Vol. 62, No. 24 (2013)
2013, 62 (24): 242901. doi: 10.7498/aps.62.242901
In the case of wake field acceleration driven by charged particle bunches, many researchers focused on adjusting parameters such as magnitude, shape and electrical properties to amplify the maximum energy which drives electrons. Comprehensively considering the existing studies, in the paper we propose a new method of acceleration in which paralleled bunches are used to excite plasma wake field and trap self-injected electrons. It is proved to be more efficient than using single drive beam. With 2.5D PIC code, the driven electrons accelerated by paralleled bunches are found to possess the advantages of acceleration in longer distance, higher energy and better quasimonoenergy. Moreover, a bunch of backflow electrons is observed in the bubble, which makes self-injected electrons well collimated.
Influence of the functional layer thickness on the light output property of tandem organic light emitting diode：a numerical study
2013, 62 (24): 247201. doi: 10.7498/aps.62.247201
Based on the principle of thin film optics, the theory of electric dipole radiation, and the principle of Fabry-Prot microcavity, an optical model is built for the tandem organic light emitting diode (OLED) with the structure of Glass/ITO/N, N0-bis(naphthalen-1-yl)-N, N0-bis (phenyl)-benzidine(NPB)/tris(8-hydroxyquinoli-ne) aluminum (Alq3)/molybdenum trioxide (MoO3)/NPB/Alq3/LiF/Al. The influence of the functional layer thickness on the light output intensity of device is systematically studied, the laws of light output intensity for tandem OLEDs under different thickness values of functional layer are obtained. This model and the results obtained in this paper can present an in-depth understanding of the working mechanism of tandem OLED and help ones fabricate high efficiency OLED.
A dissipative dynamical method based on discrete variational principle：stationary shapes of three-dimensional vesicle
2013, 62 (24): 248701. doi: 10.7498/aps.62.248701
In this article, the dissipative dynamical evolution method based on the discrete variational principle is applied to a three-dimensional (3D) vesicle system. The stationary shapes of vesicles are obtained through the minimization of membrane bending energy without any symmetric assumption, which is an outstanding merit of our method. Aiming at a 3D vesicle system with certain parameter settings, a series of axisymmetric shapes are achieved by our method. The simulation results are in good agreement with those obtained with axisymmetric assumption, which indicates that our method can describe the deformation of 3D vesicle with extremely high precision. In addition, a shape transition between two stable states with great difference is studied, which further implies the capability of our proposed method in simulating large deformations regarding the 3D membrane. We believe that we have provided a very powerful simulation method for the future studies of bio-membrane deformations.
Binomial theorem involving Hermite polynomials and negative-binomial theorem involving Laguerre polynomials
2013, 62 (24): 240301. doi: 10.7498/aps.62.240301
We propose an operator Hermite polynomial method, namely, we replace the arguments of the special function by quantum mechanical operators, and in this way we derive a binomial theorem involving Hermite polynomials and a negative-binomial theorem involving Laguerre polynomials. These two theorems will have essential applications in quantum optics calculations. This method is concise and helpful in deducing many operator identities, which may become a new branch in mathematical physics theory.
2013, 62 (24): 240501. doi: 10.7498/aps.62.240501
Focusing on the directed transport phenomena of the two-headed molecular motor, we adopt power function as the damping kernel function of general Langevin equation due to the power-law memory characteristics of cytosol in biological cells and present the model of fractional coupling Brownian motor in overdamped condition in this paper. We also discuss the influences of fractional order and coupling factor on the transport speed. From the simulation results there are found the directed transport phenomena and the inverse transport which is not seen in the conventional Brownian motor, in the overdamped fractional coupling Brownian motor. When the noise density is fixed, the generalized stochastic resonance appears when transport speed varies with the fractional order and coupling factor. In particular, the results reveal that the magnitude and direction of the directional flow are controlled by coupling the noise with the interaction force between the two heads, which is the movement characteristic of the two-headed molecular motor in the memory ratchet, rather than of the single-headed motor.
2013, 62 (24): 240502. doi: 10.7498/aps.62.240502
It is well known that time delay and random noise are universal in complex networks. However, the research on the synchronization of coupled networks that are subjected to delay-coupling and noise perturbation is very rare. In this paper, for two delay-coupled complex networks with different node dynamics, different topological structures and different numbers of nodes, under circumstance noise, the generalized projective lag synchronization between two networks is proposed for the first time. First, a more realistic theoretical framework is constructed for the drive-response network synchronization. Second, according to the LaSalle-type theorem for stochastic differential delay equations, we rigorously prove that the generalized projective lag synchronization between the drive-response networks can be achieved almost surely, by introducing an appropriate controller. Furthermore, numerical simulation is employed to verify the theoretical analysis. The results indicate that the drive-response networks can indeed achieve generalized projective lag synchronization, and that the synchronization is independent of time delay and scaling factor. Moreover, the remarkable influences of the update gain and the coupling delay on synchronization speed are revealed through the numerical results.
2013, 62 (24): 240503. doi: 10.7498/aps.62.240503
A new two-dimensional sinusoidal discrete map is achieved by nonlinearly coupling a sinusoidal map and with a cubic map. The fixed points and the corresponding eigenvalues are obtained based on this two-dimensional sinusoidal discrete map, and the stability of the system is analyzed to study the complex nonlinear dynamic behavior of the system and the evolutions of their attractors. The research results indicate that there are complex nonlinear physical phenomena in this two-dimensional sinusoidal discrete map, such as symmetry breaking bifurcation, Hopf bifurcation, period doubling bifurcation, periodic oscillation fast-slow effect, etc. Furthermore, bifurcation mode coexisting, fast-slow periodic oscillations and the evolutions of the attractors of the system are analyzed by using the bifurcation diagram, the Lyapunov exponent diagram and the phase portraits when the control parameters of the system are varied, and the correctness of the theoretical analysis is verified based on numerical simulations.
2013, 62 (24): 240504. doi: 10.7498/aps.62.240504
The stability theorem of fractional systems is the basis of controlling fractional nonlinear systems. The theorem of fractional nonlinear systems is proved by a new approach in this paper. The results show that the theorem is applicable not only to the fractional nonlinear autonomous system, but also to the fractional nonlinear nonautonomous system. Several examples are analyzed by the theorem, and simulations are carried out, whose results show the effectiveness of the theorem.
2013, 62 (24): 240505. doi: 10.7498/aps.62.240505
A nonlinear circuit model with periodic switching is established. The fold bifurcation and Hopf bifurcation sets of the subsystems are derived via the analysis of the relevant equilibrium points as well as the stabilities. Complex dynamical behaviors caused by periodic switching in various equilibrium states of subsystems are investigated. The results show that there exist two types of destabilizing cases, i.e., period-doubling bifurcation and saddle-node bifurcation, in the variation of periodic solution to the switching system with parameter, leading to different forms of chaotic oscillations correspondingly. Furthermore, by analyzing the the phase trajectory and its corresponding bifurcation, the mechanisms for different types of oscillations are presented, which can explain some phenomena of the switched dynamical system.
2013, 62 (24): 240506. doi: 10.7498/aps.62.240506
Compared with traditional two-level inverter, three-level inverter has the advantages of low output voltage harmonic distortion, and small switch voltage stress, so it attracts more and more attention in high power applications. In this paper, the bifurcation and chaos in a single-phase three-level inverter are studied. The one-dimensional discrete iterated mapping model under proportional control is established, and the stroboscopic maps in different periods of time are obtained. The bifurcation phenomena in the single-phase three-level inverter are studied when the proportional coefficient k, load resistance R, load inductance L and input voltage E are used as changing parameters. Effects of all these parameters on the system dynamical performances are analyzed on a slow scale using the bifurcation diagram and Lyapunov index spectrum. The bifurcation processes on a fast scale caused by changing values of proportional coefficient and load resistance are visually observed using folded diagram. Finally, the time-domain waveforms with different proportional coefficients are obtained by Matlab/Simulink, which corresponds with the theoretical analysis. The results show that the correct circuit parameters of single-phase three-level inverter are very important for its stable operation.
Effect of inhomogeneous distribution of ion channels on collective electric activities of neurons in a ring network
2013, 62 (24): 240507. doi: 10.7498/aps.62.240507
In this paper, we investigate the evolution and transition of collective electric activities of neurons in a ring network, induced by inhomogeneous distribution of ion channels. The local kinetics is measured by Morris-Lecar under voltage coupling type. In the numerical studies, the effect of inhomogeneous distribution of ion channels is simulated by changing the conductance in ion channels embedded in the membrane, and the potential mechanism is discussed. The effect of diversity of conductance between calcium and potassium ions on the activating of the adjacent neurons, and the dependence of developed travelling wave on the coupling intensity, are investigated in detail. The activating and waking up the nonexcitable or quiescent neurons with type I and type II excitability, are investigated, respectively. The numerical results confirm that the adjacent neurons are activated and the stable travelling wave is developed in the ring network of neurons when the conductance of calcium ions is increased beyond a certain threshold or the conductance of potassium ions is reduced below another threshold; while the propagation of the travelling wave could be slowed down or suppressed when the conductance of calcium ions is reduced or the conductance of potassium ion is increased. The development or emergence of travelling wave and propagation are greatly dependent on the increase of conductance of calcium ions and the decrease of potassium conductance.
2013, 62 (24): 240508. doi: 10.7498/aps.62.240508
To research the encryption algorithm for color joint photographic expert group (JPEG) image, by comprehensively choosing the selective encryption and the joint compression and encryption, an encryption algorithm combining with spatial domain and frequent domain for color JPEG image is proposed. The 8×8 blocks are first diffused on spatial domain, then the edge-detection method is utilized to find out the significant blocks containing abundant details. After encrypting all the direct current coefficients, part of alternating current (AC) coefficients in significant blocks are chosen to be encrypted. Finally the information about marking the significance of the blocks is embedded into AC coefficients for transmission. Theoretical analyses and experimental results show that the proposed algorithm is compatible with JPEG format. The cipher image has good visual quality and uniform color distribution. The algorithm possesses huge key space, strong key-sensitivity and good security.
Chaotic time series prediction using add-delete mechanism based regularized extreme learning machine
2013, 62 (24): 240509. doi: 10.7498/aps.62.240509
Considering a regularized extreme learning machine (RELM) with randomly generated hidden nodes, an add-delete mechanism is proposed to determine the number of hidden nodes adaptively, where the extent of contribution to the objective function of RELM is treated as the criterion for judging each hidden node, that is, the large the better, and vice versa. As a result, the better hidden nodes are kept. On the contrary, the so-called worse hidden nodes are deleted. Naturally, the hidden nodes of RELM are selected optimally. In contrast to the other method only with the add mechanism, the proposed one has some advantages in the number of hidden nodes, generalization performance, and the real time. The experimental results on classical chaotic time series demonstrate the effectiveness and feasibility of the proposed add-delete mechanism for RELM.
2013, 62 (24): 240510. doi: 10.7498/aps.62.240510
Synchronization in Newman-Watts (NW) networks is studied in a Hindmarsh-Rose neuron dynamical system. The topological structures of some optimal synchronization networks are given. Numerical simulation results show that the synchronizability of the NW network is mainly determined by the distribution of coupling points in coupling space. The network with even distribution of coupling points generally has the strongest synchronizability. For a given number of edges there may exist some optimal synchronization networks with different topological structures. Optimal synchronization network has the strongest synchronizability, homogeneous degree distribution and better symmetry. However, its symmetry is not necessarily best. In general, optimal synchronization network is an irregular network. However, in the minor cases, the regular network may be an optimal synchronization network. A new type of network, the ergodic network, is introduced in this paper, and it has strong synchronizability as well as the characteristics of the optimal synchronization network.
2013, 62 (24): 240511. doi: 10.7498/aps.62.240511
In this paper, dynamic floor field is introduced to investigate the characteristics of bicycle flow. To embody the features of bicycles and riders, the repulsive force among the riders and the velocity effect are taken into consideration in this new model. A new concept called lane-changing cost is proposed to study the effect of driving styles, and a new lane-changing rule is proposed based on the traffic environment comparison. The simulation results show that 1) the model can accurately describe the bicycle flow, and the obtained statistics accords with empirical data; 2) the capacity of the road of per unit width deceases as road-width increases; 3) aggressive driving style substantially increases the lane-changing times, which reduces the capacity of the road.
Up-conversion luminescence and temperature characteristics of Tm3+, Yb3+ co-doped CaWO4 polycrystal material
2013, 62 (24): 240701. doi: 10.7498/aps.62.240701
Tm3+, Yb3+ co-doped CaWO4 polycrystals are synthesized via the high temperature solid state method. Under excitation of a 980 nm laser, the up-conversion fluorescences based on the transitions 1G4→3H6, 1G4→3H4, 3H2→3H6 and 3H3→3H6 in the visible light region are successfully obtained. The effect of the concentration change of Yb3+ on the luminous intensity of Tm3+ is discussed. Meanwhile, the temperature properties of the red up-conversion fluorescences centered at 689 nm and 705 nm are studied in a tem-perature range between 313 and 773 K according to the fluorescence intensity ratio method. It is found that the temperature can be monitored by detecting the red up-converion fluorescences of Tm3+, Yb3+ co-doped CaWO4 polycrystals. The maximal sensitivity of temperature measurement can be achieved to be 5.7×10-4 K-1 at 458 K.
2013, 62 (24): 242801. doi: 10.7498/aps.62.242801
The real-time laser frequency scale system is developed for measuring the carbon isotopic abundance. By using confocal Fabry-Perot interferometer with different free spectral ranges, the diode laser frequency tuning characteristic is acquired in each laser frequency scanning period. During the course of measurements, two nonlinear analysis techniques are tested, i.e., linear interpolation and polynomial fitting. The absorption line center position for CO2 between 4976 and 4980 cm-1 is obtained by 100 average measurements from our experiments. A comparison between the measured absorption spectra and the spectral line positions from the HITRAN-2008 database, shows that the two method can both reach an accuracy of 10-4 cm-1 for the laser frequency scale. The best results are obtained with a linear interpolation method. The result shows that the laser frequency real-time scale system is applicable in carbon isotopic abundances measurement.
It is an important approach to preventing nuclear proliferation that comprehensive nuclear test ban treaty is reached. How to judge whether a nuclear test has happened is a key point. In this paper we investigate a method to distinguish between a nuclear test and leakage from civilian nuclear reactor by measuring inert gas xenon nuclides 133mXe, 133Xe, 135Xe, and 131mXe. The distinction between the nuclear test and the reactor leakage is found by analyzing the variation of nuclide quantity with time in a complex decay chain. Two suspicious events in North Korea are measured and analyzed. In order to verify the theoretical results, an experiment of imitating plutonium irradiated by thermal neutron is designed.
ATOMIC AND MOLECULAR PHYSICS
2013, 62 (24): 243101. doi: 10.7498/aps.62.243101
The structures of six possible native point defects (I and In vacancies, I and In antisites, I and In interstitials) that maybe exist in the orthorhombic indium iodide (InI) crystal are optimized and investigated by the first-principles calculations based on density functional theory. The levels of difficulty in forming defects in their growth processes are obtained by calculating the defect energy levels; the position of each kind of energy level of native point defect and its effect on carrier transport are analyzed via calculating the density of states. The results show that the dominant low-energy defect of In interstitial induces a recombination center and a deep hole trap: the former shortens the lifetime of the minority carriers and the latter captures the holes from the valence band, thereby reducing the mobility-lifetime product of the hole. The calculation results provide a theoretical guidance for improving the mobility-lifetime product of carriers in InI crystal and also are helpful in obtaining the excellent materials for detecting the nuclear radiation of InI crystal.
2013, 62 (24): 243201. doi: 10.7498/aps.62.243201
Based on semi-classical theory, a model of the interaction between neutral atoms and the laser standing wave field is established in this paper. The trajectories and the deposition characteristics are analyzed under channel effect, and the values of full width at half maximum of nano-grating are 0.532, 12.16 and 96.70 nm when the divergence angle, spherical and chromatic aberration are discussed, respectively. The simulation results show that the contrast of the stripe decreases with the increasing of divergence angle of the atomic beam. Also, the contrasts of the stripe are 85.2:1 and 5.33:1 when the divergence angles equal 0.1 and 0.3 mrad. When divergence angle increases over 0.5 mrad, the splitting of the stripe takes place, leading to the degradation of the deposition.
2013, 62 (24): 243601. doi: 10.7498/aps.62.243601
The geometry structures, stabilities and chemical bonding properties of the YnNO(n=1–12) clusters are studied in the generalized gradient approximation based on the density functional theory with the consideration of spin multiplicities. The results show that NO adsorption changes the basic frameworks of the corresponding Yn clusters with n=5, 7, 8, 10. The obvious elongation of N–O bond length and the attenuate vibrational frequency indicate that the adsorption of NO on Yn cluster can be regarded as the dissociative adsorption. The chemical bondings of N–Y and O–Y both simultaneously play an important role in enlarging the adsorption energy of YnNO clusters. Specially, Y3NO, Y5NO, and Y8NO have the giant adsorption energies (9.92, 9.24, and 9.82 eV) coupled with the break of the N–O bond. The calculated second-order energy differences suggest that the NO adsorption has influences on the stabilities and bonding properties of Yn clusters. The appearance of the couple electrons, arising from the sp3 hybridization of N and O atom, not only leads to the fracture of N–O bond, but also enhances the ability to form N–Y bond and O–Y bond, which has important effects on the high stabilities of Y3NO, Y5NO, and Y8NO clusters.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2013, 62 (24): 244101. doi: 10.7498/aps.62.244101
A novel method to design an ultra-thin perfect metamaterial absorber (PMA) with high quality factor (Q-factor) at microwave frequencies is proposed to improve the absorption performance. The PMA achieves a high Q-factor by appropriately loading the metal cavity based on the substrate integrated waveguide (SIW) technology and the common PMA. An ultrathin absorber with a thickness of 0.0065λ and a full-width at half-maximum of 5.8% is designed. The Q-factor of absorptivity of the absorber is 33.9, which is enhanced by 20% compared with that of the conventional PMA. Meanwhile its Q-factors for radar cross section (RCS) reductions of 1.5 and 3 dBsm respectively increase 54% and 67% higher than those of the conventional PMA. The measured results show that the proposed SIW-PMA eliminates the frequency drift between the infinite periodic array and the finite periodic array, which occurs in the conventional design process. The simulated and measured results show that the proposed PMA has high Q-factor of absorptivity and excellent effect of RCS reduction. Its RCS reduction can reach a maximum value of 14.1 dBsm at the response frequency.
2013, 62 (24): 244201. doi: 10.7498/aps.62.244201
In order to study the scattering characteristics of practical machining surfaces under the infrared laser irradiation, an experiment device is designed to measure the semispherical scattering characteristics of machining surfaces. The measuring system consists of a laser power meter with a resolution of 10 nW, a laser device of 1550 nm wavelength, and two precisely rotating platforms. The distributions of scattering power from the specimen surfaces with micro V-groove structures and different surface roughness values are measured under the irradiation of 1550 nm infrared laser. After the measured results are converted into the bidirectional reflectance distributional function, the effects of incident azimuthal angle, incident angle, and surface roughness on the scattering characteristics of surface of such a kind are comparatively analyzed, and the cause for the formation of special scattering field is also analyzed. The experimental results indicate that the surface texture, incident angle and surface roughness all affect the scattering property in a regular manner. The results contribute to the study of surface scattering characteristics and the application of laser technology to surfaces with V-groove structures.
2013, 62 (24): 244202. doi: 10.7498/aps.62.244202
A novel broadband polarization beam splitter (PBS) based on dual-core photonic crystal fiber is proposed. With a full-vector finite element method, the effects of structural parameters of fiber on the bandwidth and length of PBS are systematically investigated in detail. Numerical results indicate that an increase in the index of fluorine-doped region can not only broaden the bandwidth but also shorten the length of PBS. An increase in the diameters of air hole and hole pitch in an optical fiber can broaden the bandwidth of PBS, however, lengthen the length of PBS at the same time. Thus, it is necessary to balance the bandwidth and length of PBS when selecting the fiber structure parameters. Through optimizing the fiber structure parameters mentioned above, a kind of ultra-broadband PBS is achieved. When the extinction ratio is greater than 20 dB, the length of PBS is as short as 7.362 mm and its bandwidth is more than 600 nm.
2013, 62 (24): 244203. doi: 10.7498/aps.62.244203
In order to achieve high-resolution imaging by reducing the cost of digital holographic microscopy system, the recording and reconstruction process of image-plane digital holographic system (IPDHS) and its point spread function are analyzed. Then, the determination factor of lateral resolution of IPDHS is investigated. It is concluded that the dependence of lateral resolution of IPDHS on the photosensitive surface size of the recording device is very weak, which is completely different from the common knowledge that the lateral resolution of digital holographic imaging system can be improved by using large size recording device. Then the imaging characteristics of an IPDHS are analyzed. The results show that the information about the tested object can be completely recorded and reconstructed by image-plane digital holography, and that the lateral resolution of IPDHS is higher than that of pre-magnification digital holography. Two IPDHS with plane and spherical reference wave are built. The experimental results are in accordance with the theoretical analysis.
2013, 62 (24): 244401. doi: 10.7498/aps.62.244401
A Monte Carlo (MC) method is proposed by establishing a new model of phonon scattering processes with random sampling according to a scattering probability function. The MC scheme is used to simulate steady and transient ballistic-diffusive heat conduction in silicon nanofilm. In the MC simulations, we trace the phonon bundles that emit into media from the boundaries, and obtain the temperature profiles through statistics of the distribution of phonon bundles. It is found that the size effect of phonon transport leads to a boundary temperature jump which increases with the Knudsen number increasing. The thermal conductivity of the silicon nanofilm is calculated and the results suggest that nanofilm thermal conductivity increases with film thickness increasing, which is in good agreement with the experimental data as well as the results from the theoretical model. The temperature profiles vary with time in the transient simulations, which shows that the heat wave is related to not only time scale but also spatial scale. When the spatial scale becomes smaller, the ballistic transport is more dominant, which leads to stronger heat waves.
2013, 62 (24): 244701. doi: 10.7498/aps.62.244701
With the wave equation, the boundary integral equation with considering compressibility is deduced. Then the motion characteristics and stability of cavitation bubble driven by traveling wave are obtained. The influences of wave frequency, amplitude and initial phase on the motion of cavitation bubble are analyzed. The results show that the motion stability is enhanced with the increase of drive frequency or the reduction of drive amplitude. With appropriate frequency and amplitude, the jet will be formed at the anaphase of contraction, and the direction is the same as that of the traveling wave. With the consideration of compressibility, the time for once pulsation of the cavitation bubble is shortened and the pulsation amplitude is reduced, correspondingly the jet tip velocity and the inner pressure also decrease. With the increase of drive frequency or the reduction of drive amplitude, the pulsation amplitude and intensity of jet decrease. The variation of initial phase will lead to the changes of the initial motion state of cavitation bubble and the jet strength.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2013, 62 (24): 245201. doi: 10.7498/aps.62.245201
In this paper, we represent the radiation characteristics of electromagnetic pulse generated by Z pinch implosion. Magnetic energy which couples with motions of metallic wire arrays or solid liners driven by Z pinch can radiate away. Theoretical results indicate that the radiation power of electromagnetic pulse is determined by both load current and implosion trace. Experiments are carried on primary test stand facility at Institute of Fluid Physics where a current rising to 7 MA in (10%–90%) 65 ns is used to drive a wire array Z pinch. The measured load current and implosion trace show that the Z pinch can deliver about 1 GW, 10 ns full width, 20–70 MHz central frequency, broadband electromagnetic pulse with an energy conversion efficiency of 10-7. Parameters of electromagnetic pulse are much smaller than those of X-ray with a power of 50 TW and an energy of 0.5 MJ. In the approximation of weak relativistic case, the power of electromagnetic pulse which is proportional to sixth power of load current, dramatically increases with current increasing. Soft X-ray radiation is an important channel for dissipating a considerable fraction of energy provided by facility. The results presented here demonstrate that electromagnetic pulse emission in the case of higher load current can cause significant damage to diagnostic devices. Moreover, intense electromagnetic pulse produced by this method may have many potential applications.
2013, 62 (24): 245202. doi: 10.7498/aps.62.245202
We present a simulation model of fluorine plasma etching of silicon. A mechanism for lag effect in the silicon surface etched by an inductively coupled plasma is investigated using molecular dynamics simulation. The results show that the lag effect is popular in etching process and that the etching rate of wide grooves is higher than that of the narrow ones. A probable reason is that the wide groove is produced more easily than the narrow groove. And the escape rate of product in narrow groove is lower than in wide groove. This is because a lot of products huddle together in the groove, which causes the speed of incident ions to decrease, and thus the energy of ions reaching the surface is reduced. The etching rate increases with the decrease of energy under otherwise identical conditions. On the other hand, the incident F particles are more close to the sidewall, which leads to the fact that the incident F particles will be easier to deposit on the surface of the wall. Then the width of the groove becomes narrower and narrower. The subsequent incident particles will be more difficult to reach the bottom of the groove. The lag effect increases not only with the decrease of the width of the groove but also with the enhancement of energy, and it decreases with temperature rising.
2013, 62 (24): 245203. doi: 10.7498/aps.62.245203
A sensitivity optimized magnetic proton recoil (MPROS) spectrometer for measuring both steady and pulsed fusion neutron spectra in several complicated environments is developed. The proton energy-position projection relationship of the spectrometer is measured by utilizing an imaging plate. The energy response to neutron sensitivity of the MPROS spectrometer is studied through a single event count method on an accelerated steady DT neutron source and simulations by a three-dimensional charged particle transport code. The detection efficiency of the spectrometer to DT neutrons reaches a level of 2×10-5 cm2 through high efficiency parameter settings, therefore the experimental data of high statistic accuracy are obtained on a comparatively weak neutron source. Results from simulations and experiments, such as α particle calibrations, and DT neutron calibrations, achieve good consistency. Based on this conclusion, precise solution technique of measured spectra can be developed to increase the sensitivity and energy resolution of measurements of pulsed neutron spectroscopy.
2013, 62 (24): 245204. doi: 10.7498/aps.62.245204
The time behavior and the optimum conditions for the Xe gas capillary discharge extrem ultraviolet source are investigated theoretically and experimentally. By setting up a one-dimensional magneto-fluid-mechanics model, the plasma compressing characteristics and the emission characteristics are simulated under different gas pressures and different discharge currents. The time characteristic and the intensity of the 13.5 nm (2% bandwidth) emission are measured experimentally. The theoretical and experimental results show that there are the optimum gas pressures for different discharge currents. Meanwhile, the optimum gas pressure increases with the discharge current increasing. Moreover, the time to generate the highest 13.5 nm (2% bandwidth) emission should decrease by the increase of the discharge current. All the results should be useful to better understand the plasma condition for the discharge experiments and the EUV source. And it can be used to increase the power of the extreme ultraviolet source as well.
2013, 62 (24): 245205. doi: 10.7498/aps.62.245205
Internal charging effect and discharging effect in a dielectric material are one of the key factors threatening the spacecraft safety. Most of the spacecraft charging models could calculate only the voltage variation and electric field distribution in the spacecraft internal charging process, without estimating the consequence of discharging, such as the magnitude of voltage drop and discharging current. In this article, we present a combined model which could calculate both the charging effect and the discharging effect on polymer. The model is verified by the experimental results, and it shows that the residual voltage after discharging and the pulse width of discharging current grow with the increase of polymer resistivity. The magnitude of discharging current is determined by the breakdown electrical field.
2013, 62 (24): 245206. doi: 10.7498/aps.62.245206
This work aims at modeling the ELMing H-mode of the experimental advanced superconducting Tokamak (EAST) using edge plasma code package SOLPS5.0. The steady-state H-mode is obtained by adjusting the perpendicular radial anomalous transport coefficient according to the given upstream profiles of experimental measurements (shot 36291). The downstream divertor region can be then modeled once the simulated upstream electron density and temperature agrees well with the experimental measurements. On the basis of the H-mode simulation, the effect of the drift on divertor targets power asymmetry and that of the power damping width of scrape-off layer (SOL) on the peak power density to the divertor target are modeled, the simulation results indicate that the drift (E×B, B×▽B) is one of the main reasons for targets power asymmetry, and the increasing of power damping width of the SOL will reduce the peak power load on the divertor target sharply; besides, due to the presence of interaction between plasma and neutral gas or radiation in divertor region, the effect of the power damping width on the divertor target power load is decreased.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
Study of phase transition of HoVO4 under high pressure by Raman scattering and ab initio calculations
2013, 62 (24): 246101. doi: 10.7498/aps.62.246101
The aim of this work is to confirm that the HoVO4 has a zircon-to-scheelite phase transition as reported in ScVO4 and YVO4. Firstly, we prepare HoVO4 samples used in the experiments by solid state reaction of appropriate quantities of pre-dried Ho2O3 and V2O5. And the Raman spectra of HoVO4 are measured in a pressure range from ambient pressure to 21.25 GPa at room temperature by using a diamond anvil cell. The discontinuities on Raman mode shifts and the occurrence of new Raman bands provide strong evidence for a phase transition at 9.3 GPa. Secondly, ab initio calculations are performed and the results reveal a zircon-type (I41/amd) to scheelite-type (I41/a) structure in this phase transition. The results are compared with those previously reported for the relevant ScVO4 and YVO4 with a common zircon-to-scheelite phase transition, mainly duo to volume collapses. The results in this work may improve our understanding of the high phase and structure of HoVO4 and benefit the application of this material.
Properties of 5d atoms doped boron nitride nanotubes：a first-principles calculation and molecular orbital analysis
2013, 62 (24): 246301. doi: 10.7498/aps.62.246301
The geometry, electronic structure and magnetic property of boron nitride nanotube (BNNT), whose boron/nitride atoms are substituted by 5d atoms (B5d or N5d), are investigated by first-principles calculations based on density functional theory. The pure-BNNT and BNNT with boron vacancy (VB) or nitrogen vacancy (VN) are also investigated for comparison. Results show that the local symmetry of B5d system is similar to C3v, however the N5d system exhibits a large geometric deviation from C3v. The total magnetic moments of doped systems are different from each other, and B5d system present a strong regularity. The total density of states is presented, where impurity energy levels exist. The impurity energy levels and total magnetic moment can be explained by the molecular orbital theory under C3v symmetry.
2013, 62 (24): 246401. doi: 10.7498/aps.62.246401
The capillary evaporation process with stable liquid film along the cell wall is investigated by evaporating under constant temperature. The experimental results show that the liquid/vapor interface coincides with viscous finger, which is independent of the state of liquid film and evaporating rate. During the evaporation process, four stages are found: sharp falling rate stage, constant rate stage, linear falling rate stage and boundary effect stage, which are corresponding to different kinds of liquid film states. The evaporating takes place mainly at the tip of the liquid film region, and the stable liquid film transports the liquid from the liquid region to the capillary tube orifice.
Influence of water on the tensile properties of amorphous silica：a reactive molecular dynamics simulation
2013, 62 (24): 246801. doi: 10.7498/aps.62.246801
Humidity has an important influence on the strength of the silica (SiO2). We examine the influence of liquid water on the tensile properties of amorphous silica (a-SiO2) using reactive molecular dynamics simulation. The results of the quasi-static tension show that liquid water reduces the tensile strength of a-SiO2 significantly. The tensile strength of dry a-SiO2 is 9.4 GPa while the tensile strength of a-SiO2 in the presence of liquid water is only 4.7 GPa. The strain-stress curve of dry a-SiO2 indicates that the stiffness of the a-SiO2 structure becomes stable with the increase of strain. On the other hand, the stiffness of the a-SiO2 with liquid water is gradually reduced with the increase of tensile strain. Moreover, the strain-stress curve of a-SiO2 in a strain range of 16% to 20% in the presence of liquid water is similar to the yielding phenomenon of plastic metal. The snapshots of the a-SiO2 in the presence of liquid water during the tension show that no plastic deformation is observed. We propose that the stress-enhanced hydrolysis releases part of the stress for the rupture of the Si–O bonds, so that the stiffness of the a-SiO2 in the presence of liquid water decreases with the increase of strain.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2013, 62 (24): 247101. doi: 10.7498/aps.62.247101
Based on the density function theory, thermite reaction between Al and α-Fe2O3 at temperature 2000 K in canonical ensemble is investigated by ab initio molecular dynamics. In the simulation, with the increasing of the time, the number of the Fe–O bond decreases, while the numbers of Al–O bonds and Fe–Fe bonds increase. At the same time, the total charge quantity of Fe ions decreases and the total charge quantity of Al ions increases as time increases. In the Al/Fe2O3 thermite reaction, the redox reaction is observed, Al atoms are oxidized and the Fe ions are reduced. And then the rupture of Fe–O bonds and the formation of Al–O bonds happen at the interface. Whole redox reaction is completed in about 3 ps.
2013, 62 (24): 247102. doi: 10.7498/aps.62.247102
A first-principles plane-wave pseudopotential method based on the density functional theory is used to investigate the energies, electronic structures, and elastic properties of intermetallic compounds of Au-Sn system. The enthalpies of formation, the cohesive energies, and elastic constants of these compounds are estimated from the electronic structure calculations, and their structural stabilities are also analyzed. The results show that the Au5Sn compound is unstable with respect to other compounds, and the bonding effects of AuSn2 and AuSn4 are stronger than those of AuSn and Au5Sn, for there are the strong hybridizations between Au and Sn atoms in AuSn2 and AuSn4 compounds. The main bonding effect of AuSn is Sn–Sn bonding interaction, and due to the Au content being maximal in Au5Sn the bonding of p electrons in Sn conduction band is suppressed by the covalent bonding of Au–Au.
2013, 62 (24): 247301. doi: 10.7498/aps.62.247301
In this paper, the action exerted by a superlattice sawtooth-shaped channel on the particle is assumed to be equivalent to that exerted by a periodic field with a similar shape. In the framework of classical mechanics, by introducing the sines-quared potential, the particle motion equation is reduced to pendulum equation with a damping term and dual-frequency excitation term. The bifurcation and chaos of single-frequency excitation system are analyzed with the Melnikov method. The stability of dual-frequency excitation system is discussed by using the Lyapunov exponent. The results show that in the case of weak nonlinearity, local instability can be found in the dual frequency excitation system, and it will be expanded globally until chaos appears. The dual excitation intensity leading to chaos is far less than that of single-frequency excitation. The application of an appropriate ultrasonic field is likely to make such a sensitivity passivated, and the stability of the system improved as well.
Growth and thermoelectric properties of Ge doped n-type Sn-based type-Ⅷ single crystalline clathrate
2013, 62 (24): 247401. doi: 10.7498/aps.62.247401
Single crystalline samples of type-Ⅷ Ba8Ga16-xGexSn30 (0 ≤ x ≤ 1.0) clathrates are fabricated by the Sn flux method. The structures and thermoelectric properties of the samples at temperatures ranging from 300 to 600 K are studied. Research results show that the actual content of Ge is relatively small in single crystal. The lattice parameters of the samples decrease slightly with the increase of the doping composition of Ge. The Ge doped samples have lower carrier density and higher carrier mobility than undoped samples. The Seebeck coefficients of all the doped samples are negative, and their absolute values are smaller than those of the undoped one. However, the electrical conductivity of the sample is increased by 62% after doping Ge and the sample of x=0.5 obtains a maximum value of ZT (1.25) at about 500 K.
The effect of out-of-plane strain on the electrocaloric performances of P(VDF-TrFE) vertical heteroepitaxial film
2013, 62 (24): 247701. doi: 10.7498/aps.62.247701
Considering the boundary conditions of 1-3-type P(VDF-TrFE) composite ferroelectric film, the effect of out-of-plane strain on the electrocaloric performances of vertical heteroepitaxial film is calculated by the nonlinear thermodynamic theory. The results indicate that the out-of-plane strain can effectively regulate the ferroelectric and electrocaloric performances including the polarization, electrocaloric coefficient and adiabatic temperature change under the action of vertical electric field. In a wide temperature range, the vertical heteroepitaxial film can present higher adiabatic temperature change than the pure P(VDF-TrFE) film by controlling the out-of-plane strain. This indicates that the vertical composite heteroepitaxial film with excellent electrocaloric performances will have potential applications in the microelectronic devices such as micropowers, optical communication diodes and infrared sensors.
Optimal design of light trapping structure for broadband absorption enhancement in amorphous silicon solar cell
2013, 62 (24): 247801. doi: 10.7498/aps.62.247801
Light trapping is one of the key issues to improve the light absorption and increase the efficiency of thin film solar cell. In this paper, a novel combined light trapping structure consisting of back one-dimensional (1D) Ag nano-grating and front conformal antireflective coating is proposed for amorphous silicon (α-Si) thin film solar cell. By a numerical simulation based on the finite element method, the effect of the combination on the light absorption of α-Si solar cell is investigated, and the Ag nano-grating parameters are optimized. The results show that the combined light trapping structure can enhance broadband absorption in thin-film solar cell. For the α-Si solar cell with the combined structure at P=600 nm, H=90 nm, and W=180 nm, the integrated absorption is enhanced by 103% under AM1.5 illumination at normal incidence in a wavelength range of 300–800 nm, and the photon absorption rate is increased by 300% in a long-wavelength range of 650–750 nm compared with the reference cell. We discuss the physical mechanism of absorption enhancement in different wavelength ranges from the electrical field amplitude distributions in the solar cells. In addition, the solar cell with the combined structure is much less sensitive to the angle of incident light.
2013, 62 (24): 247802. doi: 10.7498/aps.62.247802
The properties of boron doped ZnO (BZO) films are investigated by the pulsed DC magnetron sputtering technique and the plane wave pseudo-potential method based on the density-functional theory. Highly conductive and transparent BZO thin films are prepared using a B2O3:ZnO ceramic target. The effects of the substrate temperature on the structureand electrical and optical properties are systematically investigated. The results show that by increasing the substrate temperature appropriately, the crystallinity, grain size, and carrier mobility of BZO film are improved, and the resistivity is reduced. BZO films of low resistivity (7.03×10-4 Ω·cm) and high transmittance (89%) from 400–1100 nm are achieved at an optimal substrate temperature of 200 ℃. The theoretical results show that B is doped in ZnO mainly in three forms, i.e., in the forms of substitutional BZn atoms, octahedral interstitial site (BIO), and tetrahedral interstitial site (BIT). Among them the formation energy of BZn defect is lowest, and its concentration may be the highest in all the sample concentrations. After incorporation of B, the Fermi level goes through the conduction band. The sample shows a typical n-type metallic characteristic and the optical band gap increases significantly. The carriers originate from the orbits of B 2p, O 2p and Zn 4s.
2013, 62 (24): 247803. doi: 10.7498/aps.62.247803
In this paper, spectroscopic ellipsometry with an incident photon energy range of 2.04.0 eV is used to investigate the optical properties of Mn films deposited on silicon substrates at different sputtering powers. The ellipsometric data are analyzed by Drude and Lorenz oscillators dispersion model. The results show that the refractive index of the film decreases with the increase of the sputtering power. The extinction coefficient of the Mn film increases when the energy of photons is less than 3.0 eV and decreases when the energy of photons is in a range of 3.04.0 eV, and it arrives at an extremum at about 3.0 eV. The extremum shows a red-shift with the sputting power increasing from 60 to 100 W, which is dependent on the quality of the Mn film. With the increase of sputtering power, the extinction coefficient of the film approaches to that of metal manganese. The results also imply that the voids in the film decrease with the increase of the sputtering power, which is conducive to the growth of the films.
2013, 62 (24): 247804. doi: 10.7498/aps.62.247804
Eu (1 wt%) doped 15SiO2-25B2O3-30Al2O3-30Gd2O3 oxide glasses are prepared by high-temperature melt-quenching method in air and reduced atmosphere, respectively. The properties of scintillation and photoluminescence of the two samples are compared. When the samples fabricated in reduced atmosphere are excited by the X-ray and 395 nm violet light, a 450–550 nm blue-green light beam from Eu2+ and a 615 nm red light beam from Eu3+ are obtained. The intensity ratios between Eu3+ and Eu2+ are obviously different under different excitation sources. The intensity ratio under the excitation of the 395 nm violet light is 0.02 while it increases to 1.37 under the X-ray excitation. By analyzing the phonon sideband spectra of the samples, it is found that the Eu3+ ions around the former of Si are completely reduced into Eu2+ ions, while only part of Eu3+ ions around the former of B and Al are reduced into Eu2+ ions. Further investigation about the scintillation mechanism indicates that the sensitization of Gd and the energy transfer of Eu2+→Eu3+ near the formers of B and Al increase the scintillation performances of Eu3+ ions in the reduced glass samples.
2013, 62 (24): 247901. doi: 10.7498/aps.62.247901
High current electron beam emitting from a thermal field emission cathode has an intense space charge effect. In order to investigate the mechanism for the influence of space charge effect on Nottingham effect in thermal field emission, the results of Nottingham effect with and without space charge effect at different work functions and various applied electric fields are obtained numerically on the basis of the theoretical analyses of space charge effect and Nottingham effect. The results demonstrate that the space charge effect has a significant influence on the steady electric field at the cathode surface, and thus the effect of space charge on Nottingham effect is not ignorable. When the work function is in a range of 3.0–4.52 eV and the applied electric field is in a scope of 3×109–9×109 V/m, the average energy delivered per electron in thermal field emission is in a span of 0–2.5 eV larger than that in the case without space charge effect, and the higher the cathode temperature or applied electric field, the larger the difference between them is. The average energy delivered by per electron emitting from cathode is observed to nonlinearly decrease with the increasing of applied electric field when the space charge effect is included. When the cathode temperature is high, the cooling effect in Nottingham effect can be intensified as the gap distance of diode increases.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2013, 62 (24): 248401. doi: 10.7498/aps.62.248401
Considering that the variation of nonlinear characteristics of DC-DC converter with parameter k and the fact that entropy can reflect the statistical characteristic of numerical sequence, in this paper we propose a new method to estimate the nonlinear characteristics of DC-DC converter based on the information about entropy. In this study, taking one-order DCM Buck and Boost converter for example, the distributions of numerical sequences and entropies are analysed under different values of feedback gain k and initial values of x0. The results prove that the entropy of DC-DC converter is determined by feedback gain k and irrelevant to initial value x0, and its final value is smaller than the theoretical maximum value log2N (N is the number of partition), and that entropy can discriminate the period-doubling bifurcation and chaos. So a quantifiable nonlinear dynamical behavior index is obtained which can provide theoretical basis for understanding chaos characteristic and chaos control, and perfect the theoretical analysis method in DC-DC converters.
2013, 62 (24): 248501. doi: 10.7498/aps.62.248501
For the three kinds of composite materials, i.e., Cu(111)/HfO2(001), Cu(111)/HfO2(010) and Cu(111)/HfO2(100), the first-principles method based on the density functional theory is adopted to calculate their rates of mismatching of interface model, interface adhesion energies, the electric charge densities, the electron localization functions, and the charge density differences separately. The results indicate that the rate of mismatching of the Cu(111)/HfO2(010) interface model is lowest and its interface adhesion energy is higher than the others’, which means that the Cu(111)/HfO2(010) is most stable. From the analyses of charge densities and electron localization functions of the three interfaces, it can be found that only the Cu(111)/HfO2(010) interface is able to form the connective electronic channel along the vertical direction of the Cu electrode. This indicates that electrons possess the localizabilty and connectivity along the HfO2(010) direction, which corresponds to the switching-on direction of the resistive random access memory (RRAM) device. The charge density difference analysis reveals that the charge density distributions overlap, the electrons transfer mutually and bond at the interface of the Cu(111)/HfO2(010). In addition, based on the model of Cu (111)/HfO2 (010) interface, the formation energies of the interstitial Cu at different positions are also calculated. The results show that the closer to the interface the Cu atom, the more easily it migrates into HfO2. This indicates that the electrochemical reaction takes place more easily under the applied voltage, which results in the formation and rupture of Cu conductive filaments. All the above findings will provide a theoretical guidance for improving the performances of RRAM devices.
2013, 62 (24): 248502. doi: 10.7498/aps.62.248502
One of the models for the carbon nanotube array with parallel grids is proposed. The actual electric field at the top of the carbon nanotubes and the field enhancement factor are calculated analytically with the image charge method and floated sphere model. The effects of the geometrical parameters of the device and the contact resistance on actual electric field, field enhancement factor at the top of carbon nanotubes, and the field emission current from the gated carbon nanotubes are investigated. The calculation results show that the carbon nanotube array has the best density for field emission when the intertube distance is twice the height of carbon nanotube. The actual electric field and the field emission current from gated carbon nanotube are greatly reduced by the contact resistance. When the contact resistance is larger than 800 kΩ, the emission current from carbon nanotube tends to be zero and the field emission properties are improved via modulating gate voltage.
Effect of a-Si：H interface buffer layer on the performance of hydrogenated amorphous silicon germanium thin film solar cell
2013, 62 (24): 248801. doi: 10.7498/aps.62.248801
In the light of the open circuit voltage and fill factor reduction resulting from band gap discontinuities and high defect densities at interfaces when more germanium is mixed into the intrinsic layer of hydrogenated amorphous silicon germanium solar cell, the insertion of a-Si:H buffer layer with proper band gap into PI interface not only mitigates band gap discontinuities and interface recombination, but also improves the electric field distribution by reducing the defect densities at PI interface, thus the collection efficiency of a-SiGe:H solar cell is enhanced. By inserting a-Si:H buffer layer into IN interface and designing band gap profile along the a-SiGe:H intrinsic layer further, the 8.72% conversion efficiency of single junction a-SiGe:H solar cell is achieved when only Al back reflector is added as back contact.
2013, 62 (24): 248901. doi: 10.7498/aps.62.248901
In this paper, we propose a concept of subnet of maximum controllability based on the model of network controllability, and set up the navigation search model based on the subnet of maximum controllability, called NMSMC. The strategy of adding links that is based on the subnet of maximum controllability is to solve, with the minimum cost, the terminating search, the problem that arises from no way for particles to search in the directed network. Based on the subnet of maximum controllability to deploy navigation nodes,the search time of the whole network can be made close to the average shortest path of the navigation network,which the number of navigation nodes is only 2% of the total nodes. The experimental results of the ER and SF networks show that the search efficiency is strongly correlated with the network controllability. The better the controllability, the less the adding links are, which can lead to the fact that the more the navigation nodes are distributed in the network, the more the search efficiency of the network can be enhanced.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
Experimental measurement of aluminium diffuser applied to calibration system of space-borne differential optical absorption spectrometer
2013, 62 (24): 249301. doi: 10.7498/aps.62.249301
In a space-borne differential optical absorption spectrometer, which has a large field in nadir push-broom mode the “Sun+Diffuser” method is adopted for onboard spectral calibration. Therefore the aluminium diffuser used in the space-borne spectral calibration system is required to have a good Lambert feature to ensure the full field spectral calibration accuracy of the space-borne differential optical absorption spectrometer. And it can provide a uniform source in the observing view-field of the instrument. Using bidirectional reflectance distribution function measurement instrument, bidirectional reflectance distribution function of aluminium diffuser is measured by the relative measurement method. Experimental results show that in a wavelength range of 180–880 nm and an observing view range from -70° to +70°, the bidirectional reflectance distribution function declines from middle to both sides and approximates the cosine distribution, showing that the aluminium diffuser has a good Lambert feature. The spectral calibration of the space-borne instrument is also presented with the system: high calibration accuracy is reached by the calibration system, with the maximum deviation being 0.022 nm, which meets the requirements for the accuracy better than 0.05 nm. The aluminium diffuser measured in laboratory can be chosen for the spectral calibration system.
2013, 62 (24): 249501. doi: 10.7498/aps.62.249501
The equations of thermal radiative transfer are mathematical models that describe the physical processes of photons scattering through and being absorbed in and emitted by a high-energy background material. These processes are important research objects of inertial confinement fusion (ICF). Numerical simulation is an indispensable method for this transportation equation. As an important method in the field of particle transportation, the Monte Carlo method has been widely employed in the fields of linear transportation of neutrons and high energy photons. However, traditional Monte Carlo method does not hold true when it is applied to the simulation of thermal radiation. In this research, an implicit Monte Carlo method based on calculable modeling and numerical simulation is studied. An integral transport equation that is suitable to Monte Carlo simulation is derived. A three-dimensional simulation code is developed, by which some thermal radiative transportation problems are simulated. The results of numerical experiments support that the implicit Monte Carlo method is applicable for thermal radiation transfer simulating. The work is expected to provide an important calculation method and tool for the thermal radiative transfer simulations of ICF.