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CN 11-1958/O4
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Thermal stability of MoS2 encapsulated by graphene
Liu Le, Tang Jian, Wang Qin-Qin, Shi Dong-Xia, Zhang Guang-Yu
Acta Physica Sinica, 2018, 67 (22): 226501
In-situ measurement of diamagnetic response of potassium-adsorbed multi-layer FeSe ultrathin films on SrTiO3(001) substrate
Chen Chuan-Ting, Yao Gang, Duan Ming-Chao, Guan Dan-Dan, Li Yao-Yi, Zheng Hao, Wang Shi-Yong, Liu Can-Hua, Jia Jin-Feng
Acta Physica Sinica, 2018, 67 (22): 227401
Controllable synthesis and performance of magnetic nanocomposites with core/shell structure
He Xue-Min, Zhong Wei, Du You-Wei
Acta Physica Sinica, 2018, 67 (22): 227501
Acta Physica Sinica  
  Acta Physica Sinica--2018, 67 (22)   Published: 20 November 2018
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INVITED REVIEW

Controllable synthesis and performance of magnetic nanocomposites with core/shell structure Hot!

He Xue-Min, Zhong Wei, Du You-Wei
Acta Physica Sinica. 2018, 67 (22): 227501 doi: 10.7498/aps.67.20181027
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Magnetic nanocomposites with core/shell structure are an important class of functional materials, and their comprehensive properties are affected by the microstructures of materials: they are largely dependent on the controlled sysnthesis of the composite systems. In this paper, we review the research advances in the preparation, characterization and performance of core/shell-structured magnetic nanocomposites, focusing on the following systems: 1) ferrite-based permanent-magnet/soft-magnetic (or antiferromagnetic) composite nanomaterials; 2) nanocomposites comprised of the magnetic core particles and the nonmagnetic coating layers; 3) carbon-based nanocomposites obtained by the catalytic synthesis of magnetic particles; 4) nanocomposites with exchange bias effect; 5) one-dimensional nanocomposites with coaxial core/shell structure; 6) core/shell/shell structured magnetic ternary nanocomposites. The components of these composite systems include M-type permanent-magnet ferrites, 3d transition metals (and their alloys, oxides and carbides), multiferroics, nonmagnetic (such as insulator, semiconductor and organic molecule), and carbon materials. And the emphasis is placed on the analysis of thermal stability, photoluminescence performance, photoelectrocatalytic capacity, electrochemical characteristics, microwave absorption properties, magnetoresistance effect, permanent magnetic property, high-frequency soft-magnetic properties, exchange bias effect and related phenomenology for the core/shell-structured nanocomposites. Finally, the future developing trend of the magnetic nanocomposites with core/shell structure is presented, and some fundamental researches and modified applications are also proposed.
SPECIAL TOPIC—Quantum states generation, manipulation and detection

New research progress of nuclear magnetic resonance quantum information processing

Kong Xiang-Yu, Zhu Yuan-Ye, Wen Jing-Wei, Xin Tao, Li Ke-Ren, Long Gui-Lu
Acta Physica Sinica. 2018, 67 (22): 220301 doi: 10.7498/aps.67.20180754
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In the last 20 years, there have been lots of novel developments and remarkable achievements in quantum information processing theoretically and experimentally. Among them, the coherent control of nuclear spin dynamics is a powerful tool for the experimental implementation of quantum schemes in liquid and solid nuclear magnetic resonance (NMR) system, especially in liquid-state NMR. Compared with other quantum information processing systems, NMR platform has many advantages such as the long coherence time, the precise manipulation and well-developed quantum control techniques, which make it possible to accurately control a quantum system with up to 12-qubits. Extensive applications of liquid-state NMR spectroscopy in quantum information processing such as quantum communication, quantum computing and quantum simulation have been thoroughly studied over half a century. There are also many outstanding researches in the recent several years. So we focus on the recent researches in this review article. First, we introduce the basic principle of the liquid-state NMR quantum computing and two new methods reported in the pseudo-pure state preparation which has more advantages than the traditional methods. The quantum noise-injection methods and the quantum tomography technology in liquid-state NMR are also mentioned. Then we overview Horrow-Hassidim-Lioyd algorithm, quantum support vector machine algorithm, duality quantum computing and their implementations in liquid-state NMR system. Also, we report recent researches about quantum simulations, including quantum tunneling, high-energy physics and topological sequences. Then we display the quantum cloud platform of our group. In order to let more people, either amateurs or professionals, embrace and more importantly participate in the tidal wave of quantum science, we launch our NMR quantum cloud computing (NMRCloudQ) service. Through NMRCloudQ, we offer a direct access to a real, physical spectrometer in our laboratory and encourage users to explore quantum phenomena and demonstrate quantum algorithms. Finally, we discuss the development prospects and development bottlenecks of NMR, and point out the prospects for the future development direction.

Topological quantum material simulated with superconducting quantum circuits

Yu Xiang-Min, Tan Xin-Sheng, Yu Hai-Feng, Yu Yang
Acta Physica Sinica. 2018, 67 (22): 220302 doi: 10.7498/aps.67.20181857
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During the past decades, the exploration of new topological material and the study of their novel physical properties have become a hot topic in condensed matter physics. However, it is hard to realize various topological materials and observe their physical properties that have been predicted theoretically due to the limitation of experimental techniques, such as fabrication, parameter control, and measurement. This situation makes quantum simulation a way alternative to simulating large quantum systems. In general, quantum simulation can be implemented by some controllable quantum systems. As a kind of all-solid state device, superconducting quantum circuit is an artificial quantum system that has great advantage in scalability, integration, and controllability, which provides an important scheme to realize the quantum simulator. In this paper, we review our recent results of quantum simulation in the space-time inversion symmetry protected topological semimetal bands, Hopf-link semimetal bands, and topological Maxwell metal bands with superconducting quantum circuits. These results show that the superconducting circuit is a promising system for simulating the quantum many-body system in condensed matter physics.

Research progress of semiconductor up-conversion single photon detection technology

Bai Peng, Zhang Yue-Heng, Shen Wen-Zhong
Acta Physica Sinica. 2018, 67 (22): 221401 doi: 10.7498/aps.67.20180618
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Quantum communication technology has achieved remarkable progress and development in recent years, and the single photon detector, as the receiving terminal, plays a vital role in communication systems. In this paper, we focus on the current mainstream semiconductor-based single photon detectors and review their device principle, operating mode, advantages and disadvantages. Besides, the research progress of a novel semiconductor near-infrared single photon detection technology (USPD) is introduced. The feasibility and superiority of the USPD device are demonstrated from the basic principle, device structure and key performance indicators of USPD, and the latest spatial optical coupling experiment results of the USPD are also given. The design principle of the USPD device is to utilize Si multiplication layer of the Si SPAD as a multiplication layer instead of InP in conventional InGaAs-SPAD. The Si-SPAD has a much lower dark count rate and afterpulsing effect because of high-quality material of Si. Such a characteristic design of USPD can suppress the afterpulsing probability to the same level as that of the Si-SPAD and enables it to operate in the free-running regime without sacrificing photon detection efficiency. For the same reason, the dark count rate (DCR) of USPD is also very low. The operating mechanism of USPD is to convert the infrared photons into near-infrared or visible photons and the emitted near-infrared photons can be detected by a Si SPAD, which provides us with a new idea for single photon detection.

Single-molecule electroluminescence and its relevant latest progress

Zhang Yao, Zhang Yang, Dong Zhen-Chao
Acta Physica Sinica. 2018, 67 (22): 223301 doi: 10.7498/aps.67.20181718
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Research on the interaction and interconversion between electrons and photons on an individual molecular scale can provide scientific basis for the future developing of information and energy technology. Scanning tunneling microscope(STM) can offer abilities beyond atomic-resolution imaging and manipulation, and its highly localized tunneling electrons can also be used for exciting the molecules inside the tunnel junction, generating molecule-specific light emission, and thus enabling the investigation of molecular optoelectronic behavior in local nano-environment. In this paper, we present an overview of our recent research progress related to the single-molecule electroluminescence of zinc phthalocyanine (ZnPc) molecules. First, we demonstrate the realization of single-molecule electroluminescence from an isolated ZnPc by adopting a combined strategy of both efficient electronic decoupling and nanocavity plasmonic enhancement. By further combining the photon correlation measurements via the Hanbury-Brown-Twiss interferometry with STM induced luminescence technique, we demonstrate and confirm the single-photon emission nature of such an electrically driven single-molecule electroluminescence. Second, by developing the sub-nanometer resolved electroluminescence imaging technique, we demonstrate the real-space visualization of the coherent intermolecular dipole-dipole coupling of an artificially constructed non-bonded ZnPc dimer. By mapping the spatial distribution of the photon yield for the excitonic coupling in a well-defined molecular architecture, we can reveal the local optical response of the system and the dependence of the local optical response on the relative orientation and phase of the transition dipoles of the individual molecules in the dimer. Third, by using a single molecular emitter as a distinctive optical probe to coherently couple with the highly confined plasmonic nanocavity, we demonstrate the Fano resonance and photonic Lamb shift at a single-molecule level. The ability to spatially control the single-molecule Fano resonance with sub-nanometer precision can reveal the coherent and highly confined nature of the broadband nanocavity plasmon, as well as the coupling strength and the anisotropy of the field-matter interaction. These results not only shed light on the fabrication of electrically driven nano-emitters and single-photon sources, but also open up a new avenue to the study of intermolecular energy transfer, field-matter interaction, and molecular optoelectronics, all at the single-molecule level.

Coherent manipulation of single collective excitations in a cold atomic ensemble

An Zi-Ye, Wang Xu-Jie, Yuan Zhen-Sheng, Bao Xiao-Hui, Pan Jian-Wei
Acta Physica Sinica. 2018, 67 (22): 224203 doi: 10.7498/aps.67.20181183
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Single photons are the best carriers of quantum information for long-distance transmission. Nevertheless, maximal achievable distance is limited by the exponential decay of photons as a function of link length. The protocol of quantum repeater provides a promising solution by replacing direction transmission with segmented entanglement distribution and entanglement connection via swapping. The quantum repeater necessitates a key element of quantum memory for making efficient interconnections. An atomic ensemble is very suitable for this purpose due to the collective enhanced interaction. Single photons are stored as collective excitations in an atomic ensemble. Thus a comprehensive study of the physics relating to collective excitations is crucially important for improving the quantum memory performance and its reachable applications in quantum repeater and quantum network. In this article, we review our experimental work on cold atomic ensembles in recent years, focusing on the coherent manipulation of collective excitations. We first briefly introduce the general concept of collective excitations and the preparation process through spontaneous Raman scattering, and we review our experimental work on extending the coherence time, such as suppressing motional dephasing by increasing the spin-wave wavelength, by confining atoms with a three-dimensional optical lattice. Afterwards, we discuss about the retrieval process of collective excitations and review our experiments on using a ring-cavity enhanced setup to improve the retrieval efficiency. The coherent qubit operation in a quantum memory is very useful for enabling new functionalities for a quantum network, in a subsequent section, we thus review our work on developing Raman-based coherent operations for single excitations. Afterwards, we mention our experiments on creating a pair of atom-photon entanglement by interfering two modes of a collective excitation. Improving the entanglement preparation efficiency is crucially important, and Rydberg-based interaction provides a promising solution. Our experimental work in this direction is also reviewed. Additionally, as an application in coherent manipulation with collective excitations, we show several experiments on using excitations in remote atomic memories and demonstrating basic functionality of quantum repeater and quantum network. In short, significant progress has been made in the coherent manipulation of single collective excitations in cold atomic ensembles, and further improvement will be accelerated by the Rydberg-enabled interactions; practical applications in quantum repeater and quantum network is foreseeable in the near future.

Single photon detection and circular polarized emission manipulated with individual quantum dot

Li Tian-Xin, Weng Qian-Chun, Lu Jian, Xia Hui, An Zheng-Hua, Chen Zhang-Hai, Chen Ping-Ping, Lu Wei
Acta Physica Sinica. 2018, 67 (22): 227301 doi: 10.7498/aps.67.20182049
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Studies on quantum dots (QDs) provide great opportunities in single photon detection as well as single circular polarized photon emission, which are the key technology for future quantum information processing. For single photon detection, the quantum-dot-resonant-tunneling-diode (QD-RTD) is evaluated as one of the most promising scheme but still suffering from the ultralow working temperature (~5 K) and lack the capability to discriminate photon numbers. Here we demonstrate a photon-number-resolving detector based on quantum dot coupled resonant tunneling diodes (QD-cRTD). Individual QDs coupled closely with adjacent quantum well (QW) of resonant tunneling diode operate as photon-gated switches which turn on (off) the RTD tunneling current when they trap photon-generated holes (recombine with injected electrons). With proper decision regions defined, 1-photon and 2-photon states are resolved in 4.2 K with excellent propabilities of accuracy of 90% and 98% respectively. Further, by identifying step-like photon responses, the photon-number-resolving capability is sustained to 77 K, making the detector a promising candidate for advanced quantum information applications where photon-number-states should be accurately distinguished. On the other hand, we firstly performed the magneto-optical studies on single InGaAs/GaAs self-assembled QDs. We observed the exciton Zeeman splitting and diamagnetic shift of a single QD under magnetic field, and the exciton g factor and diamagnetic coefficient was extracted by fitting the magnetic field dependent PL energies. By comparing with theories, we discussed on the effect of QD size, shape and composition on these two parameters. Based on these work, we investigated the single QD exciton-cavity mode coupling effect under external magnetic field. By first time we observed the interaction of Zeeman splitted exciton spin states with the cavity mode and realized the selective enhancement of the SE rate of the exciton state with specific spin configuration by means of magnetic manipulation of Purcell effect. In this sense, single QD emission with higher circular polarization degree under non-polarized excitation was realized. Our results have high potential to open up a way to novel quantum light sources and quantum information processing applications based on cavity quantum electrodynamics effects.

Quantum manipulation of electronic phase separation in complex oxides

Wang Wen-Bin, Zhu Yin-Yan, Yin Li-Feng, Shen Jian
Acta Physica Sinica. 2018, 67 (22): 227502 doi: 10.7498/aps.67.20182007
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Complex oxides system displays exotic properties such as high temperature superconductivity, colossal magnetoresistance and multiferroics. Owing to the strong correlation between lattice, spin, charge and orbital degrees of freedom, competing electronic states in complex oxides system often have close energy scales leading to rich phase diagrams and spatial coexistence of different electronic phases known as electronic phase separation (EPS). When the dimension of complex oxides system is reduced to the length scale of the correlation length of the EPS, one would expect fundamental changes of the correlated behavior. This offers a way to control the physical properties in the EPS system. In this paper, we review our recent works on electronic phase separation in complex oxide systems. We discovered a pronounced ferromagnetic edge state in manganite strips; by using lithographic techniques, we also fabricated antidot arrays in manganite, which show strongly enhanced metal-insulator transition temperature and reduced resistance. Moreover, we discovered a spatial confinement-induced transition from an EPS state featuring coexistence of ferromagnetic metallic and charge order insulating phases to a single ferromagnetic metallic state in manganite. In addition, by using unit cell by unit cell superlattice growth technique, we determined the role of chemical ordering of the dopant in manganite. We show that spatial distribution of the chemical dopants has strong influence on their EPS and physical properties. These works open a new way to manipulate EPS and thus the global physical properties of the complex oxides systems, which is potentially useful for oxides electronic and spintronic device applications.

Physics and devices of quanutm light emission from semicoductor self-assembled quantum Dots

Shang Xiang-Jun, Ma Ben, Chen Ze-Sheng, Yu Ying, Zha Guo-Wei, Ni Hai-Qiao, Niu Zhi-Chuan
Acta Physica Sinica. 2018, 67 (22): 227801 doi: 10.7498/aps.67.20180594
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Self-assembled semiconductor single quantum dots (QDs), as a good candidate of solid-state real single photon (SP) emitters in high purity and counting rate, have attracted great attention in recent two decades, promising for quantum information, optical quantum computation, quantum storage, and quantum coherent manipulation. To isolate single QD from the other QDs surrounding, 1) dilute QD density is well controlled during epitaxy; 2) micro-pillars or nanowires individually in space as hosts are fabricated. To enhance their uni-directional emission, GaAs/AlAs distributed Bragg reflector (DBR) planar cavity is integrated. To improve the system (i.e. confocal microscope, traditionally) stability and its optical collection efficiency, a near-field fiber coupling by adhering a micro-pillar chip to fiber facets directly is used. To enhance the coherence of QD spontaneous emission, resonant excitation technique is applied. In this article, we review our research progress in self-assembled QD SP emission, including SP emission from InAs or GaAs QDs on Ga droplet-self-catalyzed GaAs nanowires (with g2(0) of 0.031 or 0.18, respectively), SP emission from InAs/GaAs QDs coupled with high-Q (1000-5000) DBR micro-pillar cavities and their fiber-coupled device fabrication with SP fiber output rate ~1.8 MHz, single QD resonant fluorescence with inter-dot coherent visibility of 40%, strain-coupled bilayer InAs QDs to extend their emission wavelength to 1320 nm and parametric down conversion of 775 nm SP emission from single QD in nanowire to realize entangled photon pairs at 1550 nm (entanglement fidelity of 91.8%) for telecomm application, and definite quantum storage of InAs QD SPs at 879 nm in ion-doped solid (at most 100 time-bins). In future, there will be still several urgent things to do, including 1) puring the environment of a single QD (e.g. growing GaAs QDs to avoid the wetting layer, and optimizing QD growth to avoid smaller QDs) to reduce its spectral diffusion and developing a high-symmetric QD (e.g. GaAs QD) to reduce the fine structure splitting of its emission; 2) positioning single QD precisely for a good alignment of single QD to a micro-cavity or fiber cone (single mode with high numerical aperture) to increase optical excitation efficiency and SP collection efficiency; 3) developing optical quantum integrated chip, including hybrid structures of active micro-cavity and passive waveguide, and high-transmission waveguide beamsplitter or Mach-Zender interferometer to improve SP extraction (micro-cavity), collection (optical setup) and counting rate (at avalanched photon detectors and coincidence counting module).

Excited state dynamics of molecules studied with femtosecond time-resolved mass spectrometry and photoelectron imaging

Wang Yan-Mei, Tang Ying, Zhang Song, Long Jin-You, Zhang Bing
Acta Physica Sinica. 2018, 67 (22): 227802 doi: 10.7498/aps.67.20181334
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Study of quantum states of molecules, especially the evolution of excited states can help to understand their basic features and the interactions among different states. Furthermore, the information about the chemical reaction process and the interactions among several reaction channels can be obtained. Femtosecond time-resolved mass spectrometry (TRMS) and time-resolved photoelectron imaging (TRPEI), which combine pump-probe technique with time of flight mass spectrometry and photoelectron imaging, are powerful tools for detecting the molecular quantum state and for studying the molecular quantum state interaction and molecular ultrafast dynamics. With these methods, the photochemistry and photophysics mechanism of isolated molecule reaction process can be investigated on a femtosecond time scale. The principles of TRMS and TRPEI are introduced here in detail. On the basis of substantial research achievements in our group, the applications of TRMS and TRPEI are presented in the study of ultrafast internal conversion and intersystem crossing, wavepacket evolution dynamics at excited states of polyatomic molecules, energy transfer process of polyatomic molecules, ultrafast photodissociation dynamics and structural evolution dynamics of molecular excited states. In the study of ultrafast internal conversion and intersystem crossing, the methyl substituted benzene derivatives and benzene halides are discussed as typical molecular systems. In the study of wavepacket evolution dynamics at excited states of polyatomic molecules, the real-time visualization of the dynamic evolution of CS2 4d and 6s Rydberg wave packet components, the vibrational wave packet dynamics in electronically excited pyrimidine, the rotational wave packet revivals and field-free alignment in excited o-dichlorobenzene are reported. In order to discuss the energy transfer process of polyatomic molecules, the intramolecular vibrational energy redisctribution between different vibrational states in p-difluorobenzene in the S1 low-energy regime and the intramolecular energy transfer between different electronic states in excited cyclopentanone are presented. For the study of ultrafast photodissociation dynamics, the dissociation constants and dynamics of the A band and even higher Rydberg states are investigated for the iodine alkanes and iodine cycloalkanes. Structural evolution dynamics of molecular excited states is the main focus of our recent research. The structural evolution dynamics can be extracted from the coherent superposition preparation of quantum states and the observation of quantum beat phenomenon, by taking 2, 4-difluorophenol and o-fluorophenol as examples. Time-dependent photoelectron peaks originating from the planar and nonplanar geometries in the first excited state in 2, 4-difluorophenol exhibit the clear beats with similar periodicities but a phase shift of π rad, offering an unambiguous picture of the oscillating nuclear motion between the planar geometry and the nonplanar minimum. Also, the structural evolution dynamics in o-fluorophenol via the butterfly vibration between planar geometry and nonplanar minimum is mapped directly. Finally, the potential developments and further possible research work and future directions of these techniques and researches are prospected.

Novel superconducting qubits and quantum physics

Zhao Shi-Ping, Liu Yu-Xi, Zheng Dong-Ning
Acta Physica Sinica. 2018, 67 (22): 228501 doi: 10.7498/aps.67.20180845
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In the past years, superconducting quantum computation has received much attention and significant progress of the device design and fabrication has been made, which leads qubit coherence times to be improved greatly. Recently, we have successfully designed, fabricated, and tested the superconducting qubits based on the negative-inductance superconducting quantum interference devices (nSQUIDs), which are expected to have the advantages for the fast quantum information transfer and macroscopic quantum phenomenon study with a two-dimensional potential landscape. Their quantum coherence and basic physical properties have been demonstrated and systematically investigated. On the other hand, a new type of superconducting qubit, called transmon and Xmon qubit, has been developed in the meantime by the international community, whose coherence time has been gradually increased to the present scale of tens of microseconds. These devices are demonstrated to have many advantages in the sample design and fabrication, and multi-qubit coupling and manipulation. We have also studied this type of superconducting qubit. In collaboration with Zhejiang University and the University of Science and Technology of China, we have successfully fabricated various types of the coupled Xmon devices having the qubit numbers ranging from 4 to 10. Quantum entanglement, quantum algorithm of solving coupled linear equations, and quantum simulation of the many-body localization problem in solid-state physics have been demonstrated by using these devices. Also, we have made significant achievements in the studies of the macroscopic quantum phenomena, quantum dissipation, quantum microwave lasing, and some other quantum optics problems. In particular, Autler-Townes splitting under strong microwave drive, electromagnetically induced transparency, stimulated Raman adiabatic passage, microwave mixing, correlated emission lasing, and microwave frequency up-and-down conversion have been successfully studied, both experimentally and theoretically.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES

Thermal stability of MoS2 encapsulated by graphene Hot!

Liu Le, Tang Jian, Wang Qin-Qin, Shi Dong-Xia, Zhang Guang-Yu
Acta Physica Sinica. 2018, 67 (22): 226501 doi: 10.7498/aps.67.20181255
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Monolayer molybdenum disulfide (MoS2), a semiconductor material with direct band gap, is considered to be an important fundamental material for the future development of the semiconductor industry. In order to apply the material to semiconductor devices, we have to investigate the electrical, optical and thermal properties of MoS2. People have always been concerning about the electrical and optical properties, but pay little attention to the thermal properties of MoS2, especially thermal stability. It is well known that semiconductor device generates a lot of heat when it works, sometimes even running in high temperature environment. The above conditions all require the material which has good thermal stability. So we focus on how to improve the thermal stability of MoS2. In this paper, we report the construction of the van der Waals heterostructures of graphene and MoS2 by encapsulating monolayer MoS2 with graphene, and dissect the thermal stability of encapsulated MoS2 in argon (Ar) and hydrogen (H2) atmosphere respectively. The results show that in Ar atmosphere, MoS2 encapsulated by graphene keeps stable when the temperature increases to 1000 ℃, while the exposed MoS2 is decomposed almost completely at 1000 ℃. In H2 atmosphere, MoS2 encapsulated by graphene keeps stable when the temperature increases to 1000 ℃, but the exposed MoS2 is decomposed completely at 800 ℃. In conclusion, the thermal stability of MoS2 encapsulated by graphene can be improved significantly. We analyze the reason why MoS2 encapsulated by graphene gains good thermal stability. Firstly, the covered graphene provides additional van der Waals forces, which increases the decomposition energy of MoS2, making it more stable at high temperature environment. Secondly, graphene separates MoS2 from the external environment, preventing MoS2 from contacting and reacting with external gas, which greatly improves the thermal stability of MoS2 at high temperature environment. Meanwhile, graphene covers the active defect site on MoS2, making it difficult to react at defects. In summary, the monolayer MoS2 devices can work normally at high temperature when MoS2 is encapsulated by graphene. In addition, our work also provides a feasible approach to improving the thermal stability of other two-dimensional materials.

Orientation effect of asymmetric diatomic molecules in transport diffusion

Jiang Zhi-Liang, Chen Pei-Rong, Zhong Wei-Rong, Ai Bao-Quan, Shao Zhi-Gang
Acta Physica Sinica. 2018, 67 (22): 226601 doi: 10.7498/aps.67.20181376
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Non-equilibrium transport is an important research area in statistical physics. The influences of the structures of polyatomic molecules on their transport have attracted the attention of researchers. Up to now, most of researchers deemed that temperature gradient is the main factor for molecular orientation and neglected the effect of the chemical potential gradient on the molecular orientation. To make up the deficiency in the study of chemical potential gradients, we build a non-equilibrium system with both chemical potential gradient and temperature gradient, and study the transport diffusion behavior of asymmetric diatomic molecules by using molecular dynamics and Monte Carlo methods. It is found that the diatomic molecules implement the orientation effect during non-equilibrium transport. Under the chemical potential gradient, the molecular orientation effect leads to the fact that the large atom tends to be in the direction of low concentration particle bath, while the small atom tends to be in the direction of high concentration particle bath. The molecular orientation is opposite to the direction of the flow. Under the temperature gradient, the molecular orientation effect leads to the fact that the large atom tends to be in the direction of high temperature particle bath, while the small atom tends to be in the direction of low temperature particle bath. The molecular orientation is the same as the direction of the flow. The orientation direction caused by concentration gradients is opposite to that caused by temperature gradients and it appears as a competitive relationship. At the same time, the influence of the asymmetry of the molecule itself on the molecular orientation is also studied. The larger the asymmetry of the molecule itself (σB/σA), the more obvious the molecular orientation effect is. When σB/σA>1.6, the influence of the asymmetry of the molecule itself on the orientation effect is gradually saturated. When σB/σA=1, which is also for a symmetric molecule, even if neither the temperature gradient nor the chemical potential gradient is zero, no molecular orientation occurs. We explain the physical mechanism of orientation through the principle of minimum entropy production. This work is of theoretical significance for in depth understanding the relationship between mass transport and molecular structure under non-equilibrium conditions.

Different silicon crystal face index of inverted pyramid structure

Chen Quan-Sheng, Liu Yao-Ping, Chen Wei, Zhao Yan, Wu Jun-Tao, Wang Yan, Du Xiao-Long
Acta Physica Sinica. 2018, 67 (22): 226801 doi: 10.7498/aps.67.20181275
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As a kind of important semiconductor material, crystalline silicon has vast applications in many industries, such as integrated circuits and solar cells. With anisotropic etching method, including alkali etching and copper assisted catalytic etching, pyramid or inverted pyramid structure on the surface of silicon can be formed due to different crystal face indices of the silicon wafer, which is especially for multi-crystalline silicon wafers, because there are many different crystal faces on the surface. The proportion of different crystal faces has a high reference value for controlling the quality of multi-crystalline silicon. In this paper, the mathematical model of the inverted pyramid structure is established by making use of the relationship between the silicon crystal indices (abc) and {111} crystal plane. The inverted pyramid structures with different crystal face index (abc) values are divided into three possible cases for discussion, which are 0≤ab<c, 0≤a<b=c, a=b=c. The inverted pyramid structure on which the crystal face index (abc) satisfies 0≤ab<c is of a pentahedron composed of five points and has a quadrangular cross section. The inverted pyramid structure in which the crystal face index (abc) satisfies 0≤a<b=c is of a heptahedron composed of eight points and has a hexagonal cross section. The inverted pyramid structure whose crystal plane index (abc) satisfies a=b=c=1 is also of a heptahedron and has a hexagonal cross section but is composed of nine points. In general, the cross section of the (111) crystal face inverted pyramid is similar to an equilateral triangle because three of the edges are easier to etch away. The scanning electron microscopy image results show that the crystal indices are (100), (110) and (111), thereby demonstrating the correctness of the theoretical calculations. The index of crystal face has a one-to-one correspondence relationship with the inverted pyramid structure. Therefore, according to the inverted pyramid structure after anisotropic etching, we can measure the index of Si crystal face.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES

In-situ measurement of diamagnetic response of potassium-adsorbed multi-layer FeSe ultrathin films on SrTiO3(001) substrate Hot!

Chen Chuan-Ting, Yao Gang, Duan Ming-Chao, Guan Dan-Dan, Li Yao-Yi, Zheng Hao, Wang Shi-Yong, Liu Can-Hua, Jia Jin-Feng
Acta Physica Sinica. 2018, 67 (22): 227401 doi: 10.7498/aps.67.20181522
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A single-unit-cell layer FeSe ultrathin film grown on SrTiO3(001) substrate exhibits remarkable high-temperature superconductivity, which has aroused intensive research interest. Electron transfer from the substrate to the FeSe layer has been shown to play an indispensable role in enhancing the extraordinary superconductivity. With this idea, researchers have tried to search for new high-temperature superconducting material systems including K-adsorbed multi-layer FeSe ultrathin films, on which superconducting-like energy gaps have been observed with scanning tunneling spectroscopy and photoelectron spectroscopy. However, the high-temperature superconductivity of the multi-layer FeSe ultrathin films has not yet been confirmed by directly observing the zero resistance or Meissner effect. With a self-developed multi-functional scanning tunneling microscope (STM+), which enables not only usual STM functionality, but also in situ two-coil mutual inductance measurement, we successfully observe the diamagnetic response of a K-adsorbed multilayer FeSe ultrathin film grown on a SrTiO3(001) substrate, and thus determine its transition temperature to be 23.9 K. Moreover, we calculate the penetration depth of the film from the measured results and find that its low-temperature behavior exhibits a quadratic variation, which strongly indicates that the order parameter of the superconducting K-adsorbed multi-layer FeSe ultrathin film has an S± pairing symmetry.

Multipactor susceptibility chart of coaxial transmission lines with stationary statistical modeling

Lin Shu, Xia Ning, Wang Hong-Guang, Li Yong-Dong, Liu Chun-Liang
Acta Physica Sinica. 2018, 67 (22): 227901 doi: 10.7498/aps.67.20181341
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Multipactor breakdown is a detrimental electromagnetic phenomenon caused by resonant secondary electron emissions synchronizing with field oscillation, which frequently takes place in powerful microwave devices and accelerating structures. Regarded as the principal failure mode of space microwave systems, multipactor may cause the performance to degenerate or even hardware operation to deteriorate catastrophically, thus multipactor becomes a major limitation in promoting the further development of space communication technology. Meanwhile, higher power capacity and volume integration accordingly lead to continuously growing multipactor hazard. In order to prevent multipactor from occurring, the accurate predictive technique to determine multipactor susceptibility has become a key issue for the mechanical design and performance optimization of microwave devices in the ground stage. Compared with the existing approaches to investigating the multipactor, statistical theories are able to conduct multipactor threshold calculation and mechanism analysis, with the stochastic nature of secondary emission fully considered from the probabilistic perspective. Currently, stationary statistical theory of multipactor has been developed for efficient multipactor threshold analysis of the parallel-plate geometry. However, it has not been further extended to the coaxial geometry which is commonly involved in radio frequency (RF) systems. For this reason, the stationary statistical modeling of the coaxial multipactor with all influencing factors considered is detailed in this paper. Due to the field nonuniformity and the secondary emission randomness, analytic equation of electron trajectories in the coaxial geometry is approximately derived by using the perturbation approach. Based on the implicit correlation between electron emission velocity and transit time, the joint probability density function is constructed for the calculation of the probability density distribution of electron transit time. Afterwards, a system of integral equations for depicting electron multiplication process in the coaxial geometry is formulated and solved with a novel and general iteration method. Finally, this stationary statistical theory is applied to the full multipactor susceptibility chart of coaxial transmission lines with typical coating materials in space engineering, such as silver, copper, alumina and alodine. A comparison shows that the calculation results are in reasonable agreement with the experimental measurements provided by the Europe Space Agent. What is more, there exists significant difference between multipactor susceptibility curves of the parallel-plate geometry and the coaxial geometry. This research is of great significance for optimizing the mechanism design and material selection of multipactor-free microwave devices.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY

An interconnected state observer for lithium-ion battery based on reduced electrochemical model

Pang Hui, Zhang Xu
Acta Physica Sinica. 2018, 67 (22): 228201 doi: 10.7498/aps.67.20181429
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The accurate estimation of the solid concentration distribution in anode and cathode, and state-of-charge (SOC) for a Li-ion battery cell is significantly important for developing the real-time monitoring algorithm of the Li-ion cell's working operation, and further establishing an efficient and reliable advanced battery management system (BMS). Firstly, according to the porous electrode theory and concentration theory, in this article we present a systematic optimized model and a method of identifying the key internal state parameters based on a Li-ion cell's enhanced single-particle-model (ESPM), in which, an appropriate parameter vector is identified in the typical hybrid-pulse-power-characterization (HPPC) operation scenario by using the parameter sensitivity analysis method, and then this parameter optimization problem is evaluated by genetic algorithm. It is verified that the maximum relative errors of the cell's output voltage for ESPM are 1.92%, 3.18% and 2.86% under HPPC, 1C-discharge and urban dynamometer driving schedule (UDDS) current profiles, respectively. Secondly, by introducing some assumptions and reduction techniques, the battery ESPM is further reduced and then a novel interconnected state observer is proposed through the combination of the reduced ESPM and H robust control theory framework, which can realize the concurrent estimation of solid concentration and SOC in anode and cathode. Finally, the comparative validation and analysis study are conducted by using the experimental data acquired in HPPC and UDDS condition to demonstrate the effectiveness and feasibility of the proposed interconnected observer. The results show that the maximum relative errors of output voltage for the ESPM, the single-electrode concentration observer (Obsv-1) and the proposed interconnected observer (Obsv-2) of Li-ion cell are 2.0%, 3.8% and 2.6%, respectively, under HPPC operation at 23 ℃; under the same input current profile and operating condition, the maximum relative errors of SOC estimation are 2.4%, 4.7% and 3.4%, respectively. Moreover, the maximum relative errors of cell's output voltage for ESPM, Obsv-1 and Obsv-2 model are 1.9%, 3.2% and 2.1%, respectively, and the maximum relative errors of SOC estimation values for these three mathematical models are 2.1%, 4.4% and 3.2%, respectively. It is concluded that the proposed robust observer for a Li-ion cell can accurately predict the output voltage and SOC, and can also improve the dynamic performance and robust stability of Li-ion cell, which provides a solid theoretical foundation for developing the BMS.

Mechanism of magnetic radial vortex under effect of interfacial DzyaloshinskiiMoriya interaction

Dong Dan-Na, Cai Li, Li Cheng, Liu Bao-Jun, Li Chuang, Liu Jia-Hao
Acta Physica Sinica. 2018, 67 (22): 228502 doi: 10.7498/aps.67.20181392
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Recently, the topological magnetic textures, such as magnetic vortex, skyrmion, meron, have attracted wide attention. Siracusano et al. [Siracusano G, Tomasello R, Giordano A, et al. 2016 Phys. Rev. Lett. 117 087204] found a new topological magnetic configuration, named a magnetic radial vortex. The magnetic radial vortex state is a stable topological magnetic texture. The magnetization in the center of the magnetic radial vortex, namely the radial vortex polarity, points upward or downward. The in-plane component of the magnetization, namely, the radial vortex radial chirality, orientates radially outward or inward. The magnetic radial vortex has become another emerging research hotspot after skyrmion, which can be attributed to its better thermal stability and lower driven current density. In this paper, we investigate the nucleation mechanism of magnetic radial vortex under the effect of interfacial Dzyaloshinskii-Moriya interaction (IDMI) by using the micromagnetic simulation. The results indicate that the smaller the diameter of the soft magnetic nanodisk, the more easily the wider range of the intensity of IDMI is created. When the thickness of the disk is increased by one order of magnitude, the magnetic radial vortex can be formed stably. Therefore, the intensity of IDMI can be further reduced by appropriately choosing the disc size. The magnetic radial vortex can be nucleated no matter whether the initial magnetization configuration is circular vortex or uniform state. However, if the initial state is uniform, the magnetization component along the z-axis direction is prerequisite. In the magnetic radial vortex nucleation process, the nucleation time of the uniform state is significantly longer than that of circular vortex, and the energy variation time of circular vortex is longer than that of the uniform state. In the process of the formation of magnetic radial vortex, the variation of magnetic moment, skyrmion number and energy are determined by different initial magnetization configurations. This work contributes to the understanding of the mechanism of magnetic radial vortex and provides a theoretical guideline for choosing reasonable disc size and IDMI strength. Moreover, the above-mentioned conclusions contribute to the practical applications of magnetic radial vortex in spin electric devices.

Preparation and performance of high-efficient hole-transport-material-free carbon based perovskite solar cells

Fan Wei-Li, Yang Zong-Lin, Zhang Zhen-Yun, Qi Jun-Jie
Acta Physica Sinica. 2018, 67 (22): 228801 doi: 10.7498/aps.67.20181457
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Carbon based perovskite solar cells (C-PSCs) have attracted much attention because of their high stability and low-cost of production. However, due to the high interfacial resistance and the low energy level matching between perovskite and carbon electrodes, the maximum power conversion efficiency (PCE) is less than that of the metal-based perovskite solar cells. In this paper, a carbon-based perovskite solar cell is fabricated with the device structure of FTO/c-TiO2/m-TiO2/CH3NH3PbI3/Carbon. The perovskite films and carbon based perovskite solar cells are characterized by scanning electron microscope, atomic force microscope, X-ray diffraction (XRD), UV-Vis absorption spectrum, the steady-state spectrum, the time-resolved PL (TRPL) spectrum, and an electrochemical workstation. In addition, the internal mechanism of the efficiency improvement of carbon-based perovskite solar cell is discussed in depth. Then, the rotation speeds of mesoporous TiO2 layer (TiO2 paste diluted by ethanol with mass ratio of 1:4) are 1500, 1600, 1700 and 1800 r/min and the speeds of perovskite layer (CH3NH3I and PbI2 at a 1:1 molar ratio are stirred in a mixture of DMF and DMSO (9:1, v/v)) are 2000, 3000, 4000 and 5000 r/min; When the speed of m-TiO2 layer is 1700 r/min and the speed of perovskite layer is 4000 r/min, the mesoporous TiO2 layer thickness is about 500 nm, Thickness of CH3NH3PbI3 capping layer is about 400 nm. The cooperation of these two layers eventually leads to the high-quality perovskite with enlarged grain size, prolonged photoluminescence lifetime, lowered defect density, increased carrier concentration, and the finally enhanced photovoltaic performance. The device obtains the highest PCE of 11.11% with an open circuit voltage (Voc) of 0.93 V, a current density (Jsc) of 21.75 mA/cm2 and fill factor (FF) of 55%. At the same time, the stability of the carbon-based perovskite solar cell is also studied. The XRD is used for initial perovskite and the perovskite after 15 days to investigate the photo- and humidity stability of the full cells without encapsulation. The device exhibits excellent air stability with only 5% degradation when aged in ambient air at room temperature with 40%-50% humidity without any encapsulation after 15 days, which is better than the metal based perovskite solar cell. Our results open the way for making cost-efficient and stable PSCs toward market deployment.
NUCLEAR PHYSICS

Laser fast neutron measured by bubble detector

Zhao Lei, Xu Miao-Hua, Zhang Yi-Hang, Zhang Zhe, Zhu Bao-Jun, Jiang Wei-Man, Zhang Xiao-Peng, Zhao Xu, Tong Bo-Wei, He Shu-Kai, Lu Feng, Wu Yu-Chi, Zhou Wei-Min, Zhang Fa-Qiang, Zhou Kai-Nan, Xie Na, Huang Zheng, Zhong Jia-Yong, Gu Yu-Qiu, Li Yu-Tong, Li Ying-Jun
Acta Physica Sinica. 2018, 67 (22): 222101 doi: 10.7498/aps.67.20181035
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Neutron source has broad application prospects in crystallography, neutron irradiation, neutron therapy for cancer, and so on. As a new scheme to produce bright pulsed neutron source, the laser-driven neutron has attracted wide interest. In recent years, laser driven neutron sources have been extensively studied and the great progress has been made. Short pulsed laser driven neutron sources could be a compact and relatively cheap way to produce quasi-monoenergetic neutrons. The yields and the angular distributions of the laser-driven neutron sources are important in the research of laser-driven neutron sources and relevant applications. We conduct experimental investigation of this respect by using the XingGuang-Ⅲ high intense laser facility, which delivers synchronized picosecond and nanosecond laser pulses. The picosecond laser energy is 100 J, the pulse width is 1 ps, and the focusing spot diameter is 20 μm. At this time, the corresponding laser power density reaches 3×1019 W/cm2. A high-energy deuterium ion beam is produced by focusing the picosecond laser on a deuterated polyethylene foil, and the deuterium ion beam is incident on a secondary deuterated polyethylene planar target to activate the D-D reaction to obtain the neutron beam. In the experiment, the neutron yield and its angular distribution are measured by the different-sensitivity BD-PND bubble detectors, which are placed in the target chamber around the target. The emission of the neutron beam is found to be non-uniform. A maximum intensity of 5.13×107 n/sr is observed in the forward direction. The angular distribution of the neutron beam is theoretically calculated by taking into account the energy-angle cross section, the angular and energy distribution of the incident deuterium ion beam. The probability of the neutron energy-angle distribution in the laboratory system is obtained by the coordinate transformation from the probability in the center of mass frame. The results show good agreement with the experimental measurements. This experiment has a certain reference value in the practical application of D-D reaction neutron source.
ATOMIC AND MOLECULAR PHYSICS

Molecular structure and electronic spectrum of C12H4Cl4O2 under external electric field

Du Jian-Bin, Feng Zhi-Fang, Han Li-Jun, Tang Yan-Lin, Wu De-Qi
Acta Physica Sinica. 2018, 67 (22): 223101 doi: 10.7498/aps.67.20181454
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Various environmental poisons have caused damage to human production and life, and dioxin has seriously harmed human health. The C12H4Cl4O2(2, 3, 7, 8-tetrachlorodibenzo-p-dioxin, TCDD) is currently the most toxic compound. In order to study the influence of external electrical field on molecular structure and spectrum, herein the density functional theory (DFT) at a B3LYP/6-31+g (d,p) level is employed to calculate the geometrical parameters of the ground state of TCDD molecule under external electric fields ranging from 0 to 0.025 a.u. (0-1.2856×1010 V/m). Based on the optimized structure, time-dependent DFT at the same level as the above is adopted to calculate the absorption wavelengths and the molar absorption coefficients for the first twenty-six excited states of TCDD molecule under external electric fields. The results show that the most absorption band located at 221 nm with a molar absorption coefficient of 54064 L·mol-1·cm-1 in the UV-Vis absorption spectrum appears in the E belt, which originates from the benzene electronic transition from π to π*. In addition, a shoulder peak at 296 nm appears in the B belt, which is the characteristic absorption of aromatic compounds' electron transition from π to π*. Compared with the data in the literature, the wavelength of the shoulder is blue-shifted only 9 nm. The molecular geometry parameters are strongly dependent on the external field intensity, and the total energy decreases with external field intensity increasing. With the enhancement of external electric field, the electrons in the molecule have an overall transfer, which makes the big bond of benzene ring weakened, the energy of the transition decreases, and the wavelength of the transition increases, that is, the absorption peak is red-shifted. When the external electric field increases to 0.02 a.u., the electron cloud migration phenomenon of occupied and transition orbits of TCDD molecule are obvious, and the absorption peak red shift phenomenon is also very significant. With the enhancement of external electric field, the overall transfer of electrons in the molecule also reduces the density of the benzene rings and the surrounding electron cloud, reduces the number of electrons in the transition from π to π*, and also reduces the molar absorption coefficient. When the external electric field is enhanced to 0.02 a.u., the molar absorption coefficient decreases significantly. This work provides a theoretical basis for studying the TCDD detection and degradation method, and also has implications for other environmental pollutants detection methods and degradation mechanisms.

Regulation of sensitivity of Yb concentration to power-dependent upconversion luminescence colors

Gao Dang-Li, Li Lan-Xing, Feng Xiao-Juan, Chong Bo, Xin Hong, Zhao Jin, Zhang Xiang-Yu
Acta Physica Sinica. 2018, 67 (22): 223201 doi: 10.7498/aps.67.20181167
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Controlling the power density of exciting light is a widely applied technological approach to dynamically tuning emission spectra to yield desirable luminescence properties, which is essential for various applications in laser devices, cancer cell imaging, biomarker molecule detections, thermometers and optoelectronic devices. However, most of upconversion systems are insensitive to power regulation. In this study, a series of Yb/Ho doped NaYF4 microrods with different Yb concentrations was synthesized by using a sodium citrate-assisted hydrothermal method. The dependence of upconversion characteristics of NaYF4:Yb/Ho microrods on Yb concentration and excitation power density are investigated in detail by a laser confocal microscopy system. The emission spectra exhibit discrete upconversion emission characteristic peaks that can easily be assigned to 5F35I8 (at about 488 nm), 5F4, 5S25I8 (at about 543 nm), 3K7, 5G45I8 (at about 580 nm) and 5F55I8 (at about 648 nm) transitions of Ho, respectively. The upconversion spectra and synchronous luminescence imaging patterns show that the luminescence ratio of red to green is not only dependent on the Yb concentration, but also sensitive to the excitation power. With Yb concentration increasing from 5% to 60%, the sensitivity of the power-controlled red to green luminescence ratio largely increases from 0.1% to 13.0%, corresponding to a clear luminescent color modification from green to red. These results indicate that the power-tuned red-to-green-luminescence ratio can be used as a method of measuring and evaluating Yb doping concentration. We attribute the sensitivity tuned by Yb concentration to the differences in population approach and upconversion mechanism for the red and green luminescence. By recording the slope of luminescence intensity versus exciting power density in a double-logarithmic presentation, we detect a small slope for the green emission relative to that for the red emission, especially at a high Yb concentration. These results indicate that the red upconversion process may be a three-photon process. The exciting power induced color adjusting is therefore explained by preferential three-photon population of the red emission due to the high 5S25G4 excitation rate, which is verified by down-conversions of emission spectra. Our present study provides a theoretical basis for the spectral tailoring of rare-earth micro/nano materials and supplies a foundation for the applications in rare-earth materials.

Molecular dynamics simulations on scattering of Ar molecules on smooth and rough surfaces

Zhang Ran, Chang Qing, Li Hua
Acta Physica Sinica. 2018, 67 (22): 223401 doi: 10.7498/aps.67.20181608
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Molecular dynamics method is used to investigate the scattering characteristics of Ar molecule on smooth and rough Pt(100) surface. In this paper, a velocity sampling method is proposed to obtain the tangential momentum accommodation coefficients (TMACs) and the sticking probabilities of gas molecules on smooth and rough surface under different temperature conditions. The results show that the TMAC and the sticking probability decrease with increasing temperature under smooth surface condition. The results of our work are in excellent agreement with the results of the reference for a three-dimensional gas flow in a nanochannel. Unlike the scenario of smooth surfaces, the roughness of rough surfaces greatly promotes the accommodation of tangential momentum between the gas molecules and surfaces. When the roughness becoming larger, the TMAC approaches to 1.0 and the sensitivity to temperature decreases gradually. Unlike the relationship between TMAC and roughness, although the sticking probability of gas molecules increases with roughness increasing, its dependence on temperature does not change. Furthermore, the beam method where the incident velocity and angle are determined is used to quantitatively analyze the scattering characteristics of gas molecules on different surfaces. According to the number of collisions between gas molecule and the surface, we classify the scattering of gas molecules on a smooth surface into two types: single collision scattering and multiple collision scattering. For those gas molecules that experience one collision, their average tangential momentum decreases to a certain extent, however, the gas molecules scattered after multiple collisions tend to maintain the original tangential momentum. For gas molecules reflected from the smooth surface, their velocity distribution exhibits a typical bimodal distribution. The position of the first peak appears at the incident velocity value, and the position of the second peak appears at a velocity value of zero. Regarding rough surfaces, the existence of roughness changes the mode of exchange of momentum and energy between gas molecules and walls, resulting in a significant decrease in the average tangential momentum of gas molecules scattered on rough surfaces. Besides, the more the gas molecules colliding on the surface, the more severe the energy loss after scattering will be. For gas molecules reflected from the rough surfaces, their velocity distribution conforms to the characteristics of Gaussian distribution.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS

Broadband reconfigurable reflective polarization convertor

Yu Hui-Cun, Cao Xiang-Yu, Gao Jun, Yang Huan-Huan, Han Jiang-Feng, Zhu Xue-Wen, Li Tong
Acta Physica Sinica. 2018, 67 (22): 224101 doi: 10.7498/aps.67.20181041
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With the rapid evolution of radar technology and mobile communication systems, polarization conversion has received much attention from academia and industry in recent years, which has the advantages of improving system performance through eliminating multipath fading. In this paper, a novel broadband reconfigurable reflective polarization convertor is designed, which combines the idea of metamaterial and the technology of micro-electro-mechanical system (MEMS) switches. The proposed structure consists of three layers: an upper metallic patches layer, a middle dielectric layer with a thickness of 2 mm, and a bottom metal plate. There are through-holes of metal connecting the upper and bottom layers. According to the simulation using HFSS software, when the MEMS switch is on, the device works with a relative bandwidth of 57.77% from 7.78 GHz to 14.10 GHz, of which the polarization conversion ratio is larger than 80%. In addition, at 7.62 GHz and 12.56 GHz, the reflected wave is a right-hand circularly polarized wave and a left-hand circularly polarized wave, respectively. When the MEMS switch is off, the reflected wave is in the same polarization, which means the device does not convert the polarization of electromagnetic wave anymore. The electromagnetic wave are decomposed into the u-v coordinate system to further understand the wideband polarization rotation. The reflection phase and the surface current distributions of the convertor are analyzed. Then, the working principle of polarization rotation is explained by analyzing the current distributions and explaining the theory from three different viewpoints. Finally, a 1225-cell (35×35) prototype is fabricated to verify the simulation results. The measured curve has three resonant frequencies and shifts towards the lower frequency slightly. The discrepancy between simulations and measurements is mainly attributed to the restriction of fabrication and measurement condition. In general, experimental results are in agreement with the simulations: when linear polarized wave is incident, the reflected wave realizes the transition from co-polarization to cross-polarization as the switch is switched from off to on. The proposed reconfigurable polarization rotation surface has advantages of broadband, low loss and ease of fabrication, which has great potential applications in antenna radiation, reducing the radar cross section and other territories in controlling electromagnetic wave dynamically.

Doppler spectrum simulation of nonlinear ocean covered by oil film

Wang Rui, Guo Li-Xin, Zhang Ce
Acta Physica Sinica. 2018, 67 (22): 224102 doi: 10.7498/aps.67.20180184
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In recent years, marine oil spill has become an important disaster for marine environment. Marine oil spill quantity is an important indicator for evaluating the threat of oil spill. This paper focuses on the Doppler spectrum of one-dimensional (1D) nonlinear ocean covered by oil film. Oil film damps the capillary wave of the ocean, which leads to a smooth profile of the ocean covered by the film. The paper is devoted to the detailed analysis of the electromagnetic magnetic wave scattering from a sea that is covered with oil. More precisely, it focuses on the case of homogeneous oil slicks. This allows better detection of oil spills, as well as possibly an estimation of the amount of oil spilled, as the scattering coefficient depends on the layer thickness. The 1D Creamer nonlinear ocean is proposed based on the PM spectra. The Marangoni damping effect is considered for modeling the contaminated rough ocean surface. First, the influence of oil film on the ocean surface spectrum and geometrical structure are examined briefly in the present study. On this basis, the influence of oil film on the Doppler spectrum signature (in L-band) of the backscattered echo of the clean and contaminated rough ocean are studied in detail based on the iterative physical optics. The results of the Doppler spectrum signature including Doppler shift and spectral bandwidth of the backscattered echo from Creamer nonlinear ocean surface are different from those of the linear ocean surface especially at the big and moderate incident angles, which shows that it is necessary to adopt the Creamer nonlinear model in the paper. The simulation results show that the Doppler spectrum signatures including Doppler shift and spectral bandwidth of the echo from ocean covered by oil film are significantly affected by sea slicks. The influence of some important parameters, such as wind speed, oil-damping values and incident angles on Doppler spectrum signature is investigated and discussed in detail. Moreover, simulation results indicate that the Doppler spectrum signature is a promising technique for the remote sensing of oil films floating on sea surfaces.

Theoretical and experimental study of self-reconstruction property of astigmatic Bessel beam

Yang Yan-Fei, Chen Jing, Wu Feng-Tie, Hu Run, Zhang Hui-Zhong, Hu Han-Qing
Acta Physica Sinica. 2018, 67 (22): 224201 doi: 10.7498/aps.67.20181416
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In this paper, the self-reconstruction property of astigmatic Bessel beam is studied experimentally and theoretically. Based on the Fresnel diffraction integral theory and Babinet principle, the general expression of the intensity distribution of astigmatic Bessel beams passing through a circular obstacle is derived. The cross-section light intensity at transmission distance of, 10, 30, and 80 mm after astigmatism of the astigmatic Bessel beam are occluded by circular obstacles. The self-reconstruction process of the light field is observed and verified by using an specially designed experimental setup. In the experiment, we choose He-Ne laser as a light source, collimate and expand the beam through a telescope system, and a zero-order astigmatic Bessel beam is generated by a beam vertically incident on the tilted axicon after the diaphragm. A circular obstacle with a radius of 0.2 mm is placed at a distance of 200 mm behind the axicon. Finally, the cross-section intensities at different distances are observed and recorded by a microscope. The experimental phenomena are in good agreement with the theoretical prediction. The results show that the reconstruction of the zero-order astigmatic Bessel beams will occur after passing through the on-axis and off-axis obstacles. And as the transmission distance increases, the outer contour size of the astigmatic Bessel beam becomes larger, and the number of central spot arrays increases, and the complete beam is gradually reconstructed. Particularly, this feature is different from the behavior of the non-diffracting Bessel beam, which maintains the light field unchanged during transmission and has a single central spot. It is expected to be applied to multi-layer multi-particle control. And a new optical property is discovered in the experiments: the reconstruction speed of the beam in the horizontal and vertical direction are not consistent in the reconstruction process, and there is a certain speed difference. Further, we add a spiral phase plate between the diaphragm and the axicon to produce a high-order astigmatic Bessel beam. And it is verified that the high-order astigmatism Bessel beam has the same self-reconstruction characteristics after being shielded by obstacles. Compared with the zero-order aperture system, the high-order beam can not only expand the operating range, but also use the orbital angular momentum carried by the beam to achieve light rotation, which makes the particle manipulation more flexible. The research proves the self-reconstruction characteristics of astigmatic Bessel beams theoretically and experimentally, and broadens the research range of astigmatic Bessel beams. The research results have practical significance and application value in the field of optical micro-manipulation.

All-optical imaging and tracking technology for rectilinear motion targets through scattering media

Jia Hui, Luo Xiu-Juan, Zhang Yu, Lan Fu-Yang, Liu Hui, Chen Ming-Lai
Acta Physica Sinica. 2018, 67 (22): 224202 doi: 10.7498/aps.67.20180955
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Light scattering is a main factor that restricts optical transmission and deteriorates optical imaging performance. All-optical imaging for moving targets through complex scattering media is one of the most challenging techniques. In this paper, a method for real-time tracking of moving targets through scattering medium is presented by utilizing optical memory-effect and autocorrelation of speckle difference. In the experiment on imaging through a scattering medium, an object is hidden at a distance u behind a highly scattering medium. The object is illuminated by a spatially incoherent pseudothermal light source. The light is diffused through the scattering medium. Camera placed at a distance u0 on the other side of the medium records the pattern of the scattered light. According to the theory of optical memory-effect, the process of scattering imaging is a convolution process of point spread function (PSF) and object. In the procedure of object moving, the scattered signals from two frames are captured. The background noise could be removed by subtracting the two captured image. Then, the autocorrelation operation calculates the speckle difference, and hidden targets can be effectively reconstructed with the phase retrieval algorithm. The experiment demonstrates the imaging of targets with different speeds. The results have shown that the faster the speed, the worse the imaging quality is. High-speed moving objects can be imaged by using a high frame rate camera to reduce the exposure time or by disambiguating the speckle pattern. In subsequent experiments, the distance of the target movement is calculated with the magnification of the system. The collected two frames of speckle must be within the same memory effect angle. Only in this way can the calculation accuracy of the motion distance be guaranteed. With the moving of the target, the cross-correlation information of the target appears at different positions of the speckle difference autocorrelation map. Finally, according to the cross-correlation of the target at different locations, the real-time tracking of the moving target can be realized. Due to the Gaussian distribution of the laser beam, the cross-correlation intensity of the speckle difference autocorrelation map decreases with the object moving further. Therefore the target moving range is limited by the laser beam diameter, intensity distribution and camera field angle. It is verified experimentally that the imaging and tracking of moving targets which are hidden behind the ground glass can be achieved successfully by using this method. This kind of imaging and real-time tracking technology for targets moving through the scattering medium has important potential applications in biomedicine and other fields.

Structured beam designed by ray-optical Poincaré sphere method and its propagation properties

Zhang Shu-He, Shao Meng, Zhou Jin-Hua
Acta Physica Sinica. 2018, 67 (22): 224204 doi: 10.7498/aps.67.20180918
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Structured beam plays an important role in optical communication, microscopy and particle manipulations. Traditionally, structured beam can be obtained by solving Helmholtz wave equation. This method involves complex mathematical procedures, and the properties of solved light beam are obscure. It is worth noting that the structured beam can also be constructed by ray-optical Poincaré sphere method: this method is a rather intuitive and convenient for designing the structured beam with novel properties. This method also provides a ray-based way to study the propagation properties of structured beam. In this paper, the ray-optical Poincaré sphere method combined with plum-blossom curve is used to build a family of structured beams. The optical field distributions on beam waist, including intensity and phase, are calculated by the ray-optical Poincaré sphere method. The shape of inner and outer caustics of optical field are also detailed in order to demonstrate the self-healing or non-diffraction features of beams. By using angular spectrum diffraction, the free space evolutions of such structured beams are demonstrated. The results show that the structured beam turns to be the well-known Laguerre-Gaussian beam when the leaf number of plum-blossom curve is 0. While the leaf number equals 1, the structured beam has non-diffraction property, for its inner caustic concentrates onto two points. In geometrical optics sight, all light rays are tangent to the inner caustic, and the optical fields carried by rays interfere near the caustic, leading the beam to possess a self-healing capacity. The self-healing property is demonstrated in terms of rays. With the beam's propagating, rays which launch from the inner side of beam gradually reach the outer side of beam. On the contrary, the rays launching from the inner side of beam arrive at the outer side of beam. When the center of beam is blocked, the inner rays are also blocked. After propagating, outer side rays will reach the inner side, fill up the hole of beam, and recover the injury of optical field. Furthermore, we demonstrate the structured beam with a 5leave plum-blossom curve. In this case, the inner caustic of this beam turns into a decagonal star structure; our simulation results show that this beam has relatively strong self-healing capability. Theoretically, one can simply change the parameters of plum-blossom curve or choose other kind of Poincaré sphere curve to create more complex structured beams.

Design and simulation analysis of spectroscopic system for synchronous atmospheric three-phase water detection based on Raman lidar

Wang Yu-Feng, Zhang Jing, Tang Liu, Wang Qing, Gao Tian-Le, Song Yue-Hui, Di Hui-Ge, Li Bo, Hua Deng-Xin
Acta Physica Sinica. 2018, 67 (22): 224205 doi: 10.7498/aps.67.20180644
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Water is the only atmospheric parameter with three-phase states. The study on distribution and variation in three-phase water is of great scientific significance for understanding cloud microphysics, cloud precipitation physics, and water circulation, especially in the fields of artificial weather process. In the Raman lidar detection technology of three-phase water, it is necessary to solve the problem of high-spectral spectroscopic technique to ensure fine extraction of the echo signal and the detection with high signal-to-noise ratio (SNR). Considering the Raman spectrum characteristics of three-phase water, the influences of filter parameters in the Raman channels on the overlapping characteristics are theoretical simulated and discussed in detail, and the SNR is investigated as well. Regarding the fact that optimal solution can be obtained for neither overlapping nor SNR at the same time, an evaluation function method based on the multi-objective programming problem is proposed to analyze the optimal filter parameters. The results show that the minimum overlapping value and the higher system SNR can be obtained when the central wavelength and bandwidth of the filters are determined to be 397.9 nm and 3.1 nm, 403 nm and 5 nm, 407.6 nm and 0.6 nm in solid water, liquid water and water vapor channel, respectively, and thus the optimal design can be realized for synchronous detection Raman spectroscopic system for three-phase water. Further simulation results show that effective detection can reach above 3.6 km in the daytime and over 4 km on sunny days under a system factor of 1800 J·mm·min for three-phase water Raman measurement in the daytime. Furthermore, the obtained overlapping values are applied to accurate retrieval theory for three-phase water profiles. The simulated profiles of atmospheric water vapor, liquid water and ice water indicate that the water vapor, liquid water and solid water content can be increased synchronously in the cloud layer, and their content, distribution characteristics and the corresponding error are also discussed. The above results validate the feasibility of highspectral spectroscopic technique for detecting the synchronous atmospheric three-phase water, and will provide technical and theoretical support for synchronous retrieval of three-phase water by Raman lidar.

Characteristic parameters of optical wave and short-term beam spreading in oceanic turbulence

Wu Tong, Ji Xiao-Ling, Li Xiao-Qing, Wang Huan, Deng Yu, Ding Zhou-Lin
Acta Physica Sinica. 2018, 67 (22): 224206 doi: 10.7498/aps.67.20181033
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In 2000, Nikishov et al. presented an analytical model for the power spectrum of oceanic turbulence, in which the stable stratification of seawater is assumed, i.e., the eddy diffusivity of temperature is equal to that of salinity, and the eddy diffusivity ratio is equal to unity. Until now, all previous studies on the light propagation through oceanic turbulence were based on the Nikishov's power spectrum model. However, the eddy diffusivity of temperature and eddy diffusivity of salt are different from each other in most of underwater environments. Very recently, Elamassie et al. established a more reasonable power spectrum model of underwater turbulent fluctuations as an explicit function of eddy diffusivity ratio. The characteristic parameters such as the spatial coherence length of optical wave in turbulent medium play an important role in characterizing the strength of turbulence, the phase correction techniques in light propagation, etc. In the present paper, based on the Elamassie's power spectrum model of oceanic turbulence, the analytical formulae of the wave structure function, the spatial coherence length of optical wave and the Fried parameter in oceanic turbulence are derived, and the correctness of each of these formulae is verified. It is shown numerically that the results obtained by using the Elamassie's power spectrum model are quite different from those obtained by using the Nikishov's power spectrum model. If the Nikishov's power spectrum model is adopted, the strength of turbulence is underestimated when oceanic turbulence is dominated by the temperature fluctuations, while the strength of turbulence is overestimated when oceanic turbulence is dominated by the salinity fluctuations. If the Elamassie's power spectrum model is adopted, it is shown that the Kolmogorov five-thirds power law of the wave structure function is also valid for oceanic turbulence in the inertial range, and 2.1 times the spatial coherence length of optical wave is the Fried parameter, which are in agreement with those in atmospheric turbulence. In addition, based on the Elamassie's power spectrum model, the semi-analytical formula of the short-term beam spreading of Gaussian beams is derived in this paper, and its correctness is also verified. It is shown that the difference in short-term beam spreading is very large, whether the stable stratification of seawater is assumed or not. The results obtained in this paper are very useful for applications in optical communication, imaging and sensing systems involving turbulent underwater channels.

Suppression of lateral vibration in rectangular ultrasonic plastic welding tool based on phononic crystal structure

Zhao Tian-Tian, Lin Shu-Yu, Duan Yi-Lin
Acta Physica Sinica. 2018, 67 (22): 224207 doi: 10.7498/aps.67.20181150
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Ultrasonic welding is one of the main applications of high-power ultrasound and is used in the automotive industry and aerospace. Transducers and tool are important parts of the ultrasonic welding system. Different tools are required for different welding objects. For larger plastic welded parts, it is necessary to weld them with large-sized welding tools. Due to the large size of the welding tool, under the excitation of the transducer, the tool will produce a coupling effect of longitudinal vibration and lateral vibration. Lateral vibration will cause the radiation surface of the tool to be non-uniformly displaced, and the working efficiency and welding results of the welding system will also be affected. So, in this paper, the phononic crystal bandgap theory and coupling vibration theory are used to study the coupled vibration of large-sized rectangular plastic ultrasonic welding tools. In order to improve the work efficiency and radiation surface's displacement uniformity of the tool, the phononic crystal structure is used to suppress the lateral vibration of the large-sized plastic ultrasonic welding tool, and the lateral vibration band gap of the phononic crystal structure is calculated. The longitudinal resonance frequency of the system is designed in the band gap range of the lateral vibration of the tool. So the lateral vibration of the tool can be effectively suppressed. The longitudinal vibration displacements on the radiation surface of the rectangular tool before and after vibration suppression are analyzed and compared with each other. The vibration mode of the ultrasonic welding system is simulated by the Comsol Multiphysics finite element software. The large-scaled tool with phononic crystal structure has a radiation surface displacement compared with the tool without phononic crystal structure, and the results show that the radiation surface displacement with phononic crystal structure will increase and tend to be uniform, greatly optimize the welding effect, improve the working efficiency of the welding system, and meet the needs of practical engineering. It is concluded that the longitudinal resonance frequency of the ultrasonic plastic welding system within the lateral vibration bandgap on the phononic crystal structure can not only suppress the lateral vibration, but also make the longitudinal displacement of the radiation surface more uniform and larger. Therefore, the work efficiency is greatly improved.

Full focal imaging of ultrasonic Lamb waves using diffuse field information

Zhang Hai-Yan, Xu Meng-Yun, Zhang Hui, Zhu Wen-Fa, Chai Xiao-Dong
Acta Physica Sinica. 2018, 67 (22): 224301 doi: 10.7498/aps.67.20181268
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In this paper, a method is presented in which that the diffuse field information of Lamb waves is used to realize the full focal imaging of the defect that is near the transducer array. The near distance means that the defect is located in the near field of ultrasonic phased array and satisfies the near field calculation formula. Near field acoustic information of the defect is obscured by the nonlinear effects of early time saturation present in a directly acquired ultrasonic inspection. The approach proposed here is to recover near filed information through cross-correlation of diffuse fields. The diffuse field is generated through multiple scattering and reflection effects after sufficiently long time transmission of ultrasonic signal in a bounded medium. The near field information is implicitly contained throughout the diffuse field. By cross-correlating the diffuse fields of ultrasonic responses recorded at two monitoring points, the Green's functions between the two points is recovered and the direct response between them is obtained. This idea is applied to the full matrix capture of ultrasonic phased array in which the full matrix is formed by sequential acquisition of responses for each transmitter-receiver pair. A virtual array of emitters and receivers is therefore established. Typically, phase delays are used in post-processing to achieve advanced imaging. Here an undelayed full matrix of inter-element responses is reconstructed through cross-correlation of a later time diffuse full matrix. In order to evaluate the applicability of the method for ultrasonic non-destructive testing, the process of full matrix reconstruction is demonstrated experimentally on an aluminium plate containing the near field defect. Combining the full focal imaging, it is shown that a hybrid full matrix formed through a temporally weighted sum of coherent and reconstructed matrices reduces the background noise and allows the effective imaging of near field defect by direct contact experimental measurements. However, the near field defect is hidden by the region of artificial noise in conventional coherent capture images. The proposed imaging method presents a theoretical guidance for detecting and imaging near field defect in plate-like configurations by using the Lamb wave nondestructive testing method.

Horizontal refraction effects of seamounts on sound propagation in deep water

Li Sheng-Hao, Li Zheng-Lin, Li Wen, Qin Ji-Xing
Acta Physica Sinica. 2018, 67 (22): 224302 doi: 10.7498/aps.67.20181480
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The seamounts usually have important effects on sound propagation in deep water. A sound propagation experiment was conducted in the South China Sea in 2016. One of the experimental goals is to investigate the three-dimensional(3D) effects of seamounts on sound propagation. Phenomena about horizontal refraction of acoustic waves are observed in different propagation tracks which go through the seamount along different directions when the source depth is 200 m. Ray methods (BELLHOP N×2D and 3D models) which can calculate sound field efficiently and show clear physical images, are used to analyze and explain the causes of the phenomena. The experimental and numerical results show that the convergent zone structures are destroyed by the direct blockage of seamount due to the multiple reflection of acoustic waves, which leads to the increase of transmission loss (TL), and horizontal-refraction zone with obvious boundaries appears behind the seamount. Some experiment phenomena cannot be explained by BELLHOP N×2D model in which the horizontal refraction effects are not taken into consideration. The experimental sound field structure behind the seamount is obviously different from N×2D model numerical result, i.e.the width of shadow zone based on the experimental data is wider than that calculated by N×2D model, and the width of strong horizontal-refraction zone from the experiment is narrower than the N×2D model result. Moreover, the TLs calculated by N×2D model is about 10 dB less than the experimental result in horizontal refraction zone. After analyzing the difference between experimental data and N×2D model numerical results by BELLHOP 3D model which contains the azimuth-coupling capability, it can be concluded that sound waves reach the receiver through horizontal refraction after the interaction with seamount when the source is located behind the seamount. The eigenrays obtained from 3D model are less than N×2D model numerical result because some of sound beams cannot reach the receiver as a result of the horizontal refraction effects, which leads to the experimental TLs larger than the numerical results calculated by N×2D model. Therefore, 3D effect of seamount has an obvious influence on sound field within a certain angle range behind the seamount, and the investigation of 3D effects of seamounts is meaningful for the sound propagation and target detection in deep water.

Phase field lattice Boltzmann model for two-phase flow coupled with additional interfacial force

Li Yang, Su Ting, Liang Hong, Xu Jiang-Rong
Acta Physica Sinica. 2018, 67 (22): 224701 doi: 10.7498/aps.67.20181230
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The phase field model has become increasingly popular due to its underlying physics for describing two-phase interface dynamics. In this case, several lattice Boltzmann multiphase models have been constructed from the perspective of the phase field theory. All these models are composed of two distribution functions: one is used to solve the interface tracking equation and the other is adopted to solve the Navier-Stokes equations. It has been reported that to match the target equation, an additional interfacial force should be included in these models, but the scale of this force is found to be contradictory with the theoretical analysis. To solve this problem, in this paper an improved lattice Boltzmann model based on the Cahn-Hilliard phase-field theory is proposed for simulating two-phase flows. By introducing a novel and simple force distribution function, the improved model solves the problem that the scale of an additional interfacial force is not consistent with the theoretical one. The Chapman-Enskog analysis shows that the present model can accurately recover the Cahn-Hilliard equation for interface capturing and the incompressible Navier-Stokes equations, and the calculation of macroscopic velocity is also more efficient. A series of classic two-phase flow examples, including static drop test, droplets emerge, spinodal decomposition and Rayleigh-Taylor instability is simulated numerically. It is found that the numerical solutions agree well with the analytical solutions or the existing results, which verifies the accuracy and feasibility of the proposed model. In addition, the Rayleigh-Taylor instability with the imposed random perturbation is also simulated, where the influence of the Reynolds number on the evolution of the phase interface is analyzed. It is found that for the case of the high Reynolds number, a row of “mushroom” shape appears at the fluid interface in the early stages of evolution. At the later stages of evolution, the fluid interface presents a very complex chaotic topology. Unlike the case of the high Reynolds number, the fluid interface becomes relatively smooth at low Reynolds numbers, and no chaotic topology is observed at any of the later stages of evolution.

Dynamic characteristics of parallel water-entry cavity based on potential flow theory

Song Wu-Chao, Wei Ying-Jie, Lu Li-Rui, Wang Cong, Lu Jia-Xing
Acta Physica Sinica. 2018, 67 (22): 224702 doi: 10.7498/aps.67.20181375
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During the water-entry of the parallel axisymmetric bodies, the water-entry cavities are asymmetrical due to the mutual interference between the cavities. In the current study of the cavity dynamic models, the relatively perfect models of axially symmetric dynamic calculation of low speed single water entry have been established. These models mainly focus on the evolution of cavitation, and thus simplifying the flow pattern. However, due to the particularity of parallel water, the fluid forms a relative flow during the evolution of the cavitation in the inner region of the axisymmetric body axis. As a result, the flow is no longer axisymmetric but develops into a complex three-dimensional flow with strong nonlinearity, making the the theoretical model more difficult to establish. In order to analyze the evolution of the parallel cavities of the parallel axisymmetric body water-entry, the flow of the water-entry cavity interference region is simplified by the existing single water-entry calculation model based on the potential flow theory. The constraint of the relative flow to the cavity is simplified into a constraint potential, and the variation of cavity shape is analyzed. Based on the nonlinear hypothesis, the influence function is introduced to establish the calculation model of three-dimensional cavity and the three-dimensional evolution characteristics of parallel cavities are analyzed. The obtained results show that the velocity potential of the axisymmetric body water-entry can be regarded as the superposition of a point source and a line source located on the axis of the cavity. The expansion of the cavity is affected mainly by the point source, while the shrinkage of the cavity is influenced mainly by the line source. During the parallel water-entry of the axisymmetric body, the evolution of the parallel cavities in space is mirror symmetric and the mutual interference between cavities can be analyzed by introducing the potential wall surface. The potential wall has an inhibitory effect on the evolution of the cavities. The variation of the parallel water-entry cavity radius with the polar angle is related to the depth of the cavity cross section. In the inhibition evolution area near the closed point, the cavity radius decreases gradually with the increase of the polar angle, and the void section radius in the inhibition evolution region far from the closed point increases with the polar angle increasing, and is opposite to the radius law. In the shallower depth of the water-entry, the excessive evolution is formed in the expansion process of the cavity, and the excessive evolution will gradually weaken and disappear in the contraction process of the cavity.

Numerical investigation of cavity formation mechanism for micron-waterdrop impact on deep pool

Pei Chuan-Kang, Wei Bing-Qian
Acta Physica Sinica. 2018, 67 (22): 224703 doi: 10.7498/aps.67.20181422
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As one of the most fundamental and iconic fluid motion, droplet impact exists widely in scientific technologies and natural environment, and the phenomenon has been studied both for fundamental mechanism and for industrial applications in aerospace engineering, inkjet printing, agricultural irrigation and hydraulic structure erosion. Therefore, it is of great significance to study such basic movements for understanding the interfacial deformation of gas and liquid flow and improving the applications of droplet impact movement in engineering. Droplet impacting on a deep liquid pool has been extensively investigated for droplets with millimeter diameter. In this article, focusing on the cavity formation mechanism during a Micron-sized waterdrop impact on a deep pool, we perform systematic numerical simulations with adaptive mesh refinement technique and volume of fluid method to study the impact of a 290 μm water droplet on a deep water pool at velocities in a range of 2.5-6.5 m/s. The free surface motion, geometric variation of the cavity, local pressure field and vorticity field at selected times are presented to identify the pool-drop water mixing, capillary wave propagation, cavity formation, vortex ring generation and bubble entrapment phenomenon, and the dynamic mechanism of cavity motion is further explored. It is found that under the premise of neglecting the surface tension effects on the cavity whose depth is in a range of h∈(D, hmax), where D is the radius of initial droplet and hmax is the maximum depth, the cavity growth time to reach its maximum depth still scales as th5/2, where t is time, but in the end, the formation of the bottom of the cavity is driven by capillary waves. There are two types of the initial cavity shapes: one is U-shape and the other is hemispherical shape, the former one generally changes into V-shape, and in the latter case, the bottom of the cavity will gradually transform into cylindrical shape, resulting in a thin jet and possible bubble entrapment. Cavity collapse is closely related to capillary wave propagation. When the impact velocity is low (Fr=567.1, Re=1595, We=121.8), the low-pressure zone is initially generated at the junction between the cavity sidewall and the bottom, a large vortex ring is then generated near the free surface and the bottom of the cavity, respectively. Under high impact velocities (Fr=792.1, Re=1885, We=170.2), the thin jet is observed, the generation of the vortex ring is suppressed. The low-pressure zone is first generated at the junction between the wave bottom and the cavity sidewall, after the cavity becomes cylindrical, the cavity collapses before the capillary wave arrives at the bottom, causing a bubble entrapment.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES

PIC/MCC simulation of breakdown dynamics inside high power microwave output window

Zuo Chun-Yan, Gao Fei, Dai Zhong-Ling, Wang You-Nian
Acta Physica Sinica. 2018, 67 (22): 225201 doi: 10.7498/aps.67.20181260
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High power microwave (HPM) has important applications in controlled thermonuclear fusion heating, microwave high-gradient accelerator, high-power radar, directed-energy weapon, super jammer, impact radar, etc. The window breakdown of HPM has been extensively studied, and some research progress in this respect has been made. However, the researches on the transition of window breakdown from multipactor discharge to rf plasma are still not enough in-depth. Especially, the influences of microwave frequency and microwave amplitude during breakdown need further studying. This paper focuses on the process of dielectric multipactor and background argon ionization during the discharge breakdown near the HPM dielectric window/vacuum interface. A one-dimensional-spatial-distribution-and-three-dimensional-velocity-distribution (1D3V) electrostatic model with using particle-in-cell simulation is adopted in present work. The model includes secondary electron emission, electrostatic field induced by the remaining positive charge on the dielectric window, the motion of charged particles under electrostatic and microwave field, and the collision process between electron and background gas, and the corresponding PIC/MCC code is also developed. We examine the effects of gas pressure, microwave frequency and microwave amplitude on discharge breakdown. It is found that there exists only electron multipactor process during the discharge breakdown on dielectric window in vacuum. At low pressures (10 mTorr, 500 mTorr) and slightly high pressure (10 Torr), electron multipactor and gas ionization are coexistent. However, at an extremely high pressure (760 Torr), the gas ionization dominates the breakdown process. At the same time, the position of plasma density peak moves away from the dielectric window as the gas pressure increases, which is the consequence of the competition between secondary electron multiplication on the dielectric window and gas ionization in the body region. It can be seen that the advantage of gas ionization gradually increases as the gas pressure increases. In addition, it is also observed that at 500 mTorr, the moment of gas ionization moves forward first and then backward with the increase of the microwave amplitudes or the microwave frequency, especially when the increment of frequency is numerically twice that of the amplitude, gas ionization occurs earliest. This phenomenon is explained by the secondary electron emission model. Meanwhile, the results show that the position of plasma density peak from gas ionization gradually approaches to the dielectric window as the microwave amplitude increases. However, with continually increasing the microwave frequency, the plasma density peak moves away from the dielectric window first and then approaches to the dielectric window.

Picosecond laser-driven proton acceleration study of SGⅡ-U device based on charged particle activation method

He Shu-Kai, Qi Wei, Jiao Jin-Long, Dong Ke-Gong, Deng Zhi-Gang, Teng Jian, Zhang Bo, Zhang Zhi-Meng, Hong Wei, Zhang Hui, Shen Bai-Fei, Gu Yu-Qiu
Acta Physica Sinica. 2018, 67 (22): 225202 doi: 10.7498/aps.67.20181504
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The laser-driven proton acceleration experiment is carried out on the SGⅡ-U device based on charged particle activation method, and the target parameters are optimized. The charged particle method is used to measure the maximum cutoff energy of proton, angular profile, total yield and conversion efficiency of laser energy to proton energy for different copper film thickness under the same laser condition. It is found that the optimal copper film thickness for the SGⅡ-U picoseond laser-driven proton experiment is 10 μm, the highest proton energy obtained is about 40 MeV, and the total yield of protons (>4 MeV) is about 4×1012, the conversion efficiency of laser energy to proton energy is about 2%. Thicker or thinner copper film can reduce the maximum cut-off energy of accelerated proton; when the target thickness is reduced to 1 μm, the pre-pulse of the laser begins to have a significant effect on the target normal sheath acceleration (TNSA) proton, proton energy drops sharply, the proton beam porfile exhibits a hollow structure; when the target thickness is increased to 35 μm, although the energy of the proton is reduced, the proton beam spot is more uniform. According to our experimental results, when using SGⅡ-U picosecond laser to generate protons as a backlight diagnostics, a thicker Cu film can be selected which can supply more uniform proton beams. When the target is too thin, the TNSA proton itself has a modulation structure which will cause interference to yield the photographic results; when the protons generated by the SGⅡ-U picosecond are used to generate neutron source, the higher proton energy and yield are required, and 10 μm Cu film is suitable. The further enhancing the TNSA accelerated proton energy and quantity of the SGⅡ-U picosecond laser requires the further improving of the laser contrast.

Effect of phase angle on plasma characteristics in electrically asymmetric capacitive discharge

Hu Yan-Ting, Zhang Yu-Ru, Song Yuan-Hong, Wang You-Nian
Acta Physica Sinica. 2018, 67 (22): 225203 doi: 10.7498/aps.67.20181400
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In addition to the separate control of the ion energy and ion flux, the so-called electrical asymmetry effect (EAE) also plays an important role in improving the plasma radial uniformity. In this work, a two-dimensional fluid model combined with a full set of Maxwell equations is used to investigate the plasma characteristics in an electrically asymmetric capacitive discharge sustained by multiple consecutive harmonics. The effects of the phase angle θn on the dc self-bias (Vdc) and on the plasma radial uniformity for different numbers of consecutive harmonics k are discussed. The simulation results indicate that the phase angles of different harmonics θn have different influences on the dc self-bias Vdc. For instance, Vdc varies almost linearly with θ1 with a period π in dual frequency discharge, and the period is 2π for other discharge conditions. Besides, the modulation of Vdc becomes less obvious by changing the phase angle of the highest harmonic θk, especially for k>3. In addition, both the axial component of the power density Pz and the radial component of the power density Pr vary with θn, thus the plasma radial uniformity can be adjusted. When the total power density at the radial edge becomes comparable to that in the discharge center, the plasma distribution becomes uniform. For instance, when k=2, the plasma radial uniformity is the best at the phase angle θ1=π/2 and θ2=π. However, for k=3, the best radial uniformity is observed at θ1=3π/2, and the nonuniformity degree α is only 0.41% under this condition. It is worth noting that at k=8, the maximum of α is seven times higher than the minimum by changing the phase angles θ1 and θ2, which means that the plasma radial uniformity can be adjusted effectively. However, the modulation induced by θk(k>3) becomes less obvious, especially for k=8. Indeed, the electron density shows an edge-high profile, and the radial uniformity is always bad for all θ8 investigated. The results obtained in this work can help us to gain an insight into the optimization the plasma process by utilizing the EAE.
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