Disclination Structures Research Articles

Topological photonic encoder based on the disclination states

Topological disclination states are highly localized and stable by means of introducing disclination, which provide a robust platform for realizing optical information transition. A photonic encoder, as a kind of optical information transition element, can record, transmit, and protect optical information. However, there is no effective methods to realize topological photonic encoders. In this work, we propose a method to realize topological photonic encoder through topological disclination states. After the introduction of a disclination in the honeycomb structure, four types of disclination states can be generated. To demonstrate the device to carry more information, nine disclination structures with different cylindrical radii are combined, and the disclination states can be denoted by digital signals 1–4 to prepare a topological photonic encoder. In addition, to improve the security of information transition, we build an encryption algorithm based on Morse code. This work provides a new idea for the construction of encoding devices and promotes the practical application of the topological disclination states.

Nondislocational Mechanisms of Strain Localization in Nickel Nanocrystals During Deformation by High-Pressure Torsion in Bridgman Anvils

Using the methods of transmission electron microscopy, new aspects of formation of reorientation nanobands with partial involvement of nondislocational deformation mechanisms are investigated in nickel nanocrystals under the conditions of its severe plastic deformation by high-pressure torsion in Bridgman anvils: local reversible (FCC→BCC→FCC) transformations of the martensitic type and quasi-viscous mass transfer by the flows of nonequilibrium point defects in the fields of high local pressure gradients. The features of disclinational structure and elastically stressed state at the nanoband propagation front are studied. A theoretical analysis of the rate of plastic deformation via the mechanisms of quasi-viscous mass transfer by the flows of nonequilibrium point defects is performed. A possibility of simultaneous realization of the martensitic and quasi-viscous deformation modes at the front of nanoband propagation is established. An analysis of the conditions and mechanisms where these modes are involved is performed as a function of the type of point defects (vacancies and interstitial atoms), deformation temperature, and peculiarities of disclination structure and elastically stressed state.

REVISITING THE NATURE OF SITES OF MARTENSITE NUCLEATION DURING STEEL HARDENING

Presence of microvolumes most prepared for the martensite emergence in austenite is discussed. Aming many works dealing with martensitic transformations, rare works are devoted to the location of martensite origin. This aspect of transformation is important, since it allows us to obtain new knowledge about scenarios for γ → α transformation development during quenching of steel. The martensite embryos are submicron austenite volumes that are most prepared for phase transition and are characterized by increased energy. Experimental results were obtained by the methods of high-temperature metallography. Steel structure observed as a result of vacuum etching was studied, as well as the surface relief caused by shear during the martensitic transformation. The resulting structural patterns made it possible to observe most of the possible places for martensite emergence: nonmetallic inclusions, twins, high-angle and small-angle grain boundaries, previously formed martensite crystals, dislocations and elements of the disclination structure. It is shown that a high dislocation density is observed in the twin area, which facilitates nucleation of martensite as a result of disappearance of part of elastic energy of the dislocation when atoms inside the embryo are rearranged. When nucleation occurs on the grain boundaries, energy is released, which is used to construct a new interphase boundary and to compensate emerging elastic energy. The relative energy of the boundaries of different types was estimated by the method of multi-beam interferometry. The depth of the grooves that were formed on the surface by thermal etching was measured. Elements of disclination structure resulting from inhomogeneous deformation were observed, which are also sites of germinal centers formation. It is noted that nanoareas with ferromagnetic order, which are present in paramagnetic austenite, may not be observed with the help of the technique used in this work. However, magnetism plays a decisive role in realization of one or another scenario of the development of phase transformation in steels. Obtaining data on the interaction of ferromagnetic areas in austenite with each other, with crystal lattice defects, the magnetic field, and data on their lifetime, number and size is an important task for future research.

Recent progresses in lyotropic chromonic liquid crystal research: elasticity, viscosity, defect structures, and living liquid crystals

ABSTRACTLyotropic chromonic liquid crystals (LCLCs) are formed by linear stacks of disc-shaped molecular in water. Combined by weak, non-covalent forces, these aggregates are reversible, flexible and polydisperse. The self-assembly nature of the basic building units gives nematic LCLCs interesting physical properties, such as very small twist constant as compared with splay and bend constants, very large splay and twist viscosities, strong temperature dependence of , , and , diverse responses to different ionic additives, and large and azimuthally asymmetric disclination cores. We discuss our experimental studies on the viscoelastic properties and the fine structure of disclinations of LCLCs and attribute their unusual properties to the fact that LCLC aggregates are not fixed in size, but vary in response to changes of temperature, concentration and ionic content in the system. We further use these properties to explain intriguing phenomena in the ‘living liquid crystals’ composed of chromonic liquid crystal and motile bacteria Bacillus Subtilis.

Composite Dislocations in Smectic Liquid Crystals.

Smectic liquid crystals are characterized by layers that have a preferred uniform spacing and vanishing curvature in their ground state. Dislocations in smectics play an important role in phase nucleation, layer reorientation, and dynamics. Typically modeled as possessing one line singularity, the layer structure of a dislocation leads to a diverging compression strain as one approaches the defect center, suggesting a large, elastically determined melted core. However, it has been observed that for large charge dislocations, the defect breaks up into two disclinations [C. E. Williams, Philos. Mag. 32, 313 (1975)PHMAA40031-808610.1080/14786437508219956]. Here we investigate the topology of the composite core. Because the smectic cannot twist, transformations between different disclination geometries are highly constrained. We demonstrate the geometric route between them and show that despite enjoying precisely the topological rules of the three-dimensional nematic, the additional structure of line disclinations in three-dimensional smectics localizes transitions to higher-order point singularities.

Vortex and disclination structures in a nematic-superconductor state

The nematic-superconductor state is an example of a quantum liquid crystal that breaks gauge as well as rotation invariance. It was conjectured to exist in the pseudogap regime of the cuprates high $T_c$ superconductors. The nematic-superconductor state is characterized by two complex order parameters: one of them is related with superconductivity and the other one describes a nematic order. It supports two main classes of topological defects: half-vortices and disclinations. In this paper we present a Ginzburg-Landau approach to study the structure of these topological defects. Due to a geometrical coupling between the superconductor and the nematic order parameters, we show that vortices are strongly coupled with disclinations. We have found a restoring force between vortices and disclinations that produces harmonic excitations whose natural frequency depends on the geometrical coupling constant and the superconductor condensation energy. Moreover, in a regime with high density of defects, we have found a structural phase transition between vortex-disclination lattices with different symmetries.

Influence of pressure on dislocation, disclination, and generalized-disclination structures of a {310}/[001] tilt grain boundary in MgO

Abstract

Large Colloids in Cholesteric Liquid Crystals

We describe a coarse-grained Landau-de Gennes model of liquid crystals (LCs) including hydrodynamics based on the Beris-Edwards equations. The model is employed to study the impact of large colloids on the long range LC defect structure in the cholesteric LC blue phases. 'Large' here means that the particle size is comparable to the cholesteric pitch, the length scale on which the LC order undergoes a helical twist. We investigate the case of a single particle, with either normal or degenerate planar anchoring, placed initially in an equilibrium blue phase LC. It is found that in some cases, well defined steady disclination structure emerges at the particle surface, while in other cases no clear steady state is reached inthesimulations,anddisclinationreorganisationappearstoproliferatethroughthebulkLC. These systems are of potential interest in the context of using LCs to template self-assembly of colloid structure, e.g., for opto-electronic devices. Computationally, we demonstrate a parallel approach using mixed message-passing and threaded model on graphical processing units allows effective and efficient progress for this problem.

Creation and manipulation of topological states in chiral nematic microspheres.

Topology is a universal concept that is encountered in daily life and is known to determine many static and dynamical properties of matter. Taming and controlling the topology of materials therefore constitutes a contemporary interdisciplinary challenge. Building on the controllable spatial properties of soft matter appears as a relevant strategy to address the challenge, in particular, because it may lead to paradigmatic model systems that allow checking theories experimentally. Here we report experimentally on a wealth of complex free-standing metastable topological architectures at the micron scale, in frustrated chiral nematic droplets. These results support recent works predicting the formation of free-standing knotted and linked disclination structures in confined chiral nematic fluids. We also demonstrate that various kinds of external fields (thermal, electrical and optical) can be used to achieve topological remote control. All this may foster the development of new devices based on topologically structured soft media.

Phase-resolved structure of a disclination in a liquid-crystal θ-cell

An experimental testing of a disclination structure in a liquid crystal was performed. The origin of a disclination is a conflict between opposite-sign twist in a planar 90° twist-cell or a θ-cell. A theoretical analysis has shown a topological defect with the charge 1/2 (π variation in the azimuth angle of a twist along a close loop around the disclination). In the experiment we have tested a disclination zone by plane-wave linearly polarized light. The presence of the Mauguin lines was revealed in a numerical modeling and direct experiment. To resolve the structure around the disclination we used interferometric technique. The theoretical predictions were confirmed by the detection of the phase π-step occurring between the Mauguin lines.

Generation of harmonics and supercontinuum in nematic liquid crystals

Nonlinear optical properties of nematic liquid crystals (NLC) have been investigated. A technique for efficient laser frequency conversion in a microscopic NLC volume deposited on an optical fibre end face is experimentally demonstrated. An efficient design of a compact NLC-based IR frequency converter with a fibre input and achromatic collimator is proposed and implemented. Simultaneous generation of the second and third harmonics is obtained for the first time under pumping NLC by a 1.56-mm femtosecond fibre laser. The second-harmonic generation efficiency is measured to be about 1 %, while the efficiency of third-harmonic generation is several tenths of percent. A strong polarisation dependence of the third-harmonic generation efficiency is revealed. When pumping NLC by a cw laser, generation of spectral supercontinua (covering the visible and near-IR spectral ranges) is observed. The nonlinear effects revealed can be due to the light-induced change in the orientational order in liquid crystals, which breaks the initial symmetry and leads to formation of disclination structures. The NLC optical nonlinearity is believed to be of mixed orientationalelectronicnature as a whole.

Stability and rewiring of nematic braids in chiral nematic colloids

Disclination lines in chiral nematic liquid crystals exhibit larger geometrical diversity than their counterparts in ordinary nematics and form rich entangled structures in systems with dispersed colloidal inclusions. Numerous metastable and stable states separated by low energy barriers allow for simple rewiring of the braids. Introducing a new visualization scheme where variations in splay, bend and twist deformations are followed separately allows us to display the effect of chirality on the director profile of the disclinations, and to uncover the mechanism of their rewiring. We demonstrate the richness of structures by performing a survey of all possible disclination structures that entangle a pair of homeotropic silica microspheres in a π-twisted cholesteric cell using numerical free-energy minimization and laser tweezers assisted microscopy. We analyze their stability against spontaneous rewiring and trace the stability differences to the local effects of chiral environment. We compare the predictions to the experimental results and discuss the differences between both approaches.

Theoretical and experimental study of the nanoparticle-driven blue phase stabilisation

We have studied theoretically and experimentally the effects of various types of nanoparticles (NPs) on the temperature stability range [Formula: see text] T (BP) of liquid-crystalline (LC) blue phases. Using a mesoscopic Landau-de Gennes type approach we obtain that the defect core replacement (DCR) mechanism yields in the diluted regime [Formula: see text] T (BP)(x) [Formula: see text] 1/(1 - xb) , where x stands for the concentration of NPs and b is a constant. Our calculations suggest that the DCR mechanism is efficient if a local NP environment resembles the core structure of disclinations, which represent the characteristic property of BP structures. These predictions are in line with high-resolution ac calorimetry and optical polarising microscopy experiments using the CE8 LC and CdSe or aerosil NPs. In mixtures with CdSe NPs of 3.5nm diameter and hydrophobic coating the BPIII stability range has been extended up to 20K. On the contrary, the effect of aerosil silica nanoparticles of 7.0nm diameter and hydrophilic coating is very weak.

Topological aspect of disclinations in two-dimensional crystals

By using topological current theory, this paper studies the inner topological structure of disclinations during the melting of two-dimensional systems. From two-dimensional elasticity theory, it finds that there are topological currents for topological defects in homogeneous equation. The evolution of disclinations is studied, and the branch conditions for generating, annihilating, crossing, splitting and merging of disclinations are given.

Structural transition of nematic liquid crystal in cylindrical capillary as a result of the annihilation of two point defects

We study the annihilation of hedgehog-antihedgehog defects in confined nematic liquid crystals using Brownian molecular dynamics simulations. After the collision, merging of defects, and building a loop disclination structure, system can experience a structural transition into another nematic structure, triggered by a nucleation of loop disclination structure. In our rough theoretical approach we calculate the size of the emerged loop structure as the function of the typical size of the confining cavity. Attention is paid also to the dynamics of the loop structure after collision.

Line defects and temperature effects in liquid crystal tunable planar Bragg gratings

Liquid crystal tunable planar Bragg Gratings produced by Direct UV Writing are capable of wavelength tuning of over 100GHz. However, such devices exhibit non-linear tuning curves with threshold points and hysteresis. We show that these effects are due to the formation of disclination structures in the liquid crystal and discuss the role of electrode defects and sample temperature on wavelength tuning.

Stability and electronic structure of BN negative disclination

We investigate, using first-principles calculations, the stability and electronic structure of disclinations, in nitride boron monolayers, with angle of θ = - 60 ∘ , called saddle-like compounds. Such structures present antiphase boundaries (APBs) that cause a reduction of the work function of the nanosaddle, relative to the BN bulk value, by as much as 3.5 eV . Our results also indicate that nanosaddles with a lower number of B–B and N–N are the most stable among the studied structures. In addition, we find that APBs with carbon can enhance their stability.

Texture modeling in carbon–carbon composites based on mesophase precursor matrices

A computational modeling study of texture formation in carbon–carbon composites based on carbon fibers and carbonaceous mesophase precursors is presented. The modeling predictions on texture formation and disclination structures are quantitatively validated with extensive experimental data. The number and type of disclinations displayed by the carbonaceous mesophase matrix is shown to be governed by the elasticity of the mesophase, the carbon fiber–mesophase interfacial energy, the size of the fibers, and positional arrangement of the fibers. The simulations provide new insights on the fundamental principles that govern texturing and disclination nucleation, and on how to control the structure of carbon–carbon composites through fiber concentration, fiber cross-section, and fiber–matrix interaction.

Elastic constant anisotropy and disclination interaction in nematic polymers II. Effect of disclination interaction

A new tensorial approach introducing unequal elastic constants in a two-dimensional lattice model is applied to simulate the influence of disclination interaction on the structures of half-wedge disclinations, for the understanding of the variation of apparent elastic anisotropy ϵ measured from different disclinations in a texture (see preceding paper, part I [1]). The apparently random variation of ϵ implies that disclination interaction has a strong effect on the structure of disclinations; as elastic anisotropy increases, its impact becomes overpowering. Nevertheless the disclination interaction still acts in the same way as in the case of equal elastic constants. Analysis of the free energy of disclination pairs shows that the structures of both +1/2 and −1/2 disclinations are changed under the influence of a neighbouring disclination, but in different ways. For +1/2 disclination, the splay and bend distortions vary with the relative orientation of its −1/2 neighbour. Meanwhile, for −1/2 disclination, the bend and splay distortions are remarkably insensitive to the relative orientation of its +1/2 disclination neighbour.

Disclination Models of Misorientation Band Generation and Propagation

The generation and propagation of misorientation bands in polycrystalline metals under large deformation are theoretically investigated and discussed. Disclination models are proposed to describe the generation of misorientation bands at grain boundary kinks and junctions. The models consider edge disclination dipole and quadrupole configurations and predict the value for the critical external shear stress τg, above which nucleation of misorientation bands takes place. The numerical estimates for τg give values of G/1000 – G/400 (G being the shear modulus), which correspond to the level of the deforming stress observed in the materials with fine grain structure. The critical stress τg is shown to be strongly dependent on the geometry and strengths of initial disclinations at grain boundary faults. The further development of the disclination structure at an isolated grain boundary fault demonstrates two main regimes of misorientation band development: stable and unstable propagation. The transition between the two regimes is controlled by another critical value for external stress τp which is several times higher than τg. To better understand the cooperative behavior of dislocations and disclinations in metals at late stages of plastic deformation, computer simulation of dislocation–disclination interaction has also been performed, using a 2D dislocation–disclination dynamics code. Some preliminary results of these calculations are reported and discussed.