准晶

一、什么是准晶及准晶的发现过去人们一直把固态物质划分为晶体(crystal)和无序体(amorphous)。晶体在短程和长程上都是有序的,突出的特点是组成晶体的原子或离子在空间排列的点阵具有周期性,即长程平移序。无序体具有短程有序,但在长程上无序,即它的点阵在空间长程范     

n元液态合金的相干势近似方程

本文用广义点阵-气体模型取极限的方法导出了n元液态合金的CPA方程。这种方法是由Yonezawa和Watabe提出用在单元液态金属上。本文考虑了不同原子之间电子跳跃能t^AB是t^AA和t^BB的几何平均。不考虑短程有序时,本文的结果与Shiba用Coherent-Locator方法得到的结果一致。 关键词：     

AB合金有序化过程的动力学(Ⅰ)

由原子在点阵中扩散的反应率过程理论,在最近邻近似下,计算了AB合金的长程有序化动力学及长程序为零时的短程有序化动力学。所得结果与前人的实验结论及计算结果相比定性地符合,定量地接近。长程有序化动力学的计算表明,有序化过程存在有孕育期,有c-曲线形式的转变曲线,s近似与exp{-αt²}正比,算得的平衡有序度与前人计算结果相近。长程序为零时的短程有序化动力学的计算表明,σ近似与exp{-βt}正比,此时有序化速率甚快,以致在最快的淬火速率下,样品中也必然存在有一定的短程序,所得的平衡短程序值也符合前人的计算结果。本文是作为继续计算短程有序化动力学的准备及基础。     

多孔氧化铝膜上自组织生长Sn纳米点阵列的研究

以多孔阳极氧化铝膜(porous anodic alumina, PAA)为基片，采用真空电子束蒸发的方法在多孔氧化铝膜上制备了高度有序度的Sn纳米点阵列.锡纳米点阵的XRD与块体锡的完全相同，扫描电镜(SEM)测试结果表明，所制备的金属Sn纳米点阵与阳极氧化铝膜的多孔阵列具有完全相同的有序结构，阵列中每个Sn纳米粒子的形状为球形的，其直径接近于PAA膜的孔直径.对Sn纳米点阵形成过程和形成机理进行了讨论.     

金属Ni原子对石墨烯生长结构的影响

基于Material Studio软件平台，利用分子动力学方法，对Ni原子与石墨烯层状结构相互作用和晶体结构变化过程进行模拟分析，得到如下结论：低浓度Ni原子会吸附在石墨烯表面层沿边缘生长，活性从中心向边缘逐渐降低，高浓度的Ni原子会溶解到内层石墨烯中.当石墨烯层数增加，附着在表层石墨烯的Ni原子生长排列范围扩大，且在石墨烯表面形成的点阵排列被破坏，附着在内层石墨烯的Ni原子比表层石墨烯Ni原子排列更散乱，同时石墨烯生长结构逐渐出现弯曲；随着层数增多和Ni原子浓度增加，石墨烯的拉伸强度也随之增加，石墨烯生长缺陷的偏转角度也随之增大.通过计算以上结构的径向分布函数(RDF),验证了石墨烯长程有序到短程有序的结构变化过程.     

科学技术简报和经验交流 倒易空间点阵用于多晶X射线衍射图象的诠释

如所周知,解释单晶X射线衍射图象时,倒易点阵是强有力的工具。由于多晶体的多晶性,有多少取向不同的晶体数,便有多少取向不同的倒易点阵数,这一复杂情况使人们不能获得象单晶情况那样良好的结果。因此,解释多晶X射线衍射图象时,倒易点阵方法未能获得广泛的应用。本文是这方面一个尝试,结果获得适用于立方晶系的新共线图。     

纳米微晶材料的结构和性质

纳米微晶材料是纳米量级晶粒所构成的多晶物质,其晶界区域中存在与长程有序晶态和短程有序非晶态结构不相同的“气体状”的结构。本文讨论了纳米微晶材料的制备方法、结构特点和奇异性质及其在材料科学中的应用。     

纳米微晶材料的结构和性质

纳米微晶材料是纳米量级晶粒所构成的多晶物质,其晶界区域中存在与长程有序晶态和短程有序非晶态结构不相同的“气体状”的结构。本文讨论了纳米微晶材料的制备方法、结构特点和奇异性质及其在材料科学中的应用。     

网格高速摄影成象特性的研究

本文叙述了网格高速摄影的成象特性,用二维脉冲序列函数进行了分析.把网格器件用于光学系统中,获得了图象点阵列照片.目标和阵列象是对应的.测量了底片上的图象点阵列的密度分布,指出了分辨率和网格节距的关系.网格高速摄影与普通高速摄影相比是降低了图象分辨率而提高了图象对比亮度.在光学系统中使用网格器件的作用是把一个具有连续轮廓的外界目标分割成网格点阵象,通过点阵象作相对移动,在同一张底片上就可以得到许多与目标相对应的图象,网格高速摄影就是利用这种点阵运动原理来实现的.     

半导体电子态理论简介(Ⅲ)

四、非晶态半导体 非晶态半导体属于无序系统的物质.无序系统包括固溶体、非晶体、准晶体等,它们都不存在晶体的平移对称性.固溶体的特点是结构有序、组分无序.非晶体的特点是短程有序,长程(结构)无序.准晶体的特点是短程有序,长程部分有序,有点群对称性但无平移对称性、无序的分布都是随机的.与有序系统相比,无序系统有一些新的特点,例如局域化. 关于局域化问题,常用紧束缚方法来讨论,哈密顿量其中　　是原子态　的能量,V是原子间的相互作用能量.对于晶体,　　是常数.如果晶体是三维简单立方,则它的能带(取　　=0)为态密度如图 11(a)所示,…     

Structure and local-equilibrium thermodynamics of the c(2x2) reconstruction of rutile TiO2 (100)

We resolve the structure of a c(2x2) reconstruction of the rutile TiO2 (100) surface using a combination of transmission electron diffraction, direct methods analysis, and density functional theory. The surface structure contains an ordered array of subsurface oxygen vacancies and is in local thermodynamic equilibrium with bulk TiO2, but not the with oxygen gas-phase environment. The transition into a bulklike (1x1) reconstruction offers insights into the time-dependent local thermodynamics of TiO2 surface reconstruction under global nonequilibrium conditions.     

Unveiling Talbot Effect under Fresnel Diffraction at a Fork-Shaped Grating

The near-field effect of diffraction image self-reproduction or self-imaging of a periodic grating illuminated by quasi-monochromatic wave is well-known as the Talbot effect. Introducing a dislocation to a periodic structure provides a fork-shaped modulation of the phase/amplitude, which produces discrete diffraction pattern in a far-field consisting of optical vortices. In this paper, Fresnel diffraction at amplitude fork-shaped grating is theoretically and experimentally studied. The coexistence of spatial ordering and local violation of translational symmetry of the structure manifests itself in a strict diffraction pattern consisting of optical vortices in the far-field, which is shown to be accompanied by formation of a spatially ordered intensity distribution in the near-field, reminiscent the Talbot carpets for periodic structures. These results demonstrate the first evidence of Talbot effect occurred under light diffraction at fork-shaped gratings, being promising for deep understanding of near-field singular optics phenomena.     

Intrinsic vacancy-induced nanoscale wire structure in heteroepitaxial Ga2Se3/Si(001)

A highly anisotropic growth morphology is found for heteroepitaxial gallium sesquiselenide (Ga2Se3) on the lattice matched substrate, arsenic-terminated Si(001). Scanning tunneling microscopy of Ga2Se3 films reveals nanoscale, wirelike structures covering the surface in parallel lines, less than 1 nm wide and up to 30 nm long. Core-level photoemission spectroscopy and diffraction reveals the local structure of buried Ga and Se atoms to reflect the bulk, defected zinc-blende structure of beta-Ga2Se3, which contains ordered 110 arrays of Ga vacancies. These ordered vacancy lines are proposed to be responsible for the observed growth anisotropy in heteroepitaxial Ga2Se3.     

Analysis of 3D structures of platinum nanoparticles by high energy X-ray diffraction and reverse Monte Carlo simulations

In this paper, the atomic-scale three-dimensional structure of Pt nanoparticles were determined using high energy X-ray diffraction techniques and reverse Monte Carlo simulations. It was found that dendrimer encapsulated Pt nanoparticles are more ordered than those stabilized by long-chain alkanes. Furthermore, Pt nanoparticles measured in ambient dry conditions exhibit higher local atomic order compared to Pt nanoparticles measured in aqueous solutions. The atomic-scale structural information presented here can offer a more comprehensive understanding of the structure–property relationship for Pt nanoparticles and can be further used in the design of nanoparticles with improved properties.     

Interaction of pulsed laser radiation with a polycarbonate-based composite

Abstract—The interaction of pulsed laser radiation with foil-clad polycarbonate containing diffraction gratings is studied. It is found that the laser beam reflected from diffraction gratings is selffocused in polycarbonate bulk and becomes a seed for changing the polycarbonate structure, which leads to the glass transition and growth of ordered structures with symmetry features, which gives rise to the light dispersion effect in composites. A probable formation mechanism of ordered structures in polycarbonate during irradiation is proposed.     

The structure of MgO-SiO2 glasses at elevated pressure

The magnesium silicate system is an important geophysical analogue and neutron diffraction data from glasses formed in this system may also provide an initial framework for understanding the structure-dependent properties of related liquids that are important during planetary formation. Neutron diffraction data collected in situ for a single composition (38 mol% SiO(2)) magnesium silicate glass sample shows local changes in structure as pressure is increased from ambient conditions to 8.6 GPa at ambient temperature. A method for obtaining the fully corrected, total structure factor, S(Q), has been developed that allows accurate structural characterization as this weakly scattering glass sample is compressed. The measured S(Q) data indicate changes in chemical ordering with pressure and the real-space transforms show an increase in Mg-O coordination number and a distortion of the local environment around magnesium ions. We have used reverse Monte Carlo methods to compare the high pressure and ambient pressure structures and also compare the high pressure form with a more silica-poor glass (Mg(2)SiO(4)) that represents the approach to a more dense, void-free and topologically ordered structure. The Mg-O coordination number increases with pressure and we also find that the degree of continuous connectivity of Si-O bonds increases via a collapse of interstices.     

Orbital correlations in the pseudocubic O and rhombohedral R phases of LaMnO3

The local and intermediate structure of stoichiometric LaMnO3 has been studied in the pseudocubic and rhombohedral phases at high temperatures (300-1150 K). Neutron powder diffraction data were collected and a combined Rietveld and high real space resolution atomic pair distribution function analysis was carried out. The nature of the Jahn-Teller (JT) transition around 750 K is confirmed to be orbital order to disorder. In the high-temperature orthorhombic (O) and rhombohedral (R) phases, the MnO6 octahedra are still fully distorted locally. More importantly, the intermediate structure suggests the presence of local ordered clusters of diameter approximately 16 A ( approximately 4 MnO6 octahedra) implying strong nearest-neighbor JT antiferrodistortive coupling. These clusters persist well above the JT transition temperature even into the high-temperature R phase.     

Supramolecular architecture of organic molecules: PTCDA and CuPc on a Cu(1 1 1) substrate

We report on the molecular self-assembly of a highly ordered mixed molecular structure of copper-phthalocyanine (CuPc) and 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) on Cu(1 1 1) starting from dense PTCDA domains. Low temperature scanning tunneling microscopy (LT-STM) data reveal the surface structure as the CuPc coverage is stepwise increased. A highly ordered mixed phase as well as a disordered mixed phase can be found, where the highly ordered mixed phase seems to be thermodynamically favored. Low energy electron diffraction (LEED) measurements aid the STM analysis. Tentative models for the formation of the highly ordered mixed phase are given.     

Structure determination of surface magic clusters

The structure of a type of surface magic cluster is determined by a combination of scanning tunneling microscopy, density-functional calculations, and dynamical low energy electron diffraction. The diffraction method is applicable because these clusters created through hierarchical self-organization of Ga deposited onto a Si(111)-7x7 surface have identical size and structure and form an ordered array with exact translational symmetry. The unprecedented detailed structure information provided by the diffraction measurement is consistent with direct microscopic imaging and theoretical calculations.     

EXAFS and diffraction

We discuss advantages and limitations of X-ray absorption fine structure (EXAFS), X-ray diffraction and anomalous X-ray diffraction as structural tools for ordered and (chemically and structurally) disordered solids.