Electrostatic Potential Anomaly in 2D Janus Transition Metal Dichalcogenides

Symmetry breaking induced exotic physical properties is an eternal topic in scientific community. Due to lack of mirror symmetry, 2D Janus transition metal dichalcogenides (TMDs) exhibit many bizarre features; however, the physical mechanisms of most of these intrinsic properties are still unclear. Herein, a generalized and effective approach is developed to disclose the physical mechanism of electrostatic potential anomaly in 2D Janus TMDs, based on fast Fourier transform and Moore–Penrose generalized inverse matrix for separating Hartree potential and ionic potential from electrostatic potential, and conversely, calculating charge density distribution through Hartree potential. Through extensive numerical simulations and theoretical analyses, the electrostatic potential anomaly is expounded successfully, which is a pending issue in 2D Janus TMDs: the electrostatic potential energy at Se atomic layer is larger than that at S and Te atomic layers, which breaks the periodic law. Such an anomaly could be attributed to the competition between Hartree potential energy and ionic potential energy that emerges as a result of asymmetric charge transfer, atomic layer distance, and atomic species. This approach possesses universality, and is proved to be a robust method in dealing with the issues related to electrostatic potential.     

Asymptotic Analysis of a Bingham Fluid in a Thin Domain with Fourier and Tresca Boundary Conditions

In this paper we prove first the existence and uniqueness results for the weak solution, to the stationary equations for Bingham fluid in a three dimensional bounded domain with Fourier and Tresca boundary condition; then we study the asymptotic analysis when one dimension of the fluid domain tends to zero. The strong convergence of the velocity is proved, and a specific Reynolds limit equation and the limit of Tresca free boundary conditions are obtained.     

Asymptotic expressions for the nearest and furthest dislocations in a pile-up against a grain boundary

In 1965, Armstrong and Head explored the problem of a pile-up of screw dislocations against a grain boundary. They used numerical methods to determine the positions of the dislocations in the pile-up and they were able to fit approximate formulae for the locations of the first and last dislocations. These formulae were used to gain insights into the Hall–Petch relationship. More recently, Voskoboinikov et al. used asymptotic techniques to study the equivalent problem of a pile-up of a large number of screw dislocations against a bimetallic interface. In this paper, we extend the work of Voskoboinikov et al. to construct systematic asymptotic expressions for the formulae proposed by Armstrong and Head. The further extension of these techniques to more general pile-ups is also outlined. As a result of this work, we show that a pile-up against a grain boundary can become equivalent to a pile-up against a locked dislocation in the case where the mismatch across the boundary is small.     

Asymptotic analysis of a vibrating cantilever with a nonlinear boundary

Nonlinear vibration of a cantilever in a contact atomic force microscope is analyzed via an asymptotic approach. The asymptotic solution is sought for a beam equation with a nonlinear boundary condition. The steady-state responses are determined in primary resonance and subharmonic resonance. The relations between the response amplitudes and the excitation frequencies and amplitudes are derived from the solvability condition. Multivaluedness occurs in the relations as a consequence of the nonlinearity. The stability of steady-state responses is analyzed by use of the Lyapunov linearized stability theory. The stability analysis predicts the jumping phenomenon for certain parameters. The curves of the response amplitudes changing with the excitation frequencies are numerically compared with those obtained via the method of multiple scales. The calculation results demonstrate that the two methods predict the same varying tendencies while there are small quantitative differences. Supported by the National Outstanding Young Scientists Fund of China (Grant No. 10725209), the Shanghai Leading Academic Discipline Project (Grant No. S30106), and Shandong Jiaotong University Science Foundation (Grant No. Z200812) Recommended by LIAO ShiJun     

多层介质膜光栅用高反射镜的严格耦合波分析

基于严格耦合波和增强透射矩阵的方法,提出了一种数值稳定的求解多层介质膜光学特性的机理模型.利用该模型计算了多层介质膜光栅用高反射镜的优化设计膜系.使用电子束热蒸发方式制备的多层介质膜光谱特性和理论设计的结果符合得很好,该严格耦合波模型是分析介质膜光学特性有效的稳定的数值求解方法.     

Asymptotic solution for a perturbed mechanism of western boundary undercurrents in the Pacific

This paper consider a class of perturbed mechanism for the western boundary undercurrents in the Pacific. The model of generalized governing equations is studied. Using the perturbation method, it constructs the asymptotic solution of the model. And the accuracy of asymptotic solution is proved by the theory of differential inequalities. Thus the uniformly valid asymptotic expansions of the solution are obtained.     

金属/介质薄膜中声子热辐射的空间和各向异性研究

基于理论、实验和仿真相结合的方式,着重研究了金属/介质(M D)薄膜中声子热辐射的空间特性和各向异性.声子是由于晶格振动产生的元激发,是物质的内在属性.尽管声子不易调控,但是声子与其他光学激发的耦合会产生奇异的光学现象.特别是红外到太赫兹范围内的光子与极性介质中的声子强耦合产生表面声子激元(SPhP).SPhP具有强局...     

Asymptotic behaviors of the stress fields in the vicinity of dislocations and dislocation segments

We obtain the singular asymptotic behavior of the stress field in the vicinity of a non-planar dislocation in three dimensions and the nearly singular behavior of the full self-force of the dislocation including both glide and climb forces, using asymptotic analysis. We also derive asymptotic formulas for the stress field in the vicinity of a curved dislocation segment. Numerical examples are presented to examine the asymptotic formulas. The obtained formulas can be used for qualitative understanding of the stress tensor associated with dislocations and efficient and accurate calculation of the stress tensor in dislocation dynamics simulations.     

流体力学拉氏程序收敛性及数值计算不确定度初探

将欧氏(Eulerian)数值模拟不确定度分析的基本概念和方法引进拉氏(Lagrangian)计算中,包括渐近收敛阶、渐近收敛域、网格收敛指标(Grid convergence index,GCI)等.给出GCI方法刻画数值计算不确定度的具体步骤.并应用于N-R格式,验证了方法的有效性.     

Polynomial Asymptotic Representation of Subharmonic Functions in a Half-Plane

Let u(z) be a subharmonic function in a half-plane such that its Riesz measure is concentrated on the finite system of rays.In the paper the connection between the behavior of u(z) and the distribution of its measure (including boundary measure) is investigated in terms of polynomial asymptotic representations.     

Analysis of Radiation Phenomena in a Wedge Ended Dielectric Slab Waveguide

Radiation phenomena observed in a wedge shape ended dielectric slab waveguide are analyzed using mode matching technique. The case of transverse electric polarization (TE) being parallel to dielectric slab waveguide is assumed. In order to describe the fields in the wedge region, a stack of dielectric plates is assumed and in each layer the fields are expanded in terms of the mixed spectrum of guided and radiated modes. A similar expansion is used in the constant thickness slab waveguide while in free space medium a continuous-radiation mode expansion is used. Then a mode matching approach is applied, incorporating the orthogonality properties of mixed spectrum modes, in order to compute the wave fields inside the dielectric slab waveguide and wedge medium. Mode matching is achieved by discretizing the continuous radiation mode spectrum leading into a numerically stable solution provided a sufficient large number of points are used to convert integrals into finite summations. Numerical computations are carried out for various wedge geometries and shapes including linear and exponential profiles.     

关于QGP中夸克屏蔽位势来自动力论方程的研究

从ＱＧＰ的动力论方程出发，计及运动检验夸克的色电场扰动，采用微扰方法，并在高温极限下得到ＱＧＰ中夸克的分布函数和色电势．给出了考虑运动效应在内的德拜长度λＤ（或屏蔽质量ωＤ）与温度Ｔ的关系，在静态极限下它们与ＱＣＤ理论相符．     

Asymptotic solution of a weak nonlinear model for the mid-latitude stationary wind field of a two-layer barotropic ocean

A weak nonlinear model of a two-layer barotropic ocean with Rayleigh dissipation is built.The analytic asymptotic solution is derived in the mid-latitude stationary wind field,and the physical meaning of the corresponding problem is discussed.     

Scatteration and Absorption Characteristics of Multilayered Gratings Embedded in a Dielectric Slab*

A rigorous and simple method of analyzing scattering and absorbing characteristics of multilayered gratings embedded in a dielectric slab has been proposed. The method combines a generalized scattering matrix with a lattice sums matrix and a T-matrix of an isolated single cylinder instead of the aggregate T-matrix. The validity of the present method has been confirmed by several numerical experiments and comparisons with other published results. Numerical examples show that the absorbing characteristics become very complex and are not linear with the decreasing of cylinders’ conductivity, when the arrays are stacked to a multilayered structure and are embedded in a dielectric slab.* Supported by the 21^st Century COE Program “Reconstruction of Social Infrastructure Related to Information Science and Electrical Engineerings”.     

Excitation of the Classical Electromagnetic Field in a Cavity Containing a Thin Slab with a Time-Dependent Conductivity

We derive an exact infinite set of coupled ordinary differential equations describing the evolution of the modes of the classical electromagnetic field inside an ideal cavity containing a thin slab with the time-dependent conductivity σ(t) and dielectric permittivity ε(t) for the dispersion-less media. We analyze this problem in connection with the attempts to simulate the so-called dynamical Casimir effect in three-dimensional electromagnetic cavities containing a thin semiconductor slab periodically illuminated by strong laser pulses. Therefore, we assume that functions σ(t) and δε(t) = ε(t) ? ε(0) are different from zero during short time intervals (pulses) only. Our main goal here is to find the conditions under which the initial nonzero classical field could be amplified after a single pulse (or a series of pulses). We obtain approximate solutions to the dynamical equations in the cases of “small” and “big” maximal values of the functions σ(t) and δε(t). We show that the single-mode approximation used in the previous studies can be justified in the case of “small” perturbations, but the initially excited field mode cannot be amplified in this case if the laser pulses generate free carriers inside the slab. The amplification could be possible, in principle, for extremely high maximum values of conductivity and the concentration of free carries (the model of an “almost ideal conductor”) created inside the slab under the crucial condition providing the negativity of the function δε(t). This result follows from a simple approximate analytical solution confirmed by exact numerical calculations. However, the evaluation shows that the necessary energy of laser pulses must be, probably, unrealistically high.     

基于北京大学4.5 MV静电加速器的核技术应用

北京大学4.5 MV静电加速器是20世纪80年代我国自主研发的静电加速器。该器端电压在0.7~3.8 MV连续可调,主要加速氢/氦同位素离子,并可通过打靶产生准单能直流/脉冲中子场,具有多条束线及多个实验终端。该器作为开放仪器多年来为国内外从事核技术研究的团队提供了实验平台。近年来,针对我国在能源、航天和国防等领域材料研究的重要需求,该器进行了多次升级改造。一方面通过产生7 MeV以下和14~19 MeV的准单能中子场,应用于(n, a)核反应截面的测量和聚变堆中子谱仪刻度;另一方面,通过温控辐照、核反应分析等实验终端,实现了材料辐照损伤及聚变堆材料元素定量分析等研究方向的功能拓展。此外,设计新增用于半导体材料电学性能测试的原位在线辐照终端和用于研究材料微观尺度元素分布的离子束综合分析实验终端。目前部分新终端已设计组装完成,相关搭建和调试工作正在进行中。     

High-Speed Quantum Transducer with a Single-Photon Emitter in a 2D Resonator

Quantum transducers can transfer quantum information between different systems. Microwave–optical photon conversion is important for future quantum networks to interconnect remote superconducting quantum computers with optical fibers. Here, a high-speed quantum transducer based on a single-photon emitter in an atomically thin membrane resonator, that can couple single microwave photons to single optical photons, is proposed. The 2D resonator is a freestanding van der Waals heterostructure (which may consist of hexagonal boron nitride, graphene, or other 2D materials) that hosts a quantum emitter. The mechanical vibration (phonon) of the 2D resonator interacts with optical photons by shifting the optical transition frequency of the single-photon emitter with strain or the Stark effect. The mechanical vibration couples to microwave photons by shifting the resonant frequency of an LC circuit that includes the membrane. Thanks to the small mass of the 2D resonator, both the single-photon optomechanical coupling strength and the electromechanical coupling strength can reach the strong coupling regime. This provides a way for high-speed quantum state transfer between a microwave photon, a phonon, and an optical photon.     

Asymptotic behavior of correlation functions for electric potential and field fluctuations in a classical one- component plasma

The correlations of the electric potential fluctuations in a classical one-component plasma are studied for large distances between the observation points. The two-point correlation function for these fluctuations is known to decay slowly for large distances, even if exponential clustering holds for the charge correlation functions. In this paper the asymptotic behavior of the generalk-point electric potential correlation functions is analyzed. Each of these correlation functions can be split into a reducible part, which is given by a sum of products of lower-order correlation functions, and a remaining irreducible part. It is shown, on the basis of an exponential clustering hypothesis for the charge correlation functions, that for allk3 the irreducible parts of the electric potential correlation functions decay faster than any inverse power of the distance, if one or more of the observation points move far away from the others. Hence, the two-point electric potential correlation function is the only one with a slow algebraic decay. The same statement holds for the correlation functions of the electric field fluctuations.