal conductivity of the PCM shows a slightly decreasing trend with pressure increasing, in a range of 0.1–0.4 W·m-1·K-1.
2013, 62 (5): 056802. doi: 10.7498/aps.62.056802
Vanadium oxide films are prepared by Sol-Gel at different annealing temperatures. Their surface morphologies, valence states, electrical and optical properties are characterized by SEM, XRD, resistance meter, UV-Vis spectrometer and FTIR, respectively. Results reveal that the optimal temperature for producing V2O5 films by Sol-gel is 430 ℃, the organics in the films cannot be decomposed completely below 430 ℃ while the V-O bonds will be broken under a higher temperature (>430 ℃). The as-prepared vanadium pentoxide films exhibit higher TCR and larger light absorption, so that they are suitable to be used as bolometric materials for uncooled infrared detectors. The growth mechanism of vanadium oxide film prepared by Sol-Gel is also presented in this paper.
2013, 62 (5): 056803. doi: 10.7498/aps.62.056803
The growing and melting of crystal nuclei in liquid Cu are investigated by molecular dynamics simulation. The critical undercooling is proportional to the reciprocle of the nanoparticle radius. The Gibbs-Thomson coefficient of Cu is 1.12× 10-7 K·m. Then the crystal-melt interfacial free energy of Cu is 0.146 J/m2 estimated from the Gibbs-Thomson coefficient, and the Turnbull coefficient of Cu is 0.416. All the values by simulation are consistent with the experimental results of Turnbull.
2013, 62 (5): 056804. doi: 10.7498/aps.62.056804
The glow discharge polymer (GDP) films each with a thickness of about 5 m are deposited by low-pressure plasma polymer apparatus. The GDP films are heat-treated at different tempertures of 280, 300, 320 and 340 ℃ in Ar atmosphere. The influence of heat treatment on the structure of GDP film is characterized by FT-IR. The optical transparency and optical band of GDP film are investigated by UV-VIS spectrum. The results show that with temperature increasing, the relative content of CH3 decreases, while the relative content values of CH2 and CH increase. The H content in GDP film decreases. The optical band gap decreases, and the transmittance in a range of more than 600nm decreases too.
Acoustic phonon transport and thermal conductance in quantum waveguide with abrupt quantum junctions modulated with double T-shapedquantum structure
2013, 62 (5): 056805. doi: 10.7498/aps.62.056805
By using the scattering matrix method, the transmission coefficient and thermal conductance of acoustic phonon through a quantum waveguide with abrupt quantum junctions modulated with double T-shaped quantum structure at low temperatures are studied. The results show that at very low temperatures, the double T-shaped quantum structure can enhance low-temperature thermal conductance; contrarily, at higher temperatures, the double T-shaped quantum structure can reduce low-temperature thermal conductance. However, in the whole low-temperature region, the low-temperature thermal conductance can be enhanced by adding the narrowest width c in the scattering region. Moreover, it is found that both the transmission coefficient and thermal conductance can be adjusted by changing the structural parameters of the the scattering region.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2013, 62 (5): 057101. doi: 10.7498/aps.62.057101
In practical applications of graphene-based electronic devices, they may have some pore defects under energetic particle bombardment, or chemical corrosion, which will inevitably affect their electrical properties. These problems have recently aroused great concern and interest. In this paper, we systematically study the influence of shape (tripartite, tetragonal and hexagonal) of hole defect on the electrical property of zigzag graphene nanoribbon (ZGNR). The results show that the influence of the shape of the pore defects on the conductance and current characteristics of ZGNRs is significant, whicl may result from electron scattering for the different shapes of the poredefect boundary. In addition, due to defects in suspension adsorbed hydrogen or nitrogen atoms, caused by defects of the pore shape changes, it also affects the electrical properties of ZGNRs. This study will supply valuable theoretical guidances for graphene-based electronic device failure analysis and the design of the graphene pore structure.
Gas-sensing properties at room temperature for the sensors based on tungsten oxide thin films sputtered on n-type ordered porous silicon
2013, 62 (5): 057102. doi: 10.7498/aps.62.057102
n-type porous silicons are prepared by the electrochemical corrosion method, on which tungsten oxide thin films with different thickness values are sputtered using DC reactive magnetron sputtering. The structures of ordered porous silicons and tungsten oxide thin films are characterized using field emission scanning electron microscope, which show that the pores are pillared and ordered and the thin films cover the porous layer loosely with many pores open to ambient air. The X-ray diffraction characterization indicates that the lattice structure of tungsten oxide thin film is mainly triclinic polycrystalline. The gas-sensing properties at room temperature for both ordered porous silicon and composite structure are studied, which indicate that the latter is much more sensitive to nitrogen dioxide than the former. And there is a critical spurting time of WO3 thin film, which in our case is 10 min. The sensing mechanism of composite structure is discussed and the probable explanation for the improvement of sensitivity to NO2 is the formation of hetero-junctions between the ordered porous silicon layer and the WO3 thin film. In addition, there exists an inversion layer on the surface of the WO3 thin film, which causes the anomalous resistance to change during the gas sensing measurements.
2013, 62 (5): 057103. doi: 10.7498/aps.62.057103
Accumulation MOS capacitor is more linear than inversion MOS capacitor and is almost independent of the operation frequency. In this paper, we present first the formation mechanism of the plateau, observed in the C-V characteristic of the strained-Si NMOS capacitor, and then a physical model for strained-Si NMOS capacitor in accumulation region. The results from the model show to be in excellent agreement with the experimental data. The proposed model can provide valuable reference for the strained-Si device design, and is has been implemented in the software for extracting the parameter of strained-Si MOSFET.
2013, 62 (5): 057201. doi: 10.7498/aps.62.057201
By using the perturbation method, an effective nonlinear direct current (DC) and alternating current (AC) response of nonlinear composites with cylindrical coated inclusions randomly embedded in a host medium is studied under the action of an external electric field Ea=E0 + E1 sinωt+E3 sin3ωt with different amplitudes and frequencies. The local potentials of composite at all harmonics are given in the inclusion core, the coated layer and the host regions. All effective nonlinear responses of composites and the relationship between the effective nonlinear responses at all harmonics are also deduced for the cylindrical coated inclusions in a dilute limit.
2013, 62 (5): 057202. doi: 10.7498/aps.62.057202
Based on semiconductor physics and the essential structure of IGBT, the turn-off mechanism of IGBT is deeply discussed regarding the problem of turn-off time changing with voltage and current. The laws of turn-off time changing with voltage and current are deduced, i.e., the turn-off time increases with voltage increasing and decreases with current increasing. The physical mechanisms of the laws are found out. The simulation results and experimental results, demonstrate that the derived and the existing law are constant, thereby proving the correctness of the derived law. It is put forward that the law of IGBT turn-off time changing with current and voltage accords with the complex law of exponent and hyperbola. For further studying the IGBT turn-off mechanism and solving the engineering puzzles including the power electronic dead time setting, the present study is significant in theory and practical application.
2013, 62 (5): 057301. doi: 10.7498/aps.62.057301
The two lowest single-particle hole states in two vertically coupled quntum dots (CQDs) are investigated by using the six-band KP model. A bonding-antibonding ground-state transition is observed with interdot distance increasing. This result is counterintuitive, for the antibonding molecular ground state has never been observed in natural diatomic molecules. By comparing the wavafunction component of hole, we verify that the reordering of bonding and antibonding orbitals with interdot distance increasing is caused by spin-orbit interaction of holes.
2013, 62 (5): 057302. doi: 10.7498/aps.62.057302
Boron-nitride graphene-like monolayer possesses a similar atomic arrangement to that of the famous graphene. However, due to the large difference in electronegetivity between boron and nitrogen atoms, the electronic properties of the two nanomaterials are different significantly. Here, we report on our theoretical investigation of the electronic structure and spin-polarization of zigzag-edged boron-nitride triangular nanoflake using a Hubbard model and the first-principles calculations within density-functional theory. Our numerical results indicate that in contrast to graphene nanoflake with spin-polarized ground state, the boron-nitride nanoflak has the zero-energy state that is either empty or fully occupied, and its ground state is thus spin-unpolarized which breaks the Lieb's law. However, the electron occupation and spin-polarization of the zero-energy state of boron-nitride nanoflake can be tuned by doping it with electrons or holes. These results are expected to offer the theoretical basis for the applications of boron-nitride nanomaterials in spintronics.
Numerical study of the superconductor-insulator transition in double-layer graphene driven by disorder
2013, 62 (5): 057303. doi: 10.7498/aps.62.057303
The kernel polynomial method is employed to study the disorder effects of impurities on the superconductivity of double-layer graphene. The Bogoliubov-de-Gennes equations are solved self-consistently by the kernel polynomial method, and the spatial fluctuations of the superconducting order parameters caused by disorder are obtained. Furthermore, we calculate the density of states, the optical conductivity and the general inverse participation ratio, and we find that the energy gap in the density of states can be constrained by increasing disorder, accompanied with the disappearance of the Drude weight in optical conductivity. We also find that the electron states are Anderson localized by disorder and the superconductor-insulator transition happens in double-layer graphene.
2013, 62 (5): 057501. doi: 10.7498/aps.62.057501
The JA-SW hybrid model is extended in order to include the strain or stress anisotropy. With this improved model, a simulation is carried out to study the effects of stress on magnetic hysteresis loss and coercivity of ferromagnetic film. It is shown that the hysteresis loss and coercivity are related to the external field orientation, the stress intensity and the stress direction. Hysteresis loss, or the coercive force does not entirely monotonially increase with the change of stress intensity. For example, when the external field is parallel to the easy axis, a valley appears in the curve. In addition, stress will cause the peak offset of the coercivity curve as the orientation angle of the external field changes. Extensive comparisons of results with literature data are made and the differences between them are explained.
Micromagnetic simulation of magnetization reversal on the annular free layer with nick in magnetic random access memory
2013, 62 (5): 057502. doi: 10.7498/aps.62.057502
Based on magnetic tunnel junctions (MTJs), the magnetic random access memory with the pseudo-spin value film model, the annular structure with slanted cuts is used as free layer and the way to vary coercivity by changing thickness is discarded. With this improvement, the area resistance of the MTJs is reduced. The analysis of the cuts on the annular layer generated from the secondary effects of deposition in the IC process, is made by the micromagnetic simulations. The magnetization reversal characteristics from the analysis reveal the properties of low crosstalk, low RA, high magnetic reluctance, and strong anti-interference.
2013, 62 (5): 057701. doi: 10.7498/aps.62.057701
In this paper, the dielectric property of CaCu3Ti4O12 ceramic is measured by Novocontrol wide band dielectric spectrometer in a temperature range of -100-100 ℃ and frequency range of 0.1 Hz-10 MHz, and the corresponding dielectric relaxation mechanism is discussed. Firstly, on the basis of quantitative analysis of macroscopic shell-core structure, the possibility of colossal dielectric constant (CDC) originating from the surface insulated layer effect is rejected. Secondly, after the analysis of the nature of classical Maxwell-Wagner sandwich polarization and its activation energy, classical Maxwell-Wagner mechanism is also abandoned. Finally, a new model of trapped electron relaxation at the boundary of Schottky barrier is proposed. The new mechanism correctly reflects the essential connection between intrinsic point defects, conductivity and dielectric constant of CaCu3Ti4O12material.
2013, 62 (5): 057801. doi: 10.7498/aps.62.057801
The refractive index of the coating is one of the essential parameters used for principal strain separation in luminescent photoelastic coating (LPC) method by oblique incidence technique. According to the theoretical analysis of the amplitude of the emitted light exciting the coating, which returns along the same path as the oblique incident excitation light, we propose a new online method of measuring the refractive index of the LPC based on optical Fresnel response of the coating. The refractive index of the coating containing Rhodamine B as luminescent dye, is measured under the excitation at a wavelength of 465 nm and an incident angle of 60. The experimental result proves the feasibility of the new method, and the present method is also applicable to the refractive index measurement of other luminescent coating.
p-ZnO thin film/n-Si heterojunction light-emitting diode fabricated by chemical vapor deposition and its characterization
2013, 62 (5): 057802. doi: 10.7498/aps.62.057802
The Sb-doped ZnO film/n-Si heterojunction is synthesized by simple chemical vapor deposition method. The quality of crystal and surface morphology of Sb-doped ZnO film are improved after annealing at 800 ℃, which exhibits effective p-type conductivity with a hole concentration of 9.56× 1017 cm-3. The properties of the p-ZnO/n-Si heterojunction photoelectric device are investigated. The resuets show that this device has good rectifier characteristics with a positive open electric of 4.0 V, and a reverse breakdown voltage of 9.5 V. The electroluminescent is realized at room temperature under the condition of forward current 45 mA. These results also confirm that the high-quality ZnO film can be prepared by the simple chemical vapor deposition method, which opens the way for simple preparation of materials applied to ZnO based opto-electronic device.
First principles study of structures, static polarizabilities and optical absorption spectra of Cun-1Au (n=2–10) clusters
2013, 62 (5): 057803. doi: 10.7498/aps.62.057803
The structures of Cun-1Au clusters are examined using the genetic algorithm, and the static polarizabilities and optical absorption spectra are investigated by first principles computations within the static and time-dependent versions of the density functional theory. The static polarizabilities decrease after being doped by one Au atom due to the strengthened screening effect of d electrons, which can also be weakened by three-dimensional structures. The optical spectra computed within the time-dependent density functional theory indicate that the screening effect also leads to the quenching of oscillator strengths. A deeper analysis of d-orbit indicates d-orbit is the main contributor in the optical excitation while its growing up is not directly influenced by the strengthened screening effect. The research on Cu6-nAun (n=0–6) clusters in a fixed size system verifies our arguments further. Our calculation results are in good agreement with the experimental data on the optical absorption spectra, which are closer to the experimental data than the earlier theoretical results.
2013, 62 (5): 057804. doi: 10.7498/aps.62.057804
Coaxial nanoring structures have attracted extensive attention in recent years due to their peculiar optical properties. In this article, we investigate two different types of resonances in plasmonic Fabry-Pérot cavities, planar surface plasmon and propagating surface plasmon. Using nanoring arrays with the same periodicity and different gaps, we can tune propagating surface plasmons and finally filter individual colors out. With large periodicities, planar surface plasmon resonance can be fixed in the near infrared range to avoid any disturbance on propagating surface plasmon resonance which is located in visible frequencies. In this work, we filter a broadband white source into different colors by using nanoring arrays with a fixed periodicity of 1200 nm and varying gaps range from 10 nm to 180 nm (in steps of 10 nm). Compared with one-dimensional nanoslits or metal-insulator-metal (MIM) nanogratings, nanoring structures present polarization independence to the incident light, leading to more functional devices and broader applications (applicable to natural light, for instance). Finite-difference time-domain (FDTD) simulations accord well with measurements, which confirms our conclusions and supports our explanations.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2013, 62 (5): 058101. doi: 10.7498/aps.62.058101
As a new group-IV semiconductor alloy, Ge1-xSnx is a very promising material for applications in photonic and microelectronic devices. In this work, high-quality germanium-tin (Ge1-xSnx) alloys are grown on Ge(001) substrates by molecular beam epitaxy, with x=1.5%, 2.4%, 2.8%, 5.3%, and 14%. The Ge1-xSnx alloys are characterized by high resolution X-ray diffraction (HR-XRD), Rutherford backscattering spectra (RBS), and transmission electron micrograph (TEM). For the samples with Sn composition x 5.3%, the Ge1-xSnx alloys each exhibit a very high crystalline quality. The ratio of channel yield to random yield (min) in the RBS spectrum is only about 5%, and the full width at half maximum (FWHM) of the Ge1-xSnx peak in HR-XRD curve is 100''. For the sample with x=14%, the crystalline quality of the alloy is degraded and FWHM is 264.6''.
2013, 62 (5): 058102. doi: 10.7498/aps.62.058102
The diamond detector is used to diagnose X-ray and has the advantages of fast response, high sensitivity, large dynamic range, flat response, high breakdown field, and outstanding radiation hardness. The X-ray diamond detector with a metal-diamond-metal structur is developed, in which the optical grade diamond is synthesized by chemical vapor deposition (CVD) method. Through the calibration of response time on the pulse laser of 8 ps width, the full width at half maximum of the CVD diamond detector is 444 ps, the rise time of the detector is 175 ps, and the lifetime of the carrier is 285 ps. The detectors are used to measure the hard X-ray for the imploding experiment on SGIII-prototype equipment. The results indicate that the measured hard X-ray flux is produced mainly by laser and target imploding, and the peak signal is in direct proportion to the laser energy and in inverse proportion to the CH thickness of target.
2013, 62 (5): 058103. doi: 10.7498/aps.62.058103
A circuit analog absorber (CA) with wire medium is presented. Both the original CA and the structure presented are analyzed. It is shown that original CA cannot have more than one resonant frequency in low frequency band, owing to the limitation by the effective resistance. The presented structure breaks the limitation using artificial plasma resonance, with the wave dispersion in the structure analyzed using the quasi-static model. The experimental results, consistent with simulation results based on the finite-difference time-domain (FDTD) method, show that the structure obtains dual-band absorption, thus broadening the absorption band in low frequency band.
2013, 62 (5): 058104. doi: 10.7498/aps.62.058104
The variations of the main positive ion components and their energies for the plasma of H2/C4H8 gas mixture under different working pressures are investigated using the glow discharge technique and plasma mass spectrometry diagnostic method, and the effects of work pressure on both the dissociation mechanism of the mixed gas H2/C4H8 and the forming process of the main positive are analyzed. The results show that both the intensity and energy of the C-H segment ions in H2/C4H8 plasma decrease with pressure increasing. The relative concentration of m/e 41(C3H5+) reaches a maximum when work pressure is 5 Pa. And when the pressure is 10 Pa, the relative concentration of m/e 39(C3H3+) is largest; when the pressures are 15 Pa and 20 Pa, the relative concentration of m/e 29 (C2H5+) is highest; when the pressure is 25 Pa, the relative concentration of m/e 57(C4H9+) is biggest. The component and its energy distribution of H2/C4H8 plasma are qualitatively analyzed. The results will serve as a reference to the optimization of parameters for glow plasma polymer coatings.
2013, 62 (5): 058105. doi: 10.7498/aps.62.058105
Different growth orientations influence the mechanical properties and behavior of quantum dots, due to the anisotropy of elasticity and surface energy of the material. In this paper, the relations of the strain energy, strain relaxation energy and free energy to growth orientation are analyzed for the self-assembled InAs/GaAs semiconductor quantum dots, based on finite element method of cubic elasticity theory. The results show that the strain relaxation of the (211) quantum dots is biggest, and that of the (100) quantum dots is smallest. These can provide the theoretical basis for the growth of quantum dots in a controlled fashion.
2013, 62 (5): 058201. doi: 10.7498/aps.62.058201
Spiral dynamics induced by spatiotemporal modulation is investigated in the generic excitable Barkley model. The coexistence of meandering spiral wave and travelling spiral wave in the same medium is discovered under proper spatiotemporal modulation. The underlying mechanism behind this coexistence phenomenon is analyzed. And the two necessary conditions which are needed to observe the coexistence of meandering spiral wave and travelling spiral wave in the excitable Barkley model are discussed.
2013, 62 (5): 058401. doi: 10.7498/aps.62.058401
Based on far-field time reversal, the super-resolution focusing property of the subwavelength array is analyzed theoretically according to signal processing in this paper. A novel subwavelength array is proposed. It is demonstrated experimentally that the array has super-resolution focusing property in the far-field zone and each element has an independent channel. With this kind of subwavelength array used in time reversal mirror, better super-resolution focusing result is achieved. The proposed subwavelength array may be useful in the applications of multi-antenna ultra-wideband (UWB) wireless communications.
Influences of the multiple electron overtaking on the bunching process of the wide-gap klystron amplifier under high power injection condition
2013, 62 (5): 058402. doi: 10.7498/aps.62.058402
Under the high power injection condition of the wide-gap klystron amplifier (WKA), a second peak current appears besides the conventional optimally bunching current (or the first peak current) in the distribution of the fundamental integral current. Considering the electron bunching theory and the Particle-in-cell (PIC) simulation results together, the formation mechanism of the second peak current is discussed. The results indicate that the second peak current has a close relation with the electron multiple-overtaking phenomenon in the case of high voltage modulation coefficient. When the diode voltage is 600 kV, beam current is 5 kA and working frequency is 3.6 GHz, the beam modulation depth of the WKA is enhanced to 92% from 80% according to the multiple-overtaking mechanism. Simultaneously, the bunching current power is improved from 2.2 GW to 2.8 GW, and a 27 percent increment is obtained.
2013, 62 (5): 058403. doi: 10.7498/aps.62.058403
In order to increase the filling factor of conductor in superconducting coils and improve the mechanical stability of superconducting magnet, the pre-tension is always applied to the conductor during winding the coils. Because the winding pre-tension has a great effect on the quench and degradation performance of superconducting magnet, it is necessary to analyze the mechanical stress caused by the fabrication. First, the winding physical process of conductor is analyzed. Then the theoretical model is developed to calculate the winding stress of superconducting coils based on some reasonable assumptions and approximations. And some formulas used for stress and strain are derived from the theory of elastic mechanics. Two kinds of superconducting coils (one consists of one type of wire, and the other one consists of two types of wires.) are researched according to the model. The effects of winding pre-stress and material anisotropy on radial stress and hoop stress in superconducting coils are also analyzed. On the basis of the analyzed results, one can further research the stress and strain of superconducting coils under the effect of multiphysics and give some theoretical suggestions for the design and construction of superconducting coils.
Electronic conductivity effective masses along arbitrary directional channel in uniaxial strained Si(001)
2013, 62 (5): 058501. doi: 10.7498/aps.62.058501
Electronic conductivity effective mass is one of the key parameters studing electron mobility enhancement in unixial strained Si material. Its in-depth study has the significant theoretical and practical values. In this paper, we first establish the E-k relation for conduction band in a unixial strained Si material. And the model of electronic conductivity effective mass along an arbitrary directional channel in the uniaxial strained Si (001) is obtained. Our concluding results are described as follows. 1) Tensile stress should be used to enhance electron mobility for unixial trained Si. 2) In the case of tensile stress application, both /(001) and /(001) directions are the desirable ones from the evaluation of electronic conductivity effective mass. And /(001) direction should be preferable when the density of state effective mass is taken into consideration. 3) If  direction becomes the channel direction under /(001) uniaxial strain, the further electron mobility enhancement will occur. The results above can provide valuable reference for the conduction channel design related to stress and orientation in the strained Si nMOS device.
2013, 62 (5): 058502. doi: 10.7498/aps.62.058502
During serving in orbit, spacecraft will be affected by the radiation environment of the space high-energy charged particles, leading to the performance degradation or even malfunctions of electronic components. The complementary metal oxide semiconductor (CMOS) devices are sensitive to ionization damage. Therefore, it is valuable to research the mechanism of radiation effects on CMOS devices, and is significant to engineering and theory. The CC4013 CMOS integrated circuits are irradiated with 60 MeV Br ions, 5 MeV protons and 1 MeV electrons. Based on the data calculated by Geant4 code, the ionizing absorbed dose induced by 60 MeV Br ions is greatest, and the ionizing absorbed dose induced by 1 MeV electrons is lowest. The degradation of CC4013 device during the irradiation test is in-situ measured with Keithley 4200-SCS semiconductor characteristic system. From the experimental results, the threshold voltage degradation in CC4013 under an exposure of 1 MeV electrons is greatest at the same dose, a little lower under 5 MeV protons, and lowest under 60 MeV Br ions.
2013, 62 (5): 058701. doi: 10.7498/aps.62.058701
The formation of target wave in the network of Hodgkin-Huxley neuron with four variables is investigated by inputing a distribuled current. A stimulus current (I1) is input in to a local square area, and another stimulus current (I2) is input into the nodes of the network thus the distributed current is depicted. The development of target wave is measured by changing the coupling intensity, the size (number of the controlled neurons) of the local area into which current I1 is input, the gradient current (I =I1-I2). It is found that higher gradient current (I) is necessary to induce target wave when local area into which current I1 is input is smaller in size and the coupling intensity is higher in value. Finally, the potential mechanisms of the stimulus current and target wave formation are discussed in brief. Eextensive numerical results confirm that the developed target wave is robust to a certain channel noise.
Photothermal response of extracellular solution to the near-infrared laser irradiation determined by its optical absorption properties
2013, 62 (5): 058702. doi: 10.7498/aps.62.058702
Photothermal effect has been proved to mediate the interaction of near-infrared laser with biological tissue. However, the generation and transformation mechanism of the photothermal effect is still unclear. In this paper, we combine a patch clamp technique with the laser simulation to figure out the chromophores, which are responsible for the photothermal effect generation. This method is based on the fact that temperature dependence of solution can be measured as resistance changes. A dual-wavelength infrared light irradiating the open pipette in extracellular solution is designed to study the relation between the photothermal effect and the absorption property of solution. The principle is based on that the nearly ten times difference in the magnitude of the optical absorption coefficient in water (0.502 cm-1 at 980 nm and 0.0378 cm-1 at 845 nm), makes the corresponding proportional absorption-driven temperature rise. The photothermal effect in laser-tissue interaction can be assessed in two stages: the establishment and the dissipation of the temperature rise. In the establishment stage, an open pipette method is employed to measure the temperature rise by fabricating a glass pipette which is filled with electrolyte solution. In the dissipation stage, the electrophysiological function of a living neuron cell is studied based on a patch clamp. Theoretical calculation and experimental results show that the optical absorption properties of solution determine the photothermal effect. The results can be used to study the photothermal effect in laser-tissue interaction.
2013, 62 (5): 058801. doi: 10.7498/aps.62.058801
Highly conductive p-type microcrystalline silicon thin layer is inserted between the front layer (ZnO:B) and the window layer(p-a-SiC) in a p-i-n amorphous silicon solar cell, and the inserted layer is found to be able to eliminate the non-ohmic contact, which is caused by the difference in the work function between the ZnO:B and p-a-SiC. The properties of the p-type microcrystalline silicon are studied by varying layer thickness, hydrogen dilution ratio and B2H6/SiH4 ratio. The optimized p-type microcrystalline silicon film can have a dark conductivity as large as 4.2 S/cm at a thickeness of 20 nm. The p-i-n type amorphous silicon solar cell with the p-type microcrystalline silicon is shown to have a good open circuit voltage and fill factor compared with without the p-type microcrystalline silicon layer.
2013, 62 (5): 058901. doi: 10.7498/aps.62.058901
Previous research has shown that the community structure of the network well significantly affect information transmission, and the obvious community structure will significantly reduce the network transmission performance. To address the problem, first we define the link importance to communities, which is based on the spectrum of network adjacency matrix. Then we propose a topological management strategy called community weaken control strategy (CWCS) to enhance traffic capacity, which weakens the community structures by logically closing or cutting some links with great link importance. We implement the scheme in both a global shortest-path routing strategy and local routing strategy, and compare it with the previous scheme HDF that removes the links among hub nodes. The simulation results show that the traffic capacity can be greatly enhanced and the average transport time is effectively reduced under the shortest path routing strategy. Under the local routing strategy, the traffic capacity can also be greatly enhanced when the tunable parameter lies in a range from 0 and 2.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2013, 62 (5): 059201. doi: 10.7498/aps.62.059201
By analyzing the fractal feature of 120 cloud-to-ground lightning signals and 77 intracloud lightning signals obtained by the fast antenna system in Qinghai area during the summer of 2009, the results show fractal dimension of cloud-to-ground lightning signal is obviously different from that of intracloud lightning signal. Then 5 characteristic values of fractal dimension are used to recognize the discharge types of lightning signal via support vector machine, and the recognition rate is higher than 95%. The construction of cloud-to-ground lightning time series signal fractal dimension trajectory map shows that the fractal dimension minimum value corresponds to the return stroke of the original time series signal, which can be used to quickly and accurately detect the return stroke of lightning signal, and the detection rate can reach 100%.The fractal dimension is a discriminatively physical property which can be used for intelligently analyzing and automatically processing the lightning signal.
2013, 62 (5): 059202. doi: 10.7498/aps.62.059202
In recent years, critical slowing down phenomenon has shown great potentials in disclosing whether a complex dynamic tends toward critical cataclysm. Based on the concept of critical slowing down, the observed data of temperature in different regions in China which have different noises are processed in this article to study the precursory signal of abrupt climate change. First, Mann-Kendall(M-K)method is used to find the locations of the abrupt climate change in different regions, then the autocorrelation coefficient which can characterize critical slowing down is calculated; the appearance-time moments of early warning signals of abrupt climate change under the influence of different noises are also stadied. The results show that for different signal-to-noise ratios, the critical slowing down phenomenon has appeared in the data 5-10 years before the abrupt climate change took place, which indicateds that critical slowing down phenomenon is a possible early warning signal for abrupt climate change and the noise has less influence on the test results for early warning signals of abrupt climate change. Accordingly, it demonstrates the reliability of critical slowing down phenomenon to test the precursory signals of abrupt climate change, which provideds an experimental basis for the wide applications of the present method in real observation data.
Influence of radiation electron energy on deep dielectric charging characteristics of low density polyethylene
2013, 62 (5): 059401. doi: 10.7498/aps.62.059401
The interactions between high-energy charged particles and spacecraft insulating materials can cause deep dielectric charging and discharging, leading to spacecraft anomalies. In this paper, we establish a unipolar charge transport physical model of deep dielectric charging, according to the charge distribution and energy deposition of incident electrons and nonlinear dark conductivity and radiation induced conductivity (RIC) of material. Under the irradiation of electrons with different energies (from 0.1 to 0.5 MeV), the charge transport process of low density polyethylene (LDPE) can be obtained through solving the charge continuity equation and Poisson's equation. The calculation results show that the maximum electric field decreases with the increase of radiation electron energy. When radiation electron energy is less than 0.3 MeV, the distribution of the maximum electric field is similar to the change of the electron beam density. When the electron beam density is more than 3×10-9 A/m2, the maximum electric field will be greater than breakdown threshold (about 2×107 V/m), and it has higher risk of electrostatic discharge (ESD). With the increase of incident electron energy, the critical electron beam density will increase. When the radiation electron energy is 0.4 MeV, the critical electron beam density is 6×10-8 A/m2. When the radiation electron energy is more than 0.5 MeV, it seems that no electrostatic discharge (ESD) will occur in a range from 10-9 to 10-6 A/m2. The physical model has the great significance for further studying deep dielectric charging, evaluating the charged degree of spacecraft in space environment and designing protection devices.
2013, 62 (5): 059701. doi: 10.7498/aps.62.059701
Radiative pressure is an important physical factor and can affect the structure and evolution of the massive star. The Roche lobe, three Lagrangian points and the corresponding Roche potentials are calculated according to the asynchronous rotational Roche potential which includes the radiative pressure. They are compared with the corresponding synchronous rotational Roche potentials. It is found that the centrifugal force greatly reduces the gravitational acceleration at the equator while the radiative pressure can reduce the gravitational acceleration of the massive star. Both the asynchronous rotation and the radiative pressure have an obvious influence on the Roche lobe, the positions of three Lagrangian points, the Roche potentials and the time of mass overflow. Therefore, it is very important to calculate the asynchronous rotational Roche potential which includes the radiative pressure in the massive close binaries.