Study Of Defects In Crystals Research Articles

General teleparallel metrical geometries

In the conventional formulation of general relativity, gravity is represented by the metric curvature of Riemannian geometry. There are also alternative formulations in flat affine geometries, wherein the gravitational dynamics is instead described by torsion and nonmetricity. These so-called general teleparallel geometries may also have applications in material physics, such as the study of crystal defects. In this work, we explore the general teleparallel geometry in the language of differential forms. We discuss the special cases of metric and symmetric teleparallelisms, clarify the relations between formulations with different gauge fixings and without gauge fixing, and develop a method of recasting Riemannian into teleparallel geometries. As illustrations of the method, exact solutions are presented for the generic quadratic theory in 2, 3 and 4 dimensions.

HRTEM study of crystal defects in gold nanoparticles

HRTEM study of crystal defects in gold nanoparticles

Developing image‐contrast theory and analysis methods in high‐resolution electron microscopy

AbstractIt is reviewed that the structural information obtained from high‐resolution electron microscope images can be greatly improved by utilizing a group of analysis methods developed based on an image‐contrast theory derived particularly to show the image‐intensity change with the sample thickness. It is introduced that the experimental images that do not represent the structures of examined samples can be transformed into the structure maps by deconvolution processing with the structure resolution enhanced up to the information limit of the microscope, or further, for perfect crystals, to the electron diffraction limit by subsequently combining the corrected electron‐diffraction data and utilizing the structure analysis methods developed in X‐ray crystallography, and it becomes possible to recognize atoms with different atomic weights by analyzing the image‐contrast change with the sample thickness, etc. The derivation of the image‐contrast theory, the principle and functions of the developed image‐analysis methods are demonstrated. Typical examples of applications to the ab initio crystal‐structure determination and studies of crystal defects at atomic level are given.

Three-dimensional interactive Molecular Dynamics program for the study of defect dynamics in crystals

The study of crystal defects and the complex processes underlying their formation and time evolution has motivated the development of the program ALINE for interactive molecular dynamics experiments. This program couples a molecular dynamics code to a Graphical User Interface and runs on a UNIX-X11 Window System platform with the MOTIF library, which is contained in many standard Linux releases. ALINE is written in C, thus giving the user the possibility to modify the source code, and, at the same time, provides an effective and user-friendly framework for numerical experiments, in which the main parameters can be interactively varied and the system visualized in various ways. We illustrate the main features of the program through some examples of detection and dynamical tracking of point-defects, linear defects, and planar defects, such as stacking faults in lattice-mismatched heterostructures. Program summary Title of program: ALINE Catalogue identifier:ADYJ_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/ADYJ_v1_0 Program obtainable from: CPC Program Library, Queen University of Belfast, N. Ireland Computer for which the program is designed and others on which it has been tested: Computers:DEC ALPHA 300, Intel i386 compatible computers, G4 Apple Computers Installations:Laboratory of Computational Engineering, Helsinki University of Technology, Helsinki, Finland Operating systems under which the program has been tested:True64 UNIX, Linux-i386, Mac OS X 10.3 and 10.4 Programming language used:Standard C and MOTIF libraries Memory required to execute with typical data:6 Mbytes but may be larger depending on the system size No. of lines in distributed program, including test data, etc.:16 901 No. of bytes in distributed program, including test data, etc.:449 559 Distribution format:tar.gz Nature of physical problem:Some phenomena involving defects take place inside three-dimensional crystals at times which can be hardly predicted. For this reason they are difficult to detect and track even within numerical experiments, especially when one is interested in studying their dynamical properties and time evolution. Furthermore, traditional simulation methods require the storage of a huge amount of data which in turn may imply a long work for their analysis. Method of solution:Simplifications of the simulation work described above strongly depend also on the computer performance. It has now become possible to realize some of such simplifications thanks to the real possibility of using interactive programs. The solution proposed here is based on the development of an interactive graphical simulation program both for avoiding large storage of data and the subsequent elaboration and analysis as well as for visualizing and tracking many phenomena inside three-dimensional samples. However, the full computational power of traditional simulation programs may not be available in general in programs with graphical user interfaces, due to their interactive nature. Nevertheless interactive programs can still be very useful for detecting processes difficult to visualize, restricting the range or making a fine tuning of the parameters, and tailoring the faster programs toward precise targets. Restrictions on the complexity of the problem:The restrictions on the applicability of the program are related to the computer resources available. The graphical interface and interactivity demand computational resources that depend on the particular numerical simulation to be performed. To preserve a balance between speed and resources, the choice of the number of atoms to be simulated is critical. With an average current computer, simulations of systems with more than 10 5 atoms may not be easily feasible on an interactive scheme. Another restriction is related to the fact that the program was originally designed to simulate systems in the solid phase, so that problems in the simulation may occur if some particular physical quantities are computed beyond the melting point. Typical running time:It depends on the machine architecture, system size, and user needs. Unusual features of the program:In the program, besides the window in which the system is represented in real space, an additional graphical window presenting the real time distribution histogram for different physical variables (such as kinetic or potential energy) is included. Such tool is very interesting for making demonstrative numerical experiments for teaching purposes as well as for research, e.g., for detecting and tracking crystal defects. The program includes: an initial condition builder, an interactive display of the simulation, a set of tools which allow the user to filter through different physical quantities the information—either displayed in real time or printed in the output files—and to perform an efficient search of the interesting regions of parameter space.

The Onset of Plasticity of NiAl Single Crystals: Changes in the Electrical Resistance

This note reports experiments in which in situ electrical resistance measurement was used to monitor the onset of plastic deformation in NiAl single crystals during tensile tests. The aim of this research was to probe the formation and evolution of dislocations in single crystals undergoing plastic deformation. The electrical resistance allows the probing of dislocations in crystals based on the scattering of the conduction electrons by dislocations. While the electrical resistivity has been used for a long time for the study of crystal defects, most past researches consisted of post mortem measurements of deformed specimens. Where the resistance was measured in situ, the specimens studied were polycrystalline. In this research the electrical resistance was measured in situ in the sense that changes in the specimens resistance caused deformation were recorded with load and displacement while the single crystal specimens were being deformed in tension at a constant strain rate. Another feature of this research is the use of NiAl single crystals in the special orientation [001]. In NiAl both the grown-in dislocations and those introduced by slip under usual test conditions have as the Burgers vector.

Hartree-Fock perturbed-cluster treatment of local defects in crystals III. The use of local symmetry

This paper is the third of a series to illustrate the characteristics of EMBED, a computer program for the ab-initio Hartree-Fock study of defects in crystals. We describe here the algorithms which allow the symmetry of the imperfect crystal to be identified, and exploited thoroughly. Much of the discussion concerns the generation of orthogonalized symmetry orbitals (OSO) in the cluster subspace: they are classified according to the different rows of the different irreducible representations (IRREP's) of the point group of the system. Using the OSO basis set inside the cluster, and the usual atomic orbital basis set in the surrounding crystal permits us to factorize completely the problem by the different IRREP's with important savings in computer time and storage requirements. This benefit is demonstrated by considering the example of a lithium impurity in bulk MgO, with lithium at the center of a cubic interstice or slightly off-center.

Hartree-Fock perturbed-cluster treatment of local defects in crystals II. The energy dependent coupling matrices

This paper is the second of a series to illustrate the characteristics of EMBED, a computer program for the ab-initio Hartree-Fock study of defects in crystals. The subjects are the energy-dependent coupling matrices M(e), which are a key quantity of the perturbed-cluster theory, since they are used for coupling the cluster pseudo-eigen-function to the host-crystal solution. A finite-temperature formulation is introduced for these matrices: they are defined as an integral over the energy axis (λ) of the projected density of states (PDOS), ϱ(λ), times a “thermal function”, [ f( λ) − f( e)]/( e − λ). Different approximations for the latter function are proposed, in order to allow the integral to be performed analytically, yet accurately enough. The influence of the computational parameters (“temperature”, degree of the polynomial expansion of the PDOS, position of the Fermi energy) are discussed with reference to the problem of interstitial atomic hydrogen in bulk lithium.

Study of crystal defects in natural diamond

To study the crystal defects in diamonds, a specimen of natural diamond crystals selected from northeast China was tested by means of Lang X-ray topography and double-crystal X-ray topography. As a result, several types of crystal defect, such as twin boundaries, dislocations, growth bands and substructures existing in the specimen were found. The morphologies of the twin boundaries have been analysed and a detailed discussion of the determination of its characteristic vectors is given. The distribution characteristics of the various crystal defects as well as their relation to the twin boundaries are presented.

Problems and prospects for anAB-InitioHartree-FockPerturbed-Cluster treatment of local defects in ionic solids

Abstract This paper presents a new theoretical technique for the study of defects in crystals. The methods is based on an ab-initio Hartree-Fock perturbed-cluster (PC) scheme1. A prerequisite for this technique is the availability of a model for the host, perfect crystal, that must be obtained with the same Hamiltonian and the same computational conditions (basis set, accuracy etc.). This information is provided by CRYSTAL, an ab-initio periodic Hartree-Fock code2.

TEM study of crystal defects of the silicon phase in AlSi eutectics

TEM study of crystal defects of the silicon phase in AlSi eutectics

In Pursuit of the Lattice Vacancy 1990 MRS Fall Meeting Von Hippel Award Address

The Von Hippel Award is especially significant to me for several reasons. The name Von Hippel is legendary in the halls of Building 13 at the Massachusetts Institute of Technology, where I have worked for the last 12 years. Professor Von Hippel was one of the first to appreciate the interdisciplinary nature of materials research and engineering, and his insulation laboratory at MIT provided an early prototype for the National Science Foundation-supported system of interdisciplinary materials research laboratories in existence today. These laboratories, both at MIT and earlier at Cornell, have helped me in my research in many ways over the years.Also I have been a recent councillor of the Materials Research Society and have been privileged to witness the phenomenal growth of MRS at first hand. The stature of the Von Hippel Award must be increasing at least in proportion to this growth. I, therefore, feel very honored and grateful indeed! I would also like to add that much of my work has been done in collaboration with valued colleagues and students. They deserve a substantial share of this award.Over the years, my research has been devoted mainly to the experimental study of crystal defects and their behavior. These defects can be classified according to their dimensionality as shown in the accompanying table, and they are involved in one way, or another, with a host of phenomena in crystalline materials. Since I address a rather diverse audience, it might be appropriate to discuss some of the history of lattice vacancy research and some of the details of my personal contribution to it.

Shot-noise simulations of HREM images of polymer crystals

High Resolution Electron Microscopy (HREM) is a very powerful technique for the study of crystal defects at the molecular level. Unfortunately polymer crystals are beam sensitive and are destroyed almost instantly under the typical HREM imaging conditions used for inorganic materials. Recent developments of low dose imaging at low magnification have nevertheless permitted the attainment of lattice images of very radiation sensitive polymers such as poly-4-methylpentene-1 and enabled molecular level studies of crystal defects in somewhat more resistant ones such as polyparaxylylene (PPX) [2].With low dose conditions the images obtained are very noisy. Noise arises from the support film, photographic emulsion granularity and in particular, the statistical distribution of electrons at the typical doses of only few electrons per unit resolution area. Figure 1 shows the shapes of electron distribution, according to the Poisson formula :

Study of crystal defects using ion channelling and channelling radiation

The channelling technique to study crystal defects is described and its applications to various kind of defects to study their atomistic nature have been reviewed. Special emphasis has been placed on the applications to extended defects like dislocations. Finally a related new technique being developed for the last few years, namely the channelling radiation technique has been discussed along with its applications to study the dislocations.

Problems and prospects in the ab initio treatment of pure and defective crystals

Accurate Hartree-Fock LCAO calculations for moderately complex crystalline systems are now feasible; a number of important applications may be envisaged in the areas of material science and technology. Some critical aspects of the corresponding computer schemes are discussed which are of fundamental importance in determining the cost of the computation. Data are provided concerning actual computations which are indicative of the kind of periodic systems that can (or cannot) be treated at present. The result of a perfect-crystal ab initio HF study can be used as an input for treating with the same approximation local-defect problems, by use of suitable embedding techniques. A scheme of this kind is presented, and its computational implications are discussed: due to the intrinsic complexity of this problem, it may be foreseen that the study of defects in crystals will be a typical application of supercomputers in the area of quantum chemistry.

CONVERGENT-BEAM ELECTRON DIFFRACTION STUDY OF DISLOCATIONS

Convergent-beam electron diffraction near a dislocation in silicon shows that some of the higher-order Laue zone lines and Kikuchi lines are split. The splitting of crystal-lographically equivalent reflections is different. These can be explained in terms of the theory of diffraction contrast of imperfect crystals. The splitting and unsplitting of the reflections correspond to the visibility and in visibility of the dislocation. Convergent-beam electron diffraction provides a powerful means for the study of crystal defects with high spatial resolution.

Preliminary studies of crystal defects in cadmium sulphide by high-resolution transmission electron microscopy

Abstract Structural details of a 60° dislocation, a stacking fault, a partial dislocation, a low-angle and high-angle grain boundary have been revealed at a 0.3 nm level in CdS.

Recollections of the early days of dislocation physics

Other branches of solid state physics had other leaders, and Mott himself had other interests, but in the study of crystal defects and the so-called ‘structure-sensitive’ properties of solids, Nevill Mott was the acknowledged leader. I first met him in 1937, when I wrote my degree examinations in physics, and Mott was the external examiner. Mott must have had reports on my promise as an experimental physicist from T. C. Kelley, for he advised me to spend a year reading mathematics and then to join him in Bristol as a theoretician. This I did, and he set me the problem of devising the theory of magnetic coercivity, the mechanism being the pinning of Bloch walls by lattice defects. I made some progress with this, and then the black day came. As Mott has already described, one of the characteristics of the Bristol school of that time was an almost total ignorance of the German language. As I slowly ploughed my way through a German conference report, I came across the theory of coercivity on much the same lines as mine, but far more fully developed.

High resolution electron microscope studies of defects in crystals

High resolution electron microscope studies of defects in crystals

The study of defects in crystals by positron annihilation

The study of defects in crystals through their interaction with thermalized positrons is usually based on the trapping model. The theoretical foundation of the trapping model, based on the theory of diffusion-limited reactions, is reviewed with particular emphasis on short-range interactions between positrons and trapping defects. For this case the analytical properties of the trapping model are developed in some detail, including the effects of the initial pair distribution on the trapping rate, the effects of detrapping, and the effects associated with more than one type of trap. The assumptions that are usually made in determinations of monovacancy formation enthalpies from positron annihilation experiments are critically discussed. The need for additional experiments capable of providing information on the temperature dependence of the various parameters involved is, stressed. As an illustration we treat in detail monovacancies with finite detrapping rate plus divacancies, both in thermal equilibrium. In the final section a few remarks are added on cases with long-range interactions between positrons and traps, such as electrically charged point-defects in insulators or semiconductors, and dislocations.

X ray diffraction studies of defects in crystals

The study of perfect crystals and of crystals with a variety of defects is of great importance both to industry and fundamental research. A review is made of those X ray techniques which have proved to be most useful for investigating defects in single crystals