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Quantum error rejection and fault tolerant quantum communication
Deng Fu-Guo, Li Xi-Han, Li Tao
Acta Physica Sinica, 2018, 67 (13): 130301
Overview and outlook of magnetic skyrmions
Liu Yi-Zhou, Zang Jiadong
Acta Physica Sinica, 2018, 67 (13): 131201
Axial multi-particle trapping and real-time direct observation
Wang Yue, Liang Yan-Sheng, Yan Shao-Hui, Cao Zhi-Liang, Cai Ya-Nan, Zhang Yan, Yao Bao-Li, Lei Ming
Acta Physica Sinica, 2018, 67 (13): 138701
Current Issue Accepts In Press Earlier Issues Top Downloaded SCI Top Cited
  Acta Physica Sinica--2018, 67 (13)   Published: 05 July 2018
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INVITED REVIEW

Quantum error rejection and fault tolerant quantum communication Hot!

Deng Fu-Guo, Li Xi-Han, Li Tao
Acta Physica Sinica. 2018, 67 (13): 130301 doi: 10.7498/aps.67.20180598
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Quantum communication utilizes the quantum state as information carrier. The transmission of quantum states is therefore a precondition for various quantum communication protocols. Photons play a central role in quantum communication since they are fast, cheap, easy to control and interact weakly with the environment. However, the widely used polarization degree of freedom of photons is vulnerable to the noise during the transmission. In this article, we review two main methods to deal with the channel noise, i.e., the quantum error rejection scheme and fault tolerant quantum communication. To transmit an arbitrary single-photon state, Li and Deng proposed two faithful state transmission schemes only by resorting to passive linear optics. The success probability can be (2N+1-1)/2N+1 by introducing a wave splitter composed of N unbalance interferometers. Compared with other quantum error rejection schemes, these two scheme are practical both in maneuverability and resource consumption. They are not only suitable for single-photon pure state transmission but also able to be used for transmitting mixed state, which makes them useful for one-way quantum communication. The success probability of error rejection is usually less than 100% since some error cases are rejected. To realize complete fault tolerant quantum communication, decoherence free subspace can be used to encode quantum information. In 2008, Li et al. proposed two efficient quantum key distribution schemes over two different collective-noise channels. The noiseless subspaces are made up of two Bell states and the spatial degree of freedom is introduced to form two nonorthogonal bases. Although entangled states are employed, only single-photon measurements are required to read the information. Later, the scheme is generalized to an efficient one which transmits n-1 bits information via n Einstein-Podolsky-Rosen pairs and many fault tolerant quantum communication schemes were proposed. We compare the practicality of different anti-noise schemes based on maneuverability and resource consumption and a perspective of these two research directions is given in the last section.

SPECIAL TOPIC-Magnetic skyrmions

Nanoscale magnetic field sensing and imaging based on nitrogen-vacancy center in diamond

Wang Cheng-Jie, Shi Fa-Zhan, Wang Peng-Fei, Duan Chang-Kui, Du Jiang-Feng
Acta Physica Sinica. 2018, 67 (13): 130701 doi: 10.7498/aps.67.20180243
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Magnetic field measurement and imaging with nanometer resolution is a key tool in the study of magnetism. There have been several powerful techniques such as superconducting quantum interference device, hall sensor, electron microscopy, magnetic force microscopy and spin polarized scanning tunneling microscopy. However, they either have poor sensitivity or resolution, or need severe environment of cryogenic temperature or vacuum. The nitrogen-vacancy color center (NV center) in diamond, serving as a quantum magnetic sensor, has great advantages such as long decoherence time, atomic size, and ambient working conditions. The NV center consists of a substitutional nitrogen atom and an adjacent vacancy in diamond. Its electronic structure of ground state is a spin triplet. The spin state can be initialized to mS=0 state and read out by laser pulse, and coherently manipulated by microwave pulse. It is sensitive to the magnetic field by measuring the magnetic Zeeman splitting or quantum phase in quantum interferometer strategies. By using dynamical decoupling sequence to prolong the decoherence time, the sensitivities approach to nano tesla for a single NV center and pico tesla for the NV center ensemble, respectively. As a sensor with an atomic size, it reaches single-nuclear-spin sensitivity and sub-nanometer spatial resolution. Combining with scanning microscopy technology, it can accomplish high-sensitivity and high-resolution magnetic field imaging so that the stray field can be reconstructed quantitatively. The magnetic field is calculated from the two resonant frequencies by solving the Hamiltonian of NV center in order to obtain the value of stray field. Recently, this novel magnetic imaging technique has revealed the magnetization structures of many important objects in magnetism research. The polarity and chirality of magnetic vortex core are determined by imaging its stray field; laser induced domain wall hopping is observed quantitatively with a nanoscale resolution; non-linear antimagnetic order is imaged in real space by NV center. It was recently reported that magnetization of the magnetic skyrmion is imaged by NV center. The magnetization distribution is reconstructed from stray field imaging. With the topological number limited to one, the Néel type magnetization is uniquely determined. These results show that the magnetic imaging method has great advantages to resolve the emerging magnetic structure materials. The magnetic imaging technology based on the NV center will potentially become an important method to study magnetic materials under continuous development.

Overview and outlook of magnetic skyrmions Hot!

Liu Yi-Zhou, Zang Jiadong
Acta Physica Sinica. 2018, 67 (13): 131201 doi: 10.7498/aps.67.20180619
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Magnetic skyrmions are topologically protected nano-scale spin textures. They normally exist in chiral magnets and magnetic thin films with broken inversion symmetry. The size of skyrmion ranges from 1 nm to several hundred nanometers, depending on the material parameters. The spins of skyrmion wrap around the unit sphere exactly once, thus facilitating the unit topological charge of a skyrmion. Due to their non-trivial topology, skyrmions exhibit exotic physics such as the topological Hall effect (THE) and the emergent electrodynamics. Skyrmions show particle-like dynamics and can be driven with ultra-low current density. Furthermore, they can be created, annihilated, manipulated and detected by all-electric methods, making skyrmion a promising candidate for next-generation information storage and processing technologies. On the other hand, combining skyrmions with superconductors and topological insulators may also lead to intriguing physics and applications such as the topological quantum computing. Over the past few years, the creation, annihilation and detection of skyrmion at room temperature have already been demonstrated, but the precise control of single skyrmion with size below 10 nm is still a challenge. In this paper, we first review the fundamental physics of skyrmion, from its topology to its emergent dynamics. Physical mechanisms of the Dzyaloshinskii-Moriya interaction, the emergent electrodynamics and the THE are discussed. Then the skyrmion material systems, including chiral magnets, magnetic thin films, artificial skyrmion systems, frustrated magnets, bi-skyrmion materials and antiskyrmion materials, are comprehensively summarized. The optimizations of materials and potential new skyrmion materials are also proposed for different material systems. Methods of creating, annihilating and detecting skyrmions, which also cover potential application methods other than electrical methods, are discussed from both theoretical and experimental point of view. The energy efficiencies and reliabilities of different creation and annihilation methods and the sensitivities of different detection methods are still unclear, these current bottlenecks and possible avenues towards skyrmion-based spintronics are described. Finally, we address some possible future directions of skyrmion research, such as the antiferromagnetic skyrmion and skyrmions in topological insulators, which may lead to the discovery of peculiar topological quantum physics and materials.

Topological Hall effect in ferromagnetic/non-ferromagnetic metals heterojunctions

Meng Kang-Kang, Zhao Xu-Peng, Miao Jun, Xu Xiao-Guang, Zhao Jian-Hua, Jiang Yong
Acta Physica Sinica. 2018, 67 (13): 131202 doi: 10.7498/aps.67.20180369
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In a magnetic system, the spin orbit coupling can combine with the exchange interaction to generate an anisotropic exchange interaction that favors a chiral arrangement of the magnetization. This is known as the Dzyaloshinskii-Moriya interaction (DMI). Contrary to the Heisenberg exchange interaction, which leads to collinear alignment of lattice spins, the form of DMI is therefore very often to cant the spins by a small angle. If DMI is strong enough to compete with the Heisenberg exchange interaction and the magnetic anisotropy, it can stabilize chiral domain wall structure such as skyrmion. When a conduction electron passes through a chiral domain wall, the spin of the conduction electron will experience a fictitious magnetic field (Berry curvature) in real space, which deflects the conduction electrons perpendicular to the current direction. Therefore, it will cause an additional contribution to the observed Hall signal that is termed topological Hall effect (THE). The THE has attracted much attention since it is a promising tool for probing magnetic skyrmions. Recent extensive experiments have focused on the the THE in the ferromagnetic/non-ferromagnetic metal heterojunctions due to the inherent tunability of magnetic interactions in two dimensions. We firstly review the THE in ferromagnetic multilayers, in which the domain wall energy with interfacial DMI can be written as σ=4√AKD, where Dis the effective DMI energy constant, A the exchange constant, K the anisotropy constant. For the most favorable chirality, it lowers the energy. The limit of this situation is when σ goes to zero, which defines the critical DMI energy constant Dc=4√AK/π. Therefore, the domain wall energy would be negative and the chiral domain walls should proliferate if D > Dc, and the methods that can modulate D and Dc to reduce σ have been explored. We have also reviewed the THE in MnGa/heavy metal bilayers. The largest THE signals have been found based on the MnGa films with smallest Dc, which correspondingly results in the smallest σ. The large topological portion of the Hall signal from the total Hall signal has been extracted in the whole temperature range from 5 to 300 K and the magnitude of fictitious magnetic field has been determined.

In situ electron holography of magnetic skyrmions in nanostructures

Li Zi-An, Chai Ke, Zhang Ming, Zhu Chun-Hui, Tian Huan-Fang, Yang Huai-Xin
Acta Physica Sinica. 2018, 67 (13): 131203 doi: 10.7498/aps.67.20180426
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Understanding the correlations between magnetic skyrmions and the microstructural characteristics of the crystals that host skyrmions is a key issue for fundamental research and practical applications of novel type of magnetic materials. Magnetic skyrmion has received great attention due to its non-trivial topological properties and stability. Here we focus on two important points:1) dimensional confinement effects on magnetic skyrmions in magnetic nanostructures, specifically, the magnetic evolution, its related topological properties and energetic stability in confined nanostructured geometries; 2) effects of crystallographic defects on magnetic skyrmions, such as the pinning effect of magnetic skyrmion by crystal defects, and the effect of crystallographic-magnetic chirality reversal at crystal grain boundaries. For the study of dimensional effects on skyrmions in confined nanoscale geometries, we use state-of-the-art electron holography to directly image the morphology and nucleation of magnetic skyrmions in a wedge-shaped FeGe nanostripe that has a width in a range of 45-150 nm. Our experimental results reveal that geometrically-confined skyrmions are able to adopt a wide range of sizes and ellipticity in a nanostripe, which are not existent in thin films nor bulk materials and can be created from a helical magnetic state with a distorted edge twist in a simple and efficient manner. We further perform micromagnetic simulations to confirm our experimental results. The flexibility and ease of formation of geometrically confined magnetic skyrmions may help to optimize the design of skyrmion-based memory devices. For studying the effects of crystallographic defects on magnetic skyrmions, we use in situ Lorentz microscopy and off-axis electron holography to investigate the formation and characteristics of skyrmion lattice defects and their relationship to the underlying crystallographic structure of a B20 FeGe thin film. The measurements of spin configurations at grain boundaries reveal the crystallographic and magnetic chirality across adjacent grains, resulting in the formation of interface spin stripes at the grain boundaries. In the absence of material defects, our results show that skyrmion lattices possess dislocations and domain boundaries, in analogy to atomic crystals. Moreover, the distorted skyrmions can flexibly change their size and shape to accommodate local geometry, especially at sites of dislocations in the skyrmion lattice. These findings offer an insight into the elasticity of topologically protected skyrmions and their correlation with underlying material defects. Our electron holography results provide a quantitative determination of the fine skyrmionic spin textures in magnetic nanostructures. The resolved spin textures will be correlated with the material microstructures to provide important information about the relationship between the magnetic functions and the material microstructures. Our experiments also highlight the applicability of state-of-the-art electron holography for the study of complex spin textures in nanostructures.

Skyrmions in magnetic thin film heterostructures

Li Wen-Jing, Guang Yao, Yu Guo-Qiang, Wan Cai-Hua, Feng Jia-Feng, Han Xiu-Feng
Acta Physica Sinica. 2018, 67 (13): 131204 doi: 10.7498/aps.67.20180549
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Magnetic skyrmion is expected to function as an ideal information carrier for ultra-high density magnetic storage and logic functional device in the future due to its superior properties, such as topological protection, small size, and low driving current density for motion. In order to meet the basic requirements for writing and reading information in devices, one needs to be able to accurately generate, manipulate, and probe skyrmion at room temperature. Given that the history and latest developments of the skyrmion research will be reviewed comprehensively in other articles, in order to avoid repetition, in this article we briefly review a series of recent research advances we have made in magnetic multilayer materials in recent years, and discuss the advantages of relevant device applications and problems that need to be solved. They are included in three aspects as follows. 1) The room temperature skyrmion was observed in a wedge film Ta (5 nm)/Co20Fe60B20 (CoFeB) (1 nm)/Ta (t)/MgO (2 nm)/Ta (2 nm) by a polar magneto-optical Kerr microscope. Results showed that skyrmion can be created at room temperature by controlling the perpendicular magnetic anisotropy of magnetic thin film. In the following, we designed a thin film heterojunction containing an antiferromagnetic layer IrMn. The introduction of antiferromagnetic material can produce an exchange bias field in the magnetic layer, which can play the same role as an external magnetic field, making it possible to realize zero-field skyrmion. In this study, we have successfully observed a stable skyrmion at room temperature and zero magnetic field. 2) The spin-orbit torque generated by the current proved to be able to be used to manipulate the created skyrmion. In the fourth part of this review, we discuss the dynamic process of skyrmion driven by spin-orbit torque in IrMn/CoFeB heterojunctions, and the chirality of skyrmion can be deduced by the direction of its longitudinal motion driven by an applied current. Finally, a principle device based on the skyrmion is further fabricated. In this device, a set of binary data was recorded in the “track” in the presence and absence of skyrmion. Generating and manipulating numbers of skyrmions were realized by using a series of pulse currents with different amplitudes and widths. The detection of a skyrmion can be achieved by using a magnetic tunnel junction at the right end of the device. 3) The advantages of skyrmion as a storage device and the problems that need to be solved for practical applications were discussed.

Overview and advances in skyrmionics

Zhao Wei-Sheng, Huang Yang-Qi, Zhang Xue-Ying, Kang Wang, Lei Na, Zhang You-Guang
Acta Physica Sinica. 2018, 67 (13): 131205 doi: 10.7498/aps.67.20180554
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Microelectronic technologies have been developing rapidly in the past half-century following the famous Moore's Law. However, this tendency is beginning to break down due to the thermal effects induced by the leakage current and data traffic. Spintronics sheds light on eliminating this bottleneck by using the spin degree of electron, which attracts great attention from both the academia and industry. The magnetic skyrmion is a particle-like spin texture with topological protection, envisioned as an energy efficient spintronic information carrier due to its nanoscale size, ultra-low driven energy, and high thermal stability. Recent research progress shows that the nucleation, transportation, and detection of skyrmion in room temperature, which affirm its potential application in electronics, lead to a new research field called skyrmionics.
In this review article, we first introduce the fundamental concepts and recent progress of magnetic skyrmions, from both the theoretical and experimental point of view. Different types of magnetic skyrmions have different properties due to their physical dynamics. We only focus on the skyrmions stabilized by Dzyaloshinskii-Moriya interaction (DMI) in the ultra-thin film structures as their small size, high mobility and room temperature stability can provide the perspectives for electronic devices. The skyrmions have already been extensively investigated from both the theoretical and experimental aspects in recent years. Micromagnetic simulation is the main approach to theoretically studying the dynamics of skyrmions and their applications. Most of the innovative skyrmionic devices have first been demonstrated by this method. Experimentally, skyrmions can be measured by various methods, such as the neutron scattering, Lorentz transmission electron microscopy, scanning X-ray transmission microscopy, polar magneto-optical Kerr effect microscope, etc.
In the third part of this paper, we present four basic functions of skyrmionic devices ranging from nucleation, motion, detection, to manipulation. The nucleation of skyrmions, corresponding to the information writing in skyrmionic devices, has been widely investigated. A skyrmion can be nucleated by conversion from domain wall pairs, local spin injection, local heating, and spin waves. Then, we focus on the current induced skyrmion motion and compare the two different torques:the spin transfer torque and the spin orbit torque. To read the data, it is necessary to detect skyrmions electrically. One way is to measure the topological Hall effect in a Hall bar. More commonly, skyrmions can be detected through magnetoresistance effects, i.e., giant magnetoresistance/anisotropic magnetoresistance, tunnel magnetore sistance, and non-collinear magnetoresistance, in a junction geometry. For manipulation, it is mainly demonstrated by the voltage controlled magnetic anisotropy (VCMA).
Finally we discuss several representative skyrmionic nano-devices in memory, logic, and neuromorphic applications. The magnetic tunnel junction and the racetrack are two common designs for skyrmionic memory devices. The former can store multiple values in one bit, and the latter can realize fast and efficient data transmission. To control the skyrmionic data in these memories, the VCMA effect is one of the promising approaches, which is used in several designs. For the skyrmionic logic devices, they can be divided into two main types:the transistor and the logic gate. However, until now, these ideas are only demonstrated in simulation, and more efforts in experiment are needed. Besides, novel devices such as artificial synapses and neurons can be realized more naturally by skyrmion due to its particle-like property.
In summary, skyrmionics is promising in several aspects, including performance improvement, emerging function and architecture design, and bio-inspired computing. Remarkable progress has been made in the past few years, however the device integration, the materials, and the data transmission still restrict its application. We hope this overview article may present a clear picture about skyrmionics and receive more attention, thus promoting its fast research and development in the future.

Magnetoelastic phenomena and mechanisms of magnetic skyrmion crystal

Hu Yang-Fan, Wan Xue-Jin, Wang Biao
Acta Physica Sinica. 2018, 67 (13): 136201 doi: 10.7498/aps.67.20180251
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Recently, a novel two-dimensional spin structure with non-trivial topological properties, called magnetic skyrmion, has been found in many chiral magnets. In most cases, magnetic skyrmions assemble spontaneously and form a lattice structure, called magnetic skyrmion crystal (SkX). SkX, as a novel macroscopic magnetic phase, may interact with different types of external fields through the intrinsic multi-field coupling of the material, resulting in many peculiar physical phenomena. It is found that due to the intrinsic magnetoelastic coupling of chiral magnets, SkX not only influences the mechanical properties of the materials, but also has “emergent elastic properties” when subjected to external forces. In this review, we first introduce and categorize various types of SkX-related magnetoelastic phenomena, and then introduce a unified theoretical framework to analyze these magnetoelastic phenomena. Specifically, we establish the Landau-Ginzburg free energy functional with a comprehensive description of the magnetoelastic effect for B20 chiral magnets obtained through symmetry analysis, and prove that SkX should be described by a Fourier series due to its wave nature. We show quantitative agreement between theoretical results and experimental results for three types of phenomena:1) the temperature-magnetic field phase diagrams of MnSi suffering uniaxial compression, it is found that uniaxial compression in the direction[0, 0, 1]T constricts the stable region of the skyrmion phase in the phase diagram, while uniaxial compression in the direction[1, 1, 0]T extends the stable region of the skyrmion phase in the phase diagram; 2) the emergent elastic behavior of SkX, it is found that this property derives from the magnetoelastic effect of the underlying material, and the linear constitutive equation (with coefficient matrix λ) which determines the emergent deformation of SkX, is briefly introduced; 3) the variations of elastic coefficients C11, C33, C44, and C66 with the external magnetic field for MnSi, and the predictions of the variation of C12 and C13 are provided by the theory. Based on the theoretical framework, the analytical solutions of the eigenstrain problems for chiral magnets hosting SkX and the surface configuration of SkX in a half-space magnet are introduced. In this process, we show how to use the theoretical framework to deal with different problems. Finally, we make a summary and suggest several directions for the future development of this field.

Dzyaloshinsky-Moriya interaction in δ-(Zn, Cr)S(111) surface: First principle calculations

Li Xiao-Ying, Huang Can, Zhu Yan, Li Jin-Bin, Fan Ji-Yu, Pan Yan-Fei, Shi Da-Ning, Ma Chun-Lan
Acta Physica Sinica. 2018, 67 (13): 137101 doi: 10.7498/aps.67.20180342
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According to density functional theory calculations, we elucidate the atomic and electronic structure of δ-(Zn, Cr)S(111) surface. The magnetic interaction between Cr atoms is via S atoms close to the Cr layer. This interaction is shown by the analysis of spin charge contour plot and partial density of states (DOS) of each atom. The DOSs of other S atoms are non magnetic and have no magnetic exchange with the Cr layer. E(q) and E(-q) are the dispersions between energy E and wave vector q of spin spiral in the opposite directions. They are calculated with generalized Bloch equations and all the magnetic moments of Cr atoms are arranged in the plane perpendicular to the δ-(Zn, Cr)S(111) film. The differences between E(q) and E(-q) are caused by the interface of δ-(Zn, Cr)S(111), where the symmetry of space perpendicular to the film is broken. Effective Heisenberg exchange interaction (HBI) and Dzyaloshinsky-Moriya interaction (DMI) parameters between different neighbors (Ji and di) are derived by well fitting the ab initio spin spiral dispersion E(q) to HBI with DMI model and E(q)-E(-q) to DMI model, respectively. The J2 plays a major role with a large negative value of -9.04 meV. The J1 is about 2/5 of J2, and J3 is about 1/4 of J2 with positive value. The DMI d1 is -0.53 meV, and d2 is 0.07 meV. With these HBI parameters, E(0) is the largest one at which δ-(Zn, Cr)S(111) has no ferromagnetic interface. The E(q) has its lowest energy with the q at M=b1/2 in the first Brillouin zone. Hence, δ-(Zn, Cr)S(111) is an M-type antiferromagnetic (AFM) material. In this type of AFM configuration, magnetic moments of Cr atom in a line along b2 are parallel to each other, and antiparallel to the magnetic moments in adjacent lines. The E(q) at K=b1/2+ b2/2 is almost as large as that at Γ point. The value of DMI parameter d1 is about 1/5 of that on Co/Pt3 interface and 1/2 of Co/graphene. However, it is a negative number, which shows the clockwise chirality. The δ-(Zn, Cr)S(111) interface has obvious DMI, and skyrmion may be formed at this transition-metal/semiconductor (TM/S) interface. It is a good option to search for DMI in different kinds of TM/S heterojunctions. The material that combines the advantage of heterojunction, and DMI may have new magnetic phenomenon, which is usefulfor the magnetic storage. This paper enriches the research on DMI.

Critical behaviors of helimagnetic ordering systems relating to skyrmion

Zhang Lei
Acta Physica Sinica. 2018, 67 (13): 137501 doi: 10.7498/aps.67.20180137
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Study of critical phenomena plays a key role in developing the theory of phase transition. In this article, we mainly review some new experimental results about the critical phenomena reported recently in the helimagentic ordering materials. These materials exhibit a kind of a vortex-like spin texture so-called skyrmion phase. The skyrmion phase has great potential applications in the new spin-based storage due to the topologically protected stability, nanometric size, and current-driven motion. Generally, the skyrmion state exists in a helimagentic system due to the DzyaloshinskiiMoriya (DM) interaction which forms in the crystal structure without inversion symmetry. It usually emerges just below the helimagentic phase transition temperature TC under a certain temperature and magnetic field. In this review article, firstly, we introduce some basic concepts about the phase transition, such as critical phenomenon, critical exponents, scaling law, and universality. Secondly, we discuss two different methods which can help us to obtain the critical exponents, i.e., the iteration method based on the isothermal dc-magnetization and the fitting technique based on the magnetic entropy change. Both methods are extensively used in the current study of critical phenomena Thirdly, we analyze and outline some latest studies of critical behaviors and critical exponents for several typical helimagnetic systems with skyrmion state, such as MnSi, FeGe, Cu2OSeO3, Fe1-xCoxSi, and Fe1.5-xCoxRh0.5MoN. The B20 compound MnSi is a typical skyrmion material, which undergoes a paramagnetic-to-helimagnetic phase transition at ~30.5 K and the skyrmion phase appears just below TC as an appropriate external magnetic field is applied. Investigations show that critical exponents of MnSi belong in the universality class of a tricritical mean-field model, implying the existence of a long-rang magnetic interaction in this system. The critical behavior of MnSi reveals that its first-order phase transition can be driven into a second-order phase transition by the action of external magnetic field, where a field-induced tricritical point is found among the helimagnetic, conical, and paramagnetic phases in MnSi system. Unlike MnSi, the critical exponent of the near-room-temperature skyrmion system FeGe, which undergoes a helimagentic phase transition at ∼278 K, belong to the three-dimensional Heisenberg model. The critical behavior of Cu2OSeO3 is similar to that of FeGe, which indicates that the magnetic interactions in these two systems are dominated by the short-range nearestneighbor isotropic magnetic coupling. In addition, studies revealed that magnetic interaction and critical behavior of the skyrmion system can be effectively modulated by doping. The critical exponents of Fe1-xCoxSi and the newly founded skyrmion system of Fe1.5-xCoxRh0.5MoN indicated that the doping concentration of Co can change and affect their critical behaviors. In addition, it was demonstrated that the doping of Co enhanced the anisotropic magnetic coupling in Fe1-xCoxSi while it suppressed that in Fe1.5-xCoxRh0.5MoN. Fourthly, according to the universality and the scaling equations, we proposed a method to construct the detailed H-T phase diagram around the phase transition temperature in the system exhibiting field-induced phase transition. Finally, we make a brief summary and suggest our perspectives of the study of critical phenomena in helimagentic system. The results of critical behaviors indicate that although all these helimagentic systems exhibit a similar skyrmion phase, their essential magnetic interactions belong in different universality classes, indicating different types of magnetic coupling in these systems. Furthermore, the results also suggest that magnetic coupling can also be effectively tuned by the external modulation.

Modulation of skyrmion diameter in centrosymmetric frustrated magnet

Chi Xiao-Dan, Hu Yong
Acta Physica Sinica. 2018, 67 (13): 137502 doi: 10.7498/aps.67.20172709
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Magnetic skyrmions were first observed in a bulk B20 chiral magnet where the unit cell of the crystal lacks inversion symmetry, i. e. it is noncentrosymmetric, due to the Dzyaloshinskii-Moriya interaction (DMI). The breaking of structural inversion symmetry can also be achieved artificially in extremely thin FM layers adjacent to heavy elements, to induce a nonzero DMI. Many skyrmion properties in the DMI-based system are revealed such as the skyrmion diameters simply inversely proportional to the DMI constant. On the contrary, the triangular lattice, providing a simple realization of a high-symmetry system with six equivalent orientations for the helix, is centrosymmetric. In a two-dimensional triangular lattice magnet with the magnetocrystalline anisotropy perpendicular to the film plane, the magnetic frustration can arise from the coexistence of a nearest -neighbor ferromagnetic exchange interaction and a third-neighbor antiferromagnetic exchange interaction. When an external magnetic field is applied parallelly to the anisotropy, the non-coplanar alignments of spins are favored and even the topologically protected magnetic skyrmions also appear. Based on the Monte Carlo simulation, the dependence of magnetic-field-induced magnetic phase transitions in such magnetic frustrated magnets, including the magnetic phase of skyrmion crystals, and the skyrmion diameters on competing exchange interaction and magnetic field is studied. The results indicate that the diameters of magnetic skyrmions strongly depend on the competing exchange interactions and external magnetic field. Like the diameter features of magnetic skyrmions observed in the conventional DMI-based chiral magnets, the external magnetic field can magnetize the skyrmion periphery spins to reduce the skyrmion diameters. However, the enhanced antiferromagnetic exchange interaction can compress the entire skyrmions. In the framework of the spin wave theory and Monte Carlo simulation results, the diameters of magnetic skyrmions in exchange-interaction-frustrated systems are quantified. The skyrmion diameter decreases linearly with the increase of magnetic field for weak antiferromagnetic exchange interaction. With the increase of antiferromagnetic exchange interaction, the decrease of the skyrmion diameter with increasing magnetic field becomes slow, while the strong magnetic fields may rapidly reduce the skyrmion diameter. With the increase of antiferromagnetic exchange interaction, the maximum and median skyrmion diameters decrease to level-off roughly, while the minimum skyrmion diameters show a rapid decrease first and a great fluctuation later. The phenomena are explained through discussing the variations of configurations and magnetic energies of skyrmions. This work demonstrates the adjustability of skyrmion diameter in centrosymmetric frustrated magnet, which not only improves the understanding of origin of skyrmions, but also supports theoretically the development of new generation of skyrmion-based storage and logic devices.

Control of skyrmion movement in nanotrack by using periodic strain

Xuan Sheng-Jie, Liu Yan
Acta Physica Sinica. 2018, 67 (13): 137503 doi: 10.7498/aps.67.20180031
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Magnetic skyrmions are a topologically stable and particle-like chiral spin configuration. They are appealing because of their potential applications in racetrack memory and other spintronic devices. These applications are strongly dependent on the skyrmion motion in confined geometry. Therefore, it is important to study the moving behaviors of skyrmions in a nanotrack to make them have more practical applications. Mechanical strain and stress have been demonstrated theoretically and experimentally to be able to effectively control the skyrmion phase. It can stabilize the skyrmion lattice in a broad range, and change the shape of the skyrmion crystal. In this paper, we study the moving behaviors of ferromagnetic skyrmions and antiferromagnetic skyrmions under the action of sinusoidally distributed strain in a nanotrack by using micromagnetic simulation. We assume that strain is uniaxial and perpendicular to the plane of the nanotrack. Its strength varies sinusoidally along the x-axis. Meanwhile, we apply an in-pane current along the nanotrack to drive the skyrmion moving towards the right side. We first find that there is a threshold current density that is defined as the minimum current that can drive skyrmion moving continuously. When the current density is larger than the threshold current density, the skyrmion can move continuously in the nanotrack. The threshold current density increases with the amplitude of strain increasing, but decreases with the period of strain increasing. Second, we find that the trajectory of skyrmion changes under the action of the sinusoidal distributed strains. For ferromagnetic skyrmion, its trajectory changes from straight line to periodic wavy line. Also, we find that the longitudinal velocity of skyrmion is affected by the boundary of the nanotrack. When the skyrmion is close to the upper boundary of the nanotrack, the longitudinal velocity increases sharply and it will form a peak in the velocity curve, but when the skyrmion is close to the lower boundary of the nanotrack, the longitudinal velocity decreases and it will form a valley in the velocity curve. The transverse velocity of skyrmion relates to the strain gradient. It is inversely proportional to the strain gradient. For antiferromagnetic skyrmion, we find that the movement trajectory of antiferromagnetic skyrmion does not change under the stress control. However, its diameter and velocity change periodically. Its velocity can vary between 103 m/s and 0. Our results demonstrate that the sinusoidal strain can control the skyrmion motion. This work may provide guidance in designing and developing of the spintronic devices based on magnetic skyrmions.

Research progress of micromagnetic magnetic skyrmions and applications

Jin Chen-Dong, Song Cheng-Kun, Wang Jin-Shuai, Wang Jian-Bo, Liu Qing-Fang
Acta Physica Sinica. 2018, 67 (13): 137504 doi: 10.7498/aps.67.20180165
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Magnetic skyrmion, as a quasi-particle, with topologically protected property has received wide attention. In this article, We first review the existence conditions and transport characteristics of magnetic skyrmions theoretically, then view recent micromagnetic simulation researches on creation and controlling as well as the device design, which includes racetrack memories, spin transfer nano-oscillators, transistors and logic gates. We hope this paper can provide a reference for the applications of magnetic skyrmions in the future.

Overview of magnetic skyrmion-based devices and applications

Xia Jing, Han Zong-Yi, Song Yi-Fan, Jiang Wen-Jing, Lin Liu-Rong, Zhang Xi-Chao, Liu Xiao-Xi, Zhou Yan
Acta Physica Sinica. 2018, 67 (13): 137505 doi: 10.7498/aps.67.20180894
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Magnetic skyrmions possess topologically non-trivial particle-like nanoscale domain wall structures, which have reasonably good stability and unique dynamic properties and can be controlled by magnetic fields, electric fields, and electric currents. Therefore, magnetic skyrmions are expected to be used as novel information carriers in the next-generation high-density, low-energy-consumption, and non-volatile information storage and logic computing devices. Since the first experimental observation of magnetic skyrmions in 2009, a number of skyrmion-based device prototypes have been proposed. In this article, we review the recently proposed skyrmion-based devices and applications, including skyrmion-based racetrack memory, logic computing device, transistor-like functional device, and nano-oscillator. We first discuss advantages of skyrmion-based racetrack memory and solutions for some problems we are facing currently. We then introduce the duplication and merging of magnetic skyrmions and the skyrmion-based logic OR and AND gates. We also introduce the switch function of skyrmion-based transistor-like functional device. The switch function is realized via a voltage gate and controlled by the applied voltage as well as the driving spin current. Besides, a brief introduction of the skyrmion-based nano-oscillator is given. In addition, we introduce several possible methods to encode binary information in skyrmion-based devices. Finally, we discuss some possible future novel applications based on magnetic skyrmions.

Research progress on topological properties and micro-magnetic simulation study in dynamics of magnetic skyrmions

Kong Ling-Yao
Acta Physica Sinica. 2018, 67 (13): 137506 doi: 10.7498/aps.67.20180235
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Skyrmions, as a nontrivial topological magnetic structure, have the advantages of topological stability, small size and low driving electrical current, showing potential applications in spintronic memory device. There are several mechanisms for skyrmion formation in magnets. One major mechanism is, in chiral-lattice ferromagnets, the competition between the Dzyaloshinskii-Moriya and ferromagnetic exchange interactions, due to the lack of spatial inversion symmetry. The combination of topology and condensed physics demonstrates various new topological phenomena of skyrmions, which also determine their dynamics. In this review, recent progress on the topological physics foundation of Skyrmions, as well as their dynamics of application in spintronics devices, is reviewed.
The topological physics foundations of skyrmions is introduced. Firstly, the structure of skyrmions, which shows a special nontrivial topology in the real space, is presented accompanied with the formation of skyrmions caused by Dzyaloshinskii Moriya interactions in chiral magnets. Secondly, due to the importance of the describable method of the topology of a skyrmion, the topological charge, that characterize the topology, as well as the calculation method are introduced. Also, the arising topological stability is discussed here. Then, the typical topological effects arising from the topology of a skyrmion, including topological Hall effect and the skyrmion Hall effect are reviewed. The next is the introduction of the helical and the spiral spin configuration, the alternatives for Bloch and Néal type skyrmions respectively, which show up under lower external magnetic field with the same interaction. Also the phase transition of the helical/spiral state to skyrmions and the Monte Carlo method to simulate the spin configuration of a chiral magnet are introduced. At last, the spin orbital torque and the spin transfer torque, that describe the driven effect of a skyrmion by an electrical current or a thermal field, are reviewed. The consequence dynamics of skyrmions, the Landau-LifshitzGilbert equation, are also introduced.
The recent progress of typical dynamics of skyrmions on several concerned problems in practical applications are reviewed. The applications in spintronics memory require skyrmions have steady transportation driven by electrical current and controllable creation and annihilation process. Firstly, skyrmion can be generated by the spatial nonuniform electric current with a certain geometry constrain. Especially for the Néal type skyrmion, nonuniformity of the spin orbital torque, come from the non-uniform electric current, play an important role in the skyrmion generation process. Secondly, skyrmion moves with a perpendicular velocity under an electrical current, because of the skyrmion Hall effect. So the elimination of skyrmion Hall effect is practically concerned to make the transportation steady. The anti-ferromagnetic skyrmion and antiferromagnetic coupled skyrmion bilayer are found with no skyrmion Hall effect by have two opposite component cancel out. Finally, with topological stability, skyrmions are hard to convert from and to a nontrivial topological spin configuration at low temperature. So the manipulation of skyrmion creation and annihilation are discussed accompanied with their difference of Bloch and Néal type skyrmiom.

Multi-field control on magnetic skyrmions

Dong Bo-Wen, Zhang Jing-Yan, Peng Li-Cong, He Min, Zhang Ying, Zhao Yun-Chi, Wang Chao, Sun Yang, Cai Jian-Wang, Wang Wen-Hong, Wei Hong-Xiang, Shen Bao-Gen, Jiang Yong, Wang Shou-Guo
Acta Physica Sinica. 2018, 67 (13): 137507 doi: 10.7498/aps.67.20180931
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The concept of skyrmion is proposed by Tony Skyrme, a British particle physicist, to describe a state of particles as a topological soliton. Magnetic skyrmion is a novel spin structure with topological behavior, whose size is on a nanometer scale. The space between skyrmions is tunable from a few nanometers to micrometer. Magnetic skyrmion can be stable in a large temperature range, from lower temperatures, to room temperature, and even to higher temperature. The materials with magnetic skyrmions include not only low temperature B20-type ferromagnets with centrosymmetry breaking and weak ferromagnets with helical magnetic ordering, but also the hexagonal MnNiGa alloy and ferromagnetic multilayers over room temperature. By using topological spin structure of skyrmions, an electrical current can be applied to driving or flipping the skyrmions, similar to the spin transfer torque effect in spin-valves and magnetic tunnel junctions. The critical current density is on the order of 102 A/cm2, which is five orders lower than that in magnetic multilayered structures such as 107 A/cm2. This critical value is much lower than the channel current density in Si-based semiconductor technology, thus leading to great potential applications in the future magnetic information devices. In this review paper, we first introduce the discovery, a brief development history of magnetic skyrmions. Then, we summarize the materials with skyrmion spin structures, focusing on the key physical properties. Finally, we mention the recent progress of the multi-field (such as magnetic field, electrical current, and temperature) control on magnetic skyrmions in hexagonal MnNiGa alloy and Pt/Co/Ta magnetic multilayers, together with the creation, annihilation, and dynamic behavior of skyrmions.

Magnetic domain chirality and tuning of skyrmion topology

Xu Gui-Zhou, Xu Zhan, Ding Bei, Hou Zhi-Peng, Wang Wen-Hong, Xu Feng
Acta Physica Sinica. 2018, 67 (13): 137508 doi: 10.7498/aps.67.20180513
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Owing to the topologically protected properties, magnetic skyrmions possess high stability and small critical driving current, thus making them potentially applied to future racetrack memory devices. Skyrmions have been identified in several material systems. One large class contains the centrosymmetric materials, where skyrmions emerge as the competition between perpendicular magnetic anisotropy and magnetic dipolar interactions. The recently reported skyrmion host includes La-Sr-Mn-O, hexagonal MnNiGa, Fe3Sn2, etc. In these systems, due to the isotropic characteristic of the dipolar interaction, magnetic bubble can exhibit various topologies and helicities. The common types of bubbles existing in the materials are the trivial one with n=0 (n is the topological charge) and the non-trivial one with n=1, and the latter is taken to be equivalent to magnetic skyrmion. In this article, we investigate the formation of skyrmions under various magnetic parameters and the role of stripe domain chairity in tuning the bubble topology. The main method we use here is micromagnetic simulation with the Object Oriented MicroMagnetic Framework (OOMMF) code. Also some recent experimental results on MnNiGa and Fe3Sn2 are exhibited and compared with the simulation prediction. Under a fixed magnetization (Ms), by tuning the exchange constant A and magnetic anisotropy Ku, we find that the domains can evolve into a bubble state under a moderate anisotropy value, and to some extent, large anisotropy favors the formation of n=1 topological skyrmion. In the case of the stripe domains, it is found that different initial configuration can lead to different domain wall charity and further change the process of skyrmion formation. When the magnetization in the domain wall orients in the same direction, n=0 bubble will form upon applying magnetic field. While the magnetization in the wall orients alternatively up and down, a topological skyrmion is directly formed. In the stripe domains with inversed 180° Bloch wall, in-plane magnetization dominates and no bubble or skyrmion can form. In addition, the tilt of the magnetic field and uniaxial anisotropy can also change the morphology and topology of the skyrmions, which has been verified in our experiments. According to the above results, we propose to tune the topology of skyrmions in centrosymmetric material through adjusting the ground magnetic state, magnetic anisotropy and in-plane components, which can be realized by element doping at different sites and appropriately designing the sample.

Observation of new-type magnetic skymrions with extremerely high temperature stability and fabrication of skyrmion-based race-track memory device

Hou Zhi-Peng, Ding Bei, Li Hang, Xu Gui-Zhou, Wang Wen-Hong, Wu Guang-Heng
Acta Physica Sinica. 2018, 67 (13): 137509 doi: 10.7498/aps.67.20180419
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Nanoscle magnetic skyrmions are topologically protected vortex-like spin textures that have been regarded as a promising candidate for the transport of information in further spintronic applications based on the racetrack memory concept due to their nanoscale dimension, stable particle-like feature, and an ultralow threshold for current-driven motion. Recently, most of the skyrmions are observed in chiral magnetic materials, such as MnSi, FeGe, Co-Mn-Zn, where the Dzyaloshinskii-Moriya interaction is active. However, their overall low thermal stability is one of the major factors hindering the practical applications. In this paper, we report the observation of a new-type magnetic skyrmion with extremerely high temperature stability in the centrosymmetric frustrated magnet Fe3Sn2, and the fabrication of skyrmion-based race-track memory device based on Fe3Sn2 by using focused ion beam. This compound is a rare example of ferromagnetic frustrated magnet that exhibits a high Curie temperature Tc up to 640 K. As the temperature decreases from 640 K to 100 K, it undergoes a spin reorientation during which the easy axis rotates gradually from the c-axis to the ab-plane. The Fe3Sn2 has a layered rhombohedral structure with the alternate stacking of the Sn layer and the Fe-Sn bilayer along the c-axis. By a high-temperature flux method, we grow high-quality Fe3Sn2 single crystal. The in-situ Lorentz transmission electron microscopy (LTEM) observations demonstrate that this compound can host skyrmions at room temperature (RT). In contrast to the skyrmions of the chiral magnets, they possess various spin textures and are transformed from topologically trivial bubbles under a high external magnetic field of 800 mT. By using the FIB technique, we fabricate a geometrically confined nanostripe with a width of 600 nm and thickness of 250 nm. The in-situ LTEM observations demonstrate that a single chain of skyrmions with uniform spin textures can be created at RT. The investigations on the temperature stability of the single skyrmion chain reveal that it shows an extremerely high temperature stability that the size of and the distance between the skyrmions in the chain can keep unchanged at temperatures varying from RT up to a record-high temperature of 630 K. The observation of a highly stable single skyrmion chain in the geometrically confined Fe3Sn2 nanostripe can be attributed to (1) the weak temperaturedependent magnetic anisotropy Ku of the Fe3Sn2 crystal, and (2) the formation of edge states at the boundaries of the nanostripes. The observation of new-type magnetic skymrion with extremerely high temperature stability and the fabrication of skyrmion-based race-track memory devices are very important steps towards the applications in skyrmionbased spintronic devices.

Skyrmions-based magnetic racetrack memory

Liang Xue, Zhao Li, Qiu Lei, Li Shuang, Ding Li-Hong, Feng You-Hua, Zhang Xi-Chao, Zhou Yan, Zhao Guo-Ping
Acta Physica Sinica. 2018, 67 (13): 137510 doi: 10.7498/aps.67.20180764
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Magnetic skyrmions are topologically stable spin configurations with small size, which can be driven into motion by a small current. They are widely regarded as building blocks for next-generation magnetic storage. The main advantage of skyrmions lies in their particular dynamic behaviors, especially in their ability to move stably in racetrack under the action of small spin-polarized currents. The writing, driving and reading methods of skyrmions in racetrack are reviewed in detail in this paper, including the most recent research findings. The review focuses on the most commonly used driving method, i.e., driving skyrmions by applying spin-polarized currents. The clogging and annihilation of skyrmions in racetrack are analyzed, with the skyrmion Hall effect discussed which may lead skyrmion signals to lose. Methods to avoid skyrmion Hall effect are introduced and hence the optimized designs for skyrmion-based racetrack are also reviewed. Finally, some challenges of skyrmion-based racetrack memory are discussed.

CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES

First-principles study of optical properties of germanium doped with phosphorus and bismuth

Huang Lei, Liu Wen-Liang, Deng Chao-Sheng
Acta Physica Sinica. 2018, 67 (13): 136101 doi: 10.7498/aps.67.20172680
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Using first-principles calculations based on density functional theory, we investigate the electronic structures and optical properties of germanium doped by phosphorus and bismuth with different concentrations. By analyzing the electronic structures and optical properties of the doped systems, we can theoretically analyze and predict the optical and electrical practical applications of N-doped germanium semiconductors. By analyzing and comparing the densities of electronic states before and after doped, we can draw some conclusions. The conclusions show that the Fermi level moves in the direction of conduction band after being doped. Although germanium is an indirect band gap luminescent material, the doped systems all become direct band gap luminescence. Doping more or less affects various optical properties in different energy ranges. In a low energy range, the dielectric function and refractive index of the doped systems are affected. When the doping concentration is 2.083%, the dielectric function and refractive index of the doped system both have a special change. And the absorption of the doped system is changed in the high energy. As the energy increases after the absorption peak, the absorption of the doped system drops faster. The reflectance of the doped system is affected in all the energy ranges. The reflectance of the doped system increases in medium energy. And the reflectance of the doped system is reduced in low energy and high energy range. However, when the doping concentration is 2.083% and the energy is less than 1.7 eV, the reflectance of the doped system is higher than that of the undoped system. The conductivity of the doped system forms two peaks, adding a peak in low energy. The additional peaks in the systems where the doping concentrations are 1.563% and 2.083% are obvious. The peak of the loss function increases after being doped. However, as the doping concentration increases, the increment of the loss function decreases. As the doping concentration increases, the peak is formed at a higher energy. The conclusions are of significance for guiding the optical applications of N-type doped germanium. According to the conclusions, we can adjust the doping concentration and energy range in the optical applications of N-doped germanium.

Method of picking up carbon nanotubes inside scanning electron microscope

Yang Quan, Ma Li, Yang Bin, Ding Hui-Yang, Chen Tao, Yang Zhan, Sun Li-Ning, Toshio Fukuda
Acta Physica Sinica. 2018, 67 (13): 136801 doi: 10.7498/aps.67.20180347
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In this paper a promising method of recognizing spatial contact state between carbon nanotubes (CNTs) and atomic force microscope (AFM) probe inside scanning electron microscope (SEM) is proposed. The CNTs can be picked up simply and effectively by van der Waals force without knowing depth information of SEM images by using this method. And a micro-nanorobotic manipulation system with 16 DOFs, which allows the automatic pick-up of CNTs based on visual feedback, is presented. The micro-nanorobotic manipulators are assembled into 4 units with 4 DOFs individually. Namely, a manipulator has 4 DOFs i.e., three linear motions and a rotational motion. Manipulators are actuated by picomotors with better than 30 nm linear resolution and less than 1 micro-rad rotary resolution. The van der Waals force mechanics model between CNTs and AFM probe in the picking up manuplation is established. In reality, the van der Waals force is the main attractive force under the vacuum condition inside SEM when the influence of staticelectricity is ignored. It is shown that the van der Waals force under horizontal (sphere-plane) contact model is significantly larger with appropriate overlapping length. Though the positions in both x and y directions of the CNTs and AFM cantilever are acquired, the relative positions of those two objects in the z direction remain unclear. In the gradually ascending process of AFM cantilever to contact the CNTs, the CNTs abruptly drop on the surface of AFM probe due to the van der Waals force. According to the relative coordinate system of SEM visual feedback images, the detection of contact state between carbon nanotubes and AFM probe are completed by using the inclination changing value of fitting line. The experimental results suggest that the abrupt contact between CNTs and AFM probe happens when the inclination changing value of the regression line is found to be 3.0263°. The spatial contact state between carbon nanotubes and AFM probe includes line contact (Model a) and point contact (Model b, Model c). Then the dynamic difference method is introduced to identify the spatial contact model of CNTs and AFM probe. The results demonstrate that contact model of CNTs and AFM probe is line contact when the dynamic difference is approximately zero. The position of carbon nanotubes is corrected by moving AFM cantilever automatically underneath the CNTs. The picking-up of CNTs from substrate under line contact model is completed by choosing the optimum contact angle, contact length and pickup speed.

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

Growth of Tl-1223 superconducting thin films by rapidly heating-up sintering technology

Xie Qing-Lian, Su Ling-Ling, Jiang Yan-Ling, Tang Ping-Ying, Liu Li-Qin, Yue Hong-Wei, Chen Ming-Xian, Huang Guo-Hua
Acta Physica Sinica. 2018, 67 (13): 137401 doi: 10.7498/aps.67.20172753
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Owing to high critical temperature (125 K) and high upper critical field, TlBa2Ca2Cu3O9 (Tl-1223) superconductor is a kind of superconducting power transmission material working at liquefied natural gas temperature, and it has a great potential application value in the strong and weak electric field. In this work, the Tl-1223 superconducting films are fabricated by rapidly heating-up sintering technology (RHST) on (00l) lanthanum aluminate substrates. The Tl-Ba-Ca-Cu-O target is used as a sputtering source to deposit the precursor films by the radio-frequency magnetron sputtering technique. The Tl-contained pellets, named annealing targets, are fabricated by the solid-state reaction of stoichiometric quantities of Tl2O3, BaO2, CaO and CuO powders with an initial cation ratio of m Tl:Ba:Ca:Cu=0.4-1.8:2:2:3. The amorphous precursors together with the annealing target providing Tl source are sealed in a silver foil and annealed at 820℃ for 5 min in argon atmosphere, then converted into Tl-1223 superconducting phase. The heating rates are set at 2.5℃/s from room temperature to 350℃, 5℃/s from 350℃ to 650℃, and 35℃/s from 650℃ to 820℃, respectively. The prepared films are characterized by X-ray diffraction and scanning electron microscope. In the conventional low heating rate process, all of the precursor films sintered together with the annealing targets containing different Tl content are first converted into Tl-2212 superconducting phase. That is because the sample residence time in the phase transition temperature range of Tl-2212 is longer, while the phase-formed temperature of Tl-2212 is lower than that of Tl-1223. In the RHST, when the metal ion molar ratio of Tl to Ba in the annealing target is 1.8:2, the main phase of the film is (00l)-oriented Tl-2212. In addition, the film also contains a small number of Tl-2223 grains. On reducing the ratio to 1:2, the film is composed of Tl-1212, Tl-2212, Tl-1223 and Tl-2223 grains. As the ratio decreases to 0.8:2, the film contains the (00l)-oriented Tl-1223 grains and traces of Tl-2223 grains. With the ratio decreasing to 0.4:2, purely c-axis oriented Tl-1223 film is obtained. The critical transition temperature Tc onset of the as-grown film is only 103 K. The film annealed again in oxygen gas has a dense crystal structure and excellent electrical properties. The Tc onset of the sample is about 116 K, and the critical current density Jc is about 1.5 MA/cm2 (77 K, 0 T). The experimental results show that the new sintering process to grow Tl-based films has several advantages such as the short processing cycles, less raw-material consumption, and low production cost.

INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY

Axial multi-particle trapping and real-time direct observation Hot!

Wang Yue, Liang Yan-Sheng, Yan Shao-Hui, Cao Zhi-Liang, Cai Ya-Nan, Zhang Yan, Yao Bao-Li, Lei Ming
Acta Physica Sinica. 2018, 67 (13): 138701 doi: 10.7498/aps.67.20180460
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The optical tweezers with the special advantages of non-mechanical contact and the accurate measurement of positions of particles, are a powerful manipulating tool in numerous applications such as in colloidal physics and life science. However, the standard optical tweezers system uses a single objective lens for both trapping and imaging. As a result, the trapping and imaging regions are confined to the volume near the focal plane of the objective lens, making it difficult to track the trapped particles arranged in the axial direction. Therefore, multiple trapping along axial direction remains a challenge. The three-dimensional imaging technology can realize the monitoring of the axial plane, but neither the laser scanning microscopy nor the wide-field imaging technology can meet the requirement of the real-time imaging. To address this issue, we propose a modified axial-plane Gerchberg-Saxton (GS) iterative algorithm based on the Fourier transform in the axial plane. Compared with the direct algorithm such as the Fresnel lens method, the modified axial-plane GS iterative algorithm has a higher modulation efficiency, and the generated axial distribution has a sharper intensity. In theory, the traps generated each have an ideal Gaussian intensity distribution independently, which is proved by the simulation of reconstructed field. With such an iterative algorithm, we can directly create multiple point-trap array arranged along the axial direction. We also develop an axial-imaging scheme. In this scheme, the particles are trapped and a right-angled silver-coated 45° reflector is used to realize axial-plane imaging. The scheme is verified by imaging silica particles in an axial plane and a lateral plane simultaneously. Furthermore, we combine the axial-plane imaging technique with holographic optical tweezers, and demonstrate the simultaneous optical trapping in 2×2 trap array and the monitoring of multiple silica particles in the axial plane. The trap stiffness of traps array in axial plane is calibrated by measuring the Brownian motion of the trapped particles in the axial trap array with digital video microscopy. The proposed technique provides a new perspective for optical micromanipulation, and enriches the functionality of optical micromanipulation technology, and thus it will have many applications in biological and physical research.

NUCLEAR PHYSICS

Electronic structures of stable Cu-centered Cu-Zr icosahedral clusters studied by density functional theory

Jiang Yuan-Qi, Peng Ping
Acta Physica Sinica. 2018, 67 (13): 132101 doi: 10.7498/aps.67.20180296
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Cu-Zr alloy system,as a representative of transition metal-transition metal (TM-TM) metallic glass (MG),has attracted considerable attention due to its high glass-forming ability in a wide range of compositions.Many researchers have realized that the GFA of Cu-Zr alloy is intimately related to Cu-centered Cu-Zr icosahedral atomic cluster in supercooled liquid and rapidly solidified into amorphous solid.And lots of molecular dynamics simulations have shown that Cu-centered Cu-Zr icosahedral clusters not only affect the thermo-dynamical properties of metal or alloy melts,but also exhibit excellent structural stability and configuration heredity ability during the rapid solidification.Hereof a model of the metallic glass structure based on like icosahedron has become widely accepted,which plays an important role in the glass transition and its strong kinetic constraint on nucleation.However,though more and more standard and distorted Cu-Zr icosahedral clusters have been found and reported in Cu-Zr metallic glass,the fundamental understanding of these Cu-Zr icosahedral clusters of MGs is still lacking.More essential properties of Cu-centered Cu-Zr icosahedral cluster, especially on the electronic structure are still unclear.Based on this,as a further step towards in depth understanding the electronic structures of those icosahedral clusters,we will investigate the electronic structures of the stable Cucentered CunZr13-n (n=6,7,8,9) icosahedral clusters in this work,and consider all the possible atomic configurations for given chemical composition in view of originate in theory And a DMol3 molecular orbital package based on density functional theory (DFT) is adopted to calculate the energetics and electronic structures of Cu-centered Cu-Zr icosahedral clusters.During optimization and total energy calculation,electronic exchange-correlation energy functions in reciprocal space with the Perdew-Burke-Emzerhof type under general gradient approximate are used.A double-numerical basis set together with d-polarization functions (DNP) is chosen to describe the electronic wave functions of Cu and Zr atoms. And only core electrons described by the DFT Semi-core Pseudopots are calculated.All atomic positions in Cu-centered CunZr13-n (n=6,7,8,9) icosahedral clusters are relaxed by geometry optimization under a root mean square (RMS) force of 0.002 Ha/Å and RMS displacement of 0.005 Å.The calculations of total energy and electronic structure are followed by the geometry optimization with self-consistent field tolerance of 1×10-5 Ha.It is found that homogeneous atoms in the shell of clusters with low binding energy prefer to bond to each other.In this case,the results of electronic structures reveal this segregation at low energy and stable configurations can be attributed to their low N (EF) at EF to some extent.A further analysis of Mulliken'population shows that these 4s and 4p of shell Cu atoms are all donees in the formation of icosahedral cluster,different from the donations of 3d and 4s of core Cu atoms and 5s of shell Zr atoms, and this charge transfer tendency does not change with order parameter nor chemical composition of Cu-centered Cu-Zr icosahedral cluster.In addition,calculating the infrared vibration spectrum of Cu-Zr icosahedral cluster is a new idea for accurately characterizing the cluster structure.

ATOMIC AND MOLECULAR PHYSICS

Dependence of peak width of energy distribution on profile of combined field

Lin Nan-Sheng, Han Lu-Xue, Jiang Miao, Li Ying-Jun
Acta Physica Sinica. 2018, 67 (13): 133401 doi: 10.7498/aps.67.20172656
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In this paper, we use the quantum field theory to solve the generation process of particle-anti-particle pairs (PAPs), and study the generation characteristics of PAPs by changing the profile of the field combining an oscillating field and a static electric field. We find a way to increase the generation of PAPs and change the energy distribution. As the field strength of the oscillating field increases, the quantity of particle pairs generated increases. Increasing the field strength of a static electric field yields higher energy pairs of particles. If the frequency of the oscillating field becomes higher, the peak of the energy distribution shifts to higher energy but the width of the peak remains unchanged. The reduction of the field width of the oscillating field increases the generated quantity of PAPs on the one hand, and reduces the peak width of the energy distribution on the other hand. Therefore, we can obtain a narrower range of the energy distribution and more PAPs at less energy cost. Meanwhile, the relationship among the generation yield, the width of energy distribution and the width of the oscillation field is obtained. The width of the oscillating field only significantly narrows the peak width of the energy distribution in a range and reaches a limit after that. This provides useful details for future experiments, and suggests an appropriate width of the oscillating field to produce enough quantity of PAPs with concentrated energy distribution. According to previous studies, varying field width will inevitably lead to the change in the intensity of the electric field. It will be shown that the concentrating of the energy distribution is induced by narrowing the oscillating field instead of increasing the electric field intensity. Therefore, more concentrated PAPs will be obtained and their mutual annihilation will lead to the generation of γ -ray, which can be used as a γ -ray in experiments that follow. We suggest reducing the width of the oscillating field to improve the energy concentration of both particles and anti-particles while their quantities are still large enough.

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

Weighting inversion of dynamic light scattering based on particle-size information distribution character

Xu Min, Shen Jin, Huang Yu, Xu Ya-Nan, Zhu Xin-Jun, Wang Ya-Jing, Liu Wei, Gao Ming-Liang
Acta Physica Sinica. 2018, 67 (13): 134201 doi: 10.7498/aps.67.20172377
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In particle sizing with dynamic light scattering (DLS) technique, the determination of particle size distribution (PSD), via inversing the autocorrelation function (ACF) of scattering light, is usually limited by the inherently low particle size information in ACF data and, the lack of targeted inversion on the noise restriction and the particle size information utilization. For the ACF data in DLS measurement, most of particle size information is centrally contained in the decay section and the larger noise is contained in the larger delay section. However, no consideration of the particle size information distribution in the ACF data for the routine inversion method increases the difficulty of the accurate PSD inversion, especially the broad and bimodal PSDs. Until now, it is still a difficult problem to obtain an accurate recovery of the broad and bimodal PSDs, specifically the bimodal PSD with a peak position ratio less than 2:1 and containing large particles (>350 nm). In this paper, a character-weighted constrained regularization (CW-CR) method is proposed, in which, the particle size information distribution in the ACF as the base and the adjustment parameter as the exponent are used to weight the ACF. By using the weighting coefficients corresponding to the particle size information distribution along the delay time in ACF, the CW-CR method can enhance the utilization of the particle size information in ACF data, and effectively weaken the effect of noise at large delay time. With this method, the closely spaced bimodal PSD (with nominal diameters of m 350 nm:500 nm in simulation, m 300 nm:502 nm in experiment) is recovered successfully at a high noise level of 0.01. It shows that the CW-CR method, combined with the multiangle DLS (MDLS) measurement, can effectively make the best use of the particle size information hiding in the noisy ACF data, and improve the resolution of bimodal PSD as well as the capability of noise suppression. So it can make the advantages of MDLS more highlighted than the routine method in the recovery of the broad and bimodal PSDs.

High sensitivity quantum Michelson interferometer

Zuo Xiao-Jie, Sun Ying-Rong, Yan Zhi-Hui, Jia Xiao-Jun
Acta Physica Sinica. 2018, 67 (13): 134202 doi: 10.7498/aps.67.20172563
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Michelson interferometer can be applied to not only the building block of the fundamental research of physics, but also the precise measurement, such as the direct observation of gravity wave signal. Therefore, high performance Michelson interferometer is the key step towards the implementation of direct observation of weak gravity wave signal. Recently, the vacuum noise was reduced by injecting squeezed vacuum into the unused port of Michelson interferomter, and the phase signal optical field in Mach-Zender interferometer is amplified based on the four-wave mixing in hot Rubidium atom. Here we study high sensitivity quantum Michelson interferometer. In the Michelson interferometer, the linear optical beam splitter is replaced by a non-degenerated optical parametric amplifier to realize the splitting and combining of optical fields, and the squeezed vacuum is also injected into the unused port of interferomter, so that the high signal-to-noise ratio and high sensitivity of phase measurement can be realized. Due to the inevitable optical losses, the losses inside and outside the Michelson interferometer are considered in our theoretical model. We investigate the influences of the losses inside and outside the Michelson interferometer on the sensitivity of phase measurement. By theoretical calculation, we analyze the dependence of sensitivity of phase measurement on system parameters, such as intensity of optical fields for phase sensing, gain factor of non-degenerated optical parametric amplifier, the losses inside and outside the Michelson interferometer, and the squeezing parameter of input squeezed vacuum, and thus the condition of high sensitivity nonlinear Michelson interferometer can be obtained. In a broad system parametric range, the quantum Michaleson interferometer can surpass standard quantum limit, and the nonlinear Michaleson interferometer with squeezed state injection can provide the optimal sensitivity for phase measurement. The nonlinear Michelson interferometer with squeezed state is suitable for weak signal measurement. While the gain factor of non-degenerated optical parametric amplifier is large enough, the nonlinear Michelson interferometer without injecting the squeezed vacuum can still reach the optimal sensitivity, which reduces the use of quantum resources. When the phase sensing optical field is strong, the linear Michelson interferometer with injecting the squeezed vacuum can also reach the optimal sensitivity, and the sensitivity is robust for both losses inside and outside the interferometer. All the kinds of interferometers are more sensitive to the loss inside the interferometer than outside the interferometer, and the sensitivity of phase measurement can be improved by reducing the loss inside the interferometer. Our result provides direct reference of experimental implementation of high performance interferometer for high precision quantum metrology.

Generation of Bessel-Gaussian vortex beam by combining technology

Yu Tao, Xia Hui, Fan Zhi-Hua, Xie Wen-Ke, Zhang Pan, Liu Jun-Sheng, Chen Xin
Acta Physica Sinica. 2018, 67 (13): 134203 doi: 10.7498/aps.67.20180325
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Bessel beam is an important member of the family of non-diffracting beams and has some unique properties which can be used in many areas, such as micro particle manipulating, material processing and optical communication. However, the source of Bessel beam generated by the existing methods can be used only in a short distance due to its low power. In this paper, according to the coherent combining technology, we propose a method to generate a second-order Bessel-Gaussian (BG) beam by loading discrete vortex phase on specific spatially distributed Gaussian beam array. The coherent combining technology can enhance the output power by increasing the number of beams and use the phase-locking technique to maintain the beam quality. The experimental scheme is described as follows. The expanded Gaussian beam is first split by an amplitude-based spatial light modulator, then the Gaussian beam array is incident on a phase-only spatial light modulator to load the discrete vortex phase, and finally the Gaussian beam array loaded with phase can synthesize BG beam in free space. Due to the diffraction effect of the sub-beams, the optical field distribution between the adjacent sub-beams which are loaded with phase differences, are superimposed. As a result, the optical field distribution of the approximate beam can be obtained by coherent synthesis in free space. After that, the degree of similarity between simulated results and theoretical data is analyzed by correlation coefficient, including the comparison of light intensity between experiment and simulation, and the power-in-the-bucket is used to evaluate beam quality. In addition, the topological charge of the synthesized BG beams is verified by the interference method. By studying the number of beams, the waist radius and the radius of the ring, we find some interesting results which are summarized as follows. Firstly, the closed arrangement of Gaussian beam arrays can improve the quality of the synthesized BG beam. Secondly, the smaller the phase difference between the sub-beams, the more easily the discontinuous piston phase approaches to the vortex phase. Therefore, increasing the number of sub-beams can significantly improve the beam quality of the synthesized BG beam and obtain a higher order synthetic BG beam. Finally, we define the parameter k to represent the tightness of a circular array of Gaussian beams. The present study shows that when the parameter k is close to 1, the best experimental results can be obtained. Therefore, the proposed method has important guidance in generating various vortex beams or enhancing the vortex beam power.

Stability switching behavior of thermoacoustic oscillation in Rijke tube

Dang Nan-Nan, Zhang Zheng-Yuan, Zhang Jia-Zhong
Acta Physica Sinica. 2018, 67 (13): 134301 doi: 10.7498/aps.67.20180269
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Large-amplitude self-excited thermoacoustic oscillations arising due to the interaction between unsteady heat release and acoustic pressure fluctuations have been encountered in many thermal devices. These oscillations may lead to unwanted structural vibrations and efficiency reduction while emitting loud noises, and thus the predicting of such oscillations is very important. Physically, oscillation is a kind of instability, so stability analysis can be applied to understanding such a phenomenon. The present work focuses on the role of time delay between unsteady heat release and flow perturbation in the stability of thermoacoustic system. To this end, one-dimensional Rijke tube model with both open ends is numerically investigated. In the Rijke tube model, an electric heater is located at the first quarter of the Rijke tube and its unsteady heat release rate is modeled by an empirical model proposed by Heckl. Non-dimensional momentum equation and energy equation of the acoustic perturbation are derived and solved in time domain by using the Galerkin technique. The time evolution of the thermoacoustic oscillations with continuous increase in the time delay is calculated in two different acoustic damping cases, namely the heavily damped case and the weakly damped case, while other parameters are fixed. It is found that in both the heavily damped case and the weakly damped case, the system stability switches between stability and instability as the time delay increases, which is called stability switching and is a typical nonlinear phenomenon in a delay-dependent system. However, compared with in the heavily damped case, in the weakly damped case, the stability region is enlarged and the amplitude of the limit cycle oscillation is increased. Besides, in the weakly damped system, the dominating mode of system shifts in the first three modes instead of keeping in the first mode during increasing the time delay, which suggests that for the weakly damped system, the higher modes cannot be neglected and the system cannot be analyzed with a single-mode model either. Further, the bifurcation plots for the variation of the time delay for these two cases show that the system stability changes with time delay for a period of two, which is equal to the period of the first acoustic mode. As a conclusion, the results of present work indicate that the time delay between unsteady heat release and flow perturbations plays a critical role in generating thermoacoustic oscillations, and the findings of stability switching can help to understand the nonlinear phenomena in thermoacoustic systems.

PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES

Charging mechanism and application of lunar dust grains

Xue Dan, Liu Jin-Yuan, Li Shu-Han
Acta Physica Sinica. 2018, 67 (13): 135201 doi: 10.7498/aps.67.20180047
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Since the moon has an extremely rarefied atmosphere, the full spectrum of the electromagnetic radiation of the sun reaches the surface, charging the surface dust and affecting its current charge state. Lunar surface dust thus remains electrostatically charged at all times. Charged lunar dust will adversely affect the operations of most mechanical systems required by manned and unmanned exploration missions. Charged dust will also stubbornly adhere to solar panels and thermal radiators, thus reducing their efficiencies. Researches on the charged lunar dust can help to investigate lunar dusty environment as well as to solve those particle-induced problems by both simulation and experiment in laboratory. In this work, two different charging processes of charged lunar dust in the environment of electron beam and the radiation of ultraviolet source are considered. The computer numerical simulation method is used to analyze these two different charging processes of lunar dust, to explore the charging mechanisms of lunar dusts, and to choose an appropriate way of charging for the lunar environment simulation device in laboratory. On the basis of the classic dust charging equation, the charging equation of a dust in pure electron environment is given for the first time in this work. Meanwhile, the charging process under ultraviolet radiation is discussed and combined with the specific application of charging dusts. A solver of fourth-order Runge-Kutta algorithm is made to solve differential equations under two different irradiation sources. The main simulation results show that:1) in electron environment, the surface dust charge number increases as the particle size and the current intensity of electron guns increase, while the charge number increases as the beam spot radius of electron guns decreases; 2) under ultraviolet radiation, the dust charge number increases with the particle size and irradiance increasing, but charging efficiency is slow. A great dust charge number needs a long time radiation from sun (equivalent to 74 deuterium lamps), which means that more ultraviolet radiation sources are essential to speeding up the experiment in laboratory. Although the calculated efficiency of ultraviolet radiation is lower than electron irradiation, the secondary-electron emission, the scattering and the transmission process of electron irradiation are ignored, which can greatly reduce the efficiency of charging by energetic electron guns in the actual experiment. Therefore, comparing these two charging mechanisms and considering the actual design requirements for the space environment simulation device, charging by lots of ultraviolet radiation is an appropriate scheme for electrification of lunar dusts.

Acta Physica Sinica
Accepts
Note: The papers published below will continue to be available from this page until they are assigned to an issue. To see an article, click its [PDF] link. To review many abstracts, check the boxes to the left of the titles you want, and click the 'Selected articles' button. To see one abstract at a time, click its [Abstract] link.
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The Effect of Collision Parameter on a Magnetized Electronegative Plasma Sheath Structure

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

null
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

null
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

null
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

null
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

null
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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     No.7      No.8      No.9      No.10      No.11      No.12
2000 Vol.49      No.1      No.2      No.3      No.4      No.5      No.6
     No.7      No.8      No.9      No.10      No.11      No.12
1999 Vol.48      No.1      No.2      No.3      No.4      No.5      No.6
     No.7      No.8      No.9      No.10      No.11      No.12
     No.13
1998 Vol.47      No.1      No.2      No.3      No.4      No.5      No.6
     No.7      No.8      No.9      No.10      No.11      No.12
1997 Vol.46      No.1      No.2      No.3      No.4      No.5      No.6
     No.7      No.8      No.9      No.10      No.11      No.12
1996 Vol.45      No.1      No.2      No.3      No.4      No.5      No.6
     No.7      No.8      No.9      No.10      No.11      No.12
1995 Vol.44      No.1      No.2      No.3      No.4      No.5      No.6
     No.7      No.8      No.9      No.10      No.11      No.12
1994 Vol.43      No.1      No.2      No.3      No.4      No.5      No.6
     No.7      No.8      No.9      No.10      No.11      No.12
1993 Vol.42      No.1      No.2      No.3      No.4      No.5      No.6
     No.7      No.8      No.9      No.10      No.11      No.12
1992 Vol.41      No.1      No.2      No.3      No.4      No.5      No.6
     No.7      No.8      No.9      No.10      No.11      No.12
1991 Vol.40      No.1      No.2      No.3      No.4      No.5      No.6
     No.7      No.8      No.9      No.10      No.11      No.12
1990 Vol.39      No.1      No.2      No.3      No.4      No.5      No.6
     No.7      No.8      No.9      No.10      No.11      No.12
1989 Vol.38      No.1      No.2      No.3      No.4      No.5      No.6
     No.7      No.8      No.9      No.10      No.11      No.12
1988 Vol.37      No.1      No.2      No.3      No.4      No.5      No.6
     No.7      No.8      No.9      No.10      No.11      No.12
1987 Vol.36      No.1      No.2      No.3      No.4      No.5      No.6
     No.7      No.8      No.9      No.10      No.11      No.12
1986 Vol.35      No.1      No.2      No.3      No.4      No.5      No.6
     No.7      No.8      No.9      No.10      No.11      No.12
1985 Vol.34      No.1      No.2      No.3      No.4      No.5      No.6
     No.7      No.8      No.9      No.10      No.11      No.12
1984 Vol.33      No.1      No.2      No.3      No.4      No.5      No.6
     No.7      No.8      No.9      No.10      No.11      No.12
1983 Vol.32      No.1      No.2      No.3      No.4      No.5      No.6
     No.7      No.8      No.9      No.10      No.11      No.12
1982 Vol.31      No.1      No.2      No.3      No.4      No.5      No.6
     No.7      No.8      No.9      No.10      No.11      No.12
1981 Vol.30      No.1      No.2      No.3      No.4      No.5      No.6
     No.7      No.8      No.9      No.10      No.11      No.12
1980 Vol.29      No.1      No.2      No.3      No.4      No.5      No.6
     No.7      No.8      No.9      No.10      No.11      No.12
1979 Vol.28      No.1      No.2      No.3      No.4      No.5      No.6
1978 Vol.27      No.1      No.2      No.3      No.4      No.5      No.6
1977 Vol.26      No.1      No.2      No.3      No.4      No.5      No.6
1976 Vol.25      No.1      No.2      No.3      No.4      No.5      No.6
1975 Vol.24      No.1      No.2      No.3      No.4      No.5      No.6
1974 Vol.23      No.1      No.2      No.3      No.4      No.5      No.6
1973
1972
1971
1970
1969
1968
1967
1966 Vol.22      No.1      No.2      No.3      No.4      No.5      No.6
     No.7      No.8      No.9
1965 Vol.21      No.1      No.2      No.3      No.4      No.5      No.6
     No.7      No.8      No.9      No.10      No.11      No.12
1964 Vol.20      No.1      No.2      No.3      No.4      No.5      No.6
     No.7      No.8      No.9      No.10      No.11      No.12
1963 Vol.19      No.1      No.2      No.3      No.4      No.5      No.6
     No.7      No.8      No.9      No.10      No.11      No.12
1962 Vol.18      No.1      No.2      No.3      No.4      No.5      No.6
     No.7      No.8      No.9      No.10      No.11      No.12
1961 Vol.17      No.1      No.2      No.3      No.4      No.5      No.6
     No.7      No.8      No.9      No.10      No.11      No.12
1960 Vol.16      No.1      No.2      No.3      No.4      No.5      No.6
     No.7      No.8
1959 Vol.15      No.1      No.2      No.3      No.4      No.5      No.6
     No.7      No.8      No.9      No.10      No.11      No.12
1958 Vol.14      No.1      No.2      No.3      No.4      No.5      No.6
1957 Vol.13      No.1      No.2      No.3      No.4      No.5      No.6
1956 Vol.12      No.1      No.2      No.3      No.4      No.5      No.6
1955 Vol.11      No.1      No.2      No.3      No.4      No.5      No.6
1954 Vol.10      No.1      No.2      No.3      No.4
1953 Vol.9      No.1      No.2      No.3      No.4
1952
1951 Vol.8      No.1      No.2      No.3
1950 Vol.7      No.5      No.6
1949 Vol.7      No.4
1948 Vol.7      No.3
1947 Vol.7      No.1      No.2
1946 Vol.6      No.2
1945 Vol.6      No.1
1944 Vol.5      No.1      No.2
1943
1942
1941
1940 Vol.4      No.1
1939 Vol.3      No.2
1938
1937 Vol.3      No.1
1936 Vol.2      No.1      No.2
1935 Vol.1      No.3
1934 Vol.1      No.2
1933 Vol.1      No.1
物理学报
· Numerical simulation of soliton trapping of the supercontinuum in photonic crystal fiber [2012, No.12:124203-124203] (38474)
· Large-eddy simulation and experimental study of deflecting oscillation of planar opposed jets [2013, No.8:84704-084704] (38079)
· Effect of concentration of heavy oxygen vacancy in rutile and anatase (TiO2) on electric conductivity performance studied by simulation and calculation [2013, No.23:237101-237101] (30633)
· Quasiparticle band structure calculation for SiC using self-consistent GW method [2012, No.13:137103-137103] (28901)
· Proximity-effect-induced superconductivity by granular Pb film on the surface of Bi2Te3 topological insulator [2013, No.16:167401-167401] (26970)
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