bluuebunny/crossref_metadata_2025 huggingface dataset:
Time-reversal-symmetry broken quantum spin Hall in Lieb lattice
Authors: Geng Hu  , Ji Qing-Shan  , Zhang Cun-Xi  , Wang Rui  ,   Department of Electronic Information Science and Engineering, Zhejiang Ocean University, Zhoushan 316022, China;,   Department of Donghai Science and Engineering, Zhejiang Ocean University, Zhoushan 316000, China
Published: December 2020

In this paper, the time-reversal (TR) symmetry broken quantum spin Hall (QSH) in Lieb lattice is investigated in the presence of both Rashba spin-orbit coupling (SOC) and uniform exchange field. The Lieb lattice has a simple cubic symmetry, and it has three different sites in each unit cell. The most distinctive feature of this model is that it contains only one Dirac-cone in the first Brillouin zone, where the upper dispersive band and the lower dispersive band touch the middle zero-energy band at M point and form a cone-like dispersion. The intrinsic SOC is essentially needed to open the full energy gap in the bulk. When the intrinsic SOC is nonzero, all the band structures are separated everywhere in the Brillouin zone and can be characterized by some topological invariants. The exact QSH first put forward by Kane and Mele in 2005 is characterized by the z2 number. The protection from the TR symmetry ensures the gapless crossing in the surface state in the bulk gap. In our model, the presence of the exchange field breaks the TR symmetry, which results in opening a small gap in the crossing point and the z2 topological order is not suitable for the system. This kind of state is a TR symmetry broken QSH, which is characterized by the spin Chern numbers. The spin Chern numbers have a much wider scope of application than z2 index. It is suitable for both TR symmetry system and the TR symmetry broken system. For Lieb lattice ribbons, the spin polarization and the wave-function distributions are obtained numerically. There exists a weak scattering between the counter-propagating states in the TR symmetry broken QSH, and the spin transport along the boundary with a low dissipation replaces the dissipationless spin current in a TR symmetry system. In experiment, such a system can be realized by the two-dimensional Fermi gases in optical lattice with Lieb symmetry. The above conclusions are expected to give theoretical guidance in the spin device and the quantum information.
Composition formulas for Mg-Al industrial alloy specifications
Authors: Qian Sheng-Nan  , Dong Chuang  ,   Key Laboratory of Materials Modification, Ministry of Education, School of Material Science and Engineering, Dalian University of Technology, Dalian 116024, China
Published: December 2020

Mg-Al alloys are the most widely used Mg-based industrial alloys, but their composition rules behind the apparent industrial specifications are largely unknown, which hinders the development of new alloys. As is well known, industrial alloys often undergo the process of a high-temperature solution treatment, and the final structures originate from the single-phase solid solution parent state. Since solid solutions are characterized by short-range chemical orders, necessarily the optimum alloy composition should be related to the presence of a certain short-range chemical structure unit. In the present paper, by introducing our cluster-resonance model for short-range-order structure description of solid solutions, a chemical structure unit of Mg-Al binary solid solution is established,[Al-Mg12]Mg1, which represents the characteristic short-range-order structure, with the bracketed part being the nearest-neighbor cluster centered by Al and shelled by 12Mg and with one glue atom Mg located between the clusters. Because of the existence of other alloying elements besides Al, a general formula[(Al, A)1-Mg12]-(Mg, B) is then proposed, where A represents the elements showing a negative mixing enthalpy with Mg, while B showing a positive one. This formula is used to explain the multi-component Mg-Al industrial alloys. Based on this chemical formula, typical Mg-Al industrial alloy specifications in ASTM handbook are well explained. For instance, cast AZ63A alloy is formulated as[Al0.78Zn0.16-Mg12]Mg1.04Mn0.02, cast AZ81A as[Al0.97Zn0.03-Mg12]Mg0.98Mn0.02, and wrought AZ80A as[Al1.02n0.03-Mg12]Mg0.94Mn0.01. The deviations from the ideal chemical structure unit in different Mg-Al alloys are well correlated to their corresponding alloy performances. Those alloys, where the numbers of center atoms are close to ones in their cluster formulas, exhibit excellent comprehensive mechanical performances in both strength and plasticity. While the alloy with less than one center atom only shows good plastic performance with a relatively poor strength, and the one with more than one center atom shows just the reverse tendency. Among cast Mg-Al alloys, AZ81A, whose cluster formula completely matches the stable chemical structure unit, exhibits the optimized combination of strength (275 MPa) and plasticity (elongation 15%). Among wrought Mg-Al alloys, AZ61A and AZ80A, whose cluster formulas show minor deviations of -0.11 and 0.05 in the center site from the ideal chemical structure unit, also have good comprehensive mechanical properties, respectively with the strengths of 310 MPa and 380 MPa, and the elongations of 16% and 7%. Based on the results in the present paper, the simple composition rule behind the complex industrial alloy specifications as unveiled here, can be a powerful approach to the development of Mg-Al alloys.
High efficiency cross-polarized wave filter for non-vacuum transmission
Authors: Li Rong-Feng  , Xue Xing-Tai  , Zhao Yan-Ying  , Geng Yi-Xing  , Lu Hai-Yang  , Yan Xue-Qing  , Chen Jia-Er  ,   State Key Laboratory of Nuclear Physics and Technology, School of Physics, Peking University, Beijing 100871, China
Published: December 2020

Development of high-peak power laser system encounters difficulties in producing the pulses with high temporal contrast. To increase the pulse temporal contrast ratio, a nonlinear filter based on crossed-polarized wave (XPW) generation is proposed. The XPW generation relies on a third-order nonlinear process occurring in a nonlinear medium, such as barium fluorite (BaF2) crystal. The XPW process is quite straightforward:a linearly polarized laser pulse is focused on BaF2 crystal positioned between two orthogonally polarizers, high power main pulses due to nonlinear polarization rotation can pass through the second polarizer, while low power unconverted pre-and post-pulses are filtered by the second polarizer. With the XPW technique, pulse contrast can be enhanced by several orders of magnitude. Furthermore, XPW spectrum can be broaden by a factor with respect to the initial spectrum. This efficient pulse cleaner presents many advantages and has proved to be a simple and reliable pulse filter operating in a double chirped pulse amplification system.
Most of previous XPW experiments utilize short focal systems or work off focus due to an intensity limit in the crystal (BaF2). These drawbacks result in a lower conversion efficiency (lower than 10%) when using a single crystal. Dual crystal setup is capable of achieving efficiency more than 20%, yet the configuration restricts the crystal separation to a millimeter level. The use of long focus lens in the XPW device is capable of reaching higher efficiency, with BaF2 crystal positioned in the focal plane. Hence for milljoule pulses, the setup distance increases to tens of meters, resulting in a complicated system and cumbersome configuration.
Considering these limitations, a compact, highly efficient and stable XPW generation using dual-lens system suitable for non-vacuum transmission is presented. The measured nonlinear accumulated phase shows little deterioration of pulse quality. With a compact dual lens system, we realize an excellent XPW conversion of above 22% (internal efficiency of 30%) with using double BaF2 crystals, while a femtosecond laser pulse can experience a spectrum broadening up to a factor of 1.78. The dual-lens configuration overcomes the crystal separation limit, and conversion efficiency exceeds 20% for a crystal separation from 13 cm to 22 cm, which is conducible to flexibility and robustness. The stability for the setup to generate shorter pulses with very high contrast or compensate for spectral gain narrowing in the preamplifier is ensured due to the dual-lens focusing system.
Quantum trajectory simulation for nonadiabatic molecular dynamics
Authors: Li Xiao-Ke  , Feng Wei  ,   Department of Physics, Tianjin University, Tianjin 300350, China
Published: December 2020

The mixed quantum-classical (MQC) molecular dynamics (MD) approaches are extremely important in practice since, with the increase of atomic degrees of freedom, a full quantum mechanical evaluation for molecular dynamics would quickly become intractable. Moreover, in some cases, the nonadiabatic effects are of crucial importance in the proximity of conical intersection of potential energy surfaces (PESs), where the energy separation between different PESs becomes comparable to the nonadiabatic coupling. In the past decades, there has been great interest in developing and improving various nonadiabatic MQC-MD protocols. The widely known nonadiabatic MD proposals include the so-called Ehrenfest or time-dependent-Hartree mean-field approach, the trajectory surface-hopping method, and their mixed scheme. Among the trajectory-based surface hopping methods, the most popular one is Tully's fewest switches surface hopping approach. In this approach, the nonadiabatic dynamics is treated by allowing hops from one PES to another, with the hopping probability determined by a certain artificial hopping algorithm. In our present work, we extend the study of a recent work on the nonadiabatic MQC-MD scheme, which is based on a view that the nonadiabatic MQC-MD actually implies an effective quantum measurement on the electronic states by the classical motion of atoms. The new protocol, say, the quantum trajectory (QT) approach, provides a natural interface between the separate quantum and classical treatments, without invoking artificial surface hopping algorithm. Moreover, it also connects two widely adopted nonadiabatic dynamics methods, the Ehrenfest mean-field theory and the trajectory surface-hopping method. In our present study, we implement further the QT approach to simulate several typical potential-surface models, i.e., including the single avoided crossing, dual avoided crossing, extended coupling, dumbbell and double arch potentials. In particular, we simulate and compare three decoherence rates, which are from different physical considerations, i.e., the frozen Gaussian approximation, energy discrimination and force discrimination. We also design simulation algorithms to properly account for the energy conservation and force direction change associated with the surface hopping. In most cases, we find that the QT results are in good agreement with those from the full quantum dynamics, which is insensitive to the specific form of the decoherence rate. But for the model involving strong quantum interference, like other nonadiabatic MQC-MD schemes, the QT approach cannot give desirable results. Developing better method should be useful for future investigations in this research area.
Plasma characteristics of helical streamers induced by pulsed discharges
Authors: Zou Dan-Dan  , Cai Zhi-Chao  , Wu Peng  , Li Chun-Hua  , Zeng Han  , Zhang Hong-Li  , Cui Chun-Mei  ,   East China Jiaotong University, Nanchang 330013, China;,   Jiangxi Electric Power Design Institute, Nanchang 330096, China;,   Hefei University of Technology, Hefei 230009, China
Published: December 2020

The plasma discharge channel in three-dimensional helical shape induced by pulsed direct current (DC) discharge without external stable magnetic field is discovered experimentally. It can be observed by intensified charge-coupled device camera that a luminous plasma structure fast propagates along a helical path in the form of guided streamer (ionization wave). And the propagation of the streamer is stable and repeatable. We take this streamer which propagates along the helical discharge path as the study object, and explain its mechanism by constructing an electromagnetic model. The result shows that the helical shape plasma plumes can exhibit two different chiral characteristics (right-handed and left-handed helical pattern). While the discharge parameters such as pulse frequency, boundary condition, etc. can all affect the propagating characteristics of helical streamers. The electromagnetic radiation driven by pulsed DC power inside the dielectric tube which forms the wave mode is an important source of the poloidal electrical field. The helical steamers form when the poloidal electrical field is close to the axial electrical field. The velocities of the propagation in poloidal and axial direction are estimated respectively, and the hybrid propagation modes involving the interchangeable helical pattern and the straight-line pattern propagating plasmas are explained from the viewpoint of multi-wave interaction.
Optical transparency radial distribution of ion thruster
Authors: Long Jian-Fei  , Zhang Tian-Ping  , Li Juan  , Jia Yan-Hui  ,   Science and Technology on Vacuum Technology and Physics Laboratory, Lanzhou Institute of Physics, Lanzhou 730000, China
Published: December 2020

The optical system is one of the main components of an ion thruster, which consists of electrically biased multi-aperture grids. The grid design is critical to the ion thruster operation since its transparency has an important influence on the thruster efficiency and thrust. To further optimize the optical system performance and evaluate effectively the efficiency of ion thruster, the optical transparency radial distribution of ion thruster is analyzed and discussed in experiment and simulation. The process of beam extraction is simulated by the particleincell-Monte Carlo collision (PIC-MCC) method, and the movement of the ions is investigated by the PIC method while the collisions of particles are handled by the MCC method. Then the interdependency among the transparency of screen grid, the accelerator grid, optics system and the number of ion extracted is analyzed. Taking into account the distribution of ion density at the exit of discharge chamber, the radial distribution of the screen grid transparency, accelerator grid transparency and optical system transparency are acquired. An experiment is performed to verify the simulation based derivation, indicating the good agreement between experimental and simulation results. The results show that the radial distribution of screen grid transparency increases gradually along the radial direction and has a good central axial symmetry, and its minimum value is located in the center of the thruster while the maximum value is near the margin region of screen gird. The radial distribution of accelerator grid transparency is opposite to that of the screen grid transparency, which decreases along the radial direction, and its maximum value is located at the axis of the thruster. The radial distribution of optical system transparency is the same as that of the screen grid transparency. And its minimum value is in the center of optics system, which indicates that the effect of accelerator grid transparency on the optical system transparency is little. In addition, the study also finds that the total optical transparency of ion thruster decreases slowly as the beam current increases. This work will provide a lot of support for the optimal design of ion thruster optics system.
Oxygen vacancy induced band gap narrowing of the low-temperature vanadium dioxide phase
Authors: Gu Yan-Ni  , Wu Xiao-Shan  ,   Zhangjiagang Campus, Jiangsu University of Science and Technology, Zhangjiagang 215600, China;,   Laboratory of Solid State Microstructures and School of Physics, Nanjing University, Nanjing 210093, China
Published: December 2020

Switching of vanadium dioxide (VO2) from low-temperature insulating phase to high-temperature rutile phase can be induced by photons with a certain energy. Photoinduced insulator-metal transition is found experimentally in VO2 polycrystalline film by photos with energy even below 0.67 eV. However, insulator-metal transition in single crystal can only be induced when photo energyis above 0.67 eV. In order to understand these experimental phenomena, we make a first-principle study on low-temperature non-magnetic M1 phase of VO2 with oxygen vacancy by density functional theory calculations based on the Heyd-Scuseria-Ernzerhof screened hybrid functional. According to symmetry, M1 phase has two kinds of different oxygen vacancies, O1 and O2 vacancies. Calculations are made on structures and electronic properties of nonmagnetic M1 phases with O1 and O2 vacancies, respectively. The present theoretical results show that neither the short vanadium-vanadium (VV) bond length near O1 or O2 vacancy nor the lattice parameters almost change but the long VV bond length near O1 or O2 vacancy decreases due to the oxygen vacancy. The long VV bond lengths near O1 and O2 vacancies are about 2.80 and 2.95 , respectively, but the long VV bond length is 3.17 in pure M1. The insulating band gap is opened between V 3d bands, and hybridization happens between V 3d and O 2p orbitals. Furthermore, the present theoretical results demonstrate that the band gap of pure nonmagnetic M1 is 0.68 eV while M1 with O1 vacancy, O2 vacancy, and two oxygen vacancies including O1 and O2, have band gaps of 0.23 eV, 0.20 eV, and 0.15 eV, respectively. The band gap decreases probably because oxygen vacancy results in the decease of the long VV bond length near it. The present results can explain the experimental results well.
Monte-Carlo tree search for stable structures of planar clusters
Authors: He Chang-Chun  , Liao Ji-Hai  , Yang Xiao-Bao  ,   Department of Physics, South China University of Technology, Guangzhou 510640, China
Published: December 2020

Illustrated by the case of the planar clusters, we propose a new method to search the possible stable structures by combining the structural identification and Monte-Carlo tree algorithm. We adopt two kinds of model-potential to describe the interaction between atoms:the pair interaction of Lennard-Jones potential and three-body interaction based on the Lennard-Jones potential. Taking the possible triangular lattice fragment as candidates, we introduce a new nomenclature to distinguish the structures, which can be used for the rapid congruence check. 1) We label the atoms on the triangular lattice according to the distances and the polar angles. where a given triangular structure has a corresponding serial number in the numbered plane. Note that the congruent structures can have a group of possible serial numbers. 2) We consider all the possible symmetrical operations including translation, inversion and rotation, and obtain the smallest one for the unique nomenclature of the structure. In conventional search of magic clusters, the global optimizations are performed for the structures with given number of atoms. Herein, we perform the Monte-Carlo tree search to study the evolution of stable structures with various numbers of atoms. From the structures of given number of atoms, we sample the structures according to their energy with the importance sampling, and then expand the structures to the structures with one more atom, where the congruence check with the nomenclature is adopted to avoid numerous repeated evaluations of candidates. Since the structures various numbers of atoms are correlated with each other, a searching tree will be obtained. In order to prevent the over-expansion of branches, we prove the “tree” according to energy to make the tree asymmetric growth to retain the low energy structure. The width and depth of search is balanced by the control of temperature in the Monte-Carlo tree search. For the candidates with lower energies, we further perform the local optimization to obtain the more stable structures. Our calculations show that the triangular lattice fragments will be more stable under the pair interaction of Lennard-Jones potential, which are in agreement with the previous studies. Under the three body interaction with the specific parameter, the hexagonal lattice fragments will be more stable, which are similar to the configurations of graphene nano-flakes. Combining the congruence check and Monte-Carlo tree search, we provide an effective avenue to screen the possible candidates and obtain the stable structures in a shorter period of time compared with the common global optimizations without the structural identification, which can be extended to search the stable structure for materials by the first-principles calculations.
Flow unit model in metallic glasses
Authors: Wang Zheng  , Wang Wei-Hua  ,   Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
Published: December 2020

Metallic glass is a promising metallic material with many unique properties, and also considered as a model system to study the mysteries of amorphous materials. Recently, many experimental and simulation results supported the existence of “flow unit” in metallic glass. In this paper, we review the background, the theoretical and experimental evidences of flow unit model. Flow units are considered as those loosely packed regions embedded inside the elastic matrix and behave like viscous liquid. Compared with the matrix, flow unit regions have low modulus and strength, low viscosity, high atomic mobility and stand in the saddle points on energy landscape. Therefore, flow units can be treated as dynamical defects in metallic glass. The feature, activation and evolution process of flow unit region in metallic glass as well as their correlation with property in metallic glass are also reviewed. Through dynamical mechaincal methods like dynamical mechanical spectra and stress relaxation, flow unit region and its properties can be distinguished and studied. A three-parameter physical model is proposed to describe the mechnical behaivors of flow units. The activations and evolutions of flow unit under different temperature and strain conditions are studied. A three-stage evolution process is found and the relation with mechanical performance and relaxation behavior is established. The characteristics of flow units are also related to various properties of metallic glass, like plasticity, strength, fracture and boson peaks. By using the thermal, mechanical and high pressure aging procedues, the properties of metallic glass can be manipulated as desired through adjusting the density of flow units. We show that the flow unit model not only helps to understand the mechanism behind many long-standing issues like deformation, glass transition dynamic relaxations, and the connection between strucutre and properties and performance of metallic glasses, but also is crucial for tuning and designing the properties of metallic glasses.
Fabrications and mechanical behaviors of amorphous fibers
Authors: Yi Jun  ,   Laboratory for Microstructures, Institute of Materials, Shanghai University, Shanghai 200444, China
Published: December 2020

Mechanical properties of micro- and nanoscale fibers are superior to their bulk counterparts, and their mechanical behaviors are different from each other. Homogeneous amorphous fibers with smooth surfaces and controllable sizes can be continuously drawn from supercooled liquid. Compared with the preparing of crystalline fibers, the manufacturing of amorphous fibers saves much energy and time. Furthermore, amorphous materials have excellent mechanical properties due to their short-ranged ordered and long-ranged disordered structures. Therefore, amorphous fibers have wide engineering applications and research interest. In this paper we review the fabrication and mechanical behaviors of amorphous fibers with excellent mechanical properties including oxide glass fibers and amorphous alloy fibers.There are continuous and discontinuous oxide glass micro-fibers. Discontinuous oxide glass micro-fibers can be fabricated by techniques in which a thin thread of melt flowing from the bottom of a container is broken into segments. Continuous oxide micro-fibers can be fabricated by techniques in which a filament of supercooled liquid is drawn from melt. However, oxide glass nano-fibers can be fabricated by chemical vapor deposition, laser ablation, sol-gel, and thermal evaporation methods. Fabrication techniques of amorphous alloy fibers are very different from those of oxide glass fibers. These techniques adopt in-rotating-water spinning method, melt-extraction method, Taylor method, nanomoulding method, fast drawing method, melt drawing method, and gas atomization method.Microscale oxide glass fiber has a facture strength as high as 6 GPa. The fracture strength of nanoscale oxide glass fiber can reach 26 GPa which is close to the theoretical strength of 30 GPa. On the other hand, the plasticity of microscale amorphous alloy fibers is mediated by shear banding. The shear band spacing decreases with reducing sample size in bending. However, there is no tensile plasticity in microscale amorphous alloy fibers. When the sample size is smaller than the size of shear band core (500 nm), inhomogeneous plastic deformation transforms into homogeneous plastic deformation. The tensile plasticity of amorphous alloy is significantly improved. The homogeneous plastic deformation is mediated by catalyzed shear transformation. The catalyzed shear transformation may be the origin of hardening behaviors of nanoscale amorphous alloy fibers.Fianlly, we summary the unsolved problems in the fabrications and mechanical behaviors of amorphous fibers, and discuss the prospect of amorphous fibers.
Recent progress of graphene-like germanene
Authors: Qin Zhi-Hui  ,   School of Physics and Electronics, Hunan University, Changsha 410082, China
Published: December 2020

With tremendous progress of graphene and with the consideration of the compatibility with semiconductor industry, the construction of analogous two-dimensional crystalline systems-new two-dimensional honeycomb and layered materials composed of elements other than carbon, the group IV (Si, Ge) analogs of graphene and the investigation of their fascinated electronic properties have become the frontier topics of condensed matter physics. Theoretical calculation predicts that unlike the planar structure of graphene, the germanene has stable, two-dimensional, low-buckled, honeycomb structure similar to that of silicene, but has much higher spin-orbit band gap than silicene, which is certainly of crucial importance in future electronics. The influences of atomic structures and the buckling of the low-buckled geometry on local electronic structure of the fabricated germanene are also reviewed from the atomic point of view. As theoretical studies on germanene are rapidly increasing, now the major challenge in this field is the preparation of high-quality germanene. Compared with silicene, the germanene has larger Ge-Ge interatomic distance which can weaken the orbital overlaps, resulting in the big difficulty in constructing germanene. In this work we review the recent progress of experimental epitaxial growth of germanene on surfaces, with emphasis on metal surfaces. The growth of quasi-freestanding germanene and its potential applications in nanoelectronics in the future are also discussed.
Logical integration device for two-dimensional semiconductor transition metal sulfide
Authors: Li Wei-Sheng  , Zhou Jian  , Wang Han-Chen  , Wang Shu-Xian  , Yu Zhi-Hao  , Li Song-Lin  , Shi Yi  , Wang Xin-Ran  ,   National Laboratory of Solid State Microstructures, Collaborative Innovation Center of Advanced Microstructures, School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, China
Published: December 2020

The semiconductor industry has experienced exponential growth for more than 50 years, following the Moore's Law. However, traditional microelectronic devices are currently facing challenges such as high energy consumption and the short-channel effect. As an alternative, two-dimensional layered materials show the ability to restrain the carriers in a 1 nm physical limit, and demonstrate high electron mobility, mutable bandgap, and topological singularity, which will hopefully give birth to revolutionary changes in electronics. The transition metal dichalcogenide (TMDC) is regarded as a prospective candidate, since it has a large bandgap (typically about 1-2 eV for a monolayer) and excellent manufacture compatibility. Here in this paper, we review the most recent progress of two-dimensional TMDC and achievements in logic integration, especially focusing on the following key aspects:charge transport, carrier mobility, contact resistance and integration. We also point out the emerging directions for further research and development.
Electron scattering studies of NO2 radical using R-matrix method
Authors: Zhu Bing  , Feng Hao  ,   School of Science, Xihua University, Chengdu 610039, China
Published: December 2020

Nitrogen dioxide molecule plays an important role in modeling atmospheric process. It is a toxic gas and considered as an atmospheric pollutant due to its involvement in reactions that produce ground-level ozone. The electron scattering of NO2 molecule has been extensively studied, specifically at intermediate and high energies. The discrepancies between previous theoretical studies and experimental data at low impact energies (below 4 eV) suggest that the in-depth research should be carried out. The target optimized equilibrium geometry is computed at the highly accurate coupled cluster singles, doubles and perturbative triples[CCSD(T)] level in this study. The ab initio R-matrix method is employed to study the integral and momentum transfer cross sections of low-energy electron scattering from NO2 radical up to 10 eV. Two models including static-exchange and close-coupling approximation are used to reveal the dynamic interaction. The electronic excitation cross sections are computed from ground state to seven electronically allowed excited states. All target states whose vertical excitation energies are below 20 eV are included in the close-coupling expansions of the scattering system. In our CC model, six electrons are in the core orbitals 1a1, 2a1 and 1b2, and the remaining 17 electrons are free to occupy the 4a1, 5a1, 6a1, 7a1, 1b1, 2b1, 3b2, 4b2, and 1a2 orbitals. The aug-cc-pVTZ dunning basis sets are used to optimize the target structure and electron scattering. A Born closure procedure is used to account for the contribution of partial waves higher than l=4 to obtain cross sections. Two shape resonances found at 0.76 eV and 1.82 eV in this study are lower than previous theoretical calculations, but the comparisons with other theoretical calculations and experimental data show that the present R-matrix study not only agrees well with the experiments but also corrects the overestimations of total cross sections of some other theoretical data in the very low energy regions. To study the influence of electron correlations, 21, 82 and 107 target electronic configurations are used in the close coupling model calculations, respectively. The comparisons of integrated cross sections indicate that it is very important to include more target electronic configurations to obtain the converged scattering cross sections, which reveals the importance of electron correlations.
Electronic structures and ferroelectric properties of Ba-doped ZnO
Authors: Xu Jia-Nan  , Chen Huan-Ming  , Pan Feng-Chun  , Lin Xue-Ling  , Ma Zhi  , Chen Zhi-Peng  ,   School of Physics and Electronic-Electrical Engineering, Ningxia University, Yinchuan 750021, China
Published: December 2020

Wurtzite ZnO has long been considered to be a promising candidate material for photovoltaic application due to its high power conversion efficiency. More interestingly and very recently, some research results suggested that the ferroelectric property of the photovoltaic material introduced by chemical elements doping can promote its power conversion efficiency significantly. Therefore, in order to understand the effect of Ba doping on the electronic structure and the ferroelectric properties of ZnO and to reveal the potentially optoelectronic properties of Zn1-xBaxO, the energy band structure, the density of states, and the polarizability and the relative dielectric constant of the bulk Ba-doped ZnO supercell system, in which the Zn atoms are partly and uniformly substituted by the Ba atoms, are investigated by using the first-principles method based on the density functional theory and other physical theory. The norm-conserving pseudopotentials and the plane-wave basis set with a cut-off energy of 600 eV are used in the calculation. The generalized gradient approximation refined by Perdew and Zunger (GGA-PBE), the local density approximation (LDA) and the local density approximation added Hubbard energy (LDA+U) are employed for determining the exchange-correlation energy respectively. Brillouin zone is set to be within 4×4×5K point mesh generated by the Monkhorst-Pack scheme. The self-consistent convergence of total energy is at 2.0×10-6 eV/atom. Additionally, in order to obtain a stable and accurate calculation result, the cell structure is optimized prior to calculation. The calculated results suggest that the bulk Ba-doped ZnO semiconductor system is still a semiconductor with a direct wide band gap. The band gap of Zn1-xBaxO increases gradually with Ba atom doping percentage increasing from 12.5% to 87.5%. Consequently, the ferroelectric polarization properties and the dielectric properties of the bulk Ba-doped wurtzite ZnO materials are tailored by doping Ba atoms. It indicates that the polarizability of Zn1-xBaxO system increases with Ba doping atomic percentage increasing, especially, the polarizability reaches to a maximum when the atomic percentage of doping is 75%. Meanwhile, the relative dielectric constant inversely decreases with Ba atomic percentage increasing. This is attributed to the effective contribution of Ba atoms to the density of state at the bottom of the valence band. The diagonalized components of polarizability imply that there are possible micro-domains in the supercell while applying externally electric field to it. And the supercell presents a nearly isotropic polarizability macroscopically due to the strong interaction among the electric dipole moments existing in the different domains.
Bursting oscillations and mechanism of sliding movement in piecewise Filippov system
Authors: Zhang Zheng-Di  , Liu Ya-Nan  , Li Jing  , Bi Qin-Sheng  ,   Faculty of Science, Jiangsu University, Zhenjiang 212013, China;,   Faculty of Civil Engineering and Mechanics, Jiangsu University, Zhenjiang 212013, China
Published: December 2020

Since the wide applications in science and engineering, the dynamics of non-smooth system has become one of the key research subjects. Furthermore, the interaction between different scales may result in special movement which can be usually described by the combination of large-amplitude oscillation and small-amplitude one. The influence of multiple scale on the dynamics of non-smooth system has received much attention recently. In this work, we try to explore the bursting oscillations and the mechanism of non-smooth Filippov system coupled by different scales in the frequency domain. Taking the typical periodically excited Duffing's oscillator for example a Filippov system coupled by two scales in the frequency domain is established when the difference in order between the excited frequency and the system natural frequency is obtained by introducing the piecewise control into the state variable and choosing suitable parameters. For the case in which the exciting frequency is far less than the natural frequency, the whole exciting term can be considered as a slow-varying parameter, also called slow subsystem, which leads to a generalized autonomous system, i.e., the fast subsystem. The equilibrium branches and the bifurcations of the fast subsystem along with the variation of the slow-varying parameter in different regions divided according to non-smooth boundary, can be derived. Two typical cases are taken into consideration, in which different distributions of the equilibrium branches and the relevant bifurcations of the fast subsystem may exist. It is pointed out that the variations of the parameters may influence not only the properties of the equilibrium branches, but also the structures of the bursting attractors. Furthermore, since the governing equation alternates between two subsystems located in different regions when the trajectory passes across the non-smooth boundary, the sliding movement along the non-smooth boundary of the trajectory can be observed under the condition of certain parameters. By employing the transformed phase portrait which describes the relationship between the state variable and the slow-varying parameter, the mechanisms of different bursting oscillations and sliding movements are investigated. The results show that bursting oscillations may exist in a non-smooth Filippov system coupled by two scales in the frequency domain. The alternations of the governing equation between different subsystems located in the two neighboring regions along the non-smooth boundary may result in a sliding movement of the trajectory along the non-smooth boundary.
Financial complex network model based on textual mutual information
Authors: Sun Yan-Feng  , Wang Chao-Yong  ,   College of Computer Science and Technology, Jilin University, Changchun 130012, China;,   School of Information Engineering, Jilin Engineering Normal University, Changchun 130021, China
Published: December 2020

Complex networks are widely used in many problems of the financial field. It can be used to find the topological structure properties of the financial markets and to embody the interdependence between different financial entities. The correlation is important to create the complex networks of the financial markets. A novel approach to incorporating textual mutual information into financial complex networks as a measure of the correlation coefficient is developed in the paper. We will symbolize the multivariate financial time series firstly, and then calculate correlation coefficient with textual mutual information. Finally, we will convert it into a distance. To test the proposed method, four complex network models will be built with different correlation coefficients (Pearson's and textual mutual information's) and different network simplification methods (the threshold and minimum spanning tree). In addition, for the threshold networks, a quantile method is proposed to estimate the threshold automatically. The correlation coefficients are divided into 6 equal parts. And the midpoint of the 4th interval will be taken as the threshold according to our experience, which can make the MI methods and Pearson methods have the closest number of edges to compare the two methods. The data come from the closing prices of Chinese regional indexes including both Shanghai and Shenzhen stock market. The data range from January 4, 2006 to December 30, 2016, including 2673 trading days. In view of node correlation, the numerical results show that there are about 20% of the nonlinear relationships of the Chinese regional financial complex networks. In view of the network topology, four topological indicators for the regional financial complex network models will be calculated in the paper. For average weighted degree, the novel method can make the reserved nodes closely compared with Pearson's correlation coefficient. For network betweenness centralization, it can improve the betweenness importance of reserved nodes effectively. From the perspective of modularity, the novel method can detect better community structures. Finally, in dynamic network topology features, the data of regional indexes will be equally divided yearly for constructing complex network separately. The simplification method used in the section is the threshold method. The numerical results show that the proposed methods can successfully capture the two-abnormal fluctuation in the sample interval with the dynamics of the small-world and the network degree centralization. In addition, we find that the proposed regional financial network models follow the power-law distribution and are dynamically stable. Some developing regions are more important than the developed ones in the regional financial networks.
Wettability and morphology of liquid gallium on graphene surface
Authors: Wang Jun-Jun  , Li Tao  , Li Xiong-Ying  , Li Hui  ,   Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, Ji'nan 250061, China;,   Department of Physics, Changji University, Changji 831100, China
Published: December 2020

Liquid gallium and its alloy with low melting point, low toxic and high electrical conductivity are used extensively in burgeoning microfluidic and flexible electronic devices. The key to producing these devices is to effectively control the wettability and morphology of liquid metal on the solid interface in different manufacturing processes. Based on the Lennard-Jones (L-J) potential describing the solid-liquid interaction, the wettabilities of liquid gallium film on the smooth and rough graphene surfaces are effectively investigated by molecular dynamics simulation which is an available and powerful option in this field. Different regimes of wetting are discovered by changing the depth of the L-J potential, and the stable contact angle increases with Ga-C potential depth decreases. The results show that the equilibrium contact angle and the retraction velocity increase with the decrease of the L-J potential between the gallium and graphene, showing that some properties change from complete wetting to hydrophilic and to hydrophobic. The L-J potential depth obtained from the simulation results can be effectively employed to describe the interaction between the liquid gallium and the substrate because the resulting wetting angle is extremely close to the experimental value. When employing the most appropriate L-J potential, it is found that although the initial retraction velocity increases with the proportional decrease of the thickness of the liquid Ga film, there are a few of differences in equilibrium contact angle and final retraction velocity in virtue of the competition between the surface tension of the Ga film and Ga-C interaction. It means that for the wetting state the film thickness is not the crux for changing the equilibrium contact angle and retraction velocity based on a similar conversion of potential energy into kinetic energy. Finally, we investigate the effects of the L-J potential on three rough surfaces which are patterned into three types of nanopillars with different top morphologies respectively. Specifically, it is shown that in spite of similar surface roughness, the wetting morphologies of liquid gallium deposited on various nano-textured graphene surfaces range from hydrophobic to dewetting state, suggesting that not only the roughness but also the morphology of surface can exert an available influence on the wettability of liquid. The wetting transition between the wetting and dewetting state can be achieved dynamically by adjusting the morphologies of nanopillars involved although we still need to go into more detail on the configurable way to fulfill the changing requirements.
Numerical simulation of deformation and rupture process of bubble in an oil film impacted by an oil droplet
Authors: Zhou Jian-Hong  , Tong Bao-Hong  , Wang Wei  , Su Jia-Lei  ,   School of Mechanical Engineering, Anhui University of Technology, Maanshan 243032, China;,   Institute of Tribology, Hefei University of Technology, Heifei 230009, China
Published: December 2020

Impact of oil droplet on oil film usually takes place in the lubrication process of rotating mechanical parts and machinery which can easily lead to bubble entrainment. Bubbles have important influences on the motion process of the oil droplet impacting on the oil film and also on the formation quality of the oil film layer. An oil droplet impacting on the oil film which contains a bubble is simulated numerically based on the coupled level set and the method of determining volume fraction. The bubble deformation process in the oil film during an oil droplet impacting on the oil film is investigated by the simulation method. The influences of the bubble size and the bubble position on the bubble deformation characteristic are also analyzed. The dynamic mechanism of the bubble rupture is discussed. The numerical results show that as the oil droplet impacts on the oil film, the bubble may rupture on the free surface, presenting stable deformation, or rupture in the oil film, which is greatly influenced by the bubble size. When the bubble diameter is in a range between 10 m and 20 m, the bubble deformation becomes more serious with the increase of bubble diameter, and the rupture of bubble on the free surface may occur over time. When the bubble diameters are in a range between 20 m and 30 m, the bubble rupture occurs in a short time after the bubble has reached a maximum deformation, and there is no obvious relationship between the maximum bubble deformation and the bubble diameter. The diameter of 20 m is a critical value for a bubble to rupture on a free surface or inside an oil film, with which a bubble can keep stable in an oil film layer. As the bubble position changes, the bubble deformation process changes correspondingly. Under the same impact conditions, bubbles at the top of the oil film are more likely to deform than those in the center of the oil film. Bubbles at the bottom of the oil film have the smallest total deformation and finally attach to the wall. The bubble rupture is caused by the instability of the gas-liquid interface and the surface tension. The viscous shear force also plays an important role when the bubble rupture takes place in the oil film.