Topological classification of defects in crystals

Here we review the topological approach to the classification of defects in crystals, which are deviations from crystallinity. First, we illustrate this approach using the simpler case of dislocations. We show how the fact that dislocations are completely characterized by their Burgers vectors only, is related via topological considerations to the fact that a perfect crystal is exactly reproduced by translations through a lattice vector. Later, we present the developments of the French school and that of Rogula, comparing and contrasting them. These are attempts to generalize the above idea to include consideration of other transformations that leave a perfect crystal invariant. Finally, we discuss briefly some of the problems that still remain with this approach.     

Functionalisation of cholesteric liquid crystals by direct laser writing

Cholesteric liquid crystals selectively reflect circularly polarised light with the same handedness as the helix. Because of their sensitivity to external stimuli, such as heat and electrical fields, various applications utilising their tunability have been proposed. Tuning is usually performed in the bulk, meaning that cholesteric liquid crystals usually possess a single pitch throughout the medium. However, when the helical structure is locally modulated, different optical properties arise, such as tunable photonic defect modes and multiple reflection bands. Here we show a technique to locally modulate the helical structure of cholesteric liquid crystals on a submicron scale, based on two-photon excitation direct laser lithography. Two examples of cholesteric liquid crystal structures with modulated helical structures will be presented.     

The investigation of the kinking process with help of cholesteric liquid crystals

Process of kinking in CsI crystals has been studied with help of CLC (cholesteric liquid crystals) sensitive to deformation and temperature. It is shown that kinking formation is accompanied by local heat in the kink band. The velocity of deformation is the more the larger the quantity of heat in the kink band.     

Investigations of surface of glasses by nematic and cholesteric liquid crystals

The relatively strong interaction between the surface of the glass and the nematic liquid crystals is demonstrated. The same texture of liquid crystals is observed after every renewed cycle of warming and cooling. Defects, which are invisible without an electrical field, become visible by the dynamic scattering effect. The thermografic results, which are received by cholesteric liquid crystals, are able to demonstrate only strong defects.     

Chiral SmC* liquid crystals obtained by mixing of SmC and cholesteric phase

The binary mixtures of the substance providing cholesteric phase exhibit a chiral SmC* phase for certain concentration and temperature range. This phase was verified to be ferroelectric. The temperature dependences of the spontaneous polarization and the coercive field were determined for three concentrations. The pitch of the helical structure is approximately indirectly proportional to the molar concentration of the cholesteric substance and varies from 3 μm to about 25 μm in the concentration range from 20 mol% to 5 mol% of the cholesteric substance. For the lower concentrations the sample is unwound in planar samples due to the surface anchoring. Two possible unwound planar configurations can be switched by electric field.     

Effect of various external factors and pretransitional phenomena on structural transformations in cholesteric liquid crystals

The publications on low-molecular thermotropic liquid crystals with cholesteric structures have been reviewed. The effect of an applied electric field on cholesteric structures is studied. Bistability of the cholesteric-nematic transition, electric field-induced color textures of cholesteric mixtures, and electro-optics of amorphous cholesteric structures are considered as well as pretransitional (blue and TGB) phases, discotic cholesterics, PDLC films, flexoelectric electro-optics, and photostimulated switching in cholesterics.     

Diffuse scattering from crystals with point defects

The analytical expressions for calculating the intensities of X-ray diffuse scattering from a crystal of finite dimensions and monatomic substitutional, interstitial, or vacancy-type point defects have been derived. The method for the determination of the three-dimensional structure by experimental diffuse-scattering data from crystals with point defects having various concentrations is discussed and corresponding numerical algorithms are suggested.     

Topological approach to investigation of acoustic axes in crystals

The concept of topological degree of a map is generalized to the case of discontinuous maps. The numerical value of such a degree may be a rational number. The representations developed are used for topological interpretation of the characteristics of special directions of propagation of acoustic waves in crystals (specifically, acoustic axes). The Euler theorem is generalized to the case of singularities with rational indices, and this result is applied to the set of acoustic axes in crystals.     

Reversible LED controlled optical activity of a cholesteric liquid crystal layer

Abstract

A thin layer of the photosensitive cholesteric liquid crystal possessing a high value of the optical activity is studied. Reversible change in the optical activity controlled by two light emission diodes (LEDs) with different emission wavelengths is applied for information recording. The behavior of this structure under exposure to UV and blue light is investigated. A smooth decrease in optical activity caused by the UV light resulting in a change in the color of the structure between crossed polarizers is obtained. The reverse process occurs under exposure to the blue light. On the basis of the obtained results, two methods of optical information recording in such a layer have been demonstrated.     

Diffusion of intrinsic defects in dielectric and semiconductor crystals with impurities

This paper reports on the results of detailed theoretical investigations into the diffusion of intrinsic defects in impurity crystals doped with mixed-valence ions. The special case of diffusion stimulated by variations in the redox properties of the atmosphere at the crystal boundary during high-temperature annealing is analyzed. The major consideration is given to the following fundamental problems: (i) the dynamics of valence transitions and the structure of the chemical reaction zone, (ii) the possibility of determining the type of chemical reaction at the crystal-atmosphere interface and the type of diffusing defects, (iii) the effect of dilatation mechanical stresses arising in the reaction zone on the reaction-zone structure and on the dynamics of diffusion processes, and (iv) the determination of the diffusion parameters of intrinsic defects and the constants of their interaction with impurity centers.     

Primary converters for optical sensors of physical values based on polymer dispersed cholesteric liquid crystal

The investigations of system based on cholesteric liquid crystal (CLC) sensors of physical quantities with an information transfer optical channel are carried out in the paper. The results of experimental investigations of spectral characteristics of polymer dispersed cholesteric liquid crystals (PDCLC) are represented. The application such material as primary transducer of a temperature sensor is proposed. The structure of an optical fiber temperature sensor with successive allocations of primary transducers and shifted areas of selective reflection band has been designed.     

Growth of NiAl shape memory alloy single crystals

As a potential candidate of high temperature Shape Memory Alloy (SMA), inter metallic compound NiAl has been studied for more than 30 years. In order to understand better the mechanism of the Shape Memory Effect(SME) in β -NiAl phase, its bulk single crystal have been prepared with an improved magnetic levitation cold crucible Czoehralski(CZ) technique in our lab. The crystal growth procedure and the β -NiAl phase stability are discussed in this paper.     

Growth of dysprosium garnet single crystals with spiral shape using Czochralski technique

The growth conditions of dysprosium garnets single crystals, to obtain a long and good spiral shape crystal, have been investigated using the conventional Czochralski technique along the [111] pulling direction. The good spiral shape of Dy₃Ga₅O₁₂ single crystal with 40 mm in length is grown with its spiral pitch and spiral diameter of 20 mm and 15 mm, respectively.     

The edge-defined film-fed growth of controlled shape crystals

The growth of shaped crystals is examined in the various meniscus-controlled growth processes such as Czochralski, floating zone, Stepanov, and edge-defined film-fed growth (EFG). The basic physical processes which shape the crystal are the same in these techniques; they involve the interaction of the three interfaces at the crystal–liquid–vapor junction. Specifically, for a crystal of constant dimensions, the angle ø between the meniscus and the growth axis must be ø_o (a constant; for silicon, ø_o 11°). The degree of crystal shape control and the range of cross-sectional shapes which can be grown in a stable manner by the different techniques are shown to depend on the details of the meniscus shape and of the heat flow in the systems. The use of a die shaper which constrains the meniscus distinguishes the EFG and Stepnov processes from the other methods. The use of a wetted die in EFG versus a non-wetted die in Stepnov growth is shown to have an additional effect on the ability to control the crystal shape and dimensions. The role of the die shaper is examined in detail from the points of view of die material selection (e.g., wettability and chemical compatibility) and die design. The advantages and disadvantages of using wetted and non-wetted dies in the shaping process are discussed from both the theoretical and practical points of view. Specific numerical examples in the paper deal with the growth of silicon ribbons.