Point defects on graphene on metals

Understanding the coupling of graphene with its local environment is critical to be able to integrate it in tomorrow's electronic devices. Here we show how the presence of a metallic substrate affects the properties of an atomically tailored graphene layer. We have deliberately introduced single carbon vacancies on a graphene monolayer grown on a Pt(111) surface and investigated its impact in the electronic, structural, and magnetic properties of the graphene layer. Our low temperature scanning tunneling microscopy studies, complemented by density functional theory, show the existence of a broad electronic resonance above the Fermi energy associated with the vacancies. Vacancy sites become reactive leading to an increase of the coupling between the graphene layer and the metal substrate at these points; this gives rise to a rapid decay of the localized state and the quenching of the magnetic moment associated with carbon vacancies in freestanding graphene layers.     

基于Matlab分析负荷对多联机IPLV的影响

多联机有其特殊的优点,但是不同部分负荷的组合对IPLV的影响仍需进一步研究。文中建立IPLV与部分负荷的相关公式,运用MATLAB软件分析不同卸载级负荷与IPLV的关系,从而确定不同部分负荷的最优组合。     

Remarks on KdV-type flows on star-shaped curves

We study the relation between the centro-affine geometry of star-shaped planar curves and the projective geometry of parametrized maps into RP¹. We show that projectivization induces a map between differential invariants and a bi-Poisson map between Hamiltonian structures. We also show that a Hamiltonian evolution equation for closed star-shaped planar curves, discovered by Pinkall, has the Schwarzian KdV equation as its projectivization. (For both flows, the curvature evolves by the KdV equation.) Using algebro-geometric methods and the relation of group-based moving frames to AKNS-type representations, we construct examples of closed solutions of Pinkall’s flow associated with periodic finite-gap KdV potentials.     

Note on duality on polarized symplectic manifolds

In this note we use some of the results of [3] to derive a general duality theorem for the cohomologies of foliated structures on a manifold. The result is applied to the special case of a symplectic manifold M on which the foliation is given by a complex polarization F in the sense of geometric quantization. We obtain, for example, a rigorous proof of the fact that for a smooth function ƒ on M whose Hamiltonian vector field leaves F invariant, the spectrum of the corresponding prequantization operator v(ƒ) coincides with the spectrum of its transpose, under the above duality. This latter result was obtained by Simms in [12] under certain hypotheses. Proofs of the validity of those hypotheses are now available in the literature; cf. [3] and [7].     

Space-charge effects on multipactor on a dielectric

Analyzes the effects of space charge shielding on the steady state of a multipactor discharge on a dielectric. Analytic methods are used to obtain an exact function for the potential in the discharge, assuming a Maxwellian distribution of emitted electrons. An equation for the amount of power deposited on the dielectric by the multipactoring electrons, for a given saturation level, is given. A simple method for obtaining the saturation level, for a given material, is obtained     

五边形石墨烯负载Pt单原子稳定性能的应变调控

基于密度泛函理论的第一性原理计算方法,研究了Pt原子在五边形石墨烯(PG)上的吸附与动力学行为.研究结果表明,单个Pt原子在PG上虽然具有较大的吸附能及较高的扩散势垒,却不能够在衬底形成均匀分散的单原子.这是因为,随Pt原子数增加,Pt_n(n=1, 2, 3)在PG上的平均结合能也逐渐增加,更倾向于形成团簇,该发现有效否定了之前的报道称Pt能在PG上形成稳定的单原子催化剂这一结论(Phys. Chem. Chem. Phys. 21, 12201 (2019)).基于此,我们考虑对PG施加双轴应变,随着拉伸应力增加,Pt金属原子间的平均结合能逐渐降低,当拉伸应变施加至12%左右时,单个Pt在衬底上的结合能与Pt₂在衬底上的平均结合能相等,从而实现PG上均匀分散的Pt单原子催化剂.该结果对五边形石墨烯基材料应变调控实现单原子催化剂提供理论借鉴.     

Efficient terahertz coupling lens based on planar photonic crystals on silicon on insulator

We present a promising coupling device, namely, a terahertz (THz) planar photonic crystal (PhC) lens based on the effective refractive-index contrast between the PhC and the surrounding unpatterned area. Three-dimensional finite-difference time-domain calculations show a 90% power transfer from a 100-microm silicon waveguide to a 10-microm waveguide, and 45% coupling efficiency is confirmed experimentally. These results demonstrate the utility of the PhC lens as an effective approach to coupling into PhC THz circuits.     

基于光纤Bragg光栅位移测量的研究

介绍了一种基于布喇格光纤光栅技术的位移测量方法。利用宽带光源经不同的布喇格光栅得到一种由温度稳定器控制波长稳定性的合成光源。由于系统设计的合成光源波长比常规光源的波长长，所以允许的光程差测量范崮提高了几百倍，从而使得布喇格光纤光栅的波长准确性提高了干涉仪位移测量的准确性。此外，利用光外差法马赫一曾德尔干涉仪进行了位移测量实验，得到了很好的效果。     

基于BOTDA的堤坝健康监控的研究 

针对堤坝坍塌,为避免伤亡和经济损失,需要对堤坝进行健康监控,及时提供预警; 通过BOTDA(Brillouin optical time domain analysis)技术,设计并搭建堤坝实验平台,研究用分布式光纤传感器检测不同水流量下堤坝的形变;堤坝实验平台由恒流量装置和堤坝两部分组成,恒流量装置能精准的为堤坝提供所需的水流量,不同的水流量会给堤坝施加不同的压力,堤坝将在不同的水流量下产生相应的变形;将分布式光纤传感器埋入堤坝中,当堤坝发生形变,光纤会随着堤坝的变形而被拉伸,运用BOTDA技术对分布式光纤传感器进行测量分析,可得光纤的布里渊频移得数据,相应的反映了堤坝的变形;研究表明,光纤的布里渊频移能正确反映出堤坝形变,BOTDA技术应用于堤坝健康监控具有良好的监控预警效果。     

Superlattice based on graphene on a strip substrate

A graphene-based superlattice formed due to the periodic modulation of the band gap has been investigated. Such a modulation is possible in graphene deposited on a strip substrate made of silicon oxide and hexagonal boron nitride. The advantages and some possible problems in the superlattice under consideration are discussed. A model describing such a superlattice is proposed and the dispersion relation between the energy and momentum of carriers has been obtained using the transfer matrix method within this model.     

用超高速阴影摄影技术研究微喷射现象

以转镜式高速分幅相机作为记录载体,设计并建立了高速阴影系统。用该摄影系统研究在爆轰加载下金属表面的微喷射现象,以及由此产生的波阵面发展过程。通过获取的阴影图像观察到了带有特定形状缺陷（矩形槽、盲孔）锡自由表面的微喷射演变过程,给出了微射流的形状,冲击波阵面以及喷射物头部的运动速度等物理信息。     

基于Contourlet变换的图像智能分割方法研究

由于获取图像的设备不同,不同设备所拍摄图像的质量、空间分布特性差异较大。采用当前图像分割方法对图像噪声进行分割时,容易产生过分割和奇异扩散现象。为此,提出一种基于Contourlet变换的图像智能分割方法。该方法先采用方向滤波器组实现图像各个方向纹理分离,利用小波变换替换拉普拉斯变换进行图像多个方向子带分解,计算图像多尺度似然函数,并依据图像最大似然函数估计准则获得图像的初始分割,在此基础上采用自适应上下文结构从图像粗尺度的分割结果融合至最细尺度,获得最终的图像智能分割结果。仿真实验结果表明,所提方法能够有效消除噪声对图像分割的影响,使图像分割精度更高,且运行时间更短,该方法在图像获取设备中具有较高的实践价值。     

TVC effects on flow separation on slender cylinders

Occurrence of flow separation within the boundary-layer is primarily attributed to the growth of adverse pres-sure gradient within the boundary-layer. Such a separating boundary-layer can, however, be made to stay attached to the bounding surface due to various remedies where the surface transverse curvature (TVC) is one of them while the body contour is another. Assistive role of the surface transverse curvature is utilized in delaying the boundary-layer separation in a retarded flow past the slender bodies by assuming different body configurations. Axially-symmetric bodies of revolution of different shapes having a varying cross section in x, following the power-law form, have been considered in this study. It is shown that the point of separation is strongly dependent upon the surface curvature parameter k and the body-shape index n, which in fact control the flow separation. This fact ensures that an increase in the surface curvature parameter k and the body-shape index n leads to an increase in the wall shear stress which consequently delays the flow separation. Significant delay in flow separation is observed on cylinders having sufficiently large transverse curvature. Percent increase in the separation length corresponding to various values of the curvature parameter and the body-shape index has been calculated and reported in the form of Tables.