Vol. 66, No. 18 (2017)
2017-09-20
GENERAL
2017, 66 (18): 180203.
doi: 10.7498/aps.66.180203
Abstract +
Zero-determinant strategy can set unilaterally or enforce a linear relationship on opponent's income, thereby achieving the purpose of blackmailing the opponent. So one can extort an unfair share from the opponent. Researchers often pay attention to the steady state and use the scores of the steady state in previous work. However, if the player changes his strategy frequently in daily game, the steady state cannot attain easily. It is necessary to attain the transient income if there is a difference in income between the previous state and the steady state. In addition, what will happen if evolutionary player encounters an extortioner? The evolutionary results cannot be proven, just using the simulations in previous work. Firstly, for the iterated game between extortioner and cooperator, we introduce the transient distribution, the transient income, and the arrival time to steady state by using the Markov chain theory. The results show that the extortioner's payoff in the previous state is higher than in the steady state when the extortion factor is small, and the results go into reverse when the extortion factor is large. Furthermore, the larger the extortion factor, the harder the cooperation will be. And the small extortion factor conduces to approaching the steady state earlier. The results provide a method to calculate the dynamic incomes of both sides and give us a time scale of reaching the steady state. Secondly, for the iterated game between extortioner and evolutionary player, we prove that the evolutionary player must evolve into a full cooperation strategy if he and his opponent are both defectors in the initial round. Then, supposing that the evolutionary speed is proportional to the gradient of his payoff, we simulate the evolutionary paths. It can be found that the evolutionary speeds are greatly different in four initial states. In particular, the evolutionary player changes his strategy into cooperation rapidly if he defects in the initial round. He also gradually evolves into a cooperator if he cooperates in the initial round. That is to say, the evolutionary process relates to his initial behavior, but the result is irrelevant to his behavior. It can be concluded that the zero-determinant strategy acts as a catalyst in promoting cooperation. Finally, we prove that the set of zero-determinant strategy and fully cooperation is not a Nash equilibrium.
2017, 66 (18): 180204.
doi: 10.7498/aps.66.180204
Abstract +
Computer simulation is always an important means for studying laser, while laser theory is the basis of simulation. Although the semi-classical laser theory can accurately describe the generation process of laser, its complexity leads to a need of huge resources and time for computation. However, in particular cases, the influence of some factors on the laser system can be neglected. If a simpler model is employed to describe the laser system, the time of simulation can be shortened significantly.
In order to simulate the laser system more efficiently, a simulation model of Q-switched solid-state laser is proposed in this paper. In this model, the time-domain function of Q switch is introduced, which represents the modulation of Q switch loss over time. Because the cross section of the Nd:YAG rod is circularly shaped, the resonator eigenmodes are assumed to be a Laguerre-Gaussian beam for simplicity. Then, any other laser beam can be formed by superposition of the eigenmodes of the resonator. These series of resonator eigenmodes are coupled with the rate equations of laser crystals. Finally, the distribution of pump light field inside the laser crystal is approximated as super Gaussian distribution.
Based on this physical model, the influence of pump power and pump light field distribution on the output beam of multimode Q-switched solid-state laser is investigated. The simulation results are in good agreement with the experimental data, which explains the validity of the proposed model. For instance, with the increase of pump power, the output power of the laser increases, but the overall slope efficiency decreases. This is because the diffraction loss m,n of the lower order mode is less than the diffraction loss of higher order mode. When the pumping power increases, the higher order mode that starts to oscillate has lower utilization efficiency of pump energy. Therefore, the overall slope efficiency of the laser is reduced. In order to analyze the mode competition in the multimode Q-switched solid-state laser more comprehensively, the processes of laser pulse generation, relaxation oscillation and continuous oscillation are calculated as one full cycle. The laws of pulse power and beam quality factor versus time are obtained. For example, the maximum instantaneous output power of the relaxation oscillation is about 30 times the steady continuous output power. This law has a certain reference value when analyzing the damage threshold of laser optical element. In the pulse generation stage, the beam quality factor is close to 1, which explains the fact that the pulse field composition is nearly the fundamental mode of the laser. In the relaxation oscillation, the value of the beam quality factor changes irregularly with time, because mode competition is in a non-equilibrium state at this time. When stable continuous oscillation occurs, the mode competition achieves dynamic equilibrium, which means that the proportion of each mode is no longer changed in the output light field.
2017, 66 (18): 180301.
doi: 10.7498/aps.66.180301
Abstract +
With the development of broadband radar technology, transient composite scattering from a target and a randomly rough surface has aroused a great interest in oceanic remote sensing, target identification, and military applications. Time-domain integral equation (TDIE) is an effective numerical method of analyzing transient and broadband electromagnetic problems. However, the high computational complexity of numerical methods restricts its applications in analyzing the electrically large rough surfaces. To improve computational efficiency, hybrid methods have been developed by combining an analytical method with a numerical algorithm, and used to solve the electromagnetic scattering of a composite model. In these hybrid methods, numerical methods are used to calculate the scattering from a target, and analytical methods are employed to solve the scattering from a rough surface. To our knowledge, most of the hybrid methods for composite electromagnetic scattering are frequency-domain algorithms and used to investigate composite scattering from a rough surface with a target above it. Few papers have been published on the analysis of transient scattering from a rough surface with a target by using the time-domain hybrid methods. In the present paper, an efficient time-domain hybrid method that combines time-domain Kirchhoff approximation (TDKA) with TDIE is first designed to investigate the transient electromagnetic scattering from a ship located on a randomly rough sea surface. In this hybrid method, the ship and its adjacent sea surface are chosen as TDIE region and the rest of the rough surface is TDKA region. Considering the interactions between the TDIE region and the TDKA region, the hybrid TDIE-TDKA formula is derived and solved with an iterated marching-on-in-time method. Initially, the induced currents of the TDIE region are acquired by solving TDIE. Then, the currents in the TDKA region are obtained via TDKA method. The interactions between the currents in the TDKA region are neglected. The efficiency and accuracy of the hybrid TDIE-TDKA method depend on the size of the TDIE region. The minimum length of sea surface in the TDIE region is at least the size of the ship due to the strong interactions between the ship and its adjacent sea surface. Numerical results show that the hybrid TDIE-TDKA method presented in this paper is accurate and efficient compared with the full TDIE. Moreover, the influences of the ship size, the wind speed, the incident angle, and the depth of the ship immersing in sea surface on the backscattered far magnetic field are discussed in detail.
2017, 66 (18): 180303.
doi: 10.7498/aps.66.180303
Abstract +
Controllable quantum secure direct communication is an important branch of quantum communication. In this paper, we propose a controlled quantum secure direct communication protocol with channel capacity controllable based on a five-particle cluster state. To start with, the sender Alice prepares the five-particle cluster state sequence and inserts decoy photon randomly, and then sends two parts of the particle sequence to the receiver Bob and the controller Cindy, and meanwhile keeps one part of the particle sequence himself. After Bob and Cindy receive the particle sequence, Alice performs a Z-based single-particle measurement and publishes the measurement results and the position information of the decoy photon through the classical channel. According to the information published by Alice, Bob and Cindy remove the decoy photon and perform a Bell-state measurement to their own part particle sequence. Three sides of communication complete the first safety examination of the channel by checking the bit error rate of the measurement results. After that, the controller Cindy determines the channel capacity by selecting the measurement base (Z basis or X basis) to measure its own particle sequence, and then announces the measured results with classical channel. The sender Alice inserts decoy photon and codes the information by doing a unitary transformation to its own particle sequence and then sends the receiver Bob and tells him the position information of the decoy photon with classical channel. Combining the information published by Cindy with the information transmitted by Alice, Bob can complete the second safety examination of the channel and decode the information Alice has sent by removing decoy photon and performing a Bell-state measurement of his own two groups of particle with appropriate rules. Through an analysis of the entangled structural properties of the five-particle cluster state, it has been confirmed that this protocol is designed to make full use of the entanglement properties of the five-particle cluster in different entangled structures. Therefore the protocol can obviously be generalized into the two-way controlled quantum secure direct communication by simply changing the rules of the measurement and the particles used for unitary coding. Through analyzing the security of this protocol, it reveals that this protocol can effectively both prevent eavesdroppers from eavesdropping useful information and monitor this kind of act, and therefore the controlled quantum secure direct communication can theoretically be established in a certain noise environment.
2017, 66 (18): 180701.
doi: 10.7498/aps.66.180701
Abstract +
The terahertz polarization converter has potential applications in the field of terahertz spectroscopy and imaging. A broadband and high conversion rate of terahertz linear polarization converter based on the metasurface of resonant ring is proposed. The designed structure consists of three layers, i.e., the top layer, which is a metasurface of resonant ring; the bottom layer, which is a metal film of aluminum; a dielectric layer of polyethylene terephthalate, which is sandwiched in between. In order to obtain the best performance, the simulation and optimization are performed by using CST microwave studio. At the same time, the preparation conditions are also taken into account. The optimized geometric parameters of device are obtained. The samples are prepared by using the photolithography and wet etching. The performance of the designed polarization converter is demonstrated experimentally by using the terahertz time domain spectroscopy. The experimental results show that the proposed device can rotate 90° the polarization state of incident terahertz wave of linear polarization in a frequency range from 0.59 THz to 1.24 THz. The polarization conversion rate is more than 80%. The experimental result is in good agreement with the simulated one. By calculating the polarization angle and elliptical angle of the reflected terahertz wave, it is proved that this device can achieve a high-efficiency linear polarization conversion in a wide frequency range. The distributions of surface currents and electric fields are simulated at the frequency with the high polarization conversion rate. The mechanism of high polarization conversion rate is analyzed based on the distribution of surface currents. The performances of broadband and high conversion rate of the designed structure are derived from the third-order electromagnetic resonance. At the same time, the dependence of the polarization conversion rate on incident angle and polarization angle is stimulated and analyzed. The results show that this device has a good polarization conversion performance in an incidence angle range of 0°-30° and a polarization angle range of-10°-10°.
2017, 66 (18): 180201.
doi: 10.7498/aps.66.180201
Abstract +
In electromagnetic radiative transfer calculation, the accuracy and the computation time are usually determined by the representation of single-scattering phase function. Accurate calculation is time consuming even for spherical particle, thus, an analytic representation is commonly adopted to approximate the exact phase function and then accelerate the calculation. Most widely used single-scattering phase functions are the Henyey-Greenstein phase function and modified Henyey-Greenstein phase function (Henyey-Greenstein*). Although the Henyey-Greenstein phase function and the Henyey-Greenstein* phase function can represent the forward-scattering peak of Mie-scattering phase function well, they fail to reproduce the backscattering behavior, limiting the accuracy of the calculation. In order to better fit exact calculations and simulate the backward-scattering peak, we develop a new analytic expression based on the fundamental theory of electromagnetic scattering and radiation transmission. This phase function is an algebraic expression with one single free parameter (asymmetry factor), and can be expanded into Legendre polynomials. The new phase function converges to the Rayleigh phase function when the asymmetry factor approximates to 0, and it can approach to the Henyey-Greenstein phase function as the asymmetry factor is about 1. We compare the Henyey-Greenstein phase function, the Henyey-Greenstein* phase function, and the new phase function for different asymmetry factors, and find that the new phase function provides a more realistic description for the unpolarized light scattering from small particles. Furthermore, the calculated value for the ratio of the scattering intensity at 90 degree to that in the backward direction is more reasonable. We also investigate the effectiveness by approximating the scattering from polydispersed particles through comparing the new phase function, the Henyey-Greenstein* phase function, and the Mie-scattering phase function for three types of Derimendjian's polydispersions. Results show that the new phase function fits the Mie-scattering phase function much better than the Henyey-Greenstein* phase function. For the new phase function, the root-mean-square error is small for 73.3% data. By contrast, only 26.7% data fit the Mie-scattering phase function well for the Henyey-Greenstein* phase function. Similarly, the effectiveness of new function is most significant when calculating the ratio of the scattering intensity at 90 degree to that in the backward direction. In summary, the new Henyey-Greenstein* phase function provides a more accurate calculation for the scattering intensity in the backward direction, and is conducive to electromagnetic radiative transfer calculation. Furthermore, because the proposed phase function has the same basic form as the Heny-Greenstein phase function, reformatting radiative transfer model in terms of the new phase function should require relatively little effort.
2017, 66 (18): 180202.
doi: 10.7498/aps.66.180202
Abstract +
Data compression is crucial for resource-constrained signal acquisition and wireless transmission applications with limited data bandwidth. In such applications, wireless data transmission dominates the energy consumption, and the limited telemetry bandwidth could be overwhelmed by the large amount of data generated from multiple sensors. Conventional data compression techniques are computationally intensive, consume large silicon area and offset the energy benefits from reduced data transmission. Recently, compressed sensing (CS) has shown potential in achieving compression performance comparable to previous methods but it has simpler hardware. Especially, one-bit CS theory proves that the signs of compressed measurements contain sufficient information about signal reconstruction, gives that the signals are sparse or compressible in specific dictionaries, thus demonstrating its potential in energy-constrained signal recording and wireless transmission applications. However, the sparsity assumption is too restrictive in many actual scenarios, especially when it is difficult to seek sparse representation for signals. In this paper, a novel one-bit CS method is proposed to reconstruct the signals that are difficult to represent with traditional sparse models. It is capable of recovering signal with comparable compression ratio but avoiding the dictionary selection procedure.The proposed method consists of two parts. 1) The block sparse model is adopted to enforce the structured sparsity of the signals. It not only overcomes the drawbacks of conventional sparse models but also enhances the signal representation accuracy. 2) The probabilistic model of one-bit CS procedure is constructed. Because of the existence of logistic function in probabilistic model of one-bit CS, the Bayesian inference cannot be used to proceed, and the variational Bayesian inference algorithm is developed to reconstruct the original signals from one-bit measurements.Various experiments on different quantities of compressed measurements and iterations are carried out to evaluate the recovery performance of the proposed approach. The photoplethysmography (PPG) signals recorded from subject wrist (dorsal locations) by using PPG sensors built in a wristband are selected as the validation data because they are difficult to represent with traditional sparse dictionaries. The experimental results reveal that the proposed approach outperforms the state-of-the-art one-bit CS method in terms of both reconstruction accuracy and convergence rate.Compared with prior method on one-bit CS, the proposed method shows competitive or superior performance in three aspects. Firstly, by adopting the block sparse model, the proposed method improves the capability to compress signals that are difficult to represent with traditional sparse models, thus making it more practical for long term and real applications. Secondly, by embedding the statistical properties of the one-bit measurements into the recovery algorithm, the proposed method outperforms other one-bit CS methods in terms of both reconstruction performance and convergence speed. Finally, energy and computational efficiency of the proposed method make it an ideal candidate for resource-constrained, large scale, multiple channel signal acquisition and transmission applications.
2017, 66 (18): 180302.
doi: 10.7498/aps.66.180302
Abstract +
The quantum phase transitions of one-dimensional period-two anisotropic XY models in a transverse field with the Hamiltonian
where the anisotropy parameters i take and alternately, are studied. The Hamiltonian can be reduced to the diagonal form by Jordan-Wigner and Bogoliubov transformations. The long-range correlations Cx and Cy are calculated numerically. The phase with Cx Cy0 (or Cy Cx0) is referred to as the ferromagnetic (FM) phase along the x (or y) direction, while the phase with Cx=Cy=0 is the paramagnetic (PM) phase. It is found that the phase diagrams with the ratio -1 and =-1 are different obviously.
For the case with -1, the line h=hc1=1-[(1-)/2]2 separates an FM phase from a PM phase, while the line =0 is the boundary between a ferromagnetic phase along the x direction (FMx) and a ferromagnetic phase along the y direction (FMy). These are similar to those of the uniform XY chains in a transverse field (i.e., =1). When =-1, the FMx and FMy phases disappear and there appears a new FM phase. The line h=hc2=1-2 separates this new FM phase from the PM phase. The new phase is gapless with two zeros in single particle energy spectrum. This is due to the new symmetry in the system with =-1, i.e., the Hamiltonian is invariant under the transformation 2ix 2i+1y,2iy 2i+1x. The correlation function between the 2i-1 and 2i lattice points along the x (y) direction is equal to that between the 2i and 2i+1 lattice points along the y (x) direction. As a result, the long-range correlation functions along two directions are equivalent. In order to facilitate the description, we call this gapless phase the isotropic ferromagnetic (FMxx) phase.
Finally, the relationship between quantum entanglement and quantum phase transitions of the system is studied. The scaling behaviour of the von Neumann entropy at each point in the FMxx phase is SL~1/3log2L+ Const, which is similar to that at the anisotropic phase transition point of the uniform XY model in a transverse field, and different from those in the FMx and FMy phases.
ELECTROMAGNETISM,OPTICS,ACOUSTICS,HEAT TRANSFER, CLASSICAL MECHANICS,AND FLUID DYNAMICS
2017, 66 (18): 184101.
doi: 10.7498/aps.66.184101
Abstract +
In laser proton acceleration, the inevitable transverse divergence of proton beam restricts its applications in many fields. In this paper, a structured target with a properly wide channel attached to the backside of a foil is proposed, and the interaction of the ultra-short laser pulse with the structured channel target is investigated via two-dimensional particle-in-cell simulation. The simulations show that for the structured channel target, electrons on the front surface are heated by the incident high-intensity laser pulse and then the induced hot electrons transport through the target to the rear surface, building an electrostatic field in the longitudinal direction to accelerate the protons to high energies as the typical target normal sheath acceleration scheme. In the case of the structured channel target, the simulation results indicate that a strong transverse electrostatic field is created by charge separation along the inner surface of the channel while hot electrons propagate along the channel side walls under the guidance of self-induced magnetic and electric fields, which can focus the emitted proton beam transversely, leading to a smaller divergence. By comparing the channel target case with the traditional foil target case under the same conditions, it is found that the divergence angle of the proton beam from the channel target is reduced significantly. Protons with energies above 3 MeV have a divergence angle of 5.3° at the time of 500 fs in the channel target case, while the value is 17.1° in the foil case for a laser intensity of 5.4×1019 W/cm2. Additionally, the effect of the channel target on the maximum proton energy is considered. The simulation results of the energy spectra reveal that the maximum proton cut-off energy of the channel target is about 1 MeV lower than that of the foil target. This small energy loss is due to the refluxing of the cold electrons on the channel walls, which suppresses the increasing of the sheath potential. Therefore, it is concluded that the focusing electric field can work on the proton beam effectively, leading to a better collimation with conserving the proton energy by using the proposed channel target. Especially when the inner diameter of the channel target is comparable to the laser focal spot size, the proton beam can be confined to a small divergence, and a relatively higher laser energy conversion efficiency can be ensured as well.
2017, 66 (18): 184201.
doi: 10.7498/aps.66.184201
Abstract +
Lidar-radar by using an radio frequency modulated (RF-modulated) laser transmitter is a powerful technique for applications involving remote sensing. The method is based on the use of an optically carried RF signal in order to acquire the merits of both the directivity of the optical beam (lidar) and the accuracy of RF signal processing (radar). Compared with single-frequency coherent lidars, lidar-radars are less sensitive to atmospheric turbulence and the speckle noise induced by target roughness. For long range detection, pulsed operation is usually required because of the high peak power. In order to meet the requirement for long range detection, an RF-modulated pulse train based on an all-fiber frequency shifted feedback loop is proposed in this paper. A continuous-wave single-frequency fiber laser (seed laser) is coupled into a fiber link and an acousto-optic chopper is used as a frequency shifter and beam chopper. A Yb3+-doped fiber amplifier is used to compensate for the loss of the signal in the fiber loop. The pulse duration is determined by the open time of acousto-optic chopper, which is fixed at 110 ns. A square wave generated by an arbitrary waveform generator is used as a trigger signal of the acousto-optic chopper. The RF within the pulse results from the interference of frequency shifed pulse with the seed laser. By inserting a 10 km fiber in the loop and accurately controlling the trigger cycle of the acousto-optic chopper equal to the roundtrip time of the loop, the pulse train generated by acousto-optic chopper can circulate in the loop, leading to the generation of RF-modulated pulse with about 20 kHz repetition rate and 110 ns width. The gain provided by fiber amplifier in the loop partially compensates for the loss. By adjusting the gain of fiber amplifier, the modulation depth of RF within the pulse can be continuously adjusted and the maximum modulation depth is 0.67. We also present an time-delayed scalar interference model which includes the loop length, trigger cycle, frequency-shift, and the gain. According to the theoretical model, the RF-modulated pulse affected by trigger cycle and fiber amplifier is numerically simulated. The experimental results accord well with theoritical predictions. The RF-modulated pulse has the advantage of high pulse-to-pulse coherence, which provides potential applications in lidar-radar detection. Besides, with an additional frequency doubling stage one can obtain a source for underwater detections and communications. Extension of the scheme to the 1.5 μm telecommunication window is straightforwardfor various radio-over-fiber applications.
2017, 66 (18): 184202.
doi: 10.7498/aps.66.184202
Abstract +
An accurate aerosol optical property can be obtained by a high spectral resolution lidar (HSRL) technique, which employs a narrow spectral filter to suppress Mie scattering in the lidar return signal. The ability for filter to suppress Rayleigh scattering is critical for the HSRL. In the HSRL system, Rayleigh scattering signal is obtained and aerosol scattering is suppressed at least by a factor of 10-5 through using the narrow filter. Usually, an atomic absorption filter can reach this level. While, the gaseous absorption lines do not exist at many convenient laser wavelengths, thus restricting the development of multi-wavelength HSRL instrument. A new and practical filtering method is proposed to realize the precise detection of atmospheric optical parameters by using the reflection field of Fabry-Perot (FP) interferometer. An optical splitting system with high spectral resolution is designed and its spectral characteristics are analyzed. Based on the characteristic of hyper-spectral lidar detection signal, the variations of spectral separation ratio and Rayleigh signal transmittance with reflectivity and cavity length are discussed. Spectral separation ratio is the transmittance ratio of aerosol scattering signal to molecular scattering signal through the spectral filter. With the increases of FP cavity length and surface reflectivity, the spectral separation ratio decreases and the Rayleigh signal transmission increases. The high spectral separation ratio and Rayleigh signal transmittance can be achieved by the reflection field of FP interferometer when the FP cavity length and reflectivity parameter can be chosen reasonably. We design an FP interferometer with a cavity length of 36 mm and reflectivity of 0.4. Its spectral separation ratio is affected by the echo divergence and incidence angle. The spectral separation ratio keeps unchanged when the beam divergence angle is within 3 mrad and the incident angle of the beam is within 0.5 mrad. In addition, a simulation analysis model is established based on the error propagation. An observed actual Mie-scattering profile is used for analyzing the errors. Moreover, the influences of the divergence angle and the incident angle of the echo beam on detection results are also discussed. The results show that the proposed FP interferometer can achieve fine spectral separation of Mie and Rayleigh scattering signal, and the error of detection result is not sensitive to laser divergence angle. Fine aerosol optical parameters can be achieved when the divergence and incidence angles are controlled within 10 mrad and 1.5 mrad, respectively.
2017, 66 (18): 184301.
doi: 10.7498/aps.66.184301
Abstract +
Aiming at the passive impulse wideband source range problem in shallow water waveguides, a passive source range method with single hydrophone which is applied to the shallow water waveguide with a bottom of liquid semi-infinite space is presented in this paper by combining the group delay theory and warping transformation. The receive signal is composed of several normal modes, and each mode represents many characteristics of the waveguide environment. Warping transformation is a good tool which can achieve the separation and extraction of normal modes from the received signal, and it is also an unitary and reversible transformation, so the warped signal of each normal mode can be recovered completely. The dispersion curves of normal modes can be extracted by warping transformation, and the relation between arrival time and frequency of each order normal mode can also be calculated, and then the time delay of arriving hydrophone between arbitrary two different normal modes is obtained. According to the group delay theory, different order normal mode has different arrival time at the same frequency, and the arrival time of normal mode is determined at its group speed when the distance between the source and hydrophone is certain. So the propagation range can be estimated when the time delay and the slow group speed difference between two different normal modes are known. When the waveguide environmental parameters are known, the slow group speed difference of arbitrary two normal modes can be calculated by KRAKEN. However, when the bottom parameters are unknown, the bottom reflection phase shift parameter is an important parameter describing the acoustic parameters of the bottom, and it contains nearly all the bottom information, what is more, the bottom reflection phase shift parameter is also a parameter that can be extracted by some experimental data easily. When the depth and the average sound speed of the water column are known, the slow group speed difference between two order normal modes can be represented by the seafloor phase shift parameter. Therefore, the source range can be represented by the bottom reflection phase shift parameter, the sea depth and the mean sound speed in the waveguide, and under this condition, the source location can be estimated by one single hydrophone. The effectiveness and accuracy of the method are proved by the numerical simulation results and sea experimental data processing, in which the signals are both received by a single hydrophone. The sea experimental data contain linear frequency modulation impulse source signal and explosion sound source signal, and the mean relative error of range estimation is less than 10%.
2017, 66 (18): 184701.
doi: 10.7498/aps.66.184701
Abstract +
Referring to the construction of shear stress transport-improved delayed detached-eddy simulation (SST-IDDES) method, a variant of IDDES method based on the Speziale-Sarkar-Gatski/Launder-Reece-Rodi (SSG/LRR)-ω Reynolds-stress model (RSM) as Reynolds-averaged Navier-Stokes (RANS) background model, is proposed. Through combining high-order weighted compact nonlinear scheme (WCNS), the SSG/LRR-IDDES method is applied to three aeronautic cases and compared with traditional methods:SST-unsteady Reynolds-averaged Navier-Stokes (URANS), SSG/LRR-URANS, and SST-IDDES. To verify the SSG/LRR-IDDES method in simulating airfoil stalled flow, NACA0012 airfoil is adopted separately at attack angles of 17°, 45° and 60°. At the attack angle of 17°, SST-URANS, SSG/LRR-URANS, and SST-IDDES methods each predict a higher lift coefficient than the experimental data, while the SSG/LRR-IDDES method obtains a better lift coefficient result and a higher fidelity vortical flow structure. It indicates that the RSM can improve the prediction of RANS-mode for pressure-induced separations on airfoil surfaces in detached-eddy simulation. At the attack angles of 45° and 60°, the SSG/LRR-IDDES method captures the massively separated flow with three-dimensional vortical structures and obtains a good result, which is the same as that from the traditional SST-IDDES method. To indicate the improvement of the SSG/LRR-IDDES method in simulating airfoil trailing edge separation, NACA4412 airfoil is adopted. At the attack angle of 12° (maximum lift), the trailing edge separation is mainly induced by pressure gradient. The SSG/LRR-IDDES method can predict the separation process reasonably and obtains a good lift coefficient and location of separation compared with experimental results. However, none of other methods can predict trailing edge separation. It confirms that when RSM is adopted as RANS background model in detached-eddy simulation, the ability to predict pressure-induced separation on airfoil surface is improved. For further verifying the SSG/LRR-IDDES method for simulating three-dimensional separated flow, blunt-edge deltawing at the attack angle of 24.6° is adopted. At this attack angle, the primary vortex will break, which is difficult to predict by using the SST-URANS method. For the SSG/LRR-URANS method, it predicts the vortex breakdown successfully, but the breakdown process does not show any significant unsteady characteristic. The SST-IDDES and the SSG/LRR-IDDES methods both predict a significant unsteady vortex breakdown. But in terms of the accuracy of surface pressure and the fidelity of unsteady flow, the result obtained by the SSG/LRR-IDDES method is better than by the SST-IDDES method.
2017, 66 (18): 184702.
doi: 10.7498/aps.66.184702
Abstract +
For the drainage under the gravity of a vertical foam film containing insoluble surfactant, an improved concentration-dependent disjoining pressure model is formulated based on the published experimental results. The lubrication theory is used to establish the evolution equations of the film thickness, the surface concentration of insoluble surfactant, and the surface velocity, and the evolution characteristics of the film under different disjoining pressures are simulated numerically. The results show that the drainage process of a vertical liquid film generally undergoes two stages:the first stage is the thick film stage and the gravity plays a leading role in the drainage process; the subsequent stage is the thin film stage, the effects of capillary pressure and disjoining pressure increase gradually, and the disjoining pressure dominates the evolution of the film. The disjoining pressure effect is closely related to surfactant type and the correlation strength between the surfactant concentration and electrostatic repulsion force of disjoining pressure. For the ionic surfactant, electrostatic repulsion force increases with the increase of the surfactant concentration, but it is opposite for the nonionic surfactant. It is likely that the free hydroxide ions, which are considered to render the surface negatively charged, are partly adsorbed by the nonionic surfactant. So the surface charge of the foam film decreases as the concentration of the nonionic surfactant increases, resulting in a decrease in electrostatic repulsion. Therefore, some ionic surfactants can improve the stability of liquid film drainage and slow down the drainage process, while the effects of some nonionic surfactants are opposite. When the disjoining pressure is positively correlated with surfactant concentration, with the increase of correlation strength coefficient α, the thinning and drainaging processes of the film tend to slow down, hence the stability of the film is enhanced. When the disjoining pressure is negatively correlated with surfactant concentration, with the increase of the absolute value of α, the drainage process of the film is accelerated and the risk of film rupture is augmented. The results obtained in this paper are consistent with some of the experimental results, indicating that the concentration-dependent disjoining pressure is indeed an important factor in maintaining the stability of foam film containing some certain anionic or nonionic surfactants. The improved concentration-dependent disjoining pressure model established in this paper could not explain the phenomena of parts of cationic nor non-ionic surfactant film in drainage experiments. It can be inferred that the structure of surfactant molecule, the more detailed disjoining pressure model and the coupling of the disjoining pressure and surface elasticity should be considered in the future work.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2017, 66 (18): 185201.
doi: 10.7498/aps.66.185201
Abstract +
The radio-frequency (RF) emissions in a range from 30 MHz to 800 MHz from the plasma, which is produced by the nanosecond laser (532 nm, 8 ns) induced breakdown of atmospheric air, are presented. A spectrum analyzer which can scan over a spectral range of 9 kHz-26.5 GHz is used to record the RF-range radiation intensities of the emission from the plasma. RF electromagnetic radiations from the laser induced breakdown of atmospheric air are obtained for different input laser energies. A half-wave plate and a Glan prism are used to vary the input laser energy. Experimental results show that the intensities of RF radiation in a range of 30-200 MHz increase with the increase of laser energy, but the intensities of RF radiation in a 360-600 MHz frequency range decrease. To study the effect of input laser polarization on the RF radiation, we adopt the input lasers with vertical and horizontal polarization respectively. When the polarizations of the input laser and the antenna are the same, the RF radiation intensity is relatively high, and the frequency lines are relatively abundant. The changing relationship between the total power of RF radiation and the energy of the input laser is calculated and analyzed. It is observed that the total power of RF radiation first increases and then decreases with the increase of input laser energy. The influences of the plasma electron density on the plasma frequency and the plasma attenuation coefficient are investigated to explain the relationship between the total power of the RF radiation and the laser energy. The RF radiation is caused by the following processes. The generated electrons and ions are accelerated away from the core by their thermal pressures. This leads to charge separation and forming the electric dipole moments. These oscillating electric dipoles radiate electromagnetic waves in the RF range. Furthermore, the interactions of electrons with atomic and molecular clusters within the plasma play a major role in RF radiation, and the low frequency electromagnetic radiation takes place from the plasma that is far from fully ionized state. Further study of the characteristics of RF electromagnetic radiation is of great significance for understanding the physical mechanism of the interaction between laser and matter.
2017, 66 (18): 185202.
doi: 10.7498/aps.66.185202
Abstract +
By using pulsed laser induced detachment technique assisted with a Langmuir probe, the electronegative characteristics of the capacitively coupled Ar plasma doped with 5% O2 are studied in this paper. We first focus on the electrical signal of the probe after laser pulse has induced negative ion detachment, and then analyze characteristics of the probe signal with the probe bias below or above the plasma space potential. When the bias is set to be lower than the plasma potential, the probe signal usually shows a downward surge signal. As the bias is higher than the plasma potential, the main characteristics of the signal takes on an upward wide wave packet. The evolution behavior of the probe signal with bias from the downward surge valley to the upward wide wave packet might be due to the potential difference between the plasma space potential and the probe bias voltage. Furthermore, it shows that the position of the upward peak appears later than that of the downward surge valley, which may be related to the changing of the rate of the electron diffusion flux and the electric field drift flux. According to the dependence of probe collection signal on bias, the electronegativity describing the Ar+O2 plasma electronegative property is defined as saturation ratio of electron current after pulsed laser radiation to that of collection probe at a potential above plasma spatial potential. Plasma electronegativity is diagnosed with discharge pressure, radio-frequency (RF) input power and axial position. The experimental results show that the electronegativity of plasma decreases with input RF power increasing. As the gas pressure is kept at 12.0 Pa, the plasma electronegativity decreases from 5.05 to 0.98 with RF input power increasing from 50 to 300 W. It also shows an increasing trend of electronegativity with plasma discharge pressure increasing. Due to asymmetrical distribution of electrodes, the plasma electronegativity also takes on asymmetric one with respect to the axial position. In our experiments, the electronegativity near the power electrode shows about 1-4 times higher than that near the ground electrode, the lowest point of the plasma electronegativity seems to be located in the center of the plasma discharge. This may be related to the dynamics of the secondary electrons emitted from electrode and the competition processes between negative ion production in collisional dissociation of oxygen molecules and the losses of high energy electron and negative ion in collisional detachment of negative ion with oxygen molecule.
2017, 66 (18): 185203.
doi: 10.7498/aps.66.185203
Abstract +
The characteristics of the electrical explosion of aluminum wire in a vacuum and in the air are investigated.The process of energy deposition is derived from the typical voltage and current waveforms.The energy deposited into the aluminum wire at the instant of voltage breakdown is very important for estimating the state of the metal wire.Energy of~2.8 eV/atom is deposited into the aluminum wire in a vacuum at the instant of voltage breakdown.However,the current flowing through the load for the electrical explosion of aluminum wire in the air decreases to zero gradually after the onset of the phase explosion,coming into the dwell stage.Energy of about 6 eV/atom is deposited into the wire at the instant of voltage breakdown for exploding aluminum wire in the air.Temperatures of 0.9 eV and 0.4 eV are estimated for exploding aluminum wires in a vacuum and in the air according to the experimental data combined with the transport coefficient model.The dwell stage is a significant feature for exploding aluminum wires in the air.The dependence of the dwell time on the initial charging voltage of the primary energy-storage capacitor is derived.The dwell time decreases from 95 ns to 17 ns with the increase of the initial voltage from 13 kV to 17 kV.The optical diagnostic equipment with high spatial and temporal resolution is constructed by a 532 nm,30 ps laser probe.The shadowgram demonstrates the expansion trajectories of the high-density products in different media.The expansion velocities of the high-density core for exploding aluminum wire in a vacuum and in the air are 1.9 km/s and 3 km/s,respectively.The energy deposition into the aluminum wire near the electrode region is slightly higher than in the middle region due to the polarity effect, which is analyzed by the distribution of the radial electric field on the wire surface.Because the explosive emission of the electrons is suppressed substantially by the air,the structure of the energy deposition for exploding aluminum wire in the air is more homogeneous.The structures of the energy deposition and the expansion trajectory of the shock wave are analyzed.The schlieren diagnostic is used to translate the exploding products with different refractivities.The schlieren images for exploding aluminum wire in a vacuum show that the metal wire is exploded into two-phase structure,i.e.,the low-density high-temperature corona plasma surrounding the high-density low-temperature core.However,the schlieren images for exploding aluminum wire in the air demonstrate that in addition to the core-corona structure,the channels of shock wave and compressed air layer are formed.The expansion trajectory of the shockwave front is derived according to the optical diagnostics.
CONDENSED MATTER:ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2017, 66 (18): 187101.
doi: 10.7498/aps.66.187101
Abstract +
As is well known, hydrogen plays an important role in degrading mechanical properties of steel due to hydrogen embrittlement behavior. Thus, much attention should be paid to the interaction between hydrogen atom and Fe matrix especially in theoretical calculation and mechanism study. In this paper, the site occupations of hydrogen atom under different single axis strains in interstitial of α-Fe atoms are studied by the first principles calculation based on the spin-polarized density functional theory. The Kohn-Sham equations are solved under periodic boundary conditions, by using revised Perdew-Burke-Ernzerhof version of the generalized gradient approximation to account for the electron exchange and correlation. The total energy of the steady state crystal, binding energy, solution energy, density of states, charge density difference and charge population are calculated. The analyses of solution energy and density of states indicate that the hydrogen atom preferentially occupies the tetrahedral interstitial of α-Fe atoms under different single axis strains. With increasing tensile strain or reducing compressive strain, hydrogen atom prefers to occupy the site of tetrahedral interstitial. The analyses of charge population and charge density difference reveal that the hydrogen atom collects charges from Fe atoms, leading to electron density redistribution. The tensile strain reduces the charge transfer slightly while the compressive stress promotes the transfer process. The LST/QST (linear synchronous transit/quadratic synchronous transit) transition state search method is used to investigate the diffusion of hydrogen atom between two tetrahedral interstitials along the direction perpendicular to strain. Diffusion of hydrogen atom goes through transition state where the hydrogen atom is coordinated at octahedral interstitial. The minimum energy pathway for hydrogen diffusion under strainless state indicates the diffusion activation energy with a value of 0.58 eV. It is noticeable that the diffusion activation energy and the strain conforms to linear relation and are consistent with the fitting formula, Q=0.508+2.6ε. The diffusion activation energy increases with reducing compressive strain or increasing tensile strain. According to the calculation process and analysis results, we introduce the interaction between hydrogen atom and α-Fe at a level of electronic structure systematically and figure out the diffusion of hydrogen influenced by different states of stress.
EDITOR'S SUGGESTION
2017, 66 (18): 187301.
doi: 10.7498/aps.66.187301
Abstract +
In the past decades, the interface between two oxides LaAlO3 (LAO) and SrTiO3 (STO) has attracted much attention since a quasi-two-dimensional electron gas (q2DEG) at the interface was observed. It is generally believed that polar discontinuity at polar/non-polar oxide interface is responsible for the emergence of q2DEG at the interface. Recently, how to modulate the q2DEG at the interface is becoming a new research focus. Capping other oxide thin layer on LAO layer is one of alternative approaches to controlling the generation of q2DEG at interface. However the mechanism or origin for tuning q2DEG at capped LAO/STO interface has not yet completely understood. Using the first-principles calculations within the density functional theory, the electronic properties of La1-xSrxMnO3-capped LaAlO3/SrTiO3 heterointerfaces with different doping concentrations of Sr atoms are investigated. The system is composed of four layers of La1-xSrxMnO3 (LSMO), three layers of LAO and four layers of STO, denoted as 4LSMO/3LAO/4STO. The interface is normal to the[001] direction of cubic phase, namely (La1-xSrxO) layer and (MnO2) layer appear alternately at LSMO, and (LaO) layer and (AlO2) layer appear alternately at LAO. In the absence of LSMO layers, q2DEG does not appear at the LAO/STO interface. It is found that the electronic structure of 4LSMO/3LAO/4STO can be tuned significantly by capping LSMO layers. For concentration of doped Sr atoms less than 1/3, a q2DEG at LAO/STO interface is observed. In this case, a strong polarization existing in LSMO, together with the polarization in LAO, forces the electrons to be redistributed, thus inducing the q2DEG at LAO/STO interface. With the increase of the concentration of Sr atoms, the polarization in LSMO becomes weaker and weaker. When the concentration is higher than 1/3, the polaried electric field fails to make the electrons redistributed, thus the q2DEG disappears from interface.#br#Another interesting feature of the present work relates to the distribution of Sr atoms in LSMO. It is found that the electronic structure of 4LSMO/3LAO/4STO changes little with respect to the distribution of Sr atoms in LSMO. The system does not undergo the conductor-to-insulator transition for Sr atoms doping at different sites as long as the concentration of Sr does not change. The reason could be understood as follows. The LSMO layer is in a metallic state, the extra electrons, which are generated due to substituting La with Sr, will be delocalized rather than localized at each doped Sr atom. It is reasonable to expect that the electronic structure of the system should be less sensitive to the specific doping site of Sr in LSMO.
2017, 66 (18): 187102.
doi: 10.7498/aps.66.187102
Abstract +
Graphite phase carbon nitride (g-C3N4) quantum dots have received much attention due to its good stability, water solubility, biological compatibility, non-toxicity as well as strong fluorescence characteristics. In order to enhance the light absorption and improve photocatalytic activities of the g-C3N4 quantum dots, theoretical studies are carried out on the O and S atoms doped (g-C3N4)6 quantum dots. First-principles calculations based on the density functional theory and time dependent density functional theory are performed to investigate the geometries, electronic structures and ultraviolet visible absorption spectra of O and S atoms doped (g-C3N4)6 quantum dots. The results show that the highest electron occupied molecular orbital-the lowest electron unoccupied molecular orbital (HOMO-LUMO) energy gap of doped (g-C3N4)6 quantum dots is significantly reduced though the CN bond lengths closely related to the impurities only change slightly. The calculated formation energies indicate that the O-doped (g-C3N4)6 quantum dots are more stable, and the O atom tends to substitute for N atom at the N3-site, while the S atoms prefer to substitute for N atom at the N8-site. The simulated spectra indicate that the doping of O and S in (g-C3N4)6 could improve the light absorption. Not only the absorption peaks are extended from the UV to the infrared region (e.g. 200-1600 nm), but also the corresponding absorption intensities are enhanced significantly by doping the O or S atoms with the appropriate concentration. The increase of proper impurity concentration will lead to a pronounced red shift in light absorption. The effect of doping site on the optical absorption property of (g-C3N4)6 quantum dots shows that the absorption intensity is mainly affected in the visible range, however, besides the influence on the absorption intensity, the light absorptions of some structures are also affected beyond 800 nm. Overall, the O atoms and S atoms have a substantially similar effect on the light absorption of the (g-C3N4)6 quantum dots, while the effects of these impurity atoms are different in the long wavelength region. Oxygen doping is better than sulfur doping in the absorption of (g-C3N4)6 quantum dots by comparing the doping of O and S. These first-principles studies give us a method to effectively improve the light absorption of g-C3N4 quantum dots, and could provide a theoretical reference for tuning its electronic optical properties and applications.
2017, 66 (18): 187801.
doi: 10.7498/aps.66.187801
Abstract +
Verdet constant is one of the key parameters to characterize the material magneto-optical properties, and dependent on wavelength and temperature. In order to thoroughly analyze the influence mechanisms of the incident wavelength and temperature on the Verdet constant and then uncover its essence, both the advantages and disadvantages of the classical electronic dynamics theory and quantum theory are discussed on account of basic theories and test data. However, neither of the two theories can be separately used to fully explain the Verdet constant and the correlative test data. Therefore, based on the essential property of the magneto-optical effect, the interactions between the incident light and magnetic matter in a magnetic field are studied, and then a hypothesis which suggests that the Faraday effect result from the combination of various factors is proposed. Furthermore, a theory of wave-transition contribution to the Verdet constant is deduced by adopting the theory of wave-particle duality. That is, the Faraday effect is caused by two different contributions simultaneously. One is the wave contribution, which is the interaction between the wave aspect of light and the magneto-optical medium, and the other refers to the transition contribution, which comes from the electronic transition. When the light enters into a deflection angle, the wave contribution is positive while the transition contribution is negative. In a diamagnetic material, since the wave contribution is greater than the transition contribution, the diamagnetic Verdet constant is positive while in a paramagnetic material, on the contrary, the transition contribution is much larger than the wave contribution, so the paramagnetic Verdet constant is negative. According to the above-mentioned theory, the diamagnetic Verdet constant model and the paramagnetic Verdet constant model are proposed by combining the two parts together. Taking the typical diamagnetic material ZF1 and the typical paramagnetic terbium gallium garnet for example, the influences of the incident wavelength and the temperature on the Verdet constant are analyzed, and the deduced theory together with the corresponding models is tested and verified by analyzing the relevant parameters and the test data. Accordingly, the research turns out that the theoretical results correspond to the real values, which proves the rationality of the hypothesis and the authenticity of the deduced theory. Compared with the traditional theories, the wave-transition contribution theory and its model are superior in the sense of precisely describing the material Verdet constant.
EDITOR'S SUGGESTION
2017, 66 (18): 187901.
doi: 10.7498/aps.66.187901
Abstract +
The research of micro-region emission state for thermal dispenser cathode surface,especially in-situ observation and analysis,is an important subject in the field of thermal cathode.A newly developed instrument aiming at meeting the special operation requirements of thermal dispenser cathode is used to carry out this research.This instrument combines the functions of deep ultraviolet laser photo-emission electron microscope and thermal-emission electron microscope,so it is called DUV-PEEM/TEEM.In this paper,its basic principle is introduced emphatically.In addition,the actual applications of the microscope system to the electron emission investigation of thermal dispenser cathode are displayed. This system is equipped with the heating unit,which is used for activating the thermal dispenser cathode sample,and the temperature of sample can reach 1400℃.The system has three imaging modes,namely,photoemission electron imaging, cathode thermal emission electron imaging,and united imaging by integrating cathode thermal emission electron and photoemission electron.By applying new microscope system to traditional thermal dispenser cathode,we acquire the photoemission electron images of impregnated barium aluminate cathode surface at room temperature.In the heating process,we observe the thermal electron emission phenomenon originating from thermal dispenser cathode and record the variation process with temperature change.A high emission cathode which we developed before,is also studied with DUV-PEEM/TEEM.Fortunately,we find that some bright stripes appear on the surface of high emission cathode when the cathode temperature reaches 800℃.The widths of these bright stripes are about 100 nm.We calculate the thermal emission electron imaging resolution of this system by using these thermal electron emission stripes and the obtained resolution reaches 28 nm.Conveniently,the emission performance and uniformity of this high emission cathode are compared with those of traditional impregnated barium aluminate cathode directly at same temperature. Using united imaging mode of the system,in-situ observation and analysis of thermal electron emission spots on high emission cathode surface are carried out successfully.The results indicate as follows.For thermal dispenser cathode,the deep ultraviolet laser photoemission electron imaging can be used to show the surface fundamental micro-morphology of cathode;cathode thermal emission electron imaging is suitable for revealing the intrinsic emission uniformity of the thermal dispenser cathode;with the united imaging by integrating cathode thermal emission electron and photoemission electron,the positions of effective emission points on cathode surface can be fixed accurately.Based on these applications and findings,we believe that DUV-PEEM/TEEM also has ability to investigate the processes of cathode poisoning and recovery.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2017, 66 (18): 188501.
doi: 10.7498/aps.66.188501
Abstract +
In recent years, infrared (IR) photodetector has been extensively used and played an important role in environmental control, medical diagnostics, and satellite remote sensing. Therefore, the priority should be given to how to stimulate the development of imaging detection of weak IR signal. Up-conversion IR photodetector has an ability to detect quite weak IR signal in the large plane array focal plane, so it has civil and military significance. However, the poor light extraction efficiency due to total reflection severely restricts the overall efficiency of the up-conversion device, which has become one of the bottlenecks in improving the device efficiency.#br#In this work, we propose that the light-extraction efficiency of up-conversion IR photodetector can be improved by a self-assembled monolayer of SiO2 sphere. Thereby, the up-conversion efficiency can be enhanced. The up-conversion IR photodetector emits the light mainly from the silicon nitride (SiNx) passivation layer. And the hexagonal closely-packed SiO2 sphere monolayer is formed on the SiNx layer. In order to study the effect of the size of nanosphere on the light-extraction efficiency, we prepare the SiO2 spheres with diameters of 300, 450, 750, and 1000 nm respectively.#br#Results indicate that the devices with and without SiO2 nanospheres exhibit similar IR responses and dark currents, while the emission of device with SiO2 spheres obviously increases. And the light extraction efficiency increases up to an optimal level when the average size (750 nm) of SiO2 sphere approximates to the wavelength (770 nm) of light source. Taking into consideration other factors relating to external quantum efficiency, the light extraction efficiency of the device with 750-nm-sized SiO2 spheres on surface increases 2.6 times. In order to explain the physical mechanism for the light-extraction enhancement, we carry out the three-dimensional finite difference time-domain simulation, thereby calculating the transmission spectrum of the device with 750-nm-sized SiO2 spheres. Simulation results show that the incident light beyond critical angle can be partly extracted when the surface of up-conversion IR photodetector has a SiO2 sphere monolayer, leading to an enhanced light-extraction efficiency. So the SiO2 sphere monolayer acts as a two-dimensional diffraction grating, which behaves as a light scattering medium for the light propagating in a waveguiding mode within the up-conversion IR photodetector. Therefore it can be concluded that this is a simple and cost-effective method of improving the efficiency of up-conversion IR photodetector. The finding in this paper can also be applied to improving the light extraction efficiency of other semiconductor devices.
EDITOR'S SUGGESTION
2017, 66 (18): 188802.
doi: 10.7498/aps.66.188802
Abstract +
In this paper, for the ITO-SiOx (In, Sn)/n-Si photovoltaic device, the molecular coacervate of In–O–Si bonding and two kinds of quantum states for indium-grafted in amorphous silicon oxide a-SiOx (In, Sn) layers are predicted by molecular dynamics simulation and density function theory calculation, respectively. The results show that the SiOx layers are the result of the inter-diffusion of the In, Sn, O, Si element. Moreover, In–O–Si and Sn–O–Si bonding hybird structures existing in the SiOx layers are found. From the result of formation energy calculations, we show that the formation energies of such an In–O–Si configuration are 5.38 eV for Si-rich condition and 4.27 eV for In-rich condition respectively, which are both lower than the energy (10 eV) provided in our experiment environment. It means that In–O–Si configuration is energetically favorable. By the energy band calculations, In and Sn doping induced gap states (Ev+4.60 eV for In, Ev+4.0 eV for Sn) within a-SiO2 band gap are found, which are different from the results of doping of B, Al, Ga or other group-Ⅲ and V elements. The most interesting phenomena are that there is either a transition level at Ev+0.3 eV for p-type conductive conversion or an extra level at Ev+4.60 eV induced by In doping within the dielectric amorphous oxide (a-SiOx) model. These gap states (GSⅡ and GSIS) could lower the tunneling barrier height and increase the probability of tunneling, facilitate the transport of photo-generated holes, strengthen the short circuit current, and/or create negatively charged defects to repel electrons, thereby suppressing carrier recombination at the p-type inversion layer and promoting the establishment of the effective built-in-potential, increasing the open-circuit voltage and fill factor. Therefore, the multi-functions such as good passivation, built-in field, inversion layer and carriers tunneling are integrated into the a-SiOx (In, Sn) materials, which may be a good candidate for the selective contact of silicon-based high efficient heterojunction solar cells in the future. This work can help us to promote the explanations of the electronic structure and hole tunneling transport in ITO-SiOx/n-Si photovoltaic device and predict that In–O–Si compound could be as an excellent passivation tunneling selective material.
2017, 66 (18): 188902.
doi: 10.7498/aps.66.188902
Abstract +
The k-shell decomposition method of identifying the influential nodes which accelerate spread or hinder propagation, plays an important role in analyzing the spreading performance of complex network, but it is too coarse in terms of ranking granularity. Recent study shows that the accuracy of the k-shell decomposition method in determining node coreness is significantly affected by the mutually densely connected local structures. Existing approach tries to filter out the confusion of the classical k-shell decomposition method, caused by such densely local structures, through redefining a diffusion importance value which is the number of out-leaving links at/from the nodes connected by a link. This value is used to quantify the potential influence of a link in spreading process. However, the existing approach is not suitable for ubiquitously weighted networks. In this paper, we develop a new ranking approach (filter-core) to identify the node spreading influence in weighted network. Here, we concern that the redundant links, although they are less important in the spreading process, form mutually densely connected local structures, which lead to the classical k-shell decomposition method unable to accurately determine the coreness of node in network. By redefining a new diffusion importance value for each link based on the weights of its connected nodes and the weight distribution, we filter out the redundant links which have a relatively low diffusion importance in the spreading process. After filtering out all redundant links and applying the classical k-shell decomposition method to the residual network, we obtain a new coreness for each node, which is more accurate to indicate spreading influence of node in the original network. Our approach is applied to three real weighted networks, i.e., the international trading network, the neural network of C. elegans, and the coauthorship network of scientists. And the susceptible-infected-recovered epidemic spreading model is used to make a comparison of performance between our approach and other three k-shell methods (including the weighted degree decomposition method, the s-core decomposition method, and the weighted k-shell method) in terms of four quantitative indices, i.e., the imprecision function, the standard deviation of infected fraction of max coreness node, the spreading time, and the number of recovered nodes at the end of spreading process. The experimental results show that our proposed approach is more accurate to identify the influential spreaders than the weighted degree decomposition method, the s-core decomposition method, and the weighted k-shell method, and also helps to more accurately decompose the network core structure for an optimal analysis in weighted network.
2017, 66 (18): 188702.
doi: 10.7498/aps.66.188702
Abstract +
In order to measure diffusivity of carbon/carbon composite,a nonlinear fitting method for data processing of transmission pulsed infrared thermography is proposed.It is a kind of method of comparing the experimental data with the theoretical values under a fitting parameter and obtaining the optimal result by an iteration method.Traditional half rise time method calculates the diffusivity through searching the half maximum temperature rise time,which is very difficult when a long capture time is required or a big temperature rise is needed.Unlike the traditional half rise time method,the nonlinear data fitting method can effectively eliminate the capture time restriction and weaken the badsignal-to-noise ratio effects.Before applying this method to carbon/carbon composite examination,a common stainless steel 304 specimen that has reliable diffusivity indicated in the literature,is employed to evaluate the measurement accuracy and confirm the effect of fitting length on the fitting results.The examination results illustrate that the measurement accuracy of stainless steel 304 is as high as 0.3%,and the influence is very small if the fitting data length keeps no less than 1/5 that of half rise time method (t1/2).Specifically,the fitting result changes less than 1% when the fitting length varies from 1 to 4 times of t1/2.With this evaluation result,the nonlinear fitting method is further applied to testing 6 carbon/carbon composite specimens from both sides of each specimen.Furthermore,the diffusivity differences among the specimens and the uniformities of the materials are analyzed through the thermal diffusivity results gained from the examination.The results demonstrate that average diffusivity values of both sides are similar,but the diffusivities among the specimens are different greatly.Of the diffusivities of specimens,the diffusivity value 5.125 is the smallest,while the diffusivity value 6.915 is the biggest.The gap between them is nearly 30% of their mean value. Some nonuniformity areas are also examined from the diffusivity images of carbon/carbon composite samples.So we can obtain not only diffusivity values but also uniformity information of the carbon/carbon composite from this nonlinear fitting transmission thermography examination.
2017, 66 (18): 188901.
doi: 10.7498/aps.66.188901
Abstract +
Previous studies on multilayer networks have found that properties of multilayer networks show great differences from those of the traditional complex networks. In this paper, we derive strictly the spectra of the Supra-Laplace matrix of three-layer star networks and multilayer star networks through unidirectionally coupling by using the master stability method to analyze the synchronizability of these two networks. Through mathematical analyses of the eigenvalues of the Supra-Laplace matrix, we explore how the node number, the intra-layer coupling strength the inter-layer coupling strength, and the layer number influence the synchronizability of multilayer star networks through unidirectionally coupling in two different ways. In particular, we focus on the layer number and the inter-layer coupling strength between the hub nodes, and then we conclude that the synchronizability of networks is greatly affected by the layer number. We find that when the synchronous region is unbounded, the synchronizability of the two different coupling multilayer star networks is related to not only the intra-layer coupling strength or the inter-layer coupling strength between the leaf nodes of the entire network, but also the layer number. If the synchronous region of two different coupling multilayer star networks is bounded, and the intra-layer coupling strength is weak, the synchronizability of the two different coupling multilayer star networks is different with the changing of the intra-layer coupling strength and the inter-layer coupling strength between the leaf nodes and the layer number. If the synchronous region of two different coupling multilayer star networks is bounded, and the inter-layer coupling strength between the hub nodes is weak, the two different coupling multilayer star networks are consistent with the changing of the intra-layer coupling strength and the layer number while different from the inter-layer coupling strength between the leaf nodes and the inter-layer coupling strength between the hub nodes. We find that the node number has no effect on the synchronizability of multilayer star networks through coupling from the hub node to the leaf node. The synchronizability of the network is directly proportional to the layer number, while inversely proportional to the inter-layer coupling strength between the hub nodes. Finally, the effects of the coupling strength, the layer number and the node number on the synchronizability of the two different coupling star networks can be extended from three-layer network to multilayer networks.
2017, 66 (18): 188201.
doi: 10.7498/aps.66.188201
Abstract +
The Li1.4Al0.4Ti1.6(PO4)3(LATP) nanocrystal powder is synthesized by citric acid assisted sol-gel method.The LATP powder is crystalized at 850℃ for 4 h,and the X-ray diffraction patterns show that the NASICON structure is obtained without any impurity phase.The LATP films are prepared by tape casting method through using as-synthesized LATP powder and subsequently recrystalized at various temperatures for 5 h.The impedance spectra of LATP film recrystalized at various temperatures indicate that the film sintered at 950℃ has the highest lithium ionic conductivity. Meanwhile,it is demonstrated that no impurity exists in LATP film recrystalizated at 950℃,and its lattice parameters are a=b=8.50236 Å and c=20.82379 Å.The high-purity LATP-epoxy films are prepared by modification with epoxy resin.The water permeation test proves that the LATP-epoxy film can prevent water from penetrating for 15 d,which indicates that epoxy resin fills the holes in LATP film.The fracture surface topography of LATP-epoxy film shows its dense structure with grain sizes from nano-scale to micro-scale.The energy dispersive X-ray spectrometer mapping of the fracture of LATP-epoxy film indicates that the carbon elements are uniformly distributed in grain boundary,which means that epoxy resin is soaked into LATP film.The relative density of 89.5% is obtained for LATP film,which is increased to 93.0% for LATP-epoxy (the nominal density is around 2.9624 g/cm3).The difference in relative density between LATP film and LATP-epoxy film indicates that the epoxy resin is immersed in LATP film already.The total,bulk,and grain boundary lithium ionic conductivities for the LATP film at 25℃ are 8.70×10-4 S·cm-1,2.63×10-3 S·cm-1 and 1.30×10-3 S·cm-1,respectively.The total,bulk,and grain boundary lithium ionic conductivities for the LATP-epoxy film at 25℃ are 3.35×10-4 S·cm-1,1.84×10-3 S·cm-1 and 4.09×10-4 S·cm-1,respectively.The decrease in the total conductivity of the LATP-epoxy film may be caused by the increase in its grain boundary resistance and its exposure to the atmosphere during modification with epoxy resin.The high lithium ionic conductivity for both LATP film and LATP-epoxy contributes to homogeneous mixture at sol-gel process and the decreasing of grain boundary impedance for this special structure.The activation energies for LATP film and LATP-epoxy film are 0.36 eV and 0.34 eV,respectively, based on Arrhenius equation.The water-impermeable high lithium ion conducting solid electrolyte of LATP modified with epoxy resin is likely to be used as protective film for lithium metal electrode of novel high energy density batteries.
2017, 66 (18): 188401.
doi: 10.7498/aps.66.188401
Abstract +
As the application frequency is increasingly high, it becomes difficult to design joint estimators for the frequencies and directions of arrival (DOAs) under the spatial-temporal undersampling condition. Specifically, on one hand, the temporal Nyquist theorem requires that the sampling rate be at least twice the highest frequency, which is unfordable for the existing analog-to-digital converters; on the other hand, the spatial Nyquist theorem also requires that each inter-element spacing be less than or equal to half the wavelength, which inevitably results in severe mutual coupling among sensors. To solve these intractable problems, in this paper, we propose a joint estimator based on a co-prime sparse array. Firstly, based on the combination of this sparse array and the closed-form robust Chinese remainder theorem (CRT), the theoretical model for the proposed frequency and DOA joint estimator is built up. Secondly, at each sensor, a frequency estimate for the source object can be calculated through implementing the closed-form robust CRT on two frequency remainders, which are generated by applying the Tsui spectrum correction to the discrete Fourier transform results of two receiver sequences. Then, averaging these estimates at all sensors yields the final frequency estimate. Lastly, on the basis of frequency estimation, the final DOA estimate can be calculated through implementing the closed-form robust CRT on all phase-difference remainders, which are also derived from the Tsui spectrum correction. It needs to be emphasized that the proposed joint estimator possesses two attractive merits. One merit is that due to the fact that the proposed array allows a high sparsity of element-spacings, both the hardware cost and the mutual coupling among sensors can be considerably reduced; the other merit is that compared with the existing estimators, the proposed joint estimator achieves high estimation precision even in the single-and-parallel undersampling condition (i.e., multi-time undersampling is bypassed in each sensor element, leading to a high data processing efficiency). In particular, this high accuracy attributes to two aspects:1) the Tsui spectum corrector incorporated in the proposed joint estimator can provide high-accuracy remainders for the CRT recovery; 2) the closed-form robust CRT itself is unbiased and thus the CRT recovery brings no extra system errors. Numerical results verify that the proposed joint estimator possesses both strong noise robustness and high estimation accuracy, which presents a vast potential application in several passive sensing fields such as radar and remote sensing.
2017, 66 (18): 188701.
doi: 10.7498/aps.66.188701
Abstract +
G-quadruplex (G4) is a DNA structure which commonly exists in human genome, and it is considered as an important structure in DNA metabolism such as replication, transcription and homologous recombination. The G-quadruplex helicases have been widely investigated these years. Of them, the Bloom (BLM) helicase is most thoroughly studied. However, there are some basic problems that are still unclear. Most of previous studies of G4 are performed by single molecule fluorescence resonance energy transfer technique. The G4 is in a free state in these experiments, which is different from the physiological environment in cells. The traditional magnetic tweezers have a limitation of spatial resolution in a low force circumstance. Thus here we use high resolution magnetic tweezer under the illumination of total internal reflection fluorescence to study the process of BLM resolving G4. Our modification of magnetic tweezer is to separate the measurements of force and distance of magnetic tweezer in order to improve the spatial resolution, which allows us to observe the unfolding process of G4. With a 2-3 pN force we find that the process of BLM unfolding G4 in low ATP concentration is stepwise, and the G4 is mainly in the state between G-quadruplex and G-triplex. We also find that the BLM could interact with G4 for a long time. Our apparatus is also able to obtain the long time observation results compared with the single molecule fluorescence technique. So we perform experiments with a nearly saturated ATP concentration. We find that the BLM has two ways to maintain G4 dissolution in this condition. The BLM could unfold the G4 repetitively in a long period and it could also keep the G4 in unfolding state for a long time after it has opened the G4. Finally, we also perform single molecule fluorescence resonance energy transfer experiment in the same condition, and we find that the 2-3 pN force in magnetic tweezers has a rare influence on the process of BLM interacting with G4. The results of single molecule fluorescence resonance energy transfer experiments are corresponding to the results of magnetic tweezer in the same conditions. All of our experimental results show that ATP dependent BLM has a high affinity with G4 and BLM has a different way to resolve G4 in high ATP concentration. These results could provide new ideas of the mechanism of BLM resolving G4. Our modified magnetic tweezer shows its capacity in G4 single molecule study, and it could be a useful tool in the future single molecule studies.
2017, 66 (18): 188801.
doi: 10.7498/aps.66.188801
Abstract +
In recent years, with the development of solar cell technology, the conversion efficiency of the lattice-matched Ga0.51In0.49P/In0.01Ga0.99As/Ge triple-junction solar cell has achieved 30% under AM0 spectrum. As is well known, it is difficult to further improve the efficiency due to the limited bandgap combination. Therefore, an inverted metamorphic triple-junction solar cell is designed by replacing the Ge subcell with a 1.0 eV InGaAs subcell. The efficiency could be increased with the open-circuit voltage increasing, while the short circuit current maintains a similar value.#br#In this paper, the inverted metamorphic GaInP/GaAs/In0.3Ga0.7As triple-junction solar cells are grown on 4-inch GaAs substrates via metal organic chemical vapor deposition. Optimizing the epitaxy process, AlInGaAs graded buffer shows nearly 100% relaxation by the reciprocal space mapping of the high-resolution X-ray diffraction and low average threading dislocation density~5.4×106/cm2 evaluated from the cathodoluminescence image. Finally, the inverted metamorphic triple-junction solar cell with 24 cm2 area shows a conversion efficiency of 32% with an open-circuit voltage of 3.045 V and a short-circuit current of 404.5 mA under one sun, AM0 spectrum, 25℃ conditions, which is 5% higher than the lattice-matched GaInP/InGaAs/Ge triple-junction solar cell. Under 1 MeV electron irradiation test, the degradations of the external quantum efficiency and I-V characteristics of inverted metamorphic triple-junction solar cell are exhibited each as a function of fluence, and finally the end-of-life efficiency is 27.2% with a degradation of 15% under 1×1015/cm2 fluence. More experiments mainly focusing on the lattice quality of AlInGaAs graded buffer and the structure of In0.3Ga0.7As subcell, will be carried out to improve the efficiency and enhance the radiation hardness.
CONDENSED MATTER:STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2017, 66 (18): 186101.
doi: 10.7498/aps.66.186101
Abstract +
According to the microstructure of amorphous crystal, the percolation theory, which is a theoretical approach to dealing with the inhomogeneous physical systems or random fractals, is used to describe the plastic flows of amorphous alloys under shear yielding. In order to understand in depth the critical problems about the shear band initiations in amorphous alloys, a percolation model for shear transformations of these alloys is established by combining with the existing free volume model and shear transformation zone model. Taking the binary amorphous alloy Cu25Zr75 for example, the percolation threshold for the shearing instability of the atomic clusters prone to producing plastic flows in the shear transformation zone is calculated when a shear band comes into being. In addition, the size of the above-mentioned cluster is also roughly estimated. The calculated results show that the percolation threshold of the shearing instability is similar to the critical reduced free volume value (xC) of~2.4% for the onset of yielding in amorphous alloy although this threshold is closely related to the dispersity of free volume. The present study may provide a new idea and method of studying the ductile-brittle transition in amorphous alloy.
2017, 66 (18): 186102.
doi: 10.7498/aps.66.186102
Abstract +
Cerium hexaboride (CeB6) as a heavy fermion compound displays a number of interesting low-temperature physical properties such as dense Kondo behavior and a complex magnetic phase diagram due to the interaction between itinerant and local electrons. Recently, the electron emission property of CeB6 has received much attention because it has potential applications in replacing the commercial LaB6 cathode and serving as new-generation thermal cathodes. In addition, by comparison with other metal cathodes, it also possesses some advantages, such as a low work function, low volatility, high brightness, thermal stability and high mechanical strength. However, so far the thermionic emission properties of CeB6 single crystal surfaces except for the (100) surfaces have been rarely reported. Whether the different crystal surfaces of CeB6 contribute to the various interesting emission properties is main research purpose of the present work. In this paper, the (110), (111), (210) and (310) crystal surfaces of single crystal CeB6 are determined by the X-ray Laue diffraction method, and their thermionic emission current densities are measured at different temperatures and applied voltages. As a result, the maximum emission current densities of the (110), (111), (210) and (310) crystal surfaces at 1873 K are 38.4, 11.54, 50.4 and 20.8 A/cm2, respectively. When their cathode temperatures are all 1773 K, their maximum emission current densities are 15.2, 5.43, 28.0 and 11.44 A/cm2. In addition, when the cathode temperature decreases to 1673 K, their maximum emission current densities are 4.24, 0.9, 6.2 and 2.43 A/cm2. It means that the thermionic emissions are strongly anisotropic for the different crystal surfaces. In general, the maximum emission current density of (100) crystal surface of LaB6 single crystal is about 10 A/cm2 at 1700 K. By comparing the emission current density of CeB6 single crystal at 1773 K with that of LaB6 at 1700 K, it is found that the emission properties of (210) crystal surface are maybe close to those of LaB6. The work function values of the (110), (111), (210) crystal surfaces calculated by the Richardson-Dushman formula are 2.64, 2.71 and 2.40 eV, respectively. Among these, the (210) crystal surface possesses the smallest value of the work function, which is hopeful for being used as an electron source of scanning electron microscopy. Zero-field magnetoresistivity measurments confirm the transition temperatures of TQ=3.3 K and TN=2.4 K. Field-angle dependent magnetoresistivity measurments show that the electrical resistivity varies between 69 μupΩ·cm and 73 μupΩ·cm when the crystal rotates from the[001] to the[011] direction. This indicates that the electrical resistivity in a magnetic field is also anisotropic.
2017, 66 (18): 186201.
doi: 10.7498/aps.66.186201
Abstract +
When a shock wave reflects from the free surface of a solid sample, fragments may be emitted from the surface. Understanding the process of the fragments mixing with gas is an important subject for current researches in inertial confinement fusion and high pressure science. Particularly, obtaining the fragments size and distribution is important for developing or validating the physical fragmentation model. At present, the reported quantitative data are less due to the great challenges in the time-resolved measurements of the fragments.#br#Recently, high-power laser has appeared as a promising shock loading means for fragment investigation. The advantages existing in such means mainly include small sample (~μm to mm-order), convenient dynamic diagnosis and soft recovery of fragments. Our group has performed the dynamic fragmentation experiments under laser shock loading metal. The ejected fragments under different loading pressures are softly recovered by low density medium of poly 4-methy1-1-pentene (PMP) foam. The sizes, shapes and penetration depths of the fragments are quantitatively analyzed by X-ray micro-tomography and the improved-watershed method.#br#This paper mainly reports the research advances in the process of the fragments mixing with gas. The laser-driven shock experiments of tin sample are performed at Shenguang-Ⅲ prototype laser facility. Under two typical loading pressures, the fragments mixed with gas (N2) are recovered by PMP foam with a density of 200 mg/cm3, and the pressure of gas is 1 atm. The high resolution reconstructed images of the recovered fragments provided by X-ray micro-tomography and computed tomography reconstruction show that the shapes of the fragments are almost homogeneous, and their sizes are in a range of about 1-20 micron. These images are very different from the images of the fragments recovered in vacuum under similar loading pressures. The observed fragments under loading pressure less than 10 GPa in vacuum are some thin layers, while the loading pressure is increased up to more than 30 GPa, a large number of small spherical particles are observed in the front of the recovery fragments, thin layers in the middle, and these spherical particles have diameters ranging from one dozen to several hundreds of micrometers. The sizes and number of fragments are analyzed by the improved watershed method. The resulting distribution of the fragments mixed with gas follows bilinear exponential distribution. Comprehensive analyses of former simulations and our experimental results show that the secondary fragmentation should occur in the process of the fragments mixing with gas.
2017, 66 (18): 186202.
doi: 10.7498/aps.66.186202
Abstract +
Functional materials have received much attention in the development of scientific technology. Macroscopic function of material is usually linked to the microscopic properties. In order to understand the relationship between structure and function, it is necessary to observe transient structural change of material in real time. In the earlier experimental work femtosecond optical probes were used to measure associated modulation in optical properties like transmissivity or reflectivity and extract the information about structural dynamics through sophisticated theoretical modeling. Since the development of laser-based ultrafast X-ray sources, there has been extensive work on femtosecond X-ray diffraction measurements. The coupling of sensitive X-ray with time-resolved pump-probe technique provides a way to directly monitor the time-dependent lattice structural changes in condensed matter. Recent researches are devoted to the study of non-thermal melting and coherent acoustic photons. The classical continuous elastic equation can only provide a limited view of structural dynamics. So, simulation of structural dynamics at an atomic level and comparison of such simulation with time-resolved X-ray diffraction data are necessary.#br#In this paper, we use the one-dimensional chain model to study the effect of thermal stress on the lattice due to the inhomogeneity of temperature distribution after ultrafast laser heating. It is developed from the classic continuous elastic equation by considering a nanometer film as a chain of point mass connected by springs. The simulation can directly reveal the positon of each point mass (atom) as a function of time for a given temperature (stress) profile. The simulation results accord very well with experimental data obtained with femtosecond X-ray diffraction. Compared with simulation results, the ultrafast X-ray diffraction experimental results are not enough to distinguish the compression near the zero time, but the characteristic time (~123 ps) and broadening of the diffraction peak are clearly observed. The simulation and experimental study of the lattice structural response are of great help for understanding the direct relationship between the lattice responses caused by ultrafast laser excitation, the generation and propagation of strain, one-dimensional chain model has important applications in studying the recoverable ultrafast lattice dynamics of metals, semiconductors and other materials.
