bluuebunny/crossref_metadata_2025 huggingface dataset:
Hybrid improper ferroelectricity and multiferroic in Ruddlesden-Popper structures
Authors: Liu Xiao-Qiang  , Wu Shu-Ya  , Zhu Xiao-Li  , Chen Xiang-Ming  ,   School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China
Published: December 2020

Hybrid improper ferroelectricity (HIF) is a secondary ferroelectric ordering induced by the coupling between oxygen octahedral in-plane rotation and out-of-plane tilt in a metal-oxide containing the perovskite structure units. Investigation of HIF will greatly extend the connotation and denotation of ferroelectric physics and material science, and it is expected to develop the room temperature single phase multiferroic material with large polarization and strong magnetoelectric coupling, owing to its intrinsic characteristic of the electric-field control of magnetism through HIF in magnet. In the present paper, the recent primary progress of HIFs and the multiferroics with Ruddlesden-Popper structures is reviewed, and the perspective of the future development is also presented.
First principle study of weak Dzyaloshinsky-Moriya interaction in Co/BN surface
Authors: Huang Can  , Li Xiao-Ying  , Zhu Yan  , Pan Yan-Fei  , Fan Ji-Yu  , Shi Da-Ning  , Ma Chun-Lan  ,   College of Science, Nanjing University of Aeronautics and Astronautics, Najing 210006, China;,   School of Mathematics and Physics, Suzhou University of Science and Technology, Suzhou 215009, China
Published: December 2020

Based on density functional theory calculations, we elucidate the atomic and electronic structures of Co atom of hexgonal BN (Co/h-BN). The interaction between magnetic moments of Co atoms is realized through Co-N_-B_ grid, which is indicated by the analysis of spin charge contour plot and partial density of states of each atom, where and denote the site of B or N atom close to and away from Co atom, respectively. Then the dispersion relations E(q) and E(-q) (q denotes the direction vector of spin spiral) between energy and wave vector of spin spiral in the opposite directions are calculated with generalized Bloch equations. In the incommensurate spin spiral calculations, all the magnetic moments of Co atom are arranged in the same plane that is perpendicular to the Co/h-BN film. The difference between E(q) and E(-q) is caused by the interface of Co/h-BN, where the symmetry of space perpendicular to the film is broken. Moreover, the effective Heisenberg exchange interaction (HBI) and Dzyaloshinsky-Moriya interaction (DMI) parameters between different neighbors (Ji and di) are derived by well fitting the ab initio magnon dispersion E(q) to HBI with DMI model and E(q)-E(-q) to DMI model, respectively. The J1 has a negative value and plays a major role, J3 is one order of magnitude smaller than J1, and other parameters are close to zero. Hence, Co/h-BN is triangular antiferromagnetic material with the q at k point in the first Brillouin zone. However, the spin spiral with the q at M point is only 2 meV larger than the basic state with the only negative J1 and smaller positive J2. The DMI is not shown in this interface with d1 and d2 close to zero. Based on the non DMI character and its stability in air, h-BN can be capped on other DMI interfaces. The reason that the DMI in Co/h-BN is much smaller than in Co/Gra is much larger height between Co and h-BN. It is 0.192 nm for h-BN but it is 0.156 nm for Co/Gra. We may reduce the height to enhance the DMI by other ways, such as adding electrical and magnetic fields in the future.
Surface-enhanced Raman scattering effect of silver nanoparticles array
Authors: Cheng Zi-Qiang  , Shi Hai-Quan  , Yu Ping  , Liu Zhi-Min  ,   Department of Applied Physics, School of Science, East China Jiaotong University, Nanchang 330013, China
Published: December 2020

The Raman signal of adsorbed Raman probe molecule can be significantly enhanced by using metallic nanostructures with high-density hot spots as surface enhanced Raman scattering (SERS) substrates. A great effort has been devoted to the improving of the SERS detection sensitivity and reproducibility by preparing ordered metal nanostructure arrays with controlled particle size, shape and hot spot position, which are used as SERS substrates. In this paper, we prepare high-density Ag nanoparticle arrays by electrochemical deposition in anodic aluminum oxide (AAO) templates. The particle size and the nanogap between the adjacent particles can be adjusted by changing the deposition time. The structures and surface plasmons of Ag nanoparticle arrays are characterized by scanning electron microscopy and reflectance spectra. The size of the gap between the particles significantly affects the plasmon resonance and the plasmon coupling between the particles. The SERS properties of Ag nanoparticle arrays are investigated by using 1, 4-benzenedithiol (1, 4-BDT) as Raman probe molecule. The Ag nanoparticle arrays with high SERS detection sensitivity and high reproducibility (uniformity) are prepared by optimizing the deposition time (the nanogap between the adjacent particles), and the detection limit of the 1, 4-BDT can reach 10-13 mol/L. The relative standard deviation of the SERS signal intensity randomly measured from 20 spots on the Ag nanoparticle array substrate is 5.35%. The finite-difference time domain simulations confirm that the plasmon coupling between nanoparticles is strong, and that the coupling between the nanoparticles will increase as the nanogap decreases. Additionally, the local field is enhanced at the bottom of the nanoparticle and the gap between the Ag nanoparticle and the AAO template is larger. These results show that Ag nanoparticle array can be used as a high-efficiency SERS substrate.
New research progress of nuclear magnetic resonance quantum information processing
Authors: Kong Xiang-Yu  , Zhu Yuan-Ye  , Wen Jing-Wei  , Xin Tao  , Li Ke-Ren  , Long Gui-Lu  ,   State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, China;,   Collaborative Innovation Center of Quantum Matter, Beijing 100084, China
Published: December 2020

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.
New progress on FeSe-based superconductors: high-quality and high-critical-parameter (Li, Fe)OHFeSe thin film
Authors: Dong Xiao-Li  , Yuan Jie  , Huang Yu-Long  , Feng Zhong-Pei  , Ni Shun-Li  , Tian Jin-Peng  , Zhou Fang  , Jin Kui  , Zhao Zhong-Xian  ,   National Lab for Superconductivity, Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China;,   Key Laboratory for Vacuum Physics, University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

High-quality superconducting thin films play an important role in the application and basic research of high-Tc superconductivity. In these aspects, iron-based superconductors feature the merits of rich physical phenomena and high superconducting critical parameters (including the transition temperature Tc, the upper critical field Hc2 and the critical current density Jc). The recently discovered high-Tc (Li,Fe)OHFeSe superconductor proves to be an important material for the studies of the mechanism and application of unconventional high-Tc superconductivity. However, due to the hydroxyl ion inherent in the compound, none of the conventional high-temperature synthesis methods is applicable for (Li,Fe)OHFeSe materials in bulk and thin film forms. Recently, by developing a hydrothermal ion-exchange technique, we have synthesized for the first time big and high-quality single crystals of (Li,Fe)OHFeSe (2015 Phys. Rev. B 92 064515). Here in this paper, we brief our most recent progress on growing a high-quality single-crystalline superconducting film of (Li,Fe)OHFeSe (2017 Chin. Phys. Lett. 34 077404). The film has been prepared on a LaAlO3 substrate by a hydrothermal epitaxial method. The high crystalline quality of the film is verified by X-ray diffraction (XRD). The XRD measurements show a single (001) orientation with a small crystal mosaic of 0.22 in terms of the full width at half maximum of the rocking curve, as well as an excellent in-plane orientation revealed by the -scan of (101) plane. Its bulk superconducting transition temperature Tc of 42.4 K is determined by both zero electrical resistance and diamagnetism measurements. Based on systematic magnetoresistance measurements, the upper critical field Hc2 is estimated to be 79.5 T and 443 T for the magnetic field perpendicular and parallel to the ab plane, respectively. Moreover, a large critical current density Jc of a value over 0.5 MA/cm2 is achieved at ~20 K. Such a (Li,Fe)OHFeSe film therefore is not only important for the fundamental research for understanding the high-Tc mechanism, but also promising for the applications in high-performance electronic devices and large scientific facilities such as superconducting accelerator.
Multi-pole dark solitons in nonlocal and cubic-quintic nonlinear medium
Authors: Zheng Yi-Fan  , Huang Guang-Qiao  , Lin Ji  ,   Department of Physics, Zhejiang Normal University, Jinhua 321004, China
Published: December 2020

In this paper, we mainly simulate the characteristics of the ground state dark soliton and the multipole dark soliton in the nonlocal and cubic-quintic nonlinear medium. Firstly, the influences of the degree of nonlocality on the amplitude and the width of the dark soliton in the self-defocusing cubic-and self-focusing quantic-nonlinear medium are studied. Secondly, we find the nonlinear parameters affecting the amplitude values of solitons, but the refractive index induced by the light beam is always a fixed value. The numerical results show that the ground state dark soliton can be propagated stably alone the z axis, and the stable states of the dipole soliton and the dark tri-pole and quadru-pole solitons are stable. However, some quadru-pole dark soliton is unstable after propagating the remote distance. Furthermore, we also discuss the characteristics of the ground state dark soliton and the dark dipole soliton in the local cubic-nonlinear and nonlocal quantic nonlinear media. Both the amplitude and the beam width of the dark ground state soliton and dark dipole soliton are also affected by the degree of nonlocality and nonlinearity. Two boundary values of the induced refractive index change with the variations of the three nonlinear parameters. The dark soliton and the dipole dark soliton are more stable in the self-focusing cubic nonlinear and the nonlocal self-defocusing quantic nonlinear medium than those in the self defocusing cubic nonlinear and nonlocal self-focusing quantic nonlinear medium. The powers of single dark soliton and dark tri-pole soliton decrease monotonically with the increase of propagation constant when the cubic-quintic nonlinearities are certain values and these degrees of nonlocalities are taken different values. Furthermore, we also analyze linear stabilities of various nonlocal spatial dark solitons. And we find that the dipole dark soliton is unstable when the propagation constant is in the region[-0.9,-1.0]. These properties of linear stabilities of other multi-pole dark solitons are the same as their propagation properties.
Research progress of single crystal growth for topological semimetals
Authors: Yi Chang-Jiang  , Wang Le  , Feng Zi-Li  , Yang Meng  , Yan Da-Yu  , Wang Cui-Xiang  , Shi You-Guo  ,   Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China;,   School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

Topological semimetals have attracted much attention and become a hot subject in condensed matter physics, and single crystal growth is the basis of the physical investigation on these materials. At present, the research of topological materials has formed a cooperation circle:presenting materials by theoretical calculation; single crystal growth; verification by experiments on single crystals. Single crystal growth has become a bridge between theory and experiment. Here in this paper, we introduce the single crystal growth of the topological semimetals presented in recent years, including topological Dirac semimetals, Weyl semimetals, Node-Line semimetals and other new classes of topological materials. The detailed growth methods are summarized in this paper for each material.
Quantum entanglement and cosmological Friedmann equations
Authors: Wang Can-Can  ,   Department of Physics, Shanghai University, Shanghai 200444, China
Published: December 2020

Quantum entanglement the most important part of quantum information theory, represents the intrinsic property of quantum states. It is a magical physical phenomenon in the form of nonlocality in the multi quantum system. The entanglement entropy as a measure of quantum information, has become an important tool, which provides a new research method for various subjects in physics. The study of the notion of quantum entanglement can provide a tool for understanding the cosmological features. In this work, we consider the cosmological applications of the entanglement in order to understand the cosmological dynamics from the entanglement point of view. The relation between the quantum information theory and the cosmology is studied. Employing Fermi normal coordinates (FNC) and conformal Fermi coordinates, we establish a relation between Friedmann equations of Friedmann-Lemaitre-Robertson-Walker universe and entanglement. Assuming that the entanglement entropy in a geodesic ball is maximized in a fixed volume and the entanglement is the basic element of the spacetime, we derive Friedmann equations from the first law of entanglement. Friedmann equations are first derived in the Fermi normal coordinate system, where the diamond size l is much smaller than the local curvature length, but still much larger than Planck scale lp. If the diamond size is comparable to the UV scale lUV, the quantum gravity effect becomes strong. Then we extend the discussion about the area deficit of the geodesic ball so that a freely falling observer can report observations and local experiments. In the cosmological context, the FNC are only valid on a scale much smaller than the Hubble horizon. Then we relax the small ball limitation by introducing conformal Fermi coordinates (CFCs). In the CFC system, we mainly focus on the flat universe with vanishing curvature of the space k=0. The Friedmann equations are derived in the CFC system. From the first law of entanglement the emergence of gravity can be described by the change in entanglement SA caused by matter HA angle. In this paper, we study the cosmology in a new framework with the viewpoint that spacetime geometry is viewed as an entanglement structure of the microscopic quantum state, and derive the Friedmann equations for the universe from the first law of entanglement We also briefly review the first law of entanglement. The study shows that there is a basic relation between the gravitation and quantum entanglement, which is valid for the solution of the gravitational field equation.
Structured beam designed by ray-optical Poincaré sphere method and its propagation properties
Authors: Zhang Shu-He  , Shao Meng  , Zhou Jin-Hua  ,   Department of Biomedical Engineering, Anhui Medical University, Hefei 230032, China
Published: December 2020

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.
Multi-field control on magnetic skyrmions
Authors: Dong Bo-Wen  , Zhang Jing-Yan  , Peng Li-Cong  , He Min  , Zhang Ying  , Zhao Yun-Chi  , Wang Chao  , Sun Yang  , Cai Jian-Wang  , Wang Wen-Hong  , Wei Hong-Xiang  , Shen Bao-Gen  , Jiang Yong  , Wang Shou-Guo  ,   Department of Materials Physics and Chemistry, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China;,   State Key Laboratory of Magnetism, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
Published: December 2020

The concept of skyrmion is proposed by Tony Skyrme, a British particle physicist, to describe a state of particles as a topological soliton. Magnetic skyrmion is a novel spin structure with topological behavior, whose size is on a nanometer scale. The space between skyrmions is tunable from a few nanometers to micrometer. Magnetic skyrmion can be stable in a large temperature range, from lower temperatures, to room temperature, and even to higher temperature. The materials with magnetic skyrmions include not only low temperature B20-type ferromagnets with centrosymmetry breaking and weak ferromagnets with helical magnetic ordering, but also the hexagonal MnNiGa alloy and ferromagnetic multilayers over room temperature. By using topological spin structure of skyrmions, an electrical current can be applied to driving or flipping the skyrmions, similar to the spin transfer torque effect in spin-valves and magnetic tunnel junctions. The critical current density is on the order of 102 A/cm2, which is five orders lower than that in magnetic multilayered structures such as 107 A/cm2. This critical value is much lower than the channel current density in Si-based semiconductor technology, thus leading to great potential applications in the future magnetic information devices. In this review paper, we first introduce the discovery, a brief development history of magnetic skyrmions. Then, we summarize the materials with skyrmion spin structures, focusing on the key physical properties. Finally, we mention the recent progress of the multi-field (such as magnetic field, electrical current, and temperature) control on magnetic skyrmions in hexagonal MnNiGa alloy and Pt/Co/Ta magnetic multilayers, together with the creation, annihilation, and dynamic behavior of skyrmions.
(1-x)Sr3Sn2O7+xCa3Mn2O7 ceramics and their photo-electric characteristics
Authors: Huang Yu-Tian  , Wang Yu  , Zhu Min-Min  , Lü Ting  , Yang Hong-Chun  , Li Xiang  , Wang Xiu-Zhang  , Liu Mei-Feng  , Li Shao-Zhen  ,   School of Mathematics and Physics, Hubei Polytechnic University, Huangshi 435003, China;,   Institute for Advanced Materials, Hubei Normal University, Huangshi 435002, China;,   School of Materials Science and Engineering, Hubei Polytechnic University, Huangshi 435003, China
Published: December 2020

In order to obtain new hybrid improper ferroelectricity, the (1-x)Sr3Sn2O7+xCa3Mn2O7 (0 x 0.2) samples each with a Ruddlesden-Popper phase are prepared by the conventional solid-state reaction method. The phase purity is characterized by X-ray diffraction through using the GSAS refinement. The results indicate that (1-x)Sr3Sn2O7+xCa3Mn2O7 sample has an orthorhombic structure and its cell constants decrease with content x increasing. At room temperature, the dielectric constant decreases with frequency increasing. Sr3Sn2O7 exhibits diamagnetism. With increasing the content of Ca3Mn2O7, the sample presents diamagnetism, paramagnetism, weak ferromagnetic and paramagnetism in sequence. And the sample with x=0.1 has a weak ferromagnetic property. All these pave a way for selecting Ruddlesden-Popper multiferroic materials.
Precise measurement of 6Li transition frequencies and hyperfine splitting
Authors: Wu Yue-Long  , Li Rui  , Rui Yang  , Jiang Hai-Feng  , Wu Hai-Bin  ,   State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200062, China;,   National Time Service Center, Chinese Academy of Sciences, Xi'an 710600, China
Published: December 2020

In this paper, we report a precision measurement of hyperfine splitting and absolute frequency of D1 line in cold 6Li atoms. The gray molasses is realized in the experiment and the tempreature is cooled to about 50 μK, which is lower than the Doppler cooling limit, 140 μK. By use of an optical comb, the absolute frequencies and corresponding hyperfine splitting are measured. We obtain frequencies of 446789503.080(35) MHz, 446789529.198(36) MHz, 446789731.316(50) MHz and 446789757.476(29) MHz for the D1 line. The results are in reasonable agreement with the theoretical calculations and consistent with earlier measurements. They could provide an important foundation for future frequency measurement, α constant and nuclear radius.
Broadband reconfigurable reflective polarization convertor
Authors: Yu Hui-Cun  , Cao Xiang-Yu  , Gao Jun  , Yang Huan-Huan  , Han Jiang-Feng  , Zhu Xue-Wen  , Li Tong  ,   Graduate College, Air Force Engineering University, Xi'an 710077, China;,   Information and Navigation College, Air Force Engineering University, Xi'an 710077, China
Published: December 2020

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.
Regression analysis of coincidence measurements for determinating the neutron emission rate of 252Cf spontaneous fission
Authors: Li Yong-Ming  , Wang Liang  , Chen Xiang-Lin  , Ruan Nian-Shou  , Zhao De-Shan  ,   Institute of Materials, China Academy of Engineering Physics, Mianyang 621907, China
Published: December 2020

The 252Cf isotope sources have a recommended standard neutron spectrum of spontaneous fission, and have been widely used in scientific researches, such as the detection efficiency calibration of neutron detectors, the characterization of neutron dose equivalent meters, the active analysis of special nuclear materials, etc. However, it is often necessary to correct the neutron emission rate due to its short half-life of 2.645 years. As the source age increases the contributions from 250Cf and 248Cm spontaneous fission become more significant, thus the neutron emission rate cannot be calculated simply according to the 252Cf decay law. In addition, the indirect measurement method by manganese bath activation needs a long period more than 8 hours; and it will have a large uncertainty while the source strength is lower than 104 n/s. In order to develop a more portable measurement method for larger suitable dynamic range, the comprehensive algorithms based on the neutron multiplicity counting are studied in this paper. On the basis of the measurement equations under the point model assumption, the neutron coincidence counting rate is correlated with the total neutron counting rate, and then the regression analyses with different coincidence gates and different source locations in the counter are performed. On the assumption that the average neutron die-away time is constant in the sensitive range of detection system, therefore the characteristic coefficient from the changing process can be extracted, and two kinds of methods of measuring the neutron strength are established, which are independent of the efficiency variation. The verification experiments are carried out by the JCC-51 neutron coincidence counter. It is shown that the values measured by the two regression methods are consistent with the corrected results of the nominal value within 2% deviation. Furthermore, the detection efficiency is inversed by dividing the total neutron counting rate with the neutron emission rate when the source is placed at the central axis, which accords with the result of Monte Carlo simulation by using the MCNPX code well. It can provide an accurate method of determining the neutron emission rate of 252Cf spontaneous fission, and also an approach to calibrating the detection efficiency of neutron coincidence counter while the source strength is unknown.
High-resolution magnetometry based on nitrogen-vacancy centers in diamond
Authors: Peng Shijie  , Liu Ying  , Ma Wenchao  , Shi Fazhan  , Du Jiangfeng  ,   Chinese Academy of Sciences Key Laboratory of Microscale Magnetic Resonance and Department of Modern Physics, University of Science and Technology of China, Hefei 230026, China;,   Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, China;,   Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei 230026, China
Published: December 2020

Magnetism is one of the most important physical phenomena. The precision measurement of magnetism gives impetus to science and technology. Various techniques, including Hall sensors, superconducting quantum interference devices, and magnetic resonance, are used for trying to improve the resolution and the sensitivity of magnetometry. In recent years, nitrogen-vacancy (NV) centers in diamond have been investigated extensively. This solid-state spin system is convenient to initialize, manipulate, and read out. It has been applied to the experimental study of quantum information and computation, and more importantly, it has displayed enormous potential applications in magnetometry. With various techniques such as dynamical decoupling and correlation spectroscopy that are being applied to NV centers, the microscopic magnetic resonance with high resolution and sensitivity has been implemented. Typical examples of these achievements are the nuclear magnetic resonance and electron paramagnetic resonance of nanoscale samples, and even of single molecules or single spins. The NV centers can also be used for precisely measuring the microwave and radiofrequency field. The issues mentioned above will be outlined in this review.
Comparison between axial residual stresses measured by Raman spectroscopy and X-ray diffraction in SiC fiber reinforced titanium matrix composite
Authors: Huang Hao  , Zhang Kan  , Wu Ming  , Li Hu  , Wang Min-Juan  , Zhang Shu-Ming  , Chen Jian-Hong  , Wen Mao  ,   AECC Beijing Institute of Aeronautical Materials, Beijing 100095, China;,   Department of Materials Science, State Key Laboratory of Superhard Materials, Key Laboratory of Automobile Materials, MOE, Jilin University, Changchun 130012, China
Published: December 2020

Accurate measurement and analysis of residual stress state in the SiCf/Ti composites are crucial to optimizing their fabrication process and to understanding their failure mode, but they are still a challenge. In this work, SiCf/C/Ti17 composites with~48% fiber volume fraction, consisting of W-core SiC fibers (~100 m in diameter), turbostratic C coating (~2.5 m in thickness) and Ti17 matrix, are prepared by consolidating precursor wires fabricated by matrix-coated fiber method through hot isostatic pressing at 920℃/120 MPa/2 h; these samples are used for measuring their stresses. It is noted that turbostratic C coating, a necessary diffusion barrier layer between SiC fiber and Ti17 alloy matrix, bears the residual stress caused by the mismatch of thermal expansion coefficients between fiber and matrix during consolidation. It is found that the graphene planes are almost parallel to the axial direction of SiC fibers in the turbostratic C coating revealed by high magnification transmission electron microscope, and thus G peak position of C coating would be sensitive to stress state. Accordingly, micro-Raman spectroscopy is first used to measure the G peak positions of C coating under stress and stress-free state in the SiCf/C/Ti17 composite, respectively. Based on the position shift of G band caused by residual stress, the axial residual compressive stress of SiC fiber in SiCf/C/Ti17 composite is calculated to be~705.0 MPa. For comparison, X-ray diffraction method is also adopted to measure the interplanar spacing values of the Ti17 alloy matrix in different directions to obtain the spatial strains. During measurement, -Ti (213) high-angle diffraction peak is chosen to reduce test error, and then the different interplanar spacing values of -Ti (213) are obtained by varying the values of in three different directions at =0, 45 and 90. As three-axis-stress model is employed, the residual tensile stress of Ti17 alloy matrix in the axial direction of SiCf/C/Ti17 composite is~701.3 MPa, which is transformed through linear elastic theory into the residual compressive stress of SiC fiber of~759.4 MPa. The similar results confirm that it is reliable to characterize the residual stress in the SiCf/C/Ti17 composite with high-texture turbostratic carbon by both the Raman spectroscopy and the X-ray diffraction method.
Phase field lattice Boltzmann model for two-phase flow coupled with additional interfacial force
Authors: Li Yang  , Su Ting  , Liang Hong  , Xu Jiang-Rong  ,   College of Science, Hangzhou Dianzi University, Hangzhou 310018, China
Published: December 2020

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.
Different silicon crystal face index of inverted pyramid structure
Authors: Chen Quan-Sheng  , Liu Yao-Ping  , Chen Wei  , Zhao Yan  , Wu Jun-Tao  , Wang Yan  , Du Xiao-Long  ,   Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China;,   School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

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≤a≤bc, 0≤ab=c, a=b=c. The inverted pyramid structure on which the crystal face index (abc) satisfies 0≤a≤bc 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≤ab=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.
Potential energy surface and cold collision dynamics of Xe-NH(X3∑-) system
Authors: Qiao Zheng1\2  , Wang Ya-Li  , Wu Ming-Wei  , Feng Er-Yin  , Huang Wu-Ying  ,   College of Physics and Electronic Information, Anhui Normal University, Wuhu 241000, China;,   School of Common Courses, Wannan Medical College, Wuhu 241002, China
Published: December 2020

Sympathetic cooling is one of the most promising techniques for producing ultracold molecules from precooled molecules. Previous researches have shown that it is inadequate to use the ultracold alkali-metal atoms as coolant for sympathetic cooling. To explore the possibility of ultracold alkali-earth-metal atoms as coolant, in this paper a theoretical investigation is performed of the cold collision dynamics for Xe-NH(X3∑-) system in magnetic fields. The interaction potential energies of Xe-NH complex are calculated respectively by using the single and double excitation coupled-cluster theory with the noniterative treatment of triple excitations[CCSD(T)] method and complete basis set limit extrapolated method. An analytic express of potential energy surface (PES) is given for the first time. A single global minimum value occurs at R=7.14a0, θ=102.76° with an energy of-153.54 cm-1, and the PES has a weak anisotropy. Combine the ab initio PES with quantum scattering theory, then the cold collisional dynamics of Xe-NH system in a magnetic field will be studied. The elastic and inelastic transition cross sections and their ratios of NH molecules in the lowest low-field following state (n=0, mj=1) under different magnetic fields and collisional energies are calculated. The results show that the elastic cross section is independent of magnetic field, and the inelastic cross section changes with magnetic field, especially at an ultracold temperature. A common rule of thumb is that to successfully implement cooling, the ratio of elastic cross section to inelastic cross section needs to reach 100 at least. The results suggest that it is likely to be a challenging work to perform sympathetic cooling of NH molecule by ultracold Xe atom.
Measurement of low-frequency signal based on quantum-enhanced fiber Mach-Zehnder interferometer
Authors: Cheng Jian  , Feng Jin-Xia  , Li Yuan-Ji  , Zhang Kuan-Shou  ,   1. State Key Laboratory of Quantum Optics and Quantum Optics Devices, Institute of Opto-Electronics, Shanxi University, Taiyuan 030006, China;,   2. Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China
Published: December 2020

Generation of squeezed state at telecommunication wavelength has been recently a very interesting issue due to the lowest optical power attenuation of light at a wavelength of 1550 nm in a standard telecommunication fiber. The low-frequency vacuum squeezed state at 1550 nm in combination with fiber based interferometer offers the possibility to implement quantum precision measurement beyond standard quantum limit (SQL). In this paper, we experimentally realize a quantum-enhanced fiber Mach-Zehnder interferometer (FMZI) for measuring the low-frequency phase modulation signal by using low-frequency vacuum squeezing at 1550 nm. Firstly, the low-frequency vacuum squeezed state at the telecommunication wavelength of 1550 nm is generated by using a degenerate optical parametric oscillator (DOPO). The DOPO is a semi-monolithic construction based on a type I periodically poled KTiOPO4 (PPKTP) crystal and a concave mirror. The pump threshold of DOPO is 270 mW. When the pump power is 120 mW that is below the pump threshold of DOPO and the temperature of PPKTP is controlled at 34.8℃, a vacuum squeezing of 3 dB is generated at an analysis frequency range from 10 kHz to 500 kHz. The quadrature phase vacuum squeezing is obtained by locking the squeezed quadrature angle through using a coherent control scheme, in which two acousto-optic modulators are used to shift the frequency and produce the auxiliary beam acting as a coherent control field. Based on the constructed FMZI, a quantum-enhanced FMZI is realized by injecting the generated low-frequency vacuum squeezed state at 1550 nm into the vacuum channel of FMZI. The relative phase between two injected light fields is locked at π by using the Pound-Drever-Hall (PDH) locking technology, and the relative phase between light fields of its arms in FMZI is also locked at π/2 by using the PDH locking technology. When a phase modulation signal at the frequency of 500 kHz is loaded in the signal arm of FMZI, the noise power spectrum of the output from FMZI is measured by a balance homodyne detect system. A 2 dB quantum improvement beyond shot-noise-level at the frequency of 500 kHz is obtained experimentally by using the quantum-enhanced FMZI. The experimental results demonstrate a potential application in quantum precision measurement beyond the SQL based on fiber sensor technique.
Influence of leading-edge curvature on excited unsteady cross-flow vortices in three-dimensional boundary-layer
Authors: Lu Chang-Gen  , Shen Lu-Yu  ,   School of Marine Sciences, Nanjing University of Information Science and Technology, Nanjing 210044, China
Published: December 2020

Three-dimensional boundary-layer receptivity is the first stage of the laminar-turbulent transition in a three-dimensional boundary layer, and also a key issue for predicting and controlling the laminar-turbulent transition in the three-dimensional boundary layer. At a high turbulence level, the three-dimensional boundary-layer instability in the transition is caused mainly by the unsteady cross-flow vortices. And the leading-edge curvature has a significant influence on three-dimensional boundary-layer receptivity. In view of this, the direct numerical simulation is utilized in this paper to study the mechanism of receptivity to exciting unsteady cross-flow vortices in the three-dimensional (swept-plate) boundary layer with various elliptic leading edges. In order to solve the Navier-Stokes equation numerically, a modified fourth-order Runge-Kutta scheme is introduced for discretization in time; high-order compact finite difference schemes are utilized for discretization in the x-and y-direction; and Fourier transform is used in the z-direction. The pressure Helmholtz equation is solved by a fourth-order iterative scheme. Additionally, the numerical calculation is performed in the curvilinear coordinate system via Jaccobi transform. And the elliptic equation technique is used to gene-rate the body-fitted mesh. The effect of leading-edge curvature on the propagation speed and direction, distribution and receptivity coefficient of the excited unsteady cross-flow vortex wave packet, and the amplitude, dispersion relation and growth rate of the extracted unsteady cross-flow vortex are revealed. In addition, the inner link among the receptivity to unsteady cross-flow vortex, intensity, and direction of free-stream turbulence is established. Furthermore, the receptivity to anisotropic free-stream turbulence is also analyzed in detail. The numerical results indicate that the more intense receptivity to the unsteady cross-flow vortex wave packets is triggered with a smaller leading-edge curvature; whereas, the less intense receptivity is triggered with a greater leading-edge curvature. The receptivity to the unsteady cross-flow vortex wave packets in different curvatures are also found to vary with the angle of free-stream turbulence. Moreover, the anisotropic degree of free-stream turbulence can affect the excitation of the unsteady cross-flow vortex obviously. Through the above study, a further step can be taken to understand the prediction and control of laminar-turbulent transition in the three-dimensional boundary layer and also improve the theory of the hydrodynamic stability.