bluuebunny/crossref_metadata_2025 huggingface dataset:
Superconducting gap of quasi-one-dimensional Cr-based superconductor RbCr3As3
Authors: Li Qing  , Wang Min-Xiang  , Liu Tong  , Mu Qing-Ge  , Ren Zhi-An  , Li Shi-Yan  ,   State Key Laboratory of Surface Physics, Department of Physics, Laboratory of Advanced Materials, Fudan University, Shanghai 200438, China;,   Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
Published: December 2020

Since the discovery of high-temperature superconductivity in cuprates, finding more unconventional superconductors and understanding their superconducting pairing mechanism has been an important theme in condensed matter physics. Recently, ternary Cr-based superconductors A2Cr3As3 (A=K, Rb, Cs) and ACr3As3 (A=K, Rb) were reported, which own quasi-one-dimensional crystal structure, containing[(Cr3As3)-] linear chains. A2Cr3As3 belongs to P6m2 space group, and ACr3As3 crystallizes in a centrosymmetric structure with the space group P63/m. Many experiments, such as nuclear magnetic resonance, London penetration depth, show that A2Cr3As3 is an unconventional superconductor. However, these A2Cr3As3 compounds are extremely unstable in air. Here, we study the superconducting gap of the air-stable RbCr3As3 single crystal, using ultralow-temperature thermal conductivity measurement. The resistivity of RbCr3As3 single crystal shows a superconducting transition temperature Tczero at 6.6 K. The normal-state resistivity data from 20 K to 8 K are fitted to (T)=0 + AT2, which gives a residual resistivity of 0=781 cm. Then, the thermal conductivity of RbCr3As3 single crystal is measured at temperature down to 80 mK and in magnetic fields up to 9 T. In zero field, residual linear term 0/T=7.5 WK-2cm-1 is observed, which is about 24% of its normal-state value, suggesting nodes in the superconducting gap. At low field, the 0/T of RbCr3As3 shows a relatively faster field dependence than single-gap s-wave superconductors. These results reveal that RbCr3As3 is likely an unconventional superconductor with superconducting gap nodes, although the exact superconducting gap symmetry and structure for this quasi-one-dimensional superconductor needs further investigation.
Coherent synthesis of ultrashort pulses based on balanced optical cross-correlator
Authors: Huang Pei  , Fang Shao-Bo  , Huang Hang-Dong  , Hou Xun  , Wei Zhi-Yi  ,   1. State Key Laboratory of Transient Optics and Photonics, Xi'an Institute of Optics and Precision Mechanics of CAS, Xi'an 710119, China;,   2. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China;,   3. University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

Coherent synthesis of laser pulses is a major trend in the generation of ultrafast pulse field. There is no good way to compensate for the whole spectrum when the spectrum of ultrashort pulses is wide enough to reach an octave, so it is difficult to realize a sub-cycle pulse in a single-path laser system even if the spectrum range is wide enough. In this paper, 0.8 mJ, 30 fs laser pulses at 1 kHz repetition rate with 790 nm center wavelength from a Ti:sapphire chirped pulse amplifier (CPA) system are focused into hollow fiber with an inner diameter of 250 μm and a length of 1 m to produce an octave-spanning white-light supercontinuum (450-950 nm). Using this supercontinuum, we conduct two sets of comparative experiments. 1) We split the supercontinuum into two pulses with different spectrum ranges (450-750 nm and 650-1000 nm) by a dichroic mirror (HR, 500-700 nm; HT, 700-1000 nm), and we compress the two pulses by the double-chirped mirrors and wedge pairs to generate two few-cycle pulses:the long and short wavelength yielding pulses are 7.9 fs and 6.1 fs, respectively. Then we coherently synthesize two pulses by using another dichroic mirror, and controlling the relative time delay between the two pulses, and thus we synthesize a pulse of 4.1 fs. 2) We directly compress the supercontinuum by the double-chirped mirrors and wedge pairs, and obtain an optimization result of 5.3 fs, of which the pulse duration is wider than that in experiment 1. In these comparative experiments, the advantage of coherent synthesis for shorter pulse duration is preliminarily verified. Besides, the balanced optical cross-correlator technique is used to lock the relative time delay between two pulses. The root-mean-square value of relative time delay drift is less than 80 as in the case with feedback control loop, which ensures the stability of coherent synthesis system. This scheme can be adopted to accurately compensate for the dispersion and obtain the sub-cycle synthesized pulse, which is useful for generating the high harmonic and atto-second pulse.
Topological quantum material simulated with superconducting quantum circuits
Authors: Yu Xiang-Min  , Tan Xin-Sheng  , Yu Hai-Feng  , Yu Yang  ,   School of Physics, Nanjing University, National key Laboratory of Microstructures, Nanjing 210093, China
Published: December 2020

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.
Quantum manipulation of electronic phase separation in complex oxides
Authors: Wang Wen-Bin  , Zhu Yin-Yan  , Yin Li-Feng  , Shen Jian  ,   Institute for Nanoelectronics Devices and Quantum Computing, Fudan University, Shanghai 200433, China;,   State Key Laboratory of Surface Physics, Department of Physics, Fudan University, Shanghai 200433, China;,   Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, China
Published: December 2020

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.
Energy loss of surface plasmon polaritons on Ag nanowire waveguide
Authors: Wang Wen-Hui  , Zhang Nao  ,   School of Science, Xi'an Jiaotong University, Xi'an 710049, China
Published: December 2020

Metal nanostructures can support surface plasmon polaritons (SPPs) propagating beyond diffraction limit, which enables the miniaturizing of optical devices and the integrating of on-chip photonic and electronic circuits. Various surface plasmon based optical components have already been developed such as plasmonic routers, detectors, logic gates, etc. However, the high energy losses associated with SPPs' propagation have largely hampered their applications in nanophotonic devices and circuits. Developing the methods of effectively reducing energy loss is significant in this field. In this review, we mainly focus on the energy losses when SPPs propagate in Ag nanowires (NWs). Researches on energy loss mechanism, measurement approaches and methods of reducing energy loss have been reviewed. Owing to their good morphology and high crystallinity as well as low loss in visible spectrum, chemically synthesized Ag NWs are a promising candidate for plasmonic waveguides. The energy losses mainly arise from inherent Ohmic damping, scattering process, leaky radiation and absorption of substrate. These processes can be influenced by excitation wavelength, the geometry of NW and the dielectric environment, especially the effect of substrate, which is discussed in the review. Longer excitation wavelength and larger NW diameter can induce decreased mode confinements and smaller Ohmic loss. The experimental methods to measure the energy loss have been summarized. Researches on reducing energy loss have been reviewed including applying dielectric layer or graphene between NW and substrate, replacing commonly used substrate with a dielectric multilayer substrate, introducing gain materials, and forming hybrid waveguides by using the semiconductor or dielectric NW. Specifically, the leaky radiation can be prevented when an appropriate dielectric layer is placed between NW and substrate, and the mode confinement can be reduced which leads to decreased Ohmic loss. The gain materials can be used to compensate for the energy loss during propagation. Compared with metal waveguides, semiconductor or dielectric NWs suffer lower energy losses while decreased field confinement. Then the hybrid waveguides constructed by metal and dielectric NWs can combine their advantages, which possesses reduced propagation loss. In addition, the plasmon modes in NWs in a homogeneous medium and a substrate are briefly discussed respectively, followed by the introduction to fundamental properties of SPPs propagation. Finally, perspectives of the future development of reducing energy loss are given. The researches on reducing energy loss are crucial for designing and fabricating the nanophotonic devices and integrated optical circuits.
Search for Majorana zero mode in the magnetic vortex of artificial topological superconductor
Authors: Li Yao-Yi , Jia Jin-Feng ,   Key Laboratory of Artificial Structures and Quantum Control (Ministry of Education), Shenyang National Laboratory for Materials Science, School of Physics and Astronomy, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China,   Tsung-Dao Lee Institute, Shanghai 200240, China
Published: December 2020

The search for new states that exhibit topological order is currently a very active and exciting area of research. Like a topological insulator, superconducting order can also exhibit topological order, which is different from that of a conventional superconductor. This superconductor is so-called " topological superconductor”, which has a full pairing gap in the bulk and gapless surface state. Majorana Fermions obey non-Abelian fractional statistics, and have been proposed to construct topological qubits, so there is a great prospect of scientific research and application in topological quantum computing. It is very interesting that Majorana Fermions are predicted to exist in topological superconductors. However, natural topological superconductor is very rare. Inspired by the realization of topological insulators, theoretical physicists have proposed that via the fabrication of the s-wave superconductor/topological insulator heterostructure, Majorana Fermions may exist in the superconducting topological insulator induced by proximate effect. Due to various kinds of topological insulators and conventional s-wave superconductors, heterostructures constructed by this method can greatly increase the variety of artificial topological superconductors. In this paper we review the experimental progress in the heterostructure composed of the Bi2Te3-type topological insulator and the conventional s-wave superconductor NbSe2. Using molecular beam epitaxy, atomically flat topological insulator film can be fabricated at the top of superconductor substrate. The spatial distribution of Majorana Fermions on the surface of topological insulator can be directly observed by in situ scanning tunneling microscopy/spectroscopy. In the center of a magnetic vortex, Majorana Fermions will appear as the Majorana zero mode, a zero-energy peak inside the superconducting gap. Although the energy gap between low energy quasiparticle excitation and the Majorana zero mode is very small, the evidences such as zero bias conductance anomaly, Y-shape splitting of zero-bias conductance, spin-selective Andreev reflection are self-consistent and reveal that the Majorana zero mode exists in the center of a magnetic vortex. These experiments have led to a new insight into superconductivity. It may open a door to probing the novel physics of Majorana fermions.
Experimental study on fission reaction rate induced by D-T neutron in depleted uranium shell
Authors: Han Zi-Jie , Zhu Tong-Hua , Lu Xin-Xin , Qin Jian-Guo , Wang Mei , Jiang Li , Yang Bo ,   Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, China,   Institute of Applied Physics and Computational Mathematics, China Academy of Engineering Physics, Beijing 100094, China
Published: December 2020

Fission reaction rate is an important index for validating and checking the neutron transportation and fission power in nuclear engineering. The experimental data can be used in benchmark validation of cross sections, and in studying the correlation of fission power with the thickness of uranium sphere shell. There are five assemblies of depleted uranium shells used in this work, the inner radii of which are all fixed at 13.1 cm, while their outer radii are 18.1, 19.4, 23.35, 25.4 and 28.5 cm, respectively. The D-T neutron source is generated in the center of the assemblies, the yield of which is about 3 × 1010−4 × 1010 s–1. In horizontal plane across the center of the assemblies, the fission rates at positions along the radial direction are measured in the direction with 45° inclining with respect to the incident D+ beam. Due to the disturbance to assemblies and neutron field, the activation foil of uranium is a suitable choice rather than fission chamber or capture detector. The material of activation foil is the same as that in the experimental assemblies. Considering the accurate fission yield of 143Ce, the objective nuclides are selected. The total fission yield of 143Ce is contributed by 238U and a little 235U. For calculating the total fission yiled of 143Ce, the neutron energy range of 0−15 MeV is divided into eight subranges. By measuring the 293 keV gamma rays from the fission product 143Ce in activation foils with a TRANS-SPEC-DX100 HPGe detector, with a relative efficiency 40%, the fission rates and the trends at positions along the radial direction in the five assemblies are obtained based on the 143Ce fission product yield. The fission rate ranges from 5.28 × 10–29 to 7.58 × 10–28 sn-1·nuclide–1, with the relative uncertainty in a range from 6% to 11%. The Monte Carlo transport code MCNP5 and continuous energy cross section library ENDF/BV.8 are used for analyzing the fission rate distribution in the assemblies, and the experiemtal configuration, including the wall of the experimental hall is described in detail in the model. The calculated results are compared with the experimental ones and their agreement is found to be in an uncertainty range.
A spectral coarse graining algorithm based on relative distance
Authors: Yang Qing-Lin , Wang Li-Fu , Li Huan , Yu Mu-Zhou ,   School of Control Engineering, Northeastern University at Qinhuangdao, Qinhuangdao 066004, China
Published: December 2020

As a key approach to understanding complex systems (e.g. biological, physical, technological and social systems), the complex networks are ubiquitous in the whole world. Synchronization in complex networks is significant for a more in-depth understanding of the dynamic characteristics of the networks, where tremendous efforts have been devoted to their mechanism and applications in the last two decades. However, many real-world networks consist of hundreds of millions of nodes. Studying the synchronization of such large-scale complex networks often requires solving a huge number of coupled differential equations, which brings great difficulties to both computation and simulation. Recently, a spectral coarse graining approach was proposed to reduce the large-scale network into a smaller one while maintaining the synchronizability of the original network. The absolute distance between the eigenvector components corresponding to the minimum non-zero eigenvalues of the Laplacian matrix is used as a criterion for classifying the nodes without considering the influence of the relative distance between eigenvector components in an original spectral coarse graining method. By analyzing the mechanism of the spectral coarse graining procedure in preserving the synchronizability of complex networks, we prove that the ability of spectral coarse graining to preserve the network synchronizability is related to the relative distance of the eigenvector components corresponding to the merged nodes. Therefore, the original spectral coarse graining algorithm is not satisfactory enough in node clustering. In this paper, we propose an improved spectral coarse graining algorithm based on the relative distance between eigenvector components, in which we consider the relative distance between the components of eigenvectors for the eigenvalues of network coupling matrix while clustering the same or similar nodes in the network, thereby improving the clustering accuracy and maintaining the better synchronizability of the original network. Finally, numerical experiments on networks of ER random, BA scale-free, WS small-world and 27 different types of real-world networks are provided to demonstrate that the proposed algorithm can significantly improve the coarse graining effect of the network compared with the original algorithm. Furthermore, it is found that the networks with obvious clustering structure such as internet, biological, social and cooperative networks have better ability to maintain synchronization after reducing scale by spectral coarse-grained algorithm than the networks of fuzzy clustering structure such as power and chemical networks.
First principles study of effect of vaiable component Al on HfO2 resistance
Authors: Dai Guang-Zhen , Jiang Yong-Zhao , Ni Tian-Ming , Liu Xin , Lu Lin , Liu Qi ,   College of Electrical Engineering, Anhui Polytechnic University, Wuhu 241000, China,   Institute of Microelectronics of Chinese Academy of Sciences, Beijing 100029, China
Published: December 2020

In order to improve the resistance properties of HfO2 and increase the consistency and uniformity of conductive filaments formed by oxygen vacancies (VO), the first-principles calculation method based on density functional theory is used to study the micro-properties of Al-doped HfO2 resistive materials. The results show that the interval Al (Int-Al) is more suitable for being incorporated into HfO2, and the closer to the relative position of VO the Int-Al, the faster the convergence rate of the resistive material tends to be stable, and the smaller the formation energy. The effects of different Int-Al concentrations on the formation of HfO2 supercells with VO defects show that when the concentration of doped Int-Al is 4.04%, the fractional charge state density map can form relatively good charge channels. The maximum and critical equipotential surface values are highest, which is conducive to improving the consistency and uniformity of the formation of conductive filaments in HfO2 resistive materials. The calculation of energy formation shows that the change is slow when the concentration of Int-Al is lower than 4.04%. When the concentration of Int-Al is higher than 4.04%, the abnormal increase occurs, which indicates that the defect system becomes more and more difficult to form with the increase of the concentration of Int-Al.

The introduction of the impurity and the VO defect destroy the original complete crystal structure, which causes the position of the atoms around the impurity to shift, and the valence electron orbit and the energy level of the crystal are changed, and the distribution of the internal charges of the HfO2 defect system is affected. In order to study the effect of the change of the lattice structure on the formation of the VO conductive filament, the VASP software package is used to calculate the relative ratio of the atoms in the lattice structure of the HfO2 defect system as the reference and the relative ratio of the HfO2 defect system after the optimizing the lattice structure. Further study of the change of lattice structure, when the concentration of doped Int-Al is 4.04%, shows that the defect formation energy decreases significantly, which is conducive to the formation of perfect conductive channel. The conductive channel has a certain reference significance for improving the performance of HfO2 based resistive variable memory materials.
Preparation and thermophysical properties of graphene nanoplatelets-octadecane phase change composite materials
Authors: Cai Di , Li Jing , Jiao Nai-Xun ,   Key Laboratory of Low-Grade Energy Utilization Technologies and Systems of Ministry of Education, Chongqing University, Chongqing 400044, China,   School of Energy and Power Engineering, Chongqing University, Chongqing 400044, China
Published: December 2020

 Latent heat storage mainly uses the latent heat of phase change material (PCM) to realize thermal energy storage and utilization, which is the most important thermal energy storage method at present. However, most of PCMs have the disadvantage of low thermal conductivity, which greatly restricts the thermal response rate and system efficiency of the thermal energy storage system. With the development of nanotechnology, it is expected to improve the thermal conductivity of traditional PCMs by adding high thermal conductivity nanoparticles. In this paper, a novel two-dimensional carbon nanomaterial, graphene is selected as an additive for PCM.

 In this paper, graphene nanoplatelets-octadecane phase change composite materials are prepared with a two-step method and the mass fractions of graphene nanoplatelets are 0%, 0.5%, 1%, 1.5%, and 2%. Their microstructures, morphologies and thermophysical properties are characterized by scanning electron microscopy (SEM), infrared spectroscopy (IR), differential scanning calorimetry (DSC), and thermal conductivity analysis. The effects of the addition quantity of graphene nanoplatelets on the phase transition temperature, enthalpy, specific heat capacity, thermal conductivity and thermal stability of the composite PCM are compared. The experimental results show that the dispersion stability of the graphene nanoplatelets in the composite system is greatly improved by the addition of dispersant, and the system does not produce obvious agglomeration nor sedimentation after multiple phase transformation cycles. The graphene nanoplatelets still maintain good lamellar structure and homogeneous dispersion in the n-octadecane matrix, and no chemical reaction occurs in the composite process. Comparing with the n-octadecane, the melting point of the composite phase change material decreases slightly, and the freezing point increases slightly. With the increase of graphene nanoplatelets, the latent heat value of graphene nanoplatelets-octadecane composite phase change material decreases gradually. For the composite phase change material with 2.0 wt.% graphene nanoplatelets, the melting enthalpy and solidified enthalpy are reduced by 6.01% and 7.35%, respectively. When the mass fractions of graphene nanoplatelets are 0.5%, 1%, 1.5%, and 2%, the thermal conductivity values of phase change composite materials are nearly 32.4%, 77.4%, 83.1%, and 89.4% higher than the thermal conductivity value of pure octadecane, respectively. Comparing with the significant increase in thermal conductivity, the addition of graphene nanoplatelets has little effect on the phase transition temperature and latent heat of PCM, and still exhibits the good heat storage performance.
Analytical method for four wave mixing in space-frequency multiplexing optical fibers
Authors: Wan Feng , Wu Bao-Jian , Cao Ya-Min , Wang Yu-Hao , Wen Feng , Qiu Kun ,   Key Laboratory of Optical Fiber Sensing and Communication, Ministry of Education, School of Information and Communication Engineering, University of Electonic Science and Technology of China, Chengdu 611731, China
Published: December 2020

In recent years, the transmission capacity of wavelength division multiplexing (WDM) communication systems has gradually approached to the nonlinear Shannon limit. To meet the increasing demand for communication capacity, space division multiplexing (SDM) has become one of the most concerned technologies. In this paper, the four-wave mixing process (FWM) in fibers is considered from the frequency domain to the mode division multiplexing (MDM) spatial domain under pump depletion and the exact analytical solution to the FWM coupled-mode equations in the space-frequency domain is in detail deduced. The analytical method is verified by numerically calculating the amplitude and phase evolution of the idler wave in non-degenerate four-wave mixing. We discuss three new applications of the analytical solution as follows. 1) Using the phase matching condition we select the terms in the multi-wave coupling equation, and only retain the coupling term that plays a major role. According to the analytical solution in this paper, the phase matching percentage parameter is introduced to determine the FWM coupling terms necessary for multi-wave coupling equations, thus simplifying the multi-wave coupling problem in the study. 2) Combining the analytical solution with the numerical calculation results, we find the initial phase relationship between the output idler and the input guided wave for phase-insensitive FWM, and we provide the analytical expression for a theoretical basis to efficiently design the FWM-based phase arithmetic devices in parallel operating at WDM and MDM systems. 3) We propose a nonlinear compensation algorithm based on analytical solution, which can be used in the few-mode transmission system. The algorithm can fast evaluate or compensate for the fiber nonlinearity by taking into account the pump depletion of the FWM effect. Compared with the traditional digital back propagation (DBP) algorithm, our algorithm has the advantage of lower complexity.
Complex exponential reconstruction algorithm accelerated by cascadic multigrid method
Authors: Zhao Wang , Dong Li-Zhi , Yang Ping , Wang Shuai , Xu Bing ,   Key Laboratory on Adaptive Optics, Chinese Academy of Sciences, Chengdu 610209, China,   Institute of Optics and Electronics, Chinese Academy of Sciences, Chengdu 610209, China,   University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

When laser beam propagates through the turbulent atmosphere, there are branch points in wavefront, which are caused by deep turbulence or long propagation distance. Conventional least-square reconstruction algorithms cannot restore the discontinuous wavefront, which severely limits correction performance of an adaptive optics system. If the incoming wavefront contains a branch cut, there is \begin{document}$ {\rm{2}}n{\text{π}} $\end{document} difference between the measured phase difference and the principle phase difference, which is the reason why conventional least-square reconstruction algorithms cannot reconstruct wavefront with branch points. The complex exponential reconstructor is developed to restore the discontinuous wavefront with phase difference replaced by complex exponents. However, thousands of iterations are required by the complex exponential reconstructor before converging to an acceptable solution. In order to speed up the iterative calculation, the cascadic multigrid method (CMG) is introduced in the process of wavefront reconstruction. The proposed method can be used to restore discontinuous wavefront with lower residual error similar to those reconstructed by the direct iteration. The number of float point multiplications required by the CMG method is nearly 2 orders of magnitude lower than that required by the direct iteration. The acceleration of the CMG method increases with the number of subapertures increasing. The performance of CMG method to recover continuous wavefront is also investigated and compared with conventional wavefront reconstruction algorithm based on successive over-relaxation. It is shown that the CMG method has good capability for wavefront reconstruction with high precision and low computation cost no matter whether it is applied to discontinuous or continuous wavefront. Furthermore, the CMG method is used in the adaptive optics for correcting the turbulence aberration. The direct slope wavefront reconstruction algorithm based on the assumption that the measured slope and the control voltage satisfy the linear relationship cannot restore the wavefront with branch points. As a result, the adaptive optics system with the CMG method doubles the correction quality evaluated by the Strehl ratio compared with that with the direct slope wavefront reconstruction algorithm.
On the derivation of local mean pressure tensor for nonuniform systems and the analysis of uniform fluid
Authors: Cui Shu-Wen , Liu Wei-Wei , Zhu Ru-Zeng , Qian Ping ,   Department of Physics and Information Engineering, Cangzhou Normal University, Cangzhou 061001, China,   State Key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Science, Beijing 100190, China,   Key Laboratory of Microgravity, Institute of Mechanics, Chinese Academy of Science, Beijing 100190, China,   Department of Physics, University of Science and Technology Beijing, Beijing 100083, China
Published: December 2020

The expression of equilibrium pressure tensor for uniform systems derived from Virial theorem can be divided into two parts: dynamic pressure tensor and configuration pressure tensor. The local mean pressure tensor is obtained by physical analysis of the equilibrium non-uniform system. In this paper, this expression is derived in a more concise way. The theoretical relationship, with undetermined parameters, between the three parts (the volume contribution, the surface contribution and the line contribution) of the average configuration pressure of a homogeneous fluid system and the local average size L* with the atomic diameter as length unit greater than 8 is given. Taking argon as an example, molecular dynamics simulation is carried out by using the Lennard-Jones potential between atoms at temperature 180 K and density 0.8 (atomic diameter)–3. The simulation curves of configuration pressure and its three parts are given, and the undetermined coefficients in the theoretical relationship under the condition of L* > 8 are determined. It is found that for L* > 2, With the increase of L*, the volume contribution decreases monotonously from the positive pressure to the negative total configuration pressure, while the surface contribution and the line contribution both increase monotonously and tend to zero, but the line contribution tends to zero fastest. The complex characteristics of various behaviors are explained physically. It is concluded that the surface contribution and the line contribution can be neglected only if L* are large enough. In nanoscale, the neglect of the surface contribution and the line contribution, i.e., the neglect of the boundary effect, will bring obvious errors to the calculation. Finally, molecular dynamics simulation shows that the configuration pressure increases with the increase of temperature. These conclusions are significant for optimizing the selection of pressure tensor in molecular dynamics simulation.
Thermal analysis on crystal phase synthesis of iron nitride film and its magnetic properties
Authors: Lu Qi-Hai , Tang Xiao-Li , Song Yu-Zhe , Zuo Xian-Wei , Han Gen-Liang , Yan Peng-Xun , Liu Wei-Min ,   Key Laboratory of Sensor and Sensing Technology of Gansu Province, Institute of Sensor Technology, Gansu Academy of Sciences, Lanzhou 730000, China,   State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China,   Institute of Nano-materials Application Technology, Gansu Academy of Sciences, Lanzhou 730000, China,   Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
Published: December 2020

The phase transition law of Fe-N system is very important for efficiently synthesizing single-phase γ'-Fe4N thin films. The γ"-FeN thin films are deposited on silicon wafers via DC reactive magnetron sputtering; some of them are stripped from the silicon wafers and measured by using the synchronous thermal analysis (TG-DSC) for studying the phase transition law of Fe-N system. The results of TG-DSC show that at a heating rate of 10 ℃/min, the Fe-N system has five phase transitions in a temperature range between room temperature (RT) and 800 ℃, i.e. I (330−415 ℃): γ''-FeN→ξ-Fe2N with an endothermic value of 133.8 J/g; II (415−490 ℃): ξ-Fe2N→ε-Fe3N with no obvious latent heat of phase change; III (510−562 ℃): ε-Fe3N→γ'-Fe4N with an exotherm value of 29.3 J/g; IV (590−636 ℃): γ'-Fe4N→γ-Fe with an exotherm value of 42.6 J/g; V (636−690 ℃): γ-Fe→α-Fe with an endothermic value of 14.4 J/g. According to the phase transition law of Fe-N system, the crystal phase of iron nitride thin film is effectively regulated by vacuum annealing. The x-ray diffraction pattern (XRD) results show that the iron nitride thin film obtained by direct-sputtering in pure N2 is a single-phase γ"-FeN film, and it becomes a single-phase ξ-Fe2N film after being annealed at 350 ℃ for 2 h, a single-phase ε-Fe3N film after being annealed at 380 ℃ for 2 h, and a single-phase γ'-Fe4N film after being annealed at 430 ℃ for 7 h. The annealing temperature for the phase transition of Fe-N thin film is generally lower than that predicted by the TG-DSC experimental results, because it is affected by the annealing time too, that is, prolonging the annealing time at a lower temperature is also effective for regulating the crystal phase of Fe-N thin film. The magnetic properties of the Fe-N thin film are also studied via vibrating sample magnetometer (VSM) at room temperature. The γ'-Fe4N polycrystalline thin film shows an easy-magnetized hysteresis loop for the isotropic in-plane one, but a hard-magnetized hysteresis loop with a large demagnetizing field for the out-of-plane one, which belongs to the typical magnetic shape anisotropy. However, their saturation magnetizations are really the same (about 950 emu/cm3) both in the plane and out of the plane.
Movement of ablated high-Z plasmas
Authors: Liu Zhong-Heng , Meng Guang-Wei , Zhao Ying-Kui ,   Institute of Applied Physics and Computational Mathmatics, Beijing 100094, China
Published: December 2020

The energy leaking through a slot in the hohlraum filled with low-Z foams is a typical dynamic problem of the ablated high-Z plasmas. In this paper, we develop a simplified one-dimensional model to study the expansion-reverse process of the ablated Au plasmas, which corresponds to the closing-reopening process of a slot. Our work shows that its physical mechanism is the ablation pressure competing with radiation pressure difference and the material pressure of low-Z foams. The analytical formulas for the reverse time and reverse distance of the Au plasma are deduced, respectively, indicating that the cubic value for each of both peak temperatures is proportional to the density of the low-Z foams. The main conclusions of analytic theory are verified by numerical simulation through using the modified radiation-hydrodynamic program MULTI. It is shown that the power exponents of scaling law in high-Z plasma ablation keep unchanged in a wide range of density of low-Z foams. The range of validity of the model is discussed.
Watt-level passively Q-switched mode-locked Tm: LuAG laser with graphene oxide saturable absorber
Authors: Sun Rui , Chen Chen , Ling Wei-Jun , Zhang Ya-Ni , Kang Cui-Ping , Xu Qiang ,   Institute of Physics and Optoelectronics Technology, Baoji University of Arts and Sciences, Baoji 721016, China,   Institute of Laser Technology, Tianshui Normal University, Tianshui 741001, China,   School of Arts and Sciences, Shaanxi University of Science and Technology, Xi'an 710021, China
Published: December 2020

A watt-level passive Q-switched mode-locked operation in Tm: LuAG all-solid-state laser is realized for the first time by using graphene oxide (GO) saturable absorber as a mode-locked starting element. The laser is pumped by a wavelength tunable Ti: sapphire laser operating at 794.2 nm. In this experiment, the maximum continuous-wave (CW) output power of 1440 mW, 2030 mW and 2610 mW are obtained by 1.5%, 3% and 5% output coupled (OC) mirrors respectively, in which the corresponding slope efficiencies are 22.3%, 32.6% and 40.6%, respectively. When the GO is inserted into the cavity, the laser bump threshold is further increased due to more intracavity loss. With a 1.5% OC mirror, the absorbed pump threshold is as low as 325 mW, the maximum output power is 787 mW, and the corresponding slope efficiency is 12.5%. With a 3% OC mirror, the absorbed bump threshold is 351 mW, the maximum output power is 1740 mW, and corresponding slope efficiency is 30.3%. With a 5% OC mirror, the QML operation is not realized due to the increase of intracavity loss. Although the laser pump threshold power of 3% OC mirror differs from that of 1.5% OC mirror by 26 mW, the output power is more than twice higher than that of 1.5% OC mirror. For these reasons, we use a 3% OC mirror in our experiment. In this case, a stable QML operation with a threshold of 3420 mW is obtained. When the pump power reaches 8.1 W, the corresponding maximum output power is 1740 mW, the central wavelength is 2023 nm, the repetition frequency is 104.2 MHz, the maximum single pulse energy is 16.7 nJ, and the modulation depth is close to 100%. According to the symmetrical shape of the mode locked pulse and considering the definition of rise time, we can assume that the duration of the pulse is approximately 1.25 times the pulse rise time. So the width of the mode locked pulse is estimated at about 923.8 ps. The results show that the GO is a promising high power saturable absorber in 2 μm wavelength for the QML solid-state laser. In the next stage, we will increase the pump power, optimize the quality of the GO material, and compensate for the dispersion in the cavity. It is expected to achieve a CW mode-locked operation and femtosecond pulse output.
Density functional theory calculation of structure and electronic properties in N-methane hydrate
Authors: Luo Qiang , Yang Heng , Guo Ping , Zhao Jian-Fei ,   College of Science, Southwest Petroleum University, Chengdu 610500, China,   State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, China
Published: December 2020

As a clean and efficient unconventional energy source, natural gas hydrate has been highly valued and vigorously developed by many countries in recent years. In order to solve the problem that the existing hydrate structure symmetry is not high, which leads the theoretical research to be restricted, it is imperative to explore a new type of methane hydrate structure with high symmetry. Using the first-principles method which is based on the density functional theory (DFT), the structure and electronic properties of N-methane hydrate are calculated in the generalized gradient approximation (GGA) for Grimme dispersion correction. The obtained results are shown below. 1) The water cage structure of N-methane hydrate is a truncated octahedron (4668), which is composed of 8 regular hexagons and 6 squares, and the average length of the hexagons and the average length of the squares are both 2.723 Å. The average bond length of water molecules is optimized to be 1.056 Å, and the average bond angle of water molecules is 107.738°. The average bond length of methane molecules is 1.0973 Å. The average distance from methane molecules to water molecules is 4.2831 Å that is longer than the distance in the I- methane hydrate. So N-methane hydrate can accommodate larger volumes of gas molecules. The symmetric group is \begin{document}${\rm{IM}}\bar 3{\rm{M}}$\end{document} for N-methane hydrate, which has a simple and strict periodic stable structure. 2) The lattice parameter of N-methane hydrate is 7.70 Å, and the density is 0.903 g/cm3, which is greater than I-, II- and H-type hydrate density. 3) The x-ray diffraction(XRD) pattern of N-methane hydrateis calculated and is close to that of of I-methane hydrate, while the water cage of N-methane hydrate is larger. 4) The interaction between methane molecules and the water cage is van der Waals force, and the formation energy of N- methane hydrate is –0.247 eV, which indicates that the N-methane hydrate is easy to form. Both the density of states and partial density of states indicate that the interaction between methane and water cage is weak, and it relies on molecular force. 5) In addition, N-methane hydrate is an insulator material with the energy gap greater than 5 eV.
Dual-wavelength external-cavity surface-emitting laser
Authors: Qiu Xiao-Lang , Wang Shuang-Shuang , Zhang Xiao-Jian , Zhu Ren-Jiang , Zhang Peng , Guo-Yu He-Yang , Song Yan-Rong ,   College of Physics and Electronic Engineering, Chongqing Normal University, Chongqing 401331, China,   College of Applied Sciences, Beijing University of Technology, Beijing 100124, China
Published: December 2020

Dual-wavelength laser sources have important applications in the interferometry and the nonlinear-frequency-conversion generated mid-infrared or terahertz-band coherent radiation. Vertical-external-cavity surface-emitting lasers own outstanding advantages such as high output power, good beam quality and flexible emission wavelength, which make them very suitable for dual-wavelength running. In this paper, we employ a collinear Y-type cavity to produce a dual-wavelength laser. There are two semiconductor gain chips in the resonant cavity, one has an active region of In0.185Ga0.815As/GaAs strained multiple quantum wells and a designed wavelength of 960 nm, and the other has an active region of In0.26Ga0.74As/GaAsP0.02 strained multiple quantum wells and a target wavelength of 1080 nm. The peak wavelength of the photoluminescence of chip 1 is 950 nm, which is 10 nm shorter than the designed wavelength under weak pump, and the peak wavelength of the photoluminescence of chip 2 is 1094 nm, which is 14 nm longer than the target wavelength under low pump. When the pump power is increased, the peak wavelengths of the photoluminescence of two gain chips are both red-shifted. The oscillating laser wavelengths are centered at 953 nm and 1100 nm, the corresponding full width at half maximum (FWHM) values of the laser spectra are 1.1 nm and 2.7 nm, respectively. The wavelength spacing of the dual-wavelength is 147 nm, and the related mid-infrared coherent radiation is about 7.1 μm on the assumption that the dual-wavelength laser is used for difference frequency generation. When the absorbed pump power of each gain chip is 5.8 W, the total output power of the dual-wavelength laser reaches 293 mW at room temperature.
Software realization of physical property calculation model based on effective medium theory
Authors: Sun Nan-Nan , Shi Zhan , Ding Qi , Xu Wei-Wei , Shen Yang , Nan Ce-Wen ,   College of Materials, Xiamen University, Xiamen 361005, China,   School of Aerospace Engineering, Xiamen University, Xiamen 361102, China,   School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
Published: December 2020

In this paper, a composite physical property calculation software—Composite Studio is developed based on the modified effective medium theory. The computing kernel of the software is written in C++ language and its GUI is designed by Qt. With the development of the computation technique, the material genome project is proposed, which tries to shorten the period of the material design by high-throughput computation, data mining and property database establishment. On a mesoscopic scale, there are several kinds of the models to calculate the physical properties of the composite materials. However, these models usually have the formula in quite a lot of kinds of forms. A general commercial software for physical property calculation on a mesoscopic scale is still leaking. The software uses Green’s function to solve the constitutive equations. It calculates the effects of microstructural factors on physical properties. These factors include volume fraction, aspect ratio of reinforce particles, orientation distribution, and macroscopic orientation. It can obtain more than 10000 composites by freely combining four microstructure factors. The operation process of software includes 5 steps. The first step is to choose the materials of matrix and reinforcement. The second step is to select the shape type of reinforcement. The third step is to set the range of values for the microstructure factors of the composite materials. The fourth step is to choose the calculation model and start calculations. The last step is to plot and analyze the results. In addition, researchers can directly have the calculation results through the single point analysis module of the software. We use several two-dimensional line plots to display multi-dimensional calculation results. This is convenient and efficient for researchers to observe and analyze the results. Until now, two calculation modules were developed in Composite Studio, i.e. the elastic modulus calculation module and the dielectric constant calculation module. The software can be applied to different computer systems. In the future, the Composite Studio can be used as a general-purpose calculation tool embedded into an server platform for popular composite design.