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Traceable trans-scale heterodyne interferometer with subnanometer resolution
He Yin-Zhu, Zhao Shi-Jie, Wei Hao-Yun, Li Yan
Acta Physica Sinica, 2017, 66 (6): 060601
Theoretical study on the electronic structure and transition properties of excited state of ZnH molecule
Zhao Shu-Tao, Liang Gui-Ying, Li Rui, Li Qi-Nan, Zhang Zhi-Guo, Yan Bing
Acta Physica Sinica, 2017, 66 (6): 063103
Molecular field origin for magnetic ordering of magnetic materials
Qi Wei-Hua, Li Zhuang-Zhi, Ma Li, Tang Gui-De, Wu Guang-Heng, Hu Feng-Xia
Acta Physica Sinica, 2017, 66 (6): 067501
Current Issue Accepts In Press Earlier Issues Top Downloaded SCI Top Cited
  Acta Physica Sinica--2017, 66 (6)   Published: 20 March 2017
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CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES

Influence of nano-scaled roughness on evaporation patterns of colloidal droplets

Zhang Yong-Jian, Ye Fang-Xia, Dai Jun, He Bin-Feng, Zang Du-Yang
Acta Physica Sinica. 2017, 66 (6): 066101 doi: 10.7498/aps.66.066101
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Evaporation of colloidal droplets often leads to various deposited patterns which are not only interesting but also provide a very simple and useful method to fabricate functional materials. The patterns induced by the evaporation can be tuned via several factors, among which the roughness of the substrate is an important one. However, the effect of nano-scaled roughness is scarcely studied and far from being fully understood. In this work, the evaporation and pattern formation of SiO2 colloid droplets are studied on smooth substrate and nano-rough substrate, respectively. The aim of this work is to clarify how the evaporation dynamics and patterns are influenced by nano-scaled roughness. The roughness of the substrate is analyzed by using a scanning electron microscope and an atomic force microscope, the evaporation process and pattern formation are monitored via an in-situ microscope observation. The obtained deposited patterns are analyzed by using stylus profiling. It is found that the evaporation of droplets is accompanied by an obvious “coffee ring effect” on smooth substrate and the deposition patterns are bowl-shaped. However, uniform thickness evaporation patterns are obtained through evaporation on rough substrate, moreover, the crack density increases obviously. The analysis shows that nano-roughness is able to inhibit the circumfluence of droplets along the substrate, which greatly weakens the compensation for capillary flow, leading to particles gathering at air-droplet interface and formulating a particle layer. This prevents the “coffee ring effect”, and eventually results in the formation of evaporation patterns with uniform thickness.

Theoretical improvement on the determination of effective elasticity charges for charged colloidal particles

Wang Lin-Wei, Xu Sheng-Hua, Zhou Hong-Wei, Sun Zhi-Wei, Ouyang Wen-Ze, Xu Feng
Acta Physica Sinica. 2017, 66 (6): 066102 doi: 10.7498/aps.66.066102
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According to the existing shear modulus-pair potential relationship model for colloidal crystal comprised of highly charged colloidal particles, the calculated shear moduli of colloidal crystals are much larger than the measured values by the torsional resonance spectroscopy (TRS). Moreover, by using the relationship model, the effective surface charge of colloidal particles, obtained by fitting values of shear moduli measured by TRS (effective elasticity charge), is smaller than that obtained through the experimental method of conductivity-number density relationship (effectively transported charge). So far there has been no practical explanation to this discrepancy. Our analysis shows that this discrepancy is because the existing relationship model is for the perfect crystals and does not include the defects such as voids which can result in the decrease of mechanical properties of materials. The existing shear modulus-pair potential model will be improved by introducing the effect of voids, which is inspired from the Gibson-Ashby model in the study of cellular solid. The Yukawa potential, which considers Coulomb repulsions between colloidal particles and is usually used in the model expressions, will be substituted by Sogami-Ise potential, which considers a long-range attraction in addition to that Coulomb repulsions and accepts the existence of voids inside the colloidal crystals. For five different kinds of highly charged colloidal particles, the shear moduli with different volume fractions are measured by TRS. Then the fitted effective surface charges using the original and improved model respectively are compared with each other. It can be concluded that the effective elastic charge obtained by the improved model is more suitable and much closer to the renormalized charge obtained from Alexander's method. It is also clear that neither the effectively transported charge nor the Alexander's renormalized charge can be used to evaluate the shear moduli of colloidal crystals with voids inside. These results can also let us further understand and use the effective surface charge in the colloid studies.

Ductile and electronic properties of La-doped gamma-TiAl systems based on density functional theory

Song Qing-Gong, Zhao Jun-Pu, Gu Wei-Feng, Zhen Dan-Dan, Guo Yan-Rui, Li Ze-Peng
Acta Physica Sinica. 2017, 66 (6): 066103 doi: 10.7498/aps.66.066103
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Because of the low density, high specific strength and excellent performance at high temperature, γ-TiAl based alloy has become a new generation of materials in the aeronautic field. However, its poor ductility at room temperature set a limitation to its wide applications. In this paper, the crystal structures, stabilities and ductilities of La-doped γ-TiAl systems are investigated by using first principles method based on density functional theory, in which Ti or Al is substituted by La and the impurity content values are 1.85 at.%, 2.78 at.%, 4.17 at.%, 6.25 at.%, 8.33 at.% and 12.5 at.%, respectively. The results show that all of the La-doped alloys have good energy stabilities, namely they can be prepared experimentally, when the impurity concentration x of system is less than or equal to 12.5 at.%. And the density of the La-doped system is less than 4.6 g·cm-3. La doping induces the lattice parameters and the axial ratio of the alloy system to change. The axial ratio of La-doped system with low impurity concentration (x≤6.25 at.%) is closer to 1, which is very beneficial to improving the ductility of the materials. It is predicted that the system Ti11LaAl12 would have the best ductility among those of the investigated systems, for its axial ratio is the closest to 1. The electronic effect about the ductility of La-doped system is discussed through the comparisons of the populations, charge densities and densities between the states of systems Ti11LaAl12 and Ti12Al12. It is found that the system Ti11LaAl12 presents a state of electron redistribution in valence electron orbitals of Al and Ti due to an atom of titanium substituted with that of lanthanum. The charge numbers of Ti-d and Al-p orbitals and the numbers of electrons can be delocalized by reducing the p-d orbital hybridization. Thus, the intensity of p-d orbital hybridization is weakened, the resistance of dislocation movement is reduced, and the ductility of TiAl systems can be improved. Actually, the new electron redistribution shows different properties of some chemical bonds, in which some of covalent Al–Ti bonds are replaced by ionic Al–La bonds and some of covalent Ti–Ti bonds are replaced by metallic Ti–La bonds. Therefore, the covalent and directional properties of chemical bonds are reduced distinctly while the metallic properties of materials are strengthened. The average intensity of Al–Al bonds decreases and those of Al–Ti and Ti–Ti bonds are increased in the La-doped γ-TiAl system (Ti11LaAl12). As a result, the differences between the three kinds of chemical bonds diminish and the degree of isotropy of the crystal structure increases, which can greatly improve the ductility of γ-TiAl alloy.

Comparative study of irradiation swelling in monocrystalline and polycrystalline silicon carbide

Zang Hang, Huang Zhi-Sheng, Li Tao, Guo Rong-Ming
Acta Physica Sinica. 2017, 66 (6): 066104 doi: 10.7498/aps.66.066104
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Silicon carbide (SiC) is considered as one of the most promising structural and coating materials for advanced nuclear applications, due to its low neutron capture cross section and excellent irradiation resistance. The difference in swelling behavior between monocrystalline and polycrystalline SiC is experimentally investigated by heavy ion irradiation at room temperature (RT). In this work, single crystal hexagonal (6H) SiC and polycrystalline chemically vapor-deposited (CVD) SiC are irradiated by 1.5 MeV Si ions with the fluences of 1×1014-2×1016 cm-2 and 1×1015-2×1016 cm-2, respectively, at RT. The step height of irradiation swelling is measured by a white light interferometer and the lattice expansion of the damage layer is characterized by using X-ray diffraction (XRD) spectrometry, in addition, the actual irradiation swelling is obtained by dividing the height of swelling step by the depth of damage layer. The XRD profiles show that the lattice expansion in the damage layer increases with the increase of irradiation fluence, and the new diffraction peak relating to the lattice structure of damage layer disappears in a fluence of 2×1015 cm-2, which means that the damage layer is completely amorphous at this time and the threshold dose of amorphization at RT in single crystal 6H-SiC is less than 0.8 dpa. The direct-impact model is used to fit the swelling step heights of CVD SiC and 6H-SiC irradiated by 1.5 MeV Si, and the swelling results show that the amorphization threshold dose of polycrystalline CVD SiC is larger than that of single crystal 6H-SiC. In the present work, three distinct stages are found in the heavy-ion irradiation swellings between monocrystalline and polycrystalline SiC. i.e., low-fluence region, intermediate-fluence region, and high-fluence region stage. 1) In the low-fluence region, the swellings are similar to each other, since the swelling is mainly contributed to by point defects in this region, and the micron sized grains in polycrystalline CVD SiC are of single crystal structure. 2) In the intermediate-fluence region, the irradiation swelling of the polycrystalline CVD SiC is smaller than that of the single crystal 6H-SiC, since the irradiation-induced amorphousness in polycrystalline CVD SiC is relatively hard to occur due to the existence of grain boundary in this region. 3) The irradiation swellings of 6H-SiC and CVD SiC are almost the same at the high-fluence region stage, since the irradiation swelling is caused by amorphization in this region, and the swelling depends on the difference between densities before and after irradiation. In addition, in the irradiation swelling analysis of SiC materials, XRD swelling measurement method is suitable for irradiation swelling induced by point defects, especially for neutron irradiation experiments.

Investigation on the large tensile deformation and mechanical behaviors of graphene kirigami

Han Tong-Wei, Li Pan-Pan
Acta Physica Sinica. 2017, 66 (6): 066201 doi: 10.7498/aps.66.066201
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One of the main challenges in developing future stretchable nanoelectronics is the mismatch between the hard inorganic semiconductor materials and the ductility requirements in the applications. This paper shows how the kirigami architectural approach, inspired by the ancient Japanese art of cutting and folding paper applied on macroscale, might be an effective strategy to overcome this mismatch on nanoscale. In this work, the tensile large deformation and mechanical behaviors of armchair and zigzag graphene kirigami with rectangles and half circles cutting patterns are investigated based on classical molecular dynamics simulations. The effects of three non-dimensional geometric parameters that control the cutting patterns on the mechanics and ductility of graphene kirigami are also studied systematically. The results indicate that the enhancement in fracture strain can reach more than five times the fracture strain of pristine graphene. The defined three parameters can be adjusted to tailor or manipulate the ductility and mechanical behaviors of graphene. These results suggest that the kirigami architectural approach may be a suitable technique to design super-ductile two-dimensional nanomaterials and potentially expand their applications to other strain-engineered nanodevices and nanoelectronics.

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

Modification of the photocatalytic properties of anatase TiO2 (101) surface by doping transition metals

Su Qiao-Zhi, Han Qing-Zhen, Gao Jin-Hua, Wen Hao, Jiang Zhao-Tan
Acta Physica Sinica. 2017, 66 (6): 067101 doi: 10.7498/aps.66.067101
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Exploring new types of photocatalysts and modifying the photocatalytic activity have attracted more and more extensive attention in many research fields. Anatase TiO2, a promising photocatalyst widely studied, can only absorb the ultraviolet light and thus only make little use of the power in visible light. Therefore, it is an urgent task to make theoretical and experimental investigations on the photocatalytic mechanism in anatase TiO2 and then improve its visible light response so as to utilize more visible light. Now, in the present paper, we carry out a systematic theoretical investigation on modifying the photocatalytic properties of the anatase TiO2 (101) surface via doping transition metal neutral atoms such as Fe, Ni, Pd, Pt, Cu, Ag, and Au by using the plane wave ultrasoft pseudopotential method of the density functional theory. The dependence of the macroscopic catalytic activity on electronic structure and optoelectronic property is uncovered by making a comparative analysis of the geometric structures, the electronic structures, and the optical properties of the undoped and doped anatase TiO2 (101) surfaces. Our numerical results show that doping certain transition metals can suppress the band gap or induce extra impurity energy levels, which is beneficial to improving the visible light response of the TiO2 (101) surface in different ways. In most cases, the new impurity energy levels will appear in the original band gap, which comes from the contribution of the d electronic states in the transition metal atoms. Moreover, the photocatalytic activity of the TiO2 (101) surface can be changed differently by doping different transition metal atoms, which is closely dependent on the bandgap width, Fermi energy, the impurity energy level, and the electron configuration of the outermost shell of the dopants. This research should be an instructive reference for designing TiO2 (101) photocatalyst and improving its capability, and also helpful for understanding doping transition metal atoms in other materials.

Determination of dislocation density of a class of n-GaN based on the variable temperature Hall-effect method

He Ju-Sheng, Zhang Meng, Pan Hua-Qing, Zou Ji-Jun, Qi Wei-Jing, Li Ping
Acta Physica Sinica. 2017, 66 (6): 067201 doi: 10.7498/aps.66.067201
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An analytical model for electron mobility in a class of wurtzite n-GaN, whose carrier concentration is over 1018 cm-3 (Mott's critical limit), is developed. With the dislocation density and two donor levels serving as the important parameters, the proposed model can accurately predict the electron mobility as a function of temperature. The edge and screw dislocation densities in two samples, which are respectively grown on sapphire (001) by metal organic chemical vapor deposition and hydride vapor phase epitaxy, are determined by using this model which is discussed in detail. It is shown that the data-fitting of μH-T characteristic curve is a highly suitable technique for accurately determining the edge and screw dislocation densities in n-GaN films. Quantitative analyses of donor concentration and donor activation energy indicate that the impurity band occurs when the carrier concentration is under 1017 cm-3, much lower than the critical carrier concentration of Mott transition (1018 cm-3). Such a behavior can also be confirmed by the results from solving the Boltzmann transport equation by using the Rode iterative method. Another anomaly is that the dislocation density in Mott transition material perhaps is lower than that of material with carrier concentration under 1018 cm-3. This fact indicates that the cause of Mott transition should not be the shallow donor impurities around dislocation lines, but perhaps the deeper donor impurities or other defects. In the theoretical model calculation, two transition characteristics together with the donor distribution and its energy equilibrium are taken into account. Based both on the Mott transition and the H-like electron state model, the relaxation energies for the shallow-donor defects along the screw and edge dislocation lines are calculated by using an electrical ensemble average method. Besides, an assumption that should be made is that there are 6 shallow-donor defect lines around one dislocation line. The research results show that the Hall mobility should be taken as the live degree of the ionizing energy for the shallow-donor defects along the dislocation line. The experimental results indicate that our calculation function can be best fit by the experimental curve, with the values of dislocation density being between our model and others determined by X-ray diffraction or by chemical etching method, which are all in good agreement with each other. The method reported can be applied to the wurtzite n-GaN films grown by various preparation technologies under any condition, with the peak-mobility temperature about or over 300 K, whose Hall mobility near 0 K perhaps is over 10 cm2/(V·s) and even 100 cm2/(V·s).

First-principles study of Al-doped and vacancy on the magnetism of ZnO

Hou Qing-Yu, Li Yong, Zhao Chun-Wang
Acta Physica Sinica. 2017, 66 (6): 067202 doi: 10.7498/aps.66.067202
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There is a controversy over the magnetic source and mechanism of the coexistence of Al-doping and Zn vacancy or Al doping and O vacancy in ZnO systems. In order to solve the problem, the combined influence mechanism of Al doping and Zn vacancy or Al doping and O vacancy on magnetism of ZnO is studied by using the first-principle calculation in this work. The coexistence of Al doping and Zn vacancy can achieve Curie temperature higher than room temperature. Moreover, the magnetism of the doping system of Al doping and Zn vacancy is mainly contributed by electron exchange interaction through O 2p and Zn 4s states near the Zn vacancy through taking carrier as medium. However, the system of Al doping and O vacancy is non-magnetic. Meantime, in the coexistence of Al doping and Zn vacancy or O vacancy, a close relative distance between doping and vacancy will reduce the formation energy of the doping system, increase the easiness of accomplishment of doping and vacancy, and enhance the stability of the doping system.

Photoelectron characteristics of ZnSe quantum dots-sensitized mesoporous La-doped nano-TiO2 film

Ren Lun, Li Kui-Ying, Cui Jie-Yuan, Zhao Jie
Acta Physica Sinica. 2017, 66 (6): 067301 doi: 10.7498/aps.66.067301
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In the paper, the core-shell ZnSe quantum dots (QDs)-sensitized mesoporous La-doped nano-TiO2 thin film is prepared by a direct adsorption method. Photoelectron characteristics, photogenerated carriers transport mechanism, and microstructure of the QDs-sensitized nano-TiO2 thin film are probed via the stationary surface photovoltaic (SPV) and the transient photovoltaic technologies, supplemented by the Brunauer-Emmet-Teller adsorption isotherm technique, scanning electron microscope, Fourier transform infrared (FT-IR) absorption spectrum, and ultraviolet-visible (UV-VIS) absorption spectrum. The experimental results confirm that the surface of the nano-TiO2 film is covered with the ZnSe QDs with smaller particles by a chemical absorbing way, resulting in denser composite film of the QDs and the mesoporous nano-TiO2 than the nano-TiO2 film. In our experiment, the adsorption quantity of ZnSe QDs on nano-TiO2 film can be controlled effectively. The results show that ligand L-Cys capped at the outer layer of ZnSe QDs plays an important role in the sensitization process. Specifically, the peak of νS–H in the ligand disappears at 2552 cm-1 in the FT-IR spectrum of the ZnSe QDs capped by the ligand as a stabilizer. This indicates that the S–H bond is broken. In the meantime, the peak of the C–S stretching vibration in the ligand shifts from 638 cm-1 to 663 cm-1 due to the formation of Zn–S bond. These imply that the core-shell ZnSe/ZnS/L-Cys QDs are obtained. On the other hand, according to the peak of –COOH stretching vibration disappearing at 1600 cm-1 in the FT-IR spectrum of the core-shell QDs-sensitized mesoporous nano-TiO2 film, the unsaturated Ti atoms on the surface of the TiO2 film are bonded to carboxy groups from the ligand capped at the QDs. That is, the ligand acts as a bridge between the QDs and the nano-TiO2 film for achieving the sensitization. Some excellent photovoltaic characteristics of the composite film are found as follows. 1) The SPV responses of the QDs-sensitized film appear in a wavelength region of 300 nm to 800 nm (UV-VIS-Near-IR), causing the region of SPV response to enlarge about 200 nm over that of the ZnSe QDs, and 400 nm over that of the nano-TiO2 thin film. 2) The QDs-sensitized film displays an n-type photovoltaic characteristic that is different from that of the QDs. This may be more favorable for transferring those carriers from the film surface to the photo-anode material. 3) Both the separation rate and the diffusion length of photogenerated electron-hole pairs are obviously increased, and the lifetime of free charge carriers in the ZnSe QDs-sensitized film prolongs about an order of magnitude over that of the nano-TiO2 film and ZnSe QDs.

Effect of Mo doping concentration on the physical properties of ZnO studied by first principles

Jia Xiao-Fang, Huo Qing-Yu, Zhao Chun-Wang
Acta Physica Sinica. 2017, 66 (6): 067401 doi: 10.7498/aps.66.067401
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The experimental results of red-shift and blue-shift in absorption spectrum of Mo-doped ZnO are in mutual contradiction, and this phenomenon has not been explained rationally so far. For explaining this phenomenon, we analyze the energy band structure, state density, and absorption-spectrum distributions for each of Zn0.9583Mo0.0417O, Zn0.9375Mo0.0625O and Zn14Mo2O by first-principles calculation. The results show that within a limited doping amount range of 2.08 at%-3.13 at%, the higher Mo doping amount results in higher doping system volume, higher formation energy, lower system stability, and more difficult to dope. Meanwhile, all doping systems are converted into n-type degenerate semiconductors. Compared with the band gap of pure ZnO, the band gap of each doping system becomes narrow and the absorption spectrum shows red-shift. The higher the Mo doping amount, the weaker the narrowing of band gap becomes and the weaker the red-shift in absorption spectrum as well as the lower the electronic effective mass and the lower the electronic concentration; the lower the electronic mobility, the lower the electronic conductivity is; the lower the electronic magnetic moment is. The Curie temperature of doping system can reach a temperature higher than room temperature.

Molecular field origin for magnetic ordering of magnetic materials Hot!

Qi Wei-Hua, Li Zhuang-Zhi, Ma Li, Tang Gui-De, Wu Guang-Heng, Hu Feng-Xia
Acta Physica Sinica. 2017, 66 (6): 067501 doi: 10.7498/aps.66.067501
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In 1907, Weiss proposed that there is a molecular field to explain the magnetic ordering of magnetic materials. However, it has not been clarified where the molecular field comes from so far. In recent decades, the magnetic ordering of metals and alloys were explained by using the direct exchange interaction of between electrons on neighboring atoms, while magnetic ordering of oxides were explained by using the super exchange interaction and double exchange interaction models. The intrinsic relation between those exchange interactions has not been well explained. This resulted in the fact that there are many puzzles for magnetic ordering of the magnetic materials. For example, what role the Cr cations play in spinel ferrite CrFe2O4; why the calculated molecular magnetic moment (3.85μB) for La0.85Sr0.15MnO3 by using double exchange interaction model is lower than its experimental value (4.20μB); whether there is a relation between the average atom magnetic moment and their electrical resistivity for each of Fe, Co and Ni metals. These several puzzles have been explained recently by our group through using an O 2p itinerant electron model for magnetic oxides and a new itinerant electron model for magnetic metals. In this paper, a model for the molecular field origin is proposed. There are three states for the electrons rotating with high speed at the outer orbits of two adjacent ions of magnetic oxides or metals and alloys. 1) There is a probability with which form the electron pairs with opposite spin directions and a certain life time, named Weiss electron pairs (WEP); the static magnetic attraction energy between two electrons of WEP is the elementary origin of Weiss molecular field. 2) There is a probability with which two electrons with the same spin direction exchange mutually. 3) If there are two electrons at the outer orbit of an ion, then for its adjacent ion whose orbit has only one electron, the excess electron will itinerates between the ions. Furthermore, the energy equation of WEP, equilibrium distance, re0, and maximum distance, rem, between electrons of WEP are derived. The probability with which WEP forms in each of several perovskite manganites is investigated. For perovskite manganites La0.8Ca0.2MnO3, La0.75Ca0.25MnO3, La0.70Sr0.30MnO3, the crystal cell constants increase linearly with temperature when the temperature is much lower than the Curie temperature, TC, while they show a rapid increase nonlinearly near TC. We then calculate the difference in Mn–O bond length at TC between the linear and the nonlinear variation, △dobs. Obviously, when the distance between the two electrons of WEP, re, is larger than the rem, WEP and the magnetic ordering energy both disappear. Assuming △dobs=rem-re0, the probabilities with which WEP appears in La0.8Ca0.2MnO3, La0.75Ca.25MnO3, La0.70Sr0.30MnO3, are calculated to be 0.07%, 0.31% and 3.13%, respectively. These results indicate that the WEP model for the magnetic ordering energy is qualitatively reasonable.

Optimization of magnetoelectricity in thickness shear mode LiNbO3/magnetostrictive laminated composite

Xin Cheng-Zhou, Ma Jian-Nan, Ma Jing, Nan Ce-Wen
Acta Physica Sinica. 2017, 66 (6): 067502 doi: 10.7498/aps.66.067502
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Magnetoelectric (ME) composites have recently attracted much attention and triggered a great number of research activities, owing to their potential applications in sensors and transducers. Many researches have focused on the enhancement of ME coefficient by choosing suitable composite material and vibration mode based on the coupling between stress and strain. Besides normal stress, another vibration mode, shear mode, is further discussed as a potential high-frequency resonant device for a high frequency magnetic field detector, and it is useful to optimize the shear ME coefficient to broaden the application scope of the compositions. In this paper, an elasticity method is used to calculate ME coefficients of thickness shear mode LiNbO3/magnetostrictive laminated composites for various crystal orientations of LiNbO3, magnetostrictive materials and material sizes. The stretch-shear structure and shear-shear modes of the composite with considering the boundary condition are both discussed and further optimized. According to the structure design of stretch-shear mode composite from the literature, we design a new structure to achieve the uniform and pure shear ME effect, which changes the magnetostrictive phase on the bonding part into rigid material to avoid stretch deformation. We find that in the shear-shear ME composite, the structure should not move in the in-plane direction in order to realize the parallelogram deformation under shear stress, but should be free in the thickness direction to meet the change of thickness with shear deformation. For the stretch-shear mode Metglas/LiNbO3 [(xzlt) x°/y°], the shear ME coefficient αE15 as a function of orientation of LiNbO3 shows that the maximum αE15 is 235.1 mV/(cm·Oe) when x=0 and y=30. The results indicate that optimal shear ME coefficient is obtained at (xzt) 30° LiNbO3, resulting from the maximum shear piezoelectric coefficient dp15. By changing the material size in stretch-shear composite, the shear ME coefficient increases with the increase of thickness of magnetostrictive phase, because the stretch force increases with the increase of the cross-sectional area of magnetostrictive phase. The maximum values of αE15 are, respectively, 24.13 V/(cm·Oe) in the stretch-shear mode Terfenol-D/LiNbO3 and 11.46 V/(cm·Oe) in the shear-shear mode Metglas/LiNbO3 by the optimization of material sizes. Experimental results are in accordance with calculation results. It is confirmed that LiNbO3 (xzt) 30° is the best choice for achieving the largest shear ME effect, and thicker Terfenol-D can help to achieve a larger ME coefficient in this stretch-shear composite. This work provides a design method to choose the structure and crystal orientation of shear LiNbO3-based ME laminated composite, which shows a prospect of applications in high-mechanical-quality factor Qm and high-frequency magnetic detectors with shear resonant devices.

Trap distribution and direct current breakdown characteristics in polypropylene/Al2O3 nanodielectrics

Ma Chao, Min Dao-Min, Li Sheng-Tao, Zheng Xu, Li Xi-Yu, Min Chao, Zhan Hai-Xia
Acta Physica Sinica. 2017, 66 (6): 067701 doi: 10.7498/aps.66.067701
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Polypropylene (PP) is widely used as capacitor films due to its better dielectric, mechanical, and thermal performance. In order to reduce the cost and size of capacitor, high energy density for PP dielectric is pursued. Since energy density is in quadratic proportion to direct current (dc) breakdown strength for linear dielectric, the enhancement of dc breakdown strength for PP dielectric is a primary choice to improve the energy density. Considering that the incorporation of nano-Al2O3 is an effective method to improve the dc breakdown strength for polymer, it is required to study the dc breakdown strength of PP/Al2O3 nanodielectric.
In order to explore the breakdown mechanism, PP/Al2O3 nanodielectrics with different nano-particle contents are prepared by melt blending, and the samples are prepared by hot pressing. Their microstructures are observed by scanning electron microscopic. Isothermal surface potential decay, bulk resistivity, and dc breakdown strength of the samples are also measured. The experimental results show that the energy and density of deep traps, bulk resistivity, and dc breakdown strength first increase and then decrease with the increase in nano-Al2O3 content. The maximum values are obtained at a filer content value of 0.5 wt%, where dc breakdown strength can be increased by about 27%. Based on interface model, the relation between microstructure and trap is investigated. In view of space charge breakdown theory, the mechanism of dc breakdown for PP/Al2O3 nanodielectric is explored by trap parameters. It is indicated that the interface can provide more deep traps in PP/Al2O3 nanodielectric, while the decrease in the energy and density of deep traps can be attributed to the overlap of interfaces in electrical double layer. The increase in the energy and density of deep traps makes more carriers trapped near the injecting contact, thus reducing the effective field for carrier injection due to the internal field generated by the trapped carriers. The reduction of carrier injection can moderate the distortion of field in PP dielectric, consequently, resulting in enhancing the dc breakdown strength.

Transient characteristics of discharge of polymer sample after electon-beam irradiation

Feng Guo-Bao, Cao Meng, Cui Wan-Zhao, Li Jun, Liu Chun-Liang, Wang Fang
Acta Physica Sinica. 2017, 66 (6): 067901 doi: 10.7498/aps.66.067901
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Charging effect of dielectric material due to electron beam irradiation has a significant influence on the microdischarge phenomenon of dielectric microwave component by multipactor. The discharge process caused by internal electron leakage can relieve this undesirable charging effect. In this paper, we study the transient discharge characteristics of a dielectric sample after being irradiated by electron beam through numerical simulation. Both the charging and discharging processes of a dielectric sample are considered with a comprehensive model. The Monte-Carlo method is used to simulate the interaction between primary electrons and material atoms before the irradiation is interrupted, including elastic scattering and inelastic scattering. The elastic scattering is calculated with the Mott scattering model, and the inelastic scattering is simulated with the fast secondary electron model or Penn model according to electron energy. Meanwhile, the transport process of internal charges in the sample during the discharge period is simulated including the charge diffusion under the force of charge density gradient, the drift due to built-in E-field, and the trap caused by material defect. In this work, the discharge process is taken to begin at the very moment of charging reaching saturation, with the internal charges kept almost unchanged. A polymer material widely used in advanced component is considered in this work due to its remarkable charging effects. Distributions of internal charges of the sample during the discharge process are simulated, and influences of sample parameters, including sample thickness, electron mobility and trap density in the discharge process, are analyzed. The results show that internal charges move to the bottom of the sample during the discharging, leading to the surface potential reaching an ultimate state which is determined by trap density of the material. The position corresponding to the maximum internal charge density shifts towards the grounded bottom. Although a sample with a larger electron mobility means a faster discharge process, fewer free electrons in this sample result in less discharge quantity. The time constant of discharge process decreases with the increase of sample electron mobility in the form of similar linearity. Although a sample with a larger thickness can hold more internal charges, the increase of sample thickness may increase the distance of internal charges leak yet. Hence, the quantity of discharge first increases and then decreases with the increase of sample thickness. In addition, a larger trap density of a dielectric sample makes charge leak harder, resulting in a lower discharge quantity. Finally, the proportion of discharge quantity in saturated charge quantity decreases from 1 to 0 exponentially with the increase of sample trap density. As a conclusion, those sample parameters have their corresponding effects on discharge characteristics by means of different physical mechanisms. Sample electron mobility determines the discharge time constant obviously by affecting the electron transport speed. The sample thickness affects the discharge quantity by shifting the charging balance mode, and material defect impedes part of discharge quantity from trapping internal free electrons. This simulation method and results can help to recede the charging effect and estimate the evolution charging and discharging states of dielectric material during and after electron beam irradiation.

Femtosecond laser pulse energy accumulation optimization effect on surface morphology of black silicon

Tao Hai-Yan, Chen Rui, Song Xiao-Wei, Chen Ya-Nan, Lin Jing-Quan
Acta Physica Sinica. 2017, 66 (6): 067902 doi: 10.7498/aps.66.067902
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Arrays of sharp conical spike microstructures are created by repeatedly irradiating silicon surfaces with focused femtosecond laser pulses in SF6. The absorbance of light is increased to approximately 90% in a wavelength range from the near ultraviolet (0.25 μm) to the near infrared (2.5 μm) by the microstructured silicon surface. The microstructured surface presents pitch-black because of enhanced absorption with a broad wavelength range, which is called black silicon. The unique microstructure morphology of black silicon surface formed by femtosecond laser can also bring a lot of other surface functions, for example, self-cleaning and field emission. These functions make black silicon highly desirable in solar energy, detectors and other fields. Therefore, the forming mechanism and conditions of fabrication optimization for black silicon microstructure have always been the focus of research. In our work, the sample is moved by motor-controlled stage while the laser beam is fixed. In the case of laser beam scanning, arrays of sharp conical spikes on the silicon are manufactured in 70 kPa SF6. The aim of the experiment is to find how to optimize the distribution of the laser energy in a number of laser accumulation pulses (the combination of single pulse energy and pulse number) to control the surface morphology of the black silicon. Experimental results show that there appears a bottleneck effect of morphology size growth with the increase of laser irradiation (improving the single pulse energy or increasing pulse accumulation number). Excessive energy accumulation brings no extra effect on optimizing and controlling of microstructure morphology on the surface. Based on theoretical results obtained from a physical model we proposed, we find that the reason for this phenomenon is that the microstructure morphology induced by former sequence pulse modulates the laser energy absorption of current laser pulse, and changes the laser ablation efficiency of the current pulse. According to this physical mechanism, we propose a new way of optimizing surface morphology, with fixing the total laser irradiation energy. And the size and distribution of surface morphology can be achieved by optimizing the distribution of the laser energy in a number of laser accumulation pulses. This approach can not only improve the efficiency of silicon surface preparation of microstructures but also reduce the surface defects and damage. Furthermore, the proposed method can reduce the energy consumption in the process of femtosecond machining. It is of great significance for the engineering application of black silicon.

Quantum efficiency for reflection-mode varied doping negative-electron-affinity GaN photocathode

Qiao Jian-Liang, Xu Yuan, Gao You-Tang, Niu Jun, Chang Ben-Kang
Acta Physica Sinica. 2017, 66 (6): 067903 doi: 10.7498/aps.66.067903
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As a new kind of ultraviolet photocathode material, the negative-electron-affinity (NEA) GaN photocathode needs to further improve its photoemission performance and the stable performance in practical applications. Under the limit of GaN photocathode material growth level, how to further improve the quantum efficiency of cathode is an important problem. The varied doping technology can help to solve the problem under such circumstances. According to the photoemission mechanism of varying doping NEA GaN photocathode material, the built-in electric field formulas and the quantum efficiency formulas for reflection-mode varied doping NEA GaN photocathode are given. The preliminary structure of varied doping NEA GaN photocathode is designed. The varied doping material sample is divided into four layers according to the doping concentration. Using the self-developed experimental equipment, the varied doping GaN photocathode sample is activated with Cs/O. The activation process and the change characteristics of photocurrent for varied doping NEA GaN photocathode are discussed. At the beginning, the photocurrent is increased steady with the introduction of Cs, then the “Cs kill” phenomenon appears in the presence of excessive Cs. After the introduction of O, the photocurrent value starts to rise again. The spectral response of varied doping GaN photocathode is tested in situ after activation, and the quantum efficiency values ranging from 240 nm to 354 nm are obtained. On the basis of the obtained experimental results of quantum efficiency, combining to the typical quantum efficiency curve from University of California, the characteristics of quantum efficiency curves are analyzed. The results show that the quantum efficiency value for reflection-mode varied doping NEA GaN photocathode can reach 56% at 240 nm because of the built-in electric field, yet the quantum efficiency maximum value for uniform doping GaN photocathode is only 37% at 230 nm. The tested quantum efficiency maximum value of varied doping NEA GaN photocathode is improved much more than that of the uniform doping GaN photocathode. In a wider range of the incident light wavelength, the quantum efficiency of varied doping NEA GaN photocathode is relatively stable, and the excellent properties of varied doping GaN photocathode are confirmed. The reason why the value of quantum efficiency decreases with the increase of incident light wavelength is given. First, the photon energy decreases with the increase of incident light wavelength. Second, the incident light is absorbed from the front surface of cathode for reflection mode. In addition, the quantum efficiency curves of varied doping GaN photocathode show obvious sharp cut-off characteristics near the threshold, and the sharp cut-off characteristic is necessary for high detection sensitivity. The property of negative electron affinity for varied doping GaN cathode material after successful activation is also proved by the sharp cut-off feature.

INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY

Liquid-solid phase transition of Cu-Zr eutectic alloy under microgravity condition

Chen Ke-Ping, Lü Peng, Peng Wang
Acta Physica Sinica. 2017, 66 (6): 068101 doi: 10.7498/aps.66.068101
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Eutectic phase transition involves the competitive nucleation and coupled growth of two solid phases within one liquid phase. Phase selection especially under unequilibrium condition, may result in novel microstructures and thus affects the performances of eutectic alloys. Liquid Cu-10 wt.% Zr hypoeutectic, Cu-12.27 wt.% Zr eutectic and Cu-15 wt.% Zr hypereutectic alloys are rapidly solidified in the containerless process in a 3 m drop tube. During the experiments, the Cu-Zr alloys are heated by induction heating in an ultrahigh vacuum chamber and further overheated to 200 K above their liquidus temperatures for a few seconds. Then the liquid alloys are ejected out from the small orifice and dispersed into tiny droplets after adding the argon gas flow. The solidified samples are analyzed by Phenom Pro scanning electron microscope and HXD-2000 TMC/LCD microhardness instrument. The competitive nucleation and growth among (Cu) dendrite, Cu9Zr2 dendrite and (Cu+Cu9Zr2) eutectic phase become more and more intensive as droplet diameter decreases. The layer spacing in Cu-12.27 wt.% Zr eutectic alloy decreases when the undercooling increases. And the microstructural transition takes place from lamellar eutectic to anomalous eutectic. The microstructure of Cu-10 wt.% Zr hypoeutectic alloy is characterized by (Cu) dendrite and lamellar eutectic. Whereas the microstructure in Cu-15 wt.% Zr hypereutectic alloy consists of Cu9Zr2 dendrite and lamellar eutectic. For the Cu-10 wt.% Zr hypoeutectic alloy, with the decrease of droplet size, the primary (Cu) phase transforms from coarse dendrites into equiaxed grains, and the volume fraction of (Cu) dendrite becomes larger and larger. As for Cu-15 wt.% Zr hypereutectic alloy, the primary Cu9Zr2 intermetallic compound grows in a band manner, and with the decrease of droplet size and increase of cooling rate, the solidified microstructure transforms from band Cu9Zr2 dendrite plus lamellar eutectic into spherical cell structure. The three alloys reach maximal undercooling at 177 K, 156 K and 204 K, respectively. The Trivedi-Magnin-Kurz and Lipton-Kurz-Trivedi/Boetinger-Coriell-Trivedi models are used to analyze the dendritic and eutectic growth as a function of undercooling. Theoretical analysis indicates that both dendritic growth and eutectic growth are controlled by solute diffusion during liquid-solid phase transition. To further investigate the effects of cooling rate and undercooling on the mechanical properties of Cu-Zr eutectic alloys, the microhardness of each of different phases is determined. The microhardness of the primary (Cu) phase within Cu-10 wt.% Zr hypoeutectic alloy is strengthened with the increase of cooling rate. The microhardness of eutectic within the three alloys also increases with increasing the cooling rate and the initial alloy composition of the alloy.

Afterdepolarlizations induced by wave pattern in human ventricular tissue

Wang Xiao-Yan, Wang Peng, Tang Guo-Ning
Acta Physica Sinica. 2017, 66 (6): 068201 doi: 10.7498/aps.66.068201
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Recently, arrhythmogenic condition has attracted special attention of scientists in the field of different disciplines because sudden cardiac death is often caused by cardiac arrhythmia. Arrhythmias can have different underlying causes. But the underlying mechanism of arrhythmia is not fully understood due to cardiac complexity. As is well known, one particular group of arrhythmias is often associated with the afterdepolarizations. So far, afterdepolarizations have been studied mainly in isolated cardiac cells. The question how the afterdepolarization is produced at a tissue level has not been widely studied yet. In this paper, we use the model of human heart to study how spiral wave or other wave patterns induces the afterdepolarizations in two-dimensional myocardial tissue. We try to obtain the instantaneous spatial distribution of afterdepolarizations by changing the L-type calcium and fast potassium conductance. In order to avoid bringing in afterdepolarizations, the applied parameters avoid evoking the afterdepolarizations at a single-cell and one-dimensional tissues level. The numerical simulation results show that spiral wave and other wave patterns can cause the phase II and III early afterdepolarizations, the delayed afterdepolarization, the enhanced automaticity, the delayed excitation and the delayed enhanced automaticity to occur. Moreover, we observe the weak oscillation of the membrane potential during the phase I of action potential. The afterdepolarizations generally occur in the spiral-wave core. They are generated by the phase singularity of spiral wave. The afterpolarizations can also appear in other region of spiral wave pattern. The afterpolarization is characterized by scattered distribution. When parameters are appropriately chosen, we observe the outbreaks of different afterpolarizations under the state of spiral wave. The corresponding spatial and temporal distributions of the early afterdepolarizations, the delayed afterdepolarizations, and the enhanced automaticity become spiral line distributions, which exhibits memory effect. It is shown that the outbreaks of afterdepolarizations in the system do not necessarily lead to the breakup of spiral wave. By observing the changes of different ion currents we find that when sodium current exciting cell is very small, the weak excitation with small sodium current can cause the L-type calcium current and the sodium calcium exchange current to increase, and the slow potassium current and rapid potassium current to decrease, leading to the occurrences of various afterdepolarizations. Therefore, increasing sodium current can effectively suppress the occurrences of afterdepolarizations.

Barrier growth temperature of InGaAs/AlGaAs-quantum well infrared photodetector

Huo Da-Yun, Shi Zhen-Wu, Zhang Wei, Tang Shen-Li, Peng Chang-Si
Acta Physica Sinica. 2017, 66 (6): 068501 doi: 10.7498/aps.66.068501
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The InGaAs/AlGaAs quantum wells have been extensively applied to quantum well infrared photodetector of mid-wavelength. In this letter, four samples of 2.4 nm In0.35Ga0.65As/40 nm Al0.34Ga0.66As multi-quantum wells are grown by molecular beam epitaxy with the InGaAs wells growing all at a temperature of 465℃ but the AlGaAs wells growing at temperatures of 465℃, 500℃, 545℃, and 580℃ respectively. The dependence of InGaAs quantum well strain relaxation on the AlGaAs growth temperature is systematically studied by photoluminescence spectroscopy and X-ray diffraction and then the thermal-induced relaxations of three key-stages are clearly observed in the following temperature ranges. 1) 465-500℃ for the stage of elastic relaxation: the phase separation begins to take place with a low defect density; 2) 500-545℃ for the transition stage from elastic relaxation to plastic relaxation: the phase separation will be further intensified with defect density increasing; 3) 545-580℃ for the fast stage dominated by elastic relaxation and the defect density will sharply increase. Especially when AlGaAs temperature increases to 580℃, a very serious plastic relaxation will take place and the InGaAs quantum well will be dramatically destroyed.

Intermuscular coupling characteristics based on variational mode decomposition-coherence

Du Yi-Hao, Qi Wen-Jing, Zou Ce, Zhang Jin-Ming, Xie Bo-Duo, Xie Ping
Acta Physica Sinica. 2017, 66 (6): 068701 doi: 10.7498/aps.66.068701
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Intermuscular coupling is defined as the interaction, correlation and coordination between different muscles during the body movement, which could be revealed by the synchronization analysis of surface electromyogram (sEMG). The multiscaled coherence analysis of sEMG signals could describe the multiple spatial and temporal functional connection characteristics of intermuscular coupling, which could be helpful for understanding the multiple spatial and temporal coupling mechanism of neuromuscular system. Furthermore, the coupling characteristics in frequency band of sEMG generally reflect the functional connection between muscles which relate to motion control and coordinative mechanism of the central nervous system (CNS).
In this paper, we combine variational mode decomposition (VMD) and intermuscular coherence (IMC) analysis to propose a new method named VMD-IMC to quantitatively describe the muscular coupling characteristics in the corresponding frequency bands. First, sEMG data of flexor digitorum superficialis (FDS), flexor carpi ulnaris (FCU) and extensor digitorum (ED) are recorded simultaneously from twenty healthy subjects (25±3 years) who perform the designed grip task at sustained 20% maximum voluntary contraction under the static load. Then, the VMD approach is employed to adaptively decompose sEMG into several intrinsic mode functions to describe the information about different time-frequency scales. Furthermore, the coherence on different time-frequency scales between different sEMG signals is analyzed, and the significant coherent area index is calculated to quantitatively describe the functional coupling characteristics of the feature bands. And combining VMD with Hilbert transform, we calculate root mean square and mean instantaneous frequency (MIF) to describe the variations of energy and frequency of each muscle. The results show that coupling strengths increase with time, respectively, in beta (15-30 Hz) and gamma (30-45 Hz) band between two muscles (FDS vs FCU, FDS vs ED) during the sustained static force with low load. In addition, compared with the coupling between FDS and ED, the couplings between FDS and FCU in beta and gamma band under the condition of fatigue present more significant changes and similar trend in MIF variation with time. The obtained results reveal that the congenerous muscle is coordinated by CNS in a more synchronous way during the sustained isometric fatiguing contraction.

GENERAL

Partial component consensus of leader-following multi-agent systems

Wu Bin-Bin, Ma Zhong-Jun, Wang Yi
Acta Physica Sinica. 2017, 66 (6): 060201 doi: 10.7498/aps.66.060201
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Consensus problems, as basic topics in distributed coordination of multi-agent systems, have drawn a great deal of attention from different research fields. Generally, consensus refers to the asymptotic convergence of state variables of all agents with time evolution. In this paper, a concept on partial component consensus in multi-agent system is first given, which is a weaker dynamic behavior of group than the consensus in general, and then the problem of partial component consensus in leader-following first-order multi-agent system with the directed network topology is discussed. By designing an appropriate pinning control protocol and building corresponding error system, partial component consensus in multi-agent system is transformed into the partial variable stability of the error system. Using matrix theory and stability theory, a sufficient condition is given to realize partial component consensus in multi-agent system. Numerical simulations are given to illustrate the theoretical results.

Day-to-day dynamical evolution of network traffic flow with elastic demand

Liu Shi-Xu, Chen Wen-Si, Chi Qi-Yuan, Yan Hai
Acta Physica Sinica. 2017, 66 (6): 060501 doi: 10.7498/aps.66.060501
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Network traffic flow is an aggregated result of a huge number of travelers' route choices, which is influenced by the travelers' choice behaviors. So day-to-day traffic flow is not static, but presents a complex and tortuous day-to-day dynamic evolution process. Studying day-to-day dynamic evolution of network traffic flow, we can not only know whether the traffic network equilibrium can be reached and how the process is achieved, but also can know what phenomenon will occur in the evolution of network traffic flow if the equilibrium is not reached. In a real traffic system, taking day as scale unit, the day-to-day network traffic demand is variable and changes with everyday's traffic network state. The travelers' route choices are also influenced by the previous day's behaviors and network state. Then, will the day-to-day network traffic flow evolution be stable? If it is unstable, when will bifurcation and chaos occur? In this paper we discuss the day-to-day dynamic evolution of network traffic flow with elastic demand in a simple two-route network. The dynamic evolution model of network traffic flow with elastic demand is formulated. Based on a nonlinear dynamic theory, the existence and uniqueness of the fixed point of dynamic evolution model are proved, and an equilibrium stability condition for the dynamic evolution of network traffic flow with elastic demand is derived. Then, the evolution of network traffic flow is investigated through numerical experiments by changing the three parameters associated with travelers, which are the sensitivity of travelers' travel demand to travel cost, the randomness of travelers' route choices, and travelers' reliance on the previous day's actual cost. Our findings are as follows. Firstly, there are three kinds of final states in the evolution of network traffic flow: stability and convergence to equilibrium, periodic motion and chaos. The final state of the network traffic flow evolution is related to the above three parameters. It is found that under certain conditions the bifurcation diagram of the network traffic flow evolution reveals a complicated phenomenon of period doubling bifurcation to chaos, and then period-halving bifurcation. Meanwhile, the chaotic region is interspersed with odd periodic windows. Moreover, the more sensitive to cost the travelers' travel demand the more likely the system evolution is to be stable. The smaller the randomness of travelers' route choices, the less likely the system evolution is to be stable. The lower the degree of travelers' reliance on the previous day's actual cost, the more likely the system evolution is to be stable.

Improved algorithm of spectral coarse graining method of complex network

Zhou Jian, Jia Zhen, Li Ke-Zan
Acta Physica Sinica. 2017, 66 (6): 060502 doi: 10.7498/aps.66.060502
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Complex network as a key approach to understanding many complex systems, such as biological, chemical, physical, technological and social systems, is ubiquitous in nature and society. Synchronization of large-scale complex networks is one of the most important issues in network science. In the last two decades, much attention has been paid to the synchronization of complex dynamic networks, especially the meso-scale networks. However, many real networks consist of even hundreds of millions of nodes. Analyzing the synchronization of such large-scale coupled complex dynamic networks often generate a large number of coupled differential equations, which may make many synchronization algorithms inapplicable for meso-scale networks due to the complexities of simulation experiments. Coarse graining method can map the large-scale networks into meso-scale networks while preserving some of topological properties or dynamic charac-teristics of the original network. Especially, the spectral coarse-graining scheme, as a typical coarse graining method, is proposed to reduce the network size while preserving the synchronization capacity of the initial network. Nevertheless, plenty of studies demonstrate that the components of eigenvectors for the eigenvalue of the coupling matrix, which can depict the ability to synchronizing networks, distribute unevenly. Most of the components distribute concentrically and the intervals are small, while some other components distribute dispersedly and the intervals are large, which renders the applications of original spectral coarse graining method unsatisfactory. Inspired by the adaptive clustering, we propose an improved spectral coarse graining algorithm, which clusters the same or the similar nodes in the network according to the distance between the components of eigenvectors for the eigenvalue of network coupling matrices, so that the nodes with the same or the similar dynamic properties can be effectively clustered together. Compared with the original spectral coarse graining algorithm, this method can improve the accuracy of the result of clustering. Meanwhile, our method can greatly reduce algorithm complexity, and obtain better spectral coarse graining result. Finally, numerical simulation experiments are implemented in four typical complex networks: NW network, ER network, BA scale-free network and clustering network. The comparison of results demonstrate that our method outperforms the original spectral coarse graining approach under various criteria, and improves the effect of coarse graining and the ability to synchronize networks.

Predictability of forced Lorenz system

Li Bao-Sheng, Ding Rui-Qiang, Li Jian-Ping, Zhong Quan-Jia
Acta Physica Sinica. 2017, 66 (6): 060503 doi: 10.7498/aps.66.060503
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In recent years, the actual atmospheric predictability has attracted widespread attention. Improving our understanding of weather predictability is vital to developing numerical models and improving our forecast skill in weather and climate events. Given that the atmosphere is a complex and nonlinear system, taking the Lorenz system as an example is a better way to understand the actual atmosphere predictability. Up to now, some predictability problems of the Lorenz system have been investigated, such as the relative effects of the initial error and the model error. Previous advances in the research of predictability mainly focus on the relationship between the predictability limit and the initial error. As is well known, the external forcing can also result in the change of the predictability. Therefore, it is significant to investigate the predictability changing with the external forcing. The nonlinear local Lyapunov exponent (NLLE) is introduced to measure the average growth rate of the initial error of nonlinear dynamical model, which has been used for quantitatively determining the predictability limit of chaos system. Based on the NLLE approach, the influences of external forcing on the predictability are studied in the Lorenz system with constant forcing and Lorenz system with quasi-periodic forcing in this paper. The results indicate that for the Lorenz systems with constant and quasi-periodic forcings respectively, their predictability limits increase with forcing strength increasing. In the case of the same magnitude but different directions, the constant and quasi-periodic forcing both show different effects on the predictability limit in the Lorenz system, and these effects become significant with the increase of forcing strength. Generally speaking, the positive forcing leads to a higher predictability limit than the negative forcing. Therefore, when we consider the effects of positive and negative elements and phases in the atmosphere and ocean research, the predictability problems driven by different phases should be considered separately. In addition, the influences of constant and quasi-periodic forcings on the predictability are different in the Lorenz system. The effect of the constant forcing on the predictability is mainly reflected in the linear phase of error growth, while the nonlinear phase should also be considered additionally for the case of the quasi-periodic forcing. The predictability of the system under constant forcing is higher than that of the system under quasi-periodic forcing. These results based on simple chaotic model could provide an insight into the predictability studies of complex systems.

Traceable trans-scale heterodyne interferometer with subnanometer resolution Hot!

He Yin-Zhu, Zhao Shi-Jie, Wei Hao-Yun, Li Yan
Acta Physica Sinica. 2017, 66 (6): 060601 doi: 10.7498/aps.66.060601
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In order to realize the traceable trans-scale displacement measurements with high resolutions in the fields of fundamental scientific research and ultra-precision machining, we demonstrate a trans-scale heterodyne interferometer with a sub-nanometer resolution, through assembling a compact iodine-stabilized laser at 532 nm. Using modulation transfer spectroscopy, the green laser is traced back to the transition line R(56)32-O(a10), which is one of the recommended spectral lines for ‘meter’ redefinition. The Allan standard deviation of the laser frequency is 1.3×10-12 within an average time of 1 s. Compared with most He-Ne lasers, the green laser has a short wavelength and good stability, which leads to a higher resolution. We use two acoustic-optic modulators driven by a two-channel acoustic-optic driver sharing the same crystal oscillator to separate input beams spatially. The frequency of one beam is shifted by 80 MHz and the other is shifted by 82 MHz, which results in a beat frequency of 2 MHz. As a result, the nonlinearity caused by source mixing substantially is reduced. The phase noises of the fibers and two acoustic-optic modulators are well compensated. In order to minimize the difficulty in adjusting the optical path and the error of the measurement, we integrate the interferometry components and design a monolithic prism. The optical resolution of the interferometer reaches to λ/4. The experiment is carried out in a vacuum environment to reduce the influence of the refractive index of air. High-precision phase measurement technology is used to improve the accuracy of the interferometer. The errors of the interferometer can be classified as random and systematic errors. Random errors include the error from the frequency instability of the laser and the error due to environmental effects. Systematic errors include the phase measurement error and the nonlinearity error. To verify the performance of the interferometer, these errors must be evaluated. In a span of 100 mm, the measurement uncertainties caused by laser wavelength uncertainty, the air refractive index uncertainty, the phase measurement uncertainty and the nonlinearity error are 3 pm, 300 pm, 6.3 pm and 118 pm, respectively. Finally, the performance evaluation shows that the combined uncertainty of the interferometer reaches 322 pm in a span of 100 mm, which is mainly due to the refractive index of air. The heterodyne interferometer meets the requirements for traceable trans-scale measurement with a sub-nanometer resolution, which can be widely used in instrument calibration, length standard making, and geometric measurement.

NUCLEAR PHYSICS

A method of evaluating the relative light yield of ST401 irradiated by pulsed neutron

Yao Zhi-Ming, Duan Bao-Jun, Song Gu-Zhou, Yan Wei-Peng, Ma Ji-Ming, Han Chang-Cai, Song Yan
Acta Physica Sinica. 2017, 66 (6): 062401 doi: 10.7498/aps.66.062401
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High speed imaging technique is an effective method to test the information about pulsed neutron source. Imaging system is usually composed of a pinhole, a scintillator, an image intensifier and a charge-coupled device (CCD) camera. ST401 plastic scintillator is widely used to convert the neutron image into visible light image since it has features of high conversion efficiency and fast time response. When testing a pulsed neutron source of wide energy spectrum, we should evaluate the light yields of ST401 irradiated by neutrons with different energies and make the CCD camera exposed to the light appropriately. A 0.3 MeV pulsed X-ray source is often used to calibrate the imaging system because of its low cost than the D-T fusion neutron source. In this work, a method of evaluating the relative light yield of ST401 irradiated by 0.1-16 MeV neutron to 0.3 MeV X-ray is proposed.
Geant4 Monte Carlo software is used to simulate the transport performances of neutrons and X-rays. The software package can simulate the transport process of photons. But the conversion factor of ray energy deposition into photons is unknown. It is difficult to calculate the number of photons generated in ST401 accurately. In this article, we calculate the relative light yield according to the energy of charged particles produced in ST401. Firstly, all information about the particle type, energy deposition, kinetic energy is monitored on event-by-event basis in GEANT4. Secondly, the complete history of the tracks is then used to calculate the light output from the scintillator according to the neutron response functions. Thirdly, the light output caused by charged particles going out of ST401 is deducted. Ratios of average light yield of 1 mm, 3 mm, 5 mm, 1 cm, 2 cm, 3 cm, 5 cm thick ST401 irradiated by 0.1-16 MeV neutron to 0.3 MeV X-ray are given. To confirm the correctness of the simulated result, validation experiment is carried out on IVA pulsed X-ray source and SGIII pulsed neutron source. The simulated ratio of average light yield of ST401 irradiated by one single 14 MeV neutron to 0.3 MeV X-ray has a discrepancy of less than 10% compared with the measured value. Compared with the results of experiment conducted on a constant current source, the simulated results have a maximum discrepancy of less than 44%. If CCD camera exposure 10%-90% of the full scale, the image will have high contrast and information loss can be avoided. According to the simulated results and the neutron yield, exposure can be easily set to be 60% of the full scale by adjusting the gain of the image intensifier. Assume that the simulated results have a 44% discrepancy, the actual exposure will be in a range of 34%-86% of the full scale. Underexposure and overexposure can be avoided by presetting the imaging system sensitivity appropriately based on the simulated results. It implies that the method proposed is effective in predicting the imaging system response to pulsed neutron with wide energy spectrum.

ATOMIC AND MOLECULAR PHYSICS

Structures and properties of Np(NO3)nq (n=1–6, q=-2–+3) coordination compound

Ma Lei, Yin Yao-Peng, Ding Xiao-Bin, Dong Chen-Zhong
Acta Physica Sinica. 2017, 66 (6): 063101 doi: 10.7498/aps.66.063101
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In the process of nuclear waste disposal, the valuable uranium and plutonium are recycled and separated by dissolving the spent fuel in nitric acid. However, transuranic Np greatly influences the process of separation and recovery. Therefore, it is vital to study the structure and properties of nitrate, which is combined with neptunium ions and nitric acid. Furthermore, there are few researches about nitrate formed by tetravalent neptunium ions. So in this article, by using B3LYP hybrid method of density functional theory, the Gaussian 03 program is used to optimize the geometric construction of the coordination compounds Np(NO3)nq (n=1-6, q=-2-+3) formed by the tetravalent neptunium ions (Np4+) and nitrate ion (NO3-). Under the relativistic effective core potential model, the structure parameters and properties are reported. It is found that NO3- coordinates to Np4+ as a bidentate ligand, and the Np–N and Np–O bonds are the shortest in Np(NO3)22+, while the binding energy of the Np(NO3)4 is the largest. The infrared spectra of Np(NO3)4 are calculated in the gas and liquid phase. Comparing with the available experimental data, the reliability of the calculation results in this work is confirmed.

Electronic structures, magnetic properties and spin-orbital coupling effects of aluminum nitride monolayers doped by 5d transition metal atoms: possible two-dimensional long-range magnetic orders

Yang Ming-Yu, Yang Qian, Zhang Bo, Zhang Xu, Cai Song, Xue Yu-Long, Zhou Tie-Ge
Acta Physica Sinica. 2017, 66 (6): 063102 doi: 10.7498/aps.66.063102
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The magnetism of two-dimensional material is an important research topic. In particular, the long-range magnetic order of two-dimensional material is of great significance in theoretical research and practical application. According to the Mermin-Wagner theory, the isotropic Heisenberg model in a two-dimensional system cannot produce long-range magnetic orders at non-vanishing temperatures. Considering the existence of strong magnetic anisotropy, possible two-dimensional long-range magnetic orders may exist in 5d atom doped two-dimensional aluminum nitride (AlN) monolayer. This research is performed by first-principles calculations based on the density functional theory. Geometries, electronic structures, magnetic properties, and magnetic anisotropy energies from spin-orbital coupling effects in AlN monolayers doped by 5d transition metal atoms (Hf, Ta, W, Re, Os, Ir, Pt, Au, and Hg) are calculated. Four kinds of supercells are used in the calculation, i.e, 2×2, 3×3, 4×4, and 5×5, with one aluminum atom substituted by one 5d atom. Projection augmented wave method is used to describe the interaction between the valence electrons and the ions. The plane wave is used to expand the wave function of the valence electron. For an optimized geometry, the bond length between the 5d metal atom and the nearest N atom is the largest in Hg-doped supercells, which is 2.093 Å, followed by the Au, Hf, Pt, Ta, and Ir according to the order of bond length magnitude. For the densities of states (DOSs), obvious impurity energy levels appear in the forbidden bands. For all the supercells, spin-up and spin-down DOSs of Ta and Ir doped systems are symmetric, indicating non-magnetic states. DOSs of Hf, W, Re, and Os doped systems are asymmetric, indicating magnetic states. For Pt, Au, and Hg, DOSs are symmetric in 2×2 supercells, but asymmetric in the 3×3, 4×4, and 5×5 supercells. Total magnetic moments and the spin densities are also given. In 5×5 supercells, they are 1.00, 0.00, 0.39, 1.99, 1.17, 0.00, 1.00, 2.00, and 1.00 for Hf, Ta, W, Re, Os, Ir, Pt, Au, and Hg, respectively. The magnetic moment is mainly concentrated in the vicinity of the 5d atoms. The energy differences between ferromagnetic and antiferromagnetic states are calculated. For Hf, Re, Pt and Au systems, the differences in 4×8 supercells reach the maximum values of -187.2563 meV, 286.2320 meV, -48.0637 meV and -61.7889 meV, respectively. The results indicate that there is a strong interaction between the magnetic centers. Magnetic anisotropy energy originating from spin-orbital effect is calculated in the 4×4 supercells. For the Re system, it is the highest, reaching 11.622 meV. For W, Os, and Au, the values are larger than 1 meV, showing strong magnetic anisotropies. The magnetic anisotropy can produce a spin wave energy gap, resulting in long-range magnetic orders. Based on the results above, it is predicted that with appropriate 5d atoms and suitable doping concentration, two-dimensional long-range magnetic orders may exist in 5d transition metal atom doped AlN monolayers.

Theoretical study on the electronic structure and transition properties of excited state of ZnH molecule Hot!

Zhao Shu-Tao, Liang Gui-Ying, Li Rui, Li Qi-Nan, Zhang Zhi-Guo, Yan Bing
Acta Physica Sinica. 2017, 66 (6): 063103 doi: 10.7498/aps.66.063103
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The potential energy curves (PECs) associated with the lowest four dissociation limits, i.e., Zn(1Sg)+H(2Sg), Zn(3Pu)+H(2Sg), Zn+(2Sg)+H-(1Sg) and Zn(1Pu)+H(2Sg), are calculated by using a high-level configuration interaction method. The Davidson correction, scalar relativistic effect and spin-orbit coupling effect are taken into account in calculation. On the basis of our calculated PECs of Λ-S and Ω states, the spectroscopic constants including Te, ωe, ωeχe, Be and Re are evaluated by numerical solution of one-dimensional Schrödinger equation. The computed spectroscopic constants are reasonably consistent with previous experimental results. The dipole moment curves of the 7 Λ-S states are presented, and the influences of the variation of electronic configuration on the dipole moment and bonding property are discussed. The computational results reveal the ionic character of the C2Σ+ state. The variation of Λ-S component for Ω state near the avoided crossing point is illuminated, which is used to explain the change of transition dipole moment (TDM) around the avoided crossing point. Based on the TDMs, Franck-Condon factors and the transition energies, the radiative lifetimes of v'=0-2 vibrational levels of (2)1/2, (3)1/2, (4)1/2 and (1)3/2 states are predicted, which accord well with the available experimental values.

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

Propagation matrix of plane wave incident obliquely on stratified lossy chiral medium

Wang Fei, Wei Bing
Acta Physica Sinica. 2017, 66 (6): 064101 doi: 10.7498/aps.66.064101
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The real and imaginary parts of the eigen complex wave vector in a stratified lossy chiral medium for the case of oblique incidence are derived by using the phase-matching condition. Due to the fact that the real and imaginary parts are nonparallel, the eigen wave propagating in the medium is inhomogeneous. Then the refraction angle of the eigen wave can be deduced via the real part of the wave vector. Finally the propagation matrix of the obliquely incident wave in a stratified lossy chiral medium is derived based on the boundary conditions and the field equations of eigen wave in each region. By using the proposed method, the reflection, transmission, and propagation characteristics of plane wave with arbitrary incident angle in a stratified chiral medium can be analyzed.

Broadband circularly polarized high-gain antenna design based on linear-to-circular polarization conversion focusing metasurface

Li Tang-Jing, Liang Jian-Gang, Li Hai-Peng, Niu Xue-Bin, Liu Ya-Qiao
Acta Physica Sinica. 2017, 66 (6): 064102 doi: 10.7498/aps.66.064102
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A single-layer reflecting element is proposed based on the principle of linear-to-circular polarization conversion focusing metasurface, which can independently control the phases of x-polarized and y-polarized reflecting waves and operate in a broadband of 10-14 GHz. Following the generalized Snell's laws of reflection, a super cell is designed with a phase-gradient of -60° for x-polarized waves and 60° for y-polarized waves, and the simulation results show the well wideband anomalous reflection as expected. In the design of the multifunctional metasurface, the 13×13 unit cells are used to satisfy the parabolic profile and the focal-distance-to-diameter ratio is set to be 0.5. The phase compensation for forming a constant aperture phase is provided by the individual reflected elements with different structure parameters and φx-φy=90° is used to realize polarization conversion. The designed sample is simulated in CST Microwave Studio and the results show that both of the x-polarized and y-polarized plane waves are well focused through the reflection of the focusing metasurface in a broadband of 10-14 GHz. Traditionally, multi-layer element is used to broaden phase coverage and bandwidth, the single-layer design in this paper greatly reduces the cost, processing difficulty and thickness of the lens. For further application, a linearly polarized Vivaldi antenna with a highest gain of 10 dB is located at the focal point of metasurface and the angle included between its polarization direction and x-axis is 45° in order to acquire right-handed circularly polarized reflecting wave. According to the reversibility principle of electromagnetic wave propagation, the spherical wave radiated by the feed antenna is converted into plane wave by the reflection of the focusing metasurface so that the antenna gain is remarkably enhanced. Simultaneously, the linearly polarized wave can be transformed into circularly polarized wave. Finally, the feed antenna and the metasurface are fabricated, assembled and measured. Numerical and experimental results are in good agreement with each other, which shows that the -1 dB gain bandwidth of the high-gain antenna is 24% (11-14 GHz) and the 3 dB axial ratio bandwidth is 29.8% (10-13.5 GHz). In addition, the gain at 12 GHz reaches a highest value of 19.6 dBic, and the aperture efficiency is more than 54%. The good performances indicate that the proposed broadband high-gain circularly polarized antenna has a well promising application in various communication systems. It is worth noting that the horizontally polarized, vertically polarized, right-handed circularly polarized and left-handed circularly polarized high-gain antenna can be realized with the rotation of feed antenna. In this case the idea is more versatile and valuable for designing the polarization reconfigurable antenna systems.

Modeling the single-mode thermally guiding very-large-mode-area Yb-doped fiber amplifier

Cao Jian-Qiu, Liu Wen-Bo, Chen Jin-Bao, Lu Qi-Sheng
Acta Physica Sinica. 2017, 66 (6): 064201 doi: 10.7498/aps.66.064201
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The very-large-mode-area (VLMA) fiber is of great importance for suppressing the nonlinear effects which are considered as main limitations to the power scaling-up of high-power fiber lasers and amplifiers. The thermally guiding (TG) VLMA fiber is a novel VLMA fiber, the waveguide of which is formed by the thermal lens effect. Then, a low numerical aperture can be realized, which is promising to achieve the expanding of mode area with a high-quality beam. In order to study the performance of TG VLMA fiber in a fiber amplifier, we present a rate-equation model of the single-mode ytterbium-doped TG VLMA fiber amplifier, which consists of the steady-state rate equations and thermal transferring equations. With this model, the forward-pumped single-mode TG VLMA fiber amplifier is numerically studied. It is found that the diameter of fundamental mode field rises with the increase of the signal power, which shows the superiority of the TG VLMA fiber in suppressing the nonlinear effect in the fiber amplifier. The optimum fiber length and pertinent physical mechanism are also investigated. It is revealed the optimum fiber length is related to the input pump power, and it decreases with the increase of input pump power. However, when the input pump power is large enough, such a variation of optimum fiber length will become weakened. The numerical results also illuminate that the thermal load at the optimum length of TG VLMA fiber should not change too much with the input pump power. Moreover, the mode of output optical field is also discussed. It is found that the thermal load at the optimum length may not be large enough to realize a core-confined mode. In order to ensure that the core-confined mode can be output, the thermal load at the end of the fiber amplifier should be larger. It requires that the fiber length used in the amplifier should be shorter than the optimum fiber length, which will induce the decrease of the output signal power to some extent. In spite of that, the numerical results reveal that the decrease of output signal power should not be much, and the pertinent slope efficiency is not obviously lowered, either. Thus, it is verified that the core-confined mode with a VLMA can be obtained from the TG VLMA fiber amplifier with high slope efficiency. The pertinent results have significant guidance in the design of TG VLMA fiber amplifier.

Wave coupling theory of nonlocal linear electro-optic effect in a linear absorbent medium

Wu Dan-Dan, She Wei-Long
Acta Physica Sinica. 2017, 66 (6): 064202 doi: 10.7498/aps.66.064202
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Being an important optical phenomenon, the linear electro-optic effect has diverse applications in the optical modulation and optical switching. The refractive index ellipsoid theory has been widely used to study the linear electro-optic effect for a long time. Despite of its visualization such a theory has limitations and cannot deal with a lot of cases in which the linear absorption cannot be neglected, or the electric displacement vector has a nonlocal response to electric field, etc. To overcome such shortcomings, in 2001 a wave coupling theory of linear electro-optic effect was developed by She and Lee (She W, Lee W 2001 Opt. Commun. 195 303). And in 2016 we generalized this wave coupling theory to the treatment of nonlocal linear electro-optic effect in which the displacement vector has a nonlocal response to electric field.
In this paper, we use this wave-coupling theory to investigate how the linear absorption influences the linear electro-optic effect in a nonlocal medium. Starting from Maxwell's equations and considering the linear absorption and the nonlocality of the susceptibility tensors, we obtain two coupling equations for two orthogonal linear polarized waves and also analytical solutions of the resulting equations, which can be used to describe the nonlocal linear electro-optic effect for a light beam propagating along any direction, with an external direct current electric field applied along an arbitrary direction in a linear absorbent crystal. With such solutions, we study the influences of the linear absorption on the phase, amplitude, shape of the output beam, as well as the half-wave voltage and the extinction ratio of electro-optic modulation. The results show that no matter whether there exists linear absorption, the Rayleigh distance of the Gaussian beam in the crystal will be shortened as a result of the nonlocality of χ(1). When linear-absorption coefficients α11 and α22 are equal, the linear absorption damps equally the amplitudes of the two polarized output beams with keeping their phases and shapes unchanged. So in the case of α11=α22, just as in a lossless medium, the phenomenon that the output beam is no longer a Gaussian beam in an electro-optic amplitude modulation scheme can be considered as a possible signal of the nonlocal response of χ(2). More interestingly, when α11α22, the linear absorption not only reduces the amplitudes of output beams, but also changes their phases and shapes. In such a case one need to measure the nonlocal characteristic length of χ(2) to judge whether χ(2) has a nonlocal response. Finally, in the case of α11α22, as a result of linear absorption, the extinction ratio is reduced, but the half-wave voltage keeps nearly unchanged in an electro-optic amplitude modulation scheme. Besides the discussion on the influence of the linear absorption, we also make suggestions of how to measure the nonlocal characteristic lengths of χ(1) and χ(2) and the absorption coefficients α11 and α22.

A wideband coding reflective metasurface with multiple functionalities

Chen Wei, Gao Jun, Zhang Guang, Cao Xiang-Yu, Yang Huan-Huan, Zheng Yue-Jun
Acta Physica Sinica. 2017, 66 (6): 064203 doi: 10.7498/aps.66.064203
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A controllable wideband multifunctional reflective metasurface is presented. First of all, a polarization-rotating unit cell is proposed by combing micro-electromechanical system (MEMS) technology with reflective metasurface design. The proposed unit cell is characterized by wideband, low loss and controllable properties. Each unit cell is integrated with two MEMS switches. When the two switches operate in different states, the unit cell shows different responses to plane wave incidence, and the corresponding working states can be denoted by “0” or “1”. It is worth noting that a 180 degree reflection phase difference is generated for the two working states. Then, the proposed unit cell is periodically arranged to construct a metasurface. Based on different coding matrixes, multiple functionalities can be obtained by using the proposed metasurface. When all the unit cells are controlled to operate in on- or off-state, polarization-rotating function is obtained. Besides, the agility scattering field performance is also presented by using “chessboard” and “random” codings. A series of equations is derived to reveal the relationship between reflection coefficient of the unit cell and radar cross section (RCS) reduction of the chessboard reflective surface, which is also verified by full-wave simulations. Finally, four prototypes consisting of 576-cells, which correspond to the “all 0”, “all 1”, “chessboard” and “random” coding, are fabricated and measured. The measured results demonstrate that the proposed reflective metasurface shows polarization-rotating performance in a frequency range of 8.9-13.2 GHz when all unit cells operate in “0” or “1” state. The measured results of the “chessboard” and “random” coding metasurface manifest remarkable RCS reduction compared with the same size metal plane. Good agreement between simulations and measurements is obtained. Owing to the ability to control polarization and beam shape of the reflected wave dynamically, the proposed reflective metasurface has potential applications in the field of intelligent stealth.

Asymmetric waveguide and the dual-wavelength stimulated emission for CdS/CdS0.48Se0.52 axial nanowire heterostructures

Li Dan, Liang Jun-Wu, Liu Hua-Wei, Zhang Xue-Hong, Wan Qiang, Zhang Qing-Lin, Pan An-Lian
Acta Physica Sinica. 2017, 66 (6): 064204 doi: 10.7498/aps.66.064204
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Semiconductor axial nanowire heterostructures are important for realizing the high-performance nano-photonics and opto-electronics devices. Although different IV and III-V semiconductor axial nanowire heterostructures have been successfully prepared in recent decade, few of them focused on the optical properties, such as the waveguide, due to their low light emission efficiencies. The II-VI semiconductor nanowires grown by chemical vapor deposition strategy, such as CdS, CdSe and their alloys, can act as nanoscale waveguide, nanolasers, etc., because of their high optical gains and atomically smooth surfaces. However, it is still a challenge to growing the high-quality II-VI semiconductor axial nanowire heterostructures, owning to the poor controllability of the vapor growth techniques. Here, the CdS/CdSSe axial nanowire heterostructures are prepared with well controlled CVD method under the catalysis of annealed Au nanoparticles. The scanning electron microscope characterization shows that the wires have smooth surfaces with Au particles at the tips, indicating the vapor-liquid-solid growth mechanism for the nanowire heterostructures. The microscope images of the dispersed wires illuminated with a 405 nm laser show that the red and the green segment align axially with a sharp interface, demonstrating the axial alignment of CdS and CdSSe segments. The position related micro-photoluminescence spectra exhibit near band edge emissions of CdS and CdSSe without obvious emission from defect states, which suggests that the wires have highly crystalline quality. The waveguide of the nanowire heterostructures is studied through respectively locally exciting the two ends of the wire with a focused 488 nm laser. The local illuminations at both the CdS end and the CdSSe end result in red emission at the corresponding remote ends of the wires, with the emission intensity of the former being one order lower than that of the later, which is caused by the reabsorption of the green light emission (from CdS segment) in the CdSSe segment. This indicates the asymmetric waveguide in these heterosturctures, which implies that the CdS/CdSSe nanowire heterostructures have the potential applications in the photodiode. Under the pumping of 470 nm femtosecond laser, dual-color (red and green) lasing is realized based on these wires, with the lasing threshold of red light lasing being lower than that of the green one, which results from the larger round-trip loss for the green light arising from the self-absorption in CdSSe segment. To prove that the light can be transfer between the two segments with different refractivities, the waveguide of the nanowire heterostructure is simulated by the COMSOL. The result shows that the light can effectively propagate between CdS and CdSSe segments, which ensures the light-matter interaction in the axial CdS/CdSSe nanowire heterostructures as discussed above. These high-quality CdS/CdSSe axial nanowire heterostructures can be found to have the potential applications in photodiodes, dual-color nanolasers and photodetectors.

Low frequency band gap characteristics of double-split Helmholtz locally resonant periodic structures

Jiang Jiu-Long, Yao Hong, Du Jun, Zhao Jing-Bo, Deng Tao
Acta Physica Sinica. 2017, 66 (6): 064301 doi: 10.7498/aps.66.064301
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A double-split Helmholtz periodic structure with the characteristic of local resonance is designed and constructed in this paper. The double-split periodic structural cell which can be divided into internal and external cavities is adopted in structure. In such a kind of structure, the resonating area is remarkably expanded while the inner cavity is continuously enlarged. Thus, a satisfactory feature of low frequency resonance can be obtained. At the same time, the adjustability of band gap is achieved by the designed adjustment of the arc length of the inner cavity, therefore, the effect of sound insulation in a specific low frequency band can be achieved. In the analyses of the mechanism and factors of the generation of low frequency band gap, the mathematical model of the upper and lower limits of the band gap is established by using the electric circuit analogy. And some comparative analyses between the methods of electric circuit analogy and finite element method are carried out. The result suggests that a satisfactory feature of low frequency band gap is presented, and the first band-gap ranges from 86.9 Hz to 138.2 Hz. The low frequency band gap can be influenced by the arc length of inner cavity, the space between inner and outer cavities, and the interaction of the structural cells in the periodic arrangement. The longer the arc length of the inner cavity, the lower the low frequency band gap will be; the longer the distance between inner and outer cavities, and the higher the frequency of band gap, the worse the low frequency effect will be; the lower limit of low frequency band gap cannot be influenced by reducing the space between individual structures, on the contrary, the width of low frequency band gap can be sharply increased. Plenty of practical and theoretical support in the field of low frequency noise reduction is offered in the research.

Numerical simulation of convergence effect on shock-bubble interactions

Liang Yu, Guan Ben, Zhai Zhi-Gang, Luo Xi-Sheng
Acta Physica Sinica. 2017, 66 (6): 064701 doi: 10.7498/aps.66.064701
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The shock-bubble interaction is a basic configuration for studying the more general case of shock-accelerated inhomogeneous flows. In previous studies, a planar shock wave interacting with a spherical gas bubble was extensively investigated, in which the effects of shock intensity, Atwood number and secondary shock on the bubble development were considered and elucidated. However, in most of practical applications, such as inertial confinement fusion, a converging shock wave is generally involved. It is therefore of fundamental interest to explore the perturbation growth under converging shock conditions. Due to the difficulties encountered in generating a perfectly converging shock wave in laboratory, experimental investigation on the converging shock-accelerated inhomogeneous flows was seldom carried out previously. The preliminary study on the development of a gas bubble impacted by a converging shock wave showed that a large discrepancy exists compared with the planar counterparts. Because of the intrinsic three-dimensional (3D) features of this problem, the current experimental techniques are inadequate to explore the detailed differences between planar and converging shocks accelerating gas bubbles. As a result, numerical simulations become important and necessary. In this work, evolution of an SF6 spherical gas bubble surrounded by air accelerated by a cylindrical converging shock wave and a planar shock wave is numerically investigated by a 3D program, focusing on the convergence effect on the interface evolution. Multi-component compressible Euler equations are adopted in the 3D program and the finite volume method is used. The MUSCL-Hancock scheme, a second-order upwind scheme, is adopted to achieve the second-order accuracy on both temporal and spatial scales. Compared with planar shock wave, a cylindrical converging shock wave has curvature, and as the converging shock wave moves forward, the shock strength and the wall effect both increase, which will result in the diversity of the flow field after shock impact. The numerical results show that the vortex rings formed under converging shock condition are sharper than those under planar shock condition which may be associated with geometric contraction effect of the tube and reflected shock from the wall. Besides, the peak pressure generated in the vicinity of the downstream pole of the bubble under converging shock condition is higher than that of planar shock wave, and, therefore, the jet induced by high pressures moves faster under converging shock condition. Due to the variations of shock curvature and shock intensity, the distribution law and amplitude of vorticity generated by converging shock wave at the interface is changed. Comparison between circulation and gas mixing rate indicates that the converging shock is beneficial to promoting vorticity generation and gas mixing. From the present work, it can be concluded that the convergence effect plays an important role in interface evolution.

Experimental and numerical investigation on the flow structure and instability of water-entry cavity by a semi-closed cylinder

Lu Zhong-Lei, Wei Ying-Jie, Wang Cong, Cao Wei
Acta Physica Sinica. 2017, 66 (6): 064702 doi: 10.7498/aps.66.064702
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The purpose of this present study is to address instability flowing characteristics and mechanism of the water-entry cavity created by a semi-closed cylinder. For this purpose, an experimental study and a numerical study of the water-entry of a semi-closed cylinder are carried out. According to the results of the experiments and comparison, the cavitating flows between the semi-closed cylinder water entry and the sealing cylinder water entry, and the fluctuation flow pattern form of the semi-closed cylinder cavitation is found around the body. The flow characteristics of the cavity shape are gained by analyzing the image data. A further insight into the mechanisms of perturbation to the flow structure and the cavity fluctuation by the air in the opening cell are studied based on the law of conservation of energy in water entry. According to the results of simulation and comparison with the cavity visualization of experiment, three instability flow phenomena of cavity are formed during the different stages of water-entry, i.e., flow separation destroyed, local flow transformed near cavity, and unique cavity shedding pattern. A further insight into the characteristics of the flow and the distribution of pressure and velocity during the stage of the cavity unstabilized flow is gained. Finally, the formation mechanism of the cavity unstabilized flow is studied based on the boundary layer theory and Helmhotz vortex theory. The obtained results show that the water poured into the cell of cylinder after the opening end has impacted free surface causes the internal air to compress and expand, and as a consequence of these effects, periodic disturbances of flow structure occur around the cavity, then the cavity presents an identical periodic wave flow with air piston motion and the flow stability of cavity is destroyed. At the eve of impacting, the opening end approaches the free surface, which leads to the inflow velocity attenuation rapidly and the pressure increasing in the cell, which creates an initial pressure higher than ambient pressure. Because of the high pressure, air efflux from the cell forms a gas jet injected into the cavity for the first air expansion stage, then the detaching flow is destroyed and the cavity extension diameter is enlarged. The flow in the gas-liquid mixing domain of cavity is seen as an approximate boundary layer flow pattern where favorable pressure gradient on the upwind side and adverse pressure gradient on the lee side appear alternately. As this flow pattern, re-entrant flow acting on the trough of wave cavitation results in the fact that the laminar-turbulent transition is weakened in the trough field and the local gas-liquid mixing domain is thickened to be involved in unstabilized structure as cloud cavitation. The wave cavity presents a partial and multiple shedding pattern occurring at the trough positions in sequence. There is no mutual interference between shedding cavity and the main cavity. Following the cavity shedding, vortex shedding is formed. The vorticity concentrates on the inside of shedding cavity, and the pressure and velocity present a coherent structure.

Experimental progress of laser-driven flyers at the SG-III prototype laser facility

Shui Min, Chu Gen-Bai, Xi Tao, Zhao Yong-Qiang, Fan Wei, He Wei-Hua, Shan Lian-Qiang, Zhu Bin, Xin Jian-Ting, Gu Yu-Qiu
Acta Physica Sinica. 2017, 66 (6): 064703 doi: 10.7498/aps.66.064703
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Laser-driven flyers have unique advantages of high flyer velocity, low cost, simple facility compared with the flyers driven by other conventional dynamic high-pressure loading techniques. With the fast development of laser technique, launching hypervelocity flyers with high-intensity laser pulse has become more and more prevalent. In this paper, we introduce the recent experiments of laser-driven flyers at the SG-III prototype laser facility.
Three ways of launching hypervelocity flyers are developed and introduced, respectively. In the first way, multilayered aluminum flyers are gradually accelerated to a terminal velocity of 8 km/s, which is measured by optical velocimetry, without melting and vaporization. The pressure distribution within the flyer shows that the temporally ramped pulse ablation generates a compression wave, and the flyer is accelerated by this wave and its reverberation within the flyer. In the second way, a strong laser ablates the low-density reservoir foil and generates strong shock in the foil. The shock wave is strong enough, and when the shock breaks out from the free surface, the foil will unload as plasma towards the flyer with a density profile. The plasma decelerates upon colliding the flyer, and the single-layered flyer is gradually accelerated by the momentum transition. In our experiments, single-layered aluminum foil and single-layered tantalum foil are accelerated to 11.5 km/s and 6.5 km/s, respectively. According to the pressure distribution within the flyer, the flyer is also accelerated by the compression wave produced by the plasma collision, which is similar to the case of direct ablation by temporally ramped pulse. However, the way of plasma collision could better reduce X-ray and electron preheat and obtain cleaner flyers. In the last way, the flyers are launched by direct strong short-laser ablation. The multi-layered aluminum foil is accelerated to a high average velocity of 21.3 km/s by using a 3-ns quadrate laser pulse at 351 nm after spatial homogenization. A line-velocity interferometer system for any reflect (VISAR) is employed to monitor the processes of flyer launch and flight in a vacuum gap and the shock velocity associated with phase change in fused silica target after flyer impact is inferred. The reflectivity variations of the VISAR fringe pattern and the shock velocity in the fused silica suggest that the flyer owns a density gradient characteristic. Furthermore, specifically designed multi-layered flyers (polyimide/copper) are accelerated by shock impedance and reverberation techniques to a super high averaged velocity of 55 km/s, which is much faster than recently reported results. Light-emission signals of shock breakout and flyer impact on flat or stepped windows are obtained, which indicates the good planarity and integrity for the flyer. Compared with single-layer flyers, multi-layered flyers have a good planarity, and a high energy conversion efficiency from laser to flyers.
In this paper, we give a comprehensive analysis and comparison of the experimental designs, technique means and data results about laser-driven flyers. This would provide a reference for further experimental study of laser-driven flyers and also verify that the SG-III prototype laser facility is a very promising facility for studying the hypervelocity flyers launching field.

GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS

A single-element interferometer for measuring parallelism and uniformity of transparent plate

Lan Bin, Feng Guo-Ying, Zhang Tao, Liang Jing-Chuan, Zhou Shou-Huan
Acta Physica Sinica. 2017, 66 (6): 069501 doi: 10.7498/aps.66.069501
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The transparent plates (such as organic glass, plastic plate) are widely used in the construction industry, high-tech products and scientific research applications, and its parallelism and uniformity measurement in the manufacture and quality control become more and more inevitable. Interferometer is a label-free, high-precision, and high-efficient device that can be used in many fields. According to a single-element interferometer, we demonstrate a measurement for the parallelism and uniformity of transparent medium. Beam-splitter cube is a key component. Half of plane wave laser source passes through the measured medium and the remaining half directly passes through the air, then these two halves with different optical paths meet in the beam-splitter cube. The parallelism or uniformity is determined by calculating interference fringe shift number during rotating the measured sample. The coherent beam is divided into two parts by a beam-splitter, one passes through the lens and then arrives at a photoelectric counter, and the other arrives at the observation plane of the charge-coupled device. The photoelectric counter is used to count the integer part of fringe shift number during rotating the sample; and the decimal part can be detected by calculating the phase difference of the two interferograms captured before and after rotation. The measurement principle of the proposed device is analyzed in detail, and the numerical simulations of the fringe shift number and the gray level changing with the sample rotation angle, the thickness and the refractive index of the sample are carried out. The simulation results show that the bigger the rotation angle, thickness and refractive index of the sample, the greater the fringe shift number will be. Therefore, the measurement accuracy can be improved by increasing the rotation angle and the thickness of the sample. In addition, we also simulate the measurement processes of two kinds of samples, which are unparallel and inhomogeneous transparent plates. The simulation results prove the feasibility and high accuracy of the proposed method. Finally, the optical experiment is conducted to demonstrate the practicability of the present device. The parallelism of a cuvette used for more than one year, is tested by our device. The results show that the difference in thickness between the cuvettes is on a micron scale, the peak-valley (PV) value is 9.92 μm, and the root mean square (RMS) value is 2.2 μm. And the difference between the contrast test results and the results from the proposed method is very small, the PV value is 0.569 μm, and the RMS value is 0.131 μm. The stability and repeatability of the proposed setup are tested in the experimental condition. The mean value and standard deviation of the fringe shift number during 30 min are 0.0012 and 0.0008, respectively. These results further testify the high accuracy and stability of our method. In conclusion, the performance of our measurement method is demonstrated with numerical simulation and optical experiment.

Interfacial properties and morphological evolution of liquid Ag film on the modified graphene

Zhao Zhen-Yang, Li Tao, Li Xiao-Yin, Li Xiong-Ying, Li Hui
Acta Physica Sinica. 2017, 66 (6): 069601 doi: 10.7498/aps.66.069601
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The interfacial wettability and morphological evolution of liquid on a solid substrate, as natural phenomena, have received great attention in recent years. Although much work has been done to study this process, existing studies mainly focus on the wetting properties of water. Therefore, in this paper, we use molecular dynamics simulation method to study the interfacial phenomena of the nanoscale liquid silver on graphene, whose surface has been modified. By making different comparisons, such as Lennard-Jones (LJ) potential parameters, the surface structures of substrates, the thickness values of films and the shapes of films, the influences of these variables on wetting properties of liquid silver on graphene are studied. The results show that the dewetting of liquid silver occurs on graphene, implying that the wettability of liquid silver is weak, and that the potential parameters, the surface structure of substrates, the thickness of film and the shape of film have great influences on the wettability and morphology evolution of film: the change of these factors can affect the dewetting properties of liquid silver, which is evident by the detachment time and detachment speed. With the increase of LJ potential parameters, the detachment time is larger while the contraction speed and the detachment speed are smaller. Compared with the detachment times on different carbon-based substrates, the detachment time is small on the pillared graphene, followed by the vertical carbon nanotube, and the detachment time is large on the graphene. With increasing the thickness of the film, the detachment time becomes larger. The detachment time of the circle film is smaller than those of the regular hexagon film and square film, manifesting that the films with smooth boundary are beneficial to separating from the substrate. Moreover, by setting a system of two liquid films, we study the formation of silver bridge of two films and the fracture or fusion of the bridge. When two liquid films initially contact each other, the liquid bridge forms. However, the growth behaviors of liquid bridges are different from each other, some liquid bridges become slim and finally fractures, other liquid bridges do not fracture and help two droplets form one bigger drop. These different behaviors mainly depend on the size of film. This study is very valuable for well understanding the superhydrophobic surfaces and the morphological evolutions of Ag films on the graphene. Furthermore, these findings can provide an effective method to control the dewetting behavior of liquid Ag and the fracture or fusion of the two liquid drops by tuning the size of the films.

Acta Physica Sinica
Accepts
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The Effect of Collision Parameter on a Magnetized Electronegative Plasma Sheath Structure

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Accept: 2016-10-11
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The structure of an electronegative plasma sheath in an oblique magnetic field is investigated. More over, the collisions between positive ions and neutral particles are taken into account. It is assumed that the system consists of hot electrons, hot negative ions and cold positive ions. Also the negative ions and the electrons are assumed to be described by the Boltzmann distributions of their own temperatures, and the accelerated positive ions are treated by means of the continuity and momentum balance equations through the sheath region. In addition, the assumption that the collision cross section has a power law dependence on the positive velocity is introduced. After theoretical derivation, an exact of sheath criterion is obtained. The numerical simulation results include the distributions of the positive ions density for different invariable ion Mach number satisfying Bohm criterion, the comparison of net space charge distributions for variable and invariable ion Mach number. Furthermore, three species of charged particles density, the net space charge and the spatial electric potential in the sheath are studied numerically for different collision parameters under the condition of the fixed ion Mach number. The results show that the ion Mach number has not only the lower limit but also the upper limit. The ion Mach number affects the sheath structure by influencing the distribution of the positive ion density, and different conclusions can be obtained because ion Mach number is adopted as variable or invariable value while discussing the effects of the other variables which can result in the variety of the ion Mach number on the sheath formation. The reason is the actual sheath structure modification brought on by the variation of a parameter can be resolved into two parts. One is the sheath formation change caused directly by the variation of the parameter, the other is the sheath formation change caused by the Bohm criterion modification which the variation of the parameter results in. Therefore, an identical ion Mach number should be adopted when researching the direct effects of a parameter variety on plasma sheath structure. In addition, it is concluded that the collisions between positive ions and neutral particles make positive ions density curve higher and electrons’ lower than the case without collisions. Negative ions density does not alter significantly whether there exists collision or not. Besides there is a peak in the profile of the net space charge while in the presence of ion-neutral collision and the net space charge peak moves toward the sheath edge. The spatial potential increases and the sheath thickness decreases on account of the presence of the collisions between ions and neutral particles.
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Calculation of Hamilton energy function of dynamical systems by using Helmholtz theorem

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Accept: 2016-10-11
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The Helmholtz theorem confirmed that any vector field could be decomposed of gradient and rotational field. The supply and transmission of energy occur during the propagation of electromagnetic wave accompanied by variation of electromagnetic field, thus the dynamical oscillators and neurons can absorb and release energy in presence of complex electromagnetic condition. Indeed, the energy in nonlinear circuit is often time-varying when the capacitor is in charged or discharged, and occurrence of electromagnetic induction is available. Those nonlinear oscillating circuits can be mapped into dynamical systems by using scale transformation. Based on mean field theory, the energy exchange and transmission between electronic field and magnetic field could be estimated by appropriate nonlinear dynamical equations for oscillating circuits. In this paper, it investigates the calculation of Hamilton energy for a class of dimensionless dynamical systems based on Helmholtz’s theorem. Furthermore, scale transformation could be used to develop dynamical equations from the realistic nonlinear oscillating circuit, so the Hamilton energy function could be approached effectively. These results could be much useful for self-adaptive control of dynamical systems.
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Ballistic thermal rectification in the three-terminal graphene nanojunction with asymmetric connection angles

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Accept: 2016-10-11
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By using the nonequilibrium Green’s function method, the ballistic thermal rectification in the three-terminal graphene nanojunction is studied. The dynamics of atoms are described by the interatomic fourth-nearest neighbor force-constant model. The nanojunction has a Y-shaped structure, created by a combination of a straight graphene nanoribbon and a leaning branch as the control terminal holding a fixed temperature. No heat flux flows through the control terminal. There exists a temperature bias between the two ends of the graphene nanoribbon served as the left and right terminals, respectively. The primary goal of this paper is to demonstrate that the ballistic thermal rectification can be introduced by the asymmetric structure with different connection angles between terminals. The control terminal has a smaller connection angle with respect to the left terminal than to the right terminal. The forward direction is defined as being from the left terminal to the right terminal. The results demonstrate that, given the same control temperature and absolute temperature bias, the heat flux in the graphene nanoribbon tends to run preferentially along the forward direction. When the difference between the connection angles increases, the rectification ratio rises. Compared to the zigzag graphene nanoribbon, the rectification ratio of the armchair nanoribbon is more sensitive to the direction the control terminal. However, the greatest rectification ratio is found in the zigzag graphene nanoribbon which has a connection angle of 30 degrees with respect to the armchair branch. In addition, the direction of the control terminal can be adjusted to raise more than 50% of the rectification ratio of the graphene thermal recti?er based on the width discrepancy between the left and right terminals. The mechanism of the ballistic thermal recti?cation is also discussed. In the three-terminal graphene nanojunction, a smaller connection angle with respect to the control terminal leads to more phonon scattering. The confirmation of this conclusion comes from a comparison of phonon transmission between different couples of terminals, which shows that, in most of the frequency spectrum, the phonon transmission between the control terminal and the left terminal is smaller than that between the control terminal and the right terminal. Given the same control terminal temperature and temperature bias, the asymmetric connection angles therefore will introduce a higher average temperature of the left and right terminals, and a larger heat flux in the forward process. Moreover, the average temperature difference between in the forward process and in the reverse process is found to be proportional to the temperature bias, and the proportionality coefficient will get bigger if the asymmetry is strengthened.
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The Propagation Properties of Vortex Beams in a Ring Photonic Crystal Fiber

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Accept: 2016-10-11
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In the last decade, the vortex beams have received lots of attention for their orbital angular momentum. When they are applied to optical fiber communication field, the data channels will increase and information propagation speed will be effectively improved. Recently, researchers have shown the capability of long length stably propagation, nonlinear frequency conversion and mode division multiplexing of vortex modes in a ring fiber. Due to the photonic crystal fiber (PCF) has very flexible design degrees of freedom, it will enable a wide range of propagation properties. In this paper, A SiO2 air-holes ring PCF is proposed for separation and propagation of optical vortex modes. By using COMSOL Multiphysics software, the vortex modes(TE01, HE_21^± and TM01) are simulated and calculated. The differences of the effective refractive index between them are 4.59×〖10〗^(-4) and 3.62×〖10〗^(-4) respectively. One can analyze the propagation properties of vortex beams in the ring PCF by changing the size of first layer air holes’ radius and air hole pitch. When the incident light wavelength of TE01 mode ranges from 1650 nm to 1950 nm, this ring PCF can achieve a total dispersion variation between 44.18 to 45.83 ps?nm^(-1)?km^(-1), which is tend to be flat. When incident light wavelength is 1550 nm, the nonlinear coefficient of TE01 mode vortex light is 1.37 W^(-1)?km^(-1); Due to the long wavelength light is easier to leakage through the cladding than the short wavelength light, the confinement loss increases with the wavelength. When incident light wavelength is 2000 nm, there is still an eight-orders-of-magnitude of the low confinement loss. Theoretically, flat dispersion and low loss vortex beams in this fiber can be beneficial to propagate stably, and the vortex modes lay the foundation for long distance propagation in the optical fiber. In the future, this ring PCF will be used in optical fiber communication field and application in aspects such as continuous spectrum research, which can make it have immense advantage to traditional fibers.
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Penta-decomposition of instantaneous field in spanwise-rotating turbulent plane Couette flow

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Accept: 2016-10-11
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Spanwise-rotating turbulent plane Couette flow (RPCF) is one of the fundamental prototypes for wall-bounded turbulent flows in the rotating reference frames. In this turbulent problem, there are large-scale roll cells, which are widely studied. In this paper, a penta-decomposition method is proposed to separate the instantaneous velocity and the total kinetic energy into five parts, including a mean part, a streamwise part and a cross-flow part of the secondary flow, and a streamwise part and a cross-flow part of the residual field, aimed to explore the energy balance and transfer among different shares of the turbulent kinetic energy in RPCF at Reynolds number Rew=Uwh/ν=1300 (here, Uw is the half the wall velocity difference, and h is half channel-height) and rotation number Ro=2Ωzh/Uw (Ωz is the constant angular velocity in the spanwise direction) in the range of 0≤Ro≤0.9. The results show that the energy is transferred between streamwise part (cross-flow part) of secondary flows and residual field through the correlation between the vorticity of secondary flows and shear stress of residual field. The rotation term acts as a bridge to transfer the energy between streamwise part and cross-flow part of secondary flows (residual field). Moreover, pressure-strain redistribution term also plays an important role in the energy transfer between streamwise part and cross-flow part in residual field. For the streamwise part of residual field, in certain rotate rates, the energy obtained from the streamwise part of secondary flows is larger than that got from mean flow, implying that the streamwise motions of secondary flows have a significant impact on the streamwise motions of residual field.
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A fast particle simulation method for calculating the multipactor threshold based on the frequency domain solutions in microwave devices

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Accept: 2016-10-11
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In order to compute the multipactor thresholds of microwave devices with high ef?ciency and precision, a novel fast particle-in-cell (PIC) method is proposed, which takes advantages of the frequency-domain (FD) electromagnetic field solver of CST Microwave Studio (MWS). At the initial stage of multipactor (when there are not many electrons in the devices), the self-consistent field generated by the electrons is much smaller than the applied electromagnetic field. Therefore it can be ignored in calculating the multipactor threshold and this will significantly reduce the computation burden. During simulations of multipactor processes, the FD fields pre-calculated by CST MWS are converted into time-domain (TD) scaling with the square root of the input power. Then the electrons are advanced by Boris algorithm. When the electrons hit the boundaries of the simulation region, where triangular facets from CST are used for discretization, the secondary electrons would be emitted. After series of simulations with variable input powers, the multipactor threshold is determined according to time evolutions of the electron number. As verifications, the multipactor thresholds in a parallel plate and a coaxial transmission line are investigated. Compared with the results of CST Particle Studio (PS), the fast method obtains almost the same thresholds, while the computational efficiency is improved more than 1 order of magnitude. Since the self-consistent field generated by the electrons is ignored in the fast method and it is considered in CST PS, the results validate that the self-consistent field can be ignored in calculating the multipactor threshold. Finally, taking a parallel plate transmission line and a stepped impedance transformer as examples, we studied the effects of the number of initial macro-particles on the calculation precision. When the initial particles are so few that it can hardly reflect the randomness of the multipactor process, it results in a higher calculated value. With the increase of the number of initial macro-particles, the calculated multipactor threshold is lower and more accurate. It is convergent when the number reaches about 2000 for the parallel plate transmission line and 4000 for the stepped impedance transformer, respectively. Taking into account other microwave devices with more complex electromagnetic field distribution, in order to ensure precision, it is recommended to select the number of initial macro-particles 8000. In addition, although CST MWS was used to obtain the electromagnetic fields and boundary information in this paper, of course, other electromagnetic software (such as HFSS) can also be adopted as an alternation.
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The effect of linear bubble vibration on wave propagation in unsaturated porous media containing air bubbles

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Accept: 2016-10-11
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Biot model is widely applied in geophysics, petroleum engineering, civil engineering and ocean engineering since it has been presented. This leads to a considerable development of the research on the wave propagation in saturated porous medium. However, fully saturated porous medium is rarely found in nature, almost all the rocks or soils contain two kinds of fluid, such as gas and petroleum. So many researches has been done on the wave propagation in unsaturated porous medium by domestic and abroad scholars. It is well known that the presence of a small volume of gas bubbles in a liquid can greatly alter the velocity and attenuation of acoustic waves in the liquid. Evidence is beginning to accumulate that the velocity and attenuation of acoustic waves in a saturated marine sediment can be affected by the presence of gas bubbles in the saturating liquid. To investigate the sound propagation in porous media when the pore water contains a small amount of air bubbles, this paper integrates the volume vibration of bubbles in pore water into the continuity equation of pore-fluid filtration in porous media based on Biot theory, so as to obtain the continuity equation of pore-fluid filtration with bubble volume vibration. On this basis, according to the relationship between the instantaneous radius of bubbles and the background pressure of the medium under the linear vibration of bubbles, as well as the equations of motion of the fluid medium and porous medium, a new displacement vector wave equation of porous media under the influence of bubbles is derived, which establishes the model for the sound velocity dispersion and attenuation prediction under the unsaturated porous media. The presence of air bubbles increases the compressibility of pore fluid, which leads to the decrease in the sound velocity of the bubbly saturated porous media. When the wave frequency equals to the resonance frequency of the bubbles, the bubbles in pore water will produce resonance; the medium will present to be highly dispersive and the velocity can greatly exceed the gas-free velocity, but these have not been measured in field data; and the absorption cross section of the air bubble can reach the maximum, which leads to the maximum attenuation of the porous media. It should be noted that the attenuation coefficient calculated with this model is related to the damping of bubble motion(radiation, thermal and internal friction) and the dissipation of the relative motion between the pore water and porous solid frame. The obtained numerical analysis is consistent with the above conclusions, which indicates that the volume concentration, the bubble size and the excitation frequency of sound field are important parameters affecting the sound wave propagation in the saturated porous media containing few bubbles.
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Ferroelectric phase transition of perovskite SnTiO3 based on first principles

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Accept: 2016-10-11
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Due to their spontaneous polarization, ferroelectric materials have excellent dielectric, piezoelectric, pyroelectric and other properties, which enable them to be used in many applications, such as capacitors, filters, sensors, detectors, and transducers, among others. In this paper, we employ a first-principles-based effective Hamiltonian method to investigate perovskite SnTiO$_3$, obtaining essential coefficients for the effective Hamiltonian via ab initio computations, which are used in subsequent Monte-Carlo simulations to predict the phase transition temperature of SnTiO$_3$, and different structural phases involved in such phase transition.
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Nonlocal Symmetries and Interaction Solutions of the (2+1)-dimensional Higher Order Broer-Kaup System

xiangpeng xin Hanze Liu Xi-qiang LIU
Accept: 2016-10-11
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The (2+1)-dimensional higher-order Broer-Kaup (HBK) system is studied by nonlocal symmetry method and consistent tanh expansion (CTE) method. In this paper, via the localization of the residual symmetries, the nonlocal symmetries are localized to Lie point symmetries and symmetry groups are also obtained. Many types of soliton solutions and interaction solutions among different nonlinear excitations such as solitons, periodic waves etc. are constructed. In order to study their dynamic behaviors, corresponding images are explicitly given.
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Development of a intranuclear-cascade code CBIM applicable to the nuclear reaction with incident particle energy above 45MeV

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Accept: 2016-10-11
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The Monte Carlo intra-nuclear cascade program CBIM has been developed for describing nuclear reactions involving protons, neutrons and pions on complex nuclei. In order to describe cascade process, several simplifications have been made in the following: firstly, neither reaction, reflection, refraction, nor ionization will be taken into account before the incident particle enters the target nucleus; secondly, target nucleus is regarded as spherical and the atom number should be greater than 2; thirdly, the knocked nucleon is determined by cross section sampling; last, in the center-of-mass frame, the scattering angle is sampled based on differential cross section distribution.. The basis physics model bases on the above assumptions and Bertini intra-nuclear cascade model; meanwhile, nucleon-nucleon angle differential distributions of INCL in the center-of-mass frame have been introduced to overcome the shortage of Bertini model. The interactions between nucleon and nucleon or between nucleon and pion, for example, elastic scattering, pion production and charge exchange, are simulated in the code. In the particles collision, the nucleon density changes with the target nucleus radius; and the interaction cross sections refer to 22 kinds of experimental cross sections in Bertini model. The intra-nuclear cascades induced by 45MeV~3500MeV neutron, proton or pion below 2500MeV can be simulated by this code. Finally, comparisons with experiment on reaction cross section over the energy range 60~378MeV, and some simulation results by MCNPX, GEANT4 and PHITS over the energy range 65~3000MeV, the CBIM results are in reasonable agreement with them over the broad energy range considered.
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Omnidirectional photonic bandgap of the one-dimensional plasma photonic crystal based on a novel Fibonacci quasiperiodic structure

Accept: 2016-10-11
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Take the binary one-dimensional plasma photonic crystal based on Fibonacci quasiperiodic structure as an object, on the basis of the photonic bandgap characteristics of the structure with different initial sequence and number of period, a novel structure of one-dimensional plasma photonic crystal is proposed in this paper to enlarge the omnidirectional photonic bandgap (OPBG). Compared with previously reported structures in literatures, this structure is simpler in configuration with fewer layers and materials, and its OPBG width is wider. The influence of the parameters of the plasma material, such as the thickness, plasma frequency and collision frequency, on the OPBG characteristics of this structure is systematically discussed and compared with that of the structure in literatures. The research results can provide important theoretical guidance for the design of novel omnidirectional reflectors.
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Quantum secure direct communication protocol based on the mixture of Bell state particles and single photons

Zheng-Wen CAO
Accept: 2016-10-11
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By studying the properties of the mixture of Bell state particles and single photons, the paper designs a quantum code scheme with high coding capacity, and proposes a novel quantum secure direct communication protocol with high transmission efficiency. Alice prepares Bell state particles and single photons, and divides Bell state particles into two sequences $S_A$ and $S_B$. $S_B$ is sent to Bob for the first security check using quantum correlation properties of particles. When the check result shows that the quantum channel is safe, using designed quantum code scheme, Alice encodes her classical message on the mixed quantum state sequence of Bell sequence $S_A$ and single photon sequence $S_S$. Then, some single photons that are used for security check are re-inserted randomly into the encoded sequence, and the order of particles is rearranged to ensure to check Eve's attack. Alice sends the new sequence to Bob. Bob delays and receives it. And then, the quantum channel is conducted security check for the second time. The transmission error rate is calculated, if the error rate is lower than the tolerance threshold, the channel is safe. Bob decodes and reads Alice's message. The first security check is to determine whether quantum channel is safe. The second security check could test whether there are eavesdroppers during information transmission. Safety analysis is done by using quantum information theory to the proposed protocol. The error rate introduced by Eve and the amount of information by Eve are calculated. It is showed that this protocol can effectively resist measurement-resend attack, intercept-resend attack, auxiliary particle attack, denial of service attack and Trojan attack. Among them, auxiliary particle attack is analyzed in details. The transmission efficiency and coding capacity are also analyzed. The transmission efficiency is 2, the quantum bit rate is 1, and the coding capacity is that a quantum state can encode three bits of classical messages. We also compare the proposed protocol to many existing popular protocols in terms of efficiency, e.g., Ping-Pong protocol, Deng,F.G. et al.'s Two-step and One-pad-time quantum secure direct communication protocol, Wang,J. et al.'s quantum secure direct communication protocol based on entanglement swapping and Quan,D.X. et al.'s one-way quantum secure direct communication protocol based on single photon. It is proved that this proposed protocol has higher transmission efficiency. In addition, complex U operation and entanglement swapping are not used, and implementation process is simplified. However, this protocol is devoted to theoretical research of quantum secure direct communication. There are still some difficulties in the practical application. For example, the storage technology of quantum states is not mature at present. It is not easy to prepare and measure Bell state particles and combine them with single photons, and so on. The implementation of this protocol depends on the development of quantum technology in the future.
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The Relationship between Dielectric Properties and Nanoparticle Dispersion of Nano- SiO2/Epoxy Composites

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Accept: 2016-10-11
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Nano-SiO2 was modified by silane coupling agent and modified nano-SiO2 powder and nano-SiO2 dispersing liquid was obtained. Unmodified and modified nano-SiO2/Epoxy composites made by “mechanical mixing method”, and modified namo-Silica/Epoxy composites made by “bubble mixing method” were prepared, respectively. The content of nano-SiO2 in the composite is 2wt%, 3wt%, 4wt%, 5wt% and 6wt%. Breakdown strength and corona-resistance characteristics of the composites were tested. The results show that, with the increase of nano-SiO2 loading, the breakdown strength and corona-resistance of nano-SiO2/Epoxy composites increase. The maximum breakdown strength of namo-Silica/Epoxy composites was appeared when the nano-Silica content is 5wt%. The SEM images of 5wt% nano-Silica loading composites were analyzed by Software Image J, and the Morisita’s Index method was used to evaluate the dispersion of nano-Silica particles in the matrix quantitatively. The best dispersion was found in the composites made by “bubble mixing method”. The relationship between dielectric properties and nano-particle dispersions of nano-Silica/Epoxy composites was discussed.
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Combined noise source identification method based on spherical microphone array with random unifrom distribution of elements

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Accept: 2016-10-11
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As the developing of techlology, noise controlling is paied wide attention in recent years. Noise source identification is the key step for noise controlling. Spherical microphone array, which can located the noise source of arbitrary direction in three dimensional space, is widely used for noise source identification in recent years. Conventional methods for noise source localization include spherical near field acoustic holography and spherical focused beamforming. The acoustic quantities are reconstructed by using spherical near field acoustic holography method to realize the noise source identification, while the noise source can also be located by using focused beamforming based on spherical harmonic wave decomposition. However, both these methods have their own limitations while being used in noise source identification. Spherical near field acoustic holography has low resolution in high frequency with far distance from noise source to measurement array for noise source identification, whereas the spherical focused beamforming has low localization resolution in low frequency. Noise source identification is discussed here and a 64-element microphone spherical array with randomly uniform distribution of elements is designed. The combination methods of noise source identification by using spherical near field acoustic holography and mode decomposition focused beamforming are researched. The performance of the proposed combination methods is simulated, and an experiment of noise source identification is carried out based on the designed spherical microphone array to test the validity of proposed method. The dividing frequency point is when selecting noise source identification methods between near field acoustic holography of spherical wave decomposition by using the spherical array designed in this paper. Research results show that high resolution of noise source identification can be obtained by using near field acoustic holography when reconstruction frequency is with a distance from noise source to the center of spherical array, while high resolution of noise source localization can be achieved by using spherical wave decomposition beamforming when signal’s frequency is with a distance from noise source to the center of spherical array. Spherical array with random uniform distribution of elements maintains stable identification ability in all bearing. Spherical near field acoustic holography has high resolution distinguish ability in near field and low frequency, while focused beamforming method has high resolution distinguish ability in far field and high frequency. Therefore the noise source can be efficiently identified by using the proposed combined method of near field holography and focused beamforming with less elements and small aperture spherical microphone array.
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Optimization design of a Gamma-to-Electron spectrometer for high energy gammas induced by fusion

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Accept: 2016-10-11
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Apart from neutrons, the fusion core produces gamma rays during one fusion reaction. The spectrum of gamma ray can provide very important information for fusion diagnosis. However, due to the gamma energy and yield in one fusion pulse, the gamma spectrometer used should have high detection efficiency and energy resolution. The concept of a Gamma-to-Electron magnetic spectrometer GEMS provides the idea to build up such a spectrometer to meet this requirement. Based on this concept design, four important parts of this facility are investigated. The first part is the gamma-electron converter. The main physics processes include Compton scattering of gamma ray with converter material generating electron, the electron Multiple Coulomb scattering (MCS) inside the converter and the electron attenuation. Affected by the thickness of convector, these processes gives a complex influence on the detection efficiency and angular-energy distribution of the electrons which are emitted from the downstream face of the convector. The Monte Carlo code Geant4 is employed to investigated the functions of Compton scattering, MCS and converter thick on the angular-energy distribution. The second one is the collimation. The collimation is used to select the forward direction election, the performance of cutoff angle of the collimator on the detection efficiency and resolutions, as well as the correlation between electron transportation direction and energy, are also studied using Geant4 code. The third part is the dipole magnetic field. There are several parameters of geometric and magnetic, therefore, a multi-thread parallelized Genetic algorithm is developed to get the best result. Both the irregular geometry (shape) and dipole magnetic field strength are optimized to achieve the best energy resolution and detection efficiency. The obtained magnetic field has intensity less than 100 Gauss, and its performance on gathering elections is also verified by Geant4 code. The last one is the location of electron detectors. The study shows that all the electron detectors should be located according to not a straight line but a quadratic curve. Then the optimized spectrometer is simulated by Geant4 to get the responses of gamma rays with various energies. For the gammas provided by fusion reaction, the simulation shows that when the neutron yield is about 2.5×1015 and 1.2×1016, the energy resolution reaches 0.5 MeV and 0.25 MeV, respectively, provided that different thick Be converters are employed. All in all, this optimized GEMS can be employed to measure the spectrum of gamma rays generated by the fusion reaction.
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Influnence of Nonspherical Effects on the Secondary Bjerknes Force in a Strong Acoustic Field

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Accept: 2016-10-11
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The secondary Bjerknes force between bubbles in an acoustic field is a well-known acoustic phenomenon. The major theoretically researches of the secondary Bjerknes force were owing to two spherical bubbles. The secondary Bjerknes force between two spherical bubbles which is calculated based on the linear equations is very small and negligible, therefore these theoretically researches did not give a well explanation for the phenomenon, such as “streamer formation” and Multi-bubble sonoluminescence (MBSL). Experiments of sonoluminescence (SL) show that bubbles in a sound field are not entirely spherical bubbles. Nonspherical effects have an important influence on the secondary Bjerknes force when two bubbles come close to each other in a strong acoustic field (>1.0×〖10〗^5 Pa). How does the shape distortion of a nonspherical bubble cause the change of the secondary Bjerknes force between two bubbles, and the secondary Bjerknes force how to affect the oscillation and movement of bubbles are major problems which we wish to solve. The of the secondary Bjerknes force between a nonspherical bubble and a spherical bubble is obtained by considering the shape oscillation of a nonspherical bubble. We numerical simulate the secondary Bjerknes force between a nonspherical bubble and a spherical bubble based on the nonlinear oscillation equations of two bubbles, and compare the secondary Bjerknes force between a nonspherical bubble and a spherical bubble to the secondary Bjerknes force between two spherical bubbles in the same condition. We discuss the influence of nonspherical effects on the secondary Bjerknes force between two bubbles. The results show that when the amplitude of driving pressure is greater than the Blake threshold of a nonspherical bubble and makes the bubble oscillate stably, the secondary Bjerknes force between this nonspherical bubble and a spherical bubble is different to the secondary Bjerknes force between two spherical bubbles in direction and magnitude. The secondary Bjerknes force between a nonspherical bubble and a spherical bubble is much bigger than that of two spherical bubbles. The interactional distance of the secondary Bjerknes force between a nonspherical bubble and a spherical bubble is further than that of two spherical bubbles. The secondary Bjerknes force between a spherical bubble and a nonspherical bubble depends on the radii of two bubbles, distance between two bubbles, shape mode of the nonspherical bubble and the amplitude of driving pressure. Our research is more close to the actual bubbles in liquid. We also prove that big mutual interaction between bubbles is mainly cause for the formation of a stable structure between bubbles. For bubbles, big mutual interaction causes the cavitation become easier. These results are important to explain the phenomenon in an acoustic field, such as “streamer formation” and Multi-bubble sonoluminescence (MBSL).
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The Principle and Application of Diagonal Reducing Method in the Complex Noise Fields

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Accept: 2016-10-11
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Acoustic environment has low signal-to-noise ratio (SNR); hence, array signal processing is always used for noise reduction and signal enhancement. Because the delay-and-sum beamforming method performs robust, so it is almost widely used, but the array gain is limited by the array aperture. The actual underwater ambient noise is complex, which includes uncorrelated noise and correlated noise. The noise power of each array element is unequal. The noise covariance matrix is not a scaled identity matrix. Consequently, the performance of array signal processing method decreases obviously. Aiming at these two problems, the diagonal reducing method of the covariance matrix in the complex noise fields is proposed. Firstly, a reducing matrix, which is defined as a diagonal matrix with unequal diagonal elements, is subtracted from the covariance matrix so as to reduce the noise, and a new matrix is obtained. Secondly, the delay-and-sum beamforming is done by using the new matrix to obtain the beaming output. The analytic solution and approximate solution of reducing matrix are obtained under the constraint condition that the output SNR attains its maximum. Thirdly, the estimation of the reducing matrix is determined by minimizing the function that is defined as the error between the covariance matrix and the estimated covariance matrix. This minimization problem is accomplished in an iterative method. Fourthly, if the noise is uniform white noise or the nonuniform white noise, this proposed method performs well. While, under the complex noise field the performance of the proposed method may be deteriorated. So the effects of the correlation of the noise field and the input SNR on the estimated error is analyzed. In fact, the weaker the correlation is, or the smaller the input SNR is, the smaller the estimated error is. Lastly, the simulation experiment and the lake trial are implemented. The simulation results show that the diagonal reducing method of the covariance matrix reduces some ambient noise, the noise output power is decreased, the output SNR is increased, and the proposed method improves performance of array signal processing. The experimental results show that the output SNR of the target using the proposed method is increased by about 14 dB. The diagonal reducing method of covariance matrix has definite value to engineering application, and is computationally attractive.
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Moving target compressive imaging based on improved row scanning measurement matrices

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Accept: 2016-10-11
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Abstract: Moving target imaging (MTI) plays an important role in practical applications. How to capture dynamic images of the targets with high quality is a front-burner issue in the field of MTI. In order to improve the reconstruction quality, a new MTI model based on compressed sensing (CS) is proposed here, applying a sampling protocol of the row-scanning together with a motion measurement matrix constructed by our own. It is proved by the simulation and the experimental results that a relatively higher quality can be achieved through this approach. Furthermore, an evaluation criterion of reconstructed images is introduced to analyze the relationship between the imaging quality and the moving speed of the target. By contrast, the performance of our algorithm is much better than that of traditional CS algorithms under the same moving speed condition. As a result, it suggests that our imaging method may have a great application prospect in the earth observation of unmanned aerial vehicles, video monitoring in the product line and other fields.
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Spatial Correlation of Underwater Bubble clouds Based on Acoustic Scattering

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Accept: 2016-10-11
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Using effective medium theory to describe acoustic scattering from bubble clouds, one of the underlying assumptions shows that the probability of an individual bubble being located at some position in space is independent of the locations of other bubbles. However, bubbles within naturally occurring clouds are usually influenced by the motion of the fluids which makes they become preferentially concentrated or clustered. According to Weber’s method, it is a useful way to importing spatial correlation function to describe this phenomenon in bubble clouds. The spatial correlation function is contained in acoustic scattering and it is important to notice that the spatial correlation should be dependent of the position and radius of each bubble due to the ‘‘hole correction’’ or the effect of the dynamics of the fluids. Because of these reasons, it is hard to invert the spatial distribution of bubble clouds using spatial correlation function in acoustic scattering. A method is described here in which bubble clouds are separated into many small subareas and the conception called effective spatial correlation function which is the statistic of spatial correlation function used to describe the correlation between each subarea of bubble clouds. Since the effective spatial correlation function is independent of bubbles’ radius and positions, the bubble clouds’ distribution and the trend of clustering can be inverted by using this function. The result of simulation indicates that the effective spatial correlation function can precisely track the position of the clustering center, even the clustering center covered by other bubble clouds can be detected. Using multi-bean sonar measuring the bubbly ship wake generated by a small trial vessel, the method is used to invert the spatial distribution and clustering centers of bubble field in the ship wake. The results show that effective spatial correlation function accurately inverts the distribution and clustering centers of bubbles in ship wake. Furthermore, the method presented in this paper could distinguish the bubble clouds caused by different reasons and detect upper ocean bubble clouds covered by other bubbles generated by wave breaking.
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Uncertainty Quantification in the Calculation of keff Using Sensitity and Stochastic Sampling method

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Accept: 2016-10-11
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In the neutronics simulation of nuclear reactor, the uncertainties associated to the integral parameters due to the uncertainties in nuclear data are usually quantified using the sensitivity and uncertainty (S/U) analysis method based on the perturbation theory. S/U analysis method is only applicable to the linear model, moreover neutronics code generally can not be directly used in sensitivity analysis. Sampling approach, which evaluating the uncertainties by performing a set of stochastic simulations, is easy to implement and the uncertainties quantified is close to exact. The function of uncertainty quantification based on sampling approach have been added to uncertainty analysis code SURE. Before applying the sampling method to the uncertainty quantification in the simulation of complex problems, it is necessary to carry out a careful verification. The uncertainties of the calculated effective neutron multiplication factor keff for two selected simple critical benchmark experimental model are quantified using SU method and sampling method respectively. The keff uncertainties due to all nuclides and reaction types nuclear data quantified by two methods are in good agreement, and the correctness of the sampling function of SURE code is verified. The keffs distributions from sampling method obey normal distribution, which embodies a linear relation between input nuclear data and output keff in the range of the uncertainty range of nuclear data, and sensitivity analysis method is adaptable to quantify uncertainty of calculated keff.
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A super-resolution infrared microscopy based on a doughnut pump beam

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Accept: 2016-10-11
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This paper presents an approach to break through the diffraction limitation in infrared microscopies. In this method, instead of Gaussian pump beam, an intensive vortex beam is firstly focused on the sample, leading to saturation absorption of the peripheral molecules in the point spread function (PSF). The vortex beam is followed by a Gaussian beam with the same wavelength, which can only be absorbed by the molecules near the center, resulting in shrunken PSF which means higher resolution. Furthermore, the PSF of a system based on this approach is numerically simulated. With an 100 nJ pulse energy vortex beam and a 0.1 nJ pulse energy probe beam, the theoretical resolution (full width at half maximum, FWHM) is measured to be about 236 nm which is 14 times better than that of the traditional infrared microscopy.
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Fast Bayesian Blind Restoration for Single Defocus Image with Iterative Joint Bilateral Filters

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Accept: 2016-10-11
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It is significant to realize effective defocus image restoration for acquiring clear image in military and geological examination field. Most of existing algorithms have the problems of large computational cost, ringing and noise sensitivity, hence a novel approach by iterative joint bilateral filtering under Bayesian framework is proposed. Firstly, it utilizes defocus image depth estimation to compute the point spread function in the Bayesian framework. Then a minimum optimization problem is built to represent the blind restoration problem. After inferring the solution procedure of the minimum optimization problem, we find that the joint bilateral filters can be used to search the optimal solution, which not only simplify the searching procedure but also reduce the computational cost. Finally, an iterative joint bilateral filtering was designed to realize the image restoration. That means the original restored image obtained from the bilateral filtering is used to design the guide image for the joint bilateral filters, and the guide image will serve as the input of the optimization problem for acquiring the better optimal result. This procedure was repeated until convergence. The experiment results indicate that this method can yield the ringing, reduce the computational cost and remove the noise. Generally speaking, the average pixel error of 85% images is under 0.03, which has improved 19% comparing with the same error rang of existing algorithms. And 78% shorter than those of compared algorithms. It can be used in the engineering practice of blind restoration for single defocus image.
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First-principles study on the thermodynamic stabilities and electronic structures of long-period stacking ordered phases in the Mg-Y-Cu alloys

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Accept: 2016-10-11
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A first-principles method based on density functional theory has been used to investigate thermodynamic stability and electronic characteristics of long-period stacking ordered (LPSO) phases 14H and 18R (18R(m),18R(t)) in Mg–Y–Cu alloys. The present calculations are performed using Vienna Ab-initio Simulation Package (VASP) with projector augmented plane wave pseudopotential, and generalized gradient approximation is used to treat with and describe the exchange-correlation interaction. The plane wave cutoff energy is set to 360 eV, the forces on all the atoms is less than 0.02 eV/?. The calculated negative enthalpies of formation show that both 14H and 18R can exist in Mg–Y–Cu system, 14H and 18R are stable with respect to the Mg, Cu and Y elements, the reaction energies indicate that 14H is more stable than 18R. The density of states (DOS) of these phases reveals that the main bonding peaks of 14H is located at energy range between -6.82 eV and 2.09 eV, those of 18R(m) at energy range between -6.82 eV and 2.02 eV, and 18R(t) at energy range between -6.82 eV and 1.98 eV. The Cu 3d orbits, Y 4d orbits, Mg 3s and Mg 2p orbits are broadly distributed in the entire region, while Cu 4s orbits, Y 4s and Y 4p orbits are very weak in whole region. For 14H,18R(m) and 18R(t) phases, the bonding originates mainly from the valence electrons of Mg 3s, Mg2p, Cu 3d and Y 4d orbits. The presence of pseudogap indicates that the bonds in 14H and 18R phases are noticeable covalent. In addition, the charge density on (0 0 0 1) plane of 14H and 18R phases are analyzed, and the results indicate that the Cu-Y bonds exhibits covalent feature in 14H and 18R, the covalent bonding of 14H phase is stronger than that of 18R phase.
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A broadband low-frequency sound insulation structure based on two-dimensionally inbuilt Helmholtz resonators

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Accept: 2016-10-11
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A man-made acoustic structure with broadband low-frequency sound insulation property is designed based on circularly inbuilt Helmholtz resonators. Beyond this structure, a two-dimensional quiet zone can be created. Being the same as the simulating model, an experimental structure is fabricated. Experiments are carried out to study its sound insulation properties. The experimental results are very coincident with the simulating one, which show that this structure has an excellent sound insulation effect in the frequency band of 680-1050Hz, and the maximum insulation sound pressure level can reach 41dB. Meanwhile, the distribution of the two-dimensional sound field above this structure is measured. The results point out that the range of the insulation area can be changed with the change of the incident frequency. In addition, the sound insulation effect is sensitive to the resonant state of the Helmholtz resonators. This work will be of help for designing new sound protection devices.
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Total Dose Dependence of Hot Carrier Injection Effect in the NMOS Devices

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Accept: 2016-10-11
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The equipments and devices which were long-time running in space were affected by space radiation effects and hot carrier injection effects at the same time which would reduce their optional times. Normally, the single mechanism test simulation method was used on the ground simulation test but the multi-mechanism effects was affected the space equipments and devices, included total irradiation dose effect, hot carrier injection effect, and so on. The total dose dependence of hot carrier injection (HCI) effect in the 0.35μm NMOS Devices was studied in this paper. Three samples were test with different conditions (sample 1# with TID and HCI test, sample 2# with TID, annealing and HCI test, sample 3# only with HCI test). The results shows that threshold voltage of NMOS devices with 5000s HCI test after 100krad (Si) total dose radiation shift negatively then positively during total dose irradiation test and HCI test,and it was more than the devices without radiation test. But the threshold voltage shift of NMOS devices with 5000s HCI test and 200hours annealing test after TID test was more than the devices without radiation test and lower than the devices without annealing test. That was, the parameters of NMOS device varied faster with the association effects (included total dose irradiation effect and HCI effect) than with single mechanism effect. It was indicated that the hot electrons were trapped by the oxide trap charges induced by irradiation effect and then became recombination centre. And then the oxide trap charges induced by irradiation effect reduced and became to negative electronic. The interface trap charges induced by irradiation effect were reduced and then increased and it was because that the electrons of hole-electron pairs in the Si-SiO2 interface were recombined by oxide traps in the oxide during the forepart of HCI test but then the electrons were trapped by interface traps in the Si-SiO2 interface because the electrons from source area were injected to interface during the HCI test. So the threshold voltage shift was positively due to the negative oxide trap charges and interface trap charges. The association effect was attributed to the reduction of oxide traps induced by recombination with hot electrons and the increase of the interface traps induced by irradiating trapped with hot electrons.
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Optoelectronic properties of N/B doped graphene

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Accept: 2016-10-11
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Since its discovery in 2004, graphene has attracted great attention because of its unique chemical bonding structure, which has excellent chemical, thermal, mechanical, electrical and optical properties. Due to the zero band gap material, graphene has limited its development in the field of Nano Electronics. Only expanding the band gap of the graphene can promote the application of graphene in Nano Electronics. In this paper, we constructed three models of intrinsic graphene, N-doped graphene and B-doped graphene. The energy band structures, electronic density of states and optical properties of N/B doped graphene with intrinsic graphene and different doping concentrations were studied. The absorption spectra, the reflection spectra, the refractive index, the conductivity and the dielectric function were studied. The study shows that the electronic states near the Fermi level of N/B doped graphene are mainly composed of C-2p and N-2p/B-2p orbitals, and N/B doping can induce the change of the Fermi level and the photoelectric properties of graphene. The conclusion of this paper can provide a theoretical basis for the application of graphene in optoelectronic devices.
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The study of the dynamic of the slow electrons transmitted through straight glass capillary and tapered glass capillary

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Accept: 2016-08-18
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It was found that the transmission rate of the electrons through insulating capillaries as a function of the time/incident charge is not the same as that for the ions. The question arouse that, by using the electrons, if the negative charge patches can be formed to facilitate the transmission of the followed electrons, substantiating that the so-called guiding effect works also for electrons. This study aims to observe the time evolution of the transmission of electrons through a straight glass tube and a tapered glass capillary. This would reveal the details that how and/or if the negative charge patches can be formed when the electron are being transported through them. In this work, a set of MCP/phosphor two-dimensional detection system based on Labview platform was developed to obtain the time evolution of the angular distribution of the transmitted electrons. The pulsed electron beams through a small hole with the diameter of 0.5 mm was obtained to test our detection system. The time evolution of the angular profile of 1.5 keV electrons transmitted through the glass tube/capillary was observed. The transmitted electrons are observed on the detector for a very short time and disappear for a time and then back again for both the glass tube and tapered glass capillary, leading to an oscillation. The positive charge patches are formed in the insulating glass tube and tapered glass capillary since the secondary electron emission coefficient for the incident energy is larger than 1. It is due to the fast discharge of the deposited charge, leading to an increase of the transmission rate, while the fast blocking of the incident electrons due to the deposited positive charge, leads to a decrease of the transmission rate. The geometrical configuration of the taper glass capillary tends to make the secondary electrons deposited at the exit part to form the negative patches that facilitate transmission of electrons, similar to the guiding of positive charged ions. This suggests that if the stable transmission needs to be reached for the production of the electron micro-beam by using tapered glass capillaries, the steps has to be taken to have the proper grounding and shielding of the glass capillaries and tubes. Our results show a difference for electrons in transmission through the insulating capillary from that of highly charged ions.
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Accept: 2016-08-18
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internuclear-distance-dependent ionization of H$_2^+$ in strong laser fields in a classical perspective

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Accept: 2016-08-18
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The enhanced ionization of H$_2^+$ in strong laser fields is studied by numerically simulating the classical Hamiltonian equation with the fix-nuclei approximation. The classical trajectory of the electron shows the electron gains energy from the laser field by circulating one electron, then passes through the interatomic barrier and move around the other nucleus before ionization. The ionization probability is maximum when the energy difference between the ground state and the the higher value of the interatomic barrier and outatomic Coulomb barrier is minimum. The classical calculation offers a perspective to inspect the intriguing phenomena in quantum systems.
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Partition and growth of convection patterns in Poiseuille-Rayleigh-Benard flow

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Accept: 2016-08-18
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In this paper, the Simple algorithm is used to numerically simulate the two-dimensional fully hydrodynamic equations. Partition of convection pattern , growth and the effect of horizontal flow on the characteristical parameters of different patterns in Poiseuille-Rayleigh-Benard flow are studied. The result indicated that flow zone is divided into three zones by the upper and lower critical Reynolds numbers , such as traveling wave zone, localized traveling wave zone, horizontal flow zone.and increase with reduced Rayleigh number. In the growth stage of the convection pattern, the growth process of three kinds of patterns with time is different, but the convection rolls grow all from downstream; Variation of characteristic parameters with time is also different, maximum vertical velocity and Nusselt number of traveling wave and localized traveling wave enter into the stable stage of the cycle variation after the exponential growth stage;and of horizontal flow pattern down to a stable constant after slow growth. and of three types of patterns decrease with increasing Reynold number, there are different rules in the different pattern areas. In this paper, formulas on variation ofandwith and formulas on variation ofandwithin different convection patterns are suggested.
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· Proximity-effect-induced superconductivity by granular Pb film on the surface of Bi2Te3 topological insulator [2013, No.16:167401-167401] (26814)
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