A Hierarchy of Defects in Biaxial Nematics

We study the topological properties of the manifolds which describe uniaxial and biaxial nematics in order to compare and classify all types of bulk and surface defects that can arise in a nematic liquid crystal. We explain how topological charges can be assigned both to isotropic defects in uniaxial nematics and to uniaxial defects in biaxial nematics; we further see how it is possible to distinguish between isolated surface defects and surface defects which extend in the bulk, and between surface defects which can relax in the bulk and defects forced to stay on the surface. Received: 28 November 1994 / Accepted: 30 September 1996     

The differential geometry of elementary point and line defects in Bravais crystals

Since line defects (dislocations) and point defects (vacancies, self-interstitials, point stacking faults) in Bravais crystals can mutually convert, only theories which comprise these two sorts of defects can be closed in the sense of general field theory. Since the pioneering work of Kondo and of Bilby, Bullough, and Smith it is clear that differential geometry is the appropriate mathematical tool to formulate a field theory of defects in ordered structures. This is done here on the example of the Bravais crystal, where the above-mentioned defects are the only elementary point and line defects. It is shown that point defects can be described by a step-counting procedure which makes it possible to include also point stacking faults as elementary point defects. The results comprise two equations with the appropriate interpretation of the mathematical symbols. The point defects are step-counting defects and are essentially described by a metric tensorg, which supplements the torsion responsible for the dislocations. The proposed theory is meant to form a framework for defect phenomena, in a similar way that Maxwell's theory is a framework for the electromagnetic world.     

一维光子晶体中多缺陷耦合导致的杂质带

利用传输矩阵方法研究了包含多个周期性分布的缺陷的一维光子晶体的透射谱.以33个周期的1/4波堆存在5个缺陷的光子晶体为例作了数值计算.结果表明,在周期性光子晶体中加入多个缺陷时,将在截止带中产生多个缺陷模.这些缺陷模构成的杂质能带依赖于缺陷在晶体中的分布.当缺陷密集时,缺陷模耦合较强,杂质带中透射峰相距较远,它们对应禁带中许多分立定域的杂质能级;当缺陷稀疏时,缺陷模耦合较弱,透射峰相距较近,分立的杂质能级趋于简并,由此形成一个很窄的通带.     

Stone-Wales defects can cause a metal-semiconductor transition in carbon nanotubes depending on their orientation

It has been shown that the two different orientations of Stone-Wales (SW) defects, i.e. longitudinal and circumferential SW defects, on carbon nanotubes (CNTs) result in two different electronic structures. Based on density functional theory we have shown that the longitudinal SW defects do not open a bandgap near the Fermi energy, while a relatively small bandgap emerges in tubes with circumferential defects. We argue that the bandgap opening in the presence of circumferential SW defects is a consequence of long-range symmetry breaking which can spread all the way along the tube. Specifically, the distribution of contracted and stretched bond lengths due to the presence of defects, and hopping energies for low-energy electrons, i.e. the 2p(z) electrons, show two different patterns for the two types of defects. Interplay between the geometric features and the electronic properties of the tubes have also been studied for different defect concentrations. Considering π-orbital charge density, it has also been shown that the deviations of bond lengths from their relaxed length result in different doping for two defect orientations around the defects-electron-rich for a circumferential defect and hole-rich for a longitudinal one. We have also shown that, in the tubes having both types of defects, circumferential defects would dominate and impose their electronic properties.     

精密表面疵病检测美标数字化评价实现算法

采用美国军用标准MIL-PRF-13830B（简称美标）对光学元件表面疵病进行数字化评价,其难点主要在于美标中表面疵病密集度的判定。针对该难点,提出了一种表面疵病密集度判定算法。该算法采用权重域叠加的方法,通过为表面疵病赋予相应的权重域来确定疵病密集圆域,并将权重域间的叠加转换成了矩阵间位置关系的判断和计算,有效的利用了矩阵运算的便利性。实验中通过对已知表面疵病信息的石英标准板进行判定,其得到的圆域即为疪病密集圆域,与预期结果一致,验证了该算法的正确性。目前该算法已经运用于光学车间精密光学元件表面疵病的美标数字化评价。     

Melting of a dust crystal with defects

The effect of defects on the melting of a two-layer dust Wigner crystal in the electrode layer of a radio-frequency gas discharge is investigated by Langevin molecular dynamics. Two types of defects have been included in the consideration: (a) point defects and dislocations and (b) particles levitating above and below a two-layer crystal (so-called strong defects). It is shown that local melting of a two-layer crystal found experimentally is explained by the occurrence of strong defects located above the crystal.     

Defect properties of CuCrO₂:A density functional theory calculation

Using the first-principles methods,we study the formation energetics properties of intrinsic defects,and the charge doping properties of extrinsic defects in transparent conducting oxides CuCrO2.Intrinsic defects,some typical acceptortype,and donor-type extrinsic defects in their relevant charge state are considered.By systematically calculating the formation energies and transition energy,the results of calculation show that,V Cu,O i,and O Cu are the relevant intrinsic defects in CuCrO2 ;among these intrinsic defects,V Cu is the most efficient acceptor in CuCrO2.It is found that all the donor-type extrinsic defects have difficulty in inducing n-conductivity in CuCrO2 because of their deep transition energy level.For all the acceptor-type extrinsic defects,substituting Mg for Cr is the most prominent doping acceptor with relative shallow transition energy levels in CuCrO2.Our calculation results are expected to be a guide for preparing promising n-type and p-type materials in CuCrO2.     

InP中深能级缺陷的产生与抑制现象

研究了原生和高温退火处理后非掺n型和半绝缘InP单晶材料中产生的缺陷.在磷气氛下退火后，n型和半绝缘InP单晶中均明显产生相当数量的深能级缺陷，而在磷化铁气氛下退火后，InP的缺陷数量明显减少.在退火过程中缺陷的产生与磷和铁的内扩散有直接关系.向内扩散的磷原子和铁原子占据晶格中铟位后，分别产生反位缺陷和铁深受主.实验结果表明，铁通过扩散充分占据了铟位，抑制了铟空位、磷反位等缺陷的形成，而磷气氛下退火后产生的缺陷有铟空位、磷反位等.对InP中的缺陷属性进行了分析. 关键词： 磷化铟 退火 缺陷     

Molecular dynamics simulation of displacement damage in 6H-SiC

ABSTRACT

As a new generation of low-loss components, 6H-SiC is widely used in optoelectronic devices, electronic devices and other fields, especially in high temperature and strong radiation environment. Radiation will lead to a great large of defects generated in the material, then forming permanent displacement damage which will result in performance down or failure. In this paper, the molecular dynamics method was used to study the spatial distribution of defects and the effect of temperature and PKA energy on the various properties of the point defects. The main contributions were the evolution of defects in 6H-SiC crystal, the influence of PKA energy and temperature on the defect distribution and the number of point defect. In this paper, the spatial distribution of point defects in 6H-SiC crystal after PKA incidence was obtained, the recombination rate under four kinds of energy PKA was calculated, and the effect of temperature and incident PKA energy on the number of defects, the rate of the vacancy cluster and the rate of vacancy defects at steady state were investigated. The results show that the number of defects at steady-state increases linearly with the increase of incident PKA energy, the effect of temperature and energy on defects producing and the rate of clusters are insignificant.     

Single and paired point defects in a 2D Wigner crystal

Using the path-integral Monte Carlo method, we calculate the energy to form single and pair vacancies and interstitials in a two-dimensional Wigner crystal of electrons. We confirm that the lowest energy point defects of a 2D electron Wigner crystal are interstitials, with a creation energy roughly 2/3 that of a vacancy. The formation energy of the defects goes to zero at melting, suggesting that point defects may be the melting mechanism and that the melting could be a continuous transition. In addition, we find that the interaction between defects is strongly attractive, so that most defects will exist as bound pairs.     

A real-space study of random extended defects in solids: Application to disordered Stone–Wales defects in graphene

We propose here a first-principles, parameter free, real space method for the study of disordered extended defects in solids. We shall illustrate the power of the technique with an application to graphene sheets with randomly placed Stone–Wales defects and shall examine the signature of such random defects on the density of states as a function of their concentration. The technique is general enough to be applied to a whole class of systems with lattice translational symmetry broken not only locally but by extended defects and defect clusters. The real space approach will allow us to distinguish signatures of specific defects and defect clusters.     

Defect solitons supported by parity-time symmetric defects in superlattices

The existence and stability of defect solitons supported by parity-time (PT) symmetric defects in superlattices are investigated. In the semi-infinite gap, in-phase solitons are found to exist stably for positive defects, zero defects, and negative defects. In the first gap, out-of-phase solitons are stable for positive defects or zero defects, whereas in-phase solitons are stable for negative defects. For both the in-phase and out-of-phase solitons with the positive defect and in-phase solitons with negative defect in the first gap, there exists a cutoff point of the propagation constant below which the defect solitons vanish. The value of the cutoff point depends on the depth of defect and the imaginary parts of the PT symmetric defect potentials. The influence of the imaginary part of the PT symmetric defect potentials on soliton stability is revealed.     

Infrared thermography and its application in the NDT of sandwich structures

Thermographic nondestructive testing (NDT) based on the thermal resistance effect of defects is developed for the inspection of delaminated and sandwiched defects embedded in composite structures. The resolution is examined for artificial delaminated defects in carbon-fiber honeycomb structures using conventional infrared radiation heating. The experimental results have demonstrated that radiation heating is effective for revealing defects in the composite structures.An experimental and computational hybrid system is developed for detecting defects in various composite structures. The system consists of an infrared thermal video system which measures the surface temperature distribution of the structure, a computer with a PIP-1024B image board which performs image processing of thermograms, and a HP ink jet XL printer. It is found that this system is readily applicable to the detection of defects located at the interface of the core and skin in honeycomb structures and delaminations in composite materials.     

Specific features of structural defects in twisted nematic liquid crystals under conditions of electrohydrodynamic instability

The influence of inhomogeneous boundary conditions (director orientation) on the specific features of the formation and evolution of structural defects in 90·-twisted nematic liquid crystals (twisted structures) is investigated in the regime of electrohydrodynamic instability. It is found that, unlike the domain structure of nematic liquid crystals with a planar orientation, in which defects with topological indices of ±1 are formed under conditions of electrohydrodynamic instability, the domain structure of twisted nematic liquid crystals contains both the above defects and defects with a topological index of 0. It is shown that structural defects with a topological index of 0 are stable and that the existence of these defects is associated with the axial velocity u_a of nematic liquid-crystal flow in the domains.     

Distribution of Topological Defects on Axisymmetric Surface

We propose a general method of determining the distribution of topological defects on axisymmetric surface,and study the distribution of topological defects on biconcave-discoid surface, which is the geometric configuration of red blood cell. There are three most possible cases of the distribution of the topological defects on the biconcave surface:four defects charged with 1/2, two defects charged with 1, or one defect charged with 2. For the four defect charged with 1/2, they sit at the vertices of a square imbedded in the equator of biconcave surface.     

Electronic transport of bilayer graphene with asymmetry line defects

In this paper,we study the quantum properties of a bilayer graphene with(asymmetry) line defects.The localized states are found around the line defects.Thus,the line defects on one certain layer of the bilayer graphene can lead to an electric transport channel.By adding a bias potential along the direction of the line defects,we calculate the electric conductivity of bilayer graphene with line defects using the Landauer-Biittiker theory,and show that the channel affects the electric conductivity remarkably by comparing the results with those in a perfect bilayer graphene.This one-dimensional line electric channel has the potential to be applied in nanotechnology engineering.     

GaN中的缺陷与杂质

GaN是一种新型的宽禁带半导体兰色发光与激光材料。GaN中的缺陷和杂质对于材料的电学输运特性和发光性能有着至关重要的影响。本文综述了近年来对于GaN中缺陷和杂质的理论与实验研究。包括对天然缺陷与非特意性掺杂的研究,以及对离子注入所产生的缺陷与深能级的研究。     

Transport by Molecular Motors in the Presence of Static Defects

The transport by molecular motors along cytoskeletal filaments is studied theoretically in the presence of static defects. The movements of single motors are described as biased random walks along the filament as well as binding to and unbinding from the filament. Three basic types of defects are distinguished, which differ from normal filament sites only in one of the motors’ transition probabilities. Both stepping defects with a reduced probability for forward steps and unbinding defects with an increased probability for motor unbinding strongly reduce the velocities and the run lengths of the motors with increasing defect density. For transport by single motors, binding defects with a reduced probability for motor binding have a relatively small effect on the transport properties. For cargo transport by motors teams, binding defects also change the effective unbinding rate of the cargo particles and are expected to have a stronger effect.     

Probing defect species on real surfaces from the analysis of the spectral profile of admolecules

The analysis of changes in the vibrational spectrum of infrared active molecules adsorbed on a ionic surface containing point or extended defects can be an efficient method to determine the nature and density of surface defects. We study the infrared response of ammonia molecules deposited on a ionic surface of MgO containing charge vacancies and dipolar defects in various concentrations and distributions and show significant changes assigned to the defects signature. A Monte Carlo approach is used to randomly deposit the probe molecules on the surface displaying random or regularly arranged defects at low temperature.