## Vol. 65, No. 14 (2016)

##### 2016-07-20

###### GENERAL

2016, 65 (14): 140201.
doi: 10.7498/aps.65.140201

Abstract +

The symmetries, conservation laws and exact solutions to the nonlinear partial differential equations play a significant role in nonlinear science and mathematical physics. Symmetry is derived from physics, and it is a mathematical description for invariance. Symmetry group theory plays an important role in constructing explicit solutions, whether the equations are integrable or not. By using the symmetry method, an original nonlinear system can be reduced to a system with fewer independent variables through any given subgroup. But, since there are almost always an infinite number of such subgroups, it is usually not feasible to list all possible group invariant solutions to the system. It is anticipated to find all those equivalent group invariant solutions, that is to say, to construct the one-dimensional optimal system for the Lie algebra. Construction of explicit forms of conservation laws is meaningful, as they are used for developing the appropriate numerical methods and for making mathematical analyses, in particular, of existence, uniqueness and stability. In addition, the existence of a large number of conservation laws of a partial differential equation (system) is a strong indication of its integrability. The similarity solutions are of importance for investigating the long-time behavior, blow-up profile and asymptotic phenomena of a non-linear system. For instance, in some circumstance, the asymptotic behaviors of finite-mass solutions of non-linear diffusion equation with non-linear source term are described by an explicit self-similar solution, etc. However, how to tackle these matters is a complicated problem that challenges researchers to be solved. In this paper, by using the symmetry method, we obtain the symmetry reduction, optimal systems, and many new exact group invariant solution of a fifth-order nonlinear wave equation. By Lie symmetry analysis method, the point symmetries and an optimal system of the equation are obtained. The exact power series solutions to the equation are provided by the power series method, such solutions can be used for numerical computations in both theory and physical applications conveniently. Finally, a lot of conservation laws of the fifth-order nonlinear wave equation are presented by using the adjoint equation and symmetries of the equation.

2016, 65 (14): 140203.
doi: 10.7498/aps.65.140203

Abstract +

Air traffic management technical support system provides communication, navigation and surveillance service for air traffic management system and air traffic controller. The failures of some facilities may lead to large delay, even affect air transportation safety. In order to improve the ability of air traffic management technical support system to respond to emergencies, a network model of air traffic management technical support system is presented. The network model of air traffic management technical support system is established according to the effective coverage of communication, navigation and surveillance facilities, the position of air traffic management technical support system and air route network. Flexibility, robustness and efficiency are used to measure the network. The measure index of air traffic management technical support system network includes degree, degree distribution, strength, clustering coefficient, network performance, betweenness centrality, average shortest path and diameter. For Beijing, Shanghai, Guangzhou, Kunming, Shenyang and Lanzhou flight information regions, the air traffic management technical support networks are built by using the data of air traffic support facilities, air route, air traffic flow, etc. The average degrees, degree distributions, degree-degree correlations, clustering coefficients, average shortest paths and diameters of these netwoks are comparatively analyzed. The results show that the degrees of most nodes are between two and four. The network has a power law distribution, which is the same as that of air transportation network. The degree-degree correlation of air traffic management technical support system network is not assortative nor disassortative mixing, which has random network characteristics. The clustering coefficients of several air traffic management technical support system networks are between 0.25 and 0.39. The clustering value is lower than that of air transportation network. The shortest paths of air traffic management technical support system networks are between 3.16 and 5.05. The average shortest paths of these networks are all 3.4, which exhibits small world characteristics. Network attack based on degrees of priority and random is conducted to several flight information regions of air traffic management technical support network, showing the network is vulnerable. The network performance decreases quickly after degree priority attack. Some key nodes play an important role in the network. The network survivability can be improved after targeted immunized key nodes. The network performance can be improved by using more satellites based air traffic management technical support system. These rules provide theoretical support for improving and expanding air traffic management technical support system, and have practical significance for reducing the influence of emergency on air traffic management system support ability and ensuring the continuous safety of air traffic.

2016, 65 (14): 140601.
doi: 10.7498/aps.65.140601

Abstract +

Large-scale and high precision absolute distance measurement is essential in aerospace technology and advanced manufacturing. Traditional method of measuring distance cannot meet this requirement. Since the advent of optical frequency comb, it has brought a revolutionary breakthrough to absolute distance measurement. In the past decade, there were proposed many methods to measure long absolute distances with high accuracy. Especially, the simple method of using adjacent pulse-to-pulse distance as a ruler for distance measurement has been widely used. The accuracy of this method depends mainly on the knowledge of relative positions of the two overlapped pulses, i.e., pulse-to-pulse alignment. In our previous study, we have proposed a heterodyne interferometer based on synthetic wavelength method with femtosecond laser. The synthetic wavelength is derived from the virtual second harmonic and the real second harmonic, and the real second harmonic is produced by a piece of periodically poled LiNbO3 (PPLN) crystal. However, the second harmonic generation system makes the system complicated, and causes a great optical energy loss. In order to solve this problem, we generate the synthetic wavelength by two spatial band-pass filters in our present study, which can simplify the system greatly. Moreover, we can reduce the optical energy loss and tune the synthetic wavelength by controlling the angle of the filter. The synthetic wavelength used in the present system is 71.39 m. The interferometric phase of the synthetic wavelength is used as a mark for the pulse-to-pulse alignment. In order to reduce the influences of air disturbance and temperature variation, we set up a thermal-insulated cover for the interferometer to stabilize the environment in the system. By using this cover, the optical path length difference of the system in 450 s can be reduced from 8.56 m to 0.21 m. To demonstrate the efficacy of the method described above, the target mirror is moved by eight steps in steps of 5 mm. We compare the measurement results with those obtained by a commercial interferometer, and the residual error is less than 100 nm. Since the measurement range is larger than our previous study, the relative accuracy is better than the previous system. In conclusion, we demonstrate a synthetic-wavelength based absolute distance measurement by using heterodyne interferometry of a femtosecond laser. Two spatial band-pass filters are used to generate the synthetic wavelength, which can simplify the system. The comparison results show that the system has an accuracy better than 100 nm in a displacement of 40 mm. The accuracy of the experimental system can be further improved by making the common-path of the two interferometers longer, locking the fceo to the atomic clock and sampling the data synchronously.

2016, 65 (14): 140701.
doi: 10.7498/aps.65.140701

Abstract +

Avidin is a common basic protein, widely used for connecting DNA and modified surface in single-molecule techniques of biophysics, and it can also be used as a DNA vector in gene therapy. Avidin is highly positively charged and can condense DNA in solution. Understanding the physical mechanism of its condensing DNA is a key factor to promote avidin-DNA complex to be used for many purposes, such as a probe of biomacromlecules, signal enhancer or carrier of disease diagnosis.In the present study, we use atomic force microscope (AFM), dynamic light scattering (DLS), and single molecular magnetic tweezers (MT) to systematically investigate the interaction between DNA and avidin and the underlying mechanism of DNA condensation by avidin. The conformation of DNA-avidin complex is observed and measured by AFM and we find that the condensation includes two types: one is toroidal condensation of DNA induced by avidin, the other is the condensing structure by avidin compaction. Quantitative analysis shows that the size of avidin-DNA complex decreases monotonically with the concentration of avidin increasing. However, when the concentration of avidin reaches up to a critical value of 2 ngL-1, the size of complex begins to increase suddenly with avidin concentration increasing. The phenomenon is also confirmed by the corresponding DLS measurements. For example, when the concentration of avidin increases from 0 to 2 ngL-1, the size of condensed avidin-DNA complex reduces from 170 nm to about 125 nm. In the mean while, its electrophoretic mobility changes from -2.76 (10-4cm2V-1s-1) to -0.1 (10-4 cm2V-1s-1). The negative charge of DNA is mostly neutralized by avidin. From their force spectroscopy measured by MT, it is found that the extension of DNA varies almost linearly and a few stairlike jumps appear occasionally. For example, its characteristic trend is quite similar to the one by histones. The condensing force of DNA by avidin grows up with the concentration of avidin increasing. The statistics of force-extension curves by MT shows that the peak of unraveling steps of avidin-DNA complex is around 160 nm, which corresponds to the typical toroidal structure of DNA.In DNA condensation by avidin, electrostatic interaction plays a key role due to the neutralization of negatively charged phosphate groups of DNA by cationic avidin. From the comprehensive data by AFM, DLS and MT, we conclude that the process of DNA condensation induced by avidin consists of two mechnisms: the predominant DNA-avidin electrostatic attraction and the ancillary avidin aggregation.

2016, 65 (14): 140501.
doi: 10.7498/aps.65.140501

Abstract +

The vehicular-to-vehicular (V2V) communications have recently received great attention due to some traffic telematic applications that make transportation safer, more efficient, and more environmentally friendly. Reliable traffic telematic applications and services require V2V wireless communication systems to be able to provide robust connectivity. To develop such wireless communication systems and standards, accurate channel models for the V2V communication systems are required. In this paper, a geometric street scattering channel model for a V2V communication system is presented under line-of-sight (LOS) and non-LOS (NLOS) propagation conditions. Starting from the geometric model, a stochastic reference channel model is developed, where the scatterers are uniformly distributed in rectangles in the form of stripes parallel to both sides of the street. A typical propagation scenario for the proposed model is presented, where the buildings and the trees can be considered as scatterers. Analytical expressions for the probability density functions (PDFs) of the angle-of-departure (AOD) and the angle-of-arrival (AOA) are derived. By obtaining the PDF of the total Doppler frequency, the Doppler power spectral density (PSD) and the autocorrelation function (ACF) of the proposed model are also investigated and computed, assuming that the mobile transmitter (MT) and the mobile receiver (MR) are moving, while the surrounding scatterers are fixed. In this respect the underlying radio channel model differs from the traditional cellular channels. We can draw the conclusion that the PDFs of AOD and AOA first increase and then decrease within a certain angle range; the Doppler power spectral density of the signal in the outdoor street environment presents the peak value in fmax. In addition, while the Rice distribution factor is larger, the value of the autocorrelation function increases relatively, the stability of the fluctuation increases correspondingly as well. To validate the reference channel model, its Doppler parameters are compared with those of a real-world measured channel for urban and rural areas. The numerical results show a good fitting of the theoretical results to the computer simulations. In the proposed geometry-based channel model, we not only study the influence of the street scatterers on the performance of V2V communication system, but also broaden the research of the channel modeling of outdoor wireless communication in turn. To evaluate the propagation characteristics of the outdoor V2V communication systems and the simulation of wireless communication system, this paper provides a powerful research tool.

## EDITOR'S SUGGESTION

2016, 65 (14): 140702.
doi: 10.7498/aps.65.140702

Abstract +

Ionized atoms widely exist in plasmas, and studies of properties of ionized atoms are the foundations of frontier science researches such as astrophysics and controlled nuclear fusions. For example, the information about the ground configurations of atoms is required for accurately calculating the physical quantities such as energy levels and dynamical processes. The configurations for different ionized atoms can be obtained with the photo-electron energy spectrum experiment, however it is very time-consuming to obtain so many data of all ions. Therefore the more economical theoretical study will be of great importance. As is well known, the configurations of neutral atoms can be determined according to Mendeleev order while those of highly ionized atoms are hydrogen-like due to the strong Coulombic potential of their nuclei. Then with the variations of ionization degree and atomic number along the periodic table, there would appear the interesting competitions between electronic orbitals. Although some theoretical results exist for ions 3 Z 118, 3 Ne 105 (where Z is the atomic number and Ne is the electron number), there are many errors in the results for highly ionized atoms. Therefore, the ground configurations of ionized atoms and their orbital competitions still deserve to be systematically studied.
Based on the independent electron approximation, we calculate the energy levels of all possible competition configurations of all the neutral and ionized atoms in the extended periodic tables (2 Z 119) by Dirac-Slater method. Then the ground configurations are determined by calculating the chosen lowest total energy. The advantages of Dirac- Slater method are as follows. 1) It has been shown that the Dirac-Slater calculation is accurate enough for studying the ground properties of atoms, such as the 1st threshold, and that higher accuracy will be obtained for highly ionized atoms, because the electron correlation becomes less important. 2) Furthermore, with Dirac-Slater method we can obtain the localized self-consistent potential, thereby we can study the orbital competition rules for different atoms. Using the three of our designed atomic orbital competition graphs, all of our calculated ground configurations for over 7000 ionized atoms are conveniently expressed. We systematically summarize the rules of orbital competitions for different elements in different periods. We elucidate the mechanism of orbital competition (i.e., orbital collapsing) with the help of self-consistent atomic potential of ionized atoms. Also we compare the orbital competition rules for different periods of transition elements, the rare-earth and transuranium elements with the variation of the self-consistent filed for different periods. On this basis, we summarize the relationship between the orbital competitions and some bulk properties for some elements, such as the superconductivity, the optical properties, the mechanical strength, and the chemistry activities. We find that there exist some abnormal orbital competitions for some lowly ionized and neutral atoms which may lead to the unique bulk properties for the element. With the ground state electronic structures of ionized atoms, we can construct the basis of accurate quasi-complete configuration interaction (CI) calculations, and further accurately calculate the physical quantities like the energy levels, transition rates, collision cross section, etc. Therefore we can meet the requirements of scientific researches such as the analysis of high-power free-electron laser experiments and the accurate measurement of the mass of nuclei.

2016, 65 (14): 140703.
doi: 10.7498/aps.65.140703

Abstract +

X-ray sources have been extensively penetrated into all aspects in daily life, such as pharmaceutical analysis, X-ray diagnostics, and radioactive static elimination. Along with the burgeoning field of deep exploration, all kinds of X-ray sources are fabricated to meet the needs of space applications. Therefore a design proposal of a transmission-type miniature micro-beam modulated X-ray source, in allusion to the space application of X-ray sources, is proposed and its theoretical model is constructed. In contrast with the traditional X-ray sources, a grid electrode is added and three focusing electrodes are chosen and used. Amplitude modulation and pulse modulation of X-rays, by controlling the voltage value of the grid electrode, are realized. Electrons are restrained to pass when the grid electrode is loaded with a negative voltage and no X-ray photons are produced. When loaded with a positive voltage, the grid electrode lets electrons get through and finally reaches up to the anode electrode. Three focusing electrodes, meanwhile, are used to make the electron beam converge and finally focus on the anode target. The thickness of the transmission-type target material is considerable, considering it to be a key parameter influencing the conversion efficiency of X-rays. If the target thickness is too thin, only a part of electrons can convert into X-ray. On the contrary, when the target thickness is too thick, the produced X-ray intensity is low too. That is because some X-ray photons are absorbed by the anode target material even though all of the electrons convert into X-ray. And the optimum target thickness, in different anode voltage values and different target materials, is simulated using charged particle optical simulation software, and the results are presented in a table. In addition, the influence of grid voltage value on electron beam trajectory is also taken into account and finally a 150-m-diameter focusing spot is obtained. The prototype model is fabricated via coating film on the anode and the single-step brazing process in a vacuum furnace. After the test platform is set up, the spectrum feature of tungsten target is attained. And it is analyzed that the X-ray intensity is related to the grid electrode voltage value. The feasibility of grid amplitude modulation and grid pulse modulation are verified in the end. As a multifunctional modulated X-ray source, it will have more important scientific significance and space application prospects, and be used in inter-satellite X-ray communication, ionization blackout area communication, planetary science, pulsar simulation and single event effect simulation of space radiation environment.

2016, 65 (14): 140202.
doi: 10.7498/aps.65.140202

Abstract +

Fluid transport is a very common phenomenon. Recently flow process in nanochannels has drawn much attention, since it differs quite much from that in macroscopic pipes. In particular, the motion of confined water molecules in nonpolar nanochannels has become a hotspot in nanotechnology, and also an important issue in biology and chemistry. Besides the experimental studies, computer simulations (e.g., molecular dynamics simulation) have also been proven to be a powerful tool to investigate such issues. Early simulations focused on the concurrent motion of all water molecules inside nanochannels such as carbon nanotubes (CNTs), where water molecules are evenly spaced in a single file and occasionally but collectively transport through CNTs. Recently, a new model of water transport in CNTs was presented, which indicates that water-density defects in the one-dimensional (1D) chain of water molecules can move as solitons. This is explained as a natural consequence of competition between water-water interactions and water-CNT interactions. While this new model is very appealing, the identification of soliton is not a trivial work (especially at not very low temperatures), since the density defects of water molecules might not be easily recognized from their thermal fluctuation. In this paper, a new method is developed to precisely identify the soliton by quenching the simulation conformations to their nearest neighboring local minima. Based on the new soliton identification method, we study the motion of water in single-walled armchair CNTs by all-atom molecular dynamics simulations. We investigate the motion of solitons in detail, which is observed as a standard 1D diffusion on a picosecond time scale. The simulations also show that the diffusion coefficient of solitons increases with temperature rising, and decreases with the number density of solitons increasing. These results are consistent with the postulation that there exists a weak repulsion between solitons.

###### NUCLEAR PHYSICS

2016, 65 (14): 142801.
doi: 10.7498/aps.65.142801

Abstract +

For the Monte Carlo simulation of the non-static transport problem, there must be many calculation steps. Because some particles cannot finish their transport in the last step, they are naturally used as the source particles of the present step. These particles are called undied particles. It is difficult to adjust the history number of each step to obtain higher efficiency because the adjusting rule is hard to find. The most direct method is to set a large enough history number for all steps. But evidently, it is unnecessary for some steps. Among all possible rules, one candidate of adjusting the history number is to check the convergence situation of Shannon entropy (corresponding to the distribution of some undied particle attributes) every some samples in each step to determine whether or not to simulate more particles. So, this method needs to calculate the Shannon entropy frequently. Because the classical method of calculating Shannon entropy in message passing parallel programming environment must reduce massive data, it is unpractical to be used in this situation for the great increasing of computation time with the high frequency of entropy calculation. In this paper, we propose an efficient method of calculating the entropy in the message passing parallel programming environment by letting each process calculate its entropy value based on the local data in each processer and calculating the final entropy by averaging all the entropy values gotten by all processes. The entropy value calculated by this method is not the same as that by the classical method when using finite history number, but the difference goes to zero when the history number goes to infinity. The most remarkable advantage of this method is the small increasing of computation time when calculating the entropy frequently. It is a suitable method of calculating Shannon entropy when adjusting the history number automatically based on the judgment of the convergence situation of Shannon entropy.

2016, 65 (14): 142501.
doi: 10.7498/aps.65.142501

Abstract +

Dileptons have large mean free paths due to their small cross sections for electromagnetic interaction in plasma. Therefore they are considered to be an important probe for the formation and evolution of the quark matter. In this work, we calculate the dilepton production of quark-gluon plasma (QGP) produced in Au197+ Au197 central collisions at relativistic heavy ion collider (RHIC) energy based on the evolution model of a chemically equilibrating viscous QGP. The evolution of the QGP system is described by a set of coupled relaxation equations containing the master equations of partons, the equation of baryon number conservation and equation of energy-momentum conservation. Solving the set of evolution equations, one can obtain the evolution of temperature T, quark chemical potential q, fugacities q for quarks and g for gluons. To discuss the shear viscosity of QGP, the contributions of the elastic scattering of quarks qqqqand gluons gggg, as well as the inelastic scattering process of gluons ggggg are included. Based on the evolution model including the viscosity, we perform a complete calculation of the dilepton production, including the processes of quark-antiquark annihilation qqll, next-order annihilation qqgll, Compton-like scattering qg qll, qg qll, multiple scattering of quarks, as well as gluon fusion gg cc, annihilation qqcc. It is found that the spectra from the quark-antiquark annihilations qqll and qqgll are dominated. The contributions from multiple scattering cannot be neglected. We also find that the dilepton yields remarkably decrease with considering an additional gluon inelastic process in the calculation compared with the results with considering only elastic scatterings of quarks and gluons. This indicates that the evolution of QGP system is accelerated and the evolution time is shortened by the inelastic scatterings of gluons.

###### ATOMIC AND MOLECULAR PHYSICS

2016, 65 (14): 143101.
doi: 10.7498/aps.65.143101

Abstract +

Terahertz (THz) technology developed rapidly in recent years. Liquid crystals (LCs) are one of the most promising base materials to construct switchable devices in THz range because of their high optical anisotropies. However, the practical applications of the devices are hampered by the relationships between birefringence, thickness and LCs switching time. Due to the long wavelength, THz device requires a larger birefringence LC than the device operated at optical frequencies. Yet, in order to design an efficient switchable LC-THz device, it is crucial to find or synthetize LC material which will still display a useful birefringence at THz frequencies. The birefringence properties of LC are determined by the molecular polarizability of the relevant material. Knowledge of the LC molecular polarizability and its dependence on the molecular structure is important for designing LC molecules with desired THz properties. The prediction of the photoelectric characteristics could save a considerable quantity of the man-power and materials needed for the design or synthesis of new LC compounds. A priori screening of materials and the prediction of the optoelectronic properties would make a vast opportunity for expanding the LC material application scope. Hence, the main purpose of the present work is to provide a theoretical method of calculating and analyzing the THz polarizability properties of LC single compounds for LC-THz device applications. In this work, the frequency dependent molecule polarizability values of liquid crystal PCH5, 5CB and 5OCB in THz range are calculated by the density functional theory method. The geometries of the studied LCs are optimized at B3 LYP levels with the standard 6-311G(d) basis set. From the optimized geometries the molecule THz polarizabilities of LCs are calculated by the M06-2x functional with 6-311++G(2d, p) basis set, and they are found to be in good agreement with experimental data. By plotting the polarizability density analysis (PDA), the spatial contributions of electrons to the longitudinal polarizability are presented. The influences of alkyl chain and core structure on the microscopic polarizability of the LC molecule are investigated and explained by using the finite field approach and PDA. The results show that the unsaturated group, such as benzene ring or cyanobenzyl, makes great contribution to the polarizability of LC. In the design process, the new type of LC molecule must be extended the length of up electron conjugated system, to reduce the energy gap between HOMO and LUMO, and hence improving LC molecule polarizabilty. We hope that the present work could give a useful guide in screening or designing LC molecules for THz applications, and offer an effective way to understand fundamental optoelectronic characteristic of LC materials in the THz frequency range.

## EDITOR'S SUGGESTION

2016, 65 (14): 143401.
doi: 10.7498/aps.65.143401

Abstract +

The radiative electron capture (REC) and subsequent radiative decay of initial hydrogen-like Xe52+ ions are studied in the collision of Xe53+ with Xe atom at a projectile energy of 197 MeV/u within the framework of the multiconfiguration Dirac-Fock method and the density matrix theory. We calculate the differential and total cross sections as well as the REC photon energies for REC to the 1snp1/2, 3/2 Jf=1 (n=2-5) levels of finally helium-like Xe53+ ions. Moreover, the transition energies and rates of the subsequent 1snp3/2 Jf=1 1s2 Jd = 0 decay as well as the angular distribution and linear polarization of the associated characteristic photons are also calculated. It is found that the REC photons are remarkably anisotropic. Through the analysis of the REC angular distribution characteristics, we find that the different configurations of the REC angular distribution are similar in quality, and they all have a peak at the 90. That is to say, the REC process can more easily occur in the direction perpendicular to the incident direction of the projectile ions. In addition, while the characteristic photons from the subsequent 1snp3/2 Jf=11s2 Jd= 0 radiative decay of Xe52+ ions exhibit an anisotropic angular distribution and strong linear polarization, their counterparts from the 1snp1/2 Jf = 1 1s2 Jd = 0 decay are almost isotropic and linearly unpolarized. The angular distribution and linear polarization of the radiation photon decay from the (1s np1/2,3/2 Jf = 0) states to the ground state（1s2 Jd = 0）both reach a maximum value at the 90, their characteristics are similar to those of the REC photons, that is to say, the deexcited process can more easily occur in the direction perpendicular to the incident direction of the projectile ions, and in this direction the decay photons have much larger polarization degree.

2016, 65 (14): 143301.
doi: 10.7498/aps.65.143301

Abstract +

A pure rotational Raman lidar has become one of valid methods of profiling atmospheric temperature. However, its proper operation generally needs a certain collocated device of atmospheric temperature to calibrate three retrieval coefficients. This fact seriously restricts the applications of pure rotational Raman lidar in the meteorology and environment fields. In order to execute the detection technique of atmospheric temperature without calibration, we present and design a pure rotational Raman lidar based on the dependence of atmosphere molecular rotational Raman spectral envelope on temperature. It is configured with a laser having a pulse energy of 300 mJ, a pulse repetition rate of 20 Hz, and a Cassegrain telescope with a clear aperture of 250 mm. A two-stage multi-channel pure rotational Raman spectroscopic filter is proposed to extract efficiently the rotational Raman spectral lines with more than 70 dB suppression to the elastic-scattering optical signals. It is configured with one blazed diffraction grating, one convex lens, one linear fiber array and seven groups of fiber Bragg gratings. The blazed diffraction grating and fiber Bragg grating are separately utilized as the primary and secondary spectroscope. The tailor-made fiber array, which is composd of ten single mode fibers of 460-HP type and one multi-mode fiber, is designed to transfer the spectral signals. One end face of multi-mode fiber lies in the focal point of telescope, and then it transfers the lidar echo signals to the pure rotational Raman spectroscopic filter. The other end face of multi-mode fiber lies in the focal point of convex lens. The ten single mode fibers are used to transfer the optical signals from the primary spectroscope to the secondary, and their end faces lie in the focal plane of convex lens. Six pure rotational Raman spectral lines of nitrogen molecule in the anti-Strokes branch are chosen under the condition of the 0.09-nm forbidden band, with the consideration of the relationship between the pure rotational Raman spectral lines of nitrogen and oxygen molecules. While the excited laser wavelength is 532 nm, their central wavelengths are 530.76 nm, 529.86 nm, 529.41 nm, 528.51 nm, 527.62 nm, and 527.17 nm, respectively. Their corresponding positions of fiber end faces on fiber array are 156 m, 407 m, 532 m, 782 m, 1031 m, and 1156 m. Compared with these pure rotational Raman spectral lines, the elastic scattering signal lies on the other side of the focal point of convex lens, which improves the spectral purity of pure rotational Raman spectral lines. A retrieval algorithm of absolute detection technique is presented based on the least square principle. The performance of this lidar is simulated based on the U. S. standard atmospheric model. Simulation results show that this designed lidar can achieve the extraction of the pure rotational Raman spectral lines of nitrogen molecules, and that the atmospheric temperature profile obtained from absolute retrieval algorithm within a measurement time of 17 min can be up to 2.0 km with less than 0.5-K deviation. This pure rotational Raman lidar without calibration will provide a new detection method and retrieval scheme for atmospheric temperature profile.

###### ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS

## EDITOR'S SUGGESTION

2016, 65 (14): 144202.
doi: 10.7498/aps.65.144202

Abstract +

Asymmetric property of wedge lens in 3 optical path which is used as frequency separation, and focusing element is considered to be an unfavourable factor for target alignment in inertial confinement fusion (ICF). Furthermore, the thickness of wedge lens in 3 optical path will lead to laser induced damage inevitably. For the purpose of scheme improvement of final optical assembly, types I and II noncollinear sum frequency generation in KDP crystal at room temperature are discussed based on nonlinear coupled wave theory. As illustrated by simulated result, in addition to type II collinear SFG used in ICF recently, 351 nm (3) waves can be generated by type I or II noncollinear SFG process. This method can realize color separations of , 2, 3 in far field without asymmetric element such as wedge lens and posses adequate tolerance of matching angle corresponding to the high efficiency conversion. As calculated, for type I SFG, when the noncollinear angle is in the interval from 0 to 19.99, phase matching condition can be satisfied in KDP crystal. The noncritical phase matching angle 3 is 90 and the corresponding noncollinear angle is about 19.99. The tolerance of mismatching angle is about 20 mrad. For type II SFG, the noncollinear angle interval that can satisfy phase matching process is about 0-13.55. Like type I SFG, there is also an noncritical solution in type II process whose matching angle is about (3) = 86.53. Because of the smaller effective nonlinear coefficient in this case, high efficiency conversion needs about 5 cm thick SFG crystal under 1 GW/cm2. Correspondingly, tolerance of mismatching angle is about 20 mrad. Because of the harsh tolerance of noncollinear angle between and 2 and for the purpose of compactness of final optical assembly, another method of noncollinear SFG is proposed: a piece of silica wedge with 12 wedged angle is mounted behind the SHG crystal in order to produce a 3.5 mrad intersection angle between and 2, and after type II noncollinear SFG process, , 2, 3 will be frequency separated in far field automatically by using thin lens. The tolerance of incident angle corresponding to high efficient conversion is about 1.0 mrad. This scheme can improve the the final optical assembly used recently.

2016, 65 (14): 144302.
doi: 10.7498/aps.65.144302

Abstract +

Acoustic environment has a low signal-to-noise ratio (SNR); hence, array signal processing is widely used for noise reduction and signal enhancement. The actual ambient noise includes uncorrelated noise and correlated noise. The received noises of the two arbitrary array elements are correlated. Consequently, the performance of array signal processing method decreases obviously. Aiming at this problem, the real-part elimination of covariance matrix method is proposed. Firstly, from a physical point of view, the noise signals can be generated by using a number of uncorrelated noise sources: the more the noise sources, the less the error between the noise from the model and the actual noise will be. Theoretically, the noise field is decomposed into the symmetrical noise field and the asymmetrical noise field. A number of noise sources generate the symmetrical noise fields; the directions of these noise sources are symmetric, and the powers of two arbitrary symmetric sources are the same. Secondly, the symmetry of the ambient noise is analyzed, as a result, the symmetrical noise can only affect the real part of the covariance matrix. Thirdly, the real part of covariance matrix is eliminated in order to reduce the noise, and then the delay-and-sum beamforming is achieved by using only the imaginary part. The advantages are that the output signal-to-noise ratio is increased and the noise output power is reduced obviously; the disadvantage is that it produces a false target. The azimuth of the actual target differs from that of the false target by 180, and the false target cannot be distinguished. Finally, to eliminate the false target, the real part of the signal covariance matrix is reconstructed by establishing a constrained optimization problem, which is solved by using the particle swarm algorithm. Then, the reconstructed covariance matrix composed of the imaginary part and the reconstruction of real part is applied to delay-and-sum beamforming, as a result, the false target is eliminated. The simulation results show that the real-part elimination of covariance matrix method reduces the symmetrical ambient noise, the noise output power is reduced, the output signal-to-noise ratio is increased, and this method improves the performance of array signal processing. The experimental results show that the output SNRs of two targets with using the imaginary part of covariance matrix are increased by 3.57 dB and 3.149 dB, respectively, and the output SNRs of two targets with using the reconstructed covariance matrix are increased by 7.027 dB and 6.985 dB, respectively. The real-part elimination of covariance matrix method is easy to implemente, and has a definite value for engineering application.

2016, 65 (14): 144101.
doi: 10.7498/aps.65.144101

Abstract +

In microwave tomography (MWT), electric-parameter distributions of the breast can be reconstructed to detect the early-breast-cancer, which has a specific physical explanation and medical diagnostic value. In time-domain, the finite-difference time-domain (FDTD) method is usually applied to these problems. However, due to the constraint of Courant-Friedrich-Levy (CFL) stability condition, the time step should be small enough to well match the small fine cells, which begets an increasing computational cost, such as the central processing unit (CPU) time. For real-time clinical, it is very important and essential to look for efficient methods to improve the computational efficiency. The alternating-direction implicit finite-difference time-domain (ADI-FDTD) method, on the other hand, provides a larger time step than that the CFL stability condition limitation could set. In order to shorten the time of imaging and improve the detection efficiency, the ADI-FDTD method is first used for the early-breast-cancer detection in this paper. MWT for breast cancer detection requires solving nonlinear inverse scattering problems. Most nonlinear inversion algorithms require solving a number of forward scattering problems followed by an optimization procedure. Therefore, we turn the inverse scattering problem into an optimization question according to the least squares criterion. The optimization procedure aims at minimizing the error between measured scattered fields and estimated scattered fields by the forward solver. Nonlinear reconstruction algorithm is used to solve an update for the scattering object properties used in our breast model. This iteration process is repeated until the convergence between the measured and estimated data is obtained. The specific process of the iteration method is divided into two steps: the forward step, which is to solve a forward problem for a scattering object with estimated electrical properties, and the backward step, which is to solve adjoint fields by introducing the Lagrange multiplier penalty function. Both the forward and backward calculations are conducted by using the ADI-FDTD method. The algorithm is evaluated for a two-dimensional (2D) semicircle breast model with tumors. We compare the imaging results obtained by the ADI-FDTD method for various time steps with the results obtained by the conventional FDTD method and the real distribution. The results agree well, the simulation results prove that the imaging time by using this ADI-FDTD method can be reduced to 23% that by the conventional FDTD method. In addition, the simulation results suggest that the ADI-FDTD method can be more efficient if higher resolution is required, thus further enhancing the clinical applicability of MWT.

2016, 65 (14): 144201.
doi: 10.7498/aps.65.144201

Abstract +

Non-diffracting beams have been a hot topic since the Airy wave packet was introduced to optics domain from quantum mechanics. Great efforts have been made to study this theme in recent years. The researches have ranged from paraxial regime to non-paraxial regime, and a series of new non-diffracting beams have been discovered. However, most of these beams are obtained under the time harmonic condition. To break this limitation, we propose a concept of time-dependent Bessel beam in this paper, which generalizes the non-diffracting beams to non-time-harmonic regime.We start from Maxwell's equations in vacuum under non-paraxial condition using the method borrowed from the half-Bessel beam. To obtain the non-time-harmonic solution, the fourth dimensional imaginary coordinate is introduced, which refers to the covariance in the theory of special relativity. By solving the wave equation without the time harmonic condition, we obtain the analytical expression for a time-dependent beam in the form of Bessel functions. Thus we call it time-dependent Bessel beam.The diffraction properties and space-time characteristics of the time-dependent Bessel beam are investigated theoretically. The transverse intensity and the intensity distribution of the beam are calculated and discussed in detail. The wave function of the time-dependent Bessel beam is in the same form as the normal Bessel beam so that it can exhibit non-diffraction in the four dimensional space-time. When propagating along a space-time hyperbolic trajectory, the intensity of the time-dependent Bessel beam remains constant and the width of the beam decreases with propagating distance and time increasing. Besides, we deduce the critical condition of the spatiotemporal characteristics of the beam, and the result agrees well with the concept of the light cone in the theory of special relativity.The method to deduce the time-dependent Bessel beam used in this paper is universal, and it will provide a valuable access to other solutions for the wave equations under different conditions. We extend the study of non-diffracting beams from time harmonic regime to non-time-harmonic regime. Furthermore, our work demonstrates the relation between the non-diffracting accelerating beams and the theory of special relativity. We believe this work will open up a new vista and give a new insight into the research of non-diffracting accelerating beams or other relevant research fields.

2016, 65 (14): 144203.
doi: 10.7498/aps.65.144203

Abstract +

The low coherence optical fiber dynamic light scattering method is used to measure the effective diffusion coefficients of nano SiO2 aggregates suspensions with different volume fractions. The single scattering component can be detected preferentially from the multiply scattered light which is backscattered from the dense suspensions by the low coherence optical fiber dynamic light scattering. Consequently, the measured single-scattering spectrum enables the analysis of the effective diffusion coefficient by the single scattering theory. The numerical calculation results of short-time diffusion dynamics for permeable particles in dense suspension show that the effective diffusion coefficient is a function of particle size and hydrodynamics shielding depth ratio , and the volum fraction . According to the corrected Brinkman theory, the permeability of the nano SiO2 aggregates is obtained. For the volume fraction = 0.01, 0.02, 0.03, 0.04, 0.05 nano SiO2 aggregate suspensions with the average particle diameter 500 nm, the measured effective diffusion coefficients are 4.140.10, 4.060.06, 3.970.06, 3.900.08, 3.800.10 (10-13 m2/s) respectively. While according to the hard sphere model of impermeable particles, which corresponds to = , the calculated effective diffusion coefficients are 3.70, 3.61, 3.52, 3.42, 3.36 (10-13 m2/s) respectively. It can be seen that the measured values are much bigger than the theoretical values of impermeable particles: their difference comes from the influence of permeability of porous aggregates on particle diffusion. It is found that the measured values are consistent with that of = 12, in which the corrsponding permeability of the nano SiO2 aggregates is k = 4.34 10-16 m2. The pixel statistic method by Photoshop CS6 is used to deal with the SEM images of SiO2 aggregates, the calculated permeability of the nano SiO2 aggregates is k = 4.55 10-16 m2, compared with the experimental result, the standard error is 4.87%. The results show that under the condition of constant temperature, the particles of permeable aggregates spread faster than the hard sphere particles. For constant temperature, particle size and permeability, the effective coefficient decreases with the increase of the volume fraction. The measured permeability of SiO2 aggregates in concentrated suspension is consistent with that obtained from the pixel statistics by Photoshop CS6. As a result, the low coherent optical fiber dynamic light scattering can effectively measure the permeability of porous nano particles in concentrated suspension, showing high potential application in the field of chemical engineering and nano materials preparation.

2016, 65 (14): 144204.
doi: 10.7498/aps.65.144204

Abstract +

Studies on the direction distribution of laser beam intensity reflected from the sea surface is important for engineering practice in the area of optoelectronic confrontation on the sea surface. In the traditional theory of electromagnetic scattering from rough surfaces, the scattered field from the sea surface can be obtained by solving the Maxwell's equations. As is well known, it is difficult to solve the Maxwell's equations. Therefore, the numerical calculation method and approximate analytical method are used to obtain the scattered field from the sea surface. However, for the numerical calculation method, it is difficult to meet the computing requirements of large electrically targets such as the sea surface. Meanwhile the approximate analytical method has certain restrictions on the parameters of rough surface in physical approximation. What is more, the inherent error is also caused by the physical approximation. In this paper, we investigate the laser beam reflection from rough sea surface with Monte Carlo method and principles of geometric optics. The rough sea surface which is simulated with the fractal method is divided into a lot of small planes, and the mathematical equations to describe the geometric characteristics of the planes are established in the sea reference coordinate system. After that, based on the simulation of Gaussian beam with Monte Carlo method, the laser beam is divided into a great number of rays and the statistical properties of the rays satisfy the propagation characteristics of Gaussian beam. Then, the laser beam reflection model from the sea surface is derived in the reference coordinate system. The direction distribution of the laser beam intensity reflected from the sea surface is simulated under a certain experiment condition with this model. The results show that the simulation results of laser beam reflection from the sea surface fit the experimental results well.

2016, 65 (14): 144205.
doi: 10.7498/aps.65.144205

Abstract +

Microsphere resonators based on chalcogenide glasses combine the superior optical properties of microsphere resonators (such as high Q-factors and small mode volumes) and excellent material properties of chalcogenide glasses in the infrared spectrum (such as good transmissivities, high refractive indices, and low phonon energies), and thus have promising applications in the fields of low-threshold infrared lasers, nonlinear Raman amplifiers/lasers, and narrow bandwidth infrared filters.In this work, the infrared microsphere resonators are built by using a novel chalcogenide glass composition of 75 GeS2-15 Ga2S3-10 CsI (Ge-Ga-S), doped with 1.3 wt% Tm. Compared with previously reported chalcogenide microsphere resonators fabricated with As2S3 and gallium lanthanum sulfide (Ga-La-S) glasses, the proposed Ge-Ga-S glass does not contain the toxic element of As nor the expensive rare earth element of La, and thus is more environmentally friendly and cost-effective for fabricators and users. We first fabricate bulk Ge-Ga-S glasses by using the facility in our laboratory. After measuring the absorption and fluorescence spectra of bulk glasses, they are crushed into powders and the powders are blown downwards through an inert-gas-filled vertical furnace (temperature set at 1000 ℃). Molten glass powders are transformed into high-quality microspheres in the furnace due to surface tension. Thousands of microspheres with diameters ranging from 50 to 200 m can be made in one fabrication process. By using optical microscopy and scanning electron microscopy, a microsphere with high surface quality is selected for further optical characterization. The selected microsphere has a diameter of 72.84 m, an eccentricity less than 1% (about 80 nm), and a Q-factor of 1.296104. A silica fiber taper with a waist-diameter of 1.93 m is fabricated as the coupling mechanism for the microsphere resonator. The coupling between the microsphere and the micro fiber taper is realized with the aid of nano-positioning stages. An 808 nm laser diode is used as a pump light source, which is sent into one end of the fiber taper and is evanescently coupled into the microsphere. Spontaneous emissions of fluorescent light are then generated in the microsphere, whose spectral characteristics are measured by using an optical spectrum analyzer. It can be clearly noted from the measurement results that the typical fluorescence spectrum of the Tm3+-doped Ge-Ga-S glass is modified by whispering gallery mode (WGM) patterns as periodic intensity peaks/valleys are apparently present in the measured spectral curves. The locations of those experimentally measured spectral peaks/valleys are in good agreement with WGM mode calculated results through using the Mie scattering theory, which verifies that the proposed Ge-Ga-S glass can be used to build high-quality infrared microsphere resonators. The largest deviation between the experimentally measured spectral peaks/valleys and theoretically calculated WGM modes is about 0.047%. Minor deviation is present because the experimentally fabricated microsphere has a small difference from an ideal sphere (with an eccentricity of about 1% in this work). Longer processing time of glass powders in the vertical furnace or a post-thermal treatment could help improve the sphericity of microspheres.

2016, 65 (14): 144301.
doi: 10.7498/aps.65.144301

Abstract +

An ultrasonic horn can radiate a strong ultrasonic wave into viscous liquid contained in a tank or cylindrical cup, and bubble clusters could be generated by the high-intensity ultrasound in the liquid. In the bubble clusters, interaction of bubbles exists because of the secondary radiation of bubbles. Therefore, the oscillations of bubbles are coupled. On the other hand, the surrounding liquid pressure of the bubbles in the cluster is influenced by the oscillations of the bubbles, which induces a pressure gradient on the boundary of the cluster. Therefore, the oscillation of a bubble inside a cluster is contracted by the formation of the cluster and its structure evolution. In this paper, a cylindrical cavitation bubble cluster is considered as a mixture drop of bubbles and liquids, and the motion of the cluster boundary is proposed with a second two-dimensional (2D) Rayleigh equation related to the difference between the inner mixture pressure and the outside liquid pressure on the boundary. Based on the bubble cluster boundary dynamical equation, a new mathematical model is developed to describe the motion of cavitation bubbles inside a cylindrical cluster when the effects of coupled oscillation are included. Comparing the new model equation with the Rayleigh-Plesset equation of single bubble in unbounded liquid, it is easy to draw the conclusion that the contraction of oscillating bubbles is strengthened by the coupled oscillation of bubbles and the boundary motion. In the cylindrical cluster, the oscillation of bubbles is suppressed, and the natural frequency of bubbles is reduced. The proposed model is used as a basis for the numerical investigation of the nonlinear acoustic response of bubbles. The suppression of the bubble oscillation is strengthened by increasing the number density of bubbles. Comparing numerical curves of the maximum radius of the oscillating bubble, it is shown that there are local peaks which are related to the resonance response of bubbles. In some unstable parameter regions, the maximum radius of the oscillating bubble varies sensitively with the tiny change of the parameters. The parameter space distribution of the unstable regions is related to the initial bubble radius and driving frequency of ultrasound. According to the numerical results related to the parameters, such as bubble number density, initial radius, driving frequency and pressure amplitude of ultrasound, it is found that the unstable acoustic response could be amplified for bubbles of smaller initial radius driven by a low-frequency ultrasound. For cavitation bubbles of initial radii ranging from 1 m to 10 m in low-frequency ultrasonic field, the unstable regions of parameter spaces related to the evolution of maximum radius become broader with the decrease of bubble initial radius and driving frequency of ultrasound. Therefore, the tiny bubbles inside cylindrical clusters have stronger nonlinear properties and the change of the parameters in the dynamical model equation has greater influence on the tiny bubbles.

2016, 65 (14): 144303.
doi: 10.7498/aps.65.144303

Abstract +

Conventional beamforming (CBF) is an important processing step in underwater array signal processing. Previous researches have shown that the sound field structure as manifested by amplitude nonhomogeneity and wave-front corrugation can reduce the array gain of CBF. The acoustic environment of the continental shelf slope area is very complex. For an underwater acoustic array in this area, the amplitude and phase of the received signals will be distortional seriously. Recently, the acoustic field correlation has been the focus of research on the array gain relations with the underwater acoustic filed. However, the attenuation of acoustic field correlation is not the only factor that induces the array gain to decline, and the analyses of the array gain in the shallow water based on normal-mode model are not applicable to the continental slope area. In this paper, the array gain relations with the structure of acoustic field in continental slop area are investigated based on the theory of underwater acoustic signal propagation. The effects of acoustic field on the signal and noise gains are considered respectively. The analytic expressions of the array gain of CBF in an isotropic noise field are derived from the primal definition of array gain, which indicates that acoustic field correlation and transmission loss in continental slope are the intrinsic factors that affect the array gain of CBF. A horizontal uniform linear array (ULA) with a wide aperture receiving signals from a source in the deep water region is considered in the upslope propagation condition. The RAM program is utilized in the numerical simulations to generate the sound field of this specific environment with given parameters. The array gains, the ATLs and the horizontal longitudinal correlation coefficients of the acoustic field corresponding to three different locations are compared. Conclusions can be drawn as follows. 1) The array gain of CBF is determined by acoustic field correlation and the acoustic average transmission loss (ATL), and its maximum is less than 10lg M as the signal waveform distortion. 2) when the ATLs corresponding to hydrophones at two different receiving locations are similar, the correlation of acoustic filed is higher, and the array gain of CBF is larger. 3) When the ATLs corresponding to hydrophones at two different receiving locations are greatly different, the relation between the array gain of CBF and the acoustic filed correlation is no longer positive. The simulation results verify the array gain of CBF relations with the acoustic filed correlation and the transmission loss in the continental slope area.

###### PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES

2016, 65 (14): 145201.
doi: 10.7498/aps.65.145201

Abstract +

Metal rapid manufacture has received great attention in recent decades. Energy source with high power density is requisite for the metal deposition. Atmospheric pressure inductively coupled microplasma jet is commonly characterized by high temperatures, which is one of excellent candidates for metal rapid manufacture on a micro scale.In this paper, we investigate the microplasma jet driven by a 150 MHz very-high-frequency power supply at atmospheric pressure. A microplasma of 3 cm in length and about 3 mm in diameter can be produced at 90 W power applied, with gas temperatures above one thousand degree centigrade. The jet length rises first, and then decreases by increasing gas flow rate, showing a transition from laminar flow to turbulence. The jet length also increases by enhancing applied power, but then keeps a maximum value with further increasing power, which is attributed to the attainment of equilibrium between the energy absorption and losses in the transport process in plasma.Copper powders are carried by the argon flowing into plasma, and melted fast by the microjet. An alumina ceramic plate is used as a substrate, which is set on the substrate holder with a precisely controlled X-Y-Z manipulator. A copper spherical cap with 2 mm in diameter and a column with 1 cm in height are fabricated in a few seconds, respectively, on the alumina ceramic substrate. The Cu spherical cap is characterized by scanning electron microscopy. Particles obtained on the sample surface are far smaller than the source powders, indicating a melting process of copper powders in plasma, as well as high gas temperature exceeding the melting point of copper. The weak peak of Cu2+1O is present besides strong copper diffraction lines in X-ray diffraction pattern, suggesting that the weak oxidation happens during rapid fabrication.

2016, 65 (14): 145202.
doi: 10.7498/aps.65.145202

Abstract +

In order to study the breakdown process of vacuum switch, we use a vacuum diode, which is composed of a cathode and an anode, to replace the vacuum switch. We find that there is wide band microwave radiation in the breakdown process of the vacuum diode. Because there is no structure of metallic bellow waveguide in the vacuum diode, the radiation mechanism of the vacuum diode is different from that of the plasma filled microwave device. It is hard to completely imitate the theory of the plasma filled microwave device. In order to clarify the mechanism of the microwave radiation from the vacuum diode, we analyze the breakdown process of the vacuum diode. When the anode plasma has been generated and the plasma closure has not occurred, the electrons emitted from the initial plasma will be incident on the anode plasma, and the vacuum diode will radiate microwave in this process. The self-generating magnetic field of the electron beam is a poloidal magnetic field. When the electron beam is incident on the plasma, the plasma will be magnetized by the poloidal magnetic field. The theory of magnetic fluid is used to analyze the problem in this paper, and the mathematical model of the vacuum diode radiation is obtained by using the simultaneous equations of the motion equations and Maxwell's equations. In this model, there is an interface between the electron beam and the magnetized plasma. The model is divided into two parts by the interface, i.e., inside of the electron beam and outside of the electron beam. The dispersion relation of the radiation generated by the vacuum diode is obtained by solving the mathematical model. Based on the dispersion relation and the experimental data, the dispersion curves are plotted for the different electron beam velocities. The dispersion curves show that the undulation of the dispersion curve becomes smaller and smaller with the decrease of the electron beam velocity, and the final dispersion curve will be approximated by a straight line. When the theoretical dispersion curves are compared with the actually measured time-frequency maps of the radiation, we find that they are well consistent with each other. Theoretical deduction and experiments indicate that the radiation generated by the vacuum diode originates from the interaction between the electron beam and the magnetized plasma.

###### CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES

2016, 65 (14): 146101.
doi: 10.7498/aps.65.146101

Abstract +

As an h.c.p crystal structure with only a few limited slipping planes, the AZ31 magnesium alloy exhibits a bad plasticity in the presence of external stress. Due to its low density, advanced damping capacity and high ratio strength and rigidity, the magnesium alloy has gradually become the focused and potential structural and functional metallic material in the diverse fields of aerospace, aviation and vehicle transportation, electronic products, etc. Therefore, it is of great importance to improve the process ability of conventional magnetism alloy as AZ31. In the past decades many approaches have been proposed in order to improve the plastic deformation capability. Among these, the diverse physical fields are regarded as the effective methods to improve the comprehensive mechanical properties of metallic materials due to their peculiar heat, force and quantum effects together with the advantageous characteristics of low pollution and high efficiency. In the paper, on the basis of previous researches, a high pulsed magnetic field is introduced into the tensile test to study the influences of magnetic field on the plasticity and microstructure of AZ31 magnesium alloy in order to explore a novel way to enhance the plastic deformation capability of alloy. As for the current experiment, the tensile test of AZ31 magnesium alloy is carried out under the coupling action of high pulsed magnetic field and external stress. The test results are compared with those processed without magnetic field. Several advanced detection methods are utilized to investigate the microstructure including the electron back scattered diffraction, X-ray diffraction and transmission electron microscopy, etc. Besides, the first principle is utilized to calculate the magnetic properties of main precipitates (Mg17Al12).The experimental results show that the tensile strength and elongation of the 3 T sample are increased by 2.2% and 28.7% in comparison to those of the 0 T sample. It highlights that when the high pulsed magnetic field is introduced into the plastic deformation process, the plasticity of the magnesium alloy can be improved without reducing the tensile strength of the material. The action mechanism of magnetic field is analyzed in detail and attributed to the magnetoplasticity effect. The calculation results on the basis of first principle show that the (Mg17Al12) phase is paramagnetic, which is helpful for performing the effect of magnetic field. The magnetic field enhances the flexibility of the dislocation movement and facilitates the proliferation of the dislocation. The dislocation and stress concentrating at the grain boundaries accelerate the formation of recrystallization, which is of great importance to the sub-grain generation and grain refinement that is beneficial to exhibiting the fine grain strengthening and enhancing the strength and toughness of alloy. Meanwhile, during the peculiar tensile process, the magnetic field induces the grain rotation. The newborn fine grains along the non-basal face weaken the (0001) basal texture of magnesium alloy. The characteristic of the texture structure is helpful for improving the plastic deformation capacity of AZ31 alloy. The plastic deformation under high magnetic field is regarded as an advanced way to improve the plasticities of similar nonmagnetic metallic materials such as aluminum, titanium and copper alloys and their composites.

2016, 65 (14): 146201.
doi: 10.7498/aps.65.146201

Abstract +

The velocity interferometer system for any reflector (VISAR) and pyrometric measurements in dynamic highpressure experiments require the use of an optical window, and Alumina (Al2O3) or sapphires is often considered as a window material due to its high shock impedance and excellent transparency. Consequently, understanding the characteristics of its transparency and refractive index change under shock loading is crucial for explaining such experimental data. Experimental studies indicate optical transparency loss in shocked Al2O3. The mechanisms for the phenomenon are some interesting issues. A first-principles study suggests that shock-induced VO+2 (the +2 charged O vacancy) defects in Al2O3 could be an important factor causing the transparency loss. Recently, the red shift of the extinction curve (i.e., the wavelength dependence of the extinction coefficient) with increasing shock pressure has been observed. It is needed to ascertain whether this behavior is also related to shock-induced vacancy point defects. In addition, up to now, information about Al2O3 refractive index at a wavelength of 532 nm under strong shock compression (the optical source wavelength in VISAR measurement is usually set at 532 nm) has been unknown, and neither the effects of structural transitions nor vacancy point defects on the refractive index of shocked Al2O3 are determined. Here, to investigate the above-mentioned questions, we perform first principles calculations of optical absorption and refractive index properties of Al2O3 crystal without and with VO+2 and VAl3 (the -3 charged Al vacancy) defects in a pressure range of 180 GPa (the calculations in CASTEP are carried out by the plane-wave pseudo potential method in the framework of the density functional theory). Our absorption data show that the observed optical extinction in shocked Al2O3 cannot be explained by only considering pressure and temperature factors, but shock-induced VO+2 should be an important source for this behavior. On the basis of these results, we may judge that 1) the transparency loss explanation for shocked Al2O3 in the view of vacancy point defects is reasonable; 2) the absorption extinction should dominate the extinction phenomenon observed in shocked Al2O3. Our calculations find that high-pressure structural transition in Al2O3 causes an obvious enhancement of its refractive index. The refractive index decreases with increasing shock pressure in corundum and Rh2O3 regions, and decreases slightly below 172 GPa and increases slowly above 172 GPa with increasing shock pressure in CalrO3 region. The VO+2 and VAl3 defects in Al2O3 have apparent influences on the shock pressure dependence of its refractive index. These results mean that the information about Al2O3 refractive index under strong shock loading cannot be obtained simply by extrapolating its low pressure data. Our prediction could be of importance for future experimental study and new window-material development.

2016, 65 (14): 146801.
doi: 10.7498/aps.65.146801

Abstract +

Hydrogenated microcrystalline silicon germanium (c-Si1-xGex:H) thin films have been developed as alternative bottom sub-cell absorbers for multi-junction thin film silicon solar cells due to their narrower band-gaps and higher absorption coefficients than conventional hydrogenated microcrystalline silicon (c-Si:H) thin films. However, since the structure complexity was increased a lot by Ge incorporation, the influences of c-Si1-xGex:H film properties on Ge composition have not been understood yet. In this work, c-Si1-xGex:H thin films with various Ge content and similar crystalline volume fraction are fabricated by radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD). The evolutions of c-Si1-xGex:H material properties by Ge incorporation are characterized by X-ray fluorescence spectrometry, Raman spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, absorption coefficient spectrum, and conductivity measurement. The results show that the properties of c-Si1-xGex:H thin films are strongly determined by Ge content. With the increase of Ge content, the absorption coefficient, (111) grain size, microstructure factor, and dark conductivity of c-Si1-xGex:H thin films increase, while the H content, (220) grain size, and photosensitivity of c-Si1-xGex:H thin film decrease. Then, c-Si1-xGex:H is used as the intrinsic layer in the single junction solar cells. The performances of c-Si1-xGex:H solar cells with different Ge content and two types of transparent conductive oxide (SnO2 and ZnO) substrates are systematically studied. The results indicate that although c-Si1-xGex:H thin films become more defective and less compact with Ge incorporation, c-Si1-xGex:H solar cells exhibit a significant improvement in near-infrared response, especially under the circumstances of thin cell thickness and inefficient light trapping structure. Meanwhile, by using ZnO substrates, initial efficiencies of 7.15% (Jsc=22.6 mA/cm2, Voc=0.494 V, FF=64.0%) and 7.01% (Jsc=23.3 mA/cm2, Voc=0.482 V, FF=62.4%) are achieved by c-Si0.9Ge0.1:H solar cell and c-Si0.73Ge0.27:H solar cell, respectively. Furthermore, the c-Si0.73Ge0.27:H solar cell is used as the bottom sub-cell of the double-junction solar cell, and a Jsc.bottom of 12.30 mA/cm2 can be obtained with the bottom sub-cell thickness as thin as 800 nm, which is even higher than that of c-Si:H bottom sub-cell with 1700 nm thickness. Finally, an initial efficiency of 10.28% is achieved in an a-Si:H/c-Si0.73Ge0.27:H double junction cell structure. It is demonstrated that by using the c-Si1-xGex:H solar cell as the bottom sub-cell in multi-junction thin film silicon solar cells, a higher tandem cell performance can be achieved with a thin thickness, which has a great potential for cost-effective photovoltaics.

2016, 65 (14): 146802.
doi: 10.7498/aps.65.146802

Abstract +

Dye pollution, one of the most serious pollutions in water, remains a challenging issue in environmental engineering. Many strategies including membrane separation, chemical oxidation, electrolysis, adsorption, etc., have been adopted to remove the dyes from water. Compared with other methods, adsorption has its own unique advantages such as low cost, low energy consumption and high efficiency. However, commercial adsorbents have low adsorption capacities and separation of absorbents/water, which hinders their practical applications. In this paper, functional tissues based on graphene oxide are fabricated through a simple immersion method. The structure, morphology and adsorption ability for each of these functional tissues are characterized and analyzed by scanning electron microscopy, Raman spectroscopy, thermal gravity analysis and UV-Vis spectrophotometer. The combination of commercial tissue and graphene oxide can solve the aforementioned problems such as low adsorption capacity, hard separation of adsorbent from water. on the one hand, abundant oxygen-containing functional groups and defects existing in graphene oxide sheets can be used as active adsorption sites, which endows the functional tissue with high adsorption capacity; On the other hand, the crosslinking of commercial tissue and graphene oxide through hydrogen bonding enables the functional tissue to be completely recycled from water after adsorption, which can avoid the secondary pollution caused by adsorbents such as pure graphene oxide. Batch tests are conducted to investigate the adsorption performance, e.g. the influences of adsorption time, initial concentration of dyes, adsorbent amount, and temperature on the adsorption performance. The results suggest that functional tissue has excellent performance for the removal of methylene blue and rhodamine B. Giving that the initial concentrations of methylene blue and rhodamine B are 40 mgL-1 and 30 mgL-1 respectively, the adsorption capacities are 54.84 mgg-1 and 21.74 mgg-1, respectively. It is noteworthy that graphene oxide sheets play a critical role in adsorbing the dyes. The adsorption capacity of functional tissue based on graphene oxide for rhodamine B totally results from graphene oxide component. Calculating the graphene oxide loading on the tissue, the adsorption capacity for rhodamine B reaches 183 mgg-1 at initial concentration of 30 mgL-1. Meanwhile, the adsorbance quantities of the functional tissue for the two dyes increase with adsorption time, initial concentration, adsorbent dosage, and temperature. Kinetic analysis reveals that the adsorption processes for methylene blue and rhodamine B are well-matched with the pseudo-second-order kinetic model, indicating the dominance of chemical adsorption in the whole adsorption process. The thermodynamic parameters indicate that the adsorption is spontaneous and endothermic in nature. In summary, a facile, inexpensive, and eco-friendly synthesis method is developed to fabricate the functional tissues based on graphene oxide. The functional tissues have high adsorption capacities for dyes. The combination of commercial tissue and graphene oxide could be explored as a new adsorbent for removing toxic organic dye pollutants from aqueous environment.

###### CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES

2016, 65 (14): 147101.
doi: 10.7498/aps.65.147101

Abstract +

The main purpose of this work is to explore the influences of microstructures on the magnetic properties, as well as the formation mechanism of -Fe2O3/NiO core/shell nanoflowers. The synthesis of nanoflower-like samples includes three processes. Firstly, Fe3O4 nanospheres are synthesized by the solvothermal reaction of FeCl3 dissolved in ethylene glycol and NaAc. Secondly, Fe3O4/Ni(OH)2 core/shell precursor is fabricated by solvothermal method through using the early Fe3O4 spheres and Ni(NO3)26H2O in an ethanol solution. Finally, the precursor Fe3O4/Ni(OH)2 is calcined in air at 300 ℃ for 3-6 h, and therefore resulting in -Fe2O3/NiO core/shell nanoflowers. Their microstructures are characterized by using XRD, XPS, SEM, HRTEM and SAED techniques. The results show that the final powder samples are -Fe2O3/NiO with typical core/shell structure. In this core/shell system, the -Fe2O3 sphere acts as core and the NiO acts as shell, which are comprised of many irregular flake-like nanosheets with monocrystalline structure, and these nanosheets are packed together on the surfaces of -Fe2O3 spheres. The calcination time of Fe3O4/Ni(OH)2 precursor has significant influences on the grain growth, the NiO content and the compactness of NiO shells in the -Fe2O3/NiO core/shell system. VSM and SQUID are used to characterize the magnetic properties of -Fe2O3/NiO core/shell nanoflowers. The results indicate that the 3 h-calcined sample displays better ferromagnetic properties (such as higher ms and smaller HC) because of their high -Fe2O3 content. In addition, as the coupling interaction between the FM -Fe2O3 and AFM NiO components, we observe that the -Fe2O3/NiO samples formed in 3 h and 6 h display certain exchange bias (HE=20 and 46 Oe, respectively). Such a coupling effect allows a variety of reversal paths for the spins upon cycling the applied field, and thereby resulting in the enhancement of coercivity (HC(FC)=252 and 288 Oe, respectively). Further, the values of HE and HC for the former are smaller than those of the latter, this is because of the AFM NiO content in 6 h-calcined sample much higher than that in 3 h-calcined sample. Especially, the temperature dependences of the magnetization M of the two samples under both ZFC and FC conditions indicate that an extra anisotropy is induced. In a word, the size effect, NiO phase content, and FM-AFM (where FM denotes the ferromagnetic -Fe2O3 component, while AFM is the antiferromagnetic NiO component) interface coupling effect have significant influence on the magnetic properties of -Fe2O3/NiO core/shell nanoflowers.

2016, 65 (14): 147302.
doi: 10.7498/aps.65.147302

Abstract +

Transversal magneto-optical Kerr effect (TMOKE) has potential practical applications, such as biosensors, magnetic imaging, and date storage. However, these potential applications have been restricted by its very weak response (about 0.1%) in natural ferromagnetic metal material such as Fe, Co and Ni. Fortunately, with the development of the nanofabrication techniques, surface plasmons (SPs) are one of the effective strategies to solve this problem due to their special ability to manipulate light on a nanoscale and concentrate the electromagnetic energy near the metal/dielectric interface. Herein, in order to enhance the TMOKE response, we propose that a periodic gold strips array is embedded into a magnetic dielectric film of bismuth iron garnet (BIG), which is supported by a quartz substrate. Using the finite element method, we numerically study the optical properties of our proposed microstructure and the corresponding evolution of the TMOKE responses due to the coupled optical modes dependent on the structural parameters. Particularly, by optimizing the embedded depth of metal grating, a dramatic enhancement of TMOKE response (about 3.6%) is achieved when the embedded depth reaches up to 80 nm, accompanied with a high transmissivity about 22.6%, which is actually three time larger than that in the case that the gold strips are just patterned on the surface of the BIG film. As the embedding depth increases further, the TMOKE response will be weak. The relationship between the TMOKE response and the coupling efficiency of LSP resonance of the gold stripes and the waveguide (WG) mode supported by the BIG film are also discussed systematically. As the embedding depth increases up to 80 nm gradually, the coupling of the WG mode in BIG film with the LSP mode of the individual gold stripe becomes much stronger and forms a highly efficient Fano resonance, which leads to the fact that most of the electromagnetic field is localized in the BIG film and strong interaction with the BIG magnetic dielectric film, and thus, an enhancement of TMOKE response can be observed. However, when the embedded depth increases further, the uniformity of BIG film will be broken. In this case, WG mode cannot be supported by BIG film very well any more at the wavelength corresponding to excitation of the LSP, which results in a weakly coupling efficiency between LSP and WG mode. In this case, the Fano resonance cannot be formed and rare electromagnetic field can be localized in the BIG film, leading to a very weak light-magnetic dielectric film interaction and the weak TMOKE response. Our study proposes a new method to realize the amplification of weak TMOKE response by utilizing the plasmonic microstructure, which might have a potential application to designing the high-efficiency magneto-optical devices.

2016, 65 (14): 147102.
doi: 10.7498/aps.65.147102

Abstract +

The site preference, electronic structure, and magnetism of Heusler alloy Fe2RuSi are investigated theoretically and experimentally. The magnetic and electronic properties of Heusler alloys are strongly related to the atomic ordering and site preference in them. Usually, the site preference of the transition metal elements is determined by the number of their valence electrons. However, the recent results suggest that some new possible factors such as atomic radius should also be considered. Here we compare the phase stabilities of several different atomic orderings like XA, L21, DO3, L21B in Fe2RuSi, in which Fe and Ru atom have 8 valence electrons each, thus the influence of number of their valence electrons can be omitted. First-principles calculations suggest that Ru atom prefers entering sites A and C in the lattice. In ground state, the most stable structure is of XA type, in which Fe and Ru atoms occupy A and C sites, respectively and the second stable structure is L21B type, in which Fe and Ru atoms occupy A and C sites randomly. With Ru atom entering into the B site, the total energy increases rapidly. Thus there is still a strongly preferable occupation of Ru though Fe and Ru atom are isoelectronic. This confirms that the valence electrons rule may be not enough to determine the site preference of the transition metal element in Heusler alloy. The preferable occupation of Ru atom in Fe2RuSi can be explained from the electronic structure. It is found that in the XA DOS, there is strong hybridization between the electrons of the nearest Ru and Si or Fe (B) atom. However, in the high energy L21 structure the hybridization between Ru and the nearest Fe (A, C) is weak, which reduces its phase stability. This is confirmed further by the charge density difference calculation. Single phase Fe2RuSi with a lattice parameter of 5.79 is synthesized successfully. Comparing the superlattice reflections (111) and (200) in the experimental XRD pattern with those in the simulated patterns for different structures, we find that Fe2RuSi crystallizes in L21B structure rather than the most stable XA one at room temperature, which mainly originates from the contribution of mixed entropy to the free energy, and its caused atomic disorder at high temperatures. This disorder can be retained during the cooling procedure, while it does not influence the conclusion that Ru atom prefers the (A, C) sites in Fe2RuSi strongly. Finally, the saturation magnetization Ms at 5 K is 4.87 B/f.u., which agrees well with the theoretical result. The large total magnetic moment mainly comes from the contributions of Fe, especially Fe magnetic moments on B sites.

2016, 65 (14): 147301.
doi: 10.7498/aps.65.147301

Abstract +

A multi-beam antenna based on spoof surface plasmon polariton (SSPP) is proposed, which is composed of 24 identical end-fire antennas rotating around the center of the circle. Thus the angle between any two end-fire antennas is 15. Every single end-fire antenna consists of feeding monopole and periodic metallic blade structure sandwiched between two identical 0.5 mm-thick F4B substrates (r=2.65, tan()=0.001). And the periodic metallic blade structure can be regarded as two regions. The first region (Region I) is a double-side corrugated metallic strips with continuous gradient height, so that the SSPP has a linear propagation constant distribution on the strips. Good matching of both impedance and wave vectors between spatial wave and SSPP waveguide ensures the conversion of high-efficiency from spatial modes into SSPP modes and that of high-efficiency radiation from SSPP modes into spatial modes. The second region (Region II) is the transition part of the SSPP wave with constant blade height. Geometric parameters are optimized by using CST Microwave Studio and the dimension of the single end-fire antenna is 111 mm15.2 mm1 mm. A prototype is fabricated and tested, showing good agreement between numerical simulation and experimental results, which proves that the electromagnetic wave of the monopole is successfully coupled and nearly completely confined on the metallic blade structure, and radiated at the end of the blade, resulting in omnidirectional radiation pattern of the monopole being mediated to directive beam steering at end fire. Rotate the 24 identical antennas around the center of the circle with respect to a cylinder, namely the proposed 360 scanning multi-beam antenna in this paper. The optimized radius of the proposed antenna cylinder is set to be 128 mm. The simulated and measured results are consistent with each other and clearly indicate that the proposed multi-beam antenna shows a scanning capability over 360 in the xoy plane with an average directivity of approximately 11.8 dBi and 3 dB angular width of 15 in operation bandwidth 9.5-10.25 GHz. Changing the geometric parameters of the blade structure, the characteristics of the gain, bandwidth, and 3 dB angular width for multi-beam antenna will be also changed. Unlike traditional multi-beam antennas, the proposed antenna based on SSPP mode coupling is no longer limited to the principle of geometrical optics, but mediates the omnidirectional radiation pattern of the monopole to directive beam by utilizing great confinement property of SSPP, which gives high degree of freedom for designing the multi-beam antennas. Besides, derived from the characteristics of deep-subwavelength and localized field enhancement for SSPPs, the proposed multi-beam antenna obtains many advantages, such as low profile, simple structure, high realizability, and important application values.

2016, 65 (14): 147701.
doi: 10.7498/aps.65.147701

Abstract +

For barium strontium titanate (Ba0.6Ti0.4TiO3, BST) films used in tunable microwave devices, they must have excellent structural characteristics and outstanding combination of dielectric properties i.e., a low loss tangent over the range of operating direct current (DC) bias voltages, a moderate dielectric constant for impedance matching purpose, a large variation in the dielectric constant with applied dc bias (high tunability, in particular high tunability at low applied dc bias), etc. To achieve such a high objective, the following two great improvements are carried out. A normal sol-gel method is modified to prepare multilayer BST films layer by layer. Each multilayer BST film is composed of six layers, where each layer is preheated at 600 ℃, thus the layers from the first layer to the sixth layer are successively preheated once to six times. Thus each BST film is smooth and dense, and contains almost no organic residues. On the other hand, as a new doped mode, Ce/Mn alternate doping is performed. For every six layer-BST films, when the odd number layers are doped with Ce, then the even number layers are doped with Mg, vice versa. The above two improvements result from the fact that Ce doping, Mg doping and Y and Mn alternate doping could make BST thin films significantly improve the dielectric tunability, reduce the dielectric loss, and improve the combination of dielectric properties, respectively. According to the above two improvements, 1 mol% Ce and 3 mol% Mg alternately doped BST thin films are prepared on Pt/Ti/SiO2/Si wafers (substrates). The prepared BST films are denoted by the doped element as follows: Ce/Mg/Ce/Mg/Ce/Mg with Ce doped BST layer is referred to as the first layer (for short Ce/Mg) and Mg/Ce/Mg/Ce/Mg/Ce with Mg doped BST layer as the first layer (Mg/Ce), and the structure and dielectric properties of the films are studied. X-ray diffraction results show that two films present cubic perovskite structures, mainly grow along (110) crystal face, and show strong crystallization. SEM results indicate that the surface morphologies of two films are greatly improved, and Ce or Mg doped BST layer as the first layer can be well matched with the substrate. The surface of the Ce/Mg film is more uniform and denser with slightly smaller grains and weaker crystallization. XPS results demonstrate that the non-perovskite structures on the surfaces of two films are significantly reduced. Each of the two films has high tunability at low applied dc bias and high figure of merit (FOM). The Mg/Ce film shows more stable combination of dielectric properties in a high frequency range. The Ce/Mg film shows more excellent combination of dielectric properties and higher dielectric strength in a low frequency range, where when the testing frequency is 100 kHz, 10 V, 20 V and 40 V applied dc bias voltages correspond to tunabilities of 47.4%, 63.6% and 71.8%, and FOMs of 71.8%, and 27.1, 77.5 and 86.5, respectively. Such good dielectric properties can fully satisfy the requirements for tunable microwave device applications. The relevant mechanisms are also analyzed.

###### INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY

2016, 65 (14): 148101.
doi: 10.7498/aps.65.148101

Abstract +

The initial magnetization curve is closely related to the stress in ferromagnetic material, thus it could be used to evaluate the stress in ferromagnetic member online. However, the initial magnetization curve measurement system recommended by the technical standard IEC 60404-4 is not suitable for online application. It is inevitable to use excitation coils to generate the excitation field and induction coils to obtain the magnetic flux density, however winding coils closely and uniformly online is not easy to operate. To obtain the initial magnetization curve easily, a calculation method for initial magnetization curve under constant magnetization based on time-space transformation is put forward in this paper. The theoretical correctness of this method is validated through simulation with the constant current coil magnetization. Considering the fact that the constant magnetic field could also be provided by permanent magnets and that magnetizing ferromagnetic members online by permanent magnets are convenient to achieve, in this paper, we put forward the measuring principle of initial magnetization curve based on a constant magnetic field excited by permanent magnets further and set up the corresponding measurement system. This system employs permanent magnetizers as the excitation magnetic source, and adopts symmetric magnetization methods to produce a constant magnetic field on a cylindrical rod-shaped member. The excited constant magnetic field changes along the axial position of the member. Under this exciting field, the axial and radial magnetic flux densities at different lift-offs from the surface of the member are measured by a testing probe including Hall chip array. Then, the axial and radial magnetic flux densities at the interface between the member and air are calculated based on the extrapolation method through utilizing polynomial function fitting and the Gauss's law for magnetism. Furthermore, the axial magnetic field strength within the member is calculated from the axial magnetic flux density at the interface according to the continuity of the tangential magnetic field strength. On the other hand, the induced magnetic flux density within the member is calculated from the radial magnetic flux density at the interface on the basis of the Gauss' law for magnetism, the basic equation of magnetization curve in Rayleigh region and the law of approach to saturation. Finally, the initial magnetization curve could be measured. System measurement results show that with no excitation coils nor induction coils, the initial magnetization curve of the cylindrical rod-shaped member can be easily obtained from the axial and radial magnetic flux densities at the interface of the member under the constant magnetic field excited by permanent magnetizers. The measurement error is less than 10%, and the standard deviation of the error is less than 0.01, which shows that the measurement repeatability is good. Therefore, this proposed system could provide a new approach to measuring the initial magnetization curve of cylindrical rod-shaped members online conveniently.

2016, 65 (14): 148901.
doi: 10.7498/aps.65.148901

Abstract +

Many infrastructure networks interact with and depend on each other to provide proper functionality. The interdependence between networks has catastrophic effects on their robustness. Events taking place in one system can propagate to any other coupled system. Recently, great efforts have been dedicated to the research on how the coupled pattern between two networks affects the robustness of interdependent networks. However, how to dynamically construct the links between two interdependent networks to obtain stronger robustness is rarely studied. To fill this gap, a global homogenizing coupled pattern between two scale-free networks is proposed in this paper. Making the final degrees of nodes distributed evenly is the principle for building the dependency links, which has the following two merits. First, the system robustness against random failure is enhanced by compressing the broadness of degree distribution. Second, the system invulnerability against targeted attack is improved by avoiding dependence on high-degree nodes. In order to better investigate its efficiency on improving the robustness of coupled networks against cascading failures, we adopt other four kinds of coupled patterns to make a comparative analysis, i.e., the assortative link (AL), the disassortative link (DL), the random link (RL) and global random link (GRL). We construct the BA-BA interdependent networks with the above 5 coupled patterns respectively. After applying targeted attacks and random failures to the networks, we use the ratio of giant component size after cascades to initial network size to measure the robustness of the coupled networks. It is numerically found that the interdependent network based on global homogenizing coupled pattern shows the strongest robustness under targeted attacks or random failures. The global homogenizing coupled pattern is more efficient to avoid the cascading propagation under targeted attack than random failure. Finally, the reasonable explanations for simulation results is given by a simply graph. This work is very helpful for designing the interdependent networks against cascading failures.

2016, 65 (14): 148701.
doi: 10.7498/aps.65.148701

Abstract +

Translocating pore of biomacromolecules is a common phenomenon in many biological processes, such as DNA transcription, cell infection of virus and transmembrane of proteins. The understanding of translocating pore of DNA is important for studying the DNA sequencing, gene therapy and virus infection. According to the coarse-grained model, we use molecular dynamics simulations to investigate the process of translocating pore of DNA under the actions of different non-uniform forces. In the present study, we consider five kinds of non-uniform forces, i.e., linearly increasing, linearly decreasing, V-type, inverted V-shaped, and periodic type. In the simulations of coarse-grained DNA, we find that the force on the pore opening palys a key role in the process of translocation of polymer. When the force is small, the probability of successful translocation of DNA is low accordingly. In the case of inverted V-shaped potential, the difference between the maximum and minimum force should be in a limited range to a probable translocation of DNA. Out of the range it might lead to clogged pores in the polymer chain. In the action of a non-uniform force, the translocating pore of DNA shows a series of complicated behaviors. For example, the end of a polymer can move faster than its head, resulting in the hole clogging and accumulation of polymers. A reversion can occasionally occur after a successful translocation of polymer. Therefore, non-uniform force leads to various scenarios of translocating pore of polymers.In summary, due to the complicated interactions between external forces and internal potential of polymer chains, particles can be clogged in the pore since the following particles overtake the leading ones in the chain. It is also found that the success of pore translation of DNA is significantly dependent on the acting force on the pore. Among all the cases of translating the pore successfully, the translation time in the case of non-uniform force is about half that in the case of uniform force. These results might provide an insight into the understanding of the complicated pore translating mechanism of DNA.

2016, 65 (14): 148801.
doi: 10.7498/aps.65.148801

Abstract +

High concentrating photovoltaic (HCPV) technology plays a more and more important role in solar power generation due to its extremely high efficiency. However, the efficiency of the HCPV module can be reduced by many factors. Especially, there are not enough researches and knowledge on the light intensity distribution and non-uniform illumination of different wavelengths of light concentrated by Fresnel lens. It is generally considered that the maximum power of multi-junction solar cell is achieved when the cell is placed on the focal plane of Fresnel lens. But it is proved to be incorrect by our research. When light beams of different wavelengths go through the Fresnel lens, their light spot distributions on the optical axis are not the same as those when they have different refractive indexes in Fresnel lens. At the same time, the triple-junction solar cell consists of three sub-cells which absorb light beams of different wavelengths respectively. Therefore, the performance of triple-junction cells would be influenced by the light distribution along the optical axis, this is exactly what we want to study in this work. The method of simulating the light tracing is used to calculate and analyze the light intensity distribution and non-uniform characteristics of different wavelengths of light concentrated by Fresnel lens. Combined with them from the circuit network model of a triple-junction solar cell, the electrical performances of triple-junction solar cell at different positions along the optical axis are studied. It is found from the simulation that the performance of cell does not reach the best state when cell is placed on the focal plane. The power of cell on the focal plane reaches only 0.41 W while the maximum point arrives at 0.69 W. The high non-uniformity of light on cell surface when cell is placed on the focal plane causes the decline of power. And an outdoor HCPV testing system with the ability to change the distance between Fresnel lens and the cell is conducted. The experimental results and the simulation results match well, therefore our simulation approach is verified. It shows that the module achieves the maximum power on either side of the focal plane, and the output power can increase more than 20% after optimization. It is a result after equilibrium between light intensity and uniformity on cell surface.

2016, 65 (14): 148802.
doi: 10.7498/aps.65.148802

Abstract +

Oscillation behaviors of oscillators consisting of defect-free multi-walled carbon nanotubes (MWCNTs) have been extensively studied, owing to the operating frequency of the nanotubes being able to reach up to gigahertz. However, there exist defects in most carbon nanotubes, which will affect the friction force between the walls of nanotubes. It is therefore critical to investigate the oscillation characteristics of the MWCNT-based oscillators containing a distorted or defective rotating tube, for the design of MWCNTs-based oscillators.
Unlike the case in the armchair carbon nanotubes (Zeng Y H, et al. 2016 Nanotechnology 27 95705), the existence of the helical rise in the zigzag-type nanotubes can induce aberrant or defective shell structures. In this paper, the oscillatory behaviors of zigzag@zigzag double-wall carbon nanotubes containing a rotating inner tube with different helical rises are investigated using the molecular dynamics method. In all the simulation modes, the adaptive intermolecular reactive empirical bond order potential is used in this work for both the covalent bond between carbon atoms and the long-range van der Waals interaction of the force field. The perfect zigzag outer tube is assumed to be fixed while the zigzag inner tube is free after it has been rotated by a torque. At the beginning of the simulation, the whole system is heat bathed at a temperature around 300 K for 60 ps, to gently increase the whole system temperature to around 300 K after the energy minimization. The total number of particles, the system volume, and the absolute temperature are kept unchanged for 60 ps. Then we apply a torque of 30 eV to the inner tube under the constant temperature. After the rotation frequency of the inner tube reaches around 300 GHz, we remove the torque of inner tube and let the whole system be under a constant energy condition. The time steps for all simulations are all chosen to be 1 fs. The total time for the simulation is 3000 ps.
It is found that the oscillatory behavior of the inner tube is dependent on the helical rise. The simulation results show that the oscillation frequency of the inner tube increases with the length of helical rise increasing. However, as the helical rise is further increased, the oscillation becomes awful because of the breakage of the inner tube with defects. Moreover, the zigzag@zigzag double-wall carbon nanotubes without any helical rise may be used as an ideal rotating actuator because the inner tube can rotate at an approximately constant rotational frequency. The influence of the system temperature on the oscillatory behavior of inner tube with a helical rise of 0.5 nm is also investigated. The results show that the oscillation amplitude of the inner tube increases with temperature increasing, but the oscillation of the inner tube is extremely unstable if the temperature is higher than a critical value.