Vol. 66, No. 6 (2017)
2017, 66 (6): 060201. doi: 10.7498/aps.66.060201
Consensus problems, as basic topics in distributed coordination of multi-agent systems, have drawn a great deal of attention from different research fields. Generally, consensus refers to the asymptotic convergence of state variables of all agents with time evolution. In this paper, a concept on partial component consensus in multi-agent system is first given, which is a weaker dynamic behavior of group than the consensus in general, and then the problem of partial component consensus in leader-following first-order multi-agent system with the directed network topology is discussed. By designing an appropriate pinning control protocol and building corresponding error system, partial component consensus in multi-agent system is transformed into the partial variable stability of the error system. Using matrix theory and stability theory, a sufficient condition is given to realize partial component consensus in multi-agent system. Numerical simulations are given to illustrate the theoretical results.
2017, 66 (6): 060502. doi: 10.7498/aps.66.060502
Complex network as a key approach to understanding many complex systems, such as biological, chemical, physical, technological and social systems, is ubiquitous in nature and society. Synchronization of large-scale complex networks is one of the most important issues in network science. In the last two decades, much attention has been paid to the synchronization of complex dynamic networks, especially the meso-scale networks. However, many real networks consist of even hundreds of millions of nodes. Analyzing the synchronization of such large-scale coupled complex dynamic networks often generate a large number of coupled differential equations, which may make many synchronization algorithms inapplicable for meso-scale networks due to the complexities of simulation experiments. Coarse graining method can map the large-scale networks into meso-scale networks while preserving some of topological properties or dynamic charac-teristics of the original network. Especially, the spectral coarse-graining scheme, as a typical coarse graining method, is proposed to reduce the network size while preserving the synchronization capacity of the initial network. Nevertheless, plenty of studies demonstrate that the components of eigenvectors for the eigenvalue of the coupling matrix, which can depict the ability to synchronizing networks, distribute unevenly. Most of the components distribute concentrically and the intervals are small, while some other components distribute dispersedly and the intervals are large, which renders the applications of original spectral coarse graining method unsatisfactory. Inspired by the adaptive clustering, we propose an improved spectral coarse graining algorithm, which clusters the same or the similar nodes in the network according to the distance between the components of eigenvectors for the eigenvalue of network coupling matrices, so that the nodes with the same or the similar dynamic properties can be effectively clustered together. Compared with the original spectral coarse graining algorithm, this method can improve the accuracy of the result of clustering. Meanwhile, our method can greatly reduce algorithm complexity, and obtain better spectral coarse graining result. Finally, numerical simulation experiments are implemented in four typical complex networks: NW network, ER network, BA scale-free network and clustering network. The comparison of results demonstrate that our method outperforms the original spectral coarse graining approach under various criteria, and improves the effect of coarse graining and the ability to synchronize networks.
In recent years, the actual atmospheric predictability has attracted widespread attention. Improving our understanding of weather predictability is vital to developing numerical models and improving our forecast skill in weather and climate events. Given that the atmosphere is a complex and nonlinear system, taking the Lorenz system as an example is a better way to understand the actual atmosphere predictability. Up to now, some predictability problems of the Lorenz system have been investigated, such as the relative effects of the initial error and the model error. Previous advances in the research of predictability mainly focus on the relationship between the predictability limit and the initial error. As is well known, the external forcing can also result in the change of the predictability. Therefore, it is significant to investigate the predictability changing with the external forcing. The nonlinear local Lyapunov exponent (NLLE) is introduced to measure the average growth rate of the initial error of nonlinear dynamical model, which has been used for quantitatively determining the predictability limit of chaos system. Based on the NLLE approach, the influences of external forcing on the predictability are studied in the Lorenz system with constant forcing and Lorenz system with quasi-periodic forcing in this paper. The results indicate that for the Lorenz systems with constant and quasi-periodic forcings respectively, their predictability limits increase with forcing strength increasing. In the case of the same magnitude but different directions, the constant and quasi-periodic forcing both show different effects on the predictability limit in the Lorenz system, and these effects become significant with the increase of forcing strength. Generally speaking, the positive forcing leads to a higher predictability limit than the negative forcing. Therefore, when we consider the effects of positive and negative elements and phases in the atmosphere and ocean research, the predictability problems driven by different phases should be considered separately. In addition, the influences of constant and quasi-periodic forcings on the predictability are different in the Lorenz system. The effect of the constant forcing on the predictability is mainly reflected in the linear phase of error growth, while the nonlinear phase should also be considered additionally for the case of the quasi-periodic forcing. The predictability of the system under constant forcing is higher than that of the system under quasi-periodic forcing. These results based on simple chaotic model could provide an insight into the predictability studies of complex systems.
2017, 66 (6): 060601. doi: 10.7498/aps.66.060601
In order to realize the traceable trans-scale displacement measurements with high resolutions in the fields of fundamental scientific research and ultra-precision machining, we demonstrate a trans-scale heterodyne interferometer with a sub-nanometer resolution, through assembling a compact iodine-stabilized laser at 532 nm. Using modulation transfer spectroscopy, the green laser is traced back to the transition line R(56)32-O(a10), which is one of the recommended spectral lines for meter redefinition. The Allan standard deviation of the laser frequency is 1.310-12 within an average time of 1 s. Compared with most He-Ne lasers, the green laser has a short wavelength and good stability, which leads to a higher resolution. We use two acoustic-optic modulators driven by a two-channel acoustic-optic driver sharing the same crystal oscillator to separate input beams spatially. The frequency of one beam is shifted by 80 MHz and the other is shifted by 82 MHz, which results in a beat frequency of 2 MHz. As a result, the nonlinearity caused by source mixing substantially is reduced. The phase noises of the fibers and two acoustic-optic modulators are well compensated. In order to minimize the difficulty in adjusting the optical path and the error of the measurement, we integrate the interferometry components and design a monolithic prism. The optical resolution of the interferometer reaches to /4. The experiment is carried out in a vacuum environment to reduce the influence of the refractive index of air. High-precision phase measurement technology is used to improve the accuracy of the interferometer. The errors of the interferometer can be classified as random and systematic errors. Random errors include the error from the frequency instability of the laser and the error due to environmental effects. Systematic errors include the phase measurement error and the nonlinearity error. To verify the performance of the interferometer, these errors must be evaluated. In a span of 100 mm, the measurement uncertainties caused by laser wavelength uncertainty, the air refractive index uncertainty, the phase measurement uncertainty and the nonlinearity error are 3 pm, 300 pm, 6.3 pm and 118 pm, respectively. Finally, the performance evaluation shows that the combined uncertainty of the interferometer reaches 322 pm in a span of 100 mm, which is mainly due to the refractive index of air. The heterodyne interferometer meets the requirements for traceable trans-scale measurement with a sub-nanometer resolution, which can be widely used in instrument calibration, length standard making, and geometric measurement.
2017, 66 (6): 060501. doi: 10.7498/aps.66.060501
Network traffic flow is an aggregated result of a huge number of travelers' route choices, which is influenced by the travelers' choice behaviors. So day-to-day traffic flow is not static, but presents a complex and tortuous day-to-day dynamic evolution process. Studying day-to-day dynamic evolution of network traffic flow, we can not only know whether the traffic network equilibrium can be reached and how the process is achieved, but also can know what phenomenon will occur in the evolution of network traffic flow if the equilibrium is not reached. In a real traffic system, taking day as scale unit, the day-to-day network traffic demand is variable and changes with everyday's traffic network state. The travelers' route choices are also influenced by the previous day's behaviors and network state. Then, will the day-to-day network traffic flow evolution be stable? If it is unstable, when will bifurcation and chaos occur? In this paper we discuss the day-to-day dynamic evolution of network traffic flow with elastic demand in a simple two-route network. The dynamic evolution model of network traffic flow with elastic demand is formulated. Based on a nonlinear dynamic theory, the existence and uniqueness of the fixed point of dynamic evolution model are proved, and an equilibrium stability condition for the dynamic evolution of network traffic flow with elastic demand is derived. Then, the evolution of network traffic flow is investigated through numerical experiments by changing the three parameters associated with travelers, which are the sensitivity of travelers' travel demand to travel cost, the randomness of travelers' route choices, and travelers' reliance on the previous day's actual cost. Our findings are as follows. Firstly, there are three kinds of final states in the evolution of network traffic flow: stability and convergence to equilibrium, periodic motion and chaos. The final state of the network traffic flow evolution is related to the above three parameters. It is found that under certain conditions the bifurcation diagram of the network traffic flow evolution reveals a complicated phenomenon of period doubling bifurcation to chaos, and then period-halving bifurcation. Meanwhile, the chaotic region is interspersed with odd periodic windows. Moreover, the more sensitive to cost the travelers' travel demand the more likely the system evolution is to be stable. The smaller the randomness of travelers' route choices, the less likely the system evolution is to be stable. The lower the degree of travelers' reliance on the previous day's actual cost, the more likely the system evolution is to be stable.
2017, 66 (6): 062401. doi: 10.7498/aps.66.062401
High speed imaging technique is an effective method to test the information about pulsed neutron source. Imaging system is usually composed of a pinhole, a scintillator, an image intensifier and a charge-coupled device (CCD) camera. ST401 plastic scintillator is widely used to convert the neutron image into visible light image since it has features of high conversion efficiency and fast time response. When testing a pulsed neutron source of wide energy spectrum, we should evaluate the light yields of ST401 irradiated by neutrons with different energies and make the CCD camera exposed to the light appropriately. A 0.3 MeV pulsed X-ray source is often used to calibrate the imaging system because of its low cost than the D-T fusion neutron source. In this work, a method of evaluating the relative light yield of ST401 irradiated by 0.1-16 MeV neutron to 0.3 MeV X-ray is proposed.Geant4 Monte Carlo software is used to simulate the transport performances of neutrons and X-rays. The software package can simulate the transport process of photons. But the conversion factor of ray energy deposition into photons is unknown. It is difficult to calculate the number of photons generated in ST401 accurately. In this article, we calculate the relative light yield according to the energy of charged particles produced in ST401. Firstly, all information about the particle type, energy deposition, kinetic energy is monitored on event-by-event basis in GEANT4. Secondly, the complete history of the tracks is then used to calculate the light output from the scintillator according to the neutron response functions. Thirdly, the light output caused by charged particles going out of ST401 is deducted. Ratios of average light yield of 1 mm, 3 mm, 5 mm, 1 cm, 2 cm, 3 cm, 5 cm thick ST401 irradiated by 0.1-16 MeV neutron to 0.3 MeV X-ray are given. To confirm the correctness of the simulated result, validation experiment is carried out on IVA pulsed X-ray source and SGIII pulsed neutron source. The simulated ratio of average light yield of ST401 irradiated by one single 14 MeV neutron to 0.3 MeV X-ray has a discrepancy of less than 10% compared with the measured value. Compared with the results of experiment conducted on a constant current source, the simulated results have a maximum discrepancy of less than 44%. If CCD camera exposure 10%-90% of the full scale, the image will have high contrast and information loss can be avoided. According to the simulated results and the neutron yield, exposure can be easily set to be 60% of the full scale by adjusting the gain of the image intensifier. Assume that the simulated results have a 44% discrepancy, the actual exposure will be in a range of 34%-86% of the full scale. Underexposure and overexposure can be avoided by presetting the imaging system sensitivity appropriately based on the simulated results. It implies that the method proposed is effective in predicting the imaging system response to pulsed neutron with wide energy spectrum.
ATOMIC AND MOLECULAR PHYSICS
Electronic structures, magnetic properties and spin-orbital coupling effects of aluminum nitride monolayers doped by 5d transition metal atoms: possible two-dimensional long-range magnetic orders
2017, 66 (6): 063102. doi: 10.7498/aps.66.063102
The magnetism of two-dimensional material is an important research topic. In particular, the long-range magnetic order of two-dimensional material is of great significance in theoretical research and practical application. According to the Mermin-Wagner theory, the isotropic Heisenberg model in a two-dimensional system cannot produce long-range magnetic orders at non-vanishing temperatures. Considering the existence of strong magnetic anisotropy, possible two-dimensional long-range magnetic orders may exist in 5d atom doped two-dimensional aluminum nitride (AlN) monolayer. This research is performed by first-principles calculations based on the density functional theory. Geometries, electronic structures, magnetic properties, and magnetic anisotropy energies from spin-orbital coupling effects in AlN monolayers doped by 5d transition metal atoms (Hf, Ta, W, Re, Os, Ir, Pt, Au, and Hg) are calculated. Four kinds of supercells are used in the calculation, i.e, 22, 33, 44, and 55, with one aluminum atom substituted by one 5d atom. Projection augmented wave method is used to describe the interaction between the valence electrons and the ions. The plane wave is used to expand the wave function of the valence electron. For an optimized geometry, the bond length between the 5d metal atom and the nearest N atom is the largest in Hg-doped supercells, which is 2.093 , followed by the Au, Hf, Pt, Ta, and Ir according to the order of bond length magnitude. For the densities of states (DOSs), obvious impurity energy levels appear in the forbidden bands. For all the supercells, spin-up and spin-down DOSs of Ta and Ir doped systems are symmetric, indicating non-magnetic states. DOSs of Hf, W, Re, and Os doped systems are asymmetric, indicating magnetic states. For Pt, Au, and Hg, DOSs are symmetric in 22 supercells, but asymmetric in the 33, 44, and 55 supercells. Total magnetic moments and the spin densities are also given. In 55 supercells, they are 1.00, 0.00, 0.39, 1.99, 1.17, 0.00, 1.00, 2.00, and 1.00 for Hf, Ta, W, Re, Os, Ir, Pt, Au, and Hg, respectively. The magnetic moment is mainly concentrated in the vicinity of the 5d atoms. The energy differences between ferromagnetic and antiferromagnetic states are calculated. For Hf, Re, Pt and Au systems, the differences in 48 supercells reach the maximum values of -187.2563 meV, 286.2320 meV, -48.0637 meV and -61.7889 meV, respectively. The results indicate that there is a strong interaction between the magnetic centers. Magnetic anisotropy energy originating from spin-orbital effect is calculated in the 44 supercells. For the Re system, it is the highest, reaching 11.622 meV. For W, Os, and Au, the values are larger than 1 meV, showing strong magnetic anisotropies. The magnetic anisotropy can produce a spin wave energy gap, resulting in long-range magnetic orders. Based on the results above, it is predicted that with appropriate 5d atoms and suitable doping concentration, two-dimensional long-range magnetic orders may exist in 5d transition metal atom doped AlN monolayers.
2017, 66 (6): 063101. doi: 10.7498/aps.66.063101
In the process of nuclear waste disposal, the valuable uranium and plutonium are recycled and separated by dissolving the spent fuel in nitric acid. However, transuranic Np greatly influences the process of separation and recovery. Therefore, it is vital to study the structure and properties of nitrate, which is combined with neptunium ions and nitric acid. Furthermore, there are few researches about nitrate formed by tetravalent neptunium ions. So in this article, by using B3LYP hybrid method of density functional theory, the Gaussian 03 program is used to optimize the geometric construction of the coordination compounds Np(NO3)nq (n=1-6, q=-2-+3) formed by the tetravalent neptunium ions (Np4+) and nitrate ion (NO3-). Under the relativistic effective core potential model, the structure parameters and properties are reported. It is found that NO3- coordinates to Np4+ as a bidentate ligand, and the NpN and NpO bonds are the shortest in Np(NO3)22+, while the binding energy of the Np(NO3)4 is the largest. The infrared spectra of Np(NO3)4 are calculated in the gas and liquid phase. Comparing with the available experimental data, the reliability of the calculation results in this work is confirmed.
Theoretical study on the electronic structure and transition properties of excited state of ZnH molecule
2017, 66 (6): 063103. doi: 10.7498/aps.66.063103
The potential energy curves (PECs) associated with the lowest four dissociation limits, i.e., Zn(1Sg)+H(2Sg), Zn(3Pu)+H(2Sg), Zn+(2Sg)+H-(1Sg) and Zn(1Pu)+H(2Sg), are calculated by using a high-level configuration interaction method. The Davidson correction, scalar relativistic effect and spin-orbit coupling effect are taken into account in calculation. On the basis of our calculated PECs of -S and states, the spectroscopic constants including Te, e, ee, Be and Re are evaluated by numerical solution of one-dimensional Schrdinger equation. The computed spectroscopic constants are reasonably consistent with previous experimental results. The dipole moment curves of the 7 -S states are presented, and the influences of the variation of electronic configuration on the dipole moment and bonding property are discussed. The computational results reveal the ionic character of the C2+ state. The variation of -S component for state near the avoided crossing point is illuminated, which is used to explain the change of transition dipole moment (TDM) around the avoided crossing point. Based on the TDMs, Franck-Condon factors and the transition energies, the radiative lifetimes of v'=0-2 vibrational levels of (2)1/2, (3)1/2, (4)1/2 and (1)3/2 states are predicted, which accord well with the available experimental values.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2017, 66 (6): 064101. doi: 10.7498/aps.66.064101
The real and imaginary parts of the eigen complex wave vector in a stratified lossy chiral medium for the case of oblique incidence are derived by using the phase-matching condition. Due to the fact that the real and imaginary parts are nonparallel, the eigen wave propagating in the medium is inhomogeneous. Then the refraction angle of the eigen wave can be deduced via the real part of the wave vector. Finally the propagation matrix of the obliquely incident wave in a stratified lossy chiral medium is derived based on the boundary conditions and the field equations of eigen wave in each region. By using the proposed method, the reflection, transmission, and propagation characteristics of plane wave with arbitrary incident angle in a stratified chiral medium can be analyzed.
2017, 66 (6): 064201. doi: 10.7498/aps.66.064201
The very-large-mode-area (VLMA) fiber is of great importance for suppressing the nonlinear effects which are considered as main limitations to the power scaling-up of high-power fiber lasers and amplifiers. The thermally guiding (TG) VLMA fiber is a novel VLMA fiber, the waveguide of which is formed by the thermal lens effect. Then, a low numerical aperture can be realized, which is promising to achieve the expanding of mode area with a high-quality beam. In order to study the performance of TG VLMA fiber in a fiber amplifier, we present a rate-equation model of the single-mode ytterbium-doped TG VLMA fiber amplifier, which consists of the steady-state rate equations and thermal transferring equations. With this model, the forward-pumped single-mode TG VLMA fiber amplifier is numerically studied. It is found that the diameter of fundamental mode field rises with the increase of the signal power, which shows the superiority of the TG VLMA fiber in suppressing the nonlinear effect in the fiber amplifier. The optimum fiber length and pertinent physical mechanism are also investigated. It is revealed the optimum fiber length is related to the input pump power, and it decreases with the increase of input pump power. However, when the input pump power is large enough, such a variation of optimum fiber length will become weakened. The numerical results also illuminate that the thermal load at the optimum length of TG VLMA fiber should not change too much with the input pump power. Moreover, the mode of output optical field is also discussed. It is found that the thermal load at the optimum length may not be large enough to realize a core-confined mode. In order to ensure that the core-confined mode can be output, the thermal load at the end of the fiber amplifier should be larger. It requires that the fiber length used in the amplifier should be shorter than the optimum fiber length, which will induce the decrease of the output signal power to some extent. In spite of that, the numerical results reveal that the decrease of output signal power should not be much, and the pertinent slope efficiency is not obviously lowered, either. Thus, it is verified that the core-confined mode with a VLMA can be obtained from the TG VLMA fiber amplifier with high slope efficiency. The pertinent results have significant guidance in the design of TG VLMA fiber amplifier.
2017, 66 (6): 064202. doi: 10.7498/aps.66.064202
Being an important optical phenomenon, the linear electro-optic effect has diverse applications in the optical modulation and optical switching. The refractive index ellipsoid theory has been widely used to study the linear electro-optic effect for a long time. Despite of its visualization such a theory has limitations and cannot deal with a lot of cases in which the linear absorption cannot be neglected, or the electric displacement vector has a nonlocal response to electric field, etc. To overcome such shortcomings, in 2001 a wave coupling theory of linear electro-optic effect was developed by She and Lee (She W, Lee W 2001 Opt. Commun. 195 303). And in 2016 we generalized this wave coupling theory to the treatment of nonlocal linear electro-optic effect in which the displacement vector has a nonlocal response to electric field. In this paper, we use this wave-coupling theory to investigate how the linear absorption influences the linear electro-optic effect in a nonlocal medium. Starting from Maxwell's equations and considering the linear absorption and the nonlocality of the susceptibility tensors, we obtain two coupling equations for two orthogonal linear polarized waves and also analytical solutions of the resulting equations, which can be used to describe the nonlocal linear electro-optic effect for a light beam propagating along any direction, with an external direct current electric field applied along an arbitrary direction in a linear absorbent crystal. With such solutions, we study the influences of the linear absorption on the phase, amplitude, shape of the output beam, as well as the half-wave voltage and the extinction ratio of electro-optic modulation. The results show that no matter whether there exists linear absorption, the Rayleigh distance of the Gaussian beam in the crystal will be shortened as a result of the nonlocality of (1). When linear-absorption coefficients 11 and 22 are equal, the linear absorption damps equally the amplitudes of the two polarized output beams with keeping their phases and shapes unchanged. So in the case of 11=22, just as in a lossless medium, the phenomenon that the output beam is no longer a Gaussian beam in an electro-optic amplitude modulation scheme can be considered as a possible signal of the nonlocal response of (2). More interestingly, when 1122, the linear absorption not only reduces the amplitudes of output beams, but also changes their phases and shapes. In such a case one need to measure the nonlocal characteristic length of (2) to judge whether (2) has a nonlocal response. Finally, in the case of 1122, as a result of linear absorption, the extinction ratio is reduced, but the half-wave voltage keeps nearly unchanged in an electro-optic amplitude modulation scheme. Besides the discussion on the influence of the linear absorption, we also make suggestions of how to measure the nonlocal characteristic lengths of (1) and (2) and the absorption coefficients 11 and 22.
Experimental and numerical investigation on the flow structure and instability of water-entry cavity by a semi-closed cylinder
2017, 66 (6): 064702. doi: 10.7498/aps.66.064702
The purpose of this present study is to address instability flowing characteristics and mechanism of the water-entry cavity created by a semi-closed cylinder. For this purpose, an experimental study and a numerical study of the water-entry of a semi-closed cylinder are carried out. According to the results of the experiments and comparison, the cavitating flows between the semi-closed cylinder water entry and the sealing cylinder water entry, and the fluctuation flow pattern form of the semi-closed cylinder cavitation is found around the body. The flow characteristics of the cavity shape are gained by analyzing the image data. A further insight into the mechanisms of perturbation to the flow structure and the cavity fluctuation by the air in the opening cell are studied based on the law of conservation of energy in water entry. According to the results of simulation and comparison with the cavity visualization of experiment, three instability flow phenomena of cavity are formed during the different stages of water-entry, i.e., flow separation destroyed, local flow transformed near cavity, and unique cavity shedding pattern. A further insight into the characteristics of the flow and the distribution of pressure and velocity during the stage of the cavity unstabilized flow is gained. Finally, the formation mechanism of the cavity unstabilized flow is studied based on the boundary layer theory and Helmhotz vortex theory. The obtained results show that the water poured into the cell of cylinder after the opening end has impacted free surface causes the internal air to compress and expand, and as a consequence of these effects, periodic disturbances of flow structure occur around the cavity, then the cavity presents an identical periodic wave flow with air piston motion and the flow stability of cavity is destroyed. At the eve of impacting, the opening end approaches the free surface, which leads to the inflow velocity attenuation rapidly and the pressure increasing in the cell, which creates an initial pressure higher than ambient pressure. Because of the high pressure, air efflux from the cell forms a gas jet injected into the cavity for the first air expansion stage, then the detaching flow is destroyed and the cavity extension diameter is enlarged. The flow in the gas-liquid mixing domain of cavity is seen as an approximate boundary layer flow pattern where favorable pressure gradient on the upwind side and adverse pressure gradient on the lee side appear alternately. As this flow pattern, re-entrant flow acting on the trough of wave cavitation results in the fact that the laminar-turbulent transition is weakened in the trough field and the local gas-liquid mixing domain is thickened to be involved in unstabilized structure as cloud cavitation. The wave cavity presents a partial and multiple shedding pattern occurring at the trough positions in sequence. There is no mutual interference between shedding cavity and the main cavity. Following the cavity shedding, vortex shedding is formed. The vorticity concentrates on the inside of shedding cavity, and the pressure and velocity present a coherent structure.
2017, 66 (6): 064703. doi: 10.7498/aps.66.064703
Laser-driven flyers have unique advantages of high flyer velocity, low cost, simple facility compared with the flyers driven by other conventional dynamic high-pressure loading techniques. With the fast development of laser technique, launching hypervelocity flyers with high-intensity laser pulse has become more and more prevalent. In this paper, we introduce the recent experiments of laser-driven flyers at the SG-III prototype laser facility. Three ways of launching hypervelocity flyers are developed and introduced, respectively. In the first way, multilayered aluminum flyers are gradually accelerated to a terminal velocity of 8 km/s, which is measured by optical velocimetry, without melting and vaporization. The pressure distribution within the flyer shows that the temporally ramped pulse ablation generates a compression wave, and the flyer is accelerated by this wave and its reverberation within the flyer. In the second way, a strong laser ablates the low-density reservoir foil and generates strong shock in the foil. The shock wave is strong enough, and when the shock breaks out from the free surface, the foil will unload as plasma towards the flyer with a density profile. The plasma decelerates upon colliding the flyer, and the single-layered flyer is gradually accelerated by the momentum transition. In our experiments, single-layered aluminum foil and single-layered tantalum foil are accelerated to 11.5 km/s and 6.5 km/s, respectively. According to the pressure distribution within the flyer, the flyer is also accelerated by the compression wave produced by the plasma collision, which is similar to the case of direct ablation by temporally ramped pulse. However, the way of plasma collision could better reduce X-ray and electron preheat and obtain cleaner flyers. In the last way, the flyers are launched by direct strong short-laser ablation. The multi-layered aluminum foil is accelerated to a high average velocity of 21.3 km/s by using a 3-ns quadrate laser pulse at 351 nm after spatial homogenization. A line-velocity interferometer system for any reflect (VISAR) is employed to monitor the processes of flyer launch and flight in a vacuum gap and the shock velocity associated with phase change in fused silica target after flyer impact is inferred. The reflectivity variations of the VISAR fringe pattern and the shock velocity in the fused silica suggest that the flyer owns a density gradient characteristic. Furthermore, specifically designed multi-layered flyers (polyimide/copper) are accelerated by shock impedance and reverberation techniques to a super high averaged velocity of 55 km/s, which is much faster than recently reported results. Light-emission signals of shock breakout and flyer impact on flat or stepped windows are obtained, which indicates the good planarity and integrity for the flyer. Compared with single-layer flyers, multi-layered flyers have a good planarity, and a high energy conversion efficiency from laser to flyers. In this paper, we give a comprehensive analysis and comparison of the experimental designs, technique means and data results about laser-driven flyers. This would provide a reference for further experimental study of laser-driven flyers and also verify that the SG-III prototype laser facility is a very promising facility for studying the hypervelocity flyers launching field.
Broadband circularly polarized high-gain antenna design based on linear-to-circular polarization conversion focusing metasurface
2017, 66 (6): 064102. doi: 10.7498/aps.66.064102
A single-layer reflecting element is proposed based on the principle of linear-to-circular polarization conversion focusing metasurface, which can independently control the phases of x-polarized and y-polarized reflecting waves and operate in a broadband of 10-14 GHz. Following the generalized Snell's laws of reflection, a super cell is designed with a phase-gradient of -60 for x-polarized waves and 60 for y-polarized waves, and the simulation results show the well wideband anomalous reflection as expected. In the design of the multifunctional metasurface, the 1313 unit cells are used to satisfy the parabolic profile and the focal-distance-to-diameter ratio is set to be 0.5. The phase compensation for forming a constant aperture phase is provided by the individual reflected elements with different structure parameters and x-y=90 is used to realize polarization conversion. The designed sample is simulated in CST Microwave Studio and the results show that both of the x-polarized and y-polarized plane waves are well focused through the reflection of the focusing metasurface in a broadband of 10-14 GHz. Traditionally, multi-layer element is used to broaden phase coverage and bandwidth, the single-layer design in this paper greatly reduces the cost, processing difficulty and thickness of the lens. For further application, a linearly polarized Vivaldi antenna with a highest gain of 10 dB is located at the focal point of metasurface and the angle included between its polarization direction and x-axis is 45 in order to acquire right-handed circularly polarized reflecting wave. According to the reversibility principle of electromagnetic wave propagation, the spherical wave radiated by the feed antenna is converted into plane wave by the reflection of the focusing metasurface so that the antenna gain is remarkably enhanced. Simultaneously, the linearly polarized wave can be transformed into circularly polarized wave. Finally, the feed antenna and the metasurface are fabricated, assembled and measured. Numerical and experimental results are in good agreement with each other, which shows that the -1 dB gain bandwidth of the high-gain antenna is 24% (11-14 GHz) and the 3 dB axial ratio bandwidth is 29.8% (10-13.5 GHz). In addition, the gain at 12 GHz reaches a highest value of 19.6 dBic, and the aperture efficiency is more than 54%. The good performances indicate that the proposed broadband high-gain circularly polarized antenna has a well promising application in various communication systems. It is worth noting that the horizontally polarized, vertically polarized, right-handed circularly polarized and left-handed circularly polarized high-gain antenna can be realized with the rotation of feed antenna. In this case the idea is more versatile and valuable for designing the polarization reconfigurable antenna systems.
2017, 66 (6): 064203. doi: 10.7498/aps.66.064203
A controllable wideband multifunctional reflective metasurface is presented. First of all, a polarization-rotating unit cell is proposed by combing micro-electromechanical system (MEMS) technology with reflective metasurface design. The proposed unit cell is characterized by wideband, low loss and controllable properties. Each unit cell is integrated with two MEMS switches. When the two switches operate in different states, the unit cell shows different responses to plane wave incidence, and the corresponding working states can be denoted by 0 or 1. It is worth noting that a 180 degree reflection phase difference is generated for the two working states. Then, the proposed unit cell is periodically arranged to construct a metasurface. Based on different coding matrixes, multiple functionalities can be obtained by using the proposed metasurface. When all the unit cells are controlled to operate in on- or off-state, polarization-rotating function is obtained. Besides, the agility scattering field performance is also presented by using chessboard and random codings. A series of equations is derived to reveal the relationship between reflection coefficient of the unit cell and radar cross section (RCS) reduction of the chessboard reflective surface, which is also verified by full-wave simulations. Finally, four prototypes consisting of 576-cells, which correspond to the all 0, all 1, chessboard and random coding, are fabricated and measured. The measured results demonstrate that the proposed reflective metasurface shows polarization-rotating performance in a frequency range of 8.9-13.2 GHz when all unit cells operate in 0 or 1 state. The measured results of the chessboard and random coding metasurface manifest remarkable RCS reduction compared with the same size metal plane. Good agreement between simulations and measurements is obtained. Owing to the ability to control polarization and beam shape of the reflected wave dynamically, the proposed reflective metasurface has potential applications in the field of intelligent stealth.
Asymmetric waveguide and the dual-wavelength stimulated emission for CdS/CdS0.48Se0.52 axial nanowire heterostructures
2017, 66 (6): 064204. doi: 10.7498/aps.66.064204
Semiconductor axial nanowire heterostructures are important for realizing the high-performance nano-photonics and opto-electronics devices. Although different IV and III-V semiconductor axial nanowire heterostructures have been successfully prepared in recent decade, few of them focused on the optical properties, such as the waveguide, due to their low light emission efficiencies. The II-VI semiconductor nanowires grown by chemical vapor deposition strategy, such as CdS, CdSe and their alloys, can act as nanoscale waveguide, nanolasers, etc., because of their high optical gains and atomically smooth surfaces. However, it is still a challenge to growing the high-quality II-VI semiconductor axial nanowire heterostructures, owning to the poor controllability of the vapor growth techniques. Here, the CdS/CdSSe axial nanowire heterostructures are prepared with well controlled CVD method under the catalysis of annealed Au nanoparticles. The scanning electron microscope characterization shows that the wires have smooth surfaces with Au particles at the tips, indicating the vapor-liquid-solid growth mechanism for the nanowire heterostructures. The microscope images of the dispersed wires illuminated with a 405 nm laser show that the red and the green segment align axially with a sharp interface, demonstrating the axial alignment of CdS and CdSSe segments. The position related micro-photoluminescence spectra exhibit near band edge emissions of CdS and CdSSe without obvious emission from defect states, which suggests that the wires have highly crystalline quality. The waveguide of the nanowire heterostructures is studied through respectively locally exciting the two ends of the wire with a focused 488 nm laser. The local illuminations at both the CdS end and the CdSSe end result in red emission at the corresponding remote ends of the wires, with the emission intensity of the former being one order lower than that of the later, which is caused by the reabsorption of the green light emission (from CdS segment) in the CdSSe segment. This indicates the asymmetric waveguide in these heterosturctures, which implies that the CdS/CdSSe nanowire heterostructures have the potential applications in the photodiode. Under the pumping of 470 nm femtosecond laser, dual-color (red and green) lasing is realized based on these wires, with the lasing threshold of red light lasing being lower than that of the green one, which results from the larger round-trip loss for the green light arising from the self-absorption in CdSSe segment. To prove that the light can be transfer between the two segments with different refractivities, the waveguide of the nanowire heterostructure is simulated by the COMSOL. The result shows that the light can effectively propagate between CdS and CdSSe segments, which ensures the light-matter interaction in the axial CdS/CdSSe nanowire heterostructures as discussed above. These high-quality CdS/CdSSe axial nanowire heterostructures can be found to have the potential applications in photodiodes, dual-color nanolasers and photodetectors.
Low frequency band gap characteristics of double-split Helmholtz locally resonant periodic structures
2017, 66 (6): 064301. doi: 10.7498/aps.66.064301
A double-split Helmholtz periodic structure with the characteristic of local resonance is designed and constructed in this paper. The double-split periodic structural cell which can be divided into internal and external cavities is adopted in structure. In such a kind of structure, the resonating area is remarkably expanded while the inner cavity is continuously enlarged. Thus, a satisfactory feature of low frequency resonance can be obtained. At the same time, the adjustability of band gap is achieved by the designed adjustment of the arc length of the inner cavity, therefore, the effect of sound insulation in a specific low frequency band can be achieved. In the analyses of the mechanism and factors of the generation of low frequency band gap, the mathematical model of the upper and lower limits of the band gap is established by using the electric circuit analogy. And some comparative analyses between the methods of electric circuit analogy and finite element method are carried out. The result suggests that a satisfactory feature of low frequency band gap is presented, and the first band-gap ranges from 86.9 Hz to 138.2 Hz. The low frequency band gap can be influenced by the arc length of inner cavity, the space between inner and outer cavities, and the interaction of the structural cells in the periodic arrangement. The longer the arc length of the inner cavity, the lower the low frequency band gap will be; the longer the distance between inner and outer cavities, and the higher the frequency of band gap, the worse the low frequency effect will be; the lower limit of low frequency band gap cannot be influenced by reducing the space between individual structures, on the contrary, the width of low frequency band gap can be sharply increased. Plenty of practical and theoretical support in the field of low frequency noise reduction is offered in the research.
2017, 66 (6): 064701. doi: 10.7498/aps.66.064701
The shock-bubble interaction is a basic configuration for studying the more general case of shock-accelerated inhomogeneous flows. In previous studies, a planar shock wave interacting with a spherical gas bubble was extensively investigated, in which the effects of shock intensity, Atwood number and secondary shock on the bubble development were considered and elucidated. However, in most of practical applications, such as inertial confinement fusion, a converging shock wave is generally involved. It is therefore of fundamental interest to explore the perturbation growth under converging shock conditions. Due to the difficulties encountered in generating a perfectly converging shock wave in laboratory, experimental investigation on the converging shock-accelerated inhomogeneous flows was seldom carried out previously. The preliminary study on the development of a gas bubble impacted by a converging shock wave showed that a large discrepancy exists compared with the planar counterparts. Because of the intrinsic three-dimensional (3D) features of this problem, the current experimental techniques are inadequate to explore the detailed differences between planar and converging shocks accelerating gas bubbles. As a result, numerical simulations become important and necessary. In this work, evolution of an SF6 spherical gas bubble surrounded by air accelerated by a cylindrical converging shock wave and a planar shock wave is numerically investigated by a 3D program, focusing on the convergence effect on the interface evolution. Multi-component compressible Euler equations are adopted in the 3D program and the finite volume method is used. The MUSCL-Hancock scheme, a second-order upwind scheme, is adopted to achieve the second-order accuracy on both temporal and spatial scales. Compared with planar shock wave, a cylindrical converging shock wave has curvature, and as the converging shock wave moves forward, the shock strength and the wall effect both increase, which will result in the diversity of the flow field after shock impact. The numerical results show that the vortex rings formed under converging shock condition are sharper than those under planar shock condition which may be associated with geometric contraction effect of the tube and reflected shock from the wall. Besides, the peak pressure generated in the vicinity of the downstream pole of the bubble under converging shock condition is higher than that of planar shock wave, and, therefore, the jet induced by high pressures moves faster under converging shock condition. Due to the variations of shock curvature and shock intensity, the distribution law and amplitude of vorticity generated by converging shock wave at the interface is changed. Comparison between circulation and gas mixing rate indicates that the converging shock is beneficial to promoting vorticity generation and gas mixing. From the present work, it can be concluded that the convergence effect plays an important role in interface evolution.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2017, 66 (6): 066104. doi: 10.7498/aps.66.066104
Silicon carbide (SiC) is considered as one of the most promising structural and coating materials for advanced nuclear applications, due to its low neutron capture cross section and excellent irradiation resistance. The difference in swelling behavior between monocrystalline and polycrystalline SiC is experimentally investigated by heavy ion irradiation at room temperature (RT). In this work, single crystal hexagonal (6H) SiC and polycrystalline chemically vapor-deposited (CVD) SiC are irradiated by 1.5 MeV Si ions with the fluences of 11014-21016 cm-2 and 11015-21016 cm-2, respectively, at RT. The step height of irradiation swelling is measured by a white light interferometer and the lattice expansion of the damage layer is characterized by using X-ray diffraction (XRD) spectrometry, in addition, the actual irradiation swelling is obtained by dividing the height of swelling step by the depth of damage layer. The XRD profiles show that the lattice expansion in the damage layer increases with the increase of irradiation fluence, and the new diffraction peak relating to the lattice structure of damage layer disappears in a fluence of 21015 cm-2, which means that the damage layer is completely amorphous at this time and the threshold dose of amorphization at RT in single crystal 6H-SiC is less than 0.8 dpa. The direct-impact model is used to fit the swelling step heights of CVD SiC and 6H-SiC irradiated by 1.5 MeV Si, and the swelling results show that the amorphization threshold dose of polycrystalline CVD SiC is larger than that of single crystal 6H-SiC. In the present work, three distinct stages are found in the heavy-ion irradiation swellings between monocrystalline and polycrystalline SiC. i.e., low-fluence region, intermediate-fluence region, and high-fluence region stage. 1) In the low-fluence region, the swellings are similar to each other, since the swelling is mainly contributed to by point defects in this region, and the micron sized grains in polycrystalline CVD SiC are of single crystal structure. 2) In the intermediate-fluence region, the irradiation swelling of the polycrystalline CVD SiC is smaller than that of the single crystal 6H-SiC, since the irradiation-induced amorphousness in polycrystalline CVD SiC is relatively hard to occur due to the existence of grain boundary in this region. 3) The irradiation swellings of 6H-SiC and CVD SiC are almost the same at the high-fluence region stage, since the irradiation swelling is caused by amorphization in this region, and the swelling depends on the difference between densities before and after irradiation. In addition, in the irradiation swelling analysis of SiC materials, XRD swelling measurement method is suitable for irradiation swelling induced by point defects, especially for neutron irradiation experiments.
2017, 66 (6): 066201. doi: 10.7498/aps.66.066201
One of the main challenges in developing future stretchable nanoelectronics is the mismatch between the hard inorganic semiconductor materials and the ductility requirements in the applications. This paper shows how the kirigami architectural approach, inspired by the ancient Japanese art of cutting and folding paper applied on macroscale, might be an effective strategy to overcome this mismatch on nanoscale. In this work, the tensile large deformation and mechanical behaviors of armchair and zigzag graphene kirigami with rectangles and half circles cutting patterns are investigated based on classical molecular dynamics simulations. The effects of three non-dimensional geometric parameters that control the cutting patterns on the mechanics and ductility of graphene kirigami are also studied systematically. The results indicate that the enhancement in fracture strain can reach more than five times the fracture strain of pristine graphene. The defined three parameters can be adjusted to tailor or manipulate the ductility and mechanical behaviors of graphene. These results suggest that the kirigami architectural approach may be a suitable technique to design super-ductile two-dimensional nanomaterials and potentially expand their applications to other strain-engineered nanodevices and nanoelectronics.
2017, 66 (6): 066101. doi: 10.7498/aps.66.066101
Evaporation of colloidal droplets often leads to various deposited patterns which are not only interesting but also provide a very simple and useful method to fabricate functional materials. The patterns induced by the evaporation can be tuned via several factors, among which the roughness of the substrate is an important one. However, the effect of nano-scaled roughness is scarcely studied and far from being fully understood. In this work, the evaporation and pattern formation of SiO2 colloid droplets are studied on smooth substrate and nano-rough substrate, respectively. The aim of this work is to clarify how the evaporation dynamics and patterns are influenced by nano-scaled roughness. The roughness of the substrate is analyzed by using a scanning electron microscope and an atomic force microscope, the evaporation process and pattern formation are monitored via an in-situ microscope observation. The obtained deposited patterns are analyzed by using stylus profiling. It is found that the evaporation of droplets is accompanied by an obvious coffee ring effect on smooth substrate and the deposition patterns are bowl-shaped. However, uniform thickness evaporation patterns are obtained through evaporation on rough substrate, moreover, the crack density increases obviously. The analysis shows that nano-roughness is able to inhibit the circumfluence of droplets along the substrate, which greatly weakens the compensation for capillary flow, leading to particles gathering at air-droplet interface and formulating a particle layer. This prevents the coffee ring effect, and eventually results in the formation of evaporation patterns with uniform thickness.
Theoretical improvement on the determination of effective elasticity charges for charged colloidal particles
2017, 66 (6): 066102. doi: 10.7498/aps.66.066102
According to the existing shear modulus-pair potential relationship model for colloidal crystal comprised of highly charged colloidal particles, the calculated shear moduli of colloidal crystals are much larger than the measured values by the torsional resonance spectroscopy (TRS). Moreover, by using the relationship model, the effective surface charge of colloidal particles, obtained by fitting values of shear moduli measured by TRS (effective elasticity charge), is smaller than that obtained through the experimental method of conductivity-number density relationship (effectively transported charge). So far there has been no practical explanation to this discrepancy. Our analysis shows that this discrepancy is because the existing relationship model is for the perfect crystals and does not include the defects such as voids which can result in the decrease of mechanical properties of materials. The existing shear modulus-pair potential model will be improved by introducing the effect of voids, which is inspired from the Gibson-Ashby model in the study of cellular solid. The Yukawa potential, which considers Coulomb repulsions between colloidal particles and is usually used in the model expressions, will be substituted by Sogami-Ise potential, which considers a long-range attraction in addition to that Coulomb repulsions and accepts the existence of voids inside the colloidal crystals. For five different kinds of highly charged colloidal particles, the shear moduli with different volume fractions are measured by TRS. Then the fitted effective surface charges using the original and improved model respectively are compared with each other. It can be concluded that the effective elastic charge obtained by the improved model is more suitable and much closer to the renormalized charge obtained from Alexander's method. It is also clear that neither the effectively transported charge nor the Alexander's renormalized charge can be used to evaluate the shear moduli of colloidal crystals with voids inside. These results can also let us further understand and use the effective surface charge in the colloid studies.
2017, 66 (6): 066103. doi: 10.7498/aps.66.066103
Because of the low density, high specific strength and excellent performance at high temperature, -TiAl based alloy has become a new generation of materials in the aeronautic field. However, its poor ductility at room temperature set a limitation to its wide applications. In this paper, the crystal structures, stabilities and ductilities of La-doped -TiAl systems are investigated by using first principles method based on density functional theory, in which Ti or Al is substituted by La and the impurity content values are 1.85 at.%, 2.78 at.%, 4.17 at.%, 6.25 at.%, 8.33 at.% and 12.5 at.%, respectively. The results show that all of the La-doped alloys have good energy stabilities, namely they can be prepared experimentally, when the impurity concentration x of system is less than or equal to 12.5 at.%. And the density of the La-doped system is less than 4.6 gcm-3. La doping induces the lattice parameters and the axial ratio of the alloy system to change. The axial ratio of La-doped system with low impurity concentration (x6.25 at.%) is closer to 1, which is very beneficial to improving the ductility of the materials. It is predicted that the system Ti11LaAl12 would have the best ductility among those of the investigated systems, for its axial ratio is the closest to 1. The electronic effect about the ductility of La-doped system is discussed through the comparisons of the populations, charge densities and densities between the states of systems Ti11LaAl12 and Ti12Al12. It is found that the system Ti11LaAl12 presents a state of electron redistribution in valence electron orbitals of Al and Ti due to an atom of titanium substituted with that of lanthanum. The charge numbers of Ti-d and Al-p orbitals and the numbers of electrons can be delocalized by reducing the p-d orbital hybridization. Thus, the intensity of p-d orbital hybridization is weakened, the resistance of dislocation movement is reduced, and the ductility of TiAl systems can be improved. Actually, the new electron redistribution shows different properties of some chemical bonds, in which some of covalent AlTi bonds are replaced by ionic AlLa bonds and some of covalent TiTi bonds are replaced by metallic TiLa bonds. Therefore, the covalent and directional properties of chemical bonds are reduced distinctly while the metallic properties of materials are strengthened. The average intensity of AlAl bonds decreases and those of AlTi and TiTi bonds are increased in the La-doped -TiAl system (Ti11LaAl12). As a result, the differences between the three kinds of chemical bonds diminish and the degree of isotropy of the crystal structure increases, which can greatly improve the ductility of -TiAl alloy.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
Determination of dislocation density of a class of n-GaN based on the variable temperature Hall-effect method
2017, 66 (6): 067201. doi: 10.7498/aps.66.067201
An analytical model for electron mobility in a class of wurtzite n-GaN, whose carrier concentration is over 1018 cm-3 (Mott's critical limit), is developed. With the dislocation density and two donor levels serving as the important parameters, the proposed model can accurately predict the electron mobility as a function of temperature. The edge and screw dislocation densities in two samples, which are respectively grown on sapphire (001) by metal organic chemical vapor deposition and hydride vapor phase epitaxy, are determined by using this model which is discussed in detail. It is shown that the data-fitting of H-T characteristic curve is a highly suitable technique for accurately determining the edge and screw dislocation densities in n-GaN films. Quantitative analyses of donor concentration and donor activation energy indicate that the impurity band occurs when the carrier concentration is under 1017 cm-3, much lower than the critical carrier concentration of Mott transition (1018 cm-3). Such a behavior can also be confirmed by the results from solving the Boltzmann transport equation by using the Rode iterative method. Another anomaly is that the dislocation density in Mott transition material perhaps is lower than that of material with carrier concentration under 1018 cm-3. This fact indicates that the cause of Mott transition should not be the shallow donor impurities around dislocation lines, but perhaps the deeper donor impurities or other defects. In the theoretical model calculation, two transition characteristics together with the donor distribution and its energy equilibrium are taken into account. Based both on the Mott transition and the H-like electron state model, the relaxation energies for the shallow-donor defects along the screw and edge dislocation lines are calculated by using an electrical ensemble average method. Besides, an assumption that should be made is that there are 6 shallow-donor defect lines around one dislocation line. The research results show that the Hall mobility should be taken as the live degree of the ionizing energy for the shallow-donor defects along the dislocation line. The experimental results indicate that our calculation function can be best fit by the experimental curve, with the values of dislocation density being between our model and others determined by X-ray diffraction or by chemical etching method, which are all in good agreement with each other. The method reported can be applied to the wurtzite n-GaN films grown by various preparation technologies under any condition, with the peak-mobility temperature about or over 300 K, whose Hall mobility near 0 K perhaps is over 10 cm2/(Vs) and even 100 cm2/(Vs).
2017, 66 (6): 067202. doi: 10.7498/aps.66.067202
There is a controversy over the magnetic source and mechanism of the coexistence of Al-doping and Zn vacancy or Al doping and O vacancy in ZnO systems. In order to solve the problem, the combined influence mechanism of Al doping and Zn vacancy or Al doping and O vacancy on magnetism of ZnO is studied by using the first-principle calculation in this work. The coexistence of Al doping and Zn vacancy can achieve Curie temperature higher than room temperature. Moreover, the magnetism of the doping system of Al doping and Zn vacancy is mainly contributed by electron exchange interaction through O 2p and Zn 4s states near the Zn vacancy through taking carrier as medium. However, the system of Al doping and O vacancy is non-magnetic. Meantime, in the coexistence of Al doping and Zn vacancy or O vacancy, a close relative distance between doping and vacancy will reduce the formation energy of the doping system, increase the easiness of accomplishment of doping and vacancy, and enhance the stability of the doping system.
2017, 66 (6): 067301. doi: 10.7498/aps.66.067301
In the paper, the core-shell ZnSe quantum dots (QDs)-sensitized mesoporous La-doped nano-TiO2 thin film is prepared by a direct adsorption method. Photoelectron characteristics, photogenerated carriers transport mechanism, and microstructure of the QDs-sensitized nano-TiO2 thin film are probed via the stationary surface photovoltaic (SPV) and the transient photovoltaic technologies, supplemented by the Brunauer-Emmet-Teller adsorption isotherm technique, scanning electron microscope, Fourier transform infrared (FT-IR) absorption spectrum, and ultraviolet-visible (UV-VIS) absorption spectrum. The experimental results confirm that the surface of the nano-TiO2 film is covered with the ZnSe QDs with smaller particles by a chemical absorbing way, resulting in denser composite film of the QDs and the mesoporous nano-TiO2 than the nano-TiO2 film. In our experiment, the adsorption quantity of ZnSe QDs on nano-TiO2 film can be controlled effectively. The results show that ligand L-Cys capped at the outer layer of ZnSe QDs plays an important role in the sensitization process. Specifically, the peak of SH in the ligand disappears at 2552 cm-1 in the FT-IR spectrum of the ZnSe QDs capped by the ligand as a stabilizer. This indicates that the SH bond is broken. In the meantime, the peak of the CS stretching vibration in the ligand shifts from 638 cm-1 to 663 cm-1 due to the formation of ZnS bond. These imply that the core-shell ZnSe/ZnS/L-Cys QDs are obtained. On the other hand, according to the peak of COOH stretching vibration disappearing at 1600 cm-1 in the FT-IR spectrum of the core-shell QDs-sensitized mesoporous nano-TiO2 film, the unsaturated Ti atoms on the surface of the TiO2 film are bonded to carboxy groups from the ligand capped at the QDs. That is, the ligand acts as a bridge between the QDs and the nano-TiO2 film for achieving the sensitization. Some excellent photovoltaic characteristics of the composite film are found as follows. 1) The SPV responses of the QDs-sensitized film appear in a wavelength region of 300 nm to 800 nm (UV-VIS-Near-IR), causing the region of SPV response to enlarge about 200 nm over that of the ZnSe QDs, and 400 nm over that of the nano-TiO2 thin film. 2) The QDs-sensitized film displays an n-type photovoltaic characteristic that is different from that of the QDs. This may be more favorable for transferring those carriers from the film surface to the photo-anode material. 3) Both the separation rate and the diffusion length of photogenerated electron-hole pairs are obviously increased, and the lifetime of free charge carriers in the ZnSe QDs-sensitized film prolongs about an order of magnitude over that of the nano-TiO2 film and ZnSe QDs.
2017, 66 (6): 067501. doi: 10.7498/aps.66.067501
In 1907, Weiss proposed that there is a molecular field to explain the magnetic ordering of magnetic materials. However, it has not been clarified where the molecular field comes from so far. In recent decades, the magnetic ordering of metals and alloys were explained by using the direct exchange interaction of between electrons on neighboring atoms, while magnetic ordering of oxides were explained by using the super exchange interaction and double exchange interaction models. The intrinsic relation between those exchange interactions has not been well explained. This resulted in the fact that there are many puzzles for magnetic ordering of the magnetic materials. For example, what role the Cr cations play in spinel ferrite CrFe2O4; why the calculated molecular magnetic moment (3.85B) for La0.85Sr0.15MnO3 by using double exchange interaction model is lower than its experimental value (4.20B); whether there is a relation between the average atom magnetic moment and their electrical resistivity for each of Fe, Co and Ni metals. These several puzzles have been explained recently by our group through using an O 2p itinerant electron model for magnetic oxides and a new itinerant electron model for magnetic metals. In this paper, a model for the molecular field origin is proposed. There are three states for the electrons rotating with high speed at the outer orbits of two adjacent ions of magnetic oxides or metals and alloys. 1) There is a probability with which form the electron pairs with opposite spin directions and a certain life time, named Weiss electron pairs (WEP); the static magnetic attraction energy between two electrons of WEP is the elementary origin of Weiss molecular field. 2) There is a probability with which two electrons with the same spin direction exchange mutually. 3) If there are two electrons at the outer orbit of an ion, then for its adjacent ion whose orbit has only one electron, the excess electron will itinerates between the ions. Furthermore, the energy equation of WEP, equilibrium distance, re0, and maximum distance, rem, between electrons of WEP are derived. The probability with which WEP forms in each of several perovskite manganites is investigated. For perovskite manganites La0.8Ca0.2MnO3, La0.75Ca0.25MnO3, La0.70Sr0.30MnO3, the crystal cell constants increase linearly with temperature when the temperature is much lower than the Curie temperature, TC, while they show a rapid increase nonlinearly near TC. We then calculate the difference in MnO bond length at TC between the linear and the nonlinear variation, △dobs. Obviously, when the distance between the two electrons of WEP, re, is larger than the rem, WEP and the magnetic ordering energy both disappear. Assuming △dobs=rem-re0, the probabilities with which WEP appears in La0.8Ca0.2MnO3, La0.75Ca.25MnO3, La0.70Sr0.30MnO3, are calculated to be 0.07%, 0.31% and 3.13%, respectively. These results indicate that the WEP model for the magnetic ordering energy is qualitatively reasonable.
Optimization of magnetoelectricity in thickness shear mode LiNbO3/magnetostrictive laminated composite
2017, 66 (6): 067502. doi: 10.7498/aps.66.067502
Magnetoelectric (ME) composites have recently attracted much attention and triggered a great number of research activities, owing to their potential applications in sensors and transducers. Many researches have focused on the enhancement of ME coefficient by choosing suitable composite material and vibration mode based on the coupling between stress and strain. Besides normal stress, another vibration mode, shear mode, is further discussed as a potential high-frequency resonant device for a high frequency magnetic field detector, and it is useful to optimize the shear ME coefficient to broaden the application scope of the compositions. In this paper, an elasticity method is used to calculate ME coefficients of thickness shear mode LiNbO3/magnetostrictive laminated composites for various crystal orientations of LiNbO3, magnetostrictive materials and material sizes. The stretch-shear structure and shear-shear modes of the composite with considering the boundary condition are both discussed and further optimized. According to the structure design of stretch-shear mode composite from the literature, we design a new structure to achieve the uniform and pure shear ME effect, which changes the magnetostrictive phase on the bonding part into rigid material to avoid stretch deformation. We find that in the shear-shear ME composite, the structure should not move in the in-plane direction in order to realize the parallelogram deformation under shear stress, but should be free in the thickness direction to meet the change of thickness with shear deformation. For the stretch-shear mode Metglas/LiNbO3 [(xzlt) x/y], the shear ME coefficient E15 as a function of orientation of LiNbO3 shows that the maximum E15 is 235.1 mV/(cmOe) when x=0 and y=30. The results indicate that optimal shear ME coefficient is obtained at (xzt) 30 LiNbO3, resulting from the maximum shear piezoelectric coefficient dp15. By changing the material size in stretch-shear composite, the shear ME coefficient increases with the increase of thickness of magnetostrictive phase, because the stretch force increases with the increase of the cross-sectional area of magnetostrictive phase. The maximum values of E15 are, respectively, 24.13 V/(cmOe) in the stretch-shear mode Terfenol-D/LiNbO3 and 11.46 V/(cmOe) in the shear-shear mode Metglas/LiNbO3 by the optimization of material sizes. Experimental results are in accordance with calculation results. It is confirmed that LiNbO3 (xzt) 30 is the best choice for achieving the largest shear ME effect, and thicker Terfenol-D can help to achieve a larger ME coefficient in this stretch-shear composite. This work provides a design method to choose the structure and crystal orientation of shear LiNbO3-based ME laminated composite, which shows a prospect of applications in high-mechanical-quality factor Qm and high-frequency magnetic detectors with shear resonant devices.
Trap distribution and direct current breakdown characteristics in polypropylene/Al2O3 nanodielectrics
2017, 66 (6): 067701. doi: 10.7498/aps.66.067701
Polypropylene (PP) is widely used as capacitor films due to its better dielectric, mechanical, and thermal performance. In order to reduce the cost and size of capacitor, high energy density for PP dielectric is pursued. Since energy density is in quadratic proportion to direct current (dc) breakdown strength for linear dielectric, the enhancement of dc breakdown strength for PP dielectric is a primary choice to improve the energy density. Considering that the incorporation of nano-Al2O3 is an effective method to improve the dc breakdown strength for polymer, it is required to study the dc breakdown strength of PP/Al2O3 nanodielectric. In order to explore the breakdown mechanism, PP/Al2O3 nanodielectrics with different nano-particle contents are prepared by melt blending, and the samples are prepared by hot pressing. Their microstructures are observed by scanning electron microscopic. Isothermal surface potential decay, bulk resistivity, and dc breakdown strength of the samples are also measured. The experimental results show that the energy and density of deep traps, bulk resistivity, and dc breakdown strength first increase and then decrease with the increase in nano-Al2O3 content. The maximum values are obtained at a filer content value of 0.5 wt%, where dc breakdown strength can be increased by about 27%. Based on interface model, the relation between microstructure and trap is investigated. In view of space charge breakdown theory, the mechanism of dc breakdown for PP/Al2O3 nanodielectric is explored by trap parameters. It is indicated that the interface can provide more deep traps in PP/Al2O3 nanodielectric, while the decrease in the energy and density of deep traps can be attributed to the overlap of interfaces in electrical double layer. The increase in the energy and density of deep traps makes more carriers trapped near the injecting contact, thus reducing the effective field for carrier injection due to the internal field generated by the trapped carriers. The reduction of carrier injection can moderate the distortion of field in PP dielectric, consequently, resulting in enhancing the dc breakdown strength.
2017, 66 (6): 067903. doi: 10.7498/aps.66.067903
As a new kind of ultraviolet photocathode material, the negative-electron-affinity (NEA) GaN photocathode needs to further improve its photoemission performance and the stable performance in practical applications. Under the limit of GaN photocathode material growth level, how to further improve the quantum efficiency of cathode is an important problem. The varied doping technology can help to solve the problem under such circumstances. According to the photoemission mechanism of varying doping NEA GaN photocathode material, the built-in electric field formulas and the quantum efficiency formulas for reflection-mode varied doping NEA GaN photocathode are given. The preliminary structure of varied doping NEA GaN photocathode is designed. The varied doping material sample is divided into four layers according to the doping concentration. Using the self-developed experimental equipment, the varied doping GaN photocathode sample is activated with Cs/O. The activation process and the change characteristics of photocurrent for varied doping NEA GaN photocathode are discussed. At the beginning, the photocurrent is increased steady with the introduction of Cs, then the Cs kill phenomenon appears in the presence of excessive Cs. After the introduction of O, the photocurrent value starts to rise again. The spectral response of varied doping GaN photocathode is tested in situ after activation, and the quantum efficiency values ranging from 240 nm to 354 nm are obtained. On the basis of the obtained experimental results of quantum efficiency, combining to the typical quantum efficiency curve from University of California, the characteristics of quantum efficiency curves are analyzed. The results show that the quantum efficiency value for reflection-mode varied doping NEA GaN photocathode can reach 56% at 240 nm because of the built-in electric field, yet the quantum efficiency maximum value for uniform doping GaN photocathode is only 37% at 230 nm. The tested quantum efficiency maximum value of varied doping NEA GaN photocathode is improved much more than that of the uniform doping GaN photocathode. In a wider range of the incident light wavelength, the quantum efficiency of varied doping NEA GaN photocathode is relatively stable, and the excellent properties of varied doping GaN photocathode are confirmed. The reason why the value of quantum efficiency decreases with the increase of incident light wavelength is given. First, the photon energy decreases with the increase of incident light wavelength. Second, the incident light is absorbed from the front surface of cathode for reflection mode. In addition, the quantum efficiency curves of varied doping GaN photocathode show obvious sharp cut-off characteristics near the threshold, and the sharp cut-off characteristic is necessary for high detection sensitivity. The property of negative electron affinity for varied doping GaN cathode material after successful activation is also proved by the sharp cut-off feature.
Modification of the photocatalytic properties of anatase TiO2 (101) surface by doping transition metals
2017, 66 (6): 067101. doi: 10.7498/aps.66.067101
Exploring new types of photocatalysts and modifying the photocatalytic activity have attracted more and more extensive attention in many research fields. Anatase TiO2, a promising photocatalyst widely studied, can only absorb the ultraviolet light and thus only make little use of the power in visible light. Therefore, it is an urgent task to make theoretical and experimental investigations on the photocatalytic mechanism in anatase TiO2 and then improve its visible light response so as to utilize more visible light. Now, in the present paper, we carry out a systematic theoretical investigation on modifying the photocatalytic properties of the anatase TiO2 (101) surface via doping transition metal neutral atoms such as Fe, Ni, Pd, Pt, Cu, Ag, and Au by using the plane wave ultrasoft pseudopotential method of the density functional theory. The dependence of the macroscopic catalytic activity on electronic structure and optoelectronic property is uncovered by making a comparative analysis of the geometric structures, the electronic structures, and the optical properties of the undoped and doped anatase TiO2 (101) surfaces. Our numerical results show that doping certain transition metals can suppress the band gap or induce extra impurity energy levels, which is beneficial to improving the visible light response of the TiO2 (101) surface in different ways. In most cases, the new impurity energy levels will appear in the original band gap, which comes from the contribution of the d electronic states in the transition metal atoms. Moreover, the photocatalytic activity of the TiO2 (101) surface can be changed differently by doping different transition metal atoms, which is closely dependent on the bandgap width, Fermi energy, the impurity energy level, and the electron configuration of the outermost shell of the dopants. This research should be an instructive reference for designing TiO2 (101) photocatalyst and improving its capability, and also helpful for understanding doping transition metal atoms in other materials.
2017, 66 (6): 067401. doi: 10.7498/aps.66.067401
The experimental results of red-shift and blue-shift in absorption spectrum of Mo-doped ZnO are in mutual contradiction, and this phenomenon has not been explained rationally so far. For explaining this phenomenon, we analyze the energy band structure, state density, and absorption-spectrum distributions for each of Zn0.9583Mo0.0417O, Zn0.9375Mo0.0625O and Zn14Mo2O by first-principles calculation. The results show that within a limited doping amount range of 2.08 at%-3.13 at%, the higher Mo doping amount results in higher doping system volume, higher formation energy, lower system stability, and more difficult to dope. Meanwhile, all doping systems are converted into n-type degenerate semiconductors. Compared with the band gap of pure ZnO, the band gap of each doping system becomes narrow and the absorption spectrum shows red-shift. The higher the Mo doping amount, the weaker the narrowing of band gap becomes and the weaker the red-shift in absorption spectrum as well as the lower the electronic effective mass and the lower the electronic concentration; the lower the electronic mobility, the lower the electronic conductivity is; the lower the electronic magnetic moment is. The Curie temperature of doping system can reach a temperature higher than room temperature.
2017, 66 (6): 067901. doi: 10.7498/aps.66.067901
Charging effect of dielectric material due to electron beam irradiation has a significant influence on the microdischarge phenomenon of dielectric microwave component by multipactor. The discharge process caused by internal electron leakage can relieve this undesirable charging effect. In this paper, we study the transient discharge characteristics of a dielectric sample after being irradiated by electron beam through numerical simulation. Both the charging and discharging processes of a dielectric sample are considered with a comprehensive model. The Monte-Carlo method is used to simulate the interaction between primary electrons and material atoms before the irradiation is interrupted, including elastic scattering and inelastic scattering. The elastic scattering is calculated with the Mott scattering model, and the inelastic scattering is simulated with the fast secondary electron model or Penn model according to electron energy. Meanwhile, the transport process of internal charges in the sample during the discharge period is simulated including the charge diffusion under the force of charge density gradient, the drift due to built-in E-field, and the trap caused by material defect. In this work, the discharge process is taken to begin at the very moment of charging reaching saturation, with the internal charges kept almost unchanged. A polymer material widely used in advanced component is considered in this work due to its remarkable charging effects. Distributions of internal charges of the sample during the discharge process are simulated, and influences of sample parameters, including sample thickness, electron mobility and trap density in the discharge process, are analyzed. The results show that internal charges move to the bottom of the sample during the discharging, leading to the surface potential reaching an ultimate state which is determined by trap density of the material. The position corresponding to the maximum internal charge density shifts towards the grounded bottom. Although a sample with a larger electron mobility means a faster discharge process, fewer free electrons in this sample result in less discharge quantity. The time constant of discharge process decreases with the increase of sample electron mobility in the form of similar linearity. Although a sample with a larger thickness can hold more internal charges, the increase of sample thickness may increase the distance of internal charges leak yet. Hence, the quantity of discharge first increases and then decreases with the increase of sample thickness. In addition, a larger trap density of a dielectric sample makes charge leak harder, resulting in a lower discharge quantity. Finally, the proportion of discharge quantity in saturated charge quantity decreases from 1 to 0 exponentially with the increase of sample trap density. As a conclusion, those sample parameters have their corresponding effects on discharge characteristics by means of different physical mechanisms. Sample electron mobility determines the discharge time constant obviously by affecting the electron transport speed. The sample thickness affects the discharge quantity by shifting the charging balance mode, and material defect impedes part of discharge quantity from trapping internal free electrons. This simulation method and results can help to recede the charging effect and estimate the evolution charging and discharging states of dielectric material during and after electron beam irradiation.
Femtosecond laser pulse energy accumulation optimization effect on surface morphology of black silicon
2017, 66 (6): 067902. doi: 10.7498/aps.66.067902
Arrays of sharp conical spike microstructures are created by repeatedly irradiating silicon surfaces with focused femtosecond laser pulses in SF6. The absorbance of light is increased to approximately 90% in a wavelength range from the near ultraviolet (0.25 m) to the near infrared (2.5 m) by the microstructured silicon surface. The microstructured surface presents pitch-black because of enhanced absorption with a broad wavelength range, which is called black silicon. The unique microstructure morphology of black silicon surface formed by femtosecond laser can also bring a lot of other surface functions, for example, self-cleaning and field emission. These functions make black silicon highly desirable in solar energy, detectors and other fields. Therefore, the forming mechanism and conditions of fabrication optimization for black silicon microstructure have always been the focus of research. In our work, the sample is moved by motor-controlled stage while the laser beam is fixed. In the case of laser beam scanning, arrays of sharp conical spikes on the silicon are manufactured in 70 kPa SF6. The aim of the experiment is to find how to optimize the distribution of the laser energy in a number of laser accumulation pulses (the combination of single pulse energy and pulse number) to control the surface morphology of the black silicon. Experimental results show that there appears a bottleneck effect of morphology size growth with the increase of laser irradiation (improving the single pulse energy or increasing pulse accumulation number). Excessive energy accumulation brings no extra effect on optimizing and controlling of microstructure morphology on the surface. Based on theoretical results obtained from a physical model we proposed, we find that the reason for this phenomenon is that the microstructure morphology induced by former sequence pulse modulates the laser energy absorption of current laser pulse, and changes the laser ablation efficiency of the current pulse. According to this physical mechanism, we propose a new way of optimizing surface morphology, with fixing the total laser irradiation energy. And the size and distribution of surface morphology can be achieved by optimizing the distribution of the laser energy in a number of laser accumulation pulses. This approach can not only improve the efficiency of silicon surface preparation of microstructures but also reduce the surface defects and damage. Furthermore, the proposed method can reduce the energy consumption in the process of femtosecond machining. It is of great significance for the engineering application of black silicon.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2017, 66 (6): 068201. doi: 10.7498/aps.66.068201
Recently, arrhythmogenic condition has attracted special attention of scientists in the field of different disciplines because sudden cardiac death is often caused by cardiac arrhythmia. Arrhythmias can have different underlying causes. But the underlying mechanism of arrhythmia is not fully understood due to cardiac complexity. As is well known, one particular group of arrhythmias is often associated with the afterdepolarizations. So far, afterdepolarizations have been studied mainly in isolated cardiac cells. The question how the afterdepolarization is produced at a tissue level has not been widely studied yet. In this paper, we use the model of human heart to study how spiral wave or other wave patterns induces the afterdepolarizations in two-dimensional myocardial tissue. We try to obtain the instantaneous spatial distribution of afterdepolarizations by changing the L-type calcium and fast potassium conductance. In order to avoid bringing in afterdepolarizations, the applied parameters avoid evoking the afterdepolarizations at a single-cell and one-dimensional tissues level. The numerical simulation results show that spiral wave and other wave patterns can cause the phase II and III early afterdepolarizations, the delayed afterdepolarization, the enhanced automaticity, the delayed excitation and the delayed enhanced automaticity to occur. Moreover, we observe the weak oscillation of the membrane potential during the phase I of action potential. The afterdepolarizations generally occur in the spiral-wave core. They are generated by the phase singularity of spiral wave. The afterpolarizations can also appear in other region of spiral wave pattern. The afterpolarization is characterized by scattered distribution. When parameters are appropriately chosen, we observe the outbreaks of different afterpolarizations under the state of spiral wave. The corresponding spatial and temporal distributions of the early afterdepolarizations, the delayed afterdepolarizations, and the enhanced automaticity become spiral line distributions, which exhibits memory effect. It is shown that the outbreaks of afterdepolarizations in the system do not necessarily lead to the breakup of spiral wave. By observing the changes of different ion currents we find that when sodium current exciting cell is very small, the weak excitation with small sodium current can cause the L-type calcium current and the sodium calcium exchange current to increase, and the slow potassium current and rapid potassium current to decrease, leading to the occurrences of various afterdepolarizations. Therefore, increasing sodium current can effectively suppress the occurrences of afterdepolarizations.
2017, 66 (6): 068701. doi: 10.7498/aps.66.068701
Intermuscular coupling is defined as the interaction, correlation and coordination between different muscles during the body movement, which could be revealed by the synchronization analysis of surface electromyogram (sEMG). The multiscaled coherence analysis of sEMG signals could describe the multiple spatial and temporal functional connection characteristics of intermuscular coupling, which could be helpful for understanding the multiple spatial and temporal coupling mechanism of neuromuscular system. Furthermore, the coupling characteristics in frequency band of sEMG generally reflect the functional connection between muscles which relate to motion control and coordinative mechanism of the central nervous system (CNS). In this paper, we combine variational mode decomposition (VMD) and intermuscular coherence (IMC) analysis to propose a new method named VMD-IMC to quantitatively describe the muscular coupling characteristics in the corresponding frequency bands. First, sEMG data of flexor digitorum superficialis (FDS), flexor carpi ulnaris (FCU) and extensor digitorum (ED) are recorded simultaneously from twenty healthy subjects (253 years) who perform the designed grip task at sustained 20% maximum voluntary contraction under the static load. Then, the VMD approach is employed to adaptively decompose sEMG into several intrinsic mode functions to describe the information about different time-frequency scales. Furthermore, the coherence on different time-frequency scales between different sEMG signals is analyzed, and the significant coherent area index is calculated to quantitatively describe the functional coupling characteristics of the feature bands. And combining VMD with Hilbert transform, we calculate root mean square and mean instantaneous frequency (MIF) to describe the variations of energy and frequency of each muscle. The results show that coupling strengths increase with time, respectively, in beta (15-30 Hz) and gamma (30-45 Hz) band between two muscles (FDS vs FCU, FDS vs ED) during the sustained static force with low load. In addition, compared with the coupling between FDS and ED, the couplings between FDS and FCU in beta and gamma band under the condition of fatigue present more significant changes and similar trend in MIF variation with time. The obtained results reveal that the congenerous muscle is coordinated by CNS in a more synchronous way during the sustained isometric fatiguing contraction.
2017, 66 (6): 068101. doi: 10.7498/aps.66.068101
Eutectic phase transition involves the competitive nucleation and coupled growth of two solid phases within one liquid phase. Phase selection especially under unequilibrium condition, may result in novel microstructures and thus affects the performances of eutectic alloys. Liquid Cu-10 wt.% Zr hypoeutectic, Cu-12.27 wt.% Zr eutectic and Cu-15 wt.% Zr hypereutectic alloys are rapidly solidified in the containerless process in a 3 m drop tube. During the experiments, the Cu-Zr alloys are heated by induction heating in an ultrahigh vacuum chamber and further overheated to 200 K above their liquidus temperatures for a few seconds. Then the liquid alloys are ejected out from the small orifice and dispersed into tiny droplets after adding the argon gas flow. The solidified samples are analyzed by Phenom Pro scanning electron microscope and HXD-2000 TMC/LCD microhardness instrument. The competitive nucleation and growth among (Cu) dendrite, Cu9Zr2 dendrite and (Cu+Cu9Zr2) eutectic phase become more and more intensive as droplet diameter decreases. The layer spacing in Cu-12.27 wt.% Zr eutectic alloy decreases when the undercooling increases. And the microstructural transition takes place from lamellar eutectic to anomalous eutectic. The microstructure of Cu-10 wt.% Zr hypoeutectic alloy is characterized by (Cu) dendrite and lamellar eutectic. Whereas the microstructure in Cu-15 wt.% Zr hypereutectic alloy consists of Cu9Zr2 dendrite and lamellar eutectic. For the Cu-10 wt.% Zr hypoeutectic alloy, with the decrease of droplet size, the primary (Cu) phase transforms from coarse dendrites into equiaxed grains, and the volume fraction of (Cu) dendrite becomes larger and larger. As for Cu-15 wt.% Zr hypereutectic alloy, the primary Cu9Zr2 intermetallic compound grows in a band manner, and with the decrease of droplet size and increase of cooling rate, the solidified microstructure transforms from band Cu9Zr2 dendrite plus lamellar eutectic into spherical cell structure. The three alloys reach maximal undercooling at 177 K, 156 K and 204 K, respectively. The Trivedi-Magnin-Kurz and Lipton-Kurz-Trivedi/Boetinger-Coriell-Trivedi models are used to analyze the dendritic and eutectic growth as a function of undercooling. Theoretical analysis indicates that both dendritic growth and eutectic growth are controlled by solute diffusion during liquid-solid phase transition. To further investigate the effects of cooling rate and undercooling on the mechanical properties of Cu-Zr eutectic alloys, the microhardness of each of different phases is determined. The microhardness of the primary (Cu) phase within Cu-10 wt.% Zr hypoeutectic alloy is strengthened with the increase of cooling rate. The microhardness of eutectic within the three alloys also increases with increasing the cooling rate and the initial alloy composition of the alloy.
2017, 66 (6): 068501. doi: 10.7498/aps.66.068501
The InGaAs/AlGaAs quantum wells have been extensively applied to quantum well infrared photodetector of mid-wavelength. In this letter, four samples of 2.4 nm In0.35Ga0.65As/40 nm Al0.34Ga0.66As multi-quantum wells are grown by molecular beam epitaxy with the InGaAs wells growing all at a temperature of 465℃ but the AlGaAs wells growing at temperatures of 465℃, 500℃, 545℃, and 580℃ respectively. The dependence of InGaAs quantum well strain relaxation on the AlGaAs growth temperature is systematically studied by photoluminescence spectroscopy and X-ray diffraction and then the thermal-induced relaxations of three key-stages are clearly observed in the following temperature ranges. 1) 465-500℃ for the stage of elastic relaxation: the phase separation begins to take place with a low defect density; 2) 500-545℃ for the transition stage from elastic relaxation to plastic relaxation: the phase separation will be further intensified with defect density increasing; 3) 545-580℃ for the fast stage dominated by elastic relaxation and the defect density will sharply increase. Especially when AlGaAs temperature increases to 580℃, a very serious plastic relaxation will take place and the InGaAs quantum well will be dramatically destroyed.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2017, 66 (6): 069501. doi: 10.7498/aps.66.069501
The transparent plates (such as organic glass, plastic plate) are widely used in the construction industry, high-tech products and scientific research applications, and its parallelism and uniformity measurement in the manufacture and quality control become more and more inevitable. Interferometer is a label-free, high-precision, and high-efficient device that can be used in many fields. According to a single-element interferometer, we demonstrate a measurement for the parallelism and uniformity of transparent medium. Beam-splitter cube is a key component. Half of plane wave laser source passes through the measured medium and the remaining half directly passes through the air, then these two halves with different optical paths meet in the beam-splitter cube. The parallelism or uniformity is determined by calculating interference fringe shift number during rotating the measured sample. The coherent beam is divided into two parts by a beam-splitter, one passes through the lens and then arrives at a photoelectric counter, and the other arrives at the observation plane of the charge-coupled device. The photoelectric counter is used to count the integer part of fringe shift number during rotating the sample; and the decimal part can be detected by calculating the phase difference of the two interferograms captured before and after rotation. The measurement principle of the proposed device is analyzed in detail, and the numerical simulations of the fringe shift number and the gray level changing with the sample rotation angle, the thickness and the refractive index of the sample are carried out. The simulation results show that the bigger the rotation angle, thickness and refractive index of the sample, the greater the fringe shift number will be. Therefore, the measurement accuracy can be improved by increasing the rotation angle and the thickness of the sample. In addition, we also simulate the measurement processes of two kinds of samples, which are unparallel and inhomogeneous transparent plates. The simulation results prove the feasibility and high accuracy of the proposed method. Finally, the optical experiment is conducted to demonstrate the practicability of the present device. The parallelism of a cuvette used for more than one year, is tested by our device. The results show that the difference in thickness between the cuvettes is on a micron scale, the peak-valley (PV) value is 9.92 m, and the root mean square (RMS) value is 2.2 m. And the difference between the contrast test results and the results from the proposed method is very small, the PV value is 0.569 m, and the RMS value is 0.131 m. The stability and repeatability of the proposed setup are tested in the experimental condition. The mean value and standard deviation of the fringe shift number during 30 min are 0.0012 and 0.0008, respectively. These results further testify the high accuracy and stability of our method. In conclusion, the performance of our measurement method is demonstrated with numerical simulation and optical experiment.
2017, 66 (6): 069601. doi: 10.7498/aps.66.069601
The interfacial wettability and morphological evolution of liquid on a solid substrate, as natural phenomena, have received great attention in recent years. Although much work has been done to study this process, existing studies mainly focus on the wetting properties of water. Therefore, in this paper, we use molecular dynamics simulation method to study the interfacial phenomena of the nanoscale liquid silver on graphene, whose surface has been modified. By making different comparisons, such as Lennard-Jones (LJ) potential parameters, the surface structures of substrates, the thickness values of films and the shapes of films, the influences of these variables on wetting properties of liquid silver on graphene are studied. The results show that the dewetting of liquid silver occurs on graphene, implying that the wettability of liquid silver is weak, and that the potential parameters, the surface structure of substrates, the thickness of film and the shape of film have great influences on the wettability and morphology evolution of film: the change of these factors can affect the dewetting properties of liquid silver, which is evident by the detachment time and detachment speed. With the increase of LJ potential parameters, the detachment time is larger while the contraction speed and the detachment speed are smaller. Compared with the detachment times on different carbon-based substrates, the detachment time is small on the pillared graphene, followed by the vertical carbon nanotube, and the detachment time is large on the graphene. With increasing the thickness of the film, the detachment time becomes larger. The detachment time of the circle film is smaller than those of the regular hexagon film and square film, manifesting that the films with smooth boundary are beneficial to separating from the substrate. Moreover, by setting a system of two liquid films, we study the formation of silver bridge of two films and the fracture or fusion of the bridge. When two liquid films initially contact each other, the liquid bridge forms. However, the growth behaviors of liquid bridges are different from each other, some liquid bridges become slim and finally fractures, other liquid bridges do not fracture and help two droplets form one bigger drop. These different behaviors mainly depend on the size of film. This study is very valuable for well understanding the superhydrophobic surfaces and the morphological evolutions of Ag films on the graphene. Furthermore, these findings can provide an effective method to control the dewetting behavior of liquid Ag and the fracture or fusion of the two liquid drops by tuning the size of the films.