Vol. 63, No. 24 (2014)
2014-12-20
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2014, 63 (24): 248105.
doi: 10.7498/aps.63.248105
Abstract +
In this paper, the in-situ membrane chips with the electrodes are fabricated with the micro-chip technique. Using a home-made in-situ holder, the fine lithography on the InAs nanowires is demonstrated by the focused electron beam at low temperature in a transmission electron microscope. It is found that the conductance of the nanowires decreases linearly with the cross section area decreasing from bigger than 10000 nm2 down to 800 nm2 by lithography. With this lithography at low temperature, a 10 nm nano-dot is fabricated on an InAs nanowire, and its electrical properties are measured at 77 and 300 K. The coulomb blockade effect is observed at 77 K due to the electron tunneling, while this phenomenon disappears at 300 K due to the stronger thermal fluctuation.
2014, 63 (24): 248503.
doi: 10.7498/aps.63.248503
Abstract +
In this paper we establish a three-dimensional (3D) numerical simulation model of domestic SiGe heterojunction bipolar transistor (SiGe HBT) by using technology computer aided design tools, to study the bias effect on single event effect (SEE) of SiGe HBT. The response relationship between SEE and the bias of SiGe HBT is identified based on the analyses of transient current peak and charge collection of each terminal. The results show that the worst biases for SEE are different for different terminals. Even for the same terminal, the worst biases for charge collection and transient current peak are different. This phenomenon is caused mainly by the influence of applied electric field and the change of carrier transport mode.
2014, 63 (24): 248101.
doi: 10.7498/aps.63.248101
Abstract +
The influence of La on the Cu precipitation in bcc-Fe is determined by first-principles density functional calculations. The binding energies of La-vacancy, Cu-vacancy pairs, and La-Cu pair are calculated, and the effects of La atoms on the diffusion activation energy of Cu atoms in bcc-Fe are considered. It is found that there exist strong attractive interactions between La atom and vacancy and between La atom and adjacent Cu atom. In addition, the formation energy of the vacancy adjacent to Cu atom increases significantly with the La addition, suggesting that vacancy is difficult to form in the La and Cu segregation zone. Meantime, we find that the migration energy of Cu atom is enhanced due to the attractions of La atom to adjacent vacancy and Cu atom. The calculated results of the vacancy formation energy and migration energy indicate that Cu atom possesses a higher diffusion activation energy after the La addition, and in turn, delays the segregation and precipitation process of the Cu rich phase in bcc-Fe.
2014, 63 (24): 248502.
doi: 10.7498/aps.63.248502
Abstract +
In this paper, a novel symmetrical double-gate strained Si single halo metal-oxide semiconductor field effect transistor with gate stack dielectric is proposed. The two-dimensional Poisson's equation is solved under suitable boundary condition by applying the parabolic potential approximation. This analytical model for the surface potential and the threshold voltage is derived. The strained Si channel is divided into two different doping regions, and the surface potential along the channel, compared with the normal double-gate device (uniform doping channel), exhibits a stepped potential variation, which can increase carrier transport speed. The influence of drain-source voltage on short channel effects (SCEs) is discussed. it is shown that threshold voltage decreases with Ge mole fraction increasing in butter layer, increases with the increase of the high-k layer dielectric permittivity of gate stack, and increases with the increase of doping concentration in the channel near the source, of which the physical mechanisms are analyzed and explained. Results show that the novel device can suppress threshold voltage drift and SCEs, which provides the basic guidance for designing the CMOS-based devices in nanometer scale.
2014, 63 (24): 248504.
doi: 10.7498/aps.63.248504
Abstract +
For an electromagnetically levitated (EML) molten droplet, there usually exist some unstable factors, such as internal fluid convection, quick spin and horizontal displacement and so on. As a result, stabilizing the droplet is very important for EML technology. In this paper, a horizontal static magnetic field is imposed on an EML Cu droplet through a U-shaped static magnetic component. The shape oscillation of a Cu droplet is recorded continuously under different magnetic field intensities using a high speed camera. The effects of static magnetic field on the oscillation frequency, amplitude and spin angle of the droplet are analyzed from the recorded data of droplet shape. The result shows that when the strength of the static magnetic field exceeds 0.3 T the solid Cu is levitated statically without any spin and horizontal movement. For molten Cu droplet, its amplitudes of the R-, A and Dmax are reduced by 25%, 76% and 60% respectively when a static magnetic field with 0.15 T is imposed. With the increase of magnetic field strength the amplitude and frequency of oscillation decease continuously. However, when the intensity of the static magnetic field is 0.3 T, its frequency is 1 Hz higher than that when the intensity of the static magnetic field is 0.2 T. Finally, the result indicates that the horizontal static magnetic field can inhibit the spin of the levitated droplets. For instance, when the strength of the magnetic field is 0.53 T the droplet spins are within a very narrow angle of 10°, which is quite smaller than in the case without static magnetic field. These results exhibit that the imposed horizontal static magnetic field can effectively improve the stability of electromagnetic levitated droplet.
2014, 63 (24): 248701.
doi: 10.7498/aps.63.248701
Abstract +
Neuronal firing activity can be changed from the resting state to firing state either through Hopf bifurcation where the firing exhibits a fixed period or through saddle-node bifurcation where the firing frequency is nearly zero. Phase noise with periodicity can induce coherence resonances near Hopf and saddle-node bifurcation points. When the period of phase noise is shorter than the internal period of firing near the Hopf bifurcation point, the phase noise can induce single coherence resonance appearing near the frequency of the phase noise. When the period of phase noise is longer than the internal period of firing near the Hopf bifurcation point, the phase noise can induce double coherence resonances. The resonance at low noise intensity appears near the frequency of the phase noise, and the one at large noise intensity occurs near the frequency of the firing near the Hopf bifurcation. The mechanism of the double resonances is explained. Unlike the Hopf bifurcation point, only a single coherence resonance can be induced near the saddle-node bifurcation point by the phase noise with long or short periods. The results not only reveal the dynamics of phase noise induced coherence resonance of the equilibrium point and identify the distinction between two types of neuronal excitabilities corresponding to two kinds of bifurcations, but also provide an explanation about the different results of phase noise induced single or double resonances simulated in recent studies.
2014, 63 (24): 248102.
doi: 10.7498/aps.63.248102
Abstract +
Two popular interpolation formulas of calculating InGaAsSb quaternary alloy band gap energy are discussed, and the calculation results from them are presented and compared. It is found, after the two formulas have been converted into equivalent formulas in the same forms, that in them there is taken into consideration only the influence of bowing parameter in the Γ valley. In this paper, the effect of the spin-orbit splitting on the valence band is considered, and a new method of calculating the InGaAsSb band gap is proposed by introducing the bowing parameter of spin-orbit splitting. The results show that the introduction of the bowing parameter of spin-orbit splitting can improve the accuracy of the calculation results compared with the above two methods. When the fraction of In is less than 0.72, the calculation obtained from our method is most accurate.
EDITOR'S SUGGESTION
2014, 63 (24): 248104.
doi: 10.7498/aps.63.248104
Abstract +
The large single crystal diamonds are successfully synthesized in a NiMnCo-C system with the zinc additive in a series of the experiments at temperatures of 1270-1400 ℃ and pressures of 6.2-6.4 GPa by the temperature gradient growth. Morphology and structural properties of the synthesized diamond are characterized by optical microscope and scanning electron microscopy. The Raman spectrum is used to investigate the crystallization of synthesized diamond. The results show that the colors of synthetic diamond crystals change from yellow to light yellow and nearly disappears with the increase of the zinc additive. There are a large number of irregular pits in the surface of diamond crystal when the zinc additive is increased up to 3.0 wt.%. the Fourier transform infrared spectroscopy spectra reveal that the nitrogen impurity in the synthetic diamond crystal is predominantly in the form of C center (single substitutional nitrogen atoms), and the nitrogen concentration decreases with the increase of zinc additive. Two possibilities that the zinc powders can be used as the nitrogen getter are given. the Raman spectrum shows that the diamond crystallization can be improved when the zinc additive is less than 3.0 wt.%. We believe that our work is greatly helpful for deeply understanding the natural diamond genesis, enriching the types of diamonds, and expanding the application areas of synthetic diamond.
2014, 63 (24): 248106.
doi: 10.7498/aps.63.248106
Abstract +
Negative thermal expansion materials Sc2(MO4)3 (M={W}, Mo) are synthesized with a CO2 laser. It is shown that the synthesis of Sc2(WO4)3 or Sc2(MoO4)3 by laser sintering is a rapid process so that a sample can be synthesized within a few or tens of seconds and has the characteristic of rapid solidification. X-ray diffraction and Raman spectrum analyses demonstrate that the synthesized Sc2(MO4)3 (M={W}, Mo) are crystallized into orthorhombic structures and each have a high purity. Temperature dependent Raman spectrum analysis suggests that the synthesized samples do not have phase transitions above room temperature but possibly have weak hygroscopicities. According to the Raman analyses of MoO3, WO3, Sc2(MoO4)3, and Sc2(WO4)3, we draw a diagram describing their phonon energy levels and the photon energy of the laser, and then assess the mechanism of the synthesis by laser sintering. Transferring the laser photon energy to phonon energy is the channel of light-heat converting. The materials react in the molten pool and are solidified rapidly, forming the final products.
2014, 63 (24): 248107.
doi: 10.7498/aps.63.248107
Abstract +
Since LuTaO4 is a scintillator host with the highest density, it is important to investigate the structure and phase transition for its single crystal preparation. The polycrystalline formed by the mixture of Lu2O3:Ta2O5 with the molar ratio 1:1 is prepared by solid state reaction method at different temperatures. The phase transitions and structures of the polycrystalline powders are investigated by X-ray diffraction and Rietveld refinement. The results show that the polycrystalline has a single phase M'-LuTaO4 when sample is prepared at 1740 ℃, it presents a mixture phase of M'-LuTaO4 and M-LuTaO4 at 1800 ℃, and it displays a single phase M-LuTaO4 at 1840 ℃. The sample is melted when the calcined temperature is 2058 ℃, the melt is quenched and the polycrystalline is the mixture of M-LuTaO4, Lu3TaO7 and Ta2O5, whose structural parameters, including the lattice parameters, atomic fraction coordinates, etc. are obtained by Rietveld refinement to their X-ray diffraction pattern, and the results show their weight ratios are 78.1%, 18.9% and 3.0%, respectively. These results are valuable for the single crystal growth of the heavy scintillators with the host LuTaO4.
2014, 63 (24): 248201.
doi: 10.7498/aps.63.248201
Abstract +
The profile surface simulation is an important method to study the ion etching mechanism. In profile surface simulation, the result of surface evolution is primarily determined by the surface evolution model and the etching yield optimization model as well. However, the currently available surface evolution model is not accurate enough. What's more, most of the data used in etching yield optimization model are based on simulation, while no factual data are used to optimize the parameters of ion etching yield model. In order to solve these problems, the accuracy of current evolution model is improved, the optimal objects of etching yield model are redefined, and the factual etching data are introduced to optimize the etching yield model for the first time. In this paper, parallel method is also adopted to speed up the optimization process, whose optimized parameters are then applied to the etching simulation process that is based on cellular automata. The experimental results show that our proposed approach does improve the accuracy of simulation and greatly shorten the optimization process.
2014, 63 (24): 248401.
doi: 10.7498/aps.63.248401
Abstract +
Pulse train (PT) control technique is a discrete, nonlinear control technique for switching converter which benefits from simple design and ultra fast transient response. As an output-power-control-based control technique, a low-frequency oscillation phenomenon occurs in voltage-mode PT controlled switching converter operating in continuous conduction mode. This phenomenon will seriously affect the steady and transient performances of switching converter. In this paper, a current-mode PT (CM-PT) controlled switching converter is studied. The normally working region is studied and the parameter conditions are estimated. Targeting the problem of CM-PT control technique, a modified control technique is proposed.
2014, 63 (24): 248403.
doi: 10.7498/aps.63.248403
Abstract +
A series of poly(3-hexylthiophene) (P3HT)/indene-C60 bisadduct (ICBA) bulk heterojunction polymer solar cells is fabricated with different cholesteric liquid crystalline (LC)-cholesteryl chloride doping concentrations. The open circuit voltage (Voc) is increased from 0.78 V to 0.65 V, meanwhile the short circuit current density (Jsc) is reduced from 4.23 mA·cm-2 to 4.10 mA·cm-2 when the LC doping concentration is 5%, and fill factor is almost unchanged. The optimized power conversion efficiency of polymer solar cells (PSCs) with LC doping is about 1.51%. The crystallinity of the active layer and the light absorption characteristics are analyzed by X-ray diffraction, photoluminescence, UV-visible absorption spectroscopy and atomic force microscope, respectively.
2014, 63 (24): 248501.
doi: 10.7498/aps.63.248501
Abstract +
The structure of the vertical multijunction detector is introduced, and the result and technologcial conditions of preparing junction by thermomigration method are presented. In addition, the insulation line method of processing electrode wire is specially introduced; the problem of photoetching electrode after thermomigration is solved; the effects of technologcial conditions on device performance are analyzed. Besides, the difficulty in connecting all P regions is solved, and the aim of zero shade for sensitive regions is achieved. Specially, in order to realize integrating multicell device, a method of widening the size of electrode wire is described. Several important parameters are described, and the math model for the relationship of the X-ray intensity with both photovoltage and photocurrent is set up. At the same time, we introduce the measurement relation between the output voltage of the device and X-ray intensity, which shows that the measurement data and theoretical results are consistent. Finally, we analyze the measurement data, and demonstrate that the device has good enough sensibility and resolution.
2014, 63 (24): 248801.
doi: 10.7498/aps.63.248801
Abstract +
The synergistic catalytic mechanism of anion, cation ions in TiF3, TiCl3 catalysts for LiBH4 has been studied by first-principles method based on density functional theory. According to the results, Ti metal doping in LiBH4 is not easy realized with respect to halogen elements. Co-doping with transition metal and elements in halogen family is achieved easier than doping with Ti alone. For TiF3 catalyst, to achieve doping with one kind of element is helpful to doping with another kind of element, which accordingly results in the increase of doping concentration. Based on the analysis of the electronic structure, we find that doping with halogen element alone can reduce the stability of LiBH4; while doping with Ti alone leads to the rise of Fermi level; the introduction of defect energy level and the weakening of B-H bond; these may be responsible for improving greatly the desorption kinetics of LiBH4 by titanium halide catalysts. The improvement of the dehydrogenating kinetics of LiBH4 with titanium halide catalyst additives is mainly due to the B-H bond weakening, which makes H atom diffuse easily. For TiF3, TiCl3 catalysts, in the reversible desorption process of LiBH4, F and Ti have synergistic action for the B-H bond weakening, but the synergistic action of Cl and Ti is not obvious, this may be the reason for the advantage of TiF3 over TiCl3 in LiBH4 catalytic reaction.
2014, 63 (24): 248901.
doi: 10.7498/aps.63.248901
Abstract +
In this paper, we propose a local routing strategy in a complex network, use the node contraction method to evaluate the node importance. The probability of the node transmit packet to its neighbors is adaptively adjusted based on the importance of neighbor nodes and the state of the network. The strategy can take advantage of key nodes when the network is in free flow state, ensure that packets can arrive at their destination faster, protect the key nodes by accurately identifying its importance, and fully use the capabilities of all nodes when the network is in a congestion state, and stop some key nodes from becoming the bottleneck of network. Simulation results show that the proposed local routing strategy can effectively control the network congestion, enhance the network capacity.
2014, 63 (24): 248103.
doi: 10.7498/aps.63.248103
Abstract +
Graphene, as a two-dimensional sp2 hybridization allotropicity of carbon element, possesses unique properties of electricity, photology, thermology and mechanics. For industrialization, graphene possessing large dimension and homogeneous property is required. It can be prepared by chemical vapor deposition technology. In this paper, based on the progress of graphene research this year, we summarize the influence factors of graphene preparation, including substrate selection and preprocessing, carbon source and assist gas flow control, chamber temperature, pressure control, cooling rate, and deposition time setting, etc. Finally, we outlook the research direction of preparing the large-dimensional and single crystal graphenes.
2014, 63 (24): 248402.
doi: 10.7498/aps.63.248402
Abstract +
The outline design problem of navigation constellation near libration point is studied for deep space exploration. Firstly, the model of circular restricted three body problem near libration point is given, and the differential correction method is provided to calculate periodic orbits, of which the stability factor is also introduced. Secondly, the concept of effective coverage is defined, and the elements and the steps to design the constellation are proposed. Then, the outline design, performance analysis and evaluation of the navigation constellation are carried out concretely from aspects of orbits selection, space coverage, geometric dilution of precision factor, signal transmission and navigation precision. Compared with the case of global position system, the feasibility of the navigation constellation in libration point orbit is proved, and the performance index is provided. By analysis, the designed navigation constellation in L3-Vertical orbit with 5 satellites is able to provide navigation service for most of the spacecrafts in the space of between the earth and the moon, and the navigation precision is better than 10 m if the orbit determination errors of the constellation are not considered.
2014, 63 (24): 248702.
doi: 10.7498/aps.63.248702
Abstract +
A beamformer method based on multiple time windows is proposed to estimate the equivalent current sources of different strength and duration from multi-channel magnetocardiography data. This technique of visualizing cardiac electric activities will be a great help to the diagnosis of relevant heart diseases. Feasibility of this beamformer method has been proved by simulation experiments of two known sources. Source estimation results and two-dimensional excitation propagation diagrams of a complete left bundle branch block and a complete right bundle branch block patients during QRS complex are taken to be compared. The revealed characteristics of their electrophysiological phenomena are analyzed.
GENERAL
2014, 63 (24): 240303.
doi: 10.7498/aps.63.240303
Abstract +
Mesoscale sandstorm is a common form of dust weather in Nevada in the US, the northern part in China, and the Middle East countries in the Asia. To investigate the influence of mesoscale sandstorm on the quantum satellite communication channels, the physical characteristics of the dust storms are analyzed first. According to the diffusion model of mesoscale storms, the relationship between the proposed scale dust features and the degree of quantum entanglement is established then. The effect of dust on properties of the quantum satellite channel is simulated finally. The results show that if the diffusion time for the dust is 12 hours, the dust particle radii are 1 and 25 μm, the entanglement degrees of quantum satellite channels are 0.6 and 0.4, the utilization rates of quantum satellite channels are 0.9 and 0.8, the capacities of quantum satellite channels are 0.95 and 0.8. The characteristic parameters of the quantum channels are closely related to sandstorms. Therefore, in order to improve the reliability of quantum satellite communications, the parameters of quantum satellite channels should be adjusted adaptively.
2014, 63 (24): 240302.
doi: 10.7498/aps.63.240302
Abstract +
Differential quotient rule of composite function operator and its applications in quantum physics, quantum statistics, operator ordering theory, matrix theory and control theory are given. The integration problem of Wigner operator and Weyl corresponding rules are studied. Two kinds of typical operator identity formulas are proved. The differential form of Wigner operator in ordered product of operators and new differential form of important functions are obtained. Finally, a Wigner operator with parameter for unifying regular order, Weyl sequencing and abnormal order is introduced.
2014, 63 (24): 240502.
doi: 10.7498/aps.63.240502
Abstract +
We study the thermodynamical properties of a noninteracting electron gas confined in one dimension by a harmonic-oscillator potential. The exact analytical expression for the thermodynamical potential is obtained by using a formula of contour integration. The magnetizations, magnetic susceptibilities, and the specific heats are then studied each as a function of the strength of the magnetic field in different regimes of the temperature and effective thickness. It is shown at low temperature, the magnetization, magnetic susceptibility, and the specific heat oscillate as the strength of the magnetic field increases. Especially, there exist two modes of oscillations for the specific heat in certain regimes of low temperature and effective thickness.
2014, 63 (24): 240503.
doi: 10.7498/aps.63.240503
Abstract +
When using the noise of n-order time derivative as an internal thermal noise to drive a generalized Langevin equation, if the nonergodicty strength of the noise satisfies b ≠ 0 and the system is in thermal equilibrium, then the stationary velocity variable of the system can be used as a non-ergodic noise. The spectra of the non-ergodic noise is infinite at zero frequency. The one-dimensional semi-unbounded coupled harmonic oscillator chains connected with the gas molecules act as the non-ergodic noise, which is an example of non-ergodic noise. Finally regarding the non-ergodic noise as an external noise to drive the particles in the titled periodic potential, it is found that there appears superballistic diffusion with the effective diffusion index exceeding a ballistic value of 2. It is also found that the velocity distribution of the system displays two motion states, the “locked state” and the “running state”. And in the “running state” there occurs a bimodal phenomenon, which is a relatively new phenomenon.
2014, 63 (24): 240508.
doi: 10.7498/aps.63.240508
Abstract +
For nonlinear stochastic systems which are excited by Gaussian white noise, an innovational regulation method is proposed to control the shape of the probability density function of state response to track a desired shape. Firstly, a polynomial feedback scheme is established, and the nonlinear part is replaced by polynomials expansion. Then the recursive equations of the moments which are related to control gain are derived under Fokker-Planck-Kolmogorov theory framework. Meanwhile, regarding the tracking requirement, an optimization problem about the moment approximation is constructed, and the gain of regulation function is obtained by solving this optimization problem using the gradient method. Furthermore, the probability density function of state response is reconstructed from the relationship of the Fourier transform pairs between the characteristic function and probability density function. Finally, two examples are given to demonstrate the effectiveness of the method developed in this paper.
2014, 63 (24): 240601.
doi: 10.7498/aps.63.240601
Abstract +
A new scheme of measuring ultra-short optical pulses, based on the photo-elastic effect and cross-phase modulation (XPM) effect in a single-mode fiber, is proposed. In this novel scheme, a variable delay is generated in the former part of the single-mode fiber, and the nonlinear effect is produced in the latter part as a nonlinear medium. The structure including only one single-mode fiber is very simple and low-cost, and needs no complex optical circuit calibration. In addition, XPM effect in the fiber can be realized easily without phase matching. Measured pulses can be retrieved by using principal component generalized projects algorithm based on matrix, and the influences of fiber length and the pulse width on amplitude and phase are studied by numerically simulating the scheme proposed. The results show that the ultra-short optical pulses can be measured when fiber length is equal to 2 km, and the accuracy of measurement is improved by increasing the fiber length. The proposed scheme can realize the measurement of optical ultra-short pulse whose pulse width is above 80 fs.
2014, 63 (24): 240504.
doi: 10.7498/aps.63.240504
Abstract +
Modeling and analyzing of the piezoelectric vibration energy harvester using permanent magnets are systematically investigated to facilitate the evaluation and optimization of such a harvester. We set up a distributed-parameter model for describing nonlinear dynamic behaviors of these harvesters，and present harmonic analytical solution by using harmonic balance method. An analysis is performed using the simulation model to determine the effects of the distance between two magnets, amplitude of acceleration, electrical load resistance on the level of the output power. The optimum resistive loads under different vibration frequencies and accelerations are also compared. The results show that the bistable configuration is applicable to a small excitation case, and the closer to the transition region the small excitation position, the more the power can be harvested. Conversely, the monostable hardening configuration is suited for the large excitation case, the corresponding optimal magnet distance is not close to the transition region. Furthermore, the large amplitude oscillation between two potential wells and small amplitude oscillation within one potential well also bring forth coexisting phenomena of high-energy response and low-energy response; the closer to the transition region the oscillation position, the more obivious the coexisting phenomenon is. It is also demonstrated that exciting frequency is a decisive factor of optimum load resistance.
2014, 63 (24): 240506.
doi: 10.7498/aps.63.240506
Abstract +
In this paper, a novel four-dimensional chaotic system for generating multi-wing chaotic attractors is proposed, and chaotic and hyper-chaotic attractors are generated in different parameters. Besides, basic dynamical properties of the chaotic system, such as equilibrium point Poincaré mapping, dissipativity, power spectrum, Lyapunov exponent spectrum, bifurcation diagram are studied numerically and theoretically. An analog oscillator circuit is designed for implementing the four-wing hyper-chaotic attractors, and the hardware circuit experimental results are shown to be in good agreement with the numerical simulation results. Finally, the four-wing hyper-chaotic system is used for hybrid image encryption of physical chaos encryption and advanced encryption standard encryption algorithm. Because physical chaos is adopted in this system, there does not exist a definitive relationship between plaintexts and ciphertexts. And the statistical characteristics of ciphertexts should be better than those of any other encryption system.
2014, 63 (24): 240501.
doi: 10.7498/aps.63.240501
Abstract +
Microblog is a social medium platform allowing users to share real-time information through the user relationship networks which are established by the mechanism of “following”. Message is spread through the user relationship networks by users' behavior of “retweeting”. The research on the spread mechanism of microblog message is very important for public sentiment analysis and rumor propagation control. In this paper, we intend to explore the process of microblog propagation through structural analysis of microblog propagation network. First, we adopt the data of Sina Microblog to establish microblog propagation network, then analyze the formation mechanism of this network, finally, derive the degree distribution model of microblog propagation network by mean-field theory. Experimental results show that the degree distribution of microblog propagation network is a time-dependent model, and it is a power-law distribution at specific time.
2014, 63 (24): 240701.
doi: 10.7498/aps.63.240701
Abstract +
A hybrid-Fabry-Perot (F-P) interferometer based on an in-fiber ellipsoidal cavity is presented, and the refractive index sensing properties are studied. The ellipsoidal air-microcavity is formed by splicing together a single-mode fiber and a photonic crystal fiber with special arc-discharge technique. The cavity loss is analyzed by using a Gaussian beam model and the ABCD law, and the physical model of electromagnetic transmission is established. According to the cavity length ratio, there are two kinds of the influences of environment refractive index on interference fringe: contrast modulation and wavelength modulation. A fiber refractive index sensor with an enclosed air cavity based on wavelength demodulation is proposed in this paper. The result of simulation shows that the sensors has no turning point in a range of 1-1.6. A wavelength interrogation technique is used to demodulate refractive-index with high sensitivity (～ 37.088 nm·RIU-1) and high resolution (～ 2.69× 10-5) and with low temperature crosstalk. Experimental results are in good agreement with the theoretical ones. The F-P fiber sensor also holds advantages such as compactness, low cost, easy fabrication, high contrast, high resolution, no turning point, and low temperature crosstalk.
2014, 63 (24): 240509.
doi: 10.7498/aps.63.240509
Abstract +
Traffic flow time series at different time scales have different fluctuation characteristics. In order to analyze the discontinuity characteristics of traffic flow, a traffic volume time series is adopted as an empiric study objective. The multi-scale sharp variation points of the sequence are obtained by using wavelet transform. The scales are divided based on the corresponding number of the zero-crossing points of wavelet transform, and the hierarchy of the scales is analyzed. The results of calculation indicate that the number of sharp variation points of the traffic flow on a mutation scale is self-similar in a certain scale if the number of the sharp variation points is used as a measure. So, sharp variations of the traffic flow in different scales are scale-free.
2014, 63 (24): 240201.
doi: 10.7498/aps.63.240201
Abstract +
A class of transfer model for femtosecond pulse laser nano metal film is investigated. Firstly, the exact solution of a typical model is obtained. And then, the arbitrary order approximate analytic solution of corresponding model is obtained by using the functional homotopic mapping method. Finally, the meaning of solution is discussed.
2014, 63 (24): 240301.
doi: 10.7498/aps.63.240301
Abstract +
Using the second-order vector potential and Lorentz reciprocity theorem, the non-axisymmetric eddy current field induced by a probe coil outside a conducting ferromagnetic pipe is obtained analytically, where the probe coil is differently oriented: its axis being along the pipe radial direction, parallel to the pipe axis, along the pipe circumferential direction. Then, the time-domain expressions of induced voltage and eddy current density in the pipe are obtained through the Laplace inverse transformation, by calculating the residues of poles. Furthermore, the diffusion process of pulsed eddy current in the pipe, and the detection sensitivity of the time-domain induced voltage signal to the wall thickness are studied. Finally, the analytical solutions are verified through the experimental results of a steel pipe. It is found that when the probe coil is positioned such that its axis is perpendicular to the pipe axis, the strongest and the most sensitive induction voltage signal for detecting the wall thickness of a ferromagnetic pipe is obtained.
2014, 63 (24): 240702.
doi: 10.7498/aps.63.240702
Abstract +
A new approach to designing planar, high numerical aperture, low loss, focusing reflectors using circular subwavelength high contrast gratings is presented. Through analyzing particular physical scene, a mathematical transformation from existing “focus line” convergent beam, which can be achieved by bar grating reflector, to the convergent beam with a “focus point”, is obtained. By changing the shape of the bar grating reflector with the mathematical transformation obtained, a circular grating reflector, which can achieve “focus point” convergent beam, is obtained. The focusing properties and reflection characteristic of the circular grating reflector are numerically studied with the finite element method. After the radially polarized light reflected from circular grating reflector with a diameter of 29.788 μm, the beam will focus at 10 μm away from the reflector, resulting in a numerical aperture of 0.8302 and a reflectivity of 0.9163. In the focal plane, the numerical simulation results present a field distribution with a full width half maximum value of 1.5548μm, which is extremely close to diffraction limit.
2014, 63 (24): 240505.
doi: 10.7498/aps.63.240505
Abstract +
Through introducing a generalized memristor and an LC absorbing network into Wien-bridge oscillator, a kind of memristive Wien-bridge chaotic oscillator is proposed. The dynamical model of the memristive Wien-bridge chaotic oscillator is established, based on which the equilibrium points and their stabilities are studied and the dynamical behaviors are further analyzed when the circuit element parameters are varied. The research findings indicate that the memristive Wien-bridge chaotic oscillator has three determinative equilibrium points and their stabilities depend on the circuit element parameters, and that there exist complex nonlinear phenomena including periodic oscillations, chaotic oscillations and fast-slow effects when the parameters are changed. The experimental circuit is simple and easy to realize. The experimental waveforms and numerical simulations are consistent with each other, which well verify the theoretical analyses.
2014, 63 (24): 240202.
doi: 10.7498/aps.63.240202
Abstract +
A critical error in numerical simulation stems from the physical parameters in the model. To better assess the accuracy of the numerical stimulation, a mthod of impoving physical parameters is urgently desired. By modifying the four-dimensional variatiaonal data assimilation 4DVAR technique, in this paper a new method is proposed based on the use of observational data to optimize initial field and subsequent physical model. Ekman boundary layer model and Lorenz model are taken for example to conduct numerical experiment. The results show that through the variations in observational data, physical parameters and initial field are improved, thus effectively enhancing the accuracy of the model. This method improves the numerical model and physical parameters.
Doppler power spectrum density and multi-antenna system performance in three-dimensional environment
2014, 63 (24): 240507.
doi: 10.7498/aps.63.240507
Abstract +
In this paper, we develop a series of exponential probability density functions for modeling different distributions of elevation angle (EA) of arrival signals in different tree-dimensional (3D) coverage area scattering environments, and implement the modeling of channel characteristics. First, by assuming that the distribution of azimuth angle is uniform, in this paper the closed-form expressions of power spectrum density (PSD) for both symmetric and asymmetric situations of EA are derived. It can be observed from the analysis results that the PSD is closely correlated to EA function and the boundary angles βmin and βmax of the arrival signals and also to the Doppler shift. Then the spatial fading correlation (SFC) of MIMO multi-antenna signals in 3D environment is derived and simulated. The results show that the SFC between MIMO multi-antenna elements is closely related to βmin and βmax, and the parameter of EA function has little effect on SFC. The exponential EA probability function which is introduced in this paper can be applied to channel parameter estimation of multiple wireless communication environments. Compared with traditional models, this model presents the parameter estimation that satisfies theoretical and empirical values, and this model also expands the modeling of statistical channel in 3D environment.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2014, 63 (24): 247301.
doi: 10.7498/aps.63.247301
Abstract +
In order to break through the limit relationship between the breakdown voltage and specific on-resistance for LDMOS (lateral double-diffused MOSFET), a new super junction LDMOS is proposed with the electric field modulation by differently doping the buffered layer in this paper for the first time based on the buffered SJ-LDMOS. The new electric field introduced by the differently doping buffered layer, owing to the electric field modulation, is brought to the surface electric field of SJ-LDMOS, which alleviates a low lateral breakdown voltage due to the uneven electric field distribution for the LDMOS affected by the vertical electric field. Through the ISE simulation, the results are obtained that the surface electric field is optimized for the proposed SJ-LDMOS when the number of differently doping buffered layers is three. The saturated breakdown voltage for the new SJ-LDMOS is increased by about 50% compared with that for conventional LDMOS, and improved by about 32% compared with that for buffered SJ-LDMOS. The lateral breakdown voltage for unit length is increased to 18.48 V/μm. For the proposed SJ-LDMOS, the specific on-resistance is 25.6 mΩ· cm2 with a breakdown voltage of 382 V, which already breaks the limit relationship of 71.8 mΩ·cm2 with a breakdown voltage of 254 V in the conventional LDMOS.
2014, 63 (24): 247803.
doi: 10.7498/aps.63.247803
Abstract +
Aluminum nanoparticles with an average diameter of about 48 nm are compressed in a cemented-carbide mold under different pressures to produce nanocrystalline aluminum by the hot-pressing technology in a high vacuum condition. The X-ray diffraction and the positron annihilation lifetime spectroscopy (PALS) are used to characterize the microscopic structures of nanocrystalline aluminum. The PALS experimental results indicate that there are three types of defects in nanocrystalline aluminum, i.e., vacancy-like defects, vacancy clusters, and microvoids, which are corresponding to three lifetime components of positrons. The pressure for compaction has a great influence on the positron annihilating behavior. The vacancy clusters transform into the vacancy-like defects with increasing the pressure when it is below 0.39 GPa. The three types of defects tend to be rapidly eliminated in a pressure range from 0.39 GPa to 0.72 GPa. When pressure is above 0.72 GPa, the defects are further eliminated in nanocrystalline aluminum. The density and microhardness of nanocrystalline aluminum increase significantly with increasing the pressure for compaction.
2014, 63 (24): 247801.
doi: 10.7498/aps.63.247801
Abstract +
Tm2O3-doped 40GeO2-35TeO2-15PbO-5Al2O3-2.5CaO-2.5SrO glasses with different Tm3+ ion concentrations are fabricated by conventional melt-quenching method. From the measurements of thermal properties it follows that the glass transition temperature is 446 ℃ and crystallization peak does not exist. The maxmium phonon energy of the host is ~ 750 cm-1. The Judd-Ofelt strength parameter Ωt (t = 2, 4, 6), the radiative transition probalities, the branching ratios and the radiative lifetime are calculated according to Judd-Ofelt theory. The emission spectra of Tm3+ doped glasses with different Tm2O3 concentrations are measured under the excitation by an 808 nm diode laser. The result shows that the emission intensity at ~ 1.8 μm reaches a maximal value when the Tm2O3-doping concentration is 1 mol% and then it decreases with the further increase of Tm2O3-doping concentration. The emission cross section of 3F4 → 3H6 is calculated based on the McCumber theory. The peak emission cross-section is 6.5 × 10-21 cm2. The non-radiative transition rate of 3F4 caused by OH is calculated quantitatively by rate equation. The results indicate that the germanate-tellurite glass is a new candidate for mid-infrared laser application.
2014, 63 (24): 247802.
doi: 10.7498/aps.63.247802
Abstract +
Nanopore alumina films (PAF) are fabricated by two-step anodic oxidation of aluminum in oxalic acid. The field emission scanning electron microscope measurement reveals the surface microstructure of PAF, and the defect formation mechanism in PAF is analyzed. The energy dispersive X ray spectroscopy and the Fourier transform infrared spectroscopy results indicate that oxalic ions are incorporated into the PAF in the synthesis process and further heating up to 500 ℃ does not cause oxalic ions to completely decompose. The photoluminescence (PL) spectra of PAF can be divided into three bands by Gaussion fitting method. The measurement results and the defects in PAF show that the PL originate from optical transitions of two kinds of different oxygen-deficient defect centers (F and F+) and oxalic impurities related defect center, PL centered at 402, 433 and 475 nm, respectively. We put forward for the first time that F centers play a leading role. The PL characteristics of the PAF prepared in oxalic acids with different concentrations suggest that three kinds of the luminescent center positions do not change with the increase of the oxalic acid concentration, but their relative intensities change with the increase of the oxalic acid concentration, i.e., F and F+ decrease, oxalic impurities related defects increase, and these will cause the PL peak position to be red-shifted. Finally, we put forward that the oxalic impurities in PAF can be changed by controlling the concentration of oxalic acid. The present experiments and results will be beneficial to the understanding of light-emitting mechanism in PAF, meanwhile, in this paper we propose a new train of thought for PAF preparation application.
2014, 63 (24): 247501.
doi: 10.7498/aps.63.247501
Abstract +
The evolution of vortex configuration for superconducting ring is simulated by the Ginzburg-Landau theory in the presence of an externally applied field. The effects of the applied field, the material parameter, the size of ring on the entrance of vortices into the ring and distributing of steady vortices are discussed. Research results show that the higher the applied field, the bigger the material parameter κ is, and the larger the width of the ring, the bigger the number of the vortices which the ring accommodates. The vortices enter into the ring only from the inner boundary when the applied field is low enough, otherwise the vortices enter into the ring first from the outer boundary and then from the inner boundary.
2014, 63 (24): 247502.
doi: 10.7498/aps.63.247502
Abstract +
Currently, the giant magneto-impedance effect of amorphous wires is usually investigated on the hypothesis that demagnetizing field can be ignored. This hypothesis is not true for amorphous wires with small size. We propose a model for calculating the static magnetization and magnetic field distribution inside CoFe-rich amorphous wires, which divides the amorphous wire into coaxial and nonoverlaped circular rings with the same width and height and different radii. Calculating the impulse response of the demagnetizing field at point r to the magnetization in each ring, the impulse response matrix is obtained. Using the impulse response matrix, the static magnetization and demagnetizing field distribution in the amorphous wire placed in an uniform or non-uniform DC applied field can be obtained.
2014, 63 (24): 247101.
doi: 10.7498/aps.63.247101
Abstract +
By using the first-principles calculation based on density functional theory, we propose some approaches to improving the efficiency for the photocatalyst Ag2ZnSnS4 from a theoretical aspect. Comparing its band edge positions with those of other similar compounds, we find that Cu, Ge codoping can adjust both the band gaps and band edge positions of Ag2ZnSnS4 at the same time, which can optimize its band structure for water splitting. In addition, Ag2ZnSnS4 has a type-Ⅱ band offset with another photocatalyst CuGaSe2. Preparation of its homojunction can also improve their efficiencies of photocatalysis hydrolyzation.
ATOMIC AND MOLECULAR PHYSICS
2014, 63 (24): 243601.
doi: 10.7498/aps.63.243601
Abstract +
The explosion dynamics of deuterated ethane clusters driven by an intense laser pulse is simulated numerically by employing a simplified Coulomb explosion model. The dependences of deuteron kinetic energy and neutron yield on cluster size are investigated respectively in the paper. It is found that the deuteron energy and neutron yield produced from 5.0 nm deuterated ethane clusters are 20.96 keV and 6.31×105 respectively, which are higher than those from 5.0 nm deuterated methane clusters. So it can be inferred that deuterated ethane clusters are superior to deuterated methane clusters as the target for the efficient laser-induced nuclear fusion reaction to achieve a higher neutron yield, which is in accordance with the reported experimental conclusion.
2014, 63 (24): 243101.
doi: 10.7498/aps.63.243101
Abstract +
The geometric structures, electronic properties, average binding energies, second-order energy differences and energy gaps of BnY (n=1-11) clusters are systematically studied using the density functional theory (DFT) TPSSh method with 6-311+G(d) basis set for B atoms and Lanl2dz relativistic effective core potential basis set for Y atom. It is found that with the size increasing, the lowest energy structures of BnY (n=1-11) clusters gradually evolve from planar shape to cubic structure. With the atoms of B increasing, the average binding energies of the ground state of BnY (n=1-11) clusters increase. The second-order energy differences and the energy gaps of the ground states of BnY (n=1-11) clusters show that B3Y, B5Y and B7Y clusters possess relatively high stabilities. The polarization and the first static hyperpolarizability studied show that the plane structures of B5Y, B4Y, B3Y and B6Y clusters have larger nonlinear optical properties.
2014, 63 (24): 243102.
doi: 10.7498/aps.63.243102
Abstract +
Using first-principles calculations based on the density functional theory, we systematically study the geometry structure, electronic structure and optical properties of the small size (n, 0)-type TiO2 nanotubes (D2 unit decreases with the diameter increasing, and the nanotubes become more stable. At a diameter of about 14 Å, a configuration change occurs. Band structure analysis shows that electronic states of TiO2 nanotubes are localized, and the conductivity is better for nanotubes with small diameters (D2 nanotubes shift from direct band gap to indirect band gap. And the band gap increases with diameter increasing, because π orbital overlap effect is greater than the quantum confinement effect. Owing to the competition between the two effects, the peaks of the dielectric function ε2(ω) will become redshifted or blueshifted. When its diameter is larger than 9 Å ((8, 0) tube), the optical absorption of TiO2 nanotubes will be significantly enhanced.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2014, 63 (24): 244204.
doi: 10.7498/aps.63.244204
Abstract +
The performances of microwave signals generated by an optically injected semiconductor laser operated at the period-one (P1) oscillation under 1/2 subharmonic microwave modulation, are investigated experimentally. The experimental results show that under suitable injection condition, the microwave signal output from an optically injected semiconductor operated at P1 oscillation can reach a frequency of 26.5 GHz limited to the experimental conditions and may have a single sideband optical spectrum structure, but the linewidth of the microwave signal is relatively wide (on the order of MHz). After adopting 1/2 subharmonic locking technique, the linewidth of the obtained microwave signal can be reduced from tens of MHz to tens of kHz. Furthermore, we analyze the influences of the power and frequency of the subharmonic microwave on the phase noise of the generated microwave signals, and further map the subharmonic microwave locking region in the parameter space of the power and frequency of the subharmonic microwave.
EDITOR'S SUGGESTION
2014, 63 (24): 244203.
doi: 10.7498/aps.63.244203
Abstract +
The long-time trap and control of neutral atoms in an optical micro-cavity is a crucial problem in cavity quantum electrodynamics (QED), which greatly restricts the coherent manipulation of the interaction process between single atom and light. In this paper, we design a strongly coupled cavity QED system based on the traditional Fabry-Perot cavity. The parameters of the cavity are 3.5 millimeters in length, about 57000 in fineness, (g0,,)=2 (1.48, 0.375, 2.61) MHz, 1.54 and 0.89 in critical photon and atom number, respectively. The system allows building the magneto-optical trap (MOT) and optical lattice directly inside the cavity, which provides the possibility of long-time trapping deterministic single neutral atom or a number of neutral atoms in the cavity. By setting up a dipole trap and atomic imaging system, the capture, detection and imaging of single atom or several atoms in the cavity can be realized. The system overcomes some difficulties in transferring atoms in the usual cavity QED and has potential applications in robust intracavity atom control for quantum information processing.
2014, 63 (24): 244205.
doi: 10.7498/aps.63.244205
Abstract +
The process of single-atom transfer in strongly coupled cavity quantum electrodynamics (QED) with free falling atoms is investigated by experiment and Monte Carlo simulation. We conduct the simulation of the whole physical process and give the corresponding experimental results. In experiment, a high finesse optical cavity is used for real-time detection of the single-atom transits from which the interaction information between single atoms and cavity can be extracted, including the transmission spectra of the cavity strongly coupled to single atoms, the interaction duration of the single atoms in the mode, the probability distribution of atom arrival time and the atomic kinetic energy distribution when arriving at the mode. All these can be completely derived from the transmission spectra of the different initial status. An intracavity far-off resonance trap (FORT) has been established and the single-atom trapping time inside the cavity is about 5 ms which is about 30 times longer than that without FORT. This study gives the detailed analysis of the whole procedure of free-falling atom transfer in cavity QED system and is helpful for optimizing the experimental parameters and design.
2014, 63 (24): 244208.
doi: 10.7498/aps.63.244208
Abstract +
The dynamics of multi-photon ionization of CH3I under strong field has been studied experimentally by femtosecond photoelectron imaging. Ultrafast optical control of the vibrational excitation in a polyatomic ion by strong field multi-photon ionization is experimentally realized. The present work enhances the intensity of the ionization beam from 1.6×1013 to 2.5×1013 W/cm2. In the order of this higher field, a new energy component is observed and attributed. From the photoelectron imaging, photoelectron kinetic energy distributions and the photoelectron angular distributions are obtained. The discussions of the previous letter are mostly based on the photoelectron kinetic energy information, and the present study emphasizes on the trend of the photoelectron angular anisotropy. More detailed dynamics on vibrational optical control is further explored.
2014, 63 (24): 244102.
doi: 10.7498/aps.63.244102
Abstract +
A wideband strong-directivity patch antenna is designed based on the metamaterial concept on the ground plane and the Yagi antenna concept of reflector and director. Due to the fact that the effective medium parameters of substrate of antenna are affected by the left-hand characteristics, the wave propagation direction of antenna is changed, which induces the strongest radiation in horizontal direction rather than the vertical direction of the conventional patch antenna. The reflector and director induce a directivity of the radiation. The simulation results show that the relative bandwidth of the designed antenna is 65.3% (6.9-13.6 GHz), the return loss is less than -10 dB, and the average gain is over 4.4 dBi in the operating range. Experimental results are in good agreement with the simulation results.
2014, 63 (24): 244103.
doi: 10.7498/aps.63.244103
Abstract +
The generation, propagation and application of vortex beams have been hot research topics in recent years. In this paper we introduce the novel multiple-ring vortex beams, including double-ring vortex beams and triple-ring vortex beams, which are generated by the coaxial superposition of multiple Laguerre-Gaussian vortex beams with different topological charge numbers and different waist parameters, and their intensity distribution is of multiple-ring. We study the generation and distribution characteristics of multiple-ring vortex beams theoretically, obtain the computer generated hologram of multiple-ring vortex beams based on conjugate symmetric extension Fourier computer generated holography, and experimentally generate quality multiple-ring vortex beams using a spatial light modulator. Excellent agreement between theoretical and experimental results is observed. The study indicates that each ring of multiple-ring vortex beams carries different orbital angular momentum, and the spatial distribution is independent. The novel multiple-ring vortex beams provide more controllable parameters and more diverse structure distributions, which enable their applications in the fields of micro-manipulation as optical tweezers or optical spanner. Furthermore, they also have potential applications as available encoding tools in optical communication.
2014, 63 (24): 244101.
doi: 10.7498/aps.63.244101
Abstract +
In this paper, we combine the modified equivalent current approximate method and graphical electromagnetic computing method to solve the electromagnetic scattering problems in the missile target covered with the thermal protective layer. The modified equivalent current approximate method is used to calculate the lossy dielectric and combining graphical electromagnetic method, and using a computer display technology, blanking and occlusion, a three-dimensional surface is projected on a computer screen, and the pixels are calculated. The calculation in three-dimensional space is converted into the calculation in two-dimensional space, thereby greatly reducing the computation time and complexity. The results show that when the incident frequency is low, the thickness of the thermal protection layer does not affect the radar cross section value. When the frequency is increased with the thickness of the thermal protection layer, the radar cross section value continuously decreases, which indicates that the thermal barrier coating is a lossy medium: the bigger the imaginary part of the dielectric, the stronger the ability to consume the energy is and the more obvious the change of missile radar cross section is. When the thermal protective layer has pores, the higher the porosity, the greater the value of the radar cross section is; when the porosity is zero, the value of the radar cross section is minimal; when the porosity is the same, the thinner the thermal protection layer, the bigger the radar cross section is. When the projectile phenomenon occurs, it does not affect the radar cross section.
2014, 63 (24): 244201.
doi: 10.7498/aps.63.244201
Abstract +
The image plane digital holography is commonly used in digital holographic measurement and imaging. The methods of the discrete Fourier transform and the frequency filtering are usually used for reconstructing the object wave in image plane digital holography, and the influence of these algorithms on phase reconstruction is discussed in this paper. Firstly, the influence of the frequency spectrum leakage on phase reconstruction error is analyzed. The results show that the error of the phase reconstruction is very small when the sampling periodicity is an integer, so high accuracy of phase reconstruction can be achieved in this case. But when the sampling periodicity is not an integer, the phase reconstruction error is increased obviously. In order to reduce the phase error caused by the frequency spectrum leakage, the Hanning window is used in the pretreatment of the digital hologram. The result shows that the Hanning window can improve the accuracy of the reconstruction phase effectively.
2014, 63 (24): 244202.
doi: 10.7498/aps.63.244202
Abstract +
In this paper we propose a method to detect the topology charge of vortex beam using digital holography. Hologram of vortex beam interference with reference beam is recorded, and the phase of vortex beam is reconstructed. The topology charge can be accurately determined by judging the periodicity of reconstructed phase distribution around the phase singularity. The phases of vortex beams generated by numerical simulation and those obtained by experiment of vortex beams with integral and fractional topology charges are compared with each other. The feasibility of the method of detecting the topology charge of vortex beam using digital holography is proved by comparing the results.
2014, 63 (24): 244206.
doi: 10.7498/aps.63.244206
Abstract +
A multi-wavelength Mie-scattering lidar is designed and established for detecting the aerosol profiles under different weather conditions. And inversion algorithm about multi-wavelength lidar signal is studied. The atmosphere observations are carried out in Xi'an city in the winter of 2013 by using the multi - wavelength lidar. The mixed-layer depth, aerosol particle size characteristics, and atmosphere extinction are studied and analyzed on haze, cloudy and sunny days. The mixed layer depth is lower on haze day and is just about 0.4 km, while it can reach 0.5-0.8 km on sunny day. The aerosol particle characteristics are discussed under different weather conditions by using two Ångström exponents, one for the short - wavelength range (355 nm/532 nm) and other for long-wavelength range (532 nm/1064 nm). The long-wavelength Ångström exponent is less than the short-wavelength Ångström exponent on haze day, and it is contrary on sunny day. The results show that there are more coase particles on pollution day. The Ångström exponents significantly decrease, and even become negative in the clouds, showing that cloud particles are relatively large.
2014, 63 (24): 244207.
doi: 10.7498/aps.63.244207
Abstract +
A refractive index sensing structure based on the Tamm state of photonic crystal with surface defect is proposed by combing the Tamm state of semi-infinite photonic crystal with the optical sensing mechanism of porous silicon, in which the efficient bearing mechanism of the porous silicon is introduced into the surface defect cavity. The existence of Tamm state is demonstrated at the edge between the defect cavity and the periodical photonic crystal structure, and the total reflection in the defect cavity is formed by adjusting the incident angle. The resonant defect peak is obtained in the reflection spectrum by adding an absorbing medium into the defect cavity in order to reduce the reflectivity of the resonant wavelength. The full width at half maximum and the quality factor (Q value) can be optimized by adjusting the parameters of photonic crystal. Based on those results, according to the relationship between Goos-Hänchen phase shift and the resonant wavelength, the model for the relationship between the resonant wavelength and the effective refractive index variation of porous silicon adsorbing layer caused by the change of the refractive index of the sample is established, and its refractive index sensing characteristics are analyzed. The numerical simulation results show that the Q value can attain to 1429 and the sensitivity is about 546.67 nm·RIU-1, which can demonstrate the effectiveness of the structure design and provide some theoretical references for designing the refractive index sensors with high Q values and sensitivities.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2014, 63 (24): 249201.
doi: 10.7498/aps.63.249201
Abstract +
Atmospheric water cycle process includes precipitation, evaporation, water vapor transport and other links. The temporal and spatial characteristics of the atmospheric water cycle over China in summer are investigated by using the ERA-Interim and MERRA reanalysis from 1979 to 2012. Comparison and performance analyses of these two reanalysis data are given. The obtained results are as follows. 1) the spatial distributions of precipitation, evaporation and precipitable water are highly similar to each other, and all gradually decrease from southeast coast to northwest inland regions. Because the differences in physical condition, precipitation and evaporation among different underlying surfaces, mutually promot or mutually restrict each other, the interactions among them have positive and negative feedback mechanisms. Precipitable water mainly concentrates in the ground to the 700 hPa height, accounts for about 75% of the total quantity. 2) All of the atmospheric water cycle elements have obvious characteristics of climate change, especially in these areas like Northwest China, Tibet and Northeast China. The zonal flux of water vapor transport significantly decreases in most parts of the northern, while the meridional flux of water vapor transport significantly increases in the northern region of Northwest China. 3)The reproduction ability to change characteristics of precipitation is better than that of evaporation based on these reanalyses. The temporal and spatial distribution characteristics of moisture transport situation and the various levels of atmospheric water vapor content are highly consistent with each other. The simulations of flux climate change trend of precipitation and water vapor transport are more accurate and more reliable. 4) Interim reanalysis is applicable to the study of the southwest, Southeast and northeast regional atmospheric water balance, water cycle elements can maintain balance well, while MERRA reanalysis is more reasonable in Southwest and northwest.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2014, 63 (24): 246101.
doi: 10.7498/aps.63.246101
Abstract +
Radiation displacement effect of BaTiO3 ferroelectric under epitaxial compressive strain is studied by using molecular dynamics simulations which is based on shell model. The numbers of defects, distributions and changes of polarization in the system are calculated before and after radiation under epitaxial compressive strains of 0, 0.4%, 0.8%, 1.2%, 1.6%, 2.0% respectively by using O atom of 1 keV and [001] direction as a primary konck-on atom (PKA). The damaged areas, the displacement distances of the defect, and migration distances of PKA under reverse applied electric field, obtained in the two cases: 2% compressive strain and no strain, are compared. The results show that the polarization of system increases almost linearly with increasing the epitaxial compressive strain, and that both the polarization amplitude and the number of defects decrease after irradiation. The displacement distance of defects under 2% compressive strain, migration distance of PKA under reverse applied electrical field and damaged area are all smaller than under no strain condition, which indicates that epitaxial compressive strain can suppress lattice irradiation damage, and the damage in BaTiO3 can be tuned by introducing epitaxial compressive strain.
EDITOR'S SUGGESTION
2014, 63 (24): 246102.
doi: 10.7498/aps.63.246102
Abstract +
Micro-voids significantly affect shock responses of brittle materials. Knowledge about the meso-scale evolution mechanism and macro-scale shock behavior will help to utilize micro-void in applications and avoid its disadvantages. A lattice-spring model, which can represent both elastic property and fracture evolution accurately, is built in this work. Simulations reveal that severe stress relaxation, which is contributed from collapse deformation induced by voids and slippage deformation induced by shear cracks extending from voids, modulates the propagation of shock wave. In a porous brittle material, the shock wave broadens into an elastic wave and a deformation wave. On a macro-scale, the deformation wave behaves as a plastic wave in ductile metal; on a meso-scale, it corresponds to the processes of collapse and slippage deformations. It is found that porosity of the sample determines the Hugoniot elastic limit of material; whereas the porosity and shock stress affect the propagation speed of the deformation wave and stress amplitude in a final state of shock. Brittle materials containing micro-voids have potential applications in complex shock loading experiments, precaution of shock induced function failure, and crashworthiness of buildings. Shock behaviors reported in this work will benefit the design and optimization of shock responses and dynamic mechanical properties of brittle materials used in specific applications.
2014, 63 (24): 246801.
doi: 10.7498/aps.63.246801
Abstract +
In this paper, we investigate the influences of the surface structure formation mechanism and the film properties on the incident titanium ion energy in the amorphous TiO2 thin film deposition process. The results show that the surface roughness of the film is reduced by increasing the energy of the incident titanium ions, and then the optical scattering loss of the film surface will decrease. It is also found that when the incident ion energy is increased, the film growth pattern changes from the “island-like” growth to the “layer-like” growth, and the surface diffusion coefficient of ions near the incident point is also significantly increased, which is conducive to the formation of more smooth film surface.
2014, 63 (24): 246103.
doi: 10.7498/aps.63.246103
Abstract +
A 60 kW electron beam is used to study the microstructure and optical property evolutions as well as laser induced damage threshold of fused silica after irradiation at room temperature. Optical microscopic results indicate that cracks appear at the surface of SiO2 after electron beam irradiation, owing to the thermal effect, and that the crack density and size increase with increasing radiation dose. The morphology of the surface cracks is analyzed by using atomic force microscope and the width of crack is about 1 μm. In addition, there are a large number of debris particles with sizes of 0.1-1 μm on the surface. From the optical absorption spectrum of each of all samples, a weak absorption peak at 394 nm is observed and the absorbance increases at the beginning then decreases with increasing electron-radiation dose. Before and after irradiation, three absorption bands at 460 nm, 496 nm and 520 nm are clearly observed and their intensities first increase and then decrease, which is consistent with the results of absorption spectra. The effect of electron dose on the laser induced damage threshold (LIDT) at 355 nm is investigated and the results indicate that the LIDT decreases with increasing dose. At the lower electron doses, the color centers are responsible for the decrease of LIDT. However, at the higher electron doses, the decrease of LIDT is due to the light modulation and absorption induced by microscale cracks and debris particles at the surface of irradiated fused silica.
